Over the last decade, lasers have been gradually employed for Si wafer dicing to replace blade dicing. Laser dicing has the potential to replace blade dicing as the future generation ultrathin wafer ...singulation method as it enables higher cutting speed, lower damage, and smaller kerf width but various technical challenges still remain to be resolved. In this article, laser ablation and dicing of Si wafers are reviewed in terms of the physics of laser-material interaction based on nanosecond, picosecond, and femtosecond pulse durations. The effects of various laser settings, dicing process parameters, and material factors on ablation rate, ablation precision and quality, and die fracture strength are discussed in detail. With the increasing usage of Cu stabilization layer on the backside of ultrathin Si wafers, we also review laser-material interaction in Cu and elaborate on recent findings on the effects of laser dicing through Si and Cu simultaneously on the microstructural and fracture strength properties of the die. Various approaches to improve the ablation rate, ablation quality, and die fracture strength are discussed.
•Laser dicing of thin Si wafers is gaining more importance in the semiconductor industry because of its cost-effectiveness compared to conventional blade dicing.•Understanding laser ablation mechanisms remains a huge challenge because of the complexity of the processes taking place, the variety of species involved, and the range of length and time scales covered.•Many challenges remain for laser dicing of Si wafers primarily in three critical areas, i.e., ablation rate, ablation precision and quality, and die fracture strength.•Optimization of the ablation rate, ablation quality, and die fracture strength will require thorough consideration of all the influencing laser and process parameters in order to meet the requirements of the end product applications.
Ultrathin silicon wafer technology is reviewed in terms of the semiconductor applications, critical challenges, and wafer pre-assembly and assembly process technologies and their underlying ...mechanisms. Mechanical backgrinding has been the standard process for wafer thinning in the semiconductor industry owing to its low cost and productivity. As the thickness requirement of wafers is reduced to below 100 μm, many challenges are being faced due to wafer/die bow, mechanical strength, wafer handling, total thickness variation (TTV), dicing, and packaging assembly. Various ultrathin wafer processing and assembly technologies have been developed to address these challenges. These include wafer carrier systems to handle ultrathin wafers; backgrinding subsurface damage and surface roughness reduction, and post-grinding treatment to increase wafer/die strength; improved wafer carrier flatness and backgrinding auto-TTV control to improve TTV; wafer dicing technologies to reduce die sidewall damage to increase die strength; and assembly methods for die pick-up, die transfer, die attachment, and wire bonding. Where applicable, current process issues and limitations, and future work needed are highlighted.
Ultrathin silicon die is a key enabler for high performance semiconductor devices and ultrathin packaging. The quality of ultrathin wafers and dies has a significant influence on packaging assembly ...yield and device reliability. The key quality characteristics of ultrathin wafers and dies are bow/warpage, total thickness variation (TTV), subsurface damage (SSD), surface roughness, and mechanical strength. Wafer and die bow/warpage cause handling and processing problems in manufacturing processes, and induce defects during various packaging assembly processes that eventually lead to device reliability issues. The wafer TTV requirement is becoming more stringent for new generations of thin and 3-D packages. SSD, surface roughness, and dicing defects have adverse effects on die mechanical strength and reliability. Therefore, characterization methods are needed for these quality characteristics to control the manufacturing processes for ultrathin wafers and dies to ensure good device performance and reliability. The following ultrathin wafer and die characterization techniques are discussed in this paper: noncontact bow/warp/TTV measurement, materialographic analysis with optical and electron microscopy, high-resolution X-ray diffraction, micro-Raman spectroscopy, scanning infrared depolarization, optical profilometry, atomic force microscopy, and uniaxial/biaxial bending tests.
