CN114486014B - Ultrasonic transducer unit and array combining PMUT with MEMS pressure sensor and manufacturing method - Google Patents

Ultrasonic transducer unit and array combining PMUT with MEMS pressure sensor and manufacturing method Download PDF

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CN114486014B
CN114486014B CN202210271083.2A CN202210271083A CN114486014B CN 114486014 B CN114486014 B CN 114486014B CN 202210271083 A CN202210271083 A CN 202210271083A CN 114486014 B CN114486014 B CN 114486014B
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layer
wafer
pmut
metal
pressure sensor
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CN114486014A (en
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李晖
尹峰
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Zhejiang Xiansheng Technology Co ltd
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Zhejiang Xiansheng Technology Co ltd
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L1/00—Measuring force or stress, in general
    • G01L1/18—Measuring force or stress, in general using properties of piezo-resistive materials, i.e. materials of which the ohmic resistance varies according to changes in magnitude or direction of force applied to the material
    • G01L1/183—Measuring force or stress, in general using properties of piezo-resistive materials, i.e. materials of which the ohmic resistance varies according to changes in magnitude or direction of force applied to the material by measuring variations of frequency of vibrating piezo-resistive material
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B81—MICROSTRUCTURAL TECHNOLOGY
    • B81B—MICROSTRUCTURAL DEVICES OR SYSTEMS, e.g. MICROMECHANICAL DEVICES
    • B81B7/00—Microstructural systems; Auxiliary parts of microstructural devices or systems
    • B81B7/02—Microstructural systems; Auxiliary parts of microstructural devices or systems containing distinct electrical or optical devices of particular relevance for their function, e.g. microelectro-mechanical systems [MEMS]
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B81—MICROSTRUCTURAL TECHNOLOGY
    • B81C—PROCESSES OR APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OR TREATMENT OF MICROSTRUCTURAL DEVICES OR SYSTEMS
    • B81C3/00—Assembling of devices or systems from individually processed components
    • B81C3/001—Bonding of two components
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer
    • H01L21/50—Assembly of semiconductor devices using processes or apparatus not provided for in a single one of the subgroups H01L21/06 - H01L21/326, e.g. sealing of a cap to a base of a container
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L23/00—Details of semiconductor or other solid state devices
    • H01L23/52—Arrangements for conducting electric current within the device in operation from one component to another, i.e. interconnections, e.g. wires, lead frames
    • H01L23/535—Arrangements for conducting electric current within the device in operation from one component to another, i.e. interconnections, e.g. wires, lead frames including internal interconnections, e.g. cross-under constructions
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B81—MICROSTRUCTURAL TECHNOLOGY
    • B81B—MICROSTRUCTURAL DEVICES OR SYSTEMS, e.g. MICROMECHANICAL DEVICES
    • B81B2201/00—Specific applications of microelectromechanical systems
    • B81B2201/02—Sensors
    • B81B2201/0264—Pressure sensors

Abstract

The invention discloses an ultrasonic transducer unit and an array of a PMUT combined MEMS pressure sensor and a manufacturing method thereof. The invention also discloses a manufacturing method of the ultrasonic transducer unit of the PMUT combined MEMS pressure sensor, which adopts the active wafer bonding and thinning technology to stack the CMOS wafer and the MEMS pressure sensor wafer so as to manufacture the PMUT-on-CMOS, realize the vertically interconnected three-dimensional structure and obviously improve the chip integration level.

Description

Ultrasonic transducer unit and array combining PMUT with MEMS pressure sensor and manufacturing method
Technical Field
The invention relates to the technical field of MEMS-on-CMOS high-density monolithic integrated semiconductor sensors, in particular to a novel structure and a processing technology for integrating a three-dimensional PMUT framework and an MEMS pressure sensor.
Background
In recent years, with the rapid development of Ultrasonic products and applications, piezoelectric microstructure Ultrasonic Transducers PMUT (Piezoelectric micro machined Ultrasonic Transducers) have been widely used. However, when the ultrasonic transducer based on the PZT piezoelectric material or the aluminum nitride piezoelectric material is used as a receiver, the PMUT signal response is low, which is not ideal. Under current PMUT unit architecture, through membrane thickness and structural layout optimization, sensitivity can obtain the improvement of certain degree, but the amplitude of promotion still remains to be improved.
On the other hand, after a traditional MEMS silicon-based piezoresistance Sensor (Piezo-Resistive Sensor) is optimized, the pressure sensitivity can exceed that of the existing PZT or aluminum nitride ultrasonic transducer. Sensitivity can be enhanced if the MEMS pressure sensor can be combined with the PMUT unit to superimpose the output signal of the MEMS pressure sensor into the output signal of the PMUT, for example, by 100-300% over the sensitivity of existing PMUTs. However, the structure and the process flow of the traditional MEMS piezoresistive sensor are greatly different from those of the prior PMUT device, and monolithic integration is difficult to realize.
