PHOTOGATE ACTIVE PIXEL SENSOR MODELING USING PSPICE

Transcription

1 PHOTOGATE ACTIVE PIXEL SENSOR MODELING USING PSPICE Victor H. Poc-Poc, Flip Gómz-Castañda, José A. Moro-Cadas Ezquil Moliar-Solis ad Hró Molia-Lozao UPIITA-IPN Av. IPN, No. 805, C.P. 0760, Mico City, Mico Abstract Photogat-typ pils ar structurs widly usd i th dsig of modr CMOS itgratd camras, but also i ral-tim CMOS visio ssory systm applicatios. This work itroducs a PSpic macromodl for CMOS photogat-typ activ pil ssors that ca b usd for lctrical simulatio. Th aim of this work is to itroduc a PSpic macromodl du to th lack of macromodls rportd i th litratur. Simulatio rsults as wll as masurmts of a tst photogat-typ activ pil ssor, fabricatd i a stadard CMOS procss, N-wll, usig. micros, dsig ruls ar prstd.. INTRODUCTION CMOS activ pil ssors (APS) tchology is mrgig as a altrativ i th solid-stat imagig tchology with rspct to th charg coupl dvic (CCD) tchology, bcaus it has sigificat advatags i trms of rductio i th dvlopmt ad fabricatio costs. Powr dissipatio, scalig to high rsolutio formats ad compatibility with camra-systm itgratio ar also importat aspcts i CMOS APS tchology []. I th dsig flow of a CMOS APS, th us of macro modls for simulatio of dvics capabl of big fabricatd i stadard CMOS IC procsss is a importat issu. A aalysis of photocurrt modls for som photodtctor dvics has b rportd [4] ad PSpic macromodls for th photodiod dtctor ca b foud i th litratur [5], howvr, thr is a lack of macromodls rportd i th litratur of photogat-typ CMOS APS, for simulatio tools lik PSpic.. CMOS PHOTOGATE PIXEL A cross-sctio of a photogat (PG) i a P-substrat is show i Fig.. A PG is a MOS capacitor that is posd to light. Wh a positiv voltag (), is applid to th polysilico gat, photo gratio of carrirs is producd i th dpltio rgio which is formd udr th gat ad i th utral rgio. Durig th itgratio tim (T it ), photos pass through th polysilico gat ad grat hol-lctro pairs. Hols ar plld from th dpltio rgio to th substrat, du to th dvlopd lctric fild i th dpltio rgio. O th othr sid, lctros ar attractd by th gat ad ar accumulatd i th silico surfac, udrath th gat. Th PG is biasd positivly, typically at V DD, thrby cratig a pottial wll i th dp-dpltd substrat. This is why th MOS capacitor ca b s lik a buckt which ca b filld with chargs; s Fig. (a-c). A trasmissio gat,, is usd to trasfr chargs from th PG pottial wll to th floatig-diffusio diod () od whr chargto-voltag covrsio is do du to its associatd dpltio capacitac (C ). Th voltag chag (δv) at th ca b prssd as: qg pi δ V N N CVF () C Whr δv is proportioal to th umbr of lctros (N ) accumulatd with tim, i th PG pottial wll. G pi is th gai voltag of th i-pil sourc followr trasistor M, ad C VF, is dfid as th charg-to-voltag covrsio factor (i V/lctro). A rst trasistor M, is usd to rst th. Th sourc voltag of M ca b slctd ad rad by usig a pass trasistor, which grally cocts th sourc trmial of M with a commo-colum tral load trasistor for radig th pil sigal. O drawback of a PG structur is that it has a poor blu spctral rspos, sic th gat absorbs most of th short wav-lgth visual spctrum. Ev mor, i modr CMOS IC procsss, silicids compots ar addd to th gat to rduc cotact ad sht rsistac, blockig almost all th visual spctrum. Som solutios to this problm hav b facd by opig som widows o th gat so that light ca pass through. I spit of th poor blu rspos of a PG structur, its C VF grows as th ara of th gat trmial icrass, compard with th photodiod, whr th C VF rmais idpdt of th photodiod ara, for a giv icidt photo flu.. OPERATION OF THE PHOTOGATE Th basic squc of opratios for a PG APS ar: sigal itgratio, rst ad charg trasfr. Durig sigal itgratio, ad RST ar biasd lowr tha th PG to provid for som atibloomig cotrol, prvtig chargs from spillig ovr from a full wll, ad avoidig to flow ito adjact pils. S Fig (a). Aftr a itgratio tim, RST sigal is pulsd to V DD to rst th to approimatly 0.75V DD. Th fial rst voltag rachd at th od is V V DD V th, whr V th is th MOSFET thrshold voltag. Th is rst immdiatly, bfor th sigal radout, bcaus this allows improvd ois rductio; s Fig (b). Nt, is pulsd to 0V, to trasfr th charg throughout oto th od. Th δv is rad as th corrspodig pil sigal; s Fig (c).

