Pirani Gauge Research Articles

A Dynamic Calibration Method for Pirani Gauges Embedded in Fluidic Networks

This paper describes in situ dynamic calibration of microfabricated Pirani gauges, which are commonly used to measure vacuum levels in microsystems that may include pumps, valves, and other pressure modulation elements. Calibration is accomplished with a configuration where two sides of the supporting membrane of a Pirani gauge are at different pressures: an interior pressure within the microsystems and an exterior ambient pressure that can be directly controlled. This particular configuration is increasingly common as Pirani gauges are embedded in complex fluidic pathways such as those found in micro-gas chromatographs and multistage gas pumps. In the dynamic calibration the ambient pressure is rapidly modulated, while the interior pressure in the sense gap of the Pirani gauge remains relatively unchanged. The exterior pressure that is equal to the interior pressure is determined by a regression model. The dynamic calibration procedure and subsequent error analysis are illustrated by application to a 162-stage monolithic Knudsen vacuum pump. For this device, dynamic calibration improves the estimated upstream pressure from 30 (as suggested by uncorrected static calibration) to 0.9 Torr, with a 95.4% confidence interval from 0.7 to 1.1 Torr, assuming normal (Gaussian) distribution. These results demonstrate that dynamic calibration can be significantly more accurate than conventional static calibration for certain types of devices.

A Si-Micromachined 162-Stage Two-Part Knudsen Pump for On-Chip Vacuum

This paper investigates a two-part architecture for a Knudsen vacuum pump with no moving parts. This type of pump exploits the thermal transpiration that results from the free-molecular flow in nonisothermal channels. For a high compression ratio, 162 stages are serially cascaded. The two-part architecture uses 54 stages designed for the pressure range from 760 to ≈ 50 Torr, and 108 stages designed for lower pressures. This approach provides greater compression ratio and speed than using a uniform design for each stage. Finite element simulations and analytical design analysis are presented. A five-mask single-wafer fabrication process is used for monolithic integration of the Knudsen pump that has a footprint of 12 × 15 mm2. The pressure levels of each stage are measured by integrated Pirani gauges. Experimental evaluation shows that, using an input power of ≈ 0.39 W, the evacuated chamber is reduced from 760 to ≈ 0.9 Torr, resulting in a compression ratio of ≈ 844. The vacuum levels are sustained during 37 days of continuous operation.

Miniature Integrated High-vacuum MEMS

Abstract For the first time, high-vacuum ion-sorption MEMS micropump has been integrated with other vacuum MEMS devices. The first integrated device consists of a Pirani vacuum gauge and a micropump, the second – consists of a field-emission electron source and a micropump. Tandem device Pirani gauge – micropump forms a single-unit vacuum meter. Tandem device electron source – micropump enables obtaining a more stable electron beam gun after residual gas removal. The preliminary results presented here confirm the possibility of fabricating new high vacuum MEMS instruments with own pumping and vacuum monitoring system on-chip.

Thin-film encapsulation technology for above-IC MEMS wafer-level packaging

This work presents a low-cost and low-temperature wafer-level packaging solution for microelectromechanical systems (MEMS) devices. Heat-sensitive polymer poly(propylene carbonate) is used as the sacrificial material to release the capping layer in a clean and fast manner. Free-standing caps made of amorphous silicon carbide films and as large as 450 µm in diameter are successfully fabricated. To demonstrate the validity of this technology, surface-micromachined Pirani vacuum gauges are fabricated as an example of MEMS devices and encapsulated. Capped Pirani gauges respond to pressure between 1 mTorr and 1 atm. The Pirani gauges are sealed with Parylene C films that exhibit near-hermetic properties and the initial sealing pressure for 300 µm diameter cavities is characterized to be in the range of tens of torr.

Single-Walled Carbon Nanotube Pirani Gauges Prepared by DEP Assembly

This paper reports the design, fabrication, and characteristics of the single-walled carbon nanotubes (SWNTs) Pirani gauge fabricated by dielectrophoretic assembly first which requires simple equipments and process, costs little, and operates at room temperature without limitation on the materials of electrode and substrate. Its theory and design were described in detail. The characteristics were measured and analyzed systematically such as current–voltage, resistance–temperature, thermal response, resistance–pressure, and power consumption properties. The results show that this SWNTs Pirani gauge has rapid thermal response of milliseconds and can operate at very low power consumption down to 15 nW. It reveals a wide linear dynamic range from 0.8 to 80 000 Pa and high sensitivity about 8 kΩ/Pa on average.

