JP3101966B2 - High thermal expansion Al2O3-SiO2-based sintered body and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION This invention is, for example, packages such as semiconductor devices - or used as an insulating material for di- material or multi-layer wiring board, than Al ₂ O ₃ quality sintered at a low temperature of room temperature to 200 DEG ° C. Al ₂ O ₃ − with high thermal expansion and low dielectric constant
The present invention relates to a SiO ₂ -based sintered body and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, integrated circuits such as LSIs are generally housed in a ceramic package (for example, a pin grid array package) using alumina as an insulating substrate which has high thermal conductivity and excellent strength, and is made of glass epoxy. It is used by being mounted on a printed circuit board.

In recent years, it has been required to increase the signal propagation speed of the LSI, and the wiring pattern has been increased in density. Therefore, it is necessary to increase the heat dissipation of the LSI and increase the signal propagation speed. . In order to improve heat dissipation, it is necessary to use C
A heat sink made of u is also attached to an alumina substrate.

[0004]

Problems to be Solved by the Invention However, room temperature to 2
At 00 ° C., the thermal expansion coefficient of the printed circuit board is 14 to 1
Whereas it is 6 × 10 ^-6 / ℃, the thermal expansion coefficient of the alumina is about 7 × 10 ^-6 / ℃, thermal stress is generated by the difference in thermal expansion coefficient between the printed circuit board and the package, the package Of the printed circuit board and the occurrence of defective bonding between the printed circuit board and the package.

Further, since the thermal expansion coefficient of Cu of the heat sink is about 18 × 10 ⁻⁶ / ° C., which is a large difference from the thermal expansion coefficient of alumina, a tensile stress is generated on a substrate or a package, and defects such as cracks occur. I do.

In order to increase the signal propagation speed, it is necessary to use a material having a small relative dielectric constant. However, since alumina has a high relative dielectric constant of about 10, the signal propagation speed is reduced. was there.

In order to solve the drawbacks of conventional alumina packages and substrates, a printed circuit board or a C
thermal expansion coefficient similar to that of u (at room temperature to 200 ° C., 1
(2 × 10 ⁻⁶ / ° C. or higher), and ceramics for packages and substrates having a small relative dielectric constant are required. Such ceramics have not been reported before.

[0008]

The inventors of the present invention have studied the above problems and found that cristobalite as a crystal phase undergoes a transformation from β-cristobalite to α-cristobalite at 180 to 270 ° C. Therefore, the average coefficient of thermal expansion from room temperature to 200 ° C. is 30 to 5
Focusing on a large thermal expansion coefficient of 0 × 10 ⁻⁶ / ° C., a method for obtaining such a dense sintered body containing cristobalite was studied.

As a result, Al ₂ mixed at a predetermined ratio
Mixed powder of O ₃ , SiO ₂ and mullite, or SiO
₂ and the mullite powder, a Group 2a element compound of the periodic table and a Group 3a element compound of the periodic table are added, and the weight ratio of the oxide equivalent of Al / the oxide equivalent of Si in the entire composition is kept constant. It has been found that by firing the mixed powder thus obtained, a high thermal expansion Al ₂ O ₃ —SiO ₂ based sintered body containing cristobalite and / or glass containing SiO ₂ as a main component in addition to the mullite main crystal can be obtained. This has led to the present invention.

That is, the high thermal expansion Al ₂ O ₃ —Si of the present invention
The O ₂ -based sintered body contains at least 92 to 99.5% by weight of at least Al and Si in terms of oxide in terms of oxides.
at least one of group a elements is 0.5 to
Al ₂ O ₃ —SiO ₂ based sintered body containing at a ratio of 8% by weight and having a weight ratio of the Al ₂ O ₃ equivalent of Al to the SiO ₂ equivalent of Si of 0.6 or more and less than 1 The sintered body has mullite as a main crystal phase, and further contains 30% by weight or more of cristobalite and / or Si.
A high-thermal-expansion Al ₂ O ₃ —SiO ₂ -based sintered body containing glass containing O ₂ as a main component and having a thermal expansion coefficient from room temperature to 200 ° C. of 12 ppm / ° C. or more. is there.

