The present disclosure relates to an image sensor, and it particularly relates to a design for a grid structure of an image sensor.
Image sensors, such as complementary metal oxide semiconductor (CMOS) image sensors (also known as CIS), are widely used in various image-capturing apparatuses such as digital still-image cameras, digital video cameras, and the like. The light-sensing units in an image sensor may detect ambient color change, and signal electric charges may be generated depending on the amount of light received by the light-sensing units. In addition, the signal electric charges generated by the light-sensing units may be transmitted and amplified, whereby an image signal is obtained.
To meet industrial demand, pixel size has continuously been reduced, while pixel definition has continuously been enhanced. In order to maintain superior levels of performance, entry light rays should be concentrated within each color filter unit for effective light reception, without interference of light rays from adjacent color filter units. Each color filter unit is compartmentalized by a grid structure, which has a lower refractive index than that of the color filter units. Since light rays tend to be directed toward mediums with higher refractive index, the grid structure can repel potential light rays from interfering adjacent color filter units. However, more innovative ways of designing the grid structure is required to accommodate pixels with continuously shrinking size.
In an embodiment, an image sensor includes: a substrate; color filter units disposed on the substrate; and a grid structure disposed on the substrate and surrounding each of the color filter units. The grid structure includes: a first partition wall, disposed on the substrate, located between the color filter units; and a second partition wall, disposed directly on the first partition wall, located between the color filter units. A top width of the second partition wall is smaller than a bottom width of the second partition wall.
In another embodiment, an image sensor includes: a substrate; color filter units disposed on the substrate; and a grid structure disposed on the substrate and surrounding each of the color filter units. The grid structure includes: a first partition wall having a vertical side surface relative to the substrate with a base width, disposed on the substrate, located between the color filter units; a second partition wall having an inclined side surface relative to the substrate, disposed directly on the first partition wall, located between the color filter units; and a third partition wall having a first width, disposed directly on the second partition wall, located between the color filter units, wherein the first width is smaller than the base width.
The disclosure can be more fully understood from the following detailed description when read with the accompanying figures. It is worth noting that, in accordance with standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.
The following disclosure provides many different embodiments, or examples, for implementing different features of the subject matter provided. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, a first feature is formed on a second feature in the description that follows may include embodiments in which the first feature and second feature are formed in direct contact, and may also include embodiments in which additional features may be formed between the first feature and second feature, so that the first feature and second feature may not be in direct contact.
It should be understood that additional steps may be implemented before, during, or after the illustrated methods, and some steps might be replaced or omitted in other embodiments of the illustrated methods.
Furthermore, spatially relative terms, such as “beneath,” “below,” “lower,” “on,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to other elements or features as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.
In the present disclosure, the terms “about,” “approximately” and “substantially” typically mean±20% of the stated value, more typically ±10% of the stated value, more typically ±5% of the stated value, more typically ±3% of the stated value, more typically ±2% of the stated value, more typically ±1% of the stated value and even more typically ±0.5% of the stated value. The stated value of the present disclosure is an approximate value. That is, when there is no specific description of the terms “about,” “approximately” and “substantially”, the stated value includes the meaning of “about,” “approximately” or “substantially”.
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It should be understood that terms such as those defined in commonly used dictionaries should be interpreted as having a meaning that is consistent with their meaning in the context of the prior art and will not be interpreted in an idealized or overly formal sense unless expressly so defined in the embodiments of the present disclosure.
The present disclosure may repeat reference numerals and/or letters in following embodiments. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.
The grid structure (or partition grid structure) conventionally separates each color filter unit (of the corresponding sensor unit) from the others, so that incident lights may be converted into the desired color of each sensor unit without being affected by adjacent sensor units. However, the market demands for image sensors with smaller pixel sizes, which indirectly raise the probability for incident light rays within each color filter unit to enter adjacent color filter unit, and this is unwanted. When light rays are not sufficiently received by the sensing unit underlying each color filter unit, the quantum efficiency of the image sensor would be undermined. Furthermore, light interference between the color filter units may also increase cross talk, which compromises the image sensor's overall performance. The present disclosure provides several innovative designs of the grid structure to address the above issues. The grid structure of the present disclosure may concentrate entry light rays within each color filter unit onto the corresponding sensing unit, and thereby enhances quantum efficiency and eliminates cross talk, resulting in an image sensor with more superior performance.
