Safe and reliable design of MEMS components requires a statistical description of the material properties that are associated with failure. To this end, a series of microscale tensile tests was performed on polysilicon MEMS structures fabricated using Sandia National Laboratories' SUMMiT Vtrade process. Tensile bars were fabricated from each of the four freestanding polysilicon layers, with gage lengths ranging from 30 to 3750 mum. A two-parameter Weibull distribution appeared to adequately characterize the observed tensile strength distributions. The strength distribution was found to be dependent on the length of the tensile structures, as expected by the Weibull size effect, and unexpectedly strongly dependent on the layer from which the tensile bar was constructed. Specifically, the topmost polysilicon layer in the deposition process (poly4) was more than twice the strength of the bottom freestanding polysilicon layer (poly1). The mechanistic source of this layer-dependent strength appears to originate, at least in part, from process-dependent surface roughness, although other factors such as layer-dependent variations in microstructure, residual stress, and doping are also considered. A fracture mechanics analysis of the strength distributions suggests that the size of the critical flaws is in the vicinity from 50 to 150 nm. Fractography revealed crack origins along the sidewalls, corners, and top surfaces. Weibull strength distributions were also established at elevated temperatures: 200, 400, 600, and 800 degC in air and nitrogen environments. These results revealed the onset of ductility and reduction in strength at elevated temperatures: at 600 degC strength was less than 40% of the room temperature value. Most of the strength was regained if the material was tested at room temperature after a high-temperature exposure. In the discussion, we briefly review concepts for incorporating these observed strength distributions into probabilistic safe design of MEMS components

Strength Distributions in Polycrystalline Silicon MEMS

Models for the oxidation of silicon

This paper reviews the application of Auger Electron Spectroscopy (AES) to the analysis of glass surfaces. Attention is drawn to experimental perturbations which may occur when analyzing glasses with absorbed and/or mobile species at the surface. Previous workers [1–4] have shown, for example, that mobile ions (e.g. Na) are very difficult to measure in glasses due to their migration under the electron beam. Special techniques developed to permit the measurement of mobile ions are discussed, and some recent applications of AES in current research at the University of Florida are presented.

Glass surface analysis by Auger Electron Spectroscopy

Oxide thickness measurements up to 120 Å on silicon and aluminum using the chemically shifted auger spectra

The Si oxidation rate at high temperatures is increased by heavy P doping. Sputter ion‐Auger techniques have been used to investigate the oxide thickness and P concentration after 900 °C oxidation with an initial P surface concentration of between 2×1018 and 3.2×1020/cm3. Phosphorus was found to pile up in the Si‐SiO2 interface region. The maximum P concentration near the interface was found to depend on doping and was (at certain P concentrations) an order of magnitude larger than that predicted by redistribution theory. A vacancy–P‐complex model is presented to explain the observed behavior. A correlation between the selective pile‐up of P at the interface and the current gain of planar bipolar transistors is reported.

Observation of phosphorus pile‐up at the SiO2‐Si interface

Phosphorus redistribution during thermal oxidation of degenerately doped silicon was investigated using Auger electron spectroscopy and ellipsometry. Concentration profiles were determined with a combination of chemical and sputter etching techniques. A substantial phosphorus pileup was observed in the oxide in a thin layer near the ellipsometrically determined Si–SiO2 interface. Calibrated against standards of known concentration, this layer was found to contain ∼2×1021 phosphorus atoms/cm3, independent of the oxidation temperature between 850 and 1000 °C.

Auger and ellipsometric study of phosphorus segregation in oxidized degenerate silicon

The growth kinetics of SiO2 films (100-18000A) on [100], 2Ωcm silicon have been investigated between 900-1100‡C with additions of 0-9 vol.% HC1 or 0-2 vol.% Cl2 to the dry oxygen ambient. The thickness-time data could best be fitted numerically to a mixed linear-parabolic equation that included a correction for fast initial growth. Equivalent amounts of chlorine (e.g., 2 vol.% HC1 or 1 vol.% Cl2) produced completely different effects on the rate of SiO2 growth. The quantitative effects of halogen additions were studied in greatest detail at 900‡C. At that temperature, the parabolic rate constant increased linearly with the HC1 concentration. At the same time, the linear rate constant remained constant. Both rate constants did change when Cl2 was used as an additive. The effect of HC1 additions on the parabolic rate constant reaches a maximum around 1000‡C. Possible mechanisms for the halogen effects are discussed,← and it is seen that the gas phase reaction 4 HC1 + 02 → 2 H2O + 2 Cl2 is not reflected in the growth kinetics.

Effects of HCl and Cl 2 additions on silicon oxidation kinetics

Preparation and properties of silicon-cadmium selenide p-n heterojunctions

Phosphorus redistribution during thermal oxidation of degenerately doped silicon was investigated using Auger electron spectroscopy and ellipsometry. Concentration profiles were determined with a combination of chemical and sputter etching techniques. A substantial phosphorus pileup was observed in the oxide in a thin layer near the ellipsometrically determined Si–SiO2 interface. Calibrated against standards of known concentration, this layer was found to contain ~2×1021 phosphorus atoms/cm3, independent of the oxidation temperature between 850°C and 1000°C.

J. G. Cahill

Papers

Auger and ellipsometric study of phosphorus segregation in oxidized degenerate silicon

Effects of HCl and Cl 2 additions on silicon oxidation kinetics

Preparation and properties of silicon-cadmium selenide p-n heterojunctions

Auger and Ellipsometric Study of Phosphorus Segregation in Oxidized Degenerate Silicon