BackgroundSummaryIntroductionSIMS and SNMSPlatter AnalysisQuantificationResultsConclusion
Analytical was founded in 1981 and is presently situated in a
2,130m2 manufacturing plant in Warrington, England with a staff of
over 50. As a privately owned company our reputation is built on creating close
and positive relationships with our clients. Many of these customers are working
at the forefront of new technology - in the fields of plasma research, surface
science, vacuum processing and gas analysis. To maintain this reputation Hiden
Analytical have, over the years, established exceptional levels of technical
expertise in these areas within our company.
Sputtered Neutral Mass Spectrometry is ideally suited to the analysis of thin
films where composition, thickness and interface condition can be determined. In
this example a hard drive platter is analyzed, showing both the thin and thick
Despite development of flash silicon memory and optical drives, magnetic hard
disks remain the mainstay of the data storage industry, providing high speed
access and high reliability.
Modern drives comprise a number of circular platters of either aluminium or
glass construction, onto which are deposited magnetic and non-magnetic
The general construction of a disk platter is shown in figure 1. In use, the
read/write head is separated from the disk surface by a cushion of air generated
by the spinning action of the platter. Uppermost on the platter surface is a
thin polymer layer which provides a low friction surface in case the disk head
makes contact. Beneath this is the magnetic layer onto which the data are
written. The fine magnetic domains of the data layer are separated from the
thicker magnetic base by a non-magnetic barrier. In order to accommodate the
highest data density the magnetic domains in the data layer must be as small as
possible, thus the base layer serves to complete the circuit (shown as the
arrowed line) and confine the field to the region of the read / write head
Figure 1: Typical platter cross section
Figure 2 shows the disk platter installed in a drive with the head and motor
assembly prior to analysis.
Figure 2: Assembled hard drive
SIMS and SNMS
Both SIMS and SNMS use a focused, mono-energetic, chemically pure
ion beam of typically 1-10 keV to sputter erode the surface under analysis. A
small fraction of the sputtered material becomes ionized due to the sputtering
process itself and, in SIMS, it is these ions that provide the sensitive information
for which the technique is known. Being a mass spectrometry technique all
elements and isotopes may be detected, and in favorable conditions the detection
limit can be in the low ppb region.
However, because the ionization mechanism for SIMS occurs
at the sample surface, it is highly dependent upon the local chemistry and the
ionized fraction can vary by many orders of magnitude. This makes SIMS ideal
for trace analysis in materials of known matrix but quantification in materials
of changing matrix can be complex.
SNMS overcomes the “matrix effect” by separating the sputtering and
ionization events. Even in high ion yielding situations the fraction of ions
rarely exceeds 1% of the sputtered material, so the neutral flux is much more
representative of the sample composition. Ionization for SNMS occurs in an
electron bombardment cell at the front of the analyzer which means that the
ionization probability is a constant and does not depend upon the sample
To quantify SIMS it is important that the reference material be as similar
to the unknown as possible and should certainly be of the same matrix
For SNMS this matrix matching of reference materials is unnecessary, as
calibration factors do not change with matrix, therefore, the required
sensitivity factors can be determined form easily available metal and ceramic
samples of published composition.
In addition, SNMS is ideal for the analysis of insulators, as the neutral
species are unaffected by sample charging, however, charge compensation is still
advisable in order to maintain consistent primary beam conditions.
The ionized secondary particles are analysed and detected in the mass
spectrometer. At very low ion beam currents analysis is confined to the top few
monolayers – excellent for detection of surface contamination. As the ion beam
dose is increased and sputtering becomes more aggressive, subsequently deeper
layers are exposed and concentration as function of depth can be determined.
Using a focused ion beam, both SIMS and
SNMS become spatially resolving and elemental images can be recorded.
The analysis presented here was made using the Hiden SIMS
workstation, a complete and highly flexible quadrupole
SIMS/SNMS instrument equipped with the IG20 gas
ion gun and MAXIM SIMS/SNMS analyzer.
The disk platter was removed from the drive and a sample area cut from it
(approximately 1cm square from the centre of the data storage area) using a
workshop guillotine. No further sample preparation was required and the piece
was mounted in a standard holder of the SIMS
As nothing was known about the composition of this particular disk surface
the first step was to acquire a mass spectrum extending through the entire
stack. This is easily accomplished by allowing a stationary ion beam (in this
case 600nA 5keV Ar ions focused to 150ìm) to sputter erode a pit for a few
minutes. As the pit edges provide simultaneous expose of all layers, the mass
spectrum gives a good indication of the materials present.
In this case Cr, Ni, and Co showed significant concentrations in the spectrum
and these elements were chosen to be followed in the subsequent depth profile
together with C (for the surface layer) and Al (for the substrate). In
principle, up to 75 individual mass channels may be acquired in a single
analysis permitting both matrix and impurity analyses to be made.
The depth profile was measured using 5 keV Ar+ ions whilst
detecting the sputtered neutrals. Secondary ions were rejected by using a high
target potential in order to give them energy in excess of that required to pass
the parallel plate analyzer of the Hiden Maxim
There are a number of stages to the quantification of an SNMS depth profile.
Initially the instrument is calibrated by determining a sensitivity factor for
each element that will be measured, relative to a chosen standard element, for
metallurgical samples this is often Fe. It does not matter whether Fe is present
in the material under analysis as the relative sensitivity factors (RSFs) will
still apply. However, as SNMS is a mass spectrometry technique the specific
isotope may be chosen to optimize the analysis and the RSF adjusted for isotopic
abundance. The detection limit of SNMS is frequently better than one part per
The important RSF’s for this particular analysis (Ni, Cr, Co) were obtained
from sample of Hastelloy C high temperature alloy (NIST SRM C2402 –
Hastelloy7C). For simplicity, it was assumed that the substrate is composed
entirely of aluminium. However, if necessary the alloy composition of the
substrate could also have been determined.
The SNMS depth profile is quantified in a very similar way to those obtained
from electron spectroscopies. It is assumed that all elements in the material
are monitored and then, after applying the RSFs, this sums to 100% atomic
concentration. Additionally, changes in the total signal reflect variation in
erosion rate, so, to a first approximation, the SNMS depth profile may be
calibrated in both concentration and depth using library values. This is a major
advantage over SIMS for the analysis of graded materials and interfaces.
For more precise depth calibration the terminal crater depth is measured
using either interference microscopy or a stylus profilometer. In this
particular case interference microscopy was employed.
The depth profiles show a structure similar to that expected. At the very
surface is a carbon rich layer which acts to protect the magnetic data layer
beneath from both the atmosphere and accidental contact by the read/write
The magnetic layer is composed of a Co (65%) Ni (7%) Cr(27%) alloy about 40
nm thick. This is separated from the magnetic base layer by a 100nm thick
interlayer of 93% Cr. Beneath the interlayer is an 8ìm Co (25%) Ni (35%) Cr
(40%) thick base layer that serves to complete the magnetic circuit from the
head. The above figures are given in atom percent, however, these can be readily
be converted to the weight percent figures most frequently given in alloy
Electron impact SNMS on the Hiden SIMS
Workstation can provide quantified elemental depth profiles of alloy
materials using easily obtained, non-matrix matched, reference materials.
Near Surface Detail of Hard Disk Platter
SNMS Depth Profile of Hard Disk Platter
Source: Hiden Analytical
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