Papers

AVS2001 Session VST-ThM: Pressure & Flow Measurement Instruments & Their Calibration

Thursday, November 1, 2001 8:20 AM in Room 125

Thursday Morning

Start	Invited?	Item
8:20 AM		VST-ThM-1 Is the Effective Accommodation Coefficient of the Spinning Rotor Gauge Temperature Dependent? K.J. Jousten (Physikalisch-Technische Bundesanstalt, Germany) The spinning rotor gauge (SRG) is a vacuum meter for the range 10^-4 Pa -to- 1Pa with good long term stability; therefore it is widely used as reference standard in calibration services. The calibration constant of the SRG is the effective accommodation coefficient σ valued around 1. This accommodation coefficient of the gas particles to the rotor is considered temperature independent for the normal operational temperatures 18°C -to- 30°C. We have carried out investigations that show that σ of an unbaked rotor (which is not unusual in calibration services) is temperature dependent. The operational temperature of the SRG could be varied from 16°C -to- 40°C. Typical relative changes of σ were 0.1% per 3 degree change of temperature, but larger changes were also observed. It will be discussed, if the changes are related to a change of H₂O coverage of the rotor.
9:00 AM		VST-ThM-3 Using Capacitance Diaphragm Gauges at Different Temperatures J.C. Greenwood (National Physical Laboratory, U.K.); U.M.E. Christmas (University of Surrey, UK) Capacitance diaphragm gauges are used in many industrial applications for absolute pressure measurements. The gauges are heated to improve stability but this procedure introduces thermal transpiration effects due to the temperature difference between the gauge head and the vacuum system. Calibration incorporates these effects although the calibration data are only valid when the gauge is operated at the same temperature as that during calibration. In practice, different temperatures are used and systematic errors are introduced. The calibration data can be represented using a thermal transpiration model but it is not clear whether such a model provides an adequate method of predicting performance at different temperatures. Two capacitance diaphragm gauges have been calibrated over the pressure range 0.02 Pa to 133 Pa at three different ambient temperatures and the calibration data at temperatures of 291K and 301K have been compared with the data predicted by applying a thermal transpiration model to data collected at 293K. The uncertainty associated with making such a prediction is evaluated.
9:40 AM		VST-ThM-5 Low-pressure Capacitiance Diaphragm Gage Measurements and Characteristics S Lu, S.A. Tison (Mykrolis formerly Millipore Microelectronics) Low-pressure measurement is critical for many applications within the vacuum community. The International Technology Roadmap for Semiconductors identifies a number of areas of process improvement required to meet the semiconductor device characteristics for the next five years. The requirements include better sidewall etch control and selectivity, processing of high and low k dielectric films and adequate process control to improve statistical process variations. Many of the critical processes occur at pressures below 100 mTorr where pressure measurement is most difficult. Traditionally, vacuum process measurements have been referenced to capacitance diaphragm gages (CDGs) sometimes called capacitance manometers. Characteristics of these instruments such as zero drift and mechanical creep, while being acceptable for higher range CDGs become more problematic for low-range CDGs where these effects become the major source of measurement error and process variability. This study evaluates the performance of three CDG designs which attempt to improve low-pressure performance. The data shows that maximizing the sensor sensitivity, may result in more mechanical drift and a balance between mechanical sensitivity and electrical gain is the optimum solution for overall instrument performance.
