Millimeter-wave Packaging Research Articles

Modeling of Practical Substrate Thickness in Multilayer Printed Circuit Board for Millimeter-Wave Packaging

This paper proposes a modeling of the practical substrate thickness so that it is possible to accurately design by predicting the dielectric thickness that varies for each circuit of a multi-layer printed circuit board (PCB) in the manufacturing process. The amount of dielectric to be filled into the empty space of the pre-stacked metal layer varies depending on the ratio of pre-stacked metal to the entire area during the high-temperature and high-pressure process, so the dielectric thickness between the inner layer and the outer layer metal varies. When the patterns are complex and non-uniform in design, the difference in the ratio of the pre-stacked metal occurs between the entire area and the partial area of the strip. For this reason, the dielectric thicknesses are manufactured differently for each circuit. Therefore, this paper proposes a practical dielectric thickness equation and a reference area to calculate the copper foil residual ratio. To find the optimal reference area for calculating the copper foil residual ratio, microstrip lines with the same width and length but different ratios of the surrounding pre-stacked metal were analyzed. Finally, a dielectric thickness prediction equation is proposed, and verified not only on a multilayer substrate with a thickness of 40 μm but also with 20 μm to show that it is a highly reliable radio frequency (RF) modeling solution.

Microwave/Millimeter-Wave Packaging for 5G and Beyond: Materials, Technologies, and Techniques [From the Guest Editors’ Desk

It is our pleasure to write this column and introduce the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">IEEE Microwave Magazine</i> reader to this focus issue “Microwave/Millimeter-Wave Packaging for 5G and Beyond.” This focus issue was inspired by past presidents of the IEEE Microwave Theory and Technology Society (MTT-S) as a means to promote the visibility of other IEEE Societies within MTT-S activities. The issue is sponsored by the MTT-S Technical Committee (TC)-16, Microwave and Millimeter-Wave Packaging, Interconnect and Integration. Advanced microwave and millimeter-wave (mm-wave) packaging is a multidisciplinary research area where transdisciplinary innovation is key to enabling the challenging applications of 5G and mm-wave. As a result, in parallel with the MTT-S, several other societies within IEEE are working to address these challenging issues.

High-density low-loss millimeter-wave package interconnects with the impact of dielectric-material surface roughness

Heterogeneous package integration and chiplet approaches are the key technology to enable next-generation high performance small form-factor packages for emerging applications. Millimeter-wave packaging for fifth-generation and upcoming sixth-generation platforms also need to meet the high-density low signal-loss interconnect specifications utilizing advanced conductor and dielectric materials. This article presents the comparison of the liquid-based photoimageable dielectric (PID) and dry-film dielectric materials in terms of interconnect path losses that are critical in mm-wave frequency bands. The conductor loss being more dominant in the frequency bands and in thinner dielectric structures, we assess daisy chains and microstrip lines on 15-μm dielectric by measuring the S-parameters to quantify the impact of the surface roughness at around 28 GHz. Measured results from the daisy chain and microstrip line structures exhibit that the smooth surface of the liquid-based PID (3 nm) leads to 8%–32% lower signal loss in the dB scale than the 325-nm rough dry-film dielectric. The study provides comprehensive experimental results that the different material forms with various surface roughness largely impact the package-level interconnect loss.

A Low-Loss Fan-Out Wafer-Level Package With a Novel Redistribution Layer Pattern and Its Measurement Methodology for Millimeter-Wave Application

A low-loss fan-out wafer-level package (FOWLP) has been proposed for millimeter-wave application. To ensure the radio frequency (RF) signal transition in a ground–signal–ground (GSG) configuration, the signal structure in the redistribution layer (RDL) is tightly surrounded by its ground. Over a wideband, the proposed RDL structure results in a low loss and a good match at the RF port. Moreover, an efficient measurement methodology has been proposed for the millimeter-wave package. By adopting a 50- $\Omega $ ceramic coplanar waveguide (CPW), performance of the millimeter-wave package is assessed by probes. This measurement method features quite a low measurement error because insertion loss of the added ceramic CPW is very small. By the proposed cascade de-embedding method, the parasitic effects resulting from the interconnections can be removed and the package performance can be achieved. The measured insertion loss for the proposed package is around 1 dB and the measured 10-dB return loss bandwidth is from 70 to 85 GHz.

