阅读说明：本技术 一种可开关式体声波滤波器 (Switchable bulk acoustic wave filter ) 是由 朱浩慎 许霄彤 薛泉 于 2020-05-30 设计创作，主要内容包括：本发明公开一种可开关式体声波滤波器,所述可开关式体声波滤波器包括若干个串联的体声波谐振器和若干个并联的体声波谐振器；所述体声波谐振器包括第一金属层、压电层和第二金属层；压电层位于第一金属层和第二金属层的中间。所述第一金属层材料为高功函数金属。所述第二金属层材料为低声学损耗金属。所述压电层采用压电半导体材料作为压电材料。本发明的压电层中压电半导体材料上添加不同禁带宽度的三元化合物构成异质结产生二维电子气。第一金属层上的偏置电压控制二维电子气的产生与耗尽,从而达到控制谐振器开启与关闭。本发明结构简单,开启电压低且具有良好的隔离度。(The invention discloses a switchable bulk acoustic wave filter, which comprises a plurality of series bulk acoustic wave resonators and a plurality of parallel bulk acoustic wave resonators; the bulk acoustic wave resonator comprises a first metal layer, a piezoelectric layer and a second metal layer; the piezoelectric layer is located intermediate the first metal layer and the second metal layer. The first metal layer is made of high work function metal. The second metal layer material is a low acoustic loss metal. The piezoelectric layer is made of piezoelectric semiconductor materials. Ternary compounds with different forbidden band widths are added on a piezoelectric semiconductor material in the piezoelectric layer to form a heterojunction, so that two-dimensional electron gas is generated. The bias voltage on the first metal layer controls the generation and the depletion of the two-dimensional electron gas, so that the resonator is controlled to be opened and closed. The invention has simple structure, low starting voltage and good isolation.)

1. A switchable bulk acoustic wave filter, characterized in that the switchable bulk acoustic wave filter comprises a number of series connected bulk acoustic wave resonators and a number of parallel connected bulk acoustic wave resonators;

the bulk acoustic wave resonator comprises a first metal layer, a piezoelectric layer and a second metal layer; the piezoelectric layer is located intermediate the first metal layer and the second metal layer.

2. A switchable bulk acoustic wave filter according to claim 1, characterized in that the first metal layer is a high work function metal and forms a schottky contact with the piezoelectric layer.

3. A switchable bulk acoustic wave filter according to claim 2, characterized in that the second layer of metal material is a metal with low acoustic losses.

4. A switchable bulk acoustic wave filter according to claim 3, characterized in that the piezoelectric layer comprises layers of piezoelectric semiconductor material and ternary compound layers with different forbidden band widths, which constitute a heterojunction.

5. The switchable bulk acoustic wave filter of claim 4, wherein the layers of piezoelectric semiconductor material having different forbidden band widths are made of GaN as the piezoelectric semiconductor material and AlGaN as the ternary compound layer.

6. A switchable bulk acoustic wave filter according to claim 5, wherein the heterojunction produces a two-dimensional electron gas, the production and depletion of which is regulated by a reverse biased Schottky junction.

7. The switchable bulk acoustic wave filter of claim 1, wherein the thickness of the piezoelectric layer is determined by the frequency of the desired filtering band of the filter, the filtering band frequency of the filter is fine-tuned by changing the thickness of the first metal layer and the second metal layer, and the center frequency of the filtering band of the filter is changed by adjusting the thickness of the first metal layer, the second metal layer and the piezoelectric layer to meet different requirements of the industry.

8. The switchable bulk acoustic wave filter according to any one of claims 2 to 7, wherein the high work function metal of the first metal layer is Ni.

9. A switchable bulk acoustic wave filter according to claim 8, characterized in that the low-acoustic-loss metal of the second metal layer is molybdenum (Mo) or tungsten (W).

Technical Field

The invention relates to a radio frequency micro-electromechanical device technology in the field of semiconductor devices, in particular to a switchable bulk acoustic wave filter.

Background

With the advent of the 5G era, wireless communication devices are continuously becoming multifunctional and miniaturized. This means that the market today demands higher integration and lower signal power consumption for the rf front-end, which is a critical part of wireless communication. According to the expectation of Skyworks corporation, the number of radio frequency front-end filters in the 5G maturation stage is increased to more than 100, and the number of switches exceeds more than 30. The existing radio frequency front-end switch or filter selector cannot cope with the high-speed development, and the research and development of a novel rapid switching device have important research value.

There are two methods of selecting the desired devices in the rf circuit: the first method is to add a selection switch between different devices, and the addition of the selection switch can increase the circuit complexity and the influence between the devices cannot be completely eliminated; the second method is to make the device itself have a switching characteristic, and this method is to be adopted in the present invention.

