Magazine autumn99 non contact

Non-Contact Copper and Cobalt Detection For 0.18 µm Technology

by 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.

This article addresses the effect of copper and cobalt use in back-end processing trace contamination on the front end. This article will discuss the results of a collaboration between Bell Laboratories, the Silicon

F i g u re 1. Qua ntox funda mental silicon m e a s u re m e n t s .

Research Department of Bell Labs at Murray Hill, and KLA-Tencor.

A unique window of opportunity was available just prior to the upgrade of the research fab line at Murray Hill. We were able to have the last lot of wafers that went through the line be intentionally contaminated. This enabled us to decide how we could measure trace contamination, whether it would F i g u re 2. Cobalt ki lls bulk lifetime.

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make any difference, and what it would do to the electrical properties of either the silicon or the SiO2 gate oxide. The contamination was introduced two different ways.

First, it was implanted using one MeV implant of either copper or cobalt on the back of the wafer after the oxide had been grown on the wafers.

We used both floatzone and epitaxial silicon. We then annealed the wafers at either 600°C or 1000°C. After processing, we used a KLA-Tencor Quantox to measure deep level transient spectroscopy (DLTS), total internal reflection X-Ray fluorescence (TXRF), and charge-tobreakdown (QBD) on polydots, for both cobalt and copper. In the case of cobalt, we did a second method, using the dip method. After we grew an oxide on our silicon that was either 40Å, 100Å, or 1000Å of SiO2, we dipped the wafers into a standard cobalt solution. We then followed that with a 900°C anneal for 30 minutes, and did similar techniques for characterization. What we found was that

the Quantox measurements proved to be the most valuable. Therefore, most of the data presented in this article will be Quantox measurements.

A Quantox measurement is a corona-oxidesilicon measurement. We deposit the charge, measuring the amount of charge deposited on top of the oxide. We then measure two parameters: the surface voltage and the surface photovoltage. There are multiple variations that allow us to look at the properties of the SiO2, as well as those of the silicon. We found in our particular case that the recombination lifetime was the most valuable measurement.

As shown in figure 1, after we deposit our corona, we eliminate the sample with a xenon light pulse, which increases the surface photovoltage. We then turn the xenon lamp off, and watch the decay of the photovoltage. The decay has two regimes: the high injection bulk recombination lifetime regime at the top part of the decay curve, and the medium injection regime at the lower part of the decay.

Figure 2 shows the high injection bulk recombination lifetimes for all the cobaltcontaminated wafers. We took five site measurements on each sample. The samples on the left side are from the dipped experiment. Of the first three columns, one was dipped. First, we grew a 40Å oxide, dipped it, then annealed at 900°C. The two controls are a 40Å oxide by itself and a 40Å oxide that also received the 900°C anneal. The 100Å and 1000Å dipped wafers are also shown in the left side of the figure. Those results clearly show that if you put cobalt on the surface and anneal at 900°C it goes through even the thickest oxide and kills the lifetime of the silicon underneath.

taken out and annealed at either 600°C or 1000°C. Our research showed that no matter how the cobalt is introduced, no matter at what temperature it is annealed, the silicon lifetime is dead. There are two important conclusions to be made. First, Quantox is a valuable tool for identifying small amounts of cobalt present in the material. This is not trivial because we have very few ways of knowing whether or not we have metal contamination introduced during the front end. The second conclusion is, rather surprisingly, that the cobalt is such a lifetime killer. In addition to the site measurements, we also did lifetime maps with the

F i g u re 3. Cob al t re d u c e s lifetime throughout bulk of FZ wafers.

High injection bulk re c o mbination lifetime (µsec) 1.5e+003 1.29e+003 1E11Co cm - 2, 600°C, 30 min

1.07e+003 857 643 429 214

600°C , 30 min contro l

0

Quantox tool, as shown in figure 3. The scale on the left is in microseconds, so we are measuring the high injection bulk recombination lifetime over the entire wafer. The control received an oxide growth of 100Å SiO2, and then was annealed at 600°C for 30 minutes in N2.

The lifetimes are relatively high, except around the edges. The other two are for the two doses of cobalt that were implanted on

1E12C o cm - 2, 600°C, 30 min

Figure

The last seven wafers in figure 2 were from the backside implant experiment. The first of those was just the plain control wafer out of the box. Then two different doses of cobalt, 1x1011 cm-2, 1x1012 cm-2, at two different anneal temperatures (600°C and 1000°C). For the last two wafers, the wafer went into the implantation machine but was not implanted — this was just to test whether the machine itself introduced any contamination; those samples were then

4.

