Download Low Temperature SCM/AFM

Introduction to Low-Temp.
SCM/AFM in High Magnetic
Fields
By Xi Yu
Abstract
 Introduction
 Introduction
 Our
to SPMs
System
 AFM/SCM Probes
 Possible Uses of The SCM/AFM
Introduction
Standard methods for charactering
semiconductors
SEM (Scanning Electron Microscopy)
TEM (Transmission Electron Microscopy)
SIMS (Secondary Ion Mass Spectroscopy)
SRP Spreading Sheet Resistance Profiling)
1D C-V (one-dimensional CapacitanceVoltage).
Introduction
With the advent of smaller device geometry and
high reliability requirements, new characterisation
tools are needed. As alternative tools, various types
of scanning probe microscopy (SPM) have been
applied to not only characterising semiconductor
devices but also monitoring semiconductor device
processes.
Introduction
We are currently building a scanning capacitance
microscope (SCM) that will operate at temperatures
between 300K and 1.5K in magnetic fields as high as
12T. The intrinsic resolution of the instrument will be
around 20nm with an ability to measure capacitance
variations as small as 10-19 F. To correlate the
capacitance measurements with the topological
features of the material under study, we are also
building an Atomic Force Microscope (AFM) to image
surface topography on the same scanning head so that
results from the two techniques can be compared.
Introduction to SPMs
 Brief
History
 SPMs’ Pictures
 Basic Operation Principles of STM
 Basic Operation Principles of AFM
 Basic Operation Principles of SCM
Brief History
STM was invented by Gerd Binning and Heinrich
Rohrer at IBM Zurich in 1981, they won Nobel Prize in
1986.
 SCM was developed from a capacitive videodisc
reader which was developed by RCA in 1982,
 Matey and Blanc showed the first demonstration of
the SCM concept in 1985.
 Another early SCM was that of Bugg and King built
in 1988.
 AFM was developed by Binnig, Quate and Gerber in
1986 using STM technology.

