How to Select the Right Polishing Pad Material for Different Sapphire CMP Stages
Core Characteristics of Sapphire Material Sapphire
As a synthetic alumina single-crystal material, possesses two significant characteristics:
First, its extremely high hardness, reaching level 9 on the Mohs hardness scale, second only to diamond (level 10), giving it excellent scratch resistance and structural stability;
Second, its high brittleness and sensitivity to mechanical stress, making it prone to cracking under external forces, posing a significant challenge to the processing. Simultaneously, sapphire also possesses excellent light transmittance (especially in the ultraviolet to infrared band), high temperature resistance, and chemical stability, making it widely used in high-end manufacturing fields.
The Necessity of Sapphire CMP
In applications such as LED wafer substrates, optical components (e.g., high-precision lenses), mobile phone cover glass, and semiconductor epitaxial substrates, sapphire substrates have stringent requirements for surface quality.
Taking LED wafers as an example, if there are tiny defects on the surface of the sapphire substrate, it will directly affect the growth quality of the subsequent epitaxial layer, leading to a decrease in LED luminous efficiency; while mobile phone cover glass requires an extremely smooth surface to ensure touch sensitivity and visual effects.
Chemical mechanical polishing (CMP) is currently the only technology that can achieve ultra-precision surface processing of sapphire substrates. It removes processing marks on the substrate surface through the synergistic effect of chemical etching and mechanical grinding, achieving high-precision flatness.
Unique Challenges of Sapphire CMP:
Low Scratch and Near-Zero Subsurface Damage
The high hardness of sapphire makes it extremely easy to generate surface scratches during processing, while its brittleness leads to subsurface damage (SSD)—microcracks, dislocations, and plastic deformations existing below the surface. These defects may remain hidden inside the material even after surface polishing, seriously affecting product performance: for optical components, it will reduce light transmittance and imaging quality; for LED substrates, it will increase the defect density of the epitaxial layer and reduce device lifespan; for semiconductor applications, it may lead to device leakage or failure.
Therefore, the core objective of sapphire CMP is to achieve an extremely low scratch rate (typically requiring near-zero scratches per square centimeter) and near-zero subsurface damage.
Polishing Pads: More Than Just Consumables, They Are Key Polishing Tools
In the sapphire CMP process, polishing pads are not simply “consumables,” but rather core components directly involved in the polishing process. Their performance directly determines three key indicators:
Material Removal Rate (MRR):
Affects processing efficiency and determines the amount of substrate that can be processed per unit time;
Surface Roughness (Ra):
Determines the smoothness of the final surface; high-end applications typically require Ra < 1nm;
Defect Density:
Includes surface scratches and subsurface damage, directly affecting product yield. As sapphire substrates become thinner (e.g., LED substrate thickness decreases from hundreds of micrometers to tens of micrometers) and higher surface quality standards are achieved, the choice of polishing pad material has an increasingly significant impact on the processing effect, becoming a key factor determining the success or failure of the CMP process.
Formation Mechanism of Scratches and Subsurface Damage (SSD) in Sapphire CMP
Causes of Scratches on Sapphire Surfaces
As a typical hard and brittle material, the mechanisms by which sapphire develops surface scratches during CMP mainly fall into three categories:
* Scratches caused by abrasive grain retention:
* If abrasive grains (such as CeO₂ and Al₂O₃) in the polishing slurry are retained by the asperities on the surface of the polishing pad, under polishing pressure, these grains will slide on the sapphire surface like “sandpaper,” forming linear scratches.
* Irregular mechanical action of the polishing pad surface:
*If there are protrusions, burrs, or locally hardened areas on the surface of the polishing pad, these irregular structures will apply uneven mechanical forces to the sapphire surface during polishing, directly causing surface scratches.
*Excessive local contact pressure:
When the hardness of the polishing pad is too high or the surface texture is uneven, the local contact pressure at the polishing interface will exceed the critical compressive strength of the sapphire, causing brittle fracture of the material and forming scratches.
