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Korean J. Chem. Eng., 38(12), 2541-2548 (2021)DOI: 10.1007/s11814-021-0915-0

INVITED REVIEW PAPER

pISSN: 0256-1115eISSN: 1975-7220

INVITED REVIEW PAPER

†To whom correspondence should be addressed.E-mail: dhyoon@ynu.ac.krCopyright by The Korean Institute of Chemical Engineers.

Modification of a low-soda easy-sintered -Al2O3 powder for the applicationin semiconductor/display production equipment

Athiruban Sakthisabarimoorthi, Min Ji Kang, and Dang-Hyok Yoon†

School of Materials Science and Engineering, Yeungnam University, Gyeongsan 38541, Korea(Received 2 June 2021 • Revised 15 July 2021 • Accepted 1 August 2021)

AbstractSemiconductor and display production equipment requires highly plasma-resistant components, where -Al2O3 plays a vital role in shielding and insulating. In the present study, the physicochemical properties of a newdomestic -Al2O3 powder were compared with those of imported ones currently used in the Korean industry. The par-ticle size distribution revealed finer particles in the domestic powder that were generated during dry milling than theimported powder resulting in a broad size distribution and high surface area. An effort was made to eliminate the fineparticles by heat treatment because they increase the viscosity of a slurry for wet processing and induce a low greenand sintered density of the final products. In addition to the effects of heat treatment on the powder characteristics, thedispersion behavior of an aqueous Al2O3 slip was examined systematically upon the addition of each polymeric pro-cessing additive for filter pressing.Keywords:-Al2O3, Heat Treatment, Powder Properties, Dispersion, Zeta Potential

INTRODUCTION

Al2O3 is used widely in the fabrication of electronics and engi-neering devices owing to its excellent mechanical, thermal, and cor-rosion resistance properties [1,2]. On the other hand, it is used in-directly to produce semiconductors and displays by shielding theequipment from highly energetic plasma [3,4]. Preventing erosionof the processing chamber from plasma etching is becoming morecritical because higher power is needed to fabricate larger devices,where the particles generated by erosion cause a significant qualityissue of the final products [4-6].

Although Y2O3 is replacing Al2O3 because of its superior CF4

plasma resistance, Al2O3 is still used widely in the semiconductorand display industries, particularly for large components [7-9]. Never-theless, the demand for highly plasma-resistant Al2O3 is increasingcontinuously considering the cost of Y2O3 powder, which is morethan four times that of Al2O3. Indeed, many Al2O3 plates with 4,000×700×80mm3 or even larger dimensions are installed in display pro-duction chambers. Filter pressing using an aqueous slip is adoptedmainly to produce large components, while dry processing tech-niques using spray-dried granules are used to fabricate smaller ones.Although Korean companies produce a large portion of the world’ssemiconductor and display devices, the plasma-resistant componentsfor industrial production equipment rely mainly on imported Al2O3

powder. According to statistics, Korea imported 128,420 tons ofAl2O3 powder in 2018, which corresponds to approximately 130million U.S. dollars, and the amount is still increasing continuously[10]. Therefore, efforts are being made to replace foreign Al2O3 pow-der with domestic powder at the instigation of the government.

Industrial grade Al2O3 is produced mainly from bauxite by theBayer process, where NaOH is added to extract sodium aluminate(2NaAlO2) from ore at 150-200 oC, followed by cooling, impurityfiltering, and washing. The precipitated Al(OH)3 is finally convertedto micron-sized Al2O3 by calcination at approximately 1,200 oC. Themean particle size is decreased to 0.5m approximately by drymilling to confer sinterability for the powder, where a uniform sizedistribution is desirable for wet processing. At the same time, thepurity of powder is also critical for applications in a plasma envi-ronment. Therefore, sodium ions, mainly in the form of Na2O origi-nating from the NaOH used in the Bayer process, need to beeliminated or reduced to 0.05 wt%. Overall, Al2O3 powder with amean particle size about 0.5m and purity of 99.9% is neededfor plasma-resistant applications. This industrial grade Al2O3, called‘low-soda easy-sintered alumina’, ensures a high sintered density of3.93 g/cm3 (98.7% theoretical density) after sintering at approxi-mately 1,600 oC [10].

