Exploitation of Quantum Photoeffects in Reducing Microscopic Defects and Processing Cycle Time in Advanced Rapid Thermal
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425 Mat. Res. Soc. Symp. Proc. Vol. 470 01997 Materials Research Society
II. EXPERIMENT Czochralski single side polished p-type , two inch silicon wafers of 1-10 ohm-cm resistivity were used in all cases. The paste used in the formation of BSF contacts by screen printing consisted of 50-80% aluminum powder, 1-5% lead borosilicate glass, 0-1% silicon dioxide and 1-5% terpineol. The constituents of the screen printed Ag/Al ohmic contacts are 4580% silver, 1-5% aluminum, solvents like lead borosilicate glass 1-10% and diethylene glycol monobutylether 15-35 %. To study the role of photo-effects in the formation of Ag/Al ohmic contacts and aluminum BSF contacts, different lamp configurations and thermal cycles were used during annealing. In the case of Ag/Al ohmic contacts three screen printed samples were annealed with the temperature profiles as shown in Figure Ia. For studying the role of phototeffects in the formation of BSF contacts, six samples were annealed using the thermal cycles shown in Figure lb. A home-built rapid thermal processor comprising of a 24kW tungsten-halogen lamp bank was used for annealing all the samples. In the case of front irradiation, samples were annealed with the contacts facing directly the tungsten -halogen lamp bank. Samples with back irradiation, on the other hand, were placed facing away from the lamp bank. (a)
(b)
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Figure 1: Thermal Cycle used for Annealing Ag/Al and Aluminum Back Surface Field Contacts In the case of aluminum BSF contact formation, a dual spectral source comprising of tungstenhalogen lamps and a UV source were used for annealing two samples. In one case the sample was placed facing the UV source and in the other, it was placed away from the UV source and towards the tungsten-halogen lamp bank. For all the samples, following rapid thermal annealing, aluminum contacts were evaporated on the front side of silicon to form Schottky diodes. The Schottky diodes were used for electrical characterization. Scanning electron microscopy (SEM)
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was used to study the metal silicon interface for all the samples. X-ray diffraction was used to study the crystal orientation of each of the Ag/Al contacts. Thermal stress was computed to study the metallic integrity of the contacts and Atomic force microscopy (AFM) was used to study the surface of the three Ag/Al contacts. III. RESULTS The data obtained from I-V characteristics of the Schottky diodes formed with the Ag/Al paste, and those formed with the aluminum BSF contacts are given in Table I and Table II respectively. Case
Lamp Configuration
n
I1(g.A)
R(Q2)
Lc (ptA)
Cycle time
A
Front
1.25
8e-10
3.5
450
122
1.4
1.8e-10
(sec)
irradiation(FI)
B
Back ir
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