Paper PS2-WeA2
In-situ Diagnostic to Measure Charging During Plasma Etching

Wednesday, October 22, 2008, 2:00 pm, Room 306

In plasma etching processes, especially those with high aspect ratios, it is known that defects can occur such as trenching, bowing, and twisting. These defects are particularly noteworthy in the manufacture of DRAM deep-trench capacitors. In order to investigate the role of charging on these phenomena an in-situ diagnostic was fabricated using photolithographic and deposition techniques. The device consists of alternating layers of conducting and insulating materials. During the construction of the device, vias are integrated into the layout, extending all the way from the top surface to the substrate. The insulating layers create discrete measurement layers, provided by the conducting layers (electrodes). The electrodes are attached to voltage measurement leads and can then be used to measure the build up of sidewall charging at different heights along the via when exposed to a plasma. To determine the effect of geometry, if any, on charging, several aspect ratios were used by maintaining the same device thickness but varying the diameter of the vias. The entire stack is less than one micron thick, with vias ranging in diameter from 1 micron to 100 nanometers, thereby producing aspect ratios of 1:1 to 10:1. In addition, a macroscopic parallel to the diagnostic was constructed in order to compare how overall size of the features affects the charging properties. The macroscopic device is on the order of 1cm thick with features on the order of 1mm in diameter. The transition from macroscopic to microscopic gives a better understanding of the transport and charging phenomena involved in constructing DRAM features and at what scale they are significant. Conducting layers for the macroscopic device are metal and the insulating layers are ceramic. Plasma and charging experiments were conducted in a commercial silicon dioxide etch chamber. Typical ion fluxes measured on the order of 10¹⁷ ions/cm²*sec. At 500W of 2.0 and 2.2 MHz power on each of the coils (1000W total), a plasma density of 2*10¹² +/- 5*10¹¹ cm^-3 and electron temperature of 3 +/- 0.3eV was measured at 0.3+/-0.2 mm about the substrate at 4.5+/-1 cm from the edge of a 20cm chuck. RF variations in the signal were observed at the two driving frequencies and at the beating frequency of 150kHz, as expected. Results from the diagnostics will be shown for various plasma conditions and compositions.