Application of silica sol in the field of chemical mechanical polishing (CMP)

At present, the rapid development direction of the advanced manufacturing industry of electronic products is high-precision (nano-level control accuracy, sub-nano-level processing accuracy), high performance (T-level storage capacity and CPU frequency), and high integration (nano-level) The line width) and reliability (less than 1/109 thousand hours failure rate), therefore, the local flatness and overall flatness of the surface of the processed workpiece have been put forward unprecedented high requirements (required to reach the sub-nanometer level of surface roughness) Degree), and at the same time poses a severe challenge to the level of ultra-precision processing technology

An important method for surface smoothing is polishing.

For example, in the processing of silicon wafers, the wafer is fixed on the polyurethane polishing pad rotating in the same direction by adsorption, and the polishing slurry flows continuously to the polishing pad through the peristaltic pump, so that it continuously flows between the wafer and the polishing pad.

Schematic diagram of chemical mechanical polishing

Chemical mechanical polishing technology is a technology that combines chemical action and mechanical action.

At present, the commonly used polishing liquids at home and abroad include SiO2 colloid (silica sol) polishing liquid, cerium oxide polishing liquid, alumina polishing liquid, nano diamond polishing liquid and so on

Several polishing fluids have different application characteristics:

01 

Silica sol polishing liquid

Because the selection of polishing slurry should meet the characteristics of easy cleaning, fast polishing rate and good polishing uniformity, silica sol, as a soft abrasive, is coated with a colorless transparent colloid on the surface of SiO2 abrasive particles to make it harder than SiO2 abrasive grains are softer, with a particle size of about 0.

Silica sol CMP polishing liquid

Because the particle size of silicon dioxide is very fine, about 0.

Silica is an important part of the silica sol polishing solution, and its particle size, density, dispersion and other factors directly affect the rate and polishing quality of chemical mechanical polishing
.

02

Cerium oxide polishing liquid

As the size of the workpiece shrinks, traditional silicon is prone to form butterfly defects at the larger integrated circuit STI (Shallow Trench Isolation).
The second-generation polishing liquid that uses cerium oxide as the abrasive particles has high selectivity and polishing end point.
The automatic stop feature, combined with rough polishing and fine polishing, can effectively solve the shortcomings of the first-generation STI process
.

Cerium dioxide has low hardness, good stability, good dispersion uniformity, and high surface quality can be obtained.
However, because commercial cerium dioxide is generally prepared by a pulverizing process, there is uneven particle size, which affects the polishing quality, so it is necessary Study the preparation of nano-sized cerium oxide with good uniformity
.
However, because cerium is a rare metal and the purification of cerium dioxide is difficult, the price of cerium dioxide is expensive, and the prepared polishing liquid is not cost-effective
.

Cerium oxide polishing liquid

03

Alumina polishing liquid

Nano-alumina is widely used in precision polishing of optical lenses, single-core fiber connectors, glass-ceramic substrates, crystal surfaces, etc.
At present, most blue and white LED chips use sapphire wafers, and Al2O3 polishing liquid is used to complete sapphire wafer grinding at one time And polishing is also very common nowadays
.

Due to the high hardness of nano-a-alumina (corundum), it causes serious damage to the surface of the workpiece during polishing; and because the surface energy of nano-alumina is relatively high, the nanoparticles are easy to agglomerate, causing scratches and pits on the polished workpiece Such as surface defects, usually need to modify the surface of nano-alumina in the application
.
At the same time, due to the high hardness of the alumina abrasive, it is easy to scratch the surface of the workpiece when polishing soft workpieces, so it is not suitable for polishing softer workpieces
.

Alumina polishing liquid

04

Nano diamond polishing liquid

Nano-diamond has the dual characteristics of diamond and nano-particles, and the carbon surface is extremely susceptible to chemical modification, and can be compatible with any polar medium.
In addition, nano-diamond has super hard, large specific surface area, spherical shape and surface Porous and other characteristics, therefore, nano-diamond can be formulated into an ideal polishing liquid, which is widely used in the ultra-precision polishing process of hard materials
.

At present, Europe, the United States, Japan, etc.
are at the forefront of nano-diamond polishing liquid preparation technology, while the domestic research on polishing liquid has just started, and there is still a lot of room for development
.
However, nano-diamonds are easy to agglomerate, and the dispersion stability in the medium is very poor.
This is because nano-diamonds have a large specific surface area and high surface energy
.
Therefore, the dispersion of nanodiamonds has become a common technical problem in the world
.

Diamond slurry

At present, the most mature application is alumina polishing liquid, but cerium oxide also has its advantages in the field of silicon wafer polishing, and the application and promotion of nano-diamond polishing liquid still needs certain development.
Silica sol polishing liquid is more advanced in today's preparation technology.
After being mature, it will undoubtedly have a broad application space
.

However, in terms of current application effects, chemical mechanical polishing with silica sol as abrasive can basically achieve lower surface roughness, but the material removal rate will be lower than that of abrasives with greater hardness, and further improvement is needed.
Modification of abrasives to enhance the performance of certain aspects of a single abrasive has also become a major trend
.

According to the characteristics of the polished workpiece, by improving the particle size, hardness and PH properties of the silica sol, the silica sol can face a wider range of application requirements.
The application prospect of the silica sol in the field of chemical mechanical polishing is very broad
.

Reference source:

1.
Research on silica sol chemical mechanical polishing fluid, Xia Lin, Henan University of Technology;

2.
Research on the influence of silica sol on single crystal SiC chemical mechanical polishing, Xu Peijie, Guangdong University of Technology;

3.
Preparation of modified silica sol and its effect on sapphire polishing performance, Zhang Lei, Hefei University of Technology;