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Patent Issued for Exposure Apparatus, Exposure Method, and Method for Producing Device with Electricity Removal Device by Adding Additive to Liquid

August 20, 2014



By a News Reporter-Staff News Editor at Journal of Engineering -- A patent by the inventor Nagahashi, Yoshitomo (Takasaki, JP), filed on January 28, 2011, was published online on August 5, 2014, according to news reporting originating from Alexandria, Virginia, by VerticalNews correspondents.

Patent number 8797502 is assigned to Nikon Corporation (Tokyo, JP).

The following quote was obtained by the news editors from the background information supplied by the inventors: "The present invention relates to a projection exposure method and an apparatus to be used for transferring a mask pattern onto a photosensitive substrate in the lithography step in order to produce a device including, for example, semiconductor devices, image pickup devices (for example, CCD), liquid crystal display devices, and thin film magnetic heads. In particular, the present invention relates to a projection exposure apparatus and a method using the liquid immersion method.

"A projection exposure apparatus is used, for example, when a semiconductor device is produced, in which an image of a pattern on a reticle as a mask is transferred to respective shot areas on a wafer (or a glass plate or the like) coated with a resist as a photosensitive substrate via a projection optical system. A reduction projection type projection exposure apparatus (stepper), which is based on the step-and-repeat system, has been hitherto frequently used as the projection exposure apparatus. However, a projection exposure apparatus, which is based on the step-and-scan system, is also widely used recently to perform the exposure by synchronously scanning the reticle and the wafer.

"As for the resolution of the projection optical system provided for the projection exposure apparatus, As the exposure wavelength to be used is shorter, the resolution becomes higher, while as the numerical aperture of the projection optical system is larger, the higher the resolution becomes higher. Therefore, the exposure wavelength, which is used for the projection exposure apparatus, is shortened year by year as the integrated circuit becomes fine and minute, and the numerical aperture of the projection optical system is increased as well. The exposure wavelength, which is dominantly used at present, is 248 nm of the KrF excimer laser. However, the exposure wavelength of 193 nm of the ArF excimer laser, which is shorter than the above, has been already practically used as well.

"When the exposure is performed, the depth of focus (DOF) is also important in the same manner as the resolution. The resolution R and the depth of focus .delta. are represented by the following expressions respectively. R=k1.lamda./NA (1) .delta.=k2.lamda./NA.sup.2 (2)

"In the expressions, .lamda. represents the exposure wavelength, NA represents the numerical aperture of the projection optical system, and k1 and k2 represent the process coefficients. According to the expressions (1) and (2), the following fact is appreciated. That is, when the exposure wavelength .lamda. is shortened and the numerical aperture NA is increased in order to enhance the resolution R, then the depth of focus .delta. is narrowed. Conventionally, in the case of the projection exposure apparatus, the surface of the wafer is adjusted to match the image plane of the projection optical system in the auto-focus manner to perform the exposure. However, it is impossible to adjust and match the wafer surface and the image plane with no error at all. Therefore, it is desirable that the depth of focus .delta. is large so that no influence is exerted on the image formation performance even when any error remains to some extent. In view of the above, for example, the phase shift reticle method, the modified illumination method, and the multilayer resist method have been hitherto suggested in order to substantially increase the depth of focus as well.

"As described above, in the case of the conventional projection exposure apparatus, the depth of focus is gradually decreased, as the exposure light beam has the shorter wavelength, and the numerical aperture of the projection optical system is increased. In order to respond to the further higher integration of the semiconductor integrated circuit, the investigation is also made to further shorten the exposure wavelength. If such a situation is continued as it is, then the depth of focus is excessively decreased, and it is feared that the margin may be insufficient during the exposure operation.

"Accordingly, the liquid immersion method has been suggested as a method for substantially shorten the exposure wavelength and increase the depth of focus. In this method, the space between the lower surface of the projection optical system and the wafer surface is filled with a liquid such as pure water or any organic solvent so that the resolution is improved and the depth of focus is magnified about n times by utilizing the fact that the wavelength of the exposure light beam in the liquid is 1/n as compared with that in the air (n represents the refractive index of the liquid, which is about 1.2 to 1.6 in ordinary cases). A technique, which is described, for example, in International Publication No. 99/49504, is exemplified as a conventional technique concerning the projection exposure apparatus and the exposure method to which the liquid immersion method is applied.

"In the liquid immersion method as described above, for example, the pure water or the organic solvent is used as the liquid with which the space between the lower surface of the projection optical system and the wafer surface is filled. Any one of the liquids, which is used in this method, has the high electric insulation. For example, the ultrapure water, which is used in the semiconductor production factory, has a specific resistance of about 15 M.OMEGA.cm which is high. The liquid, which has the high insulation as described above, tends to be charged with the static electricity due to the friction with the piping and/or the cavitation generated in the orifice provided in the piping passage when the liquid is made to flow through the piping passage. If the liquid, which is charged with the static electricity, is used for the liquid immersion method, it has been feared that the electric discharge may be caused between the liquid and the circuit pattern having been already formed on the wafer, and the circuit pattern may be destroyed. Further, if the electric discharge is caused between the liquid and any object other than the circuit pattern, it has been feared that the electric equipment, which is arranged around the projection optical system or around the wafer, may malfunction due to the electric noise generated during the electric discharge, and the projection exposure apparatus may cause any error and/or the projection exposure apparatus may be stopped. Further, the charged liquid attracts surrounding impurities by the static electricity. Therefore, the impurities may inhibit the exposure in some cases."

