LowK wafer dicing robustness considerations and laser grooving process selection

The low-k dielectric materials in wafer fabrication are widely used. The current advanced mechanical dicing is not able to meet the requirement of low-k technology, since the low-k materials are harder and brittle. Laser dicing is developed for low-k wafer dicing. There are various laser dicing techniques in the market. Depending on the wafer thickness, laser dicing can either be a though cut or a partial cut process, which is commonly known as laser grooving. In this paper, the study work mainly focus on laser grooving and mechanical dicing process for Low-k wafer. The laser grooving used in this study is a 3-pass laser beam, and the laser grooving process setup is discussed. High power optical microscope, SEM, FIB and IR were the inspection tools used in this study. After laser grooving, the characterize of laser grooving, including the die edge quality, grooving geometry, die strength, mechanical dicing and reliability results are all investigated. The die was packaged into a 19×19mm 529IO MAPBGA package. Post assembly, CSAM and electrical test were performed on the assembled parts at T0, post MSL3/260degree C, post 264h UHST (110°C/85%RH), and post TC500cycles (-55°C to 125°C). With optimized parameter, a good dicing quality was get, without metal burr, metal remaining, micro cracks on side wall etc. The packaged sample had passed all stress. No laser grooving and mechanical dicing defect related fail. It is concluded that the 3-pass laser grooving is a good solution for low-k wafer dicing.

This paper presents an investigation on the effect and optimization of machining parameters for 90 nm low-k wafer topside peeling improvement in mechanical dicing operation. It is part of the continuous improvement that performed based on current established dicing recipe. The resulted outcomes to achieve are cut quality improvement, dicing yield loss reduction and device reliability enhancement for low-k wafer dicing. The experimental studies were conducted under varying table speed, Z1 spindle rotation as well as the Z1 cut depth was examined. The settings of machining parameters were determined by using design of experiment (DOE) techniques and the critical process parameters were determined and analyzed statistically by using analysis of variance (ANOVA). Optical visual inspection was conducted on post-processed low-k test wafers and several of scribe structures which comprised of different level of metal density, for a through quantification and categorization on the peeling mode. Worst case peeling measurements and characterizations were conducted by using optical microscope, scanning electron microscopy (SEM) and focused ion beam (FIB). Electrical test and device reliability assessments were conducted to reflect and confirm the improvement of the samples diced with optimized mechanical dicing process. As a result, the optimum dicing parameters to apply in production environment was realized and established in order to overcome the quality obstacles and yield loss issue in low-k wafer dicing.

A semiconductor package has a silicon die on which an integrated circuit (IC) is fabricated. The die is singulated from a single wafer using processes like mechanical sawing or laser grooving. These processes have impact on the final condition of the silicon die after wafer singulation. This paper discusses a study on the effect of the die condition on the breaking load of a package with a dam and fills structure. The encapsulation material of this type of package has lower modulus when compared with the epoxy mold compound material used in most molded packages. The package breaking load was determined using 3-point bend test for two sets of packages. The first set of packages was assembled with silicon die produced using mechanical sawing. The second set was assembled with die produced using laser grooving. Results of the 3-point bend test showed that the breaking load of the package with die from mechanical sawing is higher compared with the package assembled with die from laser grooving. The study revealed that that the silicon die condition has significant effect on the robustness of the final package where the die is used.

This paper describes the development of a robust process to improve chipping and peeling defect on low-k/heavy metalised saw street wafer during die singulation process using laser grooving technique. After laser groove process, the sidewall tends to have voiding and become brittle, due to recast material formed with the complex material such as IMD, TEG, silicon nitrate and etc. The sidewall area will be easily fractured and lead to top surface peeling, chipping and sidewall crack. For low-k material, even a minor chipping will lead to low-k interlayer delamination and subsequently cause the unit fail at application. Hardware such as optical lens with 0 degree, 90 degree and circular polarization were used for optimization, design of experiment (DOE) were performing on the laser groove and sawing process/parameter to improve the singulation quality.

Dicing a thick, 6 metal layer low-k Cu metallization wafer (from 90nm node wafer technology) is very challenging compared to the 4 metal layer stacked wafer. Poor topside cutting responses with excessive saw chip-outs were observed with the 6 metal layer. To resolve the saw chipping quality issue, a series of dicing assessments were performed, which includes: (1) saw machine baseline calibration and verification, (2) analysis study on the blade's elements (diamond grit, diamond concentration, bond type) (3) new saw blade selection and evaluation, and (4) saw process parameter optimization and validation. This paper is focused on discussing the fundamentals of understanding each of the blade elements and its interaction on improving the topside chipping and peeling quality. Experimental studies were conducted by using various blade types and by varying the blade element composition. This includes variations in diamond grits sizes, diamond concentration (higher vs. lower diamond concentration), and bond type (softer vs. harder bond). A thorough process characterization was conducted to validate the cutting performance on the post-processed wafers. All results and data collected from the experimental studies were statistically analyzed and interpreted. In conclusion, a new saw blade with the appropriate selected blade attributes were introduced and qualified. With the optimized blade and saw parameters for the thick low-k Cu metallization wafers, topside chipping and peeling quality was significantly improved.

