Optical filter and sensor system
An optical filter having a passband at least partially overlapping with a wavelength range of 800 nm to 1100 nm is provided. The optical filter includes a filter stack formed of hydrogenated silicon layers and lower-refractive index layers stacked in alternation. The hydrogenated silicon layers each have a refractive index of greater than 3 over the wavelength range of 800 nm to 1100 nm and an extinction coefficient of less than 0.0005 over the wavelength range of 800 nm to 1100 nm.
1. A method, comprising:
forming, using a direct current (DC) sputtering technique, a plurality of hydrogenated silicon layers of an optical filter,
the plurality of hydrogenated silicon layers having a first refractive index of greater than 3 over an operating wavelength range of the optical filter of 800 nanometers (nm) to 1100 nm, and
the plurality of hydrogenated silicon layers being formed in a deposition chamber associated with a chamber pressure of less than 2 millitorr (mTorr); and
forming a plurality of lower-refractive-index layers of the optical filter,
the plurality of lower-refractive-index layers each having a second refractive index of less than 3 over the operating wavelength range of the optical filter of 800 nm to 1100 nm, and
the plurality of lower-refractive-index layers being stacked in alternation with the plurality of hydrogenated silicon layers.
2. The method of claim 1 , where the DC sputtering technique is a magnetron DC sputtering technique.
3. The method of claim 1 , where the DC sputtering technique is a pulsed DC sputtering technique.
4. The method of claim 1 , where forming the plurality of hydrogenated silicon layers comprises:
sputtering silicon to deposit the plurality of hydrogenated silicon layers onto a substrate.
5. The method of claim 4 , where sputtering the silicon comprises:
sputtering the silicon using a circular cathode with a silicon target.
6. The method of claim 5 , where a first diameter of the circular cathode is between one times (1×) a second diameter of the substrate and two times (2×) the second diameter of the substrate.
7. The method of claim 5 , where a throw distance between a first plane of the substrate and a second plane of the silicon target is between one half times (0.5×) a diameter of the substrate and two times (2×) the diameter of the substrate.
8. The method of claim 4 , where the substrate is transparent in the operating wavelength range.
9. The method of claim 1 , where the plurality of hydrogenated silicon layers are associated with an extinction coefficient of approximately 0.00055 at 800 nm.
10. The method of claim 1 , where the plurality of hydrogenated silicon layers are associated with an extinction coefficient of less than 0.0003 at 800 nm.
11. The method of claim 1 , where the plurality of hydrogenated silicon layers and the plurality of lower-refractive-index layers are a multilayer structure; and
the method further comprises:
annealing the multilayer structure at a temperature of between 250 degrees Celsius and 350 degrees Celsius.
12. The method of claim 1 , where the plurality of hydrogenated silicon layers and the plurality of lower-refractive-index layers are a multilayer structure; and
the method further comprises:
annealing the multilayer structure for a period of between 30 minutes and 90 minutes.
13. The method of claim 1 , further comprising:
providing hydrogen plasma into the deposition chamber using a plasma activation source.
14. The method of claim 1 , where the plurality of hydrogenated silicon layers and the plurality of lower-refractive-index layers are a multilayer structure; and
the method further comprises:
forming the multilayer structure on a semiconductor sensor chip using a wafer-level processing technique.
15. The method of claim 14 , where the semiconductor sensor chip includes at least one of:
a charge-coupled device (CCD) chip, or
a complementary metal oxide semiconductor (CMOS) chip.
16. The method of claim 1 , where forming the plurality of hydrogenated silicon layers comprises:
forming the plurality of hydrogenated silicon layers at a deposition rate of between 0.05 nanometers per second (nm/s) and 1.2 nm/s.
17. The method of claim 1 , where forming the plurality of hydrogenated silicon layers comprises:
forming the plurality of hydrogenated silicon layers at a deposition rate of 0.6 nanometers per second to 1.0 nanometers per second.
18. A method of fabricating an optical filter, comprising:
fabricating a plurality of hydrogenated silicon (Si:H) layers of a multilayer structure of the optical filter using a direct current (DC) sputtering technique,
the plurality of Si:H layers being fabricated in a low-pressure deposition chamber associated with a chamber pressure of 2 millitorr (mTorr), and
the plurality of Si:H layers having a first refractive index of greater than 3 over an operating wavelength range of the optical filter of 800 nanometers (nm) to 1100 nm; and
fabricating a plurality of lower-refractive-index (LI) layers,
the plurality of LI layers having a second refractive index of less than 3 over the operating wavelength range of the optical filter of 800 nm to 1100 nm, and
the plurality of LI layers being stacked in alternation with the plurality of Si:H layers.
19. The method of claim 18 , further comprising:
annealing the multilayer structure of the optical filter at a temperature of between 250 degrees Celsius and 350 degrees Celsius.
20. The method of claim 18 , where the plurality of Si:H layers are associated with an extinction coefficient of less than 0.0003 at 800 nm.