RELIABILITY OF MEMS-BASED MASS-FLOW CONTROLLERS FOR SEMICONDUCTOR PROCESSING Elizabeth D. Lawrence and Albert K. Henning Redwood Microsystems, Inc. 959 Hamilton Avenue, Menlo Park, CA 94025 650-617-0732 (EDL), 650-617-0854 (AKH), 650-326-1899 (Fax), elawrence@redwoodmicro.com, henning@redwoodmicro.com ABSTRACT Microfabricated components are finding increasing application in semiconductor processing. In this work, we report the results of detailed reliability and MTTF studies on mass-flow controllers (MFCs) created from silicon pressure sensors, microfabricated orifices for use in the flow sensor, and microvalves. Attributes studied include accuracy, response time, inboard leak rate, and particle generation, all monitored versus number of cycles. From these measurements, MTTF is calculated to be greater than 3M cycles. Failure modes are also discussed in detail. [Keywords: MEMS; silicon microvalves; silicon pressure sensors; mass-flow control; MFC; reliability; MTTF. ] INTRODUCTION The use of MEMS-based devices is becoming increasingly common. Pressure sensors [1] and accelerometers [2] for automotive applications, and ink-jet printheads for printing applications [3], are arguably the highest-volume MEMS devices in production. Despite these applications, reports on reliability of such devices in the archival literature have been relatively few. In this work, we report detailed reliability and mean-time-to-fail (MTTF) studies of MEMS-based mass-flow controllers (MFCs). The primary intended use for these MFCs is high-performance and high-purity control and distribution of gases, used in the processing of semiconductor integrated circuits and similar microfabricated devices [4]. Eleven MFCs were characterized for reliability according to semiconductor equipment standards. Parameters considered important to semiconductor processing were measured to specifications at regular intervals during the reliability cycling. MEMS-BASED MFCS Figure 1 depicts schematically the MEMS-based MFCs studied in this work. The MFC is comprised of a flow sensor, a microvalve to throttle flow, and feedback electronics to maintain the flow relative to a user-commanded setpoint. The flow sensor is comprised of a silicon micromachined orifice, a silicon IC temperature sensor, and two silicon piezoresistive pressure sensors. Calibration information relating the pressure sensor outputs to the actual flow under a variety of setpoint, inlet pressure, and temperature conditions, is stored on- board the MFC. Full details of this device are reported elsewhere [4]. For this study, the maximum (full-scale) flow rate of the MFCs was 100 standard cubic centimeter per minute (sccm), nitrogen- equivalent for seven MFCs (serial numbers 1382 through 1388) and 200 sccm for four MFCs (serial numbers 1284, 1285, 1286, 1288). Full scale is referenced to the maximum flow of the MFC. Temperature Sensor Upstream Pressure Sensor Downstream Pressure Sensor Critical Flow Orifice Normally-Open Proportional Valve Flow Flow Metal “C” Seals or Chemraz™ Seals FIGURE 1A. SCHEMATIC CROSS-SECTION OF THE MEMS-BASED MFCS USED IN THIS STUDY. Pout Px P in CO NO FIGURE 1B. FUNCTIONAL SCHEMATIC OF THE MEMS-BASED MFCS USED IN THIS STUDY. ‘NO’ IS THE NORMALLY-OPEN SILICON MICROVALVE. ‘CO’ IS A SILICON-MICROFABRICATED ORIFICE, TYPICALLY OPERATING IN THE CRITICAL (SONIC) FLOW REGIME. ‘P X ’ IS THE UPSTREAM PRESSURE SENSOR, AND ‘P OUT ’ IS THE DOWNSTREAM PRESSURE SENSOR. A B FIGURE 1C. CUT-AWAY DRAWING OF A PACKAGED MFC AND PROGRESSIVE ASSEMBLY INTO FINAL PACKAGE (A IS THE CENTERLEG, B IS THE BASE, C IS THE SEAL PLATE). TEST PROTOCOLS AND SPECIFICATIONS Protocols for testing of MFCs are well-established, published, and maintained by SEMI and SEMATECH [5]. For this study, each MFC had its setpoint cycled according to the protocol specified by SEMI E67 and SEMASPEC 92071224. The setpoint of the MFC is the input signal (commanded) expressed as percentage of full scale flow or sccm. The protocol required cycling through setpoints 100- C