H. Guckel

A simple technique for the determination of mechanical strain in thin films with applications to polysilicon

Free standing, doubly supported micromechanical beams which are fabricated from films with built‐in compressive strain fields buckle at critical geometries. Experimental determination of the onset of buckling for known geometries leads to a direct measurement of the strain level in the films. This idea is supported by appropriate theory for experimental structures which form clamped, doubly supported beams with constant cross section and varying lengths. Application to low pressure chemical vapor deposition polysilicon leads to the conclusion that strain fields of 0.2% reduce to 0.05% during annealing.

Thermo-magnetic metal flexure actuators

Deep X-ray lithography and metal plating when coupled with a sacrificial layer, SLIGA, lends itself to the fabrication of very high aspect ratio metal structures which are mechanically stiff in the substrate direction and can be very flexible in the direction parallel to the substrate. These properties can be exploited by producing a family of new flexure actuators which can produce very significant motion via thermal expansion and magnetic forces. The magnitude of thermal effects and magnetic forces are dependent on actuator geometry. An understanding of each effect allows the design of an actuator which is dominated by one or both effects. The end result is devices intended for large motion actuators in microswitch and positioning applications. They are also useful for material constant measurements of electroplated metals.<<ETX>>

Fine-grained polysilicon films with built-in tensile strain

Novel processing conditions and strain diagnostic structures are used to demonstrate that polysilicon films with built-in tensile-strain can be achieved and that any physical size limitations due to compressive-buckling in polysilicon micromechanical structures can be eliminated. >

Characteristics of polysilicon resonant microbeams

Polysilicon resonant microbeams can be used as strain-sensitive elements to replace conventional silicon piezoresistors in precision sensor applications, such as pressure sensors and accelerometers. These elements are combined with conventional silicon diaphragms or flexures with a proof mass to convert pressure or acceleration directly into a frequency output. Vacuum-enclosed resonant microbeam elements 200 or 400 μm long, 45 μm wide and 1.8 μm thick have been fabricated using LPCVD mechanical-grade polysilicon at the University of Wisconsin. Q-values determined using gain/phase analysis are typically over 25 000. Lower Q-values are primarily the result of residual gas in the cavity. Closed-loop operation from −60 to 180°C using piezoresistive sensor and electrostatic drive has been achieved with automatic gain control (AGC) to prevent overdrive. The characteristic resonance frequencies of the beams have been measured, with 550 kHz, 1.2, 2.2 and 5.2 MHz being typical of the frequencies of the one-dimensional bending modes for the 200 μm length. These measurements of the multiple resonance frequencies of a single beam provide a means of testing mathematical models of the dynamic behavior as well as determining the residual beam stress. The one-dimensional (1D) differential equation of motion of a doubly clamped single-span beam with an axial load can be solved analytically for lateral natural frequencies and mode shapes. These 1D solutions have been verified by 3D finite-element methods. In addition, the finite-element models are used to identify both lateral and torsional modes. The closed-form solutions agree closely with the numerical results and the experimental data.

Surface micromachined pressure transducers

Abstract Typical IC processing is fundamentally two dimensional; sensors are three-dimensional structures. In surface micromachining, two-dimensional IC processing is extended to sensor structures by the addition of one or more sacrificial layers which are removed by lateral etching. The resulting sensor structures involve the substrate and one or more deposited films which form the intended micromechanical component. The concepts of this type of sensor manufacturing are readily demonstrated by considering absolute pressure transducers in some detail. Absolute pressure transducers involve a vacuum-sealed cavity and a deformation sensing technique. The cavity is formed from the substrate and a low-pressure chemical vapor deposited polycrystalline silicon film. The mechanical properties of this film must be controlled well enough to allow the device to be designed. This implies morphological control during processing. Optimized films which do exhibit controlled compressive or tensile strains exclude oxygen or nitrogen and are therefore not modified by extended hydrofluoric acid etches. Their mechanical behavior is monitored by micromechanical test structures which measure Euler buckling and thereby determine the value of the built-in strain. The cavity vacuum is established by reactive sealing. Long-term vacuum integrity is achieved by a low-stress silicon nitride barrier which also acts as a dielectric isolation barrier. Sensing is accomplished via deposited polysilicon resistors. These devices behave like metal resistors in terms of their temperature coefficient of resistance and noise figure. Their piezoresistive behavior is larger than that of typical metal film structures and smaller than that of single-crystal resistors. Pressure sensors with four diaphragms, two active and two inactive, have been constructed and optimized towards manufacturability. The measured performance is excellent and agrees with the predictions of the design algorithm.

