Understanding Smart Sensors - Nomads.usp

40 Understanding Smart Sensorsoptimized CMOS process. The aluminum interconnects in the CMOS processwere replaced with tungsten, and a titanium-nickel/titanium-silicon (TiN/TiSi 2 ) diffusion layer at metal-silicon contacts was added to increase the temperatureceiling for the process. Oxide and nitride films were deposited to isolatethe CMOS from the polysilicon.Processing temperatures are a major concern for monolithic integratedsensors. With bulk micromachining, the same processing temperature constraintsapply to both the sensor and the control electronics. With surfacemicromachining techniques, the structures are dielectrically isolated, so thatleakage is not a problem at elevated temperatures. The maximum operatingtemperature is, therefore, limited by the control electronics.Approaches for integration at the component level can be totally monolithicor multichip modules with separate die for sensor and the smart interface.The Microelectronics Center of North Carolina (MCNC) has investigatedflip-chip technology (discussed in detail in Chapter 10) to add circuitry toprocessed micromachined devices with solder bumping [33]. That approachhas been studied for surface micromachining and a combined surface and bulkmicromachining process. Substrate materials, including silicon, Pyrex ® , quartz,and gallium arsenide (GaAs), have been explored.Die size and process complexity are basic drivers of cost for semiconductorprocesses and for sensors. The smaller the die size, the greater the number ofdie that fit on a particular wafer. The simpler the process, the lower the cost forprocessing and the higher the yield. Combined processes must achieve costreduction, performance advantage(s), or improved reliability to be competitivein the marketplace.2.6 Other Micromachined MaterialsThe most common IC materials [silicon (Si), silicon dioxide (SiO 2 ), aluminum(Al), and silicon nitride (Si 3 N 4 )] have played an important role in establishedmicromachining technology. However, improving the performance of a sensorfor a particular application, addressing a higher operating range, or sensing anew parameter can require other materials and sensing techniques. Table 2.1compared GaAs, silicon carbide (SiC), and diamond to silicon. Silicon beginsto exhibit plastic deformation at temperatures above 600°C. Higher operatingtemperatures are among the desirable properties that make those alternativematerials attractive for ICs and semiconductor sensors. Other semiconductormaterials, such as indium phosphide (InP), are being investigated for micromachinedsensors. Also, several metals, metal oxides, and polymer films are