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Detection of CH3 radicals in an RF CH4H2 plasma by photoionization mass spectrometry
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Vacuum/volume
Pergamon PII : s0042-207x(97)00146-2
49/number 2Ioaaes 113 to 120/1998 0 199’8 flsevier Science Ltd All rights reserved. Printed in Great Britain 0042-207X/98 $19.00+.00
Detection of CH3 radicals in an RF CH4/H2 plasma by photoionization mass spectrometry S Ando,” M Shinoharab and K Takayama,b aDepartment of Physics, School of Science, Tokai University, Kitakaname, Hiratsuka, Kanagawa 259-12 Japan; bResearch Institute of Science and Technology, Tokai University, 1117 Kitakaname, Hiratsuka, Kanagawa 259- 12 Japan
1117
received 2 1 October 1997
To investigate processes occurring during thin-film deposition by plasma chemical vapor deposition, a study was made of the radical species in an RF CHdH* plasma using photoionization mass spectrometry. The method avoids fragmentation of particles as can occur in conventional mass spectrometry by using photons for their ionization. In the present experiment, a capacitive 13.56-MHz RFplasma reactor was used and the total gas pressure was 2.7x lb Pa. Besides the CH, radical on whose detection emphasis was laid, CH,, CH, C,H, and C,H, were detected in the excited CH$H* plasma using a quadrupole mass spectrometer with a Kr or Ar resonance lamp. The lamp was microwave-operated at 2.45 GHz to produce the ultraviolet resonance radiation with two components of energies 10.03 and 10.64 eV in Kr or 11.62 and 11.83 eV in Ar. Suppression of undesirable ionization of particles by impact of accelerated photoelectrons generated by lamp radiations was achieved by voltage control of the ion-lens electrodes. The detection sensitivity of the apparatus was estimated as having a CH, number density of typically2.0 x IO” cmm3. Data were obtained on the dependence of CH, radical density upon the CH, mole fraction of the CH+JH* source gas. 0 1998 Elsevier Science Ltd. All rights reserved
Introduction Plasmas are used in many techniques for thin-film deposition and surface processing, and neutral radicals in an ionized gas may play an important role in a given process. Thus it can be essential both for understanding a process mechanism and its control to have knowledge of the nature of the radicals in the excited gas. Methods of detecting neutrals in a plasma have been of two kinds : optical spectroscopy and mass spectrometry. The former includes optical emission spectroscopy which has been widely used and is a simple method for detecting excited species without plasma disturbance. This passive method is inapplicable to nonemissive species lying in ground states or in excited states with low emissive transition probabilities. For them active methods with external radiation sources must be adopted. Laser techniques such as dye-laser absorption spectroscopy, semiconductor-laser absorption spectroscopy and laser-induced fluorescence spectroscopy are now often employed for detecting non-emissive species.’ The major advantages of the use of laser are in both high sensitivity and spatial resolution. These techniques, however, require a large-scale and expensive apparatus, and also rather advanced expertise. Fortunately, mass spectrometry is technically simple and measurement is possible on a small scale with fairly inexpensive apparatus. Moreover, it has the advantage that one can detect and identify particles irrespective of their being emissive or non-
emissive. To detect neutrals by mass spectrometry, they are usually ionized by electron impact. Thermally emitted electrons accelerated generally to - 70 eV, far larger than ionization energies, are used for it but can cause fragmentation, i.e., dissociative ionization. Threshold ionization mass spectrometry (TIMS) was developed to avoid this problem by setting the impact-electron energy in the vicinity of the ionization threshold energy. Experiments using TIMS have reports on successful detection of several radical species formed in plasmas.2,3 However, the ionization cross-section is reduced by lowering the electron energy, and the use of a hot filament may interfere with the detection through decomposition of particles in the sampling gas and/or desorption of adsorbed gas molecules on the inner wall of the ionization chamber. In photoionization mass spectrometry (PIMS), particles to be detected can be ionized without fragmentation by using photons in the ultraviolet (UV) region with an energy slightly higher than their threshold energy. An advantage of PIMS is that it is nearly free from the above-mentioned difficulties in TIMS. Two types of radiation sources, i.e., continuum and line sources, have been used in PIMS. The former requires a monochromator, by which a radiation with a desired energy is selected from a continuum from, for example, hydrogen or a rare gas such as helium. The photon energy is thus variable in the continuum region. The latter uses the radiations of the resonance lines from 113
S Ando et al: Detection of CH, radicals in an RF CHJH, plasma excited rare-gas atoms or of the Lyman-z line from hydrogen atoms. Here, the photon energy is fixed and a monochromator is unnecessary. Line spectra are on the whole more intense than continua.4 In our experiment we adopted the Kr or Ar resonance lamp as the resonance-line source. Since the introduction of the original idea of PIMS,’ the method has been applied to various purposes including determination of ionization or appearance energies of molecules and radicals, and measurement of photoionization cross-section curves. Among other applications are detection of radicals including CH, produced by chemical reactions6 detection of clusters’ and measurement concerning chemical reactions of substituted methyl radicals8 PIMS has been employed also for diagnostics of processing plasmas related to surface or thin-film problems : for example, detection of radicals such as CF, and CF of the prein a CF4/H2 etching plasma,‘.‘” and the preliminary” sent work. Besides these, however, to our knowledge only few applications of PIMS to plasmas for surface or thin-film processing have so far been published. The present paper describes a PIMS experiment detecting CH, and other radicals in a radio-frequency (RF) CH4/H, plasma. We found both the Kr and Ar resonance lamps performed similar functions except only that the latter can detect some more species without fragmentation which the former cannot ionize. Hence, in what follows, we limit the description to the results obtained by the Kr lamp. On the apparatus, a more detailed report will be published elsewhere.”
Photoionization mass spectrometry The threshold laws. We consider here the threshold laws for an elementary ionization process to understand the principle and the characteristics of the photoionization (PI) process compared with the EI process. According to the threshold lawsI for an elementary ionization process caused by an electron or photon collision, when the number of electrons leaving the collided particle is n including the colliding electron, the ionization cross-section o(E) is represented in general by
s E
a9 -
6(E-
EC)d”E,
(1)
0
where E is the energy of the incident electron or photon, EC the ionization threshold energy, and 6 represents a delta function ; s. . d”E means the n-fold integration with respect to E. When n = 0, the right-hand side of eqn (1) is equal to the integrand itself:
a(E) - 6(E-
EC).
(2)
If n = 1, eqn (1) becomes
s E
a(E) -
6(E-EC)
dE.
(3)
0
The right-hand side corresponds to a unit step function, which is equal to zero for 0 < E < EC and unity for E > EC. For n 2 2, eqn (1) is represented in the general form,
a(E) N (E-EJp’/(n114
l)!
(E > EC).
Figure 1. Threshold laws for an elementary ionization process caused by an electron or photon collision, where n is the number of electrons including the colliding electron which leave the collided particle.
