Comparison of Continuous and Pulsed Low-Power DC Sputtered Ti Thin Films Deposited at Room Temperature

Abstract

Titanium thin films with thicknesses of up to 105 nm were deposited on borosilicate glass implementing low-power continuous (25 W) and pulsed (85 W, with an ultra-low duty cycle) DC magnetron sputtering. The characteristics of the resulting films were studied via atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), VIS spectroscopy, and four-point-probe measurements. Both deposition modes yield films with low surface roughness, and AFM analysis showed no topographical features indicative of columnar-and-void structures. The films exhibited high optical reflectivity and stable transmittance and reflectance across the visible spectrum. The electric resistivity could be measured even at single nanometer thickness, emphasizing the metallic character of the films and approaching the bulk titanium value at higher film thicknesses. The low power regime of magnetron sputter deposition not only offers the possibility of studying the development of physical characteristics during the growth of ultra-thin films but also provides the advantage of extremely low heat development and no evident mechanical stress on the substrate during the coating process. These results outline a path for low-power DC sputtering as a reliable approach for studying the evolution of functional properties in ultra-thin films and for the gentle fabrication of coatings where thermal stress must be avoided, making the method compatible with temperature-sensitive applications.

1. Introduction

Titanium (Ti) has a long history as a material of choice in the aerospace industry [1] and the development of wear-resistive coatings [2] due to its excellent mechanical properties. The natural ability to form an oxide surface layer that protects the underlying material from corrosion [3] makes Ti one of the most used biocompatible materials in medical applications [4,5]. Recently, nanometer to micrometer-sized Ti–TiO₂ structures and films have been applied across a wide range of emerging technologies, including solar panels, gas sensors, plasmonic biosensing, resistive switching devices, optical filtering, hydrogen storage, and structural coloration effects [6,7,8,9,10,11,12,13,14,15,16,17,18]. Their functionalities often rely on the tunable electronic, optical, and morphological properties of titanium and its oxides, particularly when integrated as multilayer coatings or pattered nanoscale films. Besides techniques like thermal [3,19,20,21] and e-beam evaporation [22,23,24], Ti thin films can be produced by magnetron sputtering [25], both in continuous and pulsed mode, on an industrial scale, with strong adhesion to a wide range of substrates [26,27,28,29,30,31,32,33,34,35].

A novel area of application for thin films is the fabrication of sputtered VHF (very high frequency) and UHF (ultra-high frequency) flat antennas, consisting of single [36] or multilayer films [37] on various types of rigid and flexible substrates [38]. Often, the main requirement for the substrates’ feasibility in such applications is the use of lightweight, wear-resistant, inert, and deformable materials, such as polycarbonates or polyimide (like Kapton). As substances combining these properties are typically sensitive to heat and mechanical deformation, the fabrication techniques of the conductive antenna patterns are limited to methods with minimal invasion and heat development. Depending on the application, adhesion [39] or embroidering [40] are most commonly implemented. While the optical, mechanical, and chemical properties of the metallic coatings largely determine the field of use, the electrical resistivity of the employed materials is a crucial parameter in antenna design regarding efficiency and resonance frequency [39,41,42]. Hence, the decreased electrical conductivity of thin films in contrast to the corresponding metallic bulk is a significant challenge in coating such antennas. In recent years, the diversity of substrates has opened an immense field of applications for such flexible electronic devices, ranging from health monitoring and supervision [38,39] to sensor systems in extreme conditions, as in aerospace [36,43]. While magnetron sputter deposition is generally a low-cost, expeditious, high-quality, and large-scale coating technique, it is rarely used in temperature-sensitive environments due to the substantial temperature increase during deposition. Nonetheless, to profit from its numerous advantages, we propose the utilization of low-power magnetron DC sputtering where the substrate is kept at room temperature without any additional cooling during the deposition process. In this study, we compare a continuous and an ultra-low duty cycle pulsed DC sputtering method at low-power conditions to evaluate their impact on film quality and process control. Both low-power regimes guarantee a gentle deposition process that does not inflict damage on sensitive substrate materials due to unwanted substrate heating. Additionally, the pulsed mode enables precise growth of ultra-thin layers (below 5 nm) to study early-stage film formation.

Currently, most studies on Ti thin films implement high-power sputtering or evaporation techniques and rely on additional post-deposition treatment, such as annealing, to achieve the desired film properties [27,28,31,34]. While some studies investigate the regime of low-power sputtering, these works typically target thicker films or do not combine extensive characterization techniques [29,31,34,35,44,45]. Thus, there remains a lack of systematic studies that follow the evolution of structural, optical, and electrical properties across the range from sub-nanometer to >100 nm thickness regimes using a precisely controlled deposition process.

