HK1182147A - Ultra low mass transport systems for diffusion furnaces employing anti-sag arresters and side wall heaters - Google Patents

Description

Anti-sag braking device and ultra-low mass transport system of side wall heater diffusion furnace

Technical Field

The present invention relates to a continuous conveyor, multi-lane (multi-lane) diffusion furnace for processing solar cell wafers by heating using radiation resistance or/and IR lamps in the range of 700-. More particularly, the present invention relates to a solar cell diffusion furnace having one or more heating zones and an ultra low mass, low friction, transverse wire suspended alumina tube transport system, plus longitudinal sidewall radiant heaters and anti-sag brake assemblies.

Background

The fabrication of silicon-based solar cells requires the use of a "wire saw" to cut thin slices of silicon laterally from a silicon ingot to form rough solar cell wafers. These wafers (whether formed from single crystals or from multiple crystals bonded together) are then processed to form smooth wafers having a thickness in the range of 140 to 330 microns. Due to the scarcity of suitable silicon, the trend is to make the wafers thinner, typically 140-180 microns thick.

The finished raw wafer (raw wafer) is subsequently processed into functional solar cells capable of generating electricity by the photovoltaic effect. The wafer processing includes: a two-stage process called diffusion, which produces a semiconducting "p-n" junction diode; a third process follows in which silver and aluminum-based paste coatings are screen printed onto the front and back surfaces of the wafer, respectively, and then a grid of p-n junctions and a back contact layer are fired, where these layers act as ohmic collectors and ground, respectively.

The diffusion process generally includes two stages: the first stage is to apply (coat) one or more types of dopant materials to the front and/or back side of the wafer and to dry these materials, and then the second stage is to heat (bake) the coated wafer in a diffusion furnace, chamber or heated zone to diffuse the dopant composition into the silicon (or other advanced material) wafer matrix to form a p-n junction layer or back contact layer. The present invention relates to improved diffusion furnace and firing processes, processes and thermal profiles.

In the presence of various phosphorus (P) or boron (B) sources, diffusion occurs at high temperatures. Phosphorus is used to create a p-type junction on the top surface of the wafer, while boron is used to create an n-type junction on the back surface. Phosphorus doped silicon (P-dopedSi) forms the "emitter" layer of a photovoltaic cell, i.e., the layer that emits electrons when exposed to sunlight (a conventional photon source). These electrons are collected by a fine mesh of screen-printed metal contacts, as described above, which are sintered into the surface of the cell by a metallization furnace.

Phosphorus is driven into the wafer by a high temperature diffusion process. Current processes typically take 20-30 minutes. Additional "electroactive" phosphorus allows the formation of low resistance contacts. In co-diffusion, a boron compound is applied to the back surface of the wafer and a phosphorous compound is applied to the top surface of the wafer. The wafer is heated in a single firing to co-diffuse both boron and phosphorus into its corresponding bottom and top surfaces simultaneously.

After diffusion and various cleaning, laser edge ablation, and etching processes are performed to remove unwanted semiconductor junctions from both sides of the wafer, the wafer is coated with an anti-reflective coating (ARC), typically silicon nitride (SiN 3), typically by Plasma Enhanced Chemical Vapor Deposition (PECVD). After ARC coating, the cell appears dark blue in surface color (or brown depending on the coating material used). The ARC minimizes reflection of incident photons having a wavelength of about 0.6 microns. Hydrogen embedded in the silicon due to ARC formation happens to act to repair defects, especially in polycrystalline materials. The defects are traps where electron-hole pairs can recombine, reducing cell efficiency or power output.

During the subsequent IR metallization firing, the high temperatures (above 850 ℃) cause hydrogen to diffuse back out of the wafer. Therefore, to prevent the hydrogen from "outgassing" from the wafer, a shorter firing time is required. It is desirable to trap and retain hydrogen in the bulk material (especially in the case of polycrystalline materials).

The present invention relates to an improved diffusion roaster and diffusion process. Currently available IR conveyor furnaces for these diffusion firing processes have long heating chambers with multiple IR lamps spaced substantially uniformly (typically 1.5 "apart) above and below the wafer handling system (wire mesh belt or ceramic roller conveyor). The heated area is insulated from the outside environment by various forms of insulation, most commonly compression-type insulated fiberboard. An Infrared (IR) lamp raises the temperature of the incoming silicon wafer to about 700 to 950 c. This temperature is maintained for the 30 minute duration of the diffusion process, and the wafer is then allowed to cool and transported to the next downstream process operation and equipment.

Currently available diffusion furnaces typically use one of two types of wafer transport systems: 1) a high quality conveyor comprising a plurality of static (non-longitudinally moving) heavy duty (> 350 Kg) solid ceramic rotating rollers; or 2) a moving (longitudinally moving) wire mesh belt used to transport the wafers through the firing zone of the furnace. In order to minimize or prevent metal contamination of the back surface of the wafer, a static ceramic rotating roller furnace is currently preferred. These furnaces typically have a working width of 1 to 2 meters, allowing wafers to be placed side by side on a belt or roller conveyor, and are therefore referred to as "multi-lane" furnaces. A typical conventional diffusion furnace is about 400 "long with 160 IR lamps 36" wide placed above the rolls and 100 to 160 lamps placed below the rolls.

In these high quality static solid rotary roller conveyor furnaces, the IR lamps are used to raise the temperature of the furnace chamber to a diffusion temperature in the range of 700 ℃ to 950 ℃ for a long period of time. The principle of operation is obviously the following: the IR lamp beneath the roller keeps the roller hot and the contact of the wafer with the roller helps transfer heat to the wafer by conduction through thermal contact. There is a significant thermal profile along the conveyor that rises at the inlet and falls at the outlet.

As for the wire mesh belt conveyor, the belt must be supported in the middle with a quartz tube to prevent the belt from sagging and to provide a low friction sliding surface. Since the tubes cover the underside of the wafer, the tubes are arranged in a chevron pattern in order to prevent "cold" stripes from appearing on the wafer. However, the price of the tube is high, which increases the mass of the conveyor system and hinders access to the lower heating zones of the furnace.

Thus, the state of the art solid rotary ceramic roller conveyors or wire mesh belt furnaces fail to meet the need to speed up production and increase throughput while controlling capital costs. To compensate, the oven is made laterally wider so that multiple lanes of wafers can be processed in each oven zone. This in turn requires longer, more expensive lamps, which typically have a substantially shorter mean time to failure, thereby significantly increasing operating costs. Also, it is currently not possible to increase lamp power because higher output can cause lamp elements to overheat, which is caused by the thermal mass of the furnace (mainly in high quality solid ceramic roller conveyor systems). To prevent overheating, thermocouples are used to reduce the power density, but this results in substantial changes in the spectral output emitted by the IR lamp (lower luminous flux and energy output). The reduced luminous flux results in a need to either reduce the conveyor belt speed or extend the furnace length (while maintaining the original belt speed), thus slowing the process.

Therefore, there are unmet needs in the field of diffusion furnaces and diffusion firing processes as follows: significantly improves the net efficient use of the firing area, provides better control and thermal profile of the entire furnace, allows improved utilization of firing energy, improves the speed and uniformity of the diffusion process, reduces the length of the furnace while maintaining or improving production capacity, and achieves these goals with reduced furnace footprint and lower energy, operating and maintenance costs.

Disclosure of Invention

These needs in the art are met by the present invention, which is directed to a multi-region solar cell diffusion furnace having an ultra-low mass mobile transport system for transporting wafers through a plurality of heating and cooling regions comprising at least one entry baffle region, an elevated region, followed by one or more diffusion soaking regions and optionally a diffusion firing region downstream, and one or more cooling regions for front and/or back diffusion or/and co-diffusion of phosphorus or/and boron dopants to form a p-type or n-type junction and/or back contact layer in the wafer matrix.

In particular, the present invention relates to an ultra-low mass active transport system for a wafer baking furnace that includes an anti-sag brake and a sidewall heater to ensure uniform heating of wafers traversing along the outer lane of a multi-lane furnace.

As disclosed herein by way of example, the transport system of the present invention is illustrated in two alternative embodiments: A) a belt/pin drive system; and B) a roller chain/sprocket drive system, with a roller chain/sprocket drive system being the currently preferred embodiment. In both embodiments, the wafers are supported during longitudinal passage through the processing zone on a non-rotating, small diameter, hollow refractory tube carried on a suspended wire or rod spanning the width of the transport system. In a belt/pin drive system, the opposite ends of the wire are carried by brackets formed from or attached to a drive belt. In a roller chain/sprocket drive system, the ends of the wires are mounted in hollow tubes of the link pivots.

