Measurement of electron energy distribution function in an argon/copper plasma for ionized physical vapor deposition

Transcription

1 Measurement of electron energy distribution function in an argon/copper plasma for ionized physical vapor deposition Z. C. Lu, J. E. Foster, T. G. Snodgrass, J. H. Booske, and A. E. Wendt a) Engineering Research Center for Plasma-Aided Manufacturing, University of Wisconsin, Madison, Wisconsin Received 13 August 1998; accepted 22 January 1999 The electron energy distribution function EEDF has been measured under a variety of conditions in an Ar/Cu plasma for ionized physical vapor deposition. The EEDF is directly measured in a system including a direct-current magnetron sputter source for copper and a radio frequency rf induction plasma, using a Langmuir probe with a modulated bias voltage in combination with a lock-in amplifier. The experimental data indicate that at fixed rf ionization power, the electron population in the tail of the EEDF is depleted by the introduction of copper vapor, and the electron average energy decreases slightly. Observed changes in the EEDF are attributed to inelastic collisions with copper atoms, which have lower threshold energies for excitation and ionization as well as larger cross sections as compared to argon, and the resulting reduction in the measured plasma potential American Vacuum Society. S I. INTRODUCTION Conventional physical vapor deposition has shown limited performance in filling high aspect ratio depth/width features due to the isotropic velocity distribution of the sputtered neutrals. At higher aspect ratios the sputtered particles can coat the sides of a trench before filling it, thus pinching off the trench and leaving either an open circuit or a high resistivity connection. Ionized physical vapor deposition IPVD is based on in-flight ionization of atoms sputtered from target. 1 6 The metal atoms knocked out of the sputtering source by argon ions experience ionization as they pass through a high density plasma generated by an internal radiofrequency rf antenna before reaching the substrate. The electric field at the biased substrate tends to collimate this metal ion flux thereby leading to anisotropic fills. IPVD has two intrinsic advantages over conventional sputtering. First, if the substrate is biased negatively, then it will attract the metal ions causing depositing metal ions arriving at normal incidence on the substrate. Because of this directionality, the angular distribution of the metal ion flux at the substrate can be significantly reduced, thereby leading to uniform coating of high aspect ratio features. Second, the arriving energy of the depositing species is controlled. This advantage permits resputtering and reflection of the depositing flux, leading to better sidewall conformality. The viability of IPVD as a production process for copper deposition relies on the ability to optimize the ionized flux fraction of copper, i.e., cu /( cu cu ), at the substrate. In general, the larger the metal ionization fraction, the more anisotropic the fill. In these discharges, ionization of metal atoms, whether through direct electron impact ionization or through Penning ionization ionization from a metastable state, depends on processes involving energetic electrons. 7,8 Hence, knowledge of the electron energy distribution function EEDF provides insight to copper ionization rates and a Electronic mail: wendt@engr.wisc.edu other factors governing the ionized metal flux fraction. In this study, measurements of the EEDF are made under a variety of discharge operating conditions in order to evaluate electron impact ionization rate constants for copper in the plasma and better understand discharge performance. This report describes the implementation of lock-in detection techniques, to measure the EEDF in an argon/copper plasma used for ionized physical vapor deposition. Changes in the tail of EEDF and the resulting copper ionization rate constant were measured as a function of pressure 5, 10, 20, 30, and 40 mtorr and copper magnetron sputter power W at fixed rf power 300 W. II. EXPERIMENTAL SETUP The experiment was performed in an aluminum chamber of 45 cm diameter and 52 cm height as shown in Fig. 1. The chamber uses a turbopump backed by a roughing pump to achieve a base pressure of roughly 10 6 Torr. A high density background argon plasma was generated by an internal inductive coil. This single-turn copper rf antenna is 36 cm in diameter. The diameter of the tubing which made up the coil is 0.5 cm. The rf power from the power supply is coupled to an automatching network which keeps reflected power below 10% of the incident power. Copper was introduced into the discharge via a direct-current dc magnetron sputtering gun. The 7.6-cm-diam. copper target was located 5 cm above the plane of the antenna. In general, copper neutral concentration increases with increasing magnetron sputtering power. The background gas used in this experiment is Ar. Plasma measurements were made with a planar Langmuir probe located on the chamber s centerline, 5 cm below the plane of the rf antenna. The double-sided planar probe 0.32 cm diameter as shown in Fig. 2 is designed with a recess to avoid shorting of the probe by the deposited metal. To avoid rf distortion in the EEDF measurement, a tuned, resonant, rf notch filter and a low pass filter were placed in series with the probe to maximize the rf impedance of the probe tip to 840 J. Vac. Sci. Technol. A 17 3, May/Jun /99/17 3 /840/5/$ American Vacuum Society 840

