Electrification removing component - Patent Review 5612144

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrification removing component which is employed for preventing parts from electrical breakage by a rapid electric discharge which is caused in handling an electrified electronics related parts and the like.

2. Discussion of Background

There is a case wherein, in manufacturing steps of an electronics related part including a semiconductor device, the part is provided with an electric charge by a direct electrification or by an induced electrification. When the electrified part contacts an electric conductive material such as a metal and the like having a small electric resistance, a rapid discharge of an electric charge is often caused and the part is electrically broken (dielectric breakdown) by the discharge. It is an important problem to prevent the breakage of the part caused by this discharge.

As a ceramics having an electric conductive property, an electrically conductive ceramics of alumina added with an element of iron group and an element of 2A group or an element of 3A group, is disclosed, for instance, in Japanese Examined Patent Publication No. 54630/1992, and a sintered body of silicon nitride which is capable of working with an electric discharge machining and provided with a small specific resistivity, wherein a nitride or a carbide of Ti, Ta, Zr, and Hf is mixed with silicon carbide, in Japanese Examined Patent Publication No. 43699/1990. As a ceramics for the purpose of preventing electrification, a titania ceramics, a zirconia ceramics, a silicon nitride ceramics, or a silicon carbide ceramics, having a specific resistance of 10.sup.2 through 10.sup.6 .OMEGA..cm, which is attached to a holding part of metallic tweezers, is disclosed in Japanese Examined Utility Model Publication No. 2303/1993.

Further, a suction chuck is proposed in Japanese Unexamined Utility Model Publication No. 114441/1990, which is one for attracting a part for working or for measuring physical properties, and which prevents dusts from adhering on its suction face due to electrification, by employing ceramics having a specific resistivity of 10 through 10.sup.5 .OMEGA..cm. A vacuum conveyer (vacuum chuck) is proposed in Japanese Examined Patent Publication No. 42418/1986, of which surface contacting a silicon wafer is composed of a high-purity silicon carbide (of which surface resistivity is normally in a range of 10.sup.3 through 10.sup.4 .OMEGA./cm.sup.2) and which does not contaminate an object to be handled.

However, these materials do not provide reliability for components employed in removing electrification by the following reasons. The first reason is that a voltage of static electricity charged on a part and the like is often as high as above 2000 V, and it has been revealed that a material with a level of specific resistivity of not more than 10.sup.2 .OMEGA..cm is not sufficient for protecting the part from an electric breakage when the discharge is caused and for removing the electric charge without destructing the part.

It is considered pertinent for a way of representing an electric resistance of a material for removing electrification by the surface resistivity in view of a mode of utilizing the material. However, no unified representation is currently adopted. In case of a uniform material, with respect to the surface resistivity and the specific resistivity, both generally agree with each other in most cases, although there is a case wherein measured values of these are different by a decimal digit. It is considered from these knowledges, that the surface resistivity which is suitable for the purpose of removing electrification is to be around 10 .sup.6 through 10.sup.10 .OMEGA./cm.sup.2. However, a suitable material showing the surface resistivity in this range has not been found in a material having a single composition.

On the other hand, a floor sheet for removing electrification wherein a plastics is kneaded with carbon powders and the like, has been reduced into practice. However, plastics are apt to deteriorate when a strong cleaning is performed thereon and give out dusts when they are worn. When the worn plastics contact an object to be handled, the object may be contaminated by the dusts. On the other hand, generally, ceramics are chemically stable, excellent in refractoriness and corrosion resistance and provided with an advantage wherein they are hard and difficult to wear. Therefore, ceramics are preferable materials for a component directly contacting an object to be handled that abhors contamination.

However, ceramics is provided with drawbacks wherein it is a brittle material and is easy to fracture, and much labor and cost are required for working it into a product; for instance, it is necessary to perform grinding by a diamond grinding wheel in working it into a required shape.

