Why didn't mask aligner apply for a patent?

Patent publication of mask aligner. The reason why mask mask aligner didn't apply for a patent is that the patent in mask mask aligner is open. Mask aligner can be made in China long ago, which has nothing to do with patent restrictions. Three countries in the world can manufacture mask aligner, and four companies can manufacture mask aligner, Asmel in the Netherlands, Nikon and Canon in Japan, and Shanghai Microelectronics in China. Among them, the only one that can manufacture high-end euv mask aligner is Asmel in the Netherlands, and the other three are duv mask aligner. Patent restriction is only one aspect of restriction, and the biggest factor is technical problems.