Ž . Sensors and Actuators 83 2000 30–33 www.elsevier.nlrlocatersna Capacitive differential pressure sensor for harsh environments S.T. Moe ) , K. Schjølberg-Henriksen, D.T. Wang, E. Lund, J. Nysæther, L. Furuberg, M. Visser, T. Fallet, R.W. Bernstein SINTEF Electronics and Cybernetics, P.O. Box 124, Blindern, N-0314 Oslo, Norway Received 8 June 1999; received in revised form 1 November 1999; accepted 10 November 1999 Abstract A capacitive differential pressure sensor for the pressure range of 0–1 bar has been developed. The primary field of application is hydrodynamic flow measurements in hot petroleum wells. In these harsh environments the sensor has to survive high common mode pressure in the range of 1000 bar and temperatures up to 1808C. The pressure element is formed in a triple-stack of fusion-bonded silicon wafers. A bossed diaphragm etched in the upper wafer bends due to the differential pressure across it. The capacitance to the middle wafer is measured. A reference capacitor insensitive to the differential pressure enables compensation for capacitance shifts caused by ambient pressure and temperature changes. The lower silicon wafer is included to minimise the diaphragm stress from the package. An ASIC, certified for 2308C, which is developed at SINTEF, is used for signal read-out. The sensor is measured to have a low zero pressure signal drift, smaller than 2.5% of full-scale output, when the temperature is varied in the range 0–2008C and the ambient pressure in the range 0–1000 bar. The sensitivity of the sensor is sufficient for the application. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Differential pressure sensor; Capacitive sensor; Fusion bonding; Harsh environments; Oil well instrumentation 1. Introduction The capacitive differential pressure sensor is developed as part of an oil well instrumentation system. The system is created in collaboration with several SINTEF depart- ments and the Norwegian company Maritime Well Ser- vices. The aim is that an operator at the oil platform can monitor and control the fluid flow in complex petroleum wells that have several oil producing zones. Choke valves can be opened and closed in order to control the flow from each zone independently, based on measurements of pres- sures, flow rates, fluid composition and temperatures. Sev- eral different sensors have been developed for this pur- pose. The differential pressure sensor is used for flow-rate measurements based on Bernoulli’s principle. Accurate measurements in the hectopascal range are required. The size of the sensor is not crucial, it will be placed in a probe placed concentric in the oil pipeline together with other sensors. ) Corresponding author. Tel.: q 47-22-06-7300; fax: q 47-22-06-7350. Ž . E-mail address: stm@ecy.sintef.no S.T. Moe . A capacitive pressure sensor was decided to be most wx suitable for the application 1 . A pressure-sensing di- aphragm with a stiff centre is chosen for increased linear- wx ity and sensitivity of our sensor 2 . A reference capacitor is included in the sensor element to enable temperature and pressure compensation. The high temperature ASIC used for the oil well application converts the difference between the diaphragm capacitor and the reference capacitor into a frequency signal. Fusion bonding of the wafers is chosen wx because these bonds are strong and durable 3 . At the backside of the sensor elements, a covar tube is soldered onto the pressure inlet. The high ambient temperatures in the oil well cause thermal stress at the soldered interface. The lower wafer of the wafer stack is included to minimise the influence of this stress on the measurement capacitor Ž wx. see e.g., Ref. 4 . 2. Sensor element The cross section of the sensor element is illustrated in Fig. 1. A diaphragm with a stiff centre is KOH-etched in the upper wafer. The lithographic masks used for the 0924-4247r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. Ž . PII: S0924-4247 99 00374-X