The authors report the deposition of 4.5-nm-thick cobalt (II) oxide on SiO2/Si(001) and MgO(001) substrates at 180–270 °C by atomic layer deposition using bis(N-tert-butyl-N′-ethylpropionamidinato) cobalt (II) and water as coreactants. The resulting CoO film is smooth and carbon-free. CoO can be reduced to Co metal using hydrogen or deuterium gas at 400–500 °C in a vacuum furnace, but the high temperature processing causes dewetting, leading to discontinuous Co metal islands rather than continuous films. Two low temperature (∼200 °C) reduction methods are reported: deuterium atom reduction and the use of an O-scavenging Al metal film. The low temperature methods can suppress dewetting to a large extent, and the resulting metallic cobalt film is smooth and continuous.

Cobalt oxides have a wide range of applications, such as photocatalysts, electrodes, optical sensors, magnetic detectors, and catalysts.1–7 Thin films of cobalt oxides have been grown on different substrates exhibiting a variety of properties.2–6,8,9 Cobalt (II) oxide has a bandgap of 2.4 eV, which has been used for visible light driven photoelectrochemical water oxidation.2 The growth of CoO has been performed using different growth techniques, such as molecular beam epitaxy (MBE),10–12 pulsed laser deposition,13,14 chemical vapor deposition (CVD),15–17 and atomic layer deposition (ALD).2,9,18–21 Among them, ALD offers film uniformity and conformality over large area substrates, which are desired for most applications.

Compared to cobalt oxides, cobalt metal is a widely studied material for microelectronics and memory applications.22–30 Co is well known to strongly adhere to Cu, and Co and Co alloy thin films have been studied as a Cu electromigration barrier and next-generation liner material in the back end of line interconnects.22–25 Additionally, Co and Co alloys, such as CoFe and CoFeB, are used as magnetic materials for the magnetic tunnel junction (MTJ).26–30 A basic MTJ is composed of two ferromagnetic layers (known as the fixed and free layers) separated by an ultrathin insulating tunneling barrier.28 While magnetic moment orientation of the fixed layer remains unchanged, the magnetic moment orientation of the free layer can be switched during device functioning. If the magnetic moments of the two layers are parallel, it leads to a low resistance state of the MTJ, while the antiparallel configuration leads to a high resistance state. Crystallographic structure, particle size, and texture deeply affect magnetic properties, such as anisotropy, coercivity, and the magnetization reversal process. Therefore, the study of magnetic behavior of Co and Co alloys has attracted considerable attention.31–34 Cobalt metal can be used as either the fixed or free layer, depending on its structure and particle size.32–37 

The deposition of Co and Co alloy thin films has been performed by both physical vapor deposition and CVD techniques.29–37 With downscaling of electronic device feature sizes, thin film uniformity and conformality become increasingly important, and this makes ALD a very attractive technique for cobalt metal film deposition, especially ultrathin cobalt metal films in interconnects or the MTJ stack. One advantage of ALD is precise thickness control since the precursors adsorb and/or react until a monolayer forms.38 The self-limiting behavior of ALD produces smooth and conformal films to the underlying substrates. When compared to sputtered Co-based magnetic films, a promising advantage of ALD films is that patterned films can be directly grown by area-selective ALD, thus eliminating alignment and postdeposition etching steps.39–41 This has the potential to minimize the film quality degradation coming from structural or compositional damages caused by etching.

There are limited reports of cobalt metal ALD.41–49 While most Co ALD precursors are organometallic compounds, relatively high energy is needed to convert the Co2+ or Co3+ to Co0, and the relatively high energy normally comes from either a plasma or high temperature. Plasma-enhanced ALD of cobalt metal has been reported by using precursors like Co2(CO)8, CoCp2, and Co(MeCp)2 and plasmas like H2 and NH3, but a plasma may damage the substrate surface and the anisotropy of plasma bombardment compromises the conformality of ALD.42–44 Thermal ALD of cobalt metal has been achieved at a temperature above 260 °C by using bis(N,N′-di-i-propylacetamidinato)cobalt(II) and bis(N-t-butyl-N′-ethylpropanimidamidato) cobalt(II) precursors and H2, but there is incorporation of C and N in the Co film due to precursor decomposition at high temperature.41,45,46 Recently, thermal ALD of cobalt metal has been demonstrated at a relatively low temperature below 200 °C, but these processes cannot grow Co on oxide substrates due to the selective nucleation behavior of the precursors toward metal substrates.47–49 

Another strategy to produce metal films is by reducing metal oxides using reducing agents.8,9,50,51 In this study, we investigate the feasibility of producing Co metal films by reducing cobalt oxide. The reduction of CoO on oxide substrates by using hydrogen or deuterium gas at 400–500 °C reported herein likely results in discontinuous cobalt islands instead of continuous cobalt films. This dewetting phenomenon is common during high temperature processing of metals on oxide substrates, especially for ultrathin metal films.8,9,52–55 Driven by minimization of the total energy of the film-substrate interface, film surface, and substrate surface, the edges of metastable films have the tendency to shrink and form islands when enough atom mobility is offered by thermal energy.52 Dewetting normally starts at the edges of voids, which can be as small as several nanometers. During reduction, the loss of oxygen atoms creates voids within the films, which facilitates the onset of dewetting. In this work, we report a method combining a low temperature (∼180 °C) ALD process and low temperature (∼200 °C) reduction processes to grow impurity-free and ultrathin cobalt metal films that are continuous on oxide substrates.

