Graft polymers and use thereof

Solutions of graft polymers of a polymer having reactive hydrogen groups and grafted through reactive hydrogen groups an alkoxy metallic compound wherein the metal is titanium, zirconium and/or hafnium are useful as photoresist materials which are resistant to plasma. Also, compounds and graft polymers wherein the alkoxy metallic compound also contains silicon, tin or germanium are provided.

FIELD OF THE INVENTION
 The present invention relates to certain solutions of graft polymers which
 can be used as photoresists and which exhibit enhanced resistance to
 plasma and especially to Cl/O plasma used in reactive ion etching. The
 present invention is concerned with the compositions as well as their use
 in lithography. For instance, the materials of the present invention are
 suitable for use in device fabrication on all optical, e-beam, x-ray and
 ion-beam lithography tools.
 The present invention is also concerned with certain graft polymers that
 exhibit increased hydrophobicity or increased hydrophilicity depending
 upon the characteristics of the moiety grafted onto the polymer. Moreover,
 graft polymers of the present invention can be grafted so as to create a
 permanent graft of a dye thereon.
 BACKGROUND OF INVENTION
 In the manufacture of patterned devices and especially microelectric
 devices, the processes of etching different layers which constitute the
 finished product are among the most crucial steps involved. One method
 widely employed in the etching process is to overlay the surface to be
 etched with a suitable mask.
 The mask is typically created by imagewise forming a pattern of photoresist
 material over those areas of the substrate to be shielded from the
 etching. The photoresist is normally formed of a polymeric organic
 material. The pattern is formed by imagewise exposing the photoresist
 material to irradiation by photolithographic techniques. The irradiation
 employed is usually x-ray, UV radiation, or electron beam radiation.
 Photosensitive materials and/or compositions are either positive-acting
 (i.e. photosolubilizable) or negative-acting (i.e. photoinsolubilizable or
 photocrosslinkable). Positive-working (photo)sensitive compositions are
 rendered soluble (or developable) by actinic radiation (deep-near UV,
 x-ray or electron-beam) and can be removed using selective developing
 solutions leaving unexposed areas intact. Negative-working (photosensitive
 compositions are those which become insoluble upon exposure to actinic
 radiation. Selected solutions can dissolve and remove the unexposed areas
 of the composition while leaving the exposed portions intact. Development
 of such exposed materials yields negative tone images.
 Concerning positive working resists, it is well known in the art that the
 photochemical formation of carboxylic acids, be it by amplified means such
 as a catalytic acidic-decomposition of a tertiary butyl ester, or as in
 the case of the photochemical decomposition of 1,2-naphthoquinone
 diazides, can be employed to produce high resolution and high efficiency
 resists. This type of reaction is being relied upon extensively in the
 production of positive working resists.
 The manufacture of integrated circuits and other patterned devices relies
 primarily on resist materials that enable the formation of high resolution
 patterns. In the search for materials and methods for formation of
 patterns below 0.25 microns, it is recognized that such patterns require
 exposure sources based on UV radiation below 248 nm, or on x-ray, or on
 e-beams. Likewise, it is essential to employ resist materials suitable for
 use with short wavelengths sources. In the case of UV radiation, it might
 be convenient to use excimer laser sources that produce radiation at 193
 nm.
 U.S. patent application Ser. No. 08/700,348 discloses certain polymer
 compositions that when exposed to actinic light such as UV radiation below
 240 nm, or soft rays, x-ray or e-beam, undergo a photochemical reaction
 that leads to the formation of pendant carboxylic acid groups, which are
 base soluble. This photochemical reaction is very efficient and can be
 used for high resolution positive resists.
 In particular, the resists are non-amplified polymers having pendant
 recurring groups selected from the group consisting of
 --COO--CH.sub.2 --CH(OH)--(CH.sub.2).sub.x --H, wherein x is 0-20;
 --COO--CH.sub.2 --CH(OH)--(CH.sub.2).sub.y --HE--(CH.sub.2).sub.z --H; and
 mixtures thereof,
 wherein HE is O or S, and each y and z individually is 1-18, and mixtures
 thereof.
