Negative photoresist materials for 248 nm (KrF excimer laser) implant applications are of interest to research
and development recently, due to the ever-present demand to shrink lithographically-patterned device dimensions at an
affordable cost. Challenges to developing such a successful resist are the topography of the substrate and subsequent
reflectivity complexities. Substrate reflectivity control, resist profile, and critical dimension (CD) uniformity are critical
issues that must be addressed to enable robust lithography performance at high KrF numerical aperture. The design,
synthesis and characterization of a series of polymers for negative developable bottom anti-reflective coating
(NDBARC) materials suitable for KrF negative implant resists is described.
Pitch-split resist materials have been developed for the fabrication of sub-74 nm pitch semiconductor devices. A
thermal cure method is used to enable patterning of a second layer of resist over the initially formed layer. Process
window, critical dimension uniformity, defectivity and integration with fabricator applications have been explored. A
tone inversion process has been developed to enable the application of pitch split to dark field applications in addition to
standard bright field applications.
Lithographic scaling beyond the 22 nm node requires double patterning techniques to achieve
pitch values below 80nm. The semiconductor industry is focusing on the development of several process
techniques including track-only lithographic processing methods in order to reduce cost, cycle time and
defects. Initial efforts for track-only double expose processes have relied on the use of chemical freeze
materials to prevent inter-mixing of resists, and also by means of thermal curable materials. These two
techniques may be complementary, in the sense that a chemical freeze may be very robust for protection of
exposed regions, while thermal cure systems may provide strong protection of large unexposed areas.
We will describe our results with mainly the thermal-cure double patterning resist materials, and
the application of these materials to the fabrication of sub-80 nm pitch semiconductor structures. We will summarize the process window and defect capability of these materials, for both line/space and via applications.
Dual damascene processing for back end of the line (BEOL) layers can employ bilayer film stack approaches for
lithographic patterning. These bilayer resist systems are more prevalent for KrF layers and have many unique
characteristics, including silicon-containing photoresists and gap fill underlayer material that must also act as a bottom
anti-reflective coating (BARC). Bilayer resists pattern for copper deposition; as such, defect levels are a critical concern,
as any post-patterning bridging or residue defects can often times render an entire die inoperable due to electrical shorts
or breaks. Here, two such defect types were found: missing resist patterns and resist residue. Through several
experiments and with process optimization, the defect origins were elucidated and the defects themselves significantly
reduced. This work will detail the examination, root causes and eventual elimination of these significant bilayer resist
Advancing technology nodes in semiconductor manufacturing require more demanding lithographic performance for
patterning. The advent of 45 nm development necessitated dual damascene lithography moving from a KrF-based
bilayer approach to one that includes an ArF photoresist for higher resolution. There are multiple methods for an ArF
dual damascene (via first, trench last) system, including bilayer, trilayer and hard mask approaches. Flash manufacturing
demands are sensitive to process cost of ownership, so more complex approaches such as trilayer and hard mask film
stacks were not as attractive. One method examined as an ArF dual damascene solution was a so-called "modified
bilayer" approach, which is a combination of both KrF and ArF resist materials; in particular, this film stack allows for
the use of ArF silicon-containing resists along with a variety of anti-reflective and gap fill underlayer materials. The
modified bilayer approach afforded many advantages, including chemical compatibility, etch performance and process
robustness. The modified bilayer approach represents a culmination of learning that has enabled 45 nm back end of the
line (BEOL) dual damascene processing with ArF silicon-containing photoresists.
Current semiconductor manufacturing utilizes exposure wavelengths from 365 nm to 193 nm, and current research is centered on photoresist development for 157 nm. Our research group discovered the strong inhibition response in the fluorocarbon resins designed for use at 157 nm. We have been investigating dissolution inhibitors (DIs), some of which also serve as photoacid generators (PAGs), that strongly inhibit the dissolution of poly(2-(3,3,3-trifluoro-2-trifuoromethyl-2-hydroxypropyl) bicyclo[2.2.1]heptane-5-ene)(PNBHFA) (1) and the Asahi glass RS001 polymer (2). These inhibiting PAGs, in particular, result in the creation of 2-component resist systems consisting only of the resin polymer and the PAG-DI. This design enables greater ease of formulation, reduces the number of variables present in resist development, and offers improvements in sensitivity and line edge roughness. The synthetic approach has been to design transparent, inhibiting compounds for use at 157 nm. However, during our investigation of these compounds, we found that there is an inherent “backwards compatibility” for these PAGs and DIs at 193 nm, 248 nm and 365 nm. This has created the ability to effectively design dissolution inhibitors, photoactive or otherwise, that span virtually all of the wavelengths used in photolithographic processes today. Here we will present the design, development and imaging of modern dissolution inhibitors suitable for use in a wide range of photolithography technologies.
