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Iodide ion-sensitive field-effect structures
Sensors and Actuators B, IS 16 (1993) 192 194
Iodide ion-sensitive field-effect structures M. J. SchGning”, M. Bruns”, W. Hoffmamf, B. Hoffinannb and H. J. Ache” “Kernforschungszenlrum Karlsruhe GmbH. Insiitul fir Radiochemie, P.O. Box 3640, 7500 Karl&w
I (Germany)
bUniversitiit Karlsruhe, Institut fir Technologie der Elekrrotechnik, Hertzstrasse 16, 7500 Karlsruhe 21 (Germany)
A new ion-selective thin-film membrane for ion-sensitive field-effect structures has been realized. Silver iodide ( AgI) as an iodide-sensitive material has been vacuum evaporated onto semiconductor-insulator substrates. Various preconditioning and vacuum evapor!tion parameters as well as post treatment conditions were tested. Sensitivity and selectivity of these model field-effect structures were investigated by capacitance-voltage (CL’) measurements. Nernstian behaviour, good selectivity and a high lifetime comparable with that of the ion-selective electrode (ISE) could be obtained. This method of membrane fabrication is expected to be suitable for the large scale manufacture of sensor chips.
Introduction
In 1971 Ross et al. patented halide-sensitive electrochemical electrodes and the method for making them
[ 11. These ion-selective electrodes were prepared by coprecipitating the respective silver halides and silver sulfide as sensitive materials and subsequent pressing into dense, non-porous pellets. In 1970 Bergveld [2] used electronic devices for the first time for measuring chemical quantities. Such ionsensitive field-effect transistors (ISFETs) combine chemical-sensitive membranes and field-effect transistors. With respect to the membranes, there is particular interest in inorganic materials whose preparation technique is compatible to standardized semiconductor manufacturing procedures. Several kinds of ISFETs with different inorganic membranes are discussed in the literature in addition to the basic pH-sensitive insulator membranes, e.g. for the detection of sodium and potassium [3-51. With respect to halides, LaF, as an outstandingly sensitive material for F- analysis has been adapted to ISFETs [6]. For Cl- and Br- ions the respective silver halides were evaporated onto silicon-insulator substrates [7-91, but their stability was rather low or no detailed information was given. Until now an iodide-sensitive membrane has only been realized as a mixture of an organic matrix with AgI powder [lo], a method not compatible with semiconductor technology. The aim of this work was to produce solid state membranes for ion-sensitive field-effect structures by methods compatible with current techniques for microelectronic component production. Model experiments with ESMIS structures (electrolyte-Sensitive-membrane-
0925-4005/93/$6.00
insulator-semiconductor) are useful for studying sensor properties of the membrane materials. The preparation conditions and experimental results with AgI as an iodide-sensitive material are presented. Sensitivity, selectivity and lifetime are compared with the traditional iodide crystal membrane ISE.
Experimental Materials
and processing
Silicon wafers (p-type, 18-24 Q cm) with a double layer insulator of 30 nm SiO, (thermal oxidation at 1000 “C), 70 nm S&N, (LPCVD technique) and a rear ohmic contact were used. These samples were cleaned with acetone, HF (I’%), a mixture of H,SO, and H,O, (all solvents were of analytical grade) and quartz distilled water. Silver iodide as the iodide-sensitive material was vacuum evaporated from pressed AgI (Alfa Products) pellets onto the cleaned p-Si/SiOJSi,N, substrates at a pressure of less than 10m3Pa. The substrates could be heated from 25 to 350 “C. AgI films with thickness from 100 to 2000 nm were fabricated; deposition rates could be varied between 0.05 and 5 rim/s.. An ,additional adhesive layer of approximately 20nm chromium between the S&N, substrate and the silver iodide film was prepared. Measurements
To investigate the iodide sensitivity and selectivity of the model field-effect structures the samples were mounted in a CV measurement cell (Fig. 1). The applied d.c. voltage ranged from -3 to 1 V with an a.c.
