Silicon wafers are important raw materials for the manufacture of chips and various semiconductor devices. Wafers usually have a very flat, smooth surface, like a mirror. Why do manufacturers make silicon wafers so smooth?

The ultimate fate of the wafer is to be cut into die and encased in a dark box with only a few pins visible. People usually value the threshold, resistance, current value and voltage value of the die, but generally do not look at its appearance level. In the production process, we repeatedly polish and polish the wafer, mainly to meet the flatness needs in production, especially in each lithography, the surface of the wafer must be extremely flat,This is because as the size of the chip shrinks, the lens of the lithography machine must increase the numerical aperture of the lens in order to achieve the imaging resolution of the nanometer level. However, this will also lead to a decrease in focal depth, focal depth refers to the depth of focus of optical imaging, in order to ensure that the lithography image is clear without losing focus, the rise and fall of the wafer surface must fall within the range of focal depth.

Simply put, the lithography machine sacrifices the focusing ability in order to improve the imaging accuracy, like the new generation of EUV lithography machine, the numerical aperture is 0.55, but the focal depth in the vertical direction is only 45 nanometers, and the optimal imaging interval during lithography will be smaller. If the wafer placed on is not flat enough, the thickness is not average, and the surface is undulating, it will cause problems in the lithography at high and low levels.

Of course, it is not only lithography that requires the silky surface of the wafer, but also many chip making processes that need to polish the wafer, such as: grinding after wet etching to smooth the corroded rough surface, which is convenient for gluing and glue deposition. After shallow groove isolation, polish to remove excess silicon oxide and complete groove filling. After the metal is deposited, it should be polished to remove the spilled metal layer and prevent the device from short circuit. Therefore, the birth of a chip must undergo many times of grinding in the middle to reduce the roughness of the wafer and remove excess material on the surface. In addition, the process problems that may occur during the production of wafers, resulting in surface defects, often have to wait until each grinding is completed before being exposed. So, what are the polishing methods for wafers?

Polishing process can be divided into the following three categories:

1. Mechanical polishing
The principle of mechanical polishing is the same as the grinding process, but the abrasive used will be finer. The surface flatness of silicon wafers polished by mechanical polishing is generally relatively high, but the damage caused by mechanical action is more serious, and the damage layer is deep (equivalent to several times of the particles). If very fine abrasive is used, it will cause the polishing speed to become very slow, and the mechanical polishing method is generally no longer used in industry.

2.  Chemical polishing
The chemical polishing of silicon wafer is commonly carried out with HN03-HF(nitric acid, hydrofluoric acid) etch solution.  The surface of the chemically polished silicon wafer can be almost without damage, and the polishing speed is relatively high, but the flatness is poor, so chemical polishing is usually only used as a pre-treatment before polishing in industrial production, rather than as a polishing process alone.

3. Chemical Mechanical Polishing (CMP)
The first two polishing methods have their own unique advantages, if the two methods are combined, you can achieve a complementary effect in the process. Chemical mechanical polishing method uses the dual effect of polishing liquid on the surface of silicon wafer mechanical grinding and chemical corrosion, and has the advantages of both mechanical polishing and chemical polishing. CMP is one of the technologies developed by the industry to manufacture large-diameter wafers, and it is the most commonly used polishing method in the modern semiconductor manufacturing industry.

The polishing liquid used in CMP is a colloidal solution formulated by the polishing powder and sodium hydroxide solution, where the polishing powder usually uses Si02 or Zr02(zirconia), rather than the use of too hard materials.