Coherence management in lithography printing systems

Johana Bernasconi, Hans Peter Herzig, Toralf Scharf
Spie-Int Soc Optical Engineering, 2016
Conference paper

Abstract

In proximity lithography, interference and diffraction effects arise when printing small features because of the proximity gap. Different techniques are used in order to control and take advantage of theses effects. In this paper, the focus is set on the MO Exposure Optics developed to shape the angular spectrum of the exposure light. The MO Exposure Optics contains several elements including microlens arrays that have certain symmetry and sampling. The MO Exposure Optics allows to set the angle of illumination and can be used to define spatial coherence. We study here in detail the influence of different illumination settings on optical proximity correction (OPC) structures. We apply this concept for the first time to a LED illumination. The propagation of light after an optical proximity correction structure is measured by recording aerial images over a distance of up to 60 pm behind the mask with a high resolution microscope setup.(1) As an example structure, we investigate here an optical proximity correction structure that is intended to make the edge of a line sharper. Using illumination filter plates that limit the angle of illumination and increase the coherence lead to pronounced interference effects in aerial images as expected. But special settings of the illumination allow to achieve comparable results with much larger illumination angles and higher throughput. We will show examples and analyze the results.

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Johana Bernasconi, Hans Peter Herzig, Toralf Scharf
Spie-Int Soc Optical Engineering, 2016
Conference paper

Abstract

Official source

https://infoscience.epfl.ch/record/225729?ln=en

About this result

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Polymer multilayer photonic micro- and nanostructures with tailored spectral properties

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Institut de Microtechnique, Université de Neuchâtel2008

Photolithography is one of the earliest technologies used to transfer patterns to a substrate. It is also known as optical lithography since it uses light to transfer the pattern. The main exposure techniques exist in the industry are projection printing, contact printing, and proximity printing. Projection printing technology uses optical elements between mask and wafer to project the feature on the mask to the wafer. This is very expensive and delivers the highest resolution. In contact printing, the mask and wafer are in contact with each other and in proximity printing, the mask is kept at some proximity distance away from the wafer. Proximity printing is an easy and cost effective printing technique because the damage to the mask will be less and also no optical elements between mask and wafer are used. The main drawback of the proximity printing is the diffraction effect caused by the proximity gap between mask and wafer, which limits the resolution. The main objective of this thesis is to study the limitations of proximity printing and to increase its resolution. To study the limitations, different types of design strategies and verification methods are used in the thesis. First is the simulation technique which is performed with GenISys Layout LAB. This is specially designed for proximity printing. The software gives the aerial image and final resist pattern as output. The most interesting and important aspect is the second verification technique which is the experimental setup. A measurement setup has been built to study the light propagation from different masks and to study the aerial image at different proximity gaps. The setup is known as High Resolution Interference Microscopy (HRIM). The setup is basically a Mach- Zehnder interferometer having different light sources, sample plane and reference arm which are used according to the samples. The final verification is achieved using the mask aligner. Both the simulation and experiments are carried out using a special illumination optics called MO exposure optics from Süss MicroOptics. The thesis mainly focuses on the rule based optical proximity correction a technique which is a simple method for mass production. Correction structures are designed for one dimensional and two dimensional features in amplitude masks. Adding lines near the edge to improve the edge slope will be discussed as the one dimensional correction. The different intensity cutting planes and the comparison between simulation and experimental results will be discussed along with that. A unified correction structure is designed to solve corner rounding problem and will be studied as the two dimensional study. The structure is defined to print different line widths at single proximity gap on single exposure. Usually, all the structures in the amplitude mask are studied with their aerial image intensities at different proximity gaps. But, here the study extends to phase evaluation also. The measurement technique can measure both intensity and phase evolution from the mask structures. Phase evolution from amplitude correction features will be discussed and how the phase modulates the intensity patterns is also studied. The role of fundamental principles like phase singularities, phase shifts are also discussed to find its effects on proximity printing structures. The study also leads to the intensity and phase propagation from phase shifting mask (PSM) . The structure evaluated is a group of corners in PSM.

EPFL2017

Diffractionless Fourier Domain Microscopy

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Several methods have been proposed and realized to increase the lateral resolution in microscopy and ultimately to beat the actual diffraction limit as stated by Abbe. Structured illumination for example enhances the lateral resolution by virtually extending the spatial frequency support of the system transfer function. Higher spatial frequencies are in fact mapped into the range covered by the detection transfer function. The set of spatial frequencies can be further enhanced by decoupling illumination and detection and by employing concepts similar to synthetic apertures. We present a novel method for diffractionless microscopic imaging – Fourier Domain or k-Microscopy[1] (FDM). Again we decouple illumination from detection to achieve a lateral resolution that is independent of the detection numerical aperture. Similar to structured illumination we produce a set of fringes covering a band of spatial frequencies on the sample. However, instead of detecting with an area sensor we use a simple PIN detector: By tuning the spatial frequency of the illumination we obtain the Fourier coefficient from the measured total backscattered or transmitted intensity for the particular spatial frequency in the sample dimension normal to the fringe orientation. The fringe pattern is obtained from two collimated coherent beams that are superimposed. The spatial frequency of the pattern is controlled via tuning the wavelength. In order to obtain an unambiguous Fourier representation of the one dimensional sample structure it is necessary to record at least two π/2 phase shifted copies of the frequency response signal. The lateral resolution is then determined by the tuning bandwidth of the laser as well as the subtended angle and the central wavelength of the laser. This relation is in fact very similar to the concept of Fourier Domain Optical Coherence tomography, only that it acts on the transverse instead of the longitudinal resolution. We give first experimental proof of the concept in one dimension using a slowly tuning broad- bandwidth Ti:Sapph laser. We resolve the defined structure of a resolution test target. A particular feature of the method is that the reconstructed sample will appear spatially band-pass filtered due to the finite spectral support given by the tuned fringe frequencies. Hence only the edges of the test target lines are visible. We believe that the concept of FDM will greatly benefit from new developments in rapidly- wavelength tuning source technology that recently entered the field of OCT. 1. M. Geissbuehler, T. Lasser, and R. A. Leitgeb, "K- Microscopy - resolution beyond the diffraction limit," in Progress in Biomedical Optics and Imaging - Proceedings of SPIE(2008).