JPH0590892A - Thin film type saw device - Google Patents

Abstract

PURPOSE:To provide a surface acoustic wave(SAW) device that can be used in an extremely broad frequency range (specially for high frequency wave) compared with the case of using a bulk monocrystal by using a thin LiNbO3 film having high SAW velocity for a SAW device. CONSTITUTION:A sapphire substrate, specially a sapphire substrate with ((006) surface), is used to preparte a substrate for a thin film type SAW device. On the substrate, the thin LiNbO3 film whose crystal axis is unidirectionally oriented is prepared so that surface acoustic wave whose propagation rate on the thin film ranges in 4500-6600m/s, or a surface acoustic wave device using higher harmonic waves is prepared.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to surface acoustic waves (Surfa).
ce Acoustic Wave, hereinafter referred to as SAW)
SAW such as filter, SAW resonator, SAW sensor
It is about devices.

[0002]

2. Description of the Related Art A LiNbO ₃ single crystal is a ferroelectric substance having a high Curie temperature, and has very excellent properties such as large electromechanical coupling constant, electro-optic effect, and non-linear optical effect. It is a material that is put to practical use as a material for use on the one hand, but is promising as an optical element.

With the recent progress in semiconductor technology and the integration and miniaturization of electronic components, ferroelectric elements and piezoelectric elements are becoming smaller and thinner, and the demand for thinning LiNbO ₃ single crystals is increasing. Has been extensively researched. The center frequency of the SAW filter is the propagation characteristic of the SAW and the comb-shaped electrode pattern (Inter Digital Transduc) formed on the piezoelectric body.
er, hereinafter abbreviated as IDT). SA
The W speed Vs and the center frequency F ₀ of the SAW filter are determined by the equation (1).

F ₀ = Vs / λ (1) (λ; wavelength of surface acoustic wave) Generally, it is relatively easy to increase λ, but it is extremely difficult to reduce λ. Therefore, it can be said that a material having a larger Vs is more advantageous for higher frequencies and can be a filter having a wide frequency range and is an excellent material. It is known that the bulk single crystal of LiNbO ₃ has Vs = 3400 to 4000 m / s. As for the LiNbO ₃ thin film, the theoretical value of Vs = 4000 m / s in LiNbO ₃ / Al ₂ O ₃ by Wasa et al.
And LiNbO ₃ / fused quartz Vs = 3500 m /
s (NF Foster, J. Appl. hys.,
40, p420 (1969)) and the like, but none of them exceeds Vs of the bulk single crystal of LiNbO ₃ .

[0005]

An extremely useful SAW if there is a material exceeding Vs of the bulk single crystal of LiNbO ₃
It is considered to be a material for devices.

[0006]

The present invention relates to a surface acoustic wave having a propagation velocity of 5000 to 6600 m / s on a LiNbO ₃ thin film in which crystal axes on a sapphire substrate are oriented in one direction,
Further, the present invention relates to a surface acoustic wave device characterized by using a harmonic wave thereof, particularly a surface acoustic wave device characterized in that the substrate is a sapphire substrate C surface.

As a result of intensive studies by the inventors, when a LiNbO ₃ thin film having a good crystal orientation and containing no impurities such as iron is formed on a substrate, Vs exceeding the Vs of a bulk single crystal of LiNbO ₃ is obtained. It was found that a LiNbO ₃ thin film having The LiNbO ₃ thin film used here may be a film formed by any film forming method, but an example of a synthesis method that is particularly easy to reach the present invention will be described below. A laser ablation method is adopted as a film forming method, an excimer laser (ArF 193 nm) is used as a laser, a laser frequency is 5 to 100 Hz, and a laser output is 80 to 800 mJ.
And The target is a sintered body made of Nb ₂ O ₅ and Li ₂ CO ₅ as raw materials (size 15 mmΦ × 1 to 5 mm ^t )
Was used, and the distance between the target and the substrate was set to 1 to 5 cm.
Oxygen gas atmosphere, pressure range 0.0001-0.00
The substrate temperature was 500 to 850 ° C.
The film synthesized in the pressure range of 0.0001 to 0.003 torr tends to be colored, but in that case, the film is annealed immediately at 600 to 850 ° C. in an oxygen gas atmosphere (1 torr or more) for 5 minutes or more. Then, it seems that a colorless film can be obtained. Also, these treatments seem to be effective in improving the crystallinity of the film.

