RU2541927C2 - Multifrequency resonator on volume acoustic waves - Google Patents

Abstract

FIELD: physics, acoustics.

SUBSTANCE: invention relates to microwave electroacoustics and is the basis for designing stabilised spectrum generators, narrow band-pass filters, highly sensitive sensors and other microwave frequency-setting elements for communication, automation and radar equipment. The multifrequency resonator on volume acoustic waves comprises an acoustic line and an acoustic vibration source, which includes a piezoelectric crystal, top and bottom metal electrodes, wherein the acoustic vibration source is placed on one of the surfaces of the acoustic line, the acoustic line is made from an oriented diamond monocrystal with crystallographic planes of sections (100) or (110) or (111), which correspond to propagation directions of clean longitudinal modes.

EFFECT: high stability of generation frequency and minimum phase noise in a wide frequency range.

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Description

The invention relates to microwave (microwave) electroacoustics and is the basis for creating stabilized frequency grid generators, narrow-band filters, microwave generators with a low level of phase noise, highly sensitive sensors and other microwave frequency-setting elements for communications, automation and radar.

Known multi-frequency cavity resonator (MROAV) (G. D. Mansfeld, S. G. Alekseev, N. I. Polzikova, "Unique Properties of HBAR Characteristics", 2008, IEEE International Ultrasonics Symposium Proceedings, p.439-442). The device consists of two metal electrodes, between which a piezoelectric layer is placed, and a dielectric substrate, which is a sound pipe. The thickness of the sound duct is chosen much more than the thickness of the piezoelectric layer. When microwave fields are applied to the electrodes in such a layered structure at certain frequencies, standing waves of mechanical displacements s (z) appear, which are described by the expression (GD Mansfeld, SG Alekseev, NI Polzikova, “Unique Properties of HBAR Characteristics”, 2008, IEEE International Ultrasonics Symposium Proceedings , p.439-442):

ε (z) = ε ₀ cos (kz),

where z is the coordinate of the direction of wave propagation, ε ₀ is the amplitude of mechanical displacements, k is the wave vector. Such a device allows the formation of a uniform broadband grid (spectrum) of highly stable resonant frequencies with a low level of phase noise.

A well-known US patent (US 4785269, IPC H03H 9/00, publication date 11/15/1988, "Magnetically tuned high overtone bulk acoustic resonator", Adam JD, McAvoy BR), where a single crystal of yttrium iron garnet was used as a sound pipe, which allowed changing the resonance frequency in small limits using a magnetic field. The disadvantage of this resonator is the increased sensitivity to external electromagnetic interference, which can lead to instability of the generation frequency.

In the patent of the Russian Federation (RU 2468507 C1, IPC H03H 9/15, publication date 11/27/2012, “Resonator based on higher types of volumetric acoustic waves”, Kozyrev AB, Mikhailov AK, Ptashnik SV) presented a multi-frequency cavity resonator based on acoustic waves, in which the resonance frequency is controlled by changing the boundary conditions on the opposite plane of the sound duct. However, the presence of an impedance acoustic load on the opposite side of the sound duct leads to a decrease in the quality factor of the entire resonator.

US Application (US 2011/0279187 A1, IPC H03H 9/15, H03B 5/32, H03H 9/54, H01L 41/22, publication date 11/17/2011, "HBAR resonator with high temperature stability", Ballandras S., Gashon D.) in MROAV, thin LiNbO ₃ (yxl) / 36 ° slices were used as a piezoelectric, which excited the shear waves in an AT-cut single crystal quartz sound duct. The device was proposed to be used as a multi-frequency shear-wave resonator in the frequency range 0.05–20 GHz. The disadvantage of this device is the use of quartz as a sound pipe, which has significant attenuation by microwave and the sound velocity in which is about 3 times less than in diamond, and the use of shear waves, which generally reduces the capabilities of this device as a resonator in the microwave range. For example, the quality factor Q · f of this resonator in the indicated range does not exceed 5 · 10 ¹² .

