design and characterization of a planar magnetron radiofrequency glow discharge source for atomic...

SPECTROCHIMICA ACTA

PART B

E L S E V I E R Spectrochimica Acta Part B 50 (1995) 1109-1124

Design and characterization of a planar magnetron radiofrequency glow discharge source for atomic emission

spectrometry M.J. Heintz, G.M. Hieftje*

Department of Chemistry, Indiana University, Bloomington, IN 47405, USA

Received 19 September 1994; accepted 4 January 1995

Abstract

A planar magnetron radio-frequency-powered glow discharge source has been constructed and charac- terized. Electrical behavior, sputtering rates, and emission properties of the source have been studied with both conducting and insulating samples over a pressure range of 0.05 to 0.6 Torr, and over a forward- power range between 30 and 120 W. The bias voltage showed little or no dependence on power or on pressure between 0.2 and 0.6 Torr. However, at lower pressures there was a sharp increase in the voltage as pressure was dropped, signaling a change in the operational mode of the discharge. The magnetron source proved to have much higher sputtering rates for both conducting and insulating samples than a similar source without a magnet; the highest sputtering rates were found at 0.05 Torr. The dependence of emission on pressure was similar to that of previously described d.c. magnetron sources. Detection limits ranged from 1 to 50 ppm for elements in a conducting matrix.

Keywords: Equipment; Glow discharge; Magnetron; Radio frequency

1. I n t r o d u c t i o n

The glow discharge source has found widespread use in the analysis of solid samples [ 1,2]. In conjunction with atomic emission detection the glow discharge source can achieve detection limits usually at the ppm and in some cases sub-ppm levels [3,4]. Several commercial systems are available and are widely used in industry for the analysis of conducting alloys. Unfortu- nately, these commercial systems are l imited because they rely on an applied d.c. bias to create the glow discharge; consequently, the sample (cathode) must be electrically conducting. In recent years, however, there has been a large increase in the development and production of insulating materials; as a result, the demand for analyzing these samples has increased. Several methods have been used to allow the d.c. glow discharge to be used with nonconducting samples, including coating the insulating surface with a conductor [5], and pulverizing the sample and placing it in a conducting matrix [6]. A drawback of the second technique is that all information concerning the spatial characteristics of elements in the original matrix is lost.

* Corresponding author.

0584-8547/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0584-8547(95)01307-5

1110 M.J. Heintz, G.M. Hiefije/Spectrochimica Acta Part B 50 (1995) 1109-1124

Other problems are the added time invested in sample preparation and the attendant likelihood of contamination.

Radio-frequency-powered glow discharge devices have long been used in the etching and sputtering of semiconductors, and the fundamental nature of these devices has been well charac- terized [7,8]. Because the power which is usually delivered behind the sample is an a.c. signal, the insulating sample can be thought of as being the dielectric in a capacitor, which attenuates but does not block the applied a.c. voltage. A radio-frequency glow discharge (RFGD) has recently been introduced as a source for the analysis of insulating materials [9] and coupled to both atomic emission [10,11] and mass spectrometric [12,13] detection systems. A problem with the RFGD source is that there is a substantial voltage drop across insulating materials, so the power delivered to the plasma is dramatically reduced. In addition, heat can build up on the electrically insulating samples because they are usually heat insulators as well. In prac- tice, both of these problems can be alleviated by keeping the insulating sample very thin, although sample fragility can then become a concern.

In the past, several techniques have been used to enhance the performance of the d.c. glow discharge source, including microwave and r.f. boosting [14], jet-enhanced sputtering [15], and magnetic enhancement [16,17]. The addition of a magnetic field to the glow discharge volume is an attractive option because it does not require much modification to the original source configuration. The magnetic field alters the source characteristics in several ways, including enhancing its stability by moving the discharge away from the cell wall, increasing sputtering rates by changing the discharge shape and allowing operation at lower pressures, and improving excitation by slowing down the rate of electron loss in the discharge. A widely used method of magnetically modifying a glow discharge source is the application of a magnetic field anterior to the sample [17] in order to create a closed loop for trapping electrons near the sample surface. A d.c. version of this planar magnetron arrangement has been studied extensively [18-21]. There are several potential drawbacks to this magnetic field arrangement, including unequal sputtering across the sample surface and source drift caused by the changing magnetic field strength. The second drawback is not a great concern here; when insulators are analyzed there is already a slight emission drift caused by the changing capacitance of the eroding sample.

In the work presented here the design and initial characterization of a planar magnetron RFGD source are discussed. The voltage and sputtering characteristics of the magnetron source are compared to the RFGD source without any magnetic field enhancement. The features of the r.f. source are compared also to those observed with a d.c. magnetron source. In addition, preliminary detection limits will be presented.

