JP2006147631A - Semiconductor device and its evaluation method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To evaluate defective bonding in facedown mounting (surface mounting type) semiconductor device where the surface side of a semiconductor chip is connected with a mounting substrate. <P>SOLUTION: When a semiconductor chip 10 is composed of a transparent material similar to a light-emitting diode 1 made by forming gallium nitride based compound semiconductor layers 12 and 13 on a transparent sapphire substrate 11, grains 30 composed of a material harder than that of electrodes 15 and 16 are stuck to the abutting portion of a gold bump electrode 21 in the electrodes 15 and 16 formed on the surface of the semiconductor chip 10. Consequently, the junction of the electrodes 15, 16 and the gold bump electrode 21 can be evaluated from whether the grains 30 can be confirmed visually from the rear surface of the semiconductor chip 10. Consequently, chips of defective junction can be screened and forming conditions or bonding conditions of the bump electrode can be adjusted appropriately from the evaluation results. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a structure of a semiconductor device in which a semiconductor chip made of a transparent material such as a surface-mounted light emitting diode is mounted on a mounting substrate, and a method for evaluating a bump electrode which is an electrical connection portion thereof.

  Conventionally, as a structure of a light-emitting diode made of the transparent material, there is a blue light-emitting diode as disclosed in Patent Document 1. The blue light-emitting diode is one in which P-type and N-type crystals of gallium nitride, which is a transparent material, are stacked to form a PN junction and emit light. FIG. 6 is a schematic cross-sectional view of the mounting structure inferred from this paper and using the surface mounting type (also referred to as flip chip mounting).

  First, a semiconductor chip 100 of the blue light emitting diode is manufactured by growing a single crystal of gallium nitride on a sapphire substrate (transparent) 101 as a supporting substrate by MOCVD (Metal Organic Chemical Vapor Deposition) method, and its N type. A P-type gallium nitride semiconductor layer 103 is formed on the gallium nitride semiconductor layer 102 to be P-type by irradiation with an electron beam (sometimes due to a dopant). Further, the N-type gallium nitride semiconductor layer 102 is insulated. The first electrode 105 is formed on the oxide film layer 104, and the second electrode 106 is formed on the P-type gallium nitride semiconductor layer 103. Then, the first electrode 105 on the N-type gallium nitride semiconductor layer 102 side is set to − (cathode) and the second electrode 106 on the P-type gallium nitride semiconductor layer 103 side is set to + (anode). When a current is applied, blue to ultraviolet light is emitted from the junction between the N-type gallium nitride semiconductor layer 102 and the P-type gallium nitride semiconductor layer 103, as will be described later.

  The first electrode 105 is in ohmic contact with the N-type gallium nitride semiconductor layer 102, and the second electrode 106 is in ohmic contact with the P-type gallium nitride semiconductor layer 103. The insulating oxide film layer 104 under the electrode 105 may or may not be a silicon nitride film. The insulating oxide film 104 may be an N-type gallium nitride semiconductor layer. The electrodes 105 and 106 are formed by metallizing aluminum or gold. Specifically, aluminum is used as an underlayer, and the aluminum is thermally contacted by thermally diffusing the aluminum to the P-type gallium nitride semiconductor layer 103 and the N-type gallium nitride semiconductor layer 102, and gold is metalized on the aluminum, Alternatively, it is formed by directly metallizing gold into the P-type gallium nitride semiconductor layer 103 or the N-type gallium nitride layer 102 and thermally diffusing.

  On the other hand, in the mounting substrate 120 on which the circuit pattern is formed, pads 122 for electrically connecting to the bump electrodes 121 are formed on the pattern at predetermined positions. The pad 122 is composed of a copper having a lower layer 122a of several tens of μm thick and a nickel layer for preventing diffusion of copper formed thereon with a thickness of several μm, and the upper layer 122b thereon has gold of several μm thick. Generally, it is formed by sputtering or plating.

  The bump electrode 121 includes gold, solder, etc., but a soft material is better. When the bump material is gold, it is formed by a plating method or a stud bump method using a wire bonding method. The bump electrodes 121 are formed on the electrodes 105 and 106 side of the semiconductor chip 100 or on the pads 122 side of the mounting substrate 120. The electrodes 105 and 106 of the semiconductor chip 100 of the light emitting diode formed as described above and the pads 122 of the mounting substrate 120 are electrically connected via the bump electrodes 121 by face-down bonding (also referred to as the flip chip mounting). Connected.

