JP2790109B2 - Sample preparation method for component analysis from the back side - Google Patents

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 an analytical sample, and more particularly, to a sample in which a thin film made of a different material is laminated on a substrate, the concentration distribution of elements in the thin film in the depth direction is measured on the substrate side. The present invention relates to a method for preparing an analytical sample in which a substrate is made into a thin film by mechanical polishing and a chemical technique in order to investigate.

[0002]

2. Description of the Related Art Secondary ion mass spectrometry (Secondary Io)
It is known that the concentration distribution in the depth direction of an element measured by n Mass Spectroscopy (hereinafter referred to as “SIMS”) is as high as ppb order or less. On the other hand, Auger electron spectroscopy (Auger Electron Spectroscopy;
AES), X-ray photoelectron spectroscopy has a detected element concentration of several%.
About sensitivity. For this reason, SIMS measurement has become important in response to the decrease in the scale of semiconductor devices such as impurity diffusion regions of field effect transistors that become increasingly shallow. SIMS uses a primary ion beam of a predetermined energy to sputter the surface of a sample such as a semiconductor and analyzes the mass of secondary ions emitted from the sputtered surface, thereby calculating the type and concentration of elements near the sample surface. At the same time, it is possible to measure the profile of impurities in the depth direction of the sample.

Hitherto, SIMS has been studied from the substrate surface side. However, some problems arise. These problems are, for example, as described in the following items 1) and 2).

1) In the case of a sample composed of several elements such as a compound semiconductor, selective sputtering occurs and the surface is roughened, so that the depth resolution is deteriorated.

2) The element to be measured is pushed by the incident ion beam, and an accurate concentration distribution cannot be obtained (referred to as "knock-on effect").

In order to solve these problems and further increase the resolution in the depth direction, there has been proposed a method of measuring from the back side of the substrate. In this case, it is required that the surface be as thin as possible from the substrate side and that the newly exposed surface be a flat surface.

[0007] In this experimental example, the diffusion of ohmic electrode metal on a GaAs substrate, which is a problem in a high electron mobility transistor (HEMT), into the substrate is most often examined from the back surface. This is a method that has solved both the problems 1) and 2). For example, RTLareau et al.
/ August, Journal of Vacuum Science and Technology, Volume A-5, Issue 4, page 1503 (Journal of Vacuum Science and Technology, A5
(4), P.1503, Jul./Aug., 1987)).
The diffusion of the Au electrode formed on the As surface is measured from the back side of the substrate, and from the result, it is discussed whether the diffusion can be prevented by the barrier. This measurement sample is thinned from the substrate side by mechanical polishing and plasma etching.

[0008] Yoshida has proposed a method in Japanese Patent Application Laid-Open No. 6-194285 for examining the distribution of elements in the depth direction in a thin film laminated on a substrate. That is, in the same publication, an intermediate layer inactive against an etchant of a substrate is inserted between a thin film to be analyzed and a substrate, and only the substrate is removed to obtain a thin film. A thin film analysis method has been proposed in which the performance is improved and the decrease in the resolution in the depth direction is suppressed. According to this method, the intermediate layer is hardly removed when the substrate is removed by chemical etching, and the intermediate layer is thinned. Further, when the intermediate layer is removed by ion sputtering, a back surface of the thin film is obtained. At this time, in order to reduce the displacement due to the stress of the thin film portion, the sputtering is performed while leaving the peripheral end portion (that is, the central portion is flatly sputtered while the periphery of the intermediate layer is left). Thereby, the rigidity of the thin film sample is maintained, and the stress can be reduced.

[0009]

In the conventional method for measuring the concentration distribution in the depth direction of an element from the back side of a sample, as described above, the element is attached to a supporting substrate with an adhesive and mechanical polishing and etching are combined. A method of processing a sample is general. However, these methods have several problems, as described below.

1) In order to facilitate sample processing, usually, 1
A sample as large as about 0 × 10 mm ² is used. Therefore, the thinned sample is distorted due to a large difference in the distribution intensity of the stress applied thereto. The stress at this time includes an internal stress due to the adhesive and a stress generated due to a difference in thermal expansion coefficient between the supporting substrate and the sample due to heat treatment during bonding. As a result, the resolution in the depth direction is deteriorated in the SIMS measurement.

