Patent Number: 048521337
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Now, the present invention will be described in detail in conjunction with an illustrative embodiment by referring to FIGS. 1 and 2. The X-ray lithography apparatus according to the invention includes a soft X-ray generating unit 1, a gas chamber 2, a wafer stage 5 and others which can be of same structures as those of the prior art lithography apparatus such as the one disclosed in the aforementioned JP-A-57-16924 (corresponding to U.S. Pat. No. 4,403,336). As will be readily seen from the description of the prior art, if the heat generating components such as imaging unit and others can be installed outside of the gas chamber 2, cooling effect due to natural convection can be expected. However, mark detection optics 6 for allowing alignment marks of the mask and wafer to be picked up by the imaging unit 9 must be movable for adjusting its position in accordance with the sizes of masks as used by means of a positioning mechanism 10 including an X-Y stage which has a stroke greater than about 5 mm in at least one of the X- and Y-directions. Further, in view of the fact that the mask and the wafer must be positionally aligned with each other in the X-direction, Y-direction and in the angular direction 8, there are required at least three sets of the detection optics 6 and the imaging units 9, only one set of which is shown in FIG. 1 for simplification of the illustration. Additionally, a mirror 27 and an illuminating lens 28 constituting parts of illuminating optics are required to be mounted in the manner as shown in FIG. 1. Accordingly, the detection optics 6 and the imaging unit 9 are preferred to be disposed within the atmospheric gas chamber 2 in view of the hermetically sealed structure of the X-ray lithography apparatus as a whole. On the other hand, the imaging unit 9 such as television camera constitutes a major heat generating source. Accordingly, when the imaging unit 9 is disposed within the gas chamber 2, eat generated by the imaging unit is transmitted to the gas such as helium gas or the like filling the gas chamber 2, which in turn results in a temperature rise of the gas atmosphere and hence the detection optics disposed within the gas chamber 2, bringing about the problem of drift described hereinbefore. Under the circumstance, according to an aspect of the invention, a duct 13 is provided which opens in the gas chamber 2 at a position near the main heat generating source constituted by the imaging unit 9 realized in the form of a television camera, solid state imaging device such as charge-coupled device (CCD) or the like to be used for withdrawing under suction the helium gas from the gas chamber 2. The duct 13 is connected to a recharging duct 17 also opening in the gas chamber 2 through a flexible duct 14, a blower 15, a heat exchanger 16 and a flexible duct 14' so that the helium gas withdrawn through the discharge duct 13 is returned into the gas chamber 2 through the charging port 17. The temperature of the gas leaving the heat exchanger 16 is detected by a sensor 18 for controlling the heat transfer function of the heat exchanger 16 so that the temperature of the gas leaving the heat exchanger 16 remains constant. FIG. 2 shows a mounting structure of the imaging unit 9. As will be seen, the imaging unit 9 is composed of a solid state imaging device 19 and a control circuit substrate 20 and is disposed above the detecting optics 6. More specifically, the imaging unit 9 is fixedly secured to a supporting member 30 of the detecting optics 6 through an interposed heat insulation member 21. The optical axis of the detecting optics 6 is deflected upwardly by a mirror 22 so that the positioning mark image is projected onto the solid state imaging device 19. A reference numeral 25 denotes a light source for illumination which is disposed externally of the gas chamber 2. A glass fiber 26 serving for conduction of light emitted by the light source 25 has a light exit end portion which is secured on the detecting optics 9. A reference numeral 27 denotes a mirror which is so disposed that the light beam emitted from the exit end of the glass fiber 26 and reflected by the mirror 27 illuminates the alignment patterns on the mask 3 and wafer 4 in the direction inclined thereto. The control circuit substrate 20 which is a main heat generating source is enclosed by a casing 23 having an open top connected to the discharging duct 13, whereby the ambient gas surrounding the substrate 20 is withdrawn under suction into the discharging duct 13, as is indicated by arrows 24. The charging duct 17 (FIG. 1) is preferably disposed at such a position that a uniform flow of the helium gas can take place within the gas chamber 2 in the direction toward the discharging duct 13. More preferably, a flow rectifying plate member (not shown) is provided within the gas chamber 2 for ensuring more uniform gas flow taking place within the gas chamber 2. The heat generating part of the imaging unit 9 such a television camera or the like is supported by the heat insulation material 21 to thereby prevent heat from being transmitted directly to the parts constituting the detecting optics 6. Upon operation of the X-ray lithography apparatus of the structure described above, heat generated by the heat generating source within the gas chamber causes the temperature of the ambient helium gas to rise. However, the heated gas is immediately withdrawn through the discharging duct 13 and cooled down to a predetermined temperature level by means of the heat exchanger 16. Thus, accumulation of heat within the gas chamber 2 as well as the accompanying drift phenomenon of the alignment mark position detected by the optics 6 can be positively reduced to a minimum. Further, since the flexible ducts 14 and 14' are employed for incorporating the blower 15 in the gas recirculation path, vibration of the blower 15 is positively prevented from being transmitted to the detecting optics 6 and the mask 3, which would otherwise bring about fluctuation in the detected mark position and hence positional deviations of the mask 3 relative to the wafer 4. In the case of the illustrative embodiment, the heat exchanger 16 is disposed adjacent to the blower 15. However, since the heat exchanger 16 scarcely produces vibration of any significant magnitude, it can be disposed at a position adjacent to the gas chamber 2 if allowable in view of the available space, wherein the heat exchanger 16 may be connected to the blower 15 by another flexible duct 14. In a further modification, the blower 15 and the heat exchanger 16 may be disposed within the gas chamber 2, wherein the blower 15 may be supported by an appropriate anti-vibration mechanism (not shown) for preventing any vibration which would otherwise be transmitted to the detecting optics. It has been described that the amount of heat exchange is controlled or regulated in dependence on the exit temperature of the heat exchanger. However, when the heat exchanger as employed has a sufficiently large capacity, the exit temperature can be maintained constant simply by supplying a cooling water of a constant temperature to the heat exchanger. In a further version, a heater (not shown) may be additionally provided between the heat exchanger 16 and the temperature sensor 18, wherein the amount of heat generated by the heater is controlled as a function of the output signal of the sensor 18 to thereby maintain constant the temperature of the recirculated gas. For particulars of the structures of the detecting optics 6, the imaging unit 9 and others, reference may be made to U.S. patent application No. 789,778 filed on Oct. 21, 1985. As will now be appreciated from the foregoing description, it is possible according to the present invention to suppress drift of the alignment mark position detected by the detecting optics within a short time, whereby continuous operation of the X-ray lithography apparatus over day and night is rendered unnecessary. Further, the operating state of the X-ray lithography apparatus is stabilized rapidly upon alteration of the operating conditions. Thus, an enhanced working ratio of the apparatus can be accomplished.