Chemical Mechanical Planarization
Intervening early can prevent scratch defects and prevent costly yield excursions.
In advanced-node chemical mechanical planarization (CMP) applications, scratch defects are often caused by the agglomeration of slurry abrasive particles that have the potential to negatively affect process yield performance. Our technology enables customers to perform particle size analysis online and in real time, directly in the fluid stream process. Intervening early prevents costly yield excursions.
The AccuSizer® SPOS system is recognized as the most accurate and sensitive system for detecting large particle counts (LPC) in CMP slurries. While laser diffraction can measure the mean size of the distribution well, the SPOS technique is hundreds of times more sensitive to tails and LPC.
CMP Slurry Size and Count Analysis
Oversize or agglomerated particles (>1 µm) can wreak havoc on polished semiconductor wafers by creating microscratches.
Although the mean diameter of most CMP slurries is on the order of hundreds of nanometers, relatively small amounts of larger particles can render a wafer useless. Single particle optical sizing (SPOS) allows the user to individually size and count these particles with unprecedented accuracy before they cause damage to the wafer.
With SPOS, single particles flow through a sensor and pass through a focused beam of light. As the particles pass through the laser beam, light is blocked and a photodiode records a decrease in light intensity. This decrease in intensity is proportional to the cross-sectional area of the particles. Entegris offers a variety of AccuSizer systems that use SPOS technology. The online systems provide ease and accuracy while the bench top systems are perfect for smaller scale research projects.
The histogram shows the SPOS result of an agglomerated cerium oxide CMP slurry. This portion of the particle size distribution is often referred to as the distribution “tail”. This particular distribution contained roughly 7 million particles per mL greater than 1 µm and 22 thousand particles per mL greater than 2 µm. With an AccuSizer particle size analyzer, the user can obtain high resolution information about a sample that cannot be resolved with conventional dynamic light scattering or light diffraction techniques.
A recent study spiked 1 µm PSL particles into a silica CMP slurry. The slurry was analyzed on the AccuSizer FX Mini system both with and without the 1 µm PSL particles. The estimated spike concentration was ~ 5000 particles/mL. The results are shown below.
CMP Slurry Filter Testing
In an effort to reduce the number of oversize particles in CMP slurries, filter testing has been performed.
Using an AccuSizer APS system, particles were analyzed in a ceria CMP slurry before and after it was passed through a Planargard® NMB01 and NMB03 filters. Entegris applications labs use both the AccuSizer APS and the AccuSizer FX-Nano to test retention profiles for our CMP filters. The AccuSizer FX-Nano system uses focused extinction wherein the view volume through which particles pass through the sensor is drastically reduced. This reduction provides much higher sensitivity and particle coincidence limit. Therefore, the minimum particle size limit is lowered and the coincidence limit is increased. This system is ideal for an online application where predilution is not feasible.
The ceria filtration results shown in the Figure indicate a small, but discernable LPC difference up to 2 µm. After 2 µm, essentially all the LPC counts have been removed. The ability to size and count aggregates in CMP slurries gives the user invaluable knowledge that can save time and money. With the AccuSizer FX-Nano system, conventional SPOS technology is taken to a higher level and provides the sensitivity and reliability needed for online processes.
CMP Filter Monitoring
Optimizing filter changeouts saves money and prevents unnecessary downtime.
It is critical to monitor and control the health of the CMP slurry being used in the polishing process. The AccuSizer Mini particle size and concentration analyzer has been successfully used to optimize filtration operations at several points in the distribution system. The AccuSizer Mini system can be placed close to the process both downstream of the supply line filter and the point of use filters.
In one case, an AccuSizer Mini system was placed downstream of the supply line filter to determine when it was necessary to replace the filter. The results shown at right helped the customer determine that the optimal filter replacement time. This saved the customer money by replacing filters only when required while lowering the LPC in the slurry delivered to the polishing tools. The AccuSizer Mini analyzer can also be installed downstream of the POU filters for additional assurance of low LPC.
Ceria CMP Slurry Monitoring
The AccuSizer system provides excellent reproducibility and statistical confidence.
Cerium oxide (ceria) based slurries are becoming more widely used in a variety of CMP applications for integrated circuit (IC) manufacture. A short study proved how the AccuSizer FX Mini system can accurately measure the size and concentration of ceria CMP slurries and determine the presence of tails to the distribution.
Ceria A was then spiked with 1% (Figure 1) and 10% (Figure 2) of Ceria B that contains a longer tail of larger particles. The results in Figures 1 and 2 show both the sensitivity to tails and the excellent reproducibility due to the statistical confidence from measuring so many particles.
Figure 1: Ceria A and ceria A spiked with 1% ceria B
Figure 2: Ceria A and ceria A spiked with 10% ceria B