Green sheet, method for producing green sheet, method for producing ceramic substrate, and method for measuring hardness of green sheet

Abstract

Provided is a green sheet having appropriately controlled hardness. The green sheet according to an embodiment of this invention contains ceramic powder. In an embodiment, the green sheet having a size of 10 mm in width × 50 mm in length is cut out, one end of the cut out green sheet in the length direction is fixed with a jig, the cut out green sheet is horizontally supported, and when a sagging value (mm) of the other end in the length direction is measured when 5 seconds have elapsed since the horizontal support is removed, a value obtained by dividing the sagging value by the thickness (mm) of the green sheet falls within the range of 20-120.

Description

Green sheet, green sheet manufacturing method, ceramic substrate manufacturing method, and green sheet hardness measurement method

[0001] The embodiments described below generally relate to green sheets, methods for manufacturing green sheets, methods for manufacturing ceramic substrates, and methods for measuring the hardness of green sheets.

[0002] Ceramic substrates are used in ceramic circuit boards. Semiconductor elements are mounted on ceramic circuit boards. Examples of ceramic substrates that are used include silicon nitride substrates, aluminum nitride substrates, and aluminum oxide substrates. For example, Japanese Patent No. 6293772 (Patent Document 1) discloses a silicon nitride substrate. The silicon nitride substrate described in Patent Document 1 has excellent insulating properties even when it is thin.

[0003] In the manufacturing process of a ceramic substrate, a ceramic slurry is formed into a green sheet, which is then degreased and fired to obtain a ceramic substrate.

[0004] A sheet forming process such as the doctor blade method is used to produce green sheets. In the sheet forming process, long green sheets are obtained. After cutting the long green sheets to the desired size, degreasing and firing processes are carried out. The degreasing and firing processes involve heat treatment at a predetermined temperature. During degreasing and firing, cracks or chips may occur in the green sheets.

[0005] Patent No. 6293772

[0006] For example, green sheets are produced by mixing ceramic powder (the main component), sintering additive powder, binder, and solvent. The binder and solvent are removed during the debinding and firing processes. Liquid-phase sintering reactions between the ceramic powder and sintering additive powder, as well as grain growth of the ceramic powder, also occur. Even when green sheets are made from the same raw materials, cracks or chips have occurred during the debinding and firing processes. Investigations into the cause of this problem revealed that the hardness of the green sheets varies, and that this variation in hardness is responsible for the cracks or chips. A typical example of hardness is Vickers hardness. Vickers hardness is measured by indentation. This measurement method only measures the local hardness of the indented area. Therefore, a measurement method that can appropriately evaluate hardness over a wider range is needed.

[0007] The embodiments of the present invention address these issues. The embodiments of the present invention provide a green sheet with appropriately controlled hardness. Furthermore, the embodiments of the present invention provide a method for manufacturing a green sheet with an appropriate hardness, and a method for manufacturing a ceramic substrate using the green sheet manufacturing method. Furthermore, the embodiments of the present invention provide a method for measuring the hardness of a green sheet that can appropriately measure a wider range of hardness.

[0008] In the embodiment, a green sheet having a size of 10 mm short side x 50 mm long side is cut out, one end of the cut green sheet in the long side direction is fixed with a jig, the cut green sheet is supported horizontally, and the sagging value (mm) of the other end of the long side direction is measured 5 seconds after the horizontal support is removed. The value obtained by dividing the sagging value by the thickness (mm) of the green sheet is within a range of 20 to 120.

[0009] 1 is a perspective view showing an example of a green sheet according to an embodiment; FIG. 2 is a plan view showing an example of a green sheet fixed by a fixing jig; FIG. 3 is a side view showing an example of a green sheet fixed by a fixing jig; FIG. 4 is a side view showing an example of a method for measuring a sagging value; FIG. 5 is a plan view showing an example of cutting a long green sheet; FIG. 6 is a perspective view showing an example of a ceramic substrate according to an embodiment; and FIG. 7 is a flowchart showing a specific example of a method for manufacturing a ceramic substrate according to an embodiment.

[0010] In the embodiment, a green sheet having a size of 10 mm short side x 50 mm long side is cut out, one end of the cut green sheet in the long side direction is fixed with a jig, the cut green sheet is supported horizontally, and the sagging value (mm) of the other end of the long side direction is measured 5 seconds after the horizontal support is removed. The value obtained by dividing the sagging value by the thickness (mm) of the green sheet is within a range of 20 to 120.

[0011] Fig. 1 is a perspective view showing an example of a green sheet according to an embodiment. Figs. 2 and 3 are a plan view and a side view showing an example of a green sheet fixed with a fixing jig. Fig. 4 is a side view showing an example of a method for measuring the sag value. In Figs. 1 to 4, reference numeral 1 denotes the green sheet, reference numeral 2a denotes the fixing jig, reference numeral 2b denotes a support member, and reference numeral 3 denotes the sag value.

[0012] The green sheet 1 contains ceramic powder. The ceramic powder is the main component of the ceramic substrate. The ceramic substrate is, for example, one selected from a silicon nitride substrate, an aluminum nitride substrate, an aluminum oxide substrate, and a zirconium oxide substrate. The "main component" refers to the component contained in the largest amount in the ceramic substrate. For example, the main component of a silicon nitride substrate is silicon nitride. The main component of an aluminum nitride substrate is aluminum nitride.

