Nearly Ideal Unguarded Vanadium-Silicide Schottky ...

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Manuscript received July 8, 1985; revised March 3, 1986. The author is with Tektronix Inc., Beaverton, OR 97077. IEEE Log Number 8609115. F. DROBNY. IF@).
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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-33, NO. 9, SEPTEMBER 1986

Nearly Ideal Unguarded Vanadium-Silicide Schottky-*lBarrier Diodes VLADIiklIR F. DROBNY

Abstract-NearlyidealVLSI unguardedSchottky-barrierdiodes were made using a VSiJnSi junction. The reverse-current leakage of these diodes is fully explained in terms of electric-field enhancement present near the diode edges. The values of the diode quality lastor were nearly equal tounity and were identicalto those of guardeddiodes built on the same wafer. The ideality of the VSi,/nSi diode charscteristics isattributedtocontrolledoutdiffusion of silicon that o':curs around the diode perimeter.

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I. INTRODUCTION N Adrefractorymetal/silicide-type metalliza tkon is frequently used in high-performance bipolar \, 131 circuits. The reason is the high electrical conducth'ity , high electromigration resistance, and thermodynamic stability this metallization scheme offers. Gold isresponr;ible for thehighelectricalconductivityandchemicalrt:>sistance; the refractory metal forms a diffusion barrier that prevents interdiffusion of gold and silicon and provildes for adhesion of metallization to S O 2 . A Ti : W pseudoalloy having .lo-percent-weight Ti is widely used as a diffusion-barrierlayerdue toitssuperiorcorrosion n:!;istance and excellent adhesion and barrier properties [1][4]. In bipolar technology, near-noble metal silicides mch as PtSi, Pd2Si, etc., are usedto make high-quality re producible ohmic and rectifying contacts to silicon. The: use of near-noble metal silicides is generally preferred s lnce they consume less silicon and have higher barrierhei ;hts than silicides formed by refractory metals from the first transition-metal group. : Ithasbeenshownrecently,usingtheAu/Pd'Ti W/Pd2Si metallization as an example, that a combination of a Ti : W diffusion-barrier metal and near-noble rretal silicidesresults in nonideal Z-V characteristics of unguardedSchottkybarrierdiodes(USBD)[5],[6].The quality factor of these diodes significantly exceeds UI' ity, and their reverse current leakage is much greater than lexpected from ideal unguarded Pd2Si/nSi Schottky-barrier diodes. The Z-V characteristics of these diodes deg~xde markedlyafterthermalannealsat400°C.Fig.1shsws typical experimental Z- V characteristics of Au/Pd/Ti W/ Pd2Si/nSi USBD's built on (1 11) Si with Nd = 2 X 1Ol6 cm-3 before and after a 4-h anneal in forming gas. 7'he Z-V curve of a simulated ideal Pd2Si/nSi USBD shown in the same figure for reference has been calculated fro1 3

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Manuscript received July 8, 1985; revised March 3, 1986. The author is with Tektronix Inc., Beaverton, OR 97077. IEEE Log Number 8609115.

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Fig. I . Experimentalandideal I-V curves of AuiPdlTi:W/Pd,Si/nSi Schottky-barrier diodes: experimental before anneal; - - - - expergas; simuimental after 4-h anneal at 400°C in H2/N2 forming -CCC simulated guarded Pd2Si/nSi lated ideal unguarded Pd,Si/nSi; diodes using a = 2.45 nm, rj = 0.743 eV, Nd = 2 X 10l6 cm-3. ~

(1) K 2 ) is the where S is the diode area, A**(112A cm-* effective Richardson's constant, T i s the absolute temperature, k is Boltzmann's constant, Vis the applied voltage, R, is the diode series resistance,q is theelectronic charge, c $ ~ is the effective barrier height, and I, is the current contribution due to the silicide-junction radius [7].

where L is the diode perimeter, R, is the spreading resistance of the perimeter diode, rj is the junction radius,and +Be is the effective barrier height at the diode perimeter. The effective barrier height is calculated as I

where 4Bois the intrinsic silicide/silicon barrier height, a isthefield-penetration parameter, [8], [9]and E,,, and E ( r j ) are the maximum electric fields in the planar and

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perimeter diode portions, calculated from

