Real time sensor for monitoring oxygen in radio–frequency plasma applications

Real time sensor for monitoring oxygenin radio–frequency plasma

applications

V. Milosavljevica,b, R. Faulknera and M. B. Hopkinsa

a NCPST, School of Physical Sciences, Dublin City University, Dublin 9, Irelandb Faculty of Physics, University of Belgrade, P.O.B. 368, Belgrade, Serbia

[email protected]

Abstract: Real time closed loop control of plasma assisted semiconductormanufacturing processes has received significant attention in recent years.Therefore we have developed and tested a customized optical sensor basedon buffer gas (argon) actinometry which has been used to determine relativedensities of atomic and molecular oxygen in an Ar/O2 radio–frequencyICP chamber. The operation and accuracy of our optical sensor comparedfavorably with a high resolution commercial spectrometer but at lower costand exhibited improved actinometric performance over a low resolutioncommercial spectrometer. Furthermore, threshold tests have been performedon the validity of buffer gas based actinometry in Ar/O 2 ICP plasmas whereAr is no longer a trace gas through Xe actinometry. The plasma conditionsfor which this customized optical sensor can be used for closed loop controlhave been established.

© 2007 Optical Society of America

OCIS codes: (120.4570) Optical design of instruments; (300.2140) Emission; (040.5160) Pho-todetectors; (120.2440) Filters; (130.6010) Sensors

References and links1. D. Lee, G. Severn, L. Oksuz and N. Hershkowitz, ”Laser–induced fluorescence measurements of argon ion

velocities near the sheath boundary of an argon-xenon plasma”, J. Phys. D: Appl. Phys. 39 5230-5235 (2006).2. T. Lee, W. G. Bessler, C. Schulz, M. Patel, J. B. Jeffries and R. K. Hanson, ”UV planar laser induced fluorescence

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fuel-air ratio measurements”, Appl. Phys. B: Lasers & Optics 80/2,147–150 (2005).4. J. Amorim, G. Baravian, J. Jolly and M. Touzeau, ”Two–photon laser induced fluorescence and amplified spon-

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#84460 - $15.00 USD Received 25 Jun 2007; revised 15 Aug 2007; accepted 11 Sep 2007; published 8 Oct 2007

(C) 2007 OSA 17 October 2007 / Vol. 15, No. 21 / OPTICS EXPRESS 13913

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Wiley), (1994).14. S. Fujimura, K. Shinagawa, M. Nakamura and H. Yano, ”Additive Nitrogen Effects on Oxygen Plasma Down-

stream Ashing”, Jpn. J. Appl. Phys. 29/10 2165–2170 (1990) .15. A. Granier, D. Chereau, K. Henda, R. Safari and P. Leprince, ”Validity of actinometry to monitor oxygen atom

concentration in microwave discharges created by surface wave in O2–N2 mixtures”, J. Appl. Phys. 75/1 104–114(1994).

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1. Introduction

The use of plasma processes is an important step in the fabrication of semiconductor devices.The semiconductor industry requires high yield, performance and throughput of its devicesthroughout the manufacturing process. Of critical importance to the industry is the eliminationof process drift within the etch and deposition processes used to manufacture such devices,either within one chamber or across a range of identical chambers. Off–line metrology of se-lected semiconductor wafers which have passed through such chambers is commonly used todetermine process drift along a manufacturing line but this is expensive and time consuming.

Real time control of plasma assisted semiconductor manufacturing processes is seen as aviable alternative to off–line metrology and could greatly improve process yield and perfor-mance. One possible control strategy, which is referred to as the plasma parameter controlstrategy, may reduce plasma process disturbances and drifts. This strategy aims to control para-meters internal to the plasma itself e.g. electron density, ion flux to a surface or radical speciesdensity as opposed to external process variables such as gas flow, rf power, chamber pressureetc. This control methodology has yet to be demonstrated and as a first step, a proof of principleexperiment was required to be performed on a relatively simple plasma process with an Ar/O 2

mixture operated in a laboratory inductively coupled plasma chamber. In order to perform suchcontrol experiments, non–invasive plasma diagnostics that can respond quickly to changes inplasma state are required which can operate at high speeds and be compatible with the surround-ing control infrastructure and protocols. Plasma diagnostics such as laser–induced fluorescence(LIF), optical absorption spectroscopy, optical emission spectroscopy and actinometry fulfillthis control criteria to varying degrees.

