Solving Optical Microscopy Issues with Vertical Sidewalls of Deep Trenches

[phymy8]

Problem with opticalmicroscopy of features near vertical sidewall of very deep trench

Hi,

I posted my question here:

physicsforums.com/showthread.php?t=612780

but there may be more specialists here.

Essentially I have a feature right next to a very deep vertical sidewall of the deep trench. The feature is technically geometrically within line of sight.

The image cannot be focused. I believe it is related to the top part of the trench blocking off half of the light since it is being focused in from the high NA of the objective. Theres also possibly diffraction of light since half the focused light being blocked encounters an edge. Is this it?

How do you overcome this issue? Is there a way to collimate the light at the top of the trench so it directs light uniformly straight down from the top of the trench to the intended feature so light reflects back up to the collimator and re-expanded back to the rest of the microscope? Is there something like dark field imaging? Perhaps cover the other half of the top illumination not contributing to observation?

 

[BERNIE649]

A stereo microscope may not be the best tool for inspecting a narrow trench. I suggest a compound microscope with the illumination through the lens. You don't mention the dimensions, but in my experience with deep features in the micrometer or tens of micrometers range, the compound microscope allows you to move the focus and observe deep details at various levels.

 

[phymy8]

Sorry I did mean compound microscope but top down illumination and not transmission illumination. I may have incorrectly described it as a stereomicroscope.

The sample is not optically transparent so can't use transmission illumination.

I can adjust focus. My problem is I can't get it to focus because of the tall sidewalls.

The sidewall height is close to 1mm while the feature is a couple of microns right by the sidewalls. The NA is around 0.4 so the half angle is quite big.