Indian Patents. 218575:ETCHING MEDIUM FOR ETCHING OF SILICON SURFACES AND LAYERS.

Title of Invention	ETCHING MEDIUM FOR ETCHING OF SILICON SURFACES AND LAYERS.
Abstract	The present invention relates to novel etching media in the form of etching pastes for full-area and selective etching of silicon surfaces and layers, and to the use thereof.

Full Text	WO 2004/032218 PCT/EP2003/008805 Etching pastes for silicon surfaces and layers The present invention relates to novel etching media in the form of etching pastes for full-area and selective etching of silicon surfaces and 5 layers, and to the use thereof. Prior art In the photovoltaics, electronics and semiconductor industry, silicon 10 surfaces and layers are often etched by wet-chemical methods in dip baths. This full-area etching can be carried out either in an acidic medium (isotropic etching) or in an alkaline medium (anisotropic etch- ing). In acidic etching, mixtures of hydrofluoric acid and nitric acid are frequently used, in alkaline etching, strong bases, such as sodium 15 hydroxide solution, potassium hydroxide soIution, tetramethyIammonium hydroxide (TMAH), etc., are frequently used. In order to produce defined, fine etching patterns/structures (for example for buried structures) in addition to full-area etching (for 20 example polish etches, texture etches), material-intensive, time- consuming and expensive process steps, such as, for example, the photolithographic masking process known to the person skilled in the art, are necessary before the actual etching step. 25 In a masking process of this type, the starting material is a silicon wafer. A dense oxide layer is produced thereon by thermal oxidation and structured as follows. The oxide is uncovered at the desired points by coating with a photo- 30 resist, drying, exposing to UV light using a photomask, and subse- quently developing, and then removed using hydrofluoric acid. The photoresist which still remains is subsequently removed ("stripped"), for example using a solvent. The Si wafer thus provided with an oxide mask can then be etched selectively at the points not covered by the oxide in 35 a strong base, such as, for example, 30% KOH, The oxide mask is WO 2004/032218 PCT/EP2003/008805 -2- resistant to the base. After selective etching of the silicon, the oxide mask is usually removed again using hydrofluoric acid. Lithographic processes of this type are not used in the industrial pro- 5 duction of solar cells for cost reasons [1]. However, selective structur- ing/opening of the silicon surface or layer is necessary by the following procedure. In the production process for a standard silicon solar ceil, the p-n transi- 10 tion necessary for the photoelectric effect is formed on a p-doped wafer by, for example, gas diffusion in a POCI3 oven. In the process, an n-doped silicon layer with a thickness of about 500 nm is formed around the entire wafer and has to be partially opened/cut for the tater photo- voltaic application. 15 This opening can be carried out mechanically, by laser cutting or dry- etching methods, such as plasma etching. The disadvantages of mechanical cutting, for example grinding of the 20 cell edges in the final step of the production process (after metallisa- tion), consist in considerable losses of silicon material (and metal paste), the mechanical stress and the formation of crystal defects in the solar cell. 25 Plasma etching is carried out with fluorinated hydrocarbons, for example with CF4 or C2F6 gas, in expensive vacuum equipment. In this process, the cells are stacked in advance and etched at the cell edges in the plasma-etching unit. Considerable handling problems during stacking and high wafer breakage rates occur in the process. These technologi- 30 cal problems will intensify even further in the future since the aim is, owing to high material costs, to use ever thinner potycrystalline silicon starting substrates ( of 250-330 µm which are usual today. 35 Owing to the requisite linear (XY) movement of the punctiform laser source, the isolation of the p-n transition by laser is a time-consuming, WO 2004/032218 PCT/EP2003/008805 -3- throughput-limiting process. The investment costs considerable. In addition, local crystal defects are generated. In expensive processes for the production of a selective emitter, which 5 have currently only been developed and used on a laboratory scale, the lithographic oxide masking already described above is used. The oxide masks the wafer in such a way that the areas on which the contacts will later lie remain free. The masked wafer is subjected to phosphorus dif- fusion and n++-doped in the non-masked areas. After removal of the 10 oxide mask, the entire wafer is n+-doped [2]. This gives a solar cell with a selective emitter, i.e. with highly doped n++ areas with a depth of 2-3 µm (areas without oxide mask and later lying under the contacts) with a doping concentrations of about 11020 cm-3 15 and a flat (0.5-1 µm) n+-doped emitter over the entire solar cell with a doping concentration of about 11019 cm-3. The alternative to lithography is the use of screen-printed contact lines as etching mask. Both wet-chemical and plasma-chemical etching is 20 described in the literature. Disadvantages of dipping the screen-printed solar cell into a mixture of HF/HNO3 - besides the intended removal of silicon between the contact lines - are attack of the silicon beneath the contact lines and etching damage in the metal contact lines themselves. This causes rapid impairment of the fill factor [3]. 25 Plasma-chemical etching (reactive ion etching, RIE) is carried out using gases, such as, for example, SF6 or SF6/O2, in expensive vacuum equipment and with considerable technological optimisation effort for the process [4], [5], [6]. 30 Besides the formation of the selective emitter, the silicon surface here is structured (roughened, "textured") on the emitter side in such a way that the antireflection behaviour of the solar cell improves. 35 the object of the invention is to find a less expensive process with lower material fosses for opening the p-n transition in solar cells. WO 2004/032218 PCT/EP2003/008805 -4- The object of the present invention is therefore also to provide a simple, inexpensive process which can be carried out in the solar-cell industry and by means of which silicon surfaces can be etched selectively for the production of emitters and for improving the antireflection behaviour. At 5 the same time, it is an object of the present invention to provide an inexpensive etchant for carrying out the etching process. The object is achieved, in particular, by an etching medium for the etching of silicon surfaces and layers in the form of a thickened, alkaline 10 liquid, where the etching process ts carried out in the alkaline, solvent- containing liquid. The present object is therefore achieved by me provision of an inexpen- sive etching paste which can be applied rapidly and selectively, for 15 example using a screen printer or a dispenser, to the areas to be etched and thus significantly minimises firstly the consumption of etching chemicals and secondly the loss of material on the solar cell Besides the described opening of the p-n transition of a solar cell, 20 selective etching of silicon using an etching paste enables the produc- tion of a selective (also two-stage) emitter in mass production and an improvement in the antireflection behaviour of the solar cell. The.present invention is thus distinguished from processes in which an 25 alkaline viscous salt solution is applied, for example, to ceramic parts and dried (solvent evaporates), and the actual etching process is carried out in the alkaline melt at 300-400°C [7]. The present invention is a printable and dispensable etching 30 medium in the form of an etching paste which comprises a. at least one solvent b. thickeners and c. optionally additives, such as antifoams, thixotropic agents, flow- control agents, deaerators and adhesion promoters, 35 and is effective at temperatures of from 70 to 150ºC and/or can, if desired, be activated by the input of energy. WO 2004/032218 PCT/EP2003/008805 -5- This etching medium comprises, as etching component, an organic or inorganic base in a concentration of from 2 to 50% by weight, preferably from 5 to 48% by weight, based on the total amount. 5 The etching component which can be employed is at least one component selected from the group consisting of sodium hydroxide, potassium hydroxide, ammonia, ethanolamine, ethylenediamine, tetraalkylammonium hydroxide or one of the ethylene- diamine/pyrocatechol and ethanolamine/gallic acid mixtures. 10 The present invention thus relates to etching media which comprise a solvent selected from the group consisting of water, isopropanol, diethylene glycol, dipropylene glycol, polyethylene glycols, 1,2- propanediof, 1,4-butanediol, 1,3-butanediol, glycerol, 1,5 15 pentanediol, 2-ethyl-1-hexanol or mixtures thereof, or selected from the group consisting of acetophenone, methyl-2-hexanone, 2-octanone,4-hydroxy-4-methy 1-2 - pentanone, 1 -methyl-2-pyrroIId- one, ethylene glycol monobutyl ether, ethylene glycol monomethyl ether, triethylene glycol monomethyl ether, diethylene glycol mono- 20 butyI ether, dipropyIene gIycol monomethyl ether, carboxylic acid esters, such as [2,2-butoxy(ethoxy)]ethyl acetate, propylene carbonate as such or in a mixture in an amount of from 10 to 90% by weight, preferably in an amount of from 15 to 85% by weight, based on the total amount of the medium. 25 Etching media according to the invention furthermore comprise a thickener selected from the group consisting of hydroxyalkylguar. xanthan gum, cellulose and/or ethyl-, hydroxypropyl- or hydroxy- ethylcellulose, carboxymethylcellulose, sodium carboxymethyI- 30 hydroxyethylcellulose, homopolymers or copofymers based on functionalised vinyl units of acrylic acid, acrylates and alkyl meth- acrylates (C10-C30), individually or in a mixture in an amount of from 0.5 to 25% by weight, preferably from 1 to 10% by weight, based on the total amount of the etching medium. 35 WO 2004/032218 PCT/EP2003/008805 -6- Besides these components, additives selected from the group consisting of antifoams, thixotroptc agents, flow-control agents, deaerators and adhesion promoters may be present in an amount of from 0 to 2% by weight, based on the total amount. 