Patent Number: 051577003
Section: summary

FIELD OF THE INVENTION AND RELATED ART The present invention relates to a mask pattern transferring exposure apparatus used in manufacturing semiconductor devices, and more particularly to an exposure apparatus which uses as a radiation source a synchrotron radiation source. An exposure apparatus, which uses synchrotron radiation emitted when high energy electrons make a circular orbit motion, is known. The synchrotron radiation has a wide and uniform intensity distribution in a horizontal plane that is in a plane including the circular orbit. However, the beam width measured in the vertical direction is very much limited. In order to expand the beam width in the vertical direction, there are proposed various methods. In one method, a mirror is disposed in an optical path between the synchrotron radiation source and the member to be exposed to the radiation, and the mirror is swung to scan the member to be exposed (Japanese Laid-Open Patent Applications Nos. 45026/1981 and 113065/1986; "Investigation of X-ray exposure using plane scanning mirrors" J. Vac. Sci. Techol. vol. 31, (4) p.1271, 1983, by M. Bieber, H. U. Sheunemann, H. Betz, A. Heuberger). In another example, the electron orbit in the accumulation ring of the radiation source is vertically swung by application of a magnetic field ("stationary large area exposure in synchrotron radiation lithography utilizing a new arrangement of magnets applied to the storage ring", Jpn. Appl. Phys., 22; L718-L720, 1983, H. Tanio, K. Hoh). A further example is that a fixed mirror is disposed in an optical path from the radiation source to the member to be exposed to provide a large exposure area having a uniform beam intensity (variation of the beam intensity is not more than .+-.5% in an area of 3.times.3 cm.sup.2) ("Design of Stationary Troidal Mirror for Large Area Synchrotron Radiation Exposure", preprint for the 31st Applied Physics Meeting, 282, 1984, by Koji Tanino and Ohtori Koichiro, and Japanese Laid-Open Patent Application No. 84814/1985). Referring to FIG. 1, an example of an exposure apparatus using synchrotron radiation is shown. In FIG. 1, reference numerals 1, 5 and 6 are respectively a synchrotron radiation beam, a mask and a semiconductor wafer which is to be exposed to the radiation. The apparatus comprises a stationary or fixed mirror 2 for expanding the synchrotron beam, an exposure chamber 3 filled with helium gas or the like, a window 4 of the exposure chamber 3 made of beryllium, a wafer stage 7, a spring 8, a wafer stage driver 9 and a frame 10. The apparatus further comprises movable aperture steps 11 and 12, supporting members 13 and 14 for the movable aperture steps and drivers 15 and 16 for the movable aperture steps. The movable apertures 11 and 12, the supporting members 13 and 14 and the drivers 15 and 16 constitute an exposure control means. Designated by a reference 17 is an ultra-high vacuum chamber which is isolated from the exposure chamber 3 containing the helium gas by the window 4. In operation, light (radiation) 1 travels through the ultra-high vacuum chamber 17 and is expanded by the mirror 2, and is passed through the window 4. The exposure control means controlled by a shutter controller 20 limits the light 1, and the limited light is projected onto the semicondutor wafer 6 through the mask 5, by which the pattern of the mask is transferred onto the semiconductor wafer 6. SUMMARY OF THE INVENTION In the exposure apparatus using the synchrotron radiation source, the strength or intensity distribution of the radiation beam is narrow in the direction perpendicular to the plane of the accumulation ring. Therefore, it is preferable to expand the intensity distribution in this direction by one method or another. For the purpose of this expansion, Japanese Laid-Open Patent Applications Nos. 141135/1986 and 59828/1986 disclose that the synchrotron radiation is passed through a monocrystal or a diffraction grating so as to expand the divergence angle measured in the vertical plane. The beam provided thereby, as it is, involves non-uniformity and variation in the spectral distribution in the vertical direction, so that it is still not possible to project the mask pattern onto the wafer with a uniform exposure amount. Then, Japanese Laid-Open Patent Application No. 104438/1981 discloses that a shutter is driven with a control to make uniform the exposure amount over each of many shot areas on the basis of exposure beam profiles predicted beforehand. However, in the conventional art, since there is no practical sequential operation for determining a profile of the synchrotron radiation incident on the mask, the data for determining the profile is not accurate. Accordingly, the first object of the present invention is to provide an exposure apparatus wherein the exposure control is possible on the basis of synchrotron radiation profile data which is accurately obtained and which is high in the S/N ratio (signal/noise ratio). According to an aspect of the present