10:00 AM		VST-ThM-6 Static Gas Expansion Method as High-accuracy Primary Pressure Standard W. Jitschin (University of Applied Sciences, Germany) For the calibration of vacuum gauges in the range of medium and high vacuum, the method of static gas expansion is widely adopted as high-accuracy primary method. In this method, gas is expanded from a small vessel into a large one, whereby the pressure is reduced according to the volume increase. Uncertainties of the generated pressure arise from uncertainties of initial pressure, volume ratio of both vessels, real gas behavior, and thermal effects. As comes out from the uncertainty budget of a conventional expansion apparatus, the dominant contribution stems from temperature effects. For example, a typical temperature uncertainty of 0.3K gives a relative pressure uncertainty of 0.1% . In order to obtain a substantially improved accuracy, the whole apparatus was immersed in a circulated, temperature controlled water bath. This measure resulted in a tenfold improved temperature stability of the vessel walls, i.e. better than 0.03K. Furthermore, thermal effects caused by the thermodynamic process of gas expansion were investigated. In fact, the gas in the small vessel cools by as much as -200K. As our experimental and theoretical studies reveal, the thermal equilibrium between the gas and the vessel walls is reached with time constants of the order of seconds, depending on gas species and pressure. Accordingly, after a typical waiting time of about 1 minute after filling the small vessel and after expansion into the large vessel, the equilibrium is almost fully achieved even in worst case. Other disturbing effects were also investigated in detail. The uncertainty budget of the generated calibration pressures (2σ-level) gives a total uncertainty below 0.1% in the pressure range 0.1-to- 10 mbar (single step expansion) and below 0.15% in the pressure range 0.001 to 0.1 mbar (two step expansion).
10:20 AM		VST-ThM-7 Transfer Standard for Low Rates of Gas Flow R.F. Berg (National Institute of Standards and Technology) We describe an improved laminar flow meter suitable for comparing gas flow rates from 1 -to- 1000 micromol/s (about 1 to 1000 sccm) with an accuracy of approximately 0.1%. The flow impedance consists of coils of commercial quartz capillary tubing. The capillary radius is less than 0.01% of the length, which reduces corrections due to gas expansion and the pressure drop at the entrance. A small aspect ratio also was used in the flow meter recently modeled by Berg and Tison.¹ That flow meter's impedance was a machined helical duct of rectangular cross section. In contrast, the present quartz capillaries have a circular cross section, which allows the model of the flow meter to use Van Dyke's analytical expression for centrifugal effects.² Although such corrections are as large as 3% at the largest flow rates, the model requires only one free parameter, the capillary radius. A model with minimal empiricism allows the standard to be used confidently with varied gases, varied outlet pressures, and integrated measurements as well as continuous measurements. Measurements with two primary standards are described. The first set compared the transfer standard's continuous output to PVT measurements, and the second set compared the integrated output to the mass changes of a gas cylinder. ¹ AIChE J. 47, 263 (2001). ² J. Fluid Mech. 86, 129 (1979).
10:40 AM		VST-ThM-8 Overview of Two CCM Key Comparisons in Low Pressure (1 Pa - 1000 Pa) - How Good are MEMS Sensors as Transfer Standards? A.P. Miiller (National Institute of Standards and Technology) This talk will provide an overview of two recent CCM key comparisons of primary pressure standards operating in the range 1 Pa to 1000 Pa at eight National Metrology Institutes (NMIs). The objective of the two comparisons, one in absolute pressure the other in differential pressure at a nominal line pressure of 100 kPa, was to determine the degrees of equivalence of the measurement standards. An earlier comprehensive study¹ of low-pressure transducers showed that no one type of transducer could meet all requirements for the transfer standard. Consequently the transfer standard package was constructed using four high-precision pressure transducers, two capacitance diaphragm gauges to provide high resolution at low pressures and two (MEMS) resonant silicon gauges to provide calibration stability. Two nominally identical transfer packages were used to reduce the time required for the measurements, with Package A being circulated among laboratories in the European region (IMGC, NPL-UK, and PTB) and Package B being circulated among laboratories in the Asia-Pacific region (CSIRO, KRISS, MSL-NZ, and NPL-I). The results obtained with different transfer packages were normalized by using data obtained from simultaneous calibrations of the two packages at the pilot laboratory (NIST). The degrees of equivalence of the measurement standards were determined in two ways, deviations from key comparison reference values (KCRVs) and pairwise differences between these deviations. Apart from a few results identified as outliers, the measurement standards were generally found to be equivalent relative to the KCRVs, though some additional instances of nonequivalence were observed between given pairs of standards. The results revealed no significant relative bias between different measurement methods used by the NMIs to realize their standards. ¹ Miiller A. P., "Measurement performance of high-accuracy low-pressure transducers", Metrologia, 1999, 36 (6), 617-621.
