This report describes the use of a custom-built system to perform aerosol deposition of thick films of yttrium iron garnet onto sapphire substrates at RT. The deposited films are characterized using scanning electron microscopy, profilometry, and ferromagnetic resonance to give a representative overview of the capabilities of the technique.
Aerosol deposition (AD) is a thick-film deposition process that can produce layers up to several hundred micrometers thick with densities greater than 95% of the bulk. The primary advantage of AD is that the deposition takes place entirely at ambient temperature; thereby enabling film growth in material systems with disparate melting temperatures. This report describes in detail the processing steps for preparing the powder and for performing AD using the custom-built system. Representative characterization results are presented from scanning electron microscopy, profilometry, and ferromagnetic resonance for films grown in this system. As a representative overview of the capabilities of the system, focus is given to a sample produced following the described protocol and system setup. Results indicate that this system can successfully deposit 11 µm thick yttrium iron garnet films that are > 90% of the bulk density during a single 5 min deposition run. A discussion of methods to afford better control of the aerosol and particle selection for improved thickness and roughness variations in the film is provided.
Aerosol deposition (AD) is a thick-film deposition process that can produce layers up to several hundred micrometers thick with densities greater than 95% of the bulk1. The deposition process is believed to occur through a continual process of impact, fracture or deformation, adhesion, and densification of particles. Figure 1 depicts this process as a series of steps showing particle impact and densification over several steps. As shown, the particles move toward the substrate with a typical velocity of 100–500 m/sec. As the initial particles impact with the substrate they fracture and adhere to the substrate. This anchoring layer provides the mechanical adhesion between the substrate and the bulk film. As subsequent impacts occur the underlying particles are increasingly fractured, adhered, and further densified. This process of continual impact, fracture, and densification works to compact the underlying film and bond the crystallites and produce a film with a density reaching greater than 95% of the bulk material.
Figure 1. Illustration of the deposition process. Panel A shows three particles moving toward the substrate with a typical velocity of 100–500 m/sec. Panel B shows the result of impact, fracture, and adhesion of the first particle. Panels C and D show the subsequent impact of the second and third particles, which further compact the underlying film and bond the crystallites. The result is a film with density greater than 95% of the bulk material (reproduced with permission from Reference 19). Please click here to view a larger version of this figure.
The primary advantage of AD is that the deposition takes place entirely at ambient RT; thereby enabling film growth, for instance, of a high-melting-temperature material (starting powder) onto a low-melting-temperature substrate. The deposition rate can be up to several micrometers per minute and is performed at moderate vacuum conditions of 1–20 Torr in the deposition chamber. The process shows the ability to scale up to very large deposition areas and finally, it can deposit conformally.2
There are many material systems studied by AD for a wide variety of uses, such as inductors3, abrasion-resistant coatings4, piezoelectrics5, multiferroics6, magnetoelectrics7 thermistors8, thermoelectric films9, flexible dielectrics10, hard tissue implants and bioceramics11, solid electrolytes12, and photocatalysts13. For applications to microwave devices, magnetic films of several hundreds of micrometers in thickness are required that would ideally be integrated directly into the circuit board elements. One challenge to realizing this integration is the high-temperature regime needed for fabricating ferrite films (see review by Harris et al.14), such as yttrium iron garnet (YIG). For this reason AD appears to be a natural choice for realizing potential new advancements in magnetic integrated circuit technology. The low-cost operation, high deposition rate, and simplicity of AD has spurred interest by researchers in Germany, France, Japan, Korea, and now in the United States.
Figure 2 is a drawing outlining the basic setup to perform aerosol deposition. Pressure is monitored at the locations marked PAC, PDC, and PH for the aerosol chamber, deposition chamber, and pump head, respectively. The gas flow, controlled by the mass flow controller (MFC), enters the aerosol chamber and aerosolizes the powder. The deposition chamber is pumped to create the pressure difference between the two chambers, causing the flow of particles through the rectangular (0.4 mm x 4.8 mm) nozzle opening.
Figure 2. Main components in the NRL ADM system. Pressure is monitored at the locations marked PAC, PDC, and PH for the aerosol chamber, deposition chamber, and pump head, respectively. See text for details. (copyright (2014) The Japan Society of Applied Physics, reproduced from Reference 20). Please click here to view a larger version of this figure.
