The TM-1000 Scanning Electron Microscope The TM 1000 Scanning Electron Microscope is easy to use and allows magnifications of up to 10,000x. After the specimen is loaded into the scope and the chamber is evacuated, operation of the scope is controlled through a simple computer interface on a laptop. The simplicity of the TM-1000 is due to the fact that many of the imaging parameters are preset and constant. This scope has a single aperture, the working distance can be only minimally changed, and the acceleration voltage is set at 15 kV. Even with these limitations the TM1000 is a research quality SEM and is a quick and easy way to make excellent images up to about 5,000 or 6,000x, and perhaps up to 10,000x, depending upon the sample. This SEM uses backscatter electron (BSE) detectors so it is possible to introduce a limited amount of air into the chamber to reduce charging effects. In this lesson you will learn how to:
turn on the SEM mount a sample stub onto the sample holder check the height of the specimen so it can be loaded into the SEM safely load the sample into the scope evacuate the chamber ventilate the chamber to change specimens turn off the SEM
Turning on the SEM To turn on the SEM use the black power button on the front of the SEM. You should turn on the SEM before you turn on the computer. Note in this photo that simple power up and power down instructions are taped to the scope.
Opening the Sample Chamber to Insert a Specimen Scanning Electron Microscopes are stored with a vacuum in the chamber - this keeps dust and moisture out of the instrument. This also means that when you turn the SEM on, the chamber will be evacuated and you will have to allow air into it to open it. This is done by pressing the "Exchange" button on the front of the scope. Please note that this single button is used to evacuate the chamber (establish a vacuum) and also to let air back in. The SEM will not maintain a full vacuum when turned off, so when you turn on the scope you will probably see either the yellow or red light. Press the exchange button only once. If the vacuum is increasing, as indicated by the lights (see below), press it again.
Steady green = Full vacuum
Steady Red =Chamber can be opened
A series of indicator lights across the top of the SEM that lets you know the atmospheric conditions inside the scope. A steady green light means the scope has reached full vacuum. A steady red light means the scope is at atmospheric pressure - no vacuum. The sequence of lights when pulling a vacuum is as follows:
Steady red Blinking red Blinking yellow Steady yellow Blinking green Steady green
You cannot turn on the electron beam until the SEM is under full vacuum (solid green light) The sequence of lights when allowing air back into the chamber is the opposite: It takes a full 2 minutes for the scope to either reach full vacuum or to reach room pressure. Be patient.
It is futile to resist! A word about pressure. We live at the bottom of an ocean of air. We don't usually pay much attention to it, but that air has mass and when gravity acts on that mass, it causes atmospheric pressure. Imagine going outside and drawing a 1 square inch box on the sidewalk with a piece of chalk. Now imagine the column of air above that square inch that stretches to the top of the atmosphere - more than 60 miles. If you were to weigh that column of air it would weigh, on average, 14 pounds. That means there is a force of 14 pounds per square inch pushing on everything down here on the surface of the planet. The size of the door on the SEM is about 25 square inches. When under full vacuum, the force holding that door closed is 350 lbs! That is why you have to be patient and wait for air to reenter the chamber. Another unit of pressure is the Pascal. Average atmospheric pressure (at sea level) is 101,300 Pascals. The pressure inside of an SEM under full vacuum is 0.001 Pascals. Why do we need a vacuum? Air is a very poor conductor and the SEM shoots a beam of electrons at the sample. Air molecules inside the scope will decrease the number of electron from reaching the sample and also refract the electron beam which keeps us from being able to get a focused image. How is the vacuum inside the SEM made? The TM-1000 has two vacuum pumps. The first pump to operate is shown on the right. This little red pump sits on the floor and should start chugging away when you push the exchange button to pull a vacuum. Please note that you must hold the specimen draw closed when you begin evacuating the chamber until it seals. This should take 15 or 20 seconds. You can tell when the drawer is sealed because the red vacuum pump will become appreciably softer. (You can also gently tug on the drawer to see if it comes open.) Once the green indicator light begins flashing, you should be able to hear the second vacuum pump begin to work. It will sound like a jet engine spinning up. This is a turbo pump that is used to form the very high vacuum that the scope needs to operate. (By the way, the turbo pump spins so fast that it takes about 10 minutes to stop spinning after the scope is turned off.) If you get an error message saying that a vacuum could not be formed, it is because the drawer didn't seal. If that happens, start the process over again and this time, hold the drawer closed.
