A hard disk platter is a part of a hard disk drive, used to store the data. Hard drives can have one or more hard disk platters. The platters possess complex layer structures, which can be split into three separate levels. The structure’s base is a substrate material made from aluminum or glass, which forms the major portion of the platter, and provides it with structure and rigidity. A magnetic media coating is on the top of the substrate, where the magnetic impulses that symbolize the data are written.
The magnetic layers are formed by vapor deposition of a range of metallic alloys. A thin, carbon protective layer and a super-thin lubricating layer cover the surfaces of the platters. The platters’ quality and its media coating are very crucial. Any issue with the composition of any of the layers in the platter could cause the hard disk to malfunction, resulting in data loss. Therefore it is important to characterize the platter for both chemical and elemental data spanning from the surface to the substrate in order to test layer integrity and conformity. XPS depth profiling can characterize multilayer samples such as these in a simple manner.
The Thermo Scientific K-Alpha X-ray Photoelectron Spectrometer (Figure 1) offers outstanding spectroscopic performance, and provides rapid analysis times and outstanding chemical detectability. It was used to test the changes found in elemental composition of a hard disk platter from surface to substrate. Main component analysis was used to distinguish the elements present at each level of the depth profile.
Figure 1. Thermo Scientific K-Alpha XPS system
Depth profile analysis can be done using two different techniques. The first is a fast snapshot acquisition technique, to capture region data for each element. This method is used when the elements in the sample are already known. In the hard disk platter the elements were unknown, so the second method was applied. A wide scan or survey spectrum was recorded at each depth profile level. Using a wide scan across the whole spectral range all of the elements can be detected, except helium and hydrogen.
Depth profiling was performed on a piece of a hard disk platter by rastering a beam of 500 eV argon ions over a 2x4 mm area. The time taken for each etch cycle was 10 seconds, and after every etch level a survey spectrum was gathered to identify all of the possible elements at every sample depth. To attain excellent quality depth profile the sample was rotated azimuthally during each etch cycle, leading to an etch crater measuring 2 mm in diameter. Figure 2 illustrates an optical image of the etch crater recorded instantly upon conclusion of the depth profile.
Figure 2. Optical image of the circular etch crater obtained by azimuthal rotation during depth profile sample analysis.
With excellent XPS sensitivity, the K-Alpha system collects the data with superior spectral quality, within a very short timeframe. In this case, only nine seconds were needed to obtain each survey spectrum. The Thermo Scientific Avantage data system, provided with all of the Thermo Scientific surface analysis systems, has a principal component analysis (PCA) feature to review large multi-level data sets such as images and depth profiles. This was used to select the key components of the full set of 230 survey scans from the depth profile data set. The software’s other tools were used to detect the elements present in the identified PCA components, and produce the final atomic concentration profile.
Figures 3 to 5 depict the results of the analysis. Figure 3 reveals a montage of survey spectra at each level of the depth profile. Survey spectra are usually used to provide elemental data from the surface, and can be useful to detect contaminants or unidentified sample compositions.
Figure 3. Montage of depth profile survey spectra.
The Avantage data system’s integrated PCA tool was able to pick seven major components from the total set of 230 survey scans, represented as a set of seven discrete survey scans reconstructed from the data. The depth profile is illustrated in Figure 4 in terms of the relative intensities of the seven PCA components.
Figure 4. Depth Profile of hard disk platter expressed in terms of principal components
To detect the elements present in each of the seven component PCA survey spectra, an automated “Survey ID” routine was applied. A total of 14 different elements were detected throughout the seven PCA components. Then the survey spectra from the original data set of 230 scans were peak fitted at each etch level, to provide a complete atomic percent quantification across the sample depth. Finally an etch depth (nm) was derived from the etch time (seconds) using etch rate measured on a 30 nm Ta2O5/Ta standard.
The atomic percent depth profile is depicted in Figure 5, which reveals the layers present. The platter substrate is found to be glass. A buffer layer made from tantalum and nickel is placed above this to prevent the orientation of the glass, and the crystallographic structure from influencing the orientation of the other thin film layers. A thin tantalum interlayer is found immediately above this. A seed layer of chromium and titanium is placed above the tantalum layer to enhance the growth and orientation of the following deposited layers. Two pairs of non-magnetic and magnetic layers are present, the first is made of ruthenium, and the other is made of platinum, cobalt, and chromium.
Figure 5. Depth profile of hard disk platter expressed in terms of elemental atomic percentages
Data is stored in the magnetic layers, and the thin nonmagnetic layers allow the magnetic layers to be magnetized in opposite directions. This reinforces the magnetic state, and eliminates the risk of the magnetic state being lost due to thermal effects. Finally a thin top layer of carbon is added to offer resistance to corrosion and enhance its mechanical reliability.
The Thermo Scientific K-Alpha Photoelectron system offers superior XPS sensitivity, which facilitates fast depth profiling of both known and unknown samples without giving away quality of the data. Tools, such as PCA in the Avantage data system, facilitate rapid and easy data processing of large data sets. In this case, by expressing the data in terms of its main components, only seven spectra had to be tested to discover all of the different elements present throughout all of the 230 scans. This enabled a superior quality elemental depth profile of the hard disk platter sample to be produced speedily.
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.
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