Experiment B43: Scanning Electron Microscope (SEM)V-042014

Experiment B43: Scanning Electron Microscope (SEM)

Room 01.578 (Staudtstr. 7, Building B2, 2nd floor)

Contact supervisor by email: BetreuerV43@physik.uni-erlangen.de

ATTENTION: Send the report on the preparatory work to the supervisor one day in advance (before 14:00) inpdf format.

Bring a USB-Stick with free memory to copy your experimental data.

1 Preparatory work:

a) Describe the scattering processes of fast electrons in matter and the products resulting from this interac-tion. How does the scattering process depend on the energy of the electrons and the material? Describethe information you can obtain from

(i) Secondary electrons (SE)

(ii) Back scattered electrons (BSE)

(iii) Characteristic X-rays

b) Describe the basic setup of the SEM, i.a.

(i) Electron sources (thermionic emission, field emission)

(ii) Electron lenses

(iii) Detectors for SE, BSE and X-rays

(iv) Imaging with the scanning electron microscope

c) Discuss the resolution of the SEM concerning the following points:

(i) Focal length and working distance

(ii) Acceleration voltage

(iii) Aberrations (spherical, chromatic, astigmatism)

(iv) Depth of focus

d) Compare the optical imaging in a bright field microscope with the imaging in the SEM concerning

(i) Wavelength

(ii) Optical setup

(iii) Numerical aperture and therefore resolution, working distance, and depth of focus

(iv) Magnification

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Experiment B43: Scanning Electron Microscope (SEM)V-042014

2 Recommended Reading:

[1] JEOL, A Guide to Scanning Electron Microscope Observation, S. 22-32, seewww.optik.uni-erlangen.de/odem/download/v43/v43 guideSEM.pdf

[2] L. Reimer, G. Pfefferkorn, Scanning electron microscope, Chap. 1-4, German version seewww.optik.uni-erlangen.de/odem/download/v43/Reimer REM.zip

[3] JEOL, Invitation to the SEM world, seewww.optik.uni-erlangen.de/odem/download/v43/v43 BeginnersGuide.pdf

Further reading

[4] D. Drouin, A. R. Couture, D. Joly, X. Tastet, V. Aimez, R. Gauvin, CASINO V2.42-A Fast and Easy-to-use Modeling Tool for Scanning Electron Microscopy and Microanalysis Users, SCANNING Vol. 29,92-101 (2007), seewww.optik.uni-erlangen.de/odem/download/v43/v43 casino.pdf

[5] D.B. Williams, C.B. Carter, Transmission Electron Microscopy I, Basics, Plenum Press, New York, 1996;Chap. 3-7, seewww.optik.uni-erlangen.de/odem/download/v43/Wiliams-Carter TEM Basics.zip

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Experiment B43: Scanning Electron Microscope (SEM)V-042014

3 Experiment: Scanning Electron Microscope (SEM)

Inner sample holder

THz antennas

Si-sample

PCF

Coiled filamentSi-sample: Silicon with etched surface profile,

Grating p = 2µmPCF: Photonic crystal fiber,

Fused silica (SiO2) and airTHz antennas: Multi layer system

Resist (Thickness 1µm) on GaAs,Gold antenna (Thickness 200nm) on GaAs

Si-sample Si-sample (detail)

2µm

2µm

10µm

10µm

THz antenna

ATTENTION: Loading and unloading of the samples must be undertaken only in the presenceof the supervisor. Do not tilt the sample!

Default settings for SEM:

• Move sample to z position 15mm and adjust coarse focus to 15mm.

• Select aperture stop 3 (70µm), acceleration voltage Uacc = 20kV and probe current (condenser lens)6 · 10−9A.