Further, in employing the ceramics for removing electrification for a component for removing electrification, it is necessary to electrically connect the ceramics for removing electrification to an electric conductive material that is connected to the ground. When the surface resistivity of the ceramics for removing electrification is as large as not less than 10.sup.6 .OMEGA./cm.sup.2, and an electric conductive material such as a lead wire simply contacts the ceramics for removing electrification, the contact resistance is apt to be very large and unstable, and therefore, it is not possible to remove with certainty a charged static electricity from an object to be handled when the ceramics is used as the material for removing electrification.

SUMMARY OF THE INVENTION

With these conventional technologies as the background, it is an object of the present invention to provide an electrification removing component which is easy to use, and capable of certainly and instantly removing an electrified static electricity without destructing a part by an electric discharge, by directly contacting an electrification removing component to an object to be handled, in a device which handles an object that is easy to receive destruction by a rapid electric discharge of the electrified static electricity.

According to an aspect of the present invention, there is provided an electrification removing component comprising:

a ceramics for removing electrification of which surface resistivity is in a range of 2.times.10.sup.6 through 10.sup.10 .OMEGA./cm.sup.2 ; and

an electric conductive material connected to the ground;

said ceramics for removing electrification being electrically connected to the electric conductive material with an area by contact or bonding on surfaces of the ceramics for removing electrification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional diagram of a vacuum chuck which is an example of an electrification removing component according to the present invention;

FIG. 2 is a side diagram of a vacuum chuck which is another example of an electrification removing component according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In an electrification removing component of this invention, it is possible to instantly remove a static electricity with certainty from an object to be handled which is electrified for instance, to 2000 V or more, without electrically destructing the object to be handled, since the electrification removing component of this invention is provided with a ceramics for removing electrification having a surface resistivity in a range of 2.times.10.sup.6 through 10.sup.10 .OMEGA./cm.sup.2. When the surface resistivity of the ceramics for removing electrification is smaller than 2.times.10.sup.6 .OMEGA./cm.sup.2 it is possible to electrically destruct the object to be handled by elevating the voltage of the electrified static electricity. On the other hand, when the surface resistivity is larger than 10.sup.10 .OMEGA./cm.sup.2, the removing of the electrified static electricity may be difficult.

It is preferable to further restrict the range of the surface resistivity of the ceramics for removing electrification to 10 .sup.7 through 9.times.10.sup.9 .OMEGA./cm.sup.2, in view of the function in use. Further, the function as the component for removing electrification is achieved by electrically connecting the ceramics on a surface with an area to an electric conductive material that is connected to the ground, whereby an electric charge that is transferred to the ceramics for removing electrification, is led to the ground with certainty through the electric conductive material. Although a ceramics having a surface resistivity of larger than 10.sup.6 .OMEGA./cm.sup.2 is apt to be charged with static electricity, the static electricity is removed via the electric conductive material connected to the ground with certainty, dusts do not adhere to the ceramics for removing electrification because of an electrified static electricity, and the object to be handled is not contaminated by the dusts.

The ceramics for removing electrification having such a surface resistivity, can be provided, for instance, by distributing one kind or more of fine powders selected from the group of a nitride and a carbide of Ti, Zr, Hf, Nb and Ta, that are electric conductive inorganic compounds, in a ceramics whose major component is alumina or silicon nitride, that is an electric insulating material.

The electric conductive material is preferably a metal, which is easy to work. The electric conductive material and the ceramics for removing electrification are electrically connected by contact or by bonding, not with points but with an area on the surface. In this way, the component for removing electrification of this invention provides the function for removing electrification. The invented component prevents phenomena wherein an electric current is hard to flow because of the unstable contact resistance between the ceramics and an electric conductive material which is apt to be very large owing to the large surface resistivity of the ceramics for removing electrification.

The area of the contacted or bonded surface is preferably not less than 1 mm.sup.2 and more preferably not less than 2 mm.sup.2, thereby completely excluding a possibility wherein an electric contact resistance between the ceramics for removing electrification and the electric conductive material becomes unstable and may be very large or the connection therebetween is broken.

