JPH10223714A - Section observing specimen and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make feasible manufacturing of a section observing specimen making its boundary of an observed section conspicuous, without breaking the specimen at all. SOLUTION: After forming an underlying thin film 14, made of silicon nitride in rough film quality on a specimen 10 by a plasma CVD process, RF output is increased to form a silicon nitride film 15. Next, the specimen 10, the underlying thin film 14 and the silicon nitride film 15 are cleaved to expose an observing section B of the specimen 10. Next, the observing section B is etched away using a chemical for etching the silicon nitride. Through these procedures, the underlying thin film 14 between the specimen 10 and the silicon nitride film 15 is selectively etched away.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for preparing a sample for observing a cross section, and more particularly to a method for preparing a sample when observing a cross section of a semiconductor device using a scanning electron microscope.

[0002]

2. Description of the Related Art FIG. 5 shows a scanning electron microscope (Scanning electron microscope).
A cross-sectional observation sample 5 manufactured for performing cross-sectional observation of a semiconductor device (sample) using an electron microscope (SEM) is shown. (2) in the figure is a cross-sectional view taken along the line AA 'in (1). In the sample 50, a BPSG film (a silicon oxide film containing boron and phosphorus) 52 is formed on the upper surface of a silicon substrate 51.
2 on which an aluminum wiring 53 is provided.

[0003] The aluminum wiring 5 in the sample 50
When fabricating the cross-sectional observation sample 5 for observing the cross-sectional shape of the sample 3, first, a silicon nitride film 54 is formed on the surface of the sample 50. In this case, a silicon nitride film 54 is formed on the BPSG film 52 so as to cover the aluminum wiring 53. Thereafter, the observation section B of the sample 50 is exposed by cleavage. Next, the observation section B is etched using a hydrofluoric acid-based chemical. Here, etching is performed for about 10 seconds using a hydrofluoric acid aqueous solution having a concentration of about 5%, and BPS is performed by utilizing an etching rate difference depending on a film type.
The etching proceeds in the order of the G film 52≫the silicon nitride film 54> the silicon substrate 51> the aluminum wiring 53. Thereby, the sample 5 for cross-section observation is completed.

[0004] In the above-described method of manufacturing the cross-sectional observation sample 5,
By forming the silicon nitride film 54 on the surface of the sample 50, charge-up due to an electron beam irradiated during SEM observation is prevented. In addition, by performing an etching process on the observation section B using a hydrofluoric acid-based chemical, the boundaries of the layers in the observation section B are emphasized, and the cross-sectional shape can be easily grasped by SEM observation. In particular, the BPSG film 5
2 and the silicon nitride film 54 have a higher etching rate than the aluminum wiring 53, and the etching of the silicon nitride film 54 progresses at the boundary C between the aluminum wiring 53 and the silicon nitride film 54. Therefore, the boundary C is emphasized. Therefore, it becomes easy to grasp the cross-sectional shape of the aluminum wiring 53 by SEM observation.

[0005]

However, the above-mentioned method for producing a cross-sectional observation sample has the following problems. In other words, when the etching rate of the film constituting the surface layer of the sample to be observed in cross section is close to the etching rate of the silicon nitride film, the etching of the observed cross section results in the same etching of the sample surface layer and the silicon nitride film. In the observation cross section, the boundary between the sample and the silicon nitride film cannot be emphasized.

For example, as shown in FIG. 6, a silicon oxide pattern 57 is formed on a BPSG film 52 on a silicon substrate 51.
In the sample 58 in which is formed, a cross-sectional observation sample 6 for observing the cross-sectional shape of the silicon oxide pattern 57
Is manufactured according to the above method, the silicon oxide film 57 and the silicon nitride film 54 formed on the sample 58 are formed.
Observation section B
Silicon oxide pattern 57
The boundary C between the silicon nitride film 54 and the silicon nitride film 54 cannot be emphasized.

If an excessive additional etching process is performed on the observation section B to emphasize the boundary C, the sample may be deformed. FIG. 7 shows a cross-section observation sample 7 manufactured by performing the additional etching. As shown in this figure, when excessive additional etching is performed, the BPSG film 52 is excessively etched by the additional etching, and the silicon oxide pattern 57 formed on the BPSG film 52 is deformed. Cracks D also occur in the silicon film 54. Then, the prototype of the sample 58 cannot be held.

The deformation of the sample 58 and the silicon nitride film 54 due to the additional etching is three times as large as that of the normal film, depending on the type of film constituting the sample 58 and the method of cleaning and drying the sample 58 after the etching process. The probability of occurrence in a short etching time is high.