Although Al2O3 has many polymorphs, -Al2O3 is the main phaseused for industrial applications, which can be formed by the calci-nation of Al(OH)3 at 950-1,250 oC [11-13]. A dense microstructurewith a uniform grain size and high purity of the starting materialare the most significant factors ensuring plasma resistance. Hence,0.05-0.1 wt% MgO is added intentionally to ‘low-soda easy-sinteredalumina’, which acts as a sintering additive and a grain growth inhibi-tor [14]. Al2O3 with broad size distribution and irregular shape doesnot meet the stringent requirements for plasma-resistant applica-tions of the structural components formed by various techniques[15-20].

With this background, the properties of domestic Al2O3 powderwere compared with those of two imported powders that are cur-rently used in the fabrication of industrial plasma-resistant compo-nents for semiconductor and display production equipment. Basedon the powder characterization results, an attempt was made to

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reduce the high specific surface area associated with the domesticpowder by thermal treatment because of its significance for wet pro-cessing. Moreover, the zeta potential of Al2O3 particles in an aque-ous system was examined systematically upon the addition of variouspolymeric processing additives step-by-step, such as the dispersant,plasticizer, and binder to understand the dispersion mechanism ofthe particles in a slip for filter pressing.

MATERIALS AND METHOD

Domestic commercial Al2O3 (ALG-1SH, Daehan Ceramics,Korea) was used as the main powder for property improvement,while AES-11 (Sumitomo, Japan) and P162LSB (Alteo, France) wereused as references for comparison. The powder characteristics, in-cluding the particle size, morphology, phase, impurity content, sur-face area, ignition loss, tap density, and zeta potential were com-pared by various measurements. The detailed procedure for eachcharacterization is not explained in this paper to save space, whilethe same procedure was applied for the three types of Al2O3 powder.

Because ALG-1SH revealed the presence of finer particles witha higher surface area than AES-11 and P162LSB, the viscosity ofan aqueous Al2O3 slip was compared using a computer-controlledviscometer (LV DV-II+Pro, Brookfield, U.S.A.) with an SC4-31 spin-dle and a small sample adaptor. An ammonium salt of polycarboxyl-ate anionic dispersant (Cerasperse 5468-CF, San Nopco, Korea),acrylic emulsion binder (Carboset 7700, Lubrizol, U.S.A.), and poly-ethylene glycol plasticizer were used as the polymeric additives forslip preparation. The proprietary formulation of an industrial part-ner for filter pressing was used for slip preparation, followed byball milling for six hours using 5 mm alumina balls.

The zeta potential of the pure Al2O3 particles in water and afteradding each polymeric additives was examined as a function of pH

using an electroacoustic-type zeta potential analyzer (Zeta Probe,Colloidal Dynamics, U.S.A.) to understand the dispersion mecha-nism based on electrostatic repulsion. The pH of the suspensionswas adjusted using NH4OH and HCl.

Heat treatment was performed at 800-1,600 oC with an intervalof 100 oC for two hours in air to reduce the number of fine parti-cles in ALG-1SH. The evolution of particle size and surface areawas analyzed as a function of temperature. Meanwhile, green pel-lets with a diameter of 25 mm were formed by a uniaxial pressingunder 60MPa using as-received and heat-treated powder, followedby sintering at 1,600 oC for two hours. The green density was esti-mated by measuring the dimension and mass, while the sintereddensity was measured using the Archimedes method. Thermal etch-ing was performed at 1,500 oC for one hour for the polished sam-ple to examine the grain size and pore distribution.

RESULTS AND DISCUSSION

Three types of as-received powder showed a pure -Al2O3 phasebased on a Rietveld simulation using the XRD pattern. Although- and -Al2O3 were formed by the decomposition of Al(OH)3 at198-500 oC and 500-900 oC, respectively, all were eventually trans-formed to thermodynamically stable -Al2O3 at 950-1,250 oC [21,22], which is consistent with this observation.

