Technology Platform Integrates High-Performance SiP Modules

Using a high-resistivity silicon interposer, thin-film technologies and through-substrate vias, 3-D integration of high-quality passives and high-speed digital devices becomes possible.

G. Carchon and G. Posada*, IMEC, Leuven, Belgium *Also at Katholieke Universiteit Leuven, Leuven, Belgium -- Semiconductor International, 3/1/2008

3-D integration and high-density packaging become increasingly important in the development of future portable telecommunication systems. These systems use a considerable amount of high-quality 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, 3-D integration of system-in-a-package (SiP) modules comes within reach. HRSi substrates allow for the integration of through-wafer vias, which can provide vertical interconnections for RF and high-speed digital applications and enable the use of microstrip passive components and circuits.

In this article, key features of a thin-film HRSi-based SiP technology platform are presented. The integration of high-quality passive components at various frequencies — ranging from RF frequencies up to millimeter-wave frequencies — is demonstrated. Using this enabling platform in combination with related technologies for integrating (e.g., RF-MEMS and active components) is a promising route to fulfill the current miniaturization objectives.

Wireless systems

The user demand for instant voice and data availability at any place and 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: smaller sizes and weight, higher bandwidth and lower power consumption at an ever-decreasing cost. One possible showstopper for 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 because of severe size constraints, whereas discrete passive devices suffer from decreased performance, overall increased package size, and decreased reliability caused by 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 deposited multi-chip module [MCM-D]). An important advantage of such an approach is the high-precision definition of the patterns (lines, capacitors, inductors, resistors, etc.) inherent to the fabrication technology. The multilayer thin-film technology further features low-temperature process steps.

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

From glass to HRSi substrates

Thin-film technology can generally be considered as an established technology. For instance, our thin-film technology uses alternating layers of benzo-cyclobutene (BCB, k=2.65) and electroplated copper (3–20 μm thick), combined with tantalum nitride (TaN) precision resistors and high-density Ta₂O₅ capacitors. Most of the developments in past years have been based on MCM-D on AF45 glass technology — a low-cost, 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 high-performance front-end systems.¹ Several demonstrators have been developed, such as a Bluetooth RF circuit and a 5.2 GHz wireless local area network (WLAN) front-end receiver.

However, in view of today's need for 3-D integration and high-density packaging, the use of AF45 glass poses a number of drawbacks. First, it is difficult to integrate substrate vias and perform wafer thinning and micromachining. Second, 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 an HRSi (>4 k-cm) carrier substrate, allowing the integration of high-quality passive components and circuits (e.g., the performance of a 7 GHz MCM-D power splitter and a 50 GHz distributed band-pass filter has been demonstrated²).

HRSi 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 argon, in the Si-SiO₂ interface.³ It generates traps in the silicon substrate that prevent the charges from moving. The surface remains passivated even after thin-film processing.⁴ This passivation technique has been applied to the wafers that are subject of this study.

Through-substrate via technology

One step forward toward 3-D 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 3-D 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.

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

1. The platform uses a high-resistivity silicon substrate, BCB dielectric and TSVs to integrate high-Q passive circuits.

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 >180, while the BCB capacitor's quality is ~120; higher values are obtained for smaller capacitors. The 1 nH microstrip spiral inductor provides a maximum quality ~50. Because the MCM-D technology targets applications ranging from the lowest microwave frequencies up to millimeter-wave frequencies, the performance of distributed element components are decisive as well. To assess the performance of a microstrip line implemented on the MCM-D technology on HRSi, it has been compared with a CPW line implemented on AF45 glass. An important figure of merit of such a transmission line is the quality factor. The microstrip line shows a quality factor 2.5× 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 for the integration of high-quality distributed element circuits, such as filters. In this article, the integration of both a 5.2 GHz lumped band-pass filter and a 31 GHz coupled-line band-pass filter is presented. For both band-pass filters, a good agreement is found between performance measurements and simulations (Fig. 2). For the lumped band-pass filter (2.4 × 1.5 mm² in size), a 3 dB bandwidth of 10% and 4.5 dB insertion loss is measured. The microstrip coupled-line band-pass filter (3.1 × 1.2 mm²) presents a relative bandwidth of only 5.1% and an insertion loss of 2.7 dB. These results demonstrate the versatility of this technology for integrating high-performance passive circuits ranging from the lowest microwave frequencies up to millimeter-wave frequencies.

2. Measurements and simulations of a lumped-element microstrip filter at 5.2 GHz and a lumped band-pass filter at 31 GHz show good agreement.

Outlook

A small ground via (100 μm 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 5.2 GHz and distributed 31 GHz element filters proving the versatility of the technology and viability of the ground vias (Fig. 3).

3. Photograph of a microstrip coupled-line band-pass filter with transmission zeros centered at 30 GHz. The coupling generating the zeros is highlighted.

This study is a significant step forward in the development of 3-D thin-film RF modules. Microstrip-based passive components and circuits can be successfully integrated within the thin-film MCM-D technology platform. Flip-chip and wire-bond techniques can be used to include active components in the designs and implement full systems, bringing together the appropriate technologies and forming a SiP. The overall performance can be maximized by having each component of the system implemented in the most appropriate technology. The same technology platform is also the basis of our current above-IC and RF-MEMS technology development. As a final objective, the technologies for integrating thin-film passives, RF-MEMS and active devices will be combined for realizing novel highly functional modules.

Acknowledgement

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