SURFACE POTENTIAL BEHAVIOR IN ISFET BASED BIO-(CHEMICAL) SENSORS

Transcription

1 Armenian Journal of Phyic, 1, vol. 5, iue 4, pp SURFACE POTETIAL BEHAVIOR I ISFET BASED BIO-(CHEMICAL) SESORS A. V. SURMALYA Department of Semiconuctor Phyic & Microelectronic, Yerevan State Univerity, Yerevan, Armenia amunik@gmail.com Receive 3 December, 1 1. Introuction For the mot recent of 3 year, the evelopment of ion-elective fiel-effect tranitor (ISFET) are very furiou an create prie [1]. The importance of work in thi fiel iue in lat year becaue of increae ISFET application. Many work have been recently one to characterize ISFET bae on MOS technology [-4]. Among thee evice, proton-enitive ISFET are the more eeply analyze. ISFET have a very fat repone time, high enitivity, mall ize, robutne an the potential for on-chip circuit integration. Becaue of the avantage, the ISFET can be wiely ue in many area epecially in biomeical area uch a meical iagnotic, monitoring clinical or environmental ample, fermentation an bioproce control an teting pharmaceutical or foo prouct [5]. An ISFET-bae penicillin enor [6], ISFET-bae zeta potential analyzer (protein etection) [7], urea etection [8] an ISFET glucoe enor [9] are the everal example of ISFET application in meical area. The ISFET enitivity epen mainly on the choice of the gate ielectric material(). The mot commonly ue material are ilicon an metal oxie or nitrie (SiO, Si 3 4, Ta O 5, Al O 3 an TiO ). When the tranitor gate i coate with ome ion elective membrane, the ISFET can be ue for the elective etection of the variou pecie in the urrouning electrolyte, other than the hyrogen ion. Such evice are known a the CHEmically Moifie Fiel Effect Tranitor (CHEMFET) [4]. In thi work, reult of analytical invetigation of emiconuctor urface potential epenence on the hyrogen ion concentration an parameter of gate layer() in cae of complex gate ielectric material for ISFET bae bio-enor are preente.. ISFET Structure an Principle of Operation A it i known the ISFET i in fact nothing ele than a MOSFET with the gate connection eparate from the chip in the form of a reference electroe inerte in an aqueou olution which i in contact with the gate oxie. amely, the gate tructure, preente in Fig.1, conit of a reference electroe an an inulator layer between which an electrolyte i flowing. In Fig.1 S i the gate potential, i the potential of iffuion layer in the electrolyte olution, ox i the potential of

2 ISFET Bae Bio-(Chemical) Senor Armenian Journal of Phyic, 1, vol. 5, iue 4 oxie layer an i the emiconuctor urface potential. The ion concentration in the electrolyte influence the gate potential, which in turn moifie the lateral tranitor threhol voltage. In thi way, the hyrogen ion concentration exercie an electrotatic control on the rain current moe, which mean that the change of the rain current ue to the change of the ion concentration in the electrolyte i compenate for by the ajutment of the RE potential (the gate voltage) [4-6]. Therefore, the ISFET enitivity i uually expree a the gate voltage change per a ecae of the hyrogen ion concentration ph, i.e. the change of the concentration by 1 time. ote that ph enote log H, e.g. if the value of the ph i equal to 3, the concentration of the hyrogen ion amount to 3 1 mole per liter [4]. H All conieration are approving to n-channel ISFET tructure. G (RE) S D Potential Al Electrolyte Al n + n + Depletion Layer Inulator g ox S p-silicon Back contact (Al) Fig.1. ISFET tructure. 3. The Soli-Liqui Interface Moel When a ilicon oxie urface i in contact with an electrolyte olution, urface hyroxyl group (SiOH) are built up at the oxie electrolyte interface (Fig.). Thee hyroxyl group are charge epening on the ph-value of the olution. The ph-value at which the urface i neutral, i calle the point of zero charge (ph pzc ) (ph pzc. for SiO ) [1]. At a ph lower than ph pzc, the oxie urface i poitively charge an at a ph higher than ph pzc the urface i negatively charge. Conequently, at phyiological ph value ( ph 7 ), the net urface charge of ilicon oxie i negative. In thi ection, the relationhip between the urface potential S an the ph-value of the olution, the urface charge enity an the concentration of ion in the electrolyte are coniere [1,11]. 195

