Hi all, I am having some problems when trying to perform a typical
photolitographic step after a n-type difussion.
Prior to difussion an oxidation step is performed to grow a 500 nm SiO2 layer to
serve as a barrier for the difussion. Then difussion is performed, with the
following parameters:

20' at 350ºC in O2 flow
60' at 950ºC in N2 flow

We use a liquid dopant source. After this predeposition step (we don't
redistribute) remaining dopant oxide is removed by inmersion in diluted HF
(1:9). After that a typical photolitographic step has to be performed, but the
photoresist lifts-off in critical point of the geometries (the zones surrounding
the insolated places) so we can't define any geometry.
The problem is not the photolitography parameters as we only have this problem
after a difussion step.

We though the SiO2 surface could be damaged or contained impurities as a result
of the difussion step and that this could lead to photoresist lifting (we don't
understand how it coud be, but it happens), so we tried to etch the surface of
the SiO2 layer by wet etching (HF).

If any of you could give me any idea about what happens and how to avoid this
effect I would be very grateful. Thanks to everybody,

Pablo.

Pablo de la Fuente Prado
pdlfuente@ceit.es
CEIT, Sección de Microelectrónica.
Paseo de Manuel Lardizábal, 15.
20018 San Sebastián, Spain
Tel.: (34) 943 212 800
Fax.: (34) 943 213 076