Patent Application
20070046306
1. Field of the Invention
The present invention relates to a wafer holder for use in a wafer prober for inspecting the electric characteristics of a semiconductor wafer by placing a semiconductor wafer on a wafer-placing surface and pressing a probe card against the semiconductor wafer, a heater unit having the wafer holder, and a wafer prober having the heater unit.
2. Description of the Background Art
Conventionally, in a step of testing a semiconductor wafer, a heating process is performed on a semiconductor substrate (wafer) as a process target. Here, a burn-in step is performed in which a wafer is heated to a temperature higher than a normally used temperature to accelerate a defect in a semiconductor chip that is potentially defective for removal in order to prevent a defective after shipment. In this burn-in step, before a semiconductor wafer having circuits formed therein is cut into individual semiconductor chips, the electric performance of each semiconductor chip is determined while the wafer is heated, thereby removing a defective. In this burn-in step, reduction of the process time is strongly requested in order to improve a throughput.
In such a burn-in step, a heater for holding a semiconductor substrate and heating the semiconductor substrate is used. The conventionally used heater is made of a metal since the entire back surface of a semiconductor wafer should be brought into contact with a ground electrode. A semiconductor wafer having circuits formed therein is placed on a metal flat plate in order to determine the electric characteristics of a chip. In determination, a determiner called a probe card including a number of probe pins for supplying power is pressed against a wafer at a force of a few tens of kgf to a few hundreds of kgf. Therefore, if the heater is thin, the heater is deformed, possibly resulting in poor contact between a wafer and a probe pin. Thus, for the purpose of keeping the rigidity of the heater, a thick metal plate of 15 mm or thicker has to be used, which requires long time to increase and decrease the temperature of the heater and thus hinders improvement of throughput.
Moreover, in the burn-in step, the electric characteristics are determined by feeding current to a chip. With higher power of recent semiconductor chips, semiconductor chips generate much heat during determination of electric characteristics. In some cases, chips may be failed due to the generated heat from itself Therefore, chips need to be cooled rapidly after determination. In addition, thermal uniformity is required during determination. Then, copper (Cu) having high thermal conductivity of 403 W/mK has been used as a material of the metal.
Then, Japanese Patent Laying-Open No. 2001-033484 proposes a wafer prober which is less likely to be deformed and having small heat capacity, in which, in place of a thick metal plate, a thin metal layer, which is thin but highly rigid and less likely to be deformed, is formed on a surface of a ceramic substrate. According to Japanese Patent Laying-Open No. 2001-033484, the high rigidity prevents poor contact, and the small heat capacity allows for the temperature rise and drop for a short time. In addition, an aluminum alloy, stainless steel or the like may be used for a support base for installing a wafer prober.
However, as disclosed in Japanese Patent Laying-Open No. 2001-033484, when the wafer prober is supported only with the outermost circumference thereof, the pressure of the probe card may cause the wafer prober to be warped. Therefore, any other technique is necessary such as a number of pillars or the like.
Furthermore, recently, with miniaturization in semiconductor processes, the load per unit area in probing is increased and in addition, the accuracy of registration between a probe card and a prober is required. The prober usually repeats an operation of heating a wafer at a prescribed temperature, moving to a prescribed position in probing, and pressing a probe card. Here, high positional accuracy is also required of a driving system for moving the prober to a prescribed position.
However, when a wafer is heated to a prescribed temperature, that is, a temperature of about 100-200° C., the heat is transferred to the driving system, causing thermal expansion of metal parts of the driving system, resulting in deteriorated accuracy. Furthermore, with the increased load in probing, stiffness of the prober itself on which a wafer is placed is also demanded. In other words, if the prober itself is deformed by the load in probing, the pin of the probe card cannot contact with a wafer uniformly to make the testing impossible. At worst, a wafer is broken. Therefore, in order to prevent deformation of the prober, the prober is increased in size and weight. The increased weight affects the accuracy of the driving system. In addition, with the increased size of the prober, the time required to increase and cool the prober becomes extremely long, thereby reducing throughput.
In the repeated operation of heating a wafer at a prescribed temperature, moving to a prescribed position in probing, and pressing a probe card, a problem of positional accuracy also arises especially in moving a prober for 12-inch use to a prescribed position. The prober is usually moved at high speed in order to increase throughput, so that the wafer holder is subjected to extremely high acceleration in the prober movement. In order to stop the prober at a prescribed position at high accuracy, the inertial force caused by this acceleration should be restrained.
On the other hand, since the conventional wafer size is mainly 8 inches, the 8-inch wafers are mainly tested, and the prober is also adapted to 8-inch specifications. However, in recent years, for the purpose of improving throughput, 12-inch wafers are mainly used. Accordingly, the requirements of probers are also changed to meet the 12-inch specifications rather than the 8-inch specifications.
With the increased wafer size from 8 inches to 12 inches, the size of the prober for use in testing is also increased. With the increased size of the prober, a new problem becomes conspicuous. It became clear that with the increased weight of he prober itself caused by the increased size of the prober, the positional accuracy in moving the prober is reduced. In other words, with the increased inertial force in the movement resulting from the increased weight, the positional accuracy is undesirably reduced with the conventional design to suppress the inertial force to make a stop.
Moreover, in order to improve the speed of increasing and decreasing the temperature of the prober to improve throughput, a cooling mechanism is often provided. However, a conventional cooling mechanism, for example, provides air cooling as disclosed in Japanese Patent Laying-Open No. 2001-033484 or is provided with a cold plate immediately below a metal heater. In the case of the former, the cooling speed is slow due to air cooling. On the other hand, in the case of the latter, the cold plate is made of a metal and the pressure of the probe card is directly applied to this cold plate in probing, so that the cold plate is easily deformed.
The present invention is made to solve the aforementioned problems. It is an object of the present invention to provide a wafer holder, a heater unit including the wafer holder, and a wafer prober including the heater unit, in which the positional accuracy of the wafer holder is very high when the wafer holder is moved to a prescribed position in probing, and the positional accuracy is less likely to vary even with repeated movements.
A wafer holder in accordance with the present invention includes a chuck top having a chuck top conductive layer on a surface thereof and a support body supporting the chuck top. The weight of the wafer holder is at most 28000 g.
