JEOL 7800F
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| Essentials | |
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| Full name | JEOL JSM-7800F |
| Manufacturer | JEOL, Ltd. |
| Description | Scanning electron microscope |
| Location | 03.1.K08 |
| Manual | A printed version is in a blue binder by the instrument |
| Responsibility | |
| Primary | Nader |
The JEOL 7800F is a 30 kV field emission scanning electron microscope (SEM). It is equipped with two secondary electron detectors a backscatter detector, and a EDS detector. It has a eucentric stage, a range of different sample holders depending on application requirements, and can accommodate samples of up to 50 mm in diameter. A unique feature of this SEM, is its ability to place a negative bias on the sample stage in order to decelerate incoming electrons as well as to eject secondary electrons, thereby increasing the signal-to-noise ratio. This feature is known as gentle beam (GB), and is particularly useful when working at low acceleration voltages.
Raith eLine is an alternative to this tool. It does not have beam deceleration capability, but supports scripting such as unattended imaging. Repetitive imaging at predetermined locations should be carried out on that tool instead.
Overview
The different components of the SEM are illustrated in the two figures below:
- The lower electron detector (LED) is a secondary electron detector, located in the chamber, and is mostly used for overview images, or in conjunction with sample tilting or high acceleration voltages.
- The upper electron detector (UED) is an in-lens secondary electron detector located in the electron beam column. It is typically used for obtaining ultrahigh resolution iamges at low acceleration voltages. It is usually used in conjunction with the gentle beam (GB).
- The backscatter detector (BSD) measures the high energy electrons from the incident beam that are backscattered by the sample surface. Particularly useful for samples composed of different materials of contrasting densities. The detector is inserted by a pneumatic valve when needed by the user, but is otherwise retracted.
- The EDS detector measures the x-ray wavelengths of photons generated in the sample when exposed to the electron beam. Using the measured x-ray spectrum, the sample material composition may be determined.
- The chamber camera is colour sensitive, and primarily used to see the sample and sample holder in relation to the pole-piece. It can only be switched on if the gun-valve is closed, and the gun-valve cannot be opened while the camera is on.
- The loadlock camera automatically takes a picture of the sample surface when the loadlock is evacuated. However, if the user is not logged in, the image will not be taken, and there is no way to take it without venting and re-evacuating the loadlock. The overview picture can be used for rough navigation of the sample.
- There is a magnetic field sensor suspended adjacent the electron beam column, and is connected to a magnetic field cancellation unit in the corner of the room. If you notice interference, or feel that you are not reaching the image resolution you feel you should, check the field cancellation unit to see if it has been tripped. This happens once in a while due to vibration from foot traffic outside, etc, and the field cancellation may be restored by pressing the reset button on the unit.
- The SEM has a N2 dewar for cooling the sample stage.
Loading sample
- Select the appropriate sample holder: There are several different sample holders available for the SEM, each adapted for a particular sample size or function, such as a full 2 inch wafer holder, cross-sectional sample holder etc. Generally, you should use the smallest sample holder that will accommodate your sample. This will give you the greatest range of tilt. The PC-SEM software knows about the geometry of the different sample holders, and will automatically set the safe tilt angles for you, and prevent you from accidentally running the sample holder into the pole-piece.
- Attach the sample: The 2 inch wafer holder allows for the attachment of samples to the holder via clips, and the cross-sectional sample holders alow the samples to be either clamped in place or mounted with a screw. For the remaining sample holders, and adhesive must be used, e.g. carbon tape, graphite paste, or silver conducting paste. Most users use the carbon tape. It is easy to use, and to remove, but due to the elastic nature of the tape, some drift may occur at high magnification. If this is a problem, or you have fragile samples that may break when removed from the adhesive tape, you can try the graphite paste. It has lower adhesion, but also more electrically resistive. There are two pastes available, one suspended in water solution, and the other in isopropanol. Apply only a small dot to the sample holder surface and place your sample over it, press down gently on the sample and then leave it to try for about 5 minutes. Test whether the sample is well attached to the sample holder before loading. Note: Do not use for full wafers.
