Charge transfer device and solid-state image pickup device

Details Charge transfer device and solid-state image pickup device Charge transfer device and solid-state image pickup device

2000-07-21

2004-12-21

Ho, Tuan (Department: 2612)

Television

Camera, system and detail

Solid-state image sensor

C348S316000

Reexamination Certificate

active

06833870

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a charge transfer device in which two-phase driving pulses are applied to a number of two-layered transfer electrodes arranged above a transfer channel to perform a transfer operation, and a solid-state image pickup device using the same.
2. Description of the Related Art
Generally, in a solid-state image pickup device such as CCD (Charge Coupled Device) area sensor or the like, signal charges which are photoelectrically converted by a photosensor serving as an image receiving element are transferred in a vertical direction by plural vertical transfer registers, and the signal charge thus transferred by each of the vertical transfer registers is transferred in a horizontal direction by a horizontal transfer register which is driven in two-phase.
FIGS. 5A
to
5
C show the construction of a horizontal transfer register and an output portion in a conventional solid-state image pickup device (CCD area sensor), wherein
FIG. 5A
is a cross-sectional view showing the arrangement of electrodes of the horizontal transfer register,
FIG. 5B
shows the potential distribution corresponding the electrode arrangement, and
FIG. 5C
is a plan view showing the structure in the neighborhood of the last stage of the horizontal transfer register.
In
FIG. 5
, a number of two-layered transfer electrodes
51
are arranged along a charge transfer direction X above a transfer channel
50
, and a gate electrode
52
is formed so as to be adjacent to a transfer electrode
51
L located at the last stage thereof (hereinafter referred to as “last-stage transfer electrode”). Each of the two-layered transfer electrodes
51
,
51
L is constructed by a transfer electrode
51
a
,
51
La as the first layer and a transfer electrode
51
b
,
51
Lb as the second layer. Further, a two-phase driving pulse øH
1
, øH
2
is applied to the two-layered transfer electrode
51
(containing
51
L), and a gate voltage (DC voltage) VHOG is applied to the gate electrode
52
.
Besides, in the potential distribution of the transfer channel
50
, the potential level corresponding to the transfer electrode
51
a
,
51
La of the first layer is set to be deeper than the potential level corresponding to the transfer electrode
51
b
,
51
Lb of the second layer by doping an area below the transfer electrode
51
b
,
51
Lb of the second layer with such impurities as to shallow the potential level. With the doping of these impurities, in the area of the transfer channel
50
, a storage portion (1) is formed below the transfer electrodes
51
a
,
51
La of the first layer and a transfer portion (2) is formed below the transfer electrodes
51
b
,
51
Lb of the second layer.
Further, a floating diffusion area (hereinafter referred to as “FD area”) is connected through the gate electrode
52
to the last-stage transfer electrode
51
L. The FD area
53
serves to detect the charge amount of signal charges transferred by the last-stage transfer electrode
51
L and convert the charges to the voltage corresponding to the charge amount thus detected.
Next, the manufacturing process of the horizontal transfer register in the conventional solid-stage image pickup device will be described with reference to
FIGS. 6A
to
6
D.
First, as shown in
FIG. 6A
, an N-type transfer channel is formed on a semiconductor substrate, and then the electrodes
51
a
,
51
La of the first layer are formed. Subsequently, as shown in
FIG. 6B
, a predetermined portion is covered by a resist mask
54
, and then P-type impurities to shallow the channel potential are doped by an ion implantation method or the like with the electrodes
5151
a
,
51
La of the first layer as a mask.
Subsequently, as shown in
FIG. 6C
, the peripheral portions of the electrodes
51
a
,
51
La of the first layer are covered by insulating material by oxidizing the electrodes
51
a
,
51
La of the first layer or the like, and then the electrodes
51
b
,
51
Lb,
52
of the second layer are formed. Finally, as shown in
FIG. 6D
the electrode
52
at the end position is wired to form a gate electrode. Further, the other electrodes
51
a
,
51
La of the first layer and the electrodes
51
b
,
51
Lb of the second layer which are adjacent to each other are connected to each other to form the transfer electrodes
51
,
51
L of the second layer.
The maximum operating charge amount in the horizontal transfer register (hereinafter merely referred to as “operating charge amount”) increases in proportion to the potential difference between the storage portion (1) and the transfer portion (2) and the electrode length Lst and the channel width W of the storage portion (1). In other words, the operating charge amount of the horizontal transfer register is determined by two parameters of the potential difference between the storage portion (1) and the transfer portion (2) and the electrode area of the storage portion (1) (the effective electrode area corresponding to the channel width).
The FD area
53
is formed of a more minute area than the transfer electrodes
51
,
51
L in order to increase the charge-to-voltage conversion gain and thus enhance the detection sensitivity. Therefore, the transfer channel
50
of the horizontal transfer register is designed so that the channel width W is reduced from the vicinity of the last-stage transfer electrode
51
L to the FD area
53
.
In this case, the electrode area of the storage portion of the last-stage transfer electrode
51
L is reduced as the channel width W is reduced, so that the operating charge amount is reduced by the amount corresponding to the reduction of the electrode area. Therefore, it may be considered to alter the two parameters in order to prevent the reduction of the operating charge amount. However, with respect to the potential difference between the storage portion (1) and the transfer portion (2), if the potential difference is increased to ensure the operating charge amount, the amplitude at the transfer operation time is increased and thus the consumption power is increased. Therefore, in the present situation, there may be considered a method of increasing the electrode length Lst of the storage portion (1) of the last-stage transfer electrode
51
L as shown in
FIG. 7
to increase the electrode area at that place and ensure the operating charge amount.
However, if the electrode length Lst of the storage portion (1) of the last-stage transfer electrode
51
L is increased as described above, the transfer distance (Lst+Ltr) at the last-stage transfer electrode
51
L is also increased by the amount corresponding to the increase of the electrode length Lst, and thus the signal charge is hard to flow and thus the transfer efficiency is lowered. As a result, a transfer failure of the signal charge at the last-stage transfer electrode
51
L occurs (the transfer is left uncompleted or the like), and image quality deterioration (color mixture, image tear or the like) occurs.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above circumstances, and provides a charge transfer device which can secure a sufficient operating charge amount without deteriorating the transfer efficiency, and a solid-state image pickup device using the charge transfer device.
According to an aspect of the present invention, there is provided a charge transfer device having a transfer channel and plural pairs of two-layered transfer electrodes arranged along a transfer direction on the transfer channel, wherein two-phase driving pulses are applied to the plural pairs of two-layered transfer electrodes, and the transfer channel below a paired two-layered transfer electrode disposed at the last portion in the transfer direction has a first area, a second area which is provided at the downstream of the first area in the transfer direction and has a deeper potential level than the first area, and a third area which is provided at the downstream of the second area in the transfer direction and has a deeper potential level than the second area.
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