Method of fabricating a power semiconductor device

A method of fabricating a power semiconductor device, said method comprising:

• forming a gate trench in a semiconductor substrate, said gate trench including a gate electrode;
• and
• forming a field plate trench structure in said substrate separate from said gate trench, wherein forming said field plate trench structure comprises:
• forming an upper trench situated over a lower trench in said substrate, said upper trench being wider than said lower trench and extending deeper into said substrate than said gate trench, a width of said lower trench being greater than one half a width of said upper trench;
• forming a trench dielectric in said lower trench and on sidewalls of said upper trench, said trench dielectric filling completely said lower trench;
• and
• forming a field plate electrode within said trench dielectric;
• wherein said trench dielectric is formed such that a bottom thickness of said trench dielectric is greater than a sidewall thickness of said trench dielectric on said sidewalls of said upper trench.