As a method for producing such a sintered body, Al ₂ O ₃ powder, SiO ₂ powder,
A mixed powder of a powder of at least one of a group a element compound and a group 3a element compound of the periodic table, and a mullite powder, wherein the Al ₂ O ₃ powder, the SiO ₂ powder, and the mullite powder have a total of 92%. -99.5% by weight, and at least one powder of the Group 2a element compound of the periodic table and the Group 3a element compound of the periodic table is 0.1% in terms of oxide.
5 to 8% by weight, and 0.5% by weight or more of the mullite powder, the Al in the total amount of Al
The weight ratio of the amount of ₂ O ₃ converted to the amount of Si converted to SiO ₂ is 0.
72 or more and less than 1, when the mullite powder contains 10% by weight or more, the amount of Al in terms of Al ₂ O ₃ in the total amount / S
The weight ratio of i in terms of SiO ₂ is 0.6 or more and less than 1,
The mixed powder prepared so as to satisfy the conditions is molded into a predetermined shape, and then fired at 1700 ° C. or lower to make mullite into a main crystal phase and further contain 30% by weight or more of cristobalite and / or SiO ₂ as a main component. The present invention is characterized in that a sintered body containing glass to be obtained is obtained.

Still another method is as follows: SiO ₂ powder, an elemental compound belonging to Group 2a of the Periodic Table and 3a
A mixed powder of at least one powder of the group-group element compounds and mullite powder, wherein the total amount of the SiO ₂ powder and the mullite powder is 92 to 99.5% by weight; And at least one kind of powder of the Group 3a element compound of the periodic table in terms of oxides,
-8% by weight, and the mullite powder in a proportion of 10% by weight or more.
As described above, the mixed powder prepared so as to have a range of less than 1 is formed into a predetermined shape, and then calcined at 1700 ° C. or lower to obtain mullite as a main crystal phase, and further contain 30% by weight or more of cristobalite and / or SiO ₂ . It is characterized by obtaining a sintered body containing glass as a main component.

Hereinafter, the present invention will be described in detail. Al _{2 of the} present invention
According to one method for producing an O ₃ —SiO ₂ -based sintered body, Al ₂ O ₃ powder, SiO ₂ powder, a Group 2a element compound of the periodic table, and a Group 3a element compound of the periodic table are used as starting materials. Among them, at least one kind of powder and mullite powder are used. Of these starting material powders, mullite powder is S
Since the iO ₂ component and the Al ₂ O ₃ component are partially separated,
The mixed composition of the powder is controlled according to the amount of the mullite powder added.

[0014] More specifically, when the Al ₂ O ₃ powder, the SiO ₂ powder and the mullite powder are contained in a ratio of 92 to 99.5% by weight in total, but the mullite powder amount is 0.5% by weight or more, The Al ₂ O ₃ powder and the SiO ₂ powder are added so that the weight ratio of Al converted to Al ₂ O ₃ / Si converted to SiO _{2 in the} total amount is 0.72 or more and less than 1. The weight ratio of Al ₂ O ₃ / SiO ₂ was limited to the above range because the weight ratio of Al ₂ O ₃ / SiO ₂ was 0.7
If it is less than 2, the sinterability deteriorates and it breaks due to its abnormal thermal expansion. If it is 1 or more, cristobalite or SiO ₂ -based glass is little produced, and it is 12 × 10 2 from room temperature to 200 ° C. This is because a thermal expansion coefficient of ⁻⁶ / ° C. or more cannot be obtained. If the amount of the mullite powder is less than 0.5% by weight, warpage occurs in the sintered body, and there is almost no effect of accelerating the generation of a liquid phase, and a dense sintered body cannot be obtained. Mullite powder is 0.5
It is advisable to add at least 0.5% by weight, but adding a large amount increases the cost, so it is desirable to add 0.5 to 30% by weight.

When the mullite powder is 10% by weight or more, the weight ratio of Al equivalent to Al ₂ O ₃ / Si equivalent to SiO _{2 in the} total amount is 0.6 or more and 1% or more.
The amounts of the Al ₂ O ₃ powder and the SiO ₂ powder are controlled so as to be less than. In this case, the structure becomes more uniform due to the increase in the proportion of mullite, and the strength is increased.
This is because it can withstand the generated thermal stress.

