CHANNEL TO REDUCE DAMPING ASYMMETRY

Description

Technical Field

[0001] The present invention relates to the field of micro-electromechanical systems (MEMS) and more specifically to a channel structure for decreasing damping asymmetry of comb electrodes used to measure movement of components with MEMS devices.Background

[0002] In inertial MEMS sensors, such as accelerometers, movement of one or more proof masses within the sensors are measured using comb capacitors. Each electrode of the comb capacitors is generally shaped like a comb, having a plurality of comb fingers which extend in the same direction. The fingers of each electrode are interdigitated and separated by a gap, thereby forming a capacitor. One of the comb electrodes is fixed with respect to the MEMS sensor package, while the other, which is fixed with respect to the proof mass, is permitted to move along an axis parallel to the comb fingers relative to the fixed electrode. As the moveable electrode moves closer to or further away from the fixed electrode, the capacitance of the capacitor changes in relation to the movement. Thus, movement of the proof mass, e.g. due to external acceleration, can be measured by measuring the changing capacitance of the comb capacitor. However, other factors than the quantity that is being measured, e.g. acceleration, can affect the movement of the moveable electrode. One such factor is damping of the movement of the moveable electrode due to gas pressure within the gap between the electrode. When the space "above" and the space "below" the comb electrodes are not equal, movement of the moveable comb electrode causes asymmetrical pressure at the top and bottom of the comb electrodes, which leads to movement of the comb electrode out of the intended movement axis, negatively affecting the accuracy of the measurement of proof mass movement.

[0003] Document US 2009 / 007669 A1 discloses an interdigitated comb electrode in a cavity structure wherein a cap includes a concavity so that during anodic bonding, only the peripheral portion of the mass body becomes into contact with the cap. Thereby, the sticking force is decreased and sticking of the mass body to the cap can be prevented

[0004] Document US 2014 / 152325 A1 discloses D2 discloses a displacement amount monitoring electrode structure wherein a change in sensitivity of the change amount of the capacitance with respect to a displacement amount of the movable electrode increases after the displacement of the movable electrode reaches a target displacement amount.Summary

[0005] A MEMS device according to the invention described in claim 1.

[0006] According to a first aspect of the invention, a MEMS device is provided. The MEMS device comprises at least one comb capacitor and the comb capacitor comprises a moveable electrode, the moveable electrode being moveable relative to fixed components of the MEMS device, the moveable electrode comprising a plurality of comb fingers and a fixed electrode, the fixed electrode being fixed relative to other fixed components of the MEMS device, the fixed electrode comprising a plurality of comb fingers, wherein the comb fingers of the moveable electrode are interdigitated with the comb fingers of the fixed electrode. The MEMS device also comprises a handle wafer and a cap wafer, the handle wafer and the cap wafer being located on opposite sides of the moveable and fixed electrodes and extending parallel to the comb fingers of the moveable and fixed electrodes. Sides of the comb fingers of the moveable and fixed electrodes opposite the cap wafer or the handle wafer include recesses and / or the cap wafer or handle wafer includes a recess opposite the comb fingers of the movable and fixed electrodes, wherein the recess or recesses provide a channel through which gas can move into or out of the space between the moveable electrode and fixed electrode and to or from the interior space of the MEMS device surrounding the comb capacitor.

[0007] The comb fingers of the moveable and fixed electrodes may include recesses and the recesses may be positioned on every comb finger of the comb capacitor such the grooves overlap along an axis perpendicular to the longitudinal axes of the comb fingers to provide the channel, and so that the channel runs across the full width of the comb electrode fingers perpendicular to the longitudinal axes of the comb fingers.

[0008] When the gap between the comb capacitor electrodes and the cap wafer is smaller than the gap between the comb capacitor electrodes and the handle wafer, the recesses may be formed on the sides of the comb capacitor fingers opposite the cap wafer. When the gap between the comb capacitor electrodes and the handle wafer is smaller than the gap between the comb capacitor electrodes and the cap wafer, the recesses may be formed on the sides of the comb capacitor fingers opposite the handle wafer.

