US 3911176 A
A film of lithium niobate is formed on a substrate by vaporizing a lithium compound, vaporizing a niobium compound, and bringing the resultant vapors in contact with the heated substrate in an oxidizing atmosphere.
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United States Patent 11 1 1111 3,91 1,176
Curtis et al. Oct. 7, 1975 METHOD FOR VAPOR-PHASE GROWTH 3,047,424 7/1962 Suchoff 117/169 R 0 THIN FILMS OF LITHIUM NIOBATE 3,649,501 3/1972 Sadagopan 204/192 Inventors: Bernard John Curtis, Gattikon;
f runner Primary ExaminerWilliam D. Martin g both of Assistant Examinerlanyce A. Bell wltzer an Attorney, Agent, or FirmGlenn H. Bruestle; Birgit E.  Assignee: RCA Corporation, New York, NY. Morris  Filed: Jan. 2, 1974  App]. No.2 430,211
 ABSTRACT  US. Cl. 427/248; 427/; 427/162;
427 1 4; 427 377 A film of lithium niobate is formed on a substrate by 51 1m. (:1. C23C 11/08 vaporizing a lithium Compound, vaporizing a niobium  Field of Search 117/62, 107.2 R, R, Compound. and bringing the resultant vapors in 117/225 223 1 9 R, 10 R contact with the heated substrate in an oxidizing atmosp'nere.  References Cited UNITED STATES PATENTS 15 Claims, 2 Drawing Figures 3,018,194 l/l962 Norman et a1. 117/107.2 R
US. Patent METHOD FOR VAPOR-PHASE GROWTH OF THIN FILMS OF LITHIUM NIOBATE FIELD OF THE INVENTION This invention relates to a method for depositing a film of lithium niobate on a substrate by chemical vapor deposition. More particularly, this invention relates to the vapor phase deposition of lithium niobate from volatile lithium compounds and volatile niobium compounds.
BACKGROUND OF THE INVENTION It is well known that lithium niobate (LiNbO is an excellent piezoelectric, electro-optic and nonlinear optic material. Several techniques for the deposition of LiNbO films have been tried with varying success.
A successful technique for the deposition of LiNbO is sputtering. Foster, J. of Appl. Phys., 40 420 (I969) discloses the deposition of a LiNbO film by triode sputtering in an argon-oxygen gas mixture containing 510% oxygen. Fused quartz or sapphire substrates covered with thin layers of chrome and gold were utilized. Using a lithium niobate cathode and substrate temperatures between 100 and 325 C, both polycrystalline and oriented films of lithium niobate were produced.
US. Pat. No. 3,649,501 discloses a method of radiofrequency sputtering thin, single crystal films of lithium niobate. The sputtering process was carried out at substrate temperatures in a range of 450 C to 870 C in an argon atmosphere.
Miyazawa, App. Phys. Lett., 23 199 (1973) describes a method for growing single crystal films of LiNbO on a LiTaO- substrate by an epitaXial-growth-by-melting (EGM) method.
K. K. Winslow, et al., Tech. Rept. RADC-TR-67-635, l (l968) reports repeated failures in attempts to de posit LiNbO by direct vacuum evaporation. They encountered a continuing problem of dissociation of the LiNbO when the LiNbO was heated to its evaporation temperature.
A disadvantage of sputtering a LiNbO film is that the growth temperature of the film is limited since the sticking coefficient of the particles incident on the substrate decreases with increasing substrate temperature. Also, controlled doping of the deposited film can be difficult. Chemical vapor deposition (CVD) is a standard technique used to grow films at temperatures above those normally used in sputtering. With CVD it is possible to obtain better crystal quality than that obtainable by sputtering due to the increased surface mobility of the deposited species at high temperatures. To date there has been no report of a successful deposition of a film of LiNbO by CVD. I
The instant process is a method for chemical vapor deposition of LiNbO which overcomes the problems encountered in the prior art.
SUMMARY OF THE INVENTION It has been discovered that thin, uniform, transparent, electro-optic films of lithium niobate can be deposited on a substrate by vaporizing a volatile lithium compound, vaporizing a volatile niobium compound and contacting the substrate with the resultant vapors in the presence of an oxidizing atmosphere at an elevated temperature.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a sectional view of an apparatus suitable for carrying out the process of the invention.
FIG. 2 shows a sectional view of another apparatus suitable for carrying out the process of the invention.
DETAILED DESCRIPTION OF THE INVENTION The physical properties necessary for a lithium compound to be useful in this process include a vapor pressure greater than 1 mm at a temperature of less than 500 C and thermal stability up to a temperature of about 400 C. Volatile lithium compounds useful in the present process include lithium chelates of fl-diketones. These compounds have the general structure where R R are selected from the group consisting of hydrogen and straight and branched chain alkyl groups.
