JPWO2012042856A1 - Manufacturing method of semiconductor device - Google Patents

Abstract

Before applying the silica source (18) to the silicon wafer (10), the silicon wafer (10) is stored in a storage box (desiccator) (16) in which nitrogen (17) of 30 liters / minute or more is flowing. The time during which the silicon wafer (10) is exposed to the atmosphere before applying the silica source 18 is shortened. Since the time during which the silicon wafer (10) is exposed to the atmosphere is short, formation of a natural oxide film can be suppressed, and variations in device characteristics such as on-voltage can be reduced.

Description

  The present invention relates to a semiconductor device manufacturing method, and more particularly to a platinum diffusion method for controlling the lifetime of a diode.

  The pin diode 500 is frequently used as a freewheeling diode of an inverter circuit, and the lifetime is controlled so that high frequency operation can be performed.

  The lifetime is controlled by diffusion of heavy metals such as gold and platinum, electron beam irradiation and proton irradiation. If the lifetime varies within the silicon wafer 60 or between the silicon wafers 60, device characteristics such as the on-voltage, leakage current, and reverse recovery characteristics of the pin diode 500 vary, and the yield rate is reduced.

FIG. 10 shows a cross-sectional structure of the pin diode 500. The pin diode 500 includes an n ⁺ layer 51, an n layer 52 disposed on the n ⁺ layer 51, a p layer 53 disposed on the n layer 52, and a breakdown voltage structure 54 such as a guard ring disposed around the pin diode 500. And an anode electrode 55 and a cathode electrode 56. In addition, an insulating film 57 (oxide film) is formed on the surface to cover the breakdown voltage structure 54 and the outer peripheral end of the p layer 53. Further, platinum 67 is diffused to control the lifetime.

  FIG. 11 is a process flow diagram when platinum is diffused into the silicon wafer 60.

First, as shown in FIGS. 12A and 12B, a p-layer 53 serving as an anode layer constituting the pin diode 500, an n-layer 52 serving as a drift layer, an n ⁺ layer 51 serving as a cathode layer, and a breakdown voltage structure 54 such as a guard ring. After the resist film 61 is applied and cured on the surface of the silicon wafer 60 on which the insulating film 57 has been formed, the natural oxide film 62 already formed on the back surface with a hydrofluoric acid (HF) solution is removed ( Step 1).

  Next, as shown in FIG. 13, a plurality of silicon wafers 60 are accommodated in a cassette 63, and the cassette 63 is immersed in a water tank 64 containing water 65 and stirred to clean the silicon wafer 60 (step 2).

  Next, the cassette 63 containing the silicon wafer 60 is set in a spin dryer, and the silicon wafer 60 is spin-dried (step 3). This spin drying is a batch type.

  Next, the dried silicon wafer 60 is stored in the atmosphere while being stored in the cassette 63 and waits until the next process is started (process 4). There are a plurality of stored cassettes 63, which are sequentially sent to the next process.

  Next, the cassettes 63 are conveyed one by one to the silica source coating device (step 5), and the silica source 66 containing platinum is applied to the back surface of the silicon wafer 60 one by one as shown in FIGS. 14A and 14B. (Step 6). This silica source coating is a single wafer type. After one cassette 63 is completed, the next cassette 63 is conveyed to the silica source coating device. Therefore, the silicon wafer 60 stored in the last cassette 63 is exposed to the atmosphere for a long time (about 1 hour). Therefore, as the storage time (standby time) becomes longer, the natural oxide film grows and becomes thicker. The silica source 66 is a paste containing 0.1 wt% to 10 wt% of platinum.

  Next, after the silica source 66 is applied, the silicon wafer 60 is conveyed to a thermostatic chamber, and the silica source is cured (step 6).

Next, as shown in FIGS. 15A and 15B, after removing the resist film 61 on the surface of the silicon wafer 60, the silicon wafer 60 whose back surface is coated with the silica source 66 is set in a diffusion furnace, and is 800 ° C. or higher. The platinum 67 of the silica source 66 is diffused into the silicon wafer 60 at a temperature (for example, about 950 ° C.) (step 7). Due to this diffusion, the platinum 67 is distributed throughout the n ⁺ layer 51 and the n layer 52.

