Process quality control strategy for the Phase-2 Upgrade of the CMS Outer Tracker and High Granularity Calorimeter

Abstract

Between 2025 and 2027, some essential components of the CMS (Compact Muon Solenoid) detector-most notably the tracker and the calorimeter endcap-will be upgraded to prepare for HL-LHC (High Luminosity Large Hadron Collider) conditions. The upgraded CMS Outer Tracker and parts of the new CMS High Granularity Calorimeter (HGCAL) will encompass over 50,000 new silicon sensors covering a total area of about 800 m 2. The sensor series production requires a dedicated strategy to monitor the quality and stability of the production process. The strategy is based on a test structure set that provides quick and easy access to critical process parameters. These include parameters not directly accessible on the sensors (e.g. oxide charge concentration and interface trap density) and parameters requiring potentially destructive measurements (e.g. dielectric strength). The set is implemented at least twice on each production wafer. It is divided into test structures for initial evaluation of the most relevant process parameters and structures for in-depth analysis. All structures can be contacted using a 20-needle probe card and an automated positioning stage. With this system, the initial analysis of one wafer is possible in about 30 minutes. In this paper, the CMS collaboration presents the quality assurance plan for the Phase-2 Upgrade with a focus on process quality control. We cover sensor process specifics, the layout of the test structure set that will be implemented in the production runs for the CMS Outer Tracker and HGCAL, and measurement results illustrating the functionality of the included test structures.

Silicon sensors for the Phase-2 upgrade of the CMS Outer Tracker; status and early results from the production phase

“…Each sensor type is hosted on a dedicated wafer with either one 2S or two PS-s/p sensors. Additionally, each wafer hosts a number of test structures and mini sensors, located at the periphery [6]. They are identical on each wafer and are used for conducting further studies and tests, which are important for the full characterisation of the production process as well as for the characterisation of the wafer quality.…”

“…The Van der Pauw (VdP) structures are used to measure the sheet resistances (R sheet ) of all implants including the p-stop. They are produced in various geometries [6]. The layout of such a structure is given in Figure 5.…”

“…The Field Effect Transistor (FET) is a test structure which mimics the geometry between adjacent strips or macro-pixels of the sensor [6]. It contains a p-stop implantation which encircles the drain and source, illustrated in Figure 5.…”

The silicon sensors for the High Granularity Calorimeter of CMS

“…Also, irradiation campaigns of test structures are planned which allow to investigate the radiation hardness of the pstop elements [15]. Metal-Oxide-Semiconductor (MOS) test capacitors allow measuring the oxide charges via flatband voltage, which gives information about the oxide quality [6].…”

The Silicon Sensors for the High Granularity Calorimeter of CMS

“…Irradiation tests of test structures are planned. To investigate the p-stop structures, Metal-Oxide-Semiconductor Field-Effect Transistors (MOS-FETs) [9] can be used. MOS capacitor test structures allow measuring the oxide charges via flatband voltage, which gives information about the oxide quality.…”