Donis George Flagello

Polymeric Optical Waveguides

The present work describes the characterization of internally developed epoxy ridge optical waveguides which exhibit low propagation loss (0.3 dB/cm at 1.3 μm), high environmental stability (low sensitivity to moisture), have smooth walls (100 nm sidewall roughness), and high temperature stability (275°C). The techniques used to fabricate these waveguides are compatible with the planar processes used in the manufacture of high performance electronic packages.

Understanding high numerical aperture optical lithography

Abstract This paper explores fundamental imaging phenomena that become appreciable as the numerical aperture (NA) of imaging lenses increase beyond 0.5. A high NA imaging model is formulated based on plane wave decomposition of the imaging lens exit pupil. The model shows that for NAs above about 0.55, linear polarized illumination gives rise to “astigmatic” effects in the image. Photoresist exposures in polarized light on a 0.55 NA stepper show ≈1% x-y linewidth differences in experimental tests.

Practical Characterization Of 0.5 μm Optical Lithography

This paper establishes the characterization techniques that are needed to repeatedly fabricate submicron structures which are below the resolution specifications of an optical system. A two-parameter system is defined, consisting of a material or photosensitive component and an optical component, which represents the major contributors to process variability. Statistical inference and correlation analysis are used to estimate the effects and extent of the individual components on the total lithographic system. The photosensitive component, consisting of the photoresist, films and related chemistry, is shown to substantially represent the variability in the final linewidth of 0.5 μm structures. Positive and negative photoresist systems are compared and analyzed for run-to-run and within-run variability utilizing photoresist speed-point analysis. The optical component is explained by using a two-dimensional aerial image model which takes as input the numerical aperture, wavelength, coherence, aberrations and experimentally measured effects. A method for quantitatively measuring flare or background exposure is introduced.

Vector diffraction analysis of phase-mask imaging in photoresist films

A vector diffraction analysis is applied to methods of image manipulation like phase masks or off-axis illumination. When such techniques eliminate the dc order altogether, as with alternating phase masks, it becomes possible to print a given intensity pitch with only half as large an NA as with a chrome mask. Some non-scalar imaging effects are therefore reduced. For example, in present lithographic practice the depth of focus with standard masks is often little larger than the thickness of the resist film, so it is no longer accurate to neglect defocus arising during multiple reflections within the resist. The angle dependence of the film stack then becomes significant, and small film perturbations give rise to swing-curve oscillations in pupil apodization and image shape as well as exposure dose. Specific film thicknesses can, for example, have a similar effect to stopping down the lens. Other thicknesses can enhance resolution of certain patterns. We treat these film effects by representing each layer in the process stack with pupil transfer functions. Like defocus, these pupil functions posses a symmetry that eliminates relative dephasing or apodization in the two-beam fringe pattern produced by an alternating phase mask. An alternating phase mask can produce stronger vector diffraction anomalies than a standard mask, despite being able to print a given pitch with half as large a lens NA.

Three-dimensional modeling of high-numerical-aperture imaging in thin films

This paper describes a modelling technique used to explore three dimensional (3D) image irradiance distributions formed by high numerical aperture (NA > 0.5) lenses in homogeneous, linear films. This work uses a 3D modelling approach that is based on a plane- wave decomposition in the exit pupil. Each plane wave component is weighted by factors due to polarization, aberration, and input amplitude and phase terms. This is combined with a modified thin-film matrix technique to derive the total field amplitude at each point in a film by a coherent vector sum over all plane waves. Then the total irradiance is calculated. The model is used to show how asymmetries present in the polarized image change with the influence of a thin film through varying degrees of focus.

Laser beam modeling in optical storage systems

A computer model that simulates light propagating through an optical data storage system has been developed. This paper discusses a model of laser beam that originates at a laser diode, propagates through an optical system, interacts with an optical disk, reflects back from the optical disk into the system, and propagates to data and servo detectors.

A Single Expose Double Develop (SEDD) process for self-aligned lithographic applications

Abstract A Single Expose Double Develop process has been developed which incorporates the use of a tridensity mask to expose two self-aligned patterns in conventional photoresist with one masking step. This minimizes masking cycles and overlay errors which translates into increased yield. The resultant pattern can be customized for various applications, e.g. obtaining two dopant profiles with a single implant using a dual thickness photoresist pattern, transferring two patterns into underlying films with no alignment error using RIE or wet etching, etc... Experiments are shown which optimize the processing parameters of the photoresist to the density levels in the mask, utilizing characteristic curves for analysis. Process control is correlated to these curves through modelling and experiment. Examples of the applications of the SEDD process in VLSI devices and packaging are shown with SEM photographs.

Experimental verification of high-numerical-aperture effects in photoresist

This work describes an experimental setup approximating the output of a 0.85 NA reduction stepper which is used at the limits of its resolution. The experimental method concentrates on verifying the numerical predictions of vector imaging theory. Since this theory is based on a plane-wave decomposition of the vector image field, two-beam and three-beam interference are the simplest forms. Alternating phase masks, attenuated phase masks, and standard masks can be represented by this arrangement. The setup uses a periodic grating mask to obtain diffraction orders, and then substitutes mirrors for the imaging lens which results in the desired beam interference at the image plane. A unique experimental process for obtaining the record of the image distribution is presented which results in decorating the image cross- section for analysis. SEM photographs reveal that beams of high obliquity have drastically different behavior within a photoresist film for S and P polarization for the two-beam case. The addition of a third central beam, with three-beam interference, results in a reduction in the difference between S and P polarized illumination.

Comparison of scalar and vector diffraction modeling for deep-UV lithography

This paper describes investigations into scalar and vector diffraction modeling for 248 nm lithography. An experimental design approach was used to study the effects and interactions of coherence, polarization, and numerical aperture on a resist feature response. An exposure latitude response to achieve 10% linewidth control with +/- 0.3 micron of defocus was utilized. Both vector and scalar diffraction models were used to simulate process runs. Experimental comparisons were made using a variable NA, variable coherence deep-UV projection system, adapted for control of polarization at the aperture of the mask. Exposure latitude response surfaces are presented, along with details on isolated process runs.