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Growth of High Quality Double-Sided YBCO on MgO Substrates for Microwave Applications

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A customer approached PVD Products to develop a production-oriented tool to deposit high-quality Y1Ba2Cu3O7-δ (YBCO) thin films onto MgO substrates for microwave applications, specifically for microwave filters for cellular phone applications. Our team had to demonstrate our technology's capabilities that would produce the desired quality of the films. Films with a minimum quality as noted below had to be provided: 

  • Jc> 2 MA/cm2 @ 77K and 600 nm
  • Tc> 88K
  • Rs< 0.5 mΩ @10 GHz and 77 K
  • Thickness Non-Uniformity ± 2.5%

PVD Products used a PLD 3000 system with a unique black body heater that provides excellent uniformity of substrate temperature over 75 mm diameter wafers. In addition, using a large-diameter stoichiometric target and laser beam rastering provides highly uniform film thickness. The real trick, though, is to find the proper growth temperature for the transparent oxide substrates. After four runs at various growth temperatures, our team ended up producing a wafer with a double-sided film.

It is known that growing YBCO directly on MgO is difficult due to the large (~9%) lattice mismatch. However, MgO has excellent microwave properties and, under the correct conditions, very high-quality YBCO films can be deposited. 

The substrate had an outer diameter of 50.4 mm and was 0.5 mm thick. The YBCO films’ coatings were nominally 585 nm ± 20 nm thick. To determine the quality and uniformity of the superconducting films, the uniformity of the critical current density Jc and microwave surface resistance were evaluated. XRD rocking curves were also obtained from both sides of the wafer.

To test coating uniformity, Jc was measured on both sides of the substrate at a temperature of 77 K and atmospheric pressure using a mutual inductance measurement system. The localized current density for both sides was plotted and shown in Figures 1 and 2. Figure 1 is the plot of the film that was grown first (the substrate was flipped over and the film placed back into the chamber for the second film growth) and Figure 2 is the plot for the second YBCO film growth.

For each film, the current density was between 4.5 and 5 MA/cm2. The calculations are based on a film thickness of 500 nm, so the actual Jc values are ~15% lower than those indicated—or actually 4 MA/cm2 to 4.5 MA/cm2. Regardless, the Jc values are very high for large-area superconductors deposited directly on MgO, as noted in the figures, and the critical current on both sides of the wafer is uniform.

A second test of the same films measured the microwave surface resistance, Rs, at three locations across the wafer. The measurement frequency was 11.911 GHz using the films as part of a resonant circuit. The average Rsnormalized to 10 GHz was found to be 0.231 ± 1.9% for side 1 and 2.33 ± 2.2% for side 2. These are exceedingly low Rs values for YBCO films on MgO or other substrates. 

A third test determined the crystalline nature of the thin films by measuring the Full Width Half Max (FWHM) of the (005) peak from standard Ø - 2Ø scans via x-ray diffraction (XRD). For the film deposited on the side 1, the FWHM of the rocking curve was Δω =0.420°, and for the film deposited on the side 2, the FWHM was found to be Δω = 0.366°.

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Figure 3: FWHM rocking curve from the (005) peak of side 1

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Figure 4: FWHM rocking curve from the (005) peak of side 2

The high quality and uniformity of these films are due to the excellent temperature uniformity provided by the black body heater found in our larger-area PLD systems. 

Our team at PVD Products is the best of the best for standard and custom thin film deposition systems, helping to get your ducks in a row at every phase of your project. Interested in collaborating? Send us your specs to get started.

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Topics: Pulsed Laser Deposition

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