阅读说明：本技术 一种可调谐低通滤波器及制备方法 (Tunable low-pass filter and preparation method thereof ) 是由 邱文才 赵纶 田学红 林满院 于 2021-02-26 设计创作，主要内容包括：本发明公开了一种可调谐低通滤波器及制备方法,包括：至少一个慢波共面波导CPW滤波单元；CPW滤波单元中包括电感串联电路以及微电子机械系统MEMS开关并联电路,用于通过改变CPW管线实现低通滤波调谐。本发明通过带有MEMS开关的可调低通滤波器,解决现有滤波器难以集成和调谐范围有限的问题,可以调谐两次并保持切比雪夫滤波器特性,实现大调谐范围的同时具备改进的带外特性隔离的效果。(The invention discloses a tunable low-pass filter and a preparation method thereof, wherein the tunable low-pass filter comprises the following steps: at least one slow wave coplanar waveguide (CPW) filtering unit; the CPW filtering unit comprises an inductance series circuit and a micro-electro-mechanical system (MEMS) switch parallel circuit and is used for realizing low-pass filtering tuning by changing a CPW pipeline. The tunable low-pass filter with the MEMS switch solves the problems that the existing filter is difficult to integrate and the tuning range is limited, can tune twice and keep the characteristics of the Chebyshev filter, realizes a large tuning range and has an improved out-of-band characteristic isolation effect.)

1. A tunable low-pass filter, comprising: at least one slow wave coplanar waveguide (CPW) filtering unit;

the CPW filtering unit comprises an inductance series circuit and a micro-electro-mechanical system (MEMS) switch parallel circuit and is used for realizing low-pass filtering tuning by changing a CPW pipeline.

2. Tunable low-pass filter according to claim 1, characterized in that the inductive series circuit comprises a first given number of gate inductances connected in series, forming a main transmission line.

3. The tunable low-pass filter according to claim 2, characterized in that the inductance value of each gate inductance comprised in the series circuit of inductances represents the inductance value of the main transmission line.

4. The tunable low-pass filter according to claim 1, characterized in that said MEMS switch parallel circuit comprises a second given number of electrostatic drive bridges;

and each electrostatic drive bridge is fixed on the ground wire of the CPW power transmission line and is connected in parallel to form a second-order adjustable device low-pass filter.

5. The tunable low pass filter of claim 4, wherein the capacitor within the electrostatic drive bridge is a metal-insulator-metal capacitor controlled by a MEMS capacitive switch.

6. The tunable low-pass filter according to claim 4, characterized in that said second given number is 5;

the electrostatic drive bridge forms an MEMS switch through pull-down voltage, and when the MEMS switch is regulated and controlled in an on-off state, the capacitance value of a capacitor in the electrostatic drive bridge is changed along with the MEMS switch, so that the cut-off frequency of the filter is controlled and changed.

7. A method of making a tunable low pass filter, comprising:

providing a preparation substrate of a standard radio frequency micro-electro-mechanical system (MEMS) process, and paving 520 micrometer thick quartz on the preparation substrate;

by evaporation of Ti/AuThe seed layer is additionally provided with a bottom slow wave coplanar waveguide CPW line and a bias network, and a metal protection layer with the thickness of 2 microns is coated to form the CPW line;

by evaporation of Ti/AuThe seed layer is additionally provided with a buttress of the MEMS switch, and a metal protective layer with the thickness of 4 microns is coated and plated to form an MEMS bridge;

deposition ofThick PECVD silicon nitride to dielectric layers of CPW lines and MEMS bridges;

polyimide is adopted as a sacrificial layer to pattern and thermally cure the CPW line and the MEMS bridge at a high temperature of 200 ℃;

evaporating the MEMS bridge by adopting aluminum with the thickness of 2 microns, removing the sacrificial layer by adopting oxygen plasma, and releasing the MEMS switch;

obtaining a tunable low-pass filter according to any of claims 1-7.

8. The method of claim 7, wherein each MEMS switch has a length of 460 microns and a width of 160 microns.

9. The method of claim 7, wherein each MEMS switch is controlled by three separate dc pad voltages.

10. The method for manufacturing the tunable low-pass filter, according to claim 7, is characterized in that single-ended probes with the spacing of 150 microns of a ground signal ground excitation filter are used in the manufactured tunable low-pass filter.

Technical Field

The embodiment of the invention relates to the technical field of radio frequency millimeter wave low-pass filters, in particular to a tunable low-pass filter and a preparation method thereof.

