JP2005105394A - Method for forming ferroelectric thin film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for growing a ferroelectric thin film of high quality consisting of crystal grains with small and uniform sizes, with a MOCVD technique in a short period of time (with a high growth rate). <P>SOLUTION: This film-forming method includes a step of growing the ferroelectric thin film with the MOCVD technique at a growing rate of 0.05 nm/sec or less during an early growing stage between the time of starting the growth and a time when the film thickness reaches 10 nm; or includes a step of slowly increasing the growing rate from the time of starting the growth in a growing process between the time of starting the growth and a time when the film thickness reaches 10 nm. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for forming a ferroelectric thin film, more specifically, a piezoelectric microactuator or a piezoelectric sensor using the piezoelectric characteristics of the ferroelectric thin film, a pyroelectric sensor using pyroelectricity, and a non-volatile using polarization characteristics. The present invention relates to a method of forming a ferroelectric thin film useful for manufacturing a memory.

At present, ferroelectrics such as Pb (Zr, Ti) O ₃ (usually referred to as PZT) having excellent piezoelectric characteristics are widely applied as piezoelectric actuators in the form of bulk ceramics. Aiming to develop applications for microactuators, there is a strong demand for thinner ferroelectrics. This is because if the ferroelectric material is thinned, the amount of displacement with respect to the voltage value to be applied increases, so that an actuator with a large amount of displacement driven at a low voltage can be obtained.

  However, if the ferroelectric is made too thin, the force necessary for the operation of the actuator cannot be obtained. Although it depends on the type and thickness of the substrate, it is generally said that a thin film having a thickness of 2 to 10 μm is practically advantageous for the piezoelectric microactuator.

  Until now, methods of machining a sintered bulk material and methods of forming a thin film by screen printing have been mainly studied. However, in the case of machining a bulk material, there is a problem of breakage during processing or handling and a problem of variation at the time of adhesion to a base material, and microfabrication of 100 μm or less is practically difficult. In addition, since the screen printing method cannot obtain a film having a sufficient film density, there is a problem that a dielectric breakdown tends to occur when the film thickness is reduced, and it has not been put into practical use.

  Therefore, recently, active research has been conducted on the formation of ferroelectric thin films by methods such as sputtering, sol-gel, MOCVD (chemical vapor deposition), and PLD (pulsed laser deposition), which have proven results as thin film growth technologies. It has come to be.

  For example, after applying an adjusted sol, drying, degreasing, and firing (calcination temperature: 550 ° C. to 750 ° C.) to form an initial layer, an initial adjustment layer is applied on the initial layer, A method for forming a ferroelectric thin film is known in which drying, degreasing, and firing (firing temperature: 550 ° C. to 750 ° C.) are performed to form a late layer (see, for example, Patent Document 1).

  In the near future, sol-gel method and sputtering method will be put to practical use first, but in the future, considering the good film quality, high speed of film formation, and compatibility with complex shapes, MOCVD The method is expected to be widely used in piezoelectric microactuator applications.

At present, the growth of ferroelectric thin films by MOCVD method is the bubbling method in which the raw material is supplied using the vapor pressure of the organic metal raw material, or the LDS (Liquid Delivery System) in which the organic metal raw material dissolved in the solvent is supplied by the liquid supply device. The raw material is supplied using one of the methods, and the raw material is decomposed by heating to about 600 ° C. in a reactor, and a thin film is grown on the substrate. At present, a high-quality thin film having a smaller and more uniform grain size than other thin film forming methods and having excellent dielectric breakdown characteristics can be formed.
JP 2002-036442 A (paragraph numbers 0056 to 0064, FIG. 4)

  As described above, the thickness of the ferroelectric thin film is desirably 2 μm or more in order to be used for a piezoelectric actuator. Therefore, the growth rate of the thin film is desired to be high. In the MOCVD method in which the raw material is supplied by the LDS method, the raw material can be supplied regardless of the vapor pressure of the organic metal raw material, so that the raw material can be stably supplied at a high rate and the growth rate can be remarkably increased.

  However, when the growth rate is increased, there is a problem that the size of the crystal grains of the ferroelectric thin film is nonuniform and large. In such a thin film, dielectric breakdown tends to occur, and good piezoelectric characteristics cannot be obtained. If the growth rate is lowered, the crystal grain size of the ferroelectric thin film can be made uniform and small, but it is not practical because it takes a long time to grow.

  Accordingly, an object of the present invention is to solve the above problems and provide a method for growing a high-quality ferroelectric thin film having a small crystal grain size and a uniform quality in a short time (high growth rate) using the MOCVD method. There is.

