Multi-chip packages (MCPs) are now commonly used in memory and portable electronic devices to increase capacity, and this requires stacking ultra-thin silicon chips, typically produced from a single 300 mm wafer about 750 µm thick.
To produce chips that are suitably thin (100 µm but increasingly even less), the wafer, already scored into ‘dies’ and printed with circuitry, is secured in place with a holding tool, and ‘thinned’ by grinding it from its reverse side. The resulting wafer is flexible and very delicate, and must be released from the holding tool gently before it is diced. As the need for ultra-thin wafers grows, how can this release be achieved routinely and reliably? As described in this patent, while I was employed at Henkel we designed a shape-memory polymer for use as part of a novel process technology for reversible bonding. The polymer is formed in a flat film at raised temperature, which becomes its ‘remembered’ shape. Having been warmed, the film is lowered onto the wafer, with the polymer sinking into the grid pattern created by the scoring process. When the film cools, the film is thus physically locked onto the wafer, which can now be thinned with confidence. Afterwards, gentle warming commands the film to remember its original flat shape, releasing the wafer.
I once superglued a long-stemmed yellow rose onto a red rose bush as a practical joke on my wife, and she was obviously mystified by this unusual horticultural phenomenon! But it gave me an idea…
Market research revealed that industrial nurseries often use clips to secure grafts, in highly labour-intensive manual or semi-automated processes. So, during my time at Henkel, we investigated whether medical-grade cyanoacrylates (CAs), which are already approved for sutureless bonding in human and veterinary applications, could be used in a robot-enabled setup instead.
The result was a patented robot that aligned cut shoots and rootstocks, and secured the cutting with a drop of plant-grafting adhesive. This was then flash-cured, and simultaneously cooled to prevent cell damage from polymerisation exotherms. Like subscription-model inkjet printers, this is an interesting example of a process innovation for end-users that leads to demand for a product line.