Novel potentiometric silicon sensor for medical devices

B ELSEVIER

Sensors and Actuators B 34 (1996) 476-480

Novcl potentiometric silicon sensor for medical devices U w e S c h n a k e n b e r g * , T h o m a s Lisec, R a i n e r Hintsche, Ingrid Kuna, A l b r e c h t Uhlig, B e r n d W a g n e r Fraunhofer.lnstitut f~r Siliziuratechnologie, Dillenburger Sir. 53, D-14199 Berlin, Germany Accepted 25 January 1996

Abstract

The fabrication process for a potentiometricsilicon sensor is described.The sensor was fabricated in bulk silicon micromachining using double side wafer processing. A small channel was etched anisotropicallyin (100) silicon, The back side groove was metallized. To form an ion.selectiveelectrode an ion-selectivemembranewas deposited in the channel. Additionally,a novel type of Ag/AgCI/Ag referenceelectrode was integrated on the front side of the chip. The small size of the sensor allows application in catheter systems.

geywords: Potentiometricsensor; Silicon micromachining;Ag/AgCI/Agthin film referenceelectrode

1. Introduction Important chemical parameters in human blood, like glucose, potassium or lactate have been commonly analysed ex-vivo using expensive equipment up to ~mw. There is a great demand for the development of low-cost in-vivo sensors, which achieve the required stability and working times. In potentiometric measurement techniques the potential between an ion-selective electrode and a reference electrode is measured. This tyl~ of electrode is realised by an ion-selective membrane deposited on a metal electrode, Smith et al, [11 and Knoll et al. [2] have proposed potentiometric sensors fabricated in bulk silicon micromachining technology, In the sensor published by Knoll et al. [2] the ion-selective membrane was deposited into the pyramidal shaped metallized opening. This opening was etched from the back side in (100) oriented silicon by wet anisotropic etching. For measurement, me analyte was brought in contact with the front side of the chip. No Ag/AgC! reference electrode was deposited on the chip. Based on the sensor principle proposed in Refs. [!,2] we developed a novel potentiometric sensor for potassium which has several improvements and advantages due to the possibility of double side wafer processing. The openings are shaped additionally from the front side of Correspondingauthor. 0925-4005/96/$15.00 © 1996 Elsevier Science S.A. All fights reserved PIi S0925-4005(96)01854-0

the chip to get micro containments (channels) to improve the mechanical stability of the ion-selective membrane. For stable and reproducible potentiometric measurements a reference electrode was deposited on the front side of the chip close to the openings. We developed a novel evaporation process for the deposition of an Ag/AgCI/Ag layer combination serving as thin film reference electrnde. The small size of the sensor allows the localisation in the tip of a three lumen catheter for medical applications. 2, Fabrication process The sensor was fabricated in bulk silicon micromachining technology using (100) oriented 4" double sided polished silicon wafers. The chip has overall dimensions of I × 5 mm2, which is suitable for af~plications in catheter systems. The process flow, shown in principle in ~.~oss-sections in Fig. 1, started with an oxidation (Fig. 1~). The front side of the wafer was prepared first. A platinum metallization layer was deposited and structured using a lift-off technique [3] (Fig. lb). The channel f~,r embedding and mechanical stabilisation of the ion-sensitive membrane was shaped through RIE structuring of the oxide and subsequent etching of the silicon in aqueous TMAH solution [4]. Two designs were favoured: in the first a 300~m long channel was realized, whereas in the second this channel was divided into three small openings. On the

U. Schnakenberg et al. /Sensors and Actuators B34 (1996) 476--480

477

served as a reference electrode. Here, the layer was structured using a lift-off process. The reference electrode on the front side was covered with a photosensitive polyHEMA layer as described in Refs. [5,6]. This layer served as a protection against variation of the chloride concentration in the analyte. The device described can be used as a base device for amperometric and potentiometric sensors. For potentiometric measurement of potassium different cocktails of ionophore modified polymer membranes were prepared and deposited in the channel by a dispension technique. The preparation of the membrane is discussed in detail in Ref. [6].

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3. Results 3. I. Potentiometric sensor

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Fig. 2 presents the front and back views of the sensor chip. The front view of the chip (right side) shows the three small openings, the platinum metallization line with the contact pad and the A g / A g C I / A g layer on the end of the platinum line. The platinum layer and Ag/AgCI/Ag reference electrode is located behind the channel and not beside, as indicated in Fig. 1 for simplification of the

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back side of the wafer a platinum metailization layer was deposited a~d structured in the same way as on the front side. The wafer was passivated with a silicon nitrid¢ layer. The nitride layer was structured on both sides to define the contact areas. The groove on the back side was formed after structuring the passivation layers and etching in TMAH. No etch stop mechanism was used. The timecontrolled etching was carried out until openings appeared. The sidewalls of the groove were oxidized (Fig. lc). In contrast to the usual thin film Ag/AgCI reference electrode, a novel combination of AgtAgCI/Ag was developed applying evaporation techniques. The back side groove was metallized with the Ag/AgCl/Ag combination through an aperture blend. On the front side, the Ag/AgCI/Ag combination was deposited as well and

Fig. 2. Backside (left)and frontside (right)of the potentiometrcsensor chip. Chip size: I x 5 mm2. in contrastto Fig. I the Ag/AgCI/Agreference electrodeis locatedbehindthe channeland not beside.