The lithium chelate of 2,2,6,6-tetramethyl-3,5- heptanedione, hereinafter Li(thd), is particularly preferred for the present process. Li(thd) is a volatile compound having a vapor pressure of about 15 mm at 300 C and is thermally stable up to about 400 C. Li(thd) can be prepared by reacting the ligand, 2,2,6,6- tetramethyl-3,S-heptanedione (thd), sodium hydroxide and lithium chloride in a 50 per cent alcohol/water solution for about 3 hours, as shown by the following chemical equation:
H(thd) LiCl NaOH- Li(thd)l NaCl H O After filtration, the Li(thd) is dried and purified by vacuum sublimation.
The physical properties necessary for a niobium compound to be useful in this process include a vapor pressure greater than 1 mm at a temperature of less than 500 C and thermal stability up to about 400 C. Useful volatile niobium compounds include niobium alkoxides. Niobium methoxide is particularly preferred for this process. Niobium methoxide has a vapor pressure of 15 mm at 200 C and is thermally stable up to 400 C.
Substrates suitable for use in this process must be thermally stable up to about l,000 C. Most common substrates used in electro-optic technology can be used including single crystal and polycrystalline substrates. Suitable substrates include platinum foil, yttrium aluminum garnet, spinel (orientation 111), sapphire (orientation 0001, 1124 and 1102) and lithium tantalate.
The reaction may be carried out in a wide variety of apparatus and under varying reaction conditions. A carrier gas may be used or the reaction may take place at reduced pressure. Reaction temperatures, pressures and times may be varied.
An apparatus for depositing a LiNbO film on a substrate is shown in FIG. 1. Referring to FIG. 1, a carrier gas is introduced into a reaction chamber 10 via an inlet 12. The carrier gas is an inert gas such as nitrogen, argon, helium, krypton, xenon, etc. A volatile niobium compound is placed in an open container 14 in the reaction chamber 10 in the stream of the carrier gas. A volatile lithium compound is placed in an open container 16 in the reaction chamber 10 in the stream of the carrier gas. The lithium and niobium compounds are vaporized by heating the compounds to a temperature of about 100 C to 300 C.
An oxidizing gas, such as pure oxygen, is admitted into the reaction chamber 10 and into the carrier gas stream via an inlet 18. The lithium vapor, niobium vapor and oxidizing gas are transported by the carrier gas stream to a substrate 20 placed in the reaction chamber 10. The substrate 20 is heated to a temperature of at least about 400 C. After a reaction time from about 1 to 5 hours, a shiny black film 22 is deposited on the substrate 20. The exhaust vapor exits from the reaction chamber 10 via an outlet 24.
The carrier gas is then turned off and the substrate 20 and the film 22 are annealed at a temperature of about 700 to l,0O C for l to 10 hours in an oxidizing atmosphere created by the continued admission of the oxidizing gas via the inlet 18. Excess oxidizing gas and gases coming off from the film 22 exit from the reaction chamber 10 via an outlet 24. After annealing, the film 22 formed on the substrate 20 is composed of lithium niobate.
Referring now to FIG. 2, illustrating another apparatus for performing this process, an open container 26 holding the niobium compound and an open container 28 holding the lithium compound are placed in a closed reaction chamber 30. The reaction chamber 30 is evacuated via an outlet 32 connected to a vacuum system (not shown). After the reaction chamber 30 has been evacuated an oxidizing gas is introduced into the reaction chamber 30 via an inlet 34. The lithium and niobium compounds are vaporized by heating the compounds to a temperature of about 100 to 300 C. The lithium vapors and the niobium vapors react at the surface of a substrate 36. The substrate 36 is heated to a temperature of at least about 400 C. After a time of about 1 to hours a thin film 38 of LiNbO is formed on the substrate 36.
The lithium niobatefilm may be doped with other metals, such as iron, by vaporizing a suitable volatile compound of the desired metal, such as Fe(thd) in the reaction chamber concurrently with the vaporization of the lithium and niobium compounds.
The invention will be further illustrated by the following examples, but it is to be understood that the invention is not meant to be limited to the details described therein.
EXAMPLE 1 A single crystal of lithium tantalate (LiTaO was coated with a thin film of LiNbO in an apparatus as shown in FIG. 1. An argon carrier gas flowing at a rate of 1,900 cc/minute was passed over a container of lithium 2,2,6,6-tetramethyl-3,S-heptanedione and a container of niobium methoxide (Nb(OMe) The Nb(O- Me) and the Li(thd) were volatilized by heating their containers to 200 C and 300 C respectively. Oxygen gas was added to the gas mixture at a rate of cc/minute. The substrate was heated to 450 C. After reacting the substrate with the gas mixture for 2 hours, a black film formed on the substrate. Without removing the substrate from the furnace, the argon carrier gas was turned off and the film on the substrate was annealed in pure oxygen at 900 C. After 4 hours the black film formed a single crystal of lithium niobate which was aligned with the LiTaO crystal.
