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Nano-scale surface of ZrO2 ceramics achieved efficiently by peanut-shaped and heart-shaped SiO2 abrasives through chemical mechanical polishing
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Nano-scale surface of ZrO2 ceramics achieved efficiently by peanut-shaped and heart-shaped SiO2 abrasives through chemical mechanical polishing Lei Xua, Hong Leia,b,∗ a b
School of Materials Science and Engineering, Shanghai University, Shanghai, 200444, China Research Center of Nano Science and Technology, Shanghai University, Shanghai, 200444, China
ARTICLE INFO
ABSTRACT
Keywords: Chemical mechanical polishing KH550–SiO2 Zirconia ceramic Non-spherical abrasive
Zirconia ceramics are regarded as the best development target for 5G mobile phone rear covers. However, it is necessary and urgent to improve the surface quality and processing efficiency of zirconia ceramics. Non-spherical silica abrasives were prepared by the KH550 induction method and were used in chemical mechanical polishing (CMP) of zirconia ceramics for the first time. While achieving low surface roughness of 1.9 nm, it has an efficient polishing rate of 0.31 μm/h which is superior to conventional abrasives. Silica particles are peanutshaped and heart-shaped in the scanning electron microscopy image, and its distinctive morphology provides the possibility of its excellent polishing performance. X-ray photoelectron spectroscopy analysis shows that during the CMP process, silica abrasives and zirconia ceramic undergo a solid phase chemical reaction to form ZrSiO4. At the same time, the contact wear model established in combination with the coefficient of friction indicates that the two-dimensional surface contact mode of non-spherical silica abrasives on the surface of zirconia ceramics greatly improves its mechanical effect.
1. Introduction With the development of electronic information technology, the age of 5G communication has arrived [1]. The antenna structure used in communication equipment is more complicated than 4G, and the existing antenna structure cannot meet the requirements of 5G [2–4], because 5G communication uses a wireless spectrum above 3 GHz. Therefore, in the age of 5G communication, smart phones will abandon the metal rear covers that have great shielding for 5G signals, and use inorganic non-metallic rear covers instead. Nowadays, the non-metallic rear cover is mainly made of glass or ceramic. The zirconia ceramic rear cover has a dielectric constant [5] of 3 times that of sapphire and 10 times that of tempered glass. Zirconia ceramic has good signal penetration and will be the primary choice for future mobile phone rear cover. In addition, ceramics can also be doped with some rare metals, making them more resistant to falling and abrasion than glass. Moreover, ceramics are also superior to glass in terms of color brightness and touch [6,7]. Zirconia ceramic is a kind of functional material with good compressive strength, wear resistance, corrosion resistance, excellent oxidation resistance, good thermal stability, good thermal conductivity and high dielectric constant [8–11]. However, zirconia ceramics are difficult to process due to their high hardness and wear resistance. In ∗
recent years, in order to maximize the surface brilliance of zirconia ceramics, different ultra-precision machining methods have been employed. Ma et al. [12] used a laser-assisted method combined with a conventional grinding wheel to grind zirconia ceramics to achieve ductile regime grinding with a large depth-of-cut. Yang et al. [13] processed zirconia ceramics with ultrasonic vibration assisted intermittent grinding and established a grinding force prediction model to illustrate the removal mechanism. Camila et al. [14] processed zirconia ceramics with a grinding + polishing (two-step polishing system) to obtain a smoother surface. However, the above processing methods can only make zirconia ceramic surface flatness to the micron level. Chemical mechanical polishing is a nanoscale processing method that can achieve global planarization at present. It is applicable to a wide range of applications including silicon wafers [15,16], sapphire wafers [17–19], copper [20], glass [21], silicon carbide [22,23], gallium nitride [24], yttrium aluminum garnet (YAG crystal) [25], and titanium alloy [26], etc. In order to make the surface of the zirconia ceramic more smooth and bright, and to achieve a flatness of nanometer level, the chemical mechanical polishing method is adopted. The slurry as the main polishing carrier for CMP has been the focus of research for all researchers. The slurry is primarily composed of abrasives, chemical reagents (surfactants, catalysts, oxidizing agents, etc.). As the main mechanical action of CMP, abrasives not only possess
Corresponding author. School of Materials Science and Engineering, Shanghai University, Shanghai, 200444, China. E-mail address: [email protected] (H. Lei).
https://doi.org/10.1016/j.ceramint.2020.02.108 Received 25 December 2019; Received in revised form 11 February 2020; Accepted 12 February 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Lei Xu and Hong Lei, Ceramics International, https://doi.org/10.1016/j.ceramint.2020.02.108
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unique nano-friction, but also can have solid phase chemical reaction with the polished objects. Currently, abrasives mainly include silicon oxide [18,19], cerium oxide [21], aluminum oxide [27]. Most of the abrasives used in CMP experiments are colloidal silica because of the unique hardness of soft colloidal silica abrasives and no environmental pollution. However, conventional silica abrasives have been unable to further reduce the surface roughness of polished objects, and the removal efficiency during the polishing process also appears to be low. Researchers have explored this in two aspects over the past few years. First, mainly from the improvement of the polishing properties of chemical action, Lei et al. [28] proposed modification of silica abrasives by doping with elements, the unique solid phase chemical reaction between doped elements and wafer to form a new soft surface layer. The new soft layer is then removed by mechanical grinding of the silica composite abrasives to increase the material removal rate. Yin et al. [18] produced MgAl2O4 by solid phase chemical reaction between MgO and sapphire by doping MgO into silica sol, which resulted in increasing the material removal rate. However, all of the above modified silica abrasives involve the introduction of new metal elements which will complicate subsequent processes and increase costs. In addition, the spherical morphology of silica can be changed to improve the mechanical effect of the abrasives. Liang et al. [29] used cationic Ca+ induction to prepare chain-like silica, which increased the material removal rate (MRR) by about 65% compared to spherical silica abrasives. This kind of method also has problems such as elemental contamination. In addition to the preparation of silica by metal ion induction, Xu et al. [30] prepared flower-shaped silica abrasives by double-system micro-emulsion method, and the MRR was increased by about 117%. However, this method has some limitations, such as cumbersome experimental procedures and environmental pollution, and it cannot realize industrial mass production. In this work, the method of connecting multiple spherical silica seeds was used to prepare non-spherical silica easily and conveniently. An innovative non-spherical silica abrasive was prepared by using the inducer method of KH550 silane coupling agent, and the particles were connected by more stable chemical bonds. To explore the basic mechanism of zirconia ceramic polishing, various characterization methods were carried out. The synthesized non-spherical silica was characterized by scanning electron microscopy and frictionmeter. The surface quality before and after polishing was studied by metallographic microscope and surface profiler (white light interferometer), and the polishing mechanism was analyzed by X-ray photoelectron spectroscopy and modeling.
