Image sensor and method of manufacturing the same

Abstract

A diffuser disposed between a microlens and a photodiode has a structure that redirects incident light to provide a favorable angle, thereby increasing the path length through the substrate and thus improving the absorption probability of perpendicularly incident light and light tilted from the periphery of the microlens towards the center of the pixel region. The diffuser comprises multiple gratings formed by recesses in a semiconductor substrate containing the photodiode. One of the gratings is disposed in each of the four quadrants of a region centered below the microlens. Each quadrant has a centerline extending radially from the center of that region. The gratings are oriented parallel to their respective centerlines and perpendicular to the gratings of adjacent quadrants. The gratings may include trenches with a V-shaped profile. Embodiments of this application provide an image sensor and a method of manufacturing the same.

Description

Technical Field

[0001] Embodiments of this application relate to image sensors and methods of manufacturing the same. Background Technology

[0002] Many modern electronic devices, such as digital cameras and optical imaging devices, incorporate image sensors. An image sensor contains an array of photosensitive regions that act as converters, converting light into electrical charge. Examples of image sensors include charge-coupled device (CCD) image sensors and complementary metal-oxide-semiconductor (CMOS) image sensors. Compared to CCD image sensors, CMOS image sensors are favored for their low power consumption, small size, fast data processing, direct data output, and low manufacturing cost. One type of CMOS image sensor is the back-illuminated (BSI) CMOS image sensor. Summary of the Invention

[0003] Embodiments of this application provide an image sensor, comprising: a substrate having a first side and a second side opposite the first side in thickness of the substrate; a microlens disposed on the second side; and a diffuser including a recess in the second side, wherein the diffuser is located between the microlens and the substrate and is divisible into four quadrants, each quadrant including a grating having a plurality of first planar surfaces in different planes, the plurality of first planar surfaces being parallel to each other, and each grating having the same orientation as the grating in the adjacent quadrant and a different orientation from the grating in the adjacent quadrant.

[0004] Embodiments of this application provide a method for manufacturing an image sensor, the method comprising: providing a substrate having a first side and a second side opposite the first side in thickness; forming a mask over the second side, wherein the mask has a pattern of openings, the pattern of openings being a combination of four sub-patterns arranged in a 90-degree rotational symmetry, and the pattern of openings including L-shaped openings; etching through the mask by an anisotropic wet etching process to form a recessed pattern in the second side; and depositing a first dielectric within the recess, wherein the first dielectric has a refractive index lower than that of the substrate.

[0005] Embodiments of this application provide a method for manufacturing an image sensor, the method comprising: providing a substrate having a first side and a second side opposite the first side in thickness; forming a mask over the second side, wherein the mask has a pattern of openings that can be divided into four quadrants, each quadrant including a plurality of parallel slits, the slits in opposite quadrants having the same orientation and the slits in adjacent quadrants having different orientations; etching through the mask by an anisotropic wet etching process to form a recessed pattern in the second side; and depositing a first dielectric in the recess, wherein the first dielectric has a refractive index lower than that of the substrate. Attached Figure Description

[0006] The following detailed description, taken in conjunction with the accompanying drawings, will provide the best understanding of all aspects of this disclosure. In accordance with industry standard practice, the features are not drawn to scale. Furthermore, for ease of illustration or to provide emphasis, the dimensions of features in a single drawing may be arbitrarily increased or decreased relative to each other.

[0007] Figure 1A A top view of an embodiment of an image sensor with a diffuser according to this disclosure is shown.

[0008] Figures 1B to 1D It shows Figure 1A A cross-sectional view of the image sensor.

[0009] Figure 2A A three-dimensional view of a microlens that focuses light onto a diffuser is shown.

[0010] Figures 2B to 2C A three-dimensional diagram showing the interaction between light from the microlens and the grooves that are part of the diffuser disclosed herein is presented.

[0011] Figures 3 to 4 A top view of an image sensor with a diffuser according to various embodiments is shown.

[0012] Figures 5A to 6B Paired top and cross-sectional views of image sensors with diffusers according to various embodiments are shown.

[0013] Figures 7A to 9B A top view of an image sensor with a diffuser according to various embodiments is shown.

[0014] Figures 10 to 18 ( Figures 13A to 13C Except for the image sensor manufactured according to the process disclosed herein, a cross-sectional view is shown during the manufacturing process.

[0015] Figures 13A to 13C It shows that it can be used Figures 10 to 18 The mask used in the manufacturing process shown. Detailed Implementation

[0016] This disclosure provides numerous different embodiments or examples for implementing various features of this disclosure. Specific examples of elements and arrangements are described below to simplify this disclosure. Of course, these are merely examples and are not intended to be limiting. For example, in the following description, forming a first component above or on a second component may include embodiments where the first and second components are in direct contact, and may also include embodiments where an additional component may be formed between the first and second components, such that the first and second components are not in direct contact.

[0017] Spatial relative terms, such as “below,” “under,” “lower,” “above,” “upper,” etc., are used herein to describe the relationship between one element or component and another element or component shown in the accompanying drawings. These spatial relative terms are intended to cover different orientations of the device or apparatus in use or operation other than those depicted in the drawings. The device may be oriented in other ways (e.g., rotated 90 degrees or in other orientations), and the spatial relative descriptors used herein should be interpreted accordingly. The terms “first,” “second,” “third,” “fourth,” etc., are merely general identifiers and are therefore interchangeable in various embodiments. For example, while an element (e.g., an opening) may be referred to as a “first” element in some embodiments, it may be referred to as a “second” element in other embodiments.

