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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to top surface imaging processes,
and more particularly to residue free patterning in to surface imaging
resists.
2. Discussion of the Related Art
Fabrication of very large scale integrated circuits (VLSI) and ultra large
scale integrated circuits (ULSI) requires that the resist materials,
lithographic processes, and exposure tools meet necessary performance
demands for high throughput manufacturing of sub-micron feature size
devices (i.e., devices with feature sizes less than 1.0 .mu.m). In the
instance of sub-micron lithography, top surface imaging is used to
increase the resolution capability of optical exposure systems. Several
TSI processes have been developed, most notably, Roland and Coopman's
Diffusion Enhanced Silylated Resist (DESIRE) negative tone process, as
discussed in F. Coopmans and B. Roland, Proc. of SPIE, 631 (1986), 34-39;
B. Roland, R. Lombaerts, C. Jakus, and F. Coopmans, SPIE, Proc. of SPIE,
771 (1987), 69-76; and Roland, Microelectronic Eng., 13 (1991), 11-18.
Another TSI process is the positive tone Silylated Acid Hardened Resist
(SAHR) developed by Thackeray et al. as discussed in J.W. Thackeray, J F.
Bohland, E.K. Pavelchek, G.W. Orsula, A.W. McCullough, S.K. Jones, S.M.
Bobbio, Proc. of SPIE, 1185 (1989), 2-11.
Top surface imaging in general uses reactive ion etching (RIE) to dry
develop patterns after exposure and silylation of a photoresist layer. A
dry development process for top surface imaging requires high selectivity
between exposed and unexposed regions of the photoresist to maintain
critical dimensions, high anisotropy to give vertical profiles in the
patterned photoresist and should also result in no residues after etching.
A major problem with known TSI resist processes is that RIE residue, in the
form of "grass", is produced. RIE grass is a problem in both positive and
negative working systems, since residue free images are desired. The grass
is produced as a result of silicon being incorporated into regions to be
etched, such that micromasks are formed in those regions, thus preventing
the desired regions from being completely etched during etching, resulting
in the "grass"-like residue.
It is known that the RIE grass problem can be eliminated by use of a
two-step etch process for pattern development. The first step is a
non-selective and aggressive process which removes silicon from the
regions to be etched, either chemically using fluorine plasma or RIE, or
physically using ion sputtering. The second step is a high selectivity
oxygen etch to develop a grass free pattern. This two-step process is
disadvantageous since it involves multiple process steps which introduce
undesired effects, such as, process instability.
R. Lombaerts, B. Roland, A. Selino, A.M. Goethals, and L. Van den hove,
Microelectron. Eng., 11 (1990), 543-547, discusses a method for optimizing
an etch process for the DESIRE top surface imaging process using a two
step etch process. The first step comprises a C.sub.2 F.sub.6 /O.sub.2
step to remove unwanted silicon from areas to be etched. The second step
comprises a pure oxygen low power etch for pattern transfer. An alternate
etch process included a two step oxygen only RIE process to obtain grass
free results. The first step comprises a high power etch with low
selectivity to remove unwanted silicon and further having a greater
sputtering component in the etching process. The second step comprises a
low power etch with high selectivity for grass free pattern transfer.
R.S. Hutton, R.L. Kostelak, O. Nalamasu, A. Kornblit, S. McNevin, and G.N.
Taylor, "Application of Plasmask Resist and the DESIRE Process to
Lithography at 248 nm", J. Vac. Sci. Technol. B, Microelectron. Process
Phenom. (USA), Vol. 8, No. 6, Nov. Dec. 1990, p. 1502-8, discloses a
method in developing a grass free RIE etch for the DESIRE process,
employes a two-step etch wherein the first step comprises Ar.sup.+ ion
sputtering to remove unwanted silicon prior to the second oxygen RIE step
to obtain grass free images. In this process, significant amounts of
resist are sputtered away prior to the second O.sub.2 RIE in order to
minimize grass formation. This however results in patterns with
considerable edge roughness.
The above discussed processes, however, are disadvantageous in the
manufacturing environment. In particular, process stability is a major
concern when using two-step fluorine containing etches. Residual fluorine
in the O.sub.2 RIE step can have a catastrophic effect on line-width
control during pattern development which results from deposition of
varying amounts of carbon fluorine C,F polymer on the walls of the RIE
chamber. This material then keeps on being released in the etching ambient
with attendant variation in the process, resulting in an unstable process.
