KR20040058738A - CMOS image sensor with microlense having reduced radius of curvature comparing to long wavelength of incident light and the method of fabricating the same - Google Patents

Abstract

PURPOSE: A CMOS image sensor and a method for manufacturing the same are provided to improve optical absorptance and color reproducibility by reducing radius of curvature of a microlens. CONSTITUTION: A passivation layer(25) is formed on a substrate(20) with photodiodes(22). The first, the second and the third color filter(26a,26b,26c) are formed on the passivation layer. A planarization layer(27) with a groove is formed on the color filters, wherein the groove is formed on a region corresponding to the third color filter(26c) for splitting long wavelength, such as red light. The first and the second microlens(29a,29b) are formed on the planarization layer corresponding to the first and the second color filter. The third microlens(29c) with a relatively small radius of curvature is formed on the planarization layer including the groove corresponding to the third color filter.

Description

CMOS image sensor with microlense having reduced radius of curvature comparing to long wavelength of incident light and the method of fabricating the same

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image sensor, and more particularly, to an image sensor having a microlens and reducing a curvature radius of a microlens corresponding to light having a long wavelength, and to an image sensor having improved light absorption and color reproducibility, and a method of manufacturing the same.

In general, an image sensor is a semiconductor device that converts an optical image into an electrical signal. Among them, a charge coupled device (CCD) includes individual metal-oxide-silicon (MOS) capacitors. A device in which charge carriers are stored and transported in a capacitor while being in close proximity to each other. Complementary MOS image sensors use CMOS technology that uses a control circuit and a signal processing circuit as peripheral circuits. A device employing a switching scheme that creates MOS transistors as many as pixels and sequentially detects outputs using the MOS transistors.

CCD (charge coupled device) has many disadvantages such as complicated driving method, high power consumption, high number of mask process steps, complicated process, and difficult to implement signal processing circuit in CCD chip. In order to overcome such drawbacks, the development of a CMOS image sensor using a sub-micron CMOS manufacturing technology has been studied in recent years. The CMOS image sensor forms an image by forming a photodiode and a MOS transistor in a unit pixel and sequentially detects signals in a switching method, and implements an image by using a CMOS manufacturing technology, which consumes less power and uses 30 to 40 masks as many as 20 masks. Compared to CCD process that requires two masks, the process is very simple, and it is possible to make various signal processing circuits and one chip, which is attracting attention as the next generation image sensor.

An image sensor for realizing a color image has an array of color filters on the upper part of the light sensing portion that receives and receives light from the outside to generate and accumulate photocharges. The color filter array (CFA) consists of three colors: red, green, and blue, or three colors: yellow, magenta, and cyan. It is made of collar.

In addition, the image sensor is composed of a light sensing portion for detecting light and a logic circuit portion for processing the detected light as an electrical signal to make data. The ratio of the area of the light sensing portion in the entire image sensor element is increased to increase the light sensitivity. Efforts have been made to increase the fill factor, but these efforts are limited in a limited area because the logic circuit part cannot be removed.

Therefore, in order to increase the light sensitivity, a light converging technology has emerged that changes the path of light incident to the area other than the light sensing part and collects the light into the light sensing part. For this purpose, the image sensor uses a microlens on the color filter. The method of forming is used.

FIG. 1A is a cross-sectional view showing a cross section of a CMOS image sensor including a color filter and a microlens as described above. First, an element isolation film 11 defining an active region and a field region is formed on a semiconductor substrate 10. In each unit pixel, a light sensing means 12 made of a photodiode or the like is formed. In FIG. 3, the transistors constituting the unit pixel are not shown.

In this way, after the device isolation layer 11 and the related elements including the light sensing means 12 are formed, the interlayer insulating film 13 is formed on the semiconductor substrate 10, and then the final metal wiring ( 14) is formed. Although FIG. 1A illustrates the case where one metal wire 14 is used, more metal wires may be used. In an embodiment of the present invention, a case where one metal wire is used has been described as an example. It is called (14). At this time, the metal wiring is intentionally laid out so as not to block the light incident on the light sensing means 12.

