ANALYSIS OF ANOMALOUS FILM GROWTH WHEN YTTRIUM OXIDE IS EXPOSED TO VACUUM- ULTRAVIOLET LIGHT Presenter: Devon R. Mortensen Contributors: Thomas McConkie David D. Allred

The Setup  Group members took some samples to Berkeley for measurements using the Advanced Light Source.  When they got back we needed to measure the thickness of one of the Y 2 O 3 film samples. Before we could do this we needed to put it in the plasma cleaner.  Why? Because there is a sort of “gunk” composed mainly of hydrocarbons that permeates all space and likes to deposit itself on samples.  Problem: the plasma cleaner was broken.  Solution: Let’s use the excimer (VUV) lamp instead. This lamp generates photons at 7.2eV, which are so energetic that they only go about 1 cm in air before they are absorbed.

 After approximately 5 minutes under the VUV lamp, the sample became visibly thicker.  Use ellipsometry measurements to determine if this is true.  Before cleaning: nm Enter the Mystery Thick.2 =24.733±0.139 nm

Ellipsometry Measurements  After 5 min VUV: nm  As you can see this is quite a significant change. Thick.2 =31.061± nm nm nm

 Big question is: What’s going on here?  First thought: the yttrium is not fully oxidized. Search for the Answer If the yttrium were complete unoxidized expected film growth would be 3.34 nm Actually film growth was 6.12 nm

Second Theory  Is it possible that the VUV lamp is actually depositing material onto the sample?  Not possible that more Y 2 O 3 is being added to film.  Perhaps it is knocking molecules off of the support base and these molecules are finding their way to the sample.  Subject a blank silicon substrate to same VUV treatment and look for film deposition.

 Before VUV:  After 5 min VUV: MSE=5.786 Thick.1 =2.074± nm MSE=6.713 Thick.1 =2.221± nm Blank Silicon Wafer

 After 10 min VUV:  After 25 min VUV: MSE=9.096 Thick.1 =1.797±0.012nm MSE=8.125 Thick.1=1.863±0.0112nm Blank Silicon Wafer

First Set of Conclusions  No apparent change in thickness of the silicon witness.  It doesn’t seem likely that the VUV lamp is depositing material onto our film.  Only other option is that what is already on the film is somehow being altered.  It is possible that the Y 2 O 3 is catalyzing the oxidation of the silicon wafer underneath.  Let’s take a closer look at what is happening to the optical constants:

Optical Constants  At 2.4 eV (516 nm light) Y 2 O 3 has an index of refraction of  Silicon dioxide has an index of  The formation of SiO 2 should lower the total index of the material.  The data was fit using a Cauchy model, allowing the parameters to vary to fit for optical constants.

Optical Constants

Effects of the Furnace  It seems that the possibility of silicon oxidation is a viable one.  If the silicon has indeed become oxidized the thickness should not go down when placed in a furnace.  SiO 2 is quite stable and silicon wants to be oxidized anyway.

 Before furnace:  After 2 hours in furnace: MSE=17.41 Thick.2 =16.938±0.016 nm MSE=58.82 Thick.2 = ±0.168 nm Over an 80% decrease in thickness

 Maybe the sample is simply being puffed up.  Boiling water  Ozonide The Mystery Continues

 How would we be able to tell?  TEM imaging might be able to reveal what is happening to the structure of our sample.  Need to reconcile one important piece of data.  A piece of the same sample, when first put in the plasma cleaner, exhibits none of these effects.