Mark O. Neisser

Lithographic effects of mask critical dimension error

Magnification of mask dimensional error is examined and quantified in terms of the mask error factor (MEF) for line and hole patterns on three types of masks: chrome-on-glass (COG), attenuated phase-shifting mask (PSM) and alternating PSM. The MEF is unity for large features, but increases rapidly when the critical dimension (CD) is less than 0.5 (lambda) /NA for line-space patterns and 0.75 (lambda) /NA for contacts. In general dark-field spaces exhibit higher sensitivity to mask dimensional error than light-field lines. Sensitivity of attenuated PSMs is similar to COG masks, even for applications in which attenuated PSMs provide benefits in process latitude. Alternating PSMs have the lowest MEF values. Although the MEF has only a slight dependence on feature nesting for contacts, dense lines and spaces exhibit markedly higher MEF values than isolated features. The MEF of a 0.35 (lambda) /NA isolated line is 1.6 whereas that of a dense line of the same dimension is 4.3 illumination is effective in reducing the mask error sensitivity of dense lines. Dose variation causes changes in the MEF of contacts but has little effect on line-space features; focus error degrades (increases the value of) the MEF of both pattern types. A high diffusion and low contrast photoresist process also worsens the MEF. Consequences of mask CD error amplification include tightening of mask specification, design grid reduction, shift in optimal mask bias and enhanced defect printability.

Simulation and experimental evaluation of double-exposure techniques

The process windows and capabilities of double exposure techniques with binary and attenuated masks are explored using simulation and experiment, including the effects of resist properties, illumination conditions and overlay error. Here it is shown that by using a low partial coherence factor (sigma) for the two exposures, the total window is considerably improved over that obtained using higher partial coherence illumination. We call this process ORAMEX, which stands for Ordinary Resist And Multiple EXposure. It was found that the process window for nested lines and spaces using ORAMEX is considerably better than that for conventional illumination. This is shown for aerial images and for aerial images plus a resist model with contrast and diffusion length similar to that of state of the art Deep UV resists. In fact, the total process windows found for ORAMEX show good process latitudes for both dense and isolated features, with ORAMEX usually enhancing dose latitude more than single exposure off axis illumination does. Overlay errors are found not to affect the process window for individual features. However, they do affect the common window for every other line (in positive resist) but not spaces. It was also found that using attenuated masks instead of binary masks further improves the process window and resolution of ORAMEX. Experimental results agree with simulation and show a process window for 150 nm lines and spaces with over 0.4 micrometer depth of focus and 15% dose latitude in 0.6 micrometer of resist using ORAMEX and chrome on glass masks. Using attenuated masks and ORAMEX a similar process window (0.4 micrometer DOF and 16% dose latitude) was obtained for 125 nm lines and spaces. Both results were obtained on a 0.6 NA Deep UV stepper using commercial positive resist.

Advanced negative resists using novel aminoplast crosslinkers

Current negative tone resists based on poly(4- hydroxystyrene) and aminoplast crosslinkers suffer from the limited solubility of commercially available crosslinkers in the most common casting solvents. The aminoplast crosslinkers also increase the dissolution rate of the base resin in aqueous alkaline developer. The lithographic performance of these resists is often limited by microbridging at high resolution. In this paper, synthesis of a series of glycoluril based aminoplast crosslinkers is described and the lithographic performance of resist formulations incorporating such compounds is discussed.

PHS with inert blocking groups for DUV negative resist

The synthesis, characterization, and lithographic evaluation of a polyhydroxystyrene (PHS) modified with isopropyloxycarbonate groups is described. The inert blocking group is attached to the hydroxyl sites on PHS resin to slow the dissolution rate and make the resin useful in resists designed for 0.263 N TMAH developers. A negative tone resist (CGR-IP) that is formulated with the modified polymer is compatible with the industry standard 0.263 N TMAH developer and is capable of resolving 0.22 micrometer L/S features and 0.14 micrometer isolated lines on a 0.50 NA imaging system. Reaction with PHS resin occurs primarily at the phenolic sites as shown by carbon-13 NMR and 10% protection is sufficient to lower the dissolution rate to an acceptable level so that there is less than 50 angstrom film loss in exposed areas. The blocking group described here is not acid labile and reaming intact after the resist film is baked at 150 degrees Celsius.