Due to assembly processes in microelectronics packaging, semiconductor materials are under undesired stresses, which become more important at extreme thermo-mechanical operating conditions. Investigations of the performances of the semiconductors is very challenging, because each assembly is a complex structure with many materials with very different physical-chemical properties. Remarkable is the determination of local stresses trough optical techniques, like Raman spectroscopy. Focus of this study is the monitoring of stress induced on silicon chips after bonding. Three bonding processes are investigated: soldering using a eutectic AuSn preform, sintering using a commercial Ag sintering paste, sintering using an in-house developed sintering paste made by surface enhanced etched brass particles dispersed in polyethylene glycol 600. In all cases, the Si-chip is bonded to a Cu substrate. The stress distribution after AuSn-soldering presents a homogeneous pattern, while after Ag- and Cu-sintering an axial distribution along the diagonal is observed. The samples are investigated at three temperatures: -50, 20, and 180 °C. The stress phenomena are higher for the assemblies made via AuSn-soldering, with stress value above 600 MPa. The results are discussed in term of compressive and tensile strain and thermal expansion coefficients.
Integrated circuits constitute a complex mosaic, where materials with different characteristics, grown or deposited in different ways and at different temperatures, are linked together in various geometries. It is well known that during and after processing of these devices, mechanical stresses develop in the layers. These stresses may be due to thermal steps, intrinsic stresses, which are inherent in the formation process of the film, or due to the geometry of the material. For example, high stresses are present in the substrate at film edges. The presence of local residual stress has an important effect on the electrical properties of electronic devices, in particular on the reliability and the lifetime of the semiconductor components.
The present work focuses on the optical investigation of the thermomechanical stress of semiconductor materials used to realize new LED modules for front lighting application. Blue LEDs, based on gallium nitride (GaN) on sapphire, are bonded to a silicon carrier using gold silicon. Afterwards the sapphire is removed. The GaN on silicon devices are soldered by eutectic AuSn soldered on copper substrates, with different thicknesses. In the solder process different AuSn solder layer are achieved by varying the bond force. Raman spectroscopy is used to investigate the influence of the assembly process and assembly material on the local stress in the semiconductor. By that the physical, mechanical and chemical properties of the interconnect material can be analyzed. A model is developed to simulate the thermomechanical stress in the GaN LED assemblies. The Raman results validate the computational model. The phenomena are evaluated at room temperature, at -50°C and at 180°C.
In the attempt to improve optical limiting of cw lasers by exploiting the thermo-optic effect exhibited by gold nanostructures, we investigated two coupled systems consisting of either gold nanoparticles (AuNPs) or gold-silica core-shell (AuNSs), both functionalized with a thiolated-fulleropyrrolidine (C60Py). We measured the optical limiting behavior under cw illumination at 514 and 647 nm, resonant with the surface plasmon resonance at around 520 of AuNPs and at 650 nm of AuNSs, respectively. Temporal response analysis shows the variation of transmitted irradiance in a 300 milliseconds time interval, corresponding to the blinking time of the human eye. Comparing the present results we those previously obtained for AuNPs1 we demonstrate an improvement of the response of functionalized nanoparticles (AuNPs- C60Py) with respect to bare AuNPs.
Semiconductor quantum dots possess attractive optical properties useful in the photonic field. Thanks to the evolution of core-shell colloidal synthesis, quantum dots exhibit almost ideal emitting properties. Their employment in optoelectronic devices is encouraged by their size-tunable emission as well as by their high absorption cross-section and high photoluminescence quantum yield. In this work, starting from the archetypal CdSe based system, the influence of shell thickness and composition on exciton dynamics in CdSe/CdxZn1-xS core-shell QDs is investigated. By means of steadystate and transient optical spectroscopy, single- and multi-exciton recombination processes are characterized and correlated with different shell materials. The present study clarifies why graded CdSe-CdS-Cd0.5Zn0.5-ZnS core-shell represents the best performing heterostructure for optical gain applications, since such quantum dots combine the high radiative rates of CdS with the superior confinement potential of ZnS in a single dot.
The goal of this work is to study the optical limiting properties of gold nanoparticles colloidal suspensions in organic solvent and to make a comparison with nanoparticles embedded in a polymeric matrix. The idea is to realize a solid state protection device based on gold nanoparticles (AuNPs) against cw lasers. This is desirable because polymers are usually cheap, flexible, and mechanically robust and temporally stable. In particular we use polycarbonate, a common polymer used to produce high quality optical lenses with superior mechanical properties. Thermally induced nonlinear optical properties of AuNPs, characterized by a surface plasmon resonance with a peak at about 520 nm where the human eye is most sensitive, have been reported for solution samples. The z-scan analysis performed on AuNPs in organic solvent confirms the presence of a nonlinear absorption coefficient comparable with literature for water colloids. We have investigated the optical limiting behavior of AuNPs doped thin film under cw illumination at 488 nm and their temporal response. A critical parameter in assessing the effectiveness of the optical limiting action is the time response of the material at varying input powers. It gives us information on the transmitted irradiance in a 300 milliseconds time interval, corresponding to the blinking time of the human eye, therefore on the total fluence reaching the retina. Finally we also report a characterization of the optical limiting action of film at 488, 514 and 647 nm.
