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 surface chemistry of integrated circuit bond pads is crucial to correct
connection welding. SIMS imaging provides a rapid method of assessing surface
chemistry and identifying pads covered by even a few monolayers of
contamination. Understanding of the distribution of contamination, in this case
a fluorocarbon deposit, leads to better process optimization and higher product
The bond pad is of vital importance to semiconductor technology as it is the
portal through which signals pass to and from the integrated circuit. Failure of
a simple wire bond will mean early failure in service or low device yield.
The bond pad is simply a small, 20 to 100 micron, usually square, metallic
area around the periphery of a chip that is connected to the internal circuit.
It is generally composed of an aluminiumsilicon sputter deposited alloy. When
the chip is packaged, connection is made to the bond pads via fine
ultrasonically welded wires. Any contamination on the surface of the pad leads
to a poor weld and consequent failure.
Electrical testing of the chip function, using contact probes, cannot show
poor surface chemistry as the probes penetrate the surface, breaking through any
Bond pads showing contact probe penetration
Secondary Ion Mass Spectrometry uses a focused, monoenergetic, chemically
pure ion beam of typically 1- 10 keV to sputter erode the surface under
analysis. Ionized secondary particles are then analyzed 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. If the ion beam
is scanned in a raster pattern over the surface, a spatially resolved image can
be recorded. Thus the spatial distribution of an element can be mapped.
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. SIMS is the most sensitive surface analysis technique with
detection limits in the low ppb for many elements.
A flood of low energy electrons is used during analysis of insulating
samples, such as glass, to prevent the buildup of surface charge.
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, cesium ion gun and MAXIM SIMS/SNMS
The purpose of the analysis was to identify bond pads with non ideal surface
chemistry which may lead to failure during the wire bond process. It had been
noted from earlier mass spectral analysis that generally the surface contained
significant fluorocarbon contamination and this was the most likely cause of
Using 5keV Cs+ primary ions focused into a 25ìm spot of less than
2nA, mass resolved images of 27Al were collected. The Hiden
Elemental Surface Map (ESM) software has provision for fast single frame
collection, ensuring that the ion dose is maintained near to the static SIMS limit.
Images can be collected with a wide range of pixel density, from 100x100 to
4000x4000, with separate control of the pixel dwell time and data rates of over
3000 pixels per second.
The MAXIM secondary ion analyser can collect ions from an area of
up to 6mm diameter, although, in this case, the images were confined to regions
approximately 800ìm across in the region of the row of bond pads.
The image below shows the bond pad area at a resolution of 500x500 pixels.
The bond pads are approximately 80ìm across and show up clearly, together with
some of the wiring interconnect.
SIMS Mass Resolved Image (27Al) of Bond Pads
Towards the centre of the image a bond pad is clearly visible with
significantly lower surface concentration of Al than those around it. Similarly,
the two other pads at the extreme right of the picture also show a lower
Using the image for navigation, it was possible to collect mass spectra from
these pads to compare with that from the ‘normal’ pads. The spectra confirmed
that there was a significantly higher fluorine contamination on the pads
sufficient to obscure the metal, most likely residue from a reactive ion etch
step earlier in the IC fabrication process.
Imaging SIMS is the ideal tool with which to investigate the
distribution of very thin surface layer contamination. Operated with
sufficiently low beam current density and a high sensitivity analyzer like the
MAXIM, it is possible to collect images with top monolayer specificity. This
is vital when addressing bonding and adhesion issues as it is that uppermost
monolayer which will interact with other components.
Source: Hiden Analytical
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