By Nanochip Staff

Compared with CMP for 300mm wafers, planarizing 200mm wafers with thick copper layers presented new challenges. Smaller polishing head zone sizes, much thicker bulk copper processes, and wafer edge issues added up to a demanding development process. The location of the sensor in the platen had to be repositioned, and subsequently, the smaller zones meant there was higher zone-to-zone interaction.

Power devices and other products made on 200mm wafers increasingly have thick copper layers that present new planarization challenges. To meet the need for real-time profile control (RTPC) on Applied Materials 200mm Mirra® CMP tools, Applied turned to its long-time partner, SC Solutions, Inc. (Sunnyvale, California). Together, the two companies tackled a variety of technical issues (figure 1).

Figure 1. 200mm CMP polishing heads use smaller polishing pressure zones than 300mm, resulting in larger interactions among the zones. (Source: SC Solutions)

RTPC involves three essentials: a concentric multi-zone polishing head that controls pressure across the wafer; a sensitive eddy sensor in the polishing platen; and software to control the variable pressures in the polishing head. The eddy sensor in the platen reads the bulk metal profile and sends raw data to the software controller, which uses an algorithm to convert the data into a usable thickness measurement, and adjust polishing pressure in the head accordingly, on the fly. Pressure, expressed in pounds of force per square inch of area (PSI), is the variable under control.

CMP by its nature is a process that sees continuous change, due to consumable wear, and other off-tool variations. RTPC allows the system to compensate for all of those variations and provide stable performance over the entire life of the consumable set.

It might seem that scaling RTPC from the established 300mm CMP tools to the 200mm diameter would be straightforward. But that turned out to be not quite the case, said Leighton, who has been working with Applied CMP products for 18 years, the last decade specifically on 200mm and 150mm CMP.

“With 200mm CMP, everything is smaller,” Leighton said. The wafer is smaller, the head is smaller, the polishing pad is smaller. The smaller pressure zones in the head mean there is more zone-to-zone interaction. A change in pressure in zone 1 has a greater impact on zone 2 than it would in 300mm.

the RTPC software would adjust the zone 1 pressure from 5.5 to 6 PSI to increase the removal rate on that part of the wafer,” Leighton said. In a later scan, the sensor might inform the controller of a potential overshoot, resulting in a reduction of zone pressures at the wafer edge (figure 2).

Applied made improvements to its Mirra CMP system software, allowing real-time communications between the RTPC software and the Mirra control software to support sending polishing pressure updates to the system in real time. The Applied Vita™ controller is required to support the on-the-fly communications.

DEALING WITH THICKER METAL

Figure 3. Initial Results (left) during early development already show improvement of RTPC. After process tuning and optimization, the Final Results (right) show that real-time process control (with RTPC, blue dots) delivers less than 5% head-to-head variation, a sharp improvement over open-loop control (no RTPC, orange dots). (Source: SC Solutions)

SMALLER ZONE SIZES

For 200mm customers, moving from non-RTPC to RTPC requires a different polishing pad. The sensor window—a clear polyurethane opening—had to be moved to support the new RTPC sensor position in the platen. “Applied had to work with suppliers to make that happen,” de Roover said in an interview.

Also, the 200mm contour head zone sizes are smaller. Both 300mm and 200mm use five zones, but because the 200mm zones are somewhat smaller, “in theory that provides finer resolution. In reality, the way the membrane is constructed creates a stronger reaction between the zones,” de Roover said. “Thus, if you have smaller zones, the interaction becomes relatively large between the zones.”

The biggest challenge was at the wafer edge. “We spent quite a bit of time trying to figure out what happens at the edge, working with SC Solutions’ physics-based models. The edge is where the fun stuff happens in CMP,” de Roover said.

De Roover, who led the RTPC model-based control development effort for both 300mm and 200mm CMP, said that at its heart, the RTPC algorithms are the same for both wafer diameters. “Essentially, the algorithms of RTPC are common between 200 and 300; we just use different calibration models.”

UPGRADES IN THE FIELD

Applied is currently working on a beta phase project for 200mm RTPC with a major IDM customer in Europe. “We are also seeing a strong level of interest from other customers,” Leighton said. Adding RTPC to an existing 200mm Applied Mirra Mesa system in the field can be done, though it requires “significant upgrades to make it work.”

There is a shortage of used CMP tools now, and many customers seeking to add new capacity or new technologies are buying mainly new 200mm CMP tools. “It is very difficult to find an Applied Materials used CMP tool for 200mm now,” Leighton said. “Our customers’ new technologies are driving additional demand. At Applied, we are offering new capabilities, and RTPC is one of those.”

For additional information see http://www.appliedmaterials.com/products/mirra-mesa-cmp-200mm or contact jamie_leighton@amat.com

Dick de Roover, PhD (roover@scsolutions.com) is Principal Research Engineer in the Systems & Control Division of SC Solutions, Inc., Sunnyvale, California. SC Solutions provides innovative engineering solutions to clients in the fields of structural engineering and advanced process control. Its Control Engineering business provides custom process control and modeling solutions to semiconductor manufacturing, advanced materials manufacturing, energy, aerospace and defense industries.

^[1] Model-Based Profile Control for 200mm CMP: Easier than 300mm CMP or Not?, Dick de Roover, PhD, SC Solutions, US APC Conference, October 2018.