invention, the synchrotron radiation beam profile is measured or sensed when the intensity of the radiation from the synchrotron radiation generator is substantially highest, that is, when the transmissivity of the radiation through the window disposed in the radiation path from the synchrotron radiation generator to a mask in the exposure apparatus, by which the exposure control can be performed, on the basis of the radiation beam profile data having substantially the highest S/N ratio. With the increase of the pattern density in integrated circuit devices, the line width of the pattern transferred by the exposure decreases, and the control of the line width in the resist becomes more severe. As is well known, the line width formed in the resist changes greatly depending on variation in the exposure amount. Therefore, it is important that the actual amount of exposure is controlled correctly to be the desired amount. As for the system for correctly controlling the amount of exposure, Japanese Laid-Open Applications Nos. 101839/1982 and 198726/1984 disclose that the intensity of the exposure radiation during the exposure operation is detected by a detector disposed adjacent to the mask, and upon the desired amount of exposure reached, a shutter is closed. However, it is difficult to use the conventional art of the X-ray exposure for the synchrotron radiation (SOR). This is because, in the conventional apparatus, the area in which the intensity of the exposure radiation is uniform is relatively wide, and therefore, a measurement at the marginal area of the mask is not much different from the intensity in the exposure area, but in the SOR exposure, the region in which the intensity of the SOR is uniform is narrow, so that the X-ray intensity is different at the marginal measuring position than at the exposure area. To obviate this problem, it is considered that a retractable X-ray detector is advanced into the exposure area when the exposure operation is not carried out, and it detects the X-ray intensity, whereas during the exposure, the detector is retracted so as not to block the X-rays. However, when the X-ray intensity varies with time as in SOR, it is possible that the X-ray intensity is different at the time of the exposure than at the time of measurement, with the result of an additional error. Accordingly, a second object of the present invention is to provide an X-ray exposure apparatus which can determine the X-ray intensity with high accuracy even if the SOR is used. According to an aspect of the present invention, after the electron is injected into the SOR ring to generate the synchrotron radiation, the intensity of X-rays is measured, and the X-ray intensity during the exposure operation is determined on the basis of the measurement and on an attenuation curve of the injected electron. In this system, the X-ray intensity during an exposure operation for a shot can be correctly predicted both from one or more X-ray measurements after the orbit electron injection and from the attenuation curve of the orbit electron amount. Further, in order to achieve the second object, in the X-ray exposure apparatus according to an embodiment of the present invention, the X-ray or light intensities are simultaneously measured within the exposure area and outside the exposure area after the electron injection into the SOR ring, and thereafter, the exposure control is performed on the basis of an output of the measuring device disposed outside the exposure area. In the X-ray exposure apparatus, the desired amount of exposure is provided by controlling the exposure period. The following methods are considered for accomplishing this: (1) The exposure period is determined on the basis of the sensitivity, to the X-rays, of the sensitive resist applied on the wafer and the X-ray illuminance empirically determined: and (2) As disclosed in Japanese Laid-Open Patent Application 198726/1985, an X-ray detector is disposed at a proper position in the X-ray illumination area, by which the variation of the X-ray illuminance with time is detected, and on the basis of the detection, the exposure period is controlled. However, in those methods, the exposure period is uniform over the entire exposure area on the wafer surface, and therefore, when, for example, the radiation source is a synchrotron radiation source, non-uniformness results in the exposure area of the wafer, and in addition, the correction thereof is not possible. Accordingly, a third object of the present invention is to provide an exposure apparatus wherein the exposure operation can be performed with a uniform amount of exposure over the entire wafer even if the synchrotron radiation or the like is used. According to an aspect of the present invention, the exposure apparatus according to an embodiment of the present invention includes aperture means (shutter), disposed between the radiation source and the member to be exposed and movable in a direction crossing the direction of illumination of the radiation rays, control means (controlling the opening and closing of the shutter) for controlling movement of the aperture means so as to provide a uniform radiation intensity distribution on the member to be exposed, a radiation sensor for measuring the exposure amount by the radiation rays, and feed-back means for feeding the result of the measurement to the control means. By this, the exposure amount becomes uniform over the entire exposure area even if the radiation source is a synchrotron radiation source. In an illuminance measuring system for light using a semiconductor, when a high measurement accuracy is required, a chopper is used adjacent the light source to block the light pulsewisely in order to correct for dark current which is a problem from the standpoint of the measurement correctness. The chopped light is received by an illuminance sensor. The dark current component inherent to the semiconductor sensor is corrected by measuring the AC component of the illuminance sensor output. However, as for the illuminance measurement in the exposure apparatus using variable wavelength light, even if the semiconductor sensor is used, the variation in the dark current is very small, and therefore, it is practically not necessary to correct it, and it will suffice if the DC component is measured. However, in the exposure apparatus using high energy radiation such as X-rays, when the semiconductor sensor is used for the purpose of measurement of the illuminance, the temperature of the semiconductor sensor itself increases by the illumination with the exposure radiation, by which the dark current varies, with the result that the correct illuminance is not obtained. Referring to FIG. 2, there is shown a relation in a graph between the sensor output current with time when the semiconductor sensor is exposed to X-rays having a constant intensity. As will be understood from this Figure, even if the X-ray intensity does not change, the dark current increases due to the temperature rise of the sensor, and therefore, the output of the sensor is as if the X-ray intensity changes. In order to avoid this, it would be possible to use the above-described method wherein the dark current is corrected, which, however, necessitates the sensor to be provided with a chopper. This results in an inconvenience of an increased number of drivers. Accordingly, a fourth object of the present invention is to provide an exposure apparatus wherein the dark current of the semiconductor components can be corrected, and therefore, the illuminance of the exposure radiation can be measured with high accuracy without a necessity of an additional driving mechanism. According to an embodiment of the present invention, there is provided an exposure apparatus wherein a semiconductor sensor for measuring the illuminance is disposed behind the exposure shutter with respect to the radiation source; upon measurement, an exposure shutter is opened and closed, and an output of the sensor is produced in synchronism with the opening and closing of the shutter; and the output is signal-processed to calculate the illuminance of the exposure radiation. Therefore, the high measurement accuracy can be accomplished without the necessity of increasing the number of driving parts in the exposure apparatus. As disclosed in Japanese Laid-Open Patent Application No. 276223/1986, it is known that in an exposure apparatus wherein the light from a light source is reflected by a mirror and is then projected onto a mask, the mirror is vibrated to control the amount of exposure within the exposure angle in the mask to be a given or constant amount. In this case, it is necessary to determine a driving profile for controlling the swinging vibration of the mirror. The driving profile has to be produced in consideration of an intensity distribution of the light incident on the mirror, intensity variation of the emergent light due to a change in the angle of incidence, the change in the spectral distribution or the like when the light source is, for example, a synchrotron radiation source, the intensity of the radiation attenuates at least during several hours as periods, and therefore, various driving profiles have to be produced for respective shots meeting the attenuation. This places a heavy burden on the controller for controlling the exposure apparatus. Accordingly, it is a fifth object of the present invention to provide an exposure apparatus wherein the driving profile is compensated for in accordance with the radiation intensity characteristics change of the radiation source with time to assure the uniformity of exposure amount on the exposure surface without much burden imparted on the controller. According to an embodiment of the present invention, an absolute intensity of the radiation provided by the radiation source is measured, and in accordance with the change of the detected intensity, the time axis of the driving profile is proportionally expanded or contracted, thus eliminating the necessity for the process of producing a new driving profile for each change