Substrates
The properties of the substrate material are significant for most HTS applications. For example, the performance of a RF filter strongly depends on the dielectric constant and the losses at high frequency.
In below table we summarized the most important parameters of commonly available substrate materials used for HTS film deposition.
If you are not sure which substrate is best for your application, simply ask us!
Properties of common substrate materials
| Substrate Material | Maximum Size | Buffer layer | Dielectric constant | tan delta | Max film thickness |
| MgO | 3" | none | 9,7 |
2×10-6 |
3 µm |
| Sapphire (r-cut) | 200 mm | CeO2 | 11,6/9,4 | <10-6 | 350 nm |
| LaAlO3 | 3" | CeO2 | 23,6 | 10-5 | 1 µm |
| YSZ (ZrO2:Y2O3) | 2" | CeO2 or Y2O3 | 27 | 10-3 | 1 µm |
| SrTiO3 | 1" | CeO2 | 2000 | 10-3 | 1 µm |
| NdGaO3 | 2" | CeO2 | 23 | 3×10-4 | 1 µm |
| YAlO3 | 1" | CeO2 | 15,5 | 2×10-6 | < 500 nm |
| LSAT | 1" | CeO2 | 22,7 | 2×10-4 | 1 µm |
A short guide about substrates for HTS films:
First: there is no perfect substrate for all applications!
Every substrate material has pros and cons and finally it depends on the special application which substrate to choose. 90% of the substrate materials we use are r-cut sapphire, MgO and LaAlO3.
In the following table we summarize some of the most important properties of these substrate materials.
|
|
pros |
cons |
|
MgO |
Isotropic dielectric constant (DC) Low loss (10-6) Thick films (up to 1,5µm) can be grown Narrow thickness tolerance (<5µm) |
Expensive Max size 3 inch diameter |
|
r-cut sapphire |
Large area wafers available Cheaper than MgO and LAO Low loss (10-8) |
Anisotropic DC constant Max film thickness limited to 330 nm due to low TCE |
|
LaAlO3 |
High dielectric constant Low loss (10-6) Thick films (600 nm) can be grown |
DC with local variations due to twinning Medium expensive Max size 3 inch diameter |
MgO is a low loss material with isotropic dielectric constant (DC) and high thermal expansion coefficient. Therefore 600 nm YBCO films (standard thickness for RF devices) can be grown on MgO without problem. Disadvantage is high cost and limited size (up to 3 inch diameter).
LaAlO3 has a DC of 23,7 (compared to approx. 10 in case of MgO and sapphire) and its high thermal expansion coefficient also allows growth of 600 nm YBCO. The high DC can reduce the layout size of RF devices compared to MgO or on sapphire. A problem of LaAlO3 is the twinning of the crystal, which leads to local variations of the dielectric constant that can affect the centre frequency of RF devices1. The available size also is limited to 3 inch diameter.
For the growth of HTS films r-cut sapphire wafers are often used, which have a lattice that allows the epitaxial growth of HTS films. Such r-cut sapphire wafers are commonly available in large size like 200×100 mm and comparably cheap as they are used for other mass applications like LED. Disadvantage is the anisotropic dielectric constant which makes the modelling of RF devices challenging and also the low thermal expansion coefficient (TCE) that reduces the maximum YBCO film thickness to 330 nm because of cracking.
[1] D. Reagor, F. Garzon, Dielectric and optical properties of substrates for high-temperature superconductor films, Appl. Phys. Lett. 58 (1991), 2741
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