U.S. patent application number 11/046045 was filed with the patent office on 2006-10-05 for t-gate formation.
This patent application is currently assigned to Rohm and Haas Electronic Materials LLC. Invention is credited to Frank Linskens, Rudy Pellens.
Application Number | 20060223245 11/046045 |
Document ID | / |
Family ID | 34652505 |
Filed Date | 2006-10-05 |
United States Patent
Application |
20060223245 |
Kind Code |
A9 |
Pellens; Rudy ; et
al. |
October 5, 2006 |
T-gate formation
Abstract
Methods of forming T-gate structures on a substrate are provided
that use only UV-sensitive photoresists. Such methods provide
T-gate structures using two lithographic steps using a single
wavelength of radiation.
Inventors: |
Pellens; Rudy; (Overpelt,
BE) ; Linskens; Frank; (Venray, NL) |
Correspondence
Address: |
S. Matthew Cairns;Edwards & Angell, LLP
P.O. Box 55874
Boston
MA
02205
US
|
Assignee: |
Rohm and Haas Electronic Materials
LLC
Marlborough
MA
ASML Netherlands B.V.
Veldhoven
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20050202613 A1 |
September 15, 2005 |
|
|
Family ID: |
34652505 |
Appl. No.: |
11/046045 |
Filed: |
January 28, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60539932 |
Jan 29, 2004 |
|
|
|
Current U.S.
Class: |
438/167 ;
438/182 |
Current CPC
Class: |
H01L 21/28587
20130101 |
Class at
Publication: |
438/167 ;
438/182 |
International
Class: |
H01L 21/338 20060101
H01L021/338 |
Claims
1. The present invention provides a method for forming a T-gate on
a substrate including the steps of: a) providing a substrate; b)
optionally disposing an organic planarizing layer on the substrate;
c) disposing a layer of a UV-sensitive first photoresist; d)
patterning the first photoresist by exposing the first photoresist
to UV radiation through a mask and developing the exposed first
photoresist to define a first opening for a base of the T-gate; e)
transferring the pattern to the planarizing layer if present; f)
rendering the pattern insensitive to the UV-radiation; g) disposing
a layer of a UV-sensitive second photoresist, the second
photoresist being negative-acting; h) patterning the second
photoresist by exposing the second photoresist to the UV radiation
through a mask and developing the exposed second photoresist to
define a second opening for a cap of the T-gate over the first
opening; and i) depositing a conductive material within the first
and second openings to form a T-gate.
2. The method of claim 1 wherein the UV radiation has a wavelength
of 190 to 300 nm.
3. The method of claim 1 wherein the step of rendering the pattern
insensitive to the UV radiation comprises curing the first
photoresist.
4. The method of claim 1 further comprising the step of removing
the second photoresist.
5. The method of claim 1 wherein the second photoresist layer has a
thickness greater than that of the first photoresist layer.
6. The method of claim 1 wherein the organic planarizing layer is
chosen from a via-filling layer and an antireflective coating
layer.
7. A method for forming a T-gate structure comprising the steps of:
a) providing a substrate; b) disposing a layer of a UV-sensitive
first photoresist on the substrate; patterning the first
photoresist by exposing the first photoresist to UV radiation
through a mask and developing the exposed first photoresist to
define a first opening for a base of the T-gate; and curing the
patterned first photoresist; c) disposing a layer of a UV-sensitive
second photoresist on the cured first photoresist wherein the
second photoresist is negative-acting; d) patterning the second
photoresist by exposing the second photoresist to the UV radiation
through a mask and developing the exposed second photoresist to
define a second opening for a cap of the T-gate over the first
opening; e) depositing a conductive material within the first and
second openings to form a T-gate; and f) removing the first and
second photoresists.
8. The method of claim 7 wherein the first photoresist is negative
acting.
9. The method okf claim 9 wherein the UV radiation has a wavelength
of 248 nm.
