U.S. patent application number 09/161854 was filed with the patent office on 2001-07-05 for method and apparatus for resist planarization.
Invention is credited to GOLZ, JOHN, HWANG, CHORNG-LII, ZHU, JOHN.
Application Number | 20010006761 09/161854 |
Document ID | / |
Family ID | 22583052 |
Filed Date | 2001-07-05 |
United States Patent
Application |
20010006761 |
Kind Code |
A1 |
GOLZ, JOHN ; et al. |
July 5, 2001 |
METHOD AND APPARATUS FOR RESIST PLANARIZATION
Abstract
A method for planarizing a layer of photoresist on a substrate.
The layer of photoresist is exposed to wavelengths of radiation
that the photoresist is sensitive to. The radiation is directed at
the layer of photoresist at an oblique angle with respect to a
major dimension of the layer of photoresist. The photoresist is
developed.
Inventors: |
GOLZ, JOHN; (MANASSAS,
VA) ; HWANG, CHORNG-LII; (WAPPINGERS FALLS, NY)
; ZHU, JOHN; (WAPPINGERS FALLS, NY) |
Correspondence
Address: |
ERIC J. FRANKLIN
POLLACK VANDE SANDE & PRIDDY
P O BOX 19088
WASHINGTON
DC
200363425
|
Family ID: |
22583052 |
Appl. No.: |
09/161854 |
Filed: |
September 28, 1998 |
Current U.S.
Class: |
430/311 ;
257/E21.242; 430/270.1; 430/313; 430/319; 430/322; 430/323;
430/327; 438/427; 438/689; 438/691; 438/692; 438/697 |
Current CPC
Class: |
G03F 7/2026 20130101;
H01L 21/31058 20130101; G03F 7/168 20130101 |
Class at
Publication: |
430/311 ;
430/313; 430/322; 430/323; 430/327; 430/319; 430/270.1; 438/427;
438/691; 438/689; 438/692; 438/697 |
International
Class: |
G03C 005/00; H01L
021/76; H01L 021/302; H01L 021/311; H01L 021/461 |
Claims
We claim:
1. A method for planarizing a layer of photoresist on a substrate,
the method comprising the steps of: exposing the layer of
photoresist to wavelengths of radiation that the photoresist is
sensitive to, wherein the radiation is directed at the layer of
photoresist at an oblique angle with respect to a major dimension
of the layer of photoresist; and developing the photoresist.
2. The method according to claim 1, wherein the layer of
photoresist is deposited over at least one trench formed in a
substrate.
3. The method according to claim 2, wherein the at least one trench
is formed in the substrate during a process of forming a DRAM
cell.
4. The method according to claim 1, wherein the layer of
photoresist includes at least one recess in an upper surface and
wherein an edge of the recess at least partially shades portions of
the recess from exposure to the radiation.
5. The method according to claim 1, further comprising the step of:
rotating the substrate during the exposure of the photoresist to
radiation.
6. The method according to claim 5, wherein the periodicity of
rotation of the substrate should be less than the inverse of a time
that the photoresist is exposed to the radiation.
7. The method according to claim 1, wherein the radiation is
provided by a radiation source that simultaneously illuminates the
substrate from a plurality of directions around the substrate.
8. The method according to claim 7, wherein the radiation source
surrounds the substrate.
9. The method according to claim 1, wherein the radiation is
provided by a radiation source that rotates around the substrate
while exposing the substrate to radiation.
10. The method according to claim 1, further comprising the step
of: subjecting the photoresist to further processing.
11. The method according to claim 10, wherein the further
processing includes at least one process selected from the group
consisting of chemical downstream etching and isotropic developer
etching.
12. The method according to claim 1, wherein the angle of the
radiation with respect to the major dimension of the layer of
photoresist is less than forty-five degrees.
13. The method according to claim 4, wherein a depth of the recess
is less after exposing the photoresist to radiation and developing
the photoresist as compared to before exposing and developing.
14. A device for planarizing a layer of photoresist on a substrate,
comprising: a radiation source for exposing the photoresist to
radiation at an oblique angle with respect to a major dimension of
the layer of photoresist, and a rotating substrate support for
supporting and rotating the substrate during exposure to the
radiation.
