Method of making and using guardringed SCR ESD protection cell
Methods and circuits are disclosed for protecting an electronic circuit from ESD damage using an SCR ESD cell. An SCR circuit is coupled to a terminal of an associated microelectronic circuit for which ESD protection is desired. The SCR used in the ESD cell of the invention is provided with a full guardring for shielding the SCR from triggering by fast transients. A resistor is provided at the guardring for use in triggering the SCR at the onset of an ESD event. Exemplary preferred embodiments of the invention are disclosed with silicide-block resistors within the range of about 2-1000 Ohms or less.
1. A method of protecting an electronic circuit from ESD damage:
operably coupling an SCR circuit having first and second terminals to a terminal of the electronic circuit,
wherein the SCR circuit has a full guardring for shielding the SCR circuit from triggering by fast transients;
wherein the guardring includes a pair of resistors resulting in a guardring resistance for triggering the SCR at the onset of an ESD event; and
wherein the SCR circuit includes:
a lightly doped region of a first conductivity type;
a first highly doped region of a second conductivity type opposite the first conductivity type, wherein the first highly doped region is disposed within the lightly doped region at a surface of the lightly doped region, and at least a portion of the boundary between the first highly doped region and the lightly doped region forms a forward-biased SCR junction;
a second highly doped region of the first conductivity type forming the guardring disposed within the lightly doped region at the surface, wherein the guardring surrounds the first highly doped region in a plane coplanar to the surface; and
the pair of resistors dividing the guardring into first and second regions, wherein the resistors are disposed at the surface, wherein
the first region of the guardring is electrically connected to the first terminal; and
the second region of the guardring is electrically connected to the first terminal throucih the resistors.
2. The method of protecting an electronic circuit from ESD damage according to claim 1 further comprising the step of selecting the guardring resistance from the range of approximately 2-1000 Ohms.
3. The method of protecting an electronic circuit from ESD damage according to claim 1 further comprising the step of selecting the guardring resistance of less than 2 Ohms.
4. The method of protecting an electronic circuit from ESD damage according to claim 1 wherein the resistors at the guardring are formed by using a silicide block process.
5. The method of protecting an electronic circuit from ESD damage according to claim 1 further comprising the step of isolating the terminal coupling.
6. A method of forming an ESD protection cell comprising:
forming an SCR circuit having first and second terminals for coupling in an electrical path of an associated circuit to provide ESD protection for the associated circuit, comprising:
forming a lightly doped region of a first conductivity type;
forming a first highly doped region of a second conductivity type opposite the first conductivity type, wherein the first highly doped region is disposed within the lightly doped region at a surface of the lightly doped region, and at least a portion of the boundary between the first highly doped region and the lightly doped region forms a forward-biased SCR junction;
forming a second highly doped region of the first conductivity type forming a guardring disposed within the lightly doped region at the surface, wherein the guardring surrounds the first highly doped region in a plane coplanar to the surface; and
forming a pair of resistors dividing the guardring into first and second regions, wherein the resistors are disposed at the surface and the resistors result in a guardring resistance used to trigger the SCR circuit at the onset of an ESD event, wherein
the first region of the guardring is electrically connected to the first terminal; and
the second region of the guardring is electrically connected to the first terminal through the resistors.
7. The method of forming an ESD protection cell according to claim 6 further comprising the step of selecting the guardring resistance from the range of approximately 2-1000 Ohms.
8. The method of forming an ESD protection cell according to claim 6 further comprising the step of selecting the guardring resistance less than 2 Ohms.
9. The method of forming an ESD protection cell according to claim 6 wherein the resistors at the guardring are formed by using a silicide block process.
10. The method of forming an ESD protection cell according to claim 6 further comprising:
forming a third highly doped region of the first conductivity type disposed within the lightly doped region at the surface, wherein the third highly doped region is electrically connected to the second terminal; and
forming an isolation structure between the first and second terminals.
11. The method of forming an ESD protection cell according to claim 10 wherein the isolation structure comprises an N-well and an adjacent buried N layer.
12. The method of forming an ESD protection cell according to claim 10 wherein the isolation structure comprises a buried N layer bordered by two adjacent N-wells.