CHECKBOX COLUMN

Hello,

I have this columns:

"mRender": function (val, type, row) {                                                            

                    if(val == 't')
                        var sReturn =  '<input type="checkbox" name="filter" value="t" checked="checked" />';
                    else
                        var sReturn =  '<input type="checkbox" name="filter" value="f" />';
                    
                    return sReturn;
                }

But when I use the individual filter, I write F or T and the filter doesn´t work...
Why??

Thank you!