Patent Description:
Sensors, for example pressure sensors or inertial sensors (such as accelerometers and gyroscopes) are used in many applications, including inertial navigation, robotics, avionics, and automobiles. In inertial navigation applications, such sensors may be found in self-contained systems known as "inertial measurement units" (IMUs). IMUs typically contain a plurality of accelerometers and/or gyroscopes, and provide an estimate of an object's travel parameters such as angular rate, acceleration, altitude, position, and velocity, based on the outputs of gyroscope(s) and/or accelerometer(s). Each inertial sensor in an IMU is a self-contained package. An IMU typically consists of accelerometers and gyroscopes sensing in all three axes. This is normally part of an Inertial Navigation System (INS), which adds computation of velocity and position using navigation algorithms. At IMU level, the outputs are usually limited to angular rotation and velocity increments with each sample.

Microelectromechanical systems (MEMS)-based sensors, typically fabricated from a single silicon wafer, can be used e.g. to measure pressure or temperature, or linear or angular motion without a fixed point of reference. MEMS pressure sensors often work on the principle of mechanical deformation of a MEMS structure due to fluid pressure. MEMS gyroscopes, or strictly speaking MEMS angular rate sensors, can measure angular rate by observing the response of a vibrating MEMS structure to Coriolis force. MEMS accelerometers can measure linear acceleration by observing the response of a proof mass suspended on a spring in a MEMS structure. High performance MEMS inertial sensors are defined by their bias and scale factor stability.

A MEMS sensor is usually supported on an isolation layer within its package. For example, an isolation layer of silicone elastomer may be provided between the package and the lowermost glass layer of the MEMS sensor. In some examples, the MEMS sensor may be mounted on an isolation layer including a raft that is connected to the surrounding package via springs or other damping structures. The isolation layer has two main functions: to provide isolation from unwanted external vibrations; and to absorb mechanical stress due to thermal expansion differences between the MEMS sensor and the surrounding package (typically alumina or ceramic).

The stability of the isolation layer that attaches a MEMS inertial sensor to its package is important for high performance, especially when trying to achieve better than <NUM> bias stability. An elastomeric isolation layer is usually chosen to have a very low elastic modulus (e.g. silicone) to decouple the MEMS sensor from package stresses. However, such materials suffer from long term creep and ageing effects which can therefore alter sensor performance (e.g. bias and scale factor) by virtue of stress relief over a period of time in service. It is therefore difficult to achieve good isolation of an inertial sensor from package stresses and good long term stability in performance. Similar considerations apply when mounting any MEMS sensor in a package.

<CIT> describes a MEMS assembly sandwiched between and bonded to a cap and base, in which the MEMS subassembly includes at least one MEMS device element flexibly connected to the MEMS assembly.

<CIT> describes a micromechanical device having a substrate wafer, a functional layer having a mobile micromechanical structure situated above the substrate layer, and a cap. The micromechanical device has a fixed part and a mobile part, moveably connected with a spring element to the fixed part within a cavity.

<CIT> describes an interposer chip including a base portion, a chip mounting portion and one or more flexures connecting the base portion to the chip mounting portion. A first plurality of projections extends from the base portion towards the chip mounting portion, and a second plurality of projections extends from the chip mounting portion towards the base portion and extending into interstices formed by the first plurality of projections.

There remains a need for improved isolation mounting in sensor packages.

According to a first aspect of the invention, there is provided a sensor package according to claim <NUM>.

According to a second aspect of the invention, there is provided a method of manufacturing a sensor package according to claim <NUM>.

The present disclosure relates to a sensor package and a method of manufacturing a sensor package. It will be appreciated that forming one or more springs in the same material layer as the sensing structure of the sensor unit is a completely different approach to sensor isolation than the conventional way of mounting the hermetically sealed sensor unit to an external package using a different material, e.g. an elastomeric material, as an isolation layer. The springs can decouple mechanical stresses between the sensor unit and the external package wall(s) so as to reduce the long term drift of scale factor and bias. Furthermore, the effects of temperature sensitivity on bias and scale factor can also be reduced e.g. for an inertial sensor.

Some non-limiting examples of a sensor package and a method of manufacturing a sensor package are described in further detail below.

