CN111246355B

CN111246355B - MEMS device and method of forming a MEMS device

Info

Publication number: CN111246355B
Application number: CN202010237690.8A
Authority: CN
Inventors: 傅思宇; 陆晓龙; 刘国安
Original assignee: SMIC Manufacturing Shaoxing Co Ltd
Current assignee: Semiconductor Manufacturing International Shanghai Corp; Semiconductor Manufacturing Electronics Shaoxing Corp SMEC
Priority date: 2020-03-30
Filing date: 2020-03-30
Publication date: 2021-05-25
Anticipated expiration: 2040-03-30
Also published as: CN111246355A

Abstract

The present invention relates to a MEMS device and a method of forming a MEMS device. In the MEMS device, the back plate comprises a first medium layer, a second medium layer and a conducting layer which are sequentially overlapped, wherein the stress level of the first medium layer is higher than that of the second medium layer, the whole stress and rigidity of the back plate can be improved by utilizing the first medium layer, the sensitivity and accuracy of the test can be improved when the pull-in voltage test is carried out, the larger size of the back plate can be realized, and the tensile property of the back plate can be ensured. In addition, because the conductive layer is still in contact with the second dielectric layer, the first dielectric layer has less influence on the contact performance of the conductive layer. In the MEMS device, a third dielectric layer can be arranged on one side of the first dielectric layer, which is far away from the second dielectric layer, so that the first dielectric layer is protected from being corroded. The method of forming a MEMS device may be used to form the MEMS device described above.

Description

MEMS device and method of forming a MEMS device

Technical Field

The present invention relates to the field of micro-electromechanical technology, and more particularly, to a MEMS device and a method of forming a MEMS device.

Background

A Micro Electro Mechanical System (MEMS) integrates components such as a Micro sensor, a Micro actuator, a Micro Mechanical structure, a Micro power source (Micro energy), a signal processing and control circuit, a high performance electronic integrated device, an interface, and communication, and can be regarded as an independent intelligent System, and the overall size of the System is usually several millimeters or less. With the trend of miniaturization of electronic devices, electronic devices including MEMS (hereinafter referred to as MEMS devices) such as MEMS microphones, MEMS accelerometers, MEMS gyroscopes, and the like have been developed and have a wide development prospect.

The fabrication of MEMS differs from conventional machining fabrication in that MEMS can be fabricated using CMOS compatible semiconductor processes in conjunction with surface or bulk micromachining techniques. Moreover, with the continuous improvement and development of the process technology, the MEMS process research focuses on the development of improving the MEMS performance within the allowable range of the process.

Pull-in voltage (pull-in voltage) testing is an important method for detecting the pull-in performance of MEMS. According to the method, voltage is applied between the diaphragm and the back plate of the MEMS, when the applied voltage is small, the diaphragm can reach a balanced stable state through redistribution of surface charges, the diaphragm can deform greatly to lose the stable state along with the increase of the applied voltage, and the critical voltage of the diaphragm losing the stable state is the pull-in voltage.

In the prior art, the backplate is usually made of low stress nitride and a low stress conductive layer covering the low stress nitride, but for other considerations (such as improving the signal-to-noise ratio of a chip), the MEMS backplate is designed to be larger, but the larger backplate made of the low stress nitride has poor tensile properties, which may interfere with the sensitivity and accuracy of the pull-in voltage test.

Disclosure of Invention

The invention provides an MEMS device having a backplate that has an increased stress level relative to existing structures, which can improve the sensitivity and accuracy of pull-in voltage testing. The invention additionally provides a method of forming a MEMS device.

In one aspect, the invention provides an MEMS device, where the MEMS device includes a diaphragm and a backplate located above the diaphragm, a cavity is provided between the diaphragm and the backplate, and the backplate covers the cavity and includes a first dielectric layer, a second dielectric layer, and a conductive layer, which are sequentially stacked from bottom to top, where a stress level of the first dielectric layer is higher than that of the second dielectric layer.

