EUV Resist-Fundamental Research

Transcription

1 EUV Resist-Fundamental Research Akinori Saeki, Hiroki Yamamoto, Takahiro Kozawa, and Seiichi Tagawa The Institute of Scientific and Industrial Research, Osaka University CREST, Japan Science and Technology Agency, c/o Osaka University SEMATECH/ISMI Symposium Japan 2009 September 17, Hyatt Regency Tokyo

2 Contents Current Status of EUV Resists 1. Origin, meaning and solution of RLS trade-off problem nm half-pitch target Future Prospect of EUV Resists Pattern formation mechanism and simulation of EUV resists Maximization of the chemical gradient at the image boundary Four topics 1. Difference between EUV and EB Resists 2. Shot noise of chemically amplified resists 3. Difference between Molecular and Polymer Resists 4. New types of resists

3 Current Status of EUV Resists (1) The Origin of RLS Problem CARs with High Concentration of Quenchers From the early stage of the industrial use of chemically amplified resists, amines have been contained in CARs for measures against socalled post-exposure delay effects due to surface contamination from airborne amines. Recently very high concentrations of amines are required for getting high resolution CARs. High concentration of amines improves resolution and LER, but sensitivity decreases and trade-off among resolution, line edge roughness and sensitivity ( so-called RLS tradeoff) was induced.

4 Current Status of EUV Resists (2) RLS Trade-off Problem and Solution Trade-off relation among resolution, LER and Sensitivity (RLS Trade-off) was observed experimentally and simulated later under some asumption. Experiments: Brainard et al., Proc. SPIE (2004), Pawloski et al., Proc. SPIE (2004), Wallow et al., Proc. SPIE (2008) and more. Simulations(EUV reaction mechanisms were neglected): G.M. Gallatin, Proc. SPIE (2005), D. Van Steenwinckel et al., Proc. SPIE (2007), R.L. Bristol, Proc. SPIE (2007) Is the relation specific? No or Yes No fundamental differences exist among ArF, EB and EUV resists once latent acid image formation.but acid formation process doesn t obey the relation. Acid formation mechanisms are essential in solving the trade-off problem. Tagawa, 1st European Workshop on Resist Limitations, Erlangen, 14 December, (2004), Kozawa et al., J. Vac. Sci. Technol. B23 (2005) Kozawa and Tagawa, J. Appl. Phys. 99 (2006) , Tagawa, MNC Plenary Lecture (2007), Saeki, kozawa, Tagawa, Appl. Phys. Express 2(2009) LER Sensitivity Resolution Recent research on mechanisms of EUV CARs clearly shows how to solve RLS trade-off in EUV resists.

5 Current Status of EUV Resists (3) New Targets after RLS Trade-off Problem Resolution and sensitivity are getting close to the requirements. LER is still far from the requirement. The reduction of LER is the most serious problem in EUV resist development. Sensitivity is still very important problem, because the sensitivity of resists and the intensity of exposure dose are complementary. Higher sensitivity resist makes possible to use smaller light source and to decrease in operation costs.

6 Current Status of EUV Resists (4)

7 Future Prospect of EUV Resists Pattern formation mechanism and simulation of EUV resists The increase in pattern formation efficiency (chemical gradient) at the image boundary Four topics 1. Difference between EUV and EB Resists 2. Shot noise of chemically amplified resists 3. Difference between Molecular and Polymer Resists 4. New types of resists

8 Pattern Formation Mechanisms of EUV Resists (1) Exposure (Tool) Resist pattern formation processes Interaction of radiation (photon, electron etc.) with materials Acid generation Accumulated energy profile Latent acid image Processes depend on exposure tool Acid diffusion,deprotection reaction Acid catalyzed image (Latent image after PEB) Development Other Treatments: Rinse, Vapor smoothing, Hardbake, Ectch, Ozonation, etc. The improvement at each stage is required cloth to its physical and chemical limit. The integration of improvement at each stage without interference is strongly needed for steady progress in the development of next generation EUV resists. Pattern Formation Mechanisms are important and essential in escaping interference.

