Phase detection autofocus pixel

Abstract

To provide favorable use of a phase detection autofocus pixel for focusing of a camera lens.SOLUTION: A phase detection autofocus pixel 1 comprises: a photodiode 2; a plurality of microlenses 3 for concentrating, on the photodiode, light incident on each of the microlenses; and, for each microlens, a light shield 4 associated with the microlens, for blocking part of the light transmitted by the microlens towards the photodiode, where the light shields are offset from optical axes 31 of the microlenses associated with the light shields in the same offset direction X.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present invention relates to the field of phase detection autofocus pixels. [Background technology]

[0002] Phase detection autofocus (PDAF) pixels are commonly used in CMOS image sensors (CIS). PDAF pixels allow to directly measure the amount of focus (out of focus) and, in case of out of focus blur, the direction of the out of focus (front focus, rear focus). The camera lens can therefore be quickly adjusted to focus the image. PDAF pixels allow for much faster focusing than when contrast autofocus is applied, for example.

[0003] Please refer to FIG. 1A, which corresponds to FIG. 17 of US6829008B1, showing an image sensor 61 of a camera comprising a plurality of PDAF pixels 9. In the art, each PDAF pixel 9 includes a single microlens 93. For illustrative purposes, in this example, each PDAF pixel 9 includes a first photodiode S1 and a second photodiode S2, which are offset in opposite directions, i.e., directions -X and X, respectively, with respect to an optical axis 95 of the microlens 93 of the PDAF pixel 9. The camera further comprises a camera lens 5.

[0004] In the following, it is described how the PDAF pixel 9 can be used to determine whether the camera is focused. In this specification, an idealized function of the PDAF pixel 9 is assumed, ignoring the limited focusing capabilities of the microlenses, which is described later.

[0005] It may be assumed that the light from each specific point on the subject (not shown) collected by the camera lens 5 is split into a first light beam L1 and a second light beam L2. For simplicity, FIG. 1A shows the light collected by the camera lens 5 from only a single specific point on the subject. The first light beam L1 propagates through a first half of the camera lens 5 and then through a microlens 93 of one of the PDAF pixels 9 and is incident on a first photodiode S1 of that PDAF pixel 9. The second light beam L2 propagates through a second half of the camera lens 5 and then through a microlens 93 of one of the PDAF pixels 9 and is incident on a second photodiode S2 of that PDAF pixel 9.

[0006] As in FIG. 1A, when the camera is focused on the object, the camera lens 5 focuses light from each point on the object onto the microlenses 93 of the image sensor 61, thereby generating an image of the object in the plane of the microlenses 93. Thus, when the camera is focused, the two light beams L1 and L2 from each particular point on the object are focused onto a single point on the microlenses 93 of a single PDAF pixel 9. As a result, when focused, a first photodiode S1 that detects a first light beam L1 from a particular point on the object is located in the same PDAF pixel 9 as a second photodiode S2 that detects a second light beam L2 from a particular point on the object. As a result, when focused, a first image of the object formed by the L1 light beam detected by the first photodiode S1 of the image sensor 61 and a second image of the object formed by the L2 light beam detected by the second photodiode S2 of the image sensor 61 coincide on the same pixel.

[0007] 2 corresponds to FIG. 17 of US6829008B1 and shows a situation where the camera is not in focus, more specifically, in front focus. As shown in FIG. 2, when not in focus, the first light beam L1 and the second light beam L2 from each specific point on the object cross each other at a position away from the plane of the microlens 93. Thus, when not in focus, for the light beams L1 and L2 from each specific point on the object, the first light beam L1 and the second light beam L2 are incident on different microlenses 93, such that the first photodiode S1 that detects the first light beam L1 is located in a different PDAF pixel 9 from the second photodiode S2 that detects the second light beam L2. As a result, as can be seen in FIG. 3, which corresponds to FIG. 19 of US 6,829,008 B1, when out of focus, a first image of the subject detected by a first photodiode S1 of the image sensor 61 ("S1 image") is shifted with respect to a second image of the subject detected by a second photodiode S2 of the image sensor 61 ("S2 image"). As further explained in US 6,829,008 B1, the magnitude of this shift can be directly related to the distance between the point where the first and second light beams L1, L2 intersect from a particular point on the subject and the plane of the microlens 93, providing a straightforward method for determining the distance the camera lens 5 must be translated along the optical axis 50 of the camera lens 5 in order to focus on the subject.

[0008] Refer to FIG. 1B. In the above, for illustrative purposes, each PDAF pixel includes two photodiodes (i.e., a first photodiode S1 and a second photodiode S2), but in practice, a single pixel 91, 910 typically includes only a single photodiode 92, 920. Therefore, in practice, a single PDAF pixel 91, 910 cannot detect the first light beam L1 and the second light beam L2 independently of each other. This problem has been solved in the art by using pairs of PDAF pixels 91 and 910. Each pair includes a first PDAF pixel 91 for detecting the first light beam L1, which includes a microlens 93 and a light shield 94, and the first PDAF pixel 91 is offset in a first offset direction X with respect to an optical axis 95 of the microlens 93, and half of the photodiode 92 of the first PDAF pixel 91 is covered on the side of the optical axis 95 in the first offset direction X to block the second light beam L2. Each pair further includes a second PDAF pixel 910 for detecting the second light beam L2, which includes a microlens 930 and a light shield 940, and the second PDAF pixel 910 is offset in a second offset direction -X opposite to the first offset direction X with respect to an optical axis 950 of the microlens 930, and half of the photodiode 920 of the second PDAF pixel 910 is covered on the side of the optical axis 950 in the second offset direction -X to block the first light beam L1. Thereby, the first PDAF pixel 91 of the image sensor 62 substantially detects a first image (i.e., "S1 image" in FIG. 3) correspondingly measured by the first photodiode S1 in the illustrative example of FIGS. 1A and 2, while the second PDAF pixel 910 of the image sensor 62 substantially detects a second image (i.e., "S2 image" in FIG. 3) correspondingly measured by the second photodiode S2 in the illustrative example of FIGS. 1A and 2. Of course, if for each particular point on the object only the first light beam L1 or the second light beam L2 is measured, the image resolution will be slightly reduced.

