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1 REPORT DOCUMENTATION PAGE Form Approved OMB No Puli reporting urden for this olletion of informtion is estimted to verge 1 hour per response, inluding the time for reviewing instrutions, serhing existing dt soures, gthering nd mintining the dt needed, nd ompleting nd reviewing this olletion of informtion. Send omments regrding this urden estimte or ny other spet of this olletion of informtion, inluding suggestions for reduing this urden to Deprtment of Defense, Wshington Hedqurters Servies, Diretorte for Informtion Opertions nd Reports (74-188), 1215 Jefferson Dvis Highwy, Suite 124, Arlington, VA Respondents should e wre tht notwithstnding ny other provision of lw, no person shll e sujet to ny penlty for filing to omply with olletion of informtion if it does not disply urrently vlid OMB ontrol numer. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 1. REPORT DATE (DD-MM-YYYY) 3. DATES COVERED (From - To) 15 Ot 4 4. TITLE AND SUBTITLE 2. REPORT TYPE Conferene Pper POSTPRINT Moility nd trnsverse eletri field effets in hnnel ondution of wrp-round-gte nnowire MOSFETs CONTRACT NUMBER 5. GRANT NUMBER 5. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) A.K. Shrm, *S.H. Zidi, S. Luero, S.R.J. Bruek, N.E. Islm 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) AND ADDRESS(ES) Air Fore Reserh Lortory Spe Vehiles 355 Aerdeen Ave SE Kirtlnd AFB, NM *Grtings In. 27B Brodent Pkwy NE Aluquerque, NM d. PROJECT NUMBER e. TASK NUMBER RP 5f. WORK UNIT NUMBER AA 8. PERFORMING ORGANIZATION REPORT NUMBER AFRL-VS-PS-TP SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES) 1. SPONSOR/MONITOR S ACRONYM(S) 11. SPONSOR/MONITOR S REPORT NUMBER(S) 12. DISTRIBUTION / AVAILABILITY STATEMENT Approved for puli relese; distriution is unlimited. 13. SUPPLEMENTARY NOTES Pulished in IEE Proeedings Ciruits, Devies nd Systems, Vol: 151, Issue: 5, pp , 15 Ot 4 Government Purpose Rights Dept of Physis nd Astronomy, University of New Mexio, Aluquerque, NM 8716 Dept of Eletril nd Computer Engineering, University of Missouri, Columi, MO ABSTRACT The urrent ondution proess through nnowire wrp-round-gte, ~5 nm hnnel dimeter, silion MOSFET hs een investigted nd ompred with ~2 μm wide sl, ~2 nm thik silion (SOI) top-only-gte plnr MOSFET with otherwise similr doping profiles, gte length nd gte oxide thikness. The experimentl hrteristis of the nnowire nd plnr MOSFETs were ompred with theoretil simultion results sed on semiempiril rrier moility models. The SOI nnowire MOS devies were frited through interferometri lithogrphy in omintion with onventionl I-line lithogrphy. A signifint inrese (~3 ) in urrent density ws oserved in the nnowire devies ompred to the plnr devies. A numer of prmeters suh s rrier onfinement, effets of prllel nd trnsverse field-dependent moilities, nd rrier sttering due to Coulom effets, ousti phonons, impurity doping profile nd surfe roughness influenes the trnsport proess in the hnnel regions. The eletron moility in the nnohnnel inreses to ~12 m 2 /V s ompred to ~4 m 2 /V s for wide sl plnr devie of similr hnnel length. Experiments lso show tht the pplition of the hnnel potentil from three sides in the nnowire struture drmtilly improves the suthreshold slope hrteristis. 15. SUBJECT TERMS Trnsverse eletri field effets; Chnnel ondution; Condution proess; Doping profiles; gte length; gte oxide thikness; Interferometri lithogrphy; Nnowire devie 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT. REPORT Unlssified. ABSTRACT Unlssified. THIS PAGE Unlssified 18. NUMBER OF PAGES 19. NAME OF RESPONSIBLE PERSON Dvid R. Alexnder Unlimited TELEPHONE NUMBER (inlude re ode) Stndrd Form 298 (Rev. 8-98) Presried y ANSI Std