9 Resist Comparison of Energy Absorption and Reaction Mechanisms among KrF, ArF, EB and EUV CARs Energy Absorption Mechanisms Energy absorption process Main energy absorber KrF,ArF Lambert s law Acid generator EB Pattern Formation Mechanisms of EUV Resists (3) Bethe equation Energy absorption is proportinal to electron density di( z) dz = αi ( z) Mainly Polymer EUV Lambert s law Mainly Polymer di( z) = αi ( z) dz S. Tagawa, MNC2007 Plenary Lecture Resist KrF,ArF EB EUV Reaction Mechanisms Main Initial reactions of acid generation Excitation of acid generators Mainly isolated spur reaction 1. Ionization of polymers 2. Dissociative electron attachment of acid generators 3. Geminate recombination of protons and anions Mainly multi-spur reactions 1. Ionization of polymers 2. Dissociative electron attachment of acid generators 3. Geminate recombination of protons and anions

10 Pattern Formation Mechanisms of EUV Resists (5) RLS Trade-off Relation Simulated by Monte Carlo and Dissolution Simulations (a) (b) (c) z y x (d) (e) (f) Fig. 1. Snapshots of the performed Monte Carlo and dissolution simulations. (a, d) Initial spatial distribution of protons (green) and counter anion (red) produced by EB exposure. (b, e) Latent images after PEB colorized by residence time t (small t: blue, large t: red. see text for the detail. A front side of the latent image is cut to visualize the inside of the latent image). (c, f) Positivetype line pattern after development. The pictures of (a) (c) and (d) (f) correspond to the exposed line width of 5 and 10 nm, respectively. The exposure dose and development time were 50 mc/cm 2 and 60 s, respectively. The axes x, y, and z in the left side of (a) represent the directions of exposed line width, line length, and film thickness, respectively. The black wire box is a nm 3 cuboid. L Minimum LER 3σ [nm] (a) R40 Line width [nm] 60 L 1 S Exposure dose [μc/cm 2 ] Minimum LER 3σ [nm] (b) R40 Line width [nm] A. Saeki, T. Kozawa, and S. Tagawa, Appl. Phys. Express 2 (2009) S 100 Exposure dose [μc/cm 2 ] Fig. 2. RLS relationship of developed line pattern at optimized PEB and development time. The exposed line widths of (a) and (b) are 30 and 10 nm, respectively. The area enclosed by white dotted lines represents the failure in development. The trade-off RLS relationship is successfully reproduced under the optimized condition of PEB and development time.

11 (a) Minimum LER : 3σ [nm] Pattern Formation Mechanisms of EUV Resists (6) Line edge roughness after development studied by Monte Carlo and dissolution siumulation 20 s 60 s 120 s Development time Exposure dose [μc/cm 2 ] (b) Minimum LER : 3σ [nm] Increase in line width 0 nm 10 nm 20 nm 30 nm Exposure dose [μc/cm 2 ] Exposure dose dependence of minimum LER (3σ) for different (a) development times and (b) increases in line width. Resist thickness:100 nm, 75 kev EB,100μC/cm2: EUV, 20 mj/cm 2 (from calculated) (25 mj/cm 2 from experiment) In one of the ideal case, 1.2 nm LER (high freq.) is possible. A. Saeki, T. Kozawa, S. Tagawa, H. B. Cao, H. Deng, M. J. Leeson, Nanotechnology, 19(2007)15705

12 Minimum LER dependence on exposure dose. Minimum LER 3σ [ nm] (a) LW=20 nm 30 nm, β= nm, β= nm, β= (b) LW=30 nm 30 nm, β= nm, β= nm, β= Exposure dose [μc/cm 2 ] Exposure dose [μc/cm 2 ] Fig. 3. Minimum LER dependence on exposure dose. The developed line widths (LW) are (a) 20 and (b) 30 nm. The exposed line widths are 30 nm (blue diamonds), 10 nm (yellow green triangles), and 5 nm (red squares). The solid lines are fitting curve expressed by the following equation. α( ) β LER = Dose LER is proportional to the inverse of square root of exposure dose at moderate acid diffusion length. 1 LER Dose Dependence on exposure dose becomes severer with the decrease in LW. A. Saeki, T. Kozawa, and S. Tagawa, Appl. Phys. Express 2 (2009)

13 Pattern Formation Mechanisms of EUV Resists (8) Simulation Based on Resist Pattern Formation Mechanisms of EB/EUV Chemically Amplified Resists The LERs of a positive-tone CAR for EB/EUV lithographies after development were investigated by Monte Carlo simulation and a dissolution model based on a 1 nm mesh. LER(high frequency) of less than 1.2 nm is achievable by fine-tuning exposure dose, acid diffusion length (PEB time), and process conditions (development time, amine concentration, and activation energy) even though the proton and counter anion, which are attracted by each other (geminate pairs) and form an acid, are initially separated by 5.6 nm on average. The trade-off RLS relationship is successfully reproduced under the optimized condition of PEB and development time. LER is proportional to the inverse of square root of exposure dose at moderate acid diffusion length. Note that this is an ideal condition (1 nm mesh: excluding polymer effect, cluster, swelling in development etc.) in PHS base resists (not the best resist ). The improvement at each pattern formation stage is required cloth to its physical and chemical limit. The integration of improvement at each stage without interference is strongly needed for steady progress in the development of next generation EUV resists.

