In the prior art, amorphous silicon (a-Si) thin film transistors are formed by depositing a first layer of a-Si semiconductor material over a gate and gate insulator layer and then depositing a layer of highly doped silicon (e.g. n+ layer) on top of the first layer. Metal contacts for the source and drain are then formed on the highly doped layer defining a channel area in the first a-Si layer between the contacts. The highly doped layer over the channel area can then be etched away so as not to adversely affect the channel area. The low mobility in the a-Si TFT channel makes the device less demanding on contact resistance. The metal contacts formed on the highly doped area provide a low resistance (ohmic) contact.
In metal oxide thin film transistors (MOTFT) the metal contacts are formed directly on the metal oxide semiconductor layer. That is the metal oxide semiconductor material is the same under the metal contacts as it is in the channel area. For MOTFTs the lack of an n+ layer and a higher bandgap make it harder to provide a good ohmic contact. Furthermore, the high mobility of the metal oxide semiconductor material demands a lower contact resistance than in a-Si TFTs. Without a good, low resistance contact, hereinafter referred to as an ohmic contact, the high mobility of the metal oxide semiconductor material can be masked by the contact resistance. However, ohmic contacts in MOTFTs have been virtually unknown to date or are very difficult to form and/or retain.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide new and improved source/drain metal contacts in a MOTFT.
It is another object of the present invention to provide new and improved source/drain metal contacts in a MOTFT that form a low resistance ohmic contact.
It is another object of the present invention to provide a new and improved source/drain metal contacts in a MOTFT that are relatively easy and inexpensive to fabricate.
It is also an object of the present invention to provide a process for fabricating a MOTFT from a uniform metal oxide film of which portions in contact with the source/drain metal contacts have a carrier concentration greater than the carrier concentration in the channel area.
It is also an object of the present invention to provide an insulating, passivation layer shielding the channel area, which passivation layer serves as a chemical barrier under TFT storage/operation conditions and during TFT fabrication following metal oxide semiconductor layer formation. The same passivation layer also possesses sufficient mobility to oxygen and hydrogen at an annealing temperature above its glassy temperature.
It is another object of the present invention to provide an insulating, passivation layer shielding the channel area, which passivation layer includes oxygen containing groups, and which serves as an oxygen source at annealing temperatures and serves as a chemical barrier at TFT storage/operation temperatures.