Understanding Smart Sensors - Nomads.usp

238 Understanding Smart SensorsFigure 10.8 Indoor radar sensor. (Courtesy of Lawrence Livermore National Laboratory.)passivation nitride is used to mask areas other than the aluminum-copper metalcontact on the silicon die. Sputtered titanium-tungsten (TiW) and copper areused to form a base metal for copper, lead, and tin, which are plated onto thebase. Reflow in a gradually ramped oven causes a thermally induced amalgamof tin and lead to form into a ball due to surface tension. Spacing of the bumpsevenly around the die avoids stress during thermal expansion.Figure 10.9 also shows how flip-chip technology can be applied to a sensor.The sensor is represented by a diaphragm structure but could be anymicromachined structure. In one case, the sensor could be a flip-chip on a morecomplex IC such as an MCU. An alternative approach is to have the IC as aflip-chip on the micromachined structure. The requirements of the sensor mustbe considered when this type of packaging is developed.10.4.4 Wafer-Level PackagingChapter 2 discussed surface-micromachined structures sealed inside a two-layerbulk micromachined sensor package. That wafer-level “packaging” ofmicromachined sensors results in considerable cost savings over packagingmethods that use metal can or ceramic packages. The process has demonstrated