Inside the Specimen Chamber So now let's gently pull the chamber drawer open and take a look inside the scope. This image on the left shows the open specimen chamber. You can see the mounted specimen in the top right corner of the image. The gears are the mechanism used to moves the specimen around. Note the "+" and the metal pointer. When the pointer is lined up with the center of the +, the specimen is centered in the scope.
You should always center the specimen before imaging or the scope may have trouble focusing and you will have trouble finding your specimen. If you are imaging multiple, small objects, it is a good idea to draw a quick map of where everything is on the stub to that you can tell which specimen you are looking at and which direction to move to see the next specimen. Mounting a Specimen for the SEM The next step is to place your mounted specimen on the specimen holder. Specimens are mounted on an aluminum disk called a stub. A carbon-based, doublesided sticky tape called a tab holds the specimen to the stub. There is a threaded hole in the bottom of the stub that allows it to be screwed onto the specimen holder. The height of the specimen is adjusted by twisting the screw on the specimen holder. Note that there is also a smaller, locking ring that can be used to keep this height from changing. The locking ring doesn’t have to be very tight, but snugging it down will keep the stub from vibrating and turning in the SEM. How do you know the proper height? There is a handy dandy height checker!
Setting the Specimen Height
Place the mounted specimen in the height check and adjust the screw on the specimen holder up or down until it clears the bar by 2 to 3 mm. This is a very important step. If the specimen is too low, the scope will not be able to focus the beam on it and you will not see an image. If the specimen is too tall, it cannot go into the scope and, if you force it, could damage the detector - a multi thousand dollar mistake. No kidding. The good news is that this is pretty much the only way that you might damage the SEM.
The good news is that there is a built in barrier that should keep you from putting something into the scope that may be too tall. As you GENTLY close the draw, there is a metal plate that the specimen must clear before it will go into the scope.
Once the specimen is mounted and its height is checked, simply push the specimen holder into place in the drawer (push it all the way down) and GENTLY close the drawer. Putting a Vacuum in the Specimen Chamber Hold the drawer closed and push the exchange button. Wait about 20 seconds or until the red vacuum pump becomes quiet - then you can let go. In about 2 minutes you will get a steady green light and you will ready to start making images. The two knobs on the front of the SEM are used to move the stage around inside the sealed scope. As the arrows indicate, the right hand knob moves the specimen up and down on the computer screen and the left knob moves it left and right.
When You Have Finished Imaging When you have finished working with the SEM you should: 1. Remove the specimen holder and leave it in the height checker on top of the SEM so the next person can find it. DO NOT LEAVE A SPECIMEN IN THE SCOPE! 2. Gently close the chamber and push the exchange button to begin the vacuum. 3. Wait for the solid green light that tells you that the chamber is under full vacuum. Remember - this takes 2 minutes. 4. While you are waiting for the vacuum, close the TM-1000 program and shut down the lap top. 5. Once you have full vacuum, turn off the power to the SEM. PLEASE REMEMBER TO LEAVE THE SCOPE UNDER VACUUM. The Back Scatter Electron (BSE) Detector This is a look inside the specimen chamber at the pole piece. In the center you can see the opening through which the electron beam passes to strike the specimen. The four segments around this opening are the BSE detectors. The BSE detectors collect electrons coming from the sample to make the image on the laptop.
(This image was made placing a mirror inside the chamber and shining a flashlight into the mirror for illumination while also taking a picture into the mirror and was a real pain in the drain to make). It is in a very awkward and dark place in the SEM but I wanted you to see it. Never, never, never, never (get the point) reach into the scope and touch this detector. It will ruin the scope.