Optimization FIBMOS Through 2D ATLAS Monte Carlo …downloads.hindawi.com/journals/vlsi/2001/045747.pdfPublished bylicense under ... Optimization ofFIBMOSThrough2DSilvaco ATLASand2DMonteCarlo

VLSI DESIGN2001, Vol. 13, Nos. 1-4, pp. 251-256Reprints available directly from the publisherPhotocopying permitted by license only

(C) 2001 OPA (Overseas Publishers Association) N.V.Published by license under

the Gordon and Breach Science Publishers imprint,member of the Taylor & Francis Group.

Optimization of FIBMOS Through 2D SilvacoATLAS and 2D Monte Carlo Particle-based

Device Simulations

J. KANG, X. HE, D. VASILESKA* and D. K. SCHRODER

Department of Electrical Engineering and Center for Solid State Electronics Research,Arizona State University, Tempe, AZ 85287-5706, USA

Focused Ion Beam MOSFETs (FIBMOS) demonstrate large enhancements in core deviceperformance areas such as output resistance, hot electron reliability and voltage stabilityupon channel length or drain voltage variation. In this work, we describe an optimizationtechnique for FIBMOS threshold voltage characterization using the 2D Silvaco ATLASsimulator. Both ATLAS and 2D Monte Carlo particle-based simulations were used toshow that FIBMOS devices exhibit enhanced current drive capabilities when compared tonormal MOSFETs. It was also found that the device performance is very much dependentupon the FIB implant profile. High and narrow doping of the FIB implant leads to highdrain current and low hot carrier reliability, whereas low and wide doping gives rise tolower drain current and higher hot carrier reliability.

Keywords: Device scaling; FIBMOS devices; Threshold voltage characterization; Channelengineering; Hot carrier reliability

INTRODUCTION

Focused Ion Beam (FIB) implantation has re-

cently been proposed for channel engineering inhigh-performance MOSFET fabrication. Shen andhis co-workers [1] fabricated FIBMOS deviceswith a special narrow doping region implantedwith FIB on the source side of the channel. Theyshowed that this novel device structure, schemati-cally shown in Figure 1, exhibits higher outputresistance, reduced hot electron degradation, morestable threshold voltage (Vr) upon device scaling,

and higher operation frequency. However, therather asymmetric doping profile in this deviceprevents one from using the classical expressionsfor the threshold voltage since it is a rathercomplex function of the step doping width anddensity. In this work, we first describe theoptimization technique that Kang et al. [2]developed for FIBMOS threshold voltage char-acterization, based on 2D device simulations and3D Vr contour mapping. It enables one to designFIBMOS devices with a certain Vr and bestperformance in consideration of the drain current,

*Corresponding author. Tel.: / 480 965-6651, Fax: + 480 965 8058, e-mail: [email protected]

251

252 J. KANG et al.

FIGURE Schematic description of the structure and dopingprofile of a one-step FIBMOS device with gate length 0.25 tmand oxide thickness of 5 nm.

hot-electron degradation, Vx stability and maxi-mum operation frequency.

Using both 2D Silvaco ATLAS energy balancemodel and 2D Monte Carlo particle-based simula-tions, we also examine hot-electron reliability ofFIBMOS devices. This is a severe problem innormal deep-submicrometer MOSFETs in whichthe high substrate doping, used to prevent thepunch-through effect, leads to large electric fieldsand enhanced impact ionization. For n-channelMOSFETs, the electrons generated via the impactionization process tend to be injected into the gateoxide (leading to a threshold voltage shift anddegradation of the channel mobility) or are in-jected into the drain. The generated holes are sweptinto the substrate, thus giving rise to substrateleakage current and enhanced impact ionizationdue to the forward biasing of the source/substratejunction. Both the 2D Silvaco ATLAS and the 2DMonte Carlo particle-based simulator show thatthe built-in electric fields at the source side of thechannel, due to the presence of the FIB implant,lead to enhanced current-drive capabilities ofthe FIBMOS device when compared to normal

MOSFETs. Our 2D Monte Carlo particle-basedsimulations also show that (due to the relativelylow electric field at the drain end of the channel)the average electron energy in FIBMOS devicesis low which, in turn, significantly reduces theprobability for impact ionization to occur.