At present, the switchable radio frequency acoustic wave filter has no relevant research report at home, but has a certain research at foreign countries, and the reported realization schemes of the switchable radio frequency acoustic wave filter mainly comprise the following steps:

1. s, Li, Y, Lin, Z, Ren and C.T. -Nguyen, "An MSI Micromechanical differential Disk-Array Filter," TRANSDUCERS 2007 International Solid-State TRANSDUCERS, actors and Microsystems Conference, Lyon, 2007, pp. 307-. Because the filter adopts the electrostatic driving and detecting mode for transduction, the closing of the filter passband can be realized only by closing the direct current bias voltage. Although this electrostatic coupling capacitor filter has ideal switching characteristics and is compatible with silicon-based technology, its electromechanical coupling coefficient is too low compared with piezoelectric transduction, resulting in too large insertion loss and too narrow bandwidth, and therefore it cannot be applied to rf front-end filter.

2、Lee, Seungku , and A. Mortazawi . "An Intrinsically SwitchableLadder-Type Ferroelectric BST-on-Si Composite FBAR Filter."IEEE Transactions on Ultrasonics，Ferroelectrics, and Frequency Control,vol.63,pp.456-462,2016, a switchable BST-on-Si (silicon-barium strontium titanate) composite thin film bulk acoustic wave Filter (FBAR) operating at 2.5GHz is proposed with a mechanical quality factor of 971. Under the condition of DC bias of 40V in the open state, the insertion loss is 7.8dB, and the out-of-band rejection is 26 dB. In the closed state, the isolation degree of the filter is 31dB, the isolation degree of the filter in the scheme is 31dB, and the starting direct current bias voltage reaches 40V. However, the scheme adopts a special ferroelectric material, the high polarization electric field of the special ferroelectric material enables the switching of the device to be realized only by high bias voltage, and the Q value of the resonator is limited due to the high loss of the special ferroelectric material. In addition, the ferroelectric film material is not compatible with the existing integrated circuit process due to the difficulty in growth and processing, and is difficult to popularize on a large scale.

3、R. A. Schneider and C. T. -. Nguyen, "On/off switchable high-Qcapacitive-piezoelectric AlN resonators,"2014 IEEE 27th International Conference on Micro Electro Mechanical Systems (MEMS)San Francisco, CA 2014, pp. 1265 1268, doi 10.1109/MEMSYS 2014 6765879, proposes a switchable piezoelectric MEMS resonator. The electrode of the structure is suspended on the piezoelectric film, and the electrode presses the piezoelectric film to destroy resonance through electrostatic attraction so as to close the device. The quality factor of the resonator can reach 8800 at the working frequency of 300 MHz. But has a disadvantage in that the switching voltage required for electrostatic pull-in is high (more than 100V). In addition, the reliability is poor and the switching speed is limited because the switch is operated by a mechanical switch, and the reliability of long-time operation is problematic.

Disclosure of Invention

In order to realize good switching characteristic and isolation of the bulk acoustic wave filter and lower switching direct current bias voltage, the starting voltage is compatible with the voltage of a common CMOS integrated circuit (within 5V). The invention provides a switchable bulk acoustic wave filter, which utilizes the characteristics of piezoelectric semiconductor material layers and ternary compound layers with different forbidden band widths to form a semiconductor heterojunction to be used as a switching part of a control filter.

The invention is realized by at least one of the following technical schemes.

A switchable bulk acoustic wave filter comprising a number of series bulk acoustic wave resonators and a number of parallel bulk acoustic wave resonators;

the bulk acoustic wave resonator comprises a first metal layer, a piezoelectric layer and a second metal layer; the piezoelectric layer is located intermediate the first metal layer and the second metal layer.

Further, the first metal layer is a high work function metal and forms a schottky contact with the piezoelectric layer.

Further, the second layer of metal layer material is a metal with low acoustic loss.

The piezoelectric layer comprises a piezoelectric semiconductor material layer and a ternary compound layer, wherein the piezoelectric semiconductor material layer and the ternary compound layer have different forbidden band widths and form a heterojunction.

Further, the piezoelectric semiconductor material layers with different forbidden band widths select GaN (gallium nitride) as the piezoelectric semiconductor material and AlGaN (gallium aluminum nitride) as the ternary compound layer.

Further, the heterojunction generates a two-dimensional electron gas, and the generation and depletion of the two-dimensional electron gas are regulated through a reverse biased Schottky junction.

Furthermore, the thickness of the piezoelectric layer is determined by the frequency of a filter band required by the filter, the frequency of the filter band of the filter is finely adjusted by changing the thicknesses of the first metal layer and the second metal layer, and the central frequency of the filter is changed by adjusting the thicknesses of the first metal layer, the second metal layer and the piezoelectric layer so as to meet different industrial requirements.

