5.6 Trimming

For high-precision applications, trimming is sometimes used; this refers to the adjustment of element values on the chip after fabrication, by a "surgical" procedure. For example, laser beams are sometimes used to modify thin-film resistor values or to break or make connections. Or, current pulses are used to blow polysilicon fuses; this can be used to either disconnect parts of resistor or capacitor groups or to activate or deactivate MOS transistor switches, which in turn connect or disconnect parts of resistor or capacitor groups (it must, of course, be ensured that the switch resistance does not interfere with proper operation). In a different trimming process, currents can be used to cause a permanent short circuit in base-emitter zener diodes ( zener zapping), thus short-circuiting parts of a resistor or capacitor array.

An alternative to such trimming is to use a process that allows the fabrication of electrically erasable programmable read-only memory (EEPROM) devices on the same chip [ ^[49]]. Such devices use floating gates to store charges that are placed on them through high-voltage pulses, which in a sense cause a temporary breakdown for this purpose. The high-voltage pulses can be created on the chip from the normal power supply voltage through voltage multiplication. EEPROM devices can be used to store logic levels which are in turn used to close or open analog switches; thus, elements can be added to, or removed from, element arrays. The arrays can consist of capacitors, resistors, or transistors.