Badin,

The issue is definitely with the electrical field that is being
generated vs. the thinness and flexibility of the silicon, and the small
gap of the cavity.

The piston force has very little to do with it, since the electric field
generated is by far stronger than pretty much any bonding force you
could apply, anyway.

Your best bet is to try to bond with lower force.  You may have to
evaluate the dimension of the cavity as well, since you currently have a
cavity which is ~1000x as wide as it is high. If the cavity could be
made higher, or if your design allows for "pillars", either could help.

Even ramping the voltage may not help (although it would definitely be
something to try), since the Pyrex region over the cavity will not be
neutralized by the introduction of silicon, and may remain charged (due
to the migration of the Na ions). If that charge remains, the electric
field would still remain.

Best Regards,
Chad Brubaker

-----Original Message-----
From:  Badin Damrongsak
Subject: [mems-talk] Pyrex wafer anodically bonded to Thin
Silicon:Problem

Dear all,

I found a problem during an anodic bonding between thin silicon and
pyrex
wafers using EVG520/620.

On pyrex wafer, it was etched about 2-3um depth to create the cavity
which
is around 4mm in diameter. The pyrex was then bonded to a 200um thin
silicon
wafer on which has no pattern.

The problem I have found is that the silicon area over the 3um gap
cavity
was somehow bonded to the pyrex wafer. I have no clue where the problem
is;
probably too high voltage, too high force, etc. Have you guys got any
ideas
how to solve this issue? Please feel free to contact me.

The recipe of anodic bonding is as follows:
Temp = 380 degC
Force = 800 N    (probably too high, for thin silicon wafers)
V    = -1kV      (probably too high)
Time = 4 min

The above recipe works well for anodic bonding between std thickness
silicon
wafer and pyrex.