JPH01117332A - Non-volatile semiconductor memory and manufacture thereof - Google Patents

Abstract

PURPOSE:To provide a tunnel oxide film with excellent endurance characteristics by a method wherein the tunnel oxide film is constituted in a three layer structure to form nitroxide layers respectively on both surfaces of a silicon oxide film. CONSTITUTION:A chamber vacuumized, after leading-in a reaction gas, is heated up to high temperature to remove a natural oxide film and, after pressure reduction for exhaust, O2 is led in the chamber. The semiconductor wafers in this chamber are heater up to high temperature to form a silicon oxide film. Next, a nitrifying reaction gas NH3 led in is rapidly heated by a rapid heating means to be nitrified rapidly. Furthermore, nitrogen is led in the chamber to take out the semiconductor wafers. A tunnel oxide film in a three layer structure to form a nitroxide film on both surfaces of the silicon oxide film thus formed can be constituted by rapidly heating the silicon oxide film by a halogen lamp in NH3 gas atmosphere.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a non-volatile semiconductor memory and its non-volatile semiconductor memory in which the endurance (the number of repetitions of writing and erasing) characteristics and the dielectric breakdown characteristics of a tunnel oxide film are improved. Regarding the manufacturing method.

[Prior Art] In an E2 PROM, as the number of repetitions of writing and erasing increases, the threshold voltage V
This has the property of decreasing T and narrowing the VT window. The thinner the tunnel oxide film is, the less the threshold voltage VT decreases, resulting in better endurance characteristics.
When the tunnel oxide film is configured to be thin, the dielectric breakdown life tends to become weaker, which is thought to increase the defect density of the oxide film.

Conventionally, as a means of forming a tunnel oxide film, a semiconductor wafer is placed in an electric furnace, an oxide film with a thickness of about 100 mm is formed on the surface of the semiconductor wafer, and this oxide film is used as a tunnel oxide film. I'm trying to get it used. However, it is difficult to remove a poor quality natural oxide film of about 5 to 15 layers formed on the surface of a semiconductor wafer through a cleaning process using air or chemicals.

Generally, in an electric furnace, when removing a natural oxide film on the surface of a silicon semiconductor wafer using H2 (hydrogen) at high temperature, it takes a lot of time to replace the gas before introducing H2 and after the H2 treatment. IC,
Redistribution of impurities in LSIs occurs, making it difficult to accommodate highly integrated devices. Also, if you try to shorten the time for this natural oxide film removal process,
Due to the risk of explosion, the amount of time available was naturally limited.

[Problems to be Solved by the Invention] The present invention has been made in view of the above-mentioned points, and provides a method for forming a tunnel oxide film on the surface of a semiconductor wafer without taking the semiconductor wafer out of a processing chamber. In addition, it is an object of the present invention to provide a nonvolatile semiconductor memory and a method for manufacturing the same in which the tunnel oxide film can have good endurance characteristics without having to be particularly thin.

[Means for Solving the Problems] That is, in the semiconductor memory according to the present invention, the tunnel oxide film has a three-layer structure in which nitroxide layers are formed on both sides of the oxide film. For this purpose, the natural oxide film is removed with the semiconductor wafer set in the chamber, and then the chamber is evacuated and oxygen is introduced, and the surface of the semiconductor wafer is rapidly oxidized using a halogen lamp or the like. Form an oxide film.

In this way, the inside of the chamber is evacuated, a reaction gas is introduced, and the reactant gas is rapidly heated using a halogen lamp or the like to rapidly nitride the chamber.

[Function] As mentioned above, by creating a three-layer structure in which a nitrile oxide layer is formed on each side of a tunnel oxide film made of, for example, a silicon oxide film, the amount of traps in this insulating film is reduced, and the amount of oxidation It is possible to achieve superior dielectric breakdown characteristics as compared to a tunnel oxide film layer made of only a silicon film. In other words, since the amount of traps is small, the drop in threshold voltage in the endurance characteristic is reduced, and the write and erase repetition rate characteristics are excellent, so that a highly reliable E2 FROM can be obtained. It is something that becomes.

[Embodiments of the invention] Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

FIG. 1 shows the process of forming a tunnel oxide film for an E2 PROM, in which a silicon semiconductor wafer is placed in a processing chamber.

