JPH0810666B2 - Pattern formation method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a pattern forming method, and more specifically, a projection exposure apparatus is used to form a good photoresist pattern not only in the middle portion of a step but also in the upper and lower portions. Can be formed,
The present invention relates to a pattern forming method.

[Conventional technology]

In the projection exposure method, conventionally, the focus latitude of the exposure optical system strongly depends on the numerical aperture of the projection lens and the exposure wavelength. The depth of focus of a projection lens is inversely proportional to the square of its numerical aperture, and is proportional to the exposure wavelength. Therefore, the depth of focus has been decreasing by increasing the numerical aperture of the lens and shortening the wavelength of exposure light in order to improve resolution. There is. For this reason, it is becoming difficult to deal with the image plane distortion of the projection lens, the increase in the unevenness of the substrate surface and the inclination of the substrate. The step due to the relatively fine pattern has been smoothed by the multilayer resist method so far. Regarding the multi-layer resist method, for example, Journal Off Vacuum Science and Technology, B1 (4) (J.Vac.Sci.Technol.B1 (4)
(1983)) pp. 1235 to 1240.

However, even if the above method is used, it is not possible to completely flatten the step formed above and below the large area pattern. In the control system of the conventional projection exposure apparatus, exposure at the same position on the substrate is performed by fixing the image plane to one point on the optical axis. Therefore, to resolve the pattern above and below the unevenness of the substrate surface,
The exposure was performed by setting the image plane at an average position above and below the step, but in the above method, when the step height becomes larger than the depth of focus of the projection lens, resolution failure occurs above or below the step. There was a problem.

[Problems to be solved by the invention]

As semiconductor integrated circuits have become highly integrated in recent years, it has been required to cope with the miniaturization of patterns and the increase of uneven steps on the substrate surface. When the projection exposure method is used for pattern formation, a larger depth of focus is required as the exposure optical system in order to cope with an increase in uneven steps. On the other hand, in order to improve the resolution, it is necessary to increase the numerical aperture of the projection lens or to shorten the wavelength of the exposure light, so the depth of focus is conversely decreasing. Furthermore, because the image plane is not a perfect plane due to the image plane distortion of the projection lens and the substrate surface is inclined, it becomes difficult to secure the depth of focus corresponding to the uneven surface step over the entire exposure area. ing.

With the above-mentioned multilayer resist method, it is not possible to completely flatten the uneven steps of the large area pattern, and even if perfect flattening can be achieved, the image plane of the mask pattern is the surface of the substrate due to the image plane distortion of the lens. Therefore, it was difficult to deal with the above problems.

In order to prevent size reduction due to defocus in projection exposure, JP-A-58-17446 proposes a method of performing exposure while vibrating a substrate having a resist film formed on its surface in the optical axis direction. There is.

Further, US Pat. No. 4,239,790 proposes a method of performing exposure while vibrating the substrate in the optical axis direction using piezo blocking in order to prevent the occurrence of standing waves in the resist film.

However, according to the study by the present inventor, although the former was tentatively recognized to have the effect of increasing the depth of focus, the contrast was lowered and the image formation on the upper and lower portions of the step was poor, and the upper and lower portions of the step were It has been found difficult to form a good pattern on everything.

In addition, since the latter vibrates the substrate using a piezo element, not only does the contrast deteriorate as in the former, but the amplitude of the vibration is small, so that the effect of expanding the depth of focus that is practically satisfactory can be obtained. Hard to get.

The object of the present invention is to solve the above-mentioned conventional problems and to effectively provide a good pattern not only in the central part of the step existing on the substrate surface but also in the upper part and the lower part of the step having a deep depth of focus. Can be formed with sufficient contrast,
It is an object of the present invention to provide a projection exposure apparatus and a pattern forming method using the projection exposure apparatus, which are capable of coping with the increase in the unevenness of the substrate surface in recent years, the image surface distortion of the projection lens and the increase of the numerical aperture and the shortening of the wavelength of the exposure light. .

[Means for solving problems]

The increase of the effective depth of focus of the exposure optical system is achieved by superimposing a plurality of lights having different image formation points on the same optical axis. Here, the image formation point means a point on the conjugate plane of the mask pattern with respect to the exposure optical system. Therefore, when exposing a pattern on a resist-coated substrate, the conjugate plane of the mask pattern, that is, the image plane, is set at a plurality of different positions relative to the resist layer on the optical axis, and is stepwise. The above object was achieved by performing exposure.

