1. Field of the Invention
The present invention relates to a polishing apparatus and method for polishing a surface of a workpiece such as a semiconductor wafer, and more particularly to a polishing apparatus and method having a function of polishing a surface of a film formed on a substrate to a flat mirror finish and a function of polishing unnecessary metal film or the like, particularly copper (Cu) film, deposited on an outer peripheral portion of the substrate to remove such unnecessary metal film. The present invention also relates to an outer periphery polishing apparatus for polishing an outer peripheral portion of a substrate to remove unnecessary metal film or the like deposited on the outer peripheral portion of the substrate.
2. Description of the Related Art
Generally, aluminum or aluminum alloys have been used as a material for forming interconnects (interconnections or wiring) for a semiconductor substrate. However, in recent years, there has been a growing tendency to replace aluminum or aluminum alloys with copper having a low electric resistivity and a high electromigration resistance. Copper interconnects of this kind are generally formed by filling copper into minute trenches or via holes preformed in the surface of the substrate. As a means for filling copper into the minute trenches or via holes, there are various methods including CVD, sputtering, and plating, and in every method, copper is deposited over a substantially entire surface of the substrate, and then unnecessary copper is removed by a chemical mechanical polishing (CMP).
FIGS. 13A through 13C show successive steps of manufacturing copper interconnects in a substrate such as a semiconductor wafer. As shown in FIG. 13A, an oxide film 2 of SiO2 is deposited on a conductive layer 1a on a semiconductor substrate 1 on which semiconductor devices are formed. Then, a via hole 3 and a trench 4 for a predetermined interconnect pattern are formed in the oxide film 2 by lithography and etching. Thereafter, a barrier layer 5 of TiN or TaN or the like is formed, and then a seed layer 7 to supply electric current for electroplating is formed on the barrier layer 5 by film deposition.
Then, as shown in FIG. 13B, the entire surface of the semiconductor substrate 1 is coated with copper by electroplating to deposit a copper layer 6 on the entire surface, thus filling the via hole 3 and the trench 4 with copper. Thereafter, the copper layer 6 on the barrier layer 5 and the barrier layer 5 are removed by chemical mechanical polishing (CMP), thus making the copper layer 6 in the via hole 3 and the trench 4 lie flush with the oxide film 2. In this manner, an interconnect composed of the copper layer 6 is produced as shown in FIG. 13C.
In this case, the barrier layer 5 is formed so as to cover substantially the entire surface of the oxide film 2, and the seed layer 7 is also formed so as to cover substantially the entire surface of the barrier layer 5. Thus, in some cases, as shown in FIG. 14, a sputtering film of copper which is the seed layer 7 resides in a bevel (outer peripheral portion) of the substrate W, or copper is deposited on an edge (outer peripheral portion) inwardly of the bevel and remains unpolished (not shown in FIG. 14).
Copper can easily be diffused into an oxide film in a semiconductor fabrication process such as annealing, thus deteriorating the electric insulation of the oxide film and impairing the adhesiveness of the oxide film with a film to be subsequently deposited to possibly cause separation of the deposited film. It is therefore necessary to remove the remaining unnecessary copper completely from the substrate before at least film deposition. Furthermore, copper deposited on the outer peripheral portion of the substrate other than the circuit formation area is not only unnecessary, but may cause cross contamination in subsequent processes of delivering, storing and processing the substrate. For these reasons, it is necessary that the remaining deposited copper on the peripheral portion of the substrate be completely removed immediately after the copper film deposition process or the CMP process.
Here, the outer peripheral portion of the substrate is defined as an area including an edge and a bevel of the substrate, or either the edge or the bevel. The edge of the substrate means areas of the front and back surfaces within about 5 mm from the outer peripheral end of the substrate, and the bevel of the substrate means an area of the outer peripheral end surface and a curved portion in a cross section within 0.5 mm from the outer peripheral end of the substrate.
There has been generally known an edge polisher for polishing a bare silicon wafer, but there has heretofore been no polishing apparatus for polishing only an outer peripheral portion of a substrate having a film formed thereon.