That is, the chamber conditions change with each subsequent wafer.
Another disadvantage in using multi-chamber processes to improve stability
is that it slows manufacturing throughput and increases total process
time. Additionally, two step processes using sputtering to eliminate grass
formation rely on a high selectivity O.sub.2 RIE which has a low etch
rate. This low etch rate results in longer process times and a loss in
throughput.
Thus it would be desirable to provide a process for top surface imaging to
overcome the above identified problems and disadvantages. In particular,
it would be desirable to provide a top surface imaging pattern transfer
process for the fabrication of sub-micron feature size devices which is
simple, provides high etch rates, and provides residue free images.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for achieving a
vertical wall profile in a TSI resist with residue free patterns.
Another object of the present invention is to use O.sub.2 only as the
etchant gas.
Another object of the invention is to provide a method requiring a minimum
bias for TSI resist patterning.
Yet another object is to provide a method for pattern transfer using TSI
that results in high etch rates commensurate with "clustered" or high
throughput manufacturing.
According to the invention, a method for transferring a pattern through a
photoresist layer in the fabrication of submicron semiconductor device
structures comprises the steps of:
a) providing the photoresist on a substrate;
b) exposing the photoresist with a desired pattern to form exposed and
unexposed patterned areas in the top surface of the photoresist;
c) baking the photoresist to form cross-linked regions in the exposed
pattern areas of the photoresist;
d) silylating the photoresist to incorporate silicon into the unexposed
patterned areas of the photoresist, wherein some incorporation of silicon
occurs in the exposed patterned crosslinked areas of the photoresist; and
e) etching the patterned photoresist utilizing a high density, low
pressure, anisotropic O.sub.2 plasma alone to produce residue-free images
with vertical wall profiles in the photoresist.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other teachings of the present invention will become more
apparent upon a detailed description of the best mode for carrying out the
invention as rendered below. In the description to follow, reference will
be made to the accompanying drawings, in which:
FIG. 1 (a)-(d) show patterning of a photoresist layer in the fabrication of
a submicron semiconductor device according to a preferred embodiment of
the present invention.
FIG. 2 shows an SEM micrograph of a TSI resist patterned according to a
preferred embodiment of the present invention.
FIG. 3 shows an SEM micrograph of a TSI resist patterned according to the
preferred embodiment of the present invention.
FIG. 4 shows an SEM micrograph of a TSI resist patterned according to an
alternate embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
According to the present invention, a method for transferring a pattern
provided through a photoresist layer in the fabrication of submicron
semiconductor device structures comprises an O.sub.2 Reactive Ion Etching
(RIE) process which affords residue-free pattern transfer with TSI. The
method further provides vertical wall profile, minimum bias, and high etch
rates commensurate with "clustered" or high throughput manufacturing.
A typical TSI resist material for use with the method of the present
invention comprises (i) film-forming aromatic polymer resin having
functional groups which activate the resin to stabilize the resin to
electrophilic aromatic substitution, (ii) an acid catalyzable crosslinking
agent which forms a carbonium ion upon reaction with acid, and (iii) a
radiation degradable acid generator which is adapted to absorb imaging
radiation, such that, upon crosslinking, the resist composition is more
highly densified and is less permeable to the absorption of an
organometallic reagent in the crosslinked regions than it is in the
non-crosslinked regions. With respect to the resist, the functional groups
of the aromatic polymer resin can comprise phenolic hydroxy groups. The
aromatic polymer resin can likewise be selected from the group consisting
of poly(hydroxystyrene),
poly(hydroxystyrene-co-t-butyloxycarbonyl-oxystyrene-poly(hydroxy
-styrene-co-hydroxy-methylstyrene),
poly(hydroxystyrene-co-acetoxy-methylstyrene), and novolak resin. The
carbonium ion formed can comprise a benzyl carbonium ion. The acid
catalyzable crosslinking agent which forms the benzyl carbonium ion upon
reaction with acid can comprise a polyfunctional monomer. The
organometallic reagent can be in liquid or gaseous form. Such a resist is
disclosed in commonly assigned U.S. patent application Ser. No.
07/796,527, Clecak et al., filed Nov. 22, 1991 and incorporated herein by
reference.