[0009] Please refer to FIG. 4A, which shows a first PDAF pixel 91 and a second PDAF pixel 910 on which different light beams (corresponding to a first light beam L1 from different adjacent locations on the subject and propagating through the first half of the camera lens) having the same specific incidence angle α are incident. The plot on the left shows the orientation of the incidence angle α of the light incident on the microlenses 93, 930. Here, the incidence angle α is in a plane parallel to the optical axis 95, 950 of the microlenses 93, 930 and parallel to the X axis (along which the light shields 94, 940 are offset from the optical axis 95, 950 of the respective PDAF pixels 91, 910), and is with respect to normal incidence along the optical axis 95, 950 of the microlenses 93, 930. Here, the direction of the incident light with an X component pointing in the X direction has a negative incidence angle α, and the direction of the incident light with an X component pointing in the -X direction has a positive incidence angle α.

[0010] The above-mentioned idealized function of the PDAF pixels 91, 910 without intra-pixel crosstalk means perfect selectivity of the first PDAF pixel 91 towards the first light beam L1 and the second PDAF pixel towards the second light beam L2. This is achieved when the first PDAF pixel 91 detects all light beams with a positive incidence angle α (i.e., the first light beam L1) while its light shield 94 blocks all light with a negative incidence angle α (i.e., the second light beam L2) and the second PDAF pixel 910 detects all light beams with a negative incidence angle α while its light shield 940 blocks all light with a positive incidence angle α. Such good selectivity can be achieved, inter alia, when each microlens 93, 930 tightly focuses light from each incidence angle α in the plane of the light shield 94, 940, as shown in FIG. 4A, and light collected by each microlens 93, 930 and having a negative or positive incidence angle α focuses the light in a first direction X or a second direction −X, respectively, with respect to the optical axis 95, 950.

[0011] Refer to FIG. 4B, which shows quantum efficiency (QE) as a function of incidence angle α for a regular (non-PDAF) pixel (indicated by 1100), for a structurally non-optimized PDAF pixel (indicated by 1200), and for a second structurally well-optimized PDAF pixel (indicated by 1300).

[0012] Such good focusing in the plane, and correspondingly, such good selectivity, can be observed for a structurally well-optimized second PDAF pixel (indicated by 1300), which senses light with a negative angle of incidence but hardly detects light with a positive angle of incidence. In practice, it is often difficult to achieve such tight focusing in the plane of the light shield, for example, because the microlenses are not structurally well-optimized. As a result, as can be observed for a structurally not optimized PDAF pixel (indicated by 1200), the second PDAF pixel exhibits a limited focusing ability of the microlenses, which means that, undesirably, the second PDAF pixel detects a significant portion of light with a positive angle of incidence (i.e., the first light beam L1), while a significant portion of light with a negative angle of incidence (i.e., the second light beam L2) is blocked by the light shield, thereby reducing its sensitivity and accuracy. Such limited focusing ability is, of course, correspondingly exhibited by the first PDAF pixel. As the focusing capability exhibited by the microlenses of the PDAF pixels becomes more limited, the shape of the curve will of course be reminiscent of that of a regular pixel (shown by 1100) that does not exhibit any selectivity for positive or negative angles of incidence. As a result of the limited focusing capability of the microlenses, the contrast between the first and second images detected by the first and second PDAF pixels, respectively, is reduced, thus reducing the accuracy of the autofocus.

[0013] This limited focusing ability of microlenses can be particularly problematic for image sensors containing large pixels, where process capabilities are limited with respect to the shape, material, and height of the microlenses, as well as the height of the color filters and transparent planarizing layers. For example, for large pixels, the height and radius of curvature of the microlenses can be so large that known manufacturing techniques do not result in well-formed microlenses. Thus, for large pixels, it is particularly difficult to obtain tight focusing of light from each angle of incidence by the microlenses in the plane of the light shield.

[0014] Thus, there remains a need in the art for devices and methods that address at least some of the above problems. Summary of the Invention

[0015] It is an object of an embodiment of the present invention to provide a good phase detection autofocus pixel. It is a further object of an embodiment of the present invention to provide a good image sensor comprising a pair of such phase detection autofocus pixels. It is further an object of an embodiment of the present invention to provide a good camera comprising the image sensor.

[0016] It is an object of embodiments of the present invention to provide a good use of phase detection autofocus pixels for focusing a camera lens.

[0017] The above objectives are achieved by methods and devices according to embodiments of the present invention.

[0018] It is an advantage of embodiments of the present invention that the effect of the limited focusing ability of the microlens may be limited. It is an advantage of embodiments of the present invention that good contrast may be obtained between a first image generated by a first phase-detection autofocus pixel and a second image generated by a second phase-detection autofocus pixel of a pair of phase-detection autofocus pixels. Thereby, a good quality phase-detection autofocus function may be achieved.

[0019] In a first aspect, the present invention relates to a phase detection autofocus pixel. The phase detection autofocus pixel comprises a photodiode and a number of microlenses for focusing light incident on each of the microlenses onto the photodiode. The phase detection autofocus pixel further comprises, for each microlens, a light shield associated with each microlens for blocking a portion of the light transmitted by the microlens towards the photodiode. The light shields are offset in the same offset direction from the optical axis of the associated microlens of the light shield. The light shields may have vertical and horizontal edges, although slanted edges are also possible. An advantage of embodiments of the present invention is that instead of using a single large microlens per pixel as in the state of the art, where the height and the radius of curvature of the microlenses may be so large that known manufacturing techniques do not result in well-formed microlenses, multiple small lenses of optimized size can be used, especially for large pixels.

