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Atomic Force Microscopy | Dip Pen Nanolithography | Scanning Electron Microscopy

Scanning Electron Microscopy (SEM)

INTRODUCTION:

This laboratory is designed to introduce the Hitachi S-3500N Scanning Electron Microscope. You will investigate methods for sample preparation and look at the effects of condenser lens strength. In this initial laboratory, the methods for basic operation and alignment of the instrument will be covered. The purpose of this lab is to get you familiar with the standard operation of the SEM and general facility practices.

As you know from the class, SEM merely provides the means to generate and analyze myriad signals generated from electron-specimen interactions. Of all these signals, the most prolific imaging signal is from secondary electrons (SEs). Much of the imaging in this and many other labs will be done by imaging the SE signal in SEM.

» OBJECTIVE
» MATERIALS
» SPECIMEN PREPARATION
» STARTING UP THE MOCROSCOPE
» ALIGNING THE MICROSCPE COLUMN
» CONTROLLING RESOLUTION WITH THE CONDENSER LENS
» LAB PROCEDURE
» More about SEM (PDF)

OBJECTIVE:

By the end of this laboratory session, you should be able to:

1. Prepare and mount both conductive and insulating samples for examination.
2. Start and align the Hitachi S-3500N SEM and explain the effects of the alignment upon the imaging conditions.
3. Increase the resolution of the SEM image through the use of the condenser lenses and explain the optics behind this increase.

Since SEM imaging involves bombarding a material with an electron beam, the surface of the sample will accumulate charge if the electrons are not allowed to escape from the surface via a conductive path. If there is no such path, the image formed by the SEM will be very poor. Charging can also lead to excessive heating of the sample, causing material degradation. Insulating and semiconducting materials, i.e. ceramics, polymers, and organics, should be coated with a conductive material to prevent surface charging. Usually specimens are coated with either a metal or carbon.

For this laboratory you will be provided with samples of anodic aluminum oxide. One of the samples is uncoated and one sputter-coated with a platinum/palladium alloy. Since aluminum oxide is electrically insulating, you should be able to see artifacts from the sample charging with the uncoated sample.

MATERIALS:

• PDMS stamps and Masters
• Au nanoparticles
• ZnO nanorods on silicon
• e BL patterned silicon
• Hitachi 3500 SEM

SPECIMEN PREPARATION: (Click on pictures to view the videos)

Since SEM imaging involves bombarding a material with an electron beam, the surface of the sample will accumulate charge if the electrons are not allowed to escape from the surface via a conductive path. If there is no such path, the image formed by the SEM will be very poor. Charging can also lead to excessive heating of the sample, causing material degradation. Insulating and semiconducting materials, i.e. ceramics, polymers, and organics, should be coated with a conductive material to prevent surface charging. Usually specimens are coated with either a metal or carbon.

Note: SEM's are vacuum instruments. One should always wear gloves when handling pieces that go inside the microscope vacuum chamber!

	Assembly of sample holder
	Mounting the sample
	Fastening the sample
	Checking for the height!
	Adjusting the height

STARTING UP THE MOCROSCOP:

Even though SEM's are expensive and technically advanced analytical tools, don't let them be intimidating. For the SEM on which you will be working, there are only a few ways in which you can damage the scope. If you remember the following things you will be fine:

Always wear gloves when handling your sample and the SEM sample holders. This helps keep the vacuum system clean and avoids sample contamination.

Never vent the chamber without shutting off the high voltage! Doing this can cause the tungsten filament to rapidly oxidize and burn out.

When changing the accelerating voltage, follow the proper procedure and saturate the filament. To change the accelerating voltage:

1. Shut of the high voltage
2. Change the voltage level
3. Turn the voltage back on
4. Re- saturate the filament.

Make sure that the z-height is adjusted correctly and the BSE detector is withdrawn when loading and unloading the sample. If the sample height and stage positions are not correct, you run the risk of hitting the objective lens or BSEdetector. A height gauge is available to ensure that your sample is not too tall when the z-height is set to the position for loading and unloading.

Starting the SEM consists of applying an electrical potential across the electron gun and running current through the filament. The current causes the filament to heat up, and once enough energy is supplied, electrons are ejected from the filament towards the anode. The bias on the Wehnelt cylinder can be adjusted to control the amount of electrons leaving the electron gun assembly. The S-3500 software does this process automatically, but it is still important to understand what is going on in the electron gun. For operation instructions, we will refer to the S-3500 manual for standard operating procedures.

ALIGNING THE MICROSCPE COLUMN:

In order to obtain good resolution it is essential that the microscope be properly aligned. Ideal alignment is achieved when the gun, lenses, and apertures are concentric about the optic axis - an imaginary line drawn down the center of the column. Before aligning, try to get your sample in the best possible focus, and capture a digital image of your sample. Use these conditions: 20,000X magnification, ~15mm working distance, 25kV accelerating voltage, objective aperture #3 and condenser lens strength (called beam current in software) set to 50.

Align the objective aperture:

When aperture alignment in the Operation/Alignment menu is selected, the focus wobble is activated. The focus wobble automatically changes the focus of the objective lens and aperture misalignment is manifested in translation of the image. To correct the aperture misalignment, adjust the X and Y knobs on the aperture to stop the image movement. (Note: depending on the initial gun alignment, it may be necessary to repeat this step after step 2 below) Align the electron gun: The gun alignment in the S-3500 is automatic – simply select Gun Shift, press the AGA (Auto Gun Align) button, then select Gun Tilt and press AGA again. We can, however, adjust the Gun Shift and Tilt manually with the X and Y controls and look for the brightest image on the screen. Try both and see how good the automatic alignment really is.

