# Patent

## Title
High bond line thickness for semiconductor devices

## Abstract

Die attach methods used in making semiconductor devices and the semiconductor devices resulting from those methods are described. The methods include providing a leadframe with a die attach pad, using boundary features to define a perimeter on the die pad, depositing a conductive material (such as solder) within the perimeter, and then bonding a die containing an integrated circuit to the die pad by using the conductive material. The boundary features allow an increased thickness of conductive material to be used, resulting in an increased bond line thickness and increasing the durability and performance of the resulting semiconductor device.

## Background
FIELD
This application relates generally to semiconductor devices and methods of making semiconductor devices. In particular, this application relates to die attach methods used in making semiconductor devices and the die packages and semiconductor devices resulting from those methods.
BACKGROUND
Often, during manufacture of semiconductor devices, one or more dies containing the integrated circuit may be bonded to a die pad (or paddle) of a leadframe. The process of bonding the die to the leadframe is usually referred to as a die attach process. The die attach process may be accomplished using an electrically conductive material, such as an adhesive or solder, which connects the die to the leadframe, both mechanically and electrically. The thickness of this conductive material is often referred to as the bond line thickness (BLT).
In the die attach process, the conductive material must allow the bonding to occur between the die and the leadframe while minimizing the formation of voids in the bond. Also, the die attach process must also provide a consistent bond strength across the surface of the die, thereby minimizing localized stresses that may cause fracture or other failure of the semiconductor device. Any voids and inconsistent bond strength in the bond increases the stress and strain on the die, which may lead to cracks and failures in the semiconductor device. In addition, voids may result in inefficient or ineffectual electrical or heat conductivity, potentially causing failures in the semiconductor device. The conductive material, therefore, should have a viscosity sufficiently low to allow for effective bonding by avoiding both of these problems.
FIGS. 1 and 2 illustrate an exemplary die 110 that is attached to die pad 120 by conductive material 130 to form die attach package 100. As shown in FIG. 1, t1 is the BLT between the die 110 and the die pad 120. Increasing the BLT by increasing the thickness of the conductive material 130 decreases the shear stress on the die, which makes larger thicknesses more desirable. However, the low viscosity required to assure no voids usually limits the thickness to less than 3 mils. But increasing the amounts of conductive material used during die attach in effort to increase BLT can result in flow of the conductive material to other portions of the leadframe or the die, potentially causing moisture path, short circuits and problems in wire bonding and often resulting in failure of the semiconductor device.
To avoid these problems, some die attach processes use a “spanker” to flatten the conductive material during the die attach process. However, using the spanker involves extra steps that make the device fabrication process longer, less productive, and more expensive. Additionally, if too much conductive material is used to attempt to achieve a high BLT, the conductive material may be displaced by the spanker from the die pad to other portions of the leadframe, potentially causing short-circuits and other problems.
SUMMARY
This application describes die attach methods used in making semiconductor devices and the die packages and semiconductor devices resulting from those methods. The methods include providing a leadframe with a die attach pad, using boundary features to define a perimeter on the die pad, depositing a conductive material (such as solder) within the perimeter, and then attaching a die containing an integrated circuit to the die pad by using the conductive material. The boundary features allow an increased thickness of conductive material to be used, resulting in increased bond line thickness and increasing the durability and performance of the resulting semiconductor device.



BRIEF DESCRIPTION OF THE DRAWINGS
The following description can be better understood in light of Figures, in which:
FIG. 1 illustrates a perspective view of a known die package with a die bonded to a die attach pad of a leadframe;
FIG. 2 illustrates another view of the die package shown in FIG. 1;
FIG. 3 illustrates a perspective view of an exemplary die package containing boundary features;
FIG. 4 illustrates a cross-sectional view of the die package shown in FIG. 3; and
FIGS. 5 and 7 illustrate various structures that are formed in a die attach process that uses boundary features for containing a conductive material and
FIGS. 6A, 6B, and 6C illustrate boundary features containing wire bonds that have been attached to the die pad.



