Transformation of silica fume into chemical ...

Powder Technology 205 (2011) 149–154

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Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c

Transformation of silica fume into chemical mechanical polishing (CMP) nano-slurries for advanced semiconductor manufacturing M.M. Rashad a,⁎, M.M. Hessien a, E.A. Abdel-Aal a, K. El-Barawy a, R.K. Singh b a b

Central Metallurgical Research and Development Institute, P.O. Box: 87 Helwan, 11421, Egypt Materials Science and Engineering Dept, UF, Gainesville, FL32611, USA

a r t i c l e

i n f o

Article history: Received 4 August 2009 Received in revised form 15 August 2010 Accepted 4 September 2010 Available online 15 September 2010 Keywords: Silica nanopowders Chemical processing Size distribution Chemical mechanical polishing Advanced semiconductors

a b s t r a c t Silica nanoparticles have been synthesized from silica fume using alkali dissolution–precipitation process. The dissolution efficiency of 99% at a temperature of 80 °C and a time of 20 min was achieved. Sodium silicate solution was obtained by dissolving the fume with NaOH solution. Then, silica nanoparticles were precipitated using sulfuric acid. Silica nanoparticles (175 nm) were achieved using 12% sulfuric acid at pH 7 and 200 ppm sodium dodecyl sulfate (SDS). The silica morphologies appeared as a spherical shape with narrow particle size distribution. The silica samples were used for the formulation and testing of chemical mechanical polishing (CMP) slurries. The morphology of the polished wafer surface and its roughness were examined by atomic force microscope (AFM).The results indicated that the surface roughness was greatly improved after application of CMP. It was found that the surface roughness of the polished wafer is 0.226 nm at an applied pressure of 7 psi. The removal rate was found to be 1200 Å. These values confirm the quality of polished wafers. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Microelectronic circuit densities have increased 104 times in the last 20 years [1]. Local and global planarization techniques have become key technologies for the fabrication of high density integrated circuits. Chemical mechanical polishing (CMP) has emerged as preferred manufactured process for eliminating topographic variations and achieving wafer-level global planarization in ultra-large scale integrated (ULSI) circuits with 108 or more devices on a chip. Furthermore, CMP was used for semiconductors substrates, or wafers, to reduce cost and to increase the performance of electronic product [2–4]. As semiconductor chips are highly integrated and multi-layer electro-wired, more precise planarization of each layer on chips is needed. With decreasing of device dimensions, interlevel dielectric (ILD) planarization by CMP is necessary for technologies beyond the 0.35 μm complimentary metal oxide semiconductor generation [5]. CMP process typically monitors polish rate, planarization rate, and surface quality, all of which are affected in some way by the chemicals used in the slurry. CMP performances can be optimized by several process parameters such as equipment and consumables. Among the consumables for the CMP process, pad and slurry, especially, play very important roles in the removal rate (RR) and within-wafer nonuniformity (WIWNU) for global planarity of the CMP process [6].

⁎ Corresponding author. Tel.: +20 202 25010642; fax: +20 202 25010639. E-mail address: [email protected] (M.M. Rashad). 0032-5910/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2010.09.005

However, the most critical problems of the CMP process is the higher cost of the consumables (COC) such as slurry, pad, backing film and pad conditioner, which is over 70% of cost of ownership. Since a sufficient amount of slurry is required to get a higher removal rate and lower non-uniformity, the purchase of the slurry is about 50% of COC for a typical CMP process [7–11]. The market for CMP slurries used in semiconductor operations exceeded $1.1 billion in 2005 and will grow to over $1.9 billion by 2009 [12]. In the world, the amount of silica fume generated from ferrosilicon alloys and silicon metal industries is estimated as greater than 1,000,000 tons [13]. About 25,000 tons of silica fume are produced each year by the Egyptian Ferroalloys Co. (EFACO) from its ferrosilicon plant located in Edfu City. About 20,000 tons of the produced silica fume is exported and 5000 tons are used locally in special concrete manufacturing. Silica fume, also known as silica dust or microsilica, is a byproduct of carbothermic reduction of quartz and quartzite in electric arc furnaces of silicon and ferrosilicon alloy production. In general, silica fume contains 85–95% SiO2 with very fine vitreous particles and is produced in large amounts ranging from 5 to 30 wt.% of total input quartz [14]. The main objectives of the present work are to develop the technology and to increase the complexity of circuits and memories. Greater emphasis is being placed on chemical mechanical polishing (CMP), to reduce the cost of semiconductor manufacturing. Moreover, this study attempts to utilize a cheap material (i.e., silica fume byproduct) to form CMP slurries. The study answers the most challenging technical question: can the silica fume byproduct be utilized to form high quality CMP slurries which can be used for defect-free polishing of films for semiconductor manufacturing?

