WO2011146206A1 - Improved chemistries for the texturing of silicon substrates - Google Patents

Improved chemistries for the texturing of silicon substrates Download PDF

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Publication number
WO2011146206A1
WO2011146206A1 PCT/US2011/033839 US2011033839W WO2011146206A1 WO 2011146206 A1 WO2011146206 A1 WO 2011146206A1 US 2011033839 W US2011033839 W US 2011033839W WO 2011146206 A1 WO2011146206 A1 WO 2011146206A1
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solution
etching
texturing
silicon
solutions
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PCT/US2011/033839
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French (fr)
Inventor
Curtis Dove
Baljit Singh
Greg Bauer
Jackson Hu
Mehdi Balooch
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Asia Union Electronic Chemical Corporation
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Publication of WO2011146206A1 publication Critical patent/WO2011146206A1/en

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    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K13/00—Etching, surface-brightening or pickling compositions
    • C09K13/02—Etching, surface-brightening or pickling compositions containing an alkali metal hydroxide
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/02—Details
    • H01L31/0236—Special surface textures
    • H01L31/02363—Special surface textures of the semiconductor body itself, e.g. textured active layers
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy

Definitions

  • the present invention relates to improves chemistries, etching solutions and methods for texturing of silicon substrates, particularly for use as solar cells or photovoltaic cells.
  • ARC anti-reflective coating
  • Another method of reducing reflectance and improving device efficiency is to texture the silicon wafer surface using a wet-chemical etch to from small structures, usually having a pyramidal shape. These structures provide higher levels of light trapping due to multiple scattering. Based on geometrical optics, the texturing should be on a scale equal to or greater than optical wavelengths of the incident light to cause the incident light to reflect multiple times and thereby enhance the amount of absorption.
  • the texturing process is generally carried out using a mixture of KOH or NaOH and IPA in DI water as the etchant.
  • IPA serves to mask specific silicon sites, preventing etching by the solution, to thereby form the pyramidal structures. It has also been reported that a combination of IP A and aqueous alkaline ethylene glycol resulted in more uniform pyramidal texturing on highly polished silicon (100) for use in semiconductor electronic applications.
  • TMAH tetramethylammomum hydroxide
  • Texturization of the single crystal silicon substrate is the result of anisotropic etching.
  • the etch rate for silicon using alkaline solutions depends strongly on the orientation of the crystal faces exposed to the etching solution.
  • the alkaline etch rate of a Si (111) orientation is one to two orders of magnitude smaller than that of other orientations.
  • the etched structures have (111) sidewalls.
  • the substrate is a (100) crystalline wafer, the resulting structures appear as four- sided pyramids.
  • the etching process creates hydrogen bubbles that have a tendency to stick on the surface and create discoloration traces that resemble "light comets on a dark background" on the etched surface.
  • IPA and EG can help to increase the wettability of the solution which assists in the release of hydrogen bubbles.
  • IPA is not the most effective solution for creating uniform and reliable pyramidal structures when the solution is fresh. Rather, it has been found that as the solution ages, the uniformity and consistency of textured wafers improves. Because fresh etching solution does not provide uniform and consistent texturing, a significant amount of wafers may need to be discarded before the etching solution is broken in and starts to provide acceptable uniformity and consistency.
  • the present invention provides improved wet chemical solutions for texturing of silicon substrates, particularly for use as solar cells or photovoltaic devices.
  • the solutions of the present invention provide more consistent and uniform texturing over the entire life of the solution. This improvement results in fewer discarded wafers and therefore increases reliability and yield while lowering the cost.
  • Figure 1 is representative drawing of an early stage of an etching process for a silicon substrate depicting how sites on the substrate surface are protected through chemical interaction with the etching solution to aid in the creation of uniform pyramidal texturing.
  • FIG. 1 is representative drawing of a later stage of an etching process for a silicon substrate depicting the formation of uniform pyramidal texturing.