[0009]

SUMMARY OF THE INVENTION The present invention provides a method for preparing a cross-sectional observation sample and a cross-sectional observation sample which have been made to solve the above-mentioned problems. That is, in the method for manufacturing a cross-sectional observation sample in which a silicon nitride film is formed on the surface of a sample and then the observation cross section is exposed and the observation cross section is etched, the sample is formed before forming the silicon nitride film. Is characterized in that a base thin film made of silicon nitride having a film quality coarser than that of the silicon nitride film is formed on the surface of the substrate.

[0010] In the above method for manufacturing a cross-section observation sample, when an etching process is performed on the observation cross-section, the underlying thin film made of silicon nitride having a film quality coarser than that of the silicon nitride film has a higher etching rate than the silicon nitride film. Etched. Since this base thin film is formed between the sample and the silicon nitride film, the boundary of the sample in the observation cross section is emphasized by the above-described etching process.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing an embodiment of a method for producing a cross-sectional observation sample of the present invention, and a cross-sectional view taken along the line AA ′ in FIG.

FIG. 1A and FIG.
1 shows a semiconductor device (sample) 10 for performing cross-sectional observation. As shown in these figures, the sample 10 has a configuration in which a BPSG film 12 is formed on an upper surface of a silicon substrate 11, and a silicon oxide pattern 13 is formed on the BPSG film 12. In the sample 10 configured as described above, a cross-sectional observation sample for observing the cross section of the silicon oxide pattern 13 is formed as follows.

(First Embodiment) First, FIG.
As shown in (2), a base thin film 14 made of silicon nitride is formed on the surface of the sample 10 by a plasma CVD method, and subsequently, a silicon nitride film 15 is formed on the base thin film 14.
Is formed. The underlying thin film 14 is formed of a silicon nitride film 15
It is assumed that the film quality is coarser than that. Here, the term “rough film quality” means a state in which the film contains a large amount of impurities.

Here, when forming the underlying thin film 14 and the silicon nitride film 15, the film is formed by changing the RF output which is a characteristic control factor of the film formed by the plasma CVD method. R when forming the underlying thin film 14
The F output is set to a first value, and the RF output when the underground silicon film 15 is formed is set to a second value lower than the first value. Specifically, the formation of the base thin film 14 and the silicon nitride film 15 is performed as follows.

FIG. 2 is a graph showing a change over time of the RF output when the RF output rises in the film formation by the plasma CVD method. As shown in this graph, when forming the base thin film (14) and the silicon nitride film (15), the time required for the RF output to reach a predetermined output (hereinafter referred to as a rise time) is a conventional value (shown by a broken line in the figure). Shown).

Here, the predetermined output means the silicon nitride film 1
5 is the value of the RF output (that is, the second value) when the film is formed, and is 400 W as an example. The conventional rise time is a rise time when the silicon nitride film 15 is formed directly on the sample 10. The rise time is set by RFup-rate (W / sec), and the lower this value is, the longer the time it takes to reach the set output.

As described above, the RFup-rate is set low to extend the rise time of the RF output, and at this rise time, the value of the RF output (ie, the first value)
Is formed by a plasma CVD method in which is set small.
As a result, decomposition of the reaction gas at the time of film formation is suppressed to a low level, and the base thin film 14 containing a large amount of impurities such as hydrogen and moisture, that is, formed of silicon nitride having a rough film quality is formed.

Thereafter, a film is formed by the plasma CVD method while maintaining the RF output at the predetermined output (that is, the second value). This enhances the decomposition of the reactive gas during film formation,
A silicon nitride film 15 made of silicon nitride having a small impurity content, that is, a dense film quality is formed.

Next, as shown in FIG. 1C, for example, the observation section B of the sample 10 is exposed by cleaving the sample 10, the underlying thin film 14, and the silicon nitride film 15.

Thereafter, as shown in FIG. 1D, an etching process is performed on the observation section B using a chemical solution for etching silicon nitride. Here, for example, etching is performed for about 10 seconds using a hydrofluoric acid aqueous solution having a concentration of about 5%, thereby completing the sample 1 for section observation.

In the method of manufacturing a sample for cross-section observation, a base thin film 14 made of silicon nitride having a film quality lower than that of the silicon nitride film 15 is formed between the sample 10 and the silicon nitride film 15. FIG. 3 is a graph showing the relationship between the RF output and the etching rate when forming a film made of silicon nitride. As shown in this graph, when a film made of silicon nitride is formed by a plasma CVD method,
It can be seen that the higher the RF output is set for film formation, that is, the denser the film quality, the lower the etching rate.