Four-inch wafers of SiO2/Si(001) were prepared by a thermal oxidation method, which produces an amorphous 300-nm-thick SiO2 layer on Si(001) substrates. The 0.5-mm-thick wafers were then cut into 20 × 20 mm2 pieces. MgO(001) 10 mm × 10 mm × 0.5 mm substrates were purchased from MTI Corporation. The substrates were ultrasonically cleaned with acetone, isopropyl alcohol, and deionized water for 5 min each, followed by UV/ozone treatment for 15 min to remove residual carbon contamination. The substrates were loaded into a UHV system that includes an X-ray photoelectron (XP) spectrometer, a molecular beam epitaxy chamber, an ALD chamber, and the reduction chamber; this system allows in situ sample transfer between chambers.56 The ALD system used for CoO deposition is a custom-built, hot-wall, stainless steel, rectangular 20-cm-long chamber, with a reactor volume of 460 cm3.56 Ultrahigh purity argon was used as a purge/carrier gas. The substrate temperature was monitored with a reference thermocouple in the ALD chamber that was previously calibrated against an instrumented wafer.

Bis(N-tert-butyl-N′-ethylpropionamidinato) cobalt (II) (Strem, 98%) and deionized water were used as coreactants for the ALD growth. During CoO growth, the Co precursor and water were held at 80 °C and room temperature (25 °C), respectively, while the substrate was maintained at 180 °C. Water dosing was regulated using an in-line needle valve. Each ALD cycle of CoO growth consisted of a 2 s dose of Co, a 20 s purge of Ar, a 1 s dose of H2O, and a 20 s purge of Ar. The deposition of Al metal was performed in a DCA 600 MBE system with a base pressure of 5 × 10−9 Torr.57 The deuterium gas (D2) and deuterium atom reduction were conducted in a custom-built vacuum chamber. Atomic deuterium was generated by a 400-W Osram Xenophot bulb, which had part of the glass enclosure removed to expose the tungsten filament. A current of 5.2 A was supplied by a DC power supply (KEPCO, MSK10-10M) to the tungsten filament and the filament temperature was ∼1800 K, as measured by a pyrometer. At this temperature, the tungsten filament can crack molecular deuterium to generate atomic deuterium.58 The deuterium gas (Matheson, 99.999%) pressure was controlled by a leak valve. The sample was positioned approximately 4 cm above and faced toward the tungsten filament. A pyrolytic boron nitride heater from Momentive was 2 cm above the sample. The sample temperature was monitored with a reference thermocouple in the vacuum chamber that was previously calibrated against an instrumented wafer.

X-ray reflectivity (XRR) was used to determine the CoO film thicknesses and growth rate. XRR was conducted by a Panalytical X’PERT Pro diffractometer using a sealed tube Cu Kα radiation source (λ ∼ 1.5406 Å) operating at 40 kV and 30 mA. The CoO film crystallinity and orientation were determined by in situ reflection high energy electron diffraction (RHEED). RHEED was performed using a Staib Instruments RHEED gun operated at 18 keV energy and 3° grazing incidence. The VG Scienta R3000 X-ray photoelectron spectroscopy (XPS) system with a monochromated Al Kα source at 1486.6 eV was used to determine film stoichiometry and composition, and the oxidation states of cobalt. The absolute energy scale of the analyzer on the XPS system is calibrated using a two-point measurement with the Ag 3d5/2 core level at 368.26 eV and the Fermi edge of Ag at 0 eV. Because of sample charging, all XPS peak positions were shifted by taking the CoO O 1s elemental peak to be at 530 eV.59 An ex situ Veeco Icon atomic force microscopy (AFM) system is used to characterize the film morphology in tapping mode with Bruker TESPA AFM tips.

To reveal the chemical composition as function of depth, we used time of flight secondary ion mass spectrometry (TOF-SIMS), a technique capable of ultrahigh elemental and surface sensitivity. The instrument employed was a TOF.SIMS 5 (ION-TOF GmbH, 2010) equipped with a pulsed (20 ns) Bi+ analysis ion gun (30 keV ion energy, 3.5 pA measured sample current) and a Cs+ (500 eV ion energy, 40 nA measured sample current) sputtering ion beam. During depth profiling, the sputtering beam was raster scanned over an area of 300 × 300 μm2 while the analysis beam was raster scanning an area of 100 × 100 μm2 centered within the regressing sputtered area. All detected ions had negative polarity and the vacuum was maintained at 7.5 × 10−10 Torr. The instrument provided a mass resolution >5000 (m/dm) for all detected masses. Due to the insulating nature of the Al2O3 and SiO2 films, a constant energy (21 eV) electron beam was shot on the sample for charge compensation. In addition, Al foil strips were used across the sample surface to increase the charge collection efficiency.

