Characterization of ultrathin nickel films deposited by thermal laser evaporation

Characterization of ultrathin nickel films deposited by thermal laser evaporation David S. Catherall, Yifei Yan, Finley B. Donachie, Azmain A. Hossain, and Austin J. Minnicha) Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125, USA (Dated: 23 October 2025) We report the evaporation and characterization of ultrathin nickel films deposited with a home-built thermal laser evaporation system. The system employs a continuous-wave 1 kW fiber laser (1070 nm) focused to sub-millimeter diameter onto a nickel target rod mounted inside an ultrahigh-vacuum chamber. The laser heats the target to a high enough temperature to produce vapor for film deposition; for Ni, this temperature is around the melting point of 1725 K. An Arrhenius dependence of the deposition rate on laser power is observed. A 14.7±0.1 nm thick film deposited on a sapphire substrate exhibits a root mean square roughness of 1.10±0.14 nm. The room-temperature electrical resistivity was measured using the Van der Pauw method and yielded 22 ±0.2µΩ cm, a value in quantitative agreement with reports for Ni films of similar thicknesses. This work advances the growing consensus regarding the promising potential of TLE for thin film deposition and epitaxy. a)Electronic mail: [email protected] 1 I. INTRODUCTION Physical vapor deposition of thin films is performed by various well-established techniques, including thermal and electron beam evaporation (e-beam), sputtering, pulsed laser deposition (PLD), and others.1Evaporative methods rely on the condensation of vapor created by heating of source material onto a substrate. Sputtering and PLD are non-equilibrium techniques based on the ejection of material from a target by plasma ion impingement and the ablation of a target with energetic laser pulses, respectively. These techniques enable directional or conformal deposition of diverse elemental and compound materials. Film deposition by evaporation is of interest to achieve high-purity films and for epitaxial growth using molecular beam epitaxy (MBE). However, evaporation of refractory elements is a long-standing challenge owing to their low vapor pressure. For instance, elements like Ru, Nb, Ta, and Mo can technically be deposited by e-beam evaporation but typically exhibit deposition rates well below 1 ˚ A/sec. Additionally, careful monitoring and preparation are required to avoid damaging the evaporation system due to the required high powers and temperatures. Thermal laser evaporation (TLE) is a technique that has the potential to overcome these limitations.2–5TLE uses continuous-wave (CW) lasers focused onto freestanding targets (slugs or crucibles) to provide the necessary thermal energy to produce vapor for deposition. It differs from PLD (or laser-MBE) in that heating is performed using CW rather than pulsed lasers, leading to deposition by thermal evaporation rather than an ablation process.3,6,7 The quasi-stationary nature of the deposition makes the deposition process more similar to thermal PVD techniques like e-beam evaporation. TLE requires no crucible for many materials, allowing for the deposition of both refractory metals and elements that may react with a crucible. Because the energy source is located outside of vacuum, the technique is especially suitable for reactive evaporation since there are no hot filaments or resistive heaters that could be degraded by exposure to process gases (e.g. O2)4. Finally, the sourcesubstrate distance is typically an order of magnitude smaller than in e-beam evaporation or MBE, allowing for higher pressures of reactive background gases without scattering the metallic vapors. TLE was initially developed contemporaneously with MBE, electron-beam evaporation, and PLD in the late 1960s. A variety of oxides and compounds were deposited using TLE2,3, 2 as well as pure elements including C8,9, Os10, Pt11, and Te12. However, laser technology of the time was inadequate for TLE to compete with other techniques due to two difficulties. First, the most practical CW laser of the time was the CO2laser, which was then limited in power to a few hundred watts12. Further, its 10.6 µm wavelength is not well absorbed by metals, leading many investigators to focus on oxide or compound targets3,13. Second, the coating of optical elements exposed to the heated target added complexity to the energy delivery system compared to effusion cells or electron beam systems4. Although this latter drawback could be partially worked around by using mirrors and apertures4,12,14, the other techniques were