An electrostatically-actuated integrated microflow controller.

Abstract

Many semiconductor processes such as molecular beam epitaxy, low-pressure chemical vapor deposition, and reactive ion etching require precise control of ultraclean gas flows at very low flow rates. This dissertation reports an electrostatically-actuated microvalve integrated with micromachined flow channels and a differential pressure sensor to form an integrated microflow controller. A new electrostatically-actuated microvalve was designed, fabricated, and tested. The microvalve consists of a free-standing double-end-clamped beam positioned beneath a gas inlet orifice. The estimated response time is $<$100$\mu$sec. This represents an improvement of at least one order of magnitude over existing devices. The valve is capable of delivering a 14.5 $\times$ 10$\sp{-6}$SCCM pulse of gas at 100Torr assuming a minimum operating pulse width of 10msec. With a system-limited modulation period of 1 second, this microvalve can control the flow rate with 7 bits of precision. A gas flow model of the integrated microflow controller was developed. The model, which is valid for the molecular, transition, and viscous flow regimes, was verified by data obtained from fabricated controllers. The maximum calculated flow rate for eight identical microvalves and flow channels in parallel, is 0.69SCCM. The gas flow model is applicable for designing and characterizing flow controllers utilizing other types of microvalves and flow channels. The effect of stiction on microstructures was studied. Stiction constraints set a lower bound on the actuation voltage for the controller of $\sim$40V. A stiction equation was developed relating the beam dimensions to the dimensions of standoffs placed on the contacting surfaces. Standoffs reduce the total contact surface area and thus act to reduce stiction. The stiction equation is applicable to other microstructures such as resonators and microactuators. A robust Ti/Pt-Ti/Pt lead transfer process was developed, Contact yield using the process escalated from 20% to 99%. The contacts can withstand multiple bulk micromachining steps and have been temperature cycled up to 420$\sp\circ$C.