The extended gate chemically sensitive field effect transistor as multi-species microprobe

Sensors and Actuators,

4 (1983)

291 - 298

291

THE EXTENDED GATE CHEMICALLY SENSITIVE TRANSISTOR AS MULTI-SPECIES MICROPROBE*

J VAN DER

SPIEGEL,

I LAUKS,

FIELD EFFECT

P CHANand D BABE

Center for Chemzcal Electronzcs, Department of Electrzcal Unzverszty of Pennsylvanza, Phzladelphza, PA 19104 (U S A )

Engzneerzng

and

Science,

Abstract

This paper describes a multi-species microprobe structure for potentlometnc measurements and the appropriate pattemmg techmques of the chemlcally-senskve membranes The structure consists of an integrated coaxial cable whose signal line 1s connected to a high mput impedance electrometer with the shield bootstrapped m order to reduce capacitive charging effects The electrometer 1s an onchlp source follower, designed for mmmum input capacitance The coaxial line, which 1s an extension of the gate of the transistor, 1s fabricated unth a triple poly-slhcon NMOS process The whole structure 1s compatible with current IC technology and allows mtegratlon of on-chip signal condltlonmg clrcultry Units of four element probes, each covered with a dlfferent membrane, have been fabncated One important issue m multi-sensor fabrlcatlon 1s the deposltlon and patterning technique of the membranes A scheme has been developed which allows successive pattemmg of both vacuumdeposlted morganlcs and spin coated polymers and gels. This techmque has been successfully applied for a multi-species electrode of IrOz(H+), AgCl(Cl-) LaF3(F-) and Ag

Introduction

Conventional methods of chemical enwonment monltormg using ionspecific electrodes typically employ sequential analyses of single species using electrodes specific to those species, under carefully controlled solution condltlons The multi-species electrode approach such as described here differs because all measurements are made at a single time and on a single microprobe Ideally, one 1s almmg at measurements where carefu1 control and pretreatment of the analyte 1s mmlmlzed, that IS, one hopes to achieve *Based on a Paper presented at Sohd-State Transducers 83, Delft, The Netherlands, May 31 -June 3,1983 0250-6874/83/$3 00

0

Elsevler Sequola/Prmted

m The Netherlands

292

real-time analysis of real solutions The philosophy of the multi-parameter approach to real fluld analysis rests on the assertIon that m order to measure optimally any smgle component, one needs to maxmlze the knowledge of the composltlon of the fluld with respect to all other components, and other thermodynamic variables, notably temperature and pressure This assertion IS made because one understands the limits set on single measurement systems because of electrode non-ldeallty and the interrelation of component concentrations m a complex chemical bath The fact that potentlometrlc chemical sensors fabricated usmg planar processmg and other semiconductor micro-fabrlcatlon procedures could easily lead to monohthlc multi-element sensor arrays has been recognized from the outset of this technology Moss et al discussed the possrblllty of a plurality of chemically-sensltlve field effect transistors [l] Zemel [2] suggested the feaslblhty of multiple ion-controlled diode (ICD) structures and proposed a novel patternable chemically-sensitive layer technology for their lmplementatlon [3] More recently, Pace [4] speculated about a future technology for multlple devices for sensing enzymes, sugars, Ions etc , on a monohthlc structure However, no details regarding potential fabrlcatlon procedures were offered A two-species ion-sensltlve field effect transistor (ISFET) described by Esashl and Matsuo [5 ] IS a clever approach, but it lacks generality of fabncatlon, bemg suitable only for a speclflc two-species structure The ISFET array described by Ko and Fung [S] IS not a multlelement device m the sense used here m that an array of identical pH ISFETs was fabricated and tested Because there exists a technology to fabricate arrays of electromc vehicles whether they be FET, diode or thm film hybrid does not lmmedlately or obviously lead to a technology for multi-element chemically-sensltlve devices A mador difficulty lies m the fact that the chemically-sensitive membrane materials and their means of deposition are dlverse Methods for their simultaneous hthographlc patternmg have been dlscussed but to date have not been practically demonstrated m workmg devices [ 7 J In this paper we describe a new chemically-sensltlve potentlometric device, the extended gate field effect transistor (EGFET), speclflcally designed for optimal lmplementatlon as a multi-species mlcroprobe Furthermore, we outline those chemically-sensitive membrane deposltlon and patterning technologies that are needed to realize such multi-species devices The electronic vehicle of a potentlometrlc electrode IS a high input impedance electrometer The general clrcult description of any such single potentlometrlc sensor 1s shown schematically m Fig 1 The conflguratlon consists of a chemically-sensitive layer or layers deposited on the end of a wire whose other end IS connected to a high input impedance buffer The shield may be bootstrapped as shown m order to reduce the noise sensltn&,y and capacitive loadmg of the signal line The simplest conflguratlon of this type consists of an ion-selective electrode connected by a shielded wue to an op-amp buffer With high Impedance electrodes this conflguratlon can be noise sensitive The length of the shielded wu-e can be reduced to increase noise immunity to such an extent that the electrode and the buffer can be

