Group III nitride device and method of fabricating a Group III nitride-based device

1. A method of fabricating a Group III nitride-based transistor, the method comprising:

providing a substrate comprising a Group III nitride-based layer, a first passivation layer on a first major surface of the Group III nitride-based layer and a second passivation layer arranged on the first passivation layer, the second passivation layer having a different composition from the first passivation layer;

forming a first ohmic contact and a second ohmic contact on the first major surface of the Group III nitride-based layer such that the first ohmic contact and the second ohmic contact extend at least partially through the first passivation layer, wherein the first passivation layer extends between the first ohmic contact and the second ohmic contact, and wherein the second passivation layer is arranged on the first ohmic contact and on the second ohmic contact;

covering the first ohmic contact and the second ohmic contact with a first sublayer of passivation material;

planarising to form an intermediate planarized surface comprising a surface of the first ohmic contact, a surface of the second ohmic contact and the first sublayer; and

forming a second sublayer of passivation material on the planarized surface and arranged on the first ohmic contact and the second ohmic contact, the first and second sublayers forming the second passivation layer;

forming a first mask layer on the second passivation layer, the first mask layer comprising a first insulating layer arranged on the second passivation layer and a first resist layer on the first insulating layer;

forming a first opening for a gate electrode in the first mask layer, the first opening extending through the first resist layer and through the first insulating layer;

forming a second opening for a field plate in the first mask layer, the second opening extending through the first resist layer and through the first insulating layer;

removing the second passivation layer exposed by the first opening and forming a first via for the gate electrode, the first via having a base formed by the first passivation layer;

removing the second passivation layer exposed by the second opening and forming a second via for a field plate, the second via having a base formed by the first passivation layer;

removing the first resist layer;

applying a second resist layer onto the first insulating layer that covers the second opening for the field plate and leaves the first opening for the gate electrode and a region of the first insulating layer adjacent the first opening uncovered;

removing the first passivation layer exposed by the first via and increasing the depth of the first via such that the first via has a base formed by the Group III nitride-based layer;

removing the second resist layer; and

depositing an electrically conductive layer into the first via and the second via.

2. The method of claim 1 , wherein the second resist layer leaves a portion of the first insulating layer that is arranged adjacent the first via exposed.

3. The method of claim 1 , wherein the base of the first via has a width of between 50 nm and 400 nm and/or a minimum distance between the base of the first via and the second via is between 50 nm and 400 nm.

4. The method of claim 1 , wherein the first passivation layer comprises silicon nitride, the second passivation layer comprises silicon oxide and the first insulating layer comprises titanium nitride.

5. The method of claim 1 , further comprising:

forming a third opening in the first mask layer, the third opening extending through the first resist layer and the first insulating layer and being positioned above the second ohmic contact;

removing the second passivation layer exposed in the third opening and forming a third via having a base exposing a portion of the second ohmic contact;

covering the third via with the second resist layer;

after removal of the second resist layer, further depositing the electrically conductive material into the third via; and

planarising to form a planarised surface comprising isolated areas of electrically conductive material positioned in the first, second and third vias, each isolated area of electrically conductive material being surrounded by the second passivation layer.

6. The method of claim 5 , further comprising:

forming a second insulation layer on the second passivation layer and on the isolated areas of electrically conductive material;

forming a third insulating layer on the second insulation layer, the second and third insulation layers having differing compositions;

structuring the third insulation layer to form a first trench positioned above the first via, the first trench having a base spaced apart from the first via by a portion of the third insulating layer and the second insulating layer, the first trench being laterally wider than the first via;

forming a fourth via in the base of the first trench that exposes the electrically conductive material in the first via by removing the third insulation layer and the second insulation layer, the fourth via being laterally smaller than the first trench; and

forming a second electrically conductive material layer in the fourth via and in the first trench.

7. The method of claim 6 , further comprising:

structuring the third insulation layer to form a second trench above the second via and a third trench above the third via, the second trench having a base spaced apart from the second via by a portion of the third insulating layer and the second insulating layer, the second trench being laterally wider than the second via, the third trench having a base spaced apart from the third via by a portion of the third insulating layer and the second insulating layer, the third trench being laterally wider than the third via;

forming a fifth via in the second trench that exposes the electrically conductive material in the second via by removing the third insulation layer in the base of the second trench;

forming a sixth via in the third trench that exposes the electrically conductive material in the third via by removing the third insulation layer in the base of the third trench; and

depositing the second electrically conductive material layer into the second and third trenches and into the fifth and sixth vias.

8. The method of claim 1 , wherein the electrically conductive material is tungsten.