Channel electron multiplier operated on a sounding rocket without a cryogenic vacuum pump from 120 to 80 km altitude

We demonstrate that a channel electron multiplier (CEM) can be operated on a sounding rocket in the pulse-counting mode from 120km to 80km altitude without the cryogenic evacuation used in the past. Evacuation of the CEM is provided only by aerodynamic flow around the rocket. This demonstration is motivated by the need for additional flights of mass spectrometers to clarify the fate of metallic compounds and ions ablated from micrometeorites and their possible role in the nucleation of noctilucent clouds. The CEMs were flown as guest instruments on two sounding rockets to the mesosphere. Modeling of the aerodynamic flow around the payload with Direct Simulation Monte-Carlo (DSMC) code showed that the pressure is reduced below ambient in the void behind (relative to the direction of motion) an aft-facing surface. An enclosure containing the CEM was placed forward of an aft-facing deck and a valve was opened during flight to expose the CEM to the aerodynamically evacuated region behind it. The CEM operated successfully from apogee down to ∼80km. A Pirani gauge confirmed pressures reduced to as low as 20% of ambient with the extent of reduction dependent upon altitude and velocity. Additional DSMC simulations indicate that there are alternate payload designs with improved aerodynamic pumping for forward mounted instruments such as mass spectrometers.

Spinning rotor gauge based vacuum gauge calibration system at the Institute for Plasma Research (IPR)

The Steady-state Superconducting Tokamak (SST-1) is an indigenously built medium sized fusion device at IPR designed for plasma duration of 1000 seconds. It consists of two large vacuum chambers – Vacuum Vessel (16 m3) and Cryostat (39 m3) which will be pumped to UHV and HV pressures respectively using a set of turbo molecular pumps, Cryo-pumps and Roots pumps. The total as well as the partial pressure measurement in these chambers will be carried out using a set of Pirani gauges, Bayard Alpert type gauges, Capacitance manometers and Residual Gas Analyzers (RGA). A reliable and accurate pressure measurement is essential for successful operation of SST-1 machine. For this purpose a gauge calibration system is set up in SST-1 Vacuum laboratory based on Spinning Rotor Gauge which can measure absolute pressure in the range 1.0 mbar to 1.0 × 10−7 mbar. This system is designed to calibrate up to five gauges simultaneously for different gases in different operating pressure ranges of the gauges. This paper discusses the experimental set-up and the procedure adopted for the calibration of such vacuum gauges.

High-Resolution Micro-Pirani Pressure Sensor With Transient Response Processing and Time-Constant Evaluation

A micro-Pirani pressure sensor, which consists of a pressure-dependent thermoresistance gauge, is traditionally exploited using a steady-state resistance measurement. Any signal variation occurs over an offset voltage due to the imperfection of the device, which affects the sensor's sensitivity. This paper presents, for the first time, an experimental investigation of a micro-Pirani gauge based on its dynamic behavior when heated by a current step. Such processing magnifies the pressure dependence of the gauge's signal by eliminating the constant offset influences on the measurement. Furthermore, a first-order low-pass filter step response identification of the experimental transient signal strongly reduces the noise influence on the measurement. Furthermore, such identification enables a direct calculation of the time constant. This paper investigates the pressure measurement based on the time-constant evaluation, which provides a range of measurement and a maximal sensitivity significantly shifted toward the atmospheric pressure compared to that of a traditionally processed Pirani gauge. The heating step, the recording of the transient response signal, and its digital postprocessing can be easily achieved by a small-sized controller. The proposed system provides a substantial performance enhancement of the micro-Pirani pressure sensor.

