Solution Deposition Planarization as an Alternative to Electro-Mechanical Polishing for HTS Coated-Conducters

Abstract

Mechanically flexible substrates are increasingly utilized in electronics and advanced energy technologies like solar cells and high-temperature superconducting coated conductors (HTS-CCs). These substrates offer advantages, such as large surface areas and reduced manufacturing costs through reel-to-reel processing, but often lack the surface smoothness needed for optimal performance. For HTS-CCs, specific orientation and high crystalline quality are essential, requiring buffer layers to prepare the amorphous substrate for superconductor deposition. Techniques, such as mechanical polishing, electropolishing, and chemical-mechanical polishing, can help achieve an optimally levelled surface suitable for the subsequent steps of sputtering and ion-beam-assisted deposition (IBAD) necessary for texturing. This review examines Solution Deposition Planarization (SDP) as a cost-effective alternative to traditional electro-mechanical polishing for HTS coated conductors. SDP achieves surface roughness levels below 1 nm through multiple oxide layer coatings, offering reduced production costs. Comparative studies demonstrate planarization efficiencies of up to 20%. Ongoing research aims to enhance SDP’s efficiency for industrial applications in CC production.

1. Introduction

Mechanically flexible substrates for thin films are increasingly being used in electronic applications, such as displays and printed circuit boards, as well as in advanced energy technologies like solar cells, batteries, and high-temperature superconducting coated conductors (HTS-CCs) [1,2,3,4,5,6,7,8,9,10,11,12,13]. These metallic flexible substrates provide significant benefits with respect to single crystals/wafers, including large surface areas within compact volumes and weights, diverse form factors, and, crucially, a reduction in manufacturing costs when materials are processed using reel-to-reel techniques. However, they often lack the surface smoothness necessary for optimal device performance, which can hinder the realization of these cost benefits. Various techniques can be employed to achieve a planar or levelled surface, including mechanical polishing, electropolishing, and chemical mechanical polishing.

For HTS-CC applications, the situation is complicated further by the necessity of depositing superconductors with a specific orientation and high crystalline quality that cannot be achieved directly on the amorphous metal [13,14,15,16,17,18,19,20,21,22,23,24,25]. This is why the CC architecture requires several buffer layers of progressively increased flatness/texture to reach the adequate conditions for the deposition of the superconductor (see Figure 1). The standard procedure for HTS-CC preparation requires the polishing of the substrate surface with either mechanical polishing or electropolishing (or both) [26,27] and then the deposition of amorphous layers of Al₂O₃ or Y₂O₃ via sputtering to act as diffusion barrier from the substrate. The obtained template must display very low values of roughness (below 1 nm), since the following ion-beam-assisted deposition (IBAD) texturing process is under kinetic control and any defect, such as scratches or holes on the surface, would hinder ion diffusion. For IBAD texturing, MgO is used due to the material’s fast biaxial texturing capability. Indeed, only 10 nm-thick MgO is required to obtain a 7° in-plane orientation [28,29,30,31]. On top of IBAD-MgO, other epitaxial layers are deposited before growing the superconductor (See Figure 1). The main disadvantage of this standard planarization approach is that it is a two-step, multi-technique procedure, each with its own apparatus, and also the high amount of hazardous chemical waste produced in the electropolishing step.

An alternative to this multi-technique method is represented by solution deposition planarization (SDP), in which the amorphous layer is deposited via chemical solution deposition (CSD) directly on the metallic template (See Figure 1). The obtained layer(s) would serve both as a planarization layer and as a barrier against diffusion, substituting the polishing + oxide sputtering processes. SDP has the overall advantages of reducing substrate polishing costs and minimizing apparatus costs. The principle behind SDP is that, upon coating with a liquid solution, the surface tension of the liquid planarizes the rough surface, resulting in thicker regions over valleys and thinner regions over peaks (Figure 2) [32]. After drying and pyrolysis, the coating shrinks following the original substrate contours, with a decrease in roughness compared to the underlying substrate. By repeating this process a number of times, further reduction in surface roughness, R_rms, can be obtained. More in detail, in order to meet IBAD surface requirements, the number of solution coatings needed is a function of starting substrate roughness, solution shrinkage rate, and the surface tension of the metal compound solution. The main drawback of this technique is that the efficiency of planarization is low, with many coatings required to obtain R_rms < 1 nm. However, many groups have devoted much effort to the implementation of this technique and to demonstrate its applicability to the CC production industry. In the following paragraphs, general considerations of the techniques are given before the systematic review of the literature on the topic.

2. General Considerations

The substrate–In the SDP-related literature, the most widely used substrate is Hastelloy C276, an alloy of Ni, Mo, and Cr currently employed in the coated conductor industry. The templates used in the SDP studies are both polished and unpolished. Unpolished substrates show R_rms of tens of nm (20–50 nm on a scale of 10–50 μm in [1,3,4,5,11,28,33,34,35,36]), whereas polished substrates show R_rms < 10 nm on the same scale [6,37,38,39,40,41,42,43]. The polished substrates are prepared via electropolishing in most of the examples; some works make use of partially polished substrates, showing that it is possible to further decrease the roughness values with SDP [32,38,42,44]. Another possible substrate is SUS304, a stainless steel containing Ni and Cr. Only unpolished SUS304 is employed in the referenced papers [3,35,45,46], with high values of roughness reaching 50 nm on a 10 μm × 10 μm scale. The goal of the research on SDP is to obtain R_rms values below 1 nm, that is the threshold value for IBAD-MgO successful deposition. The main issues related to metallic substrates are surface roughness, of course, that is scale-dependent due to the undulations of the tape, and the diffusion of ions from the upper layers during thermal treatments, which makes the presence of buffer layers necessary. In isolated cases, buffered templates were used to test the SDP technology, such as alternating-ion-beam-assisted, yttrium-doped ZrO₂ on stainless steel (^ABADYSZ-SS) or Al₂O₃-buffered Hastelloy. The literature review reveals that SDP was studied both at a laboratory scale and a larger scale.

The coating oxide–SDP makes use of amorphous oxide layers to achieve two goals: avoid diffusion from the substrate and reduce the template roughness. Y₂O₃ is the most used oxide for SDP due to its compatibility with the metallic template [35]. However, its tendency to recrystallize at temperatures close to those necessary for pyrolysis of the precursor solution leads to some difficulties in developing the optimal heat treatment. Indeed, the treatment temperature is defined by a delicate balance between the decomposition of the organic precursors, whose residues are detrimental to the final film morphology, and oxide recrystallization, which causes grain coarsening and leads to final rougher films. Moreover, its weak bonding energy with MgO can possibly lead to delamination of the CC architecture [42] and mixed oxides such as Y₂O₃-Al₂O₃ (in various proportions [11,32,39]) or Y₂O₃-ZrO₂ [44] can be used to avoid the recrystallization during thermal treatment. Al₂O₃ was found to be a convenient alternative to make up for the high shrinkage percentage of Y₂O₃ solutions [41]. Other materials studied for this purpose are Gd-doped ZrO₂ and 5% Zr-doped CeO₂ [6,41,42].

