Printed Circuit Design & Fab - February 2009 - (Page 24)

PackaGinG FiGurE 3. (A) Single layer array and (B) Multilayer array implementation.14 FiGurE 4. Fabricated single-layer 2x2 phased array module.15 a B FiGurE 5. (A) Stack-up of multilayer communication module. The features shown on the bottom layer are actually on the backside. The cavities in the LNA package layer line up to protect the chip, wire bonds, capacitors and to open a window for DC probing; (B) Fabricated multilayer phased array 14-15. sists of a MMIC LNA, MEMS phase shifter, RF power distribution network, biasing circuits and antenna arrays. In this 3D prototype, two antenna arrays are compared; the first is implemented using a single-layer and the second with a multi-layer LCP. Both implementations are capable of 12 degrees of beam steering. The design frequency was 14 GHz and the measured return loss was greater than 12 dB for both implementations. The use of an LNA allowed for a much higher radiated power level. FiGurE 3 shows the conceptual drawing of single and more compact multi-layer approaches. The module operates by receiving an RF signal at 14GHz. The signal is amplified by an LNA and then fed to a pair of one-bit MEMS phase shifters. If the phase shifters are in phase, the antenna array radiates perpendicular to the substrate. If the phase shifters are out of phase, the radiation is steered left or right – depending on the length of the phase path. layer. A silicon nitride layer was deposited using PECVD and etched using an RIE. A 2-μm thick sacrificial photo resist layer was patterned and hard baked. An electron beam evaporator was used again to deposit a 200A-2000A-200A Ti-Au-Ti layer. Electroplating was used to increase the gold thickness of the antennas and MEMS bridges to 1.5 μm. The sacrificial photoresist layer was removed using a stripping agent and dried with a CO2 critical point dryer. The DC bias lines for the capacitive RF MEMS switches were evaporated with the first seed layer and were not plated. The ground and bias pads for integration of the MMIC LNA were added at the same time as the MEMS to prevent any additional process steps. Once the MEMS were released, the LNA and offchip capacitors were mounted onto the sample using silver epoxy and were wire bonded to connect the LNA to the DC bias and RF signal lines. FiGurE 4 shows the fabricated single layer module. Single-Layer Module The antenna array was fabricated on a 3-inch diameter circle that was precisely cut using a CO2 laser. An electron beam evaporator was used to deposit a Ti-Au 24 Multilayer Module Implementing a multilayer configuration is much more challenging than a single-layer configuration because the approach requires multilayer alignment, device packaging, substrate bonding, fabrication on two sides of a substrate and a method for transmitting the data across layers. The top substrate was fabricated in the same way as the single-layer approach without the LNA. On the backside of the top substrate, the metal layer is etched to provide the window for aperture coupling. The final fabrication stackup is shown in FiGurE 5a. There are four main layers. The top layer has the RF input, MEMS phase shifters and phased array. The bottom layer has the LNA and off-chip capacitors. The LNA package layer has laser micro-machined cavities that protect the LNA, wire bonds and off-chip capacitors. It also provides a window for accessing the LNA DC bias pads. The LNA cap layer covers the cavities to protect the components inside. The DC bias for the LNA is accessed on the backside of the antenna to minimize interference of the DC biasing wires with the antenna. The LNA was centered directly under the 2x2 array. Before the system was assembled, all of the layers were fabricated independently and then bonded together. The fabricated multi-layer antenna array is shown in FiGurE 5B. The multilayer module (FIGURE 5B) is 25% smaller in size due to the placement of the LNA in the interior layer. Another advantage of the multilayer module is that the antennas can be effectively shielded, thereby preventing radiation from other components. However, in this prototype, the multilayer implementation shows more loss due to longer RF signal length and aperature coupling. FiGurE 6a shows the measured return loss of the single layer antenna with LNA, and FiGurE 6B shows the multilayer antenna array with the LNA. Using thick, highly conFEBRUARY 2009 printEd CirCuit dESign & fAB

Table of Contents Feed for the Digital Edition of Printed Circuit Design & Fab - February 2009

Printed Circuit Design & Fab - February 2009 Contents Our Line Market Watch Around the World Happenings ROI Tip Jar BGA Bulletin Interconnect Strategies Final Finsh Forum Defects Database Embedded Active Components In Multilayer LCP Packages Simulation: The Need for Speed Advanced Registration Systems The DC Design Squeeze Ad Index Do You Really Want a Better Autorouter? Designing With Conductive Materials, Part 1 Off th eShelf Marketplace On the Forefront

Printed Circuit Design & Fab - February 2009

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