JP2010205974A - Semiconductor device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semiconductor device having a bonding pad which can be Cu wire bonded while suppressing occurrence of unbonded parts. <P>SOLUTION: The semiconductor device is provided with an active element and an electrode 1 which is electrically connected with the active element and contains aluminum as a major component. The uppermost side of the electrode 1 is covered with a layer 5 of a compound of aluminum with copper. In this manner, occurrence of unbonded parts can be suppressed even in the case of bonding using Cu wire, its resistance to heat and heat cycle is improved and it exhibits a high long-term reliability. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a semiconductor device in which an electrode for wire bonding is formed in the vicinity of an active element such as a microcomputer or a power transistor.

  Conventionally, wire bonds such as Al (aluminum), Au (gold), and Cu (copper) have been used as methods for electrically connecting electrodes of a semiconductor element and a substrate electrode or electrodes of another semiconductor element. In particular, Au wires are mainly used for small pads with a narrow pitch because of the mechanical properties of the wire material, etc., electrode layers mainly composed of Al as semiconductor element side bonding pads, and Ag ( Silver) plating or Au plating treatment has been used.

  It is known that an Au—Al alloy layer is formed at the bonding interface between the bonding pad and the Au wire of the semiconductor device from the beginning of bonding, and the diffusion proceeds with heat and the thickness thereof is increased. It is known that an alloy layer having a composition is generated and diffusion voids are generated due to the influence thereof. As a result, there is a concern that the interlayer film may be destroyed by the stress during bonding, or that the interface cracks or peels off due to the heat cycle caused by ON / OFF of the semiconductor element.

  As a method for preventing this, a method using an Au alloy wire, a method of forming a bonding pad mainly composed of an Al alloy layer containing Cu, and the like are disclosed (for example, see Patent Documents 1 and 2).

JP 2008-270371 A (page 4) JP 2001-7149 A (page 3, FIG. 4)

  However, the method using an Au alloy wire has a problem that the wire becomes too hard and damages the surface of the bonding pad, resulting in an unjoined portion. In the method using a bonding pad mainly composed of an Al alloy layer containing Cu, when the Cu wire is used, the surface of the Cu wire is likely to be oxidized. Therefore, the bonding interface between the Al alloy bonding pad and the Cu wire is used. The problem is that an oxide film (oxide) that cannot be removed is involved and an unjoined part is formed.

  The present invention has been made to solve the above-described problems, and the intent thereof is to obtain a semiconductor device having a bonding pad capable of Cu wire bonding, in which unbonded portions are suppressed. Is.

  The semiconductor device according to the present invention comprises an active element and an electrode mainly composed of aluminum electrically connected to the active element, and the outermost surface of the electrode is covered with a compound layer of aluminum and copper It is.

  According to the present invention, since the outermost surface of the electrode mainly composed of aluminum is previously coated with a compound layer of aluminum and copper, it is possible to suppress the occurrence of an unjoined portion even in bonding using a Cu wire. As a result, heat resistance and heat cycle resistance are improved, and a semiconductor device with high long-term reliability can be obtained.

1 is a schematic diagram of a semiconductor device according to a first embodiment of the present invention. It is a schematic diagram of the junction part of the semiconductor device in Embodiment 1 of this invention.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(Embodiment 1)
FIG. 1 is a schematic diagram of a semiconductor device according to a first embodiment for carrying out the present invention. First, an outline of wire bonding to a semiconductor device in this embodiment will be described.

  In FIG. 1, a semiconductor element 2 having an Al bonding pad 1 is prepared. The semiconductor element 2 includes a semiconductor substrate and an active element (not shown) formed on the semiconductor substrate. This active element is an element constituting, for example, a microcomputer, a power transistor or the like.

