Generally, an image sensor is a semiconductor device that converts an optical image to an electric signal. An image sensor can be classified as a charge coupled device (CCD) or a complementary metal oxide silicon (CMOS) image sensor.
The CCD includes a plurality of photo diodes converting an optical signal into an electric signal that are arranged in a matrix pattern, a plurality of vertical charge coupled devices formed between the photo diodes for transferring electric charges generated from the photo diodes in a vertical direction, a horizontal charge coupled device for transferring the electric charges transferred from the vertical charge coupled devices in a horizontal direction, and a sense amplifier for outputting an electric signal by sensing the electric charges transferred in the horizontal direction and outputting an electric signal.
The major drawbacks to the CCD are the complicated driving method and high power consumption. Also, the fabricating method of the CCD is complicated because a multi-level photo process is required.
In addition, it is difficult to integrate a control circuit, a signal processing circuit, an analog/digital (A/D) converter, and other circuits with the CCD. This makes it difficult to reduce a size of a product employing the CCD.
Therefore, a CMOS image sensor has been developed as a next-generation image sensor that can overcome the drawbacks of the CCD.
The CMOS image sensor is a device employing a switching method in which outputs of unit pixels are sequentially detected by MOS transistors, the number of which is identical to that of unit pixels. The MOS transistors are formed on a semiconductor substrate, and a control circuit and a signal processing circuit are used as peripheral circuits.
That is, a photodiode and a MOS transistor are formed in each unit pixel so that the CMOS image sensor realizes an image by sequentially detecting electric signals of the unit pixels using the switching method.
Since the CMOS image sensor uses CMOS fabrication technology, the power consumption is low and the fabrication process is simplified. This simplification can be due to the reduced number of photo processes.
In the CMOS image sensor, because the control circuit, the signal processing circuit, and the A/D converter circuit can be integrated with a CMOS image sensor chip, the size of the product employing the CMOS image sensor can be reduced.
Therefore, the CMOS image sensor has been widely used in a variety of applications such as digital cameras and digital video cameras.
The CMOS image sensor is classified into types according to the number of transistors, such as a 3T-type, 4T-type, 5T-type. For example, the 3T-type CMOS image sensor includes one photodiode and three transistors, and the 4T-type CMOS image sensor includes one photodiode and four transistors. The layout of the unit pixel of the 3T-type CMOS image sensor will now be described.
FIG. 1 is a lay-out of the unit pixel of a conventional 3T-type CMOS image sensor and FIG. 2 is an embodiment of a sectional view taken along line A-A′ of FIG. 1, illustrating a photodiode and a transfer transistor of the conventional CMOS image sensor.
As shown in FIG. 1, an active region 10 is defined and one photodiode (PD) 20 is formed on a wide width portion of the active region 10. Gate electrodes 30, 40, and 50 of three transistors are formed to overlap a remaining portion of the active region 10.
That is, a reset transistor (Rx) is formed by the gate electrode 30, a drive transistor. (Dx) is formed by the gate electrode 40, and a select transistor (Sx) is formed by the gate electrode 50.
Impurities are implanted into the active region 10 of the transistors except for the regions below the gate electrodes 30, 40 and 50, thereby forming a source/drain region of each transistor.
A power voltage Vdd is applied to the source/drain region between the reset transistor (Rx) and the drive transistor (Dx). The source/drain region at a side of the select transistor Sx can be connected to a readout circuit (not shown).
The gate electrodes 30, 40 and 50 are connected to respective signal lines (not shown). Each signal line can be provided with a pad connected to an external drive circuit.
According to an embodiment of the section view taken along line A-A′ of FIG. 1 as shown in FIG. 2, a P− type epitaxial layer 12 is formed on a P++ type semiconductor substrate 11 and incorporates an active region with a photodiode region and a transistor region and an isolation region. An isolation layer 13 is formed on the isolation region.
A gate electrode 15 is formed on a portion of the epitaxial layer 12 in the transistor region for the reset transistor with a gate insulation layer 14 interposed therebetween. An insulation sidewall 16 is formed on both sides of the gate electrode 15.
An n-type diffusion region 19 is formed on the epitaxial layer 12 of the photodiode region PD.
An LLD region 17 and a source/drain impurity region 18 are formed on the transistor region of the semiconductor substrate 11.
Because the CMOS image sensor is an analog device the function of the resistor is important.
For example, a resistor can function to catch a reference voltage and is formed of a poly having a relatively low temperature dependency.
A non-salicided poly silicon is formed to obtain a desired resistance. A middle resister of the conventional CMOS sensor can have a resistance of about 200-800Ω/□(ohm/sq).
In a method for forming the middle resistor, impurities are implanted after a poly gate is formed to form a sheet resistance suitable for a device property. However, according to the prior art, the photodiode 19 can be damaged when the insulation sidewall 16 is formed on the both sides of the gate electrode 15.
That is, the region of the photodiode PD may be primarily damaged by plasma during the process for forming the gate electrode 15 and secondarily damaged by the plasma during the process for forming the insulation sidewall 16.
In order to prevent the plasma damage on the photodiode region when the insulation sidewall is formed, a scheme for forming a nitride layer has been developed. In this case, since the nitride layer remains on the photodiode even when the process is finished, the remaining nitride layer deteriorates the transmittance of the light, thereby making it difficult to scale-down the device.
That is, when the device is scaled-down, the photodiode region is reduced and thus the dynamic range is reduced. As a result, an amount of light incidence is reduced and can make it difficult to reproduce the image.