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Table 1 Chemical analysis of original silica fume.

Table 2 Dissolution efficiency of silica fume according to experimental design conditions.

Constituent

%

Constituent

%

SiO2 CaO TiO2 K2O MnO Relative density, g/cm3

96.18 0.32 0.004 0.16 0.035 2.13

Al2O3 Fe2O3 P2O5 Na2O MgO L.O.I, 1000 °C

0.51 0.23 0.004 0.011 0.015 2.08

Run no.

L.O.I: loss on ignition.

2. Experimental Silica fume sample was provided from Egyptian Ferroalloys Co. (EFACO). The chemical analysis of the original sample was carried out by XRF and the results are given in Table 1. The results showed that the original sample contains mainly of SiO2 (96.18%) and low contents of impurities. The relative density of silica fume was 2.13 g/cm3, which was slightly lower than the silica fume from different companies around the world (silica fume density 2.20–2.25 g/cm3). XRD analysis was carried out on the original silica fume. The X-ray diffraction pattern is shown in Fig. 1. The results obtained indicate that amorphous silicon dioxide is the main phase in the silica fume sample. The original silica fume was also investigated by a scanning electron microscope (SEM). The results obtained are given in Fig. 2. The results showed that silica fume particles are agglomerated in an almost spherical shape and are mainly inhomogeneous which makes these particles not suitable to prepare homogenous and stable slurries for CMP applications. The agglomerated particles consist of ultra-fine 100

80 70 60 50

Time, min.

Stoichiometry

60 60 80 80 60 60 80 80 70 70 70 70 70 70 70

10 20 10 20 15 15 15 15 10 10 20 20 15 15 15

1.5 1.5 1.5 1.5 1 2 1 2 1 2 1 2 1.5 1.5 1.5

% Dissolution Efficiency 40.8 45.9 91.8 99.9 30.6 76.5 81.6 86.7 40.8 76.5 51.0. 86.7 56.1 61.2 61.2

particles and they have wide size range from less than 1 μm to about 50 μm. Therefore it is necessary to separate a size b200 nm and not larger than 400 nm for chemical mechanical polishing (CMP) slurries. The specific surface area of the silica fume particles was determined by means of the Brunauer Emmett and Teller (BET) techniques. The BET specific surface area was calculated from nitrogen adsorption data in the relative pressure range from 0.04 to 0.2. Silica fume consists mainly of fine vitreous particles with a surface area 20 m2/g. The dissolution of silica fume was carried out using alkali leaching process by sodium hydroxide. The effect of the three reaction variables (time, temperature, and NaOH: SiO2 stoichiometric ratio) on the dissolution efficiency of the silica in silica fume was determined and presented in Table 2. The optimum conditions using the experimental design have been studied. Experimental design using the Box–Behnken method (15 experiments) was designed at variables shown in Table 1. Plots of the response surface, contours, and the best predictive models for the estimate of the response variable were developed. The Box–Behnken design can fit the following model [15], and [16]

40 3

30

i=1

20 10 0 10

3

3

3

2

EðyÞ = β0 + ∑ βi xi + ∑ ∑ βij xi xj + ∑ βii xi

20

30

40

50

60

2 Theta (degree)

70

i=1 j=1

i=1

where E(y) is the sum of the response variable and y is the estimate of the response variable (particle size mean diameter) and xi's are the independent variables (sodium silicate concentration, surfactant

Fig. 1. XRD pattern of silica fume sample.

40 35

Intensity, count/sec

Intensity (Count/sec)

90

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Factors Temperature, °C

30 25 20 15 10 5 0 10

20

30

40

50

60

70

2 Theta, degree Fig. 2. SEM micrographs of original silica fume sample.

Fig. 3. XRD pattern of the produced supercritical dried silica nanoparticles with addition of 200 ppm SDS.


