An integrated mass flow sensor with on-chip CMOS interface circuitry.

Abstract

A generic merged process has been developed for the integration of multi-element micromachined diaphragm transducers and on-chip interface electronics on a single chip. This monolithic chip simultaneously measures flow velocity, direction, gas type, pressure, and temperature from the five diaphragm windows, allowing the determination of true mass flow in gas control systems. Generic thermal models have been devised to analyze the heat transfer mechanisms in planar diaphragm structures. These analytical models have been confirmed by two-dimensional finite-element (FEM) simulations and by experimental measurements. Diaphragm windows formed using 1.5$\mu m$-thick composite dielectrics achieve a thermal resistance of 7$\sp\circ C/mW$, a thermal time constant of 5msec, and a process yield exceeding 90%. A multi-sensor chip has been fabricated and characterized by integrating four distinct planar diaphragm transducers on dielectric windows separately etched on a single chip. Three thermally-based transducers are supported on dielectric windows defined by a 15$\mu m$-thick boron diffusion. The flow velocity sensor uses a thin gold/chromium (260nm/40nm) sensing film and achieves a sensitivity of 25mV/V at 3m/sec. The flow direction sensor consists of two pairs of orthogonal polysilicon-gold thermopile detectors and offers a resolution less than $\pm 5\sp\circ$. A sensor for determining the type of gas present is formed using a 10nm-thick platinum film whose conductivity changes due to gas adsorption on the surface. A polysilicon full bridge pressure sensor is formed in a separate well using a dielectric diaphragm backed by a thin layer of boron-doped silicon. This device has a sensitivity of 12ppm/mmHg and a range over 800mmHg. On-chip circuitry has been fabricated on the unetched bulk region of the chip using a standard 3$\mu m$ p-well CMOS process, modified for process compatibility with the dielectric window formation. It operates from $\pm 5V$ supply and includes offset-free amplifiers, heater controllers, an analog multiplexer, self-test circuitry, and a temperature sensor formed using parasitic bipolar transistors. The on-chip self-test circuitry monitors any changes in the diaphragm electrothermal characteristics in situ. The overall monolithic flow sensor achieves a total chip size of 3.5 $\times$ 5$mm\sp2$ in a 13 mask process. The electrothermal circuit applications of the planar diaphragm structure have also been explored. An rms-dc converter formed using his structure has achieved a conversion nonlinearity less than 1%, high sensitivity to low-level signals, and a bandwidth of over 20$MH\sb z$.