Method for forming image sensor

Abstract

The present disclosure provides a method for forming an image sensor, which includes following steps. A substrate including a first surface and a second surface opposite to each other. A first isolation structure extending from the first surface into the substrate is formed. A mask pattern is formed on the second surface of the substrate. A portion of the substrate exposed by the mask pattern is removed from the second surface of the substrate to form a trench pattern defining the pixel regions in the substrate. The trench pattern includes a first portion having a first width in a first or a second direction, and a second portion having a second width in a diagonal direction with respect to the first and second direction. A first material layer on the mask pattern is formed to fill up the first portion of the trench pattern and to form on a bottom surface and a side surface of the second portion of the trench pattern. A second material layer is formed on the first material layer, wherein the second material layer defines a first opening in the second portion of the trench pattern.

Description

Methods for forming image sensors This invention relates to a method for forming a semiconductor structure, and more particularly to a method for forming an image sensor. Image sensors are widely used in devices such as cameras, mobile phones, and automotive lenses. In recent years, complementary metal-oxide-semiconductor (CMOS) image sensors have become increasingly advantageous over charge-coupled devices (CCDs) due to their lower power consumption, smaller size, faster data processing, direct data output, and lower manufacturing cost. Therefore, CMOS image sensors have largely replaced CCD image sensors. Generally, CMOS image sensors can include front-side illuminated (FSI) image sensors and back-side illuminated (BSI) image sensors. However, as device size continues to shrink, those skilled in the art continue to improve crosstalk between adjacent pixel regions in CMOS image sensors. The present invention provides a method for forming an image sensor, wherein a first material layer fills a first portion of a trench pattern and is formed on the bottom and side surfaces of a second portion of the trench pattern, and a second material layer is formed on the first material layer and defines a first opening in the second portion of the trench pattern. This allows the first portion of the trench pattern to be masked in subsequent processes by removing a portion of the substrate downwards, using a mask pattern, a first pattern corresponding to the first material layer, and a second layer corresponding to the second material layer as a mask, so that the depth of the first portion of the trench pattern is the same as the depth of the second portion of the trench pattern, thereby improving crosstalk between adjacent pixel regions of the image sensor. An embodiment of the present invention provides a method for forming an image sensor, comprising: providing a substrate, wherein the substrate includes a first surface and a second surface opposite to each other; forming a first isolation structure in the substrate, wherein the first isolation structure extends from the first surface of the substrate into the substrate; forming a mask pattern on the second surface of the substrate; removing a portion of the substrate exposed by the mask pattern from the second surface of the substrate to form a trench pattern defining a plurality of pixel regions in the substrate, wherein the trench pattern includes a first portion having a first width in a first direction or a second direction and a second portion having a second width in a diagonal direction relative to the first direction and the second direction, the first direction intersecting the second direction and the second width being greater than the first width; forming a first material layer on the mask pattern, wherein the first material layer fills the first portion of the trench pattern and is formed on the bottom and side surfaces of the second portion of the trench pattern; and forming a second material layer on the first material layer, wherein the second material layer defines a first opening in the second portion of the trench pattern. In some embodiments, the depth of the trench pattern in the first portion is less than the depth of the trench pattern in the second portion. In some embodiments, the material of the first material layer is different from the material of the second material layer. In some embodiments, the first opening is circular when viewed from above. In some embodiments, the method of forming an image sensor further includes removing a first material layer and a second material layer above a second surface of a substrate to form a first layer and a second layer on the first layer. The first layer includes a first portion filling a first portion of a trench pattern and a second portion formed on the bottom and side surfaces of a second portion of the trench pattern. In some embodiments, the second layer is not formed in the first portion of the trench pattern. In some embodiments, the method of forming an image sensor further includes removing a first portion of the first layer and a portion of the second portion of the first layer to expose