Laser–induced fluorescence [1, 2, 3, 4] and optical absorption spectroscopy are very powerfultechniques used to determine species density in a plasma. These techniques allow measurementof the density of ground state atoms of a chosen species present in a plasma. Both techniquesare very powerful but their application in an industrial context is unlikely due to the complex-ity of the laser and optical systems required. This problem can be solved if we employ opticalemission spectroscopy (OES), which is a relatively simple technique to implement in an indus-



trial setting. However, OES measurements provide information about excited electronic states,which are not necessarily related to the density of atoms in the ground state [5, 6, 7].

Optical actinometry is an OES technique which is widely used for in–situ monitoring of spa-tial and temporal variations of atomic and molecular concentrations [8, 9, 10]. This techniqueuses the addition of a small amount of gas in the discharge e.g. argon where the intensitiesof its spectral lines are known to be representative of the excitation mechanism. Argon is fre-quently chosen as the actinometer gas because the transition 3s23p54s–3s23p54p from multiplet2[1/2]o

1–2[1/2]0 at 750.387 nm is insensitive to two step excitation [11]. The excitation of the

2[1/2]0 4p level in Ar I is 13.48 eV [12] which is higher than the excitation threshold for the4So 3p level in O I of 10.74 eV. A well known condition for actinometry is that the excited stateof the actinometer (in our case argon) should have nearly the same energy as an excited state ofthe species of interest (i.e. oxygen). However there is a 2.7 eV difference between oxygen andargon electronic thresholds and thus it is clear that this condition is not satisfied. Nevertheless,if the remaining conditions for actinometry are fulfilled [13], then a difference between the en-ergies of the excited states of argon and oxygen is not so critical and therefore Ar I 750.387 nmand O I 777.417 nm can be used for actinometry purposes [14, 15]. Comparison of the emittedline intensity for the desired species under examination (O, O2) with the intensity of an emittedline of argon, allows one to eliminate the influence of line intensity changes due to excitationconditions and evaluate the real behavior of the emitted line intensity due to the changes in thespecies ground state concentration. The relative density of atomic oxygen is monitored by cal-culating the ratio of the 3s–3p line intensity at 777.417 nm for O I and the 4s–4p line intensityat 750.387 nm for Ar. Similarly the relative density of molecular oxygen is determined by theratio of the intensity of the B3Σ →X3Σ molecular band head at 417 nm [16] and the 750.387 nmemission line for Ar. To this end, we have developed a customized optical sensor for oxygen inan Ar/O2 discharge which measures the emitted intensity at these specific wavelengths in orderto determine the density of atomic and molecular oxygen. This diagnostic can easily be usedfor the implementation of closed loop control of atomic and molecular oxygen density in thedischarge.

However, the use of a trace gas as an actinometer in an industrial setting is unlikely as the re-quirement for an additional gas line would be outweighed by cost considerations. Many plasmaprocesses in the semiconductor industry dilute the chemically reactant gases by using a buffergas such as Ar or He. The ability to use this buffer gas as the actinometric gas would negate therequirement for additional trace gases and therefore allow this technique to be applied in a man-ufacturing process. It must be stated, however, that the validity of actinometry [17] is somewhatcontroversial and the criteria for the utilization of the technique and its limits of validity mustbe verified in each case. Actinometry becomes invalid if the excited state from which emissionis being detected has not been created by electron impact excitation from the ground state. Thevalidity of actinometry using excited oxygen and argon atoms was investigated by Walkup etal [18]. They found that the actinometric determination was well correlated with the variationof atom concentration up to 50 Pa in RF O2-CF4 plasmas, but discrepancies occurred in pureO2 plasmas. In addition, it was shown by Booth et al [19] that the ratio of the intensities ofthe oxygen line to the argon line, was poorly correlated to the oxygen atom concentration inElectron Cyclotron Resonance (ECR) plasmas operated at ultra–low–pressures (0.1–0.8 Pa).

