Tsuneyuki Hagiwara

Development of aerial image based aberration measurement technique

To maintain the best imaging performance of current high NA DUV scanners, in-situ aberration measurement is becoming more important than ever. In this paper, we present an aerial image based aberration measurement technique that can measure the aberrations up to 37th Zernike polynomial term. Our aberration measurement technique uses aerial image sensor (AIS) on DUV scanners. AIS is a slit scanning type aerial image sensor that can capture the one-dimensional intensity distribution of aerial images. Unlike previous photo resist image based aberration measurement technique, presented technique does not require the three-beam interference condition or the two-beam interference condition because it utilizes the image intensity information. This can eliminates the geometrical restriction in determination of the pupil sampling points. Thus, we made optimization of pupil sampling so that it can minimize the random error propagation in each Zernike coefficients. This optimization was done on a trial and error basis and we observed that the random error propagation significantly depended on pupil sampling plan. The measured aberration was correlated to the programmed aberration induced by lens element displacement. Also the measurement repeatability was evaluated and confirmed. The overall performance of this aberration measurement technique is found to be appropriate for in-situ aberration monitor of current high NA scanners.

Aerial image sensor: in-situ scanner aberration monitor

IC manufacture has to meet stringent requirements pushing the imaging tools beyond their limits. The key performance attribute of the imaging tool is the quality of the image projected on wafer plane. The image quality is controlled by the wavefront aberrations present in the projection lens pupil. Therefore the quality of the lenses can be represented by either various image quality metrics or by the data on the lens pupil aberration residua. Projection lens quality can be quantified by interferometers capturing the lens pupil residual aberration, leading to estimates of the image quality. These various techniques can be used off-line, testing projection lenses installed on a dedicated test bench, used during or after lens manufacture, or in-situ, testing the lenses installed in the projection tools, often at the IC manufacturing floor. These techniques have inherent tradeoffs in terms their accuracy, portability, ease-of-use and completeness of the aberration and imaging metrics. Such tradeoffs determine which technique is the most appropriate for various applications ranging from lens quality control during imaging tool manufacture, to tool qualification during its installation and setup, to tool monitoring and tuning during the IC manufacture. It is acknowledged within the scanner engineering community that qualification and maintenance of tools used for critical level pattering requires in-situ lens monitoring technique. Such method would also help to select and to fine tune the imaging tools to design-specific requirements of IC critical patterns. A preferred method of aberration monitoring should be highly compatible with routine scanner operation and should be independent of resist process conditions. This paper presents aerial image-based technique to monitor and to diagnose the quality of projection lenses used in scanners. The method involves aerial image sensor, AIS. We start with a discussion of the fundamental principles of operation and the key design issues impacting the accuracy of the technique. We follow with an examples of the AIS aberration test. These tests lead to a discussion of the methods capabilities to quantify the performance of the imaging tools.

Self-calibration of wafer scanners using an aerial image sensor

To improve both the versatility and stability of leading edge wafer scanners, the functionality of an integrated aerial image sensor has been expanded. The system performance of current wafer scanners is a strong function of the quality of image formation of the projection lens. Current wafer scanners use aerial image sensors for best image plane calibration, illumination telecentricity calibration, coma aberration calibration, and distortion calibration. The aerial image sensor is used not only for a scanners self-calibration but also during the projection lens manufacturing purposes. The slit-scan type aerial image sensor is used for measurement of the intensity distribution of the aerial images. This type of the image sensor can detect the intensity distribution of the aerial image from 110nm L/ S to 6micrometers L/ S. Therefore this aerial image sensor covers most aerial image measurement requirements. In this paper we will focus on the aerial image measurement for self-calibration purposes and their actual performances. We evaluate the actual performance of illumination telecentricity and coma aberration measurement. Evaluation is based upon not only measurement repeatability but also its agreement with resist image measurement results.

Aerial image sensor for self-calibration of wafer steppers

The requirement for higher resolution is pushing up the NA of the projection lens. As a result, DOF becomes shallower, and the focus budget becomes tight. Precise measurement of best focus is becoming more and more important. A new aerial image sensor is presented that is suitable for use on leading edge wafer steppers. This sensor detects the intensity distribution of aerial images down to 0.15 micrometer isolated lines, and is currently used as a best focus calibration sensor for wafer steppers. This sensor can measure best focus using both dense and isolated patterns with a precision of < 20 nm (3(sigma) ). In actual operation, determination of best focus on a wafer stepper requires only a few minutes. The functionality of this sensor is being expanded to include additional self- calibration tasks, such as magnification, illumination telecentricity, distortion, and other aberrations.

