MA-P18 7 EUVL Symposium Design of Attenuated Phase-shift shift Mask with Absorber for Extreme Ultraviolet Lithography Hee Young Kang and Chang Kwon Hwangbo Department of Physics, Inha University, Incheon 4-751 Republic of KOREA hwangbo@inha.ac.kr
Attenuated Phase Shift Masks * Condition of Attenuated Phase Shift Mask (Att( Att-PSM): * Two types of Att-PSMs PSMs: Att-PSM in EUV: (R ) EUV <.1 ΔΦ EUV =18 o ARC and high contrast in DUV: DUV contrast > 85 % (R DUV <5%) Δt air > 8 nm Absorber Absorber [Mo/] 4 Δt air > 1 nm Additive-type type Subtractive-type type
Geometrical shadow effects A. M. Goethalsa et al., SPIE 6517, 65179 (7) EUVL projection optics is telecentric at the image side, but non-telecentric at the mask side because of oblique illumination. The thickness of absorber in additive-type PSM structure is generally over 8 nm. The illumination beam is then shadowed by the edge of the absorber, and as a result, the printed patterns are shifted and biased. The subtractive-type PSM structure should be formed by etching into the top multilayer. It may have a drawback for the geometric shadow effect due to the depth of the etched layers of about 1 nm.
Purpose Requirements of Att-PSM in EUV Att-PSM in EUV: (R ) EUV <.1 ΔΦ EUV =18 o ARC and high contrast in DUV: DUV Contrast > 9 % ( R DUV < 5% ) Small height difference for less shadow effect 1. The mask structure should not only perform 18 phase shift with low reflectance ratio (R <.1) at 13.5nm wavelength, but also have high inspection contrast (> 9%) at 57nm wavelength.. The total thickness of the absorber stacks should be retained thin enough to meet the stack height requirement to prevent the geometric shadow effect. 3. We design a hybrid-type EUV Att-PSM based on a Fabry Perot interferometer. The reflectance ratio between the absorber stack and multilayer mirror should be tuned by choosing different insertion position and thickness of spacer.
Design of Hybrid-type Att-PSM R 1, Φ 1 R, Φ Attenuator Additive type [Mo/] 4 pairs R, Φ R 1, Φ 1 Subtractive type Conditions for Att-PSM - ΔΦ=Φ 1 -Φ =18 - R <.1 It have a drawback for the geometric shadow effect due to high height difference. Ru /[Mo/] 4 Air layer Spacer () R 1, Φ 1 Spacer stack Attenuator R, Φ Absorber stack [Mo/] 4 Hybrid-type Att-PSM (HPSM) The spacer stack should have high reflectance (R 1 ) as well as should be able to control the reflection phase Φ 1 in EUV. The insertion position and thickness of the spacer in the Fabry-Perot filter enable to vary R 1 and Φ 1. Conditions for hybrid-type Att-PSM - ΔΦ=Φ 1 -Φ =18, R <.1 at EUV range - Minimum height difference (for minimizing shadow effect)
mulation condition * Optical constants 13.5 nm 57 nm material n k n k Mo.9388.643 1.71578 3.74669.999.183 1.673 3.6413 O.97364.193 1.541. Ru.88635.179 1.8359.8493.9188.678.35376.18949 The EUV exposure conditions in the simulation are 13.5 nm wavelength, s-polarization, and 6 degree incidence angle. (R 1 and R : Reflectance of the spacer stack and the absorber stack, Φ 1 and Φ : reflection phase of the spacer stack and the absorber stack) The thicknesses of Mo and in the multilayer are.8 nm and 4.15 nm, respectively. The thickness of Ru capping layer is nm. R 1, Φ 1 R, Φ R 1, Φ 1 R, Φ Ru ( nm) Air layer Ru ( nm) Ru Air layer /[Mo/] 4 [Mo/] 4 /[Mo/] 4 Spacer () [Mo/] 4 Spacer stack Absorber stack Additive-type type Att-PSM Spacer stack Absorber stack Hybrid-type Att-PSM (HPSM)
