I am doing sputtering of boron oxide thin film on a float zone Si-wafer. The sputtered layer is in the order of few nanometers thick. I am trying to measure its thickness using the ellipsometer device available (SE800 PV). I am pretty new to ellipsometery measurements. I read that Tauc-Lorentz TL model is suitable for transparent layers. Using TL Model , I obtain a good looking fit (with mean square error of 0.3) of a thickness in the predicted range. The problem is that I obtain a different band gab value (Eg = 0.0421 ev) from that in literature (Eg = 3.4 - 3.95 ev). Furthermore, the obtained peak's amplitude A value is negative instead of lying between 10-200 ev. When I fixed the Eg to 3.6 ev, the resulted fit has mean square error of 0.4 and not good looking like before and thickness is out of the predicted range. What should I do? What I do wrong?
Thanks in advance!!
Are you using model boron oxide/SiO2/Si ?
Try to vary thickness of film and put a range for band gap 2-4 instead of fixing it.
Read this article:
Buc, D., Bello, I., Caplovicova, M., Mikula, M., Kovac, J., Hotovy, I., Chong, Y.M. and Siu, G.G., 2007. Analysis of magnetron sputtered boron oxide films. Thin Solid Films, 515(24), pp.8723-8727.
Such a thin film will require doing measurements which capture as much information as possible from the sample. You should increase averaging revolutions of the analyzer, take measurements over different angles of incidence to see if the pseudo dielectric constants show any variation or not. Then when you get proper data, you proceed to develop a model for the same. Cauchy equation is used to model for the transparent layers. It gives you flexibility as you can model the dispersion behaviour with less number of parameters. Amplitude, broadening, energy of the oscillator cannot be negative and represent nonphysical case. Always keep in mind that ellipsometry involves taking proper measurements and then building an appropriate mode. Also, there should be a minimum correlation between the parameters involved in the model. Only practice will help you learn the tricks of the trade.
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