Local Laser Annealing Research Articles

Patterning Magnonic Structures via Laser Induced Crystallization of Yittrium Iron Garnet

AbstractThe fabrication and integration of high‐quality structures of Yttrium Iron Garnet (YIG) is critical for magnonics. Films with excellent properties are obtained only on single crystal Gadolinium Gallium Garnet (GGG) substrates using high‐temperature processes. The subsequent realization of magnonic structures via lithography and etching is not straightforward as it requires a tight control of the edge roughness, to avoid magnon scattering, and planarization in case of multilayer devices. In this work a different approach is described based on local laser annealing of amorphous YIG films, avoiding the need for subjecting the entire sample to high thermal budgets and for physical etching. Starting from amorphous and paramagnetic YIG films grown by pulsed laser deposition at room temperature on GGG, a 405 nm laser is used for patterning arbitrary shaped ferrimagnetic structures by local crystallization. In thick films (160 nm) the laser induced surface corrugation prevents the propagation of spin‐wave modes in patterned conduits. For thinner films (80 nm) coherent propagation is observed in 1.2 µm wide conduits displaying an attenuation length of 5 µm that is compatible with a damping coefficient of ≈5 × 10−3. Possible routes to achieve damping coefficients compatible with state‐of‐the art epitaxial YIG films are discussed.

Dynamic Tuning of Moiré Superlattice Morphology by Laser Modification.

In artificial van der Waals (vdW) layered devices, twisting the stacking angle has emerged as an effective strategy to regulate the electronic phases and optical properties of these systems. Along with the twist registry, the lattice reconstruction arising from vdW interlayer interaction has also inspired significant research interests. The control of twist angles is significantly important because the moiré periodicity determines the electron propagation length on the lattice and the interlayer electron-electron interactions. However, the moiré periodicity is hard to be modified after the device has been fabricated. In this work, we have demonstrated that the moiré periodicity can be precisely modulated with a localized laser annealing technique. This is achieved with regulating the interlayer lattice mismatch by the mismatched lattice constant, which originates from the variable density of sulfur vacancy generated during laser modification. The existence of sulfur vacancy is further verified by excitonic emission energy and lifetime in photoluminescence measurements. Furthermore, we also discover that the mismatched lattice constant has the equivalent contribution as the twist angle for determining the lattice mismatch. Theoretical modeling elaborates the moiré-wavelength-dependent energy variations at the interface and mimics the evolution of moiré morphology.

Atomic Layer MoTe2 Field-Effect Transistors and Monolithic Logic Circuits Configured by Scanning Laser Annealing.

Atomically thin semiconductors such as transition metal dichalcogenides have recently enabled diverse devices in the emerging two-dimensional (2D) electronics. While scalable 2D electronics demand monolithic integrated circuits consisting of complementary p-type and n-type transistors, conventional p-type and n-type doping in desired regions, monolithically in the same semiconducting atomic layers, remains elusive or impractical. Here, we report on an agile, high-precision scanning laser annealing approach to realizing 2D monolithic complementary logic circuits on atomically thin MoTe2, by reliably designating p-type and n-type transport polarity in the constituent transistors via localized laser annealing and modification of their Schottky contacts. Pristine p-type field-effect transistors (FETs) transform into n-type ones upon controlled laser annealing on their source/drain gold electrodes, exhibiting a mobility of 96.5 cm2 V-1 s-1 (the highest known to date) and an On/Off ratio of 106. Elucidation and validation of such an on-demand configuration of polarity in MoTe2 FETs further enable the construction and demonstration of essential logic circuits, including both inverter and NOR gates. This dopant-free, spatially precise scanning laser annealing approach to configuring monolithic complementary logic integrated circuits may enable programmable functions in 2D semiconductors, exhibiting potential for additively manufactured, scalable 2D electronics.

Silicon erasable waveguides and directional couplers by germanium ion implantation for configurable photonic circuits.

