Staffan Greek

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

Youngs 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 ...

In situ tensile strength measurement and Weibull analysis of thick film and thin film micromachined polysilicon structures

Abstract A method is introduced in which tensile tests can be performed in situ on micromachined structures. The testing equipment consists of a testing unit mounted on a micromanipulator in a scanning electron microscope. The fracture loads of micromachined beam structures made from thick and thin film polysilicon were measured, and the fracture strengths were then calculated via measurements of the fracture surface areas. Characterization of the film materials was also performed with transmission electron microscopy, atomic force microscopy and scanning electron microscopy to locate the critical defects in the materials. The statistical scatter of the fracture strength values was evaluated using Weibull statistics, which yielded the mean fracture strength and the Weibull modulus—a measure of the amount of scattering. In this study, for the first time, Weibull theory was also applied to a real micromechanical structure, i.e. standard test results were transformed into expected strength limits of a more complex structure.

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.

Deflection of surface-micromachined devices due to internal, homogeneous or gradient stresses

Abstract Devices surface-micromachined out of a thin film containing an internal stress will be deformed upon release. The stress of a film can be approximated in second order by a biaxial homogeneous stress superimposed on a linear biaxial stress gradient. In order to model and reduce the stress-induced deformation, the stress state of a thin film is evaluated with surface-micromachined test devices. The homogeneous stress component is measured with deflecting indicators, and the gradient component is measured from the curvature of cantilever beams. A finite element model of a cantilever beam is developed and the parameters influencing the stress-induced deflection are investigated. The model is then extended to calculate the deflection of a doubly supported bridge and the result is compared with experiment. Finally, by using the finite element model of the bridge, the importance of the supports is demonstrated, indicating that reducing the undercut of the supports is an effective method to reduce the stress-induced deflection.

Tensile testing of thin film microstructures

The mechanical properties of thin film microstructures depend on size and shape and on the film manufacturing process. Hence, the test structures that are used to measure mechanical properties should have dimensions of the same order of magnitude as an application structure. The microstructures are easily monitored in a scanning electron microscope (SEM), but to be handled and tested in situ a micromanipulator was developed. The parts of the micromanipulator essential to the tests are two independently moveable tables driven by electric motors. The test structures and a testing unit are mounted on the tables. A testing unit was designed to measure force and displacement with high resolution. The testing unit consists of an arm actuated by a piezoelectric element and equipped with a probe. An optical encoder measures the movement of the arm, while strain gauges measure the force in the arm. Test structures consist typically of a released beam fixed at one end with a ring at the other. The micromanipulator is used to position the probe of the testing unit in the ring. The testing unit then executed a tensile test of the beam. Test structures of polysilicon films produced under various process conditions were used to verify the possibility of measuring Youngs modulus with an accuracy of +/- 5 percent, as well as fracture strength.Youngs modulus is calculated using the difference in elongation for different beam lengths. The fracture strength of the beams was evaluated with Weibull statistics.

Young's Modulus, Yield Strength and Fracture Strength of Microelements Determined by Tensile Testing

Some mechanical properties of thin film microelements, e.g. fracture strength, depend on the manufacturing process, the load application as well as on size and shape of the microelements. Hence, the test structures that are used to determine mechanical properties should have dimensions of the same order of magnitude as an application structure, i.e. microelements must be used to accurately characterise MEMS. The fabrication of test structures must be realised in the same process as an intended application in order to give accurate results. Microelements are easily viewed in an SEM, but to be handled and tested in situ a micromanipulator was developed. Test structures were designed as released beams fixed to the substrate at one end, with a ring at the other. A high-precision testing unit was mounted on the micromanipulator next to the test structures. In the SEM, the testing unit was manoeuvred to grip the ring of the test structure beam and a tensile test of the beam was then executed. From the test data Youngs modulus and fracture strength of polysilicon and single crystalline silicon were evaluated. Relative measurement of test structures with different beam lengths enabled Youngs modulus to be evaluated with an accuracy of ±5%. Youngs modulus was determined to 172±7 GPa for polysilicon and 142±9 GPa for single crystalline silicon in the direction. The fracture surfaces were examined and compared. Youngs modulus, yield strength and fracture strength of microelements made from electroplated nickel and nickel-iron alloy were also measured. Youngs modulus was evaluated to 231±12 GPa for nickel and 155±8 GPa for nickel-iron alloy composed of 72 at% nickel and 28 at% iron.

Mechanical considerations in the design of a micromechanical tuneable InP-based WDM filter

A WDM photodetector for fiberoptic communication systems using a wavelengths around 1.55 /spl mu/m has earlier been proposed as a tuneable Fabry-Perot interferometer micromachined on InP and integrated with a photodiode. The interferometer consists of a mirror suspended on beams and is tuned by electrostatic attraction. The size of the mirror is 30 /spl mu/m/spl times/30 /spl mu/m. In order to maintain the filter performance, the mirror must be kept so flat that the maximum bending deflection of the mirror surface is below 1 nm. Four alternative filter geometries are considered. A mathematical model describing the influence of system parameters, including tuning voltage and the sizes of the filter components, is developed. The upper and lower bounds of the natural frequency of mechanical vibrations are also calculated analytically. With finite-element analysis (FEA) the response to electrostatic actuation is calculated and the flatness of the mirror surface during tuning and at changed ambient temperature conditions is investigated. The dynamic behavior is also studied using modal analysis. The results of the analytic and FEA modeling are compared and found to agree.

High Sensitive Internal Film Stress Measurement By An Improved Micromachined Indicator Structure

As one part of a joint European project called MAXIMA, where the aim is to develop and produce a monolithically integrated three-axial accelerometer, high sensitive internal film stress measurements have been made on polysilicon films. In order to produce these accelerometer structures with high precision, it is necessary to know the mechanical properties, internal stresses and stress gradients of these films and their dependence on the process parameters.

Micromechanical tensile testing

A method is described where tensile tests can be performed in situ on micromachined structures. The testing equipment consists of a testing unit mounted on a micromanipulator in a Scanning Electron Microscope (SEM). The fracture loads of micromachined beam structures made from thick and thin film polysilicon as well as from electrodeposited nickel and nickel-iron alloy were measured, and the fracture strengths then calculated via measurements of the test structures` initial cross-sectional areas. The statistical scatter of the polysilicon fracture strength values were evaluated by Weibull statistics. The mean fracture strength and the Weibull modulus, a measure of the scatter, were obtained.

Strength of indium-phosphide-based microstructures

Microoptoelectromechanical (MOEMS) systems with InP based micromechanics are proposed for devices with wide tuning ranges in the optical wavelengths where InP optoelectronics are normally used. To evaluate if these InP based micromechanical structures may be strong enough the mechanical strengths of surface micromachined epitaxial InP micro beams are evaluated. Reactive ion etching (RIE) with CH4:H2:H2Ar is used to structure the beams. A sacrificial InGaAs layer is below the InP microstructures and selectively etched by HCl:H2O2:H2O in ratios 1:1:10 to release the InP beams. Sublimation of tert-butanol is used to dry the micro structures. The RIE conditions are shown to be of large importance, since the induced surface defects are here the dominant reasons for fracture. Bending strength values up to 3.1 GPa were measured, i.e. much higher than for the strongest construction steel. Weibull statistics show that it is possible to make micromachines for typical MOEMS applications with acceptable loss in yield due to fracture probability, i.e. with a fracture probability of 0.0001 for 100 MPa maximum bending stress.