This kind of edge polisher for polishing a bare silicon wafer generally comprises a substrate holder and a polishing member. In the embodiment shown in FIG. 15, the substrate holder 304 is composed of a robot hand 302 which extends vertically and has a holding portion 300 at a lower end thereof, and is movable in vertical and horizontal directions. The substrate W such as a silicon wafer is held by the holding portion 300 of the robot hand 302 under vacuum in such a state that the substrate W is inclined with respect to the horizontal plane and the edge of the substrate W is externally exposed. The polishing member 312 comprises a cylindrical support member 306 and a polishing cloth 308, and has a polishing surface 310 which is formed by a surface of the polishing cloth 308 affixed on a side surface of the cylindrical support member 306. With this arrangement, while the edge of the substrate W is brought into contact with the polishing surface 310, at least one of the substrate holder 304 and the support member 306 is rotated, and the substrate W is simultaneously moved up and down along the polishing surface 310 by the substrate holder 304, thereby polishing the edge of the substrate W.
When the entire surface of the edge of the substrate W is polished by this kind of edge polisher, one side of the edge of the substrate W is polished, and then the substrate W is reversed and held again by the substrate holding member 304. Thereafter, the other side of the edge of the substrate W is polished.
Therefore, if the outer peripheral portion of the substrate having a film formed thereon is polished by the edge polisher for polishing a bare silicon wafer, this polishing work is required to be performed separately at a location different from the polishing work of the surface of the substrate. Thus, this polishing work is considerably troublesome. Further, in this kind of edge polisher, the outer peripheral portion of the substrate (workpiece) is polished while the outer peripheral portion of the substrate is brought into point contact with the polishing surface at a point along a circumferential direction of the substrate, and hence the polishing rate thereof is generally low and it takes much time to complete polishing.
The present invention has been made in view of the above drawbacks. It is therefore a first object of the present invention to provide a polishing apparatus and method which can perform quickly a polishing operation of an outer peripheral portion of a workpiece such as a substrate having a film (particularly metal film) formed thereon as a series of polishing operations accompanying a primary polishing work of a surface of a substrate, and which can polish the outer peripheral portion of the workpiece efficiently.
A second object of the present invention is to provide an outer periphery polishing apparatus, for polishing an outer peripheral portion of a substrate, which can polish the outer peripheral portion of the substrate efficiently and be of compact construction.
Here, a surface of a substrate means a surface of a workpiece such as a semiconductor wafer on which a film or interconnects are formed.
According to one aspect of the present invention, there is provided a polishing apparatus for polishing a workpiece, comprising: a surface polishing mechanism for polishing a surface of the workpiece, the surface polishing mechanism comprising a polishing table having a polishing surface, and a top ring for holding the workpiece and pressing the workpiece against the polishing surface of the polishing table; and an outer periphery polishing mechanism for polishing an outer peripheral portion of the workpiece.
According to the present invention, polishing of the outer peripheral portion of the workpiece can be quickly conducted by the outer periphery polishing mechanism disposed adjacent to the polishing table as a series of processes before or after a primary polishing of a surface of the workpiece is conducted by the polishing table.
In a preferred aspect of the present invention, the top ring is constructed such that the top ring is movable in a vertical direction and a horizontal direction, and the outer periphery polishing mechanism is disposed at a position to which the top ring is movable, and has an outer periphery polishing unit having an outer periphery polishing surface for polishing the outer peripheral portion of the workpiece by contacting the outer peripheral portion of the workpiece held by the top ring.
With the above arrangement, the workpiece is held by the top ring and pressed against the polishing table to thereby conduct a primary polishing of the workpiece, and then the outer peripheral portion of the workpiece is polished by pressing the outer peripheral portion of the workpiece against the outer periphery polishing surface while the workpiece is being held by the top ring. Thereafter, the polished workpiece is transferred to a next process. Further, the outer peripheral portion of the workpiece is polished by bringing the outer peripheral portion of the workpiece into contact with the outer periphery polishing surface over an entire length along a circumferential direction of the workpiece and by rotating the workpiece and the polishing surface relatively to each other, i.e. rotating at least one of the workpiece and the polishing surface. Thus, the outer peripheral portion of the workpiece can be polished efficiently at a low pressure.
