a patterned structure including a plurality of patterned sheet members which have respective configurations and which are partially superposed on each other and which cooperate with each other to define a plurality of voids . at least one of the sheet members includes a thin - walled portion whose thickness is smaller than a thickness of the other portion . the thin - walled portion is formed by etching the appropriate sheet member or members . the thin - walled portion of one sheet member engages the corresponding thin - walled portion or the other portion of the other sheet member or members .