TERM PAPER

PHYSICS (PHY102)

Topic: CONSTRUCTION,WORKING N USES OF SCANNING

ELECTRON MICROSCOPE AND TUNNELING MICROSCOPE N ADVANTAGES OF THESE MICROSCOPE OVER CONVENTIONAL MICROSCOPE

DOA:

DOR:

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Submitted to: Submitted by:

Ms. Pragya hejib Mr. Amandeep Singh Khera

Deptt. Of Physics Roll. No. RK6005A19

Reg.No. 11000597

Class K6005

ACKNOWLEDGEMENT

It acknowledges all the contributors involved in the preparation of this project. Including me, there is a hand of my teachers, some books and internet. I express most gratitude to my subject teacher, who guided me in the right direction. The guidelines provided by her helped me a lot in completing the assignment.

The books and websites I consulted helped me to describe each and every point mentioned in this project. Help of original creativity and illustration had taken and I have explained each and every aspect of the project precisely.

At last it acknowledges all the members who are involved in the preparation of this project.

Thanks AMANDEEP SINGH

ABSTRACT

After going through this paper one will encounter about the electron

microscope and scanning electron microscope and tunneling electron

microscope. One will come to know about its construction,working

and uses of these both microscope.One will encounter its advantages

over other microscopes. One will come to know about the history of

scanning electron microscope.and Difference between this

microscopes.

TABLE OF CONTENT

1. INTRODUCTION TO SEM2. HISTORY OF SEM3. CONSTRUCTION AND WORKING OF SEM4. USES OF SEM5. INTRODUCTION TO STM6. CONSTRUCTION AND WORKING OF STM7. USES OF STM8. ADVANTAGES OVER CONVENTIONAL ONE9. BIBLOGRAPHY

INTRODUCTION TO SEM

The scanning electron microscope (SEM) is a type of electron microscope that

images the sample surface by scanning it with a high-energy beam of electrons in

a raster scan pattern. The electrons interact with the atoms that make up the

sample producing signals that contain information about the sample's

surface topography, composition and other properties such as electrical

conductivity.

The types of signals produced by an SEM include secondary electrons, back-

scattered electrons (BSE), characteristic X-rays, light (cathodoluminescence),

specimen current and transmitted electrons. Secondary electron detectors are

common in all SEMs, but it is rare that a single machine would have detectors for

all possible signals. The signals result from interactions of the electron beam with

atoms at or near the surface of the sample. In the most common or standard

detection mode, secondary electron imaging or SEI, the SEM can produce very

high-resolution images of a sample surface, revealing details about less than 1 to

5 nm in size. Due to the very narrow electron beam, SEM micrographs have a

large depth of field yielding a characteristic three-dimensional appearance useful

for understanding the surface structure of a sample. This is exemplified by the

micrograph of pollen shown to the right. A wide range of magnifications is

possible, from about 10 times (about equivalent to that of a powerful hand-lens)

to more than 500,000 times, about 250 times the magnification limit of the

best light microscopes. Back-scattered electrons (BSE) are beam electrons that

are reflected from the sample by elastic scattering. BSE are often used in

analytical SEM along with the spectra made from the characteristic X-rays.

Because the intensity of the BSE signal is strongly related to the atomic number

(Z) of the specimen, BSE images can provide information about the distribution of

different elements in the sample. For the same reason, BSE imaging can

image colloidal gold immuno-labels of 5 or 10 nm diameter which would

otherwise be difficult or impossible to detect in secondary electron images in

biological specimens. Characteristic X-rays are emitted when the electron beam

removes an inner shell electron from the sample, causing a higher energy

electron to fill the shell and release energy. These characteristic X-rays are used

to identify the composition and measure the abundance of elements in the

sample.

HISTORY OF SEM

In 1931, the German physicist Ernst Ruska and German electrical engineer Max

Knoll constructed the prototype electron microscope, capable of four-hundred-

power magnification; the apparatus was a practical application of the principles of

electron microscopy. Two years later, in 1933, Ruska built an electron

microscope that exceeded the resolution attainable with an optical (lens)

microscope. Moreover, Reinhold Rudenberg, the scientific director of Siemens-

Schuckertwerke, obtained the patent for the electron microscope in May 1931.

Family illness compelled the electrical engineer to devise an electrostatic

microscope, because he wanted to make visible the poliomyelitis virus.

In 1937, the Siemens company financed the development work of Ernst Ruska

and Bodo von Borries, and employed Helmut Ruska (Ernst’s brother) to develop

applications for the microscope, especially with biologic specimens. Also in

1937, Manfred von Ardenne pioneered the scanning electron microscope. The

first practical electron microscope was constructed in 1938, at the University of

Toronto, by Eli Franklin Burton and students Cecil Hall,James Hillier, and Albert

Prebus; and Siemens produced the first commercial Transmission Electron

Microscope (TEM) in 1939. Although contemporary electron microscopes are

capable of two million-power magnification, as scientific instruments, they remain

based upon Ruska’s prototype.

