VACUUM GAUGE

Abstract

The design of a vacuum gauge utilizing a micromachined silicon vacuum sensor to measure the extended vacuum range from ambient to ultrahigh vacuum by registering the gas thermal properties at each vacuum range is disclosed in the present invention. This single device is capable of measuring the pressure range from ambient and above to ultrahigh vacuum. This device applies to all types of vacuum measurement where no medium attack silicon is present. The disclosed vacuum gauge operates with thermistors and thermal pile on a membrane of the thermal isolation diaphragm structure with a heat isolation cavity underneath.

Claims

1. A vacuum gauge contains a micromachined thermal vacuum sensor on a silicon substrate, a measurement chamber, feedthrough, control electronics, power pack, mechanical connector, and enclosure; the micromachined sensor utilizing the thermal sensing principle on a diaphragm structure measures the extended vacuum level from ambient to ultrahigh vacuum; the thermal time-of-flight, thermal conductivity, thermal convection, and thermal radiation are measured and registered at the different stages of the vacuum to realize the extended full scale of vacuum measurement.

2. A micromachined vacuum sensor on a silicon substrate with one pair or plural numbers of pairs of thermistors symmetrically or asymmetrically against a micro-heater on the membrane, each thermistor is further thermally isolated by the open window or open slot on the membrane to the underneath cavity; and a thermistor with high thermal emissivity and a thermal pile close to it, as of claim 1.

3. A micromachined vacuum sensor on a silicon substrate of claim 2 with integrated micro-heater and temperature sensing for precise control of the temperature.

4. A micromachined vacuum sensor on a silicon substrate of claim 2 with the thermistors on the membrane of a diaphragm structure that is made of platinum, tungsten, nickel, or ploy-crystalline silicon, but preferably of platinum. Each thermistor has a thickness ranging from 100 to 500 nm but most preferably 200 to 250 nm; the distance between each thermistor is from 2 μm to 150 μm, but preferably 2 μm to 60 μm, and most preferably 50 μm; the open window or slot between any two thermistors is preferably to be within 5 μm.

5. The micromachined vacuum sensor on a silicon substrate of claim 4, the thermistors on the vacuum sensor chip are operating in combined modes of thermal time-of-flight, thermal conductivity detection, and thermal convection detection to measure the pressure or vacuum from ambient to mid −10.sup.−8 Torr, but most preferably from ambient to 2×10.sup.−7 Torr; the thermal time-of-flight sensing will measure the pressure from ambient or above to 50 Torr via the measurement of the thermal diffusivity by acquiring the heat transfer of a thermally modulated microheater to a pair of thermistors next to the microheater; the thermal conductivity measurement will be used to measure the vacuum from 50 Torr to mid −10.sup.−8 Torr, but most preferably from 50 Torr to 2×10.sup.−7 Torr.

6. The micromachined vacuum sensor on a silicon substrate of claim 5, where the thermal conductivity measurement is gas-independent; the gas dependence of thermal conductivity is removed by the simultaneously acquired and registered thermal diffusivity for a specific gas under measurement.

7. A micromachined vacuum sensor on a silicon substrate of claim 2, where the thermistor with high emissivity is preferably made of tungsten, and the thermal pile is preferably made of materials with a high Seebeck coefficient such as doped poly-crystalline silicon; an open window or open slot on the membrane to the underneath cavity and between the thermistor and the thermal pile is preferably to be within 5 μm, and most preferably within 3 μm; the thermistor will be powered at high power while the thermal pile next to it will measure the change of temperature that will be further correlated to the changes in the vacuum; it will start to function at the high to ultrahigh vacuum beyond low 10.sup.−7 Torr.

8. A micromachined vacuum sensor on a silicon substrate of claim 2, where the membrane of the diaphragm is preferable to be made of thermally isolation materials such as ployimide or parylene, or for the process compatibility, thermal conductive silicon nitride together with thermally isolation materials silicon oxide, or other materials can also be used; it is most preferably that the membrane is made of parylene with a thickness preferably from 500 nm to 5000 nm but most preferably 1000 nm.

9. A micromachined vacuum sensor on a silicon substrate of claim 2 having a passivation layer on top of thermistors made of thermally conductive materials such as silicon nitride or silicon carbide but most preferably silicon nitride with a thickness preferably from 50 to 500 nm but most preferably 100 to 200 nm.

10. A micromachined vacuum sensor on a silicon substrate of claim 2 having a cavity; the cavity will be used to provide thermal isolation to achieve the desired sensitivity.