2 NANOELECTRONICS Moility nd trnsverse eletri field effets in hnnel ondution of wrp-round-gte nnowire MOSFETs A.K. Shrm, S.H. Zidi, S. Luero, S.R.J. Bruek nd N.E. Islm Astrt: The urrent ondution proess through nnowire wrp-round-gte, B5 nm hnnel dimeter, silion MOSFET hs een investigted nd ompred with B2 mm wide sl, B2 nm thik silion (SOI) top-only-gte plnr MOSFET with otherwise similr doping profiles, gte length nd gte oxide thikness. The experimentl hrteristis of the nnowire nd plnr MOSFETs were ompred with theoretil simultion results sed on semi-empiril rrier moility models. The SOI nnowire MOS devies were frited through interferometri lithogrphy in omintion with onventionl I-line lithogrphy. A signifint inrese (B3 ) in urrent density ws oserved in the nnowire devies ompred to the plnr devies. A numer of prmeters suh s rrier onfinement, effets of prllel nd trnsverse field-dependent moilities, nd rrier sttering due to Coulom effets, ousti phonons, impurity doping profile nd surfe roughness influenes the trnsport proess in the hnnel regions. The eletron moility in the nnohnnel inreses to B12 m 2 /V s ompred to B4 m 2 /V s for wide sl plnr devie of similr hnnel length. Experiments lso show tht the pplition of the hnnel potentil from three sides in the nnowire struture drmtilly improves the suthreshold slope hrteristis. 1 Introdution Sling of semiondutor devies to the nnosle regime n led to devie nd performne prmeter improvements inluding redution in operting voltge, inresed speed nd greter pkging densities. As silion is the mteril of hoie for lrge perentge of semiondutor devies, the frition, nlysis nd testing of sled down versions of existing Si devies hs een n tive reserh sujet [1, 2]. The sling of devie prmeters of mny strutures is under onsidertion nd theories for improved effetive hnnel length for fully depleted, surroundinggte MOSFET nd doule-gte SOI MOSFET hve een proposed [3, 4]. Dimension redution for urrent tehnologies, however, hs its limits set y optil lithogrphy, nd sling of MOSFETs hs omplexities of short-hnneleffets (SCEs). Sling of doule gte nd silion-oninsultor (SOI) Delt MOSFETs to the nnosle regime shows promise ut further sling hs een limited due to frition diffiulties [5]. r IEE, 24 IEE Proeedings online no doi:1.149/ip-ds:24993 Pper first reeived 1th Otoer 23 nd in revised form 5th April 24 A.K. Shrm nd S. Luero re with the Air Fore Reserh Lortory, Spe Vehiles Diretorte, Kirtlnd AFB NM 87111, USA S.H. Zidi is with Grtings In., Aluquerque, NM 8719, USA S.R.J. Bruek is with the Deprtment of Physis nd Astronomy, University of New Mexio, Aluquerque NM 8716, USA N. E. Islm is with the Deprtment of Eletril nd Computer Engineering, University of Missouri, Columi MO 65211, USA A.K. Shrm nd S.R.J. Bruek re lso with the Centre for High Tehnology Mterils, University of New Mexio, Aluquerque NM 8716, USA For MOS trnsistors, sling studies hve minly foused on deresing hnnel length to sumiron dimensions while the width hs remined severl mirons in size due to the neessity of mintining the urrent driving pility (width-to-length rtio) of the trnsistor. True nnosling requires the redution of the overll size of the trnsistor nd not just the gte length. Suh studies hve reently ttrted onsiderle reserh interest sine the eletril, optil nd thermodynmi properties of nnostrutures n e signifintly different from those of ulk mteril of the sme omposition [6]. MOSFETs with nnowire hnnel wrp-round-gte (WAG) struture hve een shown to hve signifintly improved rrier trnsport properties over onventionl devies euse of redued sttering nd etter gte ontrol. Additionl study is neessry in order to fully determine the physil proesses impting the trnsport mehnisms [7]. In this pper we demonstrte tht the urrent density is enhned in nnowire hnnel WAG MOSFETs s result of higher rrier moilities. We disuss the physil proesses ontriuting to the inresed moility, speifilly qusi-1d trnsport t the hnnel entre of suh devies. Equtions for the moility model nd physil interprettion re provided. For this study, we frited B5 nm dimeter nnowire, wrp-round-gte MOSFETs with single nd multiple prllel hnnels nd ompred their hrteristis with B2 mm wide, 2 nm thik sl, toponly-gte MOSFETs tht were identil in ll spets exept for dimensionlity of the hnnel region. In order to fous on the effets of sling the hnnel width region, the nnowire hnnel length nd the sl gte length were oth mde intentionlly long (B2 mm) for this study so tht short hnnel effets would e minimised. Simultions of rrier moility for oth nnowire WAG nd sl gte devies re presented. The devie ondution proesses re explined in 422 IEE Pro.-Ciruits Devies Syst., Vol. 151, No. 5, Otoer 24

