We have analyzed a drilling process with a femtosecond laser on a silicon surface in order to investigate the degree of the thermal effect during the dicing of a very thin silicon substrate (thickness: 50 µm). A femtosecond laser pulse (E=30–500 µJ/pulse, τ=200 fs, λ=780 nm, f=10 Hz) was focused on a thin silicon substrate using a lens with a focal length of 100 mm. An image-intensified charge-coupled device (CCD) camera with a high-speed gate of 200 ps was utilized to take images of a drilled hole during the drilling process. As a result, it was found that the smaller the pulse energy, the faster the formation of the hole. Therefore, we tried to estimate the degree of the thermal effect semi quantitatively by analyzing the rise time of the formation of the hole. By measuring the rise time in 8 kinds of metallic material, it was found that the rise time strongly correlates with the thermal conductivity in these materials. This knowledge is thought to be very important and useful for developing a dicing technique for thin silicon wafers using a femtosecond laser.
Femtosecond laser has been expected as a new tool for the industrial usage in particular to material micromachining. We are developing the dicing technique with femtosecond laser ablation for the ultra thin semiconductor substrates, which are 50 pm thick or less. In this research, we performed drilling for 50 1um thick silicon substrate with femtosecond laser (r=120 fs, 2=800 nm, F =1 kHz) as the basic experiment for dicing, focusing on the influence of a double-pulse irradiation on the processing characteristics.The double-pulse irradiation for 1 8 shots of 10 0/pulse at the pulse separation time from 10 to 20 PS showed the remarkable reduction ofthe height ofthe molten layer around the drilled hole (<0.5 pin). At the same time, however, the ablation depth was the minimum (<2.3 pin). The surface inside the hole got smooth as the pulse separation time of more than 3 ps. We supposed that the second pulse in a double-pulse should generate the another ablation on the surface and its high pressure should prevent the ablated materials by the first pulse from flying out of the hole.
We have analyzed the drilling process with femtosecond laser on the silicon surface in order to investigate a degree of thermal effect during the dicing process of the very thin silicon substrate (thickness: 50 µm). Femtosecond laser pulse (E = 30~500 µJ/pulse, τ = 150~200 fs, λ = 780nm, f = 10 Hz) was focused on the silicon substrate using a lens with a focal length of 100 mm. An image-intensified CCD camera with a high-speed gate of 5 ns was utilized to take images of drilled hole during the processing. As a result, the rise time for increasing diameter of the holes was changed at energy between 180 and 350 µJ/pulse. The width of the molten walls around the hole became wider under the conditions where the rise time became longer. So, it is said that we can estimate the degree of the thermal effect qualitatively by analyzing the rise time. These knowledge is thought to be very important and useful for developing the dicing technique of thin silicon wafers by femtosecond lasers.
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