Optics: measuring and testing – Lens or reflective image former testing
Reexamination Certificate
2000-12-06
2004-06-22
Nguyen, Tu T. (Department: 2877)
Optics: measuring and testing
Lens or reflective image former testing
Reexamination Certificate
active
06753954
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the detection of lens aberrations associated with the projection lens utilized in a lithography system and more particularly to the design, layout and application of lens-aberration monitoring structures that can be used to monitor the projection lens performance during the manufacture of semiconductor devices.
BACKGROUND OF THE INVENTION
Lithographic apparatus may employ various types of projection radiation, non-limiting examples of which include ultra-violet light (“UV”) radiation (including extreme UV (“EUV”), deep UV (“DUV”), and vacuum UV (“VUV”)), X-rays, ion beams or electron beams. Depending on the type of radiation used and the particular design requirements of the apparatus, the projection system may be for example, refractive, reflective or catadioptric, and may comprise vitreous components, grazing-incidence mirrors, selective multi-layer coatings, magnetic and/or electrostatic field lenses, etc; for simplicity, such components may be loosely referred to in this text, either singly or collectively, as a “lens”.
In a manufacturing process using such a lithographic projection apparatus, a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the images features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g., an integrated circuit (IC). Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes may be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997 ISBN 0-07-067250-4.
The current state of integrated circuit (IC) fabrication requires lithography processes to provide for patterning feature line widths to near one-half of the exposure wavelength. For the 150 nm device generation, the krypton fluoride (KrF) excimer laser (248 nm) is typically selected as the exposure source of choice. Recent research and development efforts have further demonstrated the possibility of utilizing the KrF excimer laser for the 130 nm device generation. This is achieved by combining the use of multiple resolution enhancement techniques (RET), such as, attenuated phase-shifting masks (attPSM) and off-axis illumination (OAI), in combination with optical proximity correction (OPC) techniques. One possible alternative to the foregoing techniques is to use a shorter exposure wavelength, for example, an argon fluoride (ArF) excimer laser having a wavelength of 193 nm. However, due to various complications associated with the use of the ArF excimer laser, it is likely that the KrF excimer laser will be the dominant laser of choice for fabricating the 130 nm device generation.
Regardless of the excimer laser utilized in the fabrication process, the fabrication of devices having critical dimensions of 150 nm or less requires that the near-diffraction-limited lens utilized in the fabrication process be substantially aberration free. As is known, aberrations can be caused by various sources, such as a defective lens or an aging laser which emits a beam having a frequency shifted from the desired value. Accordingly, it is desirable to verify lens performance (i.e., qualify the lens) prior to installation, and then to subsequently monitor the lens performance during use (e.g., in an IC fabrication process).
During the lens manufacturing process, the lens performance can be fully tested interferometrically. Typically, the lens is first qualified at the factory and then again during the initial installation in the field. One common practice utilized for lens qualification is to print wafers and then measure the dimensions of the minimum feature width, or the critical dimension (CD). During this qualification process, both “vertical” and “horizontal” features are measured (i.e., features extending in two orthogonal directions on the substrate plane). In some instances, the CD for 45-degree features is also measured. In order to verify lens performance, a sufficient number of CD measurements are required across the entire exposure field. The results of the CD measurements are then analyzed to determine whether or not the lens performance is acceptable.
Although the CD measurement method provides a method of evaluating the performance of the lens, it is not a simple task to correlate the CD data to the “signature” of the lens aberration. Accordingly, there have been efforts to perform a direct observation of lens aberrations. For example, an article by Toh et al. entitled “Identifying and Monitoring of Lens Aberrations in Projection Printing,” SPIE Vol. 772, pp. 202-209 (1987) described methods for measuring the effects of relatively large lens aberrations of about 0.2&lgr;, where &lgr; is the exposure wavelength. However, for today's near-diffraction-limited optics, any lens aberration is likely to be in the neighborhood of 0.05&lgr;, or smaller. For 130 nm features, a 0.05&lgr; lens aberration translates to a 12.4 nm dimensional error when utilizing the KrF exposure source. Accordingly, if the feature CD budget (i.e., error tolerance) is assumed to be ±10% of the target feature width, a 12.4 nm error consumes almost the entire CD budget.
In an article by Gortych et al. entitled “Effects of Higher-Order Aberrations on the Process Window,” SPIE Vol. 1463, pp. 368-381 (1991) it was demonstrated that higher-order lens aberrations could deteriorate lithographic process windows. Unfortunately, the higher-order lens aberrations are difficult to eliminate after the photolithography system is assembled. In an article by Brunner entitled “Impact of Lens Aberration on Optical Lithography,” INTERFACE 1996 Proceedings, pp. 1-27 (1996) simulation was utilized to demonstrate the negative impact of near-wavelength features due to several first-order lens aberrations. Specifically, it was possible to observe coma aberrations by examining how the contact features were printed when utilizing attenuated PSM. It is also known that that lens aberrations can be balanced with custom off-axis illumination. Others have attempted to make direct measurements of various kinds of lens aberrations in an effort to achieve better CD control.
An article by Farrar et al. entitled “Measurement of Lens Aberrations Using an In-Situ Interferometer Reticle,” Advanced Reticle Symposium, San Jose, Calif. (June 1999) reported the use of an in-situ interferometer reticle to directly measure lens aberration. According to Farrar, it was possible to derive lens aberrations up to 37 Zernike terms. Although Farrar claims that the method is accurate and repeatable, it involves hundreds or thousands of registration type measurements (i.e., the measuring of the shift in relation to the intended feature position). As such, while Farrar's method may be accurate and repeatable, with the need for such exhaustive measurements, the method is clearly very time consuming, and therefore likely unusable in a manufacturing-driven environment. Furthermore, it is conceivable that minute lens aberrations can drift over time due to various reasons (e.g., as a result of the periodic preventive maintenance performed o
ASML Masktools B.V.
McDermott & Will & Emery
Nguyen Tu T.
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