  The method for forming a ferroelectric thin film according to the first aspect of the present invention is such that when a ferroelectric thin film used for a nonvolatile memory, a piezoelectric element, or a pyroelectric element is grown by the MOCVD method, the film thickness starts from the growth start. The growth rate up to 10 nm is 0.05 nm / sec or less.

  According to a second aspect of the present invention, there is provided a method for forming a ferroelectric thin film, which is a film formed at least from the start of growth when a ferroelectric thin film used for a nonvolatile memory, a piezoelectric element, or a pyroelectric element is grown by MOCVD. The growth process up to a thickness of 10 nm includes a step of gradually increasing the growth rate from the start of growth.

  According to a third aspect of the present invention, in the method for forming a ferroelectric thin film according to the first aspect, after the growth up to the film thickness of 10 nm, the growth rate is increased to a final predetermined film thickness (for example, about 2 μm). It is characterized by high-speed growth.

  According to a fourth aspect of the present invention, in the method for forming a ferroelectric thin film according to the third aspect, the process of gradually increasing the growth rate is included in the high-speed growth process.

A fifth aspect of the present invention is the method for forming a ferroelectric thin film according to any one of the first to fourth aspects, wherein the ferroelectric thin film is PZT, PLZT, SBT, BIT, BST, LiNbO ₃ , SrBi ₂ Nb. It is any one of ₂ O ₉ .

  According to the present invention, when a ferroelectric thin film is grown by the MOCVD method, the initial growth from the growth start to the film thickness of 10 nm is performed at a growth rate of 0.05 nm / sec or less, or at least the growth start. In the growth process from 1 to 10 nm in thickness, a step of gradually increasing the growth rate from the start of growth is included. By the growth at the low growth rate, the ferroelectric thin film grown at the initial stage of growth becomes a high-quality thin film with uniform and small crystal grains. When the ferroelectric thin film is grown by MOCVD on the high quality thin film at a high growth rate, the high quality thin film can be successively formed by taking over the high quality crystallinity of the base.

  Therefore, according to the present invention, a high-quality ferroelectric thin film having a small crystal grain size and uniform quality can be grown in a short time (high growth rate) using the MOCVD method.

  Hereinafter, the present invention will be described based on the illustrated embodiments.

  FIG. 1 shows a method for forming a ferroelectric thin film (PZT thin film) according to an embodiment of the present invention.

  First, the growth (initial growth) of the PZT thin film from the start of the growth to the film thickness of 10 nm is performed at an initial growth rate of 0.05 nm / sec or less, which is smaller than the normal growth rate of 1.0 nm / sec. 1 (a)). By reducing the growth rate in this way, a high-quality thin film with uniform and small crystal grains can be formed.

  Thereafter, the growth rate is gradually increased until the film thickness reaches a certain thickness of 100 nm (FIG. 1B). Successive high-quality thin films are formed by taking over the high-quality crystallinity of the base.

  When the film thickness reaches a certain thickness of 100 nm, the growth rate is increased to 1.0 nm / sec (FIG. 1C). Even if the growth rate is increased in this way, the high-quality thin film is formed because the high-quality crystallinity of the base is similarly taken over.

  Thereafter, the film is grown at the high growth rate of 1.0 nm / sec until the final predetermined film thickness reaches 2 μm, and the growth is stopped (FIG. 1D). Thereby, a good quality thin film can be formed in a short time.

<Example 1 (hysteresis measurement, comparison of dielectric breakdown characteristics)>
A ferroelectric thin film as Example 1 of the present invention was formed, and a conventional ferroelectric thin film was formed as a comparative example. The hysteresis measurement and dielectric breakdown characteristics of the ferroelectric thin film were compared between the case of using the present invention and the case of using the prior art.

A Si (100) substrate having an oxide film (SiO ₂ ) on the surface was used as a base material, and an adhesion layer Ti (50 nm) and a lower electrode Pt (100 nm) were formed in that order by EB vapor deposition. On the other hand, in one (comparative example), a 2 μm PZT thin film is grown at a constant growth rate of 1.0 nm / sec using the MOCVD method as in the prior art. The initial growth rate is 0.05 nm / sec or less, and then the growth rate is gradually increased. When the film thickness reaches 100 nm, the growth rate is set to 1.0 nm / sec. . Thereafter, the film was grown at a growth rate of 1.0 nm / sec until the film thickness reached 2 μm.

  The structure of the MOCVD apparatus used for the growth of the ferroelectric thin film (PZT thin film) is shown in FIG.

As a raw material supply method, a liquid raw material supply system (LDS) capable of realizing growth at a high growth rate was used. Pb (dpm) ₂ , Zr (dpm) ₄ , Ti (dpm) ₂ (OiPr) were used as the organic metal raw material, and these were dissolved in THF (tetrahydrofuran) as a solvent and put in the organic metal cylinder 1. .