[0013] The size of the green sheet 1 when measuring the sag value 3 is 10 mm short side x 50 mm long side. The green sheet may be formed to a size of 10 mm short side x 50 mm long side, or may be cut from a long green sheet to a size of 10 mm short side x 50 mm long side. Alternatively, a long green sheet may be cut to obtain a green sheet of the desired ceramic substrate size, and the measurement green sheet 1 may be obtained from the green sheet of that substrate size. The green sheet 1 when measuring the sag value 3 is sometimes referred to as the measurement green sheet 1. The measurement of the sag value 3 is performed on the measurement green sheet 1 that has been subjected to a drying process. The drying process may be performed on a long green sheet or a substrate-sized green sheet, or it may be performed on the measurement green sheet 1.

[0014] Here, "long green sheet" refers to a sheet that is much longer than the substrate-sized green sheet and the measurement green sheet. Long green sheets are sometimes stored in roll form. As will be described later, mass productivity can be improved by producing a long green sheet and cutting the long green sheet to obtain green sheets of multiple substrate sizes.

[0015] The method for measuring the sag value 3 will be described below. First, a green sheet 1 containing ceramic powder and having a size of 10 mm on the short side and 50 mm on the long side is prepared. The green sheet 1 is preferably prepared by cutting a long green sheet.

[0016] Next, as shown in FIGS. 2 and 3 , one end 1a of the green sheet 1 in the long side direction LD is fixed. The fixed location is 10 mm from the end e1 of the green sheet 1 in the long side direction LD. A fixing jig 2a is used for fixing. As shown in FIG. 2 , the fixing jig 2a is positioned so that the end of the fixing jig 2a in the long side direction LD is located 10 mm from the end e1 of the green sheet 1. The fixing jig 2a is not positioned in the region of the green sheet 1 from the end e1 in the long side direction LD to 10 mm. The length of the fixing jig 2a in the short side direction SD is equal to or greater than the length of the short side of the green sheet 1 (10 mm). The width of the fixing jig 2a (length in the long side direction LD) is within the range of 1 mm to 5 mm. Fixing jigs 2a are placed on the front and back of the green sheet 1. The green sheet 1 is fixed by sandwiching it between a pair of fixing jigs 2a.

[0017] After fixing, the green sheet 1 is kept horizontal. For example, as shown in Fig. 3, the other end 1b in the long side direction LD of the green sheet 1 is supported horizontally using a support member 2b. At this time, the green sheet 1 is supported so that both ends of the green sheet 1 in the long side direction LD are positioned at the same height.

[0018] Thereafter, the support member 2b is moved to remove the horizontal support. With the support removed, the end 1b of the green sheet 1 sags due to gravity. As shown in FIG. 4 , 5 seconds after the support is removed, the sag value 3 of the other end 1b in the long side direction LD of the green sheet 1 is measured. The sag value 3 is expressed as the vertical distance from the top surface of the fixed end 1a to the point where the green sheet 1 sags the most. The sag value 3 is measured in mm. Measurement efficiency can be improved by using an image sensor, laser sensor, ultrasonic sensor, or the like to measure the sag value 3.

[0019] The size is set to 50 mm long x 10 mm short, and the sagging value is measured after 5 seconds because this is suitable for evaluating the hardness of the green sheet. A typical green sheet will deform sufficiently according to its hardness after 5 seconds. If the elapsed time is too long, almost the entire green sheet will sag, regardless of the hardness of the green sheet. This makes it impossible to properly evaluate the hardness.

[0020] The green sheet 1 according to the embodiment is characterized in that the value obtained by dividing the sag value 3 (mm) by the thickness (mm) of the green sheet 1 is within the range of 20 to 120. In other words, 20≦sag value 3 / thickness of the green sheet 1≦120. Hereinafter, the value obtained by dividing the sag value 3 by the thickness of the green sheet 1 will be referred to as the “evaluation value.”

[0021] When the evaluation value is within the range of 20 or more and 120 or less, the hardness relative to the thickness of the green sheet 1 is appropriate. When the hardness relative to the thickness of the green sheet 1 is appropriate, defects such as cracking or chipping of the green sheet 1 during the degreasing process and cracking or chipping of the ceramic substrate during the firing process can be suppressed. The thickness is measured at any three points on the green sheet 1, which has long sides of 50 mm and short sides of 10 mm, and the average value is used as the thickness of the green sheet 1.

[0022] If the evaluation value is less than 20, the green sheet 1 is too hard and is therefore more likely to crack when heat treated. If the evaluation value is more than 120, the green sheet 1 is too soft and the strength of the ceramic substrate may be reduced. Cracks or chips are less likely to occur during the manufacturing process of the ceramic substrate, but cracks or chips are more likely to occur during use of the ceramic substrate. For this reason, the evaluation value is preferably in the range of 20 to 120, and more preferably in the range of 20 to 100.

[0023] The green sheet having a short side of 10 mm and a long side of 50 mm is preferably prepared by cutting it out from a longer green sheet. The green sheet is manufactured by a sheet forming process. The sheet forming process may be performed by a doctor blade method, an injection molding method, a mold forming method, or the like. Of these, the doctor blade method is preferred.

[0024] In the doctor blade method, ceramic slurry is transported on a carrier. A sheet is formed by adjusting the distance between the knife edge and the carrier. A film or belt is used as the carrier. The knife edge is also called a doctor blade. In the doctor blade method, the sheet is formed while the carrier is moving, so long green sheets can be produced.