MEMS pressure sensor:
MEMS pressure sensors have found widespread use. A typical MEMS pressure sensor structure and equivalent circuit schematic is shown in fig. 1. Wherein (a) is a layout design plan view and a cross-sectional view; there are four silicon resistors, formed by P + diffusion regions in the N-well, laid out in a thin film of silicon single crystal (Membrane) over a closed cavity (Sealed cavity). Under the external pressure, the Membrane film deforms due to the space provided by the cavity body. The resistance value of the deformed P + silicon resistor changes, the change of the resistance value is in direct proportion to the external pressure, and if the design is reasonable, a linear relation is formed in a certain pressure range. (b) Is a bridge circuit diagram formed by four piezoresistors interconnected so as to maximize the output voltage signal Vout of the MEMS pressure sensor.
FIG. 2 is a three-dimensional structure of a typical semiconductor process fabricated MEMS pressure sensor: piezoresistor (R in the figure) 1 ，R 2 ，R 3 ) Is made in the silicon epitaxial thin layer and is positioned above the cavity body. Under the external pressure, the silicon thin layer on the cavity body deforms, the resistance value of the silicon material changes due to the piezoelectric effect, and the piezoelectric sensing is realized in the process. The semiconductor material is a silicon single crystal with a crystal orientation (100), a P-type silicon substrate, an N-type epitaxial layer or forms N-well isolation. The piezoresistor is formed by P + diffusion in the N trap. The position of the diffusion resistor layout is as close as possible to the position where the Membrane generates the maximum deformation, and this position can be determined by software simulation (e.g. Commol simulation). The cavity shown provides room for the Membrane to deform under pressure. The lowest air outlet hole is designed for the pressure sensor with the Membrane having great deformation, and is used for eliminating the damping effect of air. For ultrasonic applications, the deformation of the Membrane is very small and no vent holes may be used.
An ultrasonic transducer:
the Ultrasonic Transducers are divided into two types, namely PMUT (Piezoelectric micro Ultrasonic Transducers) and CMUT (Capacitive micro Ultrasonic Transducers), and the structural schematic diagram of the Ultrasonic transducer is shown in fig. 3, wherein the left side is CMUT, and the right side is PMUT. The common feature is that they are designed with movable Membrane, they are designed with hollow cavity, they are also designed with two electrodes, the difference is that the CMUT is designed with upper and lower electrodes similar to capacitance, the upper electrode can move, and the lower electrode is fixed. After alternating current is applied to two poles of the capacitor to generate different charges, the like charges repel and the opposite charges attract, so that the upper electrode vibrates. The PMUT is excited by an electric field, and the atomic lattice of the piezoelectric material is displaced, so that the material expands or contracts along the direction of the displacement of the lattice, thereby vibrating up and down in the vertical direction. The PMUT upper and lower electrodes move simultaneously.
Taking PMUT as an example, a typical PMUT piezoelectric ultrasonic transducer structure is shown in fig. 4, and includes:
a substrate material 160, which may be a silicon material or a silicon dioxide material in general;
the cavity 120, which is typically a cavity etched into the substrate material, provides space for the PMUT to mechanically vibrate up and down, transmit, or receive ultrasonic waves.
The mechanical layer 130 is used as a mechanical support of the PMUT vibratable Membrane, and ensures the service life of the PMUT. The mechanical layer 130 material (thickness, specific gravity, young's modulus, etc.) also affects the frequency of the PMUT vibrations.
Oxide layer 132 is typically a silicon dioxide layer created on a silicon surface during a CMOS process. In addition to protecting the silicon surface, the thickness of the oxide layer 132 also affects the PMUT vibration frequency.
The sandwich stack of piezoelectric layers comprises a layer 115 of piezoelectric material, associated electrode layers being arranged below and above said layer 115 of piezoelectric material, respectively: a lower metal layer 112 and an upper metal layer 114.
The most commonly used materials for the piezoelectric material layer 115 are PZT lead zirconate titanate ((Pb (ZrTi) O3, abbreviated PZT) and aluminum nitride (AlN).
The lower metal layer 112 and the upper metal layer 114 are typically platinum-gold Pt materials or a multilayer structure of platinum and titanium metals. The lower metal layer 112 and the upper metal layer 114 generate an electric field in the piezoelectric material, thereby generating expansion and compression of the material, and further generating mechanical vibration in a vertical direction, and emitting ultrasonic waves. This is the well-known piezoelectric effect.
