JPH10233497A - Charge coupling type semiconductor device and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To greatly reduce dark current in a charge coupled type semiconductor device. SOLUTION: When an SiO2 film 2 is formed on a silicon substrate 1, a number of boundary levels are generated in the boundary therebetween (A). Then, as shown in (B), the substrate 1 is placed in a plasma ion containing an inert gas ion such as helium ion (He<+> ), etc., or hydrogen ion (H<+> ) formed of neutral gas ion, silane gas (SiH4 ), etc., and a constant high-frequency electric power (e.g. 100-200W RF power/5-inch slice) is applied thereto, so that the hydrogen ion (H<+> ) 6 in the plasma gas is added with energy for activation by the high-frequency electric power and it penetrates the film 2 to the boundary of the substrate 1, and then it is coupled with a trap center (re-coupling level) in the boundary, thereby reducing the boundary level greatly.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0010]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charge-coupled semiconductor device having a transfer electrode provided on a semiconductor layer via an insulating layer, and a method of manufacturing the same.

[0020]

2. Description of the Related Art Charge coupled semiconductor devices (Charge Coupled semiconductor devices)
Device: CCD is a simple MOS (Metal Oxide Se
A functional element having a self-scanning function and a storage function with a structure, and is used for an imaging device, an analog delay element, a digital filter, and the like. CC
D is a surface channel type CCD depending on the channel formation location
And a buried channel type CCD, and further classified into a single-phase type, a two-phase type, and the like according to a transfer driving method.

In the buried channel type CCD, charge transfer is performed at a fixed depth position in the silicon substrate. Therefore, the influence of the interface between the silicon substrate and the SiO 2 film on this surface is smaller than that in the surface channel type CCD. It is said that the charge transfer efficiency and the dark current are small.

[0040]

However, in the conventional CCD, the dark current is still large in any of the above-mentioned types, and the dark current does not decrease to a practical level even in the embedded type. is there. As a countermeasure against this, silicon substrate gettering (Intrinsic Gettering) by generating crystal defects in the silicon substrate, etc.
) Or external gettering (E
Attempts have been made to reduce dark current by making full use of technologies such as xtrinsic gettering, but the effect is still insufficient.

As described above, in the conventional CCD, since the dark current does not decrease, for example, when the CCD is used as an image pickup device, the contrast, the color tone, and the like are deteriorated, which is a major obstacle in obtaining high image quality. .

The present invention has been made in view of the above problems, and has as its object to provide a charge-coupled semiconductor device in which dark current is sufficiently reduced, and a method of manufacturing the same.

[0070]

In order to achieve the above object, a charge-coupled semiconductor device according to the present invention comprises: a semiconductor substrate made of silicon; an insulating layer formed on one main surface of the semiconductor substrate; A plurality of transfer electrodes arranged at intervals from each other on the insulating layer, selectively incorporated at a high concentration via the insulating layer in a region near the main surface of the semiconductor substrate, the semiconductor substrate and Hydrogen ions bonded to dangling bonds located at the interface between the insulating layer,
Having a configuration in which the interface state at the interface between the semiconductor substrate and the insulating layer is reduced due to the bond between the hydrogen ions and the dangling bond, thereby reducing dark current. did.

The method of manufacturing a charge-coupled semiconductor device according to the present invention includes a step of forming an insulating layer on one main surface of a silicon substrate, and a step of forming a high-frequency power in plasma ions containing inert gas ions and hydrogen ions. Applying hydrogen to the interface between the silicon substrate and the insulating layer to reduce the interface level at the interface by coupling the hydrogen ions to the recombination level at the interface. And forming a plurality of transfer electrodes on the insulating layer at intervals.

[0090]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the accompanying drawings.

Referring to FIG. 1, an embodiment of an interface state reduction processing method for reducing dark current in a CCD according to the present invention will be described.

First, as shown in FIG. 1A, an SiO 2 film 2 as an insulating layer is formed on a silicon substrate 1 to a normal thickness by a thermal oxidation technique or the like, and then a CVD (Chemic)
An impurity-doped low-resistance polysilicon layer 3 is deposited by an al-vapor deposition method. The polysilicon layer 3 is patterned into stripes by photoetching, and processed into transfer electrodes (or phase electrodes).

At this point (specifically, the SiO2 film 2
(At the time of formation), at the interface between the silicon substrate 1 and the SiO2 film 2, a large number of interface levels, which are schematically indicated by crosses, occur over the entire surface.

