Current Challenges and Opportunities for EUV Lithography
Thirty years ago, the first glimmers of “Soft X-ray Projection Lithography” were achieved by ambitious and far-sighted researchers. Steady progress has overcome many difficult barriers, and the renamed “Extreme Ultra-Violet (EUV) Lithography” is on the brink of High Volume Manufacturing (HVM) applications. This presentation will consider some of the remaining challenges for the next several years. Since an EUV image has only 1/14th as many photons of a 193 image with the same power, stochastic noise is a fundamental EUV concern. This challenge is being addressed by more powerful EUV sources, improved resist processes, advanced etches and other LER smoothing processes. Edge Placement Errors must be reduced to nm levels, which will require advanced data-prep methods to overcome asymmetric 3D mask effects and lens aberrations. Mask defects remain a worrisome issue, and EUV pellicles are highly desired to protect against particles which might fall onto the mask pattern. These pellicle membranes must withstand strong EUV radiation in an H2 environment, along with the strength for normal mask handling and exposure operations. Another mask defect mitigation technique is vote-taking lithography, where several defective masks are used to form a net image with fewer defects. One of the most fundamental challenges for all successful HVM methods is economic viability, and the EUV expose tool throughput is a key metric. Future EUV lithography resolution improvements may be enabled by higher NA projection optics and other advances, but productivity roadmaps must not ignore the need for increased exposure dose to meet fundamental stochastic noise limits.