Uzodinma Okoroanyanwu

Assessing EUV mask defectivity

This paper assesses the readiness of EUV masks for pilot line production. The printability of well characterized reticle defects, with particular emphasis on those reticle defects that cause electrical errors on wafer test chips, is investigated. The reticles are equipped with test marks that are inspected in a die-to-die mode (using DUV inspection tool) and reviewed (using a SEM tool), and which also comprise electrically testable patterns. The reticles have three modules comprising features with 32 nm ground rules in 104 nm pitch, 22 nm ground rules with 80 nm pitch, and 16 nm ground rules with 56 nm pitch (on the wafer scale). In order to determine whether specific defects originate from the substrate, the multilayer film, the absorber stack, or from the patterning process, the reticles were inspected after each fabrication step. Following fabrication, the reticles were used to print wafers on a 0.25 NA full-field ASML EUV exposure tool. The printed wafers were inspected with state of the art bright-field and Deep UV inspection tools. It is observed that the printability of EUV mask defects down to a pitch of 56 nm shows a trend of increased printability as the pitch of the printed pattern gets smaller - a well established trend at larger pitches of 80 nm and 104 nm, respectively. The sensitivity of state-of-the-art reticle inspection tools is greatly improved over that of the previous generation of tools. There appears to be no apparent decline in the sensitivity of these state-of-the-art reticle inspection tools for higher density (smaller) patterns on the mask, even down to 56nm pitch (1x). Preliminary results indicate that a blank defect density of the order of 0.25 defects/cm2 can support very early learning on EUV pilot line production at the 16nm node.

Lithographic performance evaluation of a contaminated extreme ultraviolet mask after cleaning

The effect of surface contamination and subsequent mask surface cleaning on the lithographic performance of an extreme ultraviolet (EUV) mask is investigated. SEMATECH’s Berkeley microfield exposure tool printed 40 and 50 nm line and space (L/S) patterns are evaluated to compare the performance of a contaminated and cleaned mask to an uncontaminated mask. Since the two EUV masks have different absorber architectures, optical imaging models and aerial image calculations were performed to determine any expected differences in performance. The measured and calculated Bossung curves, process windows, and exposure latitudes for the two sets of L/S patterns are compared to determine how the contamination and cleaning impacts the lithographic performance of EUV masks. The observed differences between the two masks are shown to be well within the expected process variation of 10%, indicating that the cleaning process did not appreciably affect the mask performance.

Evaluation of EUV mask cleaning process

This paper presents results of the optimization of an EUV mask cleaning process and compares the results to data obtained on COG and EPSM masks using processes specifically designed for such masks. The key parameter investigated was cleaning efficiency, as measured in terms of Particle Removal Efficiency (PRE), CD shift and actinic reflectivity change. The PRE of 100%, 84%, and 80% was obtained for COG, EUV and HT-PSM masks, respectively. The CD change per clean cycle was 0.07nm. The feature damage limit was 50nm. Actinic reflectivity change in the range <0.1% per clean cycle was obtained for the process.

A lifetime study of EUV masks

Extreme Ultraviolet Lithography (EUVL) offers the promise of dramatically improved resolution at the price of introducing a complex web of new lithographic challenges. The most conspicuous departure from DUV lithography is that exposure the wavelength is reduced from 193 to 13.5nm. Under exposure at this short EUV wavelength, all materials absorb. Consequently the scanner optics and masks must be reflective and wafer exposure occurs in vacuum without a pellicle to protect the mask. This represents a dramatic shift from the current DUV mask use case. For example, the mask will have to be cleaned after exposure to remove contamination accumulated instead of being protected for its lifetime by a transparent pellicle. The impact of cycling through the exposure tool and being cleaned multiple times will be studied using particle inspection, scatterometry, reflectometry and AFM measurements. The results will be used to identify contamination modes and to propose best practices for EUVL mask exposure.

