Pressure and Flow Sensors for Biological Measurements Dr. Lynn Fuller

Transcription

1 ROCHESTER INSTITUTE OF TECHNOLOGY MICROELECTRONIC ENGINEERING Pressure and Flow Sensors for Biological Measurements Dr. Lynn Fuller Webpage: 82 Lomb Memorial Drive Rochester, NY Tel (585) Fax (585) Department webpage: pressure_flow.ppt Page 1

2 OUTLINE Introduction Current Technology Pressure Flow MEMS Technology Pressure Flow Page 2

3 INTRODUCTION Many researchers use mice or other models for investigations of various medical procedures. Several researcher use the chicken embryo (chicken egg) to study the development of the heart. The hope is to understand this development and apply it to human pediatric cardiology. These researchers need to measure blood pressure and flow during the development of the heart. The pressures are low (less than 5 mm Hg) and the flows small (less than 5 µl/min?) and the physical size of the heart and blood vessels are small (heart is few 100 s of µm and vessels ~10 s of µm. The mouse is used for mature heart investigations. Mouse embryos can be studied but is complicated by the fact that it must remain attached to the mother. Page 3

4 PICTURES OF CHICK EMBRYO Drawing Page 4

5 SERVO-NULLING PRESSURE MEASUREMENT SYSTEM Historically the servo-nulling pressure measurement system was used for many investigations. It was capable of measuring pressures up to 200 mm Hg with an accuracy of 0.5 mm Hg. The size of the measuring tip was ~1µm. The maximum frequency response was ~100 hz (not quite enough for the heart beat rate) Page 5

6 BASIC OPERATION Blood Vessel Glass Pipette coated on outside with metal 2 molar NaCl inside R P Electronics Tips can be heated-pulled and sharpened to 0.5µm diameter. The outside is coated with metal. The resistance between inside and outside is measured. As pressure at the tip increases fluid is pushed up into the pipette increasing the resistance. The increase in resistance is measured and causes an increase in the applied pressure at the large end of the pipette to push the fluid back down keeping the resistance constant. The applied pressure is measured using conventional pressure measurement systems. Page 6

7 IPM MODEL 5A SERVO-NULL PRESSURE MEASUREMENT SYSTEM Page 7

8 TRANSONIC FLOW MEASUREMENTS Transonic Systems Inc., FULL ARTICLE Page 8

9 TRANSONIC PERIVASCULAR FLOWPROBE Fig. 1: Schematic views of a Transonic perivascular ultrasonic volume flowsensor. Using wide beam illumination, two transducers pass ultrasonic signals back and forth, alternately intersecting the flowing liquid in upstream and downstream directions. The flowmeter derives an accurate measure of the "transit time" it takes for the wave of ultrasound to travel from one transducer to the other The difference between the upstream and downstream integrated transit times is a measure of volume flow rather than velocity. Transonic Systems Inc., Page 9

10 TRANSONIC PERIVASCULAR FLOWPROBE Fig. 2: The vessel is placed within a beam that fully and evenly illuminates the entire blood vessel. The transit time of the wide beam then becomes a function of the volume flow intersecting the beam, independent of vessel dimensions. Transonic Systems Inc., Page 10

11 TRANSONIC PERIVASCULAR FLOWPROBE Fig. 3: The ultrasonic beam intersects the vessel twice on its reflective path (top diagram). With each intersection, the transit time through the vessel is modified by a vector component of flow. The full transit time of the ultrasonic beam senses the sum of these two vector components, or flow. With misalignment (bottom diagram),one vector component of flow increases as the other decreases, with little consequence to their sum. Transonic Systems Inc., Page 11

12 TRANSONIC PERIVASCULAR FLOWPROBE A Transonic perivascular flowprobe (Fig. 1) consists of a probe body which houses ultrasonic transducers and a fixed acoustic reflector. The transducers are positioned on one side of the vessel or tube under study and the reflector is positioned at a fixed position between the two transducers on the opposite side. Electronic ultrasonic circuitry directs a flowprobe through the following cycles: Upstream Transit-Time Measurement Cycle An electrical excitation causes the downstream transducer to emit a plane wave of ultrasound. This ultrasonic wave intersects the vessel or tubing under study in the upstream direction, then bounces off the fixed "acoustic reflector." It again intersects the vessel and is received by the upstream transducer where it is converted into electrical signals. From these signals, the flowmeter derives an accurate measure of the "transit time" it takes for the wave of ultrasound to travel from one transducer to the other. Downstream Transit-Time Measurement Cycle The same transmit-receive sequence is repeated, but with the transmitting and receiving functions of the transducers reversed so that the flow under study is bisected by an ultrasonic wave in the downstream direction. The flowmeter again derives and records from this transmit-receive sequence an accurate measure of transit time it takes for the wave of ultrasound to travel from one transducer to the other. Just as the speed of a swimmer depends, in part, on water currents, the transit time of ultrasound passing through a conduit is affected by the motion of liquid flowing through that vessel. During the upstream cycle, the sound wave travels against flow and total transit time is increased by a flow-dependent amount. During the downstream cycle, the sound wave travels with the flow and total transit time is decreased by the same flowdependent amount. Using wide beam ultrasonic illumination, the Transonic flowmeter subtracts the downstream transit times from the upstream transit times. This difference in the integrated transit times is a measure of true volume flow. Wide Beam Illumination One ray of the ultrasonic beam undergoes a phase shift in transit time proportional to the average velocity of the liquid times the path length over which this velocity is encountered. With widebeam ultrasonic illumination (Fig. 2), the receiving transducer sums (integrates) these velocity - chord products over the vessel's full width and yields volume flow: average velocity times the vessel's cross sectional area. Since the transit time is sampled at all points across the vessel diameter, volume flow measurement is independent of the flow velocity profile. Ultrasonic beams which cross the acoustic window without intersecting the vessel do not contribute to the volume flow integral. Volume flow is therefore sensed by perivascular probes even when the vessel is smaller than the acoustic window (Fig. 2). Transonic Systems Inc., Page 12

