Bonding 340 silicon wafers for ISIS Neutron & Muon Source’s Osiris instrument

Over two years, our Composites and Materials Testing Group (CMTG) carefully bonded 340 fragile, apple-sized silicon wafers to curved aluminium backplates for a high-precision silicon analyser.

Producing the four upper and four lower silicon plates needed for this analyser was a tricky technical challenge, but, thanks to their persistence, our technicians achieved a very positive 70% success rate.

The wafers themselves began their life as ingots of lab-grown silicon, sliced into 1-mm-thick wafers and delivered to our materials workshop at Rutherford Appleton Laboratory.

Once in the hands of our skilled technicians, the wafers were examined under a microscope for any damage before being trimmed to the right size using a diamond wire saw.

Next, the polished wafers were sprayed with gadolinium (a rare-earth element, useful for neutron detection) to a fine tolerance (engineering tolerances define the acceptable variation in a design; the tighter the tolerance, the less variation is permitted).

After being checked again for defects, the delicate wafers were cleaned and placed under a vacuum to see if they would break.

Although our team didn’t want any wafers to fail, it was better that they broke before any adhesives were introduced, as this resulted in a time-consuming cleaning process.

After testing, it was on to bonding the wafers in a honeycomb pattern using a high-grade adhesive, starting from the centre and working outwards.

Held under strain in a curved shape against their aluminium plate, the brittle wafers faced another chance of breaking as they naturally tried to return to their flat position.

After successful bonding, the wafer sheets were cleaned up and sent to our dimensional metrology facility to ensure they met the desired specifications.

And, finally, they were delivered to ISIS for installation.