Surface Wave Plasma Sources

t This remark applies to all gap-type SW launchers: typical thickness of the front plate is 0.2-0.5 mm.

tt At higher frequencies, a high intensity field at the launching gap can be obtained with no rear gap, because the total length is + 4,(see Sec. 4.2) of the intrinsic coaxial line ofthe launcher is approximately 1..0 /4 where 1..0 is the free-space wavelength.

Surface Wave Plasma Sources

223

N-TYPE CONNECTOR

ALUMINIUM

/-~~~

BRASS TEFLON

Figure 16. Example of a Ro-box field applicator to be used with an LC matching network: D) = 8 nun, D 2 = 50 nun; launching gap width GL = 2 nun, field-arresting gap width ~ = 3 rnm.Pl

The equivalent circuit of this field applicator (Fig. 17a) is composed of an inductance L b , related mainly to the wire connecting its inner tube (sleeve) with the central pin of the coaxial cable connector, and a capacitance Cb that takes into account the energy stored in the electric field within the field applicator. Impedance Matching Network. HF power is supplied to the field applicator via a matching network represented as a two-port network in Fig. 17a. Impedance matching can generally be achieved with two independently variable tuning elements. These elements can be arranged as in Fig. 17b (configuration I) or as in Fig. 17c (configuration II»)42) The parameter Z~ is the impedance seen at the input connector (or in general, at a given reference plane; note the use of the mark' for this purpose) due to the gap impedance Zg combined with the impedances related to L b and C b . Equivalent Circuit. The normalized input impedances ZL (or admittance YL) of the plasma source for the above two matching network configurations can then be calculated.

224 High Density Plasma Sources

LAUNCHER

INPUT PLANE

.

: ZL

. .:,------...,

o

:---

:-GAP

o o o o o

o

LC

a

NETWORK

POWER GENERATOR, FEED LINE

IMPEDANCE MATCHING NETWORK

FIELD :APPLICATOR 0 0 0 0 0 0 0 0

·· ··

Zo

0

Xl

z:9

b

z'9

c

Figure 17. (a) Equivalent circuit of a SW plasma source using an LC Ro-box; (b) configuration I (high impedance plasma source) and (c) configuration II (low impedance plasma source) of the LC tuning elements in the impedance matching network.ls-l

Surface Wave Plasma Sources Configuration I, (X2 = 0, i.e., Z22 - Z12 =

225

°

in Fig. 14):

Eq. (19)

where, for example, b ~ = bg + jwCb , assuming that the influence of L b is negligible at low frequencies (recall from Sec. 3.3 that ZglZo = rg + jXg and ZOIZg = gg + j bg, and Xi = X/Zo)· Configuration II, (Xl

°

= 0, i.e., Z11 - Z12 = in Fig. 14):

Eq. (20)

Frequency Tuning Response. The Ro-box field applicator module can be utilized with two types of matching networks: an LC network, yielding the LC Ro-box system described above, or the HF power can be fed to the aperture through a tunable capacitive coupler (as in a surfatron, Sec. 4.2) and further tuning provided by a coaxial stub, hence the name stub Robox in this case.Pl The Ro-box field applicator module operates over a very large range of frequency. Its upper frequency limit increases with decreasing tube diameter. For example, with R = 15 mm, it is approximately 1200 MHz while with R = 4 mm, it goes up to 2.45 GHz; when employing this field applicator with an LC matching network, it is not efficient to use it above 200 MHz. As for the lower frequency limit, it is around 10 MHz when using air coils as inductances (for still lower frequencies, ferrite loaded coils should be used). Figure 18 shows the HF power reflected at the launcher input at constant incident power as a function of frequency, the so-ealledfrequency characteristic, for an LC Ro-box plasma source. It uses configuration II for the matching network at two fixed values of X 2 while X 3 is adjusted at each frequency for minimum reflected power. We observe that: (i) for a given X 2 , there exists a frequency at which the system can be perfectly matched; (ii) the lower the frequency at which zero reflected power is

226 High Density Plasma Sources required, the sharper is the tuning process. Recall that, as a rule, the broader the tuning, the less affected is the power matching with respect to changes in the discharge conditions and hence the more stable is the discharge. One may find experimentally that employing the matching configuration I instead ofIl (i.e., reversing input and output ofthe matching network) facilitates tuning. To understand this result, one has to realize that configurations I and II yield a perfect match (PR = 0) for two different domains of Z~: configuration I requires S I ("high impedance" plasma source), while configuration II demands r~ s 1 ("low impedance" plasma source). These two conditions are not compatible in general, except when r~ < < x~ and < < in which case either structure can be used to match the plasma source. t An example ofpractical realization ofan LC Ro-box plasma source is given in Sec. 5.2.

s;

b;,

s;

40 Argon: 100 mtorr

,.. :>!! 0

30

R

+ x~l)

~

w

:: 0 0-

= 15mm

20

0

x~) o

0

w .....

0

X(l) (2) < 2
w

.~

w

10

~

0 35

40

45

50

FREQUENCY (MHz) Figure 18. Observed frequency characteristics for an LC Ro-box plasma source. The matching network uses configuration IT (Fig. 17c), at two fixed values of A] while A; is adjusted at each frequency for minimum reflected power. The applicator dimensions are (refer to Fig. 16): D J = 30 mm, D2 = 75 mm, and its axial length is 65 mm.H

t Although as a rule rg andxg are smaller than unity, the corresponding impedance values seen at the plasma source input (denoted by the mark ") can be made "high" or "low" depending on the actual configuration ofthe wave launcher (see further in relation with Fig. 25).

