Hoa Truong

Chemical and mechanical properties of UV-cured nanoimprint resists and release layer interactions

UV-curable nanoimprint resist characteristics and performance are key to controlling resist-related defects formed during template removal due to cohesive failure and strong resist-template adhesion. The debonding process is governed by both the chemical bonds that form between the template and the resist during cure, and by the structure of the resist itself which determines its elastic-plastic response under load. To gain insight to contributions from resist composition to the debonding process we examine the connection between mechanical and chemical properties of a family of methacrylate polyfunctionalized polyhedral oligomeric silsesquioxane (mPSS) containing resists to their adhesion to fluoroalkyl silane release layers. We also survey debonding of one of the mPSS formulations, an acrylate formulation and a vinyl ether formulation from as series of metal oxide and metal nitride release layers. The results show that while intrinsic storage modulus of a cured material is important, interfacial segregation of reactants in fluid resists can influence adhesive properties as well. The metal-containing release layers are shown to have generally much lower adhesion to cured resists than does a fluoroalkyl silane release layer. They present a useful alternative for template release treatments.

Investigations of the adhesion of maleic anhydride/cyclic olefin alternating copolymers to silicon substrates: Improved materials for 193 NM lithography

Various synthetic variations that affect the molecular weight, yield, and composition of maleic anhydride (MAH), norbornene (Nb), and tert-butyl 5-norbornene-2-carboxylate (Nb-TBE) terpolymers have been investigated. The adhesive properties of these polymers were evaluated from work of adhesion (W adh ) values to silicon substrates (treated with hexamethyldisilazane) using the method of fractional surface free energy from contact angle measurements of water and methylene iodide on the polymer films and substrate. These measurements showed that the W adh increased with higher Nb-TBE content, and values were close to those of conventional 248 nm photoresist polymers. Furthermore, 193 nm resist formulations incorporating polymers with high Nb-TBE content showed increased imaging performance and successfully produced sharp and defined features as small as 110 nm, which were seen via scanning electron microscopy (SEM).

Customization and design of directed self-assembly using hybrid prepatterns

Diminishing error tolerance renders the customization of patterns created through directed self-assembly (DSA) extremely challenging at tighter pitch. A self-aligned customization scheme can be achieved using a hybrid prepattern comprising both organic and inorganic regions that serves as a guiding prepattern to direct the self-assembly of the block copolymers as well as a cut mask pattern for the DSA arrays aligned to it. In this paper, chemoepitaxy-based self-aligned customization is demonstrated using two types of organic-inorganic prepatterns. CHEETAH prepattern for “CHemoepitaxy Etch Trim using a self-Aligned Hardmask” of preferential hydrogen silsesquioxane (HSQ, inorganic resist), non-preferential organic underlayer is fabricated using electron beam lithography. Customized trench or hole arrays can be achieved through co-transfer of DSA-formed arrays and CHEETAH prepattern. Herein, we also introduce a tone-reversed version called reverse-CHEETAH (or rCHEETAH) in which customized line segments can be achieved through co-transfer of DSA-formed arrays formed on a prepattern wherein the inorganic HSQ regions are nonpreferential and the organic regions are PMMA preferential. Examples of two-dimensional self-aligned customization including 25nm pitch fin structures and an 8-bar “IBM” illustrate the versatility of this customization scheme using rCHEETAH.

DSA patterning options for FinFET formation at 7nm node

Several 27nm-pitch directed self-assembly (DSA) processes targeting fin formation for FinFET device fabrication are studied in a 300mm pilot line environment, including chemoepitaxy for a conventional Fin arrays, graphoepitaxy for a customization approach and a hybrid approach for self-aligned Fin cut. The trade-off between each DSA flow is discussed in terms of placement error, Fin CD/profile uniformity, and restricted design. Challenges in pattern transfer are observed and process optimization are discussed. Finally, silicon Fins with 100nm depth and on-target CD using different DSA options with either lithographic or self-aligned customization approach are demonstrated.

DSA patterning options for logics and memory applications

The progress of three potential DSA applications, i.e. fin formation, via shrink, and pillars, were reviewed in this paper. For fin application, in addition to pattern quality, other important considerations such as customization and design flexibility were discussed. An electrical viachain study verified the DSA rectification effect on CD distribution by showing a tighter current distribution compared to that derived from the guiding pattern direct transfer without using DSA. Finally, a structural demonstration of pillar formation highlights the importance of pattern transfer in retaining both the CD and local CDU improvement from DSA. The learning from these three case studies can provide perspectives that may not have been considered thoroughly in the past. By including more important elements during DSA process development, the DSA maturity can be further advanced and move DSA closer to HVM adoption.