[0020] The microlenses may be organized in any manner, for example, an array. For example, the array may comprise N×M microlenses per pixel, where N and M are integers that may be the same or different. In a preferred embodiment, the microlenses are organized in a square array. In other words, the phase detection autofocus pixel may comprise N×N microlenses, where N is an integer. Such a regular pattern may result in relatively easy manufacturing. In an embodiment, the square array of microlenses includes rows and columns of microlenses, where each row and column includes at least two microlenses (i.e., N is at least 2). For example, the square array may include 2-10 (i.e., N is 2-10), or 2-4 (i.e., N is 2-4) microlenses in rows and columns. In an alternative embodiment, the microlenses are organized in an array of N×M microlenses per pixel, where either N or M may be equal to 1, provided that the other of M or N is greater than or equal to 2.

[0021] In an embodiment, the plurality of microlenses are substantially uniform in shape and size. In an embodiment, each of the plurality of microlenses may have substantially the same aperture and substantially the same focal length. Typically, each microlens has a flat surface facing the photodiode and a convex surface facing away from the photodiode. For example, each microlens may be a spherical lens. In practice, the microlenses may rather have a cubic shape with rounded corners and edges on the side of the microlens facing away from the photodiode. However, the invention is not limited to microlenses of any shape. This shape may typically depend on the manufacturing method.

[0022] In embodiments, the light shields may be substantially uniform in shape and size. In embodiments, the distance between each light shield and its associated microlens is substantially the same for different light shields and associated microlenses. Preferably, each light shield is offset by substantially the same distance from the optical axis of the associated microlens of the light shield. An advantage of these embodiments is that interpretation of the detection signal detected by the photodiode may be simple. Nevertheless, in alternative embodiments, the shape, size and / or offset of the light shields may be non-uniform across different pixels.

[0023] In preferred embodiments, the cross section of each light shield in the plane of the light shield is rectangular. However, the invention is not so limited and the light shield may have any suitable shape. In preferred embodiments, the light shield associated with a microlens blocks substantially all light transmitted by the associated microlens that would be collected by the photodiode on the side of the optical axis of the associated microlens in the offset direction in the absence of the light shield. Furthermore, preferably, the light shield does not block substantially any light that would be collected by the photodiode on the side of the optical axis of the associated microlens opposite the offset direction in the absence of the light shield. An advantage of these embodiments is that good sensitivity and angular response of the phase detection autofocus pixel may be achieved.

[0024] Each light shield may be formed of any material capable of blocking light. For example, the material may efficiently absorb visible light and / or efficiently reflect visible light. In embodiments, each light shield may be formed of a metal, such as tungsten, aluminum, or copper. Metals are typically highly reflective and therefore may efficiently block light.

[0025] In a second aspect, the present invention relates to an image sensor comprising a pair of phase detection autofocus pixels, the pair comprising a first phase detection autofocus pixel according to an embodiment of the first aspect of the invention, the light shields of the first phase detection autofocus pixel being each offset in a first direction from an optical axis of an associated microlens of the light shield, the pair further comprising a second phase detection autofocus pixel according to an embodiment of the first aspect of the invention, the light shields of the second phase detection autofocus pixel being each offset in a second direction substantially opposite to the first direction from an optical axis of an associated microlens of the light shield, an advantage of an embodiment of the present invention is that the image sensor may provide good focusing of a camera lens.

[0026] Preferably, the first phase detection autofocus pixel and the second phase detection autofocus pixel are located close to each other. Preferably, the first phase detection autofocus pixel and the second phase detection are separated from each other by up to 10 pixels, such as up to 5 pixels, or are directly adjacent to each other, for example, in contact. This may result in a good comparison of the signals of the first and second phase detection autofocus pixels. Typically, the image sensor comprises a plurality of first phase detection autofocus pixels and a plurality of second phase detection autofocus pixels. In other words, the image sensor may comprise a plurality of pairs of phase detection autofocus pixels. In these embodiments, the first and second phase detection autofocus pixels may be substantially evenly distributed in the image sensor.

[0027] In most cases, the analysis of the two signals is performed external to the image sensor, but in certain embodiments, the image sensor includes a device configured to compare a signal generated by a photodiode of a first phase-detection autofocus pixel with a signal generated by a photodiode of a second phase-detection autofocus pixel. In embodiments with multiple pairs, the device may be configured to compare a signal generated by a photodiode of a plurality of first phase-detection autofocus pixels, i.e., a first image, with a signal generated by a photodiode of a plurality of second phase-detection autofocus pixels, i.e., a second image. It should be noted that the "image" formed by the signals of the PDAF pixels may consist of a very limited number of pixels and does not necessarily constitute an image as understood by a user of a camera that includes the pixels.

[0028] In addition, the image sensor may include at least one further pair, e.g., a plurality of further pairs, of phase detection autofocus pixels, the further pair comprising a third phase detection autofocus pixel according to an embodiment of the first aspect of the invention. In these embodiments, the light shields of the third phase detection autofocus pixel are each offset from the optical axis of the associated microlens of the light shield in a third direction that is substantially perpendicular to the first direction. In these embodiments, the further pair also comprises a fourth phase detection autofocus pixel according to an embodiment of the first aspect of the invention, the light shields of the fourth phase detection autofocus pixel are each offset from the optical axis of the associated microlens of the light shield in a fourth direction that is substantially opposite to the third direction. An advantage of these embodiments is that more accurate focusing may be obtained. The features of the third and fourth phase detection autofocus pixels may be independent, as correspondingly described for the first and second phase detection autofocus pixels.

[0029] In a third aspect, the present invention relates to a camera comprising an image sensor according to an embodiment of the second aspect of the invention, comprising a device configured to compare a signal generated by a photodiode of a first phase detection autofocus pixel with a signal generated by a photodiode of a second phase detection autofocus pixel, the camera further comprising a camera lens, the camera being configured to move the camera lens along an optical axis of the camera lens based on said comparison of the signal generated by the photodiode of the first phase detection autofocus pixel with the signal generated by the photodiode of the second phase detection autofocus pixel, the camera comprising suitable means for moving, e.g. an actuator for moving.