Further, the high work function metal of the first metal layer is Ni.

Further, the low acoustic loss metal of the second metal layer is molybdenum (Mo) or tungsten (W).

The piezoelectric semiconductor material is mainly GaN, and the piezoelectric property and the semiconductor property of the material are utilized simultaneously. The piezoelectric property of the film bulk acoustic resonator is utilized to realize the interchange of acoustic and electric signals, and the film bulk acoustic resonator is excited to generate resonance, so that the film bulk acoustic resonator is manufactured. The semiconductor characteristic of the wide-bandgap piezoelectric semiconductor material enables the material to form a heterojunction with a ternary compound (commonly used like AlGaN) with wider bandgap to generate two-dimensional electron gas. The first metal layer is made of metal with high work function, such as Ni which is commonly used, and forms Schottky contact with the piezoelectric layer, and the generation and the depletion of two-dimensional electron gas can be controlled by adding direct current bias to the metal layer, so that the function of controlling the switch of the filter is achieved. The second metal layer is a metal layer with low acoustic loss, such as molybdenum (Mo) or tungsten (W), which enables sound waves to be transmitted and converted well.

Compared with the prior art, the invention has the following advantages and beneficial effects:

1) the heterojunction acts as a switching device. The invention simultaneously utilizes the piezoelectric property and the semiconductor property of the piezoelectric semiconductor material, and ternary compounds with different forbidden band widths are added on the wide-forbidden-band piezoelectric semiconductor material of the bulk acoustic wave resonator to form a heterojunction so as to generate two-dimensional electron gas. The bias voltage on the metal layer controls the generation and the depletion of the two-dimensional electron gas, thereby achieving the purpose of controlling the opening and the closing of the resonator. The invention has simple structure and convenient processing, and the number of mask plates is not required to be increased in the manufacturing process;

2) the low voltage achieves a switching effect. The filter is closed when the metal voltage bias on the upper layer of the filter is zero, and the heterojunction is used as the switch part of the bulk acoustic wave filter when negative low direct current bias is added on the upper layer. The structure is simple, good isolation can be realized by using low direct current bias, and the method has significance for wide development and application.

3) The starting voltage of the filter is far superior to that of the existing switching filter, and the filter is easy to realize in practical work.

Drawings

Fig. 1 is a structural diagram of a closed state of a switch type bulk acoustic wave resonator according to the embodiment;

fig. 2 is a structural diagram of the on state of the switch-type bulk acoustic wave resonator of the embodiment;

FIG. 3 is a band diagram of the switch-type bulk acoustic resonator of this embodiment under zero bias voltage;

FIG. 4 is a band diagram of the switch-type bulk acoustic resonator of this embodiment under negative bias on the electrodes;

fig. 5 is a diagram showing the simulation result of the electric field when the switched-mode bulk acoustic resonator 2DEG of the present embodiment exists;

fig. 6 is a graph showing the simulation result of the electric field of the depletion of the switching type bulk acoustic resonator 2DEG according to the present embodiment;

FIG. 7 is a comparison graph of the Y parameters of the ON state and the OFF state of the present embodiment;

FIG. 8 is a schematic diagram of a first-order L-type circuit of the present embodiment;

fig. 9 is a comparison graph of the S parameters of the bulk acoustic wave filter in the on state and the off state according to the embodiment.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.

As shown in fig. 8, a switchable bulk acoustic wave filter includes a plurality of series-connected bulk acoustic resonators and a plurality of parallel-connected bulk acoustic resonators;

the bulk acoustic wave resonator comprises a first metal layer 1, a piezoelectric layer 2 and a second metal layer 3; the piezoelectric layer is located in the middle of the first metal layer 1 and the second metal layer 3.

The first metal layer 1 is made of a high work function metal, and Ni is adopted in the embodiment.

The material of the second metal layer 2 is a metal with low acoustic loss, and the present embodiment uses Mo.

The piezoelectric layer 3 includes a piezoelectric semiconductor material layer and a ternary compound layer 4 having different forbidden band widths, and the piezoelectric semiconductor material layer and the ternary compound layer 4 constitute a heterojunction, as shown in fig. 1. In fig. 1 and 2, RF denotes a radio frequency circuit and DC denotes a DC bias.

