b Shanghai Xinanna Electronic Technology Co., Ltd., Shanghai 201506, China;
c Graduate School of the Chinese Academy of Sciences, Beijing 100049, China
The shrinkage of feature size in IC industry results in higher demand of planarity of wafers [1, 2, 3, 4]. Chemical mechanical planarization (CMP) is the only and proverbial method that is currently used in semiconductor industry to achieve global planarization [2, 5]. Abrasive is a key factor influencing the CMP process. In the last half-century,SiO2 is the most commonly used universal abrasive in almost all fields that need CMP due to controllable size,narrow distribution and low cost [6, 7, 8]. However, CeO2 is gradually becoming a proper choice in STI,glass,inner metal dielectric CMP because of its low hardness,high chemical activity,high selectivity,and so on [9, 10, 11, 12].
Combining different materials to achieve synergies or new characteristic is always a hot area in material science [13, 14, 15, 16]. In recent years,the interest of combining the advantages of both SiO2 and CeO2 attracts much attention on preparation of core-shell composite particles [17, 18, 19]. Lee et al. ,prepared CeO2 coated SiO2 spherical particles with diameters of 300-400 nm resulting in the improvement of polishing performance on thermal oxide film. Zhao and Long ,synthesized SiO2/CeO2 core-shell composite particles through organic solvents and investigated their CMP performance. The experiments indicated that composite particles used as abrasives performed higher removal rate and lower roughness on silica and GaAs wafer. Chung  synthesized SiO2/ CeO2 composite particles in microemulsion,which was formed by adding surfactants to a water-oil system. Song  synthesized Ce2(CO3)3▪8H2O coated SiO2 composite particles first and then obtained CeO2 coated SiO2 particles through heat treatment.
However,there are still several challenges. For example,in order to be compatible with existing processes,organic solvents cannot always be used. Moreover,the synthesized mixtures contain both composite particles and detached CeO2 nanoparticles due to the weak attractions between CeO2 and the core SiO2. In addition,some resulting products need post-heating treatments to convert trivalent Ce3+ to quadrivalent Ce4+ compounds in some cases,which usually leads to irreversible hard aggregation and detachment of the coating. Furthermore,in most previous work,colloidal silica was synthesized by the TEOS hydrolysis method. In order to lower the cost,commercial colloid silica is a suitable choice as the core. However,commercial colloid silica is usually produced by the water glass ion exchange method with different diameters,surface conditions and solvents from silica made through the TEOS hydrolysis method,so that a new method compatible with commercial colloid silica is necessarily investigated.
PVP is a diffluent,nontoxic reagent that is commonly applied in chemosynthesis as an assisting reagent . Therefore,our group has introduced a new method to synthesize CeO2 coated SiO2 composite particles by PVP and successfully applied it in glass CMP . However the synthesis process was not thoroughly explored. In this study,facile synthesis of SiO2/CeO2 core shell nano-particles based on small particle size commercial colloid silica without postheating treatments is introduced. The various experimental parameters are optimized to produce well coated composite particles. The mechanism of the coating process is briefly investigated.2. Experimental
The alkaline colloidal silica solution supplied by Shanghai Xinanna Electronic Technology Co.,Ltd. was synthesized through an ion-exchange method and had a mean particle size determined by SEM of about 100 nm. We chose ammonium cerium nitrate ((NH4)2Ce(NO3)6) as the cerium source because it could provide Ce4+ directly with no need of further oxidation,which was better than Ce3+ sources such as CeCl3 or Ce(NO3)3 . Ammonium cerium nitrate ((NH4)2Ce(NO3)6),urea (CO(NH2)2),polyvinylpyrrolidone (PVP,K-30) were purchased from Sinopharm Chemical Reagent Co.,Ltd.,China and were directly used without any further purification. Deionized water was used for all experiments.
