CN111044800B - State-controllable symmetrical thermoelectric MEMS microwave standing wave meter and preparation method - Google Patents
State-controllable symmetrical thermoelectric MEMS microwave standing wave meter and preparation method Download PDFInfo
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Abstract
The state-controllable symmetrical thermoelectric MEMS microwave standing wave meter adopts a symmetrical directional coupler to respectively extract incident microwave power and reflected microwave power to a coupling end and an isolation end of an upper branch and a lower branch; the thermoelectric MEMS microwave power sensor is adopted to measure the power of each port of the two branches, and the average value is taken to obtain more accurate incident and reflected microwave power so as to obtain the size of standing-wave ratio; the two designed branches work independently and can realize the measurement function, so that the failure rate is reduced; in order to realize the state conversion function of the MEMS microwave standing wave meter, an MEMS microwave switch is added on an extraction branch of the MEMS microwave standing wave meter; when the MEMS microwave switch is in an off state, incident microwave power is not extracted any more and is directly transmitted to an output port, so that the loss of the microwave power when detection is not needed is reduced. The state-controllable symmetrical thermoelectric type MEMS microwave standing wave meter improves the reliability of the MEMS microwave standing wave meter, and has the characteristics of low loss, small chip area and compatibility with a gallium arsenide monolithic microwave integrated circuit process.
Description
Technical Field
The invention relates to a microwave standing wave meter for detecting standing wave ratio on line and a preparation method thereof, in particular to a state-controllable symmetrical thermoelectric MEMS microwave standing wave meter based on MEMS (Micro-Electro-Mechanical-System) technology and a preparation method thereof.
Background
In a modern microwave high-density integrated micro system, a microwave standing wave meter is a key element of a microwave system module self-detection application and is used for representing the size of a standing wave ratio in the microwave system. The microwave high-density integrated micro system requires high integration of modules such as an antenna and a TR component, and is used for micro detection, interference, frequency detection and the like. Along with the fact that the size of a microwave system is smaller and the integration degree is higher and higher, the highly integrated microwave system is large in performance difference, difficult to disassemble and measure, and prone to component failure caused by long-term work and environmental influence, and therefore the standing-wave ratio of the microwave system is extremely important to achieve on-line detection. At present, there are two main methods for measuring standing-wave ratio of microwave system: one is based on network analyzer measurements, which can provide relatively accurate and complete signal measurements, but which are only suitable for systems that are not operating, and which require manual operation of the instrument each time to complete a single test. For many modules that often need to be tested, measuring the reliability of a microwave system in this way is significantly more cumbersome. The other is based on microwave standing wave meter measurement, the method embeds a microwave standing wave meter between an amplifier and an antenna, can perform measurement when a microwave system is in an operating state, and can perform continuous test. The measurement is convenient and quick, is especially suitable for large-scale detection, and has small influence on a microsystem. Generally, a microwave standing wave meter mainly includes two parts: the microwave signal extracting part is the microwave signal extracting part, and the microwave signal detecting part is the microwave signal extracted part. For the extraction part of the microwave signal, the structure can be divided into a structure based on a multi-port annular junction, a structure based on a sampling transmission line, a structure based on a directional coupler and the like; the detection section from which the microwave signal is extracted generally employs a diode, a logic circuit, a thermistor, and the like. However, these standing wave meters exhibit large volume, active detection, single operating frequency and inability to perform state switching functions. Therefore, it is urgently needed to develop a microwave standing wave meter with miniaturization, low power consumption, integration, low cost, controllable working state and on-line detection standing wave ratio, so as to be embedded into a microwave high-density integrated micro system, thereby realizing the self-detection of the microwave micro system. With the intensive research of the MEMS technology, it becomes possible to develop a symmetrical thermoelectric MEMS microwave standing wave meter that realizes the above functions based on the MEMS technology.
Disclosure of Invention
The invention aims to overcome the defects in the prior art, and provides a state-controllable symmetrical thermoelectric MEMS microwave standing wave meter which is formed by a fully passive structure, wherein microwave signals are input from a coplanar waveguide (CPW) transmission line at a first port, transmitted to a CPW transmission line at a second port through a main transmission line of a symmetrical directional coupler and output, so that the structure does not consume direct current power and is an online device; the preparation method is compatible with the gallium arsenide-based monolithic microwave integrated circuit process; therefore, the state-controllable symmetrical thermoelectric MEMS microwave standing wave meter provided by the invention has the characteristics of miniaturization, low power consumption, integration, low cost, controllable working state and on-line detection of standing wave ratio.