11. A vacuum gauge of claim 1 having a vacuum chamber to host the micromachined vacuum sensor; the vacuum chamber will be made of stainless steel and isolated with non-outgassing materials such as glass or ceramics; the preferred measurement chamber will be in an elongated yet low profiled design with one dead-end; the distance from the surface of the vacuum sensor chip will be preferably within 1000 μm, while the dimension of the channel from the sensor chip edge to the exit will be preferably within 1.5 to 10 mm in length.

12. A vacuum measurement chamber of the vacuum gauge of claim 11, the direction of the thermistors on the vacuum sensor is most preferably aligned to the perpendicular direction of the exit for the best performance in measurement of the thermal convection.

13. A vacuum gauge comprised of a. A micromachined vacuum sensor on a silicon substrate (100) utilizing the gas thermal diffusivity, thermal conductivity, thermal convection, and thermal emissivity to register and measure the vacuum at different vacuum stages from ambient to ultrahigh vacuum; b. An elongated and low profile vacuum measurement chamber (230) with one exit having the vacuum sensor chip closely packed with non-outgassing feedthrough and the thermistors on the vacuum sensor aligned perpendicularly to the direction of the exit; c. A mechanical interface (240) to provide the option for a variety of mechanical connectors for different applications; d. An electronic printed circuitry board (332) having the control electronics for the vacuum sensor and electronic interfaces as well as a user-accessible keyboard and a local display; e. A power supply printed circuitry board (340) with an optional battery power pack for portable applications; f. An enclosure (300/310/320) to house the said components that form the completed vacuum gauge device.

Description

BRIEF DESCRIPTIONS OF THE DRAWINGS

[0017] FIG. 1 is showing the configuration of the vacuum pressure sensor with its sensing elements on a membrane.

[0018] FIG. 2 shows the package of the vacuum pressure sensor and the sensor house.

[0019] FIG. 3 shows the package of the vacuum pressure sensor and the sensor house from rotation of view angle.

[0020] FIG. 4 is the cross-section of the said vacuum pressure sensor showing the relative sensor house space and the sensor chip position.

[0021] FIG. 5 shows the explosive graph of the final vacuum gauge assembly.

[0022] FIG. 6 is the final assembled vacuum gauge.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] The preferred making of the said vacuum sensor (100) with an extended dynamic range is shown in FIG. 1. The sensor consists of thermally isolated thermistors and a thermopile sensor for the extended pressure range from above ambient down to ultra-high vacuum. It measures the pressure above ambient by utilizing the thermal time-of-flight principle that registers the pressure value via the combined thermal transfer and thermal response at a specific pressure value or gas density, for the pressure down from ambient to low vacuum the thermal conductivity measurement plays the central role. In the range of low to high vacuum, the combined thermal conductivity and thermal convection will contribute to the measurement. And at ultra-high vacuum, the vacuum measurement will be realized by the measurement of thermal radiation at a different gas density. The package of the sensor will also be critical to the sensitivity in the full extended dynamic range with the single sensor chip.

[0024] For the making of the sensor, the silicon micromachining process is preferred and the configuration is shown in FIG. 1. On the silicon substrate (102), beneath the membrane (110) is the cavity in which the silicon bulk material is removed from the backside via the standard MEMS deep dry plasma ion etching. The cavity filled with the air or gas provides the best thermal isolation for the thermistors deposited on the membrane. To ensure the flatness of the membrane, which is critical for the time-of-flight measurement, the membrane is either made of low-stress silicon nitride or a multilayer of silicon nitride and silicon dioxide such that the stress shall not play a role in the performance. In most cases, the membrane thickness is controlled within 1.5 micrometers for the commercially available process receipts. However, for the robustness, the membrane can also be made thicker such that the impact of the abrupt changes in pressure would not lead to any fracture-related failures. It is also important for selecting the materials for the membrane with a high thermal conductivity property such that it can assist the measurement when the thermal radiation will become the dominant factor.

