Surface Micromachined Test Structures Research Articles

Extraction of residual stress and dimensions from electrical measurements on surface micromachined test structures

The deposited thin films in surface micromachining have a lot of residual stress, and it is essential to measure this for both process development and monitoring. We estimate residual stress by electrical measurements on a series of fixed-fixed polysilicon beams designed to deflect laterally due to stress. To minimize errors in estimation during parameter extraction, the device dimensions also have to be measured accurately. Surface micromachining of an oxide-anchored polysilicon cantilever beam can result in beam undercut, reduction in beam thickness, and increase in the gap between the beam and the substrate. The undercut in the beam is estimated from the resonance frequency of the cantilever beam and also by using polysilicon resistors. Final device thickness is obtained by measuring the resistance in fixed-fixed beams.

Mechanical characterization of thick polysilicon films: Young's modulus and fracture strength evaluated with microstructures

Young's modulus and the fracture strength of thick polysilicon films were evaluated with surface micromachined test structures. The polysilicon films were deposited in an epitaxial reactor and were about 10.5 µm thick. Four different processing schemes of doping and annealing were used and thus four different sets of test structures were micromachined. A micromanipulator system developed at Uppsala University was used to make tensile tests. The micromanipulator is adapted to a scanning electron microscopy environment and all tests were done in situ so that the testing procedure could be monitored at high magnification. From the tensile tests Young's modulus as well as the mean fracture strength and Weibull modulus of each of the films were evaluated. Young's modulus was also evaluated from the collapse voltage of test structures consisting of an electrostatically movable plate suspended by springs. Film texture measurements by x-ray diffraction and cross sectional transmission electron microscopy revealed that the silicon grains had preferential orientations. A mean value of Young's modulus of the film was calculated based on the Young's modulus values of the preferred orientations. It was found that Young's modulus of polysilicon evaluated by the three methods presented agree with each other as well as with other reliable data.

High-sensitivity surface micromachined structures for internal stress and stress gradient evaluation

The internal stress and stress gradient of thick () and thin () polysilicon films were evaluated with surface micromachined test structures. The structure that measured internal stress consisted of actuator beams rotating an indicator through an angle corresponding to the stress. The indicator deflection was measured in an SEM. Finite element analysis (FEA) was used both to optimize the design and to calibrate the structure. A folded beam design was used to minimize the total area the structure occupied so that it could be incorporated in the wafer layout of other surface micromachined details, and used for online process diagnostics. The indicator was provided with a Vernier scale to facilitate quick evaluation in an optimal microscope. The stress gradient was measured from the deflection of long () cantilever beams. The deflection was measured in an optical microscope and the output was calibrated with FEA calculations.

Thin-film processing of TiNi shape memory alloy

The deposition and processing of thin-film Nitinol, a shape memory alloy (SMA), are discussed and demonstrated. By using sputter-deposition techniques, 0.5 to 8.0-μm-thick films of Nitinol (an alloy of nickel and titanium) were deposited, patterned and etched. Although the processing of the individual constituents of the alloy are well understood, the processing of the shape memory alloy is not a simple extension or combination of the constituent metals processing. Requisite deposition conditions for the film to retain the shape memory effect of the bulk material and avoid stress-induced cracking are discussed. An appropriate wet-etch for patterning the thin-film Nitinol is presented. Results from surface micromachined test structures are presented, which verify the shape recovery property of the thin-film Nitinol using resistive heating as the source of heat energy for the shape recovery. Finally, we discuss an isotropic, dry-etch technique developed for the release of the thin-film test structures that also has potential use in other thin-film applications.