In the symmetrical directional coupler, two sections of asymmetrical coplanar strip line (ACPS) transmission lines are symmetrically arranged on the upper side and the lower side of a main transmission line, two ends of the two sections of ACPS transmission lines are respectively connected to CPW transmission lines at three, four, five and six ports to form the symmetrical directional coupler with two coupling ends and two isolation ends, the three ports and the five ports are coupling ends, and the four ports and the six ports are isolation ends; the tail ends of the third port, the fourth port, the fifth port and the sixth port are respectively connected with a thermoelectric MEMS microwave power sensor, so that the microwave power output from the ports can be measured, and as the coupling degree of the directional coupler is far greater than the isolation degree, the magnitude of the microwave power at the isolation ends (the fourth port and the sixth port) can be analyzed to obtain the magnitude of the reflected power on a main transmission line, so that the magnitude of the standing-wave ratio when the microwave system is in a working state is obtained, and the function of detecting the standing-wave ratio on line of the symmetrical thermoelectric MEMS microwave standing-wave meter is realized; the function of controlling the connection and disconnection of the CPW transmission line can be realized by placing four same MEMS microwave switches at the connection nodes of the CPW transmission line and the ACPS transmission line of the symmetrical directional coupler; the MEMS microwave switch adopts a parallel capacitance structure, an upper polar plate and a lower polar plate of the capacitance switch are respectively an MEMS clamped beam and a CPW signal line, wherein the MEMS clamped beam spans over the CPW signal line, two ends of the MEMS clamped beam are respectively fixed on CPW ground wires at two sides of the CPW signal line through anchor areas, and two driving electric currents are symmetrically arranged at two sides of the CPW signal line below the MEMS clamped beamA electrode covered with silicon nitride (Si) on the CPW signal line and the drive electrode3N4) An insulating dielectric layer; when the standing-wave ratio needs to be measured, no direct current driving voltage is applied between the MEMS clamped beam and the driving electrode, the MEMS clamped beam is in an Up state, a transmission channel of the CPW signal line is conducted, and the thermoelectric MEMS microwave power sensor receives microwave power; when the standing-wave ratio does not need to be measured, a direct-current driving voltage with opposite polarity is applied between the MEMS clamped beam and the driving electrode, and the MEMS clamped beam bends downwards due to the action of electrostatic force and is attracted to the Si3N4On the upper surface of the insulating medium layer, at the moment, the MEMS clamped beam is in a Down state, the thermoelectric MEMS microwave power sensor cannot receive microwave signals, the transmission channel of the CPW signal line is disconnected, and input microwave power is directly transmitted to the output end, so that the loss of the microwave power under the condition of no need of detection is reduced, and the symmetrical thermoelectric MEMS microwave standing wave meter has two working states of detection and non-detection, namely the working state is controllable; by adopting the optimized material and optimizing the simulation structure for multiple times in the processing process, the state-controllable symmetrical thermoelectric MEMS microwave standing wave meter has smaller microwave loss and chip area,
in order to achieve the purpose, the method adopted by the invention is as follows: a state-controllable symmetrical thermoelectric MEMS microwave standing wave meter adopts gallium arsenide (GaAs) as a substrate, a CPW transmission line, an ACPS transmission line, a symmetrical directional coupler, four identical MEMS microwave switches, an air bridge, a connecting line and four identical thermoelectric MEMS microwave power sensors are arranged on the GaAs substrate, and an MEMS substrate membrane structure is designed near a load resistance substrate:
the CPW transmission line is horizontally arranged on the GaAs substrate and used as the input and the output of the MEMS microwave standing wave meter; and the CPW transmission line at the coupling port and the isolation port is used for realizing the transmission of the coupled microwave signals. The input port on the left side is a port one, the output port on the right side is a port two, the coupling port on the left side above is a port three, the isolation port on the right side above is a port four, the coupling port on the left side below is a port five, and the isolation port on the right side below is a port six. The CPW transmission line is composed of a signal line and two ground lines, wherein the ground lines are located on both sides of the signal line. In order to realize microwave matching between the port and the outside, the characteristic impedance of the port of the CPW transmission line is usually designed to be 50 Ω.
Two sections of ACPS transmission lines are placed on the GaAs substrate, each section of ACPS transmission line is composed of a signal line and a ground line, and the signal line of the ACPS transmission line is used as a secondary transmission line of a symmetrical directional coupler in the MEMS microwave standing wave meter. The signal lines of the two sections of ACPS transmission lines are symmetrically positioned at the upper side and the lower side of the main transmission line in the symmetrical directional coupler, and the lengths of the signal lines are quarter wavelengths. In the symmetrical directional coupler, two ends of each section of auxiliary transmission line are respectively a coupling end and an isolation end, wherein the coupling end is close to the input end, and the isolation end is close to the output end. In order to realize microwave matching in connection with the CPW transmission line, the characteristic impedance of the ACPS transmission line is designed to be 50 Ω.
The symmetrical directional coupler is positioned among input, output, coupling and isolating ports of the MEMS microwave standing wave meter and mainly comprises a main transmission line and two sections of auxiliary transmission lines, wherein the two sections of auxiliary transmission lines are positioned on the upper side and the lower side of the main transmission line, and the distances from the two sections of auxiliary transmission lines to the main transmission line are equal. The coupling degree and the isolation degree of the upper section of the auxiliary transmission line and the lower section of the auxiliary transmission line of the symmetrical directional coupler are equal. The two ports on the left side are coupling ends, and the two ports on the right side are isolation ends.
Each MEMS microwave switch mainly comprises an MEMS clamped beam, a CPW transmission line, two driving electrodes, two pressure welding blocks, a connecting line and Si3N4An insulating medium layer and an air bridge.
Four identical MEMS microwave switches are arranged at the connection nodes of the CPW transmission line and the ACPS transmission line of the symmetrical directional coupler; the MEMS microwave switch adopts a parallel capacitance structure, wherein an upper polar plate is an MEMS clamped beam, and a lower polar plate is a signal line of a CPW transmission line. The MEMS clamped beam is a suspended clamped beam structure provided with through holes at equal intervals, and spans over the signal line of the CPW transmission line, and two ends of the MEMS clamped beam are fixed on the CPW ground lines at two sides of the CPW signal line through anchor areas; the through holes on the MEMS clamped beam are beneficial to releasing the clamped beam. Each MEMS microwave switch contains two drive electrodes,the CPW signal lines are positioned on the left and right sides below the MEMS clamped beam and are Si3N4The insulating medium layer covers the driving electrode below the MEMS clamped beam and the signal line of the CPW transmission line, so that the MEMS clamped beam is isolated from the direct current of the signal line of the CPW transmission line; the connecting wires interconnect the driving electrodes with external bonding pads. The air bridge is a suspended clamped beam structure and is used for realizing interconnection of CPW ground wires separated by connecting wires.
Each thermoelectric MEMS microwave power sensor mainly comprises a CPW transmission line and Si3N4The device comprises an insulating medium layer, a pressure welding block, two load resistors, a thermopile and an MEMS substrate film structure.
Eight same load resistors are placed in pairs at the two coupling ports and the two isolation ports. The coupling port and the isolation port are designed by adopting a CPW transmission line. Each coupling port and each isolation port are connected with two load resistors in parallel, and the resistance value of each load resistor is 100 omega.
