Methods of making power semiconductor devices with thick bottom oxide layer

1. A method of of manufacturing a semiconductor device having a charge control trench and an active control trench with a thick oxide bottom comprising:

forming a drift region of a first conductivity type;

forming a well region extending above the drift region and having a second conductivity type opposite the first conductivity type;

forming an active trench extending through the well region and into the drift region;

forming a charge control trench extending deeper into the drift region than the active trench;

forming an oxide film that fills the active trench, the charge control trench and covers a top surface of the substrate;

etching the oxide film off the top surface of the substrate and inside the active trench to leave a substantially flat layer of thick oxide having a target thickness at the bottom of the active trench;

forming an electrode in the active trench; and

forming source regions having the first conductivity type in the well region adjacent the active trench.

2. A method of of manufacturing a semiconductor device having a charge control trench and an active control trench with a thick oxide bottom comprising:

forming a drift region of a first conductivity type;

forming a well region extending above the drift region and having a second conductivity type opposite the first conductivity type;

forming an active trench extending through the well region and into the drift region;

forming a charge control trench extending deeper into the drift region than the active trench;

forming a layer of pad oxide on the substrate;

depositing a thin layer of silicon nitride on the pad oxide;

performing an anisotropic etch to remove silicon nitride from horizontal surfaces leaving silicon nitride on active trench sidewalls;

depositing oxide on horizontal surfaces including the bottom of the active trench using a sub-atmospheric chemical vapor deposition process;

removing a sandwich layer of oxide-nitride-oxide from active trench sidewalls by an etch process;

forming an electrode in the active trench; and

forming source regions having the first conductivity type in the well region adjacent the active trench.

3. The method of claim 1 further comprising masking the charge control trench so that the charge control trench remains substantially filled with oxide.

4. A method of manufacturing a semiconductor device having a charge control trench and an active control trench with a thick oxide bottom comprising:

forming a drift region of a first conductivity type;

forming a well region extending above the drift region and having a second conductivity type opposite the first conductivity type;

forming an active trench extending through the well region and into the drift region;

forming a charge control trench extending deeper into the drift region than the active trench;

forming a substantially flat layer of oxide having a target thickness at the bottom of the active trench;

lining the sidewalls of the active trench with an oxide material;

depositing a first conductive layer over the thick oxide layer on the bottom of the active trench;

depositing an inter-electrode dielectric material over the first conductive layer;

depositing a second conductive layer over the inter-electrode dielectric material; and

forming source regions having the first conductivity type in the well region adjacent the active trench.

5. A method of manufacturing a semiconductor device having a charge control trench and an active control trench with a thick oxide bottom comprising:

forming a drift region of a first conductivity type;

forming a well region extending above the drift region and having a second conductivity type opposite the first conductivity type;

forming an active trench extending through the well region and into the drift region;

forming a charge control trench extending deeper into the drift region than the active trench;

forming a substantially flat layer of oxide having a target thickness at the bottom of the active trench;

forming an electrode in the active trench; and

forming source regions having the first conductivity type in the well region adjacent the active trench;

depositing in the charge control trench a plurality of conductive layers stacked vertically and separated from each other and from the trench sidewalls by dielectric material;

wherein the thickness of the plurality of conductive layers inside the charge control trench vary.

6. A method of manufacturing a semiconductor device having a charge control trench and an active control trench with a thick oxide bottom comprising:

forming a drift region of a first conductivity type;

forming a well region extending above the drift region and having a second conductivity type opposite the first conductivity type;

forming an active trench extending through the well region and into the drift region;

forming a charge control trench extending deeper into the drift region than the active trench;

forming a substantially flat layer of oxide having a target thickness at the bottom of the active trench;

forming an electrode in the active trench; and

forming source regions having the first conductivity type in the well region adjacent the active trench;

depositing in the charge control trench a plurality of conductive layers stacked vertically and separated from each other and from the trench sidewalls by dielectric material;

wherein the thickness of a first conductive layer that is deeper inside the charge control trench is smaller than the thickness of a second conductive layer that is disposed above the first conductive layer.