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In-Situ Metrology for Deep Ultraviolet Lithography POST-EXPOSURE
-1- In-Situ Metrology for Deep Ultraviolet Lithography Nickhil Jakatdar and Costas Spanos Electrical Engineering and Computer Sciences University of California at Berkeley, Berkeley, CA 94720 Ofﬁce: (510) 642-7156 Fax: (510) 642-2739 Abstract We are developing DUV metrology for measuring the thickness and the degree of bleaching simultaneously. This is done in-situ and in quasi real-time, and is used in unison with Experimental Designs to build semi-empirical models that characterize the process parameters. 1.0 Introduction In an effort to keep up with Moore’s Law, semiconductor industries are switching to the deep ultraviolet (DUV) lithography technology. However, the advances in technology should be accompanied by the high yield and efﬁciency required to stay competitive. The chemically ampliﬁed nature of DUV resists, the lack of resist bleaching after exposure and the measurement related parasitic exposure, make in-situ DUV metrology a very challenging problem. The goal of this paper is to investigate various approaches towards the development of in-situ metrology for DUV lithography that will ultimately be used for lithography process control and diagnosis. We begin by discussing the structure and mechanism of chemically ampliﬁed resists as an aid in understanding the practical difﬁculties encountered in developing in-situ metrology.This paper investigates the possible parameters to monitor and describes the control experiment to investigate the effect of the parasitic exposure due to actinic reﬂectance measurements. The latter part describes the experimental setup and the designed experiment that is used to build the equipment models. 2.0 Deep ultraviolet photoresist structure and mechanism In our experiments, we use the IBM APEX-E DUV resist. The main ingredients are the Photo-Acid Generator (PAG), a modiﬁed resin and a casting solvent. The structure of the resin, which plays an important role, is a 3:1 modiﬁcation of parahydroxystyrene (pHOST) serving as the backbone and tertbutoxyoxycarbonstyrene (tBOC) as the side chains of the polymer . EXPOSURE POST-EXPOSURE BAKE DEVELOPMENT Fig. 1 Bleaching takes place during the PEB step -2- Initiation: On exposure, the incident energy causes the PAG to produce acid containing H+ ions similar to the usual resist mechanism. Deprotection: The H+ ions attack the side chains (tBOC) of the polymer, generating more H+ ions and thus making the resist even more soluble. Quenching: The H+ ions are slowly quenched (usually after 300 odd reactions). This rate determines the amount of deprotection caused. These resists do not seem to exhibit any major change in thickness and there is a pronounced absence of actinic absorbance after the exposure step.The deprotection dependent reaction-diffusion process causes prominent and visible changes only during the Post-Exposure Bake (PEB), due to the destruction of the tBOC side chains. 3.0 Metrology The motivation for developing a metrology for the DUV lithography process comes from our previous successful experience with the i-line generation . It has been observed that, in order to keep tighter tolerances on the ﬁnal critical dimension, it is necessary to keep track of all intermediate processing steps, which include spin-coat and bake, exposure, post-exposure bake and development. Monitoring parameters must be able to characterize the process state of the wafer after each process step. In addition, they should change signiﬁcantly during the process steps, in order to facilitate metrology. On the basis of past experience, we have chosen the thickness and the degree of bleaching (the DUV equivalent of photoactive compound concentration) as the relevant parameters. The resist thickness is a good ﬁgure of merit after the prebake and post exposure bake while the degree of bleaching offers considerable insight into the amount of deprotection that takes place due to the combined effects of exposure and post exposure bake. Here, it is important to mention the fact that we use the term degree of bleaching as opposed to the photoactive compound concentration (PAC) used in i-line resists, since in this case there is no signiﬁcant change in actinic absorbance after the exposure step. We use photospectrometry as the primary tool for our metrology. Since the reﬂectance spectrograph can be assumed, to ﬁrst order, to be a function of thickness, the refractive index and the degree of bleaching, we use these parameters to construct a theoretically derived reﬂectance spectrograph , that is ﬁtted onto the actual spectrograph through an optimizer, such as Simulated Annealing . 