[0018] Complementary metal-oxide-semiconductor (CMOS) image sensors contain an array of photodiodes formed within a semiconductor substrate. When the substrate absorbs incident radiation, electron-hole pairs are generated, thus detecting the incident radiation. Pixel circuitry (containing one of the photodiodes) converts the generated charge carriers into electrical signals.

[0019] Semiconductor substrates typically contain monocrystalline silicon, primarily due to its low cost and widespread availability in the industry. However, monocrystalline silicon exhibits a relatively low absorption coefficient for near-infrared (NIR) and other long-wavelength radiation. As photodetector pixels continue to shrink, this low absorption becomes increasingly detrimental, leading to a decrease in quantum efficiency (QE).

[0020] To enhance quantum efficiency, techniques such as back-illuminated arrays, inter-pixel reflectors, and microlenses are commonly employed. A less common but effective approach involves the use of diffusers. Diffusers increase the optical path length within the substrate by redirecting incident light, thereby increasing the likelihood of photon absorption. Diffusers can be implemented by etching patterns (such as gratings or inverted pyramids) onto the back surface of the semiconductor substrate (located between the microlens and the photodiode). These etched features are typically filled with a material whose refractive index is lower than that of the semiconductor substrate.

[0021] Diffusers redirect incident light within an angular range, with the resulting angular distribution determined by both the incident light direction and the diffuser's surface geometry. For example, light entering through the center of a microlens typically strikes the pixel surface near normal incidence, while light entering through the periphery of the microlens is typically bent to a more angled path. Inverted pyramid diffusers are particularly effective at redirecting normally incident light to a more angled path, thus enhancing absorption. However, they may also deflect already angled light (originating from the periphery of the microlens) to a steeper angle, potentially reducing overall absorption efficiency.

[0022] This disclosure provides a diffuser configured to redirect incident light at an angular distribution with peak values ​​between approximately 30 and 40 degrees, a range found to be particularly effective in enhancing absorption efficiency. The diffuser comprises four gratings, each arranged in one of four quadrants in a region substantially below a microlens. Within each quadrant, the grating comprises one or more mutually parallel grooves. Each grating has the same orientation as the grating in the corresponding quadrant and a different orientation than the grating in the adjacent quadrant. The grating orientations between adjacent quadrants differ by 90 degrees. The grooves in each quadrant are substantially parallel to a radial line extending from the center of the microlens region to the corresponding quadrant. This radial line may be the centerline of the quadrant.

[0023] In some embodiments, the grating includes grooves with planar sidewalls and inverted pyramid features. A first set of sidewalls is parallel to each other and parallel to the radial lines of the corresponding quadrant. A second set of sidewalls with different orientations is also parallel to each other and aligned with the same radial lines. Each groove contains one sidewall from each set. These repeating geometries define the grating orientation within each quadrant.

[0024] The grating described herein performs comparably to the inverted pyramid in reorienting normal incident light, but exhibits superior performance when handling oblique incident light originating from the periphery of the microlens. Related inverted pyramid diffusers can improve quantum efficiency by up to approximately 38%, while the diffuser disclosed in this application achieves a quantum efficiency gain of approximately 45% or higher under similar conditions.

[0025] In some embodiments, each grating is mirror-symmetric with respect to gratings in adjacent quadrants. In some embodiments, the gratings are rotationally symmetric about the center of the region below the microlens. Furthermore, each grating may be symmetric about its respective centerline. These symmetrical configurations are typical features of gratings that are easy to manufacture, and as taught herein, by varying the orientation in the four quadrants, absorption of normal incident light and light redirected to the center by the microlens can be enhanced.

[0026] In some embodiments, the grating includes trenches formed in a semiconductor substrate and having a V-shaped cross-sectional profile. Trenches with such a profile exhibit superior light-trapping capabilities compared to trenches with vertical sidewalls or sloping sidewalls terminating at a flat bottom. The sidewalls may be tilted at an angle of about 15 degrees to about 45 degrees with respect to the vertical direction to provide a suitable trajectory for the normal incident light. In some embodiments, adjacent trenches converge at the V-shaped peak, which further enhances the light-trapping capability compared to a configuration where adjacent recesses are separated by flat regions.

[0027] In some embodiments, the semiconductor substrate has a (100) crystal orientation, and the trenches are along... <110> Crystallographic orientations extend. These orientations support the formation of V-shaped trench profiles and enable undercutting below the mask layer, allowing adjacent trenches to merge at the V-peak. The depth of the trench can be proportional to its width. In some embodiments, the trench aspect ratio is approximately 0.7, and the sidewalls are inclined at approximately 27 degrees to the vertical direction, which is typical of trenches formed by wet etching of a semiconductor substrate with a (100) crystal orientation.

[0028] In some embodiments, the diffuser includes gratings arranged in each of the four quadrants of a central region below the microlens. Each quadrant has a centerline extending radially from the center of the region. The gratings include grooves oriented parallel to their respective centerlines. In some embodiments, the grooves in adjacent quadrants intersect to form an L-shaped groove, wherein each groove arm is aligned at a right angle and contributes to an independent grating in a different quadrant. The arms of the L-shaped groove are generally of equal length. In some embodiments, each grating includes an even number of elements; in other embodiments, it includes an odd number of elements. When the gratings are periodic and contain an odd number of elements, the diffuser includes a centrally located cross-shaped groove consisting of four arms, each arm forming part of a grating in one of the four quadrants.