10. A method for forming a T-gate structure comprising the steps
of: a) providing a substrate; b) disposing an organic planarizing
layer on the substrate, disposing a layer of a UV-sensitive first
photoresist on the organic planarizing layer, patterning the first
photoresist by exposing the first photoresist to UV radiation
through a mask and developing the exposed first photoresist to
define a first opening for a base of the T-gate, transferring the
first opening to the organic planarizing layer, and removing the
first photoresist to provide a patterned organic planarizing layer;
c) disposing a layer of a UV-sensitive second photoresist on the
patterned organic planarizing layer wherein the second photoresist
is negative-acting; d) patterning the second photoresist by
exposing the second photoresist to the UV radiation through a mask
and developing the exposed second photoresist to define a second
opening for a cap of the T-gate over the first opening; e)
depositing a conductive material within the first and second
openings to form a T-gate; and f) removing the second
photoresist.
11. The method of claim 9 wherein the UV radiation has a wavelength
of 248 nm.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit under 35 U.S.C. .sctn.
19(e) of U.S. Provisional Application No. 60/539,932, filed Jan.
29, 2004, the entire contents of which are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates generally to the manufacture
of electronic devices. More particularly, the present invention
relates to the fabrication of T-gate structures used in the
manufacture of electronic devices.
[0003] A T-gate is a gate conductor structure for a semiconductor
device in which the top of the gate conductor structure is wider
than the base of the gate conductor structure. Such T-gates
include, without limitation, structures that are substantially
T-shaped, mushroom-shaped, and Y-shaped.
[0004] In general, gate structures such as T-gates have been
advantageously used in several technologies. For example, metal
semiconductor field effect transistors ("MESFETs"), high electron
mobility transistors ("HEMTs") (variant of gallium arsenide field
effect transistor technology) mainly used in satellite broadcasting
receivers, high speed logic circuits and power modules have
employed gate structures with bases smaller than the contact area.
These types of gate structures are required in field effect
transistors for operation in ultra-high frequency ranges. The
narrow base of a T-gate structure provides a short channel length
which results in increased speed and decreased power consumption.
Parasitic resistances and capacitances that limit device speed are
also reduced. The top portion of a T-gate is made wide so that the
conductance of the T-gate remains high, for example, for high
switching speeds.
[0005] Recent advances in CMOS transistor architecture make use of
T-gate structures where the polysilicon gate electrode is narrowed
in the gate regions and wider on top of the gate. This is due to
the ever increasing demand for scaling down semiconductor devices
and scaling down power consumption requirements.
[0006] Electron-beam ("e-beam") is the most commonly used technique
for T-gate fabrication. FIGS. 1A-1D illustrate a process for
forming a T-gate using e-beam. Typically, substrate 1 is coated
with a layer of first poly(methyl methacrylate)-based photoresist
2, a layer of second poly(methyl methacrylate)-based photoresist 3,
and a layer of third poly(methyl methacrylate)-based photoresist 4.
Photoresist layers 2 to 4 are then exposed to e-beam and developed
to provide a patterned photoresist stack having generally T-shaped
profile 5 as shown in FIG. 1B. A layer of a conductive material 6
is then deposited on the entire surface inclusive of the surface of
substrate 1 exposed by the patterning of the photoresist layers,
see FIG. 1C. Photoresist layers 2 to 4 are then removed,
lifting-off the conductive material layer on the surface of
photoresist layer 4 in the process, to provide T-gate structure 7
on substrate 1 as shown in FIG. 1D.
[0007] However, such e-beam techniques suffer from certain
drawbacks. For example, e-beam lithography suffers from poor
linewidth control in the multi-layered stacks used in typical
T-gate processes because the exposing e-beam must pass through
relatively thick resist films (e.g., about one micron). Further,
e-beam exposure is a direct write process which is both slow and
expensive.
[0008] Other methods of forming T-gates have been developed.
Certain of these methods utilize a number of sacrificial inorganic
layers which require various etching steps and harsher removal
processes than photoresist-based processes. Other methods utilize
multiple photoresist layers, however, these multiple photoresist
layers are imaged at different wavelengths. For example, U.S. Pat.
No. 6,387,783 (Furukawa et al.) disclose a process for forming
T-gates using a hybrid first photoresist that is imaged using
x-rays and a second photoresist that is imaged using I-line
radiation. The use of such different wavelengths requires different
exposure tools, which increase the costs and complexity of the
process. Accordingly, a need exists for improved methods of forming
T-gate structures.
