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This article addresses the effect of copperand cobalt use in back-end processing tracecontamination on the front end. This arti-cle will discuss the results of a collaborationbetween Bell Laboratories, the Silicon

Research Department of Bell Labs atMurray Hill, and KLA-Te n c o r.

A unique window of opportunity was avail-able just prior to the upgrade of the researchfab line at Murray Hill. We were able tohave the last lot of wafers that went throughthe line be intentionally contaminated. Thisenabled us to decide how we could measuretrace contamination, whether it would

make any difference, and what it would doto the electrical properties of either the sil-icon or the SiO2 gate oxide. The contami-nation was introduced two different ways.First, it was implanted using one MeV

implant of either copper orcobalt on the back of thewafer after the oxide hadbeen grown on the wafers.We used both floatzone andepitaxial silicon. We thenannealed the wafers at either600°C or 1000°C. After pro-cessing, we used a KLA-Te n c o rQuantox to measure deeplevel transient spectroscopy(DLTS), total internal reflec-tion X-Ray flu o r e s c e n c e(TXRF), and charg e - t o -breakdown (QB D) on poly-

dots, for both cobalt and copper. In thecase of cobalt, we did a second method,using the dip method. After we grew anoxide on our silicon that was either 40Å,100Å, or 1000Å of SiO2, we dipped thewafers into a standard cobalt solution. Wethen followed that with a 900°C anneal for30 minutes, and did similar techniques forcharacterization. What we found was that

Autumn 1999 Yield Management Solutions

Non-Contact Copper and Cobalt Detection For 0.18 µm Technologyby Janet Benton, Bell Labs, Lucent Technologies

This article is based on a transcription of a paper presented at the KLA-Tencor YMS seminar at SEMICON/WEST 1999.

F i g u re 1. Quantox

fundamental silicon

m e a s u re m e n t s .

F i g u re 2. Cobalt ki lls bulk

l i f e t i m e .

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the Quantox measurements proved to bethe most valuable. Therefore, most of thedata presented in this article will beQuantox measurements.

A Quantox measurement is a corona-oxide-silicon measurement. We deposit thecharge, measuring the amount of chargedeposited on top of the oxide. We thenmeasure two parameters: the surface volt-age and the surface photovoltage. Thereare multiple variations that allow us tolook at the properties of the SiO2, as well asthose of the silicon. We found in our par-ticular case that the recombination lifetimewas the most valuable measurement.

As shown in figure 1, after we deposit ourcorona, we eliminate the sample with axenon light pulse, which increases the sur-face photovoltage. We then turn the xenonlamp off, and watch the decay of the pho-tovoltage. The decay has two regimes: thehigh injection bulk recombination lifetimeregime at the top part of the decay curve,and the medium injection regime at thelower part of the decay.

Figure 2 shows the high injection bulkrecombination lifetimes for all the cobalt-contaminated wafers. We took five sitemeasurements on each sample. The sam-ples on the left side are from the dippedexperiment. Of the first three columns, onewas dipped. First, we grew a 40Å oxide,dipped it, then annealed at 900°C. Thetwo controls are a 40Å oxide by itself anda 40Å oxide that also received the 900°Canneal. The 100Å and 1000Å dippedwafers are also shown in the left side of thefigure. Those results clearly show that ifyou put cobalt on the surface and anneal at900°C it goes through even the thickestoxide and kills the lifetime of the siliconunderneath.

The last seven wafers in figure 2 were fromthe backside implant experiment. The firstof those was just the plain control waferout of the box. Then two different doses ofcobalt, 1x1011 cm-2, 1x1012 cm-2, at twodifferent anneal temperatures (600°C and1000°C). For the last two wafers, the waferwent into the implantation machine butwas not implanted — this was just to testwhether the machine itself introduced anycontamination; those samples were then

taken out and annealed at either 600°C or1000°C.

Our research showed that no matter howthe cobalt is introduced, no matter at whattemperature it is annealed, the silicon life-time is dead. There are two important con-clusions to be made. First, Quantox is avaluable tool for identifying small amountsof cobalt present in the material. This isnot trivial because we have very few waysof knowing whether or not we have metalcontamination introduced during the frontend. The second conclusion is, rather sur-prisingly, that the cobalt is such a lifetimekiller. In addition to the site measure-ments, we also did lifetime maps with the

Quantox tool, as shown in figure 3. Thescale on the left is in microseconds, so weare measuring the high injection bulkrecombination lifetime over the entirew a f e r. The control received an oxidegrowth of 100Å SiO2, and then wasannealed at 600°C for 30 minutes in N2.The lifetimes are relatively high, exceptaround the edges. The other two are for thetwo doses of cobalt that were implanted on

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F i g u re 3. Cobal t re d u c e s

lifetime throughout bulk of

FZ wafers.

