CN114136504A

CN114136504A - Capacitive flexible pressure sensor and preparation method thereof

Info

Publication number: CN114136504A
Application number: CN202111431894.6A
Authority: CN
Inventors: 王明浩; 樊晔; 徐嘉辉; 金旻逸; 程瑜华; 王高峰
Original assignee: Hangzhou Dianzi University; Hangzhou Dianzi University Wenzhou Research Institute Co Ltd
Current assignee: Hangzhou Dianzi University; Hangzhou Dianzi University Wenzhou Research Institute Co Ltd
Priority date: 2021-11-29
Filing date: 2021-11-29
Publication date: 2022-03-04
Anticipated expiration: 2041-11-29
Also published as: CN114136504B

Abstract

The invention discloses a capacitive flexible pressure sensor and a preparation method thereof. Although the existing capacitive pressure sensor obtains higher sensitivity, the structural robustness and stability of the existing capacitive pressure sensor are poor. The invention relates to monolithic micro-processing preparation and array integration of a capacitive flexible pressure sensor. The pressure sensor mainly comprises a pair of parallel electrode plates with a specific gap and an inverted trapezoidal medium layer positioned in the middle of the electrode plates. The upper and lower polar plates and the dielectric layer of the capacitor are monolithically integrated on the flexible polymer substrate by adopting a single-sided micromachining technology, so that the reliability and consistency of device preparation are improved; by adopting the structural design of the inverted trapezoidal dielectric layer, the mechanical deformation and recovery speed of the electrode plate under pressure are increased, and the sensitivity and response speed of the flexible pressure sensor are improved.

Description

Capacitive flexible pressure sensor and preparation method thereof

Technical Field

The invention belongs to the technical field of flexible MEMS pressure sensors, and particularly relates to a capacitive flexible pressure sensor and a preparation method thereof.

Background

Flexible pressure sensors are of great interest for their potential applications in the fields of human-machine interaction systems, electronic skin, health monitoring, touch screens and intelligent robots. Due to the advantages of fast response time, low power consumption, insensitivity to temperature and the like, capacitive sensors with different structures are widely researched. For example, capacitive sensors made using flexible materials may be integrated into wearable textiles for monitoring respiration and joint motion. The flexible capacitive sensor array with the ultrathin structure can be used as multifunctional touch pad equipment and can realize force and touch functions.

Capacitive pressure sensors typically have two electrodes and a dielectric layer sandwiched between the electrodes. External pressure applied to the capacitive sensor can cause the dielectric layer to deform, reducing the distance between the two electrodes, resulting in a change in capacitance. Increasing the deformability of the dielectric layer is key to increasing the sensitivity of the sensor. While using highly compressible elastomeric materials with low young's modulus, constructing the dielectric layer structure can effectively improve sensitivity and overcome hysteresis caused by the viscoelastic behavior of the material. Zhang et al, in the article "Flexible Capacitive Sensor Based on micropattered Dielectric Layer", produced a high-sensitivity Capacitive Sensor using a lotus leaf microstructure as an electrode template and microspheres as a Dielectric Layer. Guo et al, in the article "Natural Plant Materials as Dielectric Layer for high Sensitive Flexible Electronic Skin "also uses leaves and flowers as dielectric material to make high sensitivity capacitive sensor. A capacitive Pressure sensor was developed by Bao's team in the paper "Sensitive Flexible Pressure Sensors with microscopic Rubber Dielectric Layers", which uses a micro-pyramid array as a Dielectric layer, and the pyramid tips of which in contact with the electrodes are easily deformed, thereby achieving high sensitivity (0.55 kPa)^-1). Although these methods improve sensitivity, the air gap existing at the interface of the microstructure dielectric layer and the electrode plate is not uniform, which results in poor structural robustness and stability. In order to avoid the above problems, it is necessary to realize monolithic integration of the electrode plate and the dielectric layer, which will eliminate the influence of the air gap on the sensitivity.

