Patent Number: 
Section: description

Referring to FIG. 2, a membrane mask according to an embodiment of the present invention includes a silicon wafer 11, a membrane film 12 formed thereon and implemented by a first material having a relatively low atomic number, and a mask body pattern 13 formed thereon and implemented by a second material having a relatively high atomic number. The membrane film 12 has an implanted area 14 at the bottom of the mask body pattern 13 except for the opening in the mask body pattern 13. The implanted area 14 is formed by implanting or adding heavy atoms having an atomic number higher than the atomic number of the first material to the membrane film 12. The heavy atoms in the implanted area 14 have a function of scattering electron beams or absorbing X-rays in association with the mask body pattern 13. The area of the membrane film 12 other than the implanted area 14 has an inherent function for suitably passing therethrough electron beams or X-rays due to the absence of the heavy atoms therein. Examples of the first material in the membrane film 12 include silicon nitride (SiN), silicon carbide (SiC), boron nitride (BN), diamond (C) etc. The heavy atoms in the implanted area 14 of the membrane film 12 may be preferably selected from heavy metals, and more preferably selected from the heavy metals tabulated on he periodic table at the sixth period and the subsequent periods. Examples of the heavy metals include tungsten (W), tantalum (Ta), gold (Au), platinum (Pt), lead (Pb) etc. The implanted heavy atoms may include a plurality of heavy metals. The material for the mask body pattern 13 may be preferably selected from heavy metals or heavy alloys such as W. Ta, TaGe, TaReGe. The mask body pattern 13 may preferably include one or more of the heavy metals tabulated on the periodic table at the sixth period and the subsequent periods. Referring to FIGS. 3A to 3F, there is shown a fabrication process for fabricating a membrane mask according to an embodiment of the present invention. The membrane mask is used for an electron beam lithography, for example. In FIG. 3A, a silicon nitride film (SiN) 22 is deposited on the top surface of a silicon wafer 21 having a diameter of 200 mm by using a LPCVD (low pressure chemical vapor deposition) technique to a thickness of 100 nm (nanometers). It is to be noted that the thickness of the silicon nitride film 22 is preferably 150 nm or less. Subsequently, the silicon nitride film 22 is spin-coated with resin to form a resin film thereon, followed by patterning thereof using an electron beam lithographic technique to form a resist pattern 23, as shown in FIG. 3B. The resist pattern 23 has openings therein for implanted areas to be formed for scattering the electron beams. Thereafter, heavy metal ions such as tungsten or chrome ions are implanted into the silicon nitride film 22 by using a resist pattern 23 as a mask, thereby forming a heavy-metal-implanted area 24. The resist pattern 23 is then removed, as shown in FIG. 3C. A tungsten film 25 is deposited on the silicon nitride film 22 including the heavy-metal-implanted area 24 by using a sputtering or LPCVD technique to a thickness of about 10 nm, as shown in FIG. 3D. It is to be noted that the thickness of the tungsten film 25 is preferably 20 nm or less. Subsequently, the tungsten film 25 is spin-coated with resist to form a resist film thereon, followed by an electron is beam lithography thereof to form a resist pattern. The underlying tungsten film 25 is then selectively etched by using a dry-etching technique using the resist pattern, as shown in FIG. 3E. Thereafter, a mask pattern is formed on the bottom surface of the silicon wafer 21, followed by anisotropic back etching of the silicon wafer 21 by a wet etching technique using potassium hydroxide (KOH) as an etchant and the silicon nitride film 22 as an etch stopper Thus, the silicon nitride film 22 is formed as a membrane film having an implanted area 24, as shown in FIG. 3F. The wet etching step may be replaced by a dry etching step. Referring to FIGS. 4A to 4F, there is shown another fabrication process for fabricating the membrane mask of FIG. 2 according to another embodiment. The membrane mask is used for an electron beam lithography, for example. A silicon nitride film 32 is deposited to a thickness of 130 nm by using a LPCVD technique on the top surface of a silicon wafer 32 having a diameter of 200 mm. A mask having a specified opening is then formed on the bottom surface of the silicon wafer 31, followed by back etching of the silicon wafer 31 by a wet etching technique using KOH as an etchant, to thereby leave a film of the silicon wafer 31 having a thickness of 0.1 to 1 mm and underlying the silicon nitride film 32, as shown in FIG. 4A. The silicon nitride film 32 is then spin-coated with resist to form a resist film thereon, followed by patterning thereof to form a resist pattern 33, as shown in FIG. 4B. Subsequently, tungsten ions are selectively implanted into the silicon nitride film 32 by using the resist pattern 33 as a mask to form a heavy-metal-implanted area 34. The resist pattern 33 is then removed, as shown in FIG. 4C. The order of the steps may be reversed so that the back etching step of the silicon wafer 31 is conducted after the implanting of the tungsten ions. Thereafter, tungsten is sputtered onto the silicon nitride film 32 including the heavy-metal-implanted area 34, thereby forming a tungsten film 35 having a thickness of 15 nm, as shown in FIG. 4D. The tungsten film 35 is subjected to an electron beam lithographic patterning, whereby a portion of the tungsten film 35 is left on the heavy-metal-implanted area 34, as shown in FIG. 4E. Subsequently, the remaining film 31a of the silicon wafer 31 is removed by a back etching, whereby the silicon nitride film 32 is disposed as a membrane film, as shown in FIG. 4F. The final back etching step may be an isotropic etching step wherein an etching mask is not necessarily used. In the first fabrication process, there is a possibility that the silicon wafer may be subjected to a deformation due to a tensile stress applied from the membrane film after the back etching of the silicon wafer. On the other hand, in the second fabrication process, the tensile stress of the membrane film is removed to some extent before the film for the mask body is formed. In this process, the distortion of the silicon wafer after the back etching of the silicon wafer can be alleviated, whereby the membrane mask has a lower deformation. Since the above embodiments are described only for examples, the present invention is not limited to the above embodiments and various modifications or alterations can be easily made therefrom by those skilled in the art without departing from the scope of the present invention. For example, the back etching of the silicon wafer may be conducted before the deposition of the mask body film, such as before or after the implantation of the heavy metal ions. In addition, the membrane mask of the present invention can be applied to an X-ray lithography and an ion beam lithography as well as an electron beam lithography.