• Detector switch SE, SE - Collector switch on

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Experiment B43: Scanning Electron Microscope (SEM)V-042014

3.1 The SEM Jeol 840:

3.1.1 Get briefed on handling the SEM (Controls, Loading/Unloading, ...).

3.1.2 Get briefed on handling the imaging software Elphy Quantum.

3.1.3 Acquire a first image with the SEM to learn how to align brightness, contrast coarse/fine focus. Alwaysnote down the magnification and all other parameters deviating from the default settings as the softwarewill NOT save this data.

3.1.4 Acquire a well aligned image with the SEM by aligning fine focus and astigmatism iteratively. Increasethe magnification in each iteration step ending up at approximatively 90000× .

3.1.5 You may acquire more pictures than are asked for in the description. Always note down the magnificationand all other parameters deviating from the default settings.

3.2 Aberrations

3.2.1 Astigmatism: Select an appropriate structure and magnification. Misalign one (!) stigmator until youcan observe astigmatism. Acquire images with at least five different focus positions to demonstrate theeffect of astigmatism on the imaging. Realign the SEM after this !

3.2.2 Centring of objective aperture: For high resolution imaging in the SEM, the objective aperture hasto be centred. Explore the effect of a decentred objective aperture.

(i) Select an appropriate structure and magnification. Misalign one (!) fine-thread screw of the aperturepositioning system until you can observe the effects of the decentred aperture.

(ii) Acquire images with at least five different focal positions to demonstrate the effect of the decentredaperture on the imaging.

(iii) Centre the objective aperture by wobbling the focus:

• Press WOB button to periodically vary the focus. The amplitude of this variation can be changedwith the AMPLITUDE knob.

• Select appropriate magnification and AMPLITUDE. During focus variation, you should be able tofollow the movement. Defocusing should be visible distinctly.

• Carefully turn the fine-thread screw you changed before until no lateral movement is visibleduring focus variation. With a properly centred aperture, only defocusing and focusing shouldhappen during focus variation.

• Press WOB button again to stop wobbling. Realign the focus and astigmatism after that.

3.2.3 Spherical aberration: Demonstrate the influence of the objective aperture diameter on the resolution.Select an appropriate structure and magnification. Acquire images with different objective aperturediameters (Aperture 1: d = 170µm, Aperture 2: d = 110µm, Aperture 3: d = 70µm, Aperture 4:d = 50µm). While changing the aperture, take care not to decentre the aperture. If the aperture isdecentred, centre it by wobbling the focus following the above receipt.

In your report, present the effects of the different aberrations and explain how the characteristic effects inthe imaging arise from the different aberrations. Show selected images.

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Experiment B43: Scanning Electron Microscope (SEM)V-042014

3.3 Depth of focus and working distance

a) Select the default settings. Drive to the coiled filament and align the SEM.

b) Acquire images of the coiled filament with maximum and minimum objective aperture sizes (Aperture1: d = 170µm, Aperture 4: d = 50µm) and maximum and minimum working distances (WD = 15mm,WD = 48mm). Always realign the SEM at the same position. Obtain at least two images for eachcombination of WD and d with different magnifications to demonstrate, on the one hand, the change ofthe depth of focus and, on the other hand, the influence on the resolution.

In your report, demonstrate the change of the depth of focus and explain where it does arise from. Whichchanges occur while changing the aperture size? Which changes occur while changing the working distance?Show selected images.

3.4 Acceleration voltage

3.4.1 Resolution and edge effect: Explore the influence of the acceleration voltage on the imaging of aconducting sample surface.

(i) Select a position on the Si-sample, preferably with some dust additional to the sample surface struc-ture (not only dust). Select an appropriate magnification for the image acquisition.

(ii) Acquire images of the same position with different acceleration voltages covering the interval [40kV, 1kV ](at least 5 steps, e.g. 40kV , 20kV ,10kV , 5kV , 1kV ). Always optimize the filament heating and re-align the SEM (focus and astigmatism, brightness and contrast) after changing the accelerationvoltage!

(iii) Simulate the electron distribution in the sample with the help of the Monte Carlo Simulation via thesoftware CASINO (see Sec. 4.1, [4]) for all acceleration voltages. Determine the penetration depthsof the electron for all acceleration voltages.