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qNdAr(2rj Ar) (6) E(rj) = - 2% 3 where Nd is the concentration of carriers, E , is the permitin the tivity of silicon, W isthedepletionlayerwidth planar portion of the diode, and A r is the depletion-layer width near the diode perimeter. The terms aE,,, and a I E(rj)Z from (3) and (4)reflect barrier lowering due to penetration.of the wave functions of the conduction electrons in the metal-silicide into the semiconductor [8], [9]. The last term in (3) and (4) represents the barrier lowering due to the image force. The parameter a , which is a constant for a given silicide-silicon system, has been determined from reversebias characteristics of a guarded Pd2Si/nSi SBD. The value of A r has been calculated from

by employing a numerical iteration procedure [7]. Ithasbeenshownthataparasiticlow-barrier-height titanium-silicide/oxide/nSi metal/insulator/semiconductor (MIS) diode is responsible for excessive current leakage of the Au/Pd/Ti : W/Pd2Si/nSi USBD [5], [6]. This MIS diode forms around the USBD perimeter where the thickness of the oxide at the bottomof the USBD opening exceeds about 2 nm. At those places, the reaction between palladium and silicon is blocked and, after removal of the unreacted palladium and subsequent deposition of Ti : W , a tunneling Ti-MIS diode forms. This situation is illustrated in Fig. 2. The thin MIS oxide has the ability to passivate the surface of silicon and reduce the density of interfacial states, making the barrier height strongly dependent on the work functionof the metal. Thus, the barrier height of the Ti-MIS diode becomes very low and its current leakage very high.It was demonstrated by Drobny et al. [5],[6]thataparasiticTi-MISdiodeonly5 nm widesignificantlyimpacts the I-V characteristics of an Au/Pd/Ti : W/Pd2Si/nSi USBD. Similar behavior is expected from all near-noble metal silicide/Ti : W 'combinations unless enough silicide extends laterally under the perimeter oxide edge. This can occur only for very thick not used in VLSI silicides ( = 100 nm or more), which are technology. 11. VANADIUMSILICIDE USBD-THEORY During the formation of silicon-rich silicidesby refractory metals from the first transition-metal group, the silicon atoms are the major moving species [lo]. This often results in significant lateral outdiffusion of silicon into the

Fig. 2. Formation of parasitic Ti-MIS, diode.

metal surrounding the silicided contact axeas. It is shown that such outdiffusion, if well controlled, can be used to form a narrow silicide lip around the Schottky contact. This lip can protect the perimeter of a USBD from formation of the parasitic Ti-MIS diode. The perimeter current leakage can thus be eliminated, providing the current density of the silicide/oxide/nSi MIS structure formed by or lowerthanthe siliconoutdiffusioniscomparableto current density in the planar portion of the silicide/nSi diode. In the search for an ideal silicide for the lip-protected USBD structure,thebarrier-heightmagnitude,thesilicide-formation temperature, and the silicide-phase stability are important factors. The barrier heights of refractory-metal silicides to n-type silicon are lower than those 0.55 of near-noble metal silicides, with values between and 0.65 eV for ZrSiz and WSi2, respectively [ l 11. The use of silicides with higher barrier heigh.ts is preferred to minimizethereverse-currentleakage of diodesandto minimize the diode series-resistance effect at lower forward voltages. VSi2 and WSi2 have the highest reported barrier heights of all silicides formed by metals from the first transition-metals group. The barrier height of VSi2 to n-type Si is 0.64 eV as reported by Clabes [12] and by Thompson [13]. One great advantage of using VSi2 over WSi2 is the fact that, for thermodynamic reasons, vanadium is capable of decomposing thin Si02 while tungsten is not [6].This results in more predictable interfacial reaction between vanadium and silicon as compared with the tungsten-silicon system and also results in lower formation temperature. The reaction between vanadium and silicon has been found to start at 500°C as reported by Thompson et al. [13] and by Clabes et al. [12] and was confirmed by the present author. Krautle et al. found that VSi2 forms at a linear rate with timeove:r the temperature range of 570-650°C with an activation energy of 1.7 5 0 . 2 eV[14].Vanadiumformstwosilicidephases, i.e., V3Si and VSi2, which are both very stable and are never observed together [15], [16]. The reaction of V with bare silicon produces only the VSi2 phase. 111. VANADIUM SILICIDEUSBD-EXPERIMENTS The bipolar VLSItest structure used in our experiments hadbothguardedandunguardedversions of Schottkybarrier diodes on the same wafer. The testpatternand fabrication processes used to build the VSi2 devices were identical to those used to build the Au/Pd/Ti : W/Pd2Si/ nSi devices that produced the I-V characteristics shown in Fig. 1. Theonly difference between the VSi2 and Pd2Si