[0025] The thermistor (120) is preferably made with electronic-beam evaporation of metal ingots such as platinum, nickel, copper, tungsten, and other stable metal materials with large thermal conductivity but preferably platinum for its stability and process uniformity. The membrane around this thermistor (120) will be patterned and dry-etched to have open windows (125) next to both sides of this thermistor. This window connects the upper and lower space of the silicon sensing chip which is divided by the membrane. These open windows not only provide the best thermal isolation for the thermistor but they also serve as the balancer for the membrane to keep its flatness under pressure as the open window allows the fast exchange of the gas under different pressure. This thermistor is then will be used for the measurement of the thermal conductivity changes of the gases due to the change of vacuum. The thermistor is applied with the external power and it is preferred that the thermistor will be powered with the constant power mode such that it will be easier for temperature correction. The measurement of the vacuum via the thermal conductivity is gas-dependent since the thermal conductivity value is different for different gas. In practice, the vacuum sensor is calculated only against one type of gas which is normally air. To eliminate the gas-dependent measurement of the thermal conductivity, a dual thermistor (121) is displaced next to the first thermistor (120) and both thermistors are thermally isolated with the open windows. The second thermistor will be powered simultaneously but at a different value. The second thermistor will output the thermal conductivity as well as a modulated receiving signal from the first thermistor. These three measurements or parameters can then be used to derive the gas independent vacuum value by the diffusivity and thermal conductivity as well as thermal capacity data.

[0026] The vacuum measured via the thermal conductivity will theoretically become nullified for pressure at ambient and above as the thermal conductivity will then turn into a constant. Most of the commercially available vacuum gauges utilize this measurement principle (the Pirani gauge) can only start to measure at about 20 Torr. To measure the pressure at ambient and above, the thermistor (120) and (121) are running at the time-of-flight mode that a modulated heatwave is applied to the thermistor (121) and the time differences of the heat transfer recorded at the thermistor (120) is a direct measure of the thermal diffusivity which is governed by the thermal conductivity, specific heat, and the density of the gas. Since thermal conductivity and specific heat will not change at the low positive pressure regime above the ambient, the measurement will be a direct measure of the gas density which is proportional to the gas pressure. With this scheme, a measure of the pressure to 800 kPa can be realized by the disclosed method.

[0027] The minimal vacuum can be measured by thermal conductivity is also limited at the lower end when the gas density is reduced to a level where the data corresponding to the changes in thermal conductivity is no longer meaningful, which is normally at a vacuum close to 1 mTorr. However, in a defined space, the reduction of the vacuum is realized via pumping away the enclosed gas, therefore the gas mobility at any place of the defined space will not be stationary. The speed of the gas leaving the defined space will generate a mass flow that has a direct impact on the thermal convection. The convection is then measured via the temperature differences between the thermistors on the sensor chip (100). In this regime, the thermal property of a particular gas would not play a significant role such as the thermal conductivity is no longer measurable. Hence, gas dependence would not contribute to the uncertainties to the measurement. The thermal time-of-flight measurement will be preferred using the thermistors (120) and (121). With the modulated heatwave, the heat convection plus the mass flow can be measured, and both of these data can be directly correlated to the vacuum at the defined and enclosed space. The measured value can then be correlated to the vacuum level of the defined space. This measurement can extend the dynamic range of the said vacuum sensor to register the vacuum to as high as 10.sup.−8 Torr if the sensing element is being placed in a very confined space where the dimension is significantly smaller than the mean free path of the enclosed gas.

[0028] When the vacuum level of the defined space reaches to the ultrahigh vacuum regime, the gas mobility assisted thermal convection will also be difficult to detect as the amount of molecule that could contribute to the signal are extremely limited. However, thermal radiation will be the major player for the thermal transfer. The thermal radiation is also a function of the vacuum as the transfer is a direct measure of the number of molecules available. The combined thermistor (130) and the thermal pile (140) on the sensor chip (100) will be used for this purpose. Unlike the other thermistors, the thermistor (130) will not have an open window next to it for thermal isolation. Instead, it is designed to be in the closet proximity to the thermal pile (140) for the highest sensitivity. The distance between the thermistor (130) and thermal pile (140) is desired to be within sub-10 μm and preferred within 3 μm. The material that is used to make the thermistor (130) is preferred to be those with high thermal emissivity, such as tungsten. The temperature changes measured by the thermal pile (140) will be used to correlate the vacuum level in the ultrahigh vacuum regime from 10.sup.−8 Torr and higher vacuum values. To reduce the background thermal transfer noises, the thermistor will be operated at the two-stage power relay such that the background transfer can be canceled to gain the true effects due to the vacuum changes. The thermistor (150) is designed to measure the sensor chip silicon substrate temperature such that it can provide feedback to the thermistors (120, 121, and 130) for achieving a true constant power or constant temperature mode within the electrical control circuitry. Finally, connection pads (160) for each thermistor and the thermal pile provide the interface between the sensor chip and the control electrical circuitry via a wire bonding process.