Optionally, the backplane further includes a third dielectric layer disposed on a lower surface of the first dielectric layer, and a stress level of the third dielectric layer is lower than that of the first dielectric layer.

Optionally, the thickness of at least one of the second dielectric layer and the third dielectric layer is greater than the thickness of the first dielectric layer.

Optionally, the first dielectric layer, the second dielectric layer, and the third dielectric layer all include silicon nitride.

Optionally, the internal stress of the first dielectric layer is at least one order of magnitude higher than the internal stress of the second dielectric layer and/or the third dielectric layer.

Optionally, the first dielectric layer has an inner layerForce is at 10⁴In the order of MPa.

Optionally, the thickness of the first dielectric layer is less than or equal to

Optionally, the MEMS device is an acoustic sensor.

In one aspect, the present invention provides a method of forming a MEMS device, comprising the steps of:

forming a vibrating diaphragm and a sacrificial layer positioned on the vibrating diaphragm;

forming a back plate, wherein the back plate covers the sacrificial layer and comprises a first dielectric layer, a second dielectric layer and a conductive layer which are sequentially overlapped from bottom to top, and the stress level of the first dielectric layer is higher than that of the second dielectric layer;

forming a plurality of through holes in the back plate, wherein the upper surface of the sacrificial layer is exposed from the through holes; and

and releasing the sacrificial layer by using the through hole to form a cavity between the diaphragm and the back plate.

Optionally, before the first dielectric layer is formed, a third dielectric layer is formed on the surface of the sacrificial layer, and the stress level of the third dielectric layer is lower than that of the first dielectric layer.

In the MEMS device provided by the invention, the backboard comprises the first medium layer with the stress level higher than that of the second medium layer besides the second medium layer and the conductive layer, and compared with the backboard only provided with the low-stress medium layer, the whole stress and rigidity of the backboard can be improved, the sensitivity and accuracy of the test can be improved when the pull-in voltage test is carried out, the larger size of the backboard can be realized, and the tensile property of the backboard can be ensured. In addition, since the conductive layer is still in contact with the second dielectric layer at a lower stress level, the first dielectric layer has less impact on the contact performance of the conductive layer.

Considering that the first dielectric layer is easily corroded in the etching process of other materials such as the sacrificial layer, the MEMS device of this embodiment further includes a third dielectric layer, the third dielectric layer is disposed on the lower surface of the first dielectric layer, and the stress level of the third dielectric layer is lower than that of the first dielectric layer, that is, the first dielectric layer is disposed between the second dielectric layer and the third dielectric layer with lower stress, so as to form a sandwich structure, thereby effectively protecting the first dielectric layer from being corroded.

According to the method for forming the MEMS device, after the vibrating diaphragm and the sacrificial layer on the vibrating diaphragm are formed, the backboard comprising the first medium layer, the second medium layer and the conducting layer is formed, the stress level of the first medium layer is higher than that of the second medium layer, then a plurality of through holes capable of exposing the surface of the sacrificial layer are formed in the backboard, the sacrificial layer is released through the through holes, and a cavity is formed between the vibrating diaphragm and the backboard. Compared with a low-stress dielectric layer with the same thickness, the backboard formed by the method has larger integral stress, the rigidity of the backboard is improved, and the thickness of the first dielectric layer can be adjusted according to the requirement of stress improvement. The back plate added with the first medium layer is beneficial to improving the sensitivity and the accuracy of the test when the pull-in voltage test is carried out, thereby being convenient for matching with the high sensitivity requirement of the subsequent packaging process.

Drawings

FIG. 1 is a cross-sectional schematic view of a MEMS device.

FIG. 2 is a cross-sectional schematic view of a MEMS device in accordance with an embodiment of the invention.

FIG. 3 is a cross-sectional schematic view of a MEMS device in accordance with an embodiment of the invention.

FIG. 4 is a flow chart of a method of forming a MEMS device in accordance with an embodiment of the present invention.

