Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2002-01-15
2003-09-16
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C204S192110, C204S192120, C204S192150, C428S636000, C428S686000, C428S212000, C428S218000, C428S931000
Reexamination Certificate
active
06620527
ABSTRACT:
This application incorporates by reference Taiwanese application Serial No. 90113275, filed on May 31, 2001.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates in general to an aluminum (Al) wiring layer, and more particularly to an aluminum wiring layer capable of preventing hillocks and a method of forming the same.
2. Description of the Related Art
As the semiconductor manufacturing of an integrated circuit (IC) with larger scale is required, a substrate may be insufficient to provide an enough area for forming required interconnects for the IC. In order to meet the requirement of the formation of increased numbers of interconnects due to the reduction of metal oxide semiconductors (MOSs) of the IC in sizes, two or more levels of metal layers for interconnects have become a necessary technology adopted in the manufacturing of many ICs. Particularly, for some integrated circuits with sophisticated functions such as microprocessors, four or five levels of metal layers are required to implement interconnections of the components of the integrated circuits. On the other hand, in the manufacturing of a thin-film transistor liquid crystal display (TFT-LCD) panel, thin metal films are employed as electrodes and interconnects, which are also formed in a structure with multiple levels of metal layers.
In a structure with multiple levels of metal layers, there are insulating layers, such as dielectrics, formed between any two of the metal layers in order to prevent an interlayer short circuit from occurring. In addition, a pure metal or an alloy with low electric resistance is suitably used as the material for the metal layers. In general, for examples of pure metals, Al, Cu, Mo, Ta, and W can be used. As examples of alloys with low electric resistance, an aluminum alloy containing one or more selected from the other elements, such as Al—Cu, Al—Cu—Si, Al—Pd, and Al—Nd, is used. Preferably, pure aluminum is employed as the material for metal layers. It is because aluminum has considerable adhesion with the substrate, and considerable etching characteristics in manufacturing as well as low electric resistivity. Besides, the earth contains much aluminum than other metal elements. Thus, aluminum is available and low in cost.
However, it still has disadvantages to use pure aluminum, which has a melting point lower than other metals, as the element for metal layers. Referring to
FIG. 1A
, it illustrates the deposition of a metal on a glass plate. In the manufacturing of thin film transistors, firstly, crystal particles
104
are formed on a glass plate
102
by the precipitation of metal under relatively low temperature (about 150° C.) and grain boundaries
106
are formed between the crystal particles. In fact, the crystal particles will not formed regularly in the same way as shown in FIG.
1
A and the regular crystal particles shown in
FIG. 1A
are for the sake of illustration. Next, annealing is performed so that the increased vibration of the crystal particles by heating at high temperature causes the re-arrangement of the atoms of the crystal particles, thereby reducing defects of the crystal particles and re-crystallizing the crystal particles. After the re-crystallization, inner stress of the crystal particles is rapidly reduced by the reduction of the density of defects such as dislocation. If the annealing temperature is being increased and raises the crystal particles formed in the re-crystallization to a higher energy level exceeding the surface energy among the crystal particles, the crystal particles begin to grow while the smaller ones of them vanish. Consequently, the growth of the crystal particles yields larger crystal particles and the grain boundaries of the smaller crystal particles vanish. Thus, the inner stress of the crystal particles is further reduced to a lower level.
When pure aluminum is used as the wiring layer material, hillock and the like may be produced.
FIG. 1B
shows the hillock by illustrating the glass plate with pure aluminum as the wiring layer material after annealing. In the annealing, the high temperature causes the thermal expansion of Al crystal particle
104
and glass plate
102
. Since aluminum has a greater thermal expansion coefficient than the glass, a substantial compressive stress by the Al crystal particle
104
is applied to the glass plate
102
. By this compressive stress, the aluminum atoms move along grain boundary
106
to cause a hillock
110
. The hillock and the like, such as the hillock
110
, may cause the unevenness of the thickness of the other layers in the subsequent fabrication process. Besides, in the worse case, an interlayer short circuit may occur when a large hillock penetrates an insulting layer (not shown) to be formed between the underlying metal layer and the overlying metal layer, and touches the overlying metal layer.
Hence, it is necessary to solve the problem of hillock in order to use Al as the wiring material. Conventionally, there are three approaches to this problem. The first approach is to use the other element having a high melting point, such as Nd, Ti, Zr, Ta, Si, and Cu, as the wiring material.
FIG. 2A
shows that crystal particles
204
of an Al alloy formed on a glass plate
202
after annealing. As shown in
FIG. 2A
, there is no hillock formed among grain boundaries
206
of the crystal particles
204
of the Al alloy. Since the atoms of the additional element of the Al alloy cannot dissolve in Al crystal particles, as the crystal particles
204
grow, the atoms of the additional element move to the grain boundaries
206
and gradually form small particles
210
among the grain boundaries
206
. Thus, when Al atoms move along the grain boundaries
206
, the small particles
210
hinder the Al atoms from moving above the crystal particles
204
, suppressing the formation of hillock.
The second approach is to form a metal layer with high melting point covering the Al crystal particles so as to suppress the growth of hillock.
FIG. 2B
illustrates a metal layer capping the Al crystal particles. After a metal layer
212
with a high melting point is plated over the Al crystal particles
204
, annealing is performed. Since the metal layer
212
works as caps for covering the exits formed by the grain boundaries
206
among the Al crystal particles
204
, Al atoms are blocked from forming hillocks along the grain boundaries
206
. In addition, there is provided with a variant of the second approach where an Al layer in a single amorphous phase is substituted for the metal layer
212
. The term “amorphous” indicates a non-crystalline state, that is, a state where there is no regulation in the atom array of the interior of a substance. Thus, the Al layer in a single amorphous phase has no crystal particle as a core for the growth of crystal particles and can be formed on the crystal particles
204
for the suppression of the formation of hillock.
In the third approach, an additional metal layer with a thermal expansion coefficient between that of the glass plate and Al is applied as a barrier to suppress the formation of hillock. As shown in
FIG. 2C
, a metal layer
214
is sandwiched between the glass plate
202
and the Al crystal particles
204
. The metal layer
214
is first plated on the glass plate
202
and the Al crystal particles
204
are then formed on the metal layer
214
. Besides, the metal layer
213
has a thermal expansion coefficient being greater than that of the glass plate
202
but smaller than that of the Al crystal particles
204
. During annealing, the metal layer
214
acts as a buffer against the compressive stress due to a difference in thermal expansion coefficient between the glass plate and Al so as to prevent the Al atoms from moving along the grain boundaries
206
and forming hillocks.
For these three convention approaches to the problem of forming hillocks, it is the first one that is the most effective and usually employed. For example, a Japanese company, Kobelco, provides an Al—Nd alloy as the wiring material for metal layers, which
Chi Mei Optoelectronics Corp.
Koehler Robert R.
Rabin & Berdo PC
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