Ion energy distributions from electron cyclotron resonance methane plasmas

Diamond and Related Materials, 2 (1993) 378-382

378

Ion energy distributions from electron cyclotron resonance methane plasmas W. Jacob, P. Reinke and W. M611er Max-Planck-lnstitut fiir Plasmaphysik, EURATOM Association Boltzmann Strasse 2, D-8046 Garching bei Miinehen (Germany)

Abstract We measured ion energy distributions (IEDs) in an electron cyclotron resonance (ECR) plasma reactor for argon, methane and methane-hydrogen employing a retarding field analyzer. The influence of external parameters such as pressure, magnetic field configuration and gas composition on the IEDs was studied. Typical values for the energy of ions impinging on a grounded sample were between 25 and 20 eV for argon and between 60 and 20 eV for methane in the pressure range 30-170 mPa. The IEDs were very narrow with a full width at half-maximum (FWHM) ranging from 10 to 3 eV in the same pressure range. By applying a d.c. bias to the sample, the energy of the ions can be controlled without a significant change in the plasma parameters. We employed this method to investigate the growth and structure of C: H films deposited with ion energies between 30 and 200 eV. Most film properties such as density, refractive index, and hydrogen content vary linearly in the energy range investigated, but we also found a significant change in the growth rate and in the sp3-to-sp2 ratio in the energy interval 80-120 eV.

1. Introduction

2. Experimental set-up

Until now the deposition mechanisms of C : H films have been only partially understood, owing to the complexity of the processes involved. First attempts to simulate the growth of C : H films [1, 2] revealed interesting facts but suffered from lack of knowledge on the fluxes and energy distributions of the impinging particles. It is well known that the growth and structure of C : H films is to a large extent determined by the energy of ions impinging on the growing film. As a consequence most physical properties of these films vary with changing ion energy. It is therefore very desirable to gain insight into the processes defining the ion energy and to find ways of controlling them. In this paper we present data for the ion energy distributions (IEDs) in argon, methane, and methanehydrogen electron cyclotron resonance (ECR) plasmas as a function of pressure and magnetic field configuration. After a brief description of the experimental set-up we present data illustrating the influence of external parameters on the IED. These results are discussed on the basis of a simple, qualitative model. In the following section the results of the deposition of hydrocarbon films as a function of ion energy are presented. They are compared with published data of the energy dependence of hydrocarbon film deposition.

The experiments were carried out in an ECR reactor described previously I-3]. A schematic drawing of the system is depicted in Fig. 1. In brief, the system consists of a cylindrical stainless steel chamber 27 cm in diameter. It is surrounded by two sets of solenoids (sets A and B). Set A creates the magnetic resonance field (87.5 mT) and set B can be used to change the magnetic field configuration (more homogeneous or more divergent field). The

microwave input

_ _

~

magnetic coils set A

10 cm

I substrate holder or ion energy analyzer

set B

turbomolecular pump Fig. 1. Schematic drawing of the ECR microwave deposition system.

0925-9635/93/$6.00

© 1993 - - Elsevier Sequoia. All rights reserved

W. Jacob et al. / Ion energy distributions

microwave power (2.45 GHz) is fed axially into the chamber as a circularly polarized wave from the region of high magnetic field. The chamber is pumped by a turbomolecular pump to a base pressure in the low 10 -5 Pa region. Gas flows are monitored by standard gas flow controllers and the pressure is controlled by a throttle valve between the chamber and the pump. The retarding field, parallel plate type E4, 5-1 ion energy analyzer is situated at the sample position. It is of cylindrical geometry, about 2 cm in diameter, and differentially pumped to ensure collisionless motion of the ions inside the analyzer. C : H films were deposited from methane plasmas at 150 mPa with a microwave power of 180 W. The samples were fastened on a temperature-controlled substrate holder. The ion energy was varied between 30 and 200 eV by d.c. biasing the sample electrode: The effect of d.c. biasing on the lED has been investigated by Reinke et al. [3-1 and they give experimental details. The IEDs were measured for identical geometric conditions, but in separate runs. All other plasma parameters were held constant. However, with increasing d.c. bias we observed a decrease in the ion flux to the sample. This change in ion flux with varying d.c. bias is not yet understood. C : H films were deposited onto polished aluminum samples. The sp3-to-sp 2 ratio was measured by Fourier transform IR spectroscopy (FTIR) (in reflection). The carbon and hydrogen contents of the films were analyzed using proton-enhanced cross-section scattering (PES) with 1.5 MeV H + and elastic recoil detection (ERD) using 2.6 MeV 4He + beams at a scattering angle of 30 ° [-6, 7]. A profilometer was used to measure the film thickness and the density was calculated from the thickness and the areal densities of carbon and hydrogen from the PES and ERD analyses.