Preferably, the weight of the chuck top is at most 6000 g. Preferably, the weight of the support body is at most 12000 g. Preferably, the support body includes a ring-shaped portion and a base portion, and the weight of the ring-shaped portion is at most 6000 g.
Preferably, a cooling module is arranged in a space formed between the chuck top and the support body, and a weight of the cooling module is at most 15000 g.
Preferably, a material that forms the chuck top is a composite of metal and ceramic. Specifically, any of a composite of aluminum and silicon carbide, a composite of silicon and silicon carbide, and a composite of aluminum, silicon and silicon carbide is preferable.
A material that forms the chuck top may be ceramic and preferably any of alumina, mullite, silicon nitride, aluminum nitride, and a composite of alumina and mullite.
A material that forms the support body is preferably ceramic or a composite of at least two kinds of ceramics, more preferably any of mullite, alumina, aluminum nitride, silicon nitride, and a composite of mullite and alumina.
A heater unit including the wafer holder as described above and a wafer prober including the heater unit have high stiffness. Because of the increased heat insulation effect, the positional accuracy and the thermal uniformity can be improved, and in addition, the temperature of chips can be increased and decreased rapidly.
In accordance with the present invention, in the step of placing a semiconductor wafer having a diameter of 12 inches on a wafer-placing surface and pressing a probe card against the wafer to inspect the electric characteristics of the wafer, in a repeated operation of heating the wafer to a prescribed temperature, moving to a prescribed position in probing, and pressing the probe card, extremely high positional accuracy can be achieved in moving the prober to a prescribed position. As a result, the accuracy of measurement position of a probe pin is improved, allowing proper measurement with small variations. In addition, for vibration involved with repeated positional movements, poor mechanical accuracy or damage of the assembly can be suppressed.
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.
In the following, an embodiment of the present invention will be described with reference to the figures. It is noted that in the following drawings, the same or corresponding parts will be denoted with the same reference characters and description thereof will not be repeated.
An embodiment of the present invention will be described with reference to FIGS. 1 to 12.
In the step of placing a semiconductor wafer on the wafer-placing surface of the wafer holder for 12-inch use having such a configuration and pressing a probe card on the wafer to inspect the electric characteristics of the wafer, in the repeated operation of heating a wafer at a prescribed temperature, moving to a prescribed position in probing and pressing a probe card, the positional accuracy in moving a prober to a prescribed position should be maintained. As a result of the study, the following conclusion was reached.
The inventors conducted intensive researches to find that the weight of the wafer holder should be 28000 g or less. If the weight of the wafer holder exceeds 28000 g, the positional accuracy required for the testing could not be obtained due to the inertia caused by repeated probing. In addition, the repeated movement causes the assembly of the wafer holder to loosen, resulting in poor mechanical accuracy.
On the other hand, the weight of the conventional chuck top 2 is 700-4000 g for 8-inch use. However, if this is proportionally enlarged for 12-inch use, the weight becomes excessive and the inertial force also increases, so that unfortunately, a moving prober cannot stop at high positional accuracy too often. Therefore, in the present invention, a new design was found for a prober for 12-inch use that can stop at high accuracy.
More specifically, as a result of intensive researches, the present inventors found that when the weight of the chuck top was 6000 g or less, the positional accuracy in moving a prober to a prescribed position could be improved. The weight of the chuck top of 6000 g or less results in sufficient positional accuracy required for the testing due to the inertia in repeated probing. In addition, it is further prevented that the repeated movements cause the assembly of the wafer holder to loosen resulting in poor mechanical accuracy.
Furthermore, the weight of the chuck top set at 6000 g or less can reduce the heat capacity of the chuck top, resulting in the increased heating speed and cooling speed of a prober. Therefore, the temperature increasing and cooling time can be shortened.
In probing, the chuck top is under a high load, for example, of about 100 kg since it is pressed with a probe card. If this load causes distortion of the chuck top, accurate testing cannot be conducted. In other words, the chuck top needs stiffness against such a load. In order to achieve such stiffness, the weight of the chuck top is desirably 800 g or more.
Furthermore, in a new design of a 12-inch prober, the inertial force as described above also occurs in the members other than the chuck top. Therefore, a design that can realize high positional accuracy is required for the members other than the chuck top. In the present invention, means for solving the problems has been found out for such members.
First, the weight of support body 4 is preferably 12000 g or less. The weight of support body 4 of 12000 g or less can prevent the insufficient positional accuracy in measurements of the probe pin and the poor mechanical accuracy of the wafer holder, which would be caused by the inertial force in repeated movements. On the other hand, in order to achieve the stiffness, the weight of support body 4 is desirably 2000 g or more.
Preferably, support body 4 includes a ring-shaped portion and a base portion. For example, as shown in
The weights of individual members that constitute the wafer holder have been described above. Any of the members is preferably within the range as described above and the sum of weights of all the members should be 28000 g or less.
Chuck top 2 preferably includes a heater body 6. This is because in the recent semiconductor probing, a wafer often needs to be heated at a temperature of 100-200° C. Therefore, if transmission of heat of heater body 6 heating chuck top 2 to support body 4 cannot be prevented, the heat is transferred to the driving system lying below the support body of the wafer prober, and the thermal expansion difference between the parts causes deviation of mechanical accuracy, which may result in significantly deteriorated flatness and parallelism of the top face (wafer-placing surface) of chuck top 2. However, since the present structure is thermally insulating, the flatness and parallelism are not significantly degraded. In addition, because of the hollow structure, weight reduction can be achieved as compared with cylindrical shaped support body 4.
It is noted that in the present invention “wafer-placing surface” refers to the surface of chuck top conductive layer 3 in chuck top 2.
As shown in
Preferably, Young's modulus of support body 4 is 200 GPa or more. If Young's modulus of support body 4 is less than 200 GPa, the thickness of the base portion cannot be reduced and thus the volume of the cavity portion cannot be secured enough. Therefore, the heat-insulation effect cannot be expected. In addition, the space in which a cooling module described later is provided cannot be reserved. More preferable Young's modulus is 300 GPa or more. In particular, when a material having Young's modulus of 300 GPa or more is used, deformation of support body 4 is considerably reduced, so that further miniaturization and weight reduction of support body 4 are preferably achieved.
