Creating Distinctive Connections between Multifunctional Microwave Circuits and Mobile-Terminal Radio-Frequency Integrated Chips Using Integrated Passive Device Technology

Yongle Wu,Mengdan Kong,Zheng Zhuang,Weimin Wang

School of Electronic Engineering,Beijing University of Posts and Telecommunications,Beijing 100876,China

Abstract:In this review,the advanced microwave devices based on the integrated passive device(IPD)technology are expounded and discussed in detail,involving the performance breakthroughs and circuit innovations.Then,the development trend of IPD-based multifunctional microwave circuits is predicted further by analyzing the current research hot spots.This paper discusses a distinctive research area for microwave circuits and mobile-terminal radio-frequency integrated chips.

Keywords:chips; integrated passive device(IPD);multifunctional;microwave circuit

I.INTRODUCTION

In radio frequency(RF)front-end transmitter system,microwave devices including frequency select device,interconnection,and power amplifiers play the indispensable roles.A great amount of new designs of microwave devices sprang up in the last few years,as a hot research field.The fast-growing of modern mobile and wireless communication results in an increasing demand for the multifunctional microwave circuits with the smaller size and lower loss.

The frequency-selecting circuits generally cover the single-ended/balanced filters and duplexers.Their main innovations focused on the tunable frequency[1–3],bandwidth expansion[4–6],reconfigurable function[7–9],common-mode suppression[10–12],and absorptive performance[13,14].Especially for the reflectionless bandpass[13]and bandstop filters[14],[15],which have been paid much attention recently since the undesired signals can be absorbed by the reflectionless circuits,thus reducing the adjacent circuitperformance attenuation.

For multi-port interconnected circuits including power dividers,balun,couplers,and crossovers,on the one hand there’s increasing study on the filtering integration.In[16],[17],the Chebyshev network is embedded into the power divider and balun,respectively,for implementing the filtering performed by using the coupled-line structure[18–20].On the other hand,the design of interconnected microwave devices mainly concentrated on building the balanced and single-ended hybrid circuits to suppress the common-mode noises and electromagnetic interference,such as the balanced-to-unbalanced(BTU)[21–23]or unbalanced-to-balanced(UTB)[24–26]power dividers,BTU[27]or UTB[28]couplers,crossovers with balanced and single-ended ports[29],etc.

Microwave passive devices are used to transfer the desired signals,while the power amplifiers are mainly used to amplify the RF signals with high efficiency in the transmit direction.A large number of researchers have done research on this aspect.In the last decades,masses of power amplifiers have been investigated by the non-analytic or semi-analytic lowpass and bandpass transmission-line or coupled line and lumpedelement matching approaches[30–33].In order to further simplifying the design procedure,the stepimpedance broadband matching quasi-lowpass network is used in[34].Additionally,the co-design of filters and power amplifiers are also widely researched for low loss and miniaturization of RF systems[35–39].

Nowadays,it is an inevitable trend to develop miniaturized and multifunctional microwave circuits with low power loss.However,the current multifunctional passive and active devices are primarily manufactured by the PCB,cavity waveguide,SIW,and so on,which are not suitable for the applications on the mobileterminal RF systems.Fortunately,the growing maturity of integrated passive device(IPD)process makes the implementation of ultra-miniaturized multifunctional RF chips becomes possible.This paper aims to review the current IPD-based microwave circuits and further create distinctive connections between multifunctional RF devices and mobile-terminal integrated chips based on the IPD technology.In this review,the advanced IPD-based frequency-selecting circuits covering filters and diplexers are discussed in Section II firstly.And then Section III summarizes the developed power dividers,baluns,and couplers using IPD process.In Section IV,the current reported power amplifier chips embedded in IPD technology are depicted.Finally,this paper concludes the future trend for the mobile-terminal integrated RF chips in Section V after reviewing the current situation of microwave circuits.

