Patent Application
20130181331
Smooth silicon surfaces can reflect about 35% of incident light, which can cause losses in solar cells made of the silicon. Wu Meiling, Z. W., Zhang Xinqiang, Liu Hao, Jia Shiliang & Qiu Nan, Study on the SiN Anti-Reflective Coating for Nanocrystalline Silicon Solar Cells, in P
Coatings of amorphous silicon carbide (a-SiC:H), amorphous silicon nitride (a-SiN:H), and amorphous silicon carbonitride (a-SiCN:H) can be used as a single-layer ARC in photovoltaic applications. See generally M. H. Kang, D. S. Kim, A. Ebong, B. Rounsaville, A. Rohatgi, G. Okoniewska & J. Hong, The Study of Silane-Free SiCxNy Film for Crystalline Silicon Solar Cells, 156(6) J
In one aspect, the disclosure provides a process for forming a silicon-containing film on a substrate, the process comprising providing a substrate, providing a precursor comprising silicon, and reacting the precursor with a gas comprising nitrogen (N2) in a low-temperature plasma at atmospheric pressure, wherein the products of the reacting form a film on the substrate.
In another aspect, the disclosure provides an antireflection coating made by a process comprising reacting a silicon-containing precursor with a gas comprising nitrogen (N2) in a low-temperature plasma at atmospheric pressure wherein the antireflection coating has a refractive index of about 1.5 to about 2.2.
In another aspect, the disclosure provides an article having a surface comprising an antireflection coating, wherein the coating may be made by a process comprising reacting a silicon-containing precursor with a gas comprising nitrogen (N2) in a low-temperature plasma at atmospheric pressure, wherein the coating has a refractive index of about 1.5 to about 2.2.
Other aspects and embodiments are encompassed within the scope of the disclosure and will become apparent in light of the following description and accompanying Drawings.
Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways.
It also is specifically understood that any numerical value recited herein includes all values from the lower value to the upper value, i.e., all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application. For example, if a concentration range or a beneficial effect range is stated as 1% to 50%, it is intended that values such as 2% to 40%, 10% to 30%, or 1% to 3%, etc. are expressly enumerated in this specification. These are only examples of what is specifically intended.
Further, no admission is made that any reference, including any patent or patent document, cited in this specification constitutes prior art. In particular, it will be understood that, unless otherwise stated, reference to any document herein does not constitute an admission that any of these documents forms part of the common general knowledge in the art in the United States or in any other country. Any discussion of the references states what their authors assert, and the applicant reserves the right to challenge the accuracy and pertinence of any of the documents cited herein.
In a general sense, the disclosure relates to silicon-containing films with a refractive index suitable for antireflection, articles having a surface comprising the films, and atmospheric-pressure plasma-enhanced chemical vapor deposition (AE-PECVD) processes for the formation of surface films and coatings. The methods provided herein have advantages over known vacuum-based deposition methods that typically require large, expensive equipment with substantial operation and maintenance costs. See generally M. H. Kang, D. S. Kim, A. Ebong, B. Rounsaville, A. Rohatgi, G. Okoniewska & J. Hong, The Study of Silane-Free SiCxNy Film for Crystalline Silicon Solar Cells, 156(6) J
Similarly, processes employing atmospheric-pressure plasma have been used in surface cleaning and plasma polymerization, for example as a dielectric barrier discharge (Dow-corning), atmospheric-pressure plasma jet (see generally A. Schutze, J. Y. Jeong, S. E. Babayan, Jaeyoung Park; G. S. Selwyn & R. F. Hicks, The atmospheric-pressure plasma jet: a review and comparison to other plasma sources, 26(6) IEEE T
Atmospheric-pressure plasma methods also have utility in forming functional thin films. See, e.g., M. L. Hitchman, supra; Robert A. Sailer, Andrew Wagner, Chris Schmit, Natalie Klaverkamp & Douglas L. Schulz, Deposition of transparent conductive indium oxide by atmospheric-pressure plasma jet, 203(5-7) S
A “PECVD” or “plasma-enhanced chemical vapor deposition” as used herein includes any process in which a reactive gas is introduced into the reaction vessel and a plasma is created by applying an electric field across the reactive and plasma gas. In contrast to an atmospheric-pressure PECVD, in a conventional PECVD process the reaction vessel is at a pressure lower than ambient pressure. The reaction vessel in a PECVD process can be evacuated by means of vacuum pumps.
