JPH05251428A - Method for formation of insulating film and non-volatile semiconductor device using such film - Google Patents

Abstract

PURPOSE:To form a thin and yet highly reliable insulating film by oxidizing the surface of a substrate in an oxidizing gas atmosphere containing no nitrogen to form an oxide film, a first insulating film, and subsequently changing the atmosphere over to that containing nitrogen to convert the first insulating film to an oxynitride. CONSTITUTION:A silicon substrate 200 is heated in a gas atmosphere for insulating film formation in a reaction furnace to form an insulating film. Then the infrared ray irradiation is used for heating means. In the first half part of the heating process, the surface of the substrate 200 is oxidized in an oxidizing gas atmosphere containing no nitrogen (e.g. O2) to form an oxide film, a first insulating film 204; in the second half part of the process, the oxidizing gas atmosphere is changed over to that containing nitrogen (e.g. N2O), and the first insulating film 204 is converted to an oxynitride, turned into a second insulating film 206.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of forming an insulating film on a silicon underlayer and a nonvolatile semiconductor memory device (hereinafter referred to as EEPROM) using the insulating film obtained by this method.
Call).

[0002]

2. Description of the Related Art A semiconductor integrated circuit, for example, a silicon integrated circuit, uses an oxide film having a very small thickness. For example, a non-volatile memory having a design rule of 1.0 um or less,
Especially, in the EEPROM of 1M bits or more, 100
An ultrathin oxide film having a thickness of A ° (A ° is a symbol representing angstrom) or less, for example, a silicon oxide film (SiO ₂ film) is used as a tunnel oxide film. The characteristics of such an ultra-thin oxide film are extremely important factors in determining the number of rewrites and the data storage retention time in the operation of the EEPROM.

9 and 10 show a typical conventional EEP.
It is a figure explaining ROM, (A)-(C) of FIG. 9 is E.
Figure 3 shows a schematic structural diagram of an EPROM cell. Figure 9
(A) is a floating type EEPROM cell (FLO
(TOX type) is a plan view of a main part as seen from the upper surface, where S is a source region, D is a drain region, CG is a control gate, FG is a floating gate, and W is a tunnel window (region) which is an electron injection region. Is shown. Also,
9B and 9C show X ₁ -Y ₁ of FIG. 9A.
Is a cross-sectional view and X ₂ -Y sectional view showing taken along a ₂ shown taken along. This EEPROM cell is P
A pair of impurity diffusion regions (source region S,
A drain region D) and a channel region (n ⁺ region) 106 are formed, polysilicon or the like is patterned with an insulator interposed, and a control gate CG and a floating gate FG are formed. Further, one tunnel window W is provided for one cell with an ultrathin oxide film (film thickness 100 A °) 104 (also referred to as a tunnel oxide film) interposed between the floating gate FG and the drain region D.

Next, FIG. 10 is an equivalent circuit diagram for explaining the operation of the conventional EEPROM cell. In the figure, symbol T ₁ is a select transistor (select gate transistor), T ₂ is a memory transistor (floating gate transistor), and BL is a bit line extending from the drain of the select transistor T ₁ . Further, the select gate SG forms a word line. CG is a control electrode of the memory transistor T ₂ , FG is a floating electrode, and S is a source electrode.
An operation example of this conventional EEPROM cell will be described. When erasing data, the drain D is grounded, the control gate CG is set to a high voltage, and the potential of the floating gate FG is raised by capacitive coupling. As a result, electrons are injected from the drain D to the floating gate FG through the ultrathin oxide film 104 ((B) and (C) in FIG. 9). Further, in writing data, the polarity (negative electrode) of the control gate CG is changed with respect to the drain D, and electrons are emitted from the floating gate FG to lower the threshold voltage Vt. Table 1 shows the relationship between the memory operation of the conventional EEPROM and the voltage of each part. In the table, BL is a bit line, SG is a select gate electrode, CG is a control gate electrode, and S
Indicates the source electrode. Further, the voltages at the time of reading, erasing and writing with respect to the voltage of each part are shown in the table.

