Electrical Linewidth Research Articles

Investigation of Reactive Silver Ink Formula for Reduced Silver Consumption in Silicon Heterojunction Metallization

Silver is the most expensive non-silicon component in photovoltaic cells. This is particularly salient for silicon heterojunction (SHJ) cells, which rely on large quantities of low-temperature silver pastes (LT-SP). SHJ cells would benefit greatly from an industrially scalable metallization process that simultaneously offers low silver consumption, low finger resistivities (<20 μΩ·cm), and low processing temperatures. Printed reactive silver inks (RSI) are an innovative candidate to this end. This work furthers the research on RSI metallization by investigating the impact of ink formula on properties pertinent to SHJ cells, including electrical properties, line width, ink splatter, silver consumption, cell performance, and adhesion. We introduce a scalable, high-throughput flexible needle contact printing approach for metallization that solves many of the issues associated with drop-on-demand printing. The printed silver fingers are characterized using electrical measurements and top-down and cross-sectional microscopy. The best performing ink, consisting of silver acetate, ethylamine, and formic acid, achieved silver fingers with total resistivities of 3.1 μΩ·cm and contact resistivities of 3.2 mΩ·cm2 when printed at 61 °C. This ink metallized a full-sized 156 mm × 156 mm SHJ cell. This is the first reported data for RSI metallization of a full-sized SHJ cell and shows how an optimized RSI can achieve similar performances to LT-SP while consuming 80–90% less silver.

Enabling WiGig Communications Using Quantum-Dash Laser Source Under Smoky Weather Conditions

Wireless Gigabit (WiGig) is a recent wireless local area network that operates at the 60-GHz band and supports a transmission data rate of up to 20 Gbps. This paper demonstrates the generation of a V-band millimeter-wave (mmWave) signal using a new class of InAs/InP quantum-dash laser-based comb source operating in the L-band region. A 62.5-GHz mmWave signal is generated with electrical linewidth and phase noise characterization of 1 kHz and −65 dBc/Hz, respectively. Then, the transmission of the 6-Gbaud quadrature phase-shift keying (12 Gbps) signal is experimentally achieved over a hybrid radio-over-fiber (RoF) and radio-over-free-space (RoFSO) architecture comprising an 11.6-km single-mode fiber (SMF), 6-m FSO, and up to 2-m wireless link. Moreover, we also report this WiGig signal's transmission performance in terms of the measured bit error rate and error vector magnitude under various density smoke FSO channels, exhibiting a visibility range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 100 m for error-free transmission.

Modulation Characteristics of Period-One Oscillations in Quantum Cascade Lasers

Quantum cascade lasers subject to tilted optical feedback produce periodic oscillations, quasi-periodic oscillations, and low-frequency oscillations. This work presents the modulation characteristics of period-one (P1) oscillations in a quantum cascade laser with tilted optical feedback. The electrical signal at the oscillation frequency is more than 50 dB higher than the noise level, and the electrical linewidth is less than 2.0 kHz. This electrical linewidth is about four orders of magnitude narrower than the optical linewidth (around 16 MHz) of the free-running laser, which suggests that the optical sidebands induced by the P1 oscillations are highly coherent with the main optical mode. In addition, the modulation depth of the optical signal is found to be in the range of 1% to 3.5%. In addition, it is verified in the simulations that the P1 oscillations induce not only amplitude modulation but also frequency modulation due to the phase-amplitude coupling effect.

Widely Tunable RF Signal Generation Using an InP/Si3N4 Hybrid Integrated Dual-Wavelength Optical Heterodyne Source

Photonics-based techniques spearhead the generation of high-frequency signals in the millimeter-and Terahertz wave, crucial for ultrabroadband mobile wireless link development. Photonic integration is enabling to provide the photonic approach with added advantages of energy-efficiency, flexibility and scalability, in addition to signal quality. We present an optical heterodyne system based on a novel dual laser module containing two InP-Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> hybrid lasers with intracavity wavelength selective optical filters with output optical power per laser of up to 15 dBm (31 mW), wide wavelength tuning of about 60 nm, and narrow optical linewidth below 100 kHz. To the best of our knowledge, we present for the first time the continuous-wave generation of RF frequencies over a wide tuning range from C-band (4 GHz – 8-GHz) to W-band (75 GHz – 110 GHz) achieving record low RF electrical linewidth around 108 kHz and long-term drift < 12 MHz with two free-running lasers. This is the best beat-note linewidth obtained with such an integrated source in a free-runnning regime and with a wide tuning range ever reported.

