Charging of surfaces is critical in applications involving plasmas because it affects both the characteristics and the properties of the plasma. Unfortunately, it can be difficult to measure charging in many situations due to geometry, materials involved, or short timescales.
One such case is Plasma-Assisted Cleaning by Electrostatics (PACE), which is a pulsed plasma technique aimed at removing nanometer scale contamination from surfaces used in semiconductor manufacturing. Currently, a pulsed DC substrate bias is used to draw electrons in from the plasma on the postivie pulse, and to repel the now negatively charged contamination from the surface on the negative pulse. Current results show greater than 90% particle removal efficiency (PRE) of 30nm - 220 nm polystyrene latex nanoparticles from ruthenium capped Si/quartz, used to simulate an EUV photomask. A real-time surface charging diagnostic is required to monitor the amount of ion/electron flux reaching the surface of the substrate in order to optimize the pulsed DC bias for efficiency, as well as to ensure that surfaces do not become over-charged.
To address this issue, we are developing micrometer scale in-situ monitors, fabricated using semiconductor manufacturing techniques, that are able to measure charging in real-time and at the critical locations. These devices are currently being used to study sidewall charging inside features during plasma etching applications.
The devices, constructed on a silicon wafer, consist of a base layer of titanium with alternating layers of silicon dioxide and titanium. During the construction of the device, vias (holes) are integrated into the layout, extending all the way from the top surface to the substrate. The silica layers act as insulators to create discrete measurement layers, provided by the titanium layers. The titanium layers 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 on charging, several aspect ratios were used by varying the diameter of the vias. Diameters vary from less than 50 nm to 1 micron with aspect ratios ranging from 1:1 to over 15:1. |