The present disclosure relates to semiconductor devices and, more particularly, to semiconductor devices incorporating sensors.
Micro-electromechanical systems referred to herein as “MEMS” or “MEMS devices”, integrate electrical and mechanical components on the same silicon substrate using microfabrication technologies. The electrical components are fabricated using integrated circuit processes, while the mechanical components are fabricated using micromachining processes compatible with the integrated circuit processes. This combination makes it possible to fabricate an entire electromechanical system on a silicon substrate using standard manufacturing processes.
A common application of MEMS devices is the design and manufacture of sensors. These sensors have proven to be effective solutions in various applications due to the sensitivity, spatial and temporal resolutions, and lower power requirements exhibited by MEMS devices. Consequently, MEMS sensors, such as inertial sensors, gyroscopes and pressure sensors, have been developed for use in a wide variety of applications.
MEMS inertial sensors are useful for measuring both static and dynamic acceleration. In particular, MEMS inertial sensors may be used to sense the orientation and position of a device. One type of MEMS inertial sensor includes a proof mass supported above a substrate anchor by a spring. The substrate anchor defines a cavity in which the proof mass is movably positioned. The spring positions the proof mass in a neutral position within the cavity. When the substrate anchor accelerates, the proof mass moves from the neutral position relative the substrate anchor, but remains within the cavity. The spring constant of the spring, among other factors, determines the amount of movement exhibited by the proof mass in response to acceleration of the substrate anchor. The travel span of the proof mass within the cavity is referred to as a displacement range. Electrical leads may be connected to the substrate anchor and the proof mass. Acceleration of the substrate anchor can be sensed by measuring the capacitance between the substrate anchor and the proof mass.
Some known MEMS inertial sensors incorporate one or more positioning features to limit and to define the displacement range of the proof mass within the cavity. If the sensor is subjected to a force that exceeds a threshold force, the proof mass may move to a position of maximum displacement. The positioning features ensure the proof mass remains properly positioned within the cavity even after the proof mass has been moved to a position of maximum displacement. Furthermore, positioning features prevent the proof mass from moving to a position that breaks, fractures, or otherwise damages the spring.
In the past, positioning features were implemented with a flat surface on the substrate anchor and a corresponding flat surface on the proof mass. These flat surfaces contact each other when the proof mass is moved to a position of maximum displacement. Although positioning features having flat surfaces effectively limit and define the maximum displacement of the proof mass, stiction may occur as a result of contact between the flat surfaces.
The term “stiction”, as used herein, refers to the force that must be overcome to move a first object that is in physical contact with a second object. In general, stiction increases in relation to the surface area of the contact region between the first and second objects. As applied to MEMS having positioning features with flat contact surfaces, stiction may occur as a result of large external physical shocks, such as drop tests, and spring-mass resonant frequency excitation, etc. As reported by G. J. O'Brian, D. J. Monk, and L. Lin, “A Stiction Study Via Capacitance-Voltage (C-V) Plot Electrostatic Actuation/Latching”, American Society of Mechanical Engineering MEMS, Vol. 1, pp. 275-280 (1999), electrostatic self-test actuation of MEMS inertial sensors is also an event that may result in stiction.
Stiction may cause a MEMS inertial sensor to generate false measurements or become inoperable. In particular, some stiction causing events may cause the proof mass to adhere permanently to the substrate anchor, thereby resulting in the MEMS ceasing to sense acceleration. Other stiction causing events, however, may cause the proof mass to adhere temporarily to the substrate anchor, which may result in the MEMS inaccurately sensing acceleration. Therefore, both temporary and permanent adhesion of the proof mass to the substrate anchor as a result of stiction may cause a MEMS inertial sensor to generate false measurements.
Accordingly, a MEMS inertial sensor subject to less stiction between the substrate anchor and the proof mass would be beneficial.
A micro electromechanical system (MEMS) in one embodiment includes a substrate, a first curved surface located at a position above a surface of the substrate, and a second curved surface generally opposite to the first curved surface along a first axis parallel to the surface of the substrate, wherein the first curved surface is movable along the first axis in a direction toward the second curved surface.