Indium tin oxide () layer at EUV wavelength Determination of optical constants of (Indium tin oxide) at 13.5 nm : In Sn. O 3.4 [Density : 7.14 g/cm 3 (film)] N(ω ) = 1 δ(ω) + i β(ω) r λ δ( ω ) = [ π r λ β( ω ) = [ π N ρ A m In N ρ A m In In In f 1 In f In r λ ( ω ) AF ( In ) + π r λ ( ω ) AF ( In) + π N ρ A m Sn N ρ A m Sn Sn Sn f 1 Sn f Sn r λ ( ω ) AF ( Sn ) + π r λ ( ω ) AF ( Sn ) + π N ρ A m O N ρ A m O O O f f 1 O O ( ω ) AF ( O )] / total ( ω ) AF ( O )] / total AF AF Ν (ω): Optical f (ω): Real scattering form factor f constant (ω): Imaginary scattering form factor r :Classical electron 1 radius [.8 1 3 ρ :Mass density of film [ g / cm ] 15 m ] Calculation of thickness N A m : ω : Frequency λ : 3 : Avogadro' s number [6. 1 atoms / mol ] Atomic mass [ AF : Atomic g] of light [ Hz ] Wavelength of light [ nm ] Fraction (for determination reflectance R and reflection phase Φ at 13.5 nm) Re [Y] 3 1-1 - -3-4 (19.68 nm) (73.88 nm) [Mo/] 4 pairs 1 3 4 5 6 Im [Y] 57 nm wavelength [Mo/] 4 pairs (19.68 nm) (73.88 nm) R δ=.781, β=.678 n=.9188, k=.678 = 1 1 + Y Y Y : admittance Y air : (1.,.) H. A. Macleod, Thin-Film Optical Filters, 3rd ed. (IoP, Bristol, UK, 1). Reflectance : R DUV (%) 7 6 5 4 3 1 57 nm w avelength R eflectance (% ) 1 3 4 5 6 7 8 9 1 Thickness of absorber (nm )
Design of Additive Att-PSM with [] absorber Reflectivity : R E E 1..8.6.4. Absorber : (71.8 nm) Reflectivity Phase shift 36 7 18 9 Phase shift : ΔΦ=Φ 1Ε -Φ Ε (degree) 1. At 13.5 nm wavelength ΔΦ = 179.93 (R ) EUV =.3. DUV Contrast = 96.38 % @ 57 nm 3. Height difference min : 71.8 nm. 1 3 4 5 6 7 8 9 1 Thickness of absorber (nm) Height difference (or thickness of air layer) 71.8 nm [Mo/] 4 Contrast (%) 1 9 8 7 6 5 4 3 1 (71.8 nm) Additive-type Att-PSM 1 3 4 5 6 7 8 9 3 Wavelength (nm)
Basic principle for design of hybrid type Att-PSM (HPSM) - for Extreme Ultraviolet Lithography R 1, Φ 1 R, Φ Fabry-Perot interferometer Φ R Ru Air layer Attenuator () Φ R R Front /[Mo/] 4 Spacer () [Mo/] 4 Phase- control for EUVL Spacer = π φ n d S λ S Spacer stack Absorber stack Hybrid type Att-PSM R Rear Pochie Yeh, Optical Waves in Layered Media, (John Wiley & Sons, 1988). The phase-control of the spacer stack can be explained by a simplified Fabry-Perot filter. (R Front : reflectance for the front mirrors, R Rear : reflectance for the rear mirrors, n S : refractive index of the spacer, d S : thickness of the spacer) The changes reflection phase Φ R of the filter varies as the optical phase thickness (φ) of the spacer increases for the different reflectance values of R Front and R Rear
Reflectance and reflectance phase Φ 1 of spacer stack - for Extreme Ultraviolet Lithography 1 36 Reflectance : R 1 (%) 9 8 7 6 5 4 3 1 R 1 7% Position of spacer Top Middle Bottom Reflection Phase : Φ 1 (degree) 315 7 5 18 135 9 45 Position of spacer Top Middle Bottom 1 3 4 5 6 7 8 9 1 11 1 13 14 15 16 17 18 19 1 3 4 5 6 7 8 9 111113141516171819 Thickness of spacer : (nm) Thickness of spacer : (nm) R 1 exhibits high reflectance (1) at the multiple of λ/ optical thickness of the spacer regardless of the insertion position, () when the spacer is coated as the first layer on the substrate before the Mo/ multilayer stack is coated or (3) as the last layer after the Mo/ multilayer stack. The sum of the reflection phase Φ R of the filter and the reflection phase Φ air of the air layer is the reflection phase Φ 1 of the spacer stack. (Φ 1 =Φ R +Φ air ) Phase shift is defined as the difference between the reflection phase Φ 1 and Φ. (ΔΦ=Φ 1 -Φ )