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U. Schnakenber,~et al. / Sensors and Actuators B34 (1996) 476-480

presentation. The back view of the sensor chip shown on the left chip of Fig. 2 indicates the pyramidal shaped groove coated with the Ag/AgCi/Ag layer, the three small openings at the bottom of the groove, and the platinum line with the contact pad. The platinum area around the groove is totally covered with Ag/AgCI/Ag to realize the electric contact. The hazy shading around the Ag/AgCI/Ag layer results from the deposition through the aperture blend. Figs. 3 and 4 show details of the sensor chip. In Fig. 3 a SEM picture was taken from the front side of the chip under 45 ° incidence. In the front of the picture the channel with the three openings and the back side groove are shown, whereas in the back of the picture the integrated Ag/AgCi/Ag reference electrode can be seen. In Fig. 4 an SEM picture of one opening of the channel under 45 ° incidence illustrates the geometry of the anisotropically etched channel. The inside width of this channel is 39 prn, 3.2. Ag/AgCi/Ag reference electrode

For thin film Ag/AgCI reference electrodes, usually Ag is deposited by thermal evaporation process followed by chemical chlorination. These electrodes show a dissatisfied long-term stability due to leaching out of the chloride. To avoid the chlorination on each chip we developed

Fig, 3. $EM pic~uu~from the top of the sensor chip under 45° incidence. In the frontof the picture the channeland the back side groove is shown, whereas in the back the integrated Ag/AgCI/Agreference elecmxlecan he seen.

Fig. 4. $1/Mpicture fromone openingof the anisotropicetched channel under45° incidence.Phe insidewidthis 39 #m. a new wafer-scale AglAgCllAg layer deposition process using evaporation techniques. Fig. 5 shows a SEM picture from the top view of the layer combination under 45 ° incidence. On the surface grains are visible. The crosssection of the layer combination in Fig. 6 shows a homogenous phase over the whole thickness of about 1/~m. To demonstrate the working capability of the Ag/ AgCI/Ag layer combination the potential drop versus a commercial avaiiable Ag/AgCI reference electrode

Fig. 5. SEM picture fromthe top view of the Ag/AgCIIAglayer combination under45° incidence.

U. Schnakenberg et aL / Sensors ap,d Actuators B34 (1996) 476-480

(Ingold, Germany) was measured for different KCI concentrations in H20. Fig. 7 shows the dependency of the voltage drop with increasing KCI concentration for five independent runs of Ag/AgCI/Ag deposition. The mean value of the Nernstian slopes was determined to be -55.1 _+ 1 mV/pCl-. This value corresponds very well to the expected theoretical value o f - 5 9 mV/pCI-. Longterm stability measurements over 24 h show a drift of 0.2 mV/h.

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CKcl / (mol/I) During the sophisticated double side fabrication procedure several processes have to be developed: For the deposition of the Ag/AgCI/Ag layer on the back side groove the aperture blend has to be adjusted to the process wafer. Therefore, small adjustment holes were etched into the process wafer, while mesa structures were formed on the aperture blend. In this way, the two wafers were easily adjusted when brought together. For reproducible long-term stable operation of the sensor chip leakage currents must be avoided. It is known that ions and water migrates in silicon oxide [7]. For this reason silicon nitride was used as protection and pass=vat=on layer on the front side of the channel• On the other hand it was sufficient to oxidise the sidewalls of the back side groove, because no analyte solution penetrates through the ion-sensitive membrane• For potentiometric measurements of potassium, different cocktails of potassium-sensitive membrane materials

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Fig. 7. Potentialdrop of five independentdepositedAgIAgCI/Aglayer combinationsversus a commerciallyavailableAg/AgCIreferenceelectrode (lngold, Germany) with respect to KCI concentration in H2O. The mean value of the Nernstian slopes was determined to be -55. I ± I mV/pCI-. were developed, tested and optimized. The membrane was deposited in the channel by dispensien. The sophisticated preparation arid deposition techniques for the membrane material and the implementation into the catheter system, the measurement technique, and the results are discussed in detail in Ref. [6]. ~. Conclusion We developed a double side wafer process technology in silicon bulk micromachining for the fabrication of a sensor which can be used for potentiometric measurements in medical applications• The thin film preparation of the Ag/AgCI/Ag reference electrode allows a higher integration on chip level, which increases the miniaturisat=on and reproducibility of operation of the sensor. The small chip size allows the integration into a tip of a three lumen catheter. Potassium concentration can be measured using an ion-selective polymer membrane. Acknowledgements We would like to express our sincere gratitude to Kerstin Heidler, Ingrid Wichert, and Andreas Hoyer for their helpful assistance in evaluating th ,~.process steps and discussions, Martina Rothe and Sabine Seedorf for preparing the SEM pictures, and Sigun Kilinkenberg for preparing the sample for Figs. 3 and 4. The staff of our CMOS line are gratefully acknowledged for their daily support and interest in this project. References

Fig. 6. SEM picture from the cross-section of the Ag/AgCIIAglayer combination.

