PhotoResist (PR), also known as photoresist, is an organic compound whose solubility in the developing solution changes after exposure to light sources such as ultraviolet light, excimer laser, electron beam, ion beam, and X-rays. Photoresist is mainly used for microfabrication of integrated circuits and semiconductor discrete devices, and also has a wide range of applications in the production processes of flat panel displays, flip flops, magnetic heads, and precision sensors. Due to the photoelectrochemical sensitivity of photoresist, it can be used for photochemical reactions to coat semiconductors, conductors, and insulators with photoresist. After exposure and development, the remaining part protects the bottom layer, and then etched with an etchant to transfer the required fine patterns from the mask to the substrate to be processed. Therefore, photoresist is a key chemical material in microfabrication technology. One of the steps after etching is to remove the photoresist, which serves as a pattern transfer medium from the photolithography mask to the surface of the silicon wafer, as well as a barrier layer in the etched or ion implanted areas. Once etching or implantation is completed, the photoresist on the silicon wafer surface is no longer useful and must be completely removed.

PhotoResist Removal

The photoresist removal technology plays a very important role in the microelectronics industry, and the quality of photoresist removal in integrated circuit manufacturing processes directly affects the yield of products and the manufacturing costs of devices and circuits. The main methods are wet de gluing and dry de gluing. Dry stripping is the process of removing photoresist using oxygen plasma ashing. Dry de gluing has incomparable advantages over wet de gluing and is widely used in modern semiconductor industry.

Dry plasma debonding

Plasma, also known as plasma, is an ionized gaseous substance composed of atoms with partially stripped electrons and positively and negatively charged particles produced by ionization of atoms. There are always some trace amounts of free electrons in gases that accelerate under the action of an external electric field. When electrons gain sufficient energy in an external electric field and collide with gas molecules, they can ionize the gas molecules and emit secondary electrons. These secondary electrons can further collide and ionize with the gas molecules, producing more electrons and ions. At the same time, due to the reverse process of ionization and recombination between electrons and ions, these two processes will eventually reach an equilibrium state, resulting in a stable glow discharge phenomenon and the formation of a stable plasma.

During the discharge process, in addition to forming electrons and ions, molecules, atoms, and various atomic groups that can also generate excited states are collectively referred to as free radicals. These free radicals have high chemical activity because they react chemically with the surface of the corroded material to form volatile products that continuously corrode the material.

As the etched object, photoresist can be understood as a long-chain organic polymer composed of elements such as carbon, hydrogen, oxygen, and nitrogen. Dry etching mainly uses oxygen plasma to ashing the photoresist.

O2 does not significantly erode photoresist at room temperature, but in a plasma stripping machine, oxygen is ionized into reactive plasmas such as O2-, O2+, O -, O+, oxygen atoms O, ozone, etc. Among them, oxygen atoms have high energy and can rapidly oxidize the photoresist on the substrate with the assistance of a high-frequency electric field to generate CO, CO2, H2O and other volatile oxides, which are pumped away by a vacuum pump to achieve the purpose of stripping.

CxHy＋O→CO↑＋CO2↑+H2O↑

At the same time, oxygen ions physically bombard the photoresist to destroy the surface morphology of the photoresist, remove loosely bound atoms, and enhance the desorption process of etching products, accelerating the reaction between oxygen atoms and the photoresist surface.

Oxygen plasma debonding effect

Before plasma ashing

PR thickness: 3.407 µ m

1-minute plasma ashing

PR thickness: 2.514 µ m | etching depth: 0.893 µ m

2-minute plasma ashing

PR thickness: 1.505 µ m | etching depth: 1.902 µ m

5-minute plasma ashing

PR thickness: 264.6 nm | etching depth: 3.142 µ m

7-minute plasma ashing

PR thickness: 212.2 nm | etching depth: 3.195 µ m