Characteristics and Hazards of Subsurface Damage (SSD)
Subsurface damage refers to microscopic defects that exist below the surface of sapphire and are invisible to the naked eye. These mainly include microcracks (transverse and longitudinal cracks), crystal dislocations, and plastic deformation layers. These defects have the following characteristics:
Concealment:
SSDs are located a few micrometers to tens of micrometers below the surface, making them difficult to detect using conventional surface inspection methods (such as optical microscopy).
Accumulation:
If SSDs generated during pre-processing (such as grinding) are not completely removed by subsequent CMP, they will accumulate within the material, affecting the performance of the final product.
Hazards:
SSDs significantly reduce the mechanical strength of sapphire, making the substrate prone to cracking during subsequent thinning or edge polishing. Simultaneously, they scatter light, reducing the transmittance of optical components and may become channels for impurity diffusion, affecting the stability of semiconductor devices.
The Decisive Influence of Polishing Pad Material on Scratches and SSDs
The physicochemical properties of the polishing pad material directly determine the probability of scratch and SSD formation, specifically in four aspects:
Contact stress distribution:
The hardness and elastic modulus of the polishing pad determine the uniformity of polishing pressure distribution at the interface—high-hardness polishing pads are prone to generating localized high pressure, increasing the risk of scratches and SSDs;polishing pads with good elasticity can distribute pressure evenly, reducing defects;
Abrasive grain flowability:
The surface texture and pore structure of the polishing pad affect the flow state of abrasive grains at the polishing interface— If abrasive grains are easily trapped, the risk of scratches is high; if abrasive grains can flow smoothly and be carried away in time, the defect rate is reduced.
Frictional heat generation and dissipation:
During polishing, the friction between the polishing pad and the sapphire surface generates heat. Excessive temperature increases the brittleness of sapphire, making it prone to microcracks (SSDs). The thermal conductivity and porosity of the polishing pad determine the heat dissipation efficiency; a polishing pad with good heat dissipation can reduce thermally induced defects.
Microtexture stability:
If the surface texture of the polishing pad is easily worn or deformed during use, it will lead to unstable polishing, increasing the probability of scratches and SSDs.
Controlling SSDs Becomes a Core Objective in High-End Sapphire Manufacturing
With the continuous improvement of the performance requirements of sapphire substrates in the LED, semiconductor, and high-end optics markets, controlling subsurface damage has become a core competitive advantage for manufacturers. For example, in sapphire substrates used for Mini/Micro LEDs, the presence of SSDs can lead to a decrease in the adhesion between the epitaxial layer and the substrate, making it prone to cracking during subsequent chip dicing and significantly reducing product yield. Therefore, how to control SSDs by optimizing polishing pad materials has become a key direction in the research and development of sapphire CMP processes.
Mainstream Polishing Pad Materials and Performance Characteristics for Sapphire CMP
Polyurethane (PU) Polishing Pads: The Most Widely Used Choice
Polyurethane polishing pads are currently the most commonly used material in sapphire CMP. Their core advantages are reflected in the following three points:
* Excellent elasticity and microtexture consistency:
PU material has moderate elasticity (Shore hardness is typically between 60-80D), which can adapt to the slight undulations on the sapphire surface, ensuring uniform distribution of polishing pressure. Simultaneously, through precision molding processes, PU pads can form uniform microtextures (such as grooves and pores), ensuring the stability of the polishing action.
* Excellent slurry retention:
PU material has moderate hydrophilicity, and the micropores on its surface can effectively store polishing slurry, avoiding “slurry starvation” (i.e., localized lack of slurry leading to dry polishing) at the polishing interface, reducing scratches.
* Balanced MRR and low scratch rate:
Under normal polishing parameters (pressure 0.1-0.3MPa, polishing disc speed 30-60rpm), PU… The pad can achieve a high material removal rate (typically 100-300 nm/h) while maintaining a low scratch rate, meeting the processing requirements of most sapphire substrates.
