CN114199428A

CN114199428A - Local thinning process method for MEMS inner cavity and application thereof

Info

Publication number: CN114199428A
Application number: CN202111683033.7A
Authority: CN
Inventors: 王磊
Original assignee: Nanjing Xinyuan Maims Technology Co ltd
Current assignee: Nanjing Xinyuan Maims Technology Co ltd
Priority date: 2021-12-31
Filing date: 2021-12-31
Publication date: 2022-03-18

Abstract

The invention provides an MEMS inner cavity local thinning process method and application thereof.

Description

Local thinning process method for MEMS inner cavity and application thereof

Technical Field

The invention belongs to the field of design of micro-electro-mechanical system (MEMS) sensors, and relates to an MEMS processing technology and application of the technology in sensor manufacturing.

Background

An MEMS (micro Electro Mechanical system), namely a micro Electro Mechanical system, is a novel interdisciplinary high and new technical field, integrates a plurality of micro processing technologies, and can realize acquisition, processing and conversion of a plurality of information quantities. When some sensor devices are manufactured by using the MEMS technology, a thin film structure is used, in which a cavity is formed on the surface or back of a silicon wafer by using a semiconductor processing process, and a sealed cavity is formed by bonding with a substrate or filling a material, so as to form a structure of the thin film and the cavity. Firstly, anisotropic etching is carried out on the back of a silicon wafer by using a dry method or a wet method to form a strain film structure with the thickness of several microns to dozens of microns on a local substrate, and then the etched silicon wafer and glass are subjected to anodic bonding to form a sealed cavity; secondly, forming a vacuum cavity on the surface of the silicon wafer by adopting an MEMS sacrificial layer technology, specifically, depositing a sacrificial layer material on the surface of the substrate, depositing a layer of stress film material on the sacrificial layer, manufacturing holes on two sides by utilizing photoetching to release the sacrificial layer, and finally sealing the through hole to form a sealed cavity; thirdly, using two silicon wafers, wherein one silicon wafer is used as a device layer, the other silicon wafer is used as a substrate, thermally oxidizing the substrate to form a layer of silicon dioxide, manufacturing a cavity on the substrate, etching to manufacture the cavity, bonding the silicon wafer used as the device layer with the substrate with the cavity, and thinning the thickness of the device layer to a required strain film to form a structure similar to an SOI silicon wafer and a sealed cavity; fourthly, monocrystalline silicon or polycrystalline silicon is used as a substrate, shallow grooves are etched on the silicon substrate through anisotropic etching, then, after the side walls of the shallow grooves are protected, isotropic etching is carried out to generate communicated cavities, then, a sealed cavity is formed on the substrate through an epitaxial monocrystalline silicon or polycrystalline silicon mode, and finally, the surface of the silicon substrate is flat through mechanical polishing.

The piezoresistor is manufactured on the surface of the wafer by the methods of ion implantation and the like on the basis of the structure of the sealed cavity and the strain film, the resistance value change of the piezoresistor caused by the deformation of the film due to factors such as external pressure and the like is converted into the change of output voltage by applying voltage to the piezoresistor, and various types of sensors can be manufactured on the basis of the structure and the principle. When manufacturing such a high-sensitivity sensor device having a thin film structure, in addition to reducing the thickness of the strained film, the strained film structure is usually locally thinned to form a cross beam structure, so as to further improve the sensitivity of the device. The cross beam structure has the advantages that stress concentration is realized, the sensitivity of a device is improved, the integral rigidity of the structure is increased, and the linearity and the dynamic response characteristic of the sensor are improved.

However, the structures have disadvantages, the cavity and the strain film are manufactured by utilizing anisotropic etching, the consistency and the repeatability of the device are difficult to ensure due to harsh process conditions, the thickness of the strain film is difficult to accurately control, and meanwhile, the required size of the bottom of the cavity is far larger than that of the silicon strain film due to the inclination angle between the side surface of the formed cavity and the silicon strain film, which is not beneficial to the miniaturization design of the pressure sensor; in the cavity structure manufactured by using the MEMS sacrificial layer technology, a strain film of the cavity structure usually has residual stress and stress gradient, which causes the problems of low sensitivity, large hysteresis error and the like of a sensor; meanwhile, in order to improve the sensitivity, when the cross beam structure is prepared, corrosion or etching needs to be carried out on the surface of the device layer of the silicon wafer, so that the surface is uneven, and the integration of a sensor and a circuit is difficult to follow.

