Publications

Refine Results

(Filters Applied) Clear All

Comparison of gate dielectric plasma damage from plasma-enhanced atomic layer deposited and magnetron sputtered TiN metal gates

Published in:
J. Appl. Phys., Vol. 118, No. 4, 2015, 045307.

Summary

Fully depleted silicon-on-insulator transistors were fabricated using two different metal gate deposition mechanisms to compare plasma damage effects on gate oxide quality. Devices fabricated with both plasma-enhanced atomic-layer-deposited (PE-ALD) TiN gates and magnetron plasma sputtered TiN gates showed very good electrostatics and short-channel characteristics. However, the gate oxide quality was markedly better for PE-ALD TiN. A significant reduction in interface state density was inferred from capacitance-voltage measurements as well as a 1200 x reduction in gate leakage current. A high-power magnetron plasma source produces a much higher energetic ion and vacuum ultra-violet (VUV) photon flux to the wafer compared to a low-power inductively coupled PE-ALD source. The ion and VUV photons produce defect states in the bulk of the gate oxide as well as at the oxide-silicon interface, causing higher leakage and potential reliability degradation.
READ LESS

Summary

Fully depleted silicon-on-insulator transistors were fabricated using two different metal gate deposition mechanisms to compare plasma damage effects on gate oxide quality. Devices fabricated with both plasma-enhanced atomic-layer-deposited (PE-ALD) TiN gates and magnetron plasma sputtered TiN gates showed very good electrostatics and short-channel characteristics. However, the gate oxide quality was...

READ MORE

Thermal and residual excited-state population in a 3D transmon qubit

Summary

Remarkable advancements in coherence and control fidelity have been achieved in recent years with cryogenic solid-state qubits. Nonetheless, thermalizing such devices to their milliKelvin environments has remained a long-standing fundamental and technical challenge. In this context, we present a systematic study of the first-excited-state population in a 3D transmon superconducting qubit mounted in a dilution refrigerator with a variable temperature. Using a modified version of the protocol developed by Geerlings et al., we observe the excited-state population to be consistent with a Maxwell-Boltzmann distribution, i.e., a qubit in thermal equilibrium with the refrigerator, over the temperature range 35-150 mK. Below 35 mK, the excited-state population saturates at approximately 0.1%. We verified this result using a flux qubit with ten times stronger coupling to its readout resonator. We conclude that these qubits have effective temperature Teff ơ 35 mK. Assuming Teff is due solely to hot quasiparticles, the inferred qubit lifetime is 108 microns and in plausible agreement with the measured 80 microns.
READ LESS

Summary

Remarkable advancements in coherence and control fidelity have been achieved in recent years with cryogenic solid-state qubits. Nonetheless, thermalizing such devices to their milliKelvin environments has remained a long-standing fundamental and technical challenge. In this context, we present a systematic study of the first-excited-state population in a 3D transmon superconducting...

READ MORE

Fabrication process and properties of fully planarized deep-submicron Nb/Al-AlOx/Nb Josephson junctions for VLSI circuits

Published in:
IEEE Trans. Appl. Supercond., Vol. 25, No. 3, June 2015, 1101312.

Summary

A fabrication process for Nb/Al-AlOx/Nb Josephson junctions (JJs) with sizes down to 200 nm has been developed on a 200-mm-wafer tool set typical for CMOS foundry. This process is the core of several nodes of a roadmap for fully-planarized fabrication processes for superconductor integrated circuits with 4, 8, and 10 niobium layers developed at MIT Lincoln Laboratory. The process utilizes 248 nm photolithography, anodization, high-density plasma etching, and chemical mechanical polishing (CMP) for planarization of SiO2 interlayer dielectric. JJ electric properties and statistics such as on-chip and wafer spreads of critical current, Ic, normal-state conductance, GN, and run-to-run reproducibility have been measured on 200-mm wafers over a broad range of JJ diameters from 200 nm to 1500 nm and critical current densities, Jc, from 10 kA/cm^2 to 50 kA/cm^2 where the JJs become self-shunted. Diffraction-limited photolithography of JJs is discussed. A relationship between JJ mask size, JJ size on wafer, and the minimum printable size for coherent and partially coherent illumination has been worked out. The GN and Ic spreads obtained have been found to be mainly caused by variations of the JJ areas and agree with the model accounting for an enhancement of mask errors near the diffraction-limited minimum printable size of JJs. Ic and GN spreads from 0.8% to 3% have been obtained for JJs with sizes form 1500 nm down to 500 nm. The spreads increase to about 8% for 200-nm JJs. Prospects for circuit densities > 10^6 JJ/cm^2 and 193-nm photolithography for JJ definition are discussed.
READ LESS

Summary

A fabrication process for Nb/Al-AlOx/Nb Josephson junctions (JJs) with sizes down to 200 nm has been developed on a 200-mm-wafer tool set typical for CMOS foundry. This process is the core of several nodes of a roadmap for fully-planarized fabrication processes for superconductor integrated circuits with 4, 8, and 10...