The coating technique–As already mentioned, the SDP effectiveness has already been demonstrated in industrially relevant environments. Apart from a few studies that deal with specific issues and rely on spin-coating, a large part of the reviewed literature uses dip coating for short-length samples (tens of centimetres) or reel-to-reel dip coating for longer samples (tens of metres). The optimization of the deposition speed, so as to promote deposition uniformity, maximize thickness, and so on, was performed in [3]. There is general agreement on the fact that medium deposition speeds offer the best balance between thickness and edge cracking. An isolated yet successful example of SDP with inkjet printing is given in [43], together with the optimization of the inks for different types of substrates. A schematic representation of the three different techniques is shown in Figure 3.

The chemical technique–Several solution deposition techniques were studied for SDP. Metal–Organic Decomposition (MOD) and its variations are the most common. MOD has the advantage of being well-known from the studies available on chemical deposition of YBCO [15,18,47]. The most widely used precursor (for Y₂O₃) SDP is yttrium acetate Y(CH₃COO)₃·4H₂O, the source of Y³⁺ in all low-fluorine and fluorine-free derivatives of MOD YBCO preparation. The thermal decomposition behaviour of this salt is presented in [37], which is essential knowledge when designing the thermal treatment for SDP. It shows two principal mass loss events, one up to 120 °C due to loss of water, and one up to 450 °C related to the decomposition of the acetate via the two-step process.

Y(CH₃COO)₃·4H₂O → Y(CH₃COO)₃ + 4H₂O

2Y(CH₃COO)₃ + 12O₂ → Y₂(CO₃)₃ + 9CO₂ + 9H₂O

Y₂(CO₃)₃ → Y₂O₃ +CO₂

It goes without saying that the decomposition of a MOD precursor solution is complicated by the presence of the solvents and their evaporation. Y(CH₃COO)₃ is insoluble in alcohol; therefore, organic acids must be added to the solution. Even when complete dissolution was obtained, these solutions were not stable for long, leading to the precipitation of salts coming from the hydrolysis and trans-esterification reactions. Sometimes, acetylacetonates are used since they are known to be more stable regarding hydrolysis. To improve solution stability, the use of additives is a common strategy. Diethanolamine (HN(CH₂CH₂OH)₂) is by far the most common, but diethylentriamine ((NH₂CH₂CH₂)₂NH) was also reported [35]. Additives are molecules whose free electron pairs on the nitrogen atoms and can provide additional coordination to the Y³⁺ (or other Mⁿ⁺ metal) atoms with respect to the acetate ligands in the solution, keeping the metal ions from precipitating. The MOD technique applies to the growth of other oxides via the same pathway, starting from different metalorganic precursors such as gadolinium acetate Gd(CH₃CO₂)₃·xH₂O, zirconium(IV) acetylacetonate Zr(C₅H₇O₂)₄ and cerium acetate Ce(CH₃CO₂)₃·xH₂O (for Gd-Zr-O and Zr-doped CeO₂). In some cases, mixed precursors, both organic and inorganic, are used, for example, when Al₂O₃ is grown from Al(NO₃)₃. Less frequently the sol–gel technique is used, where the precursors used are not carboxylic salts as in MOD but metal alkoxides [35,48]. The main differences with MOD are the decomposition pathway and byproducts of the organic part during pyrolysis. The advantage of this approach is that it does not require any distillation step, which is common in the MOD technique. The prepared solutions are also more stable to hydrolysis with respect to MOD solutions, but they offer no advantage in terms of efficiency. Once the composition of the solution is defined, another parameter to evaluate is the solution concentration. For the goal of planarizing a rough substrate, a thick layer of amorphous material needs to be deposited. Higher solution concentrations lead to thicker films, but, as shown in the following paragraphs, it was experimentally proven that the direct correlation is not only with thickness but also with the increase in surface roughness. Therefore, lower solution concentrations are preferred, with the disadvantage of making the deposition of multilayer necessary to reach the desired surface flatness. The concept of multilayer spans from 4 to 30 layers, depending on the deposition method and related parameters, but still, the repetition of deposition/heat treatment cycles makes the whole SDP process very inefficient. The origin of this resides in the high shrinkage percentage of the deposited films: indeed, a precursor solution with large carboxylate molecules will suffer a severe mass loss after the decomposition of the organic part. Moreover, low molarity solutions will leave, after solvent evaporation, an even lower amount of material on the template surface to fill its natural asperities. It was already mentioned that higher concentrations are not a viable option, even when processing parameters, such as heating ramps, that are known to strongly influence the final film morphology, are carefully optimized. Therefore, multilayers seem to be the only possible choice.

A completely different approach to overcome this problem was successfully proposed in [38] and studied further in [49], named colloid-solution deposition planarization (CSDP), where the first layer of SDP is performed with a Y₂O₃ particle dispersion. When the particles are of the right dimension, they can drastically reduce the surface roughness of the template with only one coating. Further refinement of the surface is performed with a standard MOD solution. Thus, the total number of coatings is reduced and the efficiency of the technique, expressed as $(\%R_{rms}^{reduction}/Coatinglayers)$ with %$R_{rms}^{reduction}$ defined as $1-(R_{rms}^{final}/R_{rms}^{initial}\times 100)$, from the few units of the other referenced papers [1,4,5,28,33,34,35,36,37] to more than 20%, also on long-length samples (1 m). A summary of the chemical techniques’ main characteristics is presented in

Table 1. Main characteristics of the SDP chemical techniques.

-	MOD	Sol–Gel	CSDP
Precursors	Metal carboxilates	Metal alkoxides	Oxide nanoparticles dispersions
Processes during thermal treatments	Pyrolisys of organic components and formation of the oxide	Pyrolisys of organic components and formation of the oxide	Evaporation of solvent
Advantages	Well known chemistry of the solution	No distillation required Solutions more stable than MOD	High planarization efficiency in a single step
Disadvantages	Solution instability Low planarization efficiency	Low planarization efficiency	Further planarization via MOD is required to reach Rrms < 1nm

.

The thermal treatment–The general thermal treatment for chemical solution deposition processes consists of two steps: decomposition of the organic part and sintering of the oxide. The optimal sintering temperature is a trade-off between the carbon residue and surface smoothness. Carbon residues, detrimental for the growth of the following layers, are eliminated with high-temperature treatments, but low temperatures are required to obtain an amorphous layer. No matter how the precursor solution is prepared, if it contains organic precursors, it will manifest problems related to carbon residues in the film [48], especially when the thermal processing requires low temperatures (T < 600 °C) [50] to avoid crystallization of the oxides; therefore, these two processes are in competition. Moreover, inert atmospheres can be useful to shift the crystallization of the oxide to higher temperatures, but they also cause significant accumulation of carbon in the final film. To complicate matters further, it was shown that the heating ramps and the sintering temperature are also related to final smoothness and IBAD layer texture (see Table 2), edge cracking, that is negligible at a low temperature [49], and delamination strength, that increases at higher sintering temperatures [43]. Again, a delicate balance between positive and negative temperature-induced effects must be found.

3. Review of the Literature

In the following paragraphs, the available literature on SDP is reviewed. To simplify the reading, the literature is classified according to the oxide chosen for planarization, starting from the most common Y₂O₃. The results are summarized in Table 3.

