BACKGROUND

Field of the Disclosure

The present disclosure relates to an optical filter and a method for forming the same, and particularly relates to an optical filter having a passband at least partially overlapping with a wavelength range of 850 nm to 2000 nm and a method for forming the same.

Description of the Related Art

Nowadays, image sensors are widely used in many fields, as well as in devices such as light sensors, proximity sensors, time-of-flight (TOF) cameras, spectrometers, devices for spectral inspection, color filters, smart sensors used in the Internet of things (IOT), and sensors for advanced driver assistance systems (ADAS). In order to provide improved sensitivity as well as a satisfactory signal-to-noise ratio, narrow bandpass filters are components that are essential in realizing these applications.

Although existing narrow bandpass filters have been adequate for their intended purposes, they have not been entirely satisfactory in all respects. Therefore, there are still some problems with narrow bandpass filters that remain to be overcome.

BRIEF SUMMARY

The present disclosure relates to an optical filter and a method for forming the same. In the embodiments, with the design of the filter stacks each including a higher-refractive-index layer, a medium-refractive-index layer, and a lower-refractive-index layer, the optical filter of the present disclosure can be provided with low angle dependency and steep filter edges between the passband and the blockband. In addition, there are at least three layers, each of which has its own designed refractive index, in each of the filter stacks in accordance with the embodiments, and so the total thickness of the optical filter can be greatly reduced.

In some embodiments of the disclosure, an optical filter is provided. The optical filter includes a substrate and a plurality of filter stacks formed on the substrate. Each of the plurality of filter stacks includes a higher-refractive-index layer, a medium-refractive-index layer, and a lower-refractive-index layer. The higher-refractive-index layer has a first refractive index of higher than 3.5. The medium-refractive-index layer is disposed on the higher-refractive-index layer. The medium-refractive-index layer has a second refractive index higher than 2.9 and lower than the first refractive index. The lower-refractive-index layer is disposed on the medium-refractive-index layer. The lower-refractive-index layer has a third refractive index lower than the second refractive index.

In some embodiments of the disclosure, a method for forming an optical filter is provided. The method includes providing a substrate and forming a plurality of filter stacks on the substrate. Forming each of the plurality of filter stacks includes: forming a higher-refractive-index layer having a first refractive index of higher than 3.5; forming a medium-refractive-index layer on the higher-refractive-index layer, wherein medium-refractive-index layer has a second refractive index higher than 2.9 and lower than the first refractive index; and forming a lower-refractive-index layer on the medium-refractive-index layer, wherein the lower-refractive-index layer has a third refractive index lower than the second refractive index.

A detailed description is given in the following embodiments with reference to the accompanying drawings.

