Preparation and Performance of Ceria-based Slurry for Ultra-precision Polishing of Calcium Fluoride

doi:10.15541/jim20260041

Abstract

Abstract:

The continuous downscaling of integrated circuit nodes demands atomic-level smoothness for calcium fluoride (CaF₂) optical materials used in lithography systems. To overcome the limitations of conventional alkaline silica polishing slurries, namely low material removal rate (MRR) and corrosion issues, this study developed a high-efficiency, stable, and eco-friendly near-neutral ceria (CeO₂) slurry for chemical mechanical polishing (CMP) of CaF₂. Monodispersed spherical CeO₂ abrasives with three particle sizes were synthesized via urea precipitation, then the polishing slurries were prepared using sodium polyacrylate (PAAS) as dispersant and primary alcohol ethoxylate (AEO-9) as surfactant. Effects of calcination temperature, abrasive size, and additive content on polishing performance were systematically investigated. Optimal performance was achieved with CeO₂ calcined at 500 ℃ in the presence of 0.2% PAAS and 0.2% AEO-9 (in mass). Specifically, the slurry containing 140 nm abrasives delivered a MRR of 109 nm/min and a surface roughness (R_a) of 0.12 nm, while the slurry with 110 nm abrasives produced an ultra-smooth surface with an R_a as low as 0.09 nm, outperforming the traditional alkaline silica slurry. Laser-induced damage threshold tests revealed that CaF₂ polished with the optimized slurry reached 6.4 J/cm², which is approximately 31% higher than the 4.9 J/cm² obtained with the silica slurry. Electron paramagnetic resonance (EPR) and Fourier-transform infrared (FT-IR) spectroscopic analyses confirmed that Ce³⁺ species on the CeO₂ surface are critical for enhancing chemical reactivity. The contact angle measurements showed that PAAS and AEO-9 notably reduced contact angle and improved wettability. This study provides an efficient, eco-friendly strategy for ultraprecision machining of CaF₂.

Key words: calcium fluoride, chemical mechanical polishing, spherical ceria, slurry, ultra-precision machining

CLC Number:

TN305

JIA Wenqing, ZHANG Zhenghao, WANG Jun, WANG Mengxia, ZHANG Chuanqi, JIANG Dapeng, ZHANG Ling, KOU Huamin, WANG Wenzhong. Preparation and Performance of Ceria-based Slurry for Ultra-precision Polishing of Calcium Fluoride[J]. Journal of Inorganic Materials, 2026, 41(10): 1425-1434.

Figures/Tables 19

Table 1 Polishing parameters of CaF2

Parameter	Condition
Pressure/kPa	4.0
Polishing controller speed/(r·min^-1)	15
Polishing pad speed/(r·min^-1)	60
Slurry flow rate/(mL·min^-1)	8.0
Polishing time/min	20

Table 2 Composition of slurry for contact angle testing

Slurry	Composition (mass fraction)
S1	2% CeO₂
S2	2% CeO₂+ 0.2% PAAS
S3	2% CeO₂+ 0.2% PAAS + 0.1% AEO-9
S4	2% CeO₂+ 0.2% PAAS + 0.2% AEO-9
S5	2% CeO₂+ 0.2% PAAS + 0.5% AEO-9
S6	2% CeO₂+ 0.2% PAAS + 1% AEO-9

Fig. 1 SEM images and particle size distributions of ceria (a, d) CeO2-1; (b, e) CeO2-2; (c, f) CeO2-3

Fig. 2 XRD patterns of ceria calcined at different temperatures

Fig. 3 Zeta potential variation with (a) PAAS and (b) AEO-9 contents (with addition of 0.5% PAAS) of the polishing slurry

Fig. 4 Variation of MRR with slurry composition using CeO2-2 calcined at different temperatures as abrasive (a) Variation with PAAS content; (b) Variation with AEO-9 content with addition of 0.2% PAAS