It is noted that in the present invention, Young's modulus can be measured, for example, by a pulse method, a bending resonance method, or the like.
Preferably, the thermal conductivity of support body 4 is 40 W/mK or less. If the thermal conductivity of support body 4 exceeds 40 W/mK, the heat applied to chuck top 2 is easily transferred to support body 4, which may undesirably affect the accuracy of the driving system. Recently, a high temperature of 150° C. is requested in probing. Therefore, the thermal conductivity of support body 4 is more preferably 10 W/mK or less. More preferably, the thermal conductivity is 5 W/mK or less. With the thermal conductivity in such an extent, the amount of heat transmission from support body 4 to the driving system is significantly reduced. As a specific material of support body 4 to satisfy these conditions, mullite, alumina, or a composite of mullite and alumina (mullite-alumina composite) is preferably used. Mullite is preferable in that the thermal conductivity is small and the heat-insulation effect is high. Alumina is preferable in that Young's modulus is large and the rigidity is high. Mullite-alumina composite is generally preferable in that the thermal conductivity is smaller than that of alumina and Young's modulus is larger than that of mullite.
It is noted that in the present invention, the thermal conductivity can be measured, for example, by a laser flash method using a sliced pellet.
Alternatively, ceramic or a composite of two or more kinds of ceramic can be used as a material of support body 4. The main component of the material forming support body 4 may be any of mullite, alumina, aluminum nitride, silicon nitride, and a composite of mullite and alumina.
Preferably, the thickness of the cylindrical portion of support body 4 shaped like a cylinder with a base is 20 mm or less. If the thickness exceeds 20 mm, the amount of heat transmission from chuck top 2 to support body 4 undesirably increases. Therefore, the thickness of the cylindrical portion of support body 4 that supports chuck top 2 is preferably 10 mm or less. However, if the thickness is less than 1 mm, in testing a wafer, the pressure to press a probe card against a wafer unfortunately causes the cylindrical portion of support body 4 to be deformed or damaged at worst. The most preferable thickness is from 10 mm to 15 mm. In addition, the thickness of that portion of the cylindrical portion which is in contact with chuck top 2 is preferably 2 mm to 5 mm. The thickness in such an extent is preferable since the strength and the heat insulation of support body 4 are well balanced.
Preferably, the height of the cylindrical portion of support body 4 is 10 mm or more. If less than 10 mm, the pressure from the probe card is applied to chuck top 2 and additionally transferred to support body 4 during wafer testing. Therefore, the base portion of support body 4 may be bent thereby undesirably deteriorating the flatness of chuck top conductive layer 3.
Preferably, the thickness of the base portion of support body 4 is 10 mm or more. If the thickness of the base portion of support body 4 is less than 10 mm, the pressure from the probe card is applied to chuck top 2 and additionally transferred to support body 4 during wafer testing. Therefore, the base portion of support body 4 may be bent thereby undesirably deteriorating the flatness of chuck top 2. Then, 10 mm to 35 mm is preferable. If less than 10 mm, heat of chuck top 2 is easily transferred to the base portion of support body 4, which may cause support body 4 to be warped due to thermal expansion, thereby undesirably deteriorating the flatness and parallelism of chuck top 2. To achieve size reduction, 35 mm or less is suitable. Alternatively, the cylindrical portion and the base portion of support body 4 may not be integrated but may be separated. In this case, the separated cylindrical portion and base portion have an interface therebetween. This interface serves as a heat resistance layer, so that the heat transferred from chuck top 2 to support body 4 is blocked at this interface. Accordingly, preferably, the temperature of the base portion of support body 4 is less likely to be increased.
Preferably, a heat insulating structure is provided at the supporting surface of support body 4 which supports chuck top 2. Here, the heat insulating structure may be formed, for example, by forming a notch groove at support body 4 to reduce the contact area between chuck top 2 and support body 4. The heat insulating structure may also be formed by forming a notch groove at chuck top 2. In this case, it is necessary that Young's modulus of chuck top 2 should be 250 GPa or more. More specifically, the pressure of the probe card is applied to chuck top 2, so that with a material having small Young's modulus, the amount of deformation is increased in the presence of a notch groove. Then, the increased amount of deformation may lead to damage to a wafer and damage to chuck top 2 itself However, a notch is preferably formed at support body 4 to prevent the aforementioned problem. The shape of the notch is not specifically limited and may include, for example, a concentrically-arranged groove 21 as shown in
In addition, as the heat insulating structure, as shown in
Preferably, the material of the pillar-like member used as a heat insulating structure has thermal conductivity of 30 W/mK or less. The thermal conductivity higher than 30 W/mK is not preferable since the heat insulation effect is reduced. The material used for the pillar-like member may include heat-resistant resin such as Si3N4, mullite, a mullite-alumina composite, steatite, cordierite, stainless steel, glass (fiber), polyimide, epoxy, or phenol, or a composite thereof.
Preferably, the surface roughness Ra of the contact portion between support body 4 and chuck top 2 or the pillar-like member is 0.1 μm or more. If the surface roughness Ra is less than 0.1 μm, the contact area between support body 4 and chuck top 2 or the pillar-like member increases and in addition, the gap between them becomes relatively small. Therefore, the amount of heat transmission undesirably increases as compared with when the surface roughness Ra is 0.1 μm or more. The upper limit of surface roughness Ra is not specifically limited. However, with surface roughness Ra of 5 μm or more, it may cost much to treat the surface. The method of providing the surface roughness Ra of 0.1 μm or more may include polishing, sandblasting, or the like. In this case, the polishing conditions or sandblasting conditions need to be set properly to control the above-noted surface roughness Ra at 0.1 μm or more.
It is noted that in the present invention the surface roughness Ra refers to arithmetic mean roughness Ra defined by JIS B 0601.
