Method for improvement of tungsten chemical-mechanical polishing process

A multi-step chemical-mechanical polishing method for improving tungsten chemical-mechanical polishing (CMP) process is provided in the present invention. The method comprises following steps. First, a wafer is placed on a first pad of a CMP system, wherein a head fixes the wafer on the first pad. Then, the head is rotated and the wafer is polished on the first pad by using a tungsten slurry. Next, the wafer is transferred to place on a second pad of the CMP system, wherein the head fixes the wafer on the second pad. Following, the head is rotated and the wafer is polished on the second pad by using the tungsten slurry. Then, the wafer is cleaned on the second pad by using a de-ionic water. Next, the wafer is transferred to place on a third pad of the CMP system, wherein the head fixes the wafer on the third pad. Following, the wafer is cleaned on the third pad by using the de-ionic water. Last, the head is rotated and the wafer is polished on the third pad by using an oxide slurry, wherein a pH value of the tungsten slurry and a pH value of the oxide slurry are opposite.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to chemical-mechanical polishing, and more
 particularly using elimination solution to improve a multi-step
 chemical-mechanical polishing method during the chemical-mechanical
 polishing is operating.
 2. Description of the Prior Art
 Chemical-mechanical polishing (CMP) is conventionally used in semiconductor
 manufacturing to achieve global planarity, usually with planarity greater
 than 94%. Normally the operation of the chemical-mechanical polishing
 combines both of chemical and mechanical effects. The chemical-mechanical
 polishing generally includes rotating table, where slurry and polishing
 pad are applied. Conventionally typical polishing slurry comprises
 SiO.sub.2, alumna Al.sub.2 O.sub.3 in an alkali solution.
 There generally will be many particles existing on the surface of wafer
 after tungsten chemical-mechanical polishing WCMP process. All particles
 usually distribute onto the surface edge of wafer. Due to the inherent
 drawbacks of the chemical-mechanical polishing mechanism, the slurry
 effect is clearly observed after the chemical-mechanical polishing,
 causing serious alkali or acid solution effect including SiO.sub.2 and
 Al.sub.2 O.sub.3 slurry, which disadvantageously affects following
 manufacturing process. Normally solution effect will appear obviously if
 oxide wafer with oxide slurry buffing process is carried out. However,
 particles would not exist on tungsten wafer with oxide slurry buffing
 process. Due to the pH rate of tungsten slurry is about 2.3 and the pH
 rate of oxide slurry is about 11. Probably the neutralization of chemical
 reaction happens leading to the special morphologic particles existed as
 FIG. 1A. And FIG. 1B shows for its close-up dramatic picture.
 According to the foregoing reasons, a method is exactly needed for
 eliminating the solution effect during the chemical-mechanical polishing
 in order to improve and reduce either alkali or acid solution effecting
 the result of polishing process.
 SUMMARY OF THE INVENTION
 In one embodiment, In accordance with the present invention, an improved
 chemical-mechanical polishing (CMP) method is provided that substantially
 eliminates the solution effect during the CMP process, thereby improving
 the alkali or acid solution of polishing process.
 Therefore this method for chemical-mechanical polishing process is
 obviously disclosed. The method comprises following steps. First, a wafer
 is placed on a first pad of a CMP system, wherein a head fixes the wafer
 on said first pad. Then, the head is rotated and the wafer is polished on
 the first pad by using a tungsten slurry. Next, the wafer is transferred
 to place on a second pad of the CMP system, wherein the head fixes the
 wafer on the second pad. Following, the head is rotated and the wafer is
 polished on the second pad by using the tungsten slurry. Then, the wafer
 is cleaned on the second pad by using a de-ionic water. Next, the wafer is
 transferred to place on a third pad of the CMP system, wherein the head
 fixes the wafer on said third pad. Following, the wafer is cleaned on the
 third pad by using the de-ionic water. Last, the head is rotated and the
 wafer is polished on the third pad by using an oxide slurry, wherein a pH
 value of the tungsten slurry and a pH value of the oxide slurry are
 opposite.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 The following is a description of the present invention. The invention will
 firstly be described with reference to one exemplary structure. Some
 variations will then be described as well as advantages of the present
 invention. A preferred method of fabrication will then be discussed. An
 alternate, asymmetric embodiment will then be described along with the
 variations in the process flow to fabricate this embodiment.
