A new method to fabricate low-loss chip-scale RF inductors

Abstract - A new method is presented for integrating high performance wire-based inductors into thin, planar, chip-scale formats. The method is designed for compatibility with commonly-used rapid prototyping tools, and fabrication can be automated for volume production. The chip-scale inductors are designed for inductances in the 1-10 nH range, self-resonant frequencies above 5 GHz, and quality factors exceeding 75-100. The process can produce a stand-alone chip or can be integrated with existing multi-chip modules and integrated circuits. Index Terms - Inductor, quality factor, radio frequency, integrated passives, multi-chip module. I. INTRODUCTION Radio-frequency (RF) matching networks, RF filters, and RF trimming solutions for cellular band frequencies and above require inductors in the 1-10 nH range, self-resonant frequencies in excess of 5 GHz, and quality factors exceeding 75-100. In addition, these components must be cost-effective and miniature. Advances in microfabrication processes have yielded a variety of methods for improving quality factor in a chip-scale inductor over the past two decades. Notable successes include creating free-standing structures [1-2] and substrate cavities [3-4] to minimize eddy current losses, and using deposition or electroplating [5-6] for thick metal and 3D topologies. In addition, efforts to integrate magnetic materials have presented new opportunities and exciting results [7-8]. However, although multiple inductor fabrication methods are available, only the most advanced devices have met the required performance specifications. In particular, electroplated inductors have demonstrated extremely high quality factors at the frequencies of interest. These must compete with industry-standard CoilCraft inductors, where devices with self-resonant frequencies> 4 GHz offer L=I-13 nH and quality factors ranging from 70-100 [9]. Other vendors of high-Q wire-wound inductors include Abracon, Murata, and Bournes. Neither technology (electroplating or commercial wire-wound) is ideal in terms of integration with miniature modules: the electroplated inductors are often in the form of 3D structures that require isolation from potting materials, and the wire-wound inductors are thicker than a typical chip substrate. Despite their size, the commercial parts have remained the product of choice for many practicing engineers due to their low cost and simplicity. To understand why wound-wire inductors perform so well, we simulated inductors with round versus rectangular cross­sections. We observed that round-section wires will always have lower loss at the frequencies of interest. Rectangular cross-sections suffer from current crowding in the corners, known as the "comer effect". Additionally, the conductivity of (rectangular) thin films is highly sensitive to surface roughness, and minor imperfections can result in significant losses [10]. This motivated the design of a new process for making wire-based inductors for chip-scale formats, presented in this paper. The design of a new prototyping process for inductors was initiated, with a primary focus on leveraging the high performance of round wire, while providing a low-cost and easily-integrated component. The process is designed so that custom inductors can be made in an automated fashion, for large-scale production if required. In addition, a key goal in designing this process was that it could be implemented with equipment found in a typical prototyping laboratory. II.