3 terms of hnges in the hnnel moility, the influene of trnsverse nd prllel omponents of the hnnel eletri field, nd the impurity distriution within the hnnel s result of the frition proess. 2 Nnowire frition Interferometri lithogrphy (IL) is well-developed tehnique for inexpensive, lrge-re nnoptterning nd ws used in the nnowire frition proess [8]. In its simplest version, IL is interferene etween two oherent wves resulting in 1D periodi pttern with pith of l/2siny where l is the optil wvelength nd 2y is the inluded ngle etween the interfering ems. A typil IL onfigurtion onsists of ollimted lser em inident on Fresnel mirror (FM) mounted on rottion stge for period vrition [9]. There is no depth dependene to n IL exposure pttern, for whih the depth-of-field is limited only y the lser oherene length nd em overlps. The 1D nnosle ptterns were first formed in photoresist followed y pttern trnsfer on to the underlying sustrte using retive-ion-ething (RIE) in prllel plte retor using CHF 3 /O 2 plsm hemistry [1]. Figure 1 shows ross-setionl views of these strutures fter therml oxidtion. These nnowires form the hnnel region of wrpped-round-gte nnohnnel MOSFET s desried in Setion 3. 3 Nnowire WAG hnnel nd top-only-gte sl MOSFET frition Nnowires were frited using the proesses desried in Setion 2 in lolised hnnel/gte res of the MOSFET devies using IL, long with onventionl I-line ontt msk printing [11] for defining the devie soure nd drin regions. For omprison, we lso frited MOSFET devies with sl top-only-gte regions. Figure 2 shows proess flow sequene. A 1 22 O-persqure re silion on insultor (SOI) wfer with 2 nm thik tive Si lyer on top of 4 nm thik uried oxide (BOX) isoltion lyer ws spin oted with photoresist (PR) nd exposed to n interferene pttern s desried ove. A 3 nm thik lnket lyer of Cr ws then deposited y e- em evportion nd lift-off step ws used to rete n rry of Cr lines tht re n effetive RIE eth msk. In order to lolise the Cr lines to the regions where the wires would e produed, the wfer ws gin spin oted with PR lyer nd msk ws then used to seletively pttern the PR to protet portions of the Cr lines from hemil nnowire region tive Si lyer ox lyer Si sutrte soure/drin mes definition ox lyer Si sutrte diffusion nd gte oxidtion ox lyer Si sutrte Al ontt pds ox lyer Fig. 1 SEM mirogrphs showing results of therml oxidtion SOI sl gte region single nnowire surrounded y thermlly grown SiO 2 multiple prllel nnowires surrounded y thermlly grown SiO 2 Si sutrte d Fig. 2 Proess flow sequene for frition of nnowire wrpround-gte MOSFET (left) nd sl-top-only-gte MOSFET devie (right) IEE Pro.-Ciruits Devies Syst., Vol. 151, No. 5, Otoer

4 Cr-eth solution. One the Cr ws ethed from the unwnted regions, the PR ws removed. One gin lyer of PR ws pplied on to the smple to define the soure nd drin regions. The soure nd drin definition msk ws ligned to the Cr lines (gte pttern). After the exposure nd develop proesses, the remining struture hd soure nd drin mess msked y PR nd Cr lines msking the gte region etween the soure nd drin regions. The smples were then ethed to the BOX lyer y RIE, thus defining the hnnel nd soure/drin regions. Figure 3 shows SEM mirogrphs of sl gte nd nnowire gte strutures efore therml oxidtion. Bsilly t this point we hve two mes strutures (soure nd drin regions) onneted y wide sl or y wires, respetively. After the RIE step the PR nd the Cr eth msk lines were removed. There is onsiderle dmge left y the RIE step. In order to remove some of this dmge, two rpid therml nnel (RTA) steps were performed. The first RTA ws performed t 91C for 5 min in nitrogen environment to nnel the dmged surfe [12] followed y seond RTA step for 3 min t 451C in hydrogen environment to pssivte the Si surfes. Next thik B.5 mm thik silion nitride lyer ws deposited on to the smples to e used s diffusion msk. A lyer of PR ws spin oted on to the wfer nd windows were opened in the nitride lyer ove the soure nd drin regions using uffered oxide eth (BOE) solution. PR ws removed nd lyer of phosphorous spin on glss (PSG) ws oted on to the smple for the pre-deposition step tht ws performed t B91C for 5 min. Next the PSG nd nitride lyers were removed using BOE. The gte oxidtion nd dopnt drivein were performed in single therml step using n oxidtion furne t 11C. As therml oxide ws grown round the wire strutures it onsumed the Si, thus deresing the width of the wire. Creful hrteristions were performed in order to optimise the therml proess in order