IMAGE SENSOR WITH REDUCED SPECTRAL AND OPTICAL CROSSTALK AND METHOD FOR MAKING THE IMAGE SENSOR

Abstract

An integrated image sensor may include adjacent pixels, with each pixel including an active semiconductor region including a photodiode, an antireflection layer disposed above the photodiode, a dielectric region disposed above the antireflection layer, an optical filter disposed above the dielectric region, and a diffraction grating disposed in the antireflection layer. The diffraction grating includes an array of pads.

Claims

1. An image sensor comprising: a plurality of pixels disposed adjacent to one another, each of the plurality of pixels comprising a photodiode disposed in a semiconductor substrate, an antireflection layer disposed above the photodiode, a dielectric region disposed above the antireflection layer, an optical filter disposed above the dielectric region, and a diffraction grating disposed in the antireflection layer, the diffraction grating comprising an array of pads.

2. The image sensor of claim 1, wherein the array of pads is periodic with a fixed separation distance between adjacent pads of the array of pads.

3. The image sensor of claim 2, wherein fixed separation distance b follows the inequality: $b<\frac{}{\left(n_s+n_i*\sin \left(\right)\right)},$ where is a wavelength of an incident light on the diffraction grating, n.sub.s is a refractive index of the semiconductor substrate, n.sub.i is a refractive index of the dielectric region, and is an angle of incidence of the incident light.

4. The image sensor of claim 1, wherein a height and diameter of the array of pads define a refractive index n of the antireflection layer, wherein the refractive index n of the antireflection layer is a square root of the product of a refractive index of the semiconductor substrate and a refractive index of said dielectric region.

5. The image sensor of claim 1, wherein each of the array of pads comprises a circular shape.

6. The image sensor of claim 5, wherein the array of pads comprise silicon.

7. The image sensor of claim 1, wherein each pixel further comprises a microlens above the optical filter.

8. The image sensor of claim 1, wherein for a given wavelength of 450 nanometres, 30% of an exposed surface area of the antireflection layer is covered with the array of pads.

9. The image sensor of claim 1, wherein for a given wavelength of 540 nanometres, 49% of an exposed surface area of the antireflection layer is covered with the array of pads.

10. The image sensor of claim 1, wherein for a given wavelength of 610 nanometres, 56% of an exposed surface area of the antireflection layer is covered with the array of pads.

11. The image sensor of claim 1, wherein for a given wavelength in a range of 450 to 610 nanometres, an exposed surface area of the antireflection layer that is covered with the array of pads is in a range of 30-56%.

12. The image sensor of claim 1, wherein the image sensor comprises a front side illumination sensor.

13. The image sensor of claim 1, wherein the image sensor comprises a rear side illumination sensor.

14. An image sensor comprising: isolation trenches disposed in a semiconductor substrate; a photodiode disposed in the semiconductor substrate, the isolation trenches disposed around the photodiode; a diffraction grating disposed over the photodiode, the diffraction grating comprising an plurality of circular pads; a dielectric region disposed over the diffraction grating; a color filter disposed over the dielectric region; and a lens disposed over the color filter.

15. The image sensor of claim 14, wherein the diffraction grating is configured to high-pass filter light passing through it by attenuating or eliminating signals of wavelengths below a target wavelength.

16. The image sensor of claim 14, wherein the image sensor comprises a front side illumination sensor.

17. The image sensor of claim 14, wherein the image sensor comprises a rear side illumination sensor.

18. The image sensor of claim 14, further comprising an antireflection layer disposed under the diffraction grating.

19. A method for making an image sensor comprising: forming isolation trenches in a semiconductor substrate; forming a photodiode in the semiconductor substrate, the isolation trenches being formed around the photodiode; forming a diffraction grating over the photodiode, the diffraction grating comprising an plurality of circular pads; forming a dielectric region over the diffraction grating; forming an optical filter over the dielectric region; and forming a lens over the optical filter.

