Consumable materials essential for semiconductor production such as wafers, slurries1, and pads are disposed of after the manufacturing process. The disposal of these materials is not only costly but also impacts the environment through the emission of carbon and hazardous gases.
1Slurry: A polishing liquid applied to surfaces prone to contact. It is used to prevent scratches on the wafer’s surface and offset the instability of the process control.
In response, SK hynix has developed technology to recycle chemical mechanical polishing (CMP) pads, a consumable material used in the wafer polishing process. This article will introduce the members who created the technology: Jaewon Jo, Joowon Kang, Juncheol Lee, and Kyungwon Kim of the Cleaning and CMP (C&C) Technology department, and Heejun Kim of the Front End (FE) Procurement department. They spoke about the pad’s development process, overcoming key challenges, and their future plans.
CMP Process: The Groundwork for Producing High-Performance Chips
As semiconductors are created by stacking multiple layers of circuits and devices on a wafer, the CMP process ensures the even stacking of layers by grinding and removing any residual substances from the wafer’s surface. Without this critical step, the surface of circuits and devices would become uneven and lead to varying wiring widths and film thicknesses. This, in turn, would impede the smooth flow of electrical signals.
▲ An overview of the CMP process
Additionally, thin films deposited onto the surface of the wafer may develop bumps and curves during the manufacturing process. To remove these imperfections, the CMP process is performed to polish and flatten the wafer surface with various chemical and mechanical solutions. These include polishing pads with a rough, sandpaper-like surface that mechanically grinds the wafer’s surface. A chuck is also used to secure the wafer in place via a vacuum, while slurry is a chemical solution which polishes the wafer. Lastly, a pad conditioner minimizes frictional wear on the curved pad during the polishing process and maintains the pad’s surface by eliminating any wafer residue or slurry.
▲ Joowon Kang explains the significance of the CMP process
Technical Leader Joowon Kang emphasized the growing significance and role of the CMP process as semiconductors continue to advance in performance and miniaturization.
“As semiconductor chips become more advanced, more layers are deposited and finer circuits are created on the wafer,” said Kang. “This increases the number of polishing cycles, making it necessary to implement a more precise process.”
Recycling Pads: Preserving the Environment & Reducing Costs
As the role of the CMP process has grown, the use of consumable materials has skyrocketed and resulted in more waste. In fact, the amount of consumable waste in the semiconductor industry increased by around a quarter between 2019 and 2022.
Searching for solutions with related departments while taking into account productivity and ESG factors, Jaewon Jo, a team leader in the C&C Technology department, concluded that the most effective method to reduce this substantial waste was to focus on recycling the pads.
▲ A breakdown of the CMP materials used at SK hynix in 2022
“Pads generate a substantial amount of waste as they are the most heavily used consumable CMP material that require frequent replacements,” explained Jo. “Furthermore, the primary material of the pads is polyurethane, a polymeric compound extracted from petroleum that can take a lengthy time to degrade naturally and emits significant hazardous emissions when incinerated. In the end, it seemed that recycling the pads would minimize our environmental impact and also decrease costs by reducing the use of consumable materials.”
▲ Heejun Kim talks about the economic impact of recycling pads
According to Technical Leader Heejun Kim, SK hynix uses more than 10,000 pads every month. “We proactively explored recycling solutions as the use of pads has been on the rise in line with the semiconductor industry’s growth,” Kim said, recalling the inception of the technology’s development.
Developing a Technology for Recycling CMP Pads
▲ Jaewon Jo (center) elaborates on the required specifications for a recycled pad
There were several criteria to meet when developing the recycling pads, including high performance, equipment compatibility, environmental friendliness, and cost-effectiveness. In addition, there was a major hurdle to overcome to ensure the success of the project. It was essential for the pads to be developed using SK hynix’s very own technology.
“There was already a patent from a competitor in the market that recycled the pads,” said Jo. Recalling the early days of development, he said that his team “aimed to devise our own original solution that could be competitive in the market without infringing on existing patents.”
▲ Kyungwon Kim inspects the surface of a recycled pad sample
Accordingly, the team needed to combat an issue related to the surface pattern, which Technical Leader Kyungwon Kim identified as the pad’s most important aspect. “As the pad becomes unusable once its pattern wears out, we thought of recreating the pattern on the used pad so that it could perform at the same level as a new pad.” By recreating the pattern, the team aimed to improve the pad’s cost-effectiveness compared to competitor products.
▲ The development team discuss the specifications and possible patterns of the pads
Jo and Kyungwon Kim of the C&C Technology department collaborated with partner companies to research the specifications and methods for recreating the patterns on the recycled pads. Meanwhile, fellow team members Kang and Juncheol Lee inspected the recycled pad samples by applying them to the actual CMP process. Lastly, Heejun Kim from the FE Procurement department reviewed the economic feasibility of the recycled pads.
▲ Juncheol Lee explains the specifications of a recycled pad
In October 2023, just ten months after the start of the project, the team successfully developed recycled pads that matched the performance of new pads. “As data from the wafer test confirmed that there is no discernible difference in performance between new and recycled pads when applied to the actual mass production process, we plan to gradually introduce recycled pads to the Touch CMP2 process starting next year,” said Lee. “This process is less complex compared to other CMP processes and carries a lower risk of errors.”
2Touch CMP: A process that wipes away small particles and residue from the wafer’s surface.
Kyungwon Kim expects that using recycled pads rather than conventional disposable pads in the Touch CMP process will save about KRW 1.2 billion (around USD 1 million) in costs. “Most importantly, with pad waste projected to fall by several tons annually through this process alone, the emission of harmful gases such as nitrogen oxide during incineration is also set to decrease,” he said.
▲ The CMP pad development team proudly holding samples of the recycled pads (from left to right: Joowon Kang, Juncheol Lee, Jaewon Jo, Kyungwon Kim, and Heejun Kim)
Expanding the Project’s Horizons in 2024
Looking ahead to 2024, the team plans to build on the success of their recycled pads project. “In the coming year, we plan to explore methods to make raw and base materials out of used pads to help mitigate environmental pollution and establish a virtuous cycle of resource utilization,” Kyungwon Kim said. “We will continue solving environmental challenges with sustainable technologies.”
