JP3166152B2 - Impurity measurement method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an impurity measuring method for analyzing impurities in a semiconductor substrate by using secondary ion mass spectrometry (SIMS), and more particularly to a method for measuring the distribution of impurities to be measured existing in an extremely shallow region. The present invention relates to a method for measuring impurities that can be measured with high accuracy.

[0002]

2. Description of the Related Art In recent years, as semiconductor devices have become finer and faster, it has become important to accurately evaluate the concentration distribution in the depth direction of impurities present in an extremely shallow region of the surface. 2. Description of the Related Art Depth direction analysis of impurities in a semiconductor by secondary ion mass spectrometry is widely used. here,
Secondary ion mass spectrometry is a method in which a sample surface is sputtered with primary ions, and secondary ions that are ionized among the sputter particles that jump out of the sample surface into a vacuum are separated and detected by a mass analyzer. .

However, in secondary ion mass spectrometry, a certain time is required from the irradiation of a sample with primary ions until the secondary ions are stably emitted. Accuracy is significantly reduced.

Further, the secondary ion mass spectrometry has a problem that the secondary ion yield is different when the composition of the main component is different. For example, when analyzing the surface of a silicon substrate,
The accuracy of the profile is also significantly reduced due to the influence of the natural oxide film existing on the outermost surface.

Therefore, as a method for solving this problem,
There is known an oxygen leak method in which measurement is performed using oxygen primary ions in a state where oxygen gas is introduced into a sample chamber of a secondary ion mass spectrometer. In this oxygen leak method, the impurity concentration from the outermost surface of the sample is accurately measured by increasing the oxygen concentration on the surface of the sample and stably generating secondary ions from the surface of the sample. In addition, the introduction of oxygen gas increases the oxygen concentration on the sputtered surface, so that there is no difference in sensitivity between the measured impurity and silicon due to the influence of the natural oxide film on the outermost surface.

However, as shown in the graph of FIG.
In addition, the accelerating voltage of 1 keV and 2 keV is applied to the silicon surface.
1 x 10^Fifteencm^-2Implant boron under the conditions
When the sample is measured by secondary ion mass spectrometry,
Ron concentration is 1 × 10¹⁸atoms / cm ^-3Is the oxygen depth
Is almost the same with and without
On the other hand, acceleration voltage 0.2 keV, dose 1 × 10^Fifteencm
^-2The sample implanted with boron under the conditions
When measured by mass spectrometry, the boron concentration is 1 × 10
¹⁸atoms / cm^-3Is measured by oxygen leak.
Results shall be about 3 nm shallower than when not performed
I understood. From the above results, the second method using oxygen leak
When performing secondary ion mass spectrometry measurement,
The sputtering rate is high, so use the oxygen leak method.
Secondary ion mass spectrometry measurement
It is difficult to accurately measure impurity profiles
You can see that

As a method for solving this problem, for example, Japanese Unexamined Patent Publication No. Hei 7-181149 (hereinafter referred to as Conventional Example 1) discloses that a cap layer is formed on a semiconductor substrate or a natural oxide film on the surface of a semiconductor layer formed on the substrate. A method in which the formed sample is placed in a secondary ion mass spectrometry measurement chamber, oxygen is introduced into the measurement chamber, and elemental analysis of the semiconductor surface is performed by secondary ion analysis and a depth-direction concentration distribution in a shallow region from the surface is measured. Is disclosed.

In Japanese Patent Application Laid-Open No. 10-90205 (hereinafter referred to as Conventional Example 2), when an impurity in a depth direction of a sample having an oxide film on a substrate is measured by an oxygen leak method, an oxide film is not used. Using secondary ion mass spectrometry to segregate at the interface between the substrate and the oxide film by introducing an auxiliary impurity consisting of an alkali metal or a compound containing an alkali metal on the surface and then measuring, and specifying the interface position of the substrate A method for measuring impurities is disclosed. In this method for measuring impurities, the position of the interface can be easily specified for a sample having an oxide film on the substrate, and the influence of oxygen leak affects the depth accuracy, as shown in FIG. In a region where the influence is not exerted (a depth of 39 nm or more in FIG. 9), the sputtering rate is stabilized in the oxide film, and thus the crater generated when performing the secondary ion mass spectrometry measurement to the oxide film thickness and the substrate. It is possible to accurately measure the concentration distribution in the depth direction in the substrate if the depth of the substrate and the position where the alkali metal or the like is piled up are known.

