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An ISFET-based calcium sensor using a photopolymerized polysiloxane membrane
Sensors and Actuators
B, 4 (1991) 235-238
235
An ISFET-based Calcium Sensor Using a Photopolymerized Polysiloxane Membrane A. VAN DEN BERG and A. GRISEL Centre Suisse d’Electronique
et da Microtechnique
SA, Rue de la Maladikre
71, NeuchBtel, CH-2007
(Switzerland)
E. VERNEY-NORBERG G.I.E. Reeherche du P6le Energie Chaleur, C.E.R.C.L.E., Chemin de la Forest&e, 69130 Ecu& (France)
Groupe Lyonnaise des Eaux, L’Orke d’EcuNy,
Abstract
Introduction
In this paper an on-wafer fabrication technique for membrane-covered calcium-sensing ISFETs is presented. The complete structure consists of a standard Alz03 pH-ISFET, covered with a hydrogel membrane and a second polysiloxane-type calcium-sensing membrane. Both membranes are deposited and photolithographically structured on the wafer. The adhesion of the membranes to the ISFET surface is ensured by presilylating the surface prior to the deposition of the membranes, and has been confirmed by tests in an ultrasonic bath. The on-wafer fabrication of calcium-sensitive ISFETs with a 25 pm thick poly (hydroxyethyl methacrylate) (polyHEMA) membrane covered with a 80 pm thick polysiloxane membrane is described and illustrated. The structural membrane definition for both membranes is better than 20 pm. The calcium-sensing properties of three completed devices have been investigated. A reproducible sensitivity of 28.7 mV/pCa and a detection limit of 10-6.4 M calcium are found upon titration of CaCl, in demineralized water. The selectivity coefficients with respect to magnesium, sodium, and potassium are found to be log KgMg = -4.9, log KPO’ = -4.5, and log Kzk = - 4.1 respecCaNa tively, which are comparable with the literature values.
Up to now, the method most frequently used to fabricate ion sensors, based on ISFETs, for ions other than H+ is to coat the ISFETs with a plasticized ionophore-loaded PVC membrane. This coating is usually carried out by dipping the encapsulated ISFET one or several times in a diluted solution of the membrane material in THF. This method, published for the first time in 1975 by Moss et al. [ 11, has several disadvantages. The membranes adhere only by physical interaction to the ISFET surface, the plasticizer slowly leaks out of the membrane, the membrane-covered ISFETs are subject to CO, interference, the thickness control is rather poor and, above all, the dipping method cannot easily be applied on whole-wafer scale. For most of these problems, separate solutions have been proposed. Methods using a suspended mesh [ 21, chemically modified PVC [3] and recently the use of alternative membrane materials like silicones [4] have all resulted in a substantial improvement of the membrane adhesion and thus the device lifetime. The problem of loss of plasticizer has been overcome by either applying a photocross-linkable plasticizer [S], or by using the aforementioned intrinsically plastic silicone-type membranes [4]. The suppression of the COZ interference has been realized by using either a Ag/AgCl type MOSFET
0925-4005/91/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
236
instead of an ISFET as the underlying structure [6], or by interposing a hydrogel membrane conditioned in an appropriate buffer [7,8]. The last two problems mentioned above, however, have until now not had a satisfactory solution that also takes account of the other problems mentioned. In this paper we will describe the on-wafer fabrication of calcium-sensitive ISFETs using a photopolymerizable membrane material. The ion-sensitive material used is similar to that reported by van der Wal et al. [4], but with the important difference that it can be photolithographically patterned. In order to ensure a thermodynamically well-defined membrane-ISFET interface, an intermediate polyHEMA membrane is used. This is chemically attached to the previously functionalized ISFET surface. The ion-sensitive membrane is also attached to the sensor surface by chemical bonding. The complete sensor structure is depicted in Fig. 1.