We have discovered that high density plasma in the low pressure regime (0.5
to 10.0 mTorr) is important to achieve "grass-free" (i.e., no residue)
pattern transfer of images with TSI. This is accomplished with a single
gas, namely, oxygen in an etching chamber and requires no additional
gases, such as, CF.sub.4 or CHF.sub.3 as used in the previously described
two-step processes. We have further discovered that high gas flow rate
during etching is equally important. A gas exchange rate in the etching
chamber of 10-50 times per second (sccm) is needed. The high density
plasma used in the present invention is preferably obtained with the
assistance of a generator which is independent of the Radio Frequency (RF)
biased cathode on which the substrate to be etched is located.
In a preferred embodiment, a Radio Frequency Induction (RFI) reactive ion
etch (RIE) system (not shown) is used which comprises, more particularly,
a radio frequency induction multipole plasma processor having magnetic
confinement of a plasma. Such an RFI plasma processor is disclosed in
commonly assigned U.S. patent application Ser. No. 07/565,851, Coultas et
al., filed Aug. 10, 1990 and incorporated herein by reference. The RFI RIE
processor is an efficient processor and can be used for etching and/or
deposition processing.
Briefly, the RFI RIE processor includes a chamber for plasma processing
having an external wall for housing a work piece, the work piece having a
surface to be plasma processed. A source of an induction field is located
outside the chamber on its opposite side from the work piece. A radio
frequency (RF) induction field applied to the chamber generates the
plasma. The plasma is confined within the external wall in the chamber by
magnetic dipoles providing a surface magnetic field for confining the
plasma. The surface magnetic field is confined to the space adjacent to
the external wall. An RF generator provides an RF generated bias to the
work piece.
Additionally, the chamber of the RFI processor is lined with a material
inert to a plasma or non-contaminating to the work piece. The induction
source is in the form of a spiral or involute shaped induction coil and is
located on the exterior of the liner material on the opposite side of the
chamber from the work piece. Distribution of gas to the chamber is uniform
because a manifold (located about the periphery of the chamber) and an
orifice (formed by the surface of the chamber and the manifold) admits gas
from the manifold into the chamber at a uniform pressure about the
periphery of the cover of the chamber.
The work piece or substrate to be processed in the RFI processor is
preferably held in place by an electrostatic chuck equipped with helium
backside cooling. Such an electrostatic chuck is disclosed in commonly
assigned U.S. patent application Ser. No. 07/694,698, Logan et al., filed
May 2, 1991 and incorporated herein by reference. In short, the
electrostatic chuck includes a means for cooling the backside of the wafer
using a cooling gas as the front side of the wafer is being processed.
Referring ngw to FIGS. 1(a)-1(d), in the preferred embodiment, the method
of transferring a pattern 20 through a photoresist layer 22 in the
fabrication of a submicron semiconductor device structure 10 comprises the
following steps. A silicon wafer or substrate 24 is first O.sub.2 plasma
cleaned and surface treated with 0.1% solution of gamma-aminopropyl
triethoxysilane (A1100) in aqueous ethanol by spin application. The wafer
is then baked at 90.degree. C. for 1 minute.
A negative working resist 22, comprising the TSI resist material as
previously describe herein, is spin applied to the top surface of
substrate 24 at 3000 rpm for 30 seconds to provide a 1-2 .mu.m thick film
after baking the wafer at 90.degree. C. for 2 minutes. The resist 22
carries a crosslinker, a photo acid generator, a light-absorber, and a
host resin for film forming as described in commonly assigned U.S. patent
application Ser. No. 07/796,527.
The resist 22 is thereafter imagewise exposed to a Deep Ultra Violet (DUV)
source at 5-10 mJ/cm.sup.2 through mask 20, the mask 20 having a desired
pattern thereon. Imagewise exposing the photoresist 22 with mask 20 forms
exposed and unexposed regions 26 and 28, respectively, in a top surface 30
of resist 22 thereby forming an imagewise patterned resist 22 (FIG. 1(a)).
Imagewise patterned resist 22 is thereafter heated at
100.degree.-130.degree. C. for 2-5 minutes to produce cross-linked regions
in the exposed patterned areas 26 in top surface 30 of the photoresist 22
(FIG. 1(b)).
After the crosslinked regionsare formed, the device structure 10, and in
particular, resist 22 is silylated in a silylation tool. Resist 22 can be
silylated with a silylating gas such as dimethylanimo-trimethyl silane.