This study fills this gap by systematically analyzing the correlation between surface morphology, optical reflectance, and electrical resistivity in ultra-thin Ti films. To the best of our knowledge, no report has combined sample characterization via X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), electrical resistivity measurements, and ultraviolet-visible spectroscopy (UV-VIS) on low-power sputtered Ti thin films with thicknesses below 105 nm. Furthermore, a gentle, heat-free deposition approach offers a path toward fabricating functional coatings on temperature-sensitive substrates without requiring thermal management.

2. Materials and Methods

2.1. Substrate Preparation

Commercially available 24 × 24 mm borosilicate glass slips (haemacytometer cover glass, BRAND, Wertheim, Germany, refractive index at 546.07 nm (Frauenhofer’s mercury “e” spectral line) n_e = 1.52 ± 0.01, Abbe number at 546.07 nm v_e = 56.5 ± 0.5) with a thickness of 0.4 mm are cut with a diamond stylus into 12 × 12 mm pieces to fit the sample holders of our analysis tools. The substrates are prepared for the coating process following a stepwise cleaning protocol: (i) washing with distilled water; (ii) immersion into a laboratory cleaning agent solution (RBS 25, Carl ROTH, Karlsruhe, Germany) and washing in an ultrasonic bath for 5 min; (iii) rinsing with distilled water and immersion into high-purity EtOH (absolute EMPLURA Merck, Darmstadt, Germany, Supelco, 99.5% purity) for 10 min; (iv) rinsing with distilled water and immersion into distilled water for 10 min; (v) immersion into 35% HNO₃ (Alfa Aesar, Schwerte, Germany) for 30 s; (vi) washing with distilled water; and (vii) drying with N₂ (Messer, Gumpoldskirchen, Austria, 99.2% purity). Before deposition, the stored samples were again dried with N₂.

2.2. PVD Process

Before every deposition process, the coating apparatus is pumped to a base pressure of 10⁻⁷ mbar by a dry vacuum pump (Pfeiffer, Asslar, Germany, ACP 40) and a turbo molecular pump (Pfeiffer, Asslar, Germany, TMU 521 Y P) to minimize reactions with residual gas components during the sputtering process. Pure Ti thin films are deposited onto the glass substrates by planar DC magnetron sputtering (MAK 2 MEIVAC, MCA 750-3000 FUG Elektronik, Schechen, Germany) of a circular high-purity Ti target (Kurt Lesker, St. Leonards-on-Sea, UK, 99.995% purity) fixed at a distance of 70 mm to the substrates, using argon (Ar) (Messer, Gumpoldskirchen, Austria, 99.999% purity) as a sputtering gas. The continuously sputtered Ti films are prepared at a working pressure of 3.3 × 10⁻³ mbar with a 25 ± 1 W magnetron power. The temperature of the substrate is monitored with a Thermocouple (K type, Advent Research Materials, Eynsham, UK) wire, and a maximum increase of 20 K is observed after 20 min of substrate coating, starting at room temperature.

Ultra-low duty cycle, pulsed sputter deposition is implemented at a working pressure of 7.4 × 10⁻³ mbar Ar, with a pulsing period of 3 s and a pulse width of 150 ± 10 ms, leading to an average power of 85 ± 10 W per pulse. The power within one pulse was calculated by measuring the voltage drop during the plasma discharge and the ignition time (compare Figure 1b). A schematic circuit diagram for the pulsed magnetron is shown in Figure 1a. The low repetition rates, as well as the low duty cycle, allow for controlled film growth and no temperature change during the pulsed deposition, starting at room temperature. In pulsed mode, the reported deposition time refers to the total duration of the deposition process, including both plasma “on” and “off” periods. The duty cycle defines the relative plasma-on time, which is below 10%.

While the sputtering power and working pressure differ slightly between the continuous and the pulsed sputtering modes, each set of conditions was optimized to ensure stable plasma generation and reproducible film growth under low thermal load. This comparison is intended to describe two practically viable low-power regimes.

