- Email: [email protected]
A planar integrated chlorinated hydrocarbon gas sensor on a silicon substrate
Sensors and Actuators
B, 5 (1991)
199-203
A planar integrated chlorinated on a silicon substrate
199
hydrocarbon
gas sensor
D. Keyvani, J. Maclay and S. Lee, Microelectronics
Laboratory,
Universiiy
of Illinois at Chicago, 840 W. Taylor Street,
Chicago, IL 60607 (U.S.A.)
J. Stetter and Z. Cao Transducers
Research Inc., 999 W. Chicago Avenue, Napervilie,
IL 60540 (U.S.A.)
Abstract A two-electrode conductivity sensor with an integral heater has been fabricated on a I pm thick thermally grown oxide on a silicon wafer using microfabrication techniques. The device contains a four-terminal heater and two two-electrode conductivity sensors with spacings of 0.2 mm and 0.6 mm. A sputtered or an evaporated 5000 A thick LaF, film on top of the platinum electrodes is used as a semiconducting material that is sensitive to the presence of chlorinated hydrocarbon vapor. The 2300 8, thick platinum heater is capable of heating the desired part of the microsensor to 700 “C, when it starts glowing red. To reduce the heat lost through the substrate and to lower the power consumption of the device, the heated area of the sensor is etched anisotropically to a membrane 25 pm thick. The heater is characterized and its temperature for different applied d.c. voltages between any of its terminals measured. The system can provide information on whether or not the gas sample contains chlorinated hydrocarbon vapor. The microsensor response due to different concentrations of trichloroethylene (TCE) vapor in air has been studied at various temperatures.
1. Introduction
A conventional non-microfabricated sensor has been reported for specific detection of chlorinated hydrocarbon vapors [ 11.The sensor is based on an inner and an outer electrode separated by a rare-earth ionic semiconductor, and its characteristics have been described [2, 31. The materials of construction include mixtures of LaF, and LazO, and the operating temperature of the sensor is reported to be about 500 “C. In the presence of chlorinated hydrocarbon vapors the resistance of the materials decreases. In this study, our efforts were centered on microfabricating a sensor which would have electrodes measuring the resistance change of the sensitive materials in the presence of chlorinated organic vapors and a heater which can bring the surface temperature of the microsensor up to 700 “C in the selected area. The microfabricated sensor has definite advantages because: (1) it is smaller than the conventional sensor and can be used in situations where
the size of the device is critical; (2) precise control of the structure makes it easy to fabricate and modify the desired geometry; (3) it is less expensive; and (4) there is a potential for mass production. In order to microfabricate the sensor, the adhesion problems between the platinum film and oxidized silicon substrate were solved without using any promoting interfacial layer, such as chromium or titanium, which can lead to electrochemical corrosion, or roughening the surface, which can lead to cracks in the silicon wafer or platinum film due to hot spots on the heater [4].
2. Experimental 2. I. Characteristics of the microsensor The complete geometry of the microsensor is illustrated in Fig. 1. The sensor consists of two pairs of platinum electrodes and an adjacent four-terminal platinum heater. The sensor was shaped using photolithographic
@ 1991 -
Elsevier Sequoia,
Lausanne
200 Pt
ELECTRODES
Pt
HEATER
m
pt
m
Si
[7
SiOz
m
LaFS
6.6 x 5.7
-
Pt
HEATER
Pt
ELECTRODES
Fig. 1. Planar configuration of the microfabricated gas sensor on a silicon wafer (all dimensions are in millimeters).
