Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim Part 14 doc

APPLICATIONS 373 excitation and the SAW device. To eliminate the error from the variation of r e , a second reflector is put on the wafer. The corresponding time delay is 12- Similar to the first reflector, we have the IF corresponding to the second reflector as h(t} = B 2 cos[0(f) - 02(01 = B 2 cos[+^t 2 t + co 0 t 2 - ^ 2 t/2] (13.27) where t 2 = r e + T 2 and T 2 = 2d 2 /v (13.28) and d 2 is the distance between the IDT and the second reflector. The difference between the two phase shifts can now be written as <Pd = (<P2 - <p\) = No - M/2fe + ti)](t2 -t1) = Kr (13.29) where K = a>o - fJL/2(t 2 + t\)**a) 0 (13.30) Since O>Q ^> pL/2(t 2 + 1\ ) as can be seen from the values calculated and (13.31) where TO is the total travel time of the surface wave from the first reflector to the second and back. This time being inversely proportional to the surface wave speed is very sensi- tive to the temperature in the vicinity of the SAW device and we propose the following relationship between the travel time t and the temperature T T = T 0 [l+a(T -To)] (13.32) where a is the temperature coefficient of the time delay of the SAW device and TO is the ambient temperature. From Equation (13.29), + KT O (! - T 0 ) (13.33) = aT +b a = KaiQ and b = KT Q (\ - 7b) (13.34) If the resolution of phase shift difference in degrees is A<p, then the resolution of the temperature reading will be AT = A<p/a (13.35) The wafer is made of YZ-cut lithium niobate with a = 94xlO~ 6 /°C (13.36) 374 IDT MICROSENSORS The two reflectors are located such that the time delay at room temperature TQ is T\ = 1 us and T 2 = 1.1 us (13.37) Then, TO = 0.1 us (13.38) The transmitted FM signal is pulse-modulated with a time duration of 1760 th of a second. The carrier frequency varies linearly from 905 MHz to 925 MHz during the period. The parameters in Equation (13.22) for the FM signal are = 905 MHz (13.39) fi/2n = 1.2 x 10~ 3 MHz/us (13.40) In operation, the distance between the two antennas is 1 to 2 meters so that r e can be neglected compared with x\ or 12- The temperature variation can be in the range 0 to 200 °C. The first and second terms in Equation (13.30) can now be calculated = 2n x 905 x 10 6 - 1.2 x 10 -3 x 1.05/2 (13.41) = 2n x 905 x 10 6 So, the approximation in Equation (13.30) is clearly justified. From Equations (13.34), (13.36), and (13.38), the constant a is a = 3.06 angular degrees/°C (13.42) The resolution of the phase shift is 1° so that the resolution of the temperature reading is given by Equation (13.35) as = 0.33°C (13.43) The experimental calibration is done in a temperature-controlled chamber called a Delta 9023, (Figure 13.14). A digital hand-held thermometer (Keruco Instruments Co.) with an accuracy of ±0.2 °C is taken as the temperature standard. The temperature range in the experiment is from room temperature (near 20 °C) to a maximum of 140 °C. The straight line theory, as shown in Figure 13.15, fits well the experimental points of phase against temperature. The equation for the line in Figure 13.15 is obtained by the least mean square method and is given by ?> = 2.897-49.1 (13.44) The value of the slope coefficient a (2.89) is in good agreement with that (3.06) estimated from Equation (13.42). The root mean square (rms) error of the phase difference is equal to 0.78°, which corresponds to an rms error of 0.26 °C. APPLICATIONS 375 Antenna Figure 13.14 Calibration of the remote reading system 400 40 60 80 Temperature (°C) Figure 13.15 Phase shift difference versus temperature (after jump correction) of a SAW-IDT microsensor 13.4.3 Pressure Sensor The SAW-based pressure sensors use the strain principle described in Section 13.4,1. To illustrate this principle, let us consider a circular quartz membrane with thickness h. The deflection of the plate, w, under uniformly distributed pressure (P) is assumed to be smaller than h/5. The differential equation describing the elastic behaviour of the middle plane of a thin plate is obtained from the elementary theory of plates p D (13.45) where D is the stiffness of the plate with piezoelectric properties of quartz. In the case of simply supported edge, the deflection w is given by 376 IDT MICROSENSORS 0.00 0.20 0.40 0.60 Normalized distance 0.80 1.00 Figure 13.16 Normalised strain against the distance from the centre of the membrane P(a 2 – r 2 ) w = 64D (13.46) where a is the radius of the plate, r is measured in a coordinate system fixed to the centre of the plate, and v is Poisson's ratio. The radial strain is given by Dz d 2 w vdw r(r,z) = ~~ + ~ 8 E(h 1 (13.47) Figure 13.16 illustrates the calculated static average strain distribution as a function of the radial distance from the centre of the circular membrane. The strain is then calibrated with change of pressure (Vlassov et al. 1993). 