Effect of membrane structure on the performance of field-effect transistor potassium-sensitive sensor

ELSEVI ER

Sensorsand ActuatorsA 57 (1996) 239-243

Effect of membrane structure on the performance of field-effect transistor potassium-sensitive sensor L e e - S o o n F a r k ~'*, Y o u n g - J u n H u r a, B y u n g - K i

Sohn b

"Deparunenr of Polymer Science, KyungpookNational University. Taegu, 702-701, South Korea b Department of Electronics Engineering, Ky~mgpookNcaional University, Taegu, 702-70]. South Korea

Received18 April 1996:revised31 July 1996;accepted6 August 1996

Abstract An bET-type K *-sensitive sensor with a membrane composed of a hydmphobic inner layer and a hy&ophilic outer layer has been fabrica~ by a photolithographic process. The base chip for the sensor is a pH-ISFET with a thin Si3N4layer. Negative photoresist (OMR-83) is used as the inner layer and poly(vinyl pyrrolidinone-co-vinyl acetate) solution in tetrahydrofuran containing valinomycin and 2,6-b~s-(p-azidobenzylidene) cyclohexanone photosensitizer is used as the outer sensing membrane material. The K-ISFET sensor with double-layered membrane shows a high sensitivity (56 mV/decade) toward K + ion, rapid response ( 1-2 s) and low interference (less than 3 mV/decade) from the competing H + ion. Keywords: Field-effecttransistor sensors; Ion-sensitivefield-effecttransistors (ISFETs);PotassiumISFETs;Double-layeredmemlnanes;P h o t o ~

method

1. Introduction

Since the discovery of the ion-sensitive field-effect transistor (ISFET) in the early 1970s [ 1], chemically modified bETs for selective ion detection have become of increasing interest. Moss et al. introduced the K+-selective ionophore valinomycin in a plasticized poly(vinyl chloride) (PVC) membrane which was attached to the gate surface of the FET [ 2]. Leaching out ofionophore and detachment of membrane from the surface gate were problems when using a conventional polymer like PVC. To solve these problems, various polymers have been used as membrane materials for ISFETtype ion sensors [3-7]. The application e.f silicon integrated circuit technology to the fabrication of ISFETs has also become important for the production of small, inexpensive and uniform-performance sensors. For the biosensors, photosensitive poly(vinyl alcohol), PVA [8] or polyvinylpyrrolidinone [9] with photosensitizer were used to generate membrane patterns. For the ion-sensitive FEI's, polysiloxane [ 10] and polyacylate [ 11 ] membranes were patterned by using a functionalized photosensitive system. * Corresponding author. Phone: +82 53 950 5627. Fax: +82 53 950 6623. 0924-4247/96/$15.00 © 1996ElsevierScienceS.A. All rightsleserved PIIS0924-4247(96)01380-5

The construction of a monolayer membrane with one polymer material on an ISFET, however, exhibited problems such as the need for plasticizer, weak adhesion to the wafer, and slow response time. Some of the these problems were solved by using modified polysiloxane as the membrane material, but these materials were not c o ~ i a l l y available or were difficult to synthesize. In this study, an FET-type K+sensitive sensor with a membrane composed of a hydrophobia inner layer and a hydrophilic outer layer was fabricated by a photolithographic process with readily available photosensitive polymers and its sensing characteristics were investigated.

2. Experimental 2.1. Chemicals

Valinomycin used as the potassium ionophore and potassium tetrakis(p-chlorophenyl) borate (PTCB) used as the anionic site were obtained from Fluka Chemical Co. Poly(vinyl chloride), poly(vinyl butyral) and poly(vinyl pyrrolidinone-co-vinyl acetate) (PVP-PVAc), used as polymer matrix and photosensitizer, and 2,6-bis-(p-azidobenzyIidene)cyclohexanone (BAC) were purchased from Tokyo Chemical Ind. (Japan) and used without further purification.

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L.S. Parket al./Sensors and ActuatorsA 57 (1996) 239-243

A negative photoresist (OMR-83 from Tokyo Ohka Co.) based on partially cyclized polyisoprene was used as received. Dioctyl adipate (DOA) and dioctyl phthalate (DOP) plasticizer and solvents such as tetrahydrofuran (THF), isopropyl alcohol (IPA), dioxane and xylene were all of reagent grade purchased from Aldrich Chemical Co. All metal salts were also reagent grade. Deionized water ( ~ 18 MI1 cm) was used to prepare buffer solutions. 2.Z Fabrication o f sensors

