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.


    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.


    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: 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.


    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 .


    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

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  • 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

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  • 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 intensity of the discharge radiation.This was observed when experiments~Fig. 7! with technicalgrade and spectrally pure helium were carried out.

    In this case, the parameters being measured with respectto pressure are the radiation of the electric discharge con-verted to photovoltage and the dc oscillator current. It wasnoticed that frequency detuning of the oscillator also variedwith pressure. All these variables are output parameters andare closely connected with the primary discharge parameterssuch as the intensity of the electric field, frequency of theelectric field, gas pressure, geometric configuration, and ion-ization and excitation probabilities of the gases. The ioniza-tion and excitation probabilities depend on the gas. A de-tailed investigation of these processes reveals that thissubject is much more complicated than it seems on first con-sideration. To understand these processes in their complexityis possible but to use them for the estimation of pressure ishardly acceptable. Therefore, the indicated phenomena areonly described in a qualitative way.

    It can be found that at higher pressures the diffusionmechanism of the electrical discharge is dominant while at

    FIG. 5. Discharge characteristics at different base frequencies of the oscilla-tor vs pressure. Top:U is the photovoltage proportional to discharge radia-tion; bottom:I is the dc oscillator current.

    FIG. 6. Radiation flux of the electric discharge converted to photovoltage dependence on pressure in different gas environments.

    512 Bello, Bederka, and Haworth: Optical method for low pressure measurements 512

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  • lower pressures the development discharge and its preserva-tion depend on the secondary electron emission. Francis8

    stated that at conditions where electron collision frequency iscomparable to the frequency of the electric field, the com-plex plasma conductivity has an inductive character and theabsorbed power in plasma, dependent on the real part of theplasma load, is





    whereN is the volume electron concentration,q is the el-emental charge,E is the effective electric field,m is theelectron mass,Z is the electron collision frequency, andv isangular frequency of electric field.

    The electron collision frequency varies with pressurewhich obviously implies that the absorbed power, given bythe equation above, is a function of pressure. If the absorbedpowerP exhibits maximum then the first derivation accord-ing to the collision frequencyZ of the absorbed power inplasma equal to zero has to give the position of the maxi-mum. The calculation shows that maximum power is con-sumed when the electron collision frequency is equal to theangular frequency of the electric field~Z5v!. Using theequation of mean free path for electronsLe , the collisionfrequency isZ5v/Le . If the electron velocityv is deter-mined from the electron temperature measurements, then thepressure position of the maximum may be found. The ab-sorbed power is linked to the dc oscillator current by replac-ing all losses in plasma. This implies that the dc oscillatorcurrent has to pass through the maximum.

    It was shown that the radiation of the electric discharge,when converted to photovoltage, varies with pressure. Athigher pressures an electron mean free path is so small thatthe electrons are able to gain only a small amount of energyalong the short paths in a given electric field. With decreas-ing pressure both the electron mean free path and the elec-tron energy gain increase, which means that neutral mol-ecules are more efficiently ionized and excited by electrons.The increase of the measured photovoltage then follows. Atvery low pressures, although the electron mean free path andenergy gain are large, the ionization and excitation processesare inefficient due to the low concentration of neutrals. Thus,photovoltage must pass through a maximum and then de-crease with dropping pressure. The photovoltage curve isvery similar to that presented by Fabricant,9 who measuredradiation fluxes corresponding to the wavelengths in a mer-cury low pressure discharge. Dopel and Kuhlke10 also foundsimilar characteristics of the rotationvibration spectra ex-cited by a laser beam.

    The frequency detuning originates in a plasma impedancehaving an inductive character. The plasma impedance is ca-pacitively connected in parallel with the resonance circuits ofthe oscillator. Therefore, the oscillator frequency detuneswith the plasma impedance load.


    On the basis of experimental experience, an electrodelessvacuum gauge was made11 as a compact system. The blockdiagram is shown in Fig. 8. The rf electric power generated

    FIG. 7. Influence of impurities on photovoltage reading in dependence on pressure demonstrated on spectral pure helium~spHe! and technical gradehelium ~tHe!.

    513 Bello, Bederka, and Haworth: Optical method for low pressure measurements 513

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  • in oscillator OC is taken to probe P supplying the power intoplasma. Probe P also senses the discharge radiation by aphotosensitive element. The photoelectric signal from probeP is taken to amplifier A and the measurement device for theevaluation of the photovoltage signal. In order to be able todetermine the pressure unambiguously, the comparator IC asan indication circuit was used. This circuit comparesI m cur-rent, derived by calibrating for the gas in use and corre-sponding to the maximum of the photovoltage curve, withthe real dc current flowing through the oscillator. This way anew signal is obtained which drives a light-emitting diode~LED! indicating the l.p. side or a LED indicating the h.p.side of the photovoltage calibration curve.

