JP2005302996A - Manufacturing method of flash memory - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of a flash memory which is reduced in the amount of an organic gaseous substance in an environmental atmosphere and capable of manufacturing a quantum dot having a size and/or density within an optimum range, in the manufacturing method the a flash memory which forms the quantum dot on the insulating film of a semiconductor substrate as a floating gate. <P>SOLUTION: The manufacturing method of the flash memory retains the environmental atmosphere of the semiconductor substrate before forming the quantum dot clean by an organic gaseous substance removing means, in the manufacturing method of the flash memory which forms an insulating film on the semiconductor substrate and forms the quantum dot on the insulating film as the floating gate. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for manufacturing a flash memory in which an insulating film is formed on a semiconductor substrate and quantum dots are formed on the insulating film as a floating gate.

In recent years, in the semiconductor industry, global demand for mobile phones has become a major driving force, and the use of flash memory is rapidly expanding to storage media for PDAs and digital cameras. With such improvements in functions of various devices, the demand for power saving of flash memories has become stronger. In order to satisfy this requirement, it was necessary to further thin the 10 nm thin tunnel insulating film formed in the flash memory. However, when the tunnel insulating film is thinned, there is a risk that the risk of dielectric breakdown increases and the reliability is lowered.
In order to solve this problem, as a floating gate formed on the SiO ₂ film or Si ₃ N ₄ film serving as an insulating film, Si dots having a minute particle diameter of 5 to 10 nm are formed instead of the conventional polysilicon film The technology to do is attracting attention.
When the flash memory floating gate is formed of Si dots, the charge is held independently for each dot, so even if the tunnel insulating film is destroyed by thinning and a leak path is created in part. The memory function is not lost due to the charge held in other dots. In this way, by using the technology to form quantum dots, the insulation film of flash memory can be made much thinner than before, dramatically reducing the leakage current of flash memory, and further reducing the write / erase voltage. It becomes possible to meet the demand for power saving by lowering the voltage.

Conventionally, the environmental atmosphere in the manufacturing process of a semiconductor substrate has been cleaned using a high-performance filter such as a HEPA filter or a ULPA filter.
However, with such a high-performance filter, organic gaseous substances in the environmental atmosphere cannot be removed, organic gaseous substances adhere to the surface of the semiconductor substrate, the contact angle of the substrate surface increases, and the substrate and There has been a problem that the affinity of the resist is affected and the film thickness and adhesion deteriorate (Patent Document 1).
When forming quantum dots that have been attracting attention in recent years, the size and / or density of the quantum dots are in an appropriate range as intended in order to retain a single electron for each dot having a small particle diameter of about several nanometers to several tens of nanometers. It was necessary to form so that. For this reason, it has been easily estimated that the influence of organic gaseous substances in the environmental atmosphere that cannot be removed by a high-performance filter on the formation of quantum dots is greater than that of conventional semiconductor substrates.
Republished 01/048813

However, it has not been known how organic gaseous substances in the ambient atmosphere affect the formation of quantum dots.
For this reason, the inventors of the present invention also have HSG-Si (hemispherical grain silicon), which is a hemispherical crystal grain whose size is close to a quantum dot with a size of several tens to several hundreds of nanometers and close to a quantum dot, in an environmental atmosphere It was assumed that it was affected by the organic gaseous substance, and the influence of the organic gaseous substance in the environmental atmosphere on the formation of HSG-Si was confirmed. However, contrary to the assumptions of the present inventors, even when organic gaseous substances in the environmental atmosphere adhere to the insulating film, the size and / or density of HSG-Si should not be greatly affected. I understood. This is because HSG-Si forms a silicon amorphous film so as to cover an organic substance on the insulating film, and HSG-Si which is a hemispherical crystal grain is formed on the surface of the silicon amorphous film. It was estimated that even if the organic gaseous substance inside adhered to the insulating film, it did not affect the size and / or density of HSG-Si.
Although the quantum dots are close in size to HSG-Si, the present inventors have formed quantum dots by CVD (chemical vapor deposition) on an insulating film of a semiconductor substrate. It has been found that the size and / or density of dots is greatly affected by organic gaseous substances. This is because when the organic gaseous substance in the environmental atmosphere adheres to the insulating film of the semiconductor substrate, the quantum dot grows with the organic substance adhering to the insulating film as a nucleus, and the size and / or density of the quantum dot increases. Was guessed.
Accordingly, the present inventors have reduced the amount of organic gaseous substances in the environmental atmosphere in an appropriate range in a flash memory manufacturing method in which quantum dots are formed as floating gates on an insulating film of a semiconductor substrate based on the above knowledge. An object of the present invention is to provide a method of manufacturing a flash memory that can manufacture quantum dots having a size and / or density within the above range.