5 The present Invention also relates to a process for the etching of silicon surfaces and layers in which an etching medium according to the invention is applied over the entire area or in accordance with the etch structure mask specifically only to the areas of the surface 10 where etching is desired and is removed again after an exposure time of from 30 s to 5 min. In accordance with the invention, the etching medium acts at a temperature in the range from 70 to 150°C, If necessary, activation 15 is carried out by the input of energy, preferably by means of IR radiation. In the process according to the invention, the etching medium is applied to the surface to be etched by a screen, template, pad, 20 stamp, ink-jet or manual printing process or by a dispensing tech- nique. After the exposure time and when the etching is complete, the etching medium is rinsed off using a solvent or solvent mixture. The etching media according to the invention can be used in 25 photovoltaics, semiconductor technology, high-performance elec- tronics, in particular for the production of photodiodes, circuits, electronic components or for the etching of silicon surfaces and layers for isolation of the p-n transition in solar cells. They can also be used for the etching of silicon surfaces and layers for the pro- 30 duction of a selective emitter for solar cells, for the etching of sili- con surfaces and layers of solar cells for improving the antireflec- tion behaviour, for the etching of silicon surfaces and layers in a process for the production of semiconductor components and cir- cuits thereof, or for the etching of silicon surfaces and layers in a 35 process for the production of components in high-performance electronics. WO 2004/032218 PCT/EP2003/008805 -7- Object of the invention The object of the invention is for semiconductor surfaces and layers, in particular silicon surfaces and layers, to be etched over the entire area 5 or structured selectively using etching pastes, A technique with a high degree of automation and high throughput which is suitable for transfer of the etching paste to the area to be etched is printing and dispensing. In particular, screen, template, pad, stamp and ink-jet printing processes and the dispensing process are known to the person skilled 10 in the art. Manual application, for example by means of a brush/application roller, is likewise possible. Depending on the screen, template, klischee or stamp design or car- tridge or metering unit control, it is possible to apply the etching pastes 15 described in accordance with the invention over the entire area or selectively in accordance with the etch structure mask only to the areas where etching is desired. All masking and lithography steps are super- fluous in this case. 20 It is thus possible for structuring processes with complex masking or processes such as laser structuring to be significantly shortened and carried out less expensively or for processes which are susceptible to technological faults, such as plasma etching, to be replaced by printing and dispensing techniques. In addition, the etching process can be sig- 25 nificantly reduced with respect to the consumption of etching chemicals since the etching paste is only applied to the areas to be etched. In particular during isolation of the p-n transition in the production of sili- con solar cells, the following advantages can be achieved through the 30 useofetching pastes: • no need for expensive plasma-etching units • reduction in the high celf breakage rates which occur • minimisation of the high loss of material during mechanical separation 35 • avoidance of surface defects WO 2004/032218 PCT/EP2003/008805 -8- In the production of the selective emitter using etching pastes, it is like- wise possible to dispense with oxide masking and expensive plasma etching, in addition, selective application of the etching paste avoids underetching of the contact areas. Since masking is not required, even 5 by screen-printed metal contact lines, etching damage to the contacts is excluded. It should also be noted that, in contrast to the photolithographic, plasma-chemical and laser processes used hitherto, the production of a 10 selective emitter and the improvement in the antireflection behaviour can be made significantly shorter and simpler with the etching paste according to the invention. The wafers are uniformly n++-doped over the entire surface. The areas between the contacts are etched away by the etching paste, thus n+-doped and improved in their antireflection 15 behaviour. A plurality of process steps are thus saved. The etching operation is preferably carried out with the input of energy, for example in the form of heat radiation (IR lamp) or by means of a hotplate. When etching is complete, the etching pastes are rinsed off 20 the etched surface using a suitable solvent or solvent mixture. The etching duration can be between a few seconds and several min- utes depending on the application, desired etching depth and/or edge sharpness of the etch structures and the etching temperature set. 25 The etching pastes have the following composition: • etching components • solvents 30 • thickeners • if desired additives, such as, for example, antifoams, thixotropic agents, flow-control agents, deaerators and adhesion promoters In order to etch semiconductor elements from main group 4 of the Peri- 35 odic Table, such as silicon, strong caustic lyes are used [7]. The etching WO2004/032218 PCT/EP2003/008805 -9- action of the etching pastes described in accordance with the invention is therefore based on the use of alkaline, silicon-etching solutions. Acidic etching pastes based on HF or fluoride - as described for oxides 5 [8], [9] - d° not exhibit an etching action on silicon. The alkaline etching components used in the etching pastes described in accordance with the invention can be aqueous solutions of inorganic lyes, such as sodium hydroxide, potassium hydroxide, ammonia or 10 organic-based, alkaline etching mixtures, such as ethyJenediamine/ pyrocatechol, ethanolamine/gallic acid, tetraalkylammonium hydroxide or combinations of the two. The proportion of etching components employed is in a concentration 15 range 2 - 50% by weight, preferably 5 - 48% by weight, based on the totaL weight of the etching paste. Particular preference is given to etch- ing media in which the etching components are present in an amount of 10 - 45% by weight. Particularly suitable are etching media in which the etching component(s) is (are) present in an amount of 30 - 40% by 20 weight, based on the total weight of the etching paste, since etching rates which facilitate complete opening of the p-n transition and a high throughput and at the same time show high selectivity have been found for etching media of this type. 25 The etching components are effective in the etching pastes at 70- 150 °C. On silicon surfaces and layers, etching depths of less than 1 µm have been achieved at temperatures below 100°C and etching depths of up to 2-3 µm have been achieved at temperatures above 100°C. 30 Suitable inorganic and/or organic solvents and/or mixtures thereof may be the following: • water • simple or polyhydric alcohols (for example isopropanol, diethylene 35 glycol, dipropylene glycol, polyethylene glycols, 1,2-propanediol, 1,4- WO2004/032218 PCT/EP2003/008805 -10- butanediol, 1,3 -butanediol, glycerol, 1,5 pentanedioI, 2-ethyl-1-hexa - nol) or mixtures thereof •ketones (for example aceiophenone, methyI-2-hexanone, 2-octanone, 4-hydroxy-4- methyl-2- pentanone, 1 -methyI-2-5 pyrrolidone) •ethers (for example ethylene glycol monobutyl ether, ethylene glyool monomethyl ether, triethylene glycol monomethyl ether, diethylene glycol monobutyl ether, dipropylene glycol monomethyl ether) •carboxylic acid esters (for example [2,2-butoxy(ethoxy)]ethyl acetate) 10 • esters of carbonic acid (for example propylene carbonate) Preference is given to the use of water and solvents from the group consisting of the ethers and ketones. 15 Water has proven particularly suitable. The proportion of the solvents is in the range 10 - 90% by weight, pref- erably 15 - 85% by weight, based on the total weight of the etching paste. Particularly suitable compositions have proven to be those in 20 which solvents are present in an amount of 55 - 75% by weight, based on the total weight of the etching paste. The viscosity of the etching pastes described in accordance with the invention is set by means of network-forming thickeners which swell in 25 the liquid phase and can be varied depending on the desired area of application. Possible thickeners are crosslinked and uncrosslinked homopolymers and copolymers based on monomer units such as funciionalised vinyl 30 units, for example acrylic acid, acrylates, alkyl methacrylates (C1o-C30) and hydroxyalkylguar, xanthan gum and β-glucosidically linked glucose units, i.e. cellulose and/or cellulose derivatives, such as cellulose ethers, in particular ethyl-, hydroxypropyl- or hydroxyethylcellulose, carboxymethylcellulose and salts of the glycolic acid ether of cellulose, 35 inparticular sodium carboxymethylcellulose.The thickeners can be employed individually and/or in combination with other thickeners. Pref- WO 2004/032218 PCT/EP2003/008805 -11 - erenee is given to the use of salts of carboxymethylcellulose and cross- linked acrylic acid polymers as thickener. The sodium salt of carboxy- methylcellulose (Finnfix®) and crosslinked acrylic acid homopolymers (Carbomers®) have proven very particularly suitable for this purpose. 5 The proportion of thickeners necessary for the specific setting of the viscosity range and for the formation of a printable or dispensable paste is in the range 0.5 - 25% by weight, preferably 1 - 10% by weight, based on the total weight of the etching paste. Particularly suitable 10 compositions have proven to be those in which thickeners are present in an amount of 1.5 - 6% by weight. Additives having properties which are advantageous for the desired purpose are antifoams, for example TEGO® Foamex N (dimethylpoly- 15 siloxane), thixotropic agents, for example BYK® 410 (modified urea), Borchigel® Thixo2, flow-control agents, for example TEGO® Glide ZG 400 (polyether-siloxane copolymer), deaerators, for example TEGO® Airex 986 (polymer with silicone tip), and adhesion promoters, for example Bayowet® FT 929 (fluorosurfactant). These can positively 20 influence the printability and dispensability of the etching paste. The proportion of the additives is in the range 0 - 2% by weight, based on the total weight of the etching paste. It has been found through experiments that both the choice of the com- 25 ponents employed for the preparation of the etching media and the mixing ratio of the components to one another in the etching media is of considerable importance. Depending on the manner in which the etching media are applied to the area to be etched, the percentage ratio of the components to one another should be set differently, since, inter 30 alia, the viscosity and flowability or the thixotropy being set are consid- erably influenced by the amounts of solvent and thickener present- The amounts of solvent and thickener present in turn influence the etching behaviour. Depending on the type of use in the process according to the invention, it is therefore possible for the person skilled in the art to 35 select a correspondingly adapted composition of the etching medium. WO 2004/032218 PCT/EP2003/008805 -12- Areas of application Areas of application of the etching pastes according to the invention are in the; 5 • solar-cell Industry • semiconductor industry • high-performance electronics 10 The etching pastes according to the invention can be employed in all areas where full-area and/or structured etching of silicon surfaces or layers is desired. Thus, individual structures can be etched over the full area or selectively to the depth desired in each case in a silicon surface or layer. 