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EUV lithography (EUVL) is a candidate technology for patterning of ever shrinking featuresizes in integrated circuits. There are several challenges to high volume manufacturingof devices using EUVL in a cost-effective manner, which include limited source power, maskdefects and non-idealities in the photoresist, the imaging medium. Focus of this thesis is onphotoresists. Specifically, influence of absorption shot noise on the final LER was studiedexperimentally through comparative analysis of LER obtained with EUV (92 eV photons)and 100 keV e-beam lithography. The key contribution here is that the lithography experimentswere performed with matched imaging conditions between EUV and e-beam, whichallowed for a fair comparison between the LER values measured using the two patterningtechnologies. In scenarios where the e-beam spatial resolution was better than that for EUV,the technique of gray-scale e-beam lithography was experimentally demonstrated to result inclosely matched image gradients between e-beam and EUV patterning. It was shown that themeasurable parameter known as the exposure latitude is a good parameter to test whetherthe aerial images between two experiments have identical gradients for idential materials andprocessing conditions.With matched imaging conditions, resist materials and processing conditions, lithographicdata showed that the incident flux needed to pattern 50 nm half-pitch lines andspaces for a leading chemically amplified resist was 10.7 photons/nm2 for 92 eV photons,and 4.44 electrons/nm2 for 100 keV electrons. Measurements of absorption of 100 keV electronsestimated through an EELS measurement with 120 keV beam showed that despite havingaccess to core levels in the material (e.g., 284 eV edge in carbon), these electrons mostly justexcite the energy levels less than 100 eV in the resist, with a mean deposited energy of 35 eV.Results showed that the probability of an energy loss event in a 45 nm thick resist film with100 keV electrons was 0.4, about 2.35x larger than that for EUV (0.17). By combining theincident flux and the absorption probabilities, the absorption flux was found to be similarbetween the two patterning technologies. A possible reason is that either the secondaryelectron spectra created in the material through ionization events are similar for EUV and 100 keV e-beam exposures, or that there are only small differences which ultimately do notmatter from the standpoint of acid generation statistics. With matched imaging conditionsand matched absorption density, the mean LER for e-beam was found to be larger by about1 nm.Influence of various material contributors in determining the resist LER was also studiedfrom a modeling standpoint. Reaction/diffusion parameters in a stochastic resist model werecalibrated to resist contrast curve data and line/space patterns. With the best fit reactionand diffusion parameters, the contributions of absorption shot noise, acid generation statisticsand the base counting statistics on the resist LER were determined. Shot noise was foundto account for 46% of the total LER, while the acid generation and base loading statisticswere found to account for 22% and 32% of the LER respectively.Interactions of low energy electrons in EUV resists were studied from both experimentaland modeling standpoint. Low energy (< 92 eV) electrons are primarily responsible forinitiating chemistry that leads to image formation in EUV resists. Thus key to controllingEUV exposure efficiency is understanding low energy electron radiation chemistry efficiencyas a function of electron energy. Thickness versus exposure dose measurements were madewith incident electron energies ranging between 29 eV and 91 eV. Thickness removed wasmuch larger than the average secondary electron range and was bake temperature dependentand thus is a useful indicator of de-protection blur introduced by the bake process. Thedissolution volume per eV deposited energy was nearly similar for 29 eV to 91 eV energies,although there is some indication that incident electrons with lower energies are slightlymore effective at causing chemistry. The volume removed per eV was about 0.1 nm3 per 1nm2 area.The well-known dielectric model for inelastic scattering was used to develop a stochasticmodel for simulating trajectories traversed by secondary electrons in the resist. Electronenergy loss spectroscopy (EELS) was used to measure the dielectric function for a leadingchemically amplified resist. Analytical expressions for the Mermin dielectric functions whichaccount for energy and momentum transfer were then fit to the measurement to build acomplete dielectric model for the resist. Stochastic simulations were then performed withthe scattering parameters determined by the dielectric model to calculate energy depositionand acid generation statistics. These results were used to quantify the net acid generationblur, which was found to be between 1.8 nm and 2 nm from the point of origin of the electrons.The radial distribution of acid generation sites was fit using a Rayleigh distribution and thebest fit sigma parameters in the distributions were found to range between 1.2 nm at 30 eV and1.41 nm at 91 eV. The net acid yield calculated by the simulator was found to be 1.6 for an80 eV electron.

Line edge roughness (LER) is seen as one of the most crucial challenges to be addressed in advanced technology nodes. In order to alleviate it, several options were explored in this work for the interference-like lithography imaging conditions. The most straight forward option was to scale interference lithography (IL) for large field integrated circuit (IC) applications. IL not only serves as a simple method to create high resolution period patterns, but, it also provides the highest theoretical contrast achievable compared to other optical lithography systems. Higher contrast yields a smaller transition region between the low and high intensity parts of the image, therefore, inherently lowers LER. Two of the challenges that would prohibit scaling IL for large field IC applications were addressed in this work: (1) field size limitations, and (2) magnification correction (i.e., pitch fine-tuning) ability. Experimental results showed less than 0.5 nm pitch adjustment capability using fused silica wedges mounted on rotational stages at 300 nm pitch pattern. A detailed discussion on maximum practical IL field size was outlined by considering the subsequent trim exposures and optical path difference effects between the interfering diffraction orders. The practical limit on the IL field size was assessed to be 10 mm for the conditions specified in this work. One of the contributors of LER is the mask absorber roughness. To mitigate it, two methods were explored that are also applicable to scanners working under interference-like conditions: (1) aerial image averaging via directional translation, and (2) pupil plane filtering. Experiments on pupil plane filtering approach were performed at Imec in Leuven, Belgium, on the ASML:NXT1950i scanner equipped with FlexWAVE wavefront manipulator. Utilizing an optimized phase filter at the pupil plane and a programmed roughness mask, the transfer of 200 nm roughness period to the wafer plane was eliminated. This mitigation effect was found to be strongly dependent on the focus.

Research described in this work includes exploration of new resist systems that can be used for 193 nm lithography. Most of the conventional 193 nm resists rely on chemical amplification in which a small amount of photogenerated acid can catalyze many reactions. Even though the sensitivity is improved drastically with such a mechanism, the printed profiles are known to be suffering from line edge roughness (LER) and image blurring. Furthermore, the resolution is limited by the diffusion length of the generated acid. For sub-32 nm technology nodes, specified LER criterion cannot be satisfied by utilizing chemically amplified resists (CAR). However, improvements in laser power and lens materials enable relaxation in high sensitivity requirement for the resist materials. When low sensitive photomaterials are acceptable, chemical amplification in the resists can be reduced or can even be completely eliminated. In this research, reasons behind the need of using non-chemically amplified resists is explained and some possible solutions are explored. At the end, a two component resist system that utilizes nitrobenzyl cholate as dissolution inhibitor and poly norbornene hexafluoro alcohol as the base resin is shown to achieve dense line/space patterns at 150 nm, 120 nm and 90 nm half pitches.