Problem w/ optical stero-microscopy of features near vertical sidewall of deep trench

phymy8

Can you guys give me advice on solutions or opinion on whether I am right about the explaination of the issue behind my problem.

Problem:

Im having trouble using optical microscopy (top down illumination) to observe features that are very deep down a trench and right near the vertical straight sidewall of that trench.

The feature I want to observe is situated right next to the very deep vertical sidewall of the trench. When observed and attempt at focus, the feature becomes very blurry even though I have set it to be in focus

Reasoning behind problem?:

The last stage in the optics is focus by the high NA objective lens. When I am viewing the intended feature in center of observation, half the light is blocked by the top highest level surface of the trench, and the other half is allowed to illuminate down to the trench and reflect back to the objective.

Essentially there is obstruction of the optical paths, since the beam is focused and not a collimated beam. Theres also possibly diffraction of the light from the edge on the top level of the trench.

Solutions?:

Can you provide some solutions to the problem so I can observe the feature next to the deep sidewall?

What is the proper term for this problem?

Is there a practice to somehow collimate the light right out of the microscope near the focal point to interrogate the measured specimen so that you can observe deep features with focus?

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