In a presently preferred heating system, laterally oriented IR lamps are used in the upper heating zone, while IR lamps in combination with resistive heating are used in the lower heating zone. To improve lateral heating uniformity across the width of the furnace, the resistive heaters are placed adjacent two longitudinal side walls oriented parallel to and spaced from the centerline of the furnace, while the IR lamps are placed laterally above or below, preferably below, the longitudinal side wall resistive heating elements. The longitudinal sidewall resistance heaters are arranged in a quartz tube, a ceramic tube or a stabilized SiC tube, said tubes being arranged immediately below the conveyor. In a 5 lane or wider furnace, the heaters are used to maintain the wafers in rows 1 and 5 adjacent to the corresponding sidewalls at the appropriate diffusion temperatures. This is particularly applicable to co-diffusion processing.

An advantage of using a high intensity IR lamp isolation module is that it provides short wavelength, high flux IR light modulation, thereby facilitating faster diffusion. In the present invention, it is understood that where a high intensity IR lamp (HI-IR) is mentioned, a SiC radiant/re-radiant heating element may be used, the disclosure of the HI-IR lamp element being by way of example only.

The transport of advanced material solar cell wafers (e.g., silicon, selenium, germanium, or gallium-based solar cell wafers) is carried out in the furnace region by using an ultra-low mass, mobile (longitudinally moving), sheltered transport system comprising two or more continuous rings of laterally spaced transport elements comprising narrow width "bands" on each side of the wafer processing travel path, the bands carrying lightweight, small diameter, non-rotating, refractory tubes suspended on wires tied between the bands. The refractory tube is a thin-walled, hard ceramic or vitreous material, preferably at least one selected from alumina, silica, zirconia.

The conveyor system "belt" is implemented in a number of exemplary embodiments, the first being a laterally spaced metal, horizontally oriented flat belt or ribbon, each having a plurality of vertically extending supports spaced longitudinally along the belt. The support bears a metal wire, and a fire-resistant pipe penetrates through the metal wire. The wires extend laterally across the wafer travel path between mating pairs of supports, one on each belt. In a second presently preferred embodiment, each belt is a roller chain, e.g., a firm bicycle-type chain, having hollow tubes rather than solid link members. A refractory tube suspension wire is threaded through the link tube and the ends of the wire are supported in the link tube. In both embodiments, the transport elements or "belts" are driven synchronously by a drive system described in detail below. Synchronizing the movement of the belts keeps the wires carrying the refractory tubes parallel to each other and straight, i.e., orthogonal to the direction of travel of the wafers along the processing path. Suitable alignment rollers or guides may be used.

Importantly, the ultra low mass conveyor system of the present invention has a significant advantage in that it does not require the use of conveyor support rods and therefore does not substantially shield the bottom surface of the wafer, which makes possible efficient co-diffusion at higher transport speeds or shorter furnace lengths.

In both embodiments, the wafers are passed sequentially through several zones of the diffusion furnace while resting on annular supports spaced along the refractory tube, which results in less contamination. The pedestals can have a variety of external profile configurations, for example, conical, circular (annular), vertical sharp edge, beveled, double conical, square-topped, fins, ribs, combinations of these shapes, and the like. The refractory material is preferably selected from high temperature ceramic or vitreous materials that can be precisely configured by casting, dry pressing, extrusion or machining, and preferably comprises at least one of silica (including silica glass), alumina and zirconia.

The configuration of the elevated regions and/or firing regions is not critical to the apparatus or method of operation of the present invention, and there are various arrangements of IR lamps or resistive heating elements. For example, in the first embodiment, where the width of the furnace is narrow (narrow transport system), or where only the top surface is doped with phosphorus, all heating can be carried out in all zones using HI-IR radiant flux lamps. In these process applications, an HI-IR zone is optionally used that employs an IR lamp isolation module to raise the temperature to the preferred soaking temperature. In other embodiments, several zones including a boost (buffer) zone and a soaking zone may be separate IR lamp heated zones.

In a second embodiment, in the case of a furnace with a wide process path (a transport system of about 1 to 2 meters wide), or in a process in which the front side is doped with boron or with phosphorus and the back side is doped with boron (where the temperature is above 950 ℃, for example in the range of 1000-.

In any heating element configuration embodiment, the heating elements may be mixed, i.e., a separate IR lamp, HI-IR radiant flux isolation module, and resistive heat radiating/re-radiating element may be used. For example, the input boost (baffle) zone may be a stand-alone IR lamp, followed by an optional first HI-IR flux isolation module, followed by a refractory radiation/re-radiation element in the second firing zone and soaking zone. The soaking region may be merely an extension of the firing region. Also, the fired region may be an extension of the buffer or elevated region. That is, the name of the region is not a determining factor, and the selection of the type, location, and number of components above and below the conveyor system is made to achieve the desired firing temperature and process progress (phosphorus diffusion, boron diffusion, or both).

In the low mass transport system of the present invention, the wafer does not touch the wire mesh belt or the ceramic roller but is supported on the ceramic support, so there is no metal contamination, no hot spots are formed in the wafer, and the wafer is not biased to one side or the other as in the conventional roller conveyor system. In addition, the diffusion process of the present invention is a high radiant flux driven process, rather than a thermally conductive, longer wavelength process.

In a first embodiment of the wafer handling system, the side belt of the handling system includes drive holes that are evenly spaced longitudinally. Each belt is configured as an endless loop comprising a transport section (for forward movement, through the processing area) and a return section. The belt loops are synchronously driven by one or more pin drive rollers at the exit end of the oven.

The strip is typically a high temperature resistant metal, such as a member of the austenitic nickel based superalloy family, with a suitable embodiment being a nickel chromium alloy, i.e., a nickel/chromium alloy of 80/20. Other strips include titanium, inconel (e.g., inconel type 600), or other high temperature alloys. The belt slides in shielded channels on each side of the furnace zone, the channel members being constructed of alumina, silica, quartz or other high temperature, low friction ceramic material to minimize heating of the belt. The belt is optionally cooled by impinging ambient air or cooled compressed air onto the belt.

In the second transport embodiment, a roller chain is used instead of the belt, and a vertical stand is not required. Instead, each end of the suspension wires is mounted in a tubular bushing pivot member of the side link. Each side chain slides in a groove or channel in a low friction high temperature ceramic material slide or rides on a guide ridge on the slide and acts as a guide to maintain linear tracking and proper alignment of the chain. The chain is driven by a sprocket drive system (two laterally spaced drive sprockets on the same drive shaft) located below the exit or entrance end of the diffusion furnace. With the sprocket drive located below the entrance end of the return path, a redirecting steel idler sprocket or flanged wheel is appropriately positioned to provide a pull drive system. At the front end of the furnace, a second diverting idler, spaced laterally, diverts the chain up onto an inlet idler that redirects the chain back onto the process path, forming a loop. The drive is optionally positioned anywhere between the inlet and outlet ends of the furnace using suitable redirecting idlers and/or rollers. The chain may be cooled in a cooling section of the furnace and/or on a return path, preferably by induced ventilation air or compressed air.

For high intensity IR flux heating element embodiments (used downstream of the buffer and/or elevation region), the HI-IR region rapidly (within about 2 seconds) raises the temperature to a diffusion process set point in the range of about 700 ℃ to about 950 ℃, while the surface of the dopant-coated wafer is light-conditioned by irradiating the surface with a high intensity short wavelength IR radiation flux to make the diffusion rate faster. This embodiment utilizes the short wavelength IR radiant flux in the diffusion process, which results in a half or more reduction in diffusion processing time, resulting in doubling or greater throughput, as compared to the long wavelength radiation process. As an example, in operation, the furnace of the present invention can complete the diffusion process in 6 minutes, while the speed of the current conventional process is 12 to 14 minutes. Thus, the production capacity is doubled or greater. In addition, the resistivity of the cell p-n junction layer is not only more uniform across the wafer and consistent from wafer to wafer, but is also in the "sweet spot" of between 45-100 ohms/cm.

An important aspect of the rapid diffusion process of the present invention is that where IR lamps are used, the IR lamps are operated at substantially higher power (percentage of maximum lamp power rating) than conventional furnaces. In a first embodiment, the controller uses an empirically based algorithm to adjust the power of the lamps in each zone and top and bottom, by voltage control only, according to a preset desired temperature. In a second embodiment, the temperature in each corresponding zone is monitored by a thermocouple and the lamp voltage is adjusted by a feedback loop control algorithm. In addition, the voltage is monitored to ensure that excessive voltage is not supplied to the lamp in the event of thermocouple failure.

