- Email: [email protected]
Integrated hydrogen leak detector with a tunnel mis structure
Sensors and Actuators, 13 (1988)
INTEGRATED STRUCTURE*
HYDROGEN
KENJI MURAKA&II,
Research Institute
(Japan)
315 - 321
LEAK DETECTOR
DONG-BAI YE**
of Electronics,
315
WITH A TUNNEL MIS
and TATSUO YAMAMOTG
Shizuoka
University,
Hamamatsu,
Shizuoka
432
,
(Received January 8,1987;
in revised form June 16,1937;
accepted August 10,1987)
Abstract A simpler integrated hydrogen leak detector has been fabricated, which consists of a Pd-Si tunnel MIS diode for the hydrogen sensor, a diffused resistor layer for the inside heater and a p-n junction diode for temperature control. Measurements were carried out on the response characteristics to hydrogen at different device temperatures and hydrogen concentrations. The results show that the newly fabricated detector can be used as a practical detector for small leakage of hydrogen at a device temperature between 100 and 120 “C. It is also demonstrated that the tunnel MIS diode with a holestructure Pd layer drastically improves the hydrogen sensitivity.
Introduction In the semiconductor device industries, large amounts of various process gases, which are usually dangerous or poisonous, are consumed. Hydrogen gas also needs careful handling because it is explosive in the oxygen atmosphere. In order to avoid such accidents in factories and laboratories, simpler and smaller detectors are required to monitor the small leakage of hydrogen gas. A metal-very thin insulator-n/p+ (or p/n*) silicon (tunnel MIS) structure shows a current-controlled negative resistance when the p-n junction is forward biased [ 11. Because of its transition from a very high impedance OFF state to a low impedance ON state, this type of device can be used as a switching device [2]. In particular, a tunnel MIS diode with Pd/SiOz/Si structure has been proposed as a hydrogen-sensitive switching device [ 31. A hydrogen concentration in the atmosphere is detected through the corresponding change in switching voltage of the device. It is, however, necessary *Based on a paper presented at the 2nd International Meeting on Chemical Sensors, Bordeaux, France, July 7 - 10, 1986. **Present address: The State Science and Technology Commission of the People’s Republic of China, Beijing, China. 0250-6874/88/$3.50
0 Elsevier Sequoia/Printed in The Netherlands
316
to keep the device at an appropriate temperature because of the temperature dependence of its response characteristics. An integrated hydrogen-switching sensor with an inside heater has been reported elsewhere by the authors 141, in which a power transistor was used as the heater. In the present paper, we describe a new device that consists of a Pd-Si tunnel MIS diode for the hydrogen sensor, a diffused resistor layer for the inside heater and a p-n junction diode for temperature control The fabrication of the new detector and the experimental set-up are discussed, and experimental results are presented. Furthermore, it is demonstrated that tunnel MIS diodes with a hole-structure Pd layer improve the characteristics for hydrogen sensitivity. Device fabrication The integrated hydrogen detectors were fabricated by using photolithographic techniques on an nepitaxial layer deposited on a (111 )-oriented p+ silicon wafer. The p+-substrate and the n-epitaxial layer were 180 - 220 pm thick with 0.01 s1 cm resistivity and 4 - 6 pm thick with 2 - 3 a cm resistivity, respectively. A schematic diagram of the integrated device is shown in Fig. 1. The device is divided into three sections: the hydrogen sensor, the inside heater and the temperature diode. Figure l(b) shows the corresponding cross-sectional views for A-A’, B-B’ and C-C’ in Fig. l(a). Two tunnel MIS diodes with 200 X 200 pm2 and 400 X 400 pm* active areas consist of a silicon wafer, a 25 A oxide layer and a 200 A Pd layer. The thin oxide layer was grown in dry oxygen at 700 “C for 13 min. The inside heater, 1100 pm long and 50 pm wide, was formed by a boron diffusion into the n-epitaxial layer and its resistance was 1 - 5 kSL The boron diffusion for the resistor layer also formed the 200 X 200 grn2 p-n junction diode. These three sections were isolated by a mesa-etching process, as shown in Fig. l(b). Finally, an aluminium layer was evaporated for electrodes and the 2 X 2 mm2 integrated device was mounted on a TO-6 header. The details of the fabrication processes are described elsewhere [ 51. The current-voltage characteristics were measured with a standard transistor curve tracer in a small chamber that is evacuated by a small vacuum pump and connected to gas inlet tubing. The measurements were carried out at different temperatures ranging from room temperature to 120 “C under gas atmospheres containing hydrogen diluted in synthetic air or in nitrogen, and pure synthetic air or nitrogen. Since the device temperature is controlled by the voltage drop across the p-n diode, the temperature sensitivity of the diode was determined in an electric furnace before the measurements. Experimental results and discussions Figure 2 shows a voltage-temperature characteristic of the p-n junction diode. A good linear relation is obtained in the range from room temperature up to 150 “C. under a forward current of 100 MA, as shown in Fig. 2. The
317 SENSOR
HEATER
TEMP.
