Page 186 - Sami Franssila Introduction to Microfabrication

16

CMP: Chemical–Mechanical Polishing

Material removal from a wafer is usually done by mechanical forces acting on microstructures. This sub-
etching, but there is the alternative technology of surface damage is 5 to 10 µm deep. Grinding is used
polishing. Polishing is an established technology in when hundreds of micrometres need to be removed,
silicon-wafer manufacturing where final polishing yields as in wafer thinning. CMP removes micrometres only,
wafers with a root mean square (RMS) roughness of and the resulting surfaces are very smooth and defect
1 ˚ A, but it emerged in microfabrication only in the free. In CMP, abrasive particles of 10 to 300 nm are
late 1980s. In microfabrication, polishing and etching dispersed in a slurry. The mechanism is different from
processes can be combined to yield identical final grinding: CMP works in the atomic regime. Atomic
structures via different process sequences, as shown bonds are weakened or broken, and removal is based on
in Figure 16.1: metal lines can be made either in the the interaction between the slurry and the mechanical
following sequence: effect of the abrasive particles. Surface roughness after
CMP is in the nanometer range, while grinding results
metal deposition ⇒ metal etching in hundreds of nanometres.
⇒ oxide deposition ⇒ oxide polishing
16.1 CMP PROCESS AND TOOL
or in the sequence
The CMP tool consists of a solid, extremely flat platen,
oxide deposition ⇒ oxide etching
on which the polishing pad is glued. The wafer chuck,
⇒ metal deposition ⇒ metal polishing which holds the wafer upside down, is situated on
a spindle. A slurry introduction mechanism feeds the
The latter sequence, known as damascene, is used for slurry on the pad. Both the platen and the spindle
metals that cannot be plasma-etched, and it is the key are rotated, and the linear velocity (used in Preston’s
technology to copper metallization of ICs. equation) is the sum of two velocities (Figure 16.2).
Polishing in microfabrication is a descendant of glass There are four major elements in a CMP process:
polishing, which has been an established technology for
400 years. Abrasive particles are dispersed in a suitable • topography
liquid to create a slurry, which is fed in between a • materials
polishing pad and the piece to be polished. Elevated • polishing pad
structures are preferentially removed since the pressure • slurry.
is highest there. In the case of a blanket, wafer-surface
irregularities are smoothed out. Down force is an average force, but local pressure is
Grinding may look similar to CMP, but the two needed to understand removal mechanisms. It depends
are quite different. In grinding, abrasive particles of on the contact area, which in turn depends on both the
1 to 100 µm in size are mounted in resin, and structures on the wafer and on the pad structure. Pads are
micrometre-sized chunks of material are removed by rough, with say 50 µm roughness, and contact is made
crack propagation and brittle fracture. Grinding is fast by asperities, and the contact area is only a fraction of
but also very coarse; the substrate is damaged due to the wafer area (Figure 16.3).

Introduction to Microfabrication Sami Franssila
 2004 John Wiley & Sons, Ltd ISBNs: 0-470-85105-8 (HB); 0-470-85106-6 (PB)