  On the other hand, the specific structure of the semiconductor chip in the light emitting diode is as shown in FIG. FIG. 7 is a cross-sectional view schematically showing a general structure of the semiconductor chip 100 of the blue light emitting diode, which is shown in Patent Document 2. As shown in FIG. Portions corresponding to the configuration of FIG. 6 are given the same reference numerals. On the sapphire substrate 101 which is a support substrate, AlN which is a buffer layer 111, an N layer 102a made of an N-type gallium nitride compound semiconductor, and an N layer 102b which is a light emitting layer and a low carrier concentration N layer 102b which is an active layer. -GaN and a P layer 103 made of a P-type gallium nitride compound semiconductor are formed, a groove 112 formed from the upper surface of the P layer 103 to the N layer 102b, and an N layer 102b from the upper surface of the P layer 103. The first electrode 105 formed up to the upper surface of the P layer 103 and the upper surface of the P layer 103 by the groove 112 and the first electrode 105. The light emitting diode is formed including a second electrode 106 formed so as to be electrically insulated and separated.

In the light emitting diode configured as described above, since the first electrode 105 and the second electrode 106 are electrically insulated and separated by the groove 112, the light emitting diode is forward-biased, that is, the above-described In this way, current flows from the second electrode 106 toward the first electrode 105, and holes and electrons are combined in the low carrier concentration N layer 102b located at the interface between the P layer 103 and the N layer 102a, and light is emitted. Other recent literature relating to the surface mounting structure of semiconductor light-emitting elements (light-emitting diodes) includes the following patent documents 2-4.
Akasaki, Amano “P-type Conduction in Mg-Doped GaN Treated with Low-Energy Electron Irradiation”: European Journal of Applied Physics, VOL. 28, no. 12, ppL2112-L2114, DEC, 1989 JP-A-4-163970 JP 2003-304003 A Japanese Patent Application Laid-Open No. 2002-217459 JP-A-11-168235

  Since the semiconductor chip 100 as described above is a surface mount type that is electrically connected to the mounting substrate 120 via the bump electrodes 121, the bump electrodes 121 and the electrodes 105, 106 formed on the surface of the semiconductor chip 100, It is extremely difficult to evaluate the joint portion. The only way to confirm the electrical contact is indirectly by the connection resistance. For this reason, bonding is extremely unstable, the bonding area is very small, and even in the case of a chip that should originally be a defective product, a minute contact portion is bonded or pressed by metal thermal diffusion. In such a case, the connection resistance is low, and it may be erroneously determined that there is no problem (good product). However, in such unstable joints, when mechanical stress or thermal stress is applied in the market environment after manufacture and assembly or after commercialization, the joint part easily peels off and the electrical connection There is a problem that reliability cannot be ensured.

  Such a problem is caused by variations in the height and surface state of the bump electrode 121. Currently, in order to prevent such a problem in advance, conditions for the face-down bonding of the semiconductor chip 100 are described. It is the actual situation that controls. Specifically, the bonding temperature, load, pressing time, chip pressing amount, parallelism when the chip is pressed, and the like are main parameters, and at least one of them is controlled. However, a great deal of labor is required to establish the optimum conditions depending on the condition of the equipment (flip chip bonder), the skill of the operator, and the surrounding environment (air volume, temperature, humidity, vibration, etc.).

  SUMMARY OF THE INVENTION An object of the present invention is to provide a semiconductor device and a method for evaluating the same that can improve reliability by making it possible to evaluate a connection portion between a semiconductor chip such as a light emitting diode and a bump electrode.

  According to the semiconductor device and the evaluation method of the present invention, a semiconductor chip made of a transparent material is opposed to an electrode formed on the surface of the semiconductor chip and a pad of a wiring pattern formed on a mounting substrate, and these are placed on the pad. In a semiconductor device that is electrically connected to the mounting substrate by being electrically connected by a provided bump electrode, a grain made of a material harder than the electrode is provided at a contact portion of the bump electrode in the electrode. It is possible to evaluate the bonding state by attaching a body and confirming from the back side of the semiconductor chip whether or not the particles are embedded in the electrode by bonding the electrode and the bump electrode. It is characterized by.

