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Towards 3D integration of system-in-a-package modules
3D intergration and high-density packaging have become increasingly important in the development of future portable telecommunication systems. G. Carchon and G. Posada, IMEC, discuss the situation.

These systems use a considerable amount of highquality passive components that need to be integrated in the most optimal way. Among the various integration routes, the use of thin-film technologies applied to an interposer substrate seems most appropriate. When high-resistivity silicon (HRSi) is used as an interposer material, 3D integration of system-in-a-package modules comes within reach. HRSi substrates allow the integration of through-wafer vias, which can provide vertical interconnections for RF and high-speed digital applications and which enable the use of microstrip passive components and circuits. In this article, the key features of such a thinfilm HRSi-based system-in-a-package technology platform are presented. The integration of high-quality passive components at various frequencies, ranging from RF frequencies up to millimetre-wave frequencies, is demonstrated. Using this enabling platform in combination with related technologies for integrating e.g. RFMEMS and active components is a promising route to fulfill the current miniaturisation objectives.


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The users demand for instant voice and data availability at any place and at any time drives the development of new wireless systems enabled by today's wireless technology revolution. The overall requirements for these wireless systems however become increasingly more stringent with smaller sizes and weight, higher bandwidth and lower power consumption at an ever-decreasing cost. One of the possible showstoppers for their further evolution might be the integration of high-quality passive components, which are present in large numbers in these portable communication systems. High-quality passive components today are difficult or even impossible to integrate on chip due to severe size constraints, whereas discrete passive devices suffer from decreased performance, overall increased package size and decreased reliability due to the number of additional interconnections. A powerful solution to overcome these limitations is the use of thin-film technologies applied to the subsystem carrier substrate. Functions such as broadband couplers, filters, matching networks etc. are easily integrated by using these technologies on an interposer high-resistive substrate (such as low-cost glass or HRSi, in this configuration called RF-system-in-a-package (RF-SiP) or MCM-D).


An important advantage of such an approach is the high-precision definition of the patterns (lines, capacitors, inductors, resistors), inherent to the fabrication technology. The multilayer thin-film technology further features lowtemperature process steps.


The study presented in this article goes one step beyond by describing the development of a technology platform that enables 3D integration of SiP modules. 3D integration in combination with high-density packaging plays an increasing role in today's telecommunication systems. Key features of such an enabling technology include (1) the use of HRSi substrates, (2) Si surface passivation and (3) the realization of through-substrate vias.


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From glass towards highresistivity Si carrier substrates
Thin-film technology can generally be considered as an established technology. E.g., IMEC's thin-film technology uses alternating layers of BCB (benzocyclobutene, k=2.65) and electroplated Cu (3-20µm thick) combined with TaN precision resistors and high-density Ta2O5 capacitors. Most of the developments in the past years have been based on MCMD on AF45 glass technology, which is a low-cost and low-loss substrate, using coplanar waveguide (CPW) designs. Passive functions from RF to V-band have been successfully integrated demonstrating the viability of the technology for integrating highperformance front-end systems [1]. Several demonstrators have been developed, such as a Bluetooth RF circuit and a 5.2GHz wireless local area network (WLAN) front-end receiver. However, in view of today's need for 3D integration and high-density packaging, the use of AF45 glass poses a number of drawbacks. Firstly, it is difficult to integrate substrate vias and to perform wafer thinning and micromachining. Secondly, the low thermal conductivity of glass limits the amount of power it can handle. HRSi is an adequate alternative material that can provide these functionalities together with high performance. The MCM-D on glass technology has therefore been transferred to a HRSi (Ú>4Køcm) carrier substrate, allowing the integration of high-quality passive components and circuits. E.g., the performance of a 7GHz MCM-D power splitter and a 50GHz distributed band pass filter has been demonstrated. [2].


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High-resistivity Si surface passivation
Although HRSi has excellent properties as a substrate material, the unavoidable occurrence of free charges at the Si-SiO2 interface drastically undermines the RF properties of the bulk HRSi. One way to solve this problem is to implant a high dose of atoms, such as Ar, in the Si-SiO2 interface [3]. It generates traps in the Si substrate which prevent the charges from moving. The surface remains passivated even after thin-film processing [4]. This passiviation technique has been applied to the wafers that are subject of this further study.