2. Experimental

2.1. R . f glow discharge source

A schematic representation of the r.f. glow discharge cell is shown in Fig. 1. The discharge cell, which is grounded and acts as the anode, is constructed of stainless steel. The support gas is introduced into the cell near the window in order to minimize deposition of sample material on it. The source was viewed end-on through a 25 mm diameter sapphire window mounted in a 2.5 in. (6.35 cm) Conflat ® flange. The sample is isolated from the discharge cell by a Macor ® spacer with a 25 mm diameter opening; this large diameter allows the magnetically confined ring-shaped plasma (outer diameter is roughly 15 mm) to be separated easily from the spacer wall. The backing assembly, which holds the sample against the cell body, is also shown in Fig. 1. The r.f. power is carried to the cathode cooling block via an extension of the inner conductor of an MHV feedthrough. The aluminum cooling block is threaded into the backing assembly, which allows easy removal of the magnets. The thickness of the front surface of the cooling block was kept to 1 mm so the magnetic field would not be excessively attenuated. The ring and inner disk magnets are placed in the cooling block as shown in Fig. 2. The backing plate behind the magnets is constructed of cold drawn steel in order to increase the magnetic field strength on the sample side of the block. When the cooling block is threaded

M.J. Heintz, G.M. Hieftje/Spectrochimica Acta Part B 50 (1995) 1109-1124 Additional Port

Aluminum Cooling Block

RF C o n n e c t ~

Stainless-----.-~,~k~L~: : ! : ~ Steel insert ~

Threaded

O-ring seals 2 Vacuum ports I .(1" Kwlk Flange)

Iiii I::::

f Macor Spacer Anode Sample

Gas Inlet Port

1111

Sapphire Window (Conflat flange)

[ ] Aluminum ~ ] Stainless Steel

] Macor Fig. 1. Schematic drawing of the r.f. glow discharge cell and backing assembly.

Volume of Highest Emission Intensity

1 to5 mm :;:: ::1::::::::.. th,o samples ... : i iliiii::i

Aluminum Co Block

Optical Axis

~FMagnetic ield Liles ....

'XMaco,

~ H20 out

Cold Drawn Steel / % S t e e l ' ~ M a g n e t s Backing Plate Spring

Fig. 2. Schematic drawing of the magnetic and emission characteristics of the source.

into the feedthrough assembly, the spring (Fig. 2) imbedded in the backing plate forces the magnets against the front surface of the cooling block. The magnetic field was generated by an outer ring magnet (Dexter Corp., Grade 16 Samarium Cobalt, 19 mm o.d. x 11 mm i.d. x 6.4 mm thick) and an inner disk magnet (Jobmaster Corp., Grade 24 Samarium Cobalt, 9.5 mm dia. x 6.4 mm thick). The resulting magnetic field strength and pattern will be discussed in Section 3.1.

2.2. Support systems and power supply

Argon (Air Products, purity 99.9995%) was used as the discharge support gas. Cell pressure was regulated by varying the argon flow rate, which was controlled with a needle valve and measured with a Matheson rotameter. The cell was evacuated by means of a rotary vane pump (Balzers, model DUO 060A) and the cell pressure measured with a capacitance manometer (MKS Baratron, model 122A). The cell was evacuated through both of the vacuum ports (see Fig. 1), and the pressure was measured at one end of a 1 in. diameter Kwik flange ® Tee which was attached to one of these ports. The cooling block was maintained at 20°C with deionized

1112 M.J. Heintz, G.M. Hiefije/Spectrochimica Acta Part B 50 (1995) 1109-1124

water from a recirculating chiller (Neslab, model RTE-5B). Radio frequency power was sup- plied by an r.f. amplifier (Kalmus, model 170F), which can operate over the frequency range from 0.3 to 40 MHz at a maximum forward power of 200 W. A variable-voltage function generator/frequency synthesizer (Hewlett Packard, model 3325A) was used to drive the amplifier. Forward and reflected power were measured with a Heathkit model HM2140A r.f. power meter; the amplifier was impedance-matched to the source with a locally constructed L-type network (0-1000 pF variable capacitors and 1.1 IxH inductor). In all of these studies the source was operated at 13.56MHz. Initially (during the measurement of the source voltage characteristics) the final amplifier stage was driven with a high bias current in order to make it operate in approximately a type A amplifier mode (lower third harmonics). With this setting the reflected power could be kept below 5 W at 120 W forward power. For the remaining experiments a 30 MHz low-pass filter (Barker and Williamson, model F110/1500) was placed in the r.f. supply line after the amplifier stage; the reflected power was then below 1 W at 120 W forward power. The RG8U cable between the impedance-matching circuit and the glow discharge source was a maximum of 30 cm in length in order to minimize power loss to ground because of cable capacitance. The resulting stray capacitance to ground parallel to the source (measured with an HP 4262A LCR meter) was between 80 and 150 pF. This includes both the cable and the backing-assembly capacitance.

2.3. Atomic emission detection system

The emission from the source was f-matched to the monochromator (Hilger-Engis model 600, 0.60 m Czerny-Turner design with 1200 lines/mm grating, f/5.2). The optical system (utilizing a single quartz lens) was arranged so the zone of highest emission intensity (see Fig. 2) and was focused onto the slit at unity magnification. Unless otherwise stated, the entrance and exit slit were both 2 mm high and 20 Ixm wide. A PMT (RCA, model R446) biased at 800 V was used as the detector, and its output current was fed to a picoammeter (Keithley, model 414S) for readout. The voltage from the picoammeter was passed through a 10 Hz low-pass Butterworth filter (Krohn Hite, model 3342), to a Macintosh Ilfx computer equipped with a data acquisition-board (National Instruments Corp., board NB-MIO-16-XL). A locally written program which determines figures of merit for a system running LabVIEW II ® software (National Instruments Corp.) was used for data collection, treatment and storage [22].

2.4. Magnetic-field and voltage measurements

Magnetic fields within the source were measured by means of a digital Gaussmeter (AML Inc., model GM1A) equipped with an axial Hall probe (AML Inc., model PA70). The shape of the probe constrained the resolution (to roughly 2 mm) and the possible probe positions which can be used. The axial magnetic field measurements were performed roughly 1 mm from the cooling-chamber surface, while the parallel magnetic fields was measured at a distance of 4 - 6 mm above the chamber surface. In order to determine the electrical features of the source a high-voltage probe (Tektronix, model P6057, 1 pF input capacitance) was connected to the electrical feedthrough of the cathode block via an MHV T connector. The probe output was fed to a 250 MHz oscilloscope (Tektronix, model 485) for visual display of the source volt- ages.

