Dong-gun Lee

Progress in EUV lithography toward manufacturing

In this article the recent progress in the elements of EUV lithography is presented. Source power around 205W was demonstrated and further scaling up is going on, which is expected to be implemented in the field within 2017. Source availability keeps improving especially due to the introduction of new droplet generator but collector lifetime needs to be verified at each power level. Mask blank defect satisfied the HVM goal. Resist meets the requirements of development purposes and dose needs to be reduced further to satisfy the productivity demand. Pellicle, where both the high transmittance and long lifetime are demanded, needs improvements especially in pellicle membrane. Potential issues in high-NA EUV are discussed including resist, small DOF, stitching, mask infrastructure, whose solutions need to be prepared timely in addition to high-NA exposure tool to enable this technology.

Electro-optical system for scanning microscopy of extreme ultraviolet masks with a high harmonic generation source

A self-contained electro-optical module for scanning extreme ultraviolet (EUV) reflection microscopy at 13.5 nm wavelength has been developed. The system has been designed to work with stand-alone commercially available EUV high harmonic generation (HHG) sources through the implementation of narrowband harmonic selecting multilayers and off-axis elliptical short focal length zoneplates. The module has been successfully integrated into an EUV mask scanning microscope achieving diffraction limited imaging performance (84 nm point spread function).

Correction of Critical Dimension Uniformity on a Wafer by Controlling Back-Surface Transmittance Distribution of a Photomask

We report on a new method for correcting the critical dimension (CD) uniformity on a wafer, using a transmittance-controlled photomask with phase patterns on the back. Theoretical analysis of changes in the illumination-pupil shape with respect to the variation of the size and density of the back-surface 180°-phase contact-hole patterns and experimental results using the transmittance-controlled photomask are presented. It is shown that pattern size on the back of the photomask must be made as small as possible in order to keep the illumination-pupil shape as close as possible to the original one and to achieve as large an illumination intensity drop as possible at the same pattern density. It is demonstrated that the method is very effective for correcting the CD nonuniformity on a wafer that is induced by both CD nonuniformity of the photomask and flare in the exposure tool.

Extreme ultraviolet mask roughness: requirements, characterization, and modeling

It is now well established that extremely ultraviolet (EUV) mask multilayer roughness can lead to wafer-plane line-edge roughness (LER) in lithography tools. It is also evident that this same effect leads to sensor plane variability in inspection tools. This is true for both patterned mask and mask blank inspection. Here we evaluate mask roughness specifications explicitly from the actinic inspection perspective. The mask roughness requirement resulting from this analysis are consistent with previously described requirements based on lithographic LER. In addition to model-based analysis, we also consider the characterization of multilayer mask roughness and evaluate the validity of using atomic force microscopy (AFM) based measurements by direct comparison to EUV scatterometry measurements as well as aerial image measurements on a series of high quality EUV masks. The results demonstrate a significant discrepancy between AFM results and true EUV roughness as measured by actinic scattering.

Effect on Critical Dimension Performance for Carbon Contamination of Extreme Ultraviolet Mask Using Coherent Scattering Microscopy and In-situ Contamination System

The impact of carbon contamination on imaging performance was analyzed using an in-situ accelerated contamination system (ICS) combined with coherent scattering microscopy (CSM) which was installed at 11B extreme ultraviolet lithography (EUVL) beamline of the Pohang Accelerator Laboratory (PAL). The CSM/ICS is composed of CSM for measuring imaging properties and ICS for implementing acceleration of carbon contamination. The mask critical dimension (CD) and reflectivity were compared before and after carbon contamination through accelerated exposure. The reflectivity degradation was measured as 1.3, 2.0, and 2.8% after 1, 2, and 3 h exposure, respectively, due to carbon contamination of 5, 10, and 20 nm as measured by Zygo interferometer. The mask CD change for 88 nm line and space pattern was analyzed using CSM and a CD scanning electron microscope (SEM), and the result shows CD-SEM and CSM give large difference of 3.8 times in mask ΔCD after carbon contamination. This difference confirms the importance of using actinic inspection technique that employs exactly the same imaging condition as exposure tool.

Application of Transmittance-Controlled Photomask Technology to ArF Lithography

We report theoretical and experimental results for application of transmittance-controlled photomask technology to ArF lithography. The transmittance-controlled photomask technology is thought to be a promising technique fo critical dimension (CD) uniformity correction on a wafer by use of phase patterns on the backside of a photomask. We could theoretically reproduce experimental results for illumination intensity drop with respect to the variation of backside phase patterns by considering light propagation from the backside to the front side of a photomask at the ArF lithography wavelength. We applied the transmittance-controlled photomask technology to ArF lithography for a critical layer of DRAM (Dynamic Random Access Memory) having a 110-nm design rule and found that the in-field CD uniformity value was improved from 13.8 nm to 9.7 nm in .

Application of actinic mask review system for the preparation of HVM EUV lithography with defect free mask

We introduce an extreme ultraviolet lithography (EUVL) mask defect review system (EMDRS) which has been developing in SAMUSNG. It applies a stand-alone high harmonic generation (HHG) EUV source as well as simple EUV optics consisting of a folding mirror and a zoneplate. The EMDRS has been continuously updated and utilized for various applications regarding defect printability in EUVL. One of the main roles of the EMDRS is to verify either mask repair or mask defect avoidance (MDA) by actinic reviews of defect images before and after the process. Using the MDA, small phase defects could be hidden below absorber patterns, but it is very challenging in case of layouts with high density patterns. The EMDRS clearly verify the success of the MDA while conventional SEM could not detect the images. In addition, we emulate images of the sub-resolution assist features (SRAFs) by the EMDRS and compared them with the wafer exposure results.

EUV mask readiness for HVM (Conference Presentation)

Currently, we are supplying defect-free EUV mask for device development. This was one of the biggest challenges in the implementation of EUV lithography for high volume manufacturing (HVM). It became possible to hide all multi-layer defects by using defect avoidance technique through improvement of blank mask defectivity and development of actinic blank inspection tool. In addition, EUV pellicle is also considered as a requisite to guarantee predictable yield. Both development of mask shop tools and preparation of EUV scanner for pellicle are going well. However, still membrane needs to be much improved in terms of transmittance and robustness for HVM. At the conference, EUV mask readiness for HVM will be discussed including blank defect improvement, preparation of actinic tools and pellicle development.

EUV Research at Berkeley Lab: Enabling Technologies and Applications

The tremendous progress in the development and deployment of lab scale extreme ultraviolet (EUV) sources over the past decade has opened up the door to a wide variety of new users beyond the traditional synchrotron community. The practical use of such sources, however, is heavily dependent on the availability of EUV optical components. In this manuscript, we describe recent advances at Berkeley Lab in the development of reflective and diffractive optical structures for imaging, wavefront encoding, metrology, spectral filtering, and more.