THRESHOLD VOLTAGECHARACTERIZATION

As already discussed in the introduction, FIBMOSdevices have an asymmetric channel doping profilewhich influences the device performance. Forexample, the threshold voltage is not only afunction of the step doping density, but it is alsoa function of the step doping width. To under-stand the relationship between the doping profileof the FIBMOS device and the threshold voltage,we simulated a large number of FIBMOS deviceswith various implant step widths and doping den-sities. The Silvaco device simulator ATLAS wasused for this purpose [2]. The extracted thresholdvoltages were then mapped on a plane of stepwidth and step doping density. In other words,three-dimensional contour plots for the thresholdvoltage were generated with the step width and thestep doping density on the horizontal and verticalaxis, respectively. The extracted threshold voltageswere then rearranged for contour plots. A samplecontour plot for 0.25tm gate-length FIBMOSdevices is shown in Figure 2. From these contourplots, it is possible to find the combinations of stepdoping width and doping density that give thedesired threshold voltage. In Table I, we list fourdifferent combinations of parameters that lead tothe best device performance. One should note thatthe four combinations of doping parameters donot result in identical device performance, eventhough the threshold voltage is the same. Thedrain current is reduced as one goes from high andnarrow to low and wide doping profile. These areimportant features of FIBMOS. Therefore, if oneneeds high output current and device reliability isless important, high and narrow doping is best.

FIBMOS DEVICES 253

3.5

t_.s

i. "": ..................O .............

?.. "- %.,.:.:.5 -,.’..":"-

;2:: [...,30 40 50 60

00. ..................70 80 90 100 110 120

Step size [nm]

FIGURE 2 Contour plots for one-step FIBMOS with channellength of 0.25 gm. The numbers on the contour lines representthreshold voltages.

TABLE Parameters for step doping giving a thresholdvoltage of 0.69. The channel length of the FIBMOS deviceequals 0.25 gm.

Step doping width (nm) 60 80 100 120

Step doping density 1.95 1.61 1.43 1.32x 10TM cm- 3)

ID(A/gm) at VG= 1.8V 94.81 93.18 90.76 88.15

For high reliability, low and wide doping has to beused.

DEVICE TRANSFER AND OUTPUTCHARACTERISTICS

To examine the enhancement in the current drivecapabilities of FIBMOS devices with respect tonormal MOSFETs, we developed a 2D MonteCarlo particle-based simulator. The Monte Carlomodel, used in the transport portion of thesimulator, is based on the usual Si band-structurefor three-dimensional electrons in a set of non-parabolic A-valleys with energy-dependent effec-tive masses. The six conduction band valleys areincluded through three pairs: valley pair pointingin the (100) direction, valley pair 2 in the (010)direction, and valley pair 3 in the (001) direction.The explicit inclusion of the longitudinal and

transverse masses is important and this is done inthe program using the Herring-Vogt transforma-tion [3]. Intravalley scattering is limited toacousitic phonons. For the intervalley scattering,we include both g- and f-phonon processes. Itis important to note that, by group symmetryconsiderations, the zeroth-order low-energyf- and g-phonon processes are forbidden. Never-theless, three zeroth-order f-phonons and threezeroth-order g-phonons with various energies areusually assumed [4]. We have taken into accountthis selection rule and have considered two high-energy f- and g-phonons and two low-energyf- and g-phonons. The high-energy phonon scatter-ing processes are included via the usual zeroth-order interaction term, and the two low-energyphonons are treated via a first-order process [5].The first-order process is not really importantfor low-energy electrons but gives a significantcontribution for high-energy electrons. Thelow-energy phonons are important in achieving asmooth velocity saturation curve, especially at lowtemperatures. The phonon energies and couplingconstants in our model are determined so that theexperimental temperature-dependent mobility andvelocity-field characteristics are consistently recov-ered [6]. At present, impact ionization, surface-roughness and Coulomb scattering are not in-cluded in the model. However, these simplifica-tions do not prevent us from examining the overallperformance enhancement of FIBMOS deviceswith respect to normal MOSFETs.The gate length of both the regular MOSFET

and the FIBMOS device being investigated equ-als 0.25 gm. The source and drain extension is50 nm, and the junction depth is 36 nm. The stand-ard device has a substrate doping density of1018 cm -3. The doping density of the source anddrain regions of this device is 1019cm -3. Thesubstrate doping of the FIBMOS device equals1016 cm-3, the length of the one-step FIB region is70 nm, the depth of the FIB region is 36 nm, andthe doping of this region is 1.4 x 10TM cm -3. Thesource and drain doping of the FIBMOS device isidentical to the one used for the standard device.