3 A.V. Surmalyan Armenian Journal of Phyic, 1, vol. 5, iue 4 Fig.. Schematic illutrate the aci-bae equilibrium (chemical reaction) on ilicon oxie urface. The theoretical tuie of the phenomena occurring at the oli liqui interface in the ISFET enor (in the cae coniere here, it i the interface between the gate ielectric an the electrolyte) ha been unertaken by many author. Uually, the ISFET operation i explaine by o-calle ite-bining theory, which relate the interface potential to the concentration of the hyrogen ion in the analyze olution [1,13]. Accoring to thi theory, the ion preent in the olution react with poitively or negatively charge active ite at the ielectric urface creating hyrogen-active ite pair an conequently changing the total value of the active ite charge at the inulator urface. Thi, in turn, influence the tranitor channel current through the variation of the threhol voltage. Moreover, the active ite might react not only with the hyrogen ion but alo with other ion preent in the meaure olution, the o-calle iturbing ion. All thee chemical reaction occurring at the phae bounary are reverible an ecribe by the iociation contant k, which i temperature epenent a well [14]. Fig.3. Charge an potential itribution in the ouble layer at oli liqui interface. 196

4 ISFET Bae Bio-(Chemical) Senor Armenian Journal of Phyic, 1, vol. 5, iue 4 Becaue of the bining of the ion with the active ite, the graient of ion concentration i create in the electrolyte an, accoring to the Guy Chapman Stern theory [], the o-calle ouble layer i etablihe at the ielectric electrolyte borer a it i hown in Fig.3. The ouble-layer conit of the iffue layer an the Helmholtz layer. The Helmholtz layer comprie the layer of aorbe hyrogen ion an the common plane of aorbe anion an cation [15]. The electrical repreentation of the ouble layer i alo hown in Fig.3. The inice D,, S, C an in refer to the iffuion layer, the common plane of iturbing anion or cation, the inulator urface, the tranitor channel an the gate inulator, repectively. Bae on the theory, auming that the number of active ite on the urface of the inulator i contant, the ytem of nonlinear equation, preente further on, ecribe the relation between all the coniere quantitie. More etail on the theory of charge an potential itribution in the ouble layer an the erivation of the equation can be foun in [,4,16,17]. Yate an coauthor evelope a ite-bining moel to ecribe the charging mechanim of the oxie urface [14]. Accoring to thi moel, the charging of the oxie urface i the reult of a thermoynamic equilibrium reaction between the urface hyroxyl group (SiOH) an the H ion of the bulk electrolyte olution. The reaction are: SiOH SiOH H, SiOH SiO H, SiOH H, SiO H. SiOH SiOH Thi chemical reaction how that an originally neutral urface hyroxyl group can bin a proton from the bulk olution to become a poitive urface charge with the iociation contant a, a well a to onate a proton to the olution, leaving a negative charge on the oxie urface with the iociation contant b [1,11]. 4. Semiconuctor urface potential behavior 4.1. One inulator material Uually, the ISFET operation i explaine by Site-Bining Theory (SBT), which relate the interface potential to ph in the analyze olution [14]. It i the amphoteric nature of the oxie group at the interface, in cae of SiO thee are SiOH group, which caue the variation of the oxie urface charge at varying ph. The neutral urface hyroxyl ite can either bin ( SiOH ) or releae (SiO ) a proton epening on the olution ph. Becaue of the bining of the ion with the active ite, the graient of ion concentration i create in the electrolyte an, accoring to the 197