1 . 5 e + 0 0 3

1 . 2 9 e + 0 0 3

1 . 0 7 e + 0 0 3

8 5 7

6 4 3

4 2 9

2 1 4

0

1E11Co cm - 2, 600°C, 30 min

1E12Co cm - 2, 600°C, 30 min

High injection bulk re c o m-bination lifetime (µsec)

F i g u re 4. DLTS shows

extended defects re l a t e d

to cobalt contamination.

600°C, 30 min contro l

the concentration of iron in the sample.The bottom curve is our control and thesample does, indeed, have iron in it. Twopeaks are shown, one marked Fei, which isiron in the interstitial position. The other,marked FeiBs, is iron interstitial sittingnext to a boron atom. These are wellknown. In DLTS we can see metal contam-ination in the silicon lattice if it is in inter-stitial or a substitutional position.

Figure 4 also shows the DLTS spectra fromtwo of the cobalt samples. The sampleswere both introduced at a dose of 1x1011

cm-2. One was annealed at 1000°C, andthe other was annealed at 600°C. Whatcan be seen here is that it is not cobalt in asoluble or a substitutional position. Alsoshown is a very broad DLTS peak.Normally, broad DLTS peaks indicate anextended defect. If you have metals thatdiffuse fast and your process has a slowcool, you would expect the metals not toremain in solution, but rather to precipi-tate during the cool.

We will now detail the diffusivity of cobaltand copper. Figure 5 shows the diffusivityof most of the transition metals in silicon.The chart shows that cobalt and copper,along with nickel, are the fastest diffusersin silicon. In this case we have introduced a

the backside of the wafer. During theanneals at 600°C for 30 minutes, thecobalt has diffused from the back of the

wafer through to the front and completelykilled the carrier lifetime in the siliconsubstrate.

The question then was: in what form is thecobalt in the lattice itself? Would it be ina soluble form, or in another form? Wouldit collect at the top SiO2/Si interface?What could we find out about it? DLTShelped a little bit here, as shown in figure4. DLTS is a measurement that allows us tolook at defects in the silicon lattice whichhave specific states in the energy gap of thesilicon. So if, for instance, iron is in thesample and it is in its interstitial position,there will be a peak in the DLTS spectra.The height of this peak will be related to

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F i g u re 5. High Di ff u s i v i t y

of cobalt results in pre c i p i-

tation during the cool.

F i g u re 6. Cobalt re d u c e s

lifetime throughout bulk of

FZ wafers.

C o n t ro l

1 . 5 e + 0 0 3

1 . 2 9 e + 0 0 3

1 . 0 7 e + 0 0 3

8 5 7

6 4 3

4 2 9

2 1 4

0

1E11Cu cm - 2

1E12Cu cm - 2

High injection bulk recombination lifetime (µsec). 600°C, 30 min

1E11Co cm - 2

1E12Co cm - 2

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metal which is a very fast diffuser and has arelatively slow cool, which is the case withmost standard furnace anneals. So we doexpect the cobalt and the copper to precip-itate during the cool. That is not particu-larly surprising. What is surprising is thatprecipitated metals would have such a dra-matic effect on the lifetime of the silicon.

Figure 6 shows the lifetime maps for ourcopper results. The control is in the mid-dle, the cobalt experiments are on the left,and on the right are results from the exper-iments where copper was implanted on thebackside of the wafers. Two things areapparant. First, in the case where we com-pare copper to cobalt, it is obvious that thecobalt has a much more dramatic effect onthe lifetime than copper. But the copperdoes seem to be scaling with the amountthat we put in, since the lifetime has beenreduced for the one on the bottom right,where we implanted 1x1012 cm-2, morethan it has been reduced at the top.

The correlation is shown even more dra-matically in the set of data in figure 7.Here our experimental points are plottedagainst other experiments that KLA-Te n c o rhas performed with another fab line. Weare plotting recombination lifetime versusthe amount of copper intentionally intro-duced into our substrates. In our case weimplanted it, as shown in the data pointsfor Fab 1. In the case of Fab 2, they haddipped their sample in copper, and thendetermined the amount of copper byvapor-phased absorption, inductively cou-pled plasma spectroscopy (VPD-ICPMS).They essentially etched off the top 0.7 µmof the silicon, then checked to see howmuch copper was in the vapor that wasetched off. As shown, the more copper, thelower the lifetime. It will take more copperthan cobalt to kill the lifetime — while1x1012 cm-2 is not a lot of copper, it willreduce the lifetime.

One more result on the cobalt was particu-larly interesting. Although figure 2 showsthe cobalt diffuses readily through theSiO2, it does not seem to migrate from onewafer to another during the furnace anneal.Figure 8 shows three wafers that were putright next to each other in the furnace. Theone on the left was intentionally contami-

nated using the dip experiment. The nextone was the control that went through thefurnace right next to the dipped one. In

fact, the second wafer was turned around,so that the two polished sides were facingeach other in the furnace. The last one hadno dip and no anneal. Even though thecobalt wafer and the control wafer wereright next to each other in the furnace, wesaw no effect of cobalt migrating from thecontaminated wafer to the next one.