SUMMARY OF THE INVENTION
[0009] The inventors have found that T-gate structures may be
formed using conventional UV exposure tools using a single
wavelength and with fewer processing steps.
[0010] The present invention provides a method for forming a T-gate
on a substrate including the steps of: a) providing a substrate; b)
optionally disposing an organic planarizing layer on the substrate;
c) disposing a layer of a UV-sensitive first photoresist; d)
patterning the first photoresist by exposing the first photoresist
to UV radiation through a mask and developing the exposed first
photoresist to define a first opening for a base of the T-gate; e)
transferring the pattern to the planarizing layer if present; f)
rendering the pattern insensitive to the UV-radiation; g) disposing
a layer of a UV-sensitive second photoresist, the second
photoresist being negative-acting; h) patterning the second
photoresist by exposing the second photoresist to the UV radiation
through a mask and developing the exposed second photoresist to
define a second opening for a cap of the T-gate over the first
opening; and i) depositing a conductive material within the first
and second openings to form a T-gate.
[0011] The present invention further provides a method for forming
a T-gate structure comprising the steps of: a) providing a
substrate; b) disposing an organic planarizing layer on the
substrate, disposing a layer of a UV-sensitive first photoresist on
the organic planarizing layer, patterning the first photoresist by
exposing the first photoresist to UV radiation through a mask and
developing the exposed first photoresist to define a first opening
for a base of the T-gate, transferring the first opening to the
organic planarizing layer, and optionally removing the first
photoresist to provide a patterned organic planarizing layer; c)
disposing a layer of a UV-sensitive second photoresist wherein the
second photoresist is negative-acting; d) patterning the second
photoresist by exposing the second photoresist to the UV radiation
through a mask and developing the exposed second photoresist to
define a second opening for a cap of the T-gate over the first
opening; e) depositing a conductive material within the first and
second openings to form a T-gate; and f) removing the second
photoresist. Preferably, the organic planarizing layer is an
antireflective coating layer.
[0012] Additionally, the present invention provides a method for
forming a T-gate structure comprising the steps of: a) providing a
substrate; b) disposing a layer of a UV-sensitive first photoresist
on the substrate; patterning the first photoresist by exposing the
first photoresist to UV radiation through a mask and developing the
exposed first photoresist to define a first opening for a base of
the T-gate; and curing the patterned first photoresist; c)
disposing a layer of a UV-sensitive second photoresist on the cured
first photoresist wherein the second photoresist is
negative-acting; d) patterning the second photoresist by exposing
the second photoresist to the UV radiation through a mask and
developing the exposed second photoresist to define a second
opening for a cap of the T-gate over the first opening; e)
depositing a conductive material within the first and second
openings to form a T-gate; and f) removing the first and second
photoresists.
BRIEF DESCRIPTION OF THE DRAWING
[0013] FIGS. 1A-1D are a schematic cross-sectional illustration of
a conventional process for T-gate formation using e-beam.
[0014] FIGS. 2A-2F are a schematic cross-sectional illustration of
a process for forming a T-gate according to one embodiment of the
invention.
[0015] FIGS. 3A-3H are a schematic cross-sectional illustration of
a process for forming a T-gate according to another embodiment of
the invention.
[0016] FIG. 4 is a scanning electron micrograph of a T-gate
structure made by the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0017] As used throughout this specification, the term "T-gate"
refers to any gate conductor structure for an electronic device in
which the top of the gate conductor structure is wider than the
base of the gate conductor structure. Such T-gate structures may
have a variety of shapes including, without limitation, T-shaped,
mushroom-shaped, and Y-shaped. The articles "a" and "an" refer to
the singular and the plural. All numerical ranges are inclusive and
combinable in any order except where it is clear that such
numerical range is constrained to add up to 100%. Like reference
numerals refer to like elements.
