Nickolay Stepanenko

Immersion specific defect mechanisms : Findings and recommendations for their control

Defectivity has been one of the largest unknowns in immersion lithography. It is critical to understand if there are any immersion specific defect modes, and if so, what their underlying mechanisms are. Through this understanding, any identified defect modes can be reduced or eliminated to help advance immersion lithography to high yield manufacturing. Since February 2005, an ASML XT:1250Di immersion scanner has been operational at IMEC. A joint program was established to understand immersion defectivity by bringing together expertise from IMEC, ASML, resist vendors, IC manufactures, TEL, and KLA-Tencor. This paper will cover the results from these efforts. The new immersion specific defect modes that will be discussed are air bubbles in the immersion fluid, water marks, wafer edge film peeling, and particle transport. As part of the effort to understand the parameters that drive these defects, IMEC has also developed novel techniques for characterizing resist leaching and water uptake. The findings of our investigations into each immersion specific defect mechanism and their influencing factors will be given in this paper, and an attempt will be made to provide recommendations for a process space to operate in to limit these defects.

Surface roughness investigation of 157- and 193-nm polymer platforms using different etch conditions

A series of different fluorinated polymer platforms used for early and current 157-nm photoresists is investigated with regard to blanket etch properties and surface roughness. Besides methacrylic-based polymers applied for 193-nm lithography, fluorine containing norbornene homopolymers, fluorinated cycloolefines, and tetrafluoroethylene (TFE) norbornene copolymers are chosen. Etch rates in different plasmas used for several applications, such as poly, SiN mask open, and selective/nonselective SiO2 etch, are determined and compared to standard 193-nm platforms currently used for DRAM manufacturing. Looking at various base resins, significant differences can be found using HBr- or Cl2-based poly etch conditions and various fluorocarbon-based oxide etch chemistries. Up to 2.4 times higher etch rates in Cl2 and the different CxFy oxide etch recipes are observed for the highly fluorinated cycloolefines and the TFE norbornenes, showing a strong correlation between fluorine content and etch rate. After stress by different etch conditions, the polymer surfaces are characterized using atomic force microscopy (AFM) and scanning electron microscopy (SEM). Surprisingly, the surface roughness of the methacrylic platforms and the norbornene base resin (determined by AFM) are found to be substantially higher than that of the highly fluorinated platforms. These results can directly be correlated to pictures obtained by optical methods (SEM).

Top coat or no top coat for immersion lithography

Since the moment immersion lithography appeared in the roadmaps of IC manufacturers, the question whether to use top coats has become one of the important topics for discussions. The top coats used in immersion lithography have proved to serve as good protectors from leaching of the resist components (PAGs, bases) into the water. However their application complicates the process and may lead to two side effects. First, top coats can affect the process window and resist profile depending on the materials refractive index, thickness, acidity, chemical interaction with the resist and the soaking time. Second, the top coat application may increase the total amount of defects on the wafer. Having an immersion resist which could work without the top coat would be a preferable solution. Still, it is quite challenging to make such a resist as direct water/resist interaction may also result in process window changes, CD variations, generation of additional defects. We have performed a systematic evaluation of a large number of immersion resist and top coat combinations, using the ASML XT:1250Di scanner at IMEC. The samples for the experiments were provided by all the leading resist and top coat suppliers. Particular attention was paid to how the resist and top coat materials from different vendors interacted with each other. Among the factors which could influence the total amount of defects or CD variations on the wafer were: the materials dynamic contact angle and its interaction with the scanner stage speed, top coat thickness and intermixing layer formation, water uptake and leaching. We have examined the importance of all mentioned factors, using such analytical techniques as Resist Development Analyser (RDA), Quartz Crystal Microbalance (QCM), Mass Spectroscopy (MS) and scatterometry. We have also evaluated the influence of the pre- and pos- exposure rinse processes on the defectivity. In this paper we will present the data on imaging and defectivity performance of the resists with and without the use of top coats. So far we can conclude that top coat/resist approach used in immersion lithography needs some more improvements (i.e. process, materials properties) in order to be implemented in high volume manufacturing.

Evaluation of 193-nm immersion resist without topcoat

A production-preferred solution is 193-nm immersion resist without a topcoat. The challenge of 193-nm immersion resist is both low leaching level and high performance. We summarize the screening results of selected 193-nm immersion resists that are designed for use without top coatings. Our evaluation is divided into several phases. Leaching levels of resist samples are first tested. The leaching data are analyzed and compared to our specifications. Both binary intensity mask and alternating phase-shift mask exposures are performed to evaluate the process window, lineedge roughness, and resist pattern profile. Resist films are rinsed by deionized (DI) water prior to or after exposure, and contrast curves are measured to investigate the resist sensitivity change. The results are compared with resist systems that use developer-soluble topcoats.

Measurement and evaluation of water uptake by resists, top coats, stacks, and correlation with watermark defects

With immersion lithography approaching the insertion in production, watermarks remain as one of the main concerns for immersion specific defects. They require special attention because of their size and associated high kill-ratio, and their increasing occurrence at higher scan speeds. IMEC has been working to understand the underlying mechanism of why remaining water droplets cause these defects. This work focuses on water uptake measurements and how this parameter correlates to watermark defectivity. Ellipsometric Porosimetry (EP) is used to measure the water uptake tendencies of resist and top coat materials and stacks thereof, and investigate what parameters are affecting it. The influence of material and process parameters and the presence of a top coat on water uptake by the resist are evaluated. In parallel, the quartz crystal microbalance (QCM) technique has been used as an alternative option to measure the water uptake. Though a one-to-one comparison between the results is not straightforward, the main trends are identical for both techniques. No perfect correlation of watermark defectivity with water uptake has been found in this study. Nevertheless, the results show a tendency towards higher watermark sensitivity with higher water uptake by the film. It is recognized that the total watermark defectivity is most probably a complex interplay of different parameters with water uptake being only one of them.

