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Abstract
Today we can see a fast growing interest in silicon micromachined sensors and actuators both in the academic research world and in commercial industrial products. One reason for this evolution is the sophisticated processing techniques and production methods associated with silicon technology. Silicon micromachining offers the possibility of low-cost highly miniaturized sensors. However, the planar nature of the photolithography technique used in micromechanics makes it very difficult to realize three-dimensional (3-D) sensors. Batch and mass fabrication of true 3-D silicon structures is a key step in future micromachining technology that can enrich the world with new 3-D sensors and actuators. Such devices have broad applications from medical, military and household equipment to pure research tools.
Measurement and control of turbulent flow fields are key issues in many technical application areas, e.g. in the optimization of wing sections of aircraft and the minimization of noise generation of vehicles. The smallest scales of the fluctuating velocities in turbulent flow are of the order of 100 µm which means that the sensors have to be roughly of the same small size. Miniaturized micromachined 3-D sensors can help researchers to study physical phenomena that are not yet understood because of lack of sufficiently small sensors. The conventional triple hot-wire sensors used today are expensive and relatively large.
The motivation for this work is to fabricated robust true 3-D structures with detailed sensor features in all three directions. The final goal is to fabricate a small, fast and accurate triple hot-wire sensor for simultaneous measurements of three perpendicular velocity components in turbulent gas flows.
This thesis presents the first functional 3-D flow sensor in silicon. The dimensions of the three orthogonal hot-wires (500 x 5 x 2 µm) are small enough to measure the velocity of the small eddies in a turbulent gas flow. The smallness is possible to achieve by the development of a new robust polyimide micro-joint technique. This polyimide joint based on thermal shrinkage of polyimide in V-grooves has been carefully investigated and described in the thesis.
Extensive experiment on fabricated test structures have been made to characterize the new micro-joint. The most important features of the polyimide joint are the following:
- Both a static and a dynamic mode can be used. Static bending angles between 0° and 200° and dynamic angles up to 5° per V-groove with a cut-off frequency in the range of 1-10 Hz have been measured.
- The 3-D structures are very robust even for a bending radius as small as 60 µm for a 30 µm thick out-of-plane rotated silicon structure. Electrical interconnection wires to the assembled structure are not affected by the out-of-plane rotation.
- The fabrication technique is compatible with batch production because of the self-assembling nature of the polyimide joint and compatible with front bulk- and surface- micromachining using sacrificial etching which is demonstrated in the fabrication of a 3-D flow sensor.
- With an integrated heater in the joint large dynamic motions can be achieved, using the large thermal expansion of polyimide. As an example of actuator applications different micro-conveyers useful for moving millimeter sized objects have been fabricated and tested.
Keywords: 3-D microstructures, silicon, 3-D sensors, actuators, polyimide joints, curing, thermal shrinkage, controlled bending, out-of-plane rotation, KOH-etching, surface- and bulk micromachining, flow sensor, hot-wire, anemometer, small-scale, turbulence, micro-conveyer.
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