 U.S. patent application Ser. No. 08/717,644 discloses using certain
 copolymers for minimizing possible water absorption of the non-irradiated
 regions of the resist film. These copolymers are from monomers consisting
 essentially of:
 1) CH.sub.2 CHCOOCH.sub.2 CHOH(CH.sub.2).sub.n H and/or CH.sub.2
 C(CH.sub.3) CHOOCH.sub.2 CHOH(CH.sub.2).sub.n H, wherein n=0 to 10; and
 2) CH.sub.2 CHCOO(CH.sub.2).sub.n H and or
 CH.sub.2 C(CH.sub.3) COO(CH.sub.2).sub.n H, wherein n=1 to 10. The amount
 of 1) is about 75 to about 95, preferably about 80 to about 90, and most
 preferably about 84 to about 87 wt. % and the amount of 2) is about 5 to
 about 25, preferably about 10 to about 20 and most preferably about 13 to
 about 16 wt. %. These weight percents are based upon the total weight of
 1) and 2).
 The entire disclosures of U.S. patent applications Ser. No. 08/700,348 and
 08/717,644 are incorporated herein by reference.
 After the photoresist is developed forming the desired mask, the substrate
 and mask can be immersed in a chemical solution which attacks the
 substrate to be etched while leaving the mask intact. These wet chemical
 processes suffer from the difficulty of achieving well-defined edges on
 the etched surfaces. This is due to the chemicals undercutting the mask
 and the formation of an isotropic image. In other words, conventional
 chemical wet processes do not provide the selectivity of direction
 (anisotropy) considered necessary to achieve optimum dimensional
 consistent with current processing requirements.
 Moreover, such wet etching processes are undesirable because of the
 environmental and safety concerns associated therewith.
 Accordingly, various so-called "dry processes" have been suggested to
 improve the process from an environmental viewpoint, as well as to reduce
 the relative cost of the etching. Furthermore, these "dry processes" have
 the potential advantage of greater process control and higher aspect ratio
 images. Also, when fabricating patterns having feature sizes below 350 nm,
 dry etching processes are necessary.
 Such "dry processes" generally involve passing a gas through a container
 and creating a plasma in this gas. The species in this gas are then used
 to etch a substrate placed in the chamber or container. Typical examples
 of such "dry processes" are plasma etching, sputter etching, and reactive
 ion etching.
 Reactive ion etching provides well-defined, vertically etched sidewalls.
 However, a crucial challenge posed by the reactive ion etching relates to
 providing photoresist compositions that are sensitive to the radiation
 employed in its imaging procedure but resistant to the reactive ion
 etching. A particularly harsh environment for the resist material involves
 those RIE processes using Cl/O plasma. Most resist materials do not
 survive long enough in this environment to provide proper protection. The
 manufacture of optical masks requires a chromium etch step which can be
 done preferentially by RIE etching. This step requires Cl/O plasma.
 However, although the above discussed non-chemically amplified resists
 based upon the polymers disclosed in U.S. Ser. No. 08/700,348 and
 08/717,644 perform quite well lithographically, their resistance to
 withstand plasma environments and especially Cl/O plasma is not
 satisfactory.
 It has also been found that the available commercial e-beam resists offer
 little protection in this plasma environment.
 In co-pending application U.S. Ser. No. (YO998-421) titanates, zirconium
 and/or hafnium are added to polymers having hydroxy groups to import
 improved resistance to reactive ion etching. The addition of these
 compounds also enhances the development with TMAH, an industry standard,
 which improves film Tg and hardness.
 Furthermore, organic tetra alkyl titanates and titanate chelates such as
 the series of DuPont products available under the tradename TYZOR.RTM.
 promote crosslinking of polymers that contain active hydrogen groups such
 as hydroxy, amino, amido, carboxyl and thio groups. The reaction produces
 resins with improved hardness, solvent resistance and new electrical
 properties.
