Borofloat 33 Glass Substrates
Borofloat 33 borosilicate glass wafer substrates for MEMS, microfluidics, anodic bonding, and bio-chip applications. Available in wafer formats from 100mm to 300mm with excellent chemical resistance and thermal stability.
Borofloat 33 Borosilicate Glass Wafers
Borofloat 33 is a premium borosilicate float glass manufactured by SCHOTT AG using a proprietary micro-float process that produces glass sheets with exceptional flatness, uniform thickness, and pristine fire-polished surfaces. When fabricated into wafer formats (100mm to 300mm), Borofloat 33 delivers a unique combination of properties that make it the material of choice for anodic bonding to silicon, microfluidic device fabrication, and bio-chip applications where optical transparency, chemical durability, and thermal stability are essential.
The glass derives its name from its boron trioxide (B₂O₃) content of approximately 13%, which — together with alumina (Al₂O₃) and alkali oxides (Na₂O, K₂O) — produces a thermal expansion coefficient (CTE) of 3.25 × 10⁻⁶/K. This value is a near-perfect match to silicon (2.6 × 10⁻⁶/K), enabling low-stress hermetic anodic bonding across temperature ranges from ambient to 450°C. Unlike soda-lime glass (CTE ~ 9 ppm/K), Borofloat 33 bonded assemblies survive repeated thermal cycling without delamination or fracture.
At GINECHIP, we supply Borofloat 33 wafer substrates laser-cut and edge-ground to SEMI-standard diameters from 100mm to 300mm. Thicknesses from 200μm to 1,100μm are available with SSP or DSP surface finish — fire-polished as-float for general applications, or CMP-polished for demanding surface quality requirements. Every lot includes a full Certificate of Analysis (CoA) documenting CTE, transmission, TTV, bow, surface roughness, and chemical durability classification.
Borofloat 33 vs Silicon: Substrate Comparison for Anodic Bonding
The choice between Borofloat 33 glass and silicon as the substrate or cap wafer in a MEMS process flow depends on the device's optical, electrical, and thermal requirements. The comparison below highlights the complementary nature of these two materials — often used together in the same device stack.
Borofloat 33 Glass
Float borosilicate — CTE-matched to silicon
Float-glass borosilicate with exceptional flatness (fire-polished surface) and thermal expansion matched to silicon. The CTE of 3.25 × 10⁻⁶/K is nearly identical to silicon (2.6 × 10⁻⁶/K), enabling low-stress anodic bonding for MEMS packaging. Float production yields large-area sheets that are laser-cut and edge-ground to wafer dimensions with excellent thickness uniformity.
Silicon (Si)
Single-crystal semiconductor — active device layer
Single-crystal silicon substrates with higher mechanical strength and thermal conductivity than glass. While silicon provides the ultimate surface quality (＜ 0.2nm RMS) and crystallographic perfection, it is opaque in the visible spectrum, electrically conductive (requiring isolation layers), and requires higher anodic bonding voltages. Often used as the active device layer with Borofloat 33 as the cap or carrier wafer.
MEMS & Microfluidics: Core Application Domains
The confluence of optical transparency, chemical inertness, and silicon-matched thermal expansion positions Borofloat 33 at the center of two of the fastest-growing MEMS market segments.
Microfluidics
Wet or dry etching of microchannels, reaction chambers, and mixing structures into Borofloat 33 wafers creates the fluidic layer of lab-on-chip devices. Isotropic HF etching through Cr/Au or a-Si masks produces semicircular channel profiles ideal for capillary-driven flow; DRIE (Bosch process with SF₆/C₄F₈) enables high-aspect-ratio anisotropic structures.
Anodic Bonding
Applying 200–1,000V across a Borofloat 33–silicon stack at 300–450°C drives mobile Na⁺ ions in the glass toward the cathode, creating a space-charge region at the glass-silicon interface. The resulting electrostatic attraction pulls the surfaces into intimate contact where Si–O–Si bonds form, producing a hermetic seal with bond strength exceeding 10 MPa.
Through-Glass Vias
Laser ablation (UV ps or fs lasers) or HF-based wet etching creates high-aspect-ratio via holes (30–200μm diameter, 200–500μm depth) through Borofloat 33 wafers. Subsequent Cu electroplating or conductive-paste filling produces vertical interconnects for 3D MEMS packaging with superior RF performance vs through-silicon vias (TSVs).
The Anodic Bonding Process: Why Borofloat 33 Excels
Anodic (field-assisted) bonding is the foundational joining technology for silicon-glass MEMS devices, and Borofloat 33 was specifically engineered to optimize this process. The glass contains approximately 4% alkali oxides (primarily Na₂O) — sufficient to provide mobile cations for the bonding reaction, but low enough to maintain chemical durability and prevent excessive ionic contamination of the silicon device layer.
During bonding, the wafer stack is heated to 300–450°C on a hotplate or in a vacuum chamber. A DC voltage (200–1,000V, cathode on the glass side) is applied, causing Na⁺ ions to drift away from the glass-silicon interface toward the cathode. The resulting oxygen-rich depletion layer at the interface reacts with the silicon surface to form a permanent, hermetic Si–O–Si bond. The low CTE of Borofloat 33 ensures that upon cooling to room temperature, the residual thermal stress is minimal — typically below 5 MPa for 4″ wafer pairs — preventing warpage, delamination, or fracture.
