KR20090015652A - Unit pixel suppressing dead zone and afterimage - Google Patents

Abstract

A unit pixel suppressing dead zone and afterimage is provided to prevent generation of after image and dead area by preventing a potential barrier which is generated at the contact where two equipotential surfaces meet. In a unit pixel suppressing dead zone and afterimage, a unit pixel comprises a photo diode producing an image charge corresponding to the video signal received and comprised a transfer transistor(M1) switching the image charge with the floating diffusion area in response to the transmission control signal. The part near to the gate terminal of the transfer transistor has impurity concentration higher than that of the rest region. The equipotential surface having the highest or lowest voltage level low is near to the gate terminal of the transfer transistor of photodiode regions. The depth of the part near to the gate terminal of the transfer transistor is deeper than that of the rest photo diode region.

Description

Unit pixel suppressing dead zone and afterimage}

The present invention relates to a unit pixel, and more particularly, to a unit pixel for suppressing dead areas and afterimages.

The image sensor is an image signal conversion apparatus for converting an incident image signal into an electrical signal corresponding thereto. The image sensor uses a plurality of unit pixels for converting an incident image signal into an electrical signal in two dimensions. The image sensor includes a photo diode for generating charge corresponding to the incident image signal. . In order to generate a photodiode on a substrate using a semiconductor process, first, a region of the substrate on which the photodiode is to be formed is defined, and the impurity ions are implanted into the defined region, followed by annealing. P-type and N-type diffusion regions are formed to form a diode. A diode produced through the above process processes an image signal and is called a photodiode.

1 illustrates a process of implanting impurity ions in a process of implanting impurity ions to form a diffusion region.

Referring to FIG. 1, the type of impurity ions implanted into the substrate is determined according to the electrical properties of the diffusion region to be formed. To form the P-type diffusion region, a pentavalent donor should be implanted. In order to form an N-type diffusion region, a trivalent acceptor must be injected.

In Fig. 1, the acceptor is implanted to form an N-type diffusion region in the P ⁻ substrate. Here, minus means that the concentration of impurity ions implanted into the substrate is not high. Figure 1 shows the cut surface of the substrate, the impurity ions are represented by a line having a constant thickness, but since the impurity ions are implanted two-dimensionally (implant) to the substrate with a constant energy (plate) of a constant thickness Becomes

2 illustrates the annealing process.

Referring to FIG. 2, an annealing process is performed to diffuse the plate-type impurity ions implanted in the process of FIG. 1 onto a substrate. In the annealing process, a predetermined thermal energy is applied to the implanted impurity ions, and the impurity ions received with the thermal energy are uniformly diffused in the up, down, left, and right directions of the substrate. Referring to FIG. 2, it can be seen that the concentration N ⁺ of the diffusion region is higher than that of the substrate P ⁻ .

3 is a vertical sectional view of an equipotential surface inside the diffusion region.

Referring to FIG. 3, the voltage level of the equipotential surface (dotted line) inside the diffusion region is the lowest in the central portion of the diffusion region and increases toward the outside in the radial direction. Impurity ions are uniformly implanted in one plane, but when all the impurity ions diffuse up, down, left and right in the annealing process, impurity ions are concentrated in the center. This is because all the charges present in the same plane diffuse in the vertical and horizontal directions, but a considerable amount of the ions of impurity ions present in the four corner portions of the plate eventually converge to the center.

4 is a plan view of an equipotential surface inside the diffusion region.

Referring to FIG. 4, the voltage level of the equipotential surface (dotted line) inside the diffusion region is the lowest at the center of the diffusion region and increases toward the outside in the radial direction. This phenomenon is because the impurity ions at the four corners are most concentrated in the central portion as described above.

5 is a cross-sectional view of the completed unit pixel.

In general, essential components of a unit pixel are a photodiode and an image signal conversion circuit, and the image signal conversion circuit includes a transfer transistor, a reset transistor, and a conversion transistor. FIG. 5 shows photodiodes PD and transfer transistors TX among the essential components of a unit pixel.

Since the function and operation of the unit pixel are generally known, the transfer control signal applied to the equipotential surface (dotted line) of the P-type diffusion region N ⁺ constituting the photodiode PD and the gate terminal M1 of the transfer transistor. Only the equipotential surface (dotted line) in the lower portion of the gate terminal M1 and the photodiode PD region generated by the voltage level of (TX) will be described.