 The mechanism of the crosslinking reaction proceeds through an equilibrium
 between the alkoxy of the titanate and, as an example, the hydroxyl group
 of the polymer.
 Ti(OR).sub.4 +POH - - - POTi(OR).sub.3 +ROH
 Another group of resists is referred to as "Amplified Resists" contain a
 phenolic group, but wherein are also not RIE resistant.
 SUMMARY OF THE INVENTION
 The present invention provides solutions of graft polymers which, among
 other uses, can be used in lithography. The polymers exhibit enhanced
 resistance to plasma and especially to Cl/O plasma used in reactive ion
 etching.
 In particular, the present invention relates to solutions in an alcohol of
 a graft polymer having reactive hydrogen groups and grafted through
 reactive hydrogen groups an alkoxy metallic compound wherein metal is
 titanium, zirconium and/or hafnium.
 The present invention also relates to a method for forming a pattern of a
 photoresist which comprises:
 a) applying to a substrate a solution in an alcohol of a graft polymer
 having reactive hydrogen groups and grafted through reactive hydrogen
 groups an alkoxy metallic compound wherein the metal is titanium,
 zirconium and/or hafnium;
 b) removing the alcohol;
 c) imagewise exposing the graft polymer to irradiation; and
 d) developing the photoresist to thereby form the pattern.
 A still further aspect of the present invention relates to a method for
 forming a pattern on a substrate which comprises:
 a) providing a layer to be patterned on a substrate;
 b) applying on the layer to be patterned a solution in an alcohol of a
 graft polymer having reactive hydrogen groups and grafted through reactive
 hydrogen groups an alkoxy metallic compound wherein the metal is titanium,
 zirconium and/or hafnium;
 c) removing the alcohol;
 d) imagewise exposing the graft polymer to irradiation;
 e) developing the graft polymer to from the desired pattern; and
 f) subjecting the layer to be patterned to reactive ion etching with the
 developed graft polymer acting as a mask to thereby form the desired
 pattern on the substrate.
 Another aspect of the present invention relates to alkoxy metallic
 compounds wherein the metal is titanium, zirconium and/or hafnium and
 which further comprises silicon, tin and/or germanium.
 A still further aspect of the present invention is concerned with graft
 polymers of a polymer having reactive hydrogen groups and grafted through
 reactive hydrogen groups an alkoxy metallic compound wherein the metal is
 titanium, zirconium and/or hafnium and wherein the alkoxy metallic
 compound further comprises silicon, tin and/or germanium.
 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 description is to be
 regarded as illustrative in nature and not as restrictive.
 BEST AND VARIOUS MODES FOR CARRYING OUT INVENTION
 The graft polymers according to the present invention are obtained from a
 polymer having reactive hydrogen groups. The reactive hydrogen groups
 include hydroxy, amino, amido, carboxyl and thio groups. Examples of some
 suitable polymers are polymers of hydroxy alkyl acrylates, polymers of
 hydroxy alkyl methacrylates, Novalacs, polymers of vinyl alcohol, polymers
 of hydroxy styrene, polyamides, cellulosic polymers such as cotton and
 paper, starch, rayon and proteins.
 Examples of some polymers of hydroxyalkyl acrylates and polymers of
 hydroxyalkyl methacrylates are disclosed in U.S. Ser. No. 08/700,348 and
 08/717,644, disclosures of which are incorporated herein by reference.
 These polymers contain pendant recurring groups selected from the group
 consisting of:
 --COO--CH.sub.2 --CH(OH)--(CH.sub.2).sub.x --H, wherein x is an integer of
 0-20;
 --COO--CH.sub.2 --CH(OH)--(CH.sub.2).sub.y --HE--(CH.sub.2).sub.z --H; and
 mixtures thereof, wherein HE is O or S, and each y and z individually is
 1-18; and mixtures thereof.
