JPS60206146A - Method for analyzing impurity - Google Patents

Abstract

PURPOSE:To obtain with ease a profile of absolute values related to the concentration distribution of an impurity in a semiconductor substrate by a method wherein comparison is made between the impurity concentration in a specimen and the absolute concentration of the ions of an element under measurement driven into a part to be tested of the specimen during a measurement conducted by using secondary ion mass spectrometric analysis. CONSTITUTION:The absolute value of the peak concentration of injected ions is theoretically determined by using the acceleration voltage and the dose size. Of the two regions 1a and 1b equipped with the same profile in a semiconductor substrate 1, ions are driven only into the region 1a. The region 1a is exposed to etching while the quantity of ions or a current (y2) is determined as indicated in a mass spectrometer. The absolute value of an ion injection peak value 1c is determined in view of the depth (y1) and the concentration (x1). The absolute value of 1d is determined by relative comparison with the ion injection peak value 1c. A current 1e is obtained by changing the specimen setting. The absolute value of the current 1e is equal to the absolute value 1d. The concentration 1f that is the wanted quantity is determined by relative comparison with the current 1e.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to a method for analyzing impurities by secondary ion mass spectrometry.

(Prior art) Secondary ion mass spectrometry (hereinafter referred to as SIMS) accelerates some ions such as oxygen with a voltage of 10 to 15 KV, irradiates the sample, and collects the secondary ions released from the sample. This is a method of mass spectrometry.

SIMS has higher sensitivity to trace impurities than Auger electron spectroscopy, photoelectron spectroscopy, and the like.

B and P are typical doping impurities for silicon.

It is said that it is possible to detect As with I x 10" atoms.
Measurements with good wired performance are possible in the range of C to 10" atoms.

The advantage of SIMS is that impurities contained in a sample can be analyzed while etching. Therefore, it is possible to obtain a temporal change in the detected amount such as current corresponding to the amount of the impurity to be measured, that is, as a concentration profile in the cross-sectional direction of the sample. Figure 1 shows an example of such a case.
Depth vs. As impurity concentration when Ast is implanted into silicon
As mentioned above, 10” atoms/C
It can be seen that the linearity is good in the range of C10'' atoms/(f) or above.

Further, by adjusting the lens-based amount analysis system in the secondary ion power purchase analyzer, the content of any element can be analyzed. Furthermore, the detection sensitivity is extremely high at about 20 ppb, and in addition to being able to measure with good linearity in the above-mentioned impurity concentration range, it is also possible to measure relative values with an accuracy of several orders of magnitude.

However, when measuring concentration using SIMS, it is necessary to convert the obtained secondary ion intensity into concentration, which poses problems in accuracy and reproducibility. In other words, it is not possible to make an absolute measurement of the content of any element; this is measured as the amount of element flowing into the mass spectrometer, so the amount entering the mass spectrometry system varies depending on the adjustment state of the mass spectrometer system. Therefore, only relative measurements can be made. Therefore, there are problems with accuracy and reproducibility.

Generally, to convert secondary ion strength into concentration, a sample with a known concentration is measured, and a calibration curve of secondary ion strength and concentration is created and calculated.

However, since the secondary ion intensity in SIMS measurement varies depending on the measurement conditions, ie, the degree of vacuum in the sample chamber, the secondary ion current, etc., there is a problem in measuring the absolute concentration.

(Objective of the Invention) An object of the present invention is to obtain an impurity analysis method that can easily obtain a profile of the absolute value of the impurity concentration distribution in a semiconductor substrate or the like.

(Summary of the Invention) The main point of this invention is that in order to measure the absolute concentration of doping impurities in a silicon substrate using SIMS, an element to be measured is ion-implanted into the measurement location in advance before measurement. The objective is to determine the absolute concentration profile of the sample to be measured by determining the concentration profile of the target element as an absolute value and relatively comparing the absolute concentration of the ion implantation and the concentration of the sample to be measured.

(Example) Next, an example of the impurity analysis method of the present invention will be described. Typically, the detection limit of SIMS for B, P, and As in silicon is I x 10'' atoms/cc. For example, approximately 3 x 10'' atoms/cc.
When a silicon substrate containing B'(r of c is measured by SIMS, the results shown in FIG. 2 are obtained.

