A method of improving the etch resistance of a patterned imageable resist prior to patterning an underlying substrate layer is provided. Specifically, the method employed by the present invention comprises applying a layer of an imageable resist to a substrate layer; patterning the layer of imageable resist by removing selective areas thereof; and treating the patterned imageable resist with an atmosphere comprising molecules of a hardening agent so as to obtain a hardened resist surface which etches at a slower rate than that of the untreated resist.

FIELD OF THE INVENTION
 The present invention provides a method of decreasing the etch rate of a
 patterned resist on a substrate by treating the patterned resist with an
 atmosphere comprising molecules of a hardening agent. By incorporating the
 molecules of the hardening agent into the resist after exposure and
 development, but prior to substrate etching, the resist structure or
 pattern remains unchanged. Extensive optimization of resolution and resist
 contrast is thereby decoupled from the etching process leading to a more
 efficient development of new materials for lithography.
 BACKGROUND OF THE INVENTION
 In the field of semiconductor manufacturing, lithography is generally
 employed to provide a pattern to a substrate layer. Specifically, an
 imageable resist is applied to the substrate needing patterning, the
 imageable resist is then patterned through exposure and development, and
 the pattern is transferred to the underlying substrate by etching with a
 plasma of various reactive ion species, e.g. CF.sub.4.
 Since the etch rate of most imageable resist films is greater than that of
 the underlying substrate layer, the imageable resist etches laterally at a
 much faster rate than the underlying substrate layer. This lateral etching
 of the imageable resist severely distorts the pattern formed in the
 substrate layer and prevents the formation of a desired, small feature
 sized pattern (100 nm or less) on the substrate layer.
 Numerous attempts to overcome this problem have been developed and are now
 in use. One solution to this problem is to change the imageable resist's
 resistance to oxygen plasma by using a silylation technique. In accordance
 with prior art silylation techniques, the imageable resist is silylated
 before the film is patterned. Such silylating techniques are disclosed,
 for example, in M. Bottcher, et al. "Surface Imaging by Silylation for Low
 Voltage Electron-beam Lithography", J. Vac. Sci. Technol. B12, 3473
 (1994); C. Pierrat, et al. "PRIME Process for Deep UV and E-beam
 Lithography", Microelectronic Engineering, Vol. 11, 507 (1990); and M.
 Irmscher, et al. "Comparative Evaluation of Chemically Amplified Resists
 for Electron-beam Top Surface Imaging Use", J. Vac. Sci. Technol. B12,
 3925 (1994).
 Despite being successful in altering the etch rate of the imageable resist,
 this prior art method requires that changes be made in the resist
 chemistry and thus the exposure and development process. Such changes are
 not desirable since they introduce additional materials not typically
 employed in lithography.
 Alternative solutions to this problem are even more complex involving, for
 example, resist films made from multiple layers of various polymers. The
 use of multi-polymer film layers is disclosed, for example, in M. A.
 McCord, et al., "Electron Beam Lithography", Chapter 2 of Handbook of
 Microlithography, Micromachining and Microfabrication, Vol. 1:
 Microlithography, P. Rai-Choudhury, ed., SPIE Optical Engineering Press,
 Bellingham, Wash. (1997). Alternatively, resist polymers can be modified
 to include alicyclic compounds. These compounds increase etch resistance,
 but they also change the exposure and development properties of the film
 (R. D. Allen, et al., "Deep-UV Resist Technology", Chapter 4, ibid.).
 There is thus a need for developing a new and improved lithography method
 which can change the etch resistance of the imageable resist without
 altering any of the resist chemistry. There is also a need for developing
 a method which could shift the photolithography industry away from complex
 and expensive multilayer techniques, and perhaps streamline
 microelectronic fabrication research by decoupling etching properties from
 exposure properties.
 SUMMARY OF THE INVENTION
 One object of the present invention is to provide a method of changing the
 etch resistance of an imageable resist so that the resist etches slower
 than any untreated resist layer.
 A further object of the present invention is to provide a method of
 changing the etch rate of an imageable resist in such a fashion that the
 chemistry of the resist is unaltered and thus no changes in the
 exposure/development steps are needed.
 A still further object of the present invention is to provide a method of
 improving the etch resistance of the imageable resist so that a reliable
 pattern having a substantially small feature size can be transferred to
 the underlying substrate layer.
 These and other objects and advantages can be achieved in the present
 invention by treating a patterned layer of an imageable resist with an
 atmosphere containing a hardening agent, e.g. metalloid-containing
 compound, after the patterning step, but prior to etching. Specifically,
 the method of the present invention comprises the steps of:
 (a) applying a layer of an imageable resist to a substrate layer;
 (b) patterning the layer of imageable resist by removing selective areas
 thereof; and
 (c) treating the patterned imageable resist with an atmosphere comprising
 molecules of a hardening agent so as to obtain a hardened resist surface
 that etches at a slower rate than the untreated resist.