Low-Loss Impedance-Matched Sub-25-μm Vias in 3-D Millimeter-Wave Packages

This article presents, for the first time, low-loss small microvias in build-up layers for the next-generation highdensity high-performance fifth-generation (5G) millimeter-wave (mm-wave) antenna-integrated packages. As the operating frequency increases, the signal losses in antenna packages become more critical and need to be mitigated to obtain desired performance in beamforming and massive multi-input multioutput. As electrical wavelengths in mm-wave spectra are short, the signal losses caused by microvias dominate the loss budget. To minimize the signal losses and identify the required microvia diameter in build-up layers, this article first focuses on the modeling of small microvias for impedance matching. Based on the models and simulated characteristic impedance, test vehicles with daisy chains are fabricated in build-up layers on a core package substrate. Ultraviolet (UV) laser is utilized to drill microvias, and targeted microvia diameters are obtained through a semiadditive patterning process. High-frequency measurements are also performed to correlate with the models and simulated results in the 28-GHz band. The characterization results exhibit good model-to-hardware correlation and indicate that small microvias (20 μm) provide impedance closer to 50 Ω compared with the larger microvias. This matched microvia impedance lowers reflection and insertion loss, resulting in a 10% reduction in the signal losses caused by microvias in the 5G New-Radio n257 band around 28 GHz.

Ultrathin Antenna-Integrated Glass-Based Millimeter-Wave Package With Through-Glass Vias

This article presents the design and demonstration of a high-bandwidth antenna-in-package (AiP) module focusing on low-loss interconnects and Yagi–Uda antenna performance fabricated on a 100- $\mu \text{m}$ low coefficient-of-thermal-expansion (CTE) glass for the 28-GHz band. It shows the modeling, design, and characterization of key technology building blocks along with the process development of advanced 3-D glass packages. The building blocks include impedance-matched antenna-to-die signal transitions, Yagi–Uda antenna, and 3-D active–passive integration with backside die assembly on 100- $\mu \text{m}$ glass substrates. The design and stack-up optimization of antenna-integrated millimeter-wave (mm-wave) modules is discussed. Process development to achieve high-density interconnects and precise dimensional control in multilayered thin glass-based packages is also described. The characterization results of the key technology building blocks show an insertion loss of 0.021 dB per through-package via (TPV), leading to the whole-chain loss of less than 1 dB and a return loss lower than 20 dB. The fabricated Yagi–Uda antenna features high repeatability of wide bandwidth due to the process control enabled by glass substrates. The antenna measurements show a bandwidth of 28.2%, which covers the entire 28-GHz fifth-generation (5G) frequency bands (n257, n258, and n261). The flip-chip assembled low-noise amplifier with 80- $\mu \text{m}$ solder balls shows a maximum gain of 20 dB as desired. The performance of the glass-based package integrated antennas is benchmarked to other 5G substrate technologies, such as organic laminates or co-fired ceramic-based substrates.

A Compact Integration of a 77 GHz FMCW Radar System Using CMOS Transmitter and Receiver Adopting On-Chip Monopole Feeder

This paper presents 77-GHz CMOS radar transmitter and receiver chips equipped with monopole feeders for use in frequency-modulated continuous-wave radar systems. The on-chip monopole feeder can feed not only the aperture of waveguides but also the slots on planar circuits by simply attaching the chips to a printed circuit board using a low-cost non-conductive epoxy. With the aid of a high ratio multiplier and the proposed on-chip feeder, a radar system can easily be integrated without needing to use precise and expensive millimeter-wave packaging technologies. In order to experimentally confirm this, the chips integrated with waveguides are first measured and a full radar system integrated on the planar circuit is evaluated using the microstrip patch antennas. Finally, with the benefit of compact integration technology, the design of a five-channel 3-D environmental sensing radar for small-unmanned aerial vehicles is presented.