The frequency of the PMUT mechanical vibration is related to the materials of the layers in the sandwich, the mechanical layer 130, the oxide layer 132, the thickness of all materials, and the shape and size of the cavity 120. The mechanical stress of all materials also has an effect on the vibration frequency.
Disclosure of Invention
The invention aims to provide a novel architecture integrating an MEMS pressure sensor and a PMUT and a process flow thereof, which can remarkably improve the sensitivity of received signals by superposing the output signals of the MEMS pressure sensor and the PMUT through a specific structure. Meanwhile, three-dimensional vertical interconnection is utilized, the area of the PMUT unit is not increased, and the area of a chip is not increased.
In order to achieve the purpose of the present invention, an embodiment of the present invention provides an ultrasonic transducer unit of a PMUT combined MEMS pressure sensor, which is formed by bonding a first wafer and a second wafer, wherein a CMOS circuit and a CMOS circuit metal interconnection layer required by the PMUT are prefabricated in a silicon substrate layer of the first wafer, and an MEMS piezoresistor bridge interconnection circuit is prefabricated in a silicon substrate layer of the second wafer; a cavity body is arranged right below the center of the MEMS piezoresistor bridge type interconnection circuit layout in the substrate material layer of the first wafer or the second wafer, and the cavity body is shared by the PMUT and the MEMS pressure sensor; thinning the back of the silicon substrate layer of the second wafer to form a monocrystalline silicon thin layer, wherein the monocrystalline silicon thin layer is used as a silicon substrate of the MEMS piezoresistor and also used as a mechanical layer of the PMUT; a first metal wiring layer is arranged in a substrate material layer of the first wafer, and an MEMS pressure sensor metal interconnection layer is arranged in a substrate material layer of the second wafer; the back surface of the silicon substrate layer of the second wafer is provided with a lower metal layer, a piezoelectric material layer and an upper metal layer of the PMUT; the first wafer and the second wafer are electrically connected through vertical interconnection among the upper metal layer, the lower metal layer, the first metal wiring layer, the MEMS pressure sensor metal interconnection layer and the CMOS circuit metal interconnection layer, and the method comprises the step of superposing an output signal of the MEMS pressure sensor into an output signal of the PMUT.
Another embodiment of the present invention provides an array chip, which is characterized by comprising the ultrasound transducer unit of the PMUT combined MEMS pressure sensor.
Another embodiment of the present invention provides a method for manufacturing an ultrasonic transducer unit of the PMUT combined MEMS pressure sensor, which is characterized by comprising the following steps:
preparing a first wafer and a second wafer, respectively forming silicon substrate layers, manufacturing a CMOS circuit and a CMOS circuit metal interconnection layer on the first wafer silicon substrate layer, and manufacturing an MEMS piezoresistor bridge type interconnection circuit on the second wafer silicon substrate layer;
respectively depositing substrate material layers on the silicon substrate layers of the first wafer and the second wafer, and manufacturing a first metal wiring layer, an MEMS pressure sensor metal interconnection layer and a vertical interconnection structure for realizing the electrical connection between the inside of the first wafer and the inside of the second wafer;
manufacturing a cavity body on the substrate material layer of the first wafer or the second wafer;
bonding the first wafer and the second wafer;
thinning the back of the second wafer silicon substrate layer to form a monocrystalline silicon thin layer;
manufacturing a vertical interconnection structure for realizing electrical connection between the first wafer and the second wafer;
depositing a lower metal layer of the PMUT on the monocrystalline silicon thin layer;
depositing a piezoelectric material layer for forming the PMUT on the lower metal layer;
depositing an upper metal layer for forming the PMUT on the piezoelectric material layer;
and manufacturing a vertical interconnection structure for realizing the electrical connection between the lower metal layer, the upper metal layer and the first and second wafers.
The invention has the beneficial effects that:
the ultrasonic transducer unit of the PMUT combined MEMS pressure sensor adopts active wafers (a first wafer prefabricated with a CMOS circuit and a second wafer prefabricated with an MEMS pressure sensor) to carry out fusion bonding and thinning, then the PMUT is manufactured, an MEMS pressure resistor is positioned in a mechanical layer of the PMUT, the MEMS pressure sensor and the PMUT share a cavity body, a three-dimensional vertically interconnected three-dimensional framework is adopted, a three-dimensional monolithic set of the MEMS pressure sensor, a PMUT array and the CMOS circuit is realized, an output signal of the MEMS pressure sensor is superposed to an output signal of the PMUT, and the overall response is improved. The ultrasonic transducer is applied to the ultrasonic probe, and can effectively enhance the sensitivity of the ultrasonic probe. The invention adopts a three-dimensional vertical interconnection three-dimensional framework, does not increase the area of the PMUT unit and the area of the chip, and obviously improves the integration level of the chip.
