1730 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 37, NO. 9, SEPTEMBER 2009 Inductively Coupled Pulsed Plasmas in the Presence of Synchronous Pulsed Substrate Bias for Robust, Reliable, and Fine Conductor Etching Samer Banna, Ankur Agarwal, Ken Tokashiki, Hong Cho, Shahid Rauf, Senior Member, IEEE, Valentin Todorow, Kartik Ramaswamy, Ken Collins, Phillip Stout, Jeong-Yun Lee, Junho Yoon, Kyoungsub Shin, Sang-Jun Choi, Han-Soo Cho, Hyun-Joong Kim, Changhun Lee, and Dimitris Lymberopoulos Abstract—Inductively coupled pulsed plasmas in the presence ics for damage-free plasma etching processes with improved of synchronous pulsed substrate bias are characterized in a com- uniformity, higher selectivity, better anisotropy, and enhanced mercial plasma etching reactor for conductor etching. The syn- process throughput have stimulated an intensive research effort chronous pulsed plasma characteristics are evaluated through the among academic and industrial communities in search of novel following: 1) Ar-based Langmuir probe diagnostics; 2) Ar/Cl2 plasma modeling utilizing the hybrid plasma equipment model approaches and methods for the design and control of the and the Monte Carlo feature model for the investigation of fea- next generation of plasma processing reactors. Hence, there ture profile evolutions; 3) basic etching characteristics such as is a vital need for wider and more flexible ranges of plasma average etch rate and uniformity; 4) sub-50-nm Dynamic Random operating conditions aiming to improve the etch processes for Access Memory (DRAM) basic etching performance and profile control; and 5) charge damage evaluation. It is demonstrated that finer features. one can control the etching uniformity and profile in advanced Typically, plasma reactors use an RF power source with con- gate etching, and reduce the leakage current by varying the syn- stant average power or voltage to excite the plasma in a vacuum chronous pulsed plasma parameters. Moreover, it is shown that chamber. Such mode of operation is known as continuous-wave synchronous pulsing has the promise of significantly reducing the (CW) RF mode. For the last two decades, several researchers electron shading effects compared with source pulsing mode and continuous-wave mode. The synchronous pulsed plasma paves the have demonstrated through numerical modeling and experi- way to a wider window of operating conditions, which allows mental studies that pulsing the RF power input, i.e., pulsed new plasma etching processes to address the large number of RF mode, has the promise to increase the flexibility of plasma challenges emerging in the 45-nm and below technologies. processing by enlarging the range of operating conditions [4]– Index Terms—Inductively coupled plasma (ICP), plasma con- [42]. Two main parameters characterize the RF pulse: 1) pulse trol, plasma-induced damage (PID), plasma material-processing frequency, i.e., the frequency at which the RF power is turned applications, synchronous pulse-time-modulated plasma. on and off per second, and 2) pulse duty cycle. The latter is de- fined as the ratio between the pulse ON time and the total pulse I. INTRODUCTION duration. By varying the pulse frequency and the duty cycle, OLLOWING Moore’s law, the pace at which the micro- pulsed plasmas provide additional “control knobs” in which pri- F electronic technology is moving these days might highly mary plasma properties, such as ion/electron densities, electron be challenging with conventional device architecture. Several temperature, ion/neutral flux ratio, and plasma potential, can intrinsic limitations have triggered an extensive research ac- be controlled. Hence, transitions from electron–ion plasma to tivity seeking new materials to be implemented in the next ion–ion plasma during the after-glow phase (power-off period) generation of integrated circuits (e.g., [1]–[3]). Moreover, the might occur for electronegative plasmas [5]–[7], [28]. Further- more stringent and conflicting requirements in microelectron- more, for gate patterning applications, it was demonstrated that the pulsed plasma exhibits highly selective, highly anisotropic, notch-free, and charge-build-up damage-free polycrystalline Manuscript received December 19, 2008; revised June 30, 2009. Current silicon etching [8]–[14], [39], [40]. Undesirable profile distor- version published September 10, 2009. This work was supported in part by tions, such as microtrenching, bowing, and local side etching the Etch Division, Applied Materials, Inc., and in part by the Semiconductor R&D Center, Samsung Electronics. (notching), which are thought to be due to differential charging S. Banna, A. Agarwal, S. Rauf, V. Todorow, K. Ramaswamy, K. Collins, and in features (electron shading), may be mitigated by using a P. Stout are with the RF and Plasma Technology Group, Etch Division, Applied Materials, Inc., Sunnyvale, CA 94085 USA (e-mail: [email protected]). pulsed plasma if the negative ions can be injected into the K. Tokashiki, H. Cho, J.