2) An epoxy resin often used as an adhesive having a strong adhesive strength is corroded by an acid-based etchant. Then, the sample surface side protected by this is also etched, and the flatness of the thin film cannot be maintained. As a result, the depth resolution of the SIMS is reduced.

3) In the method proposed in JP-A-6-194285, in which an intermediate layer is provided and etched,
Finally, the thin film is given rigidity except for the peripheral end. This method is often used when thinning a measurement sample.
However, the remaining peripheral end hinders the path of the incident ion beam during SIMS measurement. When the mass spectrometer is of the sector type, an electric field is generated near the sample to apply a bias to the sample, but the distribution of this potential is disturbed due to the remaining peripheral edge, so that the incident ion Along with the beam, the course of the secondary ions also changes. By these,
Problems such as an increase in background in SIMS measurement and a decrease in depth direction resolution occur.

Therefore, the present invention solves the above problems,
An object of the present invention is to provide a method for preparing an analysis sample, in which a sample is processed by mechanical polishing and chemical etching so that the concentration distribution of elements in the depth direction can be measured from the back surface by secondary ion mass spectrometry.

[0014]

In order to achieve the above object, the present invention provides a sample comprising a substrate material and a thin film made of a different material laminated on a substrate material. In the method of removing the substrate material and analyzing from the back surface, the sample to be analyzed is attached to the supporting substrate with an adhesive, and when the substrate material is removed by polishing and chemical etching, the adhesive is not corroded by a chemical etchant. Thus, a method for preparing an analysis sample characterized by being protected by a predetermined member is provided.

Further, the present invention is characterized in that the size of the sample to be analyzed is approximately 5 × 5 mm ² or less.

Further, the present invention is characterized in that the adhesive is surrounded by wax which is not corroded by the etchant.

That is, as a method for solving the problems described in the above items 1) and 3), SIMS measurement is theoretically possible if there is a sample having a size of at least 500 × 500 μm ² . Therefore, the size of the sample was reduced to 10 ×
Make it sufficiently smaller than about 10 mm ² . Specifically, a sample of about 3 × 3 to 5 × 5 mm ² is preferably used so that a sufficient number of measurements can be ensured to confirm reproducibility. As a result, the difference in stress distribution strength is reduced, and the effect of stress on the sample can be reduced. In addition, the stress applied to the thin film sample can be sufficiently prevented without leaving the peripheral end portion, and a flat surface can be obtained.

As a method for solving the above item 2), the periphery is surrounded with wax to protect the adhesive from the etchant. At this time, a wax that does not corrode by the etchant is used.

[0019]

According to the present invention, the influence of stress on a thin film sample is reduced by attaching a sample of a size small enough to reduce stress to a supporting substrate having a small difference in thermal expansion coefficient from the sample. Therefore, the flatness of the thin film surface after the sample substrate processing can be maintained without leaving the peripheral end.

Further, at the time of chemical etching for removing the substrate, the adhesive is surrounded by wax, and the adhesive is protected from corrosion by the etchant, so that the thin film sample can be left without being broken from the sample surface side.

As a result, the resolution in the depth direction at the time of SIMS analysis can be improved with high accuracy. Also, the background can be reduced because the peripheral end is not left.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings.

FIG. 1 is a diagram showing a sample processing process according to the present invention in the order of steps.

The sample is such that a thin film 1 made of a different material is laminated on a substrate 2 (see FIG. 1A).

This sample is attached to the support substrate 4 with the epoxy resin 3 with the surface of the thin film 1 facing down (see FIG. 1B). At this time, the adhesive 3 protruding from the sample is scraped off (see FIG. 1C). Further, the size of the sample is reduced to about 5 × 5 mm ² so as to reduce the difference in stress distribution strength.

Thereafter, the sample 5 is first mechanically polished with coarse particles to about several tens μm. Further, mirror polishing is performed with fine particles to obtain a mirror surface (see FIG. 1D).

