RU2428766C1 - Contact shaping method for nanoheterostructure of photoelectric converter based on gallium arsenide - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: invention can be used in production technologies of ohmic contact systems to photoelectric converters (PC) with high operating characteristics and namely the invention refers to formation of contacts to GaAs layers of n-type conductivity, which are front layers of the number of structures of concentrator PC, which are capable of effective conversion of incident radiation with capacity of 100-200 W/cm². Contact formation method for nanoheterostructure of photoelectric converter involves pre-formation on surface of nanoheterostructure of photoelectric converter based on gallium arsenide of electron conductivity of topology of photo-sensitive areas by photolithography with the use of mask from upper photoresist layer and lower non-photosensitive resist layer, or mask from photoresist with profile of mask elements, which has broadening from surface of nanoheterostructure of photoelectric converter. Then, cleaning of mask-free surface of nanoheterostructure of photoelectric converter, subsequent sputtering of eutectic gold-germanium alloy layer 10-100 nm thick, nickel layer 10-20 nm thick and silver layer, and further removal of photresist and annealing of contact is performed.

EFFECT: invention provides the possibility of formation of multi-layer contact during one sputtering process of contact layers for nanoheterostructure of photoelectric converter based on gallium arsenide, which allows simplifying the whole contact manufacturing process.

10 cl, 9 dwg

Description

The invention relates to the field of creating radiation-sensitive semiconductor devices, in particular to the manufacture of A ³ B ⁵ semiconductors, and can be used in postgrowth technologies for the production of ohmic contact systems for photovoltaic converters (PECs) with high operational characteristics. In particular, the invention relates to the formation of contacts to n-type GaAs layers, which are frontal layers of a number of concentrator PEC structures capable of efficiently converting incident radiation with a power of 100-200 W / cm ² .

Under the conditions of the extreme operation of concentrator FEPs, increased requirements are imposed on the quality of contacts, first of all, to the transition resistance of the contacts and their series resistance. A decrease in the contact resistance is necessary to increase the current collection from devices and to reduce the heating associated with the flow of high-density currents, and, ultimately, to increase the efficiency of concentrator PECs and their durability.

A known method of forming contact to gallium arsenide (GaAs) and to AlGaAs solid solutions with electronic conductivity (see patent US 5192994, IPC H01L 21/28, published March 9, 1993) by successive sputtering of gold layers (10-200 толщиной thick), Germany (50-200 Ǻ thick), nickel (50-200 толщиной thick) and gold (200-1000 толщиной thick), followed by annealing in an atmosphere of nitrogen or hydrogen at a temperature of 350 ° C to 500 ° C; contact transient resistance after annealing is 5 · 10 ^-5 Ohm · cm ² or less.

A contact made in a known manner does not have a small enough transition resistance, which may impede its use in a number of devices, including concentrator photovoltaic converters. A disadvantage of the known method is the lack of a process of thickening ohmic contacts, necessary to reduce the series resistance of the contacts, as well as for the subsequent operation of soldering solar cells. In addition, the use of gold as the first layer in a known contact may complicate the use of explosive photolithography in the manufacture of devices, since gold does not have very good adhesion to GaAs.

A known method of forming contact with GaAs and AlGaAs solid solutions with electronic conductivity (see patent US 5309022, IPC H01L 21/28, published 05/05/1994) by sequential deposition of layers of nickel (thickness 40-200 Ǻ), germanium (thickness 150-400 Ǻ) and gold (more than 4000 толщиной thick) followed by annealing from 1 to 200 s at a temperature of 300-500 ° C for 1-200 s.

The use of Ni as the first layer to a semiconductor in Ni-Ge-Au contact systems leads to a slight decrease in contact resistance compared to Au-Ge-Ni contact systems with the first Au layer or AuGe alloy. In this case, as a rule, the penetration of the upper semiconductor layer decreases and the morphology of the contact surface improves after annealing of the contacts. However, in the known method of forming a contact, it is not possible to significantly prevent erosion of the surface of the semiconductor. The lack of thickening of ohmic contacts is also a disadvantage.