2 841 Lu et al.: Measurement of EEDF in an argon/copper plasma 841 FIG. 1. Experimental apparatus used for this IPVD study. ground. 9 In order to minimize probe contamination due to deposition of copper onto the probe tip, the probe is biased at 30 V before measurement for each discharge condition to sputter clean the probe tip s surface. A 1 V, 10 khz pure sinusoidal signal provided by a Hewlett Packard 8116A function generator is superimposed upon the dc bias of the probe. A Stanford Research two-channel digital lock-in amplifier SR510 was used to measure the current at the second harmonic of the superimposed signal as a function of probe bias voltage. The EEDF can be extracted from the electron retarding region of a Langmuir probe current voltage I V characteristic. From the Druyvesteyn relation, 10 the EEDF, f (E),is proportional to the product of the second derivative of the probe current with respect to probe voltage and the square root of the voltage difference between the plasma potential and the probe bias: f E 4 e 3 A m ee 2 d 2 I e dv b 2, E e V p V b 0, 1 where E is the electron energy, A is the probe area, e is the electron charge, m e is the electron mass, I e is the electron current to the probe, V p is the plasma potential, and V b is the probe bias voltage. Typically the second derivative is determined by numerical differentiation. The drawback of this FIG. 2. Planar Langmuir probe. method is associated largely with noise already present in the current voltage characteristic that is amplified upon differentiation. This noise tends to obscure details in the resulting EEDF especially in the tail due to the reduced signal to noise ratio. An alternative technique is the direct measurement of probe I V second derivative by superimposing a small alternating-current ac signal on top of the probe bias voltage. 11,12 This technique was introduced in 1934 by Sloane and MacGregor 13 and has been used by many other researchers A small sinusoidal signal Ṽ of frequency is superimposed onto the probe voltage, so the probe current contains both steady and oscillating components. A lock-in amplifier which performs functions of filtering and phase sensitive detection is used to measure the second harmonic, 2, component in the probe current, I 2. The amplitude of the second harmonic signal has been shown to be proportional to the second derivative of the probe current with respect to the probe bias: 11 I 2 Ṽ2 4 d 2 I dv b 2. A complete derivation can be found in Ref. 11. The amplitude of the applied ac signal voltage must be small compared with the mean electron energy E 3kT e /2; otherwise, higher order derivatives contribute to the second harmonic signal. On the other hand, the output signal is proportional to amplitude of the ac signal voltage, and therefore it is desirable to make the ac signal voltage large to achieve a higher signal-to-noise S/N ratio. The value of the superimposed ac voltage which provides an acceptable compromise between the S/N ratio and measurement distortions is usually within an interval of ( ) E. 12 The frequency of the ac signal must be much higher than that of the dc sweeping voltage to ensure a number of ac cycles can be measured in each interval of the dc voltage, but should keep lower than one tenth of the plasma frequency to prevent a distorted EEDF. 21 III. RESULTS AND DISCUSSION The results are presented in the form of electron energy probability functions EEPF, obtained by dividing the EEDF by the square root of the electron energy to facilitate comparison with Maxwellian distributions of electron energy. 22 Normalized EEPFs at different pressures as a function of dc magnetron sputter power are shown in Fig. 3. For all plots, rf coil power is fixed at 300 W. As can be seen from the plots, the distributions are non-maxwellian, and the high energy tail is sharply truncated. A Maxwellian distribution would appear as a straight line. The energy at which the high energy tail is truncated decreases with increasing magnetron sputter power. We attribute this behavior to a combination of two interrelated factors: enhanced consumption of energetic electrons by inelastic collisions with copper atoms 23,24 and a decrease in plasma potential with increasing magnetron sputter power. 2 JVST A - Vacuum, Surfaces, and Films