Background

Tunable filters have been widely used in military and military applications, for example, in multi-standard transceivers. The tunable filter significantly reduces the size of the analog front end, suppresses large signal interference and completes multi-frequency conversion. But typical tunable filters are bulky and consume a lot of power. Radio frequency micro-electro-mechanical systems (RF MEMS) technology offers the potential for low loss, near zero power consumption, high volume, and high linearity filtering comparable to integrated circuits. In recent years, some researchers have begun to apply RF-MEMS to the development of tunable filters. These filters include centralized designs and distributed designs. While these filter components are suitable for tunable designs, they typically have large dimensions, non-coplanar characteristics, and low Q values of conventional lumped elements, which are difficult to integrate with integrated circuits. Those filters can also be divided into two different types, analog and digital. Analog tuning provides continuous passband frequency variation, but has a limited tuning range.

Disclosure of Invention

The invention provides a tunable low-pass filter and a preparation method thereof, which are used for realizing twice tuning, keeping the characteristics of a Chebyshev filter, realizing large tuning range and simultaneously having improved out-of-band characteristic isolation, wherein the pass-band ripple is less than 2dB, and the maximum out-of-band rejection is better than 20 dB.

In a first aspect, an embodiment of the present invention provides a tunable low-pass filter, including: at least one slow wave coplanar waveguide (CPW) filtering unit;

the CPW filtering unit comprises an inductance series circuit and an MEMS switch parallel circuit and is used for realizing low-pass filtering tuning by changing a CPW pipeline.

Further, the inductive series circuit comprises a first given number of gate inductors connected in series, forming a main transmission line.

Further, an inductance value of each gate inductance included in the inductance series circuit represents an inductance value of the main transmission line.

Further, the MEMS switch parallel circuit comprises a second given number of electrostatic drive bridges;

and each electrostatic drive bridge is fixed on the ground wire of the CPW power transmission line and is connected in parallel to form a second-order adjustable device low-pass filter.

Further, the capacitor in the electrostatic drive bridge is a metal insulator metal capacitor controlled by a MEMS capacitive switch.

Further, the second given number is 5;

the electrostatic drive bridge forms an MEMS switch through pull-down voltage, and when the MEMS switch is regulated and controlled in an on-off state, the capacitance value of a capacitor in the electrostatic drive bridge is changed along with the MEMS switch, so that the cut-off frequency of the filter is controlled and changed.

In a second aspect, an embodiment of the present invention further provides a method for manufacturing a tunable low-pass filter, where the method includes:

providing a preparation substrate of a standard radio frequency MEMS (micro-electromechanical systems) process, and paving 520 micrometer thick quartz on the preparation substrate;

by evaporation of Ti/AuThe seed layer is additionally provided with a bottom CPW line and a bias network, and a metal protection layer with the thickness of 2 microns is coated to form the CPW line;

by evaporation of Ti/AuThe seed layer is additionally provided with a buttress of the MEMS switch, and a metal protective layer with the thickness of 4 microns is coated and plated to form an MEMS bridge;

deposition ofThick PECVD silicon nitride to dielectric layers of CPW lines and MEMS bridges;

polyimide is adopted as a sacrificial layer to pattern and thermally cure the CPW line and the MEMS bridge at a high temperature of 200 ℃;

evaporating the MEMS bridge by adopting aluminum with the thickness of 2 microns, removing the sacrificial layer by adopting oxygen plasma, and releasing the MEMS switch;

the above-described tunable low-pass filter is obtained.

Further, each MEMS switch has a length of 460 microns and a width of 160 microns.

Further, each MEMS switch is controlled by three independent dc pad voltages.

Further, single-ended probes with a spacing of 150 microns are ground signal excitation filters used in the prepared tunable low-pass filter.

The tunable low-pass filter with the MEMS switch solves the problems that the existing filter is difficult to integrate and the tuning range is limited, can tune twice and keep the characteristics of the Chebyshev filter, realizes a large tuning range and has an improved out-of-band characteristic isolation effect.

Drawings

FIG. 1 is a simplified equivalent circuit model diagram of a low-pass filtering unit according to a first embodiment of the present invention;

FIG. 2 is a schematic diagram of a low-pass filtering unit of a CPW structure according to a first embodiment of the present invention;

FIG. 3 is a flow chart of a manufacturing process of a low pass filter according to a second embodiment of the present invention;

FIG. 4a is a schematic diagram of a pier for forming a coplanar waveguide line and a MEMS switch according to a second embodiment of the present invention;

FIG. 4b is a schematic illustration showing the patterning of a dielectric layer according to a second embodiment of the present invention;

FIG. 4c is a schematic diagram of a sacrificial layer spin-on and aluminum bridge evaporation in a second embodiment of the present invention;

FIG. 4d is a schematic illustration of a sacrificial layer etching and MEMS bridge release in accordance with a second embodiment of the present invention.