Description of reference numerals:

100. 200-a substrate; 110. 210-a diaphragm; 120. 220-a back plate; 130. 230-a cavity; 10. 20-backside trench; 12. 22-a through hole; 140. 240-an oxide layer; 150. 250-a support; 121-an insulating layer; 122. 223-a conductive layer; 221-a first dielectric layer; 222-a second dielectric layer; 224-third dielectric layer.

Detailed Description

The MEMS devices and methods of forming MEMS devices of the present invention are described in further detail below with reference to the accompanying drawings and detailed description. The advantages and features of the present invention will become more apparent from the following description. It should be noted that in the following description, numerous specific details and values are set forth in order to provide a thorough understanding of the present invention, however, it will be apparent to those skilled in the art that the present invention may be practiced without one or more of these details and in other instances, some features that are well known in the art have not been described in order to avoid obscuring the present invention. It is to be understood that the drawings in the specification are in simplified form and are not to be taken in a precise scale, for the purpose of facilitating and distinctly claiming the embodiments of the present invention.

For ease of description, some embodiments of the present application may use spatially relative terms such as "above …," "below …," "top," "below," and the like, to describe the relationship of one element or component to another (or other) element or component as illustrated in the various figures of the embodiments. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements or components described as "below" or "beneath" other elements or components would then be oriented "above" or "over" the other elements or components.

FIG. 1 is a cross-sectional schematic view of a MEMS device. Referring to fig. 1, the MEMS device, taking a MEMS microphone as an example, includes, from a structural point of view, a substrate 100, a diaphragm 110 disposed on the substrate 100, and a back plate 120 disposed above the diaphragm 110. A back-side groove 10 exposing the lower surface of diaphragm 110 is provided in substrate 100. A cavity 130 is disposed between back plate 120 and diaphragm 110, and a plurality of through holes 12 are disposed in back plate 120, and through holes 12 allow air to flow into cavity 130 and impact diaphragm 110. The backplate 120 and the portion of the diaphragm 110 located at the periphery of the cavity 130 are disposed corresponding to the upper surface of the substrate 100. In addition, the MEMS device may further include an oxide layer 140 disposed on the upper surface of the substrate, and a support 150, and a portion of the back plate 120 located at the periphery of the cavity 130 overlaps the upper surface of the support 150.

Referring to fig. 1, the existing process utilizes a low-stress dielectric material to form an insulating layer 121 in the back-plate 120, and additionally deposits a conductive layer 122 on the insulating layer 121 for metallization, so that the back-plate 120 can serve as an upper electrode of the MEMS microphone, and the diaphragm 110 can include or be composed of a conductive material layer, so that the diaphragm 110 can serve as a lower electrode of the MEMS microphone, and the upper electrode and the lower electrode are led out from a peripheral region of the cavity (not shown). When the pull-in voltage test is performed, a critical voltage at which the diaphragm 110 loses a stable state is obtained by applying a gradually increasing voltage between the upper electrode and the lower electrode. In order to ensure the required sensitivity and test accuracy, and to match the high sensitivity requirements of the subsequent packaged device, the backplate 120 in the MEMS device should have sufficient stress, especially when the area of the backplate 120 over the cavity 130 is large, the stress on the backplate 120 itself is higher.

However, on the one hand, since the backplate 120 is mainly formed of a low stress dielectric material in the MEMS device as shown in fig. 1, the stress thereof is not sufficient to meet the large stress requirement required for the MEMS device, and it is necessary to improve the existing backplate; on the other hand, since the conductive layer in the backplate 120 usually has a small stress, if the backplate is made of a high-stress insulating material, the stress of the conductive layer deposited on the high-stress insulating layer cannot be well matched with the high stress of the insulating layer, which easily causes the interface bonding force between the two layers to be reduced, and is not favorable for the working stability and the long life of the MEMS device.

The MEMS device of the present invention has been proposed on the basis of the above-mentioned research. It should be noted that the MEMS device described below is mainly illustrated by taking the MEMS used for an acoustic sensor (for example, a MEMS microphone) as an example, but the MEMS described below is not limited to be used for the acoustic sensor and is not limited to be used in the structure shown in the drawings of the present specification. The backplate in the embodiments of the present invention described below is suitable for various MEMS devices that require increased backplate stress.