13 LAB PROCEDURES FOR THE MOUSE Measurement of Renal Arterial Blood Flow in the Mouse Measurement of Cardiac Output in the Mouse The mouse can be monitored for cardiac output after recovery in 3-5 days. During this time, the flowprobe will encapsulate in fibrous tiss ue to provide good signal transmission. FULL ARTICLE FULL ARTICLE Transonic Systems Inc., Page 13

14 MEMS FLOW AND PRESSURE SENSORS MEMS (Microelectromechanical Systems) devices have promise for these measurements because of the small sizes that are possible. In the measurement world smaller is usually also better for sensitivity, cost, and frequency response. The following pages will present research on: Capacitive Pressure Sensor Novel Moving Gate MOSFET Pressure Sensor Fluid Flow Sensor Page 14

15 BLOOD PRESSURE SENSOR Binary Counter Capacitor Pressure Sensor Oscillator 300µm dia round C = eoer area /d = ~2e-15 F Page 15

16 ALUMINUM DIAPHRAGM PRESSURE SENSOR Source Diaphragm Gate Drain Insulator Contact Air Gap (~1 atm) Kerstin Babbitt - University of Rochester Stephanie Bennett - Clarkson University Sheila Kahwati - Syracuse University An Pham - Si Wafer Bottom Plate Page 16

17 ALUMINUM DIAPHRAGM PRESSURE SENSOR 2000 µm Sensor C sensor 5 to 25 pf Ring Oscillator 20/100 20/100 20/ /20 100/20 100/20 VDD= -10V 40/20 600/20 GND VO R load 1 Meg C load 20 pf 0 to 5 mm Hg Pressure Range C parasitic = 10pF T=2Ntd T = period of oscillation N = number of stages td=gate delay Page 17

18 OSCILLATOR (MULTIVIBRATOR) R1 V T + - R2 +V Vo Vo +V -V t1 t C -V R Period = T = 2RC ln 1+Vt/V 1-Vt/V Bistable Circuit with Hysteresis and RC Integrator Page 18

19 CAPACITOR TO FREQUENCY Page 19

20 DESIGN EXAMPLE CAPACITOR SENSOR R1 R V -V R C - + C Vref + - -V Vo C R Square Wave Generator RC Integrator & Capacitor Sensor Buffer Peak Detector Comparator Display Page 20

21 C to DIGITAL READOUT ELECTRONICS Square Wave Generator RC Integrator Peak Detector C to Analog Voltage LED Bar Display Voltage Divider And Comparators Page 21

22 8V 3 BIT FLASH ANALOG TO DIGITAL CONVERTER +V Vin 7V 6V 5V 4V 3V 2V 1V 3.5V Comparators Segment Detector Decoding Logic Page 22

23 POLY DIAPHRAGM FIELD EFFECT TRANSISTOR Vsource Vgate 2 µm n+ Poly 1 µm space P Å Oxide Vgate Vsource 15 µm Vdrain Aluminum Plug Vdrain P+ n-type silicon 75 µm Poly Diaphragm 5x Etch Holes Contact Cut to Poly Gate Kerstin Babbitt, 1997 BSEE U of Rochester Page 23

24 POLY GATE FET PRESSURE SENSOR A B Gate Drain Stop Source 150 µm A B 200, 400, 800 µm Add alignment marks and squares in each corner for each level Page 24

25 IC GRAPH LAYOUT 700 µm 8000 µm Page 25

26 POLY DIAPHRAGM 5x 2.5 µm 25 µm Page 26

27 POLY DIAPHRAGM PRESSURE SENSOR TEST RESULTS Pressure No Pressure An Pham 1999 Page 27

28 POLY DIAPHARGM PRESSURE SENSOR PACKAGING 500 µm Page 28

29 SECOND VERSION COMPLETED DEVICES - RIT W3 W1 L1 L2 W2 W1 = 300 µm W2 = 1100 W3 = 450 L1 = 1400 L2 = µm Page µm

30 4 TH VERSION CMOS Compatible Tiny Pressure Sensor Page 30

31 4 TH VERSION CMOS Compatible Tiny Pressure Sensor Poly is sacrifical layer, XeF2 Oxide under metal is diaphragm Metal gate Page 31

32 IMPROVED MEMS PRESSURE SENSOR The moving gate MOSFET pressure sensor shows promise for meeting the specifications of small size, low cost, accuracy and high frequency. The major problem in the work presented above is the final packaging and shaping of the probe tip. Devices made on silicon-on-insulator (SOI) could be shaped (in batch mode) by an etching technique. This would give sensors of less than 2µm by 2µm as illustrated below. Page 32

33 BLOOD FLOW Photodiode Strain Sensor Page 33

34 MEMS FLOW SENSORS Heater Flow Spring 2003 EMCR 890 Class Project Upstream Temp Sensor Polysilicon Si3N4 SacOx Silicon Substrate Downstream Temp Sensor Aluminum Page 34

35 REFERENCES 1. Transonic Systems Inc., Tel: (USA); Fax ; Page 35