JPH03245875A - Application of coating solution - Google Patents

Abstract

PURPOSE:To eliminate or reduce the thickness irregularity of a membrane in forming the membrane to a large object to be coated and to enhance yield by lowering the number of rotations of the object to be coated on and after a time when the thickness of the membrane is markedly reduced by the evaporation of a solvent after the rotation of the object to be coated is started. CONSTITUTION:An object 11 to be coated is fixed on a chuck 12 and a coating solution 14 is dripped from a nozzle 15. Subsequently, a motor 13 is driven to pre-spin the object 11 to be coated and the object 11 to be coated is continuously rotated at high speed up to the number NH of rotations. After the object 11 to be coated is rotated constant in this number NH of rotations for predetermined time, the rotational speed thereof is lowered to low speed NL at the time t1 when the evaporation of a solvent begins to dominate film thickness. When the number of rotations is lowered at the time t1 when the reduction of film thickness due to the evaporation of the solvent begins to become marked, turbulent flow is made hard to generate in the vicinity of the surface of the object to be coated. Therefore, the generation of difference in film thickness between the outer peripheral part and center part of the object 11 to be coated is suppressed and a membrane having almost uniform thickness can be formed to the entire surface of the object to be coated.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a coating liquid coating method for spin coating a resist or the like onto the surface of a semiconductor wafer, for example.

[Conventional technology] Conventionally, semiconductor wafers (hereinafter simply referred to as wafers) are used.

) is a technique for forming a thin film of uniform thickness on an object to be coated, such as spin coating. This is based on the literature (Kogyo Kenkyukai Co., Ltd., published December 13, 1986, "Electronic Materials J 19
As described in the December 1989 special issue, pages 78-83), a wafer coated with a resist solution is rotated at high speed, and the resist solution is spread over the wafer by centrifugal force, and the solvent in the resist solution is evaporated. This process forms a resist film. This will be explained with reference to FIGS. 4 and 5.

Fig. 4 is a schematic configuration diagram showing a spin coater used in the conventional spin coating method, and Fig. 5 is a graph showing changes in the rotational speed of the object to be coated from the start of rotation to the end of rotation in the conventional spin coating method. be. In these figures, 1 is an object to be coated such as a wafer, 2 is a chuck that fixes the object 1 to be coated by a method such as vacuum suction, and this chuck 2 is connected to a motor 3.
It is configured to rotate at high speed. 4 is a coating liquid such as a resist liquid, and 5 is a nozzle for supplying the coating liquid 4 onto the object 1 to be coated.

In order to form a thin film such as a resist film on the object 1 to be coated using the conventional spin coater configured as described above, first, the object 1 to be coated is positioned on the chuck 2, and the coating liquid 4 is applied from the nozzle 5. Drop onto the object 1 to be coated. Next, as shown in FIG. 5, the motor 3 is driven to cause the object to be coated 1 to bliss spin, and then to rotate at high speed together with the chuck 2 for a certain period of time.

When the object to be coated 1 is rotated at high speed in this manner, the coating liquid 4 is spread over the entire upper surface of the object to be coated 1 due to centrifugal force, and the thickness of the coating liquid 4 is reduced. After that, the film freezes significantly due to the explosion of the solvent contained in the coating liquid 4.
At the same time, the viscosity of the coating liquid 4 increases, and the contribution of centrifugal force to film reduction further decreases. Finally, the decrease in the diffusion coefficient within the liquid phase makes it difficult for the solvent to move within the film, making it impossible to evaporate from the gas-liquid interface, and the film thickness decrease stops. In this way, a thin film is formed on the object 1 to be coated.

By the way, large wafers (
For example, when coating is performed using a wafer (with a diameter of 8 inches), at conventional rotation speeds (4000 rpm to 6000 rpm), the film thickness becomes thicker at the outer periphery of the wafer, resulting in a film thickness distribution within the wafer surface. The inventors have experimentally and theoretically confirmed that this occurs due to in-plane non-uniformity of solvent evaporation. The theory is explained below in Figures 6 and 7 (a
This will be explained using L(b).

Figure 6 is a schematic diagram showing the airflow near the surface of a wafer being rotated by a spin coater used in the conventional spin coating method, and Figures 7 (a) and (b) are This is a graph showing the film thickness distribution of the formed thin film, where (a) shows a non-defective product and (b) shows a defective product. In FIG. 6, reference numeral 6 indicates an airflow near the surface of the object to be coated 1, which is a wafer (surface airflow). Also, Figures 6 and 7
In the figure, the range indicated by l is the laminar region where the surface airflow becomes laminar, the range indicated by 11 is the transition region where the surface airflow transitions from laminar to turbulent, and the range indicated by iii is the turbulent region where the surface airflow becomes turbulent. shows. When the object 1 to be coated is rotated, the surface airflow 6 changes from a laminar flow to a transition flow at the wafer outer circumference r, and then changes to a transition flow r.
At the position t, the transition flow changes to turbulent flow.

