An in situ analysis of the resist pattern formation process

As Extreme ultraviolet (EUV) lithography technology shows promising results below 40nm feature sizes, TOKYO ELECTRON LTD.(TEL) is committed to understanding the fundamentals needed to improve our technology, thereby enabling customers to meet roadmap expectations. TEL continues collaboration with imec for evaluation of Coater/Developer processing sensitivities using the ASML Alpha Demo Tool for EUV exposures. The results from the collaboration help develop the necessary hardware for EUV Coater/Developer processing. In previous work, processing sensitivities of the resist materials were investigated to determine the impact on critical dimension (CD) uniformity and defectivity. In this work, new promising resist materials have been studied and more information pertaining to EUV exposures was obtained. Specifically, post exposure bake (PEB) impact to CD is studied in addition to dissolution characteristics and resist material hydrophobicity. Additionally, initial results show the current status of CDU and defectivity with the ADT/CLEAN TRACK ACT^TM 12 lithocluster. Analysis of a five wafer batch of CDU wafers shows within wafer and wafer to wafer contribution from track processing. A pareto of a patterned wafer defectivity test gives initial insight into the process defects with the current processing conditions. From analysis of these data, it's shown that while improvements in processing are certainly possible, the initial results indicate a manufacturable process for EUV.

Preliminary results of in situ analyses of dissolution of resist patterns of half-pitch (hp) lines and spaces (L/S) using high-speed atomic force microscopy are discussed. Initial experiments were conducted on an EUV-exposed 32 nm hp L/S pattern using a standard concentration (0.26 N) of tetramethylammonium hydroxide (TMAH) developer solution. This was done using a carbon nanofiber-based cantilever tip and after various tool enhancements and optimizations. Difference in the dissolution characteristics of various resist polymers such as polyhydroxystyrene (PHS)-based and hybrid (PHS-methacryl) resists was observed. These results are in agreement with the previously obtained results of measurements performed with 32 nm isolated line patterns on resist films developed with diluted developer solutions (1/20 of 0.26 N TMAH), where the PHS-based resist showed a uniformly dissolved, grain-like dissolution characteristic, while the hybrid resist exhibited resist swelling of the exposed resist film. Keywords: Resist dissolution, half pitch, lines and spaces, PHS, hybrid, high-speed atomic force microscopy

High-Speed AFM Reveals the Dynamics of Single Biomolecules at the Nanometer Scale

To reduce the influence of carbon contamination growth, which is the origin of the resist outgassing during extreme ultraviolet (EUV) exposure, it is necessary to study the relationship between the outgassing species from the resist and carbon contamination growth during EUV exposure. Thus a novel evaluation system which enable the measurement of the carbon contamination thickness on a Mo/Si multilayer witness sample by in-situ ellipsometry in the resist outgassing environment under EUV exposure was installed at the beamline in the NewSUBARU synchrotron radiation facility. The carbon contamination thickness on the witness sample measured by in-situ ellipsometry is in good agreement with those measured using an atomic microscope and by reflectivity measurement. In addition, it is found that the time dependence of the carbon growth on a witness sample is due to the difference in outgassing species from the resist samples during EUV exposure. Furthermore, for the model resist of poly(hydroxystyrene) containing the protectiive group of tertial butoxycarbonyl group with a photoacid generator (PAG) and a quencher, the time dependence of contamination thickness was mainly due to the decomposition of outgassing species of the tert-butoxycarbonyl group and the residual casting solvent, such as propyleneglycol monomethyl ether acetate. In order to evaluate the carbon contamination thickness measurement, the in-situ measurement system was moved to the 10.8-m-long undulator beamline. The EUV light intensity flux was estimated to be the same level as that required in high-volume manufacturing.