 After conducting step (c), the pattern is formed in the substrate layer by
 conventional etching such as reactive ion etching (RIE) and, thereafter
 the resist can be removed using conventional stripping techniques.

DETAILED DESCRIPTION OF THE INVENTION
 The present invention, which provides a method of improving the etch
 resistance of a patterned imageable resist, will now be described in
 greater detail by referring to the drawings that accompany the present
 application. It is noted that in the drawings like and/or corresponding
 elements are referred to by like reference numerals.
 As stated above, the present invention provides a method for improving the
 etch resistance of a patterned imageable resist whereby the etch
 resistance is improved by treating the imageable resist with a hardening
 agent after the imageable resist has been patterned, but prior to etching
 the underlying substrate layer.
 Referring to FIGS. 1(a)-(f) there is shown one embodiment of the method of
 the present invention wherein a positive tone imageable resist is
 employed. Although illustration is shown for a positive tone imageable
 resist, the present invention works well with negative tone imageable
 resists. Where a positive tone resist is employed, the exposure of the
 imageable resist renders the exposed areas more soluble than the unexposed
 areas. The exposed areas are then removed in the development step and the
 pattern is formed. Where a negative tone resist is employed, the unexposed
 areas of the resist are removed in the development step.
 FIG. 1(a) shows a structure which comprises a substrate layer 10 and a
 layer of an imageable resist 12 formed thereon. Examples of suitable
 substrates that can be employed in the present invention include, but are
 not limited to: semiconductor chips and wafers, circuit boards,
 interconnect structures, metals, inorganic dielectric materials such as
 SiO.sub.2 and Si.sub.3 N.sub.4 and other like substrates. Preferably, the
 substrate is composed of, or contains, a semiconducting material such as
 Si, Ge, GeSi, GaAs, InAs, INP and other III/V compounds. The
 semiconducting material may be doped, undoped or contain regions of both
 therein. The substrate may contain active device regions or wiring regions
 such as are typically found in integrated circuits. For clarity, such
 device or wiring regions are not shown in the drawings, but nevertheless,
 may be present in substrates used in the method of the present invention.
 It is noted that the substrate layer may be composed of two or more of the
 above mentioned materials. For example, substrate layer 10 can be
 comprised of a Si wafer having SiO.sub.2 or a metal formed therein.
 The imageable resist is formed on top of substrate layer 10 using
 conventional deposition processes or growing techniques that are well
 known to those skilled in the art. Examples of conventional deposition
 processes that can be used in the present invention include, but are not
 limited to: chemical vapor deposition (CVD), plasma vapor deposition,
 sputtering, spin coating and other like deposition techniques.
 The thickness of the imageable resist layer is determined by the etch
 selectivity and the thickness of the underlying substrate layer. The
 resist layer thickness is typically of from about 1000 to about 10,000
 .ANG., with preferred thicknesses being from about 2000 to about 9500
 .ANG..
 The imageable resist employed in the present invention is any resist that
 is sensitive to radiation. Such resists are well known to those skilled in
 the art; therefore an exhaustive list is not believed to be needed herein.
 Preferably, the imageable resist is a resist composition that includes at
 least a base polymer resin which undergoes chemical changes when subjected
 to radiation. These chemical changes alter the solubility of the exposed
 regions such that the desired pattern can be obtained after development
 with appropriate solvents. In the case of positive tone resists, the
 exposed regions are removed by the developer solvent, whereas for negative
 tone resists, the unexposed areas are removed.
 Other components that may be present in the resist include: a solvent, a
 photoacid or photobase generator, a crosslinking agent, an acid or a base,
 a photosensitizer or a surfactant. The solvents, photoacid/photobase
 generators, acid or base additives, photosentizers and surfactants
 employed in the imageable resist are selected from conventional compounds
 that are well known to those skilled in the art; therefore a detailed
 description of those components is not given herein. It should be noted
 that the solvent may be present in the resist composition on application
 to the substrate layer, but it is typically removed from the applied
 resist before radiation exposure.
 The imageable resist employed in the present method is preferably a
 chemically amplified resist. Other resists are also contemplated herein.
 The term "chemically amplified resist" is used herein to denote a resist
 wherein radiation exposure of the resist generates a catalyst that
 catalyzes a cascade of chemical transformations (with or without
 subsequent heating) to change the solubility of the exposed regions.
