Patent Number: 052767248
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1(a) illustrates the structure of an X-ray transmitting window according to the present invention. FIG. 1(b) is an enlarged perspective view of part of the X-ray transmitting window. An X-ray transmitting window 1 is a thin tube bundle in which a plurality of juxtaposed thin tubes, each having a predetermined inner diameter (D) and a length (L), are joined parallel to each other. Such an X-ray transmitting window 1 is manufactured in the manner described below: first, the surface of each of wire like members having a predetermined diameter and made of a first material removable by etching is coated by a layer made of a second material that cannot be removed by etching. The plurality of wire-like members coated by the second material are bundled parallel to each other and then processed in the form of a plate-like member having a predetermined thickness. The wire-like members in that plate-like member are selectively removed by etching to obtain the X-ray transmitting window. The plate-like member has a pair of surfaces parallel to each other and ends of the tubes each consisting of the layer of the second material respectively open on the surfaces providing an open surface area. Such a plate-like member consisting of a bundle of thin tubes is available on the market as the secondary electron amplifying member (for example, capillary-plate manufactured by Hamamatsu Photonics K.K., Japan). In the present invention, the aforementioned capillary plate is used as the X-ray transmitting window, and the conductance, transmittance and mechanical strength thereof are evaluated. FIG. 2(a) is a graph illustrating the results of the measurements of a differential pressure obtained by various X-ray transmitting windows consisting of a plurality of thin tubes having difference inner diameters (D) and lengths (L). As shown in FIG. 2(b), two vessels 32 and 33 partitioned by the X-ray transmitting window 1 consisting of the plurality of thin tubes are coupled to each other. Pressure p.sub.1 in the vessel 32 and pressure P.sub.2 in the vessel 33 are measured while the vessel 33 is evacuated by a rotary pump 34 having a pumping speed of 500 liters/min. Combinations of the inner diameters (D) and the lengths (L) of the thin tubes 2 in the various windows 1 are shown in FIG. 2(a). The length (L) corresponds to the thickness of the window 1. Since these X-ray transmitting windows have the same diameter, they have different total numbers of thin tubes 2. The number of thin tubes 2 in the window is determined by the inner diameter (D) thereof. In these windows, the opening surface area ratio of all the thin tubes 2, i.e., the ratio of the total cross-sectional open surface area of all the thin tubes 2 to the area of the window 1, is a fixed value of 63%. The inner diameter (D) and total number (N) of the thin tubes 2 which form the windows 1 having the aforementioned opening surface area ratio and an effective diameter of 20 mm are as follows: ______________________________________ D (.mu.m) N ______________________________________ 25 4 .times. 10.sup.5 12 1.75 .times. 10.sup.6 6 7 .times. 10.sup.6 ______________________________________ According to the kinetic theory of gases, the flow rate Q of the gas which flows through the tube having a conductance C and the pressures P.sub.1 and P.sub.2 at the two sides of the tube have the relationship expressed by the following equation: EQU Q=C(P.sub.1 -P.sub.2) (1) FIG. 3 is a graph in which conductance C.sub.n of gas through the thin tube 2 obtained from the results of FIG. 2(a) and from equation (1) is plotted relative to an average value P.sub.m of pressures P.sub.1 and P.sub.2, i.e., P.sub.m =(P.sub.1 +P.sub.2)/2. Combinations of the inner diameters (D) and the lengths (L) of the thin tubes 2 are shown in FIG. 3. As shown in FIG. 3, in the region of a high average pressure P.sub.m, as the pressure increases, the conductance C.sub.n increases. In the region of a low average pressure P.sub.m, the conductance C.sub.n is not substantially affected by the pressure. These are explained by the kinetic theory of gases. That is, in the high-pressure region where the gas shows the property of a viscous flow, the conductance C.sub.n is in proportion to the average pressure P.sub.m, as expressed by equation (2). In the low-pressure region where the gas shows the property of a molecular flow, the conductance C.sub.n is not dependent on the pressure, as expressed by equation (3). Conductances C.sub.6 and C.sub.24 in the viscous flow and molecular flow regions are expressed by: EQU C.sub.24 =1349.times.