3.1. Planarization with Y₂O₃

Wen et al. [37] used the MOD precursor solution with additives to coat laboratory scale samples (1 cm²) of an electropolished Hastelloy substrate, with an initial roughness of 8 nm. The precursor solution concentration varies from 0.05 to 0.4 M. They investigated, with XRD, the effect of the heat treatment temperature, showing that amorphous Y₂O₃ is obtained at 470, 500, and 530 °C. Partial crystallization of yttria is instead observed at 560 and 590 °C. The effect of temperature on final film roughness for a 3-times spin-coated 0.4 M solution is investigated via AFM, and the result is in accordance with the XRD results, with the lowest roughness obtained at the maximum temperature before crystallization (530 °C). Also, the optimization of the rotation speed during spin coating was performed, evaluating the roughness of the final films, with the identification of 3000 rpm for 30 s as the best option. With regard to the effect of solution concentration and number of coatings, the average roughness values were decreased with an increased number of coatings and with low solution concentrations. The lowest value of 0.65 nm was obtained for 6 layers of 0.1 M solution, deposited at 3000 rpm for 30 s, and treated at 530 °C, with a planarization efficiency of 15%.

In a similar way, MOD Y₂O₃ without additives was used for the planarization of 50 m Hastelloy substrates in [29]. After 15 SDP coating, the roughness value on the 5 μm × 5 μm scale goes from 50 nm to 1.2 nm (efficiency 6.5%). The subsequent deposition of IBAD-MgO and epi-MgO demonstrated the efficiency of such templates, with MgO XRD analysis throughout the length of the tape showing FWHM values of (110) Δϕ and (200) Δω in the range of 5.5°–6.0° and 3.0°–3.5°, respectively. In addition, the effect of the thickness of the epi-MgO layer on its surface roughness was presented, and a direct relationship between thickness and roughness is shown. On a 50 nm epi-MgO film, an excellent LMO film was sputtered, with the FWHM values of (110) Δϕ and (200) Δω being 3.7° and 6.8°, respectively. It should be pointed out that after MgO and LMO deposition, partial recrystallization of yttria occurs, but it does not seem to influence the quality of the final LMO layer. Such templates were used in [34], for the deposition of YBCO via MOCVD via reel-to-reel dip coating, with a resulting J_c = 1.8 MA/cm².

In [5] SPD is applied to long-length IBAD templates (about 5 m long) using MOD-derived amorphous Y₂O₃ deposited with a continuous tape loop coater. Diethanolamine was used in the precursor solution to enhance its stability. The effect of two parameters on the final film roughness was analyzed, namely the precursor solution concentration (0.08 M and 0.4 M) and the number of coatings (1–30). They found out that coating with a combination of the two solutions led to the best results in terms of roughness reduction, from the starting R_rms = 21 nm (5 μm × 5 μm scale) to the final R_rms = 0.5 nm (5 μm × 5 μm scale) after 15 coatings with the 0.4 M solution and 15 coatings with the 0.08 M solution, with a final planarization efficiency of 3% (Figure 4a,b). This template was used for the deposition of IBAD-MgO, that displayed goon in-plane and out-of-plane texture (FWHM_o-o-p$~$2°, FWHM_i-p$~$4°). YBCO deposition was carried out via both PLD and RCE. In the first case, with the aid of a SrTiO₃ buffer layer, a J_c = 2.85 MA/cm² is obtained; in the second case, without any additional buffer layer, a J_c = 4 MA/cm² is obtained. Higher solution concentrations (0.1 up to 0.6 M) were studied in [1]. The results obtained with low molarities are comparable to [5], whereas the use of 0.6 M does not give any advantage in terms of a possible reduction in the number of coatings, while it causes the formation of cracks randomly distributed on the surface of the amorphous Y₂O₃ layer. Comparable results are obtained by Nie et al. in [4].

The same system (long-length Hastelloy, MOD Y₂O₃ + additives, and the continuous dip coating) was studied in [3]. In this case, the effect of deposition speed on the single coating uniformity of the solution was analyzed by means of optical microscopy focused on the edges of the tape. The speed was varied between 30 and 70 cm/min, and the best uniformity was obtained at the intermediate speed of 60 cm/min. Using this reference speed, the number of coatings for optimal planarization was studied, the value of R_rms = 0.573 nm (5 μm × 5 μm scale) was obtained, and homogeneous values of R_rms were measured along the whole length of the tape (Figure 5a). This value, together with an initial R_rms = 11.74 nm, results in an 11% efficiency. On such samples, IBAD-MgO and LMO were then deposited. Surprisingly, the samples with lower R_rms did not lead to the best texture of the superior epi-MgO and LMO layers. Indeed, the lowest values of FWHM_o-o-p = 5.83° were obtained for the 16-coating sample. This phenomenon is in contrast with what was observed in [5], where R_rms and FWHM_o-o-p showed the same behaviour vs. the number of coatings. This template was used for YBCO deposition via MOD. The superconducting critical currents at 77 K in self-field were in the range of 80–180 A (Figure 5b).

In [34], the previously discussed system is directly compared with other planarization methods (i.e., electropolishing and mechanical polishing) using the same experimental setup. No information on the solution concentration is given, but the precursor salts and additives are the same. The solvent is a mix of ethanol and water, instead of methanol. This difference, nor the unknown solution concentration seem to lead to significantly different results, with the R_rms that decreases to 5% of the initial value (1 nm) after 16 coatings, a value comparable to those previously mentioned, with an overall efficiency of 6%. The conclusion of this work is that all three methods were demonstrated to be good processing techniques for planarization, with the main advantage of SPD of providing simultaneously a nucleation layer for MgO.

SDP with MOD Y₂O₃ was used to planarize also the intermediate layers in a stack with the following architecture Y₂O₃^CSD/MgO^IBAD/YBCO(1)/Ag/Y₂O₃^CSD/MgO^IBAD/YBCO(2)/Ag [36]. The idea behind this model is that of increasing the I_c of the coated conductor by stacking two YBCO films, with the final current density being the sum of the individual ones. The advantage of using Y₂O₃ SDP on top of the intermediate layer of Ag is that it resets the crystalline structure of the system before another YBCO deposition, thus making it independent of any defect present in the previous one and from its roughness (see Figure 6). The planarization process was successful both on the bare Hastelloy (efficiency 6.4%) and on the intermediate Ag layer, with the obtained R_rms < 1 nm. The final stacks of well-textured YBCO layers showed a total I_c of 725 A/cm on a 7 cm long sample.

The possibility of using SPD with Y₂O₃ films on stainless steel (SS) was studied in [3,45]. In [3], the SUS304 substrate was used to perform the same study carried out by the group for Hastelloy presented before. For SUS304, the optimal dip coating speed was found to be 50 cm/min, and the number of coatings for a final film roughness R_rms = 0.9 nm was 12. The initial substrate roughness was R_rms = 20.93 nm, which gave a final efficiency of 8%. On such samples, the full SUS304/Y₂O₃^CSD/MgO^IBAD/MgO^EPI/LMO/YBCO^MOD was realized, and the resulting transport properties of the tape are promising (Figure 7a). Vilardell et al. [45], instead, used SS substrates (both polished and unpolished), and inkjet printing as a deposition technique. The preparation of the ink required an adjustment of the standard MOD precursor solution, not only through the use of additives, but also of a UV varnish. The ink’s properties were thoroughly analyzed and optimized for deposition on both unpolished and polished samples, and for long-length unpolished samples (100 m). With inkjet printing, only one step of coating is required to reduce the roughness enough to deposit YSZ^ABAD with a 66% efficiency. On short samples (approx. 1 m) it was possible to prepare the full SS/Y₂O₃^CSD/YSZ^ABAD/CeO₂ ^PLD/YBCO^PLD stack and measure its properties. The optimal texture was obtained for the YSZ layer, but rather low values of I_c were obtained on both polished and unpolished substrates (Figure 7b).

