Fabrication of low-loss thin film microstrip line on low-resistivity silicon for RF applications

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Abstract

A detail fabricating process and characterization of thin film microstrip line (TFML) on low K polyimide, used for interconnects in radio frequency integrated circuits (RFICs) technology, is reported in this study. By incorporating a spin-on dielectric polyimide and sputtering of aluminum, the TFML is fabricated on low-cost low-resistivity silicon (LRS) substrate (ρ⩽10 Ω cm). The TFML with a thickness of 20 μm polyimide dielectric layer presents attenuation losses of 0.385 dB/mm at 25 GHz and 0.438 dB/mm at 50 GHz. Effective dielectric constant and attenuation of TFML on polyimide are carefully investigated and discussed.

Introduction

Recently, integrated passive components have been recognized as high-performance and low-cost elements of deep sub-micron BiCMOS circuits in radio frequency integrated circuits (RFICs) technologies [1]. The integration of a large number of passive components including the individual passive devices of transmission lines, inductors, capacitors, or functional passive devices of filters and antennas with low loss and minimal crosstalk is as important as the advancement in active transistor technology [2], [3]. However, the low-resistivity substrate used in standard CMOS processing for most RFICs has limited the integration of high-quality passive components due to the high-lossy performance of the silicon substrate [3].

Some solutions have been provided to improve the problem of lossy silicon substrates. First method is to integrate the passive components on top of the thick dielectric layer as far above the lossy Si substrate as possible [4]. Second method is to use high-resistivity silicon (HRS, ρ>2000 Ω cm) substrates to lower the carrier transmission in the substrate [5]. Third method is to use high electron voltage (MeV) proton implantation to convert low-resistivity silicon (LRS) to HRS resulting in improved quality Q(f) of proton implanted passive devices [6]. Fourth method is to apply micro-electromechanical technology to remove the backside of the carrier substrate or etching a groove shape below the conductor line to reduce the substrate loss [7], [8], [9]. However, a primary drawback of HRS technologies is their incompatibility to standard CMOS processes. MEMS technology has the problems of weak mechanical strength, complex fabrication and shorter lifetime [9]. In the past, polyimides have been used as low-loss low K dielectrics allowing for a higher propagation velocity, which is necessary for high-speed circuits. They can form excellent dielectric insulators and provide excellent step coverage. Therefore, polyimide is comprehensively used in fabricating various RF and microwave packages. Recently, in order to achieve low-cost, high dense and fully integrated interconnects for RF front-end communication systems, low dielectric constant (K) thin film microstrip lines (TFMLs) have been presented as a good candidate for integrating the passive devices on RFICs up to 50 GHz. The TFML has four primary advantages to RFICs technologies [10], [11], [12]:

  • (1)

    The thin dielectric layer acts as a substrate and is easily etched to form very small via holes for interconnects in RFICs.

  • (2)

    The compact line scale of TFML is capable of achieving very high-dense circuits.

  • (3)

    The use of ground planes between dielectric layers and the silicon substrate permits novel circuit layouts which can minimize circuit size.

  • (4)

    The dispersion effects of TFML can be effectively reduced, using very thin dielectric layers, compared to conventional microstrip substrate thicknesses above 500 μm.

In Ref. [9], Six et al. used two photosensitive benzocyclobutene (BCB) of 10 μm as the low K dielectric material layers and electroplated Ti (30 nm)/Au (3 μm) as the ground plane and the conductor line. Finally, the thin sacrificial layer (Ti/Au, 30/20 nm) was removed by wet etching. However, the isotropic etching effect may induce some parasitical effects from the non-uniform edge side of low K dielectric layer. In Ref. [10], Ponchak et al. used a lift-off process to fill via holes and define the probe pad. The 20 nm of Ti and 1.3 μm of Au were used for a conductor line and a ground layer on silicon. However, this via holes process may cause a broken polyimide dielectric layer between the LRS and the conductor line. Although the gold has higher conductivity sufficient for a conductor line and titanium can be used as the barrier layer between the gold and polyimide, the lower conductivity of the barrier layer will cause a detrimental effect on signal propagation at microwave frequencies. In addition, the detailed process parameters such as pre-bake/post-bake conditions, spin coating speed, exposure dose, and development are not described in the above reports. However, the fabricated process will influence the final microwave performance of TFML due to the dimension sensitivity.

In this paper, we fabricated low K (dielectric constant εr⩽4) dielectric polyimide as the intermediate layer on the LRS, as shown in Fig. 1(a). The low K thin film with thickness of hPI=20 μm was spin coated on top of an evaporated aluminum ground plane with 2 μm thickness which has been deposited onto a LRS. The spin-on polyimide offers a thickness range of about 10–20 μm, which favors an excellent electromagnetic isolation between the polyimide and the LRS. A thick high conductivity aluminum layer is then sputtered on the spin-on polyimide thin film to form the microstrip line with reducing the metal conductive loss. Two main goals of this study were to provide a detail fabricating process of a TFML and to evaluate high-frequency behavior of the TFML experimentally. The microwave properties of TFML on polyimide thin film are proposed to be suitable for using in low-cost and high-performance interconnects of RFICs.

Section snippets

Fabrication process of TFML

The TFML structures of this study are fabricated on low-cost LRS (ρ⩽10 Ω cm). In Fig. 1(b), TFML is constructed on 540-μm-thick p-type LRS substrate with 〈1 0 0〉 crystal orientation. In this study, a 20-μm-thick polyimide film is used for supporting the microstrip line. The reason of choosing a 20-μm-thick polyimide film for fabricating the TFML is that it has the low attenuation of TFML. The HFSS simulation is used for analyzing the topology and EM wave propagation of the TFML [18]. Fig. 2 shows

Measured results and discussions

The TFML on polyimide was measured by an HP 8510C with microwave probes. On the tested polyimide, the TFML with electrical length θ=180° at 50 GHz was used. The propagation constant of the TFML represented by γ(f)=α(f)+jω(εreff/c) is deembedded through the thru-reflect-line (TRL) calibration routine [14], [15], [16], where α(f) is the frequency-dependent attenuation constant, ω is the angular frequency, c is the velocity of light in free space, l is the length of TFML, and εeff is the effective

Conclusion

In this study, we have fabricated and characterized the TFML on polyimide with 2 μm thick aluminum conductors for RF applications. The processing details were presented and the microwave measurement results show that low attenuation can be obtained with a TFML structure regardless of substrate type due to good EM shielding between the LRS and polyimide. Using a 20 μm dielectric thickness, the attenuation of 0.438 dB/mm at 50 GHz was obtained. The low attenuation of the TFML presented here is

Acknowledgment

The authors wish to acknowledge National Nano Device Laboratories for supporting the equipment.

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