The pH-ISFET base chip used in this work had the same basic structure as described in Ref. [ 12]. A cross-sectional view of the ISFET potassium ion sensor (K-ISFET) is shown in Fig. 1. Both mono- and double-layered membranes were patterned on the gate area (20/zm × 600/zm) of the ISFET by a photolithoTaphic method. The K-ISFET with doublelayered membrane had a thin insulating layer between the Si3N4 layer of the pH-ISFET base chip and valinomycincontaining sensing membrane. For the formation of the inner insulating layer a negative photoresist (OMR-83) was diluted with toluene (OMR83:toluene = 1:2) and the solution was spin coated on the pHISFET base chip and dried at 80°C for 3 min. A typical photosensitive solution for the formation of the outer sensing layer consisted of 1000 mg of 30 wt.% PVF-FVAc solution in THF, 4 mg of vaiinomycin, 2 mg of PTCB and 8 mg of BAC photosensitizer. About 50 mg of this solution was dropped on top of the OMR-83 insulating thin film and then spin coated (3500 rpm, 30 s). The composition of the photosensitive solution was modified to study its effect on the sensor performance. The cast membrane film was heat-treated in an 80°C oven for 10 min, exposed to UV light with a mask aligner and then developed in IPA solvent and washed with dioxane. A similar procedure was used for the formation of the monolayer membrane by a photolithographic method. 2.3. Testing o f K-ISFET sensor.~

The fabricated K-ISFET sensors were tested in a constant drain current (Id=30 pA) and drain potential (Vd=3.0 V)

[ (s)

.

Fig. I. Stnlctureof sensor chip. (a) pH-ISFETba~ chip layout (IA mm×2.5 nun): D. drain;S, source;G, gate;W, p-well;WC, p-well.(b) Cross-sectionalviewof K-ISFETsensorwithdouble-layeredmembrane:l, source;2, siliconsubstrate;3. drain,4, 5, SigN,;6, SiO2;7, ahirainium;8, ion-~nsingmembrane;9, OMR-S3insulatingmembrane.

mode. The change of output potential was recorded while varying the K + ion concentration in a phosphate buffer solution at constant temperature (25 +0.2°C).

3. R e s u l t s a n d d i s c u s s i o n

3.1. K-ISFET with monolayer membrane

Various photosensitive polymer systems were used to pattern the sensing membrane of K-ISFET by a photolithographic method. First, photosensitization of plasticized poly (vinyl chloride), PVC, was attempted. The photosensitive solution was composed of 0.15 g PVC, 0.65 g DOP, 5 ml THF and 0.10 g of BAC photosensitizer. This solution was spin coated on the pH-ISFET base chip, heat-treated (80°(2, 5 min) and then irradiated with UV light through a mask. From the photomicroscopic observation of the photolithographic process, it was noted that the migration of plasticizer (DOP) during the UV irradiation caused a significant shrinkage in the PVC membrane. Therefore it was concluded that PVC could not be used as the membrane material with a photolithographic method, although it is often used as a membrane in the plasticized form with the dip-coating method. Akiyama and Niki reported that a membrane composed of a negative photoresist (OMR-83) and DOA plasticizer with valinomycin gave good characteristics as a K + ion sensor [ 13]. When fabrication of the membrane was tried by a photolithographic method with a photosensitive solution containing OMR-83 and DOA plasticizer, only poor patterning could be obtained even with an additional amount of BAC photosensitizer due to the same phenomenon of plasticizer migration as described above for the photosensitive PVC solution with DOP plasticizer. Gotoh et al. reported that poly (vinyl butyrai) ( PVB ) could be used as a membrane material in an FET-typa urea sensor by a dip-coating method without any plasticizer [ 14]. Therefore PVB was tested as a membrane material using a photolithographic method. The composition of photosensitive solution containing PVB as polymer matrix and the sensing characteristics of the K-ISFET with monolayer membrane are summarized in Table 1. PVB as a single matrix polymer in the sensing membrane of the K-ISFET gave a relatively high output voltage toward K + ion, but the sensor also responded to H + ions in the solution. This might be caused by the relatively high hydrophilicity of the PVB membrane, allowing part of the H + ions to be transported into the Si3N4 layer under the PVB membrane, which is sensitive to H + ions. PVB, interestingly, required neither plasticizer for sensing membrane nor presilylating of ISFET base chip to ensure good adhesion. Another polymer tested as matrix for the K + ion-sensing membrane was PVP-PVAc, which is a copolymer with a vinyl pyrrolidinone/vinyl chloride repeat unit of 30/70 mol.%. PVP-PVAc has a low glass transition temperature of

