- Email: [email protected]
The improved performance of GaAs micromachined power sensor microsystem
Sensors and Actuators 76 Ž1999. 241–246 www.elsevier.nlrlocatersna
The improved performance of GaAs micromachined power sensor microsystem T. Lalinsky´ a b
a,)
ˇ Hascık ˇ Mozolova´ a, E. Burian a, M. Drzık , S. ˇˇ´ a, Z. ˇ´
b
Institute of Electrical Engineering, SloÕak Academy of Sciences, DubraÕska ´ ´ cesta 9, 842 39 BratislaÕa, SloÕak Republic Institute of Construction and Architecture, SloÕak Academy of Sciences, DubraÕska ´ ´ 9, 842 20 BratislaÕa, SloÕak Republic Received 14 September 1998; received in revised form 5 January 1999; accepted 8 January 1999
Abstract Thermal effects in a 2-mm thick GaAs cantilever beam of the power sensor microsystem are investigated. The increased thermal sensitivity of the microsystem to the thermal conductance changes of the ambient atmosphere is evaluated by the experiment and simulation. A bimetallic effect in the microsystem cantilever beam is studied using both the microscopic laser optical interferometry and laser optical reflectance measurement. The cantilever beam deflections induced by the differential thermal expansion of the cantilever layers are found to be linear with the power dissipated in the microsystem MESFET-heater. The key microsystem transfer characteristics based on the bimetallic effect are obtained. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Power sensors; Galium arsenide; GaAs micromachining; Cantilever bimetallic effect
1. Introduction Transmitted power is the basic quantity measured in RF and microwave systems. The transmitted power measurement is unavoidable in research and development period of any RF and microwave equipment and for monitoring and control of RF working systems. The most precise AC power measurement is based on thermal conversion. The absorbed RF power is converted into thermal power using thermally isolated absorbing element. The resulting heat is sensed by a temperature sensor and by means of substitution or comparison technique converted into electrical DC output quantity. A major advantage of the thermal conversion is its exact physical definition and inherent broad frequency range. There are many techniques to fabricate thermally isolated matched RF termination with integrated temperature sensor. The most recent techniques use CMOS IC technology combined with oriented etching technique w1x.
)
Corresponding author. Tel.: q42-7-375-816; Fax: q42-7-378-2919; E-mail: [email protected]
Recently a new three-terminal thermoconverter ŽTTTC. technique has been successfully developed for AC power measurement w2,3x. The technique is based on active heater utilized to sense directly the transmitted power. The new approach is based on controlled impedance of active heater enabling mirroring of external transmitted power into the heater. The thermal effect of the mirrored power is then converted into DC quantity by means of classical substitution technique. The new measurement technique is principally applicable also in RF and microwave systems. A new concept of power sensor microsystem ŽPSM. was designed w3x to demonstrate the TTTC technique of the power measurement. There was GaAs chosen as the basic electronical as well as micromechanical semiconductor material because of its lower thermal conductivity, higher saturation velocity of electrons and higher temperature working conditions as compared with a Silicon. It consisted of two power-controlled GaAs MESFETs as threeterminal heaters, and Schottky diode as a temperature sensor, monolithically integrated on two 8-mm thick GaAs cantilever beams. GaAs MESFETs processing combined with a GaAs bulk micromachining were applied to fabricate the PSM. The technology permitted precise control of
0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 9 . 0 0 0 3 7 - 0
T. Lalinsky´ et al.r Sensors and Actuators 76 (1999) 241–246
242
the thickness and uniformity of the cantilever beams directly by the thickness of the MBE grown GaAs over an AlGaAs etch-stop layer. The benefit of the PSM technology developed was also verified by investigation of the basic microsystem transfer characteristics in various gaseous environments w4–6x. The advantages of GaAs based power sensors have also been demonstrated in Ref. w7x. There was a micromachined power sensor designed to perform power measurements up to millimetre Žmm.-wave and sub-mm wave regions. A concept of the power sensor was based on a thin Ž1.5 mm. undoped AlGaAsrGaAs membrane. It consisted of NiCr thin film resistors Žas heaters. and integrated GaAs based thermocouples Žas temperature sensors.. In this paper we report on new thermal effects induced in the micromachined power sensor microsystem based on 2-mm thick GaAs cantilever beams. The increased sensitivity of the microsystem to the pressure dependent thermal conductance of gases is investigated by the experiment and simulation. A bimetallic effect in the microsystem cantilever beam is studied using both the microscopic laser optical interferometry and laser optical reflectance method. Fig. 2. Front side view of the 2-mm thick GaAs cantilever beam of the microsystem.