Four identical thermopiles are placed close to, but not in contact with, the load resistors at the two coupled ports and the two isolated ports, respectively. Each thermopile is formed by connecting ten pairs of thermocouples in series, wherein each pair of thermocouples comprises a semiconductor arm and a metal arm, and are connected at one end by using a metal connecting wire. And a plurality of pairs of thermocouples are connected in series, so that the sensitivity of measuring the temperature can be increased. The semiconductor arm, the metal arm and the load resistor are covered with Si3N4And the insulating medium layer is used for playing a role in protecting in the preparation process. When the load resistor absorbs microwave power to generate heat, the temperature of one end of the thermopile close to the load resistor is increased, and the end is called the hot end of the thermopile; and the temperature of the other end of the thermopile far away from the load resistor is almost kept constant and is the ambient temperature, namely the cold end of the thermopile. When the load resistor absorbs microwave power to generate heat, the temperature of the hot end and the cold end of the thermopile is different, based on the Seebeck effect, the thermopile generates output hot voltage, and the hot voltage reflects the temperature of the load resistor. In order to improve the heat transfer efficiency from the load resistor to the hot end of the thermopile and further improve the temperature difference between the hot and cold ends of the thermopileAnd etching and thinning the GaAs substrate near the lower part of the load resistor by a body etching technology to form an MEMS substrate film structure. The temperature of the load resistor at the moment can be obtained by measuring the thermal voltage on the pressure welding blocks on the two sides of the thermopile, and the microwave signal power of the port can be further obtained.
In the mechanical structure, the CPW transmission line, the ACPS transmission line, the symmetrical directional coupler, the thermoelectric MEMS microwave power sensor, the MEMS microwave switch, the pressure welding block, the air bridge and the connecting line are positioned on the same GaAs substrate.
The state-controllable symmetrical thermoelectric MEMS microwave standing wave meter is formed by adopting a fully passive structure, the input microwave power on a main transmission line is coupled to a port III and a port V of a coupling end through two sections of ACPS auxiliary transmission lines, and the reflected microwave power is coupled to a port IV and a port VI of an isolation end; the four same thermoelectric MEMS microwave power sensors are respectively positioned at the tail ends of the third port, the fourth port, the fifth port and the sixth port and are used for measuring the coupled microwave power at the ports, wherein the four same thermoelectric MEMS microwave power sensors are completely the same and work on the basis of the microwave power-heat-electricity conversion principle, so that a symmetrical thermoelectric MEMS microwave standing wave meter is formed; the four same MEMS microwave switches are respectively arranged at the connection nodes of the CPW transmission line and the ACPS transmission line of the symmetrical directional coupler, so that the on-off states of the four branches are controlled; the MEMS microwave switch mainly comprises an MEMS clamped beam and a driving electrode; the MEMS clamped beam spans the CPW signal line, and two ends of the MEMS clamped beam are fixed on CPW ground lines on two sides of the CPW signal line below the MEMS clamped beam through anchor areas; the two driving electrodes are symmetrically distributed on two sides of the CPW signal line below the MEMS clamped beam and are connected with the pressure welding block on the outer side of the CPW through a connecting line; covering a layer of Si on the CPW signal line and the driving electrode under the MEMS clamped beam3N4And an insulating dielectric layer. When no driving voltage is applied between the MEMS clamped beam and the driving electrode, the MEMS clamped beam is in an Up state, the MEMS microwave standing wave meter is in a detection working state, and microwave power incident from the port I and reflected from the port II pass through the symmetrical directional coupler to apply microwave power in a certain proportionThe microwave power extracted from the port I is transmitted to the port III and the port V, the microwave power extracted from the port II is transmitted to the port IV and the port VI, the microwave power transmitted to the port III and the port V is equal, the microwave power transmitted to the port IV and the port VI is equal, the coupled microwave power is completely consumed by load resistors at the ports III, IV, V and VI to generate heat, the temperature around the load resistors is increased, the temperature change is detected by a thermopile arranged near the load resistors, and the thermopile is converted into output thermoelectric voltage based on the Seebeck effect, so that the measurement of the microwave power incident at the port I and the microwave power reflected at the port II is realized, and the standing wave ratio can be further obtained; in the detection state, only a certain proportion of microwave power is coupled for measurement, and most of the microwave power is available, so that the standing-wave ratio is detected on line; when a driving voltage with opposite polarity is applied between the MEMS clamped beam and the driving electrode, the height of the MEMS clamped beam can be reduced until the lower surface of the MEMS clamped beam is attracted to Si3N4On the insulating medium layer, the MEMS clamped beam is in a Down state at the moment, the capacitance between the MEMS clamped beam and the CPW signal line below the MEMS clamped beam is rapidly increased in the Down state, so that the load resistance at the tail ends of the CPW at the coupling end and the isolation end is short-circuited, the MEMS microwave standing wave meter is in a non-detection working state, and the input microwave power is directly transmitted to the output end so as to reduce the loss of the microwave power under the condition of not needing detection; the MEMS clamped beam structure is in an Up state and a Down state by not applying and applying driving voltage between the MEMS clamped beam and the driving electrode, so that the symmetrical thermoelectric MEMS microwave standing wave meter has two working states of detection and non-detection, namely the working state is controllable; the directional coupler usually has only one section of ACPS auxiliary transmission line, one coupling end and one isolation end, but the measurement accuracy of the microwave standing wave meter can be improved by adopting a multi-measurement mode through designing two symmetrical coupling branches. For example, when two branches of the symmetrical directional coupler work normally, the average value of the measured thermal voltages at two coupling ends (isolation ends) is taken as a final value; when is paired withThe upper half branch or the lower half branch of the symmetrical directional coupler can not work normally, and the other branch which works normally can still measure the standing-wave ratio, so that the reliability of the MEMS microwave standing-wave meter is improved obviously.
