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

摘要

EUV lithography (EUVL) is a candidate technology for patterning of ever shrinking feature sizes in integrated circuits. There are several challenges to high volume manufacturing of devices using EUVL in a cost-effective manner, which include limited source power, mask defects and non-idealities in the photoresist, the imaging medium. Focus of this thesis is on photoresists. Specifically, influence of absorption shot noise on the final LER was studied experimentally 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 experiments were performed with matched imaging conditions between EUV and e-beam, which allowed for a fair comparison between the LER values measured using the two patterning technologies. 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 in closely matched image gradients between e-beam and EUV patterning. It was shown that the measurable parameter known as the exposure latitude is a good parameter to test whether the aerial images between two experiments have identical gradients for idential materials and processing conditions.;With matched imaging conditions, resist materials and processing conditions, lithographic data showed that the incident flux needed to pattern 50 nm half-pitch lines and spaces for a leading chemically amplified resist was 10.7 photons/nm 2 for 92 eV photons, and 4.44 electrons/nm2 for 100 keV electrons. Measurements of absorption of 100 keV electrons estimated through an EELS measurement with 120 keV beam showed that despite having access to core levels in the material (e.g., 284 eV edge in carbon), these electrons mostly just excite 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 with 100 keV electrons was 0.4, about 2.35x larger than that for EUV (0.17). By combining the incident flux and the absorption probabilities, the absorption flux was found to be similar between the two patterning technologies. A possible reason is that either the secondary electron 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 matter from the standpoint of acid generation statistics. With matched imaging conditions and matched absorption density, the mean LER for e-beam was found to be larger by about 1 nm.;Influence of various material contributors in determining the resist LER was also studied from a modeling standpoint. Reaction/diffusion parameters in a stochastic resist model were calibrated to resist contrast curve data and line/space patterns. With the best fit reaction and diffusion parameters, the contributions of absorption shot noise, acid generation statistics and the base counting statistics on the resist LER were determined. Shot noise was found to account for 46% of the total LER, while the acid generation and base loading statistics were found to account for 22% and 32% of the LER respectively.;Interactions of low energy electrons in EUV resists were studied from both experimental and modeling standpoint. Low energy (< 92 eV) electrons are primarily responsible for initiating chemistry that leads to image formation in EUV resists. Thus key to controlling EUV exposure efficiency is understanding low energy electron radiation chemistry efficiency as a function of electron energy. Thickness versus exposure dose measurements were made with incident electron energies ranging between 29 eV and 91 eV. Thickness removed was much larger than the average secondary electron range and was bake temperature dependent and thus is a useful indicator of de-protection blur introduced by the bake process. The dissolution 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 slightly more effective at causing chemistry. The volume removed per eV was about 0.1 nm3 per 1 nm2 area.;The well-known dielectric model for inelastic scattering was used to develop a stochastic model for simulating trajectories traversed by secondary electrons in the resist. Electron energy loss spectroscopy (EELS) was used to measure the dielectric function for a leading chemically amplified resist. Analytical expressions for the Mermin dielectric functions which account for energy and momentum transfer were then fit to the measurement to build a complete dielectric model for the resist. Stochastic simulations were then performed with the scattering parameters determined by the dielectric model to calculate energy deposition and acid generation statistics. These results were used to quantify the net acid generation blur, 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 the best fit sigma parameters in the distributions were found to range between 1.2 nm at 30 eV and 1.41 nm at 91 eV. The net acid yield calculated by the simulator was found to be 1.6 for an 80 eV electron.