It is because of this cautionary note that extensive validation measurements have been per-formed on our system which uses argon as the buffer gas. The validation procedure uses theactinometric technique itself as trace amounts of Xe gas have been added to the Ar/O 2 dischargeto confirm the applicability of the technique for a range of Ar/O 2 discharges. The validity ofusing the buffer gas as the actinometer should be manifested as an agreement between the argonand xenon actinometry data under a range of plasma conditions. Once this validation procedure



is complete, we no longer use trace gas Xe in the process and can revert back to actinometrymeasurements using the buffer gas of the process (argon).

2. Experiment

2.1. Basic radio-frequency inductive source

The plasma chamber used in this campaign is the BARIS (BAsic Radio-frequency InductiveSource) system. The BARIS discharge chamber consists of a water-cooled helical antennawhich is isolated from the discharge by a quartz dielectric tube and is centered co-axially insidea cylindrical stainless steel discharge chamber as shown in Fig. 1. The antenna is driven at 13.56MHz using a radio–frequency generator at powers from 25–300W and uses a standard ’L’-typeimpedance matching unit in automatic matching mode to ensure maximum power transfer fromthe generator to the plasma.

The discharge chamber itself is a stainless steel cylindrical vacuum chamber of internal di-ameter 200 mm and length 860 mm and has several vacuum ports to allow for diagnosticaccess. The chamber is evacuated using a turbomolecular pump and a rotary vane pump whichgives a base pressure of the order of 5× 10−5 Pa (5× 10−7). Chamber pressures from 1.33Pa(10 mTorr) to 13.3Pa (100 mTorr) are achieved using a stepper motor driven gate valve posi-tioned above the turbomolecular pump. Gas flow into the chamber is controlled via digital massflow controllers (MFCs) which precisely determine gas content in the chamber. At these pres-sures, electron densities of the order of 1010 cm−3–1011cm−3 are common in this system forrf input powers from 25–300 W. In fact the large parameter space required of this work meantthat a Design of Experiment (DOE) had to be implemented i.e RF power: 25–300 W, Ar gasflow rate (7–350 sccm), O2 gas flow rate: (2–100 sccm) and chamber pressure: 10–100 mTorr.The application of this DOE reduced the number of experimental runs to 48.

Fig. 1. Schematic diagram of the BARIS chamber and associated diagnostics.

2.2. The customized optical device

The requirement for a real time, practical and easy to use sensor for oxygen detection in aplasma processing environment resulted in the construction of the customized optical devicedetailed schematically in Fig. 2. This device may be applied to process chambers in the semi-



conductor industry as it is much cheaper to construct and easier to use than many high res-olution spectrometers available on the market. Relatively cheap low resolution spectrometersare available but, as is discussed later, such devices can have a limited range of operation foractinometry purposes.

The actinometric technique applied here requires the recording of the intensity of emissionlines at three specific wavelengths i.e. 750.387 nm for Ar I, 777.417 nm for O I and 417 nm forO2. Our customized optical device operates on the principle that the polychromatic light emittedfrom the plasma enters the device and is split into the three wavelengths of interest which fallwith approximately equal intensity onto their associated optical filters/photodiode detectionchannel. Each optical filter (OF), manufactured by LOT Oriel, has a very narrow bandwidth(FWHM: OF1=0.67 nm, OF2=0.71 nm, OF3=0.57 nm), centered at a specific wavelength asshown on Fig. 2 and is therefore only sensitive to its particular species’ emission line. Theanalog voltage from each photodiode detection channel is proportional to the emitted intensityat that wavelength which can be amplified via the photodiode’s amplifier circuitry. The plasmalight was collected by this device from a region about 2 cm in diameter and its entrance portwas positioned about 16.5 cm above the antenna itself.

