MEMS micromirror devices with anti-reflective structures
View Patent ↗Diffractive patterns are disposed on a MEMS substrate in the gaps between the MEMS micromirrors to reduce backreflection of light leaking through the gaps and reflected by the MEMS substrate. The diffractive patterns are silicon surface-relief diffraction gratings or silicon oxide gratings on silicon substrate. Sub-wavelength gratings are used to suppress higher orders of diffraction; 50% duty cycle surface relief gratings on a substrate having index of refraction close to 3 are used to suppress both reflected and transmitted zero orders of diffraction simultaneously. The gratings have lines running parallel or at a slight angle to the gaps, to prevent the diffracted light from re-entering the gaps.
1. A MEMS micromirror device comprising:
a MEMS substrate having at least one layer, wherein the MEMS substrate has a top surface;
a plurality of micromirrors for reflecting light impinging thereon, wherein said light has a central wavelength λ 0 ,
wherein the micromirrors are supported by the MEMS substrate and disposed over the top surface of the MEMS substrate, and
wherein gaps exist between the micromirrors; and
a diffractive pattern for suppressing backreflection of light leaking through the gaps between the micromirrors and impinging onto the top surface of the MEMS substrate, by diffracting said leaking light,
wherein the diffractive pattern is disposed on or within the top surface of the MEMS substrate, and
wherein the diffractive pattern encompasses at least 80% of an area formed by a normal projection of the gaps between the micromirrors onto the MEMS substrate.
2. A MEMS micromirror device of claim 1 , wherein the diffractive pattern is constructed to suppress said backreflection by directing the diffracted light away from the gaps between the micromirrors.
3. A MEMS micromirror device of claim 1 , wherein the diffractive pattern comprises lines running parallel to the gaps.
4. A MEMS micromirror device of claim 1 , wherein the diffractive pattern comprises lines running perpendicular to the gaps.
5. A MEMS micromirror device of claim 1 , wherein the diffractive pattern is a surface-relief diffraction grating.
6. A MEMS micromirror device of claim 1 ,
wherein the at least one layer is a first silicon layer, the top surface of the MEMS substrate having a common surface area with a top surface of said first silicon layer;
wherein the diffractive pattern is disposed in said common surface area; and
wherein the diffractive pattern comprises silicon oxide structures on the top surface of said first silicon layer.
7. A MEMS micromirror device of claim 6 , wherein the silicon oxide structures have a height of 0.823+−0.05 micron as measured from the top surface of the first silicon layer, and wherein a geometric area occupied by the silicon oxide structures on said top surface is 68%+−10% of a total geometric area occupied by the diffractive pattern on said top surface of the first silicon layer.
8. A MEMS micromirror device of claim 6 , wherein said MEMS substrate further comprises a buried silicon oxide layer disposed under the first silicon layer, wherein the buried silicon oxide layer has a thickness of mλ 0 /4n, wherein m is an integer odd number and n is refractive index of the buried silicon oxide layer at λ 0 .
9. A method of manufacturing of a MEMS micromirror device of claim 6 , comprising:
(a) providing the MEMS substrate;
(b) growing a silicon oxide layer of a pre-determined thickness on the first silicon layer of said MEMS substrate;
(c) patterning and etching said silicon oxide layer, so as to create the silicon oxide structures;
(d) masking the silicon oxide structures using an overlayer mask on top of the silicon oxide structures before processing the MEMS substrate any further; and
(e) fabricating the plurality of micromirrors and placing the micromirrors over the top surface of the MEMS substrate.
10. A method of claim 9 , wherein in step (a), the MEMS substrate is a silicon-on-insulator substrate having a buried silicon oxide layer having a thickness of mλ 0 /4n, wherein m is an integer odd number, and n is refractive index of the buried oxide layer at λ 0 .
11. A method of manufacturing of a MEMS micromirror device of claim 6 , comprising:
(a) providing a silicon-on-insulator (SOT) substrate for use as the MEMS substrate, wherein said SOI substrate has a buried silicon oxide layer;
(b) etching the SOI substrate, so as to expose the buried silicon oxide layer;
(c) upon completion of step (b), patterning and etching said silicon oxide layer, so as to create the silicon oxide structures; and
(d) fabricating the plurality of micromirrors and placing the micromirrors over the top surface of the MEMS substrate.
12. A method of manufacturing of a MEMS micromirror device of claim 6 , comprising:
(a) providing the first silicon layer;
(b) growing a silicon oxide layer of a pre-determined thickness on the top surface of the first silicon layer;
(c) patterning and etching said silicon oxide layer, so as to create the silicon oxide structures on the top surface of the first silicon layer;
(d) bonding a second silicon layer to the silicon oxide layer, thereby forming a stack of the first silicon layer at a bottom of the stack, the silicon oxide layer, and the second silicon layer at a top of the stack;
(e) exposing the silicon oxide structures by etching through the second silicon layer, thereby forming the MEMS substrate; and
(f) fabricating the plurality of micromirrors and placing the micromirrors over the top surface of the MEMS substrate.
13. A MEMS micromirror device of claim 1 , wherein the diffractive pattern is a sub-wavelength grating having a period smaller than λ 0 .
14. A MEMS micromirror device of claim 13 , wherein the sub-wavelength grating has a thickness of mλ 0 /4n, wherein m is an integer odd number and n is effective refractive index of the sub-wavelength grating at λ 0 .
15. A MEMS micromirror device of claim 13 , wherein the sub-wavelength grating has a duty cycle varying along the top surface, so as to suppress said backreflection by directing the diffracted light away from the gaps between the micromirrors.
16. A MEMS micromirror device of claim 1 , wherein said diffractive pattern is for suppressing light diffracted into a zeroth reflective order of diffraction.
17. A MEMS micromirror device of claim 16 , wherein said diffractive pattern is also for suppressing light diffracted into a zeroth transmissive order of diffraction.
18. A MEMS micromirror device of claim 17 , wherein said diffractive pattern has a feature size that is capable of providing a first order of diffraction of said light, and wherein the diffractive pattern is constructed to make the light diffracted into said first order propagate in a direction pointing away from the gaps between the micromirrors.
19. A MEMS micromirror device of claim 18 , wherein said diffractive pattern is a surface-relief diffraction grating on the top surface of the MEMS substrate having a refractive index of 2.5 to 3.5 at λ 0 , wherein said surface-relief grating has a relief depth of mλ 0 /4, wherein in is an integer odd number.
20. A MEMS micromirror device of claim 19 , wherein the MEMS substrate is a silicon substrate.