A novel technique for realization of configurable/one-time programmable (OTP) silicon photonic circuits is presented. Once the proposed photonic circuit is programmed, its signal routing is retained without the need for additional power consumption. This technology can potentially enable a multi-purpose design of photonic chips for a range of different applications and performance requirements, as it can be programmed for each specific application after chip fabrication. Therefore, the production costs per chip can be reduced because of the increase in production volume, and rapid prototyping of new photonic circuits is enabled. Essential building blocks for the configurable circuits in the form of erasable directional couplers (DCs) were designed and fabricated, using ion implanted waveguides. We demonstrate permanent switching of optical signals between the drop port and through the port of the DCs using a localized post-fabrication laser annealing process. Proof-of-principle demonstrators in the form of generic 1×4 and 2×2 programmable switching circuits were fabricated and subsequently programmed.

Trimming of silicon-on-insulator ring-resonators via localized laser annealing.

We propose a post-fabrication trimming method for the silicon-on-insulator photonic platform based on localised laser annealing of hydrogen silsesquioxane (HSQ) cladding. The technique is fast, does not degrade the device performance, does not require additional fabrication steps, and can therefore be implemented at minimal cost. Here we experimentally demonstrated how the spectrum of a ring resonator can be shifted by over 1 nm by annealing a section of the device as short as 30 µm, corresponding to a change in the effective refractive index of ∼10-2. Modifications of both the HSQ refractive index and its chemical structure as a function of the annealing temperature are also discussed. Trimming of multi-ring resonators indicate that this technique can be effectively used for post-fabrication reconfiguration of complex photonic circuits or to compensate for the fabrication tolerances of a typical CMOS process.

Local laser annealing of 3C-SiC film deposited on the silicon substrate by CVD

In this work, we try to suite an approach, which concerns of epitaxial graphene growth by laser irradiation of 3C-SiC (111) film deposited on silicon substrate (111) by chemical vapor deposition method. Laser treatment was performed by pulsed 1064 nm laser with 20 Hz repetition rate and 15 ns pulse duration, the fluency was varied 0-1.5 J/cm2 . Raman spectroscopy studies show that for fluence above 0.8 J/cm2 2D band is noticeable revealing formation of high crystallographic quality graphitic (graphene) film.

Towards fast nanopattern fabrication by local laser annealing of block copolymer (BCP) films

The nanopatterning of surfaces and thin films with pattern dimensions of less than 100 nm is challenging for laser processing in particular in the case of large-area, low-cost fabrication. Self-assembly processes, however, provide a mechanism of pattern generation in this dimensional range offering an alternative fabrication method. The current work focuses on high-temperature, short-time laser annealing of PS-b-PMMA block copolymer (PS-b-PMMA BCP: poly(styrene-block-methyl methacrylate)) films on fused silica samples to achieve self-assembly into vertical lamellas with periods of approximately 50 nm. BCP samples were irradiated with a focussed CO2-laser beam for studying the influence of the laser power PL and the scanning speed vs on the lamellae formation in BCP films. The formation of lamellae is observed in the centre of the laser track at sufficient laser irradiation (PL: 2 to 15 W, vs: 1 to 250 mms−1). With increasing laser irradiation, first the quality of the lamellas improves to a certain point but thereafter a partial degradation of the BCP and dewetting of the BCP film occurs. The partial degradation of the PMMA micro-phase of the ordered BCP results in a local self-developing process reducing the processing steps for nanopattern formation. The experimental results on laser-induced local self-assembly with irradiation times below 0.1 s are discussed in relation to laser-induced temperature field simulations. Combining self-assembly capabilities of BCP with local heating by laser beams can provide a tool for direct writing of hierarchical nano-/microscale patterns that is useful for various applications mimicking bio-inspired structures.

Ion Implantation in Silicon for Trimming the Operating Wavelength of Ring Resonators

In recent years, we have presented results on the development of erasable gratings in silicon to facilitate wafer scale testing of photonics circuits via ion implantation of germanium. Similar technology can be employed to control the operating wavelength of ring resonators, which is very sensitive to fabrication imperfections. Ion implantation into silicon causes radiation damage resulting in a refractive index increase, and can therefore, form the basis of multiple optical devices. In this paper, we discuss design, modeling, and fabrication of ring resonators and their subsequent trimming using ion implantation of germanium into silicon, followed by either rapid thermal annealing or localized laser annealing. The results confirm the ability to permanently tune the position of the resonant wavelength to any point inside the free spectral range of the ring resonator, thus, greatly reducing the amount of power required for active tuning of these devices.