techniques. Two different conduction paths of 0.2 mm and 0.6 mm separations are available. In the case of resistive material like LaF, , the shorter path may be preferred. Any two terminals of the four-terminal heater which include the middle resistor of the heater can be used to heat the microsensor by applying a d.c. voltage between them. The resulting temperature is obtained by measuring the change of the platinum film resistance of the other two terminals. The heated area, which is around the sensing electrodes, is etched anisotropically to a thin membrane 25 ,um thick to reduce the heat lost through the substrate and to reduce heating at the wiring connections. To increase thermal isolation and ensure electrical insulation, a 1 pm thick Si02 film was grown thermally on a low doped p-type (100) silicon wafer using a wet oxidation technique before depositing the platinum film. 2.2. Fabrication of the microsensor The silicon wafer was etched using the wet etching technique in a selected region to make a 25 pm membrane area upon which the sensor was fabricated. The anisotropic silicon etchant, CsOH, etches the silicon crystal with a higher rate along the (100) direction than along the (111) direction [ 51. The silicon etching rate using CsOH solution in water is very temperature dependent and decreases as the temperature decreases. A vacuum evaporation system was used to deposit the 2300 A thick platinum film on the substrate by the electron-beam evaporation tech-
nique. To improve the adhesion between the platinum film and the oxidized silicon wafer, the substrate was annealed in a furnace at 600 “C for fifty minutes in the presence of nitrogen gas immediately after the platinum film deposition. After producing a platinum film well adhered to the substrate, photolithographic techniques were used to shape the sensor. The platinum etchant consisted of 40% HCl, 40% HNO, and 20% Dl water and the solution was heated to 40 “C. A sputtered or an evaporated LaF, film was deposited over the electrodes as the sensing material. A shadow mask was made for this purpose by etching a silicon wafer all the way through utilizing the photomask prepared for making the thin membrane. The thickness of the LaF, film was varied from 2500 to 5000 A. It was noticed that the sputtered film adheres to the substrate much better than the evaporated film, especially after significant use at elevated temperatures or long usage of the microsensor. However, depending on the sputtering rate and argon pressure, the sputtered LaF, film has a different atomic composition (X = [F]/[La]) from that of the evaporated film [6]. 2.3. Experimental apparatus Since this device is a gas sensor, it is necessary to convert the aqueous trichloroethylene (TCE) sample into a vapor sample suitable for analysis by the gas sensor. As is illustrated in Fig. 2, a small air pump is used to push the carrier gas, air, toward the sensor. Flow meter # 1 is used to dilute the vapor sample with air and a three-way solenoid valve is used for switching the air from going to the aqueous sample or by-passing it and providing clean zero-TCE-concentration air. The two-way valve is used to prevent diffusion of vapor into the microsensor when the carrier gas is by-passing the test solution. A heater is used to heat incoming air to a constant temperature in order to eliminate the cooling effect of the air flow on the sensor. As is shown in Fig. 2, the sensor is operated with an isolated d.c. power supply for the heater and electrodes.
201
Fig. 2. Experimental
apparatus used to expose the sensor to TCE vapor.
3. Results and discussion 3.1. Characteristics of the heater The integral 2300 A thick platinum film heater with sheet resistivity of 0.43 Q/square has the capability of providing enough thermal energy to increase the surface temperature of the desired part of the microsensor from 23 to 600 “C in 15 s. To have a stable and uniform heat distribution (which is the base of having a long-lifetime heater), a very smooth substrate surface and a uniform sheet resistivity over the entire platinum film are essential. Also, having a thick platinum film reduces the effects of mismatching the coefficients of thermal expansion of the substrate and the platinum film, and eliminates the related problems. The resistance of the platinum resistor increases slightly after the first one or two heating and cooling cycles, but it remains constant thereafter. The temperature coefficient of resistance (TCR) of the platinum film was found to be about 0.0033 f 0.0003 K-’ for the temperature range 23 to 120 “C using R = R,,(l + c( AT) or
(1)
a = (R - R,)/(R, AT) where o! = temperature coefficient of resistance (K-l). The TCR value for bulk platinum is about 0.003 K-l, which is about 18% higher than
Fig. 3. The resistance of the platinum perature.
heater vs. tem-
the value we measured experimentally [7]. Figure 3 shows the resistance change of the platinum film resistor versus temperature by heating the substrate using its integral heater. Equation ( 1) does not hold for high temperatures where the R versus T relation is not linear. The heater’s temperature is controlled by the applied voltage between any of its two terminals ( 1,2 or 3,4 or 1,4 or 2,3 as shown in Fig. 1). The surface temperature of the heated area was monitored by using a constantan-copper thermocouple and measuring the resistance change of the platinum film resistor utilizing the other two terminals of the four-terminal heater. The applied voltage can be set to maintain a constant temperature even though there may be a systematic error in the temperature measurement itself. Figure 4 shows the relation between applied voltage
202
9.76
E
-
9.72
W z
9.70
B
9.66
9.66
Fig. 4. Temperature-voltage characteristic of the heater. The numbers on the experimental points give the temperatures in “C.
and resulting temperature for this device. The surface temperature gradient on the substrate from the hottest area to the farthest point was measured by a constantan-copper thermocouple to be less than l/2. For instance, when the temperature in the heated area is about 600 “C, the temperature on the wired connections is about 260 “C. Also the temperature gradient in the etched area (thin membrane) is about 15 “C from the hottest point to the coolest point. The heater I- V characteristic curve is shown in Fig. 5. 3.2. Microsensor response The response of the sensor is measured as a change in the voltage across a 10 kR dropping resistor connected in series with the platinum electrodes by a Keithley 617 electrometer as shown in Fig. 2. A potential of 30 V
9.64
I,.....
