Effects of Thin Film Depositions on the EUV mask Flatness Kyoung-Yoon Bang, Jinback Back, Hwan-Seok Seo, Dongwan Kim, DongHoon Chung, SeongSue Kim, Sang-Gyun Woo, and HanKu Cho Photomask Team Semiconductor R & D Center Samsung Electronics Co. Ltd 1
Introduction Image Placement (IP) errors due to mask non-flatness have become a serious issue for EUV lithography. Mask non-flatness is induced during complex mask fabrication. Mask non-flatness combined with non-telecentric reticle plane causes errors in focus and X shift at wafer plane. 2
Introduction Flatness relationships between mask blanks and thin films should be understood for EUV mask flatness control strategy. EUV Capping Mo/Si LTEM Conductive layer ARC Absorber Buffer EUV mask consists of many thin film layers: Mo/Si multilayers, capping layer, buffer layer, absorber, ARC, and backside conductive layer. Since they can change mask flatness during film deposition, relationship between material thickness and mask flatness should be investigated. In addition, trace of flatness variation in each deposition step can give a useful information for EUV mask flatness control strategy. 3
Experimental instruments Ion-beam sputtering (film depositions) IBD has been used to deposit films in EUVL masks since it shows lower defect density and lower film stress than conventional PVD. Depositions of Si, Mo, Ru (capping layer), TaN (absorber) films are available. Interferometer (mask flatness measurements) Near normal incidence interferometry ( Zn) 2 RMS = n TIR = Max Min Spectroscopic Ellipsometry (thickness & refractive index measurements) Δ = δ p δ s ψ = tan 1 r r p s 4
Experimental background [ Flatness from interferometer ] After deposition Before deposition Flatness difference by deposition = [ Thin film thickness from Spectroscopic ellipsometer ] Reflection from a single layer film Incident Light Air Film Substrate Reflected Light 1 2 3 n2, k2 t1, n1, k1 n0, k0 R R p s p r 12 + r = p 1+ r 12r s r 12 + r = s 1+ r 12r p 23 p 23 s 23 s 23 d β = 2π n~2 cos φ2 λ d = film thickness exp( j2β ) exp( j2β ) exp( j2β ) exp( j2β ) Thin film thickness at 11x11 point S10 S8 S6 S4 S2 1 2 3 4 5 6 7 8 9 10 11 S1 9.4-9.6 9.2-9.4 9-9.2 8.8-9 8.6-8.8 8.4-8.6 5
Experiments Preparation of individual EUV mask film (Si, Mo, Ru, TaN) using IBD. Thin film (50 nm) / thermal SiO 2 (100 nm) / c-si wafer Thin film (10 nm) / Qz blank Measurements of thickness, uniformity, and refractive indexes by Spectroscopic ellipsometry. Characterization of mask flatness using interferometer. Investigation of relationship between mask flatness and film thickness of EUV mask materials. Preparation of EUVL mask blank film stacks using IBD. Trace of mask flatness variations in each deposition step and comparing to individual film results. 6
Results and Discussion Refractive index by SE: Thin film (50 nm) / thermal SiO 2 (100 nm) / wafer Refractive index (n, k) Refractive index (n, k) 5 4 3 2 1 0 5 4 3 2 a-si 200 400 600 800 1000 TaN n k Measured Table n k Measured Table 200 400 600 800 1000 Wavelength (nm) Refractive index (n, k) Refractive index (n, k) 7 5 4 3 2 1 6 5 4 3 2 1 200 400 600 800 1000 200 400 600 800 1000 Wavelength (nm) To analyze thin film thickness, R.I. of films are calculated. Fitted R.I. values are similar to values in n-k table. Mo Ru n k Measured Table n k Measured
Uniformity of thin film thickness S10 a-si S10 Mo S8 S8 Low S6 S4 10.8-11 10.6-10.8 10.4-10.6 Low S6 S4 9.4-9.6 9.2-9.4 9-9.2 8.8-9 8.6-8.8 8.4-8.6 S2 S2 1 2 3 4 5 6 7 8 9 10 11 S1 1 2 3 4 5 6 7 8 9 10 11 S1 Thickness Range 3Sigma Average S10 S8 TaN S10 S8 Ru a-si 0.36 nm 0.184 nm 10.774 nm Mo 0.66 nm 0.479 nm 9.006 nm High S6 S4 10.2-10.3 10.1-10.2 10-10.1 9.9-10 9.8-9.9 Low S6 S4 11.5-12 11-11.5 10.5-11 Ru 0.72 nm 0.573 nm 11.525 nm TaN 0.28 nm 0.213 nm 10.148 nm S2 S2 1 2 3 4 5 6 7 8 9 10 11 S1 1 2 3 4 5 6 7 8 9 10 11 S1 Thickness uniformities are in range of ~1 nm with circular distributions. 8