To enable direct comparison between DC signals from different photodiodes and opticalfilters with different transmission coefficients (OF1=0.77, OF2=0.74, OF3=0.64), a calibrationprocedure was performed on this optical device. The calibration has been carried out using aGigahertz–Optik BN–0102–1 Reference Standard source [20].

Fig. 2. Schematic diagram of the customized optical device: BS - Beam splitter, M - mirror,OF# - Optical filter (central wavelength), L - Lens (focal distance), B - Batteries, PD -Photodiode

Calibration procedures have also been performed to compare the operation of our customizedoptical device with two commercial spectrometers. The first spectrometer is a Carl Zeiss PGS-22 m focal length spectrometer with a 1302 lines/mm grating which operates from 200 to1400 nm and has an intensified–CCD (iCCD) camera system, which is sensitive from 190 nmto 900 nm, positioned at its focal plane. The PGS-2/iCCD optical system has very high reso-lution of 9.17 pm/pixel at λ=200 nm and 7.5 pm/pixel at λ=900 nm at the optimum entranceslit width of 15μm. Although the PGS-2/iCCD optical system has very high resolution, it isbulky, difficult to operate and align and has just 5 nm across the CCD detector at any one time.An alternative spectrometer is the Ocean Optics USB2000 fibre optic spectrometer which isa low resolution spectrometer with a focal length of 42 mm, a 600 lines/mm grating which



operates from 200 to 875 nm. The resolution of the USB2000 is 0.555 nm/pixel at λ=200 nmand 0.536 nm/pixel at λ=875 nm. This compact spectrometer is easy to operate with no opticalalignment required and can display the entire spectrum from 200 to 875 nm at any one time.Both spectrometers use the same multimode fibre with core diameter of 200 μm and lengthof 2 m positioned at its slit entrance. A collimator lens at the other end of the fibre is used tocollect light from plasma.

Actinometry experiments were performed in Ar/O2 plasmas under the 48 different plasmaoperating conditions determined by the DOE calculation. Actinometry threshold tests wereperformed with xenon as a trace gas (2% of total pressure) in the Ar/O 2 plasmas to identify theplasma conditions for which buffer gas actinometry with argon is no longer valid as the argoncontent in the plasma has exceeded a validity threshold.

3. Results and Discussion

3.1. Comparison of the low and high resolution spectrometers for actinometry

If the USB2000 low resolution spectrometer were to be used for actinometry measurementsin an Ar/O2 plasma, it would need to exhibit the same behavior as the PGS-2 high resolu-tion spectrometer after the appropriate calibration procedures [20]. Fig. 3 shows a typical op-tical spectrum recorded by both spectrometers in the region of the actinometric lines for O I(777.417 nm) and Ar I (750.387 nm) for an Ar/O 2 plasma. As can be seen in the plots on theleft side of Fig. 3, there are additional lines around the actinometric wavelengths of interestas measured by the PGS-2 spectrometer which are summed and appear as one broad emissionpeak in the USB2000 spectrometer shown on the right side of Fig. 3.

Fig. 3. O I spectral lines recorded by the PGS–2 spectrometer (top left) and by the USB2000spectrometer (top right). Ar I spectral lines recorded by the PGS–2 spectrometer (bottomleft) and by the USB2000 spectrometer (bottom right).

For comparative purposes, we have selected a reduced range of plasma operating conditionsfrom the original DOE dataset of 48. The reduced dataset of 18 encompasses the extremes of rfpower/pressure in the BARIS system allowing us to investigate in more detail how the actinom-etry peak intensities at 777.417 nm and 750.387 nm behave as measured by both spectrometers.



