Change request

[Patrick Kearney]

David, I'm using 5.02, so far so good. 

 

I was using IMD to take a fit from a ML at 13.5nm and compare the calculated XRR scan at 1.5418A.  I noticed my behavior near the total external reflection shelf was wrong and thought I probably had some reaction in my top layer that was changing the top surface index of refraction.  I thought, no problem, I'll just vary the index of refraction (n,k) of the layer to try and see what it was.  (I'd always wanted to try it anyway, but never had.) 

 

To my suprise, I could only vary n(&k) by percentages of the value in the file.  That works well for k, but for variation in the real part, I'd prefer to change delta (1-n) by percentages.  An example will help communicate this better. 

 

I have a top layer with an n of .999.  Say I wanted to vary it from .998 to 1.0.  I need to set the index percentage to 99.899%-100.1001% to get that range.  If the percentages worked on delta then I'd need to use 0%-200%.  I find 0-200% a lot more intuitive than 99.899-100.1001%!  And the values I need to enter don't depend on the value of n!

 

I guess being an x-ray head, I always assumed it worked on delta, not n.  Boy was I suprised.  At first I thought I'd just have to use excel to calculate the n ranges.  Then I thought, IMD is used primarily for x-rays so the delta solution is more useful for those that use the code. 

 

Anyway, my request is to allow the user to change delta by percentages either instead of or in addition to the ability to change n by percentages.  I actually can't think of a situation in the x-ray where changing n would be better.  But you and the other users may have a different opinion. 

 

Some other options would be:

Just enter the min and max n value, but then how would you handle the wavelength variation in cases where energy is not constant. 

Using the percentage change in delta is just like changing the density of the film when calculating from f1,f2.  I usually use the included .nk files so there is no density.  If you added a density option to the .nk reading routines where "density" means density relative to the film that was measured. Then you could just have that density as a varible. 

My guess is that my request is the easiest way to go, but you know the code. 

What do you think?

Pat

[David Windt]

Hi Pat,

You actually can specify (1-n) as a fitting parameter, but not as an independent variable. It's been a while since I worked on that part of the code, so I honestly don't remember if there's some reason why I didn't allow (1-n) to be an independent variable. Anyway, assuming for the moment that it's do-able, I'll add it to the list of new features for a future release. In the meantime, maybe you can accomplish what you want by using atomic scattering factors instead of nk files, and adjust density?

David