The average size of an individual YIG particle in this work is 0.5 µm. The effect of agglomeration causes these small particles to form much larger agglomerates that range in size from about 10 µm to about 400 µm. Control of the agglomerate size and delivery rate is essential to achieving a dense well-formed film. This necessitates configuration of an aerosol chamber that allows size selection and uniform particle flux into the deposition chamber. The powder is pre-sieved to remove any agglomerates larger than 53 µm prior to being loaded into the aerosol chamber. The aerosol chamber configuration used in this work is illustrated in Figure 3. Nitrogen gas enters through four inlet nozzles (two are shown in Figure 3) located at the bottom sides of the chamber. The gas interacts with the YIG powder (shown in green) to produce an aerosol comprised of a distribution of agglomerated particle sizes less than 53 µm. An agitator at the base of the aerosol chamber made of a stainless steel plate is vibrated continuously to keep the powder moving into the gas flow. The agglomerates impact a 45 µm filter, allowing only agglomerates sized less than 45 µm to enter the nozzle inlet. Upon entering the nozzle inlet the agglomerates are accelerated to a large velocity and ejected into the deposition chamber (not shown) to perform the deposition. A stainless steel rod connects the bottom of the filter to the base of the agitator (not shown) to aid in de-clogging the filter.
Figure 3. Illustration of the internal aerosol chamber configuration, with filter, inlet nozzles, and YIG powder shown. See text for details.
This report details the experimental procedure to perform AD using the custom-built system described above to produce dense films of YIG. Representative results for an 11 µm thick film produced in this system are presented using scanning electron microscopy (SEM), thickness profiles, and ferromagnetic resonance (FMR). The results presented are not intended to be an in-depth study of the magnetic properties or material structure of the film, but as a demonstration of the films produced by this technique. Please click here to view a larger version of this figure.
1. Powder Preparation
2. Substrate Preparation
3. Performing Aerosol Deposition
4. Post-deposition Inspection
After the deposition is complete, the coated substrates are removed from the deposition chamber and inspected using an optical stereo microscope. Samples are typically brushed and washed with isopropanol to remove excess powder that remained during re-pressurization to atmosphere. Film characterization was performed on the representative results presented here using scanning electron microscopy to assess the morphology of the film, profilometry to assess the film thickness, uniformity, and roughness, and ferromagnetic resonance to assess the magnetic properties of the film (see Reference 20 for additional details on the characterization methods). In this study, a 3 mm x 3 mm sapphire substrate was coated using the above protocol and system setup. The deposition sweep was set to raster across the film at 0.65 mm/sec and to cover a total area of 75 mm2.
Figure 4 is an SEM image of the top surface of the film showing many small grains that are much smaller than the starting size of the individual particles (0.5 µm). It is evident from the image that the film has formed a somewhat rough, well-compacted surface with very few voids. For films of similar results presented here we have measured the density to between 90%–96% of the theoretical density for YIG (5.17 g/cm3). The cross-sectional view of the film shown in Figure 5 also supports the dense nature of the film. The main image in Figure 5 shows the edge of the as-deposited sample as formed during deposition, i.e., it is not a cleaved section of the film. The inset shows a magnified view of the cross-section of the film indicating the dense nature of the film. We have also performed X-ray diffraction on the as-deposited films and starting powder and found that the crystal structure does not change upon deposition (data not shown; see also References 3 and 20).
Figure 6 is a plot of the step height of the film. The total area of the sample is 3 mm x 3 mm (the total deposition area was 75 mm2). The step was created by removing a portion of the film along one edge of the substrate. The red line overlaying the data indicates the average film thickness of 10.93 µm by averaging three profile scans across the film. The rms roughness Rq = 1.37 µm. The deposition time to form this film was 337 sec, resulting in a deposition rate of 1.95 µm/min.
Figure 7 is a plot of the FMR absorption derivative taken at RT: data shown in black. A Lorentzian derivative lineshape fit to the data is shown in red. The linewidth of the data is 330 Oe and the resonance field is 2,810 Oe. The signal location and shape is comparable to a typical spectrum for polycrystalline YIG grown by other methods, e.g., pulsed laser deposition or rf magnetron sputtering15,16. The Lorentzian lineshape gives a good fit to the data suggesting a uniform film17,18.
Figure 4. SEM image of the top surface of the film post-deposition. The image shows a film with many grains that are densely compacted and much smaller than the 0.5 µm individual starting particle size. Please click here to view a larger version of this figure.
Figure 5. SEM image of the cross-section of the film on the sapphire substrate. The inset is a magnified view of the cross-section of the film. Please click here to view a larger version of this figure.
Figure 6. A representative step profile of the film in Figure 4 and Figure 6 showing the variation in thickness across 2.25 mm of the 3 mm total length of the sample. The average thickness of 10.93 µm is shown as the red line overlaying the data. Please click here to view a larger version of this figure.
Figure 7. Ferromagnetic resonance derivative curve of the film taken at RT. The spectrum is well fit by a Lorentzian function indicating a uniform homogenous film. The linewidth is 330 Oe. Please click here to view a larger version of this figure.
The SEM image in Figure 4 indicates that significant fracture and densification is occurring during the deposition process. The image is taken of the top surface of the film, which shows a small number of voids and grains. The observable region is the last of the material to be deposited and therefore does not benefit from the further impact and densification process of subsequent particles as illustrated by impact from particle 2 and 3 in Figure 1. The film density within the volume of the sample can be seen by the cross-sectional image in Figure 5. This image, along with the higher magnification inset of the cross-section, provides further evidence for a dense film.
