Optical method for low pressure measurements

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  • Optical method for low pressure measurementsI. Bello, S. Bederka, and L. Haworth Citation: Journal of Vacuum Science & Technology A 13, 509 (1995); doi: 10.1116/1.579775 View online: http://dx.doi.org/10.1116/1.579775 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/13/3?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Method of measuring low pressures within evacuated, sealed glass tubes Rev. Sci. Instrum. 51, 1573 (1980); 10.1063/1.1136102 Simultaneous measurements of acoustic pressure and particle velocity by an optical holographic method J. Acoust. Soc. Am. 65, S107 (1979); 10.1121/1.2016922 Measurement of Sound Pressure Amplitude by Optical Methods J. Acoust. Soc. Am. 32, 940 (1960); 10.1121/1.1936576 Measurement of Sound Pressure Amplitude by Optical Methods J. Acoust. Soc. Am. 32, 926 (1960); 10.1121/1.1936502 Optical Methods for the Measurement of the Sound Pressure in Liquids J. Acoust. Soc. Am. 31, 24 (1959); 10.1121/1.1907607

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  • Optical method for low pressure measurementsI. Bello and S. Bederkaa)Department of Material Engineering and Surface Science Western, University of Western Ontario,London, Ontario N6A 5B9, Canada

    L. HaworthDepartment of Electrical Engineering, University of Edinburgh, Edinburgh EH9 3JL, Scotland

    ~Received 24 October 1994; accepted 14 March 1995!

    Pressure measurements from 1023 to 103 Pa were performed by an optical method. In this method,a radio-frequency electrodeless discharge was initiated in a small glass chamber and the dischargeradiation was sensed by a photosensitive element. The radiation intensity, when converted tophotovoltage, was found to increase initially with increasing pressure but, above a certain pressurethreshold, it decreased. Thus, two pressure values were associated with each photovoltage reading.However, it was also found that the direct current~dc! current passing through the radio-frequencyoscillator, which is related to the loss in the electric discharge, could indicate whether thephotovoltage~radiation! calibration data were taken above or below the deflection point pressurein the photovoltage versus pressure curve. Hence, simultaneous measurements of the radiationintensity and dc oscillator current give a unique pressure. This optical, contactless method could beparticularly useful for the determination of pressure in chemically aggressive environments and toprovide automatic pressure stabilization in a radio frequency plasma system. 1995 AmericanVacuum Society.

    I. INTRODUCTION

    A number of methods have been developed for the mea-surement of low pressures. There is, however, no universalmethod of pressure measurement which covers the wholevacuum range from atmospheric pressure down to the lowestachievable pressure in the ultravacuum range. To satisfy thevarious requirements of low pressure measurements, differ-ent physical principles have to be used. In spite of the rela-tively large number of these methods, one could still findgreat difficulties in using them in chemically aggressive en-vironments. The development of a new method or the im-provement of any existing method for the measurement oflow pressures would be an important contribution to vacuumscience and technology.

    The reported optical method for the measurement of lowpressures in a range from 103 to 1023 Pa is based on a mea-surement of the radiation intensity emitted by a radio-frequency discharge. This method belongs to a group of in-direct methods corresponding to discharge gauges. Thisgroup includes quite a few reliable methods, which are de-rived from the original design of Penning.1 Some of thesemethods cover a wide pressure range with very reasonablesensitivity, as reported by Beck and Brisbane.2 Others allowpressure measurements in the ultravacuum range such as thegauge configurations designed by Hobson and Redhead3 andRedhead.4 The present reported method, based on sensingradiation from an electrodeless discharge, is closely relatedto the concept of discharge tubes5 or discharges initiated byTesla transformers. However, the presented optical methodpossesses distinctive features which could be used in somespecial applications.

