Micromachined Test Structures Research Articles

Correlation between Q-Factor and Residual Stress in Epitaxial 3C-SiC Double-Clamped Beam Resonators

In this work, we investigate the correlation between tensile residual stress and Q-factor of double-clamped beams fabricated on epitaxial 3C-SiC layers grown on both <100> and <111> silicon substrates, using a completely optical measurement setup to measure the Q-factor of the resonators and the residual stress of the layers by means of purposely designed micromachined test structures. From the measurements, a clear correlation appears between the residual stress of the SiC layer and the Q-factor of the resonators, with Q-factor values above half a million for resonators fabricated on <111> substrates, showing residual stress around 1 GPa.

An On-Chip Micromachined Test Structure to Study the Tribological Behavior of Deep-RIE MEMS Sidewall Surfaces

An on-chip micro-mechanical test structure is presented to investigate the tribological behavior of deep reactive ion etching (DRIE) sidewall surfaces of microelectromechanical systems (MEMS) devices. The proposed test structure is fabricated on silicon on insulator (SOI) wafer using a standard surface micromachine process. Test structure consists of two orthogonally placed electrostatic comb-drive actuators, one is used to align a contact with the friction surfaces under a certain normal load, and another one is used to generate the tangential motion on contacted sidewall surfaces. To assess the frictional behavior of DRIE sidewall surfaces, both static and dynamic friction coefficients were studied for different DRIE process parameters. Wear analysis was carried out to extract the performance and reliability of MEMS sidewall surfaces during their operation. From the experiment results, it is found that with the increment of normal load, the static friction coefficient exhibits a nonlinear dependence, and however, it has less effect on the dynamic friction coefficient. DRIE process parameters have a significant influence on static and dynamic friction coefficients, i.e., variation in asperity size changes the real contact area between the contact pairs during their operation. From the wear analysis, it is found that the friction coefficient was very high initial value, then it drops with each subsequent cycle for a few cycles and finally reaches steady-state value.

Characterizing Metal-Insulator-Transition (MIT) Phase Change Materials (PCM) for RF and DC Micro-switching Elements

Abstract Metal-insulator transition (MIT) phase-change materials (PCM) are material compounds that have the ability to be either conductors or insulators depending on external stimuli. A micromachined test structure for applying external electric fields across MIT wire segments was designed and fabricated. Using this novel test structure, Germanium Telluride (GeTe) and Vanadium Oxide (VOx) were successfully transitioned from a conductor to an insulator. The resistivity of the GeTe wire segments increased three to five orders of magnitude with ∼40 V applied to the parallel plates of the test structure. The VOx wires exhibited an order of magnitude transition in resistivity with ∼20 V applied. Characterization of both RF and DC switching performance of these MIT wire segments was completed and GeTe and VOx appear to be viable materials for micro-switching.

Electrolyte and Cathode Studies for Micro-Solid Oxide Fuel Cell

AbstractSputter deposited YSZ thin films were studied for the application as electrolyte membrane in micro solid oxide fuel cells. A new micro-machined test structure was developed to test 200 μm fuel cell membranes that are integrated onto silicon substrates. The membranes are liberated by means of deep silicon dry etching from the backside, and both contacts are situated on the front side of the wafer. Annular Pt electrodes provide contacts to specific anode and cathode layers. Preliminary tests are reported for the situation without specific electrodes. At 450 °C, an OCV of 570 mV, and a maximal power of about 0.55 mW/cm2 is obtained. LaSrMnO electrodes deposited by pulsed laser deposition were evaluated by electrical impedance spectroscopy. The dense films yielded too high ASR values. Interestingly, these could be reduced by applying a DC bias voltage.

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.

A measurement technique for thermoelectric power of CMOS layers at the wafer level

We propose a method aimed at executing accurate thermoelectric power measurements at the wafer level on micromachined test structures. In order to compensate for instrumental offsets and sensitivity limits typically existing in a standard wafer-level test instrumentation, a special purpose extraction technique is applied. The influence of air-convection and heating/cooling effects on the measurement is also discussed by carefully evaluating the results of finite-element simulations of heat exchange in the test structure. In order to validate the technique, test measurements on p and n-doped polysilicon layers are presented and compared with other results from the literature. Moreover, the accuracy of the measurement technique and its temperature resolution are discussed.

