JP7692008B2 - Fused spherical silica powder - Google Patents

Description

The present invention relates to a new fused spherical silica powder and a method for producing the same. More specifically, the present invention relates to a fused spherical silica powder with a low bubble content that is suitable for use as a filler for semiconductor encapsulation, and a method for producing the same.

Silica is used for a variety of purposes, one of which is as a filler in semiconductor encapsulation materials. When used as a filler in semiconductor encapsulation materials, it is required to have high thermal conductivity and low thermal expansion in addition to electrical insulation, and in order to satisfy these physical properties, it is desirable to increase the filler loading.

To achieve high packing properties, it is better for the filler to have a certain degree of particle size distribution rather than a uniform particle size. Furthermore, larger particle sizes tend to result in a higher packing rate.

At the same time, high moldability is required, and the flowability, i.e., low viscosity, of the resin filled with the filler (temperature: 25°C, shear rate: 1s ^-1 , viscosity: 1000 Pa s or less) is desired. In order to obtain such flowability, in recent years, spherical silica is usually used as a filler.

Furthermore, as semiconductors continue to become thinner and more miniaturized, and as large quantities of semiconductors are encapsulated at the wafer level, the maximum allowable particle size of the filler is becoming smaller, making it difficult to achieve high packing levels, and there is an increasing need to maintain a high packing rate of the filler.

Fused silica is superior to silica produced by other methods in terms of its packing properties due to its spherical shape and moderate particle size distribution, and in terms of its relatively low manufacturing costs.

Known methods for producing fused silica include (1) a method of melting and oxidizing silicon powder, (2) a method of melting fine silica powder in a flame and fusing multiple fused particles to grow and become spherical, and (3) a method of burning and oxidizing a compound containing silicon atoms in a flame to produce fine silica, which is then melted in the flame as is and allowed to grow and become spherical through the fusing of fused particles. For example, in Patent Document 1, fine silica particles obtained by burning an organic silane compound are further grown in a flame to obtain fused spherical silica powder with an average particle size of 0.05 to 5 μm. In Patent Document 2, fumed silica is melted in a flame to obtain fused silica containing a large amount of particles with a particle size of 3 μm or less.

JP 2003-137533 A JP 2000-191316 A

By the way, if bubbles exist inside the silica particles, in the semiconductor manufacturing process, when the sealing material portion is cut or ground after sealing the semiconductor, the silica particles filled in the sealing material are cut or ground, and the bubbles (voids) inside the particles are exposed. For this reason, recesses may occur on the cut surface or ground surface of the sealing material. FIG. 1 shows a schematic plan view of the ground surface of the sealing body. Silica particles are usually dense particles 1 and do not have voids on the ground surface, but in hollow particles 2 having bubbles inside the particles, the bubbles are exposed by grinding, and voids 3 occur on the ground surface. FIG. 2 shows a cross-sectional view of line A-A in FIG. 1. As shown in FIG. 2, the voids 3 on the ground surface become recesses. The occurrence of such recesses is particularly problematic when manufacturing semiconductor products by FOWLP (Fan-Out wafer level package). FOWLP is manufactured, for example, as follows. A plurality of individualized semiconductor chips are placed on a substrate such as glass with the electrode surface facing up. Next, the semiconductor chips are encapsulated together. Then, the encapsulant is ground to expose the electrodes. Then, photoresist is applied, exposed, and developed, and a conductive metal is deposited on the part where the photoresist has been removed to form a rewiring layer. Finally, the encapsulated semiconductor chips are cut into individual chips to obtain a FOWLP type semiconductor product. If the silica particles contained in the encapsulant contain air bubbles, the air bubbles are exposed when the encapsulant is ground, and a recess is generated on the ground surface. If a photoresist is provided in this recess, a recess is also generated in the photoresist, and a conductive metal is unevenly deposited in the recess in the next process. As a result, problems such as a decrease in product yield due to defects in the formation of the rewiring layer and a decrease in the long-term reliability of the semiconductor product may occur.

However, as described above, the production of fused silica involves grain growth due to the melting and fusion of tiny silica particles in the flame, which entrains air bubbles during the fusion process, making it unavoidable that the silica produced contains air bubbles.