“SiC,” “SiN,” and “SiCN” as used herein represent materials that contain the indicated elements in various proportions. For example, “SiCN” is a material that comprises silicon, carbon, nitrogen, and, optionally, other elements. “SiC,” “SiN,” and “SiCN” are not chemical stoichiometric formulae per se and thus are not limited to materials that contain particular ratios of the indicated elements. Furthermore, “silicon carbide,” “silicon nitride,” and “silicon carbonitride” as used herein include both stoichiometric, such as, for example, Si3N4 for silicon nitride, and non-stoichiometric type materials.
A “substrate” as used herein includes one or more materials that are able to, or adapted to, receive a film or coating layer and can include at least one surface layer(s) upon which film is to be formed, such as, for example, a semiconductor wafer substrate of silicon.
“Plasma conditions” and “deposition parameters” as used herein include pressure, temperature, reactive gas concentration, and any other standard parameter that may affect the film quality and properties.
A “reactive gas” or “reactant gas” as used herein refers to the gas or gases being deposited in the CVD process.
Referring to
In general any compound having a formula Rx—Si, wherein R is selected from N-alkyl or C-alkyl, or any combination of alkyl groups, and x is an integer from selected from 1, 2, 3, or 4, can be used as the precursor for producing a silicon-based film, for example, silicon carbide, silicon nitride, silicon carbonitride, and the like, as described herein. In embodiments, the method comprises reacting or contacting a silicon-containing precursor in a plasma afterglow. In some embodiments, the silicon-containing precursor can comprise any suitable silane (Si—C) or silizane (Si—N) compound such as, for example, any branched or linear C1-C6 di-, tri-, or tetra-alkyl silane or silazane. Some non-limiting examples of such precursors include cyclohexasilane, dimethylsilane, trimethylsilane, tetramethylsilane, diethylsilane, triethylsilane (TES), tetraethylsilane, dipropylsilane, tripropylsilane, tetrapropylsilane, and the like. In some embodiments the precursors can include, for example, bis(tertiarybutylamino)silane, 1,1,3,3-tetramethyldisilazane, hexamethylcyclotrisilazane, tris(dimethylamino)methylsilane and bis(dimethylamino)methylsilane. In further embodiments precursor molecule can comprise one or more silicon-nitrogen (SiN) bonds (e.g., a silazane compound). In some embodiments, the precursor is liquid at room temperature. In further embodiments, the precursor is a volatile compound.
In some embodiments, the precursor is heated, for example in an oven, to a temperature of about 33° C. The temperature can be suitably higher or lower depending upon the precursor. For example, a cyclohexasilane precursor can be heated to about 55° C. to increase the vapor pressure. A carrier gas can be bubbled through the heated precursor to carry the heated precursor into a reaction vessel. The carrier gas can be helium, argon, nitrogen, or a combination thereof. In addition to the carrier gas, a reactive gas is flowed into the reaction vessel. The reactive gas includes nitrogen and optionally helium, argon, or hydrogen, ammonia, or a combination thereof. In embodiments, the reactive gas can include nitrogen in an amount of about 0.01% to about 100.00% and other optional gases (e.g., helium, argon, hydrogen) in an amount of 0.00% to about 99.99% by volume. In some embodiments, the reactive gas comprises nitrogen with 0% to about 5% hydrogen by volume. In some embodiments, the reactive gas can comprise about 45% or more, about 50% or more, about 55% or more, about 60% or more, about 65% or more, about 70% or more, about 75% or more, about 80% or more, about 82% or more, about 84% or more, about 86% or more, about 88% or more, about 90% or more, about 91% or more, about 92% or more, about 93% or more, about 94% or more, about 95% or more, about 96% or more, about 97% or more, about 98% or more, or about 99% or more by volume nitrogen. In some embodiments, the other optional gas comprises about 5% hydrogen by volume. In some embodiments, the reactive gas used in the disclosed method can be substantially free of ammonia. In other embodiments, the precursor includes cyclochexasilane and the reactive gas comprises ammonia. The reactive gas can comprise 0% to about 5% ammonia by volume.
