JP7468769B2 - Manufacturing method of silicon nitride sintered substrate - Google Patents

Description

The present invention relates to a method for producing high-strength, small-warping silicon nitride sintered substrates with high yield.

Circuit boards used in power semiconductor modules and the like are composed of a silicon nitride sintered substrate, which has high insulation properties, mechanical strength, thermal conductivity, etc., and a metal circuit board and heat sink joined to it by brazing or direct bonding (DBC). In the case of a semiconductor module, a semiconductor chip is joined to the circuit board. In order to efficiently dissipate heat from the semiconductor chip during operation, the silicon nitride sintered substrate is required to have high thermal conductivity. Of course, the silicon nitride sintered substrate is also required to have high insulation properties (electrical resistivity).

WO 2010/002001 (Patent Document 1) discloses a method for producing a silicon nitride sintered substrate with high strength, high thermal conductivity, and excellent thermal shock resistance, by heating a sheet-shaped molded body consisting of silicon nitride powder, magnesium oxide powder, and rare earth oxide powder from 1650°C to a temperature of 1800-2000°C at a rate of 300°C/hr or less, sintering it by holding it at that temperature for 2-10 hours, and then cooling it to 1500°C at a rate of 100°C/hr or more.

WO 2013/146789 (Patent Document 2) discloses a method for producing a silicon nitride sintered substrate having a uniform distribution of grain boundary phases, suppressed Mg segregation, suppressed warping and waviness, and sufficient mechanical strength and thermal conductivity, by holding a sheet-shaped molded body containing silicon nitride powder and a sintering aid powder containing Mg and at least one rare earth element in a first temperature range (1650-2000°C) and a second temperature range (1400-1700°C) and then cooling at a rate of 100°C/hr or more.

However, even in the methods of Patent Documents 1 and 2, when the number of sheet-shaped bodies put into the firing furnace is increased, it becomes impossible to obtain a high-strength, warp-suppressed sintered silicon nitride substrate with good yield. Therefore, even when the number of sheet-shaped bodies put into the firing furnace is increased to increase productivity (for example, when the total volume of the sheet-shaped bodies in the firing furnace is 2000 ^cm3 or more), it is desired to develop a method that can obtain a high-strength, warp-suppressed sintered silicon nitride substrate with good yield.

WO 2010/002001 Publication WO 2013/146789

The object of the present invention is therefore to provide a method for producing silicon nitride sintered substrates that can produce sintered substrates with little warping and high strength with a good yield, even if the number of sheet-shaped bodies placed in the sintering container in the sintering furnace is increased.

A method for manufacturing a silicon nitride sintered substrate, comprising a stacking step of stacking a plurality of green sheets in a separable manner to form a plurality of green sheet stacks, and a sintering step of sintering the plurality of green sheet stacks to form a plurality of silicon nitride sintered substrates, wherein the total number of the green sheets is 80 or more, and 70% or more of the plurality of silicon nitride sintered substrates have a warp of 3.2 μm/mm or less, and 70% or more of the plurality of silicon nitride sintered substrates have a three-point bending strength of 700 MPa or more.

The method of the present invention can produce sintered substrates with high yield and efficiency even when the number of green sheets sintered at one time in the sintering furnace is increased.

1 is a flowchart showing a method for manufacturing a silicon nitride sintered substrate. FIG. 2 is a cross-sectional view showing a stack of a plurality of green sheets. FIG. 11 is a cross-sectional view showing how a weight plate is placed on the upper surface of the green sheet stack. FIG. 1 is a cross-sectional view showing a double-structure firing container that houses a multi-stage frame on which a green sheet stack with a weight plate placed thereon is placed. 10 is a cross-sectional view showing a state in which one green sheet stack is placed on the lower plates of the inner container and the outer container via a placing plate. FIG. 6 is a cross-sectional view showing a state in which another mounting plate is placed on the mounting plate shown in FIG. 5 via a vertical frame member, and a second green sheet stack is placed thereon. FIG. FIG. 2 is a cross-sectional view showing a small firing furnace. FIG. 2 is a cross-sectional view showing a large-scale firing furnace. This is a cross-sectional view taken along line AA in FIG. 8. 1 is a graph showing a temperature profile when a silicon nitride sintered substrate is produced using a small firing furnace. 1 is a graph showing a temperature profile when a silicon nitride sintered substrate is produced using a large-scale firing furnace. 1 is a cross-sectional view showing how a surface of a silicon nitride sintered substrate placed on a surface plate is irradiated with laser light to measure warpage. FIG. 1 is a plan view showing how a surface of a silicon nitride sintered substrate placed on a surface plate is irradiated with laser light along three scanning lines. FIG. FIG. 1 is a cross-sectional view showing a method for determining the warpage of a silicon nitride sintered substrate. 4 is a graph showing temperature patterns of a second temperature holding zone _P2 , a first cooling zone _P3 , and a second cooling zone _P4 in the sintering step of Example 1.

The embodiments of the present invention will be described in detail below with reference to the drawings, but the present invention is not limited thereto and can be modified as appropriate within the scope of the technical concept of the present invention. The description of each embodiment also applies to other embodiments unless otherwise specified.

[1] Temperature control method in a sintering vessel The method of the present invention is a temperature control method in a sintering vessel during a process in which a sintering vessel containing a green sheet stack obtained by stacking a plurality of green sheets formed from a slurry of raw material powder is placed in a sintering furnace and sintered, and a process in which the sintered body obtained is cooled. The method of the present invention is particularly suitable for producing a large and thin silicon nitride sintered substrate with each side having a length of 100 mm or more and a thickness of 0.7 mm or less.