To ensure the reproducibility of the coating process, the deposition rate is monitored with a water-cooled quartz crystal sensor (Inficon Switzerland Bad Ragaz, STM-2XM monitor, cool drawer single sensor, 6 MHz gold-coated crystal) and kept constant. Prior to the deposition on the substrates, the titanium target is cleaned by sputtering at working conditions for several minutes until the deposition rate on the quartz crystal sensor is constant and further target poisoning can be excluded. Samples with a film thickness of between 0.5 nm and 105 nm are produced by varying the deposition times. Two substrates mounted closely next to each other are simultaneously coated in each deposition process to provide two sets of identical samples, one for in-house characterization and one for external XPS analysis. After coating, the samples are taken from the vacuum vessel and oxidized in ambient air at room temperature.

2.3. Film Characterization

The elemental composition and oxidation state of the continuously sputtered Ti films are analyzed using an XPS system (Thermo Scientific, Waltham, MA, USA, Multilab 2000) with a hemispherical analyzer (Thermo Scientific, Waltham, MA, USA, Alpha 110) and a twin-crystal monochromator (Thermo Scientific, Waltham, MA, USA, XR5). Spectra are recorded in constant analyzer energy mode using the Al Kα radiation (1486.6 eV) with a focus spot of 650 μm in diameter. The depth profiles of the Ti films are recorded via Ar sputtering (Thermo Scientific, Waltham, MA, USA, EX06 Ion gun) in an area of 2 × 2 mm² at a sputtering rate of approximately 0.025 ± 5 nm/s, calibrated using nickel (Ni) and Chromium (Cr) thin films by the NIST calibration standard.

The topology of all the films is analyzed by AFM in phase imaging mode (Nanosurf, Liestal, Switzerland, NaioAFM) using a probe (BudgetSensors Sofia, Bulgaria, TAP300AL-G cantilever) with a tip radius smaller than 10 nm, a resonance frequency around 300 kHz, and a force constant of about 40 N/m. 1 × 1 µm² images are recorded on three different spots for each sample. A setpoint of 70% at 300 mV free vibration amplitude with PID settings of P = 1200, I = 1200, and D = 0 and a scanning speed of 1 s per line (1024 lines per image) are chosen as the scanning parameters. Image processing is carried out in Gwyddion using mean plane subtraction, polynomial background subtraction, and FFT filtering. The root mean square surface roughness S_q is calculated as

$$S_q=\sqrt{{\displaystyle \frac{1}{N}}{\sum }_{n=1}^N{(z_n-\overline{z})}^2},$$

(1)

with $\overline{z}$ being the mean value of the measured height z, and N the total number of measured height values in the respective area. Step-height measurements are performed on each sample with the AFM to verify the film thickness and compared to the quartz balance data, confirming their agreement. Electrical characterization of the films is performed with a four-point probe (Ossila, Sheffield, UK, 1.27 mm probe spacing) over three differently orientated lines in the center of each sample. All the measurements are performed in current auto range mode at positive polarity, with probe currents ranging from 1 to 12 mA at voltages of 0.290–4 V for the outer and 0.02–0.41 V for the inner probes, respectively. The sheet resistance R_S is calculated as

$$R_S={\displaystyle \frac{\pi }{\mathrm{l}\mathrm{n}\left(2\right)}}{\displaystyle \frac{\Delta V}{I}}C$$

(2)

where $\Delta V$ is the voltage drop between the inner probes, $I$ the probe current applied on the outer probes, and $C$ = 0.9237 is the geometric correction factor for the 12 × 12 mm² quadratic shape of the coated substrates [46].

The optical properties of the samples are investigated using a halogen light source (OSRAM, Munich, Germany, Halostar 67431 SST 2Y) and a UV-VIS spectrometer (B&W Tek, Lübeck, Germany, i-trometer, 0.6 nm resolution). Transmission and reflectance at a 45° incident angle are recorded in a wavelength range of 350 nm to 950 nm with an exposure time of 45 ms averaging over 40 spectra. To obtain the relative intensities, the recorded transmittance and reflectance spectra are referenced to the measured spectrum of the light source.

