Reliability Aspects of Microelectromechanical Systems for Space Applications

Microelectromechanical systems (MEMS) are an essential ingredient in many technological innovations and a source of game-changing inventions in the automotive industry, space exploration, consumer electronics and medical applications due to its capability to control mechanical effects on a micrometer scale. Especially where accessibility, connectivity, system size and energy supply are limited, microsystems can be an enabling technology for sensing, actuation and data transmission. However, their applicability depends on their ability to operate reliably even in extremely hazardous situations such as in space, where the devices are exposed to high levels of radiation, large temperature variations and accelerations. Improving the design and increasing the lifetime of devices requires the identification and detailed understanding of failure modes. This thesis was aimed at deepening the understanding of fabrication-related effects and space-relevant environmental hazards on the mechanical properties of MEMS on the material and the systems level. Although countless microsystems have been developed in recent years, several vital questions related to the reliability of MEMS systems and MEMS materials remain open to date. Two topics which merit special attention were at the center of this study: First, the influence of packaging on the functioning and the reliability of microsystems and second, the reliability of microsystems and their materials under the harsh environmental conditions imposed by space applications. Out of this area of interest the following questions were studied: Strain analysis in MEMS package by HRXRD: Packaging offers protection to the microsystem and aids to maintain a stable environment. However, it also influences the strain distribution in the system, and the energy losses in resonant structures due to air being squeezed in thin gaps between the resonator and the package. The distribution of residual stresses and bonding stresses in a MEMS wafer-level package was analyzed by high-resolution x-ray diffraction (HRXRD) and the possibility of nondestructive investigation of the strain distribution in sub-surface structures was demonstrated. Air damping in MEMS packages: An improved framework for the analysis of air-damping in microresonators and their packages has been presented. Understanding the pressure sensitivity allows determining the admissible pressure levels in resonator packages, above which the performance is deteriorated by losses through air-damping. In addition, monitoring the resonance frequency and quality factor variations over time can be used to determine the hermeticity and leak rates in very small packages where the traditional helium leak test method fails because the relevant leak rates are below its sensitivity. Reliability of MEMS under space-relevant environmental hazards: The performance of resonant microstructures is directly linked to the Youngâs modulus of the utilized material and therefore they are very sensitive to variations thereof. The susceptibility of microfabricated structural materials (SU-8 and silicon) to proton irradiation was investigated. In order to isolate the influence of radiation on the materials themselves, single material resonators with contactless actuation and readout were tested. The experimental results showed that single crystal silicon and SU-8 were tolerant to high doses of proton radiation and are hence very well suited for MEMS in space applications.

Theoretical and Experimental Study of Piezoelectric Modulated AlN Thin Films for Shear Mode BAW Resonators