13.4.4 Humidity Sensor There is a need for the development of a remote, wireless, and passive sensor system for humidity measurement that is more accurate than conventional methods. Here, such a system is discussed on the basis of a SAW device. The following sections describe a wireless sensor system that can remotely interrogate a passive SAW sensor for the measurement of relative humidity (RH) (Hollinger et al. 1999). The principle of operation of the wireless SAW-based sensor system was described earlier. The FM generator continuously emits pulses with duration of 16.7 ms that are linearly frequency modulated from 905 to 925 MHz. The original FM signal is expressed as = Acos[0(t)] (13.48) APPLICATIONS 377 where 0(t) = (w 0 + ut)t+0 0 (13.49) Consider a simplified SAW device with only two reflectors. The echo from the first (S\) input to the mixer is the same as the original FM signal, but with a time delay t\ and a different amplitude (13.50) 0 1 (0 = [w 0 + u(t — t 1 )/2](t - t 1 ) + 0 0 (13.51) The IF corresponding to S 1 (t) is expressed as I 1 (0 = B 1 cos[ut 1 t + w 0 t 1 - ut 2 1 / 2] (13.52) Both the frequency ut 1 and the phase shift <p1 = (w n t 1 — ut 1 2 )/2 depend on the time delay t 1 . Once again, because w n is usually much greater than ut 1 , the phase shift is more sensitive than the variation of the frequency. The total delay t\ depends not only on the travel time of the surface wave that is a function of the physical parameter being sensed but also the RF propagation time, which is a function of separation between the sensor and the reader. To eliminate this error, a second reflector is used that has a different time delay t 2 and results in the following IF. I 2 (t) = B 2 cos[ut 2 t + w 0 t 2 —ut 2 /2] (13.53) The difference between the two phase shifts can be written as <p d = [w 0 — u /2(t 2 + t 1 )](t 2 — t 1 ) = KT (13.54) where K = w 0 — u/2(t 2 + t 1 )~ W 0 (13.55) because W 0 » U / 2(T 2 + t 1 ) for the values of the present system. Now r = T 2 - t 1 = 2d /v (13.56) where r is the total travel time of the surface wave from the first reflector to the second and back, d is their physical separation, and v is the SAW velocity. For the design of the humidity sensor, the difference in delay between two reflectors (r) is 1.40 us. The differential phase shift can be written as (13.57) v The change in the differential phase shift due to a change in humidity is then u1 U0 V0 (13.58) 378 IDT MICROSENSORS From preliminary measurements of humidity, it is seen that — = 3.05 x 10 –6 x RH(%) (13.59) Although the change in velocity is small, we can replace v 1 in the denominator of the last term in Equation (13.58) by u 0 . Then, using Equations (13.56), (13.58), and (13.59) we have –6 x 10 Using a frequency of 905 MHz (i.e. W 0 = In x 905 x 10 6 rad/s), value (1.40 us) for TO, and by converting from radians to degrees, we get = 1.39 x RH(%) in degrees (13.60) the above (13.61) Therefore, because the wireless system has a phase difference measurement resolution of 1°, this system should provide a resolution of 0.72 percent RH for measurements. The substrate material for the SAW device is made of YZ-LiNbO 3 , which is a Y-axis cut and Z-axis-propagating lithium niobate crystal. The size of the piezoelectric substrate is approximately 4.3 mm by 8 mm and 0.5 mm thick. The IDTs and reflectors are made of aluminum and are deposited by sputtering using appropriate masks. Because the SAW sensor response can be affected by both the temperature and humi- dity, a design has been developed that allows for the simultaneous measurement of temperature and humidity. As shown in Figure 13.17, four IDTs are arranged in a stag- gered manner along the centre of the piezoelectric substrate with reflectors on both sides of the IDTs. On one side, between the IDTs and the right set of reflectors, a moisture- sensitive coating (SiO 2 in this case) is deposited. The substrate between the IDTs and the left set of reflectors is left bare, as lithium niobate shows very little response to changes Reflectors IDTs Bus Reflectors Lithium niobate substrate Figure 13.17 Wireless SAW sensor design for simultaneous temperature and humidity measure- ment at 915 MHz APPLICATIONS 379 in humidity. Therefore, the uncoated side is used for measuring the temperature, which is then used to compensate for temperature changes in the humidity measurement from the coated side. There are up to three phase shifters (not shown) in front of each reflector. The unique arrangement of these phase shifters gives the sensors their unique identifica- tion number, which can also be determined