Speaker
Niladri Sarkar
Title
Single-Chip Scanning Probe Microscopes
Abstract
Scanning probe microscopes (SPMs) are the highest resolution imaging instruments available today and are among the most important tools in nanoscience. Conventional SPMs suffer from several drawbacks owing to their large and bulky construction and to the use of piezoelectric materials. Large scanners have low resonant frequencies that limit their achievable imaging bandwidth and render them susceptible to disturbance from ambient vibrations. In addition, the long mechanical path from the tip to the sample contributes to thermal drift. Furthermore, intrinsic properties of piezoelectric materials result in creep and hysteresis, which contribute to image distortion. The tip-sample interaction signals are often measured with optical configurations that require large free-space paths, are cumbersome to align, and add to the high cost of state-of-the-art SPM systems. These shortcomings have stifled the widespread adoption of SPMs by the nanometrology community. Array approaches have been used to alleviate the bandwidth bottleneck; however as arrays are scaled upwards, the scanning speed must decline to accommodate larger payloads. Tiny, inexpensive, fast, stable and independent SPMs that do not incur bandwidth penalties upon array scaling would therefore be most welcome.
The present research demonstrates, for the first time, that all of the mechanical and electrical components that are required for the SPM to capture an image can be scaled and integrated onto a single CMOS chip. Principles of microsystem design are applied to produce single-chip instruments that acquire images of underlying samples on their own, without the need for off-chip scanners or sensors. Furthermore, it is shown that the instruments enjoy a multitude of performance benefits that stem from CMOS-MEMS integration and volumetric scaling of scanners by a factor of 1 million.
This dissertation details the design, fabrication and imaging results of the first single-chip contact-mode AFMs, with integrated piezoresistive strain-sensing cantilevers and scanning in three degrees-offreedom (DOFs). Static AFMs and quasi-static AFMs are both reported.
This work also includes the development, fabrication and imaging results of the first single-chip dynamic AFMs, with integrated flexural resonant cantilevers and 3 DOF scanning. Single-chip Amplitude Modulation AFMs (AM-AFMs) and Frequency Modulation AFMs (FM-AFMs) are both shown to be capable of imaging samples without the need for any off-chip sensors or actuators.
A method to increase the quality factor (Q-factor) of flexural resonators is introduced. The method relies on an internal energy pumping feedback mechanism that is based on the interplay between electrical, mechanical, and thermal effects. To the best of the author's knowledge, the present devices that are designed to harness these effects possess the highest effective Q-factors reported for flexural resonators operating in air; Q is enhanced from an intrinsic value of ~50 to an effective value of ~50,000 in one exemplary device. A physical explanation for the underlying mechanism is proposed.
The
design,
fabrication,
imaging,
and
tip-based
lithographic
patterning
with
the
first
single-chip
Scanning
Thermal
Microscopes
(SThMs)
are
also
presented.
In
addition
to
3
DOF
scanning,
these
devices
possess
integrated,
thermally
isolated
temperature
sensors
to
detect
heat
transfer
in
the
tip-sample
region.
Imaging
is
reported
with
thermocouple-based
devices
and
patterning
is
reported
with
resistive
heater/sensors.
An
"isothermal
electrothermal
scanner"
is
designed
and
fabricated,
and
a
method
to
operate
it
is
detailed.
The
mechanism,
based
on
electrothermal
actuation,
maintains
a
constant
temperature
in
a
central
location
while
positioning
a
payload
over
a
range
of
>35μm,
thereby
suppressing
the
deleterious
thermal
crosstalk
effects
that
have
thus
far
plagued
thermally
actuated
devices
with
integrated
sensors.
In
the
thesis,
models
are
developed
to
guide
the
design
of
single-chip
SPMs
and
to
provide
an
interpretation
of
experimental
results.
The
modelling
efforts
include
lumped
element
model
development
for
each
component
of
single-chip
SPMs
in
the
electrical,
thermal
and
mechanical
domains.
In
addition,
noise
models
are
developed
for
various
components
of
the
instruments,
including
temperature-based
position
sensors,
piezoresistive
cantilevers,
and
digitally
controlled
positioning
devices.