The entrapment of air bubbles has been reduced by improving the combustion conditions during manufacturing. As a result, it is now possible to reduce the amount of air bubbles to a level that cannot be detected by conventional inspection methods. However, when attempting to use it for applications such as filler for encapsulation of WLP-type semiconductors, which involves a process of cutting or grinding the encapsulation material as described above, the level of entrapment is still such that problems may arise. In the manufacture of WLP-type semiconductor products, the grinding process is almost the final process, and the occurrence of defects at this stage leads directly to increased costs.

Of course, air bubbles larger than the particle size cannot exist, so the above problem does not occur if large particles are completely eliminated from the powder. Therefore, when the silica particles are small in size, the negative effects of air bubbles are small. However, as mentioned above, in order to achieve a high filling rate, it is better for some particles to be large in size. The larger the particle size, the more likely it is that air bubbles will be included.

Therefore, the present invention aims to provide a new silica powder that contains a certain amount of large particles, but reduces the amount of such bubbles to a level that is substantially problem-free when used for applications such as filler for sealing materials for WLP-type semiconductors.

The inventors of the present invention have conducted extensive research in light of the above problems. They have discovered that the above problems can be solved by specifying the raw silica powder and combustion conditions and limiting the particle size of the fused silica to be recovered in a method for producing fused spherical silica powder by fusing and spheroidizing fine silica powder in a flame, and have completed the present invention.

That is, the present invention provides a fused spherical silica powder having a cumulative volume 95% diameter (d95) in the range of 5 μm to 30 μm when measured by laser diffraction,
The fused spherical silica powder and an epoxy resin are mixed and cured in a mass ratio of 1:1, and a portion of the cured product is polished. When the exposed cross section of the silica is observed under a microscope at 1,000x magnification, the number of bubbles having a maximum diameter of 5 μm or more that can be detected is 50 or less per 10 ^cm2 of the polished surface of the cured product.

The fused spherical silica powder of the present invention has an extremely small amount of bubbles. Therefore, when used as a filler for sealing materials for WLP type semiconductors, it has the effect of improving the product yield and long-term reliability of the semiconductors.

FIG. 1 shows a schematic plan view of the ground surface of the encapsulant. FIG. 2 shows a cross-sectional view taken along line AA of FIG.

The fused spherical silica powder of the present invention has a cumulative volume 95% diameter (d95) in the range of 5 μm to 30 μm when measured by laser diffraction. If the d95 is too small, it is difficult to obtain a high filling rate in a resin composition when used as a filler. On the other hand, if the d95 is too large, there are problems such as poor penetration into narrow parts of the resin composition when used as a filler. Preferably, the d95 is 20 μm or less.

In terms of excluding coarse particles, it is preferable that the particles exceeding 100 μm in size are 0% by mass when measured by laser diffraction, more preferably that the particles exceeding 75 μm in size are 0% by mass, and particularly preferably that the particles exceeding 50 μm in size are 0% by mass.

Details of the laser diffraction measurements will be explained in the examples below.

Furthermore, when the silica powder of the present invention is measured under the above conditions, the cumulative volume 50% particle size (d50) is preferably in the range of 1 to 20 μm, and more preferably in the range of 3 to 15 μm. From the viewpoint of eliminating coarse particles, when the fused spherical silica powder of the present invention is measured with a wet sieve, the amount of 106 μm remaining particles is preferably 0 mass%, and the amount of 45 μm remaining particles is preferably 0.1 mass% or less, more preferably 0.05 mass% or less, and particularly preferably 0.01 mass% or less. Note that "106 μm remaining" refers to the proportion of particles that do not pass through a mesh with a 106 μm opening and remain on the mesh.

By having such particle size and particle size distribution, it is possible to obtain high fluidity and good penetration into narrow spaces when used as a filler for semiconductor encapsulation, particularly as a filler for liquid encapsulation for applications such as WLP type semiconductors.

The silica powder of the present invention is spherical. Therefore, when used as a filler for various resins, the resin composition has excellent fluidity. Here, spherical means that the Wadell practical circularity (circle equivalent diameter/maximum diameter) is 0.7 to 1.0. Here, the above circle equivalent diameter is the diameter of a circle having an area equal to the projected cross-sectional area of the particle, and the maximum diameter is defined as the maximum distance between any two points on the projected periphery of the particle. The preferred Wadell practical circularity is 0.8 to 1.0. In general, silica produced by the fusion method is spherical.