(D.sup.4 /L).times.(P.sub.1 +P.sub.2)/2(2) EQU C.sub.6 =131.times.D.sup.3 /L (3) In FIG. 3, the theoretical curves corresponding to the equations (2) and (3) are shown by the broken line. The region between the viscous flow and molecular flow is a so-called intermediate region. The general conductance of the X-ray transmitting window consisting of a bundle of n thin tubes each having the conductance C.sub.n is nC.sub.n. Since the open surface area ratio of the thin tubes 2 in the various windows 1 employed in the above experiments is fixed, the total sum of the cross-sectional areas of the n thin tubes 2 is a fixed value of n.pi.D.sup.2 /4. Expressing this constant by A, we have EQU n=4A/.pi.D.sup.2 (4) Thus, the general conductance nC.sub.n of the window 1 in the viscous flow region and the molecular flow region is expressed as follows: EQU nC.sub.n =nC.sub.24 =k.sub.1 (D.sup.2 /L).times.(P.sub.1 +P.sub.2)/2(5) EQU nC.sub.n =nC.sub.6 =k.sub.2 .times.(D/L) (6) where k.sub.1 and k.sub.2 are constants. It is apparent from equations (5) and (6) that the general conductance of the window 1 decreases in proportion to the square of the inner diameter (D) of the thin tube 2 in the viscous flow region and in proportion to the inner diameter (D) in the molecular flow region. Thus, in the windows 1 having the same open surface area ratio, the smaller the inner diameter (D) of the thin tube 2, the larger the pressure difference. This is shown in FIGS. 2(a) and 3. The inner diameter (D) and length (L) of the window tubes may be selected to provide gas conductance of C.sub.o such that total gas flow through the window is G.gtoreq.nC.sub.o, so as to allow a desired pressure P.sub.2 to be maintained in the vessel 33 by a pump having an evacuation rate G. As mentioned above, the transmittance of Be having a thickness of 0.2 mm (200 .mu.m) to 10 .ANG. X-rays is about 0.0001. The transmittance of the X-ray transmitting window according to the present invention is equal to the opening surface ratio thereof. Therefore, even if it is estimated that the opening surface ratio of the thin tubes 2 in the window 1 is 50%, the transmittance of the X-ray transmitting window according to the present invention is 5000 times as high as the transmittance of the conventional one made of Be. The mechanical strength of the X-ray transmitting window 1 according to the present invention was examined. The window 1 having dimensions of 30 mm.times.30 mm and a thickness of 1 mm was mounted, as a lid, on a pressure device. After the one surface of the window 1 was covered by an aluminum foil, compressed nitrogen gas was directed toward the aluminum foil. The window 1 broke when the pressure difference on the two surfaces thereof reached 2.8 atm. It can be inferred that such a rupture strength is proportional to the thickness of the window 1. This means that a window having a thickness of 0.5 mm and used in the experiments described in connection with FIG. 2 has a sufficient strength to the pressure difference of 1 atm. When the X-ray transmitting window according to the present invention is employed, the pressure after the window is reduced to about 1/10th of the atmospheric pressure. Thus, it is necessary for the differential evacuating device, such as that shown in FIG. 4, to be provided between the high-vacuum X-ray source and the X-ray transmitting window. That is, the X-ray transmitting window 1 according to the present invention is provided between a vessel 3 where a sample (which may be a semiconductor wafer ) 4 to which X-rays are illuminated is accommodated and a final stage (having the highest pressure) 5 of the differential evacuating device. The vessel 3 is filled with helium (He) gas of, for example, 1 atm. Part of He passes through the window 1 into the final stage 5. He in the final stage 5 is evacuated by, for example, a rotary pump 6. Thus, He is constantly supplied into the vessel 3 so that the amount of He in the