 Typical of such polymers are poly(2-hydroxyalkyl acrylate) and
 poly(2-hydroxyalkyl methacrylate) polymers.
 The alkyl groups are straight or branched chain saturated hydrocarbon
 having from 1 to 22 carbon atoms.
 Some examples of these poly(2-hydroxyalkyl acrylate) and
 poly(2-hydroxyalkyl methacrylate) polymers are:
 Poly(2-hydroxyethyl methacrylate)
 Poly(2-hydroxypropyl methacrylate)
 Poly(2-hydroxyethyl acrylate)
 Poly(2-hydroxypropyl acrylate)
 Poly(2-hydroxybutyl acrylate)
 Poly(2-hydroxypentyl acrylate).
 Other suitable homopolymers include those having as backbone a
 polyacrylate, a polymethacrylate, a polymaleate, and a polyvinyl alcohol.
 In addition, copolymers employed according to the present invention are
 from:
 1) at least one monomer selected from the group consisting of
 2-hydroxyalkyl methacrylate, 2-hydroxyalkyl acrylate, and mixtures thereof
 wherein the alkyl has 1-10 carbon atoms; and
 2) at least one monomer selected from the group consisting of
 alkylacrylate, alkylmethacrylate, and mixtures thereof wherein the alkyl
 has 1-10 carbon atoms.
 The amount of 1) is about 75 to about 95, preferably about 80 to about 90,
 and most preferably about 84 to about 87 wt. %; and the amount of 2) is
 about 5 to about 25, preferably about 10 to about 20 and most preferably
 about 13 to about 16 wt. % These weight percents are based upon the total
 weight of 1) and 2).
 Examples of suitable copolymers are copolymers of 2-hydroxyalkyl
 methacrylate and/or 2-hydroxyalkyl acrylate with alkylacrylate and/or
 alkylmethacrylate. As used in describing the resins which may be used in
 this invention, the term alkyl in the ester moiety refers to a straight or
 branched chain hydrocarbon having from 1 to 10 carbon atoms and containing
 no unsaturation. Specific copolymers include: copolymers of 2-hydroxyethyl
 methacrylate and methylmethacrylate; copolymers of 2-hydroxypropyl
 methacrylate and methylmethacrylate; copolymers of 2-hydroxyethyl
 methacrylate and t-butylmethacrylate; copolymers of 2-hydroxypropyl
 methacrylate and t-butylmethacrylate; copolymers of 2-hydroxyethyl
 acrylate and t-butylmethacrylate; copolymers of 2-hydroxypropyl acrylate
 and t-butylmethacrylate; copolymers of 2-hydroxyethyl methacrylate and
 methylacrylate; copolymers of 2-hydroxypropyl methacrylate and
 methylacrylate; copolymers of 2-hydroxyethyl acrylate and
 methylmethacrylate; copolymers of 2-hydroxypropyl acrylate and
 methylmethacrylate; copolymers of 2-hydroxyethyl acrylate and
 methylacrylate; copolymers of 2-hydroxypropyl acrylate and methylacrylate;
 copolymers of 2-hydroxypropyl acrylate and methylacrylate; copolymers of
 2-hydroxybutyl methacrylate and methylmethacrylate; copolymers of
 2-hydroxybutyl acrylate and methylmethacrylate; and copolymers of
 2-hydroxypentyl acrylate and methylmethacrylate.
 The polymers typically have weight average molecular weight of about
 10.sup.4 to about 10.sup.6, more typically about 50,000 to about 650,000,
 an example being about 300,000. The polymers typically have Tg of up to
 about 70.degree. C.
 It has been found according to the present invention that alkoxy titanates,
 alkoxy zironates and/or alkoxy hafnates can be grafted to above polymers
 by equilibrium reaction through an alkoxy group and active hydrogen of the
 polymer. In particular, the reactive can be carried out by merely mixing
 the reactants, and heat and catalysts are not required.
 However, in order to avoid crosslinking, the reaction is carried out in
 solution in a low boiling point alcohol and preferably in ethanol. Other
 alcohols include methanol, isopropanol and n-butanol.
