JPH11160209A - Preparation of sample for transmission electron microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a method in which the image quality of a transmission electron microscope is enhanced when a sample for the transmission electron microscope is created, by a method wherein a process in which an anodic oxide film is formed on the sample is installed and a process in which the anodic oxide film is removed is installed. SOLUTION: A defect in a specific position on a semiconductor substrate is indicated by a mark X. A sample is shaved down to a width of, e.g., 200 mm by a mechanical polishing operation using an abrasive. The sample near the mark X is shaved by a focused ion beam(FIB) working apparatus, and the sample is made thin down to a thickness of 300 to 500 nm. The sample is turned by 90 deg. to the front, an anodic oxide film is formed on the surface of the sample by a jig, and the anodic oxide film is then removed. At this time, the semiconductor substrate is retreated by a thickens of 50 nm. After that, an electron beam is transmitted to directions of arrows inside a transmission electron microscope, and the sample is observed. When the anodic oxide film is formed and removed in this manner and when a damage layer on the surface of the sample is shaved, an image of better quality can be observed when the image by the transmission electron microscope is observed. That is to say, a lattice image having a most adjacent atomic interval can be observed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for analyzing a semiconductor device, and more particularly to a method for preparing a sample for a transmission electron microscope.

[0002]

2. Description of the Related Art There is an ion implantation technique as a technique used in a semiconductor device manufacturing technique. Ion implantation technology is a technology that introduces necessary impurities into a semiconductor substrate with a desired implantation amount and a desired acceleration energy by ionizing an introduced impurity element, selecting the same by a mass analyzer, and accelerating with an electric field energy. is there. After the ion implantation, heat treatment is performed at a predetermined temperature and time to form an impurity distribution of a desired conductivity type and conductivity.

At this time, the defect formed by the ion implantation causes a junction leak failure of the semiconductor device. Therefore, it is necessary to evaluate the density and the electrical characteristics of the implantation defect caused by the ion implantation.

[0004] As a method for evaluating the implantation defects generated in the ion implantation layer, there is a transmission electron microscope method. For thinning a specific part of the sample necessary for transmission electron microscope observation,
FIB (focused ion beam) processing is used. Also,
Ion milling is also used to reduce the thickness of non-specific locations. Ion milling is a technique in which a low-pressure atmosphere gas is turned into plasma by glow discharge and collides with a sample serving as a cathode, thereby scattering particles to be sputtered and shaving the sample surface. However, in this ion milling, it is difficult to reduce the thickness of a specific portion of the sample.

As a conventionally known method of measuring the amount of ion implantation, there is a method of measuring the sheet resistance of a sample made of a semiconductor substrate after ion implantation and heat treatment.
In this method, four probes are electrically connected on the same line,
By passing a current between the two points at both ends and measuring the voltage drop between the two points inside, the conductivity is determined from this resistance value,
This is for grasping the ion implantation amount.

Another method for measuring the amount of ion implantation is a thermal wave method. Hereinafter, the thermal wave method will be described. When a semiconductor substrate is irradiated with a pumping laser beam whose intensity changes periodically, the temperature of the surface of the semiconductor substrate changes periodically. Therefore, the surface is periodically deformed. This periodic deformation varies depending on the density of defects introduced into the semiconductor substrate. The reason for this is that if there are many defects in the crystal, heat conduction is hindered and the temperature of the region irradiated with the pumping laser light locally rises, so that the amount of deformation increases. On the other hand, when the number of defects in the crystal is small, the heat conduction is good, so that the temperature of the region irradiated with the pumping laser light is small and the amount of deformation is small.

In general, since the density of defects and the amount of implanted ions have a proportional relationship, the amount of deformation, ie, the amount of ion implantation, is measured by measuring the amount of deformation using a laser interferometer using a probing laser beam. can do.

[0008] As a conventional ion implantation distribution measurement technique in the depth direction (one-dimensional), secondary ion mass spectrometry (SI
MS) is widely used.