Magnetic property measurements were carried out with a Quantum Design physical property measurement system combined with a vibrating sample magnetometry (VSM) option. Magnetization hysteresis loops were measured with the magnetic field applied parallel to the plane of the thin films. The samples were at room temperature (25 °C) for all measurements.

Figure 1(a) shows the Co 2p XP spectra of 4.5-nm-thick CoO films grown on SiO2/Si and on MgO(001) at 180 °C. The binding energies of the main peaks are at 780.5 and 796.5 eV for Co 2p3/2 and Co 2p1/2 levels, respectively, and at 786.4 and 803.0 eV for the Co 2p3/2 and Co 2p1/2 for satellite peaks, respectively. The 2p binding energy position in conjunction with the very strong satellite at ∼6 eV higher binding energy is consistent with Co being in the +2 valence state with high spin.60 Figures 1(b) and 1(c) show RHEED images of CoO thin films grown on SiO2/Si and MgO(001) substrates, respectively. The centered-ring patterns [as shown in Fig. 1(b)] indicate that the CoO thin films grown on SiO2/Si have polycrystalline microstructure. The RHEED images in Fig. 1(c) show clear dotted streaks, which means that the CoO films grown on single crystal MgO(001) are epitaxial with the underlying substrate and have some surface roughness. The growth rate of CoO was found to be ∼0.3 Å/cycle on both SiO2/Si and MgO(001) substrates.

Fig. 1.

(a) Co 2p XP spectra of 4.5 nm CoO films on SiO2/Si (upper spectrum) and MgO(001) (lower spectrum) substrates grown by ALD at 180 °C. RHEED images of a 4.5-nm-thick CoO film grown on (b) SiO2/Si and a 4.5-nm-thick CoO film grown on (c) MgO(001).

Fig. 1.

(a) Co 2p XP spectra of 4.5 nm CoO films on SiO2/Si (upper spectrum) and MgO(001) (lower spectrum) substrates grown by ALD at 180 °C. RHEED images of a 4.5-nm-thick CoO film grown on (b) SiO2/Si and a 4.5-nm-thick CoO film grown on (c) MgO(001).

Close modal

The formation of CoO films on SiO2/Si and MgO(001) is observed over the 170–270 °C temperature range. At temperatures higher than 305 °C, cobalt metal formation was observed instead of CoO along with carbon incorporation into the cobalt metal. Figure 2(a) displays Co 2p XP spectra of 200-cycle CoO films grown at 270, 305, and 345 °C. While the film grown at 270 °C shows the presence of CoO, the films grown at 305 °C and 345 °C clearly show the presence of Co metal. Figures 2(b) and 2(c) represent RHEED images of Co metal grown on the MgO(001) substrate at 345 °C and 305 °C, respectively. The ring-centered patterns shown in Figs. 2(b) and 2(c) indicate that the thin Co films grown at 305 °C and 345 °C are polycrystalline on MgO(001). However, the spotted patterns are well ordered, meaning that the Co films have a certain orientation preference.

Fig. 2.

(a) Co 2p XP spectrum of 200-cycle CoO films grown on single crystal MgO(001) substrates using the ALD process ∼270 °C (upper spectrum at 780 eV), 305 °C (lower spectrum at 780 eV), and 345 °C (middle spectrum at 780 eV). (b) and (c) are RHEED images of 200-cycle CoO films grown on MgO(001) at 345 °C and 305 °C, respectively.

Fig. 2.

(a) Co 2p XP spectrum of 200-cycle CoO films grown on single crystal MgO(001) substrates using the ALD process ∼270 °C (upper spectrum at 780 eV), 305 °C (lower spectrum at 780 eV), and 345 °C (middle spectrum at 780 eV). (b) and (c) are RHEED images of 200-cycle CoO films grown on MgO(001) at 345 °C and 305 °C, respectively.

Close modal

Figure 3 presents the C 1s spectra for films deposited at 270–345 °C. There is ∼31% carbon incorporated into the 305 °C and 345 °C cobalt films as estimated using XPS (Fig. 3). The C 1s XPS intensity remains the same after a 15 min vacuum anneal at 500 °C [as shown in Fig. 3(b)]. This is different from the residual C (<2%) detected on the surface of as-deposited CoO films, which disappears after a slight vacuum anneal, as shown in Fig. 3(a). Carbon incorporation into Co films has also been reported using the same precursor [bis(N-tert-butyl-N′-ethylpropionamidinato) cobalt (II)] and hydrogen as a reducing agent.45,46

Fig. 3.

(a) C 1s XP spectrum of a 200-cycle CoO film grown on MgO(001) at 270 °C as-deposited (upper spectrum) and after a 15 min vacuum anneal at 500 °C (lower spectrum). (b) C 1s XP spectrum of a 200-cycle CoO film grown on MgO(001) at 305 °C as-deposited and after a 15 min vacuum anneal at 500 °C. Note the spectra overlap. (c) C 1s XP spectrum of an as-deposited 200-cycle CoO film grown on MgO(001) at 345 °C.

