Submicron Emitter Research Articles

Supercritical Fluid Nanospray Mass Spectrometry.

Supercritical fluids are typically electrosprayed using an organic solvent makeup flow to facilitate continuous electrical connection and enhancement of electrospray stability. This results in sample dilution, loss in sensitivity, and potential phase separation. Premixing the supercritical fluid with organic solvent has shown substantial benefits to electrospray efficiency and increased analyte charge state. Presented here is a nanospray mass spectrometry system for supercritical fluids (nSF-MS). This split flow system used small i.d. capillaries, heated interface, inline frit, and submicron emitter tips to electrospray quaternary alkyl amines solvated in supercritical CO2 with a 10% methanol modifier. Analyte signal response was evaluated as a function of total system flow rate (0.5-1.5 mL/min) that is split to nanospray a supercritical fluid with linear flow rates between 0.07 and 0.42 cm/sec and pressure ranges (15-25 MPa). The nSF system showed mass-sensitive detection based on increased signal intensity for increasing capillary i.d. and analyte injection volume. These effects indicate efficient solvent evaporation for the analysis of quaternary amines. Carrier additives generally decreased signal intensity. Comparison of the nSF-MS system to the conventional SF makeup flow ESI showed 10-fold signal intensity enhancement across all the capillary i.d.s. The nSF-MS system likely achieves rapid solvent evaporation of the SF at the emitter point. The developed system combined the benefits of the nanoemitters, sCO2, and the low modifier percentage which gave rise to enhancement in MS detection sensitivity.

Conducting and characterizing femto flow electrospray ionization.

Femto flow electrospray ionization (ESI) with flow rates ranging from 240 fL min-1 to the low pico level (<10 pL min-1) was conducted and measured using a submicron emitter tip and relay ESI configuration. Signature analyte ion current intensities and profiles were observed. The obtained flow rate and ionization current enabled size calculation for initial charged nanodroplets.

Submicron Emitters Enable Reliable Quantification of Weak Protein-Glycan Interactions by ESI-MS.

Interactions between carbohydrates (glycans) and glycan-binding proteins (GBPs) regulate a wide variety of important biological processes. However, the affinities of most monovalent glycan-GBP complexes are typically weak (dissociation constant (Kd) > μM) and difficult to reliably measure with conventional assays; consequently, the glycan specificities of most GBPs are not well established. Here, we demonstrate how electrospray ionization mass spectrometry (ESI-MS), implemented with nanoflow ESI emitters with inner diameters of ∼50 nm, allows for the facile quantification of low-affinity glycan-GBP interactions. The small size of the droplets produced from these submicron emitters effectively eliminates the formation of nonspecific glycan-GBP binding (false positives) during the ESI process up to ∼mM glycan concentrations. Thus, interactions with affinities as low as ∼5 mM can be measured directly from the mass spectrum. The general suppression of nonspecific adducts (including nonvolatile buffers and salts) achieved with these tips enables ESI-MS glycan affinity measurements to be performed on C-type lectins, a class of GBPs that bind glycans in a calcium-dependent manner and are important regulators of immune response. At physiologically relevant calcium ion concentrations (2-3 mM), the extent of Ca2+ nonspecific adduct formation observed using the submicron emitters is dramatically suppressed, allowing glycan affinities, and the influence of Ca2+ thereon, to be measured. Finally, we show how the use of submicron emitters and suppression of nonspecific binding enable the quantification of labile (prone to in-source dissociation) glycan-GBP interactions.

Space-Charge Limited Current From a Finite Emitter in Nano- and Microdiodes

We simulate numerically the classical charge dynamics in a microscopic, planar, vacuum diode with a finite emitter area and a finite number of electrons in the gap. We assume electrons are emitted under space-charge limited conditions with a fixed potential applied to the diode. The Coulomb interaction between all electrons is included using the method of molecular dynamics. We compare our results to the conventional two-dimensional Child-Langmuir and explain how it is limited in applicability for sub-micron diameter emitters. Finally, we offer some simple relations for understanding space-charge limited flow from very small emitters.

Compact Multistage Plasma-Based Accelerator Design for Correlated Energy Spread Compensation.