The Semiconductor Industry Association (SIA) has placed stringent requirements on Next GenerationLithography mask critical dimension (CD) control. A new chemically amplified (CA) positive resist,Shipley XP2040D was evaluated for mask making application. This resist exhibited an extraordinary postexposure bake (PEB) sensitivity, less than 0.6 nm/ C, along with a sub-50 nm resolution. The PEB stabilityin ambient was larger than 4.5 hours and the line-edge-roughness (LER) was less than 2.9 nm. 1. INTRODUCTION Critical dimension (CD) control and uniformity for a SCALPEL mask must be kept within 7 nm (3 ) inorder to meet the SIA Roadmap requirement for 70 nm NGL mask fabrication [1]. Chemically amplified(CA) resists have been considered for NGL mask patterning because of their high resolution, high electronexposure sensitivity and etch selectivity. However, CD variation due to the PEB sensitivity of CA resistcomplicates the mask making process and needs to be minimized.Previous Finite Element Analysis modeling indicates there are PEB temperature gradients across a 200 mmSCALPEL mask and within each membrane of the mask. The thermal variations are due to the combinedcomplex three-dimensional mask structure coupled with the thermal gradients associated with the bakechamber environment. These gradients were experimentally confirmed through the measurement of criticalfeatures across various membranes on the mask array. A CD uniformity of 8-10 nm for inter-membrane and4-7 nm for intra-membrane was observed using a CA negative-tone resist, NEB22[2]. This CD variationwas mainly attributed to the large PEB sensitivity of the resist ( ~8 nm/ C).In order to meet the 7 nm SIA mask budget, CD non-uniformity due to thermal variations must be reduced.One way to minimize the influence of this thermal effect is to use low PEB sensitivity resists. This paperreports on the performance of Shipley XP2040D positive CA resist for 200 mm SCALPEL maskfabrication using the Leica VB6HR 100 KeV e-beam exposure system.

In this work, the effect of exhaust condition during Post Exposure Bake (PEB) on the critical dimension (CD) performance has been investigated for different DUV resists. Two kinds of PEB chamber cover designs have been tested to see the influence of exhaust flow on CD. It has been found that cleaning and adjusting of PEB exhaust flow will change the resist CD. To confirm the impact of PEB exhaust condition on production, resists for both lines and contact holes are verified by scanning electron microscope (SEM).

Implementation of extreme ultra violet lithography (EUVL) for printing features of dimensions around 32nm and below has faced multiple challenges that have been overcome mostly through tool and process engineering. Currently, the main concern for implementation of EUVL in high volume manufacturing is related to stochastic defects, such as line bridges or breaks, which may occur randomly and have an impact on device performance.1 Photon shot noise, in addition to the inherent process variability and material non-homogeneities can lead to defects on the final patterned structures. As the pitch and critical dimension (CD) approach 32nm pitch and below, the dependency of stochastics defects increases exponentially. This paper fits within imec’s EUV stochastics and defectivity program, which investigates the key parameters for the generation and mitigation of defects from both a theoretical and practical approach. Initially, a reference process has been established and a methodology has been developed to capture and characterize bridge-type defects after pattern transfer into relevant layers for metrology and applications. Optical inspection and scanning electron microscopy (SEM) review are used to collect information on the number and types of defects present over large patterned areas (in the order of cm2). With a well-defined inspection technique, experiments related to fundamentals as well as materials and processes are continuously run. In the past years, imec has been leading the fundamental work on stochastic defects.2,3 De Bisschop et al. identified the “defect cliff”, or the exponential dependency of defect density as a function of space CD for multiple parameters, such as resist formulation and exposure/imaging conditions. More recently, the focus has changed to the investigation of the “defect floor”, which as the name suggests, is a flat response of defects to CD. The defect floor suggests that defectivity cannot be further improved, making one of the most relevant problems to solve for implementation of EUV in high volume manufacturing (HVM). Our results show a decomposition of the potential contributors to explore the effect of processing conditions related to scanner, track and pattern transfer on final defect density for a chemically amplified resist and attempt a comparison to a metal-oxide resist. Goal is to understand the contributors to the defect floor and identify an effective route to avoid/minimize defect formation.

