JPH074952A - Measuring method for surface area - Google Patents

Abstract

PURPOSE:To measure the surface area of an uneven polysilicon film nondestructively and extremely simply. CONSTITUTION:A polysilicon film in which uneven parts have been formed on the surface is deposited on a substrate 20. After that, when water is supplied to the surface of the film from an ultrapure water source 9 at 170 K in a vacuum, only a monomolecular layer is adsorbed to the whole face. After that, when the layer is set to room temperature, water molecules which have been adsorbed are all desorbed. The amount of desorbed water is measured by a quadrupole mass spectrometer 10. Since the water molecules which are desorbed are equal to the amount of water which has been adsorbed as the monomolecular layer, the amount desorbed is equal to the surface area of the polysillcon film.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for measuring the surface area of an object such as a charge storage polysilicon electrode.

[0002]

2. Description of the Related Art Conventionally, JP-A-3-272165,
As seen in JP-A-3-139882,
Polysilicon, which has a very large surface area with fine irregularities on the surface, is used for the charge storage electrode. If uneven polysilicon is used, the storage capacity can be increased in the same cell area, the α-ray error resistance is improved, and the memory retention time is also increased. Further, if the storage capacity is the same, the cell size can be reduced and the semiconductor memory can be highly integrated. Japanese Unexamined Patent Publication No. 3-272165 discloses that, when vapor deposition of silicon is carried out at a transition temperature at which the crystalline state of a deposited film changes from amorphous to polycrystal, irregularities with a diameter of about several tens nm are densely formed on the surface. The formed silicon film is formed. JP-A-3
Japanese Patent Laid-Open No. 139882 discloses that when a polysilicon film to which phosphorus is added at a high concentration is etched with a heated phosphoric acid solution, crystal grain boundaries are etched particularly quickly, and as a result, irregularities are formed on the film surface.

The performance of this charge storage depends on the size of its surface area. Therefore, it is important to measure the surface area, but conventionally, the polysilicon film formed on the substrate is divided together with the substrate and the cross section of the film is observed by a scanning electron microscope.

[0004]

However, the surface of such uneven polysilicon has a wide variety of surfaces. On the other hand, the polysilicon that can be inspected by the above-mentioned conventional measuring method is only a part of this wide distribution. Therefore, in order to estimate the surface area of a polysilicon film with various shape distributions from a microscopic observation, it is necessary to observe a large number of microscopic observations at various places constituting the electrode, and the inspection efficiency becomes extremely poor. . Furthermore, in this inspection method, the inspection sample must always be broken, resulting in a destructive inspection.

The above-mentioned problems are not limited to the charge storage polysilicon electrode, but are common to the semiconductor integrated circuit or more generally when measuring the surface area of fine irregularities formed on the surface of an object.

An object of the present invention is to provide a non-destructive and highly efficient method for measuring the surface area of an object, which was impossible by the conventional methods.

[0007]

According to the present invention, a film of an adsorbed portion is obtained by adsorbing a gas on a surface of an object at a low temperature in a vacuum and then raising the temperature and measuring the amount of the gas desorbed at that time. It is characterized by measuring the surface area of.

As the adsorbed gas, it is preferable to use a gas that adsorbs a monolayer on the surface. Water molecules can be used as the gas. In the case of water, adsorption is performed within a temperature range of about 120 K to 200 K, desorption is performed at room temperature, and the amount is measured.