FIG. 2 is a cross-sectional schematic view of a MEMS device in accordance with an embodiment of the invention. Referring to fig. 2, in an embodiment, the MEMS device includes a diaphragm 210 and a backplate 220 located above the diaphragm 210, a cavity 230 is disposed between the diaphragm 210 and the backplate 220, the backplate 220 covers the cavity 230 and includes a first dielectric layer 221, a second dielectric layer 222 and a conductive layer 223, which are sequentially stacked from bottom to top, wherein a stress level of the first dielectric layer 221 is higher than that of the second dielectric layer 222.

Specifically, the MEMS device of this embodiment further includes a substrate 200 disposed below the diaphragm 210 (i.e., on a side away from the backplate 220), where the substrate 200 may be made of silicon, germanium, silicon carbide, gallium arsenide, gallium phosphide, indium arsenide, indium antimonide, or the like, or may be made of silicon-on-insulator (SOI) or germanium-on-insulator (GOI), or may be made of another material, such as GaAs, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, or GaInAsP, or may be made of a combination of the above materials. Substrate 200 may include a doped epitaxial layer, a graded semiconductor layer, and a semiconductor layer (e.g., a silicon layer on a silicon germanium layer) overlying other semiconductor layers of different types. Transistors, field effect transistors, MEMS, or other suitable components may also be provided integrally on the same substrate 200. In this embodiment, an oxide layer 240 is formed on the front surface of the substrate 200, and a back-side groove 20 exposing the lower surface of the diaphragm 210 is formed in the substrate 200 corresponding to the position of the cavity 230.

The diaphragm 210 is formed on one side of the upper surface of the substrate 200, a portion of the diaphragm 210 located at the periphery of the cavity 230 overlaps the oxide layer 240 on the surface of the substrate 200, and a portion of the diaphragm exposed to the cavity 230 is suspended above the back-side groove 20 of the substrate 200. The diaphragm may also be provided with holes therein. In this embodiment, the diaphragm 210 serves as a lower electrode of the MEMS device, and may be made of a conductive material or have a conductive film formed on a surface thereof. By way of example, the diaphragm 210 is formed of polysilicon containing a dopant. In other embodiments, the diaphragm 210 may comprise a metal, an alloy, a metal nitride, a metal oxide, or the like.

The backplate 220 is disposed on a side of the diaphragm 210 away from the substrate 200, and the backplate 220 and the diaphragm 210 are separated by a cavity 230, and for the MEMS microphone, air vibration can impact the diaphragm through the through hole 22 on the backplate 220 and the cavity 230, and thus can be used for implementing an acoustic sensing function. The extension of the cavity 230 on the diaphragm 210 may be defined according to the range and height of the support 250 disposed on the substrate 200. The support 250 is disposed on the diaphragm 210, and may include an insulating material such as silicon nitride.

In this embodiment, the backplate 220 serves as the other electrode, i.e., the top electrode, of the MEMS device. The backplate 220 and the lower electrode formed by the diaphragm 210 respectively form two plates of a parallel plate capacitor of the MEMS microphone, and in operation, sound waves passing through the through hole 22 in the backplate 220 into the cavity 230 cause the diaphragm 210 to vibrate, and the vibration of the diaphragm 210 causes the distance between the diaphragm and the backplate 220 to change, thereby causing the capacitance of the parallel plate capacitor to change. The conductive layers on the diaphragm 210 and backplate 220 may be brought out from the respective edge regions and connected to the drive and processing circuitry of the MEMS device.