[0030] In a preferred embodiment, the comparison is performed between a signal representing a first image generated by the photodiodes of the first plurality of phase detection autofocus pixels and a signal representing a second image generated by the photodiodes of the second plurality of phase detection autofocus pixels. If the first image is detected to be shifted with respect to the second image, the camera lens can be moved so that the first image is aligned with the second image.

[0031] Alternatively, the means for moving the camera lens may be configured to reduce, and preferably minimize, a difference between a signal from a photodiode of the first phase-detection autofocus pixel and a signal generated by a photodiode of the second phase-detection autofocus pixel. For example, the comparison may include determining whether a signal generated by a photodiode of a first phase-detection autofocus pixel of a pair of phase-detection autofocus pixels has a predefined shift compared to a signal generated by a photodiode of a second phase-detection autofocus pixel of the pair of phase-detection autofocus pixels. If not, the camera lens may be moved until the signal exhibits the predefined shift.

[0032] In a fourth aspect, the present invention relates to the use of a phase-detection autofocus pixel according to an embodiment of the first aspect of the invention for focusing a camera lens.

[0033] Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

[0034] The above and other characteristics, features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention. The reference figures quoted below refer to the attached drawings. [Brief description of the drawings]

[0035] [Figure 1A] FIG. 1 is a schematic diagram of a vertical cross section of a camera lens and an image sensor with a phase-detection autofocus pixel having a first photodiode S1 and a second photodiode S2 according to the prior art, where the camera lens focuses light from a point on an object onto a microlens of the image sensor. [Figure 1B] 1 is a schematic diagram of a vertical cross section of a camera lens and an image sensor with phase-detection autofocus pixels with a light shield according to the prior art, where the camera lens focuses light onto microlenses of the image sensor. [Diagram 2] FIG. 2 is a schematic diagram of a vertical cross section of a camera lens and an image sensor with a phase-detection autofocus pixel comprising a first photodiode S1 and a second photodiode S2 according to the prior art, the camera being unfocused. [Diagram 3] 1 is a plot of "S1 image" and "S2 image," which are images produced by first and second light beams incident on a plurality of first photodiodes S1 and a plurality of second photodiodes S2, respectively. [Figure 4A] 1 is a schematic diagram of a vertical cross section of a pair of phase-detection autofocus pixels according to the prior art, where a light beam having a particular angle of incidence is incident on a microlens of the phase-detection autofocus pixel, the inset on the left indicates the orientation of the angle of incidence. [Figure 4B] 1 is a plot of regular (non-PDAF) pixel quantum efficiency of a prior art, non-structurally optimized phase-detection autofocus pixel and a prior art, structurally optimized phase-detection autofocus pixel as a function of incidence angle. [Diagram 5]2 is a schematic diagram of a top view of a portion of an image sensor, in accordance with an embodiment of the present invention; [Figure 6] 2 is a side cross-sectional view of a portion of an image sensor according to an embodiment of the present invention. [Figure 7] 1 is a schematic diagram of a vertical cross-sectional view of a camera, in accordance with an embodiment of the present invention; [Figure 8] 1 is a graph illustrating modeling results for a range of phase detection autofocus pixels, including state-of-the-art phase detection autofocus pixels and phase detection autofocus pixels according to embodiments of the present invention.

[0036] In the different figures, the same reference signs refer to the same or similar elements. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are schematic and non-limiting. In the drawings, the size of some of the elements may be exaggerated for illustrative purposes and may not be drawn to scale. The dimensions and relative dimensions do not correspond to actual reductions to the practice of the invention.

[0038] The terms first, second, third, etc. in the description and claims are used to distinguish between similar elements and are not necessarily used to describe arrangement either in time, space, ranking, or in any other manner. It will be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operating in arrangements other than those described or illustrated herein.

[0039] Moreover, the terms "top," "bottom," "upper," "lower," and the like in this description and claims are used for descriptive purposes and not necessarily to describe relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in orientations other than those described or illustrated herein.

[0040] It should be noted that the term "comprising" as used in the claims and in the present description should not be interpreted as being limited to the subsequently recited means, and does not exclude other elements or steps. It is therefore interpreted as specifying the presence of the mentioned features, integers, steps, or components, but does not exclude the presence or addition of one or more other features, integers, steps, or components, or groups thereof. The term "comprising" therefore encompasses the situation where only the mentioned features are present, as well as the situation where these features and one or more other features are present. The word "comprising" according to the present invention therefore also includes, as an embodiment, the absence of further components. The scope of the expression "a device comprising means A and B" should therefore not be interpreted as being limited to a device consisting only of components A and B. This means that in the context of the present invention, the only relevant components of the device are A and B.

[0041] Similarly, it should be noted that the term "coupled" should not be interpreted as being limited to only direct connections. The terms "coupled" and "connected" may be used along with their derivatives. It should be understood that these terms are not intended as synonyms for each other. Thus, the scope of the phrase "device A coupled to device B" should not be limited to devices or systems in which the output of device A is directly connected to the input of device B. This means that there is a path between the output of A and the input of B, which may be a path that includes other devices or means. "Coupled" may mean that two or more elements are in direct physical or electrical contact, or that two or more elements are not in direct contact with each other, but still cooperate or interact with each other.

[0042] References throughout this specification to "one embodiment" or "an embodiment" mean that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification may, but do not necessarily all refer to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments, as would be apparent to one of ordinary skill in the art from this disclosure.

[0043] Similarly, in describing exemplary embodiments of the invention, it should be understood that various features of the invention may be grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in understanding one or more of the various aspects of the invention. However, this method of disclosure should not be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, aspects of the invention include less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim being claimed as a separate embodiment of the invention.

