Understanding electron energy loss mechanisms in EUV resists using EELS and first-principles calculations

Transcription

1 Understanding electron energy loss mechanisms in EUV resists using EELS and first-principles calculations Robert Bartynski Sylvie Rangan Department of Physics & Astronomy and Laboratory for Surface Modification Rutgers University

2 Secondary Electrons and PAG Activation EUV photon absorption primary and secondary electrons inelastic scattering energy deposited into resist The resulting low energy electrons interact with the PAGs to produce photoacids 2 Brainard et al. Proc. of SPIE Vol. 6923, (2008)

3 Objective of This Study We aim to answer the following questions: What are the mechanisms by which EUV-generated electrons deposit energy into a resist? What are the relevant resulting molecular excitations in the resist? How does the probability of those excitations depend upon electron energy? How can this information be used to model electron scattering in EUV resists? Can this information be used to guide EUV resist formulation? 3

4 Experimental and Theoretical Approach Understand individual EUV resist components (monomers, polymers, PAGS, ) Electron spectroscopic techniques (XPS, UPS, EELS) First principles (DTF) calculations origin of spectral features Measure electron energy loss (EELS) spectra of resists and components. Perform theoretical calculations (TD-DFT) of excitation spectrum and compare to EELS results. Identify the molecular excitations that energetic electrons cause in the resist. Via SEMATECH, transfer this knowledge to collaborators: Modelers, synthetic chemists, 4

5 Resist components PMMA Polymer 1 Polymer 1 Polymer 2 S-PAG Resist 1 (OS 4) Resist 3 (OS 14) I-PAG Resist 2 (OS 2) Resist 4 (OS 12) Polyhydroxystyrene Polymer 2 Pure S PAG Pure I PAG Goal: Establishing predictive tool for low energy loss processes TBAH

6 Electron energy Electron energy Principle of techniques Photoemission: UPS or XPS e - Reflection Electron Energy Loss Spectroscopy (REELS) e - e - e - e - E F Primary elastic peak Electrons with less kinetic energy due to energy loss CB Electrons counts CB Electrons counts VB E F VB e - e - Energy loss Core level Core level UPS: UV photoemission XPS: x-ray photoemission 6

7 Example: Polyhydroxystyrene XPS and UPS Spectra (Do we have what we think we have?)

8 Polyhydroxystyrene XPS core level spectra: local chemical environment C 1s core level XPS DFT Calc. O 1s core level DFT Calculation XPS Density Functional Theory of simple model successfully describes core level spectra 8

9 Polyhydroxystyrene UPS valence band spectra: Identify molecular orbitals Polyhydroxystyrene Valence Band UPS Spectrum DFT Calculation StyrOHsg Valence band is well described by DFT calc. of StyrOHsg 9

10 Polyhydroxystyrene EELS Spectra Ground state successfully modeled Now investigate energy loss mechanisms

11 Absorption spectra calculations Use Time Dependent DFT (TD-DFT) to determine excited states. Excited states are linear combinations of the ground state orbitals LUMO LUMO+1 LUMO HOMO HOMO HOMO-1 First excited state Second excited state First excited state for this molecule is due to a HOMO to LUMO transition 11

12 REELS Polyhydroxystyrene HOMO HL LUMO p p* Exp tal spectrum (E p = 30 ev) HOMO H L+1 LUMO+1 H L+1 Calculated spectrum HL HOMO-1 p p* LUMO Good agreement between measured loss features and calculated spectrum. Identified origin of EELS features. 12

13 Our approach worked for a homopolymer (polyhydroxystyrene) What about a multicomponent resist polymer? (Polymer 1)

14 Simulation of Polymer 1 Polymer /20/15 StyrOHsg Styrsg TBA Molecular models Model polymer 1 as sum of 3 monomers 14

15 Polymer 1 XPS Core level Spectra Exp t 65/20/15 Valence Band Spectrum Sum UPS VB TBA Styr StyrOH Theory Core levels and valence band are well described by weighted sum of contribution from each individual components This model is a good starting point for excited states determination 15

16 Polymer 1 Sum p p* REELS Theory Polymer 1 65/20/15 Styr StyrOH TBA H L+1 HL + + StyrOH Styr TBA The absorption is dominated by p p* transition of benzene ring 16

17 Comparison: Polyhydroxystyrene vs Polymer 1 REELS Theory Overall shape of theoretical curves match experimental spectra. Polymer 1 and polyhydroxystyrene loss spectra very similar. p p* REELS Experiment The absorption is dominated by the p p* transition on benzene ring. H L+1 HL 17

18 PMMA

19 PMMA XPS Core level Spectra UPS Valence Band Spectrum PMMA successfully simulated using a simple molecular model 19

20 PMMA REELS HOMO-n LUMO HOMO-1 LUMO+1 HOMO-LUMO Loss features well described by the calculated absorption spectrum 20

21 REELS: Energy Dependence

22 Polyhydroxystyrene: Cross Section Effects 50 ev 110 ev Polyhydroxystyrene REELS p HL O Excitation energy-dependent intensity variation of loss features 30 ev? p Intensity (cross section) variation of losses Features p and HL described by model Origin of low energy losses (O) currently under investigation HL O 22

23 Polymer 1: Cross Section Effects Polymer 1 REELS 65/20/15 p HL O? p Intensity (cross section) variation of loss features as function of primary energy Features p and HL described by model Origin of low energy losses (O) currently under investigation HL O 23