  The bonding pad 1 of the semiconductor element 2 is electrically connected to the active element. The bonding pad 1 is obtained by patterning a part of the Al wiring 3 with a resist 4 such as polyimide. Further, a Cu layer of about 100 nm is formed on the surface of the bonding pad 1 by using a sputtering apparatus or the like on the surface of the bonding pad 1. Thereafter, the Cu—Al compound layer 5 is formed on the outermost surface of the bonding pad 1 by heating at 120 ° C. or higher in an environment where oxidation is difficult such as high vacuum or nitrogen, hydrogen, or a mixed gas atmosphere thereof. To do. Here, the heat treatment temperature may be any temperature at which Cu and Al diffuse together, and is preferably 200 ° C. to 450 ° C. from the viewpoint of workability and heat resistant temperature of parts. If Cu remains due to insufficient heat treatment, it is desirable to remove the Cu portion of the outermost layer by reverse sputtering, argon plasma or the like, and expose the Cu—Al compound layer 5 below the surface.

  Thereby, the structure by which the outermost surface of the electrode (Al wiring 3) which has Al as a main component was coat | covered with the Cu-Al compound layer 5 is obtained.

  FIG. 2 is a schematic diagram when a Cu wire is bonded to a bonding pad of a semiconductor device in the present embodiment. In FIG. 2, a ball portion 7 is formed by heating and melting at the joint portion of the Cu wire 6 and is brought into contact with the Cu—Al compound layer 5 formed on the outermost surface of the bonding pad 1. Next, a tool 8 of a wire bonding apparatus is brought into contact with the wire ball portion 7 and a load and ultrasonic vibration are applied to bond the Cu wire 6 and the bonding pad 1 together.

  In the semiconductor device according to the present embodiment, the wire bonding method will be described in more detail below.

As the Cu wire 6, for example, TPCW manufactured by Tanaka Electronics Co., Ltd., having a diameter of 0.25 mm and a Cu purity of 99.99% was prepared. As the semiconductor element 2, a TEG (Test Element Group) chip manufactured by Hitachi Ultra LSI Co., Ltd. was prepared. The specification of the TEG chip was a 10 mm □ × t 0.25 mm chip having a Daisy pattern with an opening of φ70 μm at a pitch of 150 μm, and bonding pads were SiO ₂ / TiN / Al—Si = 150 nm / 150 nm / 1 μm).

  In addition, a Cu frame prepared by applying 5 μm of Ag plating to oxygen-free copper having a thickness of 0.5 mm was prepared. An Sn-3Ag-0.5Cu solder paste was printed on the surface of the Cu frame with a stainless steel mask having a thickness of 12 mm and having a thickness of 0.2 mm, and the semiconductor chip was die-bonded by hot plate heating and then washed with acetone. The sample after the acetone cleaning will be referred to as a semiconductor chip sample.

Next, a stainless steel mask having a thickness of 100 μm having an opening pattern of φ75 μm at a pitch of 150 μm is placed on the semiconductor chip so that the bonding pad and the mask are aligned with each other. A Cu film was formed on the surface of the substrate with an aim of 100 nm. Thereafter, heat treatment was performed at 450 ° C. for 5 hours in a vacuum annealing furnace under the condition of 10 ⁻⁴ Pa. Thereafter, the semiconductor chip bonding pad portion was embedded with resin, and then the cross-sectional polishing was performed. When the cross-sectional analysis of the semiconductor chip bonding pad portion was performed with an electron microscope and EPMA (Electron Probe Micro Analyzer), the Cu film did not remain, All were Cu-Al compounds. When the ratio of Cu to Al in the diffusion layer was identified at five locations by wavelength dispersion type characteristic X-rays, the atomic ratio of Cu to Al was 3: 7 at almost all points. From the system phase diagram, it was estimated that the bonding pad surface was almost entirely the θ phase (Cu ₃ Al ₇ ).

  Next, as a means for bonding the Cu wire to the bonding pad of the semiconductor chip of the semiconductor chip sample, for example, a full auto wire bonder FB-880 manufactured by Kaijo Corporation can be used. The semiconductor chip sample is placed on this flip chip bonder, the Cu wire is set, and a mixed gas of 4% by volume hydrogen and nitrogen is sprayed near the workpiece and the wire at a rate of 5 L / min. At a setting of 100 μm, the ultrasonic power (US) knob and the load (F) knob are aligned so that a good bond is obtained to the extent that ball peeling does not occur when broken with tweezers after bonding. We decided to join it with power and full auto. The bonding temperature at this time was 200 ° C.