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings and examples. It is to be understood that the specific embodiments described herein are merely illustrative of the invention and are not limiting of the invention. It should be further noted that, for the convenience of description, only some of the structures related to the present invention are shown in the drawings, not all of the structures.

Example one

Fig. 1 is a simplified equivalent circuit model diagram of a low-pass filtering unit according to an embodiment of the present invention, where the tunable low-pass filter provided in this embodiment specifically includes:

at least one CPW filtering unit 10.

The CPW filtering unit 10 includes an inductor series circuit 11 and a MEMS switch parallel circuit 12, and is configured to implement low-pass filtering tuning by changing a CPW pipeline.

As shown in fig. 1, the MEMS switch parallel circuit 12 and the inductance series circuit 11 are main circuit parameters in an equivalent circuit, and C1, C2, and C3 are metal-insulator-metal (MIM) capacitors controlled by MEMS capacitance switches. L1, L2, L3 are inductance values of the transmission line, respectively, L3 is a gate inductance, and the calculation formula of the linear or strip inductance can be written as:

where L is the segment inductance in nanohenries, L, w, t are the segment length, width and thickness, each in centimeters. Further, the upstream and downstream capacitances can be expressed as:

wherein A is the plate area of the top of the center conductor, d is the distance between the bridge and the CPW line in the upward state, and t_dIs the thickness of the dielectric material at the top of the CPW line, ε_rIs the dielectric constant, ε, of the material used₀Is the dielectric constant in vacuum. C_frngIs an additional capacitance generated by the edge field effect, t_roughIs an effective thickness taking roughness into account.

In the present embodiment, the inductor series circuit 11 comprises a first given number of gate inductors connected in series, forming a main transmission line.

In particular, the first given number may be an integer greater than 1. Preferably, as shown in fig. 1, the CPW filtering unit 10 includes an inductor series circuit 11, and the inductor series circuit 11 is composed of 6 gate inductors.

Further, the inductance value of each gate inductance included in the inductance series circuit 11 represents the inductance value of the main transmission line.

In the present embodiment, the MEMS switch parallel circuit 12 includes a second given number of electrostatic drive bridges; and each electrostatic drive bridge is fixed on the ground wire of the CPW power transmission line and is connected in parallel to form a second-order adjustable device low-pass filter.

Specifically, the MEMS switch parallel circuit 12 includes an electrostatic drive bridge therein. Preferably, as shown in fig. 1, the MEMS switch parallel circuit 12 includes 5 parallel electrostatic drive bridges.

Further, the capacitor in the electrostatic drive bridge is a metal-insulator-metal capacitor controlled by the MEMS capacitive switch.

Further, the second given number is 5; the electrostatic drive bridge forms an MEMS switch through pull-down voltage, and when the MEMS switch is regulated and controlled in an on-off state, the capacitance value of a capacitor in the electrostatic drive bridge is changed along with the MEMS switch, so that the cut-off frequency of the filter is controlled and changed.

Specifically, when the amplitude of the input signal is kept unchanged, the frequency is changed to reduce the output signal to 0.707 times of the maximum value, and the point-3 dB is the cut-off frequency expressed by the frequency response characteristic. The cut-off frequency of the filter is determined by the values of C1, C2, C3, L1, L2 and L3. Once the MEMS bridge is pulled down, the switch will be set and the capacitance value will be changed, and the cut-off frequency of the filter will be changed at the same time.

Fig. 2 is a schematic diagram of a low-pass filter unit with a CPW structure according to a first embodiment of the present invention, as shown in fig. 2, the low-pass filter includes 5 electrostatic driving bridges 121, and when a signal is input, the filter is shown in fig. 2.

In this embodiment, the Ansoft HFSS software is used to build a filter model, so that parameters in various states can be simulated. Table 1 shows the parameters of the filter, and as shown in table 1, it is obvious that the frequency of the filter can be adjusted twice, the range can be as large as 30% and 105%, and when the insertion loss is less than 1.2dB, the maximum isolation can be better greater than 27 dB.

TABLE 1 parameters of the Filter

Parameter	State a	State b	State c
				3dB cut-off frequency	7.1	9.2	14.55
Insertion loss (dB)	<1.2	<0.79	<1.01
				Maximum isolation (dB)	27.1	27.9	63

In this example, the characteristics of the tunable filter were measured using an agilent PNA E8363B vector network analyzer. By measuring the 3dB cut-off frequency of the tunable low-pass filter, the capacitance of the filter changes when the switch is switched to the "on" or "off" state in the manner described previously, with the 3dB cut-off frequencies of these three states being 8.2GHz, 10.5GHz and 16.8GHz, respectively. The insertion loss is less than 2dB and the out-of-band isolation of the three states is better than 20 dB. Due to the parasitic effect of the MEMS and the dielectric surface roughness layer, the measured 3dB cut-off frequency, the insertion loss and the out-of-band isolation of the three states are not completely consistent with the simulation result, and the driving voltage of the MEMS switch is about 40V.

