Applied and SC Solutions Partner on Real-Time Profile Control for 200mm CMP
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.
Jamie Leighton, CMP product line manager at Applied Materials, said that until now Applied 200mm CMP tools have lacked the RTPC capabilities that have been an integral feature of 300mm metal CMP. The copper (Cu) interconnects for leading-edge devices made on 300mm wafers have long required RTPC functionality to control within-wafer non-uniformity (NU%). However, CMP for applications made on 200mm wafers “is a bit of a different story.” There, Cu CMP has always been able to make do with wafer-to-wafer endpoint controls like iScan and full-scan ISRM.
That picture changed when some automotive ICs, mixed-signal products made on bipolar, CMOS, DMOS (BCD) processes, and particularly power devices, moved to thicker copper layers with up to 8 microns of copper overburden.
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.
|Figure 2. Adapting real-time profile control to 200mm CMP involved repositioning the window through which the sensor detects the copper thickness. (Source: Applied Materials)|
The position of the RTPC eddy sensor also needed adjustment. “We learned very early on that the position of the sensor on the platen was not ideal. When the polishing head would sweep, we were seeing dead zones, and if the head swept out as far as it could go to the edge of the platen, there would be no sensor coverage at all on the wafer. We had to move the sensor out from four inches to five inches from the center of the platen to eliminate those dead zones.
“RTPC is a continuous, closed-loop process,” Leighton explained, citing an example of a wafer with a 6-micron Cu overburden. Every second or so, the eddy sensor in the platen passes under the wafer and takes a reading on the Cu thickness profile. The thickness data is sent to the software controller, which determines whether the rate of polishing on one area of the wafer is a bit too slow or too fast and makes a polishing pressure adjustment on the fly to the current polish recipe for the wafer.
“In the case of zone 1, at the very edge of the wafer, the sensor might see that the rate of polishing is too slow, so 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
|Dick de Roover,|
SC Solutions, which specializes in model-based control, tuning and optimization, has been working with Applied on rapid thermal processing (RTP) modeling and control since 1997, and on CMP RTPC control models since 2002. Dick de Roover, principal research engineer at SC Solutions,developed the original RTPC software used on Applied’s 300mm CMP tools, and was also closely involved with the 200mm RTPC project. He presented at the 2018 Advanced Process Control (APC) conference, outlining the challenges encountered on the 200mm CMP project. De Roover said for logic devices with multiple Cu interconnect layers made on 300mm wafers, the CMP steps start with bulk copper with thicknesses ranging from 1,000 angstroms (0.1 microns) to 2 microns, and the CMP step can be measured in seconds. However, for power devices made on 200mm wafers, the starting Cu thickness of up to 6 microns would require polishing for five minutes or more. Without RTPC, “with such a long polish, and the variability in the process conditions wafer-to-wafer, the chances that the process will not produce a perfectly uniform profile are high. Pad conditions readily affect uniformity, especially at the wafer’s edge,” he said (figure 3).
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 firstname.lastname@example.org
Dick de Roover, PhD (email@example.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.
 Model-Based Profile Control for 200mm CMP: Easier than 300mm CMP or Not?, Dick de Roover, PhD, SC Solutions, US APC Conference, October 2018.