JP3309464B2 - Charge transfer device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to relates to a charge transfer device having an output circuit for converting the detected and signal voltage transferred becoming the signal charge from the photoelectric charge transferring unit such as a solid-state imaging device, in particular a source of a plurality of stages charge with the output circuit Do that from follower
It relates to a transfer device .

[0002]

2. Description of the Related Art As a characteristic of a recent CCD solid-state image pickup device, in order to prevent the sensitivity at low illuminance from being reduced even if the image pickup device is downsized, a small signal charge is combined with a structure of a sensor section to reduce a signal voltage. Conversion ability to convert to is important. In order to enhance the voltage conversion capability of the signal charge, it is necessary to reduce the parasitic capacitance of the charge detection unit as much as possible. In order to reduce the parasitic capacitance, the capacitance between the gate and the channel of the MOS transistor which first receives the signal charge may be reduced, that is, the gate area may be reduced.

[0003]

SUMMARY OF THE INVENTION However, MOS
When the gate area of the transistor is reduced, it depends on the gate area of the MOS transistor constituting the output circuit.
/ F noise will increase. For this reason, the image quality is degraded at the time of low illuminance, which hinders maintaining and improving the S / N at the level of the conventional product when downsizing the CCD solid-state imaging device.

The present invention has been made in view of the above problems, and has as its object to reduce 1 / f noise and maintain low illuminance while maintaining a high level of voltage conversion capability of minute signal charges. Image quality and sensitivity characteristics
To provide a charge transfer device having the output circuit.

[0005]

To achieve the above objects resolving means for the ## present invention, in the charge transfer device having an output circuit for converting the detected and signal voltage transferred becoming the signal charge from the charge transfer section, the M receiving the signal charge in the output circuit
The gate oxide film of the OS transistor is thinner than the gate oxide films of other MOS transistors in the output circuit .

[0006]

When the gate oxide film of a MOS transistor receiving a signal charge is made thin, the 1 / f noise generated from the output circuit is proportional to the square of the gate oxide film thickness of the MOS transistor. it can. Therefore, the image quality and sensitivity characteristics at low illuminance can be improved by reducing the 1 / f noise while maintaining the voltage conversion capability of the minute signal charge at a high level by reducing the gate area of the MOS transistor.

[0007]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a circuit diagram showing one embodiment of an output circuit of a CCD solid-state imaging device according to the present invention, and shows a case where the present invention is applied to, for example, a floating diffusion amplifier. In FIG. 1, for example, a three-stage source follower circuit 1 including a driving MOS transistor Q _nD and a load MOS transistor Q _nL constitutes an amplifier section of the present output circuit.

In the three-stage source follower circuit 1, the gates of the load-side MOS transistors Q _1L , Q _2L and Q _3L are commonly biased by the DC power supply 2, and the first-stage drive-side MOS transistor Q _1D Is connected to a floating diffusion (FD) 3 constituting a charge detection unit of the present output circuit. Signal charges are transferred to the floating diffusion 3 from a horizontal transfer register (not shown) of the CCD solid-state imaging device. The floating diffusion 3 is reset by a reset switch 4 at a predetermined cycle.

By the way, in the output circuit having the above configuration,
In order to increase the voltage conversion capability of signal charges, it is necessary to minimize the parasitic capacitance of the charge detection unit. However, in this output circuit, the driving-side MOS transistor of the first-stage source follower that receives signal charges first Q _{1D is} also a target. For this reason, the driving side MO of the first stage source follower
In the S transistor Q _1D , the gate area is reduced to sufficiently reduce the parasitic capacitance, and the oxide film under the gate is thinned to reduce 1 / f noise depending on the gate area.

More specifically, in FIG. 2 showing the cross-sectional structure of each of the driving-side MOS transistors Q _1D and Q _2D of the first-stage and second-stage source followers, FIG.
The gate oxide film 5 of the S transistor Q _1D is thinned to about 程度 to ３ of the gate oxide film 6 of the driver MOS transistor Q _2D of the second-stage source follower. FIG. 2 shows a comparison with the driving MOS transistor Q _2D of the second-stage source follower. However, in actuality, in the source follower circuit 1, the first-stage load-side MOS transistor Q _1L and the MOS transistors of the second and subsequent stages are used. The transistors Q _2D , Q _2L , Q _3D ,
The gate oxide films of _Q3L are formed to have the same thickness, and the gate oxide film of the first-stage drive-side MOS transistor _Q1D is made thinner than these gate oxide films.