The use of yttrium acetate precursor in the standard MOD procedure requires, as already mentioned, the use of additives to avoid the precipitation of unwanted byproducts of the hydrolysis in the solution. Another group [35] proposed a mixed alkoxide precursor solution as an alternative, in which the metal ions from different precursors are kept in the solution using chelating agents (Polyethylenglycole, diethylentriamine). Thermogravimetric analysis of such solutions is provided. The SDP was carried out on both Hastelloy and SUS304 substrates with good results in terms of final film roughness. Highly aligned IBAD-MgO layers were successfully grown on these samples, which resulted in high I_c GdBCO films (300–420 A/cm). The planarization efficiencies are slightly above those of standard MOD (4.5% on Hastelloy and 6.5% on SUS304). However, information regarding the length of the deposited samples is lacking, as well as further studies on the applicability of this technique for scale-up.

Another different but more promising approach was used in [38,49] where a colloidal solution of Y₂O₃ nanoparticles was used to planarize laboratory scale samples of Hastelloy, also in combination with the MOD technique. The idea was to make up for the lack of efficiency of SDP with standard MOD, that requires many coatings steps to obtain the desired R_rms values. The use of a colloidal solution is considered a faster planarization process, since the particles fill the macroscopic defects of the substrate in one single coating, thanks to the lower shrinkage percentage of the colloid deposition layer. Afterwards, another few steps with MOD lead to further refinement of the surface in [38], with the reduction of R_rms of mechanically polished Hastelloy from 4.0 nm to 0.3 (Figure 8) with an efficiency of 24%. Moreover, the RHEED pattern of IBAD-MgO on Hastelloy substrate with such coating indicates that the MgO layer was monocrystalline, confirming the efficiency of this technique. This work on colloidal solutions was taken further by Wang et al. [49], who investigated the effect of various parameters, such as Y₂O₃ concentration, dip coating pulling rate, viscosity, and heat treatment temperature on the edge cracking phenomenon on long-length Hastelloy. They also provide TGA analysis of the Y₂O₃ colloidal solution, that identifies the temperature where the unwanted crystallization of Y₂O₃ starts. The planarization efficiencies for the samples prepared with Y₂O₃ dispersions are, also in this case, far higher than for standard MOD (22.5%). This makes the technique very promising for low-cost production of HTS-CCs.

3.2. Planarization with Y₂O₃ + M_xO_y

One of the main issues connected to the use of pure Y₂O₃ in SDP is related to its recrystallization during thermal treatment, which can cause an increase in surface roughness. For this purpose, mixed oxides can be used as an alternative [33]. Kim et al. [44] used a mix of Y₂O₃ and ZrO₂ with the MOD + additives technique. Two solution concentrations were studied (0.1 M and 0.2 M), and the substrate surface roughness was significantly reduced (from 50 nm to 3.8 nm) after 20 dip coatings with the 0.1 M solution. Good in-plane and out-of-plane textures of the IBAD-MgO and LMO layers are observed, and the final GbBCO film shows a critical current of I_c = 400 A/cm (77k, self-field).

Xue and co-workers, instead, focused on the Y₂O₃-Al₂O₃ (YAlO) composite for SDP. In [11,32], they prepared both the templates and the full YBCO-CC stack using SDP. The precursor solution was prepared with mixed organic/inorganic precursors, and their decomposition behaviour was analyzed. The Y:Al ratio effect on the surface of the SDP layer and on the texture of the MgO layer was analyzed, and the optimal value was found to be 9:1. The efficiency of planarization in this case is 4%. During the deposition of the LMO layer and YBCO layer, partial recrystallization of Y₂O₃ is visible, but optimal properties are obtained for the complete YBCO-CC with J_c (77 K, sf) of 3.2 MA/cm².

More complex compositions were studied by Qiao et al. [39], the details of which were not, unfortunately, disclosed. The solutions were made by dissolving organic yttrium, aluminium, and other metal precursors into a solvent with additives, which adjusted the viscosity of the solution. The efficiency of planarization for this method is high, 19%, and the final Y₂O₃ film roughness is comparable to that of the electropolished Hastelloy. On short planarized samples, the full stack MgO^IBAD/MgO^EPI/LMO/YBCO^MOCVD was deposited, and an I_c of 140 A on a 0.4 μm-thick YBCO was measured, corresponding to a J_c of 3.5 MA/cm². Most importantly, in this case, the technology was also transferred to long-length tapes (20 m) with a uniform I_c of 160 A on 1 μm-thick YBCO (Figure 9).

Matias et al. also attempted to use mixed oxides for SDP, namely Y₂O₃ + 10% Al₂O₃ [51]. On such a sample, the final roughness after 15 coating was excellent (efficiency 6%) and the typical FWHM for the MgO layer on top was 4°–5° in-plane and 1.5° out-of-plane. Reactive co-evaporation of YBCO lead to a 1 μm sample with J_c of 4 MA/cm² (75K, self-field). The magnetic field dependence of J_c for this sample is comparable to the state-of-the-art undoped YBCO, as well as its angular dependence, with the correlated peak parallel to the ab-planes, and a smaller feature at θ = 0 like other undoped films. When 5 μm YBCO is deposited on such a template, an I_c = 600 A/cm is obtained.

3.3. Planarization with Gd-Zr-O

In a series of dedicated works [6,41,42]. Chu et al. thoroughly analyzed the possibility of applying SPD to Gd-doped ZrO₂. This material, having a high melting point, has good barrier performance on IBAD substrates when deposited via sputtering, and it can also serve as the seed layer for MgO. The investigation in [42] started with the thermogravimetric analysis of the MOD precursor solution to identify the right temperature for the heat treatment. Consequently, the effect of the molarity of the solution and the heating ramp rates’ effect on the final film roughness was evaluated, and the combination of 0.1 M and 5 °C/min led to the lowest value of R_rms = 0.4 nm. In the following paper [41], the effect of SDP on two types of metallic templates with different types of defects was studied. More specifically, grain boundaries and particle segregation are present on Hastelloy electro-polished tapes, while rolling marks are mainly visible on the rolled tapes. With SDP, it was possible to planarize the mirror-rolled Hastelloy to reach R_rms = 5 nm on 50 μm × 50 μm with an efficiency of 5%. The IBAD-MgO layer deposited on top of the template displayed a sharp texture, and it was enabled to achieve J_c = 4.17 MA/cm² (at 77 K, self-field) in the YBCO layer. In [6], a study on the optimization of the temperature for Gd-Zr-O deposition is given. Like Y₂O₃, the temperature choice must balance the grain coarsening phenomenon that occurs at high temperatures and the minimization of the carbon residue that increases at low temperatures, which reduces compatibility with the IBAD-MgO layer. Furthermore, the effect of the thickness of the Gd-Zr-O layer on the IBAD-MgO texture was analyzed, changing the precursor solution concentration. It was shown that higher concentrations lead to an IBAD-MgO layer with no texture or weak texture (see Table 2). In conclusion, the TEM cross-section observation shows that the 130 nm-thick Gd-Zr-O monolayer from the 0.8 M solution, under optimal heat-treatment.

