Image sensor

An image sensor includes a substrate, transparent layers covering the substrate and delimiting an exposition surface exposed to light, separate photosensitive areas at the substrate level and, for each photosensitive area, a first optical means capable of deviating towards the photosensitive area light reaching a central region of a portion of the exposition surface. The sensor further includes, for each photosensitive area, a second optical means, separate from the first optical means, capable of deviating towards the photosensitive area light reaching a peripheral region of the portion of the exposition surface surrounding the central region.

TECHNICAL FIELD

An embodiment relates to an image sensor, and in particular, to a CMOS-type image sensor formed of an array of photosensitive cells arranged in lines and columns.

BACKGROUND

FIG. 1schematically shows a cross-section of two adjacent photosensitive cells10,12of a conventional CMOS-type image sensor formed on a substrate13. Such a sensor corresponds, for example, to the sensor sold by STMicroelectronics under the trade name “CMOS Image Sensor Module VS6552”. Each photosensitive cell10,12is associated with a portion of the surface of substrate13which, in a top view, generally has the shape of a square or of a rectangle. Each photosensitive cell10,12includes an active photosensitive area14,16, generally corresponding to a photodiode adapted to storing a quantity of electric charges according to the received light intensity. Substrate13is covered with a stacking of insulating and transparent layers18, for example, formed of silicon oxide. Conductive tracks20, formed on the surface of substrate13and between adjacent insulating layers, and conductive vias22, formed through insulating layers18, especially enable addressing photosensitive areas14,16and collecting electric signals provided by photosensitive areas14,16. Conductive tracks20and conductive vias22are generally formed of reflective or absorbing materials. In a color sensor, a color filter element, for example, an organic filter24,26, is arranged at the surface of the stacking of insulating layers18at the level of each photosensitive cell10,12. The elements of color filter24,26are generally covered with a planarized equalizing layer27which defines an exposition surface28exposed to light.

Photosensitive area14,16generally does not cover the entire surface of substrate13associated with photosensitive cell10,12. Indeed, a portion of the surface is reserved to devices for addressing and reading from photosensitive area14. A photosensitive area14generally covers approximately 30% of the surface of substrate13associated with photosensitive cell10,12. To increase the light intensity reaching the photosensitive area of a photosensitive cell, a microlens29,30is arranged on equalizing layer27, opposite to photosensitive area14,16to focus the light beams towards photosensitive area14,16. The paths followed by three light beams R1, R2, R3are schematically shown as an example in stripe-dot lines for photosensitive cells10,12. Conductive tracks20and conductive vias22are arranged to avoid hindering the passing of the light beams.

Microlenses29,30are generally obtained by covering equalizing layer27with a resin, etching the resin to define separate resin blocks, each resin block being formed substantially opposite to a photosensitive area14,16, by heating the resin blocks. Each resin block then tends to deform by reflow, the center of the block inflating and the lateral walls collapsing, to obtain a convex external surface32,34. The external surface32,34desired to ensure an optimal focusing of the light beams towards a photosensitive area corresponds to a portion of a sphere having its radius varying proportionally to the distance separating a microlens29,30from the associated photosensitive area14,16. As an example, for a photosensitive cell10,12with a 4-micrometer side and for a distance on the order of from 8 to 10 micrometers between a microlens29,30and the associated photosensitive area14,16, the maximum thickness of the microlens29,30is approximately ½ micrometer.

The previously-described method of manufacturing microlenses29,30, however, does not enable obtaining a microlens29,30filling the entire portion of the exposition surface associated with the photosensitive cells. Indeed, the resin blocks from which microlenses29,30are formed must be separated from one another by separation regions36surrounding each resin block, the minimum width of which especially depends on the used etch techniques and on the used resin type. For conventional etch techniques, separation regions36have a minimum width from approximately 0.4 to 0.5 micrometer, which substantially corresponds to 10% of the side of a photosensitive cell. Separation regions36are maintained after forming microlenses29,30. A circular resin block enables obtaining a microlens29,30having an external surface substantially corresponding to a spherical portion. However, to reduce separation regions36to a minimum while keeping an external microlens surface relatively close to a spherical portion, a resin block having, as seen from above, the shape of a square or of a rectangle with tapered angles, is generally used. The light arriving at the level of separation regions36associated with a photosensitive cell is not focused towards photosensitive area14,16, which reduces the sensor's sensitivity.

A solution to increase the light intensity focused towards the photosensitive area of a photosensitive cell is to provide an additional so-called “top-coating” step, which includes the conformal deposition of a transparent material (not shown), for example, silicon nitride, on microlenses29,30. The external surface of the conformal deposition follows the shape of microlenses29,30and forms the light-focusing surface. The conformal deposition then provides a focusing surface including dished areas at the level of each microlens29,30. Two adjacent dished areas are separated by a minimum distance less than the minimum width of the separation region between the two associated microlenses. When the conformal deposition has a sufficient thickness, the dished surfaces can be contiguous.

