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  • Writer's pictureQingzhou Cui

60 Years of Photoresist Materials Part 7: the End of Optical Photolithography and Outlook

Updated: Mar 29, 2020

After delayed many times, EUV (@13nm) photolithography finally arrived and new EUV steppers from ASML are being installed for major chip makers including TSMC, Samsung, and Intel. It is believed that the next process nodes including 7nm and 5nm will be based on these new EUV tools. But I do believe that as the new EUV technology takes the driver seat for the semiconductor industry, the DUV technology and even I-line lithography will hang around for quite a while. This is partially due to the murky economic future associated with the high cost from the new EUV technology in addition to a number of technical challenges.

The development of new EUV resists remains to be one of such issues. There are several material candidates at this stage. Traditional photoresist suppliers are more comfortable with the polymer territory using chemical amplification mechanism. But one issue is that PAG diffusion is even more challenging to overcome at this smaller patterning scale. So PAG-binding polymers makes a lot of technical sense to limit PAG diffusion. In addition, large polymer molecules may be no longer preferred due to thinner film thickness and smaller printing features. Smaller molecules will be needed compared to those traditional photoresist materials. Irresistible materials has been working on small molecules for years as new EUV photoresists. In order to reduce line edge roughness, PAG diffusion length need to be controlled, as may require polymer resin with higher Tg, shorter diffusion PAG, and more efficient Quencher.

EUV photon energy is very high so it certainly opens up some new possibilities for new EUV photoresists. Inorganic materials become possible photoresist candidates. Nanoparticles based photoresists are being investigated by Inpria. The answer to the question on "which material will emerge as the winner for the new EUV photoresist" will be unfolding in the next few years as the EUV exposure tool become mature.

Will the EUV technology become the mainstream lithography? The answer is "of course". However, technology complexity and prohibitive cost remains to be obstacles for completed EUV adoption by the industry. For the foreseeable future, we can safely imagine that the 193nm immersion photolithography will still be dominant for the IC industry. The EUV technology may be used only for some cut-edge fabrication processes.

The EUV photolithography will take us to the smaller world of for the nodes at 7nm and 5nm. I often couldn't help wondering what it will be after the EUV technology. It seems very clear to me that the great photolithography will come to the end. The 13.5nm EUV photolithography will conclude the great optical lithography age. The Moore’s Law has died. As a matter of fact, it has been dead for many years since further scaling couldn't keep up the further demand from technology world. We all are living in post-Moore age.

In the post-Moore age, new technology innovations will no longer be from the traditional front-end process. Instead, new technologies such as wafer level, fan-out, and 3D packaging will be the dominant forces for technology revolution. As a result, the focus of the semiconductor industry is shifting to the packaging process. We may see this transition more clearly in the next 5 years. New packaging technologies will play more and more vital roles and become the key for the success for the industry. A clear trend that we seeing now that these large chip-makers are shifting their resource and R&D manpower to new packaging technologies. They are gaining advantages in developing new packaging technologies over those traditional packaging houses owing to their better position in the IC supply chain.

New photo-imageble polymeric dielectric materials (as photoresists) will be the key element for the success of these new packaging technologies. It will remain to be an active area for material scientists and engineers for the next decade to come. The development of new photo-imageble polymeric dielectric materials will become more critical than new photoresist innovation, which is new to the electronic material world.

In summary, we have discussed the main-stream photoresist materials for semiconductor industry in this series of blogs. We must also know that there are many other photo-imageble materials available for processes such as PCB, IC, MEMS, display industries, etc. These photoresists were usually developed from epoxy, acrylic, phenolic, silicone polymers, etc. They are being applied for electroplating, lift-off, adhesion process, etc. As non-mainstream photoresists involved from the four generations of photoresists (as we discussed), they take significant market share and certainly contribute to form this versatile photoresist material family.

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