The extreme electromagnetic fields sustained by plasma-based accelerators could drastically reduce the size and cost of future accelerator facilities. However, they are also an inherent source of correlated energy spread in the produced beams, which severely limits the usability of these devices. We propose here to split the acceleration process into two plasma stages joined by a magnetic chicane in which the energy correlation induced in the first stage is inverted such that it can be naturally compensated in the second. Simulations of a particular 1.5-m-long setup show that 5.5GeV beams with relative energy spreads of 1.2×10^{-3} (total) and 2.8×10^{-4} (slice) could be achieved while preserving a submicron emittance. This is at least one order of magnitude below the current state of the art and would enable applications such as compact free-electron lasers.

Plasma wakefield acceleration experiments at FACET II

During the past two decades of research, the ultra-relativistic beam-driven plasma wakefield accelerator (PWFA) concept has achieved many significant milestones. These include the demonstration of ultra-high gradient acceleration of electrons over meter-scale plasma accelerator structures, efficient acceleration of a narrow energy spread electron bunch at high-gradients, positron acceleration using wakes in uniform plasmas and in hollow plasma channels, and demonstrating that highly nonlinear wakes in the ‘blow-out regime’ have the electric field structure necessary for preserving the emittance of the accelerating bunch. A new 10 GeV electron beam facility, Facilities for Accelerator Science and Experimental Test (FACET) II, is currently under construction at SLAC National Accelerator Laboratory for the next generation of PWFA research and development. The FACET II beams will enable the simultaneous demonstration of substantial energy gain of a small emittance electron bunch while demonstrating an efficient transfer of energy from the drive to the trailing bunch. In this paper we first describe the capabilities of the FACET II facility. We then describe a series of PWFA experiments supported by numerical and particle-in-cell simulations designed to demonstrate plasma wake generation where the drive beam is nearly depleted of its energy, high efficiency acceleration of the trailing bunch while doubling its energy and ultimately, quantifying the emittance growth in a single stage of a PWFA that has optimally designed matching sections. We then briefly discuss other FACET II plasma-based experiments including in situ positron generation and acceleration, and several schemes that are promising for generating sub-micron emittance bunches that will ultimately be needed for both an early application of a PWFA and for a plasma-based future linear collider.

Protein-Glass Surface Interactions and Ion Desalting in Electrospray Ionization with Submicron Emitters.

Theta glass electrospray emitters can rapidly mix solutions to investigate fast reactions that occur as quickly as 1μs, but emitters with submicron tips have the unusual properties of desalting protein ions and affecting the observed abundances of some proteins as a result of protein-surface interactions. The role of protein physical properties on ion signal was investigated using 1.7 ± 0.1 μm and 269 ± 7 nm emitters and 100 mM aqueous ammonium acetate or ammonium bicarbonate solutions. Protein ion desalting occurs for both positive and negative ions. The signal of a mixture of proteins with the 269 nm tips is time-dependent and the order in which ions of each protein is observed is related to the expected strengths of the protein-surface interactions. These results indicate that it is not just the high surface-to-volume ratio that plays a role in protein adsorption and reduction or absence of initial ion signal, but the small diffusion distance and extremely low flow rates of the smaller emitters can lead to complete adsorption of some proteins and loss of signal until the adsorption sites are filled and the zeta potential is significantly reduced. After about 30 min, signals for a protein mixture from the two different size capillaries are similar. These results show the advantages of submicron emitters but also indicate that surface effects must be taken into account in experiments using such small tips or that coating the emitter surface to prevent adsorption should be considered. Graphical Abstract.

Fabrication and Characterization of Self-Aligned InAlAs/InGaAsSb/InGaAs Double Heterojunction Bipolar Transistors

A trilevel resist system was employed to fabricate self-aligned, submicron emitter finger In0.52Al0.48As/In0.42Ga0.58As0.77Sb0.23/ In0.53Ga0.47As double heterojunction bipolar transistors (DHBTs). Selective wet-etchants were used to define the emitter fingers and to form an InGaAs guard-ring around the emitter fingers. Due to the low energy bandgap of the InGaAsSb base layer and type II base-collector junction, a low turn-on voltage of 0.38 V at 1 A/cm2 and a high dc current gain of 123.8 for a DHBT with a 0.65 x 8.65 µm2 emitter area were obtained. A unity gain cut-off frequency (fT) of 260 GHz and a maximum oscillation frequency (fmax) of 485 GHz at JC = 302 kA/cm2 were achieved.

