CN114061797A

CN114061797A - A kind of double bridge structure MEMS piezoresistive pressure sensor and preparation method thereof

Info

Publication number: CN114061797A
Application number: CN202111361809.3A
Authority: CN
Inventors: 黄晓东; 张鹏飞; 王广猛; 李宇翱; 熊强
Original assignee: Southeast University
Current assignee: Southeast University
Priority date: 2021-11-17
Filing date: 2021-11-17
Publication date: 2022-02-18
Anticipated expiration: 2041-11-17
Also published as: CN114061797B

Abstract

The invention discloses a MEMS piezoresistive pressure sensor with a double bridge structure and a preparation method thereof. Four piezoresistors are respectively prepared on the upper surface and the lower surface of a sensitive film, and the piezoresistors on the upper surface and the lower surface are respectively positive The four piezoresistors on the same surface are respectively facing the center of each edge of the sensitive film. Two ends of the four varistors on the lower surface of the sensitive film are respectively electrically connected with the electrode layers on the upper surface of the structure through metal leads, and an insulating layer is provided on the metal leads. Two sets of varistors on the upper and lower surfaces form two sets of Wheatstone bridges. On the premise of not reducing the thickness of the sensitive film of the sensor and not increasing the area of the sensitive film of the sensor, the invention effectively improves the sensitivity of the sensor, helps to improve the reliability and performance of the sensor, improves the integration of the sensor, and reduces the area and size of the sensor. Cost, simple wiring further ensures that the accuracy of the sensor is not compromised.

Description

MEMS piezoresistive pressure sensor with double bridge structures and preparation method thereof

Technical Field

The invention relates to an MEMS (Micro-Electro-Mechanical System) piezoresistive pressure sensor and a preparation method thereof.

Background

The MEMS piezoresistive pressure sensor utilizes the piezoresistive effect of semiconductor materials, and a Wheatstone bridge is formed by interconnection of piezoresistors to convert the ambient pressure into an electric signal so as to realize the measurement of the pressure. Compared with other types of MEMS pressure sensors such as a capacitive pressure sensor, a piezoelectric pressure sensor and the like, the MEMS piezoresistive pressure sensor has good linearity and reliability, and therefore, the MEMS piezoresistive pressure sensor is widely applied to the fields of aerospace, industrial control, environmental monitoring, medical electronics and the like. The current common MEMS piezoresistive pressure sensor mainly uses a sensitive film structure, and piezoresistors are distributed on the surface of the film. In order to improve the sensitivity of the sensor, the thickness of the sensitive film is designed to be thinner and thinner, and the area is designed to be larger and larger, so that larger deformation and stress can be obtained under the same pressure condition, and the resistance value change and the electrical output of the piezoresistor are increased. However, reducing the thickness of the sensitive film and increasing the area of the sensitive film will reduce the range of the sensor, reduce the lifetime and reliability of the sensor, and reduce the integration of the sensor and increase the cost of the sensor. In addition to the above problems, the piezoresistors need to be connected by using interconnection lines to form a wheatstone bridge, the existing interconnection lines are complex in arrangement, which affects the stress distribution on the surface of the sensitive film, thereby affecting the accuracy of the sensor, and in addition, the complexity of the interconnection line arrangement is significantly improved along with the increase of the number of the piezoresistors.

Disclosure of Invention

The purpose of the invention is as follows: aiming at the prior art, the MEMS piezoresistive pressure sensor with the double bridge structure and the preparation method thereof are provided, and the sensitivity and the precision of the MEMS piezoresistive pressure sensor are effectively improved on the premise of ensuring the range and the reliability of the sensor.

