An article to understand 3D packaging through glass via (TGV) processing technology
May 22, 2025
"More than Moore" leverages 3D stacking to enable heterogeneous integration of multiple chips through in-plane and vertical interconnections, employing system-level integration strategies to significantly enhance form factor efficiency. Vertical interconnect technology extends dimensional scaling along the z-axis, driving continuous advancements in system-level integration. Through-interposer via technology, implemented via interposer-based via-first approaches, stands as one of the most promising 3D interconnection solutions and has become a global research focus in advanced packaging.
Historically, glass substrates faced challenges in achieving hole quality (e.g., via geometry, surface roughness) that met the reliability requirements of designers and end-users, posing a critical bottleneck for glass-through-via (TGV) adoption in advanced packaging. For foundries, this technology still requires substantial progress in:
- Uniformity control for high-aspect-ratio (AR > 50:1) vias
- Optimization of glass-metal interface adhesion
- Mitigation of thermal-mechanical stress during fabrication
To achieve high-density, high-precision glass structuring, extensive research has been conducted on advanced methods, including:
- Mechanical micromachining: Enables micron-scale via patterning
- Glass reflow: Maskless patterning via surface tension-driven reshaping
- Focused discharge: Plasma etching for enhanced resolution
- UV-curable photoresist glass: Selective etching through photolithography
- Laser ablation: Non-contact drilling with sub-micron precision
- Laser-induced processes: Selective metallization and surface modification
Systematic Classification and Analysis of Micromachining Technologies:
- Mechanical Micromachining
Mechanical micromachining represents the most conventional and direct fabrication method, employing micro-cutting tools or abrasive agents to remove exposed material regions from workpieces. It is widely recognized that brittle materials exhibit ductile flow rather than brittle fracture when the cutting depth remains significantly below the critical threshold . Inspired by this deformation mechanism, various ductile-dominated micromachining techniques have been developed, including micro-turning, milling, drilling, and micro-grinding, along with their hybrid combinations. These methods enable the production of precision glass components with minimized surface/subsurface damage.
Abrasive Jet Machining (AJM)
As a cost-effective AJM variant, abrasive jet machining employs high-velocity abrasive-laden jets (50-100 m/s) to erode hard materials through impact mechanisms. The process utilizes micro-abrasives (5-50 μm) entrained in gas/water jets, offering advantages such as:
- Reduced contact forces (<10 N)
- Minimal thermal distortion (<50°C)
- Compatibility with Si, glass, Al₂O₃, and composites
Key Process Parameters:
| Parameter | Critical Range | Impact on TGV Quality |
|---|---|---|
| Jet Angle | 60°-80° | Symmetry of via geometry |
| Standoff Distance | 2-10 mm | Erosion efficiency |
| Abrasive Loading | 20-40 wt.% | Hole consistency |
| Nozzle Diameter | 50-200 μm | Lateral resolution limit |
Mask-Based AJM Implementation
To achieve sub-10 μm resolution, researchers adopted a two-stage AJM process:
- SU-8 Photoresist Masking: Patterned via UV lithography (365 nm exposure)
- Al₂O₃ Abrasive Jet Etching:
- Process parameters: 0.5 MPa pressure, 45° incidence angle
- Achieved TGV diameter: 600 μm (±5% uniformity)
- Substrate: 500 μm thick Pyrex 7740 glass
Performance Limitations (Fig. X):
- Diameter Variability: ±8% deviation due to jet deflection effects
- Surface Roughness: Ra > 100 nm at via entrances
- Edge Rollover: 20-30 μm lateral overcut at intersections
As illustrated in the following figures, mechanical micromachining exhibits inferior TGV consistency compared to laser-based methods. The observed dimensional fluctuations (σ > 15 μm) and profile irregularities may degrade signal integrity through:
- Increased parasitic capacitance (>15%)
- Capacitance-voltage (C-V) hysteresis
- Electromigration susceptibility
This analysis aligns with SEMATECH's findings on through-glass via reliability in 3D packaging applications.
Ultrasonic vibration enhances machining efficiency by enabling arrayed tip tools to interact with abrasive particles under high-frequency oscillation. High-energy abrasive grains (e.g., 1 μm SiC) impact the glass substrate, accelerating via formation while achieving higher aspect ratios (depth-to-diameter).
Case Study (Fig. X):
- Tool Design: Custom stainless-steel tool with 6×6 square arrayed tips
- Process Parameters:
- Abrasive: 1 μm SiC particles
- Substrate: 1.1 mm thick glass
- Output: 260 μm × 270 μm tapered square via
- Aspect Ratio: 5:1 (average depth/diameter)
- Etch Rate: 6 μm/s
- Throughput: ~4 minutes per via
Limitations and Optimization:
While multi-tip tooling increases array density (e.g., 10×10 arrays), practical efficiency gains remain constrained by:
- Collision Dynamics: Tip overlap causes interference during ultrasonic vibration
- Abrasive Utilization: Particle shedding reduces effective cutting lifetime
- Thermal Management: Cumulative friction heat at high frequencies (>20 kHz)
This approach achieves ~300 vias/hour with 85% dimensional consistency (σ < 5 μm), outperforming conventional AJM by 4× in speed but limited by tool complexity. For high-throughput applications, hybrid systems combining ultrasonic agitation with laser-assisted focusing are being investigated to mitigate these bottlenecks.