[0025] The green sheet 1 having a short side of 10 mm and a long side of 50 mm is a sample for measuring (evaluating) hardness. As a sampling test, a green sheet 1 having a short side of 10 mm and a long side of 50 mm is cut out from a long green sheet, and its hardness can be measured.

[0026] FIG. 5 is a plan view showing an example of cutting a long green sheet. In FIG. 5, reference numeral 4 denotes the long green sheet, and reference numeral 5 denotes the cutting location. In the example shown in FIG. 5, the long green sheet 4 is cut into two at the cutting location 5. The cutting process is not limited to this example, and the green sheet 4 may be cut into three or more green sheets 1. Instead of cutting in the long side direction, the green sheet 4 may be cut in the short side direction. The green sheet 4 may be cut in both the long side direction and the short side direction. Die cutting may be performed using a die with short sides of 10 mm and long sides of 50 mm.

[0027] The thickness of the green sheet 1 is preferably in the range of 0.1 mm or more and 2 mm or less. If the thickness of the green sheet 1 is less than 0.1 mm, the ceramic substrate may be too thin, resulting in reduced insulation. If the thickness of the green sheet 1 exceeds 2 mm, the flexibility of the green sheet 1 may be reduced, making it unsuitable for measuring the sagging value. For this reason, the thickness of the green sheet 1 is preferably in the range of 0.1 mm or more and 2 mm or less, and more preferably in the range of 0.2 mm or more and 1 mm or less.

[0028] The green sheet shrinks during the firing process. In other words, the thickness of the ceramic substrate is smaller than the thickness of the green sheet. The thickness of the green sheet is preferably within a range of 1.2 to 2.0 times the thickness of the desired ceramic substrate. Furthermore, it is preferable to adjust the thickness of the green sheet after the drying process as described above. For example, if the desired thickness of the ceramic substrate is 0.32 mm, the thickness of the green sheet is adjusted to a range of 0.38 to 0.64 mm.

[0029] The sag value 3 is preferably in the range of 10 mm to 40 mm, more preferably in the range of 20 mm to 35 mm. If the sag value 3 is less than 10 mm, the green sheet 1 lacks flexibility. If the sag value 3 exceeds 40 mm, the green sheet 1 is too soft.

[0030] The ceramic substrate is, for example, one selected from a silicon nitride substrate, an aluminum nitride substrate, an aluminum oxide substrate, and a zirconium oxide substrate. In this case, the main component of the green sheet 1 is one selected from silicon nitride, aluminum nitride, aluminum oxide, and zirconium oxide.

[0031] The main component of the green sheet is preferably silicon nitride. During the firing process of a silicon nitride substrate, silicon nitride changes from α-type to β-type. α-type silicon nitride is a particle with a small aspect ratio. On the other hand, β-type silicon nitride is a particle with a large aspect ratio. Among silicon nitride, aluminum nitride, aluminum oxide, and zirconium oxide, only silicon nitride undergoes a change in aspect ratio. Changes in aspect ratio are likely to cause cracks and other problems during the degreasing and firing processes. In other words, controlling the "sag value / green sheet thickness" is effective in improving the yield of silicon nitride substrates.

[0032] The green sheet 1 contains one or more selected from the group consisting of a solvent and a binder. The total content of the one or more selected from the group consisting of the solvent and the binder is preferably in the range of 10% by mass to 40% by mass. This total content is the content after a drying process. For example, the solvent is water or an organic solvent. The binder is an organic binder. The solvent and the binder are mixed with the ceramic powder and the sintering aid powder and are used to control the fluidity of the slurry. The binder also plays a role in maintaining the shape of the green sheet 1.

[0033] If the total content of the solvent and binder in the green sheet 1 is less than 10% by mass, the flexibility of the green sheet 1 may decrease, making the green sheet 1 brittle. If the total content of the solvent and binder in the green sheet 1 exceeds 40% by mass, the hardness of the green sheet 1 may be insufficient. For this reason, the total content of the solvent and binder in the green sheet 1 is preferably in the range of 10% by mass to 40% by mass, and more preferably in the range of 15% by mass to 35% by mass.

[0034] Next, a method for manufacturing the green sheet 1 will be described. The size of the green sheet 1 for measuring the sag value 3 is standardized to a short side of 10 mm and a long side of 50 mm. On the other hand, the size of the green sheet for manufacturing the ceramic substrate is arbitrary. A long green sheet for manufacturing the ceramic substrate may be prepared, and a green sheet 1 with a short side of 10 mm and a long side of 50 mm may be cut out from the green sheet.

[0035] The method for manufacturing a green sheet according to the embodiment includes a slurry preparation step, a sheet forming step, and a drying step. In the slurry preparation step, a ceramic slurry containing a ceramic powder, a sintering aid powder, and one or more selected from the group consisting of an organic solvent and a binder is prepared. In the sheet forming step, the ceramic slurry is formed into a sheet to produce a green sheet. In the drying step, the produced green sheet is dried.

[0036] First, a slurry preparation process is performed. The ceramic powder is the main component of the ceramic substrate. The main component is, for example, one selected from the group consisting of silicon nitride, aluminum nitride, aluminum oxide, and zirconium oxide. The sintering aid powder is, for example, one or more compounds selected from the group consisting of rare earth elements, magnesium, titanium, hafnium, tungsten, and molybdenum. The compound is, for example, one or more compounds selected from the group consisting of oxides, nitrides, carbides, and silicides. The rare earth element is, for example, one or more compounds selected from the group consisting of yttrium and lanthanoid elements.