5 A.V. Surmalyan Armenian Journal of Phyic, 1, vol. 5, iue 4 Gouy Chapman Stern theory the ouble layer i etablihe at the inulator electrolyte interface [-4]. By utilizing the Gouy Chapman Stern theory [15], it can be hown that Here g an kt C g q 1 h inh. wktc are the gate potential an electrolyte iffuion layer potential, the charge enity in the iffue layer, h (1) C i C h an C are the Helmholtz layer capacitance per unit area an the olution concentration [4], repectively, an w i the water relative permittivity, i the vacuum ielectric permittivity. The conition of charge neutrality for the invetigate tructure (ee Fig.1) give: where i the charge enity of the urface ite, given by mo, () mo i the charge enity in the emiconuctor q q n q q mo ktp 1exp 1exp kt kt p kt kt i the emiconuctor relative permittivity, n an p are the equilibrium concentration of electron an hole, repectively [3]. The charge enity i taken poitive for an negative for. Otherwie can be written a [3] where H H il nit H H H q i the total number of available urface bining ite, ilanol ite an primary amine ite per unit area, repectively; 1 (3), (4) il an nit are the number of i are the iociation contant for the chemical reaction at the inulator interface, an H i the concentration of proton at the inulator urface. For we have q kt r H 8 inh kt, (5) where r i the electrolyte relative ielectric permittivity, i the cation concentration at the common plane. The olution of the above et of equation (1) (5) lea to the computation of the epenence of the emiconuctor urface potential on the hyrogen ion concentration (ph): 198

6 ISFET Bae Bio-(Chemical) Senor Armenian Journal of Phyic, 1, vol. 5, iue 4 kt 1 H H kt n p q il q nit q H H H 8. q g kt kt w H kt r wc H (6) Here w i the water permittivity, r i the electrolyte relative electric permittivity, i the vacuum electric permittivity, C i the olution concentration. 4.. Two inulator material Many work ha been one to emontrate the feaibility of uing ISFET for meauring ph an other ion in the electrolyte. In explaining interaction of electrolyte ion with oxie urface, ome reearcher have emphaize the tructural an the phyical apect of the itribution of olute. Other have tree the pecific interaction of olute with oxie urface an olution. However, no comprehenive moel exite to illutrate urface potential behavior in ISFET with two inulator material. A we have got an equation for epenence of the emiconuctor urface potential on the hyrogen ion concentration (Eq.6), we can propoe that exitence of the econ ielectric layer can change of enitivity of ISFET. By changing the ielectric material or ielectric thickne we can obtain the urface potential characteritic capable of ph. Aitionally we have got an equation for the cae of two ifferent inulator ielectric material 1 q H il q H nit 1 1, h h H H h C E E C C C H where C 1 i the two ielectric ouble-layer capacitance per unit area, E 1 an E are electric fiel intenitie in the firt an econ ielectric, correponingly, an , C (8) where 1, an 1, are the ielectric relative permittivity an thickne of ielectric material, (7) repectively, 4 kt q C r H g w. (9) ph A we ee in Fig.4, in preence of econ ielectric the range of enitivity change an -enitivity increae. For the ame value of the urface potential the hyrogen ion concentration i ecreae. So, taking into account Eq.(8), we can aume that by changing the ielectric thickne or ielectric material type we can obtain the urface potential characteritic capable of ph. 199

7 A.V. Surmalyan Armenian Journal of Phyic, 1, vol. 5, iue 4 V SiO /Ta O 5 SiO Fig.4. ph-epenence of emiconuctor urface potential in cae of one an two inulator. V V -epenence in cae of two inulator pair (a) SiO /Al O 3 an (b) SiO /Si 3 4 in cae of one (1) an two () ielectric material. Fig.5. ph Depenence in cae of two inulator pair SiO /Al O 3 an SiO /Si 3 4 are hown in Fig.5. In thee cae alo ifferent characteritic of urface potential behavior an a little change of enitivity are preent. A you can ee in both cae the enitivity range of tructure compare to one ielectric material cae i nearly the ame. So SiO /Al O 3 an SiO /Si 3 4 pair have the ame enitivity range in theoretical reult. Maybe they are much more ifferent in experimental reult. Depenence in Fig. 5 are plotte uing following parameter for Si, inulator an electrolyte at.7v gate potential an 3 [18-]: 11.7, w 78.3, r 5, , 16 Si 1 cm ( p A cm 3, 4 n cm 3 ), 1 6, R, 5 cm 1, h 1.1 ev, il 1 nit 1 cm, 15 a.6 1 cm 3, C h F cm, 1 cm 3, 6 1 cm,.1 cm 3,.1 cm 3.