In the last years inorganic semiconductor (particularly CdSe and CdS) quantum dots (QDs) have received great attention
for their important optical properties. The possibility to tune the emission wavelength, together with their high
fluorescence quantum efficiency and photostability, can be exploited in photonic and optoelectronic technological
applications. The design of DFB devices, based on QDs as active optical material, leads to the realization of compact
laser systems. In this work we explore the use of an inorganic/organic hybrid material composed of CdSe-ZnS
semiconductor quantum dots doped into a zirconia sol-gel matrix for optical gain applications. Through the use of soft
lithography on a sol-gel germania-silica hybrid, large scale distributed feedback gratings can be created. Used in
conjunction with the CdSe-ZnS/ZrO2 hybrids, these gratings can act as microcavities and allow for the realization of true
lasing action. The lasing properties within these devices are characterized in the femtosecond regime by both one- and
two-photon excitation. From experimental data the value of the optical gain of the core-shell quantum dot samples has
been estimated. Moreover, one- and two-photon lasing threshold and stability are reported.
Two and three dimensional structures with micron and submicron resolution have been achieved in commercial resists,
polymeric materials and sol-gel materials by several lithographic techniques. In this context, silicon-based sol-gel
materials are particularly interesting because of their versatility, chemical and thermal stability, amount of embeddable
active compounds. Compared with other micro- and nano-fabrication schemes, the Two Photon Induced Polymerization
is unique in its 3D processing capability. The photopolymerization is performed with laser beam in the near-IR region,
where samples show less absorption and less scattering, giving rise to a deeper penetration of the light. The use of
ultrashort laser pulses allows the starting of nonlinear processes like multiphoton absorption at relatively low average
power without thermally damaging the samples.
In this work we report results on the photopolymerization process in hybrid organic-inorganic films based
photopolymerizable methacrylate-containing Si-nanobuilding blocks. Films, obtained through sol-gel synthesis, are
doped with a photo-initiator allowing a radical polymerization of methacrylic groups. The photo-initiator is activated by
femtosecond laser source, at different input energies. The development of the unexposed regions is performed with a
suitable solvent and the photopolymerized structures are characterized by microscopy techniques.
In this work we report on the study of the photopolymerization process in hybrid organic-inorganic films containing photopolymerizable acrylic and methacrylic groups and. The films are doped with a proper photo-initiator for radical polymerization of (meth)acrylic units and are prepared using the sol-gel technique.
The photo-initiator is activated by using continuum (single-photon polymerization) or pulsed (two-photon polymerization) laser sources at different wavelengths. After the development of the unexposed regions with a suitable solvent, the photopolymerized structures are observed with microscopy techniques. The effects of the composition of the photopolymerizable mixture, the irradiation parameters (laser power and exposure time) and the external atmosphere in which the photopolymerization is performed are investigated.
The fabrication of 3D microstructures using multiphoton absorption processes is a promising technique that involves low amount of incident exposure dose with potentially high spatial resolution.
Organic-inorganic hybrid materials, composed of inorganic oxide structures and interpenetrated cross-linked organic
polymers, are promising candidates for electro/optical applications, combining the most important glasslike and
polymerlike properties. This is particularly true when large laser power density is used: these materials show high laser
damage resistance compared with that of polymeric systems.
A deep study of effects and causes of laser damage has never been done, especially for hybrid materials. The
mechanisms of optical damage depend on different factors like laser experimental parameters, such as pulse duration,
beam size and wavelength, or the microstructural characteristics and defects of the material.
Hybrid materials possessing desired shapes and optical and mechanical properties are well synthesized by the sol-gel
technique. The use of Glycidoxypropyltrimethoxysilane (GPTMS) allows preparing heterogeneous and resistant
materials, with good optical properties.
Different sol-gel matrices have been prepared in order to study their laser damage resistance. The possibility of varying
the catalysts and precursors or the synthesis protocol allows obtaining materials with similar chemical composition and
different microscopical properties. By this way, it is possible to study the laser damage threshold of these samples and to
find the way to enhance and optimize the laser damage resistance, useful in non-linear optical devices.
A hybrid organic-inorganic sol-gel material based on 3-(glycidoxypropyl)-trimethoxysilane (GPTMS), an organically modified alkoxide containing an epoxy ring in the organic functional group, and titanium isopropoxide Ti(OPri)4 has been synthesised. The presence of Ti alkoxide is important in order to increase the refractive index and to improve the mechanical and physical properties of the film. A basic catalysis has been used to perform the hydrolysis/condensation reactions of the inorganic network and to preserve the presence of unreacted epoxy rings. The epoxy groups are thus available in the final solid state films for the UV-photopolymerization process. The photopolymerization allows one to increase the refractive index and to pattern the film directly.
