Introduction to Low-Temp. SCM/AFM in High Magnetic Fields By Xi Yu

Abstract  Introduction  Introduction to SPMs  Our 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 Capacitance- Voltage).

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 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  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  Another early SCM was that of Bugg and King built in  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 semi- conductor.

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 semi- conductor.

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=  A/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 f 0 as it interacts with the sample. Signal’s proportional to the change in f 0 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 f 0 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  The 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

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 = mV

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  The 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)

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 wires grown by cleaved edge overgrowth.

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 …