Pushing the ALD Envelope

May 1, 2010
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ALD enables films of a range of materials to be grown on substrates of just about any material and topography with digital layer-by-layer thickness control.

Table 1.

Following the seminal work of Aleskovskii1 and Suntola2 in establishing the field of atomic layer deposition (ALD) science, the subject has rapidly expanded in both academic and commercial arenas to encompass a variety of applications. This technology has proliferated mainly due to the advantages offered by the deposited film quality and conformity that result from the inherent growth mechanism found in ALD.

Films are deposited by sequentially pulsing appropriate precursor materials into the ALD reaction chamber, with purge cycles of an inert gas interspersed between precursor pulses. A key element of the deposition is the self-limiting nature of the process, which allows repeatable monolayer-by-monolayer growth. Because of this unique reaction mechanism, films deposited via ALD offer several benefits, including:
  • Ability to coat inside pores, trenches and high-aspect-ratio features
  • Digital thickness control to atomic level since the film is deposited one atomic layer at a time
  • Pinhole-free films, even over very large areas
  • Ideal material properties (n, Ebd, k, etc.) guaranteed by near-100% film density
  • Excellent repeatability (wide process windows; ALD is not sensitive to temperature or precursor dose variations)
  • Low defect density
  • Amorphous or crystalline materials, depending on substrate and temperature
  • Digital control of sandwiches, heterostructures, nanolaminates, mixed oxides, graded index layers and doping
  • Growth underneath dust, pushing it upward
A large selection of materials is available for growth via ALD, several of which are common within the ceramic industry. While most elements have an ALD process associated with them, some of the more commonly available materials are listed in Table 1.

Figure 1. Thermal Al2O3 growth using trimethyl aluminum (TMA) and water (H2O) to illustrate a typical ALD process.

Overview of ALD

The general ALD reaction sequence can be described in five steps: pulse first precursor; purge; pulse second precursor; purge; repeat. The process can be best illustrated by examining the growth of alumina (Al2O3) using trimethylaluminum Al(CH3)3 (TMA) and water, as shown in Figure 1. Figure 1a depicts a substrate surface saturated with hydroxyl (-OH) groups and the introduction of the TMA precursor. In Figure 1b, the TMA precursor reacts with the surface hydroxyl group to create an Al-O chemical bond and liberate a methane (CH4) molecule. Figure 1c depicts the completion of the TMA pulse step as all of the surface hydroxyl groups have been reacted with TMA molecules.

The deposition system is then purged of excess TMA and CH4 reaction products before the second precursor (water) is introduced into the system, as shown in Figure 1d. Water reacts with the methyl (-CH3) groups on the chemisorbed Al(CH3)2 species to form oxygen bridges between adjacent Al atoms and new hydroxyl groups, as depicted in Figure 1e. Figure 1f shows the resulting monolayer from the first Al2O3 ALD cycle. The surface of the monolayer is again saturated with hydroxyl groups and is ready for the next TMA pulse, as shown in Figure 1a.

The reaction mechanism for other materials (e.g., nitrides and sulfides) is similar, given the reaction of the second precursor with the chemisorbed surface species. However, the ALD of certain metals (e.g., platinum or ruthenium films) proceeds primarily though a combustion mechanism. In such cases, the second reactant is usually oxygen or ozone, which helps convert the organic ligands of the precursor into volatile carbonates (CO and CO2) and the reduction of the precursor into a metallic state.

Figure 2. Natural fiber, non-woven substrate coated with about 550 Å of Al2O3. This photograph was taken on an optical microscope in dark-field mode at 200x magnification.

ALD on Sacrificial Substrates

It is possible to deposit thin films using ALD onto sacrificial substrates; the subsequent removal of the sacrificial material then leaves the ALD film behind as a hollow shell. If the starting template is a spherical particle, then one is left with a hollow sphere, whereas a rod-like template will result in hollow rods. Materials used as sacrificial materials for generating hollow spheres and rods with ALD include block copolymers,3 polyvinyl alcohol4 and biological systems.5 More structurally complicated templates, such as filter paper,6 can be used as well, with the resulting ALD material uniformly coating the substrates' topography.

Creating hollow structures using sacrificial substrates has been investigated for a variety of applications, including microfluidic applications,6 electronic applications,7 semiconductors7 and drug delivery.5 The material and processing flexibility available for depositing ALD materials and the size, shape and material range of structures available for use as sacrificial templates suggest the application space for hollow, nano-scale ALD-derived materials has only begun to be investigated.

