Daniel Tanzil

EUV pellicle development for mask defect control

The absence of a reliable non-removable pellicle is a significant obstacle in the development of EUV lithography. In this paper we present analytical and experimental results of a pellicle concept. The concept is based on the development of an EUV transmissive film supported with a wire-mesh. The form factor of the proposed solution is not different from a standard pellicle application, thus this would not require dramatic tool design changes. Results from developmental studies of two materials, silicon (Si) and ruthenium (Ru), are presented. As expected, Si shows oxidation on both surfaces of the thin film, while the less transmissive Ru has excellent resistance to oxidation. Spectral analysis at EUV wavelengths of pellicle coupons agrees very well with the theoretical analysis.

Effects of soft pellicle frame curvature and mounting process on pellicle-induced distortions in advanced photomasks

Lithography registration errors induced by the attachment of soft pellicles on reticles can significantly affect wafer overlay performance for sub-90 nm lithography chip manufacturing. Intel Corporation, Mitsui Chemicals, and the University of Wisconsin Computational Mechanics Center (UW-CMC) have conducted an extensive experimental study to quantify and minimize the pellicle-induced distortions in order to meet advanced mask manufacturing requirements. A comprehensive design of experiment was elaborated to evaluate the effects of frame curvature, adhesive gasket compliance, and mounting load on pellicle-induced distortions for soft pellicle systems. A frame curvature measurement tool was custom-made at the UW-CMC, employing an MTI Instruments capacitive sensor. A TA Instruments dynamic mechanical analyzer was used to determine the elastic modulus of the adhesive gasket materials. Registration measurements were conducted by Intel on test reticles on a 21 × 21 array of grid points, before and after pellicle attachment, to obtain pellicle-induced distortion results. Results characterize the influence of attachment process, type of adhesive gasket, frame curvature, reticle guiding plate configuration, and attachment load on pellicle-induced distortions.

Improving photomask surface properties through a combination of dry and wet cleaning steps

No Abstract Available.

ArF lithography reticle crystal growth contributing factors

The formation of photoinduced crystals and haze has become a challenge for 193nm photolithography high volume manufacturing (1-6). Extensive work has been performed to develop alternative to piranha chemistry for photomask cleaning processes in an attempt to eliminate the incidence of clean induced ammonium sulfate crystal formation (9-13). However, additional factors are impacting 193nm reticle optical quality. Sources of molecular contaminants such as environmental factors, outgasing from pellicle and reticle storage material can generate varieties of photoinduced crystals over the reticle useable lifetime (5-6). This paper will quantify and rank contributing factors for crystals generated under high energy UV exposure. A broad range of analytical and metrology techniques (FTIR, IC, TD-GC/MS, Inorganics impinger, AIMSTM, KLA Starlight, UV 172nm) and improvements in technique sensitivity were developed in order to identify crystal structure, quantify photogenerated contaminants levels and assess wafer printability impact. Engineering systems aimed at minimizing organic and inorganic molecular contaminants levels will be suggested.

Numerical and experimental studies of pellicle-induced photomask distortions

Meeting the stringent error budget of 157-nm lithography for manufacturing devices in the sub-100 nm regime requires that all mask-related distortions be minimized, corrected, or eliminated. Sources include the pellicle system, which has been previously identified as a potential cause of image placement error. To characterize pellicle-induced distortions, finite element (FE) models have been developed to simulate system fabrication, including soft pellicles as well as prototype fused silica (hard) pellicles. The main sources of distortions are: (a) temperature variations, (b) initially distorted components, and (c) sag-induced refraction. Temperature variations are an issue if pellicle mounting and exposure take place at different temperatures. Sources of attachment-induced distortions include the initial frame curvature, initial reticle shape, attachment method (mounting tools-induced), frame and gasket materials, and the hard pellicle bow. These attachment-induced distortions were modeled using experimentally measured values of Youngs modulus for adhesive gaskets. Refraction aberration is an issue with bowed hard pellicles which act as optical elements and induce image degradation. These effects were assessed and found to be significant. Results from the experiments and comprehensive FE simulations have characterized the relative importance of the principal sources of pellicle-induced photomask distortions for 157-nm lithography.

