Technical Insight
Simplifying gold-free technology for nitride HEMTs
A gold-free metal stack enables HEMT processing that is compatible with silicon fabs
A partnership between researchers at MIT, Aixtron and the Singapore-MIT Alliance for Research and Technology have developed a gold-free process for forming ohmic contacts that involves an annealed Ti/Al/Ni/Pt stack
By developinga gold-free process for forming ohmic contacts, an international research team has simplified the heterogeneous integration of GaN transistors with silicon CMOS digital integrated circuits.
This work is important because the unification of GaN and silicon technologies promises to enable a new level of circuit design, but if chips are to be made in silicon fabs, they must avoid using gold in any processes. “Gold is very easy to diffuse into silicon,” explains Zihong Liu from the Singapore-MIT Alliance for Research and Technology. “It generates deep acceptor and donor levels, and thereby degrades silicon device performance.”
Liu, along with co-workers from MIT and Aixtron, have developed a process that involves the deposition of a Ti/Al/Ni/Pt stack of metals, which are then annealed under nitrogen.
This team is not the first to develop a gold-free process, but it claims that its approach has advantages over rival methods, mainly in terms of simplicity.
One alternative is to create a recess prior to the metallization of the ohmic contacts. Liu and his co-workers have already succeeded with this approach, forming a Ti/Al/W stack with a contact resistance of just 0.5 Ω mm.
However, having to perform recess and metallization steps increases process complexity.
Another option is to use a very thin AlGaN barrier. But Liu says that this has to be capped with SiN, which is not found in commonly used GaN HEMTs.
There is also a process involving implantation of ions in the AlGaN/GaN materials underneath the ohmic metals. This can realise a contact resistance of just 0.5 Ω mm, but it requires dopant activation at typically 1200 °C, and this high temperature can create defects and degrade device performance.
Finally, there are processes involving tantalum that can realise a contact resistance of just 0.5 Ω mm. However, the evaporation temperature for this metal is very high.
“For many e-beam evaporators, for example the BOC Edwards Auto 306, the power of the e-gun is not high enough to melt and evaporate tantalum,” explains Liu. He adds that melting tantalum is possible with some high-power e-guns, but during evaporation the wafers tend to get very hot, and that makes subsequent lift-off processes challenging.
The latest gold-free process developed by Liu and his co-workers avoids high temperatures. It has been applied to devices formed on an epiwafer featuring a 1.5 µm-thick buffer layer, a 1 nm AlN spacer, a 20 nm undoped Al0.25Ga0.75N barrier and a 3 nm GaN cap.
Inductively couple plasma etching formed mesa structures in this epiwafer, which was grown by MOCVD on a
150 mm high-resistivity silicon substrate. Deposition of a Ti/Al/Ni/Pt (20 nm/ 60 nm/ 40 nm/50 nm) stack, followed by rapid thermal annealing for 30 s at 975 °C in nitrogen gas, created source and drain contacts.
Measurements with the transition line method determined a contact resistance of 0.6 Ω mm. Liu and his co-workers believe that this ohmic contact is formed in the same way as that with Ti/Al/Ni/Au, with TiAlN penetrating into the AlGaN/AlN/GaN structures.
“But without gold, the Ti/Al/Ni/Pt needs a higher annealing temperature and a re-optimised Ti/Al ratio for TiAlN to penetrate into the AlGaN/AlN/GaN to form a good ohmic contact,” says Liu.
One of the features of the Ti/Al/Ni/Pt stack is its smoothness. Atomic force microscopy images of this contact have a root-mean-square surface roughness of just 4.6 nm, compared with 13.9 nm for the conventional gold-containing stack.
According to Liu and his co-workers, this greater smoothness should aid the fabrication of high-frequency devices with very short gate spacings. What’s more, they should suppress current crowding effects and avoid reliability issues in high power devices.
Comparing the HEMTs with identical devices, aside from having a gold-free contact, revealed that the change of metal stack did not make a considerable difference to DC performance. The gold-free HEMT had a maximum drain current of 700 mA and a maximum transconductance of 140 mS/mm.
“Next we will try to develop a gold-free ohmic contact method with a low annealing temperature,” reveals Liu. Cutting the temperature will make this process compatible with a gate-first process and a silicon-CMOS first, GaN-on-silicon integration process.
Z. Liu et. al. Appl. Phys.
Express 6 096502 (2013)