Next, in order to protect the epoxy resin 3 and the thin film sample 1 from the etchant, the periphery is surrounded by wax 6 (see FIG. 1E). At this time, the wax 6 is not applied to the substrate 2 so as not to leave the peripheral end.

Then, the completed sample 7 is immersed in a beaker 9 filled with an etchant 8 having a high selectivity of the substrate 2 to the thin film 1 and chemically etched (FIG. 1).
(F)). At this time, the etchant 8 having the highest selectivity is selected.

As a result of this etching, only the thin film 1 remains (see FIG. 1 (G)). Finally, wax with alcohol 6
Is dissolved and removed (see FIG. 1 (H)). This is SIM
A thin film sample having a flat surface suitable for S measurement is obtained.

[0030]

Embodiments of the present invention will be described below. FIG.
FIG. 3 is a view showing a material of a compound semiconductor used for a laser, a light emitting diode, and the like. As shown in FIG. 2, the sample was InP
A multiple quantum well structure (MQW) 203 sandwiched between InGaAsP layers 202 and 204 is laminated on a substrate 201. The MQW 203 has a configuration (laminated structure of InGaAs and InGaAsP) shown enlarged on the right side of FIG. Further, the In on the MQW 203
On the GaAsP layer 204, the InP layer 205 is
It is epitaxially grown with n. Using this sample, processing was performed according to the process shown in FIG.

More specifically, the sample shown in FIG.
An epoxy resin (petropoxy 154 (Palouse Petro Products) is applied to a Si substrate (4.15 × 10 ⁻⁶ K ⁻¹ ) having a small difference in thermal expansion coefficient from P (thermal expansion coefficient: 4.5 × 10 ⁻⁶ K ⁻¹ ).
)). Then, after mechanically polishing the sample, the sample is chemically etched using a phosphoric acid: hydrochloric acid = 1: 1 solution having a high selectivity of InP to InGaAsP. At this time, since the petropoxy was corroded by this etchant, it was surrounded by electron wax. InGa
When the etching proceeds to the AsP layer and a mirror surface is visible, it is removed. After that, when the electron wax is dissolved and removed by ultrasonic cleaning with alcohol, a sample for SIMS measurement is completed.

FIG. 3 shows the flatness of the surface of the thin film sample for measurement after the chemical etching. The flatness of the surface is measured using a stylus type surface roughness meter (ALPHA-STEP 20 manufactured by Tencor Instrument).
0) was measured. From the measurement results in FIG. 3, it is understood that the flatness of the thin film sample surface is maintained.

FIG. 4 shows, as a comparative example, the state of the back surface of the thin film sample when the electron wax for protecting petropoxy was not used. The scale in the depth direction is
10 times that of FIG. The adhesive is eroded by the etchant, and the surface InP protected by the adhesive is
The layer is etched. As a result, the thin film is broken. As shown in FIG. 4, large holes exceeding 1 μm are observed.

FIG. 5 shows the result of measurement of the thin film sample prepared according to the present embodiment by SIMS (CAMECA3f) from the back side. In the As + Cs ⁻ ion profile, there is a portion where the intensity increases after a certain intensity and stabilizes. This is from the InGaAsP layer to the MQW InGaAs
It indicates that it has moved to the s layer. That is, I on the substrate side
It can be confirmed that the etching has stopped at the nGaAsP layer 202 (see FIG. 2).

On the other hand, in the Zn profile, first, In
Diffusion into the GaAsP layer (substrate side) 202 and the MQW 203 is observed, and thereafter, the MQW 203 / InGaAsP
The intensity of Zn at the interface of the layer (surface side) 204 increases sharply. This indicates that the measurement was performed with good resolution in the depth direction. From this, it can be seen that the surface of the thin film sample is flattened without being distorted due to stress or the like when the thin film sample is attached to the Si support substrate.

