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Thread: Physics of Thermal Evaporation

Created on: 10/20/09 12:00 AM

Replies: 4

ndrawlinson





Joined: 10/20/09

Posts: 1

Physics of Thermal Evaporation
10/20/09 7:01 PM

I would like some help understanding the parameters that determine the grain size of an thermally evaporated Al film deposited on a glass substrate. For example, how does the chamber pressure, deposition rate, source to substrate distance, source temperature, and substrate surface temperature affect the grain size? Which of these parameters are most dominant in determining the grain size? I need a very compact film with a small grain size. I was under the impression that at a fixed distance, a low temperature deposition at a slow rate would give the most compact film (smallest grains). Also, by decreasing the source to substrate distance you would be able to deposit at a lower temperature, thus decreasing the grains. However, my films look pretty bad right now, and I need to change some parameters and would appreciate some guidance. Any help would be great!

Thank you.

ndrawlinson

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rswisher





Joined: 10/30/09

Posts: 1

RE: Physics of Thermal Evaporation
10/30/09 5:56 PM

Hello,
I would suggest that if you are making the coating in a coating machine of average vacuum levels that the following practices may help you.
Heat the glass to around 100 C to drive off water, methane and other stuff adsorbed onto the surface.
If you are doing this in a bell jar, use a shutter to cover the substrate.
Start your Al evaporator (a filament, I am guessing) and as the rate becomes large, quickly open the shutter, so that only high rate of arrival of Al is allowed. Al is a getter pump as well as a coating. The slower you shoot in average vacuums, the more oxides you form in your coating. Usually fast is best.
Good Luck


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wolfgangd





Joined: 12/09/09

Posts: 1

RE: Physics of Thermal Evaporation
12/09/09 9:46 AM

Hi,

in addition to ****'s comments I would also suggest to look at the substrate itself. The grain size of a thin layer is - to a large degree - determined by number and density of the nucleation sites on the substrates. The more sites you have the finer the grain as you start growing more crystals. You can influence this by either special cleaning, ever so slight surface etching or by plasma treatment.


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drippere





Joined: 02/05/10

Posts: 1

RE: Physics of Thermal Evaporation
02/05/10 4:28 PM

There are many factors which will have an effect on the grain size of Aluminum films. You did not indicate why you require this condition. The application will possibly change some of or add to the following:
My experiences in making high Ultraviolet reflecting, low scatter Aluminum films have shown the following to be important (not ranked in any order):
Very low chamber pressure (the greater the throw distance, the lower the pressure), combined with low partial pressures of Oxygen and water vapor, no air leaks or oil backstreaming.
Room temperature substrates. They can be heated, but must be allowed to cool back down.
Use of a resistance source and high purity aluminum (both fresh each deposition). Use enough Al so that source doesn't run dry during deposition.
Extremely high deposition rate (like in the previous reply), and a controlled end thickness(the thicker the film, the rougher the surface).
Use of polished substrates (not float glass), which have not been cleaned using etchants, ultrasonics or vapor drying techniques. The substrate final cleaning should be immediately prior to loading into the chamber and pumping it out.
If a blow-off step is desired, used ultra clean filtered dry gas, not compressed plant air, and do the blow off in a clean environment (like a clean bench).


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vacdepman





Joined: 06/05/10

Posts: 11

RE: Physics of Thermal Evaporation
06/05/10 9:54 AM

If you just use thermal evaporation alone the best way to get a small grain size is to rapidly deposit on a very cold surface. I have used a liquid nitrogen "cold finger" for example. Use a shutter to establish the evaporation rate before you begin deposition. Keep the deposition flux as normal to the surface as possible - off-normal deposition gives a more columnar film. If you are using sputter deposition the best way to get small grains is by cooling the substrate and energetic bombardment during deposition. The energetic bombardment can come from high energy neutrals reflected from the sputtering cathode (need low sputtering pressure)or a negative bias on the substrate to attract positive ions (either a self-bias or an applied bias [RF bias on glass]).


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