METHODS FOR PREPARING THIN FILMS USING SUBSTITUTED PYRROLYL-METAL PRECURSORS

[0001] This patent claims priority to U.S. provisional patent application serial No. 61/083,750 filed on 25 July 2008 and to U.S. provisional patent application serial No. 61/017,399 filed on 28 December 2007. The disclosure of each of the applications identified in this paragraph is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0002] The present invention relates to methods of preparing thin films by atomic layer deposition (ALD) using substituted pyrrolyl-metal precursors.

BACKGROUND OF THE INVENTION

[0003] ALD is a known method for the deposition of thin films. It is a self- limiting, sequential unique film growth technique based on surface reactions that can provide atomic layer control and deposit conformal thin films of materials provided by precursors onto substrates of varying compositions. In ALD, the precursors are separated during the reaction. The first precursor is passed over the substrate producing a monolayer on the substrate. Any excess unreacted precursor is pumped out of the reaction chamber. A second precursor is then passed over the substrate and reacts with the first precursor, forming a monolayer of film on the substrate surface. This cycle is repeated to create a film of desired thickness.

[0004] ALD processes have applications in nanotechnology and fabrication of semiconductor devices such as capacitor electrodes, gate electrodes, adhesive diffusion barriers and integrated circuits. Further, dielectric thin films having high dielectric constants (permittivities) are necessary in many sub-areas of microelectronics and optelectronics. The continual decrease in the size of microelectronics components has increased the need for the use of such dielectric films.

[0005] U.S. Patent No. 7,108,747 to Leskela, et al. report the use of cyclopentadienyl compounds of strontium or barium to grow amorphous oxide thin films by atomic layer epitaxy, also known as ALD.

[0006] Schumann H., et al. report the synthesis and structure of mono-THF adducts of bis(2,5-di-te/t-butylpyrrolyl)calcium and bis(2,5-di-tert-butylpyrrolyl)strontium. Schumann H., et al. Chem Commun. 1999. 2091-2092.

[0007] Current precursors for use in ALD do not provide the required performance to implement new processes for fabrication of next generation devices, such as semiconductors. For example, improved thermal stability, higher volatility, increased deposition rates and a high permittivity are needed.

SUMMARY OF THE INVENTION

[0008] There are now provided methods of preparing a metal oxide thin film by atomic layer deposition. The methods comprise delivering at least one precursor, or an adduct thereof, to a substrate, wherein the at least one precursor corresponds in structure to Formula I:

M[(R) _n pyr*] ₂ (Formula I) wherein:

M is Sr, Ba or Ca;

R is independently Q-Cio-alkyl or Q-Cio-alkoxy; and n is 1, 2, 3 or 4.

[0009] Other embodiments, including particular aspects of the embodiments summarized above, will be evident from the detailed description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 is a representation of the X-ray structure of bis(2,5-di-tert- butylpyrrolyl)barium(THF)2.

[0011] Figure 2 is a graphical representation of thermogravimetric analysis (TGA) data demonstrating % weight loss vs. temperature of bis(2,5-di-tert- butylpyrrolyl) strontium derived from its THF adduct.

DETAILED DESCRIPTION OF THE INVENTION

[0012] In various aspects of the invention, methods are provided that utilize substituted pyrrole-based precursors to form thin metal oxide films. [0013] The methods of the invention are used to create or grow metal oxide thin films which display high dielectric constants. A dielectric thin film as used herein refers to a thin film having a high permittivity.

[0014] As used herein, the term "precursor" refers to an organometallic molecule, complex and/or compound which is delivered to a substrate for deposition to form a thin

film by ALD.

[0015] In a particular embodiment, the precursor may be dissolved in an appropriate hydrocarbon or amine solvent. Appropriate hydrocarbon solvents include, but are not limited to aliphatic hydrocarbons, such as hexane, heptane and nonane; aromatic hydrocarbons, such as toluene and xylene; aliphatic and cyclic ethers, such as diglyme, triglyme and tetraglyme. Examples of appropriate amine solvents include, without limitation, octylamine and N,N-dimethyldodecylamine. For example, the precursor may be dissolved in toluene to yield a 0.05 to IM solution.