DLTS

sh ows

extended defects re l a t e d to cob alt contamina tion.

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the backside of the wafer. During the anneals at 600°C for 30 minutes, the cobalt has diffused from the back of the F i g u re 5. Hi gh Di ff u s i v i t y of cobalt results in pre c i p itation during the c ool.

wafer through to the front and completely killed the carrier lifetime in the silicon substrate.

The question then was: in what form is the cobalt in the lattice itself? Would it be in a soluble form, or in another form? Would it collect at the top SiO2/Si interface?

What could we find out about it? DLTS helped a little bit here, as shown in figure 4. DLTS is a measurement that allows us to look at defects in the silicon lattice which have specific states in the energy gap of the silicon. So if, for instance, iron is in the sample 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 F i g u re 6. Cobalt re d u c e s

the concentration of iron in the sample.

The bottom curve is our control and the sample does, indeed, have iron in it. Two peaks are shown, one marked Fei, which is iron in the interstitial position. The other, marked FeiBs, is iron interstitial sitting next to a boron atom. These are well known. In DLTS we can see metal contamination in the silicon lattice if it is in interstitial or a substitutional position.

Figure 4 also shows the DLTS spectra from two of the cobalt samples. The samples were both introduced at a dose of 1x1011 cm-2. One was annealed at 1000°C, and the other was annealed at 600°C. What can be seen here is that it is not cobalt in a soluble or a substitutional position. Also shown is a very broad DLTS peak.

Normally, broad DLTS peaks indicate an extended defect. If you have metals that diffuse fast and your process has a slow cool, you would expect the metals not to remain in solution, but rather to precipitate during the cool.

We will now detail the diffusivity of cobalt and copper. Figure 5 shows the diffusivity of most of the transition metals in silicon.

The chart shows that cobalt and copper, along with nickel, are the fastest diffusers in silicon. In this case we have introduced a

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

lifetime throughout bul k of FZ wafers.

1E11 Co cm - 2

C o n t ro l

1E11Cu cm - 2

1.5e+003 1.29e+003 1.07e+003 857 643 429 1E12Co cm - 2

214 0

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1E12Cu cm - 2

metal which is a very fast diffuser and has a relatively slow cool, which is the case with most standard furnace anneals. So we do expect the cobalt and the copper to precipitate during the cool. That is not particularly surprising. What is surprising is that precipitated metals would have such a dramatic effect on the lifetime of the silicon.

Figure 6 shows the lifetime maps for our copper results. The control is in the middle, the cobalt experiments are on the left, and on the right are results from the experiments where copper was implanted on the backside of the wafers. Two things are apparant. First, in the case where we compare copper to cobalt, it is obvious that the cobalt has a much more dramatic effect on the lifetime than copper. But the copper does seem to be scaling with the amount that we put in, since the lifetime has been reduced for the one on the bottom right, where we implanted 1x1012 cm-2, more than it has been reduced at the top.

The correlation is shown even more dramatically in the set of data in figure 7.

Here our experimental points are plotted against other experiments that KLA-Tencor has performed with another fab line. We are plotting recombination lifetime versus the amount of copper intentionally introduced into our substrates. In our case we implanted it, as shown in the data points for Fab 1. In the case of Fab 2, they had dipped their sample in copper, and then determined the amount of copper by vapor-phased absorption, inductively coupled plasma spectroscopy (VPD-ICPMS).

They essentially etched off the top 0.7 µm of the silicon, then checked to see how much copper was in the vapor that was etched off. As shown, the more copper, the lower the lifetime. It will take more copper than cobalt to kill the lifetime — while 1x1012 cm-2 is not a lot of copper, it will reduce the lifetime.

One more result on the cobalt was particularly interesting. Although figure 2 shows the cobalt diffuses readily through the SiO2, it does not seem to migrate from one wafer to another during the furnace anneal.

Figure 8 shows three wafers that were put right next to each other in the furnace. The one on the left was intentionally contami-

nated using the dip experiment. The next one was the control that went through the furnace right next to the dipped one. In F i g u re 7. Concentration of cop p er

c or r el a te s

with

reduction in bulk lifetime.

fact, the second wafer was turned around, so that the two polished sides were facing each other in the furnace. The last one had no dip and no anneal. Even though the cobalt wafer and the control wafer were right next to each other in the furnace, we saw no effect of cobalt migrating from the contaminated wafer to the next one.

