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Detecting Large Particles Counts in CMP Slurries

In this interview, Mark Bumiller, Technology Manager for Entegris talks to AZoM about using the Accusizer to detect large particles counts in CMP slurries.

Please could you give us an introduction to the company, who you are and what you do?

Entegris started as a materials company and has grown to become a multi-billion-dollar supplier of pure solutions, mostly to the semiconductor industry. Entegris acquired Particle Sizing Systems a year ago, mostly interested in our Mini for monitoring large particles in the semiconductor industry, specifically within the CMP area.

However, Entegris Sizing Systems is only a small part of a very big company that provides global solutions to the semiconductor industry.

Oversized or agglomerate particles can cause havoc on polished semiconductor wafers by creating micro-scratches. Although they are relatively small please can you explain the consequences of LCP in CMP slurries?

As the CMP process progresses the surface must be planarized, as it has to be very flat for the next step. As it is flattened, anything that could cause a scratch on the surface has to be avoided.

For instance, if there was a piece of debris on the wafer as you planarize it, it could cause a broad scratch across the surface. Even a single large particle of it could go through the CMP planarization process and cause a small scratch.

The dimensions are so small that even small scratches will cause a defect in that area on the wafer, and that could be multiple chips. As the various processes occur, these wafers become more and more valuable. Any small defect along the way has a deleterious effect on the final yield of the overall wafer, which is critical to the prompt ability of the factory.

The Accusizer is recognized as the most accurate and sensitive system for detecting large particles counts (LPC) in CMP slurries. Why is it the preferred product?

As they are delivered, the CMP slurries have a defined particle size distribution. This distribution can be very small, but the mean size can be found by using a technique such as dynamic light scatter or laser diffraction.

The concern with yield is associated with these few large particles. To catch a few large particles is tricky because it is a statistical game. In order to define a distribution, a large number of particles has to be measured, and the number of particles needed to be measured depends on the breadth of the distribution.

If an outlying population needs to be caught, it becomes much trickier. Many particles need to be looked at in order to make sure that any outlier particles will be caught.

That is why the AccuSizer is the tool most frequently used to look for these outlying particles. With the AccuSizer, only the large particle counts are going to be measured and not the mean size. In order to find these large particle counts, this very concentrated slurry needs to be taken and put through a sensor, and statistically enough particles measured in order to catch the large ones.

In order to do that, this very concentrated slurry needs to be diluted. We have various dilution fluidics, which allow us to find the right concentration without changing the nature of the slurry. The slurry is carefully diluted before it is brought into the sensor to make sure that those large particles are still there.

Entegris has a range of three different sensors depending on the kind of slurry that is going to be measured. For example, the LE sensor could be used for silica slurries, the FX sensor for ceria slurries, or the FX-Nano sensor for clean slurries with very few large particles.

A range of sensors has been developed that we use depending on the kind of CMP slurry that is going to be measured. It is a combination of the dilution fluidics, choosing the right sensor, and knowing how to package this into a system that can be brought online so that the same great statistical results can be delivered online as there have always been in the lab.

What sets this product apart from others on the market?

This product is set apart due to its sensitivity to the outlying populations. Since there are a great number of particles, we look at the statistics to assure that we find outlying particles that other techniques might not catch.

If you only look at 100 particles, you are not going to find the outliers. However, if you look at 200,000 particles you are going to find them. Our product is the tool that is going to give the statistical accuracy to make sure that outliers that could cause trouble are found.

How can you reduce the number of oversize particles in CMP slurries?

In order to keep yields up, we have to make sure these large particles do not get out into the fab area, and we want to make sure they do not get out of the CMP delivery area. The way to do that is usually through filtration. Entegris is also the leading supplier of filtration for the CMP delivery systems.

We have a series of filter solutions, both for the CMP delivery system and for point of use. The first step of protection is the CMP filters, and then the final protection step is the use of the AccuSizer Mini, which can alert you if you ever fall outside of the range that you should be in.

The Accusizer allows you to perform particle analysis in real-time and online. Which applications will really see the benefit of this?

A very expensive fab has been created. Its job is to manufacture as many wafers as it can in order to obtain as many integrated circuits as possible. One of the keys to profitability is the yield, and as the steps progress, any defects need to be minimized in order to keep yields up.

Filtration and the AccuSizer Mini allows yields to be kept high by keeping the large particle count low, and maximization of yields is the most important factor.

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