JP2001319954A - Sample processing method with focused ion beam - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to analyze a detective portion accurately on a surface layer of a single crystal wafer, by preventing a damaged layer during ion beam irradiation or a variation of surface state of the Si single crystal wafer like mingling of Ga or the like during observation of a scanning ion microscope image (SIM image) when a sample is processed for analyzing the surface of Sir single crystal wafer with a focused ion beam(FIB). SOLUTION: When the sample is processed for analyzing the surface of Si single crystal wafer through shape observation or composition analyze with a focused ion beam(FIB), a protective film is applied to the surface of Si single crystal wafer previously and then SIM observation at the desired observation position and the FIB process are carried out. After that, the protective film is removed with HF and the like, the damaged layer caused by FIB or a variation of surface state of the Si single crystal wafer like mingling of Ga or the like can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for processing a sample for analyzing a defect in a surface layer portion of a silicon single crystal wafer by using a focused ion beam.

[0002]

2. Description of the Related Art With the increase in integration and performance of LSIs, the influence of minute defects in the surface layer of a silicon (Si) single crystal wafer represented by COP (Crystal Originated Particles) and dislocations on device characteristics becomes apparent. There is a need for techniques for elucidating and reducing these generation mechanisms. As a typical analysis method of a defect portion in a surface layer portion of a Si single crystal wafer, a scanning electron microscope (hereinafter abbreviated as SEM), a transmission electron microscope (hereinafter abbreviated as TEM), and an atomic force microscope (hereinafter abbreviated as AFM) are used. Examples include shape observation, impurity concentration profile measurement by secondary ion mass spectrometry (hereinafter abbreviated as SIMS), composition analysis by energy dispersive X-ray spectroscopy (hereinafter abbreviated as EDX), Auger electron spectroscopy (hereinafter abbreviated as AES), and the like. When using a focused ion beam (hereinafter abbreviated as FIB) to analyze a defective portion in the surface layer portion of a Si single crystal wafer using these techniques, it is possible to easily observe a cross section of a specific portion and process a sample thereof. be able to. This method is described in, for example,
No. 5981, and the like.

According to the above-mentioned conventional method, after a mark that can define a desired observation position is processed on a sample surface by FIB,
By subjecting a window hole that enables observing a cross section from an oblique direction to a square hole, a cross section of a semiconductor device or the like can be accurately obtained. Similarly, when observing a defective portion of a semiconductor device or a cross section such as a crystal defect in a silicon single crystal wafer using a TEM, a thin film sample at a desired observation position can be manufactured by using the FIB. The method for preparing a cross-sectional TEM observation sample using FIB is described in Transmission Electron Microscope Sample.
Preparation Usinga Focused Ion Beam (J.Electron Mic
rosc.43, pp.322-326, 1994).

[0004] When FIB is used, EDX, AES
A mark for discriminating a desired defect position can be easily and accurately formed by an analyzer such as the above or a sample processing device such as a dicing saw used for precision processing of a TEM observation sample. The ion beam used for the FIB is usually gallium (G) having an acceleration voltage of about 30 keV.
a) It is an ion beam. The aperture is switched, and the lens conditions such as a focusing lens and an objective lens are changed.
The focused ion beam can be switched from a small current beam having a beam current of about several tens pA to a large current beam of about several tens nA. By scanning the sample with a Ga ion beam, processing by ion sputtering or a scanning ion microscope image (hereinafter referred to as S
The shape of the sample surface can be observed using an IM image.

A procedure for processing a sample for analyzing a defect portion of a Si single crystal wafer using the conventional FIB will be described below with reference to the drawings. 4 (a) and 4 (b) show the defect 2
FIG. 2 is a view of a Si single crystal wafer 1 including the above as viewed from the surface. First, the SIM of the surface of the silicon single crystal wafer in the FIB device
While observing the image, the sample is moved to the vicinity of a desired observation point. Next, the FIB processing area 7 is set on the SIM image,
The region is scanned with a Ga ion beam, and the surface of the silicon single crystal wafer is etched to a depth at which the cross section can be observed and the mark can be determined, thereby determining a cross section observation sample at a specific location and a defect position in the apparatus. The mark was being processed.