to result in Si nnowire region dimeter of B5 nm. This proess grew gte oxide tht ws B6 nm thik round the nnowires. Similrly B6 nm gte oxide ws grown on the sl gte devies in order to minimise ny oxide pitne (C ox ) effets in our finl nlysis. The time for the oxidtion to yield B5 nm dimeter Si wires drove some of the dopnt into the hnnel region. Figure 4 shows proess simultion of the predited phosphorous impurity ontent in the wires. The resultnt doping profile resulted in n n + n n + struture s shown in the proess simultion (Fig. 4 4), whih shows the modelled impurity profiles efore nd fter the oxidtion/diffusion proess. Next lyer of PR ws deposited nd ptterned for metllistion. A 3 nm thik Al lyer ws e-em deposited using multiple shdow evportions nd liftoff in order to hieve onforml gte overge. The smples were lened nd nneled t B431C in rpid therml nnel (RTA) to rete ohmi ontts t the soure nd drin region. Completely frited single nd multiple nnowire hnnel WAG MOSFETs nd top-only-gte sl MOSFET re shown in Fig Current voltge mesurements, modelling, nd nlysis Fig. 3 SEM mirogrphs of sl struture nd preursors to single nd multiple nnowire gte strutures Sl struture onneting top soure mes region nd ottom drin mes region Preursor to nnowire gte struture onneting soure nd drin mes regions Preursor to multiple nnowire gte struture onneting soure nd drin mes regions Experimentl I V plots for oth the nnowire nd sl devies re shown in Fig. 6. The drin urrent (I d )s funtion of drin-to-soure voltge (V ds ) for vrious gte ises ws mesured using digitl urve trer. As seen from the plots, for similr doping profiles, gte length nd gte oxide thiknesses the urrent voltge hrteristis of the nnowire nd sl MOSFET re onsiderly different. The drin urrent in the nnowire is rther flt in the sturtion region (V ds 41 V) ompred to the signifint slope in the sl devie. This is due to the geometry of the sl MOSFET, in whih the fringing fields t the edges of the sl signifintly impt the drin urrent. As the hnnel pprohes pinh-off, the rrier hrge drops t the drin end nd the lterl fringing field inreses t the edges of the sl. Further inresing V ds uses the highfield region t the drin end to widen the hnnel enough to ommodte the dditionl potentil drop, thus resulting in further inrese in the drin urrent. In ontrst, when 424 IEE Pro.-Ciruits Devies Syst., Vol. 151, No. 5, Otoer 24

5 impurity onentrtion, m 3 impurity onentrtion, m phosphorous impurities nnohnnel depth, nm 1 7 eptor 1 5 donor hnnel length entreline, µm 1 19 impurity onentrtion, m eptor donor hnnel length entreline, µm Fig. 4 Simultion plots of impurity onentrtions Impurity onentrtion through the nnowire dimeter width from the se of the nnowire (x ¼ nm) to the top surfe (x ¼ 5 nm) Pre-diffusion impurity onentrtion of eptors (sustrte doping) nd donors (phosphorous doping) throughout the soure gte drin regions Post-diffusion impurity onentrtion of oth eptors nd donors throughout the soure gte drin regions the nnowire devie is ised in the sturtion region, the effetive hnnel length of the nnowire devie is essentilly unffeted sine the depletion region t the drin terminl is physilly restrited to B5 nm. This effet, known s hnnel length modultion, is well-known phenomenon in onventionl trnsistor designs [13]. This phenomenon is more dominnt in onventionl short hnnel devies. Suppressing suh effets in the B1V ds nnowire devie (s is the se here) is of signifint enefit, speifilly in the development of low voltge iruit pplitions. Figure 6 shows tht in the nnowire MOSFET the urrent is n order of mgnitude less thn for the sl devie. Sling to the ross-setionl re shows tht the nnowire devie urrent density is three times higher thn tht of the plnr sl devie. From the experimentl dt the resultnt ondutivities for the sl nd nnowire devies re s sl B9 1 3 A/V m 2 nd s wire B3 1 4 A/ Vm 2. This mens tht we n otin the sme mount of Fig. 5 SEM mirogrphs of vrious MOSFET strutures Sl-MOSFET where the entre eletrode is the gte surrounded y the soure nd drin eletrodes Single nnowire-mosfet Multiple nnowire-mosfet totl urrent driving pility in nnowire hnnel devies tht hve muh smller ross-setions y onfiguring severl nnowires in prllel. In order to understnd nd improve the urrent hrteristis of the nnodevie, we lso modelled the urrent voltge hrteristis of the trnsistors. We re not wre of ny reported stndrd nnowire hnnel wrp-round-gte MOSFET drin urrent-voltge (I d V ds ) reltions nd in order simulte the results we developed very simple model sed on the 2-D sketh of Fig. 7. Poisson s eqution in ylindril oordintes n e written s ¼ e s ð1þ IEE Pro.-Ciruits Devies Syst., Vol. 151, No. 5, Otoer

