Patent Number: 
Section: claims

1. A membrane mask comprising wafer, a membrane film including a first material and supported by said wafer, and a mask body overlying said membrane film, said mask body having a mask pattern including an opening, said membrane film having a first area underlying said mask body other than said opening, said first area being formed by addition of atoms having an atomic number higher than an atomic number of said first material. 2. The membrane mask as defined in  claim 1 , wherein said first material is selected from the group consisting of SiN, SiC, BN, and diamond. claim 1 3. The membrane mask as defined in  claim 1 , wherein said atoms include a heavy metal. claim 1 4. The membrane mask as defined in  claim 3 , wherein said heavy metal is tabulated on a periodic table at a sixth period or one of subsequent periods. claim 3 5. The membrane mask as defined in  claim 1 , said mask body has a thickness of 20 nanometers or below. claim 1 6. The membrane mask as defined in  claim 1 , wherein said membrane film has a thickness of 150 nanometers or below. claim 1 7. A method for fabricating a membrane mask comprising the steps of: forming a membrane film supported on a wafer;  forming a resist mask on said membrane film;  selectively implanting atoms into said membrane film by using said resist mask to form an implanted area, said atoms having an atomic number higher than an atomic number of a material included in said membrane film;  forming a mask body film on said membrane film including said implanted area; and  patterning said mask body film to have a pattern including an opening exposing said membrane film other than said implanted area. 8. The method as defined in  claim 7  further comprising, after said patterning step, the step of etching said wafer at a bottom surface thereof to expose said membrane film. claim 7 9. The method as defined in  claim 7  further comprising, between said membrane film forming step and said resist mask forming step, the step of etching said wafer at a bottom surface thereof to expose said membrane film. claim 7 10. The method as defined in  claim 7  further comprising, between said selectively implanting step and said mask body forming step, the step of etching said wafer at a bottom surface thereof to expose said membrane film. claim 7 11. The method as defined in  claim 7  further comprising: the step of etching a bottom portion of said wafer so that said membrane film is not exposed, between said membrane film forming step and said resist mask forming step; and the step of etching a remaining portion of said wafer to expose said membrane film after said mask body film patterning step. claim 7 12. The method as defined in  claim 7  further comprising: the step of etching a bottom portion of said wafer so that said membrane film is not exposed, between said selectively implanting step and said mask body film forming step; and the step of etching a remaining portion of said wafer to expose said membrane film after said mask body film patterning step. claim 7