Through-glass via substrates are increasingly used in glass interposers, wafer-level packaging, MEMS packaging, RF modules, optical devices and heterogeneous integration. By creating electrically conductive vertical connections through an insulating glass substrate, TGV technology can support high-density interconnection while maintaining low electrical loss, dimensional stability and compatibility with wafer- or panel-level processing.
However, a purchase specification that only states “TGV glass substrate with copper-filled vias” is not sufficient.
Two substrates with the same external dimensions may differ significantly in glass composition, via taper, sidewall quality, copper filling, warpage, electrical resistance and reliability. These differences can affect redistribution-layer processing, chip bonding, soldering, assembly yield and long-term package performance.
A complete TGV procurement specification should define five major areas:
- Glass material and substrate dimensions
- Via diameter, pitch and geometry
- Metallization and copper filling
- Flatness, bow and warpage
- Inspection and acceptance criteria
This guide explains the information semiconductor buyers, packaging engineers and research teams should include in a TGV glass substrate RFQ.
What Is a TGV Glass Substrate?
A through-glass via substrate is a glass wafer or panel containing vertical holes that pass through the complete substrate thickness. The via sidewalls are normally metallized, after which the holes may be fully filled with copper or coated with a conductive copper layer.
Additional structures may include:
- Redistribution layers
- Copper pads
- Under-bump metallization
- Solder bumps
- Cavities
- Microfluidic channels
- Optical windows
- Segni di allineamento
- Thin-film resistors
- Ground structures
Glass is attractive for advanced packaging because its coefficient of thermal expansion can be adjusted through composition, while its electrical insulation, optical transparency and dimensional stability support RF, photonic and high-density packaging applications. Research has also shown that borosilicate glass properties such as thermal expansion and dielectric behavior can be modified through composition rather than treated as fixed values for all glass grades.
Why TGV Procurement Specifications Are Difficult
A TGV substrate combines several manufacturing technologies:
- Precision glass production
- Via formation
- Sidewall cleaning
- Adhesion and seed-layer deposition
- Copper electroplating
- Surface copper removal
- Redistribution-layer fabrication
- Trattamento termico
- Dimensional and electrical inspection
A defect introduced during one step may not become visible until a later process.
Ad esempio:
- A rough or cracked via sidewall can weaken the glass.
- An incomplete seed layer can cause a copper filling void.
- Excess copper stress can increase substrate warpage.
- Incorrect pitch can prevent alignment with the die or package.
- Copper protrusion can interfere with bonding.
- Poor adhesion can cause delamination during thermal cycling.
The buyer should therefore specify the entire functional structure rather than only the substrate diameter and via diameter.
1. Specify the Glass Material
The term “glass” covers many different substrate materials. Common options include:
- Borosilicate glass
- Silice fusa
- Quartz glass
- Vetro alluminosilicato
- Alkali-free glass
- Photosensitive glass
- Custom packaging glass
The correct material depends on electrical, thermal, mechanical and manufacturing requirements.
Information to include
The RFQ should state:
- Glass family
- Preferred manufacturer or equivalent grade
- Chemical composition restrictions
- Coefficient of thermal expansion
- Dielectric constant
- Dielectric loss tangent
- Glass transition or annealing characteristics
- Thermal conductivity
- Volume resistivity
- Alkali content
- Optical transmission, when required
- Condizioni della superficie
- Maximum allowable inclusions or bubbles
Published work on quartz and borosilicate substrates shows that different glasses can provide substantially different dielectric and thermal-expansion behavior. Buyers should therefore request an identified glass grade or a defined property range rather than accepting an unspecified “TGV glass.”
Match the CTE to the package structure
The glass coefficient of thermal expansion should be considered together with:
- Silicon dies
- Copper vias
- Redistribution-layer metals
- Solder joints
- Organic substrates
- Ceramic packages
- Adhesives and underfills
A low-CTE glass is not automatically the best choice. The required value depends on the total package stack and the temperature range.
