Paper Review 

4. femtosecond laser dicing of ULTRATHIN si wafers with cu backside layer - a fracture strength and microstructural study

Michael Raj Marks a,b, Kuan Yew Cheong b, Zainuriah Hassan c 

a Infineon Technologies (Kulim) Sdn. Bhd., Kulim Hi-Tech Park, 09000 Kulim, Kedah, Malaysia 

b Electronic Materials Research Group, School of Materials & Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia 

c Institute of Nano Optoelectronics Research and Technology (INOR), Universiti Sains Malaysia, 11800 Penang, Malaysia 

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 conducted 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 concerning the femtosecond laser processing parameters are made to improve the die sidewall frontside and backside fracture strengths, compared to previous results from nanosecond and picosecond laser dicing. 

3. A high-repetition-rate femtosecond laser for thin silicon wafer dicing 

Krishnan Venkatakrishnan1, Nitin Sudani1 and Bo Tan2 

1 Department of Mechanical and Industrial Engineering, Ryerson University, 350 Victoria Street, Toronto, M5B 2K3, Canada 

2 Department of Aerospace Engineering, Ryerson University, 350 Victoria Street, Toronto, M5B 2K3, Canada 

In this study, a high-power–high-repetition-rate femtosecond laser was investigated for singulation of silicon wafers. The femtosecond laser used for this investigation, unlike the previously used amplified system, is a compact unit that emits infrared ultrashort pulses at high repetition rates in the MHz range and an average output power of 11 W. A systematic study of the influence of the laser parameters on the kerf width, depth, and quality of machining was carried out. Some different experiments were performed using a silicon wafer of diameter 50 mm, P-type boron doped, and back grinded to a 250 µm thickness wafer with an orientation of 100. The experimental results show that the high-power–high-repetition-rate femtosecond laser can be a promising and competitive tool for thin wafer dicing. It is also the first time that the high-repetition-rate femtosecond laser has been demonstrated for real-world industrial applications for micromachining. The experiment demonstrated a cutting speed of 40 mm s−1 with acceptable quality of sidewalls, depth of cut, and kerf width, which can be considered when applying for industrial usage.

2. A HYBRID METHOD OF ULTRAFAST LASER DICING AND HIGH-DENSITY PLASMA ETCHING WITH WATER-SOLUBLE MASK FOR THIN SILICON WAFER CUTTING 

Shih-jeh Wu 

Department of Mechanical and Automation Engineering, I-Shou University, Kaoshiung City 840, Taiwan, ROC 

Future silicon wafers are getting thinner in the semiconductor industry to satisfy the requirement of miniaturizing package size in mobile device applications. This paper proposes a novel method incorporating the convenience of laser ablation and speed of plasma etching to replace traditional mechanical dicing saws for thin wafer cutting. The main idea is to use a laser to create patterns on the silicon wafer coated with water-soluble protection material. The unprotected area is exposed to the high-density plasma which is later etched through. This study includes the development of the water-soluble protecting mask material, the design of inductively coupled plasma (ICP) and microwave plasma sources, pattern scribing utilizing ultrafast laser, and the trial of an etching (Bosch) process associated with the high-density plasma chamber to create high aspect ratio trench. The polyvinyl alcohol (PVA) based protecting material has excellent solubility and is extremely easy to rinse with water. With appropriately added chemicals, optical absorption is improved for laser ablation. A 532 nm 10 pico-second laser is used to perform surface scribing with minimal recast and heat-affected zone (HAZ) at the edges. Good plasma homogeneity is demonstrated in the designed 12″ microwave and ICP chambers. With the protection coating the silicon wafer can be etched at a 7.2 µm/min rate and 10.11 etch selectivity by the microwave and ICP sources in the chamber. Finally, etching process trials are performed, and the results show that deep and high aspect ratio trenches (~15 µm wide, ~95 µm deep) can be achieved which is fully acceptable for wafers under 100 µm thickness. 

1. Investigation of through micropatterns fabrication on C194 copper foil by ultraviolet nanosecond pulsed laser micro drilling

While the lead frames of C194 copper alloy widely used in IC packages contain a high density of through micropatterns, the method for realizing high precision manufacturing of the micropatterns on copper foil with thickness down to a few hundred micrometers is highly demanded. This paper demonstrates the feasibility of manufacturing high-density through micropatterns on C194 copper foil with high precision and high uniformity using ultraviolet nanosecond pulsed laser micro-drilling. The absorption coefficient of C194 copper foil for 355 nm wavelength is experimentally derived, based on which the sublimation enthalpy of the material is calibrated by iteratively comparing predicted crater morphology by finite element simulations with experimental data. Then the multi-pulse laser ablation mechanisms of C194 copper foil are investigated jointly by finite element simulations and experiments, which derive the quantitative correlation of ablation crater depth with laser ablation parameters. Finally, high-density arrays of through micropatterns with high dimensional accuracy and high uniformity are fabricated on C194 copper foil with a thickness of 100 μm by the proposed ultraviolet nanosecond pulsed laser micro-drilling with theoretically predicted repeat scanning number. This work provides a feasible method for manufacturing copper alloy lead frames with high precision through micropatterns in an environmentally friendly manner.