 The presence of the alcohol prevents crosslinking and facilitates handling
 of the graft polymer permitting it to be readily processed such as coated
 onto a substrate. However, upon removal of the alcohol such as by
 evaporating, the polymer will then form a cross-linked material.
 Accordingly, the presence of the alcohol provide a composition that can be
 thought of as being a latent crosslinkable composition.
 Concerning the graft polymerization, the equilibrium is maintained in
 solution, and causes an increase in the viscosity of the polymer solution,
 but it does not cause precipitation of the polymer as other crosslinking
 methods do. However, upon removal of the solvent, crosslinking occurs
 resulting in a tough new resin. In lithography, it is practiced to cast
 films of the resist solution and bake the film at elevated temperatures
 where all of the solvents are driven off, precisely the conditions needed
 for crosslinking the polymers according to the present invention.
 Typically, the amount of the alkoxy metallic compound is about 0.1 to about
 15 molar % of the polymer, with about 10 molar % being an example.
 The organometallic compounds have at least one alkoxy group and up to four
 alkoxy groups. Preferably the organometallic compound has four alkoxy
 groups.
 Examples of some suitable organometallic compounds for use in the present
 invention are titanium tetramethoxide, titanium tetraethoxide, titanium
 tetrapropoxide, titanium tetrabutoxide, titanium tetrapentoxide, titanium
 tetrahexoxide, titanium tetraheptoxide, titanium tetraoctoxide, zirconium
 tetramethoxide, zirconium tetraethoxide, zirconium tetrapropoxide,
 zirconium tetrabutoxide, zirconium tetrapentoxide, zirconium
 tetrahexooxide, zirconium tetraheptoxide, zirconium tetraoctoxide, and
 hafnium ethylhexano-tert-butoxide. Mixtures of any of the compounds can be
 used if desired.
 Furthermore, the improvement in RIE retardation can be enhanced by
 providing the organometallic compounds with a silicon, tin and/or
 germanium. This can be accomplished by reacting the organometallic
 compound with a silicon, tin and/or germanium compound that has an active
 hydrogen to react through one of the alkoxy groups of the organometallic
 compound and leaving other alkoxy groups intact for subsequent grafting.
 A typical silicon compound is presented by the following formula:
EQU HO--CH.sub.2 CH.sub.2 --Si (CH.sub.3)
 Furthermore, dyes can be grafted onto polymers and especially cotton
 according to the present to create a permanent graft. For instance, a dye
 having an active hydrogen group such as an OH group is reacted through one
 of alkoxy groups of the organometallic compound while having other alkoxy
 groups intact for subsequent grafting.
 Typical dyes include acid red 73 (a dye which contains a hydroxyl group)
 and Malachite green carbinol.
 Solutions of the above grafted polymers typically include a cosolvent along
 with the alcohol such as dimethylformamide perfluoroisopropanol or
 1-methyl-1-pyrrolidinone.
 The solution can then be coated onto the desired substrate, such as by spin
 casting. Preferred substrates are those used in fabricating integrated
 circuits. Typical film thicknesses of the compositions are about 0.3 to
 about 1 micron and more typically about 0.5 to about 1 micron (dry). In
 addition, the films are typically baked after deposition at temperatures
 of about 90 to about 140.degree. C. and more typically at about 100 to
 about 120.degree. C. to cause crosslinking.
 Irradiation of crosslinked polymers according to the present invention
 produce positive working photoresists. This may be due to side chains of
 the photoresist detaching from the main chain upon irradiation thereby
 breaking the crosslinking bridges.