Fig. 5 Metallographic microscope images of polished calcium fluoride surfaces (a) After coarse polishing; (b) CeO2; (c) CeO2 + 0.2% PAAS; (d) CeO2 + 0.2% PAAS + 0.1% AEO-9; (e) CeO2 + 0.2% PAAS + 0.2% AEO-9; (f) CeO2 + 0.2% PAAS + 0.5% AEO-9

Fig. 6 3D morphologies of CaF2 surfaces polished with different slurries (a) CeO2-1 + 0.2% PAAS + 0.2% AEO-9; (b) CeO2-2 + 0.2% PAAS + 0.2% AEO-9; (c) SiO2 + 0.2% PAAS + 0.2% AEO-9, pH 11; (d) FUJIMI Compol-80 slurry. Colorful figures are available on website

Fig. 7 Laser damage curves of calcium fluoride after double-sided polishing with (a) self-made alkaline SiO2 polishing slurry and (b) CeO2-2 polishing slurry

Fig. 8 EPR spectra of CeO2 treated under different conditions and their magnified views (a, b) CeO2-2 calcined at different temperatures; (c, d) CeO2 with different particle sizes calcined at 500 ℃

Fig. 9 FT-IR spectra of CeO2-2 (calcined at different temperatures) before and after polishing

Fig. 10 Contact angles of different slurries on CaF2 surface

Fig. S1 Stability of polishing slurries with different PAAS contents over storage time (PAAS addition from left to right: 0, 0.1%, 0.2%, 0.5%, 1%)

Table S1 Results of orthogonal experiments with CeO2 particle size, PAAS content, and AEO-9 content as variables

Number	Size of CeO₂/nm	PAAS/% (in mass)	AEO-9/% (in mass)	MRR/(nm·min^-1)	Surface quality
1	110	0	0	71	1
2	110	0.1	0.1	96	3
3	110	0.2	0.2	99	5
4	110	0.5	0.5	79	2
5	110	1	1	70	2
6	110	0	0.1	74	2
7	110	0.1	0.2	100	4
8	110	0.2	0.5	89	2
9	110	0.5	1	66	2
10	110	1	0	120	3
11	140	0	0.2	67	1
12	140	0.1	0.5	80	2
13	140	0.2	1	67	2
14	140	0.5	0	129	3
15	140	1	0.1	120	3
16	140	0	0.5	56	1
17	140	0.1	1	60	1
18	140	0.2	0	120	3
19	140	0.5	0.1	122	4
20	140	1	0.2	115	5
21	210	0	1	38	1
22	210	0.1	0	95	2
23	210	0.2	0.1	109	3
24	210	0	0.2	97	5
25	210	1	0.5	89	2

Fig. S2 MRR of ceria with different particle sizes calcined at 500 ℃

Fig. S3 Polished surfaces of ceria with different particle sizes and calcination temperatures (a) CeO2-1, (b) CeO2-3 calcined at 500 ℃; CeO2-2 calcined at (c) 400 ℃, (d) 600 ℃

Fig. S4 (a) SEM image and (b) particle size distribution of the lab-synthesized silica

Fig. S5 MRR of different polishing slurries

Table S2 Comparison of the polishing parameters and performance of calcium fluoride between existing studies and this work

Polished object	Polishing method	Polishing slurry composition	Polishing parameters	Optimal polishing result	Reference
CaF₂	CMP	SiO₂, KOH, pH 9.5-10	Pressure: 44 kPa, Speed: 120 r/min	R_q=0.37 nm	[S1]
CaF₂	CMP	50 nm SiO₂, pH 11		R_q=0.18 nm	[S2]
CaF₂	CMP	SiO₂, alkaline conditions		R_a=0.68 nm	[S3]
CaF₂	Magnetorheological polishing (MRP)	6% diamond abrasive	Speed: 500 r/min, Excitation Gap:10 mm	R_a=0.03 μm	[S4]
CaF₂	MRP + Ion beam polishing (IBP)	MRP: CeO₂+ surfactant + Fe(OH)₃; IBP: Ar⁺ beam	MRP speed: 550 r/min; IBP: 400 eV	MRR: MRP=1.33 μm/min, IBP=3 nm/min, R_a=1.44 nm	[S5]
CaF₂	Single point diamond turning (SPDT)		Speed: 3000 r/min	R_a=2.84 nm	[S6]
CaF₂	Float polishing (FP)	3% 7 nm SiO₂	Speed: 78 r/min	R_q=0.08 nm	[S7]
CaF₂	CMP	2% CeO₂+ 0.2% PAAS + 0.2% AEO-9	Pressure: 7.8 kPa; Speed: 60 r/min	MRR=99 nm/min, R_a=0.09 nm	This work