  According to the above configuration, in the face-down mounting (surface mounting) type semiconductor device in which the surface side of the semiconductor chip is connected to the mounting substrate, the semiconductor chip is laminated with the transparent gallium nitrogen compound semiconductor layer on the transparent sapphire substrate. When the semiconductor chip is made of a transparent material, such as a light emitting diode, it is possible to observe the front surface side from the back surface side of the semiconductor chip. Therefore, in the present invention, particles made of a material harder than the electrode are attached to the contact portion of the bump electrode in the electrode that is a light-shielding metal layer formed on the surface of the semiconductor chip. Then, when the electrode is brought into contact with the bump electrode and heated / pressed, the particles are embedded in the electrode, part of which comes into contact with the semiconductor layer or reaches the vicinity of the semiconductor layer, and the back surface of the semiconductor chip. Therefore, the particles can be visually confirmed. On the other hand, if the particle size of the bump electrode is insufficient and there is a gap between the upper surface of the bump electrode and the electrode, or there is no gap and the contact is insufficient, Even if it is not embedded inside and confirmed from the back side of the semiconductor chip, only the color of the electrode can be confirmed.

  Therefore, check whether the bump electrode that electrically connects the electrode and the pad of the wiring pattern formed on the mounting substrate has a sufficient particle size or whether it is securely bonded The joint between the electrode and the bump electrode can be visually evaluated. As a result, defective chips can be screened, and from the evaluation results, by appropriately adjusting the formation conditions and bonding conditions of the bump electrodes, the yield can be improved and the semiconductor device with higher reliability can be obtained. Embedded products can be provided.

  In the semiconductor device of the present invention, the grain is formed in a wedge shape embedded in a tapered hole formed in the surface of the electrode, and the height of the wedge is larger than the thickness of the electrode. It is thin.

  According to said structure, the particle | grains embedded beforehand in the taper-shaped hole formed in the surface of an electrode are pointed head (pointed one) to the semiconductor layer side (support substrate side) of the said semiconductor chip. Will be directed.

  Therefore, it is easy to be pushed by the bump electrode to be joined, and even if there is an obstacle such as a foreign substance in the electrode, this can be avoided.

  Furthermore, in the semiconductor device of the present invention, the height of the particles is formed to be about 50 to 80% of the thickness of the electrode.

  According to said structure, the said wedge-shaped granule is formed in the depth of about 50%-80% from the electrode surface. If the wedge-shaped grains are low in height (shallow depth), they cannot reach the semiconductor layer. If the height is high (depth too deep), the bonding load is insufficient. Even if it is, it reaches the semiconductor layer.

  Therefore, the above range is the height of a practical wedge-shaped particle.

  In the semiconductor device of the present invention, the electrode and the bump electrode are made of gold, and the particles are made of at least one of aluminum, nickel, silver, chromium, tin, and titanium.

  According to said structure, the relationship of the hardness of the above electrodes and granule can be concretely implement | achieved, and a granule can be embedded in an electrode in a normal joining state. In addition, the particles can be color-coded from the electrodes.

  As described above, according to the semiconductor device and the evaluation method of the present invention, in the face-down mounting (surface mounting) type semiconductor device in which the surface side of the semiconductor chip is connected to the mounting substrate, the semiconductor chip is transparent to the transparent sapphire substrate. When the semiconductor chip is made of a transparent material, such as a light-emitting diode having a stack of various gallium nitrogen-based compound semiconductor layers, a bump electrode is formed on the surface of the semiconductor chip and is a light-shielding metal layer. By adhering a particle made of a material harder than the electrode to the contact part, bringing the electrode into contact with the bump electrode, and heating and pressurizing, the particle is embedded in the electrode, and a part thereof The bonded state is determined from whether the semiconductor layer contacts or reaches the vicinity of the semiconductor layer and whether or not the particles can be visually confirmed from the back surface of the semiconductor chip.

  Therefore, whether or not the bump electrode that electrically connects the electrode and the pad of the wiring pattern formed on the mounting substrate has a sufficient particle size, whether or not it is securely bonded, etc. This can be confirmed, and the joint between the electrode and the bump electrode can be visually evaluated. As a result, defective chips can be screened, and from the evaluation results, by appropriately adjusting the formation conditions and bonding conditions of the bump electrodes, the yield can be improved and the semiconductor device with higher reliability can be obtained. Embedded products can be provided.