2. Microsystem technology GaAs heterostructure layer design shown in Fig. 1 was used in the microsystem technology. A 2-mm thick GaAs layer grown by MBE at low temperature ŽLT-GaAs. on the top of AlGaAs etch-stop layer defines the thickness of the microsystem cantilever beam. A d-doped layer of Si was used for both the channel of the MESFETs and active layer of the temperature sensing Schottky diode. As mentioned already, GaAs MESFET processing combined with a GaAs bulk micromachining were applied to fabricate the microsystem. A three-dimensional patterning of two independent thin GaAs cantilever beams of the microsystem as described in detail in Ref. w8x was defined by a double side
aligned selective reactive ion etching of GaAs through the opening in masks to the AlGaAs etch-stop layer. A front side view of 2-mm thick GaAs cantilever beam of the microsystem is shown in Fig. 2. There are two power-controlled GaAs MESFETs Žas three-terminal heaters. and GaAs Schottky diode Žas a temperature sensor. clearly seen in a monolithic approach. An excellent thermal isolation of the microsystem devices can be obtained due to by a small thickness of the cantilever. This enables to achieve a very efficient power-temperature conversion in the channel of the MESFETs useful for the precise power measurement. The principle of the measuring method is to balance the unknown power dissipated by one heater by a known power on the second one while maintaining a constant sensor temperature Žabout 50–1008C. sensed by the Schottky diode. Controlling the current under constant voltage or the voltage under constant current on the second heater, a powerrcurrent or powerrvoltage converter, respectively is obtained.
3. Microsystem thermal characterization
Fig. 1. Heterostructure layer design.
The increased sensitivity of the microsystem to the thermal conductance changes of the ambient atmosphere was indicated by decrease of the cantilever beam thickness. In order to confirm the improved microsystem ther-
T. Lalinsky´ et al.r Sensors and Actuators 76 (1999) 241–246
mal performance, the key microsystem transfer characteristics were measured in various gaseous environments. Forward I–U characteristics of the Schottky temperature sensor diode as a function of power dissipation in the MESFET were measured using a semiconductor parameter analyzer ŽHP 4145B.. Power-sensor diode voltage Ž P–U . transfer characteristic under selected DC diode current biasing can be determined. By using a calibrated temperature measurements of the diode characteristics under the same selected current biasing a direct correlation between the sensor diode voltage and corresponding temperature can also be independently found. This enables to construct a direct power–temperature Ž P–T . transfer characteristic of the microsystem w4x. Typical behaviour of the microsystem P–T transfer characteristics obtained for air, argon and vacuum environments is shown in Fig. 3. There is an excellent linearity observed mainly for the air and argon atmosphere. A slope of the P–T curves determines the thermal resistance value of the microsystem. A significant influence of these environments on the thermal resistance value can also be seen. The corresponding thermal resistance values were determined to be 14 000 KrW, 17 000 KrW and 34 000 KrW for the air, argon and vacuum environments, respectively. This is consistent with the decrease of the thermal conductivity of these gaseous media. If the microsystem cantilever thickness is decreased the cantilever surface is appeared to play a significant role in the heat transfer to the ambient atmosphere. As a consequence of this increased sensitivity of the microsystem to the thermal conductance changes of the ambient gaseous environment can be found. This is demonstrated in Fig. 4, where P–T transfer characteristics of the microsystem for different gas pressures are shown. As seen, an excellent linearity is achieved in the range of transition and high pressures Ž P ) 200 Pa.. Thermal resistance values extracted from the slopes of the P–T curves as a function of
Fig. 3. P – T transfer characteristics measured in various gaseous environments.
243
Fig. 4. P – T transfer characteristics of the microsystem for different gas pressures.
gas pressures are represented in Fig. 5. There are the expressive thermal resistance changes observed in the narrow range of gas pressures Ž P ; 100–1000 Pa., while in the range of both low pressures Ž P - 100 Pa. and high pressures Ž P ) 1 kPa. decreased sensitivity is indicated. This could be explained by a different thermal conductance mechanism of the gas in the mentioned ranges of pressures as described in detail in Ref. w10x. The increased sensitivity of the microsystem cantilevers to the pressure dependent thermal conductance of gases could be useful for pressure sensing. However, the initial cantilever area Ž; 130 = 450 mm2 . should be increased to improve the pressure sensitivity of the microsystem. The thermal resistance values of the microsystem as indicated in Fig. 5 in the range of low pressures were also simulated. There was the mean free path of gas molecules assumed to be larger than the distance between the cantilever beam and the bottom of the ceramic housing Ž; 320 mm.. This distance determines the transition pressure w10x,
Fig. 5. Thermal resistance changes of the microsystem by the gas pressure.
244
T. Lalinsky´ et al.r Sensors and Actuators 76 (1999) 241–246
have found that the cantilever bending is due to the different thermal expansion coefficient of the device metallic leads ŽTirPtrAu, 350 nm. and GaAs Ž2 mm.. This was experimentally verified in Ref. w9x. There were 2-mm thick GaAs cantilever beams patterned without any metallic leads and no bending was observed. To study the microsystem cantilever beam deflection as also seen in Fig. 2, two different optical methods were applied. The first one, based on the laser optical interferometry and the second one, based on the laser optical reflectance measurement.
4.1. Laser optical interferometric method
Fig. 6. Ža. Thermal resistance changes of the microsystem at low gas pressures. Žb. Cantilever temperature difference at low gas pressures.
which is of the order of 200 Pa. The temperature in the cantilever was simulated by solving the steady state and time dependent two-dimensional heat flow equation by a finite-difference method w6x. The cantilever temperature was calculated taking into account the heat transfer to an infinite, non-convective gaseous medium around the cantilever. The simulated thermal resistance changes by the pressure at power dissipation of 4 mW are shown in Fig. 6a. As seen, a relatively good agreement with the experiment is achieved. Thermal resistance decrease by the pressure corresponds to the temperature decrease in the microsystem cantilever beam. This is simulated in Fig. 6b. It can be seen that the cantilever temperature decrease by the pressure in place of the temperature sensor Schottky diode as large as 40 K is indicated as compared with the vacuum environment. Likewise, a good correlation with the experiment is observed.
