Understanding Smart Sensors - Nomads.usp

36 Understanding Smart Sensorssidewall-etching mask. The SCREAM process can be used to fabricate complexcircular, triangular, and rectangular structures in single-crystal silicon.2.5.3 MicromillingMilling at the microlevel can be performed using tools manufactured usinganother micromachining process. A focused ion beam (FIB) micromachiningprocess has been used to precisely remove material from M42 cobalt high-speedsteel with a Rockwell C hardness of 65–70 [24]. Four-fluted, two-fluted, andtwo-fluted square micromilling tools were manufactured with FIB micromachining.The micromilling tools were subsequently used to cut narrow(4 mm), intricate trenches in polymethyl methacrylate (PMMA).2.5.4 Lasers in MicromachiningIn addition to chemical etching, lasers are used to perform critical trimmingand thin-film cutting in semiconductor and sensor processing. The flexibilityof laser programming systems allows their usage in marking, thin-film removal,milling, and hole drilling [25]. Lasers also provide noncontact residue-freemachining in semiconductor products, including sensors. The precise value ofthe thin-film resistors in interface circuits is accomplished by interactive lasertrimming. Interactive laser trimming at the die level for micromachined sensorshas been used to manufacture high-volume, interchangeable, calibrated, andcompensated pressure sensors since the mid-1980s [26].Lasers have been used to drill through silicon wafers as thick as 0.070 inch(1.78 mm) with hole diameters as small as 0.002 inch (50.8 mm) [25]. Forexample, 0.005-inch (127 mm) holes spaced on 0.010-inch (254 mm) centershave been drilled into a wafer 0.015 inch (0.381 mm) thick. The hole diametersand close spacing are achieved without causing fracturing or material degradation.Also, lasers can vaporize the material (ablation) using high-powerdensity.Lasers have also been investigated as a means of extending the bulkmicromachining process [27]. Figure 2.12 shows that either or wafers can be processed using a combination of photolithography, laser melting,and anisotropic etching. A deeper and wider etch occurs in the area thathas been damaged. The grooved shape or microchannel obtained by thisprocess has been used to precisely position fibers and spheric lenses in hybridmicrooptical devices without requiring additional bonding or capturingtechniques.