A novel method for determining Young’s modulus of thin films by micro-strain gauges

This work presents a novel method for in-situ determining Young’s modulus of thin films at the wafer level by using a set of compact micromachined test structures and without any extra load applied to test such structures. The test structures comprise of a pair of micro-strain gauges with known Young’s modulus and a cantilever beam made of the measured film. The method utilizes inexpensive and available optical measuring equipment. An analytical model is derived to extract the Young’s modulus of the measured film. A conventional surface-sacrificial layer micromachining technique is used to fabricate the structures. The micro strain gauges employed in the measurement are made of low-pressure chemical-vapor deposition (LPCVD) undoped polycrystalline silicon films produced by Semiconductor Research Center (SRC) and the measured film is made of PECVD silicon nitride for demonstration. The average value of the obtained Young’s modulus of PECVD silicon nitride SiNx is 170 ± 3 GPa by using strain gauges with a residual stress of 211 ± 10 MPa.

Thermal contact conductance of actuated interfaces

The thermal contact conductance (TCC) of microactuated mechanical interfaces has been characterized using an electronic technique, where micromachined test structures were heated with a current and the TCC was inferred from the change in resistance. For every device tested, the TCC was higher in vacuum than in air. This is in stark contrast to the behavior of bulk interfaces, and several experiments suggest that it may be the result of a decreased solid–solid contact area in air caused by the pressure of the interstitial gas. The average effective TCC of a polysilicon/nitride interface brought together by electrostatic actuation varies about values of 6.0×104 W/(K m2) in air and 9.5×104 W/(K m2) under vacuum for applied pressures of 1 MPa. These values are significantly higher than commonly reported for nonmetallic materials and probably reflect the very smooth surfaces of deposited thin films.

A high-stroke, high-pressure electrostatic actuator for valve applications

This paper presents a novel large-stroke electrostatic valve actuator for high-pressure applications. The combination of pressure balancing and flexible electrode structures ensures large flow gaps at a low actuation voltage. A simulation tool was built to evaluate the design parameters. Design specific, as well as general optimisations are performed. The model shows a 5.6 times (theoretical) performance improvement compared to earlier designs. A micromachined test structure was fabricated and evaluated. Measurement results are presented and discussed.

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.

Repeatability aspects in surface micromachining - investigations on polycrystalline silicon films

Within this paper, the repeatability of the mechanical properties of thin polysilicon films is discussed. The film thickness, mechanical stress and Young's modulus are investigated with respect to on-wafer, wafer-to-wafer and run-to-run repeatability. The repeatability is examined using micromachined test structures. Although the determined parameter variations are small compared to other publications, the effect of those variations on the characteristics of micromachined sensors is still not negligible.

Thermal and mechanical aspects for designing micromachined low-power gas sensors

Metal oxide gas sensors have to be heated during operation. Ideally they should have low power consumption and a uniform temperature distribution over the sensitive area. Thermal isolation is achieved by locating the sensitive element on thin free-standing membranes fabricated by means of micromachining of silicon. Uniformity of the temperature is helpful for discriminating different gases and, thus, increasing the selectivity. This can be achieved by careful design of the heater or by additional structures. Micromachined test structures have been fabricated, their mechanical strength and thermal behaviour have been determined and the results have been correlated with extensive FEM studies.

Effect of hydrogen termination on the work of adhesion between rough polycrystalline silicon surfaces

A novel micromachined test structure has been used to measure the work of adhesion between polycrystalline silicon surfaces. The effects of several surface treatments, including a hydrogen- and an ammonium-fluoride-induced hydrogen termination and a hydrogen peroxide chemical oxidation, have been investigated with these test structures. A reduction in the average apparent work of adhesion by a factor of 2000 has been observed on the NH4F-treated surface compared to the oxide-coated surface. By using x-ray photoelectron spectroscopy and atomic force microscopy, the observed reduction is traced to the combined effect of the surface chemistry and topography. This work demonstrates that a hydrophobic, rough surface provides a significant reduction of the apparent work of adhesion in polysilicon micromachined devices.