In order to observe the cantilever deflection the classic interferometric method was used. Taking into account the mirror-like light reflection from the surface of the cantilever such a measurement can be performed. The basic optical element of the microscopic laser interferometer set up was the splitting polarising cube made of two prisms. The interference effect is appeared in the air gap between the cantilever surface and outlying flat surface of the splitting cube. The amount of the light reflected from the glass surface of the cube is enough to create contrast interference fringes with the light from mirror-like cantilever surface. The cantilever surface was perpendicularly illuminated by He–Ne cw laser through the splitting cube fixed on the adjustable holder. The laser beam was extended using the pinhole and collimator. To secure the appropriate magnification of the interference pattern the microscopic observation of the cantilever was used. In this optical scheme the microscopic objective created the magnified image which was finally projected by means of projecting eyepiece onto the film of photographic camera. The interference pattern obtained was
4. Cantilever beam bimetallic effect The decrease of the microsystem cantilever thickness from 8 mm to 2 mm caused the cantilever bending. We
Fig. 7. Cantilever deflection shapes for different power dissipations in the microsystem heater.
T. Lalinsky´ et al.r Sensors and Actuators 76 (1999) 241–246
245
Fig. 8. Deflection changes by the power sensed on the end of cantilever beam.
evaluated by plotting of interference orders with regard to the reference plane of the glass cube. The cantilever deflection shapes obtained for different power dissipations are shown in Fig. 7. There is the deflection as large as 35 mm observed on the end of cantilever at room temperature Žwithout any power dissipation.. If the cantilever temperature increases by the power dissipated in the heater, the end cantilever deflection seems to be reduced. Fig. 8 shows the deflection changes sensed on the end of cantilever as a function of power dissipations. As seen, a reasonably linear response with an extracted sensitivity of 0.8 mm per mW is indicated. 4.2. Laser optical reflectance method The cantilever deflection changes on the end of cantilever beam were detected also by the optical probing of the inclination of mirror-like micro area on its end. The principle of this method is based on the photoelectrical sensing of transversal motions of the light spot projected by the laser beam after its reflection from the surface. The simple experimental arrangement as shown in Fig. 9 was proposed. It consists of the laser diode Toshiba Ž670 nmr10 mW., the position sensitive detector ŽPSD. and the beam splitting cube, by which the illuminating laser beam is directed onto measured area. The laser light is focusing through the objective of the collimator Ž f s 30 mm., whereby the measured area is precisely positioned in the focus by using microscopic translation stages. The position sensitive detector captures the differences in the electrical signals from two opposite photodiodes on the common substrate. Provided that the laser spot pattern after its reflection is centered on the effective area the linear proportionality exists so, U;2 Da where U is the output signal and D a is the change in the surface inclination at the reflecting point of the bimetal cantilever. The factor 2 results from the law of reflection.
Fig. 9. Experimental set up used for cantilever deflection sensing by the optical reflectance method.
Due to small angle approximation the deflection w of the cantilever is related to the inclination D a also by linear relation dw D a f tgD a s 2 dx Thus, the comparison of the results from both the interferometric and the inclination angle measurements approved the proper function of the photoelectric sensing. The transfer characteristic of the microsystem obtained from the above described optical reflectance measurement is shown in Fig. 10. An excellent linearity as expected is achieved. The response of the position-sensitive detector diode about 4.69 VrW is found. Recently, a micromachined Si cantilever beam has been designed for the first time to sense chemical reaction heats using the cantilever bimetallic effect w11x. A linear response with a measured sensitivity of ; 4.4 nm per nW has also been obtained using the optical reflectance measurements.
Fig. 10. Transfer characteristic of the microsystem obtained from the optical reflectance measurement.
246
T. Lalinsky´ et al.r Sensors and Actuators 76 (1999) 241–246
5. Conclusions Thermal effects in a 2-mm thick GaAs cantilever beam of the power sensor microsystem have been investigated. Due to a very thin cantilever a high power–temperature conversion efficiency was achieved. The thermal resistance values as high as 14 000 KrW, 17 000 KrW and 34 000 KrW were determined for the air, argon and vacuum environments, respectively. The increased microsystem sensitivity to the thermal conductance changes of the ambient atmosphere was confirmed by investigation of the thermal resistance changes in broad range of gas pressures. Thermal resistance changes obtained from simulation and electrical measurements in the range of low pressures Ž P - 200 Pa. were found to be in good qualitative agreement. Bimetallic effect in a 2-mm thick GaAs cantilever beam of the microsystem was studied. The temperature increase in the active area of the GaAs cantilever beam due to by power–temperature conversion in the GaAs MESFET Žas a heater. is detected by measuring the cantilever deflection induced by the differential thermal expansion of the cantilever layers. Two different methods were used to measure the cantilever deflection in order to study the transfer characteristics of the microsystem. The first one, based on a microscopic laser optical interferometry and the second one, based on the photoelectrical sensing of transversal motions of the light spot projected by the laser diode beam after its reflection from the cantilever surface. An excellent linearity in the microsystem transfer characteristics was obtained mainly using the laser optical reflectance measurement. The position-sensitive detector diode response of 4.69 VrW was achieved. It should be noted that no optimization of the GaAs bimetallic cantilever with respect to the microsystem sensitivity was made. The time constant of this optical sensor system is currently being performed to establish its dynamical properties and the main advantages for the possible AC–DC electrical power transfer measurements.