Table 2. Summary of the results presented in [6].

Molarity of Soln. [M]	Tsintering [°C]	Heating Ramps [°C/min]	Rrms [nm]	IBAD-MgO Texture
0.8	725	$1–$5	0.74	Strong
0.8	625	$1–$5	0.58	n.d.
0.8	425	$1–$5	0.33	n.d.
0.2	425	>100	0.41	Weak
0.2	325	>100	0.31	Weak
0.2	275	>100	0.29	Weak

Table 2. Summary of the results presented in [6].

Molarity of Soln. [M]	Tsintering [°C]	Heating Ramps [°C/min]	Rrms [nm]	IBAD-MgO Texture
0.8	725	$1–$5	0.74	Strong
0.8	625	$1–$5	0.58	n.d.
0.8	425	$1–$5	0.33	n.d.
0.2	425	>100	0.41	Weak
0.2	325	>100	0.31	Weak
0.2	275	>100	0.29	Weak

Conditions (Figure 10), can serve as the planarization and seed layer for the IBAD-MgO process, even though the final J_c of the YBCO films is low (1–2 MA/cm²). Optimization of the whole technique is, therefore, required to take full advantage of its promising efficiency values (26%). A summary of the results presented in this series of papers is given in Table 2.

In addition, Nakaoka et al. [43] studied Gd-Zr-O as the planarization layer for roughly electropolished Hastelloy. They obtained a reduction of R_rms down to 2 nm with 4–5 dip coatings of a 0.15 M MOD solution. In these conditions, the planarization efficiency reached 9%. Then, IBAD-MgO, LaMnO₃, CeO_2, and the YBCO layer were deposited, the last one via the trifluoroacetate (TFA)-MOD method. Notwithstanding the not-so-low roughness of the chemical oxide layer, the SDP method with Gd-Zr-O proved to work as a planarization layer and diffusion barrier layer. Most importantly, the delamination strength was also evaluated as a function of the sintering temperature, and it was shown that high delamination strength was obtained on those samples prepared at T > 550 °C.

3.4. Planarization with Al₂O₃

Alumina was identified as a possible alternative to Y₂O₃ to make up for the yttria solution’s high shrinkage percentage. Indeed, Al₂O₃ can act as a planarization layer with a lower number of coatings, improving the efficiency of the SDP technique. Paranthaman et al. developed an SDP coating with Al₂O₃ [40,51] using the MOD technique on polished Hastelloy. The samples were deposited via spin coating to obtain a 10–15 nm thick film in a single coat. Multiple depositions and heat treatments were performed, and a final roughness of 2.5 nm is obtained on a 2 μm $\times$ 2 μm scale, with an overall efficiency of 9%. After that, a Y₂O₃ nucleation layer followed by MgO (IBAD + Epi), LMO, and YBCO was deposited. The stack shows a self-field J_c of 3.04 MA/cm² at 77 K with excellent in-field J_c properties, which were induced by the presence of BZO pinning centres. The efficiency of Al₂O₃ as a barrier against diffusion is proven by TEM imaging since the metal elements Ni, Mn, and Cr can only be found below the Al₂O₃ layer. However, the need for a sputtered layer of Y₂O₃ makes the use of CSD less advantageous than other proposed methodologies.

3.5. Planarization with 5% Zr-CeO₂

Vlad et al. in [46] used 5% Zr-CeO₂ (CZO) for the planarization of ^ABADYSZ-SS substrate templates. In this case, the starting roughness of the template is low (1.8 nm), but the flat area fraction, defined as a deviation of less than 1.8 nm of the mean surface roughness value, is only 49%. After the deposition of a CZO layer, the overall R_rms was increased up to 3 nm, but the flatness obtained was 75%. The increase in roughness is probably due to the tendency of CZO to form a terraced surface. However, YBCO was successfully grown on top of CZO, with only a limited interfacial reaction, leading to the formation of Ba(Ce,Zr)O₃. Transport properties of the SS/YSZ^ABAD/CZO^CSD/YBCO^CSD coated conductor were analyzed, and, despite a pronounced J_c decrease at low fields, the CC performed as well as the same sample grown on a single crystal. This behaviour at μ₀H < 1.5 T is due to the higher granularity of the sample, which reflects the granularity of the substrate. These works show the possibility of eliminating the IBAD-MgO layer and additional buffers (epi-MgO, LMO…) using chemical techniques for both the CZO and the YBCO layer. In [21], these substrates are used for the growth of YBCO via the MOD technique and inkjet printing. The final CC shows J_c(77 K, sf) = 1 MA/cm², a very porous microstructure, and the presence of secondary BaO and Y₂Cu₂O₅ phases, as shown by STEM images (See Figure 11).

4. Conclusions and Prospects

SDP was proven to be a cost-effective way of reducing surface roughness in metallic flexible substrates. If we consider its application to HTS-CCs, it becomes clear how it can simplify the first stages of CC preparation by fulfilling both the task of planarization and protection from ion diffusion. The necessity of repeating the coating-drying-pyrolysis step several times does not seem to be a limiting factor since the cost, in terms of time and equipment, is probably still lower than the electropolishing + sputtering apparatus. Moreover, compared to electropolishing, SDP does not require the use of large quantities of concentrated acid solutions, with all the related issues (safety hazard, disposal…). If we compare the results presented in this review with those obtained for systems in which the standard sputtering planarization is used (Table 4), it is clear how they are perfectly comparable. However, the efficiency of the planarization process is still low, and even though some strategies have been proposed to overcome this problem, this remains the main objective of future research in this field. The most obvious idea is to start from a partially polished substrate, either mechanically or electropolished. Thus, the sputtering step is eliminated, and the number of coatings needed to reach R_rms < 1 can be lowered. On the other hand, the possibility of significantly lowering the SDP layers was demonstrated starting from the Y₂O₃ dispersion proposed in [39,50]; that is a strategy worth exploring further, not only with Y₂O₃ but also with other compatible materials less prone to shrinkage during sintering. Also, the use of technologically advanced deposition techniques, such as inkjet printing, can prove to be a successful way of integrating SDP in the CC production process, as preliminarily demonstrated in [21,46], due to the possibility to closely control the amount of deposited material and to tune the ink’s properties according to need. In the attempt to develop a new SDP process, these mentioned variations could represent a good starting point. With regard to the coating oxide, Al₂O₃, Gd-Zr-O, and mixed Y₂O₃-Al₂O₃ show higher efficiencies due to lower shrinkage rate, and their precursor solutions are more stable than those of Y₂O₃. The possibility of using more viscous solutions for deposition techniques, such as dip coating using additives with low boiling points, could be worth investigating so as to deposit more solution in a single coating on the substrate without changing the solution concentration, thus obtaining thicker films that were proven to be highly efficient in reducing surface roughness. Further refinement of the surface can be carried out with lower molarity solutions. The thermal treatment temperature should be optimized to minimize both carbon residue and oxide crystallization; heating ramps should allow for a slow removal of the solvents/evolution of gasses that would not damage the film morphology; also, cooling ramps should be controlled so as to control strain accumulation due to different thermal expansion coefficients between oxide and metal, and to avoid the formation of cracks. With respect to what has already been achieved, there is probably still room for improvement.