Referring to fig. 2, in the present embodiment, the backplane 220 includes a first dielectric layer 221, a second dielectric layer 222, and a conductive layer 223 stacked in a thickness direction in sequence, and a stress level of the first dielectric layer 221 is higher than a stress level of the second dielectric layer 222. The difference of the stress level can be realized by adjusting the material of the dielectric layer and the preparation process. In this embodiment, the stress level of the first dielectric layer 221 and the second dielectric layer 222 refers to the magnitude of stress therein, which is embodied as tensile stress. The intrinsic stress of the first dielectric layer 221 may be at least an order of magnitude higher than the intrinsic stress of the second dielectric layer 222. For example, the intrinsic stress of first dielectric layer 221 is 10⁴In the order of MPa, and the internal stress of the second dielectric layer 222 is 10²In the order of MPa. The stress ranges of the first dielectric layer 221 and the second dielectric layer 222 can be adjusted as desired. The magnitude of the stress can be obtained according to testing methods disclosed in the art (e.g., as measured using a stress gauge). In one embodiment, the first dielectric layer 221 formed of a high stress silicon-rich nitride and the second dielectric layer 222 formed of a low stress nitride are obtained by changing growth conditions (e.g., growth temperature, gas composition, growth rate, etc.). First dielectric layer 221 and second dielectric layer 222 may also be formed according to the art for high stress materials andstandard determination of low stress materials.

In one embodiment, the conductive layer 223 may include any one of a doped semiconductor material (e.g., polysilicon including a dopant), a metal and alloy, a metal nitride, a metal oxide, and the like. Due to the thin thickness of the conductive layer 223, the stress level of the back plate is closely related to the dielectric layer outside the conductive layer 223. In this embodiment, by adding the first dielectric layer 221, the overall stress of the backplane 220 can be increased, so as to increase the overall rigidity of the backplane 220, and when performing a pull-in voltage test, the sensitivity and accuracy of the test can be improved, and the increase of the stress of the backplane 220 is advantageous for the design of the large-sized cavity 230 and the large-sized backplane 220 (for example, the diameter is more than 800 μm), and the increase of the stress level is helpful for ensuring the tensile property while realizing a larger backplane size. Moreover, since the conductive layer 223 is still in contact with the second dielectric layer 222 with a lower stress level, the stress of the two layers is matched, and the contact performance between the second dielectric layer 222 and the conductive layer 223 is better.

The conductive layer 223 may be designed to face the diaphragm 210 or to face away from the diaphragm 210. That is, although fig. 2 illustrates the back plate 220, the first dielectric layer 221, the second dielectric layer 222, and the conductive layer 223 are sequentially stacked in a direction away from the substrate 200. For example, in another embodiment, the conductive layer, the second dielectric layer and the first dielectric layer are sequentially stacked along a direction away from the substrate, which is particularly related to the design and manufacturing process of the backplate. The conductive layer 223 may be a patterned layer, for example, in one embodiment, an opening in the conductive layer 223 may expose the second dielectric layer 222 (not shown).

The thicknesses of the first dielectric layer 221 and the second dielectric layer 222 may be specifically set according to design requirements, for example, the stress required by the backplate 220 may be obtained according to the overall structural design of the MEMS, and then the first dielectric layer 221 and the second dielectric layer 222 with preset thicknesses may be set according to the stress required by the backplate 220. In addition, in order to obtain the backplate 220 with the best overall performance, the thickness ratio of the first dielectric layer 221 to the second dielectric layer 222 can be adjusted. In order to reduce the existence of high stress level in first dielectric layer 221At the risk of curling (peeling), it is preferable that the thickness of first dielectric layer 221 not exceed

The thickness of second dielectric layer 221 may be greater than or equal to the thickness of first dielectric layer 221 due to the lower risk of curling. In addition, since silicon nitride has better compactness, rigidity and etching blocking capability, in the embodiment, the first dielectric layer 221 and the second dielectric layer 222 may both include silicon nitride or be formed entirely of silicon nitride.

It has been found that high stress silicon nitride is susceptible to poor etch stop capability relative to low stress silicon nitride. In particular, for a BOE (buffered hydrofluoric acid) etch used to etch silicon oxide (e.g., as a sacrificial material filling the cavity 230), there is a significant etch for high stress silicon nitride while a less significant etch for low stress silicon nitride. Therefore, when the first dielectric layer 221 in the above structure is deposited, a certain etching loss may be considered, for example, by calculating an etching rate of the sacrificial material release process to the first dielectric layer 221, the thickness of the first dielectric layer 221 deposited may be further controlled.