Align the stigmators:

In order to adequately correct for astigmatism, it is important that the stigmators are aligned along the optical axis. For both the X and Y stigmator alignments, use the X and Y controls in the software to stop the image movement (similar to aperture alignment).

Correct for astigmatism:

Once the stigmators have been aligned, you can adjust the strength of each stigmator independently to correct for astigmatism. You are looking for the sharpest image and should see no stretching of the image when changing focus.

Once the microscope has been aligned, take another picture of your sample in the same region and at the same magnification.

CONTROLLING RESOLUTION WITH THE CONDENSER LENS:

Since the SEM uses an electron probe to scan in the image of our sample point-by-point, the diameter of the electron probe contributes to the resolution of that image. That is, a smaller electron beam diameter will create an image with higher resolution. One way to control the diameter of the electron beam, or probe size, is by manipulating the condenser lenses.

The purpose of the condenser lenses is to demagnify the probe coming from the gun assembly. The ultimate effect of the condenser lenses on the probe size is illustrated in the figure below (from Goldstein et al., 52):

From the diagram, you can see that for a given working distance and objective aperture size, a stronger condenser lens will produce a smaller electron probe size (read - higher resolution). But also notice that the stronger condenser lens setting results in a lower probe current, indicated by the larger crosshatched area (more electrons are being "thrown away" from the beam). In electron imaging, this loss of probe current will be seen as a picture with more "snow" or "noise". This is the dichotomy that you will encounter over and over in using electron probe instruments - spatial resolution versus analytical sensitivity.

» Flash tutorial on the effect of condenser lens in controlling resolution

LAB PROCEDURE:

Please refer to "More about SEM" for further details concerning this lab!!

Part 1:

You should capture two images of the Au nanoparticles sample - one before alignment and the other after alignment using the following conditions:

Sample: Au nanoparticles
Accelerating Voltage: 25kV
Beam Current: 30
Detector: SE
Objective Aperture: 2
Working Distance: 10mm
Magnification: >15,000X

Part 2: Signals in the SEM

The sample for this portion of the laboratory is a silicon wafer with features patterned with electron beam lithography ( e BL). This sample was created with two separate e BL steps in order to create features with two different metal films � pure Titanium and a 60/40 Au/Pd alloy. You should capture images of patterns with each type of metal using both the SE and BSE detectors.

SEM Conditions for Part 2:

Sample: e BL patterned silicon

Accelerating Voltage: 25kV
Beam Current: 50
Detector: SE and BSE
Objective Aperture: 2
Working Distance: 15mm
Magnification: ~10,000X

Part 3: Resolution vs. Signal to Noise

Tanstaafl. There ain't no such thing as a free lunch. This axiom is quite applicable to the problems of achieving high resolution images in the SEM. For this portion of the laboratory, we will concentrate on methods for improving imaging resolution in the SEM and the trade-offs involved. There are two major aspects to achieving high resolution images: the ability to form small probe of electrons and the ability to detect high resolution signals produced by the interaction with the sample.

The sample for this portion of the laboratory is ZnO nanorods on silicon . We will explore the effects of objective aperture size and condenser lens strength on resolution and signal to noise ratio. You will capture at least 4 images for this section comparing the at least two different aperture sizes and two different condenser lens strengths (beam currents).

Sample: ZnO nanorods on silicon
Accelerating Voltage: 25kV
Beam Current: 30 and 60
Detector: SE
Objective Aperture: 1 and 4
Working Distance: 15mm
Magnification: >20,000X

Part 4:

Image the PDMS masters from lab 1.

LAB QUESTIONS: Questions for laboratory write-up

1. You may have noticed that to get a good BSE image, you need to have weak condenser lens settings and a large objective aperture. Why do you think this is the case?
2. For the Au/Pd patterns on silicon, why do the smaller features appear darker than the larger ones if they have the same composition?
3. Note that even though the image on the screen may get noisier with decreasing aperture size or increasing condenser lens strength, the photo taken may not show this. Please explain why this occurs.
4. Since SEM is essentially a surface analysis technique, why doesn't sputter coating obscure useful surface information? Can you think of any cases where the coating might become a problem?
5. Low voltage microscopy is a useful method for minimizing charging effects. What are some of the other benefits and tradeoffs? Why is cold field emission well suited for this technique?
6. If you didn't have a dedicated BSE detector in your SEM, can you think of a way to use the E-T SE detector for this purpose?
7. The image below is a pattern produced by electron beam lithography in PMMA on a silicon substrate. The PMMA was first spun onto the wafer (~150nm thick) and then the inner and outer lines of the letters in the �NUANCE' pattern were written by taking control of the SEM scan coils. After exposure to the electron beam, the PMMA becomes more soluble in the developer and can be removed. After developing, the sample was then sputter coated with a uniform thin film of Pt/Pd prior to imaging. For reference, the letters are ~2 m m tall and the lines are ~30nm wide. Why do the N, A and E have different contrast than the rest of the letters and the rest of the sample? Why are they dark? (This pattern was written several times on the sample and different letters were dark in each, so there is no difference in processing from letter to letter besides random variation .) Also, try to explain why the center of the 'A' is darker that the rest and why the 'E' is darker on the horizontal parts.

» More about SEM (PDF)