JPH05343703A - Manufacture of nonvolatile memory - Google Patents

Abstract

PURPOSE:To reduce a leakage current caused by a parasitic MOS transistor in a bird's beak region, by forming a first conductivity-type impurity layer once again after a memory gate electrode is formed. CONSTITUTION:In an element isolation region 13, impurity ions are implanted and a first impurity layer 11 is formed. A selective oxidization step is carried out to form an element isolation insulating film 15. A memory gate insulating film 19 is formed, and a memory nitride film 21 and a top insulating film 23 are formed, and moreover a memory electrode 25 is formed. After impurity ions are implanted in a semiconductor substrate 33, the memory gate electrode 25 is oxidized to form a mask oxide film 27 in a heat treatment step. At the same time, a first conductivity type impurity is diffused in the semiconductor substrate 33 by using this heat treatment step. Consequently, a second impurity layer 29, which is of the same conductivity type as that of the first impurity layer 11 and has an impurity density higher than this impurity layer 11, is formed right under a bird's beak.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a non-volatile memory, and more particularly to a method for manufacturing a non-volatile memory having improved and stable memory characteristics and high reliability.

[0002]

2. Description of the Related Art A conventional non-volatile memory manufacturing method is described below.
This will be described with reference to FIG. FIG. 3 shows a cross-sectional view of the memory transistor in the channel width direction.

First, a first conductivity type semiconductor substrate 33 is formed with a first conductivity type impurity layer 11 of the same conductivity type as the semiconductor substrate 33 in the element isolation region 13. Further, selective oxidation is performed to form an element isolation insulating film 15 made of a silicon dioxide film in the element isolation region 13. At the time of forming the element isolation insulating film 15, a bird's beak having a shape like a bird's beak is formed at the boundary between the element region 17 and the element isolation region 13.

Thereafter, in the element region 17, a memory gate insulating film 19 made of a silicon dioxide film, a memory nitride film 21 made of a nitride film, a top insulating film 23 made of a silicon dioxide film, and a polycrystalline silicon film. The memory gate electrode 25 is sequentially formed.

After that, the mask oxide film 27 is formed by thermally oxidizing the memory gate electrode 25. afterwards,
Using the memory gate electrode 25 as a mask, a second conductivity type source and drain are formed to form a nonvolatile memory element.

[0006]

In the non-volatile memory, when a sufficiently high positive voltage is applied to the memory gate electrode 25, electrons in the conduction band of the silicon single crystal of the semiconductor substrate 33 tunnel through the memory gate insulating film 19 by a tunneling phenomenon. Therefore, it is trapped in the memory nitride film 21. As a result, the threshold voltage of the memory transistor changes and the memory characteristic is obtained.

Also in the nonvolatile memory manufactured by the conventional manufacturing method, a sufficiently high positive voltage is applied to the memory gate electrode 25 to accumulate negative charges in the memory nitride film 21, as described above. At this time, the threshold voltage of the memory transistor becomes an enhancing operation.

In this state, when a voltage is gradually applied to the memory gate electrode 25, no current flows because the threshold value in the element region 17 is high, but the boundary between the element region 17 and the element isolation region 13 does not flow. Parasitic MOS in bird's beak region
A current flows through the transistor at a low voltage applied to the memory gate electrode, which causes a leak current.

An example of this is shown by a broken line 31 in the graph showing the relationship between the gate voltage and the drain current in the conventional example shown in FIG. A drain current, which is a leak current, is flowing at a low gate voltage of the memory transistor.

As shown in the graph of FIG. 2, when a leak current is generated at a low gate voltage, there arises a problem that the writing width becomes narrow in the memory characteristic.

An object of the present invention is to solve the above-mentioned problems and to provide a method for manufacturing a nonvolatile memory having stable characteristics by reducing the leak current caused by a parasitic MOS transistor in a bird's beak region. That is.

[0012]

In order to achieve this object, the manufacturing method of the nonvolatile memory of the present invention employs the manufacturing method described below.

The method of manufacturing a nonvolatile memory according to the present invention comprises a step of forming a first impurity layer of a first conductivity type in an element isolation region of a semiconductor substrate of a first conductivity type, and element isolation by performing selective oxidation. A step of forming an element isolation insulating film made of a silicon dioxide film in the region, a step of forming a memory gate insulating film made of a silicon dioxide film in the element region, and a step of forming a memory nitride film on the memory gate insulating film A step of forming a top insulating film made of a silicon dioxide film on the memory nitride film, a step of forming a memory gate electrode, and a second conductivity type second film on the semiconductor substrate in a region aligned with the memory gate electrode. A step of forming an impurity layer, a step of forming a second conductivity type source region and a drain region in the semiconductor substrate in a region where the memory gate electrodes are aligned, And having a step of forming an insulating multilayer wiring film mainly made of Con, forming a contact window in the insulating film for multilayer wiring by photolithography and etching, the step of forming a wiring metal.

[0014]

In the method of manufacturing a nonvolatile memory according to the present invention, the impurity concentration of the bird's beak portion is increased by forming the impurity layer of the first conductivity type again after forming the memory gate electrode. Therefore, by increasing the impurity concentration in the bird's beak region, the operation of the parasitic MOS transistor on the low gate voltage side is suppressed and the leak current is reduced.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. 1A and 1B are cross-sectional views in the channel direction of a memory gate electrode of a memory transistor in each step of the method for manufacturing a nonvolatile memory according to the present invention.
(C) shows a plan view of the memory transistor.