15 Areas of application are, for example: • all etching steps (synonymous with structuring steps), including sur- face cleaning/roughening of silicon surfaces and layers, which result 20 in the production of optoelectrical components, such as solar cells, photodiodes and the like, in particular isolation of the p-n transition in silicon solar ceils and the partial removal of doped layers (selective emitters) • all etching steps on silicon surfaces and layers which result in the 25 production of semiconductor components and circuits • all etching steps on silicon surfaces and layers which result in the production of components in high-performance electronics (IGBTs, power thyristors, GTOs, etc.). 30 For better understanding and for illustration of the invention, examples are given below which fall within the scope of protection of the present invention, but are not suitable for restricting the invention to these examples. 35 WO 2004/032218 PCT/EP2003/008805 -13- Examples Example 1 5 40.0 g of KOH 59.0 g of deionised water 1.5 g of ethylene gfycol monobutyl ether 4.0 g of Carbomer (acrylic acid homopolymer) 10 The chemicals were weighed into a beaker, mixed and dissolved, and the thickener was added with stirring. The mixture is transferred into containers after a short standing time. This mixture gives an etching paste with which, for example, silicon 15 surfaces and layers can be etched specifically over the entire area or in structures down to a desired depth with and/or without input of energy. The etching paste was applied to the silicon surface, for example by screen printing or using a dispenser (for example pin diameter of 20 260 µm), and etched on a hotplate for 3 min at 100°C. On production of etch structures with a fine width of about 1 mm on an n-doped (100) sili- con wafer, the etching depth determined (depending on the printing and dispensing parameters) is 0.3 - 1 µm. The etching depth can be increased by increasing the KOH concentration and the line width. For 25 line widths of 4 mm and KOH concentrations of 20 - 50% by weight, the etching depths are 2 - 3 µn. The resultant etching paste is stable on storage, easy to handle and printable- It can be removed from the printed surface or layer or from the 30 paste carrier (screen, doctor blade, template, stamp, klischee, cartridge, etc.) using a solvent, for example using water 35 WO 2004/032218 PCT/EP2003/008805 -14- Example 2 8.0 g of KOH 26.4 g of deionised water 5 4.0 g of N-methylpyrrolidone 2.3 g of the Na salt of carboxymethylcellulose (Finnfix®) The batch and processing were carried out as described in Example 1. 10 The etching paste was applied to the silicon surface using a dispenser (pin diameter 450 µm) and etched for 3 min at an etching temperature of 130°C. On production of etch structures with a line width of about 1 mm on an n-doped (100) silicon wafer, the etching depth determined (depending on the printing and dispensing parameters) is 0.2 - 1 µm. 15 Example 3 37.8% by weight of water 0.8% by weight of ethanoiamine 20 50.0% by weight of ethylene glycol 4.7% by weight of tetraethylammonium hydroxide 4.7% by weight of tetrapropylammonium hydroxide 0.3% by weight of gallic acid 25 1.8 % by weight of the Na salt of carboxymethylcellulose (Finnfix®) The batch and processing were carried out as described in Example 1. The etching paste was applied to the silicon surface by screen printing 30 or using a dispenser (pin diameter 450 µm) and etched for 3 min at an etching temperature of 130°C. The etching depth determined on pro- duction of etch structures with a line width of 1 mm is about 200 nm on a silicon wafer, 35 WO 2004/032218 PCT/EP2003/008805 - 15- Example 4 39.4% by weight of deionised water 49.3% by weight of ethylene glycol 5 4.9% by weight of tetraethylammonium hydroxide 4.9% by weight of tetrapropylammonium hydroxide 1.5% by weight of the Na salt of carboxymethylcellulose (Finnfix®) The batch and processing were carried out as described in Example 1. 10 The etching paste was applied to the silicon surface by screen printing or using a dispenser (pin diameter 450 µm) and etched for 3 min at an etching temperature of 130°C. The etching depth determined on pro- duction of etch structures with a line width of 1 mm is about 300 nm on 15 a silicon water Example 5 50% by volume of tetraethylammonium hydroxide 20 20 % by volume of ethylenediamine 20% by volume of deionised water 10% by volume of triethylenetetraamine 0.25 g of gallic acid 25 3% by weight of xanthan gum The batch and processing were carried out as described in Example 1. The etching paste was applied to the silicon surface by screen printing 30 or using a dispenser (pin diameter 450 µm) and etched for 3 min at an etching temperature of 100°C. The etching depth determined on pro- duction of etch structures with a line width of 4 mm is about 1 µm on a silicon wafer. 35 WO 2004/032218 PCT/EP2003/008805 -16- [1] W. Wettling, Phys. Bl. 12 (1997), pp. 1197-1202 [2] J. Horzel, J. Slufzik, J, Nijs, R. Mertens, Proc. 26th IEEE PVSC, (1997), pp. 139-42 5 [3] M. Schnell, R. Lüdemann, S. Schäfer, Proc. 16thEU PVSEC, (2000), pp. 1482-85 [4] D.S. Ruby, P. Yang, S. Zaidi, S. Brueck, M. Roy, S. Narayanan, Proc. 2nd World Conference and Exhibition on PVSEC, (1998), pp. 1460-63 10 [5] US 6,091,021 (2000), D.S. Ruby, W.K. Schubert, J.M. Gee, S.H Zaidi [6] US 5,871,591 (1999), D.S. Ruby, J.M. Gee, W.K. Schubert 15 [7] EP 0229915 (1986), M. Bock, K. Heymann, H.J. Middeke, D. Ten- brink [8] WO 00/40518 (1998), M. Lulyf R. Singh, C. Redmon, J. Mckown, R. Pratt 20 [9] DE 10101926 (2000), S. Klein, L. Haider, C. Zielinski, A, Kubelbeck, W. Stockum [10] A.F. Bogenschütz, Ätzpraxis für Halbleiter [Etching Practice for 25 Semiconductors], Carl Hanser Verlag, Munich 1967 30 35 WO 2004/032218 PCT/EP2003/008805 - 17- PATENT CLAIMS 1. Etching medium for the etching of silicon surfaces and layers, characterised in that 5 the etching medium is a thickened, alkaline liquid. 2. Printable etching medium according to Claim 1, characterised in that it is an etching paste which comprises a. at least one solvent b. 10 b. thickeners and optionally c. additives, such as antifoams, thixotropic agents, flow-control agents, deaerators and adhesion promoters, and is effective at temperatures as low as from 70 to 150°C and/or can, if desired, be activated by the input of energy, 15 3. Etching medium according to Claims 1 and 2, characterised in that it comprises, as etching component, an organic or inorganic base in a concentration of from 2 to 50% by weight, preferably from 5 to 48% by weight, based on the total amount. 20 4. Etching medium according to Claim 3, characterised in that it comprises, as etching component, at least one component selected from the group consisting of sodium hydroxide, potas- sium hydroxide, ammonia, ethenolamine, ethylenediamine, 25 tetraalkylammonium hydroxide or one of the ethylenediamine/ pyrocatechol and ethanofamine/gallic acid mixtures. 5. Etching medium according to Claim 2, characterised in that it a solvent selected from the group consisting of water, isopropanol, 30 diethylene glycol, dipropylene glycol, polyethylene glycols, 1,2- propanediol, 1,4-butanediol, 1,3-butanediol, glycerol, 1,5 pentanediol, 2-ethyl-1-hexanol or mixtures thereof, or selected from the group consisting of acetophenone, methyl-2-hexanone, 2-octanone, 4-hydroxy-4-methyl-2-pentanone, 1-methyl-2- 35 pyrrolidone, ethylene glycol monobutyl ether, ethylene glycol monomethyl ether, triethylene glycol monomethyl ether, di- WO2004/032218 PCT/EP2003/008805 - 18- ethylene glycol monobutyl ether, dipropylene glycol monomethyl ether, carboxylic acid esters, such as [2,2-butoxy(ethoxy)]ethyl acetate, propylene carbonate as such or in a mixture in an amount of from 10 to 90% by weight, preferably in an amount of 5 from 15 to 85% by weight, based on the total amount of the medium. 6. Etching medium according to Claim 2, characterised in that it comprises a thickener selected from the group consisting of 10 hydroxyalkylguar, xanthan gum, cellulose and/or ethyl-, hydroxy- propyl- or hydroxyethy (cellulose, carboxymethylcellulose, sodium carboxymethylhydroxyethylcellulose, homopolymers or copolymers based on functionalised vinyl units of acrylic acid, acrylates and alkyl methacrylates (C10-C30), individually or in a 15 mixture in an amount of from 0.5 to 25% by weight, preferably from 1 to 10% by weight, based on the total amount of the etching medium. 7. Etching medium according to Claim 3, characterised in that it 20 comprises additives selected from the group consisting of antifoams, thixotropic agents, flow-control agents, deaerators and adhesion promoters in an amount of from 0 to 2% by weight, based on the total amount. 25 8. Process for the etching of silicon surfaces and layers, character- ised in that an etching medium according to Claims 1-7 is applied over the entire area or in accordance with the etch structure mask specifically only to the areas of the surface where etching is desired and is removed again after an expo- 30 sure time of from 30 s to 5 min. 9. Process according to Claim 8, characterised in that the etching medium acts at a temperature in the range from 70 to 150ºC and/or, if necessary, is activated by the input of energy. WO 2004/032218 PCT/EP2003/008805 -19- 10. Process according to Claim 9, characterised in that the etching medium is activated by exposure to heat (IR lamp, hotplate). 11. Process according to Claim 8, characterised in that the etching 5 medium is applied to the surface to be etched by a screen, template, pad, stamp, ink-jet or manual printing process or by a dispensing technique. 12. Process according to Claim 8, characterised in that the etching 10 medium is rinsed off using a solvent or solvent mixture when the etching is complete. 13. Use of an etching medium according to C/aims 1-7 in photo- voltaics, semiconductor technology, high-performance elec- 15 tronics and for the production of photodiodes, circuits, electronic components. 14. Use of an etching medium according to Claims 1-7 for the etch- ing of silicon surfaces and layers for isolation of the p-n transi- 20 tion in solar cells. 15. Use of an etching medium according to Claims 1-7 for the etch- ing of silicon surfaces and layers for the production of a selec- tive emitter for solar cells, 25 16. Use of an etching medium according to Claims 1-7 for the etch- ing of silicon surfaces and fayers of solar cells for improving the antireflection behaviour. 30 17. Use of an etching medium according to Claims 1-7 for the etch- ing of silicon surfaces and layers in a process for the production of semiconductor components and circuits thereof. 18. Use of an etching medium according to Claims 1-7 for the etch- 35 ing. of silicon surfaces and layers in a process for the production of components in high-performance electronics.