[0029] Each grating may contain parallel trenches of varying lengths. In some embodiments, the longest trench is located near the grating centerline, with the trench length gradually decreasing towards both sides of the quadrant. In some structures, the outermost element of each grating is not a trench, but rather half of an inverted pyramidal recess formed in the semiconductor substrate. In some embodiments, the diffuser includes four such inverted pyramidal recesses, corresponding to the outermost elements of the four gratings. These geometries may be derived from adapting the diffuser to a shape profile (e.g., a profile defined by the pixel regions of the photodetector).

[0030] In some embodiments, the recesses forming the grating are deepest and widest near their respective center lines, gradually becoming shallower and narrower towards the outer edge of the grating. Since the center lines of the four quadrants intersect at the center of the region beneath the microlens, the deepest and widest grooves converge at the center. Making the recesses deeper at the center of the region beneath the microlens facilitates light capture.

[0031] In some embodiments, the recess forming the diffuser is confined within a central region below the microlens. The diffuser may occupy half or less of this region; in some embodiments, a quarter or less; in other embodiments, an eighth or less. When combined with a microlens that effectively focuses incident light to the central region, confining the diffuser region in this manner promotes light trapping and improves absorption efficiency.

[0032] Figure 1AA top view of an image sensor 100 is shown, which includes a diffuser 107A located within a pixel region 127 of a semiconductor substrate 113. The pixel region 127 and the diffuser 107A are located below the center of a microlens 105 (or equivalently, the microlens 105 is located above the center of the pixel region 127 and the diffuser 107A). Figures 1B to 1D They respectively show along Figure 1A A cross-sectional view taken along lines B-B', C-C', and D-D'. The diffuser 107A contains four gratings 125A, one of which is in... Figure 1A The images are marked and shaded for reference. These four gratings 125A are arranged with 90-degree rotational symmetry around the center of pixel region 127. Each grating 125A includes a set of recesses 153 formed on the back side 157 of the semiconductor substrate 113 (see...). Figures 1B to 1D Some recesses 153 are shared between adjacent gratings 125A.

[0033] The recess 153 comprises four inverted pyramids 101 and eight L-shaped grooves 103, one of which is in Figure 1A The text is marked and shaded. Both groove 103 and inverted pyramid 101 have flat sidewalls, referred to as facets 123. The intersections of these facets 123 are illustrated with different line types: solid lines indicate raised edges (V-shaped peaks 121); dashed lines 117 mark the intersections located at the bottom of groove 103; and short dashed lines 119 show the sloping intersections between facets. Each L-shaped groove 103 has two groove arms 103A, each arm forming part of a different grating 125A. The groove arms 103A have a V-shaped cross-sectional profile.

[0034] Pixel region 127 is part of semiconductor substrate 113 and is separated by isolation structures (e.g., back-side deep trench isolation (BDTI) structure 145, see...) Figure 1C The diffuser 107A is defined by a medium-refractive-index dielectric layer 111 and a low-refractive-index dielectric layer 109 that fill the BDTI structure 145 and extend across the back side 157 to fill the recess 153. In some embodiments, the BDTI structure 145 includes a metal core (not shown) to enhance reflectivity. Although the BDTI structure is depicted and has certain advantages, it can be replaced by a front-side deep trench isolation (FDTI) structure. Furthermore, a back-side metal mesh 149 may be arranged on the back side 157 above the BDTI structure 145 to further enhance light trapping and reduce optical crosstalk.

[0035] Each grating 125A is mirror-symmetrical with respect to its respective center line 115. These center lines 115 extend radially from the center of the pixel region 127, which coincides with the center of the region below the microlens 105. Adjacent gratings 125A are separated by additional radial lines, which also emanate from this center point. Thus, the diffuser 107A can be conceptually divided into eight segments by eight radial lines spaced at 45-degree intervals. Each segment contains a portion of the grating, which is symmetrical to portions of its adjacent segments by reflections across the radial lines that separate them.

[0036] Gratings 125A are oriented parallel to their respective centerlines 115, such that their trench arms 103A extend parallel to their respective centerlines 115. Within each grating 125A, the trench sidewalls include facets 123, comprising two distinct groups: facets 123A and facets 123B. Facets 123A are parallel to each other and parallel to the centerline 115 of the grating. Facets 123B, although oriented differently from facets 123A, are also parallel to each other and aligned with the same centerline 115. The pairs of facets 123A and 123B together form the angled sidewalls of the V-shaped trench arms 103A. This configuration, in which the facets 123 of the recess 153 are parallel to the centerline 115, and the centerline 115 is a radial line extending from the center of the region below the microlens 105, ensures that the grating orientation provides a consistently tilted angle for light to enter the semiconductor substrate 113, compared to other diffuser geometries that can be fabricated using related process techniques.

[0037] Figures 2A to 2C This demonstrates the advantages of the grating and facet orientation described herein. Figure 2A This is a perspective view 200, illustrating how light entering through microlens 105 is directed to diffuser 107A. As shown, light 203 entering near the center of microlens 105 tends to strike diffuser 107A with normal or near-normal incidence. In contrast, light 201 entering from the periphery of the microlens is typically refracted inward toward the center of the diffuser, resulting in a tilted angle of incidence. Grating 125A (see...) Figure 1A The gratings described herein and others are oriented to extend in a direction that is substantially the same as the bending direction of the peripheral light 201, particularly near the centerline 115.

[0038] Figure 2B This is shown at an angle approximately perpendicular to the back side 157 of the semiconductor substrate 113 (see...). Figure 1C Light 203 enters the trench arm 103A. The angled facets 123 form a V-shaped trench profile, refracting the normal incident light into a more inclined path. This reorientation increases the optical path length through the semiconductor substrate 113, thereby increasing the likelihood of photon absorption.

