1Design of Micro robotic Detector Inspiration from the fly’s eye Anshi Liang and Jie Zhou Dept. of Electrical Engineering and Computer Science University of California, Berkeley, CA 94720 ABSTRACT This paper describes the design and testing method of a micro robotic detector. The detector was implemented with capacitors. The chip employs the concept of an ultrasonic rangefinder. The fly can detect and locate objects in all directions easily. The micro robot needs to have this capability. In order to implement this, detectors was built to resemble a “soccer ball”. A robust fabrication process is required for the ultrasonic transducer. Since its resonant frequency will shift as function proportional to the membrane thickness. The fabrication process described in this paper also allows the integration of electronics into the capacitive-micromachined ultrasonic transducers [1]. The method of testing is also presented. INTRODUCTION Detection is an important issue in the design of micro robot in MEMS (micro-electro-mechanical systems) technology. Different from the detection issue in our daily life, the detection for micro robot requires a short-range, high resolution and low power consumption implementation. Inspired by the structure of fly’s eye and radar technology, we are going to implement the ultrasonic micro robot detector. Ultrasonic rangefinder is composed of a capacitor with one fixed plate and one moveable plate. A high frequency voltage is applied across the two plates of the capacitor. An ultrasonic wave is sent out from the transmitter to an object (if there is any). By calculating the time difference t, of transmitted and received signal, the distance D, of the object is located. Distance=Velocity * Time/2 (1) We choose capacitive ultrasonic transducers over piezoelectric transducers because the performance of piezoelectric transducers is limited by its strict geometric tolerances, array configurations and electrical characteristics. Micro capacitive detector has an advantage in size reduction and potential electronic integration. The successful design of capacitive acoustic transducers composed of a suspended silicon nitride membrane was reported within the past decade [8]. Instead of using a nitride membrane with a layer of aluminum as the top electrode, our design of a capacitive ultrasonic rangefinder consists of an aluminum layer, which acts as the top electrode that is suspended above a silicon bulk, which acts as the bottom electrode. The supporting structure is composed of Silicon nitride, which is a non-conducting layer. By closing the air gap between the two plates, we are able to change the output voltage. The parameters involved in the calculations are the thickness of the movable top plate, air-gap thickness and sidewall spacing. We apply a dc voltage between the two electrodes; the law of charge force predicts that the plates will attract each other, closing the air gap. However the two plates will also repel each other due to their residual stress. Ultrasound is generated by applying an ac voltage to the structure. In this paper, we report on the theory and calculations behind our design decisions and the fabrication process used to build the structure. We will also include the test structure so as to verify the performance of our design. 5 by 5 arrays of micro detectors are shown in Figure 0 as to2illustrate how they can be aligned after industrial fabrication. THEORY This Micro robot detector serves as an emitter and a receiver at the same time. On one hand, the detector needs to emit a relatively large signal, in order to transmit the signal as long as possible. Here we are going to do the analysis for both the emitter and the receiver. And we will do the verification after the fabrication. We do some approximations in the analysis. At first, we assume the restoring force is linear to the displacement. We only consider the forces between the poly-silicon layer and metal membrane and ignore others. Here we use a model consisting a spring (representing the restoring force), a mass (representing the weight of the metal membrane) and a capacitor [4]. Spring Mass Capacitor Figure 1. Simplified Model of micro-detector massspringcapacitor FFF=+ 2)0(2/2xdSVFcapacitor −=ε Here x is the displacement of the upper membrane, S is the surface area of the capacitor, and V is the voltage on the capacitor. cmF /10*854.814−=ε. V is time dependent, but in order to simplify the calculation, we use dcVtV=)( . Also, we ignore the acceleration of the upper membrane and get: kxxdSV =− )(2/02ε The breaking point is at3/0dx = , and then SkdVε27/830=. If we use 202500,2 mSmdµµ== here, and the thickness of Aluminum layer equals 2um. With Aluminum’s Young’s modulus 69Gpa. The largest allowed voltage that can be applied to the capacitor is about 125V. In our design, we set the thickness of the nitride layer to 0.5 um, but in the actual fabrication, it is hard to control. So this is the case without considering the capacitance caused by the nitride layer. The actually allowed voltage should be higher. For example, with nitride’s relative permittivity 7.5, we can consider the capacitor is air gap with d=5.75um, the maximum voltage will be 612V. Furthermore, since x is small compared to d0, we have 2022/ dSVFcapacitorε= And acdcacVVF 2∝ FABRICATION The capacitive-micromachined ultrasonic detector was fabricated by standard (semiconductor) CMOS processing techniques. To obtain good conductivity at the wafer surface, an n-type silicon wafer was doped with phosphorus gas phase drive in at 1000C. [7] And then a thin layer of LPCVD (Low-pressure chemical-vapor deposition) nitride is deposited at 800C as an etch stop in the potassium hydroxide sacrificial etching procedure. Thermal oxide is then deposited at 560C as the sacrificial layer. The sacrificial layer is later dry-etch patterned into rectangle shapes to define the active Figure 0. 5x5 array of micro-detectors3transducer regions, which will act as the gap of the capacitor with a dielectric material of air. Another layer of nitride is deposited by LPCVD at 800C to form a non-conducting layer surrounding the active transducer regions. Vias are dry etched to allow sacrificial etching in KOH at 75C. The final step is to sputter and web-etch pattern the aluminum to form the top electrode. The aluminum
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