 Preferably, the chemically amplified resist employed in the present
 invention comprises a polymer resin which contains acid-sensitive or
 base-sensitive protecting groups (i.e. positive tone resists) or a
 crosslinking functionality (negative tone resists). Typical chemical
 amplified resists are acid or base catalyzed wherein exposure of the
 resist to radiation generates a strong acid or base that catalyzes the
 deprotection (or crosslinking in the case of negative tone resists) of the
 base polymer resin. Subsequent development with an appropriate solvent
 generates the desired patterned resist.
 Positive tone resists useful in the present invention preferably contain a
 base polymer resin derived from at least one of the following: novolak,
 hydroxystyrene, acrylates, methacrylates, acrylamides, methacrylamides,
 itaconates, itaconic half esters and cycloolefins. The base polymer resin
 may be a homopolymer, a copolymer or a higher polymeric entity. When used
 as copolymers or higher polymers, monomers such as styrene,
 hydroxystyrene, acrylic acid, methacrylic acid, itaconic acid and an
 anhydride such as maleic anhydride and itaconic anhydride may be used. The
 positive tone resist can be fully or partially protected with an
 acid-sensitive or base-sensitive protecting group, such as those known in
 the art.
 Preferred negative tone resists contain a base polymer resin that is
 capable of being crosslinked. Examples of crosslinkable polymers include:
 methyl-acetoxy calixarene and hydroxystyrene; whereas examples of
 crosslinking agents include: melamines and urils.
 After forming the imageable resist on the surface of the substrate layer,
 the imageable resist is patterned. Specifically, the imageable resist is
 patterned using a two step process which includes exposure and
 development. A pre-baking step may be optionally employed as well as a
 post-baking step.
 When the optional pre-baking step is employed in the present invention, it
 occurs prior to exposure. This step is desirable if the imageable resist
 has a solvent content above 5%. When a pre-bake step is employed, the
 structure shown in FIG. 1(a) is baked, i.e. heated, to a temperature of
 from about 60.degree. C. to about 250.degree. C. for a time period of from
 about 30 to about 300 seconds. More preferably, the optional pre-bake step
 is carried out at a temperature of from about 100.degree. to about
 200.degree. C. for a time period of from about 60 to about 290 seconds.
 The unbaked or pre-baked imageable resist layer is then pattern-wise
 exposed to radiation using a masked or maskless lithographic process. The
 exposure process may be carried out using mid-UV, deep-UV (243, 193, 157
 and 129 nm), extreme-UV, e-beam, x-ray or ion beam radiation or maskless
 scanning probe lithography.
 After exposure, the imageable resist may optionally be post-baked at a
 temperature of from about 60.degree. to about 250.degree. C. for a time
 period of from about 30 to about 300 seconds. More preferably, the
 optional post-bake step is carried out at a temperature of from about
 100.degree. to about 140.degree. C. for a time period of from about 60 to
 about 120 seconds. The above post-bake conditions are preferably
 sufficient to crosslink the imageable resist in the case of negative
 resist compositions. It is noted that the post-bake step may occur before,
 during or after exposure of the resist to the hardening agent.
 A structure such as shown in FIG. 1(b) is obtained after exposure. This
 structure contains resist areas 12e that are exposed to radiation and
 resist areas 12 that are not exposed to radiation. It is noted that in the
 development step, the pattern may be formed by removing either 12e or 12,
 wherein e denotes areas exposed to radiation.
 After exposure and any optional post-bake step, the pattern shown in FIG.
 1(b) provided by areas 12e or 12 are developed using a suitable solvent.
 In the positive resist embodiment shown in the figures, the exposed areas
 12e are developed, i.e. removed, leaving unexposed areas of imageable
 resist 12 remaining on the surface of the substrate. Preferred solvents
 are aqueous alkaline developers such as aqueous tetramethylammonium
 hydroxide solutions. The resulting patterned resist structure is shown in
 FIG. 1(c). When a negative tone resist is employed, the unexposed areas 12
 of the imageable resist would be removed to obtain the desired patterned
 resist structure.
 In accordance with the method of the present invention, the patterned
 resist structure is then treated with molecules of a hardening agent which
 are capable of improving the etch resistance of the resist. The patterned
 resist structure after exposure, i.e. treatment, with a hardening agent is
 shown in FIG. 1(d), wherein the treated resist areas are now designated as
 12t.
 The term "hardening agent" is used in the present invention to denote a
 material which is capable of making portions of the imageable resist more
 etch resistant. That is, the hardening agent employed in the present
 invention is one that hardens the surface of the imageable resist so that
 the surface thereof etches at a slower rate than that of the untreated
 resist.