READ MORE

Inductance of circuit structures for MIT LL superconductor electronics fabrication process with 8 niobium layers

Summary

Inductance of superconducting thin-film inductors and structures with linewidth down to 250 nm has been experimentally evaluated. The inductors include various striplines and microstrips, their 90 degree bends and meanders, interlayer vias, etc., typically used in superconducting digital circuits. The circuits have been fabricated by a fully planarized process with 8 niobium layers, developed at MIT Lincoln Laboratory for very-large-scale superconducting integrated circuits. Excellent run-to-run reproducibility and inductance uniformity of better than 1% across 200-mm wafers have been found. It has been found that the inductance per unit length of stripline and microstrip line inductors continues to grow as the inductor linewidth is reduced deep into the submicron range to the widths comparable to the film thickness and magnetic field penetration depth. It is shown that the linewidth reduction does not lead to widening of the parameter spread due to diminishing sensitivity of the inductance to the linewidth and dielectric thickness. The experimental results were compared with numeric inductance extraction using commercial software and freeware, and a good agreement was found for 3-D inductance extractors. Methods of further miniaturization of circuit inductors for achieving circuit densities >10^6 Josephson junctions per cm^2 are discussed.
READ LESS

Summary

Inductance of superconducting thin-film inductors and structures with linewidth down to 250 nm has been experimentally evaluated. The inductors include various striplines and microstrips, their 90 degree bends and meanders, interlayer vias, etc., typically used in superconducting digital circuits. The circuits have been fabricated by a fully planarized process with...

READ MORE

Broadband magnetometry and temperature sensing with a light-trapping diamond waveguide

Published in:
Nature Phys. Lett., Vol. 11, May 2015, pp. 393-7.

Summary

Solid-state quantum sensors are attracting wide interest because of their sensitivity at room temperature. In particular, the spin properties of individual nitrogen-vacancy (NV) colour centres in diamond make them outstanding nanoscale sensors of magnetic fields, electric fields and temperature under ambient conditions. Recent work on NV ensemble-based magnetometers, inertial sensors, and clocks has employed unentangled colour centres to realize significant improvements in sensitivity. However, to achieve this potential sensitivity enhancement in practice, new techniques are required to excite efficiently and to collect the optical signal from large NV ensembles. Here, we introduce a light-trapping diamond waveguide geometry with an excitation efficiency and signal collection that enables in excess of 5% conversion efficiency of pump photons into optically detected magnetic resonance (ODMR) fluorescence--an improvement over previous single-pass geometries of more than three orders of magnitude. This marked enhancement of the ODMR signal enables precision broadband measurements of magnetic field and temperature in the low-frequency range, otherwise inaccessible by dynamical decoupling techniques.
READ LESS

Summary

Solid-state quantum sensors are attracting wide interest because of their sensitivity at room temperature. In particular, the spin properties of individual nitrogen-vacancy (NV) colour centres in diamond make them outstanding nanoscale sensors of magnetic fields, electric fields and temperature under ambient conditions. Recent work on NV ensemble-based magnetometers, inertial sensors...

READ MORE

Measurement of ion motional heating rates over a range of trap frequencies and temperatures

Published in:
Phys. Rev. A, At. Mol. Opt. Phys., Vol. 91, No. 4, April 2015, 041402.

Summary

We present measurements of the motional heating rate of a trapped ion at different trap frequencies and temperatures between ~0.6 and 1.5 MHz and ~4 and 295 K. Additionally, we examine the possible effect of adsorbed surface contaminants with boiling points below ~105 degrees C by measuring the ion heating rate before and after locally baking our ion trap chip under ultrahigh vacuum conditions. We compare the heating rates presented here to those calculated from available electric-field noise models. We can tightly constrain a subset of these models based on their expected frequency and temperature scaling interdependence. Discrepancies between the measured results and predicted values point to the need for refinement of theoretical noise models in order to more fully understand the mechanisms behind motional trapped-ion heating.
READ LESS

Summary

We present measurements of the motional heating rate of a trapped ion at different trap frequencies and temperatures between ~0.6 and 1.5 MHz and ~4 and 295 K. Additionally, we examine the possible effect of adsorbed surface contaminants with boiling points below ~105 degrees C by measuring the ion heating...

