Characterization of a high-frequency inductively coupled plasma source

Thin Solid Films 506 – 507 (2006) 469 – 473 www.elsevier.com/locate/tsf

Characterization of a high-frequency inductively coupled plasma source D.S. Lee *, H.S. Jun, H.Y. Chang Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 305-701, South Korea Available online 21 September 2005

Abstract We report the experimental results of a 27.12 MHz inductively coupled plasma source for next generation dry etching. The new source is designed to improve the plasma density uniformity for large-area processing. The key techniques of this new source are the reduction of the inductance of the antenna system in parallel connection and the induction of LC resonance with external capacitance variation. An external variable capacitor is connected in series to an outer coil to obtain a high level of plasma uniformity. We can control the radial plasma density with 5% uniformity by only varying the external capacitance. The electron density varies linearly with the discharge power and increases monotonically with pressure. The electron density in 27.12 MHz is lower than that in 13.56 MHz at the same discharge power. An essential difference was found in the effective electron temperature. The lower electron temperature in 27.12 MHz is obtained from EEPFs. The electron temperature was 1.5 eV – 2.5 eV. D 2005 Elsevier B.V. All rights reserved. Keywords: ICP; Plasma source; High frequency discharge; Parallel resonance antenna

1. Introduction With the reduction of feature size dimensions in recently developed microelectronics and flat panel display devices, a uniform high-density plasma operating at low pressures (1 – 100 mTorr) is necessary in order to obtain good anisotropic patterns and high etching throughput. In addition to producing high-density plasma, the development of a dry etcher to handle larger area wafers is required as the current 200 mm wafer diameter has increased to 300 mm. Radiofrequency discharges have been widely used for the high anisotropic etching of fine-line features in ultralarge-scale integrated devices. Many plasma sources have been studied in efforts to generate uniform high-density plasma by various discharge methods, such as inductively coupled discharge [1– 3], modified magnetron-type discharge [4], dual-frequency narrow-gap capacitive discharge [5], and ultrahighfrequency discharge using a spokewise antenna [6]. Among many plasma sources, ICP-type has many attractive aspects, including a simple apparatus without external magnetic coils, relatively efficient plasma gener* Corresponding author. E-mail address: [email protected] (D.S. Lee). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.08.057

ation, spatial uniformity, independent control over ion flux and ion energy, and scalability to large-area plasma sources [7,8]. However, a conventional ICP source using a spiral coil has some limits in extending the process area up to 300 mm. The problem is related to the coil inductance. In order to obtain larger plasma for a 300 mm wafer, the area of the coil should also be larger. As the antenna size is scaled up, the inductance of the planar coil inevitably increases and the nonuniformity of the radial plasma density increases correspondingly. In this paper, we present a new inductively coupled plasma (ICP) source (called a Parallel Resonance Antenna, PRA) with a new antenna configuration for large-area etching processing. The electron temperature could be lower depending on the discharge frequency; thus, we measured plasma parameters using a 27.12 MHz frequency.

2. Experimental setup The new source is designed such that it should have low terminal inductance and a high level of radial uniformity over a large area. Our antenna system is described schematically in Fig. 1. The source consists of four turn

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Variable capacitor, CV I.M.B

RF

(a) I.M.B

RF

Capacitor), which varied from 50 pF to 500 pF, is connected to the outer coil. Experiments are carried out in a cylindrical stainless-steel discharge chamber 30 cm in radius and 30 cm in length. The parallel resonance antenna is mounted on a top quartz plate. The chamber, shielding cap, and matching box are grounded, and the chamber is pumped with a turbomolecular pump (Leybold TP450) backed by a mechanical pump. We set the movable RF-compensated Langmuir probe to measure the EEDFs ( f(e)) at 9 cm below the dielectric window. The numerical differentiation method is used to measure the EEDFs. The detailed experimental setup is referred to another paper [10].

Outer coil

3. Experimental results Z4

Z3

Z2

Z1

Inner coils

CV

(b) Fig. 1. Schematic diagram of (a) a large-area ICP source with an external variable capacitor and (b) its equivalent circuit.

coils, and the coils are connected in parallel. Due to the parallel connection, the antenna inductance is expected to be very low compared with that of a typical spiral coil. Low inductance decreases the antenna voltage, and hence, a low capacitive field can be achieved. Scaling the antenna size up for a large-area plasma process, the parallel connection should solve the problems caused by large antenna inductance; furthermore, low inductance can make impedance matching stable during high-frequency generation. Although it can give low antenna inductance, it cannot guarantee that uniform plasma will be generated. It causes the inner coils to have small inductance compared with the outer coil and accordingly induces larger current. To obtain radially uniform plasma, the antenna current should be concentrated in a coil located near a relatively low plasma density regime; therefore, we should flow more current in the outer coil in this case. To this end, we connect the outer coil in series with an external variable capacitor to induce LC resonance. The current in each coil can be controlled by varying the capacitance, and the current in the outer coil is maximal at the LC resonance. Through the numerical calculation and experiments at 13.56 MHz, it was shown that this external capacitor serves as an external parameter to control the antenna current distribution and plasma uniformity [9,10]. The new source was made from a silver coated solid copper line of 3/8 in. The diameter of the new source is 24 cm, and the coils are spaced at 12 cm, 16 cm, 20 cm, and 24 cm apart from the center. A VVC (Vacuum Variable