To provide the electric connection between the electric conductive material and the ceramics for removing electrification by contact, it is preferable, for instance, to dispose a metal sheet which is soft and hard to stain, between the ceramics for removing electrification and the electric conductive material, and to apply a mechanical pushing force thereon by screws, bolts or nuts. Then, the surface contact with an area is provided by deforming the soft metal sheet by the pushing force.

When the electric conductive material is a metal of which surface is easy to stain, there is a danger of enhancing the contact resistance. Therefore, the electric connection is preferably to be stabilized by performing a metallizing operation on the surface of the ceramics or by further bonding the ceramics to the electric conductive material. In case of the bonding, it is possible to provide simultaneously the electric connection and the bonding strength by procuring a bonding area of not less than 1 mm.sup.2. The bonding can be performed by soldering by a solder for bonding ceramics, by brazing after a metallizing operation, by applying and baking an electric conductive paste, by applying an organic electric conductive adhesion material, or by employing a double-sided adhesion tape having an electric conductivity, in case wherein much adhesion strength is not required.

For instance, a copper lead is employed for the grounding line. The electric connection between the electric conductive material and the grounding line can be provided even by a point contact since electric resistances of both are small. However, it is preferable to connect both by screws, soldering or brazing to avoid disconnection.

A preferable ceramics for removing electrification of this invention, is a sintered body wherein the ceramics for removing electrification includes one or more kinds of electric conductive compounds which are selected from nitrides and carbides of Ti, Zr, Hf, Nb and Ta, in a matrix whose major component is alumina, and of which relative density is not less than 90% (porosity including closed pores is not more than 10%). The relative density is obtained first by measuring a bulk density, calculating the porosity from the bulk density assuming that the density of the raw material powder does not change, and by subtracting the porosity from 100%.

Alumina has been known as an electric insulating material, and is preferable as a main matrix material of ceramics for removing electrification, since it is easy to obtain a raw material powder which is easy to sinter, is fine and is provided with a high purity. The inventors succeeded in providing a ceramics for removing electrification having the surface resistivity within a range of target and the relative density of not less than 90%, by forming a raw material powder mixture of alumina powder having an average particle size of not more than 5 .mu.m and a powder of nitrides or carbides (the specific resistivity in a range of 10.sup.-4 through 10.sup.-5 .OMEGA..cm) of Ti, Zr, Hf, Nb and Ta having a similar average particle size of not less than 5 .mu.m, and by sintering the formed body.

The raw material composition for providing the target surface resistivity varies with properties of a raw material powder, sintering conditions and the like. A sintered body having the target surface resistivity can be provided comparatively easily, by employing a raw material powder having an average particle size of not more than 5 .mu.m, and by sintering a composition comprising 4 through 23 volume % of a powder of one or more electric conductive compounds selected from nitrides and carbides of Ti, Zr, Hf, Nb and Ta, and the balance of a raw material powder of a matrix whose major component is alumina.

With respect to the raw material powder of the ceramics, it is preferable to further mill the mixed raw material powder so as to obtain a dense and uniform sintered body, since the sintering performance is improved when the average particle size thereof is not more than 5 .mu.m, and more preferably not more than 2 .mu.m. It is efficient to mill the mixed raw material powder with a dispersion media of ethanol or the like, by a rotating ball mill, a vibration mill, attrition mill or the like.

The matrix of the sintered body, whose major component is alumina, may include inevitable impurities contained in a raw material powder, sintering aids such as Y.sub.2 O.sub.3, MgO and the like for accelerating the sintering operation, or an additive such as MgO, SiO.sub.2 and the like for preventing crystal growth. When the electric conductive compound is included, an effect of preventing the crystal growth of alumina can be provided. The ceramics for removing electrification having target material properties and an average crystal grain size of not more than 5 .mu.m, for instance, 2 through 3 .mu.m, can be provided stably by employing a raw material powder having an average particle size of not more than 5 .mu.m.