The buyer should specify whether the main objective is to match:
- Silicio
- Copper
- A package substrate
- A ceramic component
- An optical assembly
- Another bonded material
2. Define Substrate Size and Thickness
TGV substrates can be supplied as wafers, rectangular panels or singulated interposers.
Tra le dimensioni importanti figurano:
| Articolo | Information to Specify |
|---|---|
| Substrate format | Wafer, panel or individual interposer |
| Diameter or length and width | Nominal value and tolerance |
| Glass thickness | Nominal value and tolerance |
| TTV | Maximum total thickness variation |
| Profilo del bordo | Bevel, chamfer, rounded or custom |
| Orientation feature | Notch, flat, fiducial or none |
| Edge exclusion | Usable-area limitation |
| Finitura superficiale | Polished, lapped or as-formed |
| Rugosità superficiale | Measurement method and limit |
| Bow or warp | Maximum value and measurement condition |
Substrate thickness directly affects the required via aspect ratio. A smaller-diameter hole through thicker glass is generally more difficult to form, metallize and fill uniformly.
Published TGV demonstrations include a wide range of structures, such as 100µm vias through 300µm glass and finer-pitch structures intended for high-density packaging. These examples demonstrate process flexibility, but they should not be interpreted as universal production tolerances.
3. Specify Via Diameter Correctly
A TGV may not have one constant diameter through the complete glass thickness.
Depending on the formation process, the hole may be:
- Cylindrical
- Tapered
- Hourglass-shaped
- Double-sided tapered
- Slightly elliptical
- Irregular near the entrance or exit
Therefore, the specification should not simply state “via diameter: 50µm.”
Recommended dimensional definitions
Specify:
- Top-opening diameter
- Bottom-opening diameter
- Midpoint diameter, when necessary
- Minimum internal diameter
- Maximum internal diameter
- Via taper angle
- Via depth
- Formato
- Rotondità
- Via-center position tolerance
- Diameter measurement method
For a tapered via, both top and bottom diameters should be included on the drawing.
Via aspect ratio
The aspect ratio is generally related to substrate thickness divided by the relevant via diameter. However, the buyer and supplier should agree on whether the calculation uses:
- Top diameter
- Bottom diameter
- Minimum diameter
- Average diameter
High-aspect-ratio vias create greater challenges for:
- Sidewall cleaning
- Seed-layer continuity
- Electrolyte transport
- Additive distribution
- Gas removal
- Copper filling
- Internal inspection
The maximum practical aspect ratio depends on via shape, glass type, hole-formation method and metallization process.
4. Specify Via Pitch and Spacing
Via pitch is the center-to-center distance between adjacent vias. It should not be confused with the minimum glass spacing between their edges.
The drawing should define:
- X-direction pitch
- Y-direction pitch
- Staggered or aligned pattern
- Minimum edge-to-edge spacing
- Via-to-substrate-edge distance
- Via-to-cavity distance
- Via-to-routing-line distance
- Local via density
- Global via distribution
Why pitch affects reliability
Reducing pitch increases interconnect density, but it also reduces the amount of glass between adjacent holes.
Potential risks include:
- Cracking between vias
- Reduced local mechanical strength
- Stress interaction between copper-filled vias
- More difficult seed-layer deposition
- Increased copper volume
- Greater local thermomechanical stress
- Higher sensitivity to via-position error
Research into TGV interfacial reliability indicates that via diameter, pitch, aspect ratio, geometry and material mismatch can all influence delamination risk. In the studied structures, larger via diameter and greater thermal mismatch increased the driving force for interfacial delamination.
The supplier should review the complete via array rather than approve one isolated via dimension.
