Page 46 - Sami Franssila Introduction to Microfabrication

Micrometrology and Materials Characterization 25

radiation) or Raman spectroscopy (structural defects, Linewidth measurement by a SEM is non-contact as
implant damage, local stresses shift photon energy), opposed to stylus profiler or AFM, which make contact
must be used. with the wafer. Because full wafers are analysed in a
linewidth SEM, only top view pictures are possible, and
no cross-sectional information can be obtained.
2.14 PRACTICAL ISSUES WITH
MICROMETROLOGY
2.14.2 Blanket versus patterned wafer analysis
Many analytical methods can produce accurate results
only at the expense of great time and effort: TEM can Both in R&D and in production, analytical methods
image individual atoms but the analysis time is days (it are bound by a number of practical constraints related
consists mostly of tedious sample preparation and also to the number of data points, measurement spot size
of complicated analysis). TEM analysis costs ca. $1000 and speed of measurement. Blanket wafer measurements
to $2000 per sample if bought as a service. are simple to perform and many basic studies in film
Monitoring must be preferably so fast that whole deposition, diffusion, ion implantation, polishing or
wafer mapping can be performed for uniformity check- bonding can be done on blanket wafers but in many
ing. Mapping measurement also requires that the ana- cases structured wafers are indispensable. Linewidths
lytical equipment can handle whole wafers. Many opti- and spacings need to be identical to product wafers, but
cal and electrical measurements are suited for mapping, more amenable to probing, by optical or electron beams,
but most physical and chemical methods require wafer or by mechanical probes. Test-structure size needs to be
breakage for sample preparation. matched to design complexity: if the product chip has
Uniformity can be defined across the wafer (a.k.a. 1 000 000 contact holes, how to extrapolate from 1000
within-wafer non-uniformity, WIWNU), wafer-to-wafer hole test structure? The one-million contact test structure
(WTWNU) and lot-to-lot. The standard definitions for would probably be so large that no other test structures
uniformity are could be accommodated in the area allocated for testing.

U = (max − min)/2 × average
2.14.3 Destructive versus non-destructive analysis
U = (max − min)/(max + min) (2.11)
Cost of measurement can range from a few cents to
a few dollars per wafer, but if the measurement is
The former is applied when five measurements are taken,
◦
one at the wafer centre and four at 90 from each other wafer destructive, its cost is at least the wafer cost, or
at half-radius; the latter when the four points are at $10 to $100 per sample. Many physical measurements
wafer edges. are destructive, like SIMS, Auger depth-profiling and
Uniformity of 5% was long accepted as a typical cross-sectional SEM. But care should be made between
process performance (thin-film thickness, etch rate), wafer destructive and sample destructive measurements.
2
RBS analysis is performed on 1 cm pieces; that is,
but some processes are inherently better, for example, the wafer has to be broken for RBS analysis. But after
thermal oxidation and photoresist spinning routinely RBS analysis, other analyses can be done, for example,
produce better than 1% uniformity. On the other side,
EMPA or SIMS. But after SIMS, depth profiling the
CMP (chemical–mechanical polishing) is notoriously
sample is irrevocably lost.
non-uniform, with 10% as good uniformity.
2.14.4 Standards and reference materials
2.14.1 Contact versus non-contact measurements
Calibration standards (with traceability to NIST,
Measurements can be divided into two categories: National Institute of Standards and Technology) and
contact and non-contact (non-invasive). Both modulated reference materials (which are supplier-certified) are
photoreflectance and four-point probe can be used to available for all major wafer-level measurements:
monitor ion implant dose, but 4PP makes physical film thickness and step height, dimensions, electrical
contact to the wafer with metal (tungsten) needles, and resistivity and particles. Reference materials are enough
the wafer is deemed contaminated. It is not allowed to for daily work but they must be calibrated against
continue into high-temperature steps. traceable standards regularly.