Hot electron injection effect on the microwave performance of type-I/II AlInP/GaAsSb/InP double-heterojunction bipolar transistors

A type-I/II double heterojunction bipolar transistor (DHBT) with a layer structure of AlInP(emitter)/GaAsSb(composition graded base)/InP(collector) was designed and grown by molecular beam epitaxy technique. The band alignment of the type-I AlInP/GaAsSb emitter-base interface provides hot electron injection into the base to enhance the effective base velocity to 2.54×107 cm/s, resulting in an improved device gain and high frequency performance. We have fabricated a submicron emitter (AE=0.35×4 μm2) type-I/II DHBT with a demonstrated current gain of β=50 and a breakdown voltage of BVCEO=4.2 V, with a cutoff frequency of fT=455 GHz and fMAX=400 GHz at a collector current density, JC=10 mA/μm2.

Fabrication of InAlAs/InGaAsSb/InGaAs double heterojunction bipolar transistors

A trilevel resist system was employed to fabricate self-aligned, submicron emitter finger In0.52Al0.48As/In0.42Ga0.58As0.77Sb0.23/In0.53Ga0.47As double heterojunction bipolar transistors (DHBTs). Selective wet-etchants were used to define the emitter fingers and to form an InGaAs guard-ring around the emitter fingers. Due to the low energy bandgap of the InGaAsSb base layer and type II base-collector junction, a low turn-on voltage of 0.38 V at 1 A/cm2 and a high dc current gain of 123.8 for a DHBT with a 0.65×8.65 μm2 emitter area were obtained. A unity gain cutoff frequency (fT) of 260 GHz and a maximum oscillation frequency (fmax) of 485 GHz at JC=302 kA/cm2 were achieved.

ICP-RIE etching of self-aligned InP based HBTs with Cl 2/N 2 chemistry

We report on a simple Inductively Coupled Plasma-Reactive Ion Etching (ICP-RIE) process with Cl 2/N 2 chemistry to process InP based, self-aligned HBTs with sub-micron emitters. Since the layer to be etched is in the range of 150 nm (the thickness of emitter cap and emitter layers), a low etch rate is beneficial. On the other hand, it is also necessary to use chemistries without hydrogen to prevent any possible hydrogen passivation. Therefore, in this work, Cl 2/N 2 chemistry is selected and a plasma process providing an etch rate of 120 nm/min is optimized. Not only the etch rate but also the electrical and the surface quality of the wafers are examined. It has been illustrated that the etch rate of the optimized process is uniform over the wafer and it is reproducible. In addition to that, it has been shown with electrical measurements that there is no degradation in the material quality. To test the optimized process, sub-micron HBTs are fabricated and the RF measurements have shown an f max of 340 GHz which make them to be used in high speed communication systems. In addition to that, lower and controlled under etch gives better current gain distribution over the wafer leading better device models and resulting in better yield in MMICs.

Development of 6.00Å graded metamorphic buffer layers and high performance In0.86Al0.14As∕In0.86Ga0.14As heterojunction bipolar transistor devices

In x Al 1 − x As ∕ In x Ga 1 − x As heterojunction bipolar transistors (HBTs) with lattice parameters ranging from 6.00to6.058Å employing the use of narrow band gap InxGa1−xAs base epitaxial layers toward InAs (0.86&amp;lt;Xln&amp;lt;1) allows for the development of high speed digital and mixed signal circuits to perform at half the power required for conventional group III-V-based HBT device technologies. However, one of the key challenges inhibiting the development of low power narrow band gap HBT device circuits is the absence of semi-insulating (SI) substrates with lattice parameters towards 6.058Å. Therefore, a metamorphic InxAl1−xAs (0.52&amp;lt;Xln&amp;lt;0.86) graded buffer layer (GBL) grown on InP by molecular beam epitaxy was investigated as a means to enable a SI template with a lattice parameter of 6.00Å. The metamorphic InxAl1−xAs GBL thickness is desired to be less than a micron in order for this approach to be compatible with the aggressive design rules of high-density transistor circuits. In this study, In0.86Al0.14As∕In0.86Ga0.14As double heterojunction bipolar transistor devices were grown on 0.90, 0.45, and 0.23μm thick InxAl1−xAs GBLs to assess the role of buffer layer thickness on both defect formation and device performance. The material characterization results for the 0.90 and 0.45μm thick buffer layers exhibited a crosshatch pattern with a surface rms roughness of 4nm and threading dislocation densities of ∼106cm−2. Excellent dc and rf characteristics from metamorphic HBT devices with submicron emitter widths were observed with low turn-on voltage of 0.45V, high current gain, low reverse junction leakage(&amp;lt;1μA), and rf peak performance in the vicinity of 150GHz. Finally, preliminary circuits (dividers and delay chains) have been designed, fabricated, and demonstrated with the 6.00Å HBT technology.