Surface Wave Plasma Sources 4.2

227

Surfatron

This was the first SW launcher ofthe present family to be developedl'"! and it is still the most widely known. The term surfatron plasma is even used by some authors to designate SW sustained plasmas. It is best suited for frequencies above 500 MHz. Its overall axial length being usually more than A0I8 (Ao is the free-space wavelength), it is therefore less cumbersome to use Ro-box systems at lower frequencies. Its upper frequency limit can be as high as 2.45 GHz provided R ~ 4 mm (this is the case for the surfatron described in Sec. 5.1). The surfatron is an integrated wave launcher in the sense that a single unit performs both the field shaping and impedance matching functions. Figure 19 shows schematically a cross-section of this device in an axial plane. In terms ofHF circuit, it is a coaxial line ofcharacteristic impedance Zos and length ts + t b , terminated by a short circuit at one end and by a circular gap at the other, and supplied from a coaxial feed line via a capacitive coupler.

COAXIAL CABLE CONNECTOR "<,

Zo

INPUT PLANE

SPRING CONTACT

PLUNGJ

CAPACITIVE

LAUNCHING GAP

COUPLER'\.

~~1RfRo ............................................................................ , :::Ll.

-j

les

~ts

,

.~.

!

<, TOBE WAll

tb-1

Figure 19. Axial cross-section of a surfatron showing the field shaping structure (the coaxial line of length (b terminated by the launching gap) and the intrinsic tuning means (capacitive coupler and plunger) (after Ref. 38).

228 High Density Plasma Sources Field Shaping. SW excitation is provided by a circular gap, as in a Ro-box. Impedance Matching. HF power is supplied to the launcher via a commercial coaxial feed line terminated by an adjustable capacitive coupler (details in Sec. 5.1), which constitutes one tuning means. The second tuning means is the plunger position, which defines the length ts of the shortcircuited intrinsic coaxial line with respect to the coupler axis (Fig. 19); when limiting operation of the surfatron to a narrow frequency band, low (but generally nonzero) values of PRJ!} can nonetheless be secured with a fixed ts (see below with Fig. 22), yielding a lower cost and easier to build device. Equivalent Circuit. Figure 20a shows the equivalent circuit of the surfatron. (In this circuit and in the next one, the distributed elements are represented by bold lines). This circuit can be reduced, as shown in Fig. 20b, to that ofconfiguration I ofthe LC Ro-box (Fig. 17b). Thus, the input impedance of the surfatron is given by Eq. (19) with Eq. (21) Eq. (22) where Z~ , the gap impedance at the reference plane, is here taken at the input plane (see Fig. 19). Varying t s can be used to determine the real part of the input impedance (although it affects both parts) while varying C; through the insertion depth of the coupler (Fig. 19) affects only its imaginary part. Tuning and Frequency Characteristics. For a given rL , PR is minimum when xL = 0 (Sec. 3.3). This condition can be met by adjusting the coupler's capacitance Ce , provided the associated inductance L, is large enough. [38] An important feature which can be seen from Eqs. (19) and (21) is that, for given discharge conditions, there is only one setting of the capacitive coupler that yields XL = 0; tuning is thus simple and unambiguous. The surfatron should always be operated with XL = O. Assuming that the surfatron's coupler is always set such that XL = 0, we refer to the dependence of PR/p/ upon the plunger position ts (at constant frequency) and upon the frequency ofoperation (at constant ts ) as the tuning characteristics and the frequency characteristics of the surfatron, respectively. Examples ofthese characteristic curves are shown in Figs. 21 and 22, respectively. The existence of one or two zeroes of power means that s; s 1.£38] Note the large range of low (s 10%) reflected power in

Surface Wave Plasma Sources

229

terms of either ts or f, which ensures stability of the discharge. However, whenfis increased to higher values, the tuning characteristics narrow; past a certain value fm' one can no longer find a ts value yielding zero reflected power,[38] andfm is then considered as the maximum frequency ofoperation of the surfatron; above fm' the minimum reflected power that one can achieve increases continuously with f.

·: :,

,,, ,

··

LAUNCHER INPUT PLANE

: - COUPLER AXIS

.:-GAP a

POWER GENERATOR, FEED LINE

Zo

b

z'9

Figure 20. (a) Equivalent circuit of the surfatron (distributed elements are represented by bold lines); (b) reduced equivalent circuit showing the tuning elements and the gap impedance at the coupler's axis (matching configuration 1).142 ]

230 High Density Plasma Sources

f

CALC. (g ~

-

15

= 950 MHz

o

MEAS.

~

0 ......,

~

w

3:

-C

10

0 0a

I j

:-I

W

=0.66, b~ = 1.04 )

