This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-185606, filed Sep. 6, 2013, the entire contents of which are incorporated herein by reference.
Embodiments described herein relate generally to a MEMS (Micro Electro Mechanical Systems) device including a MEMS element.
A MEMS pressure sensor is known as one of the devices utilizing MEMS elements. The MEMS pressure sensor comprises a substrate, a fixed electrode (lower electrode), a movable electrode (upper electrode) and a diaphragm (domed thin film). The fixed electrode is formed on the substrate. The movable electrode is formed on the inner upper surface of the diaphragm.
When the diaphragm is sagged by pressure, the distance between the fixed and movable electrodes varies, thereby varying the electrostatic capacitance therebetween. The MEMS pressure sensor employs a principle that pressure is detected utilizing the relationship between pressure and electrostatic capacitance. There is a demand for high detection accuracy.
Referring now to the accompanying figures, embodiments will be described.
In general, according to one embodiment, a MEMS device is disclosed. The MEMS device comprises a substrate; a first MEMS element provided on the substrate; and a second MEMS element provided on the substrate. Each of the first and second MEMS elements comprises a fixed electrode fixed on the substrate; a movable electrode arranged above the fixed electrode and being movable vertically; a first insulating film, the first insulating film and the substrate defining a cavity in which the fixed electrode and the movable electrode are contained; and a first anchor provided on a surface of the first insulating film inside the cavity and configured to connect the movable electrode to the first insulating film. In the first MEMS element, the cavity defined by the first insulating film and the substrate is closed. In the second MEMS element, the cavity defined by the first insulating film and the substrate is opened by a through hole.
In the embodiment below, a MEMS pressure sensor will be described as an example of one of the MEMS devices. However, the embodiment is not limited to the MEMS pressure sensor. The MEMS pressure sensor of each embodiment is used as a pressure sensor for smartphones (such as a height indicator or an activity monitor), a healthcare pressure sensor, a vehicle pressure sensor (a side collision sensor, TPMS (Tire Pressure Monitoring System)).
In
An anchor 130 is provided on the inner upper surface of the diaphragm 120. A movable electrode (upper electrode) 140 is connected to the inner upper surface of the diaphragm 120 via the anchor 130. For instance, the movable electrode has a flat plate shape in parallel with the fixed electrode. Material of the movable electrode 140 is, for example, AlCu alloy, and opposed to the fixed electrode 110.
The above configuration is shared between the pressure sensing MEMS element and the reference capacitance MEMS element.
The reference capacitance MEMS element differs from the pressure sensing MEMS element in that the diaphragm 120 of the reference capacitance MEMS element has a through hole 160. Thus, in the reference capacitance MEMS element, the cavity defined between the diaphragm 120 and the substrate 100, which contains the fixed electrode 110 and the movable electrode 140, is partially opened by the through hole 160. For that reason, the opened cavity communicates with the space outside the MEMS element via the through hole 160. In other words, the opened cavity is connected to the atmosphere or external air outside the MEMS element.
In
The pressure sensing MEMS element and the reference capacitance MEMS element are formed on the same substrate by the same process, except for the presence or absence of the step of forming the through hole 160. Therefore, characteristic variations of the pressure sensing and reference capacitance MEMS elements due to variations in element manufacturing process exhibit nearly similar tendencies. This results in reducing the influence of manufacturing variations on the difference between the capacitances of the pressure sensing MEMS element and the reference capacitance MEMS element.
Further, since the pressure sensing and reference capacitance MEMS elements have substantially the same structure except for the presence or absence of the through hole 160, they exhibit substantially the same dependency of electrostatic capacitance on temperature.
Furthermore, since the diaphragm 120 of the reference capacitance MEMS element has the through hole 160, the temperature in the space (cavity) for operating the movable electrode 140 of the reference capacitance MEMS element changes when the outside atmosphere changes. The temperature dependencies of the capacitances of the pressure sensing and reference capacitance MEMS elements are nearly same, so that the influence due to temperature dependency of the capacitance of the pressure sensing MEMS element can be corrected based on the temperature dependency of the capacitance of the reference capacitance MEMS element.