Full Text

WO 2004/032218 PCT/EP2003/008805

Etching pastes for silicon surfaces and layers
The present invention relates to novel etching media in the form of
etching pastes for full-area and selective etching of silicon surfaces and
5 layers, and to the use thereof.
Prior art
In the photovoltaics, electronics and semiconductor industry, silicon
10 surfaces and layers are often etched by wet-chemical methods in dip
baths. This full-area etching can be carried out either in an acidic
medium (isotropic etching) or in an alkaline medium (anisotropic etch-
ing). In acidic etching, mixtures of hydrofluoric acid and nitric acid are
frequently used, in alkaline etching, strong bases, such as sodium
15 hydroxide solution, potassium hydroxide soIution, tetramethyIammonium
hydroxide (TMAH), etc., are frequently used.
In order to produce defined, fine etching patterns/structures (for
example for buried structures) in addition to full-area etching (for
20 example polish etches, texture etches), material-intensive, time-
consuming and expensive process steps, such as, for example, the
photolithographic masking process known to the person skilled in the
art, are necessary before the actual etching step.
25 In a masking process of this type, the starting material is a silicon wafer.
A dense oxide layer is produced thereon by thermal oxidation and
structured as follows.
The oxide is uncovered at the desired points by coating with a photo-
30 resist, drying, exposing to UV light using a photomask, and subse-
quently developing, and then removed using hydrofluoric acid. The
photoresist which still remains is subsequently removed ("stripped"), for
example using a solvent. The Si wafer thus provided with an oxide mask
can then be etched selectively at the points not covered by the oxide in
35 a strong base, such as, for example, 30% KOH, The oxide mask is

