Kevin Cummings

Projection optics for extreme ultraviolet lithography (EUVL) micro-field exposure tools (METs) with a numerical aperture of 0.5

In support of the Extreme Ultraviolet Lithography (EUVL) roadmap, a SEMATECH/CNSE joint program is under way to develop 13.5 mn R and D photolithography tools with small fields (micro-field exposure tools [METs]) and numerical apertures (NAs) of 0.5. The transmitted wavefront error of the two-mirror optical projection module (projection optics box [FOB]) is specified to less than 1 mn root mean square (RMS) over its 30 μm x 200 μm image field. Not accounting for scatter and flare losses, its Strehl ratio computes to 82%. Previously reported lithography modeling on this system [1] predicted a resolution of 11 mn with a k-factor of 0.41 and a resolution of 8 mn with extreme dipole illumination. The FOBs magnification (5X), track length, and mechanical interfaces match the currently installed 0.3 NA FOBs [2] [3] [6], so that significant changes to the current tool platforms and other adjacent modules will not be necessary. The distance between the reticle stage and the secondary mirror had to be significantly increased to make space available for the upgraded 0.5 NA illumination modules [1].

SEMATECH produces defect-free EUV mask blanks: defect yield and immediate challenges

Availability of defect-free reflective mask has been one of the most critical challenges to extreme ultraviolet lithography (EUVL). To mitigate the risk, significant progress has been made on defect detection, pattern shifting, and defect repair. Clearly such mitigation strategies are based on the assumption that defect counts and sizes from incoming mask blanks must be below practical levels depending on mask specifics. The leading industry consensus for early mask product development is that there should be no defects greater than 80 nm in the quality area, 132 mm x 132 mm. In addition less than 10 defects smaller than 80 nm may be mitigable. SEMATECH has been focused on EUV mask blank defect reduction using Veeco Nexus TM IBD platform, the industry standard for mask blank production, and assessing if IBD technology can be evolved to a manufacturing solution. SEMATECH has recently announced a breakthrough reduction of defects in the mask blank deposition process resulting in the production of two defect-free EUV mask blanks at 54 nm inspection sensitivity (SiO2 equivalent). This paper will discuss the dramatic reduction of baseline EUV mask blank defects, review the current deposition process run and compare results with previous process runs. Likely causes of remaining defects will be discussed based on analyses as characterized by their compositions and whether defects are embedded in the multilayer stack or non-embedded.

Novel resist approaches to enable EUV lithography in high volume manufacturing and extensions to future nodes

EUV lithography is needed by the semiconductor industry for both its resolution and for the process simplification it provides compared to multiple patterning. However it needs innovations to make it a success. One area where innovation is needed is resist performance. Resists that are commercially available for EUV use are typically based on conventional chemically amplified resist chemistry. So far, this has not provided the required performance at fast enough photo speed. Many innovative resist systems have been introduced in the last few years that have novel mechanisms and/or incorporate novel chemical elements with high EUV absorbance. These new systems are promising enough for EUV use that work on many of them now needs to shift to characterizing their functional parameters and optimizing their performance. For the future, new systems beyond these will have to focus on reducing the inherent noise in resist imaging. The concept of pixelated resists is introduced and it is suggested pixelated resists are one possible avenue for imaging sub 10nm features with sufficient feature size and profile control.

Update on the SEMATECH 0.5 NA Extreme-Ultraviolet Lithography (EUVL) Microfield Exposure Tool (MET)

In support of the Extreme Ultraviolet Lithography (EUVL) roadmap, a SEMATECH/CNSE joint program is underway to produce multiple EUVL (wavelength of 13.5 nm) R&D photolithography tools. The 0.5 NA projection optic magnification (5X), track length and mechanical interfaces match the currently installed 0.3 NA micro-field exposure tools (MET) projection optic [1] [2] [3]. Therefore, significant changes to the current tool platforms and other adjacent modules are not necessary. However, many of the existing systems do need upgrades to achieve the anticipated smaller exposure feature sizes [4]. To date we have made considerable progress in the production of the first of the two-mirror 0.5 NA projection optics for EUVL [5]. With a measured transmitted wave front error of less than 1 nm root mean square (RMS) over its 30 μm × 200 μm image field, lithography modeling shows that a predicted resolution of ≤12 nm and an ultimate resolution of 8 nm (with extreme dipole illumination) will be possible. This paper will present an update from the 0.5 NA EUVL program. We will detail the more significant activities that are being undertaken to upgrade the MET and discuss expected performance.