t-

~~~ 33

I I I

i I

J,--J $ i

I

65

I~

i

9

r-

O

W .....

u-

w

5

~

o 20

40

60

POSITION OF PLUNGER

80

t s (mm )

Figure 21. Measured and calculated tuning characteristics of the surfatron when sustaining a plasma column. The theoretical curve is obtained from the equivalent circuit of the surfatron (Fig. 20a), which yields the values of g~ and b~ from the two ts positions at which we observe zero reflected power. The insert shows the approximate dimensions of the launcher parts. [38J

Surface Wave Plasma Sources

--

231

20

:0.!! l :l'

w 15

3:

0 Q.

a

w .... U w .... u..

10

w

l:l'

5

600

800

1000

1200

FREQUENCY (MHz) Figure 22. Measured and calculated frequency characteristics of the surfatron when sustaining a plasma column. Fitting from the equivalent circuit equation yields g~ = 0.66 and b~ = 1.04. Launcher dimensions as in Fig. 2I.!38]

4.3

Waveguide Surfatron

This waveguide-based device should not be used much below one GHz because the large cross-section dimensions of any waveguide operating below that frequency would be cumbersome even in a research laboratory and also because the waveguide hardware for such low frequencies is costly and not readily available. This SW launcher is particularly recommended for operation with large tube diameters! and with high microwave power levels (6 kW testedj.F"

t Large diameter means here approaching or somewhat exceeding the free-space wavelength; at 2.45 GHz, Ao '" 122 mm. Indeed, most of the microwave-produced plasma columns that we are aware of at this frequency are less than 20 mm in diameter. We have achieved an 80-mm diameter plasma with a WR-430 waveguide (wide wall width 109 mm); still larger diameter plasmas can be realized at this frequency but with an hybrid version of the waveguide surfatron (Sec. 5.3).

232 High Density Plasma Sources The structure ofthis launcher consists of waveguide and coaxial line elements, as shown in Fig. 23. Microwave power is supplied by the generator to a rectangular waveguide section that is terminated, past the launching aperture, by a movable short circuit in the form of a waveguide plunger. A coaxial line section is attached perpendicularly to the wide wall of the waveguide, and its inner conductor extends into the waveguide as a sleeve around the discharge tube, forming a circular launching gap in the immediate vicinity ofthe opposite wall. The other end ofthis coaxial line is terminated by a movable short circuit (a coaxial plunger) located at a distance to from the waveguide wall (Fig. 23b).

ROD FOR MOVING COAXIAL PLUNGER

STANDARD RECTANGULAR WAVEGUIDE THINNER WALL _~--------,-;~ POWER INPUT _

WAVEGUIDE MOVABLE SHORT

a

Figure 23. (a) Overall view ofthe waveguide surfatron; (b) simplified sketch showing its internal design (after Ref. 43). (Cont'd next page.)

Surface Wave Plasma Sources

COAXIAL

COAXIAL

PLUNGER

SECTION

WAVEGUIDE SECTION

POWER INPUT

7

METAL SLEEVE

233

WAVEGUIDE PLUNGER

LAUNCHING[-L GAP

b

/ DISCHARGE TUBE

2 RO

Figure 23. (Cont'd)

Field Shaping. SW excitation is provided by a circular gap, as in Fig. 12. The gap width is usually fixed at 3-4 nun (or slightly larger to avoid arcing when operating at the kW power level). The waveguide wall around the aperture is thinned down from the outside to 0.5 nun over a circular area centered on the aperture and having a diameter close to the waveguide width (Fig. 23a). Impedance Matching. The coaxial and waveguide elements of the launcher structure serve as a wave mode converter (from the TE IO mode in the rectangular waveguide to the TEM mode in the coaxial section terminated by the field shaping structure) and as an impedance transformer. Such an impedance matching system is particularly suitable to match a low impedance load on the coaxial side of it to the characteristic impedance of the feed waveguide. Tuning the waveguide plunger position tw (Fig. 23b) for minimum reflected power is equivalent to adjusting C; with the capacitive coupler in a surfatron (it brings XL to zero) while the coaxial plunger position to plays the same role as that of ts in a surfatron, hence the name waveguide surfatron.

234 High Density Plasma Sources Equivalent Circuit. When the outer diameter of the coaxial section is small compared with the width of the waveguide, a simple equivalent circuit ofthe launcher can be constructed: all the impedances can be related to a single reference plane, coincident with the axis ofthe coaxial section. [43] Figure 24a shows such a circuit; the two distributed elements represent the short circuited coaxial and waveguide lines, of characteristic impedances Zoe and Zo and adjustable lengths tc and tw respectively, and Z, accounts for the conducting sleeve going across the waveguide. This equivalent circuit can be reduced to the matching configuration II (Fig. 17c), as shown in Fig. Then in Eq. (20) for YL, we have 24b where Z; = Zg +

z;

Eq. (23)

Eq. (24)

REFERENCE :--- ZL PLANE -: ,------, SHORT CIRCUITED COAXIAL LINE

POWER GENERATOR, FEED GUIDE

Zo

. S~Z~~~J~~U~;~~ ~ :

-,

SLEEVE

,

C?x,~z, . , ,,

b

''

Figure 24. (a) Equivalent circuit of the waveguide surfatron; (b) equivalent circuit reduced to the tuning elements and gap impedance seen at the coupler axis: matching network configuration II. The reference plane is perpendicular to the waveguide axis and runs along the tube axis.[42]