WO 2004/032218 PCT/EP2003/008805
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resistant to the base. After selective etching of the silicon, the oxide
mask is usually removed again using hydrofluoric acid.
Lithographic processes of this type are not used in the industrial pro-
5 duction of solar cells for cost reasons [1]. However, selective structur-
ing/opening of the silicon surface or layer is necessary by the following
procedure.
In the production process for a standard silicon solar ceil, the p-n transi-
10 tion necessary for the photoelectric effect is formed on a p-doped wafer
by, for example, gas diffusion in a POCI3 oven. In the process, an
n-doped silicon layer with a thickness of about 500 nm is formed around
the entire wafer and has to be partially opened/cut for the tater photo-
voltaic application.
15
This opening can be carried out mechanically, by laser cutting or dry-
etching methods, such as plasma etching.
The disadvantages of mechanical cutting, for example grinding of the
20 cell edges in the final step of the production process (after metallisa-
tion), consist in considerable losses of silicon material (and metal
paste), the mechanical stress and the formation of crystal defects in the
solar cell.
25 Plasma etching is carried out with fluorinated hydrocarbons, for example
with CF4 or C2F6 gas, in expensive vacuum equipment. In this process,
the cells are stacked in advance and etched at the cell edges in the
plasma-etching unit. Considerable handling problems during stacking
and high wafer breakage rates occur in the process. These technologi-
30 cal problems will intensify even further in the future since the aim is,
owing to high material costs, to use ever thinner potycrystalline silicon
starting substrates ( of 250-330 µm which are usual today.
35 Owing to the requisite linear (XY) movement of the punctiform laser
source, the isolation of the p-n transition by laser is a time-consuming,

WO 2004/032218 PCT/EP2003/008805
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throughput-limiting process. The investment costs considerable. In
addition, local crystal defects are generated.
In expensive processes for the production of a selective emitter, which
5 have currently only been developed and used on a laboratory scale, the
lithographic oxide masking already described above is used. The oxide
masks the wafer in such a way that the areas on which the contacts will
later lie remain free. The masked wafer is subjected to phosphorus dif-
fusion and n++-doped in the non-masked areas. After removal of the
10 oxide mask, the entire wafer is n+-doped [2].
This gives a solar cell with a selective emitter, i.e. with highly doped n++
areas with a depth of 2-3 µm (areas without oxide mask and later lying
under the contacts) with a doping concentrations of about 1*1020 cm-3
15 and a flat (0.5-1 µm) n+-doped emitter over the entire solar cell with a
doping concentration of about 1*1019 cm-3.
The alternative to lithography is the use of screen-printed contact lines
as etching mask. Both wet-chemical and plasma-chemical etching is
20 described in the literature. Disadvantages of dipping the screen-printed
solar cell into a mixture of HF/HNO3 - besides the intended removal of
silicon between the contact lines - are attack of the silicon beneath the
contact lines and etching damage in the metal contact lines themselves.
This causes rapid impairment of the fill factor [3].
25
Plasma-chemical etching (reactive ion etching, RIE) is carried out using
gases, such as, for example, SF6 or SF6/O2, in expensive vacuum
equipment and with considerable technological optimisation effort for
the process [4], [5], [6].
30
Besides the formation of the selective emitter, the silicon surface here is
structured (roughened, "textured") on the emitter side in such a way that
the antireflection behaviour of the solar cell improves.
35 the object of the invention is to find a less expensive process with lower
material fosses for opening the p-n transition in solar cells.

WO 2004/032218 PCT/EP2003/008805
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The object of the present invention is therefore also to provide a simple,
inexpensive process which can be carried out in the solar-cell industry
and by means of which silicon surfaces can be etched selectively for the
production of emitters and for improving the antireflection behaviour. At
5 the same time, it is an object of the present invention to provide an
inexpensive etchant for carrying out the etching process.
The object is achieved, in particular, by an etching medium for the
etching of silicon surfaces and layers in the form of a thickened, alkaline
10 liquid, where the etching process ts carried out in the alkaline, solvent-
containing liquid.
The present object is therefore achieved by me provision of an inexpen-
sive etching paste which can be applied rapidly and selectively, for
15 example using a screen printer or a dispenser, to the areas to be etched
and thus significantly minimises firstly the consumption of etching
chemicals and secondly the loss of material on the solar cell
Besides the described opening of the p-n transition of a solar cell,
20 selective etching of silicon using an etching paste enables the produc-
tion of a selective (also two-stage) emitter in mass production and an
improvement in the antireflection behaviour of the solar cell.
The.present invention is thus distinguished from processes in which an
25 alkaline viscous salt solution is applied, for example, to ceramic parts
and dried (solvent evaporates), and the actual etching process is carried
out in the alkaline melt at 300-400°C [7].
The present invention is a printable and dispensable etching
30 medium in the form of an etching paste which comprises
a. at least one solvent
b. thickeners and
c. optionally additives, such as antifoams, thixotropic agents, flow-
control agents, deaerators and adhesion promoters,
35 and is effective at temperatures of from 70 to 150ºC and/or can, if
desired, be activated by the input of energy.

WO 2004/032218 PCT/EP2003/008805
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This etching medium comprises, as etching component, an organic
or inorganic base in a concentration of from 2 to 50% by weight,
preferably from 5 to 48% by weight, based on the total amount.
5 The etching component which can be employed is at least one
component selected from the group consisting of sodium hydroxide,
potassium hydroxide, ammonia, ethanolamine, ethylenediamine,
tetraalkylammonium hydroxide or one of the ethylene-
diamine/pyrocatechol and ethanolamine/gallic acid mixtures.
10
The present invention thus relates to etching media which comprise
a solvent selected from the group consisting of water, isopropanol,
diethylene glycol, dipropylene glycol, polyethylene glycols, 1,2-
propanediof, 1,4-butanediol, 1,3-butanediol, glycerol, 1,5
15 pentanediol, 2-ethyl-1-hexanol or mixtures thereof, or selected from
the group consisting of acetophenone, methyl-2-hexanone,
2-octanone,4-hydroxy-4-methy 1-2 - pentanone, 1 -methyl-2-pyrroIId-
one, ethylene glycol monobutyl ether, ethylene glycol monomethyl
ether, triethylene glycol monomethyl ether, diethylene glycol mono-
20 butyI ether, dipropyIene gIycol monomethyl ether, carboxylic acid
esters, such as [2,2-butoxy(ethoxy)]ethyl acetate, propylene
carbonate as such or in a mixture in an amount of from 10 to 90%
by weight, preferably in an amount of from 15 to 85% by weight,
based on the total amount of the medium.
25
Etching media according to the invention furthermore comprise a
thickener selected from the group consisting of hydroxyalkylguar.
xanthan gum, cellulose and/or ethyl-, hydroxypropyl- or hydroxy-
ethylcellulose, carboxymethylcellulose, sodium carboxymethyI-
30 hydroxyethylcellulose, homopolymers or copofymers based on
functionalised vinyl units of acrylic acid, acrylates and alkyl meth-
acrylates (C10-C30), individually or in a mixture in an amount of from
0.5 to 25% by weight, preferably from 1 to 10% by weight, based on
the total amount of the etching medium.
35