The raw material powder used in one embodiment of the present invention contains 80 to 98.3 mass% silicon nitride (Si ₃ N ₄ ) powder as a main component, and contains 0.7 to 10 mass% (oxide equivalent) Mg compound powder and 1 to 10 mass% (oxide equivalent) of at least one rare earth element compound powder as a sintering aid. From the viewpoints of density, bending strength and thermal conductivity of the silicon nitride sintered substrate, the alpha conversion rate of the silicon nitride powder is preferably 20 to 100%.

If the silicon nitride powder is less than 80% by mass, the bending strength and thermal conductivity of the resulting silicon nitride sintered substrate will be too low. On the other hand, if the silicon nitride powder is more than 98.3% by mass, there will be a shortage of sintering aids, and a dense silicon nitride sintered substrate will not be obtained.

If the Mg compound powder is less than 0.7 mass% in terms of oxide, the liquid phase formed at low temperatures is insufficient. On the other hand, if the Mg compound powder is more than 10 mass% in terms of oxide, the amount of Mg volatilized increases, making it easier for voids to form in the silicon nitride sintered substrate. The content of the Mg compound powder (in terms of oxide) is preferably 0.7 to 7 mass%, more preferably 1 to 5 mass%, and most preferably 2 to 5 mass%.

If the rare earth element compound powder is less than 1 mass% in terms of oxide, the bond between silicon nitride particles becomes weak, and cracks easily extend through the grain boundaries, resulting in a low bending strength. On the other hand, if the rare earth element compound powder is more than 10 mass% in terms of oxide, the proportion of the grain boundary phase increases, and the thermal conductivity decreases. The content of the rare earth element compound powder (in terms of oxide) is preferably 2 to 10 mass%, more preferably 2 to 5 mass%. Therefore, the content of the Si ₃ N ₄ powder is preferably 83 to 97.3 mass%, more preferably 90 to 97 mass%. As the rare earth element, Y, La, Ce, Nd, Pm, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, etc. can be used, and among them, Y is effective for increasing the density of the silicon nitride sintered substrate and is preferable. It is preferable to use Mg and the rare earth element in the form of oxide powder. Therefore, a preferred sintering aid is a combination of MgO powder and Y ₂ O ₃ powder.

1 is a flow chart showing a preferred example of a method for manufacturing a silicon nitride sintered substrate. For the sake of simplicity, the silicon nitride powder is referred to as "Si ₃ N ₄ powder", the Mg compound powder is referred to as "MgO powder", and the rare earth element compound powder is referred to as "Y ₂ O ₃ powder". Of course, the present invention is not limited to these raw material powders.

(1) Raw material powder mixing process S
In addition to the _1Si3N4 powder, _MgO powder _, and _Y2O3 powder, a plasticizer, an organic binder, and an organic solvent (e.g., ethyl alcohol) are mixed in a ball mill or the like to prepare a slurry. The solid content concentration of the slurry is preferably 30 to 70 mass%.

(2) Molding process S2
After the slurry is degassed and viscous, it is formed into a long strip green sheet, for example, by a doctor blade method. The thickness of the green sheet is appropriately set in consideration of the thickness of the silicon nitride sintered substrate to be formed (for example, 0.7 mm or less) and the sintering shrinkage rate. The long strip green sheet is punched or cut to obtain individual green sheets of a shape and size that can obtain a silicon nitride sintered substrate with each side being 100 mm or more in length.

(3) Deposition step S3
In order to efficiently manufacture a silicon nitride sintered substrate, a plurality of green sheets 1 are stacked so as to be easily separated, as shown in Fig. 2, to form a green sheet stack 10. It is preferable to interpose a boron nitride (BN) powder layer 2 between the green sheets 1 so that they can be easily separated after sintering. The thickness of the BN powder layer 2 is preferably about 1 to 20 µm. The BN powder layer 2 can be formed by applying a slurry of BN powder to one side of each green sheet 1 by spraying or brushing.

As shown in FIG. 3, in order to suppress warping of the resulting silicon nitride sintered substrate, a weight plate 11 is placed on the top surface of the green sheet stack 10, and a load is applied to each green sheet 1. The load applied to each green sheet 1 is preferably in the range of 10 to 600 Pa. If the load is less than 10 Pa, the sintered silicon nitride sintered substrate is likely to warp. On the other hand, if the load exceeds 600 Pa, each green sheet 1 is restrained by the load, preventing smooth shrinkage during sintering, and the silicon nitride sintered substrate is likely to crack or break. The load applied to each green sheet 1 is preferably 20 to 300 Pa, more preferably 20 to 200 Pa, and most preferably 30 to 150 Pa.

If the weight of the weight plate 11 is W ₁ g, the weight and area of each green sheet 1 are W ₂ g and S cm ² , respectively, and the number of green sheets 1 in the stack 10 is n, then the load on the topmost green sheet 1a is 98×(W ₁ /S) Pa, and the load on the bottommost green sheet 1b is 98×[W ₁ +W ₂ ×(n-1)]/S Pa. For example, if a 2 mm thick BN plate is used as the weight plate 11 and the green sheet stack 10 has 10 green sheets 1, the load on the bottommost green sheet 1b is about 3 to 4 times the load on the topmost green sheet 1a. Taking this into consideration, the weight of the weight plate 11 and the number of green sheets 1 in the green sheet stack 10 are set. The weight _W1 of the weight plate 11 is preferably set so that even the bottommost green sheet 1b receives a load in the range of 10 to 600 Pa and is sintered without shrinkage being restrained and without warping.