In a typical synthesis,CeO2 nano-crystals were generated by a precipitation reaction between Ce4+ and hydrolyzed urea above 70℃,meanwhile the coating reaction was carried out between the surface of SiO2 core and CeO2 nano-crystals in the presence of PVP. In a typical synthesis: 100 mg of a PVP solution was slowly dropped into 100 g 10 wt% colloid silica under continuous strong stirring at room temperature. We tagged the mixture solution A. 0.005 mol of (NH4)2Ce(NO3)6 and 0.02 mol of CO(NH2)2 were dissolved in DI water,respectively,to form a 100 g uniform solution,which we tagged as solution B. Solution A was transferred to a round-bottom flask equipped with a mechanical stirrer,a thermometer with a temperature controller,a water-cooled reflux condenser and a heating mantle. After preheating solution A to boiling point,solution B was quickly poured into the flask. The mixture was heated at boiling point,stirred and refluxed for about 5 h. The final product was obtained by centrifugation,washing with DI water several times to remove any possible ions and remnants.
The morphology and size of SiO2/CeO2 composite particles were observed by a field emission scanning electron microscope (FESEM, Hitachi S-4700 type).3. Results and discussion
3.1. Morphology of the CeO2 coated SiO2 composite particles
The SEM image in Fig. 1 shows the morphology of uncoated colloid silica core particles and CeO2 coated SiO2 particles, respectively. It can be seen from Fig. 1a and b that the uncoated SiO2 has very smooth surface and is 100-110 nm in diameter. As expected,Fig. 1c and d displays that the size of the particles increases to about 10 nm in diameter due to a distinct layer of flocculent CeO2 surrounding the SiO2 core after coating. The other basic characterization of the composite particles could be found in our previous work .
|Fig. 1.SEM images of uncoated colloid silica (a and b) and composite particles (c and d).|
A series of SiO2/CeO2 composite particles prepared at different PVP quantity are summarized in Table 1. As discussed in our previous work ,without PVP the CeO2 particles synthesized from a precipitation reaction were suspended in the entire aqueous solution and no obvious interactions between CeO2 and SiO2 could be observed. It was hypothesized that the charge on both particle surfaces that prevented the coating reaction. The mixed solution was acidic (pH 4-5) at the beginning because NH4+ from (NH4)2Ce(NO3)6 consumed OH- in the solution,but it would gradually turn to alkaline (pH 9-10) due to the slow hydrolysis of urea. As the isolate electric point (IEP) of SiO2 is 2.4,there is no doubt that the colloid silica was always negatively charged. However the IEP of CeO2 is between 6 and 7,the pH value of the solution,the CeO2 precursor and final ceria particles were changing all the time and it was difficult to measure the surface charge of ceria during the synthesis. As most of the surface area of SiO2 core was covered by CeO2,the charge types of composite particles and ceria nano-particles were about the same. So we measured the 1 wt% composite particles at pH 9 as shown in Fig. 2,the average ZETA potential was -38.49 mV,which indicated that both CeO2 and SiO2 were negatively charged in the alkaline solution. The repulsive force prevented the particles from contacting each other.
If the PVP is added to the solution in the beginning at a lower concentration,the as synthesized CeO2 particles gather around the SiO2 core but most of them agglomerate together into a reticular structure as shown in Fig. 3a and b. Thus the CeO2 on the surface of SiO2 are not uniform enough to form a complete layer. Fig. 3b exhibits a better coating than Fig. 2a because more PVP was added.
On the other hand,Fig. 3c and d gives a distinct view of sample 4 and 5. Both the SiO2 core particles are well coated and few CeO2 nano-particles exist in the solution individually. No obvious differences of the morphology of these composite particles can be observed as long as the concentration of PVP is above a threshold value.
|Fig. 3.SEM images of different samples synthesized with different PVP quantity: 10 mg (a), 25 mg (b), 100 mg (c) and 500 mg (d).|
The comparison between composite particles synthesized at different Ce to Si molar ratios,which are 1/60 and 1/8,respectively, is exhibited in Fig. 4. A small amount of CeO2 that is synthesized at a low Ce to Si molar ratio is totally adsorbed on the SiO2 core. In contrast,as the coating on the SiO2 core is sufficiently thick,instead of the surface of SiO2 where there is no space left,the excess CeO2 has to nucleate and grow in the dispersion due to the high Ce to Si molar ratio.