[0039] Figure 2CThe diagram illustrates light 201 entering the diffuser at an angle due to refraction by microlens 105. This light 201 propagates in a direction approximately aligned with the trench arm 103A. Angled facets 123 interact with and further diffract the angled light, but in a direction that keeps the angle within a range that promotes absorption. In contrast, if the trench arm 103A were perpendicular to the optical path orientation (e.g., rotated 90 degrees), the same facets would diffract the light to a steeper angle, reducing the optical path length and absorption efficiency.

[0040] Figure 3 A top view of an image sensor 300 including a diffuser 107B is shown. The diffuser 107B includes four gratings 125B, one of which is shaded and marked. Each grating 125B includes a recess 153 formed on the back side 157 of the semiconductor substrate 113 (see...). Figure 1D The recess 153 comprises four inverted pyramids 101, eight L-shaped grooves 103, and a cross-shaped groove 301 (also shaded). The cross-shaped groove 301 has a similar structure to the L-shaped grooves 103, but comprises four groove arms 103A, each arm extending to one of the four gratings 125B.

[0041] In some embodiments, the groove arms 103A of the inverted pyramid 101, the L-shaped groove 103, and the cross-shaped groove 301 all have equal widths and form a periodic grating. Figure 1A The grating 125A in the middle contains an even number of recesses (e.g., six), while Figure 3 The grating 125B contains an odd number of recesses (e.g., seven). When the recesses are periodically spaced, the cross-shaped groove 301 will only appear when the number of recesses in each grating is odd.

[0042] In some embodiments, the sizes of diffusers 107A and 107B are designed to be equal to or larger than the spot size formed by microlens 105 on the back side 157 of the semiconductor substrate (see...). Figure 1B The spot size is typically smaller than the width W of the pixel region 127. In some embodiments, the width W is approximately 1.4 µm or less. In other embodiments with larger pixels, W can range from approximately 1.4 µm to approximately 10.1 µm. Larger pixel sizes are generally more efficient for near-infrared (NIR) imaging. The NIR wavelength range is approximately 780 nm to 2500 nm, with 940 nm being a wavelength commonly used to benchmark diffuser performance.

[0043] For NIR imaging, the depth of the recess 153 is preferably in the range of about 200 nm to 700 nm. The depth is typically about 70% of the pitch. Therefore, in some embodiments, the pitch of the recess 153 is in the range of about 280 nm to about 1000 nm. Thus, each grating 125 may contain 3 to about 35 recesses 153. In some embodiments, each grating contains five or more recesses. Generally, the number of recesses contained in each grating 125 is equal to or greater than the number shown in the figures. Gratings with fewer recesses are easier to illustrate.

[0044] Figure 4 A top view of an image sensor 400 including a diffuser 107C is shown. The diffuser 107C includes four gratings 125C arranged 90 degrees rotationally symmetrically around the center of a pixel region 127. Within each grating 125C, facets 123 are oriented parallel to their respective center lines 115. In this configuration, the center lines 115 are aligned along the diagonals of the pixel region 127, and each grating 125C occupies one of the four square boundaries of the pixel region. In contrast, in Figure 1A In this example, the center line 115 of grating 125A is aligned with the vertical and horizontal axes (rather than the diagonals), and each grating is distributed within a triangular quadrant of pixel region 127. This example demonstrates the possibility of changing the grating orientation while maintaining alignment of the gratings with the radial lines of each quadrant, and highlights the multiple ways to divide a pixel region into quadrants.

[0045] Figure 5A A top view of an image sensor 500 including a diffuser 107D is shown, and Figure 5B A cross-sectional view of the image sensor 500 is shown. Figure 5B The cross-sectional view in the middle is along Figure 5A The line B-B' shown is the cut-off point. The diffuser 107D contains four gratings 125D, arranged with 90-degree rotational symmetry around the center of the pixel region 127. (Compared to...) Figure 1A , Figure 3 and Figure 4 Unlike the periodic gratings 125A-125C, grating 125D is aperiodic. Gratings 125D are considered gratings because they contain directional and repeating features; specifically, groups of facets 123 are parallel to each other and aligned with their respective centerlines 115, thus defining the grating orientation. However, they are aperiodic because the groove arms 103A located near the centerline 115 are wider than the inverted pyramid 101 and the groove arms 103A farther from the centerline. Figure 5B As shown, the trench arm 103A is also deeper near the center of the diffuser 107D, proportional to its increased width. These deeper recesses 153 near the center define a central trap, which enhances light trapping and improves absorption efficiency.

[0046] Figure 6A A top view of an image sensor 600 including a diffuser 107E with four gratings 125E is shown, and Figure 6B A cross-sectional view of the image sensor 600 is shown. In this embodiment, the diffuser 107E occupies less than one-eighth of the total pixel area 127 and less than one-eighth of the area below the microlens 105. Figure 6B As shown, confining the diffuser to a small central region relative to the entire pixel area enhances light trapping through internal reflection (including the back side 157 of the reflective semiconductor substrate 113). This light trapping effect increases the likelihood of photon absorption, thereby improving quantum efficiency.

[0047] Figure 7A and Figure 7B Top views of image sensors 700 and 720 are shown, each containing a diffuser 107F centered below a microlens 105. Each diffuser 107F contains four gratings 125F. In image sensor 700, each pixel region 127 has four microlenses 105 and four corresponding diffusers 107F. In image sensor 720, each pixel region 127 has two microlenses 105 and two corresponding diffusers 107F. Providing multiple microlenses 105 for each pixel region 127 simplifies microlens manufacturing, especially for large pixel designs. When multiple microlenses are used within a single pixel region, each diffuser 107F is centered below its respective microlens 105, and not necessarily centered within the pixel region 127. These examples illustrate that the diffuser is centered below the corresponding microlens, regardless of whether the microlens is centered above the pixel region.

