1. Field of the Invention
The present invention relates in general to polishing methods and apparatuses, and relates in particular to a polishing method and a polishing apparatus for processing substrates, such as semiconductor wafers, glass plates and liquid crystal display panels which require a high cleanliness.
2. Description of the Related Art
In recent years, there has been a remarkable progress in the density of integrated circuit devices which leads to a narrower interline spacing of the wiring, and, in the case of using optical lithography involving less than 0.5 mm line spacing particularly, the shallow depth of focus associated with its optics demands extreme flatness at the focusing plane of the stepper. This trend means also that if a particle of a size larger than the line spacing should remain on the fabricated device, it can cause short circuiting which may lead to device failure. Therefore, it is evident that workpiece processing must produce a flat and clean workpiece. These processing requirements apply equally to other workpiece materials in general, such as glass plates for photo-masking or liquid crystal display panels.
FIG. 4 shows a conventional polishing apparatus comprising: a polishing unit 10; a loading/unloading unit 21; a transfer robot 22, and two cleaning machines 23a, 23b. FIG. 6 is a schematic illustration of the polishing unit 10 comprising a turntable 12 having a polishing cloth 11 attached thereto; and a top ring 13 for holding a workpiece 1 and pressing the workpiece (wafer) 1 onto the turntable 12.
Polishing is carried out by holding a workpiece 1 at the bottom surface of the top ring 13, and pressing the workpiece by means of a vertically movable cylinder onto the polishing cloth 11 mounted on the top surface of the rotating turntable 12. In the meantime, a polishing solution Q is supplied from a delivery nozzle 14 in such a way to retain the solution Q between the bottom surface of the workpiece 1 and the abrading surface of the polishing cloth 11.
The turntable 12 and the top ring 13 are rotated independently at their individual controlled speed. As shown in FIG. 6, the top ring 13 is positioned in relation to the turntable 12, so that the peripheral edge of the workpiece 1 is located at distances "a" and "b", respectively, from the center and the peripheral edge of the turntable 12 so that the entire surface of the workpiece 1 can be polished uniformly at some high rotational speeds. It indicates that the diameter "D" of the turntable 12 is chosen according to the following relation to be more than twice the diameter d of the workpiece 1:
D=2(d+a+b)
The polished workpiece 1 is processed in the cleaning machines 23a, 23b through several washing and drying steps, and is transferred onto the loading/unloading unit 21 to be stored in a portable workpiece cassette 24. A scrub washing is used which involves the use of brushes made of nylon or mohair, or a sponge made from polyvinylalcohol (PVA).
The conventional polishing apparatus of the type described above is satisfactory from the standpoint of achieving adequate flatness and efficiency owing to large relative displacements between the turntable 12 and the top ring 13 as well as their high relative speeds; however, surface roughness of the polished workpiece tends to be higher than desirable. To produce a polished workpiece of better surface quality, consideration may be given to using two turntables which are operated by varying the abrading qualities of the polishing cloths, rotational speeds and types of polishing solutions. However, as mentioned above, the diameter of the turntable is larger than twice that of the workpiece diameter, and each apparatus takes up a large floor space area which leads to higher facility costs. These problems becomes more ignorable as the industry seeks larger diameter substrates.
While it is possible to use one turntable to produce a superior surface quality by varying the type of polishing solution and lowering the rotational speed, for example, it is obvious that such an approach leads not only to a potential increase in the cost of polishing solution but to inevitable lowering in the production efficiency due to a prolonged operation.
The conventional method also has some problems in the cleaning process when scrubbing follows the use of abrasive particles, not only because of the inherent difficulties of removing small particles in submicron ranges but also because of the ineffective cleaning when there is a strong affinity between the workpiece and the particles.
Therefore, there has long been a need in the semiconductor device manufacturing industry for an efficient polishing method and facility which would enable to produce substrates of high surface qualities, such as flatness, smoothness and cleanliness, in a compact and low cost apparatus.