Figure 9 shows how the trace contamina-tion affects the oxide itself. The oxide,deposited polysilicon dots, was grown,then conventional CV/IV measurementswere performed. QBD data, or charge-to-breakdown data was taken, implanted and

annealed, and charge-to-breakdown datawas taken again. The controls are the firstthree on the left. The copper samples atboth doses, 1011 and 1012 cm-2, and bothtemperatures, are the next four. The cobalt

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F i g u re 7. Concentration of

copper cor rela tes with

reduction in bulk lifetime.

F i g u re 8. No evidence

of cobalt migration during

f u rnace annealing.

Co dip, 900°C 30 min

1 . 2 e + 0 0 3

1 . 0 3 e + 0 0 3

8 5 7

6 8 6

5 1 4

3 4 3

1 7 1

0

900°C, 30 min, facing Co wafer

C o n t rol: no dip,no anneal

High injection bulk re c o m-bination lifetime (µsec)

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samples are next, and then just two con-trols that went through the anneals. Weare comparing QB D before implantinganneal and after processing. It is clear thatthe copper has absolutely no effect oncharge-to-breakdown. There appears to bescatter in the data, but it is unlikely that itcan be associated with cobalt itself, becausein some cases the cobalt seems to increasethe QBD and in some cases it decreases it.

Another measurement taken using theQuantox tool is the Etunnel measurement,where a large charge is put on the sampleand the surface voltage is put into satura-tion, and then the Etunnel is measured. Thisdata is shown in figure 10 for all of thecobalt samples that had 100Å of oxide.This measurement changes with oxidethickness, so only 100Å oxides are com-pared. For the cobalt samples, the tunnel-ing voltage did not change, although we

know from the lifetime measurements thatthe cobalt diffused through the oxide. Itappears that the cobalt on the surface dif-fuses through that oxide and into the sili-con, killing the carrier lifetime, but sur-prisingly doesn’t seem to affect the qualityof the oxide.

Figure 11 shows the tunneling voltagesfor the copper samples. We have flo a t z o n eand epi. The copper was only introducedby implant on the back. Although thecopper diffuses all the way through thesample, it does not seem to diffuse out ofthe silicon and into the oxide, causing nochange in Et u n n e l.

The important thing about these results isthat they are very specific to the processconditions. Figure 12 shows the Quantoxmeasurement Qt o t, which is the totalcharge on the oxide. This was measured asa function of copper in our samples. In ourexperiment, it doesn’t seem to matter howmuch copper was in the silicon–we stillhad the same quality of oxide. Whereas, inthe case of Fab 2, the more copper put onthe surface, the bigger the change in theirQtot. Therefore, although in some cases thecopper will not cause a problem, in othercases it will. Further, it appears that it willbe a long time before we know exactlywhen copper is going to be a problem tothe front end of our system.

Conclusions for CobaltIt was clear in our experiment that it onlytakes a very small amount of cobalt to killthe lifetime. In the case of the cobalt dipexperiment, we estimate that the dose was1x109 cm-2, which is not very much cobalt.

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F i g u re 9. Ion implanted

copper or cobalt does not

change charge to bre a k-

down of oxides.

F i g u re 10. No effec t of

cobalt on oxide tunneling

v o l t a g e .

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In fact, it is below the TXRF detectionlimit. All of our samples were sent out forTXRF, both before and after our measure-ments, and in no case were we able todetect cobalt using TXRF. Therefore, youcannot count on TXRF to determinewhether there should be concern aboutcobalt contamination. In fact, even loweramounts of cobalt in your system willcause dramatic decreases in the lifetime ofyour sample.

We also saw that cobalt diffused rightthrough the oxide, whether it was 40Å,100Å, or 1000Å. It did not migrate fromone wafer to another during a heat treat-ment. The cobalt was shown not to affectthe oxide characteristics. It did not affecteither the charge-to-breakdown or theEtunnel. Our experiment showed that theQuantox turned out to be a very powerful

tool for detecting the presence of cobalt,and that the TXRF is not really adequate.Concern about cobalt contamination dic-tates the need for a tool that is more sensi-tive.

Conclusions for CopperThe copper experiment showed that traceamounts of copper also kill the carrier life-time in silicon. Further, copper introducedinto the silicon bulk after oxidation doesnot affect the oxide characteristics, at leastnot QBD or Etunnel. Lastly, we concluded thatprocessing conditions change the effect ofcopper on the device oxide.

In general, metal contamination duringsilicon device fabrication results in widevariations in electrical properties of the sil-icon and of the SiO2, and depends on theprocess flow. Therefore, careful monitoringof multiple electrical parameters isabsolutely imperative. ❈

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F i g u re 11. No ef fect of

copper on oxide tunneling

v o l t a g e .

F i g u re 12. Processing conditions change the eff e c t

of copper on total oxide charg e .

c i r cle RS#035