3.11 Processing Steps - the HBT Example
The fabrication starts with thoroughly cleaning the wafer. A liftoff lithography is followed by metalisation of the emitter as shown in step C1 of Figure 3.13. This is followed by step C2 when a mesa etch to the base is carried out after appropriate etch lithography has been done. Steps C3, C4 and C5 involve the base metalisation, mesa etch to sub-collector layer and finally the collector contact metalisation respectively. The relevant chemical etching steps have already been described in section 3.5.
Figure 3.13: Processing steps for (large geometry) conventional HBT devices
In the case of the ITO emitter contact HPTs, a few of the initial fabrication steps are either different or modified from those of the conventional contact devices. These are shown in Figure 3.14. As before the wafer is thoroughly cleaned followed by ITO deposition as shown in step T1. Step T2 shows that a film of Al is then deposited over the ITO and subsequently patterned with the emitter mesa mask for etch lithography and wet etched using the KOH solution (see section 3.6).
Figure 3.14: Additional processing steps for (large geometry) HPT devices with transparent ITO emitter ohmic contacts
The ITO is then dry etched in Ar followed by Al mask removal by dissolving in KOH as before. This is shown in step T3 of Figure 3.14. The sample is then patterned again with photoresist and etched to the base. At this point the ITO contact is annealed (step T4), followed by further liftoff lithography using the emitter metal mask. A metal is then deposited over a part of the ITO ohmic contact for bonding purposes (step T5). The remainder of the processing is the same as those shown in steps C3, C4 and C5 of Figure 3.13 for conventional HBT fabrication.
| Title | Dedication | Acknowledgements | Abstract | Symbols | Contents |
|