-Y. Lee, J. Yoon, and K. Shin are with the Semi- feature to neutralize the charge deposited by positive ions [4]– conductor R&D Center, Samsung Electronics Company Ltd., Hwasung City [20]. In addition, the pulsed RF mode is capable of reducing 445-701, Korea. S.-J. Choi, H.-S. Cho, H.-J. Kim, C. Lee, and D. Lymberopoulos are with the ultraviolet radiation damage in plasma processing using the Etch Product Business Group, Etch Division, Applied Materials, Inc., high-density plasmas and plasma-induced charge damage (PID) Sunnyvale, CA 94085 USA. [22]–[27], [45]–[51]. Moreover, pulsed plasmas have gained Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. recognition as a means to control the plasma deposition envi- Digital Object Identifier 10.1109/TPS.2009.2028071 ronment (e.g., [31] and [37]). 0093-3813/$26.00 © 2009 IEEE Authorized licensed use limited to: Applied Materials via the e-Library. Downloaded on October 12, 2009 at 13:42 from IEEE Xplore. Restrictions apply. BANNA et al.: INDUCTIVELY COUPLED PULSED PLASMAS IN THE PRESENCE OF PULSED SUBSTRATE BIAS 1731 The lack of efficient and stable coupling of RF pulsed power has substantially limited the operating window and has only allowed for a very limited number of applications to be run in mass production. There are two main ways to couple the pulsed RF power in inductively coupled plasma (ICP) reactors [43], [44]. One way is to operate the ICP source in CW RF mode and the substrate bias in pulsed RF mode. The other way is to pulse the ICP source in the presence of substrate bias in CW RF mode. The main challenge, with either way of coupling pulsed RF power, is managing the amount of reflected power. Specif- ically, in ICP reactors, a high-bias reflected power is observed for low-pressure processes (a few tens of millitorrs and below) with source pulsing, in which the time modulation of the source power is highly coupled to the bias. The high reflected power is mainly attributed to the nature of pulsed plasma, in which the plasma impedance rapidly varies within the pulse. These rapid variations cannot be tracked by a conventional commercial dynamic matching network due to the mechanical nature of its adjustment. The response time of such an adjustment is at least on the order of tens of milliseconds. Therefore, for pulse frequencies greater than 1 kHz, a high reflected power might Fig. 1. Schematic of Applied Materials, Inc., 300-mm Silicon Etch occur. Hence, even if the RF power supply operates in a load AdvantEdge G5 ICP reactor with pulsing capability. power mode, for which it compensates for the reflected power, there is a real concern of repeatability and reliability while operating at a very high reflected power level. feature profile evolution in Ar/Cl2-based chemistries. Third, Due to the aforementioned RF power coupling challenges, a basic etch performance investigation for gate applications the need for the development of new technology that renders is carried out and discussed in Section V. It includes etch pulsed plasmas production worthy is evident. As a result, the rate dependence on the synchronous pulsed plasma param- focus is on developing technology to reduce the reflected power eters. Profile control for sub-50-nm DRAM gate etching is when in pulse mode at the submillisecond scale. This paper demonstrated. Charge-build-up damage reduction and electron introduces the concept of synchronous source and bias pulsing shading control while using the synchronous pulsed plasma is with/without phase delay in ICP reactors. In this approach, both evaluated by using an in situ charge-up monitoring wafer. the ICP source and the substrate bias are pulsing at the same frequency and with the same duty cycle. II. EXPERIMENTAL SETUP Recently, Applied Materials, Inc. has modified its commer- cial ICP 300-mm silicon etch tool named AdvantEdge to fully Fig. 1 illustrates the ICP etching apparatus and the support the operation of the synchronous pulsed plasma, along Langmuir probe measurement setup that is used in this paper. with developing multiple techniques for optimizing the RF The ICP etching system is the Silicon Etch AdvantEdge G5 power delivery in pulsed RF mode, therefore providing an from Applied Materials, Inc. The ICP reactor consists of a expanded window of operating conditions. In doing so, the Yttria-coated chamber with a He-cooled chuck equipped to matching response time is reduced to a few microseconds, hold 300-mm wafers. The ICP source antenna consists of two which ensures a low reflected power level that is acceptable solenoid RF coils (inner and outer). The source RF power at for a wide variety of applications poised to benefit from pulsed 13.56 MHz is fed to the antenna coils through a matching plasmas (for details, see [42]). network and is coupled to the plasma through the dielectric win- In this paper, the basic characteristics of inductively coupled dow.