10.0
"‘20.0
TIME (set)
40.0
Fig. 6. Sensor response to 5500 and 7500 ppm of TCE vapor in air. Total flow was kept constant at 941 ml/ min.
d.c. was applied between the dropping resistor and the platinum electrodes. In the presence of the chlorinated hydrocarbon vapors, the resistance of the LaF, film decreases and that causes the current through the loop to increase, which makes the voltage drop across the 10 kR resistor increase. The sensitivity of the chlorinated hydrocarbon microsensor is highly dependent on the heater voltage that controls the temperature of the sensor. The device does not respond to trichloroethylene vapor for temperatures below 400 “C, but as the temperature increases to 550 “C the sensor responds almost immediately to the presence of the gas. For higher temperatures the signal amplitude increases slightly. However, in order to increase the lifetime of the gas sensor, a lower operating temperature of 550 “C is preferred. The sensor response to concentrations of 5500 and 7500 ppm of trichloroethylene is shown in Fig. 6.
4. Conclusions
Fig. 5. I-V characteristic of the heater. The numbers on the experimental points give the temperatures in “C.
The heated two-electrode conductivity sensor was fabricated using microfabrication techniques that offer the advantages of low
203
cost, durability, batch fabrication, the solidstate design and small size. The heater structure can heat the active part of the microsensor to operating temperature in a few seconds by applying a d.c. voltage between two of its four terminals. The resulting temperature can be determined by measuring the resistance change of the platinum resistor utilizing the other two terminals. Anisotropitally etching the silicon underneath the heated area to a 25 pm thick membrane effectively confines the heat around the sensing material. The heater resistance remains constant after one or two heating and cooling cycles. The heater lifetime has not been determined, although sometimes it was left on for more than 12 h at 550 “C. The microfabricated sensor was found to have significant responses in the high concentration range. The tested gas was 5500 or 7500 ppm of TCE vapor in air. The response is very temperature dependent and the sensor does not respond to the TCE gas at temperatures lower than 400 “C. To increase the sensitivity of the device different structures can be investigated. Some of the possible structures include the use of LaF, and La,03 combinations for the sensing material and the use of electrode configurations in the form of capacitors. A
smaller lower power device would be an obvious extension of this work.
References J. R. Stetter and Z. Cao, Gas sensor and permeation apparatus for the determination of chlorinated hydrocarbons in water, Anal. Chem., 62 (2) (1990) 182-185. Z. Cao and J. R. Stetter, A real-time monitor for chlorinated organics in water, 199OEPA/A& WMA Int. Symp. on Measurement of Toxic and Related Air Pollutants, Raleigh, NC, U.S.A., Apr. 30-May 3, 1990, pp. 836-848. Z. Cao and J. R. Stetter, A selective solid state gas sensor for halogenated hydrocarbons, Proc. Third Int. Meet. Chemical Sensors, Cleveland, OH, U.S.A., Sept. 1990, pp. 196-200. 4 J. G. Maclay, W. J. Buttner and J. R. Stetter, Microfabricated amperometric gas sensors, IEEE Trans. Electron Devices, ED-35 (1988) 793-799. 5 M. J. Declercq, L. Gezberg and J. D. Meidl, Optimiza-
tion of the hydrazine-water solution for anisotropic etching of silicon in integrated circuit technology. J. Electrochem. Sot.: Solid-State Sci. Technol., I22 (4) (1975) 545-552. 6 T. Katsube, M. Hara, I. Serizawa, N. Ishibashi, N.
Adachi, N. Miura and N. Yamazoe, MOS-type micro-oxygen sensor using LaF, workable at room temperature. Jpn. J. Appl. Phys., 29 ( 1990) 1392- 1395. 7 C. J. Smithells and E. A. Brandes, Metals Reference Rook, Butterworths, Boston, MA, 1976, pp. 941-942.
