Semiconductor device manufacturing: process – With measuring or testing – Electrical characteristic sensed
Reexamination Certificate
2000-07-25
2002-05-28
Christianson, Keith (Department: 2813)
Semiconductor device manufacturing: process
With measuring or testing
Electrical characteristic sensed
C257S252000
Reexamination Certificate
active
06395568
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to reliability and packaging electronic of 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 achieves efficient use of chip area by providing that at least a portion of each electrostatic discharge (ESD) device is positioned under a bond pad, improving device density.
2. Discussion of the Related Art
Integrated circuit devices are typically packaged. 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 e us ed to interconnect the several components on the chip surface. Many metal leads 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 “submicron” or less than 1.0 &mgr; m in width.
The wire 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. This difference in the size of these two wing structures is one of the reasons that Chip source wiring usually terminates at bond pads disposed on the chip, typically arrayed about the periphery of the chip as a pitch transformer. Traditionally of course, bond pads have been restricted to the chip periphery to avoid wire crossings. After the chip including the bond pads is formed, fine wiring in the form of bond or lead wire typically connects the bond pads to the substantial lead system (a.k.a. lead flame) that connects the package chip to the device in which is installed. One common bond pad material is aluminum, deposited dug chip fabrication. Bond wires are often gold or aluminum and are typically connected to the bond pads by means of metal balls or wedge bonds 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, generally fracture of the silicon and dielectric oxide layers. This phenomenon is sometimes referred to as “bond pad cratering”. While studies to determine exact meachanisms 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 cotributory 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
. Having reference to that Figure, there is shown a cross-section through an integrated circuit (IC) device
1
, the device formed of a plurality of layers and including at least one bond pad
2
. In this example, the layers of the device
1
include silicon substrate
4
, field oxide layer
6
, BPSG layer
8
, passivation layer
9
, and plastic encapsulant
10
. A wire bond is shown at
12
, including ball
14
. Noted in this Figure, 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 rather 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, for example as shown at
20
. With repeated thermal cycling, these microcracks can propagate, for instance as shown at
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 microdevice directly beneath a bond pad, following chemical removal of the bond wire ball, and demonstrating microcrack initiation and propagation. FIGS.
2
A′,
2
B′, and
2
C ′ are cross-sections through the same section, however with the bond pads and bond wires intact.
Having reference now to
FIG. 2
, the physical propagation of a microcrack into a full-blown pad crater is shown. At FIGS.
2
A and
2
A′ a microcrack
20
has been formed in a layer immediately beneath bond at
2
. With repeated the thermal cycling, this microcrack propagates in the direction shown at
26
in FIGS.
2
B and
2
B′. With continued thermal cycling, crack propagation moves in a generally elliptical manner (FIG.
2
B), and downward (FIG.
2
B′). It should be noted that this elliptical crater (FIG.
2
B′) is formed with its short axis aligned along a line originating substantially near the chip center.
At
FIG. 3
is shown a scanning electron microscope (SEM) image of two areas underlying bond pads of a device
1
, which failed due to bond pad cratering. This generally elliptical crater formation, and its alignment with the center of the device, as previously discussed, is clearly shown in these photomicrographs. Cratering induces a subtle and insidious reliability problem which integrated circuit devices: the craters so formed generally preclude reliable electrical contact between the chip's surface wiring and the bond pads, and hence with any device to which the package integrated circuit is electrically connected.
Moreover, this contact failure is often intermittent, rendering the chip unreliable and the defect difficult to detect.
Finally, the formation of craters is a progressive process. This means that while a pre-disposition for crater formation, in the form of the previously discussed microcracks, and attendant chip failure may be present when the chip is going through the chip test procedures during manufacturing the crater may not yet actually have formed This pre-disposition is referred to herein as “crater jeopardy”. It is only after a substantial number of thermal cycles that the crater forms, and attendant chip failure occurs.
It will be understood by those having skill in the art that the bond pad cratering phenomena previously ed are still under investigation. While it is generally believed that microcracks are initiated by stresses induced by the dynamic force of the gold ball 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 and 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 as being laborious and destructive as will now be described
A first prior art methodology for monitoring crater jeopardy is by destructive decapitation and deprocessing, including the chemical removal of the ball bonds, followed by visual inspection and high magnification. The results of one such SEM ex
Blish Richard C.
Fliesler Michael
Hatchard Colin D.
Morgan Ian
Advanced Micro Devices , Inc.
Christianson Keith
LandOfFree
Method and apparatus for achieving bond pad crater sensing... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for achieving bond pad crater sensing..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for achieving bond pad crater sensing... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2839461