Figure 9 shows how the trace contamination affects the oxide itself. The oxide, deposited polysilicon dots, was grown, then conventional CV/IV measurements were performed. QBD data, or charge-tobreakdown data was taken, implanted and

F i g u r e 8. N o e vi de nc e of cobalt migration duri ng f u rna ce a nnealin g.

High injection bulk re c o mbination lifetime (µsec) 1.2e+003 1.03e+003 900°C , 30 min, facing C o wafer

857 686 514 343 171

Co d ip, 900°C 30 min

0

annealed, and charge-to-breakdown data was taken again. The controls are the first three on the left. The copper samples at both doses, 1011 and 10 12 cm-2, and both temperatures, are the next four. The cobalt Autumn 1999

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know from the lifetime measurements that the cobalt diffused through the oxide. It appears that the cobalt on the surface diffuses through that oxide and into the silicon, killing the carrier lifetime, but surprisingly doesn’t seem to affect the quality of the oxide.

F i g u re 9. Ion im pl anted copper or cobalt does not change c har ge to bre a kdo wn of oxid es.

Figure 11 shows the tunneling voltages for the copper samples. We have floatzone and epi. The copper was only introduced by implant on the back. Although the copper diffuses all the way through the sample, it does not seem to diffuse out of the silicon and into the oxide, causing no change in Etunnel .

samples are next, and then just two controls that went through the anneals. We are comparing QBD before implanting anneal and after processing. It is clear that the copper has absolutely no effect on charge-to-breakdown. There appears to be scatter in the data, but it is unlikely that it can be associated with cobalt itself, because in some cases the cobalt seems to increase the QBD and in some cases it decreases it.

Another measurement taken using the Quantox tool is the Etunnel measurement, where a large charge is put on the sample and the surface voltage is put into saturation, and then the Etunnel is measured. This data is shown in figure 10 for all of the cobalt samples that had 100Å of oxide.

This measurement changes with oxide thickness, so only 100Å oxides are compared. For the cobalt samples, the tunneling voltage did not change, although we F i g u re 10. No effec t of cob alt on oxide tunn elin g voltage.

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The important thing about these results is that they are very specific to the process conditions. Figure 12 shows the Quantox measurement Qtot, which is the total charge on the oxide. This was measured as a function of copper in our samples. In our experiment, it doesn’t seem to matter how much copper was in the silicon–we still had the same quality of oxide. Whereas, in the case of Fab 2, the more copper put on the surface, the bigger the change in their Qtot. Therefore, although in some cases the copper will not cause a problem, in other cases it will. Further, it appears that it will be a long time before we know exactly when copper is going to be a problem to the front end of our system.

Conclusions for Cobalt It was clear in our experiment that it only takes a very small amount of cobalt to kill the lifetime. In the case of the cobalt dip experiment, we estimate that the dose was 1x109 cm-2, which is not very much cobalt.

F i g u re 11. N o ef fect of copp er on oxide tun neling voltage.

In fact, it is below the TXRF detection limit. All of our samples were sent out for TXRF, both before and after our measurements, and in no case were we able to detect cobalt using TXRF. Therefore, you cannot count on TXRF to determine whether there should be concern about cobalt contamination. In fact, even lower amounts of cobalt in your system will cause dramatic decreases in the lifetime of your sample.

tool for detecting the presence of cobalt, and that the TXRF is not really adequate.

Concern about cobalt contamination dictates the need for a tool that is more sensitive.

Conclusions for Copper The copper experiment showed that trace amounts of copper also kill the carrier lifetime in silicon. Further, copper introduced into the silicon bulk after oxidation does not affect the oxide characteristics, at least not QBD or Etunnel. Lastly, we concluded that processing conditions change the effect of copper on the device oxide.

In general, metal contamination during silicon device fabrication results in wide variations in electrical properties of the silicon and of the SiO2, and depends on the process flow. Therefore, careful monitoring of multiple electrical parameters is absolutely imperative. ❈ cir cle RS#035

F i g u r e 12. Pr ocessing conditions change t he eff e c t of copp er on total oxide cha rg e .

We also saw that cobalt diffused right through the oxide, whether it was 40Å, 100Å, or 1000Å. It did not migrate from one wafer to another during a heat treatment. The cobalt was shown not to affect the oxide characteristics. It did not affect either the charge-to-breakdown or the Etunnel. Our experiment showed that the Quantox turned out to be a very powerful Autumn 1999

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