KR20070021061A - Yttria sintered body and manufacturing method therefor - Google Patents

Abstract

In providing a yttria oxide sintered body having excellent corrosion resistance and excellent thermal shock resistance to halogen-based corrosion gas and suitable for use as a component member in a manufacturing apparatus for producing semiconductor and liquid crystal devices, in particular, a plasma processing apparatus, Containing tungsten having an average particle size of 3 μm or less dispersed in the yttrium oxide so that the ratio of tungsten to the yttrium oxide is in the range of 1 to 50% by weight, and having a porosity of 0.2% or less and 250 ° C. It provides a yttrium oxide sintered body having a thermal shock resistance by a water submersion method.

Description

Yttria oxide sintered body and its manufacturing method {Yttria sintered body and manufacturing method therefor}

The present invention relates to an yttrium oxide sintered compact having excellent corrosion resistance against halogen-based corrosive gas and plasma and suitable for use in plasma processing apparatus for manufacturing semiconductors and liquid crystal devices.

In a semiconductor manufacturing apparatus, a component member formed of silicon, quartz glass, or silicon carbide is frequently used. In principle, it consists of C, O and can be obtained with high purity. Therefore, there is an advantage of not contaminating the wafer even if the component comes into contact with the wafer or vapor of the configuration is deposited from such a component member.

However, since these materials contain defects that cause significant corrosion by halogen-based gases, in particular fluorine-based gases, etching or CVD film forming or resisting materials, in particular fluorine or chlorine It is not suitable as a member for devices used in ashing processes for removing substances carried out by plasma treatment using highly reactive halogen based corrosion gases such as

Therefore, a ceramic such as high purity alumina, aluminum nitride, yttrium oxide or YAG is used for the member exposed to the halogen plasma in such a treatment.

Among these materials, yttrium oxide is of interest because of its excellent plasma resistance.

For example, Japanese Patent Unexamined Application JP-A-2003-234300 discloses that a permeable sintered body of yttrium oxide ceramics can be used in a plasma processing apparatus.

On the other hand, in order to be used as a component member in a plasma processing apparatus, a material having excellent plasma resistance and having a volume resistivity that can be arbitrarily controlled according to the use conditions is preferable.

To reduce the volume resistivity of such ceramics, for example, alumina, powders of composites containing alkali metals or transition metals, or powders of titanium oxide are added, or titanium oxide or tungsten oxide is added to the high resistance ceramics as described above. It is conceivable to add metal carbides such as metal oxides, metal nitrides such as titanium nitride, or titanium carbide, tungsten carbide, silicon carbide showing conductivity.

JP-A-7-233434 discloses a corrosion resistant material in which particles of metal oxides such as yttrium oxide are dispersed at a volume ratio of 50% or less in a matrix of high melting metals such as tungsten. Such materials for the absence of components, such as crucibles, tend to contact these molten natural earth metals as metal resistance to corrosion by raw material earth metals such as yttrium oxide in the molten state.

The method described above, when used to reduce the volume resistivity of ceramics of high plasma resistance, such as high purity alumina, aluminum nitride, yttrium oxide or YAG, results in deterioration of plasma resistance and inclusion of impurity elements that cause contamination of the wafer. Because of this, it cannot be thought of as a practically acceptable way.

As mentioned above, in the subtitle formed by ceramics, the volume resistivity is also influenced by the porosity of the material, and a dense sintered body is preferable to reduce the volume resistivity. For this reason, sintering is required in a special method under high temperature environments such as HP (hot pressing) or HIP (hot isostatic pressing). However, fabrication of large members consistent with the increasing diameter of the wafer is expensive.

Yttrium oxide ceramics are excellent in plasma resistance as described above, but when used as a component member of a semiconductor manufacturing apparatus, have a drawback compared to other ceramics such as alumina, and have low strength defects and low thermal shock resistance. resistance) and thermal stress depending on the temperature conditions. More specifically, it can be used without difficulty under a temperature of about 50 ° C., but shows high breakdown potential when used in a high temperature range of 200 ° C. or higher.

Therefore, an yttrium oxide sintered body capable of exhibiting thermal shock resistance that can be used under high temperature conditions of 200 ° C. or higher without weakening the excellent plasma resistance of yttrium oxide for use as a component member of a plasma processing apparatus in semiconductor manufacturing or the like is desirable.