In order to improve the performance of the flexible pressure sensor, the upper and lower electrode plates and the dielectric layer of the capacitor are monolithically integrated on the flexible polymer substrate by adopting a single-sided micro-processing technology, so that the reliability and consistency of device preparation are improved; by adopting the structural design of the inverted trapezoidal dielectric layer, the mechanical deformation and recovery speed of the electrode plate under pressure are increased, and the sensitivity and response speed of the flexible pressure sensor are improved.

Disclosure of Invention

The invention aims to overcome the defects in the prior flexible pressure sensor technology, and the preparation of a wafer-level flexible pressure sensor array with high reliability and high sensitivity is realized by using a standardized micro-processing technology.

The invention relates to a capacitive flexible pressure sensor which comprises an upper electrode layer, an SU-8 inverted trapezoidal dielectric layer, an upper insulating layer, a lower electrode layer, a lower insulating layer and a filling layer covering the upper electrode layer, wherein the upper electrode layer, the SU-8 inverted trapezoidal dielectric layer, the upper insulating layer, the lower electrode layer and the lower insulating layer are sequentially stacked. The SU-8 inverted trapezoid dielectric layer comprises a plurality of dielectric strips arranged side by side. The width of the dielectric strip gradually increases from the lower electrode layer to the upper electrode layer. The lower electrode layer includes a plurality of lower electrode plates arranged side by side. The upper electrode layer comprises a plurality of upper electrode plates; the upper electrode plates are respectively arranged on the dielectric strips. The lower electrode plates and the upper electrode plates are arranged in a grid shape.

Preferably, the SU-8 inverted trapezoid dielectric layer is prepared by using a front-side photoetching process of SU-8 photoresist. The ratio of the widths of the two sides of the medium strip is adjusted by adjusting the exposure time.

Preferably, the specific exposure time of the SU-8 inverted trapezoidal dielectric layer (5) is 60% -80% of the normal exposure time. The normal exposure time represents the shortest exposure time required to form the SU-8 photoresist into a vertical step of 90 °.

Preferably, the thickness of the SU-8 inverted trapezoid dielectric layer is 10-100 microns, and the ratio of the length of the bottom side of the trapezoid with the longitudinal section close to the lower electrode layer to the length of the bottom side close to the upper electrode layer is 1/2-4/5.

Preferably, the upper insulating layer and the lower insulating layer are made of Parylene or PI, and the thickness of the upper insulating layer and the thickness of the lower insulating layer are 1-10 micrometers.

Preferably, the width of the lower electrode plate and the width of the lower electrode plate are both 100-1000 microns. The distance between two adjacent lower electrode plates and the distance between two adjacent lower electrode plates are both 100 and 1000 microns.

Preferably, the longitudinal section of the dielectric strip is an inverted isosceles trapezoid.

Preferably, the lower electrode plates and the upper electrode plates are perpendicular to each other.

The capacitive flexible pressure sensor and the preparation method thereof are as follows:

1) and depositing a layer of polymer on the surface of the silicon wafer to be used as a lower insulating layer.

2) A layer of metal is deposited on the lower insulating layer.

3) A layer of photoresist is spun on and patterned into a mask for etching the metal.

4) And corroding the metal to form a lower electrode layer.

5) And depositing a layer of polymer on the surface of the silicon wafer to be used as an upper insulating layer.

6) Spin-coating a layer of SU-8 photoresist and patterning the photoresist into an SU-8 inverted trapezoidal dielectric layer through front exposure, wherein the specific exposure time is 60-80% of the normal exposure time.

7) And depositing a layer of metal on the surface of the silicon wafer.

8) A layer of photoresist is spun on and patterned into a mask for etching the metal.

9) And corroding the metal to form an upper electrode layer.

10) A layer of photoresist is spin coated and patterned as a mask for etching the upper insulating layer.

