Page 318 - A Practical Guide from Design Planning to Manufacturing

288 Chapter Nine

any material at very similar rates. Selectivity can be achieved by using
a chemical etching mechanism instead.
Plasma etching is a chemical process like wet etching, enabling it to
be selective but it is also isotropic, cutting in all directions equally.
Plasma etching reverses another deposition method: plasma-enhanced
chemical vapor deposition (PECVD). In PECVD, plasma ions go through
a chemical reaction at the surface of the wafer that results in deposi-
tion. Plasma etching uses other chemical reactions to remove material
instead. Using plasma enables many chemical reactions that are not pos-
sible for wet etches. To create an etch process, which is both selective
and anisotropic, sputter etching and plasma etching are combined in
reactive ion etching (RIE).
RIE etches through both a physical and chemical process. A plasma of
ions that will chemically react with the material to be removed is created
and accelerated through an electric field at the wafer. The ions are accel-
erated at low enough energies that they typically will not remove mate-
rial with which they cannot chemically react, but the chemical reaction will
proceed much more quickly in the vertical direction with the ions all pro-
jected in that direction. Because it is both selective and anisotropic, RIE
is the most common form of etching for layers with very small feature size.
The limits of etching must be taken into account in creating modern
designs and fabrication processes. Imagine etching a layer of SiO that has
2
been deposited on top of a layer of wires and more SiO . It may be pos-
2
sible to find a chemical reaction to selectively etch SiO and not the metal
2
wires, but the chemical reaction cannot distinguish between the new
layer of SiO and the old layer of the same material beneath it. To help
2
allow for selective etches, etch stop layers may be added to the process
flow. These are layers deposited specifically to slow etch rates after the
layer to be removed has been cut through. A layer of silicon nitride
deposited before a new layer of SiO is applied will make it far simpler
2
to etch completely through one SiO layer without cutting into the next.
2
Etch stop layers do not perform an electrical or mechanical function, but
they make the manufacturing process itself more reliable.
Another way of aiding manufacturing is through density targets. In
any chemical etching process, the reactant chemical will tend to be con-
sumed more quickly in regions of the die with a high density of the
etched material to be removed. This causes the etch rate to be slow in
regions of high density and high in regions of low density. To help keep
the etch rate uniform across the die, density targets may be created for
each layer. Extra unneeded poly lines can be added in regions of low den-
sity to aid in manufacturing. Density targets also make polishing steps
more uniform by providing the same relative hardness across the die.
As dimensions continue to push the limits of fabrication, the design
and process flow will have to add more and more steps specifically to aid
manufacturing.