Surface Wave Plasma Sources

235

Tuning Characteristic. It represents here the dependence of PRIPJ on t ~ with tw being always tuned for minimum reflected power. Figure 25 shows a set ofcalculated tuning characteristics for the waveguide surfatron, illustrating the influence of Z~ = r~ +jx~, the gap impedance seen at the reference plane and including Z9' As a rule, we experimentally get values of r~ S 0.01 but it is possible to implement a section ofa coaxial line between the gap and the waveguide wall that acts as an impedance transformer.If-l Then, one can obtain larger values of r~, yielding flatter tuning characteristics as shown in Fig. 25, thereby achieving a plasma source much less dependent on discharge conditions. As a rule, some HF power is lost in such transformers.

'00

z

! ! ~

80

'00

r~_05

~

~ ~ ~

eo so

»: .32'

80

!

80

<1.15

loll -c

t

z

~.o

<, ~.05

005

0.1$

025

80

r~.2

~.o

80

" )

20

.32'

<1.15

~.05

10''0

r~ =0.5

r~,",o.s

x~ = ·0.5

~.O

0,15

02'

10''0

60

o

OJ

et Ii'

005

\ 20

"'-

.-/\1 z=-"--':0'7,5--::,;::--~----o<:::""'...lLJ 0.05 0.0$ 0.15 0,25

) 0.15

0-25

10''0

Figure 25. Set of calculated tuning characteristics for the waveguide surfatron, showing the influence of the real and imaginary parts of the gap impedance seen at the reference plane (Fig. 24). Clearly there exist values of Z~ = r~ +jX~ that ensure low reflected power at the waveguide surfatron input over a large portion of the tuning characteristic.I-"

5.0

TYPICAL EXPERIMENTAL ARRANGEMENTS

In the previous sections, we have presented the theory and general design principles of SW plasma sources. This section provides a detailed practical description of three kinds of SW plasma sources as examples of how to construct and operate them correctly.

236 High Density Plasma Sources Caution. Personnel must be aware that exposure to microwave energy radiated from the wave launcher and the plasma column as well as from the HF generator can be detrimental to their health. According to IEEE, [44] for "cognizant individuals" (notion ofcontrolled environmentlr'l), the maximum permissible exposure in the HF range of interest to us is as given in Table 2. To maintain RF and microwave radiations below the maximum permissible level, the plasma source should be periodically inspected and checked with a calibrated HF radiation monitor. One should also keep up with publications on the hazards of RF and microwave radiation.

Table 2. Maximum Permissible Exposure in a Controlled Environment. I44]

5.1

Frequency Range (MHz)

Power Density (E field, H field) (mW/cm 2)

3-30

900/f2, 10000/f2

30-100

1.0, 10000/f2

100-300

1.0

300-3000

1/300

Atmospheric Pressure Microwave Discharges with a Surfatron

The device presented is widely spread among analytical chemists dealing with atomic spectrometry. The sample to be analyzed is sent through an argon or helium plasma in order to dissociate its molecules, then excite and ionize the resulting atoms and finally detect these atoms through optical emission spectroscopy or mass spectrometry.l45]-[63] Recall from Sec. 2.3 that SW discharges at atmospheric pressure are stable and reproducible provided that they are achieved in a tube with a maximum inner diameter of 6-8 mm. The surfatron described below is intended for operation at the fixed ISM frequency of 2.45 GHz; a movable plunger is required because the impedance matching conditions for argon and helium are significantly different at powers below 100-200 W (this is related to variations in the resistance ~, Sec. 2.1).

Surface Wave Plasma Sources

237

Description of the Wave Launcher. Figure 26 shows two crosssectional drawings ofthe device developed by the Groupe de Physique des Plasmas at Universite de Montreal. The numbers refer to the following elements: (1) Wave launching interstice (gap)-typically 2 mm wide. (2) Front plate ofthe field shaping section-thickness: 0.5 mm. (3) Inner conducting tube (sleeve)-made of aluminium, it is anodized (Al Z03) on its non-threaded portion to reduce the possibility of its arcing with the coupler's plate (4). The central part ofthe front plate (2) is also anodized for similar reasons. (4) Plate of the capacitive coupler-round copper plate with a curvature equal to the radius of the outside diameter of the sleeve plus I or 2 mm; the coupler's axis is at 10 mm from the front plate (2). (5) Spring contacts for the capacitive coupler-it ensures that the semirigid coaxial cable constituting the coupler's feed is in good electrical contact with the body ofthe surfatron. (6) Knurled wheel to move the coupler's assembly radially. (7) Chimney preventing the coupler's semirigid cable from being stressed or bent, which would interfere with a swift and precise radial displacement of the coupler assembly. (8) Guide-pin to prevent the semirigid coaxial cable from rotating when moving the coupler's plate radially. (9) Clamping nut holding the coupler semirigid cable in placeit allows a coarse radial adjustment of the coupler's plate position with respect to the surfatron sleeve. In this way, the capacitive coupler can be used on surfatrons of different body diameters.

