Introduction

The TM3000 Scanning Electron Microscope (SEM) captures surface images of a specimen in a vacuum chamber by scanning an electron beam over a sample. Electrons bombard the specimen and secondary and backscattered electrons are formed. These electrons are detected by a secondary electron detector which produces an image of the surface of the specimen. For more information regarding how a SEM works refer to SEM Vendor Manual and Let Us Familiarize Ourselves with SEM!.

Instrument Set Up

Start Microscope

1.On the back of the instrument make sure the main power breaker is in the 'On' position

2. Flip the 'On' switch on the right side of the microscope

3.The SEM will flash the yellow 'Air' and blue 'Evac' status indicator lights and then resume the last operation it was under when turned off.

Sample Preparation

Samples can be imaged untreated, sputter coated, or with silver paint. The purpose of treating the sample is to reduce the charge on the sample. Charge on the sample attenuates the electron beam and produces fuzzy images that make it difficult to identify structures or features on the sample. It is recommended to first image an untreated sample and if too much charge is detected in the image, apply a treatment and then re-image.

All Samples

A sample can be loaded into the SEM in a variety of ways. There are two sample stub sizes and an adapter holder for thin sections that can be placed in the SEM.

Sputter Coating Treatment

Sputter coating is performed using ionized argon to create a plasma. The argon ions are accelerated by high voltage and directed toward the source via a magnet where they collide with the target and displace surface atoms. Due to this collision the surface atoms are directed toward the area below the target and coat the sample. This coating process can be set to directional or diffuse (which provides more even coating on a bigger surface and is better for fissured samples), depending on the process pressure. This also influences the coating rate (diffuse means slower rate) and the grain size (directional means finer grains).

For steps regarding to switching target sources see sections

Introduction

The Leica EM ACE200 coating system is used for precise coating of samples for subsequent examination with a scanning electron microscope (SEM). Samples are metal coated using the sputtering method where argon plasma erodes a target material. Carbon coating is achieved by carbon thread evaporation. Any material can be processed as long as it is not sensitive to vacuum, argon plasma, or the heat generated during carbon coating. Sputter coating prevents charging the specimen with an electron beam in the SEM high-voltage mode. Metal coatings are also useful for increasing signal-to-noise ratio. The ACE200 can be configured as a sputter coater, carbon coater, or both, where the two processes can be switched over from one to the other.

With the Hitachi TM3000 SEM, sputter coating is not required for all samples. Images up to a magnification of 4000x are of high quality without any sputter coating in Charge-up Reduction Mode (Settings > Observation Mode).

For detailed observations in magnifications greater than 4000x and for very delicate fossils, sputter coating before using the SEM can be used to potentially receive higher quality images.

Apparatus and Materials

Argon Gas Supply

The working gas (argon) must be supplied under a pressure of ~500 mbar ±100 mbar (5.6—8.4 psi above atmosphere). The gas should be at least 99.99% pure.

Caution! Do not use a standard two-stage regulator for the sputter coater unless it is equipped with a low-pressure delivery-side gauge. Most regulators don’t effectively pick up pressure until 10 or even 15 psi or higher. It may be necessary to put a second low-pressure-range regulator downline from the main line regulator. Make sure you have the right tool for the job!

Sputtering a Sample

Set Up Parameters

• Target material: select from a drop-down list; the instrument automatically chooses parameter settings.
• Directional mode: intended for relatively flat samples; base vacuum is 4 × 10^–2 mbar for this mode.
• Diffuse mode: intended for more topographic samples; base vacuum is 8 × 10^–2 mbar for this mode.
• Sputter current/purge cycles: automatically set when target is selected. Change under the Setup button.
• Process termination time: select the Clock icon then use the +/– buttons to select a time in seconds.
• Rotation speed: press the Rotation button, check Rotation On, and use the +/– buttons to set the speed.

Starting Measurement

1.Check the Argon levels on the regulator and confirm the level is between 5.6 - 8.4 psi

2. Wearing gloves, place sample(s) on stage. Do not put sample(s) in the center of the stage or material will not land on the sample.

3. Turn on the vacuum pump behind the Sputter Coater

4. Turn on the main power supply to the instrument and make sure the chamber door and cover are closed.

5. Select the Sputtering button.

6. Press the Pump button to start pump-down.

7. During pump-down, set the process parameters:

• Target material (last material used is presented by default)
• Mode (Directional or Diffuse) button
• Sputter current/purge cycles (preset when target was selected)
• Process termination time (e.g., 12 seconds)
• Rotation speed (e.g., 1)

8. Press the Start button (which then becomes the Stop button). The Start button can be pressed while the Sputter Coater is still achieving vacuum.

Note: if the vacuum cannot stabilize, make sure the argon supply valve is open and pressure is on the line. What happens when pump can't stabilize?

Steps During Application

1.Pump until base vacuum is reached.

2. Stabilize the plasma.

3. Pre-sputter, if target requires it (to clean oxidation from the target and to enable a stable sputter rate).

4, Start the sputter processing by opening the shutter and starting rotation (if activated).

5. Terminate sputtering by time.

6. Close the shutter.

7. Display results of the process.

8. Vent or stay under vacuum, per settings.

Silver Paint Treatment

Silver paint allows a path for electrons to discharge from the sample.

Prepare Sample

1.Dab a very small amount of paint on the side of sample and extending to the edge of the sample stub. The painted sample area will obscure surface features so it is important to use very little on the sample. 

Loading Sample in SEM

Capture Image

Uploading Data

Maintenance

Sputter Coater

Switching Target Sources

To switch between carbon thread and sputter coating, exchange the source (sputter or carbon thread) and choose the process on the main screen of the instrument. The instrument automatically detects if the correct source is installed and gives and error message if it does not match the selected process.

Loading Gold Palladium Target

There is an ACE200 youtube video that shows the loading of the target at http://www.youtube.com/watch?v=Kd1lyKIDT8.

The procedure is as follows:

1. Wear gloves and make sure the instrument is fully vented.
2. Open the source cover and unplug the connectors.
3. Unscrew the 2 sputter head screws to remove the flange.
4. Remove the bayonet ring by turning and insert the sputter target as shown in Figure 2, below.
5. Fix the target by gently tightening the bayonet ring; tighten by hand only.
6. Replace the sputter head and gently tighten the fastening screws.
7. Connect the cables and close the cover.

Loading Carbon Thread

The carbon thread can be loaded as a single thread or as a double thread. Thin layers from 1 to at least 20 nm can be achieved (there is variance in threads).

To minimize carbon thread waste, when loading a double thread, cut a piece of thread twice as long as the width of the black door frames of the coater. Fold the thread into half and load it as follows:

1. Before beginning, prepare the following on a clean surface: gloves, carbon head, Torx TX 10 key, brush, carbon thread.
2. Make sure the instrument is vented. Wear gloves.
3. Open the source cover and unplug connectors.
4. Unscrew the 2 evaporation head screws to remove the flange.
5. Loosen all 5 clamp screws with the Torx key.
6. Remove any carbon residue using the brush.
7. Loop the carbon thread around the first clamp; pull both ends gently to the left while tightening the screw.
8. Wind the thread around the other clamps, taking care that the thread slides into the clamping groove.
9. Pull the thread gently and tighten the last clamp.
10. Tighten the 3 remaining screws and trim thread on both ends as shown in Figure 3, below.
11. Replace the head, gently tighten the fastening screws, connect the cables, and close the cover.

Also note that there is an ACE200 youtube video where the loading of the thread can be seen at http://www.youtube.com/watch?v=yKd1IyKIDT8).