Dual thin film precision resistance trimming

1. A trimmable resistance structure, comprising:

an integrated circuit die having components in a semiconductor substrate;

a resistive structure in the integrated circuit die composed of a material whose resistance is permanently changeable by application of heat thereto;

a dielectric in the integrated circuit die disposed adjacent to the resistive structure;

a heater in the integrated circuit die disposed adjacent to the dielectric, the dielectric being disposed so as to separate the heater from the resistive structure; and

a switch in the integrated circuit die configured to selectively couple the heater to a voltage supply that will cause heat from the heater to heat the resistive structure to cause a permanent change in the resistive structure.

2. The structure of claim 1 wherein the resistive structure, the heater, and the dielectric material are thin films.

3. The structure of claim 1 wherein the voltage supply is capable of sourcing a voltage sufficient to increase the temperature of the heater to at least 450° C.

4. The structure of claim 1 wherein the heater is in a serpentine shape.

5. The structure of claim 1 wherein the dielectric includes Si 3 N 4 .

6. The structure of claim 5 wherein the resistive structure includes CrSi.

7. A resistor trimming method, comprising:

applying a voltage to a heater in an integrated circuit die through a switch in the integrated circuit die;

heating a resistive structure in the integrated circuit die adjacent to the heater;

permanently changing the resistance value of the resistive structure during the heating step; and

removing the voltage from the heater.

8. The method of claim 7 wherein the resistive structure, the heater, and the dielectric material are thin films.

9. The method of claim 7 wherein applying the voltage comprises applying a voltage pulse to the heater.

10. The method of claim 7 wherein heating the resistive structure includes heating the resistive structure in excess of 450° C.

11. The method of claim 10 wherein heating the resistive structure decreases the resistance of the resistive structure.

12. A circuit, comprising:

an integrated circuit die;

an amplifier in the integrated circuit die;

a trimmable thin film resistor in the integrated circuit die electrically coupled to the amplifier, a gain of the amplifier being based in part on a resistance of the trimable thin film resistor;

a thin film heater in the integrated circuit die thermally coupled to the trimmable thin film resistor;

a transistor in the integrated circuit die and electrically coupled to the thin film heater and configured to selectively pass a current through the thin film heater to increase a temperature of the trimable thin film resistor to permanently alter the resistance of the trimable thin film resistor; and

a thin film thermal separator disposed between the trimmable thin film resistor and the thin film heater.

13. The circuit of claim 12 wherein the thin film heater is a thin film resistor.

14. The circuit of claim 13 wherein the resistance of the thin film heater is less than the resistance of the trimmable thin film resistor.

15. The circuit of claim 12 wherein the resistance of the trimmable thin film resistor permanently changes with exposure to a temperature within a range of temperatures.

16. The circuit of claim 15 wherein the range of temperatures includes 450° C. to 850° C.

17. The circuit of claim 12 wherein the thin film thermal separator is a dielectric.

18. The circuit of claim 17 wherein the thin film thermal separator includes Si 3 N 4 .

19. The apparatus of claim 12 wherein the trimmable thin film resistor includes CrSi.