1. Field of the Invention
The present invention relates to an image sensor, and more specifically, to a complementary metal oxide semiconductor (CMOS) image sensor and manufacturing method thereof.
2. Description of the Related Art
Conventionally, an image sensor, as a kind of semiconductor device, transforms optical images into electrical signals. Image sensors can be generally classified into charge coupled devices (CCDs) and CMOS image sensors.
A CCD comprises a plurality of photo diodes arranged in the form of matrix to transforms optical signals into electrical signals, a plurality of vertical charge coupled devices (VCCDs) formed between the photo diodes to transmit charges generated in each photo diode in a vertical direction, a plurality of horizontal charge coupled devices (HCCDs) for transmitting charges transmitted from each VCCDs in a horizontal direction, and a sense amplifier for sensing charges transmitted in the horizontal direction to output electrical signals.
It has been generally known that CCDs have complicated operational mechanisms and high power consumption. In addition, its manufacturing method is relatively complicated, because multiple photolithographic steps are required in its fabrication. Especially, it is difficult to integrate a CCD with other devices such as control circuits, signal processing circuits, analog/digital converters. etc., in a single chip. Such disadvantages of CCD hinder miniaturization of products.
In order to overcome the above described disadvantages of CCDs, CMOS image sensors have been recently developed as the oncoming generation of image sensor. A CMOS image sensor generally comprises MOS transistors formed in a semiconductor substrate by CMOS fabrication technologies. In a CMOS image sensor, the MOS transistors are formed relative to the number of unit pixels, along with peripheral circuits such as control circuits, signal processing circuits, and the like. CMOS image sensors employ a switching mode that MOS transistors successively detect the output of each pixel.
More specifically, CMOS image sensors comprise a photo diode and a number of MOS transistors in each pixel, thereby successively detecting electrical signals of each pixel in a switching mode to express a given image.
The CMOS image sensor has advantages such as low power consumption and relatively simple fabrication process. In addition, CMOS image sensors can be integrated with control circuits, signal processing circuits, analog/digital converter, etc.. because CMOS manufacturing technologies are used, which enables miniaturization of products.
CMOS image sensors have been widely used in a variety of applications such as digital still cameras, digital video cameras, cellular phones, and the like.
Meanwhile, CMOS image sensors can also be classified into 3T, 4T, 5T types, etc., according to the number of transistors per unit pixel. The 3T type of CMOS image sensor comprises one photo diode and three transistors per unit pixel, and the 4T type comprises one photo diode and four transistors per Unit pixel. Here, a layout of unit pixel in a 4T type of CMOS image sensor is configured as follows.
FIG. 1 is a circuit diagram of a conventional 4T type of CMOS image sensor. FIG. 2 is a layout of unit pixel in the conventional 4T type of CMOS image sensor.
As shown in FIG. 1, each unit pixel 100 of the conventional CMOS image sensor comprises a photo diode 10 functioning as a photoelectric transformer, and four transistors including a transfer transistor 20, a reset transistor 30, a driver transistor 40, and a select transistor 50. In addition, the output terminal (referred to as “OUT”) of the each unit pixel 100 is electrically connected to a load transistor 60.
In FIG. 1, the reference symbol “FD” represents a floating diffusion region, “Tx” represents a gate voltage of the transfer transistor 20, “Rx” represents a gate voltage of the reset transistor 30, “Dx” represents a gate voltage of the driver transistor 40, and finally “Sx” represents a gate voltage of the select transistor 50.
As shown in FIG. 2, in the conventional CMOS image sensor, an active region is defined in a portion of each unit pixel, and an isolation layer is formed in the remaining portion of each unit pixel except for the active region. One photo diode PD is formed in a large portion of the defined active region, and gate electrodes 23, 33, 43, and 53 of four transistors are respectively formed to overlap with other portion(s) of the active region.
The gate electrode 23 constitutes part of the transfer transistor 20. The gate electrode 33 constitutes part of the reset transistor 30. The gate electrode 43 constitutes part of the driver transistor 40. And, the gate electrode 53 constitutes part of the select transistor 50.
Here, dopant ions are implanted in the active region where each transistor is formed, except for the portion of active region below each gate electrodes 23, 33, 43, and 53, to form source and drain regions of each transistor.
FIG. 3 is a cross-sectional view illustrating a CMOS image sensor manufactured according a conventional method.
As shown in FIG. 3, a P− type epitaxial layer 101 is formed on a P++ type semiconductor substrate 100 in which an active region (including a photo diode region and a transistor region) and an isolation region are defined. In addition, a field oxide layer 102 is formed in the isolation region of the substrate 100 for isolation of green, red, and blue light absorption regions. Moreover, an N− type diffusion region 103 is formed in the photo diode region of the substrate 100.
In addition, gate electrodes 105 are formed on the transistor region of the substrate 100, and gate insulating layers 104 are interposed between the substrate 100 and the gate electrodes 105. Insulating sidewalls 106 are formed on sides of each gate electrode 105. Moreover, a diffusion blocking layer 108 is formed over the entire surface of the substrate 100, covering the gate electrodes 105. An interlevel dielectric layer 109 is formed on the diffusion blocking layer 108. And, various metallization layers or wirings 110 are formed and spaced on the interlevel dielectric layer 109.
In addition, a first planarization layer 111 is formed over the entire surface of the substrate 100, covering the metallization wirings 110. Furthermore, red (R), green (Cs), and blue (B) color filter layers 112 are formed on the first planarization layer 111, respectively corresponding to each N− type diffusion region 103.
Then, a second planarization layer 113 is formed over the entire surface of the substrate 100, covering the color filter layers 112. Plural microlenses 114 are formed on the second planarization layer 113, respectively corresponding to each color filter layer 112. Here, the reference numeral “107” represents source/drain diffusion regions of each transistor.
FIG. 4a is a graph illustrating changes of absorption coefficient and penetration depth according to the wavelength of incident light, and FIG. 4b is a graph illustrating a percentage of the penetration depth to the wavelength of light incident to a photo diode region in a conventional CMOS image sensor.
As shown in FIG. 4a, red light penetrates up to 10 μm, deeper than light of other colors. In general, in the case of a RGB system, it is difficult to reproduce all colors of light in uniform proportions (i.e., 1:1:1). As a result, in color-reproduction, a CMOS image sensor rarely has an ideal proportion of 1:1:1, so that color-reproducibility characteristics are less than ideal.
In addition, as shown in FIG. 4b, the penetration region of red light having a wavelength of about 700 nm reaches up to 4000 Å˜15000 Å below a surface of a semiconductor substrate, while the penetration region of blue or green light mainly reaches to 4000 Å or less. In other words, when comparing the amounts of penetrated light detected by the conventional CMOS image sensor, red light represents 60% or more of the penetrated/detected light, green light represents about 20%˜40% of the penetrated/detected light, and blue light represents about 20% or less of the penetrated/detected light.
Especially, in the above-described conventional CMOS image sensor, a diffusion blocking layer 108 of a nitride material is formed before forming the interlevel dielectric layer 109, and the nitride layer can reduce the dynamic range of the photo diode region as the CMOS image sensor is scaled down (e.g., as the size/area of tile photo diode region decreases). Accordingly, it is more difficult to reproduce all colors in an ideal proportion, because of the relatively low transmittance of other color lights, especially, blue color light.