5. Define Via Position Tolerance
Accurate via placement is essential when TGVs must align with:
- Redistribution-layer pads
- Chip bumps
- Solder balls
- Antenna structures
- Optical channels
- MEMS cavities
- Wafer-bonding marks
- External connectors
The RFQ should specify:
- Datum system
- Segni di allineamento
- Global pattern-position tolerance
- Individual via-position tolerance
- Rotation tolerance
- Scaling tolerance across the substrate
- Inspection sampling plan
For large substrates or panels, dimensional changes caused by temperature and processing should also be considered.
A drawing should identify whether the coordinate system is referenced to:
- The substrate center
- Wafer notch
- Two perpendicular edges
- Fiducial marks
- A cavity
- A lithographic pattern
6. Define Sidewall and Hole Quality
A hole can meet its diameter requirement while still being unsuitable for metallization.
Common via-formation defects include:
- Entry chipping
- Exit chipping
- Crepe radiali
- Subsurface cracks
- Heat-affected regions
- Rough sidewalls
- Residual glass debris
- Recast material
- Non-circular openings
- Local necking
- Excessive taper
The RFQ should include acceptance limits for:
- Maximum chip width
- Maximum chip depth
- Crack allowance
- Sidewall roughness
- Hole roundness
- Residual contamination
- Entry and exit edge condition
Consider the via-formation method
TGVs may be formed through processes such as:
- Foratura laser
- Laser-induced selective etching
- Incisione a secco
- Wet chemical etching
- Powder blasting
- Electrical discharge-based processes
- Photosensitive-glass processing
Each method produces different via shapes, sidewall textures, throughputs and defect risks.
The buyer does not always need to dictate the process, but the supplier should disclose the expected via profile and inspection method.
7. Specify the Metallization Stack
Copper cannot normally be electroplated directly and uniformly onto untreated glass. A metallization structure is needed to provide adhesion and electrical continuity.
The RFQ should identify:
- Adhesion layer material
- Barrier layer material
- Seed-layer material
- Nominal layer thickness
- Minimum sidewall coverage
- Deposition method
- Allowable discontinuities
- Copper purity
- Final surface finish
Possible stack elements include combinations of:
- Titanium
- Chromium
- Tantalum
- Titanium-tungsten
- Copper
- Nickel
- Gold
The correct stack depends on the glass surface, subsequent temperature, soldering process and contamination restrictions.
Seed-layer continuity
An apparently acceptable seed layer on the front surface may still be too thin or discontinuous deep inside the via.
Discontinuous seed coverage can cause:
- Unplated regions
- High electrical resistance
- Copper seams
- Internal voids
- Weak adhesion
- Open circuits
Seed-layer acceptance should therefore address the complete via sidewall, not only the top opening.
8. Choose the Copper Structure
The buyer should specify whether the via requires:
Fully copper-filled vias
The complete internal volume is filled with copper.
Advantages may include:
- Low electrical resistance
- High current capacity
- Better vertical heat conduction
- Solid mechanical connection
Potential challenges include:
- Long plating time
- Internal voids
- Copper stress
- Copper protrusion
- Increased warpage
- Greater CTE mismatch
Conformal copper-plated vias
Copper coats the sidewall but does not fill the complete hole.
Advantages may include:
- Lower copper volume
- Reduced plating time
- Potentially lower thermomechanical stress
- Lower material usage
Limitations may include:
- Higher resistance
- Lower current capacity
- Greater sensitivity to layer discontinuity
- Need for a later plugging or filling process
The buyer should not use “copper-plated TGV” and “copper-filled TGV” interchangeably.
9. Control Copper Filling Defects
A copper-filled via can appear closed at the surface while containing a large internal defect.
Common filling defects include:
- Central voids
- Seam voids
- Keyhole-shaped voids
- Trapped gas
- Incomplete bottom filling
- Thin copper regions
- Seed-layer gaps
- Copper nodules
- Excessive overburden
- Surface recess
- Dishing
- Copper protrusion
- Copper-to-glass delamination
Research on TGV metallization has demonstrated the importance of controlled deposition and has used X-ray tomography to evaluate copper structures inside glass vias. The results show why surface appearance alone is not sufficient to verify internal filling quality.
