JP2003313094A - METHOD FOR DEPOSITING LiNbO3 THIN FILM - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for depositing a LiNbO<SB>3</SB>thin film, by which the sufficiently crystallized and highly oriented LiNbO<SB>3</SB>thin film having a large surface can be deposited on a single crystal substrate without subjecting the deposited thin film to recrystallization by heat treatment. <P>SOLUTION: In a LiNbO<SB>3</SB>thin film deposition method by an electron cyclotron resonance sputtering process using a LiNbO<SB>3</SB>target having a constant composition, the LiNbO<SB>3</SB>thin film is deposited on the single crystal substrate 101 by using a thin film deposition system constituted of a microwave output section 1, a plasma source section 2, a film deposition chamber 3, an RF bias section 4, and a turbo-molecular pump 5 and supplying oxygen under a condition that the temperature of the single crystal substrate 101 is ≥450 and ≤600°C. Thereby, the LiNbO<SB>3</SB>thin film free from deficits in Li atoms or oxygen atoms and having high crystallinity and orientation can be formed on the single crystal substrate 101. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a LiNbO ₃ thin film forming method for forming a LiNbO ₃ optical thin film used in optical waveguides and functional optical parts.

[0002]

2. Description of the Related Art LiNbO ₃ (lithium niobate) is a widely used optical material because it is transparent and exhibits various useful properties such as nonlinear effect, electro-optical effect, and optical modulation effect. In most of its applications, bulk single crystals are used from the viewpoint of composition stability, crystal perfection, crystal size, and the like. On the other hand, attempts to deposit a LiNbO ₃ thin film on a heterogeneous substrate to substitute for a bulk crystal or to use it as an optical waveguide have been made for a long time.

As a method for producing a LiNbO ₃ thin film, an RF sputtering method, a magnetron sputtering method, a sol
-Gel method, metalorganic vapor phase epitaxy method, laser sputtering method and the like can be mentioned. However, since the quality of the film does not reach that of the bulk crystal, the LiNbO ₃ thin film was substantially at the research level.

[0004]

The above-described method for producing a LiNbO ₃ thin film has the following problems. That is,
In many cases, such as RF sputtering method and magnetron sputtering method, in the deposited LiNbO ₃ film,
Li atom loss occurs. For this reason, it was necessary to use a target in which Li ₂ O and LiNbO ₃ were mixed according to the film forming conditions of the apparatus. In this case, unless a target with the optimum composition is prepared, a Li-deficient thin film (LiNbO ₃ and LiNb ₃ O ₈
Mixture) and a Li-excessive thin film (a mixture of LiNbO ₃ and Li ₃ NbO ₈ ) were obtained, and it was difficult to obtain a thin film composed only of constant composition LiNbO ₃ .

Further, in the RF sputtering method and the magnetron sputtering method, it is necessary to place the substrate at a position deviated from the center of the ion source in order to prevent the collapse of the composition due to the resputtering of the thin film by the high energy O ⁻ ions. For this reason, there is a problem in that deposition is forced at a position where the ion flux is low, and a high deposition rate of the thin film cannot be obtained.

In the sol-gel method, LiNbO having a certain thickness is used.
₃ After depositing the amorphous film on the substrate by solution reaction, the recrystallization process by heat treatment is indispensable, so that there is a problem in crystallinity. In the metal-organic vapor phase epitaxy method,
In addition to the necessity of the recrystallization process, there is a problem that organic molecules are mixed in the film. In the laser sputtering method, there are problems that the area of a film-forming region is small and foreign matter adheres to the substrate.

Further, in those methods, it is basically necessary to make a lattice match between the thin film and the substrate material.
Without a sapphire substrate, it was difficult to obtain a thin film with good orientation. Therefore, even if it is practically useful, Si whose lattice constant and lattice shape do not match
Since an amorphous film was deposited on the substrate, it was difficult to obtain an oriented thin film.

