WO2000052531A1

WO2000052531A1 - Highly plasma etch-resistant photoresist composition containing a photosensitive polymeric titania precursor

Info

Publication number: WO2000052531A1
Application number: PCT/US2000/004869
Authority: WO
Inventors: Tony D. Flaim; Douglas J. Guerrero; Michelle R. Fowler; William J. James; Vladimir Petrovsky; Harlan U. Anderson
Original assignee: Brewer Science, Inc.; University Of Missouri
Priority date: 1999-03-01
Filing date: 2000-02-25
Publication date: 2000-09-08
Also published as: TWI263865B; US6303270B1

Abstract

A composition is derived from an addition polymerizable organotitanium polymer which upon exposure to an oxygen plasma or baking in air, is converted to titanium dioxide (titania) or is converted to a mixed, titanium-containing metal oxide. The metal oxide formed in situ imparts etch-resistant action to a patterned photoresist layer. The composition may also be directly deposited and patterned into permanent metal oxide device features by a photolithographic process.

Description

HIGHLY PLASMA ETCH-RESISTANT PHOTORESIST COMPOSITION CONTAINING A PHOTOSENSITIVE POLYMERIC TITANIA PRECURSOR

Technical Field

This invention relates to light-sensitive compositions useful for defining patterns on

substrates by photolithography, particularly to new photoresist compositions especially useful

in microlithographic applications where it is desirable to form microscopically-sized patterns

which exhibit exceptional resistance to plasma etching. The present invention also relates to

precursor compositions used to form the photoresist compositions and methods for forming the

precursor compositions and the photoresist compositions.

Background Of The Invention

Semiconductor devices such as integrated circuits, solid state sensors, and flat panel

displays are produced by microlithographic processes in which a photoresist is used to form a

desired feature pattern on a device substrate. Light is passed through a patterned mask onto the

photoresist layer which has been coated onto the device substrate. A chemical change occurs in

the light-struck areas of the photoresist, causing the affected regions to become either more

soluble or less soluble in a chemical developer. Treating the exposed photoresist with the

developer etches a positive or negative image, respectively, into the photoresist layer. The

resulting pattern serves as a contact mask for selectively modifying those regions of the

substrate which are not protected by the pattern. These modifications may include, for example,

etching, ion implantation, and deposition of a dissimilar material. Plasma etching processes are being used increasingly to transfer photoresist patterns

into device substrates or underlying layers. It is important that the photoresist pattern does not

erode excessively during the plasma etching process, otherwise, the precise pattern will not be

transferred into the underlying layer. For example, if the photoresist is removed by the etching

process before the underlying layer has been fully etched, then the feature size will begin to

increase as the etching process proceeds since the photoresist is no longer protecting the area of

the substrate which it once covered. Because of the precision required in etching this is

disadvantageous.

Plasma etching processes for organic layers such as color filters used in solid state color

sensors and flat panel displays require photoresist materials with exceptional resistance to

oxygen plasma etching. Conventional photoresists cannot be used effectively since they are

primarily mixtures of organic compounds and resins which are etched more rapidly than the

color layers.

Silicon-containing photoresists and silylated photoresist products have been developed

to provide greater etch selectivity when patterning organic layers by oxygen plasma etching.

Such compositions have been described, for example, in U.S. Patent 5,250,395 issued to Allen

et al. and by F. Coopmans and B. Rola in Proceedings of the SPIE, Vol. 631, p. 34 (1986).

During the etching process, the silicon components in the photoresist are rapidly converted to

silicon dioxide which resists further etching. The in-situ formed silicon dioxide layer then

becomes the mask for etching the underlying organic layer.

Although oxygen is normally the principal plasma etchant for organic layers, it may be

admixed with various fluorinated gases such as NF₃, C₂F₆, HCF₃, and CF₄ to aid in the removal of inorganic residues arising from metallic species. The inorganic residues are often present,

for example, in color filter layers. Since, silicon dioxide is readily etched by fluorine-containing

plasma etchants, silicon-containing photoresists cannot be used effectively in plasma etching

processes where fluorine-containing gases are present.

For color filter applications, it is also desirable that the plasma etch-resistant photoresist

be left in place as a permanent part of the device structure after the etching process has been

completed. To function suitably in this respect, the remaining photoresist material must be

continuous, homogeneous, highly adherent, and optically clear so that it does not reduce the

transmissivity of the color filter assembly. Silicon-containing photoresists often exhibit poor

optical clarity after plasma etching or high temperature thermal treatments which are usually

applied to color filter layers. This problem is especially prevalent with silylated phenolic

photoresists since the phenolic components form highly colored species when heated to

above approximately 125°C.

There are other microlithographic applications where it is also desirable to leave a

processed photoresist layer in place as a permanent device structure. For example, thin barium

titanate and lead zirconate titanate layers are needed as ferroelectrics in advanced memory

devices. Presently, these complex metal oxides must be deposited by chemical vapor

deposition and then patterned in separate plasma etching processes. Considerable time and

expense could be saved if the materials could be applied in a photosensitive precursor form by

spin coating and then directly patterned by a photolithographic process, after which the

patterned layer would be calcined in air to form the desired metal oxide device features. Accordingly, there is a need for a photoresist material with greater plasma etching

resistance in processes utilizing oxygen and/or fluorinated gases as the etchant species. At the

same time, there is a need for a photoresist which is convertible to a permanent metal oxide

layer with chemical, thermal, electrical, and optical properties useful for device applications

and whose properties can be controlled by adjusting the composition of the photoresist. The

photoresist should further possess high resolution patterning capabilities and be easily

integrated into modern microlithographic processing schemes. We have now discovered that

all of these requirements suφrisingly can be met with a photoresist composition containing an

addition polymerizable organotitanium polymer as the principal constituent.