  That is, the flash memory manufacturing method of the present invention is the flash memory manufacturing method according to claim 1, wherein an insulating film is formed on a semiconductor substrate, and quantum dots are formed on the insulating film as a floating gate. The environmental atmosphere of the semiconductor substrate before the formation of the quantum dots is kept clean by an organic gaseous substance removing unit.

  According to the flash memory manufacturing method of the present invention, the environmental atmosphere of the semiconductor substrate is kept clean by the organic gaseous substance removing means before the quantum dots are formed as the floating gate of the flash memory. Since organic gaseous substances in the atmosphere are removed and organic substances adhering to the insulating film on the substrate are reduced, it becomes difficult for the quantum dots to grow using the adhering organic substances as a nucleus, and the size and / or density is within an appropriate range. It becomes possible to manufacture a quantum dot. Thus, if the size and / or density of the quantum dots can be manufactured in an appropriate range, the charge can be reliably held for each quantum dot, and the effect of a single electron transistor (SET) is exhibited. Can be made.

The method of manufacturing a flash memory according to the present invention includes: forming an insulating film on a semiconductor substrate; and forming an quantum atmosphere as a floating gate of the flash memory on the insulating film by changing an environmental atmosphere of the semiconductor substrate to an organic gas It is kept clean using the particulate matter removing means.
As the organic gaseous substance removing means, an activated carbon chemical filter or a photocatalytic unit can be used.
The environment atmosphere to be cleaned by the organic gaseous substance removing means is the environment atmosphere in the entire clean room where the semiconductor substrate on which the insulating film is formed is placed before the quantum dots are formed, or the semiconductor substrate in the sealed space. The environmental atmosphere in the stocker held in
As a stocker that forms a sealed environmental atmosphere, it is made of metal such as aluminum or stainless steel, and is made of general-purpose plastics such as ABS and acrylic, polycarbonate (P. A synthetic resin such as an engineer plastic such as C) can be used.

As the chemical filter used as the organic gaseous substance removing means, for example, a filter obtained by folding a sheet-like filter medium in which an adsorbent is sandwiched between nonwoven fabrics in a zigzag shape can be used. When such a chemical filter is used, the amount of adsorbent can be adjusted to increase or decrease the adsorption capacity of the organic gaseous substance, and the environment in which the organic gaseous substance is removed, for example, in the atmosphere of the entire clean room Use a chemical filter with an adsorbent capacity suitable for the environment, such as when removing organic gaseous substances or when removing organic gaseous substances in the atmosphere in the stocker that holds the semiconductor substrate. Is possible.
Examples of the adsorbent include organic gaseous substances in the space for producing quantum dots, that is, hydrocarbons (HC) having high adhesion (adsorption) properties to the substrate, such as —CO— and —COO— groups. Use hydrocarbons that can efficiently collect and remove hydrocarbons (for example, phthalate esters, etc.). For example, activated carbon, zeolite, alumina, silica gel, glass, fluorine compound, metal, polymer compound (styrene polymerized gold), etc. can be used. Among these adsorbents, activated carbon is an organic gas such as hydrocarbon. It is preferable as an adsorbent because the adsorbing effect of the substance is great. The adsorbent used in the chemical filter is preferably subjected to moisture removal (dehumidification) in advance in order to improve performance (adsorption power and adsorbent life).

When the activated carbon chemical filter is used as the organic gaseous substance removing means, for example, among the organic substances adsorbed on the wafer surface, bis (2-ethylhexyl) phthalate, compared to the case where the activated carbon chemical filter is not used. About (DOP) can be removed by about 100%, and the total organic carbon content (TOC) can be removed by about 90%.
The activated carbon can be used in the form of particles, fibers, nets, honeycombs, etc., but when it is sandwiched between sheet-like filter media as an adsorbent, a fiber with less pressure loss of the filter is used. preferable.