Fig. 5 shows the relationship between the position in the optical axis direction and the calculated value of the light intensity contrast of 0.7 µm line and space, in the case of a single image point, 2 µm, 3 µm, 3.5 µm and 5 µm apart.
A case where light is combined with a point as an image forming point is shown. In FIG. 5, the origin of the position in the optical axis direction is the center point of the single image formation point and the two image formation points. As shown in the figure, the light intensity contrast absolute value decreases due to the combination of lights having different image forming points compared to the case of a single image forming point, but the contrast above a certain level should be maintained in a wider range. You can Further, by setting the distance between the two image formation points to an appropriate value, a constant light intensity contrast can be obtained within a certain range in the optical axis direction. The effective increase rate of the depth of focus by this method is determined by the lower limit of the light intensity contrast that can be resolved by the material and process such as the resist, developer, and contrast enhancement material used. According to FIG. 5, the increase rate of the effective depth of focus by using the above method is 45% when the lower limit of the light intensity contrast is 0.5 when the distance between two image forming points is 3.5 μm. On the other hand, when the lower limit of the light intensity contrast is 0.4, it becomes about 70%. Further, when the lower limit is 0.3, it is possible to superimpose three lights having different image planes, which improves the effective depth of focus by about 150%.

[Action]

In order to satisfactorily resolve the pattern in the resist layer above and below the unevenness of the substrate surface over the entire exposed area,
The light intensity contrast must be maintained above a certain level within a certain range in the optical axis direction, which is determined by the image plane distortion of the projection lens, the flatness of the substrate, and the height of the unevenness of the substrate surface. On the other hand, in the projection exposure method, the contrast of the light intensity for forming the pattern on the resist layer has a value sufficient to faithfully reflect the mask pattern,
It is only in the vicinity of the conjugate plane of the mask pattern, that is, the so-called image plane, and the contrast drops sharply as the distance from the conjugate plane increases.

The contrast of the light obtained by combining the lights having the image forming points at different positions on the same optical axis is the average of the contrasts of the respective lights. Therefore, even if the desired contrast cannot be realized in the entire range where light intensity contrast is required in the optical axis direction at a single image formation point, multiple light beams having image formation points at different positions within the above range can be overlapped. As a result, it becomes possible to maintain the contrast above a certain level in the entire range.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings. First
FIG. 4 is a block diagram showing an embodiment of the projection exposure apparatus according to the present invention, FIG. 2 is a flow chart showing the operation sequence of the above embodiment, FIG. 3 is a conceptual diagram showing the operation of the above embodiment, and FIG. FIG. 5 is a timing chart showing the operation of the embodiment, and FIG. 5 is a curve diagram showing the effect of the present invention.

First Embodiment An apparatus according to a first embodiment shown in FIG. 1 includes a reticle 1, a projection optical system 2, a substrate stage composed of an XY stage 3 and a Z stage 4 (hereinafter, Z axis in the optical axis direction, X axis, (By setting the Y axis in a plane perpendicular to the optical axis), the XY sensor 5 and the Z sensor 6 that detect the position of each stage, the control system that controls the entire apparatus, and various types required for a normal projection exposure apparatus. It is composed of components. The control system specifies a computer 7 that controls the entire apparatus and an exposure shutter control system 8 that opens and closes the exposure shutter, an XY control system 9 that drives the XY stage to a designated position, and a Z stage according to a command from the computer. Z control system to drive to the specified position 1
0, a circuit including a multiplex imaging exposure control system 11 that performs movement of the substrate position only in the optical axis direction and commands the shutter control system 8, and the computer 7 and each control system are connected via a bus line. Connected.