The first SEM image was obtained by Max Knoll, who in 1935 obtained an image of silicon steel showing electron channeling contrast. Further pioneering work on the physical principles of the SEM and beam specimen interactions was performed by Manfred von Ardenne in 1937, who produced a British patent but never made a practical instrument. The SEM was further developed by Professor Sir Charles Oatley and his postgraduate student Gary Stewart and was first marketed in 1965 by the Cambridge Instrument Company as the “Stereoscan”. The first instrument was delivered to DuPont.

CONSTRUCTION AND WORKING OF SEM

When the primary electron beam interacts with the sample, the electrons lose energy by repeated random scattering and absorption within a teardrop-shaped volume of the specimen known as the interaction volume, which extends from less than 100 nm to around 5 µm into the surface. The size of the interaction volume depends on the electron’s landing energy, the atomic number of the specimen and the specimen’s density. The energy exchange between the electron beam and the sample results in the reflection of high-energy electrons by elastic scattering, emission of secondary electrons by inelastic scattering and the emission of electromagnetic radiation, each of which can be detected by specialized detectors. The beam current absorbed by the specimen can also be detected and used to create images of the distribution of specimen current. Electronic amplifiers of various types are used to amplify the signals which are displayed as variations in brightness on a cathode ray tube. The raster scanning of the CRT display is syncUhronised with that of the beam on the

In a typical SEM, an electron beam is thermionically emitted

from an electron gun fitted with a tungsten filament cathode.

Tungsten is normally used in thermionic electron guns

because it has the highest melting point and lowest vapour

pressure of all metals, thereby allowing it to be heated for

electron emission, and because of its low cost. Other types

of electron emitters include lanthanum hexaboride (LaB6)

cathodes, which can be used in a standard tungsten

filament SEM if the vacuum system is upgraded and field

emission guns (FEG), which may be of the cold-

cathode type using tungsten single crystal emitters or the

thermally-assisted Schottky type, using emitters

of zirconium oxide.

The electron beam, which typically has an energy ranging

from 0.5 keV to 40 keV, is focused by one or two condenser

lenses to a spot about 0.4 nm to 5 nm in diameter. The

beam passes through pairs of scanning coils or pairs of

deflector plates in the electron column, typically in the final

lens, which deflect the beam in the x and y axes so that it

scans in a raster fashion over a rectangular area of the

sample surface.

specimen in the microscope, and the resulting image is therefore a distribution map of the intensity of the signal being emitted from the scanned area of the specimen. The image may be captured by photography from a high resolution cathode ray tube, but in modern machines is digitally captured and displayed on a computer monitor and saved to a computer’s hard disk.

USES OF SEM

Medical Uses

A medical researcher can use a SEM to compare samples of healthy and unhealthy blood and tissue samples to determine what is causing an illness in a patient. They can also study the effect of medicine on a patient by observing the differences between the unhealthy patients and those patients given medicine.

Forensics

Police laboratories need SEM’s to examine evidence. A SEM can be used to compare such things as metal fragments, paint, and inks. Hair and fibers can also be examined to prove a person’s guilt or innocence. By comparing a sample found at the scene of a crime to a similar sample found belonging to a suspected criminal, detectives can closely determine if the samples are a match.

MetalsMetal samples can be examined with the SEM to determine strength in different conditions such as cold and heat. Experts study the metal from the frame of a crashed airplane to determine if there was a flaw in the metal that caused the crash. Studies are also done on the metal before it is used in a plane, car, train, boat or anything that would require a strong metal for reasons of safety. By studing samples from the hull of the Titanic, scientists were able to discover that the metal had shattered because the cold water had caused it to become brittle.

Scientific ResearchThere are many different areas in scientific research where the SEM can be used. Any scientist that needs to look at extremely small samples canuse the SEM. For example, Biologists use the SEM to look at plant or animal cells and tissues, Chemists examine microscopic crystals, and Material Scientists view the structure of metals, ceramics, and plastics. These are only a few of the ways a SEM can be used for scientific research.

INTRODUCTION TO STM

A scanning tunneling microscope (STM) is a powerful instrument for imaging

surfaces at the atomic level. Its development in 1981 earned its inventors,Gerd

Binnig and Heinrich Rohrer (at IBM Zürich), the Nobel Prize in Physics in 1986.[1]

[2] For an STM, good resolution is considered to be 0.1 nm lateral resolution and

0.01 nm depth resolution.[3] With this resolution, individual atoms within materials are

routinely imaged and manipulated. The STM can be used not only in ultra high

vacuum but also in air, water, and various other liquid or gas ambients, and at

temperatures ranging from near zero kelvin to a few hundred degrees Celsius.[4]

The STM is based on the concept of quantum tunneling. When a conducting tip is

brought very near to the surface to be examined, a bias (voltage difference) applied

between the two can allow electrons to tunnel through the vacuum between them.

The resulting tunneling current is a function of tip position, applied voltage, and

the local density of states (LDOS) of the sample.[4] Information is acquired by

monitoring the current as the tip's position scans across the surface, and is usually

displayed in image form. STM can be a challenging technique, as it requires

extremely clean and stable surfaces, sharp tips, excellentvibration control, and

sophisticated electronics.

