Charge neutralization in ion implanters

D.L. Smathk

*) MB

Ma&,

S.

Abstract

The control of wafer charging in high current implantation is the key to maintaining high-yield as gate oxide thicknesses decrease.The effectiveness of different charge neutralization techniques and different Faraday configurations have been evaluated using an in situ charge sensorin the Varian El000 high current ion implanter. The in situ measurementsalso aid in understanding the charging processand underscorethe plasma nature of the ion beam.

It~>reasedgate density and reduced junction depths have brought decreasinggate oxide thicknesses.Maintaining yield against the presence of device charging due to ion charge transport and secondary electron generation is thus made ever more difficult. New generation products now demand charging voltages as low as F3 V. Achieving such low levels of charging requires a comprehensive understanding of the charging processand of charge neutralization. Using an in situ charge sensorwhich emulates the implantation of a device [I], wafer charging and charge neutralization have been explored. Different charge neu?raliyation systems and different Faraday configurations have been modeled and evaluated for use in the Varian El000 and their impact on device charging has been q~!antified.The in situ sensorhas also provided invaluable insight into the cbarsing k:oc~.;sand has allowed mofelirtg of both the positive and negative charging. 2. Physics of wafer charging Wafer charging is a complex interaction between the device structureson the surface of the wafer and the beam. Charging occurs not only because of the charge transported by the ions, but also becauseof the charge removed by secondary electrons generated by the ion impact. The net effect of these secondariesis to increasethe charging by a factor, 1 f ‘y,, where ‘yS,the secondaryelectron yield, is between 1 and 7 for the speciesand energies of interest for ion implantation [Z]. Recent work has focussed on the plasma nature of the beam [l-6]. When the beam impinges

on the wafer, assumingthe wafer to be at ground potential, the potential difference between the wafer and the beam will be dropped acrossa saeath with a thickness given by the Debye length, A,, where

Here E,, is the permittivity of free space, k is the Boltzmann factor, PI,, the electron density, and T,, the electron temperature for the beam plasma. White [5] has measured beam electron temperatures in the range of 1 to 5 eV. The plasma nature of the beam has a profound effect on the charging process. In the past it was assumed that far ranging electric fields from a device charging on the wafer surface would draw neutralizing electrons into the device and, thus, given an adequate supply of electrons, the charging would beccme self regulating. The beam and the neutralizing electrons were viewed as separate entities. However, because of the beam plasma, the electric field from a device on the wafer will not extend more than a Debye length in front of the wafer and cannot penetrate the plasma. Thus, charge neutralization must be effected from the beam plasma, itself. In fact, there are three distinct contributions to the overall plasma existing in front of the wafer. The beam consists of high velocity, unidirectional and monoenergetic ions. As a result of collisions with the residual gases within the beamline, the beam creates a plasma of slow moving ions and electrons. The energy of these particles is much lower than the beam ions and these particles together with the beam ions form the beam plasma. Finally, the ion implanter frequently injects particles, which may also be a plasma, to bring about wafer charge neutralization. The interplay of these groups of charged particles is critical to an understanding of the charging process. To gain a better understanding of the physical processesin charging much recent effort has been

devoted Eowlrd the devetopment of real time in situ SalSOfB* The first rea! time sensor actually used a silicon MC% capacitor 175 Bnghmd et al, [S] and Sferlazr.o and Angel @I jnd2~ndenlly devel4qxd energy retardation probes that measurebeam potential. The slow ions generated by beam collisions are created Bt the beam potential. Ions leaving the beam and returning to ground potential have kinetic energy reflecting this potential. These probes operate by measuring the energy of the slow ions emitted from the beam. England et al. have used the same technique as a non-intt&ve prob of surface potential. Both groups have demonstrated the reduction of beam potential with external neutralization. Sferlazzo and Angel have shown the reduction in beam potential with increasing gas pressure. Although not truly a real time sensor,becauseof its inherent accuracyand unequivocal output, the CHAIXM device flcl] is certainly an important tool in understanding th,: charging process. In recent measurements Vella et al. have succeeded in fitting data taken using CHARM to a plasma model for the beam [lI]. in our own work a real time sensor, intended to emulate device charging, has been used. The sensor is a floating probe, mounted to the disk and spinning with it. To avoid stray capacitancesa high impedance, frequency compensated operational amplifier is located immediately adjacent to the sensor behind the disk. A fast V to F sonverter is used to tran&nit the signal from the spinning disk to a stationary recei\,er in the implant chamber. This sensorhas demonstrated a -barging dependence on deice size, an effect which provides a vivid demonstration of the plasma nature of the beam interaction [I]. The local environment around the sensor h:;s also been found to be critical in determining the degre:: of charging that is exhibited. 4%the

