JPS636519B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for forming a lead titanate (PbTiO ₃ ) thin film, and in particular to a new method for forming a PbTiO ₃ single crystal thin film using high frequency sputtering.

Recently, it has been proposed to utilize the optical properties of ferroelectric thin films to construct optical ICs such as optical modulators and optical switches. Since the ferroelectric thin film in this case constitutes an optical waveguide, it is preferable to form the film in the form of a single crystal rather than a polycrystal in order to avoid loss of light due to scattering or the like. Conventionally, such ferroelectric single-crystal thin films have been formed by high-frequency sputtering on, for example, SrTiO ₃ or MgO single-crystal substrates.
Epitaxial films of PLZT [(Pb, La) (Zr, Ti) O ₃ ] have been reported. However, such conventional film-forming techniques require time and effort to purify and prepare the target material, resulting in high costs and disadvantages that make them unsuitable for mass production.

In view of the above-mentioned conventional situation, this invention focuses on PbTiO ₃ , which has a high refractive index and has not been attempted to be formed into a thin single crystal film.
The aim is to provide a method for easily and reproducibly forming single-crystal thin films of PbTiO ₃ in order to improve the performance and reduce costs of electronic or optical components that apply this type of ferroelectric thin film.

Briefly stated, the gist of the present invention is to form an epitaxial film of PbTiO ₃ on a substrate crystal by high frequency sputtering using a mixed powder of PbO and TiO ₂ as a target material. In this case, sapphire is suitable as the substrate crystal.
As for film formation conditions, it is important to maintain the substrate temperature at 650°C or higher and to control the deposition rate to 20 Å/min or higher.

Preferred embodiments of the method for forming a PbTiO ₃ epitaxial thin film according to the present invention will be described below.

FIG. 1 is a schematic diagram conceptually showing the set state of a substrate and target material in a high-frequency sputtering device. In this case, a substrate crystal 1 made of sapphire is provided on the plate side with its (1102) plane as the surface. It is supported by a substrate holder 2, and a heater 3 for heating the substrate crystal 1 is installed behind it. On the other hand, as the target material 4, a mixed powder of PbO and TiO ₂ weighed to have the stoichiometric composition ratio of PbTiO ₃ to be formed is used, and this is placed in a quartz shear plate 5 with a diameter of 100 mm.
The cathode 7 is placed on top of a water-cooled cathode 7 with another quartz plate 6 interposed therebetween. The target material powder 4 has been sufficiently degassed in a vacuum.

With the substrate crystal 1 and the target material 4 set as described above and the distance between them set to 40 mm, a mixed gas of 90% Ar and 10% O ₂ 2×
High frequency sputtering was attempted by introducing at a pressure of 10 ^-2 Torr. In this case, the sputtering conditions include a substrate temperature of 520 to 700℃ and a plate voltage of 520 to 700℃.
The plate current was varied in the range of 1.2 to 2.2 KV and 120 to 235 mA.

Figure 2 shows the relationship between the substrate temperature and crystal structure during film formation on the sapphire substrate (1102) surface.The plate voltage was adjusted so that the deposition rate was approximately 25 Å/min.
It was found that when a PbTiO ₃ film with a thickness of 0.7 μm was formed at 1.8 KV, an epitaxial film with a perovskite structure oriented in the (111) crystal plane could be obtained at a substrate temperature of 650°C or higher. In other words, in Fig. 2, in the region where the substrate temperature is 550°C or less, the film is deposited in a mixed crystal form of perovskite structure and virochlore structure, and in the region of 550 to 650°C, it becomes a polycrystalline film with perovskite structure, and at 650°C
An epitaxially grown single crystal film with a perovskite structure can be obtained in the above temperature range up to 700°C. However, the upper limit of the substrate temperature in this case is appropriately determined within the range of about 800° C. depending on the characteristics of the substrate crystal and the equipment used. Here, the lattice constant of the crystal plane (111) of PbTiO ₃ is the (11) of the substrate crystal sapphire.
02) Since the mismatch with respect to the lattice constant of the plane is relatively small at 4.7% or less, it is possible to epitaxially grow a PbTiO ₃ film on the sapphire substrate. Note that it is possible to form a similar epitaxial film not only on sapphire but also on other substrate crystals whose lattice constants are similar to PbTiO ₃ .

On the other hand, the present inventors have discovered that
We have found that the epitaxial growth of PbTiO ₃ film is not only dependent on the substrate temperature but also on the sputter deposition rate. That is, the plate voltage is
If the voltage is 1.6 KV or less and the film deposition rate is 20 Å/min or less, it will not form an epitaxial film, and if the plate voltage is set to 1.6 KV or higher and the deposition rate is controlled within the range of 20 Å/min to 70 Å/min. It was found that a high refractive index epitaxial film with a perovskite structure oriented in the (111) plane could be obtained.
This indicates that the epitaxial growth of the PbTiO ₃ film is also related to the energy of the sputtered particles, but the plate voltage becomes too high and the deposition rate decreases.
If it exceeds 70 Å/min, polycrystalization will be significant and epitaxial growth will become impossible.

As an example, the substrate temperature is 660℃, the adhesion rate is 25Å/
The thickness of 0.7 μm was deposited on the saphire (1102) surface at min.
After forming the PbTiO ₃ film, it was heat-treated at 750°C in the air for 30 hours, and the lattice gap immediately after the film was deposited was 2.290.
Å, the half width of the X-ray diffraction peak on the (111) plane is 0.47
The epitaxial film was ~0.52 degrees, but after heat treatment, the lattice gap was 2.297 Å and the half width was 0.39 ~
The crystallinity was improved to 0.42 degrees. and this
The refractive index of the PbTiO ₃ single crystal thin film is 2.52 for a wavelength of 5460 Å, which is sufficiently larger than the refractive index of the substrate cipher, 1.77, and it was confirmed that it can form a good optical waveguide. This refractive index value is higher than that of conventional PLZT films and is extremely advantageous for constructing optical waveguides.

Now, as is clear from the above explanation, in short, this invention makes it possible to epitaxially grow a PbTiO ₃ thin film using a high frequency sputtering method, and uses a mixed powder of raw materials as the target material. Therefore, it is easy to prepare and can be formed at a lower cost and with better reproducibility than conventional ferroelectric single crystal thin films of this type. Since the PbTiO ₃ epitaxial thin film according to the present invention exhibits good optical properties, it can be used not only as an optical waveguide in optical ICs such as optical modulators and optical switches, but also as a substrate material for surface acoustic wave devices. It can be widely applied.

[Brief explanation of the drawing]

FIG. 1 is a diagram schematically showing a set state of a substrate crystal and a target material in a high-frequency sputtering apparatus for carrying out the present invention, and FIG. 2 is a diagram showing the relationship between substrate temperature and crystal structure. 1: Substrate crystal (sapphire), 2: Substrate holder,
3: heater, 4: target material, 5: quartz shear, 6: quartz plate, 7: cathode.

Claims (1)

[Claims] 1. Using a mixed powder of PbO and TiO ₂ as the target material, and performing high frequency sputtering at a deposition rate of 20 Å/min to 70 Å/min while maintaining the substrate temperature at 650°C or higher, a material made of sapphire is produced. on the (1102) plane of the substrate crystal
A method for forming a lead titanate thin film, characterized in that an epitaxial film of PbTiO ₃ is formed.