The threshold laws for n = O-3 are illustrated in Fig. 1. As to univalent ion formation, n = 1 in the PI process and hence the ionization cross-section curve is step-like, whereas in the EI process n = 2 and the cross-section curve is linear as seen in the figure. Necessity of photoionization. For ionization of neutral particles in mass spectrometry, the EI technique is commonly used as already noted and impact electrons are accelerated generally to - 70 eV, far higher than the ionization energy of an atom, molecule or radical ranging from several eV to -20 eV. This contributes to enhancing the ionization cross-section for an atom; however, for a molecule or a radical, it may cause fragmentation, which interferes with identification of the particle species. Fragmentation can be avoided if the impact-electron energy is reduced to the vicinity of the ionization threshold energy, as done in TIMS ; however, this results in a reduction in the ionization crosssection as seen from the linear threshold law for n = 2 (Fig. 1). In the PI process, the ionization cross-section remains relatively high even if the photon energy is reduced to the vicinity of the threshold energy since the ionization cross-section curve for this process (n = 1) is step-like. Because of this fact, particles can be detected without fragmentation with a high cross-section by the use of photons having an energy slightly higher than their ionization threshold energy. Table l3.‘4.‘5 lists ionization and dissociative-ionization energies of CH, and neutral species derived from it by dissociation. Our aim is to detect CH, and other radicals including CH, and CH using a Kr or Ar resonance-line source. In either atom, the resonance line has two components, whose energies are 10.03 and 10.64 eV in Kr and 11.62 and 11.83 eV in Ar. The major component in Kr is the lower energy one which occupies more than 80% of the total in intensity, and in Ar the higher-energy component is the major one. On the basis of the list and after due consideration of probable errors in the energy data, one can expect CH,, CH2 and CH radicals to be ionizable by the Kr resonance radiation. The Ar
SAndo
et al: Detection of CH, radicals in an RF CHJH, plasma but was fixed at 30 mm in the present experiment. The upper electrode is coupled to a 13.56-MHz RF-power supply through an LC matching circuit. The lower one is grounded and serves as a sample holder for mounting film substrates. The source gas was hydrogen-diluted methane, CH,/H,. Both component gases are individually controlled by mass flow controllers and introduced through a mixing cell into the inside of the upper electrode of hollow structure. The mixed gas is then gushed out downward through many small holes made in the bottom of the upper electrode. The gas in the bell jar is pumped out at the same time by a rotary pump to maintain a constant pressure in a range 14 x lo2 Pa (10m2-3 Torr) during plasma generation. Particles in the plasma are detected by a commercial quadrupole mass spectrometer (QMS)17 in which a resonance lamp is incorporated as the radiation source for PIMS. The QMS consists of a mass analyzer and a controller, and is backed up by a data-acquisition and processing system. For the proper operation of the QMS, the pressure in the envelope, i.e., a cylindrical container of the mass analyzer, needs to be kept at least below 10-l Pa (10m4 Torr). For this purpose, an orifice of 150-pm diam. is installed between the bell jar and the envelope, which are differentially pumped by a turbomolecular pump. The orifice is made in the center of a closed end, -200 pm in thickness, of a Pyrex tube to form an orifice unit. Pyrex was taken because of a far smaller sticking coefficient of radicals on it than on metals. The orifice unit can be removed and exchanged with others if necessary.
Table 1. Ionization and dissociative-ionization energies of CH, and neutral species derived from it by dissociation (Refs 3, 14 and 15) C+ C
CH CH,
CH, ‘=a
*11.0 20.3” 20.2” 25” G25.2
CH+
CH:
CH;
CH:
10.64b 11.4 15.5@ 22.4
10.35 15.09b 15.3
*9.81 14.4
*12.8
* Ref. 3 ; bRef. 15 ; others-Ref. 14. * Mean of values shown in the reference
resonance radiation also can ionize all these radicals. On the other hand, energies of the resonance lines are smaller than any of the dissociative-ionization energies, so that the radicals are detectable without fragmentation using the Kr and Ar resonanceline source. Apparatus Construction of apparatus. A schematic of our experimental apparatus is shown in Fig. 2. An RF plasma generatorI of conventional type is used, which consists of paired parallel-plate electrodes and a grounded stainless-steel bell jar with glass windows accommodating the electrodes. These are made of stainless steel and 100 mm in diam. ; the spacing between them is variable,
rf POWER 13.56 MHz
71
h
MATCHING BOX
DATA ACQUISITION AND PROCESSING SYSTEM
t
ORIFICE
%zI7
BELL JAR
GLASS
I
ELECTRODES
I
I
A MIXING
CELL
kid 1 \MAsS
I41 ]Z~E
I
I
-.
MANOMETER
1 IONIZATION GAUGE TUBE
11
-
I
PUMP
1 / MICROWAVE CAVITY 1 RESONANCE LAMP
t PUMP
FLOW CONTROLLERS
Figure 2. Schematic
of the apparatus 115
S Ando et al: Detection of CH, radicals in an RF CHJH, plasma In detecting neutrals, it is desirable to exclude ions from the sampling gas; this is done by a magnetic field produced by a permanent magnet which is mounted outside the neck of the envelope. Besides the differential-pumping mode, the parallel-pumping mode in the bell jar and the envelope is possible by the turbomolecular pump. In this mode, the base pressure reached to - lo-’ Pa (- lo-’ Torr). Mass analyzer and resonance lamp. Figure 3 shows the arrangement of electrodes around the ionization chamber in the mass analyzer which is housed in the envelope. The ionization chamber has two 5-mm-diam. holes positioned oppositely on its side. Through one of them, UV is introduced in the chamber from the resonance lamp ; the other serves to reduce reflection of radiations and photoemission of electrons. This arrangement allows the PI-mode operation in addition to the EI-mode operation employing a filament originally incorporated in it for thermoelectron production. Both modes can be switched to each other; in the PI mode, the filament current and acceleration voltage required in the EI mode are both turned off. The ionizer is of the B-A type equipped with a spiral grid. Ions produced in the ionization chamber are extruded from it by a voltage applied to the grid and moved into the mass filter via the ion-lens system comprised of ion-focusing and extracting electrodes. The ion current filtered is then detected and amplified by a secondary-electron multiplier (not shown in the figure) to yield an output current. The electron-multiplier voltage was continuously variable over a range of - 1.0 to -3.0 kV. Table 2 shows in magnitude standard voltages applied to the electrodes in both the PI and EI modes. The values specified for the former were determined by measurements described in the following section to secure optimum operation ; the values for the latter are those specified by the manufacturer. The Kr or Ar resonance lamp, made of Pyrex, was operated at 2.45 GHz to induce the UV resonance radiation. As Pyrex is opaque to UV, a window disk, 13-mm diam. and 1.3-mm thickness, was fitted into the lamp. The disk was cemented to the end of the discharge tube of the resonance lamp, and MgFz and LiF were used for Kr and Ar lamps, respectively; these materials have inherent UV cutoff energies of - 11 and - 12 eV, respec-
Table 2. Standard voltages applied to electrodes around chamber in the PI and EI modes (refer to Fig. 3)
the ionization
PI mode
El mode
W) (I’,,
1V,I I Vfl
Acceleration Grid voltage Ion-focusing
voltage of impact electrons voltage
0 9 0
70 12 60
tively, and act as low-pass filters. Long-term UV irradiation of window materials causes lattice defects, thereby reducing the UV transmittance and the lifetime of a resonance lamp is thus limited. However an effective lamp was made using an apparatus expressly constructed.” The method of preparing resonance lamps and their characteristics are described in detail in a published report.” Measurement A problem in practice. A typical CH, spectrum obtained in the EImode measurement is shown in Fig. 4. In this spectrum, besides a peak at the mass number M = 16, four fragment peaks are seen at M = 15-12. Even in the PI mode, however, we found that a similar spectrum with fragment peaks often occurred and interfered with identification of particle species when the measurement was made in the highest-sensitivity range. Figure 5 shows an example of such a spectrum, which was observed in the PI mode using a Kr resonance lamp. This spectrum is considered to be caused by photoelectrons which were generated inside the mass analyzer by UV and accelerated by electric fields produced by electrodes arranged there. To suppress the photoelectron acceleration, we attempted to reduce the electric field strength by voltage control of the ionlens electrodes and admitted some possible drop in QMS sensitivity. In this connection, we took data in both modes on the effect of the ion-focusing voltage upon the CH, spectrum intensity. Figure 6 shows the typical results. In the EI mode, the
IONIZATION CHAMfSER HOLEFOR INTRODUCINGLIGHT
GRID ION-FOCUSINGELECTRODE EXTRACTING ELECTRODE
QUADRUPOLE MASS FILTER
Figure 3. Electrode arrangement 116
around
the ionization
chamber
in the mass analyzer.
