Page 102 - Sami Franssila Introduction to Microfabrication

Thin-film Growth and Structure 81

The case of silicon dioxide/copper (Figure 7.6(c)) selected on the basis of their bond-forming abilities:
shows copper diffusion into the oxide. The sil- titanium and chromium are the two most widely used
icon/titanium pair will react and form silicide materials. Typical pairs of adhesion layer/noble metal
(Figure 7.6(d)). Many metals do form silicides, copper include Ti/Pt, Ti/Au and Cr/Au. Adhesion layers are also
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silicides form at very low temperatures, 200 to 300 C, useful for near-noble refractory metals like tungsten.
nickel, cobalt and titanium at successively higher tem- Barriers are additional layers between two materials.
peratures, and W, Mo and Ta will also form silicides; not Their role is to prevent reactions between adjacent
all of them, simple MeSi x compounds but complex mix- layers, be it diffusion, chemical reaction or any other
tures of various silicides, for example, Me 2 Si 5 , Me 2 Si 3 , type of unwanted interaction. Many aspects of barriers
MeSi 2 , MeSi. Aluminium reacts with tungsten and tita- are similar to adhesion layers: barriers are not needed for
nium to form Al 12 W and Al 3 Ti, respectively. device operation as such, but their presence either makes
Aluminium does not form a silicide. Annealing
at 425 C will dissolve native oxide, ensuring good the fabrication process more robust, or the resulting
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electrical contact. However, too much annealing will device more stable. Barriers are thin, like adhesion
lead to pitting: silicon is soluble in aluminium (as shown layers, with 10 to 100 nm as typical barrier thickness.
in Al-Si phase diagram, Figure 7.7), and open volume is Total barriers must prevent all fluxes through them:
left behind as the silicon atoms migrate into aluminium. atom diffusion and charge carrier transport. In the
Aluminium, on the other hand, will diffuse to fill in case of metallization, the current has to flow through
the space left by silicon dissolution. This leads to the the barrier, but atom movements must be prevented.
case depicted in Figure 7.6(e). These aluminium spikes Metallic barriers have relatively loose requirements for
can be micrometres deep, and extend beyond the pn- resistivity (the distance is <100 nm only). Most barrier
junction. To prevent junction spiking, aluminium can materials have resistivities around 100 to 500 µohm-
be alloyed with silicon: a silicon concentration of 0.5% cm, one-to-two orders of magnitude higher than the
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(wt%) will saturate aluminium at 425 C, and 1% Si will conductors. While resistivity is not a problem, contact
prevent silicon dissolution at 500 C. The other, more resistivity must be low, and barrier height considerations
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general solution is to implement a diffusion barrier. may exclude some materials.
The first barriers to be implemented were 100 nm
thick TiW films between aluminium and silicon to
7.5 ADHESION LAYERS AND BARRIERS
prevent Al-Si junction spiking. TiW grain size is ca.
Adhesion is a major issue in thin-film technology. As 100 nm: if sputtered in argon, grain boundaries offer fast
a rule of thumb, poor adhesion is the norm, and only diffusion paths, and pure TiW is not a very effective
special attention will lead to good adhesion. Some barrier. But deposition in poor vacuum led to the
materials have poor adhesion due to their chemical incorporation of oxygen and nitrogen, which passivated
nature: noble metals are noble because they do not grain boundaries. When the mechanism was elucidated,
react, and therefore they do not form bonds across the reactive sputtering of TiW in Ar + N 2 atmosphere was
substrate interface. Adhesion is also related to surface adopted. Reactive sputtering leads to 10 nm grain size
cleanliness: residues or dirt from the previous step will
and nitrogen at grain boundaries, both of which lead to
almost inevitably lead to poor adhesion. Deposition
improved barrier performance. Amorphous films would
process variables do play a role: in sputtering, energetic be preferable as barriers, and a-WN has been one
ions and atoms will kick off loosely bound atoms, but candidate. Copper metallization needs barriers not only
in evaporation, there is no inherent removal of weakly
bonded atoms. between copper and silicon, but also between copper
Adhesion layers are additional films with the role of and silicon dioxide because copper diffuses into oxide.
adhesion improvement, and, in the first approximation, Tantalum and tantalum compounds such as TaN are
have no effect on the device structure or operation. The used. Silicon nitride can be used as a dielectric barrier
thickness of the adhesion layer is in the range of 10 nm between copper and oxide because it is stable in contact
because volume properties are of no interest, but only its with both silicon and copper.
surface properties. The adhesion layer and the structural When active devices are made on glass (or on steel),
film are deposited immediately after each other in the such as thin-film transistors, the substrate has to be
same vacuum chamber: freshly formed adhesion-layer isolated from the silicon devices. Barriers like silicon
surface ensures cleanliness and thus eliminates one dioxide (both CVD oxide and spin-on-glass (SOG)) as
main factor of poor adhesion. Adhesion-layer films are well as Al 2 O 3 have been used.