Brief Decscription of the Drawings

Fig. 1 is a reaction showing the formation of an example organotitanium polymer;

Fig. 2 shows the crosslinking reaction that occurs when the polymer is applied onto the

substrate and exposed to radiation which induces addition polymerization;

Fig. 3 shows a table which displays a variation of photoresist plasma etching resistance

with baking conditions; and,

Fig. 4 is a continuation of the table of Fig. 3.

Summary Of The Invention

It is a principle object of the present invention to provide a photoresist composition with

improved resistance to plasma etching processes which utilize oxygen and/or

fluorine-containing gases, as well as noble gases, as the etchant species, with oxygen and fluorine preferred. It is a further objective to provide a photoresist composition which exhibits

the following desirable properties in addition to improved plasma etching resistance:

a) good coating quality and feature coverage when applied by spin coating onto

electronic substrates;

b) high sensitivity to ultraviolet, electron beam, and x-ray exposing radiation;

c) facile image development in aqueous alkaline developers;

d) good pre-cure adhesion to polymer, metal, and semiconductor materials;

e) high resolution, i.e., feature sizes of approximately 1 micron or smaller should be

readily obtainable; and,

f) curable to a continuous, homogeneous, and, if desired, optically clear (> 90%

transmissivity at 400-700 nm wavelengths for 0.25 micron film thickness) metal oxide layer

having good adhesion to underlying device structures.

Lastly, it is an objective of this invention to provide a method for using the photoresist

composition in a microlithographic process as a plasma etch-resistant masking layer or a

permanent device structure.

The improved photoresist composition is comprised principally of: a) an addition

polymerizable organotitanium polymer or copolymer, b) a photopolymerization initiator or

initiator system, and c) a solvent vehicle.

The photoresist composition is preferably coated onto a substrate by any of a variety of

means, with spin coating preferred, and then dried to obtain a uniform, defect-free layer, which

is then exposed to ultraviolet radiation through a patterned mask to generate a latent image. The

exposed photoresist is etched in an alkaline aqueous solution or an aqueous chelate solution, having a pH greater than 10, to leave a negative-tone image of the mask in the layer. The

patterned structure is baked in air and/or exposed to an oxygen-containing plasma to partially or

fully convert the photoresist material into a titanium-containing metal oxide layer with high

plasma etching resistance. The resulting metal oxide layer may be used as a mask for modifying

underlying layers by plasma etching, implantation, deposition, or other processes. Depending

on its final physical and chemical properties, it may be left in place as a permanent device

structure after the processing sequence has been completed.

Detailed Description Of The Invention

The improved photoresist composition of the present invention is preferably comprised

of:

a) an addition polymerizable organotitanium polymer or copolymer prepared by

reacting a poly(alkyltitanate) or poly(alkyltitanate-co-alkylmetallate) with an alcohol,

carboxyiic acid, beta-diketone, beta-ketoester, or alpha-hydroxy carboxyiic acid, acid salt, or

ester having at

least one ethylenically unsaturated double bond capable of addition polymerization, wherein the

copolymerized alkylmetallate moiety is selected from the group consisting of - (RO)Al-O-,

- (RO)₂Zr-O-, - (R)₂Si-O-, -(R) (RO)Si-O-, and - (RO)₂Si-O-,

and where R and R are monovalent organic radicals;

b) a free radical-generating photopolymerization initiator or initiator system;

c) a solvent vehicle suitable for obtaining high quality thin films on device substrates

by spin casting. The composition may additionally contain one or both of the following constituents:

d) an addition polymerizable co-monomer having at least one ethylenically unsaturated

double bond, wherein the co-monomer may contain a covalently bonded metal; and,

e) a soluble metallic compound which is stable in the presence of the other photoresist

ingredients.

It will be apparent to those skilled in the art that the organotitanium polymer or

copolymer used in the title invention, and which upon heat treatment forms a metal oxide

composition, is inherently capable of addition polymerization by virtue of the ethylenically

unsaturated double bonds present within its structure. Therefore, it is expected that the

organotitanium polymer or copolymer could be used alone in solution or in combination with

some, but not necessarily all, of the above-mentioned constituents to prepare coatings which

can be patterned by selective exposure to ionizing radiation, assuming such radiation is capable

of inducing addition polymerization in the coating. For example, the improved photoresist

composition may be exposed to an electron beam or x-ray source to form a negative-tone image

in a manner analogous to exposure to ultraviolet light. In such instances, the inclusion of a free

radical-generating photopolymerization initiator or initiator system may not be necessary for

patterning since the high energy radiation can induce crosslinking in the exposed areas of the

coating.

Components Of Composition

a. Addition polymerizable Organotitanium Polymer Organotitanium polymers suitable for use in the new photoresist composition include

the reaction products of poly(alkyltitanates) and poly(alkyltitanates-co-alkylmetallates) with

addition polymerizable alcohols, carboxyiic acids, beta-diketones, beta-ketoesters, and

alpha-hydroxy carboxyiic acids, acid salts, and esters. For example, poly(n-butyl titanate) can

be reacted with 2-hydroxyethyl acrylate to form the following polymeric titanate ester (1)

which is capable of addition polymerization:

-[Ti (OR_a)_x(OBu)_y-O] _n- (1)

where R_= -CH₂-CH₂-O-CO-CA = CH₂,

Bu= -CH₂-CH₂-CH₂-CH₃, x+y = 2, x = 0.1-2.0,

A = -H or -CH₃, and n > 2.

Poly (alkyltitanates) can also be reacted with acrylic acid or other addition

polymerizable carboxyiic acids to produce polymeric titanium acylates useful in the present

invention. The reaction of poly(n-butyltitanate) with acrylic acid, for example, results in the

following polymeric product (2) which is capable of addition polymerization:

- [ Ti (OR_b)_x (OBu)_y-O] _n- (2)

where R_b= -O-CO-CA=CH₂, Bu= -CH₂-CH₂-CH₂-CH₃,

x+y = 2, x = 0.1-2.0, A - -H or -CH₃,

and n > 2.