Dry Etching of Sub-Micron Emitter in InP Heterojunction Double Bipolar Transistor Using Inductively Coupled Plasma

not Available.

NUMERICAL INVESTIGATION OF THE EFFECT OF DOPING PROFILES ON THE HIGH FREQUENCY PERFORMANCE OF InP/InGaAs SUPER SCALED HBTs

Our simulations show that the emitter and collector doping profile engineering is very important for the transistor optimization, in particular, adjusting the low doped emitter section to the depletion length resulted in the decrease of the emitter series resistance and increased ft and fmax; decreasing of the collector doping concentration and shrinking the collector thickness reduced the collector transit time. Accounting for the lateral diffusion of hot electrons in the device with submicron emitter was found to be important in the transistor optimization process. This effect determines the effective thickness of the emitter finger and the value of the push-out current.

Submicron InP-InGaAs single heterojunction bipolar transistors with f/sub T/ of 377 GHz

Submicron InP-InGaAs-based single heterojunction bipolar transistors (SHBTs) are fabricated to achieve record-breaking speed performance using an aggressively scaled epitaxial structure coupled with a submicron emitter process. SHBTs with dimensions of 0.35 ×16 μm have demonstrated a maximum current gain cutoff frequency f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> of 377 GHz with a simultaneous maximum power gain cutoff frequency f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> of 230 GHz at the current density Jc of 650 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Typical BV/sub CEO/ values exceed 3.7 V.

Submicron emittance and ultra small beam size measurements at ATF

This paper covers improvements to a photoinjector that lead to the realization and measurement of submicron normalized emittance for a 0.5 nC electron beam at the Brookhaven Accelerator Test Facility. A fitting procedure utilizing beam size measurements from multiple beam profile monitors along the transport line was used instead of more traditional techniques. Such a small emittance beam in combination with a tight focusing scheme allowed us to achieve 10 μm RMS beam sizes at 60 MeV. The behavior and limitations of different BPM screens is compared for such beams.

Gain limitations of scaled InP/InGaAs heterojunction bipolar transistors

We investigate a current gain degradation mechanism in self-aligned InP/InGaAs heterojunction bipolar transistors. We show that surface level pinning at the emitter sidewall gives rise to a peripheral emitter injection current into the base which increases the base current due to electrons recombining at the base contacts. The surface charge at the extrinsic base surface causes the formation of a conducting channel which further enhances the electron flow from the emitter to the base contacts. Two-dimensional numerical simulations of the emitter region combined with Monte-Carlo simulations of the injection process at the abrupt base-emitter heterojunction are in good agreement with the measurements. This effect will be especially severe for submicron emitter widths where the emitter perimeter to emitter area ratio is large.

On the origin of 1/f noise in polysilicon emitter bipolar transistors

Low frequency noise in polysilicon emitter bipolar transistors (BJT) with different emitter areas (AE) was studied. In BJTs with submicron emitter area, random telegraph signals of different amplitudes and frequencies were found. Averaging of noise spectra from different submicron BJTs gives 1/f noise of the same level (normalized to the emitter area) as 1/f noise found in transistors with large emitter areas. The sheet density of traps (NT) located within the polysilicon–crystalline silicon interfacial layer and responsible for 1/f noise was estimated to be ⩾3×108 cm−2.

Submicron, fully self-aligned HBT with an emitter geometry of 0.3 μm

A new fully self-aligned heterojunction bipolar transistor (HBT) process has been developed to fabricate submicron emitter geometries for applications requiring ultra low-power consumption and very high-speed performance. In this novel process approach the emitter, base and collector ohmic contacts are all self-aligned to the emitter mesa. Furthermore, the three ohmic contacts, i.e., emitter, base, and collector are defined and deposited in a single metalization step thereby simplifying the fabrication process. Using this new process we have fabricated HBT emitter geometries as small as 0.3 /spl mu/m/sup 2/ with RF performance of over 130 GHz. To our knowledge, this is the smallest HBT ever reported.

Submicron AlGaAs/GaAs heterostructure bipolar transistor with high gain

We describe the realisation of self-aligned AlGaAs/GaAs heterostructure bipolar transistors with submicron emitter stripe width, current gain of 120 and a maximum operating current density greater than 105cm−. The high current gain was achieved by passivating the extrinsic base region with thin AlGaAs.