Surface micromachined peristaltic pumps using lead zirconate titanate film

by 1973- Hong, Eunki

In recent years, miniaturization of mass spectrometer systems has been studied for portable chemical and biological sensors. In this study, the design, fabrication and characterization of MEMS pumps which could be integrated into a MEMS mass spectrometer was investigated. The MEMS pumps were designed as peristaltic pumps with three interconnected chambers. Sequential motion of the piezoelectric diaphragms on these chambers effect pumping. Diaphragm actuators driven with ring–shaped interdigitated transducer (IDT) electrodes were required to generate deflections of several micrometers. This design used SiO2 and PZT as the passive and active layers, respectively. Zirconia films were used as barrier layers to prevent the rapid diffusion of Pb in PZT into the SiO2 at crystallization temperatures. The residual stresses of SiO2, PZT, and ZrO2 were evaluated to be -147, 100-150 and 230-270 MPa after the final film deposition. Diaphragm actuators were fabricated by bulk micromachining. The PZT in the actuators showed good dielectric and ferroelectric properties. The dielectric constants were around 660 with dielectric losses of below 2 % at 10 kHz. The remanent polarizations and coercive fields were 20 µC/cm2 and 50 kV/cm. The diaphragm actuators behaved more like membranes than plates and had a residual stress of 86 MPa. Non-180? domain motion of the PZT layer in diaphragm actuators was limited due to the residual stress and clamping by the elastic passive silicon oxide. For 980 µm diameter diaphragm actuators with an IDT spacing of 10 µm, center deflections of around 4.3 µm, larger than the thickness of the structure, were obtained at a voltage of 120 V. The deflection profiles had a funnel shape due to the relative contributions of d31 and d33 piezoelectric iii coefficients. In addition, the deflection profiles from IDT-mode diaphragm actuators were modified using annular IDT electrodes with inactive center areas. For a given voltage applied to the electrodes, the deflection decreases with increasing percentage of inactive area. However, the deflection profile is much flatter for diaphragms with a higher percentage of inactive area, resulting in a larger stroke volume for the MEMS pumps. The MEMS pumps were fabricated by surface micromachining. The fabrication steps included formation of chamber and channel structures by reactive ion etching (RIE) and subsequent release of diaphragm structures using a XeF2 process. Ion-milling was used to form portholes for the pump structure. The diaphragm actuators in the structures generated enough deflection to touch the bottom of chambers 3–4 µm deep. Sequential motion of the diaphragm actuators in a three stage peristaltic pump was demonstrated. iv