Causes of copper voids
Possible causes include:
- Poor via wetting
- Trapped air
- Contaminated sidewalls
- Discontinuous seed layers
- Inappropriate current density
- Unbalanced plating additives
- Restricted copper-ion transport
- Premature closure at the via opening
- Excessive via aspect ratio
- Irregular via shape
The supplier should state whether the process is designed for:
- Bottom-up filling
- Superconformal filling
- Double-sided filling
- Conformal sidewall plating
- Another metallization route
Copper filling acceptance criteria
The RFQ should define:
- Whether any internal void is permitted
- Maximum individual void size
- Maximum total void area or volume
- Prohibited void locations
- Minimum copper cross-sectional area
- Maximum surface recess
- Maximum protrusion
- Overburden thickness
- Copper grain or microstructure requirements, when relevant
For high-reliability applications, “void-free” should be linked to a defined inspection resolution. Otherwise, the term is difficult to verify.
10. Control Copper Overburden, Recess and Protrusion
After plating, excess copper may remain above the glass surface. It may be removed through grinding, polishing or chemical-mechanical processing.
Important surface conditions include:
- Copper flush with glass
- Controlled copper recess
- Controlled copper protrusion
- Remaining copper overburden
- Dishing
- Local glass erosion
- Graffi superficiali
- Copper smearing
The correct condition depends on the next process.
Ad esempio:
- Direct bonding may require extremely low topography.
- RDL fabrication may tolerate a controlled recess.
- Solder-pad formation may require additional copper thickness.
- Temporary handling may allow overburden that will be removed later.
The buyer should define the final delivery stage clearly.
11. Specify Warpage and Flatness
Warpage is one of the most important TGV substrate acceptance items.
Copper and glass have different thermal and mechanical properties. Copper plating, annealing, RDL processing and asymmetric metal distribution can generate stress and substrate deformation.
Experimental research on electroplated TGV substrates has identified warpage control as an important engineering challenge and has shown that heat-treatment conditions influence the final deformation of metallized glass substrates.
Factors affecting warpage
Warpage can be influenced by:
- Composizione del vetro
- Substrate thickness
- Substrate size
- Copper-filled via volume
- Via density
- Via-array symmetry
- Copper overburden
- Frontside and backside RDL balance
- Plating stress
- Annealing temperature
- Cooling rate
- Copper grain evolution
- Temporary carrier processing
The effective thermomechanical behavior of a glass substrate changes after copper vias are introduced. The combined copper-glass structure should therefore be evaluated rather than treating the glass and vias as independent features.
Define the measurement condition
A useful warpage specification should state:
- Maximum warpage value
- Measurement temperature
- Free-state or supported-state measurement
- Substrate orientation
- Measurement equipment
- Measurement grid
- Edge exclusion
- Whether local and global warpage are both controlled
- Measurement stage in the process
Warpage may need to be measured:
- Before via formation
- After via formation
- After copper filling
- After annealing
- After copper planarization
- After RDL fabrication
- After thermal reliability testing
12. Recommended Inspection Plan
A complete TGV inspection plan should combine dimensional, visual, internal, electrical and reliability measurements.
Incoming glass inspection
Inspect:
- External dimensions
- Spessore
- TTV
- Orlo e trama
- Graffi superficiali
- Chip Edge
- Bubbles
- Inclusions
- Contaminazione superficiale
- Optical transmission, when required
Via geometry inspection
Inspect:
- Top diameter
- Bottom diameter
- Minimum internal diameter
- Conico
- Rotondità
- Pitch
- Position
- Entry chipping
- Exit chipping
- Crepe
- Sidewall condition
Possible methods include:
- Optical microscopy
- Automated optical inspection
- Confocal microscopy
- Surface profilometry
- Cross-sectional microscopy
- SEM for sampled analysis
Copper filling inspection
Inspect:
- Internal voids
- Seams
- Incomplete fill
- Copper continuity
- Delaminazione
- Overburden
- Recess
- Protrusion
Possible methods include:
- 2D X-ray inspection
- X-ray computed tomography
- Destructive cross-sectioning
- Optical microscopy
- SEM
- Elemental analysis where contamination is suspected
X-ray tomography is particularly useful because it can reveal internal structures that cannot be evaluated from the via opening alone. Cross-sectioning remains valuable for detailed validation of copper morphology, seed coverage and interfaces.
