Ž . Sensors and Actuators 76 1999 133–138 www.elsevier.nlrlocatersna Novel C-MOS compatible monolithic silicon gas flow sensor with porous silicon thermal isolation G. Kaltsas, A.G. Nassiopoulou ) Institute of Microelectronics, NCSR Demokritos, P.O. Box 60228, 15310 Aghia ParaskeÕi Attikis, Athens, Greece Accepted 4 December 1998 Abstract A novel C-MOS compatible silicon gas flow sensor using porous silicon for thermal isolation has been designed and fabricated. The small sensor size combined with the very good thermal isolation by porous silicon assure fast response, with a time constant of the order of 1.5 ms. The principle of operation is based on heat transfer from a polysilicon resistor to the fluid and the detection of the flow-induced temperature difference by Alrpolysilicon thermopiles, integrated at both sides of the heater. The sensor has been evaluated in nitrogen Ž . flows from 0 to 0.4 mrs. The sensitivity per heating power is 6.0 mVr mrs W and the responsivity is 0.65 VrW. The noise equivalent power and the minimum detectable velocity are 1.5 =10 y8 WrHz 1r2 and 4.1 =10 y3 mrs, respectively. The chip size is 1.1 mm =1.5 mm. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Integrated gas flow sensor; Porous silicon; Thermal isolation 1. Introduction Flowmeters are of vital importance in many applica- tions. Industries supplying natural gas or water use flowmeters, from the accuracy of which depends the esti- mation of the amount of fluid they supply. Flowmeters play also a key role in automotive applications, in engine control and emissions, in fuel flow and consumption. In addition, environmental issues, biomedical instrumenta- tion, air conditioning and refinement of chemical products are a few more fields where flowmeters are necessary. Integrated silicon flow sensors are advantageous due to the possibility to be mass fabricated and combined with an integrated circuit on the same chip. Most of the sensors are based on the heat transfer from a heated part of the sensor to a moving fluid and they measure the induced tempera- ture gradient with various sensing elements such as diodes, transistors, thermistors, special metal resistances or ther- mocouples. However, they present some disadvantages, as is for example, their slow response, the need for a second temperature reference chip, the high power consumption ) Corresponding author. Tel.: q30-1-6533781; Fax: q30-1-6511723; E-mail: a.nassiopoulou@imel.demokritos.gr and the low signal level obtained. An alternative is to use bulk silicon micromachining in order to produce cavities, w x which offer the adequate thermal isolation 1,2 . This type of sensors is usually based on the use of microbridges, which make them fragile. In addition, they don’t allow much design freedom and they demand double side lithog- raphy and long etching times. They also use, in most of the cases, KOH for back silicon etching, which is not C-MOS compatible, or EDP which is toxic. This paper describes a new flow sensor based on a thick porous silicon layer for thermal isolation. We present the theory of operation, the design and the sensor fabrication process. We also describe the experimental setup and we give the results of the evaluation of the sensor. 2. Sensor design The flow sensor consists of two thermopiles, composed of two series of Alrp-type polysilicon thermocouples. These thermopiles are placed on both sides of a p-type polysilicon heater. The heater and the thermopiles are Ž placed on a thick porous silicon layer thickness of the . order of 40 mm . This layer provides excellent thermal w x isolation from the silicon wafer 3,4 due to the low 0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. Ž . PII: S0924-4247 98 00370-7