All in all, with so many parameters to possibly optimize, SDP holds the promise of being a beneficial integration component in the CC production process. It cannot be considered a game changer, but it can be beneficial in terms of cost reduction and production speed. Indeed, should it substitute both the polishing and sputtering steps, the capital cost and operating cost for HTS CC planarization would be significantly reduced. However, this goal can only be achieved if it really becomes competitive with the standard procedure, that is to say if the efficiency of the technique is significantly increased.

Table 3. Summary of the results presented in the literature.

Ref	Oxide	Substrate	Precursor	Deposition	Layers (Tot Thickness)	Roughness R (Scale) $(\mathit{\%}{\mathit{R}}_{\mathit{r}\mathit{m}\mathit{s}}^{\mathit{r}\mathit{e}\mathit{d}\mathit{u}\mathit{c}\mathit{t}\mathit{i}\mathit{o}\mathit{n}}$)	Final Structure	Efficiency $\frac{{\mathit{R}}_{\mathit{r}\mathit{e}\mathit{d}\mathit{u}\mathit{c}\mathit{t}\mathit{i}\mathit{o}\mathit{n}}\mathit{\%}}{{\mathit{n}}_{\mathit{c}\mathit{o}\mathit{a}\mathit{t}\mathit{i}\mathit{n}\mathit{g}\mathit{s}}}$	Length	Jc/Ic (77K, sf)
[5]	Y2O3	Hastelloy (Unpolished)	Metal–Organic + additives	Continuous Dip coating	30 (~1 μm)	<1 nm (5 μm × 5 μm) (97.6%)	Hastelloy/Y2O3CSD/MgOIBAD/YBCOPLD,RCS	3%	5 m	Jc = 2.85–4 MA/cm2
[45]	Y2O3	YZSABAD/SS (unpolished)	Metal–organic + additives (TEA) + UV Varnish	R2R inkjet	1 (90–100 nm)	5.4 nm (5 μm × 5 μm) (66%)	SS/Y2O3CSD/YSZABAD/CeO2PLD/YBCOPLD	66%		Ic = 36 A
[3]	Y2O3	Hastelloy (unpolished)	Metal–organic + additives	Continuous Dip coating	16	0.788 nm (5 μm × 5 μm) (95%)	Hastelloy/Y2O3CSD/MgOIBAD/MgOEPI/LMO/YBCOMOD	11%	10 m	Ic = 80–180 A
		SUS 304 (unpolished)			12	0.903 nm (5 μm × 5 μm) (95.7%)	SUS304/Y2O3CSD/MgOIBAD/MgOEPI/LMO/YBCOMOD	8%		Ic = 100–150 A
[28,33]	Y2O3	Hastelloy (unpolished)	MOD	Continuous Dip coating	15 (840 nm)	1.2 nm (5 μm × 5 μm) (97%)	Hastelloy/Y2O3CSD/MgOIBAD/MgOEPI/LMO	6.5%	50 m	Jc = 1.8 MA/cm2
[1]	Y2O3	Hastelloy (unpolished)	MOD + additives	R2R dip coating	30 (0.5–1.2 μm)	1.2 nm (5 μm × 5 μm) (95.9%)	Hastelloy/Y2O3CSD	3%	n.d	n.d
[37]	Y2O3	Hastelloy (e-polished)	MOD + additives	Spin coating	6	0.65 nm (5 μm × 5 μm) 92%	Hastelloy/Y2O3CSD	15%	Lab. scale	nd
[34]	Y2O3	Hastelloy (unpolished)	MOD + additives	R2R dip coating	16 (10 nm)	1.2 nm (5 μm × 5 μm) 94%	Hastelloy/Y2O3CSD/MgOIBAD/MgOEPI	6%	n.d	n.d
[38]	Y2O3	Hastelloy (mechanically polished)	Y2O3 dispersion (1° step) MOD + additives (2° step)	Spin coating	2 (1° + 2° step)	0.3 nm (5 μm × 5 μm) 95%	Hatelloy/Y2O3CSD/MgOIBAD	24%	Lab. scale	n.d
[49]	Y2O3	Hastelloy	Y2O3 dispersion	Dip coating	4	1.28 nm (5 μm × 5 μm) 90%	Hastelloy/Y2O3CSD	22.5%	1 m	nd
[35]	Y2O3	Hastelloy (unpolished)	Mixed Alkoxides + chelating agents	Dip coating	20 (1300 nm)	5 (10 μm × 10 μm) 90%	Hastelloy/Y2O3CSD/MgOIBAD/LMO/GdBCO	4.5%	n.d.	Ic = 420 A/cm
		SUS 304 (unpolished)			15 ($~$150 nm)	0.36 (10 μm × 10 μm) 98.2%	SUS304/Y2O3CSD/MgOIBAD/LMO/GdBCO	6.5%	n.d.	Ic = 300 A/cm
[36]	Y2O3	Hastelloy (unpolished)	MOD + additives	Continuous dip coating	15 ($~$1 μm)	0.7 nm (5 μm × 5 μm) 96.5%	Hastelloy/Y2O3CSD/MgOIBAD/ YBCO(1)/Ag/Y2O3CSD/MgOIBAD/YBCO(2)/Ag	6.4%	5 m	Ic = 725 A/cm (YBCO1 + 2)
[4]	Y2O3	Hastelloy (unpolished)	MOD	Continuous dip coating	24 (730 nm)	1.5 nm (5 μm × 5 μm) 96.4%	Hastelloy/Y2O3CSD/MgOIBAD/MgOEPI/LMO/YBCO	4%	n.d.	Jc = 2.5 MA/cm2
[44]	Y2O3-ZrO2	Hastelloy	MOD + additives	Dip coating	20 (440 nm)	3.8 (10 μm × 10 μm) 80%	Hastelloy/YZrOCSD/MgOIBAD/LMO/GdBCO	4%	n.d	Ic = 400 A/cm
[11]	Y2O3+ Al2O3 (YAlO)	Hastelloy (unpolished)	MOD + additives	Continuous dip coating	nd	0.2 (5 μm × 5 μm) 99.6%	Hastelloy/YAlOCSD/MgOIBAD/LMO	n.d.	n.d	n.d.
[39]	Y2O3+ Al2O3 + MOx	Hastelloy (polished)	Mixed precursors + additives	Continuous Dip coating	4 (n.d.)	0.6 (5 μm × 5 μm) 76%	Hastelloy/YAlOCSD/MgOIBAD/MgOEPI/LMO/YBCO	19%	20 m	Ic = 160 A
[32]	Y2O3+ Al2O3 (YAlO)	Hastelloy	Mixed precursors	Continuous Dip coating	24 (730 nm)	1.6–1.8 nm (5 μm × 5 μm) 96.8%	Hastelloy/YAlOCSD/MgOIBAD/MgOEPI/LMO/YBCO	4%	n.d.	Jc = 3.2 MA/cm2
[52]	Y2O3+ Al2O3	Hastelloy unpolished	N.d	N.d	15 (1.2 um)	1.5 (5 μm × 5 μm) 95.2%	Hastelloy/YAlOCSD/MgOIBAD/MgOEPI/YBCO	6%	n.d.	Ic = 600 A/cm
[40]	Al2O3	Hastelloy (polished)	MOD	Spin Coating	8 (~50 nm)	2.5 (2 μm × 2 μm) 74%	Hastelloy/Al2O3CSD/Y2O3/MgOIBAD/MgOEPI/LMO/YBCO	9%	10 cm	Jc = 3.04 MA/cm2
[41]	Gd-Zr-O	Hastelloy (E-polished)	MOD	Dip coating	9	4.2 (50 μm × 50 μm) 48%	Hastelloy/Gd-Zr-O/MgOIBAD/CeO2/YBCO	5%	n.d.	n.d.
		Hastelloy (Mirror-rolled)			9 (~300) nm	5 (50 μm × 50 μm) 83%		9%	nd	Jc = 4.17 MA/cm2
[6]	Gd-Zr-O	Hastelloy (e-polished)	MOD	Dip coating	1 (130 nm)	0.74 (1 μm × 1 μm) 26%	Hastelloy/Gd-Zr-OCSD/MgOIBAD/LMO/CeO2PLD/YBCOPLD	26%	20 cm	Jc = 1–2 MA/cm2
[42]	Y2O3 Gd-Zr-O	Al2O3-coated Hastelloy	MOD	Dip coating	1 (30–50 nm)	0.5 (1 μm × 1 μm)	Hastelloy/Gd-Zr-O/MgOIBAD/CeO2/YBCO	n.d.	20 cm	Jc = 1.65 MA/cm2
[43]	Gd-Zr-O	Hastelloy e-polished	MOD	Dip coating	4–5	2.5 - 40%	Hastelloy/Gd-Zr-OCSD/MgOIBAD/LMO/CeO2PLD/YBCOCSD	9%	n.d.	Jc = 2.5 MA/cm2
[46]	CZO	YSZABADSS	MOD	Spin coating	1 (30 nm)	3 (5 μm × 5 μm)	SS/YSZABAD/CZOCSD/YBCOCSD			Jc = 1.8 MA/cm2