11) The upper insulating layer is etched by RIE to expose the bonding pad of the lower electrode layer.

12) A layer of photoresist is spin coated and patterned as a sacrificial layer on the pads of the lower and upper electrode layers.

13) Spin coating a layer of PDMS and curing to form a filling layer.

14) Releasing the single flexible pressure sensor.

15) And placing the flexible pressure sensor into acetone to remove the glue, dissolving the sacrificial layer and exposing the bonding pads of the lower electrode layer and the upper electrode layer.

The invention has the beneficial effects that:

1. the invention improves the reliability and consistency of the device preparation by monolithically integrating the upper and lower polar plates and the dielectric layer of the capacitor on the flexible polymer substrate by adopting a single-sided micro-processing technology.

2. By adopting the structural design of the inverted trapezoidal dielectric layer, the invention increases the mechanical deformation and recovery speed of the electrode plate under pressure and improves the sensitivity and response speed of the flexible pressure sensor.

3. According to the invention, the cross section shape of the SU-8 inverted trapezoid medium layer is adjusted by adjusting the exposure time, so that medium layers with different mechanical deformation and recovery speeds can be obtained, and further flexible pressure sensor arrays with different measuring ranges and accuracies are formed.

Drawings

FIG. 1 is a schematic cross-sectional view of the present invention;

FIG. 2 is a schematic cross-sectional view of a single pressure sensing cell of the present invention;

fig. 3 is a process flow diagram of the present invention for fabricating a capacitive flexible pressure sensor.

Detailed Description

The invention is further described below with reference to the accompanying drawings.

As shown in fig. 1, the capacitive flexible pressure sensor includes an upper electrode layer 4, an SU-8 inverted trapezoidal dielectric layer 5, an upper insulating layer 6, a lower electrode layer 7, a lower insulating layer 8, and a filling layer 3 wrapping the upper electrode layer 4 and the SU-8 inverted trapezoidal dielectric layer 5. The SU-8 inverted trapezoidal dielectric layer 5 comprises a plurality of dielectric strips which are arranged side by side at equal intervals. The longitudinal section of the dielectric strip is in an inverted isosceles trapezoid, that is, the width of the dielectric strip gradually increases in the direction from the lower electrode layer 7 to the upper electrode layer 4. The lower electrode layer 7 includes a plurality of strip-shaped lower electrode plates arranged side by side at equal intervals. The upper electrode layer 4 comprises a plurality of strip-shaped upper electrode plates which are arranged side by side at equal intervals; the upper electrode plates are respectively arranged on the dielectric strips. Each lower electrode plate is vertical to each upper electrode plate. Each lower electrode plate and each upper electrode plate form a grid structure; each intersection of the grid structure forms a capacitive pressure sensor cell 2 for outputting a pressure signal at a corresponding location. The capacitive pressure sensor cells 2 distributed in a grid form together a pressure sensor array 1.

As shown in fig. 1, the SU-8 inverted trapezoidal dielectric layer 5 is prepared by a front-side photolithography process using SU-8 photoresist. By shortening the exposure time of the SU-8 photoresist, the photoresist on the surface layer is cured after being fully exposed, and the photoresist on the bottom layer can be cured only at the position with the maximum central light intensity. After development treatment, the uncured photoresist on the bottom layer is dissolved to form an inverted trapezoidal SU-8 structure.

The specific exposure time of the SU-8 photoresist is 60% -80% of the normal exposure time; the normal exposure time represents the shortest exposure time required to form the SU-8 photoresist into a vertical step of 90 °. When the specific exposure time of the SU-8 photoresist is 60% of the normal exposure time, the included angle between the two sides of the longitudinal section of the dielectric strip and the plane where the longitudinal section of the dielectric strip is located is 45 degrees. When the exposure time of the SU-8 photoresist is prolonged, the included angles between the two sides of the longitudinal section of the dielectric strip and the plane where the longitudinal section of the dielectric strip is located are gradually increased, and the deformation resistance of the obtained dielectric strip is gradually increased; thereby obtaining the pressure sensor with different detection precision and measuring range.

