3.10 Scribing, Packaging and Bonding
Before processing can begin, all wafers have to be reduced to a standard size. This is made possible by means of scribing. Scribing is also necessary for precisely separating individual devices (usually in the 100mm diameter range) for mounting onto transistor headers after all other fabricating stages have been completed.
A pneumatic scriber (Tempress Automatic Scribing Machine) was used in this work. The wafer to be cut is firmly held in position on the scriber stage by a vacuum chuck. Scribe marks are made using a diamond tip attached at the end of a mechanised arm which can be accurately positioned and moved across the wafer. Once scribing is complete, the wafer is sandwiched between two thick clean-room wipes. A heavy roller is then firmly run over it. Cracking sound confirms the cleaving action. Individual pieces are then dusted with dry N2 gas ready for packaging.
T05 transistor headers are used for mounting test samples. At first the T05 is coated with a conductive epoxy resin. A tiny drop of EPO-TEK H21D [118] is sufficient for packaging a sample several millimeters across. Suitably scribed pieces are then gently placed on the glued spot and allowed to cure at 130 șC for 2 hours.
Once the sample has been packaged, the devices are ready to be bonded to individual legs of the T05 header. A wedge bonder (Westbond model no. 7400A ) machine and 17mm diameter gold wire was used for most bondings. Bonding is usually carried out at 125 șC. This machine is also equipped with an ultrasound source which may be used for stubborn and difficult bondpads although excessive use can damage the device. A metal cap is then placed on the header to protect the devices within and prevent accidental breakage of the fine bond wires through mechanical stress.
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