In the reaction vessel, a substrate is awaiting the film deposition. In some embodiments, the substrate includes silicon. In further embodiments, the substrate is maintained at a temperature from about 25° C. to about 450° C. The substrate can be maintained at a temperature of about 25° C. or higher, about 50° C. or higher, about 75° C. or higher, about 100° C. or higher, about 125° C. or higher, about 150° C. or higher, about 175° C. or higher, about 200° C. or higher, about 225° C. or higher, about 250° C. or higher, about 275° C. or higher, about 300° C. or higher, about 325° C. or higher, about 350° C. or higher, about 375° C. or higher, about 400° C. or higher, about 425° C. or higher, or about 425° C. or higher. The substrate can be maintained at a temperature of about 450° C. or lower, about 425° C. or lower, about 400° C. or lower, about 375° C. or lower, about 325° C. or lower, about 300° C. or lower, about 275° C. or lower, about 250° C. or lower, about 225° C. or lower, about 200° C. or lower, about 175° C. or lower, about 150° C. or lower, about 125° C. or lower, about 100° C. or lower, about 75° C. or lower, or about 50° C. or lower. In some embodiments, the substrate can be maintained at a temperature of about 100° C. to about 450° C., about 200° C. to about 425° C., about 250° C. to about 425° C., or about 250° C. to about 350° C.
In order to deposit the film, an RF power or plasma power from about 40 W to about 150 W is applied to excite the plasma. In some embodiments, the plasma power is about 40 W or higher, about 50 W or higher, about 60 W or higher, about 70 W or higher, about 80 W or higher, about 90 W or higher, about 100 W or higher, about 110 W or higher, about 120 W or higher, about 130 W or higher, or about 140 W or higher. The plasma power can be about 150 W or lower, about 140 W or lower, about 130 W or lower, about 120 W or lower, about 110 W or lower, about 100 W or lower, about 90 W or lower, about 80 W or lower, about 70 W or lower, about 60 W or lower, or about 50 W or lower. In some embodiments, the plasma power is about 80 W to about 120 W, or about 110 W to about 130 W.
The disclosed method can be performed using any atmospheric-pressure plasma source with a low-temperature, or “non-thermal,” plasma. In some embodiments, the method can be performed using non-pyrophoric, non-toxic chemicals. The method can be performed in any suitable reaction vessel such as, for example, a glove box, a closed reactor or container, or in any environment that is substantially free of oxygen. In some embodiments, the reaction environment can, for example, be purged or shielded with nitrogen gas or argon in order to remove oxygen from the immediately surrounding atmosphere (e.g., a reaction environment that is free or substantially free of oxygen).
In another aspect, the disclosure relates to an antireflection coating made by a process comprising reacting a silicon-containing precursor with a gas comprising nitrogen (N2) in a low-temperature plasma at atmospheric pressure wherein the antireflection coating has a refractive index of about 1.5 to about 2.2. In some embodiments, the antireflection coating has a refractive index of about 1.1 or more, about 1.2 or more, about 1.3 or more, about 1.4 or more, about 1.5 or more, about 1.6 or more, about 1.7 or more, about 1.8 or more, about 1.9 or more, about 2.0 or more, or about 2.1 or more. The refractive index can be about 2.2 or less, about 2.1 or less, about 2.0 or less, about 1.9 or less, about 1.8 or less, about 1.7 or less, about 1.6 or less, about 1.5 or less, about 1.4 or less, about 1.3 or less, or about 1.2 or less. In some embodiments, the antireflection coating has a refractive index of about 1.6 to about 1.9, about 1.9 to about 2.2, about 2.0 to about 2.2, about 1.6 to about 1.8, about 1.6 to about 1.7, or about 1.5 to about 1.7.
In some embodiments, the disclosure relates to anti-reflection coatings including at least one of silicon nitride and silicon carbonitride, or multilayers thereof. In further embodiments, the coatings are substantially free of silicon oxide. The coatings are manufactured by methods as described herein. The coatings can be further characterized by a hardness of about 7 GPa to about 17 GPa (e.g., about 7, about 8, about 9, about 10, about 11, about 12, about 13, about 14, about 15, about 16, or about 17 GPa). In some embodiments, the coating has a hardness of about 7 GPa or more, about 8 GPa or more, about 9 GPa or more, about 10 GPa or more, about 11 GPa or more, about 12 GPa or more, about 13 GPa or more, about 14 GPa or more, about 15 GPa or more, or about 16 GPa or more.