The photocatalyst unit used as the organic gaseous substance removing means is composed of a photocatalyst body and a light source.
When the photocatalytic unit is used, about 98% and 84% of DOP and TOC adsorbed on the wafer surface can be removed, respectively, as compared with the case where the photocatalytic unit is not used.
As the photocatalyst body, for example, a breathable sheet-like photocatalyst body in which a photocatalyst is supported on a glass cloth can be used. The photocatalyst may be any as long as it is excited by light irradiation and can decompose gaseous pollutants. For example, Si, Ge, Se, etc. as elements, AlP, GaAs, InP, etc. as compounds, TiO ₂ , ZnO, etc. as oxides have been proposed, but decomposition efficiency, safety, stability From the viewpoint of degree, TiO ₂ is preferable. The TiO ₂ may be in the rutile type, anatase type, or a coexistence type thereof, but the rutile type is suitable for reacting low energy light, and the anatase type is used for enhancing the activity of the reaction. Is suitable. For the photocatalyst, addition of Pt, Ag, Pd, RuO ₂ , and Co ₃ O ₄ is preferable because the decomposition action of organic gaseous substances such as hydrocarbon (HC) is promoted.
Further, the shape of the photocatalyst body includes a cross shape, a honeycomb shape, a paper shape, etc., and any shape may be used, but an increase in pressure loss due to the addition of the photocatalyst unit is suppressed to a low level, and there is no leakage of ultraviolet rays. Since the organic gaseous substance can be safely decomposed, it is preferable to use a breathable sheet in which a photocatalyst is fixed to a mesh substrate. It is preferable to use a plurality of the air-permeable sheet-like photocatalysts stacked in a spaced manner.
As the light source used in the photocatalyst unit, an ultraviolet lamp, a low-pressure mercury lamp, a germicidal lamp, a black light fluorescent lamp, or the like is used. It is preferable because it is strong and ozoneless.

A silicon wafer can be used as a semiconductor substrate for forming a floating gate of the flash memory. The semiconductor substrate is not limited to a silicon wafer but may be an SOI (Silicon On Insulator) substrate or a semiconductor substrate formed of gallium arsenide, germanium, silicon germanium, or the like. Further, as the insulating film formed on the semiconductor substrate, a SiO ₂ film or the like can be formed by a thermal oxidation method or the like, but is not limited thereto, and a suitable insulating film is formed by a method known per se. May be. The quantum dots are not limited to the CVD method or the like, and quantum dots such as gallium arsenide, germanium, and silicon germanium suitable for a semiconductor substrate may be formed by a method known per se such as MBE (molecular beam epitaxy) method.
In addition, when a silicon wafer is used as a semiconductor substrate for forming a floating gate of a flash memory and an SiO ₂ film is formed as an insulating film, silicon dots are formed as quantum dots by a CVD method or the like.

Next, although an example of the present invention is described in detail with a comparative example, the present invention is not limited to this example.
(Example 1)
Temperature 23 ± 5 ° C., humidity 50 ± 10%, cleanliness class 10, DBP content level 0.1~0.5μg / m ^3, TOC concentration 50 to 150 / m ³ of the installed oxidizing furnace to test the clean room Then, as shown in FIG. 1, an SiO ₂ film 2 having a thickness of about 5 nm is formed on the silicon wafer 1 by thermal oxidation while supplying dryO ₂ at 1000 ° C. to form quantum dots. Thus, a semiconductor substrate 3 was obtained.
Next, as shown in FIG. 2, the semiconductor substrate 3 is placed in a storage box 4 provided with an air blower 7 for passing the air in the test clean room at 0.5 m / s and an activated carbon chemical filter 5. Stored for hours. The activated carbon chemical filter 5 has a filter pack obtained by folding a sheet-shaped filter medium having a weight of about 450 g / m ² and a thickness of 1.2 mm in a zigzag manner with palm-shell granular activated carbon sandwiched between PET spunbond nonwoven fabrics. What was assembled | attached to the 65-mm-thick aluminum filter frame was used.
Finally, the semiconductor substrate 3 is taken out from the storage 4 and heated in a CVD furnace from room temperature to 580 ° C. over 1.5 hours, and silane gas (SiH ₄ ) is divided into partial pressure (100/350) × 0.25 Torr. As shown in FIG. 3, silicon dots 8 to be a floating gate of the flash memory were formed on the SiO ₂ film 2 of the semiconductor substrate 3.

