DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] A description will now be given, with reference to the drawings, of embodiments of the present invention.

[0029] FIG. 3 is a plan view of a lead frame 10 used in a semiconductor device according to an embodiment of the present invention. FIG. 4 is an enlarged cross-sectional view of a die stage 11 of the lead frame 10 shown in FIG. 3 . The lead frame 10 used in the semiconductor device according to the present invention is formed, similar to the conventional lead frame, by processing and patterning a copper alloy plate as a base. Generally, the copper alloy for lead frames contains a very small amount of zinc (Zn), lead (Pb), chromium (Cr), etc. as additive elements. The patterning of the copper alloy plate is performed by a known technique such as stamping and etching, as usual. After patterning the copper alloy plate so as to form the configuration of the lead frame 10 , silver (Ag) plating is applied to the end of each of inner leads 12 . Thus far, the same process is performed as the conventional lead frame.

[0030] Although an organic discoloration preventing agent may be applied after the silver plating in the conventional lead frame, the lead frame 10 according to the present invention omits the discoloration preventing agent on the surface, and a thin layer of copper oxide is formed on the surface of the lead frame 10 . That is, the conventional lead frame is completed after applying a discoloration preventing agent onto the surface of the copper alloy plate, while in the lead frame 10 according to the present invention, the copper alloy, which is a base material, is oxidized by a special method mentioned later so as to form a copper oxide layer 14 as an outermost layer on the surface of the lead frame 10 (refer to FIG. 4 ).

[0031] As mentioned above, the lead frame 10 is completed after the copper oxide layer 14 is formed on the surface of the lead frame 10 , especially the surface of a die stage 11 . Then, the lead frame 10 is used for manufacture of a semiconductor device.

[0032] Fundamental composition of the semiconductor device according to the embodiment of the present invention is the same as the semiconductor device shown in FIG. 1 except for the copper oxide layer 14 being formed in the lead frame 10 . FIG. 5 is a plan view of the lead frame 10 showing a state where wire-bonding is performed after mounting the semiconductor chip 2 on the die stage 11 . FIG. 6 is an enlarged side view showing the semiconductor chip 2 mounted on the die stage 11 .

[0033] In the production process of the semiconductor device, the semiconductor chip 2 is first mounted onto the die stage 11 of the lead frame 10 with a die-bonding material 5 provided therebetween. Then, electrodes of the semiconductor chip 2 and the silver-plated parts of the inner leads 12 are connected by bonding wires 6 . Thereafter, the die stage 11 , the semiconductor chip 2 , the bonding wires 6 and the inner leads 12 are encapsulated by a seal resin 8 .

[0034] The semiconductor device according to the present embodiment uses the lead frame 10 in which the copper oxide layer 14 is formed on the surface of the base material. For this reason, even if the lead frame 10 is heated in a wire-bonding process, the copper in the base material of the lead frame 10 is not oxidized thermally. Therefore, there is no brittle layer, which may be formed between the copper alloy as the base material and the copper oxide layer due to condensation of the additive elements during the thermal oxidation process, thereby preventing package selling or cracking caused by the brittle layer.

[0035] A description will now be given, with reference to FIGS. 7A and 7B , of a formation process of the copper oxide layer 14 according to the present embodiment. FIGS. 7A and 7B are illustrations for explaining a process of forming the copper oxide layer 14 in comparison with a case where the copper oxide layer 14 is not formed. FIG. 7A shows the change of state near, the surface of the lead frame in the case where the copper oxide layer is not formed, and FIG. 7B shows the change of state near the surface of the lead frame in the case where the copper oxide layer 14 according to the present embodiment is formed.

[0036] First, a base material 21 , which constitutes a copper alloy plate, is patternized so as to from the plate into the configuration of the lead frame. In this state, as shown in FIG. 7 A-(a) and FIG. 7 B-(a), the copper alloy of the base material 21 is exposed on the surface of the lead frame.