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devices was that VSi2 replaced Pd2Si, which left the rest ofthe metallizationthe same.Unguardedand guarci.ed Schottky barrier contacts of 14.5- and 8.2-pm diamet :rs, respectively, were defined using optical-lithography and wet-etching techniques on top of a 0 . 2 5 4 cm arse ricdoped n-type epi layer. The 1000-nm-thick oxidewall tiefining the unguarded diode area was tapered by an argonion implant. In all experiments, wafers of (1 11) orie;ltation were.used. A 10-nm-thick vanadium layer was vacuum depos ted on wafers. This was followed by heat treatment in a k r nace at 600°C for 5 to 15 min in Ar/H2 ambient to f49m vanadium silicide. Next, the vanadium that remained amreacted on top of the oxide was removed by etching 11 a 4 : 1 solution of H202: NH40H.Since vanadium is a \ , a r y reactive metal, special care was taken to prevent c0nt;t.mination of the ambient by oxygen or nitrogen. The thickness of the deposited vanadium was limited to 10 nnl to minimize the consumption of silicon by silicide for~rlation. Our calculation showed that10 nm of vanadium produces 28.3 nm of VSi2, thus consuming 28.2 nm of d i con. These numbers were arrived at by assuming 5.96, 4.42, and 2.33 g cmW3 as the density values for V, V8Si2, and Si, respectively [17]. It was estimated, using the .ate of VSi2 formation data from [ 141, that approximately 4.5 min of reaction time is required to convert 10 nm oF V into VSi2 at 600°C. A slightly longer reaction time (:hetween5and15min) is typicallyusedtoallow for the USBD perimeter-protection lip to form by lateral outtiiffusion of silicon into vanadium. The amount of the 1atc:ral out-diffusion of silicon is a function of the reaction trmI? g. perature and time and of the silicon-doping level. 3(a) is an SEM micrograph of a wafer section having, a (b) 14.5-pm-diameter Schottky-barrier diode formed on top 3. (a) SEM micrograph showing silicide lip formed by lateral outdifFig. of a 2 X 10l6cm-3 arsenic-doped epi layer using ar 8fusion of silicon on walls of a Schottky diode. The silicon is doped to 2 minsilicide-formationcycle.Here,thesiliconout-llifX 10l6cm-3 by arsenic. (b) SEM micrograph of wafer area with surface fusedlaterallyapproximately 400 nm from the con act concentrations of dopants: 2 X 10l8 C I I - ~ (boron), 1 X 10'' c r K 3 (boron), and 1 x lozo cm-3 (arsenic) for upper, middle, and lower contact area to form a silicide layer that covered abouthalf of the windows, respectively. height of the1000-nm-thickdiodeoxidesidewall. l'ig. 3(b) shows an SEM micrograph of a different area on the same wafer shown in Fig. 3(a). Here, the surface concen- werebiased by aKeithley 220programmablecurrent source, and a Fluke 6502 multimeterwas used to measure trations of dopant were 2 X 10l8 cm-3 (boron), 1 X All diode cmP3 (boron), and 1 X lo2' cm-3 (arsenic) for the upper, the device-bias voltage via the Keithley 220. A middle, and lower contact windows, respectively.No !,ig- characteristics were measured under dark conditions. nificant outdiffusion is visible in this case. At locati Ins Tek 4052A computer also was used for data analysis and where the surface concentration of dopant is high, (.Nutthe calculation of the basic diode parameters such as barrier diffusion of siliconisretarded.Highertemperatures or height, diode-quality factor, and series resistance and for simulation of ideal USBD and GSBD I-V characteristics. longer formation times are needed to achieve significant For simulated diodes, the valuesof diode size, donorconoutdiffusioninheavilydopedareas. The latter effec: is beneficial, because silicide formed by the outdiffusior of centration,anddeviceoperatingtemperaturewereassumed to be the same as those for the measured devices. silicon is required to protect the perimeter of unguarrkd diodes only and is not neededor, in some cases unwanted, The parameter a was determined by curve fitting the simat the collector, emitter, base, and guarded-diode coniact ulated reverse I-V characteristics to those obtained from areas. an experimental 8.2-pm-diameter guarded vanadium-silThe I-V curves of experimental VSi2/nSi Schottky-1a - icide Schottky-barrier diode. The reverse characteristics rier diodes were measured using a Tektronix 4052Acc m- of simulated and experimental guarded diodes are shown puter-based data-acquisition system. The measured dio jes in Fig. 4. The best fit of simulated to experimental I-V