14 Resolution LER Exposure source dependent Exposure source independent Sensitivity The increase in pattern formation efficiency is required to simultaneously meet the requirements for RLS. Pattern formation efficiency = Absorption efficiency of incident energy (mainly absorption coefficient of polymer) Quantum yield of acid Efficiency of catalytic chain reaction X X Limited by side wall degradation Limited by secondary electron emission efficiency Limited by diffusion-controlled rate for chemical reaction Other factors:development and many other treatments etc..

15 Intensity of EUV (I) I = αi z Energy Absorption Process of EUV Resists Absorption coefficient (α) PHS : 3.8 μm -1 Interaction of EUV photon with CARs -spatial distribution- EUV photon (92.5 ev) photon Electron > IP Electron < IP z Inelastic mean free path <1 nm mean free path at electron with energy > IP Ionization + e - e - + Thermalization Ionization + * + Excitation Ionization e - Multi spur effect e - Thermalization e e e - e - Resist Thermalization Length 4.0 nm for PHS The number of secondary electrons is estimated expermentally. 4.2 for PHS PHS with 10 wt% TPS-tf Acid molecules per photon: 2.6 (Kozawa et al. J.Vac.Sci. Tecnnol.,B25(2007) 2481) Experimental value: 2.5 (Hirose et al.,jap.j.appl.phys,part 2(2007)

16 Pattern Formation Mechanisms of EUV Resists (4) Probability density of anion (/nm) Acid Generation in Resists by EB and EUV (a) EB Distance from ionization point (nm) Probability density of anion (/nm) (b) EUV Distance from EUV absorption point (nm) Probability density of anion generated in PHS with 10 wt% TPS-tf by (a) EB and (b) EUV Acid generation efficiency (ionization) 0.74 per ionization 0.62 per ionization G(acid) = 3.3 (3.3 acids per 100 ev) Kozawa et al. J.Vac.Sci.Tecnnol. B24,3055(2006) 2.6 acids per one photon(92.5 ev) in PHS Kozawa and Tagawa, J.Vac.Sci.Tecnnol.,B25(2007) 2481 Experimental value: 2.5 Hirose et al., Jap.J.Appl. Phys, Part 2 Let. & Express Let.,46,L979(2007) Difference between acid generation of EB and EUV resists is small, although energy absorption mechanism is different.

17 Sensitivity and Shot Noise of CAR and non-car 5 mj/nm 2 is about 3 EUV photons/(1 nm) 2. It is very difficult to overcome shot noise problem in non-car. In CAR, the recombination reaction of acids and quenchers occurs in image boundary. LER is determined at image boundary. Acid diffusion decreases shot noise in CAR. 1 nm Initial acid distribution (high E a resist) Concentration (nm -3 ) Aerial image of EUV Acid distribution after neutralization Initial acid distribution Quencher distribution after neutralization No flare nm Resolution blur Distance (nm) The aerial image of incident EUV (arb. unit) and the initial acid distribution (molecule nm -3 ).. (Jpn. J. Appl. Phys. 47 (2008) 4926). 7% flare

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20 Conclusion 1. The development of EUV resists has steadily progressed based on understanding of the patterning mechanisms of EUV CARs. Many methods for improvement of resist performance such as the increase in the PAG concentration, energy absorption, acid yield are investigated at each stages of resist patterning. 2. The sensitivity and resolution is getting close to the requirements. However, LER is still away from the target. The simulation based on reaction mechanisms of EUV CARs shows how to get 1.2 nm LER in ideal conditions (1 nm mesh). LER contains factors independent of RLS trade-off relation such as molecular interactions (clustering) and swelling in development processes. The difference between molecular and polymer resists becomes essential to LER reduction, after LER decreases the level of molecular size. 3. New ideas of EUV CAR like polymer-bound PAGs and resists with anisotropic acid diffusion enhance the resist performances. 4. Many other factors for small patterning must be investigated such as resist pattern collapse, the decrease in broading of the patterned feature size due to acid diffusion, reconsideration of limitation factors such as mobilities of counter anions and quenchers, side and competing reactions such as crosslinking of polymer radicals and trapping sites of hole, proton and electrons. 5. The improvement at each pattern formation stage is required cloth to its physical and chemical limit. The integration of improvement at each stage without interference is strongly needed for steady progress in the development of next generation EUV resists.