technically simpler to implement. Due in large part to the maturation of other PVD techniques, TLE was neglected for decades. Recent developments in laser technology, however, have made the technique increasingly viable. The key development has been the commercial availability of high-power fiber lasers, which emit at micron wavelengths and feature output powers up to multiple kilowatts. Fiber lasers are more useful than the CO2laser for evaporating most elements due to the order of magnitude higher absorption of metals at around 1 micron compared to 10.6 microns13. Recently, a group at the Max Planck Institute at St¨uttgart has developed a modern implementation of TLE using high-power CW fiber lasers and deposited many elements, oxides, and nitrides.5,15–18 However, modern TLE has only been performed by one group to date. Further, although the deposition of many elements has been reported, the properties of many of the TLE-deposited films remain to be characterized. Here, we report the deposition of ultrathin Ni films using a home-built TLE system employing a 1 kW CW fiber laser with a 1070 nm wavelength. A deposition rate of 0.35 ˚ A/sec was achieved with 177 W of CW laser power. The deposited film was 14.7±0.1 nm thick and exhibited RMS roughness Rq= 1.10 ±0.14 nm. The electrical resistivity was measured as 22 ±0.2µΩ cm, which is in quantitative agreement with prior values reported for Ni films of a similar thickness. II. EXPERIMENT A photograph of our home-built TLE system is shown in Fig. 1(a). It consists of a watercooled vacuum chamber (Kurt J. Lesker Hydra Cool™) with a turbopump providing a base pressure below 10−9Torr. Inside the chamber are a target holder and substrate holder, 3 FIG. 1 (A) Photograph of the TLE system at Caltech. At the top left is the laser delivery system. The center is the bellows mounted to the sample holder. Around the center from the left to right are a RHEED system, QCM, and shutter control, respectively. At bottom are the RGA and two bellows to which the target holders are mounted. (B) Rendering of the fiber laser delivery system. 1. Kinematic laser mount 2. Laser collimator 3. Lenses 4. Thermocouple 5. Water cooled mirror mount 6. Mirror 7. Laser window. which are both mounted on bellows to enable target alignment and sample transfer. The chamber is also equipped with a quartz-crystal microbalance (Telemark) for deposition rate monitoring and a residual gas analyzer (SRS RGA200) to monitor the vacuum conditions. The target consists of a 0.5 in long, 0.25 inch diameter Ni rod (Lesker, 99.995%) which was machined down to 0.125 inches at the base to minimize the thermal conductance to the target holder. The target is mounted in a press-fit slot, and a type-C thermocouple is mounted under the target to monitor the temperature. A 1 cm square sapphire substrate (University Wafer, C-plane, SSP) is mounted on a home-built sample holder which suspends the sample upside down approximately 3 inches above the target. All depositions in this work were performed on unheated substrates and without substrate rotation. Further details of the target and substrate holders will be given in a future publication. The laser delivery system is shown in Fig. 1(b). The laser is a 1 kW fiber laser (IPG 4 Photonics, 1070 nm) and terminates in a water-cooled free-space coupler and collimator, which is mounted in a kinematic laser mount to enable fine beam alignment. Two C-coated lenses (Thorlabs) are used to focus the beam to sub-millimeter diameter on the target. An antireflective window is used to pass the beam into vacuum (Lesker VPZL-275LYAG). The final optical element is an aluminum off-axis parabolic mirror (Thorlabs) mounted on a water-cooled rotatable flange. A type K thermocouple is placed in contact with the mirror to monitor its temperature. A water cooled beam dump is placed in the path of the beam specularly reflected by the target. The deposition procedure consisted of the following steps. First, the system was pumped down to a base pressure below 10−9Torr. No chamber bakeout was performed. A movable Ti shutter covered the sample for the entire process until deposition was initiated. The laser power was held between 119 and 144 W for approximately an hour to bake out adsorbed gases from the target and its holder. After the pressure stabilized around 10−7Torr, the power was steadily increased until a steady-state deposition rate was measured in the QCM. The shutter was then opened to allow deposition. For the present study, the deposition rate was 0.35 ˚ A/sec. III. RESULTS We first present qualitative features of the deposition process. Figs. 