(10) Semirigid cable-0.250" (6.35 mm) or 0.325" (8.25 mm). (11) N-type (female) connector to mate with the microwave coaxial cable feed. (12) Strip of metal-it serves together with screw (13) to pry the threadings in the surfatron body in order to obtain a better electrical contact, avoiding arcing in the threaded portion when operating at powers> 100 watts.

238 High Density Plasma Sources (13) Butterfly screw used with (12). (14) Discharge tube (fused silica)-minimum inner diameter tested: 0.5 mm, wall thickness tested: 0.5 to 4 mm. For a watercooled small bore tube, see Ref 63. (15) Wheel for adjusting the gap width (typically 2 mm). (16) Wheel for adjusting the plunger axial position fixing the length (s' (17) Surfatron main body. (18) Compressed air input for external cooling of the discharge tube-dry air must be used to avoid damaging the threadings. (19) Ring holding the gap thin plate (2) in position. Operating Instructions. The discharge tube must be a low-loss high temperature-resistant dielectric such as fused silica or high purity ceramic. It must fit as closely as possible the inner diameter of the sleeve (3); for example, with a 6.5 mm i.d. sleeve, we recommend using a 6 mm o.d. discharge tube. It is advisable to always have dry, oil-free compressed air flowing through (18) to provide external cooling of the discharge tube when it encloses an atmospheric pressure discharge. Water cooling ofthe surfatron body is required in the present case when operating above 150 watts. Striking the Plasma for the First Time. When initiating operation of the plasma source at atmospheric pressure, one should begin with argon (or any other easy to ionize gas) and set its flow in the range 300-600 seem (mild laminar flow). The coupler plate (4) radial position is initially adjusted with the knurled wheel (6) so it barely touches the sleeve, which means it rests approximately at 0.5-1 mm from it. The plunger is positioned with (16) so that (s :::::: 6-10 mm. The microwave generator, connected to (11) via an appropriate coaxial cable, [35] is switched on at approximately 100 W while either bringing a Tesla coil (spark generator) in front of the open end of the discharge tube or introducing into it a small wire (which will glow), to provide initial ionization at the gap level. Once the discharge is on, we adjust the capacitive coupler with the knurled wheel (6) for maximum plasma length. Then, we tune the plunger with (16) looking again for maximum plasma length. This procedure is repeated until there are no further improvements in the tuning. As a rule, one should arrive at a PRJP/ value less than 10%, as shown in Fig. 27.

~

11

D ~ ~

~

St,

~

~ ~

~

~l:::l

19

Physique des Plasmas - Universite de Montreal

I o

I 1

I 2

I 3

~

I

I

;::

4

Scm

~

Figure 26. Cross-sectional drawings of a surfatron intended for operation at 2.45 GHz with small bore discharge tubes. This device is well-suited for stable discharges at atmospheric pressures in gases as different as argon, hydrogen, and helium.

~

~

'C

240 High Density Plasma Sources

+ 4.5 mm diameter sleeve ;

40

*

6.5 mm diameter sleeve

Ill:

w

:1:

30

0

a.. C

w .... ow

20

w

10

.......

Ill:

2

4

6

8

10

12

POSITION OF PLUNGER,

14

ts

16

18

20

(mm)

Figure 27. Tuning characteristics of the surfatron described in Fig. 26 for a discharge in argon at atmospheric pressure and with 100 watts of incident power at 2.45 GHz. The two curves are for 4 and 6 mm o.d. tubes (sleeve of 4.5 and 6.5 mm i.d respectively); the gap width is 2 mm.

Striking the Plasma with Hard-to-Ionize Gases such as Helium or Nitrogen. Once the surfatron has been tuned for argon, we can use one of the two following procedures to strike a hard-to-ionize gas without damaging the surfatron and the microwave generator: 1. Mixing of the hard-to-ionize gas with argon-this gas is initially mixed with a large flow ofargon and the concentration of the latter is gradually reduced as the coupler and plunger are continuously tuned for minimum reflected power. Tuning begins to be really affected when almost no argon is flowing 2. Striking the discharge directly with the hard gas-a circulator must then be inserted between the HF generator and the surfatron to avoid damaging the feed cable and the generator. The plunger is initially set for a low power argon discharge, namely at 30-50 watts of incident power. Then, the hard-toionize gas alone is sent at a flow rate of 300-600 seem with 150-200 W of incident power; the coupler is tuned while activating the Tesla coil at the open end ofthe tube so that a stream of ionized gas reaches the gap. Faraday Cage. A small diameter Faraday cage can be attached to the front plate ring (19). It should be long enough to enclose not only the plasma column but also the dielectric tube. Otherwise, microwave radiation may be directed along the discharge tube outside the cage.

Surface Wave Plasma Sources 5.2

241

A Broadband (10-100 MHz) RF Plasma Source with an LC Ro-Box

