There are several reasons that you could get a milky aluminium thin film.

1. The evaporator may not clean. There are some other coating on the surrounding of aluminium evaporation source causing across contamination.

2. The degasing from the back side of your substrates ;

3. Aluminium source itself could cause a problem too. There are an alternative source which consume low current to get aluminium evaporation using a thermal evaporator. It is called as an alumina coated tungsten boat. We found it was very effective to overcome some problems.

Hope it helps.

Aluminium captures basically every contaminant it can get (including nitrogen). I would recommend you do an XPS or EDX on your film; when a former colleague of mine gave me his not very reflecting selfmade Al mirror, the XPS showed lots of carbon in the film. Maybe try a different crucible as well, e.g. Al2O3.

Hello Philipp Welscher ,

The book by Holland [1] mentions this problem "... a bluish tinted bloom ..." on p. 331 - Sorry, I first put down p. 351, erroneously. - in his Chapter 11 - Evaporated Aluminum Films, pp. 320-357.

[1] L. Holland; Vacuum Deposition of Thin Films; John Wiley & Sons, Inc.; 1961; p. 331.

The color of metal thin films in reflected white light is also discussed in the book by Heavens [2] as a function of the film's state of aggregation and Maxwell Garnett theory, p. 177 (text) & 206 (citation).

[2] O. S. Heavens; Optical Properties of Thin Solid Films; Dover Publications, Inc.; 1991 [Dover (New York)], 1955 [Butterworth (London)]; pp. 176-179 & 198-200.

Regards,

Thomas Cuff

Thank you for your responses.

We just measured XPS from the bluish films. It indeed shows increased oxygen and some carbon impurities. The layer on the top surface consists of about 50% Al(0) and 50% oxidized aluminum.

Yet i do net get the point, where these impuryities come from and hoe to prevent them. As said, the evaporator is inside a glovebox with an oxygen concentration below 0.1 ppm.

I cleaned the evaporator several times before evaporating.

Before the BN-crucible, we used Al2O3-crucibles. However the films ended up with a dark brown, brittle surface. With an AlN crucible the films are white and opaque, respectively.

Hello Philipp Welscher ,

I am confused. Are you saying that the aluminum thin film was never exposed to the ambient atmosphere, even when it was being photographed, or moved to where you did the XPS measurement? I mean, even a short exposure to the ambient atmosphere would immediately convert some of the aluminum, at the outside of the film, to Al2O3.

One other question: why do you use crucibles instead of performing the evaporation by hanging high-purity aluminum hooks from tungsten filaments? Holland warns against using crucibles for the evaporation of aluminum, see pp. 128-130.

Regards,

Thomas Cuff

Dear Thomas Cuff,

I didn't expose the aluminum to ambient atmosphere at any time. The picture was taken through the shield of the glovebox and i transported the films inside a nitrogen filled bomb to the XPS device.

Unfortunately, our thermal evaporator can only handle crucibles and not any baskets or boats.

However, we are thinking about exchanging the source.

Venting too soon after evaporating aluminum may cause this. We suggest waiting 15 minutes after aluminum depositions before venting the chamber.

Also, are you reusing the evaporator source?

Dear Aaron Windsor,

I usually wait until the source has cooled down below 100 °C and then vent the system. This takes about 1 h. Sometimes i even vented after up to 4 h after evaporation. However i could not see any improvement.

Yes, I am reusing the crucibles. The crucibles itself are meant to be reused and pretty expensive. From time to time i used new ones, but also without any improvement.

Quick update:

It seems that the color is determined by the layer thickness. 30 nm Al result in metallic films, while 150 nm Al produce a white, dull surface.

The bluish color from the picture above (about 100 nm) seems to be a mixture of both. However it is still unclear, whether this strange color affects the performance of the wanted OLEDs or how to prevent it.

Dear Philipp Welscher,

What was the base pressure before your evaporation? During the evaporation process, did you degasing first which means the base pressure would increase and wait to pump down. You should start your Aluminium evaporation when the pressure should be less than 3 ×10^-6 bar. Otherwise aluminium thin films are likely partially oxidized and the devices would have very poor performances.

Alternatively you could try silver electrodes to replace Aluminium which works very well with most device performances.

Best wishes

Xuhua

Dear Xuhua Wang,

Thank you for your kind suggestions.

The base pressure is usually around 2x10^-7 mbar. I then heat up and wait for the pressure to fall down to below 5x10^-7 mbar. Afterwards i heat to evaporation temperature. During the whole evaporation the pressure is around 3 - 7x10^-7 mbar. So the pressure is always below 1x10^-6 mbar.

Do you have any experience by increasing the rate during evaporation process?

Yesterday i ran an experiment where i started with a rate of 0.05 nm/s and increased it up to 0.4 nm/s. The final 80 nm film did not look perfect, but way better than before.

I also tried silver before. However, the whole evaporation device (including electronics and even the backside of the substrate) is contaminated with a thin brown to grey coating after the evaporation.

Best

Philipp Welscher

If you have multiple materials with contamination issues, you may want to use a residual gas analyzer (RGA) to monitor for chamber contaminates or clean your deposition chamber.

Dear Philipp Welscher,

Since the base pressure was good enough, the problem likely comes from the contaminated chamber which other materials were evaporated along with Aluminium. We had experienced such problems before. The best way is to clean the chamber. If there is a shielding protection, you could take the shielding to clean. I used to use one thermal evaporator with a removable steel shielding. Regularly cleaning is needed to protect the cross contamination. Alternatively you could use Aluminium foils to keep the contaminants away.

You mentioned that you have back coatings on your device substrates. This could be due to the shutter not working properly. It is likely some organic materials coatings on it.

Again keeping the evaporator cleaning is crucial for device performance.

All the best

Xuhua

I cleaned the evaporation device thouroughly two weeks ago. Afterwards the aluminium films still looked dull.

We have the contamination problem only for silver. It seems that silver particles or single atoms spread everywhere in the device. For aluminium we do not have this problem.

I currently suspect that the evaporation rate is the crucial factor for the film appearance. Last week i could prepare a few shiny films among others. Yet I do not know the exact parameters to reproduce this result.

May I ask what your rates for the evaporation of aluminium are? And do you keep the rate constant or vary it during one evaporation process?

Best regards

Philipp Welscher

Hi I am also facing same sort of problem. My aluminum layer is darker then usual. I'm using alumina coated tungsten baskets as a heat source, normally start evaporation at 30A. rate is normally start at 0.1A/s to max 1A/s. Can you suggest anything to improve it to normal?