The backplate of the MEMS device of the present embodiment is not limited to the above-mentioned double dielectric layer structure, for example, considering that the thickness of the first dielectric layer is easily etched to cause deposition when releasing the sacrificial material, and in some embodiments, the surface of the backplate facing the diaphragm may be provided with an anti-sticking protrusion structure, since the first dielectric layer 221 faces the diaphragm 210, if the first dielectric layer 221 is significantly etched when etching the sacrificial material, the protrusion structure is easily damaged, causing problems such as failure and absorption of the diaphragm. Therefore, optionally, in the MEMS device of this embodiment, the back plate may further include another low-stress dielectric layer stacked on the lower surface of the first dielectric layer 221. The concrete description is as follows.

FIG. 3 is a cross-sectional schematic view of a MEMS device in accordance with an embodiment of the invention. Referring to fig. 3, in an embodiment, the back plate 220 of the MEMS device further includes a third dielectric layer 224 in addition to the first dielectric layer 221, the second dielectric layer 222 and the conductive layer 223, and the third dielectric layer 224 are disposed on the lower surface of the first dielectric layer 221 (i.e., the surface of the side away from the second dielectric layer 222). Thus, the first dielectric layer 221 is disposed between the second dielectric layer 222 and the third dielectric layer 224 to form a sandwich structure, which can effectively protect the first dielectric layer 221 from being corroded in a process such as releasing a sacrificial material. The same reference numerals as in the MEMS devices shown in fig. 3 and 2 denote corresponding components, whose features are similar to those in the MEMS device shown in fig. 2. The stress level of the third dielectric layer 224 may be set lower than that of the first dielectric layer 221, for example, the stress level of the third dielectric layer 224 may be close to or the same as that of the second dielectric layer 222, with an intrinsic stress of about 10²In the order of MPa. Third dielectric layer 224 may comprise silicon nitride or be formed entirely of silicon nitride.

The MEMS device of the present embodiment is not limited to the MEMS structure shown in fig. 2 or fig. 3, for example, in an embodiment, the MEMS device may include two back plates respectively disposed at both sides of the diaphragm, each back plate may serve as one electrode of the MEMS device, in this embodiment, each back plate may have the first dielectric layer 221 and the second dielectric layer 222 having different stress levels, and a similar arrangement to the MEMS device may also be adopted.

The MEMS device of the present embodiment is, for example, an acoustic sensor, which may be a microphone, a receiver, a speaker, or a device including a combination of two or more of them. Referring to fig. 2 and 3, since the backplate 220 of the MEMS device has not only the second dielectric layer 222 and the conductive layer 223 with lower stress levels, but also the first dielectric layer 221 with higher stress levels, the overall stress of the backplate 220 can be increased by using the first dielectric layer 221, thereby increasing the rigidity of the backplate 220 and facilitating the design of a larger backplate size. In addition, when the pull-in voltage test is performed, the sensitivity and accuracy of the test are improved, and the influence of the first medium layer 221 on the contact performance of the conductive layer 223 is small because the conductive layer 223 is still in contact with the second medium layer 222 with a low stress level. Further, referring to fig. 3, the backplate 220 may further include a third dielectric layer 224 disposed on the lower surface of the first dielectric layer 221 opposite to the diaphragm 210 through the cavity 230, so as to protect the first dielectric layer 221 from corrosion. Therefore, the MEMS device provided by the embodiment of the invention can improve the comprehensive performance of the acoustic sensor.

The present embodiments also include a method of forming a MEMS device that can be used to fabricate the MEMS devices described in the embodiments of the present invention, but are not limited thereto, and other configurations of MEMS devices can be fabricated using the method.