Fig. 3.

(a) C 1s XP spectrum of a 200-cycle CoO film grown on MgO(001) at 270 °C as-deposited (upper spectrum) and after a 15 min vacuum anneal at 500 °C (lower spectrum). (b) C 1s XP spectrum of a 200-cycle CoO film grown on MgO(001) at 305 °C as-deposited and after a 15 min vacuum anneal at 500 °C. Note the spectra overlap. (c) C 1s XP spectrum of an as-deposited 200-cycle CoO film grown on MgO(001) at 345 °C.

Close modal

Films deposited at 180 °C, a temperature that leads to CoO, do not have hydroxyl groups incorporated in or on the films. Figure 4 presents a representative O 1s XP spectrum of the ALD CoO film grown on MgO(001) at 180 °C. The O 1s peaks at 530 and 529.5 eV for CoO and MgO, respectively, are consistent with the literature.59,61 There is no indication of a peak around 532 eV associated with OH that other groups have reported following CoO deposition.9,62

Fig. 4.

O 1s XP spectrum of a 150-cycle ALD CoO film deposited on MgO(001) at 180 °C. Deconvolution of CoO and MgO O 1s features is shown as i and ii, respectively.

Fig. 4.

O 1s XP spectrum of a 150-cycle ALD CoO film deposited on MgO(001) at 180 °C. Deconvolution of CoO and MgO O 1s features is shown as i and ii, respectively.

Close modal

CoO films were reduced using either deuterium gas, atomic deuterium, or a thin layer of Al that acted as an oxygen scavenging layer. The reduction temperature has a significant impact on the resulting film morphology and dewetting is mitigated at the lower temperatures needed with atomic deuterium or Al. Figure 5 shows the Co 2p XP spectrum of a Co film after deuterium gas reduction of 4.5-nm-thick CoO grown on SiO2/Si. The D2 reduction was conducted at 420 °C with PD2=1×103Torr for 30 min. 420 °C is approximately the threshold temperature at which the reduction occurs in our experimental apparatus and at our D2 pressure. The absolute D2 (or H2) pressure may be a factor as Väyrynen et al. have reported CoO reduction to Co at 250 °C using 10% forming gas at approximately atmospheric pressure.9 After reduction, the binding energies of the main peaks are 778.1 and 793.2 eV for Co 2p3/2 and Co 2p1/2 levels, respectively, which agrees with the peaks of Co metal.63 There is a shoulder peak at 780.5 eV, which indicates there is still some CoO in the film. Based on peak fitting of the Co 2p3/2 peak against that of CoO and metallic Co, the film contains 98% Co and 2% CoO.

Fig. 5.

(a) Co 2p XP spectrum of a Co film after a 30 min deuterium gas reduction at 420 °C on 4.5-nm-thick CoO on SiO2/Si substrate.

Fig. 5.

(a) Co 2p XP spectrum of a Co film after a 30 min deuterium gas reduction at 420 °C on 4.5-nm-thick CoO on SiO2/Si substrate.

Close modal

The D2-reduced Co film reoxidized after exposure to the ambient environment as shown in Fig. 6(a). The film that was 98% metallic cobalt before removing the film from the UHV system reoxidized after 4 h of air exposure, as confirmed by the shift of Co 2p3/2 main peak from 778.1 to 780.5 eV. In an effort to protect the cobalt from reoxidizing, we deposited a 2-nm-thick Al capping layer on a D2 gas-reduced 4.5-nm-thick CoO sample using the MBE system. The metallic Al layer (∼73 eV) oxidized to an Al2O3 (∼75 eV) capping layer upon exposure to the ambient environment, as shown in Fig. 6(b). This Al2O3 capping layer is effective in protecting the reduced Co from reoxidizing. As shown in Fig. 6(c), the capped cobalt was still predominantly composed of metallic cobalt after 48 h of air exposure.

Fig. 6.

(a) Co 2p XP spectrum of a D2 gas-reduced Co sample before air exposure (upper spectrum) and after 4 h of air exposure (lower spectrum). (b) Al 2p XP spectrum of a D2 gas-reduced Co sample with a 2 nm Al capping layer before air exposure (upper spectrum at 78 eV) and after 48 h of air exposure (lower spectrum at 78 eV). (c) Co 2p XP spectrum of a D2 gas-reduced Co sample with a 2 nm Al capping layer before air exposure (upper spectrum) and after 48 h of air exposure (lower spectrum).

Fig. 6.

(a) Co 2p XP spectrum of a D2 gas-reduced Co sample before air exposure (upper spectrum) and after 4 h of air exposure (lower spectrum). (b) Al 2p XP spectrum of a D2 gas-reduced Co sample with a 2 nm Al capping layer before air exposure (upper spectrum at 78 eV) and after 48 h of air exposure (lower spectrum at 78 eV). (c) Co 2p XP spectrum of a D2 gas-reduced Co sample with a 2 nm Al capping layer before air exposure (upper spectrum) and after 48 h of air exposure (lower spectrum).

