The present application claims priority under 35 U.S.C. 119 to Korean Patent Application No. P2006-87736 (filed on Sep. 12, 2006), which is hereby incorporated by reference in its entirety.
An image sensor is a semiconductor device used to convert optical images detected by the image sensor to electric signals. Image sensors may be classified as a charge coupled device (CCD) and a complementary metal oxide semiconductor (CMOS).
A CMOS image sensor is provided with MOS transistors whose number corresponds to the number of pixels of a semiconductor device having a control circuit and a signal processing circuit as peripheral circuits. The control circuit and the signal processing unit may be integrated together to employ a switching method that detects output through the MOS transistors.
A CMOS image sensor may be provided with a plurality of unit pixels whereby each unit pixel includes one light sensing device such as a photodiode and a plurality of MOS transistors.
As illustrated in example
As illustrated in example
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As illustrated in example
P-type ion-implantation region (PDP) and the N-type ion-implantation region (PDN) may be formed in closer spatial proximity to the upper surface of silicon substrate. Particularly, high-density ions such as boron (B) or BF2 are implanted into P-type ion-implantation region (PDP). The high-density ions of boron (B) or BF2, which have great mobility, are diffused to N-type ion-implantation region (PDN) or transfer transistor, whereby their circumferential area is changed in doping density. In order to form PN junction adjacent to the upper surface of silicon substrate (Sub), it is important to lower the doping density of P-type ion-implantation region (PDP).
Embodiment relate to a CMOS image sensor including an MOS transistor having a photodiode including a well having a first conductive type formed in a semiconductor substrate. A first ion-implantation layer formed in the semiconductor substrate having a conductive type opposite to the first conductive type of the well. A second ion-implantation layer having the first conductive type, is formed adjacent to the surface of the semiconductor substrate above the first ion-implantation layer. A conductive electrode which is transparent to visible rays, is formed on the semiconductor substrate to cover the second ion-implantation layer having the first conductive type.
In accordance with embodiments, placement of a PN junction of a photodiode adjacent to an upper surface of a silicon substrate prevent dark currents by some of the ion-implantation layer coming into a depletion region of the PN junction.
Example
Example
Example
As illustrated in example
Ions such as boron B or BF2 may be implanted adjacent to the upper surface of substrate (Sub) and may correspond to a predetermined height above first ion-implantation layer (PDN) to form P-type ion-implantation layer (PDP). Transfer transistor 101 including spacer 13 and floating diffusion region 102 are formed on and/or over substrate (Sub).
As illustrated in example
As illustrated in example
Ground potential (GND) may be applied to the conductive electrode 200 in order that positive charges (holes) of PDP region are induced to the conductive electrode 200. Meaning, the positive charges of PDP region are induced to the upper side of the ion-implantation layer, i.e., the upper surface of silicon substrate (Sub), much like the induction of positive charges to the upper surface of a substrate in a P-MOSFET.
As the positive charges are induced to the upper surface of silicon substrate (Sub) by the field effect, there is no requirement for highly-doped PDP region. Accordingly, the defective surface is not included in the depletion region of the photodiode, thereby preventing dark current.
In accordance with embodiments, while PDP region is doped with low density ions of boron or BF2, the charges may still be induced to the upper surface of substrate (Sub) by the field effect. Accordingly, there is little chance that the junction of the photodiode is moved downward due to the boron or BF2 ions having great mobility. Therefore, the PN junction may be formed adjacent to the uppermost surface of substrate (Sub).
The position of the PN junction may be determined based on the density of PDN, which is useful to the optimization of the fabrication process. In accordance to embodiments, the density of PDP region may be lower than what is typical. Thus, the current-voltage relation in the channel region of the transfer transistor may be rarely influenced by the density of PDP region.
In accordance with embodiments, a plurality of N-channel MOS transistors may be formed in a P-type semiconductor substrate. On the other hand, a plurality of P-type MOS transistors may be formed in an N-type semiconductor substrate, in which case conductive electrode 200 may be connected to a power voltage Vdd.
A CMOS image sensor in accordance with embodiments may yield a variety of advantages. For instance, by decreasing the density of the PDP region, the PN junction of the photodiode may be formed adjacent to the uppermost surface of the substrate. Current leakage caused by the defective surface of the substrate may be prevented. The energy barrier between the transfer transistor and the photodiode may also be lowered. The decrease in the energy barrier implies the decrease of time delay on reading the information of the photodiode, wand thus, enhances the efficiency of the CMOS image sensor.
Although embodiments have been described herein, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.
Number | Date | Country | Kind |
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10-2006-0087736 | Sep 2006 | KR | national |