DIODE
rpd
p'-Si I
J
CONTACT
HEATER
s”’
B-B’
J SENSOR
(a)
SENSOR
HEATER
TEMP.
HEATER
TEMP. DIODE
DIODE
(b)
’
Fig. 1. Schematic diagram of the integrated hydrogen leak detector. (a) Top view; (b) cross-sectional views.
temperature sensitivity reaches 2.5 mV/“C, which is enough to control the voltage applied to the heater. Figure 3 shows the variation of the switching voltage V,,of a typical tunnel MIS diode with temperature in the absence of hydrogen. As the temperature increases, Vthdecreases and approaches the ON state voltage V,,,, which is usually 2 - 3 V. The effects of the temperature and the hydrogen concentration on the time response of the detector are shown in Figs. 4 and 5 in the presence of 400 ppm of hydrogen in synthetic air and at 120 “C, respectively. All curves are normalized to the switching voltage V th,, inthe absence of hydrogen. The time origins are taken when the ambient gas is switched from synthetic air to hydrogen diluted in synthetic air, or vice versa. The response of the detector is faster at higher temperatures (Fig. 4) and at higher hydrogen partial pressures (Fig. 5).However, the device temperature is limited by its dynamic range, which depends on the temperature as shown in Fig. 3. This limitation is clearly demonstrated by the decrease in saturated sensitivity at 120 “C in Fig. 4. The response time is less than 30 s in the presence of 400 ppm of hydrogen at temperatures higher than 100 “C.
318
0.6 ko.1
mA
t
OIo: 50
100 Temp.(Y)
Fig. 2. Voltage-temperature
150
characteristic of the p-n junction
Fig. 3. Effect of temperature on the switching voltage of the tunnel MIS diode.
319 A
HI
4 75t 0 so’c 0 100% tt 11o*c q12oc
I
4OOPpm
I
O40 Hz-m
I
1
0
Time (min
)
1
2
on the time response of the detector
under a hydrogen
con-
1Ofwm
.
30wm
n
50Ppm
n
L
i
to Hz-off
Fig. 4. Effect of temperature centration of 400 ppm.
t
Concentration:
Temp.: 12 O°C
‘Iooppm
A 200PPm L 400PPm
1.0
-+dd -J
-4 __ Jr
--
l
I
o!i:-o” ’
I
I
2
3
Fig. 5. Effect of hydrogen
I
to
I
1
I
I
2
3
c
Tims(m%~off
concentration
on the time response of the detector at 120 “C.
