Memo Double-Side Polishing
Double-Side Polishing Memo
Overview of Double-Side Polishing
1.1 Mechanism
Simultaneous Polishing
In this method, a workpiece (such as a wafer or glass) is sandwiched between upper and lower polishing pads (or polishing plates) and polished on both sides simultaneously. A polishing fluid (slurry) is typically used.
Relative Motion
By independently rotating and vibrating the upper and lower polishing pads, the front and back of the workpiece are polished evenly while maintaining a constant polishing pressure.
1.2 Benefits
Improved Parallelism and Flatness on Front and Back
This method achieves higher parallelism, thickness uniformity, and flatness than polishing one side at a time.
Reduced Labor
Polishing both sides at once potentially reduces process time.
Reduced Damage Layer
Because a uniform force is applied simultaneously on both sides, it is less likely to develop localized distortions or cracks that can occur when excessive force is applied to only one side.
Challenges and Weaknesses of Double-Sided Polishing
2.1 Equipment Complexity and Cost
Capital Investment
Double-sided simultaneous polishing equipment tends to have a more complex structure and higher initial investment costs than single-sided polishing equipment. Furthermore, equipment maintenance is more sophisticated, which can increase operating costs.
Polishing Pad Management
The upper and lower pads must be kept in a stable condition at all times, and differences in pad thickness and surface roughness caused by wear and other factors must be managed. Pad surface flatness (dressing) also becomes more complex.
2.2 Optimizing Polishing Conditions
Slurry (Polishing Liquid) Selection
The appropriate composition, concentration, and supply rate of the slurry or chemical solution must be selected according to the type of polishing material (silicon, sapphire, glass, SiC, etc.). Failure to properly balance the chemical reaction (chemical polishing elements) and mechanical polishing elements can lead to uneven polishing and increased damage.
Pressure, Temperature, and Rotation Speed Control
Due to the double-sided nature of the equipment, it is difficult to maintain uniform pressure distribution during polishing. Even slight variations in pressure and temperature distribution can significantly affect the final flatness and surface roughness, especially for larger wafers (e.g., 12 inches) and thin-film materials.
2.3 Risk of Workpiece Damage
Edge Chipping/Cracking
When polishing pressure is applied simultaneously to both sides, force can be concentrated on the edge of the workpiece, making it more susceptible to edge chipping. The risk of damage is particularly high for thin wafers and brittle materials such as glass.
Accuracy of the Workpiece Holding Mechanism
If the carrier or holding mechanism that holds the workpiece is not highly accurate, uneven polishing and damage or scratches to the workpiece surface may occur.
Recent Trends and Opportunities
3.1 Handling Large-Diameter Wafers
In the semiconductor industry, as wafer sizes increase from 8 inches to 12 inches, increasing equipment size and ensuring polishing uniformity have become major challenges. Double-sided polishing, which can process both sides simultaneously, continues to attract attention as a technology that can achieve high precision while somewhat minimizing the process time and cost increases that come with larger wafers.
3.2 Demand for Thinner and Ultra-Thinner Materials
In fields such as smartphones, various mobile devices, and small optical devices, there is a growing need to process substrates and glass components thinner while maintaining their strength.
Double-side polishing allows for even application of pressure from both sides, even on thin workpieces, potentially enabling processing to a thin film level while minimizing distortion. However, the risk of breakage increases with ultra-thinning, making precise process control essential.
3.3 Handling High-Hardness Materials
Devices using hard and brittle materials such as sapphire, SiC, and GaN substrates are on the rise. These materials present challenges, such as time-consuming polishing and a tendency to develop micro-flakes.
Double-side polishing distributes heat and force to the substrate more effectively than single-side polishing, making it effective for planarizing these materials and reducing damage. The key to achieving this is the development of optimal slurries and pads, as well as improved control precision of the polishing machine.
3.4 Integration with CMP (Chemical Mechanical Polishing)
Double-sided polishing itself is a process that primarily involves lapping and mechanical polishing. However, there is also a trend toward combining it with the CMP process to achieve atomic-level planarization of the surface.
In the cutting-edge semiconductor field, the development of double-sided CMP equipment and research into hybrid equipment are progressing.
Future Challenges and Directions
Establishment of Advanced Process Control Technology
Practical application of a system that monitors multiple factors in real time, such as pressure, temperature, rotation speed, pad wear, and slurry concentration, and provides feedback control.
Automatic optimization using AI and IoT technologies is key.
Reduced risk of damage and improved safety
Edge protection mechanisms and rapid anomaly detection (workpiece damage monitoring using sensors such as vibration and acoustic emissions) for shutdown and control.
Further improvement of the precision of the workpiece holding mechanism.
Reduced environmental impact
Reduced slurry usage and establishment of recycling methods.
Improved systems for treating and reusing water and chemicals used in polishing.
Adaptability to a Wide Range of Materials
In addition to conventional silicon wafers, this technology supports the mass production of various wide-bandgap semiconductor substrates and specialty materials such as glass for photonics.
Expected advances include the development of polishing fluids and pad materials.
Development of Equipment that Combines Mass Production with High Precision
In the semiconductor and optical industries, achieving both mass production benefits and high performance and precision is crucial. Efforts are underway to integrate double-sided polishing into entire production lines and integrate it with other etching and cleaning processes.