[0018] T-gate structures are formed on a substrate according to the
present method using two lithographic processes, the two
lithographic processes being performed using the same wavelength of
radiation. A negative-acting photoresist is typically used as the
photoresist in the second lithographic process. An advantage of the
present invention is that a single exposure tool can be utilized
instead of multiple tools, thus reducing costs. Accordingly, the
present invention provides a method for forming a T-gate structure
including the steps of: a) providing a substrate; b) optionally
disposing an organic planarizing layer on the substrate; c)
disposing a layer of a UV-sensitive first photoresist; d)
patterning the first photoresist by exposing the first photoresist
to UV radiation through a mask and developing the exposed first
photoresist to define a first opening for a base of the T-gate; e)
transferring the pattern to the planarizing layer if present; f)
rendering the pattern insensitive to the UV-radiation; g) disposing
a layer of a UV-sensitive second photoresist, the second
photoresist being negative-acting; h) patterning the second
photoresist by exposing the second photoresist to the UV radiation
through a mask and developing the exposed second photoresist to
define a second opening for a cap of the T-gate over the first
opening; and i) depositing a conductive material within the first
and second openings to form a T-gate. The remaining photoresist
layers are then removed and function as lift-off layers by removing
the conductive material deposited on the surface of the second
photoresist. The wavelength of the UV radiation used to image the
first and second photoresists is the same.
[0019] A wide variety of substrates may be used in the present
invention. Suitable substrates are those used in the manufacture of
electronic devices. Exemplary substrates include, without
limitation, gallium arsenide ("GaAs"), silicon ("Si"), indium
gallium arsenide ("InGaAs"), aluminum gallium arsenide ("AlGaAs"),
strained silicon, silicon germanium ("SiGe"), and mixtures thereof.
Other suitable substrates are well known to those skilled in the
art. For example, the substrate may include an InGaAs/AlGaAs/GaAs
film stack grown on a semi-insulating GaAs substrate. Such films
may be grown by a variety of means, such as by molecular beam
epitaxy ("MBE"), metalorganic chemical vapor deposition ("MOCVD"),
physical vapor deposition ("PVD"), liquid phase epitaxy ("LPE"),
chemical beam epitaxy ("CBE") and atomic layer deposition ("ALD").
These film growth techniques are well known to those skilled in the
art. The substrates may include one or more additional layers of
materials. The choice of such substrates will depend upon the
particular electronic device desired and is well within the ability
of those skilled in the art.
[0020] The optional organic planarizing layers that may be used in
the present invention include, but are not limited to, one or more
layers of via-filling material, antireflective coatings and
mixtures thereof. Via-filling materials typically contain one or
more cross-linkable polymers, particularly low molecular weight
polymers such as those having a weight average molecular weight of
less than or equal to about 8000 and a value of M.sub.w/M.sub.n of
at least 1.5, one or more acid catalysts, one or more cross-linking
agents and one or more solvents. Such via-filling materials are
typically used to fill apertures to protect such apertures from
damage during subsequent processing steps. It will be appreciated
by those skilled in the art that such via-filling materials may
also function as antireflective coatings upon the inclusion of one
or more chromophores. Exemplary via-filling materials are disclosed
in U.S. Pat. No. 6,461,717.
[0021] The antireflective coating is typically an organic
antireflective coating. Such antireflective coatings contain one or
more light absorbing materials. Such materials may be bound to a
polymer or may be admixed with a polymer or incorporated by any
other suitable means. Exemplary light absorbing materials include,
without limitation, phenyl and anthracenyl, each optionally
substituted. The selection of an antireflective coating depends
upon the particular wavelength of UV radiation and the photoresist
employed. Such selection is within the abilities of those skilled
in the art. Multiple layers of antireflective coatings may
optionally be used. Exemplary antireflective coating materials are
disclosed in U.S. Pat. Nos. 6,528,235; 6,472,128; 6,451,503;
6,165,697; and 5,851,730.
[0022] The optional planarizing layer may be disposed on the
substrate by any suitable means, such as spin coating, roller
coating, curtain coating, dipping, and spraying. Spin coating is
conventionally used. The thickness of the planarizing layer is
determined by the spin speed in the case of spin coating, and by
the viscosity of the planarizing material composition. In general,
the planarizing layer, particularly an antireflective coating
layer, has a thickness of up to 500 nm, although greater
thicknesses may be used. Typically, the thickness of the
planarizing layer ranges from 100 to 500 nm, more typically from
150 to 300 nm, and still more typically from 200 to 300 nm.
Following deposition, the planarizing material is typically baked
to remove solvent and then sufficiently cured by any suitable means
to prevent intermixing with the subsequently deposited first
photoresist.