KH550 induction method. Firstly, 375 g of 40 nm SiO2 seeds (40 wt%) were weighed, placed in a four-necked flask, heated and stirred. After 5 min, the pre-hydrolyzed KH550 mixed solution (the mixture solution was composed of KH550, ethanol and H2O, with the respective proportions of KH550: 20 wt%, C2H5OH: 72 wt%, H2O: 8 wt%) was added. Then the solution was heated, condensed and refluxed for 5 min. Subsequently, 1125 ml of deionized water was added to dilute the silica sol to a solids content of 10 wt% and to maintain reflux for 20 min. Finally, 2000 ml of 2.5 wt% silicic acid (silicic acid is newly prepared from sodium silicate through a cation exchange resin) and 160 ml of 3.0 wt% NaOH solution were added dropwise. The dropping rate was controlled and the pH was maintained at about 10. The total addition was 2.5 h to complete the reaction. At last, the non-spherical silica sol was obtained. In this experiment, in order to explore the effect of KH550 content, a series of non-spherical silica abrasives with different contents of KH550 was prepared, which were composite abrasives containing 0.0 wt%, 0.5 wt%, 0.75 wt%, 1.0 wt%, 1.25 wt%, (mass ratio of KH550 to SiO2) of KH550 respectively. The spherical silica sol compared in this paper is 0.0 wt% KH550. The experimental method is described above. 2.3. Chemical mechanical polishing tests Polishing experiments were performed by spherical and non-spherical silica sol of the same solid content (TSC = 10 wt%) using the polishing machine under the polishing conditions listed in Table 1. The polishing machine was a UNIPOL-1000S CMP machine (Shenyang Kejing Instrument Co., Ltd., China) and the polishing pad was a Rodel porous polyurethane pad. 2.4. Measurement of the coefficient of friction The coefficient of friction between the zirconia ceramic, the abrasives and the polishing pad was measured with the apparatus shown in Fig. 1. The frictionmeter is a Byes-550 machine (Bangyi Precision Measuring Instrument (Shanghai) Co., Ltd., China). The polishing machine CMP environment was simulated. A polishing pad was attached to the bottom of the liquid tank, and then a certain amount of slurry was poured. The zirconia ceramic was stuck on the bottom of the weight, and the coefficient of friction was measured by pressing it on the polishing pad. The coefficient of friction data was given directly by the instrument system. 2.5. Characterizations
2. Experimental section
The morphology of non-spherical and spherical silica abrasives was characterized by 10 kV scanning electron microscopy (SEM, JSM7500F). The surface morphology of the zirconia ceramics before and after polishing was observed by a metallographic microscope (CMM-50, Shanghai Changfang Optical instrument Co.,Ltd., China) at 50 times. The average surface roughness of the polished and unpolished zirconia ceramics was measured by Ambios Xi-100 surface profiler (Ambios Technology Corp., USA) with a resolution of 0.1 Å, a focal depth of 3.0 μm, and a working distance is 4.9 mm and the measurement area is
2.1. Chemicals The chemical reagents used in all experiments involved in this paper are as follows: γ-Aminopropyl triethoxysilane KH550 (C9H23NO3Si, Analytical Reagent, Sinopharm Chemical Reagent Co., LTD, China), ammonium hydroxide (NH3·H2O, Analytical Reagent, Sinopharm Chemical Reagent Co., LTD, China) anhydrous ethanol (C2H5OH, Analytical Reagent, Sinopharm Chemical Reagent Co., LTD, China), Silica sol (SiO2, size: 40 nm, (colloidal SiO2, ZheJiang DeLiXin Micro/ Nano Science and Technology Co., LTD, China), Sodium silicate (Na2SiO3·9H2O, JiNan DeWang Chemical Industry Co., LTD, China), cation exchange resin (Sinopharm Chemical Reagent Co., LTD, China), NaOH (Sinopharm Chemical Reagent Co., LTD, China), hydrochloric acid (HCl, Analytical Reagent, Sinopharm Chemical Reagent Co., LTD, China), deionized water (DI).
Table 1 Polishing conditions.
2.2. Preparation of KH550–SiO2 The non-spherical silica was prepared via the silane coupling agent 2
Parameter
value
1. 2. 3. 4. 5. 6. 7.
Zirconia ceramic Rodel porous polyurethane pad 60 min 180 ml/min 6 Kg 30 r/min 60 r/min
Wafer type Polishing pad Polishing time Feed rate of the slurry Load pressure Upturn speed Downturn speed
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Fig. 1. Physical device image of Frictionmeter.
94.5 μm × 94.5 μm. The element compositions of KH550–SiO2 abrasives after polishing were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermofisher Scientific China., Ltd.), and the XPS spectrum was gained by a focused monochromatized Al Kα radiation, calibration by applying exogenous C 1s signal at 284.6 eV.
spherical silica seed. The specific synthesis principle is shown in Fig. 3. KH550 is first hydrolyzed and then self-condensed for a certain period of time, and multiple hydroxyl groups appear at the end of the organic macromolecule. The hydroxyl group of the organic macromolecule reacts with the hydroxyl on the surface of the spherical silica to form a hydrogen bond and condense into a Si–O–Si covalent bond, and a certain number of epitaxial layers are formed on the surface of the silica sphere. The hydroxyl group at the outer end of the epitaxial layer is then hydrolyzed and condensed with other hydroxyl groups on the surface of the spherical silica to form a Si–O–Si covalent bond. At the same time, the amino group at the outer end of the epitaxial layer forms a hydrogen bond with the hydroxyl group on the surface of other spherical silica. So the particles will connect together. Due to the steric hindrance among the particles, the repulsive force among the particles increases, and only 2–3 spherical silica particles adhere together to form a new non-spherical silica seed. In the second step, the nonspherical seed SiO2 is further grown by silicic acid, so that the texture of composite non-spherical silica abrasives is more compact. However, when the content of KH550 is too much in the first step, the attraction between particles is further increased and larger than the steric hindrance and repulsive force between each other, forming a certain degree of agglomeration.
3. Results and discussions 3.1. Morphology of the abrasives Fig. 2 (a) shows the spherical silica abrasives (SS) after growth, and Fig. 2 (b) shows the non-spherical silica abrasives (NSKS). It can be seen from the SEM image that the boundary between the individual particles in the spherical silica sol is very clear. However, there is no boundary between 2/3 particles in the non-spherical silica sol, indicating that KH550 successfully connected 2 or 3 spherical silica particles together and new particle shapes were found: peanut shape and heart shape. 3.2. Synthesis principle of non-spherical abrasives The irregular KH550–SiO2 abrasives in the paper are an innovation based on spherical SiO2 seeds. In the first step, multiple spherical silica abrasives are bonded together by the silane coupling agent to form non-
Fig. 2. SEM images of different abrasives (a) SS abrasives (b) NSKS abrasives with corresponding magnified SEM images inset.
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Fig. 3. Preparation principle of KH550–SiO2 abrasive.
3.3. CMP performance of zirconia ceramic
zirconia ceramic polished by SS and NSKS with 0.5 wt% KH550. Among them, the original Sa of zirconia ceramic is 6.7 nm, the Sa after SS polishing is 3.2 nm, and the Sa after NSKS polishing is 2.4 nm. This demonstrates that the NSKS abrasives have higher MRR and can further improve the surface quality of zirconia ceramics. In order to further determine the optimal KH550 content and study the effect of KH550 on the polishing of zirconia ceramics, different contents were analyzed. Fig. 5 shows the effects of different KH550 content on the MRR in NSKS abrasives. It can be seen from Fig. 5 that as the KH550 content increases, the MRR of KH550–SiO2 composite abrasives gradually increases. This indicates that as the KH550 content increases, the proportion of non-spherical silica particles in the slurry increases, and the chemical action and mechanical action of the abrasives also increase, thereby increasing the polishing rate. In addition, when the KH550 content is 1.0 wt%, the MRR reached the maximum (0.31 μm/h), which is 100.0% higher than that of the spherical silica. Because the proportion of peanut-shaped or heartshaped non-spherical silica in the sol at this time reaches a peak. However, after the KH550 content exceeds 1.0 wt%, the MRR begins to decrease, The main reason is that the agglomeration of the particles destroys the dispersibility of the abrasives, some deposition occurs in
The quality of the slurry for polishing zirconia ceramic depends on material removal rate and surface roughness (Sa). Therefore, it is necessary to evaluate the CMP performance of silica composite abrasives with KH550. The MRR (μm/h) is mainly calculated according to equation (1).
