Properties of Low Residual Stress Silicon Oxynitrides Used as a Sacrificial Layer
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deposited by low pressure chemical vapor deposition (LPCVD) at 900 C in a mixture of dicblorosilane, ammonia and nitrous oxide. Wafer bow, warp and radius of curvature are measured using a Tencor model FLX-2320 thin film stress monitor to determine the effect of the different sacrificial oxides upon process induced wafer warpage. The maximum wafer bow is reported, in lieu of wafer warp, to allow for differentiating between convex and concave deformation [9]. Interferometry is utilized to determine the deflection of 20 jim x 475 jim poly-Si cantilevers fabricated with either the Si0 2 or oxynitride sacrificial layers [10]. RESULTS AND DISCUSSION The residual film stress of continuous LPCVD poly-Si and TEOS Si0 2 films under post deposition N 2 anneal is shown in Fig. 1. It has been reported that in order to achieve stress relief to less than 10 MPa in released poly-Si MEMS structures it is necessary to anneal the film to a minimum of 1000 C for an extended period [2]. Under these conditions the LPCVD TEOS film, though deposited with a low level of residual stress, is observed to undergo a transition to a high state of compressive stress. For processes that deposit films on both sides of the wafer the stress forces balance and the wafer will not deform. However, for processes that deposit films on one side only the compressive stress in the TEOS oxide can lead to significant wafer deformation. Even if double-sided deposition techniques are utilized photolithographic patterning and etching of the wafer will create discontinuous films on the front side leading to an imbalance in the stress forces and possible wafer deformation.
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Anneal Temperature (°C) Figure 1. Post anneal residual stress for 2.0 jim poly-Si and TEOS Si0 2 films. Anneal time is 3 h, in N2. Figure 2 depicts the predicted wafer deformation for the film stack as each layer is grown or deposited and, at the end, annealed for 3 hours at 1100 C. The maximum wafer bow is determined from the predicted radius of curvature for each film stack, which is calculated using
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Stoney's equation [I I]. It is assumed that there is no film on the wafer back side and the wafers are initially flat. It is also assumed that the residual stress of a given film stack is given by a thickness weighted linear combination of the individual films' residual stress; this assumption has been experimentally validated [12]. For a film stack with TEOS Si0 2 sacrificial layers the wafer bow is expected to be dominated by the high residual tension of the poly-Si layers. Upon stress relieving the poly-Si via high temperature anneal, residual compression in the oxide layers dominate and the wafer bow reverses (from -120 jim to +95 gim).
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Figure 2. Predicted maximum wafer bow per process step for film stacks produced with either TEOS Si0 2 or LPCVD Oxynitride sacrificial layers. To alleviate the extreme wafer
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