Metal oxide TFT with improved source/drain contacts
A method of forming ohmic source/drain contacts in a metal oxide semiconductor thin film transistor includes providing a gate, a gate dielectric, a high carrier concentration metal oxide semiconductor active layer with a band gap and spaced apart source/drain metal contacts in a thin film transistor configuration. The spaced apart source/drain metal contacts define a channel region in the active layer. An oxidizing ambient is provided adjacent the channel region and the gate and the channel region are heated in the oxidizing ambient to reduce the carrier concentration in the channel area. Alternatively or in addition each of the source/drain contacts includes a very thin layer of low work function metal positioned on the metal oxide semiconductor active layer and a barrier layer of high work function metal is positioned on the low work function metal.
1. A method of forming ohmic source/drain contacts in a metal oxide semiconductor thin film transistor comprising the steps of:
providing a gate, a gate dielectric, a high carrier concentration metal oxide semiconductor active layer with a band gap and spaced apart source/drain metal contacts in a thin film transistor configuration, the spaced apart source/drain metal contacts defining a channel region in the active layer, and further including an etch stop passivation layer positioned in overlying relationship on the channel region of the metal oxide semiconductor active layer and partially beneath the source/drain metal contacts, the etch stop passivation layer being formed with insulating material that has a glass transition temperature (Tg), where below the Tg the insulating material functions as a chemical barrier to O 2 , H 2 O, H 2 , N 2 , and to chemicals used in following fabrication processes above the etch stop passivation layer, and above the Tg the insulating material behaves as semi-liquid with high viscosity and with sufficient mobility to oxygen, hydrogen, and nitrogen atoms;
providing an oxidizing ambient adjacent the channel region; and
heating the gate and the channel region in the oxidizing ambient to reduce the carrier concentration in the channel area.
2. A method as claimed in claim 1 wherein the step of heating the gate and the channel region includes using a radiation source, including one of a lamp or a pulsed laser light, with photon energy below the bandgap of the metal oxide layer.
3. A method as claimed in claim 1 wherein the initial carrier concentration in the metal oxide semiconductor active layer is greater than approximately 1E18/cm3.
4. A method as claimed in claim 1 wherein the carrier concentration in the channel region of the metal oxide semiconductor active layer is reduced to less than 1E18/cm3.
5. A method as claimed in claim 1 further including the steps of forming each of the source/drain metal contacts with a first portion including a low work function metal and a high work function barrier metal, the work function of the low work function metal being one of equal to and lower than a work function of the metal oxide semiconductor active layer, and the work function of the barrier material being one of equal to and greater than the work function of the metal oxide semiconductor active layer, the first portion being positioned on the metal oxide semiconductor active layer, and a second portion of conductive metal positioned on the first portion.
6. A method as claimed in claim 5 where the low work function metal has a work function lower than a work function of approximately 4 eV.
7. A method as claimed in claim 5 where the high work function barrier metal has a work function higher than a work function of approximately 4 eV.
8. A method as claimed in claim 1 further including the steps of forming each of the source/drain metal contacts with a thin layer of low work function metal positioned on the metal oxide semiconductor active layer, the work function of the low work function metal being one of equal to and lower than a work function of the metal oxide semiconductor active layer, and a barrier layer of high work function metal positioned on the low work function metal, the work function of the high work function metal being one of equal to and greater than the work function of the metal oxide semiconductor active layer.
9. A method as claimed in claim 1 where, in the step of heating the gate and the channel region, the source/drain metal contacts shield the contact portions of the high carrier concentration metal oxide semiconductor active layer on each side of the channel region from the oxidizing ambient, whereby the contact portions on both sides of the channel portion retain the high carrier concentration.
10. A method as claimed in claim 1 where, in the step of heating the gate and the channel region, the passivation layer conveys oxygen therethrough during the heating step to provide the oxidizing ambient.
11. A method as claimed in claim 1 wherein the passivation layer includes material containing oxygen, the oxygen being released during the heating step to provide the oxidizing ambient.