After the final metal wiring 14 is formed in this manner, the passivation film 15 is formed on the final metal wiring to protect the device from moisture, scratches, and the like. A color filter 16 is formed on the passivation layer 15 to implement a color image. A color filter typically uses a dyed photoresist, and one color filter 16 is formed for each unit pixel. Separates color from incident light.

Since the color filter 16 is usually formed with a step, it is necessary to eliminate the step caused by the color filter in order for the microlens to be formed in the subsequent process to be formed on the flattened surface. For this purpose, the planarization film 17 is formed on the color filter 16, and the microlens 18 is formed on the planarization film 17. The microlens 18 may form a dome-shaped microlens by flowing a rectangular photosensitive film, and a low-temperature oxide film 19 is formed on the microlens to protect the microlens.

Referring to FIG. 1A, in the conventional CMOS image sensor having such a structure, the photodiodes corresponding to the red, green, and blue color filters all have the same doping profile, and the radius of curvature of the microlenses is also formed the same.

FIG. 1B is a graph showing the relationship between the absorption rate and the penetration depth absorbed by the photodiode according to the wavelength of incident light. Referring to this, it can be seen that the penetration depth increases and the absorption rate decreases as the wavelength of incident light increases. have.

Therefore, in the case of green light or blue light having a relatively short wavelength in visible light, the penetration depth can be controlled within the photodiode region, so that relatively stable color reproduction is possible, but in the case of red light having a long wavelength, the penetration depth is deep. There is a risk of penetration in excess. In addition, such red light causes cross talk between neighboring sites, or because the depth of penetration is deep, the absorption rate is reduced, making it difficult to reproduce the color.

In addition, since the curvature radii of the microlenses corresponding to the red, green, and blue color filters are all the same, it is difficult to accurately control the color reproduction characteristics according to the wavelength of the incident light.

An object of the present invention is to provide an image sensor and a method for manufacturing the same, which improve light absorption and color reproducibility by reducing the radius of curvature of a microlens corresponding to light having a long wavelength.

1A is a cross-sectional view showing a cross-sectional structure of a conventional CMOS image sensor;

Figure 1b is a graph showing the relationship between the penetration depth and the absorption rate according to the wavelength,

Figure 1c is a view showing the change in radius of curvature according to the thickness of the microlens,

2A to 2E are cross-sectional views illustrating a method of manufacturing a CMOS image sensor according to an embodiment of the present invention;

Figure 3 is a cross-sectional view showing the structure of the CMOS image sensor according to another embodiment of the present invention.

* Description of the symbols for the main parts of the drawings *

20: substrate 21: device isolation film

22 photodiode 23 interlayer insulating film

24: final metal wiring 25: passivation film

26a: blue filter 26b: green filter

26c: red filter 27: planarization film

28: first mask 29: photosensitive film for microlenses

29a, 29b, 29c: microlenses

30: low temperature oxide film

According to the present invention for achieving the above object, a first filter for separating light having a relatively short wavelength, a second filter for separating light having a long wavelength, and for separating light having a longer wavelength than light separated by the second filter. An image sensor having a third filter, the image sensor comprising: a passivation film formed on a semiconductor substrate on which a substructure including photosensitive means is completed; First to third color filters formed on the passivation film; A planarization film formed on the first to third color filters, the flattening film having a first groove in a region corresponding to the third color filter; A first micro lens having a relatively large radius of curvature and formed on the planarization layer corresponding to the first color filter; A second micro lens having a relatively large radius of curvature and formed on the planarization layer corresponding to the second color filter; And a third microlens having a curvature radius smaller than that of the first to second microlenses and formed on the planarization film including the first groove in correspondence to the third color filter.