Comparison thickness uniformity with flatness. [Thickness] Front [Flatness] Back a-si 11 10.9 10.8 10.7 10.6 10.5 10.4 1 3 5 7 10.9-11 10.8-10.9 10.7-10.8 10.6-10.7 10.5-10.6 10.4-10.5 High Low 9 11 S1 Range 0.36nm Mo 9.5 9.4 9.3 9.2 9.1 9 8.9 8.8 8.7 8.6 8.5 8.4 1 2 3 4 5 6 7 8 9 10 S1 11 9.4-9.5 9.3-9.4 9.2-9.3 9.1-9.2 9-9.1 8.9-9 8.8-8.9 8.7-8.8 8.6-8.7 8.5-8.6 8.4-8.5 Range 0.661nm High Low Flatness Front RMS Front TIR Back RMS Back TIR a-si 20.1 nm 104.9 nm 43.0 nm 240.2 nm Mo 14.2 nm 116.6 nm 138.9 nm 668.1 nm 9
Comparison thickness uniformity with flatness. Ru [Thickness] Front [Flatness] Back 12 11.8 11.6 11.4 11.2 11 10.8 10.6 1 2 3 4 5 6 7 8 9 10 S10 S8 S6 S4 S2 11 S1 11.8-12 11.6-11.8 11.4-11.6 11.2-11.4 11-11.2 10.8-11 10.6-10.8 Range 0.729 nm High High Low Low TaN 10.25 10.2 10.15 10.1 10.05 10 9.95 9.9 9.85 9.8 1 2 3 4 5 6 7 8 9 10 11 10.2-10.25 10.15-10.2 10.1-10.15 10.05-10.1 10-10.05 9.95-10 9.9-9.95 9.85-9.9 9.8-9.85 High High Low Low S1 Range 0.288 nm Flatness Front RMS Front TIR Back RMS Back TIR Ru 60.1 nm 337.7 nm 30.8 nm 414.1 nm TaN 18.8 nm 101.6 nm 34.8 nm 184.8 nm Due to the compressive stress of deposited films, center flatness is higher than edge value on front surface. 10
Flatness vs. thickness ratio Front surface: Ru 29.31 > Mo 12.95 > TaN 10.01 > Si 9.74 Back surface: Mo 74.2 > Ru 35.94 > Si 22.3 > TaN 18.22 Back flatness is higher than front flatness in all films. F-T ratios (front surface) in mask correspond to film stresses by measuring curvature changes in wafer using FLX system. Changes in mask flatness in each deposition step are mostly coming from compressive stress induced during film deposition. Material Average Thickness Front surface Back surface Flatness (P-V) F-T ratio Flatness (P-V) F-T ratio Film stress (Compressive) Si 10.77 nm 104.9 nm 9.74 240.2 nm 22.3-1115 MPa Mo 9.0 nm 116.6 nm 12.95 668.1 nm 74.2-1563 MPa Ru 11.52 nm 337.7 nm 29.31 414.1 nm 35.94-3015 MPa TaN 10.14 nm 101.6 nm 10.01 184.8 nm 18.22-1829 MPa ML 280 nm 1142.4 nm 4.08 1184.7 nm 4.23-502 MPa 11
Flatness vs. thickness ratio after multilayer deposition F-T ratio (front side) 30 25 20 15 10 5 0 Si Mo Si F-T ratio 0 Si Mo Ru TaN Multilayer Substrate Intermixing layer 5 nm 3.0 2.0 1.5 1.0 0.5 Stress (GPa, compressive) Film stress F-T ratio values are in good agreements 2.5 with film stresses. Mo/Si multilayer has lower FT ratio than both Mo and Si films. ML front FT ratio 4.8 < Si 9.74, Mo 12.94 ML back FT ratio 4.23 < Si 22.3, Mo 74.2 SE analysis result 5 (thermal sio2_s) Coupled to #1 1.472 nm 4 poly_a_tl 4.072 nm 3 EMA (mo_l_nrd_test)/50% poly_a_tl2.514 nm 2 mo_l_nrd_test 2.441 nm 1 thermal sio2_s 100.000 nm 0 silicon_s 1 mm XTEM and SE analyses show that intermixing layer exists at the interface between Si and Mo. Lower F-T ratio in ML is primarily due to the stress relaxation by intermixing Si and Mo. 12
Trace of mask flatness during mask blank fabrication Absorber (TaN) Capping (Ru) Mo/Si (40 bilayers) Qz 80 nm 2.5 nm 280 nm 3 step flatness measurements (Qz substrate, after ML deposition, after absorber deposition). Due to the compressive stress of deposited films, convexness of front surface increases through mask blank film deposition steps. For the flatness control of EUV mask, flatness data of mask blanks should be measured and provided from suppliers. Qz After ML After absorber 3000 2500 Front surface Back surface Front P-V (nm) 2000 1500 1000 500 Qz ML Absorber Back 13
Conclusions 1. We applied spectroscopic ellipsometer to measure thickness and uniformity of thin films used in EUVL masks. Measured thickness uniformities are within 1 nm ranges. 2. Film depositions change front & back flatness. Due to the compressive stress of deposited films, center flatness is higher than edge value on front surface. 3. Flatness-vs.-thickness (F-T) ratios are in good agreements with film stresses. Changes in mask flatness are mostly coming from compressive stress induced during film deposition. 4. Mo/Si multilayer has lower F-T ratio than both Mo and Si films due to the stress relaxation by intermixing at the interface. 5. Due to the compressive stress of deposited films, convexness of front surface increases through mask blank film deposition steps. 14