(4) Degreasing step S4
Since the green sheet 1 contains an organic binder and a plasticizer, the green sheet stack 10 is degreased by heating it to 900° C. or less (preferably 400 to 800° C.) in the atmosphere before the sintering step S5. Since the green sheet 1 is brittle after degreasing, it is preferable to degrease it in the state of the stack 10.

(5) Sintering step S5
(A) Sintering container Figure 4 shows an example of a sintering container for simultaneously sintering a plurality of green sheet deposits 10. The sintering container 20 is composed of an assembly 30 in which mounting plates 21 on which the green sheet deposits 10 are placed are stacked in multiple stages, an inner container 40 that houses the assembly 30, and an outer container 50 that houses the inner container 40. The spacing between the mounting plates 21 adjacent in the vertical direction is maintained by a vertical frame member 22. By making the sintering container 20 a double structure of the inner container 40 and the outer container 50, it is possible to suppress the decomposition of _Si3N4 in the green sheet ₁ and the volatilization and decomposition of MgO, and also to suppress the decomposition of MgO contained in the packing powder described later.

Both the inner container 40 and the outer container 50 are preferably made of BN, but the outer container 50 can also be made of graphite coated with BN by CVD. In the case of the outer container 50 made of graphite coated with BN, the graphite has good thermal conductivity, making it easy to uniformize the temperature distribution during heating and cooling, and not only can it suppress warping of the silicon nitride sintered substrate, but the BN coating can also prevent the generation of a reducing atmosphere (which may decompose _Si3N4 ) due to the graphite. The inner container 40 is made of a lower plate 40a, a side plate 40b, and an upper plate 40c, _and the outer container 50 is made of a lower plate 50a, a side plate 50b, and an upper plate 50c.

If the mounting plate 21 is warped, the bottommost green sheet 1b in contact with the mounting plate 21 will have some parts that are in contact with the top surface of the mounting plate 21 and some that are not. This causes uneven shrinkage in the green sheet 1b, which is the cause of warping, since the non-contact parts of the green sheet 1b are more likely to shrink during sintering and the contact parts are less likely to shrink, resulting in uneven shrinkage in the green sheet 1b. Furthermore, the warping of the bottommost green sheet 1b will also affect the upper green sheets 1. For this reason, the top surface of the mounting plate 21 needs to be as flat as possible, and specifically, it is preferable that the warping be within 3.2 μm/mm. The warping of the mounting plate 21 can be measured using the same method as that for the warping of a silicon nitride sintered substrate.

As shown in FIG. 4, the packing powder 24 is preferably placed in the inner container 40. The packing powder 24 is preferably a mixed powder containing, for example, a powder of an oxide (MgO, etc.) or nitride (MgSiN ₂ , etc.) containing 0.1 to 50% by mass of Mg, a powder of silicon nitride (Si ₃ N ₄ ) containing 25 to 99% by mass, and a powder of boron nitride (BN) containing 0.1 to 70% by mass. The silicon nitride powder and the powder of an oxide or nitride containing Mg in the packing powder 24 volatilize at a high temperature of 1400° C. or higher, adjust the partial pressure of Mg and Si in the sintering atmosphere, and suppress the volatilization of silicon nitride and magnesia from the green sheet 1. The BN powder prevents the silicon nitride powder and the powder of an oxide or nitride containing Mg in the packing powder 24 from cohesion. In order to facilitate the handling of the packing powder 24 and to prevent it from coming into contact with the green sheet 1, it is preferable to place the packing powder 24 on the topmost loading plate 21a. In addition, a thermocouple 60 for measuring the temperature inside the firing vessel 20 is provided on the uppermost mounting plate 21a.

The amount of the packing powder is preferably 0.01 to 0.2 g/ ^cm2 per total surface area of the green sheet 1 (when there are multiple green sheets 1, the total surface area of all the green sheets 1). If the amount of the packing powder is less than 0.01 g/ ^cm2 , the decomposition of _Si3N4 and _MgO from the green sheet during sintering cannot be sufficiently suppressed, which causes a decrease in density. If the amount of the packing powder is more than 0.2 g/ ^cm2 , too much Mg is volatilized from the packing powder, which causes a decrease in strength of the silicon nitride sintered substrate, abnormal appearance (e.g., discoloration), segregation of MgO, etc.

In the illustrated example, the packing powder 24 is placed inside the inner container 40, but the packing powder 24 may be placed between the inner container 40 and the outer container 50. In that case, the scattered packing powder 24 is sintered while still adhering to the surface of the green sheet 1, which can prevent defects such as the formation of irregularities on the silicon nitride sintered substrate.

As shown in FIG. 5, the lower plate 40a of the inner container 40 is placed on the upper surface of the lower plate 50a of the outer container 50, the loading plate 21 is placed on the upper surface of the lower plate 40a, and the green sheet stack 10 and the weight plate 11 are placed on it. As shown in FIG. 6, a vertical frame member 22 is installed on the outer periphery of the loading plate 21, the loading plate 21 of the next stage is placed, and the green sheet stack 10 and the weight plate 11 are placed on it. After forming an assembly 30 with the desired stages (stage number: m) of green sheet stacks 10 and weight plates 11 placed on it, the packing powder 24 is placed on the upper surface of the loading plate 21a of the top stage. Next, the side plate 40b and the upper plate 40c of the inner container 40 are assembled, and further the side plate 50b and the upper plate 50c of the outer container 50 are assembled to complete the firing container 20 containing the stack 10. A desired number of such firing containers 20 are placed in a firing furnace (not shown).

