A series of different fluorinated polymer platforms used for early and current 157-nm photoresists is investigated with regard to blanket etch properties and surface roughness. Besides methacrylic-based polymers applied for 193-nm lithography, fluorine containing norbornene homopolymers, fluorinated cycloolefines, and tetrafluoroethylene (TFE) norbornene copolymers are chosen. Etch rates in different plasmas used for several applications, such as poly, SiN mask open, and selective/nonselective SiO2 etch, are determined and compared to standard 193-nm platforms currently used for DRAM manufacturing. Looking at various base resins, significant differences can be found using HBr- or Cl2-based poly etch conditions and various fluorocarbon-based oxide etch chemistries. Up to 2.4 times higher etch rates in Cl2 and the different CxFy oxide etch recipes are observed for the highly fluorinated cycloolefines and the TFE norbornenes, showing a strong correlation between fluorine content and etch rate. After stress by different etch conditions, the polymer surfaces are characterized using atomic force microscopy (AFM) and scanning electron microscopy (SEM). Surprisingly, the surface roughness of the methacrylic platforms and the norbornene base resin (determined by AFM) are found to be substantially higher than that of the highly fluorinated platforms. These results can directly be correlated to pictures obtained by optical methods (SEM).
We summarize our work on devising protective barrier coats for use against airborne contamination when using tert-butoxycarbonylmethyl (BOCME) capped fluoroalcohol resist resins as part of our strategy to develop a 157 nm resist platform. We will describe how a barrier coat (AZ EXP FX Coating 145) consisting of a fluoro-cyclopolymer formulation, soluble in aqueous developer, can improve the post-exposure delay (PED) latitude of 157 nm resist resists exposed under conditions or airborne contamination. Specifically, a 20 nm thick coating of AZ EXP FX Coating 145 gives a PED latitude for L/S features of at least 10 min under condition of airborne amine contamination (10 ppb amine contamination). The barrier coat, AZ EXP FX coating 145 is formulated in a solvent which is compatible with resist film coated from typical 193 nm resist spin casting solvents. Moreover, it can be easily removed as part of the normal aqueous base development scheme, no extra post-apply bake or stripping step is required.
We will describe our barrier coat approach for use in immersion 193 nm lithography. These barrier coats may act as either simple barriers providing protection against loss of resist components into water or in the case of one type of these formulations which have a refractive index at 193 nm which is the geometric mean between that of the resist and water provide, also top antireflective properties. Either type of barrier coat can be applied with a simple spinning process compatible with PGMEA based resin employing standard solvents such as alcohols and be removed during the usual resist development process with aqueous 0.26 N TMAH. We will discuss both imaging results with these materials on acrylate type 193 nm resists and also show some fundamental studies we have done to understand the function of the barrier coat and the role of differing spinning solvents and resins. We will show LS (55 nm) and Contact Hole (80 nm) resolved with a 193 nm resist exposed with the interferometric tool at the University of New Mexico (213 nm) with and without the use of a barrier coat.
The leaching of ionic PAGs from model resist films into a static water volume is shown to follow first order kinetics. From the saturation concentration and the leaching time constant, the leaching rate at time zero is obtained which is a highly relevant parameter for evaluating lens contamination potential. The levels of leaching seen in the model resists generally exceed both static and rate-based dynamic leaching specifications. The dependence of leaching on anion structure shows that more hydrophobic anions have lower saturation concentration; however, the time constant of leaching increases with anion chain length. Thus in our model system, the initial leaching rates of nonaflate and PFOS anions are identical. Investigation of a water pre-rinse process unexpectedly showed that some PAG can still be leached from the surface although the pre-rinse times greatly exceeded the times required for saturation of the leaching phenomenon, which are expected to correspond to complete depletion of leachable PAG from the surface. A model is proposed to explain this phenomenon through re-organization of the surface as the surface energy changes during the air/water/air contact sequence of the pre-rinse process.
Line-edge roughness (LER) continues to be one of the biggest challenges as the CD size shrinks down to sub 100 nm. It is shown that resist components as well as illumination conditions play a big role. Influence of resist components in both 248 and 193nm chemically amplified resist formulations has been reported but the root cause is not fully understood and may be platform or even specific formulation dependent. This paper attempts to tackle the issue from the processing side. Effects of a simple hard bake process on the LER were studied. In the hard bake process, a given resist pattern was typically baked close to the glass-transition temperature after the development process. LER improved dramatically due to melting down of the rough surface. However, the wall angle of the edge lines also started to degrade at the optimum hard bake temperature. Studies on the effects of polymer Tg, hard bake temperature and time and the issues of the process are discussed.
A detailed account will be given of work done on the Micrascan VII (NA 0.75) at INVENT in Albany with AZ EXP X20 and AZ EXP X25 resist systems based upon BOCME protected fluoroalcohol resins. These resins were examined either with a high or low level of formulated photoacid generator (PAG). Our evaluations done both with binary and alternating phase shift mask exposures. It was found in our initial studies done at relatively high amine levels (1-2 ppb) that AZ EXP X25 X with low PAG gave the best performance.