3.4.2 Probe current: Usually, a measurement of the probe current is done with the help of a Faraday Cup.This SEM is not equipped with a Faraday Cup. To perform the measurement of the probe current withlow electron loss, the gap between inner and outer sample holder is used.

a) Select the default settings and drive to the gap between inner and outer sample holder at the prede-fined position found in the software window Stage control → Command → User positions → FP

Strommessung Spalt. Press Go-button to drive to the position.

b) Adjust the sample position so that the gap is centred and increase the magnification. During increas-ing the magnification, observe the probe current. Increase the magnification until the probe currentdoes not change any more and the CRT is completely black. Note down the magnification and useit for the further probe current measurements in the gap.

c) Drive to the predefined position Stage control→ Command→ User positions→ FP Strommessung

Al and select a low magnification (< 500×). Note down the magnification and use it for the furtherprobe current measurements on the sample holder.

d) Measure the probe current for different acceleration voltages covering the interval [0.5kV, 40kV ] (atleast 5 steps, e.g. 0.5kV , 1kV , 5kV ,10kV , 40kV ) in the gap and on the sample holder. Perform themeasurements following the listed steps:

• Drive to Stage control → Command → User positions → FP Strommessung Spalt. Selectlow magnification. Change acceleration voltage. Optimize filament heating.

• Centre gap, select magnification for the gap probe current measurement.

• Measure probe current.

• Drive to Stage control → Command → User positions → FP Strommessung Al. Select mag-nification for the probe current measurement on the sample holder.

• Measure probe current on sample holder.

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Experiment B43: Scanning Electron Microscope (SEM)V-042014

e) Determine the penetration depth into aluminium for the different acceleration voltages by simulation(Sec. 4.1, [4]).

3.4.3 Multi-layer system: Drive to the THz antenna sample and select an antenna for the following experi-ment. Use a magnification lower than 500×.

a) Acquire images for different acceleration voltages covering the interval [0.5kV, 40kV ] (at least 5 steps,e.g. 0.5kV , 1kV , 5kV ,10kV , 40kV ).

b) With the help of CASINO (Sec. 4.1 [4]), simulate the multi-layer sample consisting of the two system:

(i) 200nm Au on top of a GaAs substrate

(ii) 1000nm PMMA (resist) on top of a GaAs substrate .

Simulate the acceleration voltages you used experimentally and compare the results with the exper-imental ones.

In your report, explain the influence of the acceleration voltage on the imaging of the different samples(Si-sample and multi-layer sample). Show selected images and simulation results to second your arguments.Additionally, based on your probe current measurements, calculate the percentage of electrons (SE+BSE)leaving the sample at the FP Strommessung Al position with respect to to the total probe current measuredin the gap. How does this percentage change with the acceleration voltage? How does this compare to thesimulation of the penetration depth in Al? What is the working principle of a Faraday Cup?

3.5 SE detector

The SEM uses an Everhart-Thornley-Detector as SE-detector. The detector is equipped with a mesh (Faradaycage) surrounding the scintillator crystal. Normally, the Faraday cage is set to a positive potential to attractthe low energy electrons (mainly SE). A negative voltage can be applied to the Faraday cage by changing theCOLLECTOR switch from ON to SUPPRESS. Only electrons with an energy high enough to overcome the negativepotential can reach the scintillator in this case (BSE).

Drive to the coiled filament and align the SEM. Select a magnification (lower than 500×). Acquire imagesfor positive and negative voltage applied to the Faraday cage.

In your report, explain the different image contrasts arising from positive and negative potential at theFaraday cage. Show selected images to second your arguments.

