Requesting a quote for custom glass wafers requires more than specifying diameter and thickness. Material grade, total thickness variation, bow, warp, surface roughness, edge quality, cleaning, inspection and packaging can all affect manufacturability, yield and price.
These specifications are especially important for MEMS wafer bonding, microfluidics, optical devices, sensors, semiconductor packaging and precision carrier applications. A glass wafer may have the correct nominal thickness but still create bonding, lithography or alignment problems if its TTV, bow or warp exceeds the process capability.
This guide explains the information engineers and procurement teams should include in a custom glass wafer RFQ.
Why Glass Wafer Specifications Matter
Glass wafers are used as functional substrates, carrier wafers, optical windows, sensor covers, interposers and bonding materials.
Their performance can depend on:
- Material composition
- Thermal expansion
- Wafer diameter
- Thickness
- Total thickness variation
- Bow and warp
- Surface roughness
- Flatness and parallelism
- Edge finish
- Optical transmission
- Cleaning and packaging
An RFQ that only states “100 mm glass wafer, 500 μm thick” leaves many critical requirements undefined.
The supplier still needs to know whether the wafer is intended for anodic bonding, optical transmission, temporary bonding, microfluidics, laser processing or general laboratory use.
1. Select the Glass Wafer Material
“Glass wafer” is a broad description. Different glass materials can have significantly different thermal, optical, chemical and electrical properties.
Common options include:
Borosilicate Glass
Borosilicate glass offers relatively low thermal expansion, chemical durability and good dimensional stability. Certain grades are commonly used for MEMS, microfluidics, sensor packaging and glass-to-silicon bonding.
It may be selected when the application requires:
- Anodic bonding compatibility
- Good visible transmission
- Chemical resistance
- Stable thickness control
- Cost-effective wafer processing
Fused Silica
Fused silica provides low thermal expansion, high-temperature stability and broad optical transmission. Available grades may differ in purity, OH content and UV or infrared performance.
Typical applications include:
- Optical and photonic devices
- UV systems
- Semiconductor processing
- Precision sensors
- High-temperature environments
- Low-contamination applications
Aluminosilicate Glass
Aluminosilicate glass may be selected for applications requiring improved strength, thermal stability or chemical durability.
The suitability of a particular grade depends on the process temperature, bonding method and downstream machining requirements.
Alkali-Free Glass
Alkali-free glass can be useful when sodium or other alkali contamination must be minimized. It may be considered for electronic, display, semiconductor or thin-film applications.
The RFQ should identify the exact material grade whenever it has already been qualified. If the material is still being selected, provide the application and critical properties so the supplier can recommend suitable options.
2. Specify Wafer Diameter and Shape
Common glass wafers are circular, but custom square, rectangular and irregular shapes can also be produced.
The RFQ should define:
- Nominal diameter
- Diameter tolerance
- Wafer shape
- Flat or notch requirement
- Flat or notch dimensions
- Orientation requirement
- Edge exclusion area
- Custom cutouts
- Hole or via pattern
Standard wafer diameters are generally easier to source and process. Custom diameters may require additional cutting, grinding and inspection.
If the glass wafer must fit a semiconductor tool, cassette, chuck or bonding system, include the equipment interface requirements.
3. Define Nominal Thickness
Wafer thickness affects mechanical strength, flexibility, optical path length, bonding behavior and compatibility with processing equipment.
The RFQ should identify:
- Nominal thickness
- Thickness tolerance
- TTV limit
- Edge exclusion used for measurement
- Measurement method, when critical
Thin glass wafers can be more difficult to handle and may require carrier support or specialized packaging. Thick wafers consume more material and may require additional grinding and polishing.
If multiple thickness options are acceptable, ask the supplier to recommend the most economical standard thickness.
4. What Is TTV?
TTV means total thickness variation. It is the difference between the maximum and minimum measured thickness across the wafer within the specified measurement area.
In simplified form:
TTV = Maximum Thickness − Minimum Thickness
A wafer can meet its nominal thickness tolerance while still having excessive TTV. For example, all measured points may fall within the general thickness range, but uneven thickness across the wafer can interfere with bonding or process uniformity.