By the way, since the EEPROM repeatedly performs the operations of writing and erasing data, the stress applied to the tunnel oxide film increases in proportion to the number of times of rewriting of information data. This stress has a correlation between the film thickness and the data retention period. For example, as the stress increases, the data retention characteristic (determination of erasing or writing) deteriorates. Generally, if the tunnel oxide film is thickened, the stress of the tunnel oxide film can be reduced and the data retention period can be lengthened even if the number of times of rewriting is increased.

[0006]

However, if the tunnel oxide film is thickened, a voltage of 20 V or more is required for writing and erasing. Therefore, electron injection and It becomes impossible to erase or write the information data that is released. Therefore, there is a limitation that the film thickness of the tunnel oxide film must be thin enough to obtain the tunneling effect (the film thickness is 100 A ° or less).

Further, as the number of times of rewriting information data increases, the stress applied to the tunnel oxide film increases,
For this reason, there is a problem that the data retention period is shortened and the memory operation characteristics are deteriorated, so that the reliability of the EEPROM is lowered. The deterioration of the operating characteristics mentioned here mainly appears in the form of dielectric breakdown of the tunnel oxide film and increase of leak current. Further, the deterioration of these operating characteristics strongly depends on the film thickness of the oxide film, and, for example, as the oxide film decreases, dielectric breakdown, decrease in charge amount, and increase in leak current occur.

An object of the present invention is to provide a highly reliable method of forming an insulating film which is highly reliable even if the insulating film is thin.

[0009]

In order to achieve this object, according to the present invention, a heat treatment is performed on a silicon underlayer in a gas atmosphere for forming an insulating film in a reaction furnace, and the insulating film is formed on the underlayer. It has the following characteristics in forming.

First, heating is performed by using a heating source by infrared irradiation, and in the first half of this heat treatment, the surface of the base is oxidized in a nitrogen-free oxidizing gas atmosphere to form an oxide film which is a first insulating film. Form. Subsequently, in the latter half stage of this heat treatment, the nitrogen-free oxidizing gas is switched to a nitrogen-containing oxidizing gas atmosphere, and the first insulating film is oxynitrided in this latter oxidizing gas atmosphere. To the second insulating film.

Further, according to a preferred embodiment of the present invention, it is preferable that the nitrogen-free oxidizing gas is oxygen gas and the first insulating film obtained thereby is a silicon dioxide (SiO ₂ ) film.

Further, in carrying out the present invention, preferably, the nitrogen-containing oxidizing gas is at least one selected from the gas group of nitric oxide (NO), nitrogen dioxide (NO ₂ ) and nitrous oxide (N ₂ O). It is preferable that the oxynitride film thus obtained has a (SiO-N) film structure.

Further, according to the present invention, a nonvolatile semiconductor device is formed by using the second insulating film as a tunnel film.

[0014]

According to the insulating film forming method of the present invention described above,
As can be understood from the results of secondary ion mass spectrometry (SIMS) and X-ray (XPS) analysis described later, the second insulating film formed on the silicon base has many nitrogen atoms near the interface of the silicon insulating film. Incorporated (maximum 4 × 10 ²¹
Atoms / cm ³ ) and also forms an interface by Si—N bonds. Such a second insulating film has less stress,
In addition, since it has a property of low leakage current value, it has been considered impossible with the conventional film thickness of the oxide film (SiO ₂ film).
In the lower limit region of A °, a tunnel oxide film that can be put to practical use can be provided.

[0015]

Embodiments of the insulating film forming method of the present invention will be described below with reference to the drawings. However, the apparatus and process diagrams used for the description only schematically show the shapes, sizes, and positional relationships of the respective constituent components to the extent that these inventions can be understood. First, before entering this embodiment, an apparatus for carrying out the present invention will be described.