The technical scheme is as follows: an MEMS piezoresistive pressure sensor with a double bridge structure comprises a first substrate, a second substrate is arranged below the first substrate, a cavity is arranged in the center of the upper surface of the second substrate, and a sensitive film is formed on the part, opposite to the upper surface of the cavity, of the first substrate; four first piezoresistors are arranged on the lower surface of the first substrate, the four first piezoresistors are respectively opposite to the central positions of the four edges of the cavity, and the arrangement directions of the four first piezoresistors are consistent; the upper surface of the first substrate is provided with four second piezoresistors, the four second piezoresistors are respectively over against the four first piezoresistors on the lower surface in the thickness direction of the sensitive film, and the arrangement directions of the four second piezoresistors are respectively orthogonal to the four first piezoresistors; the upper surface of the first substrate is provided with an electrode layer, two ends of the four second piezoresistors are respectively and electrically connected with the corresponding electrode layers, two ends of the four first piezoresistors are respectively and electrically connected with the corresponding electrode layers through metal leads, and the metal leads are provided with insulating layers; the four first piezoresistors positioned on the lower surface of the first substrate form a first Wheatstone bridge, and the four second piezoresistors positioned on the upper surface of the first substrate form a second Wheatstone bridge.

Further, the metal lead is composed of a lead layer located below the first substrate and a metal filling layer vertically penetrating through the first substrate.

Further, the upper surface of the first substrate is covered with an insulating layer.

Further, the thickness and the material of the insulating layer between the lead layer and the first substrate are the same as those of the insulating layer on the upper surface of the first substrate.

The preparation method of the MEMS piezoresistive pressure sensor with the double bridge structure comprises the following steps:

step 1: selecting an N-type (100) silicon wafer as a first substrate;

step 2: forming eight blind holes in the first substrate by photoetching and deep reactive ion etching;

and step 3: preparing a layer of silicon dioxide on the inner surface of the blind hole as a first insulating layer through thermal oxidation, and preparing a layer of silicon dioxide on the upper surface of the first substrate as a second insulating layer;

and 4, step 4: etching a piezoresistor area and an anode bonding area on the second insulating layer by photoetching and reactive ion etching, and completing the preparation of four first piezoresistors in the piezoresistor area by injecting boron ions;

and 5: preparing Ti and Cu on the first insulating layer in each blind hole through magnetron sputtering to serve as a barrier layer and a seed layer respectively, and filling the blind holes through electroplating Cu to form a metal filling layer;

step 6: preparing Cr and Au connected to each first piezoresistor on the second insulating layer by photoetching and magnetron sputtering to serve as lead layers;

and 7: preparing silicon nitride on the outer surface of each lead layer by photoetching and plasma enhanced chemical vapor deposition to serve as a third insulating layer;

and 8: selecting a BF33 type glass sheet as a second substrate, and forming a groove in the center of the upper surface of the second substrate through photoetching and reactive ion etching;

and step 9: tightly attaching the second substrate to the inverted first substrate in an anodic bonding mode to form a cavity;

step 10: thinning the inverted first substrate by a chemical mechanical polishing process until the metal in the metal filling layer is exposed;

step 11: preparing four second piezoresistors on the upper surface of the inverted first substrate through boron ion implantation;

step 12: preparing silicon dioxide on the upper surface of the inverted first substrate by plasma enhanced chemical vapor deposition to serve as a fourth insulating layer;

step 13: and forming a contact hole on the fourth insulating layer by photoetching, and preparing Cr and Au on the fourth insulating layer by magnetron sputtering to serve as electrode layers to finish the preparation of the device.

Has the advantages that: compared with the prior art, the invention has the following advantages:

1. compared with the traditional MEMS piezoresistive pressure sensor, the MEMS piezoresistive pressure sensor with the double-bridge structure has the advantages that the four piezoresistors are respectively arranged on the upper surface and the lower surface of the sensitive film to form two Wheatstone bridges, so that the sensitivity of the sensor is effectively improved on the premise of not reducing the thickness of the sensitive film of the sensor and not increasing the area of the sensitive film of the sensor, the reliability of the sensor is improved, the integration level of the sensor is improved, and the area and the cost of the sensor are reduced.

