Active solid-state devices (e.g. – transistors – solid-state diode – Integrated circuit structure with electrically isolated...
Reexamination Certificate
2000-07-25
2003-04-15
Cuneo, Kamand (Department: 2829)
Active solid-state devices (e.g., transistors, solid-state diode
Integrated circuit structure with electrically isolated...
C257S048000
Reexamination Certificate
active
06548881
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to reliability and packaging of electronic devices, particularly integrated circuit devices. More particularly, the present invention relates to a methodology for verifying the reliability of the interface between bond pads and other structures in integrated circuits. The present invention further resolves potential mismatches which can occur between revisions of masks used to form an electronic device and corresponding revisions of the test program used for evaluating the device.
2. Discussion of the Related Art
Integrated circuit devices manufactured on silicon wafers are typically separated into individual IC chips and assembled into packages. One of the principal functions of the package is to allow connection of the chip to a circuit board or other electronic product. Such connection can generally not be made directly from the chip to the target product due to the thin, fragile microscopic metal structure used to interconnect the several components on the chip surface. Many metal leads on the IC are typically about 0.6 &mgr;m thick and less than 1.0 &mgr;m wide. Indeed, many of the surface features of current production integrated circuits are “sub-micron” or less than 1.0 &mgr;m in width. Therefore, “bond pads” approximately 100 uM square are typically placed around the periphery of the IC chip. “Bond wires” are then used to connect the IC chip to the packaging frame. This frame is then usually “encapsulated” with either plastic or ceramic materials to complete the packaging process.
A common IC lead and bond pad material is aluminum that is deposited and patterned during chip fabrication. The gold or aluminum bond wire used to connect the bond pad to the package frame is currently available is typically in the range of 17 to 30 &mgr;m in diameter, many times larger than the integrated circuit's surface wiring. Bond wires are typically connected to the bond pads by means of metal balls (gold) or wedge bonds (aluminum) formed at the end of the bond wires and applied to the bond pad. Bond wires may be attached by thermosonic bonding, or other wire attachment methodology well known to those of ordinary skill in the art.
The first problem which occurs in some integrated circuit devices is “cratering” in the layers under the bond pads. Cratering is generally a fracture of the silicon and dielectric oxide layers under the bond pad. This phenomenon is sometimes referred to as “bond pad cratering”. While studies to determine exact mechanisms for crater initiation and propagation are still underway, an overview of some of the known mechanics of crater formation is discussed as follows.
One process which has been shown to be contributory to crater initiation is the use of thermosonic attachment methodology for attaching bond wires to bond pads. Thermosonic bonding employs ultrasonic vibration, typically about 60-120 kHz, to form the bond This dynamic is shown in FIG.
1
. It illustrates a cross-section through an integrated circuit—IC (
1
). The device is formed of a plurality of layers and includes one or more bond pads (
2
). In this example the layers of the IC (
1
) include silicon substrate (
4
), field oxide layer (
6
), BPSG layer (
8
), passivation layer (
9
) and plastic encapsulant (
10
). A wire bond (
12
) is shown including bonding ball (
14
). The center of the die is located toward the direction labeled “Z”. This listing of layers in the device is not meant to be exhaustive but is illustrative of some of the several layers of a micro-electronic device known in the art.
During the wire bonding process wire bond ball (
14
) is attached to bond pad (
2
) utilizing, for example, thermosonic bonding. The bonding process can induce microcracks (
20
). With repeated thermal cycling these microcracks can propagate (
24
) in the layers beneath the bond pad causing chip failure. Some of these mechanisms are described below.
FIGS. 2A
,
2
B, and
2
C are plan views of a section of a packaged IC directly beneath a bond pad after the chemical removal of the bond wire ball which illustrates microcrack initiation and propagation. FIGS.
2
A′,
2
B′, and
2
C′ are cross-sections through the same section with the bond pads and bond wires intact.
The physical propagation of a microcrack into a full-blown pad crater is shown in FIG.
2
. FIGS.
2
A and
2
A′ show a microcrack (
20
) that has been formed in a layer immediately beneath bond pad (
2
). With repeated thermal cycling this microcrack propagates in the direction shown (
26
) in FIGS.
2
B and
2
B′. With continued thermal cycling, crack propagation moves in a generally elliptical manner (
FIG. 2B
) and downward (FIG.
2
B′). It should be noted that this elliptical crater (FIG.
2
′) is formed with its short axis aligned along a line originating substantially near the chip center.
FIG. 3
shows a scanning electron microscope (SEM) image of two areas underlying bond pads of an IC (
1
) which failed due to bond pad cratering. This generally elliptical crater formation, and its alignment with the center of the device is clearly shown in these photomicrographs. Cratering often results in intermittent contact between the internal IC wiring and the bond pad thus inducing a subtle and insidious reliability problem by precluding reliable electrical contact between the chip's surface wiring and the bond pads which in turn precludes a reliable contact with the bond wires and the package.
Finally, the formation of craters is a progressive process. This means that while a predisposition for crater formation in the form of microcracks may be present when the chip is going through the chip test procedures during manufacturing, a crater resulting in electrical failure may not have yet formed. This predisposition is referred to herein as “crater jeopardy”. It is only after a substantial number of thermal cycles that the crater actually forms and attendant chip failure occurs.
It will be understood by those having skill in the art that the bond pad cratering phenomena previously discussed are still under investigation. While it is generally believed that microcracks are initiated by stresses induced by the dynamic force of the gold ball or aluminum wedge bond at touch-down impact, the static force applied after touch-down, the level of ultrasonic energy, mechanical vibrations before or after bonding, and/or the hardness of the gold ball in relation to the pad, the role which each of these mechanisms plays in crack/crater formation is still under investigation. Moreover, while the formation of cracks is believed to be dependent on the bonding mechanism, bond parameters, the thickness of the wire bond pad and characteristics of the wire bond material being bonded, the roles of each of these mechanisms is also under investigation. Furthermore, continued research has shown that thermal cycling and shock during the plastic encapsulation process may play a role in propagating bond pad crater formation.
While a number of mechanisms and procedures are currently being investigated to prevent bond pad crater formation and attendant chip failure, given the insidious nature of the onset of crater formation what is especially important is a practical methodology to detect microcracks under the bond pads during the manufacturing process. The methodologies previously utilized to detect bond pad crack/crater formation are insufficient, laborious and destructive as will now be described.
A first prior art methodology for monitoring crater jeopardy is by destructive decapsulation and deprocessing including the chemical removal of the ball bonds followed by visual inspection and high magnification. The results of one such SEM examination of the area under two bond pads suffering from bond pad crater formation is shown in FIG.
3
. While this monitoring for crater jeopardy is particularly effective, it is the both laborious and destructive, and renders the device inoperative and unfit for further service. Clearly, this destruct
Blish Richard C.
Hatchard Colin D.
Lewis David E.
Patel Pramod D.
Advanced Micro Devices , Inc.
Cuneo Kamand
Geyer Scott B.
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