Patent Description:
Pressure sensors or transducers are devices that convert pressure forces into electrical signals that can be interpreted to measure the pressure exerted on the pressure sensor. Some known pressure sensors incorporate micro-electro-mechanical system (MEMS) technology which allows the pressure sensors to have a small, compact size while maintaining high accuracy. However, when exposed to high vacuum conditions for extended periods of time, the accuracy of the pressure sensors suffers. For example, it has been observed that when known pressure sensors are exposed to high vacuum (e.g., very low pressure) and high temperature conditions (e.g., <NUM> Pa pressures and <NUM> degrees C), the pressure sensors experience an output shift that affects the measurement readings. The extent of the output shift and time before the output shift occurs varies, which makes it difficult to calibrate or accommodate the output shift. Furthermore, in high vacuum conditions, the construction and composition of known pressure sensors may also suffer. From <CIT>, an internal pressure simulator for pressure sensors is known. The simulator comprises a pressure sensor device that is provided with an electromagnetic coil for moving an outer diaphragm in order to simulate pressures. The pressure sensor device is provided with a support structure, a diaphragm, a sealed cavity filled with silicone oil and a MEMS sensor being arranged inside the cavity. From <CIT>, a media compatible pressure sensor is known. The pressure sensor is provided with a cavity that is closed by a diaphragm and filled with a pressure transferring medium such as mineral oil or silicone oil. Inside the cavity, a MEMS sensor is arranged. From <CIT>, an encapsulation for MEMS sensor elements and wire bonds is known. An encapsulator for encapsulating a diaphragm is made from perfluoropolyether in a semi solid-state.

The problem to be solved is to provide a pressure sensing device that can operate within high vacuum environments without experiencing an output shift that exceeds a designated tolerance threshold or range to provide accurate pressure measurements.

According to the invention, a pressure sensing device as defined in claim <NUM> is provided. The pressure sensing device including a support structure, an isolated diaphragm, a working oil, and a MEMS die sensing element. The support structure defines a portion of a sealed cavity. The isolated diaphragm is mounted to the support structure. The isolated diaphragm has in inner side that defines an end of the sealed cavity and an outer side opposite the inner side. The working oil is contained within the sealed cavity. The MEMS die sensing element is enclosed within the support structure. The MEMS die sensing element is exposed to the working oil within the sealed cavity. A pressure exerted on the outer side of the isolated diaphragm by a fluid medium is transferred via the working oil to the MEMS die sensing element to measure the pressure of the fluid medium. The working oil has a low vapor pressure and a low volatility content.

One or more embodiments presented herein disclose a pressure sensing device that includes a MEMS die sensing element. The MEMS die sensing element is isolated from a fluid medium that is to be measured via an isolated diaphragm and a working oil that transfers energy (e.g., forces) from the isolated diaphragm to the MEMS die sensing element. For example, the fluid medium exerts a pressure on the isolated diaphragm, and a resulting deflection of the isolated diaphragm is transmitted via the working oil within a cavity to the MEMS die sensing element which generates an electrical signal proportional to the force exerted by the fluid medium on the isolated diaphragm. The pressure sensing device is also configured to operate in high vacuum environments, such as at pressures as low as <NUM> Pa or lower. The working oil within the pressure sensing device has a low vapor pressure and low volatility content to avoid (or at least reduce the extent of) outgassing of the working oil (e.g., releasing vapor bubbles) in high vacuum and high temperature conditions, which alters the pressure exerted on the MEMS die sensing element and causes a shift in the electrical signal that is generated.

<FIG> is a cross-sectional illustration of a pressure sensing device <NUM> according to an embodiment. The pressure sensing device <NUM> includes a MEMS die sensing element <NUM>, an isolated diaphragm <NUM>, and a support structure <NUM>. In the illustrated embodiment, the support structure <NUM> is represented by a housing <NUM>, a header <NUM>, and a port member <NUM>. The MEMS die sensing element <NUM> (also referred to herein as sensing element <NUM>) is mounted to the header <NUM> via a bonding layer <NUM>. The bonding layer <NUM> may be an adhesive, a weld material, or the like. The pressure sensing device <NUM> according to one or more embodiments is a media isolated pressure sensor. For example, a working oil <NUM> is sealed inside the pressure sensing device <NUM> to isolate the MEMS die sensing element <NUM> from external media that is being measured.

