Patent Application
20070082313
1. Field of the Invention
The present invention relates to a wafer holder used for a wafer prober in which a semiconductor wafer is mounted on a wafer-mounting surface and a probe card is pressed onto the wafer for inspecting electric characteristics of the wafer, a heater unit including the wafer holder and to a wafer prober including the heater unit.
2. Description of the Background Art
Conventionally, in the step of inspecting a semiconductor, a semiconductor substrate (wafer) as an object of processing is subjected to heat treatment. Specifically, burn-in is performed in which the wafer is heated to a temperature higher than the temperature of normal use, to accelerate degradation of a possibly defective semiconductor chip and to remove the defective chip, in order to prevent defects after shipment. In the burn-in process, after semiconductor circuits are formed on the semiconductor wafer and before cutting the wafer into individual chips, electrical characteristics of each chip are measured and defective ones are removed while the wafer is heated. In the burn-in process, reduction of process time is strongly desired in order to improve throughput.
In the burn-in process as such, a chuck top having a heater for heating the wafer contained therein is used. Conventionally, chuck tops formed of metal have been used, as it is necessary to have the entire rear surface of the wafer in contact with the ground electrode. At the time of measurement, a wafer having circuits formed thereon is placed on the chuck top formed of metal and having the heater therein, and electric characteristics of the chips are measured. The wafer holder mounting the chuck top is moved by a driving system to a prescribed position, and the wafer is pressed to a prober referred to as a probe card, having a number of electrode pins for electric conduction, with a force of several tens to several hundreds kgf, and such operations are repeated. Therefore, when the chuck top is thin, the chuck top might possibly be deformed, resulting in contact failure between the wafer and a probe pin. Therefore, it is necessary to use a thick metal plate having the thickness of at least 15 mm, for maintaining rigidity of the chuck top and the wafer holder. As a result, it takes long time to increase and decrease temperature of the heater, which is a significant drawback in improving the throughput.
In the burn-in process, electric characteristics are measured while the chips are electrically conducted and electric characteristics are measured. As recent chips come to have higher outputs, it is possible that a chip generates considerable heat during measurement of electric characteristics, and in some situations, the chip might be broken by self-heating. Therefore, after measurement, rapid cooling is required. During measurement, heating as uniform as possible is required. In view of the foregoing, copper (Cu) having thermal conductivity as high as 403 W/mK has been used as the metal material.
In view of the foregoing, Japanese Patent Laying-Open No. 2001-033484 proposes a wafer prober having a ceramic substrate that is thin but has high rigidity and is not susceptible to deformation with a thin metal layer formed on its surface, in place of a thick metal plate, to be less susceptible to deformation and to have smaller thermal capacity. According to this reference, the chuck top has high rigidity and therefore it does not cause contact failure, and as it has small thermal capacity, it allows heating and cooling in a short period of time. It is described that as a support base for mounting the wafer prober, an aluminum alloy or stainless steel may be used.
However, recently, as the semiconductor processes have come to be miniaturized, the load applied per unit area at the time of measurement has been increased. Therefore, when the wafer prober is supported only by the outermost circumference as described in Japanese Patent Laying-Open No. 2001-033484, the prober itself may deform because of the load at the time of probing, uniform contact of the pins of probe card with the wafer would fail and inspection becomes impossible, or in the worst case, the wafer would be broken. Further, there is a problem that when the wafer is heated to a prescribed temperature, that is, to about 100 to about 200° C., the heat is transferred to the driving system, and metal components forming the driving system thermally expand, degrading positional accuracy.
Further, as the semiconductor processes have come to be miniaturized, high accuracy of registration between the probe card and the wafer holder comes to be required. There is a problem that when the wafer is heated to a prescribed temperature, that is, to about 100 to about 200° C., the heat is transferred to the driving system for moving the wafer holder, and metal components forming the driving system thermally expand, degrading positional accuracy. Because of such problems, contact failure occurs in inspection of semiconductors having particularly minute circuitry.
The present invention was made to solve the above-described problems. Specifically, an object of the present invention is to provide a wafer holder less susceptible to deformation even under high load and hence capable of effectively preventing contact failure, and having improved heat-insulating effect and hence capable of improving positional accuracy, improving thermal uniformity and rapid heating and cooling of chips, as well as to provide a heater unit including the wafer holder and a wafer prober including the heater unit.
The present invention provides a wafer holder, including a chuck top having a chuck top conductive layer on a surface and a supporter supporting the chuck top, wherein a space is formed between the chuck top and the supporter, and a support member is provided in the space.
Preferably, the support member is arranged concentric with the supporter, or arranged approximately at the center of the supporter. More preferably, the support member arranged concentric with the supporter, and the support member arranged approximately at the center of the supporter are both provided.
Preferably, the support member has a pipe-shape.
Further, preferably, thermal expansion coefficient of the support member and thermal expansion coefficient of the supporter are approximately the same. Preferably, the support member has Young's modulus of at least 100 GPa.
The present invention further provides a wafer holder, including a chuck top for mounting a wafer and a supporter supporting the chuck top, wherein a space is formed between the chuck top and the supporter, and a material of low thermal conductivity having thermal conductivity lower than that of the chuck top is inserted in the space.
Preferably, the thermal conductivity of the material of low thermal conductivity is lower than thermal conductivity of the supporter.
Further, preferably, the material of the chuck top is a composite body of metal and ceramics.
Further, preferably, the material of the chuck top is a composite body of aluminum and silicon carbide or a composite body of silicon and silicon carbide. Alternatively, the material of the chuck top may be ceramics.
It is preferred that the material of the support body is ceramics or a composite body of a plurality of ceramics. Specifically, it is preferred that the material is any of mullite, alumina, a composite body of mullite and alumina, aluminum nitride, and silicon nitride.
The present invention also relates to a heater unit including the wafer holder described above, as well as to a wafer prober including the heater unit.
The heater unit including the wafer prober as described above and the wafer prober including the heater unit have high rigidity and high heat-insulating effect, and therefore, they are capable of improving positional accuracy, improving thermal uniformity and realizing rapid heating and cooling of the chips.
According to the present invention, in the wafer holder including a chuck top for mounting and fixing a wafer and a supporter supporting the chuck top, a support member is provided in a space between the chuck top and the supporter, and therefore, deflection of the chuck top can significantly be reduced, and hence, a wafer holder less susceptible to deformation even under high load and having improved heat-insulating effect can be provided. According to the present invention, a wafer holder capable of probing even a wafer of large diameter can be provided.