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pressure (mPa) Fig. 2. (a) Mean energy E m and (b) F W H M of the IEDs from argon and methane ECR plasmas measured at the sample position. The microwave power was 180 W.

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3. Results

Figure 2 shows the mean ion energy E m and the full width at half-maximum (FWHM) of the IED in pure argon and methane plasmas as a function of pressure. The IEDs themselves are quite symmetric and examples can be found in ref. 3. For both gases the E m and F W H M decrease with increasing pressure. For pressures above 100 mPa the results are quite similar with a n E m of about 20 eV and an F W H M of about 3 eV. However, for the lower pressures E m is significantly higher for methane compared with argon. At the lowest pressure investigated (30 mPa) the energy of the methane ions is more than twice the value of argon. For both gases an F W H M of about 10 eV at the lower pressures was found. The influence of the gas composition on the ion energy in a methane-hydrogen mixture is demonstrated in Fig. 3. For two fixed total pressures the fraction of

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hydrogen fraction (%) Fig. 3. Change in the mean ion energy in a methane-hydrogen mixture for constant total pressure as a function of the hydrogen fraction.

hydrogen in the working gas was varied. With increasing hydrogen fraction the mean energy E m of the impinging ions increases. Another way of influencing the ion energy is to change the magnetic field configuration. This is shown in Fig. 4 where the mean energy for ions from a pure methane plasma is .plotted against Re; Re is the ratio of the

380

W. Jacob et al. / Ion energy distributions

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magnetic field in the resonance zone, where most of the ions are created, to the field at the position of the energy analyzer. Rc is varied by using the second set of coils (set B). In the case of a homogeneous field Rc = 1, if the field becomes more divergent Rc will increase. Figure 4 shows that the ion energy increases with increasing divergence of the magnetic field (increasing Ro). For the lowest pressure (50 mPa) this increase is rather steep. At higher pressures, however, this increase is more gradual and at the highest pressure investigated (150 mPa) no significant change in ion energy is found.

4. Discussion

Two main effects contribute to the ion energy measured in our set-up: acceleration in the sheath in front of the analyzer and ambipolar acceleration [8, 9] in a divergent magnetic field. These two contributions are not easily separated in our experiment, but in order to understand the influence of external parameters on the IED, both contributions have to be discussed. The key to a qualitative understanding of the effects is the collision cross-sections and the diffusion coefficients of the various particles in the plasma. These effects will be discussed in more detail in a separate publication [10]. We discuss our experimental findings using the following qualitative model. Charged particles are produced in a zone around the ECR region by electron impact ionization. We call this zone the "ionization zone". The microwave power is coupled to the electrons via the velocity component perpendicular to the magnetic field. In the divergent magnetic field electrons move to the region of lower magnetic field and transfer this perpendicular energy into energy parallel to the magnetic field, owing to the invariance of the magnetic moment. This loss of electrons creates an electrostatic potential