2. The MEMS piezoresistive pressure sensor with the double bridge structure utilizes the metal filling layer to realize the electrode leading-out of the first piezoresistor positioned below the sensitive film, is convenient for interconnection wire wiring and simplifies the electric connection between the piezoresistors, and the concise wiring further ensures that the precision of the sensor is not influenced.

3. The invention is prepared by adopting the MEMS technology, and the sensor has the advantages of small size, high precision, good consistency, easy batch manufacturing and low cost.

Drawings

FIG. 1 is a schematic diagram of a piezoresistor arrangement of a MEMS piezoresistive pressure sensor with a dual bridge structure according to the present invention;

FIG. 2 is a schematic diagram of a Wheatstone bridge connection of piezoresistors of the MEMS piezoresistive pressure sensor with a dual-bridge structure according to the invention;

FIG. 3 is a cross-sectional view of the MEMS piezoresistive pressure sensor with a dual bridge structure of the invention along the direction A-A' in FIG. 1;

FIG. 4 is a cross-sectional view of the MEMS piezoresistive pressure sensor with a dual bridge structure obtained in step 1;

FIG. 5 is a cross-sectional view of the MEMS piezoresistive pressure sensor with a dual bridge structure obtained in step 2;

FIG. 6 is a cross-sectional view of the MEMS piezoresistive pressure sensor with a dual bridge structure obtained in step 3;

FIG. 7 is a cross-sectional view of the MEMS piezoresistive pressure sensor with a dual bridge structure of the present invention obtained in step 4;

FIG. 8 is a cross-sectional view of the MEMS piezoresistive pressure sensor with a dual bridge structure of the present invention obtained in step 5;

FIG. 9 is a cross-sectional view of the MEMS piezoresistive pressure sensor of the present invention obtained in step 6;

FIG. 10 is a cross-sectional view of the MEMS piezoresistive pressure sensor with a dual bridge structure of the present invention, obtained in step 7;

FIG. 11 is a cross-sectional view of the MEMS piezoresistive pressure sensor of the present invention obtained in step 8;

FIG. 12 is a cross-sectional view of the MEMS piezoresistive pressure sensor of the present invention obtained in step 9;

FIG. 13 is a cross-sectional view of the MEMS piezoresistive pressure sensor of the present invention obtained in step 10;

FIG. 14 is a cross-sectional view of the MEMS piezoresistive pressure sensor of the present invention, obtained in step 11;

fig. 15 is a cross-sectional view obtained in step 12 of manufacturing a MEMS piezoresistive pressure sensor with a dual bridge structure according to the present invention.

Detailed Description

The invention is further explained below with reference to the drawings.

As shown in fig. 1 and fig. 3, the MEMS piezoresistive pressure sensor with the double bridge structure includes a first substrate 1, a second substrate 8 is disposed below the first substrate 1, a cavity 9 is disposed in the center of the upper surface of the second substrate 8, and a sensitive film 13 is formed on a portion of the first substrate 1 over the cavity 9. Four first piezoresistors 4 are arranged on the lower surface of the first substrate 1, wherein the four first piezoresistors are respectively R1 ', R2', R3 'and R4', the four first piezoresistors 4 respectively face to the central positions of the four edges of the cavity 9, and the arrangement directions of the four first piezoresistors 4 are consistent. Four second piezoresistors 10 are arranged on the upper surface of the first substrate 1, wherein the four second piezoresistors are respectively R1, R2, R3 and R4, in the thickness direction of the sensitive film 10, the four second piezoresistors 10 respectively face the four first piezoresistors 4 on the lower surface, and the arrangement directions of the four second piezoresistors 10 are respectively orthogonal to the first piezoresistors 4. The upper surface of the first substrate 1 is provided with an electrode layer 12, two ends of the four second piezoresistors 11 are respectively and electrically connected with the corresponding electrode layers 12, two ends of the four first piezoresistors 4 are respectively and electrically connected with the corresponding electrode layers 12 through metal leads, and the metal leads are provided with insulating layers. The four first piezoresistors 4 on the lower surface of the first substrate 1 form a first wheatstone bridge, and the four second piezoresistors 10 on the upper surface of the first substrate 1 form a second wheatstone bridge, as shown in fig. 2.