FIG. 4 is a flow chart of a method of forming a MEMS device in accordance with an embodiment of the present invention. Referring to fig. 4, in the present embodiment, a method of forming a MEMS device includes the steps of:

s1: forming a vibrating diaphragm and a sacrificial layer positioned on the vibrating diaphragm;

s2: forming a back plate to cover the sacrificial layer, wherein the back plate comprises a first dielectric layer, a second dielectric layer and a conductive layer which are sequentially overlapped from bottom to top, and the stress level of the first dielectric layer is higher than that of the second dielectric layer;

s3: forming a plurality of through holes in the back plate, wherein the surface of the sacrificial layer is exposed from the through holes;

s4: and removing the sacrificial layer by using the through hole to form a cavity between the diaphragm and the back plate.

Specifically, referring to fig. 2, 3 and 4, in step S1, the diaphragm 210 may be formed on the substrate 200, and in order to reduce stress, the diaphragm 210 may be formed after the oxide layer 240 is formed on the upper surface of the substrate 200. The diaphragm 210 is formed of, for example, polysilicon including a dopant, and the diaphragm 210 may serve as a lower electrode of the MEMS device to be formed. Before forming the sacrificial layer, a support 250 may be formed on the diaphragm 210, and the support 250 is used to define the range of the sacrificial layer. The sacrificial layer (not shown) is formed of, for example, silicon oxide. Next, in step S2, a back plate 220 is formed, and the back plate 220 may cover the sacrificial layer and be lapped on the support 250.

For example, when the backplate 250 is formed, a first dielectric layer 221, a second dielectric layer 222, and a conductive layer 223 may be sequentially deposited on a sacrificial layer along a direction away from the diaphragm 210, and in an embodiment, in order to avoid an adverse effect of excessive loss of the first dielectric layer 221 when the sacrificial layer is released, a low-stress dielectric layer (as a third dielectric layer 224) may be deposited before the first dielectric layer 221 is formed, so that the second dielectric layer 222 and the third dielectric layer 224 in the backplate 220 cover the first dielectric layer 221 up and down (similar to a sandwich structure). The first dielectric layer 221, the second dielectric layer 222, and the third dielectric layer 224 may be formed of silicon nitride, and the adjustment of the stress level is achieved by adjusting conditions such as growth temperature, growth atmosphere, and growth rate. Wherein the stress levels of the second dielectric layer 222 and the third dielectric layer 224 are substantially the same and are both lower than the stress level of the first dielectric layer 221. In a conventional sacrificial layer etching process (e.g., BOE etching), since low stress silicon nitride is less susceptible to corrosion than high stress silicon nitride, the use of the sandwich structure can improve the corrosion resistance of the backplate 220, which can help improve the overall performance of the MEMS device to be formed. Optionally, the thicknesses of the second dielectric layer 222, the third dielectric layer 224 and the first dielectric layer 221 may be adjusted according to the requirement of the backplane stress. Optionally, at least one of the second dielectric layer 222 and the third dielectric layer 224 has a thickness greater than that of the first dielectric layer 221. The backplate 220 serves as the upper electrode of the MEMS device to be formed. The conductive layer 223 may include a metal material or a doped semiconductor material. After depositing a desired thickness of conductive material on the second dielectric layer 222, the conductive layer 223 may be obtained by a patterning process to obtain a desired upper electrode pattern.

After the back plate 220 of the multi-layer structure is formed, step S3 is performed to form a plurality of through holes 22 in the back plate 220, so that the surface of the sacrificial layer under the back plate 220 is exposed from the through holes 22, so as to remove the sacrificial layer to obtain the cavity 230. The through-hole 22 may serve as a passage for sound waves to be transmitted between the outside of the cavity 230 and the diaphragm 210 when the MEMS device is in operation.

Then, step S4 is performed, in which the sacrificial layer is released by the through holes 22, so as to form a cavity 230 between the diaphragm 210 and the backplate 220. The sacrificial release layer may be formed using a dry or wet etching process. As an example, in the embodiment, the sacrificial layer is silicon oxide, and then wet etching may be performed by using BOE etching solution, the etching solution passes through the back plate 220 from the through hole 22, and the etched sacrificial layer material flows out from the through hole 22. After step S4 is completed, a cavity 230 is formed between the backplate 220 and the diaphragm 210. In this embodiment, the MEMS device to be formed is, for example, a MEMS microphone, and when the MEMS microphone is in operation, the airflow enters the cavity to impact the diaphragm to vibrate (or a reverse process), which can be used for sensing the sound wave. In the process of releasing the sacrificial layer, if there is a significant etching on the material of the first dielectric layer 221, the dielectric layer in the backplane 220 may adopt a sandwich structure as shown in fig. 3, and the second dielectric layer 222 and the third dielectric layer 224 cover the first dielectric layer 221 up and down, so that the risk of corrosion of the first dielectric layer 221 may be reduced.

