There could be a few causes for conductivity between the lines.

1. There is some filament bridging of the metal lines. Not all the metal
is etched out. This would be where you have steps in the topology. This
would be more a problem with a wet etch system, where even a few
angstroms of resist could retard etching. If your wafer is flat this is
much less likely. Do high power magnification inspection of topology
steps.

2. With a deposition of Titanium on a SiO2 substrate at 380 degrees C,
the titanium could reduce the SiO2 and creat a thin surface layer of a
Silicon suboxide or a Silicon Titantium suboxide thing. It would have a
low conductivity. It would not etch in a titanium wet etch well.

Aluminum can also reduce a deposited oxide with enough heat. If you take
a patterned aluminum over oxide, thermally process the wafer and then
wet etch the aluminum, you will see a pronounced pattern where the
aluminum was. The reduction of the SiO2 to a suboxide will be there and
unetched by the Aluminum etch.

Both titanium and aluminum are powerful reductants.

Plasma etch the wafers after the wet etching the pattern. Just do enough
to etch off a little of the oxide surface.

3. Your PECVD is slightly a suboxide and conductive. It can happen
easily with certain ratios of gases. In some cases the PECVD will have
more hydrogen and less oxygen and that won't be so conductive, but if it
is essentially SiO(2-X)and not SiO(2-X)HX it can be conductive. If you
put your PECVD through a furnace in O2 and then did your metal
deposition and patterning you would get a different result if this was
the cause.

4. Your PECVD has phosphorous or boron and is a little hygroscopic and
has some water absorbed on the surface. Bake dry the wafer and test it.

5. Your PECVD has pinholes and you are conducting down to the substrate
and through the substrate to the other metal line.

Lots of luck.

Ed

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Ho Yin Chan
Sent: Tuesday, March 25, 2008 4:28 PM
To: [email protected]
Subject: [mems-talk] PECVD oxide conductivity

Hi all,

  I am putting Ti/Au metal lines on top of 1um PECVD oxide at ~380C. I
found there is finite reisstance between the metal lines. Those are
parallel lines with 100um wide and 2mm long. They are seperated at a
distance of 40um. I am curious the reason behind it. I tried to measure
the resistivity of PECVD oxide. It shows > 10E14 Ohm cm.