Acknowledgements This work was supported by the COPERNICUS programme, contract No: CIPA-CT94-0197 and the Slovak Grant Agency for Science, grant no. 2r4060r97 and grant no. 95r5305r104.
References w1x D. Jaeggi, H. Baltes, D. Moser, Thermoelectric AC power sensor by CMOS technology, IEEE Electron Device Lett. 13 Ž1992. 366–368. w2x L. Grno, ˇ Thermal wattmeter with direct power conversion, IEEE Trans. Instrum. Meas. 44 Ž1995. 377–385.
ˇ Hascık, ˇ Mozolova, w3x T. Lalinsky, ´ J. Kuzmık, ´ M. Porges, S. ˇˇ´ Z. ´ L. Grno, ˇ Monolithic GaAs MESFET power sensor microsystem, Electron. Lett. 31 Ž1995. 1914–1915. w4x R.A. Adey, Ph. Renaud, MICROSIM II, Simulation and Design of Microsystems and Microstructures, Computational Mechanics Publications, Southampton, UK, 1997, pp. 43–51. ˇ Hascık, ˇ Mozolova, w5x T. Lalinsky, ´ S. ˇˇ´ Z. ´ J. Kuzmık, ´ Z. Hatzopoulos, GaAs power sensor microsystem technology and characterization, Sensors Mater. 10 Ž1998. 241–250. w6x E. Burian, D. Pogany, T. Lalinsky, ´ N. Seliger, E. Gornik, Thermal simulation and characterization of GaAs micromachined powersensor microsystems, Sensors and Actuators A 68 Ž1998. 372–377. w7x A. Dehe, ´ V. Krozer, B. Chen, H.L. Hartnagel, High-sensitivity microwave power sensor for GaAs-MMIC implementation, Electron. Lett. 32 Ž1996. 2149–2150. ˇ Hascık, ˇ Mozolova, w8x S. ˇˇ´ T. Lalinsky, ´ J. Kuzmık, ´ M. Porges, Z. ´ Fabrication of thin GaAs cantilever beams for power sensor microsystem by RIE, Vacuum 47 Ž1996. 1215–1217. ˇ Hascık, ˇ Mozolova, w9x S. ˇˇ´ T. Lalinsky, ´ Z. ´ J. Kuzmık, ´ Patterning of cantilevers for power sensor microsystem, Vacuum Ž1998. in press. w10x A.W. Van Herwaarden, D.C. Van Duyn, J. Groeneweg, Small-size vacuum sensors based on silicon thermopiles, Sensors and Actuators A 25–27 Ž1991. 565–569. w11x J.K. Gimzewski, Ch. Gerber, E. Meyer, R.R. Schlittler, Observation of a chemical reaction using a micromechanical sensor, Chem. Phys. Lett. 217 Ž1994. 589–594. Tibor Lalinsky´ was born in Bratislava, Slovakia, in 1951. He received a degree in engineering from Slovak Technical University, Bratislava in 1974, and PhD degree on GaAs FETs Technology from the Institute of Electrical Engineering, Slovak Academy of Sciences, Bratislava, in 1981. Since 1985, he has been serving as the head of Department of Microelectronic Structures at the above Institute. His research covers the pseudomorphic HFET devices at millimeter wave frequency. Since 1995, he has also headed a research team in the field of design and development of Power Sensor Microsystems based on bulk GaAs micromachining technology.
ˇ ˇ Stefan Hascık Slovakia in 1956. He received his ˇ ´ was born in Zilina, master degree in physical electronics from Comenius University in 1982. Since year 1983, he has been working at the Institute of Electrical Engineering Slovak Academy of Sciences, Bratislava. His research interests are in the field of plasma and reactive ion etching of materials for microelectronics. ˇ ´ Mozolova´ studied at the Slovak Technical University, Bratislava, Zelmıra where she received her Dipl.-Ing. degree in 1968. Since 1974, she has been with the Institute of Electrical Engineering, Slovak Academy of Sciences, Bratislava. Her current research field is processing on GaAs and related structures. Eduard Burian was born in Piest’any, Slovakia in 1967. He received his ˇ Dipl.-Ing. degree from the Slovak Technical University in 1992. Since 1995 he has been with the Institute for Electrical Engineering, Slovak Academy of Sciences, preparing for his PhD degree. His research field implies the application of numerical methods in the theory of semiconductor devices. Milan Drzık ˇ´ was born in Pastuchov, Slovakia, in 1949. In 1968–1973 he studied physics at the Comenius University Bratislava. Since 1975 he has been with the Institute of Construction, Slovak Academy of Sciences. He received his PhD degree on holographic interferometry in 1981. His research interests are aimed to advance the optical as well as optoelectronic methods of applied optics and their applications to solving the problems of mechanics of solids and fluids.
