Veljko Milanovic

Gimbal-less monolithic silicon actuators for tip-tilt-piston micromirror applications

In this paper, fully monolithic silicon optical scanners are demonstrated with large static optical beam deflection. The main advantage of the scanners is their high speed of operation for both axes: namely, the actuators allow static two-axis rotation in addition to pistoning of a micromirror without the need for gimbals or specialized isolation technologies. The basic device is actuated by four orthogonally arranged vertical comb-drive rotators etched in the device layer of an silicon-on-insulator wafer, which are coupled by mechanical linkages and mechanical rotation transformers to a central micromirror. The transformers allow larger static rotations of the micromirror from the comb-drive stroke limited rotation of the actuators, with a magnification of up to 3/spl times/ angle demonstrated. A variety of one-axis and two-axis devices have been successfully fabricated and tested, in all cases with 600-/spl mu/m-diameter micromirrors. One-axis micromirrors achieve static optical beam deflections of >20/spl deg/ and peak-to-peak resonant scanning of >50/spl deg/ in one example at a resonant frequency of 4447 Hz. Many two-axis devices utilizing four rotators were tested, and exhibit >18/spl deg/ of static optical deflection at <150 V, while their lowest resonant frequencies are above 4.5 kHz for both axes. A device which utilizes only three bidirectional rotators for tip-tilt-piston actuation achieves -10/spl deg/ to 10/spl deg/ of optical deflection in all axes, and exhibits minimum resonant frequencies of 4096 and 1890 Hz for rotation and pistoning, respectively. Finally, we discuss the preliminary results in scaling tip-tilt-piston devices down to 0.4 /spl times/ 0.4 mm on a side for high fill-factor optical phased arrays. These array elements include bonded low-inertia micromirrors which fully cover the actuators to achieve high fill-factor.

Large-displacement vertical microlens scanner with low driving voltage

We have designed, fabricated, and demonstrated large vertical displacement vertical microlens scanners with low (<10 V) driving voltage using silicon-on-insulator technology. The unique isolated and pre-engaged vertical comb-drive sets and the coupled-torsion flexure design provide both upward and downward piston motions, as well as low driving voltages. Single-directional devices demonstrate maximum static downward displacement of 8 /spl mu/m at 10 V/sub dc/. Bidirectional devices demonstrate vertical actuation from -6.5 to +9 /spl mu/m at max 12 V/sub dc/, and a vertical displacement of up to 55 /spl mu/m peak-to-peak is achieved at the resonance near 400 Hz. The lens motion shows piston motion with a small tilt angle of less than 0.034/spl deg/ and the compensation of the tilt using an isolated comb bank is demonstrated.

Micromachined Convective Accelerometers in Standard Integrated Circuits Technology

This letter describes an implementation of micromachined accelerometers in standard complimentary metal–oxide–semiconductor technology. The devices operate based on heat convection and consist of microheaters and thermocouple or thermistor temperature sensors separated by a gap which measure temperature difference between two sides of the microheater caused by the effect of acceleration on free gas convection. The devices show a small linearity error of <0.5% under tilt conditions (±90°), and <2% under acceleration to 7g(g≡9.81 m/s2). Sensitivity of the devices is a nearly linear function of heater power. For operating power of ∼ 100 mW, a sensitivity of 115 μV/g was measured for thermopile configuration and 25 μV/g for thermistor configurations. Both types of devices are operable up to frequencies of several hundred Hz.

Hybrid postprocessing etching for CMOS-compatible MEMS

A major limitation in the fabrication of microstructures as a postCMOS (complimentary metal oxide semiconductor) process has been overcome by the development of a hybrid processing technique, which combines both an isotropic and anisotropic etch step. Using this hybrid technique, microelectromechanical structures with sizes ranging from 0.05 to /spl sim/1 mm in width and up to 6 mm in length were fabricated in CMOS technology. The mechanical robustness of the microstructures determines the limit on their dimensions. Examples of an application of this hybrid technique to produce microwave coplanar transmission lines are presented. The performance of the micromachined microwave coplanar waveguides meets the design specifications of low loss, high phase velocity, and 50 /spl Omega/ characteristic impedance. Various commonly used etchants were investigated for topside maskless postmicromachining of silicon wafers to obtain the microstructures. The isotropic etchant used is gas-phase xenon difluoride (XeF/sub 2/), while the wet anisotropic etchants are either ethylenediamine-pyrocatechol (EDP) or tetramethylammonium hydroxide (TMAH). The advantages and disadvantages of these etchants with respect to selectivity, reproducibility, handling, and process compatibility are also described.

Multilevel beam SOI-MEMS fabrication and applications

A microfabrication technology has been developed and demonstrated, which enhances the capabilities and applications of high aspect ratio silicon-on-insulator microelectromechanical systems (SOI-MEMS) by enabling additional independent degrees of freedom of operation: both upward and downward vertical pistoning motion as well as bi-directional rotation. This is accomplished by applying multiple-mask high aspect ratio etches from both the front- and back-side of the SOI device layer, forming beams at different levels. The processes utilize four masks, two for front-side and two for back-side etching. As a result, single-crystal silicon beams with four different cross-sections are fabricated, and can be combined to form many additional beam cross-sections. This provides a wide variety of possible mechanical designs that can be optimized for optical and other applications. By this methodology, unique high aspect ratio micromirror devices were demonstrated with fully isolated and accurately self-aligned vertical combdrives in the SOI device layer, with initial combfinger overlap. Examples of fabricated devices are shown with performance summaries.