Table 3. Summary of the results presented in the literature.

Ref	Oxide	Substrate	Precursor	Deposition	Layers (Tot Thickness)	Roughness R (Scale) $(\mathit{\%}{\mathit{R}}_{\mathit{r}\mathit{m}\mathit{s}}^{\mathit{r}\mathit{e}\mathit{d}\mathit{u}\mathit{c}\mathit{t}\mathit{i}\mathit{o}\mathit{n}}$)	Final Structure	Efficiency $\frac{{\mathit{R}}_{\mathit{r}\mathit{e}\mathit{d}\mathit{u}\mathit{c}\mathit{t}\mathit{i}\mathit{o}\mathit{n}}\mathit{\%}}{{\mathit{n}}_{\mathit{c}\mathit{o}\mathit{a}\mathit{t}\mathit{i}\mathit{n}\mathit{g}\mathit{s}}}$	Length	Jc/Ic (77K, sf)
[5]	Y2O3	Hastelloy (Unpolished)	Metal–Organic + additives	Continuous Dip coating	30 (~1 μm)	<1 nm (5 μm × 5 μm) (97.6%)	Hastelloy/Y2O3CSD/MgOIBAD/YBCOPLD,RCS	3%	5 m	Jc = 2.85–4 MA/cm2
[45]	Y2O3	YZSABAD/SS (unpolished)	Metal–organic + additives (TEA) + UV Varnish	R2R inkjet	1 (90–100 nm)	5.4 nm (5 μm × 5 μm) (66%)	SS/Y2O3CSD/YSZABAD/CeO2PLD/YBCOPLD	66%		Ic = 36 A
[3]	Y2O3	Hastelloy (unpolished)	Metal–organic + additives	Continuous Dip coating	16	0.788 nm (5 μm × 5 μm) (95%)	Hastelloy/Y2O3CSD/MgOIBAD/MgOEPI/LMO/YBCOMOD	11%	10 m	Ic = 80–180 A
		SUS 304 (unpolished)			12	0.903 nm (5 μm × 5 μm) (95.7%)	SUS304/Y2O3CSD/MgOIBAD/MgOEPI/LMO/YBCOMOD	8%		Ic = 100–150 A
[28,33]	Y2O3	Hastelloy (unpolished)	MOD	Continuous Dip coating	15 (840 nm)	1.2 nm (5 μm × 5 μm) (97%)	Hastelloy/Y2O3CSD/MgOIBAD/MgOEPI/LMO	6.5%	50 m	Jc = 1.8 MA/cm2
[1]	Y2O3	Hastelloy (unpolished)	MOD + additives	R2R dip coating	30 (0.5–1.2 μm)	1.2 nm (5 μm × 5 μm) (95.9%)	Hastelloy/Y2O3CSD	3%	n.d	n.d
[37]	Y2O3	Hastelloy (e-polished)	MOD + additives	Spin coating	6	0.65 nm (5 μm × 5 μm) 92%	Hastelloy/Y2O3CSD	15%	Lab. scale	nd
[34]	Y2O3	Hastelloy (unpolished)	MOD + additives	R2R dip coating	16 (10 nm)	1.2 nm (5 μm × 5 μm) 94%	Hastelloy/Y2O3CSD/MgOIBAD/MgOEPI	6%	n.d	n.d
[38]	Y2O3	Hastelloy (mechanically polished)	Y2O3 dispersion (1° step) MOD + additives (2° step)	Spin coating	2 (1° + 2° step)	0.3 nm (5 μm × 5 μm) 95%	Hatelloy/Y2O3CSD/MgOIBAD	24%	Lab. scale	n.d
[49]	Y2O3	Hastelloy	Y2O3 dispersion	Dip coating	4	1.28 nm (5 μm × 5 μm) 90%	Hastelloy/Y2O3CSD	22.5%	1 m	nd
[35]	Y2O3	Hastelloy (unpolished)	Mixed Alkoxides + chelating agents	Dip coating	20 (1300 nm)	5 (10 μm × 10 μm) 90%	Hastelloy/Y2O3CSD/MgOIBAD/LMO/GdBCO	4.5%	n.d.	Ic = 420 A/cm
		SUS 304 (unpolished)			15 ($~$150 nm)	0.36 (10 μm × 10 μm) 98.2%	SUS304/Y2O3CSD/MgOIBAD/LMO/GdBCO	6.5%	n.d.	Ic = 300 A/cm
[36]	Y2O3	Hastelloy (unpolished)	MOD + additives	Continuous dip coating	15 ($~$1 μm)	0.7 nm (5 μm × 5 μm) 96.5%	Hastelloy/Y2O3CSD/MgOIBAD/ YBCO(1)/Ag/Y2O3CSD/MgOIBAD/YBCO(2)/Ag	6.4%	5 m	Ic = 725 A/cm (YBCO1 + 2)
[4]	Y2O3	Hastelloy (unpolished)	MOD	Continuous dip coating	24 (730 nm)	1.5 nm (5 μm × 5 μm) 96.4%	Hastelloy/Y2O3CSD/MgOIBAD/MgOEPI/LMO/YBCO	4%	n.d.	Jc = 2.5 MA/cm2
[44]	Y2O3-ZrO2	Hastelloy	MOD + additives	Dip coating	20 (440 nm)	3.8 (10 μm × 10 μm) 80%	Hastelloy/YZrOCSD/MgOIBAD/LMO/GdBCO	4%	n.d	Ic = 400 A/cm
[11]	Y2O3+ Al2O3 (YAlO)	Hastelloy (unpolished)	MOD + additives	Continuous dip coating	nd	0.2 (5 μm × 5 μm) 99.6%	Hastelloy/YAlOCSD/MgOIBAD/LMO	n.d.	n.d	n.d.
[39]	Y2O3+ Al2O3 + MOx	Hastelloy (polished)	Mixed precursors + additives	Continuous Dip coating	4 (n.d.)	0.6 (5 μm × 5 μm) 76%	Hastelloy/YAlOCSD/MgOIBAD/MgOEPI/LMO/YBCO	19%	20 m	Ic = 160 A
[32]	Y2O3+ Al2O3 (YAlO)	Hastelloy	Mixed precursors	Continuous Dip coating	24 (730 nm)	1.6–1.8 nm (5 μm × 5 μm) 96.8%	Hastelloy/YAlOCSD/MgOIBAD/MgOEPI/LMO/YBCO	4%	n.d.	Jc = 3.2 MA/cm2
[52]	Y2O3+ Al2O3	Hastelloy unpolished	N.d	N.d	15 (1.2 um)	1.5 (5 μm × 5 μm) 95.2%	Hastelloy/YAlOCSD/MgOIBAD/MgOEPI/YBCO	6%	n.d.	Ic = 600 A/cm
[40]	Al2O3	Hastelloy (polished)	MOD	Spin Coating	8 (~50 nm)	2.5 (2 μm × 2 μm) 74%	Hastelloy/Al2O3CSD/Y2O3/MgOIBAD/MgOEPI/LMO/YBCO	9%	10 cm	Jc = 3.04 MA/cm2
[41]	Gd-Zr-O	Hastelloy (E-polished)	MOD	Dip coating	9	4.2 (50 μm × 50 μm) 48%	Hastelloy/Gd-Zr-O/MgOIBAD/CeO2/YBCO	5%	n.d.	n.d.
		Hastelloy (Mirror-rolled)			9 (~300) nm	5 (50 μm × 50 μm) 83%		9%	nd	Jc = 4.17 MA/cm2
[6]	Gd-Zr-O	Hastelloy (e-polished)	MOD	Dip coating	1 (130 nm)	0.74 (1 μm × 1 μm) 26%	Hastelloy/Gd-Zr-OCSD/MgOIBAD/LMO/CeO2PLD/YBCOPLD	26%	20 cm	Jc = 1–2 MA/cm2
[42]	Y2O3 Gd-Zr-O	Al2O3-coated Hastelloy	MOD	Dip coating	1 (30–50 nm)	0.5 (1 μm × 1 μm)	Hastelloy/Gd-Zr-O/MgOIBAD/CeO2/YBCO	n.d.	20 cm	Jc = 1.65 MA/cm2
[43]	Gd-Zr-O	Hastelloy e-polished	MOD	Dip coating	4–5	2.5 - 40%	Hastelloy/Gd-Zr-OCSD/MgOIBAD/LMO/CeO2PLD/YBCOCSD	9%	n.d.	Jc = 2.5 MA/cm2
[46]	CZO	YSZABADSS	MOD	Spin coating	1 (30 nm)	3 (5 μm × 5 μm)	SS/YSZABAD/CZOCSD/YBCOCSD			Jc = 1.8 MA/cm2

