Experts’ Interpretation of Resolution Technology Bottleneck and Multiple Exposure Potential of 65 nm mask aligner

Experts interpret the resolution of 65 nm mask aligner.

Recently, the discussion about "65nm mask aligner" on the Internet is quite enthusiastic, but many comments lack authoritative basis, which leads to many people’s increasingly vague understanding of this technical concept. Some people explain the resolution of 65 nm mask aligner only by "8 nm alignment accuracy", and this simplified treatment deepens the confusion. Fortunately, with the promotion of popular science, the public has gradually reached a consensus: the prototype of the ArF light source mask aligner with 65 nm resolution and 8 nm overlay accuracy is about "XT:1460K" model introduced by ASML Company 20 years ago.

The next question is, can this mask aligner reach a higher level of chip manufacturing node with the help of multiple exposure technology? Looking back on an article published by IEEE in 2008, this paper discusses the challenge of 193 nm mask aligner in alignment accuracy control, and we briefly summarize it by analyzing the data in this paper.

In this paper, a series of mask aligner in ASML are listed, which evolved from dry type (such as 870G to 1400E) to the first generation submerged type (such as 1700Fi and 1900Gi), and their resolution and alignment accuracy changes are shown. In the chart, the blue line at the top represents the mask aligner resolution, which is almost equivalent to the node name of the chip in the era of dry lithography. Obviously, ASML’s XT:1400E dry ArF mask aligner matches the standards of 65nm resolution and 8nm alignment accuracy.

The orange line shows the maximum allowable deviation of registration accuracy at a certain resolution, for example, at a resolution of 65 nm, the allowable deviation is 11 nm. Therefore, the alignment accuracy of 8 nm completely meets the process standard of 65 nm. It is worth noting that before the 65 nm node, its importance has not been paid enough attention because of the large margin of overlay accuracy. When it reaches 65 nm, as dry lithography approaches the limit, the surplus space of alignment accuracy almost disappears.

It should be emphasized that the above registration accuracy standard is based on a single exposure.

In fact, ASML skipped the 65nm mask aligner and directly entered the immersion technology for the mass production of 32nm and 28nm chips. However, historical data show that the company has explored the use of a 65nm dry mask aligner with a numerical aperture of 0.93 to achieve a resolution of 40nm through double exposure technology, which was documented in the IEEE conference in 2006.

It is pointed out that for the 65nm mask aligner with a numerical aperture of 0.93, the registration accuracy required by a single exposure is 8nm, and it is required to be further improved to 5.6nm in the double exposure mode. This means that even the 8 nm alignment accuracy is not enough in the face of the demand of double exposure to 40 nm resolution. Moreover, 40 nm is only the level of a single exposure of immersion mask aligner.

According to the analysis of available historical data, mask aligner with 65nm resolution and 8nm alignment accuracy is mainly suitable for the chip manufacturing process from 65nm to 55nm, and cannot effectively enter the finer manufacturing fields such as 32nm and 28nm through double exposure technology.