Proposal of the Thin Film Pirani Vacuum Sensor Still Sensitive Above 1 Atmosphere ABSTRACT INTRODUCTION

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1 P1.11 Propoal of the Thin Film Pirani Vacuum Senor Still Senitive Above 1 Atmophere Takahima Noriaki and Kimura Mituteru Faculty of Engineering, Tohoku Gakuin Univerity 13-1, Chuo-1, Tagajo, Miyagi, , JAPAN kimura@tjcc.tohoku-gakuin.ac.jp ABSTRACT We have extended meaurable preure range of the thin film Pirani vacuum enor that i till enitive above 1 atmophere (atm). In our thin film Pirani vacuum enor, our propoed temperature difference enor of the hort circuit Seebeck-current detection type thermocouple i ued in order to get extremely high enitivity, epecially in high vacuum range. A little temperature difference, which hould be zero under very high vacuum, between the microheater and the thermally iolated heading area from the microheater of the cantilever can be meaured. We have achieved much wider meaurable preure range over 8 digit by ue of our new imple thin film Pirani vacuum enor than that of the traditional one. We could expand the preure enitivity beyond 1 atm. by adoption of the vibration of the ening cantilever due to the udden heating. INTRODUCTION Principle of the thermal vacuum enor, uch a Pirani vacuum enor, i baed on the dependence of the thermal conductivity and convection of the ambient ga on the vacuum preure, namely baed on meauring the heat lo of the hot object [1]. Author (M. Kimura) ha propoed a micro air bridge heater and uggeted that thi micro air bridge heater with a temperature enor will be ued a a flow enor, thermal vacuum enor, etc. to improve their enitivity due to larger interaction area with the air and the hot upended film [2]. Effort on improvement of the enitivity of the thermal vacuum enor, uch a Pirani vacuum enor, by ue of the hot floating membrane and thermopile a a temperature enor intead of the hot wire have been reported [3]. We have reported on the thin film vacuum enor with three pn junction diode a a high enitive temperature enor working like a thermitor (we call thi diode thermitor [4]) and a micro air bridge heater. Thi enor ha about two order or wider enitivity (2x x10 5 Pa) than the traditional thermal vacuum enor, uch a Pirani vacuum enor, and ha very fat repone and very low power conumption [5]. However, we have noticed that it i hard to compenate the effect caued by the ambient temperature change, becaue we have ued the high enitive abolute temperature enor of diode-thermitor. In thi paper we have propoed the new type thin film vacuum enor that ha a cantilever tructure with new temperature difference enor of the hort circuit Seebeck-current detection type thermocouple, which i propoed by u [8], in order to get higher enitivity in the higher vacuum range. Temperature difference, which hould be zero under the higher vacuum, between microheater and thermally iolated heading area from the microheater i meaured under a certain preure. Even a little temperature difference can be meaured in very lower preure range by the ignal amplification. Principle of the Short Circuit Seebeck-Current Type Thermocouple It i well known that the Seebeck coefficient H for ilicon i approximated a a function of electrical reitivity a a hown below [6]: H mk a ln q a % (1) 0, where a 0 ' 5x10-4 #cm, m=2.6 for ilicon (Si), k i the Boltzmann contant, and q i the electron charge. We have noticed that it i eay to decreae the reitivity three or four order, which can extremely increae the hort-circuit Seebeck current of the thermocouple, while the Seebeck coefficient H decreae only everal time [7]. If we adopt the method of the hort-circuit Seebeck current meaurement uing a ingle thermocouple with the n ++ -Si and metal film in tead of the method of the SENSOR+TEST Conference SENSOR 2009 Proceeding II 277