@ 1993 -
Elsevier Sequoia. All rights reserved
Referenceelectrode \
p-silicon
A
E’s)ctrolyte
.-_-1
7’
Contactelectrode Fig. 1. Scheme of CV measurement Ag/AgCI.
setup. Reference electrode
signal amplitude of 20 mV between 500 and 2~~0 Hz. The following solutions were tested: KI solution (concentration range 10-6-10-’ M) with ISA addition (ionic strength adjuster 2 M NaNO,, pH 7 buffered) to determine the I- sensitivity pH buffer solutions (pH 4-9 containing constant low4 M KI) to examine the pH cross sensitivity KC1 (10-4-10-’ M) and KBr solutions (10m410-l M) containing constant lO-6 M KI to detect Cland Br- interferences. Furthermore, scanning electron microscopy, Rutherford back scattering spectroscopy and X-ray diffraction investigations were performed in order to characterize the structure and the composition of the deposited membranes.
Fig. 2. Scanning electron microscopic picture of an e AgI membrane.
.ated
l
l
l
2500
-1500
-500
Fig. 3. CV characteristics for different iodide concentrations 90 days.
after
Results and discussion
Film characterization
The influence of the processing parameters on the morphology and structure of the evaporated sensitive films was investigated. At low substrate temperature during deposition only porous layers were obtained. Increasing the substrate temperature to 200 “C and adjusting the deposition rate to 1 nmjs resulted in dense non-porous layers with large grain structures as can be seen in Fig. 2. A composition with fixed stoichiometry, independent of the chosen deposition rate, was observed with RBS analysis. Using X-ray diffraction investigations the polycrystalline nature of the layers could be seen.
tration. The relationship between the logarithm of the iodide concentration and the applied voltage is about 50-55 mV per decade, i.e. nearly Nernstian. Without a chromium adhesive layer the iodide sensitivity was lost within a few hours. In the presence of the chromium interlayer, however, the lifetime was considerably increased. As shown in Fig. 4 for five probes the I- sensitivity was about 50 mV per decade, even after a three month measurement period. The CV characteristics are highly reproducible as shown in Fig. 5. Between the two measurement cycles the samples were carefully rinsed with an electrolyte solution free of iodide. Selectivity
Sensitivity and lifetime A typical set of CV curves for I- concentrations
between low5 and IO-’ M is shown in Fig. 3. The parallel shift of the curves is caused by the flatband voltage shift towards positive values with increasing iodide concen-
To determine the selectivity coefficients for interfering ions, the mixed solution method was chosen. The Cl- and Br- ion concentrations were changed by adding constant I- background solution as given above.
194 N=5
300
,g
250
200
30
0
1"
0
LO
30
40
50
60
70
80
-50
90
0
I2
Sensor -+Fig. 4. Average iodide sensitivity of five EISMIS devices during their lifetime.
*
1.Iodide cycle
11
-SSOO
-1500
-500
5
ISE
6
-
7
8
9
PX
Fig. 6. Sensitivity and selectivity of the EISMIS device compared to the ISE (ORION).
5O(
Sias[mmv] Fig. 5. Reproducibility
4
Such inorganic layers were fabricated by vacuum evaporation of silver iodide. After optimization of the preparation conditions near Nernstian behaviour for iodide sensitivity and a good selectivity against chloride and bromide ions have been obtained. These parameters are comparable to those of the conventional iodide ISE. The influence of the pH value was not significant in the investigated range. The preparation of sensitive membranes compatible with current techniques of the semiconductor industry will allow the low cost production of small, rugged and compact devices.
4 10~~
3
References
of 0’ measurements.
The Nikolsky-Eisemann equation describes the theoretical relationship between the ion (a,-) to be measured and the interfering ion (a,, X = Cl-, Br-).
3
U = L’,,- (2.3RT/zF) log[u, + K,m_x * axz~-‘=x] (K,-_x: selectivity coefficient; a, R, F, T and z have their usual meaning). An excellent agreement of the selectivity coefficients of the evaporated sensor devices was found in comparison to that of a commercial iodide ISE (ORION). In the pH range tested (pH 4-9) dependence on the pH value was negligible (about 5 mV/pH). A summary of the selectivity characteristics and the calibration curve is presented in Fig. 6.