Specifically, the material of the first substrate 1 is monocrystalline silicon, and the thickness is 10-50 μm. The material of the second substrate 8 is monocrystalline silicon or glass, preferably a BF33 type glass sheet, with a thickness of 200-1000 μm. The depth of the cavity 9 in the center of the upper surface of the second substrate 8 is set to 1 to 50 μm. The four first piezoresistors 4 are arranged on the lower surface of the first substrate 1 and face the edge center of the cavity 9, and the purpose of this is to arrange the piezoresistors at the places where the stress of the sensitive film 13 is maximum, so as to improve the sensitivity of the pressure sensor as much as possible.

The metal lead is formed by connecting a lead layer 6 positioned below the first substrate 1 and a metal filling layer 5 vertically penetrating through the first substrate 1. Eight metal filling layers 5 are symmetrically arranged on two sides of the edge of the sensitive film 13 relative to the four first piezoresistors 4 respectively, for the convenience of electroplating to fill a hole structure, the material is preferably Cu, the thickness of the material is the same as that of the first substrate 1, and the width of the material is set to be 2-10 mu m in combination with the capability of the current deep groove etching and hole filling process.

The periphery of the metal filling layer 5 is wrapped by a first insulating layer 2, a second insulating layer 3 is arranged between the lead layer 6 and the first substrate 1, a third insulating layer 7 is arranged on the lower surface of the lead layer 6, and the first insulating layer 2, the second insulating layer 3 and the third insulating layer 7 are made of silicon dioxide or silicon nitride and have the thickness of 50-200 nm. The first insulating layer 2 and the second insulating layer 3 function to achieve electrical isolation between the lead layer 6 and the metal filling layer 5 and the first substrate 1. The third insulating layer 7 functions to achieve electrical isolation between the lead layer 6 and the second substrate 8.

The upper surface of the first substrate 1 is further covered with a fourth insulating layer 11. The fourth insulating layer 11 is used to realize electrical isolation between the electrode layer 12 and the first substrate 1, and also serves as a passivation layer of the second varistor 10, so as to protect the second varistor 10 and improve the stability of the sensor. The thickness and material of the second insulating layer 3 between the lead layer 6 and the first substrate 1 are the same as those of the fourth insulating layer 11 on the upper surface of the first substrate 1, so that the stress compensation function can be realized to improve the mechanical strength and reliability of the sensor.

The material of the wiring layer 6 is preferably one of Al, Ti, Au, Cu and Pt, and the thickness is 100-500 nm. The lead layer 6 and the metal filling layer 5 are electrically interconnected, and the function of the lead layer is to lead out the electrode of the first piezoresistor 4. The electrode layer 12 is disposed above the metal filling layer 5 and the second piezoresistor 10, and the material is metal, preferably one of Al, Ti, Au, Cu and Pt, and the thickness is 100-500 nm. The electrode layer 12 is used for realizing electrode extraction of the metal filling layer 5 and the second piezoresistor 10.

