Laser Drilling Process Stages and Interaction Phenomena

The Five Stages of Laser Drilling

Stage 1: Surface Heating

The laser beam acts as a small circular heat source on the surface with a heat flux distribution largely determined by the beam intensity distribution. The rate of heating depends on the absorptivity of the metal surface, which is typically low for IR lasers. Pulse durations in laser drilling (ranging from milliseconds to nanoseconds) are shorter than the thermal response time of the material; therefore, a "steady-state" is never achieved, and heating is limited to a thin surface layer.

Stage 2: Surface Melting

If the intensity and time are sufficient, a thin layer on the surface becomes molten, but timescales are too short for significant thermal conduction into the material.

Stage 3: Vaporization

Given sufficient beam intensity, the molten surface begins to vaporize. The presence of vapor enhances absorption just above the surface, thus accelerating the vaporization process. The vaporization process causes the liquid surface to become mobile and "rough" due to:

• Surface waves
• Bubble cavities breaking the surface
• Liquid droplets being ejected

Light trapping by surface-breaking bubbles gives rise to multiple reflections and thus enhanced absorption.

Stage 4: Vapor Ejection

Mass removal now takes place due to the surface liquid boiling away.

Stage 5: Liquid Ejection

Accompanying vapor ejection, there is a reaction force on the liquid surface that pushes the liquid layer sideways out of the beam path.

Interaction Phenomena in Thermal Laser Drilling (TLD)

The saturation in TLD is considered to be due to plasma effects. Key characteristics include:

• Plasma within the "keyhole" redistributes the laser energy.
• Plasma radiation is absorbed at the walls.
• Multiple reflections inside the hole also redistribute the energy, leading to a higher overall intensity at the top of the hole.
• These factors result in an enlargement of the hole diameter.

Plasma Coupling and Oxidation Effects

This theory is supported by measurements, which show that the ratio of hole diameter to beam diameter increases with increasing laser intensity due to "plasma coupling" (independent of beam diameters and optical parameters realized by various optical systems). Furthermore, oxidation effects resulting from the use of oxygen assist gas result in wider holes compared to using argon.