Similarly, poly(alkyltitanates) can be reacted with beta-diketones, beta-ketoesters, and

alpha-hydroxy carboxyiic acids, acid salts, and esters containing addition polymerizable groups

to produce polymeric titanium chelates useful in the present invention. For example, the

reaction of poly(n-butyltitanate) with 2-acetoacetoxy ethyl methacrylate, a beta-ketoester, results in the following polymeric product (3) which is capable of addition polymerization:

where R_= -CH₂-CH₂-O-CO-C(CH₃)=CH₂

Bu= -CH₂-CH₂-CH₂-CH₃, x+y - 2, (3)

x = 0.1-2.0, and n > 2.

The reaction of poly(n-butyltitanate) with an addition polymerizable alpha-hydroxy

carboxyiic acid salt also yields a polymeric titanium chelate (4) useful in the present invention,

for example:

where B⁺ = (H₃C)₂NH⁺-R-CO-CA=CH₂,

R=-CH₂-CH₂-CH₂-NH- or -CH₂-CH₂-O-,

Bu = -CH₂-CH₂-CH₂-CH₃, x+y - 2, x = 0.1-2.0, (4)

A = -H or -CH„ and n > 2.

An example of how the organotitanium polymer is formed is shown in Fig. 1. It will be apparent to those skilled in the art that useful addition polymerizable

organotitanium polymers can also be prepared by reacting poly(alkyltitanates) with other

known titanium chelants which have been functionalized to enable free radical-initiated

polymerization. It will be further apparent that functionally equivalent, addition polymerizable

organotitanium polymers can be prepared in principle by reacting, for example, titanate

orthoesters having at least one ethylenically unsaturated double bond with a limited amount of

water to form a soluble polymeric condensation product. Lastly, it is inferred that addition

polymerizable organometallic polymers useful for the present invention can be prepared

similarly from copolymers of alkyltitanates with alkylsilicates (siloxanes) and other

alkylmetallates, notably, those of aluminum, zirconium, cerium, niobium, and tantalum.

b. Photopolymerization Initiators or Initiator Systems

All known free radical initiators or initiator systems which operate effectively at

200-500 nm exposing wavelengths can be substantially employed as the photopolymerization

initiator or initiator system for the present invention. The free radical initiator decomposes

upon exposure to ultraviolet light, forming a species which has an unpaired electron or is

capable of extracting a proton from another molecule so that the latter carries an unpaired

electron. The free radical thus formed adds readily to an unsaturated double bond, especially

acrylate-type double bonds, to generate a new free radical which then reacts with another

double bond-containing molecule, and so on, creating a polymer chain in the process. The

polymer-forming process is called addition polymerization. Examples of suitable initiators and

initiator systems include:

1) trihalomethyl-substituted triazines such as p-methoxy phenyl-2, 4-bis (trichloromethyl)-s-triazine;

2) trihalomethyl-substituted oxadiazoles such as 2-(p-butoxy styryl)

chloromethyl- 1 ,3 ,4-oxadiazole;

3) imidazole derivatives such as 2-(2'-chlorophenyl)-4, 5-diphenylimidazole dimer

(with a proton donor such as mercaptobenzimidazole);

4) hexaaryl biimidazoles such as 2,2'-bis(o-chlorophenyl)

4,4',5,5'-tetraphenylbiimidazole;

5) benzoin alkyl ethers such as benzoin isopropyl ether;

6) anthraquinone derivatives such as 2-ethylanthraquinone;

7) benzanthrones;

8) benzophenones such as Michler's ketone;

9) acetophenones such as 2,2-diethoxy-2-phenylacetophenone;

10) thioxanthones such as 2-isopropylthioxanthone;

11) benzoic acid ester derivatives such as octyl p-dimethyl aminobenzoate;

12) acridines such as 9-phenylacridine;

13) phenazines such as 9,10-dimethylbenzphenazine; and,

14) titanium derivatives such as bis(cyclopentadienyl)-bis(2,6-difluoro-3-(pyl-l-yl)

titanium.

These photopolymerization initiators may be used alone or in admixture. An example

would be combining 2-isopropylthioxanthone with octyl p-dimethylaminobenzoate. c. Solvent Vehicle and Additives

Suitable solvents for the new photoresist composition include alcohols, esters, glymes,

ethers, glycol ether, ketones and their admixtures which boil in the range 70°-180°C. Especially

preferred solvents include l-methoxy-2-propanol (PGME), 2-butoxyethanol, cyclohexanone,

2-heptanone, ethyl 3-ethoxypropionate, propylene glycol methyl ether acetate, and other

common photoresist solvents. Solvent systems containing an alcohol, such as PGME, are

preferred for obtaining improved hydro lytic stability of the photoresist composition. Solvents

such as ethyl acetoacetate and ethyl lactate may be used in the photoresist composition

provided they do not cause side reactions with the photosensitive organotitanium polymer.

The photoresist composition may be augmented with small amounts (up to 20 wt.% of

total solvents) of high boiling solvents such as N-methylpyrrolidone, gamma-butyrolactone,

and tetrahydrofurfuryl alcohol to improve the solubility of the coating components, provided

the solvents do not cause coating quality problems. Surface tension modifiers such as 3M

Company's FLUORAD® FC-171 or FC-430 fluorinated surfactants may be added at low levels

(approximately 1000 parts per million) to optimize coating quality without affecting the

lithographic properties of the photoresist.

d. Co-monomers

Co-monomers having at least one ethylenically unsaturated double bond capable of

addition polymerization may be added to the photoresist composition to improve the

photospeed, resolution, or physical and chemical properties of the photoresist layer. Preferred

comonomers carry multiple acrylate groups which participate in the addition polymerization

process described above in the initiation and initiator systems. From the standpoint of the polymerization, co-monomers are indistinguishable from the (meth)acrylate groups on the

organotitanium polymer. The comonomer can serve many puφoses for example: 1) it can

modify film properties from what would be obtained with the organotitanium polymer only,

e.g., it can make the product softer or harder, 2) it can increased the photospeed by providing a

higher concentration of polymerizable groups in the coating, or 3) it can change the

development properties of the coating by making it more or less soluble in basic developer.