In still another manufacturing method according to the present invention,
As a starting material, a mixed powder of a SiO ₂ powder, at least one powder of a Group 2a element compound of the Periodic Table and a Group 3a element compound of the Periodic Table, and a mullite powder is used. In this case, the total content of the SiO ₂ powder and the mullite powder is 92 to 99.5% by weight, and particularly the mullite powder is contained in a proportion of 10% by weight or more, and the oxide of Al in the entire composition is contained. The mixture is prepared so that the weight ratio of the reduced amount / the converted amount of the oxide of Si is 0.6 or more and less than 1.

In each of the above cases, the amount of mullite, Al ₂
If the respective conditions of the O ₃ / SiO ₂ weight ratio are not satisfied, sintering becomes insufficient in any case, and a sintered body having a desired high density and high thermal expansion cannot be obtained.

In the case of the starting material, at least one powder of the Group 2a element compound of the periodic table and the Group 3a element compound of the periodic table in any of the above cases has a powder content of 0.1% in terms of oxide. It is added at a rate of 5 to 8% by weight. The reason for limiting this amount to the above range is that
If the amount of the group a compound is less than 0.5% by weight, it is difficult to perform firing at 1700 ° C. or lower. On the other hand, if the content exceeds 8% by weight, a large amount of a liquid phase having a low melting point is generated and voids are likely to increase, and the sintering temperature becomes 1450 ° C. or less. It becomes difficult. Preferably, the content is 1 to 6% by weight.

The elements of Group 2a of the periodic table include C
There are a, Mg, Sr, Ba, and Ra. Examples of such compounds include oxides and hydroxides thereof, and salts, such as carbonates and nitrates, halides, borides and the like substantially containing these oxides and hydroxides. Further, Sc, as an element of Group 3a of the periodic table,
There are Y, La, Nd, Sm, Dy, Er, Yb and the like. Examples of such compounds include oxides and salts substantially containing these, such as hydroxides, carbonates, nitrates, halides, and borides. Is mentioned.

Next, a mixture obtained by further adding a binder such as butyral to the mixture obtained by mixing the above components is arbitrarily formed by desired molding means, for example, a die press, a cold isostatic press, an extrusion molding, a doctor blade method and the like. After being formed into the shape shown in FIG. The baking can be made dense by baking for about 10 minutes to 3 hours in a reducing atmosphere or an oxidizing atmosphere at 1700 ° C. or lower. The firing temperature is desirably 1
450-1650 degreeC is good. This is because if firing is performed at a temperature lower than 1450 ° C., simultaneous firing of the substrate and the metallized paste of W or Mo cannot be performed. If the firing temperature is higher than 1700 ° C., the cost of the firing equipment increases.

According to the above manufacturing method, when a substrate having a wiring layer is manufactured, the surface of a green sheet formed by a doctor blade method or the like is coated with W, Mo, R
e. A paste containing a high melting point metal such as e is printed by a screen printing method or the like, and in some cases, the paste is filled in a through hole in a green sheet, and these sheets are laminated and pressed. At the same time.

The sintered body obtained by the above-mentioned production method has mullite (general formula 3Al ₂ O ₃ .2SiO ₂ ) as a main crystal phase, and further contains 30% by weight or more of cristobalite and / or glass containing SiO ₂ as a main component. Is included. Components other than mullite depend on the firing temperature,
When the firing temperature is 1600 ° C. or lower, cristobalite precipitates. However, when the firing temperature exceeds 1600 ° C., cristobalite vitrifies, and so-called SiO ₂ glass containing SiO _{2 as a} main component is generated. This Si
Since O ₂ glass also has high thermal expansion like cristobalite, the thermal expansion of the whole sintered body can be increased.

The amount of the glass containing cristobalite and / or SiO ₂ as a main component depends on the starting material and the composition.
As described in Examples described later, an arbitrary amount can be precipitated at 30% by weight or more.

Thus, the high thermal expansion Al ₂ O ₃ of the present invention
-SiO ₂ quality sintered body, the thermal expansion coefficient of 200 ° C. from room temperature is important that at 12 ppm / ° C. or higher, the amount of glass 30 wt% consisting mainly of cristobalite and / or SiO ₂ in the aforementioned This is achieved by controlling as described above. If the coefficient of thermal expansion is 12 ppm / ° C. or less, it cannot be mounted on a printed circuit board, and a Cu heat sink cannot be attached.