According to another aspect of the present invention, in the wafer holder including a chuck top for mounting and fixing a wafer and a supporter supporting the chuck top, material of low thermal conductivity having thermal conductivity lower than that of the chuck top is inserted to a space formed between the chuck top and the supporter, and therefore, heat transfer from the heated chuck top to the supporter is suppressed, and the heat-insulating effect can be improved. Thus, a wafer holder can be provided, which can prevent over-heating of the bottom portion of the supporter, prevent increase in temperature of a driving system mounting the wafer holder when a semiconductor wafer having minute circuitry and requiring high accuracy is heated, and prevent over-heating of the facility itself having the heater unit attached thereon.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.
FIGS. 2 to 5 are plan views showing exemplary planar structures of the wafer holder in accordance with the present invention.
FIGS. 8 to 10 are plan views showing examples of heat-insulating structure in accordance with the present invention.
An embodiment of the present invention will be described with reference to
The wafer holder in accordance with the present invention has high heat-insulating effect, as it has the space 5. The shape of space 5 is not specifically limited, and it may have any shape that can suppress as much as possible the amount of transfer of cool air or heat generated at chuck top 2 to supporter 4. It is preferred that supporter 4 has a hollow cylindrical shape with a bottom, as the contact area between chuck top 2 and supporter 4 can be reduced and space 5 can readily be formed. When such a space 5 is formed, what lies between chuck top 2 and supporter 4 is mostly an air layer, and hence, an efficient heat insulating structure can be formed.
In a typical configuration of the present invention, a surface of support member 50 on the side of the chuck top and the contact surface of supporter 4 to chuck top 2 are formed to be approximately flush. By providing such a support member 50, the amount of deformation (deflection) of chuck top 2 caused by the high load at the time of probing can be reduced.
Preferably, support member 50 is arranged approximately at the center of supporter 4, as shown in
In the present invention, “approximately at the center” means an area of a diameter of about 50 mm at the center of the diameter of the supporter.
The shape of support member may be a pipe. Alternatively, pillar-shaped support members 50 and a pipe-shaped support member 51 may be combined as shown in
As described above, the amount of deformation of chuck top 2 is in proportion to the load applied to chuck top 2 at the time of probing, and it increases as the distance between support members increases. Therefore, by inserting the support member or members to space 5 as described above, it becomes possible to reduce the distance between the support members, and to suppress the amount of deformation of chuck top 2. As to the shape of support member, pillar or hollow cylindrical shape is only an example, and it may be a triangular pole, a quadrangular pole or a polygonal pole with any polygon as a bottom surface or a tubular body, to attain the similar effect.
In the present invention, the support member is provided as a member separate from supporter 4, and therefore, an interface exists between supporter 4 and the support member. As a result, the heat generated at chuck top 2 does not easily transferred to supporter 4, and temperature increase at the lower portion of supporter 4 can be suppressed.
The support member may be fixed on supporter 4 by brazing with an active metal, glass fixing, or screw fixing, and among these, screw fixing is particularly preferred. Screw fixing facilitates attachment/detachment, and as heat treatment is not involved at the time of fixing, deformation of the supporter or support member by the heat treatment can be suppressed.
In the present invention, it is preferred that the amount of thermal expansion of the support member and the amount of thermal expansion of supporter 4 are approximately the same over the temperature range of use of chuck top 2. Specifically, it is preferred that thermal expansion coefficient of the support member is approximately the same as that of supporter 4. By way of example, when the amount of thermal expansion of the support member is larger than the amount of thermal expansion of supporter 4, an upper surface of chuck top 2 would be protruded upward, possibly causing rattling of the wafer during probing. On the contrary, when the amount of thermal expansion of the support member is smaller than the amount of thermal expansion of supporter 4, the supporting effect by the support member during probing would decrease. Therefore, it is preferred that the materials of the support member and supporter 4 have the same thermal expansion coefficient and, further, it is desired to use the same material for the support member and supporter 4.
When the thermal expansion coefficient of the support member and the thermal expansion coefficient of supporter 4 are approximately the same, an index of the difference in thermal expansion coefficient between the support member and supporter 4 is at most 3×10−6/° C., as a particularly preferred example.
It is particularly preferred that the material of the support member is the same as the material of a hollow cylindrical portion of the supporter or a pillar member, as will be described later. When the hollow cylindrical portion, the pillar member and the support member thermally expand because of the heat from the heater body and the materials are different, a step would possibly be formed between the hollow cylindrical portion or the pillar member and the support member. As to the size of the support member, it is preferred that the radial cross-sectional area is at least 0.1 cm2. When the cross-sectional area is smaller than 0.1 cm2, the supporting effect by the support member would be insufficient, and the support member tends to deform. The cross-sectional area should preferably be at most 100 cm2. When the cross-sectional area exceeds 100 cm2, the amount of heat transferred to the driving system tends to increase.
It is preferred that the support member has Young's modulus of at least 100 GPa. Then, deformation of supporter 4 itself can be made small, and hence, deformation of chuck top 2 can further be suppressed. When a material having Young's modulus smaller than 100 GPa is used, it is possible that the support member deforms at the time of probing and the effect of reducing deflection is reduced. Specific, preferred material of the support member includes alumina, mullite, a composite body of mullite and alumina, a composite body of Si and SiC (Si—SiC), AlN, Si3N4, SiC, stainless steel, and a composite body of Al and SiC (Al—SiC). Further, for the purpose of suppressing as much as possible the heat transfer from chuck top 2 to supporter 4, among the foregoing, mullite and the composite body of mullite and alumina having low thermal conductivity are particularly preferred.
In the present invention, the thermal conductivity may be measured by a method such as laser flash method, using pelletized samples.
According to another aspect of the present invention, the wafer holder of the present invention includes a chuck top for mounting a wafer and a supporter supporting the chuck top, a space is formed between the chuck top and the supporter, and in the space, a material of low thermal conductivity having thermal conductivity lower than the chuck top is inserted. As the material of low thermal conductivity, specifically, material 11 of low thermal conductivity such as shown in a wafer holder 200 of
In the present invention, the heat-insulating effect can be improved by inserting material 11 of low thermal conductivity in space 5. Space 5 is an air layer, and when air convection occurs, the effect of heat transfer increases, and in some cases, prescribed heat-insulating effect would not be attained. Therefore, insertion of the material having low thermal conductivity is effective. Material 11 of low thermal conductivity may be inserted entirely in the space 5 or inserted to only a part of the space 5. As the thermal conductivity of material 11 of low thermal conductivity is set lower than the thermal conductivity of chuck top 2, good heat-insulating effect can be attained.