Examples of suitable comonomers include mono- and polyfunctional (meth)acrylate esters such

as 2-hydroxyethyl (meth)acrylate; ethylene glycol dimethacrylate, pentaerythritol triacrylate,

and tetraacrylate; dipentaerythritol pentaacrylate and hexaacrylate; polyester (meth)acrylates

obtained by reacting (meth)acrylic acid with polyester prepolymers; urethane (meth)acrylates;

epoxy (meth)acrylates prepared by reacting (meth)acrylic acid with epoxy resins such as

bisphenol-A type epoxy resins, bisphenol-F type epoxy resins, and novolak-type epoxy resins;

and, tris(2-acryloyloxyethyl) isocyanurate.

Suitable co-monomers also include acrylic-functional metal complexes prepared, for

example, by tranesterifying titanium or zirconium alkoxides with 2-hydroxyethyl acrylate or a

chelating organic moiety. The use of such metal-containing co-monomers is advantageous for

maintaining high metal content in the photoresist composition,

e. Non-Photopolymerizable Metallic Compounds

Non-photopolymerizable metallic compounds may be added to the photoresist

composition to increase metal content or obtain complex metal oxide compositions from the

processed photoresist layer. Suitable compounds include soluble metal carboxylates, metal

alkoxides, metal hydroxides, metal chelates, and simple metal salts such as metal chlorides or nitrates. The amount and type of metal compounds which can be added are governed by 1)

their solubility in the liquid photoresist as well as in the dried photoresist layer (an added metal

compound should not crystallize in the dried film); 2) their overall effect on the lithographic

properties of the photoresist; and, 3) their reactivity with the other photoresist components. It is

especially important that added metal compounds do not cause precipitation of the

organotitanium polymer or reduce its polymerizability through unwanted side reactions,

f. Other Compounds

Non-reactive organic compounds may be added to the photoresist composition to

modify the properties of the photoresist layer. For example, solvent-soluble dyes can be added

to the composition to prepare a patternable, permanently colored layer for light-filtering

applications. Pigments can also be dispersed in the photoresist to obtain a directly patternable

colored product. The ability to add these materials depends on their compatibility with the other

photoresist components and their impact on the lithographic properties of the coating.

Method Of Preparation

a. Preparation of Photopolymerizable Organotitanium Polymer

The addition polymerizable organotitanium polymer is preferably prepared by reacting

in solution a poly(alkyltitanate) or poly(alkyltitanate-co-alkylmetallate) with a stoichiometric

excess of an alcohol, carboxyiic acid, beta-diketone, beta-ketoester, or alpha-hydroxy

carboxyiic acid, acid salt, or ester having at least one ethylenically unsaturated double bond

capable of addition polymerization. The ester substituents on the starting polymer are

substituted by the polymerizable reactants to form the final photosensitive product. The solution may be heated to about 70°C for several hours to increase the rate and yield of the

substitution reaction. By-product alcohol may be removed continuously from the reactor by

vacuum distillation to drive the reaction to completion,

b. Formulation of Photoresist

The addition polymerizable organotitanium polymer, photopolymerization initiator(s),

and, if present, co-monomers, and non-photopolymerizable metallic and organic compounds are

combined by stirring in a portion of the solvent system and then diluted with additional

portions of the solvent system until the desired total solids level is obtained. A total solids level

of 30 wt.% is typically required in the solution to achieve a film thickness of 500-2500 A when

it is spin coated at 1000-5000 φm for 30-90 seconds and then dried at approximately 100°C.

Prior to the final dilution, the photoresist solution or its components may be treated, for

example, by ion exchange processes to remove metal ion contamination. Preferred

compositional ranges (expressed in wt.% based on total solids content) for each of the

photoresist components are summarized in the table below:

Method of Use

The improved photoresist composition can be used effectively on most ceramic, metal,

polymer, and semiconductor substrates including, for example, glass, sapphire, aluminum

nitride, crystalline and polycrystalline silicon, silicon dioxide, silicon (oxy)nitride, aluminum,

aluminum/silicon alloys, copper, platinum, tungsten, and organic layers such as color filters and

polyimide coatings. The photoresist is coated onto the substrate by any of a variety of means

including spin coating, roller coating, blade coating, meniscus or slot coating, and spray

coating. Spin coating, however, is most preferred with the photoresist applied by spin coating

at 500-5000 φm for 30-90 seconds. Spinning speeds of 1000-4000 φm are especially

preferred for obtaining uniform, defect-free coatings on 6" and 8" semiconductor substrates.

The spin-coated film is dried typically at 80-120°C for 30-120 seconds on a hot plate or

equivalent baking unit prior to exposure. The photoresist is preferably applied at a film

thickness of 0.05-1.00 micron (as-spun) and, more typically, to a film thickness of 0.10-0.50

micron by adjusting both the total solids level of the photoresist and the spinning speed and

time to give the desired layer thickness. While the before mentioned thicknesses are preferred,

the photoresists can be applied to a thickness of several microns if desired, assuming a

sufficient level of polymer solids can be supported in the photoresist solution. Film thickness

can be increased by increasing solids content, reducing the spinning speed, and formulating the

resist with faster-drying solvents.

