Abstract
A novel CMOS-process-compatible MEMSsensor formonitoring respiration is presented. This resistive flow sensor wasmanufactured by the TSMC 0.35 m CMOS/MEMS mixed-signal2P4M Polycide process. The sensor was demonstrated to be sensitiveenough to detect the respiratory flow rate, and the relationshipbetween flow rate and sensed voltage is quite linear. If one canintegrate the sensor with its sensing circuit into a single chip, thecost of a pneumotach system can be greatly reduced. Moreover, theproposed sensor is useful in both invasive and noninvasive applications.
Index Terms—CMOS/MEMS, flow sensor, respiratorymonitoring.
I. INTRODUCTION
RESPIRATORY activity is commonly monitored clinicallyto prevent sudden infant death syndrome, or torecord a patient’s physiological status for sleep studies andsports training [1], [2]. Various sensors and sensing methodshave been developed to measure respiratory rate and/or lungcapacity, including the monitoring or measurement of transthoracicimpedance, blood O CO concentration, and breathingairflow [1]–[4]. Among them, breathing airflow measurement isthe most direct method, since it measures airflow directly, ratherthan estimating it by measuring another parameter (O COconcentration or transthoracic impedance).Breathing airflow is typically detected by sensing pressure ortemperature, and the adopted sensors may be resistive, thermoelectric,pyroelectrical, or piezoelectric. In the present work, aresistive, CMOS-process-compatible flow sensor is proposed.Advances in CMOS manufacturing process techniques make itpossible to integrate the flow sensor, sensing and postprocessingcircuits into a single chip, greatly reducing the cost of a pneumotachsystem. According to the experimental data, the proposedsensor is sufficiently sensitive to detect respiratory rate.
II. SENSOR DESIGN
A. Principle of the Sensor
The flow velocity of exhaled breath is denoted . When ithits the surface of the sensor, the particles lose their momentum which is then converted to a force, , exerted on the sensorsurface(1)where is the total mass of the air particles that hit the sensorper unit time, . is given by(2)where is the density of the air particles, and is the surfacearea of the sensor. Combining (1) with (2), the exerted force isgiven by(3)The pressure applied on the sensor surface, , is(4)The sensor is deformed by the applied pressure. If the sensor isdesigned such that the deformation elongates its length and/ordecreases its cross-sectional area, then the change in the sensorresistance is proportional to the applied pressure, according tothe Ohm’s law [5](5a)(5b)where is the original sensor resistance; is its original length;is its original cross-sectional area; , , and are theirchanges due to the applied pressure, respectively. From (4) and(5b), the change in the sensor resistance is proportional to theapplied pressure and, thus, the velocity of the airflow. However,the air temperature, humidity and density changes in the inhalationand exhalation phases are not considered here.
B. Structure of the Sensor
The sensor, which is actually a miniaturized cantilever, wasmanufactured by the TSMC 0.35- m CMOS/MEMS 2P4Mpolycide mixed-signal process [6]. Fig. 1 shows the physicalstructure of the cantilever with its thickness exaggerated. Theresistance of the sensor was dominated by the ploy 2 layer, andthe underneath well was emptied. Each cantilever was 500 mlong, 30 wide, and 7 m thick. The ploy 2 was laid as a Ushape to increase the available cantilever resistance in a limitedarea.

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