Microsensors provide advantages over conventional sensors, such as increased sensitivity, smaller size and lower cost. The field of MicroElectroMechanical Systems (MEMS), however, is still developing, and microengineers are always looking for better materials and fabrication processes.
A new microaccelerometer developed at the University of Cambridge, in collaboration with Panasonic and the University of Tokyo, is based on the tunnel current effect, where a current applied across a small enough gap jumps, or tunnels, across the gap without a direct connection. Tunnel current was chosen over the more common capacitance or piezoelectric effects, because it has the potential to be more sensitive and operate in a broader bandwidth.
The accelerometer is made of a silicon-on-insulator (SOI) wafer, a multi-layer wafer composed of layers of silicon, silicon dioxide and silicon that comes bonded and polished to size. SOI wafers are becoming important in high-speed microelectronics, but are not yet common for MEMS. For MEMS to be truly electromechanical systems, compatibility between the electronics and mechanical fabrication processes is a must.
The accelerometer incorporates a novel fabrication method. In addition to a conventional isotropic etch, a focused ion beam (FIB) is used to form the primary distinguishing feature of this device – a narrow, precisely sized gap at an angle to the plane of the wafer that effectively adds a third dimension. Because FIB is regularly used for system failure analysis and prototyping of microelectronics systems, the equipment is commonly available in microelectronics fabrication facilities. Focused ion beam machining is extremely cost effective, but for cost reasons (FIB is a serial process, and relatively slow, increasing the time and cost required for manufacturing), its use for microdevice fabrication has been limited until this point. The new accelerometer was designed with both SOI and FIB in mind, and the disadvantages of each were avoided. Because the gap is small, the cutting time is relatively short and only adds £4 to the total fabrication cost.
Figure 1 shows a cross section through the device. The application of a high voltage (approx. 10V) from A to C, pulls the proof mass (A) down and closes the gap from 200nm to around 10A, close enough for tunnelling to occur. This is the neutral position. A small voltage (approx. 200mV) is then applied from A to C, causing a tunnel current across the gap. When the device is accelerated up or down, the proof mass moves, changing the gap size and thus the tunnel current. This change in tunnel current is detected and fed back as a change in voltage from A to B, returning the proof mass to its neutral position. The acceleration can be determined from the voltage required to return the proof mass.
SOI wafers have been shown to be a suitable material for MEMS, and using an SOI wafer offers the advantage of easily produced suspended structures using only a simple underetch and one lithographic mask. Suspended structures, and particularly the angled cut, allow motion to be achieved that otherwise would require several independently patterned wafers bonded together. Additionally, FIB has been successfully used in a novel way to produce otherwise difficult and complicated features. The combination of SOI and FIB has resulted in a simple and less expensive way to fabricate high-sensitivity accelerometers.
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