[0048] Figure 8 A top view of an image sensor 800 including a diffuser 107G is shown. The diffuser 107G is a single, continuous structure that performs the combined function of the four individual diffusers 107F shown in the image sensor 700 of Figure 7. The diffuser 107G includes four sets of gratings 125G, each exhibiting 90-degree rotational symmetry and located below the corresponding microlens 105. Replacing the four independent diffusers 107F with a single integrated diffuser 107G can enhance quantum efficiency by increasing the grating area and simplify fabrication.

[0049] Figure 9A and Figure 9B Top views of image sensors 900 and 920 are shown, which respectively include diffusers 107H and 107I. Similar to the previously described embodiments, these diffusers include four gratings arranged in four quadrants below the center of the microlens, each grating containing a set of facets 123 parallel to its respective centerline 115. These examples differ from the previous embodiments in that they lack certain symmetries.

[0050] Figure 9A The diffuser 107H comprises four gratings 125H, each occupying a quadrant below the microlens. Each grating 125H is mirror-symmetric with respect to the radial lines separating the quadrants with respect to its adjacent grating. However, the gratings 125H lack 90-degree rotational symmetry about the center point below the microlens 105.

[0051] Figure 9B The diffuser 107I also includes four quadrant-based gratings 125I. Although each grating is oriented parallel to its respective centerline 115, the spacing (period) of the trench arms 103A varies among the gratings 125I, producing a non-uniformity that disrupts symmetry. While 90-degree rotational symmetry naturally occurs in diffuser designs following the guidelines provided herein, Figure 9A and Figure 9B Examples demonstrate that this symmetry is not a necessary condition for achieving improved quantum efficiency. These designs still follow... Figures 2A to 2C The principle shown operates in which the orientation of the groove arm 103A and the facet 123 promotes the propagation of light at an angle, thereby increasing the light path length and enhancing absorption.

[0052] Figures 10 to 18 Views of an image sensor with a diffuser manufactured according to the process disclosed herein are shown at various manufacturing stages. Although Figures 10 to 18 It is described in conjunction with a series of operations, but it should be understood that the order of operations may be changed in some cases, and this series of operations applies to structures other than those illustrated. In some embodiments, some of these operations may be omitted entirely or partially. Furthermore, although... Figures 10 to 18 It is described in conjunction with a series of operations, but it should be understood that... Figures 10 to 18 The structure shown is not limited to the manufacturing method, but can exist as an independent structure independent of the method.

[0053] like Figure 10 As shown in cross-sectional view 1000, the method begins with a front-end process (FEOL) treatment of a semiconductor substrate 113, including a process of forming a photodiode 1001, a transistor 131, and an isolation structure 141 on the front side 161. The semiconductor substrate 113 may be or comprise, for example, a bulk silicon substrate, a silicon-on-insulator (SOI) substrate, or other suitable semiconductor substrate. In some embodiments, the semiconductor substrate 113 has a (100) crystal orientation.

[0054] The photodiode 1001 may be formed via one or more ion implantation (doping) steps. Doping may also form floating diffusion regions, pinned regions, or other such structures used in photodetectors. The isolation structure 141 may be a shallow trench isolation structure, a similar structure, or any other suitable type of isolation structure. The transistor 131 may include a transmission gate, a select gate, a reset gate, a source follower, or other such transistors used in photodetector pixel circuitry. The transistor 131 may be a planar transistor, a vertical gate transistor, a combination thereof, or any other suitable type of transistor.

[0055] like Figure 11 As shown in the cross-sectional view 1100, the method continues with a back-end process (BEOL) treatment. The BEOL treatment forms a metal interconnect structure 139 on the front side 161. The metal interconnect structure 139 includes conductive lines 135, vias 1101, and an intermetallic dielectric 1103. The conductive lines 135 may be arranged in multiple metallization layers, which are interleaved with the via layers containing the vias 1101. The conductive lines 135 and the vias 1101 form the metal interconnects that complete the photodetector pixel circuitry.

[0056] like Figure 12 As shown in cross-sectional view 1200, semiconductor substrate 113 is thinned from the back side 157. Prior to thinning, semiconductor substrate 113 is bonded to another substrate (not shown) that provides mechanical support and / or additional functionality. This other substrate may provide a portion of the photodetector pixel circuitry, provide an application-specific integrated circuit (ASIC), and may itself be attached to another substrate that provides these or other functions.

[0057] Typically, the initial thickness of the semiconductor substrate 113 is in the range of about 700 µm to about 800 µm. Thinning reduces the thickness of the semiconductor substrate 113 to the range of about 1 µm to about 100 µm. For back-illuminated CMOS image sensors, the semiconductor substrate is typically thinned to a thickness of less than 5 µm. However, using a greater thickness can increase near-infrared (NIR) absorption. Therefore, in some embodiments, the thickness is maintained above 5 µm. In some embodiments, the thickness is maintained at about 10 µm or greater.

[0058] like Figure 13 As shown in cross-sectional view 1300, a mask 1301 is formed on the back side 157 of the semiconductor substrate 113 after a thinning process. The mask 1301 may consist of photoresist, a hard mask, a combination thereof, or any other suitable material. It may be patterned using photolithography, ion beam lithography, or other suitable lithography techniques.