This paper is part of our continuing work on a new generation of more transparent, 157 nm resist platforms, which are based upon capping of fluoroalcohol-substituted, transparent perfluorinated resins (TFR) with a tert-butoxycarbonylmethyl (BOCME) moiety. Recent results indicate that by optimizing both resin structure and loading of photoacid generator and base additive a good compromise can be achieved between resolution power, dark erosion resistance, sensitivity and transparency at 157 nm. Specifically, it was found that a decrease in PAG (50% nominal loading) and base loading (75% nominal loading), coupled with optimization of the TFR resins to achieve higher transparency, gives the best compromise of properties. In this manner, resist systems with a transparency as low as 0.87 AU/micron were designed capable of resolving 60 nm 1:1 features, at a dose of 92 mJ/cm2 (non corrected for sigma), using a strong phase shift mask, and a sigma of 0.3 on a Exitech 157 nm small field mini-stepper. This type of resist material has also been imaged with a larger field tool (DUV30 Micrascan VII) to give 80 nm 1.1.5 L/S features at a dose of 135 mJ/cm2 employing using a Binary mask (σ=0.85). Finally, it was found that our BOCME-TFR based resist system can be used to transfer a 120 nm L/S pattern (imaged by 193 nm lithography) into a hardmask stack on top of silicon.
As part of a new generation of more transparent 157 nm resist platforms we are developing, a novel resist system is described that has higher transparency and contrast than AZ FX 1000P. Using a new protecting group strategy, encouraging results have been obtained with both poly(α,α-bis(trifluoromethyl)bicyclo(2.2.1)hept-5-ene-2-ethanol) and a more transparent perfluorinated resin (TFR). These new resist systems show absorbance values as low as 1 μm-1 at 157 nm, have twice the contrast (i.e., 12 instead of 7) of AZ FX 1000P, and have neither significant dark erosion nor do they switch to negative tone behavior within the dose range studied. The dry etch resistance of the TPR platform is found to be superior to APEX-E DUV resist for polysilicon but somewhat lower for oxide etches. Features as small as 50 nm lines and spaces were resolved for slightly relaxed pitches (1:1.5 micron). By adjusting the base level it is possible to improve the photospeed by a factor of more than 10 while still maintaining a resolution of 70 nm L/S features.
A statistical design of experiments for the post-applied bake and post-exposure bake temperatures for two types of resists, the commercial formulation AZ FX 1000P and an experimental resist AZ EXP 20 X, was carried out using contrast, clearing dose and dark erosion as response variables examined. It was found that for AZ FX 1000P dark erosion could be suppressed entirely and contrast improved by employing a lower PEB without significant impact on the contrast. In this manner, a substantial improvement in the image quality for AZ FX 1000P was obtained. AZ EXP 20X was not susceptible to dark erosion at higher post-applied bakes as was AZ FX 1000P. Both resists gave better imaging at lower post-exposure bake temperatures in the range of ~110°C, presumably because of excessive acid diffusion at higher temperatures, such as 150°C. Generally, the contrast achievable with AZ EXP 20 X (>16) is much higher than that possible for AZ FX 1000P (~6).
The copolymerization reaction between methyl cyanoacrylate (MCA) and a variety of cycloolefins (CO) was investigated. Cycololefin/cyanoacrylate (COCA) copolymers were obtained in good yields and with lithographically interesting molecular weights for all cycoolefins studied. Anionic MCA homopolymerization could be largely suppressed using acetic acid. Based on NMR data, the copolymerization may tend to a 1:1 CO:MCA incorporation ratio but further work with better suppression of the anionic component is needed to confirm this. Lithographic tests on copolymers of appropriately substituted norbornenes and MCA showed semi-dense and isolated line performance down to 90 nm.
The development of sufficiently transparent resin systems is one of the key elements required for a successful and timely introduction for 157 nm lithography. This paper reports on the Simple Transmission Understanding and Prediction by Incremental Dilution (STUPID) model, a quick back-of-the-envelope increment scheme to estimate the absorption of polymers at 157 nm. A number of promising candidate resins based on norbornenes are discussed, and results with a first 157 nm resin system developed at the University of Austin are presented. The new system is based on copolymers of norbornene-5-methylenehexafluoroisopropanol (NMHFA) and t-butyl norbornene carboxylate (BNC), formulated with an acetal additive obtained by copolymerization of t-butyl norbornene-5-trifluoromethyl-5-carboxylate (BNTC) with carbon monoxide. Lithographic performance of this system extends to 110 nm dense features using standard illumination and a binary mask, or 80 nm semi-dense and 60 nm isolated features with a strong phase shift mask. The dry etch resistance of this resist is found to be slightly lower than APEX-E DUV resist for polysilicon but superior to it for oxide etches.