Taro Ogata

An aberration control of projection optics for multi-patterning lithography

In order to realize further improvement of productivity of semiconductor manufacturing, higher throughput and better imaging performance are required for the exposure tool. Therefore, aberration control of the projection lens is becoming more and more important not only for cool status performance but also heating status. In this paper, we show the improvements of cool status lens aberration, including scalar wavefront performance and polarization aberration performance. We also discuss various techniques for controlling thermal aberrations including reduction of heat in the lens, simulation, compensating knob, and adjusting method with actual imaging performance data during heating and cooling.

Thermal aberration control for low-k1 lithography

For many years, we have used a lens aberration controller that works via positioning elements of the projection lens assembly. While this has worked well, its disadvantage is that controllable aberrations are only relatively low order components and not enough for the degree of compensation of thermal aberrations required by leading-edge lithography. We have developed two methods to overcome thermal aberrations specific to dipole illumination exposure. One scheme is process-dedicated aberration control by the conventional aberration controller. The other is aberration control system using infra-red irradiation. This system can compensate uniform astigmatism which is generated by asymmetric setting of illumination light sources, such as dipole illumination schemes. Theses two techniques allow us to increase productivity by reducing pattern imaging performance degradation due to thermal aberrations. These schemes are applicable not only to current systems but also to next generation very low k1 lithography systems with very high throughput.

Size-dependent flare and its effect on imaging

Flare, or unwanted light scattering, is an increasingly important phenomenon in modern lithographic lens design, operation, and testing. While it has been in the past and still frequently is characterized by a single number (“<1% flare”), it is now commonly recognized that flare is, in fact, best described by an intensity function over a spatial-frequency or scattering-length spectrum. We present a systematic study of flare as a function of scattering length. Data for a series of scanners are presented, showing the improvement in flare performance of new scanners versus previous-generation models. The effects of the entire flare spectrum are modeled, showing the effects of the flare spectrum on contrast degradation in an aerial image. Results show that experimental measurements of the flare spectrum are still too unstable for reliable assessment of the spectrum’s effects, but also that it is unlikely that low-range parts of the spectrum have a significant litho effect.

Imaging application tools for extremely low-k1 ArF immersion lithography

The k1 factor continues to be driven downward in ArF immersion lithography, even below its limit from optical theory, using various lithographic techniques such as combination of Source and Mask Optimization (SMO) and multiple patterning. Such a low k1 factor tends to lead to extremely high sensitivity tp imaging parameters such as aberrations, distortion, illumination pupilgram shape, dose, focus, etc. Therefore, fast, precise and stable settings of these parameters are crucial to make such hyper low k1 lithography practical. We introduce various kinds of imaging application tools and technique, which we have been developing, to support the imaging parameter settings and control. The application tools cover illumination pupilgram adjustment for freeform illumination, automatic aberration control, and thermal aberration parameter settings.

Scanner performance predictor and optimizer in further low-k1 lithography

Due to the importance of errors in lithography scanners, masks, and computational lithography in low-k1 lithography, application software is used to simultaneously reduce them. We have developed “Masters” application software, which is all-inclusive term of critical dimension uniformity (CDU), optical proximity effect (OPE), overlay (OVL), lens control (LNS), tool maintenance (MNT) and source optimization for wide process window (SO), for compensation of the issues on imaging and overlay. In this paper, we describe the more accurate and comprehensive solution of OPE-Master, LNS-Master and SO-Master with functions of analysis, prediction and optimization. Since OPE-Master employed a rigorous simulation, a root cause of error in OPE matching was found out. From the analysis, we had developed an additional knob and evaluated a proof-of- concept for the improvement. Influence of thermal issues on projection optics is evaluated with a heating prediction, and an optimization with scanner knobs on an optimized source taken into account mask 3D effect for obtaining usable process window. Furthermore, we discuss a possibility of correction for reticle expansion by heating comparing calculation and measurement.