The form of material 11 of low thermal conductivity may be a solid body, or it may be porous or fiber. It may be a porous ceramic body formed of alumina or silicon carbide, or it may be entangled fiber or fabric of ceramics. The fiber type one may form a convection-free space, attaining good heat-insulating effect. Further, it may be a composite body of a plurality of states, such as fibrous substance and solid body.
In order to realize better heat-insulating effect by material 11 of low thermal conductivity, it is preferred that the thermal conductivity of material 11 of low thermal conductivity is also lower than supporter 4. This is desirable, as the effect of heat insulation can be enhanced. The material 11 of low thermal conductivity may be inorganic material or organic material, and the material is not specifically limited. Though not limiting, the inorganic substance includes alumina, quartz, heat-resistant glass, porcelain, crystal, stainless steel and concrete.
It is also preferred that the present invention includes both the support member 7 and material 11 of low thermal conductivity as shown, for example, in wafer holder 300 of
Preferably, the wafer holder of the present invention includes a heater body. In the step of inspecting a semiconductor wafer, heating of a wafer may or may not be required. Recently, however, it is more often the case that heating to about 100 to about 200° C. is required. Therefore, if the transfer of heat of the heater body heating chuck top 2 to supporter 4 cannot be prevented, the heat would be transferred to the driving system provided below supporter 4 of the wafer prober, and because of difference in thermal expansion coefficient among components, machine accuracy would be deviated, possibly causing significant deterioration in flatness and parallelism of the upper surface (wafer-mounting surface) of chuck top 2. The wafer holder in accordance with the present invention, however, has a heat-insulating structure realized by the existence of space 5, and therefore, significant degradation in flatness and parallelism can be avoided. Further, as supporter 4 has a hollow structure with space 5, the weight can be reduced as compared with a supporter having a solid cylindrical shape.
As heater body 6, one having a resistance heater body 61 sandwiched by an insulator 62 such as mica, shown, for example, in
As to the method of forming heater body 6, heater body 6 may be formed by the method of forming an insulating layer through a method of thermal spraying or screen printing on a surface opposite to the wafer-mounting surface of chuck top 2 and then by forming the conductive layer in a prescribed shape through a method such as screen printing or vapor deposition, other than the method described above.
When chuck top 2 is heated by heater body 6 and inspection is done, for example, at 200° C., it is preferred that the temperature at the bottom surface of supporter 4 is at most 100° C. When the temperature exceeds 100° C., because of distortion resulting from thermal expansion of the driving system of wafer holder, accuracy deteriorates, resulting in positional deviation, warp, and biased contact of the probe caused by degraded parallelism, and therefore, contact failure possibly occurs, and accurate evaluation of the device would be impossible. When inspection is to be done at a room temperature after inspecting the wafer at 200° C., cooling takes long time and hence, throughput would be decreased.
It is preferred that supporter 4 has Young's modulus of at least 200 GPa. In this case, deformation of supporter 4 itself can be made small, and hence, deformation of chuck top 2 can further be suppressed. More preferable Young's modulus of supporter 4 is at least 300 GPa. Young's modulus of supporter 4 of 300 GPa or higher is particularly preferred, as deformation of supporter 4 can significantly be reduced and hence supporter 4 can further be reduced in size and weight.
Supporter 4 preferably has thermal conductivity of at most 40 W/mK. Then, the amount of heat transferred from chuck top 2 through supporter 4 to the driving system of the wafer holder is further reduced, and temperature increase of the driving system can effectively be prevented. Recently, a temperature as high as 150° C. is required at the time of probing, and therefore, it is more preferred that supporter 4 has thermal conductivity of at most 10 W/mK. More preferable thermal conductivity is at most 5 W/mK. With the thermal conductivity of this range, the amount of heat transfer from supporter 4 to the driving system decreases significantly.
In the present invention, the thermal conductivity may be measured by a method such as laser flash method, using pelletized samples.
As a material having good flatness, allowing processing to the shape as described above and having Young's modulus and thermal conductivity as described above as physical properties, ceramics or a composite body of a plurality of ceramics is preferred. Considering processability and cost, it is preferred that the main component is any of mullite, alumina, a composite material of mullite and alumina, aluminum nitride, and silicon nitride. Mullite is preferred as it has low thermal conductivity and attains high heat insulating effect, and alumina is preferred as it has high Young's modulus and high rigidity. Mullite-alumina composite is generally preferred as the thermal conductivity is lower than alumina and Young's modulus is higher than mullite.
In the present invention, supporter 4 typically has a hollow cylindrical shape with a bottom. It is preferred that the radial thickness of the hollow cylindrical portion of supporter 4 is at most 20 mm. When the radial thickness exceeds 20 mm, the amount of heat transferred from chuck top 2 through supporter 4 to the driving system of wafer holder may possibly increase. When the radial thickness is smaller than 1 mm, supporter 4 itself tends to be deformed or damaged by the load of the probe card. More preferable radial thickness is 10 to 15 mm. Further, that portion of the hollow cylindrical portion of supporter 4 which is in contact with chuck top 2 should preferably have the radial thickness of 2 to 5 mm, as good balance between the strength and heat-insulating characteristic of supporter 4 can be attained.
Further, it is preferred that the height of hollow cylindrical portion of supporter 4 is at least 10 mm. When the height is lower than 10 mm, the amount of heat transferred from chuck top 2 through supporter 4 to the driving system of wafer holder would increase.
The base portion of supporter 4 preferably has the thickness of at least 10 mm. When the thickness of the base portion of supporter 4 is smaller than 10 mm, supporter 4 itself may possibly be deformed or damaged by the load of the probe card, and the heat of chuck top 2 is readily transferred to the base portion of supporter 4, causing warp of supporter 4 because of thermal expansion, and the flatness and parallelism of chuck top 2 would be deteriorated. It is more preferred that the thickness is at most 35 mm. The thickness of at most 35 mm is preferred, as supporter 4 can be reduced in size. Further, supporter 4 may have a structure in which the hollow cylindrical portion and the base portion are separable from each other, rather than the integrated structure. In that case, the separated hollow cylindrical portion and the base portion have interface to each other and the interface serves as a thermal resistance layer, so that the heat transferred from chuck top 2 to supporter 4 is once intercepted by the interface, and the temperature increase of the base portion is suppressed.