Wafer edge-shot algorithm for wafer scanners

The requirement for the higher resolution is pushing up the NA of the projection lens, so the DOF becomes shallower and the focus budget becomes tight. On the other hand, the requirement for the higher through-put is still demanding. To achieve the best throughput, the alternate scanning exposure sequence is inevitable to current wafer scanners. To realize the alternative scanning exposure, it is necessary to perform precise focusing control even at the partial shot on the wafer edge region. A wafer edge stepwise focusing algorithm is developed. This algorithm utilizes multi-points focusing sensors and dynamically switches the focusing sensors during alternating scan exposure of the partial site on the wafer edge region. Thus the amount of the defocus on the wafer edge region is minimized. The actual performance of the wafer edge stepwise focusing algorithm is discussed. This algorithm can be used with or without pitching motion control of the wafer leveling stage. The influence of the pitching motion control to the focusing performance is also discussed.

Analysis of wafer flatness for CD control in photolithography

Wafer-induced focus error is investigated for analysis of our focus budget in photolithography. Using a newly developed wafer monitor, NIWF-300 (Nikon Corp.), we directly measure surface flatness of the wafer placed on wafer holder with vacuum chuck. Single site polished Si wafers were evaluated with NIWF-300 and a conventional flatness monitor. We also investigated the effect of wafer holder using a ring-shape wafer support and a pin-shape wafer support. As a result, we found wafer shape measured in a freestanding condition does not represent surface flatness of the wafer on a holder. The holder has an impact on the wafer surface. The increase of adsorption ratio between wafer and holder improves the surface flatness.

Desirable reticle flatness from focus deviation standpoint in optical lithography

We performed precise and systematic approaches to clarify what reticle flatness should be from the standpoint of focal deviation in optical lithography. The impact of reticle warpage on focus deviation was measured by an aerial image sensor to obtain any tiny shift of reticle-induced focus precisely. We clarified the criteria of reticle flatness after chucking. An optimum free-standing shape that would become the desired shape after chucking was obtained by simulation and an analytical approach. The flatness of the chucked reticle was found to be determined by both the free-standing plate shape inside the reticle holder and the shape of the plate facing the holder. Reticle flatness was redefined according to the results. Requirements with respect to the newly defined flatness for each technology node were clarified by focus budget analysis.

Method to predict CD variation caused by dynamic scanning focus errors

From 1970s IC industry has made dramatic progress due to the advancement of photo- lithography technology. At the beginning of the 21st century, photo- lithography technologies are still acting the major role of manufacturing the leading edge devices. The requirement for the higher resolution is pushing up the NA of the projection lens. As the result, DOF becomes shallower and the focus budget becomes tight. Close study for the focus error impact to CD variation becomes more and more important. A method to predict focus error induced CD variations resulting from dynamic wafer scanning has been developed. CD variations across an exposure image field are calculated from a CD lookup table that relates a CD value to the monitored focus error components.

Influence of pellicle mounting to predicted mask flatness

The shrinkage of semiconductor devices creates demand for micronization in the photolithographic process. As a result, problems are arising in photolithography in the semiconductor manufacturing process. Focus latitude in photolithography becomes smaller as micronization advances and therefore the flatness of the mask can no longer be ignored. In the previous work, we clarified what the specification of mask flatness should be from the standpoint of its warpage in vacuum chucking of an exposure tool. A two-dimensional approach was applied for the prediction of mask surface after chucking. The approach was simple analytical calculation distinguishing between x-direction and y-direction. Warpage of mask surface after chucking had two modes depending on the directions. On was leverage caused by interaction of mask surface and chucking stage. Another one was warpage along chucking stage surface. The prediction of mask flatness showed good agreement with the actual surface of chucked mask. In this study, influence of pellicle mounting to the prediction was investigated furthermore. Difference of flatness about 0.1-0.2 μm at the pellicle mounting process was observed. This value of the flatness change is not negligible in order to contrl mask flatness for hp65 nm technology node. However, the difference between the chucked mask surface with the pellicle and that without the pellicle decreased. In order to understand the cause of the change of flatness by pellicle mounting reduced by the vacuum chucking, a simulation analysis by a FEM was performed. The simulation showed that the vacuum chucking reduces the difference of flatness to permissible value. The vacuum chucking of an exposure tool negates the warpage caused by the pellicle mounting. Since the power of the leverage caused by chuck stage is overwhelmingly large as compared with the warpage power of the pellicle, this phenomenon is observed. As a conclusion, the prediction of mask flatness with the vacuum chucking has no influence of the pellicle mounting.

Desirable wafer edge flatness for CD control in photolithography

Desirable wafer edge flatness was investigated to obtain optimum free-standing wafer edge shape for photolithography. In order to obtain the criteria of free-standing edge shape, we clarified the desirable post-chuck flatness at edge sites in advance. We investigated a desirable free-standing wafer edge, taking into consideration both the wafer and wafer holder shape. Firstly, to obtain a desirable post-chuck wafer edge shape, the vicinity of wafer edge after chucking was modeled, and SFQR was simulated. Secondly, a shape in the vicinity of free-standing edge shape was modeled, and the edge flatness after chucking was simulated. And finally, the simulated flatness was compared with the desirable post-chucked wafer edge shape, and we could obtain desirable free-standing wafer edge shape. Individual measurement of the free-standing back-side and front-side surfaces as well as the thickness of the edge position was found to be necessary for accurate estimation of the post-chuck edge shape.