The present invention also includes a first filter for separating light having a relatively short wavelength, a second filter for separating light having a long wavelength, and a third filter for separating light having a longer wavelength than the light separated by the second filter. An image sensor comprising: first to third color filters formed on a semiconductor substrate on which a lower structure including a light sensing means is completed; A first groove formed on the first to third color filters, the first groove deeper than the second groove and the second groove, respectively, in the region corresponding to the second color filter and the region corresponding to the third color filter; Planarization film; A first micro lens having a relatively large radius of curvature and formed on the planarization layer corresponding to the first color filter; A second microlens having a curvature radius smaller than that of the first microlens and formed on the planarization film including the second groove corresponding to the second color filter; And a third microlens having a curvature radius smaller than that of the second microlens and formed on the planarization film including the first groove corresponding to the third color filter.

The present invention also includes a first filter for separating light having a relatively short wavelength, a second filter for separating light having a long wavelength, and a third filter for separating light having a longer wavelength than the light separated by the second filter. A method of manufacturing an image sensor, the method comprising: forming a passivation film on a semiconductor substrate on which a substructure including photosensitive means is completed; Forming first to third color filters on the passivation film; Forming a planarization film on the first to third color filters; Forming a first groove only in an area corresponding to the third color filter on the planarization film; Applying a first photoresist film for microlenses onto the entire structure including the first grooves and patterning the first photoresist film to form first to third microlenses corresponding to the first to third color filters; And forming a low temperature oxide film on the first to third microlenses.

The present invention improves the light absorption rate and color reproducibility by reducing the curvature radius of the microlenses corresponding to the long wavelength of the incident light, and the present invention provides a different curvature radius of the microlens for each wavelength of the incident light. By this, the invention enables correction and improvement of color reproducibility characteristics.

Hereinafter, the most preferred embodiments of the present invention will be described with reference to the accompanying drawings so that those skilled in the art can easily implement the technical idea of the present invention.

Figure 1c shows that the curvature radius of the microlens is changed when the coating thickness of the microlens is changed and flows under the same conditions. The thickness of the microlens decreases as the thickness of the microlens becomes thicker. have. As the radius of curvature of the microlenses decreases, the focal length of the microlenses decreases as much, which will be described later.

2A to 2E are process cross-sectional views illustrating a manufacturing process of a CMOS image sensor according to a first exemplary embodiment of the present invention, with reference to this, a CMOS image sensor and a manufacturing method thereof according to the first exemplary embodiment will be described. .

First, referring to FIG. 2A, the process of forming the color filter 26 and the flattening film 27 is the same as in the prior art. That is, after forming the isolation layer 21 defining the active region and the field region on the semiconductor substrate 20, unit pixels including the photodiode 22 are formed in a predetermined region in the substrate. In FIG. 2A, only the photodiode is shown, and each transistor constituting the unit pixel is not shown.

After the device isolation film 21, the photodiode 22, and the transistors (not shown) are formed, the interlayer insulating film 23 is formed on the semiconductor substrate 20, and the metal wiring 24 is then formed on the interlayer insulating film ( 23) form. Although FIG. 2A illustrates the case where one metal wire 24 is used, more metal wires may be used. In this case, the metal wires are intentionally laid out so as not to block the light incident on the photodiode 22.

After the final metal wiring 24 is formed in this manner, the passivation film 25 is formed on the final metal wiring to protect the device from moisture, scratches, and the like.

One embodiment of the present invention, for use in the passivation film in the lamination USG (Undoped Silicate Glass) film and the plasma-enhanced nitride film (Plasma Enhanced Nitride) film, this passivation film general oxide film (SiO ₂₎ or the general oxy-nitride film (Si ₃ There is an advantage in the dark current characteristics than the passivation film composed of N ₄ ).

Next, a color filter of blue (26a), green (26b), and red (26c) is formed on the passivation layer 25. As a constituent of the color filter, an organic photoresist is usually dyed. Is used, and a photoresist having a characteristic in which the exposed portion remains (negative characteristic) is used.

Such a color filter is formed one by one for each unit pixel through an exposure process using an appropriate reticle, and serves to separate colors from incident light.

Since the color filters 26a, 26b, and 26c are usually formed with a step, the step due to the color filter should be eliminated in order for the microlenses to be formed in the subsequent process to be formed on the flattened surface. For this purpose, the planarization layer 27 is formed on the color filter 26.

