Laser-Formed Substrate Holes With Reduced Exit-Surface Microcracks
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Solution Overview
Problem
Existing methods for forming holes in substrates using laser-damage-and-etch processes result in undesirable microcracks and voids near the exit surface, leading to high surface roughness and non-uniformity, which affect the quality of metallized through-substrate vias (TSVs) due to non-uniform metal coatings and mechanical reliability issues.
Innovation Solution
Applying an exit material with a refractive index closely matching the substrate to the exit surface of the substrate to reduce Fresnel reflections, thereby minimizing microcracks and voids, and using quasi-non-diffracting laser beams to form damage tracks that are then chemically etched to create smooth, round holes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a laser-damage-and-etch process is used to form small-diameter vias, then electrical connections through substrates can be provided, but microcracks and voids are generated near the exit surface leading to high surface roughness and non-uniformity
Solution Approach 1:
An exit material is introduced as an intermediary between the laser beam and the substrate exit surface. This exit material absorbs the harmful laser energy that would otherwise create microcracks and voids, while allowing the beneficial damage track formation to occur for hole creation. The exit material acts as a mediator that separates the harmful effects from the useful effects of laser processing.
Solution Approach 2:
The invention converts the harmful laser energy that causes microcracks and voids into a beneficial effect by using it to enhance the damage track formation in the substrate while the exit material absorbs the excess energy. The harmful back-reflected laser energy is transformed into a tool for controlling the damage pattern and improving hole quality.
2Reliability
If conventional laser processing is used without exit material, then the processing is simpler, but the interior surface roughness increases and metallization quality deteriorates
Solution Approach 1:
The exit material is applied to the substrate exit surface before the laser processing step. This preliminary action prepares the surface to prevent microcrack formation during laser processing, ensuring that the damage tracks form cleanly without subsequent defects that would compromise metallization quality.
Solution Approach 2:
The invention changes the optical parameters of the processing system by introducing a material with specific refractive index properties. This parameter change modifies how laser energy interacts with the substrate, reducing back-reflections and improving the uniformity of energy distribution during damage track formation.
3Strength
If the laser beam is focused directly into the substrate without exit material, then energy efficiency is higher, but back-reflections cause microcracks that reduce mechanical integrity
Solution Approach 1:
The exit material serves as an intermediary layer that prevents direct interaction between the laser beam and the substrate exit surface. This mediator absorbs back-reflected laser energy, preventing it from traveling back through the substrate and causing microcracks that would compromise mechanical integrity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The method produces holes with low circularity and smooth interior surfaces, enhancing the quality of metallized through-substrate vias (TSVs) by reducing microcracks and voids, ensuring consistent and reliable electrical conductivity and mechanical integrity.
Implementation Method 1
Applying an exit material with a refractive index closely matching the substrate to the exit surface of the substrate to reduce Fresnel reflections
Implementation Method 2
The quasi-non-diffracting beam generates an induced absorption within the substrate that produces a damage track within the substrate
Implementation Method 3
a damage track is initially formed in the substrate by using a laser to modify the material along the damage track
Implementation Method 4
An etching solution is then applied to the substrate. The substrate is thinned by the etching solution. Because the etching rate of the material is faster at the damage track than at non-damaged regions, the damage track is preferentially etched so that a hole is opened through the substrate
Data Source
Figure 1~2
Figure 3
Figure 4A~4B
AI summary
Methods for forming holes in a substrate by reducing back reflections of a quasi-non-diffracting beam into the substrate are described herein. In some embodiments, a method of processing a substrate having a first surface and a second surface includes applying an exit material to the second surface of the substrate, wherein a difference between a refractive index of the exit material and a refractive index of the substrate is 0.4 or less, and focusing a pulsed laser beam into a quasi-non-diffracting beam directed into the substrate such that the quasi-non-diffracting beam enters the substrate through the first surface. The substrate is transparent to at least one wavelength of the pulsed laser beam. The quasi-non-diffracting beam generates an induced absorption within the substrate that produces a damage track within the substrate.