[0059] Figure 13AA top view of a mask 1301 according to one embodiment is shown, having an opening pattern 1309A. This pattern is repeated for each photodiode 1001. The opening pattern 1309A comprises four sub-patterns 1307A arranged with 90-degree rotational symmetry about a central point. The pattern may consist of square openings 1303 and L-shaped openings 1305. Each L-shaped opening 1305 forms two slits 1311, each slit along the semiconductor substrate 113. <110> One of the crystal orientations. All slits 1311 in pattern 1309A follow these orientations. <110> Direction. This pattern can be used to form Figures 1A to 1D The diffuser 107A shown is shown.

[0060] Figure 13B A top view of mask 1301 is shown, which has an alternative opening pattern 1309B. In addition to the square opening 1303 and the L-shaped opening 1305, the pattern also includes a cross-shaped opening 1321. Similar to the previous embodiment, pattern 1309B consists of four sub-patterns 1307B, rotated symmetrically about a central point by 90 degrees. This configuration can be used for manufacturing... Figure 3 The diffuser 107B is shown.

[0061] Figure 13C Another embodiment is shown, illustrating a top view of a mask 1301 with an opening pattern 1309C. This pattern includes square openings 1303 and L-shaped openings 1305, arranged as four sub-patterns 1307C. However, unlike the previous example, the widths of the square and L-shaped openings vary between the four quadrants. Therefore, sub-patterns 1307C lack the 90-degree rotational symmetry present in patterns 1309A and 1309B. This variation enables the formation of... Figure 9A The diffuser 107I is shown.

[0062] like Figure 14 As shown in cross-sectional view 1400, a semiconductor substrate 113 is etched through an opening in mask 1301 to form a recess 153. The recess 153 may include trench arms 103A and an inverted pyramid 101. The etching process is selected to provide openings with angled sidewalls, which are facets 123. The angled sidewalls can also be formed by anisotropic dry etching; however, in some embodiments, the etching process is anisotropic wet etching.

[0063] Anisotropic wet etching can utilize alkaline solutions, such as potassium hydroxide (KOH) and tetramethylammonium hydroxide (TMAH), which provide different etching rates to different crystal planes in silicon. Specifically, these alkaline solutions exhibit significantly higher etching rates in the ⟨100> and ⟨110> crystal directions than in the ⟨111> crystal direction. The etching rate ratio can exceed 100:1. This type of etching process naturally produces inverted pyramids and trenches with V-shaped profiles. The aspect ratio of the V-shaped profiles is approximately 0.7.

[0064] In some embodiments, the etching process is allowed to proceed and undercut mask 1301 until adjacent trenches meet at V-peak 121. If the mask opening spacing is too large, or etching stops too early, mesas may be left between adjacent trenches. Retaining these mesas reduces the efficiency of the resulting diffuser. After etching, mask 1301 can be peeled off.

[0065] like Figure 15 As shown in the cross-sectional view 1500, a mask 1503 is formed and used to etch deep trenches 1501 in the back side 157. The etching of the deep trenches 1501 may stop on the shallow trench isolation structure 141. Alternatively, the deep trenches 1501 may be formed to stop before reaching the front side 161, may extend to the front side 161, or may stop on metal pads within the metal interconnect structure 139. The deep trenches 1501 form a grid within a photodiode 1001 in each cell. These cells correspond to the photodetector pixel region 127.

[0066] like Figure 16 As shown in the cross-sectional view 1600, a medium refractive index dielectric layer 111 and a low refractive index dielectric layer 109 are deposited to simultaneously fill the deep trench 1501 to form the BDTI structure 145 and fill the recess 153 to complete the formation of the diffuser 107. The low refractive index dielectric layer 109 is typically silicon dioxide (SiO2) or the like. The medium refractive index dielectric layer 111 can function as an anti-reflective coating. The medium refractive index dielectric layer 111 may comprise one or more layers of tantalum oxide (Ta2O5), titanium dioxide (TiO2), silicon nitride (Si3N4), silicon oxynitride (SiON), aluminum oxide (Al2O3), hafnium oxide (HfO2), zirconium oxide (ZrO2), or combinations thereof. These layers can be deposited by atomic layer deposition (ALD), physical vapor deposition (PVD), chemical vapor deposition (CVD), or any other suitable process.

[0067] Simultaneously filling the deep trench 1501 to form the BDTI structure 145 and filling the recess 153 to complete the formation of the diffuser 107 has the advantage of reducing the total number of process steps. However, it should be understood that these processes can be completely or partially separated to provide different compositions for forming the BDTI structure 145 and filling the recess 153 of the diffuser 107. For example, it is preferable to use one or more layers of dielectric pads to present a fixed negative charge in the deep trench 1501. Examples of such dielectrics include hafnium oxide (HfO2), aluminum oxide (AlO), etc. It may or may not be expected that the same materials will be used at the semiconductor interface of the diffuser 107.

[0068] In some embodiments, the BDTI structure 145 includes a metal core (not shown). Providing the metal core for the BDTI structure 145 involves padding a deep trench 1501 with a dielectric liner, and then filling the deep trench 1501 with metal. Excess metal can be removed from above the back side 157 by chemical mechanical polishing (CMP). The deep trench 1501 can be linerbed while filling the recess 153. In an alternative embodiment, the recess 153 is formed after the BDTI structure 145, such that the diffuser 107 can be manufactured using a completely different dielectric thickness or composition than that used for the deep trench 1501.

[0069] like Figure 17 As shown in cross-sectional view 1700, additional passivation layers and other structures may be formed on the back side 157 above the diffuser 107. These may include a back-side metal mesh 149 and color filters (not shown). The back-side metal mesh 149 may be formed by depositing and etching a composite mesh stack comprising a hard mask, metal, and dielectric layers. The color filters may be formed by filling the cells of the composite mesh with a suitable material.