which will accelerate the ions in order to maintain the quasi-neutrality of the plasma. If the system is in equilibrium, a neutralized plasma beam is ejected from the ECR plasma in which electrons and ions move with the same velocity. This effect is usually called ambipolar acceleration (in a divergent magnetic field). This effect can also be discussed on the basis of a potential that is created by the electron motion and we call this "ambipolar potential". The ambipolar potential depends on the mean energy of the electrons in the resonance zone and is therefore influenced by the plasma parameters. Furthermore, it strongly depends on the local variation in the magnetic field. In the discussion we have so far neglected the effects of collisions. With increasing pressure the probability for collisions increases and the ambipolar potential decreases. Finally, at a certain pressure which is characteristic of the geometry of the plasma chamber and the working gas, most of the particles experience at least one collision on their way to the sample or analyzer and the ambipolar potential disappears. In addition to this ambipolar potential, we still find a sheath potential in front of any surface exposed to the plasma which will accelerate ions onto the surface. The sheath potential and the ambipolar potential are both influenced by the plasma parameters and by the pressure. Both potentials contribute to the measured ion energy. At higher pressures where the plasma is dominated by collisions, the ambipolar potential disappears and the ions are only accelerated by the sheath potential. This transition is seen in Fig. 2 in the case of methane. The relatively high energy at low pressures is to a large extent caused by the ambipolar potential. Above roughly 100 mPa the ambipolar potential disappears and the ions are only accelerated in the plasma sheath. This transition is not observed for argon. The reason could be the lower diffusion coefficient of argon due to the higher mass of argon compared with methane. We assume that for argon this transition will appear at pressures below the lowest pressure investigated here. Tobinaga et al. [11] found an increase in the ion energy in an argon ECR plasma from 20 eV to 40 eV in the pressure range 50-30 mPa. Since the pressure at which the transition occurs depends on the reactor geometry, it is not possible to compare these values directly with our data, but they show that the same transition also exists for argon. The FWHM of the IED is also strongly influenced by the ambipolar acceleration in the low pressure regime. Depending on the extension of the .ionization zone and the steepness of the ambipolar potential, the IED will be broadened since the ions created~at different locations in the ionization zone travel through a different potential drop on their way to the analyzer. At higher pressure, when the ions make several collisions on their way to

W. Jacob et al. / Ion energy distributions

the analyzer, this broadening effect diminishes and the IEDs become very narrow. The increase in E m with increasing hydrogen admixture, as shown in Fig. 3, is caused by the increased ambipolar diffusion of hydrogen compared with methane and can be explained in the following way. To maintain a stable plasma the losses of charged particles have to be compensated for by the production. If, by changing the gas or the gas composition, the ambipolar diffusion of ions to the chamber walls is increased, this higher loss has to be compensated for by an increased ionization rate. The higher ionization rate is realized by an increase in the electron temperature. The increased electron temperature leads finally to an increase in both contributions to the ion energy: the sheath potential and the ambipolar potential both increase. At higher pressure we will find reduced diffusion to the walls, owing to a higher number of collisions. Therefore, we will observe a slower increase in the ion energy. This is demonstrated in Fig. 3. For a total pressure of 80 mPa E m rises from 17 to 32 eV, while it increases for 170 mPa from 14 to 23 eV. The difference in the diffusion properties is also the reason for the observed differences between argon and methane which have already been discussed. The increase in E m with increasing divergence of the magnetic field is directly caused by the ambipolar acceleration. Increasing pressure will reduce this acceleration owing to the increasing number of collisions. At pressures above 100 mPa we found no influence of the field divergence on E~ in our set-up.

5. Film deposition Figure 5 shows the sp 3 fraction in the sample as measured by FTIR (assuming [sp 3] + [sp2] = 100%) as a function of ion energy. At the pressure used for the

deposition (150 mPa) the FWHM of the IED is very small (3 eV, see Fig. 1), in particular when compared with investigations using r.f. plasmas or r.f. bias. The most striking observation is the steep decline in the sp 3 fraction in the energy interval between 80 and 120 eV, while outside this region the sp3-to-sp 2 ratio varies only slightly. This is the energy range where in general the transition from polymer-like C : H films to hard a-C:H films is observed. Unfortunately, we have no data for the mechanical properties of our films. The [H]/[-C] ratio in the film decreases from 0.6 to 0.4 in the measured energy range and the film density increases accordingly (Fig. 6). The refractive index also increases linearly from about 1.7 to 2.6 (not shown). Although the [H]/[C] ratio is reasonably well described by a straight line, it might also be compatible with a faint step around 100 120 eV, but based on the present data this interpretation is quite speculative. More experimental work needs to be done to clarify this point. A comparison of our data with published data is not straightforward since, until now, most investigations have been carried out using r.f. plasmas. In r.f. plasmas the IEDs are generally much broader and a change in ion energy is usually accomplished by increasing the discharge power which in turn changes the whole plasma. The same argument also holds for studies using an ECR plasma and employing an r.f. bias to change the ion energy [,12]. R.f. biasing will result in much broader IEDs and tend to smear out any threshold behavior. Nevertheless, our findings are in general agreement with the tendencies found in those studies. The refractive index and film density are comparable with the values given by Koidl et al. [13]. Nagai et al. [14], who used a biased ECR source, reported a change in refractive index between 1.8 and 2.2 when the bias voltage was changed from 0 to 250 V. They also used IR spectroscopy to characterize their films and found a decrease in the 0.7