[0044] Furthermore, some embodiments described herein include some features included in other embodiments but not others, but combinations of features of different embodiments are meant to be within the scope of the present invention and form different embodiments, as would be understood by one of ordinary skill in the art. For example, in the following claims, any of the claimed embodiments may be used in any combination.

[0045] Furthermore, some of the embodiments are described herein as methods or combinations of elements of methods that may be implemented by a processor of a computer system or by other means for performing functions. Thus, a processor with the necessary instructions for carrying out such a method or element of a method forms a means for carrying out the method or element of a method. Furthermore, the elements described herein of the apparatus embodiments are examples of means for carrying out the functions performed by the elements for carrying out the invention.

[0046] In the description provided herein, numerous specific details are set forth. However, it will be understood that embodiments of the present invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

[0047] The present invention will now be described by detailed descriptions of some embodiments of the present invention. Other embodiments of the present invention can be constructed according to the knowledge of those skilled in the art without departing from the technical teachings of the present invention, and it is clear that the present invention is limited only by the terms of the appended claims.

[0048] In a first aspect, the present invention relates to a phase-detection autofocus pixel comprising a photodiode and a plurality of microlenses for focusing light incident on each of the microlenses onto the photodiode. The phase-detection autofocus pixel further comprises a light shield associated with each microlens for blocking, for each microlens, a portion of light transmitted by the microlens towards the photodiode. The light shields are offset in the same offset direction from an optical axis of the light shield's associated microlens.

[0049] In a second aspect, the present invention relates to an image sensor comprising a pair of phase-detection autofocus pixels, the pair comprising a first phase-detection autofocus pixel according to an embodiment of the first aspect of the invention, wherein a light shield of the first phase-detection autofocus pixel is each offset in a first direction from an optical axis of an associated microlens of the light shield, and further comprising a second phase-detection autofocus pixel according to an embodiment of the first aspect of the invention, wherein a light shield of the second phase-detection autofocus pixel is each offset in a second direction substantially opposite to the first direction from an optical axis of an associated microlens of the light shield.

[0050] Reference is made to FIG. 5, which is a schematic diagram of a top view of a portion of an image sensor 6 according to an embodiment of the present invention. Reference is also made to FIG. 6, which is a schematic diagram of a vertical cross section of a portion of the image sensor 6 along the dashed line of FIG. 5. In the illustrated example, the portion of the image sensor 6 comprises a plurality of pixels, including two phase-detection autofocus pixels 1 and 10 according to an embodiment of the present invention, and six regular pixels 100, i.e. pixels configured to detect light for detecting an image, but not for performing phase-detection autofocus. In the illustrated embodiment, the regular pixels and the PDAF pixels have microlenses of different sizes. However, this is not necessary for the present invention, and all or substantially all microlenses may be equal to each other. Microlenses of different sizes or shapes are possible, but often make the manufacturing process more complicated.

[0051] Each of the two phase-detection autofocus pixels 1, 10 comprises a single photodiode 2, 20. The first phase-detection autofocus pixel 1 of the two phase-detection autofocus pixels 1, 10 comprises, in the example shown, four microlenses 3 for concentrating light incident on each of the microlenses 3 onto the photodiode 2 of the first phase-detection autofocus pixel 1 via a transparent planarizing layer 72 and optionally a color filter 71. The second phase-detection autofocus pixel 10 of the two phase-detection autofocus pixels 1, 10 comprises, in the example shown, four microlenses 30 for concentrating light incident on each of the microlenses 30 onto the photodiode 20 of the second phase-detection autofocus pixel 10 via a transparent planarizing layer 72 and optionally a color filter 71. Herein, the microlenses 3 and 30 in each phase-detection autofocus pixel 1, 10 are arranged in a square array of 2x2 microlenses 3, 30, i.e., the microlenses 3 and 30 are ordered into two rows 32 and two columns 33. In general, each autofocus pixel may comprise a number of regularly or irregularly ordered microlenses, preferably a rectangular array of NxM microlenses, preferably a square array of NxN microlenses.

[0052] In the illustrated example, each regular pixel 100 comprises a single microlens 300 for concentrating, e.g., focusing, light onto the photodiode 200 of the regular pixel 100 through a transparent planarizing layer 72 and optionally a color filter 71.

[0053] Each microlens 3, 30, 300 of a particular pixel 1, 10, 100 may ensure that light incident on the microlens 3, 30, 300 propagates to the photodiode 2, 20, 200 of that particular pixel 1, 10, 100, thereby preventing the light from propagating to adjacent pixels 1, 10, 100. Furthermore, as described in the background of the invention, preferably the microlens 3, 30 of a phase-detection autofocus pixel 1, 10 focuses light having a particular angle of incidence at the plane of the light shield 4, 40. In other words, preferably the microlens 3, 30 of a phase-detection autofocus pixel 1, 10 has a focal length substantially equal to the distance between the microlens 3, 30 and its associated light shield 4, 40.

[0054] Each of the regular pixels 100 comprises in the illustrated example a single microlens 300, but alternatively each of the regular pixels 100 may for example also comprise a regular array of multiple, for example four, microlenses. The number and ordering of the microlenses may be the same as or different from the number and ordering of the microlenses of the autofocus pixels. The microlenses of the regular pixels 100 may have substantially the same shape and size as the microlenses 3, 30 of the phase-detection autofocus pixels 1, 10. This may facilitate the manufacture of the image sensor 6.

[0055] The color filter 71 may block light that is not desired to be detected, e.g. light that is not in the visible spectrum but would unnecessarily result in the generation of a signal by the photodiode 2, 20, 200, from reaching the photodiode 2, 20, 200. Furthermore, the color filter 71 may only allow a limited portion of the visible spectrum to pass, e.g. red or blue or green. For example, different regular pixels 100 may have different color filters 71 to allow different portions of the visible spectrum to pass, so that the regular pixels 100 can be used as color pixels.

