7.2.1 Thickness control and uniformity
Often, final device characteristics, such as the value of a capacitor, the threshold voltage of a transistor, the resistance per square in a thin-film resistor and the resonant frequency of an acoustic wave filter, depend strongly on the thickness of a deposited layer. Therefore it must be ensured that the layer thickness is the same at all points on the wafer, and on every wafer that comes off the production line. Specifications of ±1% uniformity and reproducibility are not uncommon, and some critical applications are starting to demand thickness control to ±0.1%. This is not trivial! If the material came from a point source, we would need a chamber 75 m tall to achieve ±0.1% uniformity across a 200 mm diameter wafer – we'll have to find a much better method than that. Figure 31 illustrates the point. Also, a process lasting 2 min must stop with an accuracy of 100 ms to hit the correct thickness consistently over many process runs, despite temperature drifts, changes in chamber contamination and wall coatings, and declining stocks of material in the deposition source.
Some deposition techniques give intrinsically good uniformity by virtue of their operating mechanism. For those that don't, techniques to redistribute material more evenly include multiple or spatially distributed sources, gas scattering, shielding and wafer rotation.
Uniformity is important not only for film thickness, but for all aspects of the film's chemistry, microstructure and morphology. This is often the most difficult specification to meet, so it should be the first thing we think of, and the last to be fine-tuned once everything else is in place.