WO2002059050A1

WO2002059050A1 - A method and a device for measurement in a process for manufacturing a planar glass coating

Info

Publication number: WO2002059050A1
Application number: PCT/FI2002/000012
Authority: WO
Inventors: Juha Tikkanen; Jorma Keskinen; Markku Rajala
Original assignee: Liekki Oy
Current assignee: Liekki Oy
Priority date: 2001-01-09
Filing date: 2002-01-08
Publication date: 2002-08-01
Anticipated expiration: 2003-07-09
Also published as: FI20010036A0; FI112648B; WO2002059050A8; FI20010036L

Abstract

The invention relates to a measurement method and device to be used in connection with a manufacturing process of a planar glass coating, in which manufacturing process for example a thermal reactor (10), such as a flame or plasma is used to produce aerosol particles (13) from reactants (12), which said aerosol particles (13) are guided on the surface of a substrate (14), thus forming a coating layer. According to the invention, the electric charge conveyed on the substrate (14) by the aerosol particles (13) entering the substrate is measured by means of a current measurement means (17) during the process to monitor the coating process. The current measurement means (17) is arranged to communicate with a process control unit (18) to adjust the process conditions according to predetermined threshold value. In an embodiment of the invention, the measurement is based on the electric charge attained intrinsically by the aerosol particles (13) during their formation. If necessary, it is also possible to use a separate charger (23), for example a so-called corona charger to charge the aerosol particles (13).

Description

A METHOD AND A DEVICE FOR MEASUREMENT IN A PROCESS FOR MANUFACTURING A PLANAR GLASS COATING

The invention relates to a measurement method in a process for manufacturing a planar glass coating, especially a glass coating suitable for planar waveguides, as presented in the preamble of the appended claim 1. The invention also relates to a device according to the preamble of the appended claim 11 for implementing the aforementioned method.

An optical planar waveguide is produced by forming superimposed glass layers with different refractive indexes on the top of a suitable substrate, for example a silicon wafer. Conventionally, a so-called undercladding is first made of glass on top of the silicon wafer, on top of which a core layer having a slightly higher optical refractive index than said undercladding is formed. In this core layer, a desired waveguide pattern is formed for example by means of photolithographic methods. Furthermore, a third layer, i.e. a so-called upper cladding having a slightly lower refractive index is formed on top of the core layer. In the waveguide structure formed thereby light propagates in the core layer in the same way as in the core of an optical fibre.

Optical planar waveguides are at present quite widely used in different kinds of applications. As examples of the most important application areas of planar waveguides, it is possible to mention data processing and telecommunications technology as well as different measurement and sensor applications. Planar waveguides enable the packing of optical structures and elements on the same substrate in a small space as a compact entity, which, in addition to the small size, may have advantages such as small optical signal losses and high operating speed. In this respect it can be considered that by means of planar waveguides it is possible to attain advantages similar to the ones attained with conventional integrated circuits, in which only electric components are used within the same substrate. At present, it is, in addition to the optical components and structures, also possible to combine electrical components and structures in the same substrate in optical planar waveguides.

One known and widely used method for forming the glass layers necessary in optical planar waveguides is the so-called flame hydrosis deposition FHD.

US patent 5,622,750 discloses a method of forming the glass layers required in planar waveguides by means of the FHD method. In the method disclosed in the above-mentioned patent, the reactants necessary in the manufacture of glass are mixed together to form a solution, from which solution aerosol droplets are formed which are guided further with a carrier gas to a burner and to a flame. In the flame functioning as a thermal reactor the aerosol droplets form aerosol particles (glass soot) which are further thermophoretically guided on the substrate to be coated, thus forming a glass material coating. The substrate itself can be made of a semiconductor, glass or ceramic material. To attain a coating layer of uniform quality and thickness, the substrate is moved back and forth on a plane transverse to the flame during the coating process which may take, for example, about one hour to make one coating layer. When a suitable coating layer of the glass material has been grown on the substrate in the above-described manner, the coating layer is sintered to form a dense glass layer by heat-treating the substrate at high temperature.