Conchsions
Thin polycrystalline AgI films as membranes for ionsensitive field-effect transistors have been developed.
4
8
9
10
J. W. Ross, M. S. Frant and J. H. Riseman, US Patent No. 3563874 (1971). P. Bergveld, Development of an ion-sensitive solid-state device for neurophysiological measurements, IEEE Trans. Biomed. Eng., BME-17 (1970) 70. P. Bergveld, Development, operation and application of the ion-sensitive field effect transistor as a tool for electrophysiology, IEEE Trans. Biomed. Eng., BME-19 (1972) 342. T. Matsuo and K. D. Wise, An integrated field-effect electrode for biopotential recording, IEEE Trans. Biomed. Eng., BME-ZJ (1974) 485. S. D. Moss, J. Janata and C. C. Johnson, Potassium ion-sensitive field effect transistor, Anal. Gem., 47 (1975) 2238. W. Moritz, I. Meierhlifer and L. Miiller, Fluoride-sensitive membrane for ISFETs, Sensors and Actuafors, I5 (1988) 211. R. P. Buck, D. E. Hackleman and Y. G. Vlasov, Fabrication of a silver, chloride and bromide-responsive ion selective fieldeffect popotentiometric sensor, Anal. Chem., 51(1979)1570. L. J. Bousse, P. Bergveld and H. J. M. Geeraedts, Properties of Ag/AgCl electrodes fabricated with IC-compatible technologies, Sensors and Actuators, 9 (1986) 119. P. Fabry, F. Laurent-Yvonnov, The CF-method for characterizing ISFET or EOS devices with ion-sensitive membranes, J. EJectroanaJ. Chem., 280 (1998) 23. B. T. Shiramizu, J. Janata and S. D. Moss, ISFETs with heterogeneous membranes, Anal. Chh. Acta., JO8 (1979)161.
Iodide ion-sensitive field-effect structures M. J. SchGning”, M. Bruns”, W. Hoffmamf, B. Hoffinannb and H. J. Ache” “Kernforschungszenlrum Karlsruhe GmbH. Insiitul fir Radiochemie, P.O. Box 3640, 7500 Karl&w
I (Germany)
bUniversitiit Karlsruhe, Institut fir Technologie der Elekrrotechnik, Hertzstrasse 16, 7500 Karlsruhe 21 (Germany)
A new ion-selective thin-film membrane for ion-sensitive field-effect structures has been realized. Silver iodide ( AgI) as an iodide-sensitive material has been vacuum evaporated onto semiconductor-insulator substrates. Various preconditioning and vacuum evapor!tion parameters as well as post treatment conditions were tested. Sensitivity and selectivity of these model field-effect structures were investigated by capacitance-voltage (CL’) measurements. Nernstian behaviour, good selectivity and a high lifetime comparable with that of the ion-selective electrode (ISE) could be obtained. This method of membrane fabrication is expected to be suitable for the large scale manufacture of sensor chips.
Introduction
In 1971 Ross et al. patented halide-sensitive electrochemical electrodes and the method for making them
[ 11. These ion-selective electrodes were prepared by coprecipitating the respective silver halides and silver sulfide as sensitive materials and subsequent pressing into dense, non-porous pellets. In 1970 Bergveld [2] used electronic devices for the first time for measuring chemical quantities. Such ionsensitive field-effect transistors (ISFETs) combine chemical-sensitive membranes and field-effect transistors. With respect to the membranes, there is particular interest in inorganic materials whose preparation technique is compatible to standardized semiconductor manufacturing procedures. Several kinds of ISFETs with different inorganic membranes are discussed in the literature in addition to the basic pH-sensitive insulator membranes, e.g. for the detection of sodium and potassium [3-51. With respect to halides, LaF, as an outstandingly sensitive material for F- analysis has been adapted to ISFETs [6]. For Cl- and Br- ions the respective silver halides were evaporated onto silicon-insulator substrates [7-91, but their stability was rather low or no detailed information was given. Until now an iodide-sensitive membrane has only been realized as a mixture of an organic matrix with AgI powder [lo], a method not compatible with semiconductor technology. The aim of this work was to produce solid state membranes for ion-sensitive field-effect structures by methods compatible with current techniques for microelectronic component production. Model experiments with ESMIS structures (electrolyte-Sensitive-membrane-
0925-4005/93/$6.00
insulator-semiconductor) are useful for studying sensor properties of the membrane materials. The preparation conditions and experimental results with AgI as an iodide-sensitive material are presented. Sensitivity, selectivity and lifetime are compared with the traditional iodide crystal membrane ISE.