MRR =
m
10 4 ST
(1)
Where m is the poor quality of zirconia ceramic before and after polishing (g), is the density of zirconia ceramic (g·cm−3, zirconia = 6.10 g·cm−3), S is the area of zirconia ceramic (cm2), T is the polishing time (h). The actual size of the zirconia ceramic rear cover of smart phone is 13.8 cm * 6.7 cm. For the convenience of the experiment, the zirconia ceramics were cut into 5.5 cm *5.5 cm. The MRR of spherical SiO2 and non-spherical KH550–SiO2 is shown in Fig. 4 (a). Under the same conditions, the MRR of NSKS with 0.5 wt% KH550 is 0.18 μm/h which is larger than that of SS abrasives. Fig. 4 (b) shows the Sa of the unpolished zirconia ceramic and the Sa of the 4
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Fig. 4. (a) The MRR of SS and NSKS, (b) the Sa of unpolished and polished by SS & NSKS (0.5 wt% KH550).
Fig. 5. The MRR of different content of KH550.
Fig. 7. The Sa of different content of KH550.
the solution, and the number of effective abrasives is greatly reduced, which affects the MRR. Fig. 6 shows the state of the final sample of each content of KH550–SiO2 silica sol after standing for a period of time. It can be found that the content of KH550 is more than 1.0 wt%, the transparency degree of the silica sol is significantly reduced, and a small
amount of precipitation appears, which proves that excessive amount of KH550 indeed lead to agglomeration of abrasives. The Sa of different KH550 content abrasive polished zirconia ceramic is shown in Fig. 7. With the increase of KH550 content in the range of 0.0 wt% ~1.0 wt%, Sa of the zirconia ceramic gradually
Fig. 6. Final state of different content of KH550–SiO2 silica sol after standing for a period of time.
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Fig. 8. Micrograph of the surface of zirconia ceramics before and after polishing at 50 times: (a) unpolished, (b) after polishing by SS abrasives, (c–f) after polishing by NSKS abrasives containing 0.5 wt%, 0.75 wt%, 1.0 wt%, 1.25 wt%, respectively.
polishing with various KH550–SiO2 abrasives. As shown in Fig. 9, the polished surfaces (Fig. 9b–f) are smoother than the unpolished surface (Fig. 9a). From Fig. 9(b–e), the surface roughness gradually decreases with the increase of KH550 content, and the roughness rises again due to the scratches in Fig. 9 (f). The roughness is the smallest when the content is 1.0 wt%. In conclusion, when the KH550 content is 1.0 wt%, the polishing performance of the KH550–SiO2 composite abrasives is the best (MRR is the largest and Sa is the smallest).
decreases, and reaches a minimum of 1.9 nm. This indicates that with the increase of KH550, the increase of non-spherical silica abrasives in the slurry can further reduce the surface roughness of the zirconia ceramic. It may be due to the fact that the peanut-shaped or heartshaped non-spherical silica abrasives have more contact with the zirconia ceramic and is able to remove the rough peaks and holes on the zirconia ceramic surface. In addition, when the KH550 content is over 1.0 wt%, the Sa increases. Because when the content of KH550 is more than 1.0 wt%, the abrasives are agglomerated, and the appearance of large particles tends to cause scratches on the surface of the zirconia ceramic, thereby affecting Sa. According to the results of Fig. 5 and Fig. 7, the optimal content of KH550 was 1.0 wt%. The surface morphology of the zirconia ceramics measured by the metallographic microscope is shown in Fig. 8. As can be seen from Fig. 8 (a), the surface of the zirconia ceramic before polishing has a large number of holes, and the size of the holes is not uniform, and the large-diameter holes occupy the main body. Fig. 8 (b) shows that after polishing experiment, the number of large-diameter holes on the surface of zirconia ceramics drops sharply. Fig. 8(c–e) show that with the increase of KH550 doping amount, the number of large-diameter holes is reduced, and the small-caliber holes begin to dominate until the latter gradually disappears, indicating that the surface flatness has been definitely improved. The main reason is that the mechanical and chemical actions of the abrasives are enhanced as the relative amount of irregular silica abrasives increases. When the abrasives are in contact with the holes, the large-diameter holes are in contact with a larger number of particles, so that the grinding effect is larger, and the large-diameter holes are preferentially reduced and disappeared. In turn, the removal efficiency of the zirconia ceramic is improved, which further illustrates that the MRR is constantly increasing. As shown in Fig. 8 (f), scratches appear on the surface of the zirconia ceramic when the doping amount reached 1.25 wt%. Because the particles agglomerate to form large particles when the content of KH550 increases. Furthermore, large particles have a significant wear and tear on the surface of the zirconia ceramic, which can cause scratches on the surface. Fig. 9 shows the 3-D surface profile of the zirconia ceramic after
3.4. CMP mechanism of NSKS The CMP mechanism of NSKS abrasives on zirconia ceramics was studied from both chemical and mechanical aspects. It has been known from previous studies that silica is an acid oxide and has a certain solid phase chemical reaction with other oxides [25,28]. Therefore, it is speculated that a solid phase chemical reaction may also occur between silica and zirconia ceramic. It is well known that XPS is often used to measure changes in the valence of elements during CMP to characterize the solid phase chemical reactions that occur between the abrasives and the wafer [23,28]. At present, there is no CMP analysis on zirconia ceramics, so XPS may be used to investigate the possible chemical reactions during polishing. Fig. 10(a–c) are narrow scan spectra of the elements in the particles after 1.0 wt% NSKS abrasives polishing. Fig. 10 (a) shows that the peak of Si 2p is wider, indicating that there are multiple chemical states. There are three fitting curves in Fig. 10 (a). The binding energy (BE) of 103.59 eV [31] and 102.98 eV [32] separately corresponds to Si 2p in SiO2 state and -Si(CH2)3NH2 state, and the peak of another binding energy of 104.04 eV corresponds to Si 2p in ZrSiO4 state. Fig. 10 (b) shows the spectrum of O 1s. O 1s has three peaks, the binding energy of 532.79 eV [33] and 533.39 eV [34] correspond to the O 1s of the SiO2 state and the O 1s of the ZrO2 state, respectively. The binding energy of 531.84 eV [35] corresponds to O 1s in the bond of Zr–O–Si of ZrSiO4 state. Fig. 10 (c) shows a broad double peak of the Zr 3d, where the binding energy of 181.63 eV and 178.43 eV separately [36] correspond
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Fig. 9. Surface profiles and parameters (inset) of zirconia ceramics: (a) unpolished, (b) after polishing by SS abrasives, (c–f) after polishing by NSKS abrasives containing 0.5 wt%, 0.75 wt%, 1.0 wt%, 1.25 wt%, respectively.
to the Zr 3d in ZrO2 state. The binding energy of 180.65 eV and 176.56 eV separately [37] correspond to the Zr 3d in the ZrSiO4 state. Therefore, according to the XPS analysis of Si, O and Zr above, it can be explained that the silica and the zirconia ceramic have a solid phase chemical reaction, and the resulting reactant exists in the form of ZrSiO4. The main reaction equation is shown in equation (2). ZrO2 + SiO2= ZrSiO4
spherical silica abrasives and zirconia ceramics, a single spherical and non-spherical abrasive is taken as an example to establish a contact wear model of silica abrasives and zirconia ceramics. The mechanical action mechanism during the CMP process is shown in Fig. 11. It can be seen from Fig. 11 that the contact between the SS abrasive and the zirconia ceramic is mainly single point, while the contact between the NSKS abrasive and the zirconia ceramic is mainly multi-point contact due to the need for balance. This view is also explained in our last study [30]. In the actual polishing process, the nanoparticles have high-speed random rotation and translational motion in the flowing base fluid [39,40]. Thus, for SS abrasives, the particles first rotate in solution and then come into point contact with the wafer and undergo a certain translation, the resulting contact being a one-dimensional line contact. However, for NSKS abrasives, the particles rotate around their
(2)
Camila et al. [38] have put forward that during the polishing process, an instantaneous high temperature is generated at the moment of contact between the abrasives and the wafer. This high temperature provides a possibility for solid phase chemical reactions between SiO2 and ZrO2. In order to more vividly illustrate the contact wear between non-
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Fig. 10. The XPS narrow scan spectra of element: (a) Si, (b) O, (c) Zr from 1.0 wt% NSKS silica abrasives after polishing.
own axis in solution, because the contact with the wafer is multi-point contact, so the contact between the single spin NSKS abrasives and the wafer becomes one-dimensional coil contact. Then, the translation occurs, and the contact formed is a two-dimensional surface contact. Therefore, the contact area produced by the NSKS abrasives is much larger than the contact area produced by the SS abrasives. According to
the MRR equation (3) of established by Wang et al. [41].
