Field Effect Potentiometric Sensors Richard P. Buck” and David E. Hackleman William
R. Kenan, Jr., Laboratories of Chemistry, The University of North Carolina, Chapel Hill, North Carolina 27514
Vacuum deposition of AgBr on thin silicon dioxide films prepared on silicon substrates was shown to provide stable, rapid-response potentiometric sensors for component-ion activities. Limiting high dc impedances from impedancefrequency responses and external charge-activity measurements indicate displacement current (field-effect) as the probable cause for electrochemical responses. Implications of the mechanlm (invoking Voka potential arguments) on the processes of potential generatlon and potential communication to underlying semiconductor substrates in ISFETs and CHEMFETs are analyzed dlagrammatlcaliy and theoretically. The essential role of reference electrodes is strongly Indlcated.
Ion selective field effect transistors (ISFETs), introduced by Bergveld ( 1 , 2 ) have been the subject of both theoretical ( 3 ) a n d practical studies (4-6). The concept has also been generalized to include sensors for uncharged chemical species (CHEMFETs) via surface electron (or hole) exchange, or adsorption (7-9), within a broader category of chemically sensitive semiconductor devices (CSSDs) (3). From the theory a n d practice of MOS capacitors, gate-controlled diodes and MOSFETs ( I O ) , a necessary condition for the sensitivity of the devices is that a field be transmitted through an insulating gate and be terminated in a space charge region a t the gate/semiconductor interface. Whereas in MOSFETs the field is created by the charge on the metal gate, fields can be created in OSFETs (MOSFETs without metal) by any of the several processes of ion exchange ( I I ) , including ionization of neutral groups, such as SiOH (12,13), electron exchange, adsorption of charged species or alignment of dipoles a t the external gate surface (14). These processes can be enhanced, controlled, or made to occur by exposing the OSFET with reactive gases, solvents and electrolytes, and by coating the OSFET with reactive materials such as ion exchangers and redox-sensitive layers. In this paper, a specific MOS capacitor system is studied to demonstrate t h a t a solution-ion exchanger gate behaves electrically in a highly analogous fashion to a metal gate. T h e system uses the well-known, reversible ion exchanging substance AgBr (15, 16) on Si02-coated silicon. Modern ion selective electrodes of membrane type, and classical electrodes of first, second, and third kinds, make use of rapid, reversible charge exchange (ion or electron exchange) at each interface. Electrochemical reversibility assures development of interfacial electric fields and thermodynamically significant responses (interfacial, diffusion, and net potential differences) to bathing solution activities (11, 15-17). I n contrast, proposed ISFETs contain blocked interfaces as well as reversible interfaces of the types:
electrolyte lmembrane linsulator lmetal (reversible)
(1)
(blocked) (blocked)
and
electrolyte lmembrane linsulator lsemiconductor (11) (reversible)
(blocked)
(blocked )
In addition, totally blocked systems have been proposed as chemical sensors (14):
electrolyte linsulator lsemiconductor (blocked )
(111)
( b l o c k e d)
At ideally blocked interfaces, no net charge transfer can occur. While conventional ISEs are resistive a t dc (18-2O), cells (I-111), completed by use of reversible reference electrodes in the electrolyte, would show entirely capacitive behavior at dc. Use of an ion-exchange membrane, a chemical coating or a metal in Cell I provides the opportunity for generation of an interfacial potential difference by a charge transfer, or faradaic process (21). If the charge transfer is reversible, the generated potential difference can be Nernstian and determined within a n additive constant by the equality of electrochemical potential of the transferring species (11,22). At the other extreme of very slow, or no charge exchange, the interfacial potential difference at an ideally blocked interface depends on adsorption and dipole alignment. Interfacial potential and charge are related. The latter is established by transient external current flow, by adsorption of charged species, and by chemical generation of charged surface species (23, 24). T h e hypothesis is made that fields are generated a t a n electrolyte/ion exchanger, or electrolyte/metal interface by net ion or electron transfer. As a test, potentials at open circuit and charge accumulation for externally shorted cells have been measured. These are equilibrium measurements: the first by classical potentiometric compensation (Volta potential or flat band shifts), and the second by integration of transient current after a fresh cell has been shorted externally. In addition, cell impedances have been measured to demonstrate and confirm capacitive behavior.
