Page 639 - The Mechatronics Handbook

0066_Frame_C20.fm Page 109 Wednesday, January 9, 2002 1:46 PM

materials, and sequential steps are different depending on the MEMS which must be devised, designed,
analyzed, and optimized first.

Conductor Thin Films Electrodeposition
The conductors (microcoils to make windings) in microstructures and microtransducers can be fabricated
by electrodepositing the copper and other low resistivity metals. Electrodeposition of metals is made by
immersing a conductive surface in a solution containing ions of the metal to be deposited. The surface
is electrically connected to an external power supply, and current is fed through the surface into the
x+ −
solution. In general, the reaction of the metal ions (Metal ) with x electrons (xe ) to form metal (Metal)
x+ −
is Metal + xe = Metal.
To electrodeposit copper on the silicon wafer, the wafer is typically coated with a thin conductive
layer of copper (seed layer) and immersed in a solution containing cupric ions. Electrical contact is
2+ −
made to the seed layer, and current is flowed (passed) such that the reaction Cu + 2e → Cu occurs at
the wafer surface. The wafer, which is electrically interacted such that the metal ions are changed to
metal atoms, is the cathode. Another electrically active surface (anode) is the conductive solution to
make the electrical path. At the anode, the oxidation reaction occurs that balances the current flow at
the cathode, thus maintaining the electric neutrality. In the case of copper electroplating, all cupric ions
removed from solution at the wafer cathode are replaced by dissolution from the copper anode.
According to the Faraday law of electrolysis, in the absence of secondary reactions, the current delivered
to a conductive surface during electroplating is proportional to the quantity of the metal deposited.
Thus, the metal deposited can be controlled varying the electroplating current (current density) and
the electrodeposition time.
The hydrated Cu ions reaction is

++
(
Cu → Cu H 2 O) ++
6
and the cathode reactions are

++
−
+
−
++
+
−
+
−
+
++
Cu + 2e → Cu, Cu + e → Cu , Cu + e → Cu, 2Cu → Cu + Cu, H + e → 1
--H 2
2
The copper electroplating solution commonly used is CuSO 4 –5H 2 O (250 g/l) and H 2 SO 4 (25 ml/l).
The basic processes are shown in Fig. 20.131, and the brief description of the sequential steps and
equipment that can be used are given.
It must be emphasized that commonly used magnetic materials and conductors do not adhere well to
silicon. Therefore, as was described, the adhesion layers (e.g., titanium Ti or chromium Cr) are deposited
on the silicon surface prior to the magnetic material electroplating.
The electrodeposition rate is proportional to the current density and, therefore, the uniform current
density at the substrate seed layer is needed to attain the uniform thickness of the electrodeposit. To
achieve the selective electrodeposition, portions of the seed layer are covered with the resist (the current
density at the mask edges nonuniform degrading electroplating). In addition to the current density, the
deposition rate is also a nonlinear function of temperature, solution (chemicals), pH, direct/reverse
current or voltage waveforms magnitude, waveform pulses, duty ratio, plating area, etc. In the simplest
form, the thickness and electrodeposition time for the specified materials are calculated as
Time electroplating × Current density ×
Thickness material = -------------------------------------------------------------------------------------------------------------
Weight molecular
Faraday constant × Density material × Electron number
Thickness material × Faraday constant × Density material ×
Time electroplating = -------------------------------------------------------------------------------------------------------------------------------------------------------
Electron number
Current density × Weight molecular
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