[0023] A wide variety of UV-sensitive photoresists may be used as
the first photoresist, such as positive-acting and negative-acting
photoresists. Chemically amplified positive photoresists suitably
contain a component that has moieties that undergo a deblocking or
cleavage reaction in the presence of photogenerated acid, such as
photoacid-labile esters or acetals. Positive acting photoresists
for sub-300 nm imaging such as 248 nm include a photoacid generator
compound such as an iodonium or sulfonium salt and a polymer that
comprises phenolic units (or silsesquioxane units or both phenolic
units and silsesquioxane units) and acid-labile ester and/or acetal
moieties. Exemplary positive-acting photoresists are disclosed in,
e.g., U.S. Pat. Nos. 6,042,997; 6,090,526; 5,929,176; and
5,492,793.
[0024] Negative-acting photoresists for sub-300 nm imaging such as
248 nm typically include a phenolic resin or a silsesquioxane
polymer or blends thereof or copolymers containing both phenolic
units and silsesquioxane units, a photoacid generator compound such
as an iodonium or sulfonium salt, and a cross-linking agent.
Suitable negative-acting photoresist cross-linkers include, without
limitation, amine-based cross-linkers such as melamine resins.
Exemplary negative-acting photoresists are disclosed in, e.g., U.S.
Pat. Nos. 5,514,520; 5,340,696; and 5,210,000, and European
Published Application EP 042 391.
[0025] The term "silsesquioxane" refers to polymers, oligomers, and
partial condensates having the formula (RSiO.sub.1.5) where R is an
organic moiety such as (C.sub.1-C.sub.6)alkyl and phenyl, either
being optionally substituted. By "substituted" it is meant that one
or more hydrogens on the alkyl or phenyl group is replaced by one
or more substituent groups, such as hydroxyl,
(C.sub.1-C.sub.6)alkoxy, (C.sub.1-C.sub.6)alkyl, alkoxyalkyl,
(C.sub.7-C.sub.16)aralkyl, and the like. Exemplary phenolic resins
include, without limitation, novolak resins and hydroxystyrene
resins. Exemplary silsesquioxane-containing photoresists are those
disclosed in U.S. Pat. Nos. 4,745,169 and 5,338,818, and U.S.
patent application Ser. No. 2003/0099899.
[0026] Any UV-sensitive negative-acting photoresist may suitably be
used as the second photoresist in the present invention. Exemplary
second photoresists include, without limitation, the
negative-acting photoresists described above.
[0027] Exemplary positive-acting and negative-acting photoresists
for use in the present invention are SR.TM.2420 and UVN.TM.30,
respectively, both available from Shipley Company (Marlborough,
Mass.).
[0028] Such first and second photoresists may be imaged at a
variety of wavelengths, such as from 190 to 365 nm. Typically, the
photoresists are imaged at wavelengths of 190 to 300 nm, and more
typically at 248 nm. The first and second photoresists are
preferably selected such that they are both imaged at the same
wavelength of UV radiation.
[0029] The first and second photoresists may be disposed by any
suitable means, such as spin coating, roller coating, curtain
coating, dipping, and spraying. Spin coating is conventionally
used. The thickness of the photoresist layer is determined by the
spin speed in the case of spin coating, and by the viscosity of the
photoresist. In general, the first photoresist layer has a
thickness of up to 500 nm, although other thicknesses may be used.
Typically, the first photoresist layer thickness ranges from 100 to
500 nm, more typically from 100 to 300 nm and still more typically
from 150 to 250 nm. The second photoresist layer typically has a
thickness of up to 1 .mu.m (micron), although other thicknesses may
suitably be employed. In one embodiment, the second photoresist
layer has a thickness range of 300 nm to 1 .mu.m, more typically
from 500 nm to 900 nm, and still more typically from 600 to 800 nm.
In general, the second photoresist layer has a thickness greater
than that of the first photoresist layer.
