Image Credit: Saint-Gobain Surface Conditioning Group
Chemical mechanical planarization (CMP) slurries are liquid dispersions containing active chemicals and microabrasive grains used for chemical mechanical planarization. CMP is a surface polishing and material removal process involving both chemical attack and abrasive removal. CMP slurries are used in conjunction with CMP pads or polishing naps, which are rotated and held against a substrate or wafer surface during the planarization process.
Products and Materials Finished
The first step to CMP slurry selection is knowing which materials or products the chemicals are designed to finish. CMP slurries are used to planarize wafers, optical substrates, disk-drive components and other critical microelectronic surfaces. Wafer planarization produces a silicon substrate or semiconductor wafer with a flat, uniform surface, thereby eliminating surface roughness or form variations that would hinder chip fabrication and detract from device performance. During the semiconductor fabrication process, many different layers or materials (e.g., conductive, barrier, dielectric, semiconducting) are deposited and etched or patterned. Often CMP processes are applied between these depositions and patterning processing-steps to prevent dishing, remove excess materials, and maintain flatness and patterned wafer quality. CMP processes are essential to the damascene copper interconnect used in high-performance microprocessors.
Chemical mechanical planarization (CMP) slurries can be bought in a number of different forms. They may come as premixed slurries which can be directly applied to the application or equipment. Finishing compounds may also come in the form of a dry powder. Dry powder may be easier to buy in bulk, but it must subsequently be mixed with water, oil, or another carrier fluid before it is applied to the surface. In other cases, chemicals can come in the form of a liquid or dispersion.
Abrasive Grain Type
There are a number of different grain types used in CMP slurries depending on the needs of the application. A few of most common abrasives used include:
Aluminum oxide - Aluminum oxide is the most commonly used industrial mineral today. It occurs naturally in the form of the mineral corundum. Fused aluminum oxide is produced synthetically by melting bauxite and additives in an arc furnace to form fused aluminum-oxide ingots, which are later crushed and sized.
Ceramic - Ceramic abrasives usually consist of aluminum oxide with or without additional modifiers. They are produced in a sol-gel and sintering process. The ceramic processing method results in a hard, dense abrasive with an extremely-fine crystal size and outstanding grinding performance on a variety of workpiece materials.
- Cerium oxide - Ceria abrasives consist of cerium oxide and are used in fine polishing and lapping applications, especially in glass and optical components.
- Silica - Silica refers to abrasive compounds based on silicon dioxide or silica. Fumed silica-based CMP compounds are often used in semiconductor or silicon-wafer polishing and planarization applications.
Silicon carbide - Silicon carbide (SiC) is a synthetic abrasive that is harder than aluminum oxide, but more friable than fused aluminum-oxide grains. Typically, silicon carbide is applied to nonferrous metals such as brass, aluminum, or titanium.
- Zirconia - Alumina-zirconia abrasive grains consist of a fused alloy of aluminum oxide and zirconium oxide. The resulting fine structure and higher hardness contributes to improved grinding performance on stainless steel, titanium, and other exotic metals.
The physical properties of the CMP slurry are important when considering the best chemicals for the applications.
pH defines the acidity or alkalinity of the slurry or solution on a scale of 1 to 14, where <7 is considered acidic and >7 is considered alkaline. The measurement is usually specified as a property of the concentrate, not a diluted mixture.
Surface tension is the tendency of the liquid slurry's surface to resist external forces, measured in force per unit length (e.g. dynes/cm). The measurement is usually specified as a property of the concentrate, not a diluted mixture.
Density defines the mass per unit volume of the solution, typically in pounds per gallon (lb/gal). This property can be controlled to meet application needs.
Average particle size indicates the average diameter of the solid microabrasive particles in the slurry. Particle size determines the load applied per particle, which affects the slurry's mechanical abrasion intensity. This parameter can be controlled to meet application needs.
Boiling point indicates what temperature the slurry or solution will boil at, thus dictating the upper temperature limit of its application.
Solution stability indicates the slurry's ability to maintain functionality under extreme environments of pH, ionic strength, pressures, and temperature, in the presence of reactive additives. Stable solutions are much easier to handle and use.
Performance properties define how well CMP slurries perform in different aspects of planarization and polishing processes.
- Planarization efficiency refers to the slurry's overall effectiveness in planarizing a surface through material removal.
Removal rate defines the rate at which the CMP slurry removes material from the product surface.
Selectivity defines the tendency of a CMP slurry to remove certain materials over others, typically expressed in a ratio. For example. an oxide-nitride selectivity ratio of 100:1 means the slurry will remove oxides in a layer 100 times as fast as nitrides; this type of slurry would likely be used on an oxide layer or on an oxynitride layer where it is desired to remove only oxides. Often CMP applications will use multiple slurries with different selectivities in order to target the removal of different materials throughout the process.
Defectivity defines the average amount of defects (micro-scratches) on a surface induced by polishing, described in scratches per unit area (scratches/cm2). Increasing density and particle size typically results in a higher removal rate, but may also correspond to higher defectivity. Implementation of buffing on a soft pad and POU filter can help to reduce CMP micro-scratches.
Topography control is defined as any means used to control what and how much material is removed from a target surface. Topography can be controlled by a CMP slurry by varying pH and peroxide concentration, among other things.
The scale of the application and the system will determine the amount of slurry material that must be purchased. Chemical mechanical planarization slurries for larger industrial applications are available in standard 55 gallon drums. Smaller scale process and test applications and may prefer purchasing pails or containers from 1 to 5 gallons.
Selection Tip: When determining the appropriate purchase size of those available, consider the slurry's shelf-life and the consumption rates of the application.