- Set the sample height: The sample must be attached to the sample holder such that its surface protrudes slightly from the edge of the sample holder when viewed from the side. The height of this protrusion is known as the sample surface offset. If the sample surface is below the edge of the sample holder, it must be adjusted. In the case of the 12.5 mm sample holder (see figure), this is done by loosening the two retaining screws on the side of the holer, and then screwing the large screw on the underside of the holder. This adjusts the height of the central stub to which the sample is attached.
- Load the sample holder: Hold the VENT button for two seconds, or until you hear a valve close and the hissing sound of N2 entering the loadlock, and then release clip on the side of the loadlock. This avoids an overpressure from forming within the loadlock which could dislodge the loadlock o-ring. Once the hissing stops, open the loadlock and push the sample holder into the mounting brace. Make sure it sits snugly against the flat edge, and verify that it isn't tilted. Close the loadlock, close the brace, and then press the EVAC. The system will start rough-pumping the loadlock. Pressing EVAC will also trigger the loadlock camera to take a picture of the sample surface, which can be used for navigation later on.
- Insert the sample holder into the chamber: Once the pressure in the loadlock reaches 1.6 Pa, the SEM will automatically move the rough pumping to the back of the turbopump, and open the gate valve. This process emits three hisses, after which you can move the sample holder into the chamber using the loading rod, in accordance with the following procedure.
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Measuring sample surface offset
The SEM uses the sample surface offset value along with the geometry of the sample holder used, to set the allowed ranges for tilt, vertical position, etc. If not set correctly, you will risk running the sample holder into the pole-piece, from which the focused electron beam emerges. A secondary purpose of setting the offset, is that when the focus is linked to the z-coordinate, changing the working distance will automatically set the correct value of z, giving you a good starting focus.
Let's start by defining a few concepts:
- The distance form the edge of the pole-piece to the point at which the electron beam is focused is known as the working distance (WD).
- The z coordinate is the distance from the edge of the pole-piece to the edge of the sample holder
- The sample surface offset (o) is the the distance
Thus, it is clear that for the electron beam to be in focus at the sample surface, the condition WD + o = z must be true.
To measure the offset, do as follows:
- Make sure the ZFC button is enabled
- Move the sample holder closer to the pole-piece, but keep a safe distance from it; set WD to e.g. 10 or 15 mm, by clicking on WD in the micrograph datazone. The z will be adjusted automatically.
- Set the acceleration voltage to a reasonable value (say 10 kV).
- Once the stage has stopped moving, find something on the surface of your sample to focus on, and zoom in on it
- Now, slowly rotate the ring around the track-ball until you get a reasonably good focus.
- Increase magnification if appropriate, and refine the focus.
- Once you’re happy, the difference between the z and WD gives the offset: o = z - WD
- Click on the image of the sample holder in the lower-right quadrant of PC-SEM, and enter the offset value into the field at the bottom of the dialog.
Please note: The offset should always be measured with respect to the highest point on the sample or sample holder. For example, if you are using a sample holder with retraining pins that are held in place with a screw, the head of the screw is now the highest point and should be used to measure the offset.
Move to your region of interest. You have several options for stage navigation:
- Dragging and dropping with the mouse in the image area
- Right-clicking on objects in the image area or stage navigation image
- The trackball
- The x/y buttons on the stage console
If you have difficulties locating your structure, you can enable the LDF mode. It will give you a larger field of view and a larger depth of focus, at the expense of imaging resolution. Please note, the LDF mode works by switching off one of the focusing lenses, so if you have LDF mode on for extended periods, you may experience drift after it is turned off. This is due to the lens heating back as current passes through its focusing coils.
Imaging
Choice of acceleration voltage, working distance, detector, etc.
The choice of acceleration voltage depends on what you want to observe. That being said, the higher the acceleration voltage, the deeper into your sample the electron beam will penetrate, which can be useful e.g. for imaging through oxide layers, etc. On the other hand, if you want to perform surface imaging, you will typically want a low acceleration voltage. As a very general rule of thumb (with lots of exceptions), the working distance should match the acceleration voltage; the faster the electrons are travelling, the more difficult it is to focus them to a point over short distances away from the pole-piece.