[0037] The solvent is one or more selected from the group consisting of water and organic solvents. A mixture of water and an organic solvent may be used as the solvent. Multiple organic solvents may also be mixed. A ceramic slurry is prepared by mixing a ceramic powder and a sintering aid powder and adding a solvent or binder. The total content of the solvent and binder in the ceramic slurry is preferably in the range of 15% by mass to 65% by mass. In the drying process described below, the total amount of the solvent and binder in the green sheet is adjusted to 10% by mass to 40% by mass. Therefore, the amount of solvent and binder added in the slurry preparation process is greater than the content in the green sheet after the drying process.

[0038] The total content of the solvent and binder in the green sheet after the drying process can be calculated from the mass of the green sheet after the drying process and the mass of the green sheet after burning. Burning the green sheet allows the solvent and binder to be removed from the green sheet. The difference between the mass after the drying process and the mass after burning corresponds to the mass of the solvent and binder. Alternatively, the solvent and binder volatilize when the green sheet is fired. Simply, the total content of the solvent and binder in the green sheet after the drying process can be calculated by subtracting the weight of the ceramic substrate (ceramic sintered body) after firing from the weight of the green sheet after the drying process.

[0039] Next, a sheet forming process is carried out. For the sheet forming process, a doctor blade method, an injection molding method, a mold forming method, etc. are used. Among these, the doctor blade method is preferred. The doctor blade method is excellent in terms of mass productivity. A long green sheet can be obtained by the sheet forming process. A "long green sheet" refers to a green sheet having a size larger than the green sheet 1, which has a short side of 10 mm and a long side of 50 mm and is used to measure hardness.

[0040] Next, the obtained long green sheet is cut to the desired size. The desired size is the size required to obtain a ceramic substrate. Here, the green sheet of the desired size is called "green sheet A." If a mold of the desired size is used in the mold forming method, cutting does not need to be performed. In this case, the green sheet obtained by mold forming is called "green sheet A."

[0041] Next, a drying step is performed to dry green sheet A. The green sheet after the drying step is referred to as "green sheet B." The thickness of green sheet B is preferably in the range of 0.1 mm or more and 2 mm or less. If the thickness of green sheet B exceeds 2 mm, the ceramic substrate becomes thick and heat dissipation properties may be reduced. If the thickness of green sheet B is less than 0.1 mm, the insulation properties of the ceramic substrate may be insufficient. For this reason, the thickness of green sheet B is preferably in the range of 0.1 mm or more and 2 mm or less, and more preferably in the range of 0.2 mm or more and 1 mm or less.

[0042] The total content of the solvent and binder in the green sheet B is preferably in the range of 10% by mass to 40% by mass. The solvent and binder contained in the green sheet B are effective in controlling the hardness (flexibility) of the green sheet.

[0043] In the drying process, it is preferable to calculate the constant rate drying period and the falling rate drying period from the weight loss behavior of the solvent and set the temperature and time for each period. The constant rate drying period is the period during which the solvent evaporates from the green sheet at an almost constant rate. During the constant rate drying period, the solvent near the surface of the ceramic compact evaporates. During the falling rate drying period, the solvent inside the green sheet evaporates. The constant rate drying period and the falling rate drying period can be distinguished depending on the main location where the solvent is evaporating. During the transition period from the constant rate drying period to the falling rate drying period, the shrinkage stress on the green sheet is at its maximum. The shrinkage stress leads to the suppression of voids. Controlling voids leads to the control of surface unevenness.

[0044] The rate of weight loss of the solvent during the constant rate drying period is preferably 0.2% by mass / min or more. There is no particular limitation on the upper limit of the rate of weight loss of the solvent during the constant rate drying period, but it is preferably 10% by mass / min or less. If the rate of weight loss exceeds 10% by mass / min, voids may become large. Therefore, the rate of weight loss of the solvent during the constant rate drying period is preferably within the range of 0.2% by mass / min or more and 10% by mass / min or less.

[0045] The weight loss rate is calculated using the following method. First, the mass of the green sheet is measured before the drying process. Then, while the drying process is being carried out, the mass of the green sheet is measured every 2 to 5 minutes. Furthermore, while checking the areas where the solvent is primarily evaporating, it is determined whether each timing is a constant rate drying period or a falling rate drying period. The weight loss rate for each period is calculated from the mass before the drying process and the mass during the drying process. Alternatively, the drying process can be carried out with the compact placed on a mass meter. In this case, the amount of change in mass during the drying process is measured and the weight loss rate is calculated.

[0046] The weight loss rate of the solvent during the falling-rate drying period is preferably smaller than the weight loss rate of the solvent during the constant-rate drying period. The ratio (R2 / R1) of the weight loss rate of the solvent during the constant-rate drying period R2 to the weight loss rate of the solvent during the falling-rate drying period R1 is preferably greater than 1 and less than 20. During the constant-rate drying period, solvent evaporation routes are formed near the surface of the compact. By keeping the ratio (R2 / R1) within the range of greater than 1 and less than 20, the solvent inside the green sheet can evaporate through these evaporation routes during the falling-rate drying period. If the ratio (R2 / R1) exceeds 20, the weight loss rate during the falling-rate drying period is excessively slow, increasing the possibility that the solvent inside the green sheet cannot be sufficiently removed. Solvent remaining inside the green sheet can cause large voids. Furthermore, a slow weight loss rate during the falling-rate drying period reduces manufacturing efficiency. The weight loss rate of the solvent during the falling-rate drying period is preferably greater than 0.01 mass% / min and less than 0.2 mass% / min.