The standard
values for electrode
voltages
are given in Table 2.
SAndo
et al: Detection
of CH, radicals
in an RF CHJH, plasma
0 l
0
00 I
-60
-40
I
-20
I
0
I on-f 0cuS ing vo I tage Vf (V) Figure 6. Dependence
Mass number Figure 4. A typical CHI spectrum
observed in the EI mode. Besides the major peak at M = 16, extra peaks at M = 15-12 due to fragmentation are seen.
15 Mass number Figure 5. An example of a signal from the CH, molecule observed in the PI mode, which was caused by accelerated photoelectrons. The radiation source was a Kr resonance lamp.
spectrum intensity was represented by the major-peak height at M = 16. In the PI mode, however, the sum of the two values at M = 16 and 15 was taken as the spectrum intensity because the ratio between them was found to vary. During the measurements, the acceleration voltage was fixed at 70 V in the EI mode, and the grid voltage at 12 V in both modes ; these are the standard values for the EI mode (refer to Table 2). In the PI mode, the intensity drops sharply in a region -2O0 V with decreasing voltage in magnitude and reaches to zero for voltages near zero. Thus, one can eliminate the EI signal nearly completely by reducing the ion-focusing voltage in magnitude. In the EI mode, the intensity drops by 40% with reduction in magnitude of the ion-focusing voltage to zero. As this is regarded as due to defocusing of ions, one can conclude that 60% of the intensity drop corresponds to the net effect of avoiding photoelectron acceleration. Even after the ion-focusing voltage had been set to zero, the
of the CH, spectrum intensity observed in the PI mode (0) on the ion-focusing voltage. The spectra arose from accelerated photoelectrons. The radiation source was a Kr resonance lamp. The Elmode data (0) are also shown for comparison. The intensity is represented by the sum of peak heights at M = 16 and 15 in the PI mode (see text) and by the major-peak height at M = 16 in the EI mode. The acceleration voltage in the EI mode was 70 V and the grid voltage 12 V in either mode; these are the standard values for the EI mode.
extra peaks caused by the El mechanism were still sometimes observed. We could prevent it by reducing the grid voltage to 9 V. The voltage reduction never leads to a sensitivity drop; it is supported by the data in Fig. 7. This figure shows the dependence of the CH, spectrum intensity in the EI mode upon the grid voltage with the ion-focusing voltage reduced to zero and with the acceleration voltage of 70 V. This EI-mode result indicates that reduction in grid voltage from 12 V to 9 V causes no drop in sensitivity and suggests that it has no effect on the ion-extruding function of the grid which must be common to both modes. On the basis of the foregoing results, we have taken 0 V for the ion-focusing voltage and 9 V for the grid voltage as the standard values for optimum operation in the PI mode. These values are given in Table 2. Stability check of the apparatus. We made a series of measurements to find the performance of the apparatus. As a preliminary to it, we checked time stability of the measuring system in both modes. For such a purpose in the PI mode, nitrogen monoxide
0
4 Grid voltage
a
12
V, (V)
Figure 7. Dependence
of the CH, spectrum intensity, represented by the major-peak height at M = 16, on the grid voltage obtained in the El mode with zero ion-focusing voltage. The acceleration voltage was 70 V, the standard value. 117
S Ando et al: Detection
of CH, radicals in an RF CHJH, plasma
(NO) is useful as a probe gas. The ionization energy of NO is 9.26 eV,14 slightly lower than the energies of Kr and Ar resonance lines, Hence, the NO molecule is detectable without fragmentation using either of the Kr or Ar lamp and the spectrum consists of a single peak at A4 = 30. On the other hand, the EImode spectrum exhibits fragment peaks at A4 = 14 and 15 besides the major peak at M = 30 ; the first peak among these is the nextmajor one, but its intensity is only 7.5% of that of the major peak.19 For the stability check, we followed the peak height at M = 30 as a function of time under constant measuring conditions. The envelope pressure was - 8 x 10e4 Pa ( - 6 x 10m6 Torr) in both modes and the electrode voltages were of the standard value for either mode (refer to Table 2). First we made a long-period operation test in the EI mode. The peak height exhibited a drift for 4-5 h and then reached a constant value, which we followed over - 4 h. We are unable to judge whether the filament, which is required to turn on in the EI mode and not in the PI mode, is responsible for the drift in output intensity ; however, one can say for the PI mode that a warming-up time of - 5 h in all parts of the spectrometer except the filament is sufficient to ensure a stable operation of the QMS itself. For this reason, it was decided to routinely take a 5-6 h warming-up time before starting a PI-mode measurement. Further tests were conducted in the PI mode after the preliminary procedure, and the output was observed to vary with time, so that it was necessary to wait for a stable value to be reached. The variation in output was considered to arise from a drift in the resonance lamp, which we may ascribe partly to discharge heating of the window and to local heating of the discharge tube in the cavity regionI by taking account of the fact that the intensity recovered after some off-discharge time interval. However, the variation in output in the PI mode could be avoided by intermittent discharge, by which the result shown in Fig. 8 was obtained. The measurement consisted of the repeated run which was a 2-min discharge of the lamp for spectrum recording followed by an 8-min off-discharge period. When we actually observed particles in repeated runs under a constant condition, the S-min period was allotted for recording the background spectrum from ions which were left unexcluded from the sampling gas and for printing out the spectrum data ; when the measuring condition was altered additional time was needed. It can be seen
Time Figure 8. The output
(mid
in PIMS, represented by the NO-spectrum peak height, as a function of time which was measured with intermittent discharge of a Kr resonance lamp. The measurement was the repeated run which was a 2-min lamp discharge for spectrum recording followed by an 8-min off-discharge period. The zero point of time is taken at the start of the microwave discharge in the initial run, the output being normalized with respect to the initial value. The envelope pressure was - 8 x 10m4 Pa (- 6 x 10m6 Torr) and the microwave input power 20 W. 118
that the output intensity is almost constant with only a fewpercent scatter for a period of 300 minutes and this measurement method was adopted. Detection sensitivity. The NO peak height at M = 30 of the QMS output was measured as a function of the bell-jar pressure in the PI mode to determine the sensitivity using a Kr resonance lamp with a microwave input power 20 Wand with the plasma column laid very close to the window. The electrode voltages were of the standard value (see Table 2) and the electron-multiplier voltage was - 1.4 kV. The typical result is shown in Fig. 9. The sensitivity obtained from the slope of the straight line drawn by the least-squares method is 1.95 x lo-” A/Pa (2.60 x 10e9 A/Torr). From this result, one can obtain the limit of CH,-radical detection According to published data,20 the sensitivity for CH, in PIMS is 3.5 times that for NO. Using this factor, the sensitivity of 6.8 x lo-” A/Pa (9.1 x 10m9 A/Torr) was obtained for the CH, radical. This value should be corrected for differences between sticking coefficients of CH, and NO on both Pyrex and stainless steel which were the materials of the orifice unit and ionization chamber; however, we could not find suitable data on them. The uncorrected value obtained above is thus probably overestimated. On the other hand, the noise level in the spectrum was found under the average condition to be 7.7 x lo-l4 A, which gives the limit of signal detection. Using this value and the foregoing sensitivity, one obtains 1.1 x 10m3 Pa (8.3 x 10m6 Torr) for the CH, partial pressure corresponding to the noise level. To convert this to the number density, we assumed the gas temperature in the plasma to be equal to the surface temperature of the lower electrode during discharge, on which we have taken detailed data under various conditions using solid-liquid transition of metals/eutectics.2’ On the basis of these data, we estimated the surface temperature to be 120’C in a hydrogen plasma at a pressure 2.7 x lo2 Pa (2.0 Torr) and an RF power 30 W; these are close to average conditions for the present measurement. This estimated temperature leads to the detection limit of the CH, number density 2.0 x 10” cmm3. Detection of radicals in an RF CH,/H2 plasma. We show here an
NO pressure Figure 9. The relation between
in the bell
jar
(Pa)
the bell-jar pressure and the output, represented by the NO-peak height at M = 30, in the PI mode. A Kr resonance lamp was used as the radiation source with a microwave input power 20 W. The cavity was adjusted so that the plasma-column end might lie very close to the window. The electrode voltages were of the standard value (refer to Table 2) and the electron-multiplier voltage was - 1.4 kV. The slope of the straight line drawn by the least-squares method gives the detection sensitivity.