SUMMARY OF THE INVENTION The present invention has been made in order to solve the technical problems described above, and one of the objects of the present invention is an apparatus for producing semiconductor and liquid crystal devices, in particular, plasma, which has excellent corrosion resistance and excellent thermal shock resistance against halogen-based corrosion gases. It is to provide an yttrium oxide sintered body suitable for use as a component member in a processing apparatus and a manufacturing method thereof.

According to one aspect of the present invention, yttrium oxide and yttrium oxide constituting tungsten dispersed in yttrium oxide in such a manner that the ratio of tungsten to yttrium oxide is in the range of 1 to 50% by weight. A sintered body is provided, the porosity of the yttrium oxide sintered body is 0.2% or less, and the thermal shock resistance of the oxidized sintered body is 200 ° C in the water submersion method.

Such a sintered body is capable of improving the thermal shock resistance of the yttrium oxide sintered body without weakening the excellent plasma resistance of the yttrium oxide, and can be advantageously used as a component member of the apparatus in halogen plasma treatment even under high temperature conditions of 200 ° C or higher. Can be.

The tungsten has an average particle size of 3 μm or less.

The average particle size of tungsten as an additive of 3 μm or less facilitates obtaining a dense sintered body and easily controls the porosity at 0.2%. Therefore, when yttrium oxide ceramic is used as a component member of the halogen plasma processing apparatus, generation of particles caused from the sintered compact is suppressed.

As described above, the yttrium oxide sintered body of the present invention has excellent corrosion resistance and excellent thermal shock resistance against halogen-based corrosion gas, and is used in the manufacturing process for semiconductor and liquid crystal devices, especially as a component member in a plasma processing apparatus. Suitable material.

In addition, the member formed of the yttrium oxide sintered body suppresses the generation of particles in the halogen plasma treatment to contribute to the improvement in the production rate of the semiconductor chip or the like to be produced in the next step.

According to one of the aspects of the present invention, yttrium oxide and yttrium oxide constituting tungsten dispersed in the yttrium oxide in such a manner that the ratio of tungsten to the yttrium oxide is in the range of 1 to 50% by weight. The sintered compact is provided and the porosity of the said yttrium oxide sintered compact is 0.2% or less.

Such a sintered body used as a member in an apparatus for halogen plasma treatment can suppress the generation of particles caused by the electrostatic charge of the ceramic member, and in etching such as a wafer, achieve a uniform etching rate across the wafer surface. Do it.

The yttrium oxide sintered body preferably has a volume resistivity of 10 ⁶ Pa · cm or more and 10 ¹³ Pa · cm or less at 20 to 400 ° C.

The yttrium oxide sintered body having the volume resistivity within the above range is more effective in suppressing the generation of particles and achieving a uniform etching rate, as described above.

According to one of the aspects of the present invention, there is provided a manufacturing method for a yttrium oxide sintered body, wherein the step of the method is such that the ratio of tungsten powder to the yttrium oxide powder is in the range of 1 to 50% by weight. Adding the tungsten powder to the yttrium oxide powder,

Sintered at a temperature of 1700-2000 ° C in vacuum and reduced atmosphere to obtain an yttrium oxide sintered body having a volume resistivity of at least 10 ⁶ Pa.cm and a volume resistivity of 10 ¹³ Pa.cm at a temperature of 20-400 ° C. do.

This production method allows to obtain the yttrium oxide sintered body in an advantageous manner.

As described above, the yttrium oxide sintered body of the present invention has excellent corrosion resistance and excellent thermal shock resistance against halogen-based corrosion gas, and is used in the manufacturing process for semiconductor and liquid crystal devices, especially as a component member in a plasma processing apparatus. Suitable material.

In addition, the member formed of the yttrium oxide sintered body is to suppress the generation of particles in the halogen plasma treatment to contribute to the improvement in the production rate of the semiconductor chip to be produced in the next step.

The production method of the present invention allows to obtain the yttrium oxide sintered body of the present invention in an advantageous manner.

It should be noted that the ratio of tungsten is determined as t / s% when expressed as s (g) of yttrium oxide and t (g) of tungsten.

Next, the present invention will be described in more detail.