The silica powder of the present invention is substantially free of bubbles. Specifically, when a part of a cured product obtained by kneading and curing silica powder and an epoxy resin in a mass ratio of 1:1 is polished and the exposed silica cross section is observed under a microscope at 1,000 times, the number of bubbles having a maximum diameter of 5 μm or more that can be detected is 50 or less per 10 ^cm2 of the polished surface of the cured product.

More specifically, the evaluation method is as follows: silica powder is mixed with room temperature curing epoxy resin at 50% by mass, and kneaded until homogeneous. The kneaded mixture is then filled into a suitable mold without trapping air bubbles, and cured at room temperature. The mold for curing is preferably one that has a shape that allows a polished surface of 1 ^cm2 or more when the cured product is polished.

Once the hardened body has fully hardened, it is then polished to ensure a surface for observation. The polishing conditions are to first roughly polish the surface with diamond abrasive grains of about 1 to 3 μm, then polish the surface with colloidal silica abrasive grains for approximately 2 hours until the surface is no longer rough and has a glossy finish.

The polished surface is observed under a microscope at 1000x magnification. The microscope may be an optical microscope, a polarizing microscope, an electron microscope, or the like, but is preferably an optical microscope. In the microscope observation, an area of at least 1 ^cm2 of the polished surface is observed.

The above polishing makes it possible to observe the cross-sections (polished surfaces) of the silica particles in the cured epoxy resin, so all silica cross-sections that can be confirmed within the observation range are observed using the above microscope to determine whether or not there are any bubbles. Then, the number of bubbles whose longest diameter (the longest distance between any two points on the circumference of the object) is 5 μm or more is counted. Note that here, if one silica particle has multiple bubbles, the number of bubbles is counted as multiple, and the longest diameter of each bubble is measured.

From the number of bubbles having a maximum diameter of 5 μm or more measured by such observation and the observation area, the number of bubbles per 10 cm ² of the polished surface of the cured body can be calculated.

The number of bubbles can be measured with the naked eye, but it is more advantageous in terms of time and effort to use a digital microscope and image analysis software.

In the fused spherical silica powder of the present invention, the number of bubbles having a maximum diameter of 5 μm or more is preferably 10 or less, more preferably 5 or less, per 10 ^cm2 of the polished surface of the cured body. It is also preferable that the number of bubbles having a maximum diameter of less than 5 μm is as small as possible. However, since such minute bubbles have a strong tendency to be embedded by the resist resin when forming the rewiring layer, their presence is acceptable at the level of semiconductor wiring miniaturization at the time of filing this application.

Considering that the fused spherical silica powder of the present invention is used as a filler for semiconductor encapsulation, etc., it is preferable that the impurity content is within the following ranges: Fe is 10 ppm or less, preferably 7 ppm or less, Al is 0.7 ppm or less, preferably 0.6 ppm or less, U and Th are each 0.1 ppb or less, Na and K are each 1 ppm or less, and Cl is 1 ppm or less.

For the same reason, it is preferable that the fused spherical silica powder of the present invention does not contain ionic impurities. Therefore, it is preferable that the electrical conductivity of the aqueous dispersion of the fused spherical silica powder is low and the pH is close to neutral. Specifically, when 0.8 g of silica powder is dispersed in 80 ml of pure water, the electrical conductivity is preferably 1.5 μS/cm or less, more preferably 1.4 μS/cm or less, and even more preferably 1.3 μS/cm or less, and the pH is preferably 5.0 to 7.0, and more preferably 5.5 to 7.0.

The specific surface area as measured by the BET single point method using nitrogen is preferably 1 to 5 m ² /g, and more preferably 1.5 to 4 m ² /g.

The fused spherical silica powder of the present invention preferably has a low moisture content, specifically, preferably 0.05% by mass or less, and more preferably 0.02% by mass or less.

The fused spherical silica powder of the present invention may be treated with various surface treatment agents to improve compatibility and reactivity with resins. Examples of surface treatment agents include various silane compounds, silane coupling agents, titanate coupling agents, aluminate coupling agents, silicone oils, etc.