In accordance with another embodiment, an accelerometer for a micro electromechanical system (MEMS) includes a substrate, a proof mass at a position above a surface of the substrate and with a first curved surface located on a first side of the proof mass, and a travel stop with a second curved surface generally opposite to the first curved surface, wherein the first curved surface is movable in a direction toward the second curved surface.
In accordance with another embodiment, a method of forming an accelerometer for a micro electromechanical system (MEMS) includes, forming a substrate, forming a proof mass with a first curved surface on a first side of the proof mass at a position above a surface of the substrate, and forming a first travel stop fixed in position with respect to the substrate and including a second curved surface generally opposite to the first curved surface, wherein the proof mass is movable in a direction toward the first travel stop.
Features of the device and method disclosed herein will become apparent to those skilled in the art from the following description with reference to the figures, in which:
For the purpose of promoting an understanding of the principles of the device and method described herein, reference will now be made to the embodiments illustrated in the figures and described in the following written specification. It is understood that no limitation to the scope of the device and method is thereby intended. It is further understood that the device and method includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the device and method as would normally occur to one skilled in the art to which this device and method pertains.
An exemplary embodiment of a micro electromechanical system (“MEMS”) 100 is depicted in
The MEMS 100 includes a substrate 108, a wall 112, and a proof mass 116. The substrate 108 defines a surface 120 in an x-y plane that is generally normal, i.e. perpendicular, to the z-axis. The wall 112 is coupled to the substrate 108 and extends above the surface 120 along the z-axis. The wall 112 may be formed from the same material as the substrate 108, and, in some embodiments, may be integral with the substrate 108. Alternatively, the wall 112 may be a material different from the substrate 108 that is deposited on the surface 120 and bonded to the substrate 108. As illustrated in
The proof mass 116 is positioned in the cavity 124 to move relative the substrate 108 in response to the MEMS 100 accelerating in a direction having a component along the z-axis.
The proof mass 116 may be formed from a material different from the substrate 108. A spring 126 (
The MEMS 100 is described in further detail with reference to
The MEMS 100 includes positioning features 132 to limit the movement of the proof mass 116 relative the substrate 108 in directions along the x-axis and the y-axis. The positioning features 132 include a travel stop 134 and a follower 136. As shown in
The positioning features 132 are described in further detail with reference to
As shown in
The positioning features 132 limit the motion of the proof mass 116 along the x-axis and the y-axis to fixed distances. Specifically, the positioning features 132 on the left and right sides of the MEMS 100 limit the motion of the proof mass 116 along the y-axis to a distance 154 (
The MEMS 100 may be formed with a reduced number of positioning features 132. Referring again to
The positioning features 132 are configured to reduce stiction between the travel stop 134 and the follower 136 because a first curved surface having a radius smaller than a second curved surface necessarily contacts the second curved surface at a single point that is tangential to both curved surfaces. In particular, as shown in
Referring now to
The MEMS 200 includes positioning features 224 to limit the movement of the proof mass 212 relative the substrate 204 in directions along the x-axis and the y-axis. In particular, the MEMS 200 includes four additional positioning features 224 as compared to the MEMS 100. The additional positioning features 224 reduce the force exerted upon any one positioning feature 224 during periods of external physical shock, such as, but not limited to, drop tests, electrostatic self-test actuation, and spring-mass resonant frequency excitation.
Another embodiment of a MEMS 300 is shown in
The MEMS 300 includes positioning features 324 to limit the movement of the proof mass 312 relative the substrate 304 in directions along the x-axis and the y-axis. The positioning features 324 include a follower 328 and a post 332. Some of the followers 328 are formed on the wall 308 and some of the followers 328 are formed on the proof mass 312. Similarly some of the posts 332 are formed on the wall 308 and some of the posts 332 are formed on the proof mass 312. The MEMS 300 has the same stiction threshold reducing benefits as described with reference to the MEMS 100.
