Plasma Effects on Electrostatic Chuck Characteristics on ...

1 Plasma and Fusion Research: Letters Volume 2, 044 (2007). Plasma E ects on Electrostatic Chuck Characteristics on capacitive RF Discharge Gyu Il SHIM, Takeshi YAMAUCHI1) and Hideo SUGAI2). Department of Electrical Engineering and Computer Science, Nagoya University, Nagoya 464-8603, Japan 1). Corporate Manufacturing Engineering Center, TOSHIBA Corporation, Yokohama 235-0017, Japan 2). Department of Electronics and Information Engineering, Chubu University, Kasugai 487-8501, Japan (Received 31 July 2007 / Accepted 14 September 2007).

2 Johnsen-Rahbek Electrostatic Chuck (ESC) is installed on the cathode side of a capacitive RF discharge, and the ESC voltage-current (V-I) characteristic is measured under various conditions. First, the reference V-I curve is obtained for a grounded aluminum (Al) wafer without discharge. The observed nonlinear characteristic is attributed to the field emission of electrons at irregular contacting surfaces. When the discharge is turned on with an electrically floating wafer, the V-I curve shifts from the reference curve toward the negative direction along the Chuck voltage axis.

3 The amount of shifted Chuck voltage coincides with the self-bias DC voltage induced on the wafer. This Plasma e ect on the V-I Characteristics can be explained well in terms of the e ective Chuck voltage, taking into account the self-bias. On the other hand, the replacement of the Al wafer with a silicon (Si). wafer leads to a considerable reduction in the Chuck current. When a thin Al foil is inserted between the Si wafer and the aluminum nitride (AlN) spacer layer, the Chuck current recovers upto the reference value, suggesting that the Johnsen-Rahbek e ect is extremely sensitive to the electrical and mechanical properties of the contacting interface.

4 C 2007 The Japan Society of Plasma Science and Nuclear Fusion Research Keywords: Electrostatic Chuck , Johnsen-Rahbek, capacitive discharge, self-bias DC voltage DOI: Electrostatic Chuck (ESC) is widely used for hold- process repeatability caused by residual charges, film dam- ing silicon wafers and controlling their temperature during age induced by the Chuck current, and wafer cracking when the Plasma -assisted processing of semiconductors. There raised by lift pins. are two holding configurations of ESC: the Coulomb type To solve these problems, a deeper understanding of [1 3], using an insulating spacer layer (volume resistivity the ESC-holding mechanism is required, particularly in > 1014 -cm), and the Johnsen-Rahbek (J-R) type [4, 5], practical Plasma conditions.

5 In this paper, we present a ba- using a semiconductive spacer layer ( = 1010 -1012 -cm) sic study on J-R ESC installed in a parallel-plate discharge, between plates ( , Chuck electrode and wafer). The elec- , capacitive coupled Plasma (CCP). The voltage-current trostatic holding force in the Coulomb type stems is gen- (V-I) Characteristics of ESC is measured for a variety of erated due to the opposite polarity surface charges that ap- discharge powers, comparing the silicon wafer with the pear on the wafer and Chuck electrode at high applied volt- aluminum wafer.

6 Notable e ects of RF-induced self-bias ages. A new experiment using thin plastic films and insu- voltage on V-I Characteristics were observed, along with lating sealant has been reported on the bipolar configura- the influence of contacting material surfaces. tion of Coulomb-type ESC [6]. The experimental apparatus for a CCP discharge with In the J-R type, a very strong holding force is achieved a unipolar J-R ESC in a grounded stainless-steel cham- even at low Chuck voltages due to the high electric fields ber with a diameter of 300 mm, as shown in Fig.

7 1. An between the narrow gaps distributed over the spacer layer RF Plasma at MHz was produced in an argon at- with surface irregularities. In comparison to the Coulomb mosphere at a pressure of 100 mTorr and a flow rate of type, the J-R type is very sensitive to the following phys- 100 sccm in a 30 mm gap between a grounded anode and ical conditions of the contacting surface: electrical con- a cathode, which had a diameter of 200 mm and consisted ductivities, residual charges, surface roughness on sub- of a RF electrode, ESC, and wafer.

8 A wafer of aluminum microscopic scales, and large-scale flatness of two plates. (Al) or a silicon (Si) slab with 100-mm-radius was clamped Many questions arise on how such conditions influence next to the J-R ESC with radius r = 100 mm, where a clamping and declamping behaviors in actual ESC sys- molybdenum Chuck electrode was embedded in a 10-mm- tems. Examples of ESC-related limitations include poor thick AlN layer with separation l = mm (spacer layer thickness) from the contacting wafer surface. The vol- author's e-mail: ume resistivity of AlN used in the present experiment is 044-1.

9 C 2007 The Japan Society of Plasma Science and Nuclear Fusion Research Plasma and Fusion Research: Letters Volume 2, 044 (2007). Fig. 2 Voltage-current Characteristics for the grounded Al wafer without Plasma . Fig. 1 Schematic of CCP apparatus used with the J-R ESC sys- tem. Table 1 Chuck current I, voltage di erence V, and electric field = 5 10 -cm. The net contacting surface is reduced 10 E measured for various Chuck voltages V. to 14% of the projected area of the Chuck electrode be- cause numerous embosses of diameter 2 mm and height V (kV) I ( A) V (kV) E (MV/m).

10 H = 50 m were prepared for helium gas cooling. There- 193. fore, the total resistance R of the AlN layer between the 331. Chuck electrode and the wafer is provided by a series con- 338. nections of the base part and the embossed part as 188.. l h h - R= + (1) - S - where the Chuck area S = r2 , and by substituting the pa- rameters into Eq. (1), we get R = M . The Chuck voltage V in a range from kV to + kV is applied to the Chuck electrode through which the other hand, the Chuck current measured at high volt- the Chuck current I flows to the grounded anode and cham- ages (V > 1 kV) drastically increases, exceeding the val- ber wall, forming a current loop via the AlN layer, clamped ues predicted using Ohm's law.