[0030] The photoresists are patterned by exposing the photoresists
to suitable UV radiation through a mask, followed by development. A
wide variety of exposure tools may be used with the present
process. Suitable exposure tools are those manufactured by ASML
(Veldhoven, the Netherlands), such as a PAS5500/300 tool. The
exposure time and the exposure dose depend upon a variety of
factors, such as the particular photoresist selected, and are well
within the abilities of those skilled in the art. The photoresists
may be developed using a variety of developer solutions. Suitable
developer solutions include, but are not limited to, organic
solvents and aqueous alkaline compositions. Exemplary aqueous
alkaline compositions include, without limitation,
tetraalkylammonium hydroxide-based compositions such as 0.15 to
0.26 N tetramethylammonium hydroxide ("TMAH"). Such developers are
commercially available from a variety of sources, such as from
Shipley Company.
[0031] When an organic planarizing layer is used, the pattern from
the first photoresist layer is transferred to the planarizing
layer. Such pattern transfer may be achieved by a variety of means
such as by etching. The etching of the planarizing material may be
performed by a variety of methods including wet etching and dry
etching such as reactive ion etching, plasma etching and ion beam
milling. In general, such etching step removes the portion of the
planarizing layer exposed after patterning of the first photoresist
layer. The etching step may be stopped after removal of the
planarizing material, thus stopping at the substrate surface, or
may optionally remove a portion of the substrate. Such etching
techniques are conventional and are well within the abilities of
those skilled in the art.
[0032] Prior to disposing the second photoresist, the pattern is
rendered insensitive to the UV-radiation. The pattern may be
rendered insensitive by curing the patterned first photoresist. By
"curing" it is meant that the first photoresist is subjected to
conditions that render it less soluble, and preferably insoluble,
in the second photoresist, the developer solution used to develop
the second photoresist or both. Such curing may be accomplished by
exposure to UV radiation, heating, or a combination thereof. A
variety of exposure doses and temperatures may be used to cure the
first photoresist. For example, exposure doses of 10.times.Eo to
20.times.Eo may be used, as well as doses that are higher and lower
than this. The first photoresist may be heated at a temperature of
100 to 130.degree. C., although higher and lower temperatures may
be used. In general, the photoresist is heated for 5 to 360
seconds.
[0033] When an organic planarizing layer is used, the pattern may
alternatively be rendered insensitive to the UV radiation after
transfer of the pattern to the planarizing layer by removing the
first photoresist to provide a patterned planarizing layer. Such
planarizing layer is typically insensitive to UV radiation. The
first photoresist may be removed using any suitable resist
stripper, such as those described below.
[0034] A wide variety of conductive materials may be used in the
present invention, such as metals and conductive polymers. Metals
are preferred and include alloys. Exemplary metals and alloys
include, without limitation, gallium, arsenic, tantalum, tungsten,
molybdenum, titanium, platinum, gold, silver, germanium, aluminum,
copper, titanium-platinum-gold, and gold-germanium-platinum/gold.
Various other metals and metal alloys may be advantageously used.
Such conductive materials are deposited on the second photoresist.
When a metal and/or alloy is the conductive material, it may be
deposited by any suitable means, such as, but not limited to,
sputtering, physical vapor deposition, and chemical vapor
deposition.
[0035] In the present invention, the first and second photoresists
may be removed by any suitable technique. For example, the
photoresists may be removed by contacting them with a polymer
remover composition for a period of time and at a temperature
sufficient to remove the photoresists. Photoresists are generally
contacted with the polymer remover at a temperature of 10.degree.
to 90.degree. C. Typically, the temperature ranges from 15.degree.
to 85.degree. C. and more typically from 20.degree. to 85.degree.
C. Exemplary contact times range from 5 seconds to 15 minutes,
although shorter or longer times may be used. Typically, the
contact time is from 5 to 300 seconds and more typically from 10 to
120 seconds. The particular time and temperature will depend upon
the photoresist to be removed and the particular polymer remover
composition.
[0036] Exemplary polymer removers include, but are not limited to,
organic solvents, bases such as tetraalkylammonium hydroxides,
fluoride ion sources such as hydrogen fluoride, ammonium fluoride
and ammonium bifluoride, hydroxylamine, and alkanolamines. Such
polymer removers may contain one or more additional components such
as corrosion inhibitors, solvents, water, wetting agents,
antifreeze agents, and thickening agents. Polymer removers are
generally commercially available, such as from Shipley Company and
EKC Technologies.
