In general, an image sensor is a semiconductor device that transforms an optical image to electrical signals. The image sensor is generally classified as a charge coupled device (CCD) or a CMOS image sensor. A CCD type image sensor includes several MOS (metal oxide semiconductor) capacitors, closely positioned to one another, in which electric charge carriers are transferred to or saved in the MOS capacitors.
On the other hand, a CMOS image sensor has incorporated a switching mode by forming MOS transistors for each unit pixel with CMOS technology, and using control circuits and signal-processing circuits in conjunction with the MOS transistors to sequentially detect outputs of the photodiodes.
The CCD has various disadvantages, such as a complicated driving mode, high power consumption, impracticality of incorporating a signal processing circuit in a single chip for the CCD due to the many mask processes, and so on. Currently, in order to overcome these disadvantages, many studies have been made of the development of the CMOS image sensor using sub-micron CMOS manufacturing technology.
The CMOS image sensor obtains an image from the formation of a photodiode and a MOS transistor within a unit pixel to detect signals using a switching mode. As mentioned above, because the CMOS image sensor makes use of CMOS manufacturing technology, the CMOS image sensor has low power consumption, as well as a single manufacturing process requiring about 20 masks compared with the CCD manufacturing process requiring 30 to 40 masks. As a result, the CMOS image sensor can integrate a signal processing circuit into a single chip. Accordingly, the CMOS image sensor is currently used in various applications, such as digital still cameras (DSC), PC cameras, mobile cameras, and so forth.
The CMOS image sensor is classified as a 3T type, a 4T type or a 5T type according to the number of transistors formed in each unit pixel. The 3T type CMOS image sensor includes a single photodiode and three transistors, and the 4T type CMOS image sensor includes a single photodiode and four transistors. Hereinafter, the 4T type CMOS image sensor will be described with reference to a schematic and layout thereof.
Hereinafter, a 4T type CMOS image sensor according to the related art will be explained with reference to the accompanying drawings.
FIG. 1 is an equivalent circuit diagram of a 4T type CMOS image sensor. FIG. 2 is a layout of a unit pixel of 4T type CMOS image sensor according to the related art.
As shown in FIGS. 1 and 2, the unit pixel of the CMOS image sensor includes a photodiode 10 as a photoelectric transducer and four transistors.
Here, the four transistors include a transfer transistor (Tx/A) 20, a reset transistor (Rx/B) 30, a drive transistor (Dx/C) 40, and a select transistor (Sx/D) 50. A load transistor (not shown) is electrically connected to the drain of the select transistor (Sx/D) 50, which is an output terminal of each unit pixel.
A floating diffusion region (FD) 60 is formed between the transfer transistor (Tx/A) and the reset transistor (Rx/B) and connects to the gate of the drive transistor (Dx/C).
The construction of the CMOS image sensor may be expressed as follows. The image sensor is composed of a plurality of pixels, which are compactly aligned on a semiconductor epitaxial layer in a row and column formation. Referring to FIG. 2, the image sensor includes a photodiode P, a floating diffusion region 60, and a transfer transistor A. The photodiode P senses external light and generates photo-electrons. The floating diffusion region transfers charges from the photodiode. The transfer transistor A is disposed between the photodiode P and the floating diffusion region, and transfers the charges from the photodiode to the floating diffusion region.
The following is a description of the operation of the image sensor.
First, the reset transistor B is initially turned on so that the potential of the output floating diffusion node becomes the source voltage. At this time, the reference value is detected.
Next, when light from an exterior of the image sensor is incident to the photodiode P, an Electron-Hole Pair (EHP) is generated in proportion to the incident light.
Then, due to a signal charge generated in the photodiode P, the potential of the source node of the transfer transistor changes proportional to an amount of the generated signal charge.
Thereafter, when the transfer transistor A is turned-on, the stored signal charge is transferred to the floating diffusion region. The potential of the output floating diffusion node changes corresponding to the amount of the transferred signal charge. Simultaneously, the gate bias of the drive transistor C varies with the potential of the output floating diffusion node. In the event when the drive transistor turns on, it causes the source potential of the drive transistor C to be changed.
Next, the drive transistor D becomes in an ON state, and data is read out in a column direction.
Then, the reset transistor B is turned on to cause the output floating diffusion node potential to become VDD. This procedure then repeats.
Red, green, and blue color filter arrays are formed at an upper portion of each photodiode, and separately receive for separation red, green, and blue wavelengths. A micro lens is formed at an upper most end of a light receiving portion in order to improve the amount of light incident the photodiode. A signal from each channel is connected to an image processing circuit formed outside the light receiving portion through a plurality of metal lines, and is again combined as one phase through a signal processing procedure. As 0.18 μm and 0.13 μm technology semiconductor technology develops, a size of a pixel tends to be significantly reduced.
A dark current is a phenomenon where electrons move to a floating diffusion region in a photodiode while no light is present. The dark current is mainly incurred from various defects such as line defects and point defects, or from silicon dangling bonds at the surface of a photodiode in an active region. In other words, defects of the lattice structure functions as a trap to capture electrons, which creates the dark current. Such a dark current can cause a serious problem such as a hot pixel due to long exposure or a dark noise level non-uniformity in low illumination intensity.
Conventionally, in order to reduce the dark current, channel stop ions are implanted in an edge part of an active region and a field region. Further, so as to reduce a trap due to the dangling bond, H2 annealing is performed or a buried type photodiode is used. Here, in the buried type photodiode, a photodiode is formed under the surface of the substrate.
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