The sensing element <NUM> has a sensing side <NUM> and a mounting side <NUM> opposite the sensing side <NUM>. The mounting side <NUM> engages the bonding layer <NUM>. The sensing side <NUM> includes a diaphragm <NUM>. The diaphragm <NUM> (also referred to as a MEMS diaphragm <NUM>) is thin and is defined by a pocket <NUM> within the sensing element <NUM>. The MEMS diaphragm <NUM> includes an interior surface <NUM> that defines a portion of the pocket <NUM> and an exterior surface <NUM> opposite the interior surface <NUM>. The exterior surface <NUM> defines a portion of the sensing side <NUM> of the sensing element <NUM>. The sensing element <NUM> has resistors along the sensing side <NUM>, such as along the exterior surface <NUM> of the MEMS diaphragm <NUM>. The resistors are piezoelectric and exhibit an electrical resistance that changes based on mechanical strain applied to the sensing element <NUM>. For example, when pressure is applied across the MEMS diaphragm <NUM>, the diaphragm <NUM> flexes and the resistors that are sensitive to mechanical strain provide an electrical signal through associated circuitry. The electrical signal indicates a measure of the pressure applied across the MEMS diaphragm <NUM>.

The housing <NUM> has a top side <NUM> and a bottom side <NUM> opposite the top side <NUM>. As used herein, relative or spatial terms such as "top," "bottom," "front," "rear," "inner," and "outer" are only used to identify and distinguish the referenced elements in the orientations shown in the illustrated figures and do not necessarily require particular positions or orientations relative to gravity and/or the surrounding environment of the pressure sensing device <NUM>. The housing <NUM> is disposed between the header <NUM> and the port member <NUM>. For example, the top side <NUM> of the housing <NUM> is mounted to the header <NUM>, and the bottom side <NUM> of the housing <NUM> is mounted to the port member <NUM>. The port member <NUM> may be connected to a conduit or reservoir that contains a fluid medium to be measured by the pressure sensing device <NUM>. The port member <NUM> defines an opening <NUM> that receives the fluid medium.

The isolated diaphragm <NUM> is attached to the port member <NUM> and extends across the opening <NUM> to seal the opening <NUM>. The isolated diaphragm <NUM> may be welded, soldered, brazed, or the like to the port member <NUM>. Optionally, the isolated diaphragm <NUM> may be attached to the bottom side <NUM> of the housing <NUM> in addition to being attached to the port member <NUM> (or instead of attaching to the port member <NUM>). The fluid medium engages the isolated diaphragm <NUM>, which defines a partition that isolates the sensing element <NUM> from the fluid medium. The isolated diaphragm <NUM> has an inner side <NUM> and an outer side <NUM> opposite the inner side <NUM>. The inner side <NUM> faces towards the MEMS die sensing element <NUM>. The fluid medium engages the outer side <NUM> of the diaphragm <NUM>. The properties of the fluid medium, such as corrosivity and/or conductivity, do not affect the sensing element <NUM> due to the isolation provided by the isolated diaphragm <NUM>. The isolated diaphragm <NUM> may include one or more metals, such as stainless steel, nickel, brass, and/or the like.

The pressure sensing device <NUM> defines a cavity <NUM> that contains a working oil <NUM> for transmitting energy (e.g., forces) from the isolated diaphragm <NUM> to the sensing element <NUM>. The sensing element <NUM> extends into cavity <NUM> in the illustrated embodiment such that the sensing side <NUM> engages the working oil <NUM>. The cavity <NUM> extends through the housing <NUM> from the top side <NUM> to the bottom side <NUM>. The cavity <NUM> is vertically defined between the header <NUM> and the isolated diaphragm <NUM>. The cavity <NUM> is hermetically sealed. The working oil <NUM> is non-corrosive, non-conductive, and incompressible.

During operation, pressure applied by the fluid medium on the isolated diaphragm <NUM> within the opening <NUM> of the port member <NUM> causes the diaphragm <NUM> to slightly flex (e.g., deflect). The deflection of the diaphragm <NUM> is transferred through the working oil <NUM> to the MEMS diaphragm <NUM>. The pressure on the MEMS diaphragm <NUM> is detected by measuring the resistance (or a change in the resistance) of the piezo-resistors on the sensing element <NUM>. The pressure sensing device <NUM> is calibrated to determine the pressure of the fluid medium exerted on the isolated diaphragm <NUM> based on the measured resistance of the piezo-resistors.