  In the semiconductor device manufactured as described above, samples in which the thickness of the Cu film formed by the sputtering apparatus was changed were prepared (Examples 1 to 5). For comparison, a semiconductor device in which no Cu film was formed was also manufactured (Comparative Example 1). In the bonding portion of these semiconductor devices, the shear strength was measured using a shear tester, and the fracture mode after shear fracture was identified with an optical microscope. The shear strength was measured after wire bonding (initial strength), after holding at high temperature (after 200 ° C x 1000 hours), and after heat cycle (after 1500 cycles of -50 ° C and 150 ° C environmental temperatures). It was. In addition, the shear strength was implemented about 10 wires and evaluated by the average strength.

  Table 1 shows the thickness and shear strength of the Cu film in the present embodiment.

  From Table 1, as in Examples 1 to 5 in the present embodiment, Cu-Al compound layer 5 is formed by heat treatment after Cu is deposited to a thickness of 100 to 500 nm on the surface of the bonding pad. No decrease in shear strength was observed even after heat cycling, but the sample with Cu wire bonded to an untreated bonding pad significantly decreased in strength.

  Thus, when the Cu-Al compound layer 5 is formed and wire bonding is performed, it is estimated as follows of the effect that the shear strength does not decrease even after holding at a high temperature or after a heat cycle.

  The diffusion layer generated at the time of wire bonding on the surface of the untreated bonding pad is diffused and expanded in the surface direction of the bonding pad by holding at a high temperature or heat cycle treatment. At this time, since the oxide film and the oxide are left behind in the unjoined portion, a void is formed. This is the starting point of cracks due to the difference in thermal expansion coefficient of each member during heat retention and the heat cycle, or it becomes a promotion path for cracks generated from the unjoined part at the end, and the crack propagates to the joining interface. It is considered that the bonding strength is remarkably reduced.

On the other hand, as in the present embodiment, the outermost surface of the bonding pad coated with the Cu—Al compound layer 5 basically has a θ phase (Cu ₃ Al ₇ ) as the main phase, and is substantially the same compound. It is considered that a layer is formed. Here, the main phase represents a case where the ratio of the phase is 50% by volume or more with respect to the total volume of the Cu—Al compound layer. At this time, some unbonded portions after wire bonding become voids, but are considered to be much smaller and less in comparison with the untreated case, and it is considered that there is almost no decrease in bonding strength.

  From the above, there is an effect that the long-term reliability of the Cu wire bond portion is remarkably improved by covering the surface of the Cu—Al compound layer.

  In addition, before forming a Cu film using a sputtering apparatus, the bonding reliability is further improved by performing a process of removing the oxide film on the surface of the bonding pad 1 by reverse electrode sputtering or argon plasma. Can do.

(Embodiment 2)
In the first embodiment, the outermost surface of the bonding pad is basically a Cu—Al compound layer having a θ phase (Cu ₃ Al ₇ ) as a main phase. However, in the second embodiment, the crystal phase of the compound layer is And the shear strength after holding at high temperature.

As described in the first embodiment, the semiconductor chip bonding pad portion is filled with resin, and then the cross-sectional polishing is performed, and the cross-sectional analysis of the semiconductor chip bonding pad portion is performed with an electron microscope and EPMA, whereby a Cu—Al compound layer is obtained. Can be estimated. From the Al—Cu binary phase diagram, the Al—Cu compound layer is composed of a θ phase (Cu ₃ Al ₇ ), a η2 phase (CuAl), a ξ2 phase (Cu ₅₅ Al ₄₅ ), a δ phase (Cu ₃ Al ₂ ), It is known that γ1 phase (Cu ₁₃ Al ₇ ), α2 phase (Cu ₄ Al ₁ ) and the like are generated. Quantitative analysis can be performed with EPMA, and the product compound can be identified by changing the atomic ratio of Cu and Al.

  In the same manner as in the first embodiment, a Cu film was formed with a target film thickness of 1 μm formed on the surface of the bonding pad. Next, heat treatment is performed for 15 minutes, 30 minutes, 1 hour, 10 hours, 100 hours, and 240 hours in argon gas at 175 ° C., and the film thickness of the Cu—Al compound layer is changed by Al diffusion from the bonding pad. It was. As the heat treatment time was increased, the film thickness of the Cu—Al compound layer was increased. Finally, samples of the present embodiment in which the Cu film remaining on the surface was removed and the Cu—Al compound layer was exposed were produced by reverse sputtering in an argon gas atmosphere (Examples 6 to 11).

