Optical emission spectroscopy for analysis of broad ion beams

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1181

to 118411989

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Optical emission broad ion beams F Heinrich, H-P Stall, H-C Scheer Str 53, D- 1000 Berlin 33, FRG

Pergamon

spectroscopy and P Hoffmann,

for analysis

Fraunhofer-lnstitutftir

Press plc

of

Mikrostrukturtechnik,

Dillenburger

In sputter etching and reactive ion beam etching (RIBE), contamination and disturbance free in situ methods are desired for control of etch relevant quantities like energy and angular distribution of particles. These requirements cannot be fulfilled b y the conventional electrical methods. In this paper we show that optical emission spectroscopy is a valuable tool for in situ diagnostics of broad ion beams. We report on measurements in Argon ion beams of 300-1000 ey energy. Several emission lines of ffst and slow atoms and ions are detected in the wavelength range 4000-7000 A at a resolution of better than 0.7 A. Impact of fast particles and electrons on neutral background gas atoms are specified as main excitation mechanisms. From the Doppler-shifted emission lines we obtain information on energy and angular distribution of the beam particles.

1. Introduction cooled

Knowledge of particle energy and angular distribution is important for several applications of broad ion beams like sputter etching and reactive ion beam etching (RIBE)‘-‘. A disadvantage of the conventional electrical methods’ ’ (e.g. cup measurements) is the fact that they cannot be used without disturbing the beam. Therefore they are not adequate for in situ control of beam processing. Furthermore errors have to be taken into account due to secondary electron emission and to changing electrical detector characteristics during the ion impact. With optical emission spectroscopy we possess a means for in situ diagnostics of broad ion beams, free of disturbances and contamination.

nochromator

extraction

photomultiplier tube 1m

grid

2. Experimental The geometrical arrangement for our experiments is schematically shown in Figure 1. The measurements were carried out in a commercially available ion beam etcher (OXI-l50-LLC, Oxford Instruments). The ions produced by a Kaufman type ion source are accelerated into the process chamber by two grid optics (15 cm diameter, 3100 holes). The first grid is kept floating and the second grid has a negative voltage r/,, with respect to ground potential. The source plasma is held at a positive voltage, U,,,, above ground by a multiple ring anode concentric to the axis of the source. Hence, the ions are accelerated by a voltage of z (U,,,,, + 1U,,,j) towards the second grid and decelerated by U,,, when entering the process chamber thus leading to an ion A small correction to higher energies has to energy of z eU,,, be made to this value because the plasma potential is higher than any contacting electrode. Due to the potential situation electrons cannot escape from the source. Additional electron injection for the purpose of beam neutralization is possible via a plasma bridge neutralizer’,” with its orifice at a distance of 5 cm from the second grid. The process chamber is pumped by a turbomolecular pump (1000 I s- ‘), achieving a pressure of typically 65 mPa (0.5 mtorr) in the process chamber, depending on the gas flow into the source and the neutralizer, respectively. A sample holder with the wafer to be etched can be moved into the chamber from a separately pumped load lock. The wafer is located at a distance

deflection angle towards beam axls

L-

Figure 1.

Experimental arrangement.

of 30 cm from the second grid with its surface generally perpendicular to the beam axis. If not mentioned otherwise, source operating conditions were as follows: discharge voltage 50 V, Ua,, = - 100V, argon flow into the source 5 seem and pressure in the process chamber ~50 mPa (0.4 mtorr). Typical overall beam current was about 100 mA.

F Heinrich

et al: Optical

emission

spectroscopy

of broad

ion beams

Light emission can be observed through two tlangcs (30 mm inner diamctcr, BK7 windows) at an an& of 30 with respect to the beam axis. A I m monochromator (THR 1000, Jobin Yvon) equipped with a holographic grating of 2400 grooves mm-’ is placed at a distance of 25 cm from the window. A cooled photomultiplier tube (RCA 31034A) and photon counting technique were used for electronic detection. The spectra were taken at typical slit widths of IO-20 /cm and slit heights of 5 mm (slits perpendicular to the plane of drawing in Figure I). Due to our special geometry light is detected from an angle -((I21 < 0 c IO,] around the monochromator line of sight (the slight asymmetry is given by the different length of the Hange sides). Therefore photons reach the monochromator from a beam region roughly within a distance of IO-50 cm from the second grid. The wavelength shift A>. (small shifts, L’CCC) of the light emitted at an angle 0 is given by Aj_ = j.,, *“(’ - cos(3o-+~+cr)

I

fast

x

atoms

(1)

where i0 is the unshifted wavelength, C, the velocity of light, r, the velocity of beam particle and SIis the deflection angle. The angle 2, contains contributions from the beam divergence as defined by the extraction conditions and from momentum transfer due to collisions with neutral background atoms in the process chamber. The particle velocity, L:, is a function of the beam voltage and of momentum and energy transfer in collisions. As a consequcncc of the finite observation angle, II (/(),I + IO11 2 5 ), the Doppler-shifted emission lines are broadened by A(A,?) < 0.05 * A,i. at small angles, s(. This is comparable to the resolution of our monochromator (< 0. I A).

