JPH0722521A - Semiconductor storage device - Google Patents

Abstract

PURPOSE:To stabilize date read operation from a memory cell by enhancing the reliability of a gate insulation film of a transfer transistor. CONSTITUTION:Since a semiconductor nitriding oxide 34 is used as a gate insulation film for transfer transistors 16 and 17 forming a memory cell, the reliability of the gate insulation film of the transfer transistors 16 and 17 is higher compared to the case where a semiconductor oxide film 36 is used. Moreover, a current drive ability of the transfer transistors 16 and 17 is lower and a beta ratio is larger compared to driving transistors 12 and 13 where semiconductor oxide film 36 is used as a gate insulation film so that data reading operation is stabilized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor memory device called SRAM.

[0002]

2. Description of the Related Art FIG. 3 shows an equivalent circuit of a memory cell in a high resistance load type SRAM. The flip-flop 11 of this memory cell is composed of driving NMOS transistors 12 and 13 and load resistance elements 14 and 15, and the flip-flop 11 and the transfer NMOS are provided.
A memory cell is composed of the transistors 16 and 17.

A ground line 21 is connected to the sources of the NMOS transistors 12 and 13, and the resistance elements 14 and 15 are connected.
A power supply line 22 is connected to. Also, the word line 23
Is the gate electrode of the NMOS transistors 16 and 17, and the source / drain of one of the NMOS transistors 16 and 17 is a complementary bit line 24 or 2
5 is connected.

In the conventional memory cell of such a resistance load type SRAM, a semiconductor oxide film such as a SiO ₂ film is used as the gate insulating film of each of the NMOS transistors 12, 13, 16 and 17. It was being done.

[0005]

Generally, since the RC time constant of the word line 23 is large, the NMOS transistors 16 and 17 which are transfer transistors are subjected to a strong hot carrier stress. Further, there is an increasing demand for boosting the voltage applied to the word line 23 in order to stabilize the data read operation from the memory cell even if the power supply voltage is lowered.

From these points, high reliability is required for the gate insulating films of the NMOS transistors 16 and 17 which are transfer transistors. However, since a semiconductor oxide film such as a SiO ₂ film cannot always obtain sufficiently high reliability, the SRAM as a whole cannot obtain sufficiently high reliability.

[0007]

According to another aspect of the semiconductor memory device of the present invention, a flip-flop 11 and a transfer transistor 1 are provided.
In a semiconductor memory device in which memory cells 6 and 17 are formed, a semiconductor oxide film 3 is used as a gate insulating film for the driving transistors 12 and 13 of the flip-flop 11.
6 is used, and the transfer transistors 16 and 1 are used.
7, the semiconductor oxynitride film 34 is used as the gate insulating film.

According to another aspect of the semiconductor memory device of the present invention, the film thickness of the gate insulating film of the transfer transistors 16 and 17 is larger than the film thickness of the gate insulating film of the driving transistors 12 and 13. There is.

[0009]

In the semiconductor memory device according to the first aspect, since the semiconductor oxynitride film 34 is used as the gate insulating film of the transfer transistors 16 and 17, compared with the case where the semiconductor oxide film 36 is used, the transfer is performed. Transistors 16, 1
7. The gate insulating film of No. 7 has high reliability. Moreover, compared to the driving transistors 12 and 13 in which the semiconductor oxide film 36 is used as the gate insulating film, the transfer transistor 1
The current driving capabilities of the transistors 6 and 17 are low, and the beta ratio, which is the ratio of the current driving capabilities of the driving transistors 12 and 13 to the current driving capabilities of the transfer transistors 16 and 17, is large.

According to another aspect of the semiconductor memory device of the present invention, since the gate insulating film of the transfer transistors 16 and 17 is thicker than the gate insulating film of the drive transistors 12 and 13, the transfer transistors 16 and 17 are formed. Of the gate insulating film of the transfer transistor 16 and 17 having higher reliability
Has a lower current drive capability and a higher beta ratio.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention applied to a high resistance load type SRAM will be described below with reference to FIGS. Although FIG. 1 schematically shows the main part of the memory cell in this embodiment, a manufacturing method thereof will be described with reference to FIG.

In order to manufacture this embodiment, FIG.
As shown in, a semiconductor oxide film 27 for element isolation is formed on the surface of the semiconductor substrate 26 by a normal selective oxidation method to form element active regions such as the driving transistor section 31 and the transfer transistor section 32. Then, by performing normal wet oxidation or dry oxidation, as shown in FIG. 2B, the semiconductor oxide film 33 having a film thickness of about 10 nm is formed on both the driving transistor section 31 and the transfer transistor section 32 as elements. Form on the surface of the active region.