[0037] Next, an oxidation treatment is applied to the lead frame 10 according to the present embodiment as shown in FIG. 7 B-(b). This treatment is not applied in the case of FIG. 7A where the copper oxide layer is not formed. The oxidation treatment is performed by immersing the lead frame 10 into a solution of a strong oxidizer. Consequently, the copper in the base material 21 is oxidized by the strong oxidizer, and the copper oxide layer 14 is formed. Although the copper oxide layer 14 mainly consists of cupric (Cu ₂ O), the copper oxide layer 14 also contains cuprous (CuO).

[0038] In the oxidization by the solution of a strong oxidizer, there is no separation of additive elements between the copper oxide layer 14 and the copper alloy of the base 21 , and, thus, the brittle layer due to condensation of the additive elements is not formed. Additionally, even if the discoloration preventing agent is applied on the surface of the base material, a component of the discoloration preventing agent is not contained in the base material since the component is dissolved into the solution of the strong oxidizer, and, thus, a brittle layer is not formed.

[0039] Here, there is a so-called blackening treatment as a process for forming a copper oxide layer by immersing a copper alloy into a solution of a strong oxidizer. The blackening treatment is a process for forming a needle crystal layer of cuprous (CuO) on the surface of a copper alloy, and is called as a blackening treatment since the color of the needle crystal layer of cuprous (CuO) is black. Generally, the blackening treatment is a process to improve adhesion between a seal resin and a lead frame by configuring the surface of the lead frame into needle-like shape.

[0040] The solution of the strong oxidizer used for the blackening treatment is, for example, a mixed solution of sodium chlorite, sodium hydroxide and potassium peroxydisulfate. The needle crystal layer of cuprous (CuO) is formed by immersing the copper alloy into such a mixed solution for 3-10 minutes at a temperature of around 100° C.

[0041] Although the copper oxide layer 14 formed on the base material 21 in the present embodiment can also be formed using the mixed solution of the strong oxidizer for the above-mentioned blackening treatment, the copper oxide layer 14 is not a needle crystal layer. Namely, in the conventional blackening treatment, chemical reactions are continued until a copper oxide layer of the surface becomes a needle crystal layer of cuprous (CuO). On the other hand, the copper oxide layer 14 according to the present embodiment mainly consists of cupric (Cu2O), which is formed by taking the lead frame out of the mixed solution of a strong oxidizer prior to the formation of cuprous (CuO) which turns to the needle crystal layer.

[0042] Therefore, the time of oxidation treatment according to the present embodiment must be remarkably shorter than the time required by the conventional blackening treatment. Additionally, although the outermost layer of the lead frame, which has been subjected to the blackening treatment, is the needle crystal layer of cuprous (CuO), the lead frame 10 according to the present embodiment has the copper oxide layer 14 as the outermost layer which is not the needle crystal layer. Further, the thickness of the copper oxide layer 14 according to the present embodiment is remarkably smaller than the thickness of the needle crystal layer formed by the blackening treatment, and is sufficient in the order of about 10 to 1000 Å.

[0043] As mentioned above, since the copper oxide layer 14 according to the present embodiment can be formed by merely immersing the lead frame into a solution of a strong oxidizer for a very short time, the copper oxide layer 14 can be easily formed without increasing the manufacturing cost of the lead frame. Moreover, the copper oxide layer 14 can be very thin, and can be formed as a stable cupric (Cu ₂ O) layer.

[0044] Next, when the semiconductor device is formed using the lead frame, the lead frame is heated in a wire-bonding process. At this time, as shown in FIG. 7 A-(c), a copper oxide layer 22 is formed as shown in FIG. 7 A-(c) due to thermal oxidation of copper of the exposed base material 21 in the case where the copper oxide layer 14 is not formed as shown in FIG. 7 . On the other hand, in the case where the copper oxide layer 14 is formed in the above-mentioned oxidation treatment process, another copper oxide layer is not newly formed since the surface of the base material 21 is already covered by the copper oxide layer 14 .

