(a) Field of the Invention
The present invention relates to a complementary metal oxide semiconductor (CMOS) image sensor, and more particularly to a CMOS image sensor and a fabrication method thereof, which is adequate to reduce dark current.
(b) Description of the Related Art
In general, image sensors include a charge coupled device (CCD) image sensor and a CMOS image sensor.
Of these sensors, the CCD image sensor comprises a photo-electric conversion and charge accumulation portion for absorbing light reflected by an external irradiated object into the sensor and collecting and accumulating photocharges, a charge transport portion for transporting photocharges generated in the photo-electric conversion and charge accumulation portion, and a signal conversion portion for converting the photocharges transported from the charge transport portion into an electrical signal.
The photo-electric conversion and charge accumulation portion mainly uses a photodiode which is a device for accumulating charges generated by light in a potential well formed by a PN junction. The charges generated in the photo-electric conversion and charge accumulation portion and confined in the potential well of the photodiode can be transported to a required location by moving the potenial well. In addition, the signal conversion portion generates a voltage from the transported charges. On the other hand, after the voltage is generated by the signal conversion portion, the charges confined in the current potential well are required to be ejected for next charges. For this end, a barrier of the current potential well in the signal conversion portion is removed to eject the charges, which is referred to as “reset”.
In this way, the CCD image sensor detects signals not by means of switching by transistors in the CMOS image sensor, but charge coupling. In the CCD image sensor, since the photodiode corresponding to a pixel for detecting light extracts photocurrent after the photocurrent is accumulated for a certain time rather than extracting it instantly, there is an advantage in that a signal voltage is increased by an amount of accumulation of the photocurrent, which results in good photosensitivity and reduced noise. However, in the CCD image sensor, since the transport of photocharges is continuous, the driving method is complicated and a high voltage of about 8-10 V and high power of more than 1 W are consumed.
On the other hand, although the CMOS image sensor is inferior in electro-optic characteristics to the CCD image sensor, it is superior in low power consumption and high integration to the CCD image sensor.
Accordingly, improvement of the electro-optic characteristics, particularly a dark current characteristic acting as a factor to lower quality and reliability of the CMOS image sensor, is increasing as a subject of interest of the CMOS image sensor. The dark current characteristic is particularly affected by leakage current generated in a lateral wall of a shallow trench isolation (STI) film.
One example of the CMOS image sensor is disclosed in U.S. Pat. Nos. 6,531,725 and 6,545,302.
Hereinafter, the reason why the dark current characteristic is generated will be described in detail with reference to FIGS. 1 to 4.
FIGS. 1 and 2 show a layout of a conventional CMOS image sensor having a 3-transistor (TR) structure and an equivalent circuit diagram thereof, respectively.
A CMOS image sensor 100 comprises one photodiode PD and an aggregate of pixel units each having three transistors TR1, TR2 and TR3.
A P-type semiconductor substrate 102 has STI films 104 for device isolation formed thereon, a device region defined by the STI films 104 has a photodiode PD formed by photodiode N+ implants, and a depletion layer 106 is formed beneath the photodiode PD.
The three transistors TR1, TR2 and TR3 are respectively a reset transistor TR1, a select transistor TR2 and an access transistor TR3.
A reset signal is applied to a gate electrode of the reset transistor TR1 through a reset signal input terminal 108. The reset transistor TR1 has a drain electrode connected to a floating node 110 and a source electrode connected to a VDD terminal Vdd.
In addition, the select transistor TR2 has a gate electrode connected to the floating node 110 and a source electrode connected to the VDD terminal Vdd.
In addition, a row select signal is applied to a gate electrode of the access transistor TR3 through a row select signal input terminal 112. The access transistor TR3 has a source electrode connected to a drain electrode of the select transistor TR2 and a drain electrode connected to a column select line 114.
The photodiode PD is formed between the floating node 110 and a ground 116.
Now, operation of the CMOS image sensor as constructed above will be described.
When the reset transistor TR1 is turned on and a source node potential of the reset transistor TR1 becomes VDD, initialization is completed. At this time, a reference value is sensed.
Also, when electron-hole pairs, i.e., signal charges, are generated in the depletion layer 106 of the photodiode PD by externally incident light, potential of the floating node 110, which is the source electrode of the reset transistor TR1, or potential of the gate electrode of the select transistor TR2 is changed in proportion to the amount of the generated signal charges, and accordingly, potential of the gate electrode of the select transistor TR2 is changed and hence a bias of the source electrode of the select transistor TR2 and the drain electrode of the access transistor TR3 is changed.
In this way, while the signal charges are accumulated, the potentials of the source electrode of the reset transistor TR1 and the source electrode of the select transistor TR2 are changed. At this time, when the row select signal is applied to the gate electrode of the access transistor TR3 through the row select signal input terminal 112, a potential difference due to the signal charges generated in the photodiode PD is outputted to the column select line 114.
As described above, after a signal level is detected by the signal charges generated in the photodiode PD, while the reset transistor TR1 is turned on by the reset signal through the reset signal input terminal 108, all signal charges accumulated in the photodiode PD are reset.
However, with the CMOS image sensor as constructed above, since the photodiode is surrounded by the STI films as shown in FIGS. 3 and 4, current induced on an unstable silicon surface of a lateral wall of the STI film 104 in addition to current by electrons generated by light flows through the photodiode PD along a direction shown as an arrow in FIG. 4.
Accordingly, even in a state where there is no light, there is a problem in that a signal may be erroneously detected in the photodiode PD as if there is light. A current inducing such an error is referred to as “dark current”. This dark current may be even more greatly generated in an image sensor having a structure where devices are isolated from each other by etching a silicon wafer.