SAndo
et al: Detection
of CHB radicals
in an RF CHJH, plasma
example of neutral-radical detection in an RF CH,/H, plasma. The CH4 and H, flow rates were, respectively, 60 and 440 seem (the CH, mole fraction 12%), the total gas pressure in the bell jar being 2.7 x 10’ Pa (2.0 Torr). The RF power was 15 W, corresponding to the power density of 0.19 W/cm’ for the electrode. The radiation source was a Kr resonance lamp with the microwave input power 20 W. A set of obtained spectra is shown in Fig. IO. Here, (a) is the spectrum as observed by PIMS and (b) the background spectrum observed with the permanent magnet installed and with the resonance lamp turned off. The peaks are due to ions which were left unexcluded from the sampling gas. (c) is the net spectrum obtained as the difference between them ; here is seen a noticeable peak of the CH, radical at M = 15. In addition, small peaks of CH?, CH and C,H, radicals and of the C,H, molecule are also seen. Lack of the peak at M = 16 from CH, indicates that PIMS works effectively in this measurement. Dependence of the CH,-radical density on the CH, concentration. In Fig. 11, the CH, relative density, represented by the peak height (at M = 15) in the PI mode. is plotted against the CH, mole fraction of the source gas for three RF input powers, lo,25 and 40 W, with a total pressure kept at 2.7 x lo’ Pa (2.0 Torr). The CH, mole fraction was determined from the CH, and H2 flow rates, The radiation source was a Kr resonance lamp with the microwave input power of 40 W. The data were taken in a series of runs with intermittent discharge of the resonance lamp. The CH1 density is normalized with respect to the maximum value. We find that any of the CH, density vs CH, mole fraction curves has a maximum and its position shifts to higher CH, concentrations with increasing RF power. Acknowledgements We wish to thank Dr N. Washida of National Institute for Environmental Studies for giving us valuable guidance in the
\-
(c)
*
•.mml.m.mD.mm-~~
15
20
25
30
Mass number Figure 10. Spectra obtained in an RF CH,/H, plasma : (a) the spectrum as observed by PIMS using a Kr resonance lamp, (b) the spectrum due to ions which are left unexcluded from the sampling gas observed with the lamp turned off, and (c) the net spectrum obtained from (a) and (b) by subtraction. A noticeable peak of the CH, radical at M = 15 is seen. Small peaks of CH, (M = 14), CH (13) C2H3 (27) and C,H, (28) are also seen. The H, and CHI flow rates are, respectively, 440 and 60 seem with the total pressure of 2.7 x 10’ Pa (2.0 Torr); the RF power was 15 W, corresponding to the power density 0.19 W/cm’.
0
5
10
CH4 mole fraction
15
20
(%I
Figure 11. The CH, density in the RF CH,/H?
plasma as a function of the CH, mole fraction in the source gas for three RF powers : IO W (0.13 W/cm’) (0) 25 W (0.32 W;cm’) (0). and 40 W (0.51 W/cm’) (A). The CH, density is represented by the peak height at M = 15 and normalized with respect to the maximum value. The total pressure was kept at 2.7 x 10’ Pa (2.0 Torr) throughout the measurement. The radiation source was a Kr resonance lamp.
PIMS technique. We gratefully acknowledge contributions made by Prof. K. Sunako and Mr K. Saio in early stages of this work. We are much indebted to Mr K. Tajima and Mr Y. Inoue for their expert assistance in preparing various parts of the apparatus.
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of CH, radicals in an RF CHJH, plasma
18. Gorden, Jr., R., Rebbert, R. E. and Ausloos, P., Rare Gas Resonance Lamps, National Bureau of Standards Technical Note 496, 1969. 19. Cornu, A. and Massot, R., Compilation of Mass Spectral Data, Heyden & Son, 1966, p. 1B.
120
20. Miyoshi, A., Matsui, H. and Washida, N., J. Php. Chem., 1989, 93, 5813. 21. Shinohara, M., Tashiro, H., Saio, K., Arai, K. and Takayama, K.. J. Vat. Sci. Technol., 1990, AS,3954; 1991, AP,3183.
