Control of high-density plasma sources for CVD and etching

Vacuum/volume

48lnumber

7-g/pages 659 to 66411997 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0042-207x/97 $17.00+.00

0

Pergamon PII: s0042-207x(97l00054-7

Control of high-density etching

plasma sources for CVD and

K Suzuki, H Sugai, K Nakamura, TH Ahn and M Nagatsu, University Furo-cho, Chikusa-ku, Nagoya 464-07, Japan

Department

of Electrical

Engineering,

Nagoya

accepted in revised form 30 December 1996

Large-diameter high-density plasmas such as ECR, he/icon-, inductively-coupled, and surface-wave plasmas have been developed for thin film technologies in the next generation. Applications of such high density plasmas to etching and CVDprocesses require deeper understanding of discharge physics as well as advanced plasma control. In this paper, new findings of antenna-plasma coupling processes in a he/icon rf discharge (resonant directional wave excitation) and in a microwave discharge (standing surface-wave excitation) are described. In addition, an internal antenna system of inductive t-f discharges for production of a large-area plasma is presented. 0 1997 Elsevier Science Ltd. All rights reserved

Introduction

Recent trend in plasma processing in microelectronics industries has been diverging into two opposite directions. One is formation of thin films having ultra-fine structures, down to = 0.1 pm in ULSI technology. The other is large-area processing such as 300mm diameter wafer for the next generation etching and -0.5 m* area panel in FPD (flat panel display) technology. Such forth-coming requirements in thin films processing have pushed to develop an innovative plasma source having a large diameter (> 1 m) and high density (> lO”~rn-~) at low pressures (< 10 mTorr). To date, several types of high density sources satisfying these conditions have been developed: for example, ECR (electron cyclotron resonance) plasma,’ helicon-wave excited plasma,’ inductively coupled plasma (ICP, including TCP; transformer coupled plasma),3 and surface-wave excited plasma.4 Although these high density sources enable a high rate of deposition/etching even at low pressures, they often meet common problems. In etching processes, for example, the high density sources have problems of low etch selectivity of SiO, to Si, anomalous local side-wall etching (called notch) and charge-up damages, along with poor reproducibility.’ Several advanced etching technologies for clearing such difficulties in high-density plasma etching have been proposed: The first is downstream etching to obtain high etch selectivity in fluorocarbon ICP,6 the second is pulsedplasma etching to obtain notch-free etch profile in chlorine ECR plasma ’ and in chlorine ICP.8,9 The pulsed plasma also enhances etch selectivities. ‘w’ The third is hot wall etching to obtain high etch selectivity in fluorocarbon ICPs. ‘J’J Although these proposals appear promising, the underlying mechanisms have not been fully clarified due to poor plasma diagnostics, especially on fluorocarbon plasmas.

Recently, we have reported the space- and time-resolved measurements of neutral radicals in a chlorine ICP of pulsed etching mode and in fluorocarbon (CF.&F,) ICP of downstream etching mode and of hot waN etching mode.’ Several advanced diagnostic techniques are used such as electron density measurements by a plasma oscillation method,13 negative ion density by photodetachment combined with POM, and neutral radical density by appearance mass spectrometry. The comprehensive measurements successfully disclose the underlying physics and chemistry in the high-density plasma etching. In this paper, we present experiments to explore the discharge physics in helicon-wave and surface-wave excited plasma reactors toward better understanding of antenna-plasma coupling in a range of rf and microwave frequencies. In addition, an advantage of internal antenna system of inductively coupled plasmas for large-area plasma production is emphasized.