Drawings
FIG. 1 is a schematic diagram of a prior art MEMS pressure sensor structure and equivalent circuit, wherein (a) is a layout plan view and a cross-sectional view; (b) A bridge circuit diagram formed by interconnecting four piezoresistors;
FIG. 2 is a schematic diagram of a three-dimensional structure of a MEMS pressure sensor fabricated by a prior art semiconductor process;
FIG. 3 is a schematic view of a prior art ultrasonic transducer configuration;
FIG. 4 is a schematic diagram of a prior art PMUT structure;
fig. 5 is a structural diagram of an ultrasonic transducer unit of enhanced sensitivity of the first embodiment;
FIG. 6 is a cross-sectional view of a first wafer according to one embodiment;
FIG. 7 is a schematic cross-sectional view of a second wafer according to one embodiment;
FIG. 8 is a process flow diagram of an embodiment three CMOS circuit fabrication process;
FIG. 9 is a flowchart of an embodiment three MEMS pressure sensor fabrication process;
FIG. 10 is a schematic view illustrating bonding of a third wafer and a second wafer according to one embodiment;
FIG. 11 (a) is one exemplary illustration of a vertical interconnect structure of a first wafer and a second wafer according to an embodiment;
FIG. 11 (b) is a second exemplary diagram of the vertical interconnect structure of the first wafer and the second wafer according to the first embodiment;
FIG. 12 is a third illustration of an exemplary vertical interconnect structure for a first wafer and a second wafer of an embodiment;
FIG. 13 is a schematic diagram of the output signal of the MEMS pressure sensor in the fourth embodiment;
FIG. 14 is a schematic diagram of the output signal of the PMUT according to the fourth embodiment;
FIG. 15 is a schematic diagram of signal superposition according to the fourth embodiment;
fig. 16 is a circuit diagram for superimposing signals in the fourth embodiment.
Detailed Description
Example one
As shown in fig. 5, the present embodiment provides an ultrasonic transducer unit of PMUT combined MEMS pressure sensor, which is formed by low temperature fusion bonding a first wafer and a second wafer through a substrate material layer, where 122 is a bonding interface. The first wafer and the second wafer are active wafers, wherein a CMOS circuit 100-CMOS (as indicated in fig. 6) and a CMOS circuit metal interconnection layer 101 required for the PMUT operation are pre-fabricated in a silicon substrate layer 100 of the first wafer, for example, a high voltage pulse generation, a pulse modulation circuit, a signal amplifier, etc. required for driving the PMUT are generated, and thus the first wafer is referred to as a CMOS wafer. MEMS piezoresistor bridge interconnection circuits of the MEMS pressure sensors are prefabricated in the silicon substrate layer 300 of the second wafer, and in the figure, R-101 and R-102 represent MEMS piezoresistors, so that the second wafer is called an MEMS piezoresistor wafer. A MEMS pressure sensor unit has 4 MEMS piezoresistors, and when the layout of layout is adopted, the resistors are distributed at the position where the mechanical layer has larger mechanical deformation and are interconnected to form a resistor bridge circuit, and the bridge circuit can maximize the output voltage signal of the piezoelectric sensor.
The first metal wiring layer 102 is arranged in the substrate material layer of the first wafer, the MEMS pressure sensor metal interconnection layer 301 is arranged in the substrate material layer of the second wafer, the MEMS pressure sensor metal interconnection layer 301 and the CMOS circuit metal interconnection layer 101 are respectively and vertically interconnected with the first metal wiring layer 102, the output signal of the MEMS pressure sensor is superposed into the output signal of the PMUT, and the sensitivity of ultrasonic detection can be effectively enhanced. And thinning the back surface of the silicon substrate layer 300 of the second wafer to form a monocrystalline silicon thin layer, wherein the thickness of the monocrystalline silicon thin layer is 1-6 microns, and the monocrystalline silicon thin layer is used as a silicon substrate of the MEMS piezoresistor and a mechanical layer of the PMUT.
The ultrasonic transducer unit adopts the design of sharing a cavity body by the PMUT and the MEMS pressure sensor, the cavity body 120 is arranged right below the MEMS piezoresistor, and the cavity body not only provides a space for the mechanical vibration of a Membrane film in the PMUT, but also provides a space for the deformation of the Membrane film in the MEMS pressure sensor. The cavity body can be arranged on a substrate material layer of a first wafer and can also be arranged in a substrate material layer of a second wafer, the cavity body is located right below the layout center of the bridge-type interconnection circuit of the MEMS piezoresistors after the two wafers are bonded, and the MEMS piezoresistors are distributed on the boundary of the cavity body and the silicon substrate and located right above the cavity body. In this embodiment, the cavity is disposed in the first wafer substrate material layer, the 4 MEMS piezoresistors are symmetrically distributed around the cavity (as shown in fig. 1 (a)), and the center position of the cavity overlaps with the arrangement center positions of the four resistors.