MRR =
Vp D0 2rc
e
PS = Fs/ AC , FS =
1 (1 2v ) P + Cfb E1 1 s 2 10
1/2 Vd / Ro T
(3)
F , C = 3 (4 + v1)/8(1 µ
2v1)
Where Vp: the speed of abrasive, D0 : zero stress diffusion coefficient, rc : contact radius between the abrasive particles and the wafer, v1: Poisson's ratio of the wafer, Ps: average contact pressure between abrasive particles and wafer, Fs : Single abrasive force, AC : contact area between the abrasive particles and the wafer, µ: the coefficient of friction, C: a dimensionless parameter, fb : the fraction of the contact area where bonding occurs (~1%), E1: Young's modulus of the wafer, Vd: diffusion activation volume, Ro : gas constant, T: absolute temperature. It can be found that the MRR is proportional to the contact area, which further indicates that the polishing rate of the NSKS abrasives is higher than that of the SS abrasives. At the same time, the increase of the contact area will also accelerate the chemical reaction between substances. According to equation (3), the MRR is proportional to the coefficient of friction μ between the abrasives and the wafer. Therefore, the coefficient of friction (μs: coefficient of static friction, μk: dynamic friction factor) of silica with different KH550 contents was measured, and the results are shown in Table 2. It can be found that as the KH550 increases, the static/dynamic coefficient of friction of the silica sol is increasing, indicating that the relative amount of NSKS abrasives is increasing in the solution. And with the increase of the coefficient of friction, MRR keeps increasing, which is consistent with our previous experiments. Therefore, the polishing performance of non-spherical abrasives is better than that of spherical abrasives.
Fig. 11. Contact wear model of silica abrasives and zirconia ceramic. 8
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Table 2 Coefficient of friction of silica with different KH550 content. KH550–SiO2
0.0 wt% KH550–SiO2
0.5 wt% KH550– SiO2
0.75 wt% KH550– SiO2
1.0 wt% KH550– SiO2
1.25 wt% KH550– SiO2
μs μk
0.289 0.250
0.328 0.291
0.335 0.304
0.420 0.357
0.441 0.401
4. Conclusion
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Takeshi, Liquid electrolyte-free electrochemical oxidation of GaN surface using a solid polymer electrolyte toward electrochemical mechanical polishing, Electrochem. Commun. 97 (2018) 110–113. [25] Z. Zhang, Z. Jin, J. Guo, X. Han, Q. Mu, X. Zhu, A novel chemical mechanical polishing slurry for yttrium aluminum garnet crystal, Appl. Surf. Sci. 496 (2019) 143601. [26] Z. Zhang, Z. Shi, Y. Du, Z. Yu, L. Guo, D. Guo, A novel approach of chemical mechanical polishing for a titanium alloy using an environment-friendly slurry, Appl. Surf. Sci. 427 (2018) 409–415. [27] T. Wang, H. Lei, Y. Dong, L. Xu, S. Dai, Highly efficient removal of sapphire by composite nanoabrasive with novel inorganic polyelectrolyte as a binder, J. Alloys Compd. 782 (2019) 709–715. [28] T. Liu, H. Lei, Nd3+-doped colloidal SiO2 composite abrasives: synthesis and the effects on chemical mechanical polishing (CMP) performances of sapphire wafers, Appl. Surf. Sci. 413 (2017) 16–26. [29] C. Liang, L. Wang, W. Liu, Z. 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In this study, chemical mechanical polishing of zirconia ceramics was first carried out using novel non-spherical silica abrasives. Compared with the commonly used spherical silica sol, the non-spherical silica sol can remove holes on the surface of zirconia ceramics to achieve nanometer-scale surface roughness. Its MRR is increased by 100.0% compared to conventional silica sol. The synthesis principle of KH550–SiO2 abrasives was discussed. Through the action of the covalent bond and the hydrogen bond, a stable and non-polluting nonspherical silica sol was obtained. The polishing mechanism of the new type of non-spherical abrasives and ceramic wafer was proposed, and the solid phase chemical reaction between non-spherical silica sol and zirconia ceramic during CMP was demonstrated for the first time. At the same time, the coefficient of friction of non-spherical silica sol in CMP system was measured for the first time to demonstrate its strong mechanical properties. Moreover, the two-dimensional surface contact formed by non-spherical abrasives in the CMP process not only accelerates the chemical reaction, but also improves the actual contact area between the abrasives and zirconia ceramics, thus improving the overall MRR. The developed non-spherical silica sol provides a more convenient and efficient polishing method for the processing of 5G mobile phone ceramic rear cover. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Number 51975343). References [1] W.H. Chin, Z. Fan, R. Haines, Emerging technologies and research challenges for 5G wireless networks, IEEE Wirel. Commun. 21 (2014) 106–112. [2] E. Hossain, M. Hasan, 5G cellular: key enabling technologies and research challenges, IEEE Instrum. Meas. Mag. 18 (2015) 11–21. [3] T.S. Rappaport, S. Shu, R. Mayzus, H. Zhao, Y. Azar, K.P. Wang, G.N. Wong, J.K. Schulz, S. Mathew, F. Gutierrez, Millimeter wave mobile communications for 5G cellular: it will work!, IEEE Access 1 (2013) 335–349. [4] T.S. Rappaport, F. Gutierrez, E. Ben-Dor, J.N. Murdock, Q. Yijun, J.I. Tamir, Broadband millimeter-wave propagation measurements and models using adaptivebeam antennas for outdoor urban cellular communications, IEEE Trans. Antenn. Propag. 1 (2013) 1850–1859. [5] J. Huang, L. Huang, M.C. Tsai, M.H. Lee, M.J. Chen, Enhancement of electrical characteristics and reliability in crystallized ZrO2 gate dielectrics treated with insitu atomic layer doping of nitrogen, Appl. Surf. Sci. 305 (2014) 214–220. [6] V. Hlavacek, J.A. Puszynski, Chemical engineering aspects of advanced ceramic materials, J. Ind. Eng. Chem. Res. 35 (1996) 349–377. [7] P. Li, I.W. Chen, P.H. James, Effect of dopants on zirconia stabilization-an X‐ray absorption study: I, trivalent dopants, J. Am. Ceram. Soc. 77 (1994) 118–128. [8] M.H. Bocanegra-Bernal, S.D. De la Torre, Phase transitions in zirconium dioxide and related materials for high performance engineering ceramics, J. Mater. Sci. 37 (2002) 4947–4971. [9] M. Zhou, A. Ahmad, Synthesis, processing and characterization of calcia-stabilized zirconia solid electrolytes for oxygen sensing applications, J. Mater. Res. Bull. 41 (2006) 690–696. [10] B. Reddy, M. Patil, Promoted zirconia solid acid catalysts for organic synthesis, J. Curr. Org. Chem. 12 (2008) 118–140. [11] K. Lin, C. Lin, Interfacial reactions between Ti-6Al-4V alloy and zirconia mold during casting, J. Mater. Sci. 34 (1999) 5899–5906.