L-S. Parket al. /Sensors and Actuators A 57 (1996) 239-243

2,41

Table i Compositionof photosensitivesolutionwith PVB as singlemembranematerialand measmementof K-ISFETfabricatedby phoc~du~a~ic raethod Devicenumber pK-1

pK-2

pK-3

pK.-4

( 1) Compositionof photosensitivepolymersolution (rag) PVB/THFsolution (14.5%) BAC photosensitizer PTCB Valinomycin

1000 12 2 I

1000 12 2 2

1000 12 2 4

1000 12 2 8

(2) Measuringcharacteristicsof K-ISFET K+ ion range (tool l-t) Sensitivity (mV/decade)

I0- t-lO -s 34

I0- l-lO-S 42

I0- I-I0 -s 48

lO-t-lO -s 44

Table 2 Compositionof photosensitivesolutionwith PVP-PVAcas singlemembranematerialand measmemeatof K-ISFETfabricatedby pl~olithogtal~ ranthod Devicen u m b ~ SD-1

SD-2

SD-3

SD4

( I ) Compositionof photosensitivepolymersolution (rag) PVP-PVAcsolution BAC/THF solution Valinomycin PTCB

200 800 4 2

300 700 4 2

400 600 4 2

500 500 4 2

(2) MeasurementK-ISFET pK range Sensitivity(mV/decade) pH range Sensitivity(mV/decade)

4--2 8 9-4 55

4-2 6 9-.4 50

4--2

4-2 6 9-4 48

- 5 ° C (by differential scanning calorimetry) and also has good adhesion properties. The composition of photosensitive solution using PVP-PVAc as matrix polymer and measurement data of K-ISFET are shown in Table 2. PVP-PVAc as sensing membrane gave a larger response to the competing H + ion than the one with PVB as matrix polymer. The large response of the K-ISFET with PVP-PVAc as sensing membrane suggests that the hydrophilicity of PVP-PVAc is higher than that of P V B This was also confirmed through the developing stage of the membrane, i.e., the PVP-PVAc membrane could be developed even in water. 3.2. K-ISFETwithdouble-layeredmembrane [15]

In order to solve the problem of competing H + ion sensitivity of the K-ISFET, a thin water-repellent lipophilic film was coated under the hydrophilic sensing membrane as shown in Fig. 1. For this purpose a photosensitive OMR-83 solution was spin coated on the pH-ISFET base chip and then dried for 3 rain in an oven at 80°C. The composition of photosensitive PVP-PVAC solution for the hydrophilic sensing membrane was the same as in Table 2. This photosensitive solution was spin coated (3500 rpm, 30 s) on top of the OMR-83 layer and heat u'eated (80°C, 10 min). h'radiation

4

9-.4 50

by UV light with a mask aligner (3 rain) and development of the double-layered membrane with IPA and dioxane solvent in this order gave a well-patterned membrane. The membrane had dimensions of 15/san × 600/san and a thickness of 0.5-0.10/san (OMR-83 layer) and 2.0-3.0 /san (PVP-PVAc sensing layer). After wafer cutting, the K-ISFETs were mounted on printed circuit boards and encapsedated with RTV 3140 (Dow Coming) silicone formulation. As an initial test of the K-ISFET with the double-layered membrane, the drain current (Io) versus voltage between drain and source (Vm) was plotted at various reference electrode potentials (Va). The measurements were conducted in pH = 7 buffer solution with Ag/AgCl as reference electrode. As shown in Fig. 2, the ID versus VDs plot showed the typical current/voltage characteristic observed in FET-type sensors. The output voltage versus sensing-time profile of the KISFET with the double-layered membrane showed that very rapid response (within 1-2 s) and high sensitivity (on the average 56 mV/decade) toward K + ion and low (less than 3 mV) interference by H + ion could be obtained (Fig. 3). The calibration curve of the K-ISFET exhibited good linearity in the range 10-4-10 ° mol 1- t of K + ion concentration as shown in Fig. 4.

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L.-S. Park et al. /Sensors and Actuators A 57 (1996) 239-243

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Acknowledgements

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This research was funded by the Sensor Technology Research Center (93K4-0806-01-3) at K y u n g p o o k National University, Taegu, South Korea.

