Sunday, December 9, 2018

Adding metalurgical capability to a biological microscope - part 1

Biological microscopes among a few other things, are characterized by the fact that rely on transmitted light rather than reflected light, to produce an image of the specimen.This is adequate for most biological needs because the specimen can either be made thin enough for light to go through, or the medium and/or the specimen itself is relatively transparent to light.

However there are other applications in microscopy where it cannot be assumed that the specimen is transparent or able to be ground to a thickness small enough for light to traverse it. Some materials are opaque even at thicknesses of few microns.

The only solution in these cases is to rely on reflected light. For this to be possible, a different illumination system is required on a microscope. This type of illumination is commonly designated of epi-illumination (as opposed to trans-illumination used for biological observations). This is where a device known as vertical illuminator comes into play (picture obtained from a Olympus BHM manual):

This device provides light to the specimen by guiding a beam of light from a separate source (a tungsten filament, xenon bulb, or LED) through optics (see the principle of Köhler illumination) and into the microscope tube towards the objective. So with this approach, light travels through the objective in two different directions: the light that goes down to illuminate the specimen, and the reflected light that travels upwards into the ocular for finally being viewed by the human eye or by a camera.

As both transmitted and reflected light are mostly perpendicular to the specimen, this technique provides a very homogenous illumination of the specimen, without shadows, similar to the transmitted light microscopy.

This is ideal for example in the semiconductor industry, where very clear and consistent images of the specimens are required.

As I have documented in previous posts in this blog, I had acquired a Olympus KHC biological microscope a couple of years ago, for which I had to do some reconditioning work and an adaptation to operate with LED trans-illumination.

More recently I spent some time looking for ebay auctions in order to find a vertical illuminator suitable for this type of microscope. Waited a while until I could find a proper deal. After over a month between shipping and customs, finally it arrived.

As I expected it did not include some of the elements that constitute the complete kit, such as the light housing and filters (polarizer, ND filters, etc).

The unit came somewhat dirty, so that it required comprehensive cleaning until it could be used properly.

One of the things I first looked into were the defocusing optics near the light input. These had a considerable amount of dust, and as such decided to disassemble the part and clean the individual lenses:

Essentially there were two converging lenses separated by a spacer, and a clear glass separated by another shorter separator. From closer analysis of the clear glass, it appeared that there were traces of a coating in the edge of its surface. It appeared that this coating wore off over time, eventually due to the heating.

Also removed the tube that holds the polarizer and filters (by undoing a total of six screws -  three between the tube and an intermediate part, and another three between the later and the main vertical illuminator assembly):

The main assembly was also quite dusty inside. One thing that at a first glance worried me was the fact that the black tube that protruded from the assembly would be too long for this microscope:

Fortunately it could be unscrewed:

Therefore allowing the vertical illuminator to be assembled into the microscope. One outstanding concern would be the impact of not having this element (e.g. reflections inside the tube), but more on that later..

In order to clean the main assembly, the only option was to tear it down completely, and clean its individual mechanical components and optics. Started by the bottom, removing the threaded ring that attaches to the microscope, and the black circular part with the orifices for the light path.

With the removal of the last part, the interior becomes accessible:

The optical assembly could not be removed without first undoing the darkfield / lightfield control rod:

The optical assembly is made up of a half-mirror (for lightfield) next to a regular mirror with a clear opening in the center:

The darkfield section also includes a clear glass with a light blocker at the center. The brightfield section has a convergent lens instead:

The entire assembly is supported by a rail, through which it slides between darkfield and brightfield, as the user pulls or pushes the rod:

On the other side of the main assembly sits the ring for controlling the analyser angle (as opposed to the polarizer, which is placed in the previously described filter assembly). Also disassembled this ring, in order to clean and re-lubricate it:

It is composed of a inner black ring with the control rod, an outer silver ring that attaches to the main assembly chassis, and an outer ring which is the dial for setting the analyser angle.

After all the cleaning and lubrication was completed, assembled the device, and mounted it into the microscope. It was now time to start doing some test observations:

The only problem still, was the fact that I did not have the light housing that would mate to this vertical illuminator. Improvising would be required...

As such, grabbed the 100 Watt LED lantern (a previous DIY project) and put it close to the light input:

This allowed for doing some crude observations, for example of this Pentium processor die:

Using the 100x MPlan objective (0.90 numerical aperture), it is possible to visualize structures at roughly 1000x total magnification, revealing transistor scale details such as the traces and vias:

Or for example the recording pits on a pressed CD-ROM:

Or the laser-etched labels in the centermost area of the CD:

The next step is to continue the construction of the LED light housing for this vertical illuminator. This will provide a more convenient and consistent light source. It is still work in progress, especially from the mechanical point of view. More details to come:

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