<FIG> shows a prior art inertial sensor package <NUM> comprising an upper glass layer <NUM>, a silicon sensing layer <NUM>, a lower glass layer <NUM>, an elastomeric die bond layer <NUM>, an alumina substrate <NUM>, and a package lid <NUM>. The three layers <NUM>, <NUM> and <NUM> make up a hermetically sealed inertial sensor unit <NUM>.

The silicon sensing structure in the layer <NUM> is sensitive to some form of applied force, in this case, acceleration. This silicon sensing layer <NUM> is sandwiched between the upper glass layer <NUM> and the lower glass layer <NUM> to form a hermetically sealed inertial sensor unit <NUM>. This sealing allows for the silicon sensing structure in the layer <NUM> to occupy a space within a controlled environment. The inside of the hermetically sealed inertial sensor unit <NUM> comprises an atmosphere that is controlled to optimise sensor performance.

The hermetically sealed inertial sensor unit <NUM> is attached to the alumina substrate <NUM> by elastomeric die bond layer <NUM>. As shown, the elastomeric die bond layer <NUM> covers all of the lower glass layer <NUM> of the hermetically sealed inertial sensor unit <NUM>. The elastomeric die bond layer <NUM> is flexible. This provides good adhesion for the hermetically sealed inertial sensor unit <NUM> to the alumina substrate <NUM>, and further provides good mechanical stability under shock and vibration of the inertial sensor package <NUM>.

However, as described in the background section above, thermal and mechanical stresses across the inertial sensor package <NUM> can adversely affect the performance of inertial sensor unit <NUM>, especially later in the lifetime of the device.

Package lid <NUM> allows the hermetically sealed inertial sensor unit <NUM> to be further protected from external environmental influences, such as dirt or direct force application. Typically, this lid is a metal alloy or alumina and is soldered to the substrate <NUM>. The package formed by lid <NUM> also forms a hermetic seal, and allows for controlling the gaseous environment inside the package. This is done to ensure an optimal dry gas environment inside the package, and prevents any moisture ingress, which may negatively affect sensor performance, particularly in a capacitive-type inertial sensor.

<FIG> shows a sensor package, e.g. an inertial sensor package <NUM> in accordance with an example of the present disclosure. The boundaries of the inertial sensor package <NUM> are defined by three external walls <NUM>, and a substrate <NUM>. The inertial sensor package <NUM> comprises a silicon sensing structure <NUM> formed in a silicon material layer <NUM> for detecting an applied acceleration. The silicon sensing structure <NUM> is hermetically sealed into an inertial sensor unit <NUM>, by an upper glass layer <NUM> and a lower glass layer <NUM>. The inertial sensor package <NUM> also comprises a support structure <NUM> formed in the silicon layer <NUM>. The support structure <NUM> takes the form of a frame surrounding the hermetically sealed inertial sensor unit <NUM>. The support structure <NUM> is attached to the substrate <NUM>, through the intervening glass layer <NUM>, by a compliant or fixed mount <NUM>. The inertial sensor unit <NUM> is decoupled from the substrate <NUM>, as it is instead suspended from the support structure <NUM> by a plurality of springs <NUM>, formed in the silicon material layer <NUM>. In this example, the silicon sensing structure <NUM> is electrically connected to an external connection <NUM> via flexible wire bonds <NUM>, and through-hole vias <NUM> in the upper glass layer <NUM>.

As shown, the sensor unit <NUM> is decoupled from stresses and sudden forces applied to the package <NUM> in two ways. Primarily, the springs <NUM> that join the 'floating' sensor unit <NUM> to the support structure <NUM> compensate for any such stresses, leaving the sensor unit <NUM> free from such imbalances, decreasing long term drift of scale factor and bias. Furthermore, the effects of temperature sensitivity on bias and scale factor can also be reduced. Secondary to the effects of the springs <NUM>, the elastomeric mount <NUM> can also absorb some stresses and shocks to the package <NUM>. In these ways, sensor performance is improved.

As shown, parts of the support structures <NUM>, the springs <NUM>, and the sensing structure <NUM> are all made in the same silicon material layer <NUM>. This may have significant benefits to the manufacturing process, as streamlined development can take place, enabling the device to be mostly manufactured before needing to singulate the parts from a wafer. This batch processing of the devices may both increase throughput and decrease cost of manufacture. By manufacturing the devices this way, the sensor unit <NUM> can be conveniently decoupled from the support structures <NUM> by etching out the springs <NUM> during the same process that is used to etch out the sensing structure <NUM>.

