Page 334 - Sami Franssila Introduction to Microfabrication

Tools for Microfabrication 313

are often much simpler than direct measurements: for reaction products, such as SiF 4 or WF 6 . At the etching
example, vacuum chamber base pressure is a good end-point, free fluorine intensity increases because it
indication of vacuum quality, but mass spectrometry is not consumed by the reaction. A more selective
(usually called RGA, for residual gas analysis) can method would be the monitoring of reaction products
actually identify the residual atoms and molecules, themselves. This must be developed for every process
which can be truly significant in understanding vacuum- individually. Nitrogen signal (396 nm) is suitable for
film interactions. Molecular recognition also helps in monitoring nitride etching: there will be a sharp drop
trouble-shooting leaks. in nitrogen signal when all the nitride has been etched
Very few measurements are actually done on the away. OES does not, however, measure wafers but,
wafers during processing. This is understandable because rather, the process.
process chamber conditions are often harsh, for example, One of the oldest applications of in situ monitoring
RF-fields, corrosive gases or high temperatures. Wafer is the quartz crystal microbalance (QCM) film-thickness
temperature in RTA can be measured by pyrometry dur- control during evaporation and sputtering. The QCM
ing processing. In ultra-high vacuum conditions, sur- is placed in the same atom flux as the wafers, and
face spectroscopy can be used to monitor deposition therefore it experiences the same film deposition. Mass
processes in real time: reflection high-energy electron change is detected as a frequency change and converted
diffraction (RHEED) and low-energy electron diffrac- to film thickness. The resonance frequency of the QCM
tion (LEED) are routinely employed in MBE systems is given by
to check the crystallinity of the growing film. Unfor- f = v tr /2x (30.1)
tunately, most deposition processes are operated under
conditions in which such systems cannot be used. Film For quartz wafer of 500 µm thickness with transverse
thickness during deposition or etching can be measured wave velocity of 3340 m/s, this translates to 3.3 MHz.
by, for example, ellipsometry or interferometry, but such The frequency drop due to thickness increase is given by
systems are not commonplace. 2
f = −2f x/v tr (30.2)
Measurements can be classified into four categories
according to their immediacy: Taking into account the fact that the deposited film
density differs from that of quartz (but neglecting that
– in situ: during wafer processing in the process its elastic properties differ), we get the thickness from
chamber the frequency change:
– in-line: after wafer processing in the process tool 2
x = (v tr ρ quartz ) f/(−2f ρ film ) (30.3)
(e.g., exit load lock)
– on-line: in the wafer fab by wafer fab personnel With a 1 ppm frequency shift easily detectable, the
– ex situ: outside the analytical laboratory by expert minimum thickness change that can be seen is of the
users. order of angstroms. Temperature sensitivity of QCM
◦
is 0.5 ppm/ C, which has to be accounted for because
In situ resist development monitoring with an inter- deposition is usually accompanied by temperature rise.
ferometric end-point detector can improve linewidth In-line tools are located, for example, in load
control considerably. It can compensate for changes in locks or cool down chambers, and they measure
exposure dose, resist (de)composition, developer con- wafers immediately after, but not during, processing.
centration and temperature or resist bake drifts and Having the instrument outside the process chamber
shifts, which could easily result in 10% development helps because the ambience is usually benign: nitrogen
time differences. or vacuum atmosphere without RF-fields, plasmas or
Plasma etching is almost always monitored in real toxic gases.
time, in order to determine the end point and to prevent On-line measurements constitute the bulk of mea-
excessive etching of the substrate or the underlying surements in wafer fab. These include measurements
film. Optical emission spectroscopy (OES) is commonly of standard film-thickness (ellipsometry, reflectometry),
used: the intensity of some suitable excited species in sheet resistance, implant damage by thermal waves, step
the plasma is monitored with optical systems, including height by profilometer, and so on. Some measurements,
a wavelength selective detector. In fluorine plasmas, a such as those for sheet resistance or film thickness, are
signal at λ = 704 nm (from excited fluorine atoms) can performed in seconds; while some, such as those for
be used. During etching, the signal is small because sample preparation or pumpdown (SEM, AFM), require
there is little free fluorine: most of it is bound as a few minutes.