[0016] As used herein, the term "adduct" refers to a product formed from an addition reaction between at least (1) a precursor corresponding to Formula I and (2) an appropriate solvent, such as a Lewis base. A Lewis base, as used herein, is any molecule with an electron lone pair in a bonding orbital (also known as an electron donating group). An example of such Lewis base is tetrahydrofuran (THF).

[0017] As used herein, the term "pyr*" refers to a pyrrolyl ligand which is bound to a metal center. All four carbon atoms and the nitrogen atom of the pyrrole ligand can be bound to the metal center in η ⁵ -coordination by π bonding. Alternatively, only the nitrogen atom may be bonded to the metal center by π bonding resulting in incoordination. Other types of coordination bonding may be possible as well. [0018] The term "alkyl" (alone or in combination with another term(s)) refers to a saturated hydrocarbon chain of 1 to about 10 carbon atoms in length, such as, but not limited to, methyl, ethyl, propyl and butyl. The alkyl group may be straight-chain or branched-chain. For example, as used herein, propyl encompasses both w-propyl and iso- propyl; butyl encompasses w-butyl, sec-butyl, zso-butyl and tert-butyl. Further, as used herein, "Me" refers to methyl, and "Et" refers to ethyl.

[0019] The term "alkoxy" (alone or in combination with another term(s)) refers to an alkylether substituent, i.e., -O-alkyl. Examples of such a substituent include methoxy (- O-CH ₃ ), ethoxy, n-propoxy, isopropoxy, n-butoxy, iso-butoxy, sec-butoxy, and tert- butoxy.

[0020] In a first embodiment, a method of preparing a metal oxide thin film by atomic layer deposition is provided. The method comprises delivering at least one precursor, or an adduct thereof, to a substrate, wherein the at least one precursor

corresponds in structure to Formula I:

M[(R) _n pyr*] ₂ (Formula I) wherein:

M is Sr, Ba or Ca;

R is independently Ci-Cio-alkyl or Q-Qo-alkoxy; and n is 1, 2, 3 or 4.

[0021] In a particular embodiment, the at least one precursor according to Formula I or an adduct of the at least one precursor according to Formula I is deposited onto a substrate to form a thin film by ALD.

[0022] Examples of Lewis base solvents which may be used to form an adduct of a precursor according to Formula I include, without limitation, ethers and amines. For example, ethers corresponding to the Formula II: R ₂ O and cyclic ethers, such as THF may be used. Further, amines corresponding to the Formula III: R ₃ N may also be used. In both Formula II and III, R is Q-Qo-alkyl. In a particular embodiment, the solvent utilized to form an adduct of a precursor according to Formula I is THF. [0023] The metal center of the at least one precursor according to Formula I is comprised of an alkaline earth metal, such as barium, strontium and/or calcium. [0024] In one embodiment, the at least one precursor corresponds to Formula I, wherein R is butyl such as w-butyl, sec-butyl, iso-buty\ and tert-butyl. In a particular embodiment, butyl is tert-butyl.

[0025] In another embodiment, the at least one precursor corresponds to Formula I, wherein R is propyl such as w-propyl and iso-propyl. In a particular embodiment, propyl is iso-propyl.

[0026] In another embodiment, the at least one precursor corresponds to Formula I wherein:

M is Sr or Ba;

R is Q-Cio-alkyl; and n is 1, 2 or 3.

[0027] In another embodiment, the at least one precursor corresponds to Formula I wherein:

M is Sr;

R is methyl, ethyl, propyl or butyl; and n is 2 or 3.

[0028] In another embodiment, the at least one precursor corresponds to Formula I wherein:

M is Ba;

R is methyl, ethyl, propyl or butyl; and n is 2 or 3.

[0029] In another embodiment, the at least one precursor corresponds to Formula I wherein:

M is Ca;

R is methyl, ethyl, propyl or butyl; and n is 2 or 3.

[0030] In a further embodiment, an adduct of the at least one precursor corresponding to Formula I is: [0031] The adduct is formed by THF bonding to the metal center through its oxygen

atom as a donor/adduct bond.

[0032] In another embodiment, the method further comprises delivering for deposition at least one volatile titanium precursor and a reactive oxygen species to form a mixed metal oxide thin film, such as BaTiO ₃ , SrTiO ₃ or CaTiO ₃ . The precursors corresponding to Formula I, or adducts thereof, can also be exploited in ALD of other oxide thin films containing barium, strontium and/or calcium.