Therefore, after forming a CCD circuit having the cross-sectional structure shown in FIG. 1A, a silicon nitride film (especially an S
It has been found that when the (i3N4 film) 4 is deposited, the above-mentioned interface state is unexpectedly greatly reduced.

In this plasma CVD, for example, while supplying SiH4 (silane) gas and NH3 (ammonia) gas at a ratio of about 1: 6 onto the silicon substrate, a high-frequency power of about 600 W and a pressure of 2.0 Torr are applied. To make the reaction gas into a plasma state, thereby depositing a plasma nitride. At this time, a large amount of hydrogen ions (H
⁺ ), And these hydrogen ions are taken into the silicon nitride film 4 at a high concentration (1 × 10 ²² / cm ³ or more). Then, the hydrogen ions form the underlying polysilicon layer 3.
Through the silicon substrate 1 and the SiO2 film
2 The interface state is greatly reduced by moving to the interface with the film 2 and binding to the dangling bond of silicon (the broken bond of silicon existing at the interface) forming the above interface level. Actually, according to the measurement by the charge-pumping method described later, it is known that the interface state is 10% or less.

After performing the interface state reduction process using the plasma nitride in this manner, it is desirable to remove the silicon nitride film 4 by etching as shown in FIG. That is, if the silicon nitride film 4 is left as it is, an undesirable result is brought about as a functional element, particularly as an imaging element. For example, Si3 N4 has a refractive index of about 2.0 and absorbs blue light, thereby deteriorating imaging performance.

Here, the charge-pumping method will be described. This method is a method of measuring the substrate current due to the recombination of electrons and holes to obtain the value of the interface state of Si-SiO2.

For example, a sample having a MOS structure in which the same processing as that described with reference to FIG. 1 was applied to the interface of Si—SiO 2 was prepared, and a bias voltage (Vsd> 0) common to the source and drain regions was applied to this sample. A bias voltage (Vg) of Vg> Vsd is applied to the gate and electrons are collected on the silicon surface under the gate.

Then, switching is made to Vg <0, the holes in the silicon substrate are recombined with the electrons, and the substrate current (recombination current) flowing at this time is measured. The smaller the value of the substrate current, the more the number of electrons to be recombined, that is, S
This means that the interface state of i-SiO2 is small.

In this example, as a result of the measurement by the charge-pumping method, the interface state of Si—SiO 2 was 1 by the treatment with the plasma nitride described in FIG.
/ 2 or less. It has also been confirmed that the amount of interface trap density is reduced by about 90%.

As described above, the dark current of the CCD is １ ／.
It was found to decrease below, which corresponds well to the reduction of the interface state to 10% or less as described above. Therefore, if the interface state is reduced as in the present embodiment, the dark current can be significantly reduced correspondingly. The dark current is measured by measuring the output current with an oscilloscope while the CCD is shielded from light.

Next, referring to FIG. 2, as another embodiment, a case where the processing by the nitride is performed locally will be described.

FIG. 2A shows a state in which the transfer electrodes 3 on the SiO 2 film 2 are arranged at regular intervals, and the cross section in a direction orthogonal to the cross section in FIG. 1A is shown. Equivalent to. Also in this case, after the SiO2 film 2 and the transfer electrode 3 are sequentially formed on the silicon substrate 1 by the same method as the example of FIG.
An i3 N4 film 4 is deposited. Therefore, the Si3 N4 film 4
When depositing hydrogen, hydrogen ions (H ⁺ ) in the reaction gas
Is taken into the silicon nitride film 4 at a high concentration (1 × 10 ²² / cm ³ or more).

Next, as shown in FIG. 2B, for example, a photoresist is applied as a mask material over the entire surface and then patterned by etching, and the mask 5 is selectively formed only on the transfer electrode 3 region. leave.

Next, as shown in FIG. 2C, etching is performed using the mask 5 to selectively remove the silicon nitride film 4 in a region where the mask 5 is not provided. As a result, the transfer electrode 3
The silicon nitride film 4 is left in the same pattern.

Next, when annealing is performed at 450 ° C. in an atmosphere of N 2, hydrogen ions (H ⁺ ) try to escape by heat.
⁺ ) Escape is suppressed.

Therefore, under the same region, hydrogen ions can sufficiently move to the interface between the silicon substrate 1 and the SiO 2 film 2, and under the same region (that is, the transfer electrode 3), the interface state becomes lower for the same reason as described above. Is greatly reduced. After this process, as shown in FIG. 2D, the silicon nitride film 4 is removed by etching.

