Silicon-Based Sensor for Flourine Gas - ACS Symposium Series (ACS

Chapter 10

Silicon-Based Sensor for Flourine Gas 1

1,3

1

1

2

W. Moritz , S. Krause , Lars Bartholomäus , Tigran Gabusjan , A. A. Vasiliev , D. Yu. Godovski , and V. V. Malyshev 2

2

Downloaded by UNIV OF MINNESOTA on October 14, 2014 | http://pubs.acs.org Publication Date: April 17, 1998 | doi: 10.1021/bk-1998-0690.ch010

1

Humboldt-University Berlin, Walther-Nernst-Institute of Physical and Theoretical Chemistry, Bunsenstrasse 1, 10117 Berlin, Germany RRC "Kurchatov Institute", 123182 Moscow, Russia 2

A new Sensor for the determination of fluorine has been developed using a simple structure Si/LaF /Pt. A thin layer of the ionic conductor LaF is in direct contact to the semiconductor without intermediate insulator. The field effect in the semiconductor leads to much steeper capacitance-voltage curves than usually found for conventional MIS-structures. It was shown that a special treatment of the sensor is necessary before first use to achieve a stable response. Different ionic conductors and gate materials were tested and the preparation conditions varied. The lower limit of detection of the optimised sensor was shown to be < 0.1 ppm. It was proven that potential formation takes place at the three phase boundary LaF /Pt/gas. Oxygen can be determined with a similar sensor system but does not interfere for the fluorine sensor. 3

3

3

Fluorine is used for several industrial processes for example in the production of polymers or the preparation of nuclear fuels. On the other hand, the gas is toxic even at low concentrations. Some Fluorohydrocarbons are known to cause critical changes in the ozone concentration in the atmosphere. Consequently, substitutes are developed as for example R 134a. Therefore, there is a great demand for sensors for these gases and alarm systems controlling the environment. Murin et al. (7) have proposed to use a single crystal of LaF3 covered with a platinum film to develop a potentiometric device for the determination of fluorine. Drawbacks of this sensor are the high price of the single crystal and the formation and stability of a reversible back side contact. Low temperature oxygen sensors using the LaF /Pt contact have also been developed (2-5). Previously, we have proposed a silicon based sensor (3). The consecutive deposition of thin films of L a F and Pt on silicon wafers results in a 3

3

3

Current address: Department of Chemistry, University of Sheffield, Sheffield S10 2TN, England.

©1998 American Chemical Society In Polymers in Sensors; Akmal, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

119


120 field-effect semiconductor sensor. We have shown that for these devices a stable sensor signal can be obtained without a reversible back side contact of the LaF3. The extremely high steepness of the capacitance-voltage curves caused by the direct contact of the silicon to the ionic conductor improves the accuracy of the detection of concentrations (6,7). The mechanism of the oxygen sensor has been shown to depend on the surface treatment of the active layers (#), which have to form a three phase boundary ionic conductor/platinum/gas (9). The long-term stability of the oxygen sensor has been improved by a simple thermal activation process (10). Using this activation the increase of the response time with the measuring time is not only stopped but even a faster response is obtained. In this work we used the two different states of the sensor (reactivated or aged for some month after preparation) in combination with specific treatments of the surface to investigate the sensitivity to gases containing fluorine or flourohydrocarbons. An improvement of the electrical characteristics of the sensor was achieved by doping the LaF -layer with SrF . 3

2

Experimental The sensor used for measurements in gases was a simple thin layer structure on a silicon substrate as described in (77). The LaF3 layer (d=250nm) was prepared by thermal vapour deposition. Platinum (d=30nm) was deposited by sputtering in an Argon plasma so that a three phase boundary LaF3/Pt/gas was created. Only minor variations in the preparation conditions were necessary compared to the oxygen sensor. The sensitivity of the sensor was determined using high-frequency capacitance-voltage (CV) measurements or photo-current/voltage curves. Gas concentrations were adjusted using flow controllers controlled by a PC. A l l measurements for fluorine gas were performed at room temperature. The sensors were activated on a hot plate at a temperature of 350°C for 2 minutes or by electrical current as described below.