 The method of the present invention is applied to a broad range of
 chemical-mechanical polishing (CMP) process. The following description
 discusses several presently preferred embodiments of the WCMP of the
 present invention as implemented in CMP process, since the majority of
 currently available CMP process are used in silicon processing and the
 most commonly encountered applications of the present invention is
 involved about the slurry solution problem. Nevertheless, the present
 invention may also be advantageously employed in conventionally CMP
 process, and other semiconductor materials. Accordingly, application of
 the present invention is not intended to be limited to those devices
 fabricated in silicon semiconductor materials, but will include those
 devices fabricated in one or more of the available semiconductor
 materials.
 Moreover, while the present invention is illustrated by a number of
 preferred embodiments directed to WCMP process, it is not intended that
 these illustrations be a limitation on the scope or applicability of the
 present invention. Further, while the illustrative examples use insulated
 WCMP process, it should be recognized that the insulated gate portions
 might be replaced with other chemical-mechanical polishing process. Thus,
 it is not intended that the semiconductor devices of the present invention
 be limited to the structures illustrated. These devices are included to
 demonstrate the utility and application of the present invention to
 presently preferred embodiments.
 In one embodiment, as FIG. 2A a semiconductor wafer 21 is initially placed
 on platen 100 of a chemical-mechanical polishing system as FIG. 2,
 followed by polishing the wafer with pad 101, wherein head 102 rotates
 with respect to the platen 100. Thereafter, the wafer 21 is polished with
 pad 101, wherein head 102 rotates in the same platen. Then semiconductor
 wafer 21 is cleaned up using deionic water from slurry pine 103 in order
 to wash the retained alkali or acid solution on the surface of
 semiconductor wafer 21. The de-ionic water can be transferred from another
 individual pine of the CMP system. With respect to platen 100, thereby the
 solution effect to the surface of wafer 21 is improved.
 In another embodiment of the present invention, as FIG. 2B a semiconductor
 wafer is initially placed on first pad 101A of a chemical-mechanical
 polishing system, followed by polishing wafer 21 in the first pad 101A
 with a tungsten slurry. A head 102 fixes the wafer 21 on the pad and the
 head is rotated in the polishing process. The wafer is then transferred
 and placed on a second pad 101B of the chemical-mechanical polishing
 system, followed by polishing wafer 21 in the second pad 101B with the
 tungsten slurry. Hence wafer 21 is cleaned up using the de-ionic water
 from slurry pine 103B after head 102 completes the rotation with respect
 to second pad 101B. Thereafter, again the wafer will be transferred and
 placed on third pad 101C of the chemical-mechanical polishing system,
 followed by polishing wafer 21 with the third pad 101C with an oxide
 slurry. Also, firstly wafer 21 is cleaned up using the de-ionic water from
 slurry pine 103C and then the head will accomplish rotation.
 The duration of polishing the wafer in the second pad is approximately
 equal to duration of polishing the wafer in the third pad. Also, the
 duration of cleaning the wafer in the second pad is approximately equal to
 duration of cleaning the wafer in third pad 101C. The solution usually is
 used to the de-ionic water. For tungsten film with tungsten slurry is
 about 2 K.ANG.. Polishing remained tungsten film with tungsten slurry by
 end-point system is on first pad 101A and second pad 101B. Another for
 polishing oxide film with oxide slurry system is on third pad 101C.
 Finally for polishing oxide film, its thickness of oxide film is about 200
 to 500 .ANG. with oxide slurry system. Especially the pH value of the
 tungsten slurry and the oxide slurry is opposite due to the pH value of
 tungsten slurry is about 2.3 and oxide slurry is about 11. Therefore
 de-ionic water could eliminate retained tungsten slurry and make wafer
 clearly. Then the oxide slurry steps will consequentially follow.
 The defect that is edge distribution type will not appear after the above
 process completed because de-ionic water could remove alkali or acid
 slurry. It is sufficient not only to reduce defect count until less than
 50 ea level but also to reduce the failure rate less than 5%. When
 implementing the modified recipe, the failure rate of particle can be
 improved from 20% to less than 5%. The down-time of machine will reduce
 from 8.2% to 3.3%, therefore the available time of machine will increase
 from 65% to 85%. Finally Higher Cp Yield also will be obtained.
 Although specific embodiments have been illustrated and described, it will
 be obvious to those skilled in the art that various modifications may be
 made without departing from what is intended to be limited solely by the
 appended claims.