One of the components of a hard disk drive is a hard disk platter. All hard drives include one or more hard disk platters for storage of data. They include complicated layer structures that be can be classified into three different levels. The bulk of the platter is formed from the structure base, which is a substrate material and offers rigidity and structure and is normally made of glass or aluminium.
The magnetic media coating lies atop the substrate on which the magnetic impulses representing the data are written. Using vapour deposition of different metallic alloys, magnetic layers are created. A thin, protective carbon layer and a very thin lubricating layer cover the surface of each platter.
It is important that the platter quality and that of the media coating are really good. In case there is any issue with the platter layer composition, malfunctioning in the hard disk may occur leading to data loss. Therefore, characterizing the platter for both chemical and elemental information from the surface to the substrate becomes necessary to validate layer conformity and integrity.
In order to characterize this kind of multilayer samples, XPS depth profiling offers a simple way. The Thermo Scientific K-Alpha XPS system is an award-winning instrument that features excellent spectroscopic performance, offering rapid analysis times and very good chemical detectability.
There are two techniques of depth profile analysis. The first technique uses a rapid snapshot acquisition technique for capturing region data for each element and would be used in case the elements in the sample were already known. In the hard disk platter case, elements present were not known. Hence, a second technique was deployed.
In this method, a wide scan or survey spectrum at each depth profile level is recorded. Using a broad scan across the complete spectral range, all elements excepting helium and hydrogen can be detected.
By rastering a beam of 500eV argon ions over a 2x4mm area, a hard disk platter fragment was depth profiled. Each etch cycle was 10s and after each etch level, a survey spectrum was obtained to identify all possible elements at each sample depth. In order to obtain depth profile of very high quality, azimuthal rotation of the sample was performed during each etch cycle, causing an etch crater of 2mm diameter.
An optical image of the etch crater recorded after completing the depth profile is shown in Figure 1. The K-alpha system offers a very high XPS sensitivity, enabling data collection in a very short time period without compromising spectral quality. In this case, it just took 9s to obtain each survey spectrum.
Figure 1. Optical image of the circular etch crater obtained by azimuthal rotation during depth profile sample analysis
All Thermo Scientific surface analysis systems come with a Thermo Scientific Avantage data system that includes a principal component analysis (PCA) feature for the review of large multi-level data sets like images and depth profiles.
This was used to select the key components from the complete 230 survey scan set from the depth profile data set. Other software tools were used to determine the elements present in the identified PCA components, and generate the final atomic concentration profile.
Figures 2 to 4 provide the analysis results. A montage of survey spectra at each depth profile level is shown in Figure 2. Survey spectra are used for offering elemental information from the surface and can be used for identifying contaminants or unknown sample compositions.
Figure 2. Montage of depth profile survey spectra
The Avantage data system’s integrated PCA tool selected seven key components of the total set of 230 survey scans symbolized as a set of seven discrete survey scans recreated from the data.
The depth profile in terms of relative intensities of the seven PCA components is shown in Figure 3.
For determining the elements present in each of the seven component PCA survey spectra, an automated “Survey ID” routine was used. Across the seven PCA components, total of 15 different elements were detected.
Figure 3. Depth Profile of hard disk platter expressed in terms of principal components
The survey spectra from the original data set of 230 scans were then peak-fitted at each etch level to provide a complete atomic percent quantification all through the sample depth. The etch time was finally converted to an etch depth (nm) using the etch rate determined on a 30nm Ta2O5/Ta standard. The atomic percent depth profile and the layers present are indicated in Figure 4.
Figure 4. Depth profile of hard disk platter expressed in terms of elemental atomic percentages
Based on the depth profile in Figure 4, it is observed that glass is the platter substrate. A nickel and tantalum buffer layer is arranged above this to prevent the glass orientation and the crystallographic structure from influencing the orientation of any of the other thin film layers.
A thin tantalum interlayer is present just above this. A seed layer of titanium and chromium is present atop the tantalum layer to enhance orientation and growth of the layers deposited later on.
Next there are two pairs of non- magnetic ruthenium and magnetic platinum, cobalt and chromium layers. The magnetic layers are used for data storage and the thin non-magnetic layers enable magnetization of the magnetic layers in opposite directions that reiterates the magnetic state. The risk of loss of magnetic state due to thermal effects is also ruled out. For corrosion resistance and to enhance its mechanical reliability, a final thin top carbon layer is added.
The K-alpha system offers a very high XPS sensitivity and enables rapid depth profiling of both familiar and unfamiliar samples without a compromise on data quality. Tools in the Avantage data system, such as PCA enable rapid and convenient data processing of large data sets.
In this example, by expressing data in terms of its key components, only seven spectra need to be analyzed to determine all the different elements in all 230 scans. This allows rapid generation of a high quality elemental depth profile of the hard disk platter sample.
This information has been sourced, reviewed and adapted from materials provided by Thermo Scientific – X-Ray Photoelectron Spectroscopy.
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