6 1 N + soure wrp-round Al gte region N + drin 75 Q S Q D drin urrent, μa 5 25 = V =.5V Si nnowire SiO 2 surrounding gte oxide thikness ~6nm = 1V drin soure is, V Si wire width ~5nm 2µm length 2. gte oxide thikness ~ 6nm drin urrent, μa = V =.5V = 1V drin soure is, V Fig. 6 Experimentl plots of drin urrent s funtion of drinto-soure is for vrious gte voltges Sl-MOSFET Single nnowire-mosfet where is the eletron potentil, r is the rdius vetor, r is the hrge density per unit volume, nd e s is the dieletri onstnt of silion. We ssume tht the verge hrge in the ylindril hnnel is Q verge ðq S þ Q D Þ=2 ð2þ where Q S is the hrge per unit re in the hnnel ner the soure nd Q D ner the drin regions. The urrent density in the hnnel n e then pproximted y J d Q verge n drift ð3þ where n drift is the hnnel region rrier drift veloity. The ylindril nnowire oxide pitne C rox ner the soure nd drin regions n e written s funtion of the regionl hrge nd pplied soure, drin nd gte potentilssfollows: C rox ¼ ðq s =s V t Þ ¼ Q D =ðd V t Þ; where d ¼ s V ds ð4þ where s is the gte-to-soure potentil, V dg is the drin-togte potentil, nd V t is the threshold voltge. Sustituting Q S nd Q D into the eqution for J d results in J d C rox ½ð2s V t V ds Þ=2Šn drift ð5þ Thus the urrent density J d depends on the drift veloity, the drin nd gte ises nd the hnnel pitne. Any Si sl thikness ~ 2nm 2µm length vrition in these prmeters will e refleted in the urrent density vlue. Sine the hnnel pitne per unit re (F/m 2 ) is fixed for given oxide thikness nd euse the oxide thikness is out the sme (B6 nm) for oth the nnowire nd the ulk devies, the pitne per unit re effet my not ontriute signifintly to the urrent density. Negleting ontriutions from the terminl is effets, the mjor ontriution omes from the drift veloity omponent. The drift veloity is funtion of rrier moility while the moility itself depends on the eletri field in the hnnel. The eletri field t ny hnnel region is the vetor sum of the prllel (E O )ndtrnsverse(e > ) omponent, where E O is in the diretion of the urrent flow. It my e noted tht even without signifint hnge in tul moility, hnges in prllel nd trnsverse eletri field omponent n lter the moility (rrier veloity) omponents (nd hene the drin urrent) signifintly. For exmple, the mgnitude of the E-field t ny point n e written s ~E qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ ~E? 2 þ ~E k 2 whih n e stisfied y p vrious vlues for E > nd E O.(e.g. jej ¼ ffiffiffiffiffi 5 kv=m is stisfied for E > ¼ 5kV/m nd E O ¼ 5kV/m, or E > ¼ 1kV/ m nd E O ¼ 7 kv/m). The inrese in E O (seond se), results in n inrese in the prllel drift veloity, thus inresing urrent flow. The hypothesis stted ove ws tested through inorporting mthemtil models for moility nd eletri fields in the hnnel regions of the devies. Most moility models inorporte sturtion t high prllel field of the form s suggested y Thorner [14], n drift ðe k ; E? ; N I ; T Þ¼mðE? ; N I ; T Þ:f1 width ~2µm Fig. 7 Shemti views of nnowire WAG MOSFET nd rrier ondution Shemti onfigurtion of nnowire WAG MOSFET Crrier ondution through nnowire gte Crrier ondution through sl gte þ½mðe? ; N I ; T ÞE k =n s ðt Þ Š 1 g:e k ð6þ 426 IEE Pro.-Ciruits Devies Syst., Vol. 151, No. 5, Otoer 24

7 where N I is the lol impurity onentrtion, nd T is the solute temperture. The rrier sturtion veloity, n s in the hnnel region is ssumed independent of norml eletri field, impurity onentrtion, nd diretion of urrent flow with respet to the rystl orienttion. The fitting prmeter hs een well hrterised with vlue of 2 for oth eletrons nd holes [15]. Besides the eletri field dependene, other mehnisms suh s ousti phonon sttering, impurity sttering nd surfe sttering lso ontriute to the moility. Thus the moility n e pproximted y the sum of the following terms, [16, 17]: 1=m ¼ 1=m þ 1=m þ 1=m sr þ 1=m Coulom ð7þ where m, is the moility limited y sttering ousti phonons nd is given y [17] m ¼ qh 3 rm I =ðm m m ZA 2 k BT Þ ð8þ where q is the elementry hrge, h is Dir onstnt, m I is the sound veloity, m* ndm m is the effetive mss nd moility mss, respetively, Z A is the deformtion potentil, k B is the Boltzmnn onstnt, T is the solute temperture nd r is the rel mss density of silion. The ulk moility of silion m is given y [18] m ðn I ; T Þ¼m þðm mx ðt Þ m Þ=½1 þðn I =C r Þ Š m 1 =½1 þðc s =N I Þ Š ð9þ where N I is the lol impurity onentrtion, m mx is the ohmi (pure-lttie) eletron moility, C r nd C s re fitting prmeters nd,, m 1 re model prmeters for eletrons or holes where the vlues n e found in Msetii et l. [16] For exmple, m mx ¼ 1417 m 2 /Vs, m ¼ 52.2 nd 44.9 m 2 / Vs for eletrons nd holes respetively. m sr is the moility limited y surfe roughness sttering nd is given y [19] m sr ðe? Þ¼d=E? 2 ð1þ where E > is the trnsverse eletri field nd d is model prmeter nd is V/s nd V/s for eletrons nd holes respetively [2]. m Coulom is due to the effet of Coulom sttering, whih is minly due to oxide fixed hrge nd surfe sttes hrge nd n e found from [2] m Coulom / T =Q f ð11þ where Q f is the fixed oxide hrge nd T is the solute temperture. Detils of moility models nd prmeters for nonplnr strutures n e found in Lomrdi et l. [21] nd will not e disussed here. However, it is importnt to note tht m sr is the min reson for the higher moility in the nnowire devies, nd tht the vlues of m, m, m Coulom, re omprle for oth devies. The rrier trnsport is sed on hnges in the moility omponents due to trnsverse nd prllel eletri fields, nd is lso due to the physis desried in (8) (11), whih is refleted in the finl moility vlue nd hene the drift veloity omponent. Any redution in the trnsverse field (expeted to e minimum ner the entre of the nnowires) inreses the m sr omponent (1) nd the overll moility vlue in the Mthiesen s summtion (7). In the sl devie, the trnsverse field is direted from the top towrds the sustrte, while in the wrp round gte struture, the trnsverse field is direted from ll round nd is minimum t the entre. Thus, due to the eletri field onfigurtion ffeting the surfe intertions the moility is expeted to inrese t the entre of the nnowires. Figure 7 7 shows n expeted shemti model of the rrier trnsport mehnism for oth nnohnnel nd sl devies. For resons disussed ove, the nnowire is expeted to show enhned forwrd motion ompred to the onventionl MOSFET surfe sttering model for the sl geometry. Figures 8 nd 9 show 2D simultion plot of rrier prllel (m O ) nd perpendiulr (m > ) moilities for oth sl nd nnohnnel devies. In the sl devie (Fig. 8) the moility is lower t the Si SiO 2 interfe due to surfe intertions nd the lrge trnsverse eletri field. Further wy from the interfe in the perpendiulr diretion these effets derese nd the moility inreses nd levels out to the ulk moility vlue. The dip in m > is due to the lotion of the edge of the depletion region where the field- moility, m 2 /Vs moility, m 2 /Vs moility, m 2 /Vs sl gte, nm sl gte, nm sl gte, nm Fig. 8 Simultion plots for sl hnnel rrier moility in the prllel nd trnsverse diretions for vrious gte ises ¼ V ¼.5 V ¼ 1. V IEE Pro.-Ciruits Devies Syst., Vol. 151, No. 5, Otoer

8 15 1 moility, m 2 /Vs nnowire gte, nm 15 drin urrent, μa drin soure is, V = V =.5V = 1V moility, m 2 /Vs moility, m 2 /Vs nnowire gte, nm nnowire gte, nm Fig. 9 Simultion plots for nnowire hnnel rrier moility in the prllel nd y trnsverse diretions for vrious gte ises ¼ V ¼.5 V ¼ 1. V dependent omponent drops rpidly. The nnowire devie (Fig. 9) shows similr trends ut the moility t the hnnel entre is out three times lrger thn the sl devies. The enhned moility is due to the ft tht the trnsverse field hs n equipotentil onfigurtion in the hnnel insted of from the top only s in the sl se. This results in n improved prllel eletri field distriution through the entre of the ross-setionl hnnel region, where the surfe sttering nd Coulom effets re lso miniml. Figures 8 8 nd 9 9 show plots of the effets of pplying gte is (trnsverse eletri field) on rrier moility for oth wire nd sl devies. As n e seen from drin urrent, μa = V =.5V = 1V drin soure is, V Fig. 1 Simultion plots of drin urrent s funtion of drin soure is for vrious gte ises Sl-MOSFET; Single-nnowire MOSFET the plots the vritions in trnsverse eletri field re more