The exposure position on the substrate and the exposure mode at each exposure position are stored in advance in the computer 7. Here, the exposure mode is set when exposing the same position on the substrate.
It refers to the number of different image planes with respect to the substrate, the position on the optical axis thereof, and the exposure amount on each designated image plane. For the exposure position where the number of the image planes designated by the exposure mode is 1, the computer 7 controls the exposure by the normal step-and-repeat method, but the exposure position where the number of the image planes is 2 or more. In contrast, after the position on the substrate is moved to the exposure area of the apparatus by driving the XY stage 3, the control of the exposure sequence is transferred to the multiplex imaging exposure control system 11. FIG. 2 shows the sequence of this apparatus when the number of image formation points at all exposure positions is two. The operation of this apparatus will be described below with reference to FIG. The multiple imaging exposure control system 11 drives the Z stage 4 and sets the relative position of the imaging point with respect to the substrate to a position as shown in FIG. 3 (a), for example. At this time,
The displacement of the substrate in the XY direction due to the movement of the Z stage 4 is fed back to the XY control system 9 via the XY sensor 5 and is corrected by the drive of the XY stage 3, so that the substrate position is moved only in the optical axis direction. Become. Upon confirming that the substrate has been correctly set to the designated position, the multiple imaging exposure control system 11 instructs the exposure shutter control system 8 to perform exposure for the designated time on the relevant imaging plane position. Then, exposure 1 is performed. After confirming the end of the exposure 1, the multiple imaging exposure control system 11 again moves the substrate in the optical axis direction and sets the relative position of the imaging surface to the substrate surface to the position shown in FIG. 3 (b), for example. Exposure 2 is performed. Similarly, when the number of image forming planes designated by the exposure mode is 3 or more, the setting of the image forming plane position with respect to the substrate and the exposure are repeated for all the designated image forming planes. After all the above-mentioned operations are completed for the same position on the substrate, the computer 7 drives the XY stage 3 for the first time to move the next designated exposure position on the substrate to the exposure area of the apparatus. FIG. 4 (a) shows a timing chart of the above sequence. In this way, the multiple imaging exposure control system 11 moves the substrate position once in the XY direction (changing the exposure position).
On the other hand, it has become possible to perform the Z-direction movement and the exposure a plurality of times. For reference, a timing chart in the case where the number of image planes in the exposure mode is 1 for all exposure positions is shown in FIG. In the conventional apparatus, exposure could be performed only in the sequence shown in FIG. 4 (b).

In this embodiment, the movement of the stage in the optical axis direction and the exposure are performed at different timings.
11, it is also possible to arbitrarily perform the above timing.

Further, the control system shown in this embodiment moved to the next exposure position after performing the overexposure at a certain exposure position on the substrate, but the overexposure was performed at all the designated positions on the same substrate in the designated exposure mode. If so, there is no particular need to follow the exposure sequence of this embodiment. That is, for example, for all exposure positions on the substrate, the optical axis walk from the substrate surface first by the usual step-and-repeat method.
It is also possible to form an image at a position of m and perform exposure, and then perform image formation at a position of +2 μm in the optical axis direction from the substrate surface and perform exposure again at all designated positions.
However, the substrate must remain fixed to the stage throughout this period.

Using this device, a grid pattern of 1 μm pitch and 0.5 μm
A hole pattern having a diameter of m and the like were exposed and developed on a substrate having unevenness steps of various heights on the surface. In the conventional exposure with a single image plane, the height of the step must be 0.5 μm or less in order for each of the above patterns to be resolved above and below the unevenness of the substrate surface over the entire exposure area. . But,
By the multiple imaging exposure function of the present apparatus, exposure is performed by setting the imaging surface at two positions 3 μm apart from each other across the resist surface with respect to the substrate and exposing the entire surface of the exposure area above and below the step of 2 μm. Was able to be resolved. Further, with respect to the hole pattern, the height of a resolvable step was increased to 10 μm by setting and exposing the image forming plane at three positions relatively separated from the substrate by 3 μm. Further, the effect of the present apparatus is further enhanced by using a multi-layer resist method, a high contrast developer, a high contrast resist, a contrast enhancement material and the like. In this embodiment, the mutual position of the image plane with respect to the substrate was changed by moving the substrate stage in the optical axis direction, but the method is not necessarily limited to the above method.
Other known methods may be used. For example, there is a method of sealing the space between the lenses and injecting gas into the sealed space to change the pressure of the gas, or a method of moving the reticle or the lens in the optical axis direction.

Further, when a multiple exposure control system is combined with a projection exposure apparatus using an excimer laser as a light source, the conventional i-line, g
The effect of increasing the effective depth of focus by the multiple imaging exposure method similar to that for the ultraviolet light source such as −line was obtained, and the lack of the depth of focus peculiar to the excimer laser exposure apparatus could be overcome.

Further, by making the projection optical system telecentric, it is possible to eliminate the fluctuation of the reduction ratio due to the change of the image plane position, and the pattern dimensional accuracy on the entire surface of the chip is improved.

Second Embodiment A second embodiment is one in which a flat plate or lens made of a substance transparent to the exposure wavelength is provided in the exposure optical system of the projection exposure apparatus, which has a refractive index different from that of air and can be arbitrarily taken in and out. Here is an example. The above flat plate or lens sets or adjusts the thickness, the refractive index, and the orthogonality to the optical axis so that the image plane near the substrate can be moved only by the desired distance only in the optical axis direction when inserted. Has been done. Also, the thickness,
A plurality of flat plates or lenses having different refractive indexes can be inserted, and the combination of these allows the image plane position to be set variously.

