Understanding Smart Sensors - Nomads.usp

Mechatronics and Sensing Systems 265Sensors on the chip include a gas flow sensor, an infrared-sensing array, achemical-reaction sensor, cantilever beam accelerometers, SAW vapor sensors,a 64-element tactile sensor array, and an infrared CCD imager. The 8- by9-mm 2 chip also has MOS devices for signal conditioning, array accessing, andoutput buffering. Backside micromachining of the silicon is performed as thelast step in the fabrication process. The multielement sensor demonstrated lowprocessing cost per function.11.4.3 An Integrated Sensing SystemAn example of a fully integrated sensing system using multiple sensing elementsis a monolithic hearing aid. The hearing aid requires only two functional elementsto be a system: the sensor and the amplification. A micromachined piezoelectricmicrophone with on-chip CMOS signal conditioning has beendesigned and fabricated that has potential for hearing aid applications [14].The sensor consists of eight piezoelectric (zinc oxide) electrode pairs. Bulkmicromachining to define the microphone diaphragm is performed prior toCMOS processing. The design takes into account a 9-hour 1,150°C annealand stresses imposed by subsequent CMOS processing steps. After CMOS circuitryis completed, the 0.5-mm thick zinc oxide is magnetron sputtered ontothe diaphragm. The eight pairs of upper and lower electrodes are connected intwo series groups of four electrode pairs. The microphone has a measured sensitivityof 0.92 mV/Pa and a resonant frequency of 18.3 kHz. Preamplifier noiseof only 13 mV allows the microphone to achieve an A-weighted noise level of57-dB sound pressure level (SPL), which is roughly the level of conversationalspeech.11.5 Other System AspectsThe smart sensor system requires additional components to provide a selfcontainedunit. Current developments would allow batteries and displays to beintegrated with the sensor. Any item mentioned in Chapters 8 and 9 might alsobe part of this integration—if the packaging problems discussed in Chapter 10are solved. Those components, combined with smart sensors, digital logic froman MCU or a DSP, and smart-power ICs, will enable a number of newmechatronics approaches. One system aspect that must be addressed is the catastrophiceffect that electrostatic discharge (ESD), system voltage transients,and EMI can have on electronic components. Those factors are covered inSection 11.5.3.