Uniform-Field Over-Mode Waveguide for Spatial Power-Combining Applications

A new and effective millimeter-wave waveguide packaging technique suitable for 1-D spatial power-combining applications is presented. The transition structure using narrow corporate channels preserves the uniform field distribution when expanding the ${E}$ -plane width of a standard waveguide to several wavelengths. A Q-band test module, containing back-to-back transitions for an increased width from 2.84 to 52 mm, shows an average insertion loss of 0.34 dB in the 40–50-GHz range. The radiation patterns from an open-end module, cut in half from the original test module, confirm uniform field distribution across the over-mode guide aperture.

(Invited) SiGe BiCMOS Heterogeneous Integration Using Wafer Bonding Technologies

Heterogeneous integration has been a very attractive research topic for the last decades. Due to the limitations for further device scaling the More-than-Moore approach based on heterogeneous integration has been pointed out as the key integration path for future smart system integration [1]. Temporary and permanent wafer bonding technologies are one of the key enabling technologies for heterogeneous integration. Temporary wafer bonding is a well-established bonding technique to provide a stable handling and fabrication of ultra-thin silicon wafers. Permanent wafer bonding with various electrical isolating or conductive bonding interfaces is applied for wafer-level heterogeneous device integration [2]. Making available both temporary and permanent wafer bonding technologies together with a high performance SiGe BiCMOS technology will pave the way for new applications like millimeter-wave packaging, 3D-integration and smart sensors to realize multi-functional and miniaturized systems. The potential of temporary and permanent wafer bonding techniques for heterogeneous integration of a high performance SiGe BiCMOS technology with additional devices and technology modules will be presented by three different application examples namely BiCMOS embedded Through-Silicon Vias (TSV), silicon interposer including substrate-integrated waveguides and BiCMOS embedded microfluidics. With respect to the different applications the potential and the main technology obstacles for SiGe BiCMOS temporary and permanent wafer bonding will be highlighted. While the temporary wafer bonding process with subsequent wafer thinning and thin wafer backside fabrication is mainly limited by the robustness of adhesives against high temperature and high vacuum process conditions, the permanent wafer bonding based on SiO2-SiO2 fusion bonding is mainly limited by the demanding surface requirements in terms of roughness and wafer bow which has been optimized to fulfill the requirements. By optimizing the SiGe BiCMOS technology both temporary and permanent wafer bonding together with subsequent semiconductor fabrication becomes feasible. In summary, the development of wafer bonding application together with an industrial-level BiCMOS process will be presented. By the use of temporary and permanent wafer bonding new applications in the area of heterogeneous integration becomes feasible making SiGe BiCMOS technologies more attractive for future smart system applications.

A Flip-Chip Packaging Design With Waveguide Output on Single-Layer Alumina Board for E-Band Applications

This paper presents a millimeter-wave (mm-wave) packaging design for E-band long-haul point-to-point communications. A broadband flip-chip interconnect with standard ${\hbox{75}}~\mu{\hbox{m}}$ pad pitch is developed on an alumina substrate. To meet the application requirement, a microstrip line to WR-12 waveguide (WG) transition is proposed. The transition has no back-short cavity or modified WG involved. A back-to-back transition measurement is conducted to characterize the performance. Results show that the working bandwidth is from 69.5 to 87 GHz and the insertion loss at 77 GHz is less than 0.5 dB. A 40-nm CMOS power amplifier is packaged and measured with the proposed packaging design. The entire packaging loss is only 2.5 dB from pads to WG, including the loss of a 10-mm microstrip line. The simulated and measured results are in good agreement. The gain and peak output power are 13.4 dB and 17.6 dBm. The silicon temperature is measured as ${\hbox{44.4 }}^{\circ}{\hbox{C}}$ after saturated output for 30 min. The performance is close to probe measurement after deembedding the packaging loss. Comparison between bondwire and flip-chip packaging is discussed and both are verified in mm-wave packaging design.