151

Table 3 Optimum conditions of the precipitation of silica gel. Item pH at 12% H2SO4, 200 ppm SDS; 4 7 10 SDS, ppm surfactant at 12%H2SO4 and pH 7 Without 100 200 Sulfuric acid conc.,% at pH 7 and 200 ppm SDS 4 12 20

Surface area, m2/g 187 338 166 136 230 338 272 338 259

concentration, and sulfuric acid, %) that are known for each experimental run. Parameter β0 is the model constant, βi is the linear coefficient, βii is the quadratic coefficient and βij is the cross-product coefficient. The quality of fit of the polynomial model equation was expressed by the coefficient of determination R2. Software package, Design Expert 6.1, Stat-Ease, Inc., Minneapolis, USA, was used for regression analysis of experimental data and to plot response surface. Analysis of variance (ANOVA) was used to estimate the statistical parameters. The extent of fitting the experimental results to the polynomial model equation was expressed by the determination coefficient R2. F-test was used to estimate the significance of all terms in the polynomial equation within 95% confidence interval. Experiments were performed in triplicate and the mean values were given. The statistical design shows that the dissolution efficiency of 99.9% was achieved at a temperature of 80 °C for a time period of 20 min at NaOH:SiO2 molar ratio 1.5 with a standard deviation of 7.27%. The determination coefficient (R2 0.9508) indicates an agreement between the generated model and the experimental results. The precipitation of silica from the produced sodium silicate solution was carried out using sulfuric acid. The sodium silicate solution was stirred at 700 rpm to obtain a homogeneous solution. Then, sulfuric acid was added gradually to the solution to pH values from 4 to 10. The formed silica gel was aged for 24 h then stirred with a glass rod. After stirring, the precipitated silica gel was filtrated and washed with deionized water to remove sodium ions, until the filtrate is free from sodium ions. The effect of pH, sulfuric acid concentration, and sodium dodecyl sulfate (SDS) surfactant concentration on the precipitation of silica were systematically studied. Silica gel samples were dried using a supercritical CO2 dryer (Parr Co). Supercritical drying (SCD) was chosen as it reduces particle agglomeration by removing the solvent from the particles without the formation of capillary forces. For SCD, the samples were solvent-exchanged into isopropanol (Fisher) and concentrated via centrifugation (Beckman BH-2) at 1000 rpm. This process was repeated 3 times. For the SCD

Fig. 5. TEM micrographs of supercritical dried silica samples with addition of 200 ppm SDS.

process, slurries were poured into dialysis bags and sealed. The samples were placed in a home designed supercritical drier pressure reactor (Parr Instruments) and the solvent was exchanged completely with liquid CO2 (Air Gas Inc.). After complete solvent removal, the liquid CO2 was heated to a supercritical state and slowly released at 4 L/min. The produced silica gel was well suspended in deionized water and the silica particles were characterized by transmission electron microscope (TEM, JEOL-JM1230) for observation of the particle morphologies. The mean particle size was determined from image analysis software The sample obtained at optimum conditions was subjected to different characterization using X-ray diffraction analysis (BRUKER X-Ray diffractometer), X-ray fluorescence (XRF) (PANalytical 2005, The Netherlands), and surface area SEBT examination (model NOVA 2000 series). The CMP slurries of silica gel were dispersed in 100 mL deionized water with a different concentration of 2%. The pH of the slurries was adjusted at 10.5–11 with dilute KOH. The removal rate of the wafer after silica chemical mechanical slurry was determined using multi wave length Ellipsometer Control Module EC 110.

3. Results and discussion To transform the silica byproduct into CMP slurry, characterization of the silica byproduct, chemical dissolution of silica and controlled precipitation in the nanosize, slurry formulation, CMP testing and performance measurements have to be performed.

-40

zeta potential,mV

-45 -50 -55 -60 -65 -70

0

2

4

6

8

10

pH Fig. 4. TEM micrographs of supercritical dried silica samples without SDS addition.

Fig. 6. Zeta potential of silica sample 200 SDS addition.

