Avoiding corrosion when dry-etching aluminum

With the recent installation of our Oxford Instruments Cobra ICPRIE, it is now possible to dry-etch a much wider variety of materials at the nanoFAB, including metals.

While some common metals—such as chromium—are straightforward to etch via ICPRIE, others—such as aluminum—require more care. It is the purpose of this post to highlight one subtlety which must be considered for successful plasma etching of aluminum.

Aluminum is typically etched in a Cl₂ + BCl₃ + HBr plasma, yielding AlCl₃ as a volatile product which is pumped away. However, a significant quantity of AlCl₃ can become trapped inside the photoresist used to mask the aluminum. If the wafer is then exposed to moisture in the atmosphere, the AlCl₃ can form hydrochloric acid, causing the aluminum to corrode, even if the photoresist is afterwards stripped using conventional techniques. This corrosion is not always immediately evident, sometimes appearing only two or three days after etching:

Aluminum corrosion when photoresist is not removed before venting.
Aluminum corrosion when photoresist is not removed before venting.
Aluminum corrosion when photoresist is not removed before venting.
Corrosion on dry-etched aluminum features masked with HPR504 photoresist.
The wafer was vented to atmosphere immediately after the Al dry etch, after which the photoresist was removed using an oxygen plasma in the NGP80.

To mitigate such corrosion, the AlCl₃ should be removed as much as possible before the wafer is exposed to atmosphere. In the Cobra ICPRIE, this can be achieved by stripping the resist in an O₂ + CHF₃ plasma immediately after the Al dry etch: the addition of the CHF₃ converts residual AlCl₃ to AlF₃, which does not cause the corrosive behaviour. Following this treatment yields a dramatic reduction in corrosion:

Corrosion is absent after O₂ + CHF₃ plasma resist strip.
Corrosion is absent after an in situ photoresist strip in O₂ + CHF₃ plasma.

For more information on dry-etching aluminum or other metals using the Cobra ICPRIE, please contact Scott Munro or Aaron Hryciw.