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Complementary Techniques for High Resolution Surface InvestigationSvetlana Santer

SEM and AFM:

Freiburger Materialforschungszentrum

Freiburg, 06.07.2004

Institut fr Mikrosystemtechnik

Svetlana Santer

AFM

SEM

Freiburg, 06.07.2004

Svetlana Santer

Scanning Electron Microscopy (SEM)Vacuum: 10-4-10-10 Torr

Freiburg, 06.07.2004

Svetlana Santer

Principles of SEM ImagingWhen the electron beam hits the sample, the interaction of the beam electrons from the filament and the sample atoms generates a variety of signals. secondary electrons (produced by interaction of primary e with the loosely held outer electrons of the sample), backscattered electrons (beam electrons from the filament that bounce off nuclei of atoms in the sample(elastic-interaction of the primary electrons with the nucleus of the atom), X-rays, light, heat, transmitted electrons (beam electrons that pass through the sample). Secondary electrons: high spatial resolution, good topographic sensitivity Backscattered electorns: they have more energy and can escape from greater depths, carry some informartion of sample compositionFreiburg, 06.07.2004 Svetlana Santer

Scanning Electron Microscopy (SEM)The SEM uses a beam of electrons to scan the surface of a sample to build a three-dimensional image of the specimen.Major Components of the Scanning Electron Microscope All scanning electron microscopes consist of: 1. 2. 3. A column which generates a beam of electrons. A specimen chamber where the electron beam interacts with the sample. Detectors to monitor the different signals that result from the electron beam/sample interaction. A viewing system that builds an image from the detector signal.

4.

Freiburg, 06.07.2004

Svetlana Santer

Generating the beam of electrons

The electron gun is housed on the top of the column and generates the beam of electrons that rushes towards the sample housed in the specimen chamber. Electrons are very small and easily deflected by gas molecules in the air. Therefore, to allow the electrons to reach the sample, the column is under a vacuum. The vacuum is maintained by two vacuum pumps: a rotary pump and an oil diffusion pump which is housed inside the SEM and is water cooled. Thus, the SEM needs a water cooling line which filters the water before it cools the oil diffusion pump.Freiburg, 06.07.2004 Svetlana Santer

Generating the beam of electrons

Within the electron gun is the filament which is the source of the beam of electrons. The filament is made of tungsten and is heated to generate a fine beam of electrons. As the filament gets used, it becomes brittle and coated. If the filament is overheated or too old, it will break.

Freiburg, 06.07.2004

Svetlana Santer

Detectors of the SEM

The SEM has several detectors to view the electron signals from the sample. (1) secondary electron detector looks like a Faraday cage, and detects secondary electrons. (2) backscattered electron detector (solid state detector) is located above the sample, consists of a diode with a thin gold conductor across the front surface. Backscattered electrons have sufficient energy to pass through the front surface and produce electron hole pairs which produce a curreent in the diodeFreiburg, 06.07.2004 Svetlana Santer

Can we see electrons directly by eye?

The SEM scans its electron beam line by line over the sample. It's much like using a flashlight in a dark room to scan the room from side to side. Gradually the image is built on a TV monitor (cathode ray tube or CRT for short). The SEM has buttons on the keyboard that control the scan speed. A fast scan which takes a couple of seconds to generate an image can be very grainy - like you're looking at an object in a snow storm. A slow scan is very clear and sharp - but takes a minute or two to get a picture.Freiburg, 06.07.2004 Svetlana Santer

Sample preparationSamples have to be prepared carefully to withstand the vacuum inside the microscope. Biological specimens are dried in a special way that prevents them from shriveling. Because the SEM illuminates them with electrons, they also have to be made to conduct electricity.

Sputter coater

Freiburg, 06.07.2004

Svetlana Santer

Examples

Freiburg, 06.07.2004

Svetlana Santer

History Max Knoll and Ernst Ruska -1931electron microscopy

Freiburg, 06.07.2004

Svetlana Santer

History1938 first SEM by von Ardenne 1942 first SEM for bulk samples by Zworkin 1965 first commercila instrument (Cambridge) Resolution: 50 nm in 1942 0.7 nm todayFreiburg, 06.07.2004 Svetlana Santer

Basics of AFMAFM provides very high resolution images of various sample propertiesPSD Laser

Cantilever Tip Sample50 nm

Piezo

Three basic components: Piezoelectric scanner Cantilever with a sharp tip

Digital Instruments (DI) MultiMode Nanoscope IIIa

Position sensitive detector (PSD) coupled with a feed-back systemFreiburg, 06.07.2004 Svetlana Santer