Helicon source and antenna-plasma coupling

A helicon-wave-excited plasma is one of the innovative plasma sources suitable for low-pressure high-density plasma processing in the next generation. A high-efficiency rf power deposition enables us to obtain strongly ionized dense plasma ‘,I4 under weak magnetic fields where the wave frequency w is much lower than the electron cyclotron frequency w,. From a plasma physics point of view, however, several important problems remain: the power deposition mechanisms, the density jump process, and the possible non-existence of left-hand polarized modes. For example, the mechanism of efficient power deposition was initially attributed to electron Landau damping of helicon waves” and the Landau damping was experimentally observed under some conditions.16 On the other hand, recent experi659

K Suzuki et al: Control of high-density

plasma sources

ments’7m20 have revealed that high-energy electrons are directly ejected from underneath the antenna, which cannot be interpreted in terms of the Landau process. In addition, transverse profiles of optical emission intensities clearly show a contribution of antenna near-fields to the power deposition.” Thus, detailed investigations of antenna-plasma coupling in helicon-wave excitation are crucial for understanding the helicon discharge physics. To date, various types of antennas have been used to excite helicon waves. One is a spatially localized antenna with the Dirac delta function 6(z) which has essentially a white spectrum of wave numbers. Examples of this type are a dipole antenna’* and a flat spiral coil.23 The other is a spatially distributed antenna which preferentially excites waves of certain modes determined by the antenna geometry. Examples are a linear polarization (M = + I) antenna* and a circular polarization (A4 = + 1 or M = - 1) antenna.‘J.‘5 The latter is also called a helical antenna. Recently, the helical antenna has been found to produce a high-density helicon plasma on one side,‘6 and the high-density region is inverted to the other side of the antenna when the direction of the magnetic field is reversed. Here, we present the mechanism of such helical antenna actions based on a test wave experiment and a wave excitation model. The helical antenna preferentially excites the right-hand polarized (m = + 1)mode on one side of the antenna. The wave amplitude becomes maximum when the wavelength coincides with the antenna length or with twice the antenna length.” The experimental apparatus, which has previously been used for measurements of wave dispersion” and optical emission” of a helicon plasma, is shown in Figure I. A main power source of 13.56 MHz and 2.5 kW is coupled in the cw or pulsed mode with a helical antenna of length dO( = 15 or 24cm) surrounding a 10 cm diameter Pyrex tube under a uniform axial magnetic field B,, of 0.02 T. The helical antenna is made of a pair of spiral lines with an azimuthal rotation of n, which are terminated by two rings. The direction of rotation when moving in the positive B0 direction specifies the antenna type to be M = + 1 (right-hand polarization) or M = - 1 (left-hand polarization). For example, Figure 1 illustrates a discharge antenna of A4 = + 1; when a helical antenna is infinitely long, a pair of dc currents along the antenna generates a radial magnetic field h, on the axis which rotates counterclockwise (i.e. in the direction of ion gyromotion) with increasing distance along B,,. Thus, if the direction of B. is reversed in Figure 1, then the antenna will act as the M = - 1 antenna even though its shape is unchanged. When the rf power Par applied to the helical antenna of M = + I was increased in argon at 7 mTorr, the electron density was discontinuously increased by one order of magnitude at the

power level of - 1 kW. Similar density jumps have previously been reported by many authors. A low-density discharge mode (L mode) has been considered to be caused by electrostatic or inductive near-fields at the antenna.*’ On the other hand, helicon wave fields have been considered to be mainly responsible in the high-density mode (H mode). In the H-mode, a strong asymmetry in their profiles is observed as the plasma and the wave appear to exist only on one side of the antenna (z>O). Such one-side wave propagation is called “directional” wave excitation by a helical antenna. The direction of axial magnetic field (I?,, = 0.02T) was reversed using an identical but shorter helical antenna of do = 15 cm. Then, peaks of both plasma density and wave amplitude moved from one side of the antenna to the other. To understand this result, small-amplitude test waves are excited at various frequencies in the range of 7-28 MHz by applying a low power (< 30 W) to three types of antennas (axial length d = 12 cm): helical antennas of M = + 1 and M = - 1, and a non helical antenna labeled M = + 1. An example of M = - 1 antenna is shown in Figure 1. The center of the test exciter is at ‘.- - 43 cm and the test waves are measured by an axially magnetic probe in an interferometer method. Large signals at the discharge frequency of 13.56MHz hamper detection of small test-wave signals. To eliminate this noise, the discharge rf source having a peak power of 2.2 kW is time-modulated 100% in amplitude with a square-wave pulse, and the interferometric measurement is performed by sampling a mixer output at the afterglow time, as shown in Figure 1. The test wave measurement showed that, irrespective of the antenna type (M = + l,- l,+ I), only the right hand polalized wave is excited and the measured axial wavelengths at different frequencies satisfy the theoretical dispersion relation of helicon waves.*’ In addition, the amplitude of the excited waves dramatically changes with the frequency (7-28 MHz). Since the wave amplitude Ih,l near the antenna is proportional to the antenna current I,,, the wave excitation efficiency p is defined as /I = lb,l/l,,. This efficiency is plotted in Figure 2 as a function of the axial wavelength 1”::in the case of M = + 1 and d = 12cm. As seen in this figure, a highly efficient “resonant excitation” is found at two conditions: 1!, - dand 2,: -2d. The same phenomena were observed for a shorter antenna of d = 6cm as well. However, such resonant behavior was not observed in the case ofM= -1 and fl.