To increase the sensitivity of an image sensor, it is desirable to increase the number of photosensitive cells forming it. However, it is not desirable for the total surface area taken up by the sensor to excessively increase. It is thus desirable to decrease the surface area of a photosensitive cell. This imposes decreasing the surface area of the photosensitive area of each photosensitive cell. The sensitivity of each photosensitive cell is decreased since the photosensitive area of the photosensitive cell receives a lower and lower light intensity. The optimizing of the amount of light received by the photosensitive area of a photosensitive cell with respect to the amount of light received by the portion of the exposition surface associated with the photosensitive cell then becomes an important factor.

The performing of a conformal deposition increases the distance between each dished area and the associated photosensitive area. The more distant a dished area is from the associated photosensitive area, the higher its radius of curvature must be to ensure a proper focusing of the light beams towards the photosensitive area. This requires the forming of a microlens, itself having a high radius of curvature. The radius of curvature of a microlens is inversely proportional to the thickness of the resin block from which the microlens originates. However, the lower the thickness of a resin block, the more difficult it is to accurately control the radius of curvature of the finally-obtained microlens.

Furthermore, at small scales, it is difficult to form a perfectly conformal deposition and thus ensure for the external surface of the conformal deposition to accurately follow the convex surface of the microlenses.

SUMMARY

An embodiment provides an image sensor formed of an array of photosensitive cells enabling focusing, for each photosensitive area, as much light intensity received by the photosensitive cell as possible towards the photosensitive area of the photosensitive cell.

Another embodiment provides an image sensor including separate photosensitive areas at the level of a substrate, with an exposition surface exposed to light; and, for each photosensitive area, optical means capable of deviating towards the photosensitive area light reaching a peripheral region of a portion of the exposition surface associated with the photosensitive area.

According to another embodiment, the image sensor including a substrate; separate photosensitive areas at the substrate level; transparent layers covering the substrate and delimiting an exposition surface exposed to light; a first optical means, for each photosensitive area, capable of deviating towards the photosensitive area light reaching a central region of a portion of the exposition surface associated with the photosensitive area; and a second optical means, for each photosensitive area, capable of deviating towards the photosensitive area light reaching a peripheral region of the portion of the exposition surface surrounding the central region.

According to a further embodiment, the second optical means is arranged at an intermediary level between the exposition surface and the substrate.

According to a further embodiment, the first optical means includes a microlens arranged at the level of the central region.

According to a further embodiment, the second optical means includes refringent surfaces inclined with respect to the exposition surface delimited by a first transparent layer having a first refraction coefficient in contact with a second transparent layer having a second refraction coefficient greater than the first refraction coefficient, the first and second transparent layers being arranged at an intermediary level between the exposition surface and the substrate.

According to a further embodiment, the refringent surfaces are at least partly planar.

According to a further embodiment, the refringent surfaces are arranged, for each photosensitive area, opposite to the peripheral region.

Another embodiment provides a method for forming an image sensor, including the steps of forming separate photosensitive areas at the level of a substrate; forming a stacking of transparent layers, including a first transparent layer having a first refraction coefficient in contact with a second transparent layer having a second refraction coefficient greater than the first refraction coefficient, the first and second transparent layers delimiting at least partly planar refringent surfaces capable of deviating light towards the photosensitive areas; forming an exposition surface exposed to light, the refringent surfaces being inclined with respect to the exposition surface; and forming separate microlenses on the exposition surface, each microlens being capable of deviating light towards a photosensitive area, the microlenses being separated by separation regions arranged opposite to the refringent surfaces.

According to a further embodiment, the second transparent layer covers the first transparent layer and is planarized.

According to a further embodiment, the first transparent layer is formed of the same material as other transparent layers.

DETAILED DESCRIPTION

The following discussion is presented to enable a person skilled in the art to make and use the embodiments described in the present disclosure. Various modifications to the embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments and applications without departing from the spirit and scope of the present disclosure. Thus, the present disclosure is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.

An embodiment includes providing, in the stacking of insulating layers18, opposite to the separation regions36surrounding microlens29,30of each photosensitive cell10,12, a refringent surface capable of deviating the light beams which reach the portion of exposition surface28associated with photosensitive cell10,12towards photosensitive area14,16of photosensitive cell10,12. The light beams usually focused towards photosensitive area14,16by microlens29,30are then combined with the light beams which reach the portion of exposition surface28associated with photosensitive cell10,12at the level of separation regions36. Almost all of the light reaching the portion of exposition surface28associated with photosensitive cell10,12is then oriented towards photosensitive area14,16of photosensitive cell10,12.

FIG. 2shows a first embodiment of a sensor. A first transparent insulating layer37having a small refraction coefficient on which is formed a second transparent insulating layer38having a greater refraction coefficient is provided in the stacking of insulating layers18. As an example, layer37with a small refraction coefficient is formed of silicon oxide, the refraction coefficient of which is on the order of from 1.5 to 1.6 and layer38with a high refraction coefficient is formed of silicon nitride having a refraction coefficient on the order of 2. Low-refraction coefficient layer37may be formed of the same material as that forming insulating layers18in which are formed previously-described conductive tracks20and conductive vias22.