First, as shown in FIG. 1A, a P type semiconductor substrate 33 is formed on a semiconductor substrate 33 by chemical vapor deposition (CV).
A nitride film (not shown) having a thickness of about 75 nm by the D method).
Then, an oxidation resistant film made of a nitride film is formed only in the element region 17 by photolithography and etching.

Thereafter, using this nitride film as a blocking film for ion implantation, impurity ions of boron are implanted into the element isolation region 13 by ion implantation, for example, 4 × 10 ¹³ atoms / cm 3.
^- isolation region 1 of the semiconductor substrate 33 by ion implantation of ²
Then, the first impurity layer 11 is formed.

After that, the nitride film formed on the element region 17 is used as an oxidation resistant film, and the temperature is set to 100 in a water vapor atmosphere.
By performing selective oxidation at 0 ° C. for 160 minutes, an element isolation insulating film 15 made of a silicon dioxide film having a thickness of 750 nm is formed in the element isolation region 13. After that, the nitride film used as the oxidation resistant film is removed.

Then, an oxidation treatment is performed in an oxidizing atmosphere to form a memory gate insulating film 19 made of a silicon dioxide film having a thickness of about 3 nm on the surface of the element region 17.

After that, a memory nitride film 21 made of a nitride film is formed to a thickness of 12 n by chemical vapor deposition (CVD method).
m.

After that, the memory nitride film 21 is oxidized in a water vapor atmosphere to form a top insulating film 23 made of silicon dioxide with a thickness of about 4 nm.

After that, the memory gate electrode 2 is formed by the CVD method.
5, a polycrystalline silicon film is formed with a film thickness of about 450 nm.

Then, the memory gate electrode 25 is patterned by photolithography and etching.

Next, as shown in FIGS. 1B and 1C, the memory gate electrode 25 is used as a blocking film for ion implantation,
Impurity ions consisting of boron by the ion implantation method, the injection amount 1 × 10 ¹³ atoms / cm ^- introducing the ¹² semiconductor substrate 33 under the conditions of.

Thereafter, the temperature is 1000 ° C. in an oxidizing atmosphere.
By oxidizing the memory gate electrode 25 for 20 minutes, a mask oxide film 27 having a film thickness of about 20 nm is formed.

By utilizing the heat treatment for forming the mask oxide film 27, the boron, which is the first conductivity type impurity introduced into the bird's beak region by the ion implantation method described above, is diffused into the semiconductor substrate 33.

As a result, a second impurity layer 29 having the same conductivity type as that of the first impurity layer 11 and a higher impurity concentration than that of the first impurity layer 11 is formed in the region immediately below the bird's beak.

[0028] Next, the memory gate electrode 25 as a blocking film for ion implantation, an impurity of the semiconductor substrate 33 and the opposite conductivity type, for example phosphorus, impurity ions such as arsenic ion implantation amount ^{3 × 10 15 atoms / cm -} 12 Ion implantation is carried out under the conditions described above to form a source region and a drain region (not shown) in the semiconductor substrate 33.

In the subsequent steps, a multi-layer wiring insulating film mainly composed of silicon dioxide is formed by a general method, a contact window is formed in the multi-layer wiring insulating film by photolithography and etching, and then as a wiring metal. A nonvolatile memory is formed by forming aluminum (Al).

[0030]

As is apparent from the above description, in the method of manufacturing a nonvolatile memory of the present invention, the impurity concentration of the bird's beak region in which the parasitic MOS transistor is formed is increased to further increase the parasitic MOS transistor. The threshold voltage becomes high.

The relationship between the gate voltage and the drain current of the memory transistor of the present invention is shown in the graph of FIG. Figure 2
As shown by the solid line 35 indicating the characteristics of the present invention,
No drain current, which is a leak current at a low gate voltage, is generated. As a result, in the memory characteristics, when a positive voltage is applied to the memory gate electrode, the operation becomes more enhanced and the writing width increases. Therefore, it is possible to increase the storage retention time and shorten the programming time, improve the memory characteristics and reliability as compared with the conventional one, and obtain a non-volatile memory element having more stable characteristics.

[Brief description of drawings]

FIG. 1 is a diagram showing a method for manufacturing a nonvolatile memory according to an embodiment of the present invention.

FIG. 2 is a graph showing a relationship between a gate voltage and a drain current of a memory transistor according to the present invention and a conventional example.

FIG. 3 is a cross-sectional view showing a method for manufacturing a nonvolatile memory in a conventional example.

[Explanation of symbols]

 11 First Impurity Layer 13 Element Separation Region 17 Element Region 19 Memory Gate Insulation Film 21 Memory Nitride Film 23 Top Insulation Film 25 Memory Gate Electrode 29 Second Impurity Layer

Claims (1)

[Claims] 1. A step of forming a first impurity layer of a first conductivity type in an element isolation region of a semiconductor substrate of a first conductivity type, and element isolation comprising a silicon dioxide film in the element isolation region by performing selective oxidation. Forming an insulating film, forming a memory gate insulating film made of a silicon dioxide film in the element region, forming a memory nitride film on the memory gate insulating film, and forming a memory nitride film on the memory nitride film. A step of forming a top insulating film made of a silicon film,
Forming a memory gate electrode; forming a second impurity layer of the first conductivity type on the semiconductor substrate in a region where the memory gate electrode is aligned; and forming a second conductivity layer on the semiconductor substrate in a region where the memory gate electrode is aligned. A source region and a drain region of the mold, a step of forming an insulating film for a multi-layer wiring mainly composed of silicon dioxide, a step of forming a contact window in the insulating film for a multi-layer wiring by photolithography and etching, and a wiring A method of manufacturing a non-volatile memory, comprising a step of forming a metal.