A GEM Detector System for an Upgrade of the CMS Muon Endcaps

3.3 Single-mask GEM production technology
The production of GEM foils is based on photolithographic techniques commonly used by the printed circuit
industry. The copper-clad kapton substrate gets laminated on both sides with solid photoresist of 15 µm thickness
that the GEM hole pattern is transferred onto by UV exposure through flexible masks. In order to get good
homogeneity of the hole geometry across the foil, it is very important to keep the alignment error between the
masks on the two GEM foil sides within 10 µm. However, since both the raw material and the two masks are
made from flexible material, the manual alignment procedure becomes extremely cumbersome when the linear
dimensions of the GEM exceed 40 cm.
A natural way of overcoming this problem is the use of single-mask photolithography. In this case the GEM pattern
is transferred only to one side of the raw material, thus removing any need for alignment. The exposed photoresist
is developed and the hole pattern is used as a mask to chemically etch holes in the top copper electrode of the GEM
foil. After stripping the photoresist, the holes in the top copper electrode are in turn used as a mask to etch the
polyimide.
Single-mask photolithography (Fig.10) has been proven to be a valid manufacturing technique for making GEMs.
This technology was used to build a prototype detector for a possible upgrade of the TOTEM T1 detector. More
recently, the production process has been refined even more, giving great control over the dimensions of the GEM
holes and the size of the hole rims during the production process. Effects of the hole shape are also being explored
in simulation studies (see below). Production issues have been studied and single-mask GEMs are compatible
with industrial production using roll-to-roll equipment, which is a very important aspect of this new technique.
Consequently, a price reduction for GEM foils is expected from large-scale industrial production that is now
possible.
3.4 Test beam measurements
Figure 10: Overview of single-mask etching process for GEM foils.
The Triple-GEM prototypes were tested with a 150 GeV muon/pion beam at the CERN SPS H4 beam line during
several RD51 test beam campaigns. The detectors under test were mounted into the RD51 Triple-GEM beam
telescope as depicted in Figure 11. The telescope itself consists of three standard Triple-GEM detectors, referred
to as tracker GEMs, each with10cm×10cm active area and running with an Ar/CO2 (70:30) gas mixture. They
have 256 strips in both horizontal (y-coordinate) and vertical (x-coordinate) directions transverse to the beam, with
a pitch of 0.4 mm. The telescope detectors were always operated at a gain larger than 10 4 . This setup served as the
reference tracking device for the detectors to be tested.
The standard double-mask Triple-GEM prototypes under test were studied with different gas mixtures, Ar/CO2
(70:30, 90:10) and Ar/CO2/CF4 (45:15:40, 60:20:20) at a gas flow of about 5 l/hour corresponding to roughly 50
detector volume exchanges per hour. The single-mask Triple-GEM was operated with an Ar/CO2 mixture only.
The readout of all detectors including the tracker GEMs was done with electronics boards based on VFAT2 (Very
Forward Atlas and Totem) chips [12] developed for TOTEM [13] by INFN Siena-Pisa. The VFAT2 ASIC was
designed at CERN using radiation tolerant technology. It has a 128-channel analog front-end and produces binary
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