Why Do Voids Form at the Thin-Film Substrate Interface After Annealing?

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In summary, the conversation discusses a problem encountered with thinfilm metal stack samples after post-deposition annealing using a hotplate in clean room air. The expert recommends using rapid thermal annealing (RTA) in a forming gas atmosphere for 10-100 seconds to form good ohmic contacts. The discussion also mentions possible reasons for void formation, such as oxidation and solid state diffusion, and explains the optimal temperature and time for RTA.
  • #1
lamejane
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TL;DR Summary
Occurence of Voids after Annealing
Hello to all the experts out there!: )

I have encountered a problem on my samples after Post deposition annealing [ Please refer to the crossectional SEM picture - B below].

Sample description : Thinfilm metal stack [ Pd/Ti/Pd/Au] deposited onto GaAs substrate using PVD
Post deposition annealing : Using a Hotplate exposed to clean room air
--> 60 min @ 390°C [ Picture A]
--> additionally 70 min @350°C on same sample B [ Picture B]

My Question to the Experts :
I see occurences of Voids at the thinfilm -Substrate interface [ see picture B] after annealing at 350°C for 70 min . What effect could explain this phenomenon ?
Is it due to an increase in tensile stress at the interface because of the high annealing temperature ? or is there a strong solid state diffusion of the metal atoms in the thinfilm leading to defects ?

The thinfilm metal stack contains : Pd/Ti/Pd/Au/15nm/30nm/30nm/250nm and is meant to serve as an Ohmic contact
1613561469633.png
 
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What are the reasons for your choice of post-deposition annealing conditions (oxidizing atmosphere (air), prolonged annealing times in the range of 1 h). There could be many effects to explain the phenomenon of void forming: Oxidation, formation of new phases, Arsenic out diffusion etc.

The metal layers have thicknesses in the range of nanometers. In case of such metal-multilayer contacts, the post-deposition annealing to form good ohmic contacts is generally carried out by the rapid thermal annealing (RTA) process in a forming gas (N2/H2) atmosphere at temperatures between 300 ‒ 450 °C for 10 ‒100 sec.

See, for example: E. F. Chor and D. Zhang, “Electrical characterization, metallurgical investigation, and thermal stability studies of (Pd, Ti, Au)-based ohmic contacts”, Journal of Applied Physics 87, 2437 (2000)
 
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Lord Jestocost said:
What are the reasons for your choice of post-deposition annealing conditions (oxidizing atmosphere (air), prolonged annealing times in the range of 1 h). There could be many effects to explain the phenomenon of void forming: Oxidation, formation of new phases, Arsenic out diffusion etc.

The metal layers have thicknesses in the range of nanometers. In case of such metal-multilayer contacts, the post-deposition annealing to form good ohmic contacts is generally carried out by the rapid thermal annealing (RTA) process in a forming gas (N2/H2) atmosphere at temperatures between 300 ‒ 450 °C for 10 ‒100 sec.

See, for example: E. F. Chor and D. Zhang, “Electrical characterization, metallurgical investigation, and thermal stability studies of (Pd, Ti, Au)-based ohmic contacts”, Journal of Applied Physics 87, 2437 (2000)
Thank you for your reply.
Using the Hot plate was only for test purposes. For the the long term i would move to a RTA tool as you suggested.
Lets say the process is carried out in an RTA und N2 atmosphere , what effect does a longer annealing tme ( ~ 60 min ) have on the metal layers ? Why is 10-100 sec optimal ?
 

1. What is post-deposition annealing?

Post-deposition annealing is a process in which a thin film or coating is heated to a high temperature after it has been deposited onto a substrate. This process is used to improve the properties of the film, such as its crystal structure, density, and adhesion.

2. Why is post-deposition annealing necessary?

Post-deposition annealing is necessary because it can significantly improve the properties of thin films and coatings. It can help to remove defects, improve crystallinity, and enhance adhesion to the substrate. This can result in better performance and reliability of the film in various applications.

3. What are the different methods of post-deposition annealing?

There are several methods of post-deposition annealing, including rapid thermal annealing, furnace annealing, laser annealing, and microwave annealing. Each method has its own advantages and is suitable for different types of thin films and coatings.

4. What factors should be considered when choosing a post-deposition annealing method?

The choice of post-deposition annealing method depends on several factors, including the material of the thin film, the desired properties, and the equipment available. It is important to consider the temperature, heating rate, and atmosphere during annealing to achieve the desired results.

5. Are there any risks associated with post-deposition annealing?

Yes, there are certain risks associated with post-deposition annealing, especially when using high temperatures. These include film cracking, substrate deformation, and impurities introduced during annealing. It is important to carefully control the annealing process to minimize these risks and ensure the desired properties of the film are achieved.

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