Results Electrical Properties of the Metal/Ionic Conductor/Semiconductor Sensor. Chemical sensors based on the field effect in a semiconductor substrate can be characterised as transistor devices or using high-frequency capacitance voltage curves of simple thin layer structures. The shift of the CV-curve on the voltage axis with changing concentration of the analyte represents the voltage drop at the potential determining interface. Normally, the gate area of chemically sensitive field effect devices is covered by insulators such as S i 0 , S13N4, AI2O3 or T a 0 . While these layers are directly sensitive to the pH of a solution, additional layers on top of the insulator result in a sensitivity to other species. We have shown (6,7) that the direct contact of the semiconductor to the ionic conductor LaF^ (without insulating interlayer) leads to a field effect device too and results in a considerable improvement of the capacitance-voltage curves (compare Figure 1 curves a and b). 2

2

In Polymers in Sensors; Akmal, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

5


121

-1000

-500

0

500

1000

1500

voltage / mV

Fig. 1 Capacitance voltage curves for a) Si/Si0 /Si N /LaF /Pt; 2

b) Si/LaF /Pt and c) 3

3

4

3

Si/LaF (SrF )/Pt 3

2


122 To further improve the electrical properties of the structure, LaF and SrF were evaporated simultaneously. The doping of the thin LaF layer with 0,4% SrF leads to another significant increase in the capacitance and steepness of the C V curve of the sensor. As shown in Figures 1 and 2 a factor of more than 20 was achieved for the capacitance in the accumulation region. Impedance spectra showed that the ionic conductivity in the LaF layer doped with SrF is by two orders of magnitude higher compared to the pure LaF , i.e. for frequencies from 100 Hz to 100 kHz the impedance of the doped layer is not determined by the bulk capacitance anymore because the capacitive impedance is smaller than the ohmic impedance in parallel. Therefore, the capacitance of the sensor in accumulation and its electrical behaviour is determined by the capacities of the interface LaF /Pt and LaF /Si. For these capacities values of 0,95 mF/cm were found which corresponds to a distribution of F"-ions in a thin space charge region. 3

2

3

2

3

2

3


2

3

3

Electrical Activation of the Sensor. Using the Si/LaF /Pt sensor structure for oxygen measurements we obtained a stable sensitivity of 58 mV/Alg P Q and a response time of t9o=70s. The flat band voltage and the sensitivity were highly reproducible. However, a drawback for the application of the sensor were a decrease of the response rate as a function of time (10). While for several days after the preparation of the platinum layer reasonable times are obtained, the sensor response gets too slow for an application after a period of 2 weeks. The comparison of different methods of reactivation has shown that a thermal treatment at 350°C in air is the most promising. Using this treatment the sensor is slightly faster than a freshly prepared one and the sensor drift is reduced to 0.5 mV/h. The heating procedure can be repeated any number of times. Heating the sensor once a week, a stable response time was obtained for a period of 4 month. Even 8 month after preparation a full activation was observed. It is noteworthy that the behaviour of the reactivated sensor is similar to that of a newly prepared one but there are some differences concerning the drift and the influence of humidity on the response time (11). External heating of the sensor would be a disadvantage for sensor application. The integration of a heater is one way to a low-power on-chip reactivation. Since the sensor reaction takes place at the three phase boundary LaF /Pt/gas, only the thin platinum layer has to be be heated. This could be achieved using the platinum layer not only as an electrode but also as an electrical heater. The principle of sensor operation and activation is shown in Figure 3. The characterisation of the structure with photocurrent measurements was more convenient for these experiments because the active area was determined by the light and there was no influence of the contact areas. It was shown that the duration of the electrical heating can be reduced to very short times. Even an impulse of 100 ms gives a full reactivation of the sensor. For such short times the heat transport into the silicon becomes negligible. Hence, the temperature of the silicon chip is increased by only 0.4 °C and a measurement at room temperature is possible directly after the thermal treatment. 3