Table 1 compares the characteristics of three types of Al2O3 pow-der obtained from different suppliers. No significant difference inimpurity content and resulting powder purity could be found, show-ing a purity of approximately 99.9% for all samples. Although theAES-11 contains more MgO than the others, it was added inten-tionally as a sintering additive and a grain growth inhibitor. On theother hand, all have slight differences in particle size, showing a meanvalue (Dm) of 0.51, 0.45, and 0.55m for ALG-1SH, AES-11, and

Table 1. Physical characteristics of the three types of Al2O3 powder used in this studyPowder/Properties ALG-1SH AES-11 P162LSB

Main impurities

Fe2O3 (wt%) 0.015 0.011 0.021SiO2 (wt%) 0.008 0.030 0.032Na2O (wt%) 0.05 0.04 0.04MgO* (wt%) 0.05 0.12 0.05

Particle size

D10 (m) 0.21 0.24 0.27D50 (m) 0.44 0.40 0.48D90 (m) 0.83 0.72 0.90Dm (m) 0.51 0.45 0.55

Surface area (m2/g) 6.59 6.28 5.43Ignition loss** (wt%) 0.48 0.56 0.44Tap density (g/cm3) 1.06 1.04 1.16Purity*** (%) 99.90 99.90 99.84Green density (g/cm3) 2.22 2.21 2.18Sintered density**** (g/cm3) 3.90 3.90 3.92

*MgO was intentionally added as a sintering additive and grain growth inhibitor.**Ignition loss was measured after heat treatment at 500 oC for 2 hours.***Purity is based on the supplier’s inspection data.****The density was measured using a pellet sintered at 1,600 oC for 2 hours in air.

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P162LSB, respectively. Considering that dry milling is used to de-crease the initial particle size, it can be estimated that each supplieruses different milling conditions, such as the milling time and ballsize. The other powder characteristics, such as ignition loss and tapdensity, were similar in the three types of Al2O3 powder.

On the other hand, the domestic ALG-1SH powder had a higherspecific surface area (6.59 m2/g) than the two imported powders, asshown in Table 1. AES-11 (6.28 m2/g) had the smallest particle size.This is only possible when ALG-1SH contains a significant amountof fine particles compared to the other powder. Fig. 1 presents SEMimages of the as-received powder, where the presence of more fineparticles less than 100nm can be confirmed in ALG-1SH than AES-11 or P162LSB. These fine particles may cause difficulty in disper-sion and increase the viscosity in a slip, where a relatively low slipviscosity of around 200 cPs is desirable for filter pressing [23]. To

decrease the slip viscosity associated with ALG-1SH, a decrease inpowder loading is inevitable, which can induce the low green andsintered density of the final product. Because dry milling is usedto decrease the particle size of the initial Al2O3 synthesized fromAl(OH)3, the generation of fine particles by random collisions andshear forces during the milling process appears unavoidable. Opti-mization of the dry milling condition or the utilization of wet millingmay be considered to minimize the number of fine particles inALG-1SH. Because wet milling requires a further drying process,it is undesirable economically when the relatively low Al2O3 priceof 3-5 U.S. dollars/kg is considered.