In a conventional static rotary roller furnace, the lamps are operated at 5-20% power, and therefore the lamps are characterized by: if no effective light conditioning is performed, the flux is lower and the wavelength is longer.

In conventional metal mesh belt furnaces, the main problem is metal ion contamination of the wafers, because the wafers are located directly on the metal mesh or on metal "spots" incorporated into the belt. Even with the use of ceramic bead coatings on metal mesh ribbons, metal ion clouds can still spill out of the metal mesh of the ribbon, adversely affecting the chemistry of the solar cell layer. Attempts have been made to coat the bottom of the wafer with phosphorous dopant to reduce metal contamination from the metal mesh strip. However, this results in the formation of a p-n junction layer on the bottom of the wafer. This in turn requires an additional process step to etch away the bottom p-n junction layer. The etching step is typically a batch process, which takes additional time, thus slowing production rates.

The low quality ceramic tube transport system of the present invention addresses and solves these problems. First, metal carrying components (belts or chains) are placed on the sides of the furnace area and these components are configured to be shielded from the elements radiating [ heat ], which extends the component life. Second, the transverse wires for transport are fully shielded in a low quality ceramic tube, which is a non-rotating ceramic support with minimal contact with the wafer. The shielded drive belt or chain element in combination with the wafer support wire shielded by the ceramic tube ensures a clean atmosphere so that the firing area is substantially free of metal ion contamination. Third, the ceramic tube as a whole is much lower in mass than the roller and, although non-rotating, is still mobile, i.e. moving into and out of the furnace, so there is no large static thermal mass, which requires power reduction. In addition, because the ceramic tube is suspended from the wire, if the ceramic tube breaks vertically, the ceramic tube remains on the wire and does not need to be immediately shut down for replacement. In contrast, in a solid roll furnace, when the roll breaks, the furnace must be stopped. Finally, the wafer is not in contact with the ceramic tube, and is raised above the ceramic tube on a ceramic rib pedestal, which is preferably configured to support the wafer only at the edge.

In contrast to the 5-20% operating power rating of the conventional currently available thermal diffusion furnaces described above, process embodiments of the present invention use IR lamps operating at 40-70% power or more, as a result of which the lamp IR flux in the system of the present invention is substantially higher and the peak remains in the short IR range (below about 1.4 microns), with the IR wavelength peak typically being about 1.25 microns. The relative flux intensity produced by the IR lamp according to the process of the present invention is about 4 to 5 times greater than in the conventional thermal diffusion furnace described above. Thermal decay in the two lateral lanes (e.g., lanes 1 and 5) is avoided by using longitudinal resistive heating elements enclosed in quartz, ceramic, or stable SiC tubes.

As noted above, the furnace of the present invention in some embodiments employs HI-IR lamp modules, optionally including isolation type modules, in the elevated or/and HI-IR furnace regions. The module includes an insulating reflector element having parallel transverse (transverse to the direction of transport) cooling/reflector channels, one or more IR lamps being centered in each of the channels. The channel may optionally be covered by an IR transparent transmission window, e.g., quartz, Vicor, Pyrex, Robax, other high temperature glasses, synthetic sapphire, etc. High intensity multi-IR lamp isolation modules are arranged facing and spaced apart from each other, one module above the furnace conveyor transport system and one module optionally below the system, to define a selected IR lamp heating process firing zone between the modules, with module lamps and cooling air passages isolated from the zone. In the case of the use of the window, the channel is preferably open at its opposite ends for the entry and/or exit of a cooling air flow. Through the manifold, the cooling gas is introduced at least at one end of each channel and is exhausted at the other end or intermediate the two ends.

The transmission plate window prevents high pressure/high velocity lamp cooling air/gas from entering and interfering with the process zone through which the transport system carrying the silicon wafer is passed, while at the same time allowing the use of large volumes of cooling gas to maintain adequate cooling of the lamp quartz and the glass/quartz transmission plate. By isolation and cooling, the present invention allows the IR lamp to operate at power levels that would normally soften and buckle the lamp envelope, which would shorten the useful life.

In another embodiment, the isolation module comprises spaced IR lamps with or without channel reflectors and a Robax-type glass window spaced below the lamps, so that the windows (above and below) isolate the transporter (carrying the wafer) from the space of the lamps.

This isolation geometry, coupled with optional cooling of the IR lamp of the invention, allows the power of the lamp to be increased from current standards of 15-20% power density to the range of 40-70% or higher. This results in an increase in the heating rate in the boost and HI-IR firing zone from about 30 deg.c/sec (conventional furnace) to about 80-150 deg.c/sec using a conventional 100 watt/inch IR lamp. This effectively increases the heating rate by a factor of 2 to 4 over that of a conventional furnace without causing the lamp to be turned off, shut down or distorted. In addition, the lamp isolation/optional cooling system of the present invention allows for increased conveyor belt speeds. This results in a substantial increase in throughput or allows the length of the furnace to be shortened (with the same throughput), which reduces the footprint of the furnace.

Heat is removed from the soaking region in order to maintain a high flux for phosphorus diffusion by high lamp power density in the region to achieve high speed processing. The cooling air flow is directed from top to bottom in the soak zone to inhibit the deposition of particles on the top surface of the wafer and to remove the particles. Thus, in contrast to high quality static ceramic rotating roll conveyor systems (where the processing method does not remove heat from the processing region), in the system and process of the present invention, heat is removed so that high power density can be maintained and high flux, short wavelength IR wafer light modulation is allowed, thereby speeding up the process. While heat removal seems counterintuitive, high flux, short wavelength IR does not merely compensate for heat removal.

The power of the heating elements (whether IR lamps or resistive (SiC) radiating elements, top or bottom) is adjusted individually or in groups to achieve precise temperature gradient control in each zone. Temperature control can be achieved using thermocouple based temperature regulation, voltage controlled power regulation or hybrid systems, employing PID controllers as described above. The power regulation of the lamp is preferably voltage controlled, as this allows for facilitating the maintenance of a stable lamp power to obtain a preferably high IR intensity (radiant flux) value and an always constant spectral output. In addition, operating the lamp at higher power densities increases the IR flux and also provides a better spectral range, with the peak in place.

With respect to the co-diffusion process utilizing the transport system of the present invention, an exemplary process line is configured as follows: 1) orienting the wafer with the bottom surface facing upward; 2) applying boron dopant to an upwardly facing bottom surface of the wafer and drying the wafer; 3) flip-chip the wafer (e.g., with a rotary flip-chip) so that the top surface of the wafer is now facing up; 4) applying a phosphorous dopant to a top surface of the wafer and drying the wafer; 5) conveying the co-doped wafer into the diffusion furnace; 6) the wafers are co-fired in the furnace at once, allowed to cool and transported to downstream ARC, slurry screen printing, drying and metallization operations. Since the low mass transport system of the present invention does not significantly mask the bottom surface of the wafer, the bottom surface is properly fired so that boron diffusion occurs for subsequent formation of the back contact layer.

A transport failure protection system: as an option for the furnace, the lower half of the furnace may be provided with a droop stop assembly to prevent excessive sag of the suspended wires or rods carrying the ceramic tubes and supports in the event of failure, for example, due to wire breakage or elongation under stress or heat. In a presently preferred embodiment, the sag stopper assembly comprises a slide member, e.g., selected from the group consisting of quartz tubes, rods, or strips, aligned parallel to a centerline of the conveyor transport path that coincides with the longitudinal centerline of the furnace heating zone. The catcher slide members are spaced inwardly from the side walls of the oven chamber by a distance that aligns them with and is spaced below the path of travel of the pedestal. Each longitudinal end of the slider part of the arresting device is mounted, for example, in a transverse wall of the furnace firing zone so that the top of the slider part is above the top of the transverse wall. Thus, if the conveyor wire and tube assembly sags, at this time the pedestal will come into contact with the slide member, slide along the slide member as the conveyance progresses, and jump out of the lateral wall of the furnace without sagging and breaking, or otherwise damaging the conveyor system. For example, the spacing between the top of the stop slider member and the bottom of the support may be about 1 to 2 centimeters. In addition, because the stop slider member is spaced in alignment with the support path, the stop slider member does not cover the wafer, but is spaced from the side of the wafer.

