JPH08313244A - Method of measuring thickness of thin film - Google Patents

Abstract

PURPOSE: To provide a film-thickness measuring method in which the Auger electron image of a specific element in an exposed part is captured and in which the width in the exposed part of a thin film is measured with good efficiency by a method wherein a conductive coating is executed to the thin film, the surface of a sample is etched and the layer cross section of the thin film having a constant angle of inclination with reference to a horizontal plane is exposed. CONSTITUTION: Au is vapor-deposited 11 onto a sample 7 which is composed of a base material 13 and of a film 12, and the sample is set on a stage 8. The inside of an apparatus is set to a high vacuum, and the surface of the sample is etched by an Ar ion gun 4. The surface is removed into a conical shape, its circumference is surrounded by a wall at a cross-sectional exposure angle θ as an angle of inclination, and an exposure width W is formed with reference to a film thickness D. Then, a primary electron beam which is emitted from an electron gun 1 is converged via a converging lens 2 and an objective lens 3 so as to be incident to the sample 7. Secondary electrons which are scattered from the surface of the sample 7 are detected by a secondary-electron detector 5, and Auger electrons are detected by an Auger-electron detector 6. By using the exposure width W and the exposure angle θ which have been measured, the film thickness D is found by an expression of the film thickness D=Wtanθ.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring the thickness of a thin film, and more particularly to a method for directly exposing and measuring an inclined cross section without cutting a coating thin film having a level of several nm.

[0002]

2. Description of the Related Art As a method for measuring the thickness of a film coated on the surface, a method of measuring the thickness by a multi-beam interference method has been generally adopted as an optical method. In this method, the observation of the interference fringes is greatly affected by the sharpness, and the interference conditions are restricted by setting the distance between the mirror and the sample, improving the reflectance of the light itself, etc. Was. Further, as a method of measuring the film thickness using X-rays, there are a method of utilizing absorption of X-rays, a method of exciting fluorescent X-rays, a method of utilizing interference of total reflection X-rays, and the like.
These methods have problems in accuracy with respect to the film thickness on the order of nm, and have not been sufficiently applied.

Recently, as described above, it has been difficult in the past.
For a film thickness of the order of nm, a method that does not cause a problem in accuracy has been studied, and various methods for measuring the thickness of a thin film coated on a substrate and the like have been devised. Among them, there is a method in which a coating thin film is mechanically cut diagonally together with a substrate by using a microtome or the like, and a cross section of the cut portion is observed by using an SEM (scanning electron microscope) or the like to measure the film thickness. However, with this method, when the thin film is on the level of several nm, the cut surface is sagged, and the film to be observed cannot be confirmed, making measurement impossible.

As another method, while performing ion etching from the surface of the thin film, analysis in the depth direction by Auger electron spectroscopy is performed to monitor the element specific to the film material, and the ion etching depth when this element is no longer detected. There is a method of knowing the film thickness by calibrating the thickness. However, since the Auger electron escape depth is several nm, the depth direction resolution of Auger electron spectroscopy is several nm or more, and in this case also, the measurement cannot be performed when the thin film has a level of several nm.

As described above, it has been desired to develop a measuring method capable of simply and highly accurately measuring the thickness of a thin film of several nm level.

[0006]

In view of the above problems, an object of the present invention is to investigate a dry etching method using high-energy particles as an alternative to the mechanical cutting method regardless of whether the substrate is hard or soft. By this process, a film thickness section under a certain condition is exposed, and a film thickness measuring method for determining the film thickness by measuring the exposed portion is provided.

Another object of the present invention is to capture an Auger electron image of a specific element in the exposed portion,
Provided is a film thickness measuring method capable of efficiently measuring the width of an exposed portion of a film layer. Further, another object of the present invention is to measure the sample current value when the sample is subjected to electron irradiation, monitor the resistance value thereof, and monitor the change of the resistance value gradient at the interface between the thin film and the substrate. By recognizing, the correspondence between the dry etching rate and the integrated value of the etching amount up to that point is examined, and a film thickness method for determining the film thickness by this is provided.

[0008]

[Means for Solving the Problems] In the method for measuring the thickness of a thin film, the above object is to perform a constant treatment on a horizontal plane by applying a conductive coating on the thin film as a pretreatment of the sample and etching the surface of the sample. Exposing a thin film layer cross section having an inclination angle of, measuring a projected exposure width W of the thin film layer cross section with respect to the horizontal plane, and obtaining an exposure angle θ which is an inclination angle of the thin film layer cross section with respect to the horizontal plane. And the exposure angle θ to obtain the film thickness D from the relationship of D = W tan θ.

