Extreme ultraviolet lithography (EUVL) and three dimensional integrated circuit (3D IC) were thoroughly reviewed. Since proposed in 1988, EUVL obtained intensive studies globally and, after 2000, became the most promising next generation lithography method even though challenges were present in almost all aspects of EUVL technology. Commercial step-and-scan tools for preproduction are installed now with full field capability; however, EUV source power at intermediate focus (IF) has not yet met volume manufacturing requirements. Compared with the target of 200 W in-band power at IF, current tools can supply only approximately 40–55 W. EUVL resist has improved significantly in the last few years, with 13 nm line/space half-pitch resolution being produced with approximately 3–4 nm line width roughness (LWR), but LWR needs 2× improvement. Creating a defect-free EUVL mask is currently an obstacle. Actual adoption of EUVL for 22 nm and beyond technology nodes will depend on the extension of current optical lithography (193 nm immersion lithography, combined with multiple patterning techniques), as well as other methods such as 3D IC. Lithography has been the enabler for IC performance improvement by increasing device density, clock rate, and transistor rate. However, after the turn of the century, IC scaling resulted in short-channel effect, which decreases power efficiency dramatically, so clock frequency almost stopped increasing. Although further IC scaling by lithography reduces gate delay, interconnect delay and memory wall are dominant in determining the IC performance. 3D IC technology is a critical technology today because it offers a reasonable route to further improve IC performance. It increases device density, reduces the interconnect delay, and breaks memory wall with the application of 3D stacking using through silicon via. 3D IC also makes one chip package have more functional diversification than those enhanced only by shrinking the size of the features. The main advantages of 3D IC are the smaller form factor, low energy consumption, high speed, and functional diversification. EUVL, if adopted, will continue to enable IC performance improvement at a slower rate, but 3D IC provides an alternative way to improve the system performance. The best scenario is the adoption of both EUVL and 3D IC. However, the possible further delay of EUVL could enhance the realization of 3D IC for IC system improvement.

Extreme ultraviolet lithography and three dimensional integrated circuit—A review

Extreme ultra violet lithography (EUVL) is no longer a future technology but is going to be inserted into mass production of semiconductor devices of 7 nm technology node in 2018. EUVL is an extension of optical lithography using extremely short wavelength (13.5 nm). This short wavelength requires major modifications in the optical systems due to the very strong absorption of EUV light by materials. Refractive optics can no longer be used, and reflective optics is the only solution to transfer image from mask to wafer. This is why we need the multilayer (ML) mirror-based mask as well as an oblique incident angle of light. This paper discusses the principal theory on the EUV mask design and its component materials including ML reflector and EUV absorber. Mask shadowing effect (or mask 3D effect) is explained and its technical solutions like phase shift mask is reviewed. Even though not all the technical issues on EUV mask are handled in this review paper, you will be able to understand the principles determining the performance of EUV masks.

Mask Materials and Designs for Extreme Ultra Violet Lithography

In this report, we propose palladium oxide (PdO) as an absorber material for an EUV mask that can print line-and-space patterns with a half pitch down to 14 nm. In our simulations, because of its low refractive index (n = 0.8634) and high extinction coefficient (k = 0.0536), an attenuated phase shift mask with a very thin (~20 nm) PdO absorber can provide an EUV contrast as high as 88% at a 14 nm half pitch under dipole illumination. This results in a very limited horizontal–vertical critical dimension bias (≤2.6 nm) and a sufficiently high normalized image log slope (≥2.78) down to a 14 nm half pitch.

https://iopscience.iop.org/article/10.7567/APEX.6.076502/pdf

Very Thin Extreme Ultraviolet Mask Absorber Material for Extremely Fine Pitch Patterning

Publisher's Note: This paper, originally published on April 17, 2014, was withdrawn at the author's request on May 25, 2016

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/9048/904811/A-novel-concept-for-actinic-EUV-mask-review-tool-using/10.1117/12.2046226.pdf

A novel concept for actinic EUV mask review tool using a scanning lensless imaging method at the Swiss Light Source (Withdrawal Notice)

In semiconductor fabrication industries, photomask cleaning with no pattern damage is a critical issue, especially for advanced technology nodes. Thus, optimization of cleaning techniques adopted for photomask cleaning or development of new cleaning technique is always of great interest to meet process requirements. In this book chapter, the sources of contaminants on photomask and their impact on imprinted images are first discussed. Then a critical review is presented on the wide spectrum of cleaning techniques and strategies that are adopted for the removal of particulate contaminants and organics from the photomask. The applicability of each technique and its limitations are also examined. Because extreme ultraviolet lithography (EUVL) is considered to be a next-generation lithography, a separate section is devoted to EUVL mask-cleaning techniques.

Contamination Removal From UV and EUV Photomasks

The mask shadowing effect is a unique phenomenon caused by a mirror-based mask structure and an oblique incident angle of light in the extreme ultraviolet lithography process. This results in a horizontal–vertical (H–V) bias, an ellipticity in the contact hole pattern and, eventually, a patterning limit and critical dimension (CD) nonuniformity. Reducing the absorber thickness is the most effective method to minimize the mask shadowing effect, but this can deteriorate the mask image contrast. In this paper, an attenuated phase shift mask (PSM) is proposed as a potential method for extending the patterning limit to below 16 nm. By applying Mo as a phase shift layer, which has a refractive index (n) similar to that of the TaN absorber but with a lower absorption value, thin attenuated PSMs with various reflectivity values were obtained by controlling the Mo thickness. The proposed PSM consists of a 16.5-nm-thick TaN absorber layer and a 24-nm-thick Mo phase shifter on 2-nm-thick Ru-capped Mo/Si multilayers. This attenuated PSM results in 17.98% and 27.52% reductions in the mask error enhancement factor as well as 3.65 and 1.84 nm reductions in the H–V CD bias compared with the conventional binary intensity mask with a 70-nm-thick TaN absorber for the 22 nm line and space 1:1 vertical dense pattern under 0.25 and 0.33 numerical aperture illumination conditions, respectively. Moreover, a 10%–12% improved image contrast was obtained with 11%–17% reflectivity on the absorber stack, which corresponds to a 24–29 nm Mo thickness under 0.25 NA illumination conditions.

Inhwan Lee

Papers

Improved imaging properties of thin attenuated phase shift masks for extreme ultraviolet lithography

Carbon contamination of EUV mask and its effect on CD performance