In another aspect, the disclosure provides an article having a surface comprising an antireflection coating, wherein the coating may be made by a process comprising reacting a silicon-containing precursor with a gas comprising nitrogen (N2) in a low-temperature plasma at atmospheric pressure, wherein the coating has a refractive index of about 1.5 to about 2.2. Such articles include, but are not limited to, solar cells, protective coatings to prevent wear and corrosion, for example in opto electronic applications, and dielectric layers in microelectronics devices. The articles can also include windows and other applications that use panes of glass as substrates.
The present disclosure is illustrated by the following examples. It is to be understood that the particular examples, materials, amounts, and procedures are to be interpreted broadly in accordance with the scope and spirit of the invention as set forth herein.
A low-temperature atmospheric-pressure plasma was used with non-pyrophoric chemicals to obtain silicon-based coatings having refractive indices suitable for an antireflection coating. An atmospheric pressure plasma system by Surfx Technologies (Culver City, Calif.) was used with a triethylsilane precursor procured from Gelest Inc. (Morrisville, Pa.). The precursor was reacted with a mixture of nitrogen and hydrogen gas, and deposited on a silicon substrate that was heated to a temperature from about 250° C. to about 450° C. The refractive indices of the resulting coating were from about 1.60 to about 1.87.
A low-temperature atmospheric-pressure plasma was used in the atmospheric pressure plasma system Atomflow™ 250D by Surfx Technologies (Culver City, Calif.) (see generally M. Moravej & R. F. Hicks, supra; M. D. Barankin, E. Gonzalez II, A. M. Ladwig & R. F. Hicks, Plasma-enhanced chemical vapor deposition of zinc oxide at atmospheric pressure and low temperature, 91(10) S
The triethylsilane precursor was initially maintained in a heated bubbler at 33° C., bubbling helium gas through the triethylsilane precursor at 0.1 liter/minute. Subsequently, the triethylsilane precursor was delivered to the plasma source through delivery lines, which were maintained at 100° C. to preclude condensation. The substrate measured about 2.5 cm×2.5 cm and was maintained at a temperature from about 200° C. to about 425° C. The plasma head was held at 125° C., and at a distance of about 4 mm to about 5 mm from the substrate. Helium gas was supplied to the plasma source at about 20 liter/minute to about 30 liter/minute. Reactive gases included nitrogen with or without 5% by volume of hydrogen, at variable flow rates. Depositions were carried out by moving the heated substrate under the plasma source in a serpentine motion at a velocity of about 0.6×10−2 m·s−1. Suitable length, width, and step sizes were chosen to produce a uniform film deposition over the surface of the substrate.
To investigate the chemical bonding structure of the deposited films, Fourier transform infrared spectroscopy (FTIR) was performed with a Thermo Scientific Nicolet 8700 instrument. Spectroscopic ellipsometry was performed using an ellipsometer by J.A. Woollam Co. (Lincoln, Nebr.) to determine the film thickness, optical constant, and the reflectance. Spectroscopic ellipsometry was conducted at three different angles, namely, about 60°, about 67°, and about 74°. The measured ellipsometric parameters Ψ and Δ were fitted with the thin film model, where the thin film is assumed as Cauchy layer with silicon as the substrate. FTIR peaks were assigned based on reports available on similar coatings/precursors. See generally A. M. Wróbel, I. Blaszczyk-Lezak, A. Walkiewicz-Pietrzykowska, D. M. Bielinski, T. Aoki & Y. Hatanaka, Silicon Carbonitride Films by Remote Hydrogen-Nitrogen Plasma CVD from a Tetramethyldisilazane Source, 151(11) J. E
Referring to
Referring to
Table 1 summarizes the index of refraction, film thickness, and mechanical properties silicon-based coatings deposited at various plasma conditions and substrate temperatures. In general, the films have a refractive index lower than about 1.7 at substrate temperatures below about 300° C. Above about 300° C., the films show a refractive index higher than about 1.75 and up to about 1.86. Increase in the refractive index can help in decreasing the ARC layer thickness. Though not wishing to be bound by a particular theory, the reduced ARC thickness may in turn reduce the photon loss and the stress induced in the ARC layer.
