JP2512589Y2 - Semiconductor non-volatile memory device - Google Patents

Description

[Detailed description of the device]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrically rewritable semiconductor nonvolatile memory device, and more particularly to improvement of its memory retention.

[0002]

2. Description of the Related Art An electrically rewritable semiconductor non-volatile memory device having an insulated gate field effect transistor structure (hereinafter referred to as "E ² PROM") is designed to capture carriers spontaneously at the interface of different insulating films. Use the center. MNOS
Or, an E ² PROM having a MAOS structure or a floating type E ² PROM using the formation of an artificial potential well is generally known.

Both of them are used as a writing (erasing) means, such as a direct tunnel through a gate insulating film or a roller.
A Fowler-Nordheim tunnel current is used. Therefore, the tunnel distance and barrier height are restricted. Therefore, in order to increase tunnel efficiency, generally 2
A write (erase) voltage of 0 V or higher is inevitable.

[0004]

Therefore, for example, when the above-mentioned E ² PROM is built into the inside of a timepiece IC, the peripheral IC is composed of a thin film insulated gate.
E ² PR using a conventional write (erase) voltage of 20 V or more
With respect to the OM, it has a drawback that the peripheral IC can cause malfunction or destruction.

Therefore, in order to avoid the influence of the high voltage application on the peripheral IC, a special device of high withstand voltage design (for example, PN junction separation or substrate separation) is inevitable, which causes inconvenience in area. However, there is a problem that it is very disadvantageous for the improvement of the degree of integration.

Further, by using a high voltage generation source inside the IC
When writing (or erasing) in the E ² PROM, for example, even in the means for generating a high voltage from the clock power supply voltage of -1.55V, the booster circuit occupies a significantly large area due to problems such as boosting efficiency. It also has drawbacks.

Further, in recent years, there is a strong demand for lowering the write (erase) voltage with the miniaturization of VLSI, and the write (erase) can be performed at 10 V or less with respect to the conventional high voltage of 20 V or more.
Providing E ² PROM is an issue for the future.

Therefore, an insulated gate type electronic effect E ² PROM having an ultrathin film gate structure, which is the object of the present invention, is
Conductive gate-Silicon oxide film (Oxi
de) -silicon nitride film (Nitride) -insulating film (Insulator)
-Since it has a semiconductor structure (hereinafter referred to as "CONIS structure") and can be written (erased) at 10 V or less, it is expected as an E ² PROM used in future VLSI.

FIG. 5 shows a conventional CONIS structure E ² PRO.
FIG. 6 is an enlarged sectional view near the gate of M, and FIG.
The hysteresis curve represented by (threshold voltage) -VG (gate voltage) is shown.

In this CONIS structure, a tunnel insulating film 1 and a silicon nitride film 2 are formed in a conductive channel region (indicated by P minus) sandwiched between source and drain regions (indicated by N plus) on a semiconductor substrate S. A three-layer insulating film made of the silicon oxide film 3 is formed, and a conductive gate electrode (conductive electrode) 4 is formed on the silicon oxide film to form a gate portion.

Since the total thickness of the three-layer insulating film is thinned to about 10 nm, the tunnel probability substantially equal to that of the conventional MNOS structure with a write voltage of 20 V or more is low voltage (for example, 10 V). By forming the silicon oxide film 3 obtained in the following) and having a large barrier height on the side of the conductive gate electrode 4, the release of the trapped carriers to the conductive gate electrode 4 is suppressed and the memory retention capability is improved. It suppresses deterioration.

However, in the E ² PROM having such an ultra-thin gate, the width of logic "1" and "0" (write width) generally relates to the thickness of the gate insulating film, and the width decreases as the thickness decreases. Therefore, in comparison with the conventional E ² PROM having a thick film gate using high voltage application, there is a disadvantage that it is disadvantageous in view of long-term memory retention.
The E ² PROM having the IS structure has an ultra-thin film gate structure, and is not sufficient in view of long-term memory retention.