Next, a high temperature heat treatment for about 1 to 30 minutes in an NH ₃ atmosphere at about 900 to 1000 ° C., or 1
000 to 1200 ° C in N ₂ O atmosphere 1 to
High temperature heat treatment is performed for about 30 minutes. As a result, a small amount of nitrogen atoms are introduced into the semiconductor oxide film 33, and the semiconductor oxide film 33 becomes the semiconductor oxynitride film 34, as shown in FIG.

Next, as shown in FIG. 2D, only the transfer transistor portion 32 is covered with a resist 35, and the resist 35 is used as a mask to select only the semiconductor oxynitride film 34 of the driving transistor portion 31. To remove it. And
After removing the resist 35, normal wet oxidation or dry oxidation is performed to obtain a film thickness of 1 as shown in FIG.
The semiconductor oxide film 36 of about 0 nm is formed again on the surface of the element active region of the driving transistor section 31. This time,
The film thickness of the semiconductor oxynitride film 34 of the transfer transistor portion 32 becomes thicker than in the state of FIG.

Next, as shown in FIG. 2F, this time, only the driving transistor portion 31 is covered with the resist 37. Then, using the resist 37 as a mask, only the thickened semiconductor oxynitride film 34 of the transfer transistor portion 32 is selectively etched to reduce the thickness of the semiconductor oxynitride film 34. This balances the film thickness ratio of the gate insulating film in the driving transistor section 31 and the transfer transistor section 32.

Next, as shown in FIG. 2G, the resist 3
7 is removed, and as shown in FIG. 1, a gate electrode 41 and a source / drain N ⁺ diffusion layer 42 are formed,
NMOS transistor 12, which is a driving transistor,
13 and NMOS transistors 16 and 17 which are transfer transistors are formed. After that, the high resistance load type SRAM of the present embodiment is completed through conventionally known steps.

In the high resistance load type SRAM of the present embodiment manufactured as described above, the semiconductor oxynitride film 34 is higher in reliability such as TDDB characteristics and hot carrier resistance than the semiconductor oxide film 36 ( For example, IEDM 91-359
~ 362), the transfer NMOS transistor 16,
The high reliability required for No. 17 can be satisfied.

In the manufacturing method of the above embodiment,
The step of reducing the film thickness of the semiconductor oxynitride film 34 shown in FIGS. 2F and 2G is not always necessary, and even if the film thickness of the semiconductor oxynitride film 34 remains thicker than the film thickness of the semiconductor oxide film 36. Good. Further, the above-described embodiment is an application of the present invention to a high resistance load type SRAM, but the present invention is not limited to bulk C
It can also be applied to a MOS type SRAM, a stacked CMOS type SRAM and the like.

[0019]

According to the semiconductor memory device of the first aspect, since the reliability of the gate insulating film of the transfer transistor is high, the reliability of the semiconductor memory device is also high, and since the beta ratio is large, reading of data from the memory cell is performed. The operation is also stable.

According to another aspect of the semiconductor memory device of the present invention, since the reliability of the gate insulating film of the transfer transistor is higher, the reliability of the semiconductor memory device is higher, and the beta ratio is higher. The read operation of is also more stable.

[Brief description of drawings]

FIG. 1 is a side sectional view schematically showing a main part of a memory cell according to an embodiment of the present invention.

2A to 2C are side cross-sectional views showing a method of manufacturing a main part of a memory cell in one embodiment in the order of steps.

FIG. 3 is an equivalent circuit diagram of a memory cell in a high resistance load type SRAM to which the present invention can be applied.

[Explanation of symbols]

 11 flip-flop 12 NMOS transistor 13 NMOS transistor 16 NMOS transistor 17 NMOS transistor 34 semiconductor oxynitride film 36 semiconductor oxide film

Claims (2)

[Claims] 1. A semiconductor memory device having a memory cell composed of a flip-flop and a transfer transistor, wherein a semiconductor oxide film is used as a gate insulating film of a driving transistor of the flip-flop, and the transfer transistor. A semiconductor memory device characterized in that a semiconductor oxynitride film is used as the gate insulating film. 2. The semiconductor memory device according to claim 1, wherein the film thickness of the gate insulating film of the transfer transistor is thicker than the film thickness of the gate insulating film of the driving transistor.