PATTERNING OF SILICON OXIDE LAYERS USING PULSED LASER ABLATION
Various laser processing schemes are disclosed for producing various types of hetero-junction and homo-junction solar cells. The methods include base and emitter contact opening, selective doping, metal ablation, annealing to improve passivation, and selective emitter doping via laser heating of aluminum. Also, laser processing schemes are disclosed that are suitable for selective amorphous silicon ablation and selective doping for hetero-junction solar cells. Laser ablation techniques are disclosed that leave the underlying silicon substantially undamaged. These laser processing techniques may be applied to semiconductor substrates, including crystalline silicon substrates, and further including crystalline silicon substrates which are manufactured either through wire saw wafering methods or via epitaxial deposition processes, or other cleavage techniques such as ion implantation and heating, that are either planar or textured/three-dimensional. These techniques are highly suited to thin crystalline semiconductor, including thin crystalline silicon films.
1 . A method of ablating an electrically insulating layer on a semiconductor substrate, said method comprising:
providing a semiconductor substrate having n-type doping;
depositing a first relatively thin layer of undoped glass or undoped oxide on a surface of said semiconductor substrate;
depositing a first relatively thin semiconductor layer comprising a substance chosen from the group consisting of amorphous semiconductor, nanocrystalline semiconductor, microcrystalline semiconductor, and polycrystalline semiconductor on said relatively thin layer of undoped glass or undoped oxide;
depositing a layer of borosilicate glass or a borosilicate/undoped glass stack on said relatively thin semiconductor layer; and
selectively ablating said layer of borosilicate glass or borosilicate/undoped glass stack with a pulsed laser, said relatively thin semiconductor layer substantially protecting said semiconductor substrate from said pulsed laser.
2 . The method of claim 1 , further comprising a subsequent thermal oxidation process to oxidize said relatively thin semiconductor layer.
3 . The method of claim 1 , wherein said semiconductor comprises silicon.
4 . The method of claim 1 , wherein said first relatively thin layer of undoped glass or undoped oxide has a thickness approximately in the range of 3 to 100 nanometers.
5 . The method of claim 1 , wherein said first relatively thin semiconductor layer has a thickness approximately in the range of 3 to 30 nanometers.
6 . The method of claim 1 , wherein said laser has a pulse length of approximately 200 picoseconds or less and a wavelength of approximately 1064 nanometers or less.
7 . The method of claim 1 , further comprising process flow steps for making a thin monocrystalline semiconductor solar cell.
8 . The method of claim 7 , wherein said thin monocrystalline semiconductor solar cell comprises a thin monocrystalline silicon layer in the thickness range of 10 to 100 microns.
9 . The method of claim 7 , wherein said thin monocrystalline semiconductor solar cell comprises a back-contact/back-junction solar cell.
10 . The method of claim 1 , further comprising process flow steps for making a crystalline semiconductor based photovoltaic solar cell comprising an all-back-contact back-junction solar cell.
11 . The method of claim 1 , wherein said ablations are used to make openings to delineate base and emitter regions of an all back contact, back junction solar cell.
12 . The method of claim 10 , wherein said crystalline semiconductor based photovoltaic solar cell comprises an epitaxial silicon thin film solar substrate.
13 . The method of claim 12 , wherein said epitaxial thin film solar substrate has a thickness in the range of approximately 10 to 100 microns.
14 . The method of claim 12 , wherein said epitaxial thin film comprises a substantially planar epitaxial film formed via an epitaxial silicon liftoff process.
15 . The method of claim 12 , wherein a front surface of said epitaxial thin film comprises three-dimensional pyramids or prisms formed via a textured template liftoff process.