JPH02233174A - Coating apparatus - Google Patents

Abstract

PURPOSE:To highly control a film thickness by a method wherein monochromatic light is allowed to be incident to the surface of an object to be coated and the intensity of the reflected light from said surface is monitored during rotary coating and it is detected that a liquid film becomes a desired value, on the basis of the intensity of the reflected light. CONSTITUTION:An object 10 to be coated is irradiated with laser beam being monochromatic light from a laser 10 when or before the rotation of a motor is started and the intensity of the reflected light from said object 1 is detected by a light detector 11 and the voltage signal being the output of said detector 11 is taken in a motor control part 6. Voltage outlet V1 to be outputted at the time if a desired film thickness preliminarily calculated by experiment, the time tf required in obtaining a desired film thickness and the time t1 arbitrarily set between the time tf and the time tf-1 when voltage Vf is outputted one time ago are stored in the motor control part 6. The control part 6 continues the rotation of the motor 5 at the time of t<t1 and issues a command for the motor 5 to stop rotation at the point of time of t>t1 and V=Vf, that is, at the point of time when a desired film is obtained. By this method, the irregularity of a film thickness caused by a change of the temp. of a supplied liquid or an atmospheric condition is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a liquid coating device, and particularly to a coating device used for spin-coating photoresist or the like onto the surface of a semiconductor wafer. [Prior Art 1] Rotary coating devices have been used in various fields because it is relatively easy to uniformly coat a liquid onto a flat plate. In the semiconductor manufacturing process, it is also used to coat photoresist on silicon wafers, as shown in the separate volume of "Electronic Materials" published by Kogyo Chosenkai Co., Ltd. (December 1988 issue), pages 78-83. ing.
FIG. 7 shows a conventional coating device of this type, which is used to coat silicon wafers, etc. Apply coating liquid (2) such as photoresist to cloth body (1). (3) is the coating liquid (2)
(4) is a chuck, (5) is a mork, and (6) is a motor control unit. Next 1: The operation will be explained using the case of applying photoresist to a silicon wafer as an example. First, the motor (5
) The wafer (1) is placed on the chuck (4) connected to the chuck (4), and the wafer (1) is
is fixed to the chuck (4). Next, nozzle (3)
An appropriate amount of photoresist solution (2) is dropped onto the wafer (1). The resist liquid (2) may be dropped after the rotation of the wafer (i), which will be described below, has started. Further, the wafer (1) is rotated in conjunction with the rotation of the motor (5) via the chuck (4) according to a predetermined rotation pattern. During this time, the resist (2) spreads over the wafer (1) due to C centrifugal force while the solvent evaporates, forming a thin film. The rotation pattern such as the rotation speed and rotation time of the motor (5) is determined in advance by determining conditions based on the viscosity of the resist liquid (2) used and the desired film thickness. [Problems to be Solved by the Invention] Conventional coating apparatuses are configured as described above, and because they are uniformly operated under predetermined conditions, usually a large number of wafers are processed in succession. When coating, it is difficult to maintain a constant thickness of the resist formed on the wafer surface due to subtle changes in atmospheric conditions, supply liquid temperature, etc., resulting in variations in film thickness. there were. As a means to overcome these drawbacks, the present inventors have made an invention as outlined below (hereinafter, this invention will be referred to as the prior invention). That is, the coating apparatus shown in FIG. 8 is used to apply the coating liquid (2) to the object to be coated (1), first place the object to be coated (.1) on the chuck (4), and then apply it by vacuum suction. Fix it. After that, the coating liquid (
Drop 2) onto the object to be coated (1). In addition, the motor (
5), the object to be coated (1) and the chuck (4) are rotated at high speed for a certain period of time, and the coating liquid (2) on the object to be coated (1) is
is spread by centrifugal force to form a film of coating solution (2).
During coating, light is irradiated onto the object to be coated (1) from a light source (20), and the intensity of the reflected light is detected by a photodetector fllll. During coating, as the film thickness of the coating liquid (2) decreases, the absorption of light in the film decreases, and therefore the intensity of reflected light increases. This situation is shown in Figure 9. FIG. 5(a) is a graph showing the relationship between time t and film thickness β, and FIG. 2(b) is a graph showing the relationship between time t and reflected light intensity I. In the figure, I2r indicates the desired film thickness, and ■ and indicates the reflected light intensity when the desired film thickness is 2f. The mork rotation stop control section (12) is
As shown in Fig. 10, the intensity of reflected light from the light receiving element is determined, and the rotation of the motor (5) is stopped when the value reaches I t -e. Above, we specifically explained the preceding method.As mentioned above, by detecting that the crotch thickness has reached the desired value and stopping the rotation of the object (1) to be coated at this point, the film thickness can be increased. This makes it possible to obtain a coating device that does not cause variations in color. The present invention further advances the prior invention described above and aims to provide a can coating device that can achieve higher film thickness control accuracy.

