Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
2001-03-01
2003-07-08
Fuller, Rodney (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C349S004000, C430S311000
Reexamination Certificate
active
06590635
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to imaging of etch resistant layers, also known as “resists”. The resists may be used in the fabrication of high resolution patterns by etching or deposition. The invention has particular application in the manufacturing of integrated circuits, flat panel displays and the like using an optical stepper. A stepper is an imaging device used in the semiconductor industry to project an image of a mask onto a semiconductor wafer.
BACKGROUND OF THE INVENTION
The process of fabricating high resolution patterns, mainly on planar objects, by selective etching or deposition has been well known for centuries. In general, the layer to be shaped or patterned is covered by a protective layer known as a “resist”. Desired shapes are created in the protective layer, usually via photo-imaging. The exposed (or unexposed, if the resist is negative working) part of the image is removed, normally by using a liquid developer to expose the layer underneath. The most common resists operate by a change of solubility in a developer. The exposed layer can now be etched through the openings in the resist layer, which protects the covered area from the etching process. Etching can be by wet chemicals or by dry plasma (a process widely used in the semiconductor industry).
Instead of etching an additive process can be used. In an additive process a material is deposited through the openings in a resist to add to the layer underneath the resist. This deposition can be done in a wet process (as in the well known “additive” process for manufacturing printed circuit boards) or in a dry process, such as a vacuum deposition by evaporation or sputtering. Another way of using a resist is in allowing chemical reactions, such as oxidation, to occur only in the areas not covered by the resist. At the end of the process, any remaining resist is normally removed, or “stripped”.
In general, a resist is an imagewise mask selectively controlling a chemical or physical process and limiting the process to follow the image pattern. The term “resist” should be interpreted in this broad sense throughout this disclosure and claims. Any other layer which has suitable properties and can be patterned by light or heat can be used as a resist. Historically most resists were photoresists, i.e. activated and imaged by the photonic action of light. Because of this photonic action most photoresists operate in the UV part of the spectrum, where the photon energy is high. Some resists are exposed by other types of radiation, such as electron-beams. All photoresists and electron beam resists share one fundamental property: they respond to the total exposure, not to the momentary illumination. In optics, exposure is defined as the integral of illumination over time. For example, a photoresist can be exposed by 100 mW/cm
2
for 1 sec to yield an exposure of 100 mJ/cm
2
(100 mw×1 sec) or it can be exposed by 1000 mW for 0.1 sec (100 mW×0.1 sec=100 mW/cm
2
) to yield similar results. This law is also known as the “reciprocity law” and it is the basic law governing the exposure of photoresists. When a certain exposure is reached, a change occurs in the resist. Since exposure is a linear function of power and time, the principles of linear superposition apply.
The law of reciprocity requires that optical systems used to expose photoresists and electron beam resists provide a high contrast ratio and low stray light. For example, if an exposure system has a light leakage, or stray light, of 1% (i.e.: when exposure is “off”, the light level does not drop to zero but only to 1% of the “on” state) the effect of the stray light can be as large (or larger) than the main exposure if the photoresist is illuminated by the stray light for a long time.
An even larger problem is caused when trying to image high resolution features: the point spread function of the optical system causes a “spreading” of light from each feature. This causes light which is intended to expose one feature to overlap with adjacent features. This lowers resolution. This problem is most severe in the semiconductor industry when using steppers to image a semiconductor wafer, typically a silicon wafer.
Most integrated circuits today are fabricated using selective etching or deposition according to a master pattern known as a mask. An imaging device known as a stepper is used to image the mask onto a resist on a substrate. Steppers are also known as “step and repeat production aligners” Steppers typically include sophisticated programmable controllers capable of coordinating the selection and positioning of masks and the exposures of resists through the selected mask.
The basic elements of a stepper are shown in
FIG. 1
, where a mask
1
, containing a pattern which is to be imaged onto silicon wafer
4
is illuminated by light source
2
. Mask
1
is imaged with a reduction lens
3
to form image
5
, typically at a 5× reduction. Wafer
4
is stepped by x-y positioning system
6
and
7
. Each of a number of areas on wafer
4
is exposed with the pattern of mask
1
. A controller
20
coordinates the operation of x-y positioners
6
,
7
and light source
2
to image mask
1
onto wafer
4
at one or more locations. The exposed areas are known as dies. Typically wafer
4
is coated with photoresist before exposure, however in some cases a different layer which is capable of responding to light is used. Because of the extremely fine features (below 1 micron) in image
5
which are typically imaged on each die, lens
3
is not capable of fully resolving all detail without some distortion of features.
If a cross section of the image of the mask
1
is taken along line
8
it would look like graph
9
in FIG.
2
. If a cross section of the same image is taken at the surface of the wafer
4
along the line
8
′ (
FIG. 1
) it would look like graph
10
in FIG.
2
. For further details on microlithography in general, and operation of steppers in particular, any modern book on the subject can be consulted, such as: “Handbook of Microlithography, Micromachining and Microfabrication”, Volumes 1 and 2, Edited by P. Rai-Choudhury, SPIE Press 1997, ISBN book number 0-8194-2378-5 (V.1) and 0-8194-2379-3 (V.2).
Referring now to
FIG. 2
, mask
1
is transmits light in all of its non-opaque areas. The transmitted light forms an image on the resist-covered surface of wafer
4
. The image is made of pixels (picture elements) numbered from 1 to 16 in FIG.
2
. Graph
9
shows the light intensity distribution just under mask
1
. After passing through lens
3
, this light distribution is distorted by the limited resolution of the lens (item
3
in FIG.
1
). The resulting light distribution is shown in graph
10
.
Photoresists are formulated to have a sharp threshold. Once the exposure level crosses the threshold a chemical change occurs. The change is normally a change in the solubility of the resist in a solvent. Because of this sharp threshold a sharp image can be produced in spite of the fact that graph
10
cannot fully reproduce the details of mask
1
. As long as all desired features in mask
1
transmit sufficient light to the resist to cause the exposure at corresponding features on the resist to cross the exposure threshold the resist will change to its exposed state and an image will be formed. Line
11
represents the threshold for an exposure of a certain length.
FIG. 2
shows the use of a positive resist, which is washed away in all exposed areas. The same theory also applies to negative resists.
The resist becomes exposed in all areas of image
5
where graph
10
crosses threshold
11
. In the exposed areas, the resist is washed away from the die. The features of image
5
are imaged sharply, however, their dimensions are distorted. This distortion can be seen by comparing the pattern imaged onto wafer
4
to mask
1
in FIG.
2
. For reasons of clarity the pattern imaged onto wafer
4
and mask
1
are shown at the same size, while in most cases the pattern imaged onto wafer
4
is a reduced image. For the same r
Creo Inc.
Fuller Rodney
Oyen Wiggs Green & Mutala
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