Preferably, the surface roughness Ra of the base portion of support body 4 is 0.1 μm or more. As described above, when the surface roughness Ra of the base portion of support body 4 is coarse, the amount of heat transmission to the driving system can be reduced. When the base portion and the cylindrical portion of support body 4 can be separated from each other, the surface roughness Ra of at least one of their contact portions is preferably 0.1 μm or more. If the surface roughness Ra is less than 0.1 μm, the effect of blocking heat from the cylindrical portion to the base portion is reduced. Furthermore, the surface roughness Ra of the contact surface of the pillar-like member with support body 4 and also with chuck top 2 is also preferably 0.1 μm or more. For this pillar-like member, surface roughness Ra is increased similarly, so that the amount of heat transmission to support body 4 can be reduced. As described above, an interface is formed between members, and surface roughness Ra of the interface is 0.1 μm or more, so that the amount of heat transmission to the base portion of the support body is reduced, resulting in a reduced amount of power supply to the heating element.
Preferably, the perpendicularity of the outer circumferential portion of the cylindrical portion of support body 4 with respect to the contact surface between the support body 4 and chuck top 2, or of the outer circumferential portion of the cylindrical portion of support body 4 with respect to the contact surface between pillar-like member 23 and chuck top 2 is 10 mm or less in terms of measurement length 100 mm. For example, if the perpendicularity exceeds 10 mm, undesirably, the cylindrical portion itself is likely to be deformed when the pressure applied from chuck top 2 is applied to the cylindrical portion of support body 4.
It is noted that in the present invention the perpendicularity may be measured, for example, by three-dimensional measurement equipment.
Preferably, a metal layer is formed on the surface of support body 4. An electric field or electromagnetic wave produced from the resistance heating element for heating chuck top 2, the driving portion of the prober, or peripheral equipment affects as noise in wafer testing. However, the formation of a metal layer on support body 4 preferably blocks this electromagnetic wave. The method of forming a metal layer is not specifically limited. For example, a conductive paste of metal powder such as silver, gold, nickel, or copper with addition of glass frit may be applied by a brush and baked.
Alternatively, the metal layer may be formed by thermally spraying a metal such as aluminum or nickel. Alternatively, the metal layer may be formed by plating the surface. Alternatively, these methods may be combined. More specifically, plating with a metal such as nickel may be applied after conductive paste is baked, or plating may be formed after thermal spraying. Among these techniques, plating or thermally spraying is especially preferable. Plating is preferable because of high adhesive strength and high reliability. On the other hand, thermal spraying is preferable since the metal layer is formed at relatively low costs.
Alternatively, the metal layer may be formed by providing a conductor on at least part of the surface of support body 4. Here, the material to be used is not specifically limited as long as it is conductive. For example, stainless steel, nickel, aluminum, or the like may be used.
Furthermore, the method of providing a conductor includes attaching a ring-shaped conductor to the side surface of support body 4. A metal foil of the above-noted material may be formed in a ring shape having a size larger than the outer diameter of support body 4 and attached to the side surface of support body 4. In addition, a metal foil or a metal plate may be attached to the undersurface portion of support body 4 and connected to the metal foil attached to the side surface, thereby increasing the effect of blocking an electromagnetic wave (guard effect). Furthermore, using the space inside support body 4, a metal foil or a metal plate may be attached inside the space of the cylinder with a base and connected to the metal foil attached to the side surface and the undersurface, thereby increasing the guard effect. By employing such a technique, the aforementioned effect can be achieved relatively inexpensively as compared with the application of plating or a conductive paste. A method of fixing a metal foil and metal plate to support body 4 is not specifically limited. For example, a metal foil and a metal plate may be attached to support body 4 using a metal screw. Preferably, the metal foils or metal plates at the undersurface portion and the side surface portion of support body 4 are integrated.
In addition, as shown in
Preferably, a metal layer is formed between heater body 6 heating chuck top 2 and chuck top 2 to block (shield) an electromagnetic wave. This electromagnetic shielding electrode layer serves to block noise caused by an electromagnetic wave or an electric field produced in heater body 6 or the like, which may affect probing of a wafer. This noise does not have significant impact on the determination of normal electric characteristics but has a considerable impact on the determination of high-frequency characteristics of a wafer. This electromagnetic shielding electrode layer may be formed, for example, by inserting a metal foil between heater body 6 and chuck top 2, where chuck top 2 and heater body 6 should be insulated. In this case, the metal foil to be used is not specifically limited. However, since the temperature of heater body 6 becomes approximately 200° C., a foil of stainless steel, nickel, aluminum, or the like is preferable.
The function of the insulating layer between chuck top 2 and the electromagnetic shielding electrode layer is as follows. A capacitor is formed on an electrical circuit between chuck top conductive layer 3 formed on the wafer-placing surface of chuck top 2 and the electrode shielding layer, if chuck top 2 is an insulator, or between the chuck top itself and the electromagnetic shielding layer, if chuck top 2 is a conductor. This capacitor component may have an effect as noise during probing of a wafer. Therefore, in order to reduce the effect, an insulating layer is formed between the electromagnetic shielding layer and chuck top 2, thereby reducing the noise.
In addition, a guard electrode layer is preferably provided between chuck top 2 and the electromagnetic shielding electrode layer with an insulating layer interposed therebetween. The guard electrode layer is connected to the metal layer formed on support body 4, so that noise that affects the determination of high-frequency characteristics of a wafer may further be reduced. More specifically, in the present invention, support body 4 including the resistance heating element is entirely covered with a conductor, so that the effect of noise in determination of wafer characteristics in high frequency can be reduced. In addition, the guard electrode layer can be connected to the metal layer provided on support body 4, thereby further reducing the effect of noise.
Here, the resistance value of the insulating layer between heater body 6 and the electromagnetic shielding electrode layer, between the electromagnetic shielding electrode layer and the guard electrode layer, between the guard electrode layer and chuck top 2 is preferably 107Ω or more. If the resistance value is less than 107Ω, minute current flows toward chuck top conductive layer 3 due to the effect from the heater body, which becomes noise during probing and may affect probing. If the resistance value of this insulating layer is 107Ω or more, the above-noted minute current is preferably reduced to such an extent that it does not affect probing. In particular, since circuit patterns formed on a wafer have recently been miniaturized, the aforementioned noise should be reduced as much as possible. Thus, the resistance value of the insulating layer of 1010Ω or more can result in a more reliable structure.