Vacuum/volume
Pergamon PII : s0042-207x(97)00146-2
49/number 2Ioaaes 113 to 120/1998 0 199’8 flsevier Science Ltd All rights reserved. Printed in Great Britain 0042-207X/98 $19.00+.00
Detection of CH3 radicals in an RF CH4/H2 plasma by photoionization mass spectrometry S Ando,” M Shinoharab and K Takayama,b aDepartment of Physics, School of Science, Tokai University, Kitakaname, Hiratsuka, Kanagawa 259-12 Japan; bResearch Institute of Science and Technology, Tokai University, 1117 Kitakaname, Hiratsuka, Kanagawa 259- 12 Japan
1117
received 2 1 October 1997
To investigate processes occurring during thin-film deposition by plasma chemical vapor deposition, a study was made of the radical species in an RF CHdH* plasma using photoionization mass spectrometry. The method avoids fragmentation of particles as can occur in conventional mass spectrometry by using photons for their ionization. In the present experiment, a capacitive 13.56-MHz RFplasma reactor was used and the total gas pressure was 2.7x lb Pa. Besides the CH, radical on whose detection emphasis was laid, CH,, CH, C,H, and C,H, were detected in the excited CH$H* plasma using a quadrupole mass spectrometer with a Kr or Ar resonance lamp. The lamp was microwave-operated at 2.45 GHz to produce the ultraviolet resonance radiation with two components of energies 10.03 and 10.64 eV in Kr or 11.62 and 11.83 eV in Ar. Suppression of undesirable ionization of particles by impact of accelerated photoelectrons generated by lamp radiations was achieved by voltage control of the ion-lens electrodes. The detection sensitivity of the apparatus was estimated as having a CH, number density of typically2.0 x IO” cmm3. Data were obtained on the dependence of CH, radical density upon the CH, mole fraction of the CH+JH* source gas. 0 1998 Elsevier Science Ltd. All rights reserved
Introduction Plasmas are used in many techniques for thin-film deposition and surface processing, and neutral radicals in an ionized gas may play an important role in a given process. Thus it can be essential both for understanding a process mechanism and its control to have knowledge of the nature of the radicals in the excited gas. Methods of detecting neutrals in a plasma have been of two kinds : optical spectroscopy and mass spectrometry. The former includes optical emission spectroscopy which has been widely used and is a simple method for detecting excited species without plasma disturbance. This passive method is inapplicable to nonemissive species lying in ground states or in excited states with low emissive transition probabilities. For them active methods with external radiation sources must be adopted. Laser techniques such as dye-laser absorption spectroscopy, semiconductor-laser absorption spectroscopy and laser-induced fluorescence spectroscopy are now often employed for detecting non-emissive species.’ The major advantages of the use of laser are in both high sensitivity and spatial resolution. These techniques, however, require a large-scale and expensive apparatus, and also rather advanced expertise. Fortunately, mass spectrometry is technically simple and measurement is possible on a small scale with fairly inexpensive apparatus. Moreover, it has the advantage that one can detect and identify particles irrespective of their being emissive or non-
emissive. To detect neutrals by mass spectrometry, they are usually ionized by electron impact. Thermally emitted electrons accelerated generally to - 70 eV, far larger than ionization energies, are used for it but can cause fragmentation, i.e., dissociative ionization. Threshold ionization mass spectrometry (TIMS) was developed to avoid this problem by setting the impact-electron energy in the vicinity of the ionization threshold energy. Experiments using TIMS have reports on successful detection of several radical species formed in plasmas.2,3 However, the ionization cross-section is reduced by lowering the electron energy, and the use of a hot filament may interfere with the detection through decomposition of particles in the sampling gas and/or desorption of adsorbed gas molecules on the inner wall of the ionization chamber. In photoionization mass spectrometry (PIMS), particles to be detected can be ionized without fragmentation by using photons in the ultraviolet (UV) region with an energy slightly higher than their threshold energy. An advantage of PIMS is that it is nearly free from the above-mentioned difficulties in TIMS. Two types of radiation sources, i.e., continuum and line sources, have been used in PIMS. The former requires a monochromator, by which a radiation with a desired energy is selected from a continuum from, for example, hydrogen or a rare gas such as helium. The photon energy is thus variable in the continuum region. The latter uses the radiations of the resonance lines from 113
S Ando et al: Detection of CH, radicals in an RF CHJH, plasma excited rare-gas atoms or of the Lyman-z line from hydrogen atoms. Here, the photon energy is fixed and a monochromator is unnecessary. Line spectra are on the whole more intense than continua.4 In our experiment we adopted the Kr or Ar resonance lamp as the resonance-line source. Since the introduction of the original idea of PIMS,’ the method has been applied to various purposes including determination of ionization or appearance energies of molecules and radicals, and measurement of photoionization cross-section curves. Among other applications are detection of radicals including CH, produced by chemical reactions6 detection of clusters’ and measurement concerning chemical reactions of substituted methyl radicals8 PIMS has been employed also for diagnostics of processing plasmas related to surface or thin-film problems : for example, detection of radicals such as CF, and CF of the prein a CF4/H2 etching plasma,‘.‘” and the preliminary” sent work. Besides these, however, to our knowledge only few applications of PIMS to plasmas for surface or thin-film processing have so far been published. The present paper describes a PIMS experiment detecting CH, and other radicals in a radio-frequency (RF) CH4/H, plasma. We found both the Kr and Ar resonance lamps performed similar functions except only that the latter can detect some more species without fragmentation which the former cannot ionize. Hence, in what follows, we limit the description to the results obtained by the Kr lamp. On the apparatus, a more detailed report will be published elsewhere.”
Photoionization mass spectrometry The threshold laws. We consider here the threshold laws for an elementary ionization process to understand the principle and the characteristics of the photoionization (PI) process compared with the EI process. According to the threshold lawsI for an elementary ionization process caused by an electron or photon collision, when the number of electrons leaving the collided particle is n including the colliding electron, the ionization cross-section o(E) is represented in general by
s E
a9 -
6(E-
EC)d”E,
(1)
0
where E is the energy of the incident electron or photon, EC the ionization threshold energy, and 6 represents a delta function ; s. . d”E means the n-fold integration with respect to E. When n = 0, the right-hand side of eqn (1) is equal to the integrand itself:
a(E) - 6(E-
EC).
(2)
If n = 1, eqn (1) becomes
s E
a(E) -
6(E-EC)
dE.
(3)
0
The right-hand side corresponds to a unit step function, which is equal to zero for 0 < E < EC and unity for E > EC. For n 2 2, eqn (1) is represented in the general form,
a(E) N (E-EJp’/(n114
l)!
(E > EC).
Figure 1. Threshold laws for an elementary ionization process caused by an electron or photon collision, where n is the number of electrons including the colliding electron which leave the collided particle.