0.01 PEAK

ELI BWsM

case of a device embedded in a ~~nd~~~t~~~ e~v~~~~~rne~~~ the low enorgy ~e~~~~;d~~ry c!ectrons ;i*1entptureii by rhc &1rgillp,

devices

grea11y

rnit~~~l~~~

Xttr”

~~~F~~~~~

3, The charge mxsor as a planar Langmrair

an

probe

The beam can be regarded as a plasma and in the same vein the charge sensor and implanted devices can be modelled as a planar Langmuir probe. Consequently, the charge sensor can be used to measure a variety of beam plasma properties. The voltage measured bj: the charge sensor is the floating potentia! of the beam core plasma when the input impedance of the circuit is high enough so that no net current is drawn to the sensor.Therefore at the floating potential, V,, the ion and electron current densities are equal. Assuming that the ion current density is dominated by the beam ion current density jib, then standard probe theory 1121predicts that

where VP is the plasma potential, 9 is the electron thermal velocity, and T, is the electron temperature. This equation

1 tXW?ENT

hr

~~s~~~atin~ cnvironmen’t,,~43ndary rmissiranstill occurs but like the device, the surrounding region clso charges, so ?hat there is no potential difference to draw the secondaries into the charging device. As discussedbelow this sensur has prow3 useful for comparing charge neutralization LI,;Ff--.. * for comparing oh- impact of Faraday design orl .,JJ.~~lr..,, charge neutralization and for better undersrandicg he “.:r~ charging process.With ,egard to tha Ltte;, t:,, CL the present charge sensor, cjn be mod&6 ~5 a pianar Langmuir probe.

DENStY,

mAkm2

Fig. 1. Peakpositivechargingvoltagev5. beamcurrentdeasity.60 keV arsenicwith PIG on at 4 A and

0.4 s~~c’m.

prrdicts tkrt the di~~~r~~~e ~~t~e~n the plasma p~t~nt~~ and the fbting potential will scaleas the ~ogar~~b~~ of the icm km current density. Fiy. ; s:-*“.*-<#:,, variation of the measured ~lax~rn~~~ ctrarp ser;sLu ‘9‘!“I!/IIic !” ihc horn as the peak current den&y for B 60 keV arsenicbeam is vnried. Nrrte that the current density is plotted on a logarithmic scale. The peak current density is obtained from the beam current map and can be varied by changing ihe ovetail beam current or the proMed spot size of the beam. This can be done either by source tuning or by capitalizing on beam spacecharge and using the accel column suppression electrode for beam defocussing. The El000 uses a plasma llood gun charge neutralization system and this was turned on with an arc current of 4 A and a xenon gas flow of 0.4 seem for this data. As expected, the measured peak charging voltage is reduced by decreasing the peak current density in the ion beam, A logarithmic scaling of V, with ji,, is seen rather than with (V, - VP>, which suggeststhat the plasma potential is constant and, therefore, must be dominated by the electron temperature of the flood system. Further evidence of this fact is noted in the negative charging observed in comparing secondary electron rlooding with the plasma flooding (see below). An obvious impact of the results of Fig. 1 is the fact that implants should be performed a: the lowest current density levels that wi!l achieve the xequired throiighput.

Fig. 2. Charging

profile

vs. beam profile

with EFO.

‘~fre ClirS’t”tl

“’ :jj~ -:’ :,

L, j.lri~~i~, ‘! ,Jl#?Jl;& ;I- L.0) as

the standard charge ~~utrai~~ti{?n system. ‘Thi:; system uses a confined discharge in xenon to generate the plasma [13]. Xenon is used for its low ionization potential and for its high mass, which retards gaseous diffusion in the vacuum. As an option an electron flood gun (EFG) is also available. The electron flood gun uses 300 eV primary electronsto generate low energy secondariesin the vicinity of the wafer. This system is similar in design to the secondary electron showers widely used in many commercial impJanters. The charge sensor provides an excellient vehicle for comparing the performance of the two charge neutralizers. Fig. 2 shows charge sensor output for a 20 mA, 60 keV arsenic beam with a 2.8 mA,/cm’ beam current den&y. The figure shows a half scan, so that the charging is initially negative outside the beam and shows the typical negative-positive-negative charging when the beam actually strikes the sensor. In the El000 the wafer is part of the dose monitoring system, so that the Faraday limits the duration of the negative wings on this charging profile. Thus, the ampRude of the negative charging during beam strike is very dependent on the azimuthal extent of the

