What is chemo-mechanical polishing？

In the rapid development of the semiconductor industry, Chemical Mechanical Polishing (CMP) technology is the core process for achieving an extreme level of flatness on chip surfaces. The polishing pad, as a key consumable in CMP equipment, is extremely important. This article will explore the mechanism of action, design considerations, and the impact of CMP polishing pads on the entire process.

1. Definition and Structure of Polishing Pads

Polishing pads, as the direct contact interface in CMP systems, cover the polishing plate and serve as the direct medium for material removal from the wafer surface. They are intricately designed and typically consist of a multi-layer structure, with the outer layer being the polishing layer that directly contacts the wafer. The inner layers may include reinforcement materials, heat-dissipating layers, or adhesive layers to ensure the overall stability and durability of performance. The choice of materials for polishing pads is critical, as they must balance wear resistance, flexibility, thermal conductivity, and chemical stability to cope with the complex and variable conditions during the CMP process.

2. Working Principle of Chemical Mechanical Polishing

In the chemical mechanical polishing process, the interaction between the polishing pad and the wafer forms the basis for material removal. As the polishing pad rotates at a certain speed, the wafer is fixed to the carrier head and pressed against the rotating pad with a specific pressure. During this process, the polishing pad not only serves as a medium for physical abrasion but also achieves precise polishing of the wafer surface through its synergistic action with the chemical slurry.

3. Multifaceted Roles of Polishing Pads

Uniform Pressure Application

The flexible design of polishing pads is key to their ability to apply pressure uniformly. Under pressure, the polishing pad can deform evenly, ensuring that each area of the wafer surface experiences a consistent abrasive force. This uniformity is crucial for avoiding localized over-polishing or under-polishing, which is fundamental to achieving global flatness. Localized over-polishing can create depressions on the wafer surface, negatively impacting subsequent processes, while under-polishing can result in protrusions, both of which are detrimental to device fabrication and performance. Therefore, the elasticity and deformation characteristics of the polishing pad are important indicators for assessing its performance.

Efficient Heat Dissipation

In the chemical mechanical polishing process, the heat generated from mechanical friction and chemical reactions cannot be overlooked. If heat accumulates and causes localized temperatures to rise excessively, it can affect the physical and chemical properties of the wafer material and may accelerate the aging and wear of the polishing pad, reducing process stability and product yield. Polishing pads made from materials with good thermal conductivity (such as polyurethane) can quickly transfer heat to the polishing plate and cooling system, effectively preventing localized overheating. Additionally, the design of the polishing pad will consider optimizing airflow channels to enhance natural convection and thermal radiation effects, further improving heat dissipation efficiency.

Uniform Distribution and Delivery of Chemical Slurry

Chemical slurry, as a key chemical medium in the chemical mechanical polishing process, has its uniform distribution and effective delivery directly affecting the polishing results and efficiency. The complex pattern designs on the surface of the polishing pad (such as spiral patterns, grid patterns, and concentric circle patterns) are not only aesthetically pleasing but also essential for achieving this goal. The microchannels and grooves formed by these patterns can guide the chemical slurry to continuously and evenly flow toward the contact area between the wafer and the polishing pad, ensuring that the chemical reactions and physical abrasion occur simultaneously. Additionally, these patterns can effectively carry away and remove debris and waste generated during the polishing process, preventing them from reattaching to the wafer surface and affecting the polishing quality.

As semiconductor technology continues to evolve, the requirements for polishing pad materials are becoming increasingly stringent. The introduction of new materials such as polymer composites and nanocomposites provides polishing pads with higher wear resistance, enhanced thermal conductivity, and improved chemical stability. Furthermore, surface modification techniques, such as plasma treatment and laser engraving, can further enhance the microstructure of the polishing pads, improving their performance.