The density of sintered body is limited to be not less than 90% in relative density (hereinafter simply density), to provide strength and wear resistance, since the larger the strength of the sintered body, the tougher the sintered body is, and the larger the wear resistance of the sintered body, the less the sintered body gives out dusts, whereby the ceramics for removing electrification has excellent durability. Further, the density of the sintered body is preferably not less than 93%, to eliminate hygroscopic property, by rendering all the pores of the sintered body to closed pores.

The normally performed die pressing, slip casting, injection molding, isostatic press molding and the like can be adopted for forming the mixed powder. In case of a formed body provided by the injection molding, a debonding treatment should be performed before the final sintering, since it contains a large amount of binder. The sintering of the formed body is performed by a pressureless sintering or by a hot pressing, in a nonoxidizing atmosphere, so that the electric conductive compound is not oxidized and the conductivity is not lost.

To create the nonoxidizing atmosphere in a furnace, the gas such as nitrogen, argon, helium or the like is introduced in the furnace, and the pressureless sintering is performed by maintaining the temperature in the furnace at 1550.degree. through 1700.degree. C. for 1 through 5 hours. To provide a more dense sintered body, the pressureless sintered body having the density of not less than 90%, is further sintered in a hot isostatic press. In case of the pressureless sintering and the hot isostatic pressing, the sintering can be performed even when the formed body is provided with a complicated shape.

Another preferable ceramics for removing electrification of this invention is a sintered body having the density of not less than 90%, wherein the ceramics for removing electrification includes SiC (silicon carbide) and one or more kinds selected from nitrides of Ti, Zr, Hf, Nb and Ta in a matrix composed of Si.sub.3 N.sub.4 and sintering aids.

The ceramics for removing electrification whose major component is Si.sub.3 N.sub.4, is provided with the surface resistivity within a target range and the strength of which is higher than that of the ceramics for removing electrification whose major component is alumina. That is, the bending strength of alumina ceramics for removing electrification is at a level of 25 through 35 kg/mm.sup.2, whereas the bending strength of the silicon nitride ceramics for removing electrification can be at a level of not less than 50 kg/mm.sup.2 easily. The latter can be utilized as a structural material. The silicon nitride ceramics for removing electrification can be provided by forming and sintering a mixed powder of 0.1 through 17 weight % of SiC powder having an average particle size of not more than 5 .mu.m, 22 through 55 weight % of a powder having an average particle size of not more than 5 .mu.m of one or more kinds selected from nitrides of Ti, Zr, Hf, Nb and Ta, and the balance of Si.sub.3 N.sub.4 powder including a sintering aid or aids having an average particle size of not more than 5 .mu.m.

SiC having an intermediate specific resistivity (not more than 10.sup.2 .OMEGA..cm) is included in the ceramics for removing electrification, other than metal nitrides of Ti, Nb and the like, to facilitate the control of the surface resistivity of the sintered body. However, a formed mixed powder including SiC of more than 3 volume % is difficult to sinter densely by the pressureless sintering, and therefore, the sintering is preferably performed by hot pressing or by hot isostatic pressing.

Therefore, to enable the pressureless sintering of Si.sub.3 N.sub.4 powder, a sintering aid or aids added thereto preferably by 2 through 15 weight % in a total of one or more kinds selected from MgO, Al.sub.2 O.sub.3, MgAl.sub.2 O.sub.4 and Y.sub.2 O.sub.3, whereby a sintered body having a density of not less than 90% and a bending strength of not less than 60 kg/mm.sup.2 can easily be provided.

A preferable content of TiN is 22 through 33 weight % for providing the ceramics for removing electrification having a surface resistivity within the target range and a large strength. Further, preferable contents of ZrN and/or NbN are 27 through 40 weight %, respectively. In either case, the content of SiC is preferably 1 through 12 weight %.

