Flipping 60 GHz transistors
A TEAM of Taiwanese engineers has used flip-chip packaging to build an InGaAs HEMT delivering up to 6.5 dB of gain at 60 GHz. The researchers argue that one of the strengths of flip-chip technology is its simplicity: The circuit needs no passive components, such as metal-insulator-metal capacitors and thin-film resistors. “The main advantage of this approach is the inclusion of matching circuits on the carrier for flip-chip packaging. This provides a cost effective solution for seamless integration of the device onto the circuit,” claims team-member Edward Chang from Yuan Ze University. Silicon CMOS can also yield circuits operating at 60 GHz, which can find deployment in wireless personal area networks, wireless high-definition multimedia interfaces and wireless docking stations. However, III-Vs have the upper hand in several key areas: A lower noise figure, lower DC power consumption and superior linearity. Construction of 60 GHz devices began with MBE growth of HEMT epistructures featuring a 15 nm-thick In0.6Ga0.4As channel. Epiwafers were thinned to 100 μm and diced into die that were screened for DC and RF characteristics. Good die were then bonded to sapphire substrates, which were coated with a thin film of metal that had been processed to create transmission lines. Flip-chip packaging made no impact on the DC performance of the InGaAs HEMT, which has a drain-source current of 350 mA/mm and a transconductance of 600 mS/mm at a drain-source voltage of 0.5 V. Minimal parasitic effects at interconnections lead to a 0.7 dB reduction in the maximum achievable gain to 6.5 dB. Two-stage gain blocks were also built with the team’s flip-chip approach. At 60 GHz these devices delivered 9 dB of small signal gain when drawing just 20 mW. Cheng claims that the simplicity and lowcost of the process makes the flip-chip approach suitable for high-volume manufacturing: “It’s based on the very mature PCB printing technology.” The team is aiming to make its process even cheaper with flip-chip-on-board technology that replaces substrates with organic dielectrics.