With reference to
The MEMS 400 includes positioning features 424 to limit the movement of the proof mass 412 relative the substrate 404 in directions along the x-axis and the y-axis. The positioning features 424 include a follower 428 and a travel stop 432. The travel stop 432 includes a head 436 having a curved surface. The followers 428 include a rectangular region 440 instead of a dimple 138. The positioning features 424 limit the over travel of the proof mass 412 in directions along both the x-axis and the y-axis. Additionally, the positioning features 424 reduce the contact area between the wall 408 and the proof mass 412 when the proof mass 412 has been moved to a position of maximum displacement. For instance, when the proof mass 412 has been moved to a position of maximum displacement along the x-axis, an apex 444 of the head 436 contacts a single point of the rectangular region 440. When the proof mass 412 has been moved to a position of maximum displacement along the x-axis and y-axis, however, the head 436 contacts the rectangular region 440 at discrete contact points. Thus the stiction threshold is reduced as compared to known flat positioning features. In another embodiment (not illustrated) the travel stop 432 is on the proof mass 412 and the follower 428 having a rectangular region 440 is on the wall 408.
The MEMS 100 may be formed according to the following process. First the substrate 108 is formed. In one embodiment, the substrate 108 is formed on a silicon wafer. Next, the surface 120 is etched, or otherwise micromachined, on the substrate 108. The term “etching” as used herein, includes wet chemical etching, vapor etching, and dry etching, among other forms of etching and micromachining. While the surface 120 is shown to be in the same x-y plane as the remainder of the substrate 108, the surface 120 may also be formed below the remainder of the substrate 108 in a well.
Next, material to form the spring 126 is deposited on the surface 120. A sacrificial layer of oxide may be formed on the surface 120 around the spring 126. Thereafter, material to form the wall 112 and the proof mass 116 is deposited on the surface 120, such that the material to form the proof mass 116 is deposited on the sacrificial layer and suspended above the surface 120. The material forming the proof mass 116 bonds to the spring 126. The material forming the wall 112 bonds to the surface 120. Next, a mask is deposited over the material. The mask the outlines and defines at least the wall 112, the proof mass 116, and the positioning features 132.
Subsequently, the material unencumbered by the mask is etched away to form the wall 112, the proof mass 116, and the positioning features 132. Additionally, the sacrificial layer, if one is present, is also etched away to separate the proof mass 116 from the surface 120 and to permit the spring 126 to support the proof mass 116 in the neutral position. The process may also include forming the retaining element 128 to seal the proof mass 116 within the cavity 124. Furthermore, a first electrical lead may be coupled to the surface 120, and a second electrical lead may be coupled to the proof mass 116. In some embodiments, the spring 126 may function as the second electrical lead.
In operation, the MEMS 100 senses acceleration in a direction having a component along the z-axis. In one embodiment, acceleration is sensed by measuring the capacitance between the surface 120 and the proof mass 116 with a capacitance measuring device coupled to first and second electrical leads. In particular, the proof mass 116 is shown in the neutral position slightly separated from the surface 120, resulting in a capacitance between the proof mass 116 and the surface 120. If the MEMS 100 were to accelerate downward along the z-axis, the proof mass 116 would move upward relative the surface 120. The capacitance measured between the proof mass 116 and the surface 120 decreases as the distance between the proof mass 116 and the surface 120 increases. The change in capacitance is related to the acceleration of the MEMS 100.
An exemplary mask 500 for forming a MEMS is illustrated in
The mask 500 (
Lateral silicon etching may be used to form the MEMS tunneling tip accelerometer 700, shown in
The tips 712, 716 of the accelerometer 700 may be formed by the following process. First, mask portion 750 of
The device and method described herein have been illustrated and described in detail in the figures and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications, and further applications that come within the spirit of the device and method described herein are desired to be protected.
This application claims the benefit of U.S. Provisional Application No. 61/557,767, filed Nov. 9, 2011, the entire contents of which is herein incorporated by reference.
Number | Date | Country | |
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61557767 | Nov 2011 | US |