The improved performance of GaAs micromachined power sensor microsystem T. Lalinsky´ a b
a,)
ˇ Hascık ˇ Mozolova´ a, E. Burian a, M. Drzık , S. ˇˇ´ a, Z. ˇ´
b
Institute of Electrical Engineering, SloÕak Academy of Sciences, DubraÕska ´ ´ cesta 9, 842 39 BratislaÕa, SloÕak Republic Institute of Construction and Architecture, SloÕak Academy of Sciences, DubraÕska ´ ´ 9, 842 20 BratislaÕa, SloÕak Republic Received 14 September 1998; received in revised form 5 January 1999; accepted 8 January 1999
Abstract Thermal effects in a 2-mm thick GaAs cantilever beam of the power sensor microsystem are investigated. The increased thermal sensitivity of the microsystem to the thermal conductance changes of the ambient atmosphere is evaluated by the experiment and simulation. A bimetallic effect in the microsystem cantilever beam is studied using both the microscopic laser optical interferometry and laser optical reflectance measurement. The cantilever beam deflections induced by the differential thermal expansion of the cantilever layers are found to be linear with the power dissipated in the microsystem MESFET-heater. The key microsystem transfer characteristics based on the bimetallic effect are obtained. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Power sensors; Galium arsenide; GaAs micromachining; Cantilever bimetallic effect
1. Introduction Transmitted power is the basic quantity measured in RF and microwave systems. The transmitted power measurement is unavoidable in research and development period of any RF and microwave equipment and for monitoring and control of RF working systems. The most precise AC power measurement is based on thermal conversion. The absorbed RF power is converted into thermal power using thermally isolated absorbing element. The resulting heat is sensed by a temperature sensor and by means of substitution or comparison technique converted into electrical DC output quantity. A major advantage of the thermal conversion is its exact physical definition and inherent broad frequency range. There are many techniques to fabricate thermally isolated matched RF termination with integrated temperature sensor. The most recent techniques use CMOS IC technology combined with oriented etching technique w1x.
)
Corresponding author. Tel.: q42-7-375-816; Fax: q42-7-378-2919; E-mail: [email protected]
Recently a new three-terminal thermoconverter ŽTTTC. technique has been successfully developed for AC power measurement w2,3x. The technique is based on active heater utilized to sense directly the transmitted power. The new approach is based on controlled impedance of active heater enabling mirroring of external transmitted power into the heater. The thermal effect of the mirrored power is then converted into DC quantity by means of classical substitution technique. The new measurement technique is principally applicable also in RF and microwave systems. A new concept of power sensor microsystem ŽPSM. was designed w3x to demonstrate the TTTC technique of the power measurement. There was GaAs chosen as the basic electronical as well as micromechanical semiconductor material because of its lower thermal conductivity, higher saturation velocity of electrons and higher temperature working conditions as compared with a Silicon. It consisted of two power-controlled GaAs MESFETs as threeterminal heaters, and Schottky diode as a temperature sensor, monolithically integrated on two 8-mm thick GaAs cantilever beams. GaAs MESFETs processing combined with a GaAs bulk micromachining were applied to fabricate the PSM. The technology permitted precise control of
0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 9 . 0 0 0 3 7 - 0
T. Lalinsky´ et al.r Sensors and Actuators 76 (1999) 241–246
242
the thickness and uniformity of the cantilever beams directly by the thickness of the MBE grown GaAs over an AlGaAs etch-stop layer. The benefit of the PSM technology developed was also verified by investigation of the basic microsystem transfer characteristics in various gaseous environments w4–6x. The advantages of GaAs based power sensors have also been demonstrated in Ref. w7x. There was a micromachined power sensor designed to perform power measurements up to millimetre Žmm.-wave and sub-mm wave regions. A concept of the power sensor was based on a thin Ž1.5 mm. undoped AlGaAsrGaAs membrane. It consisted of NiCr thin film resistors Žas heaters. and integrated GaAs based thermocouples Žas temperature sensors.. In this paper we report on new thermal effects induced in the micromachined power sensor microsystem based on 2-mm thick GaAs cantilever beams. The increased sensitivity of the microsystem to the pressure dependent thermal conductance of gases is investigated by the experiment and simulation. A bimetallic effect in the microsystem cantilever beam is studied using both the microscopic laser optical interferometry and laser optical reflectance method. Fig. 2. Front side view of the 2-mm thick GaAs cantilever beam of the microsystem.