[Means for Solving the Problems] The coating device according to the present invention makes monochromatic light incident on the surface of the object to be coated, monitors the intensity of the reflected light during spin coating, and forms a liquid film based on the intensity of the reflected light. This system detects when the desired value has been reached and stops the rotation of the object to be coated. [Operation] In the present invention, the thickness of pus is controlled with high precision using a simple device.

[Example] Hereinafter, a first example of the present invention will be described with reference to FIGS. 1 to 5. In Figure l, (1) is the object to be coated, (2)
is the coating liquid, (4) is the chuck, (5) is the motor,
These are similar to those shown in FIG. The nozzle for dropping the coating liquid was omitted. Furthermore, a monochromatic light source, a laser (lO), and a photodetector (ill), such as a photodiode, are provided.

(6) is the motor control section. Next, we will explain the operation. The coating liquid (2) dropped onto the object to be coated (1) is subjected to centrifugal force due to the rotation of the object to be coated (1) due to the rotation of the motor (5), and spreads thinly on the object to be coated. 《． When the coating liquid (2) contains an evaporable solvent, such as a photoresist used in a semiconductor process, in addition to the reduction in the liquid film due to the centrifugal force, the liquid film also decreases due to solvent evaporation. In this embodiment, monochromatic laser light is irradiated onto the object to be coated (1) from a laser (IO) at the start of motor rotation or continuously before that, and the intensity of the reflected light is detected by a photodetector. It is detected by (11). As the laser 1101, a He-Ne laser (wavelength: 6328 angstroms) of several mW is used. In addition, a photodetector (Il
A metal wire is used as l, and current output is converted to voltage output across a constant resistor for detection. At this time. A voltage of ljl l O m V or more can be easily obtained as a voltage output. Furthermore, by using a photodiode, the linearity of light intensity and voltage output is excellent, and the response speed is seven times faster than feedback control for spin coating. Although there is no particular restriction on the incident angle of the laser, in this example, the angle of incidence was set to be nearly vertical.

Now, the voltage signal output from the photodetector (ill) is taken into the motor control section (6), where a control policy is determined. In this first embodiment, the moder control section (6) determines that the film thickness has reached a desired value and stops the rotation of the motor (5). Below, the basic principle for determining whether the film thickness has reached a desired value and the control contents performed by the motor control section (6) will be explained using diagrams. Fig. 2 shows the state of the laser beam when it enters the coating liquid film portion (2) present on the m coating body (1), and the mismark bt is the liquid The reflected light reflected on the film surface, arrow b2, is the repeated light that was once dropped from the liquid droplet (2) and reflected on the surface of the object to be coated (1), and the arrow b3 is the reflected light that was reflected twice. Reflected light that is reflected three or more times is omitted in the figure. Since the laser beam has a short wavelength and is coherent, the reflected lights of bl, b2, b3, . . . interfere with each other.

Of course, if the liquid film (2) is thin, interference will occur even with monochromatic light obtained by dividing natural light. At this time, due to interference, the energy intensity of the reflected light (b) is 1! i(
The thickness of 21 fluctuates periodically with changes in flooding. Figure 3 shows the above situation. There are three graphs shown in the same figure, all of which show rotation time on the horizontal axis. The vertical axis is the film thickness 2 in graph (3-1), the reflected light intensity ■ in graph (3-2), and the output voltage V of the photodetector fill in graph (3-3). Graph (3-1) shows that the liquid film (
The graph (
3-2) shows how the reflected light intensity periodically fluctuates as the thickness of the liquid layer decreases. This vibration in the reflected light intensity occurs every time the liquid film thickness decreases by a certain amount Δe. This Δ
! is given by . e is the wavelength of the laser beam, n is the coating liquid (2
) is the refractive index of 6. In this example, a He-Ne laser is used, so the wavelength is 6328. If anx is a typical value for liquids, for example 1.5, Δβ is approximately 2l00 = It becomes 0.21μm. However, 弐m is an equation for the case of vertical incidence, and for the sake of simplicity, the following description will be made assuming that the laser beam is perpendicular to the object to be coated (1). 1 in graph (3-2). ．． I. 2 shows the maximum value f and the minimum value of the periodically oscillating reflected light intensity l, and the incident light intensity ■. Equation (2) and Equation (3) respectively
is given by