The choice of detector depends on your working distance, tilt, etc:
- If you are using a high acceleration voltage, and thus a high working distance, or need to tilt your sample, use the LED. It is great for getting a quick image with minimal hassle, for overview images, etc.
- If you are using a low acceleration voltage, and a low working distance, use the UED; the lower the working distance, the more the pole-piece will shadow the LED, reducing the signal strength. The UED in invariably used in GB mode, which enhances the signal strength by accelerating secondary electrons away from the sample and towards the detector. The maximum GB bias is 2 kV. Note: when using the UED, there is no benefit in going closer than 3 mm; the sample holder will start interacting with the EM field from the pole-piece.
- Especially useful when you have samples composed of materials of contrasting atomic mass. It is typically used in conjunction with a working distance between 4 and 10 mm. Note: When using the BSD, you need a relatively slow scan speed with no frame averaging. More on that later.
- You can combine the signals from multiple detectors by selecting ADD in the detector drop down menu. For instance, you can combine the image from the BSD illustrating the material contrast of the sample, with a surface image from either the LED or UED.
Beam conditioning
Before you can image your structure, you will need to condition the electron beam. This involves
- focusing
- aligning the aperture (wobble)
- stigmating
You can focus using either
- the focus knob on the beam control console, or
- clicking-and-holding on the focus button above the imaging area (see below), then dragging the mouse.
If the imaged moved while focusing, you'll need to align the aperture. Press the wobble button; this will move the focus up and down, and cause the image to shift back and forth. Adjust the x and y knobs to minimize the wobbling – here x affects the x-movement, etc (in contrast to other systems). Once you’re satisfied, hit the wobble button again to disable it.
Fine-tune the focus. If you notice that adjusting the focus causes the image to be stretched in one direction, and then another, it is astigmated. This distortion is a result of a non-circular beam cross-section at the sample surface, and will need to be corrected using the x and y stigmators. Using mouse-focusing: move the focus back and forth from one extreme (heavily astigmated in one direction) to another ((a) and (b) in the figure below), and find the central point where the image seems equally defocused in all directions (c). Once at this point, adjust the x or y stigmator until the focus is optimized, and then repeat the procedure for the other. This should result in an improved image (d). Fine-tune the focus again, and repeat the procedure if necessary.
Scan speeds and the photo button
In the top-left of the beam-control console, you’ll find two buttons (quick, fine), that toggle the scan speed of the beam. Alternately, you can use the two buttons in the software menu. There are four speed settings available (quick1, quick2, fine1, fine2), the exact speed of which depends on your particular user settings. To change them, open the “Operation settings” dialog in the “Settings” menu (shown below).
Here, the quick1 and quick2 are set to fast scan speeds, with 16 frame averages. The lower the scan speed number, the shorter the beam dwell-time at each pixel. The scan speed number follows a logarithmic scale, as shown below.
The "Operation settings" dialog box also dictates what happens when you hit the "Freeze" button. It can either integrate for a certain number of frames depending on whether the scan speed is "Quick" or "Fine", or it can simply freeze the frame after scanning it. In the latter case, the beam will be deflected away from the sample, so carbon will not build up on the surface while the frame is frozen. For imaging at greater resolutions than the default addressing grid of 1280 x 960 pixels, the photo button must be used, and the desired resolution chosen in the "Operation settings" dialog. Alternatively, if you're fine with the default resolution, you can freeze the frame, and then press the photo button to save the image. The software will give an estimated time required to take an image, but in general, you should use the fastest scan speed which gives you an acceptable level of noise in your image, in order to avoid distortion of your image due to drift, as well as carbon contamination, etc.
Contrast and brightness
In addition to manually adjusting the contrast and brightness knobs on the beam-control console, the system also has a automatic contrast and brightness button (ACB). It generally does a reasonable job of correctly setting the contrast and brightness, but if the image is completely underexposed (black) or overexposed (white), or completely out of focus, it might fail.