2.5. Standard conditions and materials

The glow discharge was operated over a pressure range between 0.6 and 0.05 Torr. The lower limit of the pressure range was governed by the base pressure which can be achieved by the pump (5 mTorr) and by the need to maintain a reasonable argon flow rate to the cell (dependent on the conductance-limiting diameter of the pumping ports). For these reasons, the source was not operated below 0.05 Torr. The discharge behavior above 0.6 Torr was similar to that of a conventional (no magnetic field) discharge and was not explored in detail. Electro- lytic copper was used in the characterization of the source with a conducting cathode. For

M.J. Heintz, G.M. Hieftje/Spectrochimica Acta Part B 50 (1995)1109-1124 1113

the s tudy o f i n su la t ing mate r ia l s , n o n - s t a n d a r d i z e d quar tz s amp le s were emp loyed . Fo r the

d e t e r m i n a t i o n o f de t ec t ion l imi ts 2 m m th ick a l u m i n u m s tandards ( A L C O A ) were used.

3. Results and discussion

3.1. Magnetic field characteristics

The expected magnetic field lines and a schematic representation of the magnet positions are shown in Fig. 2. The electrons in the discharge follow a helical path centered on the magnetic field lines. Because of the elliptical shape of the field, the electrons approach the negatively biased sample sheath and are reflected back along their incoming path. They then exit the sheath region back along the magnetic field lines and are constrained to enter the sheath at the other intersection of the magnetic field and the negative electric field. In this manner they become "trapped" in the discharge until deflected by collision from their constrained paths.

The true pattern of the magnetic field is shown in Figs. 3(a) and 3(b). In Fig. 3(a) the magnetic field strength perpendicular to the cooling block surface is plotted versus distance from the cooling-block surface. The sharp roll-off of magnetic field strength emphasizes the need for a very thin sample (from previous work with a d.c. discharge, a magnetic field strength of at least 600-800 Gauss is needed to achieve good electron trapping [19]). The measured parallel and perpendicular magnetic field components are plotted versus radial distance from

a .

2 . S -

v 2.0

1 5 I = 1o!

0.5

0.0 0 1 2 3 4 5

Dis tance f rom the cool ing block (mm)

b.

0.5

O .t¢ v 0.3

; =

.~ 0.1

e,,

m -0.1 E 0

m -o.3 ¢0

O.

-0.5 -1.5 -1.0 -0.5 0.0 0.5 1.0

Dis tance f rom center (cm)

- 2.5 "o Q

- 1 . 5 = o.

C

-o.5 ~ 3 m

- - 0 . 5 "

W - -1.5

-z.s .5

Fig. 3. (a) Dependence of the perpendicular magnetic field strength on distance from the cooling block surface. (b) Parallel and perpendicular magnetic field strengths as the probe is moved along the plane of the cooling block surface. The parallel field was measured at a distance of 3-4 mm from the surface.

1114 M.J. Heintz, G.M. Hieftje/Spectrochimica Acta Part B 50 (1995) 1109-1124

the magnetron center in Fig. 3(b). The general trends agree with what was expected from previous studies [18,19]. The apparent asymmetry can be explained by the limited spatial resolution in the measurements and because the two magnets might not be perfectly concentric. The zone of maximum emission intensity, shown schematically in Fig. 2, coincides with the maximum (or minimum) of the component of the magnetic field that lies parallel to the sample.

3.2. Voltage characteristics

The electrical behavior of the magnetron source is dramatically different from that of a conventional glow discharge source. The electrical features of a d.c. planar magnetron have previously been studied by Sacks and co-workers [18, 19]. In brief, they found that the current- to-voltage ratio of the d.c. magnetron showed much less dependence on pressure and power than did an unenhanced source. In the pressure range studied here, magnetron discharges can operate in a regime of higher current-to-voltage ratio where an increase in r.f. power causes only a current rise.

Fig. 4 shows the dependence of the d.c. bias potential (which is slightly less than the peak r.f. voltage) on cell pressure (for an 0.8 mm thick copper sample) for r.f. powers of 50, 100 and 150 W. It can be seen that the glow discharge with no magnetic field assistance operates at a much higher voltage than does the magnetron source. This is caused by the increased probabil i ty of the magnetically trapped electrons to ionize the fill gas, which leads to a higher charge density in the plasma and thus to a greater conductance. The voltage dependence on power is extremely small because of the much higher conductivity of the magnetically enhanced discharge.

The bias voltage of the magnetron discharge does not change with pressure above 0.2 Torr; however, as the pressure is dropped below 0.2 Torr there is a small rise in the bias potential. This rise signals a change in the operational mode of the discharge. A similar change was observed also for the d.c. glow discharge [ 19] and was attributed to a switch from a premag- netron mode of operation at higher pressures to true magnetron operation at lower pressure. At higher pressures most electrons in the magnetron volume are lost via collision before they go through a single "trapped" cycle, so very few are lost to the cell wall before they collide with an atom. This high collision rate leads to a very dense discharge. As the pressure is dropped below 0.2 Torr the electrons are trapped for a longer time, and the probabili ty that they will be lost to the cell wall before they can induce an ionization event increases. This leads to a lower density discharge, and therefore to a lower conductance source.

