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THE USE OF SPIN-ON DOPED OXIDE DIFFUSION SOURCES FOR THE PRODUCTION OF SOLAR CELLS


For the production of silicon solar cells, the following solutions are recommended:

Here P or N refers to the conductivity type of the starting wafer. In these diffusion processes the side of the wafer opposite the p-n junction is coated with dopant solution of the same type as the wafer itself to prevent crossover diffusion and thereby to eliminate the need for lapping or sand blasting the reverse surface. The wafer surfaces are not polished but generally exhibit a matte finish characteristic of a lapped surface.

Wafer Surface Treatment
1.The wafer is cleaned free of lapping compound and is rinsed in trichloroethylene, followed by alcohol, and blown dry.

2.The wafer is submerged in a chromic acid-HF solution (200 grams CrO₃, 200 ml HF 48%, 700 ml H₂O) at room temperature for 15 minutes.

3.The wafer are rinsed in DI water, followed by alcohol, and blown dry.

Wafer Coating
N⁺PP⁺ CELLS
1. Several drops of Emitter Diffusion Source N-250 are applied to the center of the wafer positioned on the spinner. After allowing the solution to spread over the surface of the wafer for 1-2 seconds, the spinner is started. The spin speed should be 3000 rpm and spinning requires 5-10 seconds. The wafer, after spinning, is heated at 100^oC for 15 minutes followed by a 15 minute heat treatment at 200^oC-300^oC for 15 minutes in air. The objective of the last heating process is to sufficiently densify the film so that it is not disturbed by the contact with the spinner chuck as described in the next step.

2. The reverse surface of the wafer is then coated with Borosilicafilm 5257 in the same manner as described above.

Diffusion Process
1. The wafers are placed in slotted boats with 1/8" spacing between the slots. Care is taken to assure that surfaces coated with Emitter Source N-250 are facing each other to prevent cross doping.

2. Diffusion is carried out at 900^oC in an oxygen ambient.

The flow rate of the oxygen is not critical. The diffusion time is 1 hour. After the 1 hour diffusion interval, the wafers are cooled slowly to 400^oC in 30 minutes.

Post Diffusion
After diffusion, the doped oxides on both surfaces are removed in 10% HF solution for 5 minutes. The surfaces will be unstained and ready for coating with contact metals.

Diffusion Results
R_s on N⁺ side ----------------- 18-20 ohms/square
R_s on P side ---------------------- 60 ohms/square
Junction Depth ------------------------ .6 micron
Minority Carrier Lifetime (P)-10 microsencods


P⁺NN⁺ CELLS
The surface preparation process is the same for these wafers as for the P wafers described above.

Wafer Coating
With the wafer positioned on the spinner several drops of solution are applied to the center of the wafer. The spinner is started and spinning is carried out for 5-10 seconds at 3000 rpm. After spinning the wafers are baked at 200^oC for 10 minutes in air to densify the film.

The reverse surface of the wafer is then coated with Phosphorosilicafilm C_o = 1x1²⁰⁰ by spinning. Again the film is densified by heating at 100^oC-150^oC for several minutes.

Diffusion Process

Diffusion is carried out at 1075^oC for 15 minutes in an atmosphere consisting of 97% N₂:3% O₂. After diffusion interval the wafers are slowly cooled to 80^o0C for about 30 minutes before removal from the furnace.

Post Diffusion
After diffusion, the doped oxides are removed from both surfaces by etching in 10% HF solution for 5 minutes. The surfaces will be unstained.

Diffusion Results
R_s on P side --------------------- 13-16 ohms/square
R_s on N side --------------------- 20 ohms/square
Junction Depth ----------------------- .6 micron
Minority carrier Lifetime (N) -------- 10 microseconds

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