The roughness seen in the SEM image is also confirmed quantitatively in the profile scans. The roughness of these films may be due to the high rate of deposition (1.95 µm/min). It is possible that films formed more slowly will enable particles to fully fracture and adhere before additional particles arrive, but the increase in film roughness may also be fundamentally related to the increase in film thickness21. The drop in the thickness across the film suggests that the particle flow into the deposition chamber is not constant during the entire deposition. Possible reasons could stem from loss of powder in the aerosol, clumping of the powder at the base of the aerosol chamber, and/or filter clogging. Fully drying and sieving the powders and modifying the aerosol chamber to that shown in Figure 3 are strategies undertaken to improve and control the deposition rate and film uniformity.
The FMR spectrum for this sample indicates that the YIG film has a resonance field and lineshape that is typical of polycrystalline YIG films grown by other techniques. The linewidth of the AD deposited film is somewhat wider than other reported polycrystalline films by about 100–200 Oe16. This is likely due to the nano-crystalline make-up of the film. Note that single-crystal YIG films have a typical FMR linewidth of less than 1 Oe even for comparable thicknesses 22. Since this film is well-modeled by a Lorentzian lineshape one might conclude that the film is uniform and free from inhomogeneities. In this case, the broadening of the linewidth could be attributed to intrinsic relaxation processes such as Gilbert dampening and/or 2-magnon scattering18. The sloping thickness profile in Figure 6 is evidence that the deposition rate is not constant throughout the process, and therefore, the film is not likely to be completely free from inhomogeneities. A more in-depth study needs to be undertaken to fully implicate these mechanisms (see also discussion in Ref. 3), as well as, better refinement of the current AD system. The most likely cause of the non-uniform coverage is that the aerosol generated in the aerosol chamber may become depleted and/or the filter becomes clogged. Both of these effects would cause the amount of powder entering the deposition chamber to diminish and thereby result in a lower deposition rate.
The results presented above indicate successful deposition of a nearly 11 µm thick film at a deposition rate of nearly 2 µm/min (for a 75 mm2 area) using the AD system and protocol presented here. The film is comprised of densely compacted and randomly oriented grains that are much smaller than the starting particles due to the impact, fracture and densification mechanisms involved in this process. The film shows comparable FMR linewidth shape and resonance field to reported YIG films 15-17. Additional work to better control the flow of agglomerates into the deposition chamber is needed to ensure more uniform film growth.
Current work is underway to further improve the consistency of the deposition process. Additional drying and sieving protocols are being developed to ensure that the powder has the same consistency during each run. Designs are also underway to create an improved filtration system with less clogging. A redesign of the nozzle inlet will also aid in preventing clumped powder from collecting within the nozzle neck. More in-depth studies of the magnetic and structural properties are also underway. Some current studies include, building up films layer-by-layer to better understand variations in the magnetic properties of these films created by aerosol deposition.
The authors have nothing to disclose.
SDJ gratefully acknowledges the support of the American Association for Engineering Education/NRL Postdoctoral Fellowship Program, discussions with Konrad Bussmann (NRL) and Mingzhong Wu (Colorado State University) on the magnetic properties of materials, and Ron Holm (NRL) for his part in the design and implementation of the NRL AD system.
Ferromagnetic Resonance Spectrometer | www.bruker.com/ | 9.5 GHz Spectrometer | |
Scanning Electron Microscope | www.zeiss.com | LEO Supra 55 | |
Profilometer | www.kla-tencor.com/ | D-120 | |
Stereo Microscope | www.microscopes.com | Omano Stereo Microscope | Used for inspection directly after removal from deposition chamber |
Double-sided Copper Tape | www.2spi.com | 05085A-AB | hold-down clips or other adhesives may be used |
Nitrile Exam Gloves | www.fishersci.com | 19-130-1597D | |
2-propanol | www.fishersci.com | A451SK-4 | |
Acetone | www.fishersci.com | A11-1 | |
Yttrium Iron Garnet Powder | www.trans-techinc.com/ | Call for Product Information | Powder is custom made to order and ground to specifications |
Stainless Steel Spoon | www.fishersci.com | 14-429E | Used for scooping and transferring powder |
Alumina Boats | www.coorstek.com/ | 65580 | |
Drying Furnace | www.paragonweb.com | KM14 ceramic furnace | Furnace is connected to air during drying |
Powder Sieves | www.advantechmfg.com/ | 270SS8F | A selection of mesh openings are needed to sieve from large down to target size |
Ultra High Purity Nitrogen Gas | www.praxairdirect.com | NI 5.0UH-3K | Used as medium for aerosol. |
Air Breathing Quality | www.praxairdirect.com | AI BR-4KN | Used inside furnace during drying |
Lab Balance | www.balances.com/ | Sartorius ED224S Lab Balance | Used for weighing powder |
Sapphire Wafers | www.pmoptics.com/ | PWSP-313211 |