Sensor Fabrication The fabrication of the basic ISFET structures has been described in an earlier paper [9]. The completed wafers containing the ISFETs were presilylated with 3-( trimethoxysilyl)propyl methacrylate prior to the membrane deposition. Subsequently, polyHEMA
Esl
Silicon
0
p-Well
a
Source
c----l
Field
oxide
m
Gots
owlde
Implantation +
drain
Fig. 1. Schematic sensor.
impl.
drawing
0
Aluminium
oxlda
6
Membrane
adhesion
m
Hydrogel
IUKU
Ion-srlectiw
kz2a
Encapsulation
of the
layer
membrane membrane
complete
calcium
Fig. 2. SEM photograph of part of a wafer containing ISFET structures covered with approximately 30 pm thick pHEMA membranes.
membranes with a varying thickness between 20 and 50 ,um were deposited and photolithographically patterned onto the pH-sensitive ISFET gates as described in ref 8. In Fig. 2 a SEM photograph of part of a wafer with the polyHEMA membranes is shown. The approximate thickness of the membranes is 30 pm. After deposition of the membranes, they were conditioned in a 1:1 mixture of a pH 4 buffer and 0.1 M CaCl,. In the next step, polysiloxane membranes containing 0.7 wt.% calcium ionophore (ETH 129, Fluka) and 0.4 wt.% potassium tetrakis( 4-chlorophenyl) borate were photolithographically patterned on top of the polyHEMA-covered ISFETs. The thickness of the calcium-sensitive membrane was chosen such that it covered the underlying polyHEMA membrane completely. The completed structure is shown in Fig. 3. In this case, the polyHEMA membranes have a thickness of approximately 25 pm, whereas the polysiloxane membranes are approximately 80 ,um thick (or 55 pm on top of the polyHEMA). From the photograph it can be concluded that the structure definition is better than 20 ,um, which is quite satisfactory for the membrane thickness involved. A light-microscope photograph shows the double membrane structure over the gate of the pH-ISFET more clearly (Fig. 4).
231
-9
-8
-7
-6
-5
-4
-3
-2
-1
U
log a(Q) _
Fitted response A Ca5-2
Fig. 3. SEM photograph of part of a wafer containing ISFET structures covered with both a 25 pm thick pHEMA membrane and a 80 pm thick calcium-sensitive polysiloxane membrane.
Fig. 4. Light-microscope as in Fig. 3.
photograph of the same wafer
.
Ca5 - 1
x ca5-3
Fig. 5. Response of three different calcium-sensitive ISFETs covered with a photopolymerized pHEMA and polysiloxane membrane upon titration of CaCl, into demineralized water. T = 25 “C.
tion limit is because no calcium buffer was used in the background solution. Selectivities were measured by the separate solution method using 0.1 M unbuffered metal chloride solutions. Towards magnesium, sodium, and potassium the potentiometric selectivities were log KgMg = -4.9, log KzNa = -4.5 and log Kgk = - 4.1 respectively. These values are in good agreement with those reported in the literature for conventional calcium-sensitive PVC membranes [ lo]. The adhesion of the membranes to the sensor surface (A120g) was confirmed by tests in an ultrasonic bath in a pH 7 buffer. It was found that without chemical pretreatment, all membranes separated from the A1203 surface within five minutes, whilst there was no visible loss of membrane adhesion even after two hours when using presilylated surfaces.
Results Conclusions
The performance of the ISFETs covered with the photopolymerized calcium-sensitive membranes has been investigated for three completed devices. The calcium sensitivity was tested by titration of a demineralized, unbuffered, aqueous solution with CaCl,. The response curve of the three sensors is shown in Fig. 5. The measured sensitivity was 28.7 mV/dec and the detection limit 10-6.4 M calcium. The relatively high level of the detec-
This method of depositing and photolithographically structuring ion-sensitive polysiloxane membranes is suitable for the production of ion sensors on a whole-wafer scale. It allows the formation of thick membranes (SO- 100 pm) that can be structured into relatively well-defined surface areas (definition better than 20 pm). The calcium sensing behaviour is as good as with conventional
238
PVC-type membranes, whereas the large thickness prevents a too rapid leakage of the ionophore. Considering the excellent adhesion properties in addition, the lifetime of this ion-sensing device is expected to be comparable to that of conventional ion-selective membrane electrodes. Finally, the method offers the very attractive possibility for producing multi-ion sensors on-wafer.