[0006]

In the conventional method of processing a sample for analyzing a defect portion in a surface layer portion of a Si single crystal wafer by using FIB, a SIM image observation is performed when the sample is moved to a vicinity of a desired processing position. Sometimes, the surface of the Si single crystal wafer is irradiated with a Ga ion beam having an acceleration voltage of about 30 keV. As a result, atoms on the sample are sputtered and S
The i single crystal surface is damaged. Fig. 2 shows acceleration voltage of 30keV
Irradiating the Si surface with the Ga ion beam of
The distribution of ions is calculated by simulation using the Monte Carlo method. Thus, when observing the SIM image,
It can be seen that a region of about 50 nm from the surface layer of the i single crystal wafer is damaged by the Ga ion beam. as a result,
About 50 nm from the surface layer of silicon single crystal becomes amorphous,
For example, in the case of a crystal defect in a single crystal such as a dislocation or a stacking fault, a defect to be observed disappears during sample preparation. For this reason, there has been a problem that it is difficult to normally observe the actual state of a defect generated on the surface layer of the silicon single crystal wafer using a TEM or the like.

In order to prevent this, a method of forming a protective film such as tungsten (W) in advance on a crystal defect position by using a film forming apparatus provided in the FIB apparatus has been adopted. In this film forming method, a W film is formed by designating a film forming region with an FIB apparatus and repeatedly scanning the region with a Ga ion beam. At this time, in order to observe the SIM image on the surface of the silicon single crystal wafer and search for a film formation region, the surface layer of the silicon single crystal wafer is exposed in the same manner as when directly irradiating the silicon single crystal wafer with a Ga ion beam of about 30 keV. It will be damaged.

Japanese Patent Application Laid-Open No. 10-283971 proposes a method for setting a different acceleration voltage of a Ga ion beam between observation of a SIM image and processing of a SIM image.
However, even with the above method, in order to obtain a SIM image with sufficient resolution, it is necessary to irradiate the sample surface with a Ga ion beam at an acceleration voltage of 25 kV to 35 kV.
However, it is impossible to completely prevent the formation of a damaged layer on the sample surface layer due to the above. In addition, since Ga as an ion source is mixed into the SIM image observation region on the sample surface, the chemical composition of the sample surface layer portion changes, and an accurate defect using EDX or AES of the defect generated on the surface of the silicon single crystal wafer is used. There was also a problem that composition analysis became difficult.

An object of the present invention is to provide a sample processing method using an FIB for enabling a defect analysis of a surface layer portion of a silicon single crystal wafer, which was impossible with the prior art.

[0010]

SUMMARY OF THE INVENTION The present invention is directed to setting a processing area when processing a sample to be analyzed by FIB using a FIB for observing the shape of a defect in the surface layer of a Si single crystal wafer and analyzing the composition by composition analysis or the like. Before performing the SIM image observation of the above, as a protective film, a thin film having a shielding effect against sample damage due to a Ga ion beam and capable of being removed without damaging the surface of the Si single crystal wafer is used.
By forming the sample on the surface of the single crystal wafer in advance, it is possible to prevent a change in the surface state of the sample due to the FIB and to realize a normal analysis of a defective portion on the surface layer portion of the Si single crystal wafer.

It is necessary that the type of the protective film can be formed by a film forming method which does not affect the surface layer portion of the Si single crystal wafer and can be removed without damaging the surface of the Si single crystal wafer. is there. Typical film types include a silicon oxide film (SiO _x ) formed by applying a silica-based coating solution, and platinum palladium (P
t-Pd) film and the like.

The silica-based coating solution (hereinafter abbreviated as SOG film) is obtained by dissolving silanol [RnSi (OH) _4-n ] in an organic solvent (alcohol, ester, ketone). S
After the OG film is applied using a spin coater or the like,
By performing the heat treatment at about 0 ° C., a silicon oxide film (SiO _x ) can be easily formed on the sample surface.

The thickness of the silicon oxide film formed on the sample surface may be any thickness as long as the surface of the Si single crystal wafer is not damaged by the Ga ion beam.
As shown in the above, when the Ga ion beam at an acceleration voltage of 30 keV was irradiated on the SiO _x surface on Si, the distribution of Ga ions was calculated by a Monte Carlo simulation. As a result, Ga ions were within 100 nm from the sample surface layer. Since the SOG film is distributed over the surface of the Si single crystal wafer as a protective film with a thickness of 100 nm or more, formation of a damaged layer of the Si layer due to FIB can be prevented.