The state-controllable symmetrical thermoelectric MEMS microwave standing wave meter is prepared by adopting a gallium arsenide monolithic microwave integrated circuit and an MEMS process, and the preparation method comprises the following specific processing steps:
(1) selecting a gallium arsenide epitaxial wafer as a substrate, wherein the gallium arsenide substrate is semi-insulating, and the square resistance of the epitaxial layer is 100-130 omega; coating photoresist, removing the photoresist in the region outside the semiconductor arm of the thermocouple, etching the front epitaxial layer, and then removing the photoresist to form the semiconductor arm of the thermopile;
(2) sputtering AuGeNi/Au, forming a metal arm and a pressure welding block of the thermopile by adopting a stripping process, and then carrying out rapid thermal annealing to ensure that the semiconductor arm and the metal arm have good ohmic contact;
(3) wet etching the semiconductor arm of the thermopile to reduce the thickness of the semiconductor arm, so that the resistance of the thermopile is increased to about 300K omega;
(4) coating photoresist on the GaAs substrate obtained in the step (3), removing the photoresist at the position where the load resistor is to be manufactured, sputtering to grow TaN, and stripping to form a load resistor with the square resistance of 25 omega;
(5) coating photoresist on the GaAs substrate obtained in the step (4), removing the photoresist at the positions of preparing the CPW transmission line, the ACPS transmission line, the driving electrode, the pressure welding block and the connecting line, growing a layer of Ti/Pt/Au/Ti in an evaporation mode, and preliminarily forming the CPW transmission line, the ACPS transmission line and the pressure welding block by adopting a stripping process to completely form the driving electrode and the connecting line;
(6) growing a layer of Si 2300A thick by a plasma enhanced chemical vapor deposition process3N4Insulating dielectric layer, photoetching and etching Si3N4An insulating medium layer, and Si remained on the CPW signal line, the drive electrode and the connecting line under the semiconductor arm, the metal arm, the load resistor and the MEMS clamped beam3N4An insulating dielectric layer;
(7) spin-coating a polyimide sacrificial layer with the thickness of 1600nm, photoetching and etching the polyimide sacrificial layer, and reserving the polyimide sacrificial layer below the air bridge and the MEMS clamped beam to be prepared;
(8) evaporating the seed layer titanium/gold/titanium for electroplating, photoetching and removing the photoresist at the position to be electroplated;
(9) growing a layer of Au on the seed layer obtained in the step (8) in an electroplating mode, removing photoresist at a position where electroplating is not needed, reversely etching titanium/gold/titanium, and corroding bottom gold to completely form structures such as a CPW transmission line, an ACPS transmission line, an MEMS clamped beam, an air bridge, a pressure welding block and the like;
(10) coating a layer of photoresist on the front surface of the substrate for protection, and carrying out operation on the back surface of the GaAs substrate;
(11) coating a layer of photoresist on the back of the GaAs substrate, photoetching the photoresist, removing the photoresist below the load resistor and the hot end of the thermopile, reserving the region except the region below the load resistor and the hot end of the thermopile, and carrying out wet etching on the back of the substrate to form an MEMS substrate film structure, wherein the thickness of the MEMS substrate film structure is less than 20 microns;
(12) and (3) removing the front protection material, scribing the front of the substrate, corroding the sacrificial layer manufactured in the step (7) by using a developing solution, releasing the MEMS clamped beam and the air bridge, slightly soaking by using deionized water, dehydrating by using absolute ethyl alcohol, volatilizing at normal temperature, and airing.
Has the advantages that:
compared with the prior art, the state-controllable symmetrical thermoelectric MEMS microwave standing wave meter and the preparation method thereof provided by the invention have the following advantages:
(1) the state-controllable symmetrical thermoelectric MEMS microwave standing wave meter is formed by adopting a fully passive structure, and the measurement of the standing-wave ratio is realized by adopting a symmetrical directional coupler and a thermoelectric MEMS microwave power sensor; four identical capacitive MEMS microwave switches are added at the connecting nodes of the CPW transmission line and the ACPS transmission line of the symmetrical directional coupler to control the connection and the disconnection of the CPW transmission line, so that the MEMS microwave standing wave meter has the functions of detection and non-detection, namely a state switching function, and the loss of unnecessary power when the detection is not needed is reduced.
(2) In the structure, by designing a symmetrical directional coupler, incident microwave power is respectively coupled to a coupling end III and a coupling end V, reflected microwave power is respectively coupled to an isolation end IV and an isolation end VI, on one hand, a twice measurement mode is adopted, and the average value of thermal voltages measured at the two coupling ends (isolation ends) is taken as a final value, so that the measurement accuracy of the microwave standing wave meter is improved; on the other hand, if the upper half branch or the lower half branch of the symmetrical directional coupler can not work normally, the other branch which works normally can still carry out standing wave ratio measurement.
(3) In the structure, four same thermoelectric MEMS microwave power sensors are used for converting the extracted incident microwave power and the extracted reflected microwave power into direct-current thermal voltages respectively, so that the microwave power is measured; the method has the characteristics of zero direct current power consumption, high power, high sensitivity, good linearity and the like.
(4) The MEMS microwave standing wave meter adopts a symmetrical directional coupler based on CPW and ACPS transmission lines to replace the traditional microstrip line-based structure, realizes the extraction of incident microwave power and reflected microwave power, can ensure that the standing wave meter has lower microwave loss in higher frequency bands, and is convenient for connecting other devices in series and in parallel because a signal line and a ground line are on the same plane.
(5) The symmetrical thermoelectric MEMS microwave standing wave meter is an online device, and most microwave signals can still be used in the measurement process, so that the on-line self-detection standing wave ratio is realized; the preparation method is compatible with a gallium arsenide monolithic microwave integrated circuit.
Drawings
FIG. 1 is a schematic diagram of a state-controllable symmetrical pyroelectric MEMS microwave standing wave meter;
FIG. 2 is a cross-sectional view A-A of a state-controllable symmetrical pyroelectric MEMS microwave standing wave meter;
FIG. 3 is a B-B cross-sectional view of a state-controllable symmetrical pyroelectric MEMS microwave standing wave meter;
the figure includes: CPW transmissionLine 1, ACPS transmission line 2, symmetric directional coupler 3, thermoelectric MEMS microwave power sensing (4, MEMS microwave switch 8, Si3N4The MEMS device comprises an insulating medium layer 12, a metal arm 13, a semiconductor arm 14, a load resistor 15, a pressure welding block 16, a driving electrode 17, an air bridge 18, an MEMS clamped beam 19, a connecting wire 20, a GaAs substrate 21, an MEMS substrate membrane structure 22, a first port 31, a second port 32, a third port 33, a fourth port 34, a fifth port 35 and a sixth port 36.