The design of 157 nm photoresists is a daunting task since air, water, and most organic compounds are opaque at this wavelength. Spectroscopic studies1 led to the observation that fluorinated hydrocarbons offer the best hope for the transparency that is necessary for the design of an effective 157nm photoresist, and these classes of materials have quickly become the prominent platforms for a variety of research activities in this field. Our approach to the design of the resist polymer requires identification of a backbone that tethers the functional substituents and provides basic mechanical properties, an etch barrier that provides RIE resistance, an acidic group that permits solubility in tetramethylammonium hydroxide (TMAH) developer. Fluorocarbon polymers have been identified as promising resist candidates for 157nm material design because of their relatively high transparency at this wavelength. Numerous authors have discussed negative photoresists over the years. There are many uses for such materials at various levels in a semiconductor device. One such use is with complementary phase shift mask thus eliminating the need for a second exposure step. This paper reports our recent progress toward developing a negative 157nm resist materials based on fluoropolymers with crosslinkers that are transparent at 157nm. The authors will report on the synthesis of the polymers used in this work along with the crosslinkers and other additives used in the formulation of the photoresist. Imaging experiments at practical film thicknesses at 157nm with binary and strong phase shifting masks will be shown demonstrating imaging capabilities. Spectroscopic data demonstrating chemical mechanisms and material absorbance will be shown along with other process related information
The focus of 157 nm lithographic research is shifting from materials research to process development. Poly (2-(3,3,3-trifluoro-2-trifuoromethyl-2-hydroxypropyl) bicyclo[2.2.1]heptane-5-ene) (PNBHFA) has received a great deal of attention as a possible base resin for 157 nm lithography. The Asahi Glass RS001 polymer, which was introduced at SPIE in 2002, has also shown promise as a 157 nm base resin due to its low absorbance. Partial protection of either polymer with an acid labile protecting group is a common design for functional photoresists. We previously reported the blending of the carbon monoxide copolymers with PNBHFA copolymers to achieve the critical number of protected sites for optimum imaging performance and contrast. Our group has since studied the use of the unprotected base resin with an additive monomeric dissolution inhibitors (DIs) and a photoacid generator (PAG) to form a three component resist. Surprisingly unprotected PNBHFA was discovered to have dissolution inhibition properties that are far superior to the dissolution inhibition properties of novolac. Several DIs were prepared and tested in PNBHFA to take advantage of the resins dissolution inhibition properties. We have also recently explored the performance of a two-component resist using PAGs that also function as DIs.
Significant progress has been made in 157 nm resist technology. Material development for this emerging field is continuing at a frantic pace. Many new and interesting polymers are surfacing for these studies. Fluorine-containing polymers have become the prominent platform for a variety of research activities within this field and a tremendous amount of progress has been achieved. Since the absorbance of a variety of different organic polymers at 157 nm was first reported, a vast array of fluorine-containing materials has been proposed and designed for photolithography at this wavelength. Free radical polymerizations, metal-catalyzed addition polymerizations and metal-catalyzed copolymerizations with carbon monoxide have produced materials that have yielded positive-tone images with 157 nm exposures. Major progress has been made in decreasing the absorbance of fluoropolymers based on Tetra Fluoro Ethylene (TFE). A number of key monomers have been synthesized based on the learning this project has cataloged over the past 2-½ years. Development of these new and interesting monomers has been done with copolymerizations of TFE taken into consideration. Our project has focused on polymer synthesis efforts, learning how to maximize transparency at 157 nm with consideration to etch resistance and imaging properties of these materials. Vacuum-UV (VUV) studies and variable angle spectroscopic ellipsometry (VASE) data will be shown on numerous fluorinated compounds and synthesized polymers. Our most recent materials have an absorbance of less than 1/μm and etch resistance equal to first generation KrF materials. This paper will provide synthesis, imaging and etch studies that have been completed using a 0.60NA and 0.85NA 157nm micro exposure system.