Evaluation of novel processing approaches to improve extreme ultraviolet (EUV) photoresist pattern quality

Recently there has been a great deal of effort focused on increasing EUV scanner source power; which is correlated to increased wafer throughput of production systems. Another way of increasing throughput would be to increase the photospeed of the photoresist used. However increasing the photospeed without improving the overall lithographic performance, such as local critical dimension uniformity (L-CDU) and process window, does not deliver the overall improvements required for a high volume manufacturing (HVM). This paper continues a discussion started in prior publications [Ref 3,4,6], which focused on using readily available process tooling (currently in use for 193 nm double patterning applications) and the existing EUV photoresists to increase photospeed (lower dose requirement) for line and space applications. Techniques to improve L-CDU for contact hole applications will also be described.

Projection optics for EUVL micro-field exposure tools with 0.5 NA

In last year’s report, we discussed the design and requirements of the optical projection module (Projection Optics Box [POB]) for the 0.5-NA Micro-field Exposure Tool (MET5) and the resulting challenges. Over the course of this past year, we have completed and fully qualified the metrology of individual mirrors. All surface figure errors have been measured over seven orders of magnitude with spatial periods ranging from the full clear aperture down to 10 nm. The reproducibility of the full aperture tests measures 16 pm RMS for the M1 test and 17 pm for the M2 test with a target of 30 pm for both tests. Furthermore, we achieved excellent results on scatter and flare: For scatter, both mirrors perform about a factor of two below specification. For flare, the larger M2 mirror performs well within and the smaller M1 mirror about a factor of two below specification. In addition, we have developed processes for correcting surface figure errors for both mirrors and have successfully demonstrated high-reflectivity coatings on pathfinder mirrors. Further, we have achieved significant goals with respect to the design, assembly, metrology and alignment of the projection module. This paper reviews this progress and describes the next step in the ambitious MET5 POB development program.

First results of outgas resist family test and correlation between outgas specifications and EUV resist development

In this paper, we present the first results of witness sample based outgas resist family test to improve the efficiency of outgas testing using EUV resists that have shown proven imaging performance. The concept of resist family testing is to characterize the boundary conditions of outgassing scale from three major components for each resist family. This achievement can significantly reduce the cost and improve the resist outgas learning cycle. We also report the imaging performance and outgas test results of state of the art resists and discuss the consequence of the resist development with recent change of resist outgassing specifications. Three chemically amplified resists selected from higher outgassing materials are investigated, but no significant improvement in resist performance is observed.

In-situ optical testing of exposure tools via localized wavefront curvature sensing

We present a new form of optical testing for exposure tools based on measuring localized wavefront curvature. In this method, offset monopole illumination is used to probe localized regions of the test optic pupil. Variations in curvature manifest as focus shifts, which are measured using a photodiode-based grating-on-grating contrast monitor, and the wavefront aberrations are reconstructed using a least-squares approach. This technique is attractive as it is independent of the numerical aperture of the system and does not require a CCD or a separate interferometer branch.

Mask Technology for Optical Lithography

This PDF file contains the editorial “Mask Technology for Optical Lithography” for JM3 Vol. 3 Issue 02

Application of differential phase contrast imaging to EUV mask inspection: a numerical study

We demonstrate numerically that oblique off-axis illumination could enhance the contrast and extend the depth of focus of EUV phase defects detection. In addition to quantitative observation, it also allows us to extract the resolution-limited defect phase profiles quantitatively. This scheme can be easily implemented in both full field and scanning mask inspection tools.