[0009] When the recess is formed in the cap wafer or the handle wafer, the recess may extend across the cap wafer or handle wafer opposite the comb capacitor fingers across at least the full width of the comb capacitor fingers and perpendicular to the longitudinal axes of the comb capacitor fingers.

[0010] When the gap between the comb capacitor electrodes and the cap wafer is smaller than the gap between the comb capacitor electrodes and the handle wafer, the recess may be formed on the cap wafer.

[0011] When the gap between the comb capacitor electrodes and the handle wafer is smaller than the gap between the comb capacitor electrodes and the cap wafer, the recess may be formed on the handle wafer.

[0012] The comb fingers of the moveable and fixed electrodes may include second recesses and / or the cap wafer or handle wafer may include a second recess, wherein the second recess or second recesses provide a second channel through which gas can move in or out of the space between the moveable electrode and fixed electrode and to or from the interior space of the MEMS device surrounding the comb capacitor, and wherein the second channel extends parallel to and is offset from the first channel.

[0013] The comb capacitor may, for example, be a damping capacitor or a sense capacitor.

[0014] Both the comb fingers of the moveable and fixed electrodes may include recesses and the cap wafer or handle wafer may include a recess, and the recess in the cap wafer or handle wafer may be positioned opposite the recesses in the comb fingers of the moveable and fixed electrodes such that the recesses in the comb fingers and the recess in the cap or handle wafer form the channel.

[0015] The MEMS device may comprises a plurality of comb capacitors and two or more of the comb capacitors may comprise a moveable electrode, the moveable electrode being moveable relative to fixed components of the MEMS device, the moveable electrode comprising a plurality of comb fingers, a fixed electrode, the fixed electrode being fixed relative to other fixed components of the MEMS device, the fixed electrode comprising a plurality of comb fingers, wherein the comb fingers of the moveable electrode are interdigitated with the comb fingers of the fixed electrode, and a handle wafer and a cap wafer, the handle wafer and the cap wafer being located on opposite sides of the moveable and fixed electrodes and extending parallel to the comb fingers of the moveable and fixed electrodes; wherein the comb fingers of the moveable and fixed electrodes include recesses and / or the cap wafer or handle wafer includes a recess, wherein the recess or recesses provide a channel through which gas can move in or out of the space between the moveable electrode and fixed electrode and to or from the interior space of the MEMS device surrounding the comb capacitor.Brief Description of the Drawings

[0016] Figure 1 shows an example of the recesses in the comb electrodes. Figures 2A to 2E show the process of forming the recesses in the comb electrodes. Figure 3 shows an example of a recess in the cap wafer or handle wafer. Detailed Description

[0017] Figure 1 shows an example of a recess in the comb electrodes according to an embodiment of the present invention. Sections of a fixed comb electrode 101 and moveable comb electrode 102 are shown. The fingers of the comb electrode extend along the Y-axis, as shown in Figure 1, and are spaced apart along the X-axis. In the context of the present invention, the vertical axis is considered to be the Z-axis as depicted in Figure 1, i.e. the axis perpendicular to the axis along which the comb fingers extend and along which the comb fingers are distributed. The terms "above", "below", "top" and "bottom" refer to relative positions along the Z-axis, i.e. the vertical direction.

[0018] The moveable comb electrode 102 is configured to move perpendicular to the axis A-A, parallel to the X-axis, relative to the fixed comb electrode 101. A plurality of recesses 103 are located in each comb finger of the fixed comb electrode 101 and moveable comb electrode 102. In the embodiment of Figure 1, the recesses 103 are located top of the comb fingers of the fixed 101 and moveable 102 comb electrodes. In alternative embodiments, the recesses 103 may be located at the bottom of the comb fingers. The recesses 103 form a channel that extends through the comb fingers from one side of the comb electrode to the other. While it is not essential for the channel to run perpendicularly to the comb fingers, when the channel runs perpendicularly to the comb fingers, the symmetrical arrangement of the channel and comb fingers does not introduce asymmetric forces on the moveable comb electrode which might negatively affect measurement and control. It is therefore preferable for the recesses 103 to be arranged such that the channel they form runs perpendicularly to the comb fingers of comb electrodes 101, 102.