EXPERIMENTAL Starting materials: n ( l l 1 ) and p(lOO), 2-6 ohm-cm silicon wafers of electronic purity, “Airco” electronic grade oxygen, distilled, deionized water, electronic grade HF (48%), and reagent grade salts and solvents (AgBr, AgN03, KN03, KC1, KBr, trichloroethane, acetone, and methanol) were used. For the S O pthickness-dependence tests, n-type silicon wafers were scribed into rectangular portions of 2-cm width to fit into a tube furnace. The wafers were cleaned in trichloroethane, methanol, acetone, methanol, and blown dry with filtered nitrogen. After 1-h purge of the 1100 O C furnace with steam, water temperature was adjusted to just below boiling, and -4 ft3/h of oxygen were passed through the water and into the furnace. Thickness of oxidized films was established by known oxide growth rates for the time and conditions used (25),comparison of expected and actual color (IO),and the use of the C-V (capacitance-voltage technique) as presented by Zaininger and Heiman (26). The C-V technique also produced information of the concentration of impurities and their location in the films. The SiOz-Si pieces were scribed on a grid of 0.25-cm centers; Draftsman’s black tape (0.05-in.) protected the scribe lines from AgBr during deposition. The pieces were placed, three at a time, into the vacuum bell jar assembly of the evaporator, with approximately 0.1 g AgBr in a tantalum boat. The system was pumped down to lo4 Torr, the AgBr heated slowly to liquification, and evaporated onto the SiOz/Si substrates. This process required at least 90 min per run of three pieces. The finished pieces, with ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
2315
PINS CONNECTED TO H E A D E R WITH CONDUCTIVE PAINT
,Ag B r DEPoSiTION S I O 2 SURFACE LAYER
_REFERENCE ELECTRODE
EXTRINSIC SILICON
ELECTRODE
S E L E C T 0R
SILVER CONDUCTIVE PAINT
CON'RGL ELECTRODE SOCKET
TO-5 H E A D E R GLASS TUBING
PLEXIGLAS
V
1
ROL Enhancement
4-1
Depletion
O N E OF FIVE'
_"
ELECTRODES
-AgN03
PELLET
Ref
Figure 2. Field effect sensor test assembly D
I Ref
3'
I
Figure 1. (Upper) Experimental field effect sensor. Note: Semiconductor junction coating and Apeizon "W" wax used to seal and isolate all except the AgBr top surface from solution contact. (Lower)Idealized layouts for ISFETs and CHEMFETs sensors. V , is the applied bias between FET body and reference electrode. FETs may be n or p type using enhancement or depletion modes. Diffused wells of carrier type opposite to the underlying semiconductor are the contacts for passage of current along the semiconductor/gate Interface. An SiOp gate is shown and an AgBr sensor membrane is used in this example
c -V
Figure 3. High impedance non-inverting amplifier schematic. R: 10 KR. R,,R,: designed to allow IDS of MOSFET stages to b e near midrange at V,, = 0.OV. RB: 10 K R (1 turn) fine zero adjust. R4: 100 K fl (10 turn) gain adjust. I?,: 10 K R (10 turn) coarse zero adjust. Sa: position: (1) Input lo7 R to ground, amplifier grounded (shown). (2) Input lo' R to ground, amplifier to input. (3)Input open, amplifier to input. (4) Input open, amplifier grounded. Sb: position: (1) Operate (shown). (2)Balance. fV: f 1 5 V dc Decade concentrations of KC1, KBr, and AgN03 from lo-' to
typically 100 nm AgBr deposited, were etched free from back side SOz,and standard copper deposition solution was used to provide an ohmic contact to the silicon. Pieces separated on the scribe lines produced about 100 sensors of 0.25 X 0.25 cm from each wafer. Ten of each thickness of SiOz were mounted on transistor headers as shown in Figure 1. A second series of sensors was prepared from silicon oxidized in a muffle furnace a t 1000 OC in air. This type of oxidation provides a mixed layer of SiOz and Si3N4(27). For a series of studies involving controlled thicknesses of AgBr and constant SiOz thickness, these substrates were subjected to vacuum evaporated AgBr through a molybdenum mask to provide coated areas with 0.95-mm diameters. Thicknesses of AgBr were determined by calculation of consumed AgBr, most probable density of deposit from a distributed source (28), and use of color comparison (IO), with correction for the index of refraction of AgBr. Testing of these sensor devices of physically small size and Gigohm impedances required construction of a shielded test fixture to accommodate more than one sensor in solution a t one time. T o monitor the test solutions and proper functioning of the reference electrode with bridge, a standardized, pressed pellet electrode was tested with each set of sensors. Figure 2 shows the test mounting used with five sample sensors, one pressed pellet AgBr electrode and an Orion double junction 90-02-00 reference electrode. Protection of the wires against solution contact was accomplished by dipping all leads in melted Apiezon "W" wax. All of the sensor samples were also coated to leave only the AgBr exposed to electrolyte solutions. Transene semiconductor junction coating, Type 1, a commercial coating designed to passify and insulate semiconductor materials was used (29)underneath the Apezion "W" wax. 2316