2 open-circuit voltage meaurement uing the traditional thermopile, it i expected that our propoed ingle thermocouple with extremely low reitance can generate higher thermoelectric power W i than W v of a traditional open-circuit thermopile, where ubcript i and v correpond to current meaurement (ingle thermocouple) and voltage meaurement (traditional thermopile), repectively. We can ue the imaginary hort of the OP amplifier to meaure the hort-circuit Seebeck current of the ingle thermocouple enor a hown in Fig. 1. The combination of the ingle thermocouple enor and the OP amplifier will compoe temperature difference enor. Since the Seebeck voltage V t i given by the following equation, V % H BT, (2) t,where BT i the temperature difference between the hot junction and the cold one of the thin film thermocouple. The output voltage V o of the OP amplifier hown in Fig. 1 i given a, V % R I % R V r (3) o f f t /, where I i the hort circuit Seebeck current, R f the feedback reitance of the OP amplifier circuit and r the internal reitance of the thin film thermocouple. So we can get the temperature difference BT a follow, B T % V r / H R (4) o f If we meaure the internal reitance of the thin film thermocouple beforehand, we can ee the temperature difference BT from the above equation (4). We have meaured the Seebeck coefficient H of the phophoru diffued Si layer (n ++ -Si) at 1100 C for 2 hr, and found that it i about V/K. Thermocouple I S R f! + Op Amp V O Fig. 1: Meauring circuit for hort-circuit current of a ingle thermocouple uing imaginary hort of an OP amplifier. EXPERIMENTS 1. Structure and Fabrication Proce of the New Type Thin Film Vacuum Senor We have planned to adopt the temperature difference enor in tead of the abolute temperature enor uch a diode-thermitor in order to get the higher enitivity in the higher vacuum range. Our propoed hort circuit Seebeck-current detection type thermocouple ha more merit than the traditional thermopile due to eay fabrication, compactne and enitivity. In Fig. 2 the chematic view of the new type thin film vacuum enor compoed of the microheater and two temperature difference enor of the hort circuit Seebeck-current detection type thermocouple formed on the SOI cantilever i hown. The microheater i made of the puttering depoited chrome film and the thin film thermocouple i compoed of the phophoru diffued Si layer (n ++ -Si) and / chrome double layer. In Fig. 3 the micrograph of the ening region, which conit of the cantilever with two thin film thermocouple, of the new type thin film vacuum enor i hown. In Fig. 4 the cro ectional view at X-X line hown in Fig. 3 on the fabrication proce of the new type thin film vacuum enor i hown. We have ued the n-soi ubtrate ((100), 7.8/1.0/5508m, /40.0 #cm) a a tarting material. After initial cleaning, the cantilever pattern i formed by chemical etching of the SOI layer and the thermal ilicon dioxide film (SiO 2 film) i formed by thermal oxidation (Fig.4, tep 1). After the lithography of thermal ilicon dioxide film (about 0.58m thick) to open window for the low reitivity (heavily doped) n ++ - Si region ( <10-3 #cm), thermal diffuion uing phophorou OCD-coating diffuion ource at 1100 C for 1 hour i carried out. Then the thermal diffuion for p + region formation of pn diode i carried out uing the 278 SENSOR+TEST Conference SENSOR 2009 Proceeding II

3 Contact C Si Subtrate (n-soi) Microheater Contact A Contact B Diode-Thermitor X X" Electrode Thermocouple A Slit Thermocouple B Area of Microphotograph in Fig.3 Fig. 2: Schematic view of the new type thin film vacuum enor. Cantilever Fig.3: Micrograph of the ening region of the new type thin film vacuum enor. n-soi Microheater (A) (C) Diode-Thermitor Thermocouple A Thermocouple B (B) (D) SiO 2 Si ubtrate (n-soi) -Cr (Heater) (Electrode) Fig.4: Fabrication proce of the new type thin film vacuum enor. boron diffuion ource of the OCD at 1100 C for 1 hour (Fig.4, tep 2). The thickne of n ++ -Si diffued layer (auming an average impurity concentration