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Fig.4. Reverse I-V characteristics of 8.2-pm-diameterguardedVSiz Schottky barrier diodes: ___ ideal simulated assuming N,, = 2 X I O i 6 cm-3 and a = 1.9 nm; - - - - experimental with arsenic concentration of 2 X 1 0 ’ ~cm-3.

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Fig. 5. Reverse I-V characteristics of 14.5-pm-diameter unguarded VSiz Schottky barrier diodes: - - - - ideal simulated unguarded assuming Nd = 2 X 1OI6 cm-’, r i = 30 nm,and a = 1.9nm; * * idealsimulated guarded; -experimental unguarded with arsenic concentration of 2 X loi6 crn-j.

curves was obtained using a = 1.9 nm. The reverse characteristics of atypical14.5-pm-diameterexperimental diodeand of asimulatedvanadium-silicideUSBDare shown in Fig. 5. The Z-V characteristicsofunguarded Schottkydiodes were simulated using (1), (2), (4), and (6) [7] with the above obtained value of a. The junction radius rj = 30 nm was found to produce the best fit to the experimentaldata.Theslight differencebetweensimulated and experimental Z-V curves of USBD’s can be explained by: 1)athigherfields, present.nearthediode edges, the image-force and field-penetration barrier-lowering terms do not add in a simple manner [7]; 2) a VSi2/ Si02/nSi parasitic perimeter diode formed by silicon outdiffusion may contributeenoughcurrenttocausethis 6 small deviation; 3) field dependence of the effective Richardson’s constant [8]. The forward Z-V characteristics of the experimental and simulated USBD’s are shown in Fig. Fig. 6 . Forward I-V characteristics of 14.5-pm-dialmeter unguarded VSi2 Schottky barrier diodes: guarded ideal simulated; ideal 6. In calculations of the forward diode characteristics, the unguarded simulated; -experimental. The concentration of carriers value for the junction radius obtained from the reverseand value of a are the same as for Fig. 5. currentfittingwas used.Thediodecharacteristicsare nearly idealwith thediode qualityfactor n = 1.015, VSiz USBD’s demonstrated that the current contribution which was identical to the value obtained from the guardedfrom the lip VSiJthin Si02/nSi perimeter structure is negcounterpartmeasured onthesamewafer.ThisdemonZ-V curves of vanadium-silicide ligible.Theforward stratesthatanearlyidealunguardedSchottky-barrier USBD’s were indistinguishable fromtholse of guarded dediode can be built using vanadium silicide as the Schottkyvices built on the same wafer having a diode-quality facbarrier metal. The forward characteristics are identical to tor rz = 1.02. The reverse Z-V curves of USBD’s differed those of guarded diodes, and the current enhancement in from those of guarded diodes. This deviation is fully acthe reverse direction is dominated by the effect of field countable to the high field effects present near the diode enhancement present near the diode edges [7]. Vanadium- edges [7]. Assuming that the radius of silicide approxisilicide USBD’s subjected to anneals of up to 17-h dura- mately equals the silicide thickness, an excellent agreetionat 400°C showed nosign of degradation and their ment wasobtainedbetweenthecalculatedthickness of Z-V curves remained unchanged. silicide (28.3 nm) and the junction radius (30 nm) determined from the reverse-current Z-V curve fitting. In adIV. SUMMARY dition to their ideality, the VSi2 USBD’s were capable of It has been demonstrated that the degradation effect of surviving long anneals (tested up to 17 h) at 400°C withTi : W on the Z-V characteristics of USBD’s can be elim- out any degradation. inated by replacinganear-noblemetalsilicidesuchas The use of nearly ideal unguarded VS’i2Schottky-barPdzSi by VSi2. Controlled outdiffusion of silicon into va- rier diodes in bipolar LSI and VLSI circuits eliminated nadium forms a protective lip of VSi2 around the USBD theneed for aguardeddiodestructure..This results in perimeter. This lip prevents formationof the high-leakage significantspace savings, reduceddevicecapacitance, parasitic Ti-MIS diode. The nearly ideal Z-V curves of and lower minority-carrier injection. This is important in

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many applications, such as analog sampling circuits n.nd Schottky clamping of the collector-base junction.