The threshold laws for n = O-3 are illustrated in Fig. 1. As to univalent ion formation, n = 1 in the PI process and hence the ionization cross-section curve is step-like, whereas in the EI process n = 2 and the cross-section curve is linear as seen in the figure. Necessity of photoionization. For ionization of neutral particles in mass spectrometry, the EI technique is commonly used as already noted and impact electrons are accelerated generally to - 70 eV, far higher than the ionization energy of an atom, molecule or radical ranging from several eV to -20 eV. This contributes to enhancing the ionization cross-section for an atom; however, for a molecule or a radical, it may cause fragmentation, which interferes with identification of the particle species. Fragmentation can be avoided if the impact-electron energy is reduced to the vicinity of the ionization threshold energy, as done in TIMS ; however, this results in a reduction in the ionization crosssection as seen from the linear threshold law for n = 2 (Fig. 1). In the PI process, the ionization cross-section remains relatively high even if the photon energy is reduced to the vicinity of the threshold energy since the ionization cross-section curve for this process (n = 1) is step-like. Because of this fact, particles can be detected without fragmentation with a high cross-section by the use of photons having an energy slightly higher than their ionization threshold energy. Table l3.‘4.‘5 lists ionization and dissociative-ionization energies of CH, and neutral species derived from it by dissociation. Our aim is to detect CH, and other radicals including CH, and CH using a Kr or Ar resonance-line source. In either atom, the resonance line has two components, whose energies are 10.03 and 10.64 eV in Kr and 11.62 and 11.83 eV in Ar. The major component in Kr is the lower energy one which occupies more than 80% of the total in intensity, and in Ar the higher-energy component is the major one. On the basis of the list and after due consideration of probable errors in the energy data, one can expect CH,, CH2 and CH radicals to be ionizable by the Kr resonance radiation. The Ar
SAndo
et al: Detection of CH, radicals in an RF CHJH, plasma but was fixed at 30 mm in the present experiment. The upper electrode is coupled to a 13.56-MHz RF-power supply through an LC matching circuit. The lower one is grounded and serves as a sample holder for mounting film substrates. The source gas was hydrogen-diluted methane, CH,/H,. Both component gases are individually controlled by mass flow controllers and introduced through a mixing cell into the inside of the upper electrode of hollow structure. The mixed gas is then gushed out downward through many small holes made in the bottom of the upper electrode. The gas in the bell jar is pumped out at the same time by a rotary pump to maintain a constant pressure in a range 14 x lo2 Pa (10m2-3 Torr) during plasma generation. Particles in the plasma are detected by a commercial quadrupole mass spectrometer (QMS)17 in which a resonance lamp is incorporated as the radiation source for PIMS. The QMS consists of a mass analyzer and a controller, and is backed up by a data-acquisition and processing system. For the proper operation of the QMS, the pressure in the envelope, i.e., a cylindrical container of the mass analyzer, needs to be kept at least below 10-l Pa (10m4 Torr). For this purpose, an orifice of 150-pm diam. is installed between the bell jar and the envelope, which are differentially pumped by a turbomolecular pump. The orifice is made in the center of a closed end, -200 pm in thickness, of a Pyrex tube to form an orifice unit. Pyrex was taken because of a far smaller sticking coefficient of radicals on it than on metals. The orifice unit can be removed and exchanged with others if necessary.
Table 1. Ionization and dissociative-ionization energies of CH, and neutral species derived from it by dissociation (Refs 3, 14 and 15) C+ C
CH CH,
CH, ‘=a
*11.0 20.3” 20.2” 25” G25.2
CH+
CH:
CH;
CH:
10.64b 11.4 15.5@ 22.4
10.35 15.09b 15.3
*9.81 14.4
*12.8
* Ref. 3 ; bRef. 15 ; others-Ref. 14. * Mean of values shown in the reference
resonance radiation also can ionize all these radicals. On the other hand, energies of the resonance lines are smaller than any of the dissociative-ionization energies, so that the radicals are detectable without fragmentation using the Kr and Ar resonanceline source. Apparatus Construction of apparatus. A schematic of our experimental apparatus is shown in Fig. 2. An RF plasma generatorI of conventional type is used, which consists of paired parallel-plate electrodes and a grounded stainless-steel bell jar with glass windows accommodating the electrodes. These are made of stainless steel and 100 mm in diam. ; the spacing between them is variable,
rf POWER 13.56 MHz
71
h
MATCHING BOX
DATA ACQUISITION AND PROCESSING SYSTEM
t
ORIFICE
%zI7
BELL JAR
GLASS
I
ELECTRODES
I
I
A MIXING
CELL
kid 1 \MAsS
I41 ]Z~E
I
I
-.
MANOMETER
1 IONIZATION GAUGE TUBE
11
-
I
PUMP
1 / MICROWAVE CAVITY 1 RESONANCE LAMP
t PUMP
FLOW CONTROLLERS
Figure 2. Schematic
of the apparatus 115
S Ando et al: Detection of CH, radicals in an RF CHJH, plasma In detecting neutrals, it is desirable to exclude ions from the sampling gas; this is done by a magnetic field produced by a permanent magnet which is mounted outside the neck of the envelope. Besides the differential-pumping mode, the parallel-pumping mode in the bell jar and the envelope is possible by the turbomolecular pump. In this mode, the base pressure reached to - lo-’ Pa (- lo-’ Torr). Mass analyzer and resonance lamp. Figure 3 shows the arrangement of electrodes around the ionization chamber in the mass analyzer which is housed in the envelope. The ionization chamber has two 5-mm-diam. holes positioned oppositely on its side. Through one of them, UV is introduced in the chamber from the resonance lamp ; the other serves to reduce reflection of radiations and photoemission of electrons. This arrangement allows the PI-mode operation in addition to the EI-mode operation employing a filament originally incorporated in it for thermoelectron production. Both modes can be switched to each other; in the PI mode, the filament current and acceleration voltage required in the EI mode are both turned off. The ionizer is of the B-A type equipped with a spiral grid. Ions produced in the ionization chamber are extruded from it by a voltage applied to the grid and moved into the mass filter via the ion-lens system comprised of ion-focusing and extracting electrodes. The ion current filtered is then detected and amplified by a secondary-electron multiplier (not shown in the figure) to yield an output current. The electron-multiplier voltage was continuously variable over a range of - 1.0 to -3.0 kV. Table 2 shows in magnitude standard voltages applied to the electrodes in both the PI and EI modes. The values specified for the former were determined by measurements described in the following section to secure optimum operation ; the values for the latter are those specified by the manufacturer. The Kr or Ar resonance lamp, made of Pyrex, was operated at 2.45 GHz to induce the UV resonance radiation. As Pyrex is opaque to UV, a window disk, 13-mm diam. and 1.3-mm thickness, was fitted into the lamp. The disk was cemented to the end of the discharge tube of the resonance lamp, and MgFz and LiF were used for Kr and Ar lamps, respectively; these materials have inherent UV cutoff energies of - 11 and - 12 eV, respec-
Table 2. Standard voltages applied to electrodes around chamber in the PI and EI modes (refer to Fig. 3)
the ionization
PI mode
El mode
W) (I’,,
1V,I I Vfl
Acceleration Grid voltage Ion-focusing
voltage of impact electrons voltage
0 9 0
70 12 60
tively, and act as low-pass filters. Long-term UV irradiation of window materials causes lattice defects, thereby reducing the UV transmittance and the lifetime of a resonance lamp is thus limited. However an effective lamp was made using an apparatus expressly constructed.” The method of preparing resonance lamps and their characteristics are described in detail in a published report.” Measurement A problem in practice. A typical CH, spectrum obtained in the EImode measurement is shown in Fig. 4. In this spectrum, besides a peak at the mass number M = 16, four fragment peaks are seen at M = 15-12. Even in the PI mode, however, we found that a similar spectrum with fragment peaks often occurred and interfered with identification of particle species when the measurement was made in the highest-sensitivity range. Figure 5 shows an example of such a spectrum, which was observed in the PI mode using a Kr resonance lamp. This spectrum is considered to be caused by photoelectrons which were generated inside the mass analyzer by UV and accelerated by electric fields produced by electrodes arranged there. To suppress the photoelectron acceleration, we attempted to reduce the electric field strength by voltage control of the ionlens electrodes and admitted some possible drop in QMS sensitivity. In this connection, we took data in both modes on the effect of the ion-focusing voltage upon the CH, spectrum intensity. Figure 6 shows the typical results. In the EI mode, the
IONIZATION CHAMfSER HOLEFOR INTRODUCINGLIGHT
GRID ION-FOCUSINGELECTRODE EXTRACTING ELECTRODE
QUADRUPOLE MASS FILTER
Figure 3. Electrode arrangement 116
around
the ionization
chamber
in the mass analyzer.
The standard
values for electrode
voltages
are given in Table 2.