Further, by combining the formation of an oxide film by the anodic oxidation method and the removal of the oxide film by wet etching, a silicon crystal defect is detected (exposed), and then an atomic force microscope (AFM) or a scanning type is used. Electron microscope (SEM)
There is known a method of performing magnified observation using an image. The anodic oxidation method described here is an electrochemical oxide film forming method capable of forming a good ultrathin oxide film at room temperature. By repeating formation and removal of the anodic oxide film, the silicon substrate can be receded without damaging the silicon substrate (Japanese Patent Application No. 5-151842, or S. Fu).
Jii et al, "A Novel Atomi
c Microscopic Observation
Technique for Secondary D
effects of Ion Implantatio
n, using Anodic Oxidatio
n ", Jpn. J. Appl. Phys. v
ol 32 (1993) pp. L157-L15
9).

Hereinafter, a conventional technique for preparing a transmission electron microscope sample using FIB processing will be described with reference to the drawings. Here, a silicon substrate (p-Si (100) 10 to 15Ω ·
cm) P (phosphorus) ion implantation from above (acceleration energy:
150 keV, injection amount: 3 × 10 ¹⁵ cm ^-2 )
A sample subjected to a short-time heat treatment at 950 ° C. for 10 seconds was used.

FIG. 7 shows a method of manufacturing a transmission electron microscope sample using a step of forming an anodic oxide film on a sample, a step of removing the anodic oxide film, and a step of using focused ion beam processing. FIG. FIG. 7A shows a defect (indicated by X) at a specific position on the semiconductor substrate. FIG. 7B shows that the sample is cut down to a width of 200 mm by mechanical polishing using an abrasive.
FIG. 7 (c) shows a FIB processing device (manufactured by Seiko Denshi Kogyo: SMI-9800, Ga ion (30 Ke)
V)) shows that the thickness was reduced to a thickness of 300 nm to 500 nm. FIG. 7D shows that the sample was rotated 90 degrees in the forward direction.
FIG. 7E shows that a sample is observed by transmitting an electron beam in a direction indicated by an arrow in a transmission electron microscope.

[0012]

However, according to the conventional method as described above, when a transmission electron microscope sample is manufactured using the FIB, the G used for the FIB processing is reduced.
There was a drawback that damage was caused or an altered layer was formed by the a ion. In addition, there is a disadvantage that transmission of an electron beam is hindered by the damage and the deteriorated layer, and high-quality image observation cannot be performed. Specifically, it is not possible to observe a lattice image of the closest atomic distance (d = 2.35 °).

Further, in the sheet resistance measuring method, the thermal wave method and the secondary ion mass spectrometry, the spatial resolution at the time of measuring the ion implantation dose distribution is low, and the two-dimensional distribution of the ion implantation distribution cannot be measured. For example, the spatial resolution is about 500 mm in sheet resistance measurement, and about 1 mm in thermal wave method and secondary ion mass spectrometry. This value is completely insufficient as a method for evaluating an impurity-doped layer formed by ion implantation having a two-dimensional spatial distribution within 1 mm.

In view of the foregoing, the present invention provides a method for improving the image quality of an electron microscope when a transmission electron microscope sample is prepared by using FIB or ion milling, and also improves the spatial resolution of ion implantation dose distribution measurement. An object of the present invention is to provide a method for measuring a two-dimensional distribution of an ion implantation amount to be performed.

[0015]

According to the present invention, there is provided a method of preparing a sample for a transmission electron microscope, comprising the steps of forming an anodic oxide film on a sample and removing the anodic oxide film. is there.

Further, there is provided a method for preparing a sample of a transmission electron microscope, wherein an anodic oxide film is formed on the sample.

Further, there is provided a transmission electron microscope sample preparation method including a step of forming an anodic oxide film on a sample and a step of removing the anodic oxide film, and involving focused ion beam processing.