Further, for example, JP-A-60-60770.
As described in Japanese Patent Laid-Open Publication No. 2000-187, in the E ² PROM of the MOIOS structure in which the conductive gate electrode in the above CONIS structure is formed of metal, the silicon nitride film is
When the heat treatment is performed at a temperature of ℃ or more, there is a problem that the film quality of the silicon nitride film is changed and the memory retention property is deteriorated.

However, since the thermal oxidation rate of the silicon nitride film is slow, when the silicon oxide film is formed on the silicon nitride film, if the thermal oxidation treatment is performed at a temperature of 900 ° C. or less, the silicon oxide film will not be formed. Not only will it take too much time, but there will be problems in terms of reliability.

The invention is based on various conventional E ² P.
A low-voltage write (erase) CONIS structure E ² PR with long-term memory capacity that solves problems in ROM
The purpose is to provide OM.

[0016]

Generally, a stoichiometric silicon nitride film has a refractive index of 2.00, and it is said that the refractive index approaches the Si side as Si becomes excessive. Therefore, the invention is based on the CONIS as shown in FIG.
In an E ² PROM having a structure, a silicon nitride film forming an intermediate layer of a silicon three-layer insulating film is made into a single layer with a Si excess from a stoichiometric composition whose refractive index exceeds 2.0 and is 2.1 or less.
By forming the structure, the write width is increased with almost no decrease in the memory retention rate, and overall good long-term memory characteristics are realized.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A semiconductor nonvolatile memory device (E ²
The PROM) has a structure similar to that of the conventional CONISE structure shown in FIG. 5, but is characterized by the silicon nitride film 2. As a method of forming the silicon nitride film, for example, there is a selection of an optimum flow rate ratio of SiH ₂ Cl ₂ (dichlorosilane) or SiH ₄ (monosilane) and NH ₃ (ammonia) by a CVD method.

For example, when a nitride film is formed by a thermal CVD method at 750 ° C. using SiH ₂ Cl ₂ and NH ₃ as raw materials,
The refractive index of the film and the composition ratio of the gas have a relationship as shown in FIG. The refractive index of the silicon nitride film described here is a refractive index when the film thickness is 100 nm (1000 Å) under the same forming conditions in order to suppress the influence of the refractive index due to the formation of the lower oxide film as much as possible.

According to this invention, the silicon nitride film 2 having a single-layer structure shown by hatching in FIG. 5 is stoichiometrically adjusted so that its refractive index exceeds 2.0 and is 2.1 or less. Si is formed in excess of the theoretical composition (refractive index: 2.0).
Here, an embodiment in which the present invention is applied to a MOIO type semiconductor non-volatile memory device (E ² PROM) in which a conductive gate electrode in a CONIS structure is made of metal is improved with reference to FIG. Clear up about.

FIG. 3 shows a conventional MON formed of silicon nitride films having a refractive index near 2.0, a refractive index n = 2.06 in this embodiment, and a refractive index n = 2.14, which is Si-excessive according to the present invention.
The changes over time under bias acceleration of the IS type E ² PROM are shown for comparison.

As is clear from FIG. 3, when the silicon nitride film having a refractive index of 2.06 is used, the retention rate is the same as that of the conventional one and the write width (positive and negative threshold voltages The width is large and the effect is remarkable. When a silicon nitride film having a refractive index n exceeding 2.10 and n = 2.14 is used, the memory retention property deteriorates with time.

This is different from the case of the MNOS structure in which it is not necessary to perform the thermal oxidation treatment after the silicon nitride film is formed.
This is because, in the case of the ONIS structure, it is necessary to thermally oxidize the silicon nitride film to form the silicon oxide film, so that the film quality of the silicon nitride film changes during the thermal oxidation process. Further, when the silicon oxide film having the stoichiometric composition with the refractive index n = 2.00 is used, the writing width is smaller than that of the embodiment having the refractive index n of 2.06.

FIG. 4 shows long-term memory retention. The MONIS type E ² PROM formed of a silicon nitride film having a refractive index n = 2.08 according to the embodiment of the present invention has a conventional n = 2.0.
It can be seen from this diagram that the long-term reliability is superior to that of the silicon nitride film formed by the above method.