The method of making the silicon nitride ceramics for removing electrification is basically the same with that of the above mentioned alumina ceramics for removing electrification. However, in the method of making the silicon nitride ceramics, the sintering is performed at least in a nitrogen (N.sub.2) containing atmosphere, preferably in an atmosphere including nitrogen (N.sub.2) and silicon (Si) to prevent the sintering from hampering by the decomposition of Si.sub.3 N.sub.4 into Si and N.sub.2. The average particle size of a raw material powder is preferably finer than in case of making the alumina ceramics for removing electrification, and the raw material powder having an average particle size of not more than 5 .mu.m, preferably 0.1 through 2 .mu.m, is employed to provide a sintered body having a large density and a large bending strength.

The sintering conditions to provide the ceramics for removing electrification having the target surface resistivity and density, considerably vary with a degree of mixing the raw material powders, the particle size distribution, the forming condition and the composition. In case of performing the pressureless sintering, the sintering is performed normally at a sintering temperature of 1700.degree. through 1800.degree. C. for 1 through 10 hours.

In another preferable component for removing electrification of this invention, the ceramics for removing electrification is formed by thermal-spraying it on a metallic under coating layer which is formed on a metal substrate and the thermal expansion coefficient of the under coating layer is between those of the metal substrate and the ceramics layer.

Normally, the electric connection between an electric conductive material and another grounded electric conductive material can be performed by simply contacting them by screws or by springs. However, the electric connection by the simple contact between the ceramics for removing electrification having a surface resistivity not less than 2.times.10.sup.6 .OMEGA./cm.sup.2 and an electric conductive material is devoid of reliability, since the contact resistance is unstable and may be very large. However, in a component for removing electrification wherein a metal substrate is integrated with a ceramics layer, there is no problem of electric connection therebetween. The connection of the metal substrate to the grounding line can be performed by the simple contact. However, it is preferable to connect them by soldering or by brazing with good reliability.

Further, the working of a metal is significantly easy compared with that of a ceramics, and the manufacturing cost is more inexpensive, and therefore, components for removing electrification having various shapes can be manufactured at a low cost, when the ceramics layer is formed on metal substrates which have previously been worked to the required shapes.

When, for instance, copper, stainless steel, aluminum and the like are used for the metal substrates, the thermal expansion coefficients of these are as large as 17 through 18.times.10.sup.-6 /.degree.C. By contrast, the thermal expansion coefficients of Al.sub.2 O.sub.3 -TiO.sub.2 ceramics layer and Al.sub.2 O.sub.3 -TiN ceramics layer, are as small as 8 through 9.times.10.sup.-6 .degree.C. and the difference of the thermal expansion coefficients therebetween is considerably large.

In another ceramics for removing electrification, the ceramics layer is formed by a thermal spraying, and hence the temperature of the metal substrate is elevated by the spraying. Although the thermal stress is not present or is very small when the temperature is elevated, a strain is caused in the vicinity of the boundary of the metal substrate and the ceramics layer due to the difference between the thermal expansion coefficients of both by cooling down to the room temperature, cracks are often generated in the ceramics layer and the ceramics layer is often peeled off from the metal substrate.

Accordingly, in the component for removing electrification of this invention, the thermal strain caused around the boundary of the metal substrate having a larger thermal expansion coefficient and the ceramics layer having a smaller thermal expansion coefficient, is reduced by previously providing a metallic under coating layer having a thermal expansion coefficient the value of which is between those of both, on the metal substrate. Thereby the ceramics layer is prevented from causing cracks due to the thermal strain and peeling from the metal substrate, and the metallic under coating layer procures a good electric connection between the ceramics layer and the metal substrate.

The metallic under coating layer may be formed by the plating, flame spraying, plasma spraying, sputtering, Mo--Mn method of metallizing, vapor deposition and the like. They are, for instance, of a metal or an alloy of Mo, Ti, Ni, Nb, Ta, W, Ni--Al, Ni--Cr, Ni--Cr--Al, Ni--Cr--Al--Y, Ni--Co--Cr--Al--Y or the like.