[!] R.L. Smith and D.C. Scott, An integrated sensor for electrochemical measurements, Trans. Biomed. Eng., 33 (1985) 83-90. [2] M. Knoll, K, Cammannn, C. Dumschat, C. Sunderm¢itr and J. Eshold, Potentiometfic silicon microsensor for nitrate anti ammonium, Sensors and Actuators, B 18-19 (1994) 51-55.

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[3] K. Reimer, C. K6hler, T. Lisec, U. Schnakenberg, G. Fuhr, R. Hintsche and B. Wagner, Fabrication of electrode an'ays in the quarter micron regime for biotechnological applications, Sensors and Actuators A, 46 (1995) 66-70. [4] U. Schnakenberg., W. Benecke and P. Lange, TMAHW Etchams for silicon micromachining, Tech. Digest, 6th Int. Conf. SolidState Sensors and Actuators, Transducers '91, San Fr~acisco, CA, 1991. [5] A. van den Berg, M. Koudelka-Hep, B.H. van der Schoot and N.F. de Rooij, Miniaturised chemical analysis systems, DechemaMonogr., 126 (1993) 155-171. [6] A. Uhlig, U. Schnakenberg, E. Lindner, B. Wagner and R. Hintsche, Catheter system for potassium measurement in medical application, Tech. Digest, 8th Int. Conf. Solid-State Sensors and ActuaU)rs, Transducers '95, Stockholm, 1995. [7l W.A. Pliskin, Compmison of properties of dielectric films deposited by various methods, J. Vac. Sci. Teclmol., 14 (1977) 10641080.

Biographies Rainer Hintsche was born in Delitzsch, Germany, in 1942. He received his diploma and his Ph.D. in organic chemistry from the Universit~it of Leipzig in 1966 and 1970, respectively. From 1966 to 1969 he worked in cancer research at the Institute of Biochemistry in Berlin. From 1970 to 1983 he worked in the position of a groupleader in analytical chemistry on structure activity relationship of natural compounds in a physico-chemical centre in Berlin-Buch. From 1983 to 1992 he was groupand project-leader for chip biosensors at the Central Institute of Molecular Biology in Berlin, Since 1992 he is head of department at Fraunhofer-Institute fiir Siliziumtechnologic i,1 Berlin, He still continuing his work on silicon made chemical and biochemical sensors and completing analytical and biotechnical microsystems. Albrecht Uhlig (born 1964) received his diploma in chemistry 1990 at the Humoldt-Universitlit zu Berlin/Germany (HUB). Between 1990 and 1993 he worked at the HUB in the field of Conducting Polymers and received the Dr, rer,nat, degree, Since 1993 he has been working at the Fraunhofer Institut for Siliziumtechnology in Berlin/Germany, His main research interest is the miniatarisation of electrochemical sensors for environmental and medical applications.

Uwe Schnakenberg was born in 1960. He received the Diploma degree in physics from the RheinischWesffalischen Technischen Hochschule (RWTH) Aachen in 1986. He joined the Fraunhofer-Institut ftir Mikrostrukturtechnik (now Siliziumtechnologie) working in the field of microsystem technology focussed on the development of new technologies. He received the Dr.-Ing. degree in 1994 from the Technische Universitiit Berlin. Currently he is project coordinator of several national and industrial sponsored projects. Thomas Lisec was born in Leningrad, USSR in 1964. He received the Dipl.-Ing. degree in materials science and semiconductor technology from the Institute of precision chemical technology in Moscow in 1988. From 1988 to 1991 he worked as process engineer in the 1 MBitDRAM-Fab of the Zentrum for Mikroelectronik Dresden, Germany. In 1991 he joined the Fraunhofer-lnstitut ftir Mikrostrukturtechnik (now Fraunhofer-Institut fiir Siliziumtechnologic) in Berlin. His current fields of activity are the design of surface micromachined devices and microfluidic components and the development of micromachining fabrication technologies. lngrid Kuna was born in 1962. She received the technical assistance degree in metailography and material theory in 1984. She joined the Fraunhofer-Institut fiir Mikrostrukturtechnik (now Siliziumtechnologie) working on the development of new thin-film metallisation processes. Bernd Wagner received the M.S. and Ph.D degrees in physics from the University of Mainz, Germany, in 1982 and 1986, respectively. He joined the Fraunhofer-Institut fiir Siliziumtechnologie in Berlin, in 1988, where he is currently head of the Micromechanics/Hybrid Integrated Systems Department. His research interests are in microelectromechanical systems on the basis of silicon technology. Mechanical sensors, microactuators and hybrid integrated microsystems on silicon substrates are the focus of his current work,