Biomedical Engineering Reference
In-Depth Information
7.4.2.1 Semiconductor industry
The use of AFM in the semiconductor industry started in the mid-1990s. Since that time
several hundred atomic force microscopes have been employed by semiconductor com-
panies. The majority of the applications were never made public. The applications
presented here are some of those that were made public. Most of these applications were
associated with developing new processes, processes that most likely have a lifespan of
5-10 years.
Front-end (wafer fabrication) applications include measurement of surface texture
and dimensional metrology. The polishing of silicon wafers in the planarization step
of semiconductor manufacture can be studied by AFM. The tolerance in these
manufacturing processes is very low, and the signal-to-noise ratio of the measurements
must be very high, which is why for such applications only AFM is suitable. The noise
floor of the AFM must be below 0.5
˚
for adequate characterization. The planarization
method of chemical-mechanical polishing (CMP) was developed with the aid of AFMs
for quality control. The second major application in semiconductors is in metrology. The
measurement of feature and trench dimensions is very important, and due to its high
resolution and three-dimensional information, AFM is a powerful tool for this [730].
Specialized probes such as those in the shape of a pole, which reduce ambiguity at
feature edges, or even with flared ends that can measure line edge roughness have been
developed for these applications [731]. Such measurements do not require measuring an
entire image but only a line profile over the trench. The time required for the measure-
ment is greatly reduced when only a line profile is required. Atomic force microscopes
give excellent contrast on extremely flat surfaces, and so are very useful for thin film
characterization. Because the AFM creates a 3-D map of the surface, software algo-
rithms for measuring grain sizes are very reliable. In the case of insulating films, there is
a great advantage of AFM over SEM because the sample does not need to be coated with
a conductive film.
Back-end applications (product assembly, packaging and testing) in the semiconductor
industry include solving problems associated with packaging, thermal management,
adhesion of contacts and bonding. The thermal measurement capabilities of the AFM
have been used to help solve heat transfer problems in several generations of micropro-
cessors. Defects associated with problems in production processes can be investigated in
detail with AFM. The defects can sometimes be seen with an optical microscope, but the
AFM image can give greater insight into the source of the defect. Direct electrical
measurements with the AFM can also help in such cases. Examples of these semicon-
ductor applications are given in Figure 7.29.
7.4.2.2 Data storage
Advances in the data storage industry occur at a staggering pace. Data densities of all types
of media increase dramatically on an annual basis. Atomic force microscopes were first
used by the data storage industry to study the magnetic domains on hard disk drives in the
early 1990s. At the same time, atomic force microscopes were used to improve and study
optical disk drives. This has continued until today, and AFMs played a critical role in the
development of CDs, DVDs and their successors. Custom stages are available to hold disk
masters, stampers and replicas. With automated software it is possible to make statistical
measurements of bit dimension distributions. Common measurements are the bit width,