Electrical inspection
Possible electrical tests include:
- Continuity
- Open-circuit detection
- Daisy-chain resistance
- Individual via resistance
- Kelvin resistance
- Isolation resistance
- Leakage current
- High-frequency test-structure measurement
Resistance limits should reflect via diameter, copper structure, via length and test configuration.
Surface and dimensional inspection
Inspect:
- Final thickness
- TTV
- Global warpage
- Local flatness
- Rugosità superficiale
- Copper height
- Pad dimensions
- RDL width and spacing
- Precisione di allineamento
13. Reliability Testing
Prototype approval should not rely only on initial dimensional inspection.
Depending on the application, qualification may include:
- Cicli termici
- Thermal shock
- High-temperature storage
- Temperature-humidity exposure
- Current stressing
- Solder reflow simulation
- Bonding trials
- Shear or pull testing
- Adhesion testing
- Electrical resistance monitoring
- Repeated warpage measurement
The purpose is to identify:
- Copper-to-glass delamination
- Glass cracking
- Copper extrusion
- Resistance drift
- Pad lifting
- Interface damage
- Warpage changes
- RDL cracking
Test conditions should match the expected assembly and operating environment.
TGV Glass Substrate RFQ Checklist
The following information should be included in a quotation request.
Glass substrate
- Glass type or grade
- Wafer, panel or interposer format
- Diameter or length and width
- Thickness and tolerance
- TTV limit
- Rugosità superficiale
- Bow and warpage limit
- Profilo del bordo
- Edge exclusion
- CTE requirement
- Dielectric requirement
- Alkali or metallic impurity restrictions
TGV geometry
- Via quantity
- Via-array drawing
- Top-opening diameter
- Bottom-opening diameter
- Minimum internal diameter
- Via pitch
- Minimum glass spacing
- Via-position tolerance
- Via taper
- Roundness requirement
- Sidewall roughness
- Chipping limit
- Crack acceptance criteria
Metallization and copper filling
- Adhesion-layer material
- Barrier-layer material
- Seed-layer material
- Full copper fill or conformal plating
- Copper purity
- Minimum copper thickness
- Internal void limit
- Copper recess or protrusion
- Overburden requirement
- Surface planarization requirement
- Final surface finish
RDL and pads
- RDL layer quantity
- Copper-line width and spacing
- Pad dimensions
- RDL thickness
- Solder mask or passivation
- UBM structure
- Bump or solder requirement
- Tolleranza di allineamento
Ispezione
- Dimensional report
- Warpage map
- Optical inspection
- X-ray inspection
- X-ray CT sampling
- Cross-sectional report
- Electrical continuity test
- Resistance test
- Surface roughness report
- Material certificate
- Reliability-test requirement
Commercial information
- Prototype quantity
- Production forecast
- Required delivery format
- Packaging method
- Livello di pulizia
- Traceability requirement
- Target application
- Required delivery date
Example TGV Procurement Specification
A preliminary RFQ could be organized as follows:
| Parametro | Example RFQ Format |
| Vetro | Identified borosilicate or equivalent approved grade |
| Substrate format | Wafer or rectangular panel |
| Spessore | Nominal value with tolerance |
| TTV | Maximum acceptable value |
| Via structure | Through vias, cylindrical or specified taper |
| Via diameter | Top, bottom and minimum internal diameter |
| Via pitch | X and Y center-to-center spacing |
| Via position | Tolerance relative to defined fiducials |
| Copper structure | Fully filled or conformally plated |
| Internal voids | Acceptance limit linked to inspection resolution |
| Copper topography | Maximum recess, protrusion or overburden |
| Curvatura | Maximum value after defined process step |
| Electrical test | Continuity and resistance requirement |
| Ispezione | Optical, dimensional, X-ray and sampled cross-section |
| Imballaggio | Clean individual packaging or cassette |
| Quantità | Prototype and expected production volume |
The values should be completed after discussion with the supplier because achievable tolerances depend on the full combination of glass thickness, via diameter, pitch, array size and copper structure.
