Bit Line Contacts in 10nm-class DRAM: EUV Stochastics from Low NILS

In 10nm-class DRAM, the contacts to the bit lines are actually on the order of 10 nm in width. However, the separation between such contacts is actually several times larger. The periodicity is double the word line pitch in one direction and double the bit line pitch in the other direction. The staggered array pattern has a density of two contacts per [(2 x word line pitch) x (2 x bit line pitch)] area. Note that active area cuts in DRAM are practically an identical pattern.

With the ideal EUV quadrupole illumination, the normalized image log slope (NILS) is still much lower than 2.0, due to the low target CD/pitch ratio. The lower image contrast from lower NILS aggravates the impact from stochastic fluctuations in photon and electron density (see figure).

Electron density image for a bit line contact (target 10 nm CD) in a 56 nm x 64 nm area. The illumination is quadrupole for best depth of focus and the mask is assumed to behave as a 9% attPSM. 20 mJ/cm2 absorbed dose assumed; 7-9 electrons/photon. Pixel size = 1 nm. The numbers represent electrons/nm2. Source: 10nm DRAM bit line contact low NILS and electron blur aggravating EUV stochastics

For this reason, cross-SADP or cross-SAQP would be a more suitable method for patterning these contacts (or cuts).