Extreme Ultra-Violet Lithography (EUVL) keeps hurting my brain. Just when I can understand how it could be used in profitable commercial high-volume manufacturing (HVM) I hear something that seriously strains my brain. First it was the mirrors and mask in vacuum, then it was the resist and pellicle, then it was the source power and availability, and in each case scientists and engineers did amazing work and showed a way to HVM. Now we hear that EUVL might require fabs to park work-in-progress (WIP) lots of wafers behind a single critical tool with an idealistic 80% availability on a good day, and lots of downtime bad days. Horrors!
For “5nm-node” designs the maximum allowable edge placement-error (EPE) in patterning overlay is only 2nm. While the physics of ~13.5nm wavelength EUVL means that aberration in the reflecting mirrors appears as up to 3nm variation in the fidelity of projected patterns. This variation can be measured and compensated for at the physical mask level, but then each mask would only be good for one specific exposure tool. John Sturtevant—SPIE Fellow, and director of RET product development in the Design to Silicon Division at Mentor Graphics—briefly discussed this on February 26th during Nikon LithoVision held just before SPIE Advanced Lithography.
Sturtevant explained that the Zernike coefficients for EUV are inherently almost 1 order-of-magnitude higher than for DUV at 193nm wavelength, as detailed in the SemiMD article “Edge Placement Error Control in Multi-Patterning.” How the inherent physical sources of aberration must be tightened to avoid image distortion and contrast loss as they scale with wavelength was discussed by by Fenger et al. in 2013 in the article “Extreme ultraviolet lithography resist-based aberration metrology” (doi:10.1117/1.JMM.12.4.043001).
As discussed in my last Ed’s Threads, lithography has become patterning as evidenced by first use of Self-Aligned Quadruple Patterning (SAQP) in High Volume Manufacturing (HVM) of memory chips. Meanwhile, industry R&D hub imec has been investigating use of SAQP for “7nm” and “5nm” node finFET HVM, as reported as SPIE-AL this year in Paper 9782-12.
The specifications for pitches ranging from 18 to 24 nanometers are as follow:
7.0nm Critical Dimension (CD) after etch,
0.5nm (3sigma) CD uniformity (CDU), and
<1nm Line-Width and Line-End Roughness (LWR and LER) assuming 10% of CD.
“Pitch walk”—variation in final pitch after multi-patterning—results in different line widths, and can result in subsequent excessive etch variation due to non-uniform loading effects. To keep the pitch walk in SAQP at acceptable levels for the 7nm node, the core-1 CDU has to be 0.5nm 3sigma and 0.8nm range after both litho and etch. In other presentations at SPIE-AL this year, the best LER after litho was ~4nm, improving to ~2nm after PEALD smoothing of sidewalls, but still double the desired spec.
The team at imec developed a SAQP flow using amorphous-Carbon (aC) and amorphous-Silicon (aSi) as the cores, and low-temperature Plasma-Enhanced Atomic-Layer Deposition (PEALD) of SiO2 for both sets of spacers. Bilayer DARC (SiOC) and BARC were used for reflectivity control. Compared to SAQP schemes where the mandrels are only aSi, imec claims that this approach saves 20% in cost due to the use of aC core and the elimination of etch-stopping-layers.
Once upon a time, lithographic (litho) processes were all that IC fabs needed to transfer the design-intent into silicon chips. Over the last 10-15 years, however, IC device structural features have continued to shrink below half the wavelength of the laser light used in litho tools, such that additional process steps are needed to form the desired features. Self-Aligned Double Patterning (SADP) schemes use precise coatings deposited as “spacers” on the sidewalls of mandrels made from developed photoresist or a sacrificial material at a given pitch, such that after selective mandrel etching the spacers pitch-split. SADP has been used in HVM IC fabs for many years now. Self-Aligned Quadruple Pattering (SAQP) has reportedly been deployed in a memory IC fab, too.
An excellent overview of the patterning complexities of SAQP was provided by Sophie Thibaut of TEL in a presentation at SPIE-AL on “SAQP integration using spacer on spacer pitch splitting at the resist level for sub-32nm pitch applications.” Use of a spacer-on-spacer process flow—enabled by clever combinations of SiO2 and TiO2 spacers deposited by Atomic Layer Deposition (ALD)—requires the following unit-process steps:
1 193i litho,
2 ALD spacers,
2 wet etches, and
4 plasma etches.
Since non-litho processes dominate the transfer of design-intent to silicon, from first principles we should consider such integrated flows as “patterning.” Etch selectivity to remove one material while leaving another, and deposition dependent on underlying materials determine much of the pattern fidelity. Such process flows are new to IC fabs, but have been used for decades in the manufacturing of Micro-Electrical Mechanical Systems (MEMS), though generally on a patterning length scale of microns instead of the nanometers needed for advanced ICs. R&D labs today are even experimenting with Self-Aligned Octuple Patterning (SAOP), and based on the legacy of MEMS processing it certainly could be done.