    The power is distributed to each electronic block frompower supply PS.


    The processes of ionization, recombination, excitation,and subsequent photon emission are very complicated evenin those cases when individual emission lines are sensed bydetectors. To express the spectral radiation flux, the probabil-ity of electron transitions within atoms and the effective ex-citation cross sections have to be known in the context ofvarying electron energies with pressure change because theradiation flux is sensed by a photosensitive element having acertain relative sensitivity. Thus, the photosensitive elementdiscriminates some wavelengths of the discharge radiation.Nevertheless, it can be shown that photosensitive elementswith different spectral sensitivities to those presented givesimilar results for the reported gases. For example, a pho-totransistor having the spectral sensitivity distributed around0.85mm gave very similar results.

    The optical method of low pressure measurement hasbeen classified as an indirect method within the group ofdischarge ionization methods. The photovoltage characteris-tics have to be calibrated by absolute methods and the dcoscillator current corresponding to the maximum of the pho-tovoltage characteristics has to be found by calibration.Then, the value of the dc oscillator current corresponding to

    the maximum has to be set up for comparison with the actualdc oscillator current to determine the low or high pressureside of the calibrated photovoltage curve.

    Some indirect methods having linear calibrated character-istics, when they are once calibrated, allow one to predict thepressures of different gases using the values of relative sen-sitivities. Such an approach can hardly be used in the case ofthe optical method where the calibration curve is nonlinear.The described method is highly selective. The gas in whichthe pressure measurement takes place has to be known. Im-purities may cause the production of false data as indicatedby measurements with spectrally pure and technical gradehelium. It is obvious that impurities are the problem of allindirect methods of pressure measurement.

    It was recognized that the geometric configuration cannotbe arbitrary; otherwise the step changes and hysteresis of thecalibrated parameters may be observed. Only a properly setup configuration provides the stability of electric dischargeand photovoltage data with 3% reproducibility.

    Since the pressure measurement is a process of an inter-action between the measurement environment and device,this interaction influences the accuracy of measurement andgas composition in the measured gas environment. In dis-charge ionization gauges and also in a case of the opticalcontactless method, some irreversible processes, as creationof oxygennitrogen compounds or polymerization of hydro-carbons, may take place. In this interaction, desorption andsorption phenomena may be also observed. Thus, some prob-lems may be expected such as polymerization on the glasswall. The sediment on the wall, as a result of polymerization,may reduce the radiation intensity of the electric dischargesensed by a photosensitive detector. It should be noted thatfor reliability verification of the optical method of low pres-sure measurement only electropositive and inert gases wereused. No experiments have been carried out in a vapor envi-ronment with complicated molecules.

    The optical method of low pressure measurement couldbe used in some special applications. This optical contactlessmethod can provide low pressure measurement in perma-nently closed vacuum systems such as solar collectors orrotary vacuum systems. It could be used in chemically ag-gressive environments or for automatic pressure stabilizationin radio-frequency plasma and ion sources.

    1F. M. Penning, Physica4, 71 ~1937!.2A. H. Beck and A. D. Brisbane, Vacuum2, 137 ~1952!.3J. P. Hobson and P. A. Redhead, Can. J. Phys.36, 271 ~1958!.4P. A. Redhead, inProceedings of the 5th American Vacuum Symposium~Pergamon, Oxford, 1959!, p. 148.5A. Roth,Vacuum Technology, 3rd ed.~Elsevier, New York, 1990!, p. 310.6S. Bederka and F. Kolenic, inProceedings of the 6th CzechoslovakianConference on Microelectronic and Vacuum Physics, edited by M. Vesely,J. Breza, and R. Harman, Bratslava, August 1976~ES SVST, Bratislava,1976!, Vol. 2, p. 91.7S. Bederka and F. Kolenic, Czechoslovakian Patent No. 182 157~1980!.8G. Francis,Ionization Phenomena in Gases~Butterworth, London, 1960!.9V. A. Fabricant, Tech. Phys.5, 864 ~1938!.10E. Dopel and D. Kuhlke, Czech. J. Phys. B28, 141 ~1978!.11I. Bello, J. Hribik, and P. Pribilsky, Czechoslovakian Patent No. 219 667


    FIG. 8. The block diagram of the electrodeless vacuum gauge. OC is theoscillator, P the probe, IC the indication circuit, A the amplifier, PS thepower supplies, h.p. the LED indicating the high pressure side, l.p. the LEDindicating the low pressure side of the photovoltage calibration curve, andDCH the discharge chamber.

    514 Bello, Bederka, and Haworth: Optical method for low pressure measurements 514

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