by the wireless system based on the reflected electromagnetic signal. The bus is connected to the two terminals of each IDT and this is inductively coupled with the sensor antenna through an air gap. The SiO2 coating used on the wireless humidity sensor is 40 to 50 nm thick and was deposited using plasma-enhanced chemical vapour deposition (PECVD). A glass mask was prepared to deposit the film only on the area indicated in Figure 13.17. The coating is amorphous and porous to give it a high sensitivity to water vapour. In order to protect the IDTs and reflectors, they should be coated with a very thin (about 1 um) layer of a passivation layer that is not affected by moisture. One candidate is amorphous silicon nitride (SiN x H y ). SiN x H y has excellent passivation properties, which make it ideal for use in the semiconductor industry as an insulator and as a protective layer for silicon devices. SiN x H y can be deposited from the reaction of silane (SiH4) with either ammonia (NH 3 ) or nitrogen (N2). Using plasma to assist in the deposition, these films are deposited at much lower temperatures than by any other techniques (about 350 °C instead of more than 750 °C). Figure 13.18(a) shows the experimental setup for remote humidity measurement using wireless and passive SAW sensors. The main components of the system consist of the SAW sensor, the transceiver, central interface unit (CIU), and computer. A commercial RH sensor (Omega RH82) is also placed inside the humidity chamber for calibration and verification. SAW devices based on a LiNbO 3 substrate with part of the substrate coated with a thin layer of humidity-sensitive silicon dioxide are used. The delay line in the SAW sensor is now sensitive to changes in humidity as the silicon dioxide adsorbs moisture from the humid air. The transceiver emits RF pulses that are picked up by the SAW sensor; these pulses are converted to acoustic waves on the surface of the sensor, which are further reflected by the uniquely spaced reflectors. The reflected signal received back by the transceiver contains the sensor information. This signal passes through the CIU in which all the signal processing takes place and then the processed data is sent to the computer through the serial interface. The phase difference between two reflectors along the coated part of the propagation path can be used as a measure of the humidity. The RH of the enclosed chamber is changed by pumping nitrogen into the chamber through a bubbler. The bubbler wets the nitrogen gas, which when introduced into the chamber, causes the humidity inside the chamber to increase. The following graph, Figure 13.18(b), shows the plot between RH, as measured by the commercial humidity sensor, and the phase change in the humidity sensor, as measured by the transceiver and computer system. It can be seen that the phase change varies linearly with RH. The straight line, shown in Figure 13.18(b), has been fitted to the experimental data using least squares. The equation of the fitted line is <p = 1.4 percent RH + 75.9. There- fore, as stated earlier, because the wireless system has a phase difference measurement resolution of 1°, this system is able to provide an excellent resolution of 0.69 percent RH measurements. 380 IDT MICROSENSORS Antenna Humidity chamber t Saw Digital hygrometer Computer 190 M 180 <u 2- & 170 140 130 120 • Measured data — Linear fit (b) 30 40 50 60 70 80 Relative humidity (%RH) 90 100 Figure 13.18 (a) Setup for humidity measurement and (b) effect of relative humidity on phase change 13.4.5 SAW-Based Gyroscope In general, gyroscopes have a wide range of applications. They are • Consumer electronics - picture stabilisation in three-dimensional (3D) pointers, cam- corders, geostationary positioning system (GPS) • Industrial products - robots, machine control, guided vehicles. • Medical applications - wheel chairs, surgical tools • Military applications - smart ammunition, guided missiles and weapon systems (Soder- kvist 1994; Yazdi et al. 1998) APPLICATIONS 381 As technology matures, the market for low-cost angular sensors will extend into high volume of micromachined sensors for complete automotive pitch and roll control, which is the final step toward realisation of integrated chassis control (Eddy and Soarks 1998). This includes sensing the steering, suspension, power train and breaking systems for real- time vehicle control. All these systems require an application-specific low-cost sensor and actuator. Future air bag systems will be equipped with a low-g accelerometer and a gyroscope. Low-cost and low-g accelerometers are already in use, whereas gyroscopes that are compatible with the automotive industries' requirements, are not yet on the