I am looking at forming MOS devices using Bi4Ti3O12 thin films on silicon substrates deposited by magnetron sputtering. In the literature, they achieve this by using standard RF-magnetron sputtering with substrate heating and a Ar/O2 gas mixture. As my sputter does not have substrate heating and is limited to just Ar gas, I have to do post deposition annealing in ambient atmosphere to obtain crystalline films.
However after annealing for 1 hour at 650OC. The exposed silicon becomes oxidised and I am unable to form electrical contact with the semiconductor silicon. I have had some success annealing in vacuum, which leaves the Si conductive, but this results in the BTO film loosing it’s ferroelectric properties, due to an increase in oxygen vacancies. Does anyone know of a method I could obtain electrical contact on silicon annealing in the above conditions?
Dear David, you can remove the siliconoxide by dipping in 1% HF solution. You can protect the other part by photoresist for example. To create a good Ohmic contact you can deposit Al on the Si. An annealing step is involved after the Al deposition to form an alloy and result in low ohmic electrical contact.
Dear Daniel.
Thank you for your feed back. I have considered the SiO2 approach, but there is something I would like to inquire with you. Would I need to remove all surface SiO2 between the contact and the BTO, or would removing enough to make electrical contact be enough? I.E if I removed some SiO2 to make contact with the Si, is there likely to still be enough bulk Si under the remaining SiO2 to form a BTO/Si junction?
best wishes
David
You are making an MOS device with Bismuth titanat insulating oxide film.
1) So firstly you are making the deposition Bi4Ti3O12/Si
2) Next you have to now make the complete MOS device structure.
which means: (Metal-1) /Bi4Ti3O12 / Silicon / (Metal-2)
3) Metal -1 is a top dot electrode, prepared by evaporating through a pre-diefined mask, so that you will know the area of the electrode formed.
4) Now the metal-2 deposition is known as back metallisation of the silicon wafer.
for n-type silicon, normally aluminum is used.
Before doing this the back side of silicon wafer should be cleaned,
which means remove the SiO2 layer on it, which could have developed while you were processing your BiT film.
Clean the SiO2 layer as suggested in the previous reply.
while removing this SiO2 layer, take care that you are not spoiling your
deposited Bi4Ti3O12 layer while dipping it in H\buffered HF. You must protect the Bi4Ti3O12 deposited layer using a photo resist or black wax. Even slight vapors from the buffered HF can destroy your Bi4Ti3O12 layer. Take care.
Then deposit your aluminum layer. and anneal it in vacuum, typically the allowing temperature is around 400 deg C.
K. Sreenivas
Dear Dr Sreenivas,
Thank you for your advice. I should raise the point that the back of my silicon substrates are alkaline etched, so I am having to make electrical contact on the top side where the BTO was deposited. I have attached a quick diagram demonstrating my ideal device structure (figure 1).
I understand your description of how HF etching would work in making contact with the rear of the substrate, but I have concerns with how it would be done in my situation. Figure 2 through 5 shows the various steps I believe will be needed to form electrical contact. Some of the HF-resistant photo resist would have to cover the SiO2 to ensure the BTO is protected, meaning all surface SiO2 would not be removed. I am unsure what effect this remaining SiO2 will have on my sample, specifically I am unsure as to whether or not there is enough semi conductive Silicon beneath this layer to form the MOS structure with the BTO.
Any further advice you could provide would be greatly appreciated.
Best Regards
David Coathup
Dear David,
I don't think you should worry about the remaining bulk Silicon, this is still bulk and will not etch during HF. By the way, the SiO2 that you get during annealing will be very thin in ambient atmosphere and 650C which is very low for thermal oxidation of Si. So a short dip in a 1% HF or a bufferd HF solution will be sufficient.
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