Construction and Quality Assurance of Large Area Resistive Strip Micromegas for the Upgrade of the ATLAS Muon Spectrometer

To cope with the increased background induced hit rate of up to ~15 kHz/cm$^2$ in the innermost stations of the muon endcap system of the ATLAS experiment after the high-luminosity upgrade of the LHC, the currently used precision detectors will be replaced by resistive strip Micromegas in 2019. In the "New Small Wheel" the Micromegas will be arranged in two times four detection layers built of trapezoidally shaped quadruplets of four different sizes.The Micromegas quadruplets will consist of 5 panels, 3 drift panels and 2 readout panels, made of aluminum honeycomb core sandwiched by printed circuit boards (PCBs). To achieve 15% transverse momentum resolution for 1 TeV muons and thus a spatial resolution in a single plane of about 100 $\mu$m, each active plane has to have an accuracy of 80 $\mu$m perpendicular to the plane and the alignment of the readout strips on the individual PCBs and particularly the alignment within a quadruplet must fulfill a challenging precision of 30 $\mu$m. The required mechanical precision for the production of the components and their assembly is a key point and must be controlled during construction and integration. The readout strips are etched on PCB boards using classical photo-lithographic processes accompanied by comprehensive quality control. Depending on the type of the module 3 or 5 PCB boards need to be joined and precisely aligned to form a full readout plane. The precision in the alignment is reached either by use of precision mechanical holes as reference for precision pins or by optical alignment of masks, both referenced to the strip pattern. Assembly procedures have been developed to build the single panels with the required mechanical precision. The assembly of a quadruplet includes exact stretching and mounting on very accurate frames of the four stainless steel micro-meshes. Methods to confirm the precision of components and assembly are based on precise optical devices and X-ray or cosmic muon investigations. We will report on the construction procedures for the Micromegas quadruplets, on the quality control procedures and results, and on the assembly and calibration methods.