12

152


Table 4 Typical process conditions of CMP. Struers, Tegra Force — 5 20–25 °C 1 in.2 10.5–11 2% 50 mL/min 80 rpm 2, 3 and 5 and 7 psi 1 C 1000 pad 1 min Flat

Polisher type Polishing pad temp. Wafer Slurry pH Wt% silica in slurry Slurry flow rate Platen speed Pressures Polishing pad Polishing time Wafer carrier type

3.1. Characterization of the produced silica Fig. 3 represented XRD analysis of the produced silica. It observed that the produced silica was amorphous. The building block of silica is the SiO4 tetrahedron, four oxygen atoms at the corners of a regular tetrahedron with a silicon ion at the central cavity or centroid. The oxygen ion is much larger than the Si4+ ion that the four oxygen atoms of a SiO4 unit are in mutual contact and the silicon ion is said to occupy a tetrahedral hole. In amorphous silica, the bulk structure is determined by random packing of [SiO4]4− units, which results in a nonperiodic structure [12]. Table 3 shows the correlations between the silica precipitation parameters (sulfuric acid conc., pH and SDS conc.) and the surface area of the obtained dried silica gel. The results indicated that the optimum conditions required to achieve the highest surface area of silica particles of 338 m2/g were pH 7, SDS concentration of 200 ppm and sulfuric acid concentration of 12%. The increase in the specific surface area at smaller particle size makes the smaller silica nanoparticles suitable also for applications such as fillers in advanced composites, electronic applications and also catalyst [12]. It was clear that the surface area, i.e., the particle size of the silica was decreased by increasing SDS concentration from 0 to 200 ppm. According to the theory of homogeneous nucleation, the nucleation rate increased as the interfacial surface tension decreased. As the surfactant dramatically lower the surface tension, their presence in solution strongly increases the nucleation rate, and the particle size decreased. This means that SDS, as surfactant, is playing an important role in modifying the particle. Addition of SDS surfactant decreases the nucleation rate and increases the growth rate through networking of formed nuclei of the produced silica particles. High nucleation rate means that a high number of formed nuclei are obtained. These nuclei have a relatively low chance to grow to large particles compared to lower number of formed nuclei which grow under the same conditions. However, at high surfactant concentration, the surfactant molecules form micelles, therefore, a high attraction force was obtained [16].

Transmission electron microscope (TEM) micrographs were used to determine the shape and the particle size of the produced silica. Figs. 4 and 5 showed TEM micrographs of the supercritical dried silica samples without SDS, and with 200 ppm SDS, respectively. TEM micrographs of silica sample without addition of SDS showed large agglomeration and non-regular shape. Moreover, the particle size was 20–30 nm. On the other hand, TEM micrographs with 200 ppm SDS show some agglomeration, even after the silica gel was supercritical dried. The figures show that the produced silica has narrow size distribution (particle size ranges from 10 to 15 nm) with spherical shape. This indicates that the addition of SDS surfactant was essential to prohibit agglomeration and to attain a uniform spherical shape. The variation of the zeta potential with increasing pH of the slurry had a significant effect on the removal of particles from the wafer surface during CMP process. Zeta potential of silica gel produced using 200 ppm SDS was measured at different pH values. Fig. 6 shows that the negative charges on the particles decreased gradually with increasing the pH value up to pH 10. The zeta potential depends on the pH of the chemical solution used for CMP. However, the particle level on the wafer surface was reduced with increasing pH. The zeta potential is modified by the ionic strength. Absolute value of zeta potential reduced as the ionic strength reduced. The electrostatic interactions between the wafer and the particles were eliminated at a smaller distance in case of a thin double layer (high ionic strength) which led to a better removal efficiency. 3.2. Formulation and testing of CMP slurries The prepared silica gel was tested for CMP for silica wafer. The polisher is a rotating platen that carries the polishing pad. The wafer is pressed face down on a rotating pad by the carrier and the slurry is injected close to the center of the pad, from where the centrifugal force spreads it all over. Balance should be attained between removal rate and planarization through a combination of solution chemistry, speed, applied pressure and pad properties. The experimental conditions of the polishing process are listed in Table 4. After the polishing has been completed, the polished wafer surface was washed with deionized water. The morphology of the polished wafer surface and its roughness were examined by atomic force microscope (AFM). For calculation of the removal rate which refers to, wafer thickness that measured at three points before and after polishing using the multi wave length ellipsometer. Figs. 7 and 8 show the removal rate after polishing versus pressure psi. The results indicate that the high pressure and high wt.% of silica in the slurry led to higher removal rates. Moreover, the results indicated that the removal rate increased with increasing the pressure. The maximum average removal rate of 1200 Å was achieved at a high pressure of 7 psi.

