what is the mechanism of Titanium being an adhesive between Au and glass? what should be a thickness of Ti for the application of SAM. And I also would like to know the chemistry between gold and titanium.
Hi Syeda,
Noble metals, particularly Au, do not adhere well to glass due to their inability to form stable metal-oxide interfaces. Common oxide forming metals such as titanium or aluminum can create these oxide interfaces quite easily, as the heat of formation of the metal oxide is much more favorable.
When gold is deposited onto metallic titanium, the interface behaves more like a very thin Ti/Au alloy, which is known to have an exceptional bonding strength. You'll see that if you allow the titanium film to oxidize (venting chamber to atmosphere) before depositing the gold, the film will not adhere - again, the gold cannot form the stable metal-oxide interface required for adhesion.
For SAM work, it is quite typical to use titanium films that are anywhere from 2-5 nm thick. The films do not need to be continuous to promote the adhesion of gold, and are often kept thin to prevent the grain structure from influencing the surface roughness of the gold. Be sure to deposit titanium and gold in a single vacuum cycle to avoid the oxidation problem described above.
Titanium is a good choice for Au-SAM work; chromium is also used, but has a greater tendency to diffuse to the surface of the gold film and disrupt or influence SAM formation. To avoid a metallic adhesion promoter altogether, others have used an inter-facial SAM such as a mercaptosilane; the silane group will form a stable bond to the glassy surface and the thiol group will adhere to the gold film. This is particularly important for applications that require the gold layer to be transparent, and the additional absorption from the titanium film is not acceptable.
Let me know if this helped at all.
Cheers,
Mike Miller, PhD
Test and Process Engineering Manager
Angstrom Engineering Inc.
www.angstromengineering.com
https://www.angstromengineering.com
Hi Syeda,
Noble metals, particularly Au, do not adhere well to glass due to their inability to form stable metal-oxide interfaces. Common oxide forming metals such as titanium or aluminum can create these oxide interfaces quite easily, as the heat of formation of the metal oxide is much more favorable.
When gold is deposited onto metallic titanium, the interface behaves more like a very thin Ti/Au alloy, which is known to have an exceptional bonding strength. You'll see that if you allow the titanium film to oxidize (venting chamber to atmosphere) before depositing the gold, the film will not adhere - again, the gold cannot form the stable metal-oxide interface required for adhesion.
For SAM work, it is quite typical to use titanium films that are anywhere from 2-5 nm thick. The films do not need to be continuous to promote the adhesion of gold, and are often kept thin to prevent the grain structure from influencing the surface roughness of the gold. Be sure to deposit titanium and gold in a single vacuum cycle to avoid the oxidation problem described above.
Titanium is a good choice for Au-SAM work; chromium is also used, but has a greater tendency to diffuse to the surface of the gold film and disrupt or influence SAM formation. To avoid a metallic adhesion promoter altogether, others have used an inter-facial SAM such as a mercaptosilane; the silane group will form a stable bond to the glassy surface and the thiol group will adhere to the gold film. This is particularly important for applications that require the gold layer to be transparent, and the additional absorption from the titanium film is not acceptable.
Let me know if this helped at all.
Cheers,
Mike Miller, PhD
Test and Process Engineering Manager
Angstrom Engineering Inc.
www.angstromengineering.com
https://www.angstromengineering.com
Dr. Mike Miler has really explained very well the importance of Ti for Au metallisation.
I wish to add that Cr-Au, and Ti-Au can common bilayer metallisation systems.
Basically the adhesion layer (Cr, or Ti) and especially Ti lowves oxygen very much, and has a huge solubility for oxygen. So when it is deposited on glass it reacts and sticks very well. fopr good adhesion you need a chemical reaction at the interface.Since the Ti layer is very thin, it is a non continuous film, with lost of gaps, into which the subsequent layer Au get deposited into. Ti and Gold do form an alloy, and therefore you are able to deposit an adherrent Au metallised layer.
Dr. Miller in the previous message has rightly pointed out that you should break the vacuum between the Ti and Au deposition, and secondly if you have some substrate temperature of about 200 deg C, it helps a lot in achieving good crystallisation of the Au films.
You can understand more about such bilayer metallisations if you refer to the
classical books:
1) Handbook of thin film technology by Maissel and Glang
and many paper on bi-layer metallisation by Donal M. Mattox, although these papers are very old, but they are very useful, and are easily available in the literature
Prof. K. Sreenivas
Thanks Dr. Mike Miller and Prof. Sreenivas for your quite useful answers.
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