2. Microsystem technology GaAs heterostructure layer design shown in Fig. 1 was used in the microsystem technology. A 2-mm thick GaAs layer grown by MBE at low temperature ŽLT-GaAs. on the top of AlGaAs etch-stop layer defines the thickness of the microsystem cantilever beam. A d-doped layer of Si was used for both the channel of the MESFETs and active layer of the temperature sensing Schottky diode. As mentioned already, GaAs MESFET processing combined with a GaAs bulk micromachining were applied to fabricate the microsystem. A three-dimensional patterning of two independent thin GaAs cantilever beams of the microsystem as described in detail in Ref. w8x was defined by a double side
aligned selective reactive ion etching of GaAs through the opening in masks to the AlGaAs etch-stop layer. A front side view of 2-mm thick GaAs cantilever beam of the microsystem is shown in Fig. 2. There are two power-controlled GaAs MESFETs Žas three-terminal heaters. and GaAs Schottky diode Žas a temperature sensor. clearly seen in a monolithic approach. An excellent thermal isolation of the microsystem devices can be obtained due to by a small thickness of the cantilever. This enables to achieve a very efficient power-temperature conversion in the channel of the MESFETs useful for the precise power measurement. The principle of the measuring method is to balance the unknown power dissipated by one heater by a known power on the second one while maintaining a constant sensor temperature Žabout 50–1008C. sensed by the Schottky diode. Controlling the current under constant voltage or the voltage under constant current on the second heater, a powerrcurrent or powerrvoltage converter, respectively is obtained.
3. Microsystem thermal characterization
Fig. 1. Heterostructure layer design.
The increased sensitivity of the microsystem to the thermal conductance changes of the ambient atmosphere was indicated by decrease of the cantilever beam thickness. In order to confirm the improved microsystem ther-
T. Lalinsky´ et al.r Sensors and Actuators 76 (1999) 241–246
mal performance, the key microsystem transfer characteristics were measured in various gaseous environments. Forward I–U characteristics of the Schottky temperature sensor diode as a function of power dissipation in the MESFET were measured using a semiconductor parameter analyzer ŽHP 4145B.. Power-sensor diode voltage Ž P–U . transfer characteristic under selected DC diode current biasing can be determined. By using a calibrated temperature measurements of the diode characteristics under the same selected current biasing a direct correlation between the sensor diode voltage and corresponding temperature can also be independently found. This enables to construct a direct power–temperature Ž P–T . transfer characteristic of the microsystem w4x. Typical behaviour of the microsystem P–T transfer characteristics obtained for air, argon and vacuum environments is shown in Fig. 3. There is an excellent linearity observed mainly for the air and argon atmosphere. A slope of the P–T curves determines the thermal resistance value of the microsystem. A significant influence of these environments on the thermal resistance value can also be seen. The corresponding thermal resistance values were determined to be 14 000 KrW, 17 000 KrW and 34 000 KrW for the air, argon and vacuum environments, respectively. This is consistent with the decrease of the thermal conductivity of these gaseous media. If the microsystem cantilever thickness is decreased the cantilever surface is appeared to play a significant role in the heat transfer to the ambient atmosphere. As a consequence of this increased sensitivity of the microsystem to the thermal conductance changes of the ambient gaseous environment can be found. This is demonstrated in Fig. 4, where P–T transfer characteristics of the microsystem for different gas pressures are shown. As seen, an excellent linearity is achieved in the range of transition and high pressures Ž P ) 200 Pa.. Thermal resistance values extracted from the slopes of the P–T curves as a function of
Fig. 3. P – T transfer characteristics measured in various gaseous environments.
243
Fig. 4. P – T transfer characteristics of the microsystem for different gas pressures.
gas pressures are represented in Fig. 5. There are the expressive thermal resistance changes observed in the narrow range of gas pressures Ž P ; 100–1000 Pa., while in the range of both low pressures Ž P - 100 Pa. and high pressures Ž P ) 1 kPa. decreased sensitivity is indicated. This could be explained by a different thermal conductance mechanism of the gas in the mentioned ranges of pressures as described in detail in Ref. w10x. The increased sensitivity of the microsystem cantilevers to the pressure dependent thermal conductance of gases could be useful for pressure sensing. However, the initial cantilever area Ž; 130 = 450 mm2 . should be increased to improve the pressure sensitivity of the microsystem. The thermal resistance values of the microsystem as indicated in Fig. 5 in the range of low pressures were also simulated. There was the mean free path of gas molecules assumed to be larger than the distance between the cantilever beam and the bottom of the ceramic housing Ž; 320 mm.. This distance determines the transition pressure w10x,
Fig. 5. Thermal resistance changes of the microsystem by the gas pressure.
244
T. Lalinsky´ et al.r Sensors and Actuators 76 (1999) 241–246
have found that the cantilever bending is due to the different thermal expansion coefficient of the device metallic leads ŽTirPtrAu, 350 nm. and GaAs Ž2 mm.. This was experimentally verified in Ref. w9x. There were 2-mm thick GaAs cantilever beams patterned without any metallic leads and no bending was observed. To study the microsystem cantilever beam deflection as also seen in Fig. 2, two different optical methods were applied. The first one, based on the laser optical interferometry and the second one, based on the laser optical reflectance measurement.
4.1. Laser optical interferometric method
Fig. 6. Ža. Thermal resistance changes of the microsystem at low gas pressures. Žb. Cantilever temperature difference at low gas pressures.
which is of the order of 200 Pa. The temperature in the cantilever was simulated by solving the steady state and time dependent two-dimensional heat flow equation by a finite-difference method w6x. The cantilever temperature was calculated taking into account the heat transfer to an infinite, non-convective gaseous medium around the cantilever. The simulated thermal resistance changes by the pressure at power dissipation of 4 mW are shown in Fig. 6a. As seen, a relatively good agreement with the experiment is achieved. Thermal resistance decrease by the pressure corresponds to the temperature decrease in the microsystem cantilever beam. This is simulated in Fig. 6b. It can be seen that the cantilever temperature decrease by the pressure in place of the temperature sensor Schottky diode as large as 40 K is indicated as compared with the vacuum environment. Likewise, a good correlation with the experiment is observed.