24 PMMA: Cross Section Effects PMMA REELS O? Low energy losses near 2eV appears in PMMA, remains unexplained 24

25 Electron-induced decomposition in PMMA

26 PMMA: Electron-Induced Decomp. PMMA REELS PMMA films highly sensitive to electron beam damage! What s happening? 26

27 PMMA: Electron-Induced Decomp. XPS Core level Spectra After e-beam Before e-beam UPS Valence Band Spectrum PMMA VB and core levels modified by electron beam exposure Loss of oxygen-containing species 27

28 PMMA: Electron-Induced Decomp. Electron beam induces loss of oxygen-containing species Our experimental results compatible with literature model Model fully decomposed PMMA with oxygen-free hydrocarbon. PMMA decomp 28

29 PMMA: Electron-Induced Decomp. XPS Core level Spectra Valence Band Spectrum Degradation mechanism can be followed 29

30 PMMA: Electron-Induced Decomp. PMMA REELS 30 ev PMMA decomp Growing loss feature at 4 ev from decomp. product.? Diminishing intensity above 6 ev from less intact PMMA Loss of intensity near 2 ev from loss of O-containing species?? 30

31 Origin of ~ 2 V energy losses feature??? O Carbon, water 20Å SiO 2 20Å SiO 2 Si Si Clean Si Low energy loss ~2eV is observed on SiO 2 (and Al 2 O 3 ) films No low energy loss on film(s) where oxygen is absent 31

32 Summary Combining first-principles theory (DFT) and electron spectroscopic techniques (XPS & UPS) gives a detailed understanding of EUV resist materials. Based on that understanding, the molecular excitations produce by energetic electrons in resists can be identified using TD-DFT calculations. Phenolic resist polymers exhibit high probability for excitation in the energy range of 6 ev due to p p* transitions. PMMA exhibit high probability for excitation in the energy range of 8 ev due to HOMO-1 LUMO+1 transitions. Loss features (hence polymer excitations) have electron energydependent cross sections. PMMA is prone to decomposition by ~30 ev electron beam. Oxygen loss induces polymer hybridization new low energy excitations at ~ 4 ev. Preliminary study of oxides suggest low energy excitations related to oxygen in resist polymers. 32

33 Future Studies Polystyrene (is ~ 2eV loss band missing?) Extend studies to non-phenolic polymers (polymer 2 & components) Polymer 1 Polymer 2 S-PAG Resist 1 (OS 4) Resist 3 (OS 14) Polymer 2 Study other resist components (PAGS) Study matrix of organic resists I-PAG Resist 2 (OS 2) Resist 4 (OS 12) Extend studies to inorganic resists (polymer-coated TM oxide nanoparticles. 33

34 Acknowledgements Sylvie Rangan Funding Brainard Group CNSE SUNY Polytec (samples) Grad Student: Amrit Narasimhan 34

35 EXTRA SLIDES 35

36 Energy loss variation Polyhydroxystyrene Polymer 1 Both behavior and relative intensities seem experimentally reliable and reproducible: necessary for cross section determination (no clear sample effects: surface roughness, thickness ) 36

37 PMMA REELS cross section Similar intensity variations for both films thicknesses Lower intensity than what was observed for the polyhydroxystyrene and polymer 1 films 37

38 REELS cross section comparison PMMA Polyhydroxystyrene Main PMMA loss (7.8eV) has smaller cross section than main polyhydroxystyrene loss (6.5 ev) Loss at 2 ev is similar in magnitude for both films 38

39 Electron beam damage Experiment Left: Multiple exposures to a 30 ev electron beam does not alter the local chemistry Right: 30 ev loss spectra taken after different energy exposures. Higher beam energies slowly lead to polymer degradation. Experiment 39

40 Electron beam damage Left: Beam damage, similar to last slide. Right: Short exposure REELS spectra taken at different electron energies show intensity variations. These intensity variations are different from those obtained from beam damage. REELS contains real cross section effects information 40

41 Improved PMMA model and spectral agreement 2 CH 3 4 CH 3 MMA 3 CH CH 3 2MMA 3MMA More MMA units reduces the CH 3 contribution to the DOS 2PMMA and 3PMMA closer to measured experimental VB (black) 41

42 Investigation of energy losses below 4eV Si Al For clean films, the low energy loss feature is not visible anymore Is this feature related to oxygen content? Are low energy loss features related to oxygen? 42

43 Investigation of energy losses below 4eV Carbon, water 20Å SiO 2 40Å Al 2 O 3 Si Al REELS measurements performed on two different oxides samples: SiO 2 on Si and Al 2 O 3 on Al Low energy loss<4ev is observed on both films 43

44 How do secondary electrons affect resist? Example: Electron-induced molecular dissociation OH - desorption from n-c 8 H 18 and n-c 5 H 12 + O 2 induced by dissociative electron attachment (DEA) F - desorption induced by Dissociative electron attachment (DEA) S. Solovev, A. Palmentieri, N. D. Potekhina, and T. E. Madey, J. Phys. Chem. C 111, (2007) L. Sanche and L. Parenteau, J. Chem. Phys. 93, 7476 (1990) LOW ENERGY ELECTRON DRIVEN PROCESS Threshold energies ~ 4 5 ev Peak yields ~ 7 9 ev 44