Non-woven Polishing Pads:
Focused on Low-Defect Finishing Non-woven polishing pads are made of interwoven fibers (such as polyester fibers) bound together with adhesives (such as resins). Their soft structure and performance characteristics are as follows:
Soft Polishing Interface:
The fibrous structure of non-woven fabrics has good flexibility, resulting in lower contact pressure with the sapphire surface during polishing (typically suitable for low-pressure polishing at 0.05-0.15 MPa), minimizing surface damage from mechanical abrasion;
Excellent Scratch Inhibition:
The soft fibrous structure does not easily trap abrasive particles and buffers the impact of abrasive particles on the surface. Therefore, when used in the final finishing stage, it can significantly reduce the scratch rate, even achieving a “scratch-free” surface;
Lower Material Removal Rate:
Due to the weaker abrasive force, the MRR of non-woven pads is typically only 1/3-1/2 of that of PU pads (approximately 50-150 nm/h), making them more suitable as a final polishing step rather than a rough polishing stage.
Composite Polishing Pads: Balancing Efficiency and Stability
Composite polishing pads typically use PU as the matrix, optimizing performance by adding resin, ceramic particles, or pre-loaded abrasives. They mainly fall into two categories:
PU + Resin Composite Pads:
Epoxy or phenolic resin is added to the PU matrix, improving the pad’s hardness and wear resistance, extending its service life (20%-50% longer than ordinary PU pads), while maintaining good slurry retention. Suitable for efficient stock removal in the rough polishing stage of sapphire.
PU + Pre-loaded Abrasive Composite Pads:
Ultrafine abrasives (such as nano-sized Al₂O₃) are pre-embedded in the PU material, reducing dependence on abrasives in the polishing slurry. Suitable for scenarios sensitive to slurry costs or requiring stable polishing results. However, a drawback is that uneven abrasive distribution may increase the risk of localized scratches.
Micro-porous Polishing Pads: Optimized Slurry Distribution and Heat Dissipation
Micro-porous polishing pads, through a special foaming process, form tiny pores with a diameter of 1-10μm within a PU or nonwoven fabric matrix. Their core advantages are:
Improved slurry distribution uniformity:
The microporous structure acts like a “sponge,” storing and transporting slurry, ensuring that every point on the polishing interface receives sufficient slurry, avoiding localized scratches or uneven polishing caused by uneven slurry distribution;
Enhanced heat dissipation efficiency:
The air and slurry in the micropores quickly remove the frictional heat generated during polishing, reducing the polishing interface temperature (typically 5-10℃ lower than ordinary PU pads), reducing heat-induced SSDs and scratches;
Reduced defect density:
Uniform slurry flow and lower temperatures significantly reduce abrasive agglomeration and brittle fracture of the material, resulting in a final surface defect density reduction of over 30%.
Performance Comparison of Mainstream Polishing Pad Materials
The differences in core performance indicators of different polishing pad materials are shown in the table below (based on commonly used parameters for sapphire CMP):
| Performance Indicators | PU Polishing Pad | Non-woven Fabric Pad | Composite Pad (PU + Resin) | Microporous PU Pad |
| Shore Hardness | 60-80 | 30-50 | 75-90 | 55-75 |
| Porosity (%) | 10-20 | 40-60 | 8-15 | 25-40 |
| Material Removal Rate (nm/h) | 100-300 | 50-150 | 200-400 | 120-250 |
| Surface Roughness(Ra) | 0.5-1.5nm | 0.3-1.0nm | 1.0-2.0nm | 0.4-1.2nm |
| Scratch Rate (pieces) /cm²） | 1-5 | 0-2 | 3-8 | 0.5-3 |
| Lifespan (h) | 20-40 | 15-30 | 30-60 | 25-45 |
As can be seen from the comparison, the hardness of the polishing pad directly affects the MRR (the higher the hardness, the larger the MRR is usually), while the porosity is related to heat dissipation and slurry distribution (the higher the porosity, the better the heat dissipation and slurry distribution); the surface texture and material uniformity determine the scratch rate and the risk of SSD formation.
