Physics 366 Lab Report 1

Michael Dobbs, Sonoma State University

October 15, 2014

AbstractA film was deposited via sputtering in a vacuum with an unknown

metal. A scanning electron microscope was used to observe the thinfilm, and energy dispersive X-ray spectroscopy was utilized to obtainthe identification of the metal while micromapping calculated the lo-cation on the sample.

1 Introduction

A vacuum environment in a chamber is created by pumping air molecules outof the system irreversibly using various machines. A thin film is then platedusing molecular beam epitaxy, where a metal is heated and emits a ray ofatoms that impinge onto the substrate. This sample is observed and imagedusing a scanning electron microscope, where a beam of electrons is shot atthe sample and the energy of the backscattered electrons is used to createa 3-dimensional image. The energy dispersive x-ray spectrometer is usedto identify the sample by collecting the characteristic x-rays emitted fromthe sample. The micromapping function of the EDX uses the differences inenergy of the detected electrons to calculate the position of each element onthe sample.

2 Experimental Procedure

2.1 Vacuums

Vacuums were used to lower the pressure inside the plating chamber by re-moving air molecules. By having a very low amount of gas atoms in the

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chamber to interact with the emitted metals, the plating of a substrate oc-curred in a more ideal and pure fashion.

Mechanical Pumps use motors to force air molecules through two checkvalves and into the atmosphere. This can lower the pressure inside the cham-ber to about 10−3 Torr. The rotary vane modules must be immersed in anoil bath to cool the pump and lubricate the vanes. [1]

Figure 1: A Mechanical Pump

Diffusion Pumps use hot evaporated oil to trap and compress gases intothe bottom of the pump where they are ejected. A multistage jet assemblyand internal boiler are used to propel the oil vapor downward to attain pres-sures of 10−5 Torr. [1] These pumps only operate at pressures less than 100mT so are activated after the mechanical pumps.

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Figure 2: A Diffusion Pump Schematic

Sorption pumps trap gases by freezing them onto aluminum fins andtrapping them in a porous molecular sieve. The pumps are immersed in a2-3 gallon liquid nitrogen bath to cool. [1] The apparatus must be operatedat low pressures or the fins will quickly be saturated with molecules.

Figure 3: A Sorption Pump Schematic

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Turbomolecular Pumps are similar to mechanical pumps but use highspeed rotating blades (about 90,000 rpm) to irreversibly direct gas moleculesout of the chamber and can attain pressures of 10−6 Torr.

Figure 4: A Turbomolecular Pump Schematic

Cryopumps are similar to diffusion pumps, but use a closed-loop heliumcryogenic refrigerator to condense and absorb the gases. Cryopumps are oilfree and can operator from any orientation with high pumping speeds. [1]

Figure 5: A Cryopump Schematic

There are multiple methods to measure the pressure inside a gas cham-

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ber. A thermocouple gauge reads pressures from 1-1000mTorr using an elec-tric current, induced by the movement of the surrounding gas, that passesthrough a filament. The filament heats up to a temperature that is depen-dent on the thermal conductivity of the gas, which is then converted into apressure reading.

Figure 6: A Thermocouple Gauge Schematic

A pirani gauge is similar to a TC gauge, but the temperature is calculatedby measuring the change in resistance of the filament as it is heated. [1]

Figure 7: A Pirani Gauge Schematic

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A hot filament ionization gauge must be used below 10−3 Torr. In thisgauge, electrons are emitted from a hot filament and ionize gas molecules inthe chamber. These positively charged gas molecules will produce a currentthrough the electrode that follows the equation:

IP = IE ∗ S ∗ P (1)

where IP is the positive ion current which depends on the gas species, IE

is the electron emission current, S is the gauge sensitive, and P is the gaspressure. [1]

Figure 8: A Filament Ionization Gauge Schematic

2.2 Deposition

Once the vacuum environment is attained, the sample may now be platedin a pure fashion. Since there aren’t a high number of air molecules in thechamber, the effect on the ejected atoms will be negligible. There are multiplemethods for plating a film.

The plating process used in our experiment was thermal evaporation,where the coating material is heated and evaporated, causing the metal atomsto coat the substrate. [2]

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Sputtering deposition is a plating process where the coating material isbombarded with electrons, and the dislodged atoms are accelerated by a highvoltage toward the surface of the substrate, thereby being deposited on thesurface to form a thin film.

