Design, integration and characterization of CMOS-compatible RF varactors based on ferroelectric HfO2 thin films

Thin film varactors are widely used in modern radio frequency integrated circuits (RFICs), in matching networks, phase shifters, voltage-controlled oscillators (VCOs) and tunable filters. Ferroelectric hafnium oxide, discovered just 10 years ago, opened a whole new research direction. Compared to other ferroelectric materials, it has a decisive advantage for use in microelectronic systems and industrial complementary-metal-oxide-semiconductor- (CMOS-) compatible production as it has been used as a gate oxide in field-effect transistors (FETs) for more than a decade. When deposited as a thin film and doped with various materials, particularly zirconium, it can crystallize at temperatures suitable for its integration into industrial Back-End-of-Line (BEoL) processes. As such, it has been extensively studied as a material for ferroelectric random-access memory (FRAM) devices. These properties also make it a perfect candidate for its implementation as a ferroelectric varactor. In this work, a comprehensive study on metal-ferroelectric-metal (MFM) thin film varactors of hafnium oxide is carried out. It can be divided into two distinct paths - an investigation of the material properties of Zr-doped hafnium oxide and an investigation of its applicability as a varactor, where its property of tunable permittivity (or capacitance) was used to simulate, design and characterize passive elements such as phase shifters and demonstrate the advantages of the material over competing technologies. For the initial analysis of its properties, extensive measurements were made at low frequencies, particularly I-V , P-V and C-V characteristics. During this, the main focus was put on the influence of composition (doping), thickness, temperature and electrical treatment upon capacitance tunability. It was found that 3:5 Hf:Zr - doped thin-film varactors of 10 nm thickness have the best performance in terms of both tunability and quality factor at temperatures up to 200°C. Additionally, the broadband characteristics was conducted using a Linear Network Analysis. The influence of doping was also analyzed for RF frequencies up to 500 MHz, where the properties were comparable to low-frequency characterization. Also, the effective permittivity and loss tangent of BEoL-integrated 10 nm hafnium zirconium oxide with 1:1 doping was extracted in the frequency range between 30 MHz and 170 GHz. Finally, passive devices, particularly a phase shifter and a bandpass filter, were designed and integrated into the BEoL of 180 nm CMOS technology. The phase shifter demonstrates a phase shift up to 112° at 60 GHz with 41.3 dB insertion loss and 43.5° at 45 GHz with 14 dB insertion loss. The bandpass filter demonstrates the tuning of the center frequency between 36.6 GHz and 39.1 GHz with 3.87 dB insertion loss.