In general, that is not true. It depends on the radius of the tip, among other things. For a short discussion, you could read section 6.2 of Donna Hurley's book chapter in Applied Scanning Probe Methods Vol. XI, which is available here: http://www.nist.gov/customcf/get_pdf.cfm?pub_id=854174 If you are really interested, you could read Contact Mechanics