[0019] Not shown in Figure 1 are the cap wafer and handle wafer. The cap wafer is located above the comb electrodes 101, 102, and the handle wafer is located below the comb electrodes 101, 102. The distance between the top of the comb electrodes 101, 102 and the bottom of the cap wafer may be smaller or larger than the distance between the bottom of the comb electrodes 101, 102 and the top of the handle wafer. In conventional structures, this causes a pressure difference between the top and bottom of the comb electrodes 101, 102. In operation, such pressure difference tends to move the movable comb electrode 102 along the Z-axis and cause undesired error to the measurement, control or damping function of the comb electrode.

[0020] In the example shown in Figure 1, the bottom of the cap wafer is smaller than the distance between the bottom of the comb electrodes 101, 102 and the top of the handle wafer. However, the channel formed by recesses 103 increases the cross-sectional area of the channel between the cap wafer and the comb electrodes 101, 102 through which gas is drawn in or escapes from the changing volume between the fixed 101 and moveable 102 comb electrodes. This increased channel size decreases the effect of the asymmetry of the gaps between the comb electrodes 101, 102 and the cap wafer and between the comb electrodes 101, 102 and the handle wafer, and thereby reduces the induced movement of the moveable comb electrode 102 along the Z-axis due to the pressure difference between the top and bottom of the comb electrodes 101, 102. This improves the accuracy of the measurement of the X-axis movement of the moveable comb electrode 102 relative to the fixed comb electrode 101, when the comb capacitor is used for measurement, or improves control of the X-axis movement of the moveable comb electrode, where the comb capacitor is used for control or damping.

[0021] The sections 101, 102 and recess 103 shown in Figure 1 are small sections of larger combs that extend along the Y-axis in one or both directions. The recesses 103 extends across all of the comb fingers of the comb electrodes 101, 102 such that the channel formed by the recesses 103 is connected with the space surrounding the comb electrodes 101, 102.

[0022] The channel formed by the recesses 103 may still be used even when there is already a recess present in the cap (or handle wafer) adjacent to the combs, e.g. to allow rotation of a structure out of the plane of the cap (or handle wafer).

[0023] Figures 2A to 2E show the process of forming the recesses in the comb electrodes. At the first step, shown in Figure 2A, the comb electrodes 101, 102 are formed in the normal manner for forming comb electrodes. In Figure 2B, a resist 201 is formed over the comb electrodes. In Figure 2C an opening 202 in the resist 201 is formed. The opening extends across the entire width of the comb electrodes along the Y-axis, i.e. perpendicular to the axis along which the comb fingers extend. In Figure 2D, the comb fingers of the comb electrodes 101, 102 are etched through the opening 202 in the resist 201 to form the recesses 103. The etching process shown in Figure 2D is an isotropic etching process, which forms a roughly semi-circular recess 103 in each comb finger, the recesses extending beneath the resist 201. It is also possible to use a dry etching process, which will form a rectangular recess 103 in each comb finger, or anisotropic etching, which will form a triangular recess 103 in each comb finger.

[0024] Figure 3 shows an alternative embodiment in which the recess 103 of the embodiment shown in Figures 1 and 2 is replaced by a recess 302 which is formed in the cap wafer 301, when the distance from the top of the comb electrodes 101, 102 to the cap wafer 301 is smaller than the distance from the bottom of the comb electrodes 101, 102 to the handle wafer (not shown). Alternatively, when the separation between the comb electrodes 101, 102 and the handle wafer is smaller than the separation between the comb electrodes 101, 102 and the cap wafer, then the recess 302 may be present in the handle wafer, instead of the cap wafer 301. The recess 302 extends along the cap wafer 301 or handle wafer above or below the comb electrodes 101, 102, across the comb fingers of comb electrodes 101, 102 and opposite the comb electrodes 101, 102. While it is not essential for the recess 302 to run perpendicularly to the comb fingers, when the recess runs perpendicularly to the comb fingers, the symmetrical arrangement of the recess and comb fingers does not introduce asymmetric forces on the moveable comb electrode which might negatively affect measurement and control. Preferably, the recess 302 therefore extends across the comb fingers of comb electrodes 101, 102 in a direction perpendicular to the comb fingers.

