Electrical Misalignment Structures

For most of the structures on the wafer to operate properly, the maximum misalignment between any two layers must be less than 15 μm. For the small ring oscillator and the CP FET to work, the maximum misalignment between any two layers must be less than 7.5 μm. Rotational, or theta (θ), misalignment, will cause the x- and y-misalignments to vary across the surface of the wafer. For example, one cell might be misaligned by 4 μm, but on the other side of the wafer the misalignment might be 16 μm.

Theory

It is possible to electrically measure lithographic misalignment. The structure above determines misalignment between layers in the x- and y- directions¹. There are four structures which measure misalignment between layers 1 and 4, 1 and 5, 2 and 4, and 2 and 5. The structure is basically a voltage divider. To determine the misalignment between layers 1 and 4, current is forced through the ends, A₂ and C₂. Voltage is measured at each pad along the resistor: A₁, B₁, B₂, C₁, D₁, and D₂. The contact, B₂, is designed to be exactly halfway between A₁ and B₁. However, misalignment will cause B₂ to be off- center, and the measured voltage values can be analyzed to determine the misalignment.

We can determine the amount of misalignment based on

where L = 140 μm is the spacing betwen A₁ and B₁, and between D₁ and D₂.

Substituting V=IR

simplify

However, the diffused resistor must be operating linearly for these equations to be valid.

Measurement

Force a current through the misalignment structures and measure the voltage at each point.

Do this for each of the four structures.

References

¹Badih El-Kareh and Richard J. Bombard, Introduction to VLSI Silicon Devices, (Kluwer Academic Press, Hingham, Massachusetts, 1986), pp. 44-46.

Further Information

M. Feldman, "Alignment Techniques in Optical and X-Ray Lithography," VLSI Electronics: Microstructure Science, Lithography for VLSI, Norman G. Einspruch and R. K. Watts, Eds., (Academic Press, Orlando, FL, 1987), Vol. 16, Chap. 6, pp. 229-265.