Successful processing of thinned silicon chips thresholds and limits in mechanical properties

The share thin chips take in the total production of silicon based ICs is rising continuously. Not only that the relative thickness of wafers is reduced due to the larger diameters changing from 6 to 8 and 12 inch. Chip-thickness is also reduced in absolute numbers. The reasons therefore are: improved heat-dissipation, reduced electrical resistance and mechanical flexibility. Let us look at the main areas of applications for silicon chips: Where do we need these key-features? In lots of households you find PCs with processors inside. Memory-chips are used in every electronic module, operating for example a washing-machine. Your car is meanwhile full of power-devices and contains up to 70 chips. Smart-labels found in access-cards, tickets and labelling applications try to make our daily life easier. For processors the main advantage of thin substrates is improved heat dissipation. Stacking of memory chips needs also thin silicon. For power devices it is reduction in electrical resistance, and the gain in switching performance. For smart-cards and related applications the main feature is the flexibility of thin silicon, which makes the IC-chips capable of surviving daily use. The major influence regarding stability and flexibility of processed silicon is coming from Pre-Assembly. We investigated the main features to characterise the mechanical properties of silicon chips. We have done tests to see the relation between chip-thickness, chip-stability and chip-flexibility. Back-side surface treatments improve the mechanical performance. Several surface technologies are in use. We did several series of investigations regarding wet-etch, CMP-Polish and Plasma-etch as backside-surface treatments. Besides the features needed for the chip-applications, chip-stability is also necessary for the process chain from chip-separation to packaging. Especially die-bonding, the first back-end process, is a decisive step, stressing the chips. We have done investigations to find the stability necessary for a safe and reliable handling of the chips. Our work will demonstrate the resulting threshold for chip-stability and some process-flows, to reach this limit. The investigations will compare these flows in detail regarding flexibility, stability and backside-hardness. The results will give the differences in stability between blank silicon and processed chips. As a result we can state the stability necessary for an optimum line performance with respect to the thickness of the silicon chips processed. Thus we can show for what chip thicknesses these treatments make sense or are mandatory.