Then, the first step 1 is to depressurize and exhaust the inside of the chamber with the semiconductor wafer set inside the chamber.
Execute 1. Once the inside of the chamber has been evacuated to a vacuum state, a reaction gas such as 82, HCλ, etc. is introduced into this chamber as shown in the second step 12, and a third step is to raise the temperature inside this chamber. This third step 13 removes a poor quality natural oxide film formed on the surface of the semiconductor wafer in the air or by chemical treatment. For example, in this third step 13, processing is performed at a temperature of 1150° C. for 60 seconds.

After such natural oxide film removal processing is performed, a temperature lowering operation is performed in a fourth step 14.

Once the temperature inside the chamber is lowered in this way,
In the fifth step 15, the inside of the chamber is evacuated and the sixth step
In step 16, 02 (II element) is introduced into the chamber. Once the semiconductor wafer in the chamber is set in the oxygen atmosphere, in a seventh step 17 the temperature of the semiconductor wafer in the chamber is raised to form a silicon oxide film on the surface of the semiconductor wafer. do. In this case, the temperature raising treatment in step 17 is performed using, for example, a halogen lamp, so that the temperature is raised rapidly, and the surface of the semiconductor wafer is rapidly oxidized, so that a silicon oxide film is formed. There is.

Here, the temperature raising treatment in the seventh step 17 is performed at, for example, 1150° C. for 40 seconds, so that a silicon oxide film with a thickness of 100 μm is formed.

Once the silicon oxide film has been formed on the surface of the semiconductor sea urchin in this manner, the temperature inside the chamber is lowered in an eighth step 18, and further the vacuum is evacuated in a ninth step 19.

In the chamber in which the silicon semiconductor wafer with the silicon oxide film formed on the surface is set, there are
In step 20, a nitriding reaction gas NH3 is introduced. Then, in an eleventh step 21, the temperature is rapidly raised by a rapid heating means using, for example, a halogen lamp, and rapid nitridation is performed. This nitriding step is performed at, for example, 1150° C. for 30 seconds. After the nitriding process has been performed in this manner, a temperature-lowering process is performed in a twelfth step 22, and further, nitrogen is introduced into the chamber in a thirteenth process 23, and the semiconductor wafer is taken out.

FIG. 2 shows a schematic configuration of an apparatus for forming a tunnel oxide film as described above, in which a silicon semiconductor wafer 32 is inserted into a quartz chamber 31 and is supported. There is.

Gas introduction ports 33o and 34 are formed in this chamber 31, and N2 is introduced from the introduction port 33, and reactive gases such as NH3,02, N2, CI2, etc. are selectively introduced from the introduction port 34. ing. And this chamber 3
1 is further formed with a discharge port 35, and a vacuum pump (not shown) is used to drain the chamber 3 from this discharge port 35.
1 is selectively depressurized and evacuated.

Here, a heating mechanism using a halogen lamp 36 is provided on the outer periphery of the quartz chamber 31, and the halogen lamp 36 is used to heat the silicon semiconductor wafer 3.
2 is heated rapidly.

Although details are not shown, the above heating temperature is observed in the quartz chamber 31, and the halogen lamp 3 is set so that the heating temperature is set to the target temperature state.
6 is controlled. Further, as a heat source, an arc lamp is appropriately used instead of a halogen lamp.

FIG. 3 shows the temperature conditions and atmospheric pressure conditions in the chamber 31 at each step in the tunnel oxide film forming process shown in FIG.

FIG. 3 shows a cross-sectional structure of one memory element portion of the E2 PROM in which the tunnel oxide film is formed as described above, and the source 42 formed corresponding to the main surface of the silicon semiconductor substrate 41. A floating gate 45 made of polysilicon is formed corresponding to the region of the drain 43 with a gate insulating lI44 interposed therebetween. This floating gate 45 and the drain 4
3, a tunnel oxide film 46 is formed by a silicon oxide thin film. A control gate 48 made of polysilicon is formed on the floating gate 45 with an insulator 1047 interposed therebetween.

In the E” PROM configured in this way, 10
Data writing and erasing operations are performed by sending electrons in and out of the floating gate 45 through the tunnel oxide film 46 having a thickness of about 0.0 mm.

