Micromachining glass with a femtosecond laser

Manufacturing various types of microdevices—including microsensors, lab-on-a-chip units, and microfluid arrays—requires laser processing of glass and other transparent materials. This process, however, faces heat-affected-zone effects and a high threshold for material ablation. These problems were addressed with ultrashort, low-energy laser pulses that dramatically reduce heat-related damage, and easily ablate transparent material by breaking atomic bonds with multiphoton ionization. A numerical simulation was also developed that accurately describes the experimental results.

The experimental micromachining system described here used an ultrafast 800nm laser capable of generating 500fs, 4μJ pulses 50,000 times per second. Milling progresses by repetitiously tracing out a pattern with the laser beam, using microscope lenses and a 3D micro-positioning stage to control the focusing depth and path. Gradual vertical translation of the focusing objective can create lines, spirals, and circles in the glass as the milling area deepens.

Typically, the milling runs employed beam energies of 0.1–5μJ, feed rates of 1–100mm/min, and up to 50 pattern repeats. The Gaussian diameter of the focused beam was between 5 and 6μm, and the fluence (energy per area) was between 0.5 and 20 J/cm². On the order of a thousand laser pulses interacted with the target during 30–60ms dwell times.

The experiments used 175μm-thick glass plate. The milling pattern—starting from the center with a feed rate of 20mm/ min—consisted of a set of concentric circles with a 3μm pitch. For each sample, a different number of milling-pattern repeats was used. Every repeat of the machining pattern was focused 3μm deeper.

The beam diameter and energy at the focusing lens were 6mm and 3μJ, and the focal-spot diameter was about 6μm. These parameters correspond to a peak fluence of about 10 J/cm² and peak intensity of 2×10¹³W/cm². The damage threshold was ∼0.6μJ or a fluence of ∼2J/cm², in good agreement with published results.¹

Figure 1 illustrates the milling process at three stages of a 60-repetition pattern run. The glass surface outside the milling area shows no damage. Also, the milled area demonstrates irregular surface ablation with obvious thermal effects.

In another experiment, video was taken of the real-time dynamics of drilling a 6μm hole (see Figure 2). The ablation is not limited to the surface of the target because fragments of glass are blown away by plasma created under the surface because glass is nonhomogeneous, causing its multiphoton ionization threshold to vary from point to point. The sizes of ejected fragments range from submicrons to about 1μm. In the vicinity of the material surface, the ionization stimulates avalanche (collisional) processes. The resulting plasma plume either takes away material or causes a local breakdown, followed by the ejection of a piece of material. Additionally, internal multiphoton ionization probably causes local photochemical reactions that modify the refractive index.

A numerical model was developed to predict and understand etch rates, heat flow, and general light-matter phenomena during ultrafast laser-pulse interactions with a target. The model features include beam focusing, linear and multiphoton absorption, heat diffusion, material phase transitions, multiphoton ionization, and plasma thermal energy transfer into adjacent material.²

The heat diffusion at position and time t is defined by numerical integration of the partial-differential equation for heat energy density with a heat source :

With this model, the hole drilling corresponding to the experiments shown in Figure 1 and 2 was simulated. The simulated material removal sequence shown in Figure 3 accurately reproduces the observations, showing that the physics have been well represented in the simulation. Figure 4 reveals that temperature drops along the laser path.

This work shows the benefits of using a high-repetition rate femtosecond laser to micromachine glass, such as a small heat-affected zone confined to less than 1μm. Moreover, no cracks were observed. With the numerical model, the time-dependent process of laser milling and monitor temperature dynamics can be simulated.