3 842 Lu et al.: Measurement of EEDF in an argon/copper plasma 842 FIG. 4. Measured plasma potential at rf 300 W. FIG. 3. Normalized EEPF under different magnetron sputtering power 0, 120, and 280 W, with rf antenna power fixed at 300 W, at pressure a 40, b 30, c 20, d 10, and e 5 mtorr. The increasing gas phase copper concentration with increasing magnetron sputter power enhances the role of inelastic collisions with the copper in determining the electron energy distribution function. The energy thresholds for electron impact excitation 3 ev and ionization of copper ev are much lower than those for argon 11 ev. In addition the cross sections for electron impact excitation and ionization of copper are large compared to those for argon. As a result, the copper neutrals may act as energy absorbers in the discharge, preventing electrons from reaching energies as high as in pure argon discharges. The plasma potential shown in Fig. 4 is determined from the zero crossing point of the d 2 I/dV b A reduction in plasma potential with increasing magnetron sputter power shown in Fig. 4 also plays a role in the EEDF because it sets an energy threshold for electrostatic confinement of electrons in the discharge. The potential difference between the plasma and the wall determines the potential barrier height for electron escape. Electrons with energy high enough to overcome the barrier are not confined in the plasma, which can lead to a truncation in the tail of the distribution as observed in Fig. 3. We believe that these two factors are not independent, but rather closely related to one another. The reason for the reduction in plasma potential with increasing magnetron sputter power may in fact follow from the depletion of the tail EEDF due to inelastic collisions with copper atoms. The steady state plasma potential adjusts itself to a level such that ion and electron fluxes to the chamber walls are equal. When the tail of the EEDF is suppressed by the addition of copper atoms, fewer electrons remain at energies above the threshold to overcome the potential barrier, so that the electron flux to the walls no longer matches that of the ions. As a result, the plasma potential must decrease due to the extra negative charge in the plasma. As it decreases, the energy threshold for electron escape also decreases, allowing a greater flux of electrons to the walls. Finally a new steady state is reached with a lower plasma potential, and equal fluxes of electrons and ions to the chamber walls. This decrease in the plasma potential may itself further contribute to the depletion of the EEDF tail in the manner described in the previous paragraph. The electron average energy is calculated from the EEDF by the relation: 22 E 1 n e 0 f E EdE. As shown in Fig. 5, there is a slight decrease in average electron energy with increasing magnetron sputtering power, due to both truncation of the EEDF tail and a slight cooling of the bulk electrons. In addition to the average electron energy, the copper ionization rate constant k i was also determined from the following integral: 26 k i i e 1 n e f E i E e de. 4 Et The ionization rate constant here is a measure of the generation rate of copper ions from the electron impact ionization 3 J. Vac. Sci. Technol. A, Vol. 17, No. 3, May/Jun 1999

4 843 Lu et al.: Measurement of EEDF in an argon/copper plasma 843 FIG. 5. Electron average energy at rf 300 W. FIG. 6. At rf 300 W antenna power, Cu neutral ionization rate constant calculated by a measured EEDF, b by a Maxwellian EEDF with the same average energy. reaction: Cu e Cu 2e. In Eq. 4, E is electron energy, i is the energy dependent ionization cross section, e is the electron velocity at kinetic energy E, f (E) is the EEDF, n e is the electron density, and E t is the ionization threshold energy. When high electron densities are generated, metal atom ionization is primarily due to electron impact ionization and the Penning ionization process plays a minor role. 1,27 Based on the measured EEDF, we calculate copper ionization rate constants as a function of copper neutral concentration and inert gas background pressure. Copper ionization cross sections from the ground state used in this calculation were obtained from Lotz. 28 Recall that this rate constant is a measure of the neutral copper impact ionization rate per electron in the discharge. As can be seen in Fig. 6 a, k i decreases with increasing magnetron sputtering power. This finding suggests that the ionization efficiency of copper decreases with increasing copper concentration in IPVD. This decrease in the ionization rate constant is attributed to the reduction in the electron population with energies above the ionization threshold of copper. Such depletion of the tail also reduces the ionization and excitation efficiency of argon as well. The reduction of argon optical line emission intensity with increasing dc sputtering power has been reported by Foster et al. 24 and attributed to the EEDF depletion phenomena. It is interesting to note that this EEDF depletion is more dramatic for the higher argon pressure case. At 30 mtorr, rf antenna power 300 W, when the sputtering power increases from 0 to 280 W, the ionization rate constant decreases by nearly 95%, as compared to about 30% for the 5 mtorr case and 45% for the 10 mtorr case. This may be explained by increased thermalization of the relatively energetic copper neutrals with the argon background. 