(Example 2)
The semiconductor substrate 3 on which the SiO ₂ film 2 was formed in the same manner as in Example 1 was stored in a storage 4 provided with a photocatalytic unit in place of the activated carbon chemical filter 5 except that it was stored for 23 hours in the same manner as in Example 1. In the same manner as in Example 1, silicon dots 8 serving as a floating gate of the flash memory were formed on the SiO ₂ film 2 of the semiconductor substrate 3.
The photocatalyst unit has a light source at the center of the main casing, and three UV lamps having a wavelength of 254 nm are arranged in parallel in the depth direction of the main casing of the photocatalytic unit at equal intervals. 4 pieces of a 238 mm × 188 mm × 0.6 mm breathable sheet photocatalyst body, in which 5% by mass of the photocatalyst formed on a glass cloth made of E glass fiber having a fiber diameter of 9 μm and having an aperture ratio of 25% is held by a catalyst holder One having an outer dimension of 250 mm × 250 mm × 150 mm provided with two catalyst boxes was used.

(Comparative Example 1)
As in Example 1, the semiconductor substrate 3 on which the SiO ₂ film 2 was formed was placed in a CVD furnace without being held in the storage room 4 and exposed to the test clean room for 23 hours. 1 was formed on the SiO ₂ film 2 of the semiconductor substrate 3 to form silicon dots 8 serving as a floating gate of the flash memory.

(Comparative Example 2)
The semiconductor substrate 3 on which the SiO ₂ film 2 was formed in the same manner as in Example 1 was exposed to the test clean room for 23 hours without being held in the storage 4, and then the semiconductor in the same manner as in Example 1 in the CVD furnace. Silicon dots 8 serving as a floating gate of the flash memory were formed on the SiO ₂ film 2 of the substrate 3.

The size and density of the silicon dots formed on the SiO ₂ films of the semiconductor substrates of Examples 1 and 2 and Comparative Examples 1 and 2 were measured with an atomic force microscope (AFM). FIG. 4 shows the relationship between the silicon dot size (nm) and density (10 ¹⁰ / cm ² × nm) of each semiconductor substrate, and FIG. 5 shows two-dimensional and three-dimensional AFM photographs of the surface of each semiconductor substrate.

From the results shown in FIGS. 4 and 5, the following was found.
As shown in FIG. 4, when the silicon dots are formed immediately after the formation of the SiO ₂ film on the silicon wafer in the oxidation furnace of Comparative Example 1, the average size of the silicon dots is about 5 nm. It was uniform. For silicon wafers placed in an environmental atmosphere cleaned with the activated carbon chemical filter of Example 1, it was confirmed that silicon dots with a uniform size of about 5 nm on average were formed as in Comparative Example 1. did it. In addition, the silicon wafer placed in the environmental atmosphere cleaned with the photocatalytic unit of Example 2 is about 10 nm on average and slightly larger than Comparative Example 1, but the size is substantially uniform. It was confirmed that a silicon dot was formed.
On the other hand, the silicon dots formed on the silicon wafer exposed in the test clean room of Comparative Example 2 had a large size of about 10 nm, and the sizes were not uniform and varied.
Further, from the results shown in FIGS. 4 and 5, the peak of the density of the silicon dot of Comparative Example 1 is 0.4 (10 ¹⁰ / cm ² × nm), whereas both of Examples 1 and 2 show the silicon dot density. The density peak was slightly increased to 0.8 (10 ¹⁰ / cm ² × nm), but compared with the silicon dot density peak of Comparative Example 2 (10 ¹⁰ / cm ² × nm). It was low and it was confirmed that the growth of silicon dots was suppressed.

  The present invention provides a flash memory manufacturing method in which quantum dots are formed as a floating gate on an insulating film of a semiconductor substrate, and reduces organic gaseous substances in the ambient atmosphere of the semiconductor substrate before the quantum dots are formed. This is useful in that quantum dots having a size and / or density within the range can be manufactured.

Schematic diagram of a semiconductor substrate with an SiO ₂ film, an insulating film, formed on a silicon wafer Schematic configuration diagram of the storage showing the state of holding the semiconductor substrate Schematic diagram of a semiconductor substrate on which silicon dots are formed Diagram showing the relationship between the size and density of silicon dots 2D and 3D AFM photographs of the semiconductor substrate surfaces of Examples 1 and 2 and Comparative Examples 1 and 2

Explanation of symbols

1 Silicon wafer 2 SiO ₂ film 3 Semiconductor substrate 4 Storage 5 Activated carbon chemical filter 7 Blower 8 Silicon dot

Claims (1)

  In a flash memory manufacturing method in which an insulating film is formed on a semiconductor substrate, and quantum dots are formed on the insulating film as a floating gate, the environmental atmosphere of the semiconductor substrate before the formation of the quantum dots is removed by removing organic gaseous substances. A method of manufacturing a flash memory, characterized by being kept clean by means.