Heating the side wall of the outer wire:

in a presently preferred heating system, laterally oriented IR lamps are used in the upper heating zone, while IR lamps in combination with resistive heating are used in the lower heating zone. To improve lateral heating uniformity across the width of the furnace, the resistive heaters are placed adjacent two longitudinal side walls oriented parallel to and spaced from the centerline of the furnace, while the IR lamps are placed laterally above or below, preferably below, the longitudinal side wall resistive heating elements. The longitudinal sidewall resistance heaters are arranged in a quartz tube, a ceramic tube or a stabilized SiC tube, said tubes being arranged immediately below the conveyor. In a 5 lane or wider furnace, the heaters are used to maintain the wafers in lanes 1 and 5 adjacent to the corresponding sidewalls at the appropriate diffusion temperature. This is particularly applicable to co-diffusion processing.

The side wall heater is used to trim the furnace temperature adjacent the side wall because the IR lamp heating element has not yet reached the side wall to prevent the lamp from burning out. If the IR lamp heating element extends into the sidewall insulation, the insulation may cause the local temperature of the lamp to exceed safe operating values. The resistive longitudinal sidewall heater does not emit light as does the primary transverse IR lamp, but operates at a lower power level to adjust the temperature of the region near the sidewall, i.e., the edge temperature, high enough to compensate for the power lost at the lamp ends and any loss through the sidewall insulation to the outside of the furnace.

Due to the lower power operation, the longitudinal heaters can be placed above the level of the main IR lamps and close to the side walls, typically at a spacing of about 1 to 5 cm. In the case of a diffusion furnace using the low mass transport system of the present invention, the longitudinal heater may also be placed slightly below the conveyor plane, approximately 1 to 3 cm below the level of the ceramic tube comprising the wafer transport support elements.

Exemplary sidewall heating elements include a coiled electrical resistance heating element enclosed in an inconel jacket and insulated from the jacket by MgO powder. Subsequently inserting the element into a quartz tube, which may be transparent, translucent or opaque; the carrier tube transparency is not important because the element may not need to "glow" in or near the visible spectrum (depending on the required output power). In addition, an important safety feature provided by the use of a carrier tube is that the tube acts as an electrical shield or insulator to avoid electrical shock. Other types of heating elements may be used as the longitudinal sidewall heating elements, including simple nichrome wire air coils of 1 to 3 kilowatts of power introduced into the carrier tube. The sidewall heating elements may be controlled by standard SCR controllers.

Drawings

FIG. 1 is a schematic diagram of a first embodiment of a diffusion furnace of the present invention, depicting in side view an inlet transition zone, baffle zone or/and elevated zone, optional firing zone, at least one soaking zone, cooling zone, and low mass transport system;

FIG. 2 is an isometric view of the low mass transport system of the present invention (in this embodiment a belt-type transport) installed in several heating zones of a diffusion furnace with the front inlet end of the furnace to the right, also showing the wafers on alumina support tubes;

FIG. 3A is an isometric enlarged view of the belt transport embodiment of FIG. 2 in the area of the inlet baffle and optional HI-IR lamp, showing transport of two exemplary wafers through the furnace;

FIG. 3B is a bottom isometric view of the lower section showing the exhaust manifold for the soaking zone;

FIG. 4A is an enlarged isometric view of the belt transport exploded from FIG. 3A to show detail;

FIG. 4B is an enlarged isometric view of a carriage assembly and a bent wire tip holder for a belt carrying embodiment;

FIG. 5 is a schematic side view of the furnace of the present invention, with a temperature versus time profile developed in the corresponding zone below;

FIG. 6 graphically illustrates that spectral output is critical to the speed of the IR lamp heated diffusion process, where FIG. 6A depicts the spectral output of the low quality transport system of the present invention in relative intensity versus wavelength, while FIG. 6B depicts the comparative spectral output of the high quality solid ceramic roller system in the same relative intensity versus wavelength;

FIG. 7 is a schematic view of a second embodiment of the diffusion furnace of the present invention showing several processing zones and a chain driven wafer transport system;

FIG. 8 is an isometric enlarged view of the chain transport system in the furnace processing area, showing the transport of two exemplary wafers through the furnace;

FIG. 9 is an enlarged isometric view of the chain drive details showing how the suspension wires and ceramic tubes are installed in the hollow chain tube;

FIG. 10A is a vertical cross-sectional view through line 10A/B-10A/B of FIG. 9 showing a first embodiment of a slider plate guide, in this embodiment a channel, for a chain drive;

FIG. 10B is a vertical cross-sectional view through line 10A/B-10A/B of FIG. 9 showing a second embodiment of a slider plate guide for the chain drive, in this embodiment a rib;

FIG. 11A is a vertical cross-sectional view through a suspension wire and an alumina tube showing a third embodiment of a wafer support;

FIG. 11B is a vertical cross-sectional view through a suspension wire and an alumina tube showing a fourth embodiment of a wafer support configuration;

FIG. 12A is a transverse elevational view showing the spatial position and arrangement of the drop brake shoe components relative to the transport ceramic tube, carrier and wire assembly;

FIG. 12B is an isometric view of the diffusion furnace bottom section of the present invention equipped with a pair of drop stop blocks;

FIG. 13 is a schematic view of a process line for co-diffusing both boron and phosphorus into a wafer, such co-diffusion being part of the processing of a silicon wafer into a solar cell;

FIG. 14 is a horizontal schematic of a furnace employing the conveyor system of the present invention and employing longitudinal side wall resistance heaters in the lower motion of the soaking zone;

FIG. 15 is an alternative exemplary embodiment of the oven of the present invention employing longitudinal sidewall heaters in the upper and lower zones; and

fig. 16 is an isometric view of the furnace system of the present invention showing the placement of longitudinal side wall heaters above the IR lamps in an exemplary soaking zone.

Detailed Description

The following detailed description illustrates the invention by way of example and not by way of limitation as to the scope, equivalents, or principles of the invention. This description will clearly enable one skilled in the art to make and use the invention, and describes several embodiments, adaptations, variations, alternatives and uses of the invention, including what is presently believed to be a commercial embodiment.

Fig. 1 is a schematic view of a diffusion furnace 10 of the present invention, the diffusion furnace 10 including a frame and enclosure 12 having a lower section 14 and an upper section 16, the frame 12 optionally being configured with linear brake lifters spaced along the exterior (see fig. 2 and 3) for raising the upper section 16 relative to the lower section to allow for maintenance of the lamp assembly and transport system. The furnace 10 includes a plurality of sections or zones, from an inlet or front end 18 (in this figure, the left side) to an outlet or rear (downstream) end 20 (the right side), in that order:

IT, i.e., the inlet delivery end 18 from an upstream dopant applicator unit (not shown);

b-1, i.e., the inlet baffle area, which uses one or more compressed air knife assemblies 22; the B-l region optionally includes an elevated region containing one or more heating elements (not shown) for elevating the temperature of the wafer from ambient to about 500 ℃;

FZ, i.e., the initial firing zone, for raising the wafer temperature to a diffusion temperature in the range of about 900 ℃ to about 1100 ℃, depending on whether phosphorus, boron, or both, are diffused; FZ can be further subdivided into two or more regions, e.g.;

HI-IR region, i.e., an array of high intensity IR lamps, preferably upper or/and lower isolation reflector lamp assemblies 24-U, 24-L, for obtaining temperatures up to about 950 ℃, then;

HTZ, i.e., a high temperature region, which is heated with a resistive radiation (SiC) element 27 for obtaining temperatures up to 1100 ℃, depending on whether phosphorus, boron, or both are diffused;

s, i.e., a soaking region, having spaced apart upper and lower IR lamps or resistive elements 26-U, 26-L;

b-2, i.e., the outlet baffle area, with an air knife assembly 22;

c, i.e., a cooling region, typically without resistive elements or IR lamps; and

OT, i.e., the exit transfer area, for transferring the diffusion fired wafer to a processing equipment for screen printing the collector fingers and bus bars on the front side and the back contact layer (not shown) on the back side, followed by firing to form ohmic contacts. The exit transport zone may optionally include an upper or/and lower air knife assembly adjacent to the furnace exit (right side).

Upstream and downstream of the oven of the present invention are cryogenic conveyor belts 28-U (upstream) and 28-D (downstream) which are connected to an antireflective layer applicator, a screen printer, and then a metallization oven. These cryogenic conveyor belts 28-U and 28-D are connected to the drive system 30 of the oven 10 of the present invention.