REFERENCES

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SchottkybarrierattheV/Siinterface,” J . Vac. Sci. Technol., vol. 20, pp. 684-687, 1982. R. Thompson, M. Eizenberg, and K. N. Tu, “Schottky contacts of Gd-Pt and Gd-V alloys on n-Si and p-Si,” J . Appl. Phys., vol. 52, pp. 6763-6768, 1981. H. Krautle, M.-A. Nicolet, and J. W. Mayer, “Kinetics of silicide formation by thin films of V on Si and S i 0 2 substrates,” J . Appl. PhyS.,v01.45,pp. 3304-3308, 1974. K. N. Tu, J. F. Ziegler, and C. J. Kircher, “Formation of vanadium silicides by the interactions of V with bare and oxidized Si wafers,” Appl. Phys. Lett., vol. 23, pp. 493-495, 1973. R. J. Schultz and L. R. Testardi, “The formation of vanadium silicides at thin-film interfaces,” J . Appl. Phys., vol. 50,pp. 5773-5781, 1979. CRC Handbook of Chemistry and Physics, 60th ed., R. C. Weast and M. J. Astle, Eds. Boca Raton, FL: CRC Press, 1980.

[I41 [I] P. B. Ghate, “Metallization for very large-scale integrated circuiis,” Thin Solid Films, vol. 93, pp. 359-383, 1982. [2] D. Summers, “A process for two-layer gold IC metallization,” $?lid [15] State Technol., vol. 26, pp. 137-141, Dec. 1983. [3] S. S. Cohen, M. J. Kim, B. Gorowitz, R. Saia, T . F . McNelly, and G. Todd, “Direct W-Ti contacts to silicon,” Appl. Phys. Lett., m l , [16] 45, pp. 414-416, 1984. [4] F. Nava, C. Nobili, G. Ferla, G. Iannuzzi, G . Queirdo, and G. l k l otti, “Ti-W alloy interaction with silicon in the presence of Ptt i,” [17] J . A&. Phys., VOI. 54, pp. 2434-2440, 1983. [5] V.F.Drobny, S. C.Perino,andR.E.Rose,“The effect of ‘I’iW diffusion barrier metal on characteristics of unguardedPd,Si Scho .tky bamer diodes,” in Proc. Electron. Mater. Con5 (Santa Barbara, (:A, June 20-22, 1984 (abstracts). [6] V. F. Drobny, “The effect of T i : W barrier metal on characteriatm of palladium-silicide Schottky barrier diodes,” J . Electron Matzr., VOI. 14, pp. 283-296, 1985. Vladimir F. Drobny was born in Czechoslovakia [7] V. F. Drobny,“Relationshipbetweenjunctionradiusand rev8:rse on November 14, 1946. He received the Dipl. Ing. degreefromtheSlovakTechnicalUniversity, ZEEE Trans. Electron leakage of silicide Schottky barrier diodes,” Devices, vol. ED-31, pp. 895-899, 1984. Bratislava, Czechoslovakia, in 1969 and the Ph.D. degree from the University of British Columbia, [8] J. M. Andrews and M. P. Lepselter, “Reverse current-voltage c wacteristics of metal-silicide Schottky diodes,” Solid-State Electron., Vancouver, B.C., Canada, in 1978, both in electrical engineering. VOI. 13, pp. 1011-1023,1970. [9] J. M. Andrews, “The role of the metal-semiconductors interfac : in From 1969 to 1974, he was a Research Engisilicon-integrated circuit technology,” J . Vac. Sci. Technol., vol. 1: 1, neer at the Slovak Technical University where he wasinvolvedinGaAsresearch.From 1978 to pp.972-984, 1974. [ 101 S. P. Murarka, Silicides for VLSI Applications. Orlando, FL: )!.