Diagnostic microstructures for the measurement of intrinsic strain in thin films

Freestanding flexible microstructures fabricated from deposited thin films become mechanically unstable when internal stresses exceed critical values. A series of structures with varying geometries is used to determine the critical geometry at which buckling occurs. Observation with an optical microscope quickly reveals qualitative and quantitative information about the internal strain in the film. Strain values between +or-1.5% can be measured for a 2.0 mu m thick film using doubly-supported beams for compressive strain fields, and ring and beam structures for tensile strain fields. Parametric formulae are developed for diagnostic structure response with selected verification by finite element computations.

The application of fine-grained, tensile polysilicon to mechanicaly resonant transducers

Abstract Resonant force sensors are devices which convert axially applied forces to changes in resonant frequency. These structures are fundamentally wires or beams or more complicated structures which are in a vacuum envelope. They become interesting and useful if they can be miniaturized, can be fabricated from a single material in a cost effective manner and can be excited and read via simple techniques. The devices which are reported here satisfy most of the above criteria. The construction material involves a silicon substrate, tensile strain polysilicon films and strain-compensated silicon nitride deposits. Clamped-clamped beams of polysilicon, typically 400,μm long, 40,μm wide and 2μm thick are fabricated with an isoplanar process over an oxide filled tub. Low-pressure chemical-vapor-deposited (LPCVD) nitride is used as a second sacrificial layer which also serves to support a second polysilicon layer which is part of the vacuum envelope. Internal surface adhesion problems are avoided by freeze-sublimation procedures which remove surface tension-induced beam deflections. Passivation and sealing is accomplished via LPCVD nitride and reactive sealing. Excitation and sensing is accomplished via ion implanted resistors. Experimental results always produce quality factors, Q , above 35 000. Resonant frequencies to 750 kHz have been achieved. It is estimated that these devices can measure axially applied forces below 0.1 dyne with standard electronic interfaces.

A first functional current excited planar rotational magnetic micromotor

Complete integration and successful testing of a planar rotational magnetic micromotor have been demonstrated. The configuration is that of a three-phase variable reluctance stepping motor with 6 stator poles and 4 rotor poles. Stator and rotor heights of up to 300 mu m are available and rotor diameters of 285 mu m and 423 mu m were tested. Maximum rotational speeds which have been achieved with open loop excitation exceed 30000 rpm and show no change with operation in vacuum. After testing to more than 5*10/sup 7/ rotation cycles, deviations from initial operation are not observed. Vertical reluctance forces are used to levitate the rotor up to 50 mu m over the substrate. Position signals are available via integrated photodiodes which allow the rotor speed to be monitored and provide the groundwork for an active closed-loop system.<<ETX>>

Fabrication and testing of the planar magnetic micromotor

Planar micromotors have the potential for high-speed control applications. These systems require closed loop control and therefore involve not only the motor but also associated high-speed control circuitry. Electrostatic devices with submicron gaps and CMOS controllers offer one possible construction technique. Magnetic motors with bipolar microelectronics are an attractive alternative. Magnetic micromotors have been constructed by using deep X-ray lithography and metal plating with a modified LIGA process. The devices are designed as reluctance motors and consist of a stator that is rigidly attached to the substrate and a rotor that is fabricated as a fully released, free part. Both pieces are formed from nickel and are typically 100 mu m in thickness. The rotor is assembled onto the stator shaft with submicron tolerances. The structure is driven by an external rotating magnet or a fixed electromagnet. Rotational speeds of up to 8000 rpm have been obtained and maintained for several days.

Mechanical properties of fine grained polysilicon-the repeatability issue

Calculation and measurements of Youngs modulus, Poissons ratio, shear modulus, and internal strain for fine-grained polysilicon as a function of processing conditions are presented. Calculations are based on appropriate averaging of single-crystal silicon properties, taking into account the film morphology. Experimental data are taken from strain diagnostic and resonant beam structures. It is found that polysilicon films can be in tension and that the intrinsic quality factor is approaching 75000.<<ETX>>