Mechanical and plasma dicing of Si wafers with a backside metal layer poses serious challenges in quality and manufacturing cost. Mechanical dicing through the metal layer causes blade clogging and ...damage, which results in severe Si die chipping and cracks. The cost of plasma dicing is high as additional photolithography steps are required for etching the metal layer, in addition to the extremely slow metal etch rate. Laser dicing is promising and is currently used to singulate thin Si wafers. In this paper, picosecond laser dicing of 20 μm Si dies with 0–30 μm backside Cu was found to be feasible. Utilizing an improved three-point bending test method, the frontside and backside characteristic fracture strengths of ultrathin Si dies with varying backside Cu thickness were measured. The die types with backside Cu show an average of 17% higher frontside characteristic fracture strength, and an average of 13% higher backside characteristic fracture strength, compared to the die type without backside Cu. The die sidewall microstructures, defects, and compositions have been characterized in detail by transmission electron microscopy, and their effect on mechanical strength is discussed.
Previous studies on nanosecond and picosecond laser dicing have shown that the Si die sidewall microstructures and fracture strengths are significantly different when dicing simultaneously through Si ...and a Cu backside layer, in contrast to dicing solely through Si. This work systematically investigated the effect of femtosecond laser dicing on the fracture strength and sidewall microstructure of 20 μm Si dies with 0−30 μm Cu backside layer. Using an improved three-point bending (3PB) test method, the intrinsic fracture strengths of the die sidewall were measured. The die types with Cu backside show an average of 46.5 % higher backside characteristic fracture strength, but an average of 6.6 % lower frontside characteristic fracture strength, compared to the die type without Cu backside. Fractographic analysis by scanning electron microscopy was carried out to determine the fracture initiation behaviour in the 3PB test samples. The microstructures, phases, and defects at the sidewall were characterized by transmission electron microscopy, and their effect on fracture strength is discussed. Based on the observed microstructural features, some suggestions in relation to the femtosecond laser processing parameters are made to improve the die sidewall frontside and backside fracture strengths, in comparison to previous results from nanosecond and picosecond laser dicing.
Laser dicing of ultrathin dies is promising and is gaining importance because of its cost and quality advantages over mechanical and plasma dicing. However, the effects of laser dicing on the ...mechanical strength and microstructure of ultrathin Si dies need to be further understood, especially when dicing through Si wafers with backside Cu layer. A critical phenomenon effecting the Si die sidewall strength after nanosecond laser dicing of Si wafers with backside Cu is the formation and separation of a SiO
2
layer at the sidewall. The mechanisms behind the SiO
2
layer formation and separation were studied in this work. Si wafer samples without and with backside Cu layer were prepared by dicing with nanosecond laser using standard production parameters. The microstructure and phases formed were investigated by energy dispersive spectroscopy and nanobeam diffraction in a transmission electron microscope. In die samples without backside Cu, the sidewall consists of a thin surface layer of amorphous Si, followed by a polycrystalline Si layer, and finally an epitaxial Si layer. In die samples with backside Cu, the sidewall microstructure was observed to be vastly different. At the upper region of the sidewall, a surface layer of polycrystalline Cu was found, followed by a polycrystalline Cu
3
Si layer, a SiO
2
layer mixed with Cu
3
Si, and finally a thick SiO
2
layer. The Cu
3
Si catalyzes the growth of the SiO
2
through an oxidation step of the Cu
3
Si on the sidewall surface as well as at the SiO
2
/Si interface. In the lower region of the sidewall, the microstructure is similar to the upper region, but there is a separation of the SiO
2
layer from the crystalline Si. The SiO
2
undergoes a decomposition reaction at the SiO
2
/Si interface, releasing volatile SiO which causes microvoids to form and grow laterally at the interface. The growth and coalescence of the microvoids eventually lead to the separation of the SiO
2
layer from the crystalline Si, leaving behind a clean and rough crystalline Si surface with a peak-to-peak roughness of 100–200 nm. In the areas where the SiO
2
layer has separated from the Si die sidewall, the fracture strength of the sidewall is dependent on the material property and surface roughness of the crystalline Si, and not on the SiO
2
layer. In the sidewall region near the die frontside, the SiO
2
thickness is more than regions near the die backside, and no microvoiding and separation at the SiO
2
/Si interface were detected. This is hypothesized to be due to a higher O
2
pressure at the upper region of the narrow dicing trench which is open to the atmosphere compared to the lower regions where there could be O
2
deprivation and lower O
2
pressure.