Silylation is well known in the art and thus only briefly discussed
herein. Silylaton is a process wherein silicon is selectively incorporated
into the top surface of either the exposed or unexposed regions of the
resist, depending on the chemistry of the resist. This effectively creates
an etch barrier which, upon oxidation, is chemically resistant to an
O.sub.2 plasma. In the instant invention, silylation of the photoresist 22
predominantly incorporates silicon into the unexposed patterned areas 32
via a chemical photoresist to from silylated areas 32 via a chemical
reacton. The silylated areas 32 penetrate approximately 1000-3000 .ANG.
below the surface 30 of resist layer 22 and may further undergo swelling
above the surface 30. Since some silicon invariably is incorporated in the
crosslinked regions as well, this tends to result in micromasking in the
areas. That is, some incorporation of silicon occurs on the top surface 34
of the exposed patterned crosslinked areas 26 of the photoresist 22 (FIG.
1(c)), forming micormasks. Silylation selectivlity is the differential in
permeability of the silylating agent into the unexposed versus exposed
areas of the resist. Selectivity, or silylation contrast, can be a
critical element in the pattern transferring process of prior processes,
however, the selectivity is less critical in the process of the present
invention.
The device structure is baked again at 100.degree.-130.degree. C. for 2
minutes and transferred to the RFI tool for RIE with O.sub.2 gas alone.
Etching of the patterned photoresist 22 utilizing a high density, low
pressure, anisotropic O.sub.2 plasma alone to produce residue-free images
with vertical wall profiles 36 in the photoresist 22 is then performed
(FIG. 1(d)). That is, the wafer 24 with patterned resist 22 is placed in
the previously described RFI processor, the wafer structure being held in
place by the electrostatic chuck. The temperature of the wafer structure
can be controlled during the processing steps via the previously described
electrostatic chuck having helium backside cooling. Subsequent etching of
the patterned photoresist 22 is then performed. An etchant gas of O.sub.2
alone is used for etching the patterned photoresist 22 in combination with
the following set of etch parameters:
Inductive Power (13.56 MHz): 300-600 Watts;
RF Bias (40 MHz): 100-500 Watts;
O.sub.2 Pressure: 0.5-10.0 mTorr; and
Gas Flow: 10-50 sccm .
According to the invention, key conditions of the high density O.sub.2
plasma comprise the plasma being generated under very low pressure and
high vacuum. Plasma density is a measure of the concentration of ions per
unit volume. Under low pressure, high vacuum conditions, the mean free
path (MFP) between ions of opposite polarity in the high density plasma
large in comparison to the MFP between ions in a high density plasma
generated under high pressure, low vacuum conditions. In the case of high
pressure, low vacuum conditions, MFP between ions in the high density
plasma is small and particles or ions of opposite polarity have a greater
probability of recombining. The high density plasma generated under high
pressure, low vacuum conditions is therefore unstable and difficult to
maintain because, with the small MFP, the plasma neutralizes itself
rapidly.
In further discussion of the high density O.sub.2 plasma, it comprises an
O.sub.2 gas which is broken down into positive and negative species of
ions and radicals. Si bonds are broken by collisions with the species in
which an initial collision produces a volatile species of partially
oxidized organo silicon and subsequent collisions produce non-volatile
SiO.sub.2. Under the above described etch conditions according to the
invention, the high vacuum and low pressure conditions reduce the
possibility for the occurrence of the multiple collisions, and thus,
reduces or eliminates the possibility for the formation of "grass" in
undesired areas of the TSI resist. Thus, the desired formation of residue
free images with TSI resists is accomplished by a single-step oxygen
reactive ion etching process.
Recalling that the silylation of the imaged resist produces some unwanted
Si on the top surface of exposed cross-linked regions 26, the effect of
the high vacuum, low pressure, high density O.sub.2 plasma of the present
invention with TSI resists is to create a good (Si/no Si) differential
where it doesn't exist in the first place. That is, the effects of the
unwanted Si going into undesired areas, such as described above, are
minimized due to the high ion plasma density, low pressure, high vacuum
O.sub.2 plasma conditions.
In a preferred embodiment, the RFI processor is controlled for producing a
high density plasma in the reactor under the following preferred device
etch parameters:
Inductive Power (13.56 MHz): 500 Watts ;
RF Bias (40 MHz): 300 Watts ;
O.sub.2 Pressure: 2 mTorr ; and
Gas Flow: 20 sccm .