Close modal

The morphology of cobalt layers was studied with AFM and these measurements required the samples to be removed from the UHV system. Figure 7(a) presents AFM scans of a 4.5-nm-thick CoO film on SiO2/Si and Figs. 7(b) and 7(c) present the changes to the sample after reduction with D2. Figure 7(b) corresponds to a reduced CoO film that was exposed to ambient after which it reoxidized to CoO. Figure 7(c) corresponds to a reduced CoO film that had 2 nm of Al deposited upon it; the Al oxidized to Al2O3 and the cobalt remained reduced beneath the Al2O3. The reoxidized CoO has a root mean squared roughness (Rrms) of 3.66 nm [Fig. 7(b)] and the Al2O3-capped, reduced cobalt has a Rrms of 3.50 nm [Fig. 7(c)]. Figures 7(b) and 7(c) are presented to illustrate that morphology changes upon reduction are reasonably reflected in a reoxidized layer and the additional step of depositing 2 nm of Al may be unnecessary to study the layer morphology changes.

Fig. 7.

(a) and (b) are AFM images of the as-deposited CoO film (Rrms = 0.42 nm) and the reoxidized Co film after 30 min deuterium gas reduction at 420 °C (Rrms = 3.66 nm), respectively, (c) is the AFM image of an Al2O3 (2 nm Al)/Co (reduced by D2 gas at 420 °C)/SiO2/Si heterostructure (Rrms = 3.50 nm), and (d) is the AFM image of an Al2O3 (2 nm Al)/Co (reduced by D atom at 220 °C)/SiO2/Si heterostructure (Rrms = 0.86 nm).

Fig. 7.

(a) and (b) are AFM images of the as-deposited CoO film (Rrms = 0.42 nm) and the reoxidized Co film after 30 min deuterium gas reduction at 420 °C (Rrms = 3.66 nm), respectively, (c) is the AFM image of an Al2O3 (2 nm Al)/Co (reduced by D2 gas at 420 °C)/SiO2/Si heterostructure (Rrms = 3.50 nm), and (d) is the AFM image of an Al2O3 (2 nm Al)/Co (reduced by D atom at 220 °C)/SiO2/Si heterostructure (Rrms = 0.86 nm).

Close modal

The CoO films have a typical roughness of 0.4–0.5 nm on SiO2/Si and MgO(001). The Rrms of the SiO2/Si substrate is 0.35 nm and the CoO film [Fig. 7(a)] has a roughness of 0.42 nm. Upon reduction at 420 °C, the roughness increased substantially to 3.50 nm and a discontinuous Co layer formed. This dewetting phenomenon is common in high temperature processing of metals,52–55 and it is undesired when continuous Co films are required.

To attenuate dewetting and produce smoother and continuous Co metal films, we studied two low temperature reduction processes, D atom reduction, and use of an O-scavenging layer. Atomic deuterium is more reactive than D2 gas, therefore in principle, the threshold temperature for CoO reduction by D atoms should be lower than that by D2 gas.64 We performed a series of CoO reduction studies with D atom exposure at different temperatures with PD2=1×105Torr for 30 min and found that the threshold temperature is about 220 °C in our experimental apparatus. Figure 8(a) shows the Co 2p XP spectrum of the Co film after D atom reduction at 220 °C of a 4.5-nm-thick CoO film grown on SiO2/Si. The binding energies of the main peaks are 778.1 and 793.2 eV for the Co 2p3/2 and Co 2p1/2 levels, respectively, and there is no shoulder peak at 780.5 eV, which means 100% CoO has been reduced to Co metal. Figure 8(b) shows the C 1s XP spectrum for the Co metal film produced by D atom reduction, from which we can conclude that there is no carbon incorporation in the film.

Fig. 8.

(a) and (b) are Co 2p XP spectrum and C 1s XP spectrum of the Co film after 30 min deuterium atom reduction at 220 °C of a 4.5 nm-thick CoO film on SiO2/Si, respectively. (c) AFM image of the uncapped Co film after 30 min deuterium atom reduction at 220 °C of a 4.5 nm-thick CoO on SiO2/Si (Rrms = 0.70 nm).

Fig. 8.

(a) and (b) are Co 2p XP spectrum and C 1s XP spectrum of the Co film after 30 min deuterium atom reduction at 220 °C of a 4.5 nm-thick CoO film on SiO2/Si, respectively. (c) AFM image of the uncapped Co film after 30 min deuterium atom reduction at 220 °C of a 4.5 nm-thick CoO on SiO2/Si (Rrms = 0.70 nm).