  • the present invention relates to improved wet chemical solutions for texturing of silicon substrates, particularly for use as solar cells or photovoltaic devices.
  • the solutions of the present invention provide more consistent and uniform texturing over the entire life of the solution, resulting in fewer discarded wafers, increased reliability and yield and lower costs.
  • Figure 1 and 2 show how the presence of Si0 2 containing ions in the etching solution can aid the formation of uniform and consistent pyramidal structures on a silicon substrate.
  • an OH " containing etchant e.g. KOH, NaOH or TMAH
  • the solution reacts with silicon in the substrate to form Si0 2 (OH) 2 ⁇ that rapidly diffuses away from the etched surface because of low concentration near the surface (referred to as reaction controlled process).
  • reaction controlled process concentration near the surface
  • diffusion controlled process concentration of Si0 2 (OH) 2 ⁇ 2 increases and begins to build up near the surface, resulting in a reduction of diffusion rate (referred to as diffusion controlled process).
  • the Si0 2 (OH)2 -2 may deposit back as adsorbed Si0 2 onto the substrate surface which impedes further etching and results in a textured surface.
  • Si0 2 from the Si0 2 (OH) 2 "2 byproduct of the etching process bonds with the silicon surface and protects the area of the surface directly below from etching.
  • the OH " (from the KOH, NaOH or TMAH etching solution) continues to etch the surface around the sites where the adsorbed Si0 2 is present, or generally along the lines shown in Figure 1 , ultimately resulting in the pyramidal structures shown in Figure 2.
  • IP A in the etching solution provides some masking effect for specific silicon sites which may also help to form more uniform pyramidal structures. This is attributable to the adsorption of carbon atoms from the IP A onto the silicon surface. These carbon containing atoms are very weakly bonded with the sites on the silicon surface through hydrogen bonding of (CH 3 ) 2 CH*.
  • Si0 2 containing ions are concocted in the etching solution prior to first use to improve the anisotropic etch that creates the pyramidal texturing.
  • the present invention provides the Si0 2 containing ions in the form of an additive Si0 2 gel that is made by dissolving Si in KOH, NaOH or TMAH at high temperature, e.g. about 80° C, until all of the OH " contained in the KOH, NaOH or TMAH is consumed. The resulting solution is then cooled to room temperature and can be stored until needed. The Si0 2 gel is then dissolved into fresh etchant solution so that the etchant solution comprises KOH (or NaOH or TMAH), IPA and the additional monomelic dihydroxo anion (Si0 2 (OH) 2 "2 ).
  • silicic acid [SiO x (OH)4-2x] n can be used as the source of Si0 2 containing ions.
  • Silicic acid may be formed by acidification of silicate salts, e.g. sodium silicate, Na 2 Si0 3 also know as water glass or liquid glass, in aqueous solution. When heated the sodium silicate loses water to form silica gel, an active form of silicon dioxide that can be used as the additive.
  • a further alternative for use as the additive is ammonium silicate,
  • ethylene glycol may also be added to the etch solution.
  • the first reaction sequence is based on the use of KOH as the main etchant solution.
  • the second reaction sequence is based on the use of NaOH as the main etchant solution.
  • the third reaction sequence is based on the use of TMAH as the main etchant solution.
  • the effects of adding adsorbed Si0 2 + to the etching solutions according to the present invention are improved uniformity and consistency. In particular, superior results are achieved over the use of etching solutions that do not have silicate additives.
  • the etching solutions of the present invention provide more consistent and uniform texturing of silicon substrates over the entire life of the etching solution. This results in fewer discarded wafers, particularly when the etching solution is fresh, and therefore increases reliability and yield with a resultant lowering of cost.
  • the pyramidal structures formed using the etching solutions of the present invention are uniform and consistent the textured substrates that are formed are particularly useful for use in solar cells and photovoltaic devices.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • Electromagnetism (AREA)
  • General Physics & Mathematics (AREA)
  • Computer Hardware Design (AREA)
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Abstract

Improved wet chemical solutions for texturing of silicon substrates, particularly for use as solar cells or photovoltaic devices are described. The solutions of the present invention provide more consistent and uniform texturing over the entire life of the solution, resulting in fewer discarded wafers, increased reliability and yield and lower costs. The etching solutions are OH- containing solutions, e.g. KOH, NaOH or TMAH, optionally mixed with IPA, having an additive that provides Si02 containing ions.

