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The effects of varying film thickness on the structural and optical attributes of silicon dioxide (SiO2) and zinc oxide (ZnO) thin films, which were deposited on quartz substrates using radio-frequency magnetron sputtering, were thoroughly examined. By optimizing deposition conditions, we successfully produced stoichiometric thin films. Techniques such as X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were employed to analyze crystallite orientation, structure, and composition, while scanning electron microscopy (SEM) was utilized for surface topography analysis. The optical properties of samples with identical compositions yet differing deposition parameters, notably thickness, were measured. The optical constants (including refractive index n, extinction coefficient k, and absorption coefficient α) for SiO2 and ZnO films were extracted from transmission spectra recorded in the 190 nm range using the Swanepoel method. Additionally, the energy bandgap was assessed through absorption spectra. The impact of thickness on the optical and structural properties of the oxide films was evaluated, revealing commendable optical quality and performance, which enhances prospects for their integration into metamaterial designs.

This research presents novel findings on achieving high-performance structural and optical characteristics of oxide materials via optimized rfMS deposition conditions. The oxide materials studied have potential for future integration into advanced metamaterial systems, thereby enhancing their capabilities.

The experimental approach for the deposition of ZnO and SiO2 films signifies an evolution in the rfMS vacuum deposition methodology for dielectric layers (like ZnO and SiO2) on quartz substrates. We explored SiO2 and ZnO films with thicknesses ranging from 200 to 300 nm, and evaluated the positive implications of increased film thickness on the morphology, structure, and spectral properties of the samples. Such assessments underline the necessity to utilize these materials in the development of metamaterial structures.

Various deposition techniques can produce SiO2 and ZnO films, including matrix-assisted pulsed laser evaporation (MAPLE), sol-gel spin coating, radio-frequency magnetron sputtering (rfMS), vacuum thermal evaporation (VTE), and chemical methodologies. For instance, SiO2 and ZnO films generated through rfMS can serve dual purposes as dielectric materials in metasurface structures and as integral dielectric interfaces within metamaterial constructs.

The applications for oxide thin films are far-reaching due to their remarkable properties, including superior dielectric characteristics for metamaterial fabrication. Metamaterials offer a groundbreaking microstructural perspective on functional materials within domains like space science, public security, and sensors. SiO2 and ZnO, with vital dielectric properties, contribute to devices incorporating metasurface designs observable across the visible spectrum. Their study continues as they exhibit appealing attributes such as high transmission in the visible band and substantial energy bandgaps. Applications for these oxides extend to transparent conductive oxides and buffer layers in solar cells, alongside various sensor technologies. Moreover, materials like SiO2 and ZnO show potential electrical resistance variation dependent on temperature.

The optical transmission spectra were obtained utilizing a UV-vis-NIR Perkin-Elmer Lambda 950 Spectrophotometer, covering a measuring range of 190 nm, boasting a wavelength resolution of 0.08 nm in UV-vis and 0.3 nm in the NIR region.

SEM was employed to analyze samples at different magnifications, using a high-energy electron beam (20 keV). Surface morphology and structural quality were investigated with an FEI Co., model Inspect S50 scanning electron microscope, which includes an X-ray source and an EDX unit for elementary energy dispersion spectroscopy. Variations in magnification were applied to assess thin film quality and surface structure. Cross-sectional imaging at specific magnifications provided insights into sample thickness, while high-resolution elemental microanalysis along cross-sections was executed with adjustable tilt angles, ensuring effective dynamic focusing. The thickness was ascertained through the cross-sectional SEM technique, with a margin of error of ±5% (±5 nm) compared to standard 100 nm samples.

A combination of methods, including XPS, XRD, and SEM, were employed to characterize the deposited SiO2 and ZnO thin films' structure and thickness. Surface composition insights were acquired through ESCALAB 250+ XPS equipment, using monochromatic Al Kα radiation (1486.6 eV) and adhering to vacuum conditions within the analysis chamber. XRD assessments were conducted with a Brucker D8 Advance diffractometer, utilizing Cu Kα radiation (λ = 1.5406 Å) within a 2θ range of 25-80 degrees.