2 Poly Gat Photo Gat - Activ Pil Ssor Dpltio rgio P-Substrat D M RST YSEL VOUT Fig.. Cross-sctio of a photogat-typ activ pil, durig th photocharg itgratio priod 5V 5V.5V.5V (a) M M RST.5V M M (b) RST5V M M Ysl Ysl dpltio rgio. Oly th currt compot du to diffusio is cosidrd, sic th dpltio rgio width (X d ) is vry thi for modr CMOS IC tchologis (typically 0.5µm). Short-wavlgth photos do ot ptrat profoudly isid d substrat ad sic thy ar almost compltly absorbd by th polysilico gat matrial, thr is a gligibl amout of short-wavlgth photos arrivig to th thi dpltd rgio to grat photocarrirs. Th, it is rasoabl to cosidr that all th chargs fillig th pottial wll ar diffusig from aras outsid th dpltio rgio whr logr-wav lgth photos ca ptrat. Thrfor, th spctral modl of th PG ca b obtaid by solvig th diffusio quatio[4], i o dimsio, i th utral rgio which is formulatd as follows: p p po D + G( ) 0 () whr D is th lctro diffusio costat, p ad p0 ar th css ad quilibrium miority carrirs i th P-substrat, rspctivly, is th lctro lif-tim ad G() is th gratio rat of hol-lctro pairs, du to a icidt photo flu arrivig at th surfac of th substrat pr uit ara (φ ) giv by α G( ) φ α () whr, α is th silico absorptio cofficit ad φ is obtaid by Tgat φ φoα α (4) I (4), T gat is th gat thickss ad φ O is th photo flu pr uit ara arrivig to th gat trmial, giv by P ic ( R) φ o λ hc (5) whr P ic is th icidt light powr flu pr uit ara, R is th rflctio cofficit at th gat surfac, h is th Plack s costat, λ th icidt light wavlgth ad c is th spd of light i vacuum. Solvig (), udr th boudary coditios p () 0 ad p () po, givs: p α + po ( ) C + C + C 0 (6) 0V.5V RST.5V 4. GENERATED PHOTOCURRENT For th structur show i Fig., th gratd photocurrt, that fills th pottial wll, is composd of two compots: th drift currt, du th drift of hols ad lctros i th dpltio rgio ad th diffusio currt (I DIFF ), du to th diffusio of photocarrirs outsid th Ysl (c) Fig.. Squc of opratios for a PG APS (a) tim itgratio, t it (b) Rst priod ad (c) Charg-trasfr ad floatig-diod pottial surfac lctur. M whr C C pi po po pi + C pi pi C αpi pi αpi φα C D ( α L ) pi pi pi α α (7) (8) (9)

3 LOAD whr X pi is th thickss of th pitaial layr ad L is th lctro diffusio lgth. Th paramtr D ca b computd by usig th followig mpirical prssio for silico, which is a fuctio of th accptors impurity coctratio (N a ) for a P substrat ad th tmpratur (T) []. KT 65 [ ] (0) D q + Na /( ) whr K is th Boltzma s costat ad q is th lctro charg costat. Paramtr L is giv by D () whr is th lctro liftim for silico ad ca b obtaid by th mpirical formula [4]: () /(.45 0 N A N A ) Th absorptio cofficit α, for silico ca b computd with th mpirical prssio []: α 84.7 cm - () (i m) λ µ ad th d, is computd usig [4]: d b + b + 4aV a G, qna qnat a, b ε ε si si o (4) whr ε si is th silico dilctric prmittivity ad T o is th thickss of th thi oid. Th phocurrt dsity fillig th pottial wll is obtaid by J diff p qd d (5) Fially, substitutig (6) i (5), th prssio for th photocurrt fillig th