[0029] The package to host the said vacuum sensor for measurement will also be critical for achieving the desired extended rangeability. This is particularly important at the high and ultrahigh vacuum regime when the molecule's mean free path becomes longer and longer which reduces the possibility of the molecular interactions for thermal transfer and suppresses the signal output. In addition, to ensure the vacuum pumping efficiency, the desired package is required to eliminate any possibility of outgassing. FIG. 2 show the disclosed package of the said vacuum sensor for a vacuum gauge. The said vacuum sensor (100) is placed on the feedthrough (210) on which the sensor is connected via wire bonding to the external connector (215). The feedthrough is preferred to be made with stainless steel with high-temperature glass or ceramic sealing at the interface such that outgassing can be prevented. The vacuum sensor assembly body (230) is preferred to be made of stainless steel with the sensor assembly feedthrough housing port (220) which is also the opening to the vacuum sensing measurement chamber. The threaded ports (235) in FIG. 3 are for screws to fix the feedthrough (210) to the assembly body (235). The connector (240) is also preferred to be made of stainless steel and it provides an adapter option for various gauge port connections via the channel entrance (250) to the sensor measurement chamber. The ports (232) are made to fix the sensor assembly body to the final gauge assembly.

[0030] FIG. 4 shows the preferred cross-section of the measurement chamber (255). Since the mean free path of a specific gas is inversely proportional to the pressure value, in the high vacuum regime, the mean free path is often longer than the defined space dimension. Therefore, to boost the measurement sensitivity or the probability of molecular interaction, it is desired to confine the sensor chip within a dimension as small as possible. On the other hand, in the high to ultrahigh vacuum, the changes in thermal conductivity data are null, the thermal convection detection will prefer to have enhanced molecular directional mobility. Hence, the preferred measurement chamber will be in an elongated yet low profiled design with one dead-end as shown in FIG. 4. The distance from the surface of the sensor chip (100) shall be close to the dimension of the thermistor or within 1000 μm, while the dimension of the channel from the sensor chip edge to the exit shall be at least 5 times of the sensor chip dimension that is within 1.5 to 10 mm in length.

[0031] The structure of the complete vacuum gauge with the said vacuum sensor having the full dynamic range from ambient to ultrahigh vacuum is shown by the explosive view of the product in FIG. 5. The sensor attached to the feedthrough (210) is sealed with a copper gasket and a fixture (211) to the sensor assembly body (230). The complete unit together with the printed circuitry board having the control electronics (332), the power supply printed circuitry board (340), and an optional battery pack (345) are housed by the metal enclosure (300) which is further enclosed with the top cover (310) via screws (312) and bottom cover (320) via screws (325). The battery pack is designed for use as a standalone handheld device for low vacuum measurement applications. The material of the enclosure is preferred to be made of anodized aluminum alloy or stainless steel. On the printed circuitry board with the control electronics (332), a local display (335) is preferred to be an organic light-emitting diode (OLED) that provides a clear display even under the strong environment lighting. It will be placed at the position of a transparent window (302) in the enclosure (300), and the window materials can be either glass or plastics. The display and the window can be optional in the applications as for many of the vacuum equipment the vacuum gauge is used to control the pumping and process that will be most preferably via the digital data interface and the display will not be required. The electrical interface (330) opts with any industrial standard protocols. The keyboard (334) on the printed circuitry board of the control electronics and their access (304) on the enclosure is used to set the control point where the pump or the process can be triggered for the desired actions. The printed circuitry board of the control electronics is fixed via screws (331) to the sensor assembly body (230). The connector (240) provides a flexible port for mechanical interfaces. The example shown in the figure is a tri-clamp mechanical interface (350) which is engaged to the port (240) via a threaded connection.

[0032] FIG. 6 shows the assembled final product which is constituent of the assembled vacuum gauge enclosure (400) having the sensor and all electronics and measurement chamber, the exchangeable mechanical interface (350), the electrical interface (432 and 434) made typically of Modbus, Fieldbus, DeviceNet, ProfiBus or other industry-standard protocols. An optional wireless module such as LoRa, NB-IoT, or Bluetooth LE or other type modules is also a desired one for the remote data applications. The electrical interface will also provide the power supply in particular when the remote data communication is enforced. The LED lights (435) will provide visual operational statuses such as setpoints, error codes, and other user-defined functions. The display (302) will provide the digital readings of the measured vacuum values, digital error codes, and setpoint values and other operational data specified by the user. The display will be optional for equipment users but mandatory for handheld devices. The keyboard (304) includes functions such as power on/off, setpoints, set alarm, change vacuum units, check the status, and other user-defined functions.