The pressure sensing device <NUM> is configured to withstand harsh environments including high vacuum and high temperature conditions while maintaining accurate measurements within a designated tolerance threshold or range. For example, the designated tolerance range may be <NUM>% of span at vacuum level. Known pressure sensors experience an output shift greater than the tolerance range when exposed to a high vacuum and high temperature environment for an extended period of time. For example, applying a negative pressure, such as <NUM> Pa, over an extended time period at a high temperature, such as at or above <NUM>° C, causes the tested pressure sensors to experience an output shift. The output shift can exceed <NUM> mV, and it has been observed that greater vacuum conditions and higher temperatures accelerate the output shift. The accuracy of the pressure measurements suffer as a result of the output shift.

The pressure sensing device <NUM> according to the embodiments presented herein is able to withstand extended exposure to <NUM> Pa pressure and <NUM>° C temperature without having an output shift greater than <NUM>% span. As a result, the pressure sensing device <NUM> maintains measurement sensitivity and accuracy even in harsh environments. Due to the robustness of the pressure sensing device <NUM>, the pressure sensing device <NUM> can be utilized in various harsh environment applications. For example, the pressure sensing device <NUM> can be utilized in semiconductor manufacturing applications, such as for flash memory production. The pressure sensing device <NUM> may be installed in a mass flow controller for semiconductor applications.

The working oil <NUM> of the pressure sensing device <NUM> has a lower vapor pressure and a lower volatility content (e.g., low concentration of volatile molecules), relative to oils utilized in known pressure sensors. The inventors of the present application have discovered that the composition of the oil within the pressure sensors is a contributing factor in the observed output shift when exposed to high vacuum and high temperature. For example, at high vacuum and high temperature conditions, some of the molecules of the oil transition to the gas phase, creating gas bubbles. The addition of the gas phase within the cavity <NUM> affects the pressure exerted on the MEMS die sensing element <NUM>, causing or at least contributing to an output shift.

Even if oil is degassed by temperature and moderate vacuum conditioning of the oil prior to entering a known pressure sensor, the conditioning may not remove all volatile molecules that may outgas in high vacuum conditions. The volatility content of the working oil is an inherent material property. Known oils used in typical pressure sensors may have a large amount of volatile content with varying molecular sizes. Small volatile molecules of the oil may outgas during the conditioning process, which allows the oil to maintain the liquid state (e.g., without vapor bubbles) when the pressure sensor is subjected to moderate vacuum and temperature conditions. However, the conditioning process may not generate sufficient energy to cause larger volatile molecules within the oil to outgas, such that the larger volatile molecules remain within the oil. The larger volatile molecules that remain after the conditioning may outgas in high vacuum and high temperature conditions (e.g., pressures at or less than <NUM> Pa and temperatures at or greater than <NUM>° C), producing the vapor bubbles that contribute to the output shift. The oils used as the working oil <NUM> of the pressure sensing device <NUM> according to the embodiments described herein have low vapor pressures and low volatility contents, which eliminates or at least reduces the occurrence of volatile molecules becoming vapors at harsh conditions (e.g., high vacuum and high temperature).

The working oil <NUM> is preferably a perfluoropolyether (PFPE) oil having a low vapor pressure being less than <NUM> Pa (e.g., <NUM> Torr) at temperatures less than or equal to <NUM>° C. The PFPE oil may have a minimum viscosity of <NUM> Pa*s at <NUM> rad/s. In non-limiting examples, the PFPE oil may be ECO-<NUM>/<NUM> PFPE diffusion pump oil, available from Enparticles Diffusion Pump Oil.

The working oil <NUM> may, in the alternative to the PFPE oil mentioned above, be a silicone oil that has a volatility content (e.g., volatile molecule count) that is less than <NUM> × <NUM><NUM> per <NUM>µL. For example, the silicone oil representing the working oil <NUM> may have a volatility content of approximately <NUM> × <NUM><NUM> per <NUM>µL. A second silicone oil that is commonly used in known pressure sensors was measured to have a greater volatility content than the other silicone oil, referred to as the first silicone oil. For example, the volatility content of the second silicone oil was measured to be greater than <NUM> × <NUM><NUM> per <NUM>µL, and roughly double the content of the first silicone oil. During testing, the use of the second silicone oil within a pressure sensor resulted in an output shift greater than the designated tolerance range of <NUM>% span, while the use of the first silicone oil (with the lower volatility content) in the pressure sensor did not cause an output shift greater than the tolerance range. Thus, the first silicone oil with the lower volatility content may represent the working oil <NUM> within the pressure sensing device <NUM>, while the second silicone oil does not represent the working oil <NUM>.