II.ADVANCEDIPDFREQUENCYSELECTING CIRCUITS

2.1 IPD-based Filters

In mobile-terminal RF systems,filters serve as the key components for selecting transmission signals in the specific frequency and largely rejecting the interference signals.Facing the 2G-to-5G mobile terminal,a great deal of sub-6GHz bandpass filters(BPFs)[40–47]based on the IPD technology has been reported in the past.As illustrated in Figure 1,literatures[45]and[46]show the latest IPD-based 5G BPF designs.All lumped-element topology designs are based on the spiral inductors and MIM capacitors on the silicon or GaAs substrates,which display good stopband rejection and low loss.Moreover,in literature[47],a balanced BPF operating at 4.9 GHz is constructed for the first time with the broadband common-mode suppression.

Figure 1.On-chip 5G NR bandpass filters based on IPD technology at(a)N77[45]and(b)N78 bands[46].

Figure 2.On-chip millimeter-wave bandpass filters based on IPD technology using(a)coupling resonators[49]and(b)coupled-line structures[52].

Figure 3.On-chip diplexer based on IPD technology[53].

Figure 4.IPD-based narrowband interconnected circuits including(a)900 MHz power divider[58],(b)2.4 GHz balun[59],and(c)5 GHz coupler[60].

In addition to the sub-6GHz filters,IPD technology also shows great potential in millimeter-wave(MMW)band applications.There are two different approaches to realize MMW BPFs,as shown in Figure 2.On the one hand,the MMW BPFs can be designed by the coupling resonators[48–50].On the other hand,the transmission-line and coupled-line theories can also be applied to design the IPD-based microwave circuits in MMW band[51],[52].

Figure 5.IPD-based(a)wideband[63]and(b)dual-band[66]power divider and coupler.

2.2 IPD-based Diplexers

The diplexer is a three-port passive device performing the frequency domain multiplexing,which is composed of the low-frequency,high-frequency,and common ports carrying the multiplexed signals.Due to the superiorities in low loss and small circuit size,IPD-based diplexers are also extensively studied by designing lumped-element networks[53–57].In literature[53],an on-chip miniaturized diplexer operating at 2.6 GHz and 5.8 GHz is achieved by the synthesized lumped-element low band and high band coplanar waveguides,as shown in Figure 3.Comparing with the designed diplexers using LTCC,the IPDbased ones display a lower return loss,a higher operating frequency,and a smaller chip size.

The current reported IPD-based filters generally have a good in-band performance and miniaturized size,but the frequency selectivity is not very high due to the limitation of quality factor.Thus,the major challenge of the current IPD process is to improve quality factor of inductor largely under the condition of maintaining the effectiveness of frequencyselecting circuits in the presence of stronger interference.

III.ADVANCED IPD INTERCONNECTED CIRCUITS

The microwave interconnect circuits are the fundamental RF components,in which the power at input port is divided into dual- or multi-path equal or unequal power with different phase differences at output,including 0◦power dividers,90◦couplers,and 180◦baluns.They are often applied to power amplifiers,mixers,phased-array antennas,and feedback networks,etc.In recent years,IPD process is also employed to implement the power divider,balun,and coupler,which is widely studied on narrowband[58–60],wideband[61–65],dual-band[66],and MMW band operation[67–70].

For example,based on theπ-type lumped-element networks,the 900 MHz Wilkinson power divider[58],2.4 GHz balun[59],and 5 GHz coupler[60]are presented using IPD technology,respectively,as depicted in Figure 4.From Figure 5a,the new designed transmission-line cells are periodically utilized to realize the broadband IPD-based power divider and coupler[63],effectively extending the bandwidth.Besides,in Figure 5b the dual-band power divider at 2.4/5.5 GHz and coupler at 2.45/5.8 GHz are designed by the bridged-T coils,respectively[66].The presented bridged-T coils consist of the serial-parallel inductances and capacitance,and grounded capacitance to perform the different operating frequencies.For MMW circuits,the co-design method of transmissionline theory and lumped elements is usually adopted,such as the designed example in literature[69](shown in Figure 6).

Figure 6.IPD-based millimeter wave coupler[69].

Figure 7.Applications of IPD process on power combiners of(a)910 MHz[73]and(b)5 GHz CMOS power amplifier chips[74].

The current researches about the IPD-based power divider,balun,and coupler are mainly directed to the different bandwidth,such as wideband and dual-band operation.For the development trend of the higher integration,multifunctional microwave circuits have been much needed.For example,the co-design of filter and interconnect components can improve the frequency-selectivity and harmonic suppression,thus relaxing the use of filters.The interconnect components with the different output phase can avoid the use of the phase shifter.