Further, the flow rate ratio is changed to change the refractive index of the silicon nitride film, and the writing width (memory window width (V)) and retention rate (retention (V / log) are set with respect to the refractive index.
The result of measuring T)) is shown in FIG. As shown in this diagram, a MONIS type E ² P formed by forming a silicon nitride film so that its refractive index n exceeds 2.0 and is 2.1 or less.
As compared with the conventional MONIS type E ² PROM, the ROM can increase the writing width while hardly changing the memory retention.

The reason why the write width can be increased is that the trap density is increased by using the silicon nitride film having an excessive amount of silicon Si. As a result, the long-term memory retention of the MONIS type E ² PROM can be maintained, and a semiconductor nonvolatile memory device having high memory retention characteristics and electrically rewritable at a low voltage can be obtained.

In the above embodiment, a silicon oxide film having a thickness of about 2 nm (20 Å) was used as the tunnel insulating film 1 shown in FIG. 5, but in addition to this, a thermal nitride film or an oxynitride film obtained by thermally nitriding an oxide film is used. Even when is used as the tunnel insulating film, the same effect can be obtained.

Although an example of a MOINS type E ² PROM using a metal film as a conductive gate electrode has been described, a semiconductor non-volatile semiconductor having a CONIS structure in which another conductive film such as a silicon thin film is formed as a conductive gate electrode. It is needless to say that the same effect can be obtained when the present invention is applied to the sex memory device.

[0028]

As described above, according to the present invention, an insulated gate field effect semiconductor nonvolatile memory device (E ² P) having a CONIS structure capable of writing (erasing) at a low voltage is provided.
In ROM), the write width (memory window) can be increased without lowering the storage retention rate, and overall good long-term storage characteristics can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a refractive index, a memory window and a retention rate of a MONIS E ² PROM.

FIG. 2 shows the refractive index of a silicon nitride film formed by a thermal CVD method at 750 ° C. and SiH ₂ Cl ₂ (dichlorosilane) and NH.
FIG. 3 is a diagram showing the relationship of the composition ratio of ₃ (ammonia).

[FIG. 3] The refractive index of the silicon nitride film is 2.0, 2.06.
Is a diagram showing the time course of biassing under MONIS type E ² PROM in each case 2.14.

FIG. 4 is a diagram showing the long-term memory retention of a MONIS type E ² PROM when the silicon nitride film has a refractive index n of 2.0 and 2.08.

FIG. 5 is an enlarged cross-sectional view near the gate of a conventional CONIS structure E ² PROM to which the present invention is applied.

6 is a diagram showing a hysteresis curve of Vth (threshold voltage) -VG (gate voltage) of the E ² PROM shown in FIG.

[Explanation of symbols]

S Semiconductor substrate 1 Tunnel insulating film 2 Silicon nitride film 3 Silicon oxide film 4 Conductive gate electrode (conductive electrode) n Refractive index

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tatsuo Tsuchiya 840 Takeno, Shimotomi, Tokorozawa, Saitama Pref., Technical Research Institute, Citizen Watch Co., Ltd. (72) Seiichi Ishihara 840, Taketomi, Tokorozawa, Saitama Citizen Watch Co., Ltd. Technical Research Institute Examiner Ryo Ikebuchi (56) References JP-A-48-103277 (JP, A) JP-A-60-60770 (JP, A) JP-A-59-229871 (JP, A) JP 58-34978 (JP, A)

Claims (1)

(57) [Scope of utility model registration request] 1. A tunnel insulating film capable of injecting charges on a conductive channel region sandwiched by source and drain regions on a semiconductor substrate, and a silicon nitride film on the tunnel insulating film.
A silicon oxide film formed by thermally oxidizing the silicon nitride film on the silicon nitride film, and forming a conductive gate electrode on the silicon oxide film, and storing by using the conductive gate electrode. In an insulated gate field effect semiconductor non-volatile memory device having a structure for operating, the silicon nitride film has a single layer structure, and its refractive index exceeds 2.0.
2. A semiconductor nonvolatile memory device characterized by being 2.1 or less.