invention, wherein FIG. 3A is a front view and FIG. 3B is a cross sectional view along line A-A in FIG. 3A;FIGS. 4A and 4B are a cross sectional view along line B-B in FIG. 3A and a side view;FIG. 5 is a cross sectional view along line C-C in FIG. 3A;FIG. 6 is a cross sectional view along line D-D in FIG. 3B;FIG. 7A and FIG. 7B are cross sectional views shown for explaining an operating condition