[labnetwork] Bismuth evaporation

[Timothy J Gilheart]

Hi Aimee,

I fielded a similar request some years ago from a student user who only wanted to deposit relatively thin (~ few nm) layers of Bi as a seed layer for some 2-D materials synthesis experiments.

During my review at the time, I reached a similar conclusion about potential cross-contamination, but I was also on the verge of a shield change in my open chamber (less restricted materials) evaporator, so I gave it some additional consideration, thinking I could swap the shields out soon after if it looked like the Bi caused a problem. 

I also wondered about toxicity, due to some notes about the safety concerns for Bi vapors I found in old thin film deposition guides.

As I dug into it further, I settled on the following additional conclusions:

- Bi is the “heavy metal” least likely to cause heavy metal poisoning issues, and is most dangerous when dealing with its vapors. The evaporator in question is cryopumped, so I confirmed with my cryopump service provider that Bi was not a problem for them, and knew that I’d want to snorkel exhaust the cyropump during subsequent regeneration cycles.

- Bi alloys pretty readily with Ti (see attached phase diagram), and the melting point increases quite nicely with increasing Ti fraction, so I thought a good abatement layer of Ti (50-100 nm) deposited on the chamber immediately after (and before venting, with substrate shutter closed to protect the student samples) would help mitigate any cross-contamination issues quite a bit.

In the end, I let the student try it, and no related issues presented in the aftermath. For a thicker layer of Bi or maybe a different vacuum system configuration, I might have declined the request, but a few nm of Bi and a lot of Ti abatement worked out okay. We weren’t doing much compound semiconductor work on that machine at the time, so your mileage with a similar approach could vary.

Also: when that same student's teammate came by a few weeks later wanting to evaporate bismuth telluride on the same machine, I gave them a firm “no” because Te toxicity didn’t seem like my idea of fun.

Hope this helps in your decision process,

-- 
Tim Gilheart, Ph.D.
Research Scientist - Nanofabrication Cleanroom Manager,
Shared Equipment Authority (SEA), Rice University
Cell: 832-341-5488 | Office: 713-348-3159 | gilheart at rice.edu



> On Jun 24, 2022, at 3:43 PM, Price, Aimee <price.798 at osu.edu> wrote:
> 
> Hi everyone,
> We have had a request to ebeam evaporate Bismuth.  We do not have any experience with this material but is a group V and has a relatively low melting point.  We have two evaporators and one we strictly limit the materials allowed within, including sources, substrates, and adhesives (none).  Our other one is a bit less restricted but we are still very careful about what we allow.  An example is that we do allow SiO2 deposition in the less restrictive evaporator. If approved, Bismuth would only be allowed in the less restrictive tool.  We are concerned about the low melting point and deposition on the chamber, potentially allowing for redeposition at a later date.
>  
> Does anyone have any experience with Bismuth evaporations?  Are there any considerations that you could share?  Our main concerns are safety and cross contamination, specifically mobile and deep level traps in compound semiconductors. 
>  
> We do mostly compound semiconductor work but we do have some Si based devices/structures as well.  Below is from Lesker’s website, which generally has terrific information on evaporation and sputter source materials. 
>  
> (https://www.lesker.com/newweb/deposition_materials/depositionmaterials_evaporationmaterials_1.cfm?pgid=bi1 <https://urldefense.com/v3/__https://www.lesker.com/newweb/deposition_materials/depositionmaterials_evaporationmaterials_1.cfm?pgid=bi1__;!!BuQPrrmRaQ!mI63h_8Guu-7k8HK073CzKfATF1rHRCElzW-u0nGPJLzDjFNPtLuuib4HZCEoWp-CAT4aAWhcDBNJjnt5wtE6qPcX2AL4w$>
> Melting point: 271C
> 10-8:  330C
> 10-6:  410C
> 10-4:  520C
>  
> Thanks in advance.
>  
> Best,
> Aimee Bross Price
>  
> Manager, Nanofabrication 
> The Ohio State University
> Nanotech West Lab
> Institute for Materials Research
> 1381 Kinnear Road
> Suite 100
> Columbus, OH 43212
> 614-292-2753
>  
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