There’s a lot of confusion on using F90 and F extensions for FORTRAN program. Here’s a little explanation that I have encountered in a forum that sheds some light on the issue.
If you use free form source Fortran, I suggest to use the extension .f90, if you use fixed format, to use .f. Those seem to be the most commonly used file extensions.
For most compilers, .f or .f90 does not tell anything about the language standard, it just distinguishes between the two source forms.
Thus .f90 can be a Fortran 90, 95, 2003, or 2008 program – and .f can be a Fortran 66/77/ 90/95/ 2003/2008 program.
Hope this clears the fog a bit!!