Patent Description:
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.

Document <CIT> 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.

Document <CIT> 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.

A MEMS device according to the invention described in claim <NUM>.

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

<FIG> shows an example of a recess in the comb electrodes according to an embodiment of the present invention. Sections of a fixed comb electrode <NUM> and moveable comb electrode <NUM> are shown. The fingers of the comb electrode extend along the Y-axis, as shown in <FIG>, 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 <FIG>, 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.

The moveable comb electrode <NUM> is configured to move perpendicular to the axis A-A, parallel to the X-axis, relative to the fixed comb electrode <NUM>. A plurality of recesses <NUM> are located in each comb finger of the fixed comb electrode <NUM> and moveable comb electrode <NUM>. In the embodiment of <FIG>, the recesses <NUM> are located top of the comb fingers of the fixed <NUM> and moveable <NUM> comb electrodes. In alternative embodiments, the recesses <NUM> may be located at the bottom of the comb fingers. The recesses <NUM> 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 <NUM> to be arranged such that the channel they form runs perpendicularly to the comb fingers of comb electrodes <NUM>, <NUM>.

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

In the example shown in <FIG>, the bottom of the cap wafer is smaller than the distance between the bottom of the comb electrodes <NUM>, <NUM> and the top of the handle wafer. However, the channel formed by recesses <NUM> increases the cross-sectional area of the channel between the cap wafer and the comb electrodes <NUM>, <NUM> through which gas is drawn in or escapes from the changing volume between the fixed <NUM> and moveable <NUM> comb electrodes. This increased channel size decreases the effect of the asymmetry of the gaps between the comb electrodes <NUM>, <NUM> and the cap wafer and between the comb electrodes <NUM>, <NUM> and the handle wafer, and thereby reduces the induced movement of the moveable comb electrode <NUM> along the Z-axis due to the pressure difference between the top and bottom of the comb electrodes <NUM>, <NUM>. This improves the accuracy of the measurement of the X-axis movement of the moveable comb electrode <NUM> relative to the fixed comb electrode <NUM>, 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.

The sections <NUM>, <NUM> and recess <NUM> shown in <FIG> are small sections of larger combs that extend along the Y-axis in one or both directions. The recesses <NUM> extends across all of the comb fingers of the comb electrodes <NUM>, <NUM> such that the channel formed by the recesses <NUM> is connected with the space surrounding the comb electrodes <NUM>, <NUM>.

The channel formed by the recesses <NUM> 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).

<FIG> show the process of forming the recesses in the comb electrodes. At the first step, shown in <FIG>, the comb electrodes <NUM>, <NUM> are formed in the normal manner for forming comb electrodes. In <FIG>, a resist <NUM> is formed over the comb electrodes. In <FIG> an opening <NUM> in the resist <NUM> 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 <FIG>, the comb fingers of the comb electrodes <NUM>, <NUM> are etched through the opening <NUM> in the resist <NUM> to form the recesses <NUM>. The etching process shown in <FIG> is an isotropic etching process, which forms a roughly semi-circular recess <NUM> in each comb finger, the recesses extending beneath the resist <NUM>. It is also possible to use a dry etching process, which will form a rectangular recess <NUM> in each comb finger, or anisotropic etching, which will form a triangular recess <NUM> in each comb finger.

<FIG> shows an alternative embodiment in which the recess <NUM> of the embodiment shown in <FIG> and <FIG> is replaced by a recess <NUM> which is formed in the cap wafer <NUM>, when the distance from the top of the comb electrodes <NUM>, <NUM> to the cap wafer <NUM> is smaller than the distance from the bottom of the comb electrodes <NUM>, <NUM> to the handle wafer (not shown). Alternatively, when the separation between the comb electrodes <NUM>, <NUM> and the handle wafer is smaller than the separation between the comb electrodes <NUM>, <NUM> and the cap wafer, then the recess <NUM> may be present in the handle wafer, instead of the cap wafer <NUM>. The recess <NUM> extends along the cap wafer <NUM> or handle wafer above or below the comb electrodes <NUM>, <NUM>, across the comb fingers of comb electrodes <NUM>, <NUM> and opposite the comb electrodes <NUM>, <NUM>. While it is not essential for the recess <NUM> 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 <NUM> therefore extends across the comb fingers of comb electrodes <NUM>, <NUM> in a direction perpendicular to the comb fingers.

The recess <NUM> extends at least the full width of the comb electrodes <NUM>, <NUM>, i.e. the recess <NUM> extends over every comb finger of the comb electrodes <NUM>, <NUM>, and may extend further than the full width of the comb electrodes <NUM>, <NUM> in order to increase the size of the opening between the cap wafer <NUM>/handle wafer and the comb electrodes <NUM>, <NUM> into the space surrounding the comb electrodes <NUM>, <NUM>.

In a further embodiment of the invention, both the comb fingers comprise recesses <NUM> and the cap/handle wafer comprises a recess <NUM>. The recesses <NUM> in the comb fingers are located opposite to and parallel to the recess <NUM> 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 <NUM>, <NUM>.

In a still further embodiment of the invention, both the comb fingers comprise recesses <NUM> and the cap/handle wafer comprises a recess <NUM>. The recesses <NUM> in the comb fingers are located parallel to the recess <NUM> in the cap/handle wafer but offset from the recess <NUM>, 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 <NUM>, <NUM>.

Indeed, a single comb capacitor may have multiple recesses <NUM> in the comb fingers or a recesses <NUM> in the cap wafer/handle wafer above/below the comb fingers, in which case the recesses <NUM>/<NUM> 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.

Claim 1:
A MEMS device comprising at least one comb capacitor, the comb capacitor comprising:
a moveable electrode (<NUM>), 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 (<NUM>), 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 (<NUM>) are interdigitated with the comb fingers of the fixed electrode (<NUM>); 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 (<NUM>, <NUM>) and extending parallel to the comb fingers of the moveable and fixed electrodes (<NUM>, <NUM>);
wherein a gap between the moveable and fixed electrodes (<NUM>, <NUM>) and the handle wafer is a first gap, and a gap between the moveable and fixed electrodes (<NUM>, <NUM>) 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 (<NUM>, <NUM>) opposite one of the cap wafer or the handle wafer with a smaller gap include similar recesses (<NUM>) 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 (<NUM>) opposite the comb fingers of the movable and fixed electrodes (<NUM>, <NUM>);
wherein the recess or recesses (<NUM>) provide a channel through which gas can move into or out of the space between the moveable electrode and the fixed electrode (<NUM>, <NUM>) and to or from the interior space of the MEMS device surrounding the comb capacitor.