Furthermore, the dielectric constant of the insulating layer is preferably 10 or less. If the dielectric constant of the insulating layer exceeds 10, undesirably, electric charges are more likely to be accumulated in the electromagnetic shielding layer, the guard electrode layer and chuck top 2 having the insulating layer interposed therebetween, which may cause noise. In particular, since the recent miniaturization of a wafer circuit as described above requires noise reduction, the dielectric constant is preferably 4 or less, and in particular, 2 or less. Preferably, the reduced dielectric constant can reduce the thickness of the insulating layer required to ensure the insulation resistance value or capacitance, thereby reducing thermal resistance by the insulating layer.
If chuck top 2 is an insulator, between chuck top conductive layer 3 and the guard electrode layer and between chuck top conductive layer 3 and the electromagnetic shielding electrode layer, if chuck top 2 is a conductor, between chuck top 2 itself and the guard electrode layer and between chuck top 2 and the electromagnetic shielding electrode layer, the capacitance is preferably 5000 pF or less. If the capacitance exceeds 5000 pF, the effect of the insulating layer as a capacitor is increased, which may affect probing as noise. In particular, considering the recent miniaturization of wafer circuits as described above, the capacitance is preferably 1000 pF or less in order to realize good probing.
As described above, the noise which may affect probing can be reduced significantly by controlling the resistance value, dielectric constant and capacitance of the insulating layer within the ranges as described above. The thickness of this insulating layer is preferably 0.2 mm or more. For the reduced size of the device and good heat conduction from heater body 6 to chuck top 2, a thinner insulating layer is better. However, the thickness of less than 0.2 mm may cause a defect in the insulating layer itself or affect the durability. Ideally, the thickness of the insulating layer is 1 mm or more. The thickness in such an extent is preferable since the durability can be assured and the heat conduction from heater body 6 is good. Although the upper limit of the thickness is not specifically limited, 10 mm or less is preferable. If the thickness exceeds 10 mm, noise is blocked effectively. However, it takes much time for the heat generated in heater body 6 to conduct to chuck top 2 and the wafer, so that it may become difficult to control the heating temperature. The thickness is preferably 5 mm or less, although depending on the probing conditions, since the temperature control may become relatively easy.
Although not specifically limited, the thermal conductivity of the insulating layer is preferably 0.5 W/mK or more in order to realize good heat conduction from heater body 6 as described above. If 1 W/mK or more, the heat transmission preferably becomes better.
The specific material of the insulating layer is not specifically limited as long as it satisfies the characteristics as described above and has heat resistance such that it can endure the temperature in probing. Ceramic or resin may be employed. Among others, preferably, the resin may be, for example, silicone resin, the resin with a filler dispersed therein, ceramic such as alumina, or the like. The filler dispersed in resin serves to increase heat conduction of silicone resin. The material is not specifically limited as long as it is not reactive with the resin, and may include, for example, boron nitride, aluminum nitride, alumina, or silica.
The region in which the insulating layer is formed is preferably equivalent to the region in which the electromagnetic shielding electrode layer, the guard electrode or heater body 6 is formed. If the formation region is small, noise may intrude from the portion that is not covered with the insulating layer.
Description will be made below with reference to an example. For example, silicone resin including boron nitride dispersed therein is used as the insulating layer. The dielectric constant of this material is 2. When the silicone resin including boron nitride dispersed therein is sandwiched as an insulating layer between the electromagnetic shielding layer and the guard electrode layer, between the guard electrode layer and chuck top 2, the insulating layer can be formed to have a diameter of 300 mm, if chuck top 2 corresponds to a 12-inch wafer. Here, if the thickness of the insulating layer is 0.25 mm, the capacitance can be 5000 pF. If the thickness of the insulating layer is 1.25 mm or more, the capacitance can be 1000 pF. Since the volume resistivity of this material is 9×1015 Ω·cm, the resistance value can be about 1×1012Ω with a diameter of 300 mm and a thickness of 0.8 mm or more. Furthermore, since this material has thermal conductivity of about 5 W/mK, if the thickness, which can be selected depending on the conditions of probing, is 1.25 mm or more, both the capacitance and the resistance value can be adequate.
If the warping of chuck top 2 is 30 μm or more, the pin of the prober is in improper contact during probing, so that the characteristics cannot be evaluated or a failure determination is made due to poor contact. Undesirably, the yields are reduced due to unduly poor evaluation. On the other hand, it is not preferable that the parallelism between the surface of wafer-placing layer 3 and the undersurface of the base portion of support body 4 is 30 μm or more, because poor contact may take place similarly. Even if the warping of chuck top 2 and the parallelism is as good as 30 μm or less at room temperature, it is not preferable if the warping and the parallelism are 30 μm or more during probing at 200° C. The same applies to probing at −70° C. In other words, it is preferable that both of the warping and the parallelism are 30 μm or less throughout the temperature range for probing.
It is noted that the warping and parallelism as described above can be measured by measurement equipment such as three-dimensional measurement equipment.
Chuck top conductive layer 3 is formed on the wafer-placing surface of chuck top 2. Chuck top conductive layer 3 is formed to serve to protect the chuck top from corrosive gas, acid or alkaline chemicals, organic solvent or water that are usually used in semiconductor manufacturing and to serve to establish a ground in order to block noise from below chuck top 2 to a semiconductor wafer placed on chuck top 2.
The method of forming chuck top conductive layer 3 is not specifically limited and may include applying a conductive paste by screen printing followed by baking, vapor deposition or sputtering, thermal spraying or plating, or the like. Of these techniques, thermal spraying and plating are especially preferable. These techniques do not involve a heat treatment in forming chuck top conductive layer 3 and thus do not cause warp in chuck top 2 itself, which would be caused by the heat treatment. In addition, because of relatively low costs, a conductive layer can be formed inexpensively with excellent characteristics. In particular, preferably, a thermal sprayed film is formed on chuck top 2 and a plating film is formed thereon. The adhesiveness between the thermal sprayed film and ceramic and metal-ceramic is superior as compared with a plating film. This is because the sprayed material, for example, aluminum, nickel or the like produces some amount of oxide, nitride or oxynitride during thermal spraying, and the produced compound reacts with the surface layer of chuck top 2 and strongly adheres thereto.