[Embodiment 1]
1 and 2 are cross-sectional views schematically showing the structure of a light-emitting diode 1 which is a semiconductor device according to a first embodiment of the present invention. The light-emitting diode 1 is configured by bonding a blue light-emitting diode semiconductor chip 10 face-down to a mounting substrate 20. FIG. 1 shows a state after joining, and FIG. 2 shows a state before joining.

  In the semiconductor chip 10, a single crystal of gallium nitride is grown on a transparent sapphire substrate 11, which is a support substrate, by MOCVD or the like. On the N-type gallium nitride semiconductor layer 12, a groove 17 is used as a boundary. On the other hand, a P-type gallium nitride semiconductor layer 13 made into a P-type by irradiation with an electron beam (which may be due to a dopant) is formed, and on the other side, an insulating oxide film layer 14 is formed, and a first electrode 15 is formed. The second electrode 16 is formed on the P-type gallium nitride semiconductor layer 13 and formed. Then, the first electrode 15 on the N-type gallium nitride semiconductor layer 12 side is set to − (cathode), and the second electrode 16 on the P-type gallium nitride semiconductor layer 13 side is set to + (anode). When a current is applied, blue to ultraviolet light is emitted from the junction between the N-type gallium nitride semiconductor layer 12 and the P-type gallium nitride semiconductor layer 13.

  The first electrode 15 is in ohmic contact with the N-type gallium nitride semiconductor layer 12, and the second electrode 16 is in ohmic contact with the P-type gallium nitride semiconductor layer 13. The insulating oxide film layer 14 below the electrode 15 may or may not be a silicon nitride film. The insulating oxide film 14 may be an N-type gallium nitride semiconductor layer. The electrodes 15 and 16 are formed by metallization using aluminum or gold. Specifically, aluminum is used as an underlayer, and the aluminum is thermally diffused to the P-type gallium nitride semiconductor layer 13 and the N-type gallium nitride semiconductor layer 12 to make ohmic contact, and metallization is performed on the aluminum using gold. Alternatively, it is formed by directly metallizing the P-type gallium nitride semiconductor layer 13 or the N-type gallium nitride layer 12 using gold and thermally diffusing.

  On the other hand, on the mounting substrate 20 on which the circuit pattern is formed, pads 22 for electrical connection with the gold bump electrodes 21 are formed on the pattern at predetermined positions. The pad 22 is composed of copper having a lower layer 22a of several tens of μm thick and a nickel layer for preventing diffusion of copper formed thereon with a thickness of several μm, and the upper layer 22b thereon has gold of several μm thick. Generally, it is formed by sputtering or plating.

  The gold bump electrode 21 is formed on the pad 22 side of the mounting substrate 20 by a plating method or a stud bump method applying a wire bonding method. The electrodes 15 and 16 of the semiconductor chip 10 of the light-emitting diode 1 and the pads 22 of the mounting substrate 20 formed in this way are subjected to the face-down bonding described above at, for example, about 200 ° C. in the atmosphere or nitrogen atmosphere. Thus, it is electrically connected through the gold bump electrode 21. The above configuration is the same as that of the light emitting diode shown in FIG. 6 described above, and the specific structure related to light emission of the light emitting diode 1 may be as shown in FIG. 7 described above.

  In the light-emitting diode 1 configured as described above, it should be noted that in the present invention, the support substrate 11 is a transparent sapphire substrate, and the gallium nitride semiconductor layers 12 and 13 are also transparent. 10 on the surface of the semiconductor chip 10 by utilizing the fact that the surface side can be observed from the back side, and at the contact portion of the gold bump electrode 21 in the electrodes 15 and 16 which are light-shielding metal layers. As shown in FIG. 2, before joining, the particles 30 made of a material harder than the electrodes 15 and 16 are adhered.