In order to observe the cantilever deflection the classic interferometric method was used. Taking into account the mirror-like light reflection from the surface of the cantilever such a measurement can be performed. The basic optical element of the microscopic laser interferometer set up was the splitting polarising cube made of two prisms. The interference effect is appeared in the air gap between the cantilever surface and outlying flat surface of the splitting cube. The amount of the light reflected from the glass surface of the cube is enough to create contrast interference fringes with the light from mirror-like cantilever surface. The cantilever surface was perpendicularly illuminated by He–Ne cw laser through the splitting cube fixed on the adjustable holder. The laser beam was extended using the pinhole and collimator. To secure the appropriate magnification of the interference pattern the microscopic observation of the cantilever was used. In this optical scheme the microscopic objective created the magnified image which was finally projected by means of projecting eyepiece onto the film of photographic camera. The interference pattern obtained was
4. Cantilever beam bimetallic effect The decrease of the microsystem cantilever thickness from 8 mm to 2 mm caused the cantilever bending. We
Fig. 7. Cantilever deflection shapes for different power dissipations in the microsystem heater.
T. Lalinsky´ et al.r Sensors and Actuators 76 (1999) 241–246
245
Fig. 8. Deflection changes by the power sensed on the end of cantilever beam.
evaluated by plotting of interference orders with regard to the reference plane of the glass cube. The cantilever deflection shapes obtained for different power dissipations are shown in Fig. 7. There is the deflection as large as 35 mm observed on the end of cantilever at room temperature Žwithout any power dissipation.. If the cantilever temperature increases by the power dissipated in the heater, the end cantilever deflection seems to be reduced. Fig. 8 shows the deflection changes sensed on the end of cantilever as a function of power dissipations. As seen, a reasonably linear response with an extracted sensitivity of 0.8 mm per mW is indicated. 4.2. Laser optical reflectance method The cantilever deflection changes on the end of cantilever beam were detected also by the optical probing of the inclination of mirror-like micro area on its end. The principle of this method is based on the photoelectrical sensing of transversal motions of the light spot projected by the laser beam after its reflection from the surface. The simple experimental arrangement as shown in Fig. 9 was proposed. It consists of the laser diode Toshiba Ž670 nmr10 mW., the position sensitive detector ŽPSD. and the beam splitting cube, by which the illuminating laser beam is directed onto measured area. The laser light is focusing through the objective of the collimator Ž f s 30 mm., whereby the measured area is precisely positioned in the focus by using microscopic translation stages. The position sensitive detector captures the differences in the electrical signals from two opposite photodiodes on the common substrate. Provided that the laser spot pattern after its reflection is centered on the effective area the linear proportionality exists so, U;2 Da where U is the output signal and D a is the change in the surface inclination at the reflecting point of the bimetal cantilever. The factor 2 results from the law of reflection.
Fig. 9. Experimental set up used for cantilever deflection sensing by the optical reflectance method.
Due to small angle approximation the deflection w of the cantilever is related to the inclination D a also by linear relation dw D a f tgD a s 2 dx Thus, the comparison of the results from both the interferometric and the inclination angle measurements approved the proper function of the photoelectric sensing. The transfer characteristic of the microsystem obtained from the above described optical reflectance measurement is shown in Fig. 10. An excellent linearity as expected is achieved. The response of the position-sensitive detector diode about 4.69 VrW is found. Recently, a micromachined Si cantilever beam has been designed for the first time to sense chemical reaction heats using the cantilever bimetallic effect w11x. A linear response with a measured sensitivity of ; 4.4 nm per nW has also been obtained using the optical reflectance measurements.
Fig. 10. Transfer characteristic of the microsystem obtained from the optical reflectance measurement.
246
T. Lalinsky´ et al.r Sensors and Actuators 76 (1999) 241–246
5. Conclusions Thermal effects in a 2-mm thick GaAs cantilever beam of the power sensor microsystem have been investigated. Due to a very thin cantilever a high power–temperature conversion efficiency was achieved. The thermal resistance values as high as 14 000 KrW, 17 000 KrW and 34 000 KrW were determined for the air, argon and vacuum environments, respectively. The increased microsystem sensitivity to the thermal conductance changes of the ambient atmosphere was confirmed by investigation of the thermal resistance changes in broad range of gas pressures. Thermal resistance changes obtained from simulation and electrical measurements in the range of low pressures Ž P - 200 Pa. were found to be in good qualitative agreement. Bimetallic effect in a 2-mm thick GaAs cantilever beam of the microsystem was studied. The temperature increase in the active area of the GaAs cantilever beam due to by power–temperature conversion in the GaAs MESFET Žas a heater. is detected by measuring the cantilever deflection induced by the differential thermal expansion of the cantilever layers. Two different methods were used to measure the cantilever deflection in order to study the transfer characteristics of the microsystem. The first one, based on a microscopic laser optical interferometry and the second one, based on the photoelectrical sensing of transversal motions of the light spot projected by the laser diode beam after its reflection from the cantilever surface. An excellent linearity in the microsystem transfer characteristics was obtained mainly using the laser optical reflectance measurement. The position-sensitive detector diode response of 4.69 VrW was achieved. It should be noted that no optimization of the GaAs bimetallic cantilever with respect to the microsystem sensitivity was made. The time constant of this optical sensor system is currently being performed to establish its dynamical properties and the main advantages for the possible AC–DC electrical power transfer measurements.