It is preferred that the support surface of supporter 4 supporting chuck top 2 has a heat insulating structure. As the heat insulating structure, one formed by forming a notch in supporter 4, thereby reducing the contact area between chuck top 2 and supporter 4 may be used. It is also possible to form the heat insulating structure by forming a notch in chuck top 2. In that case, it is preferred that chuck top 2 has Young's modulus of at least 250 GPa. As the pressure from the probe card acts on chuck top 2, the amount of deformation of chuck top 2 would inevitably increase if a notch exists and Young's modulus of the material is small, and when the amount of deformation increases, damage to the wafer or damage to chuck top 2 itself may possibly result. Formation of the notch in supporter 4 is preferred, because such a problem can be avoided.
As to the shape of the notch, by way of example, concentric circular trench 21 such as shown in
As the heat insulating structure, it is preferred to provide a plurality of pillar members 23 between chuck top 2 and supporter 4, as shown in
As a material for pillar members 23 used in the heat insulating structure of supporter 4 described above, one having thermal conductivity of at most 30 W/mK is preferred. When the thermal conductivity is higher, the heat insulating effect tends to decrease. As the material of pillar members 23, Si3N4, mullite, mullite-alumina composite, steatite, or cordierite, stainless steel, glass (fiber), or heat resistant resin such as polyimide, epoxy or phenol may be used, or a composite thereof may be used.
At the contact surface between supporter 4 and chuck top 2, it is preferred that the surface roughness Ra of both supporter 4 and chuck top is at least 0.1 μm. When the surface roughness Ra is at least 0.1 μm, thermal resistance at the contact surface between supporter 4 and chuck top 2 increases, and the amount of heat transferred to the driving system of the wafer holder can be reduced. There is no specific upper limit of the surface roughness Ra. As for the method of realizing surface roughness Ra of at least 0.1 μm, for example, polishing process or sand blasting may be used.
Preferably, in addition to the contact surface between supporter 4 and chuck top 2, the surface roughness Ra at the contact surface between the bottom surface of supporter 4 and the driving system, the contact surface between the base portion of supporter 4 and the hollow cylindrical portion or the pillar member 23 when the base portion of supporter 4 and the hollow cylindrical portion or pillar member 23 are made separable, and at the contact surface between the hollow cylindrical portion and the plurality of pillar members 23 when the hollow cylindrical portion and the plurality of pillar members 23 are used in combination should also be at least 0.1 μm as in the foregoing, because the thermal resistance is increased and the amount of heat transferred to the driving system of the wafer holder can be reduced. Reduction in heat quantity transferred to the driving system, attained by the increased thermal resistance, leads to reduction of power supply to the heating body.
Perpendicularity at a contact surface between an outer circumferential portion of the hollow cylindrical portion of supporter 4 and chuck top 2, and at a contact surface between an outer circumferential portion of the hollow cylindrical portion of supporter 4 and pillar member 23 should preferably be at most 10 mm, with the measured length converted to 100 mm. When perpendicularity is larger than that and the pressure applied from chuck top 2 acts on the hollow cylindrical portion of supporter 4, the hollow cylindrical portion itself may possibly deform more easily.
It is preferred that a metal layer is formed on the surface of supporter 4. Electric field or electromagnetic wave generated by heater body 6 for heating chuck top 2, a prober driving unit or by peripheral apparatuses may affect wafer inspection as noise. Formation of the metal layer on supporter 4 is preferred, as it can intercept (shield) the electromagnetic wave. The method of forming the metal layer is not specifically limited. By way of example, a conductive paste prepared by adding glass frit to metal powder of silver, gold, nickel or copper may be applied using a brush and burned.
Alternatively, metal such as aluminum or nickel may be thermally sprayed to form the metal layer. The metal layer may be formed by plating on the surface of supporter 4. Further, combination of these methods is also possible. Specifically, metal such as nickel or the like may be plated after burning the conductive paste, or plating may be done after thermal spraying. Among these methods, plating is preferred, as it has high contact strength and is highly reliable. Further, thermal spraying is preferred as it allows formation of the metal film at a relatively low cost.
As another method, a conductor having a circular tube shape may be attached on a side surface of supporter 4. The material used here is not specifically limited, as long as it is a conductor. By way of example, metal foil or a metal plate of stainless steel, nickel, aluminum or the like may be formed to have a circular tube shape of a size larger than the outer diameter of supporter 4, and it may be attached on the side surface of supporter 4. Further, at the bottom surface portion of supporter 4, metal foil or a metal plate may be attached, and by connecting this to the metal foil or metal plate attached to the side surface of supporter 4, the effect of shielding the electromagnetic wave can be enhanced. Further, utilizing the space inside supporter 4, the metal foil or metal plate may be attached inside the space 5, and by connecting this to the metal foil or metal plate attached to the side surface and the bottom surface of supporter 4, the effect of shielding the electromagnetic wave can be enhanced. Adopting such method is preferred, because the electromagnetic wave can be shielded at a lower cost than when plating is done or a conductive paste is applied. Though the method of fixing the metal foil or the metal plate to supporter 4 is not specifically limited, the metal foil or metal plate may be attached to supporter 4 using, for example, metal screws. Further, the metal foil or the metal plates on the bottom surface and on the side surface may be integrated beforehand and then fixed on supporter 4.
It is preferred that the electromagnetic shield layer for shielding the electromagnetic wave is also formed between heater body 6 heating chuck top 2 and chuck top 2. For forming the electromagnetic shield layer, the method of forming a metal layer on the surface of supporter 4 such as described above may be used, and, by way of example, metal foil may be inserted between heater body 6 and chuck top 2. The material of metal foil to be used is not specifically limited, and stainless steel, nickel or aluminum may be used.