This paper describes the development work on single and multi beam laser grooving technology for 40nm node low-k/ULK semiconductor device. A Nd:YAG ultraviolet (UV) laser diode operating at a wavelength of 355 nm was used in this study. The effects of single and multi beam laser micromachining parameters, i.e. laser power, laser frequency, feed speed, and defocus amount were investigated. The laser processed die samples were thoroughly inspected and characterized. This includes the die edge and die sidewall grooving quality, the grooving shape/profile and the laser grooving depth analysis. Die strength is important and critical. Die damage from thermal and ablation caused by the laser around the die peripheral weakens the mechanical strength within the die, causing a reduction in die strength. The strength of a laser grooved die was improved by optimizing the laser process parameter. High power optical microscopy, Scanning Electron Microscopy (SEM), and focused ion beam (FIB) were the inspection tools/methods used in this study. Package reliability and stressing were carried out to confirm the robustness of the multi beam laser grooving process parameter and condition in a mass production environment. The dicing defects caused by the laser were validated by failure analysis. The advantages and limitations of conventional single beam compared to multi beam laser grooving process were also discussed. It was concluded that, multi beam laser grooving is possibly one of the best solutions to consider for dicing quality and throughput improvements for low-k/ULK wafer dicing. The multi beam laser process is a feasible, efficient, and cost effective process compared to the conventional single beam laser ablation process.

This paper describes the development work of enabling a multi beam laser grooving technology for 40nm node low-k/ULK semiconductor device. A Nd:YAG ultraviolet (UV) laser diode operating at a wavelength of 355 nm was used in the study. The effects of multi beam laser micromachining parameters, i.e. laser power, laser frequency, feed speed, and defocus amount were investigated. The laser processed die samples were thoroughly inspected and characterized, which included the die edge and die sidewall grooving quality, the grooving shape/profile and the laser grooving depth examination. Die strength is important and critical. Die damage from thermal and ablation caused by the laser around the die peripheral weakens the mechanical strength within the die, causing a reduction in die strength. The strength of a laser grooved die was improved by optimizing the laser process parameter. High power optical microscopy, scanning electron microscopy (SEM), and focused ion beam (FIB) are the inspection tools/methods used in this study. Package reliability and stressing were carried out to confirm the robustness of the multi beam laser grooving process parameter and condition in a mass production environment. The dicing defects caused by the laser were validated by using failure analysis. The advantages and limitations of conventional single beam compared to multi beam laser grooving process were also discussed. It is shown that, multi beam laser grooving is possibly one of the best solutions to choose for dicing quality and throughput improvements for low-k/ULK wafer dicing. The multi beam laser process is a feasible, efficient, and cost effective process compared to the conventional single beam laser ablation process.

The purpose of this study is to present three-dimensional (3D) simulations using the finite-element (FE) method of thermo-mechanical stresses developed in the laser cutting process. The CO2 laser beam is assumed to be in Gaussian mode and the calculation for prediction of material removal rate and stress values in the target was done on St.37 steel. The analysis allows the investigation of groove depth and stress field for a variety of laser power, cutting speed, and pulse frequency. The laser that is used for this study is 0.15 mm in beam radius with a 10.6 µm wavelength at an intensity range of 200–1000 W and repetition rate of 5000–10 000 Hz. The pulse duration is 10 μs and cutting speed varies from 1 to 2 m/min. Thermal properties of the material are assumed to be temperature dependent and the birth and death technique of elements was employed for simulation of material removal. Therefore, during the cutting process, if the temperature of any element is greater than the melting point, then the element is deactivated and does not participate in the calculation. Applicability of the thermal model under consideration was verified by comparing the result of simulation with experimental data in the relevant literature. The results show the validity of the simulation and applicability of the developed FE model for modelling of the laser cutting process.

An overview and emerging challenges in mechanical dicing of silicon (Si) wafer are discussed from view point of narrow kerf, thin wafer, chip strength enhancement, Copper (Cu)/low-k wafers and dicing challenges for emerging technologies.

Ultrashort pulse lasers offer the possibility to process materials with minimal heat input and high precision. However, the X-ray emission at high laser peak intensities represents an undesirable side effect. A laser system (1030 nm wavelength, 40 W maximum average laser power, 925 fs pulse duration, 100 μJ maximum pulse energy, 400 kHz pulse repetition frequency) was used to analyze the X-ray skin dose rates for applied peak intensities between 9.2 × 1012 W/cm2 and 1.3 × 1014 W/cm2 for mild steel, stainless steel and tungsten during groove processing. The lowest laser peak intensity at which X-ray emission could be measured was 9.2 × 1012 W/cm2 for processing of stainless steel. The X-ray emission showed a characteristic evolution in dependence on the number of overscans at the same line positions. For low peak intensities, a measurable X-ray emission only begins after a few passes. The number of scans to reach the maximum dose rate increases with lower laser peak intensity. Analysis of the evolving surface topography by white light interference microscopy in connection with ray-tracing simulations led to the interpretation that the increase in X-ray emission is due to the local intensity redistribution by multiple reflection within the laser-generated grooves which is promoted by a particular choice of the linear laser polarization.

One of the challenges in using very thin die, which is needed in thin package, is how to dice it cleanly without chipping, delamination of fragile low-k insulation material and contamination of bonding pads. A narrow scribe line is also highly preferable for high wafer area utilization. The authors developed a novel "grooving and stealth laser process" to satisfy all of these criteria. Grooves are formed on test pads in die sawing area by a grooving laser beam, and then a stealth laser beam is focused into the bulk silicon, causing defect regions in the bulk silicon. Dicing tape is expanded so that the wafer breaks into separated dice.

Novel UV-USP Laser Grooving and Plasma Dicing Separation Schemes for Next Generation Advanced Packaging