Detailed Description
The invention relates to a state-controllable symmetrical thermoelectric MEMS microwave standing wave meter, which comprises the following specific implementation scheme:
the GaAs substrate 21 is provided with a CPW transmission line 1, an ACPS transmission line 2, a symmetrical directional coupler 3, four identical MEMS microwave switches 8, an air bridge 18, a connecting line 20 and four identical thermoelectric MEMS microwave power sensors 4, and an MEMS substrate membrane structure 22 is designed near the substrate of the load resistor 15.
The CPW transmission line 1 is horizontally arranged on the GaAs substrate 21 and used as the input and the output of the MEMS microwave standing wave meter; and the CPW transmission line 1 at the coupling port and the isolation port is used for realizing the transmission of the coupled microwave signals. The input port on the left side is port one 31, the output port on the right side is port two 32, the coupled port on the left side above is port three 33, the isolated port on the right side above is port four 34, the coupled port on the left side below is port five 35, and the isolated port on the right side below is port six 36. The CPW transmission line 1 is composed of a signal line and two ground lines, wherein the ground lines are located at both sides of the signal line. In order to realize microwave matching of the port with the outside, the port characteristic impedance of the CPW transmission line 1 is generally designed to be 50 Ω.
Two sections of ACPS transmission lines 2 are placed on a GaAs substrate 21, each section of ACPS transmission line 2 is composed of a signal line and a ground line, wherein the signal line of the ACPS transmission line 2 is used as a secondary transmission line of a symmetrical directional coupler 3 in the MEMS microwave standing wave meter. The signal lines of the two sections of ACPS transmission lines 2 are symmetrically positioned at the upper side and the lower side of the main transmission line in the symmetrical directional coupler 3, and the lengths of the signal lines are quarter wavelengths. In the symmetric directional coupler 3, two ends of each section of secondary transmission line are respectively a coupling end and an isolation end, wherein the coupling end is close to the input end and the isolation end is close to the output end. In order to realize microwave matching in connection with the CPW transmission line, the characteristic impedance of the ACPS transmission line is designed to be 50 Ω.
The symmetrical directional coupler 3 is positioned among input, output, coupling and isolating ports of the MEMS microwave standing wave meter, and mainly comprises a main transmission line and two auxiliary transmission lines, wherein the two auxiliary transmission lines are positioned on the upper side and the lower side of the main transmission line, and the distances from the two auxiliary transmission lines to the main transmission line are equal. The coupling degree and the isolation degree of the upper and lower sections of the auxiliary transmission lines of the symmetrical directional coupler 3 are equal. The two ports on the left side are coupling ends, and the two ports on the right side are isolation ends.
Each MEMS microwave switch 8 mainly comprises an MEMS clamped beam 19, a CPW transmission line 1, two driving electrodes 17, two pressure welding blocks 16, a connecting line 20 and Si3N4An insulating dielectric layer 12 and an air bridge 18.
Four identical MEMS microwave switches 8 are arranged at the connection nodes of the CPW transmission line 1 and the ACPS transmission line 2 of the symmetrical directional coupler 3; the MEMS microwave switch 8 adopts a parallel capacitance structure, wherein an upper polar plate is an MEMS clamped beam 19, and a lower polar plate is a signal line of the CPW transmission line 1. The MEMS clamped beam 19 is a suspended clamped beam structure provided with through holes at equal intervals, and spans over the signal line of the CPW transmission line, and two ends of the MEMS clamped beam 19 are fixed on the CPW ground lines at two sides of the CPW signal line through anchor areas; the through holes in the MEMS clamped beam 19 facilitate the release of the clamped beam. Each MEMS microwave switch 8 comprises two driving electrodes 17 which are positioned at the left and right sides of the CPW signal line below the MEMS clamped beam 19, and Si3N4The insulating medium layer 12 covers the driving electrode 17 below the MEMS clamped beam 19 and the signal line of the CPW transmission line 1, so that the MEMS clamped beam 19 is isolated from the direct current of the signal line of the CPW transmission line 1; the connecting wires 20 interconnect the driving electrodes 17 with the external pads 16. The air bridge 18 is a suspended clamped beam structure for interconnecting the CPW ground lines separated by the connecting lines.
Each thermoelectric MEMS microwave power sensor 4 mainly comprises a CPW transmission line 1 and Si3N4Insulating dielectric layer 12, pressSolder bumps 16, two load resistors 15, a thermopile and a MEMS substrate membrane structure 22.
Eight identical load resistors 15 are placed two by two at the two coupled ports and the two isolated ports. The coupling port and the isolation port are designed by adopting a CPW transmission line 1. Each of the coupling port and the isolation port is connected in parallel with two load resistors 15, and a resistance value of each load resistor 15 is 100 Ω.
Four identical thermopiles are placed close to, but not in contact with, the load resistors 15 placed at the two coupled and two isolated ports, respectively. Each thermopile is formed by connecting ten pairs of thermocouples in series, each pair of thermocouples comprising a semiconductor arm 14 and a metal arm 13 and being connected at one end by a metal connecting wire. And a plurality of pairs of thermocouples are connected in series, so that the sensitivity of measuring the temperature can be increased. The semiconductor arm 14, the metal arm 13 and the load resistor 15 are covered with Si3N4And the insulating medium layer 12 is used for protecting in the preparation process. When the load resistor 15 absorbs the microwave power to generate heat, the temperature of the end of the thermopile close to the load resistor 15 rises, and the end is called the hot end of the thermopile; while the temperature at the other end of the thermopile, remote from the load resistor 15, remains almost constant at ambient temperature, called the cold end of the thermopile. When the load resistor 15 absorbs microwave power to generate heat, the temperature of the hot end and the cold end of the thermopile can be caused to be different, based on the Seebeck effect, the thermopile generates output hot voltage, and the hot voltage reflects the temperature of the load resistor 15. In order to improve the efficiency of heat transfer from the load resistor 15 to the hot end of the thermopile and further increase the temperature difference between the hot and cold ends of the thermopile, the GaAs substrate near the lower side of the load resistor 15 is thinned by etching using a bulk etching technique to form the MEMS substrate film structure 22. By measuring the thermal voltage on the bonding pads 16 on both sides of the thermopile, the temperature of the load resistor 15 at that time can be obtained, and the microwave signal power of the port can be obtained.
