Control of photoresist etch rate via a gradient magnetic field in biased inductively coupled plasma

Abstract

Axial gradient magnetic fields were applied to a planar biased inductively coupled plasma (ICP) to achieve a controllable plasma spatial distribution with high electron density and electron temperature. The magnetic field gradient was controlled by adjusting the currents in the upper and lower coils of the Helmholtz coil, while maintaining the magnetic field required for electron cyclotron resonance near the axial center of the chamber. The direction from the antenna to the bias electrode was defined as the negative z-axis (-z). Two opposite gradient magnetic field configurations were considered: one with decreasing magnetic field strength along -z (magnetic field lines diverging) and one with increasing magnetic field strength along -z (magnetic field lines converging). Experiments were conducted under the conditions of a 27.12 MHz, 300 W ICP power, a 2 MHz, 0-100 W bias power, and 0.67 Pa argon pressure. A Langmuir probe was employed to measure the electron energy probability functions in the magnetized biased ICP, and the radial distribution of electron density was analyzed. The results showed that the application of bias power did not weaken the ability of the gradient magnetic field to control the radial electron density distribution. To further evaluate its applicability in material processing, photoresist (PR) etching was performed in oxygen plasma at a bias power of 100 W and a pressure of 0.67 Pa. A floating harmonic probe was used to measure the radial ion flux distribution. The results indicated that the magnetic field with converging field lines produced a center-high, edge-low etching rate distribution, while the one with diverging magnetic field lines led to a center-low, edge-high etching rate distribution. This is because the magnetic field confines the radial diffusion of electrons, causing them to diffuse along the field lines, thereby altering the plasma spatial distribution. In summary, applying an appropriate gradient magnetic field in the presence of bias power can effectively control the PR etching distribution while maintaining a high etching rate, providing a promising approach for material processing.