Specific examples of silane compounds and silane coupling agents include hexamethyldisilazane, methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, n-propyltrimethoxysilane, hexyltrimethoxysilane, decyltrimethoxysilane, phenyltrimethoxysilane, dimethyldiethoxysilane, dimethoxydiphenylsilane, 1,6-bis(trimethoxysilyl)hexane, trifluoropropyltrimethoxysilane, methyltrichlorosilane, dimethyldichlorosilane, trimethylchlorosilane, 3-chloropropyltrichlorosilane, vinyltrimethoxysilane, vinyltriethoxysilane, 3-(2-aminoethylamino)propyltrimethoxysilane, 3-(2-aminoethylamino)propylmethyldimethoxysilane, N-phenyl-3-aminopropyltrimethoxysilane, 3-triethoxysilyl-N-(1,3-dimethyl- butylidene)propylamine, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, p-styryltrimethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropyltriethoxysilane, 3-acryloxypropyltrimethoxysilane, tris-(trimethoxysilylpropyl)isocyanurate, 3-ureidopropyltrialkoxysilane, 3-mercaptopropylmethyldimethoxysilane, 3-mercaptopropyltrimethoxysilane, 3-isocyanatepropyltriethoxysilane, etc.

The method for producing the fused spherical silica powder of the present invention is not limited, but it can be suitably produced by the following method: That is, using a multi-tube burner, hydrophobically treated fumed silica is supplied from the central tube of the multi-tube burner together with oxygen gas or/oxygen-containing gas so that the ratio of the fumed silica is 0.3 ^kg /Nm3 to ³ kg/Nm3, melted and spheroidized at 1400°C to 1700°C in the flame, and then fused silica of 0.01 μm to 100 μm is recovered.

The fumed silica (also called pyrogenic silica) used as the raw material is hydrophobized. The inventors have found it difficult to obtain the fused spherical silica powder of the present invention using hydrophilic fumed silica. The degree of hydrophobization is sufficient if the fumed silica does not completely disperse in pure water, but preferably the hydrophobicity (M value) measured by methanol titration is 25% by volume or more, and more preferably 30% by volume or more. Fused spherical silica can also be obtained using Si powder or quartz powder as the raw material, but the use of these as raw materials tends to increase the impurity content in the resulting fused spherical silica.

A suitable method for hydrophobizing the surface of fumed silica is to treat the surface with silanes such as those mentioned above (specifically, hexamethyldisilazane, methyltrichlorosilane, dimethyldichlorosilane, trimethylchlorosilane, etc. are suitable).

The particle properties of the silica used as the raw material are not particularly limited, but the specific surface area measured by the nitrogen adsorption BET single point method is preferably 80 to 250 ^m2 /g, more preferably 100 to 230 ^m2 /g. In order to reduce the amount of metal impurities in the fused spherical silica powder, it is preferable that the amount of impurities in the raw material silica is small, and specifically, the contents of Fe, Al, U, Th, Na and K are preferably equal to or less than the amounts of metal impurities in the fused spherical silica powder.

In the method for producing fused spherical silica powder of the present invention, the hydrophobic raw material fumed silica is melted in a flame, and the molten particles are fused together to cause particle growth and spheroidization.

For melting in a flame, a multi-tube burner is used, and oxygen gas or/and an oxygen-containing gas is supplied from the central tube of the multi-tube burner together with the raw material fumed silica so that the ratio of the raw material fumed silica is 0.3 kg/ ^Nm3 to ³ kg/Nm3. The multi-tube burner may be a double-tube burner, a triple-tube burner, or the like.

If the proportion of raw fumed silica fed into the flame is too small, the proportion of particles of 1 μm or less in the obtained fused spherical silica powder will be too high, and the powder will tend to have a particle size distribution that is not suitable for high loading. If the amount of raw fumed silica fed into the flame is too large, the powder will tend to contain unfused fumed silica particles and particles that are not sufficiently fused and have low sphericity. The proportion of the raw fumed silica is preferably 0.5 kg/Nm ³ to 2.0 kg/Nm ^3. Air is suitable as the oxygen-containing gas to be entrained.

When using a multi-tube burner, such as a triple-tube burner, it is preferable to supply oxygen from the second annular tube and hydrogen from the outermost tube.

The flame temperature should be 1400°C to 1700°C, preferably 1500°C to 1600°C. Below 1400°C, the powder will not be sufficiently melted and spheroidized, while above 1700°C, the process temperature will rise, making it necessary to increase the cooling capacity or increase fuel consumption, resulting in higher production costs. In addition, the higher the temperature, the greater the tendency for the proportion of small particles to increase.