cio keV, 20 mA arsenic

with a beam density

of 2.8 mA/cmZ.

-a- -

I

-4

I .

I I

/ -5 -j1.51

2.25

2

I .75

I

Fig. 3. Charging profile vs. beam profile with PFG. 60 keV, 20 mA arsenic with 2 beam density of 2.9 mA/cm’.

PFC Arc Current 2

Charge Senror Voltage

-20

3

W 5

4

6

I- ,1

-25

-30

-35 Cl

1iNl

200

300

El% Primary

400

Cwrent

Fig. 4. Charging vo!tuge in the absence of beam: EFG vs. PFG.

500

WV

hrr.,,,. uIuIlii s&~;~~o yy,llll

u s.. Iv the Width Of the Faraday. lil liiiji c3iielii ihe

electron flood system has been set trp to give roughly equal negative and positive charging on the basis that both art aquahq ‘.*po,l.tant. This requires approximateiy 190 mA of primary en~:ir~r.rICKP~‘;~, giving a net disk current (beam plus secondary electrons) aE roughly 1 mA. The equality of the charging peaks is not perfect so that the negative charging reaches - 17 V and the positive chargkg reaches f34 V. Fig. 3 shows the same test with the plasma flood system. The beam current and energy are identical to those in Fig. 2 ;ind the beam current density, at 2.9 mA/cm*, is nearly identical. Again the PFG current at 4 A was set to give roughly equal positive and negative charging. However, the plasma flood system results in much lower charging levels, with positive charging capped at + 6.5 V and negative charging at - 4.0 V. Both figures show both the charging profile and the beam intensity profile in the scan direction. The latter is indicated by the dashed curves and is measured automatically by the ElOOO.In the case of the electron flood system, the charging profile is quite a bit wider than the measuredbeam profile (note that both profiles measurethe extent of the beam in the slow scan direction). However, the beam profile is measured with the beam off the wafer sensor.The sensor for both measurementswas surrounded by an insulating area (anodizing) with a 7.5 cm radius. With the beam in this area considerable charging of the insulating surface is likely to occur, and a degree of beam “blow up” is to be expected. This is believed to be the reason that the charging profile is wider than the beam profile. The agreement between the two profiles is much closer in the case of the plasma flooding system, undoubt-

edly, becauseof its superior charge neutraiization. At ISO keV the charging profifes and the beam profifes are even more closely matched. This is becauseof the lower charge de&y and Iower space charge in the ‘XL :

The EFG and PFG systemswere also compared without beam. The intent of this test was to compare the electron energy distributions with the two systems. The charge sensor site actually con!ains two sensorswiih an analog switch to allow viewing the output of first one sensor then the other. For this test the sensorswere equal except that one sensor included an operational amplifier with the standard 70 MC8 input impedance while the second had a 30 Ma impedance. Fig. 4 shows the results. In the case of the plasma flood gun the negative charging is very low, only 3 to 4 V, and the measured charging is essentially independent of the sensor input impedance. Note also that the negative charging without beam is identical to that with beam, again suggesting that the plasma from the PFG dominates the beam plasma. In contrast, the electron flood system shows considerable nei@ive charging and the charging is strongly dependent on the sensor input impedance. This result is easily uederstood. Without beam and with the EFG, only electrons are presenl in quantity. As the sensor charges negatively, few electrons will be able to strike the sensor. Thus,

Sensor Input Resistance @IQ)

Fig. 5. Variationin peaknegativevoltagewith sensar impedance.

where K’ is a constant and n(E) is the electron energy distribution. Thus, negative charge voltage in this test samples the electron energy distribution. In the case of secondary electron emission [14], the energy distribution function has a long tail, which is roughly exponential. Substituting n(E) = K” e-kE