Low TTV is important for:
- Wafer bonding
- Lithography
- Thin-film deposition
- Grinding and polishing
- Optical alignment
- Temporary carrier applications
- Wafer-level packaging
- Precision chucking
The RFQ should state the maximum acceptable TTV and the edge exclusion used during measurement.
5. What Is Bow?
Bow describes the overall curvature of the wafer’s median surface relative to a reference plane, generally measured while the wafer is in a free and unclamped condition.
A wafer with bow may appear slightly concave or convex.
Excessive bow can cause:
- Poor contact during bonding
- Uneven chucking
- Focus variation during lithography
- Alignment difficulty
- Edge lifting
- Non-uniform coating or deposition
- Handling problems
Bow may be affected by material stress, polishing, thickness, thermal processing and coatings.
The RFQ should specify whether positive and negative bow are treated equally and identify the required measurement standard if the result is process-critical.
6. What Is Warp?
Warp represents the overall peak-to-valley deformation of the wafer’s median surface relative to a reference plane.
Unlike bow, which mainly describes general curvature, warp captures more complex deformation across the wafer.
A wafer can have relatively low bow but still have significant warp because of local shape variation.
Warp is particularly important for:
- Wafer bonding
- Vacuum chucking
- Lithography
- Inspection equipment
- Automated wafer handling
- Thin-film processes
- Advanced packaging
The measurement result can depend on equipment, wafer orientation, support method and edge exclusion. Therefore, the RFQ should identify the applicable measurement method or customer standard.
TTV, Bow and Warp Comparison
| Parameter | What it measures | Common process impact |
|---|---|---|
| Thickness tolerance | Acceptable range around nominal thickness | Equipment and assembly compatibility |
| TTV | Difference between maximum and minimum thickness | Bonding uniformity and process consistency |
| Bow | General curvature of the median surface | Chucking, focus and edge contact |
| Warp | Total peak-to-valley deformation | Handling, bonding and lithography |
| Flatness | Surface deviation from a reference plane | Optical contact and precision assembly |
| Parallelism | Relationship between front and back surfaces | Optical path and thickness uniformity |
These terms should not be used interchangeably.
7. Specify SSP or DSP Surface Finish
Glass wafers may be supplied with different surface configurations.
Single-Side Polished
A single-side polished wafer has one polished functional surface, while the opposite side may be lapped, ground or finished to a less demanding condition.
SSP wafers may be suitable when:
- Only one side performs an optical function
- One side receives a coating
- The wafer is bonded from one surface
- Lower cost is important
Double-Side Polished
A double-side polished wafer has polished front and back surfaces.
DSP wafers may be selected for:
- Through-wafer optical transmission
- Precision bonding
- Lithography on both sides
- Double-side processing
- Low TTV requirements
- Interferometric inspection
The RFQ should define which surface is considered the front side and whether both sides require the same roughness and surface quality.
8. Surface Roughness
Surface roughness affects bonding, optical transmission, coating adhesion and thin-film uniformity.
The required roughness depends on the application. A general mechanical carrier may tolerate a different surface condition from a direct-bonding or optical wafer.
The RFQ should state:
- Roughness parameter, such as Ra or RMS
- Maximum roughness value
- Measurement area
- Measurement instrument or method
- Front-side requirement
- Back-side requirement
Avoid specifying extremely low roughness unless the process requires it. More demanding surface finishes can increase polishing time, inspection cost and production risk.
9. Surface Quality and Defects
Surface roughness does not describe every type of surface defect.
The RFQ may also need to define limits for:
- Scratches
- Digs
- Pits
- Chips
- Cracks
- Stains
- Haze
- Polishing marks
- Inclusions
- Bubbles
- Edge defects
- Embedded particles
For optical wafers, specify the applicable scratch-dig or cosmetic inspection requirement.
For semiconductor and bonding applications, define the maximum allowable particle size, edge chips and critical-surface defects.
If only part of the wafer is functional, identify the clear aperture or active area.
10. Flatness, Parallelism and Wedge
Flatness describes how much a surface deviates from a reference plane. Parallelism describes the relationship between the front and back surfaces.
Wedge is the angular difference between two surfaces and can influence optical beam deviation or bonding contact.