Monolithically Integrated Microwave Frequency Synthesizer on InP Generic Foundry Platform

A photonic integrated circuit for microwave generation is proposed and experimentally validated. On the microchip, two tunable monochromatic lasers spectrally separated by 0–10.7 nm are monolithically integrated with one high-speed photodiode in heterodyne configuration for enabling continuous RF synthesis from 2 to 42 GHz. Under free-running operation, the two lasers with 20–40 MHz optical linewidth produce RF beat note with $\sim$ 90 MHz electrical linewidth at the on-chip photodiode. This is the first demonstration of such a fully integrated microwave photonic generator developed within an open-access generic foundry platform.

1 GHz InP on-chip monolithic extended cavity colliding-pulse mode-locked laser.

A record low-repetition rate from an on-chip monolithic InP extended cavity colliding-pulse mode-locked laser is experimentally reported. The device, fabricated in generic InP-based active-passive integration technology, makes use of integrated mirrors to enable its use as a building block within a photonic integrated circuit. This structure allows us to generate an electrical frequency comb with mode spacing of 1GHz, determined by the 40.5mm long resonator. Passive and hybrid mode-locking regime conditions are experimentally demonstrated. In the passive regime, an electrical beat tone at the fundamental repetition rate with an electrical linewidth (LW) of 398kHz and a signal-to-noise ratio (SNR) >30 dB is measured. In the hybrid regime, the optical comb is locked to a continuous wave signal source, improving the LW of the generated signal and the SNR>40 dB.

Square-wave oscillations in a semiconductor ring laser subject to counter-directional delayed mutual feedback.

Square-wave (SW) switching of the lasing direction in a semiconductor ring laser (SRL) is investigated using counter-directional mutual feedback. The SRL is electrically biased to a regime that supports lasing in either counter-clockwise (CCW) or clockwise (CW) direction. The CCW and CW modes are then counter-directionally coupled by optical feedback, where the CCW-to-CW and CW-to-CCW feedback are delayed by τ1 and τ2, respectively. The mutual feedback invokes SW oscillations of the CCW and CW emission intensities with a period of T≈τ1+τ2. When τ1=τ2, symmetric SWs with a duty cycle of 50% are obtained, where the switching time and electrical linewidth of the SWs can be reduced to, respectively, 1.4 ns and 1.1 kHz by strengthening the feedback. When τ1≠τ2, asymmetric SWs are obtained with a tunable duty cycle of τ1/(τ1+τ2). High-order symmetric SWs with a period of T=(τ1+τ2)/n can also be observed for some integer n. Symmetric SWs of order n=13 with a period of T=10.3 ns are observed experimentally.

Comparison of Monolithic Optical Frequency Comb Generators Based on Passively Mode-Locked Lasers for Continuous Wave mm-Wave and Sub-THz Generation

In this paper, two different Passive Mode-Locked Laser Diodes (PMLLD) structures, a Fabry-Perot cavity and a ring cavity laser are characterized and evaluated as monolithic Optical Frequency Comb Generators (OFCG) for CW sub-THz generation. An extensive characterization of the devices under study is carried out based on an automated measurement system that systematically evaluates the dynamic characteristics of the devices, focusing on the figures of merit that define the optimum performance of a pulsed laser source when considered as an OFCG. Sub-THz signals generated with both devices at 60 GHz and 90 GHz are presented and analyzed in terms of electrical linewidth to compare such components for mm-Wave and sub-THz photonic generation. This work offers a systematic comparison of PMLLD devices for OFCG operation and provides reference information of the performance of two different device topologies that can be used for the implementation of photonic integrated sub-THz CW generation.

Observation of phase noise reduction in photonically synthesized sub-THz signals using a passively mode-locked laser diode and highly selective optical filtering

A Continuous Wave (CW) sub-THz photonic synthesis setup based on a single Passively Mode-Locked Laser Diode (PMLLD) acting as a monolithic Optical Frequency Comb Generator (OFCG) and highly selective optical filtering has been implemented to evaluate the phase noise performance of the generated sub-THz signals. The analysis of the synthesized sub-THz signals up to 120 GHz gives as a result an effective reduction of the electrical linewidth when compared to direct harmonic generation that begins at 50 GHz and becomes greater as the frequency increases. The phase noise reduction offered by the setup, along with its integration potential, cost and bandwidth, make it a promising candidate to the development of an integrated and high performance low phase noise local oscillator in the sub-THz range.

