Page 252 - Sami Franssila Introduction to Microfabrication

Structures by Deposition 231

and the reaction is self-limiting: WF 6 cannot diffuse interacts with gaseous species, which then deposit
through the growing tungsten layer. Tungsten deposition on the wafer. In a photothermal process, the laser
is continued by silane reduction of tungsten hexafluoride heats the surface and elevated local temperature drives
on tungsten according to chemical reactions, but often both elements are present
simultaneously. The chemical reactions are the same as
WF 6 (g) + 2SiH 4 (g) −→ those in traditional CVD deposition; for example, silane
source gas for (poly)silicon deposition.
W (s) + 3H 2 (g) + 2SiHF 3 (g) (23.2)
It is possible to fabricate 3D structures by changing
This reaction, however, is transport limited and difficult the focal point of the focused beam in space. Electron
to control. Additionally, it faces problems when contact microscopes and FIB systems have been used in many
holes of different depths have to be filled: some are 3D-deposition applications. Structures such as out-of-
underfilled, some are overfilled (Figure 23.11). plane nanoneedles and microcoils with ca. 10 µm-
Plug fill can be achieved by continuing deposition in wire diameter and 50 µm coil diameter have been
hydrogen reduction mode: made by electron beam-induced CVD of carbon. In
stereomicrolithography, a laser beam solidifies polymer
WF 6 (g) + 3H 2 (g) −→ W (s) + 6HF (g) (23.3) at the focal spot. After a single layer has been drawn,
focus shifts up and the next level of polymer is
There is always the problem of selectivity loss. It is solidified. Elaborate 3D shapes can be drawn, but like
usually connected with residues from preceding process all direct writing techniques, stereomicrolithography has
steps, for instance, incomplete resist removal. Selective low throughput.
deposition processes are rare in volume manufacturing
even though they sometimes offer enormous simplifica-
tions in process integration. 23.4.3 Microstructure-assisted local processing
Electrical and thermal modification by microstructure-
assisted processing is also possible in the field after
23.4.2 Localized deposition by external excitation
the device processing has been completed, whereas
Localized deposition depends on some sort of local beam processes are done in wafer fab at wafer level
excitation, thermal, ion beam or photon flux, and is or chip level.
used to induce growth just at a localized spot. There are Heat dissipation in microstructures is not very
three regimes for heating: in adiabatic regime, thermal amenable to macroworld intuition because surface-to-
energy is limited to a few micrometres on wafer surface volume ratios in microstructures are very different from
because there is no time for heat diffusion; in thermal macroscopic objects. A silicon wire sandwiched between
◦
flux regime, the bulk of the wafer heats up but wafer glass wafers and heated up to 1400 C will lead to a
◦
backside is still at ambient temperature; in isothermal 40 C temperature rise 15 µm away.
regime, the wafer is in thermal equilibrium. Microfuses are one-time programmable elements that
Focused beams can be used either directly or can be used to store chip identity data or calibration
indirectly. In photomask writing, they draw the pattern curves, to trim resistors or to cut off malfunctional
in resist film, which then serves as a mask for chrome circuit blocks and to connect redundant spare blocks.
etching, however, now we are interested in beam Both normally-on and normally-off fuses exist. A
interaction with the wafer (and the surrounding gaseous normally-on fuse has a thin metallic/conductive part that
atmosphere) to form the pattern directly. can be broken. The mechanism for breakage differs:
Focused ion beam (FIB) can be used to etch features chemical reaction can turn the metal film into an
on a wafer, for example, to remove erroneous chrome insulator, a phase change can alter its resistivity or
spots from the photomask or to deposit films in the electromigration can create a void in the wire. Antifuses
presence of suitable source gases. Repair of missing can be made, for example, of high-resistivity undoped
features on the photomask can be done by depositing amorphous silicon that will crystallize and become
tungsten according to conductive when a programming pulse is driven through
it. Gigaohm versus 100 ohm off- and on-resistances (10 7
W(CO) 6 (g) −→ W (s) + 6CO (g) (23.4) on–off ratio) are possible.
Local (chip-scale) sealing of cavities has been
There are two mechanisms in laser-CVD: photolytic demonstrated with a microfabricated polysilicon resistor
and photothermal. In photolytic deposition, laser light on the wafer supplying energy for CVD of the sealing