Riki Ogawa

A novel defect detection optical system using 199-nm light source for EUVL mask

Lithography potential expands for 45nm node to 32nm device production by the development of immersion technology and the introduction of phase shift mask. We have already developed the mask inspection system using 199nm wavelength with simultaneous transmitted illumination and reflected illumination optics, and is an effectual candidate for hp 32nm node mask inspection. Also, it has high defect sensitivity because of its high optical resolution, so as to be utilized for leading edge mask to next generation lithography. EUV lithography with 13.5nm exposure wavelength is dominant candidate for the next generation lithography because of its excellent resolution for 2x half pitch (hp) node device. But, applying 199nm optics to complicated lithography exposure tool option for hp2x nm node and beyond, further development such as image contrast enhancement will be needed. EUVL-mask has different configuration from transmitted type optical-mask. It is utilized for reflected illumination type exposure tool. Its membrane structure has reverse contrast compared with optical-mask. This nature leads image profile difference from optical-mask. A feasibility study was conducted for EUV mask pattern defect inspection using DUV illumination optics with two TDI (Time Delay Integration) sensors. To optimize the inspection system configuration, newly developed Nonlinear Image Contrast Enhancement method (NICE) is presented. This function capability greatly enhances inspectability of EUVL mask.

Development of advanced mask inspection optics with transmitted and reflected light image acquisition

The lithography potential of an ArF (193nm) laser exposure tool with high numerical aperture (NA) will expand its lithography potential to 65nm node production and even beyond. Consequently, a mask inspection system with a light source, whose wavelength is nearly equal to 193nm, is required so as to detect defects of the masks using resolution enhancement technology (RET). Wavelength consistency between exposure tool and mask inspection tool is strongly required in the field of mask fabrication to obtain high defect inspection sensitivity. Therefore, a novel high-resolution mask inspection platform using DUV wavelength has been developed, which works at 198.5nm. This system has transmission and reflection inspection mode, and throughput using 70nm pixel size were designed within 2 hours per mask. In this paper, transmitted and reflected light image acquisition system and high accuracy focus detection optics are presented.

257-nm wavelength mask inspection for 65-nm node reticles

We have developed a new photomask inspection method which has capability for inspecting 65nm technology node reticles using 257nm wavelength light source. This new method meets the requirement for the current mask inspection system using KrF inspection light source to be employed even in the fabrication of photomasks for 65nm technology node by the appearance of immersion technology using ArF wavelength. This paper discusses the detection capability of the 257nm wavelength inspection system for the defects on the 6% ArF attenuated phase shifting masks for 65nm node, using DSM based test pattern mask.

Advanced mask inspection optical system (AMOS) using 198.5-nm wavelength for 65-nm (hp) node and beyond: system development and initial state D/D inspection performance

A novel high-resolution mask inspection platform using DUV wavelength has been developed. This platform is designed to enable the defect inspection of high quality masks for 65nm node used in 193nm lithography. In this paper, newly developed optical system and its performance are reported. The system is operated at wavelength of 198.5nm, which wavelength is nearly equal to 193nm-ArF laser exposure tool. Some defect image data and defect inspection sensitivity due to simulation-base die-to-die (D/D) inspection are shown on standard programmed defect test mask. As an initial state D/D inspection performance, 20-60 nm defects are certified. System capabilities for 65nm node inspection and beyond are also discussed.

DUV inspection tool application for beyond optical resolution limit pattern

Mask inspection tool with DUV laser source has been used for Photo-mask production in many years due to its high sensitivity, high throughput, and good CoO. Due to the advance of NGL technology such as EUVL and Nano-imprint lithography (NIL), there is a demand for extending inspection capability for DUV mask inspection tool for the minute pattern such as hp4xnm or less. But current DUV inspection tool has sensitivity constrain for the minute pattern since inspection optics has the resolution limit determined by the inspection wavelength and optics NA. Based on the unresolved pattern inspection capability study using DUV mask inspection tool NPI-7000 for 14nm/10nm technology nodes, we developed a new optical imaging method and tested its inspection capability for the minute pattern smaller than the optical resolution. We confirmed the excellent defect detection capability and the expendability of DUV optics inspection using the new inspection method. Here, the inspection result of unresolved hp26/20nm pattern obtained by NPI-7000 with the new inspection method is descried.