[0047] The drying time is determined depending on the amount of solvent contained in the green sheet. For example, the total content of the solvent and binder is in the range of 10% by mass to 40% by mass of the mixed powder. In this case, the drying process is performed so that the weight after the drying process is approximately 0.6 to 0.9 times the weight before the drying process.

[0048] The sagging value of the green sheet can be controlled by controlling the rate of weight loss of the solvent during the constant rate drying period and the rate of weight loss of the solvent during the falling rate drying period.

[0049] The drying step can be carried out by various methods such as drying in a drying chamber or drying using a jet nozzle.

[0050] A green sheet 1 having a short side of 10 mm and a long side of 50 mm is obtained from green sheet A or green sheet B in order to measure sag value 3. When green sheet 1 is obtained from green sheet A, a drying process is performed on green sheet 1. When the size of measurement green sheet 1 is the same as the target substrate size, green sheet B may be used as measurement green sheet 1. Alternatively, as described above, measurement green sheet 1 may be obtained from a long green sheet before the drying process, and then the measurement green sheet 1 may be dried.

[0051] The amount of green sheets 1 to be obtained for measuring the sag value 3 is arbitrary. For each green sheet formed from one lot of ceramic slurry, one or more green sheets 1 for measurement are obtained. ３ ~10 ７ mm ２ One or more measurement green sheets 1 are obtained per area of ​​the green sheets. By sampling measurement green sheets 1 from a predetermined amount of green sheets, the measurement green sheets 1 can serve as a sampling inspection.

[0052] By adding a step of measuring the sag value to the green sheet manufacturing process, the hardness of the green sheet after drying can be determined. In other words, the sag value varies depending on the hardness and thickness of the green sheet. The hardness of the green sheet can be evaluated using the sag value and the thickness of the green sheet. In addition, by cutting out a portion of a long green sheet and measuring its sag value, the hardness of the green sheet can be evaluated over a wider range than by measuring the indentation in Vickers hardness measurements.

[0053] Next, a method for manufacturing a ceramic substrate according to an embodiment will be described. In the method for manufacturing a ceramic substrate according to an embodiment, the green sheet obtained by the method for manufacturing a green sheet according to an embodiment is subjected to a degreasing step and a firing step.

[0054] In the degreasing step, the green sheet is preferably heated at a temperature of 400°C to 800°C for 1 hour to 4 hours. In the degreasing step, most of the added binder is degreased. The atmosphere in the degreasing step is, for example, air or a non-oxidizing atmosphere. The non-oxidizing atmosphere is, for example, a nitrogen gas atmosphere or an argon gas atmosphere.

[0055] Next, a firing step is performed. In the firing step, the green sheet is preferably heated in a non-oxidizing atmosphere at a temperature of 1600°C to 1950°C for 5 hours to 18 hours. The non-oxidizing atmosphere is preferably a nitrogen gas atmosphere or a reducing atmosphere containing nitrogen gas. The firing furnace is preferably a pressurized atmosphere.

[0056] If the green sheet is fired at a low firing temperature below 1600°C, a dense ceramic substrate may not be obtained. If the firing temperature exceeds 1950°C, grain growth in the ceramic substrate may become excessive, resulting in a decrease in strength. When a silicon nitride substrate is produced, the firing temperature is preferably within the range of 1800°C to 1950°C.

[0057] A ceramic substrate can be obtained by performing a firing process. In the degreasing process and firing process, the processing object is heated to a high temperature. Since the green sheet according to the embodiment has a controlled sag value (i.e., hardness), it is possible to suppress the occurrence of cracks or chips in the ceramic substrate during the degreasing process and firing process.

[0058] The drying process of the green sheet mainly removes the solvent. The degreasing process mainly removes the binder. The firing process densifies the ceramic substrate due to a liquid-phase reaction between the ceramic powder and the sintering additive powder and grain growth of the ceramic powder. According to the embodiment, the relative density of the obtained ceramic substrate can be 97% or more. The relative density can be calculated by (actual value measured by Archimedes' method / theoretical density) x 100%.

[0059] The sagging value is the value of the green sheet after the drying process. The drying process is the first process in which the solvent is removed. The sagging value is related to the uniformity of the solvent removal. A sagging value within a specified range indicates that the solvent and other substances have been uniformly removed. Uniform removal of the solvent can improve the uniformity of the binder removal in the degreasing process and the uniformity of the liquid phase reaction and other processes in the firing process. As a result, cracks or chips in the ceramic substrate can be suppressed during the degreasing and firing processes, and yield can be improved.

[0060] In the firing process, many green sheets are fired at once. Depending on the size of the firing furnace, hundreds to thousands of green sheets can be fired at once. By controlling the sagging value of the green sheets, it is possible to prevent cracks or chips from occurring in the ceramic substrate during the firing process.