The number of green sheets in the stack 10 can be, for example, 10 to 20. For example, if each stack 10 is made up of 10 green sheets 1, the number of stages m can be 8 to 18 stages (80 to 180 green sheets), and further, for example, the number of stages m can be 10 to 16 stages (100 to 160 green sheets). For example, if the stack 10 is made up of 20 green sheets, m is, for example, about 10 (200 green sheets).

(B) Sintering furnace (1) Small-sized sintering furnace As shown in FIG. 7, the small-sized sintering furnace 70 in which one sintering container 20 is placed is equipped with a heater (not shown) and a carbon-made cylindrical body 72 surrounding the sintering container 20 on a base plate 71. In the small-sized sintering furnace 70, the temperature in the sintering container 20 can quickly follow the temperature in the small-sized sintering furnace 70 (the temperature between the inner wall 70a of the small-sized sintering furnace 70 and the outer wall 72a of the cylindrical body 72), so that the temperature in the sintering container 20 is considered to be almost equal to the temperature in the small-sized sintering furnace 70. Therefore, in the present invention, the temperature in the sintering container 20 is represented by the temperature in the small-sized sintering furnace 70. The temperature in the small-sized sintering furnace 70 can be measured, for example, by measuring the temperature of a target (not shown) placed near the outer wall 72a of the cylindrical body 72 using a radiation thermometer 80. It is noted that the difference between the temperature of the green sheet deposit 10 measured by the thermocouple 60 provided on the topmost mounting plate 21a and the temperature inside the small sintering furnace 70 measured by the radiation thermometer 80 is small. Therefore, in the manufacturing process of the silicon nitride sintered substrate, which undergoes sintering temperatures higher than the heat resistance temperature of the thermocouple 60, it is preferable to represent the temperature of the green sheet deposit 10 by the temperature inside the sintering vessel 20 (by the temperature inside the small sintering furnace 70).

(2) Large-scale baking furnace As shown in Figures 8 and 9, a large-scale baking furnace 90 in which multiple baking containers 20 are arranged includes an outer shell 91, an insulating layer 92 forming a space inside the furnace, a heater (not shown), a carbon cylindrical body 93 placed in the insulating layer 92, a support plate 94 fixed to the cylindrical body 93, a base plate 95 on which multiple baking containers 20 are placed and placed on the support plate 94, a cooling pipe 96 penetrating the insulating layer 92, a valve 96g provided on the cooling pipe 96, a cooler 97 that supplies cooling gas to the cooling pipe 96, an atmospheric gas supply pipe 98 having a valve 98g, and an atmospheric gas exhaust pipe 99 having a valve 99g. A thermocouple 60 for measuring the temperature inside the baking container 20 is provided on the topmost placement plate 21a inside the baking container 20. Since the temperature inside the firing vessel 20 cannot quickly follow the temperature inside the firing furnace 90 , the temperature inside the firing vessel 20 deviates from the temperature inside the firing furnace 90 by a relatively large amount.

Since the correlation between the temperature in the firing container 20 and the temperature in the firing furnace 90 differs for each firing furnace 90, it is necessary to determine the temperature correlation for each firing furnace 90. For example, (a) measure the temperature change in the firing furnace 90 with a radiation thermometer 80 and measure the temperature change in the firing container 20 with a thermocouple 60, and (b) use the correlation obtained from that to control the temperature change in the firing furnace 90 so that the temperature change in the firing container 20 becomes a predetermined temperature change. Taking into account the heat resistance temperature of the thermocouple, the temperature change of the firing container 20 in a predetermined high temperature range may be extrapolated from the temperature change of the firing container 20 in a temperature range lower than the predetermined high temperature range (heat resistance temperature range of the thermocouple). In the case of the large firing furnace 90, as in the small firing furnace 70, there is a deviation between the temperature of the green sheet deposit 10 and the temperature in the firing container 20, but the difference between the two is small. Therefore, the temperature of the green sheet stack 10 is expressed by the temperature inside the firing vessel 20 (obtained from the temperature inside the firing furnace 90).

(C) Temperature Profile The temperature profile of one embodiment of the present invention when manufacturing a silicon nitride sintered substrate preferably has a step of raising the temperature to a temperature range of 1680 to 2000 ° C., a first temperature holding region P ₁ in which the temperature is held in the temperature range of 1680 to 2000 ° C., a second temperature holding region P ₂ in a temperature range lower than the first holding temperature P ₁ and exceeding 1400 ° C., and a cooling process after the temperature holding process (first cooling region P ₃ and second cooling region P ₄ ). The temperature profile of the cooling process is different between the small firing furnace 70 and the large firing furnace 90. FIG. 10 shows a preferred temperature profile P when the small firing furnace 70 is used, and FIG. 11 shows a preferred temperature profile P when the large firing furnace 90 is used (an enlargement of the first temperature holding region P ₁ and after). In the graph of FIG. 10, the temperature shown on the vertical axis is the temperature in the small firing furnace 70 measured by the radiation thermometer 80, but the temperature in the firing container 20 is represented by the temperature in the small firing furnace 70. Furthermore, since the temperature of the green sheet deposit 10 quickly follows the temperature inside the firing vessel 20 , the temperature on the vertical axis may be regarded as being substantially the same as the temperature of the green sheet deposit 10 .

