Observing Features Near Vertical Trench Sidewalls: Solutions?

[phymy8]

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

Can you guys give me advice on solutions or opinion on whether I am right about the explanation 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?

 

[Valerie Park]

One solution would be to tilt the sample surface so that the feature is no longer right next to the vertical sidewall, and then use a lower NA objective lens. This should reduce the amount of light blocked by the top level and also reduce diffraction from the edge of the trench. Additionally, you could use a different type of illumination such as dark field illumination or differential interference contrast (DIC) microscopy for better contrast and resolution. You could also use a higher NA objective lens, if available, to increase the resolution and contrast. Finally, you could use a laser scanning confocal microscope to achieve higher resolution imaging with less distortion.