Effect of PEB exhaust on resist CD for DUV process

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.

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

The Global 450mm Consortium (G450C), which is located at the SUNY Poly campus in Albany, NY was created to develop and evaluate a manufacturing tool set for 450mm wafers. The Lithography cell at G450C consists of a Nikon NSR-S650D 193nm immersion scanner and a SCREEN SOKUDO DUO DT-4000 track. The Lithography cell was installed and qualified in 2015, and with over a year of tool availability we have been able to perform extensive testing on the system to determine the equipment readiness for volume manufacturing. For the purposes of this paper we are focusing on the Edge Placement Error (EPE) [1] contributors of Critical Dimension Uniformity (CDU) and Overlay [2]. We will show the initial results as well as the improvements that have been made since tool acceptance. The 450mm results will be compared to 300mm tools in production today, as well as against the seven nanometer node (N7) expected requirements. Lastly, we plan to demonstrate the Nikon scanner’s ability for focus control on stressed or bowed wafers, which are characteristic challenges of large silicon substrates. This paper will showcase the current 450mm lithography performance for CDU on both Line/Space (LS) and Contact Hole (CH) patterns. We will demonstrate the process window for LS and CH features on multiple resists specially formulated for 450mm. Both Post Exposure Bake (PEB) tuning on the SCREEN track as well as CDU Master (CDUM) Corrections from the Nikon Turnkey Solution software suite will be utilized for performance improvements on 450mm wafers. The G450C goal is to drive CDU down to less than 1nm 3σ across the entire wafer with 1.5mm edge exclusion zone.” In addition to our test masks, G450C has designed a three layer mask set and with these masks we gathered “on product” CDU performance on a Back End Of Line (BEOL) metal stack. In the current reality of high volume manufacturing, multi-patterning is used to achieve the required Critical Dimension (CD) and pitch combination. The largest contributor to EPE is scanner overlay performance. We will demonstrate the Single Machine Overlay (SMO) performance as well as some Mix and Match Overlay (MMO) results. The lithography cell at G450C is the only 450mm linked lithography cell in the world. In order to create MMO wafers we were required to expose the first print at the Nikon factory in Japan and etch them at G450C to generate an align-to layer. As the wafers’ size scales, so do some of the process effects including film stress and wafer bow. The current G450C BEOL integrated process has measured wafer bow of up to 350um. We will demonstrate how the S650D measures the wafer topography and adjusts the exposure to compensate for wafer bow.

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.

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.

A new method for the measurement of acid diffusion parameters in negative-tone e-beam chemically amplified resists and its application for proximity effect correction is presented. It is based on the measurement of the diameter of lithographic features (pillars) obtained from single-pixel e-beam exposures. Generally, long post exposure bake (PEB) times or low post apply bake (PAB) temperatures lead to wider features compared with short PEB times and high PAB temperatures, due to enhanced acid diffusion. The acid diffusion coefficient was found to depend exponentially on cross-linking density. Additionally, proximity effect correction parameters (triple Gaussian parameters) are easily calculated. These parameters are t PEB dependent and in the limit (t PEB→0) are almost equal to the corresponding values for non-amplified resists. The method has been applied to SAL-601 (Shipley), and the calculated proximity parameters were used successfully to correct the proximity effect of complicated layouts with a 0.15 µ m critical dimension.

The temperature distribution on the surface of the mask, during post exposure bake (PEB) of the X-ray mask after electron beam writing (EBW), is very important for controlling the mask critical dimension(CD). In this paper, three-dimensional finite element (FE) thermal model of the X-ray mask has been set up. The results of the numerical simulation indicate that it will take 2.16 seconds for PEB to get thermal steady when the boundary conditions of the top surface of the resist are natural convections, and that the temperature distribution of the mask is non-uniform, and the maximum temperature difference is 10.19°C, which will most likely make the resist at the high temperature region baked excessively. By using insulation measures, the boundary conditions of the top surface of the resist are changed to adiabatic conditions, and the temperature distribution of the mask is very uniform, which will reduce the influence on the absorber CD from PEB.

Model-based breakdown of resist and mask contributions to local CDU for sub-30-nm contact holes in EUV lithography

In this paper, the 'pattern formation' characteristics of a polyhydroxystyrene (PHS)-acryl hybrid extreme ultraviolet (EUV) resist was investigated using a high speed atomic force microscope. This analysis covers the dissolution or development, rinse and drying processes that are conventionally applied after EUV exposure and post exposure bake. As a result, it was found that during the development process, the EUV-exposed resist film exhibited swelling characteristics. This was assumed to be the effect of the balance / imbalance in dissolvability introduced by the hydrophobic acryl and hydrophilic PHS components of this hybrid resist. During the rinse process with de-ionized water, the critical dimension (CD) of the resulting line pattern after development was observed to increase with rinse time. This was attributed to a possible permeation of water into the resist line pattern causing the CD to swell. Lastly, for the drying process, the same line pattern CD was found to shrink similar to pre-rinse sizes and taper at the top edges. This suggests evaporation of the rinse water imbedded into the resist line pattern.

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.

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.