However, since the sprayed film includes these compounds, the conductivity of the film becomes low. By contrast, the plating can form almost pure metal, so that a conductive layer excellent in conductivity can be formed although the adhesive strength with chuck top 2 is not so high as a thermal sprayed film. Therefore, when a thermal sprayed film is underlaid and a plating film is formed thereon, preferably, the plating film has good adhesive strength with the thermal sprayed film, which is a metal, and also provides good electric conductivity.
Preferably, surface roughness Ra of chuck top conductive layer 3 on chuck top 2 is 0.5 μm or less. If surface roughness Ra exceeds 0.5 μm, in determination of the electric characteristics of a device generating a large amount of heat, the heat generated by self-heating of the device itself cannot be dissipated from the chuck top during probing, thereby increasing the temperature of the device itself and possibly resulting in thermal breakdown. Surface roughness Ra of 0.02 μm or less is preferable in that heat can be dissipated more efficiently.
When the resistance heating element for chuck top 2 is heated for probing, for example, at 200° C., the temperature of the undersurface of support body 4 is preferably 100° C. or lower. If the temperature exceeds 100° C., the driving system for the prober lying below support body 4 is distorted due to the thermal expansion coefficient difference and is thus degraded in accuracy, thereby causing inconvenience such as misalignment in probing or improper contact of the probe pin due to warping and reduced parallelism. Accordingly, device evaluation cannot be made properly. In addition, when the determination is conducted at the temperature increased to 200° C. followed by determination at room temperature, it takes much time to cool from 200° C. to room temperature, thereby degrading throughput.
Preferably, Young's modulus of chuck top 2 is 250 GPa or more. If Young's modulus is less than 250 GPa, the load applied to the chuck top during probing causes bending of the chuck top, thereby significantly deteriorating the flatness and parallelism of the upper surface of chuck top 2. Therefore, poor contact of probe pins is caused to make accurate testing impossible. Moreover, wafers may be broken. Therefore, Young's modulus of chuck top 2 is preferably 250 GPa or more, more preferably 300 GPa or more.
Preferably, the thermal conductivity of chuck top 2 is 15 W/mK or more. If less than 15 W/mK, undesirably, the temperature distribution of a wafer placed on chuck top 2 may become worse. Therefore, if the thermal conductivity is 15 W/mK or more, such thermal uniformity can be obtained that is acceptable to probing. The material having such thermal conductivity may include alumina at a purity of 99.5% (thermal conductivity 30 W/mK). More preferably, the thermal conductivity is 170 W/mK or more. The material having such thermal conductivity may include aluminum nitride (170 W/mK), Si—SiC composite (170 W/mK to 220 W/mK), or the like. With the thermal conductivity in such an extent, the chuck top can be excellent in thermal uniformity.
Preferably, the thickness of chuck top 2 is 8 mm or more. If the thickness is less than 8 mm, the load applied to the chuck top during probing causes bending of chuck top 2, thereby significantly deteriorating the flatness and parallelism of the upper surface of chuck top 2. Thus, because of the poor contact of the probe pin, accurate testing becomes impossible. In addition, wafers may be broken. Therefore, the thickness of chuck top 2 is preferably 8 mm or more, more preferably 10 mm or more.
The material that forms chuck top 2 is preferably a metal-ceramic composite, ceramic or metal. Here, a metal-ceramic composite is preferably any of a composite of aluminum and silicon carbide, a composite of silicon and silicon carbide, and a composite of aluminum, silicon and silicon carbide, which have relatively high thermal conductivity and provide thermal uniformity when a wafer is heated. Among these, especially, a composite of silicon and silicon carbide is preferable because of high Young's modulus and high thermal conductivity.
Furthermore, since these composite materials are conductive, the heating element can be formed, for example, by forming an insulating layer on the surface opposite to the wafer-placing surface by thermal spraying, screen printing or any other technique and screen-printing a conductive layer thereon or forming a conductive layer in a prescribed pattern by vapor deposition or any other technique.
Alternatively, the heating element can also be formed by forming a metal foil made of stainless steel, nickel, silver, molybdenum, tungsten, chrome, or an alloy thereof in a prescribed heating element pattern by etching. In this technique, the insulation from chuck top 2 can be formed by the similar technique as described above. Alternatively, for example, an insulative sheet may be inserted between the chuck top and the heating element. In this case, preferably, an insulating layer can be formed remarkably cheaply and easily as compared with the above-noted technique. The resin to be used in this case includes a mica sheet, epoxy resin, polyimide resin, phenol resin, silicone resin or the like in view of heat resistance. Among those, especially mica is preferable. The reason is that mica is excellent in heat resistance and electric insulation, easily processed, and moreover cheap.
Ceramic as a material of chuck top 2 is relatively usable since it does not require formation of an insulating layer as described above. In this case, the method of forming a heating element can be selected from the similar techniques as described above. Among the ceramic materials, alumina, aluminum nitride, silicon nitride, mullite, or an alumina and mullite composite is preferable. These materials are especially preferable since they have relatively high Young's modulus, thereby reducing deformation caused by pressing a probe card. Among them, alumina is most excellent because of relatively low costs and excellent electric characteristics at high temperature. In particular, alumina at purity of 99.6% or more has high insulation property at high temperature. More preferably, the purity is 99.9% or more. More specifically, silicon oxides, alkaline-earth metal oxides, or the like are added to alumina in order to reduce the sintering temperature when a substrate is sintered. This reduces the electric characteristics of pure alumina, such as electrical insulation at high temperature. Therefore, the purity is preferably 99.6% or more, more preferably 99.9% or more.
Alternatively, a metal may also be used as a material of chuck top 2. In this case, tungsten, molybdenum, or an alloy thereof may be used, which have high Young's modulus. Specifically, the alloy includes a tungsten and copper alloy or a molybdenum and copper alloy. These alloys can be prepared by impregnating copper in tungsten or molybdenum. Since these metals are conductive similar to the aforementioned ceramic-metal composite, chuck top 2 can be used by applying the technique as described above as it is to form chuck top conductive layer 3 and a heating element.
When a load of 3.1 MPa is applied to chuck top 2, the amount of bending thereof is preferably 30 μm or less. A number of probe pins for testing a wafer are pressed against a wafer from a probe card, so that that pressure may also affect chuck top 2 causing some bending of chuck top 2. Here, if the amount of bending exceeds 30 μm, undesirably the pins of the probe card cannot press a wafer uniformly, so that wafer testing may become impossible. More preferably, the amount of bending with application of the pressure is 10 μm or less.