As shown in fig. 1, the upper insulating layer 6 and the lower insulating layer 8 are made of Parylene or PI (polyimide) film with a thickness of 1-10 μm. The thickness of the SU-8 inverted trapezoid dielectric layer is 10-100 micrometers, and the ratio of the length of the bottom side of the trapezoid with the longitudinal section, which is close to the lower electrode layer 7, to the length of the bottom side of the trapezoid, which is close to the upper electrode layer 4, is 1: 2. The width of the lower electrode plate and the width of the lower electrode plate are both 100-1000 microns. The distance between two adjacent lower electrode plates and the distance between two adjacent lower electrode plates are both 100 and 1000 microns.

As shown in fig. 1, the capacitive flexible pressure sensor operates according to the following principle: when pressure is applied to the upper and lower electrode plates of the pressure sensor array 1, the SU-8 inverted trapezoidal dielectric layer 5 is mechanically deformed, so that the distance d between the upper electrode layer 4 and the lower electrode layer 7 is changed. According to the functional relationship between the capacitance C and the plate spacing d: c ═ S/d, the single-valued functional relationship between capacitance C and spacing d can be obtained; wherein ε is a dielectric constant; s is the sectional area of the pressure sensor unit 2; and then, by combining the single-value function relationship between the pressure F and the distance d, the pressure F at different positions of the pressure sensor array 1 can be reflected according to the capacitance C, so that the pressure measurement is realized.

Example 1

The specific preparation steps of the capacitive flexible pressure sensor are as follows:

1) the single polished silicon substrate was ultrasonically cleaned in acetone, ethanol and deionized water for 5 minutes, blown dry with nitrogen and baked on a 180 degree hot plate for 15 minutes for use.

2) As shown in FIG. 2-1, a parylene C film with a thickness of 10 μm is deposited on the surface of the silicon wafer as a lower insulating layer using CVD (chemical vapor deposition).

3) As shown in FIG. 2-2, a layer of metal Cr/Au is deposited on the parylene C lower insulating layer as a conductive layer of the lower electrode layer, and the thickness of Cr/Au/Cr is 20/200/20 nm respectively.

4) As shown in fig. 2-2, a 5 μm thick positive photoresist was spin-coated on the conductive layer obtained in step 3) and patterned into a mask for etching metal.

5) And as shown in fig. 2-2, wet etching is adopted to etch the conductive layer Cr/Au/Cr, and the lower electrode plate with mutually independent lower electrode layers, the bonding pad and the interconnection line are formed.

6) As shown in fig. 2-3, a parylene C film with a thickness of 5 μm is deposited on the surface of the silicon wafer as an upper insulating layer.

7) As shown in fig. 2-4, a 40 micron thick SU-8 photoresist was spin coated on the upper insulating layer and patterned into an SU-8 inverted trapezoidal dielectric layer 5 by front underexposure.

8) As shown in FIGS. 2-5, a layer of metal Cr/Au/Cr is deposited on the surface of the silicon wafer as the conductive layer of the upper electrode layer, and the thickness of Cr/Au/Cr is 20/200/20 nm respectively.

9) As shown in fig. 2-5, a 5 micron thick positive photoresist was spin coated on the conductive layer resulting from step 8) and patterned as a mask to etch the metal Cr/Au.

10) As shown in fig. 2-5, the metal layer Cr/Au/Cr is etched by a wet process to form the upper electrode plates of the upper electrode layer and the pads and interconnections thereof.

11) As shown in fig. 2-5, a 5 micron thick layer of positive photoresist was spun on and patterned as a mask for etching the upper insulating layer.

12) As shown in fig. 2 to 5, RIE (reactive ion etching) is used to etch the upper insulating layer to expose the pad of the lower electrode layer.