Experimental Materials
and processing
Silicon wafers (p-type, 18-24 Q cm) with a double layer insulator of 30 nm SiO, (thermal oxidation at 1000 “C), 70 nm S&N, (LPCVD technique) and a rear ohmic contact were used. These samples were cleaned with acetone, HF (I’%), a mixture of H,SO, and H,O, (all solvents were of analytical grade) and quartz distilled water. Silver iodide as the iodide-sensitive material was vacuum evaporated from pressed AgI (Alfa Products) pellets onto the cleaned p-Si/SiOJSi,N, substrates at a pressure of less than 10m3Pa. The substrates could be heated from 25 to 350 “C. AgI films with thickness from 100 to 2000 nm were fabricated; deposition rates could be varied between 0.05 and 5 rim/s.. An ,additional adhesive layer of approximately 20nm chromium between the S&N, substrate and the silver iodide film was prepared. Measurements
To investigate the iodide sensitivity and selectivity of the model field-effect structures the samples were mounted in a CV measurement cell (Fig. 1). The applied d.c. voltage ranged from -3 to 1 V with an a.c.
@ 1993 -
Elsevier Sequoia. All rights reserved
Referenceelectrode \
p-silicon
A
E’s)ctrolyte
.-_-1
7’
Contactelectrode Fig. 1. Scheme of CV measurement Ag/AgCI.
setup. Reference electrode
signal amplitude of 20 mV between 500 and 2~~0 Hz. The following solutions were tested: KI solution (concentration range 10-6-10-’ M) with ISA addition (ionic strength adjuster 2 M NaNO,, pH 7 buffered) to determine the I- sensitivity pH buffer solutions (pH 4-9 containing constant low4 M KI) to examine the pH cross sensitivity KC1 (10-4-10-’ M) and KBr solutions (10m410-l M) containing constant lO-6 M KI to detect Cland Br- interferences. Furthermore, scanning electron microscopy, Rutherford back scattering spectroscopy and X-ray diffraction investigations were performed in order to characterize the structure and the composition of the deposited membranes.
Fig. 2. Scanning electron microscopic picture of an e AgI membrane.
.ated
l
l
l
2500
-1500
-500
Fig. 3. CV characteristics for different iodide concentrations 90 days.
after
Results and discussion
Film characterization
The influence of the processing parameters on the morphology and structure of the evaporated sensitive films was investigated. At low substrate temperature during deposition only porous layers were obtained. Increasing the substrate temperature to 200 “C and adjusting the deposition rate to 1 nmjs resulted in dense non-porous layers with large grain structures as can be seen in Fig. 2. A composition with fixed stoichiometry, independent of the chosen deposition rate, was observed with RBS analysis. Using X-ray diffraction investigations the polycrystalline nature of the layers could be seen.
tration. The relationship between the logarithm of the iodide concentration and the applied voltage is about 50-55 mV per decade, i.e. nearly Nernstian. Without a chromium adhesive layer the iodide sensitivity was lost within a few hours. In the presence of the chromium interlayer, however, the lifetime was considerably increased. As shown in Fig. 4 for five probes the I- sensitivity was about 50 mV per decade, even after a three month measurement period. The CV characteristics are highly reproducible as shown in Fig. 5. Between the two measurement cycles the samples were carefully rinsed with an electrolyte solution free of iodide. Selectivity
Sensitivity and lifetime A typical set of CV curves for I- concentrations
between low5 and IO-’ M is shown in Fig. 3. The parallel shift of the curves is caused by the flatband voltage shift towards positive values with increasing iodide concen-
To determine the selectivity coefficients for interfering ions, the mixed solution method was chosen. The Cl- and Br- ion concentrations were changed by adding constant I- background solution as given above.
