The importance of CMP process in wafer manufacturing

1. CMP is a key process for global planarization of wafers

Wafer manufacturing process can be broadly divided into two stages: front-end and back-end. Front-end processes are carried out in wafer fabs, mainly responsible for wafer processing and manufacturing; back-end processes are carried out in packaging and testing plants, mainly responsible for chip packaging and testing. Chemical Mechanical Planarization (CMP) is a key process for achieving global planarization of wafers. It refers to the efficient removal of excess material from the wafer surface and global nanometer-level planarization through the synergistic effect of chemical etching and mechanical polishing. It is a crucial process technology required in the front-end process of advanced integrated circuit manufacturing and advanced packaging.

In the front-end processing field: CMP is mainly responsible for planarizing the wafer surface.

The front-end processing stage of wafer manufacturing mainly includes seven independent processes: lithography, etching, thin film deposition, diffusion, ion implantation, chemical mechanical polishing, and metallization. CMP is mainly used to connect different thin film processes. According to the process segment, it can be divided into front-end-of-line (FEOL) and back-end-of-line (BEOL). The front-end processes mainly include STI-CMP and Poly-CMP, while the back-end processes mainly include interlayer dielectric ILD-CMP, IMD-CMP, and metal layer W-CMP, Cu-CMP, etc.

In the back-end packaging field: CMP technology is also gradually being used for polishing in advanced packaging, such as through-silicon via (TSV) technology, fan-out technology, 2.5D interposers, and 3D IC packaging technologies. These technologies require smaller and finer lead sizes, so etching and lithography processes are introduced. CMP, as a polishing process between each process, is also widely used in advanced packaging.

If nanometer-level global planarization cannot be achieved during wafer manufacturing, it is impossible to repeatedly perform key processes such as lithography, etching, thin film deposition, and doping, and it is also impossible to reduce the process node to the nanometer-level advanced field.

With the continuous miniaturization of line width in ultra-large-scale integrated circuit manufacturing and the continuous development of manufacturing processes towards advanced process nodes, the accuracy requirements for planarization are also continuously increasing, and the number of CMP steps will also increase, thus greatly stimulating the demand for CMP equipment procurement and upgrades from integrated circuit manufacturers.

CMP (Chemical Mechanical Planarization) has higher processing accuracy and speed compared to traditional methods.

Traditional mechanical polishing and chemical polishing methods cannot meet the mass production needs of advanced chips in terms of removal rate and polishing quality. CMP technology utilizes the "soft grinding hard" principle in abrasion, that is, using softer materials for polishing to achieve high-quality surface polishing. It avoids surface damage caused by pure mechanical polishing and the slow polishing speed, poor surface flatness, and poor polishing consistency caused by pure chemical polishing. It is currently the only polishing technology that can take into account both global and local surface planarization and is widely used in advanced integrated circuit manufacturing.

2. CMP Process Technology Principle

CMP equipment mainly relies onThe chemical-mechanical dynamic coupling principle of CMP technologyThrough the synergistic effect of chemical etching and mechanical polishing, it achieves efficient removal of excess material from the wafer surface and global nanometer-level planarization (ultra-high flatness with a global flatness difference of less than 5nm). The CMP polishing process can be divided into chemical and physical processes.

CPM Process Analysis

The chemical process refers to the chemical reaction between the chemical components in the polishing slurry and the materials on the silicon wafer surface. This process converts insoluble substances into soluble substances or softens high-hardness substances, generating substances that are easier to remove. The physical process refers to the mechanical physical friction between the abrasive particles in the polishing slurry and the materials on the silicon wafer surface, removing these chemical reaction products from the silicon wafer surface and carrying them away dissolved in the flowing liquid.

Specific steps of CMP:
  Step 1: Fix the silicon wafer at the bottom of the polishing head, and place the polishing pad on the polishing pad;
  Step 2: The rotating polishing head presses on the rotating polishing pad with a certain pressure. A flowing polishing slurry (composed of submicron or nano-abrasives and chemical solutions) is added between the silicon wafer surface and the polishing pad. The polishing slurry is evenly applied under the action of the polishing pad's transmission and centrifugal force, forming a liquid film between the silicon wafer and the polishing pad;
  Step 3: Planarization is achieved through an alternating process of chemical and mechanical film removal.

