Every prior generation of lithography used a transmissive mask: light passed through a quartz plate patterned with an absorber, projecting the pattern onto the wafer. EUV breaks that model because 13.5 nm photons are absorbed by glass, by air, by almost everything. The mask has to work in reflection instead.
GlobalFoundries' grant US10802393B2 (issued October 13, 2020; GLOBALFOUNDRIES INC.; CPC G03F 1/24 for EUV masks, plus G03F 1/58 and G03F 1/80) claims the structure of such a reflective EUV mask. The G03F 1/24 classification is the EUV-mask-specific bucket - this is not a generic photomask patent.
Physically, an EUV mask is a multilayer Bragg reflector - dozens of alternating molybdenum and silicon layers tuned so their reflections add constructively at 13.5 nm - topped with a patterned absorber. Get the layer thicknesses wrong and the mask reflects too little light; get the absorber wrong and the printed pattern smears. The IP in this space is about controlling exactly those stacks.
The chokepoint framing matters. EUV scanners are an ASML monopoly, but the mask ecosystem - blanks, absorbers, pellicles - is its own constrained supply chain, and the companies printing leading-edge logic need IP positions across all of it. A foundry like GlobalFoundries filing on mask structure is staking out part of that chain.
Note the assignee and the date together. GlobalFoundries stepped back from the leading edge around this period, yet was still filing EUV mask IP in 2020 - a reminder that patent portfolios outlive product roadmaps, and that mask know-how has value independent of whether you run the latest node yourself.
For the equipment-and-materials reader, the lesson is to follow the mask, not just the scanner. The scanner gets the headlines, but a scanner prints nothing without a reflective mask engineered to survive and steer 13.5 nm light - and that engineering is its own dense patent field.