The sintered body of the present invention is also characterized by having an elongation bending point between 200 ° C. and 300 ° C. in the thermal expansion curve. This is due to the transformation from β cristobalite to α cristobalite. By having such a bending point, the thermal stress generated at the time of mounting can be reduced, and the low thermal expansion W
And Mo can be reduced in thermal stress generated by simultaneous firing with a high melting point metal.

According to the Al ₂ O ₃ —SiO ₂ sintered body of the present invention, a known pigment component,
For example, Mo, W, Re, Fe, Cr, Co, Ni, C
u and Mn may be added in the form of metals, oxides, hydroxides, salts, halides, borides and the like, in which case a sintered body colored in a desired color is obtained. Further, by adding 3 to 20% by weight of ZrO ₂ containing a stabilizing material such as Y ₂ O ₃ to disperse tetragonal or monoclinic ZrO ₂ , it is possible to increase the strength of the sintered body. it can. further,
By adding ZnO, B ₂ O ₃ or the like, the firing temperature can be further lowered. In this case, it is necessary to make the conductive metal in the wiring board Ni, Co, Au, Ag, Cu, or the like.

[0027]

According to the present invention, an Al ₂ O ₃ powder, a SiO ₂ powder, and a mixed powder with a compound of Groups 2a and 3a of the periodic table,
Alternatively, mullite is added at a predetermined ratio to a mixed powder of SiO ₂ powder and a compound of Groups 2a and 3a of the Periodic Table, whereby mullite is formed of Al ₂ O ₃ , SiO ₂ , Group 2a and 3a of the Periodic Table. Dissolved in the liquid phase
Sintering due to the reactive liquid phase caused by increasing the liquid phase having good wettability with O ₂ and Al ₂ O ₃ occurs. As described above, the generation reaction of the liquid phase is promoted, and the sintering by the reactive liquid phase occurs smoothly. As a result, a glass mainly composed of cristobalite and / or SiO ₂ is deposited in addition to the mullite phase, and a temperature of 200 ° C. It is possible to obtain a printed circuit board having a coefficient of thermal expansion at 12 ° ^C./° C. or higher of 12 ° ^C./° C. or a dense sintered body having a coefficient of thermal expansion substantially equal to that of Cu.

As a result, when this Al ₂ O ₃ —SiO ₂ sintered body is used as an insulating substrate, a multilayer wiring board having a wiring layer or a package containing a semiconductor element is mounted on a printed circuit board. This makes it possible to provide a stable mounting structure for a long period of time by suppressing the occurrence of poor electrical connection with the printed circuit board.

[0029]

【Example】

Example 1 Al ₂ O ₃ powder having an average particle size of 0.6 μm, SiO ₂ powder having an average particle size of 0.8 μm, and mullite powder having an average particle size of 1.0 μm (Al ₂ O ₃ / SiO ₂ = 72 / 28, weight ratio) and carbonates of Group 3a and 2a elements of the Periodic Table having an average particle size of 1.5 μm were weighed and mixed so as to have the composition shown in Table 1, and a known solvent, binder and plasticizer were used. And the like were formed into a thickness of 0.5 mm by a doctor blade method, and then a sheet was heated and pressed to form a laminate. The laminate was fired under the firing temperature conditions shown in Table 1. In addition, Mg in the table
The amounts of O and CaO are both converted from MgCO ₃ and CaCO ₃ . Further, the crystal phase of the obtained sintered body was identified by X-ray diffraction measurement. Room temperature to 20
The coefficient of thermal expansion at 0 ° C. was measured.

The sintered body has a diameter of 60 mm and a thickness of 2 mm.
And the relative dielectric constant was determined by the method of JISC2141. The measurement was performed using an LCR meter (YHP4284A), the capacitance at 25 ° C. was measured under the conditions of 1 MHz and 1.0 Vrsm, and the relative dielectric constant at 25 ° C. was measured from the capacitance.

Further, the composition of the sintered body was analyzed by a calibration curve method using an X-ray microanalyzer, the amounts of mullite and cristobalite precipitated were measured by an internal standard method of X-ray diffraction, and cristobalite and the remaining glass were measured. Are shown in Table 2.