After the planarization film 27 is formed in this manner, the planarization film 27 is hardened through an ultraviolet violet baking process. This is to prevent the planarization layer from being removed in the subsequent photo resist strip process and to easily etch the planarization layer 27 in the subsequent etching process.

Next, after applying the first photoresist 28 having a positive characteristic on the planarization film 27, a reticle (not shown) that was used to form the red filter 26c as shown in Figure 2b Is used as it is and only the area corresponding to the red filter 26c is exposed.

Since the first photoresist 28 has a positive characteristic as described above, only the exposed portion is removed so that the first mask 28 exposing the surface of the planarization film 27 corresponding to the red filter 26c is provided. Is formed. As described above, in one embodiment of the present invention, since only one mask process is added without adding a new reticle, there is an advantage of simplifying the process and reducing the cost.

Subsequently, when the planarization film 27 is etched at about 500 to 1000 mV using the first mask 28 as an etching barrier, grooves in which the planarization film 27 corresponding to the red filter 26c is removed are formed. This groove is for forming a thick microlens corresponding to the red filter.

Next, as shown in FIGS. 2C to 2D, the photoresist film 29 for forming a microlens is coated on the entire structure and patterned to form a rectangular microlens (corresponding to each of the color filters 26a, 26b, and 26c). 29a, 29b, 29c) are formed. Referring to FIG. 2D, unlike the other microlenses, the microlens 29c corresponding to the red filter 26c is formed in the groove from which the planarization film 27 is removed, so that the thickness of the microlens 29c is higher than that of the other microlenses. Able to know.

Next, as illustrated in FIG. 2E, a rectangular microlens is flowed to form a dome microlens.

Referring to FIG. 1C, when the microlenses having different thicknesses are flowed under the same conditions, the radius of curvature of the thick microlenses is small. Therefore, even in the case shown in Fig. 2E, since the thickness of the microlens 29c corresponding to the red filter is the thickest, the radius of curvature of the microlens 29c corresponding to the red filter is the smallest.

The long wavelength red light has a deep penetration depth and sometimes penetrates beyond the photodiode region, and also has a low disadvantage of absorbance. However, if the curvature radius of the microlens condensing red light is reduced as in the embodiment of the present invention, The focal length can be reduced to compensate for the above disadvantages.

That is, when the radius of curvature is reduced, the depth of focus at which light is focused becomes shallow, thereby improving the absorption rate of light having a deep penetration depth, such as red light, which is a long wavelength, and the light having a long wavelength, such as red light. Penetration beyond the area can be prevented.

In addition, when the microlenses are formed in the grooves formed by etching the planarization layer 27 to a predetermined thickness, a bridge phenomenon in which adjacent microlenses and microlenses stick to each other during the subsequent flow process can be suppressed. There is also an advantage.

After the microlenses 29a, 29b and 29c are formed in this manner, a low temperature oxide film 30 for protecting the microlenses is further formed on the microlenses 29a, 29b and 29c.

3 is a diagram illustrating a structure of an image sensor according to another exemplary embodiment of the present invention, in which the curvature radius of the microlenses corresponding to the green filter is changed in addition to the microlenses corresponding to the red filter to improve the characteristics of the device.

That is, referring to FIG. 3, grooves are formed not only on the surface of the flattening film corresponding to the red filter 26c but also on the surface of the flattening film corresponding to the green filter 26b, where the grooves are formed at a depth d1 <d2. Are different from each other.

That is, since green light has a shorter wavelength than red light, the depth of penetration is so shallow that, considering the optimum radius of curvature, the thickness of the microlenses corresponding to the green filter 26b is greater than the thickness of the microlenses corresponding to the red filter 26c. It should be thin.

Accordingly, grooves are formed not only on the surface of the flattening film corresponding to the red filter, but also on the surface of the flattening film corresponding to the green filter, and the depth d1 at which the groove is formed is equal to that of the groove formed on the surface of the flattening film corresponding to the red filter. It can be seen that it is shallower than the depth d2.

