Kiminori Yoshino

Advanced lithography: wafer defect scattering analysis at DUV

Considerable effort is directed towards the development of next-generation lithography processes, addressing the need for transistor densification to meet Moores Law. The aggressive design rule shrinkage requires very tight process windows and induces various types of pattern failure with lithography process variations. Since the lithography process is critical in the wafer fabrication process, the requirements for high sensitivity defect detection in the lithography process becomes tighter as design rules shrink. Analysis of the root cause of the defects and of their interaction with various light sources and optics systems configurations for wafer inspection is essential for understanding the detection limits and requirements from advanced inspection systems targeting future lithography inspection applications. In this work, we present an analysis of wafer defects light scattering and detection for a variety of 3xnm design rule resist structures with various polarizations and optics configurations, at the visible, at UV and at DUV wavelengths. The analysis indicates on the defect scattering and inspection performance trends for a variety of resist structures and defect types, and shows that control of the polarization of the optical inspection system is critical for enhanced scattering and detection sensitivity. The analysis is performed also for the 2xnm and 1xnm design rules showing the advantages of polarized DUV illumination over unpolarized and visible illumination.

Novel inspection technology for half pitch 55 nm and below

In the automatic macro inspection, a diffraction light method is very effective. However, this method needs a shorter wavelength illumination for finer wafer patterns. A wavelength of 193 nm will be needed for half pitch 55 nm. Light source and optics for such shorter wavelength is large and expensive, and chemical clean environment is needed. Therefore, the equipment size and costs will increase dramatically. In order to solve this problem and to comply with the process of half pitch 55 nm and below, we have developed the breakthrough technology. The key is the image of polarization fluctuation caused by a wafer pattern structure. The polarized light is affected by the variation of the wafer pattern structure due to a dose or focus shift. The new technology converts the polarization fluctuation into the gray level of the image. At a result, the sensitivity for the dose or focus shift was enough to detect process errors.

Novel CD inspection technology leveraging a form birefringence in a Fourier space

A new technology was developed to detect Critical Dimension (CD) variations in a Fourier space. The detection principle is a form birefringence of the wafer. Utilizing this principle, CD and Pattern Edge Roughness (PER) variations are detected as a polarization fluctuation and converted into light intensity. We have achieved high resolution and high sensitivity by combining a form birefringence with a novel optical system. This system detects the light intensity in a Fourier space with a high NA objective, enabling the detection of various lights with different incident angles and polarization states at a time. We have confirmed through simulations that this system has high sensitivity toward CD variations. Furthermore, in partnership with Toshiba Corporation, and through the evaluation of wafers fabricated at Toshiba, we conclude that the light intensity detected by the new system strongly correlates with CD values, and that the new system is capable of detecting CD variations in sufficient sensitivity.

Novel technology of automatic macro inspection for 32-nm node and best focus detection

As the semiconductor design rules shrink down, process margins are getting narrower, and thus, it is getting more important than ever to monitor pattern profile and detect minor structure variation. A breakthrough technology has been introduced as a solution to this concern. The new technology converts the fluctuation of polarization ingredient, which is caused by form birefringence, into light intensity variations as an optical image. This technology, which is called Pattern Edge Roughness (PER) inspection mode, is proved to be effective for 55nm production process. We also studied the possibility of the macro inspection method for half pitch 32nm technology node through FDTD method.

New exposure tool management technology with quick focus measurement in half pitch 22nm generation

We have developed the new technology to measure focus variations in a field or over the wafer quickly for exposure tool management. With the new technology, 2-dimensional image(s) of the whole wafer are captured with diffraction optics, and by analyzing the image signal(s), we are able to get a focus map in an exposure field or over the entire wafer. Diffraction-focus curve is used instead of a CD-focus curve to get the focus value from the image signal(s). The measurements on the production patterns with the production illumination conditions are available. We can measure the field inclination and curvature from the focus map. The performance of the new method was confirmed with a test pattern and production patterns.

New Measurement Technology for CD and Pattern Profile Variation using Optical Fourier Space

As well as measuring CD, monitoring pattern profile is becoming important for semiconductor metrology. Illuminating the wafer and detecting the reflective light, reflective light intensity in the Fourier space includes the information of CD and pattern profile variation by form birefringence effect. CD change and profile variation could be detected separately for the actual wafer. Mathematical simulation is presented the background of our unique approach. The detail results of CD and pattern profile monitor is shown in this paper.

New Inspection Technology for Hole Pattern by Fourier Space on hp 4x-nm Generation

We tried to detect the CD variation of the 4x generation hole pattern using the diffraction light on Fourier space with the polarized light and the modified illumination. The new technology named DD (Dual Diffraction) method has been developed based on the optical simulation and the experimental approaches. We introduce the case of detection for the diameter variation on a multi-layered hole pattern with new method.

Hole inspection technology using Fourier imaging method

There are two kinds of critical dimension (CD) management tools; CD-SEM and Optical CD (OCD). OCD is preferable to other existing measurement tools, because of its higher throughput and lower photoresist damage. We have developed an Automated Pattern profile Management (APM) systems based on the OCD concept. For the monitoring thin line, APM detects light intensity from an optical system consisting of a polarizer and an analyzer set in a cross- Nicol configuration as a polarization fluctuation. This paper reports our development of monitoring technology for hole. In the case of hole management, APM detects light intensity from diffraction intensity fluctuation. First of all, the best conditions for hole management were designed from simulations. The best conditions were off-axis aperture and S polarizer. In our evaluation of wafers without underlayer, we obtained a good correlation with CD-SEM value. From the simulation, we consider the APM system to be very effective for shrinking hole process management of the next generation from the simulation.

Development of optical simulation tool for defect inspection

Much effort has been done to detect the defects of interest (DOI) by optical inspection systems because the size of the DOI shrinks according to the design rule of a semiconductor device. Performance of the inspection system is dependent on complicated optical conditions on illumination and collection systems including wavelength and polarization filter. Magnitude of defect signal for a given optical condition was estimated using a simulation tool to find a suitable optical condition and technologies required in the future. This tool, consisting of a near-field calculation using Finite Difference Time Domain (FDTD) methods and an image formation calculation based on Fourier optics, is applicable not only to Köhler illumination system but also to confocal system and dark field system. We investigated defect inspection methods for the 45 nm and the next technology nodes. For inspection of various defects, the system using several wavelengths is suitable. For inspection of a specific defect, the system with polarization control is suitable. Our calculation suggests that the defect detection sensitivity for the 1X nm technology node should be increased by more than 10 times compared to the 45 nm technology node.

Two-dimensional dose and focus-error measurement technology for exposure tool management in half-pitch 3x generation

As design rule of semiconductor device is shrinking, pattern profile management is becoming more critical, then high accuracy and high frequency is required for CD (Critical Dimension) and LER (Line Edge Roughness) measurements. We already presented the technology to inspect the pattern profile variations of entire wafer with high throughput [1] [2]. Using the technology, we can inspect CD&LER variations over the entire wafer quickly, but we could not separate the signal into CD and LER variations. This time, we measured the Stokes parameters, i.e., polarization status, in the reflected light from defected patterns. As the result, we could know the behavior of the polarization status changes by dose & focus defects, and we found the way to separate the signal into CD&LER variations, i.e. dose errors and focus errors, from S2 & S3 of Stokes parameters. We verified that we were able to calculate the values of CD&LER variations from S2 & S3 by the experiments. Furthermore, in order to solve the issue that many images are needed to calculate S2 & S3 values, we developed the new method to get CD&LER variations accurately in short time.