20. The method of claim 19, further comprising forming an antireflection layer under the diffraction grating.

21. The method of claim 20, wherein for a given wavelength in a range of 450 to 610 nanometres, an exposed surface area of the antireflection layer that is covered with the plurality of circular pads is in a range of 30-56%.

22. The method of claim 19, wherein the plurality of circular pads comprise polysilicon.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Other advantages and characteristics will become apparent upon examining the detailed description of non-limiting embodiments and the appended drawings, in which:

[0023] FIG. 1 is a schematic block diagram of an image sensor in accordance with the prior art.

[0024] FIG. 2 is a graph illustrating curves of quantum efficiency of a set of pixels such as in the image sensor of FIG. 1 forming a Bayer pattern in accordance with the prior art.

[0025] FIG. 3 is a schematic cross-sectional diagram of a pixel of a CMOS image sensor with front side illumination in accordance with an example embodiment.

[0026] FIG. 4 is perspective view of the antireflection layer of the pixel of FIG. 3 in greater detail.

[0027] FIG. 5 is a schematic cross-sectional view of the antireflection layer of the pixel of FIG. 3 in greater detail.

[0028] FIG. 6 is a graph illustrating the transmittance of the antireflection layer for an incident luminous signal in the device of FIG. 3.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0029] FIG. 3 illustrates a schematic section through a pixel 5 of a CMOS image sensor with front side illumination, which illustratively includes an antireflection layer. The pixel is laterally insulated from the neighboring pixels by a deep trench isolation (DTI) 21. The pixel includes, above a substrate 6, an active layer 7 (e.g., P types conductivity) including a buried zone 8 (e.g., N type conductivity) in proximity to the upper side of the layer 7, e.g., forming a photodiode.

[0030] The upper side of the layer 7 supports, for example, a transfer transistor 9 and an antireflection layer 13 including pads 24 situated above the photodiode 8. The transistor 9 and layer 13 are wrapped in or covered by an insulating layer 14, e.g., PreMetal Dielectric (PMD). This insulating layer includes a first dielectric material, e.g., silicon dioxide.

[0031] The PMD layer 14 is surmounted or covered by an interconnection part or portion 15, commonly referred to as the Back End Of Line (BEOL). The part 15 includes various metallization levels. In the present example, there are three levels M1, M2 and M3, each comprising electrically conducting tracks 16 as well as vias 18 shrouded in a dielectric region comprising a second dielectric material 17, commonly referred to as InterMetal Dielectric (IMD).

[0032] The pixel 5 also illustratively includes a color filter 19 situated above the last metallization level M3 facing the photodiode 8. This filter is configured to allow through only certain wavelengths of a luminous signal. For example, here the wavelengths close to 540 nanometers corresponding to the color green are allowed to pass.

[0033] A collimation lens 20 is advantageously disposed above the optical filter 19. The lens directs the incident rays towards the photodiode 8 to the maximum extent possible.

[0034] Referring to FIGS. 4 and 5, the antireflection layer 13 is illustrated in greater detail. The layer 13 is produced or formed on the substrate 7 and includes the array of pads 24, e.g., circular polysilicon pads. The shape of the pads 24 is not limited to a circular shape, rather the pads may be various different shapes. However, a circular shape makes it possible to have a symmetric structure of the antireflection layer, and thus isotropic behavior. The pads 24 are shrouded in the silicon oxide of the insulating layer 14, which is represented as transparent in FIG. 3 for simplification.

[0035] The pads 24 may, for example, be formed after the deposition of the insulating layer 14 by etching the layer 14 at the location of the pads, then by filling the orifices thus obtained with polysilicon according to the desired pad thickness. The orifices above the polysilicon pads may then be plugged with silicon oxide. Finally, a step of chemical-mechanical planarization (CMP) of the structure is applied.

[0036] The refractive index n of the antireflection layer disposed between the substrate 7 (which has a refractive index ns) and the layer 14 (which has a refractive index ni) will ideally satisfy the formula

n={square root over (n.sub.i*n.sub.s)}

[0037] For silicon in the wavelengths close to green, for example, theoretically ns is equal to about 4 and the theoretical index ni of the dielectric PMD is equal to about 1.5.

