In the last post in this series, we discussed how photoresist strip processes can be grouped into two different broad categories:
(1) Processes such as RCA Clean or Piranha Strip that involve chemically modifying the resist via oxidation in order to convert the resist material into water-soluble products, and
(2) Processes that rely on solvation coupled with chemical reaction with amines to form new bonds in order to dissolve the resist and related residue.
That blog post focused on the solvation/bond formation (via amines) route to bulk photoresist and etch residue removal on aluminum with Gen 1 and Gen 2 removers. This post will discuss ash and post-etch residue removal on aluminum and copper substrates via fluoride chemistry; that is, Gen 3 removers.
As previously discussed, Gen 1 removers consisted of a polar organic solvent and a small amount of an amine and possibly a surfactant. This type of formulation resulted in good removal of organic material, but not of inorganic residue. Meanwhile, Gen 2 removers have as their primary component an amine (hydroxyl amine or equivalent), a secondary component of a polar organic solvent, water as the primary formulary solvent, and a corrosion inhibitor. We discussed the cost advantages to Gen 1 given small CDs and defect concerns were of low priority. The more advanced Gen 2 can remove both organic residue and various inorganic/metallo-organic residues. While Gen 2 was an advance over the Gen 1 removers, there were some problems not yet resolved, or only partially so, for example: copper substrate compatibility, improved post-etch residue removal, single-wafer tool processing capability, and reduced dependence on water content. These problems were addressed by the Gen 3 removers and are the topic of this post.
Gen 3 removers have moved away from amine chemistry toward a fluoride-based chemistry, thus removing whatever anxiety remained in the industry concerning the single source of hydroxyl amine manufacture. However, and more to the point, the use of fluoride chemistry has very effectively solved the other problems of water concentration, processing tool type, and improved inorganic residue removal. The Gen 3 formulations generally consist of five parts: polar organic solvent, a fluoride salt, an organic acid, an organic salt (buffering agent), and water. The organic solvent and water serve the same function in Gen 3 as in Gen 2: organic residue dissolution and inorganic salts dissolution, respectively. The new features are the fluoride salt, which mildly reacts with oxides of aluminum, and the organic acid which also mildly reacts with oxides of aluminum and also of copper. In these two latter cases, the mild reaction enables dissolution into water. One important caveat about Gen 3: there is the assumption with their use that much of the bulk removal has already taken place. Gen 3 removers are designed primarily as an etch- and ash-residue removal product.
Some examples of Gen 3 removers are Air Products’ ACT NE, XT and EZStrip 500 Series removers. Like the Gen 1 and 2 removers, these Gen 3 removers can be used in bench/batch immersion and batch spray, but significantly, also in single wafer track processes. An example of Air Products’ ACT NE-14 remover in an immersion process and one from a multi-wafer spray process (Semitool Spray Solvent Tool) are shown below. It is very clear how effective this remover is in stripping away the etch residue shown in the “Before” picture, regardless of the processing conditions used.
The use of such high quality/highly effective strippers can save you time and improve the defect levels of your manufacturing operations.