The above object can also be achieved by a method for measuring the film thickness of a thin film, in which the exposure angle θ is obtained by an optical means such as a light reflection method. Further, it can be achieved by a method for measuring the film thickness of a thin film in which the exposure angle θ is obtained by the secondary electron emission amount measuring method. Further, the above-mentioned object is to set the sample in a scanning electron microscope having an ion etching device in the method of measuring the thickness of a sample provided with a thin film, and irradiate an electron beam while performing ion etching at a constant etching rate. Then, the current value flowing in the sample is measured moment by moment, the resistance value of the sample obtained from the current value is monitored, and the time point at which the rate of decrease in the resistance value indicates a bending point with respect to the etching time is recognized, until that time point. It can also be achieved by a method for measuring the thickness of a thin film, which is characterized in that the thickness of the thin film is obtained from the integrated value of the etching amount.

[0010]

In the present invention, the etching region is in the shape of a shallow mortar surrounded by slanted walls on four sides, and the thin film layer cross section is cut out and exposed at a low angle of 1 ° or less at the end thereof. If the horizontal projection width W and the exposure angle θ of the exposed thin film layer cross section are measured, the film thickness D can be obtained from the relationship of D = Wtan θ. The exposure width W is obtained from the Auger electron image of the element peculiar to the film, and the exposure angle θ is obtained by an optical means or a secondary electron emission amount measuring method. With this method, it is possible to measure the film thickness of a thin film at the level of several nm because the thin film cut-out cross section at an extremely low angle is used.

Further, according to the present invention, one electric circuit is formed by the sample and the apparatus by irradiating with an electron beam. At this time, since the thin film and the base material have different resistances, the point where the gradient of the resistance value changes, that is, the bending point at the rate of change is the interface between the thin film and the base material. Therefore, if the etching time up to this point is measured, this etching time is proportional to the film thickness, and therefore the total etching depth can be converted into the film thickness. Unlike the Auger electron, this method does not affect the escape depth, so it is possible to measure even a film thickness of several nm.

The present invention will be described below in detail with reference to the accompanying drawings of the embodiments.

[0013]

【Example】

Example 1 An example of the ultra-low-angle section cutting method of the first invention of the present invention will be described. FIG. 1 is a diagram showing an example of the apparatus of this embodiment. The high-energy primary electron beam emitted from the electron gun 1 is extremely narrowed down by the converging lens 2 and the objective lens 3 and is incident on the sample 7 set on the stage 8. At this time, the electrons scattered as secondary electrons from the sample surface are detected by the secondary electron detector 5.
An Auger electron detector 6 detects an Auger electron whose emission process is different from that of the secondary electron and whose spectrum is generally composed of a plurality of complex peaks.
In addition to the above basic structure, an ion gun 4 and a laser light source 9
And a screen 10.

The ultra-low-angle cross-section cutting method of this embodiment will be described in the order of steps. As shown in FIG. 4A, the sample 7 including the base material 13 and the film 12 is subjected to Au vapor deposition 11 and set on the stage 8. Ar ion gun 4 after high vacuum in the equipment
The sample surface is etched by. In FIG. 4B, the surface is shaving like a mortar, and the four edges are surrounded by walls having a cross-sectional exposure angle θ having an inclination angle, and an exposure width W with respect to the film thickness D is obtained.
The overall width B carved into a mortar shape is approximately 1 mm.

The end portion of the etching region is observed by SEM (low magnification) as a substantially concentric circle shape from directly above as shown in FIG. 4 (c). Since the cross section of the film thickness layer is exposed, SEM
After determining the site with the image, an Auger electron image (high magnification) of the film-specific element is captured. At this time, the element peculiar to the film is grasped in advance by qualitative analysis. The film cross-section exposure width W is measured by the image shown in FIG. From this result, the film thickness D is given by the following equation.

D = Wtan θ (1) Next, the process of measuring the exposure angle θ will be described. FIG.
The light reflection method as the optical method shown in (e) will be described.
The stage 8 is directed toward the laser light source 14 and the exposed cross section is irradiated with laser light. This laser light is reflected by the incident light with a difference of 2θ and returns to the screen. At this time, the distance 1 from the light source to the return point is measured, and θ is calculated by the following formula.

Θ = 1/2 Tan ⁻¹ ( l / L) (2) Further, as another measuring method, the secondary electron amount measuring method may be used. This method is a method of obtaining from the relationship between the secondary electron emission efficiency and the inclination angle, and as shown in FIG.
This is a method of obtaining θ from the change in the secondary electron emission efficiency δ when the initial stage angle α is changed to α + θ.

By the above-described measurement, the film thickness D is calculated by substituting θ obtained by the equation (2) into the equation (1). In addition,
The ion gun of this embodiment preferably uses a conventional, for example, liquid metal ion source as the ion source. In addition to Au vapor deposition, Pt, C, or Ag may be used for light. Further, the laser is preferably an integrated laser such as a distributed feedback type and distributed flag reflector laser.