The homogenous quality of the glass layers contained in the planar waveguide (desired thickness of the glass layer, desired composition of the glass layer, local flaws contained in the glass layer) has a very central meaning in the optical properties of the planar waveguide and further in the usability of the planar waveguide. In addition to the optical properties, the mechanical properties of different glass layers of the planar waveguide, for example the thermal expansion properties, must be adapted so that they are suitable for each other to attain durable structures. All the aforementioned factors require a very precise control of the manufacturing process. In the FHD method disclosed in the patent US 5,622,750, as well as in other methods of prior art in which a flame, plasma, or another thermal reactor is used to produce nanometre-scale aerosol particles from the reactants, the formation of the glass coating can be affected by several process parameters, the most important of which will be shortly described hereinbelow.

The control parameters relating to the thermal reactor itself are for example the mixture ratios and flow rates of combustion gases necessary in the production of a flame, or when plasma is used, the electric parameters (for example input current and input voltage determining the electrical power) affecting the flame arc (or the like) as well as the mixture ratios and flow rates of plasma gases. The formation of the glass material itself (aerosol particles) in the thermal reactor is affected by the mixture ratios and feed rates of the gaseous and/or liquid reactants necessary in the formation of the glass as well as by the process used in the formation of the aerosol droplets. The feed rate of the carrier gas/gases used for feeding reactants in the thermal reactor is also a central control parameter in the process, as well as the control of the vapour pressure of the reactants when gaseous reactants are used. The aforementioned vapour pressure is affected for instance by the temperature of the reactants, as well as the prevailing pressure affecting the same. By stabilizing all the aforementioned factors in accordance with predetermined reference values and other conditions affecting the process as well as possible, the aim is to ensure a steady production of the aerosol particles forming the coating as well as a controlled formation of the coating.

When the formation process of the coating is adjusted and controlled in the above manner, it is in practice, however, only possible to ensure stabilized conditions for the formation of coating particles taking place in the thermal reactor. By means of the solutions of prior art it is not possible to directly verify the formation of the coating particles or to monitor the movement of the formed coating particles on the surface of the substrate. Thus, for example the setting and control parameters of the aforementioned process cannot be controlled by feedback coupling either to attain a coating of uniform quality. The main purpose of the present invention is to introduce a new realtime (on-line) measurement method to be used during the process of manufacturing a planar glass coating used especially in planar waveguides, which measurement method enables a more accurate control of the manufacturing process and adjustment of the process parameters by means of feedback coupling, when compared to prior art.

To attain this purpose, the method according to the invention is primarily characterized in what will be presented in the characterizing part of the independent claim 1.

It is also an aim of the invention to provide a device implementing the aforementioned method. The device according to the invention, in turn, is primarily characterized in what will be presented in the characterizing part of the independent claim 11.

The invention is essentially based on the idea that the measurement is implemented by means of aerosol particles accumulating on the substrate to be coated, wherein by means of the measurement it is possible to detect the changes possibly occurring both in the formation of the aerosol particles and in the movement of the same on the substrate. According to the invention, this measurement is based on determining the electric charge conveyed to the substrate by the aerosol particles forming the coating by means of sensitive current measurement.

In an embodiment of the invention, the measurement is based on the electric charge attained intrinsically by the aerosol particles during their formation. It is an advantage of this embodiment that a separate charger is not necessary for electrical charging of the aerosol particles.

In a second embodiment of the invention, a separate charger, for example a so-called corona charger is used for charging the aerosol particles. The advantage of this embodiment, in turn, is that by means of a separate charger a more precisely controlled electric charge is attained for the aerosol particles, which improves the accuracy of the measurement.

The act of determining the electric charge conveyed by the aerosol particles to the substrate according to the invention can take place by connecting a sensitive current measurement means to a substrate galvanically, which substrate is electrically isolated from a base, or the like, supporting the substrate. The measurement can also be conducted in such a manner that the current measurement means detects the charge of the substrate by means of so-called Faraday cage without a galvanic connection to the substrate itself.

By means of the present invention it is possible to attain a considerably better control of the manufacturing process when compared to methods of prior art, because by means of the measurement method it is possible to monitor the amount/mass of the aerosol particles accumulating on the substrate, and thereby the amount/mass of glass material accumulating on the substrate during the process. In the method according to the invention, by means of one measurement of the electric charge of the substrate it is, in principle, possible to detect a change in any such process factor that affects either the formation of the aerosol particles and/or the movement of the aerosol particles on the substrate.