Further, Japanese Patent Application Laid-Open No. Hei 8-96740 (hereinafter referred to as Conventional Example 3) discloses, as a pretreatment of a sample, an oxide film layer on the surface of a semiconductor substrate to be analyzed is removed by wet chemical etching. There is disclosed a secondary ion mass spectrometry in which a surface coating layer formed of a metal thin film is formed by charging a plating solution.

[0010]

The prior art has the following problems.

(1) In general, the sputtering rate in SIMS measurement in a region 30 nm deep from the surface of the cap layer is higher than the sputtering speed in a region deeper than 30 nm, and "secondary ion mass spectrometry" ( As described in Wiley & Sons, 1989), amorphous silicon or polysilicon and single crystal silicon S
It is known that the sputtering rate in the IMS measurement has a difference of about 9% depending on the measurement conditions.

Therefore, as in Conventional Example 1, a cap layer is simply formed on a substrate to be measured, and the cap layer is formed by an oxygen leak method.
The effect of the difference in the sputtering rate is not taken into account only by performing the MS measurement, so that the depth accuracy of the obtained depth profile is not necessarily high. Further, in the conventional example 1, it is described that a polysilicon film or a single crystal silicon film is used for the cap layer (paragraph number 00).
13). However, when SIMS measurement is performed using a cap layer of a polysilicon film, the grain size and the crystal orientation of the polysilicon are non-uniform, so that the depth resolution of the impurity profile in the underlying semiconductor substrate is significantly reduced. In addition, when the single crystal silicon film is used as an epi film, a high-temperature heat treatment (a temperature at which an impurity profile changes by diffusion) is required to remove an oxide film on a substrate surface for epi growth.

(2) In Conventional Example 2, when measuring a sample in which an oxide film is not formed on a substrate in advance, the uniformity of the film thickness must be maintained under the condition that the profile of the impurity to be measured does not change due to thermal diffusion. It is difficult to form an oxide film having a high density. For example, as a method for forming an oxide film on a substrate at a temperature of 600 ° C. or lower, there is an atmospheric pressure CVD method. In this method, a reaction is performed under normal pressure and the deposition rate of the oxide film is high, so that the vicinity of the inner surface of the substrate is high. When the gas flow rate (ratio) varies, the oxide film thickness in the substrate varies. Therefore, the method of Conventional Example 2 cannot be applied to measurement of a sample in which an oxide film is not formed on a substrate in advance.

(3) In the conventional example 3, a metal film is used as the cap film. However, since the grain size of the metal is not uniform like a semiconductor, when the metal film is sputtered by SIMS measurement, the sputtering becomes uneven. This adversely affects the depth accuracy of the impurity profile.

An object of the present invention is to solve the above-mentioned problems, and can be applied to measurement of a sample in which an oxide film is not formed on a substrate in advance, and is affected by a region where the sputtering speed is high in the initial stage of measurement. It is an object of the present invention to provide an impurity measuring method capable of accurately measuring an impurity to be measured in the vicinity of the surface of a sample without using the method.

[0016]

An impurity measuring method according to the present invention is a method for measuring impurities in a sample using secondary ion mass spectrometry. Forming an auxiliary impurity layer made of an element having a different mass number from the impurity to be measured; forming a silicon film on the surface of the auxiliary impurity layer; Measuring the concentration distribution of the auxiliary impurity in the depth direction.

The depth of the concentration distribution in the depth direction of the impurity to be measured in the above sample depends on the depth of the crater, the silicon film thickness, and the profile of the auxiliary impurity generated by measuring the sample after forming the silicon film. Required based on.

[0018] The impurity measuring method of the present invention further comprises: adhering an auxiliary impurity comprising an element having a different mass number from the measured impurity to the surface of the sample having the measured impurity present on the surface;
A step of forming a silicon film on the surface of the sample, a first measurement step for determining the thickness of the silicon film, and performing SIMS measurement from the surface of the silicon film to the inside of the sample to obtain a secondary ion of the impurity to be measured and the auxiliary impurity. A second measurement step for obtaining the intensity distribution and the crater depth, and a difference between the crater depth obtained in the second measurement step and the thickness of the silicon film obtained in the first measurement step; In the depth direction of the impurity to be measured in the sample, the measurement time is converted into the measurement depth from the difference between the time indicating the maximum value of the secondary ion intensity of the auxiliary impurity obtained in the measurement step 2 and the measurement end time. Measuring the concentration distribution.

In the first measurement step, the SIMS measurement is performed from the surface of the silicon film to the time when the secondary ion intensity of the auxiliary impurity reaches the maximum value, and the depth of the crater generated by the SIMS measurement is measured. The thickness of the film may be determined.