15. The device according to claim 14, wherein the periodicity of
rotation of the substrate should be less than the inverse of a time
that the photoresist is exposed to the radiation.
16. The device according to claim 14, wherein the radiation source
simultaneously illuminate the substrate from a plurality of
directions around the substrate.
17. The device according to claim 16, wherein the radiation source
surrounds the substrate.
18. The device according to claim 14, wherein the radiation source
rotates around the substrate while exposing the substrate to
radiation.
19. The device according to claim 14, wherein the angle of the
radiation with respect to the major dimension of the layer of
photoresist is less than forty-five degrees.
20. The device according to claim 14, further comprising means for
developing the photoresist.
21. A method for forming a semiconductor device, the method
comprising the steps of: forming at least one trench in a
substrate; depositing a layer of a photoresist on the substrate and
in the at least one trench, wherein photoresist deposited in the at
least one trench results in formation of recesses in an upper
surface of the layer of photoresist; exposing the layer of
photoresist to wavelengths of radiation that the photoresist is
sensitive to, wherein the radiation is directed at the layer of
photoresist at an oblique angle with respect to a major dimension
of the layer of photoresist, and wherein the radiation is directed
at the layer of photoresist from a plurality of angles around the
substrate; developing the layer of photoresist, wherein after
developing of the photoresist the recesses are substantially
eliminated; and subjecting the photoresist to further processing.
Description
FIELD OF THE INVENTION
[0001] The invention relates to the method and apparatus for
planarizing a layer of photoresist during semiconductor device
manufacturing processes.
BACKGROUND OF THE INVENTION
[0002] Very large scale integrated circuit devices typically are
manufactured on a substrate, such as a silicon wafer, by a sequence
of material additions, such as low pressure chemical vapor
depositions, sputtering operations, among others; material
removals, such as wet etches, reactive ion etches, among others;
and material modifications, such as oxidations, ion implants, among
others. Typically, these physical and chemical operations interact
with the entire substrate. For example, if a substrate is placed
into an acid bath, the entire surface of the substrate will be
etched away. In order to build very small electrically active
devices on a substrate, the impact of these operations has to be
confined to small, well-defined, regions.
[0003] Lithography in the context of VLSI manufacturing includes
the process of patterning openings in photosensitive polymers,
sometimes referred to as "photoresists" or "resists", which define
small areas in which substrate material is modified by a specific
operation in a sequence of processing steps.
[0004] The radiation preferably causes desired photochemical
reactions to occur within the photoresist. Preferably, the
photochemical reactions alter the solubility characteristics of the
photoresist, thereby allowing removal of certain portions of the
photoresist. Photoresists can be negative photoresist or positive
photoresist materials.
[0005] A negative photoresist material is one which is capable of
polymerizing and being rendered insoluble upon exposure to
radiation. Accordingly, when employing a negative photoresist
material, the photoresist is selectively exposed to radiation,
causing polymerization to occur above those regions of the
substrate which are intended to be protected during a subsequent
operation. The unexposed portions of the photoresist are removed by
a solvent which is inert to the polymerized portion of the
photoresist. Such a solvent may be an aqueous solvent solution.
[0006] Positive photoresist material is a material that, upon
exposure to radiation, is capable of being rendered soluble in a
solvent in which the unexposed resist is not soluble. Accordingly,
when applying a positive photoresist material the photoresist is
selectively exposed to radiation, causing the reaction to occur
above those portions of the substrate which are not intended to be
protected during the subsequent processing period. The exposed
portions of the photoresist are removed by a solvent which is not
capable of dissolving the exposed portion of the resist. Such a
solvent may be an aqueous solvent solution.
[0007] Selectively removing certain parts of the photoresist allows
for the protection of certain areas of the substrate while exposing
other areas. The remaining portions of the photoresist may be used
as a mask or stencil for processing the underlying substrate. For
example, the openings in the mask may allow diffusion of desired
impurities through the openings into the semiconductor substrate.