GINECHIP supplies Borofloat 33 wafers pre-cleaned (RCA-1 or piranha) and characterized for surface roughness (＜ 1.0nm RMS for fire-polished, ＜ 0.5nm RMS for CMP), ensuring optimal bond yield with minimal void formation. Wafers are packaged in Class 100 cleanroom conditions with interleaved cleanroom paper to preserve surface quality through shipping and storage.
Bio-MEMS & Lab-on-Chip: The Biomedical Interface
Borofloat 33 has become the preferred glass substrate for biomedical MEMS and lab-on-chip devices due to its combination of optical transparency, biocompatibility, and resistance to biological fluids. Unlike PDMS (polydimethylsiloxane) — which is widely used in academic microfluidic prototyping but suffers from hydrophobicity, small-molecule absorption, and mechanical compliance issues — Borofloat 33 provides a rigid, chemically defined surface that can be silanized, PEGylated, or coated with biomolecular capture agents via well-established silane chemistry.
Key biomedical applications include: capillary electrophoresis chips where the glass surface supports stable electro-osmotic flow (EOF); droplet microfluidic devices for single-cell sequencing and digital PCR where fluorinated surface coatings prevent biofouling; organ-on-chip platforms where optical access through the transparent glass substrate enables real-time microscopy of cell monolayers cultured on porous membranes suspended between microfluidic channels; and implantable biosensors where the hermeticity of anodically bonded glass-silicon cavities protects sensitive electronics from the in-vivo environment for multi-year implant lifetimes.
Technical Specifications
| Parameter | Available Range / Values |
|---|---|
| Material | Borofloat 33 (SCHOTT), equivalent borosilicate float glass |
| Diameter | 100mm (4″), 150mm (6″), 200mm (8″), 300mm (12″) |
| Thickness | 200μm–1,100μm, standard SEMI thicknesses |
| CTE (20–300°C) | 3.25 × 10⁻⁶/K |
| Strain Point | 525°C |
| Softening Point | 825°C |
| Density | 2.20 g/cm³ |
| Refractive Index | n₅₈₉ = 1.4714 |
| Dielectric Constant | 4.6 at 1 MHz |
| Surface Roughness (AFM RMS) | < 1.0nm standard |
| TTV | ≤ 5μm |
| Bow / Warp | Bow ≤ 25μm |
| Transmission Range | > 90% from 310nm to 2,700nm |
| Chemical Resistance | Hydrolytic Class 1, Acid Class 1, Alkali Class 2 (per ISO 719 / DIN 12116) |
| Surface Finish | SSP (Single-Side Polish) or DSP (Double-Side Polish), fire-polished or CMP |
| Packaging | Interleaved cleanroom paper, single-wafer cassette, vacuum-sealed |
Applications & Market Segments
Microfluidic & Lab-on-Chip Devices
Borofloat 33 is the industry-standard glass substrate for microfluidic chips in biomedical diagnostics, drug discovery, and chemical analysis. Its optical transparency enables fluorescence and absorbance-based detection, while the chemically resistant surface withstands aggressive solvents, acids, and biological buffers without degradation or leaching.
Anodic Bonding for MEMS Packaging
The near-perfect CTE match between Borofloat 33 (3.25 ppm/K) and silicon (2.6 ppm/K) enables low-stress anodic bonding at 300–450°C with 200–1,000V applied voltage. The resulting hermetic seal is essential for inertial sensors, pressure sensors, and RF MEMS switches requiring long-term vacuum or inert-gas encapsulation.
Through-Glass Via (TGV) Interposers
Laser-drilled or wet-etched through-glass vias in Borofloat 33 wafers provide low-loss RF interposers for 2.5D and 3D packaging. The material's low dielectric constant (4.6) and low loss tangent (＜ 0.005 at 1 MHz) reduce signal attenuation compared to silicon interposers, making it attractive for mmWave and RF front-end modules.
Electrochemical & Gas Sensors
Planar electrochemical sensors, Clark-type oxygen electrodes, and solid-state gas sensors utilize Borofloat 33 as the insulating substrate onto which platinum, gold, or carbon electrodes are patterned. The glass surface provides excellent adhesion for thick-film and thin-film metallization without the parasitic capacitance of silicon substrates.
Bio-Chip & DNA Microarrays
Amino-silane or epoxy-silane functionalized Borofloat 33 surfaces serve as the solid support for oligonucleotide and protein microarrays. The low autofluorescence background — significantly better than soda-lime glass — improves the signal-to-noise ratio in fluorescence-based microarray scanners, enabling detection of low-abundance targets.
Capacitive MEMS Devices
Accelerometers, gyroscopes, and pressure sensors fabricated with Borofloat 33 as the cap or structural layer benefit from the material's electrical insulation (resistivity ＞ 10¹⁰ Ω·cm), eliminating the need for dielectric isolation layers required on conductive silicon substrates. This simplifies the fabrication process and reduces parasitic feedthrough capacitance.
Metrology & Quality Assurance
Every Borofloat 33 wafer lot is characterized at ISO 9001:2015 certified facilities using a multi-technique protocol covering thermal, optical, chemical, and dimensional properties. A comprehensive Certificate of Analysis (CoA) accompanies each shipment to ensure full traceability and lot-to-lot consistency.
Ready to Specify Borofloat 33 for Your MEMS Device?
Provide your wafer diameter, thickness, surface finish (fire-polished or CMP), and quantity — our MEMS substrate specialists will prepare a detailed quotation with metrology data and lead time within 24 hours.