These parameters may be important for:
- Optical windows
- Interferometers
- Laser systems
- Precision spacers
- Bonded assemblies
- Wafer-level packaging
- Metrology components
The drawing should identify whether the requirement applies to the entire wafer or only the functional area.
11. Edge Profile and Edge Quality
Glass wafer edges must be processed carefully because chips and microcracks can reduce handling strength and increase particle generation.
Possible edge options include:
- Ground edge
- Fine-ground edge
- Polished edge
- Rounded edge
- Beveled edge
- Chamfered edge
- Custom edge profile
The RFQ should define:
- Edge shape
- Bevel or chamfer dimensions
- Maximum chip size
- Edge exclusion
- Flat or notch condition
- Edge-polishing requirement
Thin glass wafers generally require additional attention to edge quality and packaging.
12. Holes, Vias and Custom Machining
Custom glass wafers may contain:
- Through holes
- Blind holes
- Microholes
- Slots
- Cavities
- Channels
- Counterbores
- Wafer flats
- Alignment features
- Through-glass vias
The quotation should identify:
- Feature diameter or width
- Feature depth
- Position tolerance
- Pitch
- Quantity per wafer
- Edge distance
- Sidewall quality
- Taper allowance
- Metallization requirement
- Inspection method
Dense via patterns and small features may require specialized laser, mechanical or chemical processing.
If the customer supplies a CAD file, a controlled 2D drawing should still identify critical dimensions and acceptance criteria.
13. Optical Transmission Requirements
For optical, UV and photonic applications, specify:
- Operating wavelength
- Minimum transmission
- Refractive index requirement
- Material homogeneity
- Bubble and inclusion limits
- Surface quality
- Flatness
- Parallelism
- Coating requirements
- Clear aperture
Visible transparency alone does not confirm performance at ultraviolet or infrared wavelengths.
The selected material grade and wafer thickness must be considered together when defining transmission.
14. Coating and Metallization
Glass wafers may require additional surface processing, such as:
- Antireflective coating
- Reflective coating
- Conductive coating
- Metal films
- Dielectric layers
- Adhesion layers
- Protective coating
- Patterned metallization
The RFQ should state:
- Coating material
- Coated side
- Wavelength range
- Minimum performance
- Pattern or mask file
- Edge exclusion
- Adhesion requirement
- Environmental durability
- Inspection criteria
Coating stress can affect wafer bow and warp. If post-coating geometry is critical, specify whether bow and warp limits apply before or after coating.
15. Cleaning and Packaging
Cleaning and packaging requirements depend on the application.
Possible requirements include:
- Standard optical cleaning
- Semiconductor cleaning
- Particle-controlled rinsing
- High-purity water rinse
- Cleanroom handling
- Non-contact handling
- Individual wafer packaging
- Wafer cassette packaging
- Vacuum packaging
- Double-bag packaging
- Nitrogen packaging
- Cleanroom-compatible labels
The RFQ should define whether wafers will be loaded directly into production equipment after unpacking.
For semiconductor and bonding applications, also specify particle, ionic and organic contamination requirements.
16. Inspection and Documentation
Possible inspection documents include:
- Material certificate
- Certificate of conformity
- Diameter inspection
- Thickness mapping
- TTV report
- Bow and warp report
- Surface roughness report
- Flatness report
- Scratch-dig inspection
- Optical transmission report
- Particle inspection
- Coating inspection
- First Article Inspection report
- Lot traceability record
Clarify whether every wafer requires measurement or whether sampling inspection is acceptable.
If customer-specific measurement equipment or standards are required, include them before quotation.