Generally, the interaction between water molecules and the solid surface is extremely weak, and basically water molecules are van der on the solid surface.
Adsorbs like Waals. Therefore, in vacuum, water molecules are not adsorbed on the surface unless the temperature of the surface of the substance falls below about 200K. When the substrate temperature is about 200 K or less, water molecules are adsorbed on the surface only for one molecular layer in vacuum. When the substrate temperature is about 120 K or lower, the multi-layered water molecules above the water molecules adsorbed by the one molecular layer are adsorbed. Therefore, when the temperature of the polysilicon film is kept at a temperature of 200 K or less and 120 K or more in a vacuum and water molecules are supplied onto the polysilicon film, the water molecules are adsorbed on the surface by one molecular layer. The adsorption coefficient of water molecules in this temperature region is close to 1. Therefore, in order to cover all the polysilicon surface with one molecular layer of water molecules, it is sufficient to supply an extremely small amount of water molecules of about 1 Langmuir. After that, when the polysilicon film is returned to room temperature again, all water molecules adsorbed on the surface are desorbed. The amount of water molecules released at this time is monitored by a quadrupole mass spectrometer. Since the total amount of desorbed water molecules is equal to the amount of water monomolecularly adsorbed on the surface area of the polysilicon film, the desorbed amount is proportional to the surface area of polysilicon. Therefore, the surface area of the entire polysilicon film can be evaluated by the amount of water molecules released when the polysilicon is returned to room temperature. This method is extremely simple in examining the entire surface area regardless of the shape distribution of polysilicon. This inspection method is non-destructive, and the temperature used at this time is room temperature or lower, so that the structure is not formed under the polysilicon by this inspection and the structure of the device or the like is not thermally destroyed. Further, water molecules used for inspection have no interaction such as etching the surface when adsorbed on the surface or terminating dangling bonds, and are completely desorbed from the surface when returned to room temperature in vacuum. Therefore, nondestructive and efficient inspection of polysilicon is realized.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an outline of an inspection apparatus for explaining the present invention. An uneven polysilicon film was formed on the front surface of the substrate 20 by the conventional method described above. A large number of the substrates 20 are introduced into the sample load chamber 1 by the cassette 2. Load chamber 1 after this is evacuated to a vacuum, 10 ^{- 7} Torr is moved following the sample transfer 3 UHV laboratory 5 in single wafer by opening the gate valve 4 by when it is pressure. Immediately after placing the substrate 20 on the sample table 6 in the inspection chamber, the gate valve 4 is closed. Then, the sample is cooled to 170 K by the cryohead 7 connected to the sample table 6. The substrate temperature can be quickly set to any temperature from room temperature to about 100K by combining the cryohead 7 and the heater 8. At a substrate temperature of 170 K, 1 L of water molecules are supplied from the ultrapure water source 9 to the substrate 20 on the sample stage 6 so that one molecule layer is adsorbed on the surface of the polysilicon film. After that, the substrate temperature is raised to room temperature at a rate of 20 K / sec, and the amount of water desorbed from the sample at this time is observed by the quadrupole mass spectrometer 10 as a function of the substrate temperature. An example of the thermal desorption spectrum obtained at this time is shown in FIG. The area of the temperature programmed desorption spectrum indicated by hatching in FIG. 2 corresponds to the amount of desorbed water molecules. The relationship between the surface area of the polysilicon films with different surface shape distributions prepared under various formation conditions, determined from the cross-sectional micrographs of multiple points in each film, and the area of the temperature rise spectrum of water obtained by this method. Is shown in FIG. As is clear from the figure, the surface area of the result of the microscope is proportional to the area of the thermal desorption spectrum of water by this method. Therefore, it is obvious that the surface area of polysilicon can be easily evaluated by this method.

[0011]

As described in detail above, according to the present invention, the surface area of polysilicon can be evaluated nondestructively and very easily. The present invention is also effective as a surface area evaluation method for not only silicon but also other semiconductors, metals, insulators and other substances.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of an inspection device for explaining an embodiment of the present invention.

FIG. 2 is a diagram showing a thermal desorption spectrum of water from a polysilicon film for explaining an example of the present invention.

FIG. 3 is a diagram showing a relationship between a surface area of a polysilicon film and a thermal desorption spectrum area of water, which are obtained by observing a cross-sectional microscope for explaining an example of the present invention.

[Explanation of symbols]

 1 sample loading chamber 2 sample cassette 3 sample transfer 4 gate valve 5 inspection chamber 6 sample stage 7 cryohead 8 heater 9 ultra high pure water source 10 quadrupole mass spectrometer 11 turbomolecular pump 12 rotary pump 13 computer 20 substrate

Claims (6)

[Claims] 1. A surface area of a film of an adsorbed portion is measured by adsorbing a gas on a surface of an object at a low temperature in a vacuum, then raising the temperature and measuring the amount of the gas desorbed at that time. Measuring method of surface area of object. 2. The surface area measuring method according to claim 1, wherein a gas that is adsorbed on the surface by a monolayer is used as the adsorption gas. 3. The method for measuring surface area according to claim 2, wherein water molecules are used as the gas to be adsorbed by one molecular layer. 4. The surface area measuring method according to claim 3, wherein adsorption is performed in a temperature range of about 120 K to 200 K, and desorption is performed at room temperature. 5. The surface area measuring method according to claim 1, 2, 3, or 4, wherein the surface area of a polysilicon film having irregularities on the surface is measured. 6. The surface area measuring method according to claim 5, wherein the polysilicon film is an electrode for accumulating charges.