The dependence of the d.c. bias potential on cell pressure and sample thickness is shown in Fig. 5. Not surprisingly, the thicker the sample, the weaker the magnetic field and the less efficient the electron trapping. Consequently, the bias voltage rises. From Fig. 5 it appears that the limit for sample thickness in the present source is between 2 and 3 mm; above this thickness

A 3>

m tl= k~ p.

.= 0

11. u~ t~

~5 ¢,.) c!

800

700

600 -

500

400

300

200

100

%, '1=1 1,1,

0 0.1 0.2 0.3 0.4 0.5 0.6

Pressure (torr)

Fig. 4. Dependence of the d.c. bias potential on cell pressure for several r.f. power levels with (magnetron) and without (standard) a magnetic field (0.8 mm thick copper sample): - - 4 ~ - magnetron, 50 W; - -O- - , magnetron, 100 W; --Ig--, magnetron 150 W; - - • - -, standard 50 W; - - O - -, standard 100 W; - - • - -, standard 150 W.

M.J. Heintz, G.M. Hieftje/Spectrochimica Acta Part B 50 (1995) 1109-1124 1115

500

450 - > - - . 4 0 0 - O R

350

o 300

=" 2so

aO 200

150

o . , - Ib . , . ~

100 0 011 01.2 0'.3 0~.4 0'.5 0'.6

Pressure (torr) Fig. 5. Dependence of the d.c. bias potential on the cell pressure for different sample thicknesses (50 W forward power, magnetron source unless otherwise stated): ~ 0.8 mm Cu; ~ , 1.6 mm Cu; -- • - - , 3.2 mm Cu; - - O - -, no field.

the discharge begins to assume some of the features of the unenhanced source. This behavior illustrates one of the deficiencies of the magnetron source: there may be significant drift in source characteristics as the sample is ablated and the effective magnetic field in the discharge changes. This drift would be similar to what would be expected as an insulating sample is ablated and the sample capacitance is altered.

As shown in Fig. 6, when quartz samples are used as the cathode, the r.f. voltage measured behind the sample (the d.c. bias is blocked by the insulator) shows very little dependence on cell pressure and sample thickness, but is highly dependent upon the applied power. The main path to ground is no longer through the discharge but rather is across the stray capacitance parallel to the source.

In summary, the electrical behavior of the r.f. planar magnetron is very similar to that of the d.c. version constructed by Sacks and co-workers [19]. The slightly different pressures found for the transition between the two modes of operation (low-pressure trapping and more conventional high-pressure operation) are probably due to differences in magnetic field strength, magnet geometry, and the smaller overall size of the r.f. discharge cell. The general similarity between the two discharges was expected because they both rely on electron ionization mechanisms for sustaining the discharge.

3.3. Sputtering characteristics

The effect of the magnetic field on the cathode sputtering rate (i.e. sample removal rate) is shown in Fig. 7. The nearly linear increase in sputtering rate with pressure for the source with

A

e l

0 O.

S a . M. I ¢

lOOO

900

800

700

600

500

400-

300 o

~ - - - - 0 - - - - 0 - ~ - - O - . . . . - 0

o'.1 0.2 0.3 o14 o'.s o'.s 0.7 Pressure (torr)

Fig. 6. Dependence of the r.f. peak potential on cell pressure for different sample thicknesses and discharge power levels for the magnetron source (notice offset of zero on the vertical axis): --•----, 1.8 mm quartz, 50 W; - -O- - , 3.2 mm quartz, 50 W; - - • - -, 1.8 mm quartz, 100 W.

1116 M.J. Heintz, G.M. Hieftje/Spectrochimica Acta Part B 50

10000 - A

8000 -

& 5ooo-

4000-

2 0 0 0 -

0 0

~ m m

Q u a r t z ~ % ~

no .m, - 1 I I I I I

0.1 0.2 0.3 0.4 0.5 0.6 Pressure (torr)

(1995) 1109-1124

- 3 0 0 u)

- I

- 2OO ~

15o 9. ==

100

50 { 5"

0 ~

0 . 7

Fig. 7. Dependence of the sputtering (sample removal) rate on cell pressure for four different discharge conditions (100 W forward power, magnetron source unless otherwise stated) (notice different vertical scales for Cu and quartz sputtering rates): --O---, copper, 0.8 mm thick; ---B~, copper, 3.0 mm thick; ~O----, copper, no magnets; - - O - -, quartz, 1.4 mm thick.

no magnetic field applied has already been reported [23]. As the pressure is raised there is an increase in discharge current and density, which leads in turn to a greater number of ions being accelerated across the sheath to sputter the sample. The high magnetic field discharge (0.8 mm thick Cu sample) exhibits a much different trend, with its greatest sputtering rate (10000 Ixg min -~ at 100 W) occurring near the lowest pressure attainable by our pumping system. There is a dramatic rise in sputtering rate as pressure is dropped from 0.2 to 0.1 Torr, roughly the same pressure range as where a change in the electrical characteristics of the discharge occur (see Fig. 4). The low magnetic field case, which occurs when a 3.0 mm thick copper sample is sputtered (see Fig. 3(a)), shows behavior intermediate between the other two situations.

The dependence of sputtering rate on pressure for a quartz sample is similar to that for the copper sample at high magnetic field, but the absolute sputtering rate is over an order of magnitude lower. This lower sputtering rate is partially due to the non-conductive sample being slightly thicker, but in large part is caused by the lower power actually delivered to the glow discharge when an insulating sample is employed.