In the present invention, as shown in
When the speed of cooling chuck top 2 is a higher priority, cooling module 8 may be fixed to chuck top 2. As a manner of fixing, as shown in
In any of the techniques, the fixing method is not specifically limited. For example, a mechanical technique such as screwing or clamping may be employed. When chuck top 2 and cooling module 8 and the insulation heater are fixed by screwing, the number of screws is three or more, preferably six or more, so that the contact therebetween is enhanced thereby further improving the ability of cooling chuck top 2.
Furthermore, in this structure, cooling module 8 may be mounted in the cavity of support body 4, or the cooling module may be mounted on support body 4 and chuck top 2 may be mounted thereon. In any method, chuck top 2 and cooling module 8 are fixed to each other, so that the cooling speed can be increased as compared with the movable module. In addition, since the cooling module portion is mounted at the support body portion, the contact area of cooling module 8 with chuck top 2 is increased thereby cooling the chuck top more quickly.
In this way, when cooling module 8 is fixed to chuck top 2, the temperature may be increased without feeding refrigerant into the cooling module. In this case, since no refrigerant flows in cooling module 8, the heat generated in the heating element is not removed by refrigerant and does not escape to the outside of the system. Therefore, the temperature can be increased more efficiently. However, also in this case, refrigerant is fed into cooling module 8 in cooling, so that chuck top 2 can be cooled efficiently.
Furthermore, chuck top 2 and cooling module 8 may be integrated. In this case, the material used for chuck top 2 and cooling module 8 to be integrated is not specifically limited. However, since a channel for refrigerant to flow needs to be formed in cooling module 8, the thermal expansion coefficient difference between the chuck top portion and the cooling module portion is preferably small. As a matter of course, the same material is preferable.
The material to be used in this case may include ceramic or a composite of ceramic and metal as described above as the material of chuck top 2. In this case, wafer holder 1 may be fabricated by forming chuck top conductive layer 3 on the wafer-placing surface side, forming a channel for cooling on the opposite side, and in addition, integrating a substrate made of the same material as chuck top 2, for example, by a technique such as soldering or glass sealing. As a matter of course, a channel may be formed on the substrate side to be affixed or channels may be formed in both substrates. Integration using screwing is also possible.
In this manner, chuck top 2 and cooling module 8 are integrated, so that chuck top 2 can be cooled more quickly than when cooling module 8 is fixed to chuck top 2 as described above.
When this cooling module 8 is movable or integrated in such a manner as to be fixed to chuck top 2, the weight of cooling module 8 is desirably 15000 g or less. This weight has the same effect as the weight of chuck top 2 or support body 4 would affect the positional accuracy or mechanical accuracy of wafer holder 1 due to the inertial force during movement of wafer holder 1. In other words, if the weight of cooling module 8 exceeds 15000 g, the positional accuracy required for testing cannot be achieved because of the inertia caused when a prober is frequently moved in the test step. In addition, the repeated tests cause the inertial force to be received by wafer holder 1 itself, so that the assembly of wafer holder 1 loosens and the poor mechanical accuracy results. On the other hand, in order to achieve the stiffness against screwing or the like, the weight of cooling module 8 is desirably 500 g or more.
In this technique, a metal can also be used as a material of the integrated chuck top 2. Since metal is easily processed and inexpensive as compared with ceramic or a composite of ceramic and metal as described above, a channel for refrigerant can be easily formed. However, when a metal is used for the integrated chuck top 2, the pressure applied in probing may cause bending. Therefore, as shown in
A substrate having Young's modulus of 250 GPa or more is preferably used as anti-chuck top-deformation substrate 10. This anti-chuck top-deformation substrate 10 may be housed in the cavity formed in support body 4, as shown in
In the present embodiment where the material of chuck top 2 is metal, for example, chuck top conductive layer 3 may be formed anew on the surface of the wafer-placing surface, for example, when the material of chuck top 2 easily causes oxidation or degradation of the surface or does not have high electrical conductivity. In this technique, as described above, chuck top conductive layer 3 may be formed by plating having oxidation resistance such as nickel or a combination with thermal spraying, and the surface of the wafer-placing surface may be polished.
Also in this structure, an electromagnetic shielding electrode layer or a guard electrode layer may be formed as described above as necessary. In this case, the insulated heating element is covered with metal as described above, a guard electrode layer is formed with an insulating layer interposed, and an insulating layer is formed between the guard electrode layer and chuck top 2. Additionally, anti-chuck top-deformation substrate 10 may be fixed integrally to chuck top 2.
In this structure, as a method of installing chuck top 2 integrated with cooling module 8 on support body 4, the cooling module portion may be installed in the cavity portion formed in support body 4. Alternatively, similarly to when chuck top 2 and cooling module 8 are fixed by screwing, the cooling module portion is used for installation on support body 4.
The material of cooling module 8 is not specifically limited. Preferably, aluminum or copper and an alloy thereof are used since the heat of chuck top 2 can be removed rapidly because of relatively high thermal conductivity. Alternatively, stainless steel, magnesium alloy, nickel, or any other metal material may be used. Additionally, a metal film having oxidation resistance such as nickel, gold or silver may be formed using a technique such as plating or thermal spraying in order to render cooling module 8 oxidation-resistant.
Ceramic may be used as a material of cooling module 8. The material in this case is not specifically limited. Aluminum nitride or silicon carbide is preferable since heat can be removed quickly because of relatively high thermal conductivity. Furthermore, silicon nitride or aluminum oxynitride is preferable because of high mechanical strength and high durability. Ceramic oxide such as alumina, cordierite or steatite is preferable since it is relatively inexpensive. As described above, a variety of materials can be selected for the cooling module and thus, a material may be selected depending on a purpose. Among those, aluminum with nickel plating or copper with nickel plating is especially preferable because of high oxidation resistance, high thermal conductivity, and relatively low costs.
Refrigerant may be fed inside this cooling module 8. In this manner, the heat transmitted from heater body 6 to cooling module 8 can be removed from cooling module 8 quickly, so that preferably, the speed of cooling the heater body can be further improved.