[0037] The second photoresist having a conductive material disposed
thereon functions as a lift-off layer when removed. That is,
removing the second photoresist by contacting it with a polymer
remover also removes the conductive material that has been
deposited on the surface of the second photoresist, leaving the
conductive material that has been deposited within the first and
second openings, i.e. the T-gate structure.
[0038] In a first embodiment (referred to as "double-coat
process"), T-gate structures are formed according to a method
including the steps of: a) providing a substrate; b) disposing a
layer of a UV-sensitive first photoresist on the substrate,
patterning the first photoresist by exposing the first photoresist
to UV radiation through a mask and developing the exposed first
photoresist to define a first opening for a base of the T-gate, and
curing the patterned first photoresist; c) disposing a layer of a
UV-sensitive second photoresist on the cured first photoresist
wherein the second photoresist is negative-acting; d) patterning
the second photoresist by exposing the second photoresist to the UV
radiation through a mask and developing the exposed second
photoresist to define a second opening for a cap of the T-gate over
the first opening; and e) depositing a conductive material within
the first and second openings to form a T-gate. The photoresist
layers are then removed and function as lift-off layers by removing
the conductive material deposited on the surface of the second
photoresist. One or more layers of antireflective coatings and/or
via-filling materials may be disposed between the substrate and the
first photoresist.
[0039] In the double-coat process, the second photoresist is
disposed on the first photoresist. The first photoresist is cured
prior to the deposition of the second photoresist. In the
double-coat process, the first photoresist is preferably
negative-acting. In one embodiment of the double-coat process, the
first photoresist and the second photoresist are the same. The
substrate may optionally be etched in the double-coat process
following patterning of the first photoresist layer, and preferably
etched after patterning of the second photoresist layer. Any of the
above described etching techniques may be used to etch the
substrate.
[0040] FIGS. 2A-2E illustrate the double-coat process of the
present invention. First photoresist 11, typically a
negative-acting photoresist, is disposed on substrate 10, such as
GaAs or silicon. First photoresist 11 is then exposed to
UV-radiation, such as 248 nm radiation, through a mask and
developed, such as with 0.26 N TMAH, to pattern first photoresist
11 and define first opening 12 fore the base of the T-gate
structure. See FIG. 2A. First photoresist 11 is then cured using
heat and light and a layer of second photoresist 13 is disposed on
cured first photoresist 11. See FIG. 2B. Second photoresist 13 is
then patterned to define second opening 14 for the top of the
T-gate by exposing second photoresist 13 to UV-radiation through a
mask, the UV-radiation being the same as that used to expose first
photoresist 11, and then developing second photoresist 13. The
patterning of second photoresist 13 is performed in a manner to
provide second opening 14 having an undercut profile as illustrated
in FIG. 2C. Such undercut profile assists in the lift-off of
subsequently deposited conductive material. Optionally, a portion
of substrate 10 exposed by the first and second openings is etched
as illustrated by recess 15 which is illustrated in FIG. 2D as
undercutting the opening in first photoresist 11. Gate
metallization layer 16 (conductive material) is then deposited
within the first and second openings as well as over the surface of
second photoresist layer 13. See FIG. 2E. The first and second
photoresists are then removed by contacting the device with a
polymer remover. The second photoresist functions to lift-off the
gate metallization layer covering the surface of the second
photoresist leaving T-gate structure 17 on substrate 10, as
illustrated in FIG. 2F.
[0041] In a second embodiment (referred to as "bi-layer process"),
T-gate structures are formed according to a method including the
steps of: a) providing a substrate; b) disposing a layer of a
planarizing material on the substrate, disposing a layer of a
UV-sensitive first photoresist on the planarizing material layer,
patterning the first photoresist by exposing the first photoresist
to UV radiation through a mask and developing the exposed first
photoresist to define a first opening for a base of the T-gate,
transferring the first opening to the planarizing material layer,
and optionally removing the first photoresist to provide a
patterned planarizing material layer; c) disposing a layer of a
UV-sensitive second photoresist wherein the second photoresist is
negative-acting; d) patterning the second photoresist by exposing
the second photoresist to the UV radiation through a mask and
developing the exposed second photoresist to define a second
opening for a cap of the T-gate over the first opening; and e)
depositing a conductive material within the first and second
openings to form a T-gate. The second photoresist is then
removed.