The working principle of the high-sensitivity MEMS piezoresistive pressure sensor is as follows:

the sensitive film 13 is deformed under the action of external pressure, the deformation of the sensitive film 13 causes the strain of the piezoresistors on the upper and lower surfaces of the sensitive film, based on the piezoresistive effect, the resistance values of the piezoresistors are correspondingly changed, the stress directions of the upper and lower surfaces of the sensitive film 13 are opposite, the piezoresistors corresponding to the upper and lower surfaces of the sensitive film are arranged in pairs to form 90 degrees, the piezoresistive coefficients corresponding to the piezoresistors playing the leading role are opposite in sign, so that the negative and positive effects are achieved, the magnitude and the direction of the change of the resistance values of the piezoresistors on the corresponding positions of the upper and lower surfaces of the sensitive film 13 are the same, and therefore, four resistors on the upper and lower surfaces respectively form two same Wheatstone bridges, compared with a single Wheatstone bridge formed by a single layer of piezoresistors, the double electrical output can be obtained, thereby on the premise of not changing the film structure and not reducing other performances of the sensor, the sensitivity of the sensor is effectively improved.

step 1: an N-type (100) silicon wafer 300 μm thick was selected as the first substrate 1, as shown in fig. 4.

Step 2: eight blind holes 5 μm wide and 40 μm Deep are formed in the first substrate 1 by photolithography and DRIE (Deep Reactive Ion Etching), as shown in fig. 5.

And step 3: a layer of silicon dioxide with a thickness of 200nm was prepared as a first insulating layer 2 on the inner surface of the blind via by thermal oxidation, and a layer of silicon dioxide with a thickness of 200nm was prepared as a second insulating layer 3 on the upper surface of the first substrate 1, as shown in fig. 6.

And 4, step 4: the piezoresistive region and the anodic bonding region are etched out on the second insulating layer 3 by lithography and RIE (Reactive Ion Etching), and the preparation of the four first piezoresistors 4 is completed in the piezoresistive region by boron Ion implantation, as shown in fig. 7.

And 5: 20nm Ti and 100nm Cu are prepared on the first insulating layer 2 in each blind hole by magnetron sputtering to be respectively used as a barrier layer and a seed layer, the blind holes are filled by electroplating Cu to form a metal filling layer 5, and then redundant Cu is removed by a CMP (Chemical Mechanical Polishing) process to improve the flatness of the upper surface of the first substrate 1, as shown in FIG. 8.

Step 6: 20nm of Cr and 180nm of Au connected to each first varistor 4 were prepared as lead layers 6 on the second insulating layer 3 by photolithography and magnetron sputtering, as shown in FIG. 9.

And 7: silicon nitride as the third insulating layer 7 was prepared to a thickness of 200nm on the outer surface of each lead layer 6 by photolithography and PECVD (Plasma Enhanced Chemical Vapor Deposition), as shown in fig. 10.

And 8: a BF33 type glass plate 500 μm thick was selected as the second substrate 8, and a groove having a depth of 50 μm was formed in the center of the upper surface of the second substrate 8 by photolithography and RIE, as shown in FIG. 11.

And step 9: the second substrate 8 is attached to the inverted first substrate 1 by anodic bonding to form a cavity 9, as shown in fig. 12.

Step 10: the inverted first substrate 1 is thinned to 40 μm by a CMP process, exposing the metal in the metal fill layer 5, as shown in fig. 13.

Step 11: four second piezoresistors 10 are prepared on the upper surface of the inverted first substrate 1 by boron ion implantation, as shown in fig. 14.

Step 12: silicon dioxide is prepared as a fourth insulating layer 11 on the upper surface of the inverted first substrate 1 by PECVD, as shown in fig. 15.

Step 13: contact holes are formed on the fourth insulating layer 11 by photolithography, and 20nm of Cr and 180nm are prepared as the electrode layer 12 on the fourth insulating layer 11 by magnetron sputtering, completing the preparation of the device, as shown in fig. 3.

The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.