How Polishing Pad Material Affects Scratch Control in Sapphire CMP
The Balance Between Hardness and Scratch Probability
The hardness of the polishing pad is a key factor affecting scratch formation, but its effect is not simply “the higher the hardness, the more scratches,” but rather there is a balance range:
Risks of High-Hardness Polishing Pads:
When the polishing pad hardness is too high (e.g., Shore hardness > 85D), the local contact pressure at the polishing interface will significantly increase, exceeding the critical shear strength of sapphire, leading to deeper scratches (i.e., “macro scratches,” typically > 1μm wide) formed by abrasive grains on the surface. Simultaneously, high-hardness pads are not easily deformed, and abrasive grains are easily trapped between the pad and the substrate, forming “drag scratches.”
Hazards of Low-Hardness Polishing Pads:
If the polishing pad hardness is too low (Shore hardness < 50D), although the contact pressure is dispersed and the risk of macro scratches is reduced, the soft pad surface easily traps abrasive grains (especially larger abrasive grain aggregates). These abrasive grains will repeatedly rub against the substrate surface under the elastic deformation of the pad surface, forming dense scratches. “Microscopic scratches” (width < 0.5μm);
Optimal hardness range:
For sapphire CMP, the Shore hardness of the polishing pad is usually selected between 60-75D (such as ordinary PU pads or microporous PU pads), which can ensure a certain MRR while keeping the scratch rate at a low level.
Stability of Polishing Pad Microtexture and Scratch Control
The microtexture of the polishing pad (such as the size and distribution of surface grooves and protrusions) directly determines the movement trajectory of abrasive grains and the distribution of polishing pressure, making its stability crucial:
Harmful Effects of Sharp Protrusions:
If sharp protrusions exist on the surface of the polishing pad (such as defects in the molding process or localized hardening during use), these protrusions will drag the abrasive grains like “sharp knives,” forming linear scratches on the sapphire surface;
Impact of Texture Wear:
As the polishing process progresses, the surface texture of the polishing pad will gradually wear down. If the wear is uneven (such as faster wear at the edges than at the center), it will lead to an imbalance in the polishing pressure distribution, resulting in “edge scratches” (i.e., the scratch density at the edge of the substrate is significantly higher than at the center);
Advantages of Stable Texture:
Uniform microtextures formed through precision machining (such as annular grooves with a spacing of 1-2 mm and uniform protrusions with a diameter of 5-10 μm) can guide the smooth flow of abrasive grains, preventing abrasive grain retention, while ensuring uniform distribution of polishing pressure, significantly reducing the risk of scratches.
For example, one study showed that PU pads with uniform annular groove textures have a scratch rate that is more than 60% lower than PU pads without grooves.
The Influence of Polishing Pad and Slurry Interaction on Scratches
The compatibility of the polishing pad material and the polishing slurry (e.g., hydrophilicity, slurry retention capacity) affects the dispersion of abrasive particles, thus influencing scratch formation:
Maintaining Slurry Film Thickness:
High-quality polishing pad materials (e.g., microporous PU pads) can form a stable slurry film (typically 5-10 μm thick) on the surface. This film separates the abrasive particles from the sapphire surface, preventing direct rigid contact between the abrasive particles and the substrate, reducing scratches. If the slurry film thickness is insufficient (e.g., with ordinary resin composite pads), the abrasive particles easily agglomerate and directly grind the surface, increasing scratches.
Maintaining Abrasive Particle Suspension:
PU materials have moderate hydrophilicity, allowing abrasive particles (e.g., CeO₂) to maintain good suspension in the slurry, preventing sedimentation and agglomeration. Non-woven fabric pads, due to the adsorption effect of their fiber structure, are prone to abrasive particle agglomeration (especially at higher slurry concentrations), increasing the risk of microscopic scratches.
Slurry Renewal Efficiency:
The pore structure of the polishing pad determines the slurry renewal efficiency. The pores of the microporous pad can quickly drain the old slurry after use and draw in the new slurry, reducing the repeated friction of abrasive particles at the interface; while the dense composite pad is prone to retaining old slurry, causing the abrasive particles to continue to act on the surface after wear, forming scratches.
Correlation between Frictional Heat Generation and Scratching
The frictional heat generated during polishing alters the mechanical properties of sapphire, indirectly increasing the risk of scratches:
Increased thermally induced brittleness:
When the polishing interface temperature exceeds 80℃, the brittleness of sapphire increases significantly. At this point, even small mechanical forces can cause brittle fracture, leading to deeper scratches and subsurface microcracks.