According to the present invention, when processing a sample for analyzing the shape of a defect in the surface layer of a Si single crystal wafer using FIB and analyzing the composition by a composition analysis or the like, an S film is used as a protective film.
After applying an OG film on the surface of the Si single crystal wafer in advance, performing SIM image observation and FIB processing at a desired observation location, and then removing the SOG with HF or the like, FIB
It is possible to prevent a change in the surface state of the sample, such as formation of a damaged layer and mixing of Ga. The composition of the SOG film in the present invention may be any composition as long as it is used as an interlayer insulating film in an LSI manufacturing process.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A procedure for processing a sample for observing a defect in a surface layer portion of a silicon single crystal wafer using an FIB according to the present invention will be described below with reference to the drawings. FIG. 1A is a view of a Si single crystal wafer 1 including a defect portion 2 as viewed from a cross section.
First, as shown in FIG. 1A, an SOG film 3 having a thickness of 100 nm or more is applied on a Si single crystal wafer 1. next,
A heat treatment at about 100 ° C. is performed to dry the SOG film 3. FIG. 1B is a diagram of the Si single crystal wafer 1 including the defect 2 as viewed from the surface. In the FIB apparatus, the SI on the surface of the silicon single crystal wafer is set so that the desired defect 2 is centered.
While observing the M image, the sample is moved to the vicinity of a desired observation point, a processing region is set on the SIM image, and the region is processed to form a mark 4 as shown in FIG. . The shape of the mark 4a is determined by setting the processing area on the SIM image and controlling the processing time, according to the resolution of the SEM or the optical microscope equipped in the analyzer or sample processing apparatus to be used next. What is necessary is just to set to the possible magnitude | size and depth.

FIG. 1C is a cross-sectional view of the Si single crystal wafer 1 taken along the line AA 'in FIG. 1B. Next, FIG.
As shown in (c), the SOG film 3 is removed by HF or the like. After that, shape observation by SEM, AFM, EDX,
Analysis of the defect 2 according to the purpose such as composition analysis by AES is performed.

FIGS. 1 (d) and 1 (e) are perspective views of a preliminary thin piece 5 obtained by processing the Si single crystal wafer 1 into a convex shape. Defect 2
When the cross-sectional TEM observation is performed, the SOG film 3 is
Then, the Si single crystal wafer 1 is precision-processed into a convex shape as shown in FIG. 1D using a dicing saw or the like so that the mark 4a is substantially at the center of the convex region.
A preliminary thin section 5 is prepared. Then, as shown in FIG. 1 (e), using the FIB, the convex region of the preliminary thin piece 5 is processed from both sides around the mark 4a, and the position including the defect 2 is thinned to a thickness of about 0.1 μm. Thus, a thin film sample at the observation position is prepared.

The processing of a defect observation sample on the surface of a silicon single crystal wafer using FIB in the case where the desired defect position on the surface of the Si single crystal wafer 1 is known has been described above. 2 is unknown, first, a film thickness of 100 nm is formed on the Si single crystal wafer 1.
After the above-mentioned SOG film 3 is applied, a crystal defect alignment mark 4b as shown in FIG. 1B is formed at an arbitrary position on the Si single crystal wafer 1 using FIB. The size of the area where the mark is formed is set so that several defects 2 are included in a predetermined area in consideration of the density of a desired crystal defect. Next, after the SOG film 3 is removed with HF or the like, the position of the defect 2 is determined by measuring the inside of the area where the mark is formed using a surface shape analyzer such as an AFM. What is necessary is just to process a defect observation sample.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Using the defect observation sample processing method of the present invention,
A cross-sectional TEM observation of threading dislocations in an IMOX (Separation by Implanted Oxygen) wafer was performed. The observation sample had a surface silicon layer of 60 nm and a buried oxide film layer of 12 nm.
At 0 nm, the density of threading dislocations contained in the surface silicon layer is 1E5
A cmox- ² SIMOX wafer was used. First, the sample was cut into chips of about 7 mm × 5 mm, and then the film thickness was 20 mm.
A 0 nm SOG film was applied, and heat treatment was performed at a temperature of 100 ° C. for 1 minute. Next, by etching the surface of the SIMOX wafer with the FIB, a 5 μm × 5 μm mark for alignment of crystal defects was formed at the end of the 50 μm × 50 μm region. In consideration of the threading dislocation density, 23 threading dislocations are included in the region. Next, the SIMOX wafer was immersed in 50% HF for 1 minute to remove the SOG film. Next, the inside of the area was scanned using the AFM, and a desired defect position was determined. Next, a 200 nm-thick SOG film is applied again on the sample surface,
The FIB is used in FIG. 1 so that the defect position becomes the center.
A cross-sectional TEM sample processing position mark as shown in FIG. Next, the SOG film was removed, and the vicinity of the defect position was scanned using an AFM, whereby the three-dimensional surface shape of the desired defect was observed in detail. Next, using a dicing saw, the sample was placed in the position shown in FIG.
It was precision-processed into a convex shape as shown in (d). Next, a 100 nm Pt-Pd film was deposited on the preliminary thin section in order to prevent damage to the sample surface layer due to processing of the cross-sectional TEM observation area, and then etched using FIB from both sides of the sample with the mark at the center. And thinned to a thickness of about 0.1 μm.