Evgeny Milyutin

The use of acoustic resonators for sensors application has opened a new branch in research and applications of piezoelectric materials and devices. The first generation of such sensors is constituted by the quartz crystal microbalance (QCM) based on AT-cut mono-crystalline quartz. High sensitivities of gravimetric sensing in both air and liquids were demonstrated. Since the 1980s, when the first QCM-based sensor was demonstrated for the detection of silver in a liquid solution, many other application in chemical, bio-medical and environmental sensing were realized using the same concept. QCM's exhibit a very good thermal stability. The AT-cut quartz plate leads to the excitation of shear waves when used in parallel capacitor geometry. This is important for achieving high quality factors in the immersed operation of the sensor. A second generation of gravimetric sensors is based on surface acoustic wave (SAW) structures, working for instance with Love waves in a SiO2 layer on top of a LiTaO3 single crystal. SAW devices are mainly used as RF filters in television and mobile phones. SAW sensors are a side product of the much larger telecommunication market. The evolution of thin film and MEMS technology has lead to a third generation of gravimetric sensors that is based on bulk acoustic wave (BAW) resonances in piezoelectric thin films. Again, such sensors are a side product from the large telecommunication market where such resonators are used for RF filters. With every generation, the oscillation frequency increased. While QCM's operate typically at 5 MHz, the SAW resonators work typically at a few 100 MHz, and the thin film BAW resonators (TFBAR) operate typically around 2 GHz. The increase of the frequency goes together with an increase in sensitivity, and a decrease of the thickness and mass range that can be measured. The TFBARs are in some sense miniaturized analogs of QCM's operating at much higher frequencies. They are very promising as they reach higher sensitivities. This attracted the attention of researchers, and experimental evidence of the potential of TFBARs as sensors was delivered. In addition to the higher sensitivity, the miniaturization allows for using arrays of sensors with different immobilization layers as needed for drug screening. However, the application of the same TFBARs as used in telecommunications would not allow a good performance in immersed operation. It would only be good for operation in air. The principal characteristic of TFBARs used in mobile phones is the value of the electromechanical coupling and not the resonance mode at which this coupling is achieved. But for the in-liquid operated sensors, it's rather the opposite – the mode of the resonance should be such that the surface of the resonator, which tis in contact with liquid, should move parallel to the surface (in-plane motion, or transverse motion) to minimize emission of acoustic waves into the liquid. The coupling coefficient is of secondary importance in this case. Therefore, the development of shear mode TFBARs became an interesting task that was challenged by several research groups. One of the most successful solutions is the use of c-axis inclined AlN thin films. Inclination of c-axis in a parallel plate capacitor structure enables the coupling of the electric field to the shear strain, which enables the excitation of a shear mode. Such devices were successfully applied for selective sensing of organic species, such as DNA molecules, suspended in a liquid. Even if the process of deposition of c-axis inclined AlN films doesn't require hardware modification, the quality of the process is still far from the one for deposition of (0001)AlN films used in telecommunications. So the goal of this thesis was to find a solution for the shear mode TFBAR's based on (0001)AlN films. In (0001)AlN films, the shear strain cannot be induced by the electric field produced in a parallel plate geometry as the coefficient e35 and e34 of the piezoelectric tensor are zero. But there are e15 and e24 coefficients that are not zero, meaning that in-plane electric field can be used to excite the shear waves. In the frame of this work, this concept was studied theoretically and experimentally. The in-plane electric field was generated via inter-digitated electrodes (IDE). A first device type was realized in solidly mounted resonator (SMR) design and based on uniform (0001)- oriented AlN thin films. The anti-phase of the electric field in adjacent half-periods of the IDE resulted in an anti-phase movement of the corresponding regions of the film. Finite element modeling and boundary element modeling (FEM-BEM) were carried out to clarify the kind of vibrations present in the device. A kind of shear/lingitudinal mode with elliptic motions was obtained. A device was fabricated and tested both in liquid and air. The resonance of the device was observed in the expected frequency range (1.86 GHz) and high quality factors under operations in air (Q=870) and silicon oil (Q=270) were obtained. The drop of the quality factor was explained by the up and down motion of the regions of the film located directly under the IDE electrodes. Such a motion is due to anti-phase motion of different regions of the film as mentioned above. To prevent this effect, a second device type was studied. It is again based on the use of (0001)AlN thin films, but with modulated piezoelectric properties. Having different piezoelectric properties in the regions corresponding to adjacent half-periods of IDE, breaks the mirror symmetry of the device and allows for coupling the electric field to a pure shear mode. Analytical and numerical models explaining such a device were established and evaluated. The optimal situation is found when perfect Al-polar and N-polar regions of AlN are combined. This maximizes the coupling coefficient k that is derived as being proportional to the difference of e15 coefficients. Finding a way to fabricate the AlN thin film with different piezoelectric properties was thus the next objective. Growth features to decrease the piezoelectric effect were first studied. Providing rougher regions on the otherwise smooth substrate allowed to modify locally the quality of AlN thin films. Device based on such films were fabricated and characterized. The resonance of a pure shear mode was found at the expected frequency (roughly 2GHz) when the piezoelectric effect was modulated. Devices without this modulation failed to show the resonance at the exact frequency, exactly as the theory predicted. We managed to reduce two times the d33 coefficient of the film by inducing a increased roughness to the substrate – from 5.0 pm/V, corresponding to 1.5 degree rocking curve, down to 2.4pm/V, corresponding to 7 degree of rocking curve. The final step of the thesis was the process development for the simultaneous growth of Al-polar and N-polar regions within sputter deposited (0001)AlN, in order to achieve the maximal possible "piezomodulation" effect. As the sputter deposition yielded only N-polar films, we included Al-polar films from another source as seed layers. High temperature epitaxial growth methods of GaN and AlN on Si(111) and Si(100) lead to Ga- and Al-polarities. On such films, the sputter deposited AlN copies polarity from the growth substrate. The selective polarity was then obtained by preventing the expitaxy locally through a patterned oxide layer. Wet etching tests together with PFM measurements were performed to prove the dual polarity in the sputter deposited film. Finally, the integration of this process into the process flow for device fabrication was investigated.

