Die Sidewall Research Articles

Femtosecond laser dicing of ultrathin Si wafers with Cu backside layer - A fracture strength and microstructural study

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.

An improved three-point bending test method for the investigation of nanosecond laser dicing of ultrathin Si dies with Cu stabilization layer

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.

Effect of Nanosecond Laser Dicing on the Mechanical Strength and Fracture Mechanism of Ultrathin Si Dies With Cu Stabilization Layer

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 $20~\mu \text{m}$ Si dies with 0– $30~\mu \text{m}$ 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.

Die singulation technologies for advanced packaging: A critical review

Die singulation, also known as wafer dicing, is reviewed in terms of the brief history, critical challenges, characterization of singulation quality, different singulation technologies and underlying mechanisms, and post-singulation die strength enhancement. Mechanical blade dicing has been the workhorse of die separation in the semiconductor manufacturing process. It faces growing challenges due to the adoption of copper/low-k dielectric interconnect structures, thin and ultra-thin wafers, die attach films, narrow dicing streets, and complex stacked structures on the dicing streets. Key dicing quality characteristics are chipping, delamination, kerf geometry, die side wall damage, die surface contamination, and die strength degradation. Various die singulation technologies have been developed to address these challenges and quality issues, including dicing by thinning, laser based approaches, laser and mechanical hybrid method, and plasma dicing. Die strength is a critical parameter for thin and ultra-thin dies. Post-dicing die strength enhancement is becoming the complement of most dicing technologies to achieve dies with high fracture strength. Plasma dicing has the potential to achieve much higher die strengths than all the other dicing approaches.

Laser singulation of thin wafer: Die strength and surface roughness analysis of 80 μm silicon dice

In this paper, we have studied the characteristics of silicon dice, singulated using a high-power-high-repetition-rate femtosecond laser. The die strength and surface roughness, of the die side walls, are evaluated for different laser parameters such as pulsewidth and repetition rate. Since, the 80-μm-thick wafers used in this study were polished on both sides, die-edge roughness plays a decisive factor in determining the die strength when compared to backside roughness and wafer thickness as is the case in other studies. Excellent side wall average surface roughness of 0.35 μm is obtained at pulsewidth of 214 fs and repetition rate of 4.33 MHz using an average laser power of 15.5 W. Die strength is measured via the 3-point bending test. Strength reduction, due to die side wall surface defects that are induced through the wafer dicing process, is evaluated through die strength and surface roughness analysis. Die strength of a silicon dice is characterized as the first step in prediction and prevention of die failure during the package assembly, reliability test and working life. Improvement in the die side wall surface roughness is observed with the usage of nitrogen gas assist as compared to that obtained in air.