Benzocyclobutene (BCB)-based spiral inductor for wireless application

Abstract

ABSTRACT This paper describes new type of MEMS spiral inductor us ing benzocyclobutene (BCB) for wireless applications. BCB consists of several attractive features for on-chip RF inductors, i.e., high resistivity, low dielectric constant and lower curing temperature. The fabricated inductor coils had 2.5 turns with 20µm for width, 20µm for pitch and the thickness of 2µm. The inductor coils are suspended about 12µm from the bottom BCB substrate. Our measured quality factor was 19 at 3.1GHz frequency. The measured inductance was approximately 3nH and it remained constant up to 10 GHz. The self resonant frequency (SRF) was at 13.6GHz. G 1. I NTRODUCTION In recent years, desires to have on-chip high performance RF MEMS inductors have been greatly increased to integrate RFICs with 3G wireless systems. High performance RF inductors are key components for implementing voltage controlled oscillators, impedance matching circuits, passive filters, low noise amplifiers and power amplifiers. However, most of the silicon based RF inductors have low qu ality factors due to lossy silicon substrate. Therefore, RF performance is degraded to use in high performance environments such as 3G wireless systems. In order to overcome this bottleneck, we designed and fabricated MEMS based indu ctor. To achieve successful commercial implementation of RF on-chip inductors, many conditions are necessary. Some examples are typical inductance values from 1 to 5nH, greater than 10 quality factor, self resonance frequency abov e 10GHz, and fully CMOS compatible. In open literature, numerous techniques have been reported to achieve above cond itions: i) incorporation of high conductivity or thick coil metals to decrease the resistance of the coil [1]; ii) utilization of high resistivity substrate such as GaAs, silicon on insulator (SOI), and glas s substrates to minimize the su bstrate parasitics; iii) carefully perforated ground line placement underneath the inductor to reduce the eddy current loss [2]; and iv) vertical-type inductor [3]. However, these techniques are expensive and labor intensive. They may not be used in conjunction with CMOS chips, which is the biggest problem. In this paper we report on the development of a new type of a suspended spiral inductor using benzocyclobutene (BCB) as shown in Fig. 1. As the IC industry drives into sub-micron technology, most dominant