Michael S. Baker

Post-CMOS-Compatible Aluminum Nitride Resonant MEMS Accelerometers

This paper describes the development of aluminum nitride (AlN) resonant accelerometers that can be integrated directly over foundry CMOS circuitry. Acceleration is measured by a change in resonant frequency of AlN double-ended tuning-fork (DETF) resonators. The DETF resonators and an attached proof mass are composed of a 1-mum-thick piezoelectric AlN layer. Utilizing piezoelectric coupling for the resonator drive and sense, DETFs at 890 kHz have been realized with quality factors (Q) of 5090 and a maximum power handling of 1 muW. The linear drive of the piezoelectric coupling reduces upconversion of 1/f amplifier noise into 1/f 3 phase noise close to the oscillator carrier. This results in lower oscillator phase noise, -96 dBc/Hz at 100-Hz offset from the carrier, and improved sensor resolution when the DETF resonators are oscillated by the readout electronics. Attached to a 110-ng proof mass, the accelerometer microsystem has a measured sensitivity of 3.4 Hz/G and a resolution of 0.9 mG/radicHz from 10 to 200 Hz, where the accelerometer bandwidth is limited by the measurement setup. Theoretical calculations predict an upper limit on the accelerometer bandwidth of 1.4 kHz.

An array of microactuated microelectrodes for monitoring single-neuronal activity in rodents

Arrays of microelectrodes used for monitoring single- and multi-neuronal action potentials often fail to record from the same population of neurons over a period of time for several technical and biological reasons. We report here a novel Neural Probe chip with a 3-channel microactuated microelectrode array that will enable precise repositioning of the individual microelectrodes within the brain tissue after implantation. Thermal microactuators and associated microelectrodes in the Neural Probe chip are microfabricated using the Sandias Ultraplanar Multi-level MEMS Technology (SUMMiTV) process, a 5-layer polysilicon micromachining technology of the Sandia National labs, Albuquerque, NM. The Neural Probe chip enables precise bi-directional positioning of the microelectrodes in the brain with a step resolution in the order of 8.8 /spl mu/m. The thermal microactuators allow for a linear translation of the microelectrodes of up to 5 mm in either direction making it suitable for positioning microelectrodes in deep structures of a rodent brain. The overall translation in either direction was reduced to approximately 2 mm after insulation of the microelectrodes with epoxy for monitoring multi-unit activity. Single unit recordings were obtained from the somatosensory cortex of adult rats over a period of three days demonstrating the feasibility of this technology. Further optimization of the microelectrode insulation and chip packaging will be necessary before this technology can be validated in chronic experiments.

Post-CMOS Compatible Aluminum Nitride MEMS Filters and Resonant Sensors

This paper reports post-CMOS compatible aluminum nitride (AlN) MEMS resonators, filters, and resonant sensors for the miniaturization of radio-frequency transceivers and sensor systems. Utilizing a resonator with two closely spaced modes, 2nd order MEMS filters occupying 0.06 mm2 have been realized in a single device. Methods for tuning the bandwidth and center frequency of these filters lithographically have been demonstrated. A 0.5% bandwidth, 108.4 MHz dual mode filter has a measured insertion loss of 9.4 dB with 50 Omega termination which can be reduced to 4.7 dB by terminating the filter with 75 Omega. In order to scale MEMS resonators to higher frequencies without increasing the size or impedance, resonators selectively driven at a harmonic determined by interdigitated drive and sense electrodes have been demonstrated reaching frequencies of 796 MHz with impedances of approximately 100 Omega and quality factors in excess of 750 in air. In the same process resonant sensors based on AlN double-ended tuning fork (DETF) sensing beams have been demonstrated at 727 kHz with quality factors of 2160. An oscillator based on the DETF sensing beams achieves a phase noise of -81 dBc/Hz at 275 Hz offset from the carrier. A 100 ng mass coupled to a pair of DETF sensors achieves an acceleration sensitivity of 565 mG/radicHz for accelerations from 275 to 1100 Hz.

Robust design and model validation of nonlinear compliant micromechanisms

Although the use of compliance or elastic flexibility in microelectromechanical systems (MEMS) helps eliminate friction, wear, and backlash, compliant MEMS are known to be sensitive to variations in material properties and feature geometry, resulting in large uncertainties in performance. This paper proposes an approach for design stage uncertainty analysis, model validation, and robust optimization of nonlinear MEMS to account for critical process uncertainties including residual stress, layer thicknesses, edge bias, and material stiffness. A fully compliant bistable micromechanism (FCBM) is used as an example, demonstrating that the approach can be used to handle complex devices involving nonlinear finite element models. The general shape of the force-displacement curve is validated by comparing the uncertainty predictions to measurements obtained from in situ force gauges. A robust design is presented, where simulations show that the estimated force variation at the point of interest may be reduced from /spl plusmn/47 /spl mu/N to /spl plusmn/3 /spl mu/N. The reduced sensitivity to process variations is experimentally validated by measuring the second stable position at multiple locations on a wafer.

