Michael L. Reed

Laminated high-aspect-ratio microstructures in a conventional CMOS process

Abstract Electrostatically actuated microstructures with high-aspect-ratio laminated-beam suspensions have been fabricated using a 0.8 μm three-metal CMOS process followed by a sequence of three maskless dry-etching steps. Laminated structures are etched of the CMOS silicon oxide, silicon nitride, and aluminum layers. The key to the process is the use of the CMOS metallization as an etch-resistant mask to define the microstructures. A minimum beam width of 1.2 μm, gap of 1.2 μm, and maximum beam thickness of 4.8 μm are obtained. These structural features will scale in size as the CMOS technology improves. The laminated material has an effective Youngs modulus of 61 GPa, an effective residual stress of 69 MPa, and a residual strain gradient of 2 × 10 −4 μm −1 . Multi-conductor electrostatic micromechanisms, such as self-actuating springs, x−y microstages, and nested comb-drive lateral resonators, are successfully produced. A self-actuating spring is a lateral electrostatic microactuator without a stator that is insensitive to out-of-plane curl. A spring 107 μm wide by 109 μm long excited by an 11 V a.c. signal has a measured resonance amplitude of 1 μm at 14.9 kHz. Finite-element simulation using the extracted value of Youngs modulus predicts the resonance frequencies of the springs to within 7% of the measured values.

Mating and piercing micromechanical structures for surface bonding applications

The authors have developed two types of micromechanical structures, using silicon micromachining techniques, which act as mechanical adhesives. Arrays of structures are fabricated on standard silicon wafers, with an areal density of approximately 200000 per cm/sup 2/, resulting in very strong bonds. Individual components are 4-18 mu m wide, and 4-15 mu m high above the substrate. Mating structures, which interlock with themselves, and piercing structures, which interlock with biological tissues, have been fabricated and tested. The mechanical behavior of this micromechanical velcro is in approximate agreement with the calculated strength.<<ETX>>

Morphology of etch hillock defects created during anisotropic etching of silicon

Etch hillocks are defects created during anisotropic etching of (100) silicon. From direct measurements of defect micrographs, we have determined the structure of etch hillocks to be pyramids bounded by (567) planes. The defects planes are different from those which are observed to emerge as anisotropic etch facets, such as (111), (211), (212), (133) and (411). We also describe the process by which defects are annihilated in high-concentration etchant. The relative stability of various defect edges is in agreement with calculations of the degree of backbonding of Si atoms along hillock edges. Our results suggest that the mechanisms responsible for hillock formation are distinct from those causing faceting during undercutting of convex corner masks.

Mechanisms of etch hillock formation

We have studied the formation of etch hillock defects during anisotropic etching of (100) silicon in KOH. Defect density is correlated with low etchant concentration and high etch temperature. Cathodic etch experiments indicate that hillocks form under conditions of decreased OH/sup -/ ion concentration. The activation energy for defect formation is 1.2 eV, considerably higher than the energy associated with silicon removal. We propose a mechanism to explain hillock formation that involves nucleation by silicon redeposited from the etch solution. The incidence of hillocks in this model is the result of a competition between the forward and reverse etch reactions. Examination of defects by electron microscopy suggests that growth occurs preferentially on slow-etching planes, in agreement with the model predictions.

Control of liquid bridging induced stiction of micromechanical structures

Stiction, the unwanted adhesion of micromachined components to the underlying substrate, is a vexing problem encountered during the fabrication of microelectromechanical systems. Liquid bridging of rinse liquids after release etch processing introduces significant surface tension forces which can initiate stiction. We show that liquid bridging can be substantially eliminated by appropriate design of the microstructure periphery. Antistiction tabs located at the center of doubly clamped beams, or at the ends of cantilevered beams, are shown to improve release yield significantly in both surface and bulk micromachining processes.

RF-magnetron sputtering of piezoelectric lead-zirconate-titanate actuator films using composite targets

Piezoelectric lead-zirconate-titanate (PZT) thin films of the morphotropic composition, suitable for microelectromechanical applications, are fabricated by means of RF-magnetron sputtering using a composite target, followed by an in situ annealing treatment. Increasing the thickness prevents the film from severe crack formation during the annealing process. Remnant polarization, coercive field, and relative dielectric constant of the films are 9.7 /spl mu//cm/sup 2/ 170 kV/cm and 930, respectively. The piezoelectric constant d/sub 31/ is estimated to be -49 pC/N. A practical microvalve application for volumetric gas flow measurement using the piezoelectric actuators is briefly described.

Low strain sputtered polysilicon for micromechanical structures

DC-magnetron sputtering silicon along with a post deposition annealing treatment is described as an alternative to LPCVD polysilicon which is commonly used as a structural material for surface micromachined devices. The sputtered silicon shows a small residual stress (compressive strain: less than 5.6/spl times/10/sup -5/) and a very smooth surface (average roughness: 25 /spl Aring/) which is inherently difficult to achieve with the LPCVD method. No need for pyrophoric silane and higher deposition rates are additional advantages over the conventional method.

New technologies and applications in robotics

Material transfer robots first appeared in the mid-1960s for use in traditional industrial applications. By the 1980s, robots found use in more demanding industrial applications such as welding, assembly, and inspection, with the help of vision and other sensors. It is estimated that 46,000 industrial robots have been installed in the U.S. japan has six to eight times as many robots

Fabrication and testing of a microdynamic rotor for blood flow measurements

Flow studies have been conducted, in vitro, on prototype microfabricated blood flow sensors. The envisioned measurement principle is the detection of the rotation of a micromachined polysilicon rotor of diameter 300 mu m. Repeatable rotation rates as a function of media velocity were obtained for both nitrogen and water. Fluid flow asymmetries, necessary to produce a torque on the rotor, were created by integrating a 2 mu m thick polysilicon cap over half the rotor. Blade deflection, due to intrinsic stress in the polysilicon films, was eliminated by thermal annealing. This allows the rotor, 2 mu m thick, to rotate in the 7 mu m gap between the substrate and the polysilicon cap. Operation of the device in heparinized dog blood resulted in considerable numbers of erythrocytes sticking to the polysilicon elements.

Design and modeling of a micromechanical surface bonding system

Optimization of the design of a micromechanical surface fastening system is discussed based on a simple cantilevered beam model. Theoretical estimates indicate that the bonding strength of these microstructures can be as high as 11-17 MPa, or 1500-2000 psi. The equivalent surface energy corresponding to the stored strain energy during separation of two interlocked sample pairs is 14.6 mu J/cm/sup 2/. The authors also report preliminary experimental results; a bonding strength of 1.1 MPa or 160 psi per unit interlocked area has been achieved, which is in agreement with the theoretical approximation.<<ETX>>