The shape of the equipotential surface of the photodiode PD may be referred to the description of FIG. 3.

The photodiode PD is connected to one terminal of the transfer transistor M1, and the floating diffusion region FD is connected to the other terminal. Since the floating diffusion region D is N ⁺ , the transfer transistor M1 is connected. An N channel will be formed below the gate. Therefore, the voltage level of the equipotential surface at the lower portion of the transfer terminal M1 of the transfer terminal M1, that is, the channel is formed, is the lowest and increases toward the bottom of the substrate.

The equipotential surface of the photodiode PD and the equipotential surface by the gate terminal M1 meet each other near the boundary indicated by the arrow.

6 is an energy diagram at a portion where the equipotential surface of the photodiode PD and the equipotential surface formed by the gate terminal M1 meet.

Referring to FIG. 6, a constant potential wall is present at a portion where an equipotential surface of the photodiode PD indicated by an arrow in FIG. 5 and an equipotential surface formed by the gate terminal M1 meet each other. Among the charges generated in PD), the charges inside the diagonal region having a lower energy than the barrier do not move to the right floating diffusion region FD.

Since the equipotential surface having the highest voltage level exists in the center of the photodiode PD, at the portion where the equipotential surface of the photodiode PD and the equipotential surface by the gate terminal M1 meet in the photodiode PD region. The voltage level of the equipotential surface is relatively higher than the voltage level of the equipotential surface by the gate terminal M1. Thus, as shown in Figure 6, an energy barrier is formed where the two equipotential surfaces meet.

Charges that do not pass through such barriers remain as afterimages or form dead zones. Here, the afterimage refers to a phenomenon that occurs because electric charges remaining in the current frame are transmitted in the signal of the next frame. The dead area defines an area that cannot be converted because the electric charges to be converted into the corresponding electric signal in the current frame do not have energy to exceed the barrier.

SUMMARY OF THE INVENTION The present invention has been made in an effort to provide a unit pixel for suppressing dead areas and afterimages.

Another technical problem to be solved by the present invention is to provide an image sensor for suppressing dead areas and afterimages.

Another technical problem to be solved by the present invention is to provide a method for manufacturing a unit pixel to suppress dead areas and afterimages.

The unit pixel according to the present invention for achieving the above technical problem is provided with at least one photodiode and a transfer transistor. The at least one photodiode generates an image charge corresponding to the received image signal, and the transfer transistor switches the image charge to a floating diffusion region in response to a transfer control signal. Here, the portion of the at least one photodiode region close to the gate terminal of the transfer transistor has a higher concentration of impurity ions or a higher or lowest voltage level of the equipotential surface than the remaining portion of the photodiode region. The depth of the portion of the transfer transistor, which is located close to the gate terminal, or the portion of the photodiode, which is close to the gate terminal of the transfer transistor, is deeper than the rest of the photodiode.

According to another aspect of the present invention, an image sensor includes a plurality of unit pixels including at least one photodiode and a transfer transistor. The at least one photodiode generates an image charge corresponding to the received image signal, and the transfer transistor switches the image charge to a floating diffusion region in response to a transfer control signal. Here, the portion of the at least one photodiode region close to the gate terminal of the transfer transistor has a higher concentration of impurity ions or a higher or lowest voltage level of the equipotential surface than the remaining portion of the photodiode region. The depth of the portion of the transfer transistor, which is located near the gate terminal, or the portion of the photodiode, which is close to the gate terminal of the transfer transistor, is deeper than that of the rest of the photodiode.

According to another aspect of the present invention, there is provided a method of manufacturing a unit pixel, including: manufacturing a unit pixel including a photodiode and an image signal conversion circuit, and defining a whole area of the photodiode. Using a second mask defining a portion of the region in contact with the transfer transistor, and implanting impurity ions into the portion defined by the first mask; impurity identical to the impurity ions in the portion defined by the second mask Implanting ions and performing annealing after performing the two steps.

The present invention has the advantage that the potential barrier does not occur where the equipotential surface of the photodiode meets the equipotential surface of the transfer transistor so that dead areas and afterimages do not occur.