The invention also enables an adjustment of the process parameters of the coating process by means of feedback coupling to stabilize the coating conditions, thus making it possible to manufacture for example optical planar waveguides of high quality. The significance of this is emphasized especially in such coating processes, in which the production of a desired coating layer requires a continuous maintenance of the process for a long period of time.

The following more detailed description of the invention by means of examples will more clearly illustrate, for anyone skilled in the art, advantageous embodiments of the invention as well as advantages to be achieved with the invention in relation to prior art. In the following, the invention will be described in more detail with reference to the appended drawings, in which

Fig. 1 illustrates in principle a preferred embodiment of the invention, and

Fig. 2 illustrates in principle another embodiment of the invention.

Figure 1 shows in principle an embodiment of the invention. In this embodiment the thermal reactor 10 is a flame, which is produced by burning combustible and oxidizing combustion gases 11 by means of a burner 22. It is possible to use for example hydrogen + oxygen or methane + oxygen as combustion gases 11. The reactants 12 necessary in the formation of the glass material are brought to the flame 10 for example in aerosol droplets along with a carrier gas, which said aerosol droplets are in the flame 10 further transformed into aerosol particles 13 necessary in the formation of the glass coating.

The reactants 12 necessary in the formation of the glass material can be for example silicon or germanium tetrachloride, or chlorine-free reactants such as TEOS, tetraethylortosilicate or GEOS, tetraethoxygermanium in a suitable format. To form so-called multicomponent glass, the reactants 12 can, in addition to the above- mentioned agents, also contain rare earth metals and lanthanides, such as erbium and/or neodymium, as well as aluminium, phosphorus, borium and/or fluorine in a suitable format.

In the thermal reactor 10 the aerosol particles 13 produced from the reactants 12 move from the flame towards a substrate 14, which substrate 14 can be made of silicon, quartz or another semiconductor, glass or ceramic material suitable for the purpose. Advantageously, the substrate 14 is a thin, round wafer, for example a silicon wafer, but it is also possible to use substrates of another shape.

When the aerosol particles 13 hit the substrate 14, they adhere on the substrate 14, thus forming a coating. By moving the flame 10 with respect to the substrate 14, it is possible to attain either a coating layer with a uniform thickness on the entire surface area of the substrate 14, or the thickness of the coating layer can be adjusted, if necessary, so that it is different in different sections of the surface of the substrate 14. In addition to the thickness of the coating layer, the local properties of the coating layer can also be changed, if necessary, by varying the supply/composition of the reactants 12 when the flame 10 is moved with respect to the substrate 14. The movement between the substrate 14 and the flame 10 can be attained by any suitable solution obvious for anyone skilled in the art, either by arranging the burner 22 or the base 16 of the substrate 14 movable for example by means of a stepper motor driven manipulator.

For the duration of the coating process, the substrate 14 is attached on top of the base 16. If the base 16 is made of conductive material, the substrate 14 is electrically isolated from the base 16 by means of a non-conductive layer 15. The non-conductive layer 15 can be made of for example aluminium oxide (Al₂0₃), sapphire, quartz, teflon or nylon. Advantageously, the contact between the substrate 14 and the base 16 and the non-conductive layer 15 used therebetween is of such a quality, that a good thermal conductivity is attained between the substrate 14 and the base 16. This is necessary especially in such a case if the base 16 is arranged to be heated and/or cooled to adjust the temperature of the substrate 14 and control the coating process.

According to the present invention, the electric charge produced on the substrate by the aerosol particles 13 hitting the surface of the substrate 14 is measured during the process to measure the amount of glass material accumulated on the substrate 14, and to monitor/control the coating process in real time. In Fig. 1 the aforementioned measurement is conducted by means of a sensitive current measurement means 17, a so-called electrometer. In the embodiment according to Fig. 1 the current measurement means 17 is galvanically connected to the substrate 14, said substrate 14 being electrically isolated from the base 16 by means of a non-conductive layer 15.