Other processes are known for forming devices on a substrate.
[0008] The manufacturing of VLSI chips typically involves the
repeated patterning of photoresists, followed by etch, implant,
deposition, or other operation, and ending with the removal of the
exposed photoresist to make way for the new photoresist to be
applied for another iteration of this process sequence.
[0009] Often, to help ensure uniform processing of materials, the
upper surface of a layer of a material deposited on a substrate may
be processed to lie in substantially only one plane. Such processes
are typically referred to as planarization. One example of
materials that may be planarized includes polycrystalline silicon.
However, techniques utilized to "planarize" materials have not been
applicable to photoresist.
SUMMARY OF THE PRESENT INVENTION
[0010] Aspects of the present invention provide a method for
planarizing a layer of photoresist on a substrate. The method
includes, a step of exposing the layer of photoresist to
wavelengths of radiation that the photoresist is sensitive to. The
radiation is directed at the layer of photoresist at an oblique
angle with respect to a major dimension of the layer of
photoresist. According to the method, the photoresist is then
developed.
[0011] Additional aspects of the present invention provide a device
for planarizing a layer of photoresist on a substrate. The device
includes a radiation source for exposing the photoresist to
radiation directed at the photoresist at an oblique angle with
respect to a major dimension of the layer of photoresist.
[0012] Further aspects of the present invention provide a method
for forming a semiconductor device. The method includes forming at
least one trench in a substrate. The layer of photoresist is
deposited on the substrate and in the at least one trench.
Deposition of photoresist in the at least one trench results
formation of recesses in an upper surface of the layer of
photoresist. The layer of photoresist is exposed to wavelengths of
radiation that the photoresist is sensitive to. The radiation is
directed at the layer of photoresist at an oblique with respect to
a major dimension of the layer of photoresist. Additionally, the
radiation is directed at the layer of photoresist from a plurality
of angles around the substrate. The layer of photoresist is
developed, wherein after developing, the recesses are at least
substantially eliminated. The photoresist is then subjected to
further processing.
[0013] Still other objects and advantages of the present invention
will become readily apparent by those skilled in the art from the
following detailed description, wherein it is shown and described
only the preferred embodiments of the invention, simply by way of
illustration of the best mode contemplated of carrying out the
invention. As will be realized, the invention is capable of other
and different embodiments, and its several details are capable of
modifications in various obvious respects, without departing from
the invention. Accordingly, the drawings and description are to be
regarded as illustrative in nature and not as restrictive.
BRIEF DESCRIPTION OF THE FIGURES
[0014] The above-mentioned objects and advantages of the present
invention will be more clearly understood when considered in
conjunction with the accompanying drawings, in which:
[0015] FIG. 1 represents a cross-sectional view of an example of a
layer of photoresist and a portion of a substrate on which the
photoresist has been deposited;
[0016] FIG. 2 represents a cross-sectional view of a larger portion
of the substrate such as that shown in FIG. 1, at a similar point
during the production of a semiconductor device;
[0017] FIG. 3 represents a cross-sectional view of the portion of
substrate illustrated in FIG. 2 being subjected to an embodiment of
a process according to the present invention;
[0018] FIG. 4 illustrates a reduction in effective trench size when
photoresist is exposed at an oblique angle according to an
embodiment of a process of the present invention;
[0019] FIG. 5 represents a cross-sectional view of the portion of
the substrate illustrated in FIG. 3 after undergoing an embodiment
of a process according to the present invention;
[0020] FIG. 6 represents a cross-sectional view of a portion of the
substrate illustrated in FIGS. 2, 3, and 5 after being subjected to
further processing of the photoresist;
[0021] FIG. 7 represents a graph illustrating a relationship
between the depth of recesses in the layer of photoresist with
respect to depth of trenches formed in the substrate over which the
layer of photoresist has been deposited;
[0022] FIG. 8 represents a graph illustrating a relationship
between the depth of the recesses in the layer of photoresist and a
required exposure dose according to an embodiment of a process
according to the present invention;
[0023] FIG. 9 represents an embodiment of an apparatus according
the present invention shown carrying out a process according to an
embodiment of the present invention;
[0024] FIG. 10 represents another embodiment of an apparatus
according to the present invention; and
[0025] FIG. 11 represents a further embodiment of an apparatus
according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0026] During the process of forming semiconductor devices, often,
portions of a substrate are removed to make a recess for certain
purposes. For example, portions of the substrate may be removed to
create a trench. According to one example, deep trenches are
created in a substrate during the formation of DRAM devices.