1

2

3



123

Fig.3

Scheme of sensor operation and activation


124 Sensitivity to Fluorine. The oxygen sensitive structure Si/ LaF / Pt could also be used as a fluorine sensor, but an optimisation of the preparation conditions for this application was necessary. Furthermore the influence of the thermal activation process was investigated. For a non-activated sensor aged for several month neither sensitivity for oxygen nor for fluorine was found. Therefore the thermal activation of the sensor is the key process to obtain a fluorine sensitivity. After the thermal activation of the sensor the potential was shifted for about + 800 mV in fluorine containing gases compared to measurements in oxygen/ inert gas mixtures. The response of the sensor to different concentrations of fluorine in synthetic air at room temperature is shown in Figure 4. A constant potential was reached within a few minutes after the concentration change even for low concentrations. Sensitivity measurements showed a linear relationship between the potential and the logarithm of the fluorine partial pressure. This corresponds to the Nernst equation, but sensitivities were found to be 111(±1 l)mV/lg p(F ) (Figure 5) corresponding to a formal exchange of about 0.5 electron. Furthermore, it was observed, that the response time increases with decreasing fluorine concentration. The lower limit of detection was shown to be 0.1 ppm. In contrast to the behaviour of the oxygen sensor no decrease in response rate with time was found for the measurements in fluorine gas. For the understanding of the mechanism of the sensor it is important to know that the shift in potential by about +800 mV after exposure to a fluorine containing gas is a slow process. As shown in Figure 6 the potential was increasing with the time of preconditioning and the concentration of fluorine. There were strong improvements in sensitivity, response time and a reduction of drift after a pretreatment in 1000 ppm fluorine. This behaviour is an argument for the existence of a mixed potential. A structure Si/Si0 /Si N /Pt was tested for fluorine sensitivity too. As there was no sensitivity found, the existence of the three phase boundary fluoride ionic conductor/platinum/gas was proven to be necessary. On the other hand we could substitute the LaF by BaF getting the same results in sensitivity and response time. Furthermore, for sensors exposed to fluorine no oxygen sensitivity was found in gases containing flourine or fluorine free gas mixtures.


3

2

2

3

3

4

2

Sensitivity to Fluorohydrocarbons. Beside the sensitivity to fluorine the possibility of a detection of HF in gases was shown (77). Therefore, we have been interested in the possibility of the specific detection of fluorohydrocarbons using our sensor. At temperatures above 650K the equilibrium between 1,1,2,2- and 1,2,2,2tetrafluorethane (R134) CHF -CHF 2

2

ψ

CH F-CF 2

3

(1)

can be obtained at catalysts.This should result in the intermediate existence of HF. As shown in Figure 7 the detection of R134a was possible at temperatures as low as 155 to 180°C using the Si/LaF /Pt sensor structure. 3



2000

25000

Fig.4

Sensor response to different concentrations of fluorine



126

Fig. 6 Response of the sensor after different treatment a) 5minutes 100 ppm fluorine b) 5 minutes 1000 ppm flurine c) 2 hours 1000 ppm fluorine

1460

off

off

\

\

t

ο

potential

Λ

Λ

> 1440

on 1400

I 0

10

ι

20

1 30

1

1 40

1 50

time / min

Fig.7

Response to 1,2,2,2-tetrafluorethane (R134a) in synthetic air at 180°C


127 Discussion The sensitivity to fluorine could be explained by the formation of F" according to


F

2

+

2e-

^

2F-

(2)

But this would result in a sensitivity of only 30 mV/decade. The sensitivity of the similar structure to oxygen and the very high sensitivities lead to the assumption that there might be some kind of mixed potential including a reaction with oxygen. The potential difference of about 2 Volts between the standard potentials of the fluorine and the oxygen reaction in aqueous solutions is an additional argument for this assumption. In (70) the rate determining process for the reaction at the oxygen sensor was shown to be:

0 ads 2

+

H 0 2

+

e

-

£

H0

+

2 a d s

OH"

(3)

