In the semiconductor chip manufacturing process, besides surface cleaning and activation, another major application of plasma cleaning lies in the removal of specific materials, including common organic and inorganic contaminants. Among these, photoresist stripping is one of the most frequent processes in the lithography sequence. Dry plasma photoresist stripping is a novel photoresist removal process developed as a replacement for wet stripping, and it is also an early application of low-temperature plasma in microelectronic device fabrication. Most of the products obtained after treating the photoresist with plasma are non-toxic and harmless gases such as carbon dioxide and water vapor, which reduces the process cost and environmental pollution in actual production. The dry plasma stripping process has become increasingly mature. By exciting abundant and active species such as free radicals and charged particles in the plasma, the dry stripping rate, uniformity and process reliability have surpassed those of most conventional wet processes.

The microscopic mechanism of dry plasma photoresist stripping is mainly related to the characteristics of the plasma itself, and likewise participates through both physical action and chemical reaction. Under the acceleration of the sheath electric field, charged particles produce physical sputtering on the photoresist surface; through the momentum transfer of ions, the chemical bonds on the polymer surface are broken, thereby removing the photoresist material. However, the thermal effect caused is obvious and can easily burn the material. In contrast, the removal of photoresist achieved by active free radicals combining with polymer molecular bonds through chemical reaction to generate volatile products is milder. According to the Arrhenius equation, given the high dependence of the rate constant on temperature, photoresist stripping often needs to be carried out under high substrate temperature conditions (>200 ℃), which can easily cause substrate damage and photoresist denaturation. The role of electrons in the stripping process is mainly to assist the reactions of other particles in the plasma. For example, collisions between electrons and atoms or molecules in the plasma can generate excited neutral atoms or free radicals; electrons attach to the surface of the contaminated layer, accelerating cation collisions and further triggering activation reactions. During plasma ashing, free radicals mainly participate in chemical reactions: active free radicals combine with photoresist molecules to generate new free radicals while continuously decomposing themselves, until easily volatile simple small molecules are formed. The mainstream process gas for plasma stripping is mainly O, and the stripping process is dominated by the chemical micro-etching of oxygen radicals (O). The main reaction equations are as follows: 

In most cases, the plasma stripping cycle consists of multiple process steps. According to the location of photoresist removal, the stripping process can be divided into bulk photoresist stripping (Stripping) and descum (Descum). Figure 1 intuitively shows the difference between these two processes under the photoresist removal cycle. As shown in Figure 1(a), the bulk photoresist stripping process usually targets the etch-stop photoresist layer remaining on the top of the lines after pattern transfer, and this process generally has low requirements for stripping directionality. Descum refers to the removal of residues on the sidewalls of patterns and at the bottom of trenches, including the photoresist residue left after development; this process has certain requirements for stripping directionality, especially for patterns with high aspect ratio, as shown in Figure 1(b).

Figure 1 Photoresist stripping process flow: (a) bulk photoresist stripping; (b) residue removal; (c) pattern profile after cleaning

Photoresist polymers with different structures and chemical properties also differ in their removal effect and mechanism. Polymers containing aromatic groups are more resistant to etching than aliphatic materials, while polymers containing halogens exhibit higher etch rates, because halogens are released when the polymer degrades; the F or Cl species generated in the plasma can effectively abstract H from the polymer chain, thereby promoting the reaction with O species and the removal of the polymer. Likewise, the addition of a small amount of halogen gas can significantly increase the concentration of O atoms, further improving the removal rate. Therefore, in dry plasma stripping equipment, it is very common to add F-containing molecules (CF4, SF6 and NF3) to the O2-based stripping process gas. It is worth noting that F-containing gases have an etching effect on silicon-based materials, and it is necessary to ensure that F atoms do not etch other films or substrates and damage the substrate material, so the application scenario is relatively limited. The addition of a small amount of H2O can also increase the stripping rate. Mechanistically, H2O can increase the concentration of O atoms in the plasma and at the same time dissociate to generate OH radicals; OH can abstract H from the polymer main chain in a manner similar to F. These additives can not only improve the removal rate of the photoresist, but also change the etching selectivity toward the substrate or film material beneath the photoresist.