Andrew D. Oliver

Bent-beam electrothermal actuators-Part II: Linear and rotary microengines

For Part I see L. Que, J.S. Park and Y.B. Gianchandani, ibid., vol.10, pp.247-54 (2001). This paper reports on the use of bent-beam electrothermal actuators for the purpose of generating rotary and long-throw rectilinear displacements. The rotary displacements are achieved by orthogonally arranged pairs of cascaded actuators that are used to rotate a gear. Devices were fabricated using electroplated Ni, p/sup ++/ Si, and polysilicon as structural materials. Displacements of 20-30 /spl mu/m with loading forces >150 /spl mu/N at actuation voltages 200 /spl mu/N at displacements >100 /spl mu/m were measured.

Long throw and rotary output electro-thermal actuators based on bent-beam suspensions

This paper reports on several aspects of the performance and application of bent-beam microactuators. Orthogonal pairs of p/sup +/ Si cascaded actuators with 500-1000 /spl mu/m long, 6.5 /spl mu/m thick beams designed for rotary drives are shown to produce 20-30 /spl mu/m non-resonant displacement with >150 /spl mu/N loading at less than 8 V, 300 mW. Inchworm type devices using 500-1500 /spl mu/m long, 8-14 /spl mu/m thick bent-beams are shown to produce up to 104 /spl mu/m displacement against 204 /spl mu/N loading farce with 250-750 mW DC and pulse actuation. Integrated passive locks reduce standby power to zero. Measured frequency response of bent-beam actuators and refinements in modeling that include non-linear thermal expansion coefficients and buckling are also reported.

Pulse and DC operation lifetimes of bent-beam electrothermal actuators

This paper reports on lifetime studies of polysilicon and p/sup ++/ Si electrothermal actuators designed for rectilinear displacements. Measurements show that degradation patterns for displacement amplitude can be linked to design variables and operating conditions. At low power levels (which result in average operating temperatures of 300-400/spl deg/C), both types of devices provide continuous DC actuation for >1400 min. and pulse actuation for >30 million cycles without change in amplitude. A model similar to that used for fatigue in steel is used to fit pulse test data for p/sup ++/ Si actuators. The model parameters are explored as functions of operating conditions and device geometry.

Photothermal surface-micromachined actuators

We report on the optical actuation of surface micromachines. Optical energy is absorbed by the micromachines and converted to heat, which causes differential thermal expansion between different parts of the device and up to 12 /spl mu/m of displacement. The polysilicon actuators are homogenous and electrically isolated.

Lifetime studies of electrothermal bent-beam actuators in single-crystal silicon and polysilicon

Microsystems using electrothermal bent-beam microactuators have been demonstrated for a variety of applications including optical attenuators, RF switches, and micro positioners, thus creating a need for information on the longevity of these devices. This paper reports on the dc and pulse mode lifetime testing of this class of actuators constructed using polysilicon and p/sup ++/ doped single crystal silicon. The relative temperature profile along the top surface of an actuator is experimentally verified by scanning probe microscopy. Displacement measurements are used to explore links between aging behavior and the design variables and operating conditions. At low power levels (which result in average operating temperatures of 300-400/spl deg/C) both polysilicon and p/sup ++/ Si devices provide continuous dc operation for >1400 min, in air without change in amplitude. While some types of p/sup ++/ Si devices show monotonic loss of amplitude in pulse tests, others have been operated up to 30 million cycles without degradation. The displacement for polysilicon actuators can either increase or decrease depending on the geometry of the device and operating conditions, both of which are related to temperature and stress of the structural members. Polysilicon grain transformations are observed over extended operation at high temperatures. Performance changes are correlated to material properties using SEM and TEM images.

A high-performance surface-micromachined Pirani gauge in SUMMIT V/spl trade/

This paper presents a new type of surface micromachined polysilicon Pirani gauge that is designed to monitor pressure in micro packages. This gauge can measure absolute pressure from greater than 760Torr to less than 10mTorr. The gauge utilizes a double heat sink design to maximize dynamic range while maintaining adequate pressure sensitivity (/spl sim/1/spl times/10/sup 3/(K/W)/Torr). A special feature of this gauge is that it achieves high performance using a widely available foundry process. This sensor can be integrated with other devices on the same substrate and packaged to measure leak rates several orders of magnitude smaller than traditional He leak testing.

Performance enhancement of polysilicon electrothermal microactuators by localized self-annealing

Microsystems using electrothermal bent-beam microactuators have been demonstrated for a variety of applications including optical attenuators, RF switches, and micro positioners for scanning microscopy, creating an important need for information on the longevity of these devices. This paper reports on the lifetime pulse testing results of polysilicon actuators. Devices have been operated up to 60 million cycles without failure, but over tens of millions of cycles the displacement for a given actuator design can either increase or decrease depending on the geometry of the device and operating conditions, both of which are related to temperature and stress of the structural members. In certain cases actuator displacement increased by more than 50% (up to 100%) of the initial displacement, while for other cases it decreases by more than 25%. Polysilicon grain transformations are observed over extended operation at high temperatures. Performance changes are correlated to material properties using SEM and TEM images.

Diamond scribing and breaking of silicon for MEMS die separation

We describe a post-release die separation process for 1 0 0 silicon wafers with polysilicon surface micromachines using a combination of diamond scribing and mechanical breaking. The theoretical basis of scribing is reviewed and the theoretical relationship between scribe force and median crack depth was experimentally verified. Also the relationship between the scribe angle and the median crack depth as well as the stress to fracture is described. Scribe speed and scribe wear were also investigated. The results from our experiments were used to develop a process that had yields above 80% for two types of electrostatic MEMS actuators.

Wafer Level Micropackaging of MEMS Devices Using Thin Film Anodic Bonding

A wafer level packaging technique that involves anodic bonding of Pyrex wafers to released surface micromachined wafers is demonstrated. Besides providing a hermetic seal, this technique allows full wafer release, provides protection during die separation, and offers the possibility of integration with optoelectronic devices. Anodic bonding was performed under applied voltages up to 1000 V, and temperatures ranging from 280 to 400°C under vacuum (10 -4 Torr). The quality of the bonded interfaces was evaluated using shear strength testing and leak testing. The shear strength of Pyrex-to-polysilicon and aluminum bonds was ∼10-15 MPa. The functionality of surface micromachined polysilicon devices was tested before and after anodic bonding. 100% of thermal actuators, 94% of torsional ratcheting actuators, and 70% of microengines functioned after bonding. The 70% yield was calculated from a test sample of 25 devices.

Design of a Silicon Micromachined Artifact for Hybrid Dimensional Measurement

We are developing calibration artifacts for mesoscale metrology (especially vision probing) by using silicon bulk micromachining. We evaluate these artifacts on both high accuracy coordinate measuring machines (CMMs) and on typical production vision-based measurement systems. This will improve the accuracy of vision-based measurement equipment used in production. Successful realization of these mesoscale artifacts will enhance both production metrology capabilities and reduce manufacturing costs.Copyright