 The following illustrates one of many possible fabrication sequences for
 using the composition of the present invention for microlithography. In
 this particular sequence, a pattern of chromium metal on a quartz plate is
 provided by the following steps:
 1. A thin film of chromium metal is provided on the surface of a quartz
 plate;
 2. The metal layer is coated with the resist;
 3. The resist is patterned;
 4. The plate is developed in a suitable developer;
 5. The exposed chromium film is etched either by wet etch or by dry etch;
 6. The residual resist is removed.
 An additional advantageous application of the present invention including
 rendering cotton or paper by hydrophobic by grafting an organometallic
 compound of the present invention having hydrophylic groups. These
 hydrophobic materials repel water and are extremely oil absorbent
 rendering them useful for controlling oil spills.
 Furthermore, the present invention can be used to graft extremely polar
 moieties onto cotton or paper making these materials very hydrophobic.
 This can be achieved by providing an organometallic compound of the
 present invention that also includes a polar moiety such as a polyether,
 amide or polyol group. These hydrophobic materials could be used in
 diapers and other applications where bibulous materials are needed.

The following non-limiting example 1 are presented to further illustrate
 the present invention:
 EXAMPLE 1
 To a solution of about 1 g of poly(2-hydroxyethyl methacrylate) in about 15
 ml NMP were added about 8 ml ethanol followed by a solution of 0.3 g of
 Tyzor TOT in 2 ml ethanol. The last solution was added dropwise with
 stirring. The solution was spin coated onto Si wafers at 2000 RPM. The
 wafer was baked at about 100.degree. C. for about five minutes. The final
 film thickness was 0.375 microns.
 The film was exposed with e-beam at various radiation intensities and
 developed for 2 minutes in tetraethylammonium hydroxide (TMAH) 0.265%.
 To illustrate the effectiveness of the present invention, Tyzor TOT is a
 commercially available alkoxy titanate. Its addition to the
 poly(2-hydroxyethyl methacrylate) reduced the etch rate (in a
 chlorine/oxygen plasma environment) from 2220 .ANG./min for
 poly(2-hydroxyethyl methacrylate) to only 800 .ANG./min. This compares
 favorably with the rate of Novolak (825 .ANG./min), an industry standard.
 The improved RIE resistance rate is evident from the data presented in
 Table 1.
 TABLE 1
 Material Etch Rate
 Poly-Hema (S) 2182 (.ANG./min)
 Poly-Hema/10% Ti Additive (0.86% Actual 831 (.ANG./min)
 Ti)
 Poly-Hema/20% Ti Additive (1.7% Actual Ti) 614 (.ANG./min)
 Poly-Hema 30% Ti Additive (2.6% Actual Ti) 614 (.ANG./min)
 Poly-Hema/40% Ti Additive (3.4% Actual Ti) 529 (.ANG./min)
 The e-beam sensitivity of the above-mentioned titanate films is 12
 microcoul/sq.cm. at 50 KV. This represents a very slight decrease in
 sensitivity compared to poly HEMA which is about 10 microcoul/sq.cm at 50
 KV. The resolution obtained so far for e-beam is 0.25 microns.
 EXAMPLE 2
 A mixture of tetraisopropyl titanate and 2-hydroxyethyl-1-trimethyl silane
 with a mole ratio of 1:4, was heated under nitrogen for one hour at
 100.degree. C. and fractionated in vacuum to remove isopropanol. When no
 more isopropanol distilled, the product was distilled at 0.1 mm Hg. The
 product contained silicon at a ratio that indicated that one of the
 isopropanols was replaced with 2-hydroxyethyl-1-trimethyl silane. This
 product was used as titanate additive in place of Tyzer TOT according to
 example 1 above to improve RIE resistance and resist performance.
 The resist graft polymized with the additive of this example has superior
 RIE resistance compared to resist graft polymerized with the Tyzor TOT
 additive according to example 1.
 EXAMPLE 3
 A mixture of tetraisopropyl titanate and octadecanol with a mole ratio of
 1:4, was heated under nitrogen for one hour at 100.degree. C. and
 fractionated in vacuum to remove isopropanol. When no more isopropanol
 distilled, the product was distilled at 0.1 mm Hg. The product
 incorporated the octadecyl chain. The product was used in place of Tyzon
 TOT according to example 1.
 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 mentioned above, 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.