Figure 9: Diagram of the Sputtering Mechanism

This process creates a uniform thickness of coating at a constant rate of1nm/s and operates under pressures of 1mTorr. [1]

Molecular beam epitaxy uses a ”beam” of molecules to slowly plate thesubstrate (less than .1nm/s). This process is useful for creating controlledsamples with a desired crystal structure. [2]

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Figure 10: Molecular Beam Epitaxy Schematic

Chemical vapor deposition uses chemical reactions to transform the gaseousmolecules surrounding the substrate into solid material that is then plated onthe surface of the sample. [2] This process can occur at atmospheric pressureor low pressures.

Figure 11: CVD Mechanism

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2.3 Scanning Electron Microscopy (SEM)

Once the metal is plated, an SEM is used to observe the surface of the thinfilm. A scanning electron microscope heats a metallic tungsten filament thatemits electrons to form an image.

Figure 12: Tungsten Electron Gun Schematic

Tungsten is used because it has a relatively low work function (4.5eV),so many electrons are emitted, following the function:

JC = A ∗ T 2 ∗ e−EW /k∗T (2)

where JC is the current density, k and A are constants, T is the temperature,and EW is the work function. The bias voltage needed is around -400V formaximum emission current.

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The SEM lenses are used to demagnify the beam about 5000 times. Con-denser lenses control the spot size of the electron beam while the objectivelens focuses the beam using the stigmator, aperture, and scanning coils. Byadjusting these various parameters of the snorkel objective lens, the work-ing distance of the electron beam was controlled. For the best resolution,the electron beam must diffract most effectively at an angle of about .004radians. [3] Many effects of the lens may keep us from optimizing this an-gle. Aberrations in the lens cause rays passing closer to the optical axis tofocus further away than other electrons. Chromatic aberrations cause higherenergy electrons to bend more when they pass through the magnetic lens.Astigmatism causes various angles of the sample to be viewed in a singlescan and is due to imperfections in the construction of the spherical lens.These various problems with the lens can be accounted for by adjusting thecontrast, focus, angle of view, and magnification of the beam.

After the electron hits the surface of the substrate, it can backscatterand head directly toward the detector, or it can enter the solid and scatterthrough the lattices. This causes secondary electrons to become excited andpossibly leave the solid and hit the detector. Electrons will penetrate fartherinto the specimen if they have a high accelerating voltage, high beam energy,low atomic number, and hit directly perpendicular to the surface.

Figure 13: Electron/Specimen Interaction: Path Dependence

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Once the electron hits the detector, the imaging process may begin. Asemiconductor is hit by the electrons and positive holes are created. Thisinformation is collected by the system and used to amplify and display thesignal on the CRT or TV screen. The image is magnified according to:

M = LCRT /LSCAN (3)

where LCRT is the length on the CRT screen and LSCAN is the length of thescanned specimen.

The depth of field of the beam refers to how large a distance is in focuson the screen. The quality of focus is inversely proportional to the size ofthe beam following the equation:

D =2 ∗ r

alpha(4)

where D is the depth of field, r is the radius of the beam that causes noticeabledefocusing, and alpha is the beam divergence. [3] You can adjust the depthof field by changing radius of aperture that in turn changes the divergenceof the beam.

Figure 14: Electron Depth of Field Schematic

The contrast of the image is affected by how the beam interacts withthe specimen. This interaction depends on the shape and composition of

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the specimen which determines the electron flux, the size and position ofthe detector, which captures electrons with different trajectories, and if theelectrons are backscattered or secondary, which determines the energy of theelectrons. Contrast is defined as:

C =S2 − S1

S2

(5)

where S2 and S1 are the signals detected at 2 points of interest on the image,usually the specimen and the environment, respectively.

The SEM may be set to backscattering electron imaging mode or sec-ondary electron imaging mode, each capturing different types of electrons.Each mode is used to quantify the number of electrons coming from eachtype of interaction. Resolution mode can be used to image the fine detailsat high magnification. High current mode is used for the best visibility andquality in the image. Depth of focus mode is used to see a wide range ofthe specimen in the z-direction by making alpha as small as possible. Lowvoltage mode is used to see surface details while sacrificing image resolution.

The SEM utilizes a variety of scans to construct a 3-dimensional image onthe screen. Line scan mode develops a 1-dimensionsal image of the surfaceof the sample. Area scan mode scans in x-y patterns on the sample andcontrast is created by differences in the signal intensity on various spots ofthe sample to create a 2-dimensional image. [4] Digital imaging mode storesthe image in the x-y-S array and creates an image by converting the digitalinformation to analog.

2.4 Energy Dispersive X-Ray Spectroscopy (EDX)

After the sample is imaged, the EDX may be used to determine the chemicalcomposition. When the electrons bombard the sample, they can transfertheir kinetic energy to the metal’s electrons to bump them out of orbit of thenucleus, and holes in a shell ar left behind.