11:00 AM		VST-ThM-9 Some Consideration About Uncertainty on the Primary Vacuum Measurements A. Calcatelli, M. Bergoglio (Consiglio Nazionale delle Ricerche, Italy) The need of better characterisation of high quality gauges in the various vacuum ranges represents a considerable input to accurate analyses of the available primary systems. That was connected to the need of defining ranges and related uncertainties for the international comparisons. The uncertainties of the IMGC systems are discussed. Static system: the measurand, p_i,is given by p_i=p_0iR_jT_V/T_v where the quantities p_0i, T_v, T_V and R_j represent respectively inlet pressure values, the temperatures of the involved volumes and the expansion ratio between the considered volumes. These quantities are generated by several other factors. The input quantities for R_j are correlated since the method is based on the accumulation of the gas inside one of the involved volumes and consequently on sequential measurements of set of pressures. In turn R_j is correlated to p_0i. The covariance matrix is taken into account for the evaluation of the final p_i uncertainty. Dynamic system: the measurand, p_i, is given by p_i=(Q_i/S_eff)F_ci where the quantities Q_i, S_eff, F_ci represent the values of the gas-flow-rate at pressure p_i, the effective pumping speed at conductance level and the temperature correction factor. These factors are related to several other input quantities. S_eff(C/(1+C/S_p) is a function of the conductance C and of the pumping speed through the ratio C/S_p. C is modelled mathematically and C/S_p is measured in a separated experiment and is periodically checked. Q_i is generated and measured at each desired pressure p_i in the calibration chamber and depends on several quantities. F_c, related to the temperatures in the calibration chamber and in the flow-meter is also measured at each pressure level. The uncertainty of both the systems is discussed taking into account the possible correlation among all the considered quantities.
11:20 AM		VST-ThM-10 Performance Test of Vacuum Components and System K.H. Chung (Korea Research Institute for Standards and Science) KRISS has established vacuum standards from atmospheric pressure down to ultra high vacuum of 5 x 10-7 Pa, and the leak standards in the range of 5.0 x 10-5 ~ 6.0 x 10-7 Pa-m³/s. The Ultra High Vacuum standards in the range of 5x10-7 ~ 10-3 Pa with a relative uncertainty of 2x10-2 is maintained by dynamic flow method using porous plug technique, and the High Vacuum 10-3 ~ 10-1 Pa with an uncertainty of 5 x 10-3 by dynamic flow method, and Low Vacuum 10-1 ~ 105 Pa by Ultrasonic Interferometer Manometer with an uncertainty of 10-2 ~ 10-5. Now, KRISS is developing a medium vacuum standards by volume expansion method. The market for vacuum equipments in Korea is very big since we have big semi-conductor manufacturers. But most of the vacuum equipments are imported, for our vacuum industry is very weak in technology and very small in size. We circulated Enquetes to the vacuum users, industry, universities, and research institutes, to find out why the Korean industry avoid buying Korean Vacuum products, and most people replied that the lack of reliability and credibility of our products is the most serious problem besides financial difficulties. In order to improve this situation, The Center for Vacuum Technolgy has been established in KRISS. In Center for Vacuum Technology, the integrated comprehensive test and evaluation facilities for vacuum system will be set up and certify the quality and specifications of wide ranges of vacuum gauges and mass flow controllers, all kinds of vacuum pumps, components, vacuum properties such as outgassing, thermal desorption spectroscopy and permeation and diagnostics of plasma in the system. The total testing items are 72 and number of systems is 17. The project has started in October 1999, and will continue to October 2003.