Micro electromechanical systems (MEMS) devices are generally very small mechanical devices driven by electricity. MEMS devices can also be referred to as micromachines and micro systems technology (MST) devices. In some types of MEMS devices, a proof mass, which is also referred to as a seismic mass, is permitted to movably travel within a frame, for sensing, actuation, and/or other purposes. For instance, in an accelerometer, travel of the proof mass within the frame provides for a way to detect the acceleration that the accelerometer is undergoing.
As noted in the background section, some types of micro electromechanical systems (MEMS) devices include a proof mass and a frame. The proof mass is permitted to movably travel within the frame. Existing such MEMS devices, however, typically permit the proof mass to movably travel within the frame more than fifty micron on-axis, due to limitations in known fabrication techniques to fabricating such MEMS devices.
For example, a flexure between the proof mass and the frame may be destroyed or otherwise impaired during the fabrication of such a MEMS device in accordance with a known fabrication technique that attempts to limit this distance to no more than fifty micron. As such, the MEMS device is nonfunctional and effectively unusable.
However, at the same time, permitting the proof mass to movably travel within the frame more than fifty micron can be disadvantageous. A flexure, which is a type of linear spring, is usually used to attach the proof mass to the frame of a MEMS device. When the proof mass can movably travel within the frame more than fifty micron, undue stress on the flexure can result in the premature failure of the MEMS device.
Furthermore, in general, the greater the distance that the proof mass can movably travel within the frame, the higher the acceleration that an accelerometer is undergoing that can be detected. This permits the accelerometer to be used in more scenarios than if the travel of the proof mass within the frame is limited, which is unintuitively disadvantageous. In particular, such an accelerometer may become subject to export controls and other regulations.
Disclosed herein are techniques for limiting the travel of a proof mass within a frame of a MEMS device. A MEMS device includes at least a proof mass and a frame enclosing the proof mass and within which the proof mass is able to movably travel. A proof mass bumper extends outwards from the proof mass towards the frame, and a frame bumper located at least partially opposite the proof mass bumper extends inwards from the frame towards the proof mass bumper. In one implementation, just the proof mass bumper or just the frame bumper is present. Disclosed herein are techniques to limit the distance between the bumpers and that defines the travel limit of the proof mass within the frame to no more than fifty micron, without the resulting MEMS device being nonfunctional and thus without this MEMS device being unusable.
More specifically, testing of existing fabrication techniques has demonstrated that a MEMS device in accordance with such techniques is manufactured so that the distance between the proof mass and the frame is no greater than about fifty micron, the resulting MEMS device is nonfunctional and hence unusable. In the type of MEMS device in relation to which such testing has been performed, this is particularly because a flexure between the proof mass and the frame becomes destroyed or otherwise impaired when limiting this distance to no greater than about fifty micron. By comparison, the techniques disclosed herein permit a MEMS device to be manufactured so that the distance can be limited to no greater than about fifty micron, without the resulting MEMS device being nonfunctional and thus without the resulting MEMS device being unusable.
The MEMS device 100 includes a proof mass 102 and a frame 104. The frame 104 encloses the proof mass 102 within the x-y plane of
The MEMS device 100 is depicted in
The proof mass 102 and the frame 104 can be fabricated from a proof mass wafer 106, such as a silicon wafer. The proof mass wafer 106 can be indirectly or directly attached to a substrate wafer 108, which also may be a silicon wafer. The substrate wafer 108 defines a cavity 110, so that the proof mass 102 is not in contact with the substrate wafer 108. As such, the proof mass 102 may just be in contact with the flexure 112 in a neutral position in which the MEMS device 100 is at rest and not undergoing any acceleration.
A first example technique by which the distance 124 that defines the movable travel limit is limited to no more than fifty micron is described with reference to
In each of
The difference among
The distance 124 that defines the travel limit of the proof mass 102 within the frame 104 is itself defined between the bumpers 202 and 204. The frame bumper 202 and the proof mass bumper 204 are offset from but overlap one another, as defined by a distance 206, which may be ten, twenty, or thirty microns in varying implementations. Specifically, the frame bumper portions 202A and 202B overlap different parts of the proof mass bumper 204. It has been determined that overlapping bumpers 202 and 204 permit the fabrication of the MEMS device 100 in a way that allows for decreasing the distance 124 so that the distance 124 is no greater than fifty micron. The distance 124 has been decreased to as low as ten, twenty, and thirty microns in different experimental tests.
In this respect, the MEMS device 100 differs from existing MEMS devices, in which there are either no bumpers, or the bumpers are positioned directly opposite to and aligned with one another such that they are not offset in relation to one another. It has been determined that typical fabrication of such an existing MEMS device cannot be achieved in a way that allows for decreasing the distance 124 to no greater than fifty micron. Rather, such an existing MEMS device can just have the distance 124 decreased to greater than fifty micron.
The proof mass wafer 106 is etched to define the proof mass 102, the frame 104, and the bumpers 202 and 204 (304). The definition of the bumpers 202 and 204 can occur at the same time the proof mass 102 and the frame 104 are defined. As such, the bumpers 202 and 204 are formed within the same etching process in which the proof mass 102 and the frame 104 are formed. The etching process can be a reactive ion etch or Bosch process, and/or another type of fabrication process.
