Generally, an image sensor is a semiconductor device for converting an optical image into an electrical signal, and is largely classified as a charge coupled device (CCD) image sensor or a complementary metal oxide semiconductor (CMOS) image sensor.
The CMOS image sensor includes a photodiode unit for detecting incident light, and a CMOS logic circuit for generating data by processing the detected light into an electric signal. As the amount of light received in the photodiode increases, the photosensitivity of the image sensor improves.
In order to improve photosensitivity of an image sensor, one method uses a technology for increasing the ratio (a fill factor) of the area occupied by a photodiode to the total area of the image sensor; and another method uses a light condensing technology, which changes an optical path of light incident on an area outside a photodiode in order to condense the light into the photodiode.
A typical example of a light condensing technology is to form microlenses. According to this technology, the microlenses are formed of a material having excellent light transmittance on an upper surface of a photodiode. The microlens is formed in a convex shape. This is to refract a path of the incident light such that a larger amount of light is projected into a photodiode region.
Hereinafter, a related art CMOS image sensor is described with reference to FIG. 1.
FIG. 1 is a sectional view of the related art CMOS image sensor.
The related art CMOS sensor, as illustrated in FIG. 1, includes photodiodes 12, a first planarization layer 13, red (R), green (G), and blue (B) color filter layers 14, a second planarization layer 15, and a microlens 16. In particular, at least one photodiode 12 is formed in the surface of a semiconductor substrate 11 to generate electric charges according to an amount of incident light. The first planarization layer 13 is formed on an entire surface of the semiconductor substrate 11 including photodiodes 12. The R, G, B color filter layers 14 are formed on the first planarization layer 13 to transmit light in each specific wavelength range. The color filter layers 14 are formed with a predetermined interval between each color filter. The second planarization layer 15 is formed on an entire surface of the semiconductor substrate 11 having the R, G, B color filter layers 14. The microlens is formed on the second planarization layer 15 in a convex shape having a predetermined curvature to condense light that transmits to a corresponding R, G, B color filter 14.
Here, the microlens 16 is formed of polymer-based resin, and shaped to have the predetermined curvature and size by first patterning the polymer-based resin through deposition, exposure, and development process, and then reflowing the patterned polymer-based resin by a process such as a reflow process.
That is, the optimized size, thickness, and curvature diameter of the microlens 16 are determined by the size, position, and shape of a unit pixel, the thickness of the photosensitive device, and the height, position, and size of a light blocking layer.
At this point, a shape of the pattern profile may change according to exposure conditions.
For example, process progressing conditions may change according to a thin film condition in a semiconductor substrate. In reality, the pattern formation condition is very unstable, thereby deteriorating light condensing efficiency.
The microlens 16 is formed to increase the light condensing efficiency in a process for manufacturing the related art CMOS image sensor. The microlens 16 is a very important factor determining image sensor characteristics.
The microlens 16 serves to condense a larger amount of light into the photodiode 12 through each color filter layer 14 according to a wavelength when natural light is projected onto the image sensor.
The light incident to the image sensor is condensed by the microlens 16, and the light filtered through the color filter layers 14 is incident into the photodiode 12 corresponding to the color filter layers 14 below the photodiode 12.
The related art CMOS image sensor has a following problem.
That is, since the color filter layers 14 have respective different thickness according to the filter color type, height difference occurs. To resolve this problem, the thickness of the second planarization layer 15 is formed to be more than half of the thickness of the color filter layer after forming the color filter layers 14.
Since the microlens is formed on the second planarization layer 15 having a plane shape, the range of a real image that can be captured is limited.
Additionally, light may be incident into adjacent pixels through the color filter layers 14 having a predetermined interval therebetween. Therefore, crosstalk may occur and image sensor characteristics may deteriorate.