FIG. 6 shows the same sample for comparison.
This is a result of examining the concentration distribution from the back surface when a thin film treatment is performed with a size of × 10 mm ² and a peripheral end portion left. After the rising oxide peak in the Zn profile,
It can be seen that the background has not decreased compared to FIG. This is a result of the peripheral end disturbing the electric field near the sample. From this, it can be seen that the background can be reduced in the sliced sample without leaving the peripheral end.

On the other hand, the result of SIMS measurement from the surface side of the same sample is shown in FIG. 7 as a comparative example. In this case, as shown in FIG. 7, as a result of being pushed by the incident ion beam, the diffusion distribution of Zn is wider than that in FIG. Therefore, the profile of Zn at the interface between the InP layer and the InGaAsP layer on the surface side is distorted, and the interface is not clear.

[0038]

As described above, according to the present invention,
The sample for measurement from the back surface of the concentration distribution in the depth direction of the element has remarkable effects as described in the following items.

1) By using a supporting substrate having a small difference in thermal expansion coefficient from the sample, the influence of stress from the supporting substrate due to heat treatment during bonding is reduced.

2) By making the measurement sample small enough to sufficiently reduce the stress, the flatness of the thin film sample can be maintained without leaving a peripheral end portion.

3) By protecting the periphery of the adhesive with wax, corrosion by a chemical etchant can be prevented.

As a result, according to the present invention, the flatness of the surface of the thin film sample after the chemical etching is maintained. And
SIMS measurement can be performed without deteriorating the resolution in the depth direction. Further, since there is no peripheral end, the background can be reduced and the amount of secondary ions can be sufficiently measured.

[Brief description of the drawings]

FIG. 1 is a sectional view of a sample for explaining a sample processing method according to the present invention in the order of steps.

FIG. 2 is a diagram showing a configuration of a sample used in an example of the present invention.

FIG. 3 is a profile showing a surface shape of a thin film sample processed according to an example of the present invention.

FIG. 4 is a profile showing the surface shape of a thin film when etched without protecting the adhesive with wax (Comparative Example).

FIG. 5 is a profile of a SIMS measurement result from the back surface of a sample prepared according to an example of the present invention.

FIG. 6 is a profile of a SIMS measurement result when a sample is subjected to thin film processing with a size of 10 × 10 mm ² and a peripheral end portion left, for comparison.

FIG. 7 is a profile of a SIMS measurement result measured from a surface for comparison.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Thin film laminated on substrate 2 Substrate 3 Epoxy resin adhesive 4 Support substrate 5 Whole sample adhered to support substrate 6 Wax 7 Whole etching sample 8 Etchant 9 Beaker

Claims (5)

(57) [Claims] 1. A method comprising: analyzing a distribution of a component concentration in a thin film of a sample obtained by laminating a thin film made of a different material on a substrate material from the back surface by removing the substrate material; The analysis sample is attached to a supporting substrate with an adhesive, and when the substrate material is removed by polishing and chemical etching, the adhesive is protected by a predetermined member so as not to be corroded by a chemical etchant. How to make an analysis sample. 2. The method according to claim 1, wherein the size of the sample to be analyzed is about 5 × 5 mm ² or less. 3. The method according to claim 1, wherein the adhesive is surrounded by a wax which is not corroded by the chemical etchant. 4. The method according to claim 1, wherein the sample to be analyzed is InG on an InP substrate.
The analysis sample according to claim 1 or 2, wherein the sample is a compound semiconductor in which an aAsP-based thin film is laminated, Si is used as the support substrate, and electron wax is used as the member for protecting the adhesive. How to create 5. A sample formed by laminating a thin film made of a different material on a substrate, using mechanical polishing and chemical polishing for analysis for examining the concentration distribution of elements in the thin film in the depth direction from the substrate side. In the method of preparing a sample to be sliced by a physical treatment, after attaching the sample to a support substrate with an adhesive, by surrounding the joint between the support substrate and the sample with a member for protecting the adhesive, When the substrate of the sample is removed by chemical etching, the adhesive is protected from corrosion by an etchant, and the sample, from which the substrate is removed by chemical etching, remains on the sample surface without leaving its peripheral end. A method for preparing an analysis sample, characterized in that flatness is maintained.