Stress behavior of CVD-PSG Films Depending on Deposition Methods and Hillock Suppression
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EXPERIMENTAL DETAILS For the experiments, the starting material is the p-type Si (100). 1.0 gim CMOS and 1.2 gm Bipolar processes are applied to compare the electrical parameters of the new method with those of normal method. Figure 1 shows the flow chart of sample preparation sequence to observe the hillock growth, void formation, buckling, and cracking of passivation layer. Passivation PSG films are deposited on the metal film by low pressure CVD (LPCVD), atmospheric pressure CVD (APCVD), or plasma enhanced CVD (PECVD). The conditions for annealing temperature are split into 350 *C and 400 *C. After pad patterning, electrical parameter variations are monitored by Kelvin resistance measurement and leakage current measurement using HP 4145B. li-type (100) Si Wafer•
Process ZNon-nal
cMOS/Bipolar
Metal Photo/Etch for 2nd Metal Patterning
140T
Passivation PSG
Annealing)
--
APCD
Characterization / Analysis
Figure 1. The flow chart of sample preparation sequence The hillock density is investigated at the capacitance area (size of 100 x 100 in'Ctm) test element group (TEG) pattern from 25 samples by optical microscope (OM; x400). Using surface profilometer, metal surface is scanned by 20 times over the length of 500 gm to observe the hillock height and so it is classified the hillock according to hillock height. Using stress gauge by wafer bow method the stress behavior of PSG film on the metal film under various circumstance is observed during alloy process. The stress is measured at the sample that is passivated by the PSG film of 7000 A thick on the metal film. The samples are heated up to 350 'C or 400 'C and then cool down to room temperature. RESULTS AND DISCUSSION Hillock has the spike shape extruded from metal surface and is mainly formed by vacancy diffusion mechanism. The hillock growth is due to thermally activated process, which is very slow at room temperature but becomes very fast as the temperature increase. The vacancy mechanism is one of the stress relaxation processes and the stress of metal film originates from mechanical stress by thermal expansion coefficient mismatch between metal film and overlayered or underlayered dielectric film.
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Figure 2 a) and b) show the stress behavior of PSG film on the AI-I%Si film deposited by PECVD with respect to annealing treatment after PSG deposition on metal film. In fig. 2 a), the stress of as deposition sample has compressive stress of - 3.Ox 109 dyne/cm 2 and final stress has 0.8x10 9 dyne/cm2 . The stress difference is about 2.2x109 dyne/cm 2 and the compressive stress is much reduced after annealing. Up to 120 'C the stress becomes more compressive and the stress change with respect to temperature increase is 1.7x10 7 dyne/cm 2 *C. It may be explained that the porous PSG film becomes dense film. From 120 *C to 200 'C, stress change slope is not changed 2 and with further increase of annealing temperature the stress change is 0.9x107 dyne/cm *C. It is probably explained that the PSG film is once sintered and then the impurities, mainly hydrogen
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