The pressure sensing device <NUM> containing the working oil <NUM> according to the embodiments described herein is capable of providing pressure measurements of fluid media at harsh conditions including high vacuum and high temperature without resulting in an output shift greater than a designated tolerance threshold or range. For example, the pressure sensing device <NUM> has been exposed for prolonged periods of time to a pressure of <NUM> Pa and a temperature of <NUM>° C, while maintaining the output within a tolerance range of <NUM>% of span at vacuum level. The pressure sensing device <NUM> may be operable at lower pressures (e.g., higher vacuum) and greater temperatures than the tested conditions. The pressure sensing device <NUM> is able to measure fluid media at pressures between about <NUM> kPa and about <NUM> kPa (about <NUM> psi to about <NUM> psi). The pressure sensing device <NUM> is able to measure corrosive and/or conductive fluid media because the MEMS die sensing element <NUM> is isolated from the fluid media via the isolated diaphragm <NUM>.

<FIG> is a perspective view of the pressure sensing device <NUM> according to an embodiment. The support structure <NUM> of the pressure sensing device <NUM> includes the housing <NUM> coupled between the port member <NUM> and a spacer <NUM>. The header <NUM> is not shown in <FIG>. The MEMS die sensing element <NUM> (shown in <FIG>) and the working oil <NUM> (<FIG>) are contained within the support structure <NUM> and are not visible in <FIG>. The pressure sensing device <NUM> includes metal pins <NUM> that pass through a spacer <NUM>. The pins <NUM> are electrically connected to the MEMS die sensing element <NUM> either directly or via wires. The pressure sensing device <NUM> includes a circuit board <NUM> mounted on a top side <NUM> of the spacer <NUM>. The top side <NUM> is opposite to the side of the spacer <NUM> that couples to the housing <NUM>. The pins <NUM> extend through the circuit board <NUM>. The circuit board <NUM> includes circuitry that is configured to receive and analyze the electrical signals generated by the piezo-resistors on the sensing element <NUM>. The pressure sensing device <NUM> includes conductors <NUM>, such as wires or rigid metal contacts, extending from the circuit board <NUM> to a remote device, such as a communication device, a display device, a control device, and/or a power source.

<FIG> is a cross-sectional view of the pressure sensing device <NUM> according to the embodiment shown in <FIG>. The pressure sensing device <NUM> in <FIG> may be similar to the illustrated embodiment of the pressure sensing device <NUM> shown in <FIG>. The housing <NUM> defines a recess <NUM> that is open along the top side <NUM> of the housing <NUM> and a channel <NUM> that is fluidly connected to the recess <NUM>. The channel <NUM> extends from the recess <NUM> to the bottom side <NUM> of the housing <NUM>. The channel <NUM> has a smaller diameter than the recess <NUM>. Both the recess <NUM> and the channel <NUM> define portions of the cavity <NUM> that receives the working oil <NUM>. The housing <NUM> includes stainless steel, but may include one or more other metals in an alternative embodiment. The MEMS die sensing element <NUM> is mounted to the header <NUM> via an adhesive, such as an epoxy or a roomtemperature-vulcanizing (RTV) silicone. The MEMS die sensing element <NUM> projects into the recess <NUM> of the housing <NUM> from above, and the sensing side <NUM> of the sensing element <NUM> engages the working oil <NUM>. For example, the exterior surface <NUM> of the MEMS diaphragm <NUM> is in contact with the working oil <NUM>. The sensing element <NUM> may be a silicon chip. The embodiment shown in <FIG> is a top-side MEMS die design (as well as the embodiment shown in <FIG>).

The isolated diaphragm <NUM> is mounted to the support structure <NUM> at or proximate to the interface between the housing <NUM> and the port member <NUM>. The inner side <NUM> of the isolated diaphragm <NUM> defines an end of the cavity <NUM>. The cavity <NUM> is sealed to prevent the ingress or egress of fluids. In the illustrated embodiment, the isolated diaphragm <NUM> includes stainless steel and nickel alloys, but may include one or more other metals in an alternative embodiment. The support structure <NUM> and the isolated diaphragm <NUM> may have a compact size, such that the diameter of the isolated diaphragm <NUM> may be approximately <NUM>-<NUM> (e.g., within <NUM>%, <NUM>%, or <NUM>% thereof), such as <NUM>.

The header <NUM> defines a fill hole <NUM> therethrough that is open to the cavity <NUM>. The fill hole <NUM> is used to fill the cavity <NUM> with the working oil <NUM>. After filling is complete, the fill hole <NUM> is sealed by a ball seal <NUM>. The ball seal <NUM> may be metallic, such as stainless steel, and may be sealed on the header <NUM> via welding, soldering, or the like. The pins <NUM> project from the header <NUM> to engage the circuit board <NUM> (shown in <FIG>), which is omitted in <FIG>, to provide an electrical connection between the MEMS die sensing element <NUM> and processing circuitry.