1400 1200

Removal Rate, Ao/min

5 % silica slurry

Removal Rate (A°)

1000 800 600 400 200

1200 1000

2 % slica slurry

800

1 % silica salurry

600 400 200

0 2

3

4

5

6

7

8

Pressure (psi) Fig. 7. Removal rate of the polished wafers at different applied pressures using 2% silica slurry.

0 2

2.5

3

3.5

4

4.5

5

5.5

Pressure, Psi Fig. 8. Removal rate of the polished samples using different silica slurries with 200 ppm SDS addition.


153

Section B 5

4

Section A

2

3

0

2

nm

µm

1

-1 1 -2 0 0

1

2

3

5

4

µm 120nm

Height

80

0.4

40

0.0

0

-0.4

-40 0

1

2

3

4

5

0

1

2

3

µm

µm

A Section analysis

B Section analysis

Fig. 9. AFM images of non-polished wafer.

1.0

0.6

0.5

µm

0.8

0.0 0.4

nm

1.0

-0.5 0.2 -1.0 0.0 0.0

0.2

0.4

0.6

0.8

1.0

µm

400pm

Height

Height

0.8nm

0 -400 0.0

0.2

0.4

0.6

0.8

1.0

µm Fig. 10. AFM images of the polished wafer using 2% silica gel slurry at pressure of 7 psi.

1.2

4

5


60

0.76

55

0.75

50

0.74

45

0.73

40

0.72

35

0.71

30

0.7 1.5

2

2.5

3

3.5

4

4.5

5

0.8

Roughness RMS (nm)

0.77

Roughness Rmax , nm

Roughness RMS, nm

154

0.6

0.4

0.2

0

25 5.5

2

3

4

Pressure, Psi Fig. 11. Surface roughness for different pressures using 1% dispersed silica slurry.

4. Conclusions Silica nanoparticles have been prepared through dissolution– precipitation process from silica fume provided from ferrosilicon industries. Silica fume was dissolved in sodium hydroxide to give sodium silicate solution. The produced solution was precipitated using sulfuric acid at pH 7. The results are summarized as: • The optimum conditions of the dissolution efficiency of the silica of about 99.9% was achieved at 80 °C for 20 min., and NaOH/SiO2 molar ratio 1.5 to 2. • Nanosize silica particles were completely precipitated at pH 7 using 12% sulfuric acid in the presence of 200 ppm SDS. Silica precipitated in a gel form was washed till free from sodium ions and supercritical dried. • The ultra-fine dried samples were characterized using TEM, surface area, and zeta potential techniques. It was found that the particles were spherical and the sizes were 10–30 nm. • High surface area (338 m2/g) can be achieved for the dried silica sample at optimum precipitation conditions. • The surface of the wafer after planarization using 2% silica slurry was investigated by atomic force microscope (AFM). It was found that the surface roughness of the polished wafer is 0.226 nm of an applied pressure of 7 psi. The removal rate was found to be 1200 Å. These values confirm the quality of polished wafers.

6

7

8

6

7

8

60

Roughness Rmax (nm)

AFM investigation was carried out to study the surface of the wafer after polishing. The wafer was scanned with an area of 5 μm × 5 μm using contact mode AFM. Atomic force microscope images of polished wafer using 1% silica slurry at pressures 3, 5 and 7 psi are shown in Figs. 9–12. Roughness Rmax (maximum peak-valley) and roughness root mean square (RMS) determined from atomic force microscope (AFM) were plotted against the down pressure. The roughness values are not correlative as deposits of the particles on the wafer surface change the roughness values significantly. In other words, surface roughness was greatly improved after application of CMP using formulated silica slurries. The results indicated that the surface roughness was greatly improved after application of CMP using formulated silica slurries. The average RMS and Rmax of non-polished wafer sample were 17.1 nm and (maximum 150 nm, minimum − 53 nm) respectively. However, the average RMS and Rmax of the polished wafer sample are 0.226 nm and (maximum 1.3 nm, minimum −1.2 nm) respectively using 2% silica gel slurry polished at a pressure of 7 psi.

5

Pressure (psi)

50 40 30 20 10 0 2

3

4

5

Pressure (psi) Fig. 12. Surface roughness at different pressures using 2% silica gel with 200 ppm SDS addition.

Acknowledgement This work was supported by the USA–Egypt Joint Scientific Program “ENV 9004005” project.

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