The dramatic jump in sputtering rate of both copper and quartz as the pressure is reduced below 0.2 Torr was unexpected; from the voltage data (see Fig. 4), the density of the discharge is probably less at the lower pressure. The greater sputtering is probably caused by two factors: reduced redeposition and a change in the discharge shape. Fig. 8 shows the theoretical fraction of sputtered atoms that are redeposited on the sample as a function of pressure. These values were calculated using the relation [5]

Fr 1 Fs - 4kT

1 + 3L~2 tr p

where FJFs is the ratio of redeposited atoms to sputtered atoms, k is Boltzmann's constant, T is gas temperature, L is the cathode-anode separation, tr is the collisional cross section, and p is the pressure (in Pascals). For Fig. 8 a plasma gas temperature of 400 K and a ca thode- anode separation of 5 mm were assumed. The fractional redeposition is very large at the higher pressures because of the short mean free path of atoms leaving the sample surface. This com- puted higher rate of redeposition was confirmed by the visible redeposition of thin films (at the higher pressures) on the area of the sample which is not actively sputtered.

A change in the shape of the plasma at lower pressures is evident from an increase in the sputtered area (see Fig. 8). The sputtered area was calculated by measuring the inner and outer diameter of the ring-shaped sputtering track. The measurement error is greater at the lower pressures because the track is less well defined. The greater sputtering area at low pressures can be attributed to more rapid diffusion of the ions because of their larger mean free paths. This diffusion leads to a change in the plasma geometry in which the glow is more in contact


7 5 0 ~ 1 0 0

7 0 0 - - - - " ~ - - ~ - - ~ " - ' ~

o° 6 5 0 80 " 600 5 5 0 7 0 _ol.

s 0 0 s 0

4so so

400 -4o

350 30 0 0.1 0.2 0.3 0.4 015 016 0.7

Pressure ( torr) Fig. 8. Possible reasons for the observed increase in sample sputtering rate at lower pressures (cf. Fig. 7). The theoreti-

cal redeposition rate [5] and measured dependence of the sputtering area on cell pressure are plotted (50 W forward power, 0.8 mm thick copper sample): - - O - -, calculated percent redeposition; - - I ~ - - , area sputtered.

with the sheath, causing an increase in the sputtering. This increase in sputtered area has not been reported for the d.c. magnetron, and may be dependent on the oscillating sheath thickness in an r.f. discharge.

3.4. Emission characteristics

It is useful to compare the features of the r.f.-driven planar magnetron source to the d.c.- powered planar magnetron source, which has been thoroughly analyzed [17-20]. The sputtering rates and voltage behavior of the d.c. and r.f. planar magnetron sources are very similar, so it would be expected that their emission characteristics would also be similar. In addition, the emission features of the discharge with a quartz cathode are compared below to those seen with a conductive cathode.

3.4.1. Conductive cathode Copper plates were sputtered at several cell pressures and r.f. power levels. The effect of

pressure on background at several wavelengths is shown in Fig. 9. The background was taken from a sideband region within 15 nm of the emission feature of interest. Several trends are evident: the background levels at 200 and 300 nm are much more intense than at 400 nm, there is a general drop in the background at pressures below 0.1 Torr, and the most intense background occurs at lower pressures as longer wavelengths are monitored. Because the energy of the trapped electrons is thought to be relatively high, it is not surprising that the background is more intense at the shorter wavelengths. The drop in background from 300 to 200 nm may be a result of the nonlinear response of the measurement system, caused by two factors; the

8"0 1 . ~ - - - - - - - - ~ 0 0 0.55 nm

1 . - / . \ o. o ~ 0.45 m o. ;;;,-, "" " 0.40

4.0 4 ~ ~

2.0 ! ' 0.30 0 0'.1 012 0'.3 014 0'.5 016 0.7

Pressure (torr)

Fig. 9. Dependence of background emission on the cell pressure at several wavelengths (100 W forward power, 0.8 mm thick copper sample): me--- - , 200 nm; - - O - - - , 300 nm; - - • - -, 400 nm.


spectrometer grating is blazed at 500 nm and the radiant sensitivity of the PMT falls off by at least 50% from 300 to 200 nm.

The background level exhibits a maximum with pressure somewhere between 0.1 and 0.4 Torr. This agrees with the sputtering and voltage data (see Figs. 4 and 7), because there is a change in the mode of operation of the source at lower pressures, and we expect to see the plasma density be at a maximum in this pressure range. The background probably results from a combination of emission from electron-ion recombination and from unresolved atomic transitions.

The dependence of argon neutral atom emission on cell pressure and r.f. forward power is shown in Fig. 10(a). The background exhibits a maximum at 0 .1-0.2 Torr, but the argon emission unexpectedly peaks at the lowest pressure attainable, 0.05 Torr. If the argon atom emission were from the ring plasma (generated by the ring-shaped electron-trapping magnetic field), we would expect it to show a maximum at 0.1 Torr or above because the plasma density appears to be greatest at these pressures (see Figs. 4 and 9). In addition, as is apparent in Fig. 10(a), an increase in power causes no significant change in the argon atom emission intensity. The increase in argon signal as the pressure is lowered was observed also with the d.c. magnetron [19] and was attributed to an increase in the density of a secondary discharge (the part of the discharge created by electrons outside the trapping magnetic field) at lower pressures. This secondary discharge extends from the cathode to the anode, and has lower electron energies because no trapping of primary electrons occurs in it. This may lead to a relatively

a,

3.5 - - 6 . 0

• / ~ ' - ~ . - 5 . 0 m 2.5 "] - t "' ,,,, = ; " - °

- 4 . 0 2.0 • • e-

l f - ~ e - _ ~ • = ~ 1.5 -3.o ~

-2.o ~ °"5"1 ,.-- "=" r - ~ 0.0 / I ¢ r - - - - ' ~ 1.0

o o'.1 o'.2 o'.3 o'.4 o'.5 0.6 0.7 Pressure (torr)

b.