[0025] The recess 302 extends at least the full width of the comb electrodes 101, 102, i.e. the recess 302 extends over every comb finger of the comb electrodes 101, 102, and may extend further than the full width of the comb electrodes 101, 102 in order to increase the size of the opening between the cap wafer 301 / handle wafer and the comb electrodes 101, 102 into the space surrounding the comb electrodes 101, 102.

[0026] In a further embodiment of the invention, both the comb fingers comprise recesses 103 and the cap / handle wafer comprises a recess 302. The recesses 103 in the comb fingers are located opposite to and parallel to the recess 302 in the cap / handle wafer, thereby providing an enlarged channel through which gas can pass to / from the space surrounding the comb capacitor in to or out of the space between the comb electrodes 101, 102.

[0027] In a still further embodiment of the invention, both the comb fingers comprise recesses 103 and the cap / handle wafer comprises a recess 302. The recesses 103 in the comb fingers are located parallel to the recess 302 in the cap / handle wafer but offset from the recess 302, thereby providing a second channel through which gas can pass to / from the space surrounding the comb capacitor in to or out of the space between the comb electrodes 101, 102.

[0028] Indeed, a single comb capacitor may have multiple recesses 103 in the comb fingers or a recesses 302 in the cap wafer / handle wafer above / below the comb fingers, in which case the recesses 103 / 302 form channels that run generally parallel to each other and are separated along the axis of the comb fingers. While it is not essential for the channels to run parallel to one another, or perpendicularly to the comb fingers, when the channels to run parallel to one another and perpendicularly to the comb fingers, the symmetrical arrangement of the channels and comb fingers does not introduce asymmetric forces on the moveable comb electrode which might negatively affect measurement and control.

[0029] Within a single device, such as a MEMS accelerometer, which comprises multiple comb capacitors, some or all of the comb capacitors may include recesses 103 in the comb fingers or a recess 302 in the cap wafer / handle wafer above / below the comb fingers.

Claims

1. A MEMS device comprising at least one comb capacitor, the comb capacitor comprising: a moveable electrode (102), the moveable electrode being moveable relative to fixed components of the MEMS device, the moveable electrode comprising a plurality of comb fingers; a fixed electrode (101), the fixed electrode being fixed relative to other fixed components of the MEMS device, the fixed electrode comprising a plurality of comb fingers, wherein the comb fingers of the moveable electrode (102) are interdigitated with the comb fingers of the fixed electrode (101); and a handle wafer and a cap wafer, the handle wafer and the cap wafer being located on opposite sides of the moveable and fixed electrodes (101, 102) and extending parallel to the comb fingers of the moveable and fixed electrodes (101, 102); wherein a gap between the moveable and fixed electrodes (101, 102) and the handle wafer is a first gap, and a gap between the moveable and fixed electrodes (101, 102) and the cap wafer is a second gap; wherein one of the first and second gaps is smaller than the other one; characterised in that one or both of: a) sides of the comb fingers of the moveable and fixed electrodes (101, 102) opposite one of the cap wafer or the handle wafer with a smaller gap include similar recesses (103) positioned on every comb finger of the comb capacitor such the recesses overlap a common axis; and b) one of the cap wafer or handle wafer with a smaller gap includes at least one recess (302) opposite the comb fingers of the movable and fixed electrodes (101, 102); wherein the recess or recesses (103) provide a channel through which gas can move into or out of the space between the moveable electrode and the fixed electrode (101, 102) and to or from the interior space of the MEMS device surrounding the comb capacitor.

2. The MEMS device of claim 1, wherein the sides of the comb fingers of the moveable and fixed electrodes (101, 102) include the recesses (103), and wherein the recesses (103) are positioned on every comb finger of the moveable and fixed electrodes (101, 102) such that the recesses (103) overlap along an axis perpendicular to the longitudinal axes of the comb fingers to provide the channel, and so that the channel runs across the full width of the comb fingers of the moveable and fixed electrodes (101, 102) perpendicular to the longitudinal axes of the comb fingers.

