7.6 Laser Trimmed Resistors

Laser trimming of film resistors allows integrated circuit manufacturers to very accurately control resistance values. Being an expensive process step, it is important to maximize the benefits once that option is taken [91]. Our electrical/thermal simulator allows easy characterization of different trim algorithms.

The performance of laser trimmed resistors is related to the so called Heat-Affected Zone (HAZ) [91], formed during the trimming process. The HAZ corresponds to a region along the edge that being heavily heated (but still below the vaporization temperature) after cooling suffers an alteration in its physical properties, namely in the sheet resistance, temperature coefficient and aging.

We model the sheet resistance of HAZ as in [91]. The use of parameterized layout generation facilitates the preparation of sets corresponding to different lengths of the laser cut. The simulated structures are then formed directly from the layout, and are made of a silicon substrate, a SiO$_2$ layer, the high-resistivity polysilicon resistor layer and finally, a passivation SiO$_2$ layer. The bottom of the silicon substrate is kept at a constant temperature of 300$^\circ$K. We compared the classical plunge trim and a modified L trim resistor (see Figure 7.15) with the same dimensions and geometry before trim. The target resistance is 1000$\Omega$ for both resistors.

Figure 7.14 shows the dependence of the resistance value as function of the cut-length. As near 1000$\Omega$ the sensitivity is much lower in the modified L-trim, its better accuracy is evident. In Figure 7.15 we present the temperature distribution along the resistors when dissipating the same overall power. We can see the crowding effect in the HAZ (corresponding to the maximum values in the current and temperature) and that for the same overall power dissipation, the maximum temperature is 20$^\circ$C lower in the modified L trim resistor. This predicts a much more stable aging of resistors using this trim algorithm.

Figure 7.14: Dependence of the resistance value on the laser-cut length.

Figure 7.15: Temperature distribution in two different trimmed resistor configurations adjusted to 1000$\Omega$.