13) As shown in fig. 2-6, a layer of 10 micron thick positive photoresist is spun on and patterned as a sacrificial layer over the conductive pads of the metal top and bottom plates.

14) As shown in fig. 2-6, a layer of 100 μm thick PDMS was spin-coated and cured to form a filling layer of metal upper and lower plates.

15) As shown in fig. 2-6, the wafer is laser diced, releasing individual flexible pressure sensor arrays.

16) And placing the single flexible pressure sensor into acetone to remove the glue, and exposing the conductive bonding pads of the metal upper and lower polar plates by dissolving the sacrificial layer.

Example 2

This example differs from example 1 in that: PI is used for replacing parylene C to be used as an upper insulating layer and a lower insulating layer of the lower electrode layer; Ti/Au/Ti is used for replacing Cr/Au/Cr to be used as the conductive layer of the metal upper and lower polar plates. The specific preparation steps of the capacitive flexible pressure sensor in the embodiment are as follows:

2) As shown in parts 1a and 1b of fig. 3, a layer of PMMA is spin-coated on the surface of the silicon wafer as a sacrificial layer, and then a 10 μm thick PI film is spin-coated as a lower insulating layer of the lower electrode layer.

3) As shown in fig. 3,

parts

2a and 2b, a conductive layer of metal Ti/Au/Ti as a lower electrode layer is deposited on the PI lower insulating layer, and the thickness of Ti/Au/Ti is 20/200/20 nm respectively.

4) As shown in fig. 3,

parts

2a and 2b, a 5 micron thick positive photoresist is spun onto the conductive layer and patterned as a mask to etch the metal.

5) As shown in fig. 3,

parts

2a and 2b, the metal layer Ti/Au/Ti is dry etched to form the lower electrode plate and its bonding pad and interconnection line, in which the lower electrode layers are independent of each other.

6) As shown in

parts

3a and 3b of fig. 3, a PI film with a thickness of 5 μm is deposited on the surface of the silicon wafer as an upper insulating layer of the lower electrode layer.

7) As shown in fig. 3,

sections

4a and 4b, a 40 micron thick layer of SU-8 photoresist was spin coated on the upper insulating layer and patterned into an SU-8 inverted trapezoidal dielectric layer 5 by front underexposure.

8) As shown in

parts

5a and 5b of FIG. 3, a layer of metal Ti/Au/Ti is deposited on the surface of the silicon wafer as the conductive layer of the upper electrode layer, and the thickness of Cr/Au is 20/200/20 nm respectively.

9) As shown in fig. 3,

sections

5a and 5b, a 5 micron thick layer of positive photoresist was spun on and patterned into a mask of etched metal Ti/Au/Ti.

10) As shown in fig. 3,

parts

5a and 5b, the pad and the interconnection line of the upper electrode layer are formed by dry etching the metal layer Ti/Au/Ti.

11) As shown in fig. 3,

parts

5a and 5b, a 5 micron thick layer of positive photoresist was spun on and patterned as a mask for etching the upper insulating layer.

12) As shown in

portions

5a and 5b of fig. 3, RIE (reactive ion etching) is used to etch the upper insulating layer, exposing the pad of the lower electrode layer.

13) As shown in fig. 3,

sections

5a and 5b, a layer of 10 micron thick positive photoresist is spun on and patterned as a sacrificial layer over the metal top and bottom plate conductive pads.

14) As shown in fig. 3,

parts

6a and 6b, a layer of 100 μm thick PDMS is spun on and cured to form a filling layer of metal upper and lower plates.

15) As shown in

sections

6a and 6b of fig. 3, the wafer is laser diced, releasing individual flexible pressure sensor arrays.

