1. Field of the Invention
The present invention relates to a film treatment method and apparatus, used in the manufacturing process of semiconductor elements or LCD substrates, for heating and drying a resist film or another kind of film coated on the surface of a wafer, an LCD glass plate or the like.
2. Description of the Related Art
In the manufacturing process of semiconductor devices, a circuit pattern is transferred onto a resist film on a semiconductor wafer and developed by photolithography. The resist treatment technique used in the photolithography step is important so as to ensure satisfactory functions and characteristics of a stepper (a mask aligner) and to make the most of the characteristics of dry etching subsequently performed.
A resist solution is a solution obtained by adding a solvent to a high molecular material (photosensitive resin). A resist film is formed by uniformly coating the resist solution over the surface of a semiconductor wafer.
In many cases, a spin coating method is used in the resist coating step. In the spin coating method, a resist solution is made to drop onto the center of the upper surface of a wafer, and then the wafer is rotated by means of a spinner. The resist solution is uniformly spread on the entire surface of the wafer by a centrifugal force, thereby forming a resist film on the entire surface of the wafer. In the spin coating step, the resist film is dried by the wind caused by both the rotation of the wafer and the exhaustion of splashing resist, to such an extent that the resist film loses its fluidity.
After the resist coating step, the wafer is pre-baked at a temperature of 80.degree. to 110.degree. C. for several seconds by means of a baking apparatus (a heat treatment apparatus), so as to further dry the resist film. As baking apparatuses, many types are known, such as a hot plate type, an oven type, an infrared heating type, etc. Each of these known baking apparatuses comprises exhausting means, and this exhausting means forcibly exhausts the processing chamber of the gaseous solvent which evaporates from the resist film at the time of pre-baking.
After the pre-baking step, the resist film is exposed to light for development, subjected to post baking (hard baking) and etching, and removed in the final step.
If a wafer subjected to spin coating has a number of chips on which circuit patterns are formed at different levels, a resist solution cannot be uniformly spread on the entire surface of the wafer. That is, the resist film formed on the wafer is not uniform in thickness.
The reason for this is that the surface of the wafer to be coated with the resist solution is uneven. When the resist solution is spread on the wafer, the centrifugal force causes the resist solution on each chip to flow from the center of rotation of the wafer in both the radially outward direction and in the direction opposite to the rotating direction. It should be noted that the wafer moves faster at the radially-outer portions (peripheral portions) than at the radially-inner portions (central portions). Hence, the resist solution present on the radially-outer portions of the wafer is exposed to stronger wind than the resist solution present on the radially-inner portions, and is therefore dried quickly. Further, an undried resist solution flows from the radially-inner portions of the wafer onto the dried resist solution present on the radially-outer portions.
As can be seen from the above, when resist is coated on a wafer on which a pattern is formed, the resist film formed thereby has internal stress and has different thicknesses, depending upon the radial positions of the wafer, as is shown in FIG. 6. In addition, the thickness difference of the resist film is greater in the chips B and C located in the peripheral portion of the wafer than in the chip A located in the central portion thereof. This thickness difference becomes greater in accordance with an increase in the diameter of the wafer. For example, the thickness difference is greater in an 8-inch wafer than in a 6-inch wafer.
If a wafer coated with a resist film of nonuniform thickness is heated at a high temperature (e.g., 100.degree. C. or thereabouts) in the subsequent pre-baking step, the resist film is dried and hardened in the state where the internal stress mentioned above remains (i.e., the state where the resist film has different thicknesses). This results in an obstacle both to the satisfactory functions and characteristics of the stepper and to the satisfactory characteristics of dry etching subsequently performed.