The effectiveness of Boron Carbo-Nitride in preventing oxidation by Hafnium Oxide to Germanium

1. The effectiveness of Boron Carbo-Nitride in preventing oxidation by Hafnium Oxide to Germanium By: Katie Jaycox Mentor: Ryan Fitzpatrick

2. Purpose: The purpose of this experiment was to find out if a layer of Boron Carbo-Nitride about 9 nanometers thick could protect a Germanium wafer from oxidation via a hafnium-oxide layer above it. Success on the Boron Carbo-Nitride’s part could be beneficial to computer technology.

3. Technique for Preparation: Samples of Germanium covered in Boron Carbo-Nitride (BCN) and topped with a layer of Hafnium Oxide were prepared using a technique called chemical vapor deposition. In this technique the layers of the germanium wafer were placed as gases in a vacuum and using tubes to make the desired gases travel into a spray placed right above the Germanium wafer in the vacuum. The chamber/ vacuum where the gases were sprayed had been heated previously to burn away impurities so that the vacuum could evacuate them from the chamber.

4. Ion Gun settings… 1.5 Nm settings: 9 Nm settings:

5. Technique for Analysis: • Once the sample was prepared it was transferred to a different chamber where a technique called X-Ray Photon Spectroscopy (XPS) was performed in order to take a depth profile of the sample and look for oxidation which would occur if the BCN layer had not protected the Germanium from the preceding layer of Hafnium-Oxide. This was done by exciting particles in the sample so that they would emit an electron sized photon and their energy could be measured by the XPS machine. An ion gun was used in order to execute a technique called sputtering where layers of the sample were stripped away in order to allow for further study of layers below the surface (i.e.. BCN and Germanium layers). Argon gas was used to aide in the sputtering process because it is non-reactive. A 3X3 surface was analyzed.

6. How it works: • XPS technique measures the Kinetic energy of a photon (hv) leaving the sample. Kinetic energy is associated with a specific atomic orbital and is useful in detecting and identifying what the chemical states of elements. The following equation is necessary in understanding XPS where Ke is kinetic energy and Be is binding energy: Ke = hv – Be • It is important to realize that binding energy for elements is unique and shifts in that binding energy show that elements may have bonded in the sample.

7. What was looked for: • Shifts in key elements, such as Germanium and Oxygen were looked at to see if there were any shifts in the binding energy between the two. Carbon was looked at as an indicator of charging, which can also effect shifts, because it is consistently found to be at 285. It was also used as a marker for how far into the experiment we had sputtered.

8. The Results: These are results for a depth profiling that measures the amount of Carbon in a sample with an 9 nm thick layer of BCN. There is little to no carbon found in the beginning Hafnium layers. However when the BCN layer is reached there is a large spike in Carbon that tapers down after the Germanium layer is reached. Carbon was used as an indicator of possible charging which was important in determining the nature of any shifts in our graphs.

9. This graph shows how much oxygen was in the layers measured with a 9 nm film of BCN to protect the germanium wafer. As the graph shows, Oxygen starts off strong and as the BCN does its job, it becomes less common. This makes it clear that the BCN layer is working and protecting the Germanium from oxidation.

10. This graph shows the amount of Germanium present in the sample with a 9 nm layer of BCN and a layer of Hafnium-Oxide covering it. If the Germanium had bonded with oxygen and the BCN layer had not worked there would have been a shift in the Germanium graph. This was not found so it was observed that a layer of Boron Carbo-nitride can be effective in preventing oxidation of Germanium.

11. What if we…. Once it was clear that our last sample was effective in preventing oxidation of Germanium the next step was to prove that the reason for this prevention of oxidation was because of this thick 9 nm BCN layer. Another sample with a layer of BCN only 1.5 nm thick was prepared. The rest of the sample was identical to the first. An XPS depth profile was then run on the sample with the hypothesis that such a thin (and often inconsistent) layer of BCN would be unsuccessful in protecting Germanium from oxidation by Hafnium-oxide.

12. The Results: This graph is a new Carbon graph for the thinner layer of BCN. Like before the carbon was used as an indicator of charge found as well as where we were in the thin film sample. In this sample there was a large amount of charging that took place and graphs were shifted to account for such problems.

13. This graph shows oxygen and it’s strength through out the sample. After the first 600 seconds of sputtering the sample shows steadily decreasing amounts of Oxygen until around 1140 seconds when the intensity of the oxygen found levels off. However there is more oxygen found in this sample, because of the thinner BCN layer. Charging was a problem for this sample so some lines have been offset by up to 7 points to account for this.

14. This graph shows Germanium and it’s relative intensity as well as any shifts it may have experienced from what is believed to be a bonding with oxygen. Only shifts pertaining to charging found in the other elements have altered this graph. No Germanium showed up in the sample until the 1020 second mark it is a bit hard to determine whether significant shifting occurred.

15. So…Did Oxidation occur or not? According to figures published in the Handbook of X-ray Photoelectron Spectroscopy, when Germanium oxidizes there is about a 3 to 4 Binding energy or eV shift. The shift seen in this sample was closer to 2 eV. The height of the curves measured should have been seen at approximately 126 eV and 122eV, proving that charging of the sample was about 6 or 7 eV. Charging was only assumed to be 5 eV in the sample based off of Carbon shifts. Because of this increased eV shift that might or might not be related to charging and a definite 2 eV shift during the 1140 second sputter it is assumed that Oxidation of the Germanium wafer did occur when only an irregular 1.5 nm layer of BCN was applied to the sample.

16. Percentages!!! The following are percentages of sample composition at various stages of the 9 nm sample of BCN deposits on the Germanium wafer. The purpose of this is to help analyze what chemicals were available for bonding in what quantities.

18. Conclusions: From this experiment it has become clear that BCN is an effective protector for Germanium from oxidation via Hafnium-oxide. However, it is also clear that a certain thickness is necessary in order to make the BCN layer effective against oxidation. Furthermore the layer must be consistent and cover the entire sample of Germanium completely. Currently tests are being run to find out the minimum thickness of BCN necessary to protect Germanium from oxidation by the Hafnium-oxide. Tests are being run at 4nm and 5nm at the moment and results are still being analyzed in hopes of finding the most efficient layer of BCN possible. Additionally tests are being run with silicon wafers to prove the superiority of Germanium and steps are being taken to decrease charging in the XPS chamber and insure more accurate results.