SU-8 photoresist is a negative, epoxy resin type, near ultraviolet (350nm~400nm) photoresist that can be exposed to electron beams, X-rays, especially I-line (365nm) ultraviolet light for lithography. It has low light absorption in the near ultraviolet range, so the exposure obtained by the entire photoresist layer is uniform and consistent, resulting in thick film patterns with vertical sidewall shapes and high aspect ratios. It is commonly used in low-cost UV-LIGA technology as a mold for electroplating microstructures; SU-8 adhesive also has good mechanical properties, chemical corrosion resistance, thermal stability, and biocompatibility. It can form complex patterns and can be used as a structural material for MEMS devices such as microfluidics, micro optics, and biochemical chips. It can also be used in fields such as chip packaging.

The typical process of LIGA/UV-LIGA technology based on SU-8 photoresist is: cleaning and depositing seed layers such as Ti, Cr/Au on the substrate (which may contain IC or other microfabrication components), spin coating SU-8, and a single spin coating can reach a thickness of several hundred microns. Pre drying (65 ℃, 95 ℃), X-ray or near UV exposure, post drying (PEB, 65 ℃, 85 ℃~95 ℃), development, hard drying, and die hardening (150 ℃~200 ℃, optional). Electroforming metals such as Ni, Cu, FeNi, etc., grinding flat (optional) to remove the seed layer from the SU-8 photoresist mold.

SU-8 adhesive is made by dissolving multifunctional and multi branched organic epoxy adhesive E-PONSU-8 in organic liquid (GBL or cyclopentanone) and adding a small amount of photoinitiator (triarylsulfide). When exposed, photons are absorbed and the photoinitiator undergoes a photochemical reaction, producing a strong acid. During post baking (PEB), the exposed area undergoes cross-linking under the catalytic action of strong acid, and the molecules grow in a chain like manner, expanding to form a cross-linked network; If subjected to further hard baking, a dense fully crosslinked network is formed, which is insoluble in organic developer and can resist swelling. However, at the same time, this highly cross-linked network makes it difficult to remove the SU-8 photoresist mold, especially without damaging the electroplated metal structure and other microfabrication components. This greatly restricts the integrated manufacturing capability of SU-8LIGA technology with IC or other microfabrication processes, and also has significant limitations on the processing of fine components, that is, the photoresist inside small slits and holes is also difficult to completely remove.

The problem of SU-8 debonding is a major challenge. The semiconductor photoresist removal process can generally be divided into two types: traditional wet photoresist removal and advanced dry plasma photoresist removal. Both methods remove photoresist through chemical reactions, and the reactions are isotropic.

Principle of Su-8 plasma debonding

Compared with the traditional wet de gluing method, the dry de gluing method, also known as plasma de gluing, has the advantages of high de gluing ash rate and high reliability. The characteristic of its process is that it requires vacuum chamber reaction through plasma and gas diffusion. The gas in plasma state has the following characteristics: strong chemical activity of gas in plasma state, different gases and materials have selectivity towards each other, and the selected gases and materials can react quickly and achieve chemical removal of materials.

SU-8 photoresist is an organic compound composed of elements such as C, H, O, N, etc. Their molecular structures are composed of long-chain carbon, hydrogen, and oxygen. The oxygen plasma stripping process uses oxygen ionization to generate a plasma mixture of oxygen ions, free oxygen atoms, oxygen molecules, and electrons. The free oxygen atoms with strong oxidizing ability react with the photoresist film under the action of a radio frequency source to produce CO2 and H2O, O2 → O *+O *, CxHy+O * → CO2 ↑+H2O ↑, which are then pumped away by a vacuum pump to ultimately remove the photoresist layer. This process is commonly known as ashing process.

In summary, the principle of SU-8 plasma de coating is to use oxygen introduced into the plasma de coating system to generate reactive oxygen species under the action of a high-frequency electric field, rapidly oxidizing the photoresist into volatile carbon dioxide, water, and other gases, thereby achieving the purpose of de coating. Compared to wet methods, dry methods for removing glue do not require the use of chemical reagents, are simpler, and have stronger operability.