Optical fingerprint sensor with high aspect-ratio metal aperture structures
An under-display optical fingerprint sensors employing microlens arrays (MLAs) and an opaque aperture layer includes high aspect-ratio metal aperture structures for efficient angular signal filtering and stray light control. Instead of relying on one or more opaque aperture baffle-layers, embodiments disclosed herein utilize an image sensor's inherent metal layers for filtering signals originated from unwanted angular ranges and blocking undesired stray light could achieve similar or better performance with simplified process flow and lower cost. Layers from the sensors' inherent metal layers are brought into the sensing area on purpose to form the high aspect-ratio metal aperture structure. The metal layers in the sensing area may include apertures aligned to apertures in the opaque layer, and may also be grounded.
1. An optical fingerprint sensor comprising:
an image sensor including a photodiode array, a metal aperture structure including two grounded metal aperture layers, and circuitry that reads from and controls pixels of the image sensor, the circuitry being both (i) in a same layer as and (ii) adjacent to one of the two grounded metal aperture layers;
an opaque aperture layer positioned above the image sensor, the two grounded metal aperture layers being between the opaque aperture layer and the photodiode array;
and
a microlens array (MLA) for focusing light from an object plane onto the image sensor through the opaque aperture layer and the metal aperture structure;
wherein a spot size created by a light bundle passing through a microlens reaches a minimum at a top metal aperture layer of the metal aperture structure;
wherein the metal aperture structure further includes a dielectric layer between the grounded metal aperture layers; and
wherein each of the two grounded metal aperture layers has a thickness less than or equal to a thickness of the dielectric layer.
2. The optical fingerprint sensor of claim 1 , wherein each of the grounded metal aperture layers further comprise an array of aperture stops aligned with a respective microlens of the MLA.
3. The optical fingerprint sensor of claim 2 , wherein the metal aperture structure has an aspect ratio h/d≥1, where h is a height of the metal aperture structure between a surface of the image sensor and a top surface of an uppermost grounded metal aperture layer farthest from the image sensor, and d is a diameter of the aperture stops in the uppermost grounded metal aperture layer.
4. The optical fingerprint sensor of claim 3 , wherein the diameter of the aperture stops of the uppermost grounded metal aperture layer is smaller than the diameter of the aperture stops of lower grounded metal aperture layers.
5. The optical fingerprint sensor of claim 4 , wherein the diameter of the aperture stops of each grounded metal aperture layer is smaller than the diameter of the aperture stops of next lower grounded metal aperture layer.
6. The optical fingerprint sensor of claim 1 , wherein the metal aperture structure includes two to four grounded metal aperture layers.
7. The optical fingerprint sensor of claim 1 , wherein the grounded metal aperture layers comprise an anti-reflection layer deposited upon and beneath the grounded metal aperture layers.
8. The optical fingerprint sensor of claim 7 , wherein a reflectance of the grounded metal aperture layers with anti-reflection layers is less than 5 percent.
9. The optical fingerprint sensor of claim 1 , wherein the opaque aperture layer further comprises one opaque baffle-layer having an array of aperture stops and two transparent layers, and the metal aperture structure further comprises four grounded metal aperture layers each having an array of aperture stops corresponding to the array of aperture stops in the opaque aperture layer.
10. The optical fingerprint sensor of claim 1 , wherein the opaque aperture layer further comprises two opaque baffle-layers each having an array of aperture stops and two transparent layers, and the metal aperture structure further comprises four grounded metal aperture layers each having an array of aperture stops corresponding to the arrays of aperture stops in the opaque aperture layer.
11. The optical fingerprint sensor of claim 10 , wherein the aperture stops in each of three grounded metal aperture layers above the image sensor have the same diameter.
12. The optical fingerprint sensor of claim 1 , wherein the opaque aperture layer further comprises two opaque baffle-layers each having an array of aperture stops and three transparent layers, and the metal aperture structure further comprises four grounded metal aperture layers each having an array of aperture stops corresponding to the arrays of aperture stops in the opaque aperture layer.
13. The optical fingerprint sensor of claim 12 , wherein the aperture stops in each of three grounded metal aperture layers above the image sensor have the same diameter.
14. The optical fingerprint sensor of claim 1 , wherein the opaque aperture layer further comprises three opaque baffle-layers each having an array of aperture stops and a diameter of the aperture stops in an upper opaque baffle-layer is wider than a diameter of the aperture stops in a lower opaque baffle layer.
15. The optical fingerprint sensor of claim 1 , further comprising an infrared cutoff filter (IRCF) layer.
16. The optical fingerprint sensor of claim 15 , wherein the IRCF layer is embedded between the opaque aperture layer and the metal aperture structure.
17. The optical fingerprint sensor of claim 15 , further comprising a microlens array (MLA) above the opaque aperture layer and a cover glass layer above the MLA, the IRCF layer is coated on a surface of the cover glass layer adjacent to the MLA, and an upper surface of the cover glass layer forms the object plane.