FIG. 5 (a) shows the result of cross-sectional TEM observation of a sample processed according to the embodiment of the present invention. FIG. 5 (b)
Shows a comparative example in which no SOG film is applied. FIG.
In (b), the surface silicon layer becomes amorphous due to the FIB processing damage, and normal analysis of a defective portion in the surface silicon layer becomes difficult. However, according to the embodiment of the present invention,
When the cross-sectional TEM observation sample was processed, it was confirmed that the actual threading dislocation could be observed without forming a damage layer due to FIB on the surface silicon layer as shown in FIG.

[0021]

As described above, according to the present invention, F
When processing a sample for analyzing the shape of a defect portion in the surface layer portion of a Si single crystal wafer using IB and analyzing the defect portion by composition analysis or the like, a SO
After applying a G film in advance, SIM image observation and FIB processing of a desired observation point are performed, and then the SOG film is removed by HF or the like, thereby forming a damaged layer by FIB, Si single crystal such as Ga mixing, etc. Variations in wafer surface conditions can be prevented, and S
Defects on the surface layer of the i-single-crystal wafer can be analyzed normally.

[Brief description of the drawings]

FIGS. 1A to 1E are diagrams showing a procedure for processing a sample for analyzing a defect portion in a surface layer portion of a Si single crystal wafer according to the present invention using FIB.

FIG. 2 is a view showing a distribution of Ga ions when a surface of a silicon single crystal is irradiated with a Ga ion beam of 30 keV.

FIG. 3 is a diagram showing a distribution of Ga ions when a surface of a silicon oxide film (SiO _x ) formed on a silicon single crystal is irradiated with a Ga ion beam of 30 keV.

4 (a) and 4 (b) show a conventional method of processing a sample for analyzing a defect portion in a surface layer portion of a Si single crystal wafer using FIB by processing a defect position determination mark and processing a cross-section observation sample. It is a figure shown about the case of.

FIGS. 5A and 5B show FI in an embodiment of the present invention.
9 shows the results of TEM observation of a cross section of a sample processed using B.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Single crystal silicon wafer 2 Defect part 3 SOG film 4 Defect position discriminating mark 5 Preliminary slice 6 Cross-sectional TEM observation area 7 FIB processing area

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H01J 37/317 G01N 1/28 GN

Claims (3)

[Claims] 1. A method for processing a sample using a focused ion beam for analyzing a defect portion of a surface layer portion of a silicon single crystal wafer, wherein a scanning ion microscope is used for processing the sample surface using the focused ion beam. Before observing the image, a protective film that has a shielding effect against sample damage caused by the Ga ion beam and that can be removed without damaging the surface of the silicon single crystal wafer is formed on the surface of the solid material. A method for processing a sample using a focused ion beam, comprising: a step of forming the film in advance by a film forming method which does not affect the process; and a step of removing the protective film after processing by the focused ion beam. 2. The method according to claim 1, wherein the protective film is a silicon oxide film formed by applying a silica-based coating solution to the surface of a silicon single crystal wafer. . 3. The silicon oxide film has a thickness of 100 nm.
3. The method for processing a sample using a focused ion beam according to claim 2, wherein: