The present invention relates to methods for attaching lead fingers to semiconductor dice and the structures formed thereby. More particularly, the present invention relates to methods for attaching lead fingers to semiconductor dice by using adhesive materials, and the structures formed from these methods.
The semiconductor industry has several goals for integrated circuit (IC) design and fabrication, such as increased performance, lower cost, increased miniaturization, and greater packaging density. One method of lowering the cost of designing and fabricating ICs is to reduce the amount of materials used, or use the materials more efficiently. One part of IC design and fabrication which inefficiently uses materials is processes for attaching or bonding semiconductor dice to a package or other connections such as lead frames. These processes include wirebonding, tape-automated-bonding connections, xe2x80x9clead-over-chipxe2x80x9d (xe2x80x9cLOCxe2x80x9d) connections, and a polymer dip chip used with a bumped die method. Another such process includes the process described in U.S. Pat. No. 5,585,282, the disclosure of which is incorporated herein by reference.
There are several methods for attaching a lead frame to a semiconductor die. One such method is the xe2x80x9clead-over-chipxe2x80x9d (xe2x80x9cLOCxe2x80x9d) method, such as those disclosed in U.S. Pat. Nos. 5,302,849, 5,548,160, and 5,286,679 (xe2x80x9cthe ""679 patentxe2x80x9d), the disclosures of which are incorporated herein by reference. The ""679 patent discloses an LOC method which employs an adhesive material. The method described in the ""679 patent applies a thermoplastic or thermoset adhesive to a semiconductor wafer. The adhesive layer is patterned during application to cover desired portions of the wafer by hot or cold screen/stencil printing or a dispensing process. After baking the adhesive layer on the semiconductor wafer to stabilize it, the individual dice are cut from the semiconductor wafer. During packaging, each adhesive-coated die is attached to the fingers of a lead frame by heating and pressing the fingers to the die. If the adhesive contains a thermoset material, a separate cure is then performed if the attach process does not or cannot perform the cure. The method of the ""679 patent unfortunately attaches the lead frame to the die inefficiently, i.e., it fails to achieve an adhesive material profile with enough upper surface area to efficiently attach the lead fingers.
In another LOC method, an adhesive tape (preferably insulative tape) is attached to an active surface of a semiconductor die and then lead fingers are attached to the adhesive tape. Although this method effectively attaches the fingers of the lead frame to the die, it is more expensive than using an adhesive material because the adhesive-coated tape costs more than the adhesive material. This adhesive tape method is also more expensive because of the fabrication steps required to cut individual tape segments from a larger sheet and material wasted when the tape segments are cut out. Further, the tape segments are often placed on a carrier film for transport to the die-attach site, raising both the cost and the complexity of the attachment process even more.
In yet another LOC method, an adhesive material is placed on the fingers of the lead frame rather than the semiconductor die. See, for example, co-pending U.S. Application Ser. Nos. 08/906,673, now U.S. Pat. No. 6,013,535 issued Jan. 11, 2000, U.S. Pat Nos. 08/906,578, 08/709,182, now U.S. Pat. No. 6,083,768 issued Jul. 4, 2000, U.S. Pat. No. 09/020,197, now U.S. Pat. No. 6,040,205 issued Mar. 21, 2000, and U.S. Pat. No. 08/916,931, the disclosures of which are incorporated herein by reference. In one method, a liquid adhesive material is sprayed on the inverted attachment surfaces of the lead fingers. Some adhesive materials, however, may flow down the sides of the lead fingers and collect on the reverse surfaces of the lead fingers (e.g., the surfaces to which the bond wires will be attached). The adhesive material subsequently cures on these bond wire surfaces and can interfere with subsequent wire bonding, resulting in failure of the semiconductor component. The tendency for adhesives to flow from the lead finger attachment surfaces to the bond wire surfaces increases if the lead fingers are formed by a stamping processxe2x80x94which leaves a slight curvature, or rounding, of the edges of the lead fingersxe2x80x94rather than by an etching process. When this edge curvature is proximate the lead finger attachment surface, there is less resistance to the flow of the adhesive material and more adhesive material placed on the attachment surface consequently flows to the bond wire surface.
Finally, present methods of applying adhesive materials to a surface (whether of the semiconductor die or the lead finger) tend to waste the adhesive material. Spray application wastes adhesive material because not all of the sprayed adhesive material attaches to the target surface. Patterning the adhesive material on the semiconductor die results in substantial areas of the adhesive material of the pattern not being utilized. To obtain the combination of precise application with maximum material usage often makes the attachment process complex and difficult.
The present invention provides methods for attaching lead fingers to semiconductor dice and the structures formed thereby. In particular, the present invention provides methods, and the resulting structures, for attaching a semiconductor connection component (e.g., a lead frame) to bond pads of a semiconductor die by applying an adhesive material to a downset portion thereof. The downset portion of such a component is a first portion, which is horizontally offset from a second portion of the component. By applying the material only to the downset portion of the lead finger, which will then be attached to the die, the adhesive material is precisely applied in a simple process and, therefore, little adhesive material is wasted. The adhesive material and the downset portion provide a one-step electrical/mechanical connection to bond pads of the die, thereby eliminating wire or other separate electrical connections.
The methods of the present invention are practiced by providing a flowable adhesive material, providing a semiconductor connection component having a first portion horizontally offset from a second portion, and contacting the first portion of the semiconductor connection component with the adhesive material so a portion of the adhesive material attaches to the first portion. The semiconductor connection component may be a lead frame element having a lead finger. The adhesive material is an electrically-conductive material, such as an isotropic or anisotropically-conductive material. The semiconductor connection component with the adhesive material attached to the first portion may then be contacted with a surface of a semiconductor die to attach the semiconductor connection component to the die.
The methods of the present invention are also practiced by providing a flowable, electrically-conductive adhesive material and then contacting a portion of a bottom surface of the semiconductor connection component with the adhesive material so a portion of the adhesive material attaches to the bottom surface of the semiconductor connection component. The bottom surface of the semiconductor connection component may be a first portion horizontally offset from a second portion of the component. The semiconductor connection component may be a lead frame element having a lead finger. The electrically-conductive adhesive material may be an isotropically or anisotropically-conductive material, such as a Z-axis anisotropically-conductive material. The semiconductor connection component with the adhesive material attached to the bottom surface thereof may then be contacted with a surface of a semiconductor die to attach the semiconductor connection component to the die.