    Discharge vacuum gauges use the self-sustained dis-

    charges at which the ionization and excitation processes takeplace without special ionization means such as the emittedelectrons from hot filament. The concentrations of ions, elec-trons, and excited particles depend on the concentration ofneutral molecules and thus also on pressure. The concentra-tions of charged and excited particles contribute to measur-able quantities, such as the electric current and the radiationof electric discharge. These macroscopic quantities do nothave simple linear pressure dependencies. For example, theradiation intensity of the electric discharge exhibits a maxi-mum upon varying the pressure. Although the radiation char-acteristic of the electric discharge passes through a maxi-mum, this characteristic can be made unambiguous. Ourdiscussion of the optical method of low pressure measure-ment will include the problem of the complicated ambiguousdischarge characteristics and their utilization for low pres-sure measurement. This optical method of measuring lowpressures increases the applicability of indirect measure-ments of gas pressure and makes it possible to measure pres-sures in cases where it would be impossible to measure themby convention means.

    II. EXPERIMENTAL SETUP

    Experiments were carried out in a special calibrating sys-tem evacuated using a high vacuum pumping station withtwo liquid nitrogen traps to prevent contamination from twoabsolute vacuum gauges, a McLeod gauge and aU-tube oilmanometer. The calibration unit is equipped with a gas dos-ing system which allows the pressure to be set from 53104

    to 1025 Pa. A small cylindrical discharge chamber with adiameter of 10 mm and closed at one end was installed onthe main calibration chamber.

    A radio-frequency discharge was initiated in the cylindri-cal discharge chamber by a standard Hartley oscillator using

    a!Permanent address: Microelectronics Department, Slovak Technical Uni-versity, Ilkovicova 3, 812 19 Bratislava, Slovakia.

    509 509J. Vac. Sci. Technol. A 13(3), May/Jun 1995 0734-2101/95/13(3)/509/6/$6.00 1995 American Vacuum Society Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 84.88.136.149 On: Thu, 27 Nov 2014 14:02:53

  • external ring electrodes. The Hartley oscillator operatedwithin a frequency range from 20 to 37 MHz.

    The radiation of the radio-frequency~electrodeless! dis-charge was sensed by a photoresistor located axially at theclosed end of the cylindrical discharge chamber. The currentpassing through photoresistor, which is proportional to theradiation intensity of electric discharge, was converted to aphotovoltage which was then calibrated as a function of pres-sure. The photoresistor had a relative sensitivity of 0.54 to0.58mm. The direct current~dc! current passing through theoscillator was also measured as a function of pressure. Thevariation of the plasma impedance load with pressure wasreflected in detuning of the base setup frequency of the Hart-ley oscillator. Thus, three variables, photovoltage, oscillatordc current, and frequency, were studied as functions of pres-sure. In addition, different geometric configurations, differentgases, and the influence of impurities were also investigated.

    III. PRINCIPLE

    The radiation of the radio-frequency discharge, when con-verted to photovoltage, shows a strong dependence on pres-sure as shown in Fig. 1. It can be seen from the figure thattwo values of pressure can be assigned to each value of pho-tovoltage~radiation! except for that at the maximum value.The photovoltage curve can be made unambiguous6,7 if thecurve of the dc oscillator current is used. However, the dcoscillator current also exhibits a maximum. Fortunately, thismaximum is shifted towards a boundary of the dischargequenching at higher pressure, and the photovoltage which isproportional to the discharge radiation may be used to deter-mine the pressure in the vacuum system. Thus, the measure-ment of pressure proportional to photovoltage is on the leftof the photovoltage maximum for a dc oscillator currentvalue higher thanI m . On the other hand, the pressure mea-surement is on the right of the photovoltage maximum for adc oscillator current value lower thanI m .

    IV. DISCUSSION OF THE DISCHARGEPARAMETERS

    The success of the present optical method of low pressuremeasurements depends on choosing the proper geometricconfiguration. It can be shown that using a 10 mm cylindricaldischarge chamber with capacitive coupling through two ringelectrodes separated by 40 mm and located 40 mm from theclosed end of the discharge tube gives step changes of pho-tovoltage~U!, frequency detuning~f !, and dc oscillator cur-rent ~I !. Such characteristics, as measured in a residual airatmosphere at a base frequency of 30 MHz, are shown inFig. 2. The origin of these step effects are in the creation ofthree plasmoids which increase with increasing pressure. Theplasmoids join to form one intensive plasmatic formation at a

    FIG. 1. Principle of low pressure measurement using an optical method.U isthe photovoltage andI the dc oscillator current.