Hereinafter, specific embodiments of the present invention will be described in detail with reference to the accompanying drawings.

7 is a layout of a unit pixel according to the present invention.

Referring to FIG. 7, the unit pixel includes a photodiode PD, a transfer transistor M1, a reset transistor M2, a conversion transistor M3, and a selection transistor M4.

The transfer control signal TX is applied to the gate of the transfer transistor M1, the reset control signal RX is applied to the gate of the reset transistor, and the selection control signal SX is applied to the gate of the selection transistor M4. . The gate of the conversion transistor M3 is connected to the floating diffusion region.

The area defined by the dotted rectangle shown in the overlapping portion of the photodiode PD and the transfer transistor M1 is an area used to change the shape of the equipotential surface of the photodiode from the conventional one.

FIG. 8 is a vertical cross-sectional view taken along line AA ′ of the unit pixel illustrated in FIG. 7.

Referring to FIG. 8, the photodiode is composed of a P-type diffusion region formed on a surface of a substrate and an N-type diffusion region bonded to a lower portion of the P-type diffusion region. For convenience of explanation, it is assumed that the P-type diffusion region is formed in the conventional manner. The shape of the N-type diffusion region is different from the conventional one, and this is the part to which the core idea of the present invention is applied.

Referring to FIG. 8, the equipotential surface of the N-type diffusion region indicated by the dotted line has the lowest voltage level of the equipotential surface indicated by the smallest circle, and the voltage level increases toward the outside. It can be seen that the smallest dotted circle shown in FIG. 8 has moved to the right side, that is, toward the transfer transistor, relative to the position of the smallest dotted circle shown in FIG.

In order to generate an N-type diffusion region having a shape as shown in FIG. 8, the concentration of the acceptor injected near the CC ′ defined by the dotted rectangle shown in FIG. 7 is the concentration of the acceptor injected into the remaining photodiode region. It can be higher than.

In this case, a mask MASK may be additionally used to define the dotted rectangular region illustrated in FIG. 7. That is, after injecting the same concentration of acceptor in the form of a plate into the portion defined by the photodiode by one mask, the acceptor is further injected into the portion (dashed rectangle) defined by the other mask. As described above, annealing is performed after the injection of the two acceptors to obtain an N-type diffusion region of the type shown in FIG. 8. At this time, the depth of the second injected acceptor may be deeper than the depth of the first injected acceptor. This is possible by controlling the impurity implantation energy.

FIG. 9 is a plan view of an equipotential surface of the photodiode of the unit pixel illustrated in FIG. 7.

Referring to FIG. 9, it can be seen that the center of the equipotential surface of the photodiode has moved toward the transfer transistor compared to the case shown in FIG. 4.

FIG. 10 is a vertical cross-sectional view taken along line BB ′ of the unit pixel illustrated in FIG. 7.

Referring to FIG. 10, since the center point of the equipotential surface of the photodiode moves toward the transfer transistor, a portion where the equipotential surface of the photodiode and the equipotential surface by a channel formed by a voltage applied to the gate of the transfer transistor meet each other ( In the arrow), there is no potential barrier so that charges generated in the photodiode are easily transferred along the channel to the floating diffusion region, so that no dead region or afterimage occurs.

As described above, in order to prevent the potential barrier from occurring at the part where the two equipotential surfaces meet, the amount of impurity ions to be injected, the energy of the impurity ions, and the annealing time may be obtained.

In FIG. 7, a plurality of transistors constituting a photodiode PD and an image signal conversion circuit are laid out as essential components of a unit pixel. Recently, however, a separate unit pixel technology has been proposed in which the essential components of the unit pixel are separated and implemented on two chips. The separate unit pixel technology implements a photodiode and a transfer transistor on one chip, and the other. One chip implements the remaining transistors and electrically combines them. In addition, the discrete unit pixel technology is further developed to connect the transfer transistors respectively connected to the plurality of photodiodes in common to one common floating diffusion region (FD), and to switch the plurality of transfer transistors in a time division manner, thereby Techniques have been proposed to reduce the area or increase the area allocated to the photodiode.

Therefore, in the case of a separate unit pixel, at least one photodiode and one common transfer transistor connected to the at least one photodiode in common may be essential components.