Further, the present invention is a method for preparing a sample of a transmission electron microscope, which includes a step of forming an anodic oxide film on a sample and a step of removing the anodic oxide film, and involves ion milling.

According to the present invention, as described in the above means, when a sample is thinned by a focused ion beam processing or an ion milling method in a sample manufacturing method of a transmission electron microscope,
At the finishing stage, the surface of the sample is scraped by adding a step of forming an anodic oxide film and a step of removing the anodic oxide film.

The anodic oxidation method described here forms a high-quality anodic oxide film at room temperature in an aqueous solution. Thereafter, when the anodic oxide film is removed, the sample can be scraped off without any crystallographic damage. That is, it is possible to remove the defect layer and the altered layer introduced by the focused ion beam processing or the ion milling method.

Since the thickness of the anodic oxide film is proportional to the concentration of the impurity introduced, an anodic oxide film having a thickness proportional to the amount of the impurity introduced by ion implantation is formed. Further, by removing the anodic oxide film, the sample substrate can be scraped off by a thickness proportional to the amount of impurities introduced by ion implantation.
After that, when observed by a transmission electron microscope, interference fringes of equal thickness are observed according to the thickness of the anodic oxide film and the thickness of the sample substrate. The two-dimensional distribution of the introduced impurities can be estimated from the equal thickness interference fringes.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention, in which an anodic oxide film is formed and removed to prepare a transmission electron microscope sample, will be described.

Here, a silicon substrate (p-Si (100) 10 to 15Ω) on which a thermal oxide film having a thickness of 100 nm is formed.
Cm), a sample subjected to P (phosphorus) ion implantation (acceleration energy: 150 keV, implantation amount: 3 × 10 ¹⁵ cm ⁻² ) from above, and subjected to a short-time heat treatment at 950 ° C. for 10 seconds was used.

FIG. 1 is a diagram for explaining a method of manufacturing a transmission electron microscope sample by a process of forming an anodic oxide film on a sample, a process of removing the anodic oxide film, and a process of performing focused ion beam processing. . FIG. 1A shows a defect (indicated by X) at a specific position on a semiconductor substrate. FIG.
(B) shows that this sample is cut down to a width of 200 mm by mechanical polishing using an abrasive. FIG.
(C) shows that the vicinity of the X mark was cut off by the FIB processing apparatus to reduce the thickness to 300 nm to 500 nm. FIG. 1 (d) shows that this sample is
It shows that it was rotated by degrees. FIG. 1E shows that an anodized film was formed on the sample surface by the jig shown in FIG. FIG. 1F shows that the anodic oxide film has been removed. At this time, the silicon substrate having a thickness of about 50 nm receded. Thereafter, it is also a diagram showing that a sample is observed by transmitting an electron beam in a transmission electron microscope in a direction indicated by an arrow.

FIG. 2 shows a first embodiment of the present invention.
FIG. 2 is a schematic structural view of a reaction cell used for forming an anodic oxide film.
You. In FIG. 2, reference numeral 20 denotes a cell (container), and reference numeral 21 denotes electrolysis.
Liquid, 22 is a sample holding bracket, 23 is an anode (sample), 24 is
Cathode (platinum), 25 is ammeter, 26 is voltmeter, 27 is constant
It is a voltage constant current source. As shown in FIG.
The sample was placed in the electrolyte 21. The electrolyte 21 is N-
Methylacetamide (CH _ThreeCONHCH_Three/ 300c
c) + potassium nitrate (KNO_Three/0.6g) + water (H_TwoO
/ 2 cc). Anodizing is current tight
Degree 7mA / cm^TwoUnder the constant current conditions. Between cathode and anode
In the case of a voltage rise value of 200 V, anodic acid having a thickness of about 100 nm
An oxide film was formed. Next, the anodic oxide film is formed on_FourF
Mixed solution (HF / NH_FourF = 1: 10)
Was. At this time, the silicon substrate having a thickness of about 50 nm recedes.
Was.