The intensity of each spectral peak was measured by determining the area under the line of in-terest as summed over a spectral region from -2.5 full width half maximum (FWHM) step sizeup to +2.5 FWHM step size from the spectral line peak central position. If the spectral line wasclearly isolated from other emission lines, then the intensity summation was straightforward asit is just a summation over ±2.5 FWHM step sizes from the spectral line peak central position.However, if other lines existed in close proximity to the spectral line of interest, as was the casefor the O I 777 nm triplet, then the intensity summation had to neglect the intrusive line andthe summation was performed down to the background continuum that existed in that spectralregion.

The intensity of each line of the triplet around 777 nm in O I measured with the PGS-2spectrometer exhibits similar behavior with changing rf power/pressure over the plasma oper-ating conditions tested here. A similar observation was made for each line of the Ar I ”doublet”around 750 nm. This infers that although the actual actinometry spectral line is not resolved bythe USB2000 spectrometer, the measured broad peak intensity can by multiplied by a certainfactor to give the actual intensity of the actinometric emission line within the particular unre-solved peak. This factor was calculated to be 0.31 for the O I line at 777.417 nm and 0.42 forthe Ar I line at 750.387 nm.

The ratio of the measured and inferred intensity of the Ar I line determined from the PGS-2and USB2000 spectrometers should therefore be constant over all the plasma operating condi-tions. Similarly a fixed but different ratio should also be observed for the O I line. However, asthe left side of Fig. 4 indicates, the line intensity ratio in each case is not actually constant, evenfor the restricted set of plasma operating conditions used here. This infers that one needs to bevery careful when using a low resolution spectrometer, such as the USB2000, for actinometrypurposes as it works only under certain experimental conditions. This discrepancy between thelow and high resolution spectrometers for certain plasma conditions results from the variationin the continuum emission measured by each spectrometer and the contribution of unwantedspectral lines to the peak profile recorded by the low resolution spectrometer. This continuumemission varies with pressure and rf power in the plasma leading to an inaccuracy in the factorused to determine the actual peak intensity of the desired actinometry line in the broad peakprofile.

To quantify the data comparison of the low resolution spectrometer and customized opticaldevice with the high resolution spectrometer, as shown in Fig. 4, the root mean square (RMS)value of the ratio has been calculated for each case. The RMS value for the USB2000/PGS-2ratio for Ar I was 3.8 × 10−3 and for O I was 7.4 × 10−3. The RMS value for the customizedoptical device/PGS-2 ratio for Ar I was 4.7 × 10−4 and for O I was 5.1 × 10−4. This wouldindeed indicate the data associated with the customized optical device is more reliable than alow resolution spectrometer when compared with an optical spectrometer of high resolution.

In addition, the 4p–4d spectral line of Ar I at 751.041nm (see Fig. 3) makes a significantcontribution to the broad unresolved profile at 751 nm in the argon spectrum at higher rf powers.Thus three lines are contributing to the unresolved peak profile instead of the two that werepreviously assumed to be involved when the factor was calculated. Other unwanted spectrallines are seen in the cluster of three lines around 778 nm in the top left pane of Fig. 3. Theselines are not from the oxygen spectrum and are in fact impurity lines from the plasma discharge.

The reduced operating space of the low resolution spectrometer for actinometry measurementnecessitated the development of a customized optical device which could be used for real timecontrol of O I and O2 species density in an rf ICP plasma.



Fig. 4. Comparison of the low resolution spectrometer (left) and customized optical device(right) with the high resolution spectrometer. Full squares represent the intensity ratio ofthe argon line. Open circles represent the intensity ratio for the oxygen line. The dashedlines represent the mean value of line intensity ratio in each case. The error bar is indicativeof the reproducibility of measured values under the same experimental conditions.

3.2. Comparison of the customized optical device with a high resolution spectrometer foractinometry

The limited range of use of the USB2000 spectrometer for actinometry purposes, resulted in theconstruction of the customized optical device, as depicted in Fig. 2. To test the reliability of thisdevice, a comparison was again made of its measured intensity with that of the high resolutionPGS-2 spectrometer. The ratio of the intensities of the Ar I and O I lines was calculated and isshown on the right side of Fig. 4.