When supplying the raw material, a fixed amount of the raw material was sent to the vaporizer 3 by the liquid MFC (mass flow controller) 2, vaporized at a temperature of 300 ° C. at the vaporizer 3, and supplied to the reactor 4 as it was. In addition, oxygen O ₂ was supplied from another route. In the reactor 4, the base material (substrate) 5 was heated to 550 ° C. with the heater 6, and the PZT thin film was grown on the base material 5 by thermally decomposing the raw material.

  Exhaust during growth was performed by the rotary pump 7 and the pressure in the reactor 4 was adjusted to 10 Torr.

  The growth conditions for MOCVD were exactly the same for both Example 1 and Comparative Example, and were performed at a substrate temperature of 550 ° C. and a reactor pressure of 10 Torr.

  Thereafter, an upper electrode Pt (100 nm) was formed on the ferroelectric thin film by EB (electron beam) vapor deposition, and the upper electrode was processed into a 5 mm square shape by RIE (reactive ion etching).

The result of measuring the hysteresis loop is shown in FIG. The applied electric field was 200 kV / cm. It can be seen that the product using the present invention (Example 1) has a large remanent polarization compared to the product produced by the prior art (Comparative Example). That is, in the PZT thin film prepared in the prior art with respect to the residual polarization 2Pr is 60 .mu.C / cm ^2, 2Pr in PZT thin films using the present invention is 75μC / cm ^2, the residual polarization is improved by the present invention I understand that

  The results of measuring the dielectric breakdown characteristics are shown in FIG. It can be seen that the PZT thin film using the present invention has a higher dielectric breakdown voltage than the PZT thin film produced by the prior art. Although the film thickness of PZT is different, for reference, general breakdown voltages of a PZT thin film produced by a screen printing method and a PZT thin film produced by machining a bulk material are also shown in FIG. It can be seen that the PZT thin film using the present invention has much larger dielectric breakdown characteristics than the screen printing method and the bulk machined product.

<Example 2>
Next, as Example 2, a cantilever needle-type piezoelectric actuator was prepared, and its characteristics were compared.

In the same manner as in Example 1, a substrate / Ti adhesion layer / Pt lower electrode / PZT thin film / Pt upper electrode were produced by both the conventional technique and the method of the present invention. Using them, RIE (etching gas = SF ₆ / Ar / O ₂ ) and wet etching (tetramethylammonium hydroxide) were used to fabricate a piezoelectric microactuator with a cantilever needle structure.

The piezoelectric characteristics of the piezoelectric microactuator were measured using a ferroelectric tester and a laser Doppler interferometer. From the result, the piezoelectric constant (−d ₃₁ ) of each PZT thin film was calculated. The piezoelectric constant (−d ₃₁ ) was 100 pm / V in the prior art product, whereas it was 140 pm / V in the product of the present invention. It was confirmed that the piezoelectric characteristics were greatly improved by the present invention compared to the prior art.

It is a figure which shows the formation method of the ferroelectric thin film of this invention. It is a figure which shows the structure of the MOCVD apparatus used for the formation method of the ferroelectric thin film of this invention. It is a figure which shows the result of the hysteresis loop measurement of the polarization P-electric field E of the ferroelectric thin film of this invention. It is the figure which showed the dielectric breakdown characteristic of the ferroelectric thin film of this invention compared with the case by a prior art etc.

Explanation of symbols

1 Organic metal cylinder 2 Liquid MFC
3 Vaporizer 4 Reactor 5 Base Material 6 Heater 7 Rotary Pump

Claims (5)

  A method for forming a ferroelectric thin film, characterized in that, when the ferroelectric thin film is grown by MOCVD, the growth rate from the start of growth to a film thickness of 10 nm is set to 0.05 nm / sec or less.   When a ferroelectric thin film is grown by MOCVD, a process of gradually increasing the growth rate from the start of growth is included at least in the growth process from the start of growth to a film thickness of 10 nm. Forming method. In the formation method of the ferroelectric thin film of Claim 1,
A method for forming a ferroelectric thin film, characterized in that after the film is grown to a thickness of 10 nm, the growth rate is increased and the film is grown at a high speed to a final predetermined film thickness. In the formation method of the ferroelectric thin film of Claim 3,
A method for forming a ferroelectric thin film, comprising the step of gradually increasing the growth rate in the high-speed growth process. In the formation method of the ferroelectric thin film in any one of Claims 1-4,
A method for forming a ferroelectric thin film, wherein the ferroelectric thin film is any one of PZT, PLZT, SBT, BIT, BST, LiNbO ₃ and SrBi ₂ Nb ₂ O ₉ .

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