If the pressure 150 is applied to the diagram 120 of the MEMS element of the first embodiment, the movable electrode 130 downwardly displaces in parallel without sagging, although the diagram 120 sags, as is shown in
In contrast, as is shown in
Thus, when the embodiment is compared with the comparative example in the case that the diagram 120 is sagged by same magnitude of pressure 150, the average distance between the movable electrode 130 and the fixed electrode 110 is shorter in the embodiment than in the comparative example. Therefore, the present embodiment can provide the MEMS element with greater change in capacitance even the same magnitude of pressure. This also enables the detection accuracy of the MEMS element to be higher.
The rates of change of the electrostatic capacitance in the embodiment and the comparative example will be described in more detail.
The deformation distribution of the diagram with pressure P applied thereto is given by
w(r)=P(1−r2/a2)a4/(64D) (1)
D=Eh3/{12(1−ν2)} (2)
where
r is the distance from the center of the diagram;
w(r) is the amount of sagging of the diagram at the distance r from the center of the diagram;
a is the radius of the diagram;
P is the pressure applied to the diagram;
D is the bending rigidity of the diagram;
E is the Yong's modulus of the diagram; and
ν is the Poisson's ratio
The maximum sagging amount wmax of the diagram, i.e., the sagging amount w(0) of the diagram at its center, is given by
wmax=w(0)=Pa4/(64D)=3(1−ν2)Pa4/(16Eh3) (3)
Accordingly, the electrostatic capacitance obtained when the pressure P is applied is given by
where
Cold is the sensor capacitance of the comparative example;
C0=∈πa2/g0 is the sensor capacitance (initial capacitance);
∈ is the dielectric constant of the air; and
g0 is the initial gap between the movable electrode and the fixed electrode before the application of pressure.
The rate of change of the electrostatic capacitance is given by
where
ΔCold/C0=(Cold−C0)/C0 is the rate of change of the comparative sensor.
Among the parameters and constants used in the following equations associated with the embodiment, the same parameters and constants as those used in the comparative example have the same meanings as in the comparative example.
The electrostatic capacitance obtained when the pressure P is applied is given by
Cnew=∈πa2/(g0−wmax) (6)
where Cnew is the sensor capacitance of the embodiment.
The rate of change of the electrostatic capacitance is given by
ΔCnew/C0=(Cnew−C0)/C0=wmax/(g0−wmax) (7)
where ΔCnew/C0 is the change rate of the capacitance of the sensor of the embodiment.
In
A fixed electrode (lower electrode) 202, and interconnects 203 and 204 are provided on the substrate 200. The interconnects 203 and 204 are provided outside the fixed electrode 202. The interconnect 203 is in contact with the plug 201. Anchors 207 are provided on the interconnects 203 and 204.
Diagrams 211b, 212 and 213 are provided on the substrate 100 to contain the fixed electrode 202, the interconnects 203 and 204, the anchors 207, etc.
An anchor 211a is provided on the inner upper surface of the diagrams 211b, 212 and 213. A movable electrode (upper electrode) 206 is connected to the inner upper surface of the diagrams 211b, 212 and 213 via the anchor 211a. The movable electrode 206 is arranged to be opposed to the fixed electrode 202.
Springs 208 that connect the movable electrode 206 to the anchors 207 are formed continuously from the upper surface of the movable electrode 206 onto the upper surfaces of the anchors 207.
The above configuration is shared between both the pressure sensing and reference capacitance MEMS elements. The reference capacitance MEMS element is differs from the pressure sensing capacitance MEMS element in that the diagrams 211b, 212 and 213 of the reference capacitance MEMS element has a through hole 240.