Heat dissipation capacity of polishing pads:
Polishing pads with high porosity (such as microporous pads and nonwoven pads) can dissipate heat through air and slurry convection within the pores, keeping the interface temperature below 50℃. Dense composite pads, however, have poor heat dissipation capacity, and the interface temperature easily exceeds 100℃, significantly increasing the risk of scratches and SSDs.
Practical Application Cases: Scratch Control Effects of Different Polishing Pad Materials
Case 1: Final Polishing of LED Sapphire Substrates:
A manufacturer used non-woven fabric pads (Shore hardness 45D) for final polishing. Compared to ordinary PU pads, the surface scratch rate decreased from 3 scratches/cm² to below 0.5 scratches/cm², Ra decreased from 1.2nm to 0.6nm, and the yield of the LED epitaxial layer improved by 15%.
Case 2: Rough Polishing of Sapphire Cover Plates for Mobile Phones:
Rough polishing was performed using composite PU pads (PU + resin, Shore hardness 80D), achieving an MRR of 350nm/h. Simultaneously, by optimizing the slurry concentration (CeO₂ concentration 10%), the scratch rate was controlled to within 5 scratches/cm², meeting the requirements of subsequent finishing processes.
Case 3: Polishing of High-Precision Optical Sapphire Lenses:
Microporous PU pads (porosity 30%) were used, combined with low-concentration… Al₂O₃ slurry (concentration 5%) achieved an ultra-smooth surface with Ra < 0.5 nm and a scratch rate close to zero, fully meeting the light transmission requirements of optical lenses.
The Influence of Polishing Pad Material on Subsurface Damage (SSD) Formation
Polishing Pad Compliance and Subsurface Stress Distribution
The compliance (i.e., elastic deformation capability) of the polishing pad directly determines the depth and magnitude of subsurface stress distribution, and is a core factor affecting SSD:
SSD Risk of High-Hardness Pads:
Hard pads (such as composite PU pads) have poor compliance, and the polishing pressure is mainly concentrated in the region 1-5μm below the sapphire surface, forming high shear stress and compressive stress. This easily leads to longitudinal microcracks (depths up to 5-10μm) and crystal dislocations in this region, and these defects constitute the main SSD;
SSD Suppression Effect of Compliant Pads:
Soft polishing pads (such as nonwoven fabric pads, microporous PU pads) can disperse the polishing pressure to a wider area below the surface (usually…) through elastic deformation. The surface hardness (SSD) of sapphire is reduced by 5-10 μm, lowering the local stress peak and ensuring that the subsurface stress is below the critical fracture strength of sapphire, thereby reducing the generation of microcracks. Simultaneously, the compliant pad better conforms to the sapphire surface, preventing the thickening of the plastic deformation layer caused by local stress concentration.
Relationship between Abrasive Embedding and SSDs
The retention of abrasive particles on the polishing pad surface (i.e., “abrasive embedding”) causes the abrasive particles to continuously act on the sapphire surface during polishing, increasing the risk to the SSD:
Abrasive embedding risk of hard pads:
The dense surface texture of hard pads makes it easy for abrasive particles to get stuck between the pad’s protrusions, forming “embedded abrasive particles.” Under polishing pressure, these particles can penetrate deeper below the sapphire surface, causing localized plastic deformation and microcracks, exacerbating the SSD’s degradation.
Abrasive release capability of soft pads:
The loose surface structure of nonwoven fabric pads or microporous PU pads makes it difficult for abrasive particles to embed. Furthermore, the elastic deformation of the pad can release the abrasive particles into the slurry in a timely manner, reducing continuous contact between the abrasive particles and the substrate. For example, the abrasive embedding rate (the proportion of embedded abrasive particles to total abrasive particles) of nonwoven fabric pads is only 1/5 that of hard pads, corresponding to a 60% reduction in SSD depth.
