If the Fraunhofer criterion is met, one can roughly approximate the areal image by the superposition of the individual intensities. As the period of the grating decreases, MTF decreases. Mask is separated from the wafer by a thin gas cushion 2. Factors affecting focus and resolution a. Factors affecting focus and resolution b. Depth of focus: The depth of focus can be described as the distance along the optical train that the wafer can be moved and still keep the image in focus.
Increasing the numerical aperture increases the resolution linearly, but decreases the depth of focus quadratically. Factors affecting focus and resolution c. Projection Printing: Scanning Type Scanning Type: Perkin Elmer Company avoided the problems of a full mask projection exposure in favor of a scanning technique.
It used a mirror system with a slit blocking part of the light coming from the light source. The slit allows a more uniform portion of the light to shine on the mirror system, which is in turn projected onto the wafer. Since the size of the slit is smaller than the wafer, the light beam is scanned across the wafer. They are called aligners, since the image dimensions on the mask are the same size as the intended image dimensions on the wafer surface.
Projection Printing: Stepper Stepper Type: A stepper is basically a projection printing method that uses the same technique used to make masks. A reticle, carrying the pattern of one or several chips, is aligned and exposed, then is stepped to the next site and the process is repeated. Some steppers are , that is, the image on the reticle has the same dimensions as those required on the wafer. However, most use reticles with 5 to 10 times the final dimensions.
These are called reduction steppers. Total views 3, On Slideshare 0. From embeds 0. Number of embeds 3. Downloads Shares 0. Comments 0. Likes 4. You just clipped your first slide! 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. Author s Puthankovilakam, Krishnaparvathy. Advisor s. Herzig, Hans Peter Scharf, Toralf. Pagination Date Photolithography ; proximity printing ; resolution enhancement techniques ; optical proximity correction ; GenISys Layout LAB ; high resolution interference microscopy ; MO exposure optics ; edge slope improvement ; corner rounding ; phase shifts and phase singularities.
DOI Other identifier s urn: urn:nbn:ch:bel-epfl-thesis Laboratories OPT. View Download. Optical Lithography refers to a lithographic process that uses visible or ultraviolet light to form patterns on the photoresist through printing. Printing is the process of projecting the image of the patterns onto the wafer surface using a light source and a photo mask.
There are three types of printing - contact, proximity, and projection printing, each of which will be described below. Equipment used for printing are known as printers or aligners. Patterned masks, usually composed of glass or chromium, are used during printing to cover areas of the photoresist layer that shouldn't get exposed to light. Development of the photoresist in a developer solution after its exposure to light produces a resist pattern on the wafer, which defines which areas of the wafer are exposed for material deposition or removal.
Figure 1. Example of a mask aligner from Suss; source: www. There are two types of photoresist material, namely, negative and positive photoresist. Negative resists are those that become less soluble in the developer solution when exposed to light, forming negative images of the mask patterns on the wafer.
On the other hand, positive resists are those that become more soluble in the developer when exposed to light, forming positive images of the mask patterns on the wafer. Commercial negative photoresists normally consist of two parts: 1 a chemically inert polyisoprene rubber ; and 2 a photoactive agent.
When exposed to light, the photoactive agent reacts with the rubber, promoting cross-linking between the rubber molecules that make them less soluble in the developer. Such cross-linking is inhibited by oxygen, so this light exposure process is usually done in a nitrogen atmosphere.
Positive resists also have two major components: 1 a resin ; and 2 a photoactive compound dissolved in a solvent. The photoactive compound in its initial state is an inhibitor of dissolution. Once this photoactive dissolution inhibitor is destroyed by light, however, the resin becomes soluble in the developer.
A disadvantage of negative resists is the fact that their exposed portions swell as their unexposed areas are dissolved by the developer.
This swelling , which is simply volume increase due to the penetration of the developer solution into the resist material, results in distortions in the pattern features.
This swelling phenomenon limits the resolution of negative resist processes.
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