  Next, the silicon wafer 60 is taken out from the diffusion furnace, the silica source 66 is removed, and diode electrodes (anode electrode 55 and cathode electrode 56) formed in the silicon wafer 60 are attached (step 8).

  Next, device characteristics such as the on-voltage, leakage current, and breakdown voltage of the diode are measured in the wafer state (step 9).

  Next, a chip that deviates from the specified value is marked and selected as defective when it is formed into a chip, and a non-defective pin diode 500 is completed (step 10).

  Further, Patent Document 1 describes a method of controlling the lifetime by applying a material containing platinum to silicon exposed ground and diffusing platinum by applying a heat treatment at a high temperature by a pin diode manufacturing method. Further, an N type semiconductor layer having a low impurity concentration is epitaxially grown on an N type semiconductor substrate, an oxide film having a desired pattern is formed on the surface, and the active region edge and guard ring region are formed by ion implantation using the oxide film as a mask. Form. And the part which forms an active region is exposed, the paste containing platinum is apply | coated to the back surface of a semiconductor substrate in that state, and platinum is thermally diffused. Thereby, the vicinity of the surface of the active region of the semiconductor layer is inverted to P-type, and the inverted region is formed shallow. Finally, it is described that a semiconductor device constituting a diode having high speed and sufficient soft recovery characteristics can be manufactured by forming a front electrode and a back electrode.

  Further, in Patent Document 2, when gold is introduced as a lifetime control of a diode, if an oxide film is removed with hydrofluoric acid, washed with water and dried, an extremely thin oxide film (natural oxide film) is formed and gold diffuses. However, it is described that the lifetime distribution is non-uniform and the characteristics vary. The variation is reduced by removing the natural oxide film.

  Patent Document 3 describes that the formation of a natural oxide film is suppressed by using a nitrogen atmosphere in cleaning a semiconductor substrate.

  In the manufacturing method of FIG. 11 described above, the silicon wafer 60 is exposed to the atmosphere until the silica source 66 is applied after the silicon wafer 60 is dried. The exposure time becomes longer as the cassette 63 processed later.

  This is because the drying of the silicon wafer 60 is a batch type and is batch processed by the cassette 63, whereas the silica source 66 is a single wafer type in which the wafers are processed one by one. There is a waiting time until it is done.

  Therefore, the natural oxide film formed on the silicon wafer 60 is thin in the cassette 63 where the silica source application is performed early, and the natural oxide film formed on the silicon wafer 60 is thick in the cassette 63 where the silica source application is performed late. .

  FIG. 16 is a diagram illustrating the relationship between the time during which the silicon wafer is exposed to the atmosphere and the on-voltage. From FIG. 16, the on-state voltage decreases linearly with respect to the time of exposure to the atmosphere. This is because as the time of exposure to the atmosphere increases, the thickness of the natural oxide film increases and the amount of platinum 67 diffused into the silicon wafer 60 decreases. The on-voltage of the silicon wafer 60 in the first cassette 63 is high, and the on-voltage of the silicon wafer 60 in the last cassette 63 is low. That is, the ON voltage varies. Here, the on-voltage is a value of a voltage drop between the anode electrode and the cathode electrode when a constant pure direction current (for example, rated current) is passed through the diode.

  FIG. 17 is a diagram illustrating variation in on-voltage. The vertical axis is frequency and the horizontal axis is on-voltage. From FIG. 17, the on-state voltage varies in the range of 2.57V to 2.69V.

  Since the on-voltage variation is large, a diode that deviates from the standard value is generated, and the yield rate is reduced.

  In each of the following patent documents, after removing the natural oxide film formed on the silicon wafer, the silicon wafer is temporarily stored in a storage box in a large amount of nitrogen atmosphere in order to prevent the natural oxide film from growing again. After that, a manufacturing method for thermally diffusing heavy metals that become lifetime killer is not described.

JP 2002-231968 A JP 55-44772 A JP 2007-88398 A

  An object of the present invention is to provide a method for manufacturing a semiconductor device that solves the above-described problems and can reduce variations in element characteristics such as on-voltage.

  The present invention devised to achieve the above object is to store a semiconductor wafer in a nitrogen atmosphere after removing a natural oxide film formed on the semiconductor wafer, and subsequently to add a silica source containing heavy metal to the semiconductor wafer. A method of manufacturing a semiconductor device is provided, wherein the silica source is cured after being applied to an upper surface or a lower surface of the semiconductor wafer, and then the semiconductor wafer is heat-treated.