 Suitable hardening agents employed in the present invention are ones which
 contain at least a metalloid or metallic element selected from Groups IIA,
 IVB, VIII, IB, IIIA, or IVA of the Periodic Table of Elements (CAS
 version). Suitable metalloids or metallic elements include, but are not
 limited to: Mg, Ca, Si, Ge, Sn, Ti, Zr, Fe, Co, Ni, Ru, Rh, Pd, Ir, Pt,
 Cu, Ag and Au. Alloys of the same are also contemplated herein. The
 metalloid or metallic element may be in elemental form or it can be
 present in a compound or complex.
 A highly preferred metalloid for use in the present invention is Si. When
 Si is the molecule to be incorporated into the resist, the hardening agent
 is a silylating agent such as, but not limited to: silyl amine,
 hexamethyldisilazane (HMDS), 3-aminopropyl-1-trimethoxy silane,
 3-aminopropyl-1-trimethyl silane, trimethylsilyldiethylamine,
 N,O--bis(trimethylsilyl)sulfamate, isopropyldimethylsilyl ether,
 t-butyldimethylsilyl chloride, (triphenylmethyl)dimethyl silyl bromide and
 methyldiisopropylsilyl chloride. Of these silylating agents, HMDS is
 highly preferred in the present invention.
 Molecules of the hardening agent are incorporated into the remaining
 portions of the patterned resist using a vapor of the hardening agent
 provided by pyrolysis. Specifically, when Si is to be incorporated into
 the resist, one of the above mentioned silylating agent is selected and
 incorporation occurs using conventional silylation techniques well known
 to those skilled in the art. For example, silylation may occur by
 providing a vapor of the silylating by way of pyrolyzing the hardening
 agent. Pyrolysis typically occurs at a temperature of from about
 100.degree. to about 300.degree. C. The amount of hardening agent
 incorporated into the imageable resist is preferably greater than about 1
 wt. %, more preferably, greater than 10 wt. %. Preferably, from about 1 to
 about 25 wt. % of said hardening agent is incorporated into the resist.
 After the hardening agent has been incorporated into the remaining portions
 of the resist, the exposed portions of the substrate layer are then
 patterned using conventional dry etching techniques which generally
 utilize a plasma containing a reactive ion species such as oxygen,
 fluorine, chlorine and bromine. Exemplary dry etching techniques that can
 be used to etch the substrate layer include: reactive ion etching (RIE),
 plasma etching and ion-beam etching. A highly preferred means for etching
 the exposed portions of the substrate layer is by way of RIE using
 CF.sub.4 gas as the reactive ion gas.
 The etching step of the present invention provides a pattern to the
 underlying substrate layer. Such a patterned structure is shown in FIG.
 1(e) of the present invention.
 After patterning the substrate layer, the remaining imageable resist layer
 that is treated with the hardening agent (12t) may be removed using
 standard stripping techniques well known to those skilled in the art. For
 example, oxygen RIE (reactive ion etching) can be employed in the present
 invention to remove the remaining treated areas of the imageable resist.
 The final structure is shown in FIG. 1(f).
 It is noted that the shape of the actual pattern may vary depending on the
 type of device being manufactured. Exemplary patterns include: trenches,
 contact holes, vias, shallow trench isolation regions, gate stacks and
 other like patterns. It is noted that since the present invention improves
 the etch resistance of the resist without changing the resist chemistry,
 one can use the same for providing small feature sized patterns to the
 substrate layer. Moreover, the method of the present invention allows for
 more accurate pattern transfer than heretofore possible with prior art
 lithography methods.
 The following example is given to illustrate the scope and spirit of the
 present invention. Because this example is given for illustrative purposes
 only, the invention embodied therein should not be limited thereto.
 EXAMPLE 1
 In this example, methyl-acetoxy calixarene (MAC) was employed as the
 imageable resist and HMDS was used as the hardening agent. Specifically, a
 thin (0.1 .mu.m) of MAC was patterned by exposure to electrons and
 subsequently developed in xylene. The patterned wafer was then baked at
 200.degree. C. for 40 minutes in an atmosphere of HMDS. The HMDS-treated
 film had an etch rate in CF.sub.4 plasma which was 40% lower than
 untreated MAC films. More importantly, the pattern remained unaltered by
 the hardening process of the present invention. TEM (transmission electron
 microscopy) photographs show patterns in MAC which are unchanged even for
 features on the 20 nm scale. Moreover, films and patterns in MAC resist do
 not swell or shrink when subjected to the hardening process of the present
 invention.
 While the present invention has been particularly shown and described with
 respect to preferred embodiments thereof, it will be understood by those
 skilled in the art that the foregoing and other changes in form and
 details may be made therein without departing from the spirit and scope of
 the invention. It is therefore intended that the present invention not be
 limited to the exact forms described and illustrated, but fall within the
 scope of the appended claims.