of about 1T10 20 cm -3 in thi layer) i etimated to be about 1 8m thick. The Ti/chrome/ metal (total thickne: about 0.3 8m) of a pair material in a ingle thermocouple i depoited by RF puttering and patterned uing the etching proce (Fig.4, tep 3). The thin film thermocouple i compoed of the double layer of n ++ -Si and Ti/chrome/ metal film electrically iolated via the thermal SiO 2 film (about 0.5 8m thick) formed on the SOI layer a hown in Fig. 4(tep 3 and tep 4). The backide cavity to releae the cantilever from the SOI ubtrate i formed uing the aniotropic etchant of hydrazine for Si (Fig.4, tep 4). The thickne of the puttering depoited chrome thin film of the microheater formed on the SiO 2 layer on the SOI cantilever i controlled o a to be about 50 #, and the top depoited layer of the film in the region of the microheater i etched away to increae it electrical reitance. The internal reitance r of the fabricated thin film thermocouple in thi experiment i about 57 #. Slit are formed in the SOI cantilever in order to make thermal reitance both between the ubtrate and microheater, and between the region A and B. 2. Meaurement of Vacuum Preure In Fig. 5 the meauring circuit for the new type thin film vacuum enor uing the hort Seebeckcurrent detection type thermocouple i hown. SENSOR+TEST Conference SENSOR 2009 Proceeding II 279

4 T B Region B Meaurement point P1 Thermocouple B T A Region A Thermocouple A n ++ -Si R f Subtrate T C! + Op Amp Output Meaurement point P2 Fig.5: Schematic diagram of the meaurement circuit for the propoed thin film vacuum enor. Fig.6: Output waveform obtained from the thermocouple B when microheater i driven a heating for 100 mec and it cooling for 100 mec Two thermocouple (thermocouple A and thermocouple B) of a pair of the n ++ -Si layer and Ti/chrome/ metal film (mainly thin film due to the low electrical reitance) are formed on the SOI cantilever. The thermocouple A can meaure the temperature difference between the point in the region A, on where the microheater i formed, and the Si ubtrate, and the thermocouple B can do between the region A and the point in the region B thermally iolated from the region A by the lit. The cold junction and the hot junction of the thermocouple B will be the junction formed in the region B and the junction (region) formed in the region A, repectively. The ame (Ti/chrome/) metal film lead-line i extended from the Ohmic junction point with the n ++ -Si layer in the region A to the Si ubtrate rim to form the hot junction for the thermocouple B. Temperature difference between the region A and the thermally iolated region B will be zero under the higher vacuum due to the cantilever tructure. Therefore, the Seebeck current of the thermocouple B will be zero, namely the output voltage of the OP amplifier from the thermocouple B hown in Fig. 5 will alo be zero under the high vacuum, uch a 1x10-4 Pa. The microheater i driven by contant current of about 40 ma, and the increaed temperature wa about 190 C at thi contant current under the high vacuum. In our vacuum enor, the output voltage ha remained by only about 10 mv for Rf =100 k# even under 1x10-4 Pa. Therefore, we have ubtracted the remaining output voltage value from the data obtained from the vacuum preure meaurement. Fig.6 how output waveform obtained from the thermocouple B when microheater i driven a heating for 100 mec and it cooling for 100 mec. The vacuum preure i meaured at the meaurement point P1 for lower preure than about 0.5 atm. in Fig.6. In Fig. 7 the linear caled characteritic of the output voltage V o v. the vacuum preure p, meaured at P1 in Fig.6, i hown. In Fig. 8 the logarithmical caled characteritic hown in Fig. 7 i hown. We can ee that we can meaure the wide preure range of about 2x10-3 1x10 5 Pa (1 atm.) by ue of our vacuum enor with our newly developed Seebeck-current detection type thermocouple a hown in Fig. 7and in Fig. 8. Fig.7: Linear caled characteritic of the output voltage V o v. the vacuum preure p. Fig.8: Logarithmical caled characteritic of the output voltage V o v. the vacuum preure p. 280 SENSOR+TEST Conference SENSOR 2009 Proceeding II

