Speaker
Description
The CMS Tracker Phase-2 Upgrade requires the production of new Strip-Strip (2S) and Pixel-Strip (PS) modules to cope with the requirements of the HL-LHC. All-together 47520 hybrid circuits are required to construct 8000 2S and 5880 PS modules. The hybrids pre-production phase is now completed. At first, a kick-off batch enabled the identification of different issues, that were resolved for the completion of the pre-production. The various issues and respective solutions will be reported.
Summary (500 words)
Two main module types were designed and prototyped for the Compact Muon Solenoid (CMS) Tracker Phase-2 Upgrade. The 2S modules are constructed from two co-planar strip sensors of (10 × 10) cm2, two front-end hybrids and a service hybrid. The PS modules are more complex, they consist of a strip and a macro pixelated-strip sensor of (5 × 10) cm2 assembled into a module with two front-end hybrids, a readout hybrid and a power hybrid. The PS readout hybrid ensures the communication with the back-end while the PS power hybrid hosts the DC-DC power converters and powers the electronics. In the 2S service hybrid these two functions are combined in the same circuit. The front-end hybrids are hosting the front-end readout ASICs and concentrator ASICs on both module types.
All circuits are based on a flexible substrate reinforced with carbon-fibre or FR4 stiffeners in the case of the PS power hybrid. The hybrids are using advanced High Density Interconnect (HDI) flexible circuits and integrate flip-chip ASICs.
The kick-off batch was made of subsets of hybrids enabling the assembly of 1.8 mm sensor spacing 2S modules and 2.6 mm sensor spacing PS modules.
The PS power hybrids and 2S service hybrids were designed in two different variants to minimize the noise injected into the modules, in a split-plane version, and in a regular version. The noise performance was compared, and the most suitable variants were selected. Also, oscillations in the low voltage outputs of the PS power hybrid were observed and were resolved with a modification of the output filters. The accuracy of the output voltages was also improved with the binning of the BPOL12 chips and the use of fine tolerance setpoint resistors.
Delamination was observed on the PS front-end hybrids: the design layout had to be changed towards a five layers design to eliminate this issue. The 2S and PS front-end hybrids suffered also from contamination on the bond pads: the manufacturer production processes were improved to mitigate this, and the cleanliness requirements were also reviewed for a less demanding acceptance requirement, but still enabling a reliable wire-bonding of modules.
The PS readout hybrid suffered from a brittle flexible tail caused by an excess of thermal stress during fabrication: the tail was then modified to stand the various thermal cycles at which it is exposed.
These various issues were all resolved for the continuation of the pre-production, that included now all the sensor spacing variants. Contamination issues had still to be resolved for the 4.0 mm sensor spacing 2S front-end hybrids, and the gluing of Aluminium nitride spacers also presented some weaknesses on the PS front-end hybrids.
The successful outcome of the pre-production enabled the launch of the series production, that is foreseen to complete in the first quarter of 2026.