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Characterization of linewidth variation by a three-step methodology is presented. Via electrical linewidth measurement, sources of linewidth variation with distinct spatial signatures are first isolated by spatial analysis. Causes with similar spatial signatures are then separated by contributor-specific measurements. Unanticipated components are lastly identified by examination of the residuals from spatial analysis. Significant sources include photomask error, flare, aberrations, development nonuniformity, and scan direction asymmetry. These components are then synthesized to quantify the contributions from the three modules of the patterning process: photomask, exposure system, and postexposure processing. Although these modules are independent of one another, their effects on linewidth variation may be correlated. Moreover, the contributions of the modules are found to vary with exposure tool, development track, and lithography strategy. The most effective means to reducing the overall linewidth variation depends on the relative importance between these components. Optical proximity correction is efficacious only for a well-controlled process where proximity effect is the predominant cause of linewidth variation.

Evaluation of sheet resistance and electrical linewidth measurement techniques for copper damascene interconnect

The effects of the barrier layer and dishing in copper interconnects lead to extra difficulties in measuring sheet resistance (R/sub S/) and linewidth when compared with equivalent measurements on nondamascene tracks. This paper examines these issues and presents the results of simulations that quantify the effects of diffusion barrier layers and dishing on the extraction of R/sub S/ from cross type test structures and the effect this has on linewidth measurement.

Exploring capabilities of electrical linewidth measurement (ELM) techniques

In this paper, we have investigated the linearity of the electrical linewidth measurement, and the relation to the linewidth values measured with a CD-SEM. The collected data has shown an almost constant bias between ELM and SEM values tested down to 50 nm. A comparison between poly- and α-Si lines with respect to different grain sizes was done. A further point of interest was the influence of the geometry of a line (e.g. straight vs. bent), as well as the dependence of the ELM result with regard to a changing linewidth along its length. An overview of an error analysis for the whole experimental procedure is presented.

Electrical linewidth test structures patterned in [100] silicon-on-insulator for use as CD standards

Electrical test structures known as cross-bridge resistors have been patterned in [100] epitaxial silicon material that was grown on Bonded and Etched-back Silicon-On-Insulator (BESOI) substrates. The critical dimensions (CDs) of a selection of their reference segments have been measured electrically, by scanning-electron microscopy (SEM), and by lattice-plane counting. The lattice-plane counting is performed on phase-contrast images of the cross sections of the reference segments that are produced by high-resolution transmission-electron microscopy (HRTEM). The reference-segment features were aligned with [110] directions in the BESOI surface material. They were defined by a silicon micromachining process that resulted in their sidewalls being nearly atomically planar and smooth and inclined at 54.737/spl deg/ to the surface [100] plane of the substrate. SEM, HRTEM, and electrical CD (ECD) linewidth measurements have been made on features of various drawn dimensions on the same substrate to investigate the feasibility of a CD traceability path that combines the low cost, robustness, and repeatability of ECD metrology and the absolute measurement of the HRTEM lattice-plane counting technique. Other novel aspects of the [100] silicon-on-insulator (SOI) implementation that are reported here are the ECD test-structure architecture and the making of lattice-plane counts from cross-sectional HRTEM imaging of the reference features. This paper describes the design details and the fabrication of the cross-bridge resistor test structure. The long-term goal is to develop a technique for the determination of the absolute dimensions of the trapezoidal cross sections of the cross-bridge resistors' reference segments, as a prelude to making them available for dimensional reference applications.

CD dispersion across the lens field: Influence on transistor characteristics

In this study, the lens plus mask contribution to the Critical Dimension ( CD) dispersion across the lens field is investigated by Electrical Linewidth Measurements ( ELM) at the gate level. It is shown that these CD dispersions are an important contributor to the transistor characteristic dispersion observed on the wafer. By using this technique, stepper qualification can be performed more precisely. We also show that electrical linewidth measurements can be used in a lens characterization procedure.

