Download Microscopy

LAB 6 – MICROSCOPY
FALL 2009
Objective
To further develop our understanding of optical imaging via microscopy. Several different types of
microscopy exist, and we will investigate several of these in this lab.
PRELAB
1) Design an experiment (using a laser as the light source) to determine the size of a single
transistor in a square array of identical transistors on a silicon wafer. You may not use any
lenses.
2) Review the ray-tracing diagram for the compound microscope of lab 3.
PART I
THE CONFOCAL MICROSCOPE
Discussion
In the third lab, we learned about the basic configuration of the compound microscope. In general,
an object you wish to image is 3-dimensional. Although you may wish to image one plane of a
specimen, out-of-plane light will cause distortion of your image and make it look blurry. The
confocal microscope uses a pinhole to remove out-of-focus rays from out-of-plane objects to
produce a higher-quality image. This is represented in the figure below. Instead of the point
source placed behind the specimen, you may also place a beam splitter in the beam path as shown
in the figure. The objective will then focus the illumination to a point.
Alternative Point
Source Location
Object plane
Re-Imaging
Lens
Point
Source
Camera
objective
Pinhole
TV
In-plane
Out-of-plane
Eyepiece
lens
Experiment
1)
Using a mag-lite, a microscope objective, a magnifying (eyepiece) lens, and a re-imaging lens, try
to get an image of the specimen (an insect) onto the camera (TV). Illuminate the specimen from
the back. The room lights should be off for this experiment.
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2)
Locate several planes of the specimen by adjusting the location of the specimen relative to the
objective lens.
3)
Position a pinhole (iris) in the focal plane as shown in the image. What does this do to the image?
Explain your observations. Be sure to compare a field-stopped image with that of the iris fully
open. You may wish to again move the specimen relative to the objective lens. You must
carefully align the iris.
4)
Replace the mag-lite with a fiber-coupled white light source pushed up near to the specimen (be
careful, this unit gets HOT!). Explain what happens.
PART II
FOURIER ANALYSIS OF A PROCESSED SILICON WAFER
Discussion
We may use Fourier optics principles to perform microscopy on a processed silicon wafer. Diffraction
principles learned earlier in the semester will be applied to the characterization of periodic structures on an
ECE 444 wafer.
Experiment
1)
Set a laser beam to reflect off of the processed side of the provided silicon wafer. Set a screen at the laser
end of the table to catch the far-field diffraction pattern. There should be a slight angle between the
incoming and reflected beams.
2)
Search around for the smallest features you can find on the wafer. Using Fourier optics principles,
determine the size of each individual element of this part of the wafer.
3)
If you look closely, you’ll see a pattern that looks like the one shown below. Use Fourier optics to
determine the width of the line outlined in the figure below. See the TA if you have trouble finding this on
the wafer.
The line is the space
between the two
rectangles
4)
Perform Fourier analysis on other interesting parts of the wafer.
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PART III
METROLOGY OF A SILICON WAFER
Discussion
We will use a Michelson interferometer to perform metrology of a silicon wafer. This is an interferometric
form of microscopy.
Experiment
1)
A Michelson interferometer is largely in place. Using a polished, process-ready silicon wafer in one of the
arms of the interferometer, form interference fringes on the TV screen.
2)
How uniform is the silicon wafer near the center? Near the very edges of the wafer? Try to estimate the
usable size of the overall silicon wafer. This can be your own judgment based on the quality of the fringe
pattern.
3)
What is the angular extent of the deformation near the edges of the wafer? Hint: You will need to measure
the minimum and maximum fringe spacings observed at one time on the TV screen. You can assume that
the TV screen displays exactly one row of the CCD camera (5.4mm across).
QUESTIONS FOR WRITE-UP
1)
Explain the confocal microscope in your own words.
2)
What is a disadvantage to imaging a small point on the large specimen plane, and how might you rectify
this? Hint: think about a scan.
3)
What was the smallest feature you were able to distinguish on the processed wafer? How would
manufacturing variation in the size of each transistor manifest itself in the far-field diffraction pattern?
4)
Assuming that the collimated beam in Part IV was exactly the same size as the CCD array, estimate the
spatial variation in the thickness of the silicon wafer near the edges. Express this in terms of a multiple of
the optical wavelength (for example, 1.7 λ). Assume the typical red HeNe wavelength, 632.8 nm.
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