Upper surface40of high-refraction coefficient layer38, opposite to filter elements24,26, is planarized and forms a first refringent surface. An insulating and transparent layer41may be provided between layer38and filter elements24,26. Surface42at the interface between high-refraction coefficient layer38and low-refraction coefficient layer37forms a second refringent surface. Low refraction coefficient layer37includes protuberances44which each define two inclined planar surfaces46,48of the second refringent surface42. Each protuberance44is substantially formed opposite to a separation region36between two adjacent microlenses29,30. The junction line between two inclined planar surfaces46,48is substantially arranged at the level of the separation between two adjacent photosensitive cells10,12. The light beams which reach separation region36according to a direction substantially perpendicular to exposition surface28cross filter elements24,26, layer41, and first refringent surface40without being deviated given their 90° angle of incidence. They are then deviated by one or the other of inclined planar surfaces46,48by a determined deviation angle which depends on the refraction coefficients of layers37,38and on the inclination of inclined planar surfaces46,48. The deviation angle is chosen so that all of the light beams which reach the portion of separation region36associated with a photosensitive cell are deviated by an inclined surface46,48towards photosensitive area14of photosensitive cell10,12. As an illustration, for each photosensitive cell10,12, the path followed by five light beams R1′ to R5′ are shown inFIG. 2. In the case where low-refraction coefficient layer37is formed of silicon oxide, there is no additional deviation of the light beams crossing layer37and the underlying layers formed of the same material.

Protuberances44may be obtained by a method in which layer37is formed by carrying out, in parallel, adapted steps of deposition and etch to form inclined planar surfaces46,48according to a desired inclination.

FIG. 3schematically shows a top view of the two photosensitive cells10,12and of two other adjacent photosensitive cells49,50enabling appreciating the relative positions between photosensitive areas14,16(shown in thin full lines), microlenses29,30(shown in thick full lines), and inclined planar surfaces46,48(shown in dotted lines).

FIG. 4shows an image sensor according to a second embodiment. A first trans-parent insulating layer51having a high refraction coefficient, on which is formed a second transparent insulating layer52having a lower refraction coefficient, is provided in the stacking of insulating layers18.

Surface54at the interface between low-refraction coefficient layer52and high-refraction coefficient layer51forms a first refringent surface. Lower surface56of high refraction coefficient layer51, at the interface with the stacking of insulating layers18, forms a second refringent surface. High-refraction coefficient layer51includes recesses58which each define two inclined planar surfaces60,62of the first refringent surface54. Each recess58is formed substantially opposite to a separation region36between two microlenses29,30. The junction line between two inclined planar surfaces60,62is substantially arranged at the level of the separation between two adjacent photosensitive cells. The light beams which reach separation region36according to a direction substantially perpendicular to exposition surface28cross filter elements24,26, layer41, and low refraction coefficient layer52without being deviated given their 90° angle of incidence. They are then deviated by one or the other of inclined surfaces60,62of second refringent surface54by a determined deviation angle which depends on the refraction coefficients of layers51,52and on the inclination of inclined surfaces60,62. The light beams then undergo an additional refraction (not shown) by crossing second refringent surface56.

The total deviation applied to the light beams reaching separation regions36is selected so that all of the light beams that reach the portion of separation region36associated with a photosensitive cell are deviated to photosensitive area14of the photosensitive cell. As an illustration, for each photosensitive cell10,12, the paths followed by five light beams R1″ to R5″ are shown inFIG. 4. It is advantageous to have, in the two previously-described embodiments, layers37,38,51,52with low and high refraction coefficients close to filter elements24,26. Indeed, the deviation to be applied to the light beams then is the smallest. However, if necessary, the layers with low and high refraction coefficients37,38,51,52can be arranged anywhere in the stacking of insulating layers18, with tracks20and conductive vias22being, however, likely to hinder the passing of the light beams.

It is necessary to take into account the angular deviations due to layers37,38,51,52to determine the path followed by the light beams focused by microlenses29,30. To simplify the determination of the travel of the light beams, it may be preferable for the light beams passing substantially at the level of the contour of a microlens29,30to reach, in the first embodiment, second refringent surface42outside of protuberances44and, in the second embodiment, first refringent surface54outside of recesses58.

According to a third embodiment, microlenses29,30are replaced with a layer having a refraction coefficient different from that of the underlying insulating layer and having, at the level of the central region of the portion of exposition surface28associated with a photosensitive cell10,12, a juxtaposition of planar surfaces inclined in such a way that the light beams reaching each inclined planar surface are deviated towards the photosensitive area of the photosensitive cell.

Image sensors according to the described embodiments may be utilized in a variety of different types of electronic devices, such as digital cameras, camcorders, cellular phones, personal digital assistants (PDAs), and so on.

Of course, the embodiments described in the present disclosure are likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, inclined planar surfaces for deviating the light beams towards the photosensitive area of a photosensitive cell have been described. These may, however, be more complex surfaces, for example, concave or convex surfaces.

Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present disclosure. Accordingly, the foregoing description is by way of example only and is not intended to be limiting.