In the US patent (US 7508286 B2, IPC Н03Н 9/00, published March 24, 2009, “HBAR oscillator and method of manufacture”, Rubi RC, Pang W.), a multi-frequency cavity acoustic wave resonator is used in an acoustic filter as a grid generator frequencies together with a film surround acoustic resonator emitting a given resonance frequency from this grid. The disadvantage of this device is the use of single-crystal quartz, silicon, sapphire as a sound pipe, which did not allow to obtain a Q factor of the resonator above 10000 at a frequency of 1.5 GHz.

The disadvantages of the considered resonators are that they operate in a fairly narrow frequency range (2-4 GHz) and they use sound ducts with a relatively low quality factor Q · f, not exceeding 5 · 10 ¹² .

Closest to the proposed technical solution is the device described in US patent (US 7609132 B2, IPC Н03Н 9/00, publication date 10/27/2009, "Hybrid resonant structure", Ballandras SJ, et al.) And consisting of an upper electrode, a layer piezoelectric material (aluminum nitride AlN or zinc oxide ZnO), lower electrode and sound duct. The authors believe that the electrodes should be made of well-conducting metals of the series Al, Mo, Ni, Ag, Pt, Au, W, Cu, etc .; the sound duct can be made of various materials, preferably having dielectric and mechanical properties such that the quality factor Q · f is in the range (3 ÷ 5) · 10 ¹² . It is proposed to use mainly crystalline quartz as a sound duct, as well as other possible materials: glass or silicon; La ₃ Ga ₅ SiO ₁₄ , La ₃ Ga _5.5 Nb _0.5 O ₁₄ , La ₃ Ga _5.5 Ta _0.5 O ₁₄ , GaPO ₄ , sapphire, LiNbO ₃ , KnbO ₃ , LiTaO ₃ , C, SiC etc. In order to increase the electromechanical coupling coefficient, an additional dielectric layer with high mechanical and dielectric properties is placed on the upper electrode of the piezoelectric in the device. The disadvantage of this device is the decrease in the quality factor of the resonator due to loading the piezoelectric with an additional dielectric layer, and a relatively low quality factor Q · f, not exceeding 5 · 10 ¹² .

The technical problem solved by the proposed device is the elimination of the above drawbacks and the creation of a multi-frequency cavity resonant acoustic waves with a high quality factor in a wide frequency range.

The multi-frequency cavity resonator was made in the form of a layered coplanar structure and consisted of a source of acoustic vibrations, including an upper metal electrode, a piezoelectric layer of aluminum nitride AlN and a lower metal electrode, and a sound duct, the source of acoustic vibrations being made on one of the surfaces sound duct. The electrodes were made of a metal selected from the series Cr, Mo, Al, Cu, Pt, preferably Mo was used.

The problem was solved due to the fact that in the proposed resonator, single crystals of artificial or natural origin with a crystallographic orientation of the (100) or (110) or (111) sections corresponding to the propagation directions of pure longitudinal modes were used as sound ducts. For best acoustic matching, a lower molybdenum electrode with a thickness of 100-200 nm was used.

The invention is illustrated in figures 1, 2, 3. Figure 1 shows the structure of a multi-frequency cavity resonant acoustic waves. The electrodes upper (1) and lower (3) in order to minimize electrical losses were made in the form of one or two metal layers. Such metals were Cr, Mo, Al, Cu, Pt. For the best acoustic matching between the piezoelectric (2) and the sound duct (4), the lower electrode was made of molybdenum. As a piezoelectric (2), aluminum nitride AlN was used as the material with the highest longitudinal wave propagation velocity in the group of known piezoelectric materials (ZnO, AlN, LiNbO ₃ , SiO ₂ , LiTaO ₃ ). An aluminum nitride film was deposited on top of the lower electrode (3) by magnetron sputtering.

Figure 2 and 3 shows the results of studies of the spectrum of the natural frequencies of the proposed resonator by measuring the reflection coefficient S ₁₁ in the ranges 6 ÷ 6.5 GHz and 15.5 ÷ 16 GHz, respectively, using a microwave network analyzer brand E5071C Agilent Technologies in LinMag mode at the microprobe station of the M-150 Cascade Microtech brand.