The heterojunction refers to a junction composed of two materials with different forbidden band widths and lattice constants. The wider bandgap semiconductor material is located above the narrower bandgap semiconductor material as the piezoelectric layer 3, and the simplified structure diagram and energy band diagram, as shown in figure 3,E _c is the energy level of the conduction band,E _F is a fermi level, and is,E _v is valence band energy level. Conduction band break occurs at the conduction band edge at the heterojunction interface due to the difference in energy levels of the two materials in contact, forming a narrow quantum well. Under the action of the polarization electric field, electrons in the material on the wider side of the band gap are transferred in the quantum well. Because the width of the quantum well is narrow compared with the width of the channel, the movement of electrons in the direction vertical to the interface is limited, and the electrons can only move in two dimensions along the direction of the heterojunction interface, so that a two-dimensional electron gas (2 DEG) is formed at the heterojunction interface. In this embodiment, GaN is used as the piezoelectric semiconductor material, and AlGaN is used as the ternary compound.

The heterojunction polarization effect of the GaN and the AlGaN is very strong, so that the concentration of two-dimensional electron gas can reach QUOTE under the condition of unintentional doping Magnitude. Under the condition of no bias voltage, the piezoelectric layer of the bulk acoustic wave resonator is shielded by high-concentration two-dimensional electron gas, the electric field of the piezoelectric layer is limited in a thinner AlGaN layer (with the thickness of about 20 nanometers) and cannot enter a GaN layer (with the thickness of about 1-3 micrometers and depending on the working frequency) serving as a main piezoelectric layer, so that the bulk acoustic wave resonator cannot be normally excited to generate vibration, the bulk acoustic wave resonator is in a closed state, and the internal electric field distribution is shown in FIG. 5.

The concentration of the two-dimensional electron gas is controlled by the negative pressure bias added by the metal layer, when the metal layer is added with enough negative pressure, the two-dimensional electron gas in the quantum well is exhausted, the electric field can smoothly pass through the AlGaN layer to excite the GaN piezoelectric layer, so that the GaN piezoelectric layer generates resonance through piezoelectric conversion, namely, the filter is closed when the voltage bias of the first metal layer is zero, and the filter is opened when enough negative direct current bias voltage (such as-5V) is added to the first metal layer. The present invention utilizes a heterojunction as the switching portion of a bulk acoustic wave filter. The band diagram is shown in fig. 4. The bulk acoustic wave resonator can now operate normally as shown in figure 2, with the internal electric field distribution as shown in figure 6.

The switchable bulk acoustic wave filter can be built by adopting a Ladder filter topology structure, and is composed of a plurality of bulk acoustic wave resonators through a plurality of first-order L-shaped circuits formed by connecting in series and in parallel, as shown in fig. 8, based on the bulk acoustic wave resonators, establishing a model in multi-physical-field finite element simulation software to perform frequency characteristic simulation, setting the thickness of a piezoelectric layer 3 to be 0.88 μm, selecting a material of a wide-bandgap piezoelectric semiconductor material layer to be GaN to form a GaN layer, setting the thickness of the GaN layer to be 0.85 μm, selecting a material of a ternary compound with a larger bandgap to be AlGaN to form an AlGaN layer, setting the thickness of the AlGaN layer to be 0.03 μm, selecting Ni to be a material of a first metal layer, selecting Mo to be a material of a second metal layer, setting the thickness of the first metal layer of the series bulk acoustic wave resonator to be 0.0925 μm, the thicknesses of the first metal layer and the second metal layer of the bulk acoustic wave resonator connected in parallel are both 0.1 mu m. The resonator constructed by the thickness has the center frequency of 3.5GHz in a 5G key frequency band. When the voltage of the upper metal layer, namely the first metal layer, of the switchable bulk acoustic wave filter is zero, two-dimensional electron gas is accumulated on the AlGaN and GaN interface, the piezoelectric effect is inhibited, and the resonator is closed. When the first metal layer and the second metal layer are added with negative voltage bias, the two-dimensional electron gas is exhausted, and the bulk acoustic wave resonator is started. Admittance parameter (Y-parameter) frequency response pairs for the on-state and off-state obtained by COMSOL Multiphysics software simulation are for example fig. 7.

The positions of the series resonance point and the parallel resonance point of the resonator can be changed by slightly adjusting the thickness of the first metal layer, and when the series resonance point of the parallel resonator is coincided with the parallel resonance point of the series resonator, the filter has the best filtering performance. The selectivity of the filter increases as the order increases. In order to embody good filtering properties, the simulation results of S parameters of the 4 th-order L-type filter in the on state and the off state are selected for comparison, and as shown in fig. 9, the switch isolation in an ideal state can reach about 60 dB.

The above embodiments are preferred embodiments of the present invention, but the present invention is not limited to the above embodiments, and any other changes, modifications, substitutions, combinations, and simplifications which do not depart from the spirit and principle of the present invention should be construed as equivalents thereof, and all such changes, modifications, substitutions, combinations, and simplifications are intended to be included in the scope of the present invention.