Extreme ultraviolet (EUV) lithography is crucial to enabling technology scaling in pitch and critical dimension (CD). Currently, one of the key challenges of introducing EUV lithography to high volume manufacturing (HVM) is throughput, which requires high source power and high sensitivity chemically amplified photoresists. Important limiters of high sensitivity chemically amplified resists (CAR) are the effects of photon shot noise and resist blur on the number of photons received and of photoacids generated per feature, especially at the pitches required for 7 nm and 5 nm advanced technology nodes. These stochastic effects are reflected in via structures as hole-to-hole CD variation or local CD uniformity (LCDU). Here, we demonstrate a synergy of film stack deposition, EUV lithography, and plasma etch techniques to improve LCDU, which allows the use of high sensitivity resists required for the introduction of EUV HVM. Thus, to improve LCDU to a level required by 5 nm node and beyond, film stack deposition, EUV lithography, and plasma etch processes were combined and co-optimized to enhance LCDU reduction from synergies. Test wafers were created by depositing a pattern transfer stack on a substrate representative of a 5 nm node target layer. The pattern transfer stack consisted of an atomically smooth adhesion layer and two hardmasks and was deposited using the Lam VECTOR PECVD product family. These layers were designed to mitigate hole roughness, absorb out-of-band radiation, and provide additional outlets for etch to improve LCDU and control hole CD. These wafers were then exposed through an ASML NXE3350B EUV scanner using a variety of advanced positive tone EUV CAR. They were finally etched to the target substrate using Lam Flex dielectric etch and Kiyo conductor etch systems. Metrology methodologies to assess dimensional metrics as well as chip performance and defectivity were investigated to enable repeatable patterning process development. Illumination conditions in EUV lithography were optimized to improve normalized image log slope (NILS), which is expected to reduce shot noise related effects. It can be seen that the EUV imaging contrast improvement can further reduce post-develop LCDU from 4.1 nm to 3.9 nm and from 2.8 nm to 2.6 nm. In parallel, etch processes were developed to further reduce LCDU, to control CD, and to transfer these improvements into the final target substrate. We also demonstrate that increasing post-develop CD through dose adjustment can enhance the LCDU reduction from etch. Similar trends were also observed in different pitches down to 40 nm. The solutions demonstrated here are critical to the introduction of EUV lithography in high volume manufacturing. It can be seen that through a synergistic deposition, lithography, and etch optimization, LCDU at a 40 nm pitch can be improved to 1.6 nm (3-sigma) in a target oxide layer and to 1.4 nm (3-sigma) at the photoresist layer.

Numerous investigations on extreme ultraviolet (EUV) resist outgassing have been performed using the pressure rise and gas chromatography mass spectrometry (GC–MS) method. However, the reliability of the results obtained with these methods has not been clearly discussed. In this paper, to analyze and verify the reliability of the resist outgassing results, de-protection reaction and photo acid generator's (PAG's) decomposing reaction in the resist film, which are viewed as the main source of resist outgassing, are investigated using the Fourier transform infrared spectroscopy (FT-IR). Analysis was performed on a resist film after EUV exposure, before post exposure bake (PEB). Based on the measured amount of de-protection reaction in the resist film, the amounts of resist outgassing released were calculated. It was found that the resist outgassing amount obtained through FT-IR was somewhere between those obtained using the pressure rise and GC–MS methods results. These similarities in the results substantiate the reliability of resist outgassing amounts obtained through these analysis methods.

The photoacid diffusion length is a critical issue for extreme ultraviolet (EUV) lithography because it governs the critical dimension (CD), line-edge-roughness (LER), and line-width-roughness (LWR) of photoresist materials. Laboratorybased experimental methods that complement full lithographic testing would enable a rapid screening of materials and process conditions. This paper provides an approach to characterize the photoacid diffusion length by applying a bilayer stack technique. The method involves quantitative measurements of the deprotection kinetics as well as film thickness at each process step: radiation exposure, post-exposure bake, and development. Analogous to a contrast curve, by comparing the film thickness of the bilayer before and after development, the photoacid diffusion length was deduced in a commercial EUV photoresist and compared to EUV lithography. Further, by combining the experiments with kinetics modeling, the measured photoacid diffusion length was predicted. Lastly, based upon the measured kinetics parameters, a criterion was developed that next-generation resists must meet to achieve a 16 nm photoacid diffusion length. These guidelines are discussed in terms of correlations and contributions from the photoacid and resist properties. In particular, the trapping kinetics of the photoacid provides a route to reduce LER and the CD at low dose.

