Design, Fabrication, Characterization and Modeling of CMOS-Compatible PtSe2 MOSFETs

For the last 50 years, Si metal-oxide-semiconductor field-effect transistors (MOSFETs) have undergone tremendous development under the Moore’s law. However, it has become more and more difficult to continue the scaling due to the limitation of Si quantum confinement when the gate length is less than 5 nm. Two-dimensional (2D) atomic-layered materials may replace Si in future-generation ultra-thin-body, low-power, and high-performance MOSFETs. However, for any 2D material to replace Si, it must not only have high mobility and sizable bandgap, but also be manufacturable. Graphene has high mobility but no bandgap. MoS2 has sizable bandgap but low mobility. Black phosphorus (BP) has high mobility and sizable bandgap but is unstable in air and has not been grown under reasonable pressure and temperature. By contrast, monolayer PtSe2 has high mobility, sizable bandgap, air stability, and can be synthesized below 450 °C ‒ −the thermal budget of CMOS back-of-the-line (BEOL) processes (BEOL). Additionally, bulk PtSe2 is semi-metallic to facilitate low resistance contact, a critical issue for all 2D devices. Experimentally, PtSe2 MOSFETs have been demonstrated by exfoliated flakes, chemical vapor deposited film, and thermal assisted converted film. In this dissertation, both material preparation and device fabrication are done below 450 °C. Transistors based on molecular beam epitaxy grown PtSe2 are fabricated and characterized for the first time. With 3-monolayer (ML) PtSe2 grown by molecular beam, transistor with n-type carrier has been demonstrated with on/off ratio of 43, which increases to 1600 at 80 K and is the best among n-type PtSe2 transistors fabricated on grown films. The MOSFETs are batch-fabricated by a CMOS-compatible process based on 200-mm-diameter Si substrates prepared by a state-of-the-art BiCMOS foundry. Dozens of rounds of fabrication were carried out to ensure the yield of large-scale fabrication. Photoresist residue formed on 2D material were reduced by reduced dry etching time. The poor adhesion between 2D material and the substrate was also addressed. Despite the thin PtSe2 layer, doping by contact bias lowers the contact resistance significantly and boosts the on current and on/off ratio. Temperature-dependent current-voltage characteristics show the bandgap is approximately 0.2 eV, which confirms that the semiconductor-semimetal transition of PtSe2 is not as abrupt as originally predicted. By the chip maps, performances of 66 devices are presented, which show reasonable uniformity across the 10 mm × 10 mm chip. Better MOSFET performance can be expected by growing even thinner PtSe2 uniformly and by thickening the PtSe2 in the contact regions

Continuous-wave and Transient Characteristics of Phosphorene Microwave Transistors

Few-layer phosphorene MOSFETs with 0.3-um-long gate and 15-nm-thick Al2O3 gate insulator was found to exhibit a forward-current cutoff frequency of 2 GHz and a maximum oscillation frequency of 8 GHz after de-embedding for the parasitic capacitance associated mainly with the relatively large probe pads. The gate lag and drain lag of the transistor was found to be on the order of 1 us or less, which is consistent with the lack of hysteresis, carrier freeze-out or persistent photoconductivity in DC characteristics. These results confirm that the phosphorene MOSFET can be a viable microwave transistor for both small-signal and large-signal applications.Comment: Accepted for oral presentation at IMS 201

Scanning Microwave Microscopy of Aluminum CMOS Interconnect Lines Buried in Oxide and Water

Using a scanning microwave microscope, we imaged in water aluminum interconnect lines buried in aluminum and silicon oxides fabricated through a state-of-the-art 0.13 um SiGe BiCMOS process. The results were compared with that obtained by using atomic force microscopy both in air and water. It was found the images in water was degraded by only approximately 60% from that in air.Comment: 3 pages, 5 figures, conferenc

Scanning microwave microscopy of buried CMOS interconnect lines with nanometer resolution

This paper reports scanning microwave microscopy of CMOS interconnect aluminum lines both bare and buried under oxide. In both cases, a spatial resolution of 190 ± 70 nm was achieved, which was comparable or better than what had been reported in the literature. With the lines immersed in water to simulate high-k dielectric, the signal-to-noise ratio degraded significantly, but the image remained as sharp as before, especially after averaging across a few adjacent scans. These results imply that scanning microwave microscopy can be a promising technique for non-destructive nano-characterization of both CMOS interconnects buried under oxide and live biological samples immersed in water

CMOS-compatible batch processing of monolayer MoS2 MOSFETs

Thousands of high-performance 2D metal-oxide-semiconductor field effect transistors (MOSFETs) were fabricated on wafer-scale chemical vapor deposited MoS2 with fully-CMOS-compatible processes such as photolithography and aluminum metallurgy. The yield was greater than 50% in terms of effective gate control with less-than-10 V threshold voltage, even for MOSFETs having deep-submicron gate length. The large number of fabricated MOSFETs allowed statistics to be gathered and the main yield limiter to be attributed to the weak adhesion between the transferred MoS2 and the substrate. With cut-off frequencies approaching the gigahertz range, the performances of the MOSFETs were comparable to that of state-of-the-art MoS2 MOSFETs, whether the MoS2 was grown by a thin-film process or exfoliated from a bulk crystal © 2018 IOP Publishing Ltd Printed in the UK1

Inverted Scanning Microwave Microscopy for Nanometer-scale Imaging and Characterization of Platinum Diselenide

Near-field scanning microwave microscopy (SMM) is a technique gaining popularity for the study of the electrical properties of both soft and hard matter on the nanometer scale. Despite the current focus on semiconductors, the applications of SMM on new two-dimensional materials such as platinum diselenide (PtSe 2 ) are still in an initial stage. In this work, the imaging capabilities of an innovative SMM and the analysis of the electrical properties of PtSe 2 are demonstrated