Auxiliary role of slurry filtration:
Although the polishing pad material is the main influencing factor for abrasive embedding, the use of 1μm… Slurry filters with smaller pore sizes can further reduce the embedding of large abrasive particles, reducing SSD density by another 20%-30%. 5.4 Polishing Pad Lifespan and SSD Evolution As polishing time increases, the polishing pad gradually wears down, and its performance changes lead to the evolution of the SSD:
Initial Stage (0-10h):
The polishing pad surface texture is intact, the paste retention capacity is strong, and the SSD depth and density are at their lowest levels.
Mid-Stage (10-30h):
The surface texture gradually wears down, the hardness decreases slightly, the MRR decreases, but the SSD performance remains largely unchanged. Late Stage (After 30h): The surface texture is severely worn, localized pitting and hardening occur, the polishing pressure distribution is uneven, frictional heat increases, and the SSD depth and density increase significantly. For example, after 40h of use, the SSD depth of a certain PU pad increased from the initial 2μm to 6μm, and the microcrack density increased by 3 times.
Importance of Pad Dressing:
By periodically dressing the polishing pad (e.g., using a diamond dresser to remove the surface wear layer), the surface texture and hardness of the pad can be restored, extending its effective lifespan. Experiments show that every 10h… A PU pad that has been trimmed once can maintain an SSD depth of less than 3μm throughout its entire lifespan.
The Impact of SSDs on the Performance of Final Sapphire Products
Although SSDs are concealed, they have a decisive impact on the performance of sapphire products:
Decreased Mechanical Strength:
Microcracks in SSDs become stress concentration points, significantly reducing the bending strength of the sapphire substrate. For example, a sapphire substrate with a 5μm deep SSD has 40% lower bending strength than a substrate without an SSD, making it prone to breakage during subsequent thinning processes.
Impaired Optical Performance:
Subsurface microcracks and plastic deformation layers scatter light, reducing the light transmittance of sapphire. For LED substrates, SSDs cause light emitted from the epitaxial layer to scatter within the substrate, reducing the LED’s light extraction efficiency by 10%-20%.
Reduced Product Yield:
SSDs may further expand during subsequent processing (such as cutting and coating), leading to product scrap. Statistics show that SSD-related scrap rates account for over 35% of total sapphire product scrap rates, making it the primary factor affecting yield.
Polishing Pad Material Selection Strategy for Different Sapphire CMP Stages
Rough Polishing Stage: Prioritize High MRR while Controlling Basic Defects
The core objective of rough polishing is to quickly remove the surface allowance (typically 5-10μm) left by previous processing (such as grinding). Therefore, the selection of polishing pads should prioritize high MRR while controlling the scratch rate within an acceptable range (typically <10 scratches/cm²).
Recommended Materials: Composite PU pad (PU + resin, Shore hardness 75-90D) or high-hardness PU pad (Shore hardness 80-85D);
Reasons for Selection: Composite pads have high hardness and good wear resistance, with an MRR of 200-400 nm/h, enabling rapid removal of excess material; simultaneously, their dense surface texture reduces abrasive embedding, preventing excessive deep scratches;
Matching Parameters: Polishing pressure 0.2-0.4 MPa, polishing disc speed 40-60 rpm, high-concentration CeO₂ slurry (concentration 15%-20%), particle size 500-800 nm; Notes: Extremely low Ra is not necessary in the rough polishing stage (usually controlled at 1.5-2.5 nm), but the SSD depth must be strictly controlled (recommended <5μm) to lay the foundation for subsequent polishing.
Intermediate Polishing Stage:Balancing MRR and Defect Control
The goal of intermediate polishing is to remove scratches and some SSDs left by rough polishing, while further reducing the surface allowance (typically 2-5 μm), preparing for final polishing. This stage requires finding a balance between MRR and defect control.
Recommended Materials: Medium-hardness PU pads (Shore hardness 65-75D) or microporous PU pads (porosity 25%-35%);
Reasons for Selection: Medium-hardness PU pads have a moderate MRR (120-250nm/h), ensuring processing efficiency while reducing local pressure and new scratches through moderate elasticity; the porous structure of microporous PU pads improves slurry distribution, lowers interface temperature, and effectively controls SSD depth (down to 2-3μm);
Matching Parameters:Polishing pressure 0.15-0.3MPa, polishing disc speed 35-50rpm, medium-concentration CeO₂ slurry (concentration 10%-15%), particle size 300-500nm; Quality Objectives: Surface roughness Ra controlled at 0.8-1.5nm, scratch rate <5 scratches/cm², SSD depth <3μm.