ANALYTICAL CHEMISTRY, VOL. 49, NO. 14,DECEMBER 1977
10-6M a t constant ionic strength of 10-1M adjusted with KNOB,
were used. The outer filling solution of the double junction reference electrode was lo-' M KNOB,and it was replaced daily and between runs. All solutions were saturated with AgBr by placing a known excess in each container, mixing, and allowing to settle one day before use. All concentration studies were done a t 25 i 1 "C. Decade solution tests were started a t the lowest concentration, with thorough washing between sets of solutions. Separate beakers and containers were dedicated to each ion concentration, with rinsing of beakers only in aliquots of the solution to be eventually left in each. Replicate sets of data were made on all sensors to provide a measure of data precision. Because of the extremely high impedances of the sample sensors, a simple impedance converting unity gain amplifier was constructed as shown in Figure 3. The MOSFETs used in this amplifier were of the unprotected gate variety, to enhance the input impedance, and were found to have less than 0.1 pA input current a t 10 volts (V,,). Use of an active load device extended the linear range of the otherwise nonlinear saturated mode of operation (30). The operational amplifier circuit offers a relatively constant input impedance to the output of the MOSFET stage, has greater than unity voltage gain, is non-inverting, has level offset, provides a high frequency filtering effect, and is stable to &1 mV/h (31). The long term stability of the amplifier has been established a t i 3 0 mV., for periods up to two weeks. The input impedance of the amplifier was not measurable. However, placing a cleaned 1O'O D resistor a t the input caused a discharge rate of a voltage reading to be 1000 times as fast as without it. This test indicates an input impedance on the order of l O I 3 '2. Impedance-frequency information was obtained using an impedance bridge described elsewhere (32,33). Bathing solutions
CuleftIAgrefIAg+in1 IAg'(and/or Br-)l AgBr/Br- in test ref soln soln inert
inert
supporting
supporting
electrolyte
AgBr lSiO,lSilCur~ht
-LOQ
a A g + ( A ,0
)
-Log
OB,-
b,o)
Figure 4. Folded response plots of potential (body vs. reference) as
a function of silver ion and bromide ion activities. Fiducial marks are statistical 95% confidence limit ranges for 195 sensors from different chips 0 and 0 ) ;silver bromide pressed pellet electrode response (A and A);solid line is t h e theoretical Ag/AgBr response
(IV)
and it is historically recognized, from the theory and practice of contact or Volta Potential measurements, that the probable condition for zero current is zero field across the S i 0 2 (22). T h e measurement of potential a t zero current is analogous to a classical Volta Potential measurement, provided any charge carriers in the SiOz are not in equilibrium at the AgBr and Si interfaces. By equating electrochemical potentials (35) of electrons (Fermi levels) a t each of the metal/metal and metal/semiconductor interfaces, and by equating electrochemical potentials of Ag+ at solution/Ag and solution/AgBr interfaces, then
1,
@t - @eft = --(peSi pAg + t e s t )
- JIAg+AgBr + N
+ A,$J - A$.,,,
@a)
were lo-* M AgN03 and KBr, respectively, with a reference electrode of Ag/AgBr wire. The entire assembly was thermostated at 25.0 f 0.1 "C, and allowed to attain thermal equilibrium with the sample before proceeding with measurements. A signal excitation voltage of 100 mV p-p was used. Impedance measurements from 10 KHz to lo-' Hz were made before the potentiometric study to provide an initial screening of sensors impedances. Those with less than 10' Q values at 1 Hz were rejected. In addition, impedance-frequency plots were required to show dependence at the lowest frequencies (lo-* Hz).
