Example of DMD projection display systems
If we take as an example DMD projection display systems developed by Texas Instruments, we notice that instead of using a surface micro-machining conventional process with a phosphosilicate glass for the sacrificial layer and polysilicon for micro mechanical components, we developed a low temperature process using a conventional positive resin and aluminum alloy applied by spraying. This process made possible the manufacturing of every electronic underlayer of micro mechanical components under mirrors, thus allowing to have a high space factor and small air gaps under micro mirrors.
In order to ensure a high reliability, we particularly took care of reducing the pinching between the parts in contact with each other. For this, a lubricant self-assembled monolayer is applied to reduce the Bonding Force. Moreover, we use an electronic feedback to pilot and control micro mirrors movements in order to avoid shocks and oscillations. A specific aluminum alloy has been developed to minimize micro mirrors angular drifts in time.
If we take in account that these devices have more than one millions micro-mechanical component and that the human eye is able to detect while reading a small number of defective pixels on a screen, we perfectly understand why packaging had to be made in ISO 4 cleanroom instead of ISO 7 cleanroom to eliminate the defects due to particles from outside devices.
Clamped micro beams, membranes and micro bridges are often subject to constraints, in particular when thin films are laid on their surfaces. These constraints can be intrinsic (mesh mismatch for example) or due to thermo-mechanical effects linked to differences in material thermal expansion coefficients. They often lead to structural deformations outside the mirror plan with unpredictable angles.
Researchers proposed a 2 x 2 optical switch based on a polysilicon clamped micro-beam with a pivoting micro-mirror in normal position, outside of the plan because of a thin film of a gold-chromium alloy putting the polysilicon in a compression state. The switching is actuated by applying a tension between beam and substrate, so that the micro-mirror is on the light way.
Micro-mirrors used for telecom routers are basically covered with gold, which has excellent reflective properties in infrared environment. But we have to pay attention not to contaminate with gold electronic devices on silicon integrated in structures.
We often use silicon nitride to make an optical waveguide with many techniques, among them spraying, CVD, PECVD and LPCVD. The technique used and the process precise condition both contribute to the variations of refractive index, optical losses, amplitude and nature of the residual stress. For instance, by using the PECVD technique, we can get refractive index between 1.9 and 2.2 with variable stress in compression or in tension according to the Si/N atomic ratio and the deposition conditions. It emerges that the choice of materials and implementation conditions must be done very carefully to ensure stable and reproductive structures.