(a) Temperature rise region The average temperature rise rate of the entire temperature rise region is not particularly limited, but as shown in FIG. 10, it is preferable to provide a heat-relaxing region P ₀ in the middle of the temperature rise. The heat-relaxing region P ₀ is a temperature region in which the sintering aid contained in the green sheet 1 reacts with the oxide layer on the surface of the silicon nitride particles to generate a liquid phase. In the heat-relaxing region P ₀ , the flow of the generated liquid phase is encouraged to rearrange the silicon nitride particles, and at the same time, the phase is transformed from α type to β type to be densified. As a result, a silicon nitride sintered substrate having a small pore size and porosity, and high bending strength and thermal conductivity is obtained through the first temperature holding region P ₁ and the second temperature holding region P _2. It is preferable that the temperature T ₀ of the heat-relaxing region P ₀ is in the range of 1400 to 1600 ° C., which is lower than the temperature T ₁ of the first temperature holding region P ₁ , the heating rate in the heat-relaxing region P ₀ is 300 ° C./hr or less, and the heating time t ₀ is 0.5 to 30 hours. The heating rate may include 0° C./hr, i.e., the cooling zone _P0 may be a temperature holding zone where the temperature is held constant. The heating rate in the cooling zone _P0 is more preferably 1 to 150° C./hr, and most preferably 1 to 100° C./hr. The heating time _t0 is more preferably 1 to 25 hours, and most preferably 5 to 20 hours.

(b) Temperature holding region The sintering process preferably has a first temperature holding region _P1 in the temperature range of 1680 to 2000 ° C. and a second temperature holding region _P2 in the temperature range lower than the first holding temperature and higher than 1400 ° C. The first temperature holding region _P1 is a region in which the silicon nitride particles generated in the heat-reducing region _P0 grow while rearranging in the liquid phase, and is a temperature region for further densification. In consideration of the size and aspect ratio (ratio of major axis to minor axis) of the β-type silicon nitride particles, the formation of voids due to the volatilization of the sintering aid, _etc. , it is preferable to set the temperature _T1 of the first temperature holding region P1 in the range of 1680 to 2000 ° _C. and the holding time _t1 to about 1 to 30 hours. If the temperature _T1 of the first temperature holding region P1 is less than 1680 ° C., it is difficult to densify the silicon nitride sintered body. On the other hand, if the temperature _T1 exceeds 2000° C., the volatilization of the sintering aid and the decomposition of silicon nitride become intense, making it difficult to obtain a dense silicon nitride sintered body. Note that, within the temperature range of 1680 to 2000° C., the heating temperature _T1 may be changed (for example, gradually increased) within the first temperature holding region _P1 .

The temperature _T1 in the first temperature holding zone _P1 is more preferably within the range of 1750 to 1950° C., and most preferably within the range of 1800 to 1900° C. The holding time _t1 is more preferably 2 to 20 hours, and most preferably 3 to 10 hours.

The second temperature holding zone _P2 is considered to be a temperature zone in which the liquid phase that has passed through the first temperature holding zone _P1 is maintained as is or in a solid-liquid coexisting state by holding the sintered body at a temperature _T2 slightly lower than the temperature _T1 of the first temperature holding zone _P1 . The temperature _T2 of the second temperature holding zone _P2 is preferably higher than 1400°C and lower than the temperature _T1 of the first temperature holding zone _P1 , and more specifically, is preferably higher than 1400°C and lower than 1800°C. The holding time _t2 of the second temperature holding zone P2 is preferably 0.5 to 45 hours. By providing the second temperature holding zone _P2 after the first temperature holding zone _P1 , the warpage of the silicon nitride sintered substrate can be kept within 3.2 μm/mm _.

If the temperature _{T2 in} the second temperature holding zone P2 is 1400°C or less, the grain boundary phase is likely to crystallize, and the bending strength of the resulting silicon nitride sintered substrate may decrease. The temperature _T2 is more preferably 1500 to 1700°C. The holding time _{t2 in} the second temperature holding zone P2 is more preferably 0.5 to 10 hours, and most preferably 1 to 5 hours. If the holding time _{t2 in} the second temperature holding zone P2 is less than 0.5 hours, the grain boundary phase is not sufficiently homogenized.

(c) Cooling Zone The cooling zone is a temperature zone in which the liquid phase maintained in the second temperature holding zone _P2 is solidified and the position of the resulting grain boundary phase is fixed. The temperature profile of the cooling step differs between the small sintering furnace and the large sintering furnace.

(1) In the case of a small sintering furnace When a silicon nitride sintered substrate is manufactured using a small sintering furnace 70, the temperature profile of the cooling process has a first cooling region _P3 in which the temperature in the sintering container 20 is from 1650°C to a temperature _T3 lower than the solidification temperature of the grain boundary phase, and a second cooling region _P4 from temperature _T3 to 900°C. The solidification temperature of the grain boundary phase is the temperature at which the solidification of the grain boundary phase ends, and the solidification temperature region of the grain boundary phase is up to that point, and the hardening temperature region thereafter. For example, in silicon _nitride containing 3.2 mass% MgO and 1.5 mass% _Y2O3 , with _the remainder being composed of _Si3N4 and unavoidable impurities, the solidification temperature of the grain boundary phase is about 1400°C, so the temperature _T3 is set to 1200°C. For example, the difference between the temperature _T3 and the solidification temperature of the grain boundary phase is preferably 100 to 300°C, more preferably 100 to 250°C. Taking as an example the case where the first cooling zone _P3 is 1650 to 1200° C. and the second cooling zone _P4 is 1200 to 900° C., a detailed explanation will be given below.