The test result shows that the tunability of the filter based on the MEMS capacitive switch structure is realized, and the 3dB cut-off frequency of the filter can be respectively adjusted to 8.2GHz, 10.5GHz and 16.8GHz by operating the MEMS switch. The tunable low pass filter can be tuned twice and maintain the chebyshev filter characteristics, achieving a large tuning range while having improved out-of-band characteristic isolation. In the future, the filter can be applied to a tunable radio frequency front end system of radar, a wireless and multi-frequency communication system and the like.

The tunable low-pass filter provided by the embodiment solves the problems that the existing filter is difficult to integrate and the tuning range is limited, can tune twice and keep the characteristics of the Chebyshev filter, realizes a large tuning range and has an improved out-of-band characteristic isolation effect.

Example two

Fig. 3 is a flowchart of a manufacturing process of a low-pass filter according to a second embodiment of the present invention, which is applicable to a case of manufacturing a low-pass filter, and includes the following specific steps:

step 210, providing a preparation substrate of a standard radio frequency micro-electro-mechanical system MEMS process, and laying 520 micrometer thick quartz on the preparation substrate.

Step 220, by evaporating Ti/AuThe seed layer is additionally provided with a bottom slow wave coplanar waveguide CPW line and a bias network, and a metal protection layer with the thickness of 2 microns is coated on the seed layer to form the CPW line.

Step 230, by evaporating Ti/AuThe seed layer is additionally provided with a buttress of the MEMS switch, and a metal protective layer with the thickness of 4 microns is coated and plated to form the MEMS bridge.

Step 240, depositingThick PECVD silicon nitride to the dielectric layer of the CPW lines and MEMS bridge.

Step 250, polyimide is used as a sacrificial layer to pattern and thermally cure the CPW lines and MEMS bridge at a high temperature in the range of 200 ℃.

Step 260, evaporating the MEMS bridge with 2 micron thick aluminum, removing the sacrificial layer with oxygen plasma, and releasing the MEMS switch.

Step 270, obtaining the tunable low-pass filter described in the above embodiment.

In the present embodiment, each MEMS switch has a length of 460 microns and a width of 160 microns.

Further, each MEMS switch is controlled by three independent dc pad voltages.

Further, single-ended probes with a spacing of 150 microns are ground signal excitation filters used in the prepared tunable low-pass filter.

Specifically, in the fabrication of the tunable low pass filter with MEMS switches, the low pass filter is fabricated on 520um thick quartz using a substrate 21 of standard radio frequency MEMS process using coplanar waveguide lines and MEMS switches. Bottom CPW line and bias network by evaporation of Ti/AuThen gold is plated, and the thickness of the seed layer is 2 um.

The rest 22 of the MEMS switch is made in the same way, but with a thickness of 4um, as shown in fig. 4 a. Next, as shown in figure 4b,thick PECVD silicon nitride is deposited to form the dielectric layer 23 and avoid short circuits between the MEMS bridge and the CPW lines, polyimide is used as the sacrificial layer 24, and patterned for hanging contact bars. The sacrificial layer 24 is patterned and thermally cured at an elevated temperature of about 200 deg.c. Next, as shown in FIG. 4c, the MEMS bridge is evaporated with 2 micron thick aluminum. Finally, as shown in FIG. 4d, the sacrificial layer 24 is removed using an oxygen plasma process, releasing the MEMS switch. The manufactured tunable low-pass filter consists of five MEMS switches and a CPW pipeline. MEMS switches length and width 460um and 160 um. He has a main bodyEach controlled by three independent dc bias lines, the filters are excited using Ground Signal Ground (GSG) single ended probes spaced 150um apart. The chip size of the low pass filter including the dc bias line is 2.5mm x 1.2 mm.

It is to be noted that the foregoing is only illustrative of the preferred embodiments of the present invention and the technical principles employed. It will be understood by those skilled in the art that the present invention is not limited to the particular embodiments described herein, but is capable of various obvious changes, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, although the present invention has been described in greater detail by the above embodiments, the present invention is not limited to the above embodiments, and may include other equivalent embodiments without departing from the spirit of the present invention, and the scope of the present invention is determined by the scope of the appended claims.