Heterostructure thickness

Heterostructure thickness measurement is an important QC step after deposition, alongside or instead of in-situ control. A heterostructure filmstack typically consists of repeated alternating low- and high-index layers (for example Al oxide and Zn oxide), the pair repeated tens or hundreds of times (typically 60× to 200×) - used for filters, anti-reflection coatings, LEDs and semiconductor lasers. Often optical constants are measured separately and only thickness is measured in the structure, but that isn't always possible because the materials can differ. Both thickness and optical constants can be measured by assuming identical properties for all layers of the same material. Linked layers reduce the number of independent layers; if the fit is imperfect, one material's optical constants can be added as measured parameters and then fixed while individual thicknesses flow.

How to measure a heterostructure

• Heterostructures often include thin metal-oxide layers, making MProbe 20 UVVis a good fit.
• Use linked layers to reduce the number of independent layer parameters in the first analysis step.
• If the fit is not good, add the optical constants of one material to the measured parameters.
• If the fit improves but is still imperfect, fix the latest optical-constant result and let individual thicknesses vary independently.
• Use a separate thin-layer measurement to check that the optical constants determined from the heterostructure measurement are correct.

Basic specification

Measured with: MProbe 20 UVVis (MProbe UVVis).