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L. Xu and H. Lei [38] P.Z. Camila, S.D. Kiara, P.R. Marília, K.R.P. Gabriel, C.B. Marco, F.V. Luiz, Influence of finishing/polishing on the fatigue strength, surface topography, and roughness of an yttrium-stabilized tetragonal zirconia polycrystals subjected to grinding, J. Mech. Behav. Biomed. 93 (2019) 222–229. [39] Ahuja, A. Singh, Augmentation of heat transport in laminar flow of polystyrene suspensions. II. Analysis of the data, J. Appl. Phys. 46 (1975) 3417.
[40] W. Cui, Z. Shen, J. Yang, S. Wu, Rotation and migration of nanoparticles for heat transfer augmentation in nanofluids by molecular dynamics simulation, Case Studies in Thermal Engineering 6 (2015) 182–193. [41] Y. Wang, Y. Chen, F. Qi, D. Zhao, W. Liu, A material removal model for silicon oxide layers in chemical mechanical planarization considering the promoted chemical reaction by the down pressure, Tribol. Int. 93 (2016) 11–16.
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Nano-scale surface of ZrO2 ceramics achieved efficiently by peanut-shaped and heart-shaped SiO2 abrasives through chemical mechanical polishing Lei Xua, Hong Leia,b,∗ a b
School of Materials Science and Engineering, Shanghai University, Shanghai, 200444, China Research Center of Nano Science and Technology, Shanghai University, Shanghai, 200444, China
ARTICLE INFO
ABSTRACT
Keywords: Chemical mechanical polishing KH550–SiO2 Zirconia ceramic Non-spherical abrasive
Zirconia ceramics are regarded as the best development target for 5G mobile phone rear covers. However, it is necessary and urgent to improve the surface quality and processing efficiency of zirconia ceramics. Non-spherical silica abrasives were prepared by the KH550 induction method and were used in chemical mechanical polishing (CMP) of zirconia ceramics for the first time. While achieving low surface roughness of 1.9 nm, it has an efficient polishing rate of 0.31 μm/h which is superior to conventional abrasives. Silica particles are peanutshaped and heart-shaped in the scanning electron microscopy image, and its distinctive morphology provides the possibility of its excellent polishing performance. X-ray photoelectron spectroscopy analysis shows that during the CMP process, silica abrasives and zirconia ceramic undergo a solid phase chemical reaction to form ZrSiO4. At the same time, the contact wear model established in combination with the coefficient of friction indicates that the two-dimensional surface contact mode of non-spherical silica abrasives on the surface of zirconia ceramics greatly improves its mechanical effect.
1. Introduction With the development of electronic information technology, the age of 5G communication has arrived [1]. The antenna structure used in communication equipment is more complicated than 4G, and the existing antenna structure cannot meet the requirements of 5G [2–4], because 5G communication uses a wireless spectrum above 3 GHz. Therefore, in the age of 5G communication, smart phones will abandon the metal rear covers that have great shielding for 5G signals, and use inorganic non-metallic rear covers instead. Nowadays, the non-metallic rear cover is mainly made of glass or ceramic. The zirconia ceramic rear cover has a dielectric constant [5] of 3 times that of sapphire and 10 times that of tempered glass. Zirconia ceramic has good signal penetration and will be the primary choice for future mobile phone rear cover. In addition, ceramics can also be doped with some rare metals, making them more resistant to falling and abrasion than glass. Moreover, ceramics are also superior to glass in terms of color brightness and touch [6,7]. Zirconia ceramic is a kind of functional material with good compressive strength, wear resistance, corrosion resistance, excellent oxidation resistance, good thermal stability, good thermal conductivity and high dielectric constant [8–11]. However, zirconia ceramics are difficult to process due to their high hardness and wear resistance. In ∗
recent years, in order to maximize the surface brilliance of zirconia ceramics, different ultra-precision machining methods have been employed. Ma et al. [12] used a laser-assisted method combined with a conventional grinding wheel to grind zirconia ceramics to achieve ductile regime grinding with a large depth-of-cut. Yang et al. [13] processed zirconia ceramics with ultrasonic vibration assisted intermittent grinding and established a grinding force prediction model to illustrate the removal mechanism. Camila et al. [14] processed zirconia ceramics with a grinding + polishing (two-step polishing system) to obtain a smoother surface. However, the above processing methods can only make zirconia ceramic surface flatness to the micron level. Chemical mechanical polishing is a nanoscale processing method that can achieve global planarization at present. It is applicable to a wide range of applications including silicon wafers [15,16], sapphire wafers [17–19], copper [20], glass [21], silicon carbide [22,23], gallium nitride [24], yttrium aluminum garnet (YAG crystal) [25], and titanium alloy [26], etc. In order to make the surface of the zirconia ceramic more smooth and bright, and to achieve a flatness of nanometer level, the chemical mechanical polishing method is adopted. The slurry as the main polishing carrier for CMP has been the focus of research for all researchers. The slurry is primarily composed of abrasives, chemical reagents (surfactants, catalysts, oxidizing agents, etc.). As the main mechanical action of CMP, abrasives not only possess
Corresponding author. School of Materials Science and Engineering, Shanghai University, Shanghai, 200444, China. E-mail address: [email protected] (H. Lei).
https://doi.org/10.1016/j.ceramint.2020.02.108 Received 25 December 2019; Received in revised form 11 February 2020; Accepted 12 February 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Lei Xu and Hong Lei, Ceramics International, https://doi.org/10.1016/j.ceramint.2020.02.108
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unique nano-friction, but also can have solid phase chemical reaction with the polished objects. Currently, abrasives mainly include silicon oxide [18,19], cerium oxide [21], aluminum oxide [27]. Most of the abrasives used in CMP experiments are colloidal silica because of the unique hardness of soft colloidal silica abrasives and no environmental pollution. However, conventional silica abrasives have been unable to further reduce the surface roughness of polished objects, and the removal efficiency during the polishing process also appears to be low. Researchers have explored this in two aspects over the past few years. First, mainly from the improvement of the polishing properties of chemical action, Lei et al. [28] proposed modification of silica abrasives by doping with elements, the unique solid phase chemical reaction between doped elements and wafer to form a new soft surface layer. The new soft layer is then removed by mechanical grinding of the silica composite abrasives to increase the material removal rate. Yin et al. [18] produced MgAl2O4 by solid phase chemical reaction between MgO and sapphire by doping MgO into silica sol, which resulted in increasing the material removal rate. However, all of the above modified silica abrasives involve the introduction of new metal elements which will complicate subsequent processes and increase costs. In addition, the spherical morphology of silica can be changed to improve the mechanical effect of the abrasives. Liang et al. [29] used cationic Ca+ induction to prepare chain-like silica, which increased the material removal rate (MRR) by about 65% compared to spherical silica abrasives. This kind of method also has problems such as elemental contamination. In addition to the preparation of silica by metal ion induction, Xu et al. [30] prepared flower-shaped silica abrasives by double-system micro-emulsion method, and the MRR was increased by about 117%. However, this method has some limitations, such as cumbersome experimental procedures and environmental pollution, and it cannot realize industrial mass production. In this work, the method of connecting multiple spherical silica seeds was used to prepare non-spherical silica easily and conveniently. An innovative non-spherical silica abrasive was prepared by using the inducer method of KH550 silane coupling agent, and the particles were connected by more stable chemical bonds. To explore the basic mechanism of zirconia ceramic polishing, various characterization methods were carried out. The synthesized non-spherical silica was characterized by scanning electron microscopy and frictionmeter. The surface quality before and after polishing was studied by metallographic microscope and surface profiler (white light interferometer), and the polishing mechanism was analyzed by X-ray photoelectron spectroscopy and modeling.