[0056] The transparent planarizing layer 72 may provide a uniformly planar top surface 70 on which the microlenses 3, 30, 300 are formed, such that the bottom surfaces of the microlenses 3, 30, 300 may be substantially planar, facilitating good focusing by the microlenses 3, 30, 300.

[0057] In this example, each microlens 3, 30, 300 has the shape of a rectangular parallelepiped with rounded corners, but the microlenses 3, 30, 300 may have any shape, for example each microlens 3, 30, 300 may be a spherical lens.

[0058] Each microlens 3 of the first phase-detection autofocus pixel 1 is associated with a light shield 4 for blocking a portion of light transmitted by the microlens 3 towards the photodiode 2 of the first phase-detection autofocus pixel 1. Herein, the light shield 4 of the first phase-detection autofocus pixel 1 is offset by a distance d from the optical axis 31 of the associated microlens 3 of the light shield in the same first offset direction X. In the illustrated example, the light shield 4 associated with the microlens 3 of the first phase-detection autofocus pixel 1 blocks substantially all light transmitted by the associated microlens 3 that would be collected by the photodiode 2 on that side of the optical axis 31 in the first offset direction X in the absence of the light shield 4. Similarly, each microlens 30 of the second phase-detection autofocus pixel 10 is associated with a light shield 40 for blocking a portion of light transmitted by the microlens 30 towards the photodiode 20 of the second phase-detection autofocus pixel 10. Herein, the light shield 40 is offset by a distance d from the optical axis 310 of its associated microlens 30 in the same second offset direction -X opposite to the first offset direction X. In the illustrated example, the light shield 40 associated with the microlens 30 of the second phase-detection autofocus pixel 10 blocks substantially all light transmitted by the associated microlens 30 that would be collected by the photodiode 20 on the side of that optical axis 310 in the second offset direction -X in the absence of the light shield 40. As can be observed in Figures 5 and 6, in the illustrated example, each light shield 4, 40 covers the entire portion of the photodiode 1, 10 that overlaps with half of the associated microlens 3, 30 on the side of the optical axis 31, 310 of the associated microlens 3, 30 in the respective offset direction X, -X, leaving the remaining part of the photodiode 1, 10 exposed. In the embodiment illustrated in Figures 5 and 6, all light shields have the same width. This is not necessarily the case. For PDAF pixels in the center of the image sensor, their width should be the same, while for pixels closer to the edges of the imager, the widths may differ.

[0059] In the illustrated example, each of the light shields 4, 40 has substantially the same shape and size. The light shields 4, 40 are offset by substantially the same distance d from the optical axis 31, 310 of the associated microlens 3, 30 of the light shield. Herein, the distance d may be the distance between the center of the light shield 4, 40 and the optical axis 31, 310 of the associated microlens 3, 30 of the light shield. The light shields 4, 40 are also located at the same distance from the associated microlens 3, 30. Although in FIG. 5 the light shields 4, 40 are shown separated from each other, this is not necessary. For example, the light shields 4, 40 that are in physical contact with each other may instead be integral with each other, which may facilitate manufacturing.

[0060] In the illustrated example, the light shields 4, 40 are offset in a direction along X, but the offset can be in any horizontal direction, i.e. in any direction in the plane of the image sensor 6, i.e. in a plane perpendicular to the optical axis 31, 310. For example, alternatively, the offset can be along a direction Y perpendicular to X. In that case, the light shields 4, 40 are typically rotated by 90° in the X, Y plane. The image sensor can comprise a first pair of phase-detection autofocus pixels 1, 10 with a light shield 4, 40 offset along the direction X and a second pair of phase-detection autofocus pixels with a light shield offset along a direction Y perpendicular to X. This can improve focusing accuracy. Furthermore, in yet another embodiment, the third pair of PDAF pixels can rotate their light shields in the X, Y plane by 45° compared to the orientation of the light shields of the first or second pair of PDAF pixels. In further alternative embodiments, other rotation angles for some light shields of the PDAF pixel pairs can also be implemented.

[0061] 6, the phase-detection autofocus pixels 1, 10 have color filters, as does the regular pixel 100. Note that if color filters are used for the phase-detection autofocus pixels, the PDAF pixel pairs have the same color filter (e.g., R, or G, or B).

[0062] In a third aspect, the present invention relates to a camera comprising an image sensor according to an embodiment of the second aspect of the present invention, comprising a device configured to compare a signal generated by a photodiode of a first phase detection autofocus pixel with a signal generated by a photodiode of a second phase detection autofocus pixel, the camera further comprising a camera lens, the camera configured to move the camera lens along an optical axis of the camera lens based on said comparison of the signal generated by the photodiode of the first phase detection autofocus pixel with the signal generated by the photodiode of the second phase detection autofocus pixel.

[0063] Typically, cameras according to embodiments of the present invention may function similarly and have similar features as state-of-the-art cameras, except that each phase-detection autofocus pixel according to embodiments of the present invention includes two or more microlenses instead of a single microlens, and a light shield associated with each microlens, and a light shield associated therewith for state-of-the-art phase-detection autofocus pixels.

[0064] Reference is made to Figure 7 which illustrates a camera 8 according to an embodiment of the present invention. The camera 8 comprises an image sensor 6, only a portion of which is shown. The image sensor 6 includes a plurality of phase detection autofocus pixels 1,10.

[0065] The image sensor 6 comprises a comparison device 60 configured to compare a signal generated by the photodiode 2 of a first phase-detection autofocus pixel 1 (e.g., a first image) with a signal generated by the photodiode 20 of a second phase-detection autofocus pixel 10 of a plurality of pairs of the phase-detection autofocus pixels 1,10 (e.g., a second image). In particular, the comparison device 60 is communicatively coupled to the photodiode 2 of each first phase-detection autofocus pixel 1 and to the photodiode 20 of each second phase-detection autofocus pixel 10 for receiving a signal from the photodiode 2,20 of the phase-detection autofocus pixels 1,10. Here, it is assumed that the signal generated in each photodiode 2,20 is proportional to the amount of light incident on the photodiode 2,20. The comparison device 60 may include a processor for performing the comparison.