IV.ADVANCED IPD POWER AMPLIFIERS

Power amplifier plays an important role on the RF signal amplification in the transmitter path,directly determining the signal transmission distance and quality.For the miniaturization of RF front end,the IPD technology is also embedded into the power combining and matching circuits of the various power amplifier designs including CMOS,LDMOS,GaAs,GaN,and SiGe chips[71–79].

There are two types of applications for IPD technology on the power amplifiers in general.One of them is to design the power combiner of the CMOS power amplifier[71–74].For instance,from Figure 7a a compact power combiner and spiral-shaped directional coupler using IPD process are firstly fabricated on a highly resistive substrate,respectively[73].Then,the designed IPD chip is combined with CMOS power amplifier chips by wire bonding for a mobile UHF RF identification(RFID)reader.Furthermore,a CMOS power amplifier with a high Q-factor IPDbased power combiner is implemented[74],as depicted in Figure 7b obviously improving the power added efficiency and output power.

Figure 8.Applications of IPD process on matching networks of(a)GaN[78]and(b)CMOS power amplifier chips[75].

Figure 9.The simplified block diagram of RF front end.

As illustrated in Figure 8,another one is to accomplish the input and output matching networks of power amplifiers[75–79].In literatures[76]and[78],the internal matching for LDMOS and GaN power amplifier chips is designed and fabricated using silicon-based IPD technology,which provides the lower manufacturing cost and smaller chip size.Note that the IPD process is not just for a narrow band power amplifier.In literature[75],the Ruthroff-type transformer is fabricated into a spiral transmission-line form using the IPD process to achieve the wideband output network.After,the CMOS power amplifier chip and IPD-based matching networks are integrated by the flip-chip technology,reducing the interconnection loss and circuit size.

Figure 10.Current state-of-the-art multifunctional microwave circuits based on PCB process including(a)absorptive bandstop filter[14],(b)filtering power divider[16],(c)controllable-phase coupler[80],(d)filtering power amplifier[38],(e)balanced crossover[29],and(f)balanced filtering antenna[81].

Compared with the conventional RFIC or MMIC,the IPD technology can offer the lower loss,lower cost,smaller circuit,and better package compatibility.Thereby,it can effectively improve the circuit performance of various power amplifiers under the premise of ultra-miniaturized chips.

V.FUTURE TREND FOR IPD MICROWAVE CIRCUITS AND CHIPS

As shown in Figure 9,the RF front end involves many microwave passive and active components,such as filter,power amplifier,low noise amplifier,and so on.With the abundance of spectrum resources from 2G to 5G,RF front end becomes more and more complicated for supporting more operating band.It’s clear that the high-integrated microwave components with multiple RF functions can well solve this challenge.Thus,multifunctional microwave circuits have attracted a lot of attentions in academic research in recent years.The multifunctional designs mainly include filtering integration,tunable frequency or bandwidth,arbitrary terminal impedances,arbitrary power division,arbitrary phase difference,harmonic rejection,and commonmode suppression,etc.As illustrated in Figure 10,some state-of-the-art multifunctional microwave components using PCB process have been in-depth researched.In literature[14],the using of the modified coupled-line structure and grounded resistor achieves the wideband input-absorptive bandstop filter.By the detailed analysis of generalizedS-parameter matrix,a phase-controllable unequal coupler[80]and a single-ended-to-balanced crossover[29]are built based on transmission lines,respectively.In literatures[16],[38],and[81],based on the transmission-line and coupled-line resonators,the bandpass filtering responses are embedded into the power dividers,power amplifiers,and antennas,respectively.Nevertheless,due to the limitation on size of the PCB process,these reported multifunctional microwave components are not suitable for the miniaturized mobile-terminal communication systems.Therefore,a proper microfabrication technology for mobile terminal needs to be solved urgently for accomplishing the multifunctional RFIC.

Figure 11.Simplified layer structure of the IPD technology.