Description

IMPROVED CHEMISTRIES FOR THE TEXTURING OF SILICON SUBSTRATES
FIELD OF THE INVENTION
(001) The present invention relates to improves chemistries, etching solutions and methods for texturing of silicon substrates, particularly for use as solar cells or photovoltaic cells.
BACKGROUND OF THE INVENTION
(002) It is known that the efficiency of solar cells and photovoltaic devices can be improved by reducing the reflectance of incident solar light on the device.
(003) One common method of reducing reflectance is to coat the silicon substrate with an anti-reflective coating (ARC), such as silicon oxide, silicon nitride or titanium dioxide. However, these films exhibit resonance structures that limit their
effectiveness to a small range of angles and wavelengths, such that reflectance becomes highly dependent on the angle of incidence and wavelength of the light.
(004) Another method of reducing reflectance and improving device efficiency is to texture the silicon wafer surface using a wet-chemical etch to from small structures, usually having a pyramidal shape. These structures provide higher levels of light trapping due to multiple scattering. Based on geometrical optics, the texturing should be on a scale equal to or greater than optical wavelengths of the incident light to cause the incident light to reflect multiple times and thereby enhance the amount of absorption.
(005) For single crystal Si(100) oriented substrates, the texturing process is generally carried out using a mixture of KOH or NaOH and IPA in DI water as the etchant. The addition of IPA serves to mask specific silicon sites, preventing etching by the solution, to thereby form the pyramidal structures. It has also been reported that a combination of IP A and aqueous alkaline ethylene glycol resulted in more uniform pyramidal texturing on highly polished silicon (100) for use in semiconductor electronic applications.
(006) In co-pending United States patent application 12/366, 141 , the texturing is carried out using a mixture of potassium hydroxide (KOH) or sodium hydroxide (NaOH) and isopropyl alcohol (IP A) in deionized (DI) water as the etchant. The resulting pyramidal structures are very uniform and exhibit desirable low reflectance values. Further, as set forth in this patent application, a portion of the IPA may be replaced with ethylene glycol (EG) for better surface wetting and providing even lower reflectance. In addition, in co-pending United States patent application
12/695,209, the use of tetramethylammomum hydroxide (TMAH) with IPA and EG as the etching solution is set forth. This combination of etching chemicals improves wettability and uniformity across the substrate surface.
(007) Texturization of the single crystal silicon substrate is the result of anisotropic etching. The etch rate for silicon using alkaline solutions depends strongly on the orientation of the crystal faces exposed to the etching solution. In particular, the alkaline etch rate of a Si (111) orientation is one to two orders of magnitude smaller than that of other orientations. As a result, the etched structures have (111) sidewalls. When the substrate is a (100) crystalline wafer, the resulting structures appear as four- sided pyramids.
(008) The etching process creates hydrogen bubbles that have a tendency to stick on the surface and create discoloration traces that resemble "light comets on a dark background" on the etched surface. As noted above, the use of IPA and EG can help to increase the wettability of the solution which assists in the release of hydrogen bubbles. However, IPA is not the most effective solution for creating uniform and reliable pyramidal structures when the solution is fresh. Rather, it has been found that as the solution ages, the uniformity and consistency of textured wafers improves. Because fresh etching solution does not provide uniform and consistent texturing, a significant amount of wafers may need to be discarded before the etching solution is broken in and starts to provide acceptable uniformity and consistency.
(009) Therefore, there remains a need in the art for further improvements to texturing silicon substrates, particularly for use in solar cells and photovoltaic devices. In particular, there remains a need in the art for improvements to the consistency and uniformity of production throughout the life of the etching solution.
SUMMARY OF THE PRESENT INVENTION
(010) The present invention provides improved wet chemical solutions for texturing of silicon substrates, particularly for use as solar cells or photovoltaic devices. The solutions of the present invention provide more consistent and uniform texturing over the entire life of the solution. This improvement results in fewer discarded wafers and therefore increases reliability and yield while lowering the cost.
BRIEF DESCRIPTION OF THE DRAWINGS
(011) Figure 1 is representative drawing of an early stage of an etching process for a silicon substrate depicting how sites on the substrate surface are protected through chemical interaction with the etching solution to aid in the creation of uniform pyramidal texturing.
(012) Figure 2 is representative drawing of a later stage of an etching process for a silicon substrate depicting the formation of uniform pyramidal texturing. DETAILED DESCRIPTION OF THE INVENTION
(013) The present invention relates to improved wet chemical solutions for texturing of silicon substrates, particularly for use as solar cells or photovoltaic devices. The solutions of the present invention provide more consistent and uniform texturing over the entire life of the solution, resulting in fewer discarded wafers, increased reliability and yield and lower costs.
(014) The poor uniformity obtained when using fresh etching solutions of OH or NaOH and IPA can be attributed to a relatively low concentration of dissolved Si in the solution. In particular, various studies have been done to determine the effect of Si dissolution in etching solutions. For example, one investigation that studied the effect of accumulating Si in an aqueous KOH solution found that the etch rate of (100) planes of a single crystalline silicon substrate was only minimally effected by the silicon concentration up to a level as high as 58 g liter in a 33% KOH solution. (See Dorsch et al, Transducers '97, 1997 International Conference on Solid-State Sensors and Actuators, Chicago, June 16-19, 1997). A further study used a TMAH etching solution and found inconsistencies in reflectance measurements for solar cells produced, the inconsistencies attributed to the formation of only partial pyramidal structure coverage. However, as the same solution was re-used, subsequently treated wafers showed more consistent reflectance, leading to the speculation that the difference in results between fresh and used solutions may be caused by the amount of dissolved silicon in the TMAH etching solution. (See Iencinella et al, An Optimized Texturing Process for Silicon Solar Cell Substrates Using TMAH, Solar Energy Materials & Solar Cells 87, pp 725-732, 2005).