2(a) and 2(b) show images of the Ni target before and after deposition, respectively. The remnants of the melt pool formed during deposition can be observed in Fig. 2(b). The deposition rate measured by QCM versus inverse laser power is shown in Fig. 2(c). To account for the geometric factor from the positioning of the QCM relative to the sample, the measured rates were scaled to match the measured film thickness. The deposition rate exhibits the expected Arrhenius dependence on laser power15, with a power of 202 W yielding a deposition rate of 0.98 ˚ A/sec. Next, we examine the physical characteristics and surface morphology of the films. The film thickness was measured using ex-situ ellipsometry on a J.A. Woollam M2000 with the built-in profiles for sapphire and nickel. The measurement yielded a value of 14.7±0.1 nm, which was not changed by incorporating roughness or native oxide. This value is quantitatively consistent with step height measurement via atomic-force microscopy (AFM) performed on a step formed by the sample holder (not shown). The film surface morphology 5 FIG. 2 Photograph of the nickel target mounted in the chamber (A) before deposition and (B) after deposition. A red guide laser is on in both images. Before deposition, the laser is observable from the camera due to diffuse reflections from the surface before melting. After deposition, the guide beam is not visible due to the increased specularity of the melt pool. (C) Deposition rate measured by QCM versus inverse laser power (symbols) and a fit line. The data follow an Arrhenius relationship. was characterized using a Bruker Dimension Icon AFM in PeakForce Tapping mode with a ScanAsyst-Air probe. The AFM scans of the sample are presented in Fig. 3. The scans indicate that the sample exhibits the smoothness expected of a thermally evaporated thin metal film growing by nucleation and coalescence, with RMS roughness Rq= 1.10 ±0.14 nm.19 Finally, the electrical resistivity was measured using the van der Pauw method (vdP). Wirebonding was performed using Al wires with a Westbond 7476D Wire Bonder, and the vdP measurement was performed using a Keithley 2400 SourceMeter. At room temperature, the value was measured as 22.0±0.2µΩcm. Fig. 4plots this value along with other values for Ni films of varying thickness from prior studies.20,21 It is observed that the measured value is compatible with those reported for films of a similar thickness. We conclude that 6 FIG. 3 AFM scan of the film surface. (A) 500 ×500 nm scan (256 lines, 0.3 Hz) and (B) 2×2µm scan (256 lines, 0.5 Hz). The RMS roughness Rq= 1.10 ±0.14 nm. FIG. 4 Electrical resistivity versus film thickness at room temperature. The measurement from this work is indicated as the red point (thickness of 1470 ˚ A). Referenced data from Ref. 20 (blue triangles) and Ref. 21 (green squares). the deposited films are of good quality. Future work will characterize the temperaturedependence of the resistivity and the residual resistance ratio to gain further insight into the structural quality and chemical purity of the films, as well as demonstrate deposition of other elements and compounds by TLE in our system. 7 IV. SUMMARY In summary, we have reported the deposition and characterization of ultrathin Ni films using a home-built thermal laser evaporation system. The successful demonstration of a modern implementation of TLE is the first to be reported in the United States. The films exhibit smoothness consistent with that expected of thin metallic films and electrical resistivity on par with those of films with similar thickness. Our work adds to the growing consensus regarding the promising potential of TLE for deposition of high-quality refractory films and compounds, molecular beam epitaxy, and related applications. ACKNOWLEDGEMENTS The TLE system was acquired under AFOSR Award FA9550-23-1-0731 and an award from the De Logi Foundation at Caltech. D.S.C. and A.J.M. were supported by AFOSR Award FA9550-22-1-0286. Y.Y. was supported by an Explorer Grant from the Resnick Sustainability Institute at Caltech. F.B.D. acknowledges support from the National Science Foundation Graduate Research Fellowship Program. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. 2139433. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. We gratefully acknowledge the critical support and infrastructure provided for this work by the Kavli Nanoscience Institute and the Molecular Materials Research Center of the Beckman Institute at the California Institute of Technology for the use of their facilities. 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