On the other hand, the recovery time is slower at higher temperatures and hydrogen partial pressures. In particular, the temperature dependence of the recovery time in Fig. 4 seems to be contradictory to the hydrogen-sensing model for Pdigate MOS transistors [6], which asserts that hydrogen is adsorbed at the Pd-SiO, interface, Although the results suggest that the
320
change of switching voltage is not caused only by the hydrogen adsorption, more detailed analyses are still necessary for further consideration. It is found from the experimental results that the newly fabricated integrated hydrogen leak detectors show optimal characteristics at temperatures of 100 - 120 “C for practical use; their sensitivity reaches 10 ppm or less, as shown in Fig. 5. The power dissipation of the detector, which is also of interest for practical use, is only 0.6 - 0.7 W at 100 “C and 0.7 - 0.8 W at 120 “c. Furthermore, no significant differences in the switching characteristics were found between two sizes of tunnel MIS diodes and among their locations on a wafer [5]. The results suggest that this type of hydrogen sensor is suitable for mass production in the semiconductor device industries, and that a multi-functional detector with several tunnel MIS diodes is possible. Finally, an improvement of hydrogen sensitivity was attempted with a hole-structure Pd layer. A MOS transistor with the gate structure containing holes was first introduced for carbon monoxide detectors by Dobos and Hijfflinger [7]. The hole structures with hole diameters of 10 and 20 ym used in the present study were formed by a photolithographic technique. The response characteristic is shown in Fig. 6 for the tunnel MIS diode with a hole-structure Pd layer 20 pm in diameter. In spite of the room temperature condition, the switching voltage is decreased to half of its initial value as soon as 10 ppm of hydrogen is introduced in the nitrogen atmosphere. For comparison, the characteristic of a device without hole structure is also shown in Fig. 6 for a device temperature of 100 “C in 100 ppm hydrogen. The drastic improvement is clearly demonstrated. However, the device with a hole structure 10 pm in diameter does not show any improvement in 10 ppm of hydrogen at room temperature, as shown in Fig. 6. In order to interpret these results, more detailed analyses are still necessary for various temperatures and hole diameters and in various gas atmospheres.
P 2 0.5d. 3 -
0
without
1
-CF- 1OOppm
1OO’C
with(20m
-
10
RT
wfh(l0)m-d
-
10
RT
2
3
4
TIME (mid Fig. 6. Comparison of the response characteristics for the tunnel MIS diodes with and without a hole-structure Pd layer.
321
Conclusions A simpler integrated hydrogen leak detector has been fabricated, which consists of a Pd-Si tunnel MIS diode, a diffused resistor layer and a p-n junction diode. The effects of temperature and hydrogen concentration on the time response of the switching characteristic were measured. The results show that the detector can be used for practical uses such as a hydrogen leak alarm when it is kept at a temperature between 100 and 120 “C by the inside heater. The tunnel MIS diode with a hole-structure Pd layer demonstrates a drastic improvement in the hydrogen sensitivity. The results suggest the possibility of room temperature operation of the detector. However, more detailed studies including the mechanism of the hydrogen sensitivity are still necessary.
Acknowledgements This work has been supported by the Ministry of Education, Science and Culture of Japan through a Grant-in-Aid for Scientific Research. The authors wish to thank Mr. Y. Inaki and Mr. D. Suzuki for their technical assistance.
References 1 T.
Yamamoto
and
Appl. Phys. Lett.,
M.
Morimoto,
Thin-MIS-structure
Si negative-resistance
diode,
20 (1972) 269 - 270.
2 T. Yamamoto, K. Kawamura and H. Shim+ Silicon p-n insulator-metal (p-n-I-M) devices, Solid-State Electron., 19 (1976) 701 - 706. 3 K. Kawamura and T. Yamamoto, Hydrogen-sensitive silicon tunnel MIS switching diodes, IEEE Electron Devices Lett,, EDL-4 (1983) 88 - 89. 4 M. Ogita, D.-R Ye, K. Kawamura and T. Yamamoto, An integrated hydrogenswitching sensor with a Pd-Si tunnel MIS structure, Sensors and Actuators, 9 (1986)
157 - 164. 5 K. Murakami, D.-B. Ye and T. Yamamoto, An integrated tunnel MIS hydrogen sensor, Proc. 6th Sensor Symp., Tsukuba, Japan, May 1986, pp. 195 - 200. 6 I. Lundstrtim, S. Shivaraman, C. Svensson and L. Lundqvist, A hydrogen-sensitive MOS field-effect transistor, Appl. Phys. Lett., 26 (1975) 55 - 57. 7 K. Dobos and B. HGfflinger, Pd-gate MOS transistor as CO sensor, Proc. 9th European Solid State Dev. Res. Confi, Munich, Sept. 1979, pp. 147 - 148.