pottial wll du to diffusio is J qdc qdc diff + qdαc α (6) 5. PSPICE PHOTOGATE MACROMODEL Usig th aalog bhavioral modlig (ABM) fatur of PSpic, quatio (6) was usd to costruct a PSpic macromodl. A GVALUE PSpic part was usd to dfi th photocurrt quatio giv i Sctio 4. Th proposd quivalt circuit to modl th surfac pottial (δv ) is show i Fig.. S RST.SUBCKT Idiff S I SUB +params: tit0 t0 ara0 pic0 vg0 lambda0 * (tit) Photocarg itgratio-tim * (t) Trasfr tim (it must match th S puls width) * (ara) Photogat ara, i [cm^] * (pic) Icidt powr of light, pr uit ara, i [w/cm^] * (lambda) Icidt light-wavlgth, i [um] * Modl paramtrs (IC procss dpdt) param h ; Plack s costat, i [jouls*sc].param R0.0 ; Rflctio cofficit.param pi5-4 ; Thickss of th pitaial layr, i [cm].param Nd7 ; Elctro dopig coctratio, i [/cm^].param Na ; Hols dopig coctratio, i [/cm^].param i.450 ; Silico itrisic carrir coctratio, i [/cm^].param K ; Boltzma s costat, i [jouls/klvi].param q.60-9 ; Elctro charg, i [coulombs].param mobp4. ; Surfac hol mobility, i [cm^/v*s].param mob65. ; Surfac lctro mobility, i [cm^/v*s].param ps.05- ; Silico dilctric prmittivity, i [F/cm].param c.994 ; Spd of light i vacuum, i [um/s].param Tgat-4 ; Thickss of th poly-gat, i [cm].param To ; Thickss of th thi-oid, i [cm].param T00 ; Room tmpratur *Modl prssios param t{/(.45-*na+9.5-*(na**))};elc. liftim for Si [sc].param po{(i**)/na} ; Miority carrir coctratio, i [/cm^] * Elctro diffusio costat, mpirical prssio for silico:.param D{((K*T)/q)*(65.4+(65/(+(Na/8.56)**0.7)))},i [cm^/sg].param {sqrt(d*t)} ;Elctro diffusio lgth, i [cm] * Absorptio cofficit, mpirical prssio for silico.param alfa{((84.7/lambda)-76.47)**}; Lambda i [um], but alpha i [cm]..param fio{(lambda*pic*(-r)/(h*c))} ; Optical icidt photo-flu *{[um*jouls/(s*cm^)/jouls*s *um/s]}>/(cm^*s).param fio{fio*p(-alfa*tgat)}; photo-flu at th substrat surfac * Modl prssios for th diffusio currt modl param Ct{ (q*na)/(*ps) }.param Ct{ (q*na*to)/(*ps) }.param { (-Ct+sqrt((Ct**)+4*Ct*Vg))/(*Ct) } ; dpltio width.param C{(fio*alfa* (**) )/(D*(-( (alfa**) * (**) )) )}.param Cum{ -po*p(-pi/)+c*(p(-/)*p(-alfa*pi)- + p(-pi/)*p(-alfa*))}.param Cd{ p(/)*p(-pi/) - p(pi/)*p(-/) }.param C{Cum/Cd}.param Cum{(-po*p(pi/)-C*(p(/)*p(-alfa*pi)- + p(pi/)*p(-alfa*)))}.param Cd{(p(pi/)*p(-/)-p(/)*p(-pi/))}.param C{Cum/Cd}.param jtot{abs((-q*(d/)*c*p(/)+ + q*(d/)*c*p(-/)- + q*d*c*alfa*p(-alfa*)))}.fuc switch(z){/(+p(-5*(z-.5)))} * blocks GPG if S is i low-stat (S0V) Puls Currt Sourc ( charg tractig lmt) S Idiff I M δv C5fF M Rlak.param qit{ara*jtot*tit} *Charg accumulatd durig itgratio tim (tit) * Implmts th puls currt sourc GDIFF I SUB VALUE {switch(v(st))*qit/t} *Sam charg, trasfrrd wh S is i o-stat SUB lctrical quivalt circuit Rlak: vry high rsistac Fig.. Elctrical quivalt circuit of th PG APS usd for simulatio, usig th PG puls currt sourc subcircuit, Idiff..ds Idiff Fig. 4. PSpic subcircuit dfiitio of th puls currt sourc Idiff, usd to modl th charg tractio procss of th od i a PG APS.

