Mechanical grating device

Details Mechanical grating device Mechanical grating device

1998-12-18

2001-06-26

Epps, Georgia (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C291S027000

Reexamination Certificate

active

06252697

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the field of modulation of an incident light beam by the use of a mechanical grating device. More particularly, this invention discloses a mechanical grating device which has a significant improvement in the output of the diffracted light beam.
BACKGROUND OF THE INVENTION
Advances in micromachining technology have given rise to a variety of Micro-electromechanical systems (MEMS) including light modulators for low cost display applications. Such modulators provide high-resolution, high operating speeds (KHz frame rates), multiple gray scale levels, color adaptability, high contrast ratio, and compatibility with VLSI technology. One such modulator has been disclosed in U.S. Pat. No. 5,311,360, issued May 10, 1994 to Bloom et al., entitled “Method and Apparatus for Modulating a Light Beam”. This modulator is a micromachined reflective phase grating. It consists of a plurality of equally spaced deformable elements in the form of beams suspended at both ends above a substrate thereby forming a grating. The deformable elements have a metallic layer that serves both as an electrode, and as reflective surface for incident light. The substrate is also reflective and contains a separate electrode. The deformable elements are designed to have a thickness equal to &lgr;/4 where &lgr; is the wavelength of the incident light source. They are supported a distance of &lgr;/4 above, and parallel to, the substrate. When the deformable elements are actuated (for example a sufficient switching voltage is applied), the deformable are pulled down and the incident light is diffracted. Optical systems can intercept the diffracted light. For display applications, a number of deformable elements are grouped for simultaneous activation thereby defining a pixel, and arrays of such pixels are used to form an image. Furthermore, since gratings are inherently dispersive, this modulator can be used for color displays.
U.S. Pat. No. 5,677,783, issued Oct. 14, 1997 to Bloom et al., entitled “Method of Making a Deformable Grating Apparatus for Modulating a Light Beam and Including Means for Obviating Stiction Between Grating Elements and Underlying Substrate” discloses a method of making a deformable grating apparatus for modulating a light beam and including means for obviating stiction between grating elements and underlying substrate. Referring to
FIG. 1
, a perspective cut-away view of a prior art light modulator
10
is shown. An insulating protective layer
24
is deposited on a silicon substrate
22
. This is followed by the deposition of a sacrificial silicon dioxide layer
16
. A silicon nitride layer
26
is next deposited in which is defined the deformable elements
12
. Both the thickness of the sacrificial silicon dioxide layer
16
and the silicon nitride layer
26
are critical in determining the amplitude modulation and thus the efficiency of the grating device. In order to achieve freestanding beams the sacrificial silicon dioxide layer
16
is etched away in the active area. The remaining sacrificial silicon dioxide layer
16
not removed acts as a supporting frame
14
for the deformable elements
12
. The last fabrication step provides an aluminum film
30
in order to enhance the reflectance of the beams and to provide an electrode for application of a voltage between the deformable elements
12
and the substrate
22
.
There are many problems with this prior art device. The thickness of both the sacrificial oxide layer
16
and silicon nitride layer
26
have to each be &lgr;/4. Because these thicknesses determine the grating amplitude of the modulator, their dimensions are critical. Variations in either of these thicknesses will result in unwanted diffraction of light in the off state, as well as lower diffraction efficiency in the on state, thus lower contrast ratios. There is no freedom to adjust the thickness of the deformable element
12
for optimization of its mechanical properties.
There is no defined etch stop in the device structure during removal of the sacrificial oxide layer
16
. This requires a carefully controlled time-dependent etch to ensure that the remaining sacrificial oxide layer
16
is able act as the supporting frame
14
. The profile left by the wet etch openings between the beams leaves an uneven wall below the deformable elements
12
where they contact the supporting frame
14
. Such effects will cause variations in the electromechanical properties of the devices. The etching process to remove the sacrificial oxide layer is also a wet process. During this wet processing step it has been seen that stiction tends to occur in that the deformable elements tend to adhere and remain adhered to the substrate. Special drying techniques can be used to overcome this problem but complicate the process. Removal of the sacrificial layer using a dry process is preferred.
U.S. Pat. No. 5,661,592, issued Oct. 14, 1997 to Bornstein et al., entitled “Method of Making and an Apparatus for a Flat Diffraction Grating Light Valve” discloses a method for making a deformable grating apparatus which attempts to address the problems associated with this prior art device. An insulating layer is deposited on the substrate. A phosphosilicate glass(PSG) sacrificial layer is next deposited. The phosphosilicate glass(PSG) sacrificial layer is selectively patterned removing the phosphosilicate glass(PSG) sacrificial layer except in regions where the deformable grating elements are to be formed. The phosphosilicate glass(PSG) is reflowed at high temperature to lower the angle of its sidewall. Silicon nitride is then deposited conformably over the phosphosilicate glass(PSG) and patterned into deformable elements. The phosphosilicate glass(PSG) sacrificial layer is then removed by wet etching. By selectively patterning the phosphosilicate glass(PSG) sacrificial layer the region under the beams is more uniform relying now on the uniformity of the reflow of the phosphosilicate glass(PSG) sacrificial layer. However the removal of the phosphosilicate glass(PSG) sacrificial layer is still a wet process with the corresponding disadvantages as described above. The conformal deposition of the silicon nitride over the step height formed by the patterned phosphosilicate glass(PSG) sacrificial layer region also has topography determined by the step height. In patterning the deformable elements this topography will limit the minimum spacing between the deformable elements. Increased spacing between elements will cause increased light scattering decreasing the efficiency of the grating. The use of a phosphosilicate glass(PSG) sacrificial layer also requires a high temperature reflow step that would complicate its integration with CMOS circuitry on the same substrate.
There is one problem with the prior art devices, which is, not to provide deformable ribbon elements with a constant cross-section along the entire length of the device. According to this drawback the efficiency of the diffraction grating device is lowered.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a mechanical grating device which has equal actuation conditions for the deformable elements in order to improve the diffraction efficiency of the device.
The object is achieved with a mechanical grating device comprising: a base having a surface; a spacer layer provided above the base, said spacer layer defining an upper surface and a longitudinal channel is formed in said spacer layer, said channel having a first and second opposing side wall and a bottom, said side walls being substantially vertically disposed with respect to the bottom, and said channel having a constant cross section along the entire length of the mechanical grating device; and a plurality of spaced apart deformable ribbon elements disposed parallel to each other and spanning the channel, said deformable ribbon elements are fixed to the upper surface of the spacer layer on each side of the channel.
Another object is to provide a electromechanical grating device which has equal actuation condition

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