The condition can be expressed by the Reynolds number shown in equation (1).

r; radial position (cm ) ω; angular velocity (rad/s) ν;
Kinematic viscosity coefficient (cd=/s) Condition for transition from laminar flow to transition flow (critical Reynolds number Re,)
and the conditions for transitioning from transition flow to turbulence (transition Reynolds number R
et) is shown below.

Re, = 0.88X10' ・−・−−・(21Re
t-3, 20X105...-13) By the way, the decrease in the resist liquid film thickness is caused by (1) flow due to centrifugal force and (2) solvent thinning. Transition flow and turbulence of surface air flow have a large influence on ■solvent formation, and the amount of evaporation also changes depending on the radial position. In other words, ■ Mass transfer rate α in the turbulent region. has radial position dependence, and as the radial position increases, the amount of evaporation increases, causing uneven evaporation. On the other hand, in the laminar region, there is no radial position dependence and uneven evaporation does not occur.

Accordingly, when the surface airflow over the entire surface of the wafer is in a laminar region, a film of the coating liquid 4 with a uniform thickness as shown in FIG. 7(a) without evaporation unevenness is obtained (hereinafter referred to as a good product). When coating at high rotation speeds or using large wafers, the surface airflow within the wafer surface is laminar from the inside (0<r<ri), transitional flow (r, rt), and turbulent flow (rt<r). becomes. At this time, the coating liquid flows from the inside (laminar region ■) to the outside (turbulent region ii).
As it flows towards the center, the amount of evaporation is greater there than on the inside, the viscosity of the liquid increases, the liquid accumulates, and the film thickness becomes thicker than in the center. This film of the coating liquid 4 with non-uniform film thickness distribution (hereinafter referred to as a defective product) will be explained with reference to FIG. 7(b).

In the laminar region (i), the film thickness is uniform, but in the transition region (ii) beyond r, it gradually changes to the turbulent region (ii) beyond rl.
In i), the coating film thickness increases rapidly.

If pattern line width checking in the exposure and development steps is carried out in this state, uneven pattern line widths will occur. Since the parts become defective, the yield rate decreases.

By the way, as mentioned above, in the spin coating of resist liquid,
The decrease in liquid film thickness is caused by (1) flow due to centrifugal force and (2) solvent evaporation. This situation is shown in FIG.

FIG. 8 is a graph showing changes in the film thickness of a coating liquid applied by the conventional spin coating method, in which the horizontal axis shows the logarithm of time and the vertical axis shows the logarithm of film thickness δ. In FIG. 8, the change in film thickness at the center of the object 1 to be coated 1 is shown by a solid line, and the change in film thickness at the outer periphery is shown by a broken line. Figure 7 shows the change in film thickness over time in four regions (I
~■). Region I is a region immediately after the start of rotation, where the film thickness rapidly decreases when subjected to acceleration. In region ■, the film thickness has a certain relationship with time under a constant rotation speed (
δ, ct-17Z). This indicates that in this region (2), the film thickness is reduced due to (1) centrifugal flow. Then, at a certain time (region ■), this relationship breaks down and the film thickness decreases rapidly again. This is because in this region (2), film thinning due to (2) solvent evaporation was more pronounced than in (1) centrifugal force flow. Finally, in area ■, the coating liquid becomes difficult to flow due to the rapid increase in the viscosity of the liquid due to one shot of the solvent in area ■, and a layer of resist with a low solvent concentration is formed on the surface of the liquid film. By preventing the evaporation of the solvent, there is almost no decrease in the film thickness.From the above, it can be seen that the turbulence of the air flow on the wafer surface has a large effect on the film thickness distribution after region ①. In other words, as shown in Fig. 8, at the initial stage (region I-region) when the film thickness is decreasing mainly due to centrifugal flow, there is almost no difference in film thickness between the outer periphery and the center. When the thickness decrease comes to be dominated by solvent evaporation (region ■), the film has a thickness distribution between the center (solid line) and the outer periphery (dashed line).

[Problem to be solved by the invention]

In the conventional coating method described above, when a large wafer is used, the airflow on the wafer surface transitions to turbulence at the periphery of the wafer, resulting in a difference in the amount of solvent evaporation between there and the center, resulting in the formation of The thickness of the coating liquid film is different. For this reason, there is a problem in that the object to be coated cannot be uniformly processed when etching is performed in a treatment step after the coating step.

[Means to solve the problem]

In the method for applying a coating liquid according to the present invention, the rotational speed is decreased after the rotation of the object to be coated is started and the film thickness is significantly reduced due to solvent evaporation.

[For production]

According to the method for applying a coating liquid according to the present invention, since the rotational speed is lowered when film thinning due to solvent evaporation becomes significant, it is possible to prevent turbulence from occurring near the surface of the object to be coated at this time.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to FIGS. 1 to 3.