As a suitable example, two aluminum plates are prepared, and a channel into which water flows is formed in one of the aluminum plates by machining or the like. Then, in order to improve corrosion resistance and oxidation resistance, nickel plating is applied on the front surface. Then, the other aluminum plate with nickel plating is affixed. Here, for example, an O-ring or the like is inserted around the channel in order to prevent water leakage. Then, the two aluminum plates are affixed to each other by screwing or welding.
Alternatively, two copper (oxygen-free copper) plates are prepared, and a channel in which water flows is formed in one of the copper plates. The other copper plate and a stainless steel pipe for inlet/outlet of refrigerant are joined at the same time by soldering. The joined cooling plates are nickel plated on the entire surface thereof in order to improve corrosion resistance and oxidation resistance. Alternatively, in the another embodiment, a pipe in which refrigerant flows may be attached to a cooling plate such as an aluminum plate or copper plate, resulting in a cooling module. In this case, a spot-face having a shape close to the cross sectional shape of the pipe may be formed in the cooling plate and is brought into close contact with the pipe, thereby further improving the cooling efficiency. Alternatively, in order to improve the contact of the cooling plate with the cooling pipe, thermal conductive resin, ceramic or the like may be inserted as an interposing layer.
As another embodiment, a pipe in which refrigerant can be fed may be fixed to aluminum or copper to form cooling module 8. In this case, in order to secure the contact area between the pipe and aluminum or copper, a groove having the approximately same shape as the cooling pipe may be provided in the metal plate made of aluminum, copper or the like, or a material having deformability such as resin may be sandwiched between the metal plate and the pipe. On the contrary, a planar shape may be formed in a part of the cross sectional shape of the pipe to be fixed to the metal plate. The method of fixing the metal plate and the pipe may include screwing using a metal band or welding or soldering, as a matter of course.
Furthermore, the refrigerant fed into such cooling module 8 may be, for example, liquid such as Fluorinert, Galden or water or gas such as nitrogen, air or helium. Here, the refrigerant to be fed is not specifically limited and may be selected depending on a purpose.
In the embodiment, a wafer holder for a wafer prober has been described as an exemplary wafer holder. However, the present invention is not limited thereto. Wafer holder 1 in accordance with the present invention may suitably be used for heating and testing a process target such as a wafer. For example, suitably, the present invention may be applied to a wafer prober, a handler or a tester because it is possible to take advantage of the characteristics such as high stiffness and high thermal conductivity. In particular, wafer holder 1 in accordance with the present invention is preferably applied as a holder for a wafer prober.
The heater unit in accordance with the present invention includes wafer holder 1 as described above and a known constituent member, and includes, for example, wafer holder 1, the heater body described above and a power supply to operate the heater body. The heater unit is preferably a heater unit for a wafer prober. The heater unit for a wafer prober heats a process target such as a semiconductor wafer.
The wafer prober in accordance with the present invention includes the heater unit as described above and a known constituent member and includes, for example, the heater unit as described above, a driver for driving the heater unit in the X, Y, Z directions, and a wafer transfer device. The wafer prober may additionally include a probe card and the like.
Composite substrates made of silicon and silicon carbide (Si—SiC substrate) each having a diameter of 305 mm and a thickness of 10, 20, 30 mm were prepared (namely, C-1, C-2, C-3, respectively). A concentrically-arranged groove for vacuum-chucking a wafer and a through hole were formed on the wafer-placing surface of the Si—SiC substrate, and the wafer-placing surface was nickel plated to form a chuck top conductive layer. Thereafter, the chuck top conductive layer was polished, resulting in a chuck top having the amount of warping of 10 μm as a whole and surface roughness Ra of 0.02 μm.
Then, ring-shaped mullite-alumina composites each having an outer diameter of 310 mm, inner diameter of 295 mm and a thickness of 15, 30, 45 mm were prepared as ring-shaped portions of the support bodies (namely, R-1, R-2, R-3, respectively). Disk-shaped aluminum oxide substrates each having a diameter of 310 mm and a thickness of 15, 20 45 mm were prepared as base portions (namely, P-1, P-2, P-3, respectively).
A stainless steel foil insulated by a silicone resin sheet was attached as an electromagnetic shielding electrode layer to each of the chuck tops, and additionally, a heating element sandwiched between silicone resin sheets was attached. The heating element was formed by etching a stainless steel foil in a prescribed pattern. In addition, a though hole for connecting an electrode for supplying power to the heating element was formed in the support body. Then, a metal layer was formed by thermally spraying aluminum on the side surface and the undersurface of each of the support bodies.
In order to prepare a cooling module, two copper plates each having a diameter of 285 mm were prepared. One of the copper plates was provided with a channel and thereafter welded with the other plate, resulting in a cooling module including a channel therein for refrigerant to flow. Three kinds of the cooling modules were prepared having thicknesses of 10, 20, 30 mm (namely, CL-1, CL-2, CL-3, respectively).
Then, three kinds of chuck tops, support body ring-shaped portions, support body base portions, and cooling modules as described above each were combined to form a wafer holder. The combinations and resulting weights are shown in Table 1.
A wafer was heated to 150° C. by supplying power to the heating element of the wafer holder in Table 1, and probing was carried out for two and ten consecutive hours. Then, the positional accuracy of the wafer holder was checked. In addition, the temperature increasing time from room temperature to 150° C. and the cooling time from 150° C. to room temperature were measured. The result is shown in Table 1. In Table 1, A represents that no problem arose in measurement, B represents measurement could be made but measurement values were instable, and C represents that measurement could not be made.
In accordance with the present invention, in the step of placing a semiconductor wafer on a wafer-placing surface and pressing a probe card against the wafer to inspect the electric characteristics of the wafer, in a repeated operation of heating the wafer to a prescribed temperature, moving to a prescribed position in probing, and pressing the probe card, extremely high positional accuracy can be achieved in moving the prober to a prescribed position. As a result, the accuracy of measurement position of a probe pin is improved, allowing proper measurement with small variations. In addition, for vibration involved with repeated positional movements, poor mechanical accuracy or damage of the assembly can be suppressed. Accordingly, in the test step in the semiconductor industry, tests can be performed with high throughput and stability.
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 |
|---|---|---|---|
| 2005-226203 (P) | Aug 2005 | JP | national |