Extreme ultra violet (EUV) resists have been developed to be able to print sub-30nm L/S features with EUV alpha DEMO tool (ADT), and several resists start to meet the target for 2X node DRAM application in resolution and sensitivity. However, an overall performance of EUV resists is still not comparable to that of DUV resists. At the same process condition having same process constant (k1), the imaging capability of EUV resists is poorer than that of DUV resists. The most critical issues are line width roughness (LWR) and critical dimension (CD) variation across a field. Although there are many studies to improve the LWR of EUV resist, the issue on CD variation across a field is not much explored, because the problem can be detected at full field exposure. In this paper, sources of the CD variation across a field are mainly investigated, and solutions to improve the CD uniformity are explored. Out of band (OOB) radiation and its reflectivity at REticle MAsking (REMA) unit of scanner or absorber of mask is regarded as one of the sources which aggravates imaging quality of EUV resist. In addition, the optical density of black border at EUV wavelength is also known to have an impact on this CD variation. Therefore, to improve pattern fidelity and LWR of EUV resist, the mitigation of OOB radiation impact is required. It is found that the resist sensitivity to DUV compared to EUV is important, and this property affects on CD uniformity. Furthermore, new material which can mitigate the OOB radiation impact is developed. The resolution limit of current EUV resists is also investigated to see the extendibility of it. At an exposure condition which readily resolves 16nm half pitch (HP) resolution, 20nm resolution is hardly achieved even with a champion resist. To overcome this barrier, more efforts on development of alternative resists are required from today.

Characterization for the photomask fabrication based on a high-resolution technique with a non-chemically amplified resist and a post-exposure bake

A next reticle format is considered from a view point of photomask fabrication process. In order to determine optimal reticle thickness corresponding to 230 mm size, processes feasibility for the thickness are examined. Cr dry etching is studied in terms of plasma condition and patterning accuracy. It is found that Cr dry etching with accurate critical dimension (CD) control is possible with substrate as thick as 12.7 mm. Post exposure bake (PEB) process for chemically amplified (CA) resists is also examined in terms of CD uniformity on substrates. Relationship of substrate thickness and CD uniformity is estimated with PEB controllability and CA resists characteristics. In addition to this, it is demonstrated that actual temperature uniformity for substrate with a thickness of 9 mm is equivalent to that for a present thickness of 6.35 mm. From these results in overall, it is proposed that substrate thickness of 9 mm is feasible for actual photomask production.

Extreme ultra violet (EUV) resists have been developed to be able to print sub-30nm L/S features with EUV alpha DEMO tool (ADT) having 0.25NA. However, a lithographic performance of EUV resist is not comparable to that of DUV resist. At same process constant (k1), the imaging capability of EUV resist is poor than that of DUV resists. The most critical issues are line width roughness (LWR) and critical dimension (CD) variation across a field. Although there are many studies to improve the LWR of EUV resist, the issue on CD variation across a field is not much explored, because the problem can be detected at full field exposure. In this paper, sources of the CD variation across a field are mainly investigated, and solutions to improve the CD uniformity are explored. Out of band (OOB) radiation and its reflectivity at REticle MAsking (REMA) unit of scanner or absorber of mask is regarded as one of the sources which aggravates imaging quality of EUV resist. In addition, the optical density of black border at EUV wavelength is also known to have an impact on this CD variation. Although the exact spectrum of OOB radiation is not open, LASER produced plasma (LPP) type source and discharge produced plasma (DPP) type source are believed to have the OOB radiation. Therefore, to improve pattern fidelity and LWR of EUV resist, the mitigation of OOB radiation impact is required. It is found that the resist sensitivity to DUV compared to EUV is important, and this property affects on CD uniformity. Furthermore, new material which can mitigate the OOB radiation impact is developed. This material is applied as an additional layer on conventional EUV resist film, and shows no intermixing. Process window is not changed by applying this layer. The filtering ability of OOB radiation is explored. LWR and pattern fidelity are much improved by applying this material to EUV process.

Real-time control is the trend for photoresist processing in the advanced lithography. In this paper we develop an in-situ ellipsometer integrated with a programmable thermal heating system, which could perform real-time monitoring and control for critical dimension (CD) latent image through the entire post-exposure bake (PEB) process. The inline ellipsometry measurements are fitted into an electromagnetic wave model built on the rigorous coupled-wave analysis (RCWA) theory for CD latent image characterization. Thereafter a real-time control scheme is proposed by applying the programmable thermal bake-plate. The experimental outcome demonstrates a significant improvement on the final CD result comparing to the conventional baking method.