A known method of forming contact with GaAs and with AlGaAs solid solutions with electronic conductivity (see patent US 5284798, IPC H01L 21/285, published 02/08/1994) by successive sputtering of layers of gold (thickness 10-200 Ǻ), germanium (thickness 50- 200 Ǻ), nickel (50-200 Ǻ thick) and gold (200-1000 толщиной thick), followed by annealing at a temperature of 350-500 ° C in an inert gas atmosphere.

The known method does not prevent erosion of the contact surface and uncontrolled etching of the metal-semiconductor interface during annealing. Also, the use of gold as the first layer in the proposed contact system may complicate the use of explosive photolithography in the manufacture of devices, since gold has poor adhesion to GaAs.

A known method of forming a contact for a photoelectric converter (see patent US 5924002, IPC H01L 29/45, published 07/13/1999), comprising applying layers of nickel (thickness from 5 to 15 nm), tin and AuGe alloy (thickness from 50 to the semiconductor surface) up to 200 nm) followed by annealing at a temperature of 190-300 ° C. Then, layers of titanium, platinum and gold are applied (for example, layers with a thickness of 5 nm, 10 nm and 300 nm, respectively).

The main advantage of this method is low annealing temperatures, which is a prerequisite for the manufacture of a number of devices, such as semiconductor lasers based on A ² B ⁶ compounds grown on n-GaAs substrates. However, when using the known method, the metal layers uncontrollably and nonuniformly melt the contact-semiconductor interface. During firing, severe erosion of the Ni-Sn-AuGe contact surface also occurs. In order to improve the contact surface in this method, it is proposed to spray another three layers of metal - Ti, Pt and Au, which complicates the manufacturing process of the contact structure.

A known method of forming contact with n-type GaAs (see application JP2002025937, IPC H01L 21/28, published January 25, 2002), comprising successively applying Au-Ge alloy layers to n-type GaAs (Ge content is 8-12% by weight), layer W, Ni layer and Au layer, while the ratio of the thicknesses of the layers of Au and AuGe should be in the range from 1.6 to 6.6, more preferably it should be 5.

The disadvantage of this method can be attributed to the need to apply a layer of tungsten, which requires the use of additional complex and expensive equipment.

A known method of forming a contact for a nanoheterostructure of a photovoltaic converter based on gallium arsenide, which coincides with the claimed solution for the largest number of essential features and adopted as a prototype (see application JP 58040858, IPC H01L 21/28, published 09.03.1983). The prototype method includes the sequential deposition of the layers of the eutectic alloy AuGe and Ni, AuGe and Pt, or AuGe, Ni and Au, burning of the resulting contact structure and subsequent deposition of two layers, for example Ti or Cr with a thickness of 1000 Ǻ or less, and Au or Ag.

The disadvantage of this method is the difficulty of manufacturing a multilayer contact, namely the multi-stage application of the contact layers.

The objective of the proposed technical solution was the development of such a method of forming a contact for a nanoheterostructure of a photovoltaic converter based on gallium arsenide, which would ensure the formation of a multilayer contact in the course of a single sputtering process of contact layers, which simplifies the contact manufacturing process.

The problem is solved in that the method of forming a contact for a nanoheterostructure of a photoelectric converter involves preliminarily forming on a surface of a nanoheterostructure a photoelectric converter based on gallium arsenide of electronic conductivity of the topology of the photosensitive regions by photolithography using a mask from the upper layer of the photoresist and the lower layer of the non-photosensitive resist, or a mask from the photoresist with the profile mask elements that have broadening from the surface of the nanogete ostruktury photoelectric transducer cleaning free from the mask nanoheterostructures photoelectric transducer surface, sequential plating layer of eutectic alloy of gold with germanium 10-100 nm thick, the nickel layer 10-20 nm thick and the silver layer, then removing the resist and annealing the contact.