A latent image is formed in the photoresist layer by exposing it to ultraviolet radiation

through a mask. An exposure dose of 10-1000 mJ/cm² is typically applied to the photoresist to

define the latent image. Alternatively, regions of the photoresist layer may be exposed to electron beam or x-ray radiation to form a latent negative-tone image. An example of the

radiation-induced crosslinking reation that occurs in the exposed areas is shown in Fig. 2. The

exposed photoresist layer is developed in aqueous alkali or an aqueous chelant solution to form

the final pattern.

For use as a plasma etching mask, the patterned photoresist is converted to an

etch-resistant metal oxide layer by one of two techniques. In the first, it is baked in air at

150°-300°C for 15-60 minutes to decompose the organic components and leave a

predominantly inorganic layer which is etch-resistant. The processed photoresist layer can then

be used as a mask for plasma etching an underlying layer with oxygen or a fluorinated gas

species.

In the second method, the patterned photoresist is applied and patterned over an organic

layer such as a color filter. The two-layer structure is placed directly in a plasma etching

environment where oxygen is the principal etching species. (Prior thermal decomposition is not

required). The photoresist layer is partially converted to a metal oxide film during the initial

portion of the process and then serves as an etching mask for the organic layer.

It is becoming popular to planarize surface topography during the construction of

integrated circuits in order to reduce photoresist thickness variations across the substrate and

thereby enhance feature size control. In such instances, a relatively thick, organic layer may be

applied over the device features to form a planar surface. A thin photoresist is then applied onto

the structure and used to pattern the organic planarizing layer by oxygen plasma etching. The

remaining photoresist and the planarizing layer form a composite mask, or bilayer photoresist

system, for etching or otherwise modifying the substrate. The new photoresist can be used very effectively in such processes because of its combined resistance to oxygen- and

fluorine-containing plasma etching processes. The use of the new photoresist in a bilayer

configuration essentially follows the process described above for patterning an organic color

filter. After the planarizing layer has been cleared by oxygen plasma etching, a fluorinated

etchant may be introduced to etch the substrate. Unlike silicon-containing bilayer photoresists

which erode under these conditions, the new photoresist shows less degradation, which helps to

maintain better edge acuity on the photoresist features throughout the etching process. This in

turn reduces negative etch biasing caused by lateral erosion of the bilayer photoresist features.

Once these modifications have been completed, the bilayer photoresist structure is lifted off by

dissolving the organic planarizing layer from beneath the metal oxide mask. The device

substrate is then ready for another processing cycle.

When used to deposit permanent metal oxide device features, the photoresist is applied

onto the device substrate, patterned, and then heated in air to form an metal oxide layer. The

metal oxide may be calcined at high temperatures (> 300°C) to obtain a densified

polycrystalline structure which has physical properties more suitable for device applications.

Examples

Example 1

A photoresist composition corresponding to the present invention was prepared and

used to pattern a color filter layer by a plasma etching process,

a. Preparation of Photopolymerizable Organotitanium Polymer Fifteen (15) parts by weight of poly(n-butyltitanate) obtained from Geleste Coφoration

were combined with 20 parts by weight of 2-hydroxyethyl acrylate (2-HEA) in a closed

container and heated for approximately one hour to cause substitution of the titanate ester

groups by 2-HEA. The resulting solution was used to prepare the photoresist composition

described below.

b. Photoresist Formulation

A photoresist composition was prepared by combining 25.9 g of the above polymer

solution, with 1.0 g of 2-isopropyl-9H-thioxanthen-9-one, and 3.0 g octyl

p-dimethylaminobenzoate in 70.1 g propylene glycol methyl ether to form a solution containing

29.9 wt.% total solids.

c. Patterning Trials on Silicon

The photoresist composition was spin coated onto silicon wafers at 3000 φm for 60 sec

and then dried at 100°C for 60 sec on a hot plate to obtain 1600 A-thick film specimens. The

coated specimens were exposed to a broadband ultraviolet light source through a contact mask

to form a latent negative image in the photoresist film. An exposure dose of 100 mJ/cm² was

applied. The exposed specimens were developed for 5-10 seconds in 0.26 N

tetramethylammonium hydroxide solution to form shaφly defined, isolated, and dense features

as small as 1 micron in width. The smallest features were retained at all points across the

substrate, indicating that the photoresist had excellent adhesion to the silicon substrate.

Patterned specimens were placed individually in a March plasma etching system and

exposed to an oxygen-rich plasma for periods ranging from 1-60 minutes. Comparison of film

thickness before and after exposure to the plasma showed that the starting film thickness of 1600 A quickly decreased to 1100 A, after which no further change was observed regardless of

the etching time. The results clearly indicated that the organic components of the photoresist

were rapidly removed by the oxygen plasma, leaving a titania layer which resisted further

etching.

d. Pattern Transfer Into An Organic Color Layer

The patterning process with the photoresist composition was repeated with the

photoresist film now applied onto a substrate which had been previously coated with a 1.5

micron-thick polyimide color filter containing solvent-soluble organic dyes. (The color filter

was baked at 230°C for 1 hour prior to applying the photoresist.) Microscopic inspection of the

photoresist layer immediately after patterning showed that two-micron and larger-sized features

were retained across the substrate during the development process. The specimen was placed in

a March plasma etching system and exposed to a plasma comprised of 90% O₂ and 10% CF₄.

After a 10-minute etch, the color filter was cleanly removed from those areas not protected by

the photoresist. The photoresist remained uniformly intact on the color features at all points on

the specimen.

Example 2

A photoresist composition corresponding to the present invention was prepared from an

organotitanium polymer formed by the reaction of poly(n-butyltitanate) and an addition

photopolymerizable beta-ketoester. The polymer showed improved solution stability in

comparison to the organotitanium polymer used in Example 1.

a. Preparation of Photopolymerizable Organotitanium Polymer In a 250 ml, oven-dried, round bottom flask fitted with a drying tube 29.67 g of

poly(n-butyltitanate), containing a calculated 0.282 moles of reactive n-butyl ester groups, were

combined with 66.32 g of 2-methacryloxyethyl acetoacetate (MEAA). The solution was stirred

at room temperature for about 20 minutes and then immersed in an oil bath for 24 hours at

70-80°C to cause substitution of the titanate ester groups by MEAA. The resulting polymer

solution was used to prepare the photoresist composition described in section (b) below.