As described above, by using the mask 5, S
According to the present embodiment, a part of the interface of i-SiO2 can be selectively treated with nitride. Level reduction processing can be performed.

Next, an interface state reduction processing method according to another embodiment of the present invention will be described with reference to FIGS. FIG. 3 shows a case where this processing method is applied to the entire surface, and FIG. 4 shows a case where the processing method is applied locally (particularly, at the position of the transfer electrode).

In the example of FIG. 3, when the SiO 2 film 2 is formed on the silicon substrate 1 as shown in FIG. 3A, a large number of interface states are present at the interface between the silicon substrate 1 and the SiO 2 film 2. Occurs.

Next, as shown in FIG. 3B, the silicon substrate 1 is treated with an inert gas ion such as helium ion (He ⁺ ) or a neutral gas ion and a hydrogen ion (H ⁺ ) by silane gas (SiH 4) or the like. ), And applying a constant high-frequency power (for example, RF power of 100 to 200 W per one 5-inch slice), hydrogen ions (H ⁺ ) 6 in the plasma gas are generated by the high-frequency power. Energetically activated, SiO2
It penetrates through the film 2 to the interface with the silicon substrate 1 and couples to the trap center (recombination level) at the interface, greatly reducing the interface level. That is, it has been confirmed that the interface state (trap density) is reduced by about 20% to 30%.

FIG. 3C shows a state in which the polysilicon transfer electrode 3 is provided on the SiO2 film 2 after the above-described interface state reduction processing to form a CCD circuit.

In the example of FIG. 4, the area other than the transfer electrode position is covered with a mask 7 on the SiO2 film 2 and predetermined plasma ions and high-frequency power are applied in the same manner as in the example of FIG. Hydrogen ions (H
⁺ ) 6 to make Si-SiO in the region without mask 7
2 Selectively reduce the interface state of the interface. After removing the mask 7, a polysilicon transfer electrode is formed on the region where the interface state is reduced.

Next, an embedded channel type CCD as a CCD to which the present invention is applied will be specifically described with reference to FIGS.

FIG. 5 shows a general layout of a frame transfer type imaging device. In this imaging device, a storage unit 31 is arranged adjacent to the imaging unit 30, and a signal is sent from the serial register unit 22 to the amplification unit 23.

FIG. 6 and FIG. 7 show a part of an image pickup section of a single-phase type device called a virtual phase (Virtual Phase) CCD.

In this imaging unit, a P ⁻ type semiconductor region 12 and a P type semiconductor region 13 connected to the P ⁻ type semiconductor region 12 are formed in, for example, an N type silicon layer 11 provided on a P type silicon substrate 10. Virtual electrode portion 14 in a bent pattern
(Note that reference numeral 15 in the drawing denotes a P ⁺ type channel stopper region.) The virtual electrode portion 14 is composed of the two regions 12 and 13 having different impurity concentrations, and forms a fixed potential corresponding to each region in the silicon layer 11.

On the SiO ₂ film 2 on the N-type silicon layer 11, the polysilicon transfer electrode 3 and the anti-blooming electrode 16 as described above are alternately formed on a region where the virtual electrode portion 14 is not provided. Is provided. The anti-blooming electrode 16 is provided to absorb excess carriers, but may be formed by the same process using the same impurity-doped polysilicon as the transfer electrode 3.

According to the present invention, the silicon layer 11 and the SiO _{2 in the} transfer electrode 3 and the virtual electrode 14 (regions 12 and 13) except for the region of the anti-blooming electrode 16 in the image pickup unit thus configured. The interface state reduction processing as described above is selectively applied to the interface with the film 2. In FIG. 6, the processing area 17 is indicated by oblique lines for easy understanding. Note that the processing region 17 may be provided only below the transfer electrode 3.

Referring to FIG. 8, this virtual phase C
Perform CD operation. First, at the time of imaging (at the time of light irradiation), no clock pulse (Vcl) is applied to the transfer electrode 3, and the silicon surface potential immediately below the transfer electrode 3 is fixed at the “L” level.