B, 5 (1991)
199-203
A planar integrated chlorinated on a silicon substrate
199
hydrocarbon
gas sensor
D. Keyvani, J. Maclay and S. Lee, Microelectronics
Laboratory,
Universiiy
of Illinois at Chicago, 840 W. Taylor Street,
Chicago, IL 60607 (U.S.A.)
J. Stetter and Z. Cao Transducers
Research Inc., 999 W. Chicago Avenue, Napervilie,
IL 60540 (U.S.A.)
Abstract A two-electrode conductivity sensor with an integral heater has been fabricated on a I pm thick thermally grown oxide on a silicon wafer using microfabrication techniques. The device contains a four-terminal heater and two two-electrode conductivity sensors with spacings of 0.2 mm and 0.6 mm. A sputtered or an evaporated 5000 A thick LaF, film on top of the platinum electrodes is used as a semiconducting material that is sensitive to the presence of chlorinated hydrocarbon vapor. The 2300 8, thick platinum heater is capable of heating the desired part of the microsensor to 700 “C, when it starts glowing red. To reduce the heat lost through the substrate and to lower the power consumption of the device, the heated area of the sensor is etched anisotropically to a membrane 25 pm thick. The heater is characterized and its temperature for different applied d.c. voltages between any of its terminals measured. The system can provide information on whether or not the gas sample contains chlorinated hydrocarbon vapor. The microsensor response due to different concentrations of trichloroethylene (TCE) vapor in air has been studied at various temperatures.
1. Introduction
A conventional non-microfabricated sensor has been reported for specific detection of chlorinated hydrocarbon vapors [ 11.The sensor is based on an inner and an outer electrode separated by a rare-earth ionic semiconductor, and its characteristics have been described [2, 31. The materials of construction include mixtures of LaF, and LazO, and the operating temperature of the sensor is reported to be about 500 “C. In the presence of chlorinated hydrocarbon vapors the resistance of the materials decreases. In this study, our efforts were centered on microfabricating a sensor which would have electrodes measuring the resistance change of the sensitive materials in the presence of chlorinated organic vapors and a heater which can bring the surface temperature of the microsensor up to 700 “C in the selected area. The microfabricated sensor has definite advantages because: (1) it is smaller than the conventional sensor and can be used in situations where
the size of the device is critical; (2) precise control of the structure makes it easy to fabricate and modify the desired geometry; (3) it is less expensive; and (4) there is a potential for mass production. In order to microfabricate the sensor, the adhesion problems between the platinum film and oxidized silicon substrate were solved without using any promoting interfacial layer, such as chromium or titanium, which can lead to electrochemical corrosion, or roughening the surface, which can lead to cracks in the silicon wafer or platinum film due to hot spots on the heater [4].
2. Experimental 2. I. Characteristics of the microsensor The complete geometry of the microsensor is illustrated in Fig. 1. The sensor consists of two pairs of platinum electrodes and an adjacent four-terminal platinum heater. The sensor was shaped using photolithographic
@ 1991 -
Elsevier Sequoia,
Lausanne
200 Pt
ELECTRODES
Pt
HEATER
m
pt
m
Si
[7
SiOz
m
LaFS
6.6 x 5.7
-
Pt
HEATER
Pt
ELECTRODES
Fig. 1. Planar configuration of the microfabricated gas sensor on a silicon wafer (all dimensions are in millimeters).
