1. Introduction to Surface Polishing Technologies

To achieve high-quality surface finishes, polishing processes remove material from the workpiece surface through the combined effects of mechanical action and chemical reactions. These processes typically utilize fine abrasive particles together with soft polishing tools (such as pitch, polyurethane, and cloth), chemical solutions, electric/magnetic fields, or particle beams as auxiliary methods.

According to the different material removal mechanisms involved, polishing technologies can be mainly classified into:

• Mechanical polishing
• Chemical/electrochemical polishing
• Chemical mechanical polishing (CMP)
• Other advanced polishing technologies

(1) Mechanical Polishing

Mechanical polishing removes a small amount of material from the workpiece surface through the interaction between abrasive particles and a polishing pad, producing a smooth surface. It is currently one of the most widely used polishing technologies for optical components.

The conventional mechanical polishing process is illustrated in the figure below.

During mechanical polishing, abrasive particles are pressed into the workpiece surface under the normal pressure generated by the asperities of the polishing pad. As the polishing pad moves relative to the workpiece, the abrasive particles interact with the surface through friction, plowing, and cutting actions, thereby removing material from the workpiece.

The deformation behavior of surface materials caused by abrasive particles can generally be divided into three stages:

• Elastic deformation
• Plastic deformation
• Brittle deformation

Because mechanical polishing usually employs small abrasive particles and relatively soft polishing tools, the surface material generally undergoes elastic or plastic deformation during the polishing process.

From a microscopic perspective, a mechanical interaction model between abrasive particles and the workpiece surface has been established, as shown in Figure (a).

In this model, the abrasive particle is assumed to be spherical and pressed into the workpiece surface under the pressure applied by the asperities of the polishing pad.

During mechanical polishing, whether material removal occurs depends primarily on the indentation depth of the abrasive particle.

There exists a critical indentation depth corresponding to the transition from elastic deformation to plastic deformation:

• When the indentation depth is smaller than the critical value, the surface material only undergoes elastic deformation, and no material removal occurs.
• When the indentation depth exceeds the critical value, plastic deformation occurs, resulting in effective material removal.

Different abrasive indentation depths lead to different material removal mechanisms, which determine the final wear state of the processed surface.

As shown in Figure (b):

When the indentation depth reaches the atomic scale (approximately 5 Å), the workpiece surface undergoes only elastic deformation. No atoms are removed, and no surface damage is generated.

When the indentation depth increases to approximately 10–15 Å, the dominant removal mechanism becomes plowing removal. Under the extrusion effect of abrasive particles, surface atoms accumulate and form atomic clusters, which are gradually removed as the abrasive particles move.

The number of removed atoms increases proportionally with the indentation depth.

When the indentation depth further increases to approximately 20 Å, the removal mechanism changes to cutting removal.

At this stage, a large number of surface atoms are directly cut by abrasive particles, forming chips that are removed along with abrasive movement.

During this process, abrasive particles not only push and accumulate surface atoms but also generate severe protrusions around the scratches, resulting in increased surface roughness.

Traditional mechanical polishing theory suggests that material removal does not occur during the elastic deformation stage. Instead, material removal begins only when abrasive particles induce plastic deformation through cutting action.

Therefore, mechanical polishing inevitably introduces a certain amount of surface and subsurface damage.

In mechanical polishing processes, the surface/subsurface damage of optical components is strongly influenced by process parameters, including:

• Polishing pressure
• Mechanical properties of the polishing tool
• Abrasive particle size
• Abrasive particle morphology

Figure (a) shows a BK7 optical glass surface after mechanical polishing.

The polishing process used 10 μm Al₂O₃ abrasive particles and a pitch polishing tool. Significant scratch defects caused by mechanical abrasion can be clearly observed on the surface.

Figure (b) shows the surface and subsurface damage of a CdZnTe wafer after 30 minutes of mechanical polishing observed by scanning electron microscopy.

The polishing process used 2–5 μm Al₂O₃ abrasive particles with an aluminum plate polishing tool.

Micron-scale and submicron-scale scratches were observed on the surface. The subsurface damage layer thickness was less than 1 μm and consisted mainly of a polycrystalline layer, indicating plastic deformation damage.

(2) Chemical Polishing and Electrochemical Polishing

Chemical polishing and electrochemical polishing utilize chemical or electrochemical reactions to dissolve material from the workpiece surface.

During the process, microscopic protrusions on the surface dissolve preferentially compared with recessed regions, resulting in a smoother surface.

The basic composition of chemical/electrochemical polishing solutions generally includes:

• Etchants
• Inhibitors
• Water

The functions of each component are as follows:

• Etchants: dissolve the surface material through chemical reactions.
• Inhibitors: suppress excessive corrosion and improve process controllability.
• Water: acts as a diluent and facilitates diffusion of reaction products.

For different workpiece materials, corresponding polishing solution systems must be selected according to their chemical properties.

Because optical materials generally possess excellent chemical stability, strong acids, strong alkalis, or strong oxidizing agents are often required as etchants during chemical polishing.

