Transcript Presentazione di PowerPoint - Istituto Nazionale di Fisica

External micro-PIXE analysis of the metal deposition
on a CMS pitch adapter
M. Massi, L. Giuntini, M.E. Fedi, C. Arilli, N. Grassi, P.A. Mandò, A. Migliori, E. Focardi
Dipartimento di Fisica and I.N.F.N., Firenze, Italy
The Compact Muon Solenoid (CMS) is one of the two multi-purpose detectors to be installed at the Large Hadron Collider (LHC), now
under construction at CERN, in Geneva, Switzerland.
The Pitch Adapter (PA) is a device situated between the silicon sensor and the front-end
hybrid in the silicon tracker of the CMS experiment. Its purpose is to make the micro-bonding
connections possible between the input of the readout chips and the silicon sensor.
From the silicon
sensor
(fan-out side)
Part of pitch adapter
To the readout
electronics
(fan-in side)
The PA consists in a fan of metallic strips engraved on a glass support;
the strips consist in an aluminium coating (nominally  400 mg·cm-2)
over a very thin chromium layer. Each strip ends with pads for the
micro-bonding contacts.
1 mm
Problems on micro-bonding in a pre-production series of PA…
… might be explained assuming the presence of metallic
contaminants in the aluminium deposition.
Al
Al
The Florence external micro-PIXE
facility appeared to be suitable to point
out this kind of impurities:
- good space resolution;
- simple operation (external beam);
- beam raster system integrated with a
mechanical sample scan (system
implemented by Oxford Microbeam Ltd.);
- multi-element capability and high
sensitivity.
Measurement conditions:
- proton beam energy of ~3 MeV;
- current up to 1 nA on the target;
- ~12 mm FWHM after 2 mm helium path.
Cu
Si
Cu
Si
660 mm
660 mm
Optical microscope
photograph and
elemental maps of
fan-out side
Optical microscope
photograph and
elemental maps of
fan-in side
Cr
Cr
We analysed an overall 660x660 mm2
area:
the scanned area corresponds to 2x2
adjacent square maps of 360x360 mm2
(each obtained by beam scanning) with
a 60 mm overlap for a safer linking of the
individual beam scans.
The X-ray detection system consists in
two Si(Li) detectors: one dedicated to
lower energy X-rays (a helium flow was
maintained in front of it) and the other
optimised for the higher energy X-rays.
Copper contaminates the depositions and it is absent outside;
Silicon is clearly coming only from the substrate (no
contamination is present in the deposited material);
Aluminium is a major constituent of the deposit, but seems
to be present also in minor quantity on the substrate, which is
reasonable; Chromium is the “glue” between the glass
support and the glass deposition.
We quantified the Cu contaminant from the spectra pertaining to
selected sub-areas corresponding to the pads, by comparison to
a thin standard and neglecting matrix effects in the metal deposit.
The mean copper areal density comes out to be around
180 mg·cm-2.
Contamination from the target holder of the electron gun?
We also analysed a “good” PA from a different batch, on which
the micro-bonding connections had resulted to be satisfactory:
we detected no copper in the pads (having a sensitivity limit
lower than 0.2 mg·cm-2).
The role of copper in the micro-bonding problems is clear.
BY OUR EXTERNAL MICRO-PIXE SET-UP WE HAVE BEEN ABLE TO IDENTIFY THE CAUSE OF
UNRELIABILITY OF A DEVICE TO BE EXTENSIVELY USED IN LHC EXPERIMENTS AT CERN