Chemical Mechanical Polishing (CMP) is the only key technology that can achieve wafer surface planarization, capable of reducing surface roughness to the sub‑nanometer level. The basic principle of CMP is that the slurry chemically reacts with the wafer surface to form a softened layer, which is then removed by mechanical friction,material removal and planarization of the wafer surface.
The surface structural characteristics (roughness, groove pattern, etc.) and material properties (hardness, elastic modulus, etc.) of the polishing pad are important factors affecting CMP. The polishing pad directly influences the polishing results of the wafer.
01 Surface Structural Characteristics of the Polishing Pad
● Surface Micro‑topography of the Polishing Pad
Polishing pads are usually full of micropores on the surface, which can store and transport slurry and abrasive particles, resist chemical attack from the slurry, and promptly remove polishing by‑products, thereby affecting material removal.
Based on structural features, mainstream commercial polishing pads are classified into three types: mesh type (damping cloth), fiber type (synthetic leather), and microporous type (polyurethane). Compared with the other two, the mesh pad has better compressibility and slurry‑carrying capability, and its lower hardness makes it less likely to cause scratches during fine polishing. The fiber and polyurethane types, due to their specific hardness and structure, are mostly used for rough polishing and polishing of non‑metallic materials.
The geometry and distribution of micropores affect the surface strength and density of the pad. The porosity and density characteristics are mainly reflected by Poisson's ratio (ν). Surface strength decreases with increasing porosity. Larger pore diameters improve transport capacity, but excessively large pores can reduce pad density and strength.
Changes in micropore size and structure also affect polishing performance. Optimizing the micropore structure can improve the contact state between the pad and the wafer. Exploring the synergistic relationship between mechanical loading at the interface and chemical reactions of the slurry can better control the CMP material removal rate.
● Surface Groove Textures of the Polishing Pad
Grooving on the pad surface serves two purposes: on one hand, it enhances the pad's ability to store and transport slurry, improving slurry flow; on the other hand, it modifies the friction coefficient and shear stress on the pad surface. The table below shows typical grooves of the IC series pads manufactured by Rohm Haas, which are widely used in the semiconductor industry. Comparing IC1010 with IC1000, IC1010 has deeper and denser grooves, resulting in stronger mechanical action from pad asperities and abrasive particles, as well as stronger chemical action, leading to higher material removal capability.
Common groove patterns include radial, grid, annular, and spiral logarithmic types. In the figure below, (a)‑(d) represent single‑pattern pads, while (e)‑(g) are composite‑pattern pads. Composite pads generally perform better than single‑pattern pads, and negative spiral‑logarithmic pads can significantly improve polishing rate.
● Pad Asperities
The rough protrusions on the pad surface are elastic to avoid causing excessive irreparable scratches on the wafer. The microscopic surface height profile of the pad typically follows a Gaussian or exponential distribution.
The average height of asperities (Rpk) strongly affects the material removal rate (MRR). Without pad conditioning, MRR decreases with polishing time; if Rpk is restored by conditioning, MRR returns to near its original level. Wear during polishing and subsequent conditioning cause continuous changes in actual pad roughness. Variations in roughness, asperity diameter, and roughness distribution directly affect MRR.
02 Material Properties of the Polishing Pad
The physical and chemical properties of the pad material affect the contact state and contact force at the polishing interface (wafer‑slurry‑pad), thereby influencing material removal rate and wafer surface roughness.
● Mechanical Property Parameters
Hardness and elastic modulus are two important mechanical parameters. Hard pads are used in rough polishing stages where high removal rates are required, but excessive hardness can cause surface damage and non‑uniform material removal. Soft pads are generally used in fine polishing stages with low roughness requirements. A pad with a hard top and soft bottom can improve MRR uniformity, but tends to have a larger edge exclusion zone; conversely, a pad with a soft top and hard bottom can reduce the edge exclusion zone and achieve better surface quality.
● Chemical Characteristics
The pad should have good chemical stability and hydrophilicity. Polyurethane exhibits a shape‑memory effect, with the shape recovery rate increasing with temperature.
In CMP pad production, chemical reactions mainly involve polyols and isocyanates. The main raw materials include prepolymers, chain extenders/crosslinkers, foaming agents, catalysts, and other functional additives. By controlling reactant concentrations and ratios, various material properties of the pad can be improved. Active groups in the pad, such as hydroxyl and amide groups, also play an important role – they enhance the redeposition layer of polishing materials and mitigate surface quality issues caused by mechanical wear. Additionally, physical and chemical modification of the pad material can improve its performance.
Furthermore, during CMP, the pad surface undergoes load and shear forces, causing the asperities to undergo plastic flow and become planarized – a phenomenon known as glazing. Pad conditioning can ameliorate surface glazing and porosity, maintaining stable polishing performance.
Conditioning Technologies for Polishing Pads
Currently, conventional conditioning techniques fall into two categories: non‑self‑conditioning and self‑conditioning.
Non‑Self‑Conditioning Technologies
Non‑self‑conditioning refers to the use of external forces to remove worn abrasives, debris, and other impurities from the pad surface, exposing fresh abrasives and maintaining cutting ability during polishing.
Diamond dresser conditioning
A diamond dresser mainly consists of diamond abrasives, a binder, and a substrate. Diamond abrasives are fixed on the substrate by electroplating, brazing, sintering, or chemical vapor deposition. Its conditioning performance is closely related to the pad surface condition and thus affects workpiece quality. The principle is that large‑diameter diamonds on the dresser forcibly dislodge dull diamond particles from the binder and remove the glazed layer and debris, exposing new cutting edges on the pad.
High‑pressure water‑jet conditioning
This technique uses the shear force of a water jet to wash away loose or poorly bonded abrasive particles and also breaks the glazed layer, removing impurities and improving pad performance.
Self‑Conditioning Technologies
Self‑conditioning refers to methods that allow the pad surface layer to wear at an appropriate rate so that abrasives in the sub‑surface layer are exposed continuously during processing, maintaining sustained cutting ability. The emergence of self‑conditioning has introduced new approaches to pad conditioning.
Self‑conditioning eliminates the conditioning step, avoids the impact of dresser wear on the pad, and extends pad life. For example, adding organic bases, inorganic salts, or other chemicals can enhance self‑conditioning; using prepolymers with hydrophobic groups provides self‑regulating action during polishing, stabilizing the process and preventing water swelling.