3. The MEMS device of claim 2, wherein the second gap is smaller than the first gap, and wherein the recesses (103) are formed on the sides of the comb fingers of the moveable and fixed electrodes (101, 102) opposite the cap wafer.

4. The MEMS device of claim 2, wherein the first gap is smaller than the second gap, and wherein the recesses (103) are formed on the sides of the comb fingers of the moveable and fixed electrodes (101, 102) opposite the handle wafer.

5. The MEMS device of claim 1, wherein a recess (302) is formed in the cap wafer or the handle wafer (301), and wherein the recess (302) extends across the cap wafer or the handle wafer opposite the comb fingers of the moveable and fixed electrodes (101, 102) across at least the full width of the comb fingers of the moveable and fixed electrodes (101, 102) and perpendicular to the longitudinal axes of the comb fingers of the moveable and fixed electrodes (101, 102).

6. The MEMS device of claim 5, wherein the second gap is smaller than the first gap, and wherein the recess (302) is formed on the cap wafer.

7. The MEMS device of claim 5, wherein the first gap is smaller than the second gap, and wherein the recess (302) is formed on the handle wafer.

8. The MEMS device of any preceding claim, wherein one or both of: a) the sides of the comb fingers of the moveable and fixed electrodes (101, 102) opposite the cap wafer or the handle wafer with a smaller gap include first recesses positioned on every comb finger of the comb capacitor such the first recesses overlap a common first axis and second recesses positioned on every comb finger of the comb capacitor such the second recesses overlap a common second axis; and b) the cap wafer or handle wafer with a smaller gap includes a first recess and a second recess; wherein the first recess or first recesses provide a first channel through which gas can move in or out of the space between the moveable electrode and fixed electrode (101, 102) and to or from the interior space of the MEMS device surrounding the comb capacitor, the second recess or second recesses provide a second channel through which gas can move in or out of the space between the moveable electrode and fixed electrode (101, 102) and to or from the interior space of the MEMS device surrounding the comb capacitor, and wherein the second channel extends parallel to and is offset from the first channel.

9. The MEMS device of any preceding claim, wherein the comb capacitor is a damping capacitor.

10. The MEMS device of any one of claims 1 to 9, wherein the comb capacitor is a sense capacitor.

11. The MEMS device of claim 1, wherein the sides of the comb fingers of the moveable and fixed electrodes (101, 102) include the recesses and the cap wafer or handle wafer with a smaller gap includes a recess (302), and wherein the recess (302) in the cap wafer or handle wafer is positioned opposite the recesses (103) in the sides of the comb fingers of the moveable and fixed electrodes such that the recesses in the sides of the comb fingers and the recess in the cap or handle wafer form the channel.

12. The MEMS device of any preceding claim, wherein the MEMS device comprises a plurality of comb capacitors and wherein two or more of the comb capacitors comprise: a moveable electrode (102), the moveable electrode being moveable relative to fixed components of the MEMS device, the moveable electrode comprising a plurality of comb fingers; a fixed electrode (101), the fixed electrode being fixed relative to other fixed components of the MEMS device, the fixed electrode comprising a plurality of comb fingers, wherein the comb fingers of the moveable electrode are interdigitated with the comb fingers of the fixed electrode; and a handle wafer and a cap wafer, the handle wafer and the cap wafer being located on opposite sides of the moveable and fixed electrodes and extending parallel to the comb fingers of the moveable and fixed electrodes (101, 102); wherein a gap between the moveable and fixed electrodes (101, 102) and the handle wafer is a first gap, and a gap between the moveable and fixed electrodes (101, 102) and the cap wafer is a second gap; wherein one of the first and second gaps is smaller than the other one; wherein one or both of: a) sides of the comb fingers of the moveable and fixed electrodes (101, 102) opposite the cap wafer or the handle wafer with smaller gap include recesses positioned on every comb finger of the comb capacitor such the recesses overlap a common axis; and b) one of the cap wafer or handle wafer with smaller gap includes a recess (302) opposite the comb fingers of the movable and fixed electrodes; wherein the recess or recesses provide a channel through which gas can move in or out of the space between the moveable electrode (102) and the fixed electrode (101) and to or from the interior space of the MEMS device surrounding the comb capacitor.