The intensity ratio is much flatter for both the Ar I and O I lines than that observed previouslywith the low resolution spectrometer (see Fig. 4 - left side). Therefore the intensity measure-ments made with the customized optical device are similar to the PGS–2 and thus reliableactinometry measurements can be made without the need for bulky spectrometers that are dif-ficult to align. Furthermore, the customized optical device delivers a signal that is directly pro-portional to intensity (area) of the spectral line. Thus no additional recording software, spectral”line” deconvolution procedures or theoretical line profile fitting techniques need to be appliedto the measured emission line profiles. Improved actinometric performance may be achievedwith a commercial compact spectrometer of moderate resolution which could be optimized foroperation in a specific spectral region e.g. 750-778 nm. However, even in this scenario, theoptimized compact spectrometer would still require additional and time consuming line pro-file fitting procedures to be employed when compared with our customized device. Fig. 4 alsoshows that the absolute value of ratio between our device and the PGS–2 spectrometer is higherthan that of the USB2000/PGS–2 spectrometer of Fig. 4. This infers another advantage asso-ciated with our optical device over a low resolution spectrometer as it can be used to recordspectral lines of low intensity. Obviously, spectrometers can record low intensity spectral linesby increasing integration time but this will also lead to an increase in the continuum measuredwhich would also contribute substantially to the unresolved peak profile leading to a miscalcu-lation of the peak intensity of the line under examination.

The customized optical device can be used for actinometric purposes to determine O I andO2 densities in an Ar/O2 plasma and so the next step was to investigate precisely where argonactinometry is valid in a system where Ar is no longer used in trace amounts as is the case inthese experiments. The actinometry threshold tests are detailed in the following subsection.



3.3. Threshold tests for argon actinometry

Optical actinometry with argon as the actinometer was required to determine the relative atomicand molecular oxygen concentration in an Ar/O 2 plasma where argon is no longer used in traceamounts. This is not the usual manner in which Ar actinometry is applied as it is normallyintroduced in trace amounts in the plasma under investigation. Thus another technique wasrequired to determine the validity of Ar actinometry in an environment where the content ofargon varies considerably. The technique chosen to test this principle was xenon actinometrywhere Xe is added in trace amounts (about 2% of the total pressure) to the Ar/O 2 plasma.

Fig. 5. Optical emission spectrum from an argon–oxygen plasma (bottom). Optical emis-sion spectrum from an argon–oxygen plasma with xenon as a trace gas (top). The relevantactinometry lines are indicated for clarity in each panel. The summed intensity of the Ar/O2and Ar/O2/Xe spectra differed by 15%.

It was essential that as little xenon as possible be introduced into the argon–oxygen plasmai.e. the requirement was to be able to record the xenon actinometry line at 834.7 nm whilstensuring that the entire argon–oxygen emission spectrum was not altered. There are two xenonactinometry lines that are frequently used: (1) 834.682 nm from the 6s–6p transition and (2)828.012 nm from the 6s–6p transition. We chose to use the Xe I line at 834.682 nm as theactinometry line as it does not overlap with any argon or oxygen spectral lines in that region.

The broadband spectral sensitivity of the USB2000 spectrometer meant that it could veryeasily be used to test for any disturbance in the argon–oxygen optical spectrum under all of the48 plasma conditions required as part of our original design of experiment (DOE). Two sets ofoptical data were then recorded for each of the 48 experimental runs, the first set was in Ar/O 2

plasma only and the second set with 2% of xenon added to the Ar/O 2 plasma. Fig. 5 shows therecorded spectra for one such argon–oxygen and argon–oxygen–xenon plasma and reveals howsimilar the two spectra are with the only apparent difference being the Xe I line at 834.7 nm.Hence we can be assured that xenon can reliably be used as our actinometer reference for thethreshold tests required for argon actinometry in Ar rich plasmas.