The Ro-box field applicator is simple to construct: dimensions are not critical and there are no movable parts. The main difficulty arises with the LC impedance matching network which should provide low reflected power, little heating ofthe matching network and reproducible tuning. The matching network described below was found to be efficient between 10 and 100 MHz in argon and helium discharges: (i) at reduced pressure (0.01 S psi torr) in a 26 mm i.d., 30 mm o.d. Pyrex tube; (it) at atmospheric pressure with 2, 4, and 6 mm i.d. fused silica tubes. Ro-Box Field Applicator. Dimensions are given with reference to Fig. 16. For the small diameter discharge tube, these are D 1 = 9 mm, D 2 = 30 mm and GA = 5 mm while for the 30 mm o.d. tube, D 1 = 31.5 mm, D 2 = 77 mm and GA = 6 mm. The launching gap front plate is 0.5 mm thick. As a rule, the various parts ofthe applicator are made of aluminium. AN-type (HN for higher power) female bulkhead connector is used and mounted as shown in Fig. 16. LC Matching Network. Figure 28a shows the circuit used; it corresponds to network matching configuration II in Fig. 17. The components are: C/ Continuously variablecapacitor(commercial), 32-241 pF (4500 V breakdown voltage). Ci Continuously variable capacitor (commercial), 13-74 pF (4500 V breakdown voltage). L / Three-taps inductance (laboratory made), approximately 110 nH/tum. It consists ofthree turns nominally of 1" (25.4 mm) diameter made with a II4" (6.3 mm) o.d. copper tubing. L 2: Continuously variable inductance (commercial), 4.4 mH with 12 turns.

All these components lay in a metallic box and are connected together by copper strips-I/l6" (1.6 mm) thick, II2" (12.7 mm) wide. No forced air and water cooling was used; the limit of input power was 150 W at reduced pressure while, at atmospheric pressure, with helium, it was 100 W. We also built matching networks that used water cooling of the inductances and worked up to 500 W. We initially tried employing an amateur radio matching network, which is shown in Fig. 28b. We could not match the plasma source satisfactorily with it; clearly this matching network does not mate correctly with the low impedances presented by the present Ro-box plasma source.

242 High Density Plasma Sources

L2 af I I I I I I I

,. ,, ,

c1 L]

C2

Z'9

I I

Z'9

Figure 28. (a) Elements and configuration (II in Fig. I7c) of the LC matching network of the Ro-box in the range 10-100 MHz (see text for actual values); (b) elements and configuration of the amateur radio matching network tested unsuccessfully with the Robox; note that it cannot be reduced to configuration I or II.

Operating Procedure. The HF power feeding arrangement includes the components shown in Fig. 11. When no circulator is available, one may need to replace it with a high power 3 dB attenuator in the feed line, which reduces the interaction between the plasma source and the generator. Before striking the plasma, one sets C j so P j is maximized on the directional power measuring line. Then, the discharge is struck, and C 2 and L 2 adjusted for maximum plasma length, and C, retuned until a maximum plasma length is obtained with a satisfactory reflected power level.

Surface Wave Plasma Sources 5.3

243

A Millitorr and Sub-Millitorr Magnetized Plasma Source Sustained by Surface Waves

As indicated in Sec. 2.4, when applying a static magnetic field of the appropriate intensity, it is possible to achieve a SW discharge at gas pressures as low as 0.1 mtorr in argon at 2.45 GHz. We present below the magnetized SW sustained plasma source and the associated reactor that we have been developing for anisotropic etching experiments. Description of the System. The wave launcher employed is an unconventional version of the waveguide surfatron of Sec. 4.3 because of its "overgrown" coaxial section. The reason for using such a variant is that there is no standard rectangular waveguide for connecting to a microwave generator at 2.45 GHz that would have a wide wall large enough to accommodate a tube of 150 mm o.d., as required by the present reactor design. The launching gap section of this wave launcher is shown in Fig. 29; it is similar to that ofa surfatron. Microwave power is supplied via a WR-284 waveguide, perpendicularly to the field applicator body, close to the gap as shown in Fig. 29, while a movable waveguide plunger is located opposite to it. As shown in Fig. 30, the fused silica discharge tube extends a few centimeters past the wave launching gap before entering a larger diameter (280 mm), 700 mm long steel chamber (reactor) where the substrates to be processed are placed. Both the SW plasma source and the reactor are submitted to a uniform (12 coils), axially directed static magnetic field, the intensity of which can attain 1400 gauss (0.14 T). Operating Conditions. The coaxial and waveguide plungers are set so that the reflected power is near zero and the plasma viewed axially from the reactor end is as homogeneous as possible azimuthally. This requires some minimum microwave power, typically 300-400 W for argon. The plasma glow has then a donut shape, i.e., its luminosity is maximum close to the discharge tube periphery. Such a radial profile of luminosity has been reported earlier with classical ECR discharges and is also typical of large diameter SW sustained plasmas in absence ofmagnetic field, because of the increase of the average electron energy with radius (Sec. 2.3).

244 High Density Plasma Sources

ISIDE· VIEwl

I~I~ ~

L

~

'----r:1l'7T---=r-::----=-.....,..!.r-

~

LAUNCHING GAP

ANODIZ;;-{ ROD TO MOVE COAXIAl PLUNGER

IFRONT VIEWI WR-284 SECTION

~

MICROWAVE INPUT

ALUMINIUM

Y/////.

BRASS

,0'/,;'/,;/

~

WR-284 WAVEGUIDE PLUNGER

TEFLON