Selective Fabrication of SiC/Si Heterojunction Diodes by Local Laser Annealing Process for Microwave Circuit Applications

This paper presents the RF characterization of SiC/Si heterojunction diodes fabricated using a novel localized laser process. Selective growth of SiC on Si is carried out by utilizing excimer laser-based annealing in the presence of carbon source. Use of ambient conditions (atmospheric pressure and room temperature) during growth makes this process simple and low cost. The Si wafer is first coated with carbon rich film and then locally annealed using high power laser which causes dissociation of solid carbon sources and a simultaneous melting of the silicon leading to the growth of SiC. The Raman spectrum shows peaks for acoustical and optical phonon modes for ${\beta }$ -SiC between 940 and 980 cm $^{-1}$ . The fabricated diodes show high breakdown voltage (>200 V) and low leakage current density of 3- $5~\mu \text{A}$ /cm2 (−5 V). Experimental results for the dc characteristics, microwave rectification and frequency multiplication are presented. The measured results show good RF rectification (e.g., 21 V/W at 3 GHz) up to 6 GHz using a large diode ( $70\times 70\mu \text{m}^{2}$ ). The diodes also work well as frequency doublers over a wide frequency range of 2–6 GHz. The diode area can be scaled down to design circuits operating at higher frequencies.

Microstructure and Property Modifications of Cold Rolled IF Steel by Local Laser Annealing

Laser annealing experiments are performed on cold rolled IF steel whereby highly localized microstructure and property modification are achieved. The microstructure is seen to develop by strongly heterogeneous recrystallization to provide steep gradients, across the submillimeter scale, of grain size and crystallographic texture. Hardness mapping by microindentation is used to reveal the corresponding gradients in macroscopic properties. A 2D level set model of the microstructure development is established as a tool to further optimize the method and to investigate, for example, the development of grain size variations due to the strong and transient thermal gradient. Particular focus is given to the evolution of the beneficial γ-fiber texture during laser annealing. The simulations indicate that the influence of selective growth based on anisotropic grain boundary properties only has a minor effect on texture evolution compared to heterogeneous stored energy, temperature variations, and nucleation conditions. It is also shown that although the α-fiber has an initial frequency advantage, the higher probability of γ-nucleation, in combination with a higher stored energy driving force in this fiber, promotes a stronger presence of the γ-fiber as also observed in experiments.

Low Temperature Geometrically Confined Growth of Pseudo Single Crystalline GeSn on Amorphous Layers for Advanced Optoelectronics

Highly textured Ge0.91Sn0.09 is obtained on both amorphous SiO2/Si and glass substrates at low temperatures <475 oC, which shows grain sizes up to tens of microns. Strikingly, the nucleation center spacing ranges from 0.1 to 1 mm, orders of magnitude larger than common solid state crystallization. This observation indicates an exceedingly high grain growth rate vs. a low nucleation rate. Therefore, we can control nucleation sites and fabricate geometrically confined pseudo single crystalline GeSn grain using patterning techniques, including surface Sn dots/patches, local laser annealing, and nanotaper patterns. Another remarkable result is that ~9 at.% Sn is incorporated substitutionally into Ge, far exceeding the equilibrium solubility limit of ~1 at.%. The high Sn composition, together with ~0.24% thermally induced tensile strain in the film, shifts the GeSn direct band gap to ~0.5 eV (2500nm) and converts it into a direct band gap semiconductor with significantly enhanced optoelectronic properties.

Selective and localized laser annealing effect for high-performance flexible multilayer MoS2 thin-film transistors

We report the use of ultra-short, pulsed-laser annealed Ti/Au contacts to enhance the performance of multilayer MoS2 field effect transistors (FETs) on flexible plastic substrates without thermal damage. An analysis of the temperature distribution, based on finite difference methods, enabled understanding of the compatibility of our picosecond laser annealing for flexible poly(ethylene naphthalate) (PEN) substrates with low thermal budget (< 200 °C). The reduced contact resistance after laser annealing provided a significant improvement in transistor performance including higher peak field-effect mobility (from 24.84 to 44.84 cm2·V−1·s−1), increased output resistance (0.42 MΩ at V gs − V th = 20 V, a three-fold increase), a six-fold increase in the self-gain, and decreased sub-threshold swing. Transmission electron microscopy analysis and current-voltage measurements suggested that the reduced contact resistance resulted from the decrease of Schottky barrier width at the MoS2-metal junction. These results demonstrate that selective contact laser annealing is an attractive technology for fabricating low-resistivity metal-semiconductor junctions, providing important implications for the application of high-performance two-dimensional semiconductor FETs in flexible electronics.

