A conoscopic scatterometer is based on a conoscope lens, so the best place to begin (if you haven’t already done so) is to read our page on how conoscope lenses work. Once you’ve mastered that material, it is a simple step to understand how conoscopic scatterometers work. Let’s begin with a picture:
There are very few differences between this image and the picture of a conoscope lens. Only a light source and a beam splitter have been added. Let’s discuss each in turn.
The beamsplitter serves to reflect light from the light source in the direction of the sample. Given that the beam diameter in the region of the beamsplitter can easily be over 50 mm, a pellicle beamsplitter is the best choice. A typical beamsplitter transmits half of the incident light and reflects the other 50%. For our application, this means that half of the light from the light source is lost immediately because it continues to proceed in the downward direction. Mechanically, this means that a light dump must be installed in the housing of the conoscope lens. After the light from the light source reflects or scatters from the sample, it must again hit the beamsplitter. Only the transmitted portion is retained this time, so at most 25% of the original light from the light source can get to the image sensor. Thankfully, light sources are sufficiently bright and image sensors sufficiently sensitive that the system still works well.
Much more can be said about the light source. Let’s begin with its size and position. Ideally, the light source would be positioned on the intermediate image plane. We have experience with one conoscopic scatterometer which did exactly that. The result was that there was a black region in every image where the light source obscured the light coming back from the source. To avoid this, the light source must be located outside of the beam of light scattered from the sample. This is accomplished in our schematic by the use of the beamsplitter and the Light Source Lenses.
The Light Source Lenses collect light from the light source and create an image of the source on the intermediate image plane. (See the Source Image in the picture.) This image is typically larger than the light source itself because we want to collect as much light from the source as possible (requiring a low input F/#) but we also want to roughly match the F/# of the Front End Lenses so we don’t get a lot of light scattering into the lens from the area surrounding the sample. In the illustration above, the cone entering the Light Source Lenses is roughly F/2, while the exiting cone is roughly F/12. This means that the image of the light source is 6X the diameter of the source itself. The size of this image can limit the angular resolution of the system, so it is common practice to use laser sources which are only a few microns across. If white light illumination is necessary, a small LED can be used and only a small cone of light from it is collected to avoid loss of resolution.
Would measuring the BRDF or BTDF of a sample in less than a second help you get to market faster? Call us at (651)315-8249 for a consultation or use our Contact Us page to learn how Eckhardt Optics LLC conoscopic scatterometers can help you.