Candidate: Farheen Nasir
Date: October 21, 2025
Time: 11:00am
Location: Online
Supervisor: Dr. Guo-Xing Miao
All are welcome!
Abstract:
Atomic Force Microscope (AFM) is in a family of microscopy called Scanning Probe Microscope (SPM) which also includes Scanning Tunneling Microscope (STM). AFM has become a standard characterization tool for surface topography measurements in most microfabrication. However, in order to obtain accurate representation of high aspect ratio structures such as trenches, the AFM tip must be capable of reaching the bottom of such a trench. This requires the AFM tip to have a high aspect ratio as well as sharp apex. Additionally, for application which require contact with the surface the AFM tip must have very high wear resistance, which is typically obtained using diamond AFM tips . In this work we have designed several methods to obtain high aspect ratio diamond and silicon AFM tips with sharp apex.
Chapter 3 and 4 discuss two methods to obtain high aspect ratio diamond tip with a silicon base. We have used ultra-nanocrystalline diamond grown on silicon wafer and have fabricated diamond AFM tips using Reactive Ion Etching. In the first method, we use O2/CHF3 gas to etch patterned diamond with shrinking mask. Next, we use pseudo-Bosch etching to fabricate high aspect ratio silicon pillars under the diamond tip. With this method we are able to obtain aspect ratio close to 10 and tip apex of less than 25 nm. In the second method, we are able to obtain an even smaller apex by fabricating the silicon base first before the fabrication of diamond apex using O2/C4F8 plasma. This method allows for an aspect ratio of 7.5 and apex diameter close to 10 nm.
Chapter 5 and 6 focus on the fabrication of silicon AFM tips. In chapter 5 we use (111) oriented silicon wafers, which are first dry etched to form pillars. Next, the sample is etched in alkaline solution using either TMAH or surfactant added TMAH. We have tested two non-ionic surfactants and have obtained aspect ratio close to 14 and tip apex of less than 10 nm without oxidation sharpening when 1000 ppm Triton X-405 is added to 25 wt% TMAH. We have also studied the effect of surfactant composition and concentration on silicon etching.
In Chapter 6, we have developed two methods to fabricate tetrahedral AFM tips using a combination of dry and wet anisotropic etching. The first method uses SOI wafer coated with LPCVD nitride. After patterning the silicon is dry etched to form trenches which are then oxidized. Next, the LPCVD nitride is removed to exposed un-oxidized silicon that is anisotropically etched in 25 wt% TMAH to form AFM tips with one side perpendicular to the edge of the cantilever surface. In the second method we remove the need of LPCVD nitride which may not be readily available. In this method, after fabrication of trenches, the entire silicon surface is oxidized along with trench sidewalls. However, before anisotropic wet etching, the surface oxide is selectively removed using angled ion milling. Careful recipe optimization is done in order to fully remove the surface oxide while the oxide on the trench sidewalls remain unharmed. After wet anisotropic etching, this method allows for fabrication of silicon tips with apex as low as 60 nm before oxidation sharpening.