[mems-talk] Glass to glass bonding

[Joseph Grogan]

You cannot anodically bond glass to glass with SiO2 in between them. The 
way anodic bonding works is that the glass is heated to the point where 
alkali ions (usually sodium) become mobile. When you apply a large 
voltage (maybe ~1000V) the ions are pulled to the cathode (negative 
side) leaving a depleted negative charge on the glass surface which then 
gets attracted down to the other substrate (traditionally silicon) and 
bonds. To bond glass to glass, you need to put a diffusion barrier in 
between the glass, so that the top glass has something to reach down and 
bond to, otherwise the sodium from the bottom glass just gets pulled up 
into the top glass and no bonding occurs. SiO2 (glass) does not serve as 
a diffusion barrier for sodium since the process is based on sodium 
being mobile in glass at high temperature. For a good list of usable 
films as diffusion barriers check this paper:
"Glass-to-glass anodic bonding with standard IC technology thin films as 
intermediate layers" Berthod et al, Sensors and Actuators, 2000.

I've never tried doing it, but I don't believe it's possible to bond 
quartz to quartz because there are no mobile ions to migrate around. You 
might be able to bond pyrex to quartz since the quartz has no ions to 
leech across the gap, however, you need to make sure that you have 
matched thermal expansion coefficients otherwise the thing will bond and 
then shatter when it cools.

PDMS bonding is a finiky art if you've never done it before. The key is 
low power plasma for short time. I believe the reason is that long time 
high power forms a glassy layer on the PDMS which prevents bonding. I 
found this paper to be particularly helpful for settings and cleaning 
procedures: "Three-Dimensional Micro-Channel Fabrication in 
Polydimethylsiloxane(PDMS) Elastomer" Byung-Ho Jo, Journal of 
Microelectromechanical Systems, Vol 9. No 1, 2000.

good luck,
Joe Grogan


Hyun Chul Jung wrote:
> Hello guys.
>
> I am having difficult time to bond glass to glass. Bottom substrate has
> 2um wide and deep microfluidic channels and electrodes (Ti/Au
> 200/1000A). Top substrate is just plain pyrex 7740.
> I have been trying two methods so far.
>
> First one is anodic bonding.
> I deposited SiO2 on top substrate using Ebeam evaporator. Tried to bond
> using anodic bond. It did not work.
>
> Second method is PDMS to glass bonding.
> First of all, I spined coat PDMS liquid on top substrate around 6um
> thickness and curing for 30 min at 180C on hotplate. Cool down it for
> while. Oxygen plasma treatment on both side of substrate and then place
> them together. It did not work either.
>
> If I miss some steps, please correct me. I will really appreciate if you
> guys give me any comments or recommendation.
> Thanks.