It has to be considered that there is not a real equilibrium, but the rates of the electrochemical reactions (2) and (3) determine the mixed potential. The existence of different oxygen species at the LaF surface has been proven using XPS. Specific detection of fluorohydrocarbons can be achieved assuming that the decomposition of the organic compound is followed by recognition of fluorine. It is noteworthy that the detection of the R134a was possible at a temperature 200°C lower than the temperature at which the best catalysts known for the decomposition of this substance are working. This is because the decomposition and the detection occur at the same place at the three phase boundary LaF /Pt/gas. All the results given here were obtained from measurements with capacitive devices but field effect transistors can be produced in a similar way. The use of two sensors, one in the aged state and the other activated by an electrical impulse results in a sensor system for the simultaneous determination of fluorine, hydrogen fluoride and fluorohydrocarbons. Furthermore, it is the same sensor that can be used for the determination of oxygen. Since there is no demand to determine all the gases at the same time the importance of selectivity is reduced. A sufficient selectivity for these gases is achieved by the difference in conditions of measurements as summarised in Table I. 3

3


128 Table I Differences in conditions for the determination of various gases using the Si/LaF /Pt sensor structure 3

Sensor

Oxygen

Hydrogenfluoride

Fluorine

activated

X

-

X

X

non activated

-

X

-

-

Voltage range[V]

1,2-1,5

1,2-1,5

2,0-2,5

1,0-1,5

Temperature

room temperature

room temperature

room temperature

440 Κ


Conditions

Literature Cited 1 2

I.W. Murin, O.W. Glumov, D.B. Samusik; Potentiometric sensor for the determination offluorinein gases, Zh. Prikl. Khim., 64 (10) 1991 2171 N. Miura, J. Hisamoto and N. Yamazoe, Solid state oxygen sensor using sputtered LaF films, Sensors and Actuators, 16 1989 301-310 S. Krause, W. Moritz, I. Grohmann, A low-temperature oxygen sensor based on the Si/LaF /Pt capacitive structure, Sensors and Actuators B, 9 1992, 191 J.P. Lukaszewitz, N. Miura and N. Yamazoe, Influence of water treatment of LaF crystal on LaF -based oxygen sensor workable at room temperature, Jpn. J. Appl. Pys., 30 1991 L1327 5 S. Harke, H.-D. Wiemhöfer and W. Göpel, Investigations of electrodes for oxygen sensors based on lanthanum trifluoride as solid electrolyte, Sensors and Actuators, B1 1990 188-194 J. Szeponik, W. Moritz, A New Structure for Chemical Sensor Devices, Sensors and Actuators, 2 1990 243 J. Szeponik, W. Moritz, F. Sellam, LaF Thin Films as Chemical Sensitive Material for Semiconductor Sensors, Ber. Bunsenges. Phys. Chem. 95 1991 1448 S. Krause, R. Krankenhagen, W. Moritz, I. Grohmann, W. Unger, Th. Gross, A. Lippitz, Influence of theLaF /metalinterface on the properties of 3

3

3

4

3

6 7

8

3

3

3


129

9

10

11

a low temperature oxygen sensor, Eurosensors VI, San Sebastian, Spain, 5.7.10.1992 and Sensors and Actuators, Β16 1993 252 S. Krause, W. Moritz, I. Grohmann, W. Unger, Th. Gross, A. Lippitz, Dynamic response of a low-temperature field effect oxygen sensor, International Meeting on Chemical Sensors, Tokyo, Japan, 13.-17.9.1992 and Sensors and Actuators B, 14 1993 499 S. Krause, W. Moritz, I. Grohmann, Improved long-term stability for a LaF based oxygen sensor, Proc. EUROSENSORS VII, 1993 Budapest, Hungary and Sens. and Act., Β 18 1993 148-154 W. Moritz, S. Krause, A.A. Vasiliev, D.Yu. Godovski and V.V. Malyshev, Monitoring of HF and F using a field effect sensor, The Fifth International Meeting on Chemical Sensors, Rome 11-14. July 1994 and Sens. and Act. B, 24-25 1995 194 3


2


Silicon-Based Sensor for Flourine Gas - ACS Symposium Series (ACS

Recommend Documents