Using the preferred RFI etch parameters, etching of the photoresist layer
22 was observed to be essentially complete in 1-2 minutes depending on
resist thickness and as detected by an end point. The endpoint was
detected while monitoring changes in the intensity of photoemission of
excited species. The plasma was kept on for an additional 5-10 seconds
beyond detection of the endpoint. This produces an etch rate of between
0.5 and 2.5 .mu.m per minute, wherein no detectable damage to the resist
mask 22 nor any resist image deformation was observed. The obtainable etch
rate of 0.5 to 2.5 .mu.m/min is commensurate in scope with "clustered"
manufacturing of submicron semiconductor device, enabling high throughput.
Furthermore, SEM examination of the etched patterns shows vertical wall
profiles with no evidence of any residue. For example, FIG. 2 and FIG. 3
show SEM micrographs of an etched pattern with the resist layer 22 still
in place, the pattern having 0.5 .mu.m lines and 1.0 .mu.m spacing. Thus,
the method of the present invention further provides residue-free images
with vertical wall profiles in the fabrication of submicron feature
devices.
In the above example, a 5 inch (127 mm) wafer was used. For scale-up to 8
inch (203 mm) wafer size, appropriate size pumping system should be used
to achieve the critical parameters given above.
In an alternate embodiment of the present invention, the method is similar
to the preferred embodiment except that in the alternate embodiment, the
method is carried out using an Electron Resonance Cyclotron (ECR) reactor,
such as an ECR reactor available from Sumitomo Metals of Japan (marketed
by Lam Corporation, Fremont, California, USA). ECR reactors are known in
the art and are therefore only briefly discussed herein. With the ECR
reactor, a microwave generator is used to produce a plasma which is
confined by an axial magnetic field. The magnetic field strength is chosen
to create a condition of electron cyclotron resonance (ECR) by the
formula:
w=eB/m
where w is the microwave frequency (in radians), e is the electron charge,
m is the electron mass, and B is the magnetic field strength. For typical
microwave frequencies of 2.45 GHz, a magnetic field strength of just under
1000 gauss is sufficient to achieve a resonant zone in the plasma. The
resonance creates a high density plasma due to electrons being confined to
a circular orbit defined by the crossed electric and magnetic fields. The
microwave generator of the ECR reactor is used to produce the high density
low pressure O.sub.2 plasma used in accordance with the alternate
embodiment of the present invention. The other etch parameters for
creating the desired plasma using the ECR reactor include: microwave
power: 1300 Watts; RF Bias: 50 Watts; O.sub.2 Pressure: 0.6 mTorr; and
O.sub.2 Flow Rate 40 sccm. An etch rate of up to 1.0 .mu.m per minute is
obtainable. FIG. 4 provides an SEM of a TSI resist etched according to the
alternate embodiment of the present invention.
In yet another alternate embodiment, the method of the present invention is
similar to the preferred embodiment, the difference being that the high
density, low pressure, O.sub.2 plasma is produced using a helicon based
plasma reactor or source. A helicon based plasma reactor based on the
research of Robert Boswell at the Australian National University, Canberra
has been licensed and is manufactured by Lucas-Signatone. This reactor has
been used successfully for etching of top surface photoresists according
to the present invention. Helicon sources are known in the art and
therefor only briefly discussed herein. Helicon sources rely on launching
a slow wave structure at radio frequencies. The helicon is powered by RF
driven coils mounted on opposite sides of a vacuum chamber. By proper
choice of polarity and aspect ratio, the coils launch slow moving plasma
waves. The slow waves are known as helicon waves and the resulting plasma
can be high in density, particularly if additional multipolar magnetic
field confinement is added. The helicon plasma has a very high absorption
efficiency. Typical etch parameters used in such a helicon reactor are: 2
mTorr pressure, 50 sccm O.sub.2, 2500 Watts helicon RF, 200 Watts
substrate RF bias, -50.degree. C. substrate temperature (cryogenic).
Typical etch rates under these conditions are up to 1 .mu.m per minute.
There is thus provided a method for patterning a photoresist layer in the
fabrication of submicron semiconductor devices using top surface imaging
which provides high etch rates for etching the photoresist with straight
wall profiles, and further provides a highly efficient process. The method
further provides residue free images.
While the invention has been particularly shown and described with respect
to the preferred and alternate embodiments thereof, it will be understood
by those skilled in the art that the foregoing and other changes in form
and detail may be made therein without departing from the spirit and scope
of the invention.
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