A second preparation of the same polymer was placed in a 50°C oven for two weeks to

determine its stability against gellation. The gold-yellow polymer solution exhibited a

kinematic viscosity of 19.62 Centistokes at the beginning of the period. After two weeks at

50°C, the color of the solution was unchanged and its viscosity had decreased to 18.18

Centistokes (-7.3%), indicating the solution possessed excellent stability. When a solution of

the photopolymerizable organotitanium polymer used in Example 1 was aged similarly, it

gelled within 24 hours.

b. Photoresist Formulation

A photoresist composition was prepared by combining 26.0 g of the above polymer

solution, 1.0 g of 2-isopropyl-9H-thioxanthen-9-one, and 3.0 g octyl p-dimethylaminobenzoate

in 74.1 g propylene glycol methyl ether The solution was stirred for 1 hr at room temperature

and then passed through a 0.2 μm endpoint filter to remove particulates prior to spin coating.

c. Patterning Trails on Silicon

and then dried at 100°C for 60 sec on a hot plate to obtain 1500 A-thick film specimens. The

coated specimens were exposed to a broadband ultraviolet light source through a contact mask to form a latent negative image in the photoresist film. An exposure dose of approximately 300

mJ/cm² was applied. The exposed specimens were developed for about 5 minutes in dilute

potassium carbonate solution to form sharply defined, isolated, and dense features as small as 1

micron in width. The smallest features were retained at all points across the substrate,

indicating that the photoresist had excellent adhesion to the silicon substrate.

d. Testing of Plasma Etching Resistance

The ability of the photoresists to withstand plasma etching in 90% O₂/10% CF₄ was

evaluated after applying various heat treatments (bakes) to the photoresists. It was expected

that plasma etching resistance would improve as baking temperature and/or time increased

since more of the easily etchable carbonaceous material would be removed from the photoresist

layer prior to plasma etching by the heat treatment. The resistance of the photoresist to plasma

etching in pure oxygen was determined at the same time. The results of the evaluations are

summarized in Table 1.

The data in Table 1 indicated that the film thickness of the photoresist layer was highly

dependent on the bake process applied to the layer prior to plasma etching. As bake

temperature and time increased, more of the carbonaceous components were outgassed from the

film, causing pre-etch film thickness to progressively decrease. Heat treatment of the

photoresist layer improved resistance to O₂/CF₄ etching as evidenced by the fact that post-etch

photoresist thickness increased as bake temperature and time increased. The photoresist layer

was highly resistant to plasma etching in pure oxygen regardless of the manner of heat

treatment.

Claims

What is claimed is:

1. A plasma etch-resistant photoresist composition comprised of:

(a) an organotitanium polymer or copolymer reaction product produced by reacting:

[i] a poly(alkyltitanate) or a poly(alkyltitanate-co-alkylmetallate) or mixtures

thereof, with

[ii] addition polymerizable alcohols, carboxyiic acids, beta-diketones,

beta-ketoesters, or alpha-hydroxy carboxyiic (acids, acid salts, or esters) or mixtures thereof;

(b) a free radical-generating photopolymerization initiator or initiator system; and,

(c) a solvent vehicle suitable for obtaining high quality thin films on device substrates

by spin casting.

2. The plasma etch-resistant photoresist composition of claim 1 is further

comprised of an addition polymerizable co-monomer having at least one ethylenically

unsaturated double bond.

3. The plasma etch-resistant photoresist composition of claims 1 or 2 wherein the

composition is further comprised of a soluble metallic compound which is stable in the

photoresist composition.

4. The plasma etch-resistant photoresist composition of claim 1 wherein the

co-polymerized alkylmetallate is selected from the group consisting of -(RO)Al-O-, -(RO)₂

Zr-O-, -(R')₂ Si-O-, and -(RO)₂ Si-O-, where R and R' are monovalent organic radicals.

5. The plasma etch-resistant photoresist composition of claim 1 wherein the

organotitanium polymer is the reaction product of poly(n-butyl titanate) and 2-hydroxyethyl

acrylate and forms a polymeric titanate ester having the formula; -[Ti(OR_a)_x(OBu)_y-O]_n-, where R. = -CH₂-CH₂-O-CO-CA=CH₂,

Bu = -CH₂-CH₂-CH₂-CH₃, x + y = 2, x = O.l to 2.O,

A = -H or -CH₃, and n >2.

6. The plasma etch-resistant photoresist composition of claim 1 wherein the

organotitanium polymer is the reaction product of poly(n-butyltitanate) and acrylic acid and has

the formula:

-[Ti(OR_b)_x(OBu)_y-O]_n-, where R„ is -O-CO-CA = CH₂ ,

Bu is -CH₂-CH₂-CH₂-CH₃, x + y is 2, x is 0.1 to 2.0,

A is -H or -CH₃, and n >2.

7. The plasma etch-resistant photoresist composition of claim 1 wherein the organotitanium polymer is the reaction product of poly(n-butyltitanate) and

2-acetoacetoxyethyl methacrylate having the formula:

wherein Re = -CH₂-CH₂-O-CO-C (CH₃) = CH

Bu = -CH₂-CH₂-CH₂-CH₃, x + y = 2, x = 0.1 to 2.0, and n > 2.

8. The plasma etch-resistant photoresist composition of claim 1 wherein the

organotitanium polymer is the reaction product of a poly(alkyltitanate) and a titanium chelant

which has been functionalized to enable addition polymerization.