  Here, it is preferable that nitrogen flow at 30 liters / minute or more in the nitrogen atmosphere, and it is preferable that the time for storing the semiconductor wafer in the nitrogen atmosphere is 10 minutes or more.

  According to the present invention, the silicon wafer is stored in a storage box (desiccator) with 30 liters or more of nitrogen flowing through the silicon wafer before applying the silica source, thereby shortening the time during which the silicon wafer is exposed to the atmosphere before applying the silica source. Since the time during which the silicon wafer is exposed to the atmosphere is shortened, the formation of a natural oxide film can be suppressed, and variations in device characteristics such as on-voltage can be reduced. Furthermore, by setting the storage time in the storage box to 10 minutes or more, variations in device characteristics such as on-voltage can be further reduced.

  More preferably, nitrogen flows at 100 liters / minute or more in the nitrogen atmosphere, and the time for storing the conductive wafer in the nitrogen atmosphere is 20 minutes or more.

  Further, the heavy metal contained in the silica source is preferably platinum or gold, and here, the content concentration of platinum or gold contained in the silica source is preferably 0.1 wt% to 10 wt%. .

FIG. 1 is a process flow diagram showing a method for manufacturing a semiconductor device 100 according to an embodiment of the present invention. FIG. 2A is a cross-sectional view of a silicon wafer. FIG. 2B is an enlarged cross-sectional view illustrating a main part of FIG. 2A in detail. FIG. 3 is a diagram showing a state of washing with water. FIG. 4 is a diagram of a storage box storing cassettes. FIG. 5A is a cross-sectional view in which a silica source is applied to a silicon wafer. FIG. 5B is a cross-sectional view illustrating an enlarged main part of FIG. 5A in detail. FIG. 6A is a cross-sectional view in which platinum is diffused into a silicon wafer. FIG. 6B is an enlarged cross-sectional view showing the main part of FIG. 6A in detail. FIG. 7 shows a cross-sectional structure of the semiconductor device 100 (pin diode) manufactured by the manufacturing method of FIG. FIG. 8 is a diagram illustrating variation in on-voltage. FIG. 9 is a diagram showing the relationship between the storage time in a nitrogen atmosphere and the on-voltage. FIG. 10 shows a cross-sectional structure of the pin diode 500. FIG. 11 is a process flow diagram showing a conventional method for manufacturing the pin diode 500. FIG. 12A is a cross-sectional view of a silicon wafer. FIG. 12B is a cross-sectional view illustrating an enlarged main part of FIG. 12A in detail. FIG. 13 is a diagram showing a state of washing with water. FIG. 14A is a cross-sectional view in which a silica source is applied to a silicon wafer. FIG. 14B is an enlarged cross-sectional view showing a main part of FIG. 14A in detail. FIG. 15A is a cross-sectional view in which platinum is diffused into a silicon wafer. FIG. 15B is a cross-sectional view illustrating an enlarged main part of FIG. 15A in detail. FIG. 16 is a diagram showing the relationship between the storage time in the atmosphere and the on-voltage. FIG. 17 is a diagram showing variation in on-voltage of a conventional pin diode.

  Embodiments will be described in the following examples. In the following, the semiconductor wafer is a silicon wafer, but it may not be silicon. For example, there is a semiconductor such as silicon carbide (SiC) or gallium nitride (GaN).

  FIG. 1 is a process flow diagram showing a method of manufacturing a semiconductor device according to one embodiment of the present invention. This semiconductor device is, for example, a platinum-diffused pin diode 100. Also, the difference from the conventional process is the process 4, in which the silicon wafer after the silicon wafer is dried is not stored in the air, but a large amount of nitrogen is flowed (over 30 liters / minute) in a nitrogen atmosphere. This is a point that is performed in the storage box 16. The other processes are the same as the conventional processes.

First, as shown in FIGS. 2A and 2B, a p layer 3 serving as an anode layer constituting the pin diode 100, an n layer 2 serving as a drift layer (i layer), an n ⁺ layer 1 serving as a cathode layer, a guard ring, and the like The resist film 11 is applied and cured on the surface of the silicon wafer 10 formed so that the insulating film 7 (oxide film) covers the withstand voltage structure 4 and the outer peripheral edge of the p layer 3. After that, the natural oxide film 12 already formed on the back surface of the silicon wafer 10 is removed with a hydrofluoric acid (HF) solution (step 1).