The first cooling region _P3 is considered to be the temperature region where the grain boundary phase changes from a molten state to a supercooled state, and if the cooling rate in this temperature region is low, the Mg added as a sintering aid separates and an Mg aggregate phase is generated. The second cooling region _P4 is considered to be the temperature region where the grain boundary phase changes from a supercooled state to a glass state (solid), and if the cooling rate is high, the warping of the silicon nitride sintered substrate increases. Therefore, the first average cooling rate _v1 in the sintering container 20 in the first cooling region P3 must be higher than the second average cooling rate _v2 in the sintering container 20 in the second cooling _{region P4} .

The first average cooling rate v ₁ (° C./hr) is expressed as v ₁ = (1650° C.-1200° C.)/t ₃ [where t ₃ is the time (hr) of the first cooling region P _3. ], and the second average cooling rate v ₂ (° C./hr) is expressed as v ₂ = (1200° C.-900° C.)/t ₄ [where t ₄ is the time (hr) of the second cooling region P _4. ]. The cooling rate in the cooling temperature region higher than the first cooling region P ₃ (from the second temperature holding region P ₂ to just before the first cooling region P ₃ ) may be different from the first average cooling rate v ₁ , but it is preferable that they are the same. The first average cooling rate v ₁ in the first cooling region P ₃ can be set to 300 to 600° C./hr, and the second average cooling rate v ₂ in the second cooling region P ₄ can be set to 160 to 220° C./hr.

The ratio ( _v1 / _v2 ) of the first average cooling rate _v1 to the second average cooling rate _v2 is preferably 1.3 or more, more preferably 1.5 or more. In order to obtain the first and second average cooling rates _v1 and _v2 , it is preferable to perform forced cooling in the first cooling region _P3 by supplying an atmospheric gas (for example, nitrogen gas, or nitrogen gas mixed with argon gas) as a cooling gas into the sintering furnace, and to perform furnace cooling (natural cooling with the heater stopped while the sintering furnace is closed) in the second cooling region. The forced cooling is preferably performed by (a) cooling and circulating the atmospheric gas in the small sintering furnace 70 with a cooler, or (b) increasing the flow rate of the atmospheric gas into the small sintering furnace 70. By passing through the first and second cooling regions _P3 and _P4 under such conditions, even when the number of green sheets and sintering containers is large, a silicon nitride sintered substrate having high bending strength and suppressed warping can be manufactured with a good yield.

(2) In the case of a large-scale sintering furnace When a silicon nitride sintered substrate is manufactured using a large-scale sintering furnace 90, as shown in Fig. 11, the temperature of the green sheet deposit 10 does not follow the atmospheric temperature in the large-scale sintering furnace 90 as much as possible. Therefore, (a) a temperature (forced cooling start temperature _{) T4} at which forced cooling is started is set within a range where the atmospheric temperature in the large-scale sintering furnace 90 is lower than the temperature _T2 in the second temperature holding region P2 of the sintering step and is 1000°C or higher, and (b) the third average cooling rate _v3 in the range _P5 from the temperature (forced cooling start temperature _T4 + 100°C) to the forced cooling start temperature _T4 is set smaller than the fourth average cooling rate _v4 in the range _P6 from the forced cooling start temperature _T4 to the temperature (forced cooling start temperature _T4 - 100°C).

The forced cooling is preferably performed by (a) cooling the atmosphere gas in the large-scale firing furnace 90 with a cooler and circulating it, or (b) increasing the flow rate of the atmosphere gas into the large-scale firing furnace 90. In the case of (a), the valve 96g of the cooling pipe 96 equipped with the cooler 97 is opened, and the atmosphere gas in the large-scale firing furnace 90 is cooled with the cooler 97 and circulated. In the case of (b), the opening degree of the valve 98g of the atmosphere gas supply pipe 98 is increased, and the opening degree of the valve 99g of the atmosphere gas exhaust pipe 99 is also increased, and the flow rate of the atmosphere gas flowing through the large-scale firing furnace 90 is increased. By performing the forced cooling, the cooling temperature of the green sheet deposit 10 in each firing container 20 is composed of a cooling region _P3 with a small average cooling rate and a cooling region _P4 with a large average cooling rate, as shown in FIG. 11, and is close to the cooling temperature profile when the small-scale firing furnace 70 shown in FIG. 10 is used.

[2] Silicon nitride sintered substrate By the above method, a large and thin silicon nitride sintered substrate with a side length of 100 mm or more and a thickness of 0.7 mm or less can be obtained. The silicon nitride sintered substrate has a warp of 3.2 μm/mm or less and a three-point bending strength of 700 MPa or more. Since the warp is 3.2 μm/mm or less, when a metal circuit plate or heat sink (sometimes collectively referred to as "metal plate") is bonded to the silicon nitride sintered substrate via a brazing material or the like to form a circuit board, the occurrence of voids (parts where the silicon nitride sintered substrate is not bonded to the metal plate) is suppressed at the bonding interface between the silicon nitride sintered substrate and the metal plate, and the thermal conductivity of the circuit board is improved. The warp is preferably 2.5 μm/mm or less, more preferably 1.5 μm/mm or less. The practical lower limit of the warp is about 0.1 μm/mm.