The present embodiment can also improve the detection accuracy of the MEMS device for the same reason as that of the first embodiment.
A description will be given of an example of a method for manufacturing the MEMS element of the present embodiment.
[
The plug 201 is formed in the substrate 200, and then the fixed electrode 202, and the interconnects 203 and 204 are formed on the substrate 200.
[
A first sacrificial film 205 is formed on the substrate 200, thereafter the first sacrificial film 205 is processed into a predetermined shape such that parts of the upper surfaces of the interconnects 203 and 204 are exposed. The first sacrificial film 205 is an insulating film comprising, for example, organic material such as polyimide.
[
A conductive film, which is to be processed into the movable electrode 206 and the anchors 207, is formed on the entire surface, then an unshown resist pattern is formed, and the conductive film is etched by using the resist pattern as a mask, thereby forming the movable electrode 206 and the anchors 207. The movable electrode 206 and the anchors 207 may be formed of different materials.
[
A conductive film, which is to be processed into the springs 208 for connecting the movable electrode 206 to the anchors 207, is formed, then an unshown resist pattern is formed, and the conductive film is etched by using the resist pattern as a mask, thereby forming the springs 208.
In the present embodiment, although the movable electrode 206 and the anchors 207 are formed by the same layer, and the springs 208 is formed by another layer, all of the movable electrode 206, the anchors 207 and the springs 208 may be formed by the same layer. The material of the anchors 207 and the springs 208 may be different from the material of the movable electrode 206. The material of the anchors 207 and the springs 208 may be formed from, for example, insulating film such as silicon nitride film. Moreover, the anchors 207 and the springs 208 may comprise a mixture of metal and insulating material.
[
The MEMS element obtained as a halfway product in the process up to
[
An insulating film 211, which is to be processed into an anchor (first anchor) and a first diagram, is formed on the second sacrifice film 209 so that the opening 210 is closed by the insulating film 211. Material of the film 211 is, for example, silicon oxide.
[
By using an unshown resist pattern as a mask, the film 211 is etched to form the anchor (first anchor) 211a connected to the movable electrode 206, and the first diagram 211b having a plurality of through holes. The resist pattern (not shown) is formed by ordinary photolithography process.
[
The above-mentioned unshown resist pattern is removed by ashing using oxygen gas. At this time, the oxygen gas is supplied to the second sacrifice film 209 through the through hole of the first diagram 211b. The second sacrifice film 209 is remove by the oxygen gas. When the second sacrifice film 209 is removed to expose the first sacrifice film 205, the first sacrifice film 205 is also removed by the oxygen gas. As a result, a cavity 230 as a space for permitting the movable portion of the MEMS element to work is formed.
After that, the second diagram 212 is formed on the first diagram 211b, and the third diagram 213 is formed on the second diagram 212.
The through hole of the first diagram 211b is closed by the second diagram 212. Material of the second diagram 212 is, for example, polyimide. Material of the third diagram 213 is, for example, silicon nitride.
In this manner, the pressure sensing MEMS element shown in
The reference capacitance MEMS element shown in
The present embodiment differs from the second embodiment in that the surfaces of the fixed electrode 202, the interconnects 203, 204, the movable electrode 206, the anchors 207 and the springs 208 of the present embodiment are covered with an insulating layer 300. Material of the insulating layer 300 is, for example, silicon nitride.
[
The fixed electrode 202, the interconnect 203, and the interconnect 204 are formed on the substrate 200, and thereafter the first insulating layer 300a is formed on the substrate 200, the fixed electrode 202, the interconnect 203, and the interconnect 204.
[
By using lithography process and etching, a portion of the first insulating layer 300a is removed, which is other than the first insulating layer 300a on the top surfaces and side surfaces of the substrate 200, the fixed electrode 202, the interconnect 203 and the interconnect 204.
[
The first sacrifice film 205 is formed on the entire surface of the resultant structure, and then openings are formed to expose upper surface portions of the interconnects 203, 204 by lithography process and etching.