A,/1d 0

1

2

3

1 0.8 0.6 0.4 0.2

27

&j

%=2p!iF-~~~

Figure 1. Experimental setup for helical antenna discharge and test wave excitation and detection. 660

n -0

5

10 15 20 25 30 35 Wavelength

Figure 2. Wave excitation M= +l antennawithd=

A,, (cm)

efficiency p as a function 12em.

of wavelength

1, for

KSuzuki

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Figure 3. Schematic illustration of forced oscillation at t = 0 for the M = + 1 antenna [top trace], and distributions of elementary waves (m = + 1) launched from z = d/2 and 0 [traces (a) and (b)] and oppositely launched from z = -d/2 and 0 [traces (c) and (d)]. Solid and open helical belts along the plasma column indicate loci of constant phases 0 and x, respectively.

The physical mechanism of resonant directional excitation of helicon waves by the helical antenna is schematically illustrated in Figure 3. Suppose that a forced oscillation of m = + 1 is excited at t = 0 under the M = + 1 antenna. As the time goes on, elementary waves propagate along B, from each point and overall wave interference gives the evolution of waves outside the antenna region. When d = 1,,/2, two elementary waves [(a) and (b) in Figure 31 launched from z = 0 and z = d/2 are in phase for z>d/2 while the elementary waves [(c) and (d) in Figure 31 originating from z = 0 and z = -d/2 are out of phase for i- > -d/2. As a consequence, the m = + 1 helicon wave is strongly excited in the right side of the antenna and suppressed in the left side in Figure 3. More definite analytical description has been given elsewhere.”

trostatic coupling from the antenna to the plasma” leads to an anomalous rise in the plasma potential (_ 1OOV at 100~ W rf power), which in turn causes frequent unipolar arcs and unstable discharges. Dielectric coating of the metal antenna lowers the plasma potential and yields a stable high-density plasma.‘3.28 On the other hand, a bare metal antenna can be used in a plasma by modifying the external rf circuit: two blocking capacitors are inserted in the circuit between the rf source and both ends of the antenna coil, thus floating the metal antenna. If sputtering of metal due to a large self-bias voltage should degrade the plasma processing, then a new internal antenna system is available as reported previously:29 superposing a dc current along the rf antenna can suppress the metal sputtering owing to a magnetic effect on electron loss, Anyhow, the internal antenna system has advantages of no dielectric window and a closer antenna-plasma coupling, which will in turn give rise to lower pressure discharges. Furthermore, an antenna of the same diameter is expected to give a larger and more uniform plasma in comparison with the external antenna system. Thus, a larger area plasma will be obtained using a smaller internal antenna, which makes an impedance matching much easier, especially in large-area plasma production. In this section, we present experimental evidences of such expectations. Namely, spatial distributions of plasma density were measured for different diameters of antennas immersed in a plasma at various pressures. A schematic diagram of the internal antenna system used in the present study is shown in Figure 4. A one-turn loop antenna of diameter d = 10-25 cm is coaxially set in a cylindrical stainless steel vacuum vessel 25 cm in radius and 60cm in length. The antenna loop is made of dielectric-covered copper conductor of 6mm in diameter. A 13.56 MHz power P,, of up to 1 kW is supplied to the antenna in argon of 0.47 mTorr. The plasma is confined in a multipole field (surface magneticjeld) formed by many permanent magnets (alternating N/S pole columns, - 2 cm apart) attached to the inner chamber 23 cm in radius. Positioning the axial position of the antenna loop to be z = 0, the plasma is terminated by the outer chamber end at z = -6 cm and by a axially movable end plate at z = 40 cm where both end plates are covered with the surface magnetic field. As reported previously,” the surface magnetic field helps to make the density distribution uniform and to lower the working pressures as a result of highenergy electron confinement. Inner Chamber _ (n = 23 cm)