  The grain 30 is harder than the gold bump electrode 21 and is made of at least one of materials that can be colored, such as aluminum, nickel, silver, chromium, tin, and titanium. When the bump electrode is made of a material other than gold, a material that is harder than that and can be color-coded may be used for the particles 30. Moreover, this granule 30 is not limited to a sphere as shown in FIGS. 1 and 2, but may be a polygonal shape. Furthermore, the size of the grains 30 may be appropriately designed in accordance with the face-down bonding conditions and the hardness and thickness of the electrodes 15 and 16, for example, the thickness of the electrodes 15 and 16 made of gold. Is 1 μm and the particle size of the gold bump electrode 21 is 100 μm, the particle size 30 of the aluminum particles 30 is formed to 30 μm.

  The particles 30 can be adsorbed to the tip of a charged probe and brought into contact with the electrodes 15 and 16 and then easily fixed by surface adsorption or heat treatment. . As shown in FIGS. 1 and 2, the number of the grains 30 is not limited to one for each gold bump electrode 21, and a plurality of grains 30 may be arranged as shown in FIG.

  Therefore, when the semiconductor chip 10 is pushed down from the state shown in FIG. 2, the electrodes 15 and 16 are brought into contact with the gold bump electrode 21, and heated and pressurized, the granules 30 are brought into contact with the electrode 15 as shown in FIG. , 16, part of which contacts the semiconductor layers 12, 13, or reaches the vicinity of the semiconductor layers 12, 13, and visually confirms the particles 30 from the back surface of the semiconductor chip 10. Can do. On the other hand, the particle size of the gold bump electrode 21 is insufficient, and there is a gap between the upper surface of the gold bump electrode 21 and the electrodes 15, 16. If there is, the particles 30 are not embedded in the electrodes 15 and 16, and only the colors of the electrodes 15 and 16 can be confirmed even when confirmed from the back surface of the semiconductor chip 10.

  Therefore, it can be confirmed whether or not the gold bump electrode 21 has a sufficient particle size, whether or not the gold bump electrode 21 is securely bonded, and the bonding portion between the electrodes 15 and 16 and the gold bump electrode 21 is determined. It can be visually evaluated. As a result, defective bonding chips can be screened, and from the evaluation results, by appropriately adjusting the formation conditions and bonding conditions of the gold bump electrode 21, the yield can be improved, and a more reliable semiconductor device and Products incorporating it can be provided.

[Embodiment 2]
3 and 4 are cross-sectional views schematically showing the structure of the light-emitting diode 41 according to the second embodiment of the present invention. The light-emitting diode 41 is similar to the above-described light-emitting diode 1, and corresponding portions are denoted by the same reference numerals and description thereof is omitted. It should be noted that in the light emitting diode 41, the particles 50 are embedded in advance in a tapered hole 42 formed on the surfaces of the electrodes 15 and 16, and are formed in a wedge shape. In addition, the height H1 of the particles 50 is smaller than the thickness H2 of the electrode and is formed to be about 50 to 80%.

  The wedge-shaped particles 50 are formed by, for example, digging the chip electrodes 15 and 16 made of gold, for example, in a wedge shape by using a photolithography technique, and then are harder than the electrodes 15 and 16 in the digging. It can be created by laminating (embedding) the material by a method such as sputtering, vapor deposition, or plating. The granule 50 may have a shape with a sharp tip, and may have a triangular pyramid shape, a cone shape, or a similar shape.

  Therefore, the grain 50 embedded in advance in the tapered hole 42 formed on the surface of the electrode has a pointed head (pointed side) on the semiconductor layers 12 and 13 side (support substrate 11 side) of the semiconductor chip 40. Therefore, even if there is an obstacle such as a foreign substance in the electrodes 15 and 16, this can be avoided.

  Here, the wedge-shaped granules 50 cannot reach the semiconductor layers 12 and 13 when the height H1 is low (the depth is shallow), and when the height H1 is high (the depth is too deep), Even if the grain size of the bump electrode 21 is small or the load at the time of bonding is insufficient, it reaches the semiconductor layers 12 and 13. When the bump electrode 21 is gold and the electrodes 15 and 16 are also gold, and the electrodes 15 and 16 are metallized, the thickness of the electrodes 15 and 16 on the chip surface is generally approximately determined by face-down bonding. 20% to 50% smaller. In view of this, assuming that face-down bonding is performed appropriately, the height H1 of the particles 50 is about 50 to 80% of the thickness H2 of the electrodes 15 and 16 from the surface of the electrodes 15 and 16. If formed to a depth, when the thickness H2 of the electrodes 15 and 16 is reduced by bonding, the tips of the grains 50 reach the semiconductor layers 12 and 13, and from the back side of the semiconductor chip 40, the grains The body 50 can be observed. Therefore, the above range is the practical height H1 of the granule 50.