2.21.gF__ ] ~ . ~ A r II 2.0

N P; 1.8

g /

0 01, 0'.4 o'.5 o'.6 Pressure ( tort)

- 5 . 0

- 4 . 5

- 4 . 0

- 3 . 5

-3.0 ~,

-2.5 :~

-2.0

1.5 0.7

Fig. 10. (a) Dependence of argon atom emission at 415 nm on cell pressure and r.f. power level. (b) Dependence of argon ion emission intensity at 413 nm on cell pressure and r.f. power level (0.8 mm thick copper sample): --II-- , 30 W Signal; - - IN - -, 30 W Background; --O--, 70 W Signal; - - • - -, 70 W Background; - -O-- , 110 W Signal; - - C)- -, II0W Background.


greater number of low-energy transitions. The density of this secondary discharge might increase at low pressures because of the longer mean free paths in the discharge at low pressure.

The dependence of argon ion emission at 413 nm on cell pressure and r.f. power is seen in Fig. 10(b). The trends here are similar to those observed with the d.c. magnetron [19], except for the drop in emission that occurs with the r.f. magnetron at pressures below 0.2 Torr. The ion line shows the same dependence on pressure as does the background, which is expected, because ionization occurs primarily in the ring discharge where high energy electrons are located. The similarities therefore can be attributed to the background and Ar ion emission both being dependent on ring discharge density. The large loss in Ar ion emission as pressure is dropped from 0.1 Torr to 0.05 Tort can most likely be attributed to an expansion of the ring discharge which causes the density of the discharge to decrease. This behavior is again not reported to occur for the d.c. magnetron. The background is slightly different for the atom (Fig. 10(a)) and ion (Fig. 10(b)) lines, possibly because of interference by unresolved emission lines. Although not shown in Fig. 10, the background noise is proportional to the background (multiplicative noise is dominant), so a maximum signal-to-noise ratio occurs at 0.1 Torr for the Ar 413 nm ion line.

The effect of r.f. power and cell pressure on copper atom emission at 202 nm (a resonance line) is shown in Fig. 1 l(a). The background peaks at 0.4 Torr, but the copper atom emission shows a maximum at 0.1 Torr. The emission intensity of the copper atom will depend princi- pally upon three factors: plasma density, sputtering rate and the degree of self reversal. The increase in copper emission as pressure is lowered to 0.1 Torr is probably due to the greater sputtering rate (see Fig. 7). The conformational change in the ring discharge (lowering its density) that was discussed earlier may account for the sudden loss in emission as the pressure is dropped below 0.1 Torr. The maximum signal-to-sideband-noise ratio occurs at 0.05 Torr.

The behavior of copper emission at 296 nm (a non-resonance line) is shown in Fig. l l(b), and is slightly different from that of the resonance emission at 202 nm (Fig. 1 l(a)). The copper non-resonance line emission peaks between 0.2 and 0.3 Torr (as does the background). The resulting signal-to-background-noise ratio is maximal at 0.2 Torr.

There are two factors which may account for the different optimal pressures for the two Cu (I) transitions. Although the upper-state energies are similar (44 963 cm -1 for the 296 nm transition and 49 383 cm -~ for the 202 nm line), the upper-state electron configurations are very different (3d94s(aD)4p for the 296 nm line and 3dl°(~S)5p for the 202 nm line). In previous work with a boosted glow discharge source [24] the 296 nm line and an emission line arising from a transition from the 5p level displayed quite a different response to changes in discharge conditions. This suggests that dissimilar processes could be responsible for populating the two levels.

The other possible reason for the slightly different optimal pressures is self absorption, which is probably greater for the 202 nm resonance line (Fig. l l(a)) at higher pressures. This is contrary to what would be expected from the sputtering data, because the sputtering rate is higher at the lower pressures. A greater self absorption at the higher pressures could be caused by the longer residence time of sputtered atoms in the discharge. A sputtered atom coming off the sample with relatively high velocity might not remain in the discharge at a lower pressure, so self absorption would be less. From this argument, it would seem that an analyte atom sputterd at low pressure would also have less of a chance to be excited, so excitation efficiency would be lower. The trade-off between self absorption and excitation efficiency might then lead to the observed optimal pressure of 0.1 Torr for the resonance emission at 202 nm. Because self absorption is not a problem for the 296 nm non-resonance line, its optimal pressure is slightly higher.

The dependence of copper ion emission at 199.97 nm on cell pressure and r.f. power is shown in Fig. 11(c). In this region of the spectrum the background levels at the two power settings exhibit maxima at 0.4 Torr. This is contrary to what would be expected if the energy of the electrons increased as the pressure is lowered, and may be caused by a change in the geometry of the ring plasma, as discussed earlier. The copper ion emission mirrors the pressure dependence of the background emission, much as the argon ion emission which was discussed earlier (see Fig. 10(b)). This is expected; the copper ion emission probably originates from the


a .

