UCSB NanoFab Microscope Training
We have numerous optical microscopes in our lab, each of which is a different model or manufacturer.
However, almost all of these microscopes share common features, even if the knob to enable the feature is in a different location. This "training" is intended to give the user an overview of these features, which are very useful for general observations, measurements, inspections after etching/other processing steps, distinguishing residues vs. delamination, to name a few.
The main danger on a microscope is crashing the objective into your sample!
ALWAYS begin at low magnification where the distance between the sample and objective is largest. Then gradually work your way up in magnification.
The focus knobs on all of our microscopes turn towards the user to increase separation between the sample stage and microscope objectives. Thus, when focusing on a sample, always start by turning the knob in this "safe" direction to prevent crashing the sample into an objective. If the image gets blurrier, then turn the knob the other way, and confirm that the sample is becoming less blurry.
There is usually a Coarse knob - the outer part of the knob, and a Fine knob - the inner part of the knob. Be very careful when turning the Coarse knob, and watch the distance between the sample stage & objective while turning to prevent crashing!
A useful trick before focusing is to do the following:
- Observe the microscope illumination spot on the stage (looking directly at the sample/stage, not through the eyepieces)
- Center the illumination spot on the edge of the sample, so there is a large height difference between the sample edge and the stage.
- Then, while looking through the eyepieces, turn the Coarse knob towards you and observe whether the Edge of the sample becomes more or less blurry. (You can also look at the sample/stage directly to make sure nothing is going to crash.) You should be able to see the hard edge of the sample even when it is very blurry, and focus on the top-surface using this edge.
- Then drive towards the feature on the sample surface you need to image, and the focus should be pretty close (Fine-focus only).
Depth Measurements using focus
Many microscopes have gradations marked on the fine-focus knob, indicating microns of movement of the sample stage. Using these marks, one can focus on different surfaces and use the gradations to estimate how many microns of vertical separation is present between the two surfaces. If the microscope has motorized focus, the control software may display the focus height, which can then be used to measure the depth between different focus heights.
Focusing on a sample with no features (Focus Stop)
If your sample has no features or dust to focus on, you can still make sure you are focused on the sample surface using an adjustable aperture that projects a shadow on the sample, called a “Focus Stop” (sometimes marked as F.S.). When the edge of this shadow is sharp, the surface is in focus.
Preventing a Crash
Changing objectives is the easiest way to crash into a sample, especially with motorized objective turrets. A few tips can easily prevent this from happening.
ALWAYS begin at low magnification where the working distance between the sample and objective is largest and the depth of focus is also largest.
The microscope objectives on all our microscopes are designed such that the sample is still approximately in focus when an objective is changed. Thus, one should only change objectives when there is some feature that is in focus on the sample. If you don't see anything at all, either because there are no features or because the sample is out of focus, find something to focus on before changing objectives! The “focus stop” iris (above), pieces of dirt or dust are particularly useful for this purpose, and the wafer edges often have such dirt from tweezers etc. The Field Iris / Focus Stop / Shadow for focusing mentioned above would also help.
Working Distance or WD of an objective is the distance (usually in millimeters) from the objective lens to the sample, for the sample to be in focus.
Generally, the lower the magnification, the longer the working distance.
For example, a 5x objective may have a long working distance (distance to sample) of 10mm, while the 100x may be very close at 0.2mm.
Thus it is very important that initial focus is found with a low magnification (and corresponding long working distance). Also the focal depth (range over which sample stays in focus) is wider for low mag, making it much easier to find the focal height, so you should always switch to the lowest mag. before starting to image your sample. (And switch back to lowest mag. when done, out of courtesy for the next user.)
Some objectives may actually have "WD" printed on them. For example, "WD 1.0" means that the sample will be in focus when it is approx. 1.0mm away from the objective. This is very helpful when estimating, by eye, whether you need to move the sample stage up or down to get into focus. If the objective says "WD 0.2" that means it has to be so close that you really should just start at low mag. and work your way up, keeping the sample in focus at each successive magnification!
This page is in-progress - More to come! • DIC/Nomarksi • Bright/Dark Field • Digital Cameras and measurements • Epi/Dia illum. • Filters: Green, ND