The current measurement means 17 can be implemented by means of any method known as such. One electrometer based on the use of a current-voltage converter is disclosed for example in the doctoral thesis by Mikko Moisio: Real Time Size Distribution Measurement of Combustion Aerosols, published by Tampere University of Technology, ISBN 952-15-0328-9, 1999, pp. 37 to 39.

In the embodiment of the invention shown in Fig. 1 the measurement is based on the electric charge obtained by the aerosol particles 13 during the formation of the particles. The advantage of this embodiment is that a separate charger is not necessary for electrical charging of the aerosol particles 13.

In principle, the aerosol particles 13 produced in the thermal reactor 10 are always intrinsically electrically charged. This results for example from the free ions occurring in the thermal reactor 10, for example in the flame, which also participate and/or electrically interact with the aerosol particles 13 that are being produced. The different chemical reactions occurring in the thermal reactor also produce charged ions and particles. The charge transfer between ions, particles, etc. can take place either by collisions or as a charge transfer without collisions.

Fig. 2 illustrates in principle another embodiment of the invention in which a separate charger 23 is used to electrically charge the aerosol particles 13 formed in the flame 10. The charger 23 can be for example a charger based on a corona discharge, or another known method of charging the aerosol particles that is obvious for anyone skilled in the art. The function of the charger can be based for example on the use of a so-called triode charger. One method based on the use of a corona discharge is disclosed for example in the doctoral thesis by Mikko Moisio: Real Time Size Distribution Measurement of Combustion Aerosols, published by Tampere University of Technology, ISBN 952- 15-0328-9, 1999, pp. 21 to 30.

The main advantage of the use of a separate charger 23 is that by means of the same it is possible to attain a stable and reproducible charging of the aerosol particles, which improves the measuring accuracy of the method according to the invention. In Fig. 2, the current measurement means 17 detects the charge of the substrate by means of a so-called Faraday cage 24 without a direct galvanic connection to the substrate 14. The measurement based on the Faraday cage 24 can be understood by considering the cage as an electrically closed chamber, on the wall of which the current measurement means 17 is galvanically connected. When for example a charge with the magnitude of one electrode is conveyed to the chamber, the current measurement means 17 attached to the chamber also shows a reading corresponding to the same. Thus, the charge conveyed by the aerosol particles 13 is detected by means of the current measurement means 17, as the particles enter a Faraday cage 24 and adhere on the surface of the substrate 14.

By means of the Faraday cage 24 it is possible to measure the charge also in materials which are electrically non-conductive. In other words, the Faraday cage makes it possible to measure the charge collected on the substrate 14 in a situation in which the substrate 14 and/or the coating formed on the substrate is/are made of electrically non- conductive material/materials.

The function of the current measurement means 17 and the possible charger 23 is controlled by means of a control unit 18. The control unit 18 is arranged to determine the amount of glass material brought by the aerosol particles 13 accumulated on the substrate substantially in real time by means of a signal given by the current measurement means 17, and further to adjust for example the movement of the base 16 (control 19 in Figs 1 and 2) and/or the heating of the base 16 (control 20 in Figs 1 and 2) and/or the set parameters of the thermal reactor 10 (for example the mixture ratios and/or flow rates of the combustion gases 11 and/or reactants 12 and/or the amounts of carrier gas/carrier gases in the reactants 12) to attain a coating of a desired quality on the substrate 14.

The signal given by the current measurement means 17 is primarily dependent on the amount of aerosol particles 13 accumulated on the substrate 14. Because the charge of single aerosol particles 13 is substantially directly proportional to the size of the aerosol particles 13 when a separate charger 23 is used, the signal given by the current measurement means 17 is thus also proportional to the amount of glass material accumulated on the substrate 14. On the basis of the aforementioned measurement signal it is thus for example possible to control the moving of the substrate 16 in such a manner that the same amount/mass of aerosol particles 13 is attained on different locations in the substrate 14.

The variation of the measurement signal given by the current measurement means 17 as a function of time indicates the disturbance/disturbances in the function of the process forming the aerosol particles 13 and/or in the movement of the aerosol particles on the substrate 14. When the control unit 18 detects such a disturbance, it may attempt to eliminate said disturbance by adjusting the set parameters of the process and/or the control unit 18 can also give an alarm for the user monitoring the process.