INTEGRATED STRUCTURE*
HYDROGEN
KENJI MURAKA&II,
Research Institute
(Japan)
315 - 321
LEAK DETECTOR
DONG-BAI YE**
of Electronics,
315
WITH A TUNNEL MIS
and TATSUO YAMAMOTG
Shizuoka
University,
Hamamatsu,
Shizuoka
432
,
(Received January 8,1987;
in revised form June 16,1937;
accepted August 10,1987)
Abstract A simpler integrated hydrogen leak detector has been fabricated, which consists of a Pd-Si tunnel MIS diode for the hydrogen sensor, a diffused resistor layer for the inside heater and a p-n junction diode for temperature control. Measurements were carried out on the response characteristics to hydrogen at different device temperatures and hydrogen concentrations. The results show that the newly fabricated detector can be used as a practical detector for small leakage of hydrogen at a device temperature between 100 and 120 “C. It is also demonstrated that the tunnel MIS diode with a holestructure Pd layer drastically improves the hydrogen sensitivity.
Introduction In the semiconductor device industries, large amounts of various process gases, which are usually dangerous or poisonous, are consumed. Hydrogen gas also needs careful handling because it is explosive in the oxygen atmosphere. In order to avoid such accidents in factories and laboratories, simpler and smaller detectors are required to monitor the small leakage of hydrogen gas. A metal-very thin insulator-n/p+ (or p/n*) silicon (tunnel MIS) structure shows a current-controlled negative resistance when the p-n junction is forward biased [ 11. Because of its transition from a very high impedance OFF state to a low impedance ON state, this type of device can be used as a switching device [2]. In particular, a tunnel MIS diode with Pd/SiOz/Si structure has been proposed as a hydrogen-sensitive switching device [ 31. A hydrogen concentration in the atmosphere is detected through the corresponding change in switching voltage of the device. It is, however, necessary *Based on a paper presented at the 2nd International Meeting on Chemical Sensors, Bordeaux, France, July 7 - 10, 1986. **Present address: The State Science and Technology Commission of the People’s Republic of China, Beijing, China. 0250-6874/88/$3.50
0 Elsevier Sequoia/Printed in The Netherlands
316
to keep the device at an appropriate temperature because of the temperature dependence of its response characteristics. An integrated hydrogen-switching sensor with an inside heater has been reported elsewhere by the authors 141, in which a power transistor was used as the heater. In the present paper, we describe a new device that consists of a Pd-Si tunnel MIS diode for the hydrogen sensor, a diffused resistor layer for the inside heater and a p-n junction diode for temperature control The fabrication of the new detector and the experimental set-up are discussed, and experimental results are presented. Furthermore, it is demonstrated that tunnel MIS diodes with a hole-structure Pd layer improve the characteristics for hydrogen sensitivity. Device fabrication The integrated hydrogen detectors were fabricated by using photolithographic techniques on an nepitaxial layer deposited on a (111 )-oriented p+ silicon wafer. The p+-substrate and the n-epitaxial layer were 180 - 220 pm thick with 0.01 s1 cm resistivity and 4 - 6 pm thick with 2 - 3 a cm resistivity, respectively. A schematic diagram of the integrated device is shown in Fig. 1. The device is divided into three sections: the hydrogen sensor, the inside heater and the temperature diode. Figure l(b) shows the corresponding cross-sectional views for A-A’, B-B’ and C-C’ in Fig. l(a). Two tunnel MIS diodes with 200 X 200 pm2 and 400 X 400 pm* active areas consist of a silicon wafer, a 25 A oxide layer and a 200 A Pd layer. The thin oxide layer was grown in dry oxygen at 700 “C for 13 min. The inside heater, 1100 pm long and 50 pm wide, was formed by a boron diffusion into the n-epitaxial layer and its resistance was 1 - 5 kSL The boron diffusion for the resistor layer also formed the 200 X 200 grn2 p-n junction diode. These three sections were isolated by a mesa-etching process, as shown in Fig. l(b). Finally, an aluminium layer was evaporated for electrodes and the 2 X 2 mm2 integrated device was mounted on a TO-6 header. The details of the fabrication processes are described elsewhere [ 51. The current-voltage characteristics were measured with a standard transistor curve tracer in a small chamber that is evacuated by a small vacuum pump and connected to gas inlet tubing. The measurements were carried out at different temperatures ranging from room temperature to 120 “C under gas atmospheres containing hydrogen diluted in synthetic air or in nitrogen, and pure synthetic air or nitrogen. Since the device temperature is controlled by the voltage drop across the p-n diode, the temperature sensitivity of the diode was determined in an electric furnace before the measurements. Experimental results and discussions Figure 2 shows a voltage-temperature characteristic of the p-n junction diode. A good linear relation is obtained in the range from room temperature up to 150 “C. under a forward current of 100 MA, as shown in Fig. 2. The
317 SENSOR
HEATER
TEMP.
