Substrate isolation in integrated circuits
View Patent ↗Substrate isolation trench ( 224 ) are formed in a semiconductor substrate ( 120 ). Dopant (e.g. boron) is implanted into the trench sidewalls by ion implantation to suppress the current leakage along the sidewalls. During the ion implantation, the transistor gate dielectric ( 520 ) faces the ion stream, but damage to the gate dielectric is annealed in subsequent thermal steps. In some embodiments, the dopant implantation is an angled implant. The implant is performed from the opposite sides of the wafer, and thus from the opposite sides of each active area. Each active area includes a region implanted from one side and a region implanted from the opposite side. The two regions overlap to facilitate threshold voltage adjustment.
1. A method for fabricating an integrated circuit comprising a conductive transistor gate formed on a gate dielectric formed on an active area of a semiconductor substrate, the active area having a first conductivity type, the method comprising:
(i) forming a dielectric layer on the semiconductor substrate, wherein the gate dielectric comprises at least a portion of the dielectric layer;
(ii) forming a first layer on the dielectric layer, wherein the conductive transistor gate comprises at least a portion of the first layer;
(iii) forming a mask over the first layer to define one or more substrate isolation regions to be formed adjacent to the active area;
(iv) patterning the first layer, and dielectric layer, and the semiconductor substrate to form a pattern defined by the mask, the patterning operation comprising:
(iv-a) removing a portion of the first layer;
(iv-b) removing a portion of the dielectric layer to form a dielectric layer sidewall, wherein the dielectric layer sidewall is part of the gate dielectric; and
(iv-c) removing a portion of the semiconductor substrate to form a trench at a location of each of the isolation regions, the trench having a sidewall at a boundary of the active area;
(v) introducing a dopant of the first conductivity type into the trench sidewall by ion implantation of dopant ions with the gate dielectric sidewall facing a stream of the dopant ions;
(vi) forming a dielectric over the trench sidewall, the dielectric being part of the isolation regions; and
(vii) heating the gate dielectric in one or more thermal step to anneal a damage to the gate dielectric caused by the ion implantation, wherein at least one of the thermal steps is performed with the gate dielectric not being exposed.
2. The method of claim 1 wherein at least one of the thermal steps is performed with the gate dielectric sidewall not being exposed.
3. The method of claim 1 wherein at least one of the thermal steps is performed when the dielectric formed over the trench sidewall in operation (vi) covers the gate dielectric sidewall.
4. The method of claim 1 wherein the gate dielectric comprises silicon oxide.
5. The method of claim 4 wherein at least one of the thermal steps comprises holding a semiconductor structure comprising the gate dielectric at a temperature of at least 750° C. for at least 15 minutes.
6. The method of claim 1 wherein at least one of the thermal steps comprises an oxidation step.
7. The method of claim 6 wherein the oxidation step comprises growing silicon dioxide.
8. The method of claim 1 wherein the ion stream forms an angle of 25° relative to a line normal to the semiconductor substrate.
9. The method of claim 1 wherein the conductive transistor gate is a floating gate of a non-volatile memory cell.
10. The method of claim 9 wherein the gate dielectric is a tunneling dielectric through which a charge is tunneled to change a state of the memory cell.
11. The method of claim 1 wherein the first conductivity type is type P.
12. The method of claim 11 wherein the dopant comprises boron.