It is preferable to form the metallic under coating layer especially by the plasma spraying method. The first reason is that it is possible to prevent the oxidation of a metal by spraying the metal in a nonoxidizing atmosphere. Further, by the plasma spraying method, since the temperature can be high and the selection of a metal to be undercoated is wide and it is easy to form a rather thick under coating layer or an under coating layer having a two-sublayer structure.

The thermal expansion coefficients of these metallic under coating layers are preferably in a range of 12 through 15.times.10.sup.-6 /.degree.C. More preferably, the under coating layer is constructed by plural sublayers having different thermal expansion coefficients, and the thermal expansion coefficient of an under coating sublayer on the side of the ceramics layer is more proximate to the thermal expansion coefficient of the ceramics layer than that of another under coating sublayer on the side of the metal substrate.

When the difference between the thermal expansion coefficient of the under coating sublayer on the side of the ceramics layer and that of the ceramics layer is smaller than 2.times.10.sup.-6 /.degree.C., a component for removing electrification with no apprehension of the peeling-off of the ceramics layer can be provided. As the metallic under coating layers having a small thermal expansion coefficient, there are Ti, Nb, W, Mo, Ta and alloys including these metals.

The thickness of the under coating layer may be small so far as the thermal stress due to the difference between the thermal expansions of the metal substrate and the ceramics layer can be reduced and the ceramics layer can be stably formed on the under coating layer. When the difference between the thermal expansions of both is large, a rather thick under coating layer is preferred. When the under coating layer is constructed by two sublayers, the thickness of the under coating layer is preferably 30 through 150 .mu.m, more preferably 80 through 120 .mu.m.

Further, another component for removing electrification of this invention is provided with practically sufficient strength and toughness by integrating the ceramics layer with the metal substrate, even if the strength of the ceramics layer is small and the thickness thereof is small. The component for removing electrification integrated with the metal substrate can easily be integrated with a machine since it is easy to work the metal substrate into a desired shape compared with the ceramics for removing electrification.

In forming the metallic under coating layer, it is possible to promote the adhesion strength of the under coating layer with respect to the metal substrate by previously grit-blasting the surface of the metal substrate by abrasive grains of alumina or silicon carbide into a roughened surface. Further, the adhesion strength of the metallic under coating layer is further promoted by cutting grooves or screws on the surface of the metal substrate, and further by grit-blasting the surface provided with the grooves and the screws. The thickness of the under coating layer is pertinent to be approximately 30 through 150 .mu.m, such that the ceramics layer having a good adhesion strength can be formed.

In another preferable component for removing electrification of this invention, the spraying material of the ceramics layer is selected to include alumina and titanium oxide and/or titanium nitride. The reason is that a ceramics layer having the surface resistivity within the target range can reproducibly be formed by thermal spraying. Further, it is easy to obtain a raw material powder of alumina having high-purity and a pertinent average particle size that is easy to spray, and the formed ceramics layer is chemically stable and provided with a high hardness and wear resistance.

Both of the necessary contents of a titanium oxide and a titanium nitride which are the minor components in the ceramics layer, are preferably 5 through 50 weight %, although depending on the spraying conditions. Further, it is preferable to form the ceramics layer by the plasma spraying method, since it is easy to control the surface resistivity in the narrow target range.

It is preferable to employ a sintered spray powder having a predetermined composition for the spray powder forming the ceramics layer, to form a uniform ceramics layer having a required surface resistivity. For instance, a powder having an average particle size which is easy to spray, for instance, a powder having an average particle size of 10 through 75 .mu.m, may be employed by milling a sintered body having a predetermined composition. When the average particle size is smaller than 10 .mu.m, the supply of the powder to the spraying device is difficult. When the average particle size is larger than 75 .mu.m, the powder particle is difficult to melt in the hot gas, and a dense ceramics layer is difficult to form.

It is preferable to form the cer