Structurally, the CPW transmission line 1, the ACPS transmission line 2, the symmetrical directional coupler 3, the thermoelectric MEMS microwave power sensor 4, the MEMS microwave switch 8, the pressure welding block 16, the air bridge 18 and the connecting line 20 are located on the same GaAs substrate 21.
The state-controllable symmetrical thermoelectric MEMS microwave standing wave meter is formed by adopting a fully passive structure, the input microwave power on a main transmission line is coupled to a port III and a port V of a coupling end through two sections of ACPS auxiliary transmission lines, and the reflected microwave power is coupled to a port IV and a port VI of an isolation end; four identical thermoelectric MEMS microwave power sensors 4 are respectively positioned at the tail ends of the ports three, four, five, six 33,34,35 and 36 and used for measuring the microwave power coupled at the ports, wherein the four thermoelectric MEMS microwave power sensors 4 are completely identical and work on the basis of the microwave power-thermal-electrical conversion principle, so that a symmetrical thermoelectric MEMS microwave standing wave meter is formed; the four same MEMS microwave switches 8 are respectively arranged at the connecting nodes of the CPW transmission line 1 and the ACPS transmission line 2 of the symmetrical directional coupler 3, so that the on-off states of the four branches are controlled; the main structure of the MEMS microwave switch 8 is an MEMS clamped beam 19 and a driving electrode 17; the MEMS clamped beam 19 spans the CPW signal line, and two ends of the MEMS clamped beam 19 are fixed on CPW ground lines on two sides of the CPW signal line below the MEMS clamped beam 19 through anchor areas; the two driving electrodes 17 are symmetrically distributed on two sides of the CPW signal line below the MEMS clamped beam 19, and the two driving electrodes 17 are connected with the pressure welding block 16 on the outer side of the CPW through a connecting line 20; a layer of Si is covered on the CPW signal line and the driving electrode 17 under the MEMS clamped beam 193N4An insulating dielectric layer 12. When no driving voltage is applied between the MEMS clamped beam 19 and the driving electrode 17, the MEMS clamped beam 19 is in an Up state, the MEMS microwave standing wave meter is in a detection working state, microwave power incident from the port I31 and reflected from the port II 32 couple a certain proportion of microwave power to the two coupling ends and the isolation ends through the symmetrical directional coupler 3, due to the characteristics of directionality and isolation of the coupler, microwave power extracted from the port I31 is transmitted to the port III 33 and the port V35, microwave power extracted from the port II 32 is transmitted to the port IV 34 and the port VI 36, the microwave power transmitted to the port III 33 and the port V35 are equal, the microwave power transmitted to the port IV 34 and the port VI 36 is equal, and the coupled microwave power is transmitted to loads at the ports III, IV, V, VI 33,34,35,36The resistor 15 is completely consumed and generates heat, the temperature around the load resistor 15 is increased, the thermopile placed near the load resistor 15 detects the temperature change and converts the temperature change into output thermal voltage based on the Seebeck effect, the measurement of the microwave power incident at the first port 31 and the microwave power reflected at the second port 32 is realized, and the standing-wave ratio can be obtained; in the detection state, only a certain proportion of microwave power is coupled for measurement, and most of the microwave power is available, so that the standing-wave ratio is detected on line; when a driving voltage with opposite polarity is applied between the MEMS clamped beam 19 and the driving electrode 17, the height of the MEMS clamped beam 19 will decrease until the lower surface of the MEMS clamped beam 19 is attracted to Si3N4On the insulating medium layer 12, at this time, the MEMS clamped beam 19 is in a Down state, and in the Down state, the capacitance between the MEMS clamped beam 19 and the CPW signal line below the MEMS clamped beam 19 is sharply increased, causing the load resistor 15 at the end of the CPW at the coupling end and the isolation end to be short-circuited, the MEMS microwave standing wave meter is in a non-detection working state, and the input microwave power is directly transmitted to the output end, so as to reduce the loss of the microwave power without detection; by not applying and applying driving voltage between the MEMS clamped beam 19 and the driving electrode 17, the MEMS clamped beam 19 structure is in an Up state and a Down state, so that the symmetrical thermoelectric MEMS microwave standing wave meter has two working states of detection and non-detection, namely the working state is controllable; the directional coupler usually has only one section of ACPS auxiliary transmission line, one coupling end and one isolation end, but the measurement accuracy of the microwave standing wave meter can be improved by adopting a multi-measurement mode through designing two symmetrical coupling branches. For example, when both branches of the symmetric directional coupler 3 work normally, the average value of the thermal voltages measured at the two coupling ends (isolation ends) is taken as the final value; when the upper half branch or the lower half branch of the symmetrical directional coupler 3 can not work normally, the other branch which works normally can still measure the standing-wave ratio, thereby obviously improving the reliability of the MEMS microwave standing-wave meter.
