I have bondpands patterned on flexible mica substrates (cleaved thin disks of muscovite mica, ~100um thick). The pads are usually e-beam evaporated Ti/Au or Ti/Pd metal stacks of thickness 5/100 nm or so, defined by litho+liftoff. I need to load these samples into a DIP, to which I would need to wirebond the pads. I've tried wirebonding with two different bonders: a ball bonder that uses gold-wires and a wedge bonder that uses aluminium wires. Most of the times, the wire doesn't stick to the pads on mica, and ends up ripping them off. I've tried using very little sonication and bonding power, but that still doesn't help. It would seem that the substrate buckles every time the tip makes contact with the pads, and hence there isn't enough area for adhesion.
Does anyone have experience with this? Any alternative methods? Current pad size is ~200x200 um.
Tanuj,
You may need to increase the thickness of the metal pads. This can be accomplished by electroplating of silver or cupper and post anneal them to bond them firmly to mica.
You can also deposit aluminum directly as a pad instead of the bimetal.
You may hold the flexible substrate firmly against hard substrate.like ceramic or glass.
wish you success
"... ends up ripping them off. ..."
Ripping off the bond pads ? If so, it is not about bonding but about the adherance of the bond pads to your mica substrate. Eventually a TiW layer might improve this property.
Regarding wire bonding: though there is the opinion that Pd is superior in terms of bonding, I doubt that. ... Not to be discussed futher here.
Pd at the surface is not recommendable. When using Pd, you'll still need an Au 'cap' on top of the bind pad.
On Au surfaces, I know that preheating the "chip" helps forming Au wire bonds. Unfortunately I do not know about the preheating temperature (had an academic fab who performed the production), but a bit of searching came up with values: 100 .. 150 °C (temperatures from the upper end preferable) seen to be appropriate.
http://www.amtechmicro.com/wire-bonding-guidelines/
gives further informations on Au wire bonding as well as on Al bonding.
Wish you success
Tanuj,
A simple solution is to connect the wires directly to the bonding pads using silver epoxy instead of thermocompression in ultrasound bonding. After attachment of the wires you need to dry silver paste in an oven at about 120 degree centigrade for about half an hour. However, in every case, you have to strengthen the adhesion of the bond pads to the substrate.
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