On-chip actuation of an in-plane compliant bistable micromechanism

A compliant bistable micromechanism has been developed which can be switched in either direction using on-chip thermal actuation. The energy storage and bistable behavior of the mechanism is achieved through the elastic deflection of compliant segments. The Pseudo-Rigid-Body Model was used for the compliant mechanism design, and for analysis of the large deflection flexible segments. To achieve on-chip actuation, the mechanism design was optimized to allow it to be switched using linear motion thermal actuators. The modeling theory and analysis are presented for three design iterations, with two iterations fabricated in the MUMPs process and the third in the SUMMiT process.

Spatially resolved temperature mapping of electrothermal actuators by surface Raman scattering

In this paper, we report spatially resolved temperature profiles along the legs of working V-shaped electrothermal (ET) actuators using a surface Raman scattering technique. The Raman probe provides nonperturbing optical data with a spatial resolution of 1.2 /spl mu/m, which is required to observe the 3-/spl mu/m-wide actuator beams. A detailed uncertainty analysis reveals that our Raman thermometry of polycrystalline silicon is performed with fidelity of /spl plusmn/10 to 11 K when the peak location of the Stokes-shifted optical phonon signature is used as an indicator of temperature. This level of uncertainty is sufficient for temperature mapping of many working thermal MEMS devices which exhibit characteristic temperature differences of several hundred Kelvins. To our knowledge, these are the first quantitative and spatially resolved temperature data available for thermal actuator structures. This new temperature data set can be used for validation of actuator thermal design models and these new results are compared with finite-difference simulations of actuator thermal performance.

Integrated measurement-modeling approaches for evaluating residual stress using micromachined fixed-fixed beams

Two methodologies have been developed to determine the biaxial residual stress value in thin films using electrostatically actuated fixed-fixed beam test structures. In the first, we determine the compliance matrix of the support posts using 3-D finite-element analysis. The residual stress value is then found from the best fit between the measured and modeled deflection curves, with the residual stress as the only free parameter in the model. An accuracy of /spl plusmn/0.5 MPa for the average biaxial residual stress level is evaluated from the reproducibility of independent measurements over a wide range of loadings. The key to the second methodology lies in the recognition that for a given value of residual stress, there exists a unique family of deflection curves associated with two adjacent beams of different lengths. Therefore, compliance information can be extracted directly from the deflection curves. We proceed to show that essentially the same values of residual stress are found by the two methodologies, while the latter allows much more rapid extraction of the residual stress. With the second methodology established, we find that residual stress values vary across a quarter of a six-inch diameter wafer by 2.5 MPa for three structural levels of polycrystalline silicon in our five-level surface micromachining technology.

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.

Long-Term Neural Recordings Using MEMS Based Movable Microelectrodes in the Brain

One of the critical requirements of the emerging class of neural prosthetic devices is to maintain good quality neural recordings over long time periods. We report here a novel MEMS (Micro Electro Mechanical Systems) based technology that can move microelectrodes in the event of deterioration in neural signal to sample a new set of neurons. Microscale electro-thermal actuators are used to controllably move microelectrodes post-implantation in steps of approximately 9 μm. In this study, a total of 12 movable microelectrode chips were individually implanted in adult rats. Two of the twelve movable microelectrode chips were not moved over a period of 3 weeks and were treated as control experiments. During the first 3 weeks of implantation, moving the microelectrodes led to an improvement in the average signal to noise ratio (SNR) from 14.61 ± 5.21 dB before movement to 18.13 ± 4.99 dB after movement across all microelectrodes and all days. However, the average root-mean-square values of noise amplitudes were similar at 2.98 ± 1.22 μV and 3.01 ± 1.16 μV before and after microelectrode movement. Beyond 3 weeks, the primary observed failure mode was biological rejection of the PMMA (dental cement) based skull mount resulting in the device loosening and eventually falling from the skull. Additionally, the average SNR for functioning devices beyond 3 weeks was 11.88 ± 2.02 dB before microelectrode movement and was significantly different (p < 0.01) from the average SNR of 13.34 ± 0.919 dB after movement. The results of this study demonstrate that MEMS based technologies can move microelectrodes in rodent brains in long-term experiments resulting in improvements in signal quality. Further improvements in packaging and surgical techniques will potentially enable movable microelectrodes to record cortical neuronal activity in chronic experiments.