Specific Process Knowledge/Thin film deposition/Deposition of Zinc: Difference between revisions
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==Zinc deposition== | ==Zinc deposition== | ||
Zinc is tricky to deposit in a multiuser vacuum system because it has a relatively high vapor pressure at low temperatures, which means that once Zn is inside it easily re-evaporates and contaminates films of other materials. Additionally, Zn | Zinc is tricky to deposit in a multiuser vacuum system because it has a relatively high vapor pressure at low temperatures, which means that once Zn is inside it easily re-evaporates and contaminates films of other materials. Additionally, we are not keen to deposit Zn in systems with cryo pumps, as it may clog the pumping system. A final consideration is that ZnO forms on the surface of Zn and needs a higher temperature to evaporate than the Zn itself, so once the ZnO is gone, the Zn within may evaporate rapidly, making it hard to control the deposition rate. For this reason it is better to use sputtering than evaporation to deposit Zn. | ||
'''The best solution is therefore a sputter system that is pumped by a turbo pump in a chamber where Zn contamination is not a problem for other users.''' | '''The best solution is therefore a sputter system that is pumped by a turbo pump in a chamber where Zn contamination is not a problem for other users.''' | ||
Unfortunately, at Nanolab the only physical vapor deposition system with no cryo pump is the Lesker Thermal Evaporator, so this is the only system | Unfortunately, at Nanolab the only physical vapor deposition system with no cryo pump is the Lesker Thermal Evaporator, so this is the only system we have considered for Zn evaporation. We tested it and found that Zn coats every surface in the chamber and re-evaporates during subsequent evaporation of other materials, contaminating the growing film. After a thorough chamber cleaning, we no longer detect Zn by XPS in Al or Ag films made in the evaporator. However, this kind of cleaning takes a full working day and while the chamber is used for Zn no other materials can be deposited without contamination. '''We are therefore hesitant to use the evaporator for Zn again. If you wish to deposit Zn, please first explore other options.''' | ||
Note that Zn will not adhere to | Note that Zn will not adhere to a Si wafer (probably due to SiO2 on the surface). The same is true for a Ti-coated wafer. We were able to deposit Zn on a Ti/Au coated dummy wafer. A user was able to deposit on InGaP and InP. | ||
===XPS of Al films to detect Zn=== | ===XPS of Al films to detect Zn=== | ||
''Measurements made in June/July 2018.'' | ''Measurements made in June/July 2018.'' | ||
XPS was done at Nanolab with 50 eV and 50 ms dwell per scan. The number of scans per element varied as shown below in the figure captions. We can clearly see that the Zn amount in the Al film decreased and became indetectable after cleaning the chamber. Of course we cannot guarantee that there is no Zn contamination at all, since there is a detection limit for the XPS even with many scans per level. | XPS was done at Nanolab with the XPS K-alpha using 50 eV and 50 ms dwell per scan. The number of scans per element varied as shown below in the figure captions. We can clearly see that the Zn amount in the Al film decreased and became indetectable after cleaning the chamber. Of course we cannot guarantee that there is no Zn contamination at all, since there is a detection limit for the XPS even with many scans per level. | ||
[[File:Al w Zn from 20-06-2018.png|400px|left|thumb|XPS of Al film made before chamber was cleaned after Zn deposition. 10 scans per level. Zn peaks are obvious. The Zn content was up to 10 % in some layers.]] | [[File:Al w Zn from 20-06-2018.png|400px|left|thumb|XPS of Al film made before chamber was cleaned after Zn deposition. 10 scans per level. Zn peaks are obvious. The Zn content was up to 10 % in some layers.]] | ||
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*It is better to use a crucible rather than a W boat to heat small Zn pellets, as the pellets sometimes jumped out of the boat when they got hot. | *It is better to use a crucible rather than a W boat to heat small Zn pellets, as the pellets sometimes jumped out of the boat when they got hot. | ||
*Do the deposition in manual mode as the power level for evaporation was not consistent and the rate was not constant even at a constant power level. | *Do the deposition in manual mode as the power level for evaporation was not consistent and the rate was not constant even at a constant power level. | ||
*We needed about 12-13 % power on the Lesker Thermal evaporator to get Zn evaporation. It is best to soak for a long time(e.g., ramping and soaking for more than 5-8 minutes). | *We needed about 12-13 % power on the Lesker Nano36 Thermal evaporator to get Zn evaporation. It is best to soak for a long time(e.g., ramping and soaking for more than 5-8 minutes). | ||
*The thickness /rate measurement may not have been accurate as the crystal monitor looked irregularly coated after the deposition. | *The thickness /rate measurement may not have been accurate as the crystal monitor looked irregularly coated after the deposition. | ||