Remarkable plasma-resistance performance by nanocrystalline Y2O3·MgO composite ceramics for semiconductor industry applications

Motivated by recent finding of crystallographic-orientation-dependent etching behavior of sintered ceramics, the plasma resistance of nanocrystalline Y2O3-MgO composite ceramics (YM) was evaluated for the first time. We report a remarkably high plasma etching resistance of nanostructure YM surpassing the plasma resistance of commercially used transparent Y2O3 and MgAl2O4 ceramics. The pore-free YM ceramic with grain sizes of several hundred nm was fabricated by hot press sintering, enabling theoretical maximum densification at low temperature. The insoluble two components effectively suppressed the grain growth by mutual pinning. The engineering implication of the developed YM nanocomposite imparts enhanced mechanical reliability, better cost effectiveness with excellent plasma resistance property over their counterparts in plasma using semiconductor applications.

referred reports, engineers in the semiconductor manufacturing field realized that proper control of the microstructure is the first and foremost requirement for high-performance structural ceramics 4 . In a recent study, etching rate was found to be dependent on the crystallographic orientation 25 . In other words, etching depth differs with different crystallographic plane for sintered polycrystalline ceramics. The authors are motivated by the above observation to determine the impact of grain anisotropic etching rate on ceramic surface roughness during plasma etching. Decrease in surface roughness eventually contributes in diminishing the particle generation problem during the plasma etching process 14 .
In this study, the plasma etching behavior of Y 2 O 3 ·MgO composite ceramics (YM) featuring nano-sized grains was for the first time evaluated. A comparative study with the pore-free transparent Y 2 O 3 and MgAl 2 O 4 ceramics bas been carried out. In our previous report 26 , various transparent ceramics free from light-scattering pores, which also act as the origin and/or as a magnifier of the etching reaction, were proven to exhibit plasma resistance exceeding that of single-crystal sapphire. The nanostructured YM research has been pursued to its excellent mid-wavelength IR transmittance and reliable mechanical properties for use as hypersonic infrared windows, domes and eye-safe laser [27][28][29][30][31] . The plasma resistance of YM composite ceramics has not yet been studied, although the monolithic forms of both Y 2 O 3 and MgO have demonstrated excellent plasma resistance 32 . In this work, the characteristics of the plasma etching behavior were mainly discussed in terms of the surface properties and microstructural aspects.

Experimental procedure
Commercially available oxide powders of Y 2 O 3 (99.9%, Rare Metallic, Japan), MgAl 2 O 4 (99.9%, Baikowski, France) and MgO (99.99%, Sukgyung AT, Korea) were adopted as raw materials for the fabrication of monolithic Y 2 O 3 , monolithic MgAl 2 O 4 , and the composite YM ceramics. The details of the entire fabrication process of Y 2 O 3 and MgAl 2 O 4 are available in the literature 33,34 . The process to fabricate YM composite is first 50:50 vol% of Y 2 O 3 and MgO powders were mixed via ball milling and dried to evaporate milling fluid. The powder mixtures are uni-axially pressed in steel mold and then the green compacts were sintered by hot-pressing at 1300℃ for 1 h under 40 MPa of pressure to reach nearly full theoretical density. After sintering, all samples were polished and masked on part of the top surface with polyimide tape to evaluate the etching properties. The thus-prepared nearly pore-free transparent ceramics were exposed to plasma using inductively coupled plasma. The details of the conditions used for plasma etching are listed in Table 1.
The density of the ceramic samples was determined by the Archimedes method. The theoretical density of the Y 2 O 3 , MgAl 2 O 4 and YM ceramics were 5.031, 3.58, and 4.30 g/cm 3 , respectively. The crystal structure of sintered specimens was identified by X-ray diffraction (XRD; D/Mas 2500, Rigaku, Japan). The etching depth was measured by a surface profiler (DXT-A, bruker, USA). The surface roughness and morphology were examined by the atomic force microscopy (XE-100, Park systems, Korea) and scanning electron microscopy (SU8230, Hitachi, Japan), respectively. The thermal diffusivity and heat capacity were measured by the laser flash analyzer (LFA 467, NETZSCH, Germany) and then the thermal conductivity (k) was calculated by following equation: where α, ρ and C p is the thermal diffusivity, density and heat capacity, respectively. The microhardness was measured by indentation method (HM, Mitutoyo, Japan) and the fracture toughness was determined by the Anstis Eq. 35 . Figure 1 shows the XRD patterns of sintered samples. The XRD peaks of YM composite have been indexed with the cubic Y2O3 (JCPDS 41-1105) and MgO (JCPDS 45-0946). No extra peak was noted in any sintered specimen, which confirms the phase purity of the samples.