How to Evaluate a TGV Supplier
A qualified TGV substrate supplier should be able to explain more than the nominal via diameter.
Important capabilities include:
- Multiple glass-material options
- Controlled via formation
- Crack and chipping inspection
- High-aspect-ratio metallization
- Continuous seed-layer deposition
- Copper filling or conformal plating
- Copper planarization
- RDL fabrication
- Warpage measurement
- X-ray or CT inspection
- Prove elettriche
- Prototype processing
- Lot traceability
- Clean packaging
Ask the supplier to provide:
- Cross-sectional images
- X-ray or CT examples
- Warpage data
- Electrical test results
- Inspection capability and resolution
- Sample acceptance reports
- Process-flow description
- Reliability data for a similar structure
A supplier that only provides a top-view photograph cannot fully demonstrate the condition of copper inside the vias.
Conclusione
Purchasing TGV glass substrates requires more than specifying glass size and via diameter.
The RFQ should clearly define:
- Glass grade and thermal properties
- Substrate thickness and TTV
- Top and bottom via diameters
- Via taper and aspect ratio
- Pitch and position tolerance
- Sidewall and edge quality
- Metallization stack
- Copper filling structure
- Internal void criteria
- Copper recess or protrusion
- Warpage measurement conditions
- Dimensional, internal and electrical inspection
Via diameter, pitch, copper volume and glass thickness must be evaluated as one connected design. A dimension that is achievable on a thick, widely spaced prototype may become difficult when transferred to a thin substrate with a dense via array.
Clear drawings, measurable acceptance criteria and agreed inspection methods help buyers reduce copper-filling defects, glass cracking, alignment errors and package warpage before the substrate enters expensive downstream assembly.
Domande frequenti
What information is required to quote a TGV glass substrate?
At minimum, provide the glass type, substrate size, thickness, via diameter, via pitch, via quantity, copper structure, warpage limit, inspection requirement and application.
Should TGV diameter be specified at the top or bottom opening?
Both should be specified when the via is tapered. For critical designs, also define the minimum internal diameter and taper angle.
What causes voids inside copper-filled TGVs?
Common causes include poor wetting, trapped gas, discontinuous seed layers, restricted ion transport, unsuitable plating additives and premature closure of the via opening.
Can copper-filled TGVs be inspected without cutting the substrate?
X-ray and X-ray CT can be used for nondestructive internal inspection. However, sampled cross-sectioning may still be required to confirm seed coverage, interface quality and small defects.
Why does a TGV substrate warp after copper plating?
Warpage can result from copper plating stress, copper-glass thermal mismatch, asymmetric metal distribution, annealing and differences between the frontside and backside structures.
Is a smaller via pitch always better?
A smaller pitch increases interconnect density, but it also reduces the glass spacing between vias and can increase manufacturing and reliability risks. Pitch must be evaluated together with via diameter, thickness and via-array density.
What does “void-free copper filling” mean?
The term should be linked to a defined inspection method and detection resolution. Without this information, it does not establish a measurable acceptance standard.
Is borosilicate glass always the best material for TGV substrates?
No. Borosilicate glass is widely used, but fused silica, aluminosilicate, alkali-free and other engineered glasses may be more suitable depending on CTE, dielectric, optical and process requirements.