FIG. 1 is a schematic configuration diagram showing a spin coater used in the method for applying a coating liquid according to the present invention, and FIG. 2 is a diagram showing the start and end of rotation of the object to be coated, which is rotated by the method for applying a coating liquid according to the present invention. Graph showing the change in rotation speed up to 3rd
The figure is a graph showing changes in film thickness from the start of rotation to the end of rotation of the coating liquid applied by the coating liquid coating method according to the present invention. In FIG. 1, 11 is an object to be coated such as a wafer, 12 is a chuck that fixes the object to be coated by a method such as vacuum suction, 13 is a motor that rotates this chuck 12 at high speed, and 14 is a motor that rotates the chuck 12 at high speed. The coating liquid 15 is a nozzle for supplying the coating liquid 4 onto the object 1 to be coated, and this spin coater is the same as a conventional one.

Next, a method for applying the coating liquid of the present invention will be explained. To form a thin film such as a resist film on the object 11 to be coated, first, the object 11 to be coated is fixed on the chuck 12, and the coating liquid 1 is applied.
4 onto the object 11 to be coated through the nozzle 15. Next, as shown in FIG. 2, the motor 13 is driven to pre-spin the object 11 to be coated, and then to continue to rotate at a high speed up to the rotational speed NK. After the object to be coated 11 is rotated for a predetermined period of time at this rotational speed NH, the rotational speed is reduced to a low speed NL at time t1. The time t1 is the time when solvent evaporation starts to dominate film thinning, and the time t1 is the time when solvent evaporation starts to dominate film thinning. is measured in advance through experiments or the like.

Further, in this embodiment, the rotation speed N is set to such a speed that no turbulent flow occurs near the surface of the object 11 to be coated.

Note that this rotational speed NL is desirably set to a rotational speed at which even transitional flow does not occur.

This rotational speed N is 480,000/R" Crprr+] or less, preferably 130,000, using 2(1,), (2) and (3) and substituting the kinematic viscosity coefficient of air at 20°C. /R2 [r pm) or less. Note that R indicates the radius (an) of the object 11 to be coated.

Therefore, as described above, if the rotational speed is lowered at time t1 when the decrease in film thickness due to solvent evaporation becomes noticeable, it is possible to make it difficult to generate turbulent flow near the surface of the object to be coated at this time. Therefore, the difference in film thickness between the outer periphery and the center of the object 11 to be coated can be suppressed, and a thin film with a substantially uniform thickness can be formed over the entire surface to be coated, as shown in FIG. can be formed.

〔Effect of the invention〕

As explained above, the method for applying the coating liquid according to the present invention is as follows:
The rotation speed is lowered after the coating object starts rotating and the film thickness is significantly reduced due to solvent evaporation, so the rotation speed is lowered when the film thickness decreases significantly due to solvent evaporation.
At this time, it is possible to make it difficult to generate turbulent flow near the surface of the object to be coated. Therefore, when forming a thin film on a large object to be coated, unevenness in film thickness can be eliminated or reduced, yield can be improved, and processing costs, material costs, etc. can be kept low.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram showing a spin coater used in the method for applying a coating liquid according to the present invention, and FIG. 2 is a diagram showing the start and end of rotation of the object to be coated, which is rotated by the method for applying a coating liquid according to the present invention. Graph showing the change in rotation speed up to 3rd
The figure is a graph showing changes in film thickness from the start of rotation to the end of rotation of the coating liquid applied by the coating liquid coating method according to the present invention. Fig. 4 is a schematic configuration diagram showing a spin coater used in the conventional spin coating method, and Fig. 5 is a graph showing changes in the rotational speed of the object to be coated from the start of rotation to the end of rotation in the conventional spin coating method. be. Figure 6 is a schematic diagram showing the airflow near the surface of a wafer being rotated by a spin coater used in the conventional spin coating method, and Figures 7 (a) and (b) are This is a graph showing the film thickness distribution of the formed thin film, where (a) shows a non-defective product and (b) shows a defective product. FIG. 8 is a graph showing changes in film thickness of a coating liquid applied by a conventional spin coating method. 1...Object to be coated, 2...Chuck, 4...Coating liquid. Agent Masuo Oishi 11Iusxt 11Iusxt 11Iusxt 11Iusxt 11Iusxt 11Iusxt 11Iusxt 11Iusxt 11Iusxt 11Iusxt 3D

Claims (1)

[Claims] In a method for applying a coating liquid, in which an object to be coated onto which a coating liquid has been dropped is rotated to evaporate a solvent in the coating liquid to form a coating liquid film on the surface of the object to be coated, after the rotation of the object to be coated is started. A method for applying a coating liquid, characterized in that the rotational speed is reduced after the film thickness is significantly reduced due to solvent evaporation.