Further, it is preferred that an insulating layer is provided between the electromagnetic shield layer and chuck top 2. The insulating layer serves to cut off noise that affects inspection of the wafer, such as the electromagnetic wave or electric field generated at heater body 6 and the like. The noise particularly has significant influence on measurement of high-frequency characteristics of the wafer, and the noise does not have much influence on the measurement of normal electric characteristics. Though most of the noise generated at the heater body 6 is shielded by the electromagnetic shield layer, in terms of electric circuit, a capacitor is formed between chuck top conductive layer 3 formed on the wafer-mounting surface of chuck top 2 and the electromagnetic shield layer when chuck top 2 is an insulator, or between chuck top 2 itself and heater body 6 when chuck top 2 is a conductor, and the capacitor may have an influence as a noise at the time of inspecting the wafer. In order to reduce the influence, it is preferred that the insulating layer is formed between the electromagnetic shield layer and chuck top 2.
Further, it is preferred that a guard electrode layer is formed between chuck top 2 and the electromagnetic shield layer with an insulating layer interposed. When the guard electrode layer is connected to the metal layer formed on supporter 4, the noise that affects measurement of high-frequency characteristic of the wafer can further be reduced. Specifically, in the present invention, when supporter 4 as a whole including heater body 6 is covered by a conductor, the influence of noise at the time of measuring wafer characteristics at a high frequency can be reduced. When the guard electrode layer is connected to the metal layer provided on supporter 4, the influence of noise can further be reduced.
At this time, it is preferred that the resistance value of the insulating layer described above is at least 1×107Ω. When the resistance value is smaller than 1×107Ω, small current flows to chuck top conductive layer 3 because of the influence of heater body 6, which small current possibly becomes noise at the time of probing and affects probing. It is preferred that resistance value of the insulating layer is at least 1×107Ω, as the small current described above can sufficiently be reduced not to affect probing. Recently, circuit patterns formed on wafers have been miniaturized, and therefore, it is preferable to reduce such noise as much as possible. When the resistance value of the insulating layer is at least 1×1010Ω, reliability can further be enhanced.
Further, it is preferred that the dielectric constant of the insulating layer is at most 10. When the dielectric constant of the insulating layer exceeds 10, charges tend to be stored more easily at the electromagnetic shield layer sandwiching the insulating layer, the guard electrode layer and chuck top 2, which might possibly be a cause of noise generation. Particularly, as the wafer circuits have been much miniaturized in these days as described above, it is preferable to reduce noise, and therefore, dielectric constant should preferably be at most 4 and more preferably at most 2. Setting small the dielectric constant is preferred, as the thickness of the insulating layer necessary for ensuring the insulation resistance value and the capacitance can be made thinner, and hence, thermal resistance posed by the insulating layer can be reduced.
Further, when chuck top 2 is an insulator, capacitance between chuck top conductive layer 3 and the guard electrode layer and between chuck top conductive layer 3 and the electromagnetic shield layer, or when chuck top 2 is a conductor, the capacitance between chuck top 2 itself and the guard electrode layer and between chuck top 2 itself and the electromagnetic shield layer, should preferably be at most 5000 pF. When the capacitance exceeds 5000 pF, the influence of the insulating layer as a capacitor would be too large, possibly causing noise and affecting probing. Capacitance of at most 1000 pF is preferred, as it enables inspection free of noise influence of even a miniaturized circuitry.
As described above, by controlling the resistance value, dielectric constant and capacitance of the insulating layer in the ranges described above, noise at the time of inspection can significantly be reduced.
The thickness of the insulating layer should preferably be at least 0.2 mm. In order to reduce the size of the device and to maintain good heat conduction from heater body 6 to chuck top 2, the thickness of the insulating layer should be small. The thickness of the insulating layer smaller than 0.2 mm is not preferred, as defects in the insulating layer itself or problems in durability would be generated. It is preferred that the thickness is at least 1 mm, because such a thickness prevents the problem of durability and ensures good heat conduction from the heater body 6. The thickness is preferably at most 10 mm. When the thickness exceeds 10 mm, though the noise cutting effect is good, the time of conduction of heat generated by heater body 6 to chuck top 2 and to the wafer becomes too long, and hence, it possibly becomes difficult to control the heating temperature. Though it depends on the conditions of inspection, the thickness of the insulating layer of at most 5 mm is particularly preferred, as temperature control is relatively easy.
The thermal conductivity of the insulating layer is preferably at least 0.5 W/mK, as it realizes good heat conduction from heater body 6 as described above. Thermal conductivity of at least 1 W/mK is preferred, as heat conduction is further improved.
As the material for the insulating layer, any material that satisfies the characteristics described above and has heat resistance sufficient to withstand the inspection temperature is preferred, and ceramics or resin may be available. Filler may be dispersed in the resin. Of these, resin such as silicone resin or the silicone resin having filler dispersed therein, and ceramics such as alumina, may preferably be used. The filler dispersed in the resin serves to improve heat conduction of the resin. Any material having no reactivity to the resin may be used as the filler, and by way of example, substances such as boron nitride, aluminum nitride, alumina and silica may be available.
Further, it is preferred that the area for forming the insulating layer is the same as or larger than the area for forming the electromagnetic shield layer, the guard electrode or heater body 6. When the area for forming the insulating layer is smaller than the area for forming the electromagnetic shield layer, the guard electrode or heater body 6, noise may possibly enter from a portion not covered with the insulating layer.
A specific example of the insulating layer will be described in the following. In the following, an example will be described in which silicone resin having boron nitride dispersed therein is used as the material for the insulating layer. The material has thermal conductivity of about 5 W/mK, and dielectric constant of 2. When the silicone resin with boron nitride dispersed is inserted as the insulating layer between the electromagnetic shield layer and chuck top 2, and the chuck top corresponds to a 12-inch wafer, it may be formed, for example, to have the diameter of 300 mm. Here, when the thickness of the insulating layer is set to 0.25 mm, capacitance of 5000 pF can be attained. When the thickness is set to 1.25 mm or more, capacitance of 1000 pF or lower can be attained. Volume resistivity of the material is 9×1015 Ω·cm, and therefore, when the diameter is 300 mm and the thickness is made at least 0.8 mm, the resistance value of the insulating layer of at least 1×1012Ω can be attained. Therefore, when the thickness is made at least 1.25 mm, an insulating layer having sufficiently low capacitance and sufficiently high resistance value can be obtained.
It is preferred that warp of chuck top 2 is at most 30 μm. When the warp exceeds 30 μm, contact with a needle of the probe card may possibly be biased at the time of inspection, resulting in a contact failure. Similarly, if the parallelism between the surface of the chuck top conductive layer 3 and the rear surface at the bottom portion of supporter 4 exceeds 30 μm, the contact failure possibly occurs. Therefore, the parallelism should also be at most 30 μm. It is preferred that warp and parallelism described above are at most 30 μm not only at the room temperature but in the temperature range of −70° C. to 200° C., in which probing is typically done.