According to the ultrasonic transducer unit, the output signal of the MEMS pressure sensor is superposed into the output signal of the PMUT through the vertical interconnection between the first wafer and the second wafer. The vertical interconnection structure between the first wafer and the second wafer can be realized by Through silicon Oxide vias (TOV, through Oxide Via, as the black part in the drawing), metal connection holes, metal lead holes, and the like. The vertical interconnection structure between the first wafer and the second wafer has a plurality of connection combinations, and those skilled in the art can flexibly design the vertical interconnection structure according to the circuit layout, and fig. 11 (a), 11 (b) and 12 respectively illustrate one of the possible connection combinations.
In this embodiment, a connection combination manner shown in fig. 12 is adopted, the MEMS piezoresistor bridge interconnection circuit is interconnected with the MEMS pressure sensor metal interconnection layer 301, the MEMS pressure sensor metal interconnection layer 301 is vertically interconnected with the first metal wiring layer 102 of the first wafer through the TOV, and the first metal wiring layer 102 is vertically interconnected with the CMOS circuit metal interconnection layer 101 through the metal wire hole 121, so that the output signal of the MEMS pressure sensor is superimposed into the output signal of the PMUT. The CMOS circuit of the first wafer silicon substrate layer comprises a circuit structure for realizing superposition of the output signal of the MEMS pressure sensor and the output signal of the PMUT.
As shown in fig. 5, a lower metal layer 112, a piezoelectric material layer 115 and an upper metal layer 114 of PMUT are disposed on the back side of the silicon substrate layer 300 of the second wafer, the lower metal layer 112 is vertically interconnected with the first metal wiring layer 102 through a lower metal connection hole ZBM163-2 and a TOV 400-1, and the upper metal layer 114 is vertically interconnected with the MEMS pressure sensor metal interconnection layer 301 through an upper metal connection hole ZTM 163-1 and a TOV 400-2.
The first metal wiring layer 102 may include more than one layer of metal wiring, which may be vertically interconnected through metal wire vias, as required by the CMOS circuit design. The present embodiment gives only an example in which the metal wiring layer has one layer of metal wiring.
After the bonding is completed and the second wafer is thinned, the electrical connection between the first wafer and the second wafer is realized by manufacturing the TOV, wherein necessary power lines, ground lines, signal line connections and the like are included. Firstly, etching through the second wafer, performing silicon side oxidation, further etching through silicon dioxide, and etching to the MEMS pressure sensor metal interconnection layer 301 of the second wafer and the first metal wiring layer 102 of the first wafer. According to the design requirements of circuit connection, the connections of the TOV mainly comprise a power line, a ground line, a signal line, a control line and the like.
As in fig. 11 (a), the left TOV is connected to the Ground GND (Ground) through the first metal wiring layer 102; the intermediate TOV is connected to the MEMS pressure sensor metal interconnect layer 301 as part of the piezoresistive interconnect; the right-side-test TOV is connected to the MEMS pressure sensor metal interconnection layer 301, further connected to the first metal wiring layer 102, the CMOS circuit metal interconnection layer 101, and finally connected to the power supply line VDD. The upper end of the TOV is respectively connected to an upper-layer metal connecting hole ZTM of the upper-layer metal layer of the PMUT and a lower-layer metal connecting hole ZBM of the lower-layer metal layer according to the electrical connection design.
As shown in fig. 11 (b), the left TOV is connected to the MEMS pressure sensor metal interconnection layer 301, further connected to the first metal wiring layer 102 and the CMOS circuit metal interconnection layer 101, and connected to the first wafer CMOS circuit to implement signal transmission; the right-side TOV is connected to the control terminal of the first wafer CMOS circuit in the same connection mode as the left-side TOV. The upper end of the TOV is respectively connected to the upper metal layer and the lower metal layer of the PMUT according to the electrical connection design.
As shown in fig. 12, the lower metal layer is connected to the right TOV through a lower metal connection hole ZBM, and then vertically connected to the first metal wiring layer 102, and finally connected to the CMOS circuit metal interconnection layer 101. The upper metal layer is connected to the left TOV through an upper metal connection hole ZTM, and then vertically connected to the MEMS pressure sensor metal interconnection layer 301, and the MEMS pressure sensor metal interconnection layer 301 is vertically connected to the first metal wiring layer 102 through the middle TOV, and finally connected to the CMOS circuit metal interconnection layer 101. Thereby, an electrical vertical interconnection between the first wafer and the second wafer is achieved, superimposing the output signal of the MEMS pressure sensor into the output signal of the PMUT.
