In this work, we investigate the Negative Tone Develop (NTD) process from a fundamental
materials/process interaction perspective. Several key differences exist between a negative tone develop
process and a traditional positive tone develop system. For example, the organic solvent dissolves the
unexposed material, while the deprotected resist remains intact. This causes key differences in key
patterning properties, such as pattern collapse, adhesion, remaining resist, and photoresist etch selectivity.
We have carried out fundamental studies to understand these new interactions between developer and
remaining resist with negative tone develop systems. We have characterized the dynamic dissolution
behavior of a model system with a quartz crystal microbalance with both positive and negative tone solvent
developers. We have also compared contrast curves, and a fundamental model of image collapse. In
addition, we present first results on Optical Proximity Correction (OPC) modeling results of current
Negative Tone Develop (NTD) resist/developer systems.
To extend immersion based lithography to below 32nm half pitch, the implementation of Double-Patterning
lithography requires that cost be contained by as many means possible. In addition to CDU and defectivity, simplifying
the process flow is a viable approach to helping accomplish cost containment. For Litho-Litho-Etch processes, this
entails replacing the flows that require spin-on chemical freeze with a solely thermally cured resist approach, thereby
eliminating materials and several process steps from the flow. As part of ongoing efforts to allow Double-Patterning
techniques to further scale semiconductor devices, we use DETO (Double-Expose-Track-Optimized) methods for producing pitch-split patterns capable of supporting 16 and 11-nm node semiconductor devices. In this paper we present the assessment from a series of thermal cure double-patterning resist systems; with a focus on process latitude, CDU, and resolution limit.
At the 32nm node, the most important issue for mass production in immersion lithography is defectivity control. Many methods have been studied to reduce post-exposure immersion defects. Although a topcoat process demonstrates good immersion defect prevention, a topcoat-less resist process is an attractive candidate for immersion lithography due to cost reduction from a simplified process. In this paper we took the innovative approach of chemically designing an internal self-assembling barrier material that creates a thin embedded layer which functions as a topcoat. Data will be presented on this novel self assembly concept, illustrating the control of leaching, contact angle and immersion defects. Several optimized process flows with non-topcoat resists were also studied to decrease the amount of immersion defects. This study was used to verify the capability of a topcoat-less immersion process to achieve the low-defectivity levels required for 32nm node production.
To optimize the anti-reflectant material (BARC) in 193nm resist processes requires a careful manipulation of the surface energy of the BARC. In general, the surface energy of the BARC is constant in the unexposed and exposed areas. We have developed a new material with a "switchable" contact angle (SBARC) whose key criteria are as follows: (1) High contact angle at about 70 degrees in the unexposed areas under the resist to prevent developer and water penetration; (2) Maximized adhesive of the SBARC to the resist. (3) Contact angle less than 50 degrees in the exposed areas at the BARC surface to reduce the density of satellite-type defects. The low contact angle in the exposed areas reduces the adhesive forces between the hydrophobic resist residues and the more hydrophilic SBARC surface and thus lowers defects. In addition, the hydrophilic SBARC surface can reduce water drop residues and therefore reduce watermark defects. This paper will also describe our process work to optimize the contact angle of unexposed and exposed BARC surface to reduce pattern collapse and minimize satellite defects. We will also discuss a few methods to improve the surface condition of the SBARC to maximize adhesive forces. Further optimization of the develop process and the refractive index and the absorption coefficient of the SBARC, will provide even better collapse margin for 193-nm resists than the present baseline.
Vortex masks composed of rectangles with nominal phases of 0°, 90°, 180° and 270° have been shown to print sub-100nm vias and via arrays when projected into negative resist using 248nm light. Arrays with pitches down to 210nm and CDs as small as 64nm have been reported. While promising, 248nm vortex via images showed some anomalies: The developed contacts were somewhat elliptical, with four different repeating via shapes. The common depth of focus for these four classes of via was limited by their different behaviors through focus. Phase edges in isolated vortex pair structures tended to print, also limiting the useful DOF. These issues can be ameliorated by employing 193nm illumination and a new negative-tone resist. Smaller NAs and higher coherence extend the common depth of focus and larger NAs can be used to print even more tightly spaced patterns. Advanced optical proximity correction techniques can also be applied to reduce the via ellipticity and placement error, and a more optimal choice of geometrical phase depth reduces pattern variability. Further developments and incremental improvements in vortex via design and processing may make it the method of choice for via patterning at the 45nm node.
Fluoropolymers have been shown to be one of the best materials for high transparency of 157 nm wavelength radiation. Both resists and pellicles are being designed from such materials. One of our approaches to improved transparency for 157 nm resists is based upon fluorinated variations of polymethacrylate and polyhydroxystyrene derivatives. Lithographic studies were carried out on experimental resist platforms using 157 nm and 248 nm steppers, and it was shown that, after selective modification, it is possible to use conventional resist backbones, such as acrylic or styrenic, in the design of single-layer resists for 157 nm lithography. It has been demonstrated in our studies that 157 nm absorbance of these materials can be as low as 1.5-2.0μm-1. Another approach to 157 nm resist design is based upon fluorinated backbone variations. Research will be described focusing on several new monomers having fluorine functions such as -F and -CF3 groups near a polymerizable double bond to improve transparency at 157 nm and to raise the resist glass transition temperature compared to their hydrocarbon analogues. Due to the lower electron density of the double bond, these monomers can be copolymerized with electron-rich vinyl monomers. As an extension to this strategy, we are synthesizing novel fluoropolymers having partially fluorinated monocyclic structures with radical cyclo-polymerization. These polymers have the C-F bond on the polymer main chain and also possess acid labile groups as part of a ring structure to eliminate degassing. In order to further enhance the transparency of these systolic polymers at 157 nm, we have eliminated the carbonyl group. The cyclic nature of the polymer will result in a high glass transition temperature.
Hexafluoroisopropyl alcohol-functionalized acrylate monomers and their (co)polymers were prepared as photoresist platforms for 157 nm imaging. In order to balance transparency with other desirable traits such as etch resistance, we developed several copolymer systems. One is using 2-methyl adamantyl methacrylate as a comonomer, and the copolymer system showed better dissolution contrast compared to the copolymer with tetrahydropyranyl methacrylate without sacrificing transparency. To further improve the absorption properties at 157 nm, monomers having (alpha) -trifluoromethyl group were prepared and polymerized in anionic mechanism. The product polymer was unexpectedly transparent at 157 nm (A = 1.6 micrometers -1) in spite that all the monomers contain carbonyl group. The second system is the copolymer with p-t-butoxy-tetrafluorostyrene. p-Hydroxy-tetrafluorostyrene and p-t-butoxy-tetrafluorostyrene were polymerized radically using AIBN in good yield, and the two resulting polymers showed distinctive solubility differences in aqueous base solution. Finally, this paper describes the synthesis of new monomers having fluorine (e.g CF3- group) in the vicinity of the double bond to improve transparency at 157 nm. Due to the lower electron density of the double bond, these monomers can be copolymerized with electron-rich vinyl monomers using radical initiators.