Page 47 - Sami Franssila Introduction to Microfabrication

26 Introduction to Microfabrication

The standards and references are silicon wafers with 2.15 EXERCISES
dedicated test patterns for quantities in question. One
wafer can provide a series of standards, such as different 1. The sheet resistance of a typical aluminium metalliza-
resistivity windows or steps heights. General step height tion is 0.03 ohm/sq. What is aluminium thickness?
standard is usually a quartz piece with etched steps; and 2. Resistance of 200 µm long copper lines was mea-
not a separate piece for each specific material. sured to be 40 ohm. From copper deposition pro-
cess we know that thickness is 300 nm. What is
the linewidth?
2.14.5 Devices as measuring instruments 3. AFM scan area is 1 × 1 µm, which corresponds to
512 × 512 pixels. What should the AFM-tip radius
It is not unusual that no analytical method is able to do be so that resolution is tip-limited?
a good job: either the quantities involved or the anal- 4. Estimate the analytical radius of electron micro-
ysis areas are too small. Quite often it is possible to probe (EMPA).
use devices themselves as measuring instruments: device 5. Can RBS be used to measure dopant profiles?
performance degradation is attributed to minutiae effects 6. If electron beam is focussed to a 15 nm spot, and at
that are not amenable to direct physical measurements. least 100 Auger events (electrons) must be collected
Metal Oxide Semiconductor (MOS) transistors are sen- to get a signal, what is the detection limit of Auger
sitive to metal contamination at levels below analytical microprobe?
9
detection limits (in the 10 cm −3 range). Microscopic
7. SIMS raw data is ion counts versus sputter time.
vacuum cavities are created by wafer bonding or depo-
How can you convert these to concentration versus
sition, and no pressure gauge is small enough to probe
depth data?
these cavities. But mechanical quality factor, Q, of the
8. What is the acceleration voltage of an atomic
microfabricated mechanical resonators in the cavities is
resolution TEM?
indicative of cavity pressure.
9. What are the resistivities of bcc-Ta and β-Ta in
Figure 2.8?
2.14.6 Failure analysis and reverse engineering
Analytical methods are needed not only during fabrica- REFERENCES AND RELATED READINGS
tion, but also after wafer processing has been completed.
When circuits are found malfunctional, either in test- Buchanan, M.: Scaling the gate dielectric: materials, integra-
tion and reliability, IBM J. Res. Dev., 43 (1999), 245.
ing or after field return, the causes must be identified. Diebold, A.C.: Materials and failure analysis methods and
Hard errors, that is, consistent failures are much eas- systems used in the development of and manufacture of
ier to locate and to understand than soft errors, that is, silicon integrated circuits, J. Vac. Sci. Technol., B12 (1994),
the intermittent failures that may take place only under 2768.
certain operating conditions (for example above certain Ohmi, T.: A new paradigm of silicon technology, Proc. IEEE
temperature or frequency). As in wafer-level analysis, (2001), p. 394.
non-destructive methods are tried first, and the destruc- Plummer, J.D. & P.B. Griffin: Material and process limits in
tive only afterwards. silicon VLSI technology, Proc. IEEE’ 89 (March 2001),
In reverse engineering, a chip is ‘disassembled’ step p. 240.
Runyan, W.R. & T.J. Schaffner: Semiconductor Measurements
by step, and the structures, materials and functions are and Instrumentation, McGraw-Hill, 1998.
recorded (see Figure 27.5 for IC metallization stripped Schaffner, T.J.: Semiconductor characterization and analytical
of all dielectric films). This is practised for example for
technology, Proc. IEEE’ 88 (2000), p. 1416.
competitive intelligence or patent infringement exam-
Schroder, D.K.: Semiconductor Material and Device Charac-
ination. Methods like electron beam–induced current terization, 2 nd ed., John Wiley & Sons, 1998.
(EBIC) can be used to probe electrical functions of Tong, Q.Y. & U. G¨ osele: Semiconductor Wafer Bonding, John
a circuit. Wiley & Sons, 1999.