In order to investigate potential new applications for ALD films, Al2O3 films deposited on a sheet of non-woven, natural fiber were studied.* The substrate was heated to 250°C and the reactor space was purged with 20 sccm of UHP N2. ALD processes are typically performed under low-vacuum conditions; a rotary vane pump maintains the system pressure at 0.2 Torr at these flow conditions. Cylinders of TMA and water are connected to the ALD reactor through fast-acting ALD valves that enable very fast, precise dosing of the ALD precursors to the reactor.

For the film discussed here, the TMA was pulsed for 0.015 seconds, followed by a 10-second purge to remove unreacted TMA and reaction byproducts. A 0.015-second water pulse was then introduced to the reactor, followed by another 10-second purge. This TMA/purge/water/purge cycle was repeated 500 times on the non-woven substrate. Figure 2 shows the coated material at 200x magnification (taken with an optical microscope in dark-field mode). At this resolution, samples of the uncoated and coated non-woven material are indistinguishable.

*The films were deposited in a Cambridge NanoTech S200 atomic layer deposition system.

Figure 3. A portion of the ashed sample following the initial burning of the substrate. Following the initial burn, the sample retains much of the solid combustion products of the natural fibers.

A 15.25 cm2 piece of the coated sample material was weighed and found to have a mass of 1.51 g. The sample was then burned in order to remove the natural fiber substrate. Flames and smoke were generated, and the sample was transformed into a dark material that looked like paper ash. However, the sample maintained its shape and proportions, whereas burning paper typically curls and shrinks. A small portion of this ashed material is shown in Figure 3.

Figure 4. Sample from Figure 3 after additional heating to drive off the solid combustion reaction products.

As flame was continuously applied to the ashed sample with a propane torch, all of the oxidized substrate material was eventually driven off, leaving a bright white Al2O3 sample that appears just as the original non-woven substrate. The sample from Figure 3 following the removal of the oxidized substrate material is shown in Figure 4. After fully burning the original 15.25 cm2 sample, the mass of the sample was found to be 0.05 g, a 96% reduction in the mass of the sample.

The thermal TMA/H2O Al2O3 ALD process puts down 1.1 Å/cycle. If both sides of a 15.25 cm2 sample were coated with 550 Å of Al2O3 (3.69 g/cc), as from a 500-cycle process, a mass gain of 8.5 mg would be anticipated. The surface area of a non-woven substrate would be expected to be different than a planar substrate due to the 3-D state of the overlapping fibers. A dense non-woven with many layers of fibers would have a much higher surface area than a planar substrate, while a non-woven with only a few layers of fibers and a substantial open area could have a surface area less than that of a planar sample.

Microscope pictures of the non-woven indicate a dense mesh with many layers of fibers, suggesting a higher-than-planar surface area for the sample. The 50 mg of measured Al2O3 following the removal of the substrate indicates that the non-woven material has a surface area five to six times that of a planar substrate.

Figure 5. Hollow Al2O3 tubes maintaining substrate topography following substrate removal. This photograph was taken on an optical microscope in dark-field mode at 200x magnification.

Figure 5 shows a picture of the resulting Al2O3 material after substrate removal (taken at 200x magnification with an optical microscope in dark-field mode). The fiber structure of the non-woven is clearly maintained in the resulting material. Under macroscopic as well as microscopic viewing, the end product appears to be very much identical to the original starting material, the primary difference being the 96% mass decrease in the final material.

Commercial or scientific applications for this specific application of an ALD film on a sacrificial template material are unclear at this point. These results are meant to be a demonstration of one of the possible categories of applications for thin films deposited via ALD outside the more traditional applications in the semiconductor/electronics industry.

Future Applications

ALD enables films of a range of materials, including many common to the ceramic industry, to be grown on substrates of just about any material and topography with digital layer-by-layer thickness control. While most current applications exist in the semiconductor/electronics industries, research and development efforts are being put forth to study the application of this thin film deposition technique in other industries.

Of significant interest is the use of ALD on sacrificial templates to make nanoscale hollow materials. These hollow materials may find new and diverse applications to expand ALD-enhanced products beyond semiconductor/electronic applications into other fields such as medicine and microfluidics.

For more information regarding ALD, contact Cambridge NanoTech Inc., 68 Rogers St., Cambridge, MA 02142; call (617) 674-8800; fax (617) 674-8850; e-mail info@cambridgenanotech.com; or visit www.cambridgenanotech.com.

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