The yttrium oxide sintered body of the present invention comprises tungsten dispersed in yttrium oxide in such a manner that the ratio of tungsten to yttrium oxide is in the range of 1 to 50% by weight, and the porosity equal to or less than 0.2% and 200 It is characterized by having thermal shock resistance by water submersion method equivalent to above ℃.

Therefore, the addition of tungsten having a high heat resistant metal to yttrium oxide having better plasma resistance per se improves the thermal shock resistance of the yttrium oxide sintered body.

Therefore, the yttrium oxide sintered body of the present invention has excellent resistance to corrosion by halogen plasma induced from fluorine or gluolin. In addition, the thermal shock resistance of the yttrium oxide sintered body can be controlled by the addition amount of high high resistance tungsten metal.

Therefore, as an additive to yttrium oxide, a material that impairs the excellent plasma resistance of yttrium oxide itself or a material comprising an element constituting a contaminant in semiconductor manufacturing is, for example, potassium or sodium Alkali metals, such as these, and heavy metals, such as nickel, copper, or iron, are not preferable.

On the other hand, tungsten is a material used as an electrode metal in a semiconductor manufacturing apparatus, and does not cause harmful effects as an additive in the yttrium oxide sintered body of the present invention.

In the yttrium oxide sintered body of the present invention, tungsten is dispersed in the sintered body, that is to say, tungsten particles diffuse around the yttrium oxide crystals.

Therefore, the thermal stress and strain of the yttrium oxide crystals at high temperature are alleviated by the tungsten particles, so that excellent thermal shock resistance can be realized.

The thermal shock resistance is hardly dispersed in the tungsten particles but hardly improved in the solidified state.

In the present invention, the addition amount of tungsten is in the range of 1 to 50% with respect to yttrium oxide.

In the case of an addition amount exceeding 50%, the sintered body is severely degraded in plasma resistance, and when used as a component member in a halogen plasma processing apparatus, it shows an increased particle generation due to the degradation of such component member.

On the other hand, the addition amount of 1% or less hardly improves the thermal shock resistance.

The yttrium oxide sintered body is a density sintered material having a porosity of 0.2% or less.

In the case of porosity exceeding 0.2%, when used as a component member in a halogen plasma processing apparatus, the sintered body is etched by the component member itself and produces particles due to being exposed to corrosion by plasma concentrated in the vicinity of the void. easy to do.

Tungsten added preferably has an average particle size of 3 μm or less.

In the case of an average particle size of more than 3 μm, tungsten becomes a material which hinders dense formation of the sintered body and hinders sintering to cause high porosity. When the sintered body is used as a component member in a halogen plasma processing apparatus, it tends to be etched in the vicinity of the voids by the plasma, and tends to cause particle generation.

Yttrium oxide sintered body has excellent thermal shock resistance More specifically, it has a thermal shock resistance by the precipitation method of 200 ℃ or more.

When the sintered body heated to a predetermined temperature is injected into the water, the thermal shock resistance means a limit temperature difference which produces cracks. For example, when a sintered body heated to 220 ° C. is injected into water at 20 ° C. and no crack is generated, the thermal shock resistance by the precipitation method is 220 ° C. or more.

The yttrium oxide sintered body of the present invention having a thermal shock resistance by a precipitation method of 200 ° C. or higher can be used as a component member of the plasma processing apparatus in a high temperature range of 200 ° C. or higher. The yttrium oxide sintered body has not been available.

More specifically, when used as a component member of an apparatus for halogen plasma treatment, the sintered body can suppress generation of particles by etching or breaking of such a member even under a high temperature of 200 ° C, It can contribute to the improvement of production rate.

In particular, plasma gases of halogen compounds such as CCl ₄ , BCL ₃ , HBr, CF ₄ , C ₄ F ₈ , NF ₃ or SF ₆ , or highly corrosive self-cleaning CLF ₃ gases, for example, in film forming processes on semiconductor wafers It can be advantageously used as a member for a device using or as a member exposed to etching by a strong sputtering plasma using N ₂ or O ₂ .

The yttrium oxide sintered body of the present invention can be produced by adding tungsten powder to the yttrium oxide powder and sintering the mixture at a temperature of 1700 to 2000 ° C. with reduced atmosphere or vacuum.

In the case of the sintering temperature of 1700 degrees C or less, since many space | gap turns into a sintered compact, it cannot achieve high enough density and it tends to be etched in the vicinity of a space | gap. Therefore, particle generation can be easily induced.