Table 4. Relevant characteristics of systems planarized via sputtering.

Ref	Group	Complete Buffer Structure	IBAD Substrate Rrms	Jc ReBCO
[53]	SuperPower	LMO/MgOEPI/MgOIBAD/Y2O3SPUTTERING/Al2O3SPUTTERING	1.5–2 nm	3.06 MA/cm2
[52,54]	Los Alamos National Lab.	LMO/MgOEPI/MgOIBAD/Y2O3SPUTTERING/Al2O3SPUTTERING	4–5 nm	4.25 MA/cm2
		LMO/MgOIBAD/Y2O3SPUTTERING/Al2O3SPUTTERING	6–8 nm	4.1 MA/cm2
		LMO/MgOEPI/MgOIBAD/YAlOSPUTTERING	4.5–5.5 nm	2.9 MA/cm2
[55]	Fujikura	CeO2/LMO/MgOIBAD/Y2O3SPUTTERING/Al2O3SPUTTERING	2.5 nm	1.36 MA/cm2
[56]	SuNAM	LMO/MgOEPI/MgOIBAD/Y2O3SPUTTERING/Al2O3SPUTTERING	1.6 nm	4.8 MA/cm2
[57]	Shangai Univesity SJTU	CeO2/MgOEPI/MgOIBAD/Y2O3SPUTTERING/Al2O3SPUTTERING	1.5–2 nm	4 MA/cm2
[58]	Superconductivity Research Laboratory, ISTEC	CeO2/MgOEPI/MgOIBAD/Y2O3SPUTTERING/Al2O3SPUTTERING	2 nm	2.6 MA/cm2

Table 4. Relevant characteristics of systems planarized via sputtering.

Ref	Group	Complete Buffer Structure	IBAD Substrate Rrms	Jc ReBCO
[53]	SuperPower	LMO/MgOEPI/MgOIBAD/Y2O3SPUTTERING/Al2O3SPUTTERING	1.5–2 nm	3.06 MA/cm2
[52,54]	Los Alamos National Lab.	LMO/MgOEPI/MgOIBAD/Y2O3SPUTTERING/Al2O3SPUTTERING	4–5 nm	4.25 MA/cm2
		LMO/MgOIBAD/Y2O3SPUTTERING/Al2O3SPUTTERING	6–8 nm	4.1 MA/cm2
		LMO/MgOEPI/MgOIBAD/YAlOSPUTTERING	4.5–5.5 nm	2.9 MA/cm2
[55]	Fujikura	CeO2/LMO/MgOIBAD/Y2O3SPUTTERING/Al2O3SPUTTERING	2.5 nm	1.36 MA/cm2
[56]	SuNAM	LMO/MgOEPI/MgOIBAD/Y2O3SPUTTERING/Al2O3SPUTTERING	1.6 nm	4.8 MA/cm2
[57]	Shangai Univesity SJTU	CeO2/MgOEPI/MgOIBAD/Y2O3SPUTTERING/Al2O3SPUTTERING	1.5–2 nm	4 MA/cm2
[58]	Superconductivity Research Laboratory, ISTEC	CeO2/MgOEPI/MgOIBAD/Y2O3SPUTTERING/Al2O3SPUTTERING	2 nm	2.6 MA/cm2