Claims

1. A capacitance type flexible pressure sensor comprises an upper electrode layer (4), an SU-8 inverted trapezoidal dielectric layer (5), an upper insulating layer (6), a lower electrode layer (7), a lower insulating layer (8) and a filling layer (3) covering the upper electrode layer (4), wherein the upper electrode layer, the SU-8 inverted trapezoidal dielectric layer and the filling layer are sequentially stacked; the method is characterized in that: the SU-8 inverted trapezoidal dielectric layer (5) comprises a plurality of dielectric strips arranged side by side; the width of the dielectric strip is gradually increased in the direction from the lower electrode layer (7) to the upper electrode layer (4); the lower electrode layer (7) comprises a plurality of lower electrode plates which are arranged side by side; the upper electrode layer (4) comprises a plurality of upper electrode plates; each upper electrode plate is arranged on each dielectric strip; the lower electrode plates and the upper electrode plates are arranged in a grid shape.

2. A capacitive flexible pressure sensor as claimed in claim 1 wherein: the SU-8 inverted trapezoidal dielectric layer (5) is prepared by using a front photoetching process of SU-8 photoresist; the ratio of the widths of the two sides of the medium strip is adjusted by adjusting the exposure time.

3. A capacitive flexible pressure sensor as claimed in claim 2, wherein: the specific exposure time of the SU-8 inverted trapezoidal dielectric layer (5) is 60% -80% of the normal exposure time.

4. A capacitive flexible pressure sensor as claimed in claim 2, wherein: the ratio of the length of the bottom side of the trapezoid of the longitudinal section of the dielectric strip close to the lower electrode layer (7) to the length of the bottom side close to the upper electrode layer (4) is 1/2-4/5.

5. A capacitive flexible pressure sensor according to claim 4, wherein: the longitudinal section of the medium strip is in an inverted isosceles trapezoid shape.

6. A capacitive flexible pressure sensor as claimed in claim 1 wherein: the thickness of the SU-8 inverted trapezoid dielectric layer is 10-100 micrometers.

7. A capacitive flexible pressure sensor as claimed in claim 1 wherein: the upper insulating layer (6) and the lower insulating layer (8) are both made of Parylene or PI, and the thickness of the upper insulating layer and the thickness of the lower insulating layer are both 1-10 micrometers.

8. A capacitive flexible pressure sensor as claimed in claim 1 wherein: the widths of the lower electrode plate and the lower electrode plate are both 100 and 1000 microns; the distance between two adjacent lower electrode plates and the distance between two adjacent lower electrode plates are both 100 and 1000 microns.

9. A capacitive flexible pressure sensor as claimed in claim 1 wherein: each lower electrode plate is vertical to each upper electrode plate.

10. A capacitive flexible pressure sensor and a method of making the same as claimed in any one of claims 1 to 9, wherein:

1) depositing a layer of polymer on the surface of the silicon wafer to be used as a lower insulating layer;

2) depositing a layer of metal on the lower insulating layer;

3) spin-coating a layer of photoresist and patterning into a mask for corroding metal;

4) corroding the metal to form a lower electrode layer;

5) depositing a layer of polymer on the surface of the silicon wafer to be used as an upper insulating layer;

6) spin-coating a layer of SU-8 photoresist and patterning the photoresist into an SU-8 inverted trapezoidal dielectric layer through front exposure, wherein the specific exposure time is 60-80% of the normal exposure time;

7) depositing a layer of metal on the surface of a silicon wafer;

8) spin-coating a layer of photoresist and patterning into a mask for corroding metal;

9) corroding the metal to form an upper electrode layer;

10) spin-coating a layer of photoresist and patterning the photoresist into a mask for etching the upper insulating layer;

11) etching the upper insulating layer by RIE to expose the bonding pad of the lower electrode layer;

12) spin-coating a layer of photoresist and patterning the photoresist into a sacrificial layer on a pad of the lower electrode layer and the upper electrode layer;

13) spin-coating a layer of PDMS and curing to form a filling layer;

14) releasing the single flexible pressure sensor;