Consequences of low bias frequencies in inductively coupled plasmas on ion angular distributions for high aspect ratio plasma etching

Plasma etching of high aspect ratio (HAR) features for microelectronics fabrication is becoming increasingly challenging due to the increasing aspect ratio and tighter tolerances on the required anisotropy of the angular distribution of ions onto the wafer. These HAR features include deep trench isolation which after etching are filled with dielectric material to electrically isolate devices. A strategy to increase ion energy and narrow angular distributions onto the wafer is to operate with substrate biases at low frequencies, typically below a few MHz to several hundred kHz. Maintaining desired properties of the ion energy and angular distributions (IEADs) to the edge of the wafer is also becoming more challenging, leading to edge exclusion—a portion of the wafer at its outer edge that does not yield high quality devices. Deviation of IEADs from the ideal at the edge of the wafer is usually due to a tilt from the normal. The tilt is generally associated with curvature of the sheath that accelerates ions into the wafer, transitioning from the edge of the wafer to the focus ring, a dielectric that surrounds the wafer. In this paper, we report on a computational investigation of IEADs incident on the wafer as a function of radius in an inductively coupled plasma (ICP) sustained in Ar/Cl₂/O₂ mixtures with substrate biases from 250 kHz to 5 MHz. Curvature of the sheath at the wafer edge leading to a tilt of the IEAD results, to first order, from charging of the focus ring that thins the sheath above the focus ring relative to that over the wafer. This charging is frequency dependent, with more charging and sheath curvature occurring at lower bias frequencies. The consequences on sheath curvature and tilt of the IEAD due to bias voltage, ICP power, and electrical properties of the focus ring are discussed. Limiting thinning of the sheath and reducing charging of the focus ring generally reduce sheath curvature and improve anisotropy of the IEADs in the edge region of the wafer.