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An Advanced, High-Power, GaN-chipset-based, X-band Transmit/Receive 3D Ceramic Hybrid Circuit Module with Calibration Capability
Keywords: 3D hermetic packaging, transmit/receive module, LTCC
As a follow-up of the successful demonstration of an X-band, low-footprint transmit/receive (T/R) hybrid circuit module (3DTRM) concept making use of a 3D interconnect and die placement packaging approach, the interest for carrying out additional development work took a further step aiming at achieving a three-fold module performance improvement. First of all, the possibility to perform an Active Electronically Steerable Antenna (AESA) highly accurate calibration was implemented by integrating a wide band, high directivity directional coupler, designed as a coplanar/stripline coupling structure, in the LTCC multilayer module circuitry. Secondly, the heat extraction capability of the metal-ceramic hermetic package was enhanced through a re-design of the monolithic microwave integrated circuit (MMIC)-to-sink interface. And finally, the gallium arsenide (GaAs) MMIC high power amplifier (HPA) of the first 3DTRM version was replaced by a gallium nitride (GaN) HPA MMIC, obtaining both a higher transmit output power @ 5dB of compressed gain and an improved power added efficiency (PAE) at module level. All the above was achieved together with a further module footprint reduction, needed to enable an improvement of the Synthetic Aperture Radar (SAR) azimuth scanning capability. A 3D packaging technology concept suitable for space applications, based on the assembly of active and passive dice on two different semi-modules inside the hermetic cavity of the hybrid circuit package, was already demonstrated by our joint Thales Alenia Space (TASI) and University of L’Aquila team[5]. Each semi-module constitutes a metal-ceramic LTCC Integral Substrate Package (ISP): in the first one, a core-chip MMIC for beam amplitude and phase control is assembled, while the other (the “lid” of the hermetic package) contains the low noise amplifier, single-pole double-throw (SPDT) switch and high power amplifier MMIC’s. Both semi-modules are electrically interconnected to each other along the “third” (z-axis) dimension of the module by means of an interposer board. Being the main goal of such initial work the significant reduction of the module footprint, and in order to save module real-estate and to limit its complexity, the possibility to perform calibrations both at module and at antenna instrument level was left as a potential upgrade to be addressed in the frame of future development. The work described in this paper went in the direction of filling such gap, as a first mandatory improvement before considering the use of the proposed 3D interconnect and dice placement approach on TASI antenna products. On the other hand, transfer of heat from the 3DTRM to the equipment onto which it is assembled at active antenna level is of primary concern, and means for conveying the heat from the external surface of the module to the satellite sink have to be carefully devised. But before the heat leaving the HPA MMIC reaches the external 3DTRM surface interfacing the thermal extraction harness at antenna level, all the involved materials and processes for the manufacturing of the 3DTRM itself have to be carefully engineered and optimized. In order to get closer to a demonstrator representative of the targeted improved active antenna performances, the main circuital feature needed was the implementation of the capability to perform an accurate SAR AESA calibration, without any increase in module footprint. For the purpose of sampling the RF signal at the antenna port during calibration, typically a microstrip directional coupler is employed. For this work, a challenging target of -45 dB of isolation for the coupling structure, in ideal port matching conditions, was set. Some real estate was devoted for the integration of the directional coupler, which translated in a push to the LTCC multilayer layout and routing design rules. The directional coupler was implemented in a mixed 3D stripline/coplanar configuration. With respect to a typical microstrip configuration, a significant improvement was expected from our design as a result of such coupler structure being embedded in a homogeneous dielectric media, so avoiding the unequal even and odd mode phase velocities that characterize edge-coupled microstrip lines. In order to improve the heat extraction capability of the package, the RF semi-module was designed with a through-cavity that allows for the placement of a hermetically soldered heat sink that is positioned right below the tab to which the HPA MMIC is gold-tin (AuSn) soldered. As a baseline constructive approach, such tab is attached to the heat sink by means of a high thermal conductivity epoxy resin. So, with respect to the first version of the package, the ceramic material (LTCC) between that tab and the sink has been removed, reducing in this way the total thermal resistance seen by the backside of the HPA MMIC down to the external surface of the sink from which the heat is to be extracted at the upper assembly level of the module in the SAR antenna. The above constitutes a “first step” of improvement. Another important contribution came from the use of a higher thermal conductivity tab material for the gold tin soldering of the GaN HPA MMIC, which was diamond-copper (Cu-D) composite in our case; this material has roughly half the thermal resistance of copper tungsten, which means a non-negligible increase in the heat extraction capability of the package. Finally, the last contribution to the heat drain improvement consisted in the use of AuSn alloy, in place of the conductive epoxy, for the attachment of the tab onto which the MMIC is also AuSn soldered. But to implement this additional soldering step in a reliable way, which means mainly without jeopardizing the package long-term hermeticity, the attachment of the seal ring and the thermal insert to the LTCC substrate has to be carried out using a higher melting point alloy, which was gold germanium (AuGe) in this study. As the main outcome of our research work, a high performance evolution of the advanced 3DTRM concept was successfully demonstrated. In addition to implementing two levels of MMICs and passive components assembly and using 3D interconnection techniques, such module was upgraded by integrating the calibration feature, improving the heat extraction capability and by extending the use of GaN MMIC technology to the power section of the module. In summary, successful demonstration of: •calibration feature performance in terms of excellent antenna port isolation in RX calibration mode, and low variability of coupling factor among different modules, •high compressed transmit mode (TX) power output, •wide useful bandwidth, •AuGe package manufacturing process set-up, including Cu-D heat spreader attachment with AuSn, •PAE at module level better than 30% @ 10% TX duty cycle, confirm that the evolution of the original 3D hermetic packaging and interconnect integration concept is suitable for SAR antenna space applications.
Antonio Fina, Head of Microelectronics R&D Unit
Thales Alenia Space Italia
L'Aquila, AQ

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