3. Results

3.1. Chemical Properties

The XPS survey spectra of the surfaces of the continuously deposited Ti films are shown in Figure 2a according to their deposition time. Figure 2b shows an exemplary depth profile, which is recorded to monitor the elemental composition throughout the films. Due to the limited energy resolution of the XPS used in this study, we did not attempt a detailed deconvolution of oxidation states and instead interpreted depth profiles based on the relative signal evolution of Ti 2p, Ti 2s, and O 1s. The XPS measurements confirm the presence of surface oxidation (TiO₂), as expected for titanium films exposed to ambient air. However, the depth profile reveals a transition to a predominant metallic Ti content below the surface. Even though the dominating impurity in all the samples is oxygen, the overall determination of the contributions to this O 1s peak is uncertain, as several chemical components can contribute to the signal: titanium oxide, which is especially anticipated on the surface of the films, unavoidable adsorption of oxidized carbons, hydroxyl compounds, as well as atmospheric oxygen present in the vacuum chamber of the XPS. The latter, in particular, might make a considerable contribution to the overall signal, as Ti is not only highly reactive with oxygen but is also known to be prone to the “knock-on” effect, where oxygen from the surface is implanted deeper into the film during the process of ion etching [47]. Similar phenomena of an increased O 1s signal have been observed and described previously by Jeyachandran et al. [29] and Cai et al. [22].

However, as can be seen in the exemplary depth profile in Figure 2b, the oxygen concentration in all samples shows an initial decrease with the etching depth, followed by a leveling to almost constant values before the concentration rises again together with the onset of silicon, both being predominant in the borosilicate substrates. The initial decrease in the O 1s signal can be attributed to the uppermost titanium dioxide layers formed through atmospheric oxidation, while the leveled region may be due to the aforementioned reasons.

As all samples from the continuously sputtered series showed comparable results in their XPS spectra, additional XPS analysis of the pulsed sputtered samples is omitted. This decision was based on two key observations. First, the ultra-low duty cycle pulsed deposition process was designed to minimize substrate heating and surface energy input, promoting a film growth mode similar to that observed in continuous low-power sputtering. Second, the pulsed films exhibit consistent trends in their surface characteristics, as shown in the following sections, suggesting similar surface oxidation and chemical characteristics.

3.2. Surface Morphology

Compared to other reports [15,16,22] on Ti thin films in this thickness regime, this work’s low-power sputtering processes result in considerably lower surface roughness, as shown in Figure 3 and

Table 1. Characteristics of the analyzed samples.

Title 1	Deposition Time (min)	Thickness Deposited (nm)	Roughness Sq (nm)	Sheet Resistance Rs (Ω/sq)
Continuous deposition	20	105 ± 5	1.13 ± 0.02	7.13 ± 0.03
	17	89 ± 5	0.94 ± 0.06	8.39 ± 0.03
	15	79 ± 5	0.98 ± 0.06	9.67 ± 0.03
	10	53 ± 2	0.81 ± 0.06	16.00 ± 0.06
	5	38 ± 2	0.46 ± 0.04	35.9 ± 0.3
	2	20 ± 3	0.26 ± 0.01	137.8 ± 0.9
	1	7 ± 2	0.29 ± 0.02	478 ± 5
	0.5	4 ± 1	0.24 ± 0.01	1355 ± 2
Pulsed deposition	90	70 ± 5	0.73 ± 0.05	20.1 ± 0.5
	40	30 ± 3	0.27 ± 0.03	45.3 ± 0.4
	10	10 ± 1	0.25 ± 0.02	279 ± 2
	5	5 ± 1	0.26 ± 0.02	830 ± 10
	2	4 ± 1	0.26 ± 0.05	9700 ± 500
	1	3 ± 1	0.76 ± 0.05	17,000,000 ± 2,000,000
	0.5	0.5 ± 1.0	0.7 ± 0.3	n.a.

. Generally, films with low surface roughness can be achieved either through low deposition rates or high substrate temperatures, enhancing adatom mobility during deposition [22,27,48]. Due to the absence of an increase in surface temperature in this study, the reported smooth surfaces can be mainly attributed to lower deposition rates, leading to the initial formation of relatively small, spherical grains [22,27].

AFM analysis does not reveal surface features typically associated with grain boundary voids or columnar growth. While AFM analysis is limited to the topography and does not provide direct information on subsurface morphology, the consistent comparison of the topography evolution at different film thicknesses can still offer insights into the growth process. Lacking the evidence of a distinct “grains and void” structure, as described for high-power sputtering with high deposition rates, this allows for the subsequent growth of smooth films, where the space around the initial grains is homogeneously filled with incoming material rather than the occurrence of any shadowing effects reported for larger grains. Similar to what is reported in the literature [22,35,44,45,48,49], the RMS surface roughness in Figure 3 of both the continuous and pulsed sputtered films shows a slight increase for higher film thicknesses (compare Figure 4a and Figure 5a) after an initial decrease for intermediate thicknesses. Overall, surface diffusion during the off periods may further contribute to the exceptionally smooth surfaces resulting from this mode (compare also Table 1).