An example implementation of the invention.

A 4 × 4 mm ² square plate 430 μm thick, cut from artificially grown type IIa diamond, was chosen as a sound duct. The plane of the plate was oriented in the [100] direction. After mechanical polishing, the roughness of the faces was R _a ≤10 nm, the non-parallelism was not more than 1 μm / cm.

The lower electrode (3) of molybdenum 150 nm thick was sprayed on the (100) plane of a diamond single crystal (4) in ultrahigh vacuum. For the synthesis of the piezoelectric layer of aluminum nitride in the flow of the working gas mixture Ar / N ₂ , the method of high-frequency reactive magnetron sputtering of a target made of highly pure aluminum was used. Testing with a TETA ARL X'TRA powder X-ray diffractometer showed that the misorientation of AlN crystallites relative to the normal to the surface of the sound duct did not exceed 0.3 ° (full width at half the height of the diffraction reflex (002)), which ensured high electromechanical efficiency. The thickness of the piezoelectric film was 1100 nm, which corresponded to its own resonant frequency of 5 GHz. The upper electrode (1) with a thickness of 150 nm was also made of molybdenum. The thicknesses of the electrodes are selected from the condition of the best acoustic matching and the least electrical loss. The frequency characteristics of the resonator were measured in the frequency range 0.02–20 GHz using an Agilent Technologies E5071C microwave circuit analyzer at an M-150 Cascade Microtech microprobe station. The position of the resonance frequency f _{r was} determined by the maximum of the admittance Y ₁₁ , the frequency of the antiresonance f _a by the maximum of the impedance Z ₁₁ .

Figure 2 and 3 clearly observed equidistant resonant peaks. The frequency interval between them was calculated by the formula:

$$f_0=\frac{V_L}{2d},$$

where V _L is the velocity of the longitudinal acoustic wave in the diamond in the [100] direction, d is the thickness of the diamond plate. The calculation for V _L = 17540 m / s and d = 430 μm gives a value of f ₀ = 20.395 MHz, which is quite close to the measured value of 20.330 MHz (figure 3).

The quality factor of the resonator in the frequency band 0.02–20 GHz varied from 25500 to 2600, which corresponded to a quality factor of at least 5 · 10 ¹³ . For example, the quality factor of the peak indicated by the marker (Fig. 3) was Q = 3325 at a frequency f _r = 15.475 GHz. The Q-factor was measured according to the method described in the works (S.G. Alekseev, I.M. Kotelyansky, G.D. Mansfeld, N.I. Polzikova, F.O. Sergeev, A.K. Abachev. The Q-measurement method and attenuation in composite microwave acoustic resonators. Radio engineering and electronics, No. 8, volume 52, year 2007, pp. 1013-1017; SG Alekseev, GD Mansfeld. A simple way to measure the quality factor and attenuation in acoustic resonators. Radio engineering and Radio Electronics, No. 1, Volume 53, Year 2008, pp. 122-126).

When using single crystals of diamond with a crystallographic orientation of the (110) or (111) sections, similar results were obtained.

Thus, the description of the device and the results of its work prove the achievement of the claimed technical result - the creation of a multi-frequency resonator based on volume acoustic waves with a sound duct from an oriented diamond single crystal having a high quality factor Q · f not worse than 5 · 10 ¹³ in a wide frequency range of 0.02 ÷ 20 GHz.

Claims (4)

1. A multi-frequency cavity resonator based on acoustic waves, comprising a sound duct and a source of acoustic vibrations, which includes a piezoelectric, upper and lower metal electrodes, and the source of acoustic vibrations is placed on one of the surfaces of the sound duct, characterized in that the sound duct is made of oriented single crystal diamond with crystallographic planes of sections (100), or (110), or (111) corresponding to the directions of propagation of pure longitudinal modes. 2. The resonator according to claim 1, characterized in that the sound duct is made of artificially grown single crystal diamond. 3. The resonator according to claim 1, characterized in that aluminum nitride AlN is used as a piezoelectric. 4. The resonator according to claim 1, characterized in that the lower electrode is made of molybdenum with a thickness of 100-200 nm.

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