The warpage of the silicon nitride sintered substrate 100 is measured as follows using a three-dimensional laser measuring instrument (LT-8100 manufactured by Keyence Corporation). As shown in Figs. 12 and 13, the surface of the silicon nitride sintered substrate 100 placed on the surface plate 101 is scanned with a laser beam 111 along three scanning lines _X1 , _X2 , and _X3 by a three-dimensional laser measuring instrument 110. The scanning lines _X1 and _X3 are located 10 mm inward from each side end of the silicon nitride sintered substrate 100, and the scanning line _X2 is the center line of the silicon nitride sintered substrate 100. As shown in Fig. 14, the straight line _C1 connecting both ends _A1 and _B1 of the scanning line _X1 on the surface of the silicon nitride sintered substrate 100 is made horizontal, and the height _G1 of the point _E1 most distant from the straight line _C1 and the height H1 of the point _F1 most distant from the straight line _C1 are obtained. The vertical distance ( _G1 + _H1 ) between points _E1 and _F1 is divided by the length _L1 of the scanning line _X1 to obtain the value of ( _G1 + _H1 )/ _L1 . This is also performed for the other scanning lines _X2 and _X3 to obtain the values of ( _G2 + _H2 )/ _L2 and ( _G3 + _H3 )/ _L3 . The average value of ( _G1 + _H1 )/ _L1 , ( _G2 + _H2 )/ _L2 and ( _G3 + _H3 )/ _L3 is taken as the warpage. Note that the warpage of the silicon nitride sintered substrate 100 is exaggerated in Figs. 12 and 14.

Since individual substrates are produced by cutting the silicon nitride sintered substrate, the larger the silicon nitride sintered substrate, the better the efficiency, but the greater the problem of warping. From the viewpoint of the balance between production efficiency and warping, the size of the silicon nitride sintered substrate is set to 100 mm or more in both length and width. A preferred size is 120 mm x 120 mm, and a more preferred size is 140 mm x 140 mm. The thinner the silicon nitride sintered substrate used as a heat transfer substrate for circuit elements such as semiconductors, the better, but the thinner it is, the more difficult it is to manufacture. Taking into account the performance as a heat transfer substrate and the difficulty of manufacturing, the thickness of the silicon nitride sintered substrate is set to 0.7 mm or less. The thickness of the silicon nitride sintered substrate is preferably 0.5 mm or less, and more preferably 0.4 mm or less. The lower limit of the thickness of the silicon nitride sintered substrate is 0.1 mm for practical use.

The present invention will be described in more detail with reference to the following examples, but is not limited thereto.

Example 1
A strip-shaped green sheet was formed by doctor blade method from _a slurry (solid content concentration: 60% _by mass) of raw material powders, which consisted of 3.0% by mass of MgO powder, 2.0% by mass _{of Y2O3} powder, and the remainder being Si3N4 powder and unavoidable impurities, and a green sheet 1 having a dry size of 250 mm x 200 mm x 0.42 mm was formed by punching. As shown in FIG. 2, ten green sheets 1 were stacked with BN powder between them to obtain a green sheet stack 10. A weight plate 11 was placed on each green sheet stack 10, and the stack was placed in a firing container 20 as shown in FIG. 4. The load applied by the weight plate 11 to the top green sheet 1a was 40 Pa. In the firing vessel 20, a plurality of mounting plates 21 carrying the green sheet stacks 10 were stacked in six stages to form an assembly 30, and a packing powder 24 consisting of 15 mass% magnesia powder, 55 mass% silicon nitride powder, and 30 mass% boron nitride powder was placed on the upper surface of the topmost mounting plate 21a. The warping of each mounting plate 21 was within 0.5 μm/mm. The total number of green sheets 1 per firing vessel 20 was 60, and the total volume was 1260 ^cm3 .

One sintering container 20 was placed in a small sintering furnace 70, and the green sheet ₁ was sintered by a temperature profile having a 10-hour annealing zone P ₀ at a temperature rise rate of 25° C./hr, a first temperature holding zone P ₁ at a temperature T 1 of 1860° C. for 5 hours, a second temperature holding zone P ₂ at a temperature T ₂ of 1650° C. for 1 hour, a first cooling zone P ₃ from 1650° C. to 1200° C., and a second cooling zone P ₄ from 1200° C. to 900° C. A silicon nitride sintered substrate having a thickness of 0.32 mm was produced. The average cooling rates v ₁ and v ₂ of the first cooling zone P ₃ and the second cooling zone P ₄ are shown in Table 1. The temperature pattern between the second temperature holding zone P ₂ , the first cooling zone P ₃ and the second cooling zone P ₄ is shown in FIG. 15. Table 2 also shows the number and total volume of the green sheets 1 per firing vessel, the total volume of the green sheets 1 per small firing furnace 70, and the amount of packing powder per total surface area of the green sheets 1.

Examples 2 to 5, Comparative Examples 1 and 2
A silicon nitride sintered substrate was manufactured in the same manner as in Example 1, except that the first and second average cooling rates _v1 , _v2 in the first and second cooling zones _P3 , _{P4, the} ratio ( _v1 / _{v2) of the first average cooling rate v1 to} the second average cooling rate v2, and the number of green sheets 1 per firing vessel 20 were changed as shown in Tables 1 and 2. The number and total volume of green sheets 1 per firing vessel 20, the total volume of green sheets 1 per small firing furnace 70, and the amount of packing powder per total surface area of green sheets 1 are shown in Table 2.

注：（１） 第一の冷却域Ｐ_３において１６５０℃から１２００℃まで一定の速度で冷却。
（２） 第二の冷却域Ｐ_４において１２００℃から９００℃まで一定の速度で冷却。

Notes: (1) Cool at a constant rate from 1650°C to 1200°C in the first cooling zone _P3 .
(2) In the second cooling zone _P4 , cooling is performed at a constant rate from 1200°C to 900°C.

