Wei-Cheng Wu

60 GHz Broadband MS-to-CPW Hot-Via Flip Chip Interconnects

In this letter, the microstrip-to-coplanar waveguide (MS-to-CPW) hot-via flip chip interconnect has been experimentally demonstrated to have broadband performance from dc to 67 GHz. The interconnect structures with the hot-via transitions were first designed and optimized by using the electromagnetic simulation tool. Three types of designs were investigated in this letter. The interconnect structures were then fabricated and radio frequency (RF) tested up to 67GHz. The optimized interconnect structure with the compensation design demonstrated excellent RF characteristics with the insertion loss less than 0.5dB and the return loss below 18dB over a very broad bandwidth from dc to 67GHz. This is to our knowledge the best result reported for this frequency range.

SPDT GaAs Switches With Copper Metallized Interconnects

Copper metallized AlGaAs/InGaAs pseudomorphic high-electron-mobility transistor (PHEMT) single-pole-double-throw (SPDT) switches utilizing platinum (Pt, 70nm) as the diffusion barrier is reported for the first time. In comparison with the Au metallized switches, the Cu metallized SPDT switches exhibited comparable performance with insertion loss of less than 0.5dB, isolation larger than 35dB and the input power for one dB compression (input P1dB ) of 27dBm at 2.5GHz. These switches were annealed at 250deg for 20h for thermal stability test and showed no degradation of the dc characteristics after the annealing. Also, after 144h of high temperature storage life (HTSL) environment test, these switches still remained excellent and reliable radio frequency (RF) characteristics. It is successfully demonstrated for the first time that the copper metallization using Pt as the diffusion barrier could be applied to the GaAs monolithic microwave integrated circuits switch fabrication with good RF performance and reliability

A

In this paper, delta-doped InGaP/InGaAs pseudomorphic high-electron-mobility transistors (pHEMTs) with doping-profile modifications are investigated in order to improve the device linearity. The proposed modification was based on the third-order intermodulation distortion (IM3) and the third-order intercept point (IP3) analysis using a simple equivalent circuit of the devices. The correlations of the extrinsic transconductance (Gm) with IM3 and IP3 indicate that the flatness of Gm, as a function of gate-bias causes a lower IM3 level. On the other hand, a high Gm with a flatter Gm distribution results in higher IP3 value for the device. Therefore, doping modifications that improve the flatness of the Gm distribution will also improve the device linearity. Doping modifications in the Schottky layer (Schottky layer doped) and in the channel layer (channel doped) of the conventional delta-doped InGaP/InGaAs pHEMT were investigated. It was also found that extra doping, either in the channel region or in the Schottky layer, improved the flatness of the Gm distribution under different gate-bias conditions. This achieved a lower IM3 and a higher IP3 with a small sacrifice in the peak Gm value. The power performances of these devices were tested at different drain biases. Even though it had the lowest electron mobility among the three different types of devices studied, the channel-doped device demonstrated the best overall linearity performance, the highest IP3 value, the lowest IM3 level, and the best adjacent-channel power ratio under code-division multiple-access modulation.

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In this paper, a novel transition design using vertical ldquocoaxial transitionrdquo for coplanar waveguide (CPW-to-CPW) flip-chip interconnect is proposed and presented for the first time. The signal continuity is greatly improved since the coaxial-type transition provides more return current paths compared to the conventional transition in the flip-chip structure. The proposed coaxial transition structure shows a real coaxial property from the 3-D electromagnetic wave simulation results. The design rules for the coaxial transition are presented in detail with the key parameters of the coaxial transition structure discussed. For demonstration, the back-to-back flip-chip interconnect structures with the vertical coaxial transitions have been successfully fabricated and characterized. The demonstrated interconnect structure using the coaxial transition exhibits the return loss below 25 dB and the insertion loss within 0.4 dB from dc to 40 GHz. Furthermore, the measurement and simulation results show good agreement. The novel coaxial transition demonstrates excellent interconnect performance for flip-chip interconnects and shows great potential for flip-chip packaging applications at millimeter waves.

-Doped InGaP/InGaAs pHEMT With Different Doping Profiles for Device-Linearity Improvement

A gold-free, fully Cu-metallized InGaP/GaAs heterojunction bipolar transistor using platinum as the diffusion barrier has been successfully fabricated. The HBT uses Pd/Ge and Pt/Ti/Pt/Cu for n-type and p+-type ohmic contacts, respectively, and Ti/Pt/Cu for interconnect metals with platinum as the diffusion barrier. The Ti/Pt/Cu structure was stable during annealing up to 350°C judging from the X-ray diffraction (XRD) data and sheet resistance. A current-accelerated stress test was conducted on the device with a current density JC=140 kA/cm2 for 24 h, and the current gain showed no degradation. The devices were also thermally annealed at 250°C for 24 h and showed little change. We have successfully demonstrated that an Au-free, fully Cu-metallized HBT can be realized using Pt as the diffusion barrier and Pd/Ge and Pt/Ti/Pt/Cu as the ohmic contacts.

Design, Fabrication, and Characterization of Novel Vertical Coaxial Transitions for Flip-Chip Interconnects

This study demonstrates a flip-chip interconnect with epoxy-based underfill (εr = 3.5 and tan δ = 0.02 at 10 MHz) for packaging applications up to V-band frequencies. To achieve the best interconnect performance, both the matching designs on GaAs chip and Al2O3 substrate were adopted with the underfill effects taken into consideration. The optimized flip-chip interconnect showed excellent performance from dc to 67 GHz with return loss below -20 dB and insertion loss less than 0.6 dB. Furthermore, the dielectric loss induced by the underfill was extracted from measurement and compared with the simulation results. The reliability tests including 85°C/85 % relative humidity test, thermal cycling test, and shear force test were performed. For the first time, the S-parameters measurement was performed to check the flip-chip reliability, and no performance decay was observed after 1000 thermal cycles. Moreover, the mechanical strength was improved about 12 times after the underfill was applied. The results show that the proposed flip-chip architecture has excellent reliability and can be applied for commercial applications.