<< Embodiment of Structure of Insulating Film Forming Apparatus Suitable for Use in Carrying Out the Invention >> FIG. 2 shows a main part of an insulating film forming apparatus (mainly a reactor and a reactor for carrying out the method of the present invention. It is sectional drawing which shows the structure of a heating part) schematically. Incidentally, FIG. 2 shows a state where the substrate is installed in the reaction furnace.

FIG. 3 is a diagram for explaining an embodiment of the present invention, and is a diagram schematically showing an overall structure of an insulating film forming apparatus.

As shown in FIG. 2, this insulating film forming apparatus has a reaction furnace 10 in which a substrate is installed, an exhaust means 12 for evacuating the inside of the reaction furnace 10, and a gas supply section 14.
And a heating unit 16 for performing heat treatment. An example of the structure of this device will be described below.

As shown in FIG. 2, in this embodiment, the reaction furnace (chamber) 10 is, for example, a main body 10a and a lid member 10b.
And an elevating member 10c. As the material for forming the main body 10a and the elevating member 10c, for example, stainless steel may be used, and as the material for forming the lid member 10b and the support 20 described later, for example, quartz or vice versa may be used.

The main body 10a and the elevating member 10c are separably united to form a concave portion a. A supporting body 20 for mounting the substrate 18 is provided on the elevating member 10c side of the concave portion a to provide the elevating member 10c. Support 20
The substrate 18 on which is placed is placed in the reaction furnace 10 or the reaction furnace 1
0 Allow it to be taken out. In the illustrated example, the lifting member 10
elevating member 10c for mechanically elevating and lowering c
Is connected to the lifting device 22.

The lid member 10b is detachably attached to the main body 10a.
Attach to. An airtight holding member 24 such as a Viton packing is provided between the main body 10a and the lid member 10b and the elevating member 10c. Therefore, when the inside of the reaction furnace 10 is evacuated, the airtight holding member 24 keeps the airtight state. It can be formed.

Further, a temperature measuring means 26 for measuring the surface temperature of the substrate 18 such as an optical pyrometer is provided at a position near the substrate in the recess a.

Further, in this embodiment, the heating section 16 is provided with an infrared irradiating means of any suitable structure, for example, an infrared lamp 16a.
And a supporting member 16b for supporting the means 16a. The substrate 18 serves as the infrared lamp 16a.
Is preferably a lamp that emits light in a wavelength range that can be efficiently heated, and is composed of any suitable lamp according to the substrate material. In this embodiment, a tungsten halogen lamp or any other suitable lamp is used. Preferably, the plurality of infrared lamps 16a are arranged so that the inside of the reaction furnace 10 can be heated uniformly.

Usually, the infrared lamp 16a is used in the reaction furnace 10.
Place it outside. At this time, a part of the reaction furnace 10 is made of a material that transmits infrared rays, and the infrared rays are transmitted from outside the reaction furnace 10 into the reaction furnace 10. As described above, in this embodiment, since the lid member 10b is made of quartz, infrared rays can be transmitted.

The structure and position of the heating unit 16 may be any suitable structure and position capable of performing the heat treatment described later. For example, the heating unit 16 is composed of a heater, and this heater is used as the reactor 10. It may be provided inside.

Although the disposing position of the supporting member 16b is not limited to this, in the illustrated example, the supporting member 16b is closed between the supporting member 16b and the main body 10a so that the abutting portions of the lid member 10b and the main body 10a are confined. In addition, the airtight holding member 24 is provided between the support member 16b and the main body 10 so as to be detachably attached to the main body 10a. In this way, the support member 1
By providing 6b, the vacuum tightness in the reaction furnace 10 can be improved.

In FIG. 2, reference numeral 28 denotes a gas supply pipe provided between the reaction furnace 10 and the gas supply unit 14, and 30
Indicates an exhaust pipe provided between the reaction furnace 10 and the exhaust means 12.

Next, the vacuum exhaust system and the gas supply system of this embodiment will be described with reference to FIG. The vacuum exhaust system and the gas supply system are not limited to the examples described below.