Close modal

The density difference between CoO and Co leads to a thickness contraction of 26.3% upon reducing CoO to Co if the film contracts only in the out-of-plane direction. A continuous, uniformly thick, and pinhole-free Co layer would be ∼3.3-nm-thick after complete reduction of a 4.5-nm-thick CoO film if it only contracted perpendicular to the surface. We consider Co films to be continuous when the Rrms does not exceed 1 nm, which is about one-third of the expected thickness, and the Rrms is not greater than approximately two times the CoO film Rrms. Figure 8(c) is the AFM image of the uncapped Co metal film produced by D atom reduction; the Rrms of this film is 0.70 nm after it reoxidized. We also deposited a 2 nm Al capping layer on a D atom-reduced 4.5-nm-thick CoO sample. Figure 7(d) is the AFM image of an Al2O3 (2 nm Al)/Co (reduced by D atom)/SiO2/Si sample; the Rrms of this film is 0.86 nm. Since the Rrms changes from 0.42 nm for CoO to 0.86 nm for Co, this Co metal film is likely continuous.

Similar morphology results are found upon reduction of CoO on MgO(001) substrates with D2 or atomic D. The MgO(001) substrate has an Rrms of 0.18 nm. Figure 9(a) represents the morphology of as-deposited 4.5-nm-thick CoO films on MgO(001) and the film roughness is 0.52 nm. After a 30 min D2 gas reduction at 500 °C with PD2=1×103Torr, the AFM results in Fig. 9(b) (Rrms of 2.03 nm) reveal the dewetting problem with the reoxidized Co film featuring void regions, while after a 30 min D atom reduction at 250 °C with PD2=1×105Torr to produce a Co film, the resulting reoxidized Co film is continuous with a roughness at 0.90 nm, as shown in Fig. 9(c).

Fig. 9.

(a) AFM image of an as-deposited 4.5-nm-thick CoO film grown on MgO(001) by ALD at 180 °C (Rrms = 0.52 nm) and the Co 2p spectrum before exposure to ambient. (b) AFM image of the uncapped Co film after 30 min D2 gas reduction at 500 °C of 4.5-nm-thick CoO on MgO(001) (Rrms = 2.03 nm) and the Co 2p spectrum before exposure to ambient. (c) AFM image of the uncapped Co film after 30 min deuterium atom reduction at 250 °C of 4.5-nm-thick CoO on MgO(001) (Rrms = 0.90 nm) and the Co 2p spectrum before exposure to ambient.

Fig. 9.

(a) AFM image of an as-deposited 4.5-nm-thick CoO film grown on MgO(001) by ALD at 180 °C (Rrms = 0.52 nm) and the Co 2p spectrum before exposure to ambient. (b) AFM image of the uncapped Co film after 30 min D2 gas reduction at 500 °C of 4.5-nm-thick CoO on MgO(001) (Rrms = 2.03 nm) and the Co 2p spectrum before exposure to ambient. (c) AFM image of the uncapped Co film after 30 min deuterium atom reduction at 250 °C of 4.5-nm-thick CoO on MgO(001) (Rrms = 0.90 nm) and the Co 2p spectrum before exposure to ambient.

Close modal

We also explored a second, low-temperature method to reduce ALD-grown CoO films to Co. We capped the CoO with metals that have a high affinity for oxidation, such as Al, which reacted with oxygen in CoO and formed a capping oxide on the Co metal film. A 4.5-nm-thick CoO film was grown on the SiO2/Si substrate by ALD at 180 °C and the sample was then transferred in situ to the MBE chamber where a 3-nm-thick Al metal film was deposited on it at the substrate temperature of 200 °C. The choice of 3 nm for the Al layer thickness is based on complete reduction of CoO and to ensure the Al2O3 layer would effectively prevent any underlying Co from reoxidizing.

The Co 2p XP spectrum after capping CoO with Al at 200 °C is shown in Fig. 10(a). The binding energies of the Co 2p3/2 peak at 778.1 eV and the Co 2p1/2 peak at 793.2 eV confirm a metallic cobalt formed from the cobalt oxide.60 This suggests that the Al reacts with CoO to form Al2O3 and Co. The Gibbs free energy ΔGr0 (298 K) of the reaction between 3 mol CoO and 2 mol Al is −939.8 kJ/mol,65 which means that the reaction is favored even at room temperature. The reaction between Al and CoO is also demonstrated by the Al 2p XP spectrum shown in Fig. 10(b). The small peak at a binding energy of ∼73 eV is for Al metal, while the larger peak at a binding energy of ∼75 eV is for Al2O3.66 The existence of Al metal after reaction indicates the complete reduction of CoO. There is a minor peak at a binding energy of ∼73.6 eV, which agrees with the Al 2p peak position for CoAl2O4,67 indicating the formation of a CoAl2O4 interlayer. The formation of a CoAl2O4 interlayer is reported when there is diffusion between Co and Al2O3.68 Figure 10(c) shows that the morphology of this Al2O3 (3 nm Al)/Co/SiO2/Si sample has an Rrms of 0.83 nm, from which we can infer the underlying Co film is continuous.

Fig. 10.

(a) Co 2p XP spectrum of a film stack after a deposition of a 3-nm-thick Al on an as-deposited 4.5-nm-thick ALD CoO film on SiO2/Si. (b) Al 2p XP spectrum of the film after deposition of 3-nm-thick Al on CoO/SiO2/Si. (c) AFM image of the Al2O3 (3 nm Al)/Co/SiO2/Si heterostructure (Rrms = 0.83 nm).