An electron from an outer (higher energy) shell will fall down to the lowershell (lower energy) and the difference in energy will create an X-ray withwavelength:

lambda =h ∗ c

Ef − Ei

(6)

The wavelength dispersion is continuous because an electron can lose allor part of its initial energy.

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Figure 15: The Energy of the Electron Can be Converted to an X-Ray

The emitted x-rays are then collected by the silicon band detector crys-tal. The rays are photoelectrically absorbed by the silicon and the resultingelectron is promoted to the conduction band, leaving a hole in the valenceband. The electron-hole pairs are collected by a voltage bias to form a chargepulse. This charge pulse is then converted to a voltage pulse and read by thecomputer which computes the counts of each energy (measured in keV) andcompile them. The number of charges in the detector per incident photonfollows: [5]

N =E

e(7)

where E is the energy of the initial x-ray, and e is the transition energy forSilicon (3.86eV).

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Figure 16: The Energy Lost by Shell Transitions

To convert the charge pulse to a voltage pulse, the charge is swept intoa virtual ground at the input of an operational amplifier and stored in acapacitor. [5] The capacitor is discharged with a light-emitting diode andthe signal is amplified by the pulse shaping circuitry. So as not to saturatethe capacitor and create a pulse pileup, a fast amplifier is needed to count thepulses quickly. When the amplifier gets too many pulses and cannot processthem all, it turns off and the detector endures dead time, which lowers theprecision of your measurements. If the infrared lamp is left on, the dead timebecomes very high and there will be a lot of noise in the spectra.

To determine the identity of the peak, the computer requires a high res-olution and a small standard deviation to the energy of the collected x-rays.The INCA program will output the spectra with the interpreted identificationof the elements in the sample, taking into account ZAF (atomic weight cor-rection, absorption correction, and fluorescence correction). [5] The programwill take the energy counts and compare them to the characteristic x-rayspikes of each element on the periodic table to match them correspondingly.

The exact location of the elements on the film may be found using the

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Figure 17: An X-Ray Detector Schematic

micro-mapping setting on the EDX. This program back-calculates the posi-tion of each element from the energy counts of the x-rays along the sample,and forms an image with colored dots that represent the various elements.

3 Data and Analysis

Spectra from the EDX and images from the SEM were obtained for varioussamples given to us throughout the semester. A choice few are shown:

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Figure 18: The Spectra for the Unknown Plated Metal

After plating an unknown metal using thermal evaporation, inserting thesample in the SEM and running the it through the EDX, we found the thinfilm deposited on the silicon substrate consisted of gold and palladium.

Figure 19: A Powdered Silicon Sample

A powdered silicon sample was evaluated and found to have traces of goldinside.

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Figure 20: An SEM Image of the Powdered Silicon Sample

The following micromapping sample spectrum allow us to determine thelocation of each element found on the different samples. Two samples weregiven to us to examine, both with solder inside of a metal ring. The carboncomes from the carbon fiber sticker used to hold the samples in place and thesolder inside the ring. The oxygen comes from the oxidized metal containedinside the ring that is bonded to the tin and lead.

Figure 21: Micromapping Image of Solder Inside a Metal Tube

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Figure 22: The Other Metal Tube

Figure 23: An EDX Spectra of the Donuts

4 Conclusion

A vacuum system created by a series of pumps enables the fabrication ofa pure metal coated silicon film. Deposition allows for the creation of acontrolled alloy with a desired thickness and composition. The generatedfilm may be precisely imaged by a scanning electron microscope at the nano-level. The exact elemental composition of the sample can be determined

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by utilizing the energy-dispersive x-ray spectrometer and the position of theelements on the sample is computed by the micromapping program suppliedby INCA.

References

[1] Hongtao Shi. Vacuum system and technology 1. http:http://www.phys-astro.sonoma.edu/people/faculty/shi/p366/lectures/Vacuum

[2] Hongtao Shi. Vacuum system and technology 2. http:http://www.phys-astro.sonoma.edu/people/faculty/shi/p366/lectures/Vacuum

[3] Hongtao Shi. Scanning electron microscopy 1. http:http://www.phys-astro.sonoma.edu/people/faculty/shi/p366/lectures/sem01.pdf, October2014.

[4] Hongtao Shi. Scanning electron microscopy 2. http:http://www.phys-astro.sonoma.edu/people/faculty/shi/p366/lectures/sem02.pdf, October2014.

[5] Hongtao Shi. Energy dispersive xray spectroscopy. http:http://www.phys-astro.sonoma.edu/people/faculty/shi/p366/lectures/edx.pdf, October2014.

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