An integrated vapor sensor is described which is fabricated ona silicon substrateusing conventional NMOS processing steps. It consists of a poly-El mcro- bridge, underlying electrodes for electrostatic excitation and capacitive pickup of the vibration, and an integrated NMOS detection circuit. Organic vapors are selectively sorbed by a polymer film patterned on the microbridge. The increased mass-loading perturbs the first resonant frequency f1, which causes a phase shift between the excitation and output voltages. If this device is used as the feedback element in an oscillator, the frequency of oscillation will shift in response to changing vapor concentration.
The microbridge is made from a polycrystalline silicon (poly-Si) thin film by etching an underlying oxide layer, a technique which eliminates the need for anisotropic etching of the substrate. The sensor process begins with poly-Si gate NMOS transistor fabrication. After phosphosilicate glass (PSG) deposition, a second poly-Si layer is deposited for the microbridge. Intrinsic stress in the poly-Si film is relaxed by high-temperature annealing. A double layer of hardened photoresist is applied after metallization to protect the NMOS transistors from etchant attack. Finally, a 150 nm-thick layer of negative photoresist is patterned on the poly-Si, and the PSG layer is etched in buffered EF to form the microbridge.
The prototype microbridges are 1.35 um-thick and are offset 2.0 um from the substrate, with lengths ranging from 120 um to 100 um. From frequency response measurements, the Young's modulus for poly-Si is found to be E = 4*10^10 Nm^-2. Higher quality factors are observed for apertured microbridges, due to reduced air damping. A first-order theory for the ire;,cn:: response of a driven microbridge is verified experimentally. The sensor response to xylene vapor is studied, since it is highly sorbed by negative photoresist. Saturated xylene vapor causes a phase shift of -5 for an apertured poly-Si microbridge which is driven at its nominal resonant frequency (244 kHz). In terms of frequency shift deltaf1, the sensor response is -0.3 Hz ppm, comparing favorably with that of surface-acoustic-wave vapor sensors.
May 31, 1984
Howe, R. T. (1984). Integrated Silicon Electromechanical Vapor Sensor. United States: University of California, Berkeley.