in the intensity of the radiation by the radiation source. Therefore, the controller is not given the heavy burden. More particularly, the exposure apparatus according to this embodiment includes an exposure radiation source, a stage for supporting a member to be exposed; optical control means for controlling the radiation projected from the radiation source to the member to be exposed; optical measurement means for measuring the intensity of the radiation provided by the exposure radiation source or the illuminance on the surface to be exposed; driving profile determining means for determining the driving profile of the optical control means, wherein the profile determining means is coupled with the optical control means and optical measuring means, and provides a uniform exposure amount on the surface to be exposed in consideration of the illuminance on the surface to be exposed as a function of a position of the surface to be exposed and the radiation intensity characteristics of the radiation source; and driving profile compensating means for expanding and contracting the time axis of the driving profile in accordance with a change of the radiation intensity characteristics of the radiation source. In a preferred embodiment, the exposure radiation source is a synchrotron radiation source, and the optical control means includes a movable aperture. In another preferred embodiment, the exposure radiation source is a synchrotron, and the optical control means includes an actuator for swingingly driving a mirror. Referring to FIG. 3, there is shown a movable aperture, in an enlarged scale, employed in the apparatus of FIG. 1. In this Figure, aperture limiting members 11 and 12 are movable in a y direction, and an aperture limiting member 20 is movable in an x direction constitute the movable aperture. More particularly, the edge surfaces 18 and 19 define the aperture. The synchrotron radiation passes through the limited opening or aperture in the z direction to within the view angle 21. As shown in FIG. 4, if the beam intensity at the aperture position is uniformly constant also in the y direction, the edge surface 18 of the aperture limiting member is moved in the y direction at a constant speed determined by (y.sub.z -y.sub.1)/.DELTA.T in accordance with the operation curve (1) in FIG. 5 during the time period of .DELTA.T from the point of time T1 at which the aperture limiting members 11 and 12 are contacted to close the aperture. Thereafter, it is stopped at the position indicated in FIG. 3. Then, the edge surface 19 of the aperture limiting member is moved in accordance with the operation line (2) in the +y direction at a constant speed determined by (y.sub.1 -y.sub.1)/.DELTA.T during the period of .DELTA.T from the point of time (T1+.DELTA.T). Then, it is brought into contact with the aperture limiting member 11 which has advanced and stopped. Thus, the exposure amount in the view angle 21 is constant, so that the wafer is uniformly exposed. If the strength of the beam is constant at the position of the aperture, the exposure amount of the view angle 21 is constant, and the member to be exposed is uniformly exposed. Actually, however, it is not constant, and in addition, the radiation intensity attenuates with time due to collisions of the electrons to the inside wall of the accumulation ring or gas molecules. For this reason, the exposure amount is not uniform within the angle of view, and therefore, there is a problem that the pattern of the mask is not uniformly formed on the wafer with high resolution. Accordingly, it is a sixth object of the present invention to provide an exposure apparatus wherein the exposure amount within the angle of view is made uniform, thus assuring the uniform exposure. According to an embodiment of the present invention, a driving speed of a movable member of an exposure control means for making uniform the exposure amount of the member to be exposed is determined in accordance with the attenuation characteristics with time of the illumination intensity to the member to be exposed, and the movable member of the exposure means or the stage supporting the member to be exposed is driven at the speed thus determined. More particularly, according to the embodiment of the present invention, there is provided an exposure apparatus, including an illumination source, a stage for supporting a member to be illuminated, exposure control means including a movable member for selectively limiting the light from the illumination source to the member to be illuminated, means for determining the driving speed in accordance with the attenuation (with time) characteristics of the illumination intensity to the member to be illuminated to provide a constant amount of exposure, and driving means for driving the movable member of the exposure control means or the stage at a speed thus determined. In a preferred embodiment, the exposure control means includes a movable aperture stop or a movable mirror. These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.