Fortunately,the IPD technology can be an excellent candidate approach to implement the multifunctional mobile-terminal RF integrated chips,as the obvious advantages of ultra-miniaturization,low loss,and low cost.As shown in Figure 11,IPD technology manufactures the resistor,capacitor,and inductor on a semiconducting substrate by using photolithography,thin film deposition and etching technologies[59].The three-layer metals(top,middle,and bottom metals)construct the spiral inductors with air bridge,while the top and bottom metals connected with silicon nitride(Si3N4)material can achieve the MIM capacitor.The NiCr layer with a surface impedance of approximately 25 Ω/sqacts as the thin film resistor.Thus,the using of IPD can transform the lumped-element networks into the RF integrated circuits(ICs),which is similar to low temperature co-fired ceramic(LTCC)process.Table 1 illustrates the differences including circuit performance and size of the RF filter chips between IPD and LTCC technologies.It can be seen that the IPD technology adopting thin film process can realize a thinner and smaller IC chips,since the current minimum line-width and line-spacing of IPD technology can be fabricated into 5µm.However,the quality factor of IPD technology is at a disadvantage,comparing with other integrated process,generally resulting in a poor stopband rejection and frequency selectivity.

Figure 12.IPD-based multifunctional microwave circuit chips including(a)tunable bandpass filter[86],(b)balanced bandpass filter[47](c)impedance-transforming power divider with stopband rejection[87],(d)filtering Marchand balun[88],and(e)absorptive bandstop filter[89].

Table 1.Difference Between IPD and LTCC Technologies for Filters.

Although the quality factor of IPD technology needs to be further improved,all kinds of IPDbased microwave passive and active chips in mobile phone communication systems,such as filters,power dividers,diplexers,and power amplifiers,have been developed by the designed lumped-element and transmission-line networks.In addition,IPD technology not only can be applied on sub-6GHz microwave circuits,but also to perform the millimeterwave operation.Thus,we can design the multifunctional lumped-element or transmission-line networks to manufacture the desired sub-6GHz and millimeterwave RF IC chips based on the IPD technology for ultra-miniaturized mobile terminal.

Up to now,there are few works on the IPD-based multifunctional RF integrated chips,but some researchers have started working on this,as shown in Figure 12.For example,in IPD-based multifunctional frequency-selecting circuits,literature[85]reported a miniaturized filter with the tunable stopband frequencies from 1.3 to 2.3 GHz using an IPD-based balun.By using a new capacitor with air bridge and differential inductor,an IPD-based tunable bandpass filter is presented in literature[86].Furthermore,literature[47]designs a 4.9 GHz balanced bandpass filter model using GaAs-based IPD technology,performing 20-dB common-mode suppression from DC-7.5 GHz.In IPD-based multifunctional interconnected circuits,the quasi-Chebyshev and quasi-elliptic broadband matching network is introduced to build ultra-miniaturized power divider chip(1.1×1.6mm2)integrated with the stopband-rejection and impedance-transforming functions[87].Besides,a bandpass filtering Marchand balun(1.49×1.39mm2)based on the spiral coupled lines is developed in[88].Recently,a brand-new input-absorptive bandstop filter with wide stopband based on IPD technology is reported in[89].It can be seen that the IPD technology can be used to perform multiple RF functions in one high-integrated chip from the reported works of[85–89],such as filteringintegration power dividing,tunable-band function,and impedance transformation.Moreover,the IPD-based microwave components display the significant advantages of smaller size,lower loss,lower cost,and good package compatibility.Thus,the development trend of the IPD-based multifunctional microwave circuit chips is irresistible for mobile-terminal RF systems for solving the current complicated RF front end.

VI.CONCLUSION

This article reviews the state-of-the-art microwave circuit chips using integrated passive device(IPD)technology at first.By summarizing the latest research hotspot of multifunctional microwave components,the article finally creates the distinctive connections between multifunctional microwave circuits and IPDbased integrated chips for the application on mobileterminal radio-frequency systems.

ACKNOWLEDGEMENT

This work was supported in part by Beijing Natural Science Foundation(No.JQ19018),National Natural Science Foundations of China(No.U20A20203 and No.61971052),and National Special Support Program for High-Level Personnel Recruitment(No.2018RA2131).(Corresponding author:Yongle Wu).