EPFL2011

Nanogap MEM resonators on SOI

Nicoleta-Diana Ciressan

Today's consumer electronics based on a large variety of time-keeping and frequency reference applications is based on quartz-crystal oscillators, because of their excellent performances in terms of quality factor, thermal and frequency stability. However, macroscopic size and CMOS incompatibility of quartz-crystal resonators draw a major limit on the miniaturization of wireless communication applications. For this reason, silicon microelectromechanical (MEM) resonators are considered promising candidates to replace quartz-oscillators in VLSI communication systems, due to their compactness, design flexibility and CMOS process compatibility. This work reports on the design, fabrication and characterization of MEM bulk lateral resonators with resonance frequencies in the order of tens of MHz. The need of stable, low cost and high yield processes for fabricating devices with extremely narrow (sub 100 nm) transduction gaps is a main research driver, together with the requirements of improved designs which include high quality factors and appropriate power handling. We are investigating two major designs: (1) longitudinal beam resonators and (2) novel fragmented-membrane resonators that respond to both high quality factor (Q) and low motional resistance (Rm) requirements. We propose several design optimizations for solving some of the issues which currently affect the MEM resonator performance, like the frequency stability and Q enhancement through energy loss minimization. An original fabrication process is presented, which enables the manufacturing of SOI-based, fully monocrystalline devices with 100 nm transduction gaps and aspect ratios as high as [60:1], without the need of advanced lithography techniques. We successfully validate the fabrication process on two different SOI substrates, with silicon film thicknesses of 1.5 µm and 6.25 µm. This thickness range combined to the doping level corresponds to partially depleted SOI, which can be a substrate of choice for the fabrication of future integrated hybrid MEMS-CMOS integrated circuits for communication applications. The fabricated structures are successfully characterized, demonstrating excellent resonator performance at room temperature. Q's as high as 235'000 and Rm's as low as 59 kΩ have been extracted in vacuum. Atmospheric pressure frequency response measurements of the fragmented membrane resonators show Q's of 3'600. This value is among the best reported to date, opening the possibility for atmospheric pressure applications such as mass-detection for gas sensing applications with detection done without a special package, just by direct exposure of the resonator to the environment. A novel study on temperature dependence (between 80 K and 320 K) of the fragmented membrane resonators is presented and discussed. Significant Q increase and Rm reduction are experimentally observed at cryogenic temperatures. The bulk mode resonators developed and discussed in this thesis can successfully be integrated in oscillator circuits, as we demonstrate by simulating a Pierce topology based on the calibrated equivalent circuit model of a 24.48 MHz fragmented membrane BLR. Our oscillator shows very good phase-noise performance of -142 dBc/Hz at 1 kHz offset from the carrier and the noise floor at -144 dBc/Hz, which nearly meets the GSM specifications.

EPFL2009

Carbon Nanotube Electromechanical Devices for Radio Frequency Applications

Anupama Arun

Carbon nanotubes (CNT) have been investigated extensively in the recent years for application in nano-electromechanical systems (NEMS). CNTs have extraordinary electrical and mechanical properties. They are stiff, with low density, high electrical conductivity and hence are the ideal candidates for NEMS. Beyond miniaturization, NEMS can also offer better performance than its predecessor micro-electro mechanical systems (MEMS). NEMS find application in ultra-sensitive mass and force detection, RF signal processing, low power operated switches with fast switching speeds, to name a few. Despite tremendous promise the main challenge in CNT research has been the fabrication technology to realize CNT based devices with precise control of location and properties of CNT. CNTs are one-dimensional structures with high aspect ratio. To realize devices for practical application with high signal-to-noise-ratio, it becomes necessary to operate many of them in parallel. To fabricate an array of CNTs in parallel posse's additional challenge to the fabrication technology but the resulting structures could provide benefits compared to both NEMS fabricated with one dimensional structures and MEMS. During the course of this thesis, different fabrication technologies were investigated to realize CNT array based devices. A self assembly method, dielectrophoresis and CVD growth were optimized to realize CNT based devices. A novel technique using a modified dielectrophoresis method was setup to be able to trap a single CNT. A CNT crossed junction was fabricated using the same. The electrical properties of the junction were studied. A Suspended Gate CNT array Field Effect Transistor (SG-CNT-FET) based resonator was demonstrated for the first time. SG-CNT-FET combines the excellent mechanical properties of CNTs with the gain of the solid state field effect transistor. The use of an array configuration ensures better signal-to-noise-ratio compared to device implemented with a single CNT. A novel process flow combining the fabrication of a FET and deposition of CNTs by dielectrophoresis was developed. Resonance frequency up to 150 MHz with a quality factor of approximately 50 was obtained. The resonance frequency is tuneable by modulating the tension in the CNT array by applying a DC bias. Dependence of the resonance frequency on the applied bias has been studied under the frame work of a continuum mechanics model. The small signal circuit parameters have been extracted, enabling future circuit design. Vertically grown CNT array were grown on metal lines to realize a tuneable MEM capacitor. The CNT array forms a membrane like structure which can be electro-statically actuated. Morphological and electrical characterizations were performed on the fabricated devices. DC characterization revealed the pull-in behavior like conventional MEMS. The Young's modulus of the grown vertical CNT array was extracted from the measured pull-in voltage. An effective Young's modulus of 30-100 MPa was obtained. An equivalent electrical model was developed to extract the capacitance from the S-parameters measurements, measured up to 6 GHz. The model was validated by ADS circuit simulations. A voltage controlled oscillator at 2.45 GHz has been implemented using the thus realized tuneable MEM capacitor.

EPFL2010