An electrical connection is made to the sensing structure <NUM> with a conductive e.g. metal path passing down the through-hole vias <NUM>. This connection is then carried down to the substrate <NUM>, via the flexible wire bonds <NUM>, to meet the external connections <NUM> which allow an electrical connection to be made from the outside of the sensor package <NUM> directly to the sensing structure <NUM>. The flexible wire bonds <NUM> are flexible enough to withstand any stresses or gradients within the package, for example flexing of the springs <NUM>.

The upper <NUM> and lower <NUM> glass layers form a hermetic seal around the sensor unit <NUM>. The environment within the sensor unit <NUM> can be controlled at the time of sealing, and in this example, the sensor is filled with dry Nitrogen at atmospheric pressure. This controlled environment enables tuning of the damping factor within the hermetically sealed sensor unit <NUM>.

The external walls <NUM> form a hermetic seal around the support structures <NUM> and the sensor unit <NUM>. The environment within this area can be controlled upon sealing as well. The control of this environment enables tuning of the damping factor of the squeeze film damping fingers (not shown in <FIG>), which is explained in more detail below with reference to <FIG>.

<FIG> shows a plan view of the first material layer in accordance with an example of the present disclosure. The first material layer is a silicon layer <NUM>. Silicon layer <NUM> is made from a single sheet of crystalline silicon. The silicon layer <NUM> comprises a support structure <NUM>, taking the shape of an outer frame, a plurality of springs <NUM>, and a sensing structure <NUM>. There is no residual glass in the spring area. The support structure <NUM> is a frame surrounding the sensing structure <NUM>.

As shown, the sensing structure <NUM> is suspended from the support frame <NUM> by the springs <NUM>. The springs <NUM> decouple the sensing structure <NUM> from the mechanical and other stresses experienced by the inertial sensor package <NUM>, whilst having a spring constant such that inertial movement is still transferred to the sensing structure <NUM>. The resonant frequency of the springs <NUM> is between <NUM>-<NUM>, for example <NUM>. The length and width of the springs <NUM> is designed in order to select an optimal resonant frequency. The springs <NUM> are serpentine in shape, and have a number of serpentine turns, for example between <NUM>-<NUM> turns.

The manufacture of the support structure <NUM>, springs <NUM> and sensing structure <NUM> all in a single material layer allows for a streamlined manufacturing process, saving both cost and time.

<FIG> shows a further plan view of the first material layer <NUM>, including squeeze film damping fingers <NUM>, in accordance with an example of the present disclosure. Shown in <FIG> is a support structure <NUM>, a sensing structure <NUM>, springs <NUM>, and squeeze film damping fingers <NUM>.

As in <FIG>, the springs <NUM> suspend the sensing structure <NUM> from the support structure <NUM>. The addition of the squeeze film damping fingers <NUM> assists in the decoupling of mechanical and other stresses between the inertial sensor package and support structure, and the sensing structure <NUM>. The squeeze film damping fingers <NUM> help to provide near critical damping to the inertial sensor package, preventing the sensing structure <NUM> from being damaged. The squeeze film damping fingers <NUM> do this by limiting the range of movement of the sensing structure <NUM> with respect to the support structure <NUM>.

The damping effect of the squeeze film damping fingers <NUM> can be tuned by altering the composition of the inertial sensor package environment, for example by filling it with dry Nitrogen, Neon or Argon at atmospheric pressure. The damping effect can also be tuned by adjusting the number of fingers, the lengths of the fingers, and the size of the gaps between the fingers.

<FIG> shows a process for manufacturing a sensor package in accordance with an example of the present disclosure.

<FIG> shows the first step of pre-cavitating a layer of glass <NUM> with a wet etch. The etch is defined by a mask, and the glass layer <NUM> is only etched in the region which a moving sensing structure will later occupy. The depth of the etch is typically around <NUM>.

<FIG> shows the next step of anodically bonding a silicon wafer <NUM> to the glass layer <NUM>.

<FIG> shows the next step of creating through-hole vias <NUM>. This is typically done by first applying a photomask (not shown), and powder blasting the glass layer <NUM> in order to create the through-hole vias <NUM>. The through-hole vias <NUM> go through to the silicon layer <NUM>, inside the pre-cavitated area of the glass layer <NUM>.

