Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-10-28
2001-09-11
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06287732
ABSTRACT:
FIELD OF THE INVENTION
The field of the invention is the field of lithography, and particularly photolithography for use in semiconductor, magnetic recording, and micromachining applications.
BACKGROUND OF THE INVENTION
Photolithography
This invention relates to the field of microlithography for the manufacture of integrated circuits, magnetic devices, and other microdevices such as micromachines. In this field the final product is manufactured in sequential manner in which various patterns are first produced in a “resist” material with each pattern subsequently defining a product attribute. The “resist” materials, generally polymer compositions, are sensitive to light or other forms of radiation. The patterns are formed in the resist by exposing different regions of the resist material to different radiation doses. In the bright (high dose) regions, chemical changes take place in the resist that cause it to dissolve more easily (for positive resists) or less easily (negative resists) than in dim (low dose) regions. The bright and dim regions are formed using an exposure tool which generally transfers corresponding features to the resist from a mask or reticle. The masks or reticles are formed from mask blanks, which are plates of quartz coated with an opaque material such as chrome. The chrome is etched away in a pattern to form the mask. The radiation employed may be (but is not limited to) ultraviolet light and x-rays, and the regions of the mask that are opaque and transparent form a pattern of bright and dark when illuminated uniformly. In the most common implementation of this technology, a projection lens forms an image of the mask pattern in the resist film on a planar substrate. That image comprises the high and low dose regions that produce the resist pattern. When some form of light is employed in this process, it is called photolithography.
Wavefront Engineering
The patterns formed in the resist are not identical to those on the mask, and the methods of obtaining the pattern desired for the ultimate manufactured device in spite of deficiencies in the microlithography process is called “wavefront engineering.” Among the various devices used for this purpose are phase shifting masks (PSM)s-which create desired dark regions though interference. Phase shift masks were first published by the inventor of the present invention in a paper entitled “Improving resolution in photolithography with a phase shifting mask,” M. D. Levenson, N. S. Viswanathan, and R. A. Simpson, IEEE Trans. Electron Devices ED-29, 1828-1836 (1982). Since that time, there have been hundreds of patents and thousands of papers issued containing the phrase “phase shift mask”. However, the technology is presently used only in applications such as memory chips and microprocessor chips. The inventor of the present invention has realized that the design and construction of the required lithography masks is so expensive that the investment required can not be returned on a few hundred or thousands of wafers. The present invention shows a way to produce phase shift masks in a cost-effective way, so that the same phase shift mask substrate design may be used with many different device designs by trading off maximum density of features on a device with cost for low volume runs.
There are presently two types of PSMs in use: weak-PSMs such as the Attenuated-PSM and strong-PSMs such as the Alternating-Aperture-PSM. These two differ in that the weak-PSMs have only one type of bright feature, while the strong-PSMs contain two types of bright features identical except for the optical phase, which differs by ~180°. See, for example, M. Shibuya, Japanese Patent Showa 62-50811, M. D. Levenson et. al. IEEE Trans. Elect. Dev. ED-29, 1828-1836 (1982), and M. D. Levenson, Microlithograpy World 6-12 (March/April 1992).
Alternating Aperture PSMs
“FIGS.
1
(A-C) shows plan, side elevation (along cut A), and end elevation (along cut B) views of the result of steps in construction of an alternating aperture PSM as currently implemented commercially. A substrate
10
is made of a material such as a fused quartz plate or other stable material which must be transparent to the light used in the photolithography for a transmission mask. The substrate
10
is coated with an opaque (“chrome”) film
12
in which openings
14
and
16
have bee opened by normal photoresist application, exposure, and development, followed by chrome etch, to form a conventional chrome-on-glass (COG) photomask. After stripping the original photoresist, the photomask is then recoated with a resist film (hatched areas
22
of FIGS.
2
(A-C)) and apertures
20
are opened in the resist film at the locations of apertures
14
which will be phase-shifted. The openings in this second resist film are larger an those in the underlying chrome to accommodate possible mis-registration. The photomask is then etched and the chrome
12
exposed in the resist openings is used as a mask to etch the underlying substrate
10
to a depth d below the original surface to make depressions
24
as shown in the view of FIGS.
2
(A-C) taken after etching of the substrate
10
. The depth d of the features
24
etched in the substrate
10
is carefully chosen in on the basis of the wavelength of the light to be used in the photolithography and the difference in the index of refraction of the material of the substrate and the ambient atmosphere in which the phase shift mask is used.”
A plan view of the etched substrate
10
of
FIG. 2A
with the chrome removed is shown in
FIG. 3
where the hatched areas
32
correspond to the etched phase-shifted apertures
24
in FIG.
2
. The substrate
10
etched and patterned as shown in
FIG. 2
defines a small part of a phase shift mask used to produce patterns in a photoresist. The difference in phase velocities of radiation in the air and in the substrate
10
material produces a 180° phase shift in the light passing through regions
16
and regions
20
of the phase shift mask shown in FIGS.
2
(A-C), (with photoresist removed), which result in destructive interference and which cancels the light amplitude in the region between regions
16
and
24
. The term “alternating aperture-PSM” refers to the fact that the transparent apertures on opposite sides of a dark line have alternate (0°-180°) phases. The alternation in phase between otherwise identical apertures doubles the period of the optical amplitude pattern which corresponds to a given intensity pattern. Thus, that a given projection exposure tool can create resist patterns smaller by a factor of 2 (or more) when using an alternating aperture PSM, and dramatically increase the depth of focus. In particular, robust isolated dark lines characteristic of transistor gates can be made 3× thinner, dramatically increasing circuit speed.
“
FIG. 4
shows the pattern of exposed photoresist
44
and unexposed photoresist
42
resulting when light passes through the regions
16
and
20
of the mask of FIG.
2
. The pattern shown in
FIG. 4
is typically 4 or 5 times smaller than the pattern of the mask shown in FIG.
2
A. The width
40
of the exposed areas of the photoresist is typically greater than the wavelength of the light used for exposure.”
In known art, the pattern of phase-shifting is different from that of the open (non phase shifted) apertures and must be customized for each mask of each product. Such masks require multiple customized patterns to be written on each mask substrate.
In order to ensure that the two types of aperture perform identically in an optical sense, except for the phase-shift, the substrate of the prior art may or may not be etched back laterally under the opaque film as shown in
FIG. 5
, thus possibly leaving the opaque film unsupported at the edge
50
. The non phase shift apertures
52
and
54
and the phase shift apertures
58
are noted. The trenches
56
and
58
etched in the substrate beneath the apertures are necessarily formed after the apertures are etched in the opaque layer, which is a high-cost process. The requirement to form a second custo
Hodgson Rodney T.
Rosasco S.
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