Figure 29. Field applicator and coaxial tuning system of a SW launcher sustaining plasma with diameters that exceed the width of the largest standard rectangular waveguide at 2.45 GHz. The inner diameter of the applicator sleeve is 154 mm for a 150 mm o.d. discharge tube. This wave launcher uses a waveguide plunger (not shown) as its second tuning means. The waveguide feed and plunger are of the WR-284 type (72 x 34 mm inside dimensions).

12 Solenoids Wave Launcher

Metallic Vessel (280 rnrn LD.)

•

,'£;:~!m;,~"'~:~~~iifrii!~:~r:i:I~~I~·

II .--.

Gas

InJe/~~I~~;m:j: ~ Probe

O62

Source :..

- - - .. Reactor

1200

.,.i

To Pump

.-,

Wafer Holder (water cooled)

..

~

~

~

~

...

~

~

850 r

o

.

25 Axial position (em)

~

80

Figure 30. Sketch of our magnetized SW plasma source and associated reactor for plasma etching. The axially directed static magnetic field is uniform starting from the launching gap up to 600 mm away from it in the reactor.

i

~

t:

a~ ~ tJl

246 High Density Plasma Sources The discharge can be operated in a broad range ofgas pressure. For example, in argon, we have been successful in sustaining plasma from 0.1 mtorr to 20 mtOIT. As noted in Sec. 2.4, magnetized SW discharges have a larger Bo operating domain than conventional ECR systems. More specifically, at the lower end of the pressure range, we find that a minimum B o value is required at the above-mentioned microwave power level; in argon, it is typically 700 gauss. Though we have not tested this possibility, we feel that it should be possible to decrease this minimum Bo value by increasing the wave power. We have also observed that plasma can be sustained at magnetic field intensities higher than for ECR, although above a certain B o value the discharge becomes unstable (above 1 kgauss in argon). Finally, we find that the operating values given above depend somehow on the gas used. Confinement of the Plasma Glow in the Reactor Chamber. Confinement ofthe plasma by the magnetic field can be observed visually in the reactor region: as the plasma enters the reactor, its diameter remains approximately the same as that of the discharge tube, here 15 em.

6.0

CONCLUSION

It is now an accepted fact that given the discharge conditions (nature and pressure ofthe gas, discharge vessel dimensions and material, frequency of operation and, eventually, intensity of the applied static magnetic field) and the HF power density deposited in the plasma, the plasma characteristics (electron energy distribution function, power absorbed per electron, radially averaged electric field intensity) are, to a first approximation, the same in all HF sustained discharges.Pf However, SW discharges have the advantage of the broadest operating conditions in terms of frequency, tube dimensions and shape, and gas pressure; for example they can be utilized over both the RF and microwave domains, which permits one to optimize given processes as a function of frequency (generally through changes in the electron energy distribution function). [8] A further advantage ofSW plasmas is that they are the best modeled HF plasmas, which yields a better insight into their use and operation. Finally, a major future avenue for these discharges is their operation as magnetized plasmas. The first results presented on this subject in this chapter show that these discharges are much more flexible than common ECR systems; their modeling is also highly promising.l'"

Surface Wave Plasma Sources

247

ACKNOWLEDGMENTS The work presented has been achieved thanks to the skilled technical assistance of Messieurs F. Roy, R. Lemay, R. Martel, and 1. E. Samuel. It was funded through the following agencies or programs: the Fonds pour la Formation de Chercheurs et l'Aide a la Recherche (FCAR) of the Quebec government, the France-Quebec scientific program, and the Conseil de Recherches en Sciences Naturelles et en Genie (CRSNG) of Canada.

REFERENCES 1. Tuma, D. R, Rev. Sci. Instrum., 41:1519 (1970) 2. Moisan, M., Beaudry, C., and Leprince, P., Phys. Lett. A, 50:125-126 (1974) 3. Zakrzewski, Z., Moisan, M, Glaude, V. M M, Beaudry, P., Plasma Phys., 19:77-83 (1977)

c., and

Leprince,

4. Chaker, M, and Moisan, M., J. App/. Phys., 57:91-95 (1985) 5. Moisan, M., and Zakrzewski, Z., Rev. Sci. Instrum., 58:1895-1900 (1987) 6. Moisan, M., and Zakrzewski, Z., J. Phys. D: Appl. Phys., 24:1025-1048 (1991) 7. Ricard, A, Barbeau, C., Besner, A, Hubert, J., Margot-Chaker, 1., Moisan, M., and Sauve, G., Can. J. Phys., 66:740-748 (1988) 8. Moisan, M., Barbeau, c., Claude, R, Ferreira, C. M, Margot, J., Paraszczak, 1., Sa, A B., Sauve, G., and Wertheimer, M. R, J. Vac. Sci. Techno/. B, 9:8-25 (1991) 9. Margot, 1., and Moisan, M., in Microwave Excited Plasmas, (M. Moisan and J. Pelletier, eds.), Ch. 8, Elsevier, Amsterdam (1992) 10. Moisan, M., Pantel, R, and Hubert, 1., Contrib. Plasmas Phys., 30:293-314 (1990) 11. Moisan, M, Shivarova, A, and Trivelpiece, A. W.,Plasma Phys., 24:13311400 (1992)

12. Margot, 1., and Moisan, M. ,J. Plasma Phys., 49:357-374 (1993) 13. Margot-Chaker, 1., Moisan, M., Chaker, M, Glaude V. M M, and Sauve, G., presented at the XVIII Int. Conf., Phenomena in Ionized Gases, contributed papers, 4:602-603, Adam Hilger, Bristol (1987) 14. Margot-Chaker, 1., Moisan, M., Chaker, M., Glaude, V. M. M., Lauque, P., Paraszczak, 1., and Sauve, G., J. Appl. Phys., 66:4134--4148 (1989)