ABSTRACTPhoto-acid generator (PAG) is the key component responsible for the increased sensitivity of chemically amplified resists in use today for microelectronics production. Concentration variation of PAG through the thickness of the photoresist film adversely affects materialís performance. To offset reduced acid concentration at the bottom of the resist, we have developed adhesion promoting photo acid generators, called as a class “APPAG” that enhance acid concentration at interface between the resist and the substrate. An overview on the preparation and characterization of two siloxane based APPAG materials along with a performance comparison of commercial DUV and EUV resists on APPAG is provided. Longer diffusion length photo-acid generator (APPAG 9) retained square profile of critical dimension (CD) at off-focus values and was found to give nearly a 50% improvement in depth of focus for 250nm node DUV lithography. For EUV lithography, both shorter diffusion length APPAG 6 and APPAG 9 were shown to substantially improve performance envelope for 100nm dense lines and spaces at reduced post exposure bake (PEB).

Chemically amplified resists (CARs) are the current workhorse for photolithography, where higher resolution and smaller feature size represent a continual driving force for the semiconductor industry. As the feature size decreases to sub-30 nm, LWR and gate critical dimension (CD) control become serious concerns. In order to reach the goals in the ITRS, an unprecedented level of control of photoacid diffusion while maintaining the high resist sensitivity and resolution during image formation is required. CARs require a post exposure bake (PEB), typically performed on a hot plate at 90-150°C for 30-120 seconds, to complete the resist deprotection after photoacid generation. This bake step is a primary influence on resist performance as the time/temperature profile controls both the diffusion of photogenerated acids and the deprotection of the resist backbone. Sufficient time must be provided to achieve the level of deprotection required for the solubility switching in a developer, but the seconds timeframe of conventional hot plate PEB leads to an undesirable amount of acid diffusion. As long as the activation energy of diffusion is less than that for deprotection, higher temperatures for optimized time durations will result in reduced diffusion. However, traditional hot plate PEB cannot access times shorter than a few seconds. We utilize a laser (CO₂) based scanned heating system to achieve sub-millisecond to milliseconds in heating durations with temperatures up to the thermal decomposition limit of the resist. This research is aimed at using synthetic techniques to vary the structure of the photoacid generator (PAG) in order to learn about the role of PAG size and structure on acid diffusion during sub-millisecond heating. A variety of PAGs with different anion sizes have been synthesized and tested on the CO₂ laser system, and their lithographic performance and effect on acid diffusion has been studied.

The fabrication of next-generation devices via roll-to-roll (RTR) nanoimprinting has received considerable attention due to the high throughput and resolution associated with the process. In particular, RTR nanoimprinting with ultraviolet (UV)-curable resin is expected to be useful for the fabrication of large-area devices such as nonreflective films, transparent conductive sheets, and organic solar cells. In order to improve the resolution of such devices, the roll mold must be able to produce seamless fine patterns. Generally, a roll mold for nanoimprinting is made by attaching a planar mold replicated from the master mold via a nickel electroforming technique. However, this method produces seams, which reduces the product yield. In order to resolve these issues, the authors have developed a direct writing method using electron beam (EB) lithography on a rotating cylindrical substrate. In this study, the authors examined the potential for fabricating a seamless roll mold using high-resolution inorganic EB resist and the pattern shrink effect from postexposure bake (PEB). The authors used the dip-and-pull method to form the EB resist layer on the roll mold substrate (32 mm diameter, made of brass). The cleaned roll mold was dipped in the inorganic EB resist and then pulled out at a constant speed. The resulting sample was cured at 300 °C for 1 h. Next, the sample was mounted on rotating equipment and placed in a scanning electron microscope (SEM) equipped with an EB writing system, and the roll substrate was exposed to 10 kV EB while rotating at a constant speed. The revolution speed was set to ten turns per line pattern. Subsequently, PEB was carried out at 200 °C for 10 min in air. After the sample was cooled to room temperature, the EB-exposed area of the EB resist layer was developed using buffered HF. The obtained pattern on the roll mold was examined with the same SEM used for writing. Finally, RTR nanoimprinting using the obtained roll mold was carried out using UV-photocurable resin. At that time, the sample was coated with an antisticking layer of fluorinated silane coupling agent. As a result, a seamless line pattern 190 nm wide was obtained on a polyethylene terephthalate film over a length of 10 cm.

EUV lithography has been brought into mass production. To enhance the yield, improvements in critical dimension (CD) stability, and defectivity still remain of utmost importance. In order to enhance the defectivity performance on contact hole pattern, continuous work has been executed.1 As the result of process optimizations presented at SPIE 2018 for 24 nm contact hole half pitch pattern, single- and multi-closed hole modes as caused by particles included in coating materials, called “in-film particles”, or stochastic failures are the major of defects modes. For defectivity improvement work done this year, optimization of material dispense has been carried out in an attempt to improve the defectivity of single- and multi-closed holes as caused by in-film particles. As the result, totally 19 % of defectivity improvement was obtained as compared with conventional dispense conditions. On the other hand, CD variability is comprised of several components such as wafer to wafer, field to field, within field, and local CD. In addition, not only coater/developer but also scanner, mask, and materials contribute to the variations. In this paper, CD uniformity (CDU) optimization on also contact hole 24 nm half pitch pattern has been executed to improve each component from the standpoint of coater/developer. As a result of the optimization of development process, 13.9 and 6.4 % of field to field and within field CDU improvement have been achieved, respectively.