Imaging optics setup and optimization on scanner for SMO generationprocess

Source & Mask Optimization1 (SMO) is a promising candidate to realize further reduction of k1 factor to achieve 22nm feature lithography and beyond. To make the SMO solutions feasible all imaging-related parameters should be closer to the designed parameters used in SMO process. In this paper, we discuss how we realize this in the imaging system setup on the scanner. The setup process includes freeform pupilgram generation, pupilgram adjustment and thermal aberration control. For each step the important factors are speed and accuracy.

Solutions with precise prediction for thermal aberration error in low-k1 immersion lithography

Thermal aberration becomes a serious problem in the production of semiconductors for which low-k1 immersion lithography with a strong off-axis illumination, such as dipole setting, is used. The illumination setting localizes energy of the light in the projection lens, bringing about localized temperature rise. The temperature change varies lens refractive index and thus generates aberrations. The phenomenon is called thermal aberration. For realizing manufacturability of fine patterns with high productivity, thermal aberration control is important. Since heating areas in the projection lens are determined by source shape and distribution of diffracted light by a mask, the diffracted pupilgram convolving illumination source shape with diffraction distribution can be calculated using mask layout data for the thermal aberration prediction. Thermal aberration is calculated as a function of accumulated irradiation power. We have evaluated the thermal aberration computational prediction and control technology “Thermal Aberration Optimizer” (ThAO) on a Nikon immersion system. The thermal aberration prediction consists of two steps. The first step is prediction of the diffraction map on the projection pupil. The second step is computing thermal aberration from the diffraction map using a lens thermal model and an aberration correction function. We performed a verification test for ThAO using a mask of 1x-nm memory and strong off-axis illumination. We clarified the current performance of thermal aberration prediction, and also confirmed that the impacts of thermal aberration of NSR-S621D on CD and overlay for our 1x-nm memory pattern are very small. Accurate thermal aberration prediction with ThAO will enable thermal aberration risk-free lithography for semiconductor chip production.

An intelligent imaging system for ArF scanner

The k1 factor continues to be driven downwards, even beyond its theoretical limit 0.25, in order to enable the 32 nm feature generation and beyond. Due to the extremely small process-window that will be available for such extremely demanding imaging challenges, it is necessary that not only each unit contributing to the imaging system be driven to its ultimate performance capability, but also that the final integrated imaging system apply each of the different components in an optimum way with respect to one another, and maintain that optimum performance level and cooperation at all times. Components included in such an integrated imaging system include the projection lens, illumination optics, light source, in-situ metrology tooling, aberration control, and dose control. In this paper we are going to discuss the required functions of each component of the imaging system and how to optimally control each unit in cooperation with the others in order to achieve the goal of 32 nm patterning and beyond.

Imaging control functions of optical scanners

For future printing based on multiple patterning and directed self-assembly, critical dimension and overlay requirements become tighter for immersion lithography. Thermal impact of exposure to both the projection lens and reticle expansion becomes the dominant factor for high volume production. A new procedure to tune the thermal control function is needed to maintain the tool conditions to obtain high productivity and accuracy. Additionally, new functions of both hardware and software are used to improve the imaging performance even during exposure with high-dose conditions. In this paper, we describe the procedure to tune the thermal control parameters which indicate the response of projection lens aberration and reticle expansion separately. As new functionalities to control the thermal lens aberration, wavefront-based lens control software and reticle bending hardware are introduced. By applying these functions, thermal focus control can be improved drastically. Further, the capability of prediction of reticle expansion is discussed, including experimental data from overlay exposure and aerial image sensor results.

Fractal model applied wavefront aberration for the expression of local flare

Various inputs representing exposure tool will be required for the advanced OPC in the future. Among them, local flare is difficult to account in OPC because of difficulty to make accurate model it and incorporate it in imaging simulations. In this paper we introduce a method to input the local flare in the simulations. It applies fractal model to the PSD of wavefront aberration, and it generates a model wavefront expressing local flare. This model wavefront is consistent with the disappearing pad experiments. The impact of local flare on the OPC is estimated by the imaging simulation involving the generated model wavefront.