What is polishing, and how does it differ from lapping?
The role of grinding is primarily rough processing, as it involves the use of abrasives that are coarser and harder than the workpiece. Consequently, the mechanical action of the abrasive is the dominant factor. In contrast, polishing is a finishing process that utilizes relatively fine abrasive particles and softer tools. Additionally, abrasive particles and polishing fluids that chemically react with the workpiece can also be selected.
This is due to two primary reasons: the current mainstream approach involves a combination of chemical effects.
Mechanical action alone is inefficient.
In the finishing of semiconductor materials, a completely strain-free and processing-free surface is essential.
When the polishing method is classified based on the processing mechanism, it can be categorized into five types, as illustrated in Table 1.
Note: The terminology for polishing methods that utilize chemical effects can differ among researchers. Polishing is classified based on the underlying mechanisms involved.
Chemical and mechanical polishing is a removal method that employs the combined effects of mechanical microcutting by abrasive particles and the chemical elution of processing fluids or atmospheric gases.
Chemical mechanical polishing is a method used to remove reaction products, such as oxide films and hydration films, that form on the surface of a workpiece due to atmospheric exposure. This process involves the micro-cutting action of abrasive particles.
Mechanochemical polishing is a method in which reaction products generated near the contact point adhere to abrasive particles and are subsequently removed due to the mechanical stress caused by contact with these particles.
Mechanical Polishing：MP
The fundamental principle involves the micro-cutting action of fine abrasive particles, primarily through plastic deformation.
This process removes the material and creates a mirror-like finish.
Furthermore, in the case of metallic materials, the contact point becomes locally softened due to frictional heat and is smoothed as a result of plastic flow.
For instance, the polishing of ancient inscriptions and metal mirrors is considered to fall into this category.
Even today, it is commonly used to finish precision components, including metals and ceramics.
Here are three specific examples:
Polishing the aluminum plate with fine aluminum oxide.
Ceramic and silicon carbide (SiC) using fine diamond abrasives.
Mechanical polishing (MP) of single-crystal materials, such as gallium nitride (GaN),
In commercially available silicon carbide (SiC) single crystal wafers, the carbon (C) surface is classified as a non-device surface. Mechanical polishing (MP) finishing is typically performed using diamond paste and polishing cloths.
Mechanochemical Polishing (MCP)
The fundamental principle of mechanochemical polishing (MCP) involves utilizing a solid-state reaction between the workpiece and softer abrasive particles.
Polishing is achieved by scraping away the reactants generated by high temperatures and pressures in the microscopic area where the abrasive particles make contact with the workpiece.
A common example is the use of colloidal silicon dioxide for the final polishing of sapphires.
Chemomechanical Polishing
Chemical mechanical polishing is a technique used to eliminate reaction products (such as oxide films and hydration films) that accumulate on the surface of a workpiece due to the processing atmosphere. This method utilizes the micro-cutting effect of abrasive particles. (*Note: For dry polishing, an air or oxygen atmosphere is used; for wet polishing, water or a polishing liquid is employed, among other options.)
A common example is glass polishing.
The action of water as a polishing fluid creates a soft silicate gel layer on the glass surface.
The soft gel layer is removed through the micro-cutting of abrasive particles.
When the original glass surface is exposed, a layer of silicic acid gel is formed once again under the influence of water.
The concept involves refining the process through repetition of this mechanism.
In addition, we believe that chemical-mechanical polishing may involve oxidation-assisted polishing techniques, particularly with materials such as silicon carbide, gallium nitride, and silicon nitride. These carbides and nitrides possess strong covalent bonds, making them extremely hard, heat-resistant, and chemically stable. Consequently, even when exposed to strong acids, bases, or oxidizers, no significant reactions occur at room temperature, which complicates traditional polishing methods.