254 J. KANG et al.

The conduction band edge, for applied biasV- V and Vz 0.8 V, is shown in the top panelof Figure 3. One can clearly see the presence ofa potential barrier near the source end of thechannel, as a result of the high doping of the FIBregion. Also note that the width of the barrierregion is less than 40nm, which suggests thatsource to drain tunneling might take place in thisdevice structure, thus leading to enhanced off-stateleakage currents. The x-component of the electricfield (electric field component along the channel)is shown on the bottom panel of Figure 3. Thepresence of a built-in electric field near the source

Distance [nm]Distance [nm]

oo 350

90 300

80 250

70200

150E 60100

50

30-50

20 -loo

10 450

00 50 100 150 200 250 300 350

2oo

Distance [nm]

FIGURE 3 Top panel: Conduction band edge variation forthe FIBMOS device, for Va= V and VD=0.8 V. Bottompanel: The x-component of the electric field for the same biasconditions.

end of the channel can significantly accelerate thoseelectrons that make it over the potential barrier.Similar electric field conditions exist in bipolarjunction transistors with graded base regions.

The average electron drift velocity and theaverage electron kinetic energy along the channelof the FIBMOS device, for the bias conditionsfrom Figure 3, are shown in Figure 4(a) and 4(b),respectively. Note that both the average driftvelocity and the average electron energies are lowin the front end of the FIB region. Because of thebuild-in field in the FIBMOS device, once elec-trons surmount the potential barrier at the sourceend of the channel, their energy and velocityincrease rapidly. Phonon scattering and the pre-sence of relatively low electric fields at the middle

2.5x107 ’" "’1(a) regular MOSFET2x107 FIBMOS device

>’1.5xl

" lx10

5X10

00 50 100 150 200 250 300 350

Distance [nm]

0.4

0.35

0.3

0.25

0.2

0.15

0.1

0.05

00

regular MOSFET ,gi (b)FIBMOSdevice t’’

i ;..’

50 100 150 200 250 300 350

Distance [nm]

FIGURE 4 (a) Average drift velocity and (b) average energyof the carriers along the channel of the FIBMOS device. In bothcases, the applied bias equals VG= V and V/ 0.8 V.

FIBMOS DEVICES 255

and drain end of the channel lead to a reduction inboth the average drift velocity and the averageelectron energy, thus eliminating the problem ofsubstrate leakage currents due to impact ioniza-tion. The characteristics of the regular device arequite different. The drift velocity is relatively lownear the source end of the channel which, as shownlater, leads to a factor of four smaller drain currentwhen compared to the drain current of theFIBMOS device. On the other hand, the averageelectron energy peaks at the drain end of thechannel and is about a factor of four higher thanthe average electron energy in the FIBMOS device.This gives rise to hot-carrier degradation in regularMOSFETs, and can be reduced by the introduc-tion of the lightly doped drain (LDD) regions.The transfer and the output characteristics of

the FIBMOS device and the regular MOSFET,obtained with the 2D particle-based simulator, areshown in Figures 5(a) and 5(b), respectively. Alsoshown in Figure 5(b) are the output characteristicsof both the regular and the FIBMOS deviceobtained with the Silvaco ATLAS simulator [2].Assuming that the threshold voltage equals thegate voltage for which the drain current is

laA/lam, we find that threshold voltage of theregular MOSFET is Vr=0.68V, in close agree-ment to the Silvaco ATLAS predictions. For theFIBMOS device, the threshold voltage equalsVr 0.61 V. This slightly lower value of Vr isdue to the fact that we use 1.4 x 1018 cm-3 insteadof 1.8 x 10TM cm -3 doping of the FIB region, andleads to slight overestimation of the performanceof the FIBMOS device. Both the Silvaco ATLASand the 2D Monte Carlo particle-based simula-tions suggest that the FIBMOS device beinginvestigated exhibits significantly higher drain cur-rent when compared with the normal MOSFETperformance. For example, for a gate voltageVa 1.0 V, the current of the FIBMOS device ismore than a factor of four larger than the draincurrent of the normal MOSFET. Note also that inthe Monte Carlo particle-based simulations we use1019 cm-3 doping for the source and drain regions,which leads to about 30% current degradation due