The flame temperature can be adjusted by the gas composition and supply rate of oxygen, hydrogen, nitrogen, etc. supplied to the burner.

In the method for producing fused spherical silica powder of the present invention, fused spherical silica generated in the flame is collected with a particle size in the range of 0.01 μm to 100 μm. As a collection method, it is efficient and suitable to first collect particles with an average particle size of 0.01 μm to 1000 μm using a cyclone, then classify them at a classification point of 8 μm to 100 μm using a sieve and/or an air classifier, and collect the fine particles (below the classification point). When using a sieve and/or an air classifier, the classification point is preferably set in the range of 10 μm to 50 μm. By adjusting the classification point to such a range, it becomes easy to make the d95 of the fine particles produced by the above method in the range of 5 μm to 30 μm. Note that the d95 does not fall within the range of 5 μm to 30 μm under all conditions, so it is necessary to set the conditions appropriately according to the characteristics of the classifier used.

Recovery using a sieve has the advantage that there is virtually no risk of recovering particles larger than the openings of the sieve used. On the other hand, recovery is laborious, and the industrial productivity of an air classifier is higher. However, air classifiers tend to recover small amounts of particles that exceed the classification point, so there is a risk that small amounts of particles above the desired upper limit particle size will be included. Recognizing these advantages and disadvantages, it is necessary to select the appropriate method according to the desired particle size range and productivity, etc. Note that sieve classification and classification using an air classifier may be used together. Furthermore, when classifying using a sieve, either dry classification or wet classification may be used.

When the fused spherical silica powder is surface-treated, this may be done before or after classification using the sieve and/or air classifier.

Surface treatment may be performed by a known method depending on the silane compound and silane coupling agent used, and may be dry or wet. Surface treatment, particularly wet surface treatment, may cause particle aggregation, so appropriate crushing or further classification may be performed as necessary. Treatment with a device that applies mechanical stress can provide the effects of aggregate crushing, bulk adjustment, and crushing of bubble-containing particles. Examples of devices that can be used to apply mechanical stress include free vortex centrifugal classifiers, forced vortex centrifugal classifiers, ball mills, jet mills, two-roll mills, three-roll mills, stone mill-type crushers, and rotary blade mixers.

The following examples and comparative examples are provided to more specifically explain the present invention, but the present invention is not limited to these examples.

The manufacturing conditions for the fused spherical silica in the examples and comparative examples, as well as the methods for evaluating various physical properties, are as follows:

The manufacturing method for producing fused silica is described below.

(1) Melting and spheroidization with a burner Using a triple-tube burner, raw material fumed silica and oxygen were supplied from the central tube, oxygen from the second annular tube, and hydrogen from the outermost tube. The flame temperature was adjusted by the hydrogen/oxygen ratio and the amount of silica.

(2) Classification From the fused silica obtained above, first, particles of 0.1 μm to 1000 μm were collected using a cyclone, and then the collected silica was classified at a predetermined classification point using an air classifier, and the fine particle side was collected.

The physical property evaluation methods are shown below.

(1) Hydrophobicity of raw material fumed silica (M value)
While the raw material fumed silica was floating on the surface of the pure water, methanol was added dropwise with stirring. The amount of methanol required to suspend the entire amount of silica in the pure water was calculated in volume %.

(2) Silica Concentration The weight of silica introduced into the silica supply nozzle of the melting and spheroidizing burner was divided by the volume of oxygen gas supplied to the central tube to determine the silica concentration per unit volume.

(3) Flame Temperature The burner flame temperature was calculated using the adiabatic flame temperature calculation formula based on the amounts of hydrogen, oxygen, and fumed silica introduced into the silica fused spheroidizing burner.

(4) Accumulative volume diameter Measurement was performed using a Microtrac laser diffraction scattering type particle size distribution measuring device (MT-3300EX2) with an aqueous dispersion medium, and the cumulative volume 50% diameter (d50) and 95% diameter (d95) were calculated. In addition, 250 mL of dispersion medium and 0.02 g to 0.1 g of sample were put into the sample slurry circulation tank of the measuring device. Then, while circulating the sample slurry, it was ultrasonically dispersed at 40 W for 1 minute, and d50 and d95 were measured. Here, the amount of the sample put in was adjusted according to the instruction manual of the device so that the sample slurry concentration value (Sample Loading value) displayed on the screen of the personal computer for controlling the device was between 0.85 and 0.90.