194 N=5
300
,g
250
200
30
0
1"
0
LO
30
40
50
60
70
80
-50
90
0
I2
Sensor -+Fig. 4. Average iodide sensitivity of five EISMIS devices during their lifetime.
*
1.Iodide cycle
11
-SSOO
-1500
-500
5
ISE
6
-
7
8
9
PX
Fig. 6. Sensitivity and selectivity of the EISMIS device compared to the ISE (ORION).
5O(
Sias[mmv] Fig. 5. Reproducibility
4
Such inorganic layers were fabricated by vacuum evaporation of silver iodide. After optimization of the preparation conditions near Nernstian behaviour for iodide sensitivity and a good selectivity against chloride and bromide ions have been obtained. These parameters are comparable to those of the conventional iodide ISE. The influence of the pH value was not significant in the investigated range. The preparation of sensitive membranes compatible with current techniques of the semiconductor industry will allow the low cost production of small, rugged and compact devices.
4 10~~
3
References
of 0’ measurements.
The Nikolsky-Eisemann equation describes the theoretical relationship between the ion (a,-) to be measured and the interfering ion (a,, X = Cl-, Br-).
3
U = L’,,- (2.3RT/zF) log[u, + K,m_x * axz~-‘=x] (K,-_x: selectivity coefficient; a, R, F, T and z have their usual meaning). An excellent agreement of the selectivity coefficients of the evaporated sensor devices was found in comparison to that of a commercial iodide ISE (ORION). In the pH range tested (pH 4-9) dependence on the pH value was negligible (about 5 mV/pH). A summary of the selectivity characteristics and the calibration curve is presented in Fig. 6.
Conchsions
Thin polycrystalline AgI films as membranes for ionsensitive field-effect transistors have been developed.
4
8
9
10
J. W. Ross, M. S. Frant and J. H. Riseman, US Patent No. 3563874 (1971). P. Bergveld, Development of an ion-sensitive solid-state device for neurophysiological measurements, IEEE Trans. Biomed. Eng., BME-17 (1970) 70. P. Bergveld, Development, operation and application of the ion-sensitive field effect transistor as a tool for electrophysiology, IEEE Trans. Biomed. Eng., BME-19 (1972) 342. T. Matsuo and K. D. Wise, An integrated field-effect electrode for biopotential recording, IEEE Trans. Biomed. Eng., BME-ZJ (1974) 485. S. D. Moss, J. Janata and C. C. Johnson, Potassium ion-sensitive field effect transistor, Anal. Gem., 47 (1975) 2238. W. Moritz, I. Meierhlifer and L. Miiller, Fluoride-sensitive membrane for ISFETs, Sensors and Actuafors, I5 (1988) 211. R. P. Buck, D. E. Hackleman and Y. G. Vlasov, Fabrication of a silver, chloride and bromide-responsive ion selective fieldeffect popotentiometric sensor, Anal. Chem., 51(1979)1570. L. J. Bousse, P. Bergveld and H. J. M. Geeraedts, Properties of Ag/AgCl electrodes fabricated with IC-compatible technologies, Sensors and Actuators, 9 (1986) 119. P. Fabry, F. Laurent-Yvonnov, The CF-method for characterizing ISFET or EOS devices with ion-sensitive membranes, J. EJectroanaJ. Chem., 280 (1998) 23. B. T. Shiramizu, J. Janata and S. D. Moss, ISFETs with heterogeneous membranes, Anal. Chh. Acta., JO8 (1979)161.