Method for forming trenches with wide upper portion and narrow lower portion

A method for forming a semiconductor structure includes the following steps. A hard mask layer is formed over a semiconductor region. The hard mask layer has inner portions that are thinner than its outer portions, and the inner portions define an exposed surface area of the semiconductor region. A portion of the semiconductor region is removed through the exposed surface area of the semiconductor region. The thinner portions of the hard mask layer are removed to expose surface areas of the semiconductor region underlying the thinner portions. An additional portion of the semiconductor region is removed through all exposed surface areas of the semiconductor region thereby forming a trench having an upper portion that is wider than its lower portion.