(015) Figure 1 and 2 show how the presence of Si02 containing ions in the etching solution can aid the formation of uniform and consistent pyramidal structures on a silicon substrate. When etching with an OH" containing etchant, e.g. KOH, NaOH or TMAH, the solution reacts with silicon in the substrate to form Si02(OH)2 ~ that rapidly diffuses away from the etched surface because of low concentration near the surface (referred to as reaction controlled process). However, as the etching solution is reused, the concentration Si02(OH)2 ~2 increases and begins to build up near the surface, resulting in a reduction of diffusion rate (referred to as diffusion controlled process). Finally, when high concentrations of Si02(OH)2 "2 have been obtained, the Si02(OH)2-2 may deposit back as adsorbed Si02 onto the substrate surface which impedes further etching and results in a textured surface.
(016) As shown in Figure 1 , Si02 from the Si02(OH)2 "2 byproduct of the etching process bonds with the silicon surface and protects the area of the surface directly below from etching. The OH" (from the KOH, NaOH or TMAH etching solution) continues to etch the surface around the sites where the adsorbed Si02 is present, or generally along the lines shown in Figure 1 , ultimately resulting in the pyramidal structures shown in Figure 2.
(017) As noted above, IP A in the etching solution provides some masking effect for specific silicon sites which may also help to form more uniform pyramidal structures. This is attributable to the adsorption of carbon atoms from the IP A onto the silicon surface. These carbon containing atoms are very weakly bonded with the sites on the silicon surface through hydrogen bonding of (CH3)2CH*.
(018) In order to maintain consistency and uniformity throughout the life of the etching solution, it is necessary to have adequate Si02 containing ions in the solution at the beginning or when the etching solution is fresh. Therefore, in accordance with the present invention, Si02 containing ions are concocted in the etching solution prior to first use to improve the anisotropic etch that creates the pyramidal texturing.
(019) The present invention provides the Si02 containing ions in the form of an additive Si02 gel that is made by dissolving Si in KOH, NaOH or TMAH at high temperature, e.g. about 80° C, until all of the OH" contained in the KOH, NaOH or TMAH is consumed. The resulting solution is then cooled to room temperature and can be stored until needed. The Si02 gel is then dissolved into fresh etchant solution so that the etchant solution comprises KOH (or NaOH or TMAH), IPA and the additional monomelic dihydroxo anion (Si02(OH)2 "2). Alternatively, silicic acid, [SiOx(OH)4-2x]n can be used as the source of Si02 containing ions. Silicic acid may be formed by acidification of silicate salts, e.g. sodium silicate, Na2Si03 also know as water glass or liquid glass, in aqueous solution. When heated the sodium silicate loses water to form silica gel, an active form of silicon dioxide that can be used as the additive. A further alternative for use as the additive is ammonium silicate,
(NH4)2Si03. To increase wettability, ethylene glycol may also be added to the etch solution.
(020) Chemical reactions for achieving uniform and consistent texturing in accordance with the present invention are summarized below. The first reaction sequence is based on the use of KOH as the main etchant solution. The second reaction sequence is based on the use of NaOH as the main etchant solution. The third reaction sequence is based on the use of TMAH as the main etchant solution.
Reaction Sequence 1
A) KOH→ K+ + OH"
B) Si + 20H" + 2H20→ Si02(OH)2 " + 2H2
C) Si02(OH)2 "→ Si02 + (adsorbed) + OH" (solution)
Reaction Sequence 2
A) NaOH→ Na+ + OH"
B) Si + 20H" + 2¾0→ Si02(OH)2 " + 2H2
C) Si02(0H)2 "→ Si02 ÷ (adsorbed) + OH" (solution)
Reaction Sequence 3
A) (CH3)4NOH→ (CH3)4N+ + OH"
B) Si + 20H" + 2H20→ Si02(OH)2 " + 2H2
C) Si02(OH)2 "→ Si02 + (adsorbed) + OH" (solution) (021) As noted, carbon ions from IP A assist to some degree with the uniform formation of pyramids. However, much greater consistency and uniformity are achieved from the simultaneous etching and deposition of silicon, particularly silicon ions coming out of solution and decomposing to deposit as absorbed Si02 on the substrate surface. The combination of IP A and a silicon ion source additive may be optimal for etching uniform and consistent pyramidal structures.
(022) The effects of adding adsorbed Si02 + to the etching solutions according to the present invention are improved uniformity and consistency. In particular, superior results are achieved over the use of etching solutions that do not have silicate additives. By priming the etching solutions with Si02 containing ions according to the present invention, a number of advantages are obtained. In particular, the etching solutions of the present invention provide more consistent and uniform texturing of silicon substrates over the entire life of the etching solution. This results in fewer discarded wafers, particularly when the etching solution is fresh, and therefore increases reliability and yield with a resultant lowering of cost. Further, because the pyramidal structures formed using the etching solutions of the present invention are uniform and consistent the textured substrates that are formed are particularly useful for use in solar cells and photovoltaic devices.
(023) It is anticipated that other embodiments and variations of the present invention will become readily apparent to the skilled artisan in the light of the foregoing description, and it is intended that such embodiments and variations likewise be included within the scope of the invention as set out in the appended claims.