[0066] The camera 8 further comprises a collection device (not shown) for collecting the signals generated by the photodiodes 200 of the regular pixels 100. The comparison device 60 for the comparison and the collection device for collecting the signals generated by the photodiodes 200 of the regular pixels 100 may be the same device, e.g. the same processor, or different devices, e.g. different processors.

[0067] The camera 8 further comprises a camera lens 5. Similar to the prior art and as described elsewhere in the description, the signals from the first phase-detection autofocus pixel 1 and the second phase-detection autofocus pixel 10 may be compared by a comparison device 60, which may induce an actuator 51 to move the camera lens 5 along an optical axis 50 of the camera lens 5 based on the comparison. In particular, partially repeating the description in the background of the invention, it may be assumed that light from each point 801, 802 on the object 80 is split into a first light beam L1 and a second light beam L2 propagating through different halves of the camera lens 5.

[0068] 7, when the camera 8 is said to be focused with respect to a subject 80, the light collected by the camera lens 5 from each particular point 801, 802 on that subject 80 is focused in the plane of the microlenses 3, 30, 300. In this way, a sharp image of the subject 80 is projected by the camera lens 5 onto the image sensor 6.

[0069] For simplicity, in Fig. 7 the propagation paths of light beams L1, L2 from two different adjacent points 801 and 802 on the object 80 are shown. Similar to prior art phase detection autofocus pixels, in an ideal case with optimal focusing capabilities of the microlenses, when the camera 8 is in focus, for a light beam L1, L2 (e.g., light beam L1, L2 from the first point 801) focused on the microlens 3 of one of the first phase detection autofocus pixels 1, the first light beam L1 is detected by the photodiode 2 of the first phase detection autofocus pixel 1, and the light shield 4 associated with the microlens 3 blocks the second light beam L2. For a light beam L1, L2 focused on one microlens 3 of the second phase detection autofocus pixel 10 (e.g., light beam L1, L2 from the second point 802), the second light beam L2 is detected by the photodiode 20 of the second phase detection autofocus pixel 10, and the light shield 40 associated with the microlens 30 blocks the first light beam L1.

[0070] As a result, when the camera 8 is in focus, a first image of the subject 80 generated by the first phase-detection autofocus pixel 1 has a predefined offset compared to a second image of the subject generated by the second phase-detection autofocus pixel 10. This predefined offset takes into account the physical shift between the two centers of the PDAF pixel pairs.

[0071] Instead, when the camera 8 is not focused on the object 80, the first light beam L1 and the second light beam L2 from each point 801, 802 of the object 80 are focused to a point away from the plane of the microlenses 3, 30, 300, so that the first and second light beams L1, L2 from each point 801, 802 of the object 80 are incident on different pixels 1, 10, 100. As a result, as shown in FIG. 3 and described in the background of the invention, a first image ("S1 image") generated by the first phase-detection autofocus pixel 1 detecting the first light beam L1 and a second image ("S2 image") generated by the second phase-detection autofocus pixel 10 detecting the second light beam L2 will be shifted with respect to each other by more than a predefined offset. When the camera 8 is not focused, the camera lens 5 may be moved by the actuator 51 to overlap the first and second images, so that the camera 8 is focused. 3, if the S1 image comes in front of the S2 image, a rear focus situation occurs, and if the S2 image comes in front of the S1 image (not shown), a front focus situation occurs. Depending on which situation occurs, the actuator can determine which direction the camera lens must move to achieve focus.

[0072] The inset in the upper right corner shows the orientation of the incidence angle α of light incident on the microlens 3, 30, where the incidence angle α lies in a plane parallel to the optical axis 31, 310 of the microlens 3, 30 and the X-axis, with respect to normal incidence, i.e. along the optical axis 31, 310 of the microlens 3, 30. Here, the direction of the incident light with an X component pointing in the X direction has a negative incidence angle α, and the direction of the incident light with an X component pointing in the -X direction has a positive incidence angle α.

[0073] The idealized functionality described above means perfect selectivity of the first phase-detection autofocus pixel 1 to the first light beam L1, and perfect selectivity of the second phase-detection pixel 10 to the second light beam L2. In the illustrated example, this is achieved for the first phase-detection autofocus pixel 1 when all light incident on each microlens 3 of each first phase-detection autofocus pixel 1 at an angle α less than 0° (i.e., the second light beam L2) is blocked by the light shield 4 associated with the microlens 3, and all light incident on the microlens 3 at an angle α greater than 0° (i.e., the first light beam L1) is detected by the photodiode 2 of the first phase-detection autofocus pixel 1. Similarly, in the illustrated example, for the second phase detection autofocus pixel 10, this perfect selectivity is achieved when all light incident on each microlens 30 of each second phase detection autofocus pixel 10 at an angle α greater than 0° is blocked by the light shield 40 associated with the microlens 30, and all light incident on the microlens 30 at an angle α less than 0° is detected by the photodiode 20 of the second phase detection autofocus pixel 10.