6966.0

6966.5

6965.5

( K )

wavelength

2. Emission spectrum of Ar I : transition 4s[i/2]-4p’[1!2] itt an unshifted wavelength of& = 696.i.43 A.

Figure

Ar I 3.

6965.43 i

I 6967

b

I 9 , 6965 6966

wavelength

( g

)

Figure 3. Emission spectra recorded at ditrercnt beam voltages (same transition as in Figure 2).

the source, they are present in the beam without neutralization as well, due to ionizing gas phase collisions and fast particle bombardment induced secondary electron emission. The influence of the beam voltage on the spectra is shown in

F Heinrich et al: Optical emission spectroscopy

of broad ion beams

Figure 3. In order to minimize intensity losses by sputtering of the window with chamber material we have chosen a higher scan speed (0.3 A min-‘) than in Figure 2 (resolution lower than in Figure 2). It turns out that the Doppler-shifted emissions are relatively nirrinv shaped. This reflects relatively narrow shaped differential cross-sections for collisional excitation as well. Assuming the inelastic differential cross-sections, being maximum at a scattering angle of 0” as it is well known for elastic and resonant charge transfer collisions’ ‘-16, the main particle velocity t’ can be deduced from the wavelength difference, A& of the maxima between the fast and thermal particle peaks by v=AA

&.cos

’

act

30”’

b)

Figure 4 shows the values of v obtained in this way, corresponding well to the beam voltage. As mentioned above, the energy of the ions entering the process chamber is expected to be slightly above e* U,,, (potential of the source plasma). On the other hand, a small part of the particle energy is lost in the collision process, The energy required for the excitation of the 4p’ level from the ground state is about 13 eV. These small energy differences are within the experimental error limits of +5%, mainly given by the uncertainty in the determination of the peak maxima. The broader distribution of the fast particle peak at low beam voltages (300 V) can be explained by a higher beam divergence due to the poor extraction quality. The influence of the accelerator voltage, too, on the beam divergence is clearly illustrated in Figure 5(a) and (b). Here, U,,, is varied from -500 to - 100 V at a constant beam voltage of 800 V. The shape of the fast particle peak is narrowing at lower j U,,,,(reflecting a lower divergence at these extraction conditions. This behaviour is expected, according to the theory of grid by cup measurements as well’. For optics7,” and confirmed quantitative evaluation of the beam divergence differential excitation cross-sections for heavy particle collisions are required. To our knowledge these are not available from the literature at the moment. In contrast to the behaviour of the neutral particle signals, the structure of the ion emission lines strongly depends on the

U beam= 800V U

*

act

= 1oov

I

I

6966.5

6966.0

wavelength

6965.5 I i

6965.0

)

Figure 5. Emission spectra at different accelerator voltages (same transition as in Figure 2). (a) High divergence, (b) Low divergence.

transition chosen. A more detailed presentation of these and other phenomena concerning the ions will be given in a forthcoming paper. For completion we show an ion signal (Figure 6) from the 2P-2D transition at an unshifted wavelength of 4879.86 A which yields the best intensity in our detection range.

fast

ions

At- II

4879.86

slow

.i

ions

I

AX .I”

=

c

XO~COS300

:1;o:“;o;” , 1 1oqov

3oov

5oov

803V

vubeam Figure 4. Particle velocities as derived from the spectra and theoretical values as expected from the beam voltage (abscissa scaled in the square root of the beam voltage).

wavelength

Figure 6. Emission spectrum of the ionic transition unshifted wavelength of i., = 4879.86 A.

Ar II, *P-*D at an

1183

F Hunrich

et a/: OptIcal

emission

spectroscopy

of broad

ion beams

The accuracy of the wavelength bar shown in Figure 6 is about 1070. Again we distinguish two relatively narrow shaped peaks at the red side (fast ions) and at the blue side (slow ions) of the spectrum. Similar to the neutrals. the wavelength differcnce between the maxima depends linearly on the square root ol the beam voltage.

Acknowledgement

This work has been funded by the German Ministry for Research and Technology BMFT. The authors wish to thank T Blask and K Grandorff for preparing the drawings.

4. Summary

Optical emission spectroscopy has been established as a valuable method for the diagnostics of broad ion beams. Absolute particle velocities and qualitative information on their angular distribution could be derived from the Doppler-shifted emission lines of neutral argon atoms. The information obtained from fast neutral atoms is believed to reflect the behaviour of the ions as well. since these neutrals are produced by charge exchange collisions. The excitation mechanisms arc shown to be electron impact (unshifted neutral line) and impact of fast heavy particles (Doppler-shifted emission lines) on thermal atoms of the background gas. In contrast to the Ar I lines the structure of the Ar II lines are found to be strongly dependent on the transition chosen. Optical emission spectroscopy as 3 contamination-free i/2 sircc diagnostic method will be extended to reactive ion beams in the near future.

1184

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