Formation of Thick Dense Yttrium Iron Garnet Films Using Aerosol Deposition

Summary

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.

Abstract

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.

Introduction

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 P_AC, P_DC, and P_H 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 P_AC, P_DC, and P_H 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.

Protocol

1. Powder Preparation

1. Sieve as-received yttrium iron garnet (YIG) powder to obtain 100–150 g of agglomerates sized less than 53 µm.
2. Place the sieved powder into a furnace to dry for at least 24 hr at a temperature greater than 300 °C.

2. Substrate Preparation

1. Clean a substrate of desired size, e.g., 3 mm x 3 mm using acetone then isopropanol. Dry using nitrogen gas.

3. Performing Aerosol Deposition

1. Mount the sample to the translation mounting stage.
   1. Place double-sided copper tape to the mounting stage. Place the cleaned substrate onto the copper tape.
   2. Measure the distance from the edge of the mounting stage to each edge of the sample. Record the dimensions and sample position on the sample history sheet.
   3. Check that the mounting stage is aligned parallel with the body of the translation motor by measuring the distance from the edge of the mounting stage to the edge of the motor body with calipers. Load the sample into the deposition chamber.
   4. Clamp the flange to seal the deposition chamber. Attach the 3 15-pin D-sub controller cables for the translation motors.
2. Load the powder and close up the aerosol chamber.
   1. Remove the dried powder from the furnace and place it in the aerosol chamber bottom section. Slide the filter de-clogging rod attachment onto the filter de-clogging rod.
   2. Place the main body of the aerosol chamber onto the aerosol chamber bottom section. Allow the filter de-clogging rod attachment to rest on the agitation plate as the main body is being attached to the bottom section.
   3. Clamp the main body to the bottom section. Attach the aerosol chamber pressure gauge to the side port on the main body of the aerosol chamber.
   4. Clamp the nozzle inlet section to the top port on the main body of the aerosol chamber using a QF clamp. Raise the nozzle inlet tube to the inlet port on the deposition chamber and secure the top and bottom fitting.
   5. Record the powder and aerosol chamber identification numbers on the sample history sheet.
3. Turn on the roughing pump with it isolated from the rest of the system. Turn on the deposition chamber illumination lamp. Open the constriction valve on the bypass line to begin the pump-down of the entire system.
4. Setup the pressure monitoring software with the deposition run identification.
5. Run the stage controller macro creator by typing ‘pitrans’ in the command line terminal window and enter the requested information. Create a new sheet in the run log spreadsheet and record the deposition parameters and setup notes.
6. After the system pressure has reached about 150–200 Torr, slightly open the unconstricted valve. Maintain a pump-down rate of about 1 Torr/sec. Once the pressure has dropped below 100 Torr start the pressure monitoring software and the translation stage motor controller software.
7. Once the system pressure has reached about 1 Torr close all three valves to the bypass line and open the main pumping valve. Tighten the clamp to the top cap on the deposition chamber.
8. Turn on the blower pump. Open the ultra-high purity (UHP) nitrogen gas cylinder. Monitor the pressure and record the base pressure of the system (it typically reaches 15–25 mTorr).
9. Set the distance between the nozzle and the substrate. Use the stage controller software graphical user interface window to move the mounted substrate over the nozzle. Lower the substrate until it contacts the nozzle. Move the substrate 7.5 mm in the vertical direction from this position.
10. Close the main pumping line and monitor the leak rate of the system on the pressure monitoring software. Note the initial leak rate upon shutting the valve. If this leak rate is less than 3.33 mTorr/sec continue, otherwise begin checking for leaks. A typical leak rate is less than 1.2 mTorr/sec.
11. Set the deposition chamber butterfly valve to the 500 Torr preset value. Set the mass flow controller value to 13.63 L/min (do not turn it on).
12. Move the mounting stage to the starting position for the deposition. Load the macro created in step 3.7 into the controller software.
13. Program the function generator to sweep linearly between 135 and 145 Hz every 10 sec. Turn the function generator on. Turn the nitrogen gas flow on. After a 3 sec countdown start the stage controller macro.
14. Monitor the deposition and adjust gas flow rate as necessary to keep the pressure difference at 500 ± 0.5 Torr (or as desired for the run) for the duration of the deposition.
    Note: The pressure in the deposition chamber is typically 0.65 Torr and the pressure in the deposition chamber is typically 501 Torr. Uncontrollable variations in pressure usually indicate that the UHP nitrogen is running out. A slight drop in pressure (1–2 Torr) over the duration of the run is typical. This can be remedied by increasing the gas flow rate. During the initial passes a visible film should form on the substrate, lack of film formation suggests insufficient aerosolization of the powder and/or significant filter clogging.
15. At the end of the deposition note the exact deposition run time. Shut off the nitrogen gas, the function generator, and the pumps. Open the deposition chamber butterfly valve completely.
16. Open the bypass valve located at the side of the deposition chamber. Turn the house nitrogen gas regulator to zero and redirect it into the deposition chamber. Close the main pumping valve while slowly increasing the house gas regulator pressure.
17. Home the nozzle to X = 25 mm, Y = 25 mm, and Z = 25 mm, then close the stage controller software.
18. Once the pressure in the system has risen above 100 Torr stop the pressure monitoring software. Record the total nitrogen gas used and time to complete the deposition. Adjust the house gas pressure as necessary until the system reaches atmosphere.
19. Unplug the 3 15-pin D-sub stage controller cables and unclamp the top cap. Remove the top cap from the deposition chamber and unmount the sample.

4. Post-deposition Inspection

1. Remove the sample from the mounting stage and inspect it under a microscope. If necessary, wash the sample in isopropanol to remove loose powder. Perform the planned characterizations of the film.

Results

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 re...

Discussion

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 ...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
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