The present invention has solved the above-mentioned problems and has a sufficiently crystalline highly oriented Li without the need for recrystallization by heat treatment.
An object of the present invention is to provide a method for forming a LiNbO ₃ thin film, which makes it possible to form an NbO ₃ thin film on a single crystal substrate in a large area.

[0009]

In order to solve the above-mentioned problems, the invention of claim 1 uses an electron cyclotron resonance sputtering method using a LiNbO ₃ target having a constant composition.
In the LiNbO ₃ thin film forming method, the temperature of the single crystal substrate is 4
In the method of forming a LiNbO ₃ thin film, oxygen is supplied at a temperature of 50 ° C. or higher and 600 ° C. or lower to form a LiNbO ₃ thin film on the single crystal substrate.

The invention of claim 2 is the LiNb according to claim 1.
In the method of forming an O ₃ thin film, the supply of oxygen is LiNb ₃
It is characterized in that the flow rate is lower than that in which the O ₈ phase is generated, and that the flow rate is sufficient to keep the target surface covered with a certain amount of oxygen atoms.

The electron cyclotron resonance (ECR) sputtering method has an ion flow energy of 10-30 eV.
Despite its low level, it has a high electron density, so a high-density plasma can be obtained. Therefore, the problem that the composition is remarkably collapsed by the re-sputtering of the light element due to the high energy ion bombardment, which is remarkable in the RF sputtering, does not occur. Rather, a dense and dense thin film is formed by the effect of energy transfer to the substrate surface brought about by ion flow irradiation of about 10-30 eV.

Since a high-purity LiNbO ₃ bulk material is used as the target, mixing of organic molecules into the film, which is a problem in the sol-gel method or MOCVD method, is basically impossible. By selecting appropriate film formation conditions, Li
The composition of the NbO ₃ thin film can be precisely controlled.

[0013]

Embodiments of the present invention will now be described in detail with reference to the drawings. The LiNbO ₃ thin film forming method according to the present embodiment is performed by the ECR sputtering method in the thin film forming apparatus shown in FIG. The thin film forming apparatus shown in FIG. 1 includes a microwave output unit 1, a plasma source unit 2, a film forming chamber 3, an RF bias unit 4, and a turbo molecular pump 5.

The microwave output section 1 includes a waveguide section 11, a coil 12, a connecting section 13, and a quartz window 14. The microwave output section 1 outputs the 2.45 GHz microwave applied to the waveguide section 11 to the plasma source section 2 through the quartz window 14.

The plasma source section 2 includes a container portion 21 and a coil 22. The plasma source unit 2 includes a connecting portion 1
It is connected to the microwave output section 1 by 3. Moreover, Ar gas or Xe gas is supplied to the plasma source unit 2. The plasma source unit 2 generates plasma and adds the plasma to the film forming chamber 3.

The film forming chamber 3 is provided with a container portion 31, a LiNbO ₃ target 32, and a table 33. The container portion 31 of the film forming chamber 3 is connected to the container portion 21 of the plasma source unit 2. O ₂ gas is supplied to the film forming chamber 3. The film forming chamber 3 includes Li, Nb, and O from the LiNbO ₃ target 32 generated by the plasma from the plasma source unit 2.
Depending on the atoms, LiNbO _{3 can be} added to the substrate 101 of the table 33.
Form a thin film. In addition, the table 33 is a container portion 31.
It rotates about an axis 112 that is inclined with respect to the central axis 111. As a result, when a LiNbO ₃ thin film is formed on the substrate 101, the thin film has a uniform thickness.

The RF bias unit 4 applies a high frequency bias to the LiNbO ₃ target 32 in the film forming chamber 3. The turbo molecular pump 5 evacuates a chamber formed by the connecting portion 13 of the microwave output unit 1 and the quartz window 14, the container portion 21 of the plasma source portion 2 and the container portion 31 of the film forming chamber 3.

