Ultrafast laser high precision patterning on single-crystal diamond toward micro/nano functional devices

Microwave hydrogen plasma etching on the laser-cut surface of single crystal diamond: Phases, etching morphology, stress evolution and the effect of etching on the subsequent homoepitaxial growth

ABSTRACTAs a wide bandgap (5.47 eV) semiconductor material, single crystal diamond has high electron mobility (reportedly between 2000 and 4400 cm2V-1s-1), high electron saturation velocity (2×107 cms-1), high breakdown voltage (>107 Vcm-1), and high thermal conductivity (>21 Wcm-1K-1). Diamond-based semiconductor devices offer the potential of operation at high voltages, power levels, temperatures and under extreme radiation conditions. In this work, we present our effort to grow high quality homo-epitaxial diamond films on (100)-single crystal diamond substrates by microwave plasma chemical vapor deposition (MPCVD). The growth rate can vary from 0.01 to 100 micrometers per hour, depending on growth conditions, doping, and quality; and using a “lift-off” process, free-standing homo-epi films with remarkably low p-type doping (1×1014–1×1017 cm-3) and exceptionally low compensation ∼ 1×1013 cm-3 have been made. Vertical and lateral structure high voltage diamond Schottky rectifiers have been built for frequency dependent capacitance-voltage (C-V), and current-voltage (I-V) measurements. A breakdown voltage of 8 kV at 100 μm distance and 12.4 kV at 300 μm distance is recorded for lateral structure devices without Ohmic contact (back to back Schottky contacts), while an un-optimized vertical device with an back-side Ohmic contact has demonstrated a forward voltage drop of 7 V at 18 A/cm2 in a device that can only block 600 V. New test results show 3.7 kV blocking voltage vertical devices on 20 μm freestanding MPCVD diamond film. This data shows that the quality of diamond film extremely affect the electrical properties of the built devices.

We report on the design, fabrication and optical performance of gain mirrors in single crystal diamond substrates for vertical external cavity surface emitting lasers (VECSELs). VECSELs have gained attention recently due to their potential for high emission power in single mode with low beam divergence, yet their maximum output power remains typically limited due to thermal roll-over resulting from insufficient heat dissipation. In order to increase the heat transfer, we exploit the excellent thermal conductivity of single crystal diamond, which is assembled in direct contact with the active structure. The optical cavity is hereby defined by an output coupler and a high reflection grating structure etched into the diamond surface. We here present the design and microfabrication of a diffraction grating that was optimized to reflect light into the 0th order, therefore combining the role of a gain mirror and a heatsink at the same time. Our process involved metal mask deposition onto the diamond surface, e-beam lithography and reactive ion etching. Characterization showed reflection above 95% at a center wavelength of 1550 nm, potentially allowing the integration of the diamond mirror into a vertical external cavity surface emitting laser.

Photovoltaic (PV) energy conversion systems require fast-switching power devices that are highly efficient with low semiconductor loss under harsh environmental conditions. Silicon (Si) semiconductor devices are nearing their practical limits in meeting the ever-increasing requirements of power converters. However, wide bandgap (WBG) semiconductor devices made from silicon carbide (SiC) are exceeding these limits. SiC power devices enable more efficient and higher performance power converters. This paper presents a non-isolated dc-dc boost converter based on SiC power devices optimized for use in PV systems. The performance of two otherwise identical converters is compared, one with a new SiC MOSFET/SiC Schottky diode, and one with a conventional Si MOSFET/Si diode. A comparison of switching characteristics and energy loss of each semiconductor device is performed at different switch currents. Converter total loss and overall efficiency are evaluated at different switching frequencies, input voltages, and output power levels. The results indicate that the SiC MOSFET/SiC Schottky diode in the converter is more efficient, performs better, and has reduced power loss compared to the Si MOSFET/Si diode.

