Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making electrical device
Reexamination Certificate
2000-09-01
2002-05-14
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making electrical device
C430S322000, C430S328000, C430S330000, C430S394000, C430S397000, C430S945000
Reexamination Certificate
active
06387597
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to imaging of etch resistant layers also known as “resists”. Resists exposed according to the invention may be used to fabricate high resolution patterns by etching or deposition. The invention may be applied to the manufacture of integrated circuits, flat panel displays and printed circuit boards, for example.
BACKGROUND OF THE INVENTION
Processes for fabricating high resolution patterns, mainly on planar objects, by selective etching or deposition are well known. In general, a layer to be shaped or patterned, which may be called a substrate, is covered by a protective layer known as a “resist”. In general, a resist is used as an imagewise mask for selectively controlling a chemical or physical process. The resist limits the process to follow an image pattern defined by the resist. The term “resist” should be interpreted in this broad sense throughout this disclosure and claims. Most commonly used resists operate by undergoing a change of solubility in a developer when they are exposed.
An image made up of desired shapes is created on the resist usually via photo-imaging. The exposed (or unexposed, if the resist is negative working) parts of the image are removed, normally by using a liquid developer to expose the substrate. The substrate can now be treated, for example by etching through the openings in the resist layer. The treatment is limited to areas of the substrate adjacent the openings. Portions of the substrate which remain covered by the resist are protected from the etching or other treatment.
Etching may be done, for example, by wet chemicals or by dry plasma (a process widely used in the semiconductor industry). Resists are also used in additive processes in which one or more materials are deposited through openings in the resist to add to the substrate. Deposition may 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 or CVD. Resists may also be used to permit chemical reactions, such as oxidation, to occur only in selected areas of a substrate which are not covered by the resist.
At the end of the process the remaining resist is normally removed, or “stripped”. Historically most resists were photoresists, i.e. activated and imaged by the photonic action of the light. Because of this photonic action most photoresists operate in the UV part of the spectrum, where the photon energy is high. Some resists can be 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. When a certain exposure is reached, a change occurs in the resist. The change depends upon the exposure but not upon the intensity of light used to achieve that exposure. 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). The same exposure results when the photoresist is exposed by 1000 mW for 0.1 sec with similar results (1000 mW×0.1 sec=100 mJ/cm
2
). This law, also known as the “reciprocity law”, is the basic law governing the exposure of photoresists.
The law of reciprocity requires that photoresists and other integrating resists be exposed with the use of an optical system which provides a high contrast ratio and low stray light. For example, if an exposure system has a leakage, or stray light, of 1% (e.g.: when exposure is “off”, the light level does not drop to zero but only drops to 1% of the “on” state) the effect of this stray light may be as large (or larger) than the main exposure. The effects of stray light accumulate over time and are especially significant if the photoresist is exposed for a long time to the “off” state.
An even larger problem is caused when trying to image closely-spaced high resolution features: the point spread function of any practical optical system causes a “spreading” of light from each feature. Stray light from one feature illuminates adjacent features and lowers the resolution.
FIG. 1
illustrates this problem. A first feature
1
has a light distribution
1
′ and a second feature
3
has a light distribution
3
′. Exposure curve
2
, generated by lens
8
imaging first feature
1
, is added to exposure curve
4
, generated by imaging second feature
3
, to create a curve
5
, which is the equivalent exposure. Curve
5
creates distorted images
6
and
7
of features
1
and
3
on photoresist
9
which has a threshold
10
. It makes no difference whether exposures
2
and
4
are applied simultaneously or sequentially. The photoresist will add up, or integrate, the exposures.
The problems described above can be compounded if the surface of the resist is not flat. It is known in the art to treat the surfaces of semiconductors in various ways to enhance planarity. This can increase the cost of manufacturing semiconductor devices.
FIG. 4
shows the what occurs when a prior art system is used to expose a non-planar substrate
12
coated with a photoresist
9
. The deviation from planarity need not be large in order to cause a problem. When making integrated circuits, the depth of focus is typically below 1 micron due to the large numerical aperture of the lenses used. A deviation of 1 micron can be caused by a build-up of lower layers. Today a CMP process (Chemical-Mechanical Polishing) is used to bring the silicon wafer back to planarity. If lens
8
is focused on the substrate
12
at one point, all points higher or lower than the plane of focus will be out of focus causing loss of imaging resolution. For example, narrow lines will widen and merge (or narrow gaps will disappear). It is not possible to correct this problem by repeating the exposure at a different focus setting because, when the same substrate (which obeys the law of reciprocity) is imaged again at a different focus setting, all the exposure which was absorbed but did not reach the threshold will add up with the new exposure and destroy the image.
Recently a different type of resist, known as thermoresist, has been used in the manufacturing of printing plates and printed circuit boards. A thermoresist (also known as a thermal resist or heat-mode resist) changes solubility when a certain temperature, rather than a certain accumulated exposure, has been reached. Such thermoresists are imaged using near infra-red light and therefore are also known as “IR resists”. Some exampled of thermoresists are disclosed in U.S. Pat. No. 5,340,699 (Haley); U.S. Pat. No. 5,372,907 (Haley); U.S. Pat. No. 5,372,915 (Haley); U.S. Pat. No. 5,466,557 (Haley); U.S. Pat. No. 5,512,418 (Ma); U.S. Pat. No. 5,641,608 (Grunwald); U.S. Pat. No. 5,182,188 (Cole); U.S. Pat. No. 5,314,785 (Vogel) and U.S. Pat. No. 5,328,811 (Brestel). The thermoresist described by Haley is unusual as the same composition acts as a photoresist, obeying the reciprocity law, when exposed by UV light (at low power densities) but also acts as a thermoresist, responding only to temperature, when heated up by infrared light at high power densities. Thermal resist is also available from Creo Ltd. (Lod Industrial Park, Israel), sold under the trade name “Difine 4LF”. All of the above mentioned thermoresists respond to temperature and do not follow the reciprocity law. Such resists may be called “non-integrating”. It is not possible to have a practical true thermoresist which follows the reciprocity law. Such a thermoresist would become exposed simply by long exposure to ambient temperature (just as a photoresist can be exposed by a long exposure to low levels of ambient light). While it is possible to shield a photoresist from ambient light it is not possible to shield from ambient temperature. Therefore a practical thermoresist cannot obey the reciprocity law.
Prolonged exposures to ambient temperatures below the threshold temperature has little effect o
Creo Srl
Oyen Wiggs Green & Mutala
Young Christopher G.
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