As the IC fabrication came down to 90 nanometer process node, ArF excimer laser (193nm) photolithography became the main stream of technology for IC fabrication. The t-BOS-PHOST photoresist (for KrF @248nm) could no longer meet the new photolithography requirements due to strong absorption from aromatic group. At the time, new ArF photoresist materials had been studied for years and was ready to be implemented for the new ArF process.
Before going to technical details, I want to clarify a few things on this topic. First, the ArF 193nm photolithography is further divided into dry photolithography and immersion photolithography. I am focusing on the dry 193nm photoresists in this blog and will discuss the immersion materials for the next blog. Second, the 193nm photoresists are still the “go-to” material for the smallest nodes of 14nm and 10nm. The photoresists are still under active development and the commercially available products are much more diversified compared to early generations of photoresists. I will try to jog down what I understand for the field, even though it is not the best time to write a review on the topic yet. Third, for the ArF photoresists there are still a lot of intellectual properties/trade secrets in this tight technology space. However, the product differential factor from different suppliers is rather trivial. I hope what I will write will not step on anyone’s foot. If it does happen, please let me know.
The material design ideas for 193nm photoresists were inherited from the KrF photoresists. Chemical amplification, capping agents, positive-tone are very common for major commercial ArF photoresists. High resolution, dry etching resistance, adhesion, etc. are key parameters for such ArF photoresists. Non-aromatic polymer systems such as alicyclic-modified methacrylates and cyclic olefin maleic anhydride (COMA) backboneshad been investigated. IBM designed a well-known 193nm photoresist based on terpolymer of methylmethacrylate, t-butylmethacrylate, and methacrylic acid (shown in Figure 1). Each component serves different purpose in the photoresist film: methylmethacrylate provides hydrophilicity and adhesion for the film and it also help to limit acid diffusion into dark area; t-butylmethacrylate is the active site for PAG/resist interaction and the cleavage of the protection group offers dissolution contrast for patterning; methacrylic acid may provide decent material glass transition temperature, tune formulation solubility, and adjust aqueous solubility in later development process. With adjustment of ratio of components during resin synthesis, the material property can be finely tuned, to a certain extend.
Figure 1. Polymer backbone as the general platform for ArF photoresists.
The issue for the prototype IBM material was quickly identified for its poor dry etch resistance. To circumvent the issue, other polymer bones, such as cyclo-olefin and maleic anhydride, were actively explored. However, these new polymer backbones face challenges such as too high glass transition temperatures (difficult for film forming), organometallic catalysts needed for resin synthesis, etc. On the other hand, the IBM terpolymer gained popularity after improvement. To enhance etch resistance, some carbon-rich functionalities (adamantine, norbornane, and tricyclodecyl groups as shown in Figure 2) were incorporated into the polymer backbone in form of acid-detachable groups. These new side chains improve dry etch resistance significantly. However, these carbon-rich groups usually deteriorate film adhesion due to their hydrophobic nature. To enhance film adhesion, lactone structure or alcoholic hydroxyl may be introduced.
Figure 2. Carbon-rich groups: adamantine, norbornane, and tricyclodecyl.
The methacrylate backbone by IBM becomes a platform for further development of major ArF photoresists. Different photoresist manufacturers take different material routes for side chains to improve dry-etch resistance. These side-chains are often the differential factor for ArF photoresists from different suppliers. The following figure 3 shows a few examples for different side chains used by some major photoresist suppliers. These resins, along with PAG, solvent, additives (dissolution inhibitors, surfactants, etc.), formed the most advanced photoresists for the cutting-edge IC processes for the current semiconductor industry.
Figure 3. Different polymer resins are used in commercial ArF photoresists. (this figure was taken off site due to some IP issues.)