The problem of forming a contact to gallium arsenide with electronic conductivity with low values of transition resistance is solved in the present method by using a gold-germanium alloy layer (germanium in this case is a donor impurity in GaAs) to create a highly doped degenerate semiconductor region after contact

Germanium in the composition of the Au-Ge alloy is a necessary component in the method of manufacturing the inventive contact structure, performing the function of a dopant (donor impurity) to create a contact region in n-GaAs with a high concentration of free charge carriers.

The nickel layer acts as a barrier layer between the gold-germanium alloy layer and the conductive silver layer, slowing down the diffusion of silver from the upper layer into the semiconductor and, thereby, preventing the violation of planarity of the contact-semiconductor interface.

A nickel layer with a thickness of less than 10 nm may have discontinuities (occurrence of punctures in the layer), and with a nickel layer with a thickness of more than 20 nm, the contact transition resistance increases after annealing.

The thickness of the conductive silver layer is selected, first of all, for reasons of reducing the resistance of the contact grid, as well as the cost of contact. However, in addition, the following is taken into account: when the contact thickness is less than 1-1.5 μm, the process of soldering the current leads of solar cells is difficult, and when the contact thickness is more than 5 μm, stressed layers can occur, as a result of which the adhesion of the contact to the semiconductor structure decreases and its peeling.

The surface cleaning of the nanoheterostructure of the photoelectric converter can be carried out by ion-beam etching.

The contact annealing is preferably carried out at a temperature of 360-380 ° C for a time of 10 s to several minutes in a stream of pure hydrogen or in a stream of a mixture of nitrogen and hydrogen, or in vacuum.

The nickel layer can be sputtered by magnetron sputtering.

The deposition of a gold-germanium alloy layer and a silver layer can be carried out by thermal evaporation.

The silver layer can be sprayed with a thickness of 30-9000 nm.

A barrier layer of nickel 20-100 nm thick or platinum 20-100 nm thick and a gold layer 30-200 nm thick in contact with the environment can be sprayed onto a silver layer. The top layer of gold, in addition to protecting silver from weathering, can improve the soldering process of the current outputs of photovoltaic converters.

The reproducible formation of a contact with a small transition resistance (of the order of 1 · 10 ^-5 Ohm · cm ² ) is achieved, firstly, by using the Au-Ge eutectic alloy (weight ratio 88:12) containing Ge as a donor impurity in the first layer GaAs, to create a strongly doped degenerate semiconductor region after contact The application of all layers of multilayer contact of the required thickness, up to 5-10 μm, during one deposition process is achieved by forming a mask on the surface of the semiconductor before spraying the layers of metal from the photoresist with an etching rate increasing in depth to obtain a profile of its individual elements having a broadening from the surface of the semiconductor in the perpendicular direction, or by applying a mask from the upper layer of the photoresist and the lower LOR (lift-off photoresist), i.e. a non-photosensitive layer, for example, based on polymethylglutarimide (see Popovich, Laura L; Gehoski, Kathy A .; Mancini, David P .; Resnick, Douglas J. Bilayer and trilayer lift-off processing for i-line and DUV lithography. Proc. SPIE Vol. 4691, 2002, p. 899-906). A feature of the application of LOR resists is the use of two-layer lithography. The lower layer is not photosensitive, and the upper layer is a photoresist that forms a pattern. LOR resistors provide a more stable level of explosive lithography.

When using standard photoresists, the side walls of the mask elements are not obtained strictly perpendicular to the surface of the semiconductor (they have a positive slope of the profile), which is due to the uneven absorption of radiation through the thickness of the photoresist during the process of its manifestation. Ultraviolet radiation when passing through the photoresist film is absorbed unevenly, as a result, the upper layers of the film receive a higher dose of energy than the lower ones. As a result, the upper layers of the film will dissolve faster in the developer, the profile of the photoresist becomes flat, broadened at the top and reduced below at the base. Usually this positive slope is 75-85 ° C depending on the process conditions and the characteristics of the equipment for exposure. When contact is made, the metal layer also deposits on the side walls of the mask elements from the photoresist, forming a continuous coating, which makes it difficult to subsequently remove the photoresist film by dissolution. Note that in this case, the elements of the contact grid can be partially damaged, have uneven, "torn" edges. In the known method of applying a mask with a conventional profile of a photoresist, with the same width of the mask elements in height, the maximum allowable thicknesses of the resulting contact layers in a single spraying process cannot exceed 0.3 microns. To obtain contacts with a thickness of 1-3 μm in the known method, they need additional thickening, usually gold or silver, for example by electrochemical deposition (for its implementation, an additional photolithography process is necessary to apply a photoresist mask).

