A collaboration of US and Swedish researchers presents, for the first time, radio frequency (RF) reliability studies of fully integrated complimentary metal-oxide-semiconductor (CMOS) radio frequency integrated circuits (RFICs), important for designing next generation wireless communication applications, especially those that are based on ultra-thin flexible packages. Mitra et al. have a wide spread of research areas, including such topics as communications radar, hand-held medical scanners and portable weapon scanning technology. It is, however, their research into CMOS RFICs that lead to the present paper. Mastery of CMOS technology, with its small chip size, is thought to be crucial for the continued miniaturisation of circuits, with the world of nanoelectronics providing a potential gateway to the next generation of highly integrated multifunctional devices, circuits and systems. The Radio Frequency Integrated Circuits (RFIC) Lab members at North Dakota State University. (From Left) Arka Biswas, Chitralekha Biswas, Dipankar Mitra (in the back), and Babak Hamidi. RFIC Characterisation Set-up at NDSU. The ‘thinning-down’ of integrated circuits is one step towards mastering CMOS technology, with thinner and more lightweight circuitry providing more efficient and cost-effective solutions for various applications, including wireless communications, wearable and flexible electronics and Internet of Things (IoT) devices. One of the biggest challenges for realisation of flexible and ultra-thin ICs is the effect die-substrate thinning has on the RF performance of the IC. It was this problem that triggered Mitra et al. into producing the RF reliability studies presented in Electronics Letters. Thinning of the die-substrate, the small block of semiconducting material upon which a circuit is built, is one of the main contributors to performance degradation for RFICs, whilst also being the most important for producing thinner and more flexible circuitry. In their work, Mitra et al. use a voltage-controlled oscillator (VCO) to measure the effects of die-substrate thickness on the RF performance of the IC, including frequency range, RF output power variation, phase noise variation and DC parameters. The usual thickness of die-substrate for CMOS circuits as between 250–300 micrometres. Mitra et al. used a 250 µm thickness VCO as a test case, and then thinned down the die-substrate to 50 µm, 35 µm and 25 µm. They found that deviations of frequency and output power are within 1% and 1 dB respectively, with DC parameters also shown to be fairly similar for the different substrate thicknesses. The sensitivity to phase noise when thinning down was found to be significant, which confirms the well-known fact that phase noise sensitivity increases due to leakage. Flexible electronics is changing the way we make and use electronics. Many existing applications, such as wireless communication, wearable electronics, and healthcare monitoring require flexibility to promote more efficient and cost-effective solutions. Development of these flexible electronics is driving progress in the field, which enables futuristic applications such as a network of IoT devices and connected, smart cities. Several initiatives from governments and industry have also contributed to the progress and it is now estimated that the market for flexible electronics will reach $300 billion by 2028, with expected growth from $29 billion in 2017 to over $63 billion in 2023 for printed, flexible and organic electronics alone. The high-performance of CMOS electronics will be critical to this growth in flexible electronics, as a great number of current and future electronics rely on the fast communication and computation they can provide.
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