A second example technique by which the distance 124 that defines the movable travel limit of the proof mass 102 is limited to no more than fifty micron in relation to the frame 104 is described with reference to
The frame bumper 202 extends inwards from the frame 104 towards the proof mass 102 along the x-axis 118. The proof mass bumper 204 extends outwards from the proof mass 102 towards the frame 104 along the x-axis 118. In the example of
The distance 124 that defines the travel limit of the proof mass 102 within the frame 104 is defined between the bumpers 202 and 204. As noted above, it has been determined that typical fabrication of an existing MEMS device having such a frame bumper and a proof mass bumper cannot be achieved in a way that allows for decreasing the distance 124 to no greater than fifty micron. However, fabrication pursuant to an example method described below permits fabrication of the MEMS device 100 of
The cavity 110 is formed within the substrate wafer 108 (502), and a cavity is also formed within the proof mass wafer 106 (506). The formation of the cavity 110 and the cavity within the proof mass wafer 106 can be achieved via an etching process, such as a reactive ion etch or Bosch and/or another type of fabrication process. The proof mass wafer 106 is directly attached to the substrate wafer 108 (506), such that the cavity within the proof mass wafer 106 faces the cavity 110. A through-hole extending from the bottom of the cavity within the proof mass wafer 106 is formed (508), such as via an etching process. The through-hole has a width that defines the distance 124 between the bumpers 202 and 204.
A through-hole 604 is formed within proof mass wafer 106, which defines the proof mass 102, the frame 104, and the bumpers 202 and 204. The width of the through-hole 604 corresponds to and thus defines the distance 124 between the bumpers 202 and 204. The bumpers 202 and 204 have a height 606 along the z-axis 122 that can be set according to the specifications of the particular MEMS device 100 being fabricated. Likewise, the proof mass wafer 106 can itself be ground to have a height 608 along the z-axis 122 that can be sett according to the particular specifications of the MEMS device 100 being fabricated.
It is noted that in
A third example technique by which the distance 124 that defines the movable travel limit of the proof mass 102 is limited to no more than fifty micron in relation to the frame 104 is described in relation to
The proof mass wafer 106 is provided with a buried insulating layer (702). For instance, the proof mass wafer 106 may be provided as a silicon-on-insulator (SOI) wafer. As such, the insulating layer may be a buried oxide (BOX) layer. The cavity 602 is formed within the proof mass wafer 106 (704), such as by selective etching of the wafer 106, where the cavity 602 stops at the buried insulating layer. The buried insulating layer, where exposed through the cavity 602, is removed (706), such as via etching of the exposed buried insulating layer. The cavity 110 is formed within the substrate wafer 108 (708), such as also by selective etching of the wafer 108. The proof mass wafer 106 is attached to the substrate wafer 108 (710), and the through-hole 604 is then formed within the proof mass wafer 106 (712).
The through-hole 604 is formed within the proof mass wafer 106, which defines the proof mass 102, the frame 104, and the bumpers 202 and 204. Note that the through-hole 604 is not defined within the insulating layer 802, which was previously removed. The width of the through-hole 604 corresponds to and thus defines the distance 124 between the bumpers 202 and 204. The proof mass wafer 106, including the insulating layer 802, has a height 804 along the z-axis 122 that can be set according to the particular specifications of the MEMS device 100 being fabricated.
Another, fourth example technique by which the distance 124 that defines the movable travel limit of the proof mass 102 is limited to no more than fifty micron in relation to the frame 104 is described in relation to
A difference between the third technique and the fourth technique is that in the former the cavity 602 of the proof mass wafer 106 is adjacent to the cavity 110 of the substrate wafer 108, whereas in the latter the cavity 602 is not adjacent to the cavity 110. Another difference between the third and fourth techniques is that in the former the through-hole 604 is formed after the wafers 106 and 108 being joined together. By comparison, in the latter the through-hole can be formed before the wafers 106 and 108 are joined together.
The proof mass wafer 106 is provided with a buried insulating layer 802 (902). For instance, the proof mass wafer 106 may be provided as an SOI wafer. As such, the insulating layer may be a BOX layer. The through-hole 604 is formed within the proof mass wafer 106, including through the buried insulating layer 802 (904). The cavity 110 is formed within the substrate wafer 108 (906), such as by selective etching of the wafer 108. The proof mass wafer 106 is attached to the substrate wafer 108 (908), and the cavity 602 is formed within the proof mass wafer 106 (910), such as also by selective etching of the wafer 106, where the cavity 602 stops at the buried insulating layer 802.
The cavity 602 of the proof mass wafer 106 is not adjacent to the cavity 110 of the substrate wafer 108. The through-hole 604 defines the proof mass 102, the frame 104, and the bumpers 202 and 204. Note that the through-hole 604 is defined within the insulating layer 802 as well, which was not previously removed. The width of the through-hole 604 corresponds to and thus defines the distance 124 between the bumpers 202 and 204. The proof mass wafer 106, including the insulating layer 802, has the height 804 along the z-axis 122 that can be set according to the particular specifications of the MEMS device 100 being fabricated.
Note, therefore, the differences between the MEMS device 100 of
Another difference between these two techniques is that the insulating layer 802 is removed from the bottom of the cavity 602 in the third technique of
The proof mass wafer 106 is attached to the substrate wafer 108 (1102). The proof mass 102, the frame 104, and the bumpers 202 and 204 are formed within the proof mass wafer 106 (1104). The manner by which the proof mass 102, the frame 104, and the bumpers 202 and 204 are formed can be as has been described above in relation to the method 300, 500, 700, and/or 900.
In conclusion