Refer to
In other embodiments, the substrate 100 may also be a semiconductor on insulator (SOI) substrate. The semiconductor on insulator substrate may include a base plate, a buried oxide layer disposed on the base plate, and a semiconductor layer disposed on the buried oxide layer. Furthermore, the substrate 100 may be an N-type or a P-type conductive type.
In some embodiments, the substrate 100 may include various isolation elements (not shown) to define active regions, and to electrically isolate active region elements within or above the substrate 100. In some embodiments, isolation elements may include shallow trench isolation (STI) elements, local oxidation of silicon (LOCOS) elements, other suitable isolation elements, or a combination thereof. In some embodiments, the formation of the isolation elements may include, for example, forming an insulating layer on the substrate 100, selectively etching the insulating layer and the substrate 100 to form trenches within the substrate 100, growing rich nitrogen-containing (such as silicon oxynitride) liners in the trenches, and filling insulating materials (such as silicon dioxide, silicon nitride, or silicon oxynitride) into the trenches with deposition processes, then performing annealing processes on the insulating materials in the trenches, and performing planarization processes on the substrate 100 to remove excessive insulating materials, so the insulating materials in the trenches are level with the top surface of the substrate 100.
In some embodiments, the substrate 100 may include various P-type doped regions and/or N-type doped regions (not shown) formed of, for example, ion implantation and/or diffusion process. In some embodiments, transistors, photodiodes, or the like, may be formed at the active regions, defined by the isolation elements.
The plurality of sensing units 102 are embedded in the substrate 100. In some embodiments, the plurality of sensing units 102 are photodiodes. Each of the sensing units 102 is configured to sense light and generate an intensity signal according to the intensity of the light falling thereon. The image signal is formed by the intensity signals.
The anti-reflection layer 104 is disposed on the substrate 100. In some embodiments, the anti-reflection layer 104 is configured to decrease the reflection of the light being transmitted to the plurality of sensing units 102. In some embodiments, the anti-reflection layer 104 is disposed horizontally in correspondence (or parallel with respect) to the array of sensing units 102. In some embodiments, the materials of the anti-reflection layer 104 may include SiOxNy (wherein x and y are in the range of 0 to 1). The anti-reflection layer 104 may be formed by any suitable deposition processes.
In some embodiments, the image sensor 10 may include color filter units 106 disposed on the anti-reflection layer 104 and the substrate 100, and corresponding to the array of sensing units 102. In some embodiments, the height of the color filter units 106 may be approximately between 0.3 μm and 2.0 μm. In some embodiments, the color filter units 106 may be colored red, green, blue, white, or infrared. Each of the color filter units 106 may correspond to each respective sensing unit 102 of the image sensor 10, and the color of the unit depends on the requirement of the image sensor 10. The respective sensing units 102, such as photodiodes, may convert received light signals into electric signals.
In some embodiments, each of the color filter units 106 allows a predetermined range of wavelengths of light to pass through. For example, the red color filter units allow wavelengths of light in a range from 620 nm to 750 nm (red light) to transmit to the corresponding sensing units 102, the green color filter units allow wavelengths of light in a range from 495 nm to 570 nm (green light) to transmit to the corresponding sensing units 102, and the blue color filter units allow wavelengths of light in a range from 450 nm to 495 nm (blue light) to transmit to the corresponding sensing units 102.
Refer to
The material of the grid structure 110 may include a transparent dielectric material. In some embodiments, the materials of the grid structure 110 may include silica ball and air bubble (material doped with inorganic material), or polysiloxane. At first, a grid material layer is coated on the anti-reflection layer 104. Next, a mask layer (not shown) is coated on the grid material layer. In some embodiments, the material of the mask layer is a photoresist. A photolithography process is performed on the mask layer for patterning. Next, an etching process is performed on the grid material layer by using the patterned hard mask layer. The etching process may be dry etching. After the etching process, a portion of the grid material layer is removed on the anti-reflection layer 104, and multiple openings are formed therein. The openings will subsequently be filled with the color filter units 106. According to some embodiments of the present disclosure, multiple photolithography and etching processes may be implemented to form rectangular partition walls (the first partition wall 112 and the third partition wall 116) with different widths. Furthermore, the deposition of material layers with different carbon bonds, followed by etching with etching gas of different fluorine ion concentrations may result into a trapezoidal partition wall (the second partition wall 114).