9. The plasma etch-resistant photoresist composition of claim 1 wherein the

organotitanium polymer is the reaction product of poly(n-butyltitanate) and an alpha-hydroxy

carboxyiic acid salt having the formula:

where B⁺ equals -(H₃C)₂NH⁺-R-COCA=CH₂, R equals -CH₂-CH₂-CH₂-NH- or

-CH₂-CH₂-O-, Bu equals -CH₂CH₂CH₂CH₃, x + y = 2, x is equal to between 0.1 and 2.0, A

equals -H or -CH₃, and n is greater than 2.

10. The plasma etch-resistant photoresist composition of claim 1 wherein the

organotitanium polymer is the reaction product of a titanate orthoester having at least one

ethylenically unsaturated double bond and a limited amount of water to form a soluble

polymeric condensation product.

11. The plasma etch-resistant photoresist composition of claim 1 wherein the

organotitanium polymer is the reaction product of copolymers of alkytitanates and alkylsihcates

(siloxanes) and other alkylmetallates having metals selected from the group of consisting of

aluminum, zirconium, cerium, niobium, and tantalum.

12. The plasma etch-resistant photoresist composition of claim 1 wherein the free

radical initiator or initiator system operates effectively at 200-500 nm exposure wavelenths.

13. The plasma etch-resistant photoresist composition of claim 2 wherein the

comonomer improves the photospeed, resolution, or physical and chemical properties of the

photoresist composition and is selected from the group of mono- and polyfunctional

(meth)acrylate esters consisting of 2-hydroxyethyl (meth)acrylate; ethylene glycol

dimethacrylate, pentaerythritol triacrylate and tetraacrylate; dipentaerythritol pentaacrylate and

hexaacrylate; polyester (meth)acrylates obtained by reacting (meth)acrylic acid with polyester

prepolymers; urethane (meth)acrylates; epoxy (meth)acrylates prepared by reacting

(meth)acrylic acid with epoxy resins such as bisphenol-A type epoxy resins, bisphenol-F type

epoxy resins, and novolak-type epoxy resins; and tris(2-acryloyloxyethyl) isocyanurate.

14. The plasma etch-resistant photoresist composition of claim 2 wherein the

co-monomers are selected from acrylic-functional metal complexes prepared by transesterifying

titanium or zirconium alkoxides with 2-hydroxyethyl acrylate or a chelating organic moiety

having at least one ethylenically unsaturated double bond.

15. The plasma etch-resistant photoresist composition of claim 3 wherein the

soluble metallic compound is selected from the group consisting of soluble metal carboxylates,

metal alkoxides, metal hydroxides, metal organo chelates, metal chlorides, and metal nitrates.

16. The plasma etch-resistant photoresist composition of claim 1 wherein the

photoresist composition is futher comprised of materials to enhance color, with the material

being selected from the group consisting of solvent-soluble dyes and pigments.

17. The plasma etch-resistant photoresist composition of claim 1 wherein the

solvent is selected from the group consisting of: alcohols, esters, glymes, ethers, glycol ethers,

ketones, and combinations thereof.

18. An organotitanium polymer capable of addition polymerization, wherein the

organotitanium polymer is selected from the group consisting of:

(a) -[Ti(OR₃)_x(OBu)_y-O]_n- where R, equals -CH₂CH₂OCOCA=CH₂, Bu equals

-CH₂CH₂CH₂CH₃, x + y = 2, x is equal to between 0.1 and 2.0, A equals -H or -CH₃, and n is

greater than 2;

(b) -[Ti(OR_b)_x(OBu)_v-O]_n- where R_b equals -OCOCA=CH₂, Bu equals

greater than 2;

(c)

where R_ equals -CH₂CH₂OCOC(CH₃)=CH₂, Bu equals -CH₂CH₂CH₂CH₃, x + y = 2, x

is equal to between 0.1 and 2.0, and n is greater than 2; and,

(

equals -H or -CH₃, and n is greater than 2.

19. A method for forming an organtitanium polymer capable of addition

polymerization, wherein the method is comprised of: reacting a titanium polymer or copolymer

selected from the group consisting of poly(alkyltitanates) and

poly(alkyltitanates-co-alkylmetallates) with a compound selected from the group consisting of

alpha-hydroxy carboxyiic acids, acid salts, and esters.

20. An organotitanium polymer capable of addition polymerization, wherein the

organotitanium polymer is the reaction product of:

(a) a titanium polymer or copolymer selected from the group consisting of

poly(alkyltitanates) and poly (alky ltitanates-co-alkylmetallates); and,

(b) a compound selected from the group consisting of addition

polymerizable alcohols, carboxyiic acids, beta-diketones, beta-ketoesters, and alpha-hydroxy

carboxyiic acids, acid salts, and esters.

21. A dried photoresist composition on a substrate, wherein the photoresist

composition is comprised of:

(a) an addition polymerizable organotitanium polymer added in an amount

equal to between about 15% by weight and about 90% by weight of the photoresist

composition; and,

(b) a photopolymerization initiator that operates at 200-500 nm added in an

amount equal to between about 1% and about 20% by weight of the photoresist composition.

22. The photoresist composition of claim 21 wherein the addition polymerizable

organotitanium polymer is added in an amount equal to between about 40% and about 80% by

weight of the photoresist composition.

23. The photoresist composition of claim 21 wherein the photopolymerization

initiator is added in an amount equal to between about 10% and about 15% by weight of the

photoresist composition.

