Biomedical Engineering Reference
In-Depth Information
piece. The thickness of the Ti coating is determined by the deposition rate and
contact time. The coating thickness is critical in that it is a key parameter in
determining the diffusion depth and concentration. Typically the metal will
be deposited to a height of 0.3-1.0 μm depending on the wavelength of opera-
tion and the degree of confinement of the waveguide. Ti metal will be in direct
contact with the LiNbO 3 only in those channels opened up by the photolitho-
graphic process. The next step is the lift off or stripping of the resist. The Ti
coated piece is immersed in a suitable solvent that attacks the polymerized
resist causing it to disintegrate and the Ti adhering to it to lift or wash off. The
resist's aspect ratios become important at this point in that the resist must be
substantially thicker than the Ti coating. If not, the metal coating adhered to
the LiNbO 3 can bridge over and become adhered to metal covering the resist,
which results in a phenomena known as mushrooming (see Figure 5.22). This
can result in poor line definition and ultimately in poor device performance.
Titanium indiffusion is the next key area in the fabrication of a LiNbO 3
waveguide device. Extensive research continues in this area to characterize
more fully the exact nature of Ti diffusion into the material and to more pre-
dictably control the diffusion. The current preferred process is to indiffuse
the Ti at a temperature of 980°C-1050°C for 4-12 h. The diffusion must be per-
formed under tightly controlled atmospheric conditions to prevent the out-
diffusing of LiO 3 which can accompany metal indiffusion. The first LiNbO 3
waveguide devices were fabricated by taking advantage of the outdiffusing
of LiO 3 at high temperature [55]. With the current technology this outdiffus-
ing is considered to be a major problem as it can result in the formation of
surface waveguides outside the channel regions [56].
Molecular diffusion is a means for mass transfer which results from the
thermal motion of molecules and is limited by collisions between molecules.
The first step in understanding the diffusion process is to develop an under-
standing of Fick's Law
d
d
σ
d
n
= −
Dq
d
t
(5.25)
x
Photoresist
Photoresist
Ti
Ti
LiNbO 3
LiNbO 3
(a)
(b)
FIGURE 5.22
(a) Ti coated over photoresist with and insufficient aspect ratio and (b) “Mushrooming” noted
after resist stripping.
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