As in this embodiment, when a sample of a transmission electron microscope is manufactured, a specific portion of the sample is thinned using FIB, and then a damaged layer on the sample surface is formed by forming and removing an anodic oxide film. Shaving off enables higher quality image observation during image observation with a transmission electron microscope. That is, it became possible to observe a lattice image of the closest atomic distance (d = 2.35 °).

Next, a description will be given of a second embodiment of the present invention, in which an anodized film is formed to produce a transmission electron microscope sample. Here, a silicon substrate (p-Si (100) 10 to 15) on which a thermal oxide film having a thickness of 100 nm is formed.
A sample subjected to P (phosphorus) ion implantation (acceleration energy: 150 keV, implantation amount: 3 × 10 ¹⁵ cm ⁻² ) from above, and subjected to a short-time heat treatment at 950 ° C. for 10 seconds was used.

FIG. 3 shows a method of producing a transmission electron microscope sample by performing a step of forming an anodic oxide film on a sample, a step of removing the anodic oxide film, and a step involving focused ion beam processing. FIG. FIG. 3A shows a defect (indicated by X) at a specific position on the semiconductor substrate.
FIG. 3B shows that this sample is cut to a width of 200 mm by mechanical polishing using an abrasive. FIG.
(C) shows that the vicinity of the X mark was cut off by the FIB processing apparatus to reduce the thickness to 300 nm to 500 nm. FIG. 3D shows that the sample is moved 90 degrees in the front direction.
It shows that it was rotated by degrees. FIG. 3E shows that an anodic oxide film was formed on the sample surface. Here, the method of forming the anodic oxide film is the same as that of FIG. 2 used in the description of the first embodiment of the present invention. At this time, the thickness is about 10
A nm anodic oxide film was formed. FIG. 3F shows that a sample is observed by transmitting an electron beam in a transmission electron microscope in a direction indicated by an arrow while the anodic oxide film is formed.

Next, the fact that the two-dimensional distribution of the ion implantation amount can be measured according to the second embodiment of the present invention will be described with reference to FIG. 4, reference numeral 40 denotes a gate electrode portion of a MOS transistor, 41 denotes a drain or source portion formed by ion implantation, and 42 denotes an anodic oxide film formed in FIG. 43 is an anodized film 42
Forming the drain or source portion 4
This indicates that the anodic oxide film thickness changed according to the ion implantation amount of 1. When the amount of ion implantation is converted into the thickness of the oxide film as described above, an equal thickness interference fringe is observed when observed using a transmission electron microscope. By comparing with a part measured by (SIMS), a detailed secondary distribution of the ion implantation amount can be known. FIG. 5 shows the second embodiment of the present invention.
An example measured by the method described in the embodiment will be described. Here, the arrow in FIG. 5 indicates the direction in which the secondary ion mass spectrometry was performed in this direction and the secondary distribution of the ion implantation amount was measured.

According to the second embodiment of the present invention, when fabricating a sample for a transmission electron microscope, an anodic oxide film is formed on the sample to reduce the amount of ion implantation to a certain extent. By observing with a transmission electron microscope by converting to the above, it became possible to know the secondary distribution of the ion implantation amount by observing the interference fringes of equal thickness. Although the example using the FIB has been described here, the same applies to the case where the ion milling is used.

Next, an example in which an anodic oxide film according to the third embodiment of the present invention is formed to prepare a transmission electron microscope sample will be described. Here, a silicon substrate (p-Si (100) 10 to 10) having a thermal oxide film having a thickness of 100 nm is formed.
A sample subjected to P (phosphorus) ion implantation (acceleration energy: 150 keV, implantation amount: 3 × 10 ¹⁵ cm ⁻² ) from above and a short-time heat treatment at 950 ° C. for 10 seconds was used.