Tube-shaped Pirani gauge for in situ hermeticity monitoring of SiN thin-film encapsulation

Here the integration of a tube-shaped Pirani gauge with a SiN thin-film encapsulation process is presented. The tube geometry gives the sensor a very low detection limit with a small footprint, since the tube is buried under the silicon surface. The Pirani tube is very suitable for in situ evaluation of MEMS vacuum packaging. Moreover, since the Pirani gap is all around the tube and deep below the silicon surface, the deflection of the encapsulation shell is not a concern and wafer-level measurement of the device is possible. Pirani tubes with different lengths are encapsulated inside SiN micropackages in order to measure the resulting vacuum level achieved after the encapsulation step. The longest tube shows a detection limit of 0.1 Pa for a noise level of 50 μV and it has a footprint of only 0.006 mm2. The pressure inside the sealed micropackage was extracted to be 0.7 kPa. Furthermore, the pressure is monitored over time to evaluate the hermeticity of the packages. A leak rate of 8× 10−18 Pa m3 s−1 was measured over four months time.

A Si-micromachined 48-stage Knudsen pump for on-chip vacuum

This paper describes a thermal transpiration-driven multistage Knudsen pump for vacuum pumping applications. This type of pump relies upon the motion of gas molecules from the cold end to the hot end of a channel in which the flow is restricted to the free molecular or transitional regimes. To achieve a high compression ratio, 48 stages are cascaded in series in a single chip. A five-mask, single silicon wafer process is used for monolithic integration of the designed Knudsen pump. The pump has several monolithically integrated Pirani gauges to experimentally measure the vacuum pumping characteristics of the pump. It has a footprint of 10.35 × 11.45 mm2. For an input power of 1350 mW, the fabricated pump self-evacuates the encapsulated cavities from 760 to ≈50 Torr, resulting in a compression ratio of 15. It also pumps down from 250 to ≈5 Torr, resulting in a compression ratio of 50. Each integrated Pirani gauge requires ≈3.9 mW of power consumption, and its response is sufficiently sensitive in the operating pressure range of 760–1 Torr.

An analytical model and verification for MEMS Pirani gauges

A new analytical model for the design of micromachined Pirani gauges operating in constant current mode is presented. This model expresses the pressure range as a closed-form analytical function of the design variables such as geometry and biasing. Furthermore, it yields simplified expressions for other performance parameters such as the sensitivity, output swing and power consumption. A Pirani gauge has been designed according to the presented model and has been fabricated and characterized in order to verify the validity of the model. The measurements match the theory closely. The model will be useful to designers who need to trade off performance against the costs of chip area and biasing power.

Modeling and Characterization of MicroPirani Vacuum Gauges Manufactured by a Low-Temperature Film Transfer Process

The novelty of this paper is the proof of functional microdevice fabrication using a recently developed low-temperature transfer process. The process is based on adhesion control of molded Ni microstructures on a donor wafer by using plasma-deposited fluorocarbon films. Low-temperature adhesive bonding of the microstructures on the target wafer using benzocyclobutene sealing enables mechanical tearing off from the donor wafer. Interest of this process for manufacturing microsensors is demonstrated here in the case of microbeams used as pressure sensors based on the Pirani principle. A simple analytical model is used to estimate the electrothermal behavior of the suspended microwires as a function of the ambient gas pressure. Estimations are compared to experimental measurements performed on Ni electroplated microwires of 550-1200-μm length, 10-μm width, and 0.7-7- μm thickness characterized into a vacuum chamber. These microsensors present a maximum of sensitivity in the range of 0.1-100 mbar, which is in line with standard performances of Pirani gauges. The presented results thus demonstrate the interest of a simple film transfer process for the elaboration of 3-D functional microstructures.