For example, optical components mainly composed of SiO₂, such as K9 glass and fused silica, commonly use hydrofluoric acid (HF) as the etchant.

The chemical reaction is:

$$Si{O}_2+6HF\to H_{2}Si{F}_6+2H_{2}O$$

Chemical polishing features a simple process flow and relatively easy implementation.

However, it also suffers from several limitations:

• Low processing accuracy
• Poor controllability
• Limited repeatability
• Environmental pollution issues

Therefore, it is difficult to meet the requirements of ultra-precision optical component manufacturing.

Electrochemical polishing is widely applied in:

• Metal finishing
• Metallographic sample preparation
• Precision surface treatment

Electrochemical polishing utilizes anodic dissolution of metals for surface processing.

It is considered a non-contact, stress-free machining method with advantages including:

• High material removal efficiency
• No mechanical damage
• No work-hardening layer
• No residual stress generation

However, it requires the processed material to possess good electrical conductivity, making it unsuitable for most optical materials.

Chemical and electrochemical polishing remove materials through dissolution mechanisms.

Because the removal process is independent of mechanical properties such as hardness, toughness, and strength, these methods can rapidly reduce surface roughness and achieve excellent polishing quality.

Meanwhile, since there is no mechanical friction or cutting action, the required equipment is generally simpler and less expensive compared with mechanical polishing systems.

However, chemical/electrochemical polishing technologies also have several limitations:

1) High development cost

Due to differences in chemical properties among materials, dedicated polishing solutions must be developed for different materials, requiring extensive experimental optimization of process parameters.

2) Difficult control of chemical reaction rates

The reaction rate is difficult to precisely regulate, making it challenging to maintain dimensional accuracy and geometric precision.

3) Strong dependence on material uniformity

The obtained surface quality is highly affected by the internal structure and purity of the workpiece.

Impurities and structural defects inside the material may significantly reduce the final surface quality.

(3) Chemical Mechanical Polishing (CMP) Technology

Chemical Mechanical Polishing (CMP) is a surface planarization technology that combines chemical reactions and mechanical abrasion. It has been widely applied in semiconductor manufacturing, optical component processing, and advanced material fabrication.

The development of CMP technology is closely associated with the advancement of semiconductor technology. In the early stages, polishing mainly relied on purely mechanical grinding methods, such as surface finishing of glass and metals. However, these conventional approaches were unable to satisfy the increasingly stringent requirements for high precision and nanoscale surface planarization.

In the 1950s, researchers discovered that chemical reagents, such as acids and alkaline solutions, could assist material removal and reduce mechanical damage. This discovery laid the foundation for the development of CMP technology.

In 1965, IBM introduced CMP technology for silicon wafer polishing for the first time. With the rapid development of integrated circuits (ICs), traditional dry etching processes and purely mechanical polishing methods were unable to effectively eliminate surface topography variations. As a result, CMP gradually became a critical process technology.

However, early CMP processes still faced several challenges, including:

• Poor stability of polishing slurries
• Insufficient equipment precision
• Difficulty in controlling material removal rates

In 1997, IBM successfully introduced CMP into copper interconnect polishing processes.

Copper CMP requires precise control of the balance between chemical dissolution and mechanical abrasion to suppress excessive copper oxidation and achieve uniform material removal.

With the continuous evolution of semiconductor manufacturing technology, CMP has become the only viable planarization solution when semiconductor processes entered the sub-0.25 μm technology node.

Furthermore, the introduction of low-k dielectric materials required gentler CMP processes to minimize mechanical damage and maintain structural integrity.

CMP Processing Mechanism

CMP utilizes relatively soft abrasive particles to achieve high-quality surface polishing.

During the CMP process, the workpiece is pressed against a polishing pad under controlled pressure and moves relative to the pad.

Through the synergistic interaction between nanoscale abrasive particles and chemical components in the polishing slurry, the workpiece surface gradually becomes smoother, as illustrated in the figure below.

The CMP system generally consists of a rotating carrier and a polishing platen.

Through their coordinated rotation, a complex relative motion trajectory is generated between the wafer and polishing pad.

A slurry continuously supplied by a peristaltic pump contains two functional components:

• Abrasive particles: provide mechanical polishing action.
• Oxidizing agents: induce chemical reactions on the material surface.

The CMP material removal mechanism mainly involves two sequential steps:

Step 1: Surface oxidation and chemical modification

The oxidizing agents react with the workpiece surface and generate a chemically modified or softened surface layer.

Step 2: Mechanical removal of the reacted layer

The abrasive particles remove this softened reaction layer through mechanical interaction.

This cyclic process ultimately produces an ultra-smooth surface with extremely low roughness.

It should be noted that the balance between chemical and mechanical effects in CMP plays a decisive role in determining the final processing quality.

When mechanical action dominates:

• Surface scratches may occur.
• Residual stress concentration may increase.
• Subsurface damage may be generated.

When chemical action becomes excessive:

• Surface corrosion defects may appear.
• Localized etching may occur.
• Surface morphology may deteriorate.

Therefore, optimizing the synergistic interaction between chemical reactions and mechanical abrasion is the key factor for improving CMP performance.