Fig. 10.

(a) Co 2p XP spectrum of a film stack after a deposition of a 3-nm-thick Al on an as-deposited 4.5-nm-thick ALD CoO film on SiO2/Si. (b) Al 2p XP spectrum of the film after deposition of 3-nm-thick Al on CoO/SiO2/Si. (c) AFM image of the Al2O3 (3 nm Al)/Co/SiO2/Si heterostructure (Rrms = 0.83 nm).

Close modal

Figure 11 presents XP spectra for the Co 3s feature at 100.8 eV for 4.5-nm-thick CoO films on SiO2/Si(001) substrates. Spectra are also presented after reducing films with atomic D at 220 °C [Fig. 11(a)] and D2 at 420 °C [Fig. 11(b)]. After reduction, an additional feature appears at 103.5 eV that is associated with Si 2p from the SiO2 substrate. The spectra in Fig. 11(a) are for reduction conditions that lead to Rrms < 1 nm. The Co 3s signal dominates the substrate Si 2p signal before reduction in both cases, which is consistent with electron effective attenuation length estimates of 4.5 nm CoO attenuating ∼94% of the underlying substrate signal.69 Assuming contraction of the CoO to Co metal perpendicular to the substrate, a 3.3 nm Co metal film would attenuate ∼92% of the underlying substrate signal. After reduction, the substrate Si 2p signal appears at 103.5 eV in both cases, while the Co 3s metal peak appears at 100.8 eV. In both reduction spectra, the Si 2p signal is greater than what is expected for a uniformly thick 3.3 nm Co metal film. The relative intensity of the Si 2p signal compared to the Co 3s signal is larger for D2 gas reduction, while the D atom reduction has a lower intensity. Combining these results with TOF-SIMS results discussed below indicates the D atom reduction generated a thin, nonuniform Co metal layer on the SiO2/Si(001) substrate. The increase in Si 2p signal intensity in Fig. 11(b) strongly suggests voids formed in the Co metal film reduced in D2 gas at 420 °C. It was not possible within the body of work to determine if a continuous Co network formed that did not fully cover the SiO2/Si(001) substrate in a manner similar to 16 nm Co films reported on TiN,9 or if discontinuous Co islands formed following 420 °C reduction.

Fig. 11.

(a) Co 3s and Si 2p XP spectra of 4.5 nm CoO on SiO2/Si(001) before and after D atom reduction at 220 °C for 30 min. (b) Co 3s and Si 2p XP spectra of 4.5 nm CoO on SiO2/Si(001) before and after D2 gas reduction at 420 °C for 30 min. Curves i represent the Si 2p feature after deconvoluting spectra recorded after reduction. Curves ii represent the Co 3s feature after deconvoluting spectra recorded after reduction. Curves iii are the spectra recorded before reduction showing the fit to the Co 3s feature.

Fig. 11.

(a) Co 3s and Si 2p XP spectra of 4.5 nm CoO on SiO2/Si(001) before and after D atom reduction at 220 °C for 30 min. (b) Co 3s and Si 2p XP spectra of 4.5 nm CoO on SiO2/Si(001) before and after D2 gas reduction at 420 °C for 30 min. Curves i represent the Si 2p feature after deconvoluting spectra recorded after reduction. Curves ii represent the Co 3s feature after deconvoluting spectra recorded after reduction. Curves iii are the spectra recorded before reduction showing the fit to the Co 3s feature.

Close modal

Previous work has shown that CoO ALD films grown at 170–180 °C from cobalt bis(diisopropylacetamidinate) were epitaxial and crystalline on SrTiO3-templated Si(001) and polycrystalline on anatase TiO2.2 AFM and cross-sectional scanning transmission electron microscopy revealed that surface roughness increased with film thickness and that the films were dense and pinhole-free with sharp interfaces to the substrate.2 The 4.5-nm-thick CoO films reported herein have Rrms values of ∼0.5 nm, considerably less than the 0.92 Rrms value for 8-nm-thick CoO on SrTiO3-templated Si(001). Given the absence of an identifiable Si 2p feature in Figs. 11(a) and 11(b) prior to reduction, and by extrapolation of the results reported in Ref. 2, it is reasonable to conclude the 4.5-nm-thick CoO films are smooth, continuous, and void-free.