EXAMPLE 2 A single crystal sapphire substrate was coated with a film of LiNbO in an apparatus as shown in FIG. 2. The reaction chamber was placed in a multi-zone furnace. The reaction chamber was evacuated to a pressure of 5 X 10 mm. Lithium 2,2,6,6-tetramethyl-3,5- heptanedione was vaporized by heating it to 200 C. Niobium methoxide was vaporized by heating it to 220 C. Oxygen at a pressure of 3 mm was introduced into the reaction chamber. The substrate was heated to 700 C. After 20 hours a 5 micron thick film of LiNbO formed on the sapphire substrate surface.
What is claimed is:
1. A process of depositing a film of lithium niobate on a substrate, said substrate being thermally stable up to about l,000C, which comprises:
a. vaporizing a lithium compound having a vapor pressure greater than 1 millimeter at a temperature of less than 500C and a stable gas phase up to 400C,
b. vaporizing a niobium compound having a vapor pressure greater than 1 millimeter at a temperature of less than 500C and a stable gas phase up to 400C, and
c. contacting said substrate with the vapor of said lithium compound and the vapor of said niobium compound in an oxidizing atmosphere at a temperature of at least about 400C.
2. A process according to claim 1 wherein the lithium compound is an organolithium compound.
3. A process according to claim 2 wherein the organolithium compound is a lithium chelate of a B-diketone of the formula where R, R R R R and R independently are selected from the group consisting of hydrogen and straight and branched chain alkyl groups.
4. A process according to claim 3 wherein the lithium chelate is the lithium chelate of 2,2,6,6-tetramethyl- 3,5-heptanedione.
5. A process according to claim 1, wherein the niobium compound is an organoniobium compound.
6. A process according to claim 5 wherein the organoniobium compound is a niobium alkoxide.
7. A process according to claim 6 wherein the niobium alkoxide is niobium methoxide.
8. A process according to claim 3 wherein the lithium chelate is vaporized by heating the chelate to a temperature of between about to 300 C.
9. A process according to claim 6 wherein the niobium alkoxide is vaporized by heating the niobium alkoxide to a tempereature of between about 100 to 300 C.
10. A process according to claim 1 wherein the substrate is annealed in an oxidizing atmosphere at a temperature of between about 700 to l,00O C for a period of time of between about 1 to hours after contact with the lithium compound vapor and the niobium compound vapor.
11. A process according to claim 1 wherein the lithium compound vapor and the niobium compound vapor are transported to the substrate in a stream of an inert carrier gas.
12. A process according to claim 1 wherein the oxidizing atmosphere is obtained by an oxygen-containing gas.
13. A process according to claim 1 wherein the process is carried out in an evacuated chamber.
14. A process of depositing a film of lithium niobate on a substrate, said substrate being thermally stable up to about l,000C, which comprises:
a. vaporizing the lithium chelate of 2,2,6,6-
tetramethyl-3,S-heptanedione at a temperature of from about 100 to 300 C in a stream of an inert carrier gas,
b. vaporizing niobium methoxide at a temperature of from about 100 C to 300 C,
c. heating the substrate to a temperature of at least about 400 C,
d. contacting the lithium chelate vapor and the niobium methoxide vapor with the substrate in an oxidizing atmosphere so as to deposit a film on the substrate, and
e. annealing the substrate and the deposited film at a temperature of about 700 to 1,000 C for a time of about 1 to 10 hours in an oxidizing atmosphere.
15. A process of depositing a film of lithium niobate on a substrate, said substrate being thermally stable up to about 1,000C, in an evacuated chamber which comprises:
a. vaporizing the lithium chelate of 2,2,6,6-
tetramethy1-3,S-heptanedione at a temperature of from about to 300 C,
b. vaporizing niobium methoxide at a temperature of from about 100 to 300 C,
c. heating the substrate to a temperature of at least 400 C, and
d. contacting the substrate with the lithium chelate vapor and the niobium methoxide vapor in an oxidizing atmosphere for about 1 to 20 hours.
UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,911,176
DATED October 7, 1975 I INVENTOWS) 1 Bernard John Curtis et al.
It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:
Column 2 line 20 and 132 3 Column 4 line 40 change O C Li CH O CII CR R R O C Li CH CR R R Column 2, line 57 change "1102" to 1152 o Signed and Scaled this tenth D3) Of February 1976 [SEAL] Arrest: O
RUTH c. MASON c. MARSHALL DANN Anesrmg Officer Commissioner uflalems and Trademarks