Acknowledgements This work was supported by the COPERNICUS programme, contract No: CIPA-CT94-0197 and the Slovak Grant Agency for Science, grant no. 2r4060r97 and grant no. 95r5305r104.
References w1x D. Jaeggi, H. Baltes, D. Moser, Thermoelectric AC power sensor by CMOS technology, IEEE Electron Device Lett. 13 Ž1992. 366–368. w2x L. Grno, ˇ Thermal wattmeter with direct power conversion, IEEE Trans. Instrum. Meas. 44 Ž1995. 377–385.
ˇ Hascık, ˇ Mozolova, w3x T. Lalinsky, ´ J. Kuzmık, ´ M. Porges, S. ˇˇ´ Z. ´ L. Grno, ˇ Monolithic GaAs MESFET power sensor microsystem, Electron. Lett. 31 Ž1995. 1914–1915. w4x R.A. Adey, Ph. Renaud, MICROSIM II, Simulation and Design of Microsystems and Microstructures, Computational Mechanics Publications, Southampton, UK, 1997, pp. 43–51. ˇ Hascık, ˇ Mozolova, w5x T. Lalinsky, ´ S. ˇˇ´ Z. ´ J. Kuzmık, ´ Z. Hatzopoulos, GaAs power sensor microsystem technology and characterization, Sensors Mater. 10 Ž1998. 241–250. w6x E. Burian, D. Pogany, T. Lalinsky, ´ N. Seliger, E. Gornik, Thermal simulation and characterization of GaAs micromachined powersensor microsystems, Sensors and Actuators A 68 Ž1998. 372–377. w7x A. Dehe, ´ V. Krozer, B. Chen, H.L. Hartnagel, High-sensitivity microwave power sensor for GaAs-MMIC implementation, Electron. Lett. 32 Ž1996. 2149–2150. ˇ Hascık, ˇ Mozolova, w8x S. ˇˇ´ T. Lalinsky, ´ J. Kuzmık, ´ M. Porges, Z. ´ Fabrication of thin GaAs cantilever beams for power sensor microsystem by RIE, Vacuum 47 Ž1996. 1215–1217. ˇ Hascık, ˇ Mozolova, w9x S. ˇˇ´ T. Lalinsky, ´ Z. ´ J. Kuzmık, ´ Patterning of cantilevers for power sensor microsystem, Vacuum Ž1998. in press. w10x A.W. Van Herwaarden, D.C. Van Duyn, J. Groeneweg, Small-size vacuum sensors based on silicon thermopiles, Sensors and Actuators A 25–27 Ž1991. 565–569. w11x J.K. Gimzewski, Ch. Gerber, E. Meyer, R.R. Schlittler, Observation of a chemical reaction using a micromechanical sensor, Chem. Phys. Lett. 217 Ž1994. 589–594. Tibor Lalinsky´ was born in Bratislava, Slovakia, in 1951. He received a degree in engineering from Slovak Technical University, Bratislava in 1974, and PhD degree on GaAs FETs Technology from the Institute of Electrical Engineering, Slovak Academy of Sciences, Bratislava, in 1981. Since 1985, he has been serving as the head of Department of Microelectronic Structures at the above Institute. His research covers the pseudomorphic HFET devices at millimeter wave frequency. Since 1995, he has also headed a research team in the field of design and development of Power Sensor Microsystems based on bulk GaAs micromachining technology.
ˇ ˇ Stefan Hascık Slovakia in 1956. He received his ˇ ´ was born in Zilina, master degree in physical electronics from Comenius University in 1982. Since year 1983, he has been working at the Institute of Electrical Engineering Slovak Academy of Sciences, Bratislava. His research interests are in the field of plasma and reactive ion etching of materials for microelectronics. ˇ ´ Mozolova´ studied at the Slovak Technical University, Bratislava, Zelmıra where she received her Dipl.-Ing. degree in 1968. Since 1974, she has been with the Institute of Electrical Engineering, Slovak Academy of Sciences, Bratislava. Her current research field is processing on GaAs and related structures. Eduard Burian was born in Piest’any, Slovakia in 1967. He received his ˇ Dipl.-Ing. degree from the Slovak Technical University in 1992. Since 1995 he has been with the Institute for Electrical Engineering, Slovak Academy of Sciences, preparing for his PhD degree. His research field implies the application of numerical methods in the theory of semiconductor devices. Milan Drzık ˇ´ was born in Pastuchov, Slovakia, in 1949. In 1968–1973 he studied physics at the Comenius University Bratislava. Since 1975 he has been with the Institute of Construction, Slovak Academy of Sciences. He received his PhD degree on holographic interferometry in 1981. His research interests are aimed to advance the optical as well as optoelectronic methods of applied optics and their applications to solving the problems of mechanics of solids and fluids.