Since the microlenses corresponding to the blue light having the shortest wavelength are formed on the surface of the planarization film 27 as they are, they are the thinnest. Since the blue light has the shortest penetration depth, the radius of curvature of the corresponding microlenses is increased (micro By reducing the thickness of the lens), the focal length optimized for blue light is realized.

In another embodiment of the present invention, by differently forming the curvature radii of the microlenses corresponding to the red, green, and blue color filters, the color reproducibility can be precisely controlled by optimizing the absorption ratio for each wavelength band of the incident light.

In addition to the CMOS image sensor, the present invention can be applied to a charge coupling device including a micro lens and a color filter.

As described above, the present invention is not limited to the above-described embodiments and the accompanying drawings, and the present invention may be variously substituted, modified, and changed without departing from the spirit of the present invention. It will be apparent to those of ordinary skill in Esau.

According to the present invention made as described above, by forming the curvature radius of the microlens according to the wavelength of the incident light, it is possible to optimize the color reproducibility of the image sensor, and also to prevent the crosstalk phenomenon caused by the red light.

Claims (7)

An image sensor having a first filter for separating light having a relatively short wavelength, a second filter for separating light having a long wavelength, and a third filter for separating light having a longer wavelength than the light separated by the second filter, A passivation film formed on the semiconductor substrate on which the underlying structure including the light sensing means is completed; First to third color filters formed on the passivation film; A planarization film formed on the first to third color filters, the flattening film having a first groove in a region corresponding to the third color filter; A first micro lens having a relatively large radius of curvature and formed on the planarization layer corresponding to the first color filter; A second micro lens having a relatively large radius of curvature and formed on the planarization layer corresponding to the second color filter; And A third microlens having a curvature radius smaller than that of the first to second microlenses and formed on the planarization layer including the first groove in correspondence to the third color filter; Image sensor comprising a. An image sensor having a first filter for separating light having a relatively short wavelength, a second filter for separating light having a long wavelength, and a third filter for separating light having a longer wavelength than the light separated by the second filter, First to third color filters formed on the semiconductor substrate on which the lower structure including the light sensing means is completed; A first groove formed on the first to third color filters, the first groove deeper than the second groove and the second groove, respectively, in the region corresponding to the second color filter and the region corresponding to the third color filter; Planarization film; A first micro lens having a relatively large radius of curvature and formed on the planarization layer corresponding to the first color filter; A second microlens having a curvature radius smaller than that of the first microlens and formed on the planarization film including the second groove corresponding to the second color filter; And A third microlens having a curvature radius smaller than that of the second microlens and formed on the planarization layer including the first groove corresponding to the third color filter; Image sensor comprising a. The method according to claim 1 or 2, The first groove has a depth of 500 ~ 1000Å, characterized in that the image sensor. The method according to claim 1 or 2, Each of the first to third color filters is a blue filter, a green filter, and a red filter. A method of manufacturing an image sensor comprising a first filter for separating light having a relatively short wavelength, a second filter for separating light having a long wavelength, and a third filter for separating light having a longer wavelength than the light separated by the second filter. To Forming a passivation film on the semiconductor substrate on which the substructure including the light sensing means is completed; Forming first to third color filters on the passivation film; Forming a planarization film on the first to third color filters; Forming a first groove only in an area corresponding to the third color filter on the planarization film; Applying a first photoresist film for microlenses onto the entire structure including the first grooves and patterning the first photoresist film to form first to third microlenses corresponding to the first to third color filters; And Forming a low temperature oxide film on the first to third microlenses Method of manufacturing an image sensor comprising a. The method of claim 5, wherein The first to second microlenses are formed to have a relatively large radius of curvature, And the third microlens has a smaller radius of curvature than the first to second microlenses. The method of claim 5, wherein The step of forming the first groove only in the region corresponding to the third color filter on the planarization film, Applying a positive photoresist on the planarization film; Exposing only the region corresponding to the third color filter to remove the positive photoresist of the exposed portion; And Forming a first groove in the planarization layer by using the positive photoresist from which only a region corresponding to the third color filter is removed as an etching barrier; Manufacturing method of an image sensor, characterized in that it further comprises.