the surface of the first portion of the trench pattern and form a first pattern surrounding the second layer. In some embodiments, the top surface of the first pattern, the side surface of the second portion of the trench pattern, and the surface of the second layer facing the side surface of the second portion of the trench pattern define a second opening in the substrate. In some embodiments, the second opening is annular when viewed from above. In some embodiments, the method of forming an image sensor further includes using a mask pattern, a first pattern, and a second layer as a mask, and removing a portion of the substrate downward through a first portion of the exposed trench pattern, such that the depth of the first portion of the trench pattern is the same as the depth of the second portion of the trench pattern. In some embodiments, the method of forming an image sensor further includes filling a trench pattern with a dielectric material to form a pixel isolation structure. Based on the above, in the above-mentioned method for forming an image sensor, a first material layer fills a first portion of the trench pattern and is formed on the bottom and side surfaces of a second portion of the trench pattern, and a second material layer is formed on the first material layer and defines a first opening in the second portion of the trench pattern. This allows the first portion of the trench pattern to be masked in subsequent processes by removing a portion of the substrate downwards, using a mask pattern, a first pattern corresponding to the first material layer, and a second layer corresponding to the second material layer as a mask. This makes the depth of the first portion of the trench pattern the same as the depth of the second portion of the trench pattern, thereby allowing the pixel isolation structure formed therein to avoid cross-talk between adjacent pixel regions, thus improving the performance of the image sensor. The invention is described more fully with reference to the drawings of this embodiment. However, the invention may be embodied in various different forms and should not be limited to the embodiments described herein. The thickness of layers and regions in the drawings is enlarged for clarity. The same or similar reference numerals denote the same or similar elements, which will not be repeated in the following paragraphs. It should be understood that when an element is referred to as being "on" or "connected" to another element, it may be directly on or connected to the other element, or there may be intermediate elements present. If an element is referred to as being "directly on" or "directly connected" to another element, there are no intermediate elements present. As used herein, "connection" may refer to a physical and / or electrical connection, while "electrical connection" or "coupling" may refer to the presence of other elements between two elements. "Electrical connection" as used herein may include physical connections (e.g., wired connections) and physical disconnections (e.g., wireless connections). As used herein, “about,” “approximately,” or “substantially” includes the value mentioned and the average value within an acceptable range of deviations that can be determined by one of ordinary skill in the art, taking into account the measurement under discussion and the specific amount of error associated with the measurement (i.e., limitations of the measurement system). For example, “about” may mean within one or more standard deviations of the value, or within ±30%, ±20%, ±10%, ±5%. Furthermore, the use of “about,” “approximately,” or “substantially” herein may be chosen based on the optical, etched, or other properties, and a single standard deviation may not be applicable to all properties. The terminology used herein is for illustrative purposes only and is not intended to limit the scope of this disclosure. In this context, the singular form includes the plural form unless the context otherwise requires. Figures 1A to 6B are schematic diagrams of a method for forming an image sensor according to an embodiment of the present invention. Figures 1A, 2A, 3A, 4A, and 6A are cross-sectional schematic diagrams of the method for forming an image sensor according to an embodiment of the present invention, while Figures 1B, 2B, 3B, 4B, and 6B are cross-sectional schematic diagrams taken along line A-A' of Figures 1A, 2A, 3A, 4A, and 6A, respectively. In this embodiment, the method for forming an image sensor includes the following steps. First, referring to Figures 1A and 1B, a substrate 100 is provided. The substrate 100 includes a first surface S1 and a second surface S2 opposite to each other. The substrate 100 can be any type of semiconductor substrate (e.g., silicon, SiGe, SOI, etc.) and any other type of semiconductor layer and / or epitaxial layer associated therewith. Next, a first isolation structure 102 is formed in the substrate 100, wherein the first isolation structure 102 extends from the first surface S1 of the substrate 100 into the substrate 100. The first isolation structure 102 may include one or more dielectric materials. The dielectric materials may include oxides (e.g., silicon oxide), tetraethyl orthosilicate (TEOS), nitrides (e.g., silicon nitride, silicon oxynitride, etc.), carbides (e.g., silicon carbide, silicon oxycarbide, etc.), or the like. In some embodiments, the first isolation structure 102 may be a shallow trench isolation (STI) structure. In some embodiments, the first surface S1 of the substrate 100 may be a surface on which a component layer and an interconnect structure are formed. The component layer may include active components such as transfer transistors, reset transistors, source follower transistors, select transistors, or combinations thereof. In some embodiments, the first isolation structure 102 may define an active region in the substrate 100 on which the aforementioned active components are formed. The interconnect layer may include a dielectric layer formed by front-end processing (FEOL) and / or back-end processing (BEOL) and / or a conductor layer and / or a conductive via buried therein. The dielectric layer may include a dielectric material such as an oxide (e.g., silicon oxide) or a nitride (e.g., silicon nitride). The conductor layer and the conductive via may each include a conductive material such as a metal or a metal alloy. The metal and metal alloy may be, for example, Cu, Al, Ti, Ta, W, Pt, Cr, Mo, or alloys thereof. In some embodiments, when the image sensor is a back-side illumination (BSI) image sensor, the image sensor can convert radiation (e.g., light) incident from the second surface S2 of the substrate 100 toward the first surface S1 of the substrate 100 into an electrical signal. Next, a mask pattern 110 is formed on the second surface S2 of the substrate 100. In some embodiments, the mask pattern 110 may include a nitride such as silicon nitride. Then, the portion of the substrate 100 exposed by the mask pattern 110 is removed from the second surface S2 of the substrate 100 to form a trench pattern 104 defining a plurality of pixel regions PX in the substrate 100. The trench pattern 104 includes a first portion 104a having a first width W1 in a first direction D1 or a second direction D2, and a second portion 104b having a second width W2 in a diagonal direction D3 relative to the first direction D1 and the second direction D2. The first direction D1 intersects the second direction D2, and the second width W2 is greater than the first width W1. In some embodiments, the first direction D1 and the second direction D2 are perpendicular to each other. In some embodiments, the trench pattern 104 can be formed by the following steps. First, a mask layer (not shown) and a photoresist pattern (not shown) are sequentially formed on the second surface S2 of the substrate 100. Next, a portion of the mask layer exposed by the photoresist pattern and a portion of the substrate 100 beneath that portion are removed through the photoresist pattern to form the mask pattern 110 and the trench pattern 104. In some embodiments, etching can be used to remove the portion of the mask layer exposed by the photoresist pattern and a portion of the substrate 100 beneath that portion. Then, after the trench pattern 104 is formed, the photoresist pattern is removed. In some embodiments, as shown in FIG1B, the depth d1 of the trench pattern 104 in the first portion 104a is less than the depth d2 of the trench pattern 104 in the second portion 104b. In some embodiments, the trench pattern 104 corresponds to the pattern of the first isolation structure 102 when viewed from a top view. For example, the trench pattern 104 overlaps with the first isolation structure 102 on a first surface S1 or a second surface S2 perpendicular to the substrate 100. Next, referring to Figures 1A and 1B, and Figures 2A and 2B, a first material layer 120 is formed on the mask pattern 110, wherein the first material layer 120 fills the first portion 104a of the channel pattern 104 and is formed on the bottom and side surfaces of the second portion 104b of the channel pattern 104. In this embodiment, the first width W1 of the first portion 104a of the channel pattern 104 in the first direction D1 and the second direction D2 is smaller than the second width W2 of the channel pattern 104 in the diagonal direction D3. The thickness of the first material layer 120 is controlled to fill the first portion 104a of the channel pattern 104 but not the second portion 104b of the channel pattern 104. That is, the first material layer 120 is formed on the bottom and side surfaces of the second portion 104b of the channel pattern 104, while retaining the central portion of the second portion 104b of the channel pattern 104. In some embodiments, as shown in Figure 2A, the central portion may be circular when viewed from a top view. In some embodiments, the first material layer 120 may include oxides such as silicon oxide. Then, a second material layer 130 is formed on the first material layer 120, wherein the second material layer 130 defines the first opening OP1 in the second portion 104b of the trench pattern 104. In this embodiment, the second material layer 130 may be conformally formed on the surface of the first material layer 120. Since the first portion 104a of the trench pattern 104 is filled by the first material layer 120, the second material layer 130 may be formed above the first portion 104a of the trench pattern 104 and extend into the second portion 104b of the trench pattern 104 to form above the bottom and side surfaces