3.1. Plasma parameter measurements It was found that the new plasma source with an external variable capacitor can reduce antenna inductance, due to its parallel connection, and generate uniform plasma at a driving frequency of 13.56 MHz. Variation of the coil current distribution induced by LC resonance was observed through experiments; therefore, we could generate plasma with 3% nonuniformity by varying the external capacitor [10]. First, the radial profile of the plasma density at 27.12 MHz was measured to get a uniform density profiles as the same LC resonance effect of 13.56 MHz discharge. Fig. 2(a) and (b) show the radial profile at 27.12 MHz compared with conventional ICP source at 13.56 MHz and the radial profile by using process gas. Plasma density was measured near the resonance point. The external capacitor variation verified that both the current distribution in the coil and the plasma density could be adjusted [10]. The radial profile of the plasma density can be also controlled within 5% uniformity at 27.12 MHz. Generally, the nonuniform current generates corresponding nonuniformity in an inductive field containing magnetic components and an electric component. The electron heating below the coil (in the region of the skin depth) is directly affected by the distribution of coil currents. Consequently, nonuniformity in the coil currents results in the nonuniformity in the electron temperature. Since the ionization rate is determined by the electron temperature, the ion density profile should be nonuniform. As the currents flowing in the inner coils are higher than the outer current in the parallel connection, more plasma in the center region is produced. To obtain radially uniform plasma, the antenna current should be concentrated in a coil located near a relatively low plasma density regime; therefore, we should flow more current in the outer coil in this case. As a uniform plasma density can be obtained by an appropriate capacitance at any discharge, the new plasma source could be applied to 300 mm wafer processing. The basic plasma parameters were measured to obtain the characteristics of high-frequency inductive discharge. The

D.S. Lee et al. / Thin Solid Films 506 – 507 (2006) 469 – 473

Parallel Resonance Antenna

1 mtorr 1E12

10 mtorr

15

50 mtorr

10

Electron Density (cm-3)

Plasma Density (arb. unit)

20

471

Conventional Antenna

5

0 -200

-100

0

100

1E11

1E10

200

Position (mm)

(a)

1E9

2.0x1011 1.8x1011

27.12 MHz 700W 200W

Power (W) Fig. 3. Electron density dependence on discharge power for 1 mTorr, 10 mTorr, 50 mTorr at 27.12 MHz discharge.

1.6x1011 1.4x1011 1.2x1011

200 mm

1.0x1011 8.0x1010 -150 -100

-50

0

50

100

150

position (mm)

(b) Fig. 2. (a) Radial profile of plasma density at 27.12 MHz comparison with conventional ICP (13.56 MHz). (b) Radial profile of plasma density at process gas: RF power of 27.12 MHz source with 700 W and 2 MHz bias with 200 W.

electron densities n e and the effective electron temperatures Te found from integrals of the EEPFs are given in Figs. 3 and 4. Plasma parameters are measured at r = 0 cm and z = 9 cm below the dielectric window for various discharge conditions. n e and Te are calculated using the measured EEPFs, f(e), in accordance with the following formulae: Z 2 Te ¼ be;ne ¼ f ðeÞde: 3 The plasma potential V p is the probe potential referenced to the grounded metal chamber at the zero crossing point of the second derivative (d2 I p)/(dV 2). The electron density dependence on discharge power is given in Fig. 3. The electron density varies linearly with the discharge power and increases linearly and monotonically with pressure; however, the electron density at 27.12 MHz is lower than that at 13.56 MHz [10]. The power dependence of the effective electron temperature Te is given in Fig. 4. The similar trends of the electron temperature at 13.56 MHz are observed. The decrease in

electron temperature with discharge power is a typical property of ICP discharge at high pressure (50 mTorr). As discharge power increases, electron density increases and ionization is mainly controlled by a two-step ionization process. As a consequence of two-step ionization, the ionization frequency can grow and electron temperature could become lower with plasma density at high pressure [11]. Furthermore, electron – electron collisions (m ee ” ne 3/2) are large enough to Maxwellize the low energy part of EEPF under high pressure, and we could observed the Maxwellian EEPFs over the entire discharge power range at this pressure 3.5 1 mtorr 10 mtorr 3.0

Electron Temperature (eV)

n (cm-3)

100

Nonuniformity = 3 % @ 200 mm Nonuniformity = 4 % @ 300 mm

50 mtorr

2.5

2.0

1.5

1.0 0

100

200

300

400

500

Power (W) Fig. 4. Effective electron temperature dependence on discharge power for 1 mTorr, 10 mTorr, 50 mTorr at 27.12 MHz discharge.