In the inventive method, the configuration of the photoresist mask allows the production of contact systems for photovoltaic photovoltaics of significantly greater thickness, up to 10 μm, during a single sputtering process, which is more than an order of magnitude higher than the maximum permissible thickness of the contact layers when using the explosive photolithography method with a conventional photoresist profile (with the same width by the height of the mask elements). This is due to a significant simplification of the process of removing the photoresist after deposition of the contact layers, since the contact layers on the surface of the semiconductor do not have direct contact with the lower layers of the photoresist. As a result, the manufacturing technology of photomultipliers is simplified, the laborious operation of galvanic thickening of the photoconductor contacts is eliminated, without impairing the instrument performance. The formation of a mask from the upper layer of the photoresist and the lower layer of the non-photosensitive resist, or a mask from the photoresist with increasing depth of etching of the photoresist with the profile of its individual elements having a broadening from the surface of the semiconductor in the perpendicular direction, also makes it possible to obtain smooth (with a mirror surface) walls of the contact strips , which reduces the shading of the photosensitive surface of the solar cells and, accordingly, increases its efficiency. The use of silver instead of gold as the main conductive layer leads to a decrease in the series resistance of individual elements of the contact network with the same contact geometry, since silver has a lower resistivity than gold (~ 1.5 times); replacing gold with silver in a thick conductive layer also reduces the cost of contact (this becomes noticeable in the manufacture of concentrator PECs, since their manufacture requires the formation of contacts with low resistance - a thickness of about 3-5 microns or more). In the inventive method there is no need for additional thickening of the contact. In addition, it is possible to reduce the cost of contact by partially replacing a thick layer of gold with, for example, silver or aluminum with a barrier layer of, for example, platinum or nickel.

The inventive method of forming contact for nanoheterostructure of the photoelectric Converter is illustrated by drawings, where

figure 1 shows a cross section of a single element of the mask of the photoresist obtained by the prototype method;

figure 2 shows a cross section of a single element of the mask of the photoresist with increasing in depth etching rate;

figure 3 shows a cross section of a single element of the mask from two layers of photoresists with LOR (lift-off photoresist) the lower layer;

Individual operations of the proposed method are illustrated in figure 4-figure 9:

4 shows a lower layer of a non-photosensitive resist deposited on a nanoheterostructure;

figure 5 shows the process of exposure through the template applied to the upper layer of the photoresist;

figure 6 shows the region of the etched top layer of the photoresist;

7 shows the region of the etched lower layer of the non-photosensitive resist;

on Fig depicts the process of applying metal;

figure 4 shows the contact deposited on a nanoheterostructure.

Figure 1-figure 9 marked: nanoheterostructure 1; photoresist 2 used in the prototype method; photoresist 3 with increasing in depth etching rate; bottom layer 4 non-photosensitive resist; top layer 5 photoresist; region 6 with bonds torn by ultraviolet radiation; pattern 7; ultraviolet radiation stream 8; cavity 9 of the etched top layer 5; cavity 10 of the etched lower layer 4 of the non-photosensitive resist; sprayed metal stream 11; a metal layer 12 deposited on a photoresist layer 5; formed on nanoheterostructure 1 contract 13.

