How to ensure I don't keep keep creating additional EXCEL.EXE processes each time I call a routine in C# from a loop?

I am developing a C# Windows application using VS2005.
In the following routine which I call from a loop,
it seems that I create a new EXECEL.EXE process each time this routine is called
according to the Windows Task Manager.
Is there a way to ensure that I drop the EXCEL.EXE before I call this routine the next time?

public static void ExcelFormat(string filename, string footnote)
{
            Excel.Application oXL = new Excel.Application();
            oXL.Visible = false;
            oXL.DisplayAlerts = false;

            Excel._Workbook oWB = (Excel._Workbook)oXL.Workbooks.Open(filename, 0, false, 5, Missing.Value, Missing.Value, false, Missing.Value, Missing.Value, false, false, Missing.Value, false, false, false);
            Excel._Worksheet oSheet = (Excel._Worksheet)oWB.ActiveSheet;
            oSheet.Activate();

            try
            {

                oSheet.get_Range(oSheet.Cells[2, 1], oSheet.Cells[65535, 1]).EntireColumn.NumberFormat = "#,##0";

            }
            finally
            {                
                oXL.Workbooks.Close();
                oXL.Quit();
                System.Runtime.InteropServices.Marshal.ReleaseComObject(oXL);
                System.Runtime.InteropServices.Marshal.ReleaseComObject(oSheet);
                System.Runtime.InteropServices.Marshal.ReleaseComObject(oWB);
                oSheet = null;
                oWB = null;
                oXL = null;
            }
}