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1. EP0325606 - METHOD OF FABRICATING SOLAR CELLS WITH ANTI-REFLECTION COATING.

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Claims

1. A method of fabricating a photovoltaic solar cell comprising the steps of providing a silicon substrate (2) having first and second opposite surfaces, forming a P/N junction (4) in the substrate adjacent to the first surface, forming a polysilazane coating (10) on the first surface by a plasma reaction and forming electrical contacts (10,14) on the first and second surfaces, with the first contact (20) extending through and being flanked by said polysilazane coating, said coating being etched so as to form a two-dimensional pattern for said first contacts, characterized in that before the electrical contacts are formed the first surface is subjected sequentially to an ammonia plasma treatment long enough to produce hydrogen implantation and a combined silane and ammonia plasma treatment long enough to produce additional hydrogen implantation and formation of a polysilazane coating and, after the plasma treatment, an aluminum-containing coating (14) is applied to the second opposite surface and the substrate (10) is heated to a temperature and for a time sufficient to cause the aluminum constituent of the aluminum-containing coating (14) to alloy with the silicon substrate and thereby form an aluminum electrical contact at the second surface and to drive the implanted hydrogen deep into the substrate.
  2. A method according to claim 1, characterized in that after the substrate is heated to cause alloying of aluminum the polysilazane coating has a refractive index of 2.15.
  3. A method according to claim 1 or 2, characterized in that the polysilazane coating has a thickness of between 8.4 x 10⁻⁸m and 8.9 x 10⁻⁸m (840 and 890 Angstroms) after the sequential plasma treatments.
  4. A method according to any preceding claim, characterized in that after the sequential plasma treatments the polysilazane is covered with an adherent coating of a photoresist material, the photoresist coating being exposed to radiant energy through a mask defining a predetermined two-dimensional pattern, the photoresist is chemically developed so that selected portions of the resist are removed from the polysilazane coating according to the predetermined pattern and those portions of the polysilazane coating which are not covered by the photoresist are removed so that selected portions of the first surface are exposed to the atmosphere and the conductive metal is applied to the selected portions of the first surface.
  5. A method according to claim 1 or 4, characterized in that the metal applied to the selected portions of the first surface is silver or nickel.
  6. A method according to claim 1 or 4, characterized in that a solderable metal is applied to the selected portions of the first surface and the solderable metal is sintered so that the solderable metal and the silicon of the substrate react to form a solderable metal silicide at their interface.
  7. A method according to any preceding claim, characterized in that the substrate is subjected to ammonia plasma at a temperature of between 320° C and 500° C for between 1 and 15 minutes.
  8. A method according to any preceding claim, characterized in that the silane/ammonia plasma treatment is conducted for between 1.0 and 4.0 minutes at a temperature of between 320° C and 500° C.
  9. A method according to any preceding claim, characterized in that the ammonia plasma treatment and the silane and ammonia plasma treatment are carried out in the same plasma reaction chamber.
  10. A method according to any preceding claim, characterized in that the polysilazane coating is represented by the formula Si xH yN z where Si, H and N are silicon, hydrogen and nitrogen respectively and x and z each range from about 1.0 to about 1.3 and y ranges from about 0.05 to about 0.30.
  11. A method according to any preceding claim, characterized in that the sequential plasma treatments are carried out using an RF frequency of 35 to 450 kilohertz.
  12. A method according to claim 11, characterized in that the sequential plasma treatment is carried out using 300 to 800 watts of RF power supplied to the electrodes of the plasma reaction chamber.
  13. A method according to any preceding claim, characterized in that the plasma treatments are carried out using an RF frequency of 150 kilohertz, the substrate being exposed to ammonia plasma for at least about 1.0 minute and the substrate being exposed to a combined silane and ammonia plasma for about 2.8 minutes.
  14. A method according to any preceding claim, characterized in that the polysilazane coating has a relatively fast etch rate in a buffered oxide etch such as a solution of HF and NH₄F before it is heated and a relatively slow etch rate in the same buffered oxide etch after it has been heated.
  15. A method according to claim 14, characterized in that the polysilazane coating has an etch rate of 4.0 x 10⁻⁹ to 10.0 x 10⁻⁹ m/minute (40 to 100 Angstrom units/minute) in the buffered oxide etch before it has been heated and an etch rate of 2.0 x 10⁻⁹ or less m/minute (20 or less Angstrom per minute) after it has been heated.