On the other hand, the sintering temperature exceeding 2000 ° C. causes excessively large crystal grains, which undesirably weakens the strength.

[Yes]

In the following, one of the embodiments of the present invention is described in detail by way of example, but the present invention is not limited to these examples.

[Example 2-1]

The natural resource powder for yttrium oxide of 99.9% purity was formed into small grains by a spray-dryer, and the powder of tungsten (W) had a ratio of tungsten to yttrium oxide in terms of weight 1 It was added to the yttrium oxide powder in a manner in the range of%. For example, the ratio of 1% can be derived from the case where the weight of tungsten is 1 (g) and the weight of yttrium oxide is 100 (g).

The pellets were pressurized to be molded by the CIP method under a pressure of 1.5 t / cm 2, and the obtained molded article (φ280 mm × 30 mm) was sintered at 1800 ° C. in a halogen atmosphere to obtain an yttrium oxide sintered body.

On the obtained sintered compact, the porosity was measured by the Archimedes method.

In addition, thermal shock resistance was evaluated by the following precipitation method.

From the said sintered compact, 30 tests of the magnitude | size of 3 mm x 4 mm x 40 mm were prepared by the grinding process.

Each of the 10 tests was kept at a predetermined temperature for at least 30 minutes and then submerged in the water tank for 5 minutes (temperature difference Δ (° C.) = 130, 170 or 200).

After the water was thoroughly wiped off, the sample was dried at 120 ° C. for 2 hours, then cooled to room temperature and irradiated for cracks using a fluorescent defect detection liquid.

The sintered body was subjected to grinding treatment to prepare a plasma rectifying ring.

It was mounted on a Reactive Ion Etching (RIE) etching apparatus (gases used: CF ₄ , O ₂ ), used to etch 8 inch silicon wafers, and many particles of 0.3 μm on the wafer were measured by the laser particle count. .

The results are shown in Table 1.

[Examples 1-2 to 1-6 and Comparative Examples 1-1 to 1-5]

The sintered compact was the same as in Example 1 except that the tungsten (W) added to the yttrium oxide was changed to the amounts shown in Examples 1-2 to 1-6 and Comparative Examples 1-1 to 1-5 in Table 1. Prepared in a way.

Each sintered ceramic was measured for porosity and volume resistivity in the same manner as in Table 1, and evaluation of the particle number of the wafer and evaluation of the particle yield of the wafer when the plasma rectifying ring was made was also performed.

The results obtained are summarized in Table 1.

W addition amount (wt%) Porosity (%) Thermal Shock Resistance (ΔT / Cracks) Particles (Number) 130 ℃ 170 ℃ 220 ℃   Yes 1-1 One 0.06 0 0 0 18 1-2 5 0.08 0 0 0 10 1-3 10 0.08 0 0 0 14 1-4 20 0.10 0 0 0 15 1-5 40 0.15 0 0 0 26 1-6 50 0.18 0 0 0 12   Comparative example 1-1 0 0.07 3 10 - 10 1-2 0.8 0.06 0 10 - 15 1-3 10 0.21 0 0 0 42 1-4 40 0.22 0 0 0 48 1-5 55 0.18 0 0 0 40

As apparent from Table 1, the ratio of tungsten to yttrium oxide has an added amount of tungsten in the range of 1 to 50% by weight, and has a porosity of 0.2% or less (Examples 1-1 to 1-6). It was confirmed that the yttrium oxide sintered compact had no crack in the evaluation of the thermal shock resistance by the precipitation method, and had better thermal shock resistance.

In addition, the yttrium oxide sintered bodies of Examples 1-1 to 1-6, when used as plasma rectifying rings, suppress the particle production and provide an improvement in the production rate of semiconductor devices or the like prepared using such sintered bodies as members of the apparatus. It is expected.

[Examples 1-7 to 1-9 and Comparative Examples 1-6]

The sintered body was prepared in the same manner as in Example 1-1, except that tungsten (W) powder having the particle size shown in Examples 1-7 to 1-9 and Comparative Examples 1-6 in Table 2 was used. It was added in an amount of 2% by weight based on titanium.

Each sintered ceramic was measured for porosity and density by Archimedes' method in the same manner as in Example 1-1. Particle count of the wafer was also performed when the sintered ceramic was made of a plasma rectifying ring.