(5) BET Specific Surface Area: The BET specific surface area was measured by a nitrogen adsorption BET single point method using a specific surface area measuring device (SA-1000) manufactured by Shibata Rikagaku Co., Ltd.

(6) Measurement of Fe and Al Concentration Silica particles were dissolved in fluoronitric acid, and the Fe and Al concentrations were measured by ICP emission spectrometry.

(7) Moisture The moisture content in the silica particles was measured by loss on drying method (at 110° C. for 6 hours).

(8) Measurement of pH and Electrical Conductivity An aqueous dispersion of silica particles (8.0 g silica/80 mL pure water, 25° C.) was prepared, and the pH was measured with a glass electrode pH meter and the electrical conductivity was measured with an AC two-electrode electrical conductivity meter.

(9) U Concentration Silica particles were dissolved in fluoronitric acid, and the U concentration was measured by ICP-MS.

(10) Na ⁺ and Cl ⁻ Concentrations Silica particles were immersed in pure water at 110° C. for 24 hours to prepare an eluate aqueous solution, and the Na ⁺ concentration was measured by an atomic absorption spectrometer, and the Cl ⁻ concentration was measured by ion chromatography.

(11) Resin Compound Viscosity An epoxy resin (bisphenol A/F mixed resin ZX-1059 manufactured by Tohto Kasei Co., Ltd.) and silica particles of each Example and Comparative Example were blended in a ratio of 78 silica:resin 22 (weight ratio), and the mixture was stirred for 8 minutes at 1000 rpm using a planetary mixer (AR-250 manufactured by Thinky Corporation), and further kneaded under conditions of a degassing time of 2 minutes at 2000 rpm to obtain an epoxy resin composition.

Next, the viscosity of the epoxy resin composition was measured using a rheometer viscometer (RHEOSTRESS RS600 manufactured by Haake) under conditions of a temperature of 25° C., a plate gap of 50 μm, and a shear rate of 1 s ⁻¹ .

(12) Number of Air Bubbles Silica powder was mixed into room temperature curing epoxy resin (Epocure 2 manufactured by Buehler) so that the content was 50 mass %, and the mixture was kneaded until it was uniform. The kneaded mixture was then filled into an embedding mold (plastic ring manufactured by Buehler, inner diameter 1 inch (25.4 mm)) while being careful not to trap air bubbles, and was allowed to cure sufficiently at room temperature.

Next, a part of the cured body was polished to ensure an observation surface. The polishing conditions were as follows: first, rough polishing was performed using abrasives with abrasive grain sizes of 3 μm and 1 μm (Buehler's Metadye single crystal diamond suspension aqueous/abrasive grain size 3 μm, then 1 μm was used), and then polishing was performed with a polishing abrasive for main polishing (Mastermet 2 colloidal silica) until the surface became glossy.

An area of 1 ^cm2 of the resulting polished surface was observed at 1000x magnification using an optical microscope (MICROSCOPE EVHX-5000 manufactured by KEYENCE Corporation) with epi-illumination/coaxial epi-illumination, and the number of bubbles whose longest diameter (the longest length among the distances between any two points on the circumference of the object) was 5 μm or more was counted.

In addition, when one silica particle had multiple bubbles, it was counted as multiple bubbles. This observation was performed on 10 samples, and the number of bubbles observed was totaled to calculate the number of bubbles per 10 ^cm2 of the polished cross-sectional area of the cured body.

(13) Wadell's Practical Circularity Approximately 1 mg of silica powder was placed in the center of a slide glass (2 cm x 4 cm), 2 to 3 drops of pure water were dripped to prepare a silica slurry, and a cover glass was placed on the silica slurry to prevent air bubbles from entering, to prepare a specimen for observation. The specimen was observed under a Leica optical microscope DMLB (transmitted light source, magnification 400 times), and the image of the silica particles was analyzed using an image analyzer (Leica Q500IW) to determine the circle equivalent diameter/longest diameter for each particle. Measurements were repeated while moving the observation field until the total number of measured particles reached 500 or more, and the arithmetic mean value of the measured values was taken as the value of Wadell's practical circularity of the silica powder.