Regarding the vertical connection between the metal layers in the present invention, this embodiment only illustrates a partial way of implementing the vertical connection, and according to the teaching of this embodiment, a person skilled in the art can flexibly arrange, and the implementation way is not limited to this embodiment.
Example two
The present embodiment provides an array chip, which includes a plurality of PMUT combined MEMS pressure sensor ultrasonic transducer units according to the first embodiment. The design of the array chip for product application comprises a linear array and an area array, wherein the linear array of 1X128 bits is taken as an example, 128 PMUT units are arranged, the lower metal layer of each unit is connected to a common ground wire, and the upper metal layer of each unit is connected to a 128-bit signal wire. In the transmitting operation mode, the 128-bit signal line is applied with a high level according to a designed timing, and ultrasonic waves are transmitted. In a receiving mode, a 128-bit PMUT + MEMS PRS (Piezo-Resistive-Sensor, pressure Sensor) combination unit collects piezoelectric signals respectively, and sends the piezoelectric signals to an adder for superposition and amplification.
EXAMPLE III
The present embodiment provides a method for manufacturing an ultrasonic transducer unit of a PMUT combined with a MEMS pressure sensor according to the first embodiment, the main process route includes:
the first wafer manufactures the CMOS circuitry required for the PMUT, the second wafer manufactures the MEMS pressure sensor;
face-to-face fusion bonding of the CMOS wafer and the MEMS wafer;
performing special back thinning on the bonded second wafer to form a mechanical layer required by the PMUT array and a silicon substrate of the MEMS piezoresistor, wherein the MEMS pressure sensor is arranged in the mechanical layer;
continuing the process steps required for the PMUT to fabricate the PMUT array;
and the PMUT array is vertically interconnected with the MEMS pressure sensor and the CMOS circuit to realize the MEMS-on-PMUT-on-CMOS framework.
The manufacturing method of the ultrasonic transducer unit of the PMUT combined MEMS pressure sensor specifically comprises the following steps:
step 1, preparing a first wafer, growing silicon dioxide of about 100 nm on the surface to form a silicon substrate layer, and manufacturing a CMOS circuit, wherein a schematic cross-sectional structure diagram of the CMOS circuit is shown in fig. 6, wherein:
nMOS Transistor n-channel field effect Transistor pMOS Transistor: p-channel field effect transistor
n + Source: p + Source
n + Drain terminal p + Drain terminal
p + Substrate Tap p + Substrate ground N + Well Tap N Well power supply
The CMOS circuit is fabricated using standard processes in the industry, and the process flow is shown in fig. 8.
Step 2, depositing a substrate material layer on the silicon substrate layer of the first wafer, and manufacturing a CMOS circuit metal interconnection layer, a first metal wiring layer and a vertical interconnection structure in the first wafer;
and 3, manufacturing a cavity body in the first wafer substrate material layer. The specific method comprises the following steps: a Cavity pattern is formed by photolithography, and then a substrate material is etched by plasma chemical vapor to form a Cavity, as shown in fig. 10.
Since the cavity body of the first embodiment is disposed on the first wafer, the manufacturing process of the cavity body of the first embodiment is located at the end of the first wafer preparation process. If the cavity body is disposed on the second wafer, the fabrication process of the cavity body may be correspondingly placed at the end of the second wafer fabrication process.
And 4, preparing a second wafer, forming a silicon substrate layer, manufacturing an MEMS piezoresistor bridge type interconnection circuit, depositing a substrate material layer on the silicon substrate layer of the second wafer, and manufacturing an MEMS pressure sensor metal interconnection layer and a vertical interconnection structure in the second wafer.
According to the requirements of CMOS circuit design, the first metal wiring layer can comprise more than one layer of metal wiring, and the metal wiring layers can be vertically interconnected through metal lead holes to form a multilayer metal wiring vertical interconnection architecture. The process flow of the embodiment provides great flexibility for the number of wiring layers.
FIG. 7 is a cross-sectional view of a MEMS piezoresistor fabricated on a second wafer. The P + piezoresistor is arranged in the isolated N-Well (or N type epitaxial layer), and when external pressure is applied, the P + piezoresistor changes the resistance value to generate a piezoelectric signal.
The process flow for manufacturing the second wafer is shown in fig. 9, and mainly includes the following steps:
(1) Preparing a P-type silicon material;
(2) Forming a thin silicon dioxide layer;
(3) Photoetching, ion implantation and diffusion to form N-Well;
(4) Photoetching to form a piezoresistance pattern area;
(5) P + ion implantation;
(6) Removing photoresist and performing rapid annealing;
(7) Depositing silicon oxide at low temperature;
(8) Photoetching to form a contact hole of the piezoresistor;
(9) Depositing interconnection metal, and photoetching, corroding and forming;
(10) Depositing silicon dioxide and CMP polishing.