While the continuously sputtered films show uniform morphology for all deposited thicknesses (Figure 4), the low deposition rates of the pulsed sputtered films, in particular, allow for a good observation of the evolution in surface morphology, as can be seen in Figure 5b–d.

The thickness growth is initially not linear with respect to the sputtering time for both the continuously and ultra-low duty cycle pulsed sputtered films (see Table 1 for comparison). This can be explained by the film’s initial island growth (Volmer–Weber mode) and subsequent closing to form a smooth surface. The formation of such islands leads to an initial high estimation of the growth rate. As the islands merge laterally to form a continuous film, the increase in vertical thickness temporarily slows down. Once a continuous, stable surface layer is established, the deposition rate becomes more consistent. This multi-slope behavior is thus an intrinsic consequence of the different growth regimes that occur during film formation [18,45,49].

3.3. Electrical Properties

The sheet resistance at room temperature of the Ti films deposited by continuous and ultra-low duty cycle pulsed magnetron sputter deposition is compared to values reported in the literature in Figure 6. Similar to what others observe [8,17,22], it is impossible to approach the calculated electrical resistivity of Ti bulk extrapolated for thin film thicknesses. This effect can be attributed to deviations in the films’ structures from bulk material and chemical bonding of the residual gas, contaminating the pure Ti films during deposition.

Table 1 summarizes the analyzed RMS surface roughness and sheet resistance for each Ti film according to the film thickness and deposition method.

3.4. Optical Properties

Reflectance spectra for the films deposited via continuous and pulsed magnetron sputtering are presented in Figure 7. The films considered in this study show a relatively uniform absorbance and reflectance in the visible region of the light spectrum. In contrast, an increase in the reflectance with increasing wavelengths has been reported for higher-power magnetron deposition [28,30].

Similar to what is commonly reported in the literature [27,28,30,51], the reflectance of the films increases strongly with their thicknesses, as seen in Figure 8. At low thicknesses, partial transparency and low reflectance indicate semi-continuous layers. As the films become optically denser and more reflective above ~30 nm, this behavior indicates the development of continuous metallic coverage, in agreement with AFM and electrical measurements. It is generally assumed that higher surface roughness reduces the films’ reflectivity due to multiple scattering of photons on the rough surface [28,51]. Reversely, the films prepared in this study with ultra-low-power sputtering exhibit relatively high reflectance, possibly due to their smooth surfaces (compare Figure 3). Notably, some differences in the thickness-dependent reflectance of the films can be observed when comparing the continuous and pulsed modes, especially as the 30 nm pulsed film shows higher reflectance than the 20 nm continuous film. When comparing the values for RMS surface roughness in Table 1, the films from the pulsed deposition mode maintain a lower surface roughness up to 30 nm, which likely contributes to its high reflectance. In the continuously sputtered films, the RMS roughness increases significantly at the steps between 20 nm, 38 nm, and 53 nm, suggesting that the initially smooth, coalesced film surface begins to develop larger features as growth continues. These differences highlight that optical performance at low thicknesses is not solely determined by total film thickness, but also by the growth mode and resulting microstructure. This transition corresponds to the percolation threshold, the critical film thickness at which isolated metal clusters coalesce into a continuous, compact network, enabling metallic conductivity and high optical reflectance [49,52,53].

The calculation of the absorption coefficient α

$$\alpha =-{\displaystyle \frac{1}{d}}\mathit{ln}({\displaystyle \frac{T}{1-R}})$$

(3)

with d denoting the film thickness in cm, T the transmittance, and R the reflection of the corresponding sample, allows for the presentation of the data in a Tauc plot [54], as shown in Figure 9. In the case of the thinnest films, where the uppermost oxidized $\mathrm{T}\mathrm{i}{\mathrm{O}}_2$ layer is not negligible in the overall film thickness, it is possible to observe a distinct absorption band in the Tauc plots. Consequently, a linear increase in the spectroscopic data can be observed due to absorptions above the bandgap energy. As this is only the case for materials with semi-conductive properties, the disappearance of the absorption edge indicates the increasingly metallic character of the respective film. In the continuously deposited films, the absorption band disappears at around 38 nm, while for the pulsed sputtered films, the metallic character already prevails at a film thickness above 30 nm.