In a first embodiment, the low mass drive system 30 of the furnace of the present invention comprises: a pin drive roller 32 (driven by a motor 34 and chain or belt 36 disposed at the exit (right) end of the furnace), a wafer transport belt assembly 38, an idler roller 40, and a tension system 44 including a tension roller 42. The tension system 44 includes an automatic tension compensator spring that acts as a buffer to help prevent loosening. It should be noted that with this drive geometry, the tape 38 is pulled through the various zones from left to right along the feed path F.

The furnace of the present invention comprises: a plurality of plenums that define a region interior to the housing 12; and a plurality of air manifolds comprising inlets and outlets for ambient or pressurized air flow into the respective zones, as indicated by arrows I (inlets) and E (outlets), in order to maintain the appropriate temperature in the respective zones. In addition, pressurized air is provided in channels of the reflector bodies that isolate the lamp assemblies 24-U and 24-L (where used) to cool the lamps disposed in the channels. The surface of each of the high intensity lamp assemblies 24-U and 24-L is covered by a piece of clear quartz to seal the reflector channel from the wafer undergoing light conditioning in the HI-IR region. This lamp cooling allows the lamp to operate at about 60% to 100% of maximum power, which is much higher than currently available competing units. This provides immediate thermal elevation to the wafer, as well as high intensity IR illumination, peak diffusion modulation temperature from ambient temperature to 700 to 950 ℃ (phosphorous diffusion), and high intensity light modulation.

The scale of fig. 1 is such that solid thermal insulation 46 cannot be depicted at every location, but those skilled in the art will understand that the housing includes the necessary blocks of insulation configuration. A reverse flow of air (depicted by arrows 48) is optionally provided through the insulation entry region, which is opposite the heat flow. The air upon entry extracts heat from the insulation and recirculates the heat into the zone, which is a highly efficient heat exchange operation. It should be noted that the lamps 26-U and 26-L or resistive elements 27 are illustrated as being staggered to provide a uniform heating and/or IR light field; the lamps 26-U and 26-L or the resistive elements 27 are optionally arranged so as not to be staggered.

Turning now to the low mass transport system 30, fig. 2 depicts the furnace 12 in an isometric view with the exterior panels and insulation removed so as to depict the framework of the lower section 14, the lower section 14 having an entrance end 18 (lower right) and an exit end 20 (upper left). The inlet and outlet conveyors 28-U and 28-D are not shown for clarity. Brackets and lifters 50 are depicted at the four corners of side rails 52 of the oven; these carriages and risers 50 are used to lift the upper section 16 (not shown) from the lower section 14 (not shown) for inspection, adjustment, maintenance and repair/replacement of parts (e.g., lamps, resistive heating elements, conveyor system elements, etc.). The lower section 14 includes side rails 52, sidewall insulation blocks 46-S, and insulation inner region segments 54. In order not to obscure the details of the belt 38, the front insulating block is not shown, which is located at the front of the baffle region B-1 and separates B-1 from the high intensity IR region HI-IR, and at the rear of the cooling region C.

The insulating blocks 56 form the bottom layer of several areas. These floor insulation blocks 56 typically have openings, here slots 88, which, in addition to the plenum (not shown) below the floor, allow hot exhaust air to be drawn by an ID fan (not shown). This air flow removes heat from the various regions, allowing operation of the elements (lamps, SiC rods or coils) to produce higher output, and also allowing extraction of contaminants, since the air flow is from top to bottom. This hot gas/air flow pattern results in a reduced level of contaminants in the furnace area and thus results in cleaner product.

Apertures 58 are shown in the front side rails 52 and the distal side wall insulator blocks 46-S, the apertures 58 being for mounting the resistive elements 27 or/and the high strength light tubes 26 (only one of each element is shown for clarity) and for connecting electrical leads, the elements/lamps spanning the width of the furnace area. A compressed air chamber 60 for the lower high intensity IR lamp zone 24-L feeds compressed air through line 62 into the annulus between the reflector channels and the lamps and exhausts below to the exterior, or into an adjacent downstream soaking zone, as needed.

For clarity, only a portion of the low quality live conveyor system 30 is shown on the right. The side belts 38 engage the pins in spaced idler rollers 40 of the drive system 30. The idle roller 40 and the drive roller 32 below it are only visible inside the cooling zone C. Two wafers W-l and W-2 are shown at the inlet end (right side) and are placed on a lateral alumina tube 64 at a location for transport through the furnace.

FIG. 3A is an enlarged isometric view of the inlet end 18 of the furnace 10 of FIG. 2, showing a portion of the belt transport system 30 carrying two wafers W-l and W-2. The two spaced belts 38 of the conveyor system 30 are disposed in a U-shaped channel 66, the U-shaped channel 66 being formed in the top of the left and right sidewall insulation blocks 46-S. Each belt contains precisely spaced holes 68, which holes 68 engage with pins 90 of idler roller 40. Also regularly spaced along the strip are upstanding brackets 70, said brackets 70 carrying wires 72, on which wires 72 ceramic tubes 64 are placed. An exemplary wire is a 0.080 "diameter nichrome. The standoffs 84 (here rings in this embodiment) are disposed on, mounted to, or formed in the tube 64 and are spaced laterally along the tube. During transport through the furnace 10, the wafers are positioned on the susceptor ring so that there is little contact between the back surfaces of the wafers and the transport assembly elements, see footprint 96 in fig. 4A.

In the event that tube 64 breaks or tears, wire 72 will hold tube 64 until tube 64 can be replaced. Since the holes 68 in the belts are indexed with the pins 90 in the rollers (drive and idler rollers), the supports of each belt move in parallel aligned relationship so that the alumina tube remains transverse to the direction of wafer feed travel (as indicated by arrow F).

Fig. 3B is a lower isometric view of the lower section 12 of the furnace with the frame removed for clarity. A bottom insulating block 56 spans the side frame 52, the bottom insulating block 56 having vent slots 88 (best seen in fig. 3A). The steel plate 100 is spaced from the bottom of the insulating block 56 and this spacing provides a collector plenum for the hot air. An exhaust manifold assembly 102 is connected to the plenum (the space between 56 and 100) by a collar 104 on the end of a cross tube 106. The other end of the cross tube is connected to a collector duct 108, which duct 108 discharges hot gases from an exhaust or flue pipe 110.

Fig. 4A shows the low mass transit system 30 in an isometric view, and the U-channel assembly 74 in which the belt 38 of this embodiment travels in the U-channel assembly 74. Fig. 4B is a close-up of the single leg assembly of fig. 4A.

Each belt 38 is supported on a quartz slide member 74, the slide member 74 being U-shaped in cross-section and having shorter vertical side walls. A pair of retainer strap members 76 are glued to the top of the U-shaped arms with a high temperature ceramic cement, the retainer strap members 76 being made of quartz, alumina or other high temperature fiber type ceramic material, overlapping sufficiently to physically hold the band in the channel and shield the band from contact with the heating elements, thereby keeping the band cool. The belt is optionally cooled using ambient compressed ambient air.

The bracket 70 includes a vertical leg 80 secured to a vertical tab 78, the tab 78 being formed by a perforation 92 from the strip. The leg 80 and the projection 78 may be secured together by any suitable means, such as by spot welding, riveting with aligned projections and leg holes 94, or screwing. The upper end of the leg 80 carries one or more tabs 82, each tab having a hole into which the refractory wire or rod 72 is inserted. An alumina tube 64 slides over and is carried by the wire. Each tube optionally contains a plurality of laterally spaced support members 84, here rings, with the wafer resting on the support members 84, as shown by the footprint 96 in fig. 4A.

The seat ring 84 may have a variety of cross-sectional configurations ranging from: a simple flat surface circle (as shown in fig. 4A) to a tapered profile, e.g., a profile with a cross-sectional profile that is bell-shaped and curvilinear. The various profiles are seen in fig. 10A, 10B, 11A and 11B. It should be noted that in fig. 4B, the bent tip 86 of the wire 72 fits between the two tabs, thereby locking the wire against lateral movement that would cause the other end (right side) to fall out of the tab of the right bracket. It should be noted that in fig. 4A, the right side of the wire extends sufficiently through the outer tab of its corresponding stent. The use of a wire with a free end may allow the wire length to expand and contract without falling out of the hole in the tab.

Important features of the belt and chain embodiments of the transport system of the present invention are their ultra-low mass, ease of installation, ease of keeping clean, few contact points on the wafer as it is transported through the oven, and ease of maintenance. As shown in fig. 4A, because there are so many wires spaced sufficiently closely to provide support for the wafer, broken tubes and/or wires are typically left to be replaced at scheduled shutdowns, as the loss of one tube or wire does not significantly affect product throughput.