The Figures illustrate specific aspects of the semiconductor devices and associated methods of making and using such devices. Together with the following description, the Figures demonstrate and explain the principles of the semiconductor devices and associated methods. In the drawings, the thickness of layers and regions are exaggerated for clarity. It will also be understood that when a layer is referred to as being “on” another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present. The same reference numerals in different drawings represent the same element, and thus their descriptions will not be repeated.
DETAILED DESCRIPTION
The following description supplies specific details in order to provide a thorough understanding. Nevertheless, the skilled artisan would understand that the apparatus and associated methods of using the apparatus can be implemented and used without employing these specific details. Indeed, the devices and associated methods can be placed into practice by modifying the illustrated devices and associated methods and can be used in conjunction with any other apparatus and techniques conventionally used in the industry. For example, while the description below focuses on die attach processes for semiconductor devices, the device and associated processes could be equally applied to any process of device where a die is connected to a substrate, such as a printed circuit board, MEMS devices, and the like.
One exemplary semiconductor device formed using the methods describe herein is illustrated in FIGS. 3 and 4. In FIG. 3, the semiconductor device comprises a die package 200 that contains a die 210 bonded to a die pad 220 by conductive material 230. Boundary features 240 form a perimeter on die pad 220 around die 210. As shown in FIG. 4, a conductive material 230 may be located between die 210 and die pad 220 with a bond line thickness (BLT) represented by t2.
The die 210 may comprise any type of semiconductor die known in the art. In some embodiments, the die comprises a silicon-based substrate with an integrated circuit formed therein. But the die may also be made of GaAs, SiC, GaN, or any other suitable semiconductor material. The substrate and the integrated circuit may have any configuration desired and needed to perform any desired function. For example, the die 210 may include one or more discrete transistors, diodes, integrated circuits, or other suitable semiconductor device. Thus, the die 210 may be designed to perform any number of functions, such as power regulation, memory, processing, or any other integrated circuit (IC) function. The die 210 may have any size needed for these functions. In some embodiments, the size of the die can range, for example, from about 100 μm by about 100 μm to about 20000 μm by about 20000 μm.
The die pad 220 may be a portion of any leadframe known in the art or may be a separate paddle. As well, the die pad 220 may be a single die pad of a leadframe, may be one of a plurality of die pads on a leadframe, or plurality of connected leadframes used in semiconductor manufacturing. When a leadframe is used, it is formed so that it is relatively planar in the area of the die pad 220. The leadframe serves as part of the I/O interconnection system, and also provides a thermally conductive path for dissipating the majority of the heat generated by the die.
The material of the leadframe may comprise any metal, such as copper or a copper alloy. In some instances, the leadframe can contain a layer of metal plating (not shown), if desired. The layer of metal plating may comprise an adhesion sublayer, a conductive sublayer, and/or an oxidation resistant layer. For example, the leadframe may include a leadframe plating containing an adhesion sublayer and a wettable/protective sublayer.
The die 210 and the die pad 220 are attached to each other by a conductive material (that forms a layer) 230. The conductive material 230 may be any conductive material that can attach these two components to each other. In some embodiments, the conductive material 230 comprises a solder that is configured to be used in a die bonding process. For example, the conductive material 230 may be a Pb—Sn, Au—Sn, or other solder. Other solders that may be used as conductive material 230 may be made of Sn, Ag, and/or Pb—Sn—Ag. In some embodiments, the conductive material 230 may be an adhesive configured to be used in a die bonding process. For example, an adhesive conductive material 230 may be a non-conductive or a conductive epoxy material like silver epoxy.
As shown in FIG. 4, the die package 200 contains boundary features 240 that form a perimeter on die pad 220 around the area containing the conductive material 230. The boundary features 240 function to allow an increased amount of conductive material 230 to be used in the die bonding process, resulting in a higher BLT. In other words, the boundary features 240 comprise a raised surface of a sufficient height to make a boundary for the conductive material 230.
FIG. 4 illustrates one example of the BLT that can be obtained and is represented by t2. In some embodiments, t2 may range up to 30 mils. In other embodiments, this thickness can range from about 4 to about 30 mils.
The size of the perimeter defined by boundary features 240 depend on the size of the die and, therefore, the type of semiconductor device being made. In some embodiments, the perimeter can measure from about 100 μm by about 100 μm to about 20000 μm by about 20000 μm. And while the perimeter is illustrated as being substantially rectangular in shape, the shape will also depend on the shape of the die 210 and can therefore be substantially square, circular, triangular, or polygonal.
The boundary features 240 may be a complete or partial boundary. In some embodiments, and as shown in FIGS. 3 and 4, the boundary features may be applied to form a complete perimeter on die pad 220. In other embodiments, the boundary features 240 may be applied to form a partial perimeter, such that about 75% or even 50% or more of the perimeter is defined by boundary features 240.
The boundary features 240 may be made of any material suitable that can create an elevated perimeter or raised surface around the desired location. For example, the boundary features 240 may be formed as an integral part of die pad 220, or may be deposited onto die pad 220 using any known deposition, mask, and etching processes. In another example, any known selective deposition process can be used to form the boundary feature 240.
These structures described above may be formed using any known process that will form the structures illustrate above. In some embodiments, the die 210 is manufactured by providing the various electronic components (i.e., the transistors) in a semiconductor substrate as known in the art. In other embodiments, the integrated circuits are manufactured, cut, tested, and die-bonded to a substrate as known in the art.
Next, a leadframe is formed by any known method, for example, by any metal stamping and etching processes. If desired, a layer of metal plating may be formed on the base metal used in the leadframe by processes such as electroless plating, sputtering, or electroplating. A pre-plated leadframe can also be used instead. The leadframe is made with the die pad 220 formed as part of the leadframe.
Then, as shown in FIG. 5, boundary features 240 are applied on the die pad 220 to create a perimeter. If the boundary features 240 are not already formed as part of the die pad, they can be manufactured by taking a wire of the desired material (i.e., aluminum) and forming it into the desired shape (i.e., the rectangular shape shown in FIG. 5). The shaped wire is then placed in any desired location of the die pad.
Next, the wire is attached to the die pad 220 using any known technique. In some embodiments, the boundary features 240 may be attached using a wire bonding technique that wraps a stitch around the wire and the die pad. The boundary features 240 may be connected (or stitched) to the die paid using any number of connection points. In some embodiments, and as shown in FIG. 6A, the boundary feature is connected at multiple points along the perimeter using a thick stitch having multiple attach points along the perimeter, thereby forming an almost continuous stitch. For example, one side of the perimeter as illustrated may have between about 4 and 40 stitches.
Another type of connection is depicted in FIG. 6B. In these embodiments, the boundary features 240 may be connected at along the perimeter using only discrete attachment points along the perimeter to make a sparse stitch having only a few connection points. For example, a sparse stitch perimeter of boundary features may be connected to die pad 220 with one connection on each corner to as many as four stitches on each side of the boundary feature (when it is formed as a rectangle).
Next, as shown in FIG. 7, the conductive material 230 is deposited onto die pad 220 within the perimeter defined by boundary features 240. The conductive material can be deposited using any known process, such as chemical vapor deposition or a reflow process, until the desired height is reached that will provide the desired BLT. In some embodiments, a spanker may be used to flatten conductive material 230. In other embodiments, though, no spanker is used due to the possibility of conductive material 230 being displaced or splashed out of the perimeter defined by boundary features 240.
Then, the die 210 is then placed onto the conductive material 230 using any known process in the art. The resulting structure is then heated at a sufficient time and temperature that will re-flow the conductive material 230 without changing the boundary feature(s) 240. During the re-flow process, the conductive material is forced to stay with the perimeter established by the boundary feature. After the reflow process is complete, the die 210 is attached to the die pad 220 by the reflowed conductive material 230 that has the desired height, yet has substantially no voids.
Once the die packages have been formed in this manner, further processing can be performed to make a semiconductor device. For example, electrical connections may be established between portions of integrated circuit on the die and portions of the lead fingers using wires, usually with a wire bonding process. After the wire bonding process, a resin body is formed to encapsulate the die and the wirebonds. The resulting structure may then be singulated (and optionally tested) to create a semiconductor package with leads protruding from the resin body. The lead fingers can then be connected to another electrical device, such as a printed circuit board (or PCB) so that it is connected electrically to the integrated circuit of the die.
The die packages 200 described above have several advantages. First, a higher BLT allows for more robust die packages 200, limiting the mechanical failure of die 210 due to cracking and due to voids in conductive material 230. The higher BLT also can result in increased thermal performance, limiting the failure rate of die 200.
In addition to any previously indicated modification, numerous other variations and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of this description, and appended claims are intended to cover such modifications and arrangements. Thus, while the information has been described above with particularity and detail in connection with what is presently deemed to be the most practical and preferred aspects, it will be apparent to those of ordinary skill in the art that numerous modifications, including, but not limited to, form, function, manner of operation and use may be made without departing from the principles and concepts set forth herein. Also, as used herein, examples are meant to be illustrative only and should not be construed to be limiting in any manner.