1+ RIR2+ 2JL shoulder 1+RIR2+ 2'RIR2 Here, Rl is the reflectance at the interface between sex appeal and coating liquid (2),
R, is the reflectance at the interface between the coating liquid (2) and the object to be coated (1), and is expressed by the formula! 41,151. Here, n2 is the refractive index of the coating liquid (2), and Ill and kl are the refractive index and absorption index of the object to be coated (1). Now, for example, silicon wafer (n.=3.9, k.=0.2)
Above the refractive index 1. When calculating the case where a liquid of =l,5 is applied, l. /I O =0.351, I. /
I O =0. It becomes 073. In other words, the intensity of the reflected light incident on the photodetector (fil) during spin coating changes periodically over a fairly large range of about 7% to about 35% of the laser beam intensity. As mentioned above, this periodic change occurs every time the film thickness decreases by about 2000 people. The graph (3-3) in FIG. 3 shows the time change of the voltage signal output by the photodetector (1l).
Since the photodiode has excellent linearity,
The time change of the output voltage V is similar to the time change of the reflected light intensity I. Next, control for achieving a desired liquid film thickness using the above output voltage signal will be specifically explained using FIG. 3 and FIG. 4 showing a control flowchart. Before the explanation, I will explain the symbols used in the explanation. Qt is the desired film thickness, and ■ is the voltage output that should be detected at the desired film thickness, which is determined in advance by experiment.
As you can see from the graph (3-3) in Figure 3. T.I.
The pressure VW changes to 1 many times until the desired film thickness reaches 2! j is output in return. 1+ is the time t when the desired film thickness Qr is obtained, and the time L, - when the voltage V is output one time before that.
．． It shows the time set arbitrarily between. The purpose of this sprouting is to prevent variations in film thickness caused by fluctuations in the supply liquid temperature and atmospheric conditions.
The fluctuations in 1 and t r-+ are usually small. Moreover, tr
and t. -, for example, when the desired liquid film thickness is about 1 μm as in the application of photoresist liquid in a semiconductor process, it is quite long, so it is easy to determine t1 as f. be. This is because in spin coating, the liquid film thickness changes exponentially with time, so the thinner the liquid film becomes, the longer it takes to decrease the same film thickness. In addition, in liquids containing solvents such as photoresist liquids, the viscosity of the liquid increases rapidly as the shoulder medium evaporates, making it difficult to flow. Next, the control method will be explained using Fig. 4. First, a time t is measured from the start of rotation of the motor (5). Detection of reflected light is performed at or before the start of motor rotation, and a voltage signal V is constantly taken into the motor writing section {6}. Further, the pre-given values of V and 1+ are written in the motor control section (6). During spin coating, the motor control unit (6) determines whether time t is greater or less than tl, and if 1 < 1 +
If so, the motor (5) is instructed to continue rotating. Then, when 1>1+ and V=V, that is, when the desired crotch thickness is reached, the motor (5)
An instruction is given to stop the rotation of the

Finally, we will discuss the film thickness control accuracy of this invention. As mentioned above, when applying the photoresist liquid (2) to the silicon wafer (1), the ratio between the minimum and maximum voltage output values is about 1:5. Full scale 100m
Measurement shall be made with a voltmeter in the V range. If the voltage output level is adjusted appropriately, the maximum value of 0mV. I can be set to 20+mV. Yu. Furthermore, the desired 11q
J! l! , and the refractive index of the coating liquid (2) is 02, the light source (
1) By appropriately selecting the wavelength and incident angle of light, the output voltage is set to 60 mV at Qt. FIG. 5 shows this situation. In the figure, point P is the time depending on the desired thickness, the point of the previous minimum voltage, and point Q is t.
It shows the point of maximum voltage immediately after r (of course, during operation, rotation stops at t).