Bottom-type type HPSM Reflectivity : R 1..8.6.4. Absorber : (73.88 nm) Reflectivity Phase shift. 5 1 15 5 3 35 4 45 5 55 6 65 Thickness of spacer (nm) [Mo/] 4 36 7 18 Height difference (or thickness of air layer) 65.1 nm The thickness of spacer(.5nm) 4.11 nm 9 Phase shift : ΔΦ=Φ 1 -Φ (degree) 1. At 13.5 nm wavelength Contrast (%) The thickness of spacer(63.6nm) 1 9 8 7 6 5 4 3 1 ΔΦ = 179.98 (R ) EUV =.5. DUV Contrast = 97.13% @ 57 nm 3. Height difference min : 4.11 nm (73.88 nm) Hybrid-type Att-PSM (Bottom) 1 3 4 5 6 7 8 9 3 Wavelength (nm)
Middle-type HPSM Reflectivity : R 1..8.6.4. Absorber : (71.35 nm) Reflectivity Phase shift. 5 1 15 5 3 35 4 45 5 55 6 65 Thickness of spacer (nm) [Mo/] 4 36 7 18 Height difference (or thickness of air layer) 64.99 nm The thickness of spacer(.1nm) 3.91 nm 9 Phase shift : ΔΦ=Φ 1 -Φ (degree) 1. At 13.5 nm wavelength Contrast (%) The thickness of spacer(61.9nm) 1 9 8 7 6 5 4 3 1 ΔΦ = 179.79 (R ) EUV =.4. DUV Contrast = 93. % @ 57 nm 3. Height difference min : 3.91 nm (71.35 nm) Hybrid-type Att-PSM (Middle) 1 3 4 5 6 7 8 9 3 Wavelength (nm)
Top-type HPSM Reflectivity : R 1..8.6.4. Absorber : (71.17 nm) Reflectivity Phase shift R 1E > 7%. 5 1 15 5 3 35 4 45 5 55 6 65 Thickness of spacer (nm) 36 7 18 Height difference (or thickness of air layer) 64.91 nm 51.51 nm [Mo/] 4 The thickness of spacer(.1nm) 9 Phase shift : ΔΦ=Φ 1 -Φ (degree) 1. At 13.5 nm wavelength Contrast (%) The thickness of spacer(13.51nm) 1 9 8 7 6 5 4 3 1 ΔΦ = 179.45 (R ) EUV =.3. DUV Contrast = 9.41 % @ 57 nm 3. Height difference min : 51.51 nm (71.17 nm) Hybrid-type Att-PSM (Top) 1 3 4 5 6 7 8 9 3 Wavelength (nm)
Summary of various designs Types of Att-PSM Absorber ΔΦ=Φ 1 -Φ (at 13.5nm) R (at 13.5nm) DUV Contrast (@ 57nm) Height difference Degree of difficulty in fabrication Additive (71.8 nm) 179.93.3 96.38 % 71.8 nm easy Bottom spacer (73.88 nm) 179.98.5 97.13 % 4.11 nm difficult Hybrid Middle spacer (71.35 nm) 179.79.4 93. % 3.91 nm difficult Top spacer (71.17 nm) 179.45.3 9.41 % 51.51 nm easy Att-PSMs perform 18 phase shift with low reflectance ratio (R E E <.1) at EUV wavelength, as well as have high inspection contrast (> 9%) at deep ultraviolet (DUV) wavelength. The structures with absorber have lower height difference than the existing structure with TaN absorber, the thickness of which is greater than 8 nm. The bottom- and the middle-type HPSM have small height difference. Additive-type Att-PSM and top HPSM may have advantages in fabrication.
Conclusion We have designed two types attenuated phase shift mask with absorber. The hybrid-type design work has been processed in the way that 18 phase shift and attenuated reflectance ratio for EUV are matched by the principle of the Fabry-Perot interferometer. The results show that the mask structures not only perform 18 phase shift with low reflectance ratio (R <.1) at EUV wavelength, but also have high inspection contrast (> 9%) at deep ultraviolet (DUV) wavelength. Height difference and degree of difficulty in fabrication for various Att-PSMs Att-PSMs Type Hybrid-type height difference (Shadow effect) fabrication bottom-spacer Small difficult middle-spacer Small difficult top-spacer medium easy Additive-type large easy The top HPSM with absorber has advantages in shadow effect and fabrication. Further researches on various spacer materials are needed.