Although the above description is about a photodiode formed on a P-type substrate, in the case of an N-type substrate, the positions of the P-type diffusion region and the N-type diffusion region of the photodiode may be interchanged with each other. These differences belong to the technical scope that those skilled in the art can easily use from the above description, and will not be described in detail herein.

In the above description, the technical idea of the present invention has been described with the accompanying drawings, which illustrate exemplary embodiments of the present invention by way of example and do not limit the present invention. In addition, it is apparent that any person having ordinary knowledge in the technical field to which the present invention belongs may make various modifications and imitations without departing from the scope of the technical idea of the present invention.

1 illustrates a process of implanting impurity ions in a process of implanting impurity ions to form a diffusion region.

2 illustrates the annealing process.

3 is a vertical sectional view of an equipotential surface inside the diffusion region.

4 is a plan view of an equipotential surface inside the diffusion region.

5 is a cross-sectional view of the completed unit pixel.

6 is an energy diagram at a portion where the equipotential surface of the photodiode PD and the equipotential surface formed by the gate terminal M1 meet.

7 is a layout of a unit pixel according to the present invention.

FIG. 8 is a vertical cross-sectional view taken along line AA ′ of the unit pixel illustrated in FIG. 7.

FIG. 9 is a plan view of an equipotential surface of the photodiode of the unit pixel illustrated in FIG. 7.

FIG. 10 is a vertical cross-sectional view taken along line BB ′ of the unit pixel illustrated in FIG. 7.

Claims (7)

A photodiode for generating an image charge corresponding to the received image signal; A unit pixel comprising a transfer transistor for switching the image charge to a floating diffusion region in response to a transfer control signal. The portion of the photodiode close to the gate terminal of the transfer transistor has a higher concentration of impurity ions than the rest of the photodiode region, An equipotential surface having the highest or lowest voltage level among the equipotential surfaces is located at a portion of the photodiode region close to the gate terminal of the transfer transistor, and The unit pixel, wherein a depth of the portion of the photodiode close to the gate terminal of the transfer transistor is deeper than the rest of the photodiode. A common floating diffusion region; At least one photodiode for generating an image charge corresponding to the received image signal; And A unit pixel comprising at least one transfer transistor connected to each of the at least one photodiode and switching the image charge to the common floating diffusion region, The portion of the at least one photodiode region close to the gate terminal of the transfer transistor has a higher concentration of impurity ions than the rest of the phototransistor, An equipotential surface having the highest or lowest voltage level among the equipotential surfaces is located at a portion of the photodiode region close to the gate terminal of the transfer transistor, and The unit pixel, wherein a depth of the portion of the photodiode close to the gate terminal of the transfer transistor is deeper than the rest of the photodiode. A photodiode for generating an image charge corresponding to the received image signal; And A transfer transistor for switching the image charge to a floating diffusion region in response to a transfer control signal, The portion of the photodiode close to the gate terminal of the transfer transistor has a higher concentration of impurity ions than the rest of the photodiode region, An equipotential surface having the highest or lowest voltage level among the equipotential surfaces is located at a portion of the photodiode region close to the gate terminal of the transfer transistor, and And a plurality of unit pixels having a depth of a portion near the gate terminal of the transfer transistor of the photodiode region deeper than that of the rest of the photodiode. In the unit pixel manufacturing method comprising a photodiode and an image signal conversion circuit, A first mask defining an entire area of the photodiode; And A second mask defining a portion of the photodiode in contact with the transfer transistor; Implanting impurity ions into the portion defined as the first mask; Implanting the same impurity ions as the impurity ions into the portion defined as the second mask; And And performing annealing after performing the two steps. The method of claim 4, wherein The energy of the impurity injected into the portion defined by the second mask is higher than the energy of the impurity injected into the portion defined by the first mask. The method of claim 5, When the photodiode is formed on the P-type substrate, the impurity ions are acceptor, And wherein the impurity ion is a donor when the photodiode is formed on an N-type substrate. A wafer in which a unit pixel including a photodiode and an image signal conversion circuit is implemented, A first mask defining an entire area of the photodiode; And A second mask defining a portion of the photodiode in contact with the transfer transistor; Implanting impurity ions into the portion defined as the first mask; Implanting the same impurity ions as the impurity ions into the portion defined as the second mask; And And performing annealing after the two steps are applied.

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