As mentioned previously, the present embodiment provides an innovative way of designing the grid structure 110. According to some embodiments of the present disclosure, the first width W1 of the third partition wall 116 is smaller than the base width W of the first partition wall 112 by approximately 20% to 60%, for example, approximately 20% to 50%. According to some embodiments of the present disclosure, the base width W and the first width W1 are measured in a transversal direction parallel to the substrate 100. The second partition wall 114 is disposed between the first partition wall 110 and the third partition wall 116. For example, the first partition wall 112 adjoins the bottom of the second partition wall 114, while the third partition wall 116 adjoins the top of the second partition wall 114. According to some embodiments of the present disclosure, a top width of the second partition wall 114 is equal to the first width W1 of the third partition wall 116, while a bottom width of the second partition wall 114 is equal to the base width W of the first partition wall 112. Since the first width W1 of the third partition wall 116 is smaller than the base width W of the first partition wall 112, the top width of the second partition wall 114 is thus smaller than the bottom width of the second partition wall 114. Therefore, the second partition wall 114 has an inclined side surface relative to the substrate 100, so the cross section of the second partition wall 114 appears trapezoidal. The resulting grid structure 110 of the present disclosure has a sidewall that includes both a vertical side surface and an inclined side surface, relative to the substrate 100.
In some embodiments, the light shielding structure 108 may be embedded within the grid structure 110, and the details will be described subsequently. The conventional grid structure has a single rectangular cross section. Due to application requirement, the light shielding structure may sometimes be shifted. In order to accommodate the shift design of the light shielding structure, the grid structure needs to be wide enough. However, if the grid structure becomes too wide, the dimension of the already reduced color filter units may be further compressed. When the color filter units are too small in dimension, the performance of the overall image sensor may be severely affected. By designing the grid structure 110 to have various portions of different width, not only can the process window for the light shielding structure's shift design be improved, the dimension of the color filter units may remain sufficient enough to maintain the performance of the image sensor. Furthermore, the grid structure 110 of the present disclosure causes each of the color filter units 106 to form into a funnel shape. When the incident lights are forced to “funnel” into the color filter units 106, the light rays may be gradually concentrated toward the respective sensing units 102.
As mentioned previously, the grid structure 110 of the present disclosure may enhance quantum efficiency and eliminates cross talk. In some embodiments, the quantum efficiency is the photoelectrical transferring efficiency, which is the measure of how efficient incident lights can be converted into electrical signal. The cross talk is the reading of signal of different light color interfering with the desired light color. In other words, lower quantum efficiency and higher cross talk are unwanted characteristics, as they may affect the performance of image sensors. The grid structure 110 may effectively address the above issues, leading to higher quantum efficiency and less cross talk.
However, if the grid structure 110 only includes multiple rectangular partition walls of different widths stacked together, the grid structure may be in stepped form. When incident lights are transmitted onto the horizontal stepped surface, the light rays may be reflected away from the underlying sensing unit 102. Although such reflection may eliminate cross talk, quantum efficiency may not be improved significantly. Therefore, as shown in
Refer to
In equation (1), the difference between the base width W and the first width W1 defines how much the first partition wall 112 laterally protrudes beyond opposing sidewalls of the third partition wall 116. Half of that difference then defines how much the first partition wall 112 laterally protrudes beyond from a single side of the third partition wall 116. Based on the trigonometric rules, multiplying the protrusion dimension on the single side by the tangent of the interior angle θ may obtain the height of the second partition wall 114.
From equation (1), the height of the third partition wall 116 (abbreviated herein as H_116) may be determined by the following equation:
In equation (2), the height of the third partition wall 116 can simply be calculated by subtracting the first height H1 and the height of the second partition wall 114 from the total height H of the grid structure 110.
In order to form the second partition wall 114 with inclined surfaces, the inventor exploits various characteristics on chemical reactions between different materials and different etching gas. Initially, the partition material layer may be coated to include various layers with different carbon bonds, the material of these layers may be different. According to some embodiments of the present disclosure, fluorine ions may be introduced into the etcher chamber as strong active gas, in which the fluorine ion concentration may be constantly adjusted throughout the entire etching process. In some embodiments, when fluorine ions come in contact with carbon bonds, a chemical reaction will occur to produce hardened carbon material, which is difficult to be etched.