A Pirani gauge in a high-intensity proton accelerator

We have developed a Pirani-type vacuum gauge for monitoring the back pressure of a turbo-molecular pump (TMP), which is employed as the main pump for the 3-GeV Rapid Cycling Synchrotron (RCS) at the Japan Proton Accelerator Complex (J-PARC).As vacuum components are subjected to high levels of radiation in these devices, the TMP and the vacuum gauge used for monitoring the TMP’s back pressure should have high reliability and a long life so as to minimize exposure to radiation during maintenance. Moreover, to ascertain the vacuum conditions of the ring and to monitor the back pressure, the gauge should preferably be capable of measuring pressures from 0.1 Pa to 103 Pa. To satisfy these requirements, the following measures were taken: All the components around the sensor head, including the connector and cables, were replaced with components that were guaranteed to perform satisfactorily even with a radiation dose greater than 30 MGy. Then, the semiconductor devices used in the measuring circuits were checked by using the four-point probe method and found to work satisfactorily at a distance of 150 m from the sensor. To measure higher pressures, the current control circuit was modified in such a way that the set value of the current increased in stages as the pressure increased. Further, it was ensured that the gauge showed high resistance to vibration and to an abrupt air intake as well as high radioactive resistance. The newly designed gauge head comprised a series of twelve Pt wires, each with a diameter of 100 μm. It was confirmed that pressures from 5 × 10−2 Pa to atmospheric pressure were measurable with an accuracy of less than 30%.This Pt Pirani gauge has shown good performance in monitoring the back pressure of the TMP at the J-PARC RCS, where it has been in use since Oct. 2009.

A Tube-Shaped Buried Pirani Gauge for Low Detection Limit With Small Footprint

We present a micromachined Pirani gauge that combines low detection limit and strongly reduced footprint. It consists of a tube-shaped resistor that is buried in the silicon substrate. The choice of the tube geometry gives the resistor a very high structural rigidity. This enables the fabrication of much longer resistors, thus shifting the detection limit toward lower pressures. In addition, since the resistor is buried under the silicon surface, its footprint is kept very small. The high stiffness allowed the fabrication of a 3-mm-long and 1.8-μm-thick poly-Si tube with a 1-μm gap without buckling and/or stiction problems. It shows a detection limit of 0.1 Pa for a noise level of 50 μV, and it has a footprint of only 0.012 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . This is an improvement of at least 20 times compared with Pirani gauges with the same detection limit. Pirani tubes of 1.6- and 0.4-mm lengths have also been designed, fabricated, and tested. The 0.4-mm-long tube shows a low pressure limit of 2 Pa, whereas the tube of 1.6 mm shows a low pressure limit of 0.2 Pa. The measured transfer functions correspond very well to the 1-D analytical model.

Improvement of Sensitivity near Atmospheric Pressure in a Pirani Gauge in Constant Current Mode

Rather large errors occur when measuring pressures of around 5×103-104 Pa with a Pirani vacuum gauge in constant current mode, as the change in output voltage is very small. Thus, an attempt was made to achieve more accurate pressure measurement in this range by modifying the thermal conductivity arising from the natural convection flow of gas with the shape of the envelope. The experimental results showed that the pressure range with a large change in output per relative change in pressure expanded to lower pressures when the inner diameter of the envelope increased. For envelopes with an inner diameter of 72 mm, the change |pd V/dp| was greater than 50 mV at pressures from 1.5×103 Pa to atmospheric pressure, while the change |pd V/dp| was less than 20 mV at pressures of less than 2×104 Pa for envelopes with an inner diameter of 34 mm. Thus, more accurate pressure measurement near 104 Pa was achieved with this large envelope.

Infrared Radiation Image Sensor with Function of Vacuum Pressure Detection

We propose a new detecting method of vacuum pressure utilizing reference cells with the similar structure as pixels in the IR(infrared radiation) image sensor of which the sensitivity is greatly depended on the vacuum pressure in the sensor package. This method showed the excellent correlation between the vacuum pressure and output voltage without any additional pirani gauge requiring readout circuit which is generally utilized to detect the vacuum pressure. The proposed method is understood to realize the detection of the package vacuum pressure with detecting infrared radiation as an IR image sensor.

Integrated micro Pirani gauge based hermetical package monitoring for uncooled VO x bolometer FPAs

This paper presents the design and fabrication of a micro Pirani gauge using VO x as the sensitive material for monitoring the pressure inside a hermetical package for micro bolometer focal plane arrays (FPAs). The designed Pirani gauge working in heat dissipating mode was intentionally fabricated using standard MEMS processing which is highly compatible with the FPAs fabrication. The functional layer of the micro Pirani gauge is a VO x thin film designed as a 100 × 200 μm pixel, suspended 2 μm above the substrate. By modeling of rarefied gas heat conduction using the Extended Fourier’s law, finite element analysis is used to investigate the sensitivity of the pressure gauge. Also the thermal interactions between the micro Pirani gauge and bolometer FPAs are verified. From the fabricated prototype, the measured device TCR is about −0.8% K−1 and the sensitivity about 1.84 × 10−3 W K−1 mbar−1.