DIODE
rpd
p'-Si I
J
CONTACT
HEATER
s”’
B-B’
J SENSOR
(a)
SENSOR
HEATER
TEMP.
HEATER
TEMP. DIODE
DIODE
(b)
’
Fig. 1. Schematic diagram of the integrated hydrogen leak detector. (a) Top view; (b) cross-sectional views.
temperature sensitivity reaches 2.5 mV/“C, which is enough to control the voltage applied to the heater. Figure 3 shows the variation of the switching voltage V,,of a typical tunnel MIS diode with temperature in the absence of hydrogen. As the temperature increases, Vthdecreases and approaches the ON state voltage V,,,, which is usually 2 - 3 V. The effects of the temperature and the hydrogen concentration on the time response of the detector are shown in Figs. 4 and 5 in the presence of 400 ppm of hydrogen in synthetic air and at 120 “C, respectively. All curves are normalized to the switching voltage V th,, inthe absence of hydrogen. The time origins are taken when the ambient gas is switched from synthetic air to hydrogen diluted in synthetic air, or vice versa. The response of the detector is faster at higher temperatures (Fig. 4) and at higher hydrogen partial pressures (Fig. 5).However, the device temperature is limited by its dynamic range, which depends on the temperature as shown in Fig. 3. This limitation is clearly demonstrated by the decrease in saturated sensitivity at 120 “C in Fig. 4. The response time is less than 30 s in the presence of 400 ppm of hydrogen at temperatures higher than 100 “C.
318
0.6 ko.1
mA
t
OIo: 50
100 Temp.(Y)
Fig. 2. Voltage-temperature
150
characteristic of the p-n junction
Fig. 3. Effect of temperature on the switching voltage of the tunnel MIS diode.
319 A
HI
4 75t 0 so’c 0 100% tt 11o*c q12oc
I
4OOPpm
I
O40 Hz-m
I
1
0
Time (min
)
1
2
on the time response of the detector
under a hydrogen
con-
1Ofwm
.
30wm
n
50Ppm
n
L
i
to Hz-off
Fig. 4. Effect of temperature centration of 400 ppm.
t
Concentration:
Temp.: 12 O°C
‘Iooppm
A 200PPm L 400PPm
1.0
-+dd -J
-4 __ Jr
--
l
I
o!i:-o” ’
I
I
2
3
Fig. 5. Effect of hydrogen
I
to
I
1
I
I
2
3
c
Tims(m%~off
concentration
on the time response of the detector at 120 “C.