SPMs’ Pictures
 STM
(scanning tunnelling microscope) Pictures
 AFM (atomic-force microscope) Pictures
 MFM (magnectic-force microscope) Pictures
 SCM (scanning capacitance microscope) Pictures
Basic Operation Principles of STM
First of all, the STM is the first and most
important one to introduce since it is the
ancestor/ancestrees of all the SPMs.
In general, SPMs contain the components
illustrated in the Figure 1. The differences
between them are the different probes and
different probe-sample interactions under
investigation.
Basic Operation Principles of STM
STMs use a sharpened, conducting tip with a bias
voltage applied between the tip and the sample. When
the tip is brought to within about 1nm of the sample,
electrons from the sample begin to ”tunnel” through the
1nm gap into the tip or vice versa, depending upon the
sign of the bias voltage. (See Figure 2) The resulting
tunnelling current varies with tip-to-sample gap, and it
is the signal used to create an STM image. For
tunnelling to take place, both the sample and the tip
must be conductors or semiconductors. Unlike AFMs,
STMs cannot image insulating materials.
Basic Operation Principles of STM
 Two
Modes of Operation(Figure 3)
 Constant
Height Mode
It gives higher resolution and can do faster
scanning, but it can only be applied on very flat
surface.
 Constant Current Mode
It yields quantitative information of sample
topography as a feedback loop is used to make
the tip follow changes in surface height.
Basic Operation Principles of AFM
The atomic force microscope (AFM) probes the surface of a
sample with a sharp tip, a couple of microns long and often less
than 10 nm in diameter. In conven-tional AFM, the tip is located
at the free end of a cantilever that is 100 to 500µm long. Forces
between the tip and the sample surface cause the cantilever to
bend, or deflect (shown in Figure 3). A detector measures the
cantilever detection as the tip is scanned over the sample, or the
sample is scanned under the tip. The measured cantilever de
ections allow a computer to generate a map of surface
topography. AFMs can be used to study insulators and
semiconductors as well as electrical conductors.
Basic Operation Principles of AFM
Several forces typically contribute to the detection of
an AFM cantilever (Van der Waal’s, Capillary, Coulomb
repulsion, Ionic repulsion, etc.). A typical force-distance
curve is shown in Figure 5.
Basic Operation Principles of SCM
The SCM consists of a conductive probe tip and a
highly sensitive capacitance sensor in addition to
normal AFM components. The tip in contact with an
oxidised semiconductor sample forms a MOS (Metal
Oxide Semiconductor) capacitor. The MOS capacitor
has two capacitors in series: one from the insulating
oxide layer and the other from the active depletion
layer near the oxide/silicon interface. Figure 6 depicts
the MOS capacitor formed by the tip and the semiconductor.
Basic Operation Principles of SCM
The SCM consists of a conductive probe tip and a
highly sensitive capacitance sensor in addition to
normal AFM components. The tip in contact with an
oxidised semiconductor sample forms a MOS (Metal
Oxide Semiconductor) capacitor. The MOS capacitor
has two capacitors in series: one from the insulating
oxide layer and the other from the active depletion
layer near the oxide/silicon interface. Figure 6 depicts
the MOS capacitor formed by the tip and the semiconductor.
Basic Operation Principles of SCM
The total capacitance is determined by the oxide
thickness and the thickness of depletion layer which, in
turn, depends on the carrier concentration in the silicon
substrate and the applied DC voltage between the tip
and the semiconductor. The change of capacitance
due to alternating electric field is illustrated in Figure 7.
Basic Operation Principles of SCM
In SCM, changes in capacitance (rather than
absolute capacitance) are measured. The tip-sample
capacitance is only a tiny fraction of the overall
capacitance in the system. Therefore, it is easier to
modulate a bias voltage applied to the sample (C=Q/V)
or the tip-sample distance (C=eA/d) and detect
variations in the amplitude of the resulting capacitance
modulation at that frequency. (See Figure 8, Figure 9
for the full story)
Our System
 General
View
 The AFM System
 The SCM System
 Computer Controlled Lock-In Amplifier
 Vibration Control
General View
As shown in Figure 10, a quartz piezoelectric tuning
fork with a very sharp tungsten tip will act as the
sample probe, operating in tapping mode AFM and
non-contact mode SCM. The sample will be mounted
on a piezoelectric tube for scanning. Because the tip is
very close to the sample, we need to reduce the
vibration from outside by a specially designed vibration
control system.
The AFM System
As shown in Figure 11, the Oxford Instrument’s
TOPSystems3 will control the probe to approach and
scan the sample. The tuning fork and the sharp tip is
driven by the function generator tapping the sample at
the fork’s resonant frequency. A phase-locked-loop is
used to track changes in the fork’s resonant frequency
f0 as it interacts with the sample. Signal’s proportional
to the change in f0 and amplitude (A) are then sent to
TOPS3, which uses them either as image data or to
control the tip-sample height so as to keep f0 and/or
amplitude constant.
Why Do SCM and AFM
Simultaneously?
 SPM
is based on a local interaction between a
sharp tip and a sample. In the case of SCM, a
conducting tip is scanned across a sample
and small changes in the tip-sample
capacitance are measured. These changes
can be caused by 3 interactions:
Why Do SCM and AFM
Simultaneously?
The output changes by these 3 reasons:
 Topographic changes (surface height
variations).
 Variations in dielectric constant. (won’t change
much for semiconductor)
 Local variation in carrier density. (dC/dV)
Why Use a Tuning Fork?
Some commercially available AFMs presently
employ a diode laser whose beam is reflected
from the back of a micro-machined Si cantilever
to monitor the tip-sample interaction.
Unfortunately, the use of a diode laser in AFM
introduces several problems in our case.
Problems with the Laser
 Illumination
from the diode laser may damage
the sample.
 The diode laser introduces drift into the
measurement.
 Interaction of the diode-laser illumination and
the sample topography causes spurious
modifications to the measured tip-sample
interaction.
Why Use a Phase-locked Loop?
Due to the high mechanical Q of quartz tuning
forks, there is a delay of tens to hundreds of ms
before the tuning-fork oscillations reach their
steady-state condition. As a result, it’s not fast
enough for tracking a surface at a scan rate
faster than 0.1 Hz with a 100 ms lag in its
sensor.
The SCM System
 Block
Diagram (See Figure 12)
 RF Method (See Figure13)
Computer Controlled Lock-in
Amplifier
 Labview
data collector (See Figure 14)
 Delphi programed data analyser (See Figure 15)
Vibration Control
2 meters deep cement-pit (See Figure10,
Figure 16)
 Vibration control system (four legs)
 Rotary pump is in the next door
 Pump line goes through 2 sandbox
 Electrical leads are all clamped to the table
frame
 The
AFM/SCM Probes
 General
View (See Figure 17)
 Why Using Ultra-sharp Tips? (See Figure 18)
 Making an Ultra-sharp Tip
 Tuning Fork’s Specifications
 Attaching a Tip to the Tuning Fork
 Glued Tine
Making an Ultra-sharp Tip
 Schematic
diagram (See Figure 19)
 Chemical cell
 A normal
tip we made (See Figure 20)
Tuning Fork’s Specifications
 Resontant
frequency is about 32768Hz (see the
figure 21, 22)
 Q is about 60,000 in the can
 We drive the oscillation electrically by applying
an AC voltage of typically V = 0.01- 10mV
Attaching a Tip to the Tuning Fork
 For
use in our system, we remove the tuning
forks from their casing and glue a thin
tungsten wire (0.02mm dia.) to the end of one
tine (See Figure 22).
 The Q will drop to about 10000
Glued Tine
symmetry of the prongs is broken when
one of them is subject to a tip sample
interaction.
 Epoxy resin with fillers was used to attach one
tine to a metal base.
 Cancel the capacitance between the
electrodes (See Figure)
 The
Possible uses of The SCM/AFM
Quantum Hall Effect: Profiling of edge states,
breakdown of quantum Hall effect, mapping of
localised and extended states and how quantum Hall
states evolve with magnetic field.
 Capacitance spectroscopy of single InAs quantum
dots.
 Mapping carrier concentrations in GaN and relating
the surface topology to the dopant profiles.
 Capacitance measurements on one-dimensional
wires made by electron beam lithography or
fabricated by UHV-STM. This includes quantum

Possible Uses
 Two
dimensional dopant profiles in
semiconductor structures for the
characterisation of large-scale integration
processing techniques including the
temperature variation of the profiles.
 Imaging of active regions of low dimensional
structures - e.g. characterisation of side-wall
depletion in dry-etched structures.
 Lots more …