In the furnace embodiment of the present invention, where the wafer is heated extremely quickly (within a few seconds) to within or near the diffusion temperature range, a high intensity IR lamp or isolation module is used adjacent or downstream of the inlet to the furnace of the present invention. The wafers are light conditioned with high intensity short wavelength IR radiant energy that is about 4 to 5 times greater or greater than current roller furnaces, so diffusion proceeds more rapidly. As an example, in the furnace of the present invention, the temperature reaches the diffusion temperature within a few seconds. More importantly, by using an isolation module in the HI-IR region, and because the HI-IR region and soak region components can be powered at higher voltages, the IR flux during the process is higher and diffusion is completed in less than 6 to 8 minutes, which is half to one third of the length of time of current systems.

Fig. 5 and 6 illustrate these principles. FIG. 5 is a side view of the upper portion of a schematic representation of the oven of the present invention, with a temperature versus time profile formed in the corresponding zone below; the dashed curve P is the temperature profile in the furnace of the invention when phosphorus diffusion is performed on the front side by IR lamp heating only to form the P-n junction layer. The solid curves show that boron diffusion in the furnace of the present invention to form the back contact layer occurs at temperatures about 200 c higher and requires heating of at least some regions with resistive elements such as the disclosed SiC rods. It should be noted that the extremely steep profile produced by the IR lamps or resistive elements of the furnace of the present invention, allows the wafer to reach the process temperature of phosphorous (and/or boron) diffusion quickly. A commercially available control solid ceramic roller furnace heated using IR lamps showed a phosphorus diffusion curve that substantially followed the dashed outline, marked "p.a. (prior art) on the graph. Since the rolls are already hot in the oven, the lamps are automatically adjusted to run at lower power (see fig. 6B, below), thus resulting in a substantially and significantly lower temperature profile slope and a longer time to reach temperature, about a few minutes. The most important aspect of this graph of fig. 5 is that, when using the oven of the present invention, the speed of completion of the diffusion is substantially faster (point "D end point" on the ordinate), and the wafer continues to cool and is transferred to screen printing (point "XFER" on the ordinate).

In contrast, the comparative prior art phosphorus diffusion process (dashed line in fig. 5) continues soaking at a lower power setting for a much longer time, as indicated by the arrow pointing to the right on the dashed line. The high intensity IR radiant flux phosphorus diffusion process of the present invention is typically used from 1/2 to 1/3 times of conventional thermal conduction processes. Thus, the throughput is substantially higher and the furnace volume is much smaller (less than 300 "in length and half as wide) compared to a conventional furnace (400" long by 36 "wide) of equal output.

Fig. 6A and 6B graphically illustrate that spectral output is key to improving the speed of the IR diffusion process in the light conditioning, boosting, and HI-IR regions. The spectral output of a lamp varies with the power of the lamp and can be expressed as a percentage of the maximum power capacity of the lamp. FIG. 6A depicts the spectral output curve of the low mass transport system of the present invention in terms of relative intensity versus wavelength. The upper curve is the theoretical maximum T, showing an IR peak of about 1.2 microns and a relative intensity of about 12.5. It should be noted that the visible spectrum VS is on the left, drawn with a dashed line. The lower curve labeled "invention" shows that in the ultra low quality delivery system of the present invention using the HI-IR lamp module, the IR lamp can be operated at about 40-100% of the rated maximum (here shown at about 40-70%) using the lamp voltage control system, and the intensity maximum at the peak is 8.

In contrast, fig. 6B plots comparative spectral output of a high quality solid ceramic roller system with the same relative intensity versus wavelength. In such a control system operating with a thermocouple type thermal monitoring control feedback system, the lamp must be operated at about 20% power. However, the relative intensity decreases exponentially, and at a peak intensity of about 1.8, the peak labeled "PA" shifts closer to 1.75 microns, which is less than one-fourth of the process of the present invention. Shifting to longer wavelengths, lower energy spectral profiles are also important in conventional systems.

Thus, in the system of the present invention, the lamp can be operated at a higher power, resulting in an increase in relative intensity by a factor of 4 to 5. This increased IR intensity applied more quickly to the wafer conditions the wafer to promote faster diffusion of phosphorus or/and boron into the advanced wafer material to form the corresponding junction and back contact layers. Thus, in the system of the present invention, the IR intensity is higher and the hold time is long enough for faster processing.

Fig. 7 through 10B are directed to a second embodiment of the ultra low mass transport system of the present invention which employs a pair of spaced apart chains from which wafer support wires and ceramic tubes are suspended. In fig. 7, the description of fig. 1 above applies to like-numbered parts. IT should be noted that the IT and B1 regions are combined in this embodiment into an elevated region in which the wafer temperature is elevated from room temperature to about 500 ℃ to 900 ℃, 900 ℃ in the case of an elevated region containing a HI-IR isolation lamp module. This is followed by a firing zone that raises the temperature to about 950-. As shown, the fired region employs the disclosed exemplary resistive SiC elements. The firing setpoint temperature is maintained in the soaking zone and heating elements are not shown to prevent cluttering the figure, but see fig. 1 and 5. As shown, the cooling zone is divided into two sub-zones CZ-1 and CZ-2, but CZ-2 may be external.

In fig. 7 and 8, the transport is by roller chain 112, roller chain 112 being moved by sprocket 114 powered by motor 34, motor 34 being located below the exit end of furnace 20. Idler wheels (in this case sprockets or flanged wheels) 40A-40C are arranged at the inlet, outlet and upstream of the return path R to redirect the chain in the drive loop shown. A spring biased tension and idler system 44 disposed downstream of the drive 30 provides the appropriate tension. Guide rollers 116 and skid blocks 74 are provided along the chain links to maintain the path straight. The air knives 22 provide cooled compressed air to the chain in or outside the cooling zone. In addition, a tube cooler 118 may be provided in the return section for further cooling of the chain.

Fig. 8 shows how two spaced apart chains 112-L (left side) and 112-R (right side) are supported in a groove or channel 120 in the slide 74. The transverse wire 72 supporting the ceramic tube 64 has an end 122, the end 122 passing through a tubular link bushing 124. The wires 72 are spaced about 1 "(2.5 cm) apart.

Fig. 9 is an enlarged view showing alternate link installation of wire 72. The middle link has a solid link pin 126. The free ends 122 of the wires of both link bushing 124 and link pin 126, which are covered with link rollers 128 (not shown in fig. 9; shown in fig. 10A and 10B), terminate in push-type or screw-type nuts or other types of fasteners 130, 140 to prevent the wires from falling out of bushing 124 (see fig. 10A, 10B).

Fig. 10A and 10B illustrate two embodiments of the slider plate 74. In fig. 10A, the slider plate contains a groove or channel 120, in which groove or channel 120 the chain 112 is supported. The free end 122 of the wire 72 is threaded to receive a locking nut 130. An optional spacer washer 132 is shown. The pedestal 84 has an inverted V-shaped periphery so that the wafer rests on the circumferential ridge 134. In FIG. 10B, the slider plate 74 includes ridges 136 that can have a variety of configurations including linear or curvilinear (sloped) sidewalls. The side links of the chain 112 ride on the ridges 136 and are guided by the ridges 136. In this embodiment, the slider plate 74 generally does not have an outboard block 138, and the plate is flat on both sides of the ridge, as shown by the dashed lines that define the side blocks 138. In addition, the tip 122 of the wire 72 terminates in a cap or push nut 140. In this embodiment, the profile of the abutment is a tapered roof, the cross-section being a half sine wave.

Two additional embodiments of the standoff 84 are shown in fig. 11A and 11B, with fig. 11A showing an asymmetric fin-shaped standoff having a sloped outer surface (to the right of the ridge 134) and vertical or sloped side surfaces. Figure 11B shows a currently preferred form of embodiment of the abutment, i.e. a two sided tapered cone optionally with an annular rib 134 at the apex where the two tapered sides meet. The ribs or long ramps each provide support for the wafer product as shown by the position of wafers W-1 and W-2, respectively, depending on the spacing of the standoffs 84 along the tube 64 as compared to the width of the (wafer) product. If the product is large, the bottom surface is on the rib, as shown for wafer W-l (see also 96 in FIG. 4A), while if the product is not as wide as wafer W-2, only the outer lower edge of the product is on the bevel (as shown). This support is glued to the ceramic tube 64 instead of being formed integrally. The vertical height of the pedestal can be selectively varied to accommodate various shipping and furnace designs and configurations.