The VARIAN ER EletroRava deposition machine, available at the National Institute for Laser, Plasma and Radiation Physics (INFLPR), comprises a deposition chamber featuring two magnetrons and in situ thickness monitoring. Thus, rfMS was employed for SiO2 and ZnO film deposition, delivering materials onto extensive areas while ensuring high-quality thin films for varied applications. The films were developed at room temperature on quartz substrates while retaining thicknesses ranging from 200 to 300 nm from 4-inch diameter and 0.125-inch thick disc targets. SiO2 and ZnO (99.99% purity, Lesker) were separately sintered. The chamber received controlled argon (95%) and oxygen (5%) working gases via flow meters (30 and 1.5 sccm, respectively) to regulate gas flow accurately. Two-cm square quartz (fused silica, NEGS2) slides were utilized as substrates after thorough cleaning in an ultrasonic bath to guarantee reproducibility. Substrates were mounted on a rotating metallic plate positioned above the target structures. The rfMS method enabled uniform oxide film growth and precise compositional control.

Results

X-ray diffraction analyses of oxide samples were restricted to an angle range of 25-80 degrees to ascertain the structural type (e.g., polycrystalline or amorphous) and orientation in thin films. Typical XRD patterns of ZnO layers with increased thickness, prepared via rfMS, were examined. Results indicated a peak intensity enhancement correlating with thickness, reflecting improvements in crystalline structure. Notably, ZnO samples’ crystalline phases matched the (002) and (004) planes per JCPDS standards. These patterns suggest commendable crystalline quality, specific diffraction peaks indicative of the hexagonal wurtzite structure are observed in the studied films, with a pronounced orientation of the (002) planes aligned horizontally relative to the substrate.

Observations from the diffractogram of the 200 nm thick film highlighted diffraction peaks consistent with single-phase growth. The lower intensity of the (004) diffraction lines arose from uneven distribution of growth stress in the film. The diffractogram for the 250 nm thick specimen illustrated how the film started to exhibit orientation with the c-axis perpendicular to the substrate, as expected from room temperature depositions.

Table 1

sampled (nm)(hkl)2θ (°)β2θ (mrad)D (nm)dhkl (Å)c (Å)ε2 (%)TC (hkl)ZnO(002)34.023.02....82(004)72.734.51....05ZnO(002)34.928.42....98(004)72.138.91....88ZnO(002)34.335.12....19(004)72.144.01....55Open in a separate window

The 300 nm thick ZnO film exhibited enhanced structural performance compared to other films deposited under similar conditions. This conclusion derives from the increased size of crystallites and elevated texturing coefficient values greater than 1, indicating more crystallites aligned with the (hkl) planes parallel to the substrate surface. Narrower diffraction peaks corresponded to larger crystallite dimensions. Recorded lattice parameters for the deposited films registered lower values than standard ones, attributed to the absence of thermal treatments inducing internal stress within the ZnO thin films. Despite room temperature depositions, SiO2 films predominantly displayed amorphous structures, characterized by a lack of distinct XRD reflection lines and exhibiting matte surfaces. XPS assessments of the impact of thickness on sample characteristics further demonstrated the maintained stoichiometry and purity of the preparation technique.

The XPS methodology characterizes a surface area of 2-6 nanometers, effectively capturing the contaminant layer. The chemical composition garnered from XPS may diverge slightly from that of the target material due to surface diffusion and chemical interactions with the atmosphere. XPS data were collected using photons that minimally disturbed the bombarded surface.

To avoid contamination on film surfaces, samples were not subjected to sputtering via the ion gun, as this could diminish oxygen stoichiometry within the films. Subsequently, general oxide spectra were gathered for three SiO2 samples.

The high-resolution (HR) analysis of Si 2p3 and O 1s spectra recorded for SiO2 samples is also displayed. The elemental composition derived from this analysis provided insights into the chemical states present in the SiO2 films. Although minor chemical changes occurred in the Si 2p3 spectrum, a binding energy of 103.7 eV confirmed complete oxidation of silicon within the SiO2 films, asserting excellent stoichiometry in the material.