SYNCHRONIZATION OF CHAOTIC NEURAL NETWORKS AND APPLICATIONS TO COMMUNICATIONS

Methods for synchronizing discrete time chaotic neural networks are presented with possible applications in single- or multi-user private communications. Chaotic neurons, characterized with a piecewise-linear N-shaped transfer function, are connected into Hopfield-like networks with parameters set for chaos. The networks are used as transmitter and receiver circuits in chaotic communications schemes. The first algorithm is a modification of simple chaotic masking which makes synchronization robust and insensitive to the perturbation from the added information signal. A mathematical proof and simulation results of the scheme are shown for small networks. We have verified the method experimentally, using single- and two-neuron circuits. The second algorithm utilizes modulation of the transmitting chaotic network by a binary bit stream and detection of the corresponding synchronization error at the receiver. A method for multiple-user chaotic communication is also presented, utilizing chaotic neurons and spread spectrum techniques. The effects of additive noise in the proposed communication schemes are considered and simulated. Synchronization of larger networks and possible applications are also discussed.

Laterally actuated torsional micromirrors for large static deflection

We report on the implementation of laterally electrostatically actuated, torsionally suspended silicon-on-insulator (SOI) micromirrors with a static optical deflection angle of over 40/spl deg/ peak-to-peak. Decoupling the actuator and mirror design allows for large actuator arrays, allowing large dc deflection angle and high resonant frequency to coexist in the same device. The micromirror structures are fully monolithic, micromachined from the front side and back side of an SOI wafer-device layer. In-plane actuation is transformed into out-of-plane motion and rotation, enabling integration of a wide variety of SOI-MEMS sensors, actuators, and micromirrors. When operated in resonance at 1321 Hz, a typical device measured up to 92/spl deg/ peak-to-peak optical deflection at 127 Vdc with 15 Vac amplitude.

Microrockets for Smart Dust

This paper details the design and fabrication of millimeter-scale solid propellant rockets for one-time deployment of wireless sensor platforms, known as Smart Dust. Each microrocket assembly is an integrated system, incorporating a combustion chamber, composite propellant grain, nozzle, igniter, and thermoelectric power converter. Solid propellant is advantageous for a millimeter-scale single-use device because of its simple implementation, unlike liquid propellants, which require a more elaborate system of pumps and valves. Therefore the total system volume and complexity are minimized. One type of combustion chamber was fabricated in silicon; however, thermal losses to the silicon sidewalls during combustion through a 1.5 mm2 cross section of fuel were too high to reliably maintain a burn. Successful combustion was demonstrated in cylindrical alumina ceramic combustion chambers with thermal conductivities five times lower than silicon and cross sections of 1-8 mm2. Thrusts of 10-15 mN were measured for ceramic rockets weighing under l g, with specific impulses up to 15 s. Silicon nozzles integrated with polysilicon microheaters and thermopiles for thermal power conversion were microfabricated in a single process. Fuel ignition by polysilicon microheaters suspended on a low-stress nitride (LSN) membrane was demonstrated. Microheaters require less than 0.5 W of power to ignite a propellant composed primarily of hydroxyl-terminated polybutadiene (HTPB) with ammonium perchlorate (AP) oxidizer. They are suspended for thermal isolation through bulk post-processing by a backside deep reactive ion etch (DRIE). The etched hole beneath the igniter area also serves as a nozzle through which high-velocity combustion gases exit the rocket. Thermopiles, which generate voltages proportional to hot and cold junction temperature differentials, have been fabricated in the same process as igniters, and span backside DRIE thermal isolation cavities. Ten-junction thermopiles produced a maximum power of 20 µW. With potential temperature differences of hundreds of degrees and a total of 120 thermocouple junctions fabricated on the silicon nozzle chip, hundreds of milliwatts of power could feasibly be produced during the microrockets flight and used to augment the Smart Dust power supply.

Vertical combdrive based 2-D gimbaled micromirrors with large static rotation by backside island isolation

We introduce a backside island isolation method for silicon-on-insulator (SOI)-based microelectromechanical systems technology and demonstrate vertical comb drive-based two-dimensional gimbaled micromirrors with large static rotation using the isolation method. The proposed isolation method provides electrical isolation and mechanical coupling of SOI structures without additional dielectric backfill and planarization by utilizing timed etched backside handle wafer structures. The backside island is a hidden layer beneath the gimbal and allows independent application of actuation potentials to the gimbal and inner mirror. We developed the fabrication process that accommodates the backside island isolation structures into an established vertical comb drive process, thereby allowing implementation of two-axis gimbaled structures. The maximum static optical deflections of the gimbal and mirror are 46/spl deg/ and 15/spl deg/, respectively.

Micromachining technology for lateral field emission devices

We demonstrate a range of novel applications of micromachining and microelectromechanical systems (MEMS) for achieving efficient and tunable field emission devices (FEDs). Arrays of lateral field emission tips are fabricated with submicron spacing utilizing deep reactive ion etch (DRIE). Current densities above 150 A/cm/sup 2/ are achieved with over 150/spl middot/10/sup 6/ tips/cm/sup 2/. With sacrificial sidewall spacing, electrodes can be placed at arbitrarily close distances to reduce turn-on voltages. We further utilize MEMS actuators to laterally adjust electrode distances. To improve the integration capability of FEDs, we demonstrate batch bump-transfer of working lateral FEDs onto a quartz target substrate.