course *tie PFG electrons would be expected to be in thermal equibbrium in the PFG plasma and, thus, won93 have a Maxweilian energy distribnti~n with a tow eiectron temperature. Regardless, the sensor does modei devhze charging and htdicates wet: controlled negative charging. 5, Csmparisaa

of Faraday

systems

gives vcc3ekVo=m With measurement at two values of the load resistance,R, the value of the exponent, k, can be determined. As shown in Fig. 5, this coefficient lies between 0.07 eV- r and 0.09 eV-’ ) The published data for silver has k = 0.07 eV-r, in accord with the present results, which used an aluminum target. Thus, the high negative charging and the strong rlnnenri~~c~ .---r-----

nn ___ the

sensor

imn~rlnnm= ‘-“Iv-----

are.”

hnth “V.I.

+rihtz+-F!n IS.%. I”Q%IL.- -1

to the long tail in the energy distribution. This energy distribution will be present in any neutralization system using secondary electron emission. Note from Fig. 5 that in the caseof device implantation, where the impedance to the remainder of the wafer may be extremely high, damaging negative charg.ing is easily produced. If this same analysis is used to deduce the electron distribution of the plasma flood system, again assuming an exponential energy distribution, the value of k determined is 30 to 50 times greater, indicating a very sharp cutoff. However, in the caseof the plasma flood gun the sensoris immersed in a plasma and responds not only to the electrons but also to the ions. Thus, for this measurement the probe reachesthe floating potential of the T’FQplasma and does ROEtruly IRETSWKSthe electron energy distribution. Of Peak Charging Voltage

I

” Full 10

I

Tie ED90 uses a Faraday in front of the wafer on the principle that tht, Farad,.~r*=5semblywill confine neutralizing electrons to the vicinity aE the wafer, assis:“?~,I’T~ maintaining devic:: nturralitg. Test:ng w: _ t.. _1 .i’ ct sensorconfirms the role of e&tron confinc.mzntas snow5 in Fig. 6 for the case of the electrostatic Faraday. This Faraday uses a short range (picket fence) magnet array to prevent electron escapebetween the diik and Faraday and L-l-- 22 p‘” -.FG*iim: &.&On cscapii;eat the a?: .&.:tr.&&c .j;;> ‘WA11 entrance. When the confining picket fence magnets and the suppressionelectrodes are removed frum ihe &r;imidc Faraday, the positive charging is seen to increaseby more than a factor of 2 for both the PFG and EFG. Even with the PFG there is a growth in the charging observed, when the electrons are not confined, and it is no longer possible to balance positive and negative charging. The imbalance is even more marked in the case of the EFG and even at the maximum primary current available, the charging cannot be brought down to the levels achieved with the electron confinement in place. In the caseof the magnetic Faraday, the peak charging measured by the charge sensorwith matching PFG conditions is similar to that measured for the electrostatic Faraday. The motivation for the magnetic Faraday was to

-- -=

Faradq

No Magnets

-

;No Su ppressior

-

-

I-P

PFC 4A Fig. 6. Charging

PFG 4A

EFC

139

mA

with and withoutelectrorrconfinement.As+, 18 zn& 60 IN,

EFG

500

1.2 mA/cm”.

rnA

Y Retumd

Fig. 7. Typicalsecondaryelectrontrajectoryin the ElOOOmagneticFaraday.90% ncatralized25 mA Gaussianbeam with I cm beam radius. eliminate bias ring microdischarges [IS]. The magnetic Faraday uses a transversemagnetic bottle to prevent electrons from leaving the Faraday. The Faraday is arranged so that t&e is no magnetic field (< 2 G) in the vicinity of th:b charge neutralization system and between the charge neutraii-zr and the wafer. A very steep gradient of magnetic field on both sides of the magnetic bias ring reflects electrons traveling with the beam. Monte Carlo modeling has been carried out on the r.:lgnetic field ?r?ensureproper soppressm of the Yam&y. The model does not in&de the acceleration of electrons across the plasma boundary layer at the surface of the wafer but includes all other pertinent physics. Fig. 7 shows one secondary electron ejected from the beam spot on the disk/wafer. The secondary is created within the confines of the beam and stays within the beam up to very high levels of charge neutralization (assumed uniform in the beam for simplicity). Of course, secondariesare returned not to the exact spot at which they originated, but do return to the wafer within the beam spot size. Thus, on the average the multiplier effect of the secondarieson positive charging wili be nulled out. This is a significant factor and is the reason for the superiority of the Faraday in front of the wafer. 6. Conclusions Measurementswith an in situ charge monitor have been used to study the effects or’ charge neutralization by elec-