The warp and parallelism may be measured using, for example, a three-dimensional measuring apparatus.
Chuck top conductive layer 3 formed on the wafer-mounting surface of chuck top 2 has a function of a ground electrode and, in addition, has the functions of shielding the electromagnetic noise from heater body 6 and protecting chuck top 2 from corrosive gas, acid, alkali chemical, organic solvent or water.
Possible methods of forming chuck top conductive layer 3 include a method in which a conductive paste is applied by screen printing and then fired, vapor deposition or sputtering, thermal spraying and plating. Among these, thermal spraying and plating are particularly preferred. These methods do not involve heat treatment at the time of forming the conductive layer, and therefore, warp of chuck top 2 caused by heat treatment can be avoided and chuck top conductive layer 3 can be formed at a low cost.
Particularly, a method of forming a thermally sprayed film on chuck top 2 and then forming a plated film further thereon is preferred. The material thermally sprayed (aluminum, nickel or the like) forms some compound such as oxide, nitride or oxynitride at the time of thermal spraying, and such compound reacts to the surface of chuck top 2, realizing firm contact with chuck top 2. The thermally sprayed film, however, has low electric conductivity because it contains the compound mentioned above. In contrast, plating forms an almost pure metal film, and therefore, a conductive layer of superior electric conductivity can be formed, though contact strength with the surface of chuck top 2 is not as high as that of the thermally sprayed film. The thermally sprayed film and the plated film both contain metal as the main component and, therefore, contact strength therebetween is high. Therefore, by forming a thermally sprayed film as a base and forming a plated film thereon, chuck top conductive layer 3 having both high contact strength and high electric conductivity can be provided.
It is preferred that surface roughness Ra of chuck top conductive layer 3 is at most 0.5 μm. When the surface roughness Ra exceeds 0.5 μm and a device having a high calorific value is inspected, the heat generated from the device itself cannot be radiated from chuck top 2, and the device might possibly be broken by the heat. Surface roughness Ra of at most 0.02 μm is preferred, as more efficient heat radiation becomes possible.
It is preferred that the thickness of chuck top 2 is at least 8 mm. When the thickness is smaller than 8 mm, chuck top 2 much deforms when load is applied at the time of inspection, possibly causing contact failure, and further causing damage to the wafer. It is more preferable that chuck top 2 has the thickness of at least 10 mm, as the possibility of contact failure can further be reduced.
It is preferred that chuck top 2 has Young's modulus of at least 250 GPa. When Young's modulus is smaller than 250 GPa, chuck top 2 much deforms when load is applied at the time of inspection, possibly causing contact failure, and further causing damage to the wafer. Young's modulus of chuck top 2 is preferably at least 250 GPa, and more preferably at least 300 GPa, as the possibility of contact failure can further be reduced.
Chuck top 2 preferably has thermal conductivity of at least 15 W/mK. When the thermal conductivity is lower than 15 W/mK, temperature uniformity of the wafer mounted on chuck top 2 would be deteriorated. When the thermal conductivity is not lower than 15 W/mK, thermal uniformity having no adverse influence on inspection can be attained. Thermal conductivity of 170 W/mK or higher is more preferable, as the thermal uniformity of the wafer can further be improved.
As the material of chuck top 2 having at least one of Young's modulus and thermal conductivity as described above, various ceramics and metal-ceramics composite materials may be available. Preferred metal-ceramics composite material includes composite material of aluminum and silicon carbide (Al—SiC) and composite material of silicon and silicon carbide (Si—SiC), which have relatively high thermal conductivity and easily realize thermal uniformity when the wafer is heated. Of these, Si—SiC is particularly preferred, as it has high thermal conductivity of 170 W/mK to 220 W/mK and high Young's modulus.
The composite bodies described above are conductive and, therefore, as to the method of forming heater body 6, heater body 6 may be formed by forming an insulating layer through a method of thermal spraying or screen printing on a surface opposite to the wafer-mounting surface of chuck top 2, and by forming the conductive layer in a prescribed pattern through a method such as screen printing or vapor deposition.
Alternatively, metal foil of stainless steel, nickel, silver, molybdenum, tungsten, chromium and an alloy of these may be etched to form a prescribed pattern of heater body, to form heater body 6. In this method, insulation from chuck top 2 may be attained by the method similar to that described above, or an insulating sheet may be inserted between chuck top 2 and heater body 6. This is preferable, as the insulating layer can be formed at a considerably lower cost and in a simpler manner than the method described above. The resin that can be used for the insulating sheet here includes, from the viewpoint of heat resistance, mica sheet, epoxy resin, polyimide resin, phenol resin and silicone resin. Among these, mica is particularly preferable, as it has superior heat resistance and electric insulation, allows easy processing and is inexpensive.
Use of ceramics as the material for chuck top 2 is advantageous in that formation of an insulating layer between chuck top 2 and heater body 6 is unnecessary. Among ceramics, alumina, aluminum nitride, silicon nitride, mullite, and a composite body of alumina and mullite are preferred as they have relatively high Young's modulus and hence, not much deformed by the load of the probe card. Among these, alumina is preferred as it is relatively inexpensive and has superior insulation at a high temperature. Generally, at the time of sintering, in order to lower sintering temperature, an oxide of alkali-earth metal, silicon or the like is added to alumina. If the amount of addition is reduced and purity of alumina is increased, insulation can further be improved, though the cost increases. With the purity of 99.6%, high insulation can be attained, and with the purity of 99.9%, particularly high insulation can be attained. Further, as the purity increases, alumina comes to have higher insulation and, at the same time, higher thermal conductivity, and with the purity of 99.5%, thermal conductivity of 30 W/mK can be attained. Purity of alumina may appropriately be selected in consideration of insulation, thermal conductivity and cost. Aluminum nitride is preferred as it has particularly high thermal conductivity of 170 W/mK.
As the material of chuck top 2, metal may be applied. In that case, tungsten, molybdenum or an alloy of these having particularly high Young's modulus may be used. Specific alloy may include an alloy of tungsten and copper and an alloy of molybdenum and copper. Such an alloy can be fabricated by impregnating tungsten or molybdenum with copper. Similar to the metal-ceramics composite material described above, these metals are conductors and therefore, the above-described method can directly be applied, to form chuck top conductive layer 3 and further to form heater body 6, and these metals may be used as chuck top 2.