The temperature for forming the silicon film is preferably not higher than the temperature at which the profile of the impurity to be measured does not change.

The silicon film is formed by a CVD method,
This is one of the sputtering method and the vapor deposition method.

The silicon film is an amorphous silicon film or a sputtered silicon film.

The present invention is preferably carried out by an oxygen leak method in which oxygen is introduced into a sample chamber in which a sample is placed for measurement.

According to the present invention, by forming a silicon film on a sample, the sputtering rate accompanying the primary ion irradiation is stabilized in the silicon film due to the influence of oxygen leak. Unaffected by change. Therefore, the measurement of the concentration distribution of the impurity to be measured and the total dose can be performed with high accuracy.

The difference between the sputtering rates of the silicon film and the sample is corrected using the silicon film thickness obtained by performing the first measurement step in advance.

[0026]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the drawings. (First Embodiment) FIGS. 1A and 1B are cross-sectional views for explaining a manufacturing process of a measurement sample used in a first embodiment of the present invention.

First, as shown in FIG.¹¹B
Dose 1 × 10 at 0.2 keV^Fifteencm ^-2(Setting the injection device
Value), heat-treated silicon after ion implantation
A few atomic layers of boron 2 are deposited on the surface of the substrate 1.

Next, as shown in FIG. 1B, an amorphous silicon film 3 is formed on the silicon substrate 1 at a temperature at which boron is not diffused to a thickness of 150 nm (a set value of a growth apparatus).
Prepare a measurement sample.

Next, the secondary ion intensity of ¹⁰ B is measured by the secondary ion mass spectrometry until the time when the sample reaches the peak top. FIG.
It is a graph which shows the measurement result.

As shown in FIG. 2, ¹⁰ B
By the depth secondary ion intensity distribution 4 using oxygen primary ion and oxygen leak method SIMS measurement, ^{1 0 B} of secondary ion intensity time reaches a peak top of the (A point in FIG. 2 ). Then, the depth of the crater generated by the measurement is measured by a stylus type surface roughness meter, and the thickness t0 of the amorphous silicon film 3 is obtained. As a result, t0 was 151 nm.

Next, a portion of the sample for measurement, which is approximately 1 mm away from the portion where the secondary ion mass spectrometry measurement was performed, was subjected to secondary ion mass spectrometry measurement to obtain ¹⁰ B and ¹¹ B secondary ions in the depth direction. Measure strength. FIG. 3 is a graph showing the measurement results.

As shown in FIG.¹⁰B and¹¹B depth
Direction secondary ion intensity distribution 5 and 6 ¹¹Secondary ion of B
The time when the intensity reaches the background level (B in FIG. 4)
Point), and determine the crater depth generated by the measurement.
Secondary ions by measuring with a stylus type surface roughness meter
The depth t1 generated by mass spectrometry is determined. That
As a result, t1 was 220 nm.

[0033] As a result of the measurement, the measurement time in the silicon substrate 1 is between is A 'a peak top of secondary ion intensity of ^{10 B} in Figure 3 B, the actual depth t1-t0
= 69 nm, the point A 'in FIG. 3 is set to the outermost surface of the sample (depth 0 nm), and the point B in FIG.
The measurement time is converted to depth as m.

Then, using a standard sample of known concentration, ¹¹
By converting the secondary ion intensity distribution of B in the concentration distribution, as shown in FIG. 4, the depth direction concentration distribution of ^{11 B} in the silicon substrate can be obtained. From FIG. 4, the boron concentration is 1
It can be seen that the depth of × 10 ¹⁸ atoms / cm ⁻³ is 20 nm. (Second Embodiment) FIGS. 5A to 5C are cross-sectional views for explaining a manufacturing process of a measurement sample used in a second embodiment of the present invention.

First, as shown in FIG. 5A, a 3-nm-thick thermal oxide film 12 and a 1-nm-thick
A polysilicon film 13 having a thickness of 00 nm is formed, and heat treatment is performed after ion implantation of ⁷⁵ As into the surface of the polysilicon film 13.

Next, as shown in FIG. 5B, the polysilicon film 13 and the thermal oxide film 12 are etched by chemical etching.
Is removed, a few atomic layers of boron 15 are deposited on the surface of the silicon substrate 11.

Next, as shown in FIG. 5C, a sputtered silicon film 16 is formed on the silicon substrate 11 at a temperature at which As does not diffuse, to a thickness of 100 nm (a set value of a growth apparatus) to prepare a measurement sample.