Cavity Resonators Do the Trick: A Packaged Substrate Integrated Waveguide, Dual-Band Filter

The 2015 Microwave Theory and Techniques Society (MTT-S) International Microwave Symposium in Phoenix, Arizona, played host to a total of 16 student design competitions. One of these, sponsored jointly by the MTT-12 Technical Committee on Microwave and Millimeter-Wave Packaging and Manufacturing and the MTT-8 Technical Committee on Filters and Passive Components, was based on the design of a substrate integrated waveguide (SIW) dual-band filter.

A 77-GHz FMCW Radar System Using On-Chip Waveguide Feeders in 65-nm CMOS

This paper presents a 77-GHz radar system using a transmitter (Tx) and receiver (Rx) integrated with on-chip waveguide feeders in 65-nm CMOS. The newly proposed on-chip waveguide feeder shows the insertion loss of about 2 dB and more than 30% bandwidth. Additionally, both the Tx and Rx are integrated with internal ×10 frequency multipliers. Therefore, a radar system can be easily implemented without sensitive millimeter-wave packaging technology by mounting the Tx and Rx chips on the waveguide aperture. The interconnections for the low-frequency reference and baseband signals can be realized on the low-cost FR-4 printed circuit board. The radar system shows 9-dBm output power and 13-dB down-conversion gain from the waveguide port.

High-$Q$ Inductors Embedded in the Fan-Out Area of an eWLB

We investigate high-quality (high- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> ) inductors implemented in the redistribution layer (RDL) of the fan-in and fan-out area of an embedded wafer-level ball grid array (eWLB) package. The eWLB is an innovative package technology introduced recently for wireless applications. The technology has outstanding capabilities especially for high-frequency and millimeter-wave package design. We demonstrate that inductors realized in the eWLB fan-out area have negligible substrate losses and lower parasitic capacitances compared to inductors in the eWLB fan-in area. As a result, the inductors implemented in the fan-out area offer significantly higher quality factors and higher self-resonance frequencies. We investigate the effects of the chip-to-package interconnection. We show that the chip-to-package transition creates a bottleneck for the integration of high- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> inductors. We demonstrate the advantages of inductors in the fan-out area of the eWLB on the example of a 6-GHz voltage-controlled oscillator (VCO) chip manufactured in a 65-nm complementary metal-oxide-semiconductor process and assembled in an eWLB package. For the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> tank of this example, we use a 1.1-nH high- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> differential fan-out eWLB inductor to reduce the phase noise. For comparison, we investigate a VCO fabricated with a standard on-chip inductor and assembled in the identical eWLB package. Our measurement results demonstrate lower phase noise and higher output power for all VCOs with inductors embedded in the RDL compared to the reference VCO with the on-chip inductor. The measured phase noise for the VCO with the eWLB inductor in the fan-out area is in the best case 9 dB lower than that of the reference VCO with the on-chip inductor. The presented results prove the integration concept and demonstrate the excellent potential of embedded inductors realized in the fan-out area of the eWLB package.

An RCP Packaged Transceiver Chipset for Automotive LRR and SRR Systems in SiGe BiCMOS Technology

We present a transceiver chipset consisting of a four channel receiver (Rx) and a single-channel transmitter (Tx) designed in a 200-GHz SiGe BiCMOS technology. Each Rx channel has a conversion gain of 19 dB with a typical single sideband noise figure of 10 dB at 1-MHz offset. The Tx includes two exclusively-enabled voltage-controlled oscillators on the same die to switch between two bands at 76-77 and 77-81 GHz. The phase noise is -97 dBc/Hz at 1-MHz offset. On-wafer, the output power is 2 × 13 dBm. At 3.3-V supply, the Rx chip draws 240 mA, while the Tx draws 530 mA. The power dissipation for the complete chipset is 2.5 W. The two chips are used as vehicles for a 77-GHz package test. The chips are packaged using the redistribution chip package technology. We compare on-wafer measurements with on-board results. The loss at the RF port due to the transition in the package results to be less than 1 dB at 77 GHz. The results demonstrate an excellent potential of the presented millimeter-wave package concept for millimeter-wave applications.