The deposited films can be interpreted as a TiO₂−Ti bilayer system. At low thicknesses, the films are dominated by the surface oxide layer, behaving as a wide-bandgap semiconductor [18,55], consistent with the linear regions observed in the Tauc plots of Figure 9 for thicknesses below 30 nm. As the film thickness increases, the underlying metallic Ti dominates the optical response, as seen by the rise in reflectance (Figure 8) and flattening of the adsorption edge. Additionally, the oscillations observed in the reflectance spectra of Figure 7 may result from interference effects. Low-energy light-matter interference arises throughout the overall film thickness, while higher-energy frequency modulations possibly stem from interference in the surface oxide or bi-layered subsurface structure. These effects are especially pronounced in semi-transparent films near the percolation threshold and disappear as the films become predominantly metallic. This evolution is consistent with the XPS data, confirming the bilayer structure. Both techniques hence quickly lead to a predominantly metallic film, where the influence of oxidation on the overall performance can be neglected, as is also validated by the previous analysis of the electric conductivity and spectroscopy.

4. Discussion

The low-power DC magnetron sputtering method allows for controlled growth of film thicknesses in both continuous and pulsed deposition, enabling detailed analysis of the development of thickness-dependent physical characteristics. While continuous DC sputtering is performed at 30 W magnetron power, the ultra-low duty cycle (150 ms pulse width every 3 s) results in 85 W power pulses. While the deposition substrate is herein momentarily subjected to intense ion bombardment, which can enhance adatom mobility and surface diffusion, the extended off-time between pulses allows the surface to cool and relax, minimizing cumulative heating and promoting uniform layer formation. Compared to other deposition techniques, both presented low-power sputtering modes offer the unique advantage of negligible to no increase in substrate temperature, while simultaneously resulting in comparably low RMS surface roughness and high film uniformity. As no topographical column and void structure is visible in the AFM analysis with increasing film thickness, we report the production of comparatively smooth surfaces at room temperature deposition without additional annealing processing, as described in previous studies [27,28]. This effect may be attributed to the extended time available for adatom reorganization in the low-power regime, reducing the overall surface roughness and avoiding the shadowing effects of higher deposition rates. The distinct plasma environments created by the continuous and pulsed sputtering modes likely influence ion flux, adatom mobility, and film oxidation. The higher instantaneous power and in each duty cycle in pulsed mode may enhance surface diffusion during deposition pauses, contributing to improved uniformity at the lowest thicknesses. Although direct plasma diagnostics were not performed, the trends observed in morphology and composition are consistent with known effects of deposition parameters on surface chemistry and growth dynamics.

Furthermore, spectroscopy shows constant reflectance and absorbance values as a function of the wavelength within the visible range of the photon spectrum. Additionally, the films are characterized by high reflection with increasing film thicknesses. As oxidation in air leads to the formation of a superficial $\mathrm{T}\mathrm{i}{\mathrm{O}}_2$ layer on each sample, Tauc plots can reveal the influence of this semi-conductive layer on the overall absorption of each film. A purely metallic behavior, with no visible absorption band, can be observed for films with thicknesses greater than 38 nm and 30 nm, for continuous and pulsed deposition, respectively. This highlights the overall good electrical conductivity of the thin titanium films in this study, which was confirmed by measurements using a four-point probe. A high electric conductivity, preferably approaching the one of the corresponding bulk, amounts to the most crucial requirement in the coating of various flat antennas, which is combined with the flexibility of films under 100 nm thickness in this study. The technique of low-power magnetron sputtering thus combines the advantages of conventional sputtering, being a low-cost, commonly used, and easily scalable process, with a soft coating method suitable for any temperature-sensitive substrate. This makes it an attractive alternative to conventional, more time-consuming, and invasive coating techniques, such as etching, inkjet printing, or embroidering. While we propose that these coatings could be implemented on various substrates, depending on the future application, we chose borosilicate glass to highlight the physical characteristics of the resulting thin titanium films. Mechanical properties such as flexibility, adhesion to polymer substrates, and thermal cycling behavior were not evaluated in this study and merit future investigations to explore transferability.

Despite this limitation, the approach presented here provides a valuable platform for depositing ultra-thin, conductive films with minimal thermal stress. The low-power sputtered Ti thin films combine the mechanical and optical advantages of sputtered Ti thin films with the low surface roughness, chemical resistivity, and high electrical conductivity of the bulk material. The low-power sputtering approach demonstrated in this study is highly suitable for a range of emerging applications where substrate temperature sensitivity is the limiting factor. The ability to tailor film thickness while maintaining optical and electric functionality may benefit applications of multilayer circuit design or flexible printed electronics, where polymer-based substrates, such as polyimide or PET, are widely used for thin-film interconnectors, sensors, or transparent electrodes.