Chemical Mechanical Polishing

Introduction:

Chemical mechanical polishing is often termed as Chemical Mechanical Planarization or CMP. This is one of the latest and currently one of the most widely used processes for making the surface of objects smooth by combining both the chemical and mechanical forces. Thus, it is a type of partly mechanical and partly chemical process. It can be referred to as a hybrid of chemical etching (micro-fabrication) and free abrasive polishing of the surface.

The role of CMP is multi-dimensional; it is used for a wide range of things such as, surface smoothening, material removal, planarization, uniformity control, defect reduction and more.

Principle of CMP:

The general mechanism of CMP is illustrated in following steps:

• Firstly, the surface of the metal or object is reacted with a suitable substance chemically. This is to soften the surface layer by using a chemical reaction.
• Secondly, a type of mechanical force is applied with the help of abrasive particles to remove the soft layer previously created by chemical treatment.

NOTE:

There are certain terms related to CMP that one must know to understand the procedure completely, these are:

1. Wafer: A wafer is a thin slice of semiconductor in the field of electronics, It can be a crystalline silicon (c-Si), designed in the form of a very thin disc, used for the fabrication of integrated circuits.
2. Chemical Slurry: CMP slurries typically consist of a Nano-sized abrasive powder dispersed in a chemically reactive solution to make it suitable to react with the surface of the given object.
3. Retaining Ring: A retaining ring is an integral part of the CMP process; its function is to hold the wafer under the carrier during polishing, to prevent it from slipping out of the carrier head, and to achieve uniform material removal throughout the surface.
4. Polishing Pad: It is made up of a rigid, micro-porous polyurethane material.

Working of CMP Method:

• An abrasive and corrosive chemical slurry is generally used in this process in conjunction with a polishing pad and retaining ring, which is typically of a greater diameter than the wafer itself.
• A dynamic polishing head and a plastic retaining ring helps to press the pad and wafer together to held them in place.
• The dynamic polishing head is rotated with different axes of rotation which are generally not concentric. Therefore, making the wafer flat or planar by removing material which tends to even out any irregular topography.
• The step in the above statement is necessary to set up the wafer for the formation of additional circuit elements.

The process is illustrated in the image given below, which represents the CMP Machine:

Both the chemical and mechanical processes are very crucial in this method, both of them are required because the physical grinding alone will cause huge damage to the component and might make it unfit for further use as any mechanically polished metal surface suffers a profusion of scratches, metal debris, and abrasives embedded in its surface, while, on the other hand the chemical process will not be sufficient enough to independently polish the surface completely. So a combination of both Chemical and Mechanical process is required.

NOTE: There's a lot of friction involved in this method and thereby the friction generated by these processes can produce a huge amount of heat, so the temperature needs to be maintained very carefully.

Uses and Application of CMP:

• CMP is widely used in the manufacturing of semiconductors and integrated circuits.
• It is essential in fabricating electronic Items.
• CMP is typically used to prepare a wafer for lithography.

Advantages of CMP:

• It ensures that the regions of semiconductor components are in level or planarized. It thereby reduces the rough topography of the substance to a planarized state.
• It helps in reducing surface defects by means of its subtractive nature.
• It can be used to planarize a wide range of materials starting from metals to their oxides.
• It is often an environment friendly process, as no harmful gases are produced in it as compared to the dry etching procedures.

Disadvantages of CMP:

• CMP is a costly procedure as it requires advanced equipment and expensive consumables.
• It requires frequent replacement of chemicals and higher maintenance, adding more to the cost of CMP procedure.
• Since the endpoint of the CMP is difficult to control for the desired thickness, it sometimes require an additional process development for process control, making it bit complex method.
• Sometimes, new types of defects arise due to CMP method and they get more critical.

Conclusion:

CMP has its own limitations, but despite this fact, it is a very successful method for surface polishing. Sometimes, interactions between the layers present inside the electrical equipment often lead to rough or uneven surfaces that need to be corrected and even small imperfections can bring about lots of issues such as problems in flow of current through the electric equipment, heat dissipation, component durability etc. All the defects such as grinding lines, scratches, pits and surface flaws including those damage that may affect a part's appearance, as well as its functionality are eliminated with the help of Chemical Mechanical Polishing.

CMP is a latest technology that can bring the entire surface of the given material within the depth of field requirements of a photolithography system, or selectively remove material based on its position.