Final Polishing (Fine Polishing) Stage:Pursuing the Ultimate Low Defect and Ultra-Smooth Surface
The core goal of final polishing is to completely remove scratches and SSDs left by previous processes, achieving an ultra-smooth surface (Ra < 1nm) and near-zero defects to meet the needs of high-end applications. Recommended Materials: Nonwoven fabric pads (Shore hardness 30-50D) or soft microporous PU pads (Shore hardness 55-65D);
Reasons for Selection:The soft structure of nonwoven fabric pads minimizes mechanical abrasion, achieving scratch-free polishing through a gentle chemical-mechanical synergy; soft microporous PU pads combine softness and slurry retention, achieving a certain MRR (80-150nm/h) while ensuring low defects, avoiding excessive processing time;
Supporting Parameters: Polishing pressure 0.05-0.15MPa (low-pressure polishing), polishing disc speed 25-40rpm, slurry selection: low-concentration, fine-particle slurry (e.g., CeO₂ concentration 5%-8%, particle size 100-300nm, or Al₂O₃ nano slurry);
Quality Objectives: Surface roughness Ra < 1nm (some high-end applications require Ra < 0.5nm), scratch rate < 1 /cm², SSD depth <1μm, even near-zero SSD.
Key Factors Influencing Polishing Pad Selection
Besides the CMP stage, the following factors should also be considered during selection:
Sapphire substrate thickness and strength requirements:
For thin substrates (thickness <100μm), polishing pads with good flexibility (such as non-woven fabric pads) should be selected to avoid substrate bending or cracking due to excessive pressure; for products with high strength requirements (such as aerospace optical components), microporous pads or non-woven fabric pads that can control SSD size should be prioritized;
Final optical performance standards:
If the product has extremely high light transmittance requirements (such as ultraviolet optical lenses), non-woven fabric pads that can achieve Ra <0.5nm should be selected, along with fine-grained pastes; for ordinary LED substrates, medium-hardness PU pads are sufficient;
Paste type compatibility:
Different polishing pad materials have different compatibility with pastes—PU pads are suitable for CeO₂ pastes (with strong chemical action), while non-woven fabric pads are suitable for… Al₂O₃ slurry (with weaker mechanical action), while composite pads require high-concentration slurries to achieve high MRR (Mean Reduction). If diamond slurry is used (for polishing superhard materials), a composite pad with good wear resistance must be selected.
Equipment parameter matching is also crucial: when the polishing disc speed is high (>50 rpm), a microporous pad with good heat dissipation is needed to avoid excessively high interface temperatures; when the equipment pressure adjustment range is narrow, compensation must be made through the hardness of the polishing pad (e.g., a high-hardness pad should be selected to ensure MRR when the pressure is low).
Selection Summary:
Polishing Pad Material Determines the Final Quality of Sapphire CMP
In the sapphire CMP process, the polishing pad material has a greater impact on surface quality than any other process parameter (such as pressure, rotation speed, or slurry concentration). The correct selection strategy should follow the principle of “stage adaptation and performance matching”:
Rough Polishing:
Select high-hardness, high-wear-resistance composite pads for efficient removal;
Intermediate Polishing:
Select high-hardness, high-porosity PU pads or microporous pads to balance efficiency and defects;
Final Polishing:
Select low-hardness, high-flexibility nonwoven pads or soft microporous pads to achieve low defects and an ultra-smooth surface.
Scientific selection can significantly reduce rework rates (typically by 20%-30%), improve product yield (by 15%-25%), and ensure that sapphire substrates meet the stringent standards of high-end optics, LED, and semiconductor applications. With the continuous development of sapphire processing technology, polishing pad materials will evolve towards “high flexibility, high stability, and long lifespan,” further driving the upgrade of sapphire CMP processes.