  Next, as shown in FIG. 3, a plurality of silicon wafers 10 are accommodated in a cassette 13, and the cassette 13 is immersed in a water tank 14 containing water 15 and stirred to wash the silicon wafer 10 (step 2).

  Next, the cassette 13 containing the silicon wafer 10 is set in a spin dryer, and the silicon wafer 10 is spin-dried (step 3). This spin drying is a batch type.

  Next, the plurality of cassettes 13 in which the silicon wafers 10 are stored are moved from the spin dryer to the storage box (desiccator) 16 shown in FIG. 4, and the cassette 13 is stored together in the storage box 16 and waits for the next process. The storage box 16 is configured such that, for example, an acrylic resin lid 16a and a main body 16b are hermetically sealed to each other through an O-ring (not shown). One side of the storage box 16 (on the left side of the sheet of FIG. 4) is provided with an inlet 21 made of a valve, and similarly, an air outlet 22 made of a valve is provided on the opposite side. Nitrogen 17 of 30 liters / minute or more (preferably 100 liters / minute or more) flows from the inlet 21 to the storage box 16, and flows out of the storage box 16 from the outlet 22. When the silicon wafer 10 is stored in the storage box 16, the lid 16 a and the main body 16 b are sealed, so that a large amount of nitrogen 17 can flow into the storage box 16. By flowing nitrogen in this manner, the silicon wafer 10 can be stored in a nitrogen atmosphere having a sufficiently low oxygen concentration (for example, 1% or less). The storage time (standby time) of the silicon wafer 10 may be less than 10 minutes, but preferably 10 minutes or more (step 4). There are a plurality of stored cassettes 13, which are sequentially sent to step 5.

  Next, the cassettes 13 are conveyed one by one to the silica source coating device (step 5), and the silica source 18 containing platinum is applied to the back surface of the silicon wafer 10 one by one as shown in FIGS. 5A and 5B. (Step 6). At this time, the back surface is on the upper side. This silica source coating is a single wafer type. After the application of one cassette 13 to the silicon wafer 10 is completed, the next cassette 13 is conveyed to the silica source coating apparatus. Therefore, the silicon wafer 10 stored in the last cassette 13 is in a standby state for a long time (for example, about 1 hour), but the silicon wafer 10 is contained in the storage box 16 in which a large amount of nitrogen 17 flows. The time for exposure to the atmosphere is a short time for taking out the cassette 13 from the storage box 16. Therefore, the growth of the natural oxide film is suppressed. The silica source 18 is a paste containing 0.1 wt% to 10 wt% of platinum. If the platinum content is less than 0.1% by weight, the amount introduced is too small to obtain good diode characteristics. On the other hand, if it exceeds 10% by weight, crystal defects frequently occur, and good diode characteristics cannot be obtained. The platinum content is preferably about 1% by weight.

  Next, after the silica source 18 is applied, the silicon wafer 10 is transported to a thermostat and the silica source 18 is cured (step 6).

  Next, as shown in FIGS. 6A and 6B, after removing the resist film 11 on the surface of the silicon wafer 10, the silicon wafer 10 whose back surface is coated with the silica source 18 is set in a diffusion furnace, and 800 ° C. or higher. The platinum 19 of the silica source 18 is diffused into the silicon wafer 10 at a high temperature (for example, about 950 ° C.) (step 7).

  Next, the silicon wafer 10 is taken out from the diffusion furnace, the silica source 18 is removed, and electrodes (anode electrode 5 and cathode electrode 6) are attached to the diode formed in the silicon wafer 10 (step 8).

  Next, device characteristics such as the on-voltage, leakage current, and breakdown voltage of the diode are measured in the wafer state (step 9).

  Next, a chip out of the specified value is marked and selected when the chip is formed to complete a non-defective diode as shown in FIG. 7, which is the semiconductor device 100 of the present invention (step 10).