This FIG. 2 shows the time change of the secondary ion intensity due to the -order ion 02. The horizontal axis is the sputtering time, and the vertical axis is the obtained secondary ion intensity of B+. There is a proportional relationship between secondary ion strength and concentration, and the concentration is usually calculated from the secondary ion strength. For this purpose, it is necessary to create a calibration curve using samples whose concentrations are known.

However, the calibration curve of this concentration and secondary ion strength changes depending on the measurement conditions, which is a big problem, especially when measuring oxygen in silicon.

Figure 3 shows a sample containing approximately 3 X 10" atoms/cc of B' with B+It 150 keV, I X 101
These are the results measured by SIMs after 4 cm'' ion implantation.

The peak concentration of ion implantation can be determined by calculation. The silicon semiconductor substrate concentration can be calculated from the secondary ion intensity A at the ion implantation peak position, the secondary ion intensity B inside the silicon semiconductor substrate, and the peak concentration. In this example of Figure 3, the peak concentration is 5.35.
From X 1018 atoms/cc, a silicon semiconductor substrate concentration of 3.2 X 10''atoms/CC was obtained.

Here, the absolute value of the peak concentration of ion implantation is theoretically different from the accelerating voltage and the dose amount, and the absolute value can be obtained with a large degree of precision. By monitoring the current value at an accelerating voltage of 50 KV to 100 KV and a dose of 10'' to 10' ions/era, the peak concentration of ion implantation can be determined in numbers as described above. Based on this peak concentration, the Measured objects can be compared.

Therefore, by comparing the known ion implantation peak concentration and the relative value, the absolute value of the peak concentration of the sample to be measured can be determined with high accuracy.

Note that if you want to have a different profile near the surface, as shown in FIG. While etching this portion 1a in the depth direction, the ion intensity (current) of the mass spectrometer is measured.

As a result, the absolute value of the ion implantation peak value IC can be determined, as is clear from the relationship between depth and concentration in FIG. 4(b), and from the relative comparison with this ion implantation peak value IC, the absolute value is calculated.

Next, change the setting of the sample to be measured and set it as shown in Figure 4 (c).
), find the current 1ek. The absolute value of this current 1e is equal to the absolute value 1d in FIG. 4(b).

Then, the absolute value of the concentration 1f can be determined by relatively comparing it with the current 1e.

(Effects of the Invention) As explained above, in this invention, the element to be measured is ion-implanted into the measurement location in advance, and the concentration profile of the element to be measured is determined as an absolute value. Since the absolute concentration profile of the sample to be measured is obtained by comparing the concentration of the sample to be measured relatively, it becomes possible to measure the absolute concentration of doping impurities in silicon wafers.

(1) Measuring the amount of doped impurities in the semiconductor substrate and impurities in the metal, (2) Measuring the concentration profile of a specific part by appropriately selecting the accelerating voltage for ion implantation, (3) Measuring the concentration profile of a certain part in the silicon. Measurements such as oxygen and carbon concentration can be easily performed.

[Brief explanation of the drawing]

Figure 1 shows the relationship between depth and As impurity concentration when the impurity concentration profile in the cross-sectional direction of the sample is determined while etching the sample using SIMS. Figure 2 shows the relationship between depth and As impurity concentration.
Silicon SIMS containing B of 0” atoms/cc
Figure 3 shows the results of the relationship between sputtering time and "B + secondary ion strength when measured at 3 x 101015a.
to/cc of Bt-containing silicon B”k 150 K
eV, I X l 014 cm-2 ion implantation,
Figures 4(a) to 4(c), which show the relationship between sputtering time and "B+ secondary ion strength measured by SIMS," show the impurity concentration profile near the surface of a silicon semiconductor substrate in the impurity analysis method of the present invention. Patent applicant Oki Electric Industry Co., Ltd. Figure 1 Depth: μm) Figure 2

Claims (1)

[Claims] When measuring impurity concentration using secondary ion mass spectrometry, the element to be measured is ion-implanted into a part of the measurement target region so that the concentration is higher than the concentration of the target object, and the peak value of the concentration profile due to ion implantation is An impurity analysis method characterized by measuring the concentration of an analyte.