From the silicon sensor (fan-out side). To the readout electronics (fan-in side). Al. External micro-PIXE analysis of the metal deposition on a CMS pitch adapter.
To the readout
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 constructionat 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.
Part of pitch adapter
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.
Problems on micro-bonding in a pre-production series of PA…
… might be explained assuming the presence of metallic contaminants in the aluminium deposition.
We analysedan 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.
Optical microscope photograph and elemental maps of fan-in side
Optical microscope photograph and elemental maps of fan-out side
Coppercontaminates 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
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