Pratul K. Ajmera

Nondestructive technique to measure bulk lifetime and surface recombination velocities at the two surfaces by infrared absorption due to pulsed optical excitation

The modeling and analysis of electron devices including photovoltaic devices requires the knowledge of the surface recombination velocities at the two surfaces of the wafer along with the bulk lifetime. In this paper, the work of Luke and Cheng [J. Appl. Phys. 61, 2282 (1987)] is extended to account for the asymmetric case of different surface recombination velocities at the two wafer surfaces. We present the analysis and discuss experimental procedures to extract the above three parameters. The contactless measurement technique is based on the transient behavior of infrared absorption due to the decay of optically excited excess carriers. In order to determine the surface recombination velocities at both surfaces, the measurements must be made with each side acting as the front surface. An example of parameter extraction is presented.

Measurement of bulk lifetime and surface recombination velocity by infrared absorption due to pulsed optical excitation

Knowledge of both carrier bulk lifetime τb and surface recombination velocity S are necessary to analyze and model electron devices. In this paper, the analytical results of Luke and Cheng [J. Appl. Phys. 61, 2282 (1987)] are applied to examine excess carrier decay due to pulsed Gaussian optical excitation to extract values of τb and S under low level injection. The nondestructive, contactless measurement technique utilizes a visible pulsed laser pump beam and a CW infrared laser probe beam. Values for τb and S are obtained for a number of different Si samples. The effect of varying the pump beam frequency is examined. The technique is simple to use in the laboratory and the parameter extraction from the measured data is straightforward.

Use of a photoresist sacrificial layer with SU-8 electroplating mould in MEMS fabrication

In this paper, new processes have been developed to fabricate micromechanical components and systems that utilize two different photoresists (Shipley S1813 and Hoechst AZ P4620) as sacrificial layers in conjunction with SU-8 photoresist used as an electroplating mould. The use of photoresists as sacrificial layers offers several advantages including reduction in processing steps and hence processing cost. However, the normal sacrificial layer photoresist processing cycle fails when used in conjunction with SU-8 photoresist. The latter is often used as an electroplating mould, especially for high-aspect ratio or tall microstructures in MEMS fabrication. In this paper, problems arising in the use of a photoresist sacrificial layer are discussed and new sacrificial layer processes are developed. New curing temperatures for photoresist sacrificial layers are determined, which prevent damage that otherwise occurs from the use of SU-8 as an electroplating mould. New hard baking temperatures determined are 175 °C and 200 °C for S1813 and AZ P4620 photoresist processes, respectively. A comb drive structure is utilized to demonstrate these new fabrication processes.

Simultaneous extraction of bulk lifetime and surface recombination velocities from free carrier absorption transients

Bulk lifetime and surface recombination velocities at both the front and the rear surfaces are important parameters that affect the performance of solar cells. In an earlier work [G. S. Kousik, Z. G. Ling and P. K. Ajmera, J. Appl. Phys. 72, 141 (1992)], a detailed theoretical analysis for a contactless nondestructive technique to obtain these parameters is presented. Here, the experimentally measured data using this new approach was presented. Moreover, the earlier analytical work has now been extended to also provide an estimate of the sensitivity of the extracted parameters to errors in experimental measurements. Results on measurements on single crystal silicon wafers are provided as an example.

A new synchrotron light source at Louisiana State University's Center for Advanced Microstructures and Devices

Abstract A 1.2-GeV synchrotron light source is being constructed at the Center for Advanced Microstructures and Devices (CAMD) at Louisiana State University. The expressed purpose of the center, which has been funded by a grant from the US Department of Energy, is to develop X-ray lithography techniques for manufacturing microcircuits, although basic science programs are also being established. The storage ring will be optimized for the soft-X-ray region and will be the first commercially manufactured electron storage ring in the United States. The magnetic lattice is based on a design developed by Chasman and Green and will allow up to three insertion devices to be installed for higher-energy and higher-intensity radiation. In addition to the lithography effort, experimental programs are being established in physics, chemistry, and related areas.

Characterization of vacuum grown thin films of ZnSnP2

Abstract Near stoichiometric thin films of ternary semiconductor material ZnSnP2 have been grown by the vacuum growth technique. Surface morphology of the grown films is examined. Maximum value of the absorption coefficient, α, of > 105 cm− and band gap of 1.62 eV have been observed from optical transmission analysis. X-ray diffraction studies on the grown films showed diffraction peaks appropriate for the I42d space group. The observed lattice-constant values in the grown films ranged from 5.64 to 5.67 A which compared well with the reported value of 5.651 A for single-crystal ZnSnP2.

A Laterally Movable Gate Field Effect Transistor

A laterally movable gate field effect transistor (LMGFET) device that directly couples lateral mechanical gate motion to drain current of a FET is presented in this paper. Lateral motion of the FET gate results in a change in channel width, keeping the channel length and the gap between the gate and the oxide layer constant. This results in a change in channel current that, in principle, is linearly proportional to mechanical motion. The operating principle of an LMGFET, along with details of the fabrication process for a depletion-type LMGFET device, is described. Fabricated LMGFET shows an average drain current sensitivity to gate motion of -5.8 ¿A/¿m at VDS = 20 V and VGS = 0 V for 60-¿m gate motion. A model for the fabricated LMGFET is developed based on electrical measurements. The device shows promise both as a sensor and as an actuator in MEMS and other related applications.

Chemical and electrical characteristics of low temperature plasma enhanced CVD silicon oxide films using Si2H6 and N2O

Abstract Silicon oxide films have been deposited at low temperatures in the range of 30–250 °C using Si2H6 and N2O by conventional plasma enhanced chemical vapor deposition technique. The dependencies of deposition temperatures on the film properties are studied. The leakage current and the etch rate of these low temperature films compare favorably to films deposited by silane and TEOS at higher temperatures, respectively.

A 0.8 V ultra-low power CMOS operational amplifier design

In this paper, a low-power (/spl plusmn/0.4V) operational amplifier is designed in a standard CMOS process. The amplifier has a dc gain of 7000 (77 dB), and bandwidth 30 kHz. The design incorporates a low voltage current mirror with a forward bias between source and substrate of the MOSFET to reduce the threshold voltage.

Integration of LIGA structures with CMOS circuitary

Successful direct integration of a mechanical structure fabricated by LIGA on a Si chip containing CMOS circuitry has been achieved in this work. A 1D cantilever accelerometer is chosen as a vehicle to demonstrate this integration process. The capacitive sensor element employs one electrode formed in the Si substrate during the integrated circuit fabrication. The other electrode is fabricated using the LIGA technique with sacrificial layer etching. Details of the fabrication process to achieve this integration are given. Need for careful control of stress in the deposited layers and achieving appropriate contrast in the X-ray mask are delineated.