<FIG> shows the next step of depositing a metal tracking layer <NUM> onto the glass layer <NUM>, forming a uniform thin layer <NUM> coating the glass layer <NUM>, and the inside surfaces of the through-hole vias <NUM> in the process. Alternatively, the metal tracking layer <NUM> may fill the through-hole vias <NUM>. This allows electrical connections to be made to the silicon layer <NUM>, in order to connect a sensing structure made in the silicon layer <NUM> with an external package. The metal tracking layer <NUM> is typically deposited and then patterned by photo-lithography.

<FIG> shows the next step of performing an isotropic wet etch on the glass layer <NUM>, in the regions <NUM> where the springs will be formed, in order to suspend the sensing structure later formed in the silicon layer <NUM>. A photomask (not shown) is used to protect the other areas from the wet etch. This exposes the underlying silicon layer <NUM>.

<FIG> shows the next step of performing a Deep Reactive Ion Etch (DRIE) on the underlying silicon layer <NUM> from the bottom. A standard photo mask (not shown) is used to define the etched regions of the silicon layer <NUM>. In this step, a sensing structure <NUM> is etched from the silicon layer <NUM>. This etch also defines a plurality of serpentine springs <NUM> suspending the sensing structure <NUM> from the newly defined support structure <NUM>.

Next, a lower glass layer <NUM> is pre-cavitated in moving regions of the sensing structure <NUM> in the silicon layer <NUM>. As shown in <FIG>, the lower glass layer <NUM> is then anodically bonded to the silicon layer <NUM>, forming a hermetically sealed sensor unit <NUM> containing the sensing structure <NUM>. The hermetically sealed sensor unit <NUM> is back-filled with a gas, typically dry Nitrogen, Argon or Neon at atmospheric pressure. This ensures near critical damping of the sensing structure <NUM>.

<FIG> shows the next step of performing an isotropic wet etch on the lower glass layer <NUM> to the depth of the silicon layer <NUM>. This etch is defined by a photomask (not shown), and leaves only the springs <NUM> in the regions <NUM> of <FIG>. This also releases the hermetically sealed sensor unit <NUM> from the support structure <NUM>, leaving it suspended by the springs <NUM>. This decouples the hermetically sealed sensor unit <NUM> from any large shocks or stresses experienced by the support structure <NUM>, as they will be absorbed by the springs <NUM> instead. Furthermore, it will be seen that the sensing structure <NUM> is hermetically isolated from the springs <NUM> by the anodically bonded glass layers <NUM>, <NUM>, forming the hermetically sealed sensor unit <NUM>. The device may be singulated from the wafer after this stage too. This allows for streamlining of production of the devices, as the devices are almost fully formed before they are singulated from the wafer.

<FIG> shows the next step of bonding the support structures <NUM> to a substrate <NUM> (typically made from alumina or ceramic), via one or more elastomeric mounts <NUM> (for instance). In this way, the hermetically sealed sensor unit <NUM> is decoupled from any stresses or shocks experienced by the substrate <NUM> via the springs <NUM> as well as the elastomeric mounts <NUM>.

<FIG> shows the next step of adding flexible wire bonds <NUM> to the device, thereby attaching the metal tracking layer <NUM> on the hermetically sealed sensor unit <NUM> to the (relatively) fixed support structures <NUM>. Furthermore, flexible wire bonds <NUM> are also added from the support structure <NUM> to the substrate <NUM>. The wire bonds typically have a diameter of <NUM>. An external electrical connection <NUM> is also added through the substrate layer <NUM>.

<FIG> shows the final step of adding a metal lid <NUM>, hermetically sealing the internal gas volume of the package. This is typically done using solder sealing (at -<NUM>), securing the lid <NUM> to the substrate <NUM>. The internal gas volume of the package is controlled to optimise the decoupling between the hermetically sealed sensor unit <NUM> and the support structures <NUM>, and typically comprises Argon, Neon or dry Nitrogen at atmospheric pressure - e.g. to optimise squeeze film damping.

<FIG> show a process for manufacturing a sensor package in accordance with another example of the present disclosure. The manufacturing process shown in <FIG> is similar to that shown in <FIG>, and will be described below with reference to <FIG> where appropriate.