4 Th subcircuit has thr ports. Th ports I ad SUB, corrspod to th iput-output trmials of th pulsd currt grator. A cotrol iput port S is usd for triggrig th pulsd currt grator. Th tim-width of th pulsd sigal S must b qual to th paramtr trd i th symbol placd durig schmatic captur. Th PSpic macromodl cod for this currt grator is itroducd i Fig. 4..0V.0V.0V Rst Lvl 6. EXPERIMENTAL RESULTS Th charg-trasfr procss of lctros stord i th PG pottial wll to th diod, i th last phas of opratio of th PG, ca b modld with th us of a currt sourc (I diff ) which tracts a amout of charg stord i th pottial wll, aftr th rst cycl, qual to th accumulatd charg (qn ) i th PG pottial wll, at th d of th itgratio cycl. This charg trasfr procss is assumd to b idal. A fi-tim puls width () is chos, th, th subcircuit computs th currt amplitud to tract th cssary amout of charg from th od. A fabricatd tst PG APS, dsigd for a CMOS visio chip applicatio [6] was usd to compar th simulatio rsults of th PSpic macromodl proposd i this work. Som thortical pil spcificatios ar show i Tabl I. I Fig. 5, th pil masurd rspos is show, udr complt darkss, ad th corrspodig simulatio is itroducd i Fig. 7. I Fig. 6, th pil masurd rspos is show whr a light powr icidt of P ic 0. mw/cm ad light wavlgh of λ ic 650m has b applid. Th simulatio that corrspods to this cas is itroducd i Fig. 8. Th microphotography of th tst cll is show i Fig. 9. Rst Lvl 0V 0s 5us 0us 5us 0us V(VOUT) 0.*V() Fig. 7. Pil simulatd rspos i darkss (P ic 0 mw/cm ). A.0V.0V.0V Rst Lvl Th diffrc i th fial V out lvl obtaid aftr th puls ca b attributd mostly to th rror i th icidt powr light masurmt at th surfac of th itgratd circuit. A P ic 0.mW/cm was masurd durig th tst masurmt ad it was usd for th simulatio. Tim 0V 0s 5us 0us 5us 0us V(VOUT) 0.*V() Tim Fig. 8. Pil simulatd rspos udr a powr icidt light of P ic 0. mw/cm ad λ ic650m. A δv out.9v is obtaid. A itgratio tim of 80µs has b cosidrd (Itgratio) (Trasfr) TABLE I GENERAL SPECIFICATIONS Fig. 5. Pil masurd rspos i darkss (P ic 0 mw/cm ). A itgratio tim of 80µs has b cosidrd Full-wll capacity 49 M - capacity 44 K - Voltag covrsio 7.6 µv/ - Fill factor.6 % Rst Lvl δv OUT (a) (b) Fig. 6. Pil masurd rspos udr a powr icidt light of P ic 0. mw/cm ad λ ic650m. A δv out.9v is obtaid. A itgratio tim of 80µs has b cosidrd Fig. 9. Photogat (a) layout dsig ad (b) microphotography of th tst cll fabricatd i stadard CMOS doubl poly ad mtal, with. µm dsig ruls 4

5 7. CONCLUSIONS A PSpic macromodl that simulats th photorspos of a CMOS IC photogat-typ activ pil ssor has b prstd. Th macromodl is basd o th aalog bhavioral modlig (ABM) fatur of PSpic. Th proposd subcircuit dfiitio cotais may paramtrs that ca b asily obtaid from CMOS IC data rus. Th rsults of th simulatios coductd hav b compard with th masurmts of a fabricatd PG APS tst cll, showig a good agrmt with th ral lctrical bhavior. 8. ACKNOWLEDGEMENTS This work was supportd by th Ctr for Rsarch ad Advacd Studis (CINVESTAV-IPN) ad a grat from th Program SUPERA-ANUIES-COTEPABE, through th Natioal Polytchic Istitut (IPN) of Mico. 9. REFERENCES [] E. Fossum, Activ pil imag ssors Ar CCD s disosaurs?, Proc. SPIE, vol. 900, pp. -4, Fb. 99. [] D.M. Caughy ad R.E. Thomas, Carrir mobilitis i silico mpirically rlatd to dopig ad fild, Proc, IEEE (Ltt.) 55, 9 (967). [] D. Schrodr, Smicoductor matrial ad dvic charactrizatio, Joh Wily ad Sos, Ic., Ch. 8, 990. [4] A. Moii, Visio Chips, Kluwr Acadmic Publisrs, 999, pp [5] R.J. Prry, K. Arora, Usig PSPICE to simulat th photorspos of idal CMOS itgratd circuit photodiods, Proc. IEEE SOUTHCON 96, pp , Apr. 996 [6] Victor Poc-Poc, Flip Gómz-Castañda, José A. Moro-Cadas, Luis M. Flors-Nava, Motio Dtctio Ssor Basd o CMOS Floatig-Gat Dvics, Proc. ICEEE/CIE, pp , Spt