248 High Density Plasma Sources 15. Trivelpiece, A. W., Slow-Wave Propagation in Plasma Waveguides, San Francisco Press, San Francisco (1967) 16. Sa, A. B., Ferreira, C. M., Pasquiers, S., Boisse-Laporte, C., Leprince, P., and Marec, J., J. Appl. Phys., 7:4147-4158 (1991) 17. Rakem, Z., Leprince, P., and Maroc, J., Rev. Physique Appliquee, 25: 125130 (1990) 18. Hajlaoui, Y., Pomathiod, L., Margot, J., and Moisan, M., Rev. Sci. Instrum., 62:2671-2678 (1991) 19. Moisan, M., Ferreira, C. M., Hajlaoui, Y., Henry, D., Hubert, J., Pantel, R., Ricard A., and Zakrzewski, Z., Rev. Physique Appliquee, 17:707-727 (1982) 20. Rogers, J, and Asmussen, J., IEEE Trans. Plasma Sci., PS-1O:11-16 (1982) 21. Chaker, M., Moisan, M., and Zakrzewski, Z., Plasma Chem. Plasma Process., 6:79-96 (1986) 22. Aliev, Y. M., Ivanova, K. M., Moisan, M., and Shivarova, A. P., Plasma Sources Sci. Technol., 2:145-152 (1993) 23. Ferreira, C. M., J. Phys. D: Appl. Phys., 16:1673 (1983) 24. Moisan, M., and Zakrzewski, Z., in Microwave Excited Plasmas, (M. Moisan and J. Pelletier, eds.), Ch. 5, Elsevier, Amsterdam (1992) 25. Margot, J., Moisan, M., and Ricard, A., Appl. Spectrosc., 45:260-271 (1991) 26. Kapitza, P. L., Sov. Phys.- JETP, 30:973 (1970) 27. Massey, J. T., and Cannon, S. M., J. Appl. Phys., 36:361-372 (1965) 28. Massey, J. T., J. Appl. Phys., 36:373 (1965) 29. Ricard, A., Moisan, M., and Hubert, J., presented at theXVIlI Int. Conf Phenomena in Ionized Gases, contributed papers, p. 741, Budapest (1985) 30. Rowley, A. T., presented at the 43rd Annual Gaseous Electronics Conf, Abstract K-36, Champaign-Urbana, Illinois (1990) 31. Levy, D., presented atthe 43rdAnnuai Gaseous Electronics Conf, Abstract GB-2, Champaign-Urbana, Illinois (1990) 32. Beneking, c., and Anderer, P., J. Phys. D: Appl. Phys., 25:1470-1482 (1992) 33. Stangeby, P. C. and Daoud, M. A., Can. J. Phys., 53:1443-1448 (1975) 34. Margot, J., Johnston, T. W., and Musil, J., in Microwave Excited Plasmas, (M. Moisan and J. Pelletier, eds.), Ch. 6, Elsevier, Amsterdam (1992) 35. Hubert, 1., Moisan, M., and Zakrzewski, Z., Spectrochim. Acta, 4IB:205215 (1986)

Surface Wave Plasma Sources

249

36. Ferreira, C. M., Moisan, M., and Zakrzewski, Z., in Microwave Excited Plasmas, (M. Moisan and 1. Pelletier, 008.), Ch. 2, Elsevier, Amsterdam (1992) 37. Barlow, H. M., and Brown, 1., Radio Surface-Waves, Clarendon, Oxford (1962) 38. Moisan, M., Zakrzewski, Z., and Pantel, R., J. Phys. D: Appl. Phys. 12:219-237 (1979) 39. Moutoulas, C., Moisan, M., Bertrand, L., Hubert, J., Lachambre, 1. L., and Ricard, A., Appl. Phys. Lett., 46:323-325 (1985) 40. Moisan, M., Zakrzewski, Z., Pantel, R., and Leprince, P., IEEE Trans. Plasma Sci., PS-12:203-214 (1984) 41. Moisan, M. and Pelletier, 1., in Microwave Excited Plasmas, (M. Moisan and 1. Pelletier, eds.), Appendix 1.1, Elsevier, Amsterdam (1992) 42. Zakrzewski, Z., Moisan, M., and Sauve, G., in Microwave Discharges: Fundamentals and Applications, (C. M. Ferreira and M. Moisan, eds.) pp. 117-140, Plenum, New York (1993) 43. Moisan, M., Chaker, M., Zakrzewski, Z., and Paraszczak, 1., J. Phys. E: Sci. Instrum. 20:1356-1361 (1987) 44. IEEE Standard for safety levels with respect to human exposure to radio frequency electromagnetic fields, 3 kHz to 300 GHz, IEEE C95.1-1991 (revision of ANSI C, 95:1-1982), IEEE, New York (1992) 45. Hubert, J., Moisan, M., and Ricard, A., Spectrochim: Acta, 41B:205 (1979) 46. Moisan, M., Pantel, R., Hubert, 1., Bloyet, E., Leprince, P., Marec, 1., and Ricard, A., J. Microwave Power, 14:57 (1979) 47. Hanai, T., Coulombe, S., Moisan, M., and Hubert, 1., in Developments in A tomic Plasma SpectrochemicalAnalysis, (R. Barnes, ed.) London, Heyden (1981) 48. Zander, A., and Hieftje, G. M., Appl. Spectrosc., 35:357 (1981) 49. Abdallah, M. H., Coulombe, S., Merrnet, 1. M., and Hubert, 1., Spectrochim. Acta, 37B:583 (1982) 50. Chevrier, G., Hanai, T., Tran, K. C., and Hubert, 1., Can. J. Chem., 60:898 (1982) 51. Matousek, 1. P., Orr, B. 1., and Selby, M., Prog. Anal. Atom. Spectros., 7:275 (1984) 52. Deruaz, D., and Merrnet, 1. M.,Analysis, 14:707 (1986) 53. Takigawa, Y, Hanai, T., and Hubert, 1., J. High Resol. Chrom., 9:698 (1986) 54. Selby, M., and Hieftje, G. M., Spectrochim. Acta, 42B:285 (1987)

250 High Density Plasma Sources 55. Selby, M., Rezaaiyaan, R., and Hieftje, G. M., Appl. Spectrosc., 41:749 (1987) 56. Riviere, B., Mennet, 1. M., and Deruaz, D., J. Anal. Atom. Spectrosc., 2:705 (1987) 57. Galante, L. J., Selby, M., and Hieftje, G. M., Appl. Spectrosc., 42:559 (1988) 58. Besner, A., Hubert, 1., and Moisan, M., J Anal. Atom. Spectrosc., 3:863 (1988) 59. Lauzon, C., Tran, K. C., and Hubert, 1., J Anal. Atom. Spectrom., 3:901 (1988) 60. Luffer, D. R., Galante, L. J., David, P. A., NovotnyM., and Hieftje, G. M., Anal. Chem., 60:1365 (1988) 61. Galante, L. 1., Selby, M., Luffer, D. R., Hieftje, G. M., and Novotny, M., Anal. Chem., 60:1370 (1988) 62. Sing, R. L. A., Lauzon, C; Tran, K. C., and Hubert, 1., Appl. Spectrosc., 46:430 (1992) 63. Hubert, 1., Sing, R., Boudreau, D., Tran, K. C., Lauzon, C., and Moisan, M., in Microwave Discharges: Fundamentals and Applications, (C. M. Ferreira and M. Moisan, eds.), pp. 509-530, Plenum, New York (1993) 64. Zakrzewski, Z., Moisan, M., and Sauve, G., in Microwave Excited Plasmas, (M. Moisan and J. Pelletier, eds.), Ch. 4, Elsevier, Amsterdam (1992)