293 CHEMICALLY

(b)

SENSITIVE

LAYER

7

F

Fig 1 (a) Clrcult representation of the potentlometrlc metric chemical sensor usmg MOS source follower buffer

chemical sensor

(b) Potentlo-

placed m close proxlmlty on a smgle structure such as m a hybnd conflguratlon Further reduction m signal line length places the chemically-sensltlve electrode region, the coaxial cable and the buffer on a single mtegrated structure When the buffer 1s a field effect transistor, the resulting devrce 1s the EGFET A still further reduction results, m the hmlt of an mfmltely short signal lme, m the placement of the chemically-sensltlve region directly over the buffer, such as m the ISFET and ICD structures With the active electronics of the buffer and any other on-chip electromcs removed from the chemically-active region and separately hermetically sealed, the EGFET 1s snnpler to passlvate and package than the ISFET or ICD There 1s also a potential for better long-term stability because ions from the chemical environment are excluded from any region close to the gate insulator of the buffer The flexlblhty of shape of the coaxial hne allows that the sensor can take almost any geometry specified by the user’s apphcatlon The structure 1s also light msensltlve The EGFET 1s fabricated usmg standard technology and no difficulty IS experienced m reahzmg the multi-species device The design gives mmnnal cross-talk between signal channels Description and fabncatlon of the EGFET Two quite different technologies are requved to realize the multlspecies microprobe sensor the fabrlcatlon of the coaxial hne and associated integrated electronics, and the deposition of the chemlcally-sensltlve layers and the appropnate patterning techmques (a) In tegratton of substrate electronm The extended gate field effect tranastor sensor consists of an mtegrated guarded coaxial hne, connected to a high mput unpedance onchlp pream-

294

phfler (Fig l(a)) The chemIcally_sensitlve layers are deposited on the signal lme and, when contacted to the chemical environment under test, a signal 1s generated that 1s transferred over the coaxial hne to the preamplifier The shield 1s connected to the output of the voltage follower This guarding technique considerably reduces the effects of input capacitance and leakage This becomes rmportant for chemical membranes with a high impedance In Its simplest version, the preamplifier 1s an NMOS source follower This MOS-buffer has the advantage that It provrdes a very high input Impedance and an amphflcatlon close to one and that It can be easily implemented together with the guarded lme Figure l(b) gives the configuration with either an external load resistor RL or a second transistor (depletion-mode or JFET) as active load, mtegrated together with the source follower as a matched pan In this latter approach V,, = 0,providing a source follower with zero offset The threshold voltage of the depletion-mode transistors can easily be adlusted by ion-lmplantatlon, a technique fully compatible with the technology for the sensor fabrication In Fig l(b) a grounded shield IS added as a supplementary protectlon of the clrcult from the envvonment In Fig 2, a perspective view of the sensor and MOSFET 1s shown The outer shield has been omitted for clarity The top view of the 1 7 X 3 8 mm2 chip IS shown m Fig 3 The sensor 1s fabricated on a (100) orientation 5 i2 cm p-type slhcon substrate The guarded electrode consists of three layers of 500 nm thick poly-sllrcon, msulated by a thermally grown 100 nm thick layer of SIOZ The poly-silicon layers are phosphorus doped and have a sheet resistance of 30 !iZ/D The signal hne 1s the second poly-Sl layer and the guarded shield IS made from poly& layers one and three A 1 pm thick CVD S102 layer and a Sl,NG overcoatmg layer are deposited as prunary passlvatlon and are etched away on top of the chemically-sensitive region and the bonding pads of the transistors The chemically-senatlve coatings are then deposited The details

CHEMICALLY ENSITIE LAYER SIGNAL

LINE

Fig 2 Perspective view of guarded electrode and MOS follower

295 Chemically sensltrve area

Co - axtal poly

9

SCALE ‘

3 250pm

1

MOSFET

!

Bcndtng pads

Fig

3

Top anew of the extended gate field effect transistor chemical sensor

of the membrane coating steps are described below A fmal passlvatlon consists of a polynnlde layer that 1s deposited on all faces of the probes after they have been etched and separated Other techniques to ensure probe isolation are being mvestlgated, such as fabncatmg the active devices m a diffused well which 1s isolated from the substrate, or usmg &con on sapphire. These will be required to ehmmate the - 300 “C polyunlde curing step after membrane deposltlon so that a broader range of membranes, mcludmg those that are thermally unstable, may be used (b) Chemrcally-sensttrue layers Patternmg of the sequentially vapourdeposlted films was by hft-off In this method photoreslst 1s deposited and a window photodefmed over one sensor region An overhang 1s formed on the photoresist edge defmmg a window [8], and the first film 1s deposlted Removal of the film from all areas other than the window LS accomphshed by stnppmg the photoresist, causmg lifting-off of the vapourdepoated over-layer Second and subsequent films are patterned on nelghbounng sensor sites m the same way Thm films of IrO,, LaF,, AgCl and Ag,S were deposited and patterned on the four EGFETs as shown m the photograph (Fig 4)