[0045] Here, in the case of FIG. 7A where the copper of the exposed base material 21 is oxidized thermally and the copper oxide layer 22 is formed, the additive elements in the base material 21 separate and condense between the copper oxide layer 22 and the base material 21 , thereby forming a condensation layer 23 . This condensation layer 23 corresponds to the above-mentioned brittle layer. On the other hand, in the case shown in FIG. 7A where the copper oxide layer 14 is formed, a copper oxide layer due to thermal oxidation is not formed, and, thus, the condensation layer 23 is not formed.

[0046] After the wire-bonding process, the semiconductor chip 2 is encapsulated by the seal resin 8 . The semiconductor chip 2 is mounted and fixed onto the die stage 11 of the lead frame 10 , and the die stage 11 is also encapsulated together with seal resin 8 . Therefore, in the process of FIG. 7 A, the copper oxide layer 22 is covered by the seal resin 8 as shown in FIG. 7 A-(d). On the other hand, in the process of FIG. 7 B, the copper oxide layer 14 , which is forcibly formed by the oxidation process is covered by the seal resin 8 as shown in FIG. 7 B-(d).

[0047] The formation of the semiconductor device is completed after the resin encapsulation is completed. At this time, the semiconductor device functions normally both in the case of FIG. 7 A and the case of FIG. 7B . Accordingly, the semiconductor device is stored until it is used. During the time period of storage, the seal resin of the semiconductor device may absorb moisture from a surrounding atmosphere.

[0048] Then, when a product is manufactured using the semiconductor device, the semiconductor device is mounted onto a mounting substrate etc. In many cases, solder mounting is used for mounting the semiconductor device. Especially, a lead terminal type semiconductor device is mounted by soldering the outer leads to the electrode pads of the mounting substrate. In such a mounting process, the semiconductor device is subjected to the heat of the solder reflow. Since a lead-free solder has a high-melting point, the heating temperature reaches about 230-240° C.

[0049] When the semiconductor device is heated at such a temperature, a thermal stress generated in the semiconductor device (seal resin) is increased, which may cause a small crack formed in the brittle condensation layer 23 . If the moisture, which the seal resin absorbed, enters such a crack and turns into steam exfoliation may occur in the condensation layer 23 , as shown in FIG. 7 A-(e), and a problem arises in that the seal resin is swollen or broken.

[0050] On the other hand, in the case of FIG. 7B where the semiconductor device is provided with the copper oxide layer 14 so as to prevent the formation of the condensation layer 23 , there is no crack or breakage occurs near the boundary between the lead frame 10 and the seal resin 8 since there is no brittle layer, and, thus, there is no problem such as package swelling or cracking.

[0051] The inventors produced the base material 21 having the copper oxide layer 14 according to the present embodiment (in the case of FIG. 7B ) and also produced the base material 21 having the copper oxide layer 22 which is formed by thermal oxidation (in the case of FIG. 7A ), and performed tape pealing tests on both the copper oxide layers 14 and 22 . The lead frames were placed on a heater block heated at 250° C. for 3 minutes, and then the copper oxide layers 14 and 22 are attached to tapes and pealed from the read frames. As a result, the copper oxide film 22 , which is a thermal oxidation film, is exfoliated from the base material 21 in all of five test pieces. On the other hand, exfoliation did not occur in the copper oxide layer 14 according to the present embodiment in all of five test pieces. Therefore, it was proved that the copper oxide layer 14 according to the present embodiment is joined to the base material 21 of the lead frame more firmly than the copper oxide layer 22 formed by thermal oxidation.

[0052] As mentioned above, by using the lead frame 10 in which the copper oxide layer 14 according to the present embodiment is formed, package swelling or cracking due to heating of the semiconductor device in a mounting process can be prevented. Especially, even when a solder reflow is performed at 230-240° C. as in a mounting process using a lead-free solder, the semiconductor device can be prevented from being swollen or cracked.

[0053] The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.

[0054] The present application is based on Japanese priority application No. 2002-166898 filed Jun. 7, 2002, the entire contents of which are hereby incorporated by reference.