Contact hole structure and method of fabricating the same

A contact hole structure, comprising:

• a substrate having a top surface;
• an epitaxial layer embedded within the substrate;
• a contact hole disposed in the epitaxial layer, wherein the contact hole in the epitaxial layer has an inner surface comprising a convex curved portion and a concave curved portion, the convex curved portion is in a shape of a first circular arc and the concave curved portion is in a shape of a second circular arc, and wherein the contact hole has an intersection, and the intersection has an opening which is coplanar with the top surface of the substrate, and a first width of the intersection of the contact hole is larger than a third width of any part of the contact hole below the intersection thereof, and the first width and the third width are parallel to the top surface of the substrate, wherein the contact hole further has a portion thereof shaped as a nipple tip and a nipple bottom, the nipple tip is disposed below the nipple bottom within the contact hole, a width of the nipple bottom is greater than a width of the nipple tip;
• and
• a silicide layer filling in the contact hole, wherein the convex curved portion and the concave curved portion directly contact the silicide layer.