Next, the secondary ion intensity of ¹¹ B is measured by the secondary ion mass spectrometry until the time when the sample reaches the peak top. FIG.
It is a graph which shows the measurement result.

As shown in FIG. 6, in the measurement sample,¹¹B
Of secondary ion intensity distribution 17 in the depth direction of oxygen
Ion mass spectrometry measurement using oxygen and oxygen leak method
And ¹¹When the secondary ion intensity of B reaches the peak top
Between the two points (point C in FIG. 6),
The depth of the spatter is measured with a stylus type surface roughness
The thickness t'0 of the recon film 16 is obtained. As a result, t'0
Was 102 nm.

Next, a secondary ion mass spectrometry measurement was performed at a position about 3 mm away from the position where the secondary ion mass spectrometry measurement was performed on the sample for measurement to obtain a secondary ion intensity of ¹¹ B and ⁷⁵ As in the depth direction. Is measured. FIG. 7 is a graph showing the measurement results.

As shown in FIG. 7, the secondary ion intensity distributions 18 and 19 in the depth direction of ¹¹ B and ⁷⁵ As are measured until the time when the secondary ion intensity of ⁷⁵ As reaches the background level (point D in FIG. 7). Then, the depth t′1 generated by the secondary ion mass spectrometry is determined by measuring the depth of the crater generated by the measurement using a stylus type surface roughness meter. As a result, t′1 was 164 nm.

[0042] As a result of the measurement, the measurement time in the silicon substrate 11 is between C 'of D is a peak top of secondary ion intensity of ^{11 B} in FIG. 7, the actual depth t'1-
Since t′0 = 62 nm, the measurement time is converted into depth by setting point C ′ in FIG. 7 to the outermost surface of the sample (depth 0 nm) and point D in FIG. 7 to a depth of 62 nm.

[0043] Then, using a known concentration of the standard sample ⁷⁵
By converting the secondary ion intensity distribution of As into a concentration distribution, As shown in FIG.
Is obtained in the depth direction. As shown in FIG. 8, As, which has penetrated the thermal oxide film 12 from the polysilicon film 13 into the silicon substrate 11 by ion implantation or thermal diffusion, is 2 × in a region 25 nm deep from the surface in the silicon substrate 11.
It can be seen that 10 ¹⁸ atoms / cm ^-3 exists.

According to the present invention, since a silicon film such as an amorphous silicon film and a sputtered silicon film is formed on a sample, the sputtering rate accompanying primary ion irradiation is stabilized in the silicon film due to the influence of oxygen leak. The impurity profile in the sample is not affected by changes in sputtering rate. Therefore, the measurement of the concentration distribution of the impurity to be measured and the total dose can be performed with high accuracy.

The present invention can be applied to measurement of a sample in which an oxide film is not formed on a substrate in advance.

The difference between the sputtering speed of the silicon film and the sputtering speed of the sample is corrected using the previously measured silicon film thickness.

The present invention is not limited to the above embodiment, and various modifications can be made within the scope of the technical matters described in the claims. For example, the thickness measurement of a silicon film is performed by a method other than SIMS (for example, TE
M method).

[0048]

According to the present invention, since a silicon film is formed on a sample, the sputtering rate accompanying primary ion irradiation is stabilized in the silicon film due to the influence of oxygen leak. Unaffected by speed changes. Therefore, the measurement of the concentration distribution of the impurity to be measured and the total dose can be performed with high accuracy.

The present invention can be applied to measurement of a sample in which an oxide film is not formed on a substrate in advance.

[Brief description of the drawings]

FIGS. 1A and 1B are cross-sectional views for explaining a manufacturing process of a measurement sample used in a first embodiment of the present invention.

FIG. 2 is a graph showing a measurement result obtained by measuring a sample for measurement by a secondary ion mass spectrometry until a secondary ion intensity of ¹⁰ B reaches a peak top in the first embodiment of the present invention. is there.

FIG. 3 shows the first embodiment of the present invention, in which a point on the sample for measurement is about 1 mm from the point where the measurement result shown in FIG. 2 is obtained.
Graph showing the ^{1 0 B} and ¹¹ depth secondary ion intensity distribution ^B of secondary ion intensity is obtained by measuring secondary ion mass spectrometry to the time to reach the background level of the remote location ^{11 B} It is.

4 is a graph showing the depth direction concentration distribution of ^{11 B} in the first silicon substrate obtained by the embodiment of the present invention.