of the second portion 104b of the trench pattern 104. In this embodiment, the thickness of the second material layer 130 is controlled to not completely fill the aforementioned central portion to define the first opening OP1. In some embodiments, as shown in FIG2A, the first opening OP1 may be circular when viewed from a top view. In this embodiment, the material of the first material layer 120 is different from the material of the second material layer 130. For example, the first material layer 120 may include oxides such as silicon oxide, while the second material layer 130 may include nitrides such as silicon nitride. In some embodiments, the second material layer 130 may be formed by atomic layer deposition (ALD). Then, referring to Figures 2A and 2B, and Figures 3A and 3B, the first material layer 120 and the second material layer 130 above the second surface S2 of the substrate 100 are removed to form a first layer 122 and a second layer 132 on the first layer 122. In this embodiment, the first layer 122 includes a first portion 122a filling a first portion 104a of the trench pattern 104 and a second portion 122b formed on the bottom and side surfaces of the second portion 104b of the trench pattern 104. In some embodiments, the first material layer 120 and the second material layer 130 above the second surface S2 of the substrate 100 can be removed by a planarization process, such as chemical mechanical polishing (CMP), to form the first layer 122 and the second layer 132 on the first layer 122. In this embodiment, as shown in FIG3B, since the second material layer 130 is not formed in the first portion 104a of the trench pattern 104, the second layer 132 is not formed in the first portion 104a of the trench pattern 104 after the planarization process described above. Next, referring to Figures 3A and 3B, and Figures 4A and 4B, a portion of the first portion 122a and a portion of the second portion 122b of the first layer 122 are removed to expose the surface of the first portion 104a of the trench pattern 104 and form a first pattern 124 surrounding the second layer 132. In this embodiment, the top surface of the first pattern 124, the side surface of the second portion 104b of the trench pattern 104, and the surface of the second layer 132 facing the side surface of the second portion 104b of the trench pattern 104 define a second opening OP2 in the substrate 100. In some embodiments, the second opening OP2 may be annular when viewed from a top view. In the second part 104b of the ditch pattern 104, since the surface of the second part 122b of the first layer 122 is covered by the second layer 132, in the step of removing the first layer 122, only a portion of the second part 122b of the first layer 122 will be removed through the top surface exposed by the second layer 132, while the surface of the first part 122a of the first layer 122 will be removed in this step because it is not covered by the second layer 132. In this way, the bottom surface and the lower side surface of the second part 104b of the ditch pattern 104 will still be covered by the first pattern 124 and the second layer 132, while the bottom surface and the side surface of the first part 104a of the ditch pattern 104 will be exposed after this step. When the material of the first layer 122 is an oxide, an etch-back process for oxides can be used to remove a portion 122a of the first layer 122 and a portion of the second portion 122b of the first layer 122 to expose the surface of the first portion 104a of the trench pattern 104 and form a first pattern 124 surrounding the second layer 132. Then, referring to Figures 4A, 4B, and 5, using the mask pattern 110, the first pattern 124, and the second layer 132 as a mask, a portion of the substrate 100 is removed downwards by the first portion 104a of the exposed trench pattern 104, so that the depth d3 of the first portion 104a' of the trench pattern 104' is formed to be the same as the depth d2 of the second portion 104b of the trench pattern 104'. In this way, the pixel isolation structure formed in the trench pattern 104' (pixel isolation structure 140 as shown in Figure 6) has approximately the same depth in each portion, thereby helping to improve crosstalk between adjacent pixel regions PX of the image sensor. In other embodiments, in the second portion 104b of the trench pattern 104, a portion of the substrate 100 exposed by the first pattern 124 may be partially and laterally removed during the formation of the first portion 104a' of the trench pattern 104' having a depth d3, such that the second portion 104b of the trench pattern 104' shown in FIG. 5 may be formed to include a lower portion and an upper portion, wherein the width of the upper portion may be greater than the width of the lower portion. Next, referring to Figures 5, 6A, and 6B, dielectric material is filled into the trench pattern 104' to form a pixel isolation structure 140. The pixel isolation structure 140 may include one or more dielectric materials. The dielectric materials may include oxides (e.g., silicon oxide), tetraethyl orthosilicate (TEOS), nitrides (e.g., silicon nitride, silicon oxynitride, etc.), carbides (e.g., silicon carbide, silicon carbide, etc.), or the like. In some embodiments, the pixel isolation structure 140 may be a