Research has demonstrated that mechanical polishing intensity and chemical reaction rate exhibit a positive correlation.

This coupling effect directly influences material removal efficiency.

Therefore, precise control of slurry composition has become one of the major technological challenges in the development of advanced CMP processes.

To obtain a high-quality polished surface, the chemical and mechanical effects during CMP must reach an appropriate equilibrium.

If chemical action dominates over mechanical action:

• Corrosion pits
• Orange-peel-like surface patterns
• Non-uniform surface morphology

may occur.

Conversely, if mechanical action dominates:

• Scratches
• Cracks
• Subsurface damage layers

are likely to form.

2. Factors Affecting Chemical Mechanical Polishing

During CMP processing, process parameters have a significant influence on:

• Material removal rate (MRR)
• Surface roughness
• Surface integrity
• Processing uniformity

According to the Preston equation:

$$MRR=K\times P\times V$$

where:

• MRR represents the material removal rate.
• K is the Preston coefficient related to polishing conditions.
• P represents polishing pressure.
• V represents relative polishing velocity.

The polishing pressure and rotational speed jointly determine the material removal efficiency.

(1) Effect of Polishing Pressure

Polishing pressure directly determines the contact stress between the polishing pad and wafer surface.

Increasing pressure generally improves the contact between abrasive particles and the wafer, thereby increasing MRR.

However, excessive pressure may result in:

• Surface micro-scratches
• Wafer deformation
• Localized over-polishing
• Dishing defects

Therefore, an appropriate pressure range must be selected to balance efficiency and surface quality.

(2) Effect of Rotational Speed

The relative rotational speed between the polishing pad and wafer affects the hydrodynamic behavior and distribution of polishing slurry.

A large speed difference may cause excessive edge removal, leading to the phenomenon known as edge roll-off.

Increasing rotational speed enhances mechanical shear forces, which may improve material removal efficiency.

However, excessively high speed can generate localized temperature increases (typically exceeding 10°C), resulting in changes in slurry chemical activity and affecting polishing stability.

(3) Effect of Slurry Flow Rate

The slurry flow rate influences both:

• Chemical reactions between the slurry and wafer surface.
• Mechanical removal of the oxidized reaction layer.

In addition, through convection, the slurry acts as a coolant and helps dissipate heat generated at the interface between the polishing pad and wafer.

Therefore, optimizing slurry flow rate contributes to improved polishing efficiency and surface quality.

Appropriate adjustment of process parameters, including:

• Pressure
• Rotational speed
• Slurry flow rate

is essential for achieving high-performance CMP processes.

Research has shown that within an appropriate pressure range, MRR increases approximately proportionally with applied pressure.

At low pressure and low slurry flow rates:

• Insufficient oxidation reactions may occur.
• Material removal efficiency decreases.

Increasing pressure or slurry flow rate can:

• Reduce polishing pad temperature.
• Improve oxidation capability.
• Enhance mechanical removal efficiency.

As a result, MRR increases.

However, excessive pressure or slurry flow rate may cause mechanical abrasion to dominate.

Although MRR may continue increasing, surface roughness may also increase and scratch defects may appear.

Therefore, establishing a dynamic balance between oxidation reactions and mechanical removal is essential for achieving maximum polishing efficiency.

(a) Polishing Slurry Composition

Although CMP requires precise control of process parameters such as:

• Polishing pressure
• Rotational speed
• Slurry flow rate

the polishing slurry remains the most critical factor affecting CMP performance.

The key components of CMP slurry include:

• Abrasive particles
• Oxidizing agents
• pH regulators

These components significantly influence:

• Material removal rate (MRR)
• Surface roughness
• Surface defect formation

(b) Oxidizing Agents

Oxidizing agents play a critical role in CMP by promoting oxidation reactions on the material surface.

These reactions generate a relatively softer oxide layer, which can then be easily removed by mechanical abrasion.

The oxidation process effectively reduces the mechanical force required for material removal and helps minimize surface damage.

(c) pH Value

The pH value of the polishing slurry directly affects:

• Polishing rate
• Surface quality
• Material removal mechanism

By adjusting slurry acidity or alkalinity, the chemical reaction rate between the slurry and material surface can be controlled.

An optimized pH value helps achieve a balance between chemical modification and mechanical removal.

(d) Surfactants

Surfactants act as stabilizing agents in CMP slurry.

Their main functions include:

1. Maintaining slurry stability

Surfactants prevent:

• Abrasive particle aggregation
• Component separation
• Sedimentation

thereby maintaining slurry uniformity.

2. Improving slurry spreading behavior

Surfactants reduce slurry surface tension, allowing better spreading on hydrophobic wafer surfaces.

This enhances contact between:

• Abrasive particles
• Chemical components
• Material surface

leading to improved polishing uniformity and efficiency.

3. Reducing surface damage

Surfactants can regulate surface reaction rates or form protective films on the surface.

This helps balance mechanical abrasion and chemical corrosion, improving:

• Material removal efficiency
• Surface smoothness
• Subsurface damage resistance