Claims

CLAIMS What is claimed:
1. A wet chemical solution for etching silicon substrates, comprising an OH" containing compound having S1O2 containing ions dissolved therein.
2. The solution of claim 1 , wherein the OH" containing compound is KOH, NaOH or TMAH.
3. The solution of claim 1 , further including IP A.
4. The solution of claim 1, wherein the Si02 containing ions are provided in the form of Si02 gel, silicic acid or sodium silicate.
5. The solution of claim 1, further including ethylene glycol.
6. A method of etching a silicon substrate, comprising:
providing the silicon substrate; and
etching the silicon substrate with an etching solution comprising an OH" containing compound having Si02 containing ions dissolved therein.
7. The method of claim 6, wherein the OH" containing compound is KOH, NaOH or TMAH.
8. The method of claim 6, wherein the etching solution further includes IPA.
9. The method of claim 6, wherein the Si02 containing ions are provided in the form of Si02 gel, silicic acid or sodium silicate.
10. The method of claim 6, wherein the etching solution further includes ethylene glycol.
11. A textured silicon substrate formed by etching the silicon substrate with an etching solution comprising an OH" containing compound having Si02 containing ions dissolved therein.
12. The substrate of claim 11 , wherein the OH" containing compound is KOH, NaOH or TMAH.
13. The substrate of claim 11 , wherein the etching solution further includes IP A.
14. The substrate of claim 11 , wherein the Si02 containing ions are provided in the form of S1O2 gel, silicic acid or sodium silicate.
15. The substrate of claim 11 , wherein the etching solution further includes ethylene glycol.
PCT/US2011/033839 2010-05-18 2011-04-26 Improved chemistries for the texturing of silicon substrates WO2011146206A1 (en)

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US34570410P 2010-05-18 2010-05-18
US34569910P 2010-05-18 2010-05-18
US61/345,699 2010-05-18
US61/345,704 2010-05-18

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Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4137123A (en) * 1975-12-31 1979-01-30 Motorola, Inc. Texture etching of silicon: method
US20090218542A1 (en) * 2008-02-28 2009-09-03 Hayashi Pure Chemical Ind, Ltd. Anisotropic silicon etchant composition

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4137123A (en) * 1975-12-31 1979-01-30 Motorola, Inc. Texture etching of silicon: method
US20090218542A1 (en) * 2008-02-28 2009-09-03 Hayashi Pure Chemical Ind, Ltd. Anisotropic silicon etchant composition

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