The control for putting in and out the flat plate or the lens is the first
The multiple imaging exposure control system having the same function as that of the embodiment is used, and during the exposure of the same position on the substrate, the exposure is interrupted at an arbitrary timing, and the plane plate or the lens is taken in and out to form an image plane on the substrate. The position can be changed.

Using the above-mentioned projection exposure apparatus, the same experiment as in the first embodiment was carried out, and the same effect was confirmed.

Third Example The entire exposure optical system of the projection exposure apparatus was installed in a closed container, and the atmospheric pressure in the closed container was quickly changed so that it could be set to an arbitrary value. By using the above function, the exposure is interrupted during the exposure, the atmospheric pressure in the closed container is quickly changed, and then the exposure is restarted, so that the image forming plane of the mask pattern is changed before and after the interruption. It was possible to set to. The control of the atmospheric pressure in the closed container is performed by the multiplex imaging exposure control system as in the first embodiment.
An experiment similar to that of the first embodiment was conducted using the above apparatus, and the same effect could be confirmed.

〔The invention's effect〕

As described above, the projection exposure apparatus according to the present invention is a projection exposure apparatus that projects and exposes a mask pattern onto a substrate. At the same position on the substrate, at least two points designated in advance on the same optical axis with respect to the substrate are specified. Since the effective depth of focus can be increased by having multiple image forming exposure control means for setting different image forming points and exposing at respective positions of the image forming points, it is possible to increase the numerical aperture of the projection lens, It becomes possible to cope with an image surface distortion and an increase in unevenness of the substrate surface. Further, according to the present invention, since the exposure is performed without vibrating the substrate, the contrast is not significantly lowered due to the vibration of the substrate,
The depth of focus can be effectively increased, which is a great feature of the present invention. The amount of increase in the focus latitude according to the present invention depends on the lower limit of the light intensity contrast and the type of pattern that can be resolved by the material / process to be used. Approximately 70% of the pattern is set by setting two image planes that are separated by a distance equivalent to the focus latitude during exposure with that single image plane, and by setting three image planes 150
It is possible to increase the focus margin by%. Further, for the hole pattern, the focus latitude can be increased virtually unlimitedly by further increasing the number of image planes.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the projection exposure apparatus according to the present invention, FIG. 2 is a flow chart showing the operation sequence of the above embodiment, and FIGS. 3 (a) and 3 (b) show different image forming points. 4A is a conceptual diagram showing the operation of the above embodiment, FIG. 4A is a timing chart showing the operation of the embodiment, and FIG. 4B is a timing chart when the number of image planes in the exposure mode is 1 for all exposure positions. FIG. 5 is a curve diagram showing the effect of the present invention. 4 ... Z stage, 6 ... Z sensor 7 ... computer, 8 ... exposure shutter control system 10 ... Z control system 11 ... multiple imaging exposure control means

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Norio Hasegawa 1-280 Higashi Koigakubo, Kokubunji, Tokyo Inside Central Research Laboratory, Hitachi, Ltd. (72) Toshihiko Tanaka 1-280 Higashi Koigakubo, Kokubunji, Tokyo Hitachi Ltd. Central Research Laboratory of Manufacturing Co., Ltd. (56) Reference JP-A-58-17446 (JP, A) US Patent 4239790 (US, A)

Claims (5)

[Claims] 1. A plurality of projections of the same mask pattern are formed on the same region of a photoresist film formed on a substrate to be exposed having a step on the surface through an optical system, and then the photoresist film is developed to give a predetermined pattern. In the method of forming a photoresist pattern, the multiple projection exposure forms an image of the same mask pattern at a first position on the substrate to be exposed that is higher than an intermediate portion of the step, and forms the same mask. Projecting and exposing the image of the pattern while still at the first position, and forming the image of the same mask pattern at a second position of the substrate to be exposed, which is lower than the intermediate portion of the step. At least including the step of projecting and exposing the formed image of the same mask pattern at the second position, and the light intensity in the upper portion, the lower portion, and the intermediate portion of the step due to the multiple projection exposure. Pattern forming method characterized by substantially constant contrast. 2. The pattern forming method according to claim 1, further comprising a step of storing at least the first position and the second position. 3. The number of positions in the optical axis direction of the optical system at which the images of the same mask pattern are formed, respectively, is two. The described pattern forming method. 4. The number of positions in the optical axis direction of the optical system at which the images of the same mask pattern are formed are three, respectively. The described pattern forming method. 5. The images of the same mask pattern are sequentially formed at the first position and the second position by moving the substrate in the optical axis direction of the optical system. The pattern forming method according to any one of claims 1 to 4.