Fig. 12A and 12B illustrate an optional shipping fail-safe system that includes a droop brake arranged side-by-side below the seat travel path. Fig. 12A shows the spatial location and arrangement of the droop stopper slide member 144 below the conveyor assembly, which includes the drive chain 112, ceramic tube 64, pedestal 84 and wire 72.

As shown in fig. 12A, the droop stopper assembly 144 serves to prevent the suspension wire or rod 72 carrying the ceramic tube 64 and the seat 84 from sagging excessively in the event of failure, for example, due to the wire breaking or elongating under stress or heat. In an exemplary embodiment, the sag stopper comprises a slide member 144, for example, selected from a quartz tube, rod, or strip, the slide member 144 aligned parallel to a centerline of the conveyor transport path indicated by arrow F, the centerline coinciding with a longitudinal centerline of the furnace firing zone. The catcher slide members 144 are spaced inwardly from the side walls of the oven chamber by a distance that aligns the catcher slide members 144 with and is spaced below the path of travel of the selected support. Each longitudinal end of the catcher slider members is mounted in a recess 146 in the transverse wall 54 of the furnace firing zone 23 such that the top of the slider members are spaced above the top of the transverse wall 54. Thus, if the conveyor wire and tube assembly sags, at this time the pedestals 84 will come into contact with the slide members, slide along the slide members as the conveyor progresses, and jump over the lateral walls of the furnace without sagging and breaking, or otherwise damaging the conveyor system. For example, the spacing between the top of the stop slider member 144 and the bottom of the seat 134 may be about 1 to 2 centimeters. In addition, because the stop slider member is spaced in alignment with the support path, the stop slider member does not shield the wafer (see wafers W-1 and W-2 of FIG. 11B), but is spaced from the side of the wafer.

Fig. 12A also shows that a compressed air duct 148 is provided above, below, or on one side of each conveyor chain slider plate 74 in its insulating side wall block 46S to provide cooling air to the bottom of the slider 74 and to the drive chain 112 through holes 150 in the slider plate 74.

FIG. 12B shows the bottom section 14 of an exemplary diffusion furnace 10 having a width of 5 lanes, the bottom section 14 being equipped with a pair of drop stop blocks 144, each drop stop block 144 being aligned side-by-side with a series of standoffs 84 and spaced below the standoffs 84. A lateral gap guard 152 may be placed across the width of the furnace at the entrance end to ensure clearance of any sagging wire as the transport is redirected upwards and then back to the horizontal product transport level.

Fig. 13 shows a process line 154 that includes a first dopant device Do-1 for doping a bottom surface of a prepared silicon wafer with a boron doping composition (arrow B). After the wafer is doped with boron in Do-1 and dried, the wafer is inverted in a flip-chip 158 so that the top surface of the wafer faces upward. The wafer with the inverted top side up is then transported to a second dopant apparatus Do-2 where a phosphorus doping composition (arrow P) is applied to the top surface of the wafer and the wafer is dried. The wafer double coated with boron/phosphorus is then transported to the diffusion furnace 10 of the present invention, subjected to co-diffusion firing as described herein above, cooled in the cooling section of the furnace, and transported to an anti-reflective coating device ARC 158 where an anti-reflective coating, such as SiN3, is applied by plasma enhanced chemical vapor deposition. After the ARC is applied, the wafer is transported to a printing station where a printer/dryer apparatus 160 applies silver-based paste "inks" in the form of finely spaced lines onto the phosphorus-doped/diffused top surface (back contact aluminum-based paste is applied onto the boron-doped/diffused bottom surface only when needed; boron-doped bottom surfaces typically do not require aluminum-based back contact paste). The "printed" wafers are then transported to a dryer 162, where the dryer 162 burns off the organic binder in the slurry at a fixed temperature of no more than about 600-650 ℃, as shown, with VOC emissions at 164, and then the off-gas is condensed or burned off in a thermal oxidizer. The printed wafer is transferred to a metallization furnace 166, fired in the metallization furnace 166 so that the selective back contact slurry flows into the continuous layer and forms a fine grid of ohmic collectors on the front side, which is fired into the p-n junction layer, but not through the p-n junction layer. The resulting cell 168 continues through the various cleaning, trimming and testing steps, and then is formed into a ribbon and assembled into an array for lamination between glass layers to form a finished solar panel.

Fig. 14-16 illustrate the use of longitudinal sidewall heaters to improve the lateral uniformity of heating in each zone (primarily the soaking zone) so that wafers being transported in the corresponding lane closest to the sidewall of the furnace (e.g., lanes 1 and 5 in a furnace having a width of 5 lanes) are at a processing temperature that is uniform across their width. The description of the same parts and regions in fig. 1 and 7 applies to fig. 15. In this embodiment, the furnace zone is an elevated zone 19, followed by a plurality of soaking zones S1, S2 … … Sn, followed by at least one cooling zone CZ, such as C1 or/and C2. The lamps 26U and 26L in the zones provide high IR flux for conditioning and heating. The elevated regions 19 (upper and lower 19U and 19L, respectively) rapidly elevate the wafer from ambient temperature to within the range of about 500 c to 700 c or higher. The first soaking region S1 raises the front and back, top and bottom surfaces of the wafer to a desired diffusion temperature in the range of about 950 ℃ (phosphorus only doped) to about 1100 ℃ (co-doping of phosphorus and boron). The remaining soak zones S2-Sn maintain the desired diffusion process set point temperature. The wafer is then cooled in one or more cooling zones C1, C2, etc. In this embodiment, in the lower half of the regions S1, S2, and in the region C1 (when the region C1 is replaced by the soaking region Sn, Sn is S3 in this embodiment), the side wall heater 170 is disposed above the lamps.

Fig. 15 illustrates the use of sidewall heaters 170U and 170L in the upper and lower regions. In this embodiment, a sidewall heater 170L is used in the lower elevated area. Heaters are used in all soaking zones shown.

FIG. 16 shows a 3-wire track furnace that transports wafers W-1, W-2, and W-3 in direction F. The remaining part numbers are the same as in fig. 3A. In the soaking region, two sidewall heaters 170R, 170L for the right and left sidewalls, respectively, are shown adjacent to their sidewalls 46R and 46L and above the lateral lower lamp 26L. The sidewall heaters may optionally extend into the elevated regions 19 or into the optional firing regions. The sidewall heaters are positioned at a preselected distance 172 from the corresponding sidewall 46R, 46L, the distance 172 ranging from about 1 cm to about 5 cm. The sidewall heaters may be supported in notches 173 cut in the lateral zone divider blocks 54 or in through holes (not shown) in the blocks. As shown, the sidewall heater includes an outer quartz tube 174, and a rod or coil-type resistive element ITS is placed in the outer quartz tube 174. These sidewall heaters ensure that the outer edges of wafers W-L in lane 1 and wafers W-3 in lane 3 (the edges closest to sidewalls 46R and 46L) are uniformly heated edge-to-edge.

The diffusion furnace of the present invention of the present application is widely applicable to the solar cell manufacturing industry, i.e., to the following process steps: the solar cell wafer is fired to diffuse and co-diffuse phosphorus or/and boron into the wafer matrix to form a p-n junction layer or conductive back surface layer. The system is clearly an improvement over currently available furnaces, providing higher throughput due to: substantially shorter processing times, less shadowing, lower energy requirements, less process contamination of the wafer, and improved uniformity in the p-n junction layer and the boron-doped back surface layer. It is therefore apparent that the system of the present invention may potentially be used as a new standard for apparatus and methods for diffusing dopants into solar cell advanced material wafers.