And 5, performing low-temperature melt bonding on the first wafer and the second wafer to form a bonding interface 122, as shown in fig. 10.
Step 6, thinning the back of the second wafer silicon substrate layer to form a monocrystalline silicon thin layer;
after fusion bonding and thinning, the MEMS pressure sensor is left in a monocrystalline silicon thin layer, namely a single mechanical layer of the PMUT, and a cavity body is positioned right below the MEMS piezoresistor, so that a space is provided for mechanical vibration of a Membrane film in the PMUT on the one hand, and a space is provided for deformation of the Membrane film in the MEMS pressure sensor on the other hand.
And 7, performing a TOV process for vertically interconnecting the first wafer and the second wafer. There are various connection combinations of TOVs when connecting two wafers, and fig. 11 (a) and 11 (b) show two examples of connection combinations. When manufacturing the TOV, photoetching and corroding silicon at the design part of the TOV, continuously corroding the silicon dioxide connecting hole after oxygen plasma treatment, and depositing metal (common metal tungsten) to realize electrical connection after the corrosion is finished. The TOV also reserves contact sites for the PMUT array and the required power and ground lines while connecting the two wafers.
Continuing next with the process steps required for fabricating the PMUT, fabricating a PMUT array, comprising:
step 8, depositing a lower metal layer of the PMUT on the monocrystalline silicon thin layer;
step 9, depositing a piezoelectric material layer of the PMUT on the lower metal layer;
step 10, depositing an upper metal layer of the PMUT on the piezoelectric material layer;
and 11, manufacturing a metal connecting hole to realize the electrical connection between the lower metal layer and the upper metal layer and between the first wafer and the second wafer. As shown in FIG. 5, the Metal connection holes include an upper layer Metal connection hole ZTM (Z-shape Via with Top Metal) 163-1 for vertically interconnecting the upper layer Metal layer and the TOV 400-2, and a lower layer Metal connection hole ZBM (Z-shape Via with Bottom Metal) 163-2 for vertically interconnecting the lower layer Metal layer and the TOV 400-1.
Therefore, the invention realizes the three-dimensional monolithic integration of the MEMS pressure sensor, the PMUT array and the CMOS circuit by adopting the three-dimensional vertically interconnected stereo architecture, realizes the superposition of the output signal of the MEMS pressure sensor and the output signal of the PMUT unit and improves the sensitivity of the ultrasonic transducer unit.
In the PMUT array process, due to the requirement of connection and wiring of a plurality of units, an additional layer of metal wiring (using gold or aluminum copper wires) may be required above the PMUT for implementing metal interconnection between PMUT units required by the array wiring, and required power lines, ground lines, and the like.
Example four
The superposition of the output signal of the MEMS pressure sensor and the output signal of the PMUT unit belongs to the superposition of two analog small signals, and there are a variety of implementation methods.
Referring to fig. 13, a typical MEMS pressure sensor uses four piezoresistors to form a bridge circuit, a current source provides a stable bias, and the bridge circuit outputs a piezoelectric signal Vout to a low-noise amplifier circuit for amplification.
Referring to fig. 14, a block diagram of a CMOS auxiliary circuit of a pmut cell in transmit (Transmitter) and receive (Receiver) mode is shown. In a receiving mode, after receiving ultrasonic reflection waves, the PMUT unit generates small analog signals, and extracts low-voltage small analog signals V through the high-low voltage level shift circuit RX_out And the output is sent to a low noise amplifier for amplification.
In the embodiment, the output signal Vout of the MEMS pressure sensor and the output signal V of the PMUT are used RX_out The signal amplitudes are superimposed by an addition operational amplifier, and the principle of the superimposition is shown in fig. 15. When the configuration resistance value is R f ＝R 1 ＝R 2 ＝R 3 Then, an addition operation is implemented, in which case V o ＝－(V i1 +V i2 +V i3 )。
As shown in FIG. 16, the output signal Vout of the MEMS pressure sensor is stored and controlled at the first input terminal of the summing operational amplifier via the timing control signal Clk1, and at the same time, the output signal V of the PMUT RX_out After voltage conversion and filtering, the time sequence control signal Clk2 is stored and controlled at the second input end of the addition operational amplifier, the signal Vo is obtained through addition, differential superposition and amplification, and the signal Vo replaces the original signal V RX_out And the output signal of the integrated ultrasonic transducer unit is input into a low-noise amplifying circuit for amplification. Compared with the prior art in which the PMUT is used alone, the present embodimentThe addition operation circuit for realizing signal superposition is added into the CMOS circuit part supporting the PMUT in the first wafer, and corresponding circuit connection is designed, so that the piezoelectric signal output by the MEMS pressure sensor can be superposed into the output signal of the PMUT, the signal to noise ratio is improved, and the sensitivity of the PMUT is greatly improved.