When a desired layer of aerosol particles has been formed on the surface of the substrate 14, the formation/growth of the coating layer is terminated either by turning off the flame 10 entirely or by terminating the supply of reactants 12 used in the production of aerosol particles to the flame 10 and/or by moving the substrate aside 14 from underneath the jet of aerosol particles 13.

Thereafter the coating layer on the surface of the substrate 14, which at this stage is still a porous layer composed of particles that have melted together only partly, is sintered to form a dense glass material for example by transferring the substrate 14 to a separate oven, in which the sintering is conducted by means of known methods by increasing the temperature of the substrate 14 in such a manner that the glass material particles accumulated on the substrate melt together, thus forming a homogenous glass layer.

The invention is not, of course, restricted solely to the use of a flame as a thermal reactor 10, as presented in the examples above. The thermal reactor 10 can also be any other method obvious for anyone skilled in the art for producing such a high local temperature, in which corresponding reactions that generate aerosol particles are produced. Such possibilities include for example different kinds of plasmas, which can be produced for example by means of an electric current or laser light. The reactants 12 necessary in the production of a glass material can also be introduced to the process in any other way known as such.

If necessary, the thermal reactor 10 together with the substrate 14 can be isolated from the ambient atmosphere inside a chamber or a corresponding arrangement, which is shown in principle by broken lines in Figs 1 and 2. Said chamber or the like enables the act of creating such conditions for the coating process that differ from the ambient atmosphere as far as the composition of the gases and/or temperature and/or pressure is/are concerned. To prevent the aerosol particles 13 from adhering to the walls of the aforementioned chamber, said walls, as well as other parts of the device, can be arranged to be heated in sections where necessary for example by means of electric resistors. The aforementioned walls of the chamber can also be equipped with a shielding gas flow to prevent wall contamination, in which shielding gas flow the shielding gas is arranged to flow in a laminar flow in the vicinity of the wall, or the walls of the chamber are for example made of a porous metal or quartz material, the shielding gas discharged through the pores of said wall preventing the aerosol particles 13 from adhering on the walls of the chamber.

Advantageously, the thermal reactor 10 is a flame spray gun as disclosed in the Finnish patent No. 98832 and in the international patent application PCT/FI 99/00818, in which flame spray gun some of the reactants 12 necessary in the formation of the glass material can be conveyed to the flame 10 in liquid format in such a manner that said reactant/reactants are atomized into aerosol droplets only in the immediate vicinity of the flame 10, right before they are conveyed to the flame 10. The advantage attained hereby is that the single droplets supplied to the flame 10 contain the different components of the used liquid reactant precisely in the original ratio of said components, because the vapour pressures of these components that differ from each other do not have time to affect the composition of the aerosol droplets through evaporation occurring in a different manner. A further advantage is that in the case of said flame spray gun it is possible to supply larger amounts of components in the flame 10 when compared to such methods in which the aerosol droplets are formed further away from the flame 10 and conveyed to the flame 10 with the carrier gas. When large feed rates of liquid reactants 12 are used together with carrier gas, the sizes of the aerosol particles tend to grow during the conveying for example as a result of coagulation or agglomeration before the aerosol droplets end up in the flame 10. In addition, deposition occurs on the walls of the conveying channel that is being used, for example on the walls of a tubing, and in this way the conveying channel also tends to become contaminated, which makes the control of the process even more difficult.

It is, of course, obvious for anyone skilled in the art that by combining, in different ways, the methods, modes of operation and device structures presented above in connection with different embodiments of the invention, it is possible to provide various embodiments of the invention in accordance with the spirit of the invention. It is, for example, possible to use a separate charger 23 to charge the aerosol particles 13 and to measure the electric charge collected on the substrate 14 on the basis of the galvanic connection of the current measurement means 17 and the substrate 14. On the other hand, charge measurement based on the Faraday cage 24 can be used in a situation where the measurement is based on the electric charge obtained intrinsically by the aerosol particles 13 during the formation of the particles.