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(50 mTorr); however, Te increases with discharge power at low pressure (1 mTorr) and low discharge power (10 mTorr). This trend is opposite to the general case observed in high pressure discharge. This electron temperature dependence is due to the high population of the low energy electrons in the electron energy distribution. As discharge power increases, low energy electrons are heated by high energy electrons through electron – electron collisions and the excess of low energy electron disappears. Thus, the effective electron temperature increases with power at low pressure [11].

Electron Density (cm-3)

PRA 13.56 MHz PRA 27.12 MHz 1011

1010

0

100

200

300

400

500

3.2. Comparison with 13.56 MHz

Power (W)

The plasma parameters were measured to compare 27.12 MHz discharge with 13.56 MHz for a fixed pressure of 10 mTorr. The electron densities at two different frequencies (13.56 MHz and 27.12 MHz) are given in Fig. 5(a). In both cases, the electron density increases gradually with the discharge power, and the E – H mode transition is not clear. In the case of 27.12 MHz, lower electron densities were obtained, and this was caused by lower coupling efficiency from the antenna coil to the plasma. Stray capacitance always exits between the coil and the plasma, and this stray capacitance increases with driving frequency; the contribution of Emode appears to be another reason for lower electron density. Even in the inductive discharge (H-mode), the capacitive discharge (E-mode) still contributes to plasma generation. At a high frequency, the contribution of the Emode to plasma generation is larger, and it has a long lasting effect on plasma; however, the power in this paper is not dissipated power to plasma, but transferred power from an

(a) Electron Temperature (eV)

3.2

PRA 13.56 MHz

3.0

PRA 27.12 MHz

2.8 2.6 2.4 2.2 2.0 1.8 1.6 0

100

200

300

400

500

Power (W)

(b) Fig. 5. Plasma parameters at 27.12 MHz comparison with 13.56 MHz for 10 mTorr.

10 10

10 10

PRA 13.56 MHz PRA 27.12 MHz

PRA 13.56 MHz PRA 27.12 MHz 10 9

-3 cm ) -3/2

10 8

EEP F (eV

EEPF (eV -3/2 cm -3 )

10 9

10 7

10 8

10 7 10

Ne ~ 2.7x10 cm

10 6 0

5

10

Electron Energy (eV)

(a)

15

10 6 0

-3

5

10

15

Electron Energy (eV)

(b)

Fig. 6. The EEPFs (a) at a fixed discharge power of 100 W (b) at a same electron density of n e ¨ 2.7  1010 cm 3.

D.S. Lee et al. / Thin Solid Films 506 – 507 (2006) 469 – 473

RF generator to a matching network, thus, it is hard to make mention of frequency dependence on electron density. The electron temperature measurements at different frequencies are given in Fig. 5(b). An essential difference was found in the effective electron temperature: the electron temperature of 27.12 MHz is lower than that of 13.56 MHz. To elucidate the frequency dependence on the electron temperature, the EEPFs were measured for various discharge powers. Fig. 6(a) shows the EEPFs at the same power of 100 W and the EEPFs at the same electron density of n e ¨ 2.7  1010 cm 3 are given in Fig. 6(b). Both of EEPFs are very close to Maxwellian distribution. The distribution temperature, defined as Te = [(d(ln fe(e)))/ (de)] 1, is inversely proportional to the EEPF slope. Lower electron temperature can be verified clearly in Fig. 6. Te values are 1.8 eV and 2.2 eV at 27.12 MHz and 13.57 MHz, respectively. Godyak reported that the frequency dependence (from 3.39 MHz to 13.56 MHz) is caused by the finite transit time resonance [12]. For a higher frequency range, frequency dependence on the electron temperature could be applied well. More study on a wide range of frequencies (0.5 MHz– 60 MHz) is needed to solve this frequency dependence.

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sure; however, the electron density in 27.12 MHz is lower than that in 13.56 MHz at the same discharge power. A lower electron temperature at 27.12 MHz was also obtained; the electron temperatures were 1.5 –2.5 eV. The trends of the plasma densities, the effective temperatures, and the plasma potentials on discharge power and pressure are similar to the normal trends observed in inductive discharge plasma. As a result, this new source at highfrequency discharge can be a good candidate for large are plasma processing.

Acknowledgments This work was sponsored in part by the SYSTEM I.C. 2010 (Grant no. M103BY010030-03B2501-03011), the National Research laboratory (NRL, grant No. M1031800007103J0000-02911), the Engineering and Research Center (ERC, grant No. R11-2000-086-03007-0), Tera Level Nanodevices Project of MOST (TND, Grant no. M101KC01001503K0301-01520) and IMT 2000.

References 4. Conclusion It was found that the new plasma source with the external variable capacitor can reduce antenna inductance due to its parallel connection and generate uniform plasma [10]. The radial profile of the plasma density could also be controlled within 5% nonuniformity at a high-frequency discharge. As a uniform plasma density can be obtained by an appropriate capacitance at high frequency, the new plasma source could be applied to 300 mm wafer processing. The plasma parameters were obtained from the measured EEDFs using a Langmuir probe. The new source with high frequency produced high densities (1010 –1012 cm 3) in Ar plasma under a pressure range of 1 mTorr and 50 mTorr. The electron density varies linearly with the discharge power and increases monotonically with pres-

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