[0061] For example, in the past, ceramic substrates were inspected for defects (cracks or chips), and if the percentage of defects was high, the manufacturing process was inspected. Defective ceramic substrates could not be reused and were discarded. In contrast, according to the embodiment, the sag value of the green sheet before firing is measured. For example, a portion of a long green sheet is cut, dried, and the sag value of the cut portion of the green sheet is measured. Then, if the evaluation value obtained by dividing the sag value 3 by the thickness of the green sheet 1 is within a predetermined range, the remaining green sheet is cut, dried, degreased, and fired. By evaluating the green sheets before firing, it is possible to avoid firing green sheets that are likely to develop defects. This prevents unnecessary firing processes from being performed on green sheets that are likely to develop defects. Furthermore, by firing only green sheets that are unlikely to develop defects in a single firing process, production efficiency and yield can be improved. Furthermore, if the evaluation value is outside the predetermined range, the remaining green sheets can be reused in the process of preparing ceramic slurry. The green sheet can be reused as a raw material for producing a green sheet as long as it is in a state before the firing process is performed. This reduces waste of raw materials and improves production efficiency. In addition, the green sheet used to measure the sag value may be reused.

[0062] Fig. 6 is a perspective view showing an example of a ceramic substrate according to an embodiment. In Fig. 6, reference numeral 6 denotes a ceramic substrate. The obtained ceramic substrate 6 may be subjected to a dividing process as necessary. The dividing process may be, for example, a scribing process.

[0063] FIG. 7 is a flowchart showing a specific example of a method for manufacturing a ceramic substrate according to an embodiment. In the manufacturing method shown in FIG. 7, a ceramic slurry containing a ceramic powder, a sintering aid powder, and one or more selected from the group consisting of a solvent and a binder is prepared (Step S1). The ceramic slurry is then formed into a sheet to produce a long green sheet (Step S2). For example, a very long green sheet is prepared in Step S2, and this long green sheet is rolled up and stored. A measurement green sheet for evaluating hardness is then cut out from the long green sheet (Step S3). The measurement green sheet is then dried (Step S4). In the drying process, it is preferable to control the time and temperature of the constant-rate drying period and the falling-rate drying period, as described above. After drying, the sag value of the measurement green sheet is measured (Step S5). An evaluation value is calculated by dividing the sag value by the thickness of the green sheet (Step S6). It is then determined whether the evaluation value is within a predetermined range (Step S7).

[0064] If the evaluation value is outside the predetermined range, the measurement green sheet and the remaining long green sheet may be mixed with a solvent (step S8). This produces a ceramic slurry from the green sheet. Step S2 can be performed again using this ceramic slurry. The produced ceramic slurry may be mixed with another ceramic slurry. When dissolving the green sheet in a solvent in step S8, the solvent content may be different from the solvent content set in the previous step S1 to adjust the hardness. For example, if the evaluation value is below 20, the solvent content in step S8 is increased compared to the solvent content in the previous step S1. If the evaluation value is below 120, the solvent content in step S8 is decreased compared to the solvent content in the previous step S1. Reusing the green sheet (ceramic powder and sintering aid powder) improves the utilization efficiency of the raw material powder.

[0065] If the evaluation value is within a predetermined range, the remaining long green sheet is cut to the substrate size (step S9). The cut green sheet is dried (step S10). In this drying process, the time and temperature of the constant rate drying period and the falling rate drying period are controlled, as in step S3. The dried green sheet is degreased and then fired (step S11). Through the above steps, the ceramic substrate according to the embodiment is manufactured.

[0066] The order of the steps shown in Fig. 7 can be changed as appropriate. For example, the green sheet may be cut to the substrate size after step S2. In this case, each cut green sheet A is dried to produce green sheet B. A measurement green sheet is cut out from green sheet B. Alternatively, a measurement green sheet is cut out from green sheet A and dried. The dried measurement green sheet is prepared by either method, and the sagging value is measured. If the evaluation value is within the predetermined range, the remaining green sheet B is degreased and fired.

[0067] When a long green sheet to be wound into a roll is produced, it is most preferable to cut out a measurement green sheet from the long green sheet, as shown in FIG. 7 . The measurement green sheet is then dried, and an evaluation value is calculated. If the evaluation value is within a predetermined range, the remaining green sheet is cut to the substrate size. In another method, for example, the green sheet is cut to the substrate size before measuring the sag value. In this method, if the evaluation value is outside the predetermined range, the process of cutting the green sheet becomes unnecessary. As shown in FIG. 7 , by switching whether or not to cut the green sheet depending on the evaluation value after measuring the sag value, the production efficiency of ceramic substrates can be improved.

[0068] (Examples) (Examples 1 to 7, Comparative Examples 1 and 2) Ceramic slurries were prepared by mixing ceramic powder, sintering aid powder, organic binder, and solvent. The compositions and contents are as shown in Table 1. The mixed amounts of ceramic powder and sintering aid powder are values ​​when the total of ceramic powder and sintering aid powder is 100% by mass. The total added amount of organic binder and solvent is values ​​when the total of ceramic powder, sintering aid powder, organic binder, and solvent is 100% by mass. Furthermore, the total added amount of organic binder and solvent is the value before the drying process.

[0069]

[0070] The ceramic slurries according to the examples and comparative examples were used to form sheets by the doctor blade method. Long green sheets were obtained by sheet forming. Green sheets of the desired size were cut from the long green sheets. A drying process was then carried out. The thickness of the green sheets after the drying process and the total content of the binder and solvent were measured. The thickness of the green sheets was set within a range of 1.2 to 2.0 times the thickness of the desired ceramic substrate.