KH550 induction method. Firstly, 375 g of 40 nm SiO2 seeds (40 wt%) were weighed, placed in a four-necked flask, heated and stirred. After 5 min, the pre-hydrolyzed KH550 mixed solution (the mixture solution was composed of KH550, ethanol and H2O, with the respective proportions of KH550: 20 wt%, C2H5OH: 72 wt%, H2O: 8 wt%) was added. Then the solution was heated, condensed and refluxed for 5 min. Subsequently, 1125 ml of deionized water was added to dilute the silica sol to a solids content of 10 wt% and to maintain reflux for 20 min. Finally, 2000 ml of 2.5 wt% silicic acid (silicic acid is newly prepared from sodium silicate through a cation exchange resin) and 160 ml of 3.0 wt% NaOH solution were added dropwise. The dropping rate was controlled and the pH was maintained at about 10. The total addition was 2.5 h to complete the reaction. At last, the non-spherical silica sol was obtained. In this experiment, in order to explore the effect of KH550 content, a series of non-spherical silica abrasives with different contents of KH550 was prepared, which were composite abrasives containing 0.0 wt%, 0.5 wt%, 0.75 wt%, 1.0 wt%, 1.25 wt%, (mass ratio of KH550 to SiO2) of KH550 respectively. The spherical silica sol compared in this paper is 0.0 wt% KH550. The experimental method is described above. 2.3. Chemical mechanical polishing tests Polishing experiments were performed by spherical and non-spherical silica sol of the same solid content (TSC = 10 wt%) using the polishing machine under the polishing conditions listed in Table 1. The polishing machine was a UNIPOL-1000S CMP machine (Shenyang Kejing Instrument Co., Ltd., China) and the polishing pad was a Rodel porous polyurethane pad. 2.4. Measurement of the coefficient of friction The coefficient of friction between the zirconia ceramic, the abrasives and the polishing pad was measured with the apparatus shown in Fig. 1. The frictionmeter is a Byes-550 machine (Bangyi Precision Measuring Instrument (Shanghai) Co., Ltd., China). The polishing machine CMP environment was simulated. A polishing pad was attached to the bottom of the liquid tank, and then a certain amount of slurry was poured. The zirconia ceramic was stuck on the bottom of the weight, and the coefficient of friction was measured by pressing it on the polishing pad. The coefficient of friction data was given directly by the instrument system. 2.5. Characterizations
2. Experimental section
The morphology of non-spherical and spherical silica abrasives was characterized by 10 kV scanning electron microscopy (SEM, JSM7500F). The surface morphology of the zirconia ceramics before and after polishing was observed by a metallographic microscope (CMM-50, Shanghai Changfang Optical instrument Co.,Ltd., China) at 50 times. The average surface roughness of the polished and unpolished zirconia ceramics was measured by Ambios Xi-100 surface profiler (Ambios Technology Corp., USA) with a resolution of 0.1 Å, a focal depth of 3.0 μm, and a working distance is 4.9 mm and the measurement area is
2.1. Chemicals The chemical reagents used in all experiments involved in this paper are as follows: γ-Aminopropyl triethoxysilane KH550 (C9H23NO3Si, Analytical Reagent, Sinopharm Chemical Reagent Co., LTD, China), ammonium hydroxide (NH3·H2O, Analytical Reagent, Sinopharm Chemical Reagent Co., LTD, China) anhydrous ethanol (C2H5OH, Analytical Reagent, Sinopharm Chemical Reagent Co., LTD, China), Silica sol (SiO2, size: 40 nm, (colloidal SiO2, ZheJiang DeLiXin Micro/ Nano Science and Technology Co., LTD, China), Sodium silicate (Na2SiO3·9H2O, JiNan DeWang Chemical Industry Co., LTD, China), cation exchange resin (Sinopharm Chemical Reagent Co., LTD, China), NaOH (Sinopharm Chemical Reagent Co., LTD, China), hydrochloric acid (HCl, Analytical Reagent, Sinopharm Chemical Reagent Co., LTD, China), deionized water (DI).
Table 1 Polishing conditions.
2.2. Preparation of KH550–SiO2 The non-spherical silica was prepared via the silane coupling agent 2
Parameter
value
1. 2. 3. 4. 5. 6. 7.
Zirconia ceramic Rodel porous polyurethane pad 60 min 180 ml/min 6 Kg 30 r/min 60 r/min
Wafer type Polishing pad Polishing time Feed rate of the slurry Load pressure Upturn speed Downturn speed
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Fig. 1. Physical device image of Frictionmeter.
94.5 μm × 94.5 μm. The element compositions of KH550–SiO2 abrasives after polishing were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermofisher Scientific China., Ltd.), and the XPS spectrum was gained by a focused monochromatized Al Kα radiation, calibration by applying exogenous C 1s signal at 284.6 eV.
spherical silica seed. The specific synthesis principle is shown in Fig. 3. KH550 is first hydrolyzed and then self-condensed for a certain period of time, and multiple hydroxyl groups appear at the end of the organic macromolecule. The hydroxyl group of the organic macromolecule reacts with the hydroxyl on the surface of the spherical silica to form a hydrogen bond and condense into a Si–O–Si covalent bond, and a certain number of epitaxial layers are formed on the surface of the silica sphere. The hydroxyl group at the outer end of the epitaxial layer is then hydrolyzed and condensed with other hydroxyl groups on the surface of the spherical silica to form a Si–O–Si covalent bond. At the same time, the amino group at the outer end of the epitaxial layer forms a hydrogen bond with the hydroxyl group on the surface of other spherical silica. So the particles will connect together. Due to the steric hindrance among the particles, the repulsive force among the particles increases, and only 2–3 spherical silica particles adhere together to form a new non-spherical silica seed. In the second step, the nonspherical seed SiO2 is further grown by silicic acid, so that the texture of composite non-spherical silica abrasives is more compact. However, when the content of KH550 is too much in the first step, the attraction between particles is further increased and larger than the steric hindrance and repulsive force between each other, forming a certain degree of agglomeration.
3. Results and discussions 3.1. Morphology of the abrasives Fig. 2 (a) shows the spherical silica abrasives (SS) after growth, and Fig. 2 (b) shows the non-spherical silica abrasives (NSKS). It can be seen from the SEM image that the boundary between the individual particles in the spherical silica sol is very clear. However, there is no boundary between 2/3 particles in the non-spherical silica sol, indicating that KH550 successfully connected 2 or 3 spherical silica particles together and new particle shapes were found: peanut shape and heart shape. 3.2. Synthesis principle of non-spherical abrasives The irregular KH550–SiO2 abrasives in the paper are an innovation based on spherical SiO2 seeds. In the first step, multiple spherical silica abrasives are bonded together by the silane coupling agent to form non-
Fig. 2. SEM images of different abrasives (a) SS abrasives (b) NSKS abrasives with corresponding magnified SEM images inset.
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Fig. 3. Preparation principle of KH550–SiO2 abrasive.