With this thin film forming apparatus, the sputtering gas such as Ar or Xe introduced into the plasma source section 2 which is the ECR plasma source is ionized by the discharge of the microwave of 2.45 GHz from the microwave output section 1,
High-density plasma is generated in the ECR region 201. When an RF voltage is applied from the RF bias unit 4 to the LiNbO ₃ target 32 in the film forming chamber 3, the generated Ar and Xe ions are LiNb.
It collides with the O ₃ target 32, and Li, Nb, and O atoms are sputtered. When oxygen gas is introduced into the film forming chamber 3, these also become a highly excited state or ions, and oxygen atoms also adhere to the substrate 101 together with the sputtered particles, and a thin film is formed on the substrate 101 which is a single crystal substrate on the table 33. To be done.

In this embodiment, the substrate 101 is heated from behind the substrate 101 by a heater. The substrate temperature must be 45 because the amorphous component is not contained in the thin film.
0 ° C or higher is required. On the other hand, if the substrate temperature is too high, the orientation deteriorates and polycrystals are likely to occur.
Must be kept below. Especially, if the substrate temperature is 500
By setting the temperature to 550 ° C., it is possible to more stably form a highly oriented film having high crystallinity.

FIG. 2 shows an argon gas flow rate of 8 sccm and oxygen gas.
Flow rate 1 sccm, RF power 500W, microwave power 5
LiNbO deposited on Si (100) substrate at 00W_ThreeThin film
The X-ray-diffraction spectrum by CuK (alpha) ray is shown. Base during film formation
The plate temperature is 530 ° C. <0 from Si (100) substrate
04> Reflection (69 °), CuK not removed by spectroscope
Other than <004> reflection (61.7 °) due to β rays, <0
01> Oriented LiNbO _Three<006> reflection from thin film (3
8.9 °), <0012> Only reflection (83.5 °) is observed
Has been done.

This indicates that the obtained thin film has a single phase of LiNbO ₃ and is strongly oriented in the <001> direction. Thus, the formation of a highly oriented LiNbO ₃ thin film on a semiconductor substrate was made possible for the first time by the present invention. Even when grown on a Si (111) substrate under the same film forming conditions, <006> reflection (3) from the LiNbO ₃ thin film was observed.
Only 8.9 °) and <00 12 > reflection (83.5 °) are observed. Even if another single crystal substrate is used, <0 similarly.
A LiNbO ₃ thin film oriented in the 01> direction is obtained.

The film formation rate is shown to depend on the oxygen gas pressure in FIG.
As described above, the lower the oxygen gas flow rate, the larger the flow rate. this
When the oxygen gas pressure is high, the dissociated and ionized oxygen
LiNbO _ThreeStrongly covers the target surface of the target 32
However, the sputtering of Li and Nb atoms is suppressed. acid
The effect of elementary gas pressure also affects the composition of the deposited thin film.
It The X-ray diffraction spectrum of Fig. 4 shows only the oxygen gas flow rate.
Change to 3sccm, otherwise under the same conditions as the sample preparation in Figure 2.
It is from the film-formed sample. LiNbO_ThreePhase 3
LiNb with <602> orientation accompanied by a peak at 8.9 °
_ThreeO₈A peak from the phase appears at 38 °. this child
Means that Li atoms attached to the substrate become Li due to excess oxygen partial pressure.
_TwoLiNb that is partially desorbed as O and lacks Li_ThreeO₈Phase crystallizes
It means that you are doing. Also, some other
Diffraction peaks are observed, and the orientation is better than that of the sample in Figure 1.
It shows that the sex is inferior.

Therefore, in order to form a thin film composed of only the desired LiNbO ₃ phase, the oxygen flow rate must be below a certain value.

On the other hand, FIG. 5 shows the X-ray diffraction spectrum (oxygen flow rate: 0 sccm) of the thin film deposited without introducing any oxygen gas. In this case, since oxygen atoms are not supplied to the target at all, oxygen is insufficient in the vicinity of the surface of the target. As a result, the thin film deposited on the substrate has an oxygen-deficient composition. In fact, since the diffraction intensity at 38.9 ° is reduced to about 1/5 of that of FIG. 1, it is considered that the volume of the LiNbO ₃ crystal phase is also reduced to the same degree. Also, a,
Extra peaks such as b, c, d, e, and f appear, and it is considered that an amorphous compound containing Li and Nb atoms in excess is deposited on the substrate.

