Joseph P. Kirk

Phase-Only Complex-Valued Spatial Filter*

A new type of phase-only filter is described for wave-front construction, in which both the amplitude and phase information necessary to construct an arbitrary image over a limited field are encoded. It is shown that this phase-only filter can duplicate the performance of an ideal complex-valued spatial filter (a filter that controls both amplitude and phase transmittance). This phase-only filter controls the amplitude transmittance by the use of a modulated high-frequency phase carrier that diffracts a controlled amount of light into the image. This type of filter is particularly useful in the implementation of computational wave-front construction, because the maximum spatial frequency that must be plotted is associated with the image and not the carrier. The performance of the filter is examined both numerically and experimentally.

Scattered light in photolithographic lenses

Scattered light, flare, is present in the images formed by all photolithography lenses and it reduces lithographic process tolerances. It varies from lens to lens and with time, but is easily measured by observation of images of opaque objects formed in positive photoresist. The scattered light halo of a lens is modeled and the model used to estimate the flare for any reticle used with that lens.

Analysis Of Overlay Distortion Patterns

A comprehensive geometrical approach is presented for the least-squares analysis of overlay distortion patterns into useful, physically meaningful systematic distortion subpatterns and an essentially non-systematic residue. A scheme of generally useful distortion sub-patterns is presented in graphic and algorithmic form; some of these sub-patterns are additions to those already in widespread use. A graphic and geometric approach is emphasized rather than an algebraic or statistical approach, and an example illustrates the value in utilizing the pattern-detecting ability of the eye-brain system. The conditions are described under which different distortion sub-patterns may interact, possibly leading to misleading or erroneous conclusions about the types and amounts of different distortions present. Examples of typical interaction situations are given, and recommendations are made for analytic procedures to avoid misinterpretation. It is noted that the lower-order distortion patterns preserve straight-line linearity, but that higher-order distortion may result in straight lines becoming curved. The principle of least-squares analysis is outlined and a simple polynomial data-fitting example is used to illustrate the method. Algorithms are presented for least-squares distortion analysis of overlay patterns, and an APL2 program is given to show how this may easily be implemented on a digital computer. The appendix extends the treatment to cases where small-angle approximation is not permissible.

Astigmatism and field curvature from pin-bars

Astigmatism and field curvature of lithography lenses can be measured with an error less than 30 nm using an optical microscope and conventional resolution test patterns. A sequence of through-focus images, recorded in positive resist, are examined with an optical microscope to determine the smallest lines remaining after development. It is only necessary to judge existence of lines; no dimensional measurements are required. Imperfections in fabrication and design that limit the resolution of most lithography lenses are easily identified by this new technique. The method for doing the measurements, the analysis, and recent application to characterization of lithography lenses are presented.

Application of blazed gratings for determination of equivalent primary azimuthal aberrations

Aberrations are determined from wafers exposed using a test reticle having blazed gratings with orientations from 0 to 337.5 degrees in increments of 22.5 degrees. The image of the phase grating is micro stepped thorough focus on the surface of high absorption photoresist. A second blanket exposure is applied to the latent images to make the developed resist images have a surface relief linearly related to the aerial image intensity. The first order diffraction efficiency of this surface grating is directly propagational to the first harmonic of the surface relief. This diffraction efficiency is recorded as a digitized dark- field image and processed to evalute the aberrations. These images are simulated and matched to those recorded in the photoresist by adjusting the aberrations to achieve a best match. The primary azimuthal aberration contributions are separated by Fourier analysis into coma, astigmatism, 3 leaf clover, etc. and laser used to indicate lens quality. Several grating frequencies are required to separate higher order components of each azimuthal aberration. Only one grating frequency is used in the data reported here and all orders of each azimuthal aberration are lumped into equivalent primary azimuthal aberrations.

Aberration analysis using reconstructed aerial images of isolated contacts on attenuated phase-shift masks

A technique for the evaluation of scanner lens aberration is described and analyzed. The method is based on the reconstruction of aerial image distribution using a double exposure technique: A first exposure of the mask feature of interest is followed by uniform background exposure. The topdown images in resist at increasing background exposure dose are analyzed using suitable threshold algorithms to obtain a set of aerial image intensity contour lines. This technique has been applied to the analysis of aerial images formed by isolated contacts using an attenuated PSM. Of particular interest in this case is the aerial image intensity present on the first sidelobe and its angular dependence. In the absence of lens aberrations the sidelobe intensity has no angular dependence whereas the presence of aberrations in the lens generally results in a non-uniform angular sidelobe intensity distributions. A detailed theoretical analysis of the capabilities of this method is being presented: Linearity, zero response and expected results in the presence of various Zernike terms have been studied. We were not only able to separate Zernike terms based on their angular dependence but we also propose a method to assess the order of the radial component.

Measurement of microlithography aerial image quality

The aerial image formed by a microlithography lens places fundamental limits on the size and density of patterns that are manufactured by that lens. In the past, the lithographer was unable to directly measure the dose distribution in this aerial image. Highly absorbing photoresists have continuous tone relief response to dose variations and this relief is a record of the dose distribution in an image. It is shown how high resolution measurement of that relief by an atomic force microprobe gives the lithographer a sampling of the spatial distribution of dose within the image.

Aberration measurement using in-situ two-beam interferometry

A reticle with phase-only blazed gratings of varying azimuthal orientations diffracts light into only two orders, 0 & +1, discretely illuminating a lens pupil. The image of each grating is a sinusoidal interference pattern and is recorded as a surface relief in a highly absorbing photoresist. The maximum image contrast occurs when focus is set such that the RMS wavefront error over the two beams is minimized. This maximum contrast vs focus is recorded by a CCD array mounted on a dark-field optical microscope and the aberrations are obtained from an analysis of this record. Repeatability of equivalent primary aberrations of less than 0.001(lambda) RMS are achieved and used to monitor lens stability.

Measurment of astigmatism in microlithography lenses

The direction, (phi) , and magnitude, A1, of residual astigmatism in microlithography lenses used for semiconductor circuit fabrication is determined by measuring the focal position, F, of lines orientated at four values of (Theta) equals 0 degrees, 45 degrees, 90 degrees, 135 degrees. These parameters are related by F equals A0 plus A1 cos2((Theta) + (phi) ) which is solved for the four measured values of F. If the lens is axially symmetric the angle (phi) will be that of the field diameter, but real lenses have fabrication errors that may introduce non-classical astigmatism, so (phi) may have values 0 less than or equal to (phi) less than or equal to (pi) at any point in the field. It is for this reason that conventional resolution reticles with perpendicular resolution targets are inadequate to accurately measure residual astigmatism. Using such a reticle will result in under estimation of the actual astigmatism. Wafers are exposed through focus using a reticle having an array of resolution targets, each having the four orientations. Measuring the focal position of each of the four orientations by examination of their photoresist images with a dark field optical microscope enables determination of astigmatism with a standard deviation 7 nm. Application of this procedure used to evaluate the residual astigmatism in high quality lithography lens is reported.

Detection of focus and spherical aberration by use of a phase grating

A phase only grating consisting of equal lines of 0 and 90 degree phase is imaged into a highly absorbing photoresist forming a surface relief grating that is measured with a dark- field optical microscope and a CCD array. The resulting images are analyzed to determine spherical aberration, +-0.001 wave RMS, and focus variations of +-2.0 nm. This method of measurement and analysis is applied to both 248 and 193 nm photolithography lenses.