Outer Chamber (rz = 25, cm)

Inductively coupled plasma produced by an internal antenna In conventional inductively coupled plasmas (ICPs), electromagnetic energies are externally coupled to a plasma through dielectric materials (cylinder or disk) from a helical or spiral antenna placed outside the vacuum vessel. This external antenna system has several disadvantages when producing a large area plasma. First of all, the dielectric materials are mechanically weak and cause difficulties in maintenance. Especially, a recent trend toward the planar plasma with surface areas as large as 1 m2 forces using a very thick (> 5 cm) expensive dielectric disk in TCP to withstand a huge force against the atmospheric pressure. Thus, it is desirable to insert the antenna into a vacuum vessel without a dielectric window. To realize such ICPs, one may simply immerse a bare metal antenna in a plasma with one end of the antenna loop grounded. In this case, however, the elec-

/

i”l I

Parmanent Magnet

T-E-7 Generator

1

-

Figure 4. Schematic of experimental apparatus for inductive using internal antenna.

discharge

661

K Suzuki et al: Control of high-density

plasma sources parison of radial distributions obtained by different diameters of antenna at low pressure (p = 0.4mTorr), It is notable in this internal antenna system that even such small antenna as 1Ocm diameter can produce a uniform large-diameter (46 cm) plasma, at low pressures, where the plasma diffusion is only weakly disturbed by collisions.

1

8

.E X

0.8

$

0.6 o 0.4

E f

‘-< T ~:

0.2

: z=13cm

I ,,,I 0

I I ,, I,, ,,I,,

5 10 15 20 Radial Position r (cm)

\. A..\ g z

25

: . 1-.:: ,.

0.2 :

_ r=Ocm

‘0

~ ,I ”, 1.

11,,,111~ 5 10 15 20 25 30 35 40 Axial Position z (cm)

Figure 5. (a) Radial distributions at z = 13 cm and (b) axial distributions at r = 0 of normalized electron density n&z, produced at different pressures. PRF = 800 W.

A small Langmuir probe is moved radially and axially to measure the spatial distribution of electron density n, produced at different pressures. Typical examples obtained for the antenna diameter d = 18cm are shown in Figure 5 where (a) the radial density distributions at z = 13 cm and (b) the axial density distributions at Y = 0 are measured at pressure p = 0.4, 2 and 7mTorr. As seen in this figure, a uniformity of the normalized electron density is significantly improved at low pressures such as 0.4mTorr, owing to the large diffusion constant. The absolute electron density decreases with decreasing pressures but it is still as high as 10” cmd3 at 1 kW. The electron temperature is about 5 eV at 0.4mTorr. Thus, a large-diameter (>45 cm) planar (z <20cm) plasma can be produced by the internal antenna system. The most interesting result is the dependence of the density uniformity on the antenna diameter. Figure 6 shows a com-

ntenna Diameter

-0

5

Radial

10

15

Position

20

25

r (cm)

Figure 6. Radial distribution of electron density normalized at Y= 0 for different diameters of antenna. p = 0,4mTorr, P,, = 8OOW, and z = 13cm. 662

