Double patterning

The 193 nm water-based immersion lithography is the only option for the next nodes. However, its resolution limit at a maximum practical NA of 1.35 is around 40 nm. In this situation, double patterning has emerged as the semiconductor industry s chosen method. In one form of double patterning known as litho-etch-litho-etch (LELE), features are printed at a relaxed pitch that is within the capabilities of 193 nm limitation lithography, e.g., line and space patterns at a V3 critical dimension (CD) pitch ratio. Schematics 

Removing resist. Coating resist, 2nd exposure, development 

Schematic of a litho-etch-litho-etch (LELE) process for double patterning. First exposure Imaging and etch of first structure (1 3 L/S). Etch 1 Coat and image of second resist. Second exposure Second etch step. Etch 2 Final dense line structure after stripping. 

Target position First resist for 2 lines lines 

Self-aligned double-patterning (SADP) process using the resist pattern as a core. 

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The adoption of HM processes in IC device manufacture started around the 45-nm technology node they are expected to become the dominant resist process technology in technology nodes <32 nm because of their compatibility with double-patterning technology as well as for the reasons stated above. 

Relative to the other double-patterning approaches, the double exposure is the simplest to implement since it does not require additional follow-up process steps. Its main challenge is meeting alignment tolerance requirements. 

Designed to overcome the problem of printing at small pitches, double-patterning techniques typically comprise those techniques in which two cycles each of... 

Figure 17.15 Process sequence for LELE double-patterning technique.

Figure 17.16 SEM cross section of 25-nm half-pitch lines patterned with LELE double-patterning technique. The line profiles were obtained after the final pattern transfer to the underlying substrate. The second-layer patterns after the second etch step are considerably longer than the first-layer patterns because they are capped with the second HM material. (Courtesy of S. Holmes.)...

Figure 17.17 Process sequence for LFLE double-patterning technique.

Figure 17.19 Process sequence for self-aligned double-patterning technique by means of sidewall spacer formation.

The spacer double-patterning approach has some issues, the main one of which concerns placement accuracy of the features, which requires excellent dose and etch uniformity control. Equally problematic is the fact that the spacers may not stay in place at the proper location after the material to which they are attached (the HM pattern) is removed. Forming acceptable uniform spacer profiles that will yield uniform CDs is not a trivial exercise. In addition, the etch pattern transfer steps, often involving delicate removal of materials adjacent to the spacers, place unusually tight etch process tolerance on this technique, in order to have a successful outcome. 

Figure 17.20 SEM images of shallow trench isolation structures patterned with self-aligned double-patterning technique through various stages of the process flow, and ending in 22-nm half-pitch structures. The HM used is Applied Materials Corporation s advanced patterning film (APF). ...

C. Bencher, Y. Chen, H. Dai, W. Montgomery, and L. Huli, 22 nm half pitch patterning hy CVD spacer self ahgned double patterning, Proc. SPIE 6924, 69244E (2008). 

Maybe it was just as well that this added two-carbon side-chain with lowered activity was already enough to disprove the doubling pattern. 

Chemically amplified resists are quite effective in improving pattern sizes in photofabrication. However, optical limitations in resolution must be overcome to improve further progress. Several processes contribute to improving the resolution of photofabrication. In this chapter, immersion, double-patterning, multipatterning maskless, multielectron beam, and direct self-assembly lithography processes are introduced. 

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