In the method for forming the MEMS device, after the diaphragm 210 and the sacrificial layer thereon are formed, the backplate 220 including the first dielectric layer 221 is formed, then a plurality of through holes 22 capable of exposing the surface of the sacrificial layer are formed in the backplate 220, and the sacrificial layer is removed by using the through holes 22, so that a cavity is formed between the diaphragm 210 and the backplate 220. Compared with a low-stress dielectric layer with the same thickness, the backboard 220 formed by the method has larger overall stress, the rigidity of the backboard 220 is improved, and the thickness of the first dielectric layer 221 can be adjusted according to the requirement of improving the stress. When the pull-in voltage is tested, the sensitivity and the accuracy of the test can be improved, and the pull-in voltage test method is favorable for matching with the high sensitivity requirement of a subsequent packaging process.

It should be noted that, the embodiments in this specification are described in a progressive manner, and for the method disclosed in the embodiments, the features thereof correspond to the MEMS device disclosed in the embodiments, so that the description is simple, and relevant points can be understood by reference.

The above description is only for the purpose of describing the preferred embodiments of the present invention and is not intended to limit the scope of the claims of the present invention, and any person skilled in the art can make possible the variations and modifications of the technical solutions of the present invention using the methods and technical contents disclosed above without departing from the spirit and scope of the present invention, and therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention belong to the protection scope of the technical solutions of the present invention.

Claims

1. The MEMS device is characterized by comprising a vibrating diaphragm and a back plate positioned above the vibrating diaphragm, wherein a cavity is arranged between the vibrating diaphragm and the back plate, the back plate covers the cavity and comprises a first medium layer, a second medium layer and a conducting layer which are sequentially overlapped from bottom to top, and the tensile stress of the second medium layer is 10²And the tensile stress of the first dielectric layer is at least one order of magnitude higher than that of the second dielectric layer.

2. The MEMS device of claim 1, wherein the backplate further comprises a third dielectric layer disposed on a lower surface of the first dielectric layer, the third dielectric layer having a lower stress level than the first dielectric layer.

3. The MEMS device of claim 2, wherein at least one of the second dielectric layer and the third dielectric layer has a thickness greater than a thickness of the first dielectric layer.

4. The MEMS device of claim 2, wherein the first dielectric layer, the second dielectric layer, and the third dielectric layer each comprise silicon nitride.

5. The MEMS device of claim 2, wherein the intrinsic stress of the first dielectric layer is at least an order of magnitude higher than the intrinsic stress of the third dielectric layer.

6. The MEMS device of any one of claims 1 through 5, wherein the intrinsic stress of the first dielectric layer is at 10⁴In the order of MPa.

7. The MEMS device of any of claims 1 to 5, wherein the first dielectric layer has a thickness less than or equal to

8. The MEMS device, as recited in any of claims 1 through 5, wherein the MEMS device is an acoustic sensor.

9. A method of forming a MEMS device, comprising:

forming a back plate, wherein the back plate covers the sacrificial layer, the back plate comprises a first dielectric layer, a second dielectric layer and a conductive layer which are sequentially overlapped from bottom to top, and the tensile stress of the second dielectric layer is 10²The magnitude of MPa, the tensile stress of the first dielectric layer is at least one magnitude order higher than that of the second dielectric layer;

10. The method of forming a MEMS device of claim 9, wherein a third dielectric layer is formed on the surface of the sacrificial layer prior to forming the first dielectric layer, the third dielectric layer having a lower stress level than the first dielectric layer.