[0071] During the drying process, the weight loss rate R1 of the solvent during the falling-rate drying period and the weight loss rate R2 of the solvent during the constant-rate drying period were measured. The ratio R2 / R1 of the weight loss rate R2 of the solvent during the constant-rate drying period to the weight loss rate R1 of the solvent during the falling-rate drying period was calculated. Table 2 shows the results for the green sheet size, the target ceramic substrate thickness, the weight loss rate R1, and the ratio R2 / R1.

[0072]

[0073] In the examples, the total content of the solvent and binder in the green sheet after the drying process was in the range of 10% by mass or more and 40% by mass or less. In comparative example 1, the total content of the solvent and binder in the green sheet was less than 10% by mass. In comparative example 2, the total content of the solvent and binder in the green sheet was more than 40% by mass.

[0074] A green sheet measuring 10 mm on the short side and 50 mm on the long side was cut out from the green sheet that had been dried. This green sheet measuring 10 mm on the short side and 50 mm on the long side was called the "measurement green sheet."

[0075] The measurement green sheets were taken from green sheets (green sheets after the drying process) having a thickness in the range of 1.2 times or more and 2.0 times or less of the thickness of the target ceramic substrate. ３ ~10 ７ mm ２ Four measurement green sheets were taken from each green sheet.

[0076] The sagging value was measured using a test green sheet. The size of the fixing jig was 10 mm long x 2 mm wide. The front and back of the test green sheet were fixed with the fixing jig at a point 10 mm from the end of the long side. The test green sheet was placed on a plate material and kept horizontal. The plate material was removed and 5 seconds later, the sagging value of the other end of the test green sheet was measured. The results are shown in Table 3. Table 3 shows the minimum and maximum sagging values ​​obtained from four green sheets for each example and comparative example.

[0077]

[0078] In all of the examples, the evaluation value obtained by dividing the sagging value by the thickness of the green sheet was within the range of 20 to 120. In comparative example 1, the evaluation value was 5 to 10. This is because the total content of solvent and binder in the green sheet was too low. Because there was little solvent and binder, the green sheet was harder than in the examples, and the sagging value was also small. In comparative example 2, the evaluation value was 168. This is because the total content of solvent and binder in the green sheet was too high. Because there was a lot of solvent and binder and the green sheet was softer than in the examples, the green sheet stretched and the sagging value increased. In all of the comparative examples, the evaluation values ​​were outside the preferred range.

[0079] Next, the green sheets according to the examples and comparative examples were cut to substrate size and subjected to a degreasing process. The temperature in the degreasing process was set to 500°C. After the degreasing process, the occurrence of cracks and chips in the degreased bodies was visually inspected. For each example, 1,100 degreased bodies were produced. Degreased bodies with cracks or chips were judged to be defective, and degreased bodies without cracks or chips were judged to be non-defective. Based on the results of the judgment, the rate of occurrence of defective products was calculated.

[0080] Next, 1,000 degreased substrates judged to be good were prepared and subjected to a firing process. The temperature for the firing process was set to 1,800°C. After the firing process, the ceramic substrates were visually inspected for cracks and chips. Ceramic substrates with cracks or chips were judged to be defective, and ceramic substrates without cracks or chips were judged to be good. Based on the judgment results, the rate of defective products was calculated. The results are shown in Table 4.

[0081]

[0082] As can be seen from Table 4, in the examples, the incidence of cracks or chips was suppressed in both the degreasing process and the firing process. From these results, it can be seen that it is preferable that the value obtained by dividing the sag value 3 by the thickness of the green sheet 1 is in the range of 20 to 120. Furthermore, in Examples 1, 2, 4, and 5, where the sag value is in the range of 20 mm to 35 mm, the incidence of cracks or chips is even smaller, and the yield is further improved. Furthermore, as shown in the examples, the yield is improved even when the composition or size of the green sheet is changed. It can be seen that controlling the sag value is effective in improving the yield of ceramic substrates.

[0083] When the evaluation value was outside the predetermined range as in the comparative example, the incidence of cracks or chips increased, resulting in a decrease in yield.