## Claims

1. A die package for a semiconductor device, the die package comprising:
a leadframe containing a die attach pad;
a conductive layer on the die attach pad, wherein the conductive layer has a thickness up to about 30 mil and a portion of a perimeter of the conductive layer contains a boundary feature defining the perimeter, the boundary feature comprising one or more wires contacting the die attach pad, wherein both ends of each of the one or more wires contact the die attach pad of the leadframe; and
a die on the conductive layer.

2. The die package of claim 1, wherein the boundary feature completely surrounds the conductive layer.

3. The die package of claim 1, wherein the boundary feature extends for at least half of the periphery of the conductive layer.

4. The die package of claim 1, wherein the conductive layer has a thickness ranging from about 4 mil to about 30 mil.

5. The die package of claim 1, wherein the conductive layer is substantially free of voids between the die and the die pad.

6. The die package of claim 1, wherein each of the one or more wires has been stitched to the die attach pad.

7. The die package of claim 1, wherein each of the one or more wires has multiple, discrete connection points to the die attach pad.

8. A semiconductor device, comprising:
a leadframe containing a die attach pad;
a conductive layer on the die attach pad, wherein the conductive layer has a thickness up to about 30 mil and a portion of a perimeter of the conductive layer contains a boundary feature defining the perimeter, the boundary feature comprising one or more wires contacting the die attach pad, wherein both ends of each of the one or more wires contact the die attach pad of the leadframe; and
a die on the conductive layer.

9. The semiconductor device of claim 8, wherein the boundary feature completely surrounds the conductive layer.

10. The semiconductor device of claim 8, wherein the boundary feature extends for at least half of the periphery of the conductive layer.

11. The semiconductor device of claim 8, wherein the conductive layer has a thickness ranging from about 4mil to about 30mil.

12. The semiconductor device of claim 8, wherein the conductive layer is substantially free of voids between the die and the die pad.

13. The semiconductor device of claim 8, wherein each of the one or more wires has been stitched directly to the die attach pad.

14. The semiconductor device of claim 8, wherein each of the one or more wires has multiple, discrete connection points to the die attach pad.