  On the other hand, in the wedge-shaped particles 50, the base end side portion with which the bump electrode 21 abuts needs a certain wide area so as not to be buried (do not advance) in the bump electrode 21. When the particle diameter of the bump electrode 21 is, for example, φ = 50 to 100 μm and the thickness of the electrodes 15 and 16 is about several μm, the portion with which the bump electrode 21 abuts is about φ = 1 to 10 μm. Is considered necessary. The condition of the size of the portion with which the bump electrode 21 abuts is not particularly defined because it is affected by the capability of the face-down bonding apparatus, the size of the semiconductor chip 40, and the like.

  The present invention is not limited to the light emitting element as described above, and can be applied as long as a transparent semiconductor layer is formed on a transparent substrate.

  For example, Japanese Patent Application Laid-Open No. 9-312317 discloses a method of inspecting bonding defects by irradiating infrared rays from the side opposite to the bonding portion of the mounting substrate of the semiconductor chip. It is to be detected, and the bonding state between the electrodes 15 and 16 and the bump electrode 21 cannot be inspected as in the present invention.

  Japanese Laid-Open Patent Publication No. 2001-68514 discloses that a transparent dummy substrate is flip-chip mounted on a mounting substrate, and the state of the joint between the electrode and the bump electrode is visually inspected from the back surface of the substrate. A dummy substrate is used to evaluate a mounting process, and a semiconductor device as a product cannot be individually inspected as in the present invention.

It is sectional drawing which shows typically the structure of the light emitting diode which is a semiconductor device which concerns on the 1st Embodiment of this invention. It is sectional drawing which shows the state before joining of the light emitting diode shown in FIG. It is sectional drawing which shows the other structural example of the light emitting diode which concerns on the 1st Embodiment of this invention. It is sectional drawing which shows typically the structure of the light emitting diode which concerns on the 2nd Embodiment of this invention. It is sectional drawing which shows the state before joining of the light emitting diode shown in FIG. It is sectional drawing which shows typically the structure of the typical prior art light emitting diode. It is sectional drawing which shows the specific structure of the semiconductor chip in a common light emitting diode.

Explanation of symbols

1,41 Light emitting diode 10 Semiconductor chip 11 Support substrate 12 N-type gallium nitride semiconductor layer 13 P-type gallium nitride semiconductor layer 14 Insulating oxide layer 15 First electrode 16 Second electrode 17 Groove 20 Mounting substrate 21 Gold bump electrode 22 Pad 30 50 grains

Claims (5)

A semiconductor chip made of a transparent material is opposed to an electrode formed on the surface thereof and a pad of a wiring pattern formed on a mounting substrate, and these are electrically connected by a bump electrode provided on the pad. Thus, in a semiconductor device that is mounted face-down on the mounting substrate,
At the contact portion of the bump electrode in the electrode, a particle made of a material harder than the electrode is attached,
A semiconductor device characterized in that the bonding state can be evaluated by confirming from the back side of the semiconductor chip whether or not the particles are embedded in the electrode by bonding the electrode and the bump electrode.   2. The granule is formed in a wedge shape that is embedded in a tapered hole formed on the surface of the electrode, and the height of the wedge is smaller than the thickness of the electrode. The semiconductor device described.   3. The semiconductor device according to claim 2, wherein the height of the particles is formed to be about 50 to 80% of the thickness of the electrode.   The electrode and the bump electrode are made of gold, and the particles are made of at least one of aluminum, nickel, silver, chromium, tin, and titanium. The semiconductor device described. A semiconductor chip made of a transparent material is opposed to an electrode formed on the surface thereof and a pad of a wiring pattern formed on a mounting substrate, and these are electrically connected by a bump electrode provided on the pad. So, in the evaluation method of the semiconductor device that is mounted face down on the mounting substrate,
At the contact portion of the bump electrode in the electrode, a particle made of a material harder than the electrode is attached,
A bonding state can be evaluated by confirming from the back side of the semiconductor chip whether or not the particles are embedded in the electrode by bonding the electrode and the bump electrode. Evaluation methods.

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