Disclosure of Invention

The invention aims to solve the problems and provides an MEMS inner cavity local thinning process method and application thereof.

Specifically, the technical scheme of the invention is as follows:

a local thinning method for an MEMS inner cavity comprises the following steps:

selecting a first substrate comprising opposing first and second surfaces;

etching the first surface of the first substrate to form a first groove;

selecting a second substrate comprising opposing third and fourth surfaces;

etching the third surface of the second substrate to form a local thinning structure;

and bonding the first surface of the first substrate and the third surface of the second substrate to form an inner cavity comprising a local thinning structure.

Optionally, the second substrate comprises a single crystal silicon or SOI substrate.

Optionally, the method further comprises the step of further thinning the fourth surface of the second substrate.

Optionally, the third surface of the second substrate is a top silicon layer of the SOI.

Optionally, etching the third surface of the second substrate, and stopping at the buried oxide layer of the SOI substrate to form a local thinning structure.

Optionally, the local thinning structure includes a cross beam structure or a peninsula beam structure formed on the third surface of the second substrate.

A preparation method of a MEMS pressure sensor comprises the following steps:

selecting a first substrate comprising opposing first and second surfaces;

etching the first surface of the first substrate to form a first groove;

selecting a second substrate comprising opposing third and fourth surfaces;

bonding the first surface of the first substrate and the third surface of the second substrate to form an inner cavity comprising a local thinning structure;

thinning the fourth surface of the second substrate to form a strain film;

and manufacturing a piezoresistor and an ohmic contact region on the strain film.

Optionally, the piezoresistors constitute a wheatstone bridge.

A MEMS substrate comprising:

the device comprises a first substrate, a second substrate and a cavity, wherein the first substrate and the second substrate are overlapped, and the cavity is sealed between the first substrate and the second substrate;

wherein the first substrate comprises a first surface and a second surface which are opposite;

the first surface of the first substrate comprises a first groove;

the second substrate comprises a third surface and a fourth surface which are opposite;

a third surface of the second substrate comprises a local thinning structure;

the first groove and the local thinning joint are oppositely arranged to form the inner cavity.

Optionally, the locally thinned structure is disposed on a top silicon layer of the SOI substrate.

Optionally, the inner cavity top surface comprises a partially exposed SOI buried oxide layer.

The invention also provides a MEMS pressure sensor which comprises the MEMS substrate.

Optionally, the display device further comprises a piezoresistor arranged on the fourth surface of the second substrate.

Optionally, the piezoresistors constitute a wheatstone bridge.

Optionally, the fourth surface is a thinned layer of a bottom silicon layer of the SOI substrate or a top silicon layer with a buried oxide layer of the SOI removed.

Compared with the prior art, the technical scheme provided by the invention at least has the following beneficial effects:

(1) compared with the traditional piezoresistive pressure sensor with a flat membrane structure, the piezoresistive pressure sensor with the local thinning process of the MEMS inner cavity has the advantages of high sensitivity and large measuring range, and the stress is concentrated to a local thinning area by arranging the local thinning structure and utilizing the stress concentration principle, so that the sensitivity of the sensor is improved;

(2) by arranging the raised mass block, the overall rigidity of the structure is increased, large deformation of the silicon strain film is restrained, the linearity and the dynamic response characteristic of the sensor are improved, and the measuring range of the sensor is improved, so that high sensitivity, high linearity and wide measuring range are realized.