In addition to the background information obtained for this patent, VerticalNews journalists also obtained the inventor's summary information for this patent: "The present invention has been made taking the foregoing viewpoints into consideration, an object of which is to provide a projection exposure apparatus which makes it possible to avoid the malfunction of the apparatus and the destruction of the circuit pattern caused by the charging of the liquid to be used for the liquid immersion method. Another object of the present invention is to provide a projection exposure method and a method for producing a device, in which it is possible to avoid the destruction of the circuit pattern and the malfunction of the apparatus.

"According to a first aspect of the present invention, there is provided a projection exposure apparatus which transfers a pattern formed on a mask onto a substrate through a liquid; the projection exposure apparatus comprising a projection optical system which projects an image of the pattern onto the substrate; and an electricity removal unit which removes electricity from the liquid to be supplied to a space between the projection optical system and a surface of the substrate.

"According to the projection exposure apparatus of the present invention, the liquid, from which the electricity has been removed, can be supplied to the space between the projection optical system and the substrate. Therefore, it is possible to prevent the circuit pattern formed on the substrate from being destroyed by the electric discharge of the static electricity. Further, it is possible to prevent the electric equipment arranged around the projection optical system and the substrate from malfunctioning due to the electric discharge of the static electricity. In this arrangement, the electricity removal unit may have an electricity-removing filter which is provided in a flow passage of a liquid supply piping for supplying the liquid to the space between the projection optical system and the surface of the substrate, and which is grounded. The electricity-removing filter may be formed of a conductive metal foam or a conductive mesh member. Accordingly, the static electricity, with which the liquid is charged, can be removed from the liquid made to pass through the electricity-removing filter. The exposure apparatus may further comprise a liquid supply unit which supplies the liquid to the space between the projection optical system and the surface of the substrate. In this arrangement, the liquid supply unit may be provided with the electricity removal unit. When the projection exposure apparatus is a step-and-repeat type projection exposure apparatus, the liquid supply unit may supply the liquid in a direction in which the substrate is subjected to stepping. On the other hand, when the projection exposure apparatus is a step-and-scan type projection exposure apparatus, the liquid supply unit may supply the liquid in a scanning direction.

"According to a second aspect of the present invention, there is provided a projection exposure apparatus which transfers a pattern formed on a mask onto a substrate through a liquid; the projection exposure apparatus comprising:

"a projection optical system which projects an image of the pattern onto the substrate; and

"an electricity removal unit which removes electricity from the liquid intervened between the projection optical system and a surface of the substrate.

"In this arrangement, the electricity removal unit may have an electrode member which is provided in an optical element of the projection optical system opposed to the substrate. The projection exposure apparatus may have an electricity-removing filter which is provided in at least one of a supply port of a liquid supply piping for supplying the liquid and a recovery port of a liquid recovery piping for recovering the liquid. Accordingly, the electricity can be removed even in a state in which the space between the optical element and the substrate is filled with the liquid. Therefore, the liquid can be prevented from being charged during the exposure and/or during the movement of the substrate.

"According to a third aspect of the present invention, there is provided a projection exposure method for irradiating a mask with an exposure light beam and projecting a pattern formed on the mask onto a substrate through a liquid with a projection optical system, the projection exposure method comprising:

"a step of removing electricity from the liquid; and

"a step of supplying the liquid to a space between the projection optical system and a surface of the substrate.

"Accordingly, the static electricity is removed from the liquid with which the space between the projection optical system and the surface of the substrate is filled. It is possible to avoid the malfunction of the projection exposure apparatus and the destruction of the circuit pattern which is feared to be caused by the electric discharge of the static electricity. The step of removing the electricity may be performed prior to the step of supplying the liquid. In this procedure, the liquid may be made to pass through an electricity-removing filter in the step of supplying the liquid to the space between the projection optical system and the surface of the substrate. The electricity-removing filter may be provided at an end portion of a liquid supply tube for supplying the liquid to the space between the projection optical system and the surface of the substrate. The liquid supplied to the space between the projection optical system and the surface of the substrate may be caused to make contact with a conductive member in the step of removing the electricity from the liquid.

"According to a fourth aspect of the present invention, there is provided a method for producing a device, comprising a lithography step, wherein the projection exposure apparatus according to any one of the aspects described above is used to perform exposure in the lithography step.

"Accordingly, it is possible to avoid the destruction of the circuit pattern which is feared to be caused by the electric discharge of the static electricity. Therefore, the yield of the device to be produced is improved, and it is possible to avoid the malfunction of the projection exposure apparatus which would be otherwise caused by the electric discharge of the static electricity. Therefore, it is possible to maintain the high processing ability."

URL and more information on this patent, see: Nagahashi, Yoshitomo. Exposure Apparatus, Exposure Method, and Method for Producing Device with Electricity Removal Device by Adding Additive to Liquid. U.S. Patent Number 8797502, filed January 28, 2011, and published online on August 5, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8797502.PN.&OS=PN/8797502RS=PN/8797502

Keywords for this news article include: Electronics, Semiconductor, Nikon Corporation.

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Source: Journal of Engineering


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