Transparent Conducting Nb-Doped TiO2 Electrodes Activated by Laser Annealing for Inexpensive Flexible Organic Solar Cells

A KrF excimer laser (λ= 248 nm) has been adopted for annealing cost-effective Nb-doped TiO2 (NTO) films. Sputtered NTO layers were annealed on SiO2-coated flexible poly(ethylene terephthalate) (PET) substrates. This local laser annealing technique is very useful for the formation of anatase NTO electrodes used in flexible organic solar cells (OSCs). An amorphous NTO film with a high resistivity and a low transparency was transformed significantly into a conductive and transparent anatase NTO electrode by laser irradiation. The 210 nm anatase NTO film shows a sheet resistance of 50 Ω and an average optical transmittance of 83.5% in the wavelength range from 450 to 600 nm after annealing at 0.25 J/cm2. The activation of Nb dopants and the formation of the anatase phase contribute to the high conductivity of the laser-annealed NTO electrode. Nb activation causes an increase in the optical band gap due to the Burstein–Moss effect. The electrical properties are in agreement with the material characteristics determined by X-ray diffraction (XRD) analysis and secondary ion mass spectrometry (SIMS). The irradiation energy for the NTO electrode also affects the performance of the organic solar cell. The laser annealing technique provides good properties of the anatase NTO film used as a transparent electrode for flexible organic solar cells (OSCs) without damage to the PET substrate or layer delamination from the substrate.

Laser annealing of thin film electroluminescent devices deposited at a high rate using high target utilization sputtering

This paper presents the photoluminescent (PL) and electroluminescent (EL) characteristics of ZnS:Mn deposited at room temperature using high target utilization sputtering (HiTUS). Significant improvements in PL intensity are seen when ZnS:Mn is deposited using HiTUS instead of conventional RF magnetron sputtering. When incorporated as part of a complete EL device with yttrium oxide forming the dielectric layers and indium tin oxide used as the top contact electrode, localized laser annealing of the ZnS:Mn phosphor layer is shown to provide enhancement of the EL characteristics.

Measuring stresses in thin metal films by means of Raman microscopy using silicon as a strain gage material

AbstractMechanical stresses in microelectronics and micro‐electromechanical systems may influence the reliability of applications and devices. The origin of the stresses can be because of the joining of dissimilar materials with regard to the thermal expansion coefficient, electromigration or the deposition process utilized. Stresses can lead to delamination, crack formation and stress migration and therefore to a failure of the device. Identifying the locations of highest stresses in a device is crucial for reliability improvement. Currently, both Laue X‐ray micro diffraction and convergent‐beam electron diffraction are able to locally determine the stresses in thin metal films. Here, we propose a modified method of indirect Raman microspectroscopy to measure stresses with a lateral resolution in the submicrometer range at a laboratory scale. The method encompasses the crystallization of an amorphous silicon layer by local laser annealing and its subsequent usage as a strain gage. Stresses in an aluminum thin film were determined as a function of temperature. In addition to the average stress, the stress distribution could be monitored. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Residual stress in Si nanocrystals embedded in a SiO2 matrix

Multiple quantum wells consisting of alternating Si and SiO2 layers were studied by means of Raman scattering. The structures were fabricated by the remote plasma enhanced chemical vapor deposition of amorphous Si and SiO2 layers on quartz substrate. The structures were subjected to a rapid thermal annealing procedure for Si crystallization. The obtained results suggest that the Si layers consist of nanocrystals embedded in an amorphous Si phase. It was found that the silicon nanocrystals inside 2nm thin layers are under high residual compressive stress. Moreover, the metastable Si III phase was detected in these samples supporting the presence of large compressive stresses in the structures. The compressive stress could be relaxed upon local laser annealing.

