  The upgrade of the CMS detector for the high luminosity LHC (HL-LHC) will
include gas electron multiplier (GEM) detectors in the end-cap muon
spectrometer. Due to the limited supply of large area GEM detectors, the Korean
CMS (KCMS) collaboration had formed a consortium with Mecaro Co., Ltd. to serve
as a supplier of GEM foils with area of approximately $0.6\,m^{2}$. The
consortium has developed a double-mask etching technique for production of
these large-sized GEM foils. This article describes the production, quality
control, and quality assessment (QA/QC) procedures and the mass production
status for the GEM foils. Validation procedures indicate that the structure of
the Korean foils are in the designed range. Detectors employing the Korean
foils satisfy the requirements of the HL-LHC in terms of the effective gain,
response uniformity, rate capability, discharge probability, and hardness
against discharges. No aging phenomena were observed with a charge collection
of $82\,mC/cm^{2}$. Mass production of KCMS GEM foils is currently in progress.