Quantitative measurement of resist outgassing during exposure

Determination of both the identity and quantity of species desorbing from photoresists during exposure at any wavelength - 248nm, 193nm and EUV - has proved to be very challenging, adding considerable uncertainty to the evaluation of risks posed by specific photoresists to exposure tool optics. Measurements using a variety of techniques for gas detection and solid film analysis have been reported but analytical results have not in general been easy to compare or even in apparent agreement, in part due to difficulties in establishing absolute calibrations. In this work we describe two measurement methods that can be used for any exposure wavelength, and show that they provide self-consistent quantitative outgassing data for 2 all-organic and 2 Si-containing 193 nm resists. The first method, based upon gas collection, uses two primary chromatographic techniques. Organic products containing C, S and Si are determined by collection of vapors emitted during exposure in a cold trap and analysis by Gas Chromatography-Flame Ionization Detector-Pulsed Flame Photometric Detector-Mass Spectrometry (GC-FID-PFPD-MS). Inorganic products such as SO2 are identified by adsorbent bed with analysis by Gas Particle-Ion Chromatography (GP-IC). The calibration procedure used provides reasonable accuracy without exhaustive effort. The second method analyzes the elemental concentrations in resist films before and after exposure by secondary ion mass spectrometry technique (SIMS), which requires only knowledge of the resist compositions to be quantitative. The extent of outgassing of C and S determined by the two methods is in good agreement for all 4 resists, especially when taking their fundamentally different characters into account. Overall, the gas collection techniques yielded systematically lower outgassing numbers than did SIMS, and the origins of the spread in values, which likely bracket the true values, as well as detection limits will be discussed. The data for Si were found to differ significantly, however, and we show that the discrepancy is due to photo-induced reactions at the polymer surface with the gas atmosphere present above the resist during exposure. For example, photolytic oxidation of the C-Si bonds in air causes volatile Si-containing products to be formed from an otherwise stable polymer, showing it is important to take the gas environment during exposure into account when designing resist polymers for low Si outgassing.

EUV mask line edge roughness

Extreme ultraviolet (EUV) mask fabrication faces many unique challenges, including more stringent line edge roughness (LER) requirements. EUV mask absorber LER will need to be reduced to reliably meet the 2013 International Roadmap for Semiconductors line width roughness target of 3.3 nm. This paper will focus on evaluating resists modified and deployed specifically to reduce LER on EUV masks. Masks will be built, and the final mask absorber LER reported considering multiple imaging and analysis techniques. An assessment of best methods for mask LER analysis will be provided and used to judge resist performance.

Impact of curing kinetics and materials properties on imprint characteristics of resists for UV nano-imprint lithography

UV curable resist formulations for nanoimprint must satisfy criteria for cure rate, volatility, viscosity, cohesion of the cured material and release from the template in addition to being successfully imprintable. We describe an investigation of the properties of a series of formulations comprising polyhedral oligomeric silsesquioxane and selected diluents as candidates for imprintable dielectrics. Although all have low viscosity and volatility and are successfully imprinted, significant variations in cure rate, mechanical and adhesion properties with resist composition are found. The trends observed are not all predictable from the literature, indicating that formulation optimization for this application requires a focus on the fundamentals of both materials and processes.

A new class of low bake resists for 193-nm immersion lithography

We report here, new non-acetal containing low bake (PEB < 100° C ) resists that are suitable for immersion lithography. These resists are based on novel low activation energy (low-Ea) tertiary ester protecting groups. One major obstacle to imaging in the sub-50 nm regime using chemically amplified resists is the diminished image integrity in the pattern (image blur) due to photo-generated acid diffusion into unexposed regions. Low processing temperatures are predicted to decrease the degree of photoacid diffusion and, in turn, decrease the image blur. Even though many low bake resist compositions have previously been reported, they are all based on acetal/ketal protecting groups. Unfortunately, these materials require a stoichiometric amount of water for the photoacid-catalyzed deprotection reaction to proceed. It is usually assumed that the water for the reaction comes from the environment in the bake station. However, fluctuations in humidity could affect the performance of the resist. Furthermore, acetal/ketal-based resists generally lack storage stability. For these reasons, acetal/ketal-based resists did not receive widespread acceptance in the lithography community. With the introduction of water based immersion lithography, acetal/ketal-based resists are expected to have further performance difficulties. Therefore, we targeted the development of new low blur resists for 193nm lithography that do not contain acetal/ketal protecting groups.

The limitations of high-index resists for 193-nm hyper-NA lithography

This paper will investigate the potential benefits and limitations of increasing the refractive index of the photoresist for water and high-index immersion based lithography. The primary potential benefits are increased exposure latitude due to restoration of the TM polarization component and improved depth of focus due to a delay in the onset of image-induced top-loss. After first understanding the physical origins of these effects, a series of simulation studies will probe the level of impact they may have for the 32nm and 22nm technology nodes. It is concluded that, although they may provide some process latitude relief, the benefits are minimal for 1.35NA water immersion, especially when weighed against the likely required development effort and cost. The benefits are slightly more compelling for high-index immersion (>1.5 NA), but a high index resist does not appear to be critical, provided the resist is at least as large as the immersion fluid index. A comparable benefit can be achieved with a conventional resist by using polarized illumination (a trend already happening for various reasons) and thinning the resist by ~9% for 1.35NA water immersion and ~15% for 1.55NA high-index immersion. Additionally, increasing the refractive index is typically accompanied by a corresponding increase in absorption. This will be addressed, concluding the limitations of absorption are likely chemical and not optical in nature. High absorption is likely tolerable, provided the chemistry can be engineered to account for exponential intensity decay. The level of difficulty in doing so is addressed.