techniques. Two different conduction paths of 0.2 mm and 0.6 mm separations are available. In the case of resistive material like LaF, , the shorter path may be preferred. Any two terminals of the four-terminal heater which include the middle resistor of the heater can be used to heat the microsensor by applying a d.c. voltage between them. The resulting temperature is obtained by measuring the change of the platinum film resistance of the other two terminals. The heated area, which is around the sensing electrodes, is etched anisotropically to a thin membrane 25 ,um thick to reduce the heat lost through the substrate and to reduce heating at the wiring connections. To increase thermal isolation and ensure electrical insulation, a 1 pm thick Si02 film was grown thermally on a low doped p-type (100) silicon wafer using a wet oxidation technique before depositing the platinum film. 2.2. Fabrication of the microsensor The silicon wafer was etched using the wet etching technique in a selected region to make a 25 pm membrane area upon which the sensor was fabricated. The anisotropic silicon etchant, CsOH, etches the silicon crystal with a higher rate along the (100) direction than along the (111) direction [ 51. The silicon etching rate using CsOH solution in water is very temperature dependent and decreases as the temperature decreases. A vacuum evaporation system was used to deposit the 2300 A thick platinum film on the substrate by the electron-beam evaporation tech-
nique. To improve the adhesion between the platinum film and the oxidized silicon wafer, the substrate was annealed in a furnace at 600 “C for fifty minutes in the presence of nitrogen gas immediately after the platinum film deposition. After producing a platinum film well adhered to the substrate, photolithographic techniques were used to shape the sensor. The platinum etchant consisted of 40% HCl, 40% HNO, and 20% Dl water and the solution was heated to 40 “C. A sputtered or an evaporated LaF, film was deposited over the electrodes as the sensing material. A shadow mask was made for this purpose by etching a silicon wafer all the way through utilizing the photomask prepared for making the thin membrane. The thickness of the LaF, film was varied from 2500 to 5000 A. It was noticed that the sputtered film adheres to the substrate much better than the evaporated film, especially after significant use at elevated temperatures or long usage of the microsensor. However, depending on the sputtering rate and argon pressure, the sputtered LaF, film has a different atomic composition (X = [F]/[La]) from that of the evaporated film [6]. 2.3. Experimental apparatus Since this device is a gas sensor, it is necessary to convert the aqueous trichloroethylene (TCE) sample into a vapor sample suitable for analysis by the gas sensor. As is illustrated in Fig. 2, a small air pump is used to push the carrier gas, air, toward the sensor. Flow meter # 1 is used to dilute the vapor sample with air and a three-way solenoid valve is used for switching the air from going to the aqueous sample or by-passing it and providing clean zero-TCE-concentration air. The two-way valve is used to prevent diffusion of vapor into the microsensor when the carrier gas is by-passing the test solution. A heater is used to heat incoming air to a constant temperature in order to eliminate the cooling effect of the air flow on the sensor. As is shown in Fig. 2, the sensor is operated with an isolated d.c. power supply for the heater and electrodes.
201
Fig. 2. Experimental
apparatus used to expose the sensor to TCE vapor.
3. Results and discussion 3.1. Characteristics of the heater The integral 2300 A thick platinum film heater with sheet resistivity of 0.43 Q/square has the capability of providing enough thermal energy to increase the surface temperature of the desired part of the microsensor from 23 to 600 “C in 15 s. To have a stable and uniform heat distribution (which is the base of having a long-lifetime heater), a very smooth substrate surface and a uniform sheet resistivity over the entire platinum film are essential. Also, having a thick platinum film reduces the effects of mismatching the coefficients of thermal expansion of the substrate and the platinum film, and eliminates the related problems. The resistance of the platinum resistor increases slightly after the first one or two heating and cooling cycles, but it remains constant thereafter. The temperature coefficient of resistance (TCR) of the platinum film was found to be about 0.0033 f 0.0003 K-’ for the temperature range 23 to 120 “C using R = R,,(l + c( AT) or
(1)
a = (R - R,)/(R, AT) where o! = temperature coefficient of resistance (K-l). The TCR value for bulk platinum is about 0.003 K-l, which is about 18% higher than
Fig. 3. The resistance of the platinum perature.
heater vs. tem-
the value we measured experimentally [7]. Figure 3 shows the resistance change of the platinum film resistor versus temperature by heating the substrate using its integral heater. Equation ( 1) does not hold for high temperatures where the R versus T relation is not linear. The heater’s temperature is controlled by the applied voltage between any of its two terminals ( 1,2 or 3,4 or 1,4 or 2,3 as shown in Fig. 1). The surface temperature of the heated area was monitored by using a constantan-copper thermocouple and measuring the resistance change of the platinum film resistor utilizing the other two terminals of the four-terminal heater. The applied voltage can be set to maintain a constant temperature even though there may be a systematic error in the temperature measurement itself. Figure 4 shows the relation between applied voltage
202
9.76
E
-
9.72
W z
9.70
B
9.66
9.66
Fig. 4. Temperature-voltage characteristic of the heater. The numbers on the experimental points give the temperatures in “C.
and resulting temperature for this device. The surface temperature gradient on the substrate from the hottest area to the farthest point was measured by a constantan-copper thermocouple to be less than l/2. For instance, when the temperature in the heated area is about 600 “C, the temperature on the wired connections is about 260 “C. Also the temperature gradient in the etched area (thin membrane) is about 15 “C from the hottest point to the coolest point. The heater I- V characteristic curve is shown in Fig. 5. 3.2. Microsensor response The response of the sensor is measured as a change in the voltage across a 10 kR dropping resistor connected in series with the platinum electrodes by a Keithley 617 electrometer as shown in Fig. 2. A potential of 30 V
9.64
I,.....