pronouned in the sl gte devies ompred to the nnowire hnnel devies. Figure 1 shows simulted I V hrteristis for the B5 nm hnnel devie t different gteises.theresultsreingreementwiththemesured I V for the frited single hnnel devie s shown in Fig. 6. The lose mth etween the simulted nd mesured I V plots is n indition of the vlidity of the model nd mteril prmeters hosen for the simultion. The simultion lso shows tht the nnowire hnnel devie hs hrteristis tht re somewht similr to uried hnnel MOSFET [22]. Our simultions show tht the I V hrteristi of the devie is very sensitive to the doping profile of the nrrow B5 nm thik hnnels. The est fit is for the se where n n-type dopnt is ner the upper surfe nd dereses monotonilly in the y- diretion (Fig. 4). This profile is expeted sine during the frition proess the soure nd drin were diffused t 11C with n n-type dopnt with pek onentrtion of 1 18 m 3. It is very likely tht diffusion into the hnnel region lso took ple. The net effet is the retion of devie very similr to normlly-on n-uried hnnel MOSFET. The hnnel doping profile prior to nd following the diffusion proess is different s is evident from the proess simultion in Fig. 4 [23]. As n e ntiipted, during devie opertion the onduting hnnel is the n-region rther thn n inversion lyer t the Si SiO IEE Pro.-Ciruits Devies Syst., Vol. 151, No. 5, Otoer 24

9 interfe s would normlly e the se if the p-type hnnel were the dominnt dopnt. Buried n-lyer MOSFETs hve een nlysed nd the physis of the devie I V hrteristis for the liner nd sturtion regions re different from tht for n inversion lyer formed t the Si-SiO 2 interfe [24]. 5 Suthreshold urrents The suthreshold region performne is prtiulrly importnt when evluting the suitility of MOSFETs for low voltge, low power pplitions, suh s when the MOSFET is used in high it rte swithing pplitions. The suthreshold urrents s funtion of gte is re shown in Fig. 11 for single WAG nnohnnel nd sl devies. As n e seen from the figure the urrent drops out three dedes for smll (DVB.1 V) vrition in the gte is. In ontrst, the suthreshold urrent in the sl gte devie did not drop one dede for muh lrger vrition in gte is. Typil nnohnnel devies hrterised hd B5 nm width Si surrounded y B6 nm oxide with n Al wrp-round-gte eletrode nd the sl devies hd B2 nm thik Si tht ws B2 mm wide with B6 nm thik oxide with top only Al gte. Both nnohnnel nd sl hnnels were B2 mm long.from Fig. 11 we n lulte the gte voltge swing S using the stndrd definition given y (12), whih results in suthreshold l ds, μa suthreshold l ds, μa Nnowire MOSFET Vgs, V V ds =.1V V ds =.1V Vgs, V Fig. 11 Mesured suthreshold drin urrent plots s funtion of gte is Nnowire WAG MOSFET Sl top only gte MOSFET suthreshold slope of B1 mv/dede for the nnowire MOSFET. S ¼ ln1½dv G =dðlni D ÞŠ ð12þ Sine the suthreshold urrent did not drop even one dede we extrpolted the suthreshold slope to e B4V/ dede for the sl gte devies where the urrent nnot e effetively turned off. This is due to the ft tht these devies do not hve typil n-hnnel MOSFET doping profile ut re essentilly uried hnnel devies with n n + n n + doping profile in whih there re suffiient rriers in the hnnel for ondution even t te is. Considering we hve similr doping profile for the nnowire hnnel devie tht hs gte oxide thikness of B6 nm, the suthreshold performne is truly remrkle. These results lso demonstrte tht nrrow hnnels provide etter ontrol over the hnnel potentil due to the enhned gte ontrol from ll sides in ontrst with only the front surfe overge in onventionl plnr trnsistors, even for these unonventionl devie-doping profiles. 6 Anlysis nd onlusion Through experiments nd simultion, we hve explined the rrier trnsport properties of Si nnowire hnnel wrp round gte MOSFET. Nnowire WAG MOSFET devies were frited long with sl top-only-gte MOSFETs for omprtive study. Interferometri lithogrphy ws used to define the nnowire in the hnnel region nd onventionl lithogrphy ws used to define the soure nd drin regions. Devies hrterised hd B5 nm dimeter wire hnnels nd B2 nm thik, B2 mm wide sl regions tht were oth B2 mm inlength. In future studies we pln to investigte shorter devies to understnd non-equilirium effets s funtion of lterl nd trnsverse dimensions. A semi-empiril rrier moility model for non-plnr silion strutures ws used to model the urrent voltge hrteristis. Simultion results show good greement with experiment. Anlysis showed tht the urrent density is out B3 time higher in the nnowire hnnel WAG devies s then in the sl devies. This is primrily due the verge higher rrier moilities in the nnowire hnnel devies s well s onforml uniform eletri flux densities in the wire devies. Study lso shows tht MOSFET width sling is possile while mintining the urrent driving pility high y integrting multiple nnowires in prllel. The experimentl results showed tht the urrent ws liner funtion of the numer of wires. 