RESULTS Activity-Potential d c Responses. Measured potentials, qjrt - 41eft,for a wide range of silver ion activities are summarized in Figure 4. Not surprisingly, for all thicknesses of S i 0 2between 50 and 1300 nm with 100-nm coating of AgBr, the potential responses a t each activity lie within *20 mV of the mean value of a 95% confidence limit. Any other thickness dependence in these data would require explanation by a transport process through the oxide or mixed oxide-nitride layer. A line with theoretical Nernstian slope, but displaced about 40 mV negatively, passes through the experimental points for all data obtained in excess Ag' solutions, and most of the data obtained in excess Br- solutions. For comparison, the theoretical Nernstian response for an equilibrium Ag/AgBr electrode is illustrated. T h e theoretical curve obeys: ft
+ A@J- A@ref= 0.557 + 0.059 log aAg++ A@J (volts a t 2 5 C)
(3)
where - a's are the electronic and ionic work functions, because the outer potentials a t the S i 0 2 interfaces cancel out. This derivation follows Lange's treatment as modified by Parsons (22). Because of the internal equilibrium between ions and electrons (36) in AgBr,
(4) the more conventional expression for the cell potential is:
- @left = ,$A g / A g t
(1)
A folded plot is used t o cover the wide range of silver ion activities in the concentration range 0.1 M AgN03 to 0.1 M KBr. Experimental points should lie parallel to the normal electrochemical, Nernstian response but displaced by the work function difference - meS1 + a e A g as derived below. Contact potential measurements ( 3 4 ) ,give a range of values for the difference (-0.01 to -0.27 V); photoelectric effect work functions give a comparable range. Despite the inevitable scatter in literature work function data, the Volta Potential measurements should be more negative than the equilibrium, electrochemical measurements. The data in Br- solutions are noisier, less reproducible, and deviate from theoretical values a t high activities. T h e potentiometric sensor, or gate section of an ISFET, can be written out as a cell in the form:
In the usual case that the AgBr is saturated with silver metal, @t - ,$left = - q S i
=
+ %Ag+
OAg/Ag+
+ A@j - 40ref (6a)
- V,(Pot)
(6b)
where VG(P0t) is the measured, zero current potentiometric cell voltage. In the application of this type of cell t o the sensor portion of an ISFET, V G is considered as an adjustable bias applied such that VG > VG(P0t). Since the reversible interface potential differences retain constant values regardless of bias, all of the excess applied voltage appears as voltage drops across the SiOz and across the space charge created at the AgBr/SiO, and SiO,/Si interfaces. Any additional charge induced by bias ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
2317
is presumed to be linearly additive to the total charge. The additional voltage drops are written in terms of space charge: r i
1
0 r-
(7)
u
3
where Q is the total space charge (a negative quantity) in the depletion layer of the silicon. The denominators in the right side of Equation 7 are Gouy-Chapman capacitances of the space charge regions in AgBr and silicon, respectively, while C(oxide) is the geometrical capacitance of the oxide film. K'S are Debye thicknesses and 6 is the permittivity of free space multiplied by the appropriate dielectric constant. I n a practical device, VG - VG(Pot) also contains terms of the same form as Equation 7 resulting from any fixed charge (surface states) a t t h e SiOz interfaces. This term can be added into Equation 7 for completeness, if necessary. T h e magnitude of VG is chosen, such that Q is negative enough to cause the surface to be inverted (charge carriers at the surface with concentrations opposite t o that observed in the bulk). The required minimum potential drop between i.e., the SiOz/Si interface and the bulk Si is ( l / F ) (EF- EL), the difference between the Fermi levels in the actual piece of doped silicon and in an intrinsic piece of silicon. This quantity is computed for the condition that n(surf) = p(surf) = n, or p t , the intrinsic concentrations. This threshold condition is rather weak and a stronger condition is ( 2 / F ) (EF - E J , where n(surf) = p(bu1k) = NAand p(surf) = n,'/NA, and N A is the acceptor concentration. The threshold bias for an arbitrary minimum detectable level of Ag+ in the test solution is written by combining Equations 6 and 7 :
rLKE(AgBr) l +
3
Lo-
P"
3
3 0
3
d
3 3
3 3
M
3
-
d
3
3 3 3
3 -2
jc
I -1
I I 1 30 2 33 1 OG FSEWCMCV
'2
OS
I
3 10
9 4 35
Figure 5. Impedance-frequency response of a field effect sensor. Electrode No. 117 at 25 OC using 100 mV p-p. Electrode used n-type silicon with 1000 nm SO, and 100 nm AgBr. Electrolyte: 0.01 M AgN03 -6.
-4.
.
'?
T
-10\
C(oxide)
\
'
\
,
-!I
0
1
2
3
4
l o g t o X l O E (nm)
where Qmin is the generated space charge (and fixed charge) a t "surface inversion" and @Ag,Ag+min is the value a t the lowest activity level of Ag+ to be detected. By fixing VG, the external bias, charges a t interfaces, particularly Q a t the SiOz/Si interface, are established in relation to the silver ion activity. This result follows by eliminating Vc(Pot) between Equations 6 and 7, and taking the difference between the resulting expression for VG and VT using Equation 7.