References S. D. Moss, J. Janata and C. C. Johnson, Potassium ion-sensitive field effect transistor, Anal. Chem., 47 (1975) 2238. G. F. Blackburn and J. Janata, The suspended mesh ion selective field effect transistor, J. Electrochem. Sot., 129 (1982) 2580. D. J. Harrisson, A. Teclemariam and L. L. Cunningham, Proc. 4th Znt. Conf. Solid-State Sensors and Actuators (Transducers ‘87), Tokyo, Japan, June 2-5, 1987, p. 768.
P. D. van der Wal, M. Skowronska-Ptasinska, A. van den Berg, P. Bergveld, E. J. R. Sudhiilter and
D. N. Reinhoudt, New membrane materials for potassium-selective ion-sensitive field-effect transistors, Anal. Chim. Acta, 231 (1990) 41. D. J. Harrisson, A. Teclemariam and L. Cunningham, Photopolymerization of plasticizer in ion-sensitive membranes on solid-state sensors, Anal. Chem., 61 (1989) 246.
K. Nagy, T. A. Fjeldly and J. S. Johannessen, Proc. J53rd Ann. Meet. Electra. Chem. Sot.,
78 (1978)
Abstr. 108. E. J. R. Sudholter, M. Skowronska-Ptasinska, P. D. van der Wal, A. van den Berg and D. N. Reinhoudt, Eur. Pat. Applic. No. 258 951 (1986). E. J. R. Sudholter, P. D. van der Wal, M. Skowronska-Ptasinska, A. van den Berg, P. Bergveld and D. N. Reinhoudt, Modification of ISFETs by covalent anchoring of (poly)hydroxyethyl methacrylate) hydrogel. Introduction of a thermodynamically defined semiconductor-sensing membrane interface, Anal. Chim. Acta. 230 (1990) 59.
R. L. Smith and D. C. Scott, An integrated sensor for electrochemical measurements, IEEE Trans. Biomed. Eng., BME-33
(1986) 83.
10. U. Schefer, D. Ammann, E. Pretsch, U. Oesch and W. Simon, Neutral carrier based Ca*+ selective electrode with detection limit in the sub-nanomolar range, Anal. Chem., 58 (1986) 2282.
B, 4 (1991) 235-238
235
An ISFET-based Calcium Sensor Using a Photopolymerized Polysiloxane Membrane A. VAN DEN BERG and A. GRISEL Centre Suisse d’Electronique
et da Microtechnique
SA, Rue de la Maladikre
71, NeuchBtel, CH-2007
(Switzerland)
E. VERNEY-NORBERG G.I.E. Reeherche du P6le Energie Chaleur, C.E.R.C.L.E., Chemin de la Forest&e, 69130 Ecu& (France)
Groupe Lyonnaise des Eaux, L’Orke d’EcuNy,
Abstract
Introduction
In this paper an on-wafer fabrication technique for membrane-covered calcium-sensing ISFETs is presented. The complete structure consists of a standard Alz03 pH-ISFET, covered with a hydrogel membrane and a second polysiloxane-type calcium-sensing membrane. Both membranes are deposited and photolithographically structured on the wafer. The adhesion of the membranes to the ISFET surface is ensured by presilylating the surface prior to the deposition of the membranes, and has been confirmed by tests in an ultrasonic bath. The on-wafer fabrication of calcium-sensitive ISFETs with a 25 pm thick poly (hydroxyethyl methacrylate) (polyHEMA) membrane covered with a 80 pm thick polysiloxane membrane is described and illustrated. The structural membrane definition for both membranes is better than 20 pm. The calcium-sensing properties of three completed devices have been investigated. A reproducible sensitivity of 28.7 mV/pCa and a detection limit of 10-6.4 M calcium are found upon titration of CaCl, in demineralized water. The selectivity coefficients with respect to magnesium, sodium, and potassium are found to be log KgMg = -4.9, log KPO’ = -4.5, and log Kzk = - 4.1 respecCaNa tively, which are comparable with the literature values.
