D. Lowney

Evaluation of mechanical stresses in silicon substrates due to lead-tin solder bumps via synchrotron X-ray topography and finite element modeling

Solder-based flip-chip packaging has prompted interest in integrated circuit (IC) packaging applications due to its many advantages in terms of cost, package size, electrical performance, input/output density, etc. The ball grid array (BGA) is one of the most common flip-chip packaging techniques used for microprocessor applications. However, mechanical stresses induced by the flip-chip process can impact adversely on the reliability of products. Synchrotron X-ray topography (SXRT), a non-destructive technique, has been employed to investigate the spatial extent of strain fields imposed on the underlying silicon substrate for Intel® Pentium® III microprocessors due to the lead-tin solder bump process for BGA packaging. Large area and section back-reflection SXRT images were taken before and after a simulation of the reflow process at 350 °C in atmosphere. The presence of induced strain fields in the Si substrate due to the overlying bump structures has been observed via the extinction contrast effect in these X-ray topographs. In addition, orientational contrast effects have also been found after the reflow process due to the severe stresses in the underlying silicon beneath the lead bumps. The estimated magnitudes of stress, |σ|, imposed on the underlying silicon were calculated to be of the order of 100 MPa. The spatial strains in the underlying silicon were relieved dramatically after the lead bumps were removed from the wafer, which confirms that the bumps are indeed a major source of strain in the underlying Si. Finite element modeling (FEM) has also been performed in two-dimensional (2-D) plane strain mode. The magnitudes and spatial distribution of the stresses after the reflow process are in good agreement with the SXRT results.

Investigation of strain induced effects in silicon wafers due to proximity rapid thermal processing using micro-Raman spectroscopy and synchrotron x-ray topography

Thermal stress induced by rapid thermal oxidation (RTO) and rapid thermal doping (RTD) of 001 silicon wafers was analysed using micro-Raman spectroscopy and synchrotron x-ray topography. The RTO wafers exhibited elevated stress levels as the process time was increased. The maximum magnitude and topographical distribution of the strain was found to agree with theoretical predictions. A maximum compressive strain of 320 MPa was observed after 166 s of RTO. The introduction of boron into the silicon lattice via the RTD process enhanced the rate at which the stress in the wafer exceeded the yield stress. Stress relief was subsequently accomplished through the formation of slip and misfit dislocations. The thermally induced stress and dislocation density increased with the time spent at the process peak temperature.

Examination of mechanical stresses in silicon substrates due to lead–tin solder bumps via micro-Raman spectroscopy and finite element modelling

Due to the fact that semiconductor devices have decreased significantly in geometry and increased enormously in electronic design complication, flip-chip packaging technology was launched to increase input/output count, improve electrical performance, reduce packaging size and be cost effective. The Intel®Pentium®III microprocessor uses the popular ball grid array (BGA) packaging technique. BGA is one of the most common flip-chip packaging techniques used for microprocessor applications. However, mechanical stresses induced by the flip-chip process are major concerns for the reliability of such devices. Micro-Raman spectroscopy (μRS) is a powerful technique for investigating the spatial extent of strain fields in microelectronic devices. In this study, the strain fields imposed on the underlying silicon substrate due to the lead–tin solder bump process in BGA packaging have been investigated in pre- and post-reflowed samples using μRS and finite element modelling (FEM). For pre-reflowed samples, an approximate uniaxial compressive stress of 200 MPa is developed near the edge of the under bump metallization (UBM). However, a tensile stress up to ~300 MPa is found for post-reflowed samples. Two-dimensional (2D) plane strain FEM has also been performed. The magnitudes and spatial distribution of the stresses after the reflow process are in good agreement with the micro-Raman results.

Determination of crystal misorientation in epitaxial lateral overgrowth of GaN

Epitaxial lateral overgrowth of GaN on Al2O3 using a SiO2 mask with different fill factors (ratio of stripe opening width to stripe period) is examined with synchrotron X-ray topography (SXRT) and X-ray diffraction (XRD) techniques. The crystal misorientation in the lateral overgrown region (wing) and the normal region (window region and beneath the seed layer) is determined with SXRT. The wings tilt asymmetrically around the window and the tilts increase as the fill factor increases. XRD measurements confirm the same wing tilt tendency as the fill factor changes. The average wing tilt reaches approximately 1600 arcsec measured using the X-ray rocking curve method at a fill factor of 0.625, but the maximum wing tilts can reach values as large as 2400 arcsec measured by SXRT when the fill factor is only 0.571. The significance of this is explained. The crystal misorientation in the normal region is approximately an order of magnitude less than the wing tilt. r 2002 Elsevier Science B.V. All rights reserved.

