I received my B.Sc. degree in Electronics Engineering at University of Tabriz in 2017. I entered Sharif University of Technology in September 2017 for M.Sc. studies in the field of Micro and Nano-Electronic Devices, and I was working on implementing Scanning Probe Lithography (SPL) systems as my master thesis. I received my M.Sc. degree in January 2020.
One of the main challenges in implementation of
SPM/SPL systems is the fabrication of the tip due to the direct
affect of it on microscopy and lithography resolution. Here,
Tungsten STM tips are used as a probe to interact with the
surface.
Electrochemical etching is the most common method to fabricate
STM tips. In this method, a part of Tungsten wire is placed in
an Alkali solution and a counter electrode is placed in the
solution, shown in figure. By applying the bias voltage between
the tip and the counter electrode, the Tungsten dissolves in the
solution and a neck is produced at the interface of the liquid
and air, and the neck becomes thinner and thinner by further
etching. Finally, the wire breaks and the lower part breaks off.
The etching should be stopped at this time in order to prevent
the tip from being blunted by further etching, and this is done
by a controlling circuit.
SEM images of some fabricated tips are shown in the following
figures.
In order to control the aspect ratio of the fabricated tip, the wire was attached to the nanopositioner and
was pulled up while etching process. This gives us an
opportunity to control the physical shapes of the
fabricated tip. This should be kept in mind that if the aspect ratio of the
fabricated tip is high, the mechanical strength and robustness
of it is low, and an optimized physical shape should be
fabricated depending on the application. An SEM image of
fabricated tip using this method with three pulling up steps is
shown in the figure.
An SPL system based on STM was implemented. In this setup by
applying a bias voltage between the tithe tunneling current
between the conductor tip and sample is applied and the
tunneling current is measured using a high-accuracy
electrometer. A MATLAB code run in PC collects the current date
from the electrometer and controls the nanopositioner in order
to control the distance between the tip and the sample. A
photoresist layer is coated on a conductor substrate used as the
sample. In tunneling ranges, the tip is almost in contact with
the conductor layer; therefore, by moving the nanopositioner in
horizontal direction the desired pattern is transferred to the
photoresist layer. In this setup controlling the distance
between the tip and the sample in the lithography process was
closed-loop. The implemented setup's schematic and an SEM image of a
scratched line on the photoresist layer are shown in the following
figures.
Silicon tips are widely used as tips in SPM methods. One
common method to fabricate these tips is using KOH solution that
has anisotropic etch rates in different directions of Silicon
crystals. As shown in the following figure, a (100) wafer is
used as substrate, and after a photolithography process, the
wafer is placed in KOH solution and the tips are fabricated.
An optical setup was implemented in order to characterize the
nanopositioning stage. In this setup, a He-Ne laser is used, and
the light is reflected from a rotating mirror that has
mechanical contact with the nanopositioner. A small displacement
in the nanopositioner cause a bigger displacement in the sensor;
therefore, smaller displacement (in the order of 10 nm)
could be detected using this setup. In addition, another ray was
reflected to another sensor from a fixed mirror in order to
reduce the environmental effects. In this setup a quadrant
sensor is used and one side used to detect displacement of the
nanopositioner, and the other side to detect the ray reflected
from fixed mirror. By differentially amplifying the sensor's
signals, the displacement is detected. The schematic of the
setup are shown in the following figures.
Furthermore, a Michelson Interferometer setup was implemented to
characterize the nanopositioner. In the Michelson
Interferometer, the laser beam is divided into two rays using a
beam splitter, and these rays are then reflected from a fixed
and a movable mirror to the beam splitter. The beam splitter
combined these two rays, and an interferogram pattern is created
on the photodetector. The pattern is changed with the length of
the arm between beam splitter and the movable mirror. The
schematic of the setup and an interferogram pattern are shown in the
following figures.