For this reason, when the etching process is performed on the observation section B, the underlying thin film 14 made of silicon nitride whose film quality is coarser than that of the silicon nitride film 15 is changed to the silicon nitride film 15 or an etching rate similar thereto. Is etched at a faster etching rate than the silicon oxide pattern 13 having Therefore, the base thin film 14 between the sample 10 and the silicon nitride film 15 is etched without performing excessive additional etching, and the boundary C of the sample 10 in the observation section B can be emphasized without destroying the cross-sectional shape. it can.

(Second Embodiment) Here, FIG. 1 (2),
When the underlying thin film 14 and the silicon nitride film 15 shown in (3) are formed by the CVD method, the set value of the RF output is set in two stages to prepare the sample 1 for cross-sectional observation. That is, FIG.
As shown in the graph, in the initial stage of the film formation by the plasma CVD method, the RF output is set to the first value of about 200 W to form the base thin film (14). Next, R
A film is formed by the plasma CVD method with the F output set at a second value of about 400 W, and a silicon nitride film (15) is formed on the upper surface of the base thin film (14).

As described above, the cross-section observation sample 1 in which the underlying thin film 14 and the silicon nitride film 15 are formed is the same as the cross-section observation sample 1 in the method of the first embodiment.
Can be produced. In addition, quantitative control of the film quality and film thickness of the base thin film 14 becomes possible. However, the present invention is a method for producing a cross-sectional observation sample to the last,
Since quantitative management of the film quality and thickness of the base thin film 14 is not always necessary, there is no problem in the procedure of the first embodiment.

In this embodiment, it is not always necessary to form the base thin film 14 and the silicon nitride film 15 by a continuous plasma CVD method.

In the first and second embodiments, the production of a cross-sectional observation sample for observing the cross-sectional shape of the silicon oxide pattern 13 is taken as an example. However, the method for producing a cross-sectional observation sample of the present invention is not limited to this. For example, the method for producing a cross-sectional observation sample for observing the cross-sectional shape of an aluminum wiring as described in the related art may be used. Applicable.

Further, the method of forming the underlying thin film 14 having a rough film quality and the silicon nitride film 15 having a higher film quality is not limited to the above-described plasma CVD method.

[0028]

As described above, according to the method for manufacturing a cross-sectional observation sample of the present invention, a silicon nitride film is formed on a surface of a sample via a base thin film made of silicon nitride having a rough film quality. Thus, it is possible to selectively etch the underlying thin film while leaving the silicon nitride film for preventing charge-up during SEM observation. Therefore, it is possible to produce a cross-sectional observation sample in which the boundary of the sample in the observation cross section is emphasized without breaking the sample.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a method for manufacturing a cross-sectional observation sample.

FIG. 2 is a graph illustrating film formation by a plasma CVD method in the first embodiment.

FIG. 3 is a graph showing a relationship between an RF output and an etching rate during film formation.

FIG. 4 is a graph illustrating film formation by a plasma CVD method in a second embodiment.

FIG. 5 is a diagram illustrating a conventional sample for cross-sectional observation.

FIG. 6 is a diagram (part 1) for explaining a conventional problem.

FIG. 7 is a diagram (part 2) for explaining a conventional problem.

[Explanation of symbols]

 1 Cross section observation sample 10 Sample 14 Underlayer thin film 15 Silicon nitride film B Observation cross section

Claims (3)

[Claims] After forming a silicon nitride film on a sample,
A method for manufacturing a cross-sectional observation sample, in which an observation cross section of the sample and the silicon nitride film is exposed and then the observation cross section is etched using a chemical solution for etching silicon nitride, wherein the silicon nitride film is formed. Forming a base thin film made of silicon nitride having a higher film quality than the silicon nitride film on the surface of the sample. 2. The film formation of the base thin film is performed by a plasma CVD method using an RF output value as a first value, and the film formation of the silicon nitride film uses an RF output value as a second value. The method for producing a section observation material according to claim 1, wherein the method is performed by a plasma CVD method. 3. A sample in which a silicon nitride film is formed on a surface and a cross-sectional observation sample in which an observation cross section of the silicon nitride film is subjected to an etching treatment using a chemical solution for etching silicon nitride. A sample for cross-sectional observation, wherein a base thin film made of silicon nitride having a film quality coarser than that of the silicon nitride film is provided between the silicon nitride film and the sample.