  In step 4, by increasing the flow rate of nitrogen 17 flowing to the storage box 16 to 30 liters / minute or more (preferably 100 liters / minute or more), the lid 16a of the storage box 16 is opened and the cassette 13 is taken in and out. Even in this case, the silicon wafer 10 can always be kept in a nitrogen 17 atmosphere. As a result, the natural oxide film formed on the silicon wafer 10 is kept thin regardless of the standby time (storage time). Further, if the storage time is 10 minutes or longer (preferably 20 minutes or longer), the thickness of the natural oxide film is kept thin and constant, so that variations in device characteristics such as on-voltage are further reduced.

  FIG. 8 is a diagram illustrating variation in on-voltage. This figure is a histogram of on-voltage, the horizontal axis is on-voltage, and the vertical axis is frequency (number). The storage conditions were a nitrogen flow rate of 100 liters / minute and a storage time of 20 minutes or more. The on-voltage variation is in the range of 2.60 V to 2.63 V, which is 25% of the conventional variation width, so that the on-voltage variation width can be greatly reduced. This is the same variation when the flow rate of nitrogen is 30 liters / minute and the storage time is 10 minutes or more. Further, even when the flow rate of nitrogen is 30 liters / minute and there is no storage time, it is smaller than the conventional variation in on-voltage in FIG. As a result, the yield rate is improved. The storage time corresponds to the standby time.

  FIG. 9 is a diagram showing the relationship between the storage time in a nitrogen atmosphere and the on-voltage. It is the data which experimented by changing the storage conditions in the process of FIG. The vertical axis is on-voltage, and the horizontal axis is storage time. The flow rate of nitrogen 17 flowing through the storage box 16 was three types of 10 liters / minute, 30 liters / minute, and 100 liters / minute. In the case of 30 liters / minute and 100 liters / minute, when the storage time exceeds 10 minutes, the decrease in the on-voltage is rapidly suppressed. This is because when the growth of the natural oxide film exceeds 10 minutes, it tends to be saturated. When the flow rate is 10 liters / minute, the rate of decrease in the on-voltage is larger than that in the case of the above-described flow rate, and thus the variation in the on-voltage is increased. Moreover, the condition of the storage time that suppresses the decrease in the ON voltage is 20 minutes or more, which is slower than the case of 30 liters / minute.

  As described above, when the flow rate of the nitrogen 17 is increased, the decrease in the on-voltage is reduced. This is because the amount of air (oxygen) substituted for nitrogen 17 when the silicon wafer 10 is taken out from the storage box 16 decreases as the flow rate of nitrogen 17 increases. The small decrease in the ON voltage means that the variation in the ON voltage of the diodes between the silicon wafers 10 is small.

  From the above, by storing the silicon wafer 10 in the storage box 16 in which the flow rate of nitrogen 17 flows in at least 30 liters / minute before applying the silica source, the growth of the natural oxide film is suppressed, and the on-voltage variation is reduced. Can be small.

  Further, by setting the storage time to 10 minutes or longer, the growth of the natural oxide film is saturated (stopped), and the dependence of the on-voltage on the storage time becomes extremely small. As a result, variation in element characteristics can be further reduced.

  However, in mass production, based on the above experiment, taking into account variations in storage conditions, the storage conditions may be such that the flow rate of nitrogen 17 is 100 liters / minute or more and the storage time is 20 minutes or more. As a result, as shown in FIG. 8, the variation in the ON voltage can be extremely reduced. The upper limit of the flow rate of nitrogen 17 is, for example, 1000 liters / minute, but a higher flow rate may be used. As for the upper limit of the storage time, as shown in FIG. 9 above, if the storage time is 20 minutes or more, the ON voltage variation can be suppressed. Therefore, if the storage time is shorter than 120 minutes, all process leads The effect of increasing time is small.

  The shape or the like of the storage box 16 is not limited to that shown in FIG. In short, the lid 16a and the main body 16b may be sealed so that the outside air does not enter the storage box 16, and the inflow port 21 and the blowout port 22 for the replacement gas such as nitrogen 17 may be provided. About this inflow port 21 or the blower outlet 22, it is preferable that the direction of the flow of substitution gas can be controlled by opening and closing of a valve. Regarding the material of the storage box 16, for example, the lid 16a and the main body 16b may be mainly composed of stainless steel or glass, in addition to resin such as acrylic. Of course, the part where the cassette 13 containing the silicon wafer 10 is taken in and out may be a door formed on the side surface of the storage box 16 instead of the lid 16a. Moreover, in order to improve the sealing property of a lid | cover or a door, you may fasten to a main body using a snap lock (Draw Latch) etc.