References 33

[1]	WAGNER C, HARNED N. Lithography gets extreme. Nature Photonics, 2010, 4(1): 24. DOI
[2]	SANDERS D P. Advances in patterning materials for 193 nm immersion lithography. Chemical Reviews, 2010, 110(1): 321. DOI PMID
[3]	LI Y, LI Y L, LIU X H, et al. A study on cross-level interconnection of metal layers under 193 immersion lithography conditions. 2024 Conference of Science and Technology for Integrated Circuits (CSTIC), Shanghai, 2024: 1.
[4]	NAGAI T, NAKAGAWA H, NARUOKA T, et al. Novel high sensitivity EUV photoresist for sub-7 nm node. Journal of Photopolymer Science and Technology, 2016, 29(3): 475. DOI URL
[5]	LI X, HU C, LIU Q, et al. Fluoride transparent ceramics for solid-state lasers: a review. Journal of Advanced Ceramics, 2024, 13(12): 1891. DOI URL
[6]	MOUHOVSKI J T. Control of oxygen contamination during the growth of optical calcium fluoride and calcium strontium fluoride crystals. Progress in Crystal Growth and Characterization of Materials, 2007, 53(2): 79. DOI URL
[7]	GEORGE S A, NAULLEAU P P, MOCHI I, et al. Extreme ultraviolet mask substrate surface roughness effects on lithographic patterning. Journal of Vacuum Science & Technology B, 2010, 28(6): C6E23.
[8]	SRINIVASAN R, DANDU P V, BABU S V. Shallow trench isolation chemical mechanical planarization: a review. ECS Journal of Solid State Science and Technology, 2015, 4(11): 5029. DOI
[9]	LUO Q F, WEN H L, LU J. Sol-gel polishing technology for extremely hard semiconductor substrates. The International Journal of Advanced Manufacturing Technology, 2022, 120(3): 1415. DOI
[10]	XU N, ZHANG G S, ZHANG J, et al. Research progress of CeO₂- based polishing slurry in SiC-CMP. Journal of Materials Science, 2025, 60(44): 21786. DOI
[11]	JOHANSEN H, KASTNER G. Surface quality and laser-damage behaviour of chemo-mechanically polished CaF₂ single crystals characterized by scanning electron microscopy. Journal of Materials Science, 1998, 33(15): 3839. DOI
[12]	GUO J, GONG J, SHI P F, et al. Study on the polishing mechanism of pH-dependent tribochemical removal in CMP of CaF₂ crystal. Tribology International, 2020, 150: 106370. DOI URL
[13]	YIN G J, LI S Y, XIE X H, et al. Ultra-precision process of CaF₂ single crystal. 7th International Symposium on Advanced Optical Manufacturing and Testing Technologies: Advanced Optical Manufacturing Technologies, Harbin, 2014, 9281: 92811I.
[14]	SAR S, SAMUELSSON C, ENGSTRÖM F, et al. Experimental study on the dissolution behavior of calcium fluoride. Metals, 2020, 10(8): 988. DOI URL
[15]	JIA T Q, WANG J, PENG L H, et al. Angle-resolved ellipsometric analysis of polishing-induced subsurface damages in calcium fluoride crystals for ultra-precision manufacturing. Applied Surface Science, 2025, 696: 162997. DOI URL
[16]	WANG Y X, SHI Y H, CHENG J B, et al. Recent advances in CeO₂ based abrasives for chemical mechanical polishing. Physical Chemistry Chemical Physics, 2025, 27(25): 13213. DOI URL
[17]	MA J H, XU N, CHENG J, et al. A review on the development of ceria for chemical mechanical polishing. Powder Technology, 2024, 444: 119989. DOI URL
[18]	XU G H, ZHANG Z Y, MENG F N, et al. Atomic-scale surface of fused silica induced by chemical mechanical polishing with controlled size spherical ceria abrasives. Journal of Manufacturing Processes, 2023, 85: 783. DOI URL