Through-substrate via technology
One step forward towards 3D integration of SiP modules is the ability of generating through-substrate vias. Integrated vias can provide vertical interconnections for RF or high-speed applications, which is a critical building block within the 3D SiP concept. They further allow the use of microstrip-based passive components and circuits within the thin-film MCM-D platform. The use of microstrip components has some significant advantages over coplanar waveguide transmission lines (as used in MCM-D on glass technology), as will be further highlighted.


Fig 1 shows the cross section of the technology in which 100µm thick HRSi wafers are used enabling the implementation of substrate vias. On the front side of the wafer, low-loss dielectrics and copper metallisations are implemented, allowing the integration of high-quality passive components. Tantalum nitride (TaN) resistors are available with a typical value of 25ø/sq as well as medium-density capacitors. On the backside of the wafer a Cu layer is used for the ground plane of the microstrip components, via metallisation and possible interconnection to other components of the SiP. The vias have a bottom diameter of about 100µm and a top diameter of 50µm. All standard passive component types can be integrated in this technology platform. Flip-chip and wirebond techniques can be used to include active components in the designs and implement full systems.


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Demonstrators
High-quality thin-film resistors, capacitors, inductors and transmission lines have been integrated on a microstrip configuration [5]. The Ta2O5 capacitor has a quality factor higher than 180, while the BCB capacitor's Q is around 120, higher values are obtained for smaller capacitors. The 1nH microstrip spiral inductor provides a maximum Q around 50. Since the MCM-D technology targets applications ranging from the lowest microwave frequencies up to millimetre-wave frequencies, the performance of distributed element components are decisive as well. In order to assess the performance of a microstrip line implemented on the MCM-D technology on HRSi, it has been compared to a CPW line implemented on AF45 glass. An important figure of merit of such a transmission line is the quality factor, the value of which can be viewed in figure 3. The microstrip line shows a quality factor 2.5 times higher than a CPW line with comparable dimensions implemented on AF45 glass. This is an important advantage of using microstrip type circuits.


The good performance of the microstrip transmission line allows the integration of high-quality distributed element circuits such as filters. Here, the integration of both a 5.2GHz lumped band pass filter and a 31GHz coupled line band pass filter is presented. For both band pass filters, a good agreement is found between performance measurements and simulations. For the lumped band pass filter, being 2.4x1.5mm2 in size, a 3dB bandwidth of 10% and 4.5dB insertion loss is measured. The microstrip coupled line band pass filter, measuring 3.1x1.2mm2, presents a relative bandwidth of only 5.1% and an insertion loss of 2.7dB. These results demonstrate the versatility of this technology for integrating highperformance passive circuits ranging from the lowest microwave frequencies up to millimetre-wave frequencies.


Conclusion and outlook
A small ground via (100µm diameter) is demonstrated through the integration of passive components and circuits in a microstrip configuration. The integration of microstrip inductors, high-density capacitors, resistors and transmission lines are demonstrated together with lumped 5.2GHz and distributed 31GHz element filters proving the versatility of the technology and the viability of the ground vias.


This study means a significant step forward in the development of 3D thinfilm RF modules. Microstrip-based passive components and circuits can be successfully integrated within the thinfilm MCM-D technology platform. Flipchip and wirebond techniques can be used to include active components in the designs and implement full systems bringing together the appropriate technologies forming a system in a package.


Having each component of the system implemented in the most appropriate technology, the overall performance can be maximised. The same technology platform is also the basis of IMEC's current above-IC and RF-MEMS technology development. As a final objective, the technologies for integrating thin-film passives, RF-MEMS and actives will be combined for realising novel highly-functional modules.


These results have been obtained in the frame of IMEC's APIC program that pursues R&D on advanced packaging.


 


REFERENCES
[1] G. Carchon et al, ‘Multi-layer thin-film MCM-D for the integration of high-performance RF and microwave circuits', IEEE Trans. CPT, Vol. 24, 3, 2001.
[2] G. Posada et al, ‘Low-loss coupled line filters with transmission zeros in multi-layer thin-film MCM-D technology, IEEE MTT-S Digest, IEEE, 2004, pp. 1471-1474.
[3] A.B.M. Jansman et al, ‘Elimination of accumulation charge effects for high-resistive silicon substrates', Europ. S-SDR, pp. 3-6, 2003.
[4] G. Posada et al, ‘High-resistivity silicon surface passivation for the thin-film MCM-D technology', SiRF, pp. 46-49, 2006.
[5] G. Posada et al, ‘Microstrip thin-film MCM-D technology on high-resistivity silicon with integrated through-substrate vias', European Microwave Week, Munich, Germany, October 2007.


 



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