The field of diamond photonics is reviewed, with a focus on recent experimental demonstrations of photonic integrated devices in a single crystal diamond. This field leverages the outstanding material properties of diamond with the aim to establish large-scale integrated photonics for applications in sensing, information and communication technologies, and optomechanics. Accordingly, this review introduces recent progress in scalable micro- and nano-fabrication techniques for single crystal diamond photonic integrated devices, and provides quantitative comparative evaluation of the performance of the state of the art devices. The review concludes with an outlook of the potential of photonic integrated circuits in single crystal diamond.

Recent advances in the design, fabrication, robustness and reliability of high-voltage vertical and lateral smart power devices in wide and extreme bandgap semiconductors (particularly SiC and GaN) [1] are presented and discussed. The basic device structures and their refinements, with respect to their potential and experimental performance, are evaluated and compared to their Si or GaAs counterparts. Key processing technologies (such as MOS and heterojunction) needed for the performance and manufacturing of these power devices are examined with their compatibility with commercial silicon foundry capabilities. The robustness and reliability of present commercial discrete power transistors (like threshold stability, current collapse, HTRB) are assessed. Future directions and challenges for the widespread adoption of this emerging power device technology are identified.[1] T.P. Chow, I. Omura, M. Higashiwaki, H. Kawarada, and V. Pala, IEEE Trans. Electron Devices, 64, 856 (2017).

The introduction of next generation mobile systems and alternative applications demands significant improvement in efficiency and linearity of the power amplifier. It is essential to understand the limitations inherent to the power transistor, and improve it for emerging applications. The package containing the power transistor is built up of various components; MOS capacitors, bond wire arrays and ESD protection circuitry common in commercial power transistors in addition to the semiconductor transistor die. The objectives of this work are to develop multiphysics modelling components which are needed for CAD-based design optimisation. An electrothermal bond wire array model is proposed which combines the electrical and thermal characteristics at high-frequency, reproducing the non-uniform distribution seen in measurements. The model returns the temperature distribution over 2000-times faster, in a fraction of a second, compared to an equivalent 3D multiphysics simulation which takes at least 23 minutes, making it well suited to circuit modelling for CAD-design and optimisation. Fundamentally, the foundations of the power transistor is the semiconductor technology that the package holds. Extraction of compact transistor models is time consuming, requiring a multitude of measurement steps and specialised equipment. Behavioural modelling techniques simplify the measurement extraction procedure, while conserving the accuracy and convergence needed for circuit modelling. A primary limiting factor of the X-parameter behavioural model is that it lacks thermal memory effects that are prevalent in most applicable transistor technologies. This work extends the extraction of X-parameters to include dynamic thermal memory effects. Thermal hysteresis is demonstrated and shown to introduce spectral asymmetry during simulations using two-tone and QAM-16 modulation stimulus. Furthermore, distributed multiphysics modelling techniques co-simulate electromagnetics, thermodynamics and semiconductor device physics - the coupled solution providing enhanced insight into the performance and limitations of the transistor compared to lumped modelling approaches. A distributed electrothermal model was developed which enables significant design improvements by optimisation of the thermal and electrical layouts. This is demonstrated at 28~GHz for a GaN pHEMT transistor where a 12% increase in linear output power and 54% increase in PAE is achieved relative to the peak efficiency of the transistor (24%). The multiphysics modelling components developed in this work aim to improve power transistors for next generation mobile systems and applications requiring the efficient application of high-power microwaves.

To develop the capability to create diamond electrodes with greater functionality, covalent modification was carried out on homoepitaxial single-crystal diamond electrode surfaces. (100) and (111) single-crystal boron-doped diamond electrodes were first prepared homoepitaxially, then subjected to oxidative treatments, and the functional groups on the oxidized surfaces were analyzed by employing X-ray photoelectron spectroscopy (XPS). Based on the results, we conclude that both singly and doubly bonded oxygen groups and were generated on the anodically treated (100) diamond electrode surfaces, whereas only singly bonded oxygen groups were generated on the (111) surfaces. Second, selective surface modifications of anodically treated (100) and (111) diamond surfaces with 2,4-dinitrophenylhydrazine and 3-aminopropyltriethoxysilane moieties were carried out. Based on the tendencies of these surface modifications, together with the XPS results, we propose that carbonyl and hydroxyl groups are generated on anodically treated (100) diamond surfaces, and hydroxyl groups are mainly generated on anodically treated (111) diamond surfaces. The study of chemical modification of single-crystal diamond electrode surfaces should be useful, not only for creating new types of functional electrodes, but also for better understanding the properties of polycrystalline diamond electrodes and their possible applications. © 2004 The Electrochemical Society. All rights reserved.