FIGS. 5A to 5C are cross-sectional views illustrating a process for manufacturing a measurement sample used in a second embodiment of the present invention.

FIG. 6 is a graph showing a measurement result obtained by measuring a measurement sample by a secondary ion mass spectrometry until the secondary ion intensity of ¹¹ B reaches a peak top in the second embodiment of the present invention. is there.

FIG. 7 shows a measurement test according to a second embodiment of the present invention.
Approximately 3 mm from the place where the measurement results shown in FIG.
Distant places⁷⁵As secondary ion intensity of As background
Secondary ion mass spectrometry measurement until time reaches
Obtained by ⁷⁵As and¹¹B depth direction 2
It is a graph which shows a secondary ion intensity distribution.

FIG. 8 is a graph showing a concentration distribution in the depth direction of As in a silicon substrate obtained according to a second embodiment of the present invention.

[Fig. 9] With oxygen leak (●), no oxygen leak (▲)
Of the implantation acceleration voltage and the B concentration at which the concentration of B reaches 1 × 10 ¹⁸ atoms / cm ^-3 when performing ion implantation of ¹¹ B at a dose of 1 × 10 ¹⁵ cm ⁻² obtained by performing secondary ion mass spectrometry measurement with It is a graph which shows a relationship.

[Explanation of symbols]

1 silicon substrate 2 boron 3 amorphous silicon film 4 ¹⁰ B of the secondary ion intensity distribution 5 ¹⁰ B of the secondary ion intensity distribution 6 ¹¹ B of the secondary ion intensity distribution 11 silicon substrate 12 thermal oxide film 13 a polysilicon film 15, boron 16 Sputtered silicon film 17 ¹¹ B secondary ion intensity distribution 18 ¹¹ B secondary ion intensity distribution 19 ⁷⁵ As secondary ion intensity distribution

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-9-260448 (JP, A) JP-A-7-58304 (JP, A) JP-A-5-72152 (JP, A) JP-A-3- 89152 (JP, A) JP-A-7-181149 (JP, A) JP-A-10-90205 (JP, A) JP-A 8-96740 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01N 23/225 JICST file (JOIS)

Claims (8)

(57) [Claims] 1. A method for measuring impurities in a sample using secondary ion mass spectrometry, comprising: measuring, on a surface of a sample having an impurity to be measured on its surface, an element having a different mass number from the impurity to be measured. Forming an auxiliary impurity layer, forming a silicon film on the surface of the auxiliary impurity layer, and measuring a concentration distribution in a depth direction of the impurity to be measured and the auxiliary impurity from the surface of the silicon film. And a method for measuring impurities. 2. The depth of the concentration distribution of impurities to be measured in the sample in the depth direction is determined by measuring the sample after forming the silicon film, the depth of a crater, the silicon film thickness and the silicon film thickness. The impurity measuring method according to claim 1, wherein the impurity measuring method is obtained based on a profile of the auxiliary impurity. 3. A step of adhering an auxiliary impurity comprising an element having a different mass number from the measured impurity to a surface of the sample having the measured impurity present on the surface, and forming a silicon film on the surface of the sample. A first measurement step for determining the thickness of the silicon film; and performing SIMS measurement from the surface of the silicon film to the inside of the sample to determine a secondary ion intensity distribution of the measured impurity and the auxiliary impurity and a crater depth. A second measurement step to be sought;
The difference between the crater depth determined in the second measurement step and the thickness of the silicon film determined in the first measurement step, and the secondary of the auxiliary impurity determined in the second measurement step. Measuring the depth-direction concentration distribution of the impurity to be measured in the sample by converting the measurement time to the measurement depth from the difference between the time indicating the maximum value of the ionic strength and the measurement end time, Impurity measurement method. 4. The method according to claim 1, wherein the first measuring step performs a SIMS measurement from a surface of the silicon film to a time when a secondary ion intensity of the auxiliary impurity reaches a maximum value, and measures a crater depth generated by the SIMS measurement. 4. The method according to claim 3, wherein the thickness of the silicon film is determined. 5. The impurity measuring method according to claim 1, wherein the temperature at which the silicon film is formed is a temperature at which the profile of the impurity to be measured does not change. 6. A method for forming a silicon film, comprising:
The impurity measuring method according to claim 1, wherein the method is any one of a sputtering method and a vapor deposition method. 7. The impurity measuring method according to claim 1, wherein said silicon film is an amorphous silicon film or a sputtered silicon film. 8. The method for measuring impurities according to claim 1, wherein oxygen is introduced into a sample chamber in which the sample is placed for measurement.