For example, the carbon bonds of the partition material layer may be arranged to have the highest amount at the bottom, and gradually decrease toward the top, this can be achieved by a consecutive deposition of material layers with a decreasing order of carbon bonds. During etching, the fluorine ion within the active gas may begin at the highest concentration for etching the topmost material layer with the lowest amount of carbon bonds. As the etching proceeds to etching the underlying material layers with increasing carbon bonds, the fluorine ion within the active gas may be correspondingly lowered. Under those conditions, a larger area of the grid material may be etched away at first (from the top) due to less hardened carbon material generated. However, a minimal area of the grid material may be etched away toward the end (at the bottom) due to more hardened carbon material generated. By accurately calculating the fluorine ion concentration and carbon bonds, and precisely controlling the etching time using the material etching rate, the inclined side surface of the second partition wall 114 may be formed. Please note that the amount of carbon bonds does not affect the refractive index of the material of the second partition wall 114.
According to a specific embodiment of the present disclosure for forming the second partition wall 114, approximately 10% to 30% of grid material layers may be sequentially deposited. The bottommost material layer may include air silica ball (material doped with inorganic material), with the carbon bond concentration between about 40% and 80%. The topmost material layer may include polysiloxane, propylene glycol monomethyl ether, 3-methoxy-1-butanol, with the carbon bond concentration between about 20% and 60%. In some embodiments, the etching process may begin from etching the topmost material layer, during which the etching gas may include a fluorine ion concentration between about 15% and 30%. In other embodiments, etching gas of CH2F2, CHF3, CH3F, CO2, O2, H2, Ar, or the like, or a combination thereof may also be used. When etching the material layer directly underlying the topmost material layer, the fluorine ion concentration may be reduced by approximately 30% to 50%. When etching the bottommost material layer, the fluorine ion concentration may be adjusted to zero.
Refer to
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As shown in
As shown in
In Table 1, Items 1-3 are the quantum efficiency peak data of red light, green light, and blue light, respectively. Items 4-6 are cross talk data, in which the image sensor 10 illustrates significantly reduced cross talk in comparison with the conventional image sensor. In Item 5, the ratio of blue light cross talk and red light cross talk is measured at 530 nm. Please note that, 530 nm is in a wavelength range where the green light belongs, thus in an ideal situation, the blue light and the red light readings should not exist. In Item 6, the ratio of blue light cross talk and green light cross talk is measured at 650 nm. Please note that, 650 nm is in a wavelength range where the red light belongs, thus in an ideal situation, the blue light and the green light readings should not exist. Therefore, a reduced cross talk can improve the overall performance, as the image sensor 10 displays. Please also note that, the green color filter units often occupies about 50% of an entire image sensor, while the red color filter units and the blue color filter units each occupies about 25% of the entire image sensor. On that basis, the green color filter units may be affected by the red light cross talk and the blue light cross talk the most, as shown in Item 5 of Table 1.
Refer to
In equation (3), it can be noted that the formula in the braces (displayed as “0”) within the inverse tangent parenthesis is in fact the content of equation (2), or the calculation of the height of the third partition wall 116. The inverse tangent of the ratio of half the first width W1 and the height of the third partition wall 116, followed by a multiplication of 2 may result in an upper limit of the top angle θtop, or a final value larger than the top angle θtop.
Refer to
Refer to
The foregoing outlines features of several embodiments so that those skilled in the art will better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. Therefore, the scope of protection should be determined through the claims. In addition, although some embodiments of the present disclosure are disclosed above, they are not intended to limit the scope of the present disclosure.
Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present disclosure should be or are in any single embodiment of the disclosure. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present disclosure. Thus, discussions of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment.
Furthermore, the described features, advantages, and characteristics of the disclosure may be combined in any suitable manner in one or more embodiments. One skilled in the prior art will recognize, in light of the description herein, that the disclosure can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the disclosure.
This application claims priority of provisional application of U.S. Patent Application No. 63/048,865 filed on Jul. 7, 2020, the entirety of which is incorporated by reference herein.
Number | Date | Country | |
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63048865 | Jul 2020 | US |