Search

The Nikon Metrology Laser Radar system focuses a beam from a fiber to a target object and receives the light scattered from the target through the same fiber. The system can, among other things, make highly accurate measurements of the position of a tooling ball by locating the angular position of peak signal quality, which is related to the fiber coupling efficiency. This article explores the relationship between fiber coupling efficiency and focus condition.

Illumination has been a critical factor to producing good imaging in microlithography since the beginning. Early lithographers used very simple illumination schemes, but as they improved their understanding of how the illumination influences image fidelity and resolution they moved into more complex partially coherent illumination conditions. Today, lithographers deploy many tricks with coherence and polarization that are tightly coupled with the patterns being imaged to help achieve seemingly impossible resolutions well below the classical definition. In this talk we review the progress of illumination engineering in microlithography up to the present.

This excerpt gives a succinct explanation of on-axis irradiance behind a circular aperture.

This excerpt gives a succinct explanation of coherent and incoherent point spread function.

This excerpt gives a succinct explanation of interference of two plane waves (different frequency).

This excerpt gives a succinct explanation of Fresnel diffraction between confocal surfaces.

This excerpt gives a succinct explanation of Interference of two plane waves (same frequency).

Typical focus sensors in conventional integrated circuit lithographic equipment have long been known to exhibit errors due to the nature of patterns printed on the wafer. One such error can be characterized as being derived from thin film effects where the derivative of the phase on reflection by the wafer introduces a shift in the beam that is basically identical to a shift that would be generated by a change in wafer height. In this paper we present a theoretical investigation into the nature and magnitude of the focus offset produced by this wafer effect under current typical process parameters.

An original approach to measurement accuracy of a typical focus sensor in conventional integrated circuit lithographic equipment is introduced. Causes of measurement error in the focus sensor are theoretically analyzed and found to be generated mainly from interactions between imperfections of the optical system and the actual surface of processed wafers. We derive mathematical formulations describing these errors, which are confirmed by the experimental results performed by using an optical setup composed of the focus sensor and samples on which the wafer surface condition is reproduced. Furthermore, several novel techniques that are intended to reduce those measurement errors are successfully demonstrated.

As first generation extreme ultraviolet lithography (EUVL) systems go into the hands of lithographers, the obstacles to full production are reducing in number and scope. Chief among these remaining obstacles, however, is the source power. Although source manufacturers have made remarkable recent progress, power levels are still far below what is needed for production tools. This makes the importance of an efficient illumination system even more so as Moore's law pushes the industry to resolutions requiring EUVL. This paper describes some of the special issues for EUVL illumination systems, discusses several modeling tools appropriate to EUVL illumination systems, with examples based on fly's eye and debris mitigation systems found in the literature.

Geometrical limitations can sometimes preclude the accurate measurement of the bulk index of refraction. A novel dual interferometer system for measuring the bulk index of thin transparent optical materials is presented.

A long-standing goal of optical metrology is testing aspherics without the need for part specific nulls lenses. The problem involves increasing the measurement dynamic range while preserving accuracy. The Shack-Hartmann wavefront sensor offers an interesting alternative to interferometry where the dynamic range is tied to the wavelength of light. Because the Shack-Hartmann wavefront sensor is a geometric test, the lenslet array can be designed in a way that trades sensitivity for dynamic range making it possible to test, without a null, aspheres that would otherwise require null optics. However, a system with this much dynamic range will have special calibration issues. Shack-Hartmann wavefront sensors are widely used in feedback control systems for adaptive optics. In that application, calibration is not a serious problem as the system drives the correction to a null; calibration errors slow the rate of convergence. For metrology applications, the calibration of the Shack-Hartmann wavefront sensor must be absolute. This presentation will discuss issues related to the design and calibration of a Shack-Hartmann metrology system including the design of an appropriate lenslet array, methods for dealing with induced aberrations, vignetting and spatial resolution limitations.