market. The construction of the gyroscope is based on integration of a SAW resonator (Bell and Li 1976; Staples 1974) and a SAW sensor (White 1985; Ballantine et al 1997) that operates primarily in the Rayleigh mode. The Rayleigh wave is a SAW that has its energy concentrated within one wavelength of the substrate surface (Achenbach 1973). The displacement of particles near the surface, due to the Rayleigh wave, has out-of- surface motion that traces an elliptical path (Slobodnik 1976). The Rayleigh wave can be generated at the surface of a piezoelectric material by applying a voltage to an IDT patterned on the substrate (White and Voltmer 1965). Lao (1980) derived theoretically the dependence of SAW velocity on the rotation rate of the wave-propagating medium and it is established that, for an isotropic medium, the rotation rate is a function of Poisson's ratio. Kurosowa et al. (1998) proposed a SAW gyroscope sensor based on an equivalent circuit simulation. He concluded that the angular velocity could not be detected because of the mismatch of resonant frequencies. Whereas Varadan et al. (2000a,b) presented the design, proof of concept through fabrication, and performance evaluation of a SAW gyro- scope using a two-port resonator and a sensor. In this section, we present the equivalent circuit model analysis and experimental evaluation of a SAW gyroscope (Varadan et al. 2000b). The SAW resonator is designed and optimised using the coupled-mode theory. The gyroscope is optimised using a cross-field circuit model for numerical simulation. This gyroscope has the added capability that it can be used as a wireless gyroscope that can be easily integrated onto a SAW accelerometer (Subramanian et al. 1997). 13.4.5.1 Principle of SAW-based gyroscope Any mechanical gyroscope must have a stable reference vibrating motion (V) of a mass (m) such that when subjected to a rotation, the angular rotation (£2) perpendicular to the reference motion would cause Coriolis forces at the frequency of the reference motion. The strength of the Coriolis force F is a measure of the rotation rate and is given by F = 2mV X& (13.62) It is well known that in a standing Rayleigh surface wave, the particle vibration will be perpendicular to the surface. This particle vibration can be cleverly utilised for the creation of a reference vibratory motion for the gyroscope. The concept of utilising a SAW for gyroscopic motion is illustrated in Figure 13.19. It consists of IDTs, reflectors, and a metallic dot array within the cavity, which are fabricated through microfabrication techniques on the surface of a piezoelectric substrate. The resonator IDTs create a SAW that propagates back and forth between the reflectors and forms a standing wave pattern within the cavity because of the collective reflection 382 IDT MICROSENSORS from the reflectors. A SAW reflection from individual metal strips adds in phase if the reflector periodicity is equal to half a wavelength. For the established standing wave pattern in the cavity, shown in Figure 13.19, a typical substrate particle at the nodes of the standing wave has no amplitude of deformation in the z-direction. However, at or near the antinodes of the standing wave pattern, such particles experience large amplitude of vibration in the z-direction, which serves as the reference vibrating motion for this gyroscope. To amplify acoustically the magnitude of the Coriolis force in phase, metallic dots (proof mass) are positioned strategically at the antinode locations. The rotation (ft, x-direction) perpendicular to the velocity (V in ±z-direction) of the oscillating masses (m) produces Coriolis force (F = 2m V x ft, in +y-direction) in the perpendicular direction, as illustrated in Figure 13.19. This Coriolis force establishes a SAW in the y-direction with the same frequency as the reference oscillation. The metallic dot array is placed along the j-direction such that the SAW, because of the Coriolis forces, adds up coherently. The generated SAW is then sensed by the sensing IDTs placed in the v-direction. The operating frequency of the device is determined by the separation between the reflector gratings, periodicity of reflectors, and the IDTs. The separation between reflectors was chosen as an integral number of half-wavelengths such that standing waves are created between both reflectors. The periodicity of IDT was chosen as a half-wavelength (A./2) of the SAW. Therefore, for a given material, the SAW velocity in the material and the desired operating frequency f 0 define the periodicity of the IDT. The substrate used for the present gyroscope is 128YX LiNbO 3 because of its high electromechanical coupling coefficient. For such materials, the wave velocities in the Figure 13.19 Working principle of a MEMS SAW gyroscope [...]