Results and discussion
The measured mass loss and etching depth of the ceramic specimens as a function of the plasma etching time are plotted in Fig. 2. With an increase in the plasma exposure time, both mass loss and etching depth monotonically increased for all specimens. The inversion of the magnitude of the mass loss and etching depth between Y 2 O 3 and MgAl 2 O 4 can be understood by considering the density differences, which were 5.031 g/cm 3 for Y 2 O and 3.58 g/cm 3 for MgAl 2 O 4 . Unlike the result of a medium level of mass loss, MgAl 2 O 4 showed the deepest etching depth at all etching times. This tendency was noted to be similar to the earlier work 26 and herein, based on the mass loss and etching depth with Y 2 O 3 and MgAl 2 O 4 ceramics, particle contamination caused by the Y 2 O 3 ceramics is postulated to be more reducible than that by MgAl 2 O 4 ceramics, but further work is necessary for sufficient clarification. It is known that the etching depth of sintered polycrystalline Y 2 O 3 is roughly half that www.nature.com/scientificreports/ of single-crystal sapphire 20,32 , while a previous report by the authors using pore-free transparent Y 2 O 3 found best-ever results with a record low 20% etching depth compared to that of sapphire 26 . It is surprising that the developed YM showed an even lower etching depth compared to that of transparent-grade Y 2 O 3 , which will be discussed later with the results of microstructural analysis of the etched surface morphology. The images in the inset of Fig. 2 Figure 3 depicts AFM images of the etched surface profiling results. Topographical features after etching were clearly different in the three samples. The Y 2 O 3 and YM ceramics tended to be homogeneously eroded by plasma exposure, resulting in an extremely smooth surface with R a = 4.0 and 2.3 nm, respectively. On the other hand, the surface of MgAl 2 O 4 was deeply eroded and, relatively rough; i.e., R a = 6.6 nm. Generally, the surface roughness of conventional ceramics after plasma etching is reported to have a large range, from 100 nm to several microns, and these results have mainly been attributed to residual pores in the ceramics 4,8,20 . For transparent ceramics having nearly pore-free microstructures, smooth erosion (plateau-like surface morphology) was found to be possible without selective erosion caused by ion bombardment around the pores 4,25,26 . Given that the R a value of the as-polished surface was under 2 nm, the AFM images in Fig. 3(c)    www.nature.com/scientificreports/ out 21 . With an increase in the surface roughness, the condensed particle size simultaneously increased and the particles accumulated at the edge without being pumped out. The remaining large particles in the chamber may attach to the wafers, resulting in current shorts in integrated circuits 37 . From this point of view, YM ceramics yielding the most smoothly etched surface is expected to be used to create promising plasma-resistant materials, leading to reduced particulate contamination. Furthermore, in an atmosphere of fluorine-based plasma, the chemical reaction of Y 2 O 3 and MgO are YF 3 and MgF 2 , respectively. Their Gibbs free energy was estimated as -747 and -517 kJ/mol 38,39 . Therefore, a higher fluorine-plasma stability of MgO can explain a higher plasma resistance of YM composites than that of Y 2 O 3 ceramics. In order to elucidate the mechanism of the smooth surface roughness of the as-etched YM ceramics, the microstructural features of the Y 2 O 3 and the YM ceramics were compared, as shown in Fig. 4. The grain sizes of the Y 2 O 3 and YM were approximately 13 μm and 300 nm, respectively. Clearly, the etching depth of the coarser Y 2 O 3 grain appears to differ depending on each grain, as shown in Fig. 4(a). This phenomenon can be explained by the different plasma etching behaviors with respect to the grain orientation in polycrystalline Y 2 O 3 ceramics 25 . In contrast, the fine-grained microstructure of polycrystalline YM ceramics presented different etching behavior, as shown in Fig. 4(b). The schematics of the etched surface morphology in Fig. 4(c) and (d) specifically explain this. Based on the orientation-dependent erosion mechanism demonstrated by Kindelmann et al. 25 , each coarse Y 2 O 3 grain started to erode specifically on certain crystal planes, such as the (001) or (011) plane, resulting in higher surface roughness and deeper etching depths relative to the fine-grained YM ceramics. That is, the etching speed is always rate-controlled by the subsequently emerging slow-etch-rate oriented Y 2 O 3 grains upon the etching away of both the fast-etch-rate Y 2 O 3 grains and all MgO upper grains. Therefore, the greater plasma resistance of YM over Y 2 O 3 ceramics is primarily considered to originate from the effect of nano-sized grains, as smaller grain size is effective during exposure with a slow etch rate of Y 2 O 3 grains on the surface. As for the monolith Y 2 O 3 it is postulated that etch-rate might decrease with decreasing grain size by the same analogy. It should be emphasized that the theory of ceramic processing on making the composite between two insoluble components (Y 2 O 3 and MgO) was successfully utilized to fabricate pore-free nano-sized grain microstructure.
Moreover, given that physical and chemical etching simultaneously occur during plasma etching 40 , another possible reason for the high plasma-resistance of YM ceramics is the higher mechanical reliability compared   Table 2. The results of mechanical and thermal properties of specimens indicate that the inherently inferior physical properties of Y 2 O 3 ceramics can be overcame by adopting YM composite with the highest facture toughness and thermal conductivity. Kasashima et al. also reported a newly developed ceramic composite that showed enhanced plasma resistance given its improved mechanical properties and after an adjustment of the electrical resistivity 32 . The crucial microstructural aspects pertaining to the high strength of YM ceramics are the nano-sized Y 2 O 3 and MgO grains, combining both suppressed grain growth and reaches full densification.

Conclusion
A nanocrystalline YM composite have been mainly studied to focus on its excellent IR transmittance and mechanical properties so far. The results here shed light for the first time on the feasibility of applying YM composite to parts requiring plasma resistance in the semiconductor processing industry. Specifically, the YM composite exhibited superior plasma resistance, including both a lower etching rate and smoother as-etched surface roughness relative of the transparent Y 2 O 3 and Mg-spinel ceramic; originating from the effect of nanosized grains with very carefully controlled microstructural defects, i.e., residual pores. Therefore, this novel trial with the YM composite for use in plasma environments is believed to set an important milestone to mitigate the particulate contamination issue in the semiconductor industry.   www.nature.com/scientificreports/