MULTIPLE LASER TRIMMED RESISTORS

Publishing Venue

Motorola

Related People

Abstract

As circuit densities continue to increase, so does the desire to built in volumetric rather than planer man- ner. One area in which this may be feasible is in resistor networks used for biasing and padding. One of the main problems with multi-layer resistors is trimming the resistors on top and underneath without hurting the other. This can be controlled through the layout, making it possible to trim resistors independently or in a ratio to each other. Still more resistors can be stacked on to make pads that can be trimmed with a single laser cut, yet maintain their initial ratio.

MOTOROLA Technical Developments Volume 9 August 1989

MULTIPLE LASER TRIMMED RESISTORS

by Randy Grunwell

   As circuit densities continue to increase, so does the desire to built in volumetric rather than planer man- ner. One area in which this may be feasible is in resistor networks used for biasing and padding. One of the main problems with multi-layer resistors is trimming the resistors on top and underneath without hurting the other. This can be controlled through the layout, making it possible to trim resistors independently or in a ratio to each other. Still more resistors can be stacked on to make pads that can be trimmed with a single laser cut, yet maintain their initial ratio.

The examples shown below are discrete devices, but the same ideas can be an integral part of the substrate circuitry.

   The basic premise on the geometries and their trimming is that resistance increases drastically if resistor material is removed perpendicular to the flow of current, while it only moves slightly when trimmed in the same direction. Figure 1 shows these two conditions.

LASER CUT \ LASER CtJl

DRASTIC CHANGE SLIGHT CHANGE

FIGURE 1

   By placing a resistor on top of and perpendicular to another resistor, it is possible to trim the resistors fairly independently. This is due to the laser cut being perpendicular to the current flow in one resistor while being parallel to the current flow in the other. This is shown in Figure 2.

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BACK FIRST METAL

AND FIRST RESISTOR

DIELECTRIC SECOND METAL

AND SECOND RESISTOR

EXPLOSION OF FRONT SIDE METALS, RESISTORS,

AND DIELECTRIC

SCHEMATIC

COMPOSITE COMPOSITE TRIMS

WITHOUT SHOWN DIELECTRIC

F...