A micro-machined Pirani gauge for vacuum measurement of ultra-small sized vacuum packaging

Quite a few MEMS devices need vacuum packaging technology to guarantee the desirable performance. Developing an absolute vacuum environment for those devices is indispensable. However, it is difficult to monitor the pressure change in vacuum chamber in on-line and real-time mode. A surface micro-machined Pirani gauge for measuring vacuum pressure inside vacuum packaging in wafer level was presented in this paper. It was designed with a simplified structure and did not need complex circuit. Only a simple Wheatstone bridge circuit is needed, which could be manufactured by conventional CMOS processes. Preliminary tests on this device were conducted. The experimental results show that the Pirani gauge is capable of measuring pressures from atmospheric value to 1 Pa and has a very good linearity in the range from1 Pa to 300 Pa. It demonstrates that the micro-machined Pirani gauge has great potential to be used in wafer level vacuum packaging. Also, the Pirani gauge is able to be the packaging hermeticity detector.

Use of nanoporous columnar thin film in the wafer-level packaging of MEMS devices

This paper presents a new packaging technology that uses a nanoporous columnar thin film to seal microelectromechanical system (MEMS) devices at the wafer level. In the proposed packaging process, the processing temperature is 350 °C. The process is relatively inexpensive compared to wafer level packaging processes, because the wafer-bonding step is eliminated and the die size is shrunk. In the suggested approach, a sputtered columnar thin film at room temperature forms vertical nanopores as etch holes, and an air cavity is formed by the removal of a sacrificial layer through the nanopores in the columnar membrane. Subsequent hermetic vacuum packaging of the cavity is achieved by depositing thin films over the membrane under low pressure. The hermeticity of the packaging was verified by using an optical surface morphology microscope to measure the deflection change of the sealing membrane before and after breaking of the vacuum through an interconnected membrane. The long-term hermeticity was monitored by measuring the maximum central deflection of the PECVD sealing layer over a period of 170 days. The precise pressure (0.7 Torr) and short-term (30 days) pressure change inside the cavity were measured by encapsulated Ni Pirani gauges, representing packaged freestanding MEMS devices.

The Development of a Pirani Vacuum Gauge with a Platinum Wire in the J-PARC 3-GeV Rapid Cycling Synchrotron

The back pressure of Turbo-Molecular Pumps (TMPs) is constantly monitored using Pirani gauges at J-PARC (Japan Proton Accelerator Complex) RCS (3-GeV Rapid Cycling Synchrotron) where they are used not only in rough pumping but also evacuations during beam operations. The gauge head needs to be very resistant to vibration and abrupt air inlet etc. in minimizing exposure to radiation during maintenance and hence a 50 μm in diameter W wire was adopted as the filament. This type of Pirani gauge has worked well in monitoring the back pressure of the TMP but it has become difficult to measure the low pressure of less than several Pa with the gauge, which may have been due to changes in the emissivity of the W surface. An attempt was therefore made to develop a gauge head made of Pt wire in allowing pressures as low as 0.1 Pa to be measured. Platinum is one of the best possible materials to use because it is very stable against oxidization. However, ordinary Pt gauge heads are rather weak when it comes to vibrations and abrupt air inlet due to its low tensile strength. In order to improve its toughness the filament was composed of twelve 100 μm in diameter Pt wires that were 65 mm long, resulting in it being capable of enduring a force of 25 N. All the wires were welded in series on metal poles in two separate glass plates, with the poles being electrically insulated. This resulted in the filament, 78 cm long and about 10 Ω at room temperature, being containable in a 5 cm in diameter and 10 cm long cylindrical envelope. The output from the gauge head was then examined as a function of pressure under constant current as the plan was for it to be controlled using the constant current method. Confirmation then took place that the pressures of 0.1 Pa up to 103 Pa were measurable with the gauge using current control in such way that the set value increased with pressure increases in three stages.