Now, considering the case where electrons serving as majority carriers are transferred in the silicon layer 11 in the direction of the arrow 18, the electron [−] existing at the position of the virtual electrode portion 13 at the time of imaging is
When the surface potential under the anti-blooming electrode 16 is alternately switched between a high level and a low level by a clock pulse (Vabg), a part thereof is captured at a high level as indicated by an arrow 19. Then, at the next switching to the low level, the holes are recombined with holes [+] generated by light irradiation and disappear. This causes excess electrons to disappear (absorb) under the anti-blooming electrode 16, and
The adverse effects (particularly halation in the image pickup tube) due to the excess carriers can be prevented.

Next, at the time of carrier transfer, the surface potential under the anti-blooming electrode 16 is fixed to the level shown by the broken line 20, and the transfer electrode 3 alternately switches between high level "H" and low level "L". Clock voltage (Vcl)
Is applied, the electrons as carriers are transferred below the transfer electrode 3 as shown by the broken line 21 (at the “H” level), and further transferred to the left in the drawing at the next “L” level. It is important that the surface potential under the transfer electrode 3 has a step shape, which can be realized by making the surface impurity concentration different.

As described above, in the imaging section,
Imaging and carrier transfer are performed. In particular, since the interface state of Si—SiO ₂ in the transfer electrode 3 and the virtual electrode portion 14 is reduced in advance, carriers are not emitted (or at the time of carrier transfer) during light irradiation. The dark current leaking through the interface state is greatly reduced. Therefore, for example, a signal faithfully corresponding to the image of the subject can be satisfactorily extracted.

Note that the above-described storage unit 11 (FIG. 5) may have a structure as shown in FIG. That is,
Compared to the imaging unit shown in FIG.
6 is the same except that 6 is not provided.

Although the present invention has been illustrated, the above-described embodiment can be further modified based on the technical idea of the present invention.

The above-described process of reducing the interface state needs to be performed at least in a region where dark current is a problem in the CCD. However, the same process can be performed on the entire surface depending on the type of CCD to be applied. The material of each layer and each film described above may be changed, and the conductivity type of the semiconductor and the polarity of the carrier may be changed. In the above example, a single-phase CCD
However, the present invention can of course be applied to other driving systems such as a two-phase system, a three-phase system, and a surface channel type other than the embedded type.

[0460]

As described above, according to the present invention,
At the interface between the silicon substrate and the SiO2 film, the dangling bonds and hydrogen ions are bonded to reduce the interface state, thereby significantly reducing the dark current. , And high image quality can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a manufacturing process of a CCD according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view illustrating a manufacturing process of a CCD according to another embodiment.

FIG. 3 is a cross-sectional view illustrating a manufacturing process of the CCD according to the embodiment of the present invention.

FIG. 4 is a cross-sectional view illustrating a manufacturing process of a CCD according to another embodiment.

FIG. 5 is a schematic plan view showing a layout of a CCD imaging device according to an embodiment.

FIG. 6 is a plan view showing a main part of an imaging unit of the CCD device in the embodiment.

FIG. 7 is a sectional view taken along line IX-IX of FIG. 6;

FIG. 8 is a diagram for explaining the operation of the imaging unit of the CCD device in the embodiment.

FIG. 9 is a cross-sectional view illustrating a main part of a storage unit of the CCD device according to the embodiment.

[Explanation of symbols]

1,10 silicon substrate 2 SiO ₂ film 3 polysilicon layer (transfer electrode) 4 plasma nitride film 6 hydrogen ions 12 P ^- -type semiconductor region 13 P-type semiconductor region 14 the virtual electrode portion 16 anti-blooming electrode 17 interface level reduction process Area 30 Imaging unit 31 Storage unit

Claims (2)

[Claims] A semiconductor substrate made of silicon; an insulating layer formed on one main surface of the semiconductor substrate; a plurality of transfer electrodes arranged on the insulating layer at a distance from each other; Hydrogen ions that are selectively incorporated at a high concentration in a region near the main surface via the insulating layer and are bonded to dangling bonds located at an interface between the semiconductor substrate and the insulating layer. Wherein the interface state at the interface between the semiconductor substrate and the insulating layer is reduced by the bonding of the hydrogen ions and the dangling bonds, thereby reducing the dark current. Combined semiconductor device. 2. A step of forming an insulating layer on one main surface of a silicon substrate; and applying high-frequency power in plasma ions containing inert gas ions and hydrogen ions to convert the hydrogen ions into the silicon substrate. A step of invading an interface between the insulating layer and the semiconductor layer to reduce an interface state of the interface by coupling to a recombination level at the interface; Forming a charge transfer type semiconductor device.