Antenna Design of 60-GHz Micro-Radar System-In-Package for Noncontact Vital Sign Detection

A circularly polarized sequential-rotation 2 × 2 patch antenna array integrated with a 60-GHz Doppler radar system is implemented on the low-temperature co-fired ceramic (LTCC) substrate. The LTCC technology with multiple metal layers is able to satisfy the different dielectric thickness requirements of antennas and microstrip feedlines, and it also provides a compact millimeter-wave packaging option for the CMOS radar chip. Compared to linearly polarized single-patch antenna, the 10-dB bandwidth of the antenna array on LTCC increases from 1.3 (2.4%) to 7 GHz (12.8%) at 55 GHz, and narrow 50- Ω feedline is achieved to realize flip-chip transition. As the modeling and manufacturing variations are often present in the millimeter-wave systems, wide antenna bandwidth is able to cover the possible frequency drift and increase the yield of system-in-package. The circular polarization also provides better isolation against other sensors in multiple radars applications.

Novel Wideband Transition Between Coplanar Waveguide and Microstrip Line

A novel wideband vertical transition for connecting the coplanar waveguide (CPW) to the microstrip line is proposed. This transition can be very useful for millimeter-wave packaging and vertical interconnects. It is multilayered, partly tapered, and consists of only one via interconnect. Two different transitions are designed. The first transition allows connectivity of a CPW with Zc=50 Ω to a microstrip line with Zc=16 Ω with a bandwidth of 10-60 GHz. The second transition has the same characteristic impedance, Zc=50 Ω, at the two ports. In this case, the operating frequency is from 40 MHz to 60 GHz. The return losses of both transitions are generally lower than -10 dB over their indicated frequency ranges, while the maximum measured insertion losses are 1.8 and 2.4 dB for the first and second transition, respectively. To extract the S-parameters of the transitions, a new thru-line technique, based on the standard thru-reflect-line two-tier calibration is introduced. Simulation and experimental results, showing good agreement, are presented and discussed.

Experimental Analysis of the Water Absorption Effects on RF/mm-Wave Active/Passive Circuits Packaged in Multilayer Organic Substrates

The feasibility of using near-hermetic organic materials for microwave and millimeter-wave packaging is investigated. The effects of moisture on both passive and active components on/inside the nonhermetic materials are tested. Microstrip patch antennas on three low moisture absorption substrates (0.1% or less) are subjected to immersion tests until saturated and the corresponding measured frequency shifts are found to be less than 0.25% for two of the three materials tested. The recovery time from saturation to normal operation is found to occur from natural evaporation at room temperature in 10 min or less. One of the three materials, a promising organic laminate material called liquid crystal polymer (LCP), showed no measurable change in weight or in antenna resonance frequency between the ambient (air) state and that after extended submersion in water. In addition, a novel laminated LCP package for a 13-25 GHz monolithic microwave integrated circuit (MMIC) formed from seven thin layers of laser-machined LCP is subjected to immersion testing. The seal for the embedded chip cavity (formed by the LCP conforming around the feeding transmission lines) is found to be robust for protecting the embedded chip. Standard hermetic materials are suggested to be a potentially unnecessary requirement for reliable, long-lifetime, high-performance RF systems.