3.3. CMP performance of zirconia ceramic
zirconia ceramic polished by SS and NSKS with 0.5 wt% KH550. Among them, the original Sa of zirconia ceramic is 6.7 nm, the Sa after SS polishing is 3.2 nm, and the Sa after NSKS polishing is 2.4 nm. This demonstrates that the NSKS abrasives have higher MRR and can further improve the surface quality of zirconia ceramics. In order to further determine the optimal KH550 content and study the effect of KH550 on the polishing of zirconia ceramics, different contents were analyzed. Fig. 5 shows the effects of different KH550 content on the MRR in NSKS abrasives. It can be seen from Fig. 5 that as the KH550 content increases, the MRR of KH550–SiO2 composite abrasives gradually increases. This indicates that as the KH550 content increases, the proportion of non-spherical silica particles in the slurry increases, and the chemical action and mechanical action of the abrasives also increase, thereby increasing the polishing rate. In addition, when the KH550 content is 1.0 wt%, the MRR reached the maximum (0.31 μm/h), which is 100.0% higher than that of the spherical silica. Because the proportion of peanut-shaped or heartshaped non-spherical silica in the sol at this time reaches a peak. However, after the KH550 content exceeds 1.0 wt%, the MRR begins to decrease, The main reason is that the agglomeration of the particles destroys the dispersibility of the abrasives, some deposition occurs in
The quality of the slurry for polishing zirconia ceramic depends on material removal rate and surface roughness (Sa). Therefore, it is necessary to evaluate the CMP performance of silica composite abrasives with KH550. The MRR (μm/h) is mainly calculated according to equation (1).
MRR =
m
10 4 ST
(1)
Where m is the poor quality of zirconia ceramic before and after polishing (g), is the density of zirconia ceramic (g·cm−3, zirconia = 6.10 g·cm−3), S is the area of zirconia ceramic (cm2), T is the polishing time (h). The actual size of the zirconia ceramic rear cover of smart phone is 13.8 cm * 6.7 cm. For the convenience of the experiment, the zirconia ceramics were cut into 5.5 cm *5.5 cm. The MRR of spherical SiO2 and non-spherical KH550–SiO2 is shown in Fig. 4 (a). Under the same conditions, the MRR of NSKS with 0.5 wt% KH550 is 0.18 μm/h which is larger than that of SS abrasives. Fig. 4 (b) shows the Sa of the unpolished zirconia ceramic and the Sa of the 4
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Fig. 4. (a) The MRR of SS and NSKS, (b) the Sa of unpolished and polished by SS & NSKS (0.5 wt% KH550).
Fig. 5. The MRR of different content of KH550.
Fig. 7. The Sa of different content of KH550.
the solution, and the number of effective abrasives is greatly reduced, which affects the MRR. Fig. 6 shows the state of the final sample of each content of KH550–SiO2 silica sol after standing for a period of time. It can be found that the content of KH550 is more than 1.0 wt%, the transparency degree of the silica sol is significantly reduced, and a small
amount of precipitation appears, which proves that excessive amount of KH550 indeed lead to agglomeration of abrasives. The Sa of different KH550 content abrasive polished zirconia ceramic is shown in Fig. 7. With the increase of KH550 content in the range of 0.0 wt% ~1.0 wt%, Sa of the zirconia ceramic gradually
Fig. 6. Final state of different content of KH550–SiO2 silica sol after standing for a period of time.
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Fig. 8. Micrograph of the surface of zirconia ceramics before and after polishing at 50 times: (a) unpolished, (b) after polishing by SS abrasives, (c–f) after polishing by NSKS abrasives containing 0.5 wt%, 0.75 wt%, 1.0 wt%, 1.25 wt%, respectively.
polishing with various KH550–SiO2 abrasives. As shown in Fig. 9, the polished surfaces (Fig. 9b–f) are smoother than the unpolished surface (Fig. 9a). From Fig. 9(b–e), the surface roughness gradually decreases with the increase of KH550 content, and the roughness rises again due to the scratches in Fig. 9 (f). The roughness is the smallest when the content is 1.0 wt%. In conclusion, when the KH550 content is 1.0 wt%, the polishing performance of the KH550–SiO2 composite abrasives is the best (MRR is the largest and Sa is the smallest).
decreases, and reaches a minimum of 1.9 nm. This indicates that with the increase of KH550, the increase of non-spherical silica abrasives in the slurry can further reduce the surface roughness of the zirconia ceramic. It may be due to the fact that the peanut-shaped or heartshaped non-spherical silica abrasives have more contact with the zirconia ceramic and is able to remove the rough peaks and holes on the zirconia ceramic surface. In addition, when the KH550 content is over 1.0 wt%, the Sa increases. Because when the content of KH550 is more than 1.0 wt%, the abrasives are agglomerated, and the appearance of large particles tends to cause scratches on the surface of the zirconia ceramic, thereby affecting Sa. According to the results of Fig. 5 and Fig. 7, the optimal content of KH550 was 1.0 wt%. The surface morphology of the zirconia ceramics measured by the metallographic microscope is shown in Fig. 8. As can be seen from Fig. 8 (a), the surface of the zirconia ceramic before polishing has a large number of holes, and the size of the holes is not uniform, and the large-diameter holes occupy the main body. Fig. 8 (b) shows that after polishing experiment, the number of large-diameter holes on the surface of zirconia ceramics drops sharply. Fig. 8(c–e) show that with the increase of KH550 doping amount, the number of large-diameter holes is reduced, and the small-caliber holes begin to dominate until the latter gradually disappears, indicating that the surface flatness has been definitely improved. The main reason is that the mechanical and chemical actions of the abrasives are enhanced as the relative amount of irregular silica abrasives increases. When the abrasives are in contact with the holes, the large-diameter holes are in contact with a larger number of particles, so that the grinding effect is larger, and the large-diameter holes are preferentially reduced and disappeared. In turn, the removal efficiency of the zirconia ceramic is improved, which further illustrates that the MRR is constantly increasing. As shown in Fig. 8 (f), scratches appear on the surface of the zirconia ceramic when the doping amount reached 1.25 wt%. Because the particles agglomerate to form large particles when the content of KH550 increases. Furthermore, large particles have a significant wear and tear on the surface of the zirconia ceramic, which can cause scratches on the surface. Fig. 9 shows the 3-D surface profile of the zirconia ceramic after
3.4. CMP mechanism of NSKS The CMP mechanism of NSKS abrasives on zirconia ceramics was studied from both chemical and mechanical aspects. It has been known from previous studies that silica is an acid oxide and has a certain solid phase chemical reaction with other oxides [25,28]. Therefore, it is speculated that a solid phase chemical reaction may also occur between silica and zirconia ceramic. It is well known that XPS is often used to measure changes in the valence of elements during CMP to characterize the solid phase chemical reactions that occur between the abrasives and the wafer [23,28]. At present, there is no CMP analysis on zirconia ceramics, so XPS may be used to investigate the possible chemical reactions during polishing. Fig. 10(a–c) are narrow scan spectra of the elements in the particles after 1.0 wt% NSKS abrasives polishing. Fig. 10 (a) shows that the peak of Si 2p is wider, indicating that there are multiple chemical states. There are three fitting curves in Fig. 10 (a). The binding energy (BE) of 103.59 eV [31] and 102.98 eV [32] separately corresponds to Si 2p in SiO2 state and -Si(CH2)3NH2 state, and the peak of another binding energy of 104.04 eV corresponds to Si 2p in ZrSiO4 state. Fig. 10 (b) shows the spectrum of O 1s. O 1s has three peaks, the binding energy of 532.79 eV [33] and 533.39 eV [34] correspond to the O 1s of the SiO2 state and the O 1s of the ZrO2 state, respectively. The binding energy of 531.84 eV [35] corresponds to O 1s in the bond of Zr–O–Si of ZrSiO4 state. Fig. 10 (c) shows a broad double peak of the Zr 3d, where the binding energy of 181.63 eV and 178.43 eV separately [36] correspond
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Fig. 9. Surface profiles and parameters (inset) of zirconia ceramics: (a) unpolished, (b) after polishing by SS abrasives, (c–f) after polishing by NSKS abrasives containing 0.5 wt%, 0.75 wt%, 1.0 wt%, 1.25 wt%, respectively.