(3) Compared with the traditional piezoresistive pressure sensor with the front cross beam structure, the locally thinned structure controls the size of the cavity to be equal to that of the strain film by arranging the cavity on the first substrate, so that the problem that the bottom surface area of the cavity is larger than that of the strain film due to anisotropic corrosion is solved, and the area of a chip is reduced;

(4) the surface of the second substrate with the local thinning structure is bonded with the first substrate with the cavity to form a base with an inner cavity local thinning structure, the thickness of the strain film can be controlled by thinning the non-bonding surface of the base, so that the thickness of the strain film is not limited by the thickness of the top silicon of the SOI sheet, and compared with the traditional wet etching method for forming the cavity, the precision is higher, and the consistency is better; in addition, no corrosion process is adopted in the subsequent process after bonding, the surface of the substrate is kept flat, and the integration of a circuit and a sensor is facilitated.

Drawings

One or more embodiments are illustrated by way of example in the accompanying drawings, which correspond to the figures in which like reference numerals refer to similar elements and which are not to scale unless otherwise specified.

FIG. 1 is a schematic top view of a piezoresistive pressure sensor manufactured by the local thinning process of the MEMS inner cavity of the present invention;

FIGS. 2-13 are schematic cross-sectional views of the local thinning process of the MEMS inner cavity along the A-A 'direction and the B-B' direction in FIG. 1;

FIGS. 14-19 are schematic cross-sectional views of a process flow along the direction B-B' in FIG. 1 for fabricating a piezoresistive pressure sensor from a MEMS substrate according to the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention more apparent, embodiments of the present invention will be described in detail below with reference to the accompanying drawings. However, it will be appreciated by those of ordinary skill in the art that numerous technical details are set forth in order to provide a better understanding of the present application in various embodiments of the present invention. However, the technical solution claimed in the present application can be implemented without these technical details and various changes and modifications based on the following embodiments. The following embodiments are divided for convenience of description, and should not constitute any limitation to the specific implementation manner of the present invention, and the embodiments may be mutually incorporated and referred to without contradiction.

Fig. 1 is a schematic top view of a piezoresistive pressure sensor manufactured by the local thinning process of the MEMS inner cavity of the present invention.

Wherein, in FIGS. 2-10 and 13, (a) is a schematic sectional view along A-A 'direction, and (B) is a schematic sectional view corresponding to B-B' direction; fig. 14-19 correspond to a schematic sectional view in the direction B-B'.

Example 1

The embodiment provides a method for locally thinning an inner cavity of an MEMS, as shown in fig. 2 to 6, including the following steps:

selecting a first substrate 1, the first substrate 1 comprising a first surface 101 and a second surface 102 opposite;

selecting a second substrate 2, the second substrate 2 comprising a third surface 201 and a fourth surface 202 opposite to each other, as shown in fig. 2;

the first substrate in this embodiment may be, for example, a silicon wafer, glass, or ceramic, and has a thickness of, for example, 50 to 800 μm; the second substrate of this embodiment may be selected from an N-type monocrystalline silicon wafer, a P-type monocrystalline silicon wafer, or an soi (silicon on insulator) wafer.

In this embodiment, the first substrate 1 is exemplified by a silicon wafer, and the second substrate 2 is exemplified by a single crystal silicon wafer of an N-type (100) crystal plane.

Etching the first surface 101 of the first substrate 1 to form a first groove 103;

specifically, in order to etch the first groove 103, first, the first substrate 1 is subjected to surface thermal oxidation, and a layer of silicon oxide 3 is grown on each of the front and back sides of the first substrate 1, as shown in fig. 3;

the first surface 101 of the first substrate 1 is patterned and a first recess 103 is etched by a bulk silicon etching process, as shown in fig. 4.

specifically, the third surface 201 of the second substrate 2 is patterned and lithographically patterned to form a local thinning structure 203, as shown in fig. 4;

the local thinning structure 203 may be a cross beam structure or a silicon island structure.

Bonding the first surface 101 of the first substrate 1 and the third surface 201 of the second substrate 2, of course, the first surface 101 of the first substrate 1 further includes an oxide layer 3 thereon, forming an inner cavity 4 including a local thinning structure 203, as shown in fig. 5.