Surface-wave excited plasma and the mode structure To date, many experimental and theoretical works on surface wave plasmas have been reported as found in the latest review?’ However, most of them concern the production of a small-diameter long plasma columns, typically 5 cm in diameter and 30 cm in length. A recent trend toward large-area plasma processing has driven the development of a planar surface-wave plasma of diameter > 20 cm. Some attempts of such plasma production have been reported using a microwave launcher of large aperture formed on a waveguide,” a dielectric line,4 and a slotted antenna.3’-‘4 However, propagation of surface waves in the planar geometry has not been identified. Here an experimental evidence of standing surface waves in the microwave excited plasma is presented.” The experimental apparatus has been described elsewhere.35 Briefly, the 2.45 GHz1 kW microwave launched through a quartz window from a pair of slot antennas gives a large-diameter (22cm diameter) highdensity (- 2 x 10L2cm-‘) plasma in argon at pressures 0. l-l Torr. The radial distribution of electron density at 1 Torr is uniform within +5% over 20cm in diameter, at the axial position 3 = 20cm from the slot antenna, in a downstream chamber of 35 x 35 x 35cm3. An intense optical emission from the argon plasma with azimuthally periodic mode pattern was observed near the plasma boundary just below the quartz window. Figure 7(a) shows a typical photograph taken at 0.3Torr and 400W discharge. Twelve bright regions in the azimuthal direction and two bright regions in the radial direction clearly suggest the mode pattern of (m,n) = (6,2). In the same conditions, the azimuthal distribution of the radial component I&I of the microwave field was measured, using an antenna rotatable azimuthally at r = 5cm where the strong optical emission was observed as seen in Figure 7(a). The polar plot of the radial electric field intensity l&1* measured at z = 7mm is shown in Figure 7(b). A comparison of two figures in Figure 7 leads to that the azimuthal location of high microwave fields coincides with the location of strong optical emission. This suggests that the intense microwave field accelerates electrons which locally excite argon atoms, thus yielding the localized optical emission. Next, the axial distribution of the microwave field lE,(’ was measured for the fixed azimuthal angle 8. The data taken at 0 = 0’ (the high amplitude) and 0 = - 15” (the node) are shown in the upper figure in Figure 8 where the radial position is r = 5 cm and the mode pattern of (m,n) = (6,2) is present in the same experimental conditions as in Figure 7. It should be noted that the mode of the surface wave is M = 0 in a region of r < 8 mm. Subtraction of the data at 6 = - 15” from 8 = 0” gives the hump peaked at r - 12 mm as indicated by closed circles in the lower figure in Figure 8. Thus, the m = 6 mode is considered to be localized around the resonant layer (w = wP) in the electron density profile near the quartz wall. The detailed theoretical analysis of surface wave propagation in a plane geometry will be reported elsewhere.‘6

K Suzuki ef al: Control

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Optical

(4

(b)

Emission

Microwave

Field

plasma sources Intensity

Intensity

Axial Position Z(mm) 90”

Figure 8. Axial distributions of radial electric field intensity l&l’ at 0 = 0” (0) and 0 = - 15” (0) in the upper figure. The closed circles in the lower figure correspond to the difference between two data in the upper figure while the open circles denote the same data as in the upper figure.

1

is attributed to a surface wave satisfying a dispersion relation for a plane geometry. Various advantages using an internal antenna in inductive rf discharges for a large-area plasma production are presented. Especially, one can considerably reduce the antenna size which in turn makes the impedance matching relatively easy.

180”

Acknowledgements

270”

Figure 7. (a) Optical emission intensity profile on the r-0 plane and (b) polar plot of radial electric field intensity l&l2 for the same discharge power of 400 W and p = 0.3 Torr.

The authors would like to thank H Ohkubo, G Xu and I P Ghanashev for their help in a course of experiments. This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture in Japan, and partly by Toshiba Co. Ltd and Samsung Electronics Corporation. References

Summary A recent development plasma was reviewed. plasma processing are discharge physics and

of production of high-density large-area Some problems common to high-density pointed out. The following topics on the the plasma control are presented.