First, the vacuum exhaust system will be described. In this embodiment, the exhaust means 12 is, for example, a turbo molecular pump 12a and a rotary pump 12b connected to the pump 12a.
And with. For example, the reaction means 10 is provided via an exhaust pipe 30 and a valve in which the exhaust means 12 is arranged as shown in the figure.
Connect to connect with.

In FIG. 3, 32a to 32d are exhaust pipes 30.
Is a vacuum gauge (or pressure gauge) that is connected to the
The vacuum gauges 32a and 32d are baratron vacuum gauges (or Pirani vacuum gauges) used for pressure measurement in the range of, for example, 1 to 10 ^-3 (10 to the third power) Torr, and the vacuum gauges 32b and 32c are, for example, 10 ^-3 to. An ion gauge used for pressure measurement in the range of 10 ⁻⁸ (10 ⁻⁸ ) Torr. An automatic opening / closing valve 34 for protecting the vacuum gauge 32b is provided between the vacuum gauge 32b and the exhaust pipe 30.
^-3 Automatically control the opening and closing of the valve 34 so as not to apply a pressure of ³ Torr or more. Reference numerals 36a to 36f are automatic opening / closing valves provided between the exhaust means 12 and the reaction furnace 10. By appropriately opening / closing these valves 36a to 36f, respectively, the pressure inside the reaction furnace 10 is controlled to an arbitrary suitable pressure. Then, a low vacuum exhaust state and a high vacuum exhaust state are formed in the reaction furnace 10.

Further, 38 is a needle valve for pressure adjustment and 40 is a relief valve. The valve 40 is automatically opened when the pressure in the reaction furnace 10 exceeds atmospheric pressure, for example, 760 Torr. The gas supplied from the gas supply unit 14 into the reaction furnace 10 is exhausted.

Next, the gas supply system will be described. In this embodiment, the gas supply unit 14 is connected to a first oxidizing gas source such as O 2.
₂ gas source 14a, second oxidizing gas source, for example N ₂ O gas source 14b, auxiliary gas source 14c and inert gas source 1
4d. The gas supply unit 14 is connected to the reaction furnace 10 through a supply pipe 28 and a valve arranged as shown in the drawing so as to communicate with the reaction furnace 10.

In FIG. 3, 42 is a gas supply system, 44 is a valve, 46a to 46d, 48a and 48b are automatic opening / closing valves, and 50a and 50b are automatic gas flow controllers relating to the gas introduced from the gas supply unit 14 into the reactor gas. Is.

Valves 44, 48a, 48b, 46a-4
A desired gas can be supplied from the gas supply unit 14 to the reaction furnace 10 by opening / closing each of 6d arbitrarily and suitably.

<< Description of Embodiments of Insulating Film Forming Method of the Present Invention >> Next, the insulating film forming method of the present invention will be described. FIG. 1 is a diagram for explaining a heat treatment step used for explaining the present invention. The horizontal axis of the figure plots time, and the vertical axis plots temperature and pressure.

Further, FIGS. 4A to 4C are process diagrams for explaining an embodiment of the insulating film forming method of the present invention, and each of the diagrams shows the cross section of the structure obtained in the main process steps. The cut is schematically shown. The following description will be given with reference to FIGS. 1 and 4 as appropriate.

In the embodiment of the present invention, first,
It is preferable to prepare the silicon substrate 200 as a base and pre-clean the substrate 200 with a chemical agent, pure water or the like as a conventional pretreatment. In the embodiment of the present invention, first, after washing in a mixed solution (1: 4) of sulfuric acid (H ₂ SO ₄ ) and hydrogen peroxide (H ₂ O ₂ ),
Cleaning with pure water is performed to remove organic substances on the substrate. Then, the natural oxide film 202 is removed in a 1% hydrofluoric acid (HF) solution, and then pure water cleaning and drying are performed.