Partial magnetization reversal using laser annealing in patterned NiFe/FeMn film

We have studied local magnetization reversal by laser annealing in exchange biased NiFe/FeMn bilayer. Local magnetization reversal was performed by using the Nd:YAG laser under external magnetic field. When the laser illuminated the patterned film with the power of above 300 mW during 15 min, a magnetoresistance (MR) curve with symmetric peaks at the opposite field was obtained due to the local reversal of exchange biasing. A similar result was observed in NiFe/FeMn/NiFe trilayer. As the exposed area expanded, the intensity of opposite MR peak increased. The direction of exchange anisotropy in the partially reversed region can be restored by local laser annealing under alternating magnetic field, even if its MR peak was reduced by the damage and interdiffusion. The magnetic new domain structures of the partially reversed region was generated by laser annealing near the exposed area.

FeMn/NiFe에서 Laser 열처리에 의한 자구연구

교환결합된 NiFe/FeMn 이중박막에서 Laser 열처리를 통하여 국소적인 자화반전을 연구하였다. 600 G의 외부 자기장을 시편에 가하면서 DPSS Laser로 자화 반전시켰으며, 300 ㎽에서 15분 동안 국소적인 부분을 조사하였다. 국소적으로 자화반전 시킨 박막을 다시 원래 방향으로 역 반전시켜 자기저항(MR)곡선 피크가 감소된 것을 관찰하였다. 이는 직접적으로 Laser를 가한 박막의 손상과 계면에서 내부 확산에 의한 것으로 생각된다. 국소적인 반전 자화의 자구구조는 MFM을 통하여 관찰하였다. 자화반전 영역에서 새로운 자구벽을 형성하였으며, Laser가 조사된 영역근처에서 자구벽이 형성된 것을 관찰하였다.

Raman study of HgBa 2Ca n−1 Cu nO 2 n+2+ δ ( n=1,2,3,4 and 5) superconductors

Polarized micro-Raman scattering measurements have been performed on the five members of the HgBa 2Ca n−1 Cu n O 2 n+2+ δ ( n=1,2,3,4 and 5) high- T c superconductor family using different laser frequencies. Local laser annealing measurements were carried out to investigate the variation of the Raman spectra with the excess oxygen content, δ. A systematic evolution of the spectra, which display mainly peaks near 590, 570, 540 and 470 cm −1, with increasing number of CuO 2 layers has been observed; its origin has been shown to lie in the variation of the interstitial oxygen content. In addition to confirming that the 590 cm −1 mode represents vibration of apical oxygens in the absence of neighboring excess oxygen, the 570 cm −1 mode, which may be composed of some finer structures, has been assigned to the vibration of the apical oxygen modified by the presence of the neighboring excess oxygens. The 540 and 470 cm −1 modes may represent the direct vibration of excess oxygens. The implication of possible different distribution sites of excess oxygens is discussed. All other observed lower-frequency modes are also assigned.

Raman spectra of Hg-based superconductors: Effect of oxygen defects

Polarized micro-Raman scattering measurements have been performed on the five members of the $\mathrm{Hg}{\mathrm{Ba}}_{2}{\mathrm{Ca}}_{n\ensuremath{-}1}{\mathrm{Cu}}_{n}{\mathrm{O}}_{2n+2+\ensuremath{\delta}}$ ($n=1, 2, 3, 4, \mathrm{and} 5$) high-${T}_{c}$ superconductor family. A systematic evolution of the spectrum, which mainly involves oxygen-related phonons around 590, 570, 540, and 470 ${\mathrm{cm}}^{\ensuremath{-}1}$, with an increasing number of Cu${\mathrm{O}}_{2}$ layers, has been observed. Local laser annealing measurements clearly demonstrate that all these phonons are closely related to interstitial oxygen in the $\mathrm{Hg}{\mathrm{O}}_{\ensuremath{\delta}}$ planes. The origin of the spectrum evolution with the number of Cu${\mathrm{O}}_{2}$ layers lies in the variation of interstitial oxygen content.