SAndo
et al: Detection
of CH, radicals
in an RF CHJH, plasma
0 l
0
00 I
-60
-40
I
-20
I
0
I on-f 0cuS ing vo I tage Vf (V) Figure 6. Dependence
Mass number Figure 4. A typical CHI spectrum
observed in the EI mode. Besides the major peak at M = 16, extra peaks at M = 15-12 due to fragmentation are seen.
15 Mass number Figure 5. An example of a signal from the CH, molecule observed in the PI mode, which was caused by accelerated photoelectrons. The radiation source was a Kr resonance lamp.
spectrum intensity was represented by the major-peak height at M = 16. In the PI mode, however, the sum of the two values at M = 16 and 15 was taken as the spectrum intensity because the ratio between them was found to vary. During the measurements, the acceleration voltage was fixed at 70 V in the EI mode, and the grid voltage at 12 V in both modes ; these are the standard values for the EI mode (refer to Table 2). In the PI mode, the intensity drops sharply in a region -2O0 V with decreasing voltage in magnitude and reaches to zero for voltages near zero. Thus, one can eliminate the EI signal nearly completely by reducing the ion-focusing voltage in magnitude. In the EI mode, the intensity drops by 40% with reduction in magnitude of the ion-focusing voltage to zero. As this is regarded as due to defocusing of ions, one can conclude that 60% of the intensity drop corresponds to the net effect of avoiding photoelectron acceleration. Even after the ion-focusing voltage had been set to zero, the
of the CH, spectrum intensity observed in the PI mode (0) on the ion-focusing voltage. The spectra arose from accelerated photoelectrons. The radiation source was a Kr resonance lamp. The Elmode data (0) are also shown for comparison. The intensity is represented by the sum of peak heights at M = 16 and 15 in the PI mode (see text) and by the major-peak height at M = 16 in the EI mode. The acceleration voltage in the EI mode was 70 V and the grid voltage 12 V in either mode; these are the standard values for the EI mode.
extra peaks caused by the El mechanism were still sometimes observed. We could prevent it by reducing the grid voltage to 9 V. The voltage reduction never leads to a sensitivity drop; it is supported by the data in Fig. 7. This figure shows the dependence of the CH, spectrum intensity in the EI mode upon the grid voltage with the ion-focusing voltage reduced to zero and with the acceleration voltage of 70 V. This EI-mode result indicates that reduction in grid voltage from 12 V to 9 V causes no drop in sensitivity and suggests that it has no effect on the ion-extruding function of the grid which must be common to both modes. On the basis of the foregoing results, we have taken 0 V for the ion-focusing voltage and 9 V for the grid voltage as the standard values for optimum operation in the PI mode. These values are given in Table 2. Stability check of the apparatus. We made a series of measurements to find the performance of the apparatus. As a preliminary to it, we checked time stability of the measuring system in both modes. For such a purpose in the PI mode, nitrogen monoxide
0
4 Grid voltage
a
12
V, (V)
Figure 7. Dependence
of the CH, spectrum intensity, represented by the major-peak height at M = 16, on the grid voltage obtained in the El mode with zero ion-focusing voltage. The acceleration voltage was 70 V, the standard value. 117
S Ando et al: Detection
of CH, radicals in an RF CHJH, plasma
(NO) is useful as a probe gas. The ionization energy of NO is 9.26 eV,14 slightly lower than the energies of Kr and Ar resonance lines, Hence, the NO molecule is detectable without fragmentation using either of the Kr or Ar lamp and the spectrum consists of a single peak at A4 = 30. On the other hand, the EImode spectrum exhibits fragment peaks at A4 = 14 and 15 besides the major peak at M = 30 ; the first peak among these is the nextmajor one, but its intensity is only 7.5% of that of the major peak.19 For the stability check, we followed the peak height at M = 30 as a function of time under constant measuring conditions. The envelope pressure was - 8 x 10e4 Pa ( - 6 x 10m6 Torr) in both modes and the electrode voltages were of the standard value for either mode (refer to Table 2). First we made a long-period operation test in the EI mode. The peak height exhibited a drift for 4-5 h and then reached a constant value, which we followed over - 4 h. We are unable to judge whether the filament, which is required to turn on in the EI mode and not in the PI mode, is responsible for the drift in output intensity ; however, one can say for the PI mode that a warming-up time of - 5 h in all parts of the spectrometer except the filament is sufficient to ensure a stable operation of the QMS itself. For this reason, it was decided to routinely take a 5-6 h warming-up time before starting a PI-mode measurement. Further tests were conducted in the PI mode after the preliminary procedure, and the output was observed to vary with time, so that it was necessary to wait for a stable value to be reached. The variation in output was considered to arise from a drift in the resonance lamp, which we may ascribe partly to discharge heating of the window and to local heating of the discharge tube in the cavity regionI by taking account of the fact that the intensity recovered after some off-discharge time interval. However, the variation in output in the PI mode could be avoided by intermittent discharge, by which the result shown in Fig. 8 was obtained. The measurement consisted of the repeated run which was a 2-min discharge of the lamp for spectrum recording followed by an 8-min off-discharge period. When we actually observed particles in repeated runs under a constant condition, the S-min period was allotted for recording the background spectrum from ions which were left unexcluded from the sampling gas and for printing out the spectrum data ; when the measuring condition was altered additional time was needed. It can be seen
Time Figure 8. The output
(mid
in PIMS, represented by the NO-spectrum peak height, as a function of time which was measured with intermittent discharge of a Kr resonance lamp. The measurement was the repeated run which was a 2-min lamp discharge for spectrum recording followed by an 8-min off-discharge period. The zero point of time is taken at the start of the microwave discharge in the initial run, the output being normalized with respect to the initial value. The envelope pressure was - 8 x 10m4 Pa (- 6 x 10m6 Torr) and the microwave input power 20 W. 118
that the output intensity is almost constant with only a fewpercent scatter for a period of 300 minutes and this measurement method was adopted. Detection sensitivity. The NO peak height at M = 30 of the QMS output was measured as a function of the bell-jar pressure in the PI mode to determine the sensitivity using a Kr resonance lamp with a microwave input power 20 Wand with the plasma column laid very close to the window. The electrode voltages were of the standard value (see Table 2) and the electron-multiplier voltage was - 1.4 kV. The typical result is shown in Fig. 9. The sensitivity obtained from the slope of the straight line drawn by the least-squares method is 1.95 x lo-” A/Pa (2.60 x 10e9 A/Torr). From this result, one can obtain the limit of CH,-radical detection According to published data,20 the sensitivity for CH, in PIMS is 3.5 times that for NO. Using this factor, the sensitivity of 6.8 x lo-” A/Pa (9.1 x 10m9 A/Torr) was obtained for the CH, radical. This value should be corrected for differences between sticking coefficients of CH, and NO on both Pyrex and stainless steel which were the materials of the orifice unit and ionization chamber; however, we could not find suitable data on them. The uncorrected value obtained above is thus probably overestimated. On the other hand, the noise level in the spectrum was found under the average condition to be 7.7 x lo-l4 A, which gives the limit of signal detection. Using this value and the foregoing sensitivity, one obtains 1.1 x 10m3 Pa (8.3 x 10m6 Torr) for the CH, partial pressure corresponding to the noise level. To convert this to the number density, we assumed the gas temperature in the plasma to be equal to the surface temperature of the lower electrode during discharge, on which we have taken detailed data under various conditions using solid-liquid transition of metals/eutectics.2’ On the basis of these data, we estimated the surface temperature to be 120’C in a hydrogen plasma at a pressure 2.7 x lo2 Pa (2.0 Torr) and an RF power 30 W; these are close to average conditions for the present measurement. This estimated temperature leads to the detection limit of the CH, number density 2.0 x 10” cmm3. Detection of radicals in an RF CH,/H2 plasma. We show here an
NO pressure Figure 9. The relation between
in the bell
jar
(Pa)
the bell-jar pressure and the output, represented by the NO-peak height at M = 30, in the PI mode. A Kr resonance lamp was used as the radiation source with a microwave input power 20 W. The cavity was adjusted so that the plasma-column end might lie very close to the window. The electrode voltages were of the standard value (refer to Table 2) and the electron-multiplier voltage was - 1.4 kV. The slope of the straight line drawn by the least-squares method gives the detection sensitivity.