deep trench isolation (DTI) structure. In summary, in the above-described method for forming an image sensor, a first material layer fills a first portion of the trench pattern and is formed on the bottom and side surfaces of a second portion of the trench pattern. A second material layer is formed on the first material layer and defines a first opening in the second portion of the trench pattern. This allows the first portion of the trench pattern to be masked in subsequent processes by removing a portion of the substrate downwards using a mask pattern, a first pattern corresponding to the first material layer, and a second layer corresponding to the second material layer. This makes the depth of the first portion of the trench pattern the same as the depth of the second portion of the trench pattern, thereby allowing the pixel isolation structure formed therein to avoid crosstalk between adjacent pixel regions, thus improving the performance of the image sensor. 100: Base; 102: First isolation structure; 104, 104': Trench pattern; 104a, 104a': First part; 104b: Second part; 110: Mask pattern; 120: First material layer; 122: First layer; 122a: First part; 122b: Second part; 124: First pattern; 130: Second material layer; 132: Second layer; 140: Pixel isolation structure; d1, d2, d3: Depth; D1: First direction; D2: Second direction; D3: Diagonal direction; OP1: First opening; OP2: Second opening; PX: Pixel area; S1: First surface; S2: Second surface; W1: First width; W2: Second width. Figures 1A to 6B are schematic diagrams of a method for forming an image sensor according to an embodiment of the present invention. 100: Base 102: First isolation structure 104a: Part 1 104b: Part Two 110: Curtain Pattern 120: First material layer 130: Second material layer OP1: First opening PX: Pixel area

Claims

1. A method for forming an image sensor includes: A substrate is provided, wherein the substrate includes a first surface and a second surface that are opposite to each other; A first isolation structure is formed in the substrate, wherein the first isolation structure extends from the first surface of the substrate into the substrate; A mask pattern is formed on the second surface of the substrate; the portion of the substrate exposed by the mask pattern is removed from the second surface of the substrate to form a trench pattern defining a plurality of pixel regions in the substrate, wherein the trench pattern includes a first portion having a first width in a first direction or a second direction and a second portion having a second width in a diagonal direction relative to the first direction and the second direction, the first direction intersecting the second direction, and the second width being greater than the first width; A first material layer is formed on the cover pattern, wherein the first material layer fills the first portion of the trench pattern and is formed on the bottom and side surfaces of the second portion of the trench pattern; and a second material layer is formed on the first material layer, wherein the second material layer defines a first opening in the second portion of the trench pattern. The method as described in claim 1, wherein the depth of the ditch pattern in the first portion is less than the depth of the ditch pattern in the second portion. The method as described in claim 1, wherein the material of the first material layer is different from the material of the second material layer. The method described in claim 1, wherein the first opening is circular when viewed from a top view. The method described in request item 1 further includes: Remove the first material layer and the second material layer above the second surface of the substrate to form a first layer and a second layer on the first layer, wherein the first layer includes a first portion filling the first portion of the trench pattern and a second portion formed on the bottom and side surfaces of the second portion of the trench pattern. The method as described in claim 5, wherein the second layer is not formed in the first portion of the trench pattern. The method described in request item 5 further includes: Remove the first portion of the first layer and a portion of the second portion of the first layer to expose the surface of the first portion of the trench pattern and form a first pattern surrounding the second layer. As described in claim 7, wherein the top surface of the first pattern, the side surface of the second portion of the trench pattern, and the surface of the second layer facing the side surface of the second portion of the trench pattern define a second opening in the substrate. The method described in claim 8, wherein the second opening is annular when viewed from above. The method described in request item 7 further includes: Using the mask pattern, the first pattern, and the second layer as a mask, a portion of the substrate is removed downwards by the first portion of the exposed trench pattern, such that the depth of the first portion of the trench pattern is the same as the depth of the second portion of the trench pattern. The method described in request item 10 further includes: Dielectric material is filled into the trench pattern to form a pixel isolation structure.

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