tron flood gun and plasma flood gun techniques. The charging measured for the plasma flood gun is found to be half of that observed for the electron flood gua. Yield results on test devices also confirm the superiority of the plasma flood gun [16]. The use of xenon in the plasma flood gun is shown to lead to the lowest charging voltages. A Faraday or other appropriately designed eleciron confiaemer.! scheme in front of the wafer can significantly mitigate waft.- charging. The variation of the positive charging voirage with beam current density is in good agreement with the predictions of the planar Langmuir probe theory description of the charge sensor.

Acknowkdgements The authors are grateful to Mary Lou Pascucci and Dave Prisby for the data collection work and Peter Barschall and Phil Corey for their work on the charge sensor electronics.

References

[l] ME. Mack,P. Barschall,P. Corey,S. Satohand S. Wnlther, Nucl. Instr. and Meth. B 74 (1993)287. [2] ME. &lack, Handbookof Ion ImplantationTechnology,ed. J.F.Ziegler,section3.2.3(North Holland,Amsterdam1992). [3] ME.

Mack, Charge Neutralization, U.C. Extension Short

Course, Wafer Charging Effects in ion hnplaatalion Processmg, Dallas, June 1993 [4] MC. Vega, Role of the Beam/Plasma Potential in Charging Damage, U.C. Extension Short Course, Wafer Charging Effects in Ion Implantation Processing, Dallas, June 1993. [5] N. White, Nucl. Instr. and Meth. B 55 (1991) 287. [G] H. Ito, F. Plumb, J. England, .I. Fothcringhwm and P. Kindersley, ion Implantation Technology-92, cds. D.F. Downey, M. Farley, KS. Jones and G. Ryding (Elsevier, Amsterdam, 1993) &I”609. [7] V. Beneveniste, HE. Friedman, ME. Mack and F. Sinclair, Nucl. Instr. and Meth B 37/38 (1389) 568. [8] J. England, N. Bryan, H. Ito, D. Armour, J. Van den Berg, I. Fotheringham and P. Kinderslep, Ion Implantation Technology-92, eds. D.F. Downey, M. Farley, KS. Jones and G. Ryding (Elsevier, Amsterdam, 1993) p. 613; J. England, Ion Spectroscopy Probes of Ion Beams, DC. Extension Short Course, Wafer Charging Effects in Ion !mplant&ion Processing, Dallas, June, i993. [9] P. Sferlazzo and G. Angel, Ion Implantation Technology-92, eds. D.F. Downey, M. Farley, KS. Jones and 6. Ryding, (Elsevier, Amsterdam 1993) p. 651. [lo] W. Lukaszek, W. Dixon, E. Qttek, W. Weisenberger and S. Ho, Ion Implantation Technology-92, eds. D.F. Downey, M. parley, K.S. Jones and G. Ryding (Elsevier, Amsterdam, 14331 p. 301;

W. Lukaszck and G. Angel, ion ~nlpla~t~t~~~r~ Tcshnotogy-92: eds. D.F. Downey, M. Far&. KS. Jones and G. Ryding iElsevier, Amsterdam, 199.3) p. 645; W. Lukaszek, EEPRDM Based ~~on~tar~~g Devices. UC. Extension Short Course, Wafer Charging Effects in Ion Implantation Processing, Dallas, June, 1993. [ll] MC. V&la, 15’. Lukaszek, MI. Current and NH. Tripsas, these Proceedings (IlT’94], Nucl. fnstr. rind Metb, B 96 (1995) 48. il’] F.F. Chen, Ir.. PII%!J “i mnostic Rcifhniqxs, eds. RX. Huddlestone and S L. LAomud I&ademic Press, New “e: k. 1965). [13] S. Kikuchi, S. Satoh, T. Sakae, S. W&XZ:~ R.B. ticbe,. &YD ME. Ma& Ion Implantation Technology-!+L, eds. D.F. Downey, M. Farley, KS. Jones and G. Ryding (EIsevier, Amsterdam 1993) p. 641; W. Lukaszek and G. Angel, Ion Implantation Technology-92, eds. D.F. Downey, M. Farley, KS. Jones and G. Ryding (Elsevier, Amsterdam. 19931 p. 645. [14] E. Rudberg, Proc. R. Sot. London A 127 11939) 113; Phys. Rev. 4 (1934) 964. [1.5] ME. Mack, G.C. Angel, M.L. Pascucci and D. P&by, these Proceedings (III”941 Nucl. Instr. and Meth. B 96 t’1995.l X0. [16] S. Mehta, R. Nee, S. Walther and R. Eddy, Part IP of these Proceedings fIIT’94) (North-Hollznd, Amsterdam, 19951.

1. ADVANCED IMPLANTERS/PRGCESS

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