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382

W. Jacob et al. / Ion energy distributions

spa-to-sp 2 ratio with increasing bias voltage. Tamor et al. [15] investigated C : H films with nuclear magnetic resonance and also found a decrease in the sp3-to-sp 2 ratio with increasing ion energy. However, until now no threshold behavior like that in our experiments has been reported. This could be attributed to the much narrower IEDs in our experiment. Tamor et al. [15] and Koidl et al. [13] also report a decreasing hydrogen content of the films with increasing ion energy, in accordance with our results. The decrease in the hydrogen content with increasing ion energy is also reproduced in the computer simulation by Mrller [1] which is based on the TRIM and TRIDYN computer codes (see references in ref. 1). The problem in these simulations is that so far no experimental data for the particles fluxes to the growing film have been available. Results of the model calculations which predict the sp3-to-sp 2 ratio by preferential displacement of sp 2 hybridized carbon atoms by Mrller [2] are in disagreement with the results presented here. The modeling predicts an increase in the sp 3 fraction with increasing ion energy whereas we found a significant decrease. This implies that preferential displacement alone is not sufficient to explain the s p 3 - t o - s p 2 ratio in plasma grown C : H films.

6. Conclusions We performed systematic measurements on the energy distribution of ions arriving at the sample surface in an ECR plasma. The distributions are very narrow and typical energies are between 20 and 60 eV. We showed how external parameters such as pressure, gas composition, and magnetic field configuration influence the IED and discussed these results on the basis of a simplified, qualitative model.

C : H films were deposited from the ECR plasma with ion energies between 30 and 200 eV. The variation in the physical properties of the films is in agreement with published data, but we found a threshold behavior in the sp3-to-sp 2 ratio of the C : H films.

Acknowledgment The Max-Planck-Institut fiir Plasmaphysik is affiliated with EURATOM. The authors gratefully acknowledge the technical help of G. Kerkloh.

References I W. MSller, in R. E. Clausing, L. L. Horton, J. C. Angus and P. Koidl (eds.), Diamond and Diamond-Like Films and Coatings, NATO-ASI Series B 266, Plenum, New York, 1991, p. 229. 2 W. MSller, Appl. Phys. Lett., 59 (1991) 2391. 3 P. Reinke, S. Schelz, W. Jacob and W. MSller, J. Vac. Sci. Technol. A, 10 (1992) 434. 4 S. M. L. Prokopenko, J. G. Laframboise and J. M. Goodings, J. Phys. D, 5 (1972) 2152. 5 S. M. L. Prokopenko, J. G. Laframboise and J. M. Goodings, J. Phys. D, 7 (1974) 355. 6 D. Boutard, W. MSller and B. M. U. Scherzer, Phys. Rev. B, 38 (1988) 2988. 7 D. Boutard and W. MSller, Surf Coat. Technol., 45 (1991) 353. 8 M. Matsuoka and K. Ono, Appl. Phys. Lett., 50 (1987) 1864. 9 M. Matsuoka and K. Ono, J. Vac. Sci. Technol. A, 6 (1988) 25. I0 P. Reinke, W. Jacob and W. Mrller, in preparation. 11 Y. Tobinaga, N. Hayashi, H. Araki, S. Nakayama and H. Kudoh, J. Vac. Sci. Technol. B, 6 (1988) 272. 12 Y. H. Shing and F. S. Pool, Vacuum, 41 (1990) 1368. 13 P. Koidl, Ch. Wild, B. Dischler, J. Wagner and M. Ramsteiner, Mater. Sci. Forum, 52-53 (1989) 41. 14 I. Nagai, A. Ishitani, H. Kuroda, M. Yoshikawa and N. Nagai, J. Appl. Phys., 67 (1990) 2890. 15 M. A. Tamor, W. C. Vassel and K. R. Carduner, Appl. Phys. Lett., 58 (1991) 592.