[0027] After creation of a recess, such as a deep trench, the
substrate, including the recess may be covered with a layer of
photoresist. Due to the deposition of material in the recess, the
top surface of the layer of photoresist may be depressed in an area
in the vicinity of the trench, resulting in a recess being formed
in the top surface of the layer of photoresist. This problem is
particularly evident when coating substrates that include high
aspect ratio trenches due to the high volume consumption of resist
into the trenches.
[0028] Additionally, a typical layer of photoresist can typically
only be optimized to provide consistent resist thickness across the
wafers on planar structures but not on top of trenches. The
nonplanar profile of photoresist deposited over trenches and
nonuniformity of the upper surface of a layer of photoresist across
the wafers may translate down to resist recess depth and become a
control problem for photoresist recess processes.
[0029] The present invention provides method and apparatus for
planarizing photoresist prior to photoresist recess processes. The
present invention method and apparatus for planarizing a layer of
photoresist may be particularly useful for defining three
dimensional structures and high aspect ratio trenches for DRAM and
DRAM derivatives. Polycrystalline silicon recess processes may be
applied when trenches are ready to be filled with a conducting
material without requiring stripping of the polysilicon out of the
trenches. However, with structures such as buried plates, vertical
transistors, and collar oxide formation prior to node dielectric,
trenches typically need to remain empty after the intended
processes. In these latter cases, polymer materials, such as
photoresist, that can easily be stripped off without harming other
portions of the structures maybe be required as recess
materials.
[0030] Throughout this application, the term "resist recess" will
be utilized to refer to processes carried out on a layer of
photoresist utilizing stripable polymer materials. This is to be
differentiated from depressions that may result in the upper layer
of photoresist due to deposition of the photoresist in the
trenches.
[0031] The present invention provides a method and apparatus for
polymerizing photoresist. Unlike polysilicon, for which polysilicon
chemical mechanical polishing (CMP) planarization processes are
available for planarization prior to resist recess, only one
possible alternative method for planarizing photoresist exists, and
this method is not in practice, but only under study. The method
utilizes CMP to planarize photoresist.
[0032] Advantages of the present invention include that it is
inexpensive. Along these lines, the present invention may require
only a light source and developer. Additionally, the present
invention could be smoothly integrated into existing lithography
coating tracks. Furthermore, the present invention is clean. On the
other hand, for example, CMP produces particles and requires extra
cleaning that is not necessary according to the present invention.
The present invention also produces good uniformity. On the
contrary, for example, CMP has an associated dishing problem at the
array edge. CMP may also consume materials under the photoresist.
The present invention does not result in any such material
consumption. Still further, the present invention, as compared to
CMP has an shorter associated raw process time, resulting in a
better throughput.
[0033] A typical photoresist coating over wafers including trench
structures or other recesses typically results in a top surface
profile as illustrated in FIG. 1. The structure illustrated in FIG.
1 includes a layer of photoresist 1 deposited on a substrate 9.
Prior to deposition of the layer photoresist 1, an array 5 has been
formed in the substrate. The array illustrated in FIG. 1 includes
trenches 3. The array extends to an array edge 7. Commonly,
substrate 9 will be a semiconductor substrate. A common example of
a semiconductor substrate is silicon. Silicon carbide is another
example of a semiconductor substrate.
[0034] The layer photoresist 1 illustrated in FIG. 1 includes a
depression 11 over the array 5, resulting from the consumption of
photoresist by the trenches 3. As a result, over portions of the
array, the layer of photoresist 1 includes a portion 13 having a
reduced height h. The photoresist typically has a greater thickness
in areas 15 outside of the array 5. Additionally, the height of the
resist over the trench array may be difficult to control and may be
nonuniform.