  Further, regarding the replacement gas, it is important that the oxygen content is sufficiently small and the humidity is low. For example, in addition to the nitrogen 17 in the embodiment of the present invention, an inert gas such as helium and argon, or a combination thereof. It doesn't matter.

  Further, the atmospheric pressure inside the storage box 16 may be reduced below the atmospheric pressure. However, when the wafer is taken in and out, the atmospheric pressure inside the storage box 16 is returned to the atmospheric pressure so that dust, particles, etc. do not flow into the storage box 16.

  On the other hand, in the embodiment of the present invention, the silicon wafer 10 is stored in the storage box 16 in the nitrogen atmosphere. However, the storage box 16 is not necessarily required, and the storage atmosphere may be a nitrogen atmosphere. For example, the natural oxide film 12 of the silicon wafer 10 is removed with a hydrofluoric acid (HF) solution, the silicon wafer 10 is washed with water 15, dried with a spin dryer, and then the silica source 18 from the spin dryer. The silicon wafer 10 is transported to a coating apparatus that coats. At this time, the cassette 13 on which the silicon wafer 10 is placed is transported by a transport device using a belt conveyor, and nitrogen is flowed into the transport device at a flow rate of 30 liters / minute or more to form a nitrogen atmosphere. A space between the spin dryer and the transport device and between the transport device and the silica source coating device is also a nitrogen atmosphere. And the conveyance time of the silicon wafer 10 in a conveyance apparatus shall be 10 minutes or more. By doing so, it is possible to obtain the same effect as storing all the silicon wafers 10 in a nitrogen atmosphere for 10 minutes or more without using the storage box 16.

  The embodiment described above is an application of the present invention to lifetime control of a diode. However, the present invention is not an diode but an insulated gate transistor (MOSFET) that is a majority carrier device, or a minority carrier device. The same applies to lifetime control of an insulated gate bipolar transistor (IGBT), or a thyristor or gate turn-off thyristor. For example, as is well known, a MOSFET incorporates a parasitic diode, and this parasitic diode may perform a reverse recovery operation. In order to increase the speed of the reverse recovery operation, lifetime control may be performed by introducing a heavy metal such as platinum or gold. In the process of introducing the heavy metal (for example, platinum) from the back surface (drain side) of the MOSFET, if the method of the present invention is used, the variation in the lifetime is small, and the electrical characteristics such as the forward voltage drop and the reverse recovery time of the built-in diode. MOSFETs with small variations can be manufactured.

  Moreover, although the Example demonstrated above is an example which uses platinum as a lifetime killer, the same effect can be acquired by applying this invention also in the case of gold | metal | money.

1,51 n ⁺ layer 2,52 n layer 3,53 p layer 4,54 breakdown voltage structure 5,55 anode electrode 6,56 cathode electrode 7,57 insulating film 10,60 silicon wafer 11,61 resist film 12,62 natural Oxide film 13,63 Cassette 14,64 Water tank 15,65 Water 16 Storage box 16a Lid 16b Body 17 Nitrogen 18,66 Silica source 19,67 Platinum 21 Inlet 22 Outlet

Claims (6)

After removing the natural oxide film formed on the semiconductor wafer, store the semiconductor wafer in a nitrogen atmosphere,
Subsequently, the silica source containing a heavy metal is applied to the upper or lower surface of the semiconductor wafer, and then the silica source is cured,
Subsequently, the semiconductor wafer is subjected to a heat treatment.   2. The method of manufacturing a semiconductor device according to claim 1, wherein nitrogen flows at a rate of 30 liters / minute or more in the nitrogen atmosphere.   2. The method of manufacturing a semiconductor device according to claim 1, wherein a time for storing the semiconductor wafer in the nitrogen atmosphere is 10 minutes or more.   2. The method of manufacturing a semiconductor device according to claim 1, wherein nitrogen flows at 100 liters / minute or more in the nitrogen atmosphere, and a time for storing the semiconductor wafer in the nitrogen atmosphere is 20 minutes or more. .   The method for manufacturing a semiconductor device according to claim 1, wherein the heavy metal contained in the silica source is platinum or gold.   6. The method for manufacturing a semiconductor device according to claim 5, wherein the content concentration of platinum or gold contained in the silica source is 0.1 wt% to 10 wt%.

Images