1. Field of the Invention
The present invention relates to a coating method and a resist coater for coating a resist film on a wafer. More particularly, the invention relates to a coating method and a resist coater for coating a resist film having a thickness of 5500 .ANG. or less on a wafer having a large diameter of 8 inches or more by a spin coating process, wherein the resist film is free from unevenness in the coating of the resist film and favorable in an in-plane evenness in film thickness and film quality. Incidentally, the resist film in this specification refers to a wide concept including a photoresist film.
2. Description of the Related Art
In a manufacturing process of a semiconductor device, when patterning is performed by etching, a resist film mask having a desired pattern Is formed on a layer to be etched, and the layer is etched by the use of the mask.
A resist film for forming a mask is normally formed on a layer to be etched by coating a resist on a wafer using a resist coater and baking the resist.
By the way, according to a higher integration of a semiconductor device, each elements in the semiconductor device including wirings is made more and more fine. In the generation of the fine pattern rule of 0.35 .mu.m or less, the thickness of a resist film which is coated on a wafer is made thinner and thinner for reasons in the etching processing. In particular, in an resist coating process which requires a coating of a reflection preventing film, in order to secure evenness in film thickness and prevent generation of unevenness in reflection as can be seen in pin holes, it is required to set the thickness of the reflection preventing film to 2000 .ANG. or more.
On the other hand, in order to improve precision in dry etching for a resist resolution pattern, and in order to prevent the neighboring resist patterns from being connected, it is required to set the total thickness of the reflection preventing film and the resist film to 7000 .ANG. or less. Consequently, it is inevitably required to set the thickness of the resist film to 5500 .ANG. or less.
Before explaining a conventional coating method, a resist coater for coating a resist on a wafer will first be explained with reference to FIG. 1. FIG. 1 is a sectional view showing a structure of a conventional resist coater of a spin coat type.
As shown in FIG. 1, a resist coater 10 has a wafer holding table 12 for rotating a wafer W around an axis which is perpendicular to the wafer W while holding the wafer W with a wafer surface thereof directed upwardly. A resist supply nozzle 14 supplies resist to the wafer W which is held on the wafer holding table 12. A coater cup 16 surrounds the wafer holding table 12 and the resist supplying nozzle 14.
The wafer holding table 12 is attached on an upper end of a rotation shaft 18 which penetrates a bottom portion of the coater cup 16 and which is rotated with a rotation device (not shown). The wafer holding table 12 is provided on its wafer holding surface with a chuck mechanism for vacuum-sucking the wafer W, and it is rotated together with the rotation shaft 18 around an axis which is perpendicular to the wafer holding surface.
The coater cup 16 is provided for trapping resist particle which are scattered from the wafer W by a centrifugal force due to rotation of the wafer W, and the coater cup 16 has an outer cup 20 for trapping resist particles which are scattered in a sidewise and an upward directions, and an inner cup 22 for guiding resist particles which are scattered in a downward direction from the wafer W to a bottom portion of the outer cup 20.
The outer cup 20 has at an upper portion thereof an opening 24 for introducing the resist supply nozzle 14 or the like, and for taking in and taking out the wafer W. Furthermore, the inner cup 22 is provided on a lower portion of the outer cup 20, and the inner cup 22 has a cylindrical portion 22a, and an umbrella portion 22b which spreads toward the inside of the coater cup 16 from an upper opening thereof.
The resist supply nozzle 14 is lowered from the above via the opening 24 of the coater cup 16, and is arranged at a position facing a resist coating surface of the wafer W. Also, in order to clean an edge to the back surface of the wafer W, a first cleaning nozzle 26 for spraying rinse agent is provided to be directed toward the edge of the back surface of the wafer W through the bottom portion of the coater cup 16. Furthermore, in order to clean an edge of the surface of the wafer W, a second cleaning nozzle 28 for spraying rinse agent is provided to be directed toward the edge of the wafer W via the upper opening 24 of the coater cup 16.
Exhaust pipes 30 connected to an exhaust device (not shown) for exhausting the inside of the coater cup 16, and a drain pipe 32 for exhausting the resist trapped in the coater cup 16 are connected to the bottom portion of the coater cup 16.
Following, a conventional method for coating a resist film by the use of the above-mentioned resist coater 10 will be explained by giving an example of forming a resist film having a thickness of 5500 .ANG. on an 8 inch wafer. FIG. 2 is a graph showing a rotating speed of a wafer of each step in the conventional method.
When a resist film having a thickness of 5500 .ANG. or less, for example, 5000 .ANG., is coated on an 8 inch wafer, the coating process is conventionally performed in the following procedure.
Resist having, for example, a relatively low viscosity on the order of 7 cp is used for a resist film. The wafer is held on the wafer holding table 12 and the resist is first dripped on a wafer from the resist supply nozzle 14 while rotating the wafer at a low rotation speed of 1000 rpm, and the resist is spread on the whole surface of the resist coating surface of the wafer for 4 seconds.
Next, the dripping of resist is suspended, and the wafer is rotated for 19 seconds at a rotation speed of 5000 rpm, so that a resist film having a desired thickness, i.e., of 5000 .ANG. is formed.
Subsequently, rinse agent is sprayed from the first cleaning nozzle 26 for 20 seconds, and the back surface edge of the wafer is cleaned. Also, rinse agent is sprayed from the second rise nozzle 28, and the surface edge of the wafer is cleaned. Thus, the coating step is terminated.
Subsequently, the process proceeds to the following baking step, where the residual solution in the resist is completely removed.
However, in the case where a resist film having a thickness of 5500 .ANG. or less on a 8 inch or more wafer in accordance with the conventional coating method, an in-plane unevenness regarding film thickness and film quality are not favorable, so that variations occur in film thickness and film quality. For example, when a high speed rotational coating is performed on an 8 inch or more wafer, variations in the resist film thickness occur in the in-plane of the wafer in a range of 90 .ANG. or more.
As a consequence, problems are caused in a photolithography stop and an etching step which follow the coating step. For example, there arise a problem that unevenness occurs in wiring width which is obtained in the etching, which results in difficulty in improvement in product yield.
Specifically, on a wafer having a large diameter of 8 inches or more, when the wafer is rotated for 20 seconds or more at a rotation speed of 5000 rpm after dripping the resist agent, a coating unevenness occurs and generated at a constant cycle on a peripheral portion of the wafer having a width of 20 mm from an outer periphery thereof inwardly. For example, in an example according to the method described above, the coating unevenness occurs on the peripheral portion of the wafer, and a variation in the resist film thickness is caused between a central portion of the wafer and the peripheral portion thereof due to the coating unevenness. Thus, as shown in FIG. 3, variations in the fill thickness exceeding 100 .ANG. are generated within the in-plane of the wafer.
With respect to a 6 inch wafer, for reference, even when the wafer is rotated for 20 seconds or more at a rotation speed of 6000 rmp after dripping the resist agent on the wafer, it never happens that the coating unevenness occurs in the peripheral portion of the wafer.