The wave excitation mechanism in a helicon discharge was investigated. It is shown that helicon waves are resonantly launched in one side of the helical antenna when the wavelength satisfies the definite relations with respect to the antenna length. Standing waves having the radial and azimuthal structures are observed at the surface of a planar plasma produced by a microwave discharge using a slot antenna. The observed mode

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11. Hikosaka, Y.. Nakamura, M. and Sugai, H., Proceedings of 2nd Int Coqfon Reacfive Plnsmas, Yokohama, Japan (1994) p 67. 12. Sugai, H., Nakamura, K., Hikosaka, Y. and Nakamura, M., J Vat Sri Techol, 1995, A13, 887. 13. Shirakawa, T., Toyoda, H. and Sugai, H., Jup J Appl Phys, 1990,29, L1015. 14. Boswell, R. W., Proc2lst Int CocfPhenomena IonizedGases, Bochum (1993) p 118. 15. Chen, F. F., Plasma Phys Controlled Fusion, 1991,33, 339. 16. Komori, A., Shoii, T.. Miyamoto, K. and Kawai, J., Kawai, Y., Phys Fluids, 1991, B3,“893. 17. Zhu. P. and Boswell. R. W.. Phvs Fluids. 1991. B3. 869 18. Abe: T., Nakazawa, S., Koshikawa, N., Sakawa, Y. and Shoji, T., Proc 2nd Int Conf Reactive Plasmas, Yokohama (1994) p 63 19. Sugimoto, M., Tanaka, M., Komori. A. and Kawai, Y., Proc 2nd Int Coqf Reactice Plasmas, Yokohama (1994) p 557. 20 Ellingboe, A. and Boswell, R. W., &II Am Phys Sot, 1994.39, 1460. 21. Nakamura, K., Suzuki, K. and Sugai, H., Jap J Appl Ph_~s,1995,34, 2152. 22. Sugai, H., Sato, M and Ido, K., Takeda, S., J Phys Sac Jap, 1978, 44. 1953. 23. Stevens, J. E.. Sowa, M. J. and Cecchi, J. L., J Vat Sci Technol, 1995, A13,2476. 24. Shoji, T., Sakawa, Y., Nakazawa, S. and Kadota, K., Sato, T., Plasma Sources Sci Technol, 1993,2, 5.

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25. Chen, F. F., J Vat Sci Technol, 1995, AlO, 1389. 26. Sudit, I. D., Light, M. and Chen, F. F., &tli Am Phys Sac, 1994,39, 1461. 27. Suzuki, K., Nakamura, K. and Sugai, H., Jap J Appl Phys, 1996,35, 4044. 28. Sugai, H., Nakamura, K. and Suzuki, K., Jup J Appl Phys, 1994.33, 2189. 29. Nakamura, K., Kuwashita, Y. and Sugai, H., Jap J Appl Phys, 1995, 34, L1686. 30. Moisan, M. and Zakzewsky, Z., Microwave Excited Plasmas, Moisan, M. and Pelletier, J. (Eds). Elsevier, Amsterdam (1992) p 123. 31. Fujimura, S., Shinagawa, K., Suzuki, M. and Nakamura, M., J Val Sci Technol, 1991, B9, 357. 32. Werner, F., Korzec, D. and Engelmann, J., Plasma Sources Sci Technol, 1994, 3,473. 33. Bluem. E.. Be&u. S.. Boisse-Laoorte. C.. Leurince, P. and Marec, J., J Phys,‘1995, I&: 1529. I 34. Tamura, H., Otsubo, T., Sasaki, I., Ohara, K., Yamagutchi, Y. and Kato, S., in Proc 41s/ National Symp Am Vat Sot, Denver (1994) p 191. 35. Nagatsu, M., Xu, G., Yamage, M. and Kanoh, M., Sugai, H., Jap J Appl Phvs, 1996, 35, L341. 36. Ghanashev, I. P., Nagatsu, M. and Sugai, H., J Appl Phys, 1997,36, 337-344.