Quality Assessment of Sapphire Wafers for X-Ray Crystal Optics Using White Beam Synchrotron X-Ray Topography

The white beam Synchrotron X-Ray Topography (SXRT) technique was used to assess the quality of sapphire wafers grown by the Heat-Exchanger Method (HEM) and the Modified Czochralski Method (MCM). Sapphire is a potential new material for X-ray crystal optics, especially for use as Bragg backscattering mirrors for X-rays and Mossbauer radiation. The dislocation distribution, dislocation density and Burgers vector of selected dislocations and stacking faults in the sapphire wafers were studied. A correlation between the sapphire quality and its performance as an X-ray backscattering mirror was established in this paper. The results reveal the high quality of the inspected HEM sapphire wafers and their subsequently improved performance as Bragg backscattering mirrors.

Mapping of mechanical, thermomechanical and wire-bond strain fields in packaged Si integrated circuits using synchrotron white beam X-ray topography

Thermal processing steps used during the production of packaged integrated circuits can lead to severe thermomechanical stresses. In addition, the process of bonding wires to contact pads can also lead to strain field generation. A feasibility study using the application of white beam synchrotron x-ray topography to packaged erasable programmable read-only memory (EPROM) Si integrated circuits (ICs) has been undertaken in order to produce maps of the strain fields induced by such processing steps. This technique provides depth-resolved mapping with spatial resolutions currently in the region of 5-10 /spl mu/m throughout the entire mapping volume. Furthermore, the use of different experimental geometries allows the user to nondestructively probe the strain fields present at the wafer surface right through to the back side.

On the use of total reflection x-ray topography for the observation of misfit dislocation strain at the surface of a Si/Ge–Si heterostructure

Synchrotron X-Ray Topography has been used in Total Reflection Topography (TRT) mode to observe strain induced surface bumps due to the presence of underlying misfit dislocations in strained layer SiGe on Si epitaxial heterostructures. In these experiments the x-rays approached the sample surfaces at grazing incident angles below the total external reflection critical angles for a number of reflections and hence surface strain features nominally less than a few tens of Ångstroms from the sample surface have been observed. These are similar to the surface bumpiness observed by Atomic Force Microscopy, albeit on a much larger lateral length scale. The fact that TRT mode images were taken was confirmed by the observation of clear and conventional back reflection topographic images of misfit dislocations in all samples when the grazing incidence angle became greater than the critical angle.

Epitaxial Lateral Overgrowth of GaN on Sapphire – An Examination of Epitaxy Quality Using Synchrotron X-Ray Topography

Section transmission white beam X-ray topography was applied to the evaluation of the growth of GaN on sapphire using the epitaxial lateral overgrowth (ELO) technique. Using openings in 100 nm thick SiO 2 windows, a new GaN growth took place, which resulted in nominal overgrowth thicknesses of 6.8 μm. Measurements of the recorded Laue section topographs revealed misorientation between the epilayer and the substrate. Neglecting the misorientation contribution due to the 30° rotation of the epilayer with respect to the substrate, the misorientation mechanism was found to be a consequence of lattice rotation and dilatation. In the case of the ELO sample, these parameters varied with the stripe/window dimensions. The quality of the ELO epilayer is improved when compared to the non-ELO sample, though some local deviations from lattice coherence were observed. These results were complemented by observations made using X-ray diffraction and transmission electron microscopy.

Dynamical diffraction imaging of voids in nearly perfect silicon

X-ray diffraction topographs of extremely pure and perfect silicon single crystals are made using low-energy undulator radiation from a positron storage ring. Typical defect images observed are rather large round images having a black-white contrast and a diameter of about 40 µm. Applying the dynamical theory of x-ray diffraction, the defect contrast is explained by tensile strain in the lattice around voids close to the exit surface. This discovery of void-like microdefects explains, at least in part, the reduced density of the crystal intended to be used for a redefinition of the unit of mass, the kilogram.

Femtosecond versus nanosecond laser micro-machining of InP : a nondestructive three-dimensional analysis of strain

Ultra-fast femtosecond laser micro-machining can lead to improved surface morphology and a reduction in the heat-affected zone. In this paper, synchrotron x-ray topography (SXRT) and micro-Raman spectroscopy have been used as nondestructive tools to compare the residual strain in InP substrates after femtosecond (fs) and nanosecond (ns) laser processing. Two-dimensional stain distributions with varying probing depth and cross-section images across the four laser machined grooves were obtained. The recrystallized poly-InP layer on the laser machined groove surface has been found to be highly strained in tension, and the stress magnitude is much bigger than the shear stress introduced by crystal distortion underneath. After comparing the simulation results of SXRT orientation contrast with the topography images, the nature of the crystal plane distortion induced by both fs and ns laser machining methods was elucidated.