The inventive method of forming contact for nanoheterostructure of the photoelectric Converter is as follows. In one embodiment of the method, a lower non-photosensitive (LOR) resist layer 4 is applied to the nanoheterostructure 1 (see FIG. 4), a photoresist upper layer 5 is applied, and UV light is applied through the pattern 7 (see FIG. 5). Areas 6 with the bonds broken by ultraviolet radiation are etched, forming cavities 9 of the etched upper layer 5 (see Fig. 6). Through the obtained cavities 9 of the photoresist 5, the lower (LOR) layer 4 of the non-photosensitive resist is etched by the liquid method, forming cavities 10 and thereby forming a mask for metal deposition 11 (see Fig. 7). Due to the presence of lateral etching, the mask will have areas in which the sprayed material will not enter. Immediately before the deposition process of the contact layers of contact 13, the front surface of the nanoheterostructure 1 is cleaned. The surface of the nanoheterostructure 1 can be cleaned in an aqueous HCl solution at a volume ratio of HCl and H ₂ O 1: (1-6) or by ion-beam etching. The surface cleaning of the nanoheterostructure 1 by ion beam etching is carried out to a depth of 0.005-0.3 μm. The removal of the surface layer is necessary to improve the adhesion of the metal to nanoheterostructure 1 and to reduce the transition contact resistance. When etching to a depth of less than 0.005 μm, the removal of surface contaminants and oxides is not effective enough, when etching to a depth of more than 0.3 μm, the structural defect increases. An Au-Ge eutectic alloy 10-100 nm thick is applied as the first layer with a dopant impurity of contact 13, a nickel layer 10-20 nm thick is used as a barrier layer, and a silver layer 30-9000 nm thick is used as a conductive layer. If the photoelectric converter is designed to operate in the presence of traces of an aggressive environment (for example, hydrogen sulfide), it is necessary to add a contacting upper layer, for example, a gold layer 30–30 nm thick with an intermediate Ni, Ti, or Pt barrier layer 20–100 nm thick. In this case, the metal 11 forms both the contact 13 and the metal layer 12 on the photoresist layer 5 (see Fig. 8). Then, the photoresist layer 5 and the non-photosensitive resist layer 4 are removed and the contact 13 is annealed. As a result, metal strips of the correct geometry (up to the mirror walls) are obtained, which is especially important when creating a contact grid with a small element size (for example, the width of the contact strips in concentrator PECs is ~ 10 μm). This greatly facilitates the process of removing the photoresist (its "explosion") after spraying the contact. When spraying thick layers, it is possible to exclude the stage of galvanic amplification of the contact.

When testing the manufacturing technology of the contacts, the standard method of measuring the contact resistance of the contacts TLM (transmission line method) was used, using a set of identical rectangular contact pads located parallel to each other at different distances.

Example 1. The contact was formed on a GaAs layer doped with silicon, grown by the method of MOS hydride epitaxy on a semi-insulating GaAs substrate. The concentration of free charge carriers (doping level) was ~ 5 · 10 ¹⁸ cm ^-3 . Before the contact metals were deposited on the surface of the nanoheterostructure, a mask was formed from the lower non-photosensitive resist layer and the upper photoresist layer, the structure surface was cleaned by ion-beam etching (100 Ǻ of the surface layer was removed). The multilayer contact consisted of a 50 nm thick Au-Ge alloy, 15 nm nickel, and 900 nm silver. Contact transient resistance after annealing of the multilayer contact structure at temperatures of 360 ° C, 370 ° C, and 380 ° C for 30 seconds was, respectively, 2.9 · 10 ^-5 Ohm · cm ² , 1.6 · 10 ^-5 Ohm · cm ² and 3.7 · 10 ^-5 Ohm · cm ² (according to the method of measuring the transition resistance of contacts TLM (transmission line method)); the depth of the interface (interface) GaAs contact was within 100 nm.

Example 2. The contact structure was formed on the same n-type GaAs layer as in Example 1. Before applying the contact structure, a mask was formed on the surface of the semiconductor. Before the contact metals were deposited on the surface of the nanoheterostructure, a mask was formed from the lower non-photosensitive resist layer and the upper photoresist layer, the structure surface was cleaned by ion-beam etching (100 Ǻ of the surface layer was removed). The multilayer contact structure consists of a 50 nm thick Au-Ge alloy, 25 nm nickel, and 920 nm thick silver. Contact transient resistance after annealing of the multilayer contact structure at temperatures of 360 ° C, 370 ° C, and 380 ° C for 30 seconds was, respectively, 3.9 · 10 ^-4 Ohm · cm ² , 2.2 · 10 ^-4 Ohm · cm ² and 3.1 · 10 ^-4 Ohm · cm ² (according to the TLM method); the depth of the interface (interface) GaAs contact was within 100 nm.