Further, the fourth surface 202 of the second substrate 2 may be further thinned to form a strained film, thereby completely forming a base with a locally thinned inner cavity structure for subsequent sensor applications, as shown in fig. 6.

Example 2

With respect to embodiment 1, the second substrate 2 of the present embodiment employs an SOI substrate, in which the second substrate SOI includes: a bulk silicon layer 204, a buried oxide layer 205, and a top silicon layer 206, as shown in fig. 7.

Wherein the thickness of the bulk silicon layer 204 is, for example, 50-800 μm, the thickness of the buried oxide layer 205 is 1-50 μm, the thickness of the top silicon layer 206 is 1-50 μm,

the second substrate SOI top silicon layer is lithographically and patterned to etch a local region of the SOI top silicon layer 206 to form a top silicon layer local thinning structure 203, as shown in fig. 8.

The steps and processes for etching the first substrate 1 to form the first groove 103 and bonding the first substrate 1 and the second substrate 2 to form the base with the cavity 4 are the same as those of embodiment 1, and are not described again here.

Further, the fourth surface 202 of the second substrate 2 may be further thinned to form a strained film, and in this embodiment, the thinning may be started from the bulk silicon layer 204 of the SOI substrate to the buried oxide layer 205, so as to completely form a base with a locally thinned inner cavity structure for subsequent device applications such as sensors, as shown in fig. 9 to 10.

Example 3

With respect to embodiment 1, the second substrate 2 of the present embodiment employs an SOI substrate, in which the second substrate SOI includes: a bulk silicon layer 204, a buried oxide layer 205, and a top silicon layer 206.

Compared with embodiment 2, in this embodiment, when the top silicon layer local thinning structure 203 is prepared, the top silicon layer 206 is etched to reach the buried oxide layer 205.

Wherein the projection of the top silicon layer local thinning structure 203 in the first recess 103 is smaller than in the area of the first recess 103, i.e. the local thinning structure 203 is located in the area of the first recess 103, as shown in fig. 11.

Further, the fourth surface 202 of the second substrate 2 may be further thinned to form a strained film, and in this embodiment, the thinning may be started from the bulk silicon layer 204 of the SOI substrate to the buried oxide layer 205, so as to completely form a base with a locally thinned inner cavity structure for subsequent device applications such as sensors, as shown in fig. 12.

Example 4

Compared with embodiment 2, in this embodiment, when the top silicon layer local thinning structure 203 is prepared, the top silicon layer is etched to the buried oxide layer.

Wherein the projection of the top silicon layer local thinning structure 203 in the first recess 103 is smaller than in the area of the first recess 103, i.e. the local thinning structure 203 is located in the area of the first recess 103. This is the same as in example 3 and will not be described again.

Further, the fourth surface 202 of the second substrate 2 may be further thinned to form a strained film, and in this embodiment, the thinning may be started from the bulk silicon layer 204 of the SOI substrate until the remaining thickness of the bulk silicon layer 204 is 0.1-50 μm, so as to completely form a base with a locally thinned inner cavity structure for subsequent device applications such as sensors, as shown in fig. 13.

Example 5

The present implementation provides a MEMS pressure sensor and a method of manufacturing the same, the MEMS pressure sensor using the MEMS substrate provided in embodiments 1 to 4 as a substrate, comprising the steps of:

on the substrate obtained in embodiments 1 to 4, the periphery of the strained film 10 is sequentially subjected to photolithography and ion implantation, so as to fabricate a lightly doped varistor 5 and a heavily doped ohmic contact region 6, where the ohmic contact region 6 is mainly used for interconnecting the varistor 5 with a subsequent metal lead 7 and interconnecting the varistors 5, as shown in fig. 14;

depositing an oxide layer with a thickness of 0.01-1 μm and a nitride layer with a thickness of 0.03-3 μm by APCVD, PECVD or LPCVD, etc., thereby forming an insulating dielectric layer 8, as shown in FIG. 15;

