Page 209 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

TWO-PHOTON MSL 189

The resolution of the super-IH process is excellent and is typically less than 1 um. The
fabrication speed can be increased by operating the galvano scanning mirror and X-Y-Z
stages together. The disadvantages are that the optics system is more expensive than both
the IH and mass-IH processes, and specialised monomer systems need to be developed
for each application.

7.4 TWO-PHOTON MSL

As mentioned earlier, conventional MSL is limited in terms of the minimum thickness
of the resin layers possible because of viscosity and surface tension effects. In contrast,
the two-photon MSL process (like the super-IH) does not have this problem because the
resin does not need to be layered.
When a laser beam is focused on a point with a microscope objective lens as shown
in Figure 7.22 (Maruo and Kawata 1998), the density of photons decreases with the
distance away from the focal plane, but the total number of photons in the beam at every
cross section remains the same (see Figure 7.22(b)). Thereafter, the resin is solidified
completely in the illuminated region even beyond focal point, leading to a poor resolution;
this means that the linear response of the materials to the light intensity based on a
single-photon absorption does not have optical sectioning capability. On the other hand,
if the material response is proportional to the square of the photon density, the integrated
material response is enhanced greatly at the focal point (see Figure 7.22(c)), and, therefore,
the polymerisation based on two-photon absorption occurs only in a small volume within
the focal depth. Normally, the beam power of the laser has to be extremely high (several
kilowatts) to obtain two-photon absorption.
A two-photon MSL apparatus is shown in Figure 7.23 (Maruo and Kawata 1998).
The beam is generated by a mode-locked titanium sapphire laser and is directed by two
galvanic scanning mirrors. The beam is then focused with an objective lens into the resin.
A charge-coupled device (CCD) camera is used to aid focusing and monitor the forming
of the microstructure. A Z-stage moves the resin container along the optical axis for
multilayer fabrication. The objective lens used by Maruo had a numerical aperture of
0.85 (magnification of 40). The accuracy of the galvano-scanner set (General scanning)

Integration of the Integration of the squared
intensity of laser beam intensity of laser beam
Optical

Focused laser beam
(a) (b)

Figure 7.22 Two-photon absorption and one-photon absorption generated by a focused laser: (a)
schematic diagram of a focused laser beam; (b) total one-photon absorption per transversal plane,
which is calculated by integrating the intensity over the plane versus the optical axis; and (c) total
two-photon absorption per transversal plane, which is calculated by integrating the intensity squared
over the plane