Claims

1. A double-bridge structure MEMS piezoresistive pressure sensor, characterized in that it comprises a first substrate (1), a second substrate (8) is provided below the first substrate (1), and the second A cavity (9) is provided in the center of the upper surface of the substrate (8), and a sensitive film (13) is formed on the part of the first substrate (1) facing above the cavity (9); Four first piezoresistors (4) are arranged on the lower surface of the bottom (1), the four first piezoresistors (4) are respectively facing the center positions of the four edges of the cavity (9), and the four first piezoresistors (4) The arrangement directions of the varistors (4) are consistent; four second varistors (10) are arranged on the upper surface of the first substrate (1), and four second varistors (10) are arranged along the thickness direction of the sensitive film (10). The second varistors (10) are respectively facing the four first varistors (4) on the lower surface, and the arrangement direction of the four second varistors (10) is respectively the same as that of the first varistors (4) Orthogonal; an electrode layer (12) is provided on the upper surface of the first substrate (1), two ends of the four second varistors (11) are respectively electrically connected to the corresponding electrode layer (12), and the four second varistors (11) are electrically connected Two ends of a varistor (4) are respectively electrically connected to the corresponding electrode layers (12) through metal leads, and the metal leads are provided with an insulating layer; four varistors located on the lower surface of the first substrate (1) The first varistor (4) constitutes a first Wheatstone bridge, and the four second varistors (10) located on the upper surface of the first substrate (1) constitute a second Wheatstone bridge.

2. The double bridge structure MEMS piezoresistive pressure sensor according to claim 1, characterized in that the metal leads are formed by a lead layer (6) located under the first substrate (1) and vertically passing through The metal filling layer (5) of the first substrate (1) is connected to form.

3. The double bridge structure MEMS piezoresistive pressure sensor according to claim 1, wherein the upper surface of the first substrate (1) is further covered with an insulating layer (11).

4. The double bridge structure MEMS piezoresistive pressure sensor according to claim 3, wherein the thickness of the insulating layer (3) between the lead layer (6) and the first substrate (1) is equal to The materials are the same as the insulating layer (11) on the upper surface of the first substrate (1).

5. The preparation method of the double-bridge structure MEMS piezoresistive pressure sensor according to any one of claims 1-4, characterized in that, comprising:

Step 1: select an N-type (100) silicon wafer as the first substrate (1);

Step 2: forming eight blind holes in the first substrate (1) by photolithography and deep reactive ion etching;

Step 3: prepare a layer of silicon dioxide on the inner surface of the blind hole by thermal oxidation as the first insulating layer (2), and prepare a layer of silicon dioxide on the upper surface of the first substrate (1) as the second insulating layer (3) );

Step 4: The varistor region and the anode bonding region are etched on the second insulating layer (3) by photolithography and reactive ion etching, and four first varistor regions are completed by boron ion implantation in the varistor region. Preparation of resistor (4);

Step 5: preparing Ti and Cu on the first insulating layer (2) in each blind hole by magnetron sputtering as a barrier layer and a seed layer respectively, and filling the blind hole with Cu by electroplating to form a metal filling layer (5);

Step 6: preparing Cr and Au connected to each first varistor (4) on the second insulating layer (3) by photolithography and magnetron sputtering as a lead layer (6);

Step 7: preparing silicon nitride as the third insulating layer (7) on the outer surface of each lead layer (6) by photolithography and plasma enhanced chemical vapor deposition;

Step 8: selecting a BF33 type glass sheet as the second substrate (8), and forming a groove in the center of the upper surface of the second substrate (8) by photolithography and reactive ion etching;

Step 9: The second substrate (8) is closely attached to the inverted first substrate (1) by means of anodic bonding to form a cavity (9);

Step 10: thinning the inverted first substrate (1) through a chemical mechanical polishing process to expose the metal in the metal filling layer (5);

Step 11: preparing four second varistors (10) on the upper surface of the inverted first substrate (1) by implanting boron ions;

Step 12: preparing silicon dioxide as the fourth insulating layer (11) on the upper surface of the inverted first substrate (1) by plasma enhanced chemical vapor deposition;

Step 13: forming contact holes on the fourth insulating layer (11) by photolithography, and preparing Cr and Au as electrode layers (12) on the fourth insulating layer (11) by magnetron sputtering to complete the device preparation.