The manufacturing process shown in <FIG> is the same as that shown in <FIG>.

<FIG> shows the next step of performing an isotropic wet etch on the glass layer <NUM>, in the regions <NUM> where the springs will be formed, in order to suspend the sensing structure. A photomask (not shown) is used to protect the other areas from the wet etch. This exposes the underlying silicon layer <NUM>.

<FIG> shows the next step of performing a DRIE on the underlying silicon layer <NUM> from the bottom. A standard photo mask (not shown) is used to define the etched regions of the silicon layer <NUM>. In this step, a sensing structure <NUM> is etched from the silicon layer <NUM>. This etch also defines the serpentine springs <NUM> suspending the sensing structure <NUM> from newly defined support structure <NUM>.

<FIG> shows the next step of depositing a metal tracking layer <NUM> onto the silicon layer <NUM> and the glass layer <NUM>, forming a uniform thin layer <NUM> coating the glass layer <NUM>, and the inside surfaces of the through-hole vias <NUM> in the process. Alternatively, the metal tracking layer <NUM> may fill the through-hole vias <NUM>. This allows electrical connections to be made to the silicon layer <NUM>, in order to connect a sensing structure made in the silicon layer <NUM> with an external package. This step also defines metal tracking down the faces of the isotropically etched glass layer <NUM>, and across the surface of the springs <NUM>. This provides an electrically conductive path from the sensing structure <NUM> and the through-hole vias <NUM>, to the edge of the glass layer <NUM> where the support structure <NUM> surrounds the sensing structure <NUM>. The metal tracking layer <NUM> is typically deposited, and then patterned by photo-lithography.

<FIG> shows the next step of performing an isotropic wet etch on the lower glass layer <NUM> to the depth of the silicon layer <NUM>. This etch is defined by a photomask (not shown), and leaves only the springs <NUM> and the corresponding metal tracking in the regions <NUM> of <FIG>. This also releases the hermetically sealed sensor unit <NUM> from the support structure <NUM>, leaving it suspended by the springs <NUM>. This decouples the hermetically sealed sensor unit <NUM> from any large shocks or stresses experienced by the support structure <NUM>, as they will be absorbed by the springs <NUM> instead. Furthermore, it will be seen that the sensing structure <NUM> is hermetically isolated from the springs <NUM> by the anodically bonded glass layers <NUM>, <NUM>, forming the hermetically sealed sensor unit <NUM>. The device may be singulated from the wafer after this stage too. This allows for streamlining of production of the devices, as the devices are almost fully formed before they are singulated from the wafer.

<FIG> shows the next step of bonding the support structure <NUM> to a substrate <NUM> (typically made from alumina or ceramic), via one or more elastomeric mounts <NUM> (for instance). In this way, the hermetically sealed sensor unit <NUM> is decoupled from any stresses or shocks experienced by the substrate <NUM> via the springs <NUM> as well as the elastomeric mounts <NUM>.

<FIG> shows the next step of adding a flexible wire bond <NUM> to the device, attaching the metal tracking layer <NUM> at the support structure <NUM> to the substrate layer <NUM>. The wire bonds typically have a diameter of <NUM>. An external electrical connection <NUM> is also added through the substrate layer <NUM>. As previously mentioned, this allows for an external electrical connection to be made to the sensing structure <NUM>, but in this example across the electrically conductive paths carried by the springs <NUM> between the flexible wire bond <NUM> and the sensor unit <NUM>.

<FIG> shows the final step of adding a metal lid <NUM>, hermetically sealing the internal gas volume of the package. This is typically done using solder sealing (at -<NUM>), securing the lid <NUM> to the substrate <NUM>. The internal gas volume of the package is controlled to optimise the decoupling between the hermetically sealed sensor unit <NUM> and the support structure <NUM>, and typically comprises Argon, Neon or dry Nitrogen at atmospheric pressure - e.g. to optimise squeeze film damping.

It will be appreciated that forming one or more springs in the same material layer as the sensing structure provides for ease of manufacture while also decoupling mechanical and thermal stresses between the sensor unit and the external package wall(s). More generally, some examples of a sensor package and a method of manufacturing a sensor package according to the present disclosure are provided below.