The obtained films showed properties that are comparable with a-SiCN:H films obtained using vacuum PECVD. See, e.g., I. Blaszczyk-Lezak, A. M. Wrobel & D. M. Bielinski, Remote nitrogen microwave plasma chemical vapor deposition from a tetramethyldisilazane precursor. 2. Properties of deposited silicon carbonitride films, 497(1-2) T
In order to determine the stability of the a-SiCN:H coatings for high temperature Ag metal firing process that is most commonly used in Si solar manufacturing processes, a-SiCN:H sample was subjected to a rapid thermal annealing (RTA) at about 700° C. for about 60 seconds. Relevant industrial standards may vary from about 750° C. to about 835° C. for about 1 second to a few seconds. The material properties measured before and after the rapid thermal annealing are summarized in the Table 2 &
Antireflection coatings were made by reacting a triethylsilane precursor in a glove box by Surfx Technologies (Culver City, Calif.). The triethylsilane precursor was initially maintained in a bubbler, bubbling helium gas through the triethylsilane precursor at variable flow rates. Helium gas was supplied to the plasma source at about 30 liter/minute. The gases listed in Table 3 were used as the reactive gas at the respectively listed flow rates. The substrate was heated to about 260° C. The plasma head was held at a distance of about 4 mm to about 5 mm from the substrate, at a fixed plasma power of about 120 W to about 140 W. Depositions were carried out by moving the heated substrate under the plasma source in a serpentine motion at a velocity of about 0.6×10−2 m·s−1. Varying the precursor bubbler flow did not materially alter the refractive index of the antireflection coating.
a-SiNx:H thin films were fabricated using a cyclohexasilane (CHS) Si6H12 precursor such as is described in U.S. Pat. No. 5,942,637, incorporated by reference herein. The precursor was reacted with nitrogen in the plasma at atmospheric pressure, leading to the formation of a good SiNx:H thin films at a substrate temperature of about 200° C. to about 350° C.
The CHS precursor that was contained in the bubbler was heated to about 55° C. to increase the vapor pressure. Helium was used as the carrier gas at 0.9 liter/min through the bubbler. Helium gas was supplied to the plasma source at about 20 liters/minute. Nitrogen was used as the reactive gas at a flow rate of about 500 sccm. The substrate temperature was varied between about 100° C. to about 450° C. in the steps of 50° C. The remaining conditions were the same as in previous examples.
The a-SiNx:H thin films deposited at different substrate temperatures on intrinsic silicon substrates were examined using FTIR spectroscopy. The resulting spectra are depicted in
Surface morphology of the films was investigated using atomic force microscopy.
The refractive index, film thickness, and density of the obtained films are tabulated in Table 3. Films deposited at and above about 250° C. have a refractive index above about 1.9. Films with such refractive index values and a suitable thickness can provide excellent anti-reflective properties suitable for crystalline silicon solar cells. Increasing the substrate temperature between about 150° C. to about 300° C. additionally decreased the film thickness. Above about 300° C., an increase in thickness was observed. The measured film density of about 2.80 kg/m3 to about 2.89 kg/m3 was in agreement with a-SiNx:H deposited using other vacuum-based techniques.
Mechanical properties such as hardness and Young's modulus of the coatings were determined using a nanoindenter.
It is understood that the disclosure may embody other specific forms without departing from the spirit or central characteristics thereof. The disclosure of aspects and embodiments, therefore, are to be considered in all respects as illustrative and not restrictive, and the claims are not to be limited to the details given herein. Accordingly, while specific embodiments have been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention and the scope of protection is only limited by the scope of the accompanying claims.
This application claims the benefit of priority to U.S. Provisional Patent Application No. 61/387,256, filed Sep. 28, 2010, the content of which is incorporated herein by reference in its entirety.
Activities relating to the development of the subject matter of this invention were funded at least in part by the U.S. Government, Department of Energy Grant Nos. DOE-PV-DS-43500 and DE-FC36-08G088160. The United States Government has certain rights in this invention.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US11/53624 | 9/28/2011 | WO | 00 | 3/27/2013 |
| Number | Date | Country | |
|---|---|---|---|
| 61387256 | Sep 2010 | US |
