Algorithm and Experiment of Silicon Wafer Multifocus Laser Stealth Dicing

Abstract

Objective: Chips are commonly used in scientific research as well as everyday life. In practice, separating a single chip from a large silicon wafer is often necessary. Laser ablation and blade dicing are two older means for dicing wafers, both of which have major limitations such as heat and debris contamination. Laser stealth dicing is to focus the laser inside the silicon and realize high-quality separation, which is a completely dry process with low kerf loss. A large numerical aperture focusing lens is used to generate a peak power density, and a nonlinear absorption effect results in extremely high absorption at localized points, resulting in cracks. Only the focus within the wafer can be machined without damaging the front and back surfaces. When the wafers are thin, a single scan is sufficient for dicing. However, for thick wafers, creating stacked stealth dicing layers by repeat scanning of laser beam at different depths in the wafer is necessary. The efficiency can be increased by simultaneously generating multifocus along the axis inside the wafer. This paper proposes and applies a simple multifocus algorithm with a large numerical aperture to the silicon wafer laser stealth dicing experiment. Methods: In this paper, the multifocus silicon wafer laser stealth dicing is studied. First, the multifocus phase diagram under a large numerical aperture is optimized. The phase difference between different focal lengths is used as a variable, and the original function satisfying the Fourier series requirements is solved iteratively. Each variable is nonlinearly mapped to produce the target phase diagram. Then, the phase diagram is loaded onto a spatial light modulator (SLM) made of liquid crystals on silicon. The SLM modulate the phase of the incident light and reflect it. The displacement table is adjusted to focus the laser inside the silicon wafer. The nonlinear absorption effect is then generated just in the focal point inside the wafer by optimizing the optical system parameters and adjusting the moving speed of the displacement stage, resulting in laser stealth dicing. Then, the wafer is examined under a microscope to determine the cut quality and foci position. Notably, silicon materials exhibit low absorption in the wavelength range of 1.26.5 μm. The wavelength of the laser should be selected within this range because stealth dicing needs to focus the laser at different depths inside the material. Results and Discussions: The proposed algorithm realizes the distribution of different types of axial multifocus light fields. The axial distributions of three-focus with equal energy (Fig.4), five-focus with equal energy (Fig.5), and five-focus with varying energies (Fig.6) are simulated. The multifocus stealth dicing experimental setup is schematically depicted in Fig.7. In the stealth dicing experiment, a nanosecond laser with a center wavelength of 1.342 μm is selected. By adjusting the laser power to 1.2 W, the pulse repetition rate to 40 kHz, the single pulse energy to 30 μJ, and the moving speed of the displacement table to 200 mm/s, the one-time three-focus laser stealth dicing is realized. An optical microscope is used to examine the side views of diced chips (Fig.8). The processing depth centers are located approximately 35.0, 105.2, and 176.0 μm from the upper surface of the silicon wafer, which is consistent with the initial design. The number of foci does not continue to increase because of the limitation of the energy of a single pulse. The thermal effect is broad, which is due to spherical aberration caused by the difference in refractive index between the air and the silicon. And with the increase in the machining depth, the spherical aberration also increases. Conclusions: To realize the multifocus stealth dicing of silicon wafers, an axial multifocus algorithm with a large numerical aperture is proposed. The number of on-axis foci, energy, and the interval between the foci can all be adjusted by changing the Fourier series coefficients. For the multifocus light field with equal energy, the simulation results show that the energy utilization rate is over 90% and the light intensity uniformity is over 90%. The phase diagram is loaded onto the spatial light modulator to simultaneously generate three focuses inside the silicon wafer. The 250 μm thick silicon wafer is successfully diced at a time by selecting the corresponding laser parameters and the movement speed of the displacement table. Because spherical aberration increases with machining depth, developing a multifocus laser dicing method for simultaneous aberration correction is essential in future work.