Study of advanced mask inspection optics with super-resolution method for next generation mask fabrication

The lithography potential of an ArF (193nm) laser exposure tool with high numerical aperture (NA) will expand its lithography potential to 45nm node production and even beyond. Consequently, a mask inspection system with a wavelength nearly equal to 193nm is required so as to detect defects of the masks using resolution enhancement technology (RET). A novel high-resolution mask inspection platform using DUV wavelength has been developed, which works at 199nm. The wavelength is close to the wavelength of ArF exposure tool. In order to adapt 199nm optics for hp2x nm node and beyond defect detection on next generation mask with appropriate condition, further development such as the illumination condition modification technique has been studied. The illumination optics has the advantageous feature that super-resolution method is applied by adding the optics. To evaluate the super-resolution effect of illumination condition control optics, the interaction of light with mask features is calculated rigorously using RCWA (Rigorous Coupled-Wave Analysis) method. In this paper, image contrast enhancement effect using newly designed super-resolution optics which is applied to transmitted and reflected light image acquisition system are presented with simulation and experiment.

Improvement of a DUV mask inspection tool to hand over the baton for next-generation tool smoothly

Various technologies such as multiple patterning (MP) are being developed to extend the current DUV optical lithography to deal with the delay of next generation lithography such as EUV and NIL. Likewise, it is necessary to continue to develop technologies for mask inspection tools for masks fabricated for the DUV optical lithography so that they can be appropriately inspected, until the next generation EB or EUV actinic inspection tools is put into practical use. To fabricate 1x nm devices with the present lithography process, the industry will likely further extend double patterning (DP) to multiple patterning (MP). For MP, the requirements for the inspection sensitivity of traditional defects such as intrusions or extrusions do not change much, but those for CD control and overlay tolerances will become more critical. In this paper, we will discuss the main features of NPI-7000, a DUV based mask inspection tool for the 1x nm node devices, and our challenges in enhancing the CD error sensitivities to enable the inspection of masks.

Development of a captured image simulator for 199 nm mask inspection tools

Recently, technologies of ArF laser exposure tools and alternating phase shifting masks (Alt-PSM) are expected to be used in actual production. To utilize such newly developed technologies, it is inevitable to develop a mask inspection technology to check them properly. But it is currently difficult to check them precisely because sufficient image contrast is hard to obtain with any conventional mask inspection tools. It is not well understood whether we can get sufficient sensitivity with conventional optical setups and wavelength with the assistance of some kind of resolution enhancement techniques (RET), or we should move toward inspection using revolutionary new optics or shorter inspection wavelength. To study precisely the sensitivity of inspection optics for common types of defects, we have made a captured image simulator based on the RCWA (Rigorous Coupled Wave Analysis) method with which we can take into account the effect of the three-dimensional structure of a mask. We tried to calculate captured images for some mask structures at two different wavelengths, namely 199 nm and 257 nm. We made certain that no significant differences were observed for larger scale defects, but that a considerable difference of image contrast was observed for small scale defects around 50 nm in size. Thus we confirmed that this simulator is effective for evaluating and designing optical systems of mask inspectors, in order to achieve a high sensitivity for the detection of small defects in Alt-PSMs.

Development of a captured image simulator for the differential interference contrast microscopes aiming to design 199 nm mask inspection tools

Recently, technologies of ArF laser exposure tools and alternating phase shifting masks (Alt-PSM) are expected to be used in actual production. To utilize such newly developed technologies, it is inevitable to develop a mask inspection technology to check them properly. But it is currently difficult to check them precisely because sufficient image contrast is hard to obtain with any conventional mask inspection tools. Among many observation methods, the differential interference contrast (DIC) is one of a few methods that can be used to observe a differentiated phase shift of transmitted light of an object with high resolution. To study precisely the performance of this optical configuration, we built a new captured image simulator in which Wollaston prisms were modeled as a kind of phase modulation plates. We built this simulator as an extension of the captured image simulator we reported formerly), which is based on Rigorous Coupled- Wave Analysis (RCWA) to calculate diffractions; this enables us to properly treat effects of polarization, high NA, and 3-dimensional mask structures. We applied this simulator to see sensitivities of DIC against bumps and divots with various sizes. We found that the image contrast for small phase defects 20 to 50 nm in sizes is much higher in DIC microscopes than in conventional optical setup with coherence factor less than 1. We also found the dependence of captured images on polarizations and optical axis directions. We expect our simulator to be a useful tool for studying, designing, and developing mask inspection tools.

Characteristics of an autofocus system on a grating with period smaller than the focus-beam wavelength

In this paper, characteristics of an autofocus system on a grating with period smaller than the focus beam wavelength are investigated. The through-the-lens autofocus system, which has a visible light source of 670nm wavelength and light radiation mechanism for causing light to be obliquely incident on a sample surface, has been prepared for experiments. It is shown in experiments that the focusing error for a grating with 0.6-micrometer period is larger than 0.2 micrometer, and polarization of reflected light is changed from circular to elliptic. By performing RCWA simulations, the qualitative correspondence of theoretical expectations with experimental results is obtained. Based on the result of experiments and simulations, methods of reducing the focusing error are proposed. One of the methods is to use the polarization information to correct the focusing error. The method is evaluated experimentally and is shown to achieve autofocus accuracy of ± 0.1 micrometer.