It is preferred that chuck top 2 deflects at most by 30 μm when a load of 3.1 MPa is applied to chuck top 2. A large number of pins for inspecting the wafer press the wafer from the probe card to chuck top 2, and therefore, the pressure also acts on chuck top 2, and chuck top 2 deflects to no small extent. When the amount of deflection at this time exceeds 30 μm, it becomes impossible to press the pins of the probe card uniformly onto the wafer, and inspection of the wafer might fail. More preferably, the amount of deflection when the load described above is applied is at most 10 μm.
In the present invention, as shown in wafer holder 400 of
As the material of cooling module 8, aluminum, copper and an alloy of these are preferred, because they have high thermal conductivity and capable of removing heat quickly from chuck top 2. It is also possible to use stainless steel, magnesium alloy, nickel or other metal materials. An oxidation-resistant metal film such as nickel, gold, silver or the like may be formed by the method of plating, thermal spraying or the like, to add oxidation resistance to cooling module 8.
Ceramics may also be used as the material for cooling module 8. Among ceramics, aluminum nitride and silicon carbide are preferred as they have high thermal conductivity and are capable of removing heat quickly from chuck top 2. Further, silicon nitride and aluminum oxynitride are preferred, as they have high mechanical strength and superior durability. Oxide ceramics such as alumina, cordierite and steatite are preferred as they are relatively inexpensive. As described above, the material for cooling module 8 may be selected appropriately in consideration of the intended use, cost and the like. Among these materials, nickel-plated aluminum or nickel-plated copper is particularly preferred, as it has superior oxidation resistance and high thermal conductivity and is relatively inexpensive.
A coolant may be caused to flow in cooling module 8. Causing the coolant flow is preferred, as the heat transferred from chuck top 2 to cooling module 8 can quickly be removed from the cooling module 8 and the cooling rate of chuck top 2 can be improved. Types of the coolant may be selected from liquid such as water, Fluorinert or Galden, or gas such as nitrogen, air or helium. When the temperature of use is always 0° C. or higher, water is preferred considering magnitude of specific heat and cost, and when it is cooled below zero, Galden is preferred considering specific heat.
As the method of forming the passage for the coolant flow, two plates may be prepared, for example, and the passage may be formed by machine processing or the like on one of the plates. In order to improve corrosion resistance and oxidation resistance, entire surfaces of the two plates may be nickel-plated, and thereafter, the two plates may be joined by means of screws or welding. At this time, a sealing member such as an O-ring may be inserted around the passage, to prevent leakage of the coolant.
As another method of forming the flow passage, a pipe through which the coolant flows may be attached to a cooling plate. Here, in order to increase contact area between the cooling plate and the pipe, the cooling plate may be processed to have a trench of an approximately the same cross-sectional shape as the pipe and the pipe may be arranged in the trench, or a flat-shaped portion may be formed on a portion of the cross-sectional shape of the pipe and that flat portion may be fixed on the cooling plate. As to the method of fixing the cooling plate and the pipe, screw fixing using a metal band, welding or brazing may be available. By inserting a deformable substance such as resin between the cooling plate and the pipe, tight contact between the two is attained and cooling efficiency can be enhanced.
At the time of heating chuck top 2, if cooling module 8 can be separated from chuck top 2, efficient temperature elevation of chuck top 2 becomes possible, and therefore, it is preferred that cooling module 8 is movable. As a method of realizing mobile cooling module 8, an elevating mechanism such as an air cylinder may be used. In this case, cooling module 8 does not bear the load of probe card, and therefore, it is free from the problem of deformation caused by the load.
When the cooling rate of chuck top 2 is of high importance, cooling module 8 may be fixed on chuck top 2. Specifically, as shown in wafer holder 400 of
In any of the arrangements described above, the method of fixing is not specifically limited, and by way of example, they may be fixed by a mechanical method such as screw fixing or clamping. When chuck top 2, cooling module 8 and heater body 6 are to be fixed together by screws, three or more screws are preferred as tight contact between each of the members can be improved, and six or more screws are more preferable.
Further, cooling module 8 may be mounted inside the space 5 of supporter 4, or cooling module 8 may be mounted on supporter 4 and chuck top 2 may be mounted thereon. No matter which method is adopted, cooling rate can be increased as compared with the example in which cooling module is movable, as chuck top 2 and cooling module 8 are firmly fixed together. When cooling module 8 is mounted on supporter 4, contact area between cooling module 8 and chuck top 2 is increased, and it becomes possible to cool chuck top 2 in a shorter time period.
When cooling module 8 fixed on chuck top 2 can be cooled by a coolant, it is preferred that the flow of coolant to cooling module 8 is stopped when the temperature of chuck top 2 is increased or when it is kept at a high temperature, because the heat generated by heater body 6 is not removed by the coolant, and whereby efficient temperature increase or maintenance of high temperature becomes possible. Naturally, chuck top 2 can be cooled efficiently by causing the coolant to flow again at the time of cooling.
Further, the chuck top itself may be formed as the cooling module, by providing a passage through which the coolant flows inside the chuck top. In that case, the time for cooling can further be reduced than when cooling module 8 is fixed on chuck top 2. As the material for the chuck top, ceramics and metal-ceramics composite body similar to the above may be used. As for the structure, for example, the chuck top conductive layer may be formed on one surface of a member I to be the wafer-mounting surface, and a passage for the coolant flow may be formed on the opposite surface, and a member II may be integrated by brazing, glass fixing or screw fixing, on the surface having the passage formed thereon. Alternatively, a passage may be formed on one surface of member II, and the member may be integrated with member I on the surface having the passage formed thereon, or passages may be made both on members I and II, and the members may be integrated on the surfaces having the passages formed thereon. It is preferred that the difference in thermal conductivity of members I and II is as small as possible, and ideally, the members are preferably formed of the same material.
When the chuck top itself is formed as the cooling module, metal may be used as the material. Metal is advantageous as it is less expensive as compared with the ceramics or composite material of ceramics and metal and it allows easy processing so that formation of the passage is easier. However, it is susceptible to deformation under the load from the probe card, and therefore, a plate-shaped member may be provided for preventing deformation of the chuck top, as the plate for preventing deformation, on the side opposite to the wafer-mounting surface of the chuck top. It is preferred that the plate for preventing deformation has Young's modulus of at least 250 GPa, as in the case where ceramics or metal-ceramics composite material is used as the material for the chuck top.