Main technical parameters of CMP:
  Polishing rate: the thickness of material removed per unit time;
  Planarity: the difference in height between the step before and after CMP at a certain point on the silicon wafer / the step height before CMP * 100%;
  Polishing uniformity: includes intra-wafer uniformity and inter-wafer uniformity. Intra-wafer uniformity = standard deviation of polishing rate of the same wafer / polishing rate; Inter-wafer uniformity = standard deviation of polishing rate under the same conditions of different wafers / average polishing rate
  Defect quantity: surface defects of the silicon wafer caused by the CMP process, generally including scratches, dents, erosion, residues, and particle contamination, which directly affect the yield.

3. CMP equipment and materials have a key impact on process effects

CMP processes cannot be separated from equipment and materials, including polishing pads and polishing slurries. Equipment and materials have a key impact on process effects. The main influencing factors of CMP effects are as follows:
  Equipment parameters: polishing time, polishing pad rotation speed, polishing head rotation speed, polishing head swing, back pressure, down pressure, etc.;
  Polishing slurry parameters: abrasive size, abrasive content, abrasive agglomeration, acidity, oxidant content, flow rate, viscosity, etc.;
  Polishing pad parameters: hardness, density, pore size, elasticity, etc.;

CMP object film parameters: type, thickness, hardness, chemical properties, pattern density, etc. CMP materials mainly include polishing liquid, polishing pad, diamond disc, cleaning liquid, etc., which all have a key impact on the CMP process effect.
1. CMP polishing pad: The main functions are to store and transport polishing liquid, remove abrasive debris, and maintain a stable polishing environment;
2. CMP polishing liquid: It is a mixture of abrasive materials and chemical additives, which can create an oxide film on the wafer surface, which is then removed by the abrasives in the polishing liquid to achieve the purpose of polishing.
3. CMP diamond disc: It is an indispensable consumable in the CMP process, used to maintain a certain roughness of the polishing pad surface, usually used in conjunction with the CMP polishing pad.
4. CMP cleaning liquid: Mainly used to remove residual micro-dust particles, organic matter, inorganic matter, metal ions, oxides and other impurities from the wafer surface, meeting the extremely high cleanliness requirements of integrated circuit manufacturing, and playing an important role in the yield of wafer production.

CMP equipment is the carrier of CMP technology application, integrating advanced technologies from multiple fields such as tribology, surface/interface mechanics, molecular dynamics, precision manufacturing, chemical engineering, and intelligent control. It is one of the more complex and difficult-to-develop equipment in integrated circuit manufacturing equipment. At the same time, due to the widespread use of copper interconnects in microprocessor production, CMP equipment, which is the only equipment capable of polishing copper metal layers, has become an essential tool for chip manufacturers.

CPM equipment structure diagram

CMP equipment is mainly divided into polishing and cleaning parts. The polishing part consists of a polishing head, a grinding disc, etc., and the cleaning part consists of a cleaning brush, a liquid supply system, etc.

• Polishing head: Usually has a vacuum adsorption device for adsorbing wafers to prevent wafer displacement during polishing, while applying downward pressure.
• Grinding disc: Plays a supporting role for the wafer, supporting the polishing pad and driving its rotation, and controlling the pressure, rotation speed, and switching actions of the polishing head.
• Cleaning brush: Used in the post-CMP cleaning stage to remove particles and other chemical contaminants after CMP. It is divided into cleaning, rinsing, and drying stages to ensure that the wafer is dry in and dry out.

Endpoint detection equipment: Endpoint detection equipment is used to detect whether the CMP process has polished the material to the correct thickness, avoiding the negative impact of being too thin (not achieving the polishing effect) and too thick (loss of the underlying material). It usually uses electrical performance and optical measurement methods.

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