Up to now, the method most frequently used to fabricate ion sensors, based on ISFETs, for ions other than H+ is to coat the ISFETs with a plasticized ionophore-loaded PVC membrane. This coating is usually carried out by dipping the encapsulated ISFET one or several times in a diluted solution of the membrane material in THF. This method, published for the first time in 1975 by Moss et al. [ 11, has several disadvantages. The membranes adhere only by physical interaction to the ISFET surface, the plasticizer slowly leaks out of the membrane, the membrane-covered ISFETs are subject to CO, interference, the thickness control is rather poor and, above all, the dipping method cannot easily be applied on whole-wafer scale. For most of these problems, separate solutions have been proposed. Methods using a suspended mesh [ 21, chemically modified PVC [3] and recently the use of alternative membrane materials like silicones [4] have all resulted in a substantial improvement of the membrane adhesion and thus the device lifetime. The problem of loss of plasticizer has been overcome by either applying a photocross-linkable plasticizer [S], or by using the aforementioned intrinsically plastic silicone-type membranes [4]. The suppression of the COZ interference has been realized by using either a Ag/AgCl type MOSFET
0925-4005/91/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
236
instead of an ISFET as the underlying structure [6], or by interposing a hydrogel membrane conditioned in an appropriate buffer [7,8]. The last two problems mentioned above, however, have until now not had a satisfactory solution that also takes account of the other problems mentioned. In this paper we will describe the on-wafer fabrication of calcium-sensitive ISFETs using a photopolymerizable membrane material. The ion-sensitive material used is similar to that reported by van der Wal et al. [4], but with the important difference that it can be photolithographically patterned. In order to ensure a thermodynamically well-defined membrane-ISFET interface, an intermediate polyHEMA membrane is used. This is chemically attached to the previously functionalized ISFET surface. The ion-sensitive membrane is also attached to the sensor surface by chemical bonding. The complete sensor structure is depicted in Fig. 1.
Sensor Fabrication The fabrication of the basic ISFET structures has been described in an earlier paper [9]. The completed wafers containing the ISFETs were presilylated with 3-( trimethoxysilyl)propyl methacrylate prior to the membrane deposition. Subsequently, polyHEMA
Esl
Silicon
0
p-Well
a
Source
c----l
Field
oxide
m
Gots
owlde
Implantation +
drain
Fig. 1. Schematic sensor.
impl.
drawing
0
Aluminium
oxlda
6
Membrane
adhesion
m
Hydrogel
IUKU
Ion-srlectiw
kz2a
Encapsulation
of the
layer
membrane membrane
complete
calcium
Fig. 2. SEM photograph of part of a wafer containing ISFET structures covered with approximately 30 pm thick pHEMA membranes.
membranes with a varying thickness between 20 and 50 ,um were deposited and photolithographically patterned onto the pH-sensitive ISFET gates as described in ref 8. In Fig. 2 a SEM photograph of part of a wafer with the polyHEMA membranes is shown. The approximate thickness of the membranes is 30 pm. After deposition of the membranes, they were conditioned in a 1:1 mixture of a pH 4 buffer and 0.1 M CaCl,. In the next step, polysiloxane membranes containing 0.7 wt.% calcium ionophore (ETH 129, Fluka) and 0.4 wt.% potassium tetrakis( 4-chlorophenyl) borate were photolithographically patterned on top of the polyHEMA-covered ISFETs. The thickness of the calcium-sensitive membrane was chosen such that it covered the underlying polyHEMA membrane completely. The completed structure is shown in Fig. 3. In this case, the polyHEMA membranes have a thickness of approximately 25 pm, whereas the polysiloxane membranes are approximately 80 ,um thick (or 55 pm on top of the polyHEMA). From the photograph it can be concluded that the structure definition is better than 20 ,um, which is quite satisfactory for the membrane thickness involved. A light-microscope photograph shows the double membrane structure over the gate of the pH-ISFET more clearly (Fig. 4).
231
-9
-8
-7
-6
-5
-4
-3
-2
-1
U
log a(Q) _
Fitted response A Ca5-2
Fig. 3. SEM photograph of part of a wafer containing ISFET structures covered with both a 25 pm thick pHEMA membrane and a 80 pm thick calcium-sensitive polysiloxane membrane.
Fig. 4. Light-microscope as in Fig. 3.
photograph of the same wafer
.