The results obtained are summarized in Table 2.

W Average Particle Size (㎛) Density (g / cm 3) Porosity (%) Particles (Number)  Yes 1-7 0.5 5.102 0.06 16 1-8 1.2 5.050 0.06 13 1-9 2.8 4.963 0.19 32 Comparative example 1-6 3.5 4.922 0.25 61

As is apparent from Table 2, a dense sintered body was difficult to obtain when the added tungsten had an average particle size exceeding 3 µm, and the sintered body of Comparative Examples 1-6 had a porosity exceeding 0.2%, and the plasma rectifying ring Large particles were produced when used as.

In the following, another aspect of the present invention is described in more detail.

The yttrium oxide sintered body of the present invention is characterized by containing tungsten dispersed in an amount of 1 to 50% with respect to yttrium oxide and having a porosity equivalent to 0.2% or less.

Therefore, adding tungsten with high melting point to yttrium oxide with plasma resistance allows to reduce volume resistivity.

Therefore, the sintered body of the present invention has excellent resistance to corrosion by halogen plasma induced from fluorine or gluolin. In addition, the volume resistance can be adjusted by the addition amount of tungsten having a high melting point.

Therefore, such a sintered body, when used as a component member of an apparatus used for halogen plasma treatment, suppresses the generation of particles caused by the charge of the ceramic member, and maintains a uniform etching rate across the wafer surface in an etching treatment such as a wafer. By doing so, the production rate of the semiconductor chip to be produced can be improved as compared with the case of using the previous insulating ceramic.

In the present invention, the addition amount of tungsten is in the range of 1 to 50% with respect to yttrium oxide, as described above.

When the addition amount exceeds 50%, the sintered body is significantly weakened by the plasma resistance, and when used as a component member in a halogen plasma processing apparatus, it shows an increased particle generation by the deterioration of such a component member.

On the other hand, the addition amount of 1% or less hardly reduces the volume resistivity.

In addition, the yttrium oxide sintered body has a porosity equivalent to 0.2% or less and is a dense texture sintered material.

In the case of the porosity exceeding 0.2%, when used as a component member in a halogen plasma processing apparatus, the sintered body is exposed to corrosion by plasma concentrated in the vicinity of the void, so that the component member thereof is etched and the particles are etched. Tends to produce.

Further, the yttrium oxide sintered body has a volume resistivity of 10 ⁶ Pa · mm or more and 10 ¹³ Pa · mm or less at a temperature of 20 to 400 ° C.

When the volume resistivity is 10 ¹³ Pa · mm or more, the sintered ceramic member tends to be electrically charged easily, and when used as a component member in a halogen plasma processing apparatus, it is difficult to suppress particle generation. In addition, the iron sheath directly formed on the wafer is unbalanced to cause an uneven etching rate in the wafer surface.

On the other hand, when the volume resistivity is 10 ⁶ Pa · mm or less, sufficient insulation characteristics cannot be provided, and the above effect of suppressing particle generation and obtaining a uniform etching rate cannot be obtained.

The yttrium oxide sintered body of the present invention is produced by adding tungsten powder to yttrium oxide powder in an amount of 1 to 50% relative to yttrium oxide, and sintering the mixture at a temperature of 1700 to 2000 ° C. with reduced atmosphere or vacuum. Can be.

On the other hand, when the sintering temperature exceeds 2000 ° C, excessively large crystal particles are attracted, so that the strength is undesirably weakened.

[Yes]

Next, the embodiment of the present invention will be more apparent by way of example, but the present invention is not limited by these examples.

[Example 2-1]

5% with respect to yttrium oxide was added for a 99.9% purity yttrium oxide natural powder, and the mixture was made into small grains by a spray dryer.

The pellets were pressure molded by the CIP (Cold Isostatic Pressure) method under a pressure of 1500 kgf / cm 2, and the obtained mold body (φ280 mm × 30 mm) was sintered at 1800 ° C. in a halogen atmosphere to obtain a sintered body.

On the obtained sintered body, the porosity was measured by the Archimedes method and measured at room temperature (25 ° C.) by the for-terminal method and the double ring method.

The results of these measurements are shown in Table 3.

The sintered compact was ground to prepare the plasma rectifying ring.