Low-Loss Integrated-Waveguide Passive Circuits Using Liquid-Crystal Polymer System-on-Package (SOP) Technology for Millimeter-Wave Applications

In this paper, we show a low-loss integrated waveguide (IWG), microstrip line-to-IWG transition, IWG bandpass filter (BPF), and system-on-package (SOP) using a liquid-crystal polymer (LCP) substrate, which can be used toward SOP technology for millimeter-wave applications. The proposed IWG can be used as a low-loss millimeter-wave transmission line on this substrate. The measured insertion loss of the IWG is -0.12 dB/mm and the measured insertion loss of two microstrip line-to-IWG transitions is -0.14 dB at 60 GHz. The evaluated IWG filter is demonstrated as the pre-select filter for RF front-end modules at the millimeter-wave band. The fabricated three-pole BPF at a center frequency of 61.1 GHz has specifications: a 3-dB bandwidth of approximately 13.4% (~8.4 GHz), an insertion loss of -1.8 dB at the center frequency of 61.1 GHz, and a rejection of >15 dB over the passband. The proposed IWG can also be used as a low-loss millimeter-wave feed-through transition and interconnection between the monolithic microwave integrated circuit and the module instead of the vertical via structure. In terms of a SOP on LCP for millimeter-wave applications, the top face of the IWG does not have any electromagnetic effects, and a package lid can be attached to provide a hermetic sealing. These low-loss IWG circuits on LCP can easily be used in many millimeter-wave packaging applications

Signal Integrity Characterization of Microwave XFP ASIC BGA Package Realized on Low-K Liquid Crystal Polymer (LCP) Substrate

With the development of low-K nanometer devices, the need for compatible packaging material is ever increasing. Liquid crystal polymer (LCP) is emerging as a promising material for RF, microwave, and millimeter-wave packaging. Its coefficient of thermal expansion can be matched to that of low-K die to ensure mechanical reliability. This paper, for the first time, characterizes the electrical performance of a wire bonded application-specific integrated circuit (ASIC) ball grid array (BGA) package based on LCP substrate technology for application in 10 Gb/s small form factor pluggable module (XFP) optical communication systems. Specifically, it compares the electrical performance of LCP to that of traditionally used FR4/spl I.bar/epoxy (FR-4) and Polyimide (PI) substrate materials. Findings show that at 10 GHz, insertion loss was decreased as much as 31% and 15% compared to FR-4 and PI, respectively. In particular, mode conversion was decreased by 66% and 42% compared to FR-4 and PI, respectively. Time delay was decreased by 10 and 4 ps compared to FR-4 and PI. No significant differences in power, ground coupling, and simultaneously switching output (SSO) noise at 10 GHz were observed. Based on the package structure used in this study, it was concluded that LCP offers superior electrical performance compared to FR/spl I.bar/4, PI, and is qualified as next generation substrate material for high data rate XFP BGA packaging.

60-GHz direct-conversion gigabit modulator/demodulator on liquid-crystal polymer

In this paper, we demonstrate the first implementation of the integrated system-on-package (SOP) 60-GHz gigabit modulator and demodulator on liquid-crystal polymer (LCP). LCP provides an organic low-cost low dielectric-constant platform suitable for millimeter-wave passive design and packaging. Firstly, we demonstrate a 60-GHz planar bandpass filter and RF/baseband duplexer as the building blocks of the integrated module. Measurement results show /spl sim/3-dB insertion loss in the bandpass filter, as well as the RF path of the duplexer, and a higher than 30-dB isolation between the baseband and RF outputs. We utilize those building blocks in the design and implemented a hybrid 60-GHz antiparallel diode-pair-based 4/spl times/ sub-harmonic mixer suitable for direct up or down conversion. Measurement results indicate a better than 17-dB insertion loss with 1.25-GHz baseband bandwidth for the 4/spl times/ mixer. Input 1-dB compression point of -2 dBm has been measured. Two subharmonic mixers are integrated back-to-back to perform the simultaneous binary phase-shift keying modulation and demodulation of the pseudorandom binary sequences. Eye diagrams show a better than 13-dB SNR for a data rate up to 1.5 Gb/s. 40-GHz versions of the 4/spl times/ subharmonic mixer and the back-to-back chain have also been implemented. Hence, we present the first integrated millimeter-wave gigabit SOP modulator and demodulator on LCP.