Maarten P. de Boer

Demonstration of an in situ on-chip tensile tester

Polycrystalline silicon (polysilicon) strength data reported in the literature usually present results from only a limited number of trials because of the difficulties in applying high forces to the high-strength specimens. These forces are most often applied by off-chip actuators, which can pose cumbersome alignment issues. Here we demonstrate a compact on-chip tester using a thermal actuator to apply stress to a self-aligning tensile specimen via a prehensile grip mechanism. Preliminary characteristic strength and Weibull modulus values of 3.05 GPa and 12.8, respectively, are reported, in good agreement with other literature data. By querying the fracture strain of the material, this distinct measurement approach complements other methods of testing the strength of brittle polysilicon. Instrinsic test time is 5 min or less, and the area occupied is relatively small compared to other on-chip tensile test devices. This will enable many trials for high confidence in polysilicon strength distribution in future work.

Impact of Contact Materials and Operating Conditions on Stability of Micromechanical Switches

Nano and micromechanical switches are of great interest in applications that require high speed, low-power consumption and high electrical isolation. There is strong evidence that airborne hydrocarbon accumulation on the contact surfaces of the switch is a key cause for device failure. Relatively unexplored contact materials such as RuO2 are of interest because they are believed to be less prone to hydrocarbon deposit accumulation than more commonly used materials such as Pt and Au. Here, we measure the reliability of RuO2 and Pt-coated microswitches in hydrocarbon-rich environments with N2 and N2:O2 background gases. The RuO2 material performs very poorly in contaminated N2, but very well in contaminated N2:O2. Furthermore, RuO2 performs much better than Pt in the contaminated N2:O2. It is demonstrated that the deposit, initially being an insulator, can be electrically broken-down, thereby substantially lowering switch resistance. It is further shown that the passage of electrical current through the contacts augments deposit accumulation.

Rate-state friction in microelectromechanical systems interfaces: Experiment and theory

A microscale, multi-asperity frictional test platform has been designed that allows for wide variation of normal load, spring constant, and puller step frequency. Two different monolayer coatings have been applied to the surfaces—tridecafluorotris(dimethylamino)silane (FOTAS, CF3(CF2)5(CH2)2 Si(N(CH3)2)3) and octadecyltrichlorosilane (OTS, CH3(CH2)17SiCl3). Static friction aging was observed for both coatings. Simulating the platform using a modified rate-state model with discrete actuator steps results in good agreement with experiments over a wide control parameter subspace using system parameters extracted from experiments. Experimental and modeling results indicate that (1) contacts strengthen with rest time, exponentially approaching a maximum value and rejuvenating after inertial events, and (2) velocity strengthening is needed to explain the shorter than expected length of slips after the friction block transitions from a stick state. We suggest that aging occurs because tail groups in the monolayer coatings reconfigure readily upon initial contact with an opposing countersurface. The reconfiguration is limited by the constraint that head groups are covalently bound to the substrate.

Stress mapping of micromachined polycrystalline silicon devices via confocal Raman microscopy

Stress mapping of micromachined polycrystalline silicon devices with components in various levels of uniaxial tension was performed. Confocal Raman microscopy was used to form two-dimensional maps of Raman spectral shifts, which exhibited variations on the scale of the component and on the scale of the microstructure. Finite element analysis models enabled direct comparison of the spatial variation in the measured shifts to that of the predicted stresses. The experimental shifts and model stresses were found to be linearly related in the uniaxial segment, with a proportionality constant in good agreement with calculations based on an opto-mechanical polycrystalline averaging analysis.

Growth of Silicon Carbide Nanoparticles Using Tetraethylorthosilicate for Microelectromechanical Systems

Silicon carbide (SiC) is a wide bandgap semiconductor of interest in microelectromechanical systems. We demonstrate the growth of SiC nanoparticles using silicon dioxide (SiO 2 ) films deposited from tetraethylorthosilicate [TEOS, Si(OC 2 H 5 ) 4 ]. High-temperature annealing allows residual carbon to diffuse to and react with the silicon substrate to form SiC nanoparticles (rather than a uniform SiC film). The growth of the SiC nanoparticles can be controlled by annealing conditions or eliminated by flowing oxygen during the film deposition. These particles are revealed by a standard wet etchant and provide an effective method to reduce adhesion between micromachined surfaces.