5 Fig.9: Schematic ide view of the thin film Pirani vacuum enor with vibration of the cantilever ening region. Fig.10: Improvement of the preure enitivity above 1 atm. due to the vibration of the cantilever. In Fig. 9 the chematic ide view of the thin film Pirani vacuum enor developed for high preure meaurement, in which the ening cantilever i vibrating due to heating and cooling of the microheater, i hown. Thi meaurement method i baed on the air convection cooling due to the vibration of the thin film cantilever compoed of double layer of a SOI layer and a BOX (SiO 2 ) layer, on which a microheater and two temperature-difference enor are formed. The cantilever will vibrate when it i heated up and cooled down baed on it on and off tate, like a bimetal thermal bending, becaue double layer of the cantilever have very large difference in their thermal expanion coefficient. In our experiment the microheater i driven o a to be heating for 15 mec and cooling alo for 20 mec in order to repeatedly heat up to about 190 C of the aturated temperature at 100mW under the higher preure than near 1 atm. In thi experiment with ening-cantilever vibration the maximum amplitude of the output voltage Vo at P2 hown in Fig.6, which i correponding to the maximum temperature difference between the A and the B thermocouple, are meaured after the microheater i turned off, becaue we can acquire the table and reproducible preure data due to vibration effect of the ening cantilever. In Fig.10 the output characteritic of the thin film Pirani vacuum enor with the cyclic vibration mode and the non vibration mode are hown. We can ee that the output voltage in high preure range near 1 atm how aturation characteritic a hown in Fig.7, however, a hown in Fig.10, increae in the output voltage Vo of the vibration mode i oberved a increaing the preure p, while the decreae in that of the non vibration mode i oberved up to the preure of 2.6 atm. Thee effect with the vibration mode may be due to heat tranfer by air convection through the urrounding high preure air. CONCLUSIONS We have propoed the thin film vacuum enor that ha a cantilever tructure with new temperature difference enor of the hort circuit Seebeck-current detection type thermocouple. In our experiment uing the prototype enor, very wide vacuum preure range between 10-3 Pa and 10 5 Pa can be reproducibly meaured. We could expand the preure enitivity range from 1x10 5 Pa to 2.6x10 5 Pa by adoption of vibration of the ening cantilever. REFERENCES [1] R. Puer, S. Reyntjen and D. De Bruyker, The Nano Pirani an extremely miniaturized preure enor fabricated by focued ion beam rapid prototyping, Senor and Actuator A: Phyical, Vol , pp (2002). [2] M. Kimura, Microheater and microbolometer uing microbridge of SiO 2 film on ilicon, Electron. Letter, Vol.17, No.2, pp (1981). [3] A. W. Van Herwaarden, P. M. Sarro and H. C. Meijer, Integrated vacuum enor, Senor and Actuator,Vol. 8, No. 3, pp (1985). [4] M. Kimura and K. Tohima, Thermitor-like pn junction temperature-senor with variable enitivity and it combination with a micro air bridge heater, Senor and Actuator, A108, pp (2003). [5] Mituteru Kimura, Fumitohi Sakurai, Hirao Ohta and Tomoyuki Terada, Propoal of A New Structural Thermal Vacuum Senor with Diode-thermitor Combined with a Micro-Air-Bridge Heater, Microelectronic J. Vol.38, pp (2007). [6] A. W. Van. Herwaarden, and P. M. Sarro, Thermal enor baed on the Seebeck effect, Senor and Actuator A Vol. 10, pp (1986). [7] Seung Seoup Lee, Mituteru Kimura, Short-circuit meaurement by Seebeck current detection of a ingle thermocouple and it application, Senor and Actuator, A139, pp (2007). SENSOR+TEST Conference SENSOR 2009 Proceeding II 281