5 . 5 q , . , . , . , , , , , 0 , . _ _

4.5-1 ~ i r ~, , 4 ~ ,

,01 - . . 3.5d I •

• ] I I I I I 0 0.1 0.2 0,3 0.4 0.5 0.6

Pressure (tort)

- 6 . 0

-5.0 m 60 o pc

( o -4.0 a c

Q.

-3.0 o

-2.o ~

1.0 0.7

b.

600 -

500 -

? i 400 -

300 -

200 -

100

-8 .0

i / c~ ' ~" ~ 7.0

" ' S " . . . . ' ~ ' ~ " " ,~ 6.0 s ~ . ~ "% "e 5.0

2.0 ,>>

1.0

0.0 0.1 012 013 014 0'.5 016 0.7

Pressure (torr)

A < ?

o v -

( l l c o l

12.0-

10.0 -

8.0-

6.0-

4 . 0 "

2.0 0

Y -7.0

-6.0 O l m

-5.o g- ta~ 3

-4 .0 ~ o .

-3.0 ~

-2 .0

1.0 011 o12 o13 01, 015 o15

Pressure (torr)

Fig. 11. (a) Dependence of copper neutral-atom emission at 202 nm (resonance line) on cell pressure and r.f. power level. (b) Dependence of copper neutral-atom emission at 296 nm (non-resonance line) on cell pressure and r.f. power level. (c) Dependence of copper ion emission at 199.97 nm on cell pressure and r.f. power level (0.8 mm thick copper sample). --I~----, 70 W Signal; - - • - -, 70 W Background; --O---, 110 W Signal; - - O - -, 110 W Background.

r ing d i s cha rge b e c a u s e o f the h i g h e r e n e r g y e l ec t rons tha t are p resen t there and the fact tha t

the c o p p e r spec ies o r ig ina t e at the c a t h o d e surface , c lose to the r ing emiss ion .

Overa l l , the e m i s s i o n fea tures o f the r.f. p l ana r m a g n e t r o n are s imi la r to those o b s e r v e d wi th

the d.c. p l ana r m a g n e t r o n . Th i s is p r o b a b l y b e c a u s e b o t h re ly p r imar i ly on e l ec t ron exci ta t ion .

The d i f f e rences tha t ex i s t c an poss ib ly be a t t r ibu ted to a c h a n g e in the p l a s m a c o n f o r m a t i o n .

Th i s c h a n g e in p l a s m a shape m a y be due to the osc i l l a t ing p l a s m a shea th or to a d i f f e rence

in the phys i ca l d i m e n s i o n s o f the m a g n e t s and cel l u sed wi th the two sources .


3.4.2. Insulating samples The difficulty inherent in analyzing insulating materials (higher required voltage, higher

impedances, and limited physical strength) is exacerbated by the fact that they are often molecular solids. The presence of high concentrations of possible interferents (such as oxygen in quartz) and the difficulty of producing single atoms make the emission behavior even more unpredictable. The goal of these studies was to determine whether the emission trends changed appreciably when the sample was changed from a conductor to an insulator.

The effect of cell pressure on argon atom and ion emission, at 415 and 413 nm, respectively, with a quartz sample as the cathode is shown in Figs. 12(a) and 12(b). These measurements were carded out with a slit width of 75 Ixm and a slit height of 4 mm because of the low emission levels. The background near the two lines is markedly different from that seen when a copper (conductive) cathode is used (cf. Fig. 10). This change in background with cathode material is not surprising and has been observed also with conventional glow discharges and with the d.c. magnetron [19]. The dependence of the background on cathode material could be caused by a change in the discharge gas makeup (possibly from a greater abundance of molecular gas species, especially oxides, when quartz is analyzed) due to the high relative population of analyte ions to argon ions (estimated at around 1%) in the discharge. It may also be that a non-conductive cathode produces a much lower plasma density (and electron temperature) because it attenuates the useful r.f. power. Calculations show that stray capacitance causes only about 10-20% of the power to be delivered to the glow discharge when insulating samples are used, and a comparison of the relative background levels (between discharges with insulating and conducting samples) suggests that the discharge density is con-

a .

2.8 2.6 2.4 2.2- 2.0- 1.8 1.6 1.4 1.2

0.0 011 l ] l l i

0.2 0.3 0.4 0.5 0.6 Pressure (torr)

-2.8 -2.6 -2.4 !N -2.2 - 2.0 ~

1.8 1.6 1.4

0.7

b.

A ,< 0 ,e--

8"0 7 ~,

6.0-

5.0 t ~ ' ~ ~ 4.0

3.0 i 0.0 0.1 0'.2 0'.3 0'.4 0'.5 0'.6

Pressure (torr)

-2.4 -2.2

-2.0 !o -1.8 -1.6

1.4 " 1.2 1.0

0.7

Fig. 12. (a) Dependence of argon atom emission at 415 nm on cell pressure. (b) Dependence of argon ion emission at 413 nm on cell pressure (50 W forward power, 1.4 mm thick quartz sample): mOB, Signal; - - O - -, Background.


-7.0 100t, 9.o -6.o ~

-5.0 ~ 7.0 R

-4.0 "~

..o 1 -.,x,, - 3 . 0 >

4 . 0 1 , , , ~ , T 2.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Pressure (tort)

Fig. 13. Dependence of silicon atom emission at 288 nm (non-resonance line) on cell pressure (50 W forward power, 1.4 nun thick quartz sample): --O--, Signal; - - O - -, Background.

siderably lower with the quartz cathode. The argon atom and ion emission lines exhibit the same general dependence on pressure for the quartz as they did for the copper sample (see Fig. 10). The argon atom-to-ion emission ratio, calculated from the data in Figs. 12(a) and 12(b), is lower for the quartz-cathode discharge than the discharge with the conducting cathode; this difference is probably due to the lower delivered power with the quartz cathode, which causes the plasma to be "cooler".