In addition, several methods have been proposed in the past for the final polishing of silicon carbide using chromium oxide abrasive particles in conjunction with oxidants such as hydrogen peroxide and permanganate. This consideration blurs the distinction between chemomechanics and mechanochemistry. However, since all these methods are fundamentally based on mechanochemical phenomena, it may not be necessary to impose a strict classification on them.
Chemical-Mechanical Polishing：CMP
The fourth type is chemical mechanical polishing, commonly abbreviated as CMP.
This method employs the synergistic action of mechanical micro-cutting by abrasive particles and the chemical elution of polishing fluid to effectively remove material.
In other words, the polishing fluid has an etching (dissolution) effect on the workpiece.
If the material is metallic, the dissolution process can be accelerated by the addition of acids or oxidizing agents.
At this time, the pH value of the polishing liquid must be maintained at a sufficiently low level to prevent dissolved metal ions from binding with hydroxyl groups (OH-) and subsequently settling or adhering to the surfaces of the abrasive and polishing pad. The addition of complexing agents is effective in mitigating this issue. In the case of silicon or silicon oxide films, dissolution is enhanced by the incorporation of alkali. Additionally, a "REDOX potentiogram" (Pube diagram), which illustrates the relationship between pH and REDOX potential, can serve as a reference for predicting the dissolved state of specific chemical substances. A typical example of this process is the final polishing of silicon, which includes primary, secondary, and final polishing stages. Among these methods, using an abrasive solution composed of silica particles suspended in an alkaline solution is known to produce a mirror-like finish with exceptional smoothness and crystallinity.
The abrasive particles utilized in this process are colloidal silicon dioxide and zirconia, with a particle size ranging from 10 to 80 nm. The processing liquid consists of inorganic alkalis, such as potassium hydroxide (KOH), and various organic amines. To achieve high efficiency, it is relatively straightforward to enhance mechanical factors, including processing pressure, abrasive particle size, and concentration, as well as chemical factors such as alkali concentration and pH, or a combination of both. Typically, the treatment pressure is maintained between 20 and 30 kPa, the pH ranges from 10 to 13, and the temperature is kept between 20 and 25°C. Under these conditions, the treatment efficiency for silicon is approximately 0.1 to 1 μm/min.
However, if the mechanical action is too intense, there is a risk of processing damage, such as scratching. Conversely, if the chemical action is overly strong, the damage may be less severe, but it can lead to adverse effects, such as bumps and bruises. Therefore, it is crucial to strike a balance between mechanical and chemical effects.
The above describes Chemical Mechanical Polishing (CMP) performed on bare silicon wafers. CMP also refers to the polishing process used in silicon semiconductor manufacturing to smooth out the bumps and steps on the surface of the device wafer. Through the miniaturization of wiring and the advancement of multi-layer wiring technology, the performance of semiconductor devices has significantly improved. However, during this process, the accuracy of pattern shapes can deteriorate if irregularities beyond the focus depth are present during the exposure of the wiring pattern.
Therefore, the processing technology for the bumps and steps on the flat wafer surface became crucial, leading to the introduction of Chemical Mechanical Polishing (CMP) in the 1990s.
In recent years, various materials, including ruthenium, have emerged, and a combination of abrasive materials and additives is utilized based on the specific properties of each material.
The smoothing removal margin is minimal, approximately 1 μm, making it essential to balance the mechanical action of the particles with the chemical action of the additives.
In addition, to prevent contamination from impurities and the degradation of additive components, abrasive particles are composed of ultrafine metal oxide particles that possess nanoscale dimensions and are free from impurities.
In order to enhance the performance of semiconductor devices in the future, the chemical mechanical polishing (CMP) of silicon wafers is transitioning from nanoscale control to angstrom control, necessitating further advancements in processing technology.
Other new technical methods
Magnetic Energy-Assisted Polishing Method
Polishing methods that utilize magnetic energy can be broadly categorized into two main approaches.
How to Use Magnetic Force to Control Magnetic Abrasive Particles
A method for magnetically controlling abrasive particles using a functional fluid, which is a general term for a fluid that exhibits specific properties and functions in response to external stimuli. These properties and functions can be applied in various industrial applications.