Parts list (this parts list is provided for convenience of review and may be truncated if permitted)

Claims (10)

1. A continuous conveyor diffusion furnace for processing solar cell wafers, said diffusion furnace comprising in operable combination:

a) a plurality of heating and cooling zones, said zones being oriented in a furnace inlet to furnace outlet order,

i) said zones being arranged in abutting relation so as to define a continuous longitudinal process conveyor path therethrough, said path being oriented in a generally horizontal plane, each of said zones comprising spaced apart outer side walls between which a process volume is defined, said process volume being divided into a zone upper half and a zone lower half along a horizontal plane generally parallel to said process path plane;

ii) the heating zone comprises a heating element selected from an IR lamp and a resistive radiation element oriented transverse to the longitudinal process conveyor path for heating the process volume;

iii) the transversely oriented heating elements are arranged in at least some of the heating zones in at least one of the upper half of the zone and the lower half of the zone;

iv) at least one pair of resistive heating elements disposed in at least one of the lower or/and upper half of the zone, the resistive radiating elements oriented parallel to the longitudinal process conveyor path, one of each pair disposed adjacent to one of the outer side walls;

b) a low mass conveyor system for receiving and moving solar cell wafers along the longitudinal process path from the furnace entrance through the zone to the furnace exit, the conveyor system comprising:

i) a plurality of spaced apart refractory wires oriented transverse to the longitudinal process path, the wires having lengths defining a useful wafer transport width through the furnace region between the sidewalls of the conveyor system;

ii) a small diameter, thin-walled, non-rotating refractory tube suspended from the wire to provide support to the wafers as they are transported through the furnace region by the conveyor system and to substantially completely shield the wafers from metal vapors emanating from the wire;

iii) the refractory tube extends for at least a majority of the length of the wire and is positioned on the wire so as to expose only short opposite lateral ends of the wire;

iv) a pair of spaced apart transport members, one transport member disposed adjacent each end of the wire, each of the transport members forming a continuous loop that traverses the longitudinal process path from a furnace entrance to the furnace exit and then returns to the entrance on a return path outside the furnace area;

v) each of the transport members includes a plurality of receiving members evenly spaced along each of the successive loops of the transport members, each of the receiving members being configured to removably retain the short side end of the wire arranged to be suspended between the transport members across the transport width; and

c) a drive system arranged outside the furnace region, the drive system being arranged to engage the two transport members to effect synchronous movement through the region as the transport members carry the plurality of refractory tubes and wires suspended between the receiving members, the wafers being transported across the region on the refractory tubes for processing thereof during furnace operation.

2. The continuous conveyor diffusion furnace of claim 1, wherein the conveyance system comprises at least one of:

a) an endless member selected from a belt and a roller chain;

b) wherein in use the strap comprises a vertically extending bracket in which the wire ends are received;

c) wherein when a roller chain is used, the roller chain includes a tubular pivot link in which the wire end is received.

3. The continuous conveyor diffusion furnace as claimed in claim 1 wherein said lower half zone sections are mounted immovably in a frame, said furnace including a plurality of powered vertical lift members arranged to be connected to said upper and lower half furnace zone sections, said vertical lift members arranged to lift said upper half zone sections relative to said fixed lower half zone sections to expose and allow access to the interior of the furnace heating zones for inspection, adjustment, maintenance and repair as required.

4. The continuous conveyor diffusion furnace as claimed in claim 1 wherein said upper half is mounted immovably in a frame, said furnace including a plurality of powered vertical lift members arranged to be connected to said upper and lower half furnace zone portions, said vertical lift members arranged to lower said lower half zone portions relative to said fixed upper half zone portions to expose and allow access to the interior of said furnace heating zones for inspection, adjustment, maintenance and repair as required.

5. The continuous conveyor diffusion furnace of claim 1 including droop detent assemblies mounted in lower halves of at least some of the furnace zones and arranged spaced below and in side-by-side alignment with the pedestals.

6. A method of continuous diffusion or co-diffusion layer firing of a Photovoltaic (PV) solar cell wafer, the wafer having a bottom surface and a top surface, the method comprising the steps of:

a) applying at least one dopant composition to at least one of the bottom wafer surface and the top wafer surface to create a plurality of wafers doped with a layer of dopant composition on at least one of the top and bottom surfaces;

b) transporting the plurality of co-doped wafers sequentially from a furnace entrance to a furnace exit through a plurality of heating and cooling zones arranged in abutting relationship so as to define a continuous longitudinal process conveyor path, the path oriented in a substantially horizontal plane and the wafers oriented with the top surface facing upwardly;

c) supporting the wafers in the transporting step on a low mass conveyor system, the conveyor system comprising a non-rotating refractory tube of small diameter supporting the wafers, the refractory tube suspended on wires spanning the wafer processing path from a first longitudinally outward side of the furnace to a second longitudinally outward side of the furnace, and the wafers advancing continuously through the zone at a selected rate;

d) heating the wafer in the heating zone directly by at least one of IR lamp radiation and thermal resistance radiation or re-radiation impinging on the top and bottom surfaces for a time sufficient to facilitate diffusion of the dopant from the coating layer into the wafer matrix material to complete formation of at least one of a p-n junction top surface layer and a back contact bottom surface layer; and

e) maintaining uniform heat across the wafer processing path by resistive radiation applied from a longitudinal resistive heating element disposed in at least one of the heating zones adjacent to and parallel to the outer sidewall.

7. The method of claim 6, wherein the heating zone is divided into an upper heating zone portion and a lower heating zone portion along a plane that is substantially parallel to a horizontal plane of the processing path, and heat is applied from resistive heating elements disposed in at least one lower heating zone portion.

8. An apparatus for thermal processing of a silicon wafer in a diffusion and metallization step, the wafer having a top surface and a bottom surface, the apparatus comprising in operable combination:

a) at least one dopant module for applying a doping composition selected from at least one of a boron dopant composition and a phosphorous dopant composition;

b) a continuous conveyor IR lamp heated diffusion furnace receiving doped silicon wafers from said doper module, said diffusion furnace having an elongated heating zone horizontally divided into an upper section and a lower section at the conveyor level, and said diffusion furnace including a lifting device allowing relative movement of said upper and lower sections so that access to the interior of said heating zone is gained, said furnace being adapted for diffusion firing doped silicon wafers;

c) an anti-reflective coating module disposed downstream of the diffusion furnace to receive a diffusion fired silicon wafer from the diffusion furnace and comprising means for applying an anti-reflective coating (ARC) to at least a top surface of the silicon wafer;

d) a printer/dryer module disposed downstream of the anti-reflective coating module to receive the ARC coated silicon wafer, the printer/dryer module comprising means for applying a back contact paste to the bottom surface and printing a fine collector line on the top surface to produce a printed wafer;

e) a dryer module for receiving the printed wafer and heating the printed wafer up to about 650 ℃ in an IR lamp heated zone to burn off volatile organic binder from the slurry and lines on the top and bottom surfaces of the printed silicon wafer; and

f) a metallization furnace having elongated heating zones divided horizontally into upper and lower sections at the conveyor level, and including lifting means allowing relative movement of the upper and lower sections so that access to the interior of the heating zones is gained, the furnace being adapted for IR lamp heated metallization firing of printed silicon wafers to within the range of about 750 ℃ to about 1100 ℃; thereby producing silicon wafers that can be processed into solar panel arrays, including cleaning, testing, and lamination.

9. The apparatus for thermal processing of silicon wafers as set forth in claim 8, wherein the diffusion furnace comprises:

a) a plurality of heating and cooling zones, said zones being oriented in a furnace entrance to furnace exit order, said zones being arranged in an abutting relationship so as to define a continuous longitudinal process conveyor path therethrough, said path being oriented in a generally horizontal plane;

b) a low mass conveyor system for receiving and moving solar cell wafers along the longitudinal process path from the furnace entrance through the zone to the furnace exit, the conveyor system comprising:

i) a plurality of spaced apart refractory wires oriented transverse to the longitudinal process path, the wires having a length that defines a useful wafer transport width of the conveyor system through the furnace region;

ii) a small diameter, thin-walled, non-rotating refractory tube suspended from the wire to provide support to the wafers as they are transported through the furnace region by the conveyor system and to substantially completely shield the wafers from metal vapors emanating from the wire;

iii) the refractory tube extends for at least a majority of the length of the wire and is positioned on the wire so as to expose only short opposite lateral ends of the wire;

iv) a pair of spaced apart transport members, one transport member disposed adjacent each end of the wire, each of the transport members forming a continuous loop that traverses the longitudinal process path from a furnace entrance to the furnace exit and then returns to the entrance on a return path outside the furnace area;

v) each of the transport members includes a plurality of receiving members evenly spaced along each of the successive loops of the transport members, each of the receiving members being configured to removably retain the short side end of the wire arranged to be suspended between the transport members across the transport width; and

c) a drive system arranged outside the furnace area, said drive system being arranged to engage said two transport members to effect synchronous movement through said area as said transport members carry said plurality of refractory tubes on which said wafers are transported through said area during furnace operation and a wire suspended between said receiving members.

10. The apparatus for thermal processing of silicon wafers as set forth in claim 9 comprising: a first dopant module for applying a boron dopant compound to the bottom surface of the wafer; and a second dopant module for applying a phosphorous dopant compound to the top surface of the wafer, the first dopant module feeding the doped wafer to a flip-chip module having a means for inverting the wafer so that the top surface faces upward, and the wafer with the inverted top surface facing upward is transferred to the second dopant module.