[0033] Examples of volatile titanium precursors include, without limitation, titanium halide, titanium alkoxide, titanium nitrate, an alkylamino complex of titanium, a cyclopentadienyl complex of titanium, a silylamido complex of titanium, a silylamido complex of titanium, titanium dialkyldithiocarbamate and a titanium-β-diketonate. [0034] In another embodiment, two or more precursors according to Formula I can be used to form a mixed metal oxide film.

[0035] The precursors corresponding to Formula I, or adducts thereof, can be deposited onto the substrate in pulses alternating with pulses of an oxygen source, such as a reactive oxygen species. Examples of such oxygen source include, without limitation, H ₂ O, O ₂ and/or ozone.

[0036] The film can also be formed by the at least one precursor corresponding to Formula I, or adduct thereof, independently or in combination with a co-reactant. Examples of such co-reactants include, but are not limited to hydrogen, hydrogen plasma, oxygen, air, water, H ₂ O ₂ , ammonia, hydrazines, allylhydrazines, boranes, silanes, ozone or any combination thereof.

[0037] A variety of substrates can be used in the methods of the present invention. For example, the precursors according to Formula I, or adducts thereof, may be delivered for deposition on substrates such as, but not limited to, silicon, silicon oxide, silicon nitride, tantalum, tantalum nitride, or copper.

[0038] The ALD methods of the invention encompass various types of ALD processes. For example, in one embodiment conventional ALD is used to form a metal oxide film of the invention. For conventional and/or pulsed injection ALD process see for example, George S. M., et. al. J. Phys. Chem. 1996. 100:13121-13131. [0039] In another embodiment, liquid injection ALD is used to form a ruthenium- containing film, wherein a liquid precursor is delivered to the reaction chamber by direct

liquid injection as opposed to vapor draw by a bubbler. For liquid injection ALD process see, for example, Potter R. J., et. al. Chem. Vap. Deposition. 2005. 11(3): 159. [0040] Examples of liquid injection ALD growth conditions include, but are not limited to:

(1) Substrate temperature: 160-300 ⁰ C on Si(IOO)

(2) Evaporator temperature: about 175 ⁰ C

(3) Reactor pressure: about 5mbar

(4) Solvent: toluene, or any solvent mentioned above

(5) Solution concentration: about 0.05 M

(6) Injection rate: about 2.5μl pulse ^"1 (4 pulses cycle ^"1 )

(7) Inert gas flow rate: about 200 cm ³ min ^"1

(8) Pulse sequence (sec.) (precursor/purge/HiO/purge): will vary according to chamber size. Number of cycles: will vary according to desired film thickness.

[0041] In another embodiment, photo-assisted ALD is used to form a ruthenium- containing film. For photo-assisted ALD processes see, for example, U.S. Patent No. 4,581,249.

[0042] Thus, the organometallic precursors according to Formula I, or adducts thereof, utilized in these methods may be liquid, solid, or gaseous. Particularly, the precursors or adducts thereof are liquid at ambient temperatures with high vapor pressure for consistent transport of the vapor to the process chamber.

[0043] In particular embodiments, the method of the invention is utilized for applications such as dynamic random access memory (DRAM) and complementary metal oxide semi-conductor (CMOS) for memory and logic applications, such as on silicon chips.

[0044] ALD relies substantially on chemical reactivity and not thermal decomposition. Therefore, there are fundamental differences in the characteristics desirable for a suitable precursor (or adduct thereof). The precursor must be thermally stable at the temperatures employed and should be sufficiently volatile to allow deposition onto the substrate or sufficiently soluble in a desired solvent if the process of d.l.i. (direct liquid injection) is to be used. Further, when depositing a metal oxide film, a

fast and complete chemical reaction is necessary between the metal precursor and the oxide source. However, the reaction should only take place at the substrate surface so as not to damage the underlying structure, and by-products, such as carbon and nitrogen species, should be removed readily from the surface.