The dependence of silicon atom emission at 288 nm (a non-resonance line) on pressure is shown in Fig. 13. There is a maximum Si emission signal at 0.1 Torr and a rapid decrease as the pressure is lowered to 0.05 Torr. This trend is similar to what was seen for copper atom emission (see Fig. 1 l(a)) and is probably dependent on the same plasma parameters.

3.5. Optimization and determination of preliminary detection limits

Preliminary detection limits of the r.f. planar magnetron were determined using the 0.5 m monochromator described earlier. It would be expected that detection limits would be at least an order of magnitude better with a high-resolution system; this possible improvement will be investigated in a future publication. It is anticipated that the background will be much lower with a high-resolution system because minor lines can then be resolved from the background (see discussion in Section 3.4.1.) This background-structure problem is illustrated in the spectrum shown in Fig. 14, in which the iron line is barely resolved from nearby interfering lines. Close examination of the background indicates that there are very few spectral regions which show no line structure. Therefore, the first consideration when choosing which emission lines to study was whether the line was free from spectral interferences. Detection limits for several elements in an aluminum matrix were calculated using the formula of Boumans and Vrakk- ing [25]. The aluminum was cut to 2 mm thickness; thinner samples might give higher signal- to-noise values but would be fragile and might not withstand the required pressure differential.

3.5-

3 . 0 - -

2.0-

~ 1.5-

~ 1.0-

0.5-

0.0 I I I I I l l l i [ a l l l l l a l l l l l l l |

350 355 360 365 370 375

Wavelength (nm)

Fig. 14. Emission spectrum of an aluminum standard (70 W forward power, 0.2 Tort).

M.Z 1123 Heintz, G.M. Hieftje/Spectrochimica A cta Part B 50 (1995) 1109-1124

Table 1 Detection limits of several elements in an aluminum matrix

Element Wavelength/nm Detection limit/ppm

Beryllium 234.9 1 Chromium 425.4 27 Iron 372.0 45 Magnesium 285.2 1.7 Manganese 403.1 35 Silicon 251.4 40

The signal and signal-to-background-noise ratio (S[Nb) w e r e linear with concentration at all pressures (0.05-0.4Torr) over the three orders of magnitude concentration range (100- 20000 ppm) of the minor constituents that were studied. For each element the three most intense emission lines were selected from the NBS tables [26]. Although the lines in the NBS tables were not measured with a glow discharge source, it was felt that the tables would give a good indication of the relative line strengths in the glow dicharge (ion lines, resonance lines, and high energy transitions were disregarded). The SINb ratio of these lines were measured at the various pressures and the line for each element with the best SINb value and its corresponding detection limit are reported here. The chosen line was optimized for pressure (in 0.1 Torr steps) and power (in 20 W steps). Large steps were used because we wished to determine the relative detection limits only to an order of magnitude; a larger array of samples will be needed to conduct a more thorough study. It was found that there was only slight improvement in SINb values when r.f. power levels above 70 W were used, so all detection limits were determined with a 70 W plasma to minimize sample deposition on the walls of the discharge chamber. The SINb values of all emission lines of the minor constituents were found to be greatest at cell pressures between 0.2 and 0.3 Torr. The detection limits for several elements are listed in Table 1 and range from 1-45 ppm.

The source is currently being modified so insulating standards can be analyzed. Soda-lime quartz standards proved too brittle to withstand the pressure differential and heat. Preliminary work with quartz leads us to expect detection limits roughly an order of magnitude worse than those reported here for conducting samples.

4. Conclusions

The addition of a magnetic field in a planar magnetron configuration enhances sample ablation rates and analyte line emission intensities over those from an unenhanced (conventional) r.f. glow discharge. The emission characteristics of the r.f. magnetron and d.c. magnetron sources appear to be very similar over the pressure regions studied here. This similarity was expected because excitation in both sources is probably similar and depends primarily on electron collisions. Sputtering rates of 10000 ixgmin -~ (0.05 Torr, 100W) for a 0.8 mm thick copper sample and 250 IJ, g min-1 (0.05 Tort, 70 W) for a 1.4 mm thick quartz sample were achieved. The dependence of sputtering rates on pressure appeared to be similar in the r.f. and d.c. discharges also, although the sputtering profiles (i.e. the shape and area of the sputter track) were not the same. In particular, the sputtered area in the r.f. magnetron increases with a drop in pressure. Preliminary detection limits in an aluminum matrix range from 1-45 ppm, and estimated detection limits in a quartz matrix are an order of magnitude worse.

Further studies of the sputtering profile in the magnetron source need to be carried out in order to determine its ability to perform depth profiling. A systematic determination of detection limits on a high-resolution spectrometer for both insulating and conducting samples also must be carded out. At present, the background noise is suspected to be dependent to some degree upon unresolved atomic emission lines.

1124 M.J. Heintz, G.M. Hieflje/Spectrochimica Acta Part B 50 (1995) 1109-1124

Acknowledgements

This work was suppor ted by the Nat ional Sc ience Founda t ion through grant CHE90-20631

and by the Nat ional Inst i tutes o f Heal th through grant RO1 GM46853.

References

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design and characterization of a planar magnetron radiofrequency glow discharge source for atomic...

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