[0070] like Figure 18 As shown in the cross-sectional view 1800, a microlens 105 is formed above the pixel region 127 and the diffuser 107. The microlens 105 is located at the center above the diffuser 107. Each diffuser 107 may correspond to one microlens 105. The microlens 105 may be formed of a dielectric, polymer, or any other suitable material, and may be formed by any suitable process. The process of forming the microlens 105 may include, for example, depositing a lens material, followed by etching to define a lens shape. In some embodiments, the forming process includes a thermal reflow process, etc.

[0071] The microlens 105 can have any suitable focal point. In some embodiments, the focal point is located on the back side 157. This provides the highest quantum efficiency for an ideal lens. In some embodiments, the focal point is located below the back side 157 within the semiconductor substrate 113. This design is less sensitive to manufacturing variations and provides the highest quantum efficiency in practical applications.

[0072] Certain aspects of this disclosure relate to a diffuser in an image sensor. This image sensor is of the type having a photodiode in a semiconductor substrate, a metal interconnect structure disposed on the front side, and a microlens disposed on the back side. The diffuser is located between the microlens and the photodiode on the back side. The diffuser is located at the center below the microlens and includes four gratings formed by a recess on the back side. The gratings are arranged with 90-degree rotational symmetry about the center point.

[0073] In some embodiments, the semiconductor substrate has a (100) crystal orientation, and the grating and <110> Crystal orientation alignment. In some embodiments, the recess has an opening that is cross-shaped, L-shaped, or square, and has sloping sidewalls that narrow as the depth into the semiconductor substrate increases. In some embodiments, a recess with a cross-shaped or L-shaped opening includes a trench with a V-shaped profile, and a recess with a square opening forms an inverted pyramid. In some embodiments, the diffuser includes four inverted pyramids.

[0074] In some embodiments, two recesses are laterally adjacent and their openings meet at a V-shaped peak. In some embodiments, the openings are L-shaped or square. In some embodiments, one of the openings is cross-shaped. In some embodiments, all openings have equal width. In some embodiments, each of the four gratings is symmetrical about its respective centerline, and the recess closest to the centerline is deeper and wider than the recess furthest from the centerline. In some embodiments, the photodiode is located within a pixel region defined by an isolation structure extending into the semiconductor substrate, and the diffuser occupies less than half of the pixel region.

[0075] Certain aspects of this disclosure relate to diffusers in image sensors. The image sensor is of the type having photodiodes in a semiconductor substrate, a metal interconnect structure disposed on the front side, and microlenses disposed on the back side. The diffuser includes a recess in the back side. The diffuser can be divided into four quadrants located centrally below the microlenses, each quadrant having a center line, and each quadrant containing a grating having a plurality of first planar surfaces located in different planes, the first planar surfaces being parallel to each other and parallel to their respective center lines.

[0076] In some embodiments, each grating includes one or more grooves extending parallel to its respective centerline. In some embodiments, a plurality of first planar surfaces located in different planes include first planar surfaces in at least three different planes. In some embodiments, each grating is mirror-symmetrical to gratings in each adjacent quadrant. In some embodiments, each grating includes a plurality of second planar surfaces located in different planes, the second planar surfaces being parallel to each other and parallel to their respective centerlines, wherein the second planar surfaces have an orientation different from the first plurality of planar surfaces. In some embodiments, the first and second planar surfaces form sidewalls of grooves extending parallel to their respective centerlines.

[0077] Certain aspects of this disclosure relate to a method for manufacturing an image sensor. The method includes: forming a photodiode within a semiconductor substrate; forming a metal interconnect structure on a front side of the semiconductor substrate; bonding the semiconductor substrate to a second substrate; thinning the semiconductor substrate from the second side; forming a mask on the second side; etching through the mask using an anisotropic wet etching process to form a recessed pattern in the second side; depositing a first dielectric within the recess, wherein the first dielectric has a refractive index lower than that of the semiconductor substrate; and forming a microlens above the second side, the microlens being located at the center above the recessed pattern. The mask has an opening pattern consisting of four sub-patterns arranged with 90-degree rotational symmetry about a central point, and the opening pattern includes an L-shaped opening.

[0078] In some embodiments, the semiconductor substrate has a (100) crystal orientation, and the arm of the L-shaped opening is along... <110> The crystal direction extends. The method also includes etching a deep trench in the second side, wherein depositing a first dielectric within the recess will deposit the first dielectric within the deep trench.

[0079] Embodiments of this application provide an image sensor, comprising: a substrate having a first side and a second side opposite the first side in thickness of the substrate; a microlens disposed on the second side; and a diffuser including a recess in the second side, wherein the diffuser is located between the microlens and the substrate and is divisible into four quadrants, each quadrant including a grating having a plurality of first planar surfaces in different planes, the plurality of first planar surfaces being parallel to each other, and each grating having the same orientation as the grating in the opposite quadrant and a different orientation from the grating in the adjacent quadrant.

[0080] In some embodiments, the substrate has a (100) crystal orientation, and the grating is... <110> Crystal orientation alignment. In some embodiments, the recess has an opening, which is cross-shaped, L-shaped, or square, and has sloping sidewalls such that the opening narrows with increasing depth into the substrate. In some embodiments, the recess comprises four inverted pyramids. In some embodiments, two of the recesses are laterally adjacent and have openings that meet at a V-shaped peak. In some embodiments, the openings have equal widths. In some embodiments, each of the four gratings is symmetrical about its respective centerline, and the recess closest to the centerline is deeper and wider than the recess furthest from the centerline. In some embodiments, the diffuser is disposed above a photodiode within the substrate. In some embodiments, the plurality of first planar surfaces in different planes includes first planar surfaces in at least three different planes. In some embodiments, each of the gratings has a plurality of second planar surfaces in different planes and parallel to each other, and the first planar surfaces and the second planar surfaces form the sidewalls of a trench. In some embodiments, the image sensor further includes: a photodiode within the substrate, wherein the substrate is a semiconductor substrate and the photodiode is located within a pixel region of the semiconductor substrate; and an isolation structure extending into the semiconductor substrate adjacent to the boundary of the pixel region; wherein the diffuser occupies less than half of the pixel region.