29,30 The sputtered atoms, depending on the pressure and the distance between the cathode and the substrate, may undergo from zero to many hundreds of collisions. Each collision changes the velocity and direction of the sputtered atom, and usually results in a general cooling thermalization of the sputtered atom, while at the same time increasing the average temperature of the background gas. Because copper atoms cooled through collisions with argon have a longer residence time, and thus increased likelihood of inelastic collisions with electrons, thermalization of copper enhances its effect on the EEDF. This effect becomes more pronounced at higher argon pressure because copper atom collisions with argon become more frequent as the argon concentration increases, enhancing the thermalization effect. The ionization rate constant k i from a Maxwellian energy distribution function with the same average energy (T e is found from kt e 2/3 E ) is shown for comparison in Fig. 6 b. The ionization rate constant k i determined by Eq. 4, with the upper limit of integration chosen to be 20 ev, high enough so that truncation of the integral has negligible effect. It can be seen that there is a big difference between k i and k i, suggesting that the assumption of a Maxwellian distribution could lead to inaccuracies in predicting discharge parameters. The ionization rate constant calculated assuming a Maxwellian energy distribution tends to overestimate the real value, 27 especially at the higher pressure and higher dc sputtering power cases, where the high energy electron depletion effect is most significant. In our study, the overestimate is 16% at 10 mtorr, dc sputtering power 60 W, while 6200% at 40 mtorr, dc sputtering power 280 W. JVST A - Vacuum, Surfaces, and Films

5 844 Lu et al.: Measurement of EEDF in an argon/copper plasma 844 IV. CONCLUSION Using the lock-in technique, it is observed that the tail of the electron energy probability function in an Ar/Cu plasma used for ionized physical vapor deposition is depleted with increasing magnetron sputter power. This depletion of the electron energy tail is attributed to inelastic collisions with copper, which has lower inelastic excitation and ionization energy thresholds than argon. A consequence of this EEDF depletion is a reduced average electron energy and a reduced ionization rate constant. These effects appear to be greater at higher pressures, most likely due to the shorter inelastic collision path lengths associated with increased metal vapor thermalization at the elevated pressures. ACKNOWLEDGMENT This research is supported by the National Science Foundation under Grant No. EEC J. Hopwood, Phys. Plasmas 5, S. M. Rossnagel and J. Hopwood, Appl. Phys. Lett. 63, S. M. Rossnagel and J. Hopwood, J. Vac. Sci. Technol. B 12, S. M. Rossnagel, J. Vac. Sci. Technol. B 16, P. F. Cheng, S. M. Rossnagel, and D. N. Ruzic, J. Vac. Sci. Technol. B 13, M. Yamashita, J. Vac. Sci. Technol. A 7, R. L. Rhoades and S. M. Gorbatkin, J. Appl. Phys. 80, C. Doughty, R. L. Rhoades, S. M. Gorbatkin, and L. A. Berry, in AIP Conference Proceedings 392, edited by J. L. Duggan and I. L. Morgan American Institute of Physics, New York, 1997, pp A. P. Paranjpe, J. P. McVittie, and S. A. Self, J. Appl. Phys. 67, M. J. Druyvesteyn, Z. Phys. 64, J. D. Swift and M. J. Schwar, Electrical Probes for Plasma Diagnostics Iliffe, London, 1970, Chap V. A. Godyak, in Plasma-Surface Interactions and Processing of Materials, edited by O. Auciello et al. Kluwer, Dordrecht, The Netherlands, 1990, pp R. H. Sloane and E. I. R. MacGregor, Philos. Mag. 18, R. L. F. Boyd and N. D. Twiddy, Proc. R. Soc. London, Ser. A 250, N. A. Vorobeva, Yu. M. Kagan, and V. M. Milenin, Sov. Phys. Tech. Phys. 8, G. R. Branner, E. M. Friar, and G. Medicus, Rev. Sci. Instrum. 34, A. I. Kilvington, R. P. Jones, and J. D. Swift, J. Sci. Instrum. 44, K. Wiesemann, Ann. Phys. Leipzig 23, S. W. Rayment and N. D. Twiddy, Br. J. Appl. Phys. 2, K. F. Schoenberg, Rev. Sci. Instrum. 51, H. Amemiya and K. Shimizu, J. Phys. E 12, M. A. Lieberman and A. J. Lichtenberg, Principle of Plasma Discharge and Material Processing Wiley, New York, 1994, p M. Dickson, F. Qian, and J. Hopwood, J. Vac. Sci. Technol. A 15, J. E. Foster, A. E. Wendt, W. Wang, and J. H. Booske, J. Vac. Sci. Technol. A 16, V. A. Godyak, in Plasma-Surface Interactions and Processing of Materials, edited by O. Auciello et al. Kluwer, Dordrecht, The Netherlands, 1990, pp. 119 and M. A. Lieberman and A. J. Lichtenberg, Principle of Plasma Discharge and Material Processing Wiley, New York, 1994, p J. Hopwood, J. Appl. Phys. 78, W. Lotz, Z. Phys. 220, S. M. Rossnagel, J. Vac. Sci. Technol. A 6, S. M. Rossnagel, J. Vac. Sci. Technol. B 16, J. Vac. Sci. Technol. A, Vol. 17, No. 3, May/Jun 1999