    FIG. 2. Pressure threshold at a base frequency of 30 MHz.U is the photovoltage proportional to radiation flux,I the dc oscillator current, andf the frequency.

    510 Bello, Bederka, and Haworth: Optical method for low pressure measurements 510

    J. Vac. Sci. Technol. A, Vol. 13, No. 3, May/Jun 1995 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 84.88.136.149 On: Thu, 27 Nov 2014 14:02:53

  • certain pressure threshold. This way, the plasma load is sud-denly changed and consequently all other parameters followthe change of the new plasmatic formation. Measurement inthe opposite direction, from the higher pressure to lower

    pressure, gives a similar step effect but at a lower pressurethreshold. Thus, within a relatively narrow pressure region,hysteresis~Fig. 3! may be observed.

    A properly set up configuration provides stable dischargeparameters without observation of the step effect or hyster-esis. Only a single plasmatic formation is observed whichfluently increases and decreases with pressure. For example,the combination of a ring electrode located at the closed endof a discharge chamber and a cylindrical electrode 18 mmlong and spaced 55 mm from the first electrode gives stabledischarge conditions in a tube with diameter of 10 mm. Themeasured characteristics~Fig. 4! of photovoltage, frequencydetuning, and dc oscillator current are smooth. The hollowcircles, triangles, and squares in Fig. 4 represent measure-ments from lower pressure~l.p.! to higher pressure~h.p.!,from higher pressure to lower pressure and, after 24 h, fromlower pressure to higher pressure, respectively. The repro-ducibility of the photovoltage measurement corresponding tothe discharge radiation is within 3%.

    Using a higher base frequency of the oscillator leads to ahigher intensity of photovoltage, steeper characteristics ofthe dc oscillator current, and larger detuning of the basesetup frequency by the plasma load~see Fig. 5!.

    Another important variable of the electric discharge is theintensity of the electric field or radio-frequency voltage am-plitude. Higher voltage amplitude corresponds to higher ra-diation intensity of the electric discharge and therefore tohigher sensed photovoltage. The plot of photovoltage withrespect to radio-frequency voltage amplitude give a set oflinear dependencies with various slopes at different constantpressures.

    This configuration, which provides the stable dischargeconditions as described above, was used for measurements

    FIG. 3. Photovoltage hysteresis vs pressure. The dimensions in millimetersof the used geometrical configuration are seen in figure. DC is the dischargechamber, RE the external ring electrodes, R the photoresistor, and P theplasmoid.

    FIG. 4. Discharge characteristics dependent on pressure measured at a properly set up geometrical configuration with indication of reproducibility.U is thephotovoltage,I the dc oscillator current, andf the frequency. Direction of measurement: circles, l.p.h.p.; triangles, h.p.l.p.; and squares, l.p.h.p. after24 h. The geometrical configuration used, with millimeter dimensions, is given within figure. DC is the discharge chamber, RE the ring electrode, CE thecylindrical electrode, R the photoresistor, and P the plasmoid.

    511 Bello, Bederka, and Haworth: Optical method for low pressure measurements 511

    JVST A - Vacuum, Surfaces, and Films Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 84.88.136.149 On: Thu, 27 Nov 2014 14:02:53

  • with different gases. Technical grade gases and spectrallypure neon and helium were used in these measurements. Fig-ure 6 shows that the photovoltage obtained was the highest

    when spectrally pure neon was used. We observed this inspite of the fact that the neon ionization potential~21.5 V! isthe highest of all gases used except for helium. Neon atomsare excited to resonance and metastable states by relativelylow energy electrons, but these resonance states are so ener-getic that radiated resonance energy quanta may cause sec-ondary electron emission from the discharge chamber walls.These secondary electrons are new excitation and ionizationagents. Adding even a small amount of impurities to puregases may cause a change in the electron energy distributionwhich may affect the inten...

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