The issue of continuous versus discontinuous Co layers forming during CoO reduction was examined with TOF-SIMS. A 4.5 nm CoO film was reduced using atomic D at 220 °C for 60 min at PD2=1×106Torr and capped with 2 nm of Al to prevent reoxidation of the Co after removing the sample from the growth system. (Note this reduction condition is for a longer time and at a lower pressure than the D-atom-reduced results presented herein.) This film has a Rrms of 0.98 nm. Continuity of the Co metal film can be probed by monitoring when the Al2O3 capping layer disappears relative to where the Co layer disappears. The depth profiles (as reflected with the sputtering time) of the species of interest are shown in Fig. 12. Presenting a strong signal at the surface, Al2O3 was selected as a proxy for the Al film, thus demonstrating the full Al layer oxidation. Co and Si were selected as markers for the Co and SiO2 layers, respectively. Al2O3 and Co secondary ions were chosen as the TOF-SIMS species markers for Al and Co, respectively, due to the lack of nearby peaks that might have convoluted the ion signal. The log representation of the profiles shows intermixing between the Al2O3 and Co layers, which proves that there is a discontinuity in the Co layer. Depth profiling revealed the Al2O3 signal attenuated faster than the Co signal. Further, the Co signal remains past the sputtering time the Al2O3 signal reaches very low values. Thus, the Co metal film is continuous, but nonuniform in thickness. Additional deconvolution of the layer corrugation and sputtering effects is further required to comment on the Co film nonuniform thickness (ongoing work). Based on the TOF-SIMS and XPS results, we suggest 4.5 nm CoO films that are reduced under mild conditions and generate films with Rrms ≤ 1 nm are continuous and fully cover the substrate surface.

Fig. 12.

Secondary ion signals vs sputtering time for a 4.5 nm CoO film on SiO2/Si(001) that had a Rrms of 0.98 after deuterium atom reduction to Co.

Fig. 12.

Secondary ion signals vs sputtering time for a 4.5 nm CoO film on SiO2/Si(001) that had a Rrms of 0.98 after deuterium atom reduction to Co.

Close modal

Motivated by potential use of Co layers in devices, such as the MTJ, we explored the magnetic characteristics of the Co layers that were formed under conditions that led to excessive dewetting, i.e., high temperature reduction with D2, and that were designed to minimize dewetting, i.e., reduction with atomic D or an Al capping layer. Figure 13 displays the magnetization hysteresis loops of three samples as a function of magnetic field applied parallel to the substrate surface. All the measurements were conducted within one day after the samples were removed from the UHV system and the Co films stayed reduced when they were measured.

Fig. 13.

(a)–(c) are the magnetization hysteresis loops of samples A, B, and C, respectively, measured by VSM. The magnetic field is applied parallel to the plane of the thin films. The insets in (b) and (c) are enlarged versions of the corresponding loops around zero field. The x axis range of the inset in (b) is from −1000 to 1000 Oe. The x axis range of the inset in (c) is from −300 to 300 Oe. Background signals from the substrate and the capping layer have been subtracted from the measured loops.

Fig. 13.

(a)–(c) are the magnetization hysteresis loops of samples A, B, and C, respectively, measured by VSM. The magnetic field is applied parallel to the plane of the thin films. The insets in (b) and (c) are enlarged versions of the corresponding loops around zero field. The x axis range of the inset in (b) is from −1000 to 1000 Oe. The x axis range of the inset in (c) is from −300 to 300 Oe. Background signals from the substrate and the capping layer have been subtracted from the measured loops.

Close modal

Sample A is a 4.5 nm CoO film grown on SiO2/Si and then reduced by D2 gas at 420 °C and finally covered by a 2 nm Al capping layer. Sample B is a 4.5 nm CoO film grown on SiO2/Si and then reduced by atomic D at 220 °C and finally covered by a 2 nm Al capping layer. Sample C is a 4.5 nm CoO film grown on SiO2/Si and then reduced by a 3 nm Al capping layer at 200 °C. The measured coercivities of samples A, B, and C are 480, 90, and 20 Oe, respectively. The two samples, B and C, produced by low temperature reduction processes that lead to Rrms ≤ 1 nm have much smaller coercivity than sample A, which is produced by high temperature D2 gas reduction. This may be explained by microstructure differences between the continuous nature of Co films produced by the low temperature reduction processes and the nature of the Co films produced by the high temperature reduction process.70 The difference between the coercivity of the Co film produced by deuterium atom reduction and the coercivity of the cobalt film produced by an oxygen scavenger layer reduction may come from the presence of a potentially diffusion-induced CoxAlyOz interlayer. The deterioration of the coercivity due to the diffusion-induced interface layer is demonstrated in a hard/soft multilayer in literature.71 

We report the growth of carbon-free CoO on SiO2/Si and MgO(001) substrates by ALD in a temperature range of 180–270 °C. While the CoO films grown on SiO2/Si are polycrystalline, the CoO films grown on single crystal MgO(001) are crystalline and epitaxial. High temperature thermal reduction of CoO causes severe dewetting. We demonstrate the reduction of CoO to form smooth and continuous carbon-free Co metal films by two low temperature processes, using a deuterium atom source and using an Al metal layer as oxygen scavenger. The Co films produced by the low temperature reduction processes have smaller coercivity than the cobalt films produced by the high temperature thermal reduction. These results provide a process to form Co metal films from ALD-grown CoO and indicate routes to tune the microstructure and magnetic properties through the reduction method and temperature.

This work was supported by the National Science Foundation (NSF) (Grant No. EEC-1160494). The magnetic characterization was supported by the NSF (No. NNCI-1542159) at the Texas Nanofabrication Facility. The authors wish to thank A. Dolocan for assistance with the TOF-SIMS analysis.

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