The nature of residual stress, defects, and device characteristics for thick single-crystalline Si films on oxidized Si
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of Electrical Engineering, Princeton University, Princeton, New Jersey 08544.
514
J. Mater. Res. 3 (3), May/Jun 1988
http://journals.cambridge.org
oxidized to a 4 /um depth, with parallel seed windows, spaced 3.5 mm apart, patterned in the oxide. A 50/zm thick low-pressure chemical vapor deposited (LPCVD) poly-Si layer is then deposited on the patterned thermal oxide. Finally, a 2.5 /um LPCVD SiO2 capping layer is deposited. The stationary lamp furnace and procedure used to melt and recrystallize the 50 /um thick poly-Si film have also been described.3 The radiant heat flux from the lamps incident on the poly-Si side of the wafer allows melting and seeded recrystallization of this layer without wafer meltdown, since a thermal gradient is imposed across the wafer thickness. Three methods were used to analyze stress in the recrystallized Si films: (1) x-ray microdiffraction, (2) Raman microprobe, and (3) radius-of-curvature analysis. In (1) a Rigaku high brilliance reflection mode microdifFractometer8 was used with CuKa incident radiation generated by a 40 kV, 50 mA source. The x-ray beam was incident on the SiO2-covered Si film surface. The penetration/sampling depth in the Si was about 5 fim, with irradiated area approximately 100 /um diam. The signal versus 20 data were collected in the vicinity of the Si (400) peak reflection and compared with an unstressed single-crystal Si control sample as well as the American Society for Testing and Materials (ASTM) data. Stress is directly related to a shift in the measured peak position from the standard unstressed position and is then calculated. The Raman microprobe consisted of a standard Raman scattering arrangement coupled to an optical microscope.9 The probe beam was a 3 mW, 6471 A line of the K r + ion laser focused to a 1 /um spot on the oxidecapped Si film surface, with a Si penetration/sampling depth of 3 /urn. The phonon frequency resolution is about 0.2 cm~'. Stress affects the Si Raman-Stokes line
0003-6951/88/030514-07S01.75
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Trimble eta/.: Stress, defects, device characteristics of Si films on Si wafers
by shifting the central frequency from the characteristic, unstressed value. The frequency shift Aco is then used to calculate stress. The radius-of-curvature analysis consisted of first cutting a "beam" or strip from the center of a recrystallized wafer. Two beams, one parallel to the seed stripes ( 5 x 8 7 mm) and one perpendicular ( 7 x 9 8 mm) were studied to determine if any stress anisotropy existed due to the crystal growth (seed pattern) direction. The beams were protected on the back side with wax, and immersed in an etchant to remove first the capping SiO2 layer and then the recrystallized Si film layer. The etchants used were buffered oxide etch (7:1, 40% NH 4 F solution/49% HF solution) for SiO2 and aqueous KOH solution (25% by weight) at 85 °C for Si. The curvature of the strip was determined using a topograph instrument before and after the removal of ea
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