Ca5 - 1
x ca5-3
Fig. 5. Response of three different calcium-sensitive ISFETs covered with a photopolymerized pHEMA and polysiloxane membrane upon titration of CaCl, into demineralized water. T = 25 “C.
tion limit is because no calcium buffer was used in the background solution. Selectivities were measured by the separate solution method using 0.1 M unbuffered metal chloride solutions. Towards magnesium, sodium, and potassium the potentiometric selectivities were log KgMg = -4.9, log KzNa = -4.5 and log Kgk = - 4.1 respectively. These values are in good agreement with those reported in the literature for conventional calcium-sensitive PVC membranes [ lo]. The adhesion of the membranes to the sensor surface (A120g) was confirmed by tests in an ultrasonic bath in a pH 7 buffer. It was found that without chemical pretreatment, all membranes separated from the A1203 surface within five minutes, whilst there was no visible loss of membrane adhesion even after two hours when using presilylated surfaces.
Results Conclusions
The performance of the ISFETs covered with the photopolymerized calcium-sensitive membranes has been investigated for three completed devices. The calcium sensitivity was tested by titration of a demineralized, unbuffered, aqueous solution with CaCl,. The response curve of the three sensors is shown in Fig. 5. The measured sensitivity was 28.7 mV/dec and the detection limit 10-6.4 M calcium. The relatively high level of the detec-
This method of depositing and photolithographically structuring ion-sensitive polysiloxane membranes is suitable for the production of ion sensors on a whole-wafer scale. It allows the formation of thick membranes (SO- 100 pm) that can be structured into relatively well-defined surface areas (definition better than 20 pm). The calcium sensing behaviour is as good as with conventional
238
PVC-type membranes, whereas the large thickness prevents a too rapid leakage of the ionophore. Considering the excellent adhesion properties in addition, the lifetime of this ion-sensing device is expected to be comparable to that of conventional ion-selective membrane electrodes. Finally, the method offers the very attractive possibility for producing multi-ion sensors on-wafer.
References S. D. Moss, J. Janata and C. C. Johnson, Potassium ion-sensitive field effect transistor, Anal. Chem., 47 (1975) 2238. G. F. Blackburn and J. Janata, The suspended mesh ion selective field effect transistor, J. Electrochem. Sot., 129 (1982) 2580. D. J. Harrisson, A. Teclemariam and L. L. Cunningham, Proc. 4th Znt. Conf. Solid-State Sensors and Actuators (Transducers ‘87), Tokyo, Japan, June 2-5, 1987, p. 768.
P. D. van der Wal, M. Skowronska-Ptasinska, A. van den Berg, P. Bergveld, E. J. R. Sudhiilter and
D. N. Reinhoudt, New membrane materials for potassium-selective ion-sensitive field-effect transistors, Anal. Chim. Acta, 231 (1990) 41. D. J. Harrisson, A. Teclemariam and L. Cunningham, Photopolymerization of plasticizer in ion-sensitive membranes on solid-state sensors, Anal. Chem., 61 (1989) 246.
K. Nagy, T. A. Fjeldly and J. S. Johannessen, Proc. J53rd Ann. Meet. Electra. Chem. Sot.,
78 (1978)
Abstr. 108. E. J. R. Sudholter, M. Skowronska-Ptasinska, P. D. van der Wal, A. van den Berg and D. N. Reinhoudt, Eur. Pat. Applic. No. 258 951 (1986). E. J. R. Sudholter, P. D. van der Wal, M. Skowronska-Ptasinska, A. van den Berg, P. Bergveld and D. N. Reinhoudt, Modification of ISFETs by covalent anchoring of (poly)hydroxyethyl methacrylate) hydrogel. Introduction of a thermodynamically defined semiconductor-sensing membrane interface, Anal. Chim. Acta. 230 (1990) 59.
R. L. Smith and D. C. Scott, An integrated sensor for electrochemical measurements, IEEE Trans. Biomed. Eng., BME-33
(1986) 83.
10. U. Schefer, D. Ammann, E. Pretsch, U. Oesch and W. Simon, Neutral carrier based Ca*+ selective electrode with detection limit in the sub-nanomolar range, Anal. Chem., 58 (1986) 2282.