Electrical linewidth standards for advanced lithography characterization

Electrical linewidth standards for advanced lithography characterization

Extraction of sheet resistance from four-terminal sheet resistors replicated in monocrystalline films with nonplanar geometries

This paper describes limitations of conventional methods of extracting sheet resistance from four-terminal sheet resistors incorporated into electrical linewidth test structures that are patterned in [110] monocrystalline silicon-on-insulator (SOI) films. Nonplanar sections of these structures render the extraction of sheet resistance by conventional techniques subject to systematic errors. The errors are addressed here by algorithms incorporating the results of finite-element current flow analysis. The intended end application is to facilitate the use of the uniquely high repeatability and low cost of electrical critical dimension (CD) metrology as a secondary reference in a traceability path for CD-reference artifacts.

Determination of the Critical Dimension and Proximity Bias Variations Across the Lens Field by Electrical Linewidth Measurements

Electrical critical dimension (CD) measurements are now widely used as a characterization technique in optical lithography. In this paper this technique is applied to the determination of the CD and proximity bias variations across the lens field. A method, based on statistical considerations, is introduced to decorrelate the contribution of the mask to the global Across Field linewidth variation (AFLV). It is shown that the mask CD dispersion has an important contribution in the total CD range observed on the wafer. By using this technique, a lens quality determination has been done and the optical contribution to the field CD dispersion has been determined. Moreover, because it is a fast and accurate technique, we show that electrical linewidth measurements can be used in a lens characterization procedure. Results obtained indicate that the proximity bias dispersion across the lens field is not negligible and the question of a dependent field position correction is raising up.

Novel methodology for postexposure bake calibration and optimization based on electrical linewidth measurement and process metamodeling

By combining electrical linewidth measurements and neural-network (NN) process metamodeling, lithography simulators can be calibrated in an efficient way. In this work we present a novel methodology for characterizing postexposure bake using a very large experimental data set, so that the calibrated model can be used as a truly predictive tool. The adoption of a special test reticle mask allowed us to collect more than 700 000 critical dimensions CDs from 24 silicon wafers for a matrix of postexposure bake (PEB) time, and temperature conditions. The lithographic patterns included isolated, semidense and dense lines for structures of 0.25, 0.20, 0.175, and 0.15 μm nominal size replicated across the exposure field and across the wafer. As a result of this particular metrology, each measured CD was associated with both topological (position on the wafer and position within the field) and process information (exposure dose, PEB time, and temperature). Database management techniques were implemented in order to extract and analyze such a massive data set. Process metamodeling (PMM) was used for the calibration of a PEB model describing the joint effect of photoacid diffusion and photoacid loss, coupled with a deprotection reaction. PMM creates a NN model of the PEB original model (a “model of a model”) so that the diffusion coefficient, the acid loss, and the deprotection rates can be estimated by inversion of the NN mapping. The comparisons between experimental and simulated data show excellent agreement that is maintained across the entire process space.

Electrical linewidth test structures fabricated in monocrystalline films for reference-material applications

The physical widths of reference features incorporated into electrical linewidth test structures patterned in films of monocrystalline silicon have been determined from Kelvin voltage measurements. The films in which the test structures are patterned are electrically insulated from the bulk-silicon substrate by a layer of silicon dioxide provided by SIMOX (Separation by the IMplantation of OXygen) processing. The motivation is to facilitate the development of linewidth reference materials for critical-dimension (CD) metrology-instrument calibration. The selection of the (110) orientation of the starting silicon and the orientation of the structures' features relative to the crystal lattice enable a lattice-plane-selective etch to generate reference-feature properties of rectangular cross section and atomically planar sidewalls. These properties are highly desirable for CD applications in which feature widths are certified with nanometer-level uncertainty for use by a diverse range of CD instruments. End applications include the development and calibration of new generations of CD instruments directed at controlling processes for manufacturing devices having sub-quarter-micrometer features.

Optical generation of mm-waves using spectrum-sliced ASE for low-cost WDM based fibre radio systems

The authors describe the optically supported generation of a 30 GHz mm-wave signal, with an electrical linewidth of &lt; 1 Hz, from a spectrum slice of the ASE noise output of an EDFA. The technique described has the potential to significantly reduce the cost of a WDM-based fibre mm-wave radio system.