[0038] Furthermore, for such an array structure, diffraction orders appear for a light ray of a given wavelength and angle of incidence when the period b of the array satisfies the equation:

[00001]$b<\frac{}{\left(n_s+n_i*\sin \left(\right)\right)}$

[0039] For example, for a pixel with a green color filter, the pads 24 are organized here as a regular array of period b equal to 100 nanometers. Their height h is 50 nanometers, and their diameter d is 70 nanometers.

[0040] One advantage of an array structure is that, by adapting the ratio between the area occupied by the pads with respect to the total area, it is possible to vary the ratio between the quantity of polysilicon of the pads and the quantity of silicon oxide of the PMD layer 14 present in the antireflection layer. It is therefore possible to adapt the refractive index of the antireflection layer by configuring the diameter d of the pads to obtain the desired refractive index n, which will be between 1.5 and 4.

[0041] By way of example, 49% of the area of the antireflection layer occupied by pads 24 leads to a configuration best adapted for the transmission of signals of wavelength close to 540 nanometers (green). In this example, and with the values of b, h, and d mentioned above, the refractive index n of the antireflection layer is in the vicinity of 2.4.

[0042] When an incident light ray 25 arrives at the antireflection layer 13, it is transmitted within the substrate 7 in the form of a transmitted ray mo, reflected by the antireflection layer in the form of a reflected ray mr and diffracted as several diffracted rays m1, m2, m3, m4 . . . of different wavelengths, each corresponding to a different mode of diffraction and order. The incident ray 25 may, for example, have previously passed through the optical filter 19 and because the filter is not perfect, includes wavelengths remote from 540 nanometers.

[0043] The transmitted light ray mo, corresponding to the wavelength for which the filter is configured (here the wavelengths close to 540 nanometers and corresponding to the color green), deviates very little, if at all. The diffracted rays of higher orders, corresponding to the signals of wavelengths below the desired wavelength, undergo a diffraction proportional to their diffraction order. Thus, the second-order ray M2 has a lower transmittance than the ray m1 of order one, and so on and so forth.

[0044] FIG. 6 schematically represents the transmittance of the antireflection layer 14 for the incident luminous signal. Here, the best transmittance corresponds to the transmitted signal mo of wavelength 540 nanometers. It may be observed that the luminous signals m1, m2, m3, m4, respectively of orders 1, 2, 3, 4, are very attenuated with respect to the main order, and appear for wavelengths below 540 nm. Thus, the antireflection layer 13 behaves as a high-pass filter, attenuating or eliminating the signals of wavelengths below the desired wavelength, here 540 nanometers.

[0045] The antireflection layer may also be adapted to the color blue, in which case 30% of the area of the layer is occupied by pads 50 nanometers in height and 55 nanometers in diameter, and the period of the array is 100 nanometers. In this example, the refractive index of the antireflection layer is in the vicinity of 3.2.

[0046] In another example embodiment, the antireflection layer may also be adapted to the color red, in which case 56% of the area of the layer is occupied by pads 50 nanometers in height and 225 nanometers in diameter, and the period of the array is 300 nanometers. In this example, the refractive index of the antireflection layer is in the vicinity of 2.7.

[0047] These various values are merely indicative and should be adapted by the person skilled in the art as a function of the transmission of the layer and of the cutoff wavelength which are desired in different embodiments. Nonetheless, using this approach it is possible to reduce the spectral and optical crosstalk by attenuating the humps G1 and R1 of FIG. 2.

[0048] Although the pixel 5 exhibited in these examples is of a type with front side illumination, the antireflection layer 13 may also be integrated into pixels of a type with rear side illumination. In this case, the pads are obtained by depositing a layer of polysilicon, for example, according to the desired thickness h, then by the etching the layer to define the geometry of the pads. Further, the dielectric material of the PMD insulating layer is deposited. A step of chemical-mechanical planarization (CMP) of the structure may then be performed.