Contamination Thresholds of Pt- and

Micro- and nano-mechanical switches are being considered as complements to solid state transistors. While several different device designs may satisfy performance requirements, contact reliability due to hydrocarbon contamination remains a critical concern in each. This issue can be addressed by identifying contact materials and environments that optimize immunity to contaminants. Here we demonstrate that RuO2, a conducting oxide, does not exhibit contaminant-induced degradation at up to 130 parts per million (PPM) benzene in a nitrogen/oxygen background, and experiences minimal electrical resistance rise at 1,300 PPM. In comparison, Pt-coated switches degrade significantly at only 0.02 PPM benzene contaminant level in nitrogen background. This paper establishes that a proper selection of materials and environment is a promising path toward achieving reliable micro- and nanoswitches.

{\rm RuO}_{2}

Two major problems associated with Si-based MEMS devices are stiction and wear. Surface modifications are needed to reduce both adhesion and friction in micromechanical structures to solve these problems. In this paper, the authors will present a process used to selectively coat MEMS devices with tungsten using a CVD (Chemical Vapor Deposition) process. The selective W deposition process results in a very conformal coating and can potentially solve both stiction and wear problems confronting MEMS processing. The selective deposition of tungsten is accomplished through silicon reduction of WF{sub 6}, which results in a self-limiting reaction. The selective deposition of W only on polysilicon surfaces prevents electrical shorts. Further, the self-limiting nature of this selective W deposition process ensures the consistency necessary for process control. Selective tungsten is deposited after the removal of the sacrificial oxides to minimize process integration problems. This tungsten coating adheres well and is hard and conducting, requirements for device performance. Furthermore, since the deposited tungsten infiltrates under adhered silicon parts and the volume of W deposited is less than the amount of Si consumed, it appears to be possible to release stuck parts that are contacted over small areas such as dimples. Results from tungsten deposition on MEMS structures with dimples will be presented. The effect of wet and vapor phase cleanings prior to the deposition will be discussed along with other process details. The W coating improved wear by orders of magnitude compared to uncoated parts. Tungsten CVD is used in the integrated-circuit industry, which makes this approach manufacturable.

-Coated Ohmic Switches

There has been a resurgence of interest in developing ohmic switches to complement transistors in order to address challenges associated with electrical current leakage and lowering power consumption. A critical limitation is the reliability of their electrical contacts, which are prone to wear and hydrocarbon-induced contamination. These phenomena progressively inhibit signal transmission, eventually leading to device failure. We report on progress made towards converting the contamination into a highly conductive material. We show that Pt-coated microswitch contacts operating in the presence of O2 experience limited contaminant accumulation even in hydrocarbon-rich environments. We then demonstrate that devices that have experienced contamination can recover their original performance when operated in a clean N2:O2 environment. Auger and Raman spectroscopy indicate that this resistance recovery is associated with the structural transformation of the contaminant as opposed to its removal and that the transformed contaminant may shield the Pt coating from wear.

Chemical vapor deposition coating for micromachines

Ohmic micro- and nanoswitches are of interest for a wide variety of applications including radio frequency communications and as low power complements to transistors. In these switches, it is of paramount importance to maintain surface cleanliness in order to prevent frequent failure by tribopolymer growth. To prepare surfaces, an oxygen plasma clean is expected to be beneficial compared to a high temperature vacuum bakeout because of shorter cleaning time (<5 min compared to ~24 h) and active removal of organic contaminants. We demonstrate that sputtering of the electrode material during oxygen plasma cleaning is a critical consideration for effective cleaning of switch surfaces. With Ti electrodes, a TiO x layer forms that increases electrical contact resistance. When plasma-cleaned using graphite electrodes, the resistance of Pt-coated microswitches exhibit a long lifetime with consistently low resistance (<0.5 Ω variation over 300 million cycles) if the test chamber is refilled with ultra-high purity nitrogen and if the devices are not exposed to laboratory air. Their current–voltage characteristic is also linear at the millivolt level. This is important for nanoswitches which will be operated in that range.

Oxygen-induced graphitization of amorphous carbon deposit on ohmic switch contacts improves their electrical conductivity and protects them from wear

Coefficient of thermal expansion (CTE) is an important thin film property that is typically measured using multiple whole wafers. Here, the authors show how CTE can be locally extracted on a single substrate using out-of-plane deflection measurements of freestanding fixed–fixed beams versus temperature. Residual strain information is simultaneously extracted. Results for aluminum/0.5% copper thin film CTE, 37.4 ppm/u2009°C, and tensile residual strain, −161 μe, are in good agreement with previously published values.