[0035] The thickness of the photoresist in areas 15 outside of the
array may be optimized to be substantially uniform across the
substrate. As a result, it will be beneficial to have a
planarization process to reduce a nonplanar profile in the upper
surface of the photoresist and resist height variation over
trenches and across a substrate. Without a planarization step, a
nonplanar upper surface of the photoresist and height variation
will translate into resist recess depth nonuniformity.
Nonuniformity across an entire substrate could use up the process
window.
[0036] The method and apparatus of the present invention provides a
simple and controllable way to planarize photoresist prior to
resist recess processing. The present invention facilitates the
creation of uniform resist recess profile and helps to control
resist recess depth by eliminating or at least substantially
reducing the effect of nonuniformity of photoresist to resist
recess processing.
[0037] FIG. 2 illustrates an example of a typical structure that
the present invention may be utilized with. The structure shown in
FIG. 2 includes a substrate 17 with a layer of photoresist 19
formed thereon. A plurality of arrays 21 have been formed in
substrate 17. Each array 21 includes plurality of trenches 23
formed in the substrate. As a result of photoresist being deposited
in the trenches as compared to on the surrounding surfaces, the
layer of photoresist 19 includes a plurality of depressions 25 over
the trenches.
[0038] To planarize the photoresist, the present invention includes
a method wherein the layer of photoresist is exposed to wavelengths
of radiation that the photoresist is sensitive to, wherein the
radiation is directed at the layer of photoresist at an oblique
angle with respect to a major dimension of the layer of
photoresist. In other words, the angle of the radiation is oblique
with respect to most of the upper surface of the photoresist. FIG.
3 illustrates a photoresist-covered substrate undergoing an
embodiment of a method according to the present invention. In the
embodiment illustrated in FIG. 3, the structure is illuminated with
radiation 27 produced by radiation source 29.
[0039] The radiation directed at the photoresist is angled with
respect to the substrate and the layer of photoresist. The angle is
measured with respect to the major dimension of the substrate and
the layer of photoresist deposited thereon. In other words, the
angle is measured with respect to a plane parallel to the substrate
and the layer photoresist. In FIG. 3 the angle is represented by
.theta..
[0040] The angle 31 (.theta.) of the radiation with respect to the
photoresist may depend upon the depth and width of the depressions
25 in the upper surface of the layer of photoresist. By exposing
photoresist coated trenches to radiation at an oblique angle will
cause the resist outside of the trenches to be exposed and become
removable by developers. On the other hand, the resist in the
depressions will not be exposed as much. Therefore, less of the
resist within the trenches will be removed, thereby equalizing, or
planarizing, the level of the upper surface of the photoresist.
[0041] FIG. 4 illustrates the principle upon which the present
invention operates. By illuminating the photoresist covered
trenches at an oblique angle, the effective size 33 of the trench
opening is reduced. As illustrated in FIG. 4, on edge of
depressions in the photoresist may at least partially shade
portions of the resist in the depressions over the trenches from
being exposed to the radiation at an oblique angle. According to
the present invention, the depth of depressions in the upper
surface of the photoresist maybe expected to be reduced
tremendously.
[0042] On the other hand, if the photoresist in the trenches were
exposed to light surfaces directed vertically down, the resist in
the trench as well as the resist outside the trench would be
exposed by the radiation and become removable by developers. As a
result, the photoresist would still be recessed in the trenches.
The present invention solves this problem by illuminating the
resist at an angle.
[0043] Various parameters relating to the exposure of the
photoresist may be varied, depending upon the application. For
example, the time of the exposure may be varied. The angle of the
radiation with respect to the photoresist may also be varied.
Typically, the angle of the radiation with respect to the
photoresist is less than about 45.degree.. However, the angle may
be as low as necessary to result in the desired degree of
planarization. The angle may depend upon the size of depressions in
the photoresist surface, which may in turn depend upon the trench
size. The exposure dose may be optimized to minimize the recess
into the trenches.
