Mechanical Properties of Thermally Crystallized Boron-Doped Silicon Thin Films
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379 Mat. Res. Soc. Symp. Proc. Vol. 403 01996 Materials Research Society
reflected light beams. The resolution of the film stress measurements was about ±10 MPa. Because of the very low difference in thermal expansion coefficient between the a-Si:H films and the c-Si substrates, the thermal stress remained substantially smaller than the intrinsic stress. During wafer curvature monitoring as a function of temperature of a-Si:H films under a N 2 atmosphere, the furnace temperature was increased at 10'C/min up to 575°C, then maintained at this temperature for several hours, and finally decreased back to room temperature. The hardness and elastic modulus E/(1-vf2) of the films were measured from the load/ penetration data obtained from nanoindentation experiments. All the indentations were performed using a Nanoindenter in the constant displacement rate mode whereby a diamond Berkovich pyramid was driven into the specimen at a constant velocity [2]. The maximal indentation depths were between 50 nm and 150 nm, depending on the thickness of the indented film. In all cases, a series of ten indents were performed on each sample. FTIR spectroscopy was used for analysis of hydrogen-bonding configurations in the aSi:H films in the transmission mode. IR absorption measurements were made relative to the uncoated (100) silicon wafer with a BioRad-Digilab FTIR spectrometer in the range 4000 cm' from the integrated intensity 400 cm 1 . The total bonded-hydrogen concentration was calculated of the wagging band of Si-H bonds centered around 640 cm 1 [3]. The amount of hydrogen in (Si-H 2 )nbonding configurations and clustered monohydrides on internal surface of microvoids was estimated from the integrated band intensity of the IR-stretching mode centered around 2100 cm-1. RESULTS AND DISCUSSION Average values of the compressive residual stress in the a-Si:H films are reported in figure 1. We noted a linear dependence of the residual stress on the bonded-hydrogen concentration for B 2H6/SiH 4 ratios comprised between 0 and 5.10-3 (fig. 2). During the film deposition, energetic hydrogen ions not incorporated at the growing surface may diffuse into the sub-surface region and transform weak Si-Si bonds into Si-H bonds. The accomodation of this additional hydrogen in the random rigid network tends to increase the bond angle distribution, and to contribute thus to a compressive stress [4]. For B 2H6/SiH 4 = 10-2 and 2.10-2, the compressive residual stress in the a-Si:H films does not follow anymore a linear dependence on the hydrogen content, but increases with the doping level. In this case, the doping efficiency in a-Si:H is very low, of the order of 10% [5]. Excess boron atoms may either segregate at interstitial sites, or be passivated forming B-H complexes [6]. Then, the network expansion induced by these configurations will contribute to the compressive (i.e negative) stress.
445
475
545
60
40
455
38
a.€)
a.) °-o
-240 -620 0
10-4
-610 10-
3
-20 a-Si:H
EJ poly-Si 3.10-3 5.10-3 B2H6 / Sil4
2.10-3
380
-365
-385
1
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