Abrading – Precision device or process - or with condition responsive... – By optical sensor
Reexamination Certificate
2001-07-05
2003-06-17
Hail, III, Joseph J. (Department: 3723)
Abrading
Precision device or process - or with condition responsive...
By optical sensor
C451S008000, C451S041000
Reexamination Certificate
active
06579150
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to a method for end point detection in chemical mechanical polishing and more particularly, relates to a dual detection method for end point in chemical mechanical polishing by utilizing both an optical method and an acoustical method.
BACKGROUND OF THE INVENTION
In the fabrication of a semiconductor devices, such as silicon wafers, a variety of different semiconductor equipment and/or processing tools are utilized. One of those processing tools is used for polishing thin, flat semiconductor wafers to obtain a planarized surface. A planarized surface is highly desirable on a shadow trench isolation (STI) layer, on an inter-layer dielectric (ILD) or on an inter-metal dielectric (IMD) layer which are frequently used in modern memory devices. The planarization process is important since in order to fabricate the next level circuit, a high resolution lithographic process must be utilized. The accuracy of a high resolution lithographic process can only be obtained when the process is carried out on a substantially flat surface. The planarization process is therefore an important processing step in the fabrication of a semiconductor device.
A global planarization process can be carried out by a technique known as chemical mechanical polishing or CMP. The process has been widely used on ILD or IMD layers in fabricating modem semiconductor devices. A CMP process is performed by using a rotating platen in combination with a pneumatically actuated polishing head. The process is used primarily for polishing the front surface or the device surface of a semiconductor wafer for achieving planarization and for preparation of the next level processing. A wafer is frequently planarized one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer can be polished in a CMP apparatus by being placed on a carrier and pressed face down on a polishing pad covered with a slurry of colloidal silica or aluminum.
A polishing pad used on a rotating platen is typically constructed in two layers overlying a platen with a resilient layer as an outer layer of the pad. The layers are typically made of a polymeric material such as polyurethane and may include a filler for controlling the dimensional stability of the layers. A polishing pad is typically made several times the diameter of a wafer while the wafer is kept off-center on the pad in order to prevent polishing a non-planar surface onto the wafer. The wafer itself is also rotated during the polishing process to prevent polishing a tapered profile into the wafer surface. The axis of rotation of the wafer and the axis of rotation of the pad are deliberately not co-linear, however, the two axes must be parallel. It is known that uniformity in wafer polishing by a CMP process is a function of pressure, velocity and concentration of the slurry used.
In a process for polishing a dielectric layer, the goal is to remove typography and yet maintain good uniformity across the entire wafer. The amount of the dielectric material removed informally between about 5000 Å and about 10,000 Å. The uniformity requirement for ILD or IMD polishing is very stringent since non-uniform dielectric films lead to poor lithography and resulting window etching or plug formation difficulties. The CMP process has also be applied to polishing metals, for instance, in tungsten plug formation and in embedded structures. A metal polishing process involves a polishing chemistry that is significantly different than that required for oxide polishing.
The important component needed in a CMP process is an automated rotating polishing platen and a wafer holder, which both exert a pressure on the wafer and rotate the wafer independently of the rotation of the platen. The polishing or the removal of surface layers is accomplished by a polishing slurry consisting mainly of colloidal silica suspended in deionized water or KOH solution. The slurry is frequently fed by an automatic slurry feeding system in order to ensure the uniform wetting of the polishing pad and the proper delivery and recovery of the slurry. For a high volume wafer fabrication process, automated wafer loading/unloading and a cassette handle are also included in a CMP apparatus.
As the name implies, a CMP process executes a microscopic action of polishing by both chemical and mechanical means. While the exact mechanism for material removal of an oxide layer is not known, it is hypothesized that the surface layer of silicon oxide is removed by a series of chemical reactions which involve the formation of hydrogen bonds with the oxide surface of both the wafer and the slurry particles in a hydrolyzation reaction; the formation of hydrogen bonds between the wafer and the slurry; and finally, the breaking of the oxide bond with the wafer or the slurry surface when the slurry particle moves away from the wafer surface. It is generally recognized that the CMP polishing process is not a mechanical abrasion process of slurry against a wafer surface.
While the CMP process provides a number of advantages over the traditional mechanical abrasion type polishing process, a serious drawback for the CMP process is the difficulty in end point detection. The CMP process is frequently carried out without a clear signal about when the process is completed by using only empirical polishing rates and timed polish. Since the calculation of polish time required based on empirical polishing rates is frequently inaccurate, the empirical method fails frequently resulting in serious yield drops. Attempts have been made to utilize in end point mechanism including those of capacitive measurements and optical measurements. However, none of these techniques have been proven to be satisfactory in achieving accurate control of the dielectric layer removed.
Another method for achieving end point detection is marketed by the Applied Materials Corporation of Santa Clara, Calif. In a MIRRA® CMP device. In the MIRRA® device, a system of in-situ remote monitor (ISRM) is provided to determine end point by the concept of a periodic change of optical interference. In the MIRRA® device, signals received from a patterned wafer surface are processed by digital filtering algorithms by a PC programmable filter such that an optical interference intensity changes periodically with the thicknesses of removed surface material. For instance, a built-in laser source which is fixed at 6700 Å wavelength is utilized to cause interference at a wafer surface and thus producing a waveform received by a laser detector. The waveform generated by such a technique is shown in FIG.
1
. Similarly, a reflectance vs time curve obtained in a LAM® CMP apparatus is shown in FIG.
4
.
FIG. 1
illustrates four cycles of a waveform with each cycle corresponds to a removed material layer thickness of approximately 2437 Å. The technique is adequate to detect an end point in a polishing process wherein only a relatively thin layer, for instance, of only 2000 Å is removed. When a large thickness of material such as an IMD oxide layer having a thickness of at least 4000 Å is to be removed, the method frequently produces faulty results since the laser detector cannot distinguish which one of the waveform cycles the end point falls on. The wafer surface can therefore be either over-polished or under-polished by 2400 Å thickness. For instance,
FIG. 5A
shows an over-polish condition of a present layer wherein Cu residue causes a short across metal lines, while
FIG. 5B
shows an over-polish of a previous layer under a present layer which leads to high resistance due to dishing/erosion, and furthermore, hard-to-remove Cu residue is left on the next layer.
U.S. Pat. No. 6,071,177, assigned to the common assignee of the present invention discloses a method and apparatus for determining end point in a chemical mechanical polishing process by utilizing a dual wavelength interference technique. The patent further discloses a method and apparatus
Hail III Joseph J.
Shakeri Hadi
Taiwan Semiconductor Manufacturing Co. Ltd
Tung & Associates
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