The state-controllable symmetrical thermoelectric MEMS microwave standing wave meter is prepared by adopting a gallium arsenide monolithic microwave integrated circuit and an MEMS process, and the preparation method comprises the following specific processing steps:
(1) selecting a gallium arsenide epitaxial wafer as a substrate, wherein the gallium arsenide substrate is semi-insulating, and the square resistance of the epitaxial layer is 100-130 omega; coating photoresist, removing the photoresist in the region outside the semiconductor arm of the thermocouple, etching the front epitaxial layer, and then removing the photoresist to form the semiconductor arm 14 of the thermopile;
(2) sputtering AuGeNi/Au, forming a metal arm 13 and a pressure welding block 16 of the thermopile by adopting a stripping process, and then carrying out rapid thermal annealing to ensure that the semiconductor arm 14 and the metal arm 13 have good ohmic contact;
(3) etching the semiconductor arm 14 of the thermopile to reduce the thickness thereof, so that the resistance of the thermopile is increased to about 300K omega;
(4) coating photoresist on the GaAs substrate 21 obtained in the step (3), removing the photoresist at the position where the load resistor 15 is to be manufactured, sputtering to grow TaN, and stripping to form the load resistor 15 with the square resistance of 25 omega;
(5) coating photoresist on the GaAs substrate 21 obtained in the step (4), removing the photoresist on the positions of the CPW transmission line 1, the ACPS transmission line 2, the driving electrode 17, the pressure welding block 16 and the connecting line 20 to be manufactured, growing a layer of Ti/Pt/Au/Ti in an evaporation mode, and preliminarily forming the CPW transmission line 1, the ACPS transmission line 2 and the pressure welding block 16 by adopting a stripping process to completely form the driving electrode 17 and the connecting line 20;
(6) growing a layer of Si 2300A thick by a plasma enhanced chemical vapor deposition process3N4Insulating dielectric layer, photoetching and etching Si3N4An insulating medium layer, and Si remained on the CPW signal line and driving electrode 17 and the connecting line 20 under the semiconductor arm 14, the metal arm 13, the load resistor 15 and the MEMS clamped beam 193N4An insulating dielectric layer 12;
(7) spin-coating a polyimide sacrificial layer with the thickness of 1600nm, photoetching and etching the polyimide sacrificial layer, and reserving the polyimide sacrificial layer below the air bridge 18 and the MEMS clamped beam 19 to be manufactured;
(8) evaporating the seed layer titanium/gold/titanium for electroplating, photoetching and removing the photoresist at the position to be electroplated;
(9) growing a layer of Au on the seed layer obtained in the step 8 in an electroplating mode, removing photoresist at places where electroplating is not needed, reversely etching titanium/gold/titanium, and corroding bottom gold to completely form structures such as a CPW transmission line 1, an ACPS transmission line 2, an MEMS clamped beam 19, an air bridge 18, a pressure welding block 16 and the like;
(10) coating a layer of photoresist on the front surface of the substrate for protection, and carrying out operation on the back surface 21 of the GaAs substrate;
(11) coating a layer of photoresist on the back of a GaAs substrate 21, removing the photoresist below the load resistor 15 and the hot end of the thermopile, reserving the region except the region below the load resistor 15 and the hot end of the thermopile, and performing wet etching on the back of the substrate to form an MEMS substrate membrane structure 22, wherein the thickness of the MEMS substrate membrane structure is less than 20 microns;
(12) and removing the front protection material, scribing the front of the substrate, corroding the sacrificial layer manufactured in the step 7 by using a developing solution, releasing the MEMS clamped beam 19 and the air bridge 18, slightly soaking by using deionized water, dehydrating by using absolute ethyl alcohol, volatilizing at normal temperature, and airing.
The criteria for distinguishing whether this structure is present are as follows:
(1) the transmission of microwave signals is realized by adopting a CPW transmission line 1 and an ACPS transmission line 2 which are horizontally arranged;
(2) coupling a main transmission line to two sections of ACPS auxiliary transmission lines by adopting a symmetrical directional coupler 3, coupling input signals to two coupling ends, and coupling reflected signals to two isolation ends;
(3) arranging four MEMS microwave switches 8 at the connecting node of the CPW transmission line 1 and the ACPS transmission line 2 of the symmetrical directional coupler;
(4) four MEMS microwave power sensors 4 are respectively arranged at the tail ends of the two coupling ends and the two isolation ends;
(5) at the MEMS microwave switch 8 structure: the MEMS clamped beam 19 spans the CPW signal line, two ends of the MEMS clamped beam are fixed on the CPW ground line through anchor areas, two driving electrodes 17 are arranged on two sides of the CPW signal line below the MEMS clamped beam 19, and the driving electrodes are connected with the pressure welding blocks 16 on the outer side of the CPW transmission line 1 through connecting wires 20;
(6) at the MEMS microwave power sensor 4 structure: the tail end of the CPW transmission line is provided with a load resistor 15, a thermopile structure consisting of a metal arm 13 and a semiconductor arm 14 is arranged close to the outer side of the load resistor 15, pressure welding blocks 16 are arranged at two ends of the thermopile, and an MEMS substrate film structure 22 is designed at the load resistor 15 and the hot end of the thermopile;
(7) si is covered on the load resistor 15, the metal arm 13, the semiconductor arm 14, the CPW signal line 1 and the driving electrode 17 below the MEMS clamped beam3N4An insulating dielectric layer 12;
the structure satisfying the above conditions is regarded as the state-controllable symmetrical thermoelectric MEMS microwave standing wave meter of the present invention.
Claims (7)
1. The utility model provides a controllable symmetry type thermoelectric type MEMS microwave standing wave meter of state which characterized in that: the MEMS microwave power sensor comprises a GaAs substrate (21), wherein a CPW transmission line (1), an ACPS transmission line (2), a symmetrical directional coupler (3), four MEMS microwave switches (8), a pressure welding block (16), a connecting line (20), an air bridge (18) and four thermoelectric MEMS microwave power sensors (4) are arranged on the GaAs substrate (21); the symmetrical directional coupler (3) comprises a CPW transmission line (1) and an ACPS transmission line (2); the CPW transmission line (1) is composed of a signal line and two ground wires, wherein the signal line of the CPW transmission line (1) forms a main transmission line of the symmetrical directional coupler (3); the ACPS transmission line (2) is composed of a signal line and a ground line; two signal lines of the two sections of ACPS transmission lines form two auxiliary transmission lines of the symmetrical directional coupler (3), the two auxiliary transmission lines are positioned on two sides of the main transmission line in parallel and have equal distance to the main transmission line, two ends of each section of auxiliary transmission line are respectively a coupling end and an isolation end, wherein the coupling end is close to the input end, and the isolation end is close to the output end; microwave power is transmitted to a main transmission line of the symmetrical directional coupler (3) from an input port, incident microwave power of a port I (31) is coupled to a port III (33) and a port V (35) through two auxiliary transmission lines respectively, and reflected microwave power of a port II (32) is coupled to a port IV (34) and a port VI (36) respectively;two coupling ends and two isolation ends of the symmetrical directional coupler (3) are respectively connected with a thermoelectric MEMS microwave power sensor (4); each MEMS microwave switch (8) comprises a CPW transmission line (1), a driving electrode (17) and an MEMS clamped beam (19); the four MEMS microwave switches (8) are arranged at the connecting nodes of the CPW transmission line (1) and the ACPS transmission line (2) of the symmetrical directional coupler (3) to control the connection or disconnection of the CPW transmission line (1); the MEMS microwave switch (8) adopts a parallel capacitance type structure, an upper polar plate and a lower polar plate of the MEMS microwave switch (8) are respectively an MEMS clamped beam (19) and a CPW signal line, wherein the MEMS clamped beam (19) spans over the CPW signal line, two ends of the MEMS clamped beam (19) are respectively fixed on the CPW ground lines at two sides of the CPW signal line through anchor areas, two driving electrodes (17) are symmetrically arranged at two sides of the CPW signal line below the MEMS clamped beam (19), and the CPW signal line and the driving electrodes (17) are covered with Si3N4An insulating dielectric layer (12); when direct current driving voltage is not applied and applied between the MEMS clamped beam (19) and the driving electrode (17), the structure of the MEMS clamped beam (19) is respectively in an Up state and a Down state, so that the symmetrical thermoelectric MEMS microwave standing wave meter has two working states of detection and non-detection, namely the working states are controllable.