[0074] See FIG. 8, which is a plot of modeled quantum efficiency as a function of the incidence angle α for different, large, first phase-detection autofocus pixels, where the photodiodes of the different pixels are of equal size. The simulation data was obtained through TCAD simulations (Ansys Lumerical optoelectronic device simulator). The curve annotated with diamond shapes is the angle-dependent quantum efficiency of a large, first phase-detection autofocus pixel, which according to the state of the art includes only a single microlens with a height of 2.7 μm and a single light shield associated with it. Due to the size of the state of the art phase-detection autofocus pixel (7.2 μm for the pixel in the simulation), the formation of the pixel's components is done at the edge of processing power. For example, the shape of the microlens is particularly poor, since the height and the radius of curvature of the microlens are so large that known manufacturing techniques do not result in well-formed microlenses in this case. As a result, the focusing of light for each incidence angle by the microlens in the plane of the light shield is poor. In fact, as is evident from Figure 8, light incident at an incidence angle α below zero is not blocked at all as efficiently by the light shield of the state-of-the-art first phase-detection autofocus pixel as compared to light incident at an incidence angle α above zero. This poor selectivity results in poor contrast between the first and second images produced by such large state-of-the-art first and second phase-detection autofocus pixels, and thus poor autofocus functionality.

[0075] For comparison, the unannotated curve shows the angular dependent quantum efficiency of a regular pixel with a single microlens with a height of 2.7 μm, without a light shield. The regular pixel is therefore equally sensitive to light with positive and negative angles of incidence, in other words it does not show any selectivity.

[0076] In FIG. 8, the curve annotated with black squares corresponds to a large phase-detection autofocus pixel (7.2 μm wide) according to an embodiment of the present invention, with an array of 2×2 microlenses, each with a height of 2.7 μm, each microlens with an associated light shield that blocks all light propagating towards a part of the photodiode of the phase-detection autofocus pixel in a first direction of the optical axis of the microlens. It is noted that the single microlens and the array of 2×2 microlenses used in the different simulations have the same thickness of 2.7 μm. The same thickness of the microlenses on a smaller pitch results in a larger curvature of the microlenses and therefore a higher focusing ability, as the smaller microlenses (for a given planarization and color filter thickness) are better able to focus the light with the light shield. The curve annotated with triangles shows the situation of a similar phase-detection autofocus pixel according to an embodiment of the present invention, but with 4×4 microlenses (with associated light shields), each microlens with a height of 1.8 μm. The curve annotated with circles shows the situation for a similar phase-detection autofocus pixel according to an embodiment of the present invention, but with 3×3 microlenses (with associated light shields), each with a height of 1.2 μm. It can be observed that each of the exemplary first phase-detection autofocus pixels according to an embodiment of the present invention has very good selectivity for light beams with positive angles of incidence α, with almost all light with negative angles of incidence α being blocked by the light shield, and a good signal being obtained from light with positive angles of incidence α.

[0077] Thus, according to embodiments of the present invention, when using phase detection autofocus pixels, each comprising multiple microlenses and each microlens having an associated light shield, even in the case of large pixels and correspondingly large photodiodes, the processing steps used to form the microlenses and light stacks can be performed in the range for which they are optimized. Thus, even for image sensors with large pixels, and therefore large phase detection autofocus pixels, good focusing of light for each angle of incidence by the microlenses in the plane of the light shield can be obtained, and the selectivity and therefore the autofocus function can be good.

[0078] Although preferred embodiments, specific structures and configurations, and materials have been discussed herein for devices in accordance with the present invention, it should be understood that various changes or modifications in form and detail may be made without departing from the scope of the invention. Steps may be added or deleted to the methods described within the scope of the invention.

Claims

1. A phase detection autofocus pixel (1), comprising: A photodiode (2); a plurality of microlenses (3) for concentrating light onto the photodiode (2), the light being incident on each of the microlenses (3); and for each microlens (3), a light shield (4) associated with the microlens (3) to block a portion of the light transmitted by the microlens (3) towards the photodiode (2), the light shield (4) being offset in the same offset direction (X) from an optical axis (31) of the microlens (3) with which the light shield (4) is associated.

2. The phase-detection autofocus pixel (1) of claim 1, wherein the plurality of microlenses (3) are organized in a square array.

3. 3. The phase-detection autofocus pixel (1) of claim 2, wherein the square array of microlenses (3) comprises rows and columns of microlenses, each row (32) and column (33) comprising at least two microlenses (3).

4. The phase detection autofocus pixel (1) of claim 1, wherein the plurality of microlenses (3) are substantially uniform in shape and size.

5. 2. The phase detection autofocus pixel (1) of claim 1, wherein the light shield (4) associated with a microlens (3) blocks substantially all light transmitted by the associated microlens (3) that would be collected by the photodiode (2) on the side of the optical axis (31) in the offset direction (X) in the absence of the microlens (3).

6. 2. The phase-detection autofocus pixel (1) of claim 1, wherein each light shield (4) is made from metal.

7. 2. The phase detection autofocus pixel (1) of claim 1, wherein the light shields (4) have substantially the same dimensions and are offset by substantially the same distance from the optical axis (31) of their associated microlenses (3).

8. An image sensor (6) comprising a pair of phase-detection autofocus pixels (1, 10), A first phase-detection autofocus pixel (1) according to any one of claims 1 to 7, wherein light shields (4) of the first phase-detection autofocus pixel (1) are each offset in a first offset direction (X) from an optical axis (31) of an associated microlens (3) of the light shield (4); 11. An image sensor (6) comprising: a second phase-detection autofocus pixel (10) according to any of the preceding claims, wherein a light shield (40) of the second phase-detection autofocus pixel (10) is each offset from an optical axis (310) of an associated microlens (30) of the light shield (40) in a second offset direction (-X) substantially opposite to the first offset direction (X).

9. 9. The image sensor (6) of claim 8, further comprising a device (60) configured to compare a signal generated by a photodiode (2) of the first phase detection autofocus pixel (1) with a signal generated by a photodiode (20) of the second phase detection autofocus pixel (10).

10. A camera (8) comprising an image sensor (6) according to claim 9, further comprising a camera lens (5), The camera (8), wherein the camera is configured to move the camera lens (5) along an optical axis (50) of the camera lens (5) based on the comparison between the signal generated by the photodiode (2) of the first phase detection autofocus pixel (1) and the signal generated by the photodiode (20) of the second phase detection autofocus pixel (10).

Images