to the Zr 3d in ZrO2 state. The binding energy of 180.65 eV and 176.56 eV separately [37] correspond to the Zr 3d in the ZrSiO4 state. Therefore, according to the XPS analysis of Si, O and Zr above, it can be explained that the silica and the zirconia ceramic have a solid phase chemical reaction, and the resulting reactant exists in the form of ZrSiO4. The main reaction equation is shown in equation (2). ZrO2 + SiO2= ZrSiO4
spherical silica abrasives and zirconia ceramics, a single spherical and non-spherical abrasive is taken as an example to establish a contact wear model of silica abrasives and zirconia ceramics. The mechanical action mechanism during the CMP process is shown in Fig. 11. It can be seen from Fig. 11 that the contact between the SS abrasive and the zirconia ceramic is mainly single point, while the contact between the NSKS abrasive and the zirconia ceramic is mainly multi-point contact due to the need for balance. This view is also explained in our last study [30]. In the actual polishing process, the nanoparticles have high-speed random rotation and translational motion in the flowing base fluid [39,40]. Thus, for SS abrasives, the particles first rotate in solution and then come into point contact with the wafer and undergo a certain translation, the resulting contact being a one-dimensional line contact. However, for NSKS abrasives, the particles rotate around their
(2)
Camila et al. [38] have put forward that during the polishing process, an instantaneous high temperature is generated at the moment of contact between the abrasives and the wafer. This high temperature provides a possibility for solid phase chemical reactions between SiO2 and ZrO2. In order to more vividly illustrate the contact wear between non-
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Fig. 10. The XPS narrow scan spectra of element: (a) Si, (b) O, (c) Zr from 1.0 wt% NSKS silica abrasives after polishing.
own axis in solution, because the contact with the wafer is multi-point contact, so the contact between the single spin NSKS abrasives and the wafer becomes one-dimensional coil contact. Then, the translation occurs, and the contact formed is a two-dimensional surface contact. Therefore, the contact area produced by the NSKS abrasives is much larger than the contact area produced by the SS abrasives. According to
the MRR equation (3) of established by Wang et al. [41].
MRR =
Vp D0 2rc
e
PS = Fs/ AC , FS =
1 (1 2v ) P + Cfb E1 1 s 2 10
1/2 Vd / Ro T
(3)
F , C = 3 (4 + v1)/8(1 µ
2v1)
Where Vp: the speed of abrasive, D0 : zero stress diffusion coefficient, rc : contact radius between the abrasive particles and the wafer, v1: Poisson's ratio of the wafer, Ps: average contact pressure between abrasive particles and wafer, Fs : Single abrasive force, AC : contact area between the abrasive particles and the wafer, µ: the coefficient of friction, C: a dimensionless parameter, fb : the fraction of the contact area where bonding occurs (~1%), E1: Young's modulus of the wafer, Vd: diffusion activation volume, Ro : gas constant, T: absolute temperature. It can be found that the MRR is proportional to the contact area, which further indicates that the polishing rate of the NSKS abrasives is higher than that of the SS abrasives. At the same time, the increase of the contact area will also accelerate the chemical reaction between substances. According to equation (3), the MRR is proportional to the coefficient of friction μ between the abrasives and the wafer. Therefore, the coefficient of friction (μs: coefficient of static friction, μk: dynamic friction factor) of silica with different KH550 contents was measured, and the results are shown in Table 2. It can be found that as the KH550 increases, the static/dynamic coefficient of friction of the silica sol is increasing, indicating that the relative amount of NSKS abrasives is increasing in the solution. And with the increase of the coefficient of friction, MRR keeps increasing, which is consistent with our previous experiments. Therefore, the polishing performance of non-spherical abrasives is better than that of spherical abrasives.
Fig. 11. Contact wear model of silica abrasives and zirconia ceramic. 8
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Table 2 Coefficient of friction of silica with different KH550 content. KH550–SiO2
0.0 wt% KH550–SiO2
0.5 wt% KH550– SiO2
0.75 wt% KH550– SiO2
1.0 wt% KH550– SiO2
1.25 wt% KH550– SiO2
μs μk
0.289 0.250
0.328 0.291
0.335 0.304
0.420 0.357
0.441 0.401
4. Conclusion
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In this study, chemical mechanical polishing of zirconia ceramics was first carried out using novel non-spherical silica abrasives. Compared with the commonly used spherical silica sol, the non-spherical silica sol can remove holes on the surface of zirconia ceramics to achieve nanometer-scale surface roughness. Its MRR is increased by 100.0% compared to conventional silica sol. The synthesis principle of KH550–SiO2 abrasives was discussed. Through the action of the covalent bond and the hydrogen bond, a stable and non-polluting nonspherical silica sol was obtained. The polishing mechanism of the new type of non-spherical abrasives and ceramic wafer was proposed, and the solid phase chemical reaction between non-spherical silica sol and zirconia ceramic during CMP was demonstrated for the first time. At the same time, the coefficient of friction of non-spherical silica sol in CMP system was measured for the first time to demonstrate its strong mechanical properties. Moreover, the two-dimensional surface contact formed by non-spherical abrasives in the CMP process not only accelerates the chemical reaction, but also improves the actual contact area between the abrasives and zirconia ceramics, thus improving the overall MRR. The developed non-spherical silica sol provides a more convenient and efficient polishing method for the processing of 5G mobile phone ceramic rear cover. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Number 51975343). References [1] W.H. Chin, Z. Fan, R. Haines, Emerging technologies and research challenges for 5G wireless networks, IEEE Wirel. Commun. 21 (2014) 106–112. [2] E. Hossain, M. Hasan, 5G cellular: key enabling technologies and research challenges, IEEE Instrum. Meas. Mag. 18 (2015) 11–21. [3] T.S. Rappaport, S. Shu, R. Mayzus, H. Zhao, Y. Azar, K.P. Wang, G.N. Wong, J.K. Schulz, S. Mathew, F. Gutierrez, Millimeter wave mobile communications for 5G cellular: it will work!, IEEE Access 1 (2013) 335–349. [4] T.S. Rappaport, F. Gutierrez, E. Ben-Dor, J.N. Murdock, Q. Yijun, J.I. Tamir, Broadband millimeter-wave propagation measurements and models using adaptivebeam antennas for outdoor urban cellular communications, IEEE Trans. Antenn. Propag. 1 (2013) 1850–1859. [5] J. Huang, L. Huang, M.C. Tsai, M.H. Lee, M.J. Chen, Enhancement of electrical characteristics and reliability in crystallized ZrO2 gate dielectrics treated with insitu atomic layer doping of nitrogen, Appl. Surf. Sci. 305 (2014) 214–220. [6] V. Hlavacek, J.A. Puszynski, Chemical engineering aspects of advanced ceramic materials, J. Ind. Eng. Chem. Res. 35 (1996) 349–377. [7] P. Li, I.W. Chen, P.H. James, Effect of dopants on zirconia stabilization-an X‐ray absorption study: I, trivalent dopants, J. Am. Ceram. Soc. 77 (1994) 118–128. [8] M.H. Bocanegra-Bernal, S.D. De la Torre, Phase transitions in zirconium dioxide and related materials for high performance engineering ceramics, J. Mater. Sci. 37 (2002) 4947–4971. [9] M. Zhou, A. Ahmad, Synthesis, processing and characterization of calcia-stabilized zirconia solid electrolytes for oxygen sensing applications, J. Mater. Res. Bull. 41 (2006) 690–696. [10] B. Reddy, M. Patil, Promoted zirconia solid acid catalysts for organic synthesis, J. Curr. Org. Chem. 12 (2008) 118–140. [11] K. Lin, C. Lin, Interfacial reactions between Ti-6Al-4V alloy and zirconia mold during casting, J. Mater. Sci. 34 (1999) 5899–5906.
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