SAndo
et al: Detection
of CHB radicals
in an RF CHJH, plasma
example of neutral-radical detection in an RF CH,/H, plasma. The CH4 and H, flow rates were, respectively, 60 and 440 seem (the CH, mole fraction 12%), the total gas pressure in the bell jar being 2.7 x 10’ Pa (2.0 Torr). The RF power was 15 W, corresponding to the power density of 0.19 W/cm’ for the electrode. The radiation source was a Kr resonance lamp with the microwave input power 20 W. A set of obtained spectra is shown in Fig. IO. Here, (a) is the spectrum as observed by PIMS and (b) the background spectrum observed with the permanent magnet installed and with the resonance lamp turned off. The peaks are due to ions which were left unexcluded from the sampling gas. (c) is the net spectrum obtained as the difference between them ; here is seen a noticeable peak of the CH, radical at M = 15. In addition, small peaks of CH?, CH and C,H, radicals and of the C,H, molecule are also seen. Lack of the peak at M = 16 from CH, indicates that PIMS works effectively in this measurement. Dependence of the CH,-radical density on the CH, concentration. In Fig. 11, the CH, relative density, represented by the peak height (at M = 15) in the PI mode. is plotted against the CH, mole fraction of the source gas for three RF input powers, lo,25 and 40 W, with a total pressure kept at 2.7 x lo’ Pa (2.0 Torr). The CH, mole fraction was determined from the CH, and H2 flow rates, The radiation source was a Kr resonance lamp with the microwave input power of 40 W. The data were taken in a series of runs with intermittent discharge of the resonance lamp. The CH1 density is normalized with respect to the maximum value. We find that any of the CH, density vs CH, mole fraction curves has a maximum and its position shifts to higher CH, concentrations with increasing RF power. Acknowledgements We wish to thank Dr N. Washida of National Institute for Environmental Studies for giving us valuable guidance in the
\-
(c)
*
•.mml.m.mD.mm-~~
15
20
25
30
Mass number Figure 10. Spectra obtained in an RF CH,/H, plasma : (a) the spectrum as observed by PIMS using a Kr resonance lamp, (b) the spectrum due to ions which are left unexcluded from the sampling gas observed with the lamp turned off, and (c) the net spectrum obtained from (a) and (b) by subtraction. A noticeable peak of the CH, radical at M = 15 is seen. Small peaks of CH, (M = 14), CH (13) C2H3 (27) and C,H, (28) are also seen. The H, and CHI flow rates are, respectively, 440 and 60 seem with the total pressure of 2.7 x 10’ Pa (2.0 Torr); the RF power was 15 W, corresponding to the power density 0.19 W/cm’.
0
5
10
CH4 mole fraction
15
20
(%I
Figure 11. The CH, density in the RF CH,/H?
plasma as a function of the CH, mole fraction in the source gas for three RF powers : IO W (0.13 W/cm’) (0) 25 W (0.32 W;cm’) (0). and 40 W (0.51 W/cm’) (A). The CH, density is represented by the peak height at M = 15 and normalized with respect to the maximum value. The total pressure was kept at 2.7 x 10’ Pa (2.0 Torr) throughout the measurement. The radiation source was a Kr resonance lamp.
PIMS technique. We gratefully acknowledge contributions made by Prof. K. Sunako and Mr K. Saio in early stages of this work. We are much indebted to Mr K. Tajima and Mr Y. Inoue for their expert assistance in preparing various parts of the apparatus.
References 1. Donnelly, V. M.. Optical diagnostic techniques for low pressure plasmas and plasma processing. In Plusnm Diugmsric~s, Vol. 1, ed. 0. Auciello and D. L. Flamm. Academic Press. London, 1989, p. I. 2. Robertson, R.. Hils. D.. Chatham. H. and Gallagher, A.. Appl. P/I~x. Left.. 1983, 43, 544. i Sugai, H. and Toyoda. H.. J. Vnc. %i. Tw/md.. 1992, AlO, 1193. 4: Samson, J. A. R., Techniques of’ Vuc~uum C’lt~arrokr Sprcmwop~~. John Wiley. New York, 1967. 5. Lossing. F. P. and Tanaka, I., J. Chrm. Phys., 1956. 25, 1031. 6. Jones. I. T. N. and Bayes, K. D., J. Anf. Clt?nt. Sot.. 1972. 94, 6869. 7. Shinohara. H., Nishi, N. and Washida, N.. J. c‘I7m. PIzys.. 1985. 83, 1939. 8. Masaki, A., Tsunashima, S. and Washida, N.. J. Phj,s. Ckw., 1995, 99, 13126. 9. Hayashi. T., Miyamura. M. and Komiya. S.. Jp/t. J. Appt. P/I),.\., 1982,21, L755. 10. Hayashi, T., Kikuchi, M., Fujioka, T. and Komiya, S.. Proccedinys of htrrnationul Ion Engineering Corlgre.~s~ISIAT’83 & IPA T’83. Kyoto. 1983. p. 1611. 11. Ando, S., Shinohara. M., Sunako. K. and Takayatna. K.. Proceediqys of’the 3rd Asia-Paific Confmwce on Pkrsnm Scietzce & TechnolqAPCPST’96. Tokyo, 1996, p. 173. 12. Ando. S., Shinohara. M. and Takayama. K., Pmceedings of the School of’.Sbenw of’To,kui C’nkersit,v, 1998, 33 (to be published). 13. Collin, J. E., Physical meaning of photon and electron impact ionization process. in Muss Spccfrorn~tr~~, ed. R. I. Reed. Academic Press, London. 1965. p. 201. 14. Levitt. R. D. and Lias, S. G.. /oni:a/ior~ Pofemid md Appeorcuw Potentiul MwsuwIHPIII.s, 1971-1981, NSRDS-NBS 71, 1982. H. M.. Draxl, K.. Steiner, B. W. and Herron, J. T.. 15. Rosenstock, Enwg:c/cticsof Guseous Ions. J. Pky~. c‘hm. Ref: Dofu, 1977, 6(Suppl. 1). by Samco International Inc., 36 Wara16. Model PD-ZS, manufactured yacho, Takeda, Fushimi-ku. Kyoto 612 Japan, and partially moditied by the authors. by ULVAC Corporation, 2500 17. Model MSQ-15OA manufactured Hagizono. Chigasaki, Kanagawa 253 Japan. 119
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of CH, radicals in an RF CHJH, plasma
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