Claims (9)

1. An ultrasonic transducer unit of a PMUT combined MEMS pressure sensor is characterized by being formed by bonding a first wafer and a second wafer, wherein a CMOS circuit and a CMOS circuit metal interconnection layer (101) required by the PMUT are prefabricated in a silicon substrate layer (100) of the first wafer, and an MEMS piezoresistor bridge type interconnection circuit is prefabricated in a silicon substrate layer (300) of the second wafer; a cavity (120) is arranged in a substrate material layer of the first wafer or the second wafer right below the center of the MEMS piezoresistor bridge-type interconnection circuit layout, and the cavity (120) is shared by the PMUT and the MEMS pressure sensor; thinning the back of a silicon substrate layer (300) of the second wafer to form a monocrystalline silicon thin layer, wherein the monocrystalline silicon thin layer is used as a silicon substrate of the MEMS piezoresistor and also used as a mechanical layer of the PMUT; a first metal wiring layer (102) is arranged in the substrate material layer of the first wafer, and a MEMS pressure sensor metal interconnection layer (301) is arranged in the substrate material layer of the second wafer; the back side of a silicon substrate layer (300) of the second wafer is provided with a lower metal layer (112), a piezoelectric material layer (115) and an upper metal layer (114) of the PMUT; the first wafer and the second wafer are electrically connected through vertical interconnection among the upper metal layer (114), the lower metal layer (112), the first metal wiring layer (102), the MEMS pressure sensor metal interconnection layer (301) and the CMOS circuit metal interconnection layer (101), and the output signal of the MEMS pressure sensor is superposed into the output signal of the PMUT.
2. The PMUT-coupled MEMS pressure sensor ultrasound transducer cell of claim 1, wherein the structure implementing the vertical interconnect comprises through silicon oxide vias, metal connection holes, metal wire vias.
3. The PMUT-MEMS pressure sensor-bonded ultrasound transducer cell of claim 1, wherein the MEMS pressure sensor metal interconnect layer (301) and the first metal wiring layer (102) are vertically interconnected by a through-silicon-oxide via, the first metal wiring layer (102) and the CMOS circuit metal interconnect layer (101) being vertically interconnected by a metal wire via (121).
4. The PMUT-MEMS pressure sensor-bonded ultrasound transducer cell of claim 1, wherein the lower metal layer (112) is vertically interconnected with the first metal routing layer (102) and the upper metal layer (114) is vertically interconnected with the MEMS pressure sensor metal interconnect layer (301).
5. The PMUT-bonded MEMS pressure sensor ultrasonic transducer cell of claim 1, wherein the first metal routing layer (102) comprises at least one layer of vertically interconnected metal routing.
6. The PMUT-coupled MEMS pressure sensor ultrasonic transducer unit of claim 1 wherein the thin layer of single crystal silicon has a thickness of 1-6 microns.
7. An array chip comprising an ultrasound transducer cell of a PMUT-bonded MEMS pressure sensor according to any of claims 1 to 6.
8. Method for manufacturing an ultrasonic transducer unit of a PMUT in combination with a MEMS pressure sensor according to any of claims 1 to 6, characterized in that it comprises the following steps:
preparing a first wafer and a second wafer, respectively forming silicon substrate layers, manufacturing a CMOS circuit and a CMOS circuit metal interconnection layer on the first wafer silicon substrate layer, and manufacturing an MEMS piezoresistor bridge type interconnection circuit on the second wafer silicon substrate layer;
respectively depositing substrate material layers on the silicon substrate layers of the first wafer and the second wafer, manufacturing a first metal wiring layer and a metal interconnection layer of the MEMS pressure sensor, and respectively manufacturing vertical interconnection structures for realizing the electrical connection inside the first wafer and inside the second wafer;
manufacturing a cavity body on the substrate material layer of the first wafer or the second wafer;
bonding the first wafer and the second wafer;
thinning the back of the second wafer silicon substrate layer to form a monocrystalline silicon thin layer;
manufacturing a vertical interconnection structure for realizing electrical connection between the first wafer and the second wafer;
depositing a lower metal layer of the PMUT on the monocrystalline silicon thin layer;
depositing a piezoelectric material layer for forming the PMUT on the lower metal layer;
depositing an upper metal layer for forming the PMUT on the piezoelectric material layer;
and manufacturing a vertical interconnection structure for realizing the electrical connection among the lower metal layer, the upper metal layer, the first wafer and the second wafer.
9. The method of claim 8, wherein the first wafer and the second wafer are bonded by low temperature melting.
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