WO 2004/032218 PCT/EP2003/008805
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Besides these components, additives selected from the group
consisting of antifoams, thixotroptc agents, flow-control agents,
deaerators and adhesion promoters may be present in an amount
of from 0 to 2% by weight, based on the total amount.
5
The present Invention also relates to a process for the etching of
silicon surfaces and layers in which an etching medium according to
the invention is applied over the entire area or in accordance with
the etch structure mask specifically only to the areas of the surface
10 where etching is desired and is removed again after an exposure
time of from 30 s to 5 min.
In accordance with the invention, the etching medium acts at a
temperature in the range from 70 to 150°C, If necessary, activation
15 is carried out by the input of energy, preferably by means of IR
radiation.
In the process according to the invention, the etching medium is
applied to the surface to be etched by a screen, template, pad,
20 stamp, ink-jet or manual printing process or by a dispensing tech-
nique. After the exposure time and when the etching is complete,
the etching medium is rinsed off using a solvent or solvent mixture.
The etching media according to the invention can be used in
25 photovoltaics, semiconductor technology, high-performance elec-
tronics, in particular for the production of photodiodes, circuits,
electronic components or for the etching of silicon surfaces and
layers for isolation of the p-n transition in solar cells. They can also
be used for the etching of silicon surfaces and layers for the pro-
30 duction of a selective emitter for solar cells, for the etching of sili-
con surfaces and layers of solar cells for improving the antireflec-
tion behaviour, for the etching of silicon surfaces and layers in a
process for the production of semiconductor components and cir-
cuits thereof, or for the etching of silicon surfaces and layers in a
35 process for the production of components in high-performance
electronics.

WO 2004/032218 PCT/EP2003/008805
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Object of the invention
The object of the invention is for semiconductor surfaces and layers, in
particular silicon surfaces and layers, to be etched over the entire area
5 or structured selectively using etching pastes, A technique with a high
degree of automation and high throughput which is suitable for transfer
of the etching paste to the area to be etched is printing and dispensing.
In particular, screen, template, pad, stamp and ink-jet printing
processes and the dispensing process are known to the person skilled
10 in the art. Manual application, for example by means of a
brush/application roller, is likewise possible.
Depending on the screen, template, klischee or stamp design or car-
tridge or metering unit control, it is possible to apply the etching pastes
15 described in accordance with the invention over the entire area or
selectively in accordance with the etch structure mask only to the areas
where etching is desired. All masking and lithography steps are super-
fluous in this case.
20 It is thus possible for structuring processes with complex masking or
processes such as laser structuring to be significantly shortened and
carried out less expensively or for processes which are susceptible to
technological faults, such as plasma etching, to be replaced by printing
and dispensing techniques. In addition, the etching process can be sig-
25 nificantly reduced with respect to the consumption of etching chemicals
since the etching paste is only applied to the areas to be etched.
In particular during isolation of the p-n transition in the production of sili-
con solar cells, the following advantages can be achieved through the
30 useofetching pastes:
• no need for expensive plasma-etching units
• reduction in the high celf breakage rates which occur
• minimisation of the high loss of material during mechanical separation
35 • avoidance of surface defects

WO 2004/032218 PCT/EP2003/008805
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In the production of the selective emitter using etching pastes, it is like-
wise possible to dispense with oxide masking and expensive plasma
etching, in addition, selective application of the etching paste avoids
underetching of the contact areas. Since masking is not required, even
5 by screen-printed metal contact lines, etching damage to the contacts is
excluded.
It should also be noted that, in contrast to the photolithographic,
plasma-chemical and laser processes used hitherto, the production of a
10 selective emitter and the improvement in the antireflection behaviour
can be made significantly shorter and simpler with the etching paste
according to the invention. The wafers are uniformly n++-doped over the
entire surface. The areas between the contacts are etched away by the
etching paste, thus n+-doped and improved in their antireflection
15 behaviour. A plurality of process steps are thus saved.
The etching operation is preferably carried out with the input of energy,
for example in the form of heat radiation (IR lamp) or by means of a
hotplate. When etching is complete, the etching pastes are rinsed off
20 the etched surface using a suitable solvent or solvent mixture.
The etching duration can be between a few seconds and several min-
utes depending on the application, desired etching depth and/or edge
sharpness of the etch structures and the etching temperature set.
25
The etching pastes have the following composition:
• etching components
• solvents
30 • thickeners
• if desired additives, such as, for example, antifoams, thixotropic
agents, flow-control agents, deaerators and adhesion promoters
In order to etch semiconductor elements from main group 4 of the Peri-
35 odic Table, such as silicon, strong caustic lyes are used [7]. The etching

WO2004/032218 PCT/EP2003/008805
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action of the etching pastes described in accordance with the invention
is therefore based on the use of alkaline, silicon-etching solutions.
Acidic etching pastes based on HF or fluoride - as described for oxides
5 [8], [9] - d° not exhibit an etching action on silicon.
The alkaline etching components used in the etching pastes described
in accordance with the invention can be aqueous solutions of inorganic
lyes, such as sodium hydroxide, potassium hydroxide, ammonia or
10 organic-based, alkaline etching mixtures, such as ethyJenediamine/
pyrocatechol, ethanolamine/gallic acid, tetraalkylammonium hydroxide
or combinations of the two.
The proportion of etching components employed is in a concentration
15 range 2 - 50% by weight, preferably 5 - 48% by weight, based on the
totaL weight of the etching paste. Particular preference is given to etch-
ing media in which the etching components are present in an amount of
10 - 45% by weight. Particularly suitable are etching media in which the
etching component(s) is (are) present in an amount of 30 - 40% by
20 weight, based on the total weight of the etching paste, since etching
rates which facilitate complete opening of the p-n transition and a high
throughput and at the same time show high selectivity have been found
for etching media of this type.
25 The etching components are effective in the etching pastes at 70-
150 °C. On silicon surfaces and layers, etching depths of less than 1 µm
have been achieved at temperatures below 100°C and etching depths
of up to 2-3 µm have been achieved at temperatures above 100°C.
30 Suitable inorganic and/or organic solvents and/or mixtures thereof may
be the following:
• water
• simple or polyhydric alcohols (for example isopropanol, diethylene
35 glycol, dipropylene glycol, polyethylene glycols, 1,2-propanediol, 1,4-