[0044] Depth of depressions in the photoresist maybe affected by a
plurality of factors. Among these factors maybe trench size,
wavelength of radiation utilized and optical dose. FIGS. 7 and 8
illustrate a relationship between the depth of depressions formed
in the upper surface of the layer of photoresist with respect to
trench size and exposure dose, respectively. As illustrated in FIG.
7, depth of the depressions in the photoresist may increase as the
size of the trenches increases. For example, the depth of the
depressions may increase as depth of the trenches increases. The
processes, results of which are illustrated in FIG. 7, included
fixed dose and wavelength of radiation.
[0045] FIG. 8 illustrates the relationship between the depth of
depressions in the photoresist and exposure dose. As can be seen,
the longer the exposure dose, the greater the depth. However, just
as with trench size, depth dose, level off at some point such that
with increased dosage or trench size depth of the recess will only
increase slightly.
[0046] To increase the uniformity of the exposure of the
photoresist and portions of the side walls of the depressions in
the photoresist that may be exposed by the angled radiation, the
radiation preferably maybe directed from various angles around the
substrate. To accomplish this, the substrate can be rotated as it
is being exposed to the radiation at an oblique angle.
Alternatively, the radiation source may be rotated around the
substrate. Furthermore, both the substrate and the radiation source
could be rotated. According to another embodiment, the plurality of
radiation sources are arranged about the substrate. According to a
still further embodiment, a circular radiation source is utilized
to expose the photoresist. One or more radiation sources could also
be rotated around the substrate.
[0047] FIG. 9 illustrates an embodiment of an apparatus according
to the present invention that includes a rotating substrate support
38. The rotation is indicated by arrow 37. The apparatus shown in
FIG. 9 includes a radiation source 39. Both the substrate and the
radiation source are controlled by control unit 41.
[0048] FIG. 10 illustrates an embodiment of an apparatus according
to the present invention that includes a rotating radiation source
43 and a stationary substrate support 47. The passage of the
radiation source is indicated by the broken line 45 in FIG. 10. The
embodiment shown in FIG. 10 also includes a control unit (not
shown). In embodiments where a substrate and/or a single radiation
source are rotated, the periodicity of rotation of the substrate
and the radiation source with respect to each other maybe less than
the inverse of a time that the photoresist is exposed to the
radiation.
[0049] FIG. 11 illustrates an embodiment of an apparatus according
to the present invention that includes a radiation source 47 that
entirely surrounds the substrate being exposed. In all of FIGS.
9-11, the path of the radiation is indicated by arrows 49.
[0050] After exposure of the photoresist to radiation directed at
an oblique angle, the photoresist maybe developed according to
standard procedures for developing the photoresist. FIG. 5
illustrates an example of a layer of photoresist that has been
planarized according to the present invention. As illustrated in
FIG. 5, only very slight recess may remain in the photoresist after
being subjected to a process according to the present invention.
According to some embodiments, the depression may be entirely
eliminated. According to typical embodiments of the present
invention, depressions in the photoresist may be reduced by about
80% to about 100% as compared to their original depth. The depth of
the depressions may also be of non-uniform depth as compoared to
each other and among different samples treated according to the
present invention.
[0051] After the undergoing the above-described planarization
process, the photoresist may be processed according to resist
recess processes, such as chemical downstream etch and isotropic
developer etch, among others.
[0052] The foregoing description of the invention illustrates and
describes the present invention. Additionally, the disclosure shows
and describes only the preferred embodiments of the invention, but
as aforementioned, it is to be understood that the invention is
capable of use in various other combinations, modifications, and
environments and is capable of changes or modifications within the
scope of the inventive concept as expressed herein, commensurate
with the above teachings, and/or the skill or knowledge of the
relevant art. The embodiments described hereinabove are further
intended to explain best modes known of practicing the invention
and to enable others skilled in the art to utilize the invention in
such, or other, embodiments and with the various modifications
required by the particular applications or uses of the invention.
Accordingly, the description is not intended to limit the invention
to the form disclosed herein. Also, it is intended that the
appended claims be construed to include alternative
embodiments.
* * * * *