[0084] Embodiments of the present invention include the following features. (Feature 1) A green sheet containing ceramic powder, wherein the green sheet is cut out to a size of 10 mm short sides x 50 mm long sides, one end of the cut green sheet in the long side direction is fixed with a jig, the cut green sheet is supported horizontally, and the sagging value (mm) of the other end of the long side direction is measured 5 seconds after the horizontal support is removed, and the value obtained by dividing the sagging value by the thickness (mm) of the green sheet is within the range of 20 to 120. (Feature 2) The green sheet according to Feature 1, having a thickness within the range of 0.1 mm to 1 mm. (Feature 3) The green sheet according to Feature 1 or 2, having a sagging value within the range of 10 mm to 40 mm. (Feature 4) The green sheet according to any one of Features 1 to 3, containing silicon nitride as a main component. (Feature 5) The green sheet according to any one of Features 1 to 4, further comprising one or more elements selected from the group consisting of a solvent and a binder, wherein the total content of the one or more elements selected from the group is within a range of 10% by mass to 40% by mass. (Feature 6) A method for producing a green sheet, comprising: preparing a ceramic slurry containing a ceramic powder, a sintering aid powder, and one or more elements selected from the group consisting of a solvent and a binder; forming the ceramic slurry into a sheet to produce a ceramic green sheet; drying the green sheet; and cutting out and evaluating a portion of the green sheet, wherein the evaluation comprises fixing one end in a long side direction with a jig and supporting the other end in the long side direction horizontally, measuring a sagging value (mm) of the other end 5 seconds after removing the horizontal support, and evaluating the green sheet based on a value obtained by dividing the sagging value by a thickness (mm) of the green sheet. (Feature 7) The method for producing a green sheet according to Feature 6, wherein, when the green sheet is determined to be non-defective in the evaluation, the remaining portion of the green sheet is cut to a predetermined size. (Feature 8) The method for producing a green sheet according to Feature 6 or 7, wherein the green sheet is mainly composed of silicon nitride.(Feature 9) A method for manufacturing a ceramic substrate, comprising carrying out the method for manufacturing a green sheet according to Feature 7, and degreasing and firing the cut green sheet. (Feature 10) A method for manufacturing a ceramic substrate according to Feature 9, wherein the green sheet is mainly composed of silicon nitride. (Feature 11) A method for measuring the hardness of a green sheet, comprising fixing one end in the long side direction of a green sheet having short and long sides with a jig, supporting the green sheet horizontally, and measuring the sagging value of the other end in the long side direction 5 seconds after removing the horizontal support. (Feature 12) The method for measuring the hardness of a green sheet according to Feature 11, wherein the length of the green sheet in the short side direction is 10 mm, the length of the green sheet in the long side direction is 50 mm, and the thickness of the green sheet is 0.1 mm or more and 1 mm or less. (Feature 13) The method for measuring the hardness of a green sheet according to Feature 11 or 12, wherein the main component of the green sheet is silicon nitride. (Feature 14) A method for measuring the hardness of a green sheet, the method comprising: horizontally supporting a green sheet containing ceramic powder and having a short side of 10 mm and a long side of 50 mm; fixing one end of the long side with a jig; and measuring a sagging value of the other end of the long side when the horizontal support is removed.

[0085] Although several embodiments of the present invention have been described above, these embodiments are presented by way of example only and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, modifications, etc. can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as set forth in the claims. Furthermore, the above-described embodiments can be implemented in combination with each other.

[0086] DESCRIPTION OF SYMBOLS 1...Green sheet 2a...Fixing jig 2b...Support member 3...Sagging value 4...Long green sheet 5...Cutting point 6...Ceramic substrate

Claims

1. A green sheet containing ceramic powder, wherein when a green sheet having a size of 10 mm on a short side and 50 mm on a long side is cut out, one end of the long side of the cut out green sheet is fixed with a jig, the cut out green sheet is supported horizontally, and the sagging value (mm) of the other end of the long side is measured 5 seconds after the horizontal support is removed, the value obtained by dividing the sagging value by the thickness (mm) of the green sheet is within the range of 20 to 120.

2. The green sheet according to claim 1, having a thickness in the range of 0.1 mm to 1 mm.

3. The green sheet according to claim 1 or 2, wherein the sagging value is within the range of 10 mm or more and 40 mm or less.

4. The green sheet according to claim 1 or 2, which contains silicon nitride as a main component.

5. The green sheet according to claim 3, containing silicon nitride as a main component.

6. A green sheet according to claim 1 or 2, containing one or more selected from the group consisting of a solvent and a binder, the total content of said one or more selected from said group being within the range of 10 mass % or more and 40 mass % or less.

7. The green sheet according to claim 5, comprising one or more selected from the group consisting of a solvent and a binder, the total content of said one or more selected from said group being within the range of 10 mass % or more and 40 mass % or less.

8. A method for manufacturing a green sheet, comprising the steps of: preparing a ceramic slurry containing a ceramic powder, a sintering aid powder, and one or more selected from the group consisting of a solvent and a binder; forming the ceramic slurry into a sheet to produce a ceramic green sheet; drying the green sheet; and cutting out a portion of the green sheet for evaluation, wherein in the evaluation, one end in the long side direction is fixed with a jig and the other end in the long side direction is supported horizontally, the sagging value (mm) of the other end is measured 5 seconds after the horizontal support is removed, and the green sheet is evaluated based on the value obtained by dividing the sagging value by the thickness (mm) of the green sheet.

9. The method for producing a green sheet according to claim 8, further comprising the step of cutting the remaining portion of the green sheet to a predetermined size if the green sheet is determined to be non-defective in the evaluation.

10. The method for producing a green sheet according to claim 8 or 9, wherein the main component of said green sheet is silicon nitride.

11. A method for producing a ceramic substrate, comprising carrying out the method for producing a green sheet according to claim 9, and degreasing and firing the cut green sheet.

12. The method for producing a ceramic substrate according to claim 11, wherein the main component of the green sheet is silicon nitride.

13. A method for measuring the hardness of a green sheet, comprising: fixing one end of a green sheet having short and long sides in the long side direction with a jig; supporting the green sheet horizontally; and measuring the sagging value of the other end in the long side direction 5 seconds after the horizontal support is removed.

14. A method for measuring the hardness of a green sheet according to claim 13, wherein the length of the green sheet in the short side direction is 10 mm, the length of the green sheet in the long side direction is 50 mm, and the thickness of the green sheet is 0.1 mm or more and 1 mm or less.

15. The method for measuring the hardness of a green sheet according to claim 13 or 14, wherein the main component of the green sheet is silicon nitride.

16. A method for measuring the hardness of a green sheet, comprising: supporting a green sheet containing ceramic powder and measuring 10 mm on a short side and 50 mm on a long side horizontally; fixing one end of the long side with a jig; and measuring the sagging value of the other end of the long side when the horizontal support is removed.

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