vessel 3 remains the same. The final stage 5 of the differential evacuating device and the X-ray source 8 evacuated to produce a high vacuum are connected each other through a plurality of intermediate stages 7.sub.1, 7.sub.2, . . . The intermediate stages 7.sub.1, 7.sub.2, . . . are evacuated by, for example, turbo pumps TP.sub.1, TP.sub.2, . . . of a high pumping speed, respectively. The final stage 5, the intermediate stages 7.sub.1, 7.sub.2, . . . and the X-ray source 8 are separated from each other by partitioning walls 9 each of which has an orifice of 5 mm.times.20 mm. The orifices of the partitioning walls 9 and the X-ray transmitting window 1 are disposed on a straight line. How the X-ray source 8 evacuated to produce a vacuum of about 10.sup.-8 Pa is connected to the final stage 5 evacuated to produce a vacuum of about 1/10th of the atmospheric pressure has been described. Reference numeral 10 denotes an exposure mask. A case in which an X-ray transmitting window made of a Be plate and having a thickness of about 10 .mu.m is used in place of the X-ray transmitting window 1 according to the present invention will be considered. In that case, the pressure in the final stage 5 must be made substantially equal to the atmospheric pressure so as to prevent breakage of the Be window. To achieve this, the number of stages of the differential evacuating device provided between the final stage 5 and the intermediate stage 7.sub.1 must be increased. This increases the installation area of the X-ray illumination device and production cost thereof, and makes the practical use thereof difficult. The aforementioned thin 10-.mu.m thick Be window can be used without increasing the number of stages of the differential evacuating device if it is combined with the X-ray transmitting window according to the present invention. Such a structure will be described with reference to FIG. 5. An X-ray illuminating device shown in FIG. 5 has substantially the same structure as that of the device shown in FIG. 4 with the exception that a thin Be window 20 is provided at a position of an orifice 9.sub.1. As mentioned above, the pressure of the final stage 5 of the differential evacuating device is 1/10th of the atmospheric pressure. Thus, the Be window 20 is enough to withstand a pressure different of about 1/10th of the conventional one. Under such conditions, a Be window 20 having dimensions of 30 mm.times.30 mm and a thickness of 10 .mu.m can be used. This allows for the opening area of the orifices 9.sub.2, 9.sub.3. . . to be increased to 30 mm.times.30 mm. In such a structure, it is not necessary for the intermediate stages 7.sub.1, 7.sub.2, . . . to serve as the differential evacuating device, and the structure can be simplified as compared with that shown in FIG. 4. In the structure shown in FIG. 5, the X-ray illumination area of the sample 4 can be increased as compared with that of the case which employs the conventional Be window, thus increasing the throughput of the X-ray lithography. FIG. 6 illustrates another embodiment of the X-ray transmitting window according to the present invention. That is, in the X-ray transmitting window shown in FIG. 6 each of thin tubes 17 which constitute the X-ray transmitting window has an inner diameter which gradually decreases in one direction. In other words, the inner surface of the thin tube 17 is tapered. Normally, the X-rays reflected by a mirror provided in the X-ray source expands by about 6 milliradian (mrad) in the direction transverse to the direction of travel. Thus, the X-rays which pass through the thin tube 17 whose inner diameter increases toward the X-ray incident side are condensed as they are directed toward the emitting side thereof, and the illumination intensity of the X-rays is thus increased. If the inner surface of the thin tube 17 is coated by a reflecting material, such as platinum (Pt), the transmission efficiency of the window is enhanced, and the loss caused by the tapered thin tube 17 or variations in the degree of parallelization of the plurality of thin tubes 17 is reduced. Such a coating is also effective for protection of the window material. The material of the X-ray transmitting window according to the present invention is not limited to those described in the above embodiments.