Processing Lab
UNC-Charlotte
Intgrated Force Arrays(IFAs) are flexible metalized membranes which may be patterned using the techniques of VLSI electronics and which undergo substantial deformation when voltage is applied. They may be configured as macroscopic actuators or used in highly articulated systems of great complexity. The theory of operation, the methods of construction, and the observational results for IFA tests structures will be presented.
The IFA produces motion on a practical scale by adding the response of many microscopic elements acting under an electrostatic force. The IFA resembles a thin , flexible membrane. When voltage is applied , the membrane contracts by about 30% in one dimension, producing large macroscopic motion with high efficiency. In a sense, the IFA are a form of artificial muscle tissue controlled by an electrical voltage, where the microscopic structures of the tissue are fabricated using methods derived from VLSI electronics.
The IFA will have immediate application in engineering solutions to low-weight, high-efficiency actuators problems and will revolutionize the way many machines are built in the future. Integrated force array devices offer advantages such as greatly reduced power consumption, the absence of sliding friction, operation under a wider range of external conditions, precise positioning capabilities, and up to three orders of magnitude reduction in weight when used as direct substitutes for existing mechanisims such as solenoids. The IFA structures need not be configured for simple one-dimensional motion,but may be arranged in highly articulated arrays which will find applications in a wide range of areas from integrated optical and flow control systems to biomedical actuators and prosthetics.
As development proceeds, the economic and technical impact of the force array technology could equal or exceed that of VLSI microelectronics. In fact, in a substantive and quite natural way, the force array technology completes VLSI development by providing mechanical systems that are capable of using the enormously detailed control possibilities inherent in modern electronics to form systems of such complexity that the only current comparisons are biological rather than items of manufacture.
Force array technology is based on electrically deformable flexible membranes. The membranes are composed of microscopic force elements arranged in arrays containing 10e5 elements for a relatively modest device to more than 10e10 elements for advanced systems. When powered, the arrays may produce both substantial forces and displacements.For example, a 1.5e6 element that is 1 cm on a side and 2um thick will deform by 0.3cm (30%) and produce an average force equivalent to 0.4g. Once suitable large scale manufacturing methods have been developed, larger systems could produce pounds of force over inches of displacement.
The IFA array structure is constructed of polyimide and is robust enough to stand alone as an unsupported membrane, yet sufficently flexible to allow the deformation. The plates are thin chromium (800 Angstroms)
that has been directionally evaporated upon the polyimide and which conform to the polyimide during the deformation without cracking. The choice of chromium is motivated by its excellent adhesion properties.
Substantial effort is currently being focussed on micromachined solicon structures. Miniature electrostatic motors and comb structures have been constructed. Such devices have definite advantages. By comparison to the polyimide IFAs, the structural rigidity of silicon would lead to more inherent precision and the high spring constant of the material would allow higher frequency operation. Silicon micromachines would certainly equal the IFAs in their integratability with electronic drivers. On the other hand hand, polyimide versions of the IFAs are more compliant and less fragile than comparable silicon structures and, because of this, they may be configured in large and ultra scale to form compound free-standing or conformoble devices which may be more readily suited to macroscopic motion.
The cells are basically deformable capacitors in which the force between the plates is proportional to the plate area divided by the square of the separation, and thus it is independent of the scale of dimension. This independance of scale is the reason for making the cells small. In an array, the greater the number of cells, the greater the net force and range of compression.
A second major advantage for making the cells small is reduced operating voltage. The test structures begin to compress at 2V and have been taken as high as 100V with no electrical breakdown of the polyimide structure.
The IFA consume electrical power only when they are moving. As the plates close, the capacitance is increased and, in order to maintain constant voltage, the power source must supply current. When the structure is fully compressed, the current flow and the power consumption are zero. Once the voltage has produced some compression, the plate separation is reduced and the attractive force is increased. This causes more compression and still greater force until the plates are fully closed. On the other hand, if the structure is more resilient, the inherent spring force acts against the capacitive force. This results in a stable equilibrium position that is continuously resolvable as a function of the applied voltage. The stiffer structure corresponds to a device whose state of compression is controllable.
In order to realize a number of advantages, two modifications to the basic IFA design have been included in the test structure. Both modifications involve arranging the array elements in subarrays which are surrounded by a more substantial supporting structure. These modifications lead to overall improvement in robustness, improved load distribution, and amelioration of irregularities in cell performance. These arrays are called modular
and fractal
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The first step in testing IFAs is to remove them from the supported silicon. A lift off procedure in an HF solution (49 wt % HF) is used to float them free.
Electrical testing is done using a conventional IC probe station. Of the structures tested, 60% show functionality.
The IFAs which have been successfully tested exhibit a large amplitude motion. Some compression may be seen with the microscope at voltages as low as 2.0 volts, while compression of the entire structure which can be seen with the unaided eye typically occurs at 20.0 V.
We are currently constructing a testing apparatus to measure the forces associated with this motion.
SEM Photo of IFA plates
At this point the IFA tecnology is in an early stage of development. Even so, the large scale of motion already evidenced by the test structures indicates that the overall concept is viable. Once the post-VLSI processing has been systematized and the various designs more fully evaluated, it will be possible to progress to the fabrication of larger structures. In the course of the next year we anticipate scaling-up by a factor of ten and constructing 1cmx1cm devices, containing approximately 3x10E6 cells and a number of applications have already been identified.
The future applications of IFAs include:
- Planar shutter array for neural fluxes.
- Reconfigurable board-level crossbar switch array for making up to 10,000 high speed electrical interconnects.
- Wrap-around endoscope.
The first large scale volumetric structure to be constructed will contain approximately 2X10E8 elements and serve as a solenoid replacement. It will be made of 22 planar arrays ech about 1cm X 3cm. Buffer polyimide sheets are used to insulate and protect the arrays from each other. The IFA based devices will be two orders of magnitude smaller and three orders of magnitude lighter then the solenoid it replaces. In addition, The IFA device would be capable of proportional control and use no power at the control points of the motion.