[
A second insulating layer 300b is formed on the first sacrifice film 205 to cover the side and bottom surfaces of the openings in the first sacrifice film 205. Material of the second insulating layer 300b is the same material as the first insulating layer 300a.
[
The portions of the second insulating layer 300b on the bottoms of the openings in the first sacrifice film 205 are removed by lithography process and etching, and thereafter the movable electrode 206, the anchors 207 and the springs 208 are formed as in the case shown in
[
A third insulating film 300c is formed on the entire surface of the resultant structure, and thereafter the third insulating layer 300c which is exposed on the top surface of the first sacrifice film 205, and the second insulating layer 300b which is below the exposed third insulating layer 300c, are selectively removed by using lithography process and etching. Material of the third insulating layer 300c is the same material as the second insulating layer 300b. After that, the same steps as those shown in
In the third embodiment, all of the fixed electrode 202, the interconnects 203, 204, the movable electrode 206, the anchors 207 and the springs 208 are covered with the insulating layer 300, but only one or some of these members may be covered with the insulating layer 300. For instance, only the surfaces of the fixed electrode 202 and the movable electrode 206 may be covered with the insulating layer 300. The members covered with the insulating layer 300 are protected from corrosion.
The present embodiment differs from the second embodiment in that the material of the first diagram 211b and the material of the third diagram 213b are the same. The material of the first and third diagrams 211b, 213b is, for example, silicon oxide.
In the present embodiment, since the symmetrical property of the structure (material) of the diagrams 211b, 212, 213b is improved, warpages that occurs in the diagrams 211b, 212 and 213b due to thermal expansion difference are canceled, whereby the temperature dependency of their electrostatic capacitances can be reduced.
In the present embodiment, the fixed electrode 202, the interconnects 203 and 204, the movable electrode 206, the anchors 207 and the springs 208 may be covered with the insulating layer 300, as in the third embodiment.
The present embodiment differs from the second embodiment in that the through hole 240 is not provided in the diagrams 211b, 212 and 213b, but is provided in the substrate 200.
In the first to third embodiments, the pressure sensing MEMS element differs from the reference capacitance MEMS element in that it has no through holes in the diaphragm. In contrast, in the present embodiment, the pressure sensing MEMS element and the reference capacitance MEMS element are identical to each other in structure (shape and dimensions), which can further reduce the slight difference of the temperature dependency that occurs due to the electrostatic difference between the pressure sensing MEMS element and the reference capacitance MEMS element.
In the present embodiment, the material of the first diagram 211b and the material of third diagram 213b may be same material, as in the second embodiment.
Further, in the present embodiment, at least one of the fixed electrode 202, the interconnects 203 and 204, the movable electrode 206, the anchors 207 and the springs 208 may be covered with the insulating layer 300, as in the third embodiment.
While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.
Number | Date | Country | Kind |
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2013-185606 | Sep 2013 | JP | national |
Number | Name | Date | Kind |
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6631645 | Satou | Oct 2003 | B1 |
8004053 | Miyagi | Aug 2011 | B2 |
8309858 | Kojima | Nov 2012 | B2 |
20130234263 | Ikehashi | Sep 2013 | A1 |
Number | Date | Country |
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2000-131173 | May 2000 | JP |
Entry |
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Hitchaci, Capacitance-Type Physical Sensor, JP 2000-131173. |
Y. Zhang, et al.: “A High-Sensitive Ultra-Thin MEMS Capacitive Pressure Sensor”, IEEE, Jun. 5-9, 2011, pp. 112-115 (in English). |
Klaus Kasten, et al.: “Capacitive pressure sensor with monolithically integrated CMOS readout circuit for high temperature applications”, Sensors and Actuators A, 97-98 (2002), pp. 83-87 (in English). |
Number | Date | Country | |
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20150068314 A1 | Mar 2015 | US |