Additionally, the general spectra of ZnO samples revealed notable peaks corresponding to C 1s, O 1s, and Zn 2p across all tested samples. These data indicated the presence of photoelectron signals from ZnO, as well as carbon contamination at a binding energy of 284.8 eV in the contaminated films.

The carbon peak represented surface-absorbed carbon, utilized for finalizing energy calibration across other spectra. It is noteworthy that the carbon contamination layer diminishes with increasing film thickness, which may explain the reduction of carbon peaks observed. Notably, no metallic components were detected; instead, they existed solely in oxide bonds like those found in oxide compounds.

The identified carbon contamination clearly diminishes as film thickness increases, and a rise in Zn 2p and O 1s intensities is concurrently noted. HR spectra analyzed the electron distribution levels at Zn 2p and O 1s for the ZnO samples.

As illustrated, the Zn 2p spectrum showcases a doublet with binding energy values of 1.1 and 1.4 eV corresponding to the Zn 2p3 and Zn 2p1 lines, respectively. The absence of metallic zinc within the general spectrum confirms the presence of solely oxidized forms of zinc within the films—a finding consistent with existing literature. SEM analyses utilized magnifications of 2,000x to reveal subtle structural changes in the quality of SiO2 and ZnO films correlating to increased thickness. Cross-sectional SEM images displayed films with a compact, homogeneous texture. Furthermore, SEM surface measurements indicated smooth surface finishes, exhibiting minor variances in morphology between thinner and thicker oxide films.

Regarding ZnO samples, a discernible trend of self-structuring was observed, substantiating findings illustrated in SEM imagery.

Verification of sample morphology was achieved through depth penetration techniques performed via SEM, confirming the compact nature of analyzed films. Additionally, increased granulation corresponding to growing thickness was noted.

Measured thickness values of the oxide films (supplied later) aligned closely with established standards. EDS distribution across all investigated samples signified the presence of elemental species including Zn L, Si K, and O K content.

Table 2

Sampled (nm)ElementWeight %Atomic %SiO2 O K35..38Si K64..62SiO2 O K47..64Si K52..36SiO2 O K50..88Si K49..11ZnO O K17..73Zn L82..27ZnO O K18..52Zn L81..48ZnO O K25..38Zn L74..62Open in a separate window

Analysis of the 200 nm thick SiO2 sample’s composition confirmed its stoichiometry alongside a noted depletion of oxygen (48.38%) in the oxide film. In contrast, the 250 nm SiO2 sample demonstrated increased O K content, with a corresponding decrease in Si K, alongside carbon contamination impacts (as derived from XPS findings). Silicon atomic concentration marginally declined from 51.62% to 34.11%. EDS data revealed SiO2 films exhibited enhanced oxygen levels as film thickness escalated, while silicon mass concentration diminished from 64.64% to 49.50%, paralleled by oxide content growth from 35.36% to 50.50%. Despite targeting the same chemical composition and maintaining good stoichiometry, deposited films reflected values in close proximity to target specifications, especially with thicker samples. For the ZnO films, increased thickness culminated in augmented atomic oxide (from 45.73% to 49.38%), derivatively linked to plasma-chemical processes.

Optical properties were characterized through light absorption analysis in ZnO and SiO2 dielectric films, with optical constants derived from transmission spectra deemed pivotal for integration into metamaterial structural designs. The optical characteristics of these films were constructed based on transmission spectra outputs, as per individual thickness variations.

Observations of transmission spectra maxima and minima for SiO2 and ZnO thin films facilitated the acquisition of optical constants through Swanepoel's envelope method. Thinner samples maintained transmission rates between 78-92%, while thicker variations reported 82-65% at a 600 nm wavelength.

Both interference maxima and minima identified (TM and Tm) were delineated across two transmission spectra curves termed envelopes defined by continuous functions, demonstrating the relationship between refractive analyses and thickness.