Historical steps of development

1981-invention of STM 1985-invention of AFM 1986-Nobel Price Christoph GerberIBMs Zurich Research Center in Rschlikon

It Vt

I t ( z ) = I 0 e 2 kz , k = 2m / h ~ 1 o 1

Restriction: conductive samplesFreiburg, 06.07.2004 Svetlana Santer

General components and their functions

Freiburg, 06.07.2004

Svetlana Santer

Control of the cantilever deflectionOptical Lever

STM Tunneling sensor (Binnig, Rohrer) tip

I Vt t

Optical interferometer detection system

Piezoresistive detection

special design of cantilever changing of resistivity with the applied stress

Freiburg, 06.07.2004

Svetlana Santer

Piezoelectric scannerSPM scanners are made from a piezoelectric material that expands and contracts proportionally to an applied voltage Whether they expand or contract depends upon the polarity of the applied voltage. Digital Instruments scanners have AC voltage ranges of +220 to 220V +V -V 0V

No applied voltage

Extended

Contracted

In some versions, the piezo tube moves the sample relative to the tip. In other models, the sample is stationary while the scanner moves the tipPZTCantilever

Solenoid

Freiburg, 06.07.2004

Svetlana Santer

Piezoelectric scanner Material PropertiesPiezoelectric ceramics are a class of materials that expand or contract when in the presence of a voltage gradient Lead (plumbum) zicronate titanate (PZT) crystallites exhibit tetragonal or rhombohedric structure Due to their permanent electrical and mechanical asymmetry, they exhibit spontaneous polarization and deformation

Perovskite-type PZT unit cell (1) in the symmetric cubic state, (2) distorted

Poling, an intense electric field (>2000V/mm) is applied

Freiburg, 06.07.2004

Svetlana Santer

Geometry of PZT scannerTube scanner The tripod

Not stable

The outer electrode is segmented in four equal sectors of 90 degrees The inner electrode is driven by the z signalModel Bipolar configuration A E J Scan Size 0.4 m 10 m 0.4 m 10 m Vertical Range 0.4 m 2.5 m 5 m Svetlana Santer

x = KV , K ~ 3nm / V

125 m 125 m Freiburg, 06.07.2004

Triangular patternFast scan speed

v f = 2lv Hzl v Hz N

Slow scan speed v s = Fast scan direction

Slow scan direction

Freiburg, 06.07.2004

Svetlana Santer

Feedback loop

Freiburg, 06.07.2004

Svetlana Santer

AFM Probe ConstructionLow spring constant (k - 10-2 to 102 N/m) Sharp protruding tip (r=5-50 nm) High resonance frequency

1 = 2

k m

Three common types of AFM tipnormal supertip ultralever

Freiburg, 06.07.2004

Svetlana Santer

A few requisites for cantilevers1. Must be soft

F = kz

Minimize k

For rectangular cantilevers Example: t=10 m, w=1mm, l=4mm k (C-C stretch.)~500N/m k~1N/m

k (C-C-H bend)~50 N/m

2. Must be insensitive to external vibrations

Maximize eigenfrequencies

=

k m

Minimize m L=140m, w=40m, t=1.5 m, k~0.7 N/m, ~60 kHzFreiburg, 06.07.2004 Svetlana Santer

Ex: Si or Si3N4

Common types of cantilevers Si3N4 Si

Diamond

Freiburg, 06.07.2004

Svetlana Santer

Fabrication of cantilevers

Freiburg, 06.07.2004

Svetlana Santer

Calibration of cantileverTheoretical method

E=300 GPa

Static method

Dynamic method

E=238 GPa

Measuring of thermal response of the cantilever Measuring of the change of resonance frequency caused by the addition of known masses

(Z

t'

Z t k s = Z t kcFreiburg, 06.07.2004 Svetlana Santer

)

Superposition of two geometries

Freiburg, 06.07.2004

Svetlana Santer

Reconvolution of the tip shape I II

r

d D=dreal D

D2 d= 4rFreiburg, 06.07.2004 Svetlana Santer

Deconvolution of the tip shapeTobacco Mosaic Virus (TMV) d~18 nm r-?

Freiburg, 06.07.2004

Svetlana Santer

AFM Tip Artifacts

We start off with an example of a good AFM image of 300 nm polystyrene spheres.....

Freiburg, 06.07.2004

Svetlana Santer

AFM Tip Artifacts

Similar spheres imaged with a supposedly sharp tip

Freiburg, 06.07.2004

Svetlana Santer

AFM Tip Artifacts

This image should only contain images of large polysterene spheres

Freiburg, 06.07.2004

Svetlana Santer

Blind ReconstructionAFM profile of a single bump

What does this single scan li

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