7 Aknowledgment The uthors would like to thnk Mr Rihrd Mrqurdt for ssisting in the experimentl setup for the suthreshold response, Dr P. Vrngis for ssisting in the ritil frition steps of the devies, nd Mr George Jzeremes for setting up the omputer simultions. 8 Referenes 1 Wind, S.J., et l.: Lithogrphy nd frition proesses for su- 1nm sle omplementry metl-oxide-semiondutor, J. V. Si. Tehnol. B, 1995, 13, (6), pp Fritze, M., Astolfi, D., Liu, H., Chen, C.K., Sunthrlingm, V., Prele, P., nd Wytt, D.W.: Su-1 nm KrF lithogrphy for omplementry metl-oxide-semiondutor iruits, J. V. Si. Tehnol. B, 1999, 17, (2), p Auth, C.P., nd Plummer, J.D.: Sling theory for ylindril, fullydepleted surrounding-gte MOSFET s, IEEE Eletron Devie Lett., 1997, 18, (2), p IEE Pro.-Ciruits Devies Syst., Vol. 151, No. 5, Otoer

10 4 Suzuki, K., Tnk, T., Tosk, Y., Horie, H., nd Arimoto, Y.: Sling theory for doule-gte SOI MOSFETs, IEEE Trns. Eletron Devies, 1993, 4, (12), p Colinge, J.P., Go, M.H., Rodriguez, A.R., nd Cleys, C.: Silionon-insultor gte-ll-round devies, IEDM Teh. Dig., 199, pp Persns, P. D., Lurio, L.B., Pnt, J., Lin, G.D., nd Hyes, T.M.: Zn inorportion in Cds nno prtiles in glss, Phys. Rev., 21, 63,pp (1) 11532(8) 7 Leondung, E., Gu Gin, Lingjie, G., nd Cho, S.Y.: Wire-hnnel nd wrp-round-gte metl-oxide semiondutor field-effet trnsistors with signifint redution of short hnnel effets, J. V. Si. Tehnol., B, 1997, 5, (6), p Chen, X., nd Bruek, S.R.J.: Imging interferometri lithogrphy: wvelength division multiplex pproh to extending optil lithogrphy, J. V. Si. Tehnol., 1998, B16, pp Zidi, S.H., nd Bruek, S.R.J.: Multiple exposure interferometri lithogrphy, J. V. Si. Tehnol., 1993, B11, p Zidi, S.H., nd Bruek, S.R.J.: Si-texturing with su-wvelength strutures, Pro. 26th IEEE Photovolti Speilists Conf. PVSC, 1997, 26, pp Zidi, S.H., Bruek, S.R.J., Hill, T., nd Shgm, R.N.: Mix nd mth interferometri nd optil lithogrphies for nnosle struturing, Pro SPIE, 1998, 3331, p Wolf, S., nd Tuer, R.N.: Silion Proessing for the VLSI Er (Lttie Press, 1986), Vol. 1, pp Gry, P.R., nd Meyer, R.G.: Anlysis nd Design of Anlog Integrted Ciruits (John Wiley & Sons, In., 1993, 3rd edn.), pp Thorner, K.K.: Reltion of drift veloity to low-field moility nd high-field sturtion veloity, J. Appl. Phys., 198, 51, pp Msetti, G., Severi, M., nd Solmi, S.: Modeling of rrier moility ginst rrier onentrtion in rseni-, phosphorous-, nd orondoped silion, IEEE Trns. Eletron Devies, 1983, ED-3, pp Sh, C.T., Ning, T.H., nd Tshopp, L.L.: Sttering of eletrons y surfe oxide hrges nd y lttie virtions t the silion-silion dioxide interfe, Surf. Si., 1972, 32, pp Deye, P.P., nd Conwell, E.M.: Eletril properties of n-type germnium, Phys. Rev., 1954, 93, pp Msetti, G., Severi, M., nd Solmi, S.: Modeling of rrier moility ginst onentrtion in rseni-, phosphorous-, nd oron-doped silion, IEEE Trns. Eletron Devies, 1983, ED-3, pp Fng, F.F., nd Fowler, A.B.: Eletron sttering in inverted silion surfes, Phys. Rev. B, 1968, 169, (3), pp Goodnik, S.M., Gnn, R.G., Ferry, D.K., Wilmsen, C.W., nd Krivnek, O.L.: Surfe roughness indued sttering nd nd tiling, Surf. Si., 1982, 113, pp Lomrdi, C., Mnzini, S., Sporito, A., nd Vnzi, M.: A physilly sed moility model for numeril simultion of nonplnr devies, IEEE Trns. Computo-Aided Design, 1988, 7, (11) 22 Sze, S.M.: Physis of Semiondutor Devies (John Wiley nd Sons, NY, 1981, 2nd edn.), Chp Proess simultion softwre, Athen, Silvo Interntionl Softwre, Users Mnul, 2 24 Merkel, G.: Ion Implnted MOS Trnsistors-Depletion Mode Devies, in Engle, F. nd Jespers, W.L. (Eds.) Proess nd Devie Modeling for IC Design (Noordhoff, Leyden, 1977) 43 IEE Pro.-Ciruits Devies Syst., Vol. 151, No. 5, Otoer 24