For example, during a data write operation in which electrons are input to the floating gate 45, the control gate 48
A voltage of 18 to 25 V is applied to and the drain 43, source 42, and substrate 41 are set to Ov. Furthermore, in order to extract electrons from the 70-ring gate 45 for erasing, the control gate 48, source 42 and substrate 41 are set to 0, and a voltage of 18 to 25 V is applied to the drain 43. .

In order to smoothly perform data writing and erasing device operations in such an E"FROM by controlling the application of high voltage, tunnel oxidation 1g!46 is required.
becomes important.

The tunnel oxide film 43 formed as described above has a structure similar to that of a band tie gellumn shown in FIG. 5, and the surface of the tunnel oxide film and the interface side of the silicon substrate are , resulting in a nitroxide structure. Therefore, as shown by the broken line in this figure, this tunnel oxide film is made of silicon oxide (StO”
) The barrier hydride of the tunnel film at the surface and interface of the tunnel oxide film is lowered than when using only III.

Such a tunnel oxide film having a three-layer structure in which nitrooxide films are formed on both sides of the silicon oxide film is obtained by converting the silicon oxide film formed in the seventh step 17 to the tenth step. 20 Furthermore, as shown in the 21st step 21, it can be constructed by rapid heating with a halogen lamp in an NHi gas atmosphere.

For example, the literature (Yasushi Na1to at
at, J.

Vac, Technol, 85 (3), Ma.
c/Jun 1987, P2S5), the above-mentioned nitriding method is shown, and when the nitriding time is short, nitroxide is formed on the surface and interface of the silicon oxide film, and oxidation increases over time. The entire film becomes nitroxide. This was also confirmed by the inventors of the present invention.

If the tunnel oxide film has a 3Iil structure with nitroxide layers on both sides as shown in FIG. It was confirmed that the dielectric breakdown characteristics were better than when

Figure 6 shows the results obtained from the experiment. In this example, the current density aJ is "J-64mA/cm
! ``This is a case where the thickness Tox of the tunnel oxide film is 105 people.

That is, as is clear from FIG. 6, the example shown by the track mA is an example in which the rapid nitriding time is set to "0 seconds", and is an example in which the rapid nitriding process is not substantially performed. Voltage vg increases with time. Here, the voltage ■9 is
This corresponds to the amount of charge trapped in the tunnel oxide film.

On the other hand, when rapid nitriding is performed at a temperature of 1150'C for 10 seconds and when rapid nitridation is performed at the same temperature for 30 seconds, VQ hardly increases as shown by curves B and C in this figure. However, as shown by the curved line, when the rapid nitriding treatment is performed for 100 seconds, ■g rapidly increases.

Figure 7 shows the results of the constant voltage 811TDDB dielectric breakdown life test, where A, B, and C are each 1150°C.
D and E are examples in which the rapid nitriding times were set to 0 seconds, 10 seconds, and 30 seconds, respectively, and D and E are examples in which the rapid nitriding times were set to 100 seconds and 300 seconds, respectively.

i.e. 3 m with a layer of nitroxide on both sides.
structure, the amount of trapped electrons is kept small, and the threshold voltage drop in the endurance characteristics is small, even if the number of data writing and erasing cycles increases. Its writing and erasing characteristics are kept stable. Furthermore, as is clear from the results in Figure 7, when the tunnel oxide film has a 311i structure, the dielectric breakdown characteristics are good and the dielectric breakdown life is excellent. It becomes like this.

Figure 8 shows the state of the endurance characteristics. In this figure, A has a tunnel oxide film thickness of 110 mm.
This is a case of rapid nitriding at 1150° C. for 30 seconds, and the threshold voltage V↑ hardly decreases even if writing and erasing are repeated. On the other hand, when the rapid nitriding indicated by B is not performed and there is no nitroxide layer, the threshold voltage VT decreases as the shadowing and erasing operations are repeated, and the 7th window becomes narrower.

In the explanation so far, the temperature of rapid nitriding treatment was set at 1150°C.
This describes the case where However, even if the conditions for rapid nitriding are changed, a tunnel oxide film with a three-layer structure is still formed, and a layer of nitroxide is formed on the surface side and the interface side of the silicon oxide film. The conditions for forming the three-layer structure, that is, the nitriding temperature and rapid nitriding time, are within the range shown by diagonal lines in FIG.

If the silicon oxide film is rapidly nitrided to form a three-layer tunnel oxide film with nitroxide layers on both sides of the oxide film, the endurance characteristics and dielectric breakdown life characteristics will be improved. This seems to be due to the following reasons.

That is, when the tunnel oxide film portion is composed of only a silicon oxide film, there is a strain that is said to be the cause of the so-called 5i-0 trap in the vicinity of the interface between the S+a& board and the SiO2 that constitutes the tunnel oxide film. Although a bond exists, it is thought that when rapid nitriding is performed and a certain amount of nitroxide III (nitroxide) is formed near the interface, the strain at this interface decreases and a trap reduction phenomenon occurs. . Furthermore, this nitroxide has a lower barrier bite than the oxide film,
Therefore, although the tunnel film is thick as a whole, it appears to be a thin film, and it is thought that the number of traps is further reduced in addition to the reduction in traps due to the above-mentioned reason. However, when the entire structure is converted to night O oxide, the strain increases and the amount of traps increases significantly, indicating that an optimum range exists.

[Effects of the Invention] As described above, in the nonvolatile semiconductor memory according to the present invention, the tunnel oxide film is sandwiched between 311! By adopting this structure, the endurance characteristics are effectively improved, and the dielectric breakdown life characteristics are also improved, resulting in a long-life and highly reliable E.
Furthermore, the tunnel oxide film with the above-mentioned 3111 structure can be formed in a series of steps without taking it out of the processing chamber even once, making it possible to obtain this type of semiconductor memory. This simplifies the manufacturing process and has a significant effect on improving product reliability.

[Brief explanation of the drawing]

FIG. 1 is a diagram illustrating the manufacturing process, particularly for the tunnel oxide film portion, of a nonvolatile semiconductor memory according to an embodiment of the present invention; FIG. Figure 3 is a diagram showing the temperature and pressure conditions inside the chamber over time during the above treatment process, and Figure 4 is a diagram showing the state of the temperature and pressure inside the chamber in the above processing step.
FIG. 5 is a diagram showing the band diagram of the above-mentioned device. FIG. 6 is a diagram showing the results obtained by experimenting with the state of the amount of traps. FIG. 7 is also an insulating diagram. Figure 8 is a diagram showing the results of a destructive life test; Figure 8 is a diagram showing the endurance characteristics of the memory according to the present invention in comparison with a conventional example; Figure 9 is a diagram showing the nitriding temperature and rapid FIG. 3 is a diagram showing the relational conditions with nitriding time. 31...Quartz chamber, 32...Semiconductor wafer, 3
6... Halogen lamp, 4.1... Silicon semiconductor substrate, 42... Source, 43... Drain, 44...
・Insulating film, 45 ・Floating gate, 4B・
...Tunnel oxide film, 48...Control gate. Applicant's agent Patent attorney Takehiko Suzue Figure 1 Figure 2 Figure 3 Figure 4 Figure 4 5i02 Accumulation cough Figure 5 District 6 Figure 7

Claims (3)

[Claims] (1) A process of depressurizing and evacuating the inside of the chamber where the semiconductor wafer is set, and removing a natural oxide film formed on the surface of the semiconductor wafer by introducing a reaction gas into the depressurized and evacuated chamber and raising the temperature. A step of evacuating the chamber containing the semiconductor wafer from which the natural oxide film has been removed and introducing oxygen, and a step of rapidly oxidizing the surface of the semiconductor wafer in the chamber into which oxygen has been introduced. a step of forming a film; a step of evacuating the chamber and introducing a nitrogen-based reaction gas into the chamber; and heating the surface of a semiconductor wafer set in the chamber into which the nitrogen-based reaction gas has been introduced. nitriding step, and in the nitriding step, the surface of the semiconductor wafer is rapidly heated to form a tunnel layer in which a layer of nitroxide is formed on both sides of the oxide film. A method for manufacturing a non-volatile semiconductor memory. (2) The method for manufacturing a nonvolatile semiconductor memory according to claim 1, wherein in the heating step, a heat source such as a halogen lamp or an arc lamp is used. (3) A tunnel oxide film layer with a three-layer structure in which nitroxide layers are formed on both sides of the oxide film is formed between the drain and floating gate formed on the semiconductor substrate. Characteristics of non-volatile semiconductor memory.