Example 3. The contact structure was formed on the same n-type GaAs layer as in Example 1. Before applying the contact structure, a mask was formed on the surface of the semiconductor. Before the contact metals were deposited on the surface of the nanoheterostructure, a mask was formed from the lower non-photosensitive resist layer and the upper photoresist layer, the structure surface was cleaned by ion-beam etching (100 Ǻ of the surface layer was removed). The multilayer contact structure consists of a 10 nm thick Au-Ge alloy, 10 nm nickel, and 1070 nm thick silver. The contact transient resistance after annealing the multilayer contact structure at temperatures of 360 ° C, 370 ° C, and 380 ° C for 30 seconds was, respectively, 3.3 · 10 ^-5 Ohm · cm ² , 1.2 · 10 ^-5 Ohm · cm ² and 4.1 · 10 ^-5 Ohm · cm ² (according to the TLM method); the depth of the interface (interface) GaAs contact was within 80 nm.

Example 4. The contact structure was formed on the same n-type GaAs layer as in Example 1. Before applying the contact structure, a mask was formed on the surface of the semiconductor. Before the contact metals were deposited on the surface of the nanoheterostructure, a mask was formed from the lower non-photosensitive resist layer and the upper photoresist layer, the structure surface was cleaned by ion-beam etching (100 Ǻ of the surface layer was removed). The multilayer contact structure consists of a 100 nm thick Au-Ge alloy, 20 nm thick nickel, and 960 nm thick silver. The contact transition resistance after annealing of the multilayer contact structure at temperatures of 360 ° C, 370 ° C, and 380 ° C for 30 seconds was, respectively, 4.3 · 10 ^-5 Ohm · cm ² , 3.1 · 10 ^-5 Ohm · cm ² and 4.9 · 10 ^-5 Ohm · cm ² (according to the TLM method); the depth of the interface (interface) GaAs contact was within 120 nm.

Claims (10)

1. A method of forming a contact for a nanoheterostructure of a photoelectric converter based on gallium arsenide, comprising pre-forming on the surface of a nanoheterostructure a photoelectric converter based on gallium arsenide of electronic conductivity of the topology of photosensitive regions by photolithography using a mask from the upper layer of the photoresist and the lower layer of the non-photosensitive resist or mask from the photoresist with the profile mask elements having a broadening from the surface of a nanoheterostructure photoelectric converter, cleaning the mask-free surface of the nanoheterostructure of the photoelectric converter, sequentially sputtering a layer of a eutectic gold alloy with germanium 10-100 nm thick, a nickel layer 10-20 nm thick and a silver layer, subsequent removal of the photoresist and annealing of the contact. 2. The method according to claim 1, characterized in that the surface cleaning of the nanoheterostructure of the photoelectric converter is carried out by ion beam etching. 3. The method according to claim 1, characterized in that the contact is annealed at a temperature of 360-380 ° C for a time from 10 s to several minutes. 4. The method according to claim 1, characterized in that the contact is annealed in a stream of pure hydrogen. 5. The method according to claim 1, characterized in that the contact is annealed in a stream of a mixture of nitrogen and hydrogen. 6. The method according to claim 1, characterized in that the contact is annealed in vacuum. 7. The method according to claim 1, characterized in that the deposition of the Nickel layer is carried out by magnetron sputtering. 8. The method according to claim 1, characterized in that the spraying of the gold-germanium alloy layer and the silver layer is carried out by thermal evaporation. 9. The method according to claim 1, characterized in that the silver layer is sprayed with a thickness of 30-9000 nm. 10. The method according to claim 9, characterized in that a silver layer of nickel 20-100 nm thick and a gold layer 30-200 nm thick are sprayed onto the silver layer.

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