40

30

20- 10

,,,, ’J, "" ,,I,,

0.4 0.5 0.6 0.7 0.8 0.900.3

100

80

60

40

20

01.1

Gate voltage VG

[V]

0 0.2 0.4 0.6 0.8

Drain voltage Vo [V]

FIGURE 5 (a) Transfer characteristics for Vz) 0.2 V.(b) Output characteristics for gate bias Va V.

to series resistance effect [7]. This, in a waycompensates for the fact that Coulomb andsurface-roughness scattering are not being con-sidered in the present model. The source and draindoping used in the Silvaco ATLAS simulationsequals 10= cm -3, thus leading to significantlysmaller source and drain series resistance.

CONCLUSIONS

We have proposed and used an optimizationtechnique for threshold voltage extraction of a

one-step FIBMOS device. Using 3D contour plotsfor a range of step doping width and dopingdensity, we have determined the doping profilesthat set the threshold voltage at the targeted value

256 J. KANG et al.

for best device performance. We find that if thesystem requirements are high current drive or low-power operation, one needs to use narrow FIBimplants with high doping. On the other hand, ifthe system requires long device lifetime, then wideand low-doping profiles for the FIB implant needto be used. Both the 2D Monte Carlo particle-based simulations and the Silvaco ATLAS simu-lations suggest performance enhancement ofFIBMOS devices with respect to normalMOSFETs. An increase in the drain current ofabout a factor of four at the highest drain voltagewas found for the FIBMOS device when comparedto the regular MOSFET device. Similar trendshave been observed in Ref. [8].

Acknowledgments

This work was supported in part by the Micro-electronics Research Lab under Contract No.MDA 904-97-1-0113 and by ONR under ContractNo. N00014-99-1-0318.

References

[1] Shen, C.-C., Murguia, J., Goldsman, N., Pecckerar, M.,Melngailis, J. and Antoniadis, D. A. (1998). "Use offocused-ion-beam and modeling to optimize submicronMOSFET characteristics", IEEE Transactions on ElectronDevices, 45(2), 453-459.

[2] Kang, J. and Schroder, D. K., Proceedings of the 3rdInternational Conference on Modeling and Simulation ofMicrosysterns (Computational Publications, Boston, 2000),p. 380.

[3] Herring, C. and Vogt, E. (1956). "Transport and Defor-mation-Potential Theory for Many-Valley Semiconductorswith Anisotropic Scattering," Phys. Rev., 101, 944.

[4] Jacoboni, C. and Reggiani, L. (1983). "The Monte CarloMethod for the Solution of Charge Transport in Semi-conductors with Applications to Covalent Materials," Rev.Modern Phys., 55, 645.

[5] Ferry, D. K. (1976). "First-Order Optical and Inter-valley Scattering in Semiconductors," Phys. Rev. B, 14,1605.

[6] Gross, W. J., Vasileska, D. and Ferry, D. K. (2000). "3DSimulations of Ultra-Small MOSFETs with Real-SpaceTreatment of the Electron-Electron and Electron-IonInteractions," VLSI Design Special Issue, 10, 437.

[7] Xiao Jiang He, Masters Thesis, Arizona State University,May, 2000.

[8] Stockinger, M. and Selberherr, S., "Automatic devicedesign optimization with TCAD frameworks," Proceedingsof the 3rd International Conference on Modeling andSimulation of Microsysterns (Computational Publications,Boston, 2000), p. 1.

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Optimization FIBMOS Through 2D ATLAS Monte Carlo …downloads.hindawi.com/journals/vlsi/2001/045747.pdfPublished bylicense under ... Optimization ofFIBMOSThrough2DSilvaco ATLASand2DMonteCarlo