the insulating dielectric layer 8 above the ohmic contact region 6 is etched and etched away to form a lead hole 12, as shown in fig. 16;

sputtering one or more layers of metal material such as Au, Al, Ti, etc. to a thickness of 0.1-4 μm and photolithography to form metal leads 7 and metal pads 15, as shown in fig. 17;

forming a protective layer 11 by depositing an oxide layer having a thickness of 0.01 to 1 μm and a nitride layer having a thickness of 0.03 to 3 μm by APCVD, PECVD, LPCVD, or the like, as shown in fig. 18;

the resist 11 over the metal pad 15 is photo-etched and removed, so that the metal pad 15 is exposed as a metal pad 15 for testing, as shown in fig. 19.

In this example, the substrate obtained in example 1 was used as a substrate to prepare a MEMS pressure sensor, and the substrates obtained in other examples were used to prepare MEMS pressure sensors in the same manner, and the description thereof will not be repeated.

Has the advantages that:

It will be understood by those of ordinary skill in the art that the foregoing embodiments are specific examples for carrying out the invention, and that various changes in form and details may be made therein without departing from the spirit and scope of the invention in practice.

Claims

1. A local thinning method for an MEMS inner cavity is characterized by comprising the following steps:

selecting a first substrate comprising opposing first and second surfaces;

etching the first surface of the first substrate to form a first groove;

selecting a second substrate comprising opposing third and fourth surfaces;

2. The local thinning method for the MEMS inner cavity according to claim 1, characterized in that: the second substrate includes a single crystal silicon or an SOI substrate.

3. The local thinning method of the MEMS inner cavity according to claim 1 or 2, characterized in that: further comprising the step of further thinning the fourth surface of the second substrate.

4. The local thinning method for the MEMS inner cavity according to claim 2, characterized in that: the third surface of the second substrate is a top silicon layer of the SOI.

5. The local thinning method for the MEMS inner cavity is characterized by comprising the following steps of: and etching the third surface of the second substrate, and stopping at the buried oxide layer of the SOI substrate to form a local thinning structure.

6. The local thinning method for the MEMS inner cavity according to claim 1, characterized in that: the local thinning structure comprises a cross beam structure or a peninsula beam film structure formed on the third surface of the second substrate.

7. A preparation method of a MEMS pressure sensor is characterized by comprising the following steps:

selecting a first substrate comprising opposing first and second surfaces;

etching the first surface of the first substrate to form a first groove;

selecting a second substrate comprising opposing third and fourth surfaces;

thinning the fourth surface of the second substrate to form a strain film;

8. The method of manufacturing a MEMS pressure sensor as claimed in claim 7, wherein: the second substrate includes a single crystal silicon or an SOI substrate.

9. The method of manufacturing a MEMS pressure sensor as claimed in claim 7, wherein: the piezoresistors form a Wheatstone bridge.

10. A MEMS substrate, comprising:

the first surface of the first substrate comprises a first groove;

a third surface of the second substrate comprises a local thinning structure;

11. A MEMS substrate according to claim 10 wherein: the second substrate includes a single crystal silicon or an SOI substrate.

12. A MEMS substrate according to claim 11 wherein: the local thinning structure is arranged on the top silicon layer of the SOI substrate.

13. A MEMS substrate according to claim 10 wherein: the local thinning structure comprises a cross beam structure or a peninsula beam film structure formed on the third surface of the second substrate.

14. A MEMS substrate according to claim 11 wherein: the top surface of the inner cavity comprises a partially exposed SOI buried oxide layer.

15. A MEMS pressure sensor comprising the MEMS substrate of any one of claims 10-14.

16. The MEMS pressure sensor of claim 15, further comprising a piezo-resistor disposed on a fourth surface of the second substrate.

17. The MEMS pressure sensor of claim 16, wherein the piezoresistors form a wheatstone bridge.

18. The MEMS pressure sensor of claim 16, wherein the fourth surface is a thinned layer of a bulk silicon layer of an SOI substrate or a top silicon layer with an SOI buried oxide layer removed.