Therefore, the above-presented examples must not be interpreted as restrictive to the invention, but the embodiments of the invention can be freely varied within the scope of the inventive features presented in the claims hereinbelow.

It is, of course, obvious for anyone skilled in the art that the appended drawings are only intended for illustration of the invention, and thus the structures and components presented therein are not drawn to scale.

Claims

Claims:

1. A method in connection of a process for manufacturing a planar glass coating, suitable to be used especially in planar waveguides, in which manufacturing process of the glass coating aerosol particles (13) are produced from reactants (12), which said aerosol particles (13) move on the surface of a substrate (14), thus forming a coating layer, characterized in that the electric charge conveyed on the substrate (14) by the aerosol particles (13) entering said substrate (14) is measured during the process to monitor and/or adjust the manufacturing process.

2. The method according to claim 1 , characterized in that the electric charge obtained intrinsically by said aerosol particles (13) during their formation is measured.

3. The method according to claim 1 , characterized in that the aerosol particles (13) are electrically charged by means of a separate charger (23) before the aerosol particles (13) enter the substrate (14).

4. The method according to any of the preceding claims, characterized in that the current measurement means (17) used in the measurement of the electric charge of the substrate (14) is galvanically connected to the substrate (14).

5. The method according to any of the preceding claims 1 to 3, characterized in that the current measurement means (17) used in the measurement of the electric charge of the substrate (14) is galvanically connected to a Faraday cage (23), which Faraday cage (23) is formed around the substrate (14).

6. The method according to any of the preceding claims 1 to 5, characterized in that the substrate (14) is electrically isolated from its base (16) by means of a non-conductive layer, which non-conductive layer can be made of for example aluminium oxide (Al₂0₃), sapphire, quartz, teflon, nylon or another suitable electrically non-conductive material.

7. The method according to any of the preceding claims, characterized in that on the basis of the measurement information obtained from the electric charge of the substrate (14) one or more process parameters in the coating process are adjusted to optimize the process conditions so that they correspond to predetermined conditions.

8. The method according to any of the preceding claims, characterized in that the method is applied in connection with such a coating process in which the thermal reactor (10) used in the coating process is a flame spray gun, in which flame spray gun some of the reactants (12) necessary in the formation of the glass material are conveyed to the flame (10) in liquid format in such a manner that said reactant/s (12) is/are atomized into aerosol droplets only in the immediate vicinity of the flame (10), right before they are conveyed to the flame (10).

9. The method according to any of the preceding claims, characterized in that the method is applied in connection with such a coating process in which the reactant (12) necessary in the formation of the glass material in the coating process or one of the components of said reactant is inorganic or organic compound of silicon or germanium, such as silicon tetrachloride or germanium tetrachloride, TEOS (tetraethylortosilicate), or GEOS (tetraethoxygermanium).

10. The method according to any of the preceding claims, characterized in that the method is applied in connection with such a coating process in which coating process the reactant (12) necessary in the formation of multicomponent glass in the production of glass material or one of the components of said reactant contains erbium, neodymium, another lanthanide, aluminium, phosphorus, borium and/or fluorine.

11. A device for in connection of a process for manufacturing a planar glass coating suitable to be used especially in planar waveguides, in which manufacturing process of the glass coating aerosol particles (13) are produced, which said aerosol particles (13) move on the surface of a substrate (14), thus forming a coating layer, characterized in that the device comprises at least a current measurement means (17) for measuring during the process the electric charge conveyed on the substrate (14) by the aerosol particles (13) entering said substrate (14) and for monitoring and/or adjusting the coating process.

12. The device according to claim 11 , characterized in that the device also comprises a separate charger (23) for electrically charging the aerosol particles (13) before the aerosol particles (13) enter the substrate (14).

13. The device according to the preceding claim 11 or 12, characterized in that the current measurement means (17) is galvanically connected to the substrate (14).

14. The device according to the preceding claim 11 or 12, characterized in that the device also comprises a Faraday cage (23) formed around the substrate (14), which Faraday cage is galvanically connected to the current measurement means (17).

15. The device according to any of the preceding claims 11 to 14, characterized in that the current measurement means (17) is arranged to communicate with a process control unit (18) to adjust the process conditions to correspond to predetermined conditions.