[0032]

[Table 1]

[0033]

[Table 2]

From Tables 1 and 2, it can be seen that Samples Nos. 4 and 22 in which no mullite powder was added could not be sintered either, and the sintered bodies fired by adding other mullite powders were dense. there were. However, the amount of mullite added was 0.5
In Sample No. 6 having less than the weight%, warpage occurred due to insufficient sintering. Even when mullite is added in an amount of 0.5% by weight or more,
Sample No. 1 having an Al ₂ O ₃ / SiO ₂ ratio of less than 0.72,
No. 2 could not be densified, and sample No. 1 had cracks. Therefore, if mullite is added at 0.5% by weight, Al ₂
It can be seen that a favorable sintered body is obtained when the O ₃ / SiO ₂ weight ratio is in the range of 0.72 or more. However, it can be seen that when the amount of mullite is 10% by weight or more, the Al ₂ O ₃ / SiO ₂ weight ratio can be made dense with 0.6 or more.

Further, when the weight ratio of Al ₂ O ₃ / SiO ₂ is 1
In the above sample No. 23, the coefficient of thermal expansion was small, and the coefficient of thermal expansion from room temperature to 200 ° C. was less than 12 × 10 ⁻⁶ / 1 at less than 1.
° C or higher, and the coefficient of thermal expansion of the printed circuit board or Cu (12 to
18 × 10 ⁻⁶ / ° C.), which indicates that surface mounting on a printed circuit board and use of a Cu heat sink are possible.

Further, the sintered body of the present invention has a relative dielectric constant of 5.8 or less, which is lower than that of alumina, and can improve the signal propagation speed.

FIG. 1 is a diagram showing a thermal expansion curve of elongation with respect to temperature of sample No. 8 in Table 1. From FIG. 1, it can be seen that by containing cristobalite, the coefficient of thermal expansion is large only in the range of room temperature to 200 ° C., and the coefficient of thermal expansion is small on the high temperature side, so that ordinary metallization of W and Mo is possible. The bending point of thermal expansion is 200 to 300.
° C, the average coefficient of thermal expansion from room temperature to 200 ° C is 12x
It turns out that it is 10 ^<-6 > / degreeC or more.

As shown in FIG. 1, all of the samples containing cristobalite as in Sample No.
It had an inflection point in the thermal expansion curve of elongation at about 300 ° C.

Example 2 As raw materials, SiO ₂ powder having an average particle size of 0.8 μm, mullite powder having an average particle size of 1.0 μm (Al ₂ O ₃ / SiO ₂ = 72/28, weight ratio), and average particle size 1.5 μm periodic table 3a,
Using a carbonate of a Group 2a element, the components were weighed and mixed into the composition shown in Table 3 and molded by a doctor blade method to produce a green sheet. A plurality of the green sheets were laminated and fired as shown in Table 3. It was fired under temperature conditions. Further, a crystal phase was identified for the obtained sintered body in the same manner as in Example 1 above, and the coefficient of thermal expansion and relative dielectric constant at room temperature to 200 ° C.
The composition of the sintered body and the amount of cristobalite + SiO ₂ glass phase precipitation were measured. The results are shown in Table 4.

[0040]

[Table 3]

[0041]

[Table 4]

According to Tables 3 and 4, mullite was 10% by weight.
Even with the above addition, cracks occurred in the sintered body in Sample No. 24 in which the Al ₂ O ₃ / SiO ₂ ratio was less than 0.6.

On the other hand, all sintered bodies within the range of the present invention, which were baked with an Al ₂ O ₃ / SiO ₂ weight ratio of 0.6 or more and a mullite powder of 10% by weight or more, were dense. And the coefficient of thermal expansion from room temperature to 200 ° C. is 12 × 10 ⁻⁶
/ ° C or higher, and the thermal expansion coefficient (12
-18 × 10 ⁻⁶ / ° C.), which indicates that surface mounting on a printed circuit board and use of a Cu heat sink are possible. Furthermore, in the sintered body of the present invention, the relative dielectric constant is 5.8 or less, which is lower than that of alumina, and the signal propagation speed can be improved.