Recent advances in fluorinated resists for application at 157 nm

This paper is part of our continuing work on a new generation of more transparent, 157 nm resist platforms, which are based upon capping of fluoroalcohol-substituted, transparent perfluorinated resins (TFR) with a tert-butoxycarbonylmethyl (BOCME) moiety. Recent results indicate that by optimizing both resin structure and loading of photoacid generator and base additive a good compromise can be achieved between resolution power, dark erosion resistance, sensitivity and transparency at 157 nm. Specifically, it was found that a decrease in PAG (50% nominal loading) and base loading (75% nominal loading), coupled with optimization of the TFR resins to achieve higher transparency, gives the best compromise of properties. In this manner, resist systems with a transparency as low as 0.87 AU/micron were designed capable of resolving 60 nm 1:1 features, at a dose of 92 mJ/cm2 (non corrected for sigma), using a strong phase shift mask, and a sigma of 0.3 on a Exitech 157 nm small field mini-stepper. This type of resist material has also been imaged with a larger field tool (DUV30 Micrascan VII) to give 80 nm 1.1.5 L/S features at a dose of 135 mJ/cm2 employing using a Binary mask (σ=0.85). Finally, it was found that our BOCME-TFR based resist system can be used to transfer a 120 nm L/S pattern (imaged by 193 nm lithography) into a hardmask stack on top of silicon.

Evaluation of 193nm immersion resist without topcoat

193nm immersion resist without topcoat is production preferred solution. The challenge of 193nm immersion resist is both low leaching level and high performance. This paper summarizes the screening results of selected 193nm immersion resists which are designed for use without top coatings. Our evaluation is divided into several phases. Leaching levels of resist samples were first tested. The leaching data were analyzed and compared to our specifications. Both binary image mask and alternating phase-shift mask exposures were done to evaluate the process window, line-edge roughness, and resist pattern profile. Resist films were rinsed by DI water prior to or after exposure, and contrast curves were measured to investigate the resist sensitivity change. The results are compared with resist systems which use developer-soluble topcoats.

Study of barrier coats for protection against airborne contamination in 157-nm lithography

We summarize our work on devising protective barrier coats for use against airborne contamination when using tert-butoxycarbonylmethyl (BOCME) capped fluoroalcohol resist resins as part of our strategy to develop a 157 nm resist platform. We will describe how a barrier coat (AZ EXP FX Coating 145) consisting of a fluoro-cyclopolymer formulation, soluble in aqueous developer, can improve the post-exposure delay (PED) latitude of 157 nm resist resists exposed under conditions or airborne contamination. Specifically, a 20 nm thick coating of AZ EXP FX Coating 145 gives a PED latitude for L/S features of at least 10 min under condition of airborne amine contamination (10 ppb amine contamination). The barrier coat, AZ EXP FX coating 145 is formulated in a solvent which is compatible with resist film coated from typical 193 nm resist spin casting solvents. Moreover, it can be easily removed as part of the normal aqueous base development scheme, no extra post-apply bake or stripping step is required.

Study of 157 nm resists with full field exposure tools

A detailed account will be given of work done on the Micrascan VII (NA 0.75) at INVENT in Albany with AZ EXP X20 and AZ EXP X25 resist systems based upon BOCME protected fluoroalcohol resins. These resins were examined either with a high or low level of formulated photoacid generator (PAG). Our evaluations done both with binary and alternating phase shift mask exposures. It was found in our initial studies done at relatively high amine levels (1-2 ppb) that AZ EXP X25 X with low PAG gave the best performance.

Experimental investigation of fabrication process-, transportation-, storage, and handling-induced contamination of 157-nm reticles and vacuum UV cleaning

Mask fabrication process, transportation, storage, and handling contribute to contamination of 157nm reticles and modified fused silica substrates, resulting in transmission loss. A stable VUV cleaning procedure for contaminated binary, alternating, and attenuated phase shift reticles has been developed. This cleaning procedure was verified by lithographic imaging on the 157nm ASML MS-VII exposure scanner. A point-to-point steady state dose transmission uniformity range across a batch of 25 wafers (the exposure conditions of which were equivalent to that of a 300 mm wafer, 26mm×33mm fields, 50mJ/cm2) that were exposed with a modified fused silica substrate, was found to be <0.24% for a reticle that was cleaned prior to exposure using this VUV cleaning process. In-situ laser cleaning of contaminated mask substrates during exposure in the MS-VII resulted in 1% change in transmission at doses of up to 20 J/cm2, above which transmission remains stable (<0.24% variation). The cleaning procedure involves exposing the contaminated reticle in the UVO Reticle Cleaning Station for 30 minutes, using a cleaning gas mixture of N2/O2=99%/1%. Transmission loss due to contamination within the clean room is limited to 1 - 2 % and is reversible upon VUV cleaning. Flare levels of 3% were measured on contaminated reticle relative to a clean state of the same reticle. VUV cleaning is not only good for improving and maintaining stable mask transmission, but it is also good for preventing reticle contamination-induced flare. Contamination rate and contaminant type appear to be dependent on the storage environment of mask substrates and reticles. Typical contaminants included molecular acids (halogens, sulfur, sulfates), molecular bases (ammonia, amines), molecular condensables (hydrocarbons, alcohols, ketones, fatty acids, siloxanes, phthalate), molecular dopant (boron) and molecular metals (Ca, Mg, Al, Cu). Contamination of mask substrates appears to be through a competitive adsorption phenomenon, whereby low molecular weight species with high vapor pressure and low adsorption energies are over time replaced by large molecular weight ones with low vapor pressure and high adsorption energies.