... x- and y -directions on LiNbO3 Substrate Measured velocity (m/s) Reference velocity (m/s) 128YX-LiNbO3 x-direction y-direction 3961 3656 3997 not available Source: Campbell and Jones (1968) x- and y-directions are different because of its anisotropy The effective SAW velocities were experimentally measured in the x- and y-directions because the device utilises wave propagation in these directions and. .. sensor," Smart Materials Struct., 9, 89 0-8 97 Varadan, V K et al (2000a) "Conformal MEMS- IDT gyroscopes and their performance comparison with fiber optic gyroscope," Proc SPIE Smart Electronics MEMS, 3990, 33 5-3 44 Varadan, V K et al (2000b) "Design and development of a MEMS- IDT gyroscope," J Smart Materials Struct., 9, 898–905 Varadan, V V et al (1997) "Wireless passive IDT strain microsensor," Smart Materials... narrow-band IDT sets with the same periodicity were placed in the x- and y-direction so that they included the dot array in the middle Hence, the response measured using IDTs in x- and y-directions were different for the same periodicity of these IDT sets, and the velocities of both directions were measured as 3961 m/s and 3656 m/s, respectively The difference between published values (Campbell and Jones... "Micromachined inertial sensors," Proc IEEE, 86, 164 0-1 658 14 MEMS- IDT Microsensors 14. 1 INTRODUCTION The combination of a microelectromechanical system (MEMS) device with an interdigital transducer (IDT) (surface acoustic wave (SAW)) microsensor is a relatively new concept MEMS- IDT-based microsensors can offer some significant advantages over other MEMS devices, including the benefits of excellent sensitivity,... coupling-of-modes theory," IEEE Ultrasonics Symp., 1, 277–281 Hollinger, R D et al (1999) "Wireless surface acoustic wave-based humidity sensor," Proc SPIE, 3876, pp 5 4-6 2 Hoummady, M., Campitelli, A and Wlodarski, W (1997) "Acoustic wave sensors: design, sensing mechanisms and applications," Smart Materials Struct., 6, 64 7-6 57 Kurosawa, M., Fukuda, Y., Takasaki, M and Higuchi, T (1998) "A surface-acoustic... a cross-field equivalent circuit model was chosen because then Coriolis force can be directly related to input force on the SAW sensor model Various SAW devices have been modeled using COM theory The application of coupled-mode theory on SAW devices for given geometry and material data is well documented by Cross and Schmidt (1977), Haus and Wright (1980), and Campbell (1998) Figures 13.20 and 13.21... the input-output equations The versatility of this technology is very impressive and permits a wide variety of different sensing applications, such as strain, pressure, temperature, conductivity, and dielectric constant The next chapter is dedicated to the topic of IDT -MEMS sensors that represent an exciting development combining the fields of MEMS and SAW-IDT devices The final example of a SAW-IDT device... equivalent voltages V and particle velocities at the surface v are transformed to equivalent currents / These transformations can be written in terms of a proportionality constant 0 as 390 IDT MICROSENSORS 0.6 0.4 ^ 0.2 i 1 i» 'i 5 > 0 –0.2 –0.4 -0 .6 0.1 Time (us) 0.05 0.15 0.2 1.5 1. 0- c 5. 0- £ 0.0 Q 23 0.15 0.05 02 1-5 .0 -1 . 0-1 .5 Time (us) Figure 13.25 Computed velocity of a particle at the middle... Campbell, J J and Jones, W R (1968) "A method for estimating optimal crystal cuts and propagation directions for excitation of piezoelectric surface waves," IEEE Trans Sonics Ultrasonics, 15, 20 9-2 17 Cross, P S and Schmidt, R V (1977) "Coupled surface acoustic wave resonator," Bell Syst Tech J., 56, 144 7 148 1 Datta, S (1986) Surface Acoustic Wave Devices, Ch 3, Prentice-Hall, New Jersey 4 The lift-off process... IDT MICROSENSORS Eddy, D S and Soarks, D R (1998) "Application of MEMS technology in automotive sensors and actuators," Proc IEEE, 86, 174 7-1 755 Gere, J M and Timoshenko, S P (1990) Mechanics of Materials, PWS-Kent, Kent, UK Hauden, D., Jaillet, G and Coquerel, R (1981) Temperature sensor using SAW delay line, Proc IEEE Ultrasonic Symp., Chicago, IL, USA, pp 148 –151 Haus, H and Wright, P V (1980) "The . of coupled-mode theory on SAW devices for given geometry and material data is well documented by Cross and Schmidt (1977), Haus and Wright (1980), and Campbell (1998). Figures 13.20 and . 0 'i 5 > –0.2 –0.4 -0 .6 1.5 1. 0- c 5. 0- £ 0.0 Q. 23 1-5 .0 -1 . 0- -1 .5 0.05 0.1 Time (us) 0.15 0.2 0.05 0.15 02 Time (us) Figure 13.25 Computed velocity of a particle at the middle . (m/s) Reference velocity (m/s) 128YX-LiNbO 3 x-direction 3961 3997 y-direction 3656 not available Source: Campbell and Jones (1968). x- and y-directions are different because