READ MORE

Increasing the coherence time in a magnetically-sensitive stimulated Raman transition in 85Rb

Published in:
FIO 2014: Frontiers in Optics, 14 October 2014.

Summary

We experimentally study the Ramsey, spin echo, and CPMG pulse sequences of a magnetically sensitive transition of a cold 85Rb gas. We can increase the coherence time by up to a factor of 10 by using CPMG pulse sequences as compared to Ramsey or spin echo.
READ LESS

Summary

We experimentally study the Ramsey, spin echo, and CPMG pulse sequences of a magnetically sensitive transition of a cold 85Rb gas. We can increase the coherence time by up to a factor of 10 by using CPMG pulse sequences as compared to Ramsey or spin echo.

READ MORE

Energy efficiency benefits of subthreshold-optimized transistors for digital logic

Published in:
2014 IEEE SOI-3D-Subthreshold Microelectronics Technology Unified Conf. (S3S), 6-9 October 2014.

Summary

The minimum energy point of an integrated circuit (IC) is defined as the value of the supply voltage at which the energy per operation of the circuit is minimized. Several factors influence what the value of this voltage can be, including the topology of the circuit itself, the input activity factor, and the process technology in which the circuit is implemented. For application-specific ICs (ASICs), the minimum energy point usually occurs at a subthreshold supply voltage. Advances in subthreshold circuit design now permit correct circuit operation at, or even below, the minimum energy point. Since energy consumption is proportional to the square of the supply voltage, circuit design techniques and process technology choices that reduce the minimum energy point inherently improve the energy efficiency of ICs. Previous research has shown that optimizing process technology for subthreshold operation can improve IC energy efficiency. This, coupled with the energy efficiency advantages offered by fully-depleted silicon-on-insulator (FDSOI) processes, have led to the development of a subthreshold-optimized FDSOI process at MIT Lincoln Laboratory (MITLL) called xLP (Extreme Low Power). However, to date there has not been a quantitative estimate of the energy efficiency benefit of xLP or other analagous technology for complex digital circuits. This paper will show via simulation that the xLP process technology enables energy efficiency improvements that exceed that of process scaling by one generation. Specifically, the process is shown to improve power delay product by 57% vs. the IBM 90nm low power bulk process, and by 9% vs. the IBM 65 nm low power bulk technology at 0.3V.
READ LESS

Summary

The minimum energy point of an integrated circuit (IC) is defined as the value of the supply voltage at which the energy per operation of the circuit is minimized. Several factors influence what the value of this voltage can be, including the topology of the circuit itself, the input activity...

READ MORE

Silicon photonics devices for integrated analog signal processing and sampling

Published in:
Nanophotonics, Vol. 3, No. 4-5, 1 August 2014, pp. 313-27.

Summary

Silicon photonics offers the possibility of a reduction in size weight and power for many optical systems, and could open up the ability to build optical systems with complexities that would otherwise be impossible to achieve. Silicon photonics is an emerging technology that has already been inserted into commercial communication products. This technology has also been applied to analog signal processing applications. MIT Lincoln Laboratory in collaboration with groups at MIT has developed a toolkit of silicon photonic devices with a focus on the needs of analog systems. This toolkit includes low-loss waveguides, a high-speed modulator, ring resonator based filter bank, and all-silicon photodiodes. The components are integrated together for a hybrid photonic and electronic analog-to-digital converter. The development and performance of these devices will be discussed. Additionally, the linear performance of these devices, which is important for analog systems, is also investigated.
READ LESS

Summary

Silicon photonics offers the possibility of a reduction in size weight and power for many optical systems, and could open up the ability to build optical systems with complexities that would otherwise be impossible to achieve. Silicon photonics is an emerging technology that has already been inserted into commercial communication...

READ MORE

A tunable AC atom interferometer magnetometer

Published in:
QIM 2013, Quantum Information and Measurement, 17-20 June 2013.

Summary

We demonstrate an atom interferometer designed to measure magnetic fields and field gradients. Here, we study various pulse sequences and show how they can be manipulated to filter unwanted frequencies and to enhance desired frequencies.
READ LESS

Summary

We demonstrate an atom interferometer designed to measure magnetic fields and field gradients. Here, we study various pulse sequences and show how they can be manipulated to filter unwanted frequencies and to enhance desired frequencies.

READ MORE