24. The photoresist composition of claim 21 wherein the addition polymerizable

organtitanium polymer is selected from the group consisting of:

-[Ti(OR_a)_x(OBu)_y-O]_n- where R_ equals -CH₂CH₂OCOCA=CH₂, Bu equals

greater than 2;

-[Ti(OR_b)_x(OBu)_y-O]_n- where R_b equals -OCOCA=CH₂, Bu equals -CH₂CH₂CH₂CH₃, x

+ y = 2, x is equal to between 0.1 and 2.0, A equals -H or -CH₃, and n is greater than 2;

is equal to between 0.1 and 2.0, and n is greater than 2; and,

where B⁺ equals -(H₃C)₂NH⁺CH₂CH₂OCOCA=CH₂, Bu equals -CH₂CH₂CH₂CH₃, x + y

= 2, x is equal to between 0.1 and 2.0, A equals -H or -CH₃, and n is greater than 2.

25. The photoresist composition of claim 21 wherein the photopolymerization

initiator is selected from the group consisting of: trihalomethyl-substituted triazines,

trihalomethyl-substituted oxadiazoles, imidazole derivatives, hexaaryl biimidazoles, benzoin

alkyl ethers, anthraquinone derivatives, benzanthrones, benzophenones, acetophenones,

thioxanthones, benzoic acid ester derivatives, acridines, phenazines, titanium derivatives, and

mixtures thereof.

26. The photoresist composition of claim 21 wherein the photoresist composition

includes a co-monomer added in an amount equal to between about 0 and about 80% by weight

of the photoresist composition, with the co-monomer selected from the group consisting of:

mono- and polyfunctional (meth)acrylate esters, ethylene glycol dimethacrylate, pentaerythritol

triacrylate, tetraacrylate, dipentaerythritol pentaacrylate and hexaacrylate; polyester

(meth)acrylates obtained by reacting (meth)acrylic acid with polyester prepolymers, urethane

(meth) acrylates, epoxy (meth)acrylates prepared by reacting (meth)acrylic acid with epoxy resins such as bisphenol-A type epoxy resins, bisphenol-F type epoxy resins, and novolak-type

epoxy resins, and tris(2-acryloyloxyethyl) isocyanurate.

27. The photoresist composition of claim 21 wherein the photoresist composition

includes a non-photopolymerizable metallic compound added in an amount equal to between

about 0 and about 60% by weight of the photoresist composition, with the metallic compound

selected from the group consisting of: metal carboxylates, metal alkoxides, metal hydroxides,

metal organo chelates, and metal salts.

28. The photoresist composition of claim 27 wherein the non-photopolymerizable

metallic compound is added in an amount equal to between about 0 and about 40% by weight

of the photoresist composition.

29. The photoresist composition of claim 26 wherein the co-monomer is added in an

amount equal to between about 0 and about 50% by weight of the photoresist composition.

30. A method for preparing a photoresist composition, wherein the method is

comprised of:

(a) stirring in a solvent solution an addition polymerizable organotitanium

polymer and a photopolymerization initiator; and,

(b) diluting the composition of step (a) with additional solvent to achieve a

total solids level of about 30 wt.%, thereby forming the photoresist composition.

31. The method of claim 30 wherein the solution is selected from the group

consisting of: alcohols, esters, glymes, ethers, glycol ethers, ketones, and combinations thereof.

32. The method of claim 30 wherein a co-monomer is added to the photoresist

composition.

33. The method of claim 30 wherein a non-addition polymerizable metallic

composition is added to the photoresist composition.

34. A microelectronic device structure comprising a plasma etch resistant,

titanium-containing photoresist layer coated on a substrate wherein the photoresist layer has a

film thickness ranging between about 0.05 microns and about 1.00 microns, with the

microelectronic device structure comprised of:

(a) the substrate selected from the group consisting of ceramic, metal,

polymer, and semiconductor substrates; and,

(b) the photoresist layer, with the photoresist layer comprised of an addition

polymerizable organotitanium polymer added in an amount equal to between about 15% by

weight and about 90% by weight of the photoresist composition and a photopolymerization

initiator that operates at 200-500 nm added in an amount equal to between about 1% and about

20% by weight of the photoresist composition.

35. A method for forming a microelectronic device structure having a titanium

containing metal oxide layer, comprised of:

(a) selecting a substrate material selected from the group consisting of

ceramic, metal, polymer, and semiconductor substrates;

(b) applying a photoresist composition comprised of a photopolymerizable

organotitanium polymer to the substrate by spin coating the photoresist composition onto the

substrate at a speed ranging between 500 and 5000 φm for a time equal to between about 30

seconds and 90 seconds, so that the photoresist is present on the substrate at a thickness equal to between about 0.05 microns and about 1.00 microns, thereby forming a photoresist coated

substrate;

(c) exposing the photoresist coated substrate to ultraviolet radiation passing

through a mask, wherein the ultraviolet radiation is equal to between about 10 mJ/cm² and

about 1000 mJ/cm² to form an exposed photoresist coated substrate;

(d) developing the exposed photoresist coated substrate in a solution selected

from the group consisting of an aqueous alkali solution and an aqueous chelant solution to form

a patterned photoresist; and,

(e) converting the patterned photoresist to an etch resistant metal oxide layer

on the substrate by heating, thereby forming the microelectronic device structure.

36. The method of claim 35 wherein step (e) is achieved by baking the patterned

photoresist in air at a temperature ranging between about 150° C and about 300° C for a time

period ranging between about 15 minutes and about 60 minutes so as to decompose organic

components and leave an inorganic layer that is etch resistant.

37. A plasma etch resistant radiation-sensitive resist composition comprised of:

a) organotitanium polymer or copolymer reaction product produced by reacting:

thereof, with

[ii] addition polymerizable alcohols, carboxyiic acids, beta-diketones,

and, b) a solvent vehicle suitable for obtaining high quality thin films on device substrates

by spin casting.

38. The plasma etch resistant photoresist composition of claim 37 wherein exposure

to a high energy source will crosslink the organotitanium polymer or copolymer by inducing

addition polymerization.

39. The plasma etch resistant photoresist composition of claim 38 wherein the high

energy source is selected from the group consisting of an electron beam, an x-ray source, and

combinations thereof.