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.

Texture and stress profile in thick polysilicon films suitable for fabrication of microstructures

In this paper we report on results concerning the texture and mechanical stress of thick polycrystalline silicon films, grown in an epitaxial reactor. These phosphorus doped films were subjected to different annealing treatments at temperatures at or above 1100 °C in nitrogen and oxygen ambients, respectively. The morphology and texture of the films, respectively, were obtained using TEM and X-ray diffraction analysis. The samples texture does not present any significant changes related to the used annealing treatment. The stress gradient of released micromachined test structures was determined with optical interference measurements and compared with -Raman spectroscopy stress profiles and X-ray diffraction stress data using the sin 2÷-method obtained at different depths. We show that annealing in oxygen changes the stress state in the top regions of the polycrystalline films to a more compressive value, thus causing released beams to bend downwards. On the contrary, annealing in nitrogen atmosphere shifts the stress towards tensile values and released beams bend upwards.

Stress measurement by microRaman spectroscopy of polycrystalline silicon structures

In this work, microRaman spectroscopy is applied for the stress analysis of LPCVD polysilicon films deposited on SiO2 sacrificial layers. The features of the first-order Si Raman signal (shape, width and position of maximum) are analyzed taking into account the presence of structural defects and stress distribution in the layers. These measurements are correlated with the results obtained by using micromachined test structures.

Low stress nitride as surface isolation in bipolar transistors

AbstractLow stress silicon rich nitride SiNx has been examined as surface isolation in a 15 GHz washed emitter–base (WEB) bipolar process. The resistance of SiNx to wet etching in HF simplifies the processing and improves the control of device dimensions. The WEB process allows a direct comparison of device characteristics for several SiO2 and SiNx passivation layers. Films of SiNx with strain levels in the range 4×10−4–3×10−3 were developed using a micromachined test structure. Damage of the silicon surface, which induces emitter–collector shorts, is avoided by using very low stress nitride (strain <1·8×10−3). The base leakage currents, which are associated with silicon/insulator interface traps, are not stress dependent but are characteristic of the type of isolation.MST/3222

Raman Characterization of Polycrystalline Silicon: Stress Profile Measurements

ABSTRACTRaman spectroscopy is applied for the analysis of polysilicon films deposited on SiO2 sacrificial layers. Different deposition technologies and processing parameters have been studied. The features of the first order Si Raman signal (shape, width and position of maximum) are analyzed in order to evaluate the stress average value in the scattering volume. MicroRaman measurements performed at different points on the edge of the samples allows the estimation of the stress gradient through the polysilicon layers. These measurements are correlated with the structure obtained by using micromachined test structures.

Mechanical Characterization of Thin Films by Micromechanical Techniques

Evaluating the mechanical properties of thin films on thick substrates is tricky. Some useful techniques for mechanical property measurements on macroscopic film specimens exist, e.g., internal stress measurements by x-ray diffraction or by wafer buckling. Many conventional techniques, however, such as indentation for hardness measurement or scratch testing for evaluation of adhesion or wear resistance, yield results that are difficult or impossible to evaluate in terms of more fundamental film properties. In fact, these techniques are more practical engineering tools than tools for scientific study.One solution is to miniaturize the test probe to dimensions approaching the film thickness. The nanoindentation technique for submicrohardness measurement described elsewhere in this issue by Pharr and Oliver is a recent example of such an approach. Still, for very thin films it is difficult to separate the influence of the substrate properties from the film properties, and the indentation process is inherently too complex to be well suited for determining fundamental materials parameters.The rapidly evolving field of micromechanics offers some new possibilities in thin film characterization. Important film properties such as internal stress, elastic moduli, plastic yield limit, and fracture data can be extracted from experiments with micromachined test structures in the 10–1,000 μm size range.

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.