Could somebody explain to me the purpose of the dissimilar materials function in part 3 of a step height measurement? Do I need to do this when measuring a step between a semiconductor and metal or dielectric surface? E.g. GaAs and glass or GaAs and Au. I noticed that there was an option for Si, should I use this?
The dissimilar materials function is used to compensate for the fact that a measured step height between two surfaces may appear to be slightly different from the actual step height if the top and bottom of the step are not the same material.
For images obtained using an optical profiler, the apparent surface height of any portion of a sample changes slightly, depending on the material present on that part of the surface. This happens because the phase change of light reflecting from a surface depends on the optical properties of the material.
A common type of step edge is a metal layer deposited on a dielectric resulting in metal adjacent to dielectric. If you use the dissimilar materials feature, you can specify which metal is present, resulting in a change to the reported step height by up to 40 nm, depending on the material.
You should use the dissimilar materials feature whenever measuring a step between two different materials when one of the materials is not a dielectric (has a non-zero optical extinction coefficient or k). In the case of Si, the difference in measured step height when selecting Si vs selecting Dielectric as the material is only about 2 nm, so it will only be significant for very small steps. GaAs and other semiconductors will differ slightly from Si, but will still change the measured step height by only a few nm. We can add GaAs to the list, but this will require some calibration measurements - we expect that the step height difference for GaAs vs dielectric will be slightly more than the difference for Si vs dielectric.
Something very interesting happens when a very thin metal film (<30 nm) deposited on a dielectric is measured with WLI. For some cases, the scan result is inverted and the metal appears lower than it actually is. When light reflects off a metal, the phase changes. For example, an Au surface appears to be about 35 nm lower than the glass (dielectric) surface. Now imagine an Au step of only 20 nm: 20 – 35 = - 15 nm, the Au actually appears to be below the glass surface! In order to correct it, we will need to apply the dissimilar materials function. The final result may be a negative step height, but the magnitude will be accurate