Nickhil H. Jakatdar

Specular spectroscopic scatterometry

Scatterometry is one of the few metrology candidates that has true in situ/in-line potential for deep submicrometer critical dimension (CD) and profile analysis. Most existing scatterometers are designed to measure multiple incident angles at a single wavelength on periodic gratings. We extend this idea by deploying specular spectroscopic scatterometry. Specular spectroscopic scatterometry (SS) is designed to measure the zeroth-order diffraction response at a fixed angle of incidence and multiple wavelengths. This mechanism allows the use of existing thin-film metrology equipment, such as spectroscopic ellipsometers, to accurately extract topographic profile information from one-dimensional (1-D) periodic structures. In this work, we developed the grating tool-kit (gtk), which implements several variants of rigorous coupled-wave analysis (RCWA) to accurately and efficiently simulate diffraction behavior of 1-D gratings. Theoretical simulations using this package show that specular spectroscopic scatterometry can be applied in the current semiconductor manufacturing technology, and can be easily extended to the 0.07-/spl mu/m generation. We have also applied a library-based profile extraction methodology to resist and poly focus-exposure matrices patterned using 0.25and 0.18-/spl mu/m lithography and etch technology, respectively, to extract their cross-sectional profiles. Discrepancies between CD-SEM, CD-AFM, and SSS measurements are discussed and explained.

Specular spectroscopic scatterometry in DUV lithography

Scatterometry is a one of the few types of metrology that has true in-situ potential for deep submicron CD and profile analysis. To date, commercial prototypes have been used to establish scatterometry based on single wavelength, multiple incident angle inspection. We extend this idea by deploying specular spectroscopic scatterometry (SSS). Conventional scatterometry is designed to measure either many diffraction orders or variable incident/collection angle at a single wavelength. Specular spectroscopic scatterometry is designed to measure the 0th order diffraction responses at a fixed angle of incidence. Specular spectroscopic scatterometry can make direct use of the existing spectroscopic ellipsometry equipment. We show that SSS provides an accurate, inexpensive, and non-destructive CD metrology solution.

Characterization of a chemically amplified photoresist for simulation using a modified poor man's DRM methodology

As we enter the DUV lithography generation, the developmental phase of the photolithography process is becoming crucial due to the high costs associated with the lithography equipment. Improvements in the modeling of chemically amplified resists are necessary to extract the maximum possible information from the minimum amount of experimentation. The poor mans dissolution rate monitor (drm) method has been used successfully to extract the post exposure bake (PEB) and develop rate parameters for conventional I-line photoresists and some DUV chemically amplified photoresists (CARs). However, the original method suffers from some drawbacks such as locally optimized results due to the highly non-linear nature of the Mack development model and the need for visual inspection to detect convergence of the rate data. This paper used a simulated annealing optimization engine for global optimization and uses the deprotection induced thickness loss phenomenon for the conversion of dose to m. Post-exposure bake and develop rate parameters have been extracted for Shipleys UV-5 DUV photoresist.

In-line lithography cluster monitoring and control using integrated scatterometry

In the continuous drive for smaller feature sizes, process monitoring becomes increasingly important to compensate for the smaller lithography process window and to assure that Critical Dimensions (CD) remain within the required specifications. Moreover, the higher level of automation in manufacturing enables almost real-time correction of lithography cluster machine parameters, resulting in a more efficient and controlled use of the tools. Therefore, fast and precise in-line lithography metrology using Advanced Process Control (APC) rules are becoming crucial, in order to guarantee that critical dimensions stay correctly targeted. In this paper, the feasibility of improving the CD control of a 193nm lithography cluster has been investigated by using integrated scatterometry. The target of the work was to identify if a dose correction on field and wafer level, based on precise in-line measurements, could improve the overall CD control. Firstly, the integrated metrology has been evaluated extensively towards precision and sensitivity in order to prove its benefits for this kind of control. Having a long-term repeatability of significantly better than 0.75nm 3σ, this was very promising towards the requirements for sub-nanometer CD correction. Moreover, based on an extensive evaluation of the process window on the lithography cluster, it has been shown that the focus variation is minimal and that CD control can be improved using dose correction only. In addition, systematic variations in across-wafer uniformity and across-lot uniformity have been determined during this monitoring period, in order to identify correctable fingerprints. Finally, the dose correction model has been applied to compensate for these systematic CD variations and improved CD control was demonstrated. Using a simple dose correction rule, a forty percent improvement in CD control was obtained.