Polycrystalline diamond films, single crystal bulk diamonds, and diamond powder were treated in microwave plasma of hydrogen at 1.6 torr under a negative direct-current bias of −150 to −300 V without metal catalyst. It was found that fibrous structures, uniformly elongated along the direction normal to the specimen surface, were formed on the diamond surfaces. Similar experiments for glasslike carbon resulted in conical structures with frizzy fibers at the tops. Transmission electron microscopy measurements indicated that the fibers formed on diamond consisted of randomly oriented diamond nanocrystals with diameters of less than 10 nm, while the conical structures formed on glasslike carbon consisted of graphite nanocrystals. Field emission measurements of the fibrous specimens exhibited better emission efficiency than untreated ones. The field emission electron microscopy of the fibrous glasslike carbon showed a presence of discrete electron emission sites at a density of approximately 10,000 sites/cm2.

In order to improve efficiency and reduce circuit size the switching speeds of semiconductor power devices are being reduced. This is being achieved by the use of wide band-gap semiconductor devices. Traditional current measurement techniques are unable to accurately measure these new high speed switching edges, due to a lack of bandwidth and high insertion inductance. In this paper a 1.6 GHz bandwidth, scalable current, SMD shunt based current probe is developed for switching and steady state current measurements in wide band-gap power devices. By designing this shunt with an extremely low insertion inductance of less than 10 pH it is ensured that the measurement circuit has negligible impact on switching device operation.

A new strategy coupling Fe etching with Ar+ bombardment was proposed and carried out on the single-crystal diamond (SCD) surface for enhancing the electrical properties of diamond by the dual effects of defects and impurity elements. Scanning electron microscope, energy dispersive x-ray spectrum, x-ray diffractometer, Raman spectroscopy, and Hall effects measurements were performed to investigate the modified surface structure and electron field emission (EFE) performance. The results indicated that concentric circle patterns accompanying propitious clouds were generated after the treatment, which were composed of nano-sized carbides including FeC, WC, W2C, and Fe3W3C, respectively. The Hall mobility and carrier concentration of the treated SCD were about 22.55 cm2 V−1 s−1 and 2.71 × 1021 cm−3, which are 12 times and 4 orders of magnitude higher than the pristine SCD, respectively. The EFE behavior was turned on at E0 = 6.2 V/μm, achieving a current density of 6.8 mA/cm2 at the applied field of 10 V/μm. The present study offers the possibility of using SCD as EFE material and an effective method for enhancing the EFE properties of diamond.

A novel beam position monitor, operated at zero bias voltage, based on high-quality chemical-vapor-deposition single-crystal Schottky diamond for use under intense synchrotron X-ray beams was fabricated and tested. The total thickness of the diamond thin-film beam monitor is about 60 µm. The diamond beam monitor was inserted in the B16 beamline of the Diamond Light Source synchrotron in Harwell (UK). The device was characterized under monochromatic high-flux X-ray beams from 6 to 20 keV and a micro-focused 10 keV beam with a spot size of approximately 2 µm × 3 µm square. Time response, linearity and position sensitivity were investigated. Device response uniformity was measured by a raster scan of the diamond surface with the micro-focused beam. Transmissivity and spectral responsivity versus beam energy were also measured, showing excellent performance of the new thin-film single-crystal diamond beam monitor.