2. The state-controllable symmetrical thermoelectric MEMS microwave standing wave meter as claimed in claim 1, wherein: the signal lines of the two sections of ACPS transmission lines (2) are symmetrically distributed on the upper side and the lower side of a main transmission line formed by the signal lines of the CPW transmission line (1), wherein the length of each section of ACPS transmission line (2) is one quarter wavelength.
3. The state-controllable symmetrical thermoelectric MEMS microwave standing wave meter as claimed in claim 1, wherein: each thermoelectric MEMS microwave power sensor (4) comprises a CPW transmission line (1), two load resistors (15), an MEMS substrate film structure (22), a Si3N4 insulating medium layer (12) and a thermopile; the thermopile is composed of a semiconductor arm (14) and a metal arm (13), wherein the semiconductor arm (14) is made of N + gallium arsenide, and the metal arm (13) is made of AuGeNi/Au alloy.
4. The state-controllable symmetrical thermoelectric MEMS microwave standing wave meter as claimed in claim 1, wherein: the four thermoelectric MEMS microwave power sensors (4) are the same, and the four MEMS microwave switches (8) are the same.
5. The state-controllable symmetrical thermoelectric MEMS microwave standing wave meter as claimed in claim 1, wherein: the port characteristic impedance of the CPW transmission line (1) is 50 omega.
6. The state-controllable symmetrical thermoelectric MEMS microwave standing wave meter as claimed in claim 1, wherein: the characteristic impedance of the ACPS transmission line (2) is 50 omega.
7. A method for manufacturing a state-controllable symmetrical thermoelectric MEMS microwave standing wave meter as claimed in claim 1, comprising the steps of:
(1) selecting a gallium arsenide epitaxial wafer as a substrate, wherein the gallium arsenide substrate is semi-insulating, and the square resistance of the epitaxial layer is 100-130 omega; coating photoresist, removing the photoresist in the region outside the semiconductor arm of the thermocouple, etching the front epitaxial layer, and then removing the photoresist to form the semiconductor arm (14) of the thermopile;
(2) sputtering AuGeNi/Au, forming a metal arm (13) and a pressure welding block (16) of the thermopile by adopting a stripping process, and then carrying out rapid thermal annealing to ensure that the semiconductor arm (14) and the metal arm (13) have good ohmic contact;
(3) etching the semiconductor arm (14) of the thermopile to reduce the thickness thereof, so that the resistance of the thermopile is increased to about 300K omega;
(4) coating photoresist on the GaAs substrate (21) obtained in the step (3), removing the photoresist at the position where the load resistor (15) is to be manufactured, sputtering to grow TaN, and stripping to form the load resistor (15) with the square resistance of 25 omega;
(5) coating photoresist on the GaAs substrate (21) obtained in the step (4), removing the photoresist at the positions of the CPW transmission line (1), the ACPS transmission line (2), the driving electrode (17), the pressure welding block (16) and the connecting line (20) to be manufactured, growing a layer of Ti/Pt/Au/Ti in an evaporation mode, and preliminarily forming the CPW transmission line (1), the ACPS transmission line (2) and the pressure welding block (16) by adopting a stripping process to completely form the driving electrode (17) and the connecting line (20);
(6) growing a layer of 2300-thick Si3N4 insulating medium layer by a plasma enhanced chemical vapor deposition process, photoetching and etching the Si3N4 insulating medium layer, and keeping a CPW signal line and a driving electrode (17) below a semiconductor arm (14), a metal arm (13), a load resistor (15) and an MEMS clamped beam (19) and an Si3N4 insulating medium layer (12) on a connecting line (20);
(7) spin-coating a polyimide sacrificial layer with the thickness of 1600nm, photoetching and etching the polyimide sacrificial layer, and reserving the polyimide sacrificial layer below the air bridge (18) and the MEMS clamped beam (19) to be manufactured;
(8) evaporating the seed layer titanium/gold/titanium for electroplating, photoetching and removing the photoresist at the position to be electroplated;
(9) growing a layer of Au on the seed layer obtained in the step (8) in an electroplating mode, removing photoresist at a position where electroplating is not needed, reversely etching titanium/gold/titanium, and corroding bottom gold to completely form structures of a CPW transmission line (1), an ACPS transmission line (2), an MEMS clamped beam (19), an air bridge (18) and a pressure welding block (16);
(10) coating a layer of photoresist on the front surface of the substrate for protection, and carrying out operation on the back surface (21) of the GaAs substrate;
(11) coating a layer of photoresist on the back of a GaAs substrate (21), removing the photoresist below the load resistor (15) and the hot end of the thermopile, reserving the region outside the lower parts of the load resistor (15) and the hot end of the thermopile, and performing wet etching on the back of the substrate to form an MEMS substrate film structure (22), wherein the thickness of the MEMS substrate film structure is less than 20 um;
(12) and (3) removing the front-side protective material, scribing the front side of the substrate, corroding the sacrificial layer manufactured in the step (7) by using a developing solution, releasing the MEMS clamped beam (19) and the air bridge (18), slightly soaking the MEMS clamped beam in deionized water, dehydrating by using absolute ethyl alcohol, volatilizing at normal temperature, and airing.
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