Robert L. Kostelak

Application of plasma polymerized methylsilane in an all dry resist process for 193 and 248 nm lithography

The performance of Plasma Polymerized Methylsilane (PPMS) as a single layer and bilayer resist system at 248 nm advanced lithography are exposed. PPMS based photolithographic processes are shown to be extendible for use in 193 nm lithography.

Automated layout of mask assist-features for realizing 0.5 k1 ASIC lithography

The virtues of mask-plane assist features for improving imaging performance of generic ASIC layouts in the 0.5k1 realm has been previously proclaimed. In this report we provide experimental verification and introduce a methodology to automatically deploy these features on ASIC layouts.

Focused-ion-beam repair of phase-shift photomasks

Phase-shift photolithography is emerging as an important new technology for sub-half-micron design rule circuits. Unfortunately part of the price paid for the improvements in spatial resolution and process latitude afforded by phase-shift lithography is increased mask defect printing sensitivity. The minimum printable defect size, 0.3 microns (on the mask) for I-line steppers at 0.35 microns, is roughly half that for conventional photomasks. This paper examines the issues associated with extending high resolution focused ion beam mask repair to phase-shift masks.

Analyzing deep-UV lens aberrations using aerial image and latent image metrologies

The technique of direct aerial image metrology (AIM) has been applied to characterize the performance of a microlithographic lens. AIM is potentially faster and more reproducible than measurements obtained by scanning electron microscopy. Direct measurement of the aerial image eliminates both the process variations associated with resist processing as well as the subjective nature of evaluating resist profiles. We have used AIM to evaluate some of the primary aberrations of a 248 nm stepper lens. We compare the results to those obtained with latent image scatterometry, a proven technique for measuring lens performance. We found that AIM, while providing qualitatively good results, contained some slight systematic errors that reduced the accuracy of the data. The sources of error and their remedies are discussed.

Phase-shifting structures for isolated features

The technique for improving optical projection-system resolution by phase-shifting alternate apertures of a periodic grating was introduced in 1982. This halves the frequency content of the image passing through the optics and should therefore double the effective resolution of such patterns. Unfortunately, as feature separation increases, the efficacy of this method diminishes. Previous work applying a similar approach to isolated features involves introducing minute, non-printable, phase-shifted assist slots around the desired feature. The diffraction side-lobes of these slots constructively interfere with the center lobe of the primary aperture. The resolution enhancement afforded be this technique is limited by the printability of the assist slots. This restraint also dictates 1X-size reticle feature dimensions and the employment of high contrast imaging resists. A new approach entails significantly oversizing the desired feature and introducing a phase-shifting region around the periphery. This type of structure affords substantial focus-exposure improvements and may either be fabricated in a single-level, self-aligned scheme or by a two-level exposure with conventional e-beam tools since the phase-shifting regions are on the order of 1 micrometers (reticle dimensions). Extensive modeling of this structure for isolated contact holes and spaces explores the myriad of trade- offs involved in an optimum design. Mask-fabrication tolerances, such as phase-shift uniformity, are also investigated. It is shown that the focus-exposure window enlarges as the overall structure dimensions increase. The degree of enhancement must therefore by weighed against packing density restrictions. Also, the structure suffers, to some degree, from the effect of side-lobes. However, for a given side-lobe intensity, this technique yields enhancements superior to the assist-slot approach. As is typical of phase-shifted systems, performance is improved as the partial coherence ((sigma) ) of the illuminating radiation is reduced. The decrease in throughput sometimes associated with a (sigma) reduction is, in this case, however, mitigated by the oversized aperture that produces twice the illuminating intensity as the corresponding non-phase-shifted feature. Simulated exposure-focus analysis conclude that a 0.45 (lambda) /NA contact hole may be printed with a 15% exposure and +/- .42 K2 unit focus tolerance assuming a +/- 5% CD control. A demonstration mask was patterned with a MEBES III generation reticle writer and exposure-focus latitude predictions for phase-shifting spaces are verified on an I-line, (sigma) equals0.5, 0.45 N.A. stepper.

Nonparametric Approximation Methods-A New Tool For Process Engineering

Recently developed statistical inference methods permit detailed process information to be developed from experiments without relying on particular physical process models. Generalized Cross Validation is a powerful calculational method that can be used to find the smooth function that best approximates noisy, scattered experimental data. Already employed in applications in such fields as geology, biology, and meteorology, its three principal benefits for process engineering are: - The method does not require a parameterized model of the experiment, so it can be used in situations where no model, or no sufficiently detailed model, is available. - The results of the calculation are expressed in terms of piecewise polynomial or polylogarithm functions, rather than the global polynomials, which are used by traditional Response Surface Methods, that can fail if the experimental data cannot be described well by a polynomial. - The method can be used effectively in situations where the errors in the experimental results are not known precisely. These qualities make Generalized Cross Validation (GCV) a very attractive tool for analyzing and optimizing processes. We present an overview of the GCV method with two examples that show how it can be applied to solve problems in process modeling and optimization. In our first example, we efficiently determine the best values for the GHOST proximity-effect correction parameters (defocused-beam diameter and dose) by using the GCV method to fit and interpolate experimental data. For the 50-kV electron-beam system we studied, the best values were predicted to be a defocused beam diameter of 20 μm and a dose of 34 per cent of the pattern dose. These results are in good agreement with earlier work that employed a much more time-consuming method. In our second example, the GCV method is applied to interpolate experimental data to predict the developer concentration and prebake temperature that produce maximum contrast for a particular deep-UV resist.