8 ISFET Bae Bio-(Chemical) Senor Armenian Journal of Phyic, 1, vol. 5, iue 4 5. Summary Many work have been one to emontrate the feaibility of uing ion-enitive fiel-effect tranitor for meauring ph an other ion in the electrolyte. However, no comprehenive moel exite to illutrate urface potential behavior in ISFET with two inulator material. A we have got an equation for epenence of the emiconuctor urface potential on the hyrogen ion concentration (Eq.6), we can propoe that the exitence of the econ ielectric layer can change the enitivity of ISFET. In thi paper we ugget an ISFET moel with a gate of ouble inulator. The moel prove to be appropriate for analyzing the ISFET behavior a a function of hyrogen ion concentration an bining ite enity. By utilizing the Gouy Chapman Stern theory an the conition of charge neutrality in builtup ISFET bae bio-enor we have got an equation which expree epenence of inulator urface potential on hyrogen ion concentration (ph) in the cae of built-up ouble gate ielectric material. In preence of the econ ielectric the range of enitivity change an ph-enitivity i increae. Depenence in cae of two inulator pair SiO /Al O 3 an SiO /Si 3 4 are howing that in both cae the enitivity range of tructure compare to one ielectric material cae i nearly the ame. But in cae of SiO /Ta O 5 pair in preence of the econ ielectric the range of enitivity change. We can aume that by changing the ielectric thickne or ielectric material type we can obtain the urface potential characteritic capable of ph. REFERECES 1. I.P.Bergvel, Senor Actuator B, 88, 1 (3).. M.Daniel, M.Janicki, W.Wroblewki, A.Dybko, Z.Brzozka, A.apieralki, Technical Univerity of Al. Politechniki, 11, 93 (4). 3. G.Maobrio, S.Martinoia, M.Grattarola, Simulation of Semiconuctor Device an Procee, 4, 563 (1991). 4. C.D.Fung, P.W.Cheung, W.H.o, IEEE Tran. Electron Device, ED-33, 8 (1986). 5..Y.Park, S.B.Choi, M.Lee, B..Sohn, S.Y.Choi, Senor Actuator B, 83, 9 (). 6. A.Poghoian, M.J.Schöning, P.Schroth, A.Simoni, H.Luth, Senor Actuator B, 76, 519 (1). 7. S.och, P.Woia, L..Meixner, S.Drot, H.Wolf, Bioen. Bioelectron., 14, 413 (1999). 8. W.Sant, M.L.Pourciel, J.T. Launay, A. Do Conto, Martinez, P.Temple-Boyer, Senor Actuator B, 95, 39 (3). 9..Y.Park, M.Lee, B..Sohn, Electronic Lett., 37, 8 (1). 1. L.Boue, J. Chem. Phy., 76, 518 (198). 11. R.E.G. Van Hal, J.C.T.Eijkel, P.Bergvel, Senor Actuator B, 4-5, 1 (1995). 1. W.Olthui, Senor Actuator B, 15, 96 (5). 13. D.E.Yate, S.Levine, T.W.Healy, J. Chem. Soc. Faraay Tran., I7, 187 (1974). 1

9 A.V. Surmalyan Armenian Journal of Phyic, 1, vol. 5, iue D.Yate, S.Levine, T.W.Healy, J. Chem. Soc. Faraay Tran., 7, 187 (1974). 15. P.R.Barabah, R.S.C.Cobbol, W.B.Wloari, IEEE Tran. Electron Device, ED-34, 171 (1987). 16. M.Grattarola, G.Maimo, S.Martinoia, IEEE Tran. Electron Device, 39, 813 (199). 17. M.ollar, Meaurement Science Review, 6, Section 6, no.3, D.Lanheer, G.Aer, W.R.Mcinnon, M.J.Deen, J.C.Ranuarez, J. Appl. Phy., 98, 4471 (5). 19. F.Gaparyan, V.Aroutiounian, Senor & Tranucer Journal, 137, 36 (1).. F.V.Gaparyan, B.O.Semerjyan, P.A.Varereyan, Proc. of the 8 th Int. Conf. on Semicon. Micro- & anoelectronic. July 1 3, Yerevan, 11, Armenia, pp