The actinometry results with argon as the actinometer were then compared with those with



Fig. 6. Actinometry results by argon (full lines) and by xenon (broken line). OIAr and O2Arrepresent densities of atomic oxygen and molecular oxygen, respectively, as determined byargon actinometry. OIXe and O2Xe represent densities of atomic oxygen and molecularoxygen, respectively, as determined by xenon actinometry. The error bars are indicative ofthe reproducibility of the data.

xenon as the actinometer. The validity for actinometry by argon in the Ar/O 2 plasmas is studiedby monitoring the point when the relative atomic oxygen and molecular oxygen determined byAr and Xe actinometry begins to diverge. Fig. 6 shows a selection of such actinometry measure-ments as the percentage of argon in the argon–oxygen plasma and argon–oxygen–xenonplasmais increased from 0 to 100% of total flow. This plot shows some of the limiting cases where Aractinometry is valid e.g. Ar actinometry is not valid when the total Ar flow is greater than 45%at low rf powers and pressures (25 W, 1.3 Pa). The validity of argon actinometry at higher rfpowers and pressures (300 W, 13.3 Pa) is restricted to cases where Ar flow is less than 20%of the total flow into the chamber. In fact, Fig. 6 shows that for the range of rf powers andpressures used in this experiment, Ar actinometry is only valid in Ar/O 2 plasmas when the Arflow is less than 20% of the total flow.

The atomic oxygen and molecular oxygen densities were calculated to be of the order of1018 m−3 – 1019 m−3 for the Ar/O2 plasma conditions where Ar actinometry was valid. Thesedensities were determined using all the relevant actinometry data [13, 21] and assuming a gastemperature, according to previous investigations, which changes linearly with RF power from300 K (the reactor-wall temperature) at 0 W up to 450 K at 300 W. The calculated value forthe densities of atomic and molecular oxygen are within the expected limits for the powers andpressures used in the plasma chamber and are typical for many similar RF plasmas [22].

From the above discussion it is clear that a customized optical device can be used for buffergas actinometry purposes in an Ar/O2 plasma when the Ar content is below a certain threshold.As a general rule validation of the use of a particular buffer gas for actinometry purposes isrequired. The customized optical device is a viable alternative to a high resolution spectrometer



for the measurement of atomic and molecular oxygen densities in such plasmas. It is envisagedthat this sensor will be used for real time measurement of species concentration which can beused to achieve active species control (O, O2) in a radio–frequency inductively coupled plasma.

4. Conclusions

A real time sensor for atomic and molecular oxygen in a radio–frequency inductively coupledplasma has been developed. This sensor uses buffer gas actinometry where argon is no longer atrace gas to determine relative atomic and molecular oxygen concentrations in an Ar/O 2 plasma.The validity of using the buffer gas as the actinometer has been determined by a one–off setof actinometric measurements with xenon gas used in trace amounts. This sensor exhibits im-proved performance over a low resolution spectrometer which cannot always be used for preciseactinometric measurements. This sensor is also seen as a practical alternative to more cumber-some, expensive and complex high resolution optical spectrometers for radical species densitymeasurement.

Acknowledgments

This work is a part of the projects: No. 02/IN.1/I147 founded by Science Foundations Ire-land, EURATOM contract FU06-CT-2004-00068 founded by European Union, ”Measurementof atomic Oxygen in an industrial plasma etcher” and project ”The use of non invasive OESas method for the determination of gas species concentration and the main plasma parametersin industrial plasma processing equipment” founded by Enterprise Ireland. V. Milosavljevic isgrateful to the Ministry of Science and Environment Protection of the Republic of Serbia un-der grand No.: OI141031 ”Nonlinear dynamical phenomena in photorefractive media, liquidcrystals, plasmas and left–handed materials”.



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Real time sensor for monitoring oxygen in radio–frequency plasma applications

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