|Fig. 4.SEM images of samples with different Ce/Si mole ratio: 1/60 (a) and 1/8 (b).|
It can be reasonably assumed that all the SiO2 cores and synthesized CeO2 nano-particles were standard sphere about 100 nm and 5 nm in diameter,respectively,as shown in Scheme 1a. The CeO2 shell was 5 nm thick,which was not thin enough to ignore the difference between outer and inner surface. So we defined the distance from center to the middle of the shell as the radius,r = 100/2 + 5/2 = 52.5 nm,the surface area of the shell is 4 × π × 52.52 = 34,636 nm2. If the CeO2 spheres are formed as the close-packed structure shown in Scheme 1b,each one would cover an area of the circumscribed hexagon about 21.65 nm2,so we would need at least 34,636/21.65 = 1600 CeO2 particles to cover the surface of each SiO2 core. The density of SiO2 and CeO2 is 2.2and 7.2,respectively,and the weight ratio is 1.53. Changing the molar ratio 60/1 & 8/1 to weight ratio 20.93 & 2.79,both are larger than 1.53,means the CeO2 is not enough to cover the surface even at a 1/8 molar ratio. But in fact,the specific surface area greatly decreases due to the agglomeration of colloid silica. Moreover the synthesized CeO2 particles are porous and the weight is much lighter than solid sphere with the same diameter. These two facts make the CeO2 shell lighter than the theoretical value.
|Scheme 1.(a) 100 nm SiO2 core be coated with 5 nm CeO2; (b) close-packed structure of CeO2 and the cover area of each particle.|
The CeO2 nano-particles were produced by precipitation of Ce4+ and OH- that came from hydrolyzed urea first. Then the CeO2 dispersion was mixed with PVP-modified colloid silica. After stirring,heating,refluxing,aging and washing,the final product was shown in Fig. 5. Most CeO2 particles were washed off,which indicates that the PVP could attract CeO2 precursor-hydrated cerium hydroxide,but not CeO2 itself.
Ce(OH)m▪(H2O)n(4-)++m+(4-m)NH3→CeO2▪2H2O+(m+n-4)H2O + (4-m)NH4+3.5. Influence of the surfactant
As PVP plays an important role in the reaction,we are very interested in how it works. The possible coating procedure is shown as follows. The colloid silica is synthesized by the methods of ion exchange and hydrothermal synthesis,as a result the silica surfaces are occupied by hydroxy groups. Once the PVP is added to the sol,the oxygen atom in the carbonyl groups will form hydrogen bonds with hydroxyls. As the PVP molecule has a long chain with many carbonyl groups and colloid silica has many hydroxy groups on the surface,PVP will entwine the silica surface tightly as shown in Scheme 2.
The CeO2 nano-particles are synthesized by a precipitation reaction. (NH4)2Ce(NO3)6 will fully ionized to produce Ce4+ when it is dissolved in water. CO(NH2)2 can be hydrolyzed in aqueous solution above 70℃ and slowly releases NH3,which gradually makes the solution alkaline. The Ce4+ precipitates by forming hydrated cerium hydroxide. The final product ceria is obtained by aging and dehydration of the precursor-hydrated cerium hydroxide, which interacts with the carbonyl groups of PVP through hydrogen bonds as shown in Scheme 3.
To confirm our hypothesis,we introduced cationic surfactant cetyl trimethyl ammonium bromide (CTAB) to the system. The resulting product synthesized in the presence of CTAB instead of the nonionic surfactants PVP is clearly displayed in Fig. 6. The SiO2 particles obtained under this condition are highly agglomerated with few CeO2 particles. Because of the positive charge that ammonium carries,it is strongly attracted by the negative charge of the surface hydroxyl of the colloid silica. So the long hydrophobic chains exposed to aqueous solution are forced to fold to reduce the contact area of water,which forms a protective layer that results in the separation of ceria particles and colloid silica as shown in Scheme 4.
|Scheme 4.CTAB attracted by the silica surface and the carbon chains fold.|
In summary,100 nm commercial colloid silica core coated with nano-sized CeO2 composite particles were prepared by the precipitation method using ammonium cerium nitrate and urea as a precipitator with PVP as an assistant in aqueous solution. The influence of various parameters on the formation of composite particles was discussed such as Ce/Si ratio,experimental sequence, and surfactants. It is found that the type and quantity of surfactant are the key factors of the coating process. The nonionic surfactant PVP attracts ceria and silica through hydrogen bonds and combines them together to form composite particles.
The work is supported by National Integrate Circuit Research Program of China (Nos. 2011ZX02704-002,2009ZX02030-001), National Natural Science Foundation of China (No. 51205387), Science and Technology Council of Shanghai (Nos. 11nm0500300, 10QB1403600).
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