According to one or more examples of the present disclosure, the one or more springs may have a serpentine form. The geometrical form of the springs may be designed to provide a predefined spring compliance or stiffness. In at least some examples, the one or more springs are configured to provide a spring resonance ≥ <NUM> and preferably in the range of <NUM>-<NUM>. This has been found by the inventors to give enough compliance without compromising sensor performance at lower frequency.

According to one or more examples of the present disclosure, the one or more springs preferably comprises a plurality of springs. The springs may be arranged around the sensor unit. For example, the sensor unit may be arranged centrally within the support structure and the springs may extend in multiple directions between the sensor unit and the support structure. The support structure may be in the same plane as the sensor unit or in a different plane, above and/or below the sensor unit. In one or more examples, the sensor unit may be suspended by the springs fixing the sensor unit to the support structure.

According to one or more examples of the present disclosure, the material layer in which the sensing structure is formed comprises silicon. The one or more springs may therefore be formed in the same silicon layer as the sensing structure of the sensor unit. The silicon springs can be shaped and/or dimensioned to give radial compliance to allow for stress relief between the sensor unit and the support structure. The silicon springs may conveniently be etched out during the same process that is used to etch out the sensing structure. For example, the one or more springs may be formed by etching a serpentine form in the silicon material layer.

According to one or more examples of the present disclosure, the hermetically sealed sensor unit comprises a glass layer, a silicon material layer comprising the sensing structure, and a further glass layer. Such a material structure is known as a silicon-on-glass (SOG) structure. The one or more further material layers arranged to seal the sensing structure to form a hermetically sealed sensor unit may therefore be glass layer(s).

According to one or more examples of the present disclosure, the hermetically sealed sensor unit comprises a silicon layer, a silicon material layer comprising the sensing structure, and a further silicon layer. The one or more further material layers arranged to seal the sensing structure to form a hermetically sealed sensor unit may therefore be silicon layer(s).

According to one or more examples of the present disclosure, the support structure is formed in the same material layer as the sensing structure of the sensor unit and the spring(s). In such examples the support structure is in the same plane as the material layer. This means that the support structure may be conveniently decoupled from the sensing structure by etching out the spring(s) during the same process that is used to etch out the sensing structure. The support structure may therefore be formed from silicon, the same as the sensing structure.

According to one or more examples of the present disclosure, the support structure is a frame. The frame may surround the hermetically sealed sensor unit. As mentioned above, a plurality of the springs may extend between the sensor unit and the frame e.g. suspending the sensor unit centrally within the frame.

According to one or more examples of the present disclosure, the support structure is fixed to at least one external package wall via a compliant (e.g. elastomeric) mount. Such an elastomeric mount may provide a degree of compliance, but it will be appreciated that the main decoupling between the sensor unit and the support structure is through the one or more springs. The compliant mount may require much less elastomeric material than the conventional elastomeric isolation layer used in prior art sensor packages.

According to one or more alternative examples of the present disclosure, the support structure is fixed to at least one external package wall via a rigid mount. It will be appreciated that a rigid mount may be used as the sensor unit is already decoupled from the support structure through the one or more springs. The rigid mount may comprise an adhesive e.g. epoxy bond or a metal solder joint.

According to the present disclosure relating to a sensor package, the sensor package further comprises a squeeze film damping structure arranged between the hermetically sealed sensor unit and the support structure. Such a damping structure comprises one or more gaps that are sized so as to provide a squeeze film damping effect in the gaseous atmosphere within the package, as is known in the art. For example, the squeeze film damping structure may comprise a plurality of interdigitated damping fingers. The plurality of interdigitated damping fingers may be arranged in one or more sets, for example multiple sets arranged around the sensor unit.

According to the present disclosure, the squeeze film damping structure is formed in the same material layer as the sensing structure of the sensor unit and the spring(s). This means that the squeeze film damping structure may conveniently be formed during the same process that is used to etch out the sensing structure and the spring(s). The squeeze film damping structure (e.g. interdigitated damping fingers) may therefore be formed from silicon, the same as the sensing structure.

According to one or more examples of the present disclosure, the hermetically sealed sensor unit is evacuated. According to one or more alternative examples of the present disclosure, the hermetically sealed sensor unit comprises a first gaseous environment e.g. comprising one or more of Argon, Neon or dry Nitrogen. The first gaseous environment may be at a pressure below atmospheric pressure, e.g. partially evacuated. Alternatively, the first gaseous environment may be at a pressure above atmospheric pressure. This elevated pressure may give a higher damping factor.

