Study of Line Edge Roughness and Interactions of Secondary Electrons in Photoresists for EUV Lithography

EUV lithography (EUVL) is a candidate technology for patterning of ever shrinking feature/nsizes in integrated circuits. There are several challenges to high volume manufacturing/nof devices using EUVL in a cost-effective manner, which include limited source power, mask/ndefects and non-idealities in the photoresist, the imaging medium. Focus of this thesis is on/nphotoresists. Specifically, influence of absorption shot noise on the final LER was studied/nexperimentally through comparative analysis of LER obtained with EUV (92 eV photons)/nand 100 keV e-beam lithography. The key contribution here is that the lithography experiments/nwere performed with matched imaging conditions between EUV and e-beam, which/nallowed for a fair comparison between the LER values measured using the two patterning/ntechnologies. In scenarios where the e-beam spatial resolution was better than that for EUV,/nthe technique of gray-scale e-beam lithography was experimentally demonstrated to result in/nclosely matched image gradients between e-beam and EUV patterning. It was shown that the/nmeasurable parameter known as the exposure latitude is a good parameter to test whether/nthe aerial images between two experiments have identical gradients for idential materials and/nprocessing conditions./nWith matched imaging conditions, resist materials and processing conditions, lithographic/ndata showed that the incident flux needed to pattern 50 nm half-pitch lines and/nspaces for a leading chemically amplified resist was 10.7 photons/nm2 for 92 eV photons,/nand 4.44 electrons/nm2 for 100 keV electrons. Measurements of absorption of 100 keV electrons/nestimated through an EELS measurement with 120 keV beam showed that despite having/naccess to core levels in the material (e.g., 284 eV edge in carbon), these electrons mostly just/nexcite the energy levels less than 100 eV in the resist, with a mean deposited energy of 35 eV./nResults showed that the probability of an energy loss event in a 45 nm thick resist film with/n100 keV electrons was 0.4, about 2.35x larger than that for EUV (0.17). By combining the/nincident flux and the absorption probabilities, the absorption flux was found to be similar/nbetween the two patterning technologies. A possible reason is that either the secondary/nelectron 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 not/nmatter from the standpoint of acid generation statistics. With matched imaging conditions/nand matched absorption density, the mean LER for e-beam was found to be larger by about/n1 nm./nInfluence of various material contributors in determining the resist LER was also studied/nfrom a modeling standpoint. Reaction/diffusion parameters in a stochastic resist model were/ncalibrated to resist contrast curve data and line/space patterns. With the best fit reaction/nand diffusion parameters, the contributions of absorption shot noise, acid generation statistics/nand the base counting statistics on the resist LER were determined. Shot noise was found/nto account for 46% of the total LER, while the acid generation and base loading statistics/nwere found to account for 22% and 32% of the LER respectively./nInteractions of low energy electrons in EUV resists were studied from both experimental/nand modeling standpoint. Low energy (< 92 eV) electrons are primarily responsible for/ninitiating chemistry that leads to image formation in EUV resists. Thus key to controlling/nEUV exposure efficiency is understanding low energy electron radiation chemistry efficiency/nas a function of electron energy. Thickness versus exposure dose measurements were made/nwith incident electron energies ranging between 29 eV and 91 eV. Thickness removed was/nmuch larger than the average secondary electron range and was bake temperature dependent/nand thus is a useful indicator of de-protection blur introduced by the bake process. The/ndissolution volume per eV deposited energy was nearly similar for 29 eV to 91 eV energies,/nalthough there is some indication that incident electrons with lower energies are slightly/nmore effective at causing chemistry. The volume removed per eV was about 0.1 nm3 per 1/nnm2 area./nThe well-known dielectric model for inelastic scattering was used to develop a stochastic/nmodel for simulating trajectories traversed by secondary electrons in the resist. Electron/nenergy loss spectroscopy (EELS) was used to measure the dielectric function for a leading/nchemically amplified resist. Analytical expressions for the Mermin dielectric functions which/naccount for energy and momentum transfer were then fit to the measurement to build a/ncomplete dielectric model for the resist. Stochastic simulations were then performed with/nthe scattering parameters determined by the dielectric model to calculate energy deposition/nand acid generation statistics. These results were used to quantify the net acid generation/nblur, which was found to be between 1.8 nm and 2 nm from the point of origin of the electrons./nThe radial distribution of acid generation sites was fit using a Rayleigh distribution and the/nbest fit sigma parameters in the distributions were found to range between 1.2 nm at 30 eV and/n1.41 nm at 91 eV. The net acid yield calculated by the simulator was found to be 1.6 for an/n80 eV electron.

Advisor: Neureuther, Andrew R.