From this, it is understood that there is a minimum oxygen flow rate in order to carry out sputtering while keeping the amount of oxygen on the target surface at a constant value.

Further, if the oxygen partial pressure is too low, the optical absorption of the thin film becomes large, which causes a problem in application to an optical waveguide. Further, the oxygen-deficient thin film may adhere to the inner wall of the chamber, and these may serve as a dust source.

As described above, in order to obtain a thin film having only highly oriented LiNbO ₃ crystal phase, there is an optimum value of oxygen flow rate.
That is, the flow rate is lower than that in which the LiNb ₃ O ₈ phase is generated, and the flow rate is sufficient to keep the target surface covered with a certain amount of oxygen atoms. In the apparatus in which the samples shown in FIGS. 2, 4, and 5 were produced, the optimum value was 0.5 to 1 sccm, which was determined by the size of the film forming chamber, the pumping speed of the pump, the plasma source and the film forming chamber. It is determined by the conductance, microwave power, RF power, etc., and is a value peculiar to the device.

Although the embodiment of the present invention has been described in detail above, the specific configuration is not limited to this embodiment, and there are design changes and the like within the scope not departing from the gist of the present invention. Are also included in the present invention.

[0029]

As described above, according to the present invention,
LiNbO of success_ThreeECR sputtering using a target
By law, LiNbO_ThreeSingle crystal substrate when forming a thin film
Oxygen is supplied at a temperature of 450 ° C to 600 ° C
Then, on the single crystal substrate, LiNbO_ThreeForm a thin film. Also,
The oxygen supply is LiNb_ThreeO₈Lower than the phase occurs, or
The target surface covered with a certain amount of oxygen atoms.
Make sure the flow rate is sufficient to maintain. This allows Li atoms and acids
Highly oriented LiNbO with high crystallinity without defects of elementary atoms _ThreeThin
The film can be formed on a single crystal substrate.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a configuration of a thin film forming apparatus for carrying out an embodiment of the present invention.

[Fig. 2] <001> oriented LiNb on Si (100) substrate
O ₃ is a diagram showing an X-ray diffraction spectrum of the thin film.

FIG. 3 is a diagram showing the oxygen flow rate dependency of a film formation rate.

[Fig. 4] <001> -oriented LiNb on Si (100) substrate
O ₃ phase and <602> is a diagram showing an X-ray diffraction spectrum of the oriented LiNb ₃ thin O ₈ phase are mixed.

FIG. 5 is a diagram showing an X-ray diffraction spectrum when a film is formed without introducing oxygen gas.

[Explanation of symbols]

1 Microwave Output Part 11 Waveguide Part 12 Coil 13 Connection Part 14 Quartz Window 2 Plasma Source Part 21 Container Part 22 Coil 3 Film Forming Chamber 31 Container Part 32 LiNbO ₃ Target 33 Table 4 RF Bias Part 5 Turbo Molecular Pump

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 2H047 PA04 QA03 TA44                 4G077 AA03 BC32 DA12 EA02 EA07                       SB03                 4K029 BA02 BA43 BA50 BB09 BC07                       CA05 DC05 DC27

Claims (2)

[Claims] 1. LiNbO ₃ by electron cyclotron resonance sputtering using a LiNbO ₃ target of constant composition
In the thin film forming method, the temperature of the single crystal substrate by supplying oxygen in the following state 600 ° C. 450 ° C. or higher, LiNbO ₃ thin film forming method and forming a LiNbO ₃ thin film on the single crystal substrate. 2. The supply of oxygen is lower than that in which a LiNb ₃ O ₈ phase is generated, and at a flow rate sufficient to keep the target surface covered with a certain amount of oxygen atoms. The method for forming a LiNbO ₃ thin film according to claim 1.