CONSTRUCTION AND WORKING OF STM

If the tip is moved across the sample in the x-y plane, the changes in surface

height and density of states cause changes in current. These changes are

mapped in images. This change in current with respect to position can be

measured itself, or the height, z, of the tip corresponding to a constant current

can be measured. These two modes are called constant height mode and

constant current mode, respectively. In constant current mode, feedback

electronics adjust the height by a voltage to the piezoelectric height control

mechanism. This leads to a height variation and thus the image comes from the

The components of an STM include scanning tip, piezoelectric controlled height and x,y scanner, coarse sample-to-tip control, vibration isolation system, and computer.

First, a voltage bias is applied and the tip is brought

close to the sample by some coarse sample-to-tip

control, which is turned off when the tip and sample

are sufficiently close. At close range, fine control of

the tip in all three dimensions when near the

sample is typically piezoelectric, maintaining tip-

sample separation W typically in the 4-7 Å range,

which is the equilibrium position between attractive

(3<W<10Å) and repulsive (W<3Å) interactions[4]. In

this situation, the voltage bias will cause electrons

to tunnel between the tip and sample, creating a

current that can be measured. Once tunneling is

established, the tip's bias and position with respect

to the sample can be varied (with the details of this

variation depending on the experiment) and data is

obtained from the resulting changes in current.

tip topography across the sample and gives a constant charge density surface;

this means contrast on the image is due to variations in charge density. In

constant height mode, the voltage and height are both held constant while the

current changes to keep the voltage from changing; this leads to an image made

of current changes over the surface, which can be related to charge density The

benefit to using a constant height mode is that it is faster, as the piezoelectric

movements require more time to register the change in constant current mode

than the voltage response in constant height mode. All images produced by STM

are grayscale, with color optionally added in post-processing in order to visually

emphasize important features.

In addition to scanning across the sample, information on the electronic structure

at a given location in the sample can be obtained by sweeping voltage and

measuring current at a specific location. This type of measurement is

called scanning tunneling spectroscopy (STS) and typically results in a plot of the

local density of states as a function of energy within the sample. The advantage

of STM over other measurements of the density of states lies in its ability to make

extremely local measurements: for example, the density of states at

an impurity site can be compared to the density of states far from impurities

Framerates of at least 1 Hz enable so called Video-STM (up to 50 Hz is

possible). This can be used to scan surface diffusion

USES OF STM

1. Surface Science

The invention of STM have left a great impact on surface science. Uses of STM to study metals and semiconductors surface can provide non-trivial real space information. especially in studying semiconductor such as Si(100) surface, which is the technologically important Si substrate material for microelectronics device fibrication. The STM image of Si(100) surface shown below gives direct confirmation of dimers formation during surface reconstruction, although it has been previously suggested by theoretical calculation and other experimental observations.

Here are the bias-dependent STM images, the top left image, with positive bias to the tip, the right one, with negative bias to the tip. The images directly reflect the spatial distribution of the occupied -bonding state is between the dimer atoms, while the unoccupied -antibonding states localized away from the dimer. The bottom pictures depict the model of the reconstruction of the bulk-terminated (1x 1) lattices into a (2 x 1) reconstruction via dimerization, the large dots represent atoms on top layer, while small dots are atoms of second layer.

2. Metrological Applications

The microtopography and nanotopography of a surface is crucial in many applications, such as for high precision optical components and disk drive surface roughness of machined or ground surfaces in area where such a finish is crucial.

This is the STM image of and individual turn mark on a diamond-turned Al substrate to be used for subsequent magnetic film deposition for a high capacity hard disc drive. The image obtained by scanning electron microscope (SEM) is shown to the right for comparison. The high spatial resolution of STM provides an important complement to the SEM.

3. Manipulation of Atoms

ne innovative applications of STM recently found is manipulation of atoms. Here is an example, Iron atoms are placed on Cu(111) surface at very low temperature (4K), Iron atoms are first physisorbed on the Su surface, then the tip is placed directly over a physisorbed atom and lowered to increase the attractive force by increasing the tunneling current, the atom was dragged by the tip and moves across the surface to a desired position. Then, the tip was withdrawn by lowering the tunneling current.

These STM images show the steps of "quantum corral" formation

ADVANTAGES

The biggest advantage is that they have a higher resolution and are therefore also able of a higher magnification (up to 2 million times). Light microscopes can show a useful magnification only up to 1000-2000 times. This is a physical limit imposed by the wavelength of the light. Electron microscopes therefore allow for the visualization of structures that would normally be not visible by optical microscopy.

Depending on the type of electron microscope, it is possible to view the three dimensional external shape of an object (Scanning Electron Microscope)

In scanning electron microscopy (SEM), due to the nature of electrons, electron microscopes have a greater depth of field compared to light microscopes. The higher resolution may also give the human eye the subjective impression of a higher depth of field.

BIBLOGRAPHY

1. en.wikipedia.org/wiki/Scanning_electron_microscope2. en.wikipedia.org/wiki/STM3. www.mos.org/sln/sem/ 4. www.purdue.edu/rem/rs/sem.htm5. en.wikipedia.org/wiki/Scanning_tunneling_microscope6. nobelprize.org/educational/physics/microscopes/scanning/

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