What is double patterning lithography and its impact on nanometer design?

Deep sub-wavelength lithography, i.e., using the 193nm lithography to print feature sizes/densities (e.g., for 45nm, 32nm, 22nm nodes) much smaller than the wavelength (193nm), is one of the most fundamental challenges for the nanometer CMOS scaling. Many intriguing techniques have been developed to push the limit of the 193nm lithography, from immersion lithography to computational lithography. However, even with these exotic techniques, the single patterning 193nm lithography is approaching its ultimate limit around the 22nm node. On the other hand, the so-called next generation lithography (NGL), including extreme ultraviolet (EUV) lithography, nano- imprint lithography (NIL), multiple e-beam direct-write (MEBDW) is still "next-generation" (i.e., not ready for primetime in 22nm or perhaps even the 16nm node) due to many technology challenges (not to mention the economical barriers to adopt them).