Embodiment 2 An embodiment of the second invention will be described below. In the second invention, the exposure angle θ is measured by an electric resistance measuring method. FIG. 2 shows a measurement example of this embodiment. It is the same as the basic diagram of the first embodiment except that the electron gun 1 and the stage 8 are connected as an electric circuit. The high energy electron flux is narrowed down as much as possible, and the sample 7 is irradiated. Further, high energy ions are emitted from the ion gun 4 to be etched. First, the thin film sample 7 is set on the stage 8 as it is, and the inside of the apparatus is set to a high vacuum. Between the sample and the device, when the electron beam is emitted from the electron gun 1, the sample is charged without charge up,
The electric current I
And the voltage V is measured. At this time, the secondary electron current emitted from the sample is much smaller than the sample current I (primary electron current) flowing through the circuit and can be ignored.

The resistance R is obtained from the above current I, and ion etching is continued while monitoring this R.
The relationship between the etching time and the resistance value is plotted. Figure 5
(B) shows the time when the resistance value is monitored, and the substrate 13 and the film 12 are shown.
The primary electron current is measured through the film, and as shown in FIG. 5A, the resistance value R decreases in proportion to the etching time until the etching time C, and thereafter the resistance value R decreases in proportion to the etching time. When it reaches, it shows a constant resistance value.

That is, since the resistance value decreasing gradient changes at the interface between the film and the substrate, the etching time up to this point is converted into the thickness. This method also obtains the converted thickness in the same way in the conventional AES (Auger electron spectroscopy) depth measurement,
Unlike Auger electrons, the effect of escape depth can be eliminated, and depth resolution is improved.

[0022]

The invention according to claims 1 to 3 makes it possible to cut out a thin film cross section at an extremely low angle by using a mortar-like shape formed by etching which is the end of the ion etching region. Further, by using the optical system together, it becomes possible to obtain the inclination angle of the extremely low inclined surface. Furthermore, since the invention according to claim 4 does not require the Auger electron analysis mechanism, the apparatus itself is as simple as SEM.

[Brief description of drawings]

FIG. 1 is a diagram showing an outline of a measuring apparatus according to a first embodiment of the present invention.

FIG. 2 is a diagram showing an outline of a measuring apparatus according to a second embodiment of the present invention.

FIG. 3 is a diagram showing an outline of a secondary electron emission amount measuring method according to Example 1 of the present invention.

FIG. 4 shows a measurement example according to Example 1 of the present invention, (a)
Au deposition, (b) mortar-like shape, (c) etching site,
It is a figure which shows (d) width of a film thickness layer cross section, and (e) measurement of an exposure angle.

FIG. 5 shows a measurement example according to Example 2 of the present invention, (a)
It is a figure which shows the change of resistance value and (b) measurement of a primary electron current.

[Explanation of symbols]

 1 ... Electron gun 2 ... Converging lens 3 ... Objective lens 4 ... Ion gun 5 ... Secondary electron detector 6 ... Auger electron detector 7 ... Sample 8 ... Stage 9 ... Laser light source 10 ... Screen 11 ... Au 12 ... Film 13 ... Substrate 14 ... Light / laser light source

Claims (4)

[Claims] 1. A thin film thickness measuring method, wherein a thin film layer cross-section having a constant inclination angle with respect to a horizontal plane is formed by applying a conductive coating on the thin film as a pretreatment of the sample and etching the surface of the sample. Exposed, the projected exposure width W of the cross section of the thin film layer with respect to the horizontal plane is measured, and the exposure angle θ, which is the inclination angle of the cross section of the thin film layer with respect to the horizontal plane, is obtained, and D = Wta
A method for measuring the film thickness of a thin film, characterized in that the film thickness D is obtained from the relationship of n θ. 2. The method for measuring the film thickness of a thin film according to claim 1, wherein the exposure angle θ is obtained by an optical means such as a light reflection method. 3. The method for measuring the film thickness of a thin film according to claim 1, wherein the exposure angle θ is obtained by a secondary electron emission amount measuring method. 4. A method for measuring a film thickness of a sample provided with a thin film, wherein the sample is set in a scanning electron microscope having an ion etching apparatus, and an electron beam is irradiated while performing ion etching at a constant etching rate, The current value flowing in the sample is measured moment by moment, the resistance value of the sample obtained from the current value is monitored, and the time point at which the rate of decrease of the resistance value indicates a bending point with respect to the etching time is recognized, and etching up to that time point is performed. A method for measuring the thickness of a thin film, characterized in that the thickness of the thin film is obtained from the integrated value of the quantity.