3.6 Charge-up effects

Usually, observing dielectric samples leads to charge-up effects which result in severe image distortions. Here,the charge-up effects for various acceleration voltages should be studied exemplarily at a photonic crystal fibre(insulator).

a) Reduce the acceleration voltage to 0.5kV BEFORE driving to the photonic crystal fibre (PCF) and alignthe SEM at a test structure (e.g. coiled filament).

b) Drive to the photonic crystal fibre and align the SEM at the site of fracture.

c) Increase the acceleration voltage stepwise up to 40kV (at least 5 steps, e.g. 0.5kV , 1kV , 5kV ,10kV ,40kV ), align the SEM, and observe the charge-up effects. Acquire images of the site of fracture for everystep at 300× and 30× magnification. Adjust brightness and contrast so that you can demonstrate, on theone hand, the charge-up effects of the fibre and, on the other hand, their influence on the imaging of thesurrounding area.

d) Observe the mirror effect of the charged-up fibre. Therefore, reduce the acceleration voltage to 1kV . Alignthe SEM and acquire images demonstrating the mirror effect.

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Experiment B43: Scanning Electron Microscope (SEM)V-042014

In your report, describe the different charge-up effects depending on the acceleration voltage and presentselected examples. Also, show the influence on the imaging of the surrounding area. How does the mirror effectarise? What can you do to reduce charge-up effects? What are the disadvantages of the different methods toavoid charge-up effects?

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Experiment B43: Scanning Electron Microscope (SEM)V-042014

4 Basic Operation Instructions:

4.1 Short software manual CASINO

Menu CASINO

4.1.1 Create a simulation:

(I) Setting up the Micoscope : Open the dialogue Microscope and

Simulation Properties by pressing the menu button Microscope

Setup (1):

(a) Energy of the primary electrons.

(b) Select if multiple energies should be simulated in one run.Enter energy step size Step and maximum energy End.

(c) Number of primary electrons. The larger the number, themore significant the results. Attention: A larger number ofelectrons also requires more memory and longer simulationtime.

(II) Creating the sample: Open dialogue Edit Layers by pressingthe menu button Sample Definition (2):

(a) Press to add a new layer. Add all necessary layers includingthe substrate before proceeding.

(b) By double clicking the name, the chemical composition canbe changed (see point III).

(c) To change the layer thickness, double click the thicknessvalue. The thickness is entered in [nm]. Attention: Thelowest layer should stay Substrate.

(d) Both checkboxes Use Substrate and Multi-Layer should beselected.

(III) Chemical composition: The chemical composition can be changed directly by entering the chemi-cal elements (including valence number) into the field Composition (e + f). Attention: User Defined

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Experiment B43: Scanning Electron Microscope (SEM)V-042014

Density and User Defined Distribution (g) must not be checked in this case!The composition can also be changed by selecting a material from the database (h). Enter the Name (e.g.PMMA) and the chemical composition will be taken from the database. Here, User Defined Density

and User Defined Distribution (i) have to be checked.

4.1.2 Start simulation: To start simulation, press menu icon Begin Simulation (3). The window title showsthe progress of the simulation in percentage during simulation.

4.1.3 Show data: The left panel will show a folder with the simulated data for each energy once the simulationis finished. You can choose from several data representations:

Main folder: Paths of the primaryelectrons with colour coded energy loss.

Main folder: Paths of the primaryelectrons with colour coded energy lossand all collisions. BSE are depicted red.

Distribution → ZMax: Penetration depth ofthe primary electrons

Distribution → Energy by Position:Distribution of the absorbed energy in

the sample.

The menu icon Display Options (4) opens the properties dialogue for thedisplay of the electron paths:

a) Colour coding the energy loss.

b) Show all collisions.

c) Show all electrons leaving the sample (BSE) in red.

The menu icon Distribution Display Options (6) opens the properties dialogue for the histogram.The menu icon Automatic Rescale (5) resets the displayed area.

4.1.4 Save results: Use the menu icon Save Display (7) to save the various displayed distributions as im-ages. Check that the image was saved correctly. The simple image manipulation program PaintShopPro(psp.exe) is installed on the computer.

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