[0045] Surprisingly, it has been discovered that a substituted pyrrolyl ligand metal complex demonstrates useful and improved properties for ALD processes, such as a high permittivity. Compounds comprising substituted pyrrolyl metal complexes have never been considered for oxide growth applications. The precursors corresponding to Formula I, or adducts thereof, provide an increased ability to deposit metal oxide films by ALD.

EXAMPLES

[0046] The following examples are merely illustrative, and do not limit this disclosure in any way. Example 1 - Synthesis of 2,5-di-tert-butylpyrrole

-70 ⁰ C- »-r t

3,3-Diniethyl-2-butanone (pinacolone) 2,2,7,7-tetramethyl-octan-3,6-dione 2, 5-Di-tert-butyl -pyrrole

CAS# 75-97-8 CAS # 27610-88-4 CAS# 3760-56-3

Product * P45605 Product * X43093 Product * Q39456

[0047] To a solution of N, N-diisopropylamine (243.8 mL, 1.738 mol) in THF (1.77 L) n-butyllithium (2.57 M in hexanes, 677 mL, 1.74 mol) was added dropwise at -70 ⁰ C. The reaction mixture was allowed to stir for ~ 2h and 3,3-dimethyl-2-butanone (200 mL, 1.58 mol) was added dropwise. The reaction mixture was stirred for another ~ 30 min and the solution of copper (II) chloride (238.4 g, 1.738 mol) in DMF (1.77 L) was added dropwise. The reaction mixture was stirred for ~ 30 min at ~ -75 ⁰ C and then allowed to warm up to room temperature. The reaction mixture was cooled to ~ -5 ⁰ C and HCl (1.0 M aqueous, 3.5 L, 3.5 mol) was added dropwise. Methyl t-butyl ether (3.5 L) was added then and the reaction mixture was stirred for 40 min. The organic was separated and the aqueous layer was extracted with methyl t-butyl ether (3.5 L). The organic layer was

separated and the combined organic extracts were washed with brine (3.5 L X 2). The organic layer was separated, dried (Na ₂ SO ₄ ) and concentrated to give 2,2,7,7-tetramethyl- octane-3,6-dione (131.5 g, 94% purity by GC, 79 % yield, CAS # 27610-88-4). (/. Am.

Chem. Soc. 1975. 97:2912-2914.)

[0048] Mixture of the crude 2,2,7,7-tetramethyl-octane-3,6-dione (109.9 g, 0.521 mol), ammonium acetate (80.6 g, 1.04 mol) and acetic acid (0.550 L, 9.60 mol) was refluxed for 2 hours and then allowed to cool down to room temperature (inert atmosphere of nitrogen was not required for this reaction). The reaction mixture was poured onto water (2.0 L, 110 mol) and extracted with methyl t-butyl ether (1.0 L, 8.4 mol) (X2). The combined organic layers were washed twice with sodium hydroxide solution (2.0 M aqueous, 2.00 L), dried (Na ₂ SO ₄ ) and concentrated. Purification by vacuum distillation gave 2,5-di-tert-butyl-pyrrole (81.5255 g, ~ 96 % purity by GC, 84 % yield). Spectral and physical properties matched those reported in literature. (/.

Organomet. Chem. 1992. 440:289-296. Tetrahedron. 1979. 35:687-689.)

Example 2 - Synthesis of 2,5- di-tert-butyl-3-methyl-pyrrole

[0049] To a mixture of 2,5-di-tert-butyl-pyrrole (2.00 g, 0.0112 mol) and aluminum trichloride (3.0 g, 0.022 mol) methyl iodide (3.0 mL, 0.048 mol) was added dropwise.

Reaction mixture was refluxed for 3.5 h. GC showed 77% conversion. Reaction mixture was allowed to cool down to room temperature, then diluted with methyl t-butyl ether (10 mL) and quenched slowly with water (10.0 mL). Organic layer was diluted with MTBE

(10 mL) and separated. Aqueous layer was extracted with methyl t-butyl ether (20 mL).

Combined organic extracts were washed with NaOH solution (2.0 M aqueous, 20 mL), dried (Na ₂ SO ₄ ) and concentrated.

