Complete dehydration of porous low-K material poses a major challenge to process engineers. The problem with porous low-K material is the series of deep cave-like holes that harbor moisture in an insulating material. Even when it is possible to heat the material enough to evaporate moisture on the cave walls, the steam is still present and needs to be drawn out of the cave.
Low-k Dielectric Sealing
Unfortunately, in a standard clean room, enough moisture exists to react with the pore walls and slowly produce a moisture layer that will reduce the K factor. However, if the pores are sealed against the ingress of moisture, integrity of the dielectric can be retained.
YES equipment uses a series of vacuum pulls and hot Nitrogen purges so the sidewalls of the pores are completely dehydrated. This way, when the atmosphere of the chamber is hot Nitrogen at 1 Torr, the atmosphere in all pores is at 1 Torr.
In order to successfully remove all moisture from porous low-K material, transform it into hydrophobic material, and seal it so it won’t absorb moisture again, YES recommends using either of two ovens:
- A manual vacuum oven with temperature control up to 200°C
- An automatic vacuum oven with temperature control up to 450°C
Here’s an outline of the process:
- First, pull a vacuum to 10 Torr. This allows water to boil at 11°C and actively pulls the moisture out of the cave.
- Next, fill chamber with hot nitrogen at the oven temperature to 600 Torr. This fills and heats all walls of the caves at the same time. Repeat as needed. While flat silicon surfaces typically need three nitrogen purge cycles to achieve complete dehydration, porous insulation material may require as many as six purges.
- Then, pull a vacuum to 1 Torr, dehydrate further, and prepare for wall treatment and sealing.
- Finally, using the correct volume and temperature, a wet silane (typically HMDS) is vaporized and the resulting vapor pressure of 6 to 50 Torr replaces the 1 Torr of nitrogen in the caves with active chemical to produce entirely hydrophobic surfaces. At the same time, it seals the pores so moisture can never penetrate again.
NOTE: It is possible to do a copper anneal at the same time you treat the low-K dielectric and reduce process steps.
Vapor deposition can restore low-k dielectrics that are plasma (ash) damaged.