Magnetic Polishing Method Utilizing Magnetic Abrasive Materials
The magnetic brush is created by filling a magnetic field with a magnetic abrasive material that has a particle size of approximately 30 to 40 μm.
Rotate or vibrate the workpiece within it.
The surface of the workpiece undergoes polishing, deburring, and edge finishing, among other processes.
When the workpiece is inserted, the magnetic brush is held in place between the magnetic poles by the magnetic field and exerts pressure on the surface of the workpiece.
At this time, when both rotational and axial motions are applied to the workpiece, a relative motion occurs between the magnetic brush and the workpiece, resulting in polishing. In this context, the magnetic abrasive must possess both high magnetization and excellent polishing capabilities.
For example, composite abrasives, such as iron coated with abrasive particles like alumina, are utilized.
Polishing Methods Utilizing Magnetic Fluids (MF)
Magnetic fluids are specialized functional fluids created by coating the surfaces of tiny magnetic particles, such as magnetite, which have a diameter of approximately 10 nm, with a surfactant. These particles are then stably dispersed in either water or oil.
Even when centrifugal force or a magnetic field is applied, solid-liquid separation will not occur, and the system can be considered a magnetically homogeneous liquid.
When non-magnetic abrasive particles are mixed into a magnetic fluid and placed in the presence of a permanent magnet, the magnetic fluid is drawn toward the strong magnetic field. Consequently, the non-magnetic abrasive particles are suspended in the direction of the relatively weaker magnetic field, creating a high-density layer of abrasive particles. When a workpiece is pressed against this layer and rotated, the magnetic levitation force can be utilized as the processing pressure, enabling effective polishing. This technique is known as the maglev polishing method.
In order to ensure the stability of magnetic fluid dispersion, there is an upper limit to the magnetization of the dispersed particles. A disadvantage of this limitation is the difficulty in achieving high processing pressures.
However, a method utilizing the power of "float" was subsequently developed and applied to the polishing of ceramic balls.
Polishing Method Utilizing Magnetorheological Fluid (MRF)
Magnetorheological fluid is a specialized functional fluid composed of spherical carbonyl iron powder, which has a uniform particle size of several microns. This powder is coated with a surfactant and dispersed in silicone oil.
It exhibits a very high level of magnetization, and when a magnetic field is applied, it generates yield stress, loses its fluidity, and behaves like a solid.
The abrasive particles are mixed and circulated to perform Magnetorheological Fluid (MRF) Magnetohydrodynamic (MHD) polishing using the device. The MRF is fed to a rotating wheel, where a magnetic field is applied, causing it to solidify due to the influence of the magnetic field. The workpiece, such as a lens, is pressed onto the wheel, and material is removed through shear force. As the MRF continuously circulates, it provides new abrasive cutting edges, ensuring that the shape of the MRF tool remains intact, which results in exceptionally stable machining performance.
In addition, the abrasive particles are suspended in a fluid, facilitating nanoscale machining of the unit. This process enhances shape accuracy and smoothness while effectively removing damaged layers from prior machining operations.
This polishing method can be employed to refine the shape of high-precision lenses.
Polishing Method Utilizing Magnetorheological Fluid (MRF)
Magnetorheological fluid is a specialized functional fluid composed of spherical carbonyl iron powder, which has a uniform particle size of several microns. This powder is coated with a surfactant and dispersed in silicone oil.
It exhibits a very high level of magnetization, and when a magnetic field is applied, it generates yield stress, loses its fluidity, and behaves like a solid.
The abrasive particles are mixed and circulated to perform Magnetorheological Fluid (MRF) Magnetohydrodynamic (MHD) polishing using the device. The MRF is fed to a rotating wheel, where a magnetic field is applied, causing it to solidify due to the influence of the magnetic field. The workpiece, such as a lens, is pressed onto the wheel, and material is removed through shear force. As the MRF continuously circulates, it provides new abrasive cutting edges, ensuring that the shape of the MRF tool remains intact and resulting in exceptionally stable machining performance.
