Abstract
A pressure sensor that combines two principles ofmeasurement into one integrated unit with optical and electronicparts is fabricated and tested. The sensing element for both integratedparts is an embossed silicon diaphragm that deflects underdifferential pressure. The optical part of the sensor is based onFabry–Perot interferometry; the electronic part of the sensor isbased on the piezoresistive effect in silicon. In the application ofFabry–Perot interferometry, the sensing element utilizes an opticalcavity, where interference of multiple reflections changes withmovement of the diaphragm caused by pressure. In the applicationof the piezoresistive effect, a change in the electrical resistivity of asensor material is induced by mechanical stress in the diaphragmand detected by a Wheatstone bridge circuit. The advantages ofintroducing the embossed diaphragm in sensor fabrication andits benefits for integration are discussed. The existence of a nearlyideal Fabry–Perot interferometer in the optical part of the sensoris demonstrated experimentally. Noise characteristics of theFabry–Perot part of the sensor are presented. The independentlyproduced electronic output serves to establish the quiescence point(Q-point) of the output from the optical part of the sensor.
Index Terms—Diaphragm pressure sensor, embossed diaphragm,integrated sensor, Fabry-Perot interferometry,piezoresistors.
I. INTRODUCTION
THE ability to use microelectromechanical systems(MEMS) fabrication methods in mass production of highperformance sensors at low cost has opened a wide range ofapplications for pressure sensors, which include automotive,aerospace, marine, instrumentation and industrial process control,hydraulic systems, microphones, bioscience and medicalapplications [1], [2]. Since the introduction of MEMS, piezoresistivepressure transducers have become the dominant types,owing to their high performance, stability and repeatability.Recent industrial trends indicate increased need for pressuresensors that are suitable for hazardous environments, hightemperatures and biomedical applications. Advanced applicationsinvolve requirements of small volume, high performancecharacteristics, environmental restrictions and materials compatibility.Of the various sensing mechanisms used for pressuremeasurements, optical techniques provide capabilities for smallsizes, immunity to harsh environments, remote operation andease of integration with other devices. Of particular importance is the immunity of optical sensors to electromagnetic wave interference,chemical attack, and high temperatures. The opticaltechnique of Fabry–Perot interferometry [3], [4] is selectedfor investigation in this work, owing to its high sensitivity andaccuracy.II. SENSOR DESIGN PRINCIPLESA schematic illustration of the integrated optical and electronicpressure sensor (IOEPS) studied in the work is shown inFig. 1. A pressure source is applied through a sealed port in thesensor head. The sensing elements comprise a diaphragm withan embossed structure formed in single-crystal silicon, an opticalfiber, and piezoresistors embedded in the diaphragm. Thediaphragm elastically deflects in response to differential pressureapplied across its two sides. The piezoresistors are sensitiveto the strains induced by diaphragm deflection. The endface of a single-mode glass optical fiber is positioned oppositeto the center of the diaphragm, where a small gap betweenthe two surfaces (glass and silicon) forms an optical interference(Fabry-Perot) cavity. This design structure thus providesthe means for combining two principles of measurement intoone integrated unit, using its optical (Fabry-Perot) and electronic(piezoresistive) capabilities for simultaneously sensingthe movement of the diaphragm under applied external pressure.Changes in the Fabry–Perot gap are detected with an opticalfiber-optic system interconnecting a laser source and a photodetectorwhich produces the optical output signal. Changes in theresistances of the piezoresistors are detected by a Wheatstonebridge circuit which yields the electronic output signal.


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