[0042] In one embodiment of the present bi-layer process, the first
photoresist is positive-acting. In another embodiment of the
bi-layer process, the first photoresist is negative acting. In a
further embodiment of the bi-layer process, the first photoresist
is positive-acting and contains a polymer including silsesquioxane
units.
[0043] FIGS. 3A-3H illustrate the bi-layer process of the present
invention. A layer of a polymeric antireflective coating 20, such
as AR.TM.2450 underlayer available from Shipley Company, is spin
coated to a thickness of 200 to 250 nm on substrate 10, such as
GaAs or silicon. Antireflective coating 20 contains a phenolic
moiety, a thermal acid generator and a cross-linking agent. After
spin-coating, antireflective coating 20 is baked and sufficiently
cured to prevent intermixing with subsequently applied
photoresists. First photoresist 21 is spin coated on antireflective
coating 20 to a thickness of, e.g., up to 250 nm. First photoresist
21 may be a positive-acting photoresist such as SR.TM.2420 which is
a silsesquioxane-containing photoresist available from Shipley
Company. See FIG. 3A. First photoresist 21 is patterned to define
aperture 22 by exposing first photoresist to UV radiation through a
mask using a PAS5500/300 tool and then developing the exposed first
photoresist. See FIG. 3B. Antireflective coating 20 is then etched
to form first opening 23 for the base of the T-gate and then first
photoresist 21 is removed, such as by contact with a fluoride-based
polymer remover such as PRx417 available from Shipley Company.
Negative-acting second photoresist 24 is then spin coated to a
thickness of approximately 900 nm on patterned antireflective
coating 20, as illustrated in FIG. 3D. An exemplary second
photoresist is UVN.TM.30, available from Shipley Company. Second
photoresist 24 is patterned to define second opening 25 for the top
of the T-gate by exposing second photoresist 24 to UV-radiation
through a mask, the UV-radiation being the same as that used to
expose first photoresist 21, and then developing second photoresist
24. The patterning of second photoresist 24 is performed in a
manner to provide second opening 25 having an undercut profile as
illustrated in FIG. 3E. Optionally, a portion of substrate 10
exposed by the first and second openings is etched as illustrated
by recess 26 which is shown in FIG. 3F as undercutting the opening
in antireflective coating 20. A gate metallization layer
(conductive material) 27 is then deposited within the first and
second openings as well as over the surface of the second
photoresist layer. See FIG. 3G. The second photoresist and the
antireflective coating are then removed by contacting the device
with a polymer remover. The second photoresist functions to
lift-off the gate metallization layer covering the surface of the
second photoresist leaving T-gate structure 28 on substrate 10, as
illustrated in FIG. 3H.
[0044] The following example is expected to further illustrate
various aspects of the present invention, but is not intended to
limit the scope of the invention in any aspect.
EXAMPLE
[0045] A layer of negative-acting photoresist (UVN.TM. 30, Shipley
Company) was spin coated on a gallium arsenide substrate to a
thickness of approximately 200 nm. The photoresist was patterned by
exposure to UV radiation at 248 nm through a mask using a
PAS5500/300 tool followed by development using 0.26 N TMAH for 10
seconds to provide approximately 150 nm openings for the base of
the T-gate. The photoresist layer was then cured by flood exposure
of 200 mJ (approximately 15.times.Eo) followed by heating at
115.degree. c for 60 seconds. A second layer of a negative acting
photoresist (UVN.TM. 30) was spin coated to a thickness of
approximately 700 nm on the cured first photoresist layer. The
second photoresist layer was then patterned in the same way as the
first photoresist layer (248 nm) to define a second opening for the
top (or cap) of the T-gate over the first opening. Next, a gate
metallization layer was deposited by vapor deposition. The gate
metallization layer was titanium-platinum-gold. Next, the first and
second photoresist layers were removed by contacting them with a
TMAH-containing polymer remover. The second photoresist layer acted
as a lift-off layer, removing the gate metallization layer that was
deposited over the surface of the second photoresist layer. The
resulting T-gate structure, shown in FIG. 4, had a stem CD of
approximately 200 nm, a cap CD of approximately 600 nm, and a total
height of approximately 500 nm.
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