Ohmic Contacts and Thin Film Resistors for GaN MMIC Technologies

Licentiatavhandling, 2011

Gallium nitride (GaN) based high electron mobility transistor (HEMT) technology has attracted much attention during the last decade which has resulted in a rapid development in material quality and device performance. GaN HEMT microwave electronics are currently finding its applications in wireless communication infrastructure and radar systems. This work deals with two aspects of GaN HEMTs and integrates circuits. The first part is focused on ohmic contacts for GaN HEMTs. The topic of the second part is titanium nitride- (TiN) and tantalum nitride (TaN) thin film resistors (TFRs). The common link between these two topics is the implementation in monolithic microwave integrated circuits (MMIC). The quality of the ohmic contacts affects the noise-, high power-, and high frequency performance of the HEMT. In this work tantalum (Ta) based ohmic contacts were developed. A contact resistance as low as 0.06 mm was obtained at an anneal temperature of 550 – 575 °C. Conventional titanium (Ti) based ohmic contacts on the other hand require an anneal temperature of around 800 °C. This high temperature may cause contact broadening and bad edge acuity, which prevents down-scaling of the HEMT. With the new metallization developed in this work excellent edge acuity was obtained. A high anneal temperature may also affect the sheet resistance of the GaN material. Moreover, for the sake of compatibility with the remaining MMIC process a low anneal temperature is preferable. TaN TFRs have previously been used in an in-house GaN HEMT MMIC process at Chalmers. TiN TFRs were developed since there is a need for films with a lower sheet resistance. TiN films as thick as 3500 Å were deposited by reactive sputtering without any signs of mechanical stress. The lowest sheet resistance obtained was 10 /□. The properties of TiN and TaN TFRs were compared. Measurements of composition, thermal dependence, and thermal- and electrical stress were performed. The opposite signs of the temperature coefficient of resistance of TiN and TaN were used to form temperature compensated TFRs. These were implemented in a Wheatstone bridge for on-chip temperature sensing.