296

Fig 4 Photomlcrograph

of completed

devices on the wafer

IrO, was d c reactively sputtered m 10 mtorr oxygen at 1800 V d c and 0 5 mA for 160 mmutes, resultmg m an approximately 800 a thick film 5000 a LaF, was depoated by thermal evaporation from an A1203-coated molybdenum open boat at 1 5 a s-’ 5000 a AgCl was thermally evaporated from a molybdenum open boat at 2 a s-l Ag,S was evaporated from a quartz boat Results The process and the devices were characterized by onchlp test structures MOS capacitors were used to determme the flat band voltage, the oxide charge Q, and the interface trapped charge density N,, High frequency C-V measurement gave a flat band voltage of - 1 35V and a resultmg oxide charge of a few 1O1* cm- 2 The interface trapped charge density was determined from quaslstatlc C-V measurement and was m the mid-10” cm-’ eV- l range This rather high value IS due to the absence of a proper annealing step and could be easily reduced to the low-1O1* cmb2 eV_’ range The MOS transistor has a threshold voltage of + 0 25V The transconductance, g,, IS equal to 700 PUS at a dram current of 0 5 mA The amphflcatlon of the source follower IS equal to 0 95 when used with a 55 ka load reslstor, which 1s close to the calculated value The output stablhty of the source follower was wlthm 1 mV over 12 hours

297

IO

8

6

4

2

0

PX

Fig 5 Electrode response to H+ (MI,),

Cl- ( AgCl) and F- ( LaF3)

LaF, and AgCl films were confirmed as the polycrystallme matenals by X-ray dlffractlon Irldlum oxide, IrO, was amorphous The material deposited by thermal evaporation from an Ag,S source was determined as Ag This 1s a typlcal problem encountered with thermal evaporation of multlcomponent materials that decompose m the gas phase to species with dlfferent vapour pressures Flash evaporation or sputtering are possible alternative methods for Ag,S deposltlon Electrochemical charactenzatlon was not extensive, since the purpose here was to demonstrate the feaslblhty of the fabrlcatlon methodology LaF, electrodes were characterized m 0 1M NaCl solutions with variable Fconcentration obtained by additions of NaF AgCl electrodes were characterized m solutions of vanable Cl- obtamed by NaCl addltlons IrO, electrodes were cahbrated m pH buffer solutions (pHydnon) Ag membrane electrodes were not used for electrochemical measurement Representative results of as-prepared electrode responses are summarized m Fig 5 In the case of the LaF3 membrane, electrode stablhty and speed of response were not adequate (typlcally, response times to 95% change were of the order of 10 seconds, drift was of the order of several tens of mllhvolts per hour However, Nernstlan response at [F-j > 10m3 M was obtained Europlum doping of the films 1s bemg investigated to unprove speed. Ways of stablhzmg the LaF,/poly-Sl interface are bemg considered Both IrO, and AgCl were stable over the short term, Ee , had better than a few mllhvolts per day drift Longterm stability was not mvestlgated IrO, was Nernstlan over the range 0 < pH < 14, as was obtamed previously on metal-coated IrO, electrodes [9] AgCl electrodes were Nernstlan at [Cl-] > 10e4 M

298

Conclusion This paper represents the fn-st general multi-species potentlometrlc structure based on planar &con fabrication technology A new electronic structure, the extended gate FET, and a general chemically-sensltlve membrane deposltlon and patternmg methodology are presented These devices and the coatmgs methodology represent the first step m the evolution of a technology for the lmplementatlon of a broad class of monohthlc multlspecies structures

Acknowledgements Grateful thanks go to Dr Jay N Zemel for useful dlscusslons, and to Mr T Carroll for much of the practical technology A part of this work was sponsored by the Department of Energy, Offlce of Basic Energy Science under grant number DE-AC02-82-ER12035 References S D Moss, J Janata and C C Johnson, Potassium lon-sensltlve field effect transistor Anal Chem , 47 (1975) 2238 J N Zemel, Ionsensltlve field effect transistors and related devices AnaE Chem , 47 (1975) 255A J N Zemel, US Patent No 4 302 530, November 24,198.l S Pace, Surface modlflcatlon and commercial apphcatlon, Sensors and Actuators, 1 (1982) 475 M Esashl and T Matsuo, Integrated micro multi ion sensor using field effect of semiconductor IEEE Trans Bromed Eng , BME-25 (1978) 184 W H Ko and C D Fung, VLSI and intelligent transducers, Sensors and Actuators, 2 (1982) 239 I Lauks, Multi-element thm film chemical mlcrosensors, SPIE Internatzonal Soczety for Optzeal Engrneermg Proceedzngs, Crztlcal Reviews of Technology Stratrfied Medra, Vol 387,1983 M Hatzakls, B G Canavello and J M Shaw, Single-step optical lift-off process, IBM J Res and Dev , 24 (1980) 452 T Katsube, I Lauks, J Van der Spiegel and J N Zemel, High temperature and high pressure pH sensors wrth sputtered iridium oxide films, Jup J Appl Phys , 22 (1983) 469