Next, in order to prevent a new natural oxide film from growing on the substrate 200 in the reaction furnace 10, a dry inert gas such as nitrogen gas or argon gas is previously introduced into the reaction furnace 10. deep. At this time, valves 44 and 48
b and 46d are opened, 46a to 46c are closed, and an inert gas is introduced from the inert gas source 14d.

Next, the substrate 200 is set in the reaction furnace 10. The substrate 200 is fixed on the support 20 of the elevating member 10c. The valves 44 and 46d are then closed and the substrate 20
The supply of the inert gas into the reaction furnace 10 in which 0 is installed is stopped.

Next, the inside of the reaction furnace 10 is evacuated to a high vacuum of, for example, 10 ⁻⁶ (10 ⁻⁶ power) Torr by the exhaust means 12 to remove the water in the reaction furnace 10. In order to perform this vacuum exhaust, the valves 38, 36a, 36e, 3
With 6f and 36d closed, the valves 36c and 36d are opened, the dry pump 12b is operated, and vacuum exhaust is performed while the pressure inside the reaction furnace is monitored by the vacuum gauge 32a.

Next, the inside of the reaction furnace 10 is, for example, 1 × 10
^After reaching ^-3 Torr, valves 36c and 3
6d is closed and valves 36e, 36b, and 34 are opened, and the pressure inside the reaction furnace 10 is monitored by a vacuum gauge 32b, while the reaction furnace 10 is kept until a high vacuum of, for example, 1 × 10 ⁻⁶ Torr is reached.
The inside is evacuated.

Next, in order to form a silicon oxide film which is the first insulating film on the substrate 200 by performing heat treatment in a nitrogen-free oxidizing gas atmosphere, the valves 36b and 36e are closed and the valves 44, 48a and 46a is opened, and dry oxygen gas is supplied from the first oxidizing gas source 14a to the reaction furnace 10.
Supply in. At this time, initially, the substrate temperature is set to room temperature, for example, about 25 ° C. The oxygen pressure is increased in the reaction furnace 10 up to the atmospheric pressure (760 Torr) while monitoring it with the vacuum gauge 32a. When the atmospheric pressure is reached, open the valve 36f,
The mass flow controller 50a is adjusted in order to flow oxygen gas at a constant flow rate, for example, 2 liters / minute (see H1 in FIG. 1).
O ₂ flow).

Next, the substrate 200 in the reaction furnace 10 (see FIG. 2) is subjected to heat treatment by the heating unit 16 while flowing oxygen gas.
(Corresponding to 18) is heated to about 1000 ° C.
By this heat treatment, a first insulating film, for example, a SiO ₂ film 204 is formed on the surface of the substrate (FIG. 4B). The heating time for film formation at this time is t1 (FIG. 1). The heating of the substrate 200 is performed by the infrared lamp 16a of the heating unit 16. At this time, for example, while measuring the substrate temperature by the temperature measuring means 26, for example, 50 ° C./s
ec to 200 ° C./sec at an appropriate rate, preferably a heating rate of 100 ° C./sec and a heating temperature of about 1
The temperature is raised to 000 ° C., preferably about 10 seconds, about 10
The temperature is kept at 00 ° C. (t1 in FIG. 1). In this case, it is preferable that the rate of temperature rise be constant. This is because the growth of an insulating film such as an oxide film is kept constant to form a high quality film. The heating rate is set within the above range in order to form a film with good controllability of film thickness and good quality. The heating temperature is set to about 1000 ° C. because it is a preferable minimum temperature required for forming the SiO ₂ film 204 which is the first insulating film. By heating the substrate under such conditions, a thin and high-quality oxide film having a film thickness of about 50 Å (A °) can be formed. The thickness of this oxide film is, for example, the oxidation temperature,
It can be controlled by adjusting the oxidation time and the flow rate of the oxidizing gas. The oxide film obtained through the steps up to here is also referred to as a pure oxide film.