The modification of the hybrid sol-gel TiO2 matrix microstructure during the thermal treatment and the interaction with the UV light have been investigated by FT-IR and UV-Vis spectroscopies, DTA and TGA thermal analysis. The sol-gel film thickness and roughness have been analyzed by a profilometer: the films possess high optical quality and well controlled thickness, ranging from 500 nm to 2 μm. The refractive index of sol-gel thin films was measured by m-line spectroscopy and ellipsometry. An increase of the refractive index in the range 0,02÷0,1 was achieved, depending on the UV irradiation dose.
A novel scheme for implementing the joint exploitation of different, somehow complementary mechanisms of nonlinear transmission in an optical limiting device is proposed. As active materials we have chosen the fullerene derivative FULP, as a reverse saturable absorber, and a new heterocyclic quadrupolar dye, PEPEP, with highly efficient multiphoton absorption for nanosecond pulses. The nonlinear absorption properties of PEPEP in solution are extensively investigated for both femtosecond and nanosecond pulses. When Z-scan experiments are performed with nanosecond pulses, much larger effective cross sections are measured than with femtosecond pulses and with remarkably different wavelength dispersion. This is interpreted as due to two-photon absorption followed by one-photon absorption from the excited state. Chemically modified nonlinear molecules are incorporated in a hybrid organic-inorganic sol-gel matrix. Sufficiently high concentrations are achieved to allow the assembling of thin sol-gel disks into a "tandem" limiter with a total thickness smaller than the Rayleigh range of the focused laser beam. Preliminary testing of our limiter is reported and shows encouraging results. The resistance of the FULP-doped sol-gel glass to laser damage is substantially improved and the nonlinear attenuation at high pulse energies is enhanced.
We report the synthesis of sol-gel materials based on highly efficient heterocycle-based push-pull chromophores showing second- and third-order nonlinear optical activity. We show the proper functionalization of the best performing chromophores and their incorporation into a hybrid organic- inorganic sol-gel matrix. Different types of functionalization of the active molecule have been considered, including hydroxyl and alkoxysilyl end-groups. The functionalization strategy responded to different criteria such as stability and synthetic availability of the final molecular precursors, their solubility, and the used synthetic approach to the sol-gel material. The synthesis of the sol-gel materials has been tuned in order to preserve molecular properties and control important factors such as final concentration of the active dye in the matrix. Both acid- and base-catalyzed sol-gel synthesis has been taken into account. 3-Glycidoxypropyltrimethoxysilane and 3- aminopropyltriethoxysilane have been used as the organically modified alkoxides to prepare the hybrid organic-inorganic matrix. Characterization of the spectroscopic properties of the sol-gel materials is presented.
A hybrid organic-inorganic material derived 3- (glycidoxypropyl)-trimethoxysilane (GPTMS) and fullerene derivatives has been developed for optical limiting applications. The control of the final material has been achieved by engineering the matrix-fullerene solid-state system. Different Lewis acids (Zr(OBut)4, Ti(OBut)4, BF3, SiCl4, and TiCl4) have been used as the catalysts of the epoxy ring opening of (GPTMS) and the polyethylene oxide formation, in three different syntheses, to control and optimize the matrix- fullerene interaction and the optical limiting properties. The microstructure of the hybrid matrix has been studied by Fourier Transform Infra-Red Spectroscopy. UV-vis spectra and optical limiting measurements of the doped materials showed that in the Zr(OBut)4, TiCl4, BF3 derived matrices clustering of fulleropyrrolidine molecules is avoided and optical limiting is obtained. In the Ti(OBut)4, SiCl4, and not epoxy catalyzed derived matrices, instead, the aggregation of fulleropyrrolidines degraded the optical limiting properties. The laser damage threshold in the Zr(OBut)4, TiCl4 and especially BF3 derived matrices was larger than in the other matrices. Moreover, a multilayer system has been fabricated to develop an optical limiting device.
Some recent results of a research work aimed at exploiting the potentialities of fullerene derivatives to obtain sol- gel glassy materials for use as optical limiters are reported. The solubility of fullerene derivatives in organic solvents compatible with the sol-gel process has been increased by more than two orders of magnitude by suitable functionalization of C60. Covalent linking to the glassy matrix is favored by introducing silicon alkoxide groups in the fullerenes. The optical limiting properties of these derivatives in toluene solution are comparable to those of the parent compound and exhibit enhancement in the red spectral range. Optical-quality films of various thickness are obtained by incorporating the C60 derivatives at high concentration in organically modified sol-gel matrices. These materials exhibit high resistance to laser damage and can be used to build multilayer structures optimized for optical limiting.
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