Kenneth Skrobis

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>>

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>>

Metal micromechanisms via deep X-ray lithography, electroplating and assembly

The availability of a fully functional LIGA-like processing sequence allows the construction of high aspect ratio metal structures. These structures can be used to produce assembled microelectromechanical (MEM) components. Testing of planar magnetic micromotors with gear boxes has been successfully completed. The results are encouraging enough to consider design of a MEM system: a dynamometer on a chip. Preliminary results prove that LIGA-like processing is compatible with silicon substrates which contain microelectronic components. Difficulties in magnetic winding construction have been overcome. Material issues which are primarily related to the magnetic behavior of electroplated films must be resolved to complete the study.

Micromechanics for actuators via deep x-ray lithography

Micromechanics and, in particular, micromechanics for actuators requires a processing tool that can provide 3-dimensionality and can accommodate many materials in a cost effective manner. The tool is further enhanced if high geometric resolution and compatibility with microelectronics are provided for. The desired attributes for the processing tool are in part contained in the original German LIGA process which introduced deep x-ray lithography and metal plating to micromechanics with achieved structural heights above 500 micrometer. This process can and has been extended to heights above 1 cm with improved resolution by developing a low stress photoresist application technology which is based on precut photoresist sheets and solvent bonding. Exposure of these thick layers is achieved by using 20 keV photon fluxes from the Brookhaven National Laboratory 2.6 GeV synchrotron. Application of this technology to actuator construction has taken two forms: linear, spring constrained electrostatic and magnetic actuators with large throws, and magnetic rotational machines for either high output torques or very high rotational speeds.

Preliminary results for a planar microdynamometer

Initial experiments with externally excited LIGA fabricated magnetic micromotors have shown that excitations required to overcome nickel to nickel sliding friction are reasonably low. These encouraging results have prompted an effort to integrate current excitation into the micromotor as well as to provide a test to quantitatively measure performance and frictional behavior of a magnetic micromotor. Such measurements may he made by use of a dynamometer which consists of a motor, a coupling gear train, a generator to act as an active load, and associated control electronics. Integration of such a micromechanical system requires extensions of the basic LIGA process. Problems with obtaining an electrodeposited material with reasonable magnetic properties must be resolved in order to complete the implementation.<<ETX>>

Micromechanics for actuators

Fabrication processes for microactuator construction must provide the capability to achieve 3- D geometries with a large material base while being able to benefit from the low cost economy of scale of batch processing. These attributes are provided in part by the basic LIGA process originating in Germany which is able to form high aspect ratio metal components up to 500 micrometers in thickness with very low vertical run-out and is compatible with microelectronic processing. The process has been extended to allow geometries with thicknesses up to 1 cm via a low-strain performed photoresist sheet and solvent bonding with x ray exposure via the 2.5 GeV National Synchrotron Light Source (NSLS). Such results enable batch fabrication of parts suitable for larger precision engineered actuators and mechanisms. To demonstrate the extended process capabilities a magnetic micromotor has been constructed using electroplated permalloy and assembled LIGA defined components. The low-inertia of the small rotor sizes is demonstrated by a stepping micromotor with a 150 micrometers diameter rotor which achieved maximum rotational speeds over 150,000 rpm.

Fabrication and testing of metal micromechanisms with rotational and translational motion

Microactuators ideally produce large output forces per unit chip area. This requires processing procedures which lend themselves to structures with large structural heights. Processing which also produces large edge acuities is required for low friction, low wear sliding bearing surfaces. Both attributes are accommodated in a processing sequence which uses thick photoresist technology, X-ray exposure, and metal plating together with a surface micromachined sacrificial layer. The end results are thick precision metal structures which can be assembled to achieve submicron tolerances in sliding bearing surfaces. The process has been used to fabricate rotational planar magnetic micromotors with low friction. Linear reluctance drives with spring returns have also been achieved and are in the testing phase.

Deep x-ray lithography for micromechanics

Micromechanical processing tools fall into three basic categories: bulk micromachining of single crystal materials, surface micromachining with deposited films and lateral sacrificial etching, and high aspect ratio processing of polymers and metals. The third category includes LIGA and LIGA-like processing which shares with high aspect ratio processing the need for thick polymer layers with good chemical behavior, acceptable mechanical properties and minimal built-in strain. A decal technique via solvent bonding of thick, essentially strain free polymethyl methacrylate sheets has been used as an alternative to casting and in situ polymerization to avoid the strain difficulty. Final photoresist thicknesses between 100 to 500 micrometers are achieved by precision mechanical milling, prior to x-ray exposure. The photoresist process has been used to produce structures with structural heights to 500 micrometers and run-outs of less than 0.1 micrometers per 100 micrometers of structural height in a LIGA-like processing sequence which combines the LIGA process concept with surface micromachining.