References

(a)

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4, C o n c l u s i o n s A K - I S F E T sensor with double-layered m e m b r a n e fabricated by a photolithographic method showed high sensitivity ( 5 6 m V / d e c a d e ) toward K + ion, rapid response ( 1-2 s) and low interference (less than 3 m V / d e c a d ~ ) by the competing H + ion. Readily available negative photoresist ( O M R - 8 3 ) used for the. inner layer provided both good adhesion to the ISFET base chip and a barrier to the ~ l u e o u s solution. For

[ 1] P. Bergveld, Development of an ion-sensitive solid state device for neumphysiological measurements, IEEE Trans. Biomed. Eng., BME. 17 (1970) 70--71. [2] S.D. Moss, J. Janata and C.C. Johnson, Potassium ion sensitive field effect transistors, Anal. Chem., 47 (1975) 2238-2243. [ 3 ] EJ.R. SudhOlter,P.D. Van der Wal, M. Skowmuska-Ptesinska,A. Van den Berg and D.N. Reinhoudt, Ion-sensing usingchemically-modified ISFETs, Sensors andAetuators, 17 (1989) 189-194. [4] P.D. Van der Wal, M. Skowronska-Plasinska, A. Van den Berg, P. Bergveld, EJ.R. Sudb0her and D.N. Reinhoudt0 blew membrane materials for potassium selective ion-sensitive field-effect transistors, Anal. Chim. Acta, 731 (1990) 41-52. [5] N. Jaffrezic-Renanlt, J.M. Chovelon, H. Perrot, P. Le Perehec and Y. Chevalier, lon-sensitivn field-effect transistor sensors with a covalent bound monolayer membrane: example of calcium detection, Sensors and Actuators B, 5 ( 1991 ) 67-70. [6] F. Kanffmann, B. Hoffmann, R. Erbach, L. Heiliger~ H.U. Siegmand and M. V01ker, Ca2+ sensors with amphiphilic Langmuir-Blodgett membranes, Sensors and Actuatots B, 18--19 (1994) 60-454. [7] K. Tsukada, Y. Miyaham, Y. Shibate and H. Miyagi, An integrated chemical sensor with multiple ion a n d g a s sensors, Sensors and Actuators B, 2 (1990) 291-295. [8] Y. Hanazato, M. Nakako, M. Maeda and S. Shiono, Glucose sensor based on a field-effecttransistor with a photolithographicallypatterned glucose oxiduse membrmte,Anal. Chim. Acta, 193 (1987) 87-96. [9] Y. Hanazato, M. Nakako, S. Shiono and M. Maeda. Integrated multibiosensors based on an ion-sensitive field-effect trz,~nsistorusing photolithographic techniques, IEEE Trans. E!eceron Devices, ED-36 (1989) 1303-1310. [ 10] A. Van den Berg, A. Griesel and E. Vemey-Norgherg, An ISFETbased calcium sensor using a phOtopolymerized polysihixane membrane, Sensors and Actuators B, 4 ( 1991) 235-238. [ 1i ] S.S. Levichev, A.V. Bratov and Y.G. Vlasov, Hew photecurable composition for ISFET polymer membranes, Sensors and Actuators B. 18-19 (1994) 625-628. [ 1~] C.S. Kim, S.K. Lee, H.I. Sea and B.K. Sohn, High-performanceISFET glucose ~nsor by employing electrolysis of hydrogen peroxide, Tech. Digest, 7¢h Int. Conf. Solid-State Sensors and Actuators (Tra~duners "93). Yo~hama. Japan. 7-10 June. 1993, pp, 502-304. [ 13] T. Akiyan~ and E. Niki. Ion-sensitivefield-effecttra.~sisterfor pK and pNa sensing, Pure AppL Chem., ..59(1987) 535-538. [14] M. Goteh, E. Tamiya and 1. Karube, Ut~a-FET sensor using polyvinylbatyral resin menlbmee, Sensors Mater., 1 (1988) 25-33. [ 15] Patent is pendit~gon part of this work.

L-S. Park etaL I Sensors and ActuatorsA 57 (1996/J239-243

Biographies Lee-Soon Park received a Ph.D. in polymer science from the University of Southern Mississippiin 1982. Hejoined the faculty of the Departmem of Polymer Science, Kyungpook National University, in 1987 and has served as a member of the Sensor Technology Research Center (STRC) there since 1989. His research interest is in the application of polymer materials to the sensor. Young-Jun Hur received a M.E. in polymer science from the Kyungpook National University in 1996. His research

243

interest is polymer synthesis and photo]i~ograph~ processes.

8yung-gi Sohn received a Ph.D. in physics from Kyungpook National UniwrsRy in 1981. Hc j o i n ~ Kyu~gpeek National UniversRy as a facuRy n ~ n b ~ in 1965 bec~m~ a full professor in 1976. He has s~ved as p~'si~n¢ of the Korean Sensor Society and chamama of the Sensor Technology Promotion Committee. Since 1970, his principal research interests have been in the fields of FETmicrosensors and micrcelec~ronic [n'oce~ f e c h m ~ .