how to Kill Excel Process

Dim FN As String = Server.MapPath("\esmp\temp\" & Session.SessionID & ".xls")
Dim xl As New Excel.Application
Dim wk As Excel.Workbook = xl.Workbooks.Add()
xl.Cells(2, 2) = "its ok"
wk.Close(SaveChanges:=True, FileName:=FN)
wk = Nothing
xl.Quit()
xl = Nothing
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that was the coding I wrote in the button click event. It does work fine...but it is creating a separate process every time, the event is fired...and the process from task manager could not get deleted automatically. If I click on button 5 times....5 processes named as "EXCEL.EXE" are staying in the processes of TaskManager.

So, How to make the process to close byitself after the completion of its work.