As to the position of the plate for preventing deformation, it may be housed in the space 5 formed in supporter 4, or it may be inserted between the chuck top and supporter 4. The chuck top and the plate for preventing deformation may be fixed by a mechanical method such as screw fixing, or may be fixed by brazing or glass fixing. Efficient heating and cooling is also possible, by not causing coolant to flow through the cooling module when the chuck top is heated or kept at a high temperature and causing the coolant to flow only at the time of cooling, as in the example in which a chuck top having an integrated cooling module is used or cooling module 8 is fixed on chuck top 2.
When the material of chuck top 2 is metal, chuck top conductive layer 3 may be newly formed on the wafer-mounting surface, if it is the case that the chuck top material is much susceptible to oxidation or alteration, or it does not have sufficiently high electric conductivity. As the method of forming chuck top conductive layer 3, vapor deposition, sputtering, thermal spraying or plating may be used as in the foregoing.
In the structure in which the plate for preventing deformation is provided on chuck top 2 formed of metal, formation of the electromagnetic shield layer or the guard electrode layer similar to that described above may be possible. By way of example, on the surface opposite to the wafer-mounting surface of chuck top 2, an insulated heater body 6 may be provided and covered with a metal layer, and further, the guard electrode layer may be formed with an insulating layer interposed, and between the guard electrode layer and chuck top 2, an insulating layer may be formed. Further, the plate for preventing deformation may be arranged, and chuck top 2, heater body 6 and the plate for preventing deformation may be fixed integrally.
The wafer holder in accordance with the present invention may preferably be used as a heater unit for a wafer prober, for example, when a heater body and a power source for operating the heater body are provided. The heater unit appropriately includes known components, in addition to the wafer holder. Further, the wafer holder and the heater unit in accordance with the present invention may be used as a wafer prober having a structure combined with known components such as a driving apparatus (driving system) for driving the wafer holder in X, Y and Z directions, a wafer conveying apparatus, and a probe card. Further, the wafer holder and the heater unit in accordance with the present invention may also be applied to a handler apparatus or a tester apparatus. The wafer holder, heater unit and wafer prober in accordance with the present invention allow inspection without contact failure even of a semiconductor having minute circuitry.
Wafer holder 100 shown in
Thereafter, a mullite-alumina composite body of a pillar shape having the diameter of 310 mm and thickness of 40 mm was prepared as supporter 4. The surface of supporter 4 in contact with the chuck top and the bottom surface are finished to the flatness of 0.09 mm, and the surface on the side of chuck top 2 was counter-bored to have the inner diameter of 290 mm and the depth of 20 mm, to provide the space for mounting heater body 6 shown in
Next, support members formed of materials of a mullite-alumina composite body, copper, inver alloy, alumina and aluminum were prepared. As the support member, a pipe-shaped member having an outer diameter of 10 mm and an inner diameter of 6 mm was used. On supporter 4, chuck top 2 with heater body 6 and the electromagnetic shield layer attached was mounted, and the support members were arranged as shown in Table 1, to provide wafer holders for the wafer prober.
Using the wafer probers mounting the wafer holders, wafers were heated to 100° C. and 200° C. by feeding electric power to heater body 6, and initial evaluation was done. While the entire surface of the wafer was probed, samples were evaluated as A when deflection was small, B when deflection was within tolerable extent, C when deflection affecting characteristics was observed locally, and D when deflection was significant and at many portions so that probing was impossible. The results are as shown in Table 1.
From Table 1, it can be seen that insertion of support member is preferable in order to reduce deflection at the time of probing. Further, from the results when copper and inver were used as the material for the support member, it can be seen that when the thermal expansion coefficient of the support member is much different from that of the supporter, the amount of thermal expansion differs, and therefore, probing characteristics tend to be unsatisfactory as compared with samples using support members of which thermal expansion coefficient was approximately the same as that of the support body. Further, it can be seen that when a material having low Young's modulus such as aluminum was used for the support member, the result was less satisfactory than samples using materials of higher Young's modulus. It can be seen that when a support member of alumina having the amount of thermal expansion close to that of the supporter formed of a mullite-alumina composite body and having high Young's modulus was used, relatively good probing characteristics were observed, though the materials of the supporter and the support member were not the same.
Wafer holder 200 shown in
Thereafter, a mullite-alumina composite body of a pillar shape having the diameter of 310 mm, thickness of 40 mm and thermal conductivity of 30 W/mK was prepared as supporter 4. The surface of supporter 4 in contact with the chuck top and the bottom surface are finished to the flatness of 0.09 mm, and the surface on the side of chuck top 2 was counter-bored to have the inner diameter of 295 mm and the depth of 20 mm, to enable formation of space 5 on the side of chuck top 2 after assembly. On chuck top 2, stainless steel foil insulated with mica was attached as the electromagnetic shield layer, and further, a resistance heater body sandwiched by mica was attached, to provide heater body 6. Resistance heater bodies were formed by etching stainless steel foil in a prescribed pattern. The electromagnetic shield layer and heater body 6 were arranged to be housed in space 5 provided in supporter 4. Further, a through hole for connecting an electrode for feeding power to heater body 6 was formed in supporter 4 in the form as shown in
Materials shown in Table 2 were processed to the shape of pillars having the diameter of 290 mm and the height of 19 mm, as materials 11 of low thermal conductivity.
Next, on supporters 4, materials 11 of low thermal conductivity were arranged, and chuck top 2 with heater body 6 and the electromagnetic shield layer attached was mounted, to provide the wafer holders for the wafer prober.
Using the wafer probers, chuck tops 2 were heated to 160° C. or 200° C. by feeding electric power to heater body 6, and temperature at the bottom portion of supporters 4 was measured. The results are as shown in Table 2.
As can be seen from Table 2, when the thermal conductivity of material 11 of low thermal conductivity was lower than that of chuck top 2, the temperature at the bottom portion of supporter 4 was lower than 100° C., and good heat-insulating effect was exhibited. Further, when the thermal conductivity of material 11 of low thermal conductivity was lower than that of supporter 4, the temperature at the bottom portion of supporter 4 was lower than 50° C., and better heat-insulating effect was exhibited.
Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-226202 (P) | Aug 2005 | JP | national |
| 2005-227335 (P) | Aug 2005 | JP | national |
