By: Hayato Kawashima, Masahiro Yamaji, Jun’ichi Suzuki, Shuhei Tanaka

We have developed a three-dimensional (3D) femtosecond laser processing method using a computer generated hologram (CGH) that generates a 3D structure inside transparent materials. Since 3D structure can be formed simultaneously with femtosecond laser pulses shaped by a CGH, our method is high efficient, and it is a high-throughput femtosecond laser processing method.

In this paper, we report microdrilling of silica glass sheets by femtosecond laser processing using a CGH. Especially we focus on through microholes for silica glass sheets. These are very useful for μTAS (Micro Total Analysis Systems), microfluidics, microsensors, and through-hole interconnections. Therefore mcrodrilling is an important technique for 3D micromachining.

Fig.1 shows our experimental setup for the microdrilling. The key factors to use our microdrilling method are threefold: (1) to design a CGH with long-focal-depth (LFD-CGH); (2) to fabricate a glass-hologram based on the phase distribution pattern of the LFD-CGH; and (3) to irradiate a silica glass sheet with femtosecond laser pulses using the glass-hologram.

Fig. 1: The experimental setup for microdrillng.

 

To drill through microholes, we design a LFD-CGH to extend optical damage in the depth direction resulted by femtosecond laser irradiation. Fig.2 shows optically reconstructed intensity distribution profile for the glass-hologram based on LFD-CGH using He-Ne laser beam with the wavelength of 632.8nm.

Fig. 2: Optically reconstructed intensity distribution profile for the glass-hologram based on LFD-CGH.

 

By using the LFD-CGH, we don’t need translation a sample and post-process such as wet etching or heat treatment on drilling a through mircohole. So the LFD-CGH is efficient for microdrilling. It is difficult to directly drill a through microhole in glass materials by laser processing using only focusing lenses.

Experimentally we fabricate through microholes approximately 1μm in diameter at the exit surface through 500μm silica glass sheet by femtosecond laser processing using the LFD-CGH. The drilled microholes are examined using a scanning electronic microscope (SEM). Fig.3 shows SEM images of the drilled through microhole. “Top View” in Fig.3 is the through microhole approximately 100μm in diameter at the entrance surface. “Bottom View” in Fig.3 is the through microhole approximately 1μm in diameter at the exit surface. From “Side View” in Fig.3, it is found that our microdrilling results in a tapered microhole.

Fig. 3: SEM images of the drilled microhole through the 500μm silica glass sheet.

 

As an experimental application of the through microholes, we fabricate a micro air-filter by drilling through microholes based on a dots square grid pattern in a silica glass sheet, and compose a micro air-shower with the micro air-filter. The designed micro air-filter has 625 microholes based on a square grid pattern with 200μm pitch in a 500μm-thick silica glass sheet. We drill the through microholes. Fig.4 shows the micro air-filter made in a silica glass. In Fig.4(b), most drilled microholes to form the square grid pattern are well fabricated uniformly.

Fig. 4: (a) Overview of the micro air-filter. (b) Zoomed view of the square grid pattern by through microholes.

 

We compose a micro air-shower with the micro air-filter in Fig.5. When the micro air-shower blows air into water, we can observe bubbles from the micro air-filter. Thus we verify the achievement of through microholes.

Fig. 5: Overview of the micro air-shower with the micro air-filter in Fig.4.