4.0 Control experiment for parasitic exposures PR (area 1) (area 2) Si Area subjected to exposure Area subjected to reﬂectance measurements Fig. 2 Control Experiment -3- Before we proceed with the Design of Experiments required to build our models using the above described metrology, we need to do a preliminary experiment to see the effect of the parasitic exposure due to the reﬂectance spectrograph (RS) measurements. An experiment is done where the wafer is coated with resist after which it is subjected to a RS measurement in order to extract the thickness. This is followed by the exposure step where two areas are exposed.This is then followed by the Post-Exposure Bake (PEB) step during which there are RS measurements made, for varying times, in one of the exposed areas.This is to simulate a series of consecutive measurements that could be made during the PEB step. We thus have one region that is exposed to both the exposure dose as well as the RS measurements and one region that has been subjected to only the former. Both these areas are measured for thickness change after the PEB step to see the effect of the parasitic exposure due to RS measurements. 5.0 Experimental Setup The reﬂectance measurements are made on the chill plate of the SVG8626 wafer track where we have installed a modiﬁed version of the commercially available Inspector by SC Technology. We use a Xenon light source because it provides the required energy for reﬂectance measurements at the lower wavelengths. To capture the degree of bleaching, the spectral range of interest will be 240 to 300 nm. For thickness measurements, we will look at the 600 to 800 nm range of the spectrum. It was also necessary to ensure that the photodetectors were sensitive to the near and deep ultraviolet wavelengths, a feature available only in the later versions of the Inspector. The probe, which is connected to the Xenon light source and detector using optical ﬁbers, was carefully aligned perpendicular to the wafer. A mechanical sensor has been installed under the chill plate to automatically trigger the measurements whenever a wafer comes in. The resist spin-on and prebake is done on the Cee 100CB spincoat/bake unit which is placed in a enclosed environment with ﬁlters that eliminate organic contamination, in order to avoid problems like Ttopping. This is followed by the ﬁrst of the reﬂectance measurements. The wafer is then exposed using the KrF Cymer excimer laser at 248 nm followed by the PEB on a hot plate. The period between the exposure and the PEB step is kept to a minimum due to the increased sensitivity of the resist to contamination . This is followed by the next reﬂectance measurement. Another interesting measurement, that will be done in the near future, will be to continuously monitor the change in thickness during the PEB to learn more about the dynamic behavior of deprotection, and to compile a change in thickness versus PEB time function for real time control of the DUV lithography process . n, k thickness RS (future work) n, k thickness RS spin coat prebake expose PEB Fig. 3 Flow chart of the experiment n = refractive index of APEX-E resist k = extinction coefﬁcient of APEX-E resist -4- The baseline conditions were established using past experience with the APEX-E resist. We are carrying out a two level, ﬁve parameter fractional factorial experiment that included sixteen runs with four center point runs totalling twenty runs in all. The parameters that we vary, are the spin speed, prebake time, exposure dose, PEB time and PEB temperature. We use two hot plates that are maintained at the two different temperatures, required for the two level experiment, in order to avoid temperature stability problems that arise when the temperatures are changed frequently on bake plates. Eventually, we plan to build the following models: - Thickness as a function of spin speed, prebake temperature and prebake time after the prebake step - Thickness as a function of exposure dose, PEB time and PEB temperature. - Degree of Bleaching as a function of exposure dose, PEB time and PEB temperature. 6.0 Conclusion We are developing in-situ DUV metrology for measuring photoresist thickness and the degree of bleaching. This information is being used to build equipment models that would eventually be used for lithography process control and diagnosis. This metrology allows us insight into both the physical as well as chemical properties of the resist. In the future, we hope to capture the dynamic behavior of the resist during the PEB step. We will also extend this metrology to measure multilayer stacks such as photoresist on oxide-silicon and photoresist on polysilicon-oxide-silicon systems. 7.0 Acknowledgment We would like to thank the staff of the Berkeley Microfabrication laboratory for their help during the experimental phase of the project. We are also grateful to Professor Andy Neureuther and his research group for valuable consultations and experimental support. We are also grateful to ATMEL, SVG, AMD, TI, Applied Materials, Lam Research, and to the SRC (95-FP-700) for funding this work. 8.0      References N.Eib, E.Barouch, U.Hollerbach, S.Orszag, “Characterization and Simulation of acid catalyzed DUV positive photoresist.”, Proc SPIE 1993, vol 1925. S.Leang, C.J.Spanos, “A Novel In-Line Automated Metrology for Photolithography", IEEE Trans. on Semiconductor Manufacturing, vol. 9, 101, Feb. 1996. M.Born, E.Wolf, “Principles of Optics-Electromagnetic theory of Propagation, Interference and Diffraction of Light”, 6th Edition, Pergamon Press, 1980. Lester Ingber, “Adaptive Simulated Annealing (ASA)”, ftp://alumni.caltech.edu/pub/ingber/, 1995 T.Kumada, et. al, “Study on the over-top coating supressing surface insoluble layer generation for chemically ampliﬁed resists”, Proc SPIE 1993, vol 1925.
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