Session S51: New Materials and Physics for Innovations in Telecommunications

Characterizing interconnects to 325 GHz

Circuit designers couple active device layers and passive signal transmission layers in heterogeneous integrated circuits using high frequency interconnects (ICs). ICs transmit electrical signals between layers either with (contact) or without (contactless) direct electrical contacts. The state-of-the-art procedure for characterizing ICs uses multiline thru-reflect-line (TRL) calibration kits for the transmission line on each side of the IC to translate the measurement reference planes to the IC boundaries. This approach assumes that it is possible to directly probe each layer coupled by the IC. However, it is often impossible to probe each layer in multilayer integrated circuits. Consequently, reported scattering (S)-parameter measurements of ICs (S_IC) often include the effects from the IC as well as the transmission line on one or both sides. This calibration problem results in overestimating the insertion loss and obscuring the phase of corrected devices, because the IC is included in the corrected network.

In this talk I present an IC characterization procedure that works by embedding S_IC into the error boxes of a TRL calibration and subsequently de-embedding S_IC using a separate TRL calibration. I begin by introducing an analytical model of distributed contactless ICs. This model shows excellent agreement with full-wave simulations for the case of broadside-coupled coplanar waveguides (CPWs) separated by an SU-8 spacer layer. I then present experimental results where we embed and then de-embed broadside-coupled CPWs from a TRL calibration. We find excellent agreement between our model, simulations, and experimental results. These results validate calibration schemes that embed ICs into the calibration error boxes and provide a pathway to translate measurement reference planes beyond IC boundaries in circuit topologies where certain layers are not directly probable.

    

Frequency Scaling Acoustic Resonators into Millimeter Wave Using Thin-Film Lithium Niobate

Recent studies have aimed to reproduce the success of acoustic resonators at sub-6 GHz at higher frequencies, e.g., into millimeter waves. However, such frequency scaling calls for innovation from both the material-level development and device-level design. In this talk, we will examine the fundamental limits of mm-wave acoustic resonators, then showcase transferred thin-film lithium niobate (LiNbO3) as a promising platform. More specifically, periodically poled piezoelectric film (P3F) LiNbO3 will be used for frequency scaling without losing quality factor (Q) or electromechanical coupling (k2). Finally, we report a 50.74 GHz lithium niobate (LiNbO3) acoustic resonator with a high quality factor (Q) of 237 and an electromechanical coupling (k2) of 5.17% resulting in a figure of merit (FoM, Q·k2) of 12.2. The device also shows a Q of 159, k2 of 65.06%, and FoM of 103.4 for the 16.99 GHz tone. This result shows promising prospects of P3F LiNbO3 towards mm-wave front-end filters.   

New Surface Chemistry and Adatom Kinetics Results in Novel Extreme Bandgap Semiconductor Device Opportunities

Novel, low thermal budget synthesis methods have resulted in breakthroughs in AlN and AlScN epitaxy. New surface kinetics and chemistry discoveries including enhanced adatom diffusion at low temperatures, enhanced defect and impurity control. Catalytic induced decomposition of plasma excited molecular nitrogen has resulted in ~3x increases in growth rates and is shown to be key to metal rich single-phase AlScN epitaxy at low temperatures (~400°C). Absent this effect, AlN can still be grown metal rich at ~600°C, 400-800°C less than other synthesis methods. These physics discoveries have paved the way for a variety of new devices and applications. An 80-year-old roadblock has been shattered wherein Aluminum Nitride (AlN) was successfully doped, achieving both substantial p and n-type conductivity. In breaking this barrier, commercial epitaxy tools were used in a new unexpected way, to convert AlN from merely an insulator to a viable semiconductor, the largest direct bandgap semiconductor (6.1 eV) ever discovered. Augmenting the many deep UV optoelectronic opportunities, because of AlN's large bandgap energy, AlN has – by far – the highest Johnson and Baliga figures of merit (figures of merit describing transistor viability) of any semiconductor that has a commercial substrate making it the most promising commercially viable semiconductor for high temperature, high voltage and high-power electronics. When Scandium is added to AlN, a remarkable emerging piezo/ferroelectric semiconductor is formed with acoustic applications predicted to extend to 50 GHz operation. Additionally, AlScN has been shown by Ga Tech to result in record transistor channel conductivity of 150 ohms/square implying that AlScN can augment AlN's remarkable high temperature and high voltage capability with ~2.5 times higher current capability compared to AlGaN/GaN state-of-the-art High Electron Mobility Transistors (HEMTs).   

Measuring out-of-plane permittivity of dielectric thin films at millimeter-wave frequencies

Accurate permittivity data is critical for the design of next-generation microelectronics. Whether for quantum computing or advanced communications, industry needs to measure broadband, out-of-plane permittivity up to millimeter-wave frequencies. The current state-of-the-art for measuring out-of-plane permittivity of thin films involves either scanning microwave microscopy, which has limited accuracy, or a metal-insulator-metal (MIM) capacitor method, which is limited by large uncertainty at frequencies above 5 GHz. Here, I will discuss our approach to extend the MIM capacitor method to millimeter-wave frequencies. First, I will discuss how we built on the existing method to remove parasitics, increase accuracy, and extend the applicable frequency range by updating the device design. Next, I will detail synthesis and measurements of SiN, a material with well-documented permittivity that we used to validate the method. Lastly, I will discuss data from a set of dielectric thin-film materials with different microstructures and chemical compositions. These materials provide a case study for extracting out-of-plane permittivity at millimeter-wave frequencies.