FIG. 1 shows the sample No. 26 in Table 3.
3 shows a thermal expansion curve of elongation with respect to temperature. From FIG. 1, it can be seen that by containing cristobalite, room temperature to 2
It can be seen that the coefficient of thermal expansion is large only in the range of 00 ° C., and the coefficient of thermal expansion is small on the high temperature side, so that normal metallization of W and Mo is possible. Further, the bending point of thermal expansion is between 200 and 300 ° C., and the average thermal expansion coefficient between room temperature and 200 ° C. is 12 × 10
It turns out that it is ^-6 / degreeC or more.

As shown in FIG. 1, each of the samples containing cristobalite as in Sample No.
It had a bending point in the thermal expansion curve of elongation at around 0 to 300 ° C.

[0046]

According to the method for producing an Al ₂ O ₃ —SiO ₂ sintered body of the present invention, mullite powder is added to a mixed powder of Al ₂ O ₃ and SiO ₂ , so that the reaction of forming a liquid phase is accelerated. In addition, sintering due to the reactive liquid phase occurs smoothly, and the generation of voids is suppressed, so that a dense Al ₂ O ₃ —SiO ₂ -based sintered body containing cristobalite can be obtained. Therefore, in the method for producing an Al ₂ O ₃ —SiO ₂ based sintered body of the present invention, a sintered body can be easily obtained at low cost.

[Brief description of the drawings]

FIG. 1 is a graph showing the elongation of samples Nos. 8 and 26 of the present invention with respect to temperature.

Claims (3)

(57) [Claims] 1. A total of 92 to 99.5% by weight of at least Al and Si in terms of oxides, and 0.5 to 8 in terms of oxides of at least one of Group 2a and 3a elements of the periodic table. % By weight, and the weight ratio of the Al equivalent to Al ₂ O _{3 to the} Si equivalent to SiO ₂ is 0.6.
As described above, the Al ₂ O ₃ —SiO ₂ based sintered body of less than 1 has a main crystal phase of mullite,
% By weight of cristobalite and / or SiO ₂
A high thermal expansion Al ₂ O ₃ —SiO ₂ based sintered body comprising a glass having as a main component, and having a thermal expansion coefficient from room temperature to 200 ° C. of 12 ppm / ° C. or more. 2. A mixed powder of Al ₂ O ₃ powder, SiO ₂ powder, powder of at least one of Group 2a element compound and Periodic Table 3a element compound of the periodic table, and mullite powder. And the Al ₂ O ₃ powder and the SiO ₂
Powder and the mullite powder in a total amount of 92 to 99.5% by weight, and at least one powder of the Group 2a element compound and the Group 3a element compound in the periodic table is 0.5 to 8 in terms of oxide. % Of the mullite powder and 0.5% by weight or more of the mullite powder, the weight ratio of the Al equivalent to Al ₂ O ₃ / the Si equivalent to SiO _{2 in the} total amount is 0.72 or more, When the mullite powder is contained in an amount of 10% by weight or more, the weight ratio of the amount of Al in terms of Al ₂ O ₃ / the amount of Si in terms of SiO _{2 in the} total amount is 0.6 or more and less than 1. After forming the prepared mixed powder into a predetermined shape, 170
Firing at 0 ° C. or lower to make mullite into a main crystal phase, and further, 30% by weight or more of cristobalite and / or Si
Method for producing a high thermal expansion Al ₂ O ₃ -SiO ₂ -based sintered body, characterized in that to obtain a sintered body of O ₂ comprises a glass composed mainly. 3. A mixed powder of SiO ₂ powder, at least one powder of a Group 2a element compound of the Periodic Table and a Group 3a element compound of the Periodic Table, and mullite powder,
The total of the SiO ₂ powder and the mullite powder is 92 to
99.5% by weight, 0.5 to 8% by weight in terms of oxide of at least one powder of the Group 2a element compound of the periodic table and the Group 3a element compound of the periodic table, and 10% by weight of the mullite powder. The mixed powders contained in the above proportions and prepared so that the weight ratio of the equivalent amount of Al oxide / the equivalent amount of Si oxide in the entire composition is in the range of 0.6 or more and less than 1 in a predetermined shape. After molding to 17
Baking at a temperature of not more than 00 ° C. to make mullite into a main crystal phase, and further containing at least 30% by weight of cristobalite and / or S
method for producing a high thermal expansion Al ₂ O ₃ -SiO ₂ -based sintered body, characterized in that to obtain a sintered body containing glass and iO ₂ as a main component.