In addition, the abrasive particles are suspended in a fluid, facilitating nanoscale machining of the unit. This process enhances shape accuracy and smoothness while effectively removing damaged layers from prior machining operations.
This polishing method can be employed to refine the shape of high-precision lenses.
Electric Energy-Assisted Polishing Method
Polishing methods that utilize electrical energy encompass techniques based on the phenomenon of electrolytic elution, as well as methods that regulate the movement of abrasive particles or pastes within an electric field.
Let's examine two of them here.
Other recognized methods include polishing techniques that utilize electrophoretic phenomena.
Electrolytic Polishing
This polishing method does not utilize abrasive particles.
In an electrolyte, such as sodium chloride or sodium nitrate, a direct current voltage is applied between the metal workpiece, which acts as the positive electrode, and the negative electrode.
This is a polishing method that utilizes the phenomenon of metal dissolution on the surface of the workpiece.
The smoothing mechanism is attributed to two primary factors.
During elution, the small convex portion of the surface will be eluted first.
Eluted metal ions and electrolytes combine to create an emulsion with high specific gravity, viscosity, and resistance, which fills the grooves and inhibits further elution.
Because this polishing method is a non-contact process, it does not create a damage layer, making it easy to achieve a clean and shiny surface. However, the flatness of the workpiece surface may not be optimal.
Electrolytic Abrasive Polishing
This polishing method integrates the removal of electrolytic elution with the mechanical elimination of abrasive particles.
Therefore, the device is relatively complex, utilizing a rotating disk electrode and spraying an electrolyte mixed with abrasive particles. However, it is well-suited for efficiently achieving a smooth and flat finish on large areas of workpieces, such as stainless steel.
The finished surface roughness achieved through electrolytic abrasive polishing depends significantly on the balance between electrolytic elution and the mechanical action of the abrasives. For a specific abrasive particle size, there exists an optimal current density that minimizes surface roughness. When the current density falls below this optimal value, the active surface of the workpiece rapidly develops a passivation film due to the abrasion caused by the particles. This passivation inhibits dissolution, allowing for a superior surface finish to be attained at higher current densities without the formation of passivation films.
The active surface of the workpiece is continuously affected by abrasive scratches, leading to the formation of pits that deteriorate the finished surface roughness.
Electric Field-Abrasive Controlled Polishing Method
The "electric field" abrasive control polishing method proposed here is distinct from In other words, it is unrelated to electrolytic elution. Electrolytic abrasive control technology is a machining method that applies a low-frequency, high-voltage electric field to the machining area while utilizing a slurry as the functional liquid. However, electrolytic abrasive control techniques necessitate the use of insulating oil as a solvent.
For example, when a low-frequency, high electric field is applied to a slurry composed of a mixture of insulating silicone oil, which has a dielectric constant of 2.7, and diamond abrasive particles with a dielectric constant of 5.7, the abrasive particles will actively move in response to variations in the electric field and changes in the dielectric constant.
It has been found that this process inhibits the dispersion of abrasive particles and attracts them to the polishing area, resulting in a superior finish.
On the other hand, water-based slurries, such as colloidal silicon dioxide, have a dielectric constant of 80, which is an order of magnitude higher than that of wear particles. Additionally, these slurries are electrically conductive, meaning that the wear particles do not respond to changes in polarity. Consequently, "electric field slurry control technology" was developed not to control the abrasive particles, but to manage the water itself. In a water-based slurry, the dielectric constant of water is approximately 80, while that of air is about 1. As a result, the electric field attracts water, which has a high dielectric constant, toward air, which has a low dielectric constant. By utilizing this principle, the movement of the water-based slurry can be controlled by applying a low-frequency, high-voltage electric field. This technology is gaining attention as a novel method for efficiently polishing various optical and semiconductor materials.
