Relatore
Descrizione
The CBC3.1 is the ?nal version of the readout ASIC for 2S-modules in the outer
radial region of the upgraded CMS Tracker at the High Luminosity LHC. The chip
development was completed in an engineering run in 2018. Subsequently two pre-
production lots of wafers were delivered in 2019, and large scale production deliveries
began in May 2021. So far almost 270 production wafers have been received. The
engineering run wafers and pre-production lots were tested using an automatic wafer
prober, intended for ?nal acceptance tests, and showed some variations in yield of
good chips across the wafers, whose patterns suggested non-optimal processing in the
foundry. To try to understand this better, the probe station was adapted so that
measurements could be carried out at low temperatures, down to 30?C.
Although both preproduction and production wafers have a high yield of good
chips, some unexpected behaviour was observed at low temperature. Rare memory
errors were observed when the hit data are read out. The wafers a?ected are from lots
with patterns of central yield loss, and the errors do not seem to be present in better
quality wafers. However, the rate of occurrence is so low that there should be negligible
impact on CMS track reconstruction.
The second issue is occasional corruption of some registers which store ?ne tuning
values for individual channel pedestals, following certain write operations into speci?c
registers. This also appears to be correlated with manufacturing quality and although
the impact should be minor, and probably avoidable during tracker operation, the
origin of the problem is not yet fully understood and is the subject of ongoing investi-
gations.
The status of the wafer probing will be presented with results from studies to date.
| Collaboration | CMS collaboration |
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