注：（１） 焼成容器２０当たり。
（２） 小型焼成炉７０当たり。 （３） グリーンシート１の総表面積当たり。

Notes: (1) Per 20 baking containers.
(2) Per 70 small firing furnaces. (3) Per total surface area of 1 green sheet.

The warpage and bending strength of the silicon nitride sintered substrates obtained in the examples and comparative examples were measured by the following methods, and the pass rate for warpage, pass rate for bending strength, and pass rate for both warpage and bending strength are shown in Table 3.

(1) Warpage All of the silicon nitride sintered substrates were evaluated for warpage by the method described in the above item [2] using a three-dimensional laser measuring device (LT-8100 manufactured by Keyence Corporation) shown in Figures 12 and 13. Silicon nitride sintered substrates with a warpage of 3.2 μm/mm or less were deemed to pass.

(2) Bending strength Ten test pieces (4 mm wide) were cut from random locations of each silicon nitride sintered substrate, and bending strength was measured by a three-point bending test method (support roll distance: 7 mm, crosshead speed: 0.5 mm/min). When the average bending strength of the ten test pieces of each silicon nitride sintered substrate was 700 MPa or more, the silicon nitride sintered substrate was deemed to have passed the test.

As is clear from Tables 1 to 3, the silicon nitride sintered substrates of Examples 1 to 5 obtained through the first cooling region _P3 with an average cooling rate _v1 of 300 to 900°C/hr and the second cooling region _P4 with an average cooling rate _v2 of less than 300°C/hr had small warpage and high bending strength. In contrast, the silicon nitride sintered substrate of Comparative Example 1, in which the average cooling rate _v2 of the second cooling region _P4 was 300°C/hr or more, had a significantly low pass rate for warpage. Moreover, the silicon nitride sintered substrate of Comparative Example 2, in which the average cooling rate _v1 of the first cooling region _P3 was less than 300°C/hr, had a significantly low pass rate for bending strength.

1: Green sheet 1a: Topmost green sheet 1b: Bottommost green sheet 2: Boron nitride (BN) powder layer 10: Green sheet stack 11: Weight plate 20: Firing container 21: Loading plate 21a: Topmost loading plate 22: Vertical frame member 24: Packing powder 30: Assembly 40: Inner container 40a: Lower plate 40b: Side plate 40c:
Upper plate 50...outer container 50a...lower plate 50b...side plate 50c...upper plate 60...thermocouple 70...small firing furnace 70a...inner wall of small firing furnace 71...base plate of small firing furnace 72a...outer wall of carbon cylinder 80...radiation thermometer 90...large firing furnace 91...outer shell part of large firing furnace 92...insulating layer of large firing furnace 93...carbon cylinder of large firing furnace 94...support plate of large firing furnace 95...base plate of large firing furnace 96...cooling pipe of large firing furnace 97...cooler of large firing furnace 98...atmospheric gas supply pipe 98g...valve of atmospheric gas supply pipe 99...atmospheric gas exhaust pipe 99g...valve of atmospheric gas exhaust pipe 100...silicon nitride sintered substrate 101...base plate 110...three-dimensional laser measuring instrument 111...laser light P ₁ ...first temperature holding area P ₂ ...second temperature holding area P ₃ : First cooling region _P4 : Second cooling region _P5 : Range from the temperature (forced cooling start temperature _T4 + 100 ° C.) to the forced cooling start temperature _T4 ; _P6 : Range from the forced cooling start temperature _T4 to the temperature (forced cooling start temperature _T4 - 100 ° C.); _T1 : Temperature of the first temperature holding region _P1 ; _T2 : Temperature of the second temperature holding region _P2 ; _T3 : Temperature below the solidification temperature of the grain boundary phase; _T4 : Forced cooling start temperature _v1 : First average cooling rate in the sintering vessel in the first cooling region _P3 ; _V2 : Second average cooling rate in the sintering vessel in the second cooling region _P4 ; _V3 : Third average cooling rate in the range from the temperature (forced cooling start temperature _T4 + 100 ° C.) to the forced cooling start temperature _{T4 ;} Fourth average cooling rate in the range from the temperature (forced cooling start temperature _T4 - 100 ° C.) to the temperature (forced cooling start temperature _T4 - 100 ° C.) _X1 , _X2 , _X3 : Scanning line

Claims (4)

a stacking step of separably stacking the plurality of green sheets to form a plurality of green sheet stacks;
and a sintering step of sintering the plurality of green sheet stacks to obtain a plurality of silicon nitride sintered substrates,
The total number of the green sheets is 80 or more,
Among the plurality of silicon nitride sintered substrates, 70% or more of the silicon nitride sintered substrates have a warp of 3.2 μm/mm or less, and 70% or more of the silicon nitride sintered substrates have a three-point bending strength of 700 MPa or more.
A method for manufacturing a silicon nitride sintered substrate. The plurality of silicon nitride sintered substrates each have a side length of 100 mm or more.
A method for producing the silicon nitride sintered substrate according to claim 1. The plurality of silicon nitride sintered substrates each have a thickness of 0.7 mm or less.
The method for producing the silicon nitride sintered substrate according to claim 1 or 2. The total volume of the green sheet is 1680 ^cm3 or more.
The method for producing the silicon nitride sintered substrate according to any one of claims 1 to 3.

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