For intermediate and lower absorption domains, derived expressions allowed for enhanced understanding of the material behaviors.

From synthesized relationships, further conclusions detailed refractive index variations and absorption dynamics within these materials, particularly for the series of transparent SiO2 and ZnO films.

Increased absorption in 300 nm thick films was observed, attributed to enhanced volumes within inter-crystalline regions. Evaluations indicated a strong relationship between optical constants, with significant links between k and α for transparent compositions.

Additionally, bandgap parameters were derived for all sample categories, yielding values ranging between 3.92-4.0 eV for SiO2 and 3.2-3.3 eV for ZnO films. The discerned band differences among ZnO films indicated direct transitions from the valence to conduction band, corroborating film stress influences on the bandgap.

The optically determined values for direct transitions align well within scholarly consensus. The optical properties of ZnO and SiO2 films illuminate their critical role within metamaterial frameworks. Notably, as film thickness increases, the visible transmission rate decreases alongside an observed increase in surface porosity, attributable to the absence of thermal treatments.

Analyzed refractive index distribution for the sampled films demonstrated a regular dispersion across designated spectral ranges, culminating in minimum values around 430 nm. The dielectric materials' quality is further indicated through dielectric constant values.

Using the Drude methodology, real dielectric constants were calculated, elucidating material permittivity and polarizability alongside density states analysis across the bandgap. The derived real and imaginary dielectric constants displayed August variations, particularly pronounced in the 300 nm samples, stressing optimal deposition conditions for superior performance, affirming potential integration within metamaterial architectures.

Recent investigations have underscored the innovative use of ZnO, given its dielectric features in distinct applications. Our findings showcase analogous values when compared to various SiO2-ZnO mixtures under different weight proportions.

Heightened thickness increments (e.g., from 200 nm to 300 nm) engendered improvements in sample quality by 12-15%, leading to favorable stoichiometry, enhanced crystallinity, and augmented optical and dielectric properties.

Transparent oxides represent a captivating class of plasmonic materials currently under rigorous investigation for low-loss metamaterial integrations and diverse applications within transformation optics, sensors, and imagery. Our explorations emphasized oxide thin films’ optical characteristics for potential utilization in metamaterials exhibiting low refractive indices. Findings indicate electron density redistribution’s minimal impact concerning permittivity variability. The significance of these dielectric oxide films (SiO2 and ZnO) within metamaterial configurations, such as electrically tunable or low-loss formats, stems from their superior properties.

Structures akin to Si/SiO2/ITO/Au metamaterials have been fabricated globally. This research purports a promising novel metamaterial configuration comprising dielectric (SiO2), inductive elements, and dielectric (ZnO). Future integrations of these oxide materials could extend to applications within space microsatellites.

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Silicon Dioxide (SiO2) Sputtering Targets, Fused Quartz, Indium

Purity: 99.995%, Size: 1'', Thickness: 0.125'' 

Sputtering, a well-established technique, effectively deposits thin films using various materials onto diverse substrate shapes and sizes. The sputtering target process is repeatable, with scalability from small research initiatives to larger-scale production. The reaction occurs at the target surface, during material transport, or on the substrate contingent upon process parameters. The complexity of sputter deposition allows specialists substantial control over the growth and microstructural features of films.

Applications of Sputtering Targets;

• Used in film deposition methods, sputtering yields thin films through material erosion from a target to a substrate, typically involving silicon wafers.
• In semiconductor contexts, sputtering targets assist in high-precision etching where selectivity is secondary.
• Sputtering targets are also utilized for analytical purposes by progressively etching the target material away.

For instance, in secondary ion mass spectrometry (SIMS), the target sample undergoes constant sputtering while analyzing the composition and concentration of the sputtered atoms with mass spectrometry. This mechanism enables the determination of target material composition and facilitates the detection of trace impurity levels.

Sputtering has applications in space as well, serving as a form of space weathering—a modification process affecting the physical and chemical traits of airless celestial bodies such as asteroids and the Moon.

If you seek more insights into sio2 sputtering, feel free to reach out.