Clicking the button with the triangle symbol above the LUT button will bring up a histogram of the pixel values of the current frame (shown). You can use this histogram to correctly expose your image: the brightness controls the position of the signal, the contrast controls the width of the signal, and you should aim to use the entire dynamic range of the detector, i.e. the signal should be as wide as possible while still fitting in the histogram range.
Unloading
When you're done with the SEM, turn off the beam, and retract the BSD if applicable, and then press the "Spec. Exchange" button. It will move the stage to the loading/unloading position (X,Y,R,T = 0, Z = 40 mm). Once it's done, the button will become green 
and it is safe to transfer the sample holder from the chamber to the loadlock using the loading rod as described above. Once in the loadlock, you can vent the loadlock to retrieve your sample. Leave the loadlock pumped down. Fill in the log-book, and transfer your files from the SEM PC to e.g. the ZDrive.
Tips and tricks
Stepping and sample alignment
Under the "Step Control" tab in the upper right corner of PC-SEM, you can find tools for stepping predefined distances in the X, Y directions, as well as rotation. The stepping can be defined as either a physical distance, or a percentage of the frame width. Furthermore, you can align the sample to a given axis clicking on either the horizontal or vertical ruler tools, and drawing a line along the preferred horizontal/vertical axis on the imaging area. This will rotate the stage to match your input. Furthermore, under "Step control" you can save a stage position by clicking the "Addition" button. If you later want to return to this position, you can select the position from the list, and then click on "Move".
Restoring stage and beam conditions from previously taken image
In the "Image File" tab in the lower-left corner of PC-SEM, there is a dialog box which allows you to restore beam, stage, and detector conditions, acceleration voltage, magnification etc. from a previously taken image. Press browse to select the directory in which your images (as well as the sidecar .txt files). Once the images have been loaded, right click on an given image to restore the conditions it was taken under.
Dynamic focus
When imaging samples at a high tilt, you may find that the depth of focus is not sufficient. You can compensate for this by checking the "Dynamic Focus" box in the lower middle of PC-SEM, and using the tool in the drop-menu below to set the focus in two regions of your sample surface. When scanning, the SEM will use interpolation to attempt to keep the beam focused along the sample surface.
Refocusing after inserting the BSD
At some point, you will run into a situation where you want to view your sample using the BSD, but inserting it causes your focus and beam conditions to become degraded. This is due to the fact that the BSD sits very snugly against the pole-piece, and disrupts the balance between the electrostatic and magnetic focusing. In order to restore your focus, use the "Focus Correct" slider at the bottom of the software to restore the balance between the two focusing mechanisms. Note that there is a slight hysteresis in the slider, and that the optimal focus may lie somewhere between two steps.
Probe current
The probe current can be set using the spin box below the imaging area. The lower the probe current, the smaller the spot-size of the electron beam, and thus, the smaller the features you should be able to resolve. However, decreasing the probe current comes at the cost of a reduced signal-to-noise ratio, and thus there exists a trade-off relationship between signal and resolution. You can mitigate this to an extent by decreasing the scan-speed, but then drift may become an issue. In general, use the following to guide your choice of probe current:
- ultra high resolution: 3-5 (usually only with gentle beam on)
- high resolution: 6
- standard: 8
- overview: 10
The probe current number relates logarithmically to the actual current as shown in the following graph:
Focus ranges
There are two focusing regimes in the SEM; At working distances below ~5.5 mm, the beam will be focused with both an electrostatic and magnetic lens system. Above this working distance, the electrostatic lens is switched off. There is thus a discontinuity at this switching point, and is clearly visible as a "jump" in the image, and you should thus avoid positioning your sample surface there.
Troubleshooting
- Photo button doesn't work?
- Restart PC.
- Drifty image?
- Chiller water needs to be topped up. Contact cleanroom@nbi.ku.dk.
Remote access
- TeamViewer: JEOL7800F
- LogMeIn: JEOL7800F_XFER_PC (BASEMENT)