Paper PS-WeA8
Effect of Multi-frequency Bias on Ion Energy Distribution in Inductively Coupled Plasma

Wednesday, November 2, 2011, 4:20 pm, Room 201

Inductively coupled plasma (ICP) sources, commonly used for semiconductor and conductor etching, embody the concept of functional separation between plasma production and ion energy control, wherein the inductive coupling through the coils is only responsible for the plasma generation while the bias determines the ion energies. Plasma etching of microelectronics structures at advanced technological nodes (< 3x nm), especially complicated structures such as multi-gate MOSFETs and 3D memory stacks, are placing great emphasis on control of ion energy distributions (IEDs) to finely discriminate etching thresholds.[1] Sinusoidal biases typically provide broad IEDs, making such control difficult to achieve. One promising alternative is non-sinusoidal bias waveforms, which have been demonstrated to provide such control.[2] However, there are issues associated with passing non-sinusoidal signals through the finite impedance of the match, transmission line, and the substrate.

Multi-frequency bias is compatible with current manufacturing hardware, and has been successfully used for controlling IEDs in capacitively coupled plasmas.[3] This approach may be utilized to enable IED control in ICP etchers as well. We investigate the impact of multiple bias frequencies in this paper. A high frequency applied in addition to a relatively low frequency bias causes the sheath potential to vary in a complicated manner due to the non-linear nature of the sheath. As a result, IED exhibits a complex dependence on relative bias voltages and frequencies.

In this work, the effect of applying bias at multiple frequencies will be discussed using results from a computational investigation. The 2-dimensional plasma equipment model, HPEM[4], has been modified to enable power deposition at multiple frequencies on the same electrode. Results will be discussed for Ar/Cl₂ plasma utilizing a 13.56 MHz bias in addition to a bias at a different frequency in an ICP chamber. The additional frequency is varied over a wide range and its consequences assessed on the ion and radical flux and IEDs incident on the wafer. We found that, in addition to modulating the IEDs, the flux composition is different depending on the frequency due to the secondary plasma generation by the alternate frequency.

[1] A. Nitayama and H. Aochi, ECS Trans. 18, 89 (2009).

[2] A. Agarwal and M.J. Kushner, J. Vac. Sci. Technol A 27, 37 (2009).

[3] S. Shannon, D. Hoffman, J.-G. Yang, A. Paterson, and J. Holland, J. Appl. Phys. 97, 103304 (2005).