The sidewall fracture strength of ultrathin Si dies with frontside and backside metallization layers cannot be determined directly from three-point bending (3PB) test. The complex 3PB elastoplastic ...stress distribution makes it difficult to determine the die sidewall fracture strength by analytical method or finite element analysis due to various theoretical and practical constraints. In this work, a selective chemical etching method has been developed for complete removal of the frontside and backside metallization layers on the ultrathin die without affecting the critical microstructure and strength of the die sidewall induced by laser dicing. By removing all the metallization layers on the die, the complex 3PB elastoplastic stress distribution is simplified to an elastic stress distribution. This enables a straightforward calculation of the die sidewall fracture strength using the standard 3PB tensile stress equation for a homogeneous linear elastic material with a rectangular cross section. Two chemical etchants were evaluated and found to be suitable but FeCl3 is preferred because of the much faster etching time. With this improved 3PB test method, the effect of nanosecond laser dicing on ultrathin Si dies with backside Cu layer was investigated. The die frontside and backside characteristic fracture strengths of ultrathin Si dies with varying backside Cu thickness were determined and correlated to TEM analyses of the compositions and microstructures at the fracture origins.
•A selective chemical etching method has been developed for removal of metallization layers on ultrathin Si dies for three-point bending test.•The complex elastoplastic stress distribution in the die is simplified to an elastic stress distribution.•The die sidewall fracture strength can be determined using the standard three-point bending tensile stress equation.•The characteristic fracture strengths of ultrathin Si dies were correlated to the laser-induced microstructures of the die sidewall.
Ultrathin dies require a Cu stabilization layer, which is essentially a backside Cu layer, to prevent warpage and cracks during solder die attach and wire bonding. The dicing of Si wafers with a ...backside Cu layer is challenging. Mechanical blade dicing through the Cu layer causes blade clogging and damage, which eventually results in severe die chipping and cracks. Plasma dicing is costly as it requires additional photolithography and etching steps. Laser dicing is promising and is currently used to singulate thin Si wafers. However, there is no reported work on its application for dicing ultrathin wafers with a backside Cu layer. In this paper, nanosecond laser dicing of 20 μm Si dies with 0-30 μm backside Cu was found to be feasible. The effect of nanosecond laser dicing on the die sidewall strength was evaluated with the three-point bend (3PB) test. Analytical and experimental results have shown that the Cu and AlCu layers have gone into plastic condition during the 3PB test. Comparison of the 3PB fracture loads indicates that the Si backside strength is higher than the Si frontside strength. Fractographic analysis has confirmed that the fracture initiation sites during the 3PB tests are at the die sidewall. The die sidewall defect morphologies, structures, and elemental compositions have been characterized in detail by transmission electron microscopy, and their effect on mechanical strength is discussed.
A study was performed to examine the effect of burn-in and Au-plating thickness on the shear strength of 63Sn-37Pb solder joints in ball grid array (BGA) packages. The Au-plating thicknesses of 0.3 ...mu m, 0.6 mu m, 0.9 mu m, and 1.4 mu m were evaluated. An isothermal aging temperature of 150 degree C was employed to simulate burn-in conditions. The evolution of the bulk solder microstructure and intermetallic compounds at the solder joint interface were characterized and correlated to the measured shear strength. The strength of the solder joints with 0.3- mu m Au plating was approximately three times higher than the thicker platings after aging. Solder joints with 0.3- mu m Au plating failed within the solder matrix, and their strength was dependent upon the bulk solder microstructure and composition. The weakness of the solder joints with thicker Au platings was attributed to the formation of a brittle AuSn sub(4) /Ni sub(3)Sn sub(4) interface and a ductile Pb-rich layer at the interface.
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