Monitoring reticle molecular contamination in ASML EUV Alpha Demo Tool

Molecular contamination risk to an EUV reticle exposed to up to 1600J/cm2 of 13.5 nm EUV radiation in ASML Alpha Demo Tool (ADT) is negligible. Carbon film (< 0.5 nm) deposition and oxidation (surface oxides ~1 nm) are the two main molecular contaminants observed on this EUV reticle. These results run counter to recent empirical results obtained from EUV micro-exposure tools (MET) which suggest that molecular contamination of EUV reticles, even at the very low partial pressures expected in the exposure chamber of EUV exposure tools, poses challenges in the implementation of EUV lithography in large-scale volume manufacturing of devices. To assess the molecular contamination risk to the use and lifetime of a given EUV reticle, we monitored the contamination buildup on a specially designed reticle during one year as it was exposed on ASML ADT located in Albany, New York. The reticle was analyzed with a suite of complementary surface analytical technique (such as Auger Electron Spectroscopy, AES) and chemical analytical techniques (such as Grazing Incidence Reflection Fourier Transform Infra-red Spectroscopy, GIR-FTIR), as well as imaging technique (such as Scanning Electron Microscopy). The influence of molecular contamination on the reflectivity of this reticle was measured at the Lawrence Berkeley Advanced Light Source EUV reflectometry. The differences in the contamination outcome of EUV reticles exposed in ASML ADT and MET may be related to the implementation of active contamination mitigation schemes in ADT and the lack of similar schemes in METs.

Additive-loaded EUV photoresists: performance enhancement and the underlying physics

A series of molecular glasses (MGs) protected with multiple tert-butoxylcarbonylmethyl (tBCM) groups are employed as additives to enhance extreme ultra violet (EUV) photolithographic performance of a hydroxystyrene based Environmentally Stable Chemically Amplified Photoresist (ESCAP). The tBCM groups deprotect to form carboxylic acids that are capable of hydrogen bonding with chain segments of the polymer resist. This approach enables a systematic study of the governing physics underlying the improved lithographic performance. While MGs inhibit solubility in all cases, we find that differences in the structure of the MGs can significantly affect the photoacid diffusivity. In our ongoing optimization of the structure and loading of MGs, photoacid generators (PAGs), and base quenchers, 25 nm to 30 nm resolution has been achieved. The structure-property relationships and the synergistic effects of employing small, multi-functional additives in the polymeric photoresists are studied using various characterizations.

Analysis and characterization of contamination in EUV reticles

A host of complementary imaging techniques (Scanning Electron Microscopy), surface analytical technique (Auger Electron Spectroscopy, AES), chemical analytical and speciation techniques (Grazing Incidence Reflectance Fourier-Transform Infrared Spectroscopy, GIR-FTIR; and Raman spectroscopy) have been assessed for their sensitivity and effectiveness in analyzing contamination on three EUV reticles that were contaminated to varying degrees. The first reticle was contaminated as a result of its exposure experience on the SEMATECH EUV Micro Exposure Tool (MET) at Lawrence Berkeley National Laboratories, where it was exposed to up to 80 hours of EUV radiation. The second reticle was a full-field reticle, specifically designed to monitor molecular contamination, and exposed to greater than 1600J/cm2 of EUV radiation on the ASML Alpha Demo Tool (ADT) in Albany Nanotech in New York. The third reticle was intentionally contaminated with hydrocarbons in the Microscope for Mask Imaging and Contamination Studies (MIMICS) tool at the College of Nanoscale Sciences of State University of New York at Albany. The EUV reflectivities of some of these reticles were measured on the Advanced Light Source EUV Reflectomer at Lawrence Berkeley National Laboratories and PTB Bessy in Berlin, respectively. Analysis and characterization of thin film contaminants on the two EUV reticles exposed to varying degrees of EUV radiation in both MET and ADT confirm that the two most common contamination types are carbonization and surface oxidation, mostly on the exposed areas of the reticle, and with the MET being significantly more susceptible to carbon contamination than the ADT. While AES in both surface scanning and sputter mode is sensitive and efficient in analyzing thin contaminant films (of a few nanometers), GIRFTIR is sensitive to thick films (of order of a 100 nm or more on non-infra-red reflecting substrates), Raman spectroscopy is not compatible with analyzing such contaminants because of laser-induced evaporation of the contaminant film. SEM and EUV reflectometry are effective in quantifying the impact of contamination on imaging performance and reflectivity, respectively.