On the other hand, the recovery time is slower at higher temperatures and hydrogen partial pressures. In particular, the temperature dependence of the recovery time in Fig. 4 seems to be contradictory to the hydrogen-sensing model for Pdigate MOS transistors [6], which asserts that hydrogen is adsorbed at the Pd-SiO, interface, Although the results suggest that the
320
change of switching voltage is not caused only by the hydrogen adsorption, more detailed analyses are still necessary for further consideration. It is found from the experimental results that the newly fabricated integrated hydrogen leak detectors show optimal characteristics at temperatures of 100 - 120 “C for practical use; their sensitivity reaches 10 ppm or less, as shown in Fig. 5. The power dissipation of the detector, which is also of interest for practical use, is only 0.6 - 0.7 W at 100 “C and 0.7 - 0.8 W at 120 “c. Furthermore, no significant differences in the switching characteristics were found between two sizes of tunnel MIS diodes and among their locations on a wafer [5]. The results suggest that this type of hydrogen sensor is suitable for mass production in the semiconductor device industries, and that a multi-functional detector with several tunnel MIS diodes is possible. Finally, an improvement of hydrogen sensitivity was attempted with a hole-structure Pd layer. A MOS transistor with the gate structure containing holes was first introduced for carbon monoxide detectors by Dobos and Hijfflinger [7]. The hole structures with hole diameters of 10 and 20 ym used in the present study were formed by a photolithographic technique. The response characteristic is shown in Fig. 6 for the tunnel MIS diode with a hole-structure Pd layer 20 pm in diameter. In spite of the room temperature condition, the switching voltage is decreased to half of its initial value as soon as 10 ppm of hydrogen is introduced in the nitrogen atmosphere. For comparison, the characteristic of a device without hole structure is also shown in Fig. 6 for a device temperature of 100 “C in 100 ppm hydrogen. The drastic improvement is clearly demonstrated. However, the device with a hole structure 10 pm in diameter does not show any improvement in 10 ppm of hydrogen at room temperature, as shown in Fig. 6. In order to interpret these results, more detailed analyses are still necessary for various temperatures and hole diameters and in various gas atmospheres.
P 2 0.5d. 3 -
0
without
1
-CF- 1OOppm
1OO’C
with(20m
-
10
RT
wfh(l0)m-d
-
10
RT
2
3
4
TIME (mid Fig. 6. Comparison of the response characteristics for the tunnel MIS diodes with and without a hole-structure Pd layer.
321
Conclusions A simpler integrated hydrogen leak detector has been fabricated, which consists of a Pd-Si tunnel MIS diode, a diffused resistor layer and a p-n junction diode. The effects of temperature and hydrogen concentration on the time response of the switching characteristic were measured. The results show that the detector can be used for practical uses such as a hydrogen leak alarm when it is kept at a temperature between 100 and 120 “C by the inside heater. The tunnel MIS diode with a hole-structure Pd layer demonstrates a drastic improvement in the hydrogen sensitivity. The results suggest the possibility of room temperature operation of the detector. However, more detailed studies including the mechanism of the hydrogen sensitivity are still necessary.
Acknowledgements This work has been supported by the Ministry of Education, Science and Culture of Japan through a Grant-in-Aid for Scientific Research. The authors wish to thank Mr. Y. Inaki and Mr. D. Suzuki for their technical assistance.
References 1 T.
Yamamoto
and
Appl. Phys. Lett.,
M.
Morimoto,
Thin-MIS-structure
Si negative-resistance
diode,
20 (1972) 269 - 270.
2 T. Yamamoto, K. Kawamura and H. Shim+ Silicon p-n insulator-metal (p-n-I-M) devices, Solid-State Electron., 19 (1976) 701 - 706. 3 K. Kawamura and T. Yamamoto, Hydrogen-sensitive silicon tunnel MIS switching diodes, IEEE Electron Devices Lett,, EDL-4 (1983) 88 - 89. 4 M. Ogita, D.-R Ye, K. Kawamura and T. Yamamoto, An integrated hydrogenswitching sensor with a Pd-Si tunnel MIS structure, Sensors and Actuators, 9 (1986)
157 - 164. 5 K. Murakami, D.-B. Ye and T. Yamamoto, An integrated tunnel MIS hydrogen sensor, Proc. 6th Sensor Symp., Tsukuba, Japan, May 1986, pp. 195 - 200. 6 I. Lundstrtim, S. Shivaraman, C. Svensson and L. Lundqvist, A hydrogen-sensitive MOS field-effect transistor, Appl. Phys. Lett., 26 (1975) 55 - 57. 7 K. Dobos and B. HGfflinger, Pd-gate MOS transistor as CO sensor, Proc. 9th European Solid State Dev. Res. Confi, Munich, Sept. 1979, pp. 147 - 148.