[19]	LI Y X, WANG X L, DING L M, et al. Changing the calcination temperature to tune the microstructure and polishing properties of ceria octahedrons. RSC Advances, 2022, 12(26): 16554. DOI PMID
[20]	ZHAO X Y, HAN X Y, WANG F Y, et al. Improvement of polishing performance of SiO₂/Si₃N₄ by surfactants in CeO₂ based slurry. Applied Surface Science, 2025, 710: 163978. DOI URL
[21]	OH M H, SINGH R K, GUPTA S, et al. Polishing behaviors of single crystalline ceria abrasives on silicon dioxide and silicon nitride CMP. Microelectronic Engineering, 2010, 87(12): 2633. DOI URL
[22]	VENKATARONAPPA A, BANKAITIS J, SEO J. Enhancing dispersion stability and recyclability of ceria slurry with polyacrylic acid for improved glass polishing performance. Journal of Industrial and Engineering Chemistry, 2024, 138: 623. DOI URL
[23]	XU N, LIN Y, LUO Y X, et al. Effect of surfactants with different ionizing properties on dispersion stability and PCMP properties of CeO₂ nanoparticle polishing slurry. Ceramics International, 2025, 51(1): 856. DOI URL
[24]	CLAYTON K N, SALAMEH J W, WERELEY S T, et al. Physical characterization of nanoparticle size and surface modification using particle scattering diffusometry. Biomicrofluidics, 2016, 10(5): 054107. DOI URL
[25]	LEE J, KIM E, BAE C, et al. Improvement of oxide chemical mechanical polishing performance by increasing Ce³⁺/Ce⁴⁺ ratio in ceria slurry via hydrogen reduction. Materials Science in Semiconductor Processing, 2023, 159: 107349. DOI URL
[26]	DOI H, SUZUKI M, KINUTA K. Effects of Ce³⁺ on removal rate of ceria slurries in chemical mechanical polishing for SiO₂. Proceedings of International Conference on Planarization/CMP Technology 2014, Kobe, 2015: 194.
[27]	CHOI J, SHIN C, YANG J, et al. Effect of ceria abrasive synthesized by supercritical hydrothermal method for chemical mechanical planarization. ECS Journal of Solid State Science and Technology, 2019, 8(5): 3128.
[28]	LI J Q, HU H X, QIAO G B, et al. Damage-free and ultra-smooth chemical mechanical polishing of calcium fluoride crystal surfaces. Precision Engineering, 2025, 94: 725. DOI URL
[29]	ESMAILPOUR A A, MORADI S, YUN J, et al. Promoting surface oxygen vacancies on ceria via light pretreatment to enhance catalytic ozonation. Catalysis Science & Technology, 2019, 9(21): 5979.
[30]	JAVED K, BIAN X N, REN Y F, et al. Enriched surface oxygen vacancies of CeO₂ nanosheets: surfactant free synthesis at room temperature and its enhanced anti-bacterial property. ChemistrySelect, 2024, 9(21): e202401142. DOI URL
[31]	ZHANG Y, ZHAO S N, FENG J, et al. Unraveling the physical chemistry and materials science of CeO₂-based nanostructures. Chem, 2021, 7(8): 2022. DOI URL
[32]	ZHANG R S, LIU X Y, LIANG H, et al. Probing the role of surface activated oxygen species of CeO₂ nanocatalyst during the redox cycle in CO oxidation. RSC Advances, 2022, 12(40): 26238. DOI URL
[33]	DE ARRIETA I G, DEL CAMPO L, DE SOUSA MENESES D. Infrared spectroscopy of CeO₂ nanoparticles using Bergman’s spectral representation: effects of phonon confinement and lattice strain. Physical Chemistry Chemical Physics, 2021, 23(23): 13095. DOI URL