Example 3 - Synthesis of bis(2,5-di-tert-butylpyrrolyl)M(THF), wherein M is Sr, Ba or Ca

[0050] The sodium or potassium salt of bis(2,5-di-tert-butylpyrrolyl) is reacted with strontium diiodide, barium diiodide or calcum diiodide in THF to yield mono-THF adducts of either bis(2,5-di-tert-butylpyrrolyl)strontium, bis(2,5-di-tert- butylpyrrolyl)barium or bis(2,5-di-tert-butylpyrrolyl)calcium. See Schumann H., et al.

Chem Commun. 1999. 2091-2092.

[0051] The NMR spectra of these compounds is exemplified by that of bis(2,5-di-

tert-butylpyrrolyl) strontium, derived from its THF adduct compound by heating at 120 ⁰ C under vacuum (0.1 Torr) for 2 hours in a cold finger sublimation apparatus. Adduct free compound was obtained by sublimation starting at 200-240 ⁰ C. Due to its insolubility in non-polar solvents, the NMR of bis(2,5-di-tert-butylpyrrolyl)strontium was carried out in CD ₂ Cl ₂ : 6.05 ppm (s, CH, 4H), 1.25 ppm (s, 18H, Me ₃ ), small trace of free 2,5-di-tert- butylpyrrole (5.85, 1.05 ppm).

[0052] Figure 1 illustrates, as an example of these compounds, the X-ray structure of bis(2,5-di-tert-butylpyrrolyl)barium(THF) ₂ . The results indicate that, whereas the structures of both the calcium and strontium analogues show only one coordinated THF molecule, the barium analogue has two coordinated THF molecules, presumably due to its larger size.

Example 4 - ALD using bis(2,5-di-tert-butylpyrrolyl)Sr(THF) to form SrTiO ₃ layer [0053] Strontium titanium oxide thin films are deposited in a custom-built ALD reactor. Bis(2,5-di-tert-butylpyrrolyl)Sr(THF), titanium tetraisopropoxide (Ti(O ¹ C ₃ Hv) ₄ ) and ozone are used as precursors. The strontium titanium oxide films are deposited on silicon wafer substrates. Prior to deposition, the wafer substrates are prepared by dicing the wafer (linch x ¹ A inch), and 1% HF polished.

[0054] The growth temperature is about 300 ⁰ C. The growth pressure is about 250 milliTorr. The reactor is continuously purged with 30 seem of dry nitrogen. [0055] The growth of SrTiO ₃ is implemented by using alternate Ti-O and Sr-O deposition cycles. The Ti-O cycle is made up of four steps: (i) pulse of Ti(O ¹ C ₃ Hv) ₄ , (ii) purge with inert nitrogen gas, (iii) pulse of ozone, and (iv) purge with nitrogen gas. Similarly, the Sr-O cycle is made up of four steps: (i) pulse of bis(2,5-di-tert- butylpyrrolyl)Sr(THF), (ii) purge with nitrogen gas, (iii) pulse of H ₂ O, and (iv) purge with nitrogen gas.

[0056] The composition of the film is controlled by the ratio of the Ti-O and Sr-O cycles. The alternation of the Ti-O and Sr-O cycles is implemented so that there are at maximum two similar cycles in succession. For example, the cycle ratio Ti—O/Sr— 0=1:1 is achieved by repeating the cycling formula q[(Ti— O)(Sr-O)] and the ratio Ti- O/Sr — O = 3:4 by means of the formula q[(Ti-O)(Sr-O)(Ti-O)(Sr-O)(Ti-O)(Sr-O)(Sr-O)], in which formulae q indicates how many times the said cycling was repeated. Thus q

determines the thickness of the film.

[0057] The total amount of cycles is typically 300.

Example 5 - TGA of bis(2,5-di-tert-butylpyrrolyl)strontium

[0058] Bis(2,5-di-tert-butylpyrrolyl)strontium derived from its THF adduct was heated under vacuum (0.1 Torr) in a cold finger sublimation apparatus. As illustrated in

Figure 2, the TGA shows only one step, as expected, starting at 200-240 ⁰ C and leaving

4.4% residue. This indicates that there is no THF left coordinated to the strontium. The quantity of residue was similar to that found from the THF adduct material Bis(2,5-di- tert-butylpyrrolyl)strontium(THF).

[0059] All patents and publications cited herein are incorporated by reference into this application in their entirety.

[0060] The words "comprise", "comprises", and "comprising" are to be interpreted inclusively rather than exclusively.