Fig. 2 compares the viscosity behavior of a slip prepared usingthree types of as-received Al2O3 and heat-treated ALG-1SH pow-der at different temperatures. All reveal a weak pseudo-plastic rhe-ology, showing a gradual decrease in viscosity with increasing shearrate. The slip containing as-received ALG-1SH shows the highest vis-cosity among six samples due to the significant presence of smallparticles and the resulting high surface area, as shown in Fig. 1 andTable 1, respectively. The viscosity of the slip prepared using as-received ALG-1SH, AES-11, and P162LSB after milling was 308,191, and 83 cPs, respectively, at a fixed shear rate of 50.5 s1. Thehigher slip viscosity with AES-11 than P162LSB is reasonable, con-sidering the smaller particle size of the AES-11. On the other hand,the slips with the heat-treated ALG-1SH reveal much lower viscos-ity than that with the as-received ALG-1SH, showing a value of 190,153, and 139 cPs for the powder heat-treated at 900 oC, 1,000 oC,and 1,100 oC, respectively, at the same shear rate. The slip viscosityof heat-treated ALG-1SH is comparable or even lower than that ofAES-11, depending on heat treatment temperature. This gradualdecrease in slip viscosity with the increase in heat treatment tem-perature would be attributed to the elimination of fine particles andwell-determined particle morphology in the ALG-1SH powder.

The viscosity of all slips increased by several tens of cPs afterkeeping them for 24 hours from milling, even though not shownhere. According to the industrial production engineer, this increasein viscosity by aging causes significant problems for filter pressing

Fig. 1. SEM images of the as-received Al2O3 powder: (a) ALG-1SH,(b) AES-11, and (c) P162LSB. Note the presence of more fineparticles in ALG-1SH.

Fig. 2. Viscosity of the slips prepared using the three types of as-received Al2O3 powder and the ALG-1SH heat-treated at dif-ferent temperatures.

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or slip casting when immediate casting is not possible right aftermilling. This time-dependent viscosity increase might be attributedto the weak re-agglomeration or surface hydration of Al2O3 powder.The slip for tape casting, which contains approximately 10wt% binderwith respect to the powder, usually shows a much higher degree ofpseudo-plastic behavior than that for the filter pressing shown inFig. 2. On the other hand, the slip for filter pressing or slip castingcontains a much smaller amount of binder, i.e., approximately 1wt%,and usually reveals weak pseudo-plastic behavior or almost New-tonian behavior. This almost invariable viscosity upon the changein shear rate is desirable for achieving a uniform casting rate for

filter pressing or slip casting. In contrast, strong pseudo-plastic behav-ior is beneficial for a smooth slip moving when the slip passesthrough a doctor blade at a high shear rate during tape casting[23,24].

Fig. 3 presents the zeta potential behavior of Al2O3 as a func-tion of pH for pure powder as well as after adding each polymericadditives, such as dispersant, binder, and plasticizer, which arerequired to prepare an aqueous slip for filter pressing. This study isvery important for examining any possible difference in zeta poten-tial among the three types of as-received powder and understand-ing the dispersion mechanism of the particle upon the addition of

Fig. 3. Zeta potential behavior for the three types of Al2O3 as a function of pH by adding various polymeric additives.

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polymeric additives. According to the zeta potential behavior ofthe Al2O3 powder in water shown in Fig. 3(a), all exhibit similarbehavior regardless of the supplier, showing the iso-electric point(IEP) at pH=9-10, which is consistent with the report [25]. Whenthe acrylic binder and polyethylene glycol plasticizer were addedto the powder, the IEP decreased slightly to pH=7.0-8.3, as shownin Fig. 3(b) and (c). On the other hand, the IEP of the particle de-creased significantly to pH=2.1-2.4 when the ammonium salt ofthe polycarboxylate dispersant was added, as shown in Fig. 3(d).This shift was attributed to the anionic nature of the dispersant,even though only 0.5 wt% with respect to the powder was added.The IEP of the Al2O3 powder in the final slip for the filter press-ing was at pH2, as shown in Fig. 3(e), which was affected mainlyby the anionic dispersant, regardless of the powder type. This sug-gests an adequate dispersion of the aqueous Al2O3 slip in terms ofthe electrostatic repulsion mechanism. The zeta potential of theheat-treated ALG-1SH showed similar behavior regardless of theheat-treated temperature, even though it is not presented, implyingthat heat treatment does not change the surface charge of Al2O3

