1T Bitcell

A Split Channel Device

Kilopass’ 1T antifuse bitcell patent, issued in 2005, is a split channel device consisting of a thin (core) gate and a thick (I/O) gate. The thin gate is the program gate, while the thick gate is the select gate. When a normal supply voltage such as an I/O or core voltage is applied to the gate, no current is sensed along the bitcell. The equivalent circuit is a capacitor. Since there is no current that flows along the bitline, the bitcell is “0” by default. When a large programming voltage is applied along the gate of the program transistor, a hard gate oxide breakdown occurs. A resistive path is created. The equivalent circuit for the program transistor is a resistor. Due to the thickness gradient, hard oxide breakdown occurs at the weakest link, the junction of the thick gate and thin gate.

1T bitcell is not Standard CMOS

(Diagram: 1T not standard cmos)

TEM Cross Section of Kilopass 1T Cell Manufactured in 2002(Diagram: Transmission electron microscopy (TEM) Cross Section of Kilopass 1T Cell Manufactured in 2002 using standard CMOS process and featuring shallow trench isolation (STI) and two-gate dielectric thicknesses under the same gate.)

Tradeoff

The benefit of 1T includes smaller peripheral logic since only one wordline needs to be decoded. It is a two terminal device. The tradeoff is the 1T is not a standard CMOS transistor. Foundry reliability data including time to dielectric breakdown (TDDB) cannot be used. Special high grade mask may be needed for better CD control since the poly gate and gate oxides are not self aligned. The defects due to multiple implants created at the junction of the thick gate and thin gate may cause long term reliability issues including self healing. This is not a standard foundry flow so the behaviors of the defects are not well known or monitored. The Kilopass embedded NVM products implemented in standard CMOS process, including XPM and Gusto, does not use the 1T bitcell due to these potential manufacturing issues. They use the 2T bitcell that conformed to foundry DFM and DRC rules.