Experimental and numerical studies of the effects of materials and attachment conditions on pellicle induced distortions in advanced photomasks

Lithography registration errors induced by the attachment of soft pellicles on reticles can significantly affect wafer overlay performance for sub-100 nm lithography chip manufacturing. Intel Corporation and the University of Wisconsin have conducted an extensive study to identify the various sources of pellicle-induced distortions and methods for error reduction in order to meet advanced mask manufacturing requirements. In this study, pellicle attachment processes and system materials were evaluated to determine the effects on image placement accuracy. In particular, the in-plane distortions due to the pellicle attachment technique, pellicle frame flatness, frame adhesive, and environmental temperature were characterized. At Intel, pellicles were attached to a test reticle with a 21 X 21 array of grid points. Registration measurements were conducted before and after pellicle attachment using an optical distance metrology system. A comprehensive finite element model was developed at the University of Wisconsin to assess the contributions to pellicle-induced distortions from individual components of the pellicle system. Pellicle frame flatness, frame adhesive, and temperatures were measured and used as input to the finite element model. The correlation between simulation results and experimental data was excellent. Analyses were also performed to study pellicle mounting mechanisms and pellicle frame flatness.

Pellicle-induced distortions in advanced photomasks

A comprehensive design of experiment was elaborated to evaluate the effects of frame flatness, mask adhesive compliance, and mounting load on pellicle-induced distortions for soft pellicle systems. A dynamic mechanical analyzer was used to determine the elastic modulus of the adhesives materials, and a capacitive sensor-based tool was employed to measure the pellicle frame bow prior to mounting. Registration measurements were conducted on test reticles on a 21 X 21 array of grid points, before and after pellicle attachment. Statistical analysis (Anova test) was conducted to determine if the means for each sample group were statistically discernable. Overall, the magnitude of the distortions was very low for the pellicle mounting mechanism selected. Nevertheless the results indicated that the sample group with the flexible (softer) mask adhesive material exhibited lower distortions than that with conventional (stiffer) mask adhesive. Either larger sample size and/or wider variations in initial frame bow and mounting pressure will be required to assess the impact of these parameters on pellicle-induced distortions. Flexible (softer) mask adhesives are believed to minimize photomask deformation during the mounting process, resulting in lower pellicle-induced distortions.

Photomask lifetime issues in ArF lithography

Photomask lifetime has become a challenge since the introduction of high volume manufacturing 193nm photolithograph. Photomask lifetime is being impacted by a broad range of environmental and process factors resulting in inorganics crystals and organic contaminants formation as well as pellicle lifetime issues. Extensive work has been published on strategies for reduction of inorganic crystals photoinduced defects formation mainly focusing on photomask clean process improvements. This paper will focus on identifying root causes for photoinduced contaminants forming within the pellicle space area as well as identify environmental factors which have the potential of impacting pellicle membrane longevity. Outgasing experiments coupled with 193nm laser exposure tests were conducted to decouple and rank reticle/pellicle storage materials as well as pellicle outgasing contributors to photoinduced defects and identify factors impacting pellicle membrance longevity. Analytical test were conducted to compare the relative levels of reticle storage materials and pellicle outgasing contaminants. Experiments aimed at quantifying the fab environment contribution to photoinduced defects formation and impact on pellicle membrane lifetime will be discussed. Environmental conditions minimizing external contributing factors impacting photomask front side photoinduced defects formation and pellicle membrance longevity will be suggested.

Distortions in advanced photomasks from soft pellicles

The more stringent image placement error budgets for 157-nm lithography require a total assessment of photomask distortion sources and their eventual control. This includes proposed soft pellicle systems. Similar pellicles in use today have been previously identified as a major source of distortion. To characterize the many aspects of this problem, a numerical modeling and simulation program was initiated. Finite element results are reported in this paper along with correlations from experimental measurements.

Impact of DUV exposure on reticle repairs

The reticle manufacturing process induces various defects on the mask that need to be repaired. Missing absorber or clear defects are often repaired by depositing a carbon-based material (depo) using a Focused Ion Beam (FIB) tool. Few cases of such depo repairs on defects in between nested contacts on attenuated phase shift masks were found to fail upon use in high volume wafer manufacturing factories. With the goal of first reproducing the problem in the mask shop, a controlled set of depo repairs were performed on a test reticle and sequentially exposed on a DUV flood exposure system, emulating stepper exposure. The repair AIMSTM printability and AFM height profiles were measured before and after each exposure step. With incremental exposures, AIMSTM results showed the repaired contacts gradually printing larger in size and AFM results showed the tail of the depo repair (also referred to as depo overspray or halo) correspondingly receding with exposure. This suggests that the tail of the depo presumably contributes to the correct print CD of the repaired contact, and its gradual recession with exposure was likely causing the contacts to print larger, ultimately even bridging with the neighboring nested contact in some cases. This mechanism was confirmed by checking similar repairs on several production masks already being used in the wafer factories, at different stages of exposure. Subsequently, a novel post-repair process was developed which achieves rapid overspray removal thereby avoiding any further change in these repairs and associated wafer yield impact upon prolonged use on scanners.