It was mounted on a Reactive Ion Etching (RIE) etching apparatus (gases used: CF ₄ , O ₂ ), used to etch 8 inch silicon wafers, and many particles of 0.3 μm on the wafer were measured by the laser particle count. .

In addition, chips (15 mm x 7 mm) were prepared from wafers, and the yield of these chips was evaluated.

The results are shown in Table 3.

[Examples 2-2 to 2-6 and Comparative Examples 2-1 to 2-6]

The sintered body was prepared in Example 2-1 except that tungsten (W) added to yttrium oxide was changed to the amounts shown in Examples 2-2 to 2-6 and Comparative Examples 2-1 to 2-6 in Table 3. It was prepared in the same way.

Each sintered ceramic was measured for porosity and volume resistivity in the same manner as in Table 2-1, and the evaluation of the particle count of the wafer and the chip particle yield when the plasma rectifying ring was made was also performed.

The results obtained are summarized in Table 3.

W addition amount (%) Porosity (%) Volume resistivity (Ω · cm) Particles (Number) Chip output (%)     Yes 2-1 5 0.05 7 × 10 ¹² 18 90 2-2 10 0.06 3 × 10 ¹² 10 96 2-3 20 0.08 2 × 10 ¹⁰ 14 92 2-4 30 0.08 9 × 10 ⁸ 15 90 2-5 40 0.12 9 × 10 ⁶ 26 90 2-6 48 0.11 2 × 10 ⁶ 12 91    Comparative example 2-1 0 0.07 1 × 10 ¹³ 40 82 2-2 One 0.06 1 × 10 ¹³ 39 80 2-3 10 0.26 9 × 10 ¹¹ 42 76 2-4 35 0.22 1 × 10 ⁹ 48 72 2-5 48 0.22 7 × 10 ⁵ 40 80 2-6 50 0.30 7 × 10 ⁵ 44 80

As is apparent from Table 3, an yttrium oxide sintered body having an added amount of tungsten of 1 to 50% with respect to yttrium oxide and having a porosity of 0.2% or less, when used as a plasma rectifying ring, suppresses particle formation. It has been found to provide over 90% yield on chips prepared from processed wafers (Examples 2-1 to 2-6).

Although described in connection with a preferred embodiment of the present invention, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the scope thereof. Therefore, as is intended to be classified within the true spirit and scope of this invention, it is intended that such changes and modifications be covered in all claims.

As described above, the yttrium oxide sintered body of the present invention has excellent corrosion resistance and excellent thermal shock resistance against halogen-based corrosion gas, and is used in the manufacturing process for semiconductor and liquid crystal devices, especially as a component member in a plasma processing apparatus. Suitable material.

Claims (5)

 Yttrium oxide and an yttrium oxide sintered body constituting tungsten dispersed in the yttrium oxide in such a manner that the ratio of tungsten to the yttrium oxide is in the range of 1 to 50% by weight, The porosity of the yttrium oxide sintered body is 0.2% or less, The yttrium oxide sintered body, characterized in that the thermal shock resistance of the sintered oxide is 200 ℃ or more in the water submersion method. The method of claim 1, The tungsten yttrium oxide sintered body, characterized in that having an average particle size of less than 3㎛. Yttrium oxide and an yttrium oxide sintered body constituting tungsten dispersed in the yttrium oxide in such a manner that the ratio of tungsten to the yttrium oxide is in the range of 1 to 50% by weight, The yttrium oxide sintered compact, wherein the porosity of the yttrium oxide sintered compact is 0.2% or less. The method of claim 3, A yttrium oxide sintered compact having a volume resistivity of 10 ⁶ Pa · cm or more and 10 ¹³ Pa · cm or less at 20 to 400 ° C. In the manufacturing method for a yttrium oxide sintered compact, Adding the tungsten powder to the yttrium oxide powder in such a way that the proportion of tungsten powder to the yttrium oxide powder is in the range of 1 to 50% by weight; Sintering at a temperature of 1700-2000 ° C. in a vacuum and reduced atmosphere to obtain an yttrium oxide sintered body having a volume resistivity of at least 10 ⁶ Pa · cm and a volume resistivity of 10 ¹³ Pa · cm at a temperature of 20-400 ° C. The manufacturing method for the yttrium oxide sintered compact characterized by the above-mentioned.