Sapphire Substrates
Materials, Substrates and Fabrication
Sapphire substrates are ideal for use instead of glass substrates when optical transmission is required in the ultraviolet (above 200 nm) or infrared (below 5 μm) range. Low-temperature optical measurements will also benefit from the higher thermal conductivity of sapphire substrates, and they may also be used in high temperature environments up to 2300 K.
Product Specifications
We offer sapphire substrates with three different surface finish qualities. Our 'Standard' sapphire substrates have been polished to a surface quality of 60/40 scratch-dig, have an RMS roughness of ~0.3 nm, and are suitable for most applications (including spectroscopy or thin-film deposition).
In comparison, our 'Ultra Smooth' sapphire substrates have been polished to a surface quality of 10/5 scratch-dig, have an RMS roughness of ~0.1 nm, and are suitable for applications involving atomic force microscopy, 2D materals, or any application where surface quality is crucial.
We also stock sapphire with a 'Coarse' surface finish (although still smoother than most glass substrates) with better than 80/50 scratch-dig and a slightly higher RMS roughness of ~0.7 nm. If you are not concerned about small amounts of optical scatter, these substrates are ideal.
| Properties | Standard | Ultra Smooth | Coarse |
|---|---|---|---|
| Substrate size | 20 mm x 15 mm | 20 mm x 15 mm | 20 mm x 15 mm |
| Thickness | 1.1 mm | 1.1 mm | 1.1 mm |
| Material | Synthetic sapphire | Synthetic sapphire | Synthetic sapphire |
| Surface finish (scratch-dig) | 60/40 (both sides polished) | 10/5 (both sides polished) | Better than 80/50 (both sides polished) |
| Surface roughness (RMS) | ~0.3 nm | ~0.1 nm | ~0.7 nm |
| Surface orientation | C-plane | C-plane | C-plane |
| Applications | UV/optical/NIR spectroscopy | Atomic force microscopy, 2D material substrate | UV/optical/NIR spectroscopy |
Comparison Between Float Glass and Sapphire
| Properties | Float Glass | Sapphire |
|---|---|---|
| Hardness (Mohs) | 5.5 | 9 |
| Density (g/cm3) | ~2.5 | 3.975 |
| Compressive strength (MPa) | 1000 | 2000 |
| Transmission window (nm) | ~350-2000 | ~200-5000 |
|
Refractive index n (k) 300 nm 600 nm 2000 nm 5000 nm |
1.55 (5.0×10-5) 1.52 (4.5×10-7) 1.50 (4.4×10-6) 1.39 (3.0×10-3) |
1.91 (1.7×10-8) 1.76 (2.0×10-8) 1.74 (2.5×10-8) 1.62 (3.1×10-8) |
|
Thermal conductivity (W/m.K) 30 K 300 K |
0.2 0.9 |
10000 40 |
| Melting point (K) | 950 | 2300 |
| Specific heat capacity (J/K.kg) | 870 | 750 |
Chemical Properties
Sapphire is a crystalline form of aluminum oxide (Al2O3). It is formed of Al3+ cations and O2- anions arranged in a hexagonal lattice. It is extremely unreactive and chemically-resistant to acids and alkalis, including hydrofluoric acid.
Mechanical Properties
Sapphire is exceptionally hard with a Mohs hardness of 9, second only to diamond (which has a hardness of 10). For comparison, glass has a hardness of ~5.5. This makes it extremely scratch-resistant.
Optical Properties
Sapphire is birefringent, meaning that its refractive index depends on the direction at which the light propagates through the crystal and its polarisation. While birefringence has uses in various optical elements, it is generally undesirable in a substrate used for optical measurements.
To overcome this, our sapphire is cut along the C-plane which eliminates polarisation-dependent birefringence for normally-incident light. Sapphire is transparent to wavelengths of light between 200 nm and 5 µm, making it an excellent choice for UV and near/mid-IR applications. Sapphire has a refractive index of ~1.76 in the visible spectrum.
Below is a comparison of the optical transmission between our sapphire substrates and our quartz-coated glass substrates, showing the superior UV transmission of sapphire. Note: The lower transmission of sapphire in the wavelength range from 350nm - 1000nm is due to its higher real refractive index (n) causing greater reflection of incident light at the air-substrate interface.
Thermal Properties
Sapphire has a high thermal conductivity of ~40 W/m.K at room temperature. This is almost 50 times higher than glass, and twice as high as stainless steel. This value increases to ~10000 W/m.K as temperature is reduced. This makes sapphire highly suitable for low-temperature optical measurements where thermal equilibrium between the sample and cryostat is required. It is also suitable for use in high-temperature environments up to 2300 K. Our sapphire substrates are polished to optical quality, and have an RMS roughness significantly below than that of our glass substrates.