The synthesis and characterization of several new fluoropolymers designed for use in the formulation of photoresists for exposure at 157 nm will be described. The design of these resist platforms is based on learning from previously reported fluorine-containing materials. We have continued to explore anionic polymerizations, free radical polymerizations, metal-catalyzed addition polymerizations and metal-catalyzed copolymerizations with carbon monoxide in theses studies. The monomers were characterized by vacuum-UV (VUV) spectrometry and polymers characterized by variable angle spectroscopic ellipsometry (VASE). Resist formulations based on these polymers were exposed at the 157 nm wavelength to produce high-resolution images. The synthesis and structures of these new materials and the details of their processing will be presented.
Fluorocarbon based polymers have been identified as promising resist candidates for 157nm material design because of their relatively high transparency at this wavelength. This paper reports our recent progress toward developing 157nm resist materials based on transparent dissolution inhibitors. These 2 component resist systems have been prepared and preliminary imaging studies at 157nm are described. Several new approaches to incorporating these transparent monomers into functional polymers have been investigated and are described. The lithographic performance of some of these polymers is discussed.
(alpha) -Fluoroalcohols have been proposed as transparent, base-soluble functional groups for use in the design of new 157 nm photoresist polymers. The two most common and easily prepared fluoroisopropanol groups are bis-trifluoromethyl carbinols (hexafluoroalcohol) and methyl-trifluoromethyl carbinols (trifluoroalcohol). This paper describes studies designed to assess the suitability of both of these functionalities as acidic groups. Dissolution rate studies were carried out on polystyrene films that incorporate these groups. The dissolution rates of the sample polymers were compared to that of poly(hydroxystyrene) (PHOST) to provide a reference for the measurements. It was found that the trifluoroalcohol polymers do not exhibit any solubility in basic media, while the hexafluoroalcohol polymers dissolve rapidly relative to PHOST in 0.13N TMAH. Further, it was found that the two fluoroalcohol polymers can be blended to adjust the inherent dissolution rate of the resin and that the hexafluoroalcohol polymer is sensitive to incorporation of classical dissolution inhibitors. The study concludes that hexafluoroalcohol is a promising candidate for incorporation into the design of 157 nm photoresists.
Finding materials that offer the all of the characteristics required of photoresist matrix resin polymers while trying to maintain a high level of transparency at 157 nm is a daunting challenge. To simplify this task, we have broken the design of these polymers down into subunits, each of which is responsible for a required function in the final material. In addition, we have begun collecting gas-phase VUV spectra of these potential subunits to measure their individual absorbance contributions. Progress on developing materials for each of these subunits are presented along with plans for future studies.
Top surface imaging (TSI) systems based on vapor phase silylation have been investigated for use at a variety of wavelengths. This approach to generating high aspect ratio, high resolution images held great promise particularly for 193 nm and EUV lithography applications. Several 193 nm TSI systems have been described that produce very high resolution (low k factor) images with wide process latitude. However, because of the line edge roughness associated with the final images, TSI systems have fallen from favor. In fact, top surface imaging and line edge roughness have become synonymous in the minds of most. Most of the 193 nm TSI systems are based on poly(p-hydroxystyrene) resins. These polymers have an unfortunate combination of properties that limit their utility in this application. These limiting properties include (1) High optical density (2) Poor silylation contrast (3) Low glass transition temperature of the silylated material. These shortcomings are related to inherent polymer characteristics and are responsible for the pronounced line edge roughness in the poly(p-hydroxystyrene) systems. We have synthesized certain alicyclic polymers that have higher transparency and higher glass transition temperatures. Using these polymers, we have demonstrated the ability to print high resolution features with very smooth sidewalls. This paper will describe the synthesis and characterization of the polymers and their application to top surface imaging at 193 nm. Additionally, it will describe the analysis that was used to tailor the processing and the polymer's physical properties to achieve optimum imaging.