Sapphire Substrates


Order Code: S2005A1
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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 two 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.

Properties

Standard

Ultra Smooth

Substrate size 20 mm x 15 mm 20 mm x 15 mm
Thickness 1.1 mm 1.1 mm
Material Synthetic sapphire Synthetic sapphire
Surface finish (scratch-dig) 60/40 (both sides polished) 10/5 (both sides polished)
Surface roughness (RMS) ~0.3 nm ~0.1 nm
Surface orientation C-plane C-plane
Applications UV/optical/NIR spectroscopy Atomic force microscopy, 2D material substrate

sapphire substrate AFM
Atomic force microscope images of the surface of sapphire substrates. Left: 'Standard' surface finish (scratch-dig 60/40). Right: 'Ultra Smooth' surface finish (scratch-dig 10/5).

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 aluminium 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.

 

sapphire crystal structure planes
The crystal structure of sapphire (left) has numerous symmetry planes (right), along which the properties of the material differ slightly.

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.

sapphire optical transmission spectrum
Comparison between the optical transmission of a sapphire substrate (blue line) and a quartz-coated substrate (grey line).

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.