V, - VT = constant =
@Ag,Agtrnin
-
Flgure 6. Low frequency capacitance vs. thickness of oxide. Substrates were both n- and p- type silicon
thickness of the doped Si substrate; then, the charge a t the S O z / Si interface "tracks" the external voltage. T h e source-to-drain current, which is proportional to the lateral conductivity, and therefore proportional to the number of charge carriers, is approximately given (in the linear range) by:
@Ag/Ag+ +
where VD is the source-to-drain voltage which influences the uniformity of the space charge from one well to the other (10).
2 + -(EF
F
-
E ; ) - -+ A A @ J Kf(Si)
Time-Response Dependence on Oxide Thickness. (9)
This equation expresses the theoretical result for MOSFET in this case) operation, viz., the external voltage source controls the charge at the SiOz/Si interface, i.e. the quantity
2 Q -(EF - Ei) -F Kepi) If the ISFET is designed to use (1) a surface coating with large Debye thickness, which implies a high resistance coating with low concentration of charge carriers; and (2) a thin oxide or nitride gate so that C(oxide) is large compared with the Debye 2318
ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
Although oxide thicknesses in the range 50-1300 nm show no statistically significant differences on either slope or intercept of steady state Ag+ and Br- potential responses, thickness variations have a marked effect on time responses. Impedance-frequency plots of the isolated potentiometric sensor show typical capacitive i.e., reactive impedance behavior. One run for a 1000-nm oxide coat is shown in Figure 5 . Measured capacitances, calculated from measured impedances vs. oxide thicknesses are shown in Figure 6 for both n and p-type substrates. The uncertainty in the correlation arises from the variable thicknesses of the AgBr coatings. Because the AgBr coatings are of comparable thickness with the SiOz films, and the AgBr films have much higher conductivities than the SiOz
Table I. Response Characteristics with Oxide Thickness Variation Br- Response
Ag+ Response
Slope, mV/decade
bide,
nm
63 i 60t 59i 60i
0-10 50-7 0
loot
5 11ot 5 180i 5 300i 5 380i 5 500i 5 600i 6 1300 t 50 ideal:
2 2 2
1 60i 1 60i 5
58i 59 i 59 t 58r 59
4 4 0.6 1
1%
c,,
F/mm2
I
0.2
-49
- 7 . 7 i 0.3 -7.75 -7.85
-10
-9.0 -9.8 -9.0 -8.8 -9.5 -9.9 -8.9
-10 i 0.1 -10 i 0.1
i
0.2
t i t
0.4
t
- 10.
0.6 0.5 i 0.2 i 0.2
- 10.
-9.6 -9.9 -
i
log c,, F/mmZ
Slope, mV/decade
log c2, F/mm2
0.3
11.
6
-8.4
-56t 1 -61 t 8 -62k 5
-8.7
-53i 8 -4Oi. 6 -52* 2 -57i 4 -54i 2 -56 i 1 - 59
- 9.9
t
-9.0
1%
c2,
F/mm2
i
0.6
-10
i
0.2
-9.9
i
0.8 0.1
i
- 10.
-9.4 -9.6
i i
0.6 0.1
-9.7
i
0.1
-9.9 i 0.3 - 10.
-10.2
-10.6
+. 0.1
Table 11. Response Characteristics with AgBr Thickness Variation Ag+ response 0
Slope, mV/decade
tAgBrv
-.
nm
Br- response
log C,,
F/mm2
Slope, mV/decade
log C,, F/mm2
0 c
55 i 5 77 i 7 110 i 1 0 130 i 1 0
= i c '=
60 56 60 59
7
i
+. i
6 4 12 2
-9.6
t
-9.8 -10.
i
t
0.3 0.3 0.9
-9.4
i
0.4
-57 -52
-48 -61
i t t i
1
-9.5
9 4 6
-9.7 -9.5 -9.8
i i
0.2
i
0.2 0.9
i
0.3
Table 111. Externally Transferred Charge Potential (mV) pAg+
@d- @'eft
5 4
55
-
x
60 76
-3 1 1.8
2
100
3
1
200
10
3 3
Q(Cs
J 7
g
,
-2
;:
'11
:I
1 :3
2 :3
3 20
*
3:
3 G =?E3dEVC
