Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
2001-03-27
2003-11-04
McDonald, Rodney G. (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S298360, C216S066000, C427S551000, C427S552000
Reexamination Certificate
active
06641705
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to charged particle beam milling and, in particular, to an apparatus and method for reducing differential sputter rates in crystalline and other materials.
BACKGROUND OF THE INVENTION
Focused Ion Beam (FIB) microscope systems have been produced commercially since the mid 1980's, and are now an integral part of rapidly bringing semiconductor devices to market. FIB systems produce a narrow, focused beam of charged particles, and scan this beam across a specimen in a raster fashion, similar to a cathode ray tube. Unlike the scanning electron microscope, whose charged particles are negatively charged electrons, FIB systems use charged atoms, hereinafter referred to as ions, to produce their beams. In most commercial FIB systems, the ions used are positively charged gallium ions (Ga
+
) from liquid metal ion sources, however beams of other ions can be produced. For example, materials such as silicon, indium, cesium or even gases such as argon, krypton or oxygen can be utilized as ion sources.
Modem FIB systems can produce a beam of gallium ions as narrow as approximately 5 nm in diameter. One can increase the current of ions in the beam to operate the FIB as an “atomic scale milling machine,” selectively removing materials wherever the beam is placed, and at the same time imaging the sample by correlating the known beam position with electrical signals produced as the incident beam interacts with the specimen. One skilled in the art would understand the operation of this well-known procedure.
Semiconductor devices such as microprocessors can be made up of millions of transistors, each interconnected by thin metallic lines branching over several levels and isolated electrically from each other by layers of dielectric materials. When a new semiconductor design is first produced in a semiconductor fabrication facility, hereinafter referred to as a “fab”, it is typical to find that the design does not operate exactly as expected. It is then necessary for the engineers who designed the device to test their design and “rewire” it to achieve the desired functionality. Due to the complexity of building a semiconductor device in the fab, it typically takes weeks or months to have the re-designed device produced. Further, the changes implemented frequently do not solve the problem or expose a yet further difficulty in the design. Iterating through the process of testing, re-designing and re-fabrication can significantly lengthen the time to market of new semiconductor devices.
Over the past decade, techniques have been developed to allow FIB systems to reduce the time required for this procedure of perfecting a design. FIB instruments were first used to “cut” metal lines, typically comprised of alloys of aluminum and/or tungsten, on prototype devices, thus allowing for design verification in simple cases. Further, techniques have been developed using special gas chemistries in the FIB system to permit selective deposition of thin metallic lines to connect two or more conductors, selective removal of dielectric insulators but not metallic interconnects, and selective removal of metal interconnects without removing the dielectric insulators. Techniques have yet further been developed that allow the deposition of insulating materials. Hence, these advances in FIB system technologies now allow the cutting of metal interconnect lines, the insulating of these metal interconnect lines from their surroundings and the re-wiring of the lines to another location. Essentially, these capabilities now permit prototyping and design verification in a matter of days or hours rather than weeks or months as re-fabrication would require. This FIB “rapid prototyping” is frequently referred to as “FIB device modification” or “microsurgery.” Due to its speed and usefulness, FIB microsurgery has become crucial to achieving the rapid time-to-market targets required in the competitive semiconductor industry.
Until recently, the typical metals used for metallic interconnects were primarily alloys of aluminum and/or tungsten. The above described advances in FIB system techniques for cutting and depositing metal interconnects were specifically designed for these metal alloys and their particular physical characteristics.
Polycrystalline aluminum interconnect lines are composed of small, contiguous grains of aluminum. Within each grain, the atoms share a regular array-like order, but the relative position of the arrays of atoms can vary from grain to grain. This alignment of the arrays of atoms is known as the “crystallographic orientation” of a given grain. Differences in crystallographic orientation can cause grains sputtered or milled with an ion beam to be removed at different rates, depending on the given orientation. For aluminium though, the difference between most of the slowest sputtering orientations compared to the fastest sputtering orientations is not particularly significant and hence is not a key factor in the techniques to cut and/or remove aluminum interconnects. Additionally, for aluminum, chemistries have been developed that selectively attack aluminum, causing grains of any orientation to sputter much more quickly in the presence of the gas than with just the ion beam alone. This process is well-known within the art and is commonly referred to as Gas Assisted Etching (GAE). In one particular well-known technique, chlorine gas is used to perform GAE of aluminum interconnects, cleanly removing aluminum grains, with little regard to their individual crystallographic orientation. The terms “etch” “mill” and “sputter” are used interchangeably below.
Recently, copper-based interconnects have begun to replace aluminum-based interconnects in state-of-the-art devices due to the increased transmission speeds achievable with the use of copper. Unfortunately, FIB sputtering of copper is more difficult than sputtering aluminum alloys. Firstly, aluminum atoms have a lower atomic mass and less “stopping power” than copper atoms, and simple ion beam milling of the copper atoms is less effective than the equivalent milling of aluminum. Further, a gas chemistry that permits GAE of copper has not yet been successfully developed. And yet further, the relative sputter rate between grains of different crystallographic orientations of copper can differ by a large factor, this factor being approximately 360% in some experimental tests.
Some of the difficulties that can occur when attempting to sputter copper interconnects that have different crystallographic orientations will now be described by example with reference to FIG.
1
and
FIGS. 2A through 2I
.
FIG. 1
illustrates portions of three grains in a typical section of copper interconnect, sections
2
,
3
, and
4
. The grains at each end section
2
and
4
, in this example, have similar orientations, while the grain in the center section
3
is quite different. Consider a situation where the grains at each end section
2
and
4
are “slow milling”, whereas the grain in the center section
3
is “fast milling.”
FIGS. 2A through 2I
illustrate these three grain sections
2
,
3
, and
4
in the cross-section of a semiconductor device with two levels of copper interconnect. The three grain sections from
FIG. 1
are represented in copper layer
5
in
FIGS. 2A through 2I
. As in a “real” device, they are covered with a protective dielectric material
6
. They are also isolated, in the vertical dimension, from lower level conductors by a dielectric material
7
. A second, lower layer of conductive interconnect is represented by a copper layer
8
. Subsequent lower levels of the device containing transistors, etc. are not shown for clarity. Additionally, one skilled in the art would realize that copper layers
5
and
8
would extend to the left and right of the figure to carry electrical signals necessary to the functioning of the device and that the layers and grains are not shown to scale.
For purposes of illustration,
FIGS. 2A through 2I
show the case where it is necessary to perform microsurgery
Casey, Jr. J. David
Li Jian
Noll Kathryn
Phaneuf Michael
Shuman Richard F.
FEI Company
Griner David
McDonald Rodney G.
Scheinberg Michael O.
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