[0081] Embodiments of this application provide a method for manufacturing an image sensor, the method comprising: providing a substrate having a first side and a second side opposite the first side in thickness of the substrate; forming a mask over the second side, wherein the mask has a pattern of openings, the pattern of openings being a combination of four sub-patterns arranged in a 90-degree rotational symmetry, and the pattern of openings including L-shaped openings; etching through the mask by an anisotropic wet etching process to form a recessed pattern in the second side; and depositing a first dielectric within the recesses, wherein the first dielectric has a refractive index lower than that of the substrate.

[0082] In some embodiments, the method further includes: forming a microlens over the recessed pattern. In some embodiments, the method further includes: forming a metal interconnect structure over the first side; bonding the substrate to a second substrate; and thinning the substrate from the second side. In some embodiments, the substrate is a semiconductor substrate having a (100) crystal orientation, and the L-shaped opening has along... <110> An arm extending in the crystal direction. In some embodiments, the method further includes etching a deep trench in the second side, wherein depositing the first dielectric within the recess deposits the first dielectric within the deep trench.

[0083] Embodiments of this application provide a method for manufacturing an image sensor, the method comprising: providing a substrate having a first side and a second side opposite the first side in thickness of the substrate; forming a mask over the second side, wherein the mask has a pattern of openings that can be divided into four quadrants, each quadrant including a plurality of parallel slits, the slits in opposite quadrants having the same orientation and the slits in adjacent quadrants having different orientations; etching through the mask by an anisotropic wet etching process to form a pattern of recesses in the second side; and depositing a first dielectric in the recesses, wherein the first dielectric has a refractive index lower than that of the substrate.

[0084] In some embodiments, the substrate is a semiconductor substrate having a (100) crystal orientation, and the slit is along... <110> The crystal orientation extends. In some embodiments, slits in adjacent quadrants converge to form an L-shaped opening. In some embodiments, the method further includes: forming a photodiode by implanting a dopant in the first side; forming a metal interconnect structure over the first side; bonding the substrate to a second substrate; thinning the substrate from the second side; and forming a microlens over the recessed pattern.

[0085] The foregoing has outlined features of several embodiments to enable those skilled in the art to better understand various aspects of this disclosure. Those skilled in the art should understand that they can readily use this disclosure as a basis to design or modify other processes and structures to perform the same purposes and / or achieve the same advantages as the embodiments described herein. Those skilled in the art should also recognize that such equivalent constructions do not depart from the spirit and scope of this disclosure, and that various changes, substitutions, and alterations can be made to them herein without departing from the spirit and scope of this disclosure.

Claims

1. An image sensor, comprising: A substrate having a first side and a second side opposite the first side in terms of the thickness of the substrate; Microlenses are arranged on the second side; as well as A diffuser, including a recess in the second side, wherein the diffuser is located between the microlens and the substrate and is divisible into four quadrants, each quadrant including a grating having a plurality of first planar surfaces in different planes, the plurality of first planar surfaces being parallel to each other, and each grating having the same orientation as the grating in the opposite quadrant and a different orientation from the grating in the adjacent quadrant.

2. The image sensor according to claim 1, wherein, The substrate has a (100) crystal orientation, and the grating and <110> Crystal orientation aligned.

3. The image sensor according to claim 1, wherein, The recess has an opening that is cross-shaped, L-shaped, or square, and has sloping sidewalls such that the opening narrows as the depth into the substrate increases.

4. The image sensor according to claim 1, wherein, The depression comprises four inverted pyramids.

5. The image sensor according to claim 3, wherein, The two recesses are laterally adjacent and have openings that meet at the V-shaped peak.

6. The image sensor according to claim 3, wherein, The openings have equal widths.

7. The image sensor according to claim 1, wherein, Each of the four gratings is symmetrical about its respective center line, and the recess closest to the center line is deeper and wider than the recess furthest from the center line.

8. The image sensor according to claim 1, wherein, The diffuser is positioned above the photodiode within the substrate.

9. A method for manufacturing an image sensor, the method comprising: A substrate is provided having a first side and a second side opposite the first side in the thickness of the substrate; A mask is formed above the second side, wherein the mask has an opening pattern, the opening pattern being a combination of four sub-patterns arranged in a 90-degree rotational symmetry, and the opening pattern including an L-shaped opening; The mask is etched through using an anisotropic wet etching process to form a recessed pattern in the second side; as well as A first dielectric is deposited within the recess, wherein the first dielectric has a refractive index lower than that of the substrate.

10. A method for manufacturing an image sensor, the method comprising: A substrate is provided having a first side and a second side opposite the first side in the thickness of the substrate; A mask is formed above the second side, wherein the mask has an opening pattern that can be divided into four quadrants, each quadrant including a plurality of parallel slits, the slits in opposite quadrants having the same orientation and the slits in adjacent quadrants having different orientations. The mask is etched through using an anisotropic wet etching process to form a recessed pattern in the second side; as well as A first dielectric is deposited within the recess, wherein the first dielectric has a refractive index lower than that of the substrate.

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