Optionally, the pressure sensing device <NUM> includes a ceramic insert <NUM> mounted to the header <NUM> and surrounding the MEMS die sensing element <NUM>. The ceramic insert <NUM> projects into the recess <NUM> of the housing <NUM> with the sensing element <NUM>, and engages the working oil <NUM> in the cavity <NUM>. The pressure sensing device <NUM> optionally may include a compensation board to compensate the electrical sensor output.

<FIG> is a cross-sectional view of the pressure sensing device <NUM> according to another embodiment. In the illustrated embodiment, the support structure <NUM> includes a housing <NUM>, a spacer <NUM>, and a port member <NUM>, similar to <FIG>. The support structure <NUM> lacks the header <NUM> shown in <FIG> and <FIG>. The MEMS die sensing element <NUM> is mounted to the housing <NUM>. The sensing element <NUM> is disposed within a recess <NUM> of the housing <NUM>. Unlike the recess <NUM> shown in <FIG>, the recess <NUM> does not define a portion of the cavity <NUM> that contains the working oil <NUM>. For example, the housing <NUM> defines a channel <NUM> and a fill hole <NUM> spaced apart from the channel <NUM>. Each of the channel <NUM> and the fill hole <NUM> extends from the recess <NUM> to the bottom side <NUM> of the housing <NUM>. The channel <NUM> aligns with and is fluidly connected to an aperture <NUM> in the mounting side <NUM> of the MEMS die sensing element <NUM>. The aperture <NUM> extends from the mounting side <NUM> to the pocket <NUM>. The fill hole <NUM> is plugged by a ball seal <NUM> that is within the recess <NUM>. The cavity <NUM> that contains the working oil <NUM> extends from the fill hole <NUM> along the space between the bottom side <NUM> of the housing <NUM> and the inner side <NUM> of the isolated diaphragm <NUM> through the channel <NUM> and into the aperture <NUM> to the MEMS diaphragm <NUM> of the sensing element <NUM>.

The working oil <NUM> engages the interior surface <NUM> of the MEMS diaphragm <NUM>. The embodiment shown in <FIG> is a back-side MEMS die design because the working oil <NUM> contacts the interior surface <NUM> instead of the exterior surface <NUM>, as with the top-side MEMS die design shown in <FIG>. In <FIG>, the piezo-resistors along the exterior surface <NUM> of the MEMS diaphragm <NUM> are outside of the cavity <NUM> and therefore not exposed to the working oil <NUM>. In the illustrated embodiment, the working oil <NUM> may be any of the working oils described with respect to the top-side embodiment shown in <FIG>. For example, the working oil <NUM> has a low vapor pressure and low volatility content.

The MEMS die sensing element <NUM> extends through, and is electrically connected to, a bond printed circuit board <NUM>. The pressure sensing device <NUM> optionally includes a compensation board <NUM> to compensate the electrical sensor output. The compensation board <NUM> is mounted on the top side <NUM> of the spacer <NUM> in the illustrated embodiment, such that the spacer <NUM> is stacked between the compensation board <NUM> and the housing <NUM>.

Claim 1:
A pressure sensing device (<NUM>) comprising:
a support structure (<NUM>) defining a portion of a sealed cavity (<NUM>);
an isolated diaphragm (<NUM>) mounted to the support structure (<NUM>), the isolated diaphragm (<NUM>) having in inner side (<NUM>) that defines an end of the sealed cavity (<NUM>) and an outer side (<NUM>) opposite the inner side (<NUM>);
a working oil (<NUM>) contained within the sealed cavity (<NUM>); and
a MEMS die sensing element (<NUM>) enclosed within the support structure (<NUM>), the MEMS die sensing element (<NUM>) exposed to the working oil (<NUM>) within the sealed cavity (<NUM>), wherein a pressure exerted on the outer side (<NUM>) of the isolated diaphragm (<NUM>) by a fluid medium is transferred via the working oil (<NUM>) to the MEMS die sensing element (<NUM>) to measure the pressure of the fluid medium, wherein the working oil (<NUM>) has a low vapor pressure and a low volatility content, characterized in that the working oil (<NUM>) is a perfluoropolyether (PFPE) oil having a vapor pressure less than <NUM> Pa at temperatures less than or equal to <NUM>° C or a silicone oil having a volatility content less than <NUM> × <NUM><NUM> per <NUM>µL.