According to one or more examples of the present disclosure, the sensor package comprises a second gaseous environment outside the hermetically sealed sensor unit, e.g. made up of one or more of Argon, Neon or dry Nitrogen. The second gaseous environment may be at atmospheric pressure. As a squeeze film damping structure is arranged between the hermetically sealed sensor unit and the support structure, the second gaseous environment may be chosen to provide the desired squeeze film damping effect.

According to one or more examples of the present disclosure, the sensor package further comprises flexible wire bonds electrically connecting the sensor unit to at least one of the external package wall(s). The hermetically sealed sensor unit may further comprise one or more through-hole vias for electrical connection to the sensing structure. This means that direct wire bonds may pass down the through-hole vias to provide for electrical connection of the sensing structure.

According to one or more examples of the present disclosure, the hermetically sealed sensor unit is electrically connected to at least one of the external package wall(s) by an electrically conductive path carried by the one or more springs. For example, conductive (e.g. metal) tracking may be carried by the one or more springs. The hermetically sealed sensor unit may further comprise one or more through-hole vias for electrical connection to the sensing structure. This means that direct wire bonds may pass down the through-hole vias to provide for electrical connection of the sensing structure. This connection can then be linked to the electrically conductive path carried by the one or more springs in order to provide an electrical connection from at least one of the external package walls to the sensing structure.

According to one or more examples of the present disclosure, the sensor is a MEMS sensor.

According to one or more other examples of the present disclosure, the sensor is an inertial sensor. It follows that the hermetically sealed sensor unit may be a hermetically sealed inertial sensor unit.

According to one or more examples of the present disclosure, the inertial sensor is a gyroscope. The sensing structure may comprise a proof mass in the form of a disc or ring. The gyroscope may be a vibrating structure gyroscope.

According to one or more examples of the present disclosure, the inertial sensor is an accelerometer. The sensing structure may comprise a fixed substrate and a proof mass mounted to the fixed substrate by flexible support legs.

According to one or more further examples of the present disclosure, the accelerometer is one of the following: a capacitive accelerometer, an inductive accelerometer, or a piezoelectric accelerometer. In at least some examples, the capacitive accelerometer comprises:.

In one or more examples, the method may further comprise: forming the support structure in the same material layer as the sensing structure of the inertial sensor unit and the one or more springs. As is mentioned above, this is advantageous as the support structure, spring(s) and sensing structure may all be formed from the same material layer by a common manufacturing process such as DRIE.

In one or more examples, the method may further comprise: connecting flexible wire bonds between the sensor unit and at least one of the external package wall(s).

Claim 1:
A sensor package (<NUM>) comprising:
an inertial sensor, wherein the inertial sensor comprises a sensing structure (<NUM>; <NUM>; <NUM>; <NUM>; <NUM>) formed in a material layer (<NUM>; <NUM>; <NUM>) and one or more further material layers arranged to seal the sensing structure to form a hermetically sealed inertial sensor unit (<NUM>; <NUM>; <NUM>);
a support structure (<NUM>; <NUM>; <NUM>; <NUM>; <NUM>);
one or more springs (<NUM>; <NUM>; <NUM>; <NUM>; <NUM>) flexibly fixing the hermetically sealed inertial sensor unit to the support structure;
wherein the one or more springs are formed in the same material layer as the sensing structure of the inertial sensor unit;
one or more external package wall(s) (<NUM>, <NUM>; <NUM>; <NUM>) encapsulating the inertial sensor unit, the support structure, and the one or more springs, wherein the support structure is fixed to at least one of the package wall(s); and characterised in that the sensor package further comprises:
a squeeze film damping structure (<NUM>) arranged between the hermetically sealed inertial sensor unit and the support structure, wherein the squeeze film damping structure (<NUM>) is formed in the same material layer (<NUM>; <NUM>; <NUM>) as the sensing structure (<NUM>; <NUM>; <NUM>; <NUM>; <NUM>) of the inertial sensor unit (<NUM>; <NUM>; <NUM>) and the spring(s) (<NUM>; <NUM>; <NUM>; <NUM>; <NUM>).