Glass Wafer Price Factors
| Price factor | Typical cost impact |
|---|---|
| Standard diameter and thickness | Lower |
| Custom diameter or shape | Medium to high |
| Very thin wafer | High |
| Low TTV | High |
| Tight bow and warp | High |
| Double-side polishing | Medium to high |
| Optical surface quality | High |
| Custom holes or vias | Medium to high |
| Coating or metallization | Medium to high |
| Semiconductor cleaning | Medium |
| Full inspection reports | Medium |
| Prototype quantity | Higher unit cost |
| Production volume | Lower unit cost potential |
Custom Glass Wafer RFQ Checklist
| RFQ item | Information to provide |
|---|---|
| Application | MEMS, optical, bonding, packaging or carrier |
| Material | Glass type and grade |
| Diameter | Nominal size and tolerance |
| Thickness | Nominal thickness and tolerance |
| TTV | Maximum acceptable value |
| Bow | Maximum value and measurement method |
| Warp | Maximum value and measurement method |
| Surface | SSP, DSP, lapped or ground |
| Roughness | Front and back requirements |
| Surface quality | Scratch, dig, pit and chip limits |
| Edge | Bevel, chamfer, polish, flat or notch |
| Flatness | Global or functional-area requirement |
| Optical performance | Wavelength and transmission |
| Machining | Holes, vias, slots and cavities |
| Coating | Material, pattern and performance |
| Cleaning | Standard, optical or semiconductor |
| Inspection | Required reports and sampling plan |
| Packaging | Individual, cassette or cleanroom packaging |
| Quantity | Prototype and production volume |
| Delivery | Required date and destination |
Copyable Glass Wafer RFQ Template
- Application:
- Material grade:
- Wafer diameter:
- Diameter tolerance:
- Nominal thickness:
- Thickness tolerance:
- Maximum TTV:
- Maximum bow:
- Maximum warp:
- Edge exclusion:
- Front surface finish:
- Back surface finish:
- Surface roughness:
- Surface quality:
- Edge profile:
- Flat or notch:
- Holes, vias or cavities:
- Optical wavelength:
- Coating requirement:
- Cleaning requirement:
- Inspection documents:
- Prototype quantity:
- Production quantity:
- Required delivery date:
Common RFQ Mistakes
Specifying Thickness Without TTV
Nominal thickness does not describe thickness uniformity across the wafer.
Treating Bow and Warp as the Same Parameter
Bow represents general curvature, while warp captures the full peak-to-valley deformation.
Omitting the Measurement Method
Different measurement systems, support conditions and edge exclusions can produce different results.
Using Extremely Tight Specifications Everywhere
Unnecessary TTV, bow, warp and surface requirements can increase price and reduce manufacturing yield.
Not Identifying the Functional Surface
The manufacturer needs to know which surface will be bonded, coated, patterned or used optically.
Ignoring Edge Quality
Edge chips and microcracks can cause wafer breakage and particle contamination during processing.
Frequently Asked Questions
What is the difference between TTV, bow and warp?
TTV measures thickness variation. Bow measures overall wafer curvature, while warp measures the complete peak-to-valley deformation of the wafer’s median surface.
Why is low TTV important for glass wafers?
Low TTV supports more uniform bonding, lithography, coating, polishing and wafer chucking.
Is DSP always better than SSP?
No. DSP is useful when both surfaces require precision finishing or optical transmission. SSP may be more economical when only one side is functional.
Can custom glass wafers include holes and vias?
Yes. Glass wafers can be processed with holes, slots, cavities, channels and via patterns. Feature size, pitch, taper and sidewall requirements should be included in the RFQ.
Can glass wafer bow and warp change after coating?
Yes. Coating stress and thermal processing can affect wafer geometry. Specify whether the limits apply before or after coating.
What information is needed for MEMS bonding wafers?
Important requirements may include material grade, thermal expansion, diameter, thickness, TTV, bow, warp, surface roughness, edge quality, cleanliness and packaging.
Can a supplier recommend practical tolerances?
Yes. Provide the equipment, bonding process and functional requirements. The manufacturer can identify which tolerances are critical and which can be relaxed.
Conclusion
An accurate custom glass wafer quotation requires more than diameter and nominal thickness.
TTV, bow, warp, surface roughness, edge quality, polishing, cleanliness, inspection and packaging can directly influence process compatibility and manufacturing cost.
Before submitting an RFQ, clearly define:
- Glass material and application
- Diameter and thickness
- TTV, bow and warp
- SSP or DSP finish
- Surface roughness and defect limits
- Edge profile
- Holes, vias or other machining
- Cleaning and packaging
- Inspection documents
- Prototype and production quantities
A complete specification enables the glass wafer manufacturer to evaluate manufacturability, recommend practical tolerances and provide a more accurate quotation.