Phase Profilometry for the 193 nm lithography gate stack

Phase Profilometry (PP) has been proposed for in-situ/in-line critical dimension and profile measurements. This is usually accomplished by using rigorous electromagnetic theory to simulate the optical responses of gratings with different profiles, and by using spectroscopic ellipsometry/reflectometry to measure 1-D gratings. In this paper, phase profilometry is applied to the lithography process for cross-sectional profile extraction metrology. A focus-exposure experiment was conducted using Sematechs 193 nm lithography tool. Comparison between the measurements from CD-SEM, CD-AFM and PP are discussed and explained.

Neural network approach to rapid thin film characterization

A novel approach for thin film thickness and optical constant extraction from spectral reflectance data is presented here. This methodology combines the global minimization abilities of Adaptive Simulated Annealing with the high computational efficiency of Neural Networks to solve complex characterization problems in real time. The optical constants of many thin films such as Polysilicon are a function of the processing conditions and hence the real time measurement of these parameters could possibly be used in real time or run to run process control applications.

Specular Spectral Profilometry on Metal Layers

With the advent of deep sub-micron semiconductor technology, metrology for metal interconnects becomes more critical. In addition to the line width, information about the height and the sidewall profile is needed to ensure good circuit performance. Conventional metrology tools such as CD SEMs and AFMs are either unable to measure the profile, or too slow for production process control. Scatterometry is a promising candidate as in situ, full-profile metrology tool. In this method, scattering of broadband light (240 nm to 760 nm) on periodical structures is simulated by approximating the structure with a finite series of Fourier expansion terms. By comparing the measured spectrum and the simulated spectra for various possible profiles in a precalculated library, the profile can be extracted. Previous work has shown good results on resist structures. For metal structures, however, more diffraction orders need to be included to accurately simulate light scattering. In this study, a library for 0.22 micrometer line and 0.44 micrometer space metal grating structures is generated using 31 orders. The profiles of metal grating structures of the same size are extracted using this library. Our data shows that the correlation between CD-SEM and scatterometry-based profile extraction appears to be related to the sidewall angle of the profile. These discrepancies will be analyzed and discussed.

Characterization of a positive chemically amplified photoresist for process control

Chemically Amplified Resists (CARs) are much less observable than their i-line counterparts due to the absence of photoresist actinic absorbency. CARs however, exhibit resist thinning during the Post-Exposure Bake process (PEB). A Design of Experiments (DOE) technique was employed around the exposure and the PEB temperature for a commercial DUV photoresist. A Fourier Transform Infrared (FTIR) technique was used to measure the deprotection of the CARs after the PEB step while standard interferometry techniques were used for exposed area thickness loss measurements after the PEB step. Our analysis indicates that exposed area thickness loss is strongly correlated to the deprotection of the photoresist, so that thickness loss can serve as a reliable deprotection indicator and can hence be possibly used as an observable for control of the photolithography sequence.

Physical modeling of deprotection-induced thickness loss

High activation energy, chemically amplified resist systems exhibit a 4 percent to 15 percent volume shrinkage during the post-=exposure bake process. Current lithography process simulators do not take this volume shrinkage into account, thus violating the continuity equations used to model the process. This work aims at describing the kinetics of the post-exposure bake process by tracking the volume shrinkage observed in high activation resists. A dynamic model is derived and corroborated with experimental results for Shipley UV5. A global simulation technique is then used in conjunction with the models to extract the lithography parameters for these resists.

Deep ultraviolet lithography simulator tuning by resist profile matching

TCAD simulation is very important for DUV lithography process development and control. Traditional lithography process engineering has relied on short-loop and pilot-lot experiments to understand the effects of particular process control factors. However, experiments are very expensive, and the complexity of lithographic patterns and processes is such that we must often resort to computational simulation. The availability, accuracy, and ease of use of lithography simulation are essential to the semiconductor industry. In this paper we present a methodology for DUV lithography simulator tuning by resists profile matching. A global optimization procedure is used to efficiently extract the correct values of the important fitting parameters by matching the simulated resist profiles to measured data. Results for a DUV lithography process are presented.