[0008]

EXAMPLES Next, the present invention will be described in more detail by way of examples. The LiNbO ₃ thin film was synthesized by the laser ablation method. FIG. 1 shows an outline of the device. The sintered target was irradiated with laser light to form a film on the substrate in an oxygen gas atmosphere. The crystallinity of the film was improved by performing annealing treatment in an oxygen gas atmosphere immediately after the film formation. The conditions used in the synthesis are listed below.

Synthesis conditions Single crystal substrate Sapphire C surface Substrate temperature 750 ° C. Introduced gas Oxygen reaction pressure 0.001 torr Target Li / Nb 2.0 Laser output 120 mJ Laser frequency 20 Hz Film formation time 20 to 180 minutes Annealing condition Temperature 750 ° C. Pressure A LiNbO ₃ thin film was synthesized under the condition of 1.5 torr time of 15 minutes or more.
ID on the synthesized film by photolithography process
Configured T. There are four types of IDTs, all of which are normal type electrodes, and the number of pairs of electrode fingers is 64 pairs for both input and output, and the wavelength (λ) is 4 μm, 8 μm, 10 μm, and 12 μm. The SAW filter characteristics of each sample were evaluated by these IDTs, and Vs was obtained. The results are summarized in Table 1. The crystallinity is evaluated with the half width of the X-ray rocking curve. At that time, divergence slits and scattering slits having 1/2 ° were used, and light receiving slits having 0.15 mm were used.

In each sample, two fundamental waves having different modes were obtained, and SAW velocities Vs1 and Vs were obtained.
2 is 5000 m / s or more, and LiNbO
_It was larger than 4000 m / s of ₃ bulk single crystals. In addition, harmonics (especially third-order harmonics) having a speed that is an odd multiple of Vs1 and Vs2 were obtained at the same time. No. 1 in Table 1 is shown in FIG.
1 shows the SAW filter characteristics at 1 (SAW wavelength 6 μm).

[0011]

Comparative Example As a comparative example, a bulk single crystal of LiNbO ₃ was used to form an IDT similar to that of the example and the filter characteristics were measured. SAW speed stays unchanged at 3800
m / s, which was always slower than the LiNbO ₃ thin film. When a SAW filter was designed using SAW (6 μm) having the same wavelength as that of the example, the results shown in FIG. 3 were obtained. It can be seen from comparison with FIG. 2 that the LiNbO ₃ thin film is a good SAW material having a high SAW speed and suitable for high frequencies.

[0012]

[Table 1]

[0013]

EFFECT OF THE INVENTION LiNb having a high SAW speed according to the present invention
By using the O ₃ thin film for the SAW device, it is possible to obtain a SAW device that can be used in an extremely wide range of frequencies (particularly high frequencies) as compared with the case of using a bulk single crystal.

[Brief description of drawings]

FIG. 1 is a schematic view of a synthesizer used in the present invention.

FIG. 2 Filter characteristics of LiNbO ₃ thin film

FIG. 3 Filter characteristics of bulk single crystal of LiNbO ₃

[Explanation of symbols]

 1 ArF excimer laser 2 lens 3 window 4 target 5 substrate 6 film thickness measuring device 7 gas inlet

Claims (1)

[Claims] 1. A propagation velocity of 5000-66 on a LiNbO ₃ thin film in which crystal axes are oriented in one direction on a sapphire substrate.
A surface acoustic wave device characterized by using a surface acoustic wave of 00 m / s or harmonics thereof.