The Shack-Hartmann (S-H) method is a good candidate for general aspheric metrology because the lenslet array can be designed to accommodate the dynamic range associated with wildly aspheric wavefronts. However, when the S-H method is used in this fashion several issues must be taken into consideration. First, while the sensitivity and dynamic range of the instrument can be increased by allowing the spots to shift several lenslet sub-apertures, real lenslets are not thin lenses with zero aperture so the spots will not shift in exact proportion to the average phase gradient across the lenslet as is commonly expected. Second, if the wavefront is sufficiently aspheric, any relay optics will induce additional aberrations, which can be accounted for with proper calibration and reverse raytracing. Another limitation of the S-H method is that spots cannot overlap or cross. While this is a limitation on the divergence of the phase distribution or wavefront curvature the problem can be avoided if we guarantee that the beam has no caustic between the lenslet array and detector. Finally, the single biggest problem in aspheric metrology is losing the light or vignetting. One general way to address this problem is to image the part onto the lenslet array with a large numerical aperture. In this way, rays leaving the part can have some range of angles that are guaranteed to make it through the system. This presentation will discuss these issues and methods for overcoming them. Experimental results will also be presented to demonstrate the effects.

Rapid growth in the contact lens industry towards higher levels of customization has precipitated the need for advances in the metrology techniques and instrumentation used to evaluate soft contact lenses. By measuring the transmitted wavefront, the information needed to evaluate a wide range of lens types (spherical, toric, bifocal) is obtained. A Mach-Zehnder interferometer is used with the lenses tested in saline solution. The lenses must be tested in saline solution to prevent dehydration of the lens, which results in an index change. The lenses are mounted in a cuvette, or water cell, that circulates fresh saline. Calibration of the instrument is complicated by the aspheric wavefronts produced by the lenses and the inherent aberrations picked up by the wavefront as it is imaged from immediately behind the lens to the detector. Simply removing a baseline, no test optic measurement from the measured wavefront does not satisfactorily remove the induced aberrations. Instead, removal of the induced aberrations is achieved by reverse raytracing. In reverse raytracing, the wavefront at the detector is traced back through the system to immediately behind the lens. The use of raytracing code enables theoretical wavefronts to be generated and expected-to-calculated performance evaluations to be made at the transmitted wavefront level.

Historically, the spots produced by a Shack-Hartmann wavefront sensor are expected to stay in a region immediately behind their respective lenslets. Valuable sensitivity and dynamic range can be recovered if the spots are allowed to wander. A new algorithm is presented for resolving this issue for a hex-packed lenslet geometry.

The basic problem associated with aspheric testing without the use of null optics is to obtain increased measurement range while maintaining the required measurement accuracy. Typically, the introduction of a custom-designed and fabricated null corrector has allowed the problem of aspheric testing to be reduced to that of spherical testing. Shack-Hartmann wavefront sensor have been sued for adaptive optics, but have seen little application in optical metrology. We will discus the use of a Shack-Hartmann wavefront sensor as a means of directly testing wavefronts with large aspheric departures. The Shack-Hartmann sensor provides interesting tradeoffs between measurement range, accuracy and spatial resolution. We will discus the advantages and disadvantages of the Shack-Hartmann wavefront sensor over more conventional metrology tests. The implementation of a Shack-Hartmann wavefront sensor for aspheric testing will be shown.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The basic problem associated with aspheric testing without the use of null optics is to obtain increased measurement range while maintaining the required measurement accuracy. Typically, the introduction of a custom-designed and fabricated null corrector has allowed the problem of aspheric testing to be reduced to that of spherical testing. Shack-Hartmann wavefront sensors have been used for adaptive optics, but have seen little application in optical metrology. We will discuss the use of a Shack-Hartmann wavefront sensor as a means of directly testing wavefronts with large aspheric departures. The Shack-Hartmann sensor provides interesting tradeoffs between measurement range, accuracy and spatial resolution. We will discuss the advantages and disadvantages of the Shack-Hartmann wavefront sensor over more conventional metrology tests. The implementation of a Shack-Hartmann wavefront sensor for aspheric testing will be shown.