MANHASSET, N.Y. While integrated circuits remain a staple of the annual International Electron Devices Meeting, this year's conference will look closer than ever before at the progress being made in microelectromechanical systems.
Work on MEMS is in the advanced research phase, and the presentations scheduled for this year's IEDM, to be held in Washington on Dec. 7-10, will give an inkling of what can be expected in the commercial sector over the next few years. Six MEMS presentations will describe potential solutions to knotty problems where electronics is to be linked with mechanics in an integrated package.
Analog Devices Inc. researchers have integrated a sensor and electronics in single-chip 3 x 3-mm inertial measurement units (IMUs). They were able to integrate both the sensor and the electronics in a planar-type process for high-volume, low-cost, high-reliability inertial measurement components. The researchers claim to have produced more than 100 million inertial measurement devices in this way.
The authors will highlight the process developments and design innovations that were necessary to produce what they say is the industry's first fully integrated single-chip "rate to voltage converter," or gyroscope. The same process improvements are enabling the next generation of high-accuracy integrated low-g accelerometers, now in development.
Integrating sensors with electronics is complex and expensive. All other volume producers of MEMS inertial sensors that are capable of true angular rate sensing, as well as acceleration sensing, use the initially less expensive and certainly easier-to-develop two-chip approach. But that approach requires interchip bonding, and the resultant rise in parasitic capacitance and crosstalk affects the resolution of the measurements that can be made.
The authors report that they improved the surface micromachining of the original integrated process for accelerometers in order to produce a gyroscope. The structural polysilicon layer had to be doubled in thickness to be stiff enough and yet still manufacturable in high volumes. The surface topography of the thicker "planar" structures posed a high-volume manufacturability challenge: An extra layer of interconnect beneath the structure had to be developed for both shielding and low-capacitance connections, the researchers write.
The resultant gyro mechanical structure is able reach very tight tolerances. The gyro rejects mechanical cross-coupling from movement on one axis to induced movement on an orthogonal axis to a few tens of parts per million (ppm). Based on this structure and the low parasitic integrated electronics, a single-chip rate sensor was developed with an Allan deviation of 50 degrees/hour and a dynamic range of 10,000/1. The technology is said to be able to resolve movements on the order of a few tens of femtometers.
The process has also enabled the development of a two-axis accelerometer, accurate over temperature to 50 milli-g, that will enter production soon.
Using the accelerometer with the rate sensor would enable the realization of a very compact, 6-degrees-of-freedom (DoF) IMU, ADI claims.
Researchers at South Korea's Seoul National University, meanwhile, will report on an electrical isolation method for single-crystal-silicon MEMS devices. The approach uses sacrificial bulk micromachining (SBM) to deploy horizontal dielectric layers at arbitrary depths in any desired region of a wafer.
The authors explain that the most popular way to achieve electrical isolation has been to use silicon-on-insulator (SOI) wafers. But deep silicon RIE processing on SOI wafers leads to the well-known "footing" phenomenon, resulting in the unwanted lateral and reverse-directional etching of silicon at the silicon/oxide interface. The resultant rough and distorted bottom surface can alter the design specifications of the device. A more severe problem of footing is that the silicon fragments that separate from the bottom of the structure float around and can cause electrical shorts between electrodes.
In the SBM process, a very small sacrificial gap is left over the areas that will become electrodes. The gap is then filled with thermal oxide/nitride/polysilicon films. After filling the gap, to strengthen the bonding of polysilicon layers, thermal annealing is performed in an O2 atmosphere at 1,000 degrees C. Next the films are removed from the top surface and the desired devices fabricated, and then the etch masks are removed.
A microgyroscope fabricated in this way exhibited leakage current of less than 15 pA up to the applied bias of 120 volts, which is similar to that exhibited using SOI, the team will report. But the bottom surfaces of the fabricated microgyroscope are as flat and as clean as a polished surface. The structural thickness is 21 microns; the sacrificial gap is 30 microns. In testing the gyroscope, the bandwidth was measured to be 7.3 Hz, and the output was linear within 1.2 percent for a plus/minus 50 degree/second range.
The Seoul researchers claim the proposed method holds potential for such applications as RF switches, optical mirrors and 6-DOF inertial sensors.
At the University of Michigan, Ann Arbor, MEMS researchers have developed vibrating polysilicon micromechanical ring resonators (see figure, far left) in an extensional "wineglass" shape capable of achieving lower impedance than previous UHF resonators. Resonators have been demonstrated to operate at frequencies as high as 828 MHz with Q's around 2,200 both in air and in a vacuum, and at 426 MHz with a Q of 7,700 in a vacuum.
According to the authors, the resonators operate in a special resonant mode that combines the aspects of two previously demonstrated modes: the extensional radial contour vibration mode and the wineglass disk vibration mode. The resonators are said to achieve the best of each mode, along with the geometric advantages of a ring structure.