powder.Because all slips for filter pressing reveal pH9, which is very

close to the IEP of the powder itself, there would be no electrostaticrepulsion if the pure powder was dispersed at this pH. Despitethis, the IEP of Al2O3 decreased drastically to pH=2 in the finalslip by the effects of an anionic dispersant. A highly negative zetapotential smaller than 40 mV can be obtained at around pH=9,as shown in Fig. 3(e). Industry uses an adequate dispersant con-sidering that an absolute zeta potential >40 mV is required to con-fer a sufficient electrostatic repulsion for particle dispersion [26].Moreover, steric repulsion by the adsorption of the polymeric dis-persant on the particle surface can also be achieved besides elec-

trostatic repulsion, which is called electro-steric dispersion.The domestic Al2O3 powder was compared with two types of

imported Al2O3 powder, which are currently being applied to manu-facture components for semiconductor and display productionequipment in Korea. The domestic ALG-1SH did not have any sig-nificant difficulty dispersing in an aqueous system. The only short-coming based on characterization was the presence of more fineparticles, which resulted in a high slip viscosity, as shown in Fig. 2.Because optimization of the dry milling process with a large quantityof powder demands significant effort, heat treatment of the milledpowder was performed alternatively to reduce the fine particles inALG-1SH.

Fig. 4 shows SEM images of the heat-treated ALG-1SH at somechosen temperatures, even though heat treatment was performedunder eight different conditions. Compared to the as-received par-ticles image shown in Fig. 1(a), heat treatment at 800 oC for twohours reveals a more distinct particle boundary, as shown in Fig.4(a), owing to crystallization of the amorphous particle surface.After heat treatment at 1,000 oC, the particles have a round shapewith a clear boundary, which looks effective for dispersion, as shownin Fig. 4(b). On the other hand, when the temperature was in-creased to 1,200 and 1,400 oC, a growth in particle size by Ost-wald ripening was observed, as shown in Fig. 4(c) and (d).

Fig. 5 presents the evolution of mean particle size and surfacearea of ALG-1SH as a function of heat treatment temperature. Theinitial mean particle size of 0.51m remained at 800 and 900 oCand increased to 0.52m at 1,000 oC, followed by a slight increaseto 1,300 oC. A significant increase in particle size was observed at1,400 oC. In contrast, the surface area decreased continuously withincreasing heat treatment temperature. The initial surface area of6.59 m2/g decreased to 4.76 m2/g after heat treatment at 1,000 oC

Fig. 4. SEM images of the heat-treated domestic ALG-1SH Al2O3 powder at various temperatures for two hours: (a) 800, (b) 1,000, (c) 1,200,and (d) 1,400 oC.

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and showed the lowest value of 0.02 m2/g at 1,600 oC. A some-what inconsistent relationship exists between the particle size andsurface area of ALG-1SH heat-treated at 800-1,200 oC, where theparticle size remains almost constant but the surface area decreasescontinuously. This might be due to the particle size measurementmethod based on the ‘dynamic light scattering (DLS) technique’using a commercial analyzer (Delsa Nano C, Beckman Coulter,U.S.A.). Using the Stokes-Einstein equation based on the Brown-ian motion of dispersed particles, the DLS technique has a draw-back in distinguishing small particles when those with a wide sizedistribution are analyzed [27]. Therefore, a consideration of bothparticle size and surface area behavior is desirable for the charac-terization of heat-treated ALG-1SH. Therefore, it can be concludedthat decrease in surface area with the powder heat-treated at1,000 oC is due mainly to the elimination of fine particles.

One concern with the heat-treated ALG-1SH is the possibledecrease in particle activity for sintering because the powder ex-posed at high temperatures tends to stabilize. Fig. 6 shows thegreen and sintered density of the pellets prepared using various