WO2004/032218 PCT/EP2003/008805
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butanediol, 1,3 -butanediol, glycerol, 1,5 pentanedioI, 2-ethyl-1-hexa -
nol) or mixtures thereof
•ketones (for example aceiophenone, methyI-2-hexanone,
2-octanone, 4-hydroxy-4- methyl-2- pentanone, 1 -methyI-2-5 pyrrolidone)
•ethers (for example ethylene glycol monobutyl ether, ethylene glyool
monomethyl ether, triethylene glycol monomethyl ether, diethylene
glycol monobutyl ether, dipropylene glycol monomethyl ether)
•carboxylic acid esters (for example [2,2-butoxy(ethoxy)]ethyl acetate)
10 • esters of carbonic acid (for example propylene carbonate)
Preference is given to the use of water and solvents from the group
consisting of the ethers and ketones.
15 Water has proven particularly suitable.
The proportion of the solvents is in the range 10 - 90% by weight, pref-
erably 15 - 85% by weight, based on the total weight of the etching
paste. Particularly suitable compositions have proven to be those in
20 which solvents are present in an amount of 55 - 75% by weight, based
on the total weight of the etching paste.
The viscosity of the etching pastes described in accordance with the
invention is set by means of network-forming thickeners which swell in
25 the liquid phase and can be varied depending on the desired area of
application.
Possible thickeners are crosslinked and uncrosslinked homopolymers
and copolymers based on monomer units such as funciionalised vinyl
30 units, for example acrylic acid, acrylates, alkyl methacrylates (C1o-C30)
and hydroxyalkylguar, xanthan gum and β-glucosidically linked glucose
units, i.e. cellulose and/or cellulose derivatives, such as cellulose
ethers, in particular ethyl-, hydroxypropyl- or hydroxyethylcellulose,
carboxymethylcellulose and salts of the glycolic acid ether of cellulose,
35 inparticular sodium carboxymethylcellulose.The thickeners can be
employed individually and/or in combination with other thickeners. Pref-

WO 2004/032218 PCT/EP2003/008805
-11 -
erenee is given to the use of salts of carboxymethylcellulose and cross-
linked acrylic acid polymers as thickener. The sodium salt of carboxy-
methylcellulose (Finnfix®) and crosslinked acrylic acid homopolymers
(Carbomers®) have proven very particularly suitable for this purpose.
5
The proportion of thickeners necessary for the specific setting of the
viscosity range and for the formation of a printable or dispensable paste
is in the range 0.5 - 25% by weight, preferably 1 - 10% by weight,
based on the total weight of the etching paste. Particularly suitable
10 compositions have proven to be those in which thickeners are present in
an amount of 1.5 - 6% by weight.
Additives having properties which are advantageous for the desired
purpose are antifoams, for example TEGO® Foamex N (dimethylpoly-
15 siloxane), thixotropic agents, for example BYK® 410 (modified urea),
Borchigel® Thixo2, flow-control agents, for example TEGO® Glide ZG
400 (polyether-siloxane copolymer), deaerators, for example TEGO®
Airex 986 (polymer with silicone tip), and adhesion promoters, for
example Bayowet® FT 929 (fluorosurfactant). These can positively
20 influence the printability and dispensability of the etching paste. The
proportion of the additives is in the range 0 - 2% by weight, based on
the total weight of the etching paste.
It has been found through experiments that both the choice of the com-
25 ponents employed for the preparation of the etching media and the
mixing ratio of the components to one another in the etching media is of
considerable importance. Depending on the manner in which the
etching media are applied to the area to be etched, the percentage ratio
of the components to one another should be set differently, since, inter
30 alia, the viscosity and flowability or the thixotropy being set are consid-
erably influenced by the amounts of solvent and thickener present- The
amounts of solvent and thickener present in turn influence the etching
behaviour. Depending on the type of use in the process according to the
invention, it is therefore possible for the person skilled in the art to
35 select a correspondingly adapted composition of the etching medium.

WO 2004/032218 PCT/EP2003/008805
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Areas of application
Areas of application of the etching pastes according to the invention are
in the;
5
• solar-cell Industry
• semiconductor industry
• high-performance electronics
10 The etching pastes according to the invention can be employed in all
areas where full-area and/or structured etching of silicon surfaces or
layers is desired. Thus, individual structures can be etched over the full
area or selectively to the depth desired in each case in a silicon surface
or layer.
15
Areas of application are, for example:
• all etching steps (synonymous with structuring steps), including sur-
face cleaning/roughening of silicon surfaces and layers, which result
20 in the production of optoelectrical components, such as solar cells,
photodiodes and the like, in particular isolation of the p-n transition in
silicon solar ceils and the partial removal of doped layers (selective
emitters)
• all etching steps on silicon surfaces and layers which result in the
25 production of semiconductor components and circuits
• all etching steps on silicon surfaces and layers which result in the
production of components in high-performance electronics (IGBTs,
power thyristors, GTOs, etc.).
30 For better understanding and for illustration of the invention, examples
are given below which fall within the scope of protection of the present
invention, but are not suitable for restricting the invention to these
examples.
35

WO 2004/032218 PCT/EP2003/008805
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Examples
Example 1
5 40.0 g of KOH
59.0 g of deionised water
1.5 g of ethylene gfycol monobutyl ether
4.0 g of Carbomer (acrylic acid homopolymer)
10 The chemicals were weighed into a beaker, mixed and dissolved, and
the thickener was added with stirring.
The mixture is transferred into containers after a short standing time.
This mixture gives an etching paste with which, for example, silicon
15 surfaces and layers can be etched specifically over the entire area or in
structures down to a desired depth with and/or without input of energy.
The etching paste was applied to the silicon surface, for example by
screen printing or using a dispenser (for example pin diameter of
20 260 µm), and etched on a hotplate for 3 min at 100°C. On production of
etch structures with a fine width of about 1 mm on an n-doped (100) sili-
con wafer, the etching depth determined (depending on the printing and
dispensing parameters) is 0.3 - 1 µm. The etching depth can be
increased by increasing the KOH concentration and the line width. For
25 line widths of 4 mm and KOH concentrations of 20 - 50% by weight, the
etching depths are 2 - 3 µn.
The resultant etching paste is stable on storage, easy to handle and
printable- It can be removed from the printed surface or layer or from the
30 paste carrier (screen, doctor blade, template, stamp, klischee, cartridge,
etc.) using a solvent, for example using water
35

WO 2004/032218 PCT/EP2003/008805
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Example 2
8.0 g of KOH
26.4 g of deionised water
5 4.0 g of N-methylpyrrolidone
2.3 g of the Na salt of carboxymethylcellulose (Finnfix®)
The batch and processing were carried out as described in Example 1.
10 The etching paste was applied to the silicon surface using a dispenser
(pin diameter 450 µm) and etched for 3 min at an etching temperature
of 130°C. On production of etch structures with a line width of about
1 mm on an n-doped (100) silicon wafer, the etching depth determined
(depending on the printing and dispensing parameters) is 0.2 - 1 µm.
15
Example 3
37.8% by weight of water
0.8% by weight of ethanoiamine
20 50.0% by weight of ethylene glycol
4.7% by weight of tetraethylammonium hydroxide
4.7% by weight of tetrapropylammonium hydroxide
0.3% by weight of gallic acid
25 1.8 % by weight of the Na salt of carboxymethylcellulose (Finnfix®)
The batch and processing were carried out as described in Example 1.
The etching paste was applied to the silicon surface by screen printing
30 or using a dispenser (pin diameter 450 µm) and etched for 3 min at an
etching temperature of 130°C. The etching depth determined on pro-
duction of etch structures with a line width of 1 mm is about 200 nm on
a silicon wafer,
35

WO 2004/032218 PCT/EP2003/008805
- 15-
Example 4
39.4% by weight of deionised water
49.3% by weight of ethylene glycol
5 4.9% by weight of tetraethylammonium hydroxide
4.9% by weight of tetrapropylammonium hydroxide
1.5% by weight of the Na salt of carboxymethylcellulose (Finnfix®)
The batch and processing were carried out as described in Example 1.
10
The etching paste was applied to the silicon surface by screen printing
or using a dispenser (pin diameter 450 µm) and etched for 3 min at an
etching temperature of 130°C. The etching depth determined on pro-
duction of etch structures with a line width of 1 mm is about 300 nm on
15 a silicon water
Example 5
50% by volume of tetraethylammonium hydroxide
20 20 % by volume of ethylenediamine
20% by volume of deionised water
10% by volume of triethylenetetraamine
0.25 g of gallic acid
25 3% by weight of xanthan gum
The batch and processing were carried out as described in Example 1.
The etching paste was applied to the silicon surface by screen printing
30 or using a dispenser (pin diameter 450 µm) and etched for 3 min at an
etching temperature of 100°C. The etching depth determined on pro-
duction of etch structures with a line width of 4 mm is about 1 µm on a
silicon wafer.
35