The extensional wineglass resonator design reportedly allowed the researchers to achieve a high resonance frequency, low motional impedance and higher Q. The high Q and low impedance make the device superior to its predecessors for use in the front-end RF filtering and local oscillator functions needed by present-day wireless communication devices, the researchers write. The design also retains the multiple-frequency on-chip integration advantages that electrostatically transduced micromechanical resonators have over film bulk acoustic resonators (FBARs) and other piezoelectric devices.
The resonance frequency of the wineglass resonator in extensional mode is determined primarily by the width of its ring rather than by its radius. Thus, the perimeter of the device can be made arbitrarily large to maximize transducer capacitance and drive down series motional resistance.
In addition, since the frequency of the device is determined primarily by its lateral dimensions, which are set by CAD layout, the wineglass resonator easily supports multiple frequencies on a single chip without the need for multiple film depositions, the authors state. In contrast, counterparts with frequencies determined only by thickness (e.g., FBARs) require an additional film deposition for each additional frequency.
A group at the University of Michigan's Center for Wireless Integrated Microsystems will report on a 10-MHz series resonant micromechanical resonator oscillator. The implementation uses a custom-designed, single-stage, zero-phase-shift sustaining amplifier together with a clamped-clamped (C-C) beam micromechanical resonator, designed with a relatively broad width to achieve substantially lower series motional resistance and higher power handling than previous devices.
Although widening of the resonator beam lowers its resistance-and thereby allows the use of a lower-noise, lower-power single-stage sustaining amplifier-it also seems to lower the resonator's Q from the approximately 3,000 normally seen for 8-micron-wide devices to only 1,036. Nevertheless, the authors will report, the latter Q is still more than two orders of magnitude higher than what an on-chip inductor capacitor pair can achieve.
That Q performance, together with the potential for full integration of the transistor-sustaining circuit and MEMS device onto a single silicon chip, makes the Michigan team's micromechanical resonator oscillator an attractive on-chip replacement for quartz crystal reference oscillators in communications and other applications, the researchers claim.
Researchers from the Georgia Institute of Technology (Atlanta) will report on the design, fabrication and testing of high-frequency piezoelectrically transduced, single-crystal-silicon (SCS) block resonators on SOI substrates. Thus far, the highest measured frequency is 195 MHz.
The simple, three-mask low-temperature (250 degrees C) fabrication process makes the devices feasible for post-CMOS integration. Quality factors of 5,500 and 4,700 were measured in a 50mTorr vacuum for a 480-micron-long x 120-micron-wide silicon block resonator (4 microns in thickness) at 66.6 MHz and 195 MHz, respectively.
Given the increasing demands for higher integration and lower cost in wireless and portable electronics, these high-Q resonators are choice candidates for on-chip frequency references, integrated filters and sensors, the Georgia Institute group says.
The team presented the first generation of the devices at the IEEE MEMS 2003 conference earlier this year, showing low-frequency piezoelectric clamped-clamped beam structures with resonant frequency of less than 17 MHz. The group's current work extends the frequency into the VHF range using a side-supported block resonator with in-plane length-extensional vibrations.
In their effort to reach higher frequencies, the researchers designed block structures with electrodes placed in such a way that the block can be excited longitudinally, leading to high-order modes. In structure, the resonator resembles a simple block, centrally supported by small, self-aligned tethers. Single-crystal silicon is used as the resonating element because of its high inherent mechanical quality factor and stress-free properties.
Piezoelectric sense and actuation are provided by a thin zinc-oxide film sputtered on silicon. The sense and drive electrodes are made of aluminum. The zinc-oxide film acts as an insulator between the top aluminum electrode and bottom device layer.
The researchers also observed pure extensional modes of the fabricated block resonators. For the 120 x 40-micron block resonator, the first and second extensional modes were measured at 35 MHz and 104 MHz (with a Q of 4,500)-results that they say came close to the calculated theoretical values. The highest Q measured for the block resonators, 11,600, was recorded for the first extensional mode of a 240 x 20-micron block at 17 MHz.
Finally, researchers at NTT Microsystem Integration Labs (Kanagawa, Japan) will report on a micromirror array for optical MEMS switch systems being developed for future photonic networks. Because they consist of many ICs and wires, optical MEMS switch systems are quite large. One solution is to integrate the ICs on one chip and fabricate micromirrors on LSIs. The NTT researchers worked on the structure of the control electrodes to reduce the drive voltage of the micromirror. They also developed anti-sticking technology by employing screen printing in the fabrication process.
The team will report that it achieved a maximum rotation angle of 1 degree at 30 V, which enables a micromirror array to be fabricated on the control LSI effectively and thus confirms the possibility of compact optical MEMS switch systems.