When thinning the gate oxide film 5 of the driving side MOS transistor Q _1D of the first stage source follower, first, all the MOS transistors Q _1D , Q _1L , Q _2D , Q _2L , Q ₂ constituting the source follower circuit 1 are formed. A gate oxide film having the same film thickness is formed for _3D and Q _3L , and thereafter, only the gate oxide film 5 of the first-stage drive-side MOS transistor Q _1D is subjected to, for example, 2/3 to 1/3 by dry etching.
This can be achieved by making it as thin as possible.

Here, the gate area of the MOS transistor is W · L, and the gate oxide film thickness of the MOS transistor is do.
When x, relationship equation 1 of the noise energy E _nf ² of 1 / f noise that rely on these.

[Number 1] ^{_{E nf 2 ∽ (1 / W}} · L), dox 2, K, where (1 / f ^m), K is determined by the interface level of the Si / SiO ₂ constant, f is the frequency, m is 1 to 0.8.

In the source follower circuit 1, the final stage (third stage) is constituted by MOS transistors having a size capable of flowing a current capable of sufficiently driving a buffer (not shown) external to the CCD solid-state imaging device. Also, 2
The stage is configured such that the first stage can be sufficiently driven and the last stage can be sufficiently driven, and the size of each MOS transistor is optimized so that the entire output circuit has sufficient frequency characteristics. Note that reducing the thickness of the entire output circuit reduces the electrostatic withstand voltage and increases the possibility of destruction of the circuit section, which is not suitable for mass production.

As described above, the gate oxide film 5 of the driving-side MOS transistor Q _1D of the first-stage source follower is connected to the other MOS transistors Q _1L , Q _2D , Q _2L , Q ₂ in the same circuit.
_Since the driving side MOS transistor Q _1D of the first stage source follower, which receives the signal charge first, has a small amount of current due to the thinner film than the oxide film of _3D and Q _3L , as shown in FIG. It is possible to reduce the 1 / f noise while maintaining a high level of voltage conversion capability of signal charges of minute signals by reducing the gate area without reducing the electrostatic withstand voltage even if the thickness is reduced. Can be.

FIG. 3 shows the experimental results of the present invention. FIG.
FIG. 4 is a characteristic diagram showing how noise energy changes with respect to two different values of a gate area and a gate oxide film thickness of a first-stage source follower. As is clear from the experimental results, when the gate oxide film thickness is reduced,
It can be seen that 1 / f noise is reduced even with the same gate area. That is, even if the size of the MOS transistor is reduced in order to increase the voltage conversion capability of the signal charge, the 1 / f noise can be reduced to the level of the conventional product or more by reducing the gate oxide film thickness. .

[0016]

As described above, according to the present invention,
Charges having an output circuit for detecting signal charges transferred from a charge transfer unit such as a solid-state imaging device and converting the signal charges to a signal voltage
In the transfer device, a gate oxide film of the MOS transistor receiving the signal charge in the output circuit, by which thinned than the gate oxide film of the other MOS transistor in the output circuit <br/>, generated by the output circuit Since the 1 / f noise to be generated is proportional to the square of the gate oxide film thickness of the MOS transistor, the 1 / f noise can be reduced.
By reducing the gate area of the S transistor, 1 /
By reducing the f noise, the image quality and sensitivity characteristics at low illuminance can be improved.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing one embodiment of an output circuit according to the present invention.

FIG. 2 shows each drive side MO of the first and second stage source followers.
FIG. 3 is a sectional structural view of an S transistor.

FIG. 3 is a characteristic diagram showing an experimental result according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Source follower circuit 2 Floating diffusion (FD) 4 Reset switch 5, 6 Gate oxide film

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01L 21/339 H01L 21/8234 H01L 27/088 H01L 29/762 H04N 5/335

Claims (3)

(57) [Claims] 1. A charge transfer device having an output circuit for detecting a signal charge transferred from a charge transfer unit and converting the signal charge into a signal voltage, wherein a gate oxide film of a MOS transistor receiving the signal charge in the output circuit . Is thinner than a gate oxide film of another MOS transistor in the output circuit . 2. The charge transfer device according to claim 1, wherein said output circuit includes a plurality of stages of source followers, and said MOS transistor receiving said signal charge is a driving MOS transistor constituting a first stage source follower. Dress
Place . 3. The charge transfer device according to claim 2, wherein said another MOS transistor is a MOS transistor forming a source follower of a second stage or later.