Example 1
A hydrophobized fumed silica having an M value of 47 and a BET specific surface area of 120 ^m2 /g was used, the amount of fumed silica fed to the burner was 0.7 kg/ ^Nm3 , and a flame temperature was set to 1600°C to obtain a fused silica powder. The silica powder obtained was then classified to a classification point of 10 μm and then recovered. The physical properties of the obtained fused spherical silica powder are shown in Table 1.

Examples 2 to 5
Using hydrophobized fumed silica having the M value and BET specific surface area shown in Table 1, and using the silica supply amount, flame temperature, and classification point shown in Table 1, fused spherical silica powder was produced in the same manner as in Example 1. The physical properties of the obtained fused spherical silica powder are shown in Table 1.

Comparative Example 1
Fused silica was produced in the same manner as in Example 1, except that hydrophobized fumed silica with an M value of 47 and a BET specific surface area of 126 ^m2 /g was used, the amount of fumed silica supplied to the burner was 0.3 kg/ ^Nm3 , the flame temperature was 1800°C, and the classification point was 3 µm. The physical properties of the obtained fused silica are shown in Table 1, but the flame temperature was high, resulting in a high proportion of small particle diameter particles, and the classification point during production was also too small, resulting in a small d95, which caused a significant increase in viscosity during the preparation of the resin compound, making it impossible to form a resin compound.

Comparative Example 2
Fused silica was produced in the same manner as in Example 1, except that hydrophobized fumed silica with an M value of 47 and a BET specific surface area of 115 ^m2 /g was used, the amount of fumed silica supplied to the burner was 0.7 kg/ ^Nm3 , the flame temperature was 1600°C, and the classification point was 5 μm. The physical properties of the obtained fused silica are shown in Table 1, but the d95 was small because the classification point during production was too small. Unlike Comparative Example 1, it was possible to form a resin compound, but the viscosity was extremely high.

However, it is believed that if the fused silica of Comparative Example 1 or Comparative Example 2 is mixed with the fused silica of the Examples, the viscosity of the resin compound will be lower and the number of bubbles will remain small.

Comparative Example 3
Fused silica was produced in the same manner as in Example 1, except that hydrophilic fumed silica (M value = 0) was used as the raw material. In this case, the number of bubbles was significantly large.

Comparative Example 4
Fused silica was produced in the same manner as in Example 2, except that the raw material fumed silica was hydrophilic (M value = 0) and had a BET specific surface area of 125 ^m2 /g. In this case, the bubble content was significantly high.

Comparative Example 5
A commercially available fused silica (d95: 29.5 μm, d50: 10 μm) was evaluated, but the number of bubbles was significantly high. Furthermore, by mixing it with the small-sized fused silica of Comparative Example 1 or 2, a combination of large and small particles was achieved, and although the filling characteristics were improved and the resin compound viscosity was reduced, it was determined that the number of bubbles could not be sufficiently reduced.

1 ... dense silica particle 2 ... hollow silica particle 3 ... void (recess) exposed by grinding

Claims (7)

A fused spherical silica powder having a cumulative volume 95% diameter (d95) in the range of 5 μm to 14.9 μm, a cumulative volume 50% particle diameter (d50) in the range of 3 to 7 μm, and a U (uranium) content of 0.1 ppb or less, when measured by laser diffraction;
The fused spherical silica powder is characterized in that when the fused spherical silica powder and epoxy resin are mixed and cured in a mass ratio of 1:1, a portion of the cured product is polished, and the exposed silica cross section is observed under a microscope at 1,000x magnification, the number of bubbles having a maximum diameter of 5 μm or more that can be detected is 50 or less per 10 ^cm2 of the polished surface of the cured product. The fused spherical silica powder according to claim 1, in which the particle surface is treated with a silane compound and/or a silane coupling agent. 2. The fused spherical silica powder according to claim 1, which has a BET specific surface area in the range of 1.0 ^{m 2} /g to ^{5.0 m 2} /g. 3. The fused spherical silica powder according to claim 2, which has a BET specific surface area in the range of 1.0 ^{m 2} /g to ^{5.0 m 2} /g. The fused spherical silica powder according to claim 1, which is used as a filler for liquid semiconductor encapsulation materials. The fused spherical silica powder according to claim 2, which is used as a filler for liquid semiconductor encapsulation materials. The fused spherical silica powder according to claim 3, which is used as a filler for a liquid semiconductor encapsulant.

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