Ami Chand

Strain studies in LPCVD polysilicon for surface micromachined devices

Abstract Polycrystalline silicon (polysilicon) has emerged as a preferred material for surface micromachined MEMS applications because of its compatibility with standard CMOS process. The important parameters of polysilicon films for sensor–actuator devices are the residual stress and stress gradient. For free standing microstructures, it is important to reduce the stress in the film. In order to exploit the advantage of polysilicon for MEMS applications, it is essential to develop a process to obtain low-stress polysilicon films. In the present work, we have investigated the effect of deposition parameters on 2–4 μm thick LPCVD polysilicon films using a specially designed spiral structure for strain measurements. The films were deposited in the temperature range of 580–630°C at pressures 180 to 320 mTorr. The role of post-deposition annealing at 1000°C in N2 ambient on strain reduction has been investigated using conventional furnace annealing and rapid thermal annealing (RTA). The as-deposited films show significant strain under all the deposition conditions investigated. The strain is reduced to insignificant values after 100 min of furnace annealing. In case of RTA, similar strain values are achieved in only 30 s of annealing. The overwhelming superiority of RTA over furnace annealing in terms of the thermal budget of the process has been clearly demonstrated. The effect of polysilicon doping with boron or phosphorus has also been studied for applications in electrically conducting microstructures. The final strain values after annealing are about 1×10−4, which is the limit of resolution of the strain measurement scheme used in the present investigations.

C-V characterization of MOS capacitors in SOI structures

Abstract The capacitance-voltage characterization of a MOS structure in the SOI film has been carried out and the results have been interpreted with the help of a numerical solution to the one-dimensional Laplace-Poissons equation. Various parameters characterizing the SOI MOS structures have been extracted. It has been shown that the C-V data on a simple three-terminal SOI MOS capacitor structure can yield all the information such as the thickness of the gate oxide, buried-oxide as well as the SOI film, along with the doping density in the film and the substrate.

Effect of high dose N+2 ion implantation on surface characteristics of AISI 302 stainless steel with and without an inducing strain

The effect of nitrogen ion implantation in AISI 302 stainless steel has been examined on two different surface conditions of the substrate such as strain induced (mechanically polished) and excess strain free (chemically etched). Samples were implanted with 2.5 × 1021 N+2 m−2 at 130 keV. The identification of the phase change and compounds formed was performed by glancing-angle X-ray diffraction. Diffraction patterns were recorded on polished samples prior to and after implantation, with and without chemical etching. The α′-martensite phase formed during mechanical polishing is observed on an austenitic substrate. It is completely removed by chemical etching before implantation. After implantation, in polished samples, more α′-martensite is observed together with Cr2N and ϵ-Fe2NFe3N whereas, in etched samples, only the γ-austenite phase is observed with Cr2N. The surface hardness is found to be enhanced after implantation in both cases. The possible correlation of the changes observed after implantation with increase in the surface hardness is discussed.

H2/O2 operated portable and steam-bottomed MHD power generators: an analytical study

Abstract Analytical investigations have been carried out to assess the feasibility of utilizing an H 2 /O 2 system seeded with potassium (K) as a working fluid in portable and steam-bottomed MHD power generators. Optimum values of duct inlet velocity of the fluid, combustor pressure and duct length have been obtained for maximum overall plant efficiency.

Design and development of microstructures for MEMS applications

A new approach has been proposed to realize suspended microstructures such as cantilevers, diaphragms, springs, spirals, etc. over pits of controlled depth in bulk silicon micromachining process using direct wafer bonding technology. The structures have been realized in heavily boron doped silicon and dielectric layers. The electrostatic actuation voltage of the cantilever beams have been computed as a function of physical parameters and compared with experimentally measured values. LPCVD polysilicon has been investigated with a view to obtain low stress films for use in surface micromachining technology. The effect of deposition parameters on stress in the polysilicon films has been studied. Post deposition annealing is shown to have profound effect on the stress properties of the polysilicon films. Rapid thermal annealing is found to be much more effective in reducing the stress compared to conventional furnace annealing. The advantage of direct wafer bonding technology for electrostatically actuated microstructures have been presented. The application of this technology for MEMS have been discussed.

Synthesis of buried silicon nitride layers by rapid thermal annealing

Abstract Buried layers of silicon nitride (Si3N4) were synthesised by rapid thermal annealing (RTA) of high-dose (1018 ions cm−2) nitrogen implanted (at 150 keV) silicon. The influence of RTA temperature and time on the formation of buried layers of silicon nitride was studied by IR transmission and X-ray diffraction techniques. It is found that RTA at 1200 °C for 150 s or 1250 °C for 100 s results in buried polycrystalline silicon nitride layers. The results are compared with those obtained by traditional furnace annealing.

Optimum fuel combination for MHD power generators

Various combinations of commercially available gaseous and liquid fuels, burnt with air and preheated (1400K) air, and seeded with K2CO3, have been investigated analytically to arrive at an optimum combination that gives a reasonable electrical conductivity (10 mho/m) at minimum cost. This analysis has been carried out for gaseous and liquid fuels, separately, under Indian conditions. Variation of the electrical conductivity with the ratio of the mass of the fuels has also been obtained to make them usable in the cost structure of any country. Further, it has been shown that it is not only the C/H ratio but also the calorific value of the fuel that determines the electrical conductivity of the working fluid.

Optimization of stress in LPCVD polysilicon films for MEMS applications

The influence of LPCVD process parameters on stress in polysilicon films has been investigated for surface micromachined structures. The as deposited films show a large strain which can be considerably reduced by post deposition annealing. The polycrystalline film deposited at 605 degree(s)C and 250 mTorr is found to have minimum residual stress. The rapid thermal annealing (RTA) at 1100 degree(s)C for 30 sec relieves the stress completely. Further, the RTA is shown to be a superior process compared to the conventional furnace annealing for obtaining stress free films.

Investigations of seeded combustion products of biogas/air-O2 systems

Abstract The composition, temperature and electrical conductivity of the seeded (with K 2 CO 3 ) combustion products of biogas/air-O 2 systems have been analytically evaluated. The effects of preheating and enrichment of air on the combustion temperature and electrical conductivity have been studied. Experimental measurements of the temperature and electrical conductivity of biogas/oxygen systems have been made to validate the theoretical predictions.

Electrical conductivity of seeded combustion products of water gas-air system

Abstract The dependence of the electrical conductivity of seeded combustion products of water gas-air system on preheating and oxygen enrichment of air has been analytically investigated. Optimum oxygen enrichment is estimated to be around 40% while preheating could be carried for temperatures in the range of 1100 to 1400 K. The conductivity of the system is found to increase with the addition of extra CO to the fuel.