Patent Application
20190259717
This invention relates to the packaging of a semiconductor die.
The present invention is described with reference to the attached figures, wherein like reference numerals are used throughout the figures to designate similar or equivalent elements. The figures are not drawn to scale and they are provided merely to illustrate the invention. Several aspects of the invention are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the invention. One skilled in the relevant ark, however, will readily recognize that the invention can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring the invention. The present invention is not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with the present invention.
Referring to the drawings,
Wire bonds 100 electrically conned the semiconductor die 20 to the leads 90 of the lead frame. The wire bonds 100 may be any suitable material, such as aluminum, copper, or gold. Molding compound 110 encapsulates the semiconductor die 20, the die attachment medium 80, the wire bonds 100, and portions of the die pad 70 and leads 90 of the lead frame. A surface 120 of the die pad 70 and leads 90 may be left unencapsulated so that heat generated by the semiconductor die can be dissipated and the packaged semiconductor die 10 can be electrically connected to other electrical devices (not shown).
In accordance with the example embodiment, a copper seed layer 130 is applied over the copper leads 30 of the semiconductor die 20 and a nickel alloy 140 is coated over the copper seed layer 130. The copper seed layer 130 facilitates the process of coating the nickel alloy, as explained more fully below. The nickel alloy 140 may act as a barrier against the corrosion of the copper seed layer 130 and the copper leads 30. In addition, the nickel alloy 140 may act as a barrier to the diffusion of copper.
More specifically, the nickel alloy 140 of the example embodiment includes tungsten and cerium. Nickel acts to reduce the interfacial reactions between the aluminum wire bonds 100 and the copper leads 30 of the semiconductor die 20. However, pure nickel causes the interface between the wire bonds 100 and the copper leads 30 to become brittle due to copper corrosion (resulting in reliability issues for the packaged semiconductor die 10). Therefore, cerium is added to the nickel to increase corrosion resistance (by helping to prevent the oxidation of copper leads 30). In addition, pure nickel allows copper to diffuse from the copper leads 30 towards the wire bonds 100. Therefore, tungsten is also added to the nickel alloy coating 140 to help block the copper diffusion paths from the leads 30 of the semiconductor die 20 (due to the segregation of tungsten towards the grain boundaries of nickel). The nickel alloy 140 of the example embodiment may provide cost savings. Specifically, nickel alloy 140 is less expensive than other currently used connector materials, such as alloys containing gold or palladium.
Step 210 is the provision of a fully processed semiconductor wafer 150, as shown in
The semiconductor dies 20 are spaced apart from each other on the semiconductor wafer 150 by zones 160 of unprocessed semiconductor material. These zones 160 of unprocessed semiconductor material are mostly destroyed by a rotating saw blade during the singulation process (as explained below). Therefore, the zones 160 of unprocessed semiconductor material are often called “saw streets” because they form a grid between all of the semiconductor dies 20 on the semiconductor wafer 150 that is largely destroyed by the saw during the dicing process. The semiconductor wafer 150 has a front side 170 that includes the leads 30 of the semiconductor die 20. In addition, the semiconductor wafer 150 has a back side 180 that is opposite to the front side 40.
Step 220 is the removal of the protective overcoat 50 over the copper leads 30 of the semiconductor dies. More specifically, a patterned photoresist is applied over the front surface 170 of the semiconductor wafer 150 and then an etchant is used to remove the protective overcoat 50 over the copper leads 30, as shown in
As shown in
Next, in step 240, the copper seed layer 130 is coated with a nickel alloy 140. It is to be noted that the copper seed layer 130 and the coating of nickel alloy 140 may not be conformal over the etched protective overcoat 50, as shown in
The nickel alloy 140 in the example application is NI-10 wt % W-0.5 wt % Ce. Therefore, the nickel alloy 140 is a mixture of 89.5% nickel, 10.0% tungsten, and 0.5% cerium, However, the amount of tungsten may range from 0.5% to 35.0%. Furthermore, the amount of cerium may range from 0.05% to 0.5%.
As shown in
The semiconductor wafer 150 is singulated in step 260, as shown in
A shown in
In step 280 a wire bonding process is used to form wire bonds 100 between the leads 30 of the semiconductor die 20 and the leads 90 of the lead frame, as shown in
Additional modifications to the invention as described above are within the scope of the claimed invention. As an example, it may be desirable to clean the semiconductor wafer 150 after the copper leads 30 have been exposed in the etching step 220 in order to remove any copper oxide that may have formed before the sputtering step. Similarly, it may be desirable to clean the semiconductor wafer 150 following the CMP step 250 to remove any unwanted debris created by the polishing operation.
While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Numerous changes to the disclosed embodiments can be made in accordance with the disclosure herein without departing from the spirit or scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above described embodiments. Rather, the scope of the invention should be defined in accordance with the following claims and their equivalents.