  The Cu—Al compound layer exposed on the surface was identified by EPMA, and the thickness of the δ phase, ξ1 phase, η2 phase, and θ phase was examined from the surface side. For this, the same Cu wire as in the first embodiment was wire-bonded under the same conditions, and the compound layer was identified by EPMA at the initial stage and after holding at a high temperature of 200 ° C. × 1000 hours, and the five-point average thickness was determined. It was measured. A similar sample was prepared separately and a wire bond portion shear test was performed.

  Table 2 shows the relationship between the crystal phase of the Cu—Al compound layer and the shear strength at the initial stage and after holding at high temperature in the present embodiment.

  From Table 2, in Examples 6 to 10 in which the γ1 phase is not initially detected, the initial shear strength is 18 gf or more, but in Example 11 in which the γ1 phase is detected, the initial shear strength is slightly reduced. On the other hand, in Examples 8 to 11 in which the γ1 phase was detected after holding at a high temperature, it can be seen that the shear strength decreased according to the thickness of the γ1 phase.

When the Cu film formed on the surface of the Cu—Al compound layer is thin, no compound other than the θ phase is formed even if it is diffused. However, when the film thickness exceeds 1000 nm, the diffusion of Cu proceeds. From the Cu wire to the bonding pad side, the θ phase (Cu ₃ Al ₇ ) to the η 2 phase (CuAl), ξ 2 phase (Cu ₅₅ Al ₄₅ ), δ phase (Cu ₃ Al ₂ ), γ 1 phase (Cu ₁₃ Al ₇ ), Α2 phase (Cu ₄ Al ₁ ), and another phase is newly deposited, and the thickness thereof is considered to increase. Among these, from the θ phase to the δ phase, no significant decrease in strength occurs, but since a significant diffusion void occurs between the γ1 phase and the δ phase, it is considered that the bonding area decreases. For this reason, it is considered that the share strength decreases. Actually, in Examples 8 to 12 after holding at high temperature, a void was observed at this position by electron microscope observation, and the fracture position by the shear test was also near the interface between the γ1 phase and the δ phase.

  The bonding reliability by maintaining these high temperatures is overloaded compared with the operating conditions of the actual semiconductor device. However, in the initial bonding state before bonding, the θ phase is the main phase. Since only the θ phase is detected, and only the θ phase is recognized in the first embodiment even after the heat cycle, the bonding reliability can be obtained by covering the Cu—Al compound layer with the θ phase as the main phase. The sex can be further enhanced.

  In this embodiment, Al wiring is used as an electrode containing aluminum as a main component. However, as electrodes other than pure Al, 99.5Al-0.5Cu, 99.0Al-1.0Cu, 99. The same effect can be obtained even when wiring such as 5Al-0.5Si, 9 · 0Al-1.0Si, 99.0Al-0.5Si-0.5Cu (wt%) is used.

  In this embodiment, pure copper is used as the wire material, but 99.5Cu-0.5Pd, 99.0Cu-1.0Pd, 99.5Cu-0.5Pt, 99.0Cu-1.0Pt, etc. Similar effects can be obtained by using an alloy wire mainly composed of Cu or a wire mainly composed of Cu plated with noble metal such as Pd.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  The present invention can be applied particularly advantageously to a semiconductor device in which an electrode for wire bonding is formed in the vicinity of an active element such as a microcomputer or a power transistor.

  1 Al bonding pad, 2 semiconductor element, 3 Al wiring, 4 resist, 5 Cu-Al compound layer, 6 Cu wire, 7 ball part, 8 tool.

Claims (2)

In a semiconductor device comprising an active element and an electrode mainly composed of aluminum electrically connected to the active element,
A semiconductor device, wherein the outermost surface of the electrode is covered with a compound layer of aluminum and copper.   The semiconductor device according to claim 1, wherein the compound layer of aluminum and copper has a θ phase as a main phase.

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