Immediately after the SiO ₂ film 204 is formed to a desired film thickness, the valves 46a and 36f are closed and 3
6d and 36e are opened, and the inside of the reaction furnace 10 is 1 × 10 ⁻¹ T
Evacuate to orr vacuum. This evacuation period is shown in FIG.
2 shows. In this case, the heating temperature of the substrate 200, that is, the SiO ₂ film 204 that is the first oxide film is kept at about 1000 ° C. This time t2 is set to about 10 seconds, and after evacuation, the valves 36d and 36e are closed, the valve 46b is opened, and an oxidizing gas containing nitrogen, for example, dinitrogen monoxide (N ₂ O) is placed in the reactor 10. (Fig. 1
N ₂ O flow inside).

At this time, the pressure in the reaction furnace immediately increased to 76
It will be 0 Torr. The heating temperature of the substrate 200 under this pressure is set to about 1000 ° C., and this temperature is maintained for t3 hours, for example, 30 hours.
Hold for seconds. By the heat treatment in the nitrogen-containing oxidizing gas, the first insulating film 204 is changed to the second insulating film. In this embodiment, the second insulating film 206 is an oxynitride SiO ₂ film and has a film thickness of, for example, about 60 Å (A).
°). This state is shown in FIG.

Next, the heating of the substrate 200 is stopped. With this heating stop or after the heating stop, the valve 48
Close a and 48b and stop the supply of N ₂ O gas.
In the example shown in FIG. 1, the gas flow is stopped after the heating is stopped (indicated by H2 in FIG. 1). After that, it waits for the substrate 200 to cool until the surface temperature of the substrate 200 reaches room temperature, for example, about 25 ° C. (Period indicated by t4 in FIG. 1) Next, the valves 36f and 44 are closed, and 36d and 3
6e is opened and the inside of the reaction furnace 10 is, for example, 1 × 10 ⁻³ Tor.
Close r and evacuate to vacuum.

Next, the valves 36d and 36e are closed,
The valves 44, 48b and 46d are opened, and the atmosphere in the reaction furnace 10 is replaced with an inert gas such as nitrogen gas (see FIG. 1).
N ₂ flow indicated by H3 in FIG. When the pressure in the reaction furnace 10 reaches the atmospheric pressure (760 Torr), the substrate 200 on which the second insulating film 206 is formed is taken out from the reaction furnace 10. Through such a process, Si-N is formed as the second insulating film 206.
An oxynitride SiO ₂ film containing a bond can be formed uniformly and with a very thin film thickness of about 60 A ° (angstrom).

Regarding the oxynitride film (SiO-N film) as an insulating film thus formed on the Si substrate and the pure oxide film (SiO ₂ film) as an insulating film formed on the conventional Si substrate, The distribution characteristics of nitrogen and hydrogen in the film in the depth direction were measured by secondary ion mass spectrometry (SIMS) to obtain comparative data. FIG. 5A shows experimental data for a pure oxide film and FIG. 5B shows experimental data for an oxynitride film. In both figures, the horizontal axis indicates the depth from the surface of the insulating film to the Si substrate side (n
It is shown by taking the unit of m, and the vertical axis shows the concentration (atoms / cm).
³ units). In both figures, curves I and II show characteristic curves of hydrogen and nitrogen, respectively. In the case of the ordinary oxide film of FIG.
Both the hydrogen concentration and the nitrogen concentration are maximum near the interface between the insulating film and the Si substrate, but the hydrogen concentration is higher than the nitrogen concentration, and the nitrogen concentration is 5 × 10 ¹⁹ atoms / cm at maximum.
It is only about ³ . On the other hand, in the case of the oxynitride film completed in the example of the present invention, the nitrogen concentration is significantly higher than the hydrogen concentration near the interface between the insulating film and the Si substrate even when measured under the same conditions. Therefore, it can be understood that the maximum value of the nitrogen concentration is about 4 × 10 ²¹ atoms / cm ³ . As described above, in the oxynitride film formed by the method of the present invention, it is understood that more nitrogen atoms are taken in near the boundary surface with Si (silicon) than in the conventional normal oxide film.