Al2O3 powders obtained from different suppliers or heat-treated atdifferent temperatures using ALG-1SH. The green density of thesamples was similar around 2.20 g/cm3, which corresponds to 55%theoretical density, except for the pellet prepared using the ALG-1SH heat-treated at 1,200 oC, showing the lowest green density of1.99 g/cm3. The pellet shape could not be formed with the ALG-1SH heat-treated at 1,300 oC without adding a binder because ofthe decreased adhesion by particle growth. On the other hand, theheat-treated ALG-1SH at 800 and 900 oC showed the highest sin-tered density of 3.93 g/cm3, which is even higher than that withthe as-received ALG-1SH (3.90 g/cm3) or foreign powder (3.90 g/cm3 for AES-11 and 3.92 g/cm3 for P162LSB). Furthermore, thesintered density of pellets prepared using heat-treated ALG-1SH at1,000 oC decreased gradually, showing the lowest value of 3.52 g/cm3 after heat-treatment at 1,200 oC, which is due most likely tothe decrease in powder activity. Although a pellet was used to exam-ine the effects of heat treatment on sintered density instead of a fil-ter-pressed compact, this result is encouraging by proving that theheat-treatment of ALG-1SH up to approximately 1,000 oC is effec-tive in eliminating the particles without deteriorating the powderactivity or increasing the mean particle size.

Fig. 7 presents SEM images of the sintered Al2O3 pellet surfaceafter polishing and thermal etching at 1,500 oC for one hour pre-pared by various powders obtained from different suppliers and theheat-treated ALG-1SH at different temperatures. All show the pres-ence of inter-granular and small intra-granular pores without abnor-mally grown grains. The mean grain size (PSm) is similar for allsintered samples based on the linear intercept method, showingvalues between 2.05-2.25m, indicating that heat treatment ofALG-1SH does not affect the final sintered microstructure. Over-all, heat treatment at 800-1,000 oC appears to be most effective forALG-1SH when the surface area and particle size of the importedtarget powder shown in Table 1 are considered because this treat-ment decreases the surface area effectively without changing themean particle size.

CONCLUSIONS

Replacing imported Al2O3 powder with a domestic powder forthe application in Korean semiconductor and display productionequipment is a major focus. This study examined the various pow-der characteristics of three types of Al2O3 powder. Compared totwo foreign powders, the domestic ALG-1SH contained finer par-ticles, resulting in a higher surface area and slip viscosity preparedusing this powder. On the other hand, the zeta potential of thethree types of powder shows similar behavior, revealing the iso-electric point at pH=9-10. The zeta potential decreased signifi-cantly to pH=2 by adding an anionic dispersant and remained atthis pH with the final slip. Considering the pH9 of the slip forfilter pressing, effective dispersion of particles could be achieveddue to the strong electrostatic repulsion of the slip, which shows azeta potential smaller than 40 mV, along with steric repulsion bythe adsorption of polymeric dispersant on the particle surface simul-taneously. The only shortcoming with the domestic ALG-1SH pow-der is the presence of more fine particles than the imported powders,which results in a low green and sintered density of the final prod-

Fig. 5. Evolution of the mean particle size and surface area of ALG-1SH as a function of the heat treatment temperature.

Fig. 6. Variation of green and sintered density of the pellets preparedusing various Al2O3 powder obtained from different suppli-ers or heat-treated at different temperatures using ALG-1SH.

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Korean J. Chem. Eng.(Vol. 38, No. 12)

uct due to the reduced solid loading in the slip for processing. Ac-cording to the powder characteristics after heat treatment at 800-1,600 oC, a temperature between 800-1,000 oC was chosen as anoptimal condition to eliminate small particles without increasingthe mean particle size. Moreover, heat treatment did not affect thedensity and microstructure of the sintered body. A test for indus-trial applications in a production line using heat-treated ALG-1SHis currently underway based on these results.

DECLARATION OF COMPETING INTEREST

The authors declare that they have no known competing finan-cial interests or personal relationships that could have influencedthe work reported in this paper.

ACKNOWLEDGEMENTS

This study was supported by the i-Ceramic Manufacturing Inno-vation Program (20003891) and the Technology Innovation Pro-gram (20012911) funded by the Korean Ministry of Trade, Industry

& Energy. The authors also thank the Core Research SupportCenter for Natural Products and Medical Materials (CRCNM) fortechnical support regarding surface area measurement.

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