Synthesizing single-crystal diamond by repetition of high rate homoepitaxial growth by microwave plasma CVD

Laser applications, such as cutting of medical devices (e.g. stents) and watch components (e.g. gear wheels, hands), demand highest cutting accuracy in combination with extremely high cutting edge quality and minimum cutting kerf. For single pass laser cutting of small components with high precision the application of fiber lasers is the common industrial practice [1]. In this fusion cutting process, molten material in the cutting kerf is ejected by the process gas. Besides the formation of a heat affected zone (HAZ), the weakness of the fusion cutting process is the formation of striations on the cutting surface, which increases the surface roughness of laser cut components [2].Ultra-short pulse lasers provide the possibility of material processing with minimum thermal impact on the remaining material structure [3,4]. Thus laser cutting with very small feature sizes (several microns) and negligible HAZ can be realized. This laser vaporization process using ultra-short laser pulses can replace the common fusion cutting process, if an optimization of the cutting quality and cutting precision is developed. Specifically, the influence of pulse duration on the cutting characteristics must be characterized.In this study, the laser cutting process of different materials is examined by reducing the pulse duration from 6 ps to 900 fs, then down to 400 fs. The results of the micro-cutting process of thin metal sheets with regard to pulse duration, pulse energy, laser repetition rate and cutting velocity have been determined. Quality criteria were identified for evaluating the influence of cutting parameters on the cutting results. The HAZ and micro-structural changes, the striation formation and surface roughness and the taper lead angle of the cutting kerf and surface deposition of material were chosen as criteria and have been analyzed in detail.The results show that ultra-short laser pulses enable high quality cutting with improved roughness and minimized HAZ. The highest quality and cutting speed are reached with sub-ps pulses. Furthermore the processing window for laser vaporization cutting is enlarged by using femtosecond pulses. Finally, with shorter pulses under optimal cutting conditions, an improvement of cutting efficiency and cutting quality is possible.Laser applications, such as cutting of medical devices (e.g. stents) and watch components (e.g. gear wheels, hands), demand highest cutting accuracy in combination with extremely high cutting edge quality and minimum cutting kerf. For single pass laser cutting of small components with high precision the application of fiber lasers is the common industrial practice [1]. In this fusion cutting process, molten material in the cutting kerf is ejected by the process gas. Besides the formation of a heat affected zone (HAZ), the weakness of the fusion cutting process is the formation of striations on the cutting surface, which increases the surface roughness of laser cut components [2].Ultra-short pulse lasers provide the possibility of material processing with minimum thermal impact on the remaining material structure [3,4]. Thus laser cutting with very small feature sizes (several microns) and negligible HAZ can be realized. This laser vaporization process using ultra-short laser pulses can replace the common ...

During the last few decades, the microelectronics packaging industry has moved into the 2.5D to 3D space for increased density, functionality, and speed. Similar concepts and ideas for developing 2.5D to 3D power electronics packaging are desired to achieve even greater efficiency and power density over conventional power electronics packaging methods. Wide-band gap (WBG) semiconductors, such as SiC and GaN, have accelerated the ability to shrink the volumetric size and weight of these power conversion systems, and thus improve overall power density metrics, due to their inherent high frequency, high temperature, and high voltage capabilities. WBG power semiconductor devices, with these attributes, thus make themselves excellent candidates for more aggressive packaging, compared to Si-derived packaging, in order to not only take full advantage of the WBG device ratings, but also to achieve high power densities of the overall power conversion systems. Already different/multiple power semiconductor devices are being combined by processing them together on the same die to boost electrical performance and increase power density. It can be assumed that further levels of integration will be sought after for the next levels of packaging to enable similar gains, especially with the advent of double side solderable die. The 3D stacking of die, components, and substrates creates the question of how well will each of these perform in close proximity to each other. This work focuses on the numerical simulation and experimental measurements to predict the temperature distribution of power converters built in a stacked fashion. Thermal models of a stacked power electronic switching unit — a silicon controlled rectifier and anti-parallel diode — are modeled under the assumption of equally sized die. Temperature field maps are generated for 20W to 250W of power dissipations across the power semiconductor die. Thermal models are then compared with matching experimental setups to observe the effect of switching unit placement attached to a given substrate on the die junction temperatures for various scenarios of thermal crosstalk. Results from this work are expected to aid in the development 2.5D to 3D power electronic packaging by predicting thermal performance of stacked, ultra-dense, WBG device -based packages.