As already mentioned, the output voltage cycle is approximately 2
This happens every time the number of people decreases by 000, so the difference in film thickness between point P and point () is 10008. Therefore, the film thickness measurement sensitivity S between points P and Q is approximate. Now, since we are considering measurement using a voltmeter with a full scale of 1 0 0 V, it is easy to obtain a measurement accuracy of, for example, 1 mV, and therefore it is easy to achieve a 12.5 mm accuracy. In fact, it is easy to further improve accuracy using current electrical A1 measurement technology. From the above, it can be seen that the present invention has sufficient film thickness control accuracy. The first embodiment of this invention has been described above, in which a predetermined distance and voltage output value are used to detect when the film thickness has reached a predetermined value. FIG. 6 is a flowchart showing the control of this second embodiment. In the figure, ■ represents the output voltage obtained by the reflected light intensity detector (!l), and V represents the time differential of the output voltage V. By the way, as shown in Fig. 3, in spin coating, the change in crotch thickness gradually becomes smaller with time.
- the time required to decrease and from there further l It
The time required for tm to decrease and lum to increase in thickness differs by more than five times even when there is no evaporation of the solvent and the viscosity of the liquid does not change. And as the thickness of the liquid film decreases, the reaction q+
Changes in light intensity also become more gradual over time. Therefore. Intensity V of reflected light when desired crotch thickness I2f
, is determined in advance through experiments under conditions such as a predetermined rotational speed, then it is possible to determine whether V and V are equal to vf and V, respectively. It is possible to judge whether the liquid film thickness at that time is the desired value l. In order to know the value of M from the detected value in (2), it is sufficient that the motor control section (6) has a built-in memory and a timer.

Next, another embodiment will be described in which the reliability of the coating device of the present invention is further improved. First, in the third embodiment, while detecting the reflected light intensity during spin coating, the control unit stores the maximum value Vp repeatedly output from the photodetector during spin coating. Then, the voltage output Vr when the predetermined film thickness β is achieved is determined in advance. The ratio X to Then, instead of the output voltage V, monitor x = V / V p, and this value becomes x
The criterion for stopping the motor is whether it is equal to r. Of course, since X is periodically equal to X, another criterion is required, as in the first and second embodiments. By being equipped with a control unit that can make the above decisions,
The third embodiment has the effect that it is not necessary to change the memory value of the control unit in response to long-term changes in the signal level due to, for example, fluctuations in the laser output or dirt on the detection surface of the reflected light detector. , which can further improve reliability.

In addition, as a fourth embodiment, using the same idea as the third embodiment, a local minimum value may be used instead of the local maximum value of the voltage output, or even a local maximum (direct minimum value) may be used. Furthermore, as a fifth embodiment, when the object to be coated (1) has a small absorption index k2, such as a silicon wafer, the maximum value of the reflected light intensity during coating may be determined by a coating without a liquid film. It is known that the intensity of the reflected light from the surface of the coating material is almost equal to that of the coating material, so instead of detecting the maximum value during coating, the intensity of the reflected light is detected before the coating droplets are applied and stored in the control unit. The same effect can be obtained by controlling the ratio of the value of , and the intensity of reflected light detected during coating.

[Effects of the Invention] As described above, according to the present invention, the rotation of the object to be coated can be stopped at high speed using a simple device that uses the reflection intensity of monochromatic light from the surface of the object to be coated as a motor to stop the rotation of the object to be coated. Since film thickness can be determined with high precision, it is possible to reduce variations in film thickness during spin coating, which greatly contributes to improving the quality and yield of semiconductor products, for example.

[Brief Description of the Drawings] Figures 1 to 5 show a first embodiment of the present invention, with Figure 1 being a schematic side view and Figure 2 explaining the optical path of the light ray in the liquid film section. Fig. 3 is a time variation characteristic diagram of film thickness, reflected light intensity, and output voltage, Fig. 4 is a flowchart to explain the operation, and Fig. 5 is a diagram to explain control accuracy. be. FIG. 6 is a flowchart for explaining the operation of the second embodiment. FIG. 7 is a schematic side view of a conventional coating device, FIG. 8 is a schematic side view of a coating device according to the prior invention, and FIG. Figure 10 is a flowchart for explaining the operation of Figure 8. (1) is the object to be coated, (2) is the coating liquid, (5) is the motor,
(6) is a motor control unit (control means), and (1ro) is a monochromatic light source. +11+ is a photodetector. In addition, the same reference numerals in the figures indicate the same or equivalent parts. Representative Person So Ga Michi Teruaki 1 Figure 1: 2: 6・ 10: 11: Branch coating 11 pieces 1 half night t 1 ta 5 1 lit Sap child J 1 Chit-r (I+!'ltH) ηi Hinoki divination ``Myself'' comic yf) Figure 3 (b) Leather figure 9 Shudo (τ shoulder figure 10)

Claims (1)

[Scope of Claims] A coating device that drops a coating liquid onto an object to be coated, rotates the object to be coated, and spreads the coating liquid by centrifugal force to form a film of the coating liquid on the object to be coated, A monochromatic light source that makes monochromatic light incident on the surface of the object to be coated, a photodetector that detects the reflected light intensity of this incident light, and a rotation of the object to be coated based on the light intensity detected by the photodetector during coating. A coating device characterized by comprising a control means for stopping.