WO 2004/032218 PCT/EP2003/008805
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[1] W. Wettling, Phys. Bl. 12 (1997), pp. 1197-1202
[2] J. Horzel, J. Slufzik, J, Nijs, R. Mertens, Proc. 26th IEEE PVSC,
(1997), pp. 139-42
5 [3] M. Schnell, R. Lüdemann, S. Schäfer, Proc. 16thEU PVSEC, (2000),
pp. 1482-85
[4] D.S. Ruby, P. Yang, S. Zaidi, S. Brueck, M. Roy, S. Narayanan,
Proc. 2nd World Conference and Exhibition on PVSEC, (1998), pp.
1460-63
10
[5] US 6,091,021 (2000), D.S. Ruby, W.K. Schubert, J.M. Gee, S.H
Zaidi
[6] US 5,871,591 (1999), D.S. Ruby, J.M. Gee, W.K. Schubert
15 [7] EP 0229915 (1986), M. Bock, K. Heymann, H.J. Middeke, D. Ten-
brink
[8] WO 00/40518 (1998), M. Lulyf R. Singh, C. Redmon, J. Mckown, R.
Pratt
20
[9] DE 10101926 (2000), S. Klein, L. Haider, C. Zielinski, A, Kubelbeck,
W. Stockum
[10] A.F. Bogenschütz, Ätzpraxis für Halbleiter [Etching Practice for
25 Semiconductors], Carl Hanser Verlag, Munich 1967
30
35

WO 2004/032218 PCT/EP2003/008805
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PATENT CLAIMS
1. Etching medium for the etching of silicon surfaces and layers,
characterised in that
5 the etching medium is a thickened, alkaline liquid.
2. Printable etching medium according to Claim 1, characterised in
that it is an etching paste which comprises
a. at least one solvent
b. 10 b. thickeners and optionally
c. additives, such as antifoams, thixotropic agents, flow-control
agents, deaerators and adhesion promoters,
and is effective at temperatures as low as from 70 to 150°C
and/or can, if desired, be activated by the input of energy,
15
3. Etching medium according to Claims 1 and 2, characterised in
that it comprises, as etching component, an organic or inorganic
base in a concentration of from 2 to 50% by weight, preferably
from 5 to 48% by weight, based on the total amount.
20
4. Etching medium according to Claim 3, characterised in that it
comprises, as etching component, at least one component
selected from the group consisting of sodium hydroxide, potas-
sium hydroxide, ammonia, ethenolamine, ethylenediamine,
25 tetraalkylammonium hydroxide or one of the ethylenediamine/
pyrocatechol and ethanofamine/gallic acid mixtures.
5. Etching medium according to Claim 2, characterised in that it a
solvent selected from the group consisting of water, isopropanol,
30 diethylene glycol, dipropylene glycol, polyethylene glycols, 1,2-
propanediol, 1,4-butanediol, 1,3-butanediol, glycerol, 1,5
pentanediol, 2-ethyl-1-hexanol or mixtures thereof, or selected
from the group consisting of acetophenone, methyl-2-hexanone,
2-octanone, 4-hydroxy-4-methyl-2-pentanone, 1-methyl-2-
35 pyrrolidone, ethylene glycol monobutyl ether, ethylene glycol
monomethyl ether, triethylene glycol monomethyl ether, di-

WO2004/032218 PCT/EP2003/008805
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ethylene glycol monobutyl ether, dipropylene glycol monomethyl
ether, carboxylic acid esters, such as [2,2-butoxy(ethoxy)]ethyl
acetate, propylene carbonate as such or in a mixture in an
amount of from 10 to 90% by weight, preferably in an amount of
5 from 15 to 85% by weight, based on the total amount of the
medium.
6. Etching medium according to Claim 2, characterised in that it
comprises a thickener selected from the group consisting of
10 hydroxyalkylguar, xanthan gum, cellulose and/or ethyl-, hydroxy-
propyl- or hydroxyethy (cellulose, carboxymethylcellulose,
sodium carboxymethylhydroxyethylcellulose, homopolymers or
copolymers based on functionalised vinyl units of acrylic acid,
acrylates and alkyl methacrylates (C10-C30), individually or in a
15 mixture in an amount of from 0.5 to 25% by weight, preferably
from 1 to 10% by weight, based on the total amount of the
etching medium.
7. Etching medium according to Claim 3, characterised in that it
20 comprises additives selected from the group consisting of
antifoams, thixotropic agents, flow-control agents, deaerators
and adhesion promoters in an amount of from 0 to 2% by
weight, based on the total amount.
25 8. Process for the etching of silicon surfaces and layers, character-
ised in that an etching medium according to Claims 1-7 is
applied over the entire area or in accordance with the etch
structure mask specifically only to the areas of the surface
where etching is desired and is removed again after an expo-
30 sure time of from 30 s to 5 min.
9. Process according to Claim 8, characterised in that the etching
medium acts at a temperature in the range from 70 to 150ºC
and/or, if necessary, is activated by the input of energy.

WO 2004/032218 PCT/EP2003/008805
-19-
10. Process according to Claim 9, characterised in that the etching
medium is activated by exposure to heat (IR lamp, hotplate).
11. Process according to Claim 8, characterised in that the etching
5 medium is applied to the surface to be etched by a screen,
template, pad, stamp, ink-jet or manual printing process or by a
dispensing technique.
12. Process according to Claim 8, characterised in that the etching
10 medium is rinsed off using a solvent or solvent mixture when the
etching is complete.
13. Use of an etching medium according to C/aims 1-7 in photo-
voltaics, semiconductor technology, high-performance elec-
15 tronics and for the production of photodiodes, circuits, electronic
components.
14. Use of an etching medium according to Claims 1-7 for the etch-
ing of silicon surfaces and layers for isolation of the p-n transi-
20 tion in solar cells.
15. Use of an etching medium according to Claims 1-7 for the etch-
ing of silicon surfaces and layers for the production of a selec-
tive emitter for solar cells,
25
16. Use of an etching medium according to Claims 1-7 for the etch-
ing of silicon surfaces and fayers of solar cells for improving the
antireflection behaviour.
30 17. Use of an etching medium according to Claims 1-7 for the etch-
ing of silicon surfaces and layers in a process for the production
of semiconductor components and circuits thereof.
18. Use of an etching medium according to Claims 1-7 for the etch-
35 ing. of silicon surfaces and layers in a process for the production
of components in high-performance electronics.

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Patent Number

218575

Indian Patent Application Number

00554/KOLNP/2005

PG Journal Number

14/2008

Publication Date

04-Apr-2008

Grant Date

02-Apr-2008

Date of Filing

01-Apr-2005

Name of Patentee

MERCK PATENT GMBH

Applicant Address

FRANKFURTER STRESSE 250, 64293 DARMSTADT, GERMANY

Inventors:

#	Inventor's Name	Inventor's Address
1	KLEIN, SYLKE	AUF DEM WINGERT 8, 64380 ROSSDORF, GERMANY.
2	KUBELBECK, ARMIN	AUGERTENSTRASSE 45, 64625 BENSHATM, GERMANY
3	STOCKUM, WERNER	WALDSTRESSE 59, 64354 REINHEIM, GERMANY.

PCT International Classification Number

C09K13/02

PCT International Application Number

PCT/EP2003/008805

PCT International Filing date

2003-08-08

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	102 41 300.2	2002-09-04	Germany