Using Single Particle Optical Sizing to Quantify CMP Slurry Health

Chemical Mechanical Polishing (CMP) is a key process in high-density integrated circuits production, where CMPs are utilized during various stages for normally short periods of time. Hence, CMPs need to be handled extensively and are subjected to many different environments. It is essential to prepare new batches often, from concentrate kept in storage. All of these conditions affect the stability of the slurries, eventually influencing the yield levels of chips.

A highly sensitive measurement technique capable of detecting trace amounts of out-of-specification particles as a result of aggregation from applied stress or contamination from handling samples.

This technique must have the ability to perform quality checks on fresh material and in situ monitoring of the slurry. This article demonstrates the ability of the Single Particle Optical Sizing (SPOS) technique to perform quantitative analysis on CMP slurry health.

Advantages of SPOS Technique

The ability to count single particles makes the SPOS technique to determine the presence of aggregates. The technique exhibits unprecedented sensitivity by counting and sizing particles individually. It is easily operable and can be used online or in the QC lab. Unlike ensemble techniques that have the limitation of lower sensitivity, the SPOS technology can provide quantitative information about large particle tails.

Acoustic Spectroscopy is a relatively new technique applied for acquiring particle size information using sound waves. However, it is also an ensemble technique like Laser Diffraction and is able to make measurements only at high concentrations. According to a study involving the analysis of several CMP slurries by Acoustic Spectroscopy, the detection limit of this technique was 1% relative solids content. However, SPOS measurements have revealed that CMP slurries with even 0.005% of large particle solid fractions can cause unacceptable decreases in yields.

Analysis of CMP Slurries with SPOS Technique

Chemical Effects

Here, the SPOS technique was used to test CMP slurries exposed to many different mechanical and chemical stresses. The results will demonstrate the sensitivity of the SPOS technique in detecting the differences in large particle counts brought on by handling. Figure 1 shows the results acquired for an alumina slurry concentrate diluted in H₂SO₄ and HNO₃, separately.

Figure 1. Alumina CMP diluted in Sulfuric Acid (red, circle) and Nitric Acid (blue, squares); a. Number-Weighted PSDs; b. Volume-Weighted PSDs.

In order to have identical acidity for each measurement, the pH of each diluent was maintained at 3. Nevertheless, the results were quite different. The tail observed for the slurry diluted in H₂SO₄ was much broader than the slurry diluted in HNO₃. This variation is more obvious in the Volume-Weighted particle size distributions (PSDs).

The variation may be due to the improper preparation of the mixture as slurries are prepared from concentrate prior to use, typically involving the mixing of a chemical component with an abrasive component. The improper mixing causes out-of-spec ionic strength, which might cause the slurry to become less stable. Testing for ionic strength is a much more challenging task than the testing for large particles. However, the SPOS technique easily determines the proper mixing of a new batch of slurry.

Stress Effects

Figure 2 depicts several PSDs acquired for silica slurry to test the effect of the pump on the slurry while pumping the material through a recirculation system. Any noticeable change was not observed in the PSDs of the slurry during the first 16 hours. However, after 24 hours of pumping, the particle count increased from 10,000 to 100,000 and the volume percentage contributed by the tail increased from 0.002% to 0.016%. This demonstrates the stress effects of pumping that destabilize the slurry content. These results demonstrate the superior sensitivity of the SPOS technique as any commonly used ensemble technique is not capable to detect this minute change in volume percentage.

Figure 2. a. PSDs of a Silica slurry after being pumped through a recirculation system for 40 hours; b. Graph of Percent Volume and cumulative Particle Counts while in recirculation system.

Environmental Effects

Figure 3 shows the PSDs acquired for cerium oxide slurry exposed to three different temperatures. Although Number-Weighted distributions at each temperature are nearly identical, differences can be observed. Unlike the warmed and ambient samples, the slurry sample cooled to 30°F was observed with aggregates. This is more obvious in the Volume-Weighted PSDs, showing the presence of larger particles of 20µm in the slurry sample cooled to 30°F. Additional two samples did not have particles bigger than 5µm. This is key information owing to the fact that this very low amount of aggregates caused significant wafer scratching, which is a major problem to the user. Ensemble techniques are not capable of delivering this information.

Figure 3. a. Population distributions of cerium oxide slurry at 30, 70, and 100°F. b. Volume-Weighted PSDs of cerium oxide slurry at 30, 70, and 100°F.

Conclusion

The results clearly demonstrate the ability of the SPOS technique to provide quantitative information about the extent of aggregation and contamination of CMPs slurries that are exposed to many different environments.

This information has been sourced, reviewed and adapted from materials provided by Entegris

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