Thermal treatment of silicon wafers useful for photovoltaic applications
View Patent ↗Efficiency of silicon photovoltaic solar cells is increased by an annealing process for immobilizing oxygen formed in Czochralski-grown silicon. The annealing process includes a short anneal in a rapid thermal annealing chamber at a high temperature, for example, greater than 1150° C. in an oxygen-containing ambient. More preferably, the wafer is rapidly cooled to less than 950° C. without an intermediate temperature hold, at which temperature oxygen does not nucleate and/or precipitate. Subsequent processing to form a photovoltaic structure is typically performed at relatively low temperatures of less than 1000° C. or even 875° C.
1. A process for forming a photovoltaic solar wafer, comprising the steps of:
growing a silicon ingot by the Czochralski process;
mechanically forming a plurality of silicon wafers from the ingot;
annealing at least one of the wafers at an annealing temperature of greater than 1150° C. in an oxygen-containing ambient for a time of between 1 and 60 seconds; and
then fabricating a photovoltaic solar cell structure including a semiconductor junction in the at least one wafer at processing temperatures of less than 1000° C.
2. The process of claim 1 , further comprising cleaning the at least one wafer between the forming step and the annealing step.
3. The process of claim 1 , wherein the annealing step is performed in a rapid thermal processing chamber including a plurality of incandescent heating lamps directed at one of the at least one wafer.
4. The process of claim 1 , wherein the ambient contains a partial pressure of at least 0.76 Torr of an oxygen-containing gas.
5. The process of claim 4 , wherein the oxygen-containing gas comprises oxygen gas (O 2 ).
6. The process of claim 1 , wherein the annealing temperature is in a range of 1200 to 1275° C.
7. The process of claim 6 , further comprising the step performed between the annealing and fabricating steps of cooling the wafer from the annealing temperature to less than 1150° C. at a cooling rate of at least 20° C./s.
8. The process of claim 7 , wherein the cooling step includes no intermediate holding step during which the cooling rate is less than 20° C./s.
9. The process of claim 7 , wherein the cooling step cools the wafer to no more than 950° C.
10. The process of claim 7 , wherein the cooling step cools the wafer at a cooling rate of at least 10° C./s over all parts of a temperature range extending from the annealing temperature to less than 950° C.
11. The process of claim 1 , wherein the processing temperatures are no more than 950° C.
12. The process of claim 1 , wherein the silicon ingot has an oxygen concentration of greater than 25 ppma.