Design of LCOS Microdisplay Backplanes for Projection Applications

Chapter 2: Constraints from lcos technology CH2 - 51
Finally, with the placed spacer solution, the designer must produce a dedicated mask
layout.
Figure 2-13 : placed spacer comets
The second solution consists of spraying tiny spacer balls on top of the alignment
layer. Their positions are random and differ from chip to chip. To its advantage, the
second solution does not exhibit shadows or comets.
Spacer thickness depends on the LC effect; the precision of the LC layer’s thickness
also depends on the planarity of the Si chip. Note that the visibility of spacers can
depend on the state of the LC director. It is important for good contrast ratios to have
no spacer visibility in the dark state. On the other hand, when spacers are visible in
the bright state, it will affect the brightness of the display.
2.2.1.3 Chip planarity
Some LC modes require extremely well controlled thickness of the LC layer. Others
are more tolerant to thickness variations. Fact is, without precautions, a chip's
surface topography presents 'hills' and 'dales' of the order of micrometer. This is not
surprising, considering the thickness of the patterned layers (µm range). As the
topography variations are large enough compared with the cell gap, it can be
expected that the Si planarity affects the quality of the assembled cell.
A first technique to smoothen the chip surface consists of the addition of a
planarizing poly-imide layer just below the top mirror layer. Furthermore, this layer
can be made opaque so that it results in an excellent light shield [23]. Fortunately,
advanced sub micron processes already suffer this much from excessive surface
topography that foundries anyway had to introduce a polishing technique known as
chemical mechanical polishing (CMP), [59].The processing step consists of the
deposition of an insulator layer that reduces the topography variations; next, this
layer is polished to yield a very flat surface. The CMP step here comes after the
patterning of a conductor layer (poly and/or metal layers) and covering it with an
insulator. It is incapable to yield a mathematically flat surface; it works as an
integrator that averages 'fast' topography variations over a small distance range. It
cannot cancel topography variations that are too steep on short range or that slowly
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