FIG. 6 is a diagram for explaining a method of manufacturing a transmission electron microscope sample by a process of forming an anodic oxide film on a sample, a process of removing the anodic oxide film, and a process of performing focused ion milling. It is. FIG. 6A shows that a semiconductor substrate on which a semiconductor device is formed is stuck face to face. Here, the bonding is performed using a general epoxy resin. FIG. 6B is an enlarged view of FIG. In FIG. 6B, as an example of the semiconductor device, a flash memory type including a large number of rows of gate electrodes is used. Here, 60 is a protective film, 61 is a gate electrode portion,
62 is a diffusion layer. FIG. 6C shows that the sample was hollowed out to a diameter of about 3 mm when the sample was prepared for the transmission electron microscope. FIG. 6D is a diagram of the sample viewed from the side. At this stage, the diameter is about 3 mm and the thickness is 50
mm by mechanical polishing.

FIG. 6E shows that the sample was thinned by an ion milling method (Gatan: 600 type) using argon ions. FIG. 6F shows that an anodic oxide film was formed on the sample surface. Here, the method of forming the anodic oxide film is the same as that of FIG. 2 used in the description of the first embodiment of the present invention. At this time, the thickness is about 10
An anodic oxide film having a thickness of 0 nm was formed. FIG. 6 (g) shows that the anodic oxide film has been removed. At this time, the thickness is about 5
The 0 nm silicon substrate receded.

According to the third embodiment of the present invention, when preparing a sample for a transmission electron microscope, the sample is thinned by ion milling, and then the surface of the sample is damaged by forming and removing an anodic oxide film. The removal of the layer enables higher quality image observation when observing an image with a transmission electron microscope. That is, the nearest atomic spacing (d = 2.35)
Observation of the lattice image of Å) became possible.

Although the anodic oxide film is formed and removed in an aqueous solution in the embodiment of the present invention, almost the same results can be obtained by using plasma anodic oxidation performed in a vacuum.

[0036]

According to the present invention, it is possible to improve the image quality of an electron microscope when a transmission electron microscope sample is manufactured by using FIB or ion milling. Further, the spatial resolution of the ion implantation dose distribution measurement is improved, and the two-dimensional distribution of the ion implantation dose can be measured, and the industrial effect of the present invention is great.

[Brief description of the drawings]

FIG. 1 is a diagram showing a sample preparation process of a transmission electron microscope used in a first embodiment of the present invention.

FIG. 2 is a schematic structural diagram of an anodic oxidation cell used in the first embodiment of the present invention.

FIG. 3 is a view showing a sample preparation process of a transmission electron microscope used in a second embodiment of the present invention.

FIG. 4 is a diagram illustrating anodic oxidation in a cross section of a semiconductor device according to a second embodiment of the present invention;

FIG. 5 is a diagram illustrating an example of a measurement result of a two-dimensional distribution of an ion implantation amount obtained according to a second embodiment of the present invention.

FIG. 6 is a view showing a sample preparation process of a transmission electron microscope used in a third embodiment of the present invention.

FIG. 7 is a diagram showing a sample preparation process of a conventional transmission electron microscope.

[Explanation of symbols]

 Reference Signs List 20 cell (container) 21 electrolytic solution 22 sample holding fixture 23 anode (sample) 24 cathode (platinum) 25 ammeter 26 voltmeter 27 constant voltage constant current source 40, 61 gate electrode part 41 drain part or source part 42 anodic oxide film 43 Thickness change of anodized film 60 Protective film 62 Diffusion layer

Claims (4)

[Claims] A step of forming an anodic oxide film on a sample;
Removing the anodic oxide film. 2. A method for preparing a sample for a transmission electron microscope, comprising forming an anodic oxide film on a sample. 3. The method for preparing a sample for a transmission electron microscope according to claim 1, further comprising focused ion beam processing. 4. The method for preparing a sample for a transmission electron microscope according to claim 1, further comprising an ion milling process.