On the other hand, SiO in the vicinity of the SiO ₂ / Si interface
_{For the two} films, the type and bonding state of atoms contained in the films were examined by using X-ray photoelectron spectroscopy (XPS).
The measurement result by this X-ray photoelectron spectroscopy is shown in FIG. Figure 6
In the figure, the horizontal axis represents the binding energy (eV unit), and the vertical axis represents the X-ray transmission intensity (arbitrary unit). S in this measurement
It was measured at the SiO ₂ film surface at about 10 A ° from the iO ₂ / Si interface. In this measurement, attention was paid to the 1s level of nitrogen (N). It is shown by N (1s) in the figure. In the obtained spectrum, the region indicated by I is an O—N bond, the region indicated by II is an N—H bond, and the region indicated by III is a Si—N bond, and the Si—N bond has a peak around 396 to 398 eV. You can see that it is the maximum. Nitrogen atoms are bonded to silicon atoms in the film and at the interface, and it is expected that Si dangling bonds also found in the oxide film are terminated by nitrogen atoms. It is considered that due to such an action, it has resistance to the stress caused by electron injection.

FIGS. 7A and 7B and FIGS. 8A and 8B are the same as those shown in FIG. 9 using the pure oxide film and the oxynitride film obtained by the present invention as the tunnel film. Experimental data obtained by forming an EEPROM cell having a structure and measuring the relationship between the voltage between the control electrode-drain (CG-D) and the leak current by using the injected charge amount as a parameter for this cell are shown. 7A and 7B are experimental data when a pure oxide film is used as a tunnel oxide film, and FIGS. 8A and 8B are also shown.
Are experimental data using the oxynitride film formed by the present invention as a tunnel oxide film.

In this experiment, the control electrode CG
Then, the floating electrode FG was short-circuited, and the voltage of the control electrode CG was adjusted so as to be a constant voltage between the control electrode CG and the drain D to determine the injected charge amount (= constant current density × time). The voltage between the control electrode CG and the drain D is changed while the tunnel oxide film is stressed by this implantation amount, and the current flowing between them is measured as a leak current.
Therefore, in FIGS. 7 and 8, the injected charge amount is used as a parameter, the horizontal axis represents the voltage (V) between the control electrode CG and the drain D, and the vertical axis represents the leakage current (A). is there. The measurement curve before giving the injected charge amount, that is, before applying the stress was I ₀ , and the measured curves for the injected charge amount were 1.00 C / cm ² , 0.50 C / cm ² and 0.05 C / cm ² _They are denoted by ₁ , I ₂ , and I ₃ , respectively. And, in both figures, (A) shows experimental data (positive polarity) when a positive voltage is applied to the control electrode CG more than the drain D, and (B) shows conversely control the drain D. The experimental data when a positive electrode is applied rather than the electrode CG (negative polarity) is shown.

In both cases of the pure oxide film and the oxynitride film, the tendency of the leak current is not changed by the application of the negative polarity and the positive polarity. However, in the case of the pure oxide film, as can be understood by comparing the curves I ₀ with I ₁ , I ₂ and I ₃ , it can be seen that the stress largely changes before and after the charge injection. On the other hand, according to the method of the present invention, it can be concluded that, in the case of the oxynitride film formed, there is substantially no change in stress before and after the charge injection. Moreover, when the voltage between the control electrode CG and the drain D is about 4 V or more when the polarity is negative and when the voltage is about 2 V or more when the polarity is positive, the pure oxide film is more acidic. The leak current becomes larger than that in the case of the nitrided SiO ₂ film. From this fact, the leak current value after the stress application is lower than that of the tunnel oxide film of the oxynitride film formed in the present invention in the voltage region applied when the EEPROM cell is driven.

As described above, the oxynitride film formed by the method according to this description can be used as an insulating film of higher quality than the conventional pure oxide film. Therefore, this oxynitride film is used as an electronic device, for example, FLOTOX type EEPROM. When used to form a cell, its electrical characteristics can be improved over conventional cells.