In a preferred aspect of the present invention, the outer periphery polishing unit is rotatable about its own axis, and is provided with a polishing liquid supply port around a rotational center of the outer periphery polishing unit for supplying a polishing liquid to the outer periphery polishing surface by a centrifugal force caused by the rotation of the outer periphery polishing unit.
With this arrangement, the polishing liquid can be supplied to the outer periphery polishing surface from a position below the workpiece.
In a preferred aspect of the present invention, the workpiece comprises a substrate on which copper interconnects are formed.
In this case, the polishing liquid has a polishing rate for copper which is at least ten times greater than that for an oxide film or a low-K material.
In a preferred aspect of the present invention, the outer periphery polishing surface is upwardly inclined with respect to the horizontal plane in a radially outward direction.
In this case, the inclination angle of the polishing surface can be determined on the basis or the hardness of material constituting the outer periphery polishing surface, the shape of the outer peripheral portion of the substrate, the cutting width at the outer peripheral portion, or a pressing force applied to the workpiece during polishing so that the lower half, and a portion slightly thereabove, of the outer peripheral portion of the workpiece may be brought into contact with the outer periphery polishing surface or the cutting width of the outer periphery portion of the workpiece may be freely adjusted.
According to another aspect of the present invention, there is provided a polishing method for polishing a workpiece, comprising: polishing a surface of the workpiece by pressing the surface of the workpiece against a polishing surface of a polishing table under a predetermined force; and polishing an outer peripheral portion of the workpiece before or after the polishing of the surface of the workpiece.
According to still another aspect of the present invention, there is provided an outer periphery polishing apparatus for polishing an outer peripheral portion of a substrate, comprising:
a substrate holder for holding the substrate so as to allow an outer peripheral portion of the substrate to be exposed externally; a polishing unit having a polishing surface located at a position facing the outer peripheral portion of the substrate held by the substrate holder; a rotating mechanism for rotating at least one of the substrate holder and the polishing unit; and a moving mechanism for moving at least one of the substrate holder and the polishing unit to bring the substrate holder and the polishing unit into engagement with each other and separate the substrate holder and the polishing unit from such engagement.
According to the present invention, the outer peripheral portion of the substrate is polished by bringing the outer peripheral portion of the substrate into contact with the outer periphery polishing surface over the entire length along the circumferential direction of the substrate and by rotating the substrate and the polishing surface relatively to each other. Thus, the outer peripheral portion of the substrate can be polished efficiently at a low pressure.
In a preferred aspect of the present invention, the polishing surface is upwardly inclined with respect to the plane formed by the substrate held by the substrate holder in a radially outward direction.
With this arrangement, the lower half, and a portion slightly thereabove, of the outer peripheral portion of the substrate can be brought into contact with the polishing surface, or the outer peripheral portion of the substrate can be polished at a desired cutting width. This inclination angle can be determined on the basis of the hardness of material constituting the polishing surface, the shape of the outer peripheral portion of the substrate, the cutting width at the peripheral portion, or a pressing force applied to the substrate during polishing.
The substrate holder may hold the substrate by attracting the central portion of the substrate in such a state that the surface of the substrate faces downwardly.
In a preferred aspect of the present invention, the polishing unit is rotatable about its own axis, and is provided with a polishing liquid supply port around a rotational center of the polishing unit for supplying a polishing liquid to the polishing surface by a centrifugal force caused by the rotation of the polishing unit.
Thus, the polishing liquid can be supplied to the polishing surface from a position below the substrate.
In a preferred aspect of the present invention, the substrate comprises a substrate on which copper interconnects are formed. The polishing liquid has a polishing rate for copper which is at least ten times greater than that for an oxide film or a low-K material.
In a preferred aspect of the present invention, the outer periphery polishing apparatus further comprises a pressing mechanism for pressing the outer peripheral portion of the substrate against the polishing surface of the polishing unit.
Thus, the outer peripheral portion of the substrate can be reliably pressed against the polishing surface of the polishing unit at a predetermined pressing force to thus prevent generation of a defective portion due to displacement of the substrate or random movement of the substrate.
The above and other objects, features, and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings which illustrates preferred embodiments of the present invention by way of example.