Sharp needles on the Si(111)7x7 and Si(100)2x1 surfaces are formed in a controlled manner by applying a sample bias V-s ranging from +5.0 to +10.0 V while keeping the scanning tunneling microscope (STM) tunneling current constant. The size of the needle-shaped structure is 1-20 nm in height and 1-5 nm in diameter depending on the bias voltage and duration (1-30 s), The needle structures are utilized to counter image the scanning tip with an atomic resolution (needle formation and tip imaging=NFTI). The STM resolution for the Si(111)7x7 structure is evaluated in relation to the atomic structure of the scanning tip imaged by the NFTI method. When a bias voltage of V-s = -10.0 V is used, needle formation on the scanning tip is observed. The NFTI method gives us a conventional technique to modify and monitor in situ the scanning tip of STM.