[0054]

As is apparent from the above description, according to the insulating film forming method of the present invention, the insulating film can be formed into an extremely thin film of 60 A °, which has been impossible in the past. Nevertheless, a film with less stress and a lower leak current value can be formed. Therefore, by utilizing this insulating film as a tunnel film of an EEPROM or the like, it is possible to provide a highly reliable EEPROM having higher quality electric characteristics than conventional ones. Further, the oxynitride SiO ₂ film can be used as a gate oxide film of a transistor as well as the EEPROM.

[0055]

[Table 1]

[Brief description of drawings]

FIG. 1 is a block diagram for explaining a heat treatment method used in the present invention.

FIG. 2 is a cross-sectional view for explaining a reaction furnace used for the heat treatment of the present invention.

FIG. 3 is a block diagram for explaining a gas supply system and a vacuum exhaust system used for the heat treatment of the present invention.

FIG. 4 is a process drawing for forming an insulating film of the present invention.

FIG. 5 is a curve diagram showing the distribution of nitrogen and hydrogen in the insulating film in the depth direction.

FIG. 6 is an XPS spectrum diagram near the SiO ₂ / Si interface.

FIG. 7 is a voltage / leakage current curve diagram in which a leakage current before and after applying stress to the insulating film is plotted.

FIG. 8 is a voltage / leakage current curve diagram in which a leakage current before and after applying stress to the insulating film is plotted.

FIG. 9 is a structural diagram of a conventional EEPROM cell.

FIG. 10 is a circuit diagram illustrating an operation of a conventional EEPROM cell.

[Explanation of symbols]

10: Reactor 10a: Main body 10b: Lid member 10c: Elevating member 12: Exhaust means 12a: Turbo molecular pump 12b: Dry pump 14: Gas supply unit 14a: Oxidizing gas source (O ₂ ) 14b: Oxidizing gas source ( N ₂ O) 14c: Spare gas source 14d: Inert gas source 16: Heating part 16a: Infrared lamp 16b: Support member 18: Substrate 20: Support member 22: Lifting device 24: Airtight holding member 26: Temperature measuring means 28 : Gas supply pipe 30: Exhaust pipe 32a to 32d: Vacuum gauge 34, 36a to 36f, 38, 40, 44, 46a to 4
6d, 48a, 48b: valve 42: gas supply system 50a, 50b: gas flow controller 200: silicon substrate 202: natural oxide film 204: first oxide film 206: oxynitride SiO ₂ film (tunnel oxide film)

Claims (4)

[Claims] 1. A silicon underlayer is heated in a reaction furnace in an insulating film forming gas atmosphere to form an insulating film on the underlayer. (A) The heating is performed by infrared irradiation. And (b) a step of oxidizing the surface of the base in an atmosphere of a nitrogen-free oxidizing gas in the first half stage of the treatment to form an oxide film which is a first insulating film, and (c) the heating. In the latter half of the process, the nitrogen-free oxidizing gas atmosphere is switched to the nitrogen-containing oxidizing gas atmosphere, the first insulating film is oxynitrided in the latter oxidizing gas atmosphere, 2. A method of forming an insulating film, comprising the step of changing to an insulating film. 2. The insulating film forming method according to claim 1, wherein the nitrogen-free oxidizing gas is oxygen gas, and the first insulating film obtained thereby is silicon dioxide (SiO ₂ ).
A method for forming an insulating film, which is a film. 3. The method for forming an insulating film according to claim 1, wherein the nitrogen-containing oxidizing gas is selected from a gas group of nitric oxide (NO), nitrogen dioxide (NO ₂ ) and nitrous oxide (N ₂ O). At least one kind of gas, and the oxynitride film obtained thereby has a SiOxNy film structure (provided that x
And y are values giving composition ratios, and 0 <x and 0
<A value that satisfies y and x + y ≦ 2). 4. A nonvolatile semiconductor device, wherein the second insulating film obtained by the insulating film forming method according to claim 1 is used as a tunnel film.