10.0
"‘20.0
TIME (set)
40.0
Fig. 6. Sensor response to 5500 and 7500 ppm of TCE vapor in air. Total flow was kept constant at 941 ml/ min.
d.c. was applied between the dropping resistor and the platinum electrodes. In the presence of the chlorinated hydrocarbon vapors, the resistance of the LaF, film decreases and that causes the current through the loop to increase, which makes the voltage drop across the 10 kR resistor increase. The sensitivity of the chlorinated hydrocarbon microsensor is highly dependent on the heater voltage that controls the temperature of the sensor. The device does not respond to trichloroethylene vapor for temperatures below 400 “C, but as the temperature increases to 550 “C the sensor responds almost immediately to the presence of the gas. For higher temperatures the signal amplitude increases slightly. However, in order to increase the lifetime of the gas sensor, a lower operating temperature of 550 “C is preferred. The sensor response to concentrations of 5500 and 7500 ppm of trichloroethylene is shown in Fig. 6.
4. Conclusions
Fig. 5. I-V characteristic of the heater. The numbers on the experimental points give the temperatures in “C.
The heated two-electrode conductivity sensor was fabricated using microfabrication techniques that offer the advantages of low
203
cost, durability, batch fabrication, the solidstate design and small size. The heater structure can heat the active part of the microsensor to operating temperature in a few seconds by applying a d.c. voltage between two of its four terminals. The resulting temperature can be determined by measuring the resistance change of the platinum resistor utilizing the other two terminals. Anisotropitally etching the silicon underneath the heated area to a 25 pm thick membrane effectively confines the heat around the sensing material. The heater resistance remains constant after one or two heating and cooling cycles. The heater lifetime has not been determined, although sometimes it was left on for more than 12 h at 550 “C. The microfabricated sensor was found to have significant responses in the high concentration range. The tested gas was 5500 or 7500 ppm of TCE vapor in air. The response is very temperature dependent and the sensor does not respond to the TCE gas at temperatures lower than 400 “C. To increase the sensitivity of the device different structures can be investigated. Some of the possible structures include the use of LaF, and La,03 combinations for the sensing material and the use of electrode configurations in the form of capacitors. A
smaller lower power device would be an obvious extension of this work.
References J. R. Stetter and Z. Cao, Gas sensor and permeation apparatus for the determination of chlorinated hydrocarbons in water, Anal. Chem., 62 (2) (1990) 182-185. Z. Cao and J. R. Stetter, A real-time monitor for chlorinated organics in water, 199OEPA/A& WMA Int. Symp. on Measurement of Toxic and Related Air Pollutants, Raleigh, NC, U.S.A., Apr. 30-May 3, 1990, pp. 836-848. Z. Cao and J. R. Stetter, A selective solid state gas sensor for halogenated hydrocarbons, Proc. Third Int. Meet. Chemical Sensors, Cleveland, OH, U.S.A., Sept. 1990, pp. 196-200. 4 J. G. Maclay, W. J. Buttner and J. R. Stetter, Microfabricated amperometric gas sensors, IEEE Trans. Electron Devices, ED-35 (1988) 793-799. 5 M. J. Declercq, L. Gezberg and J. D. Meidl, Optimiza-
tion of the hydrazine-water solution for anisotropic etching of silicon in integrated circuit technology. J. Electrochem. Sot.: Solid-State Sci. Technol., I22 (4) (1975) 545-552. 6 T. Katsube, M. Hara, I. Serizawa, N. Ishibashi, N.
Adachi, N. Miura and N. Yamazoe, MOS-type micro-oxygen sensor using LaF, workable at room temperature. Jpn. J. Appl. Phys., 29 ( 1990) 1392- 1395. 7 C. J. Smithells and E. A. Brandes, Metals Reference Rook, Butterworths, Boston, MA, 1976, pp. 941-942.