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ECD FUNDAMENTALS

ECD FUNDAMENTALS 16.5

9
is one mechanism thought to promote superconformal deposition. The third type of additive that is
commonly added to these baths is called a leveler. Levelers are polar (negatively charged) molecules
that perform a suppressing function. They are typically high-molecular-weight compounds with sul-
16
fonic or sulfamic acid or other nitrogen-containing functional groups. Their function is to attach to
the areas of the surface with higher current densities and decrease the local deposition rate. They also
tend to reduce the deposition rate over small features. The concentrations of all these organic addi-
tives are typically very low, ranging from ppb to 0.1 percent of the bath composition. The low con-
centrations make these compounds very susceptible to changes in the diffusion boundary conditions,
as noted earlier. Other organic components are sometimes added to copper plating baths, including
surfactants, ductility modifying agents, and bactericides.
Other parameters that affect the electrodeposition process include temperature, flow rate, agita-
tion, and substrate motion. Temperature can increase the diffusion rates of the bath components and
change the adsorption characteristics and usage rates of some of the organic additives. The flow rate,
agitation, and substrate motion parameters typically impact the process by changing the hydrody-
namic boundary layer thickness (which sets the diffusion boundary layer thickness) and the distrib-
ution of chemical constituents across the surface. It is sometimes difficult to predict all the changes
associated with modifying the process conditions, but the results can usually be explained by exam-
ining the basics noted earlier.
It is also important to pay attention to the anode in electrolytic deposition systems. There are two
classes of anodes available for copper deposition—consumable (copper) and inert (nonconsumable,
dimensionally stable, noble). The main differences between the types of anodes are in the anodic
reactions they support. Consumable anodes promote copper dissolution at the anode (Eq. (16.1b)),
while inert anodes usually involve the anodic generation of oxygen gas. Oxygen gas can lead to film
defects if not managed properly. 18 Most industrial systems utilize consumable anodes for copper
deposition on semiconductor wafers. In this case, the anode reaction is the reverse of the cathode
reaction, leading to a stable copper concentration. Any contaminants in the anode will also be dis-
solved and will lead to chemical or particulate contamination of the chemistry.
There is a large amount of information published on ECD that is beyond the scope of this short
chapter. For instance, alloys can be deposited if the reduction potentials of the metals involved are
close enough to each other. In some cases, the reduction potential must be adjusted by utilizing
complexing agents or other chemistry changes that affect the deposition overpotential. Similar
techniques can be used to affect the ability to deposit metals on certain surfaces. In addition, there
are nonaqueous systems, such as solvents or molten salts, available for the deposition of certain
materials. 19,20

16.3 BENEFITS OF COPPER DAMASCENE ECD PROCESSING

Electrochemical deposition of copper provides all the material property benefits of copper intercon-
nections at a low cost. Copper is second only to silver in elemental resistivity at 1.67 µΩ-cm and pro-
vides high electromigration resistance when compared to aluminum alloys.
Electroplating is well known as a purification method for copper (electrorefining). This same effect
can be seen in ECD films. Impurity levels in the final deposit typically range from 0 to 50 ppm (see
Fig. 16.3). However, sulfur concentrations tend to be an order of magnitude higher in concentration
due to organic additives incorporated into the deposit. The electrical resistivity of ECD copper films
21
is approximately 2.1 µΩ-cm, as deposited, because of the small grain size. After a low-temperature
anneal (200 to 400°C), the increased grain size reduces the resistivity to about 1.8 µΩ-cm. This is
far below the resistivity of the aluminum alloys that are used to provide reasonable electromigration
resistance (approximately 4 µΩ-cm).
Copper inherently provides an electromigration advantage over aluminum. Additionally, ECD
copper films are predominantly (111) in crystal orientation. 11 Copper interconnects typically
offer an order of magnitude increase in electromigration lifetime over traditional aluminum cop-
per alloys. 22

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