Semiconductor device having trench top isolation layers and method for forming the same

A method for forming a semiconductor device having trench top isolation layers. A substrate has a trench therein and a collar oxide is formed over the sidewall of the lower trench. A first conductive layer is formed in the lower trench and protrudes the collar oxide, and then a second conductive layer is formed on the first conductive layer and covers the collar oxide. A composite insulating spacer is formed over the sidewall of the upper trench where a gap is between the second conductive layer and it. The second conductive layer is thermally oxidized to form an oxide layer thereon and fills the gap. The oxide layer is removed before a reverse T-shaped trench top isolation layer is formed on the second conductive layer by low pressure chemical vapor deposition, and then the composite insulating spacer is removed. A semiconductor device having trench top isolation layers is also disclosed.