BackgroundSummaryIntroductionInstrumentationResults and DiscussionConclusion
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.
The use of SIMS Quadrupole Mass Spectrometry for End Point Detection
during ion beam etch is demonstrated. The technique can be applied to a wide
variety of materials and devices with depth resolution of just ± 5 Å and precise
control of end point, or etch stop. This article describes the principle behind
the technique and provides examples from the range of applications.
Dry (ion beam) etching is used in numerous process steps during semiconductor
device and magnetic recording media manufacture.
However, owing to the low selectivity in ion-beam processing, even when
reactive gases are used, methods must be found to ascertain depth and location
of the etch stop. Consequently test pieces of material are often processed to
assess etch rates, which can be time consuming and costly. Metals and
dielectrics are frequently patterned on semiconductor material and it is
important not to etch or damage the underlying layer. It is therefore important
to have a good selective etch or to be able to stop at, or near, the interface.
These criteria are essential for a wide range of applications involving the
presence of multilayers and interfaces.
The technique of ion beam etching employs a broad ion beam to physically
sputter and/or chemically etch the surface under consideration. During this
process secondary ions are created at the surface with a mass to charge (m/z)
ratio which is characteristic of the material at the surface. Mass spectrometry
can be used to monitor the intensity of the secondary ion signal at specific m/z
ratios, making it possible to detect when an interface has been reached and a
new type of material exposed (figure 1). The ion beam etch can then be
terminated if the new material represents the desired end point of the process.
This approach is essentially Secondary Ion Mass Spectrometry (SIMS) -
well established as the most sensitive of all UHV surface analysis
Figure 1: Ion beam etching
The SIMS process involves the detection and identification of
secondary ions formed at the surface of a sample by the impinging primary ion
beam. The ion flux density is typically a few percent of the sputtered neutral
flux density and the secondary ion energy distribution depends on the angle of
primary beam incidence, secondary ion emissionangle and target material. Both
positive and negative ions are formed at the surface along with electrons,
sputtered neutrals and scattered primary particles.
Point Detector (EPD) is a convenient, compact device and is appropriate for
secondary ion mass analysis in this application. For adequate mass resolution
EPD requires an acceptance energy range of 0-5 eV. These considerations indicate
that the ion transport system must reject high energy ions, neutrals and
electrons, and accept positive ions with a kinetic energy range of 0-5 eV.
Additionally the SIMS EPD can function as a residual gas analyser (RGA) so that
chamber gas composition can be conveniently determined. This is particularly
important when more than one gas is admitted to the chamber.
Two basic arrangements are shown schematically in figure 2.
Figure 2a: Schematic representation of the positioning of
the primary ion source, target and SIMS EPD with axial acceptance
Figure 2b: Schematic representation of the positioning of
the primary ion source, target and SIMS EPD with 30° acceptance
Secondary ions are emitted from the surface with a wide range of angles and
energies. The high transmission energy filter (A) either has an axial acceptance
as in figure 1(a) or a 30o acceptance as shown in (b). The mass filter may be
biased with respect to vacuum ground to select a range of secondary ion
energies. A dual filament RGA electron-impact ion source (B) is positioned
between the SIMS EPD energy filter and the quadrupole mass filter (C). The
mass filter has pre- and post- r.f. filter stages which serve to enhance the
The air stable continuous dynode electron multiplier detector (D) is mounted
in an off-axis position. Typical chamber pressures are in the 10-4
mbar range during etching and the SIMS EPD is
differentially pumped to maintain pressures low enough to enable the use of the
detector (~5 x 10-6 mbar). The system is designed for process
operation and can be controlled as a standalone device or alternatively may be
fully integrated with the host tool via a variety of digital I/O and software
command sets. In production environments the SIMS EPD is
tied into the production tool recipe select software for complete ‘hands free’
process integration. Layer by layer data is displayed in real time and the etch
process is controlled by a variety of user selectable endpoint methods.
In typical applications such as MRAM manufacture, devices based on Giant
Magneto-Resistant effect elements (GMR) require low primary ion energies because
the multilayer structures can be just a few atomic layers thick. In addition to
this the exposed area of pre-etched wafer devices may only be <3% of the
total open area of a 150mm diameter wafer. Hiden’s SIMS EPD
has been proven to be the only technique that produces consistently precise,
accurate and reproducible end point control in an industrial setting for such
Results and Discussion
The elements to be monitored, corresponding to the layer materials are chosen
from the drop-down periodic table (figure 3).
Figure 3: Software selection of layer materials.
In operation the intensities of the chosen elements of interest are plotted
against time. Decisions can then be made on the end point.
Data acquired from a thin film structure used in the fabrication of magnetic
film sensors is shown schematically in figure 4a and in more detail in figure
4b. The objective is to etch through the FeMn, NiFe and and Co/Cu/Ta layers
stopping on the SiO2 substrate layer with no further etching. In this example
99% of the total surface area is masked. The profile shows each element from the
various layers with excellent signal intensity and resolution. Etching is
terminated at the end of the duration of the nickel and iron signal. Data was
acquired from the aforementioned exposed sample area of only 1%, clearly
demonstrating the high sensitivity of the technique.
Figure 4a: Multilayer SIMS EPD etch data
In figure 4b the periodicity in the signal is clearly seen and is due to the
Figure 4b: Multilayer SIMS EPD etch data - detail
This can in the worst case produce false endpoints but is completely dealt
with in the advanced Hiden SIMS EPD endpoint methods by assigning the rotation period and
collecting data matched with the EPD acquisition timing.
End point methods included with the Hiden SIMS EPD
include rising and falling edge variables, the use of reference layer peaks and
layer counting. End point control variables are comprehensive and shown in the
screen shot below.
Figure 5: Endpoint variables
has been proven as the industry standard end point detector. It is used by every
major IBE tool manufacturer due to the demonstrated sensitivity, accuracy and
reliability, and due to the excellent levels of after sales service and field
support. In the most demanding IBE applications the Hiden SIMS EPD
delivers depth resolution of +/- 5 angstroms.
Source: Hiden Analytical
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