JP2017009307A - Method for evaluating semiconductor substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for evaluating a semiconductor substrate that can measure a generation life time in a short time.SOLUTION: There is provided the method for evaluating a semiconductor substrate using a MOS structure, which includes the steps of forming an oxide film on a semiconductor substrate and forming an electrode on the oxide film, thereby obtaining a MOS structure; depositing, on the electrode, an electric charge having such a polarity that a depletion layer is formed immediately below the oxide film of the semiconductor substrate, by a corona charging method; and measuring change in voltage involved with attenuation of the amount of the deposited electric charge and determining a generation life time from the measured change in voltage.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for evaluating a semiconductor substrate, and more particularly, to a method for determining a generation lifetime near a substrate surface using a MOS structure formed on a semiconductor substrate.

  In obtaining the semiconductor substrate characteristics, the MOS structure is formed, and the technique for obtaining the electrical characteristics can obtain information on many semiconductor substrates. Therefore, various measurement techniques have been studied and proposed (for example, non-patent) Reference 1). There are various parameters such as oxide film thickness, fixed charge of oxide film, oxide film / semiconductor interface state, dopant concentration, and generation and recombination lifetime. Among various evaluation analysis methods for semiconductor substrates, measurement using the generated lifetime is expected to have a close relationship with an actual device in order to evaluate the device active region, that is, the depletion layer.

  On the other hand, as described in Non-Patent Document 1, research on semiconductor evaluation methods using corona charge has been actively conducted since about 1983. Non-Patent Document 1 introduces several methods as an electrical measurement method using corona charge. Corona charging is a technique for depositing ions on an object by applying a high voltage using a probe to which a high voltage is applied in the atmosphere.

  Among various evaluation methods, the lifetime is extremely sensitive to crystal defects and metal impurities, and various measurement methods have been proposed. Patent Document 1 defines the conditions for obtaining the recombination lifetime by the μ-PCD method. As described therein, if the conditions are selected, extremely sensitive measurement is performed. Has been suggested to be possible. Further, Patent Documents 2 and 3 are examples of measurement using a corona charge. These are measurements that combine a Kelvin probe, light modulation light reflectance measurement, and the like. In either case, no MOS structure is formed, and the surface is passivated with chemicals or oxide films for measurement.

  The measurement of the generation lifetime using the MOS structure is well known by the Zerbst method shown in Non-Patent Document 1, which is obtained from the change in capacitance of the MOS structure. However, in the Zerbst method, it is necessary to perform measurement until the capacitance change reaches a steady state, and there is a concern that the measurement may take a very long time, particularly in a sample having a long generation lifetime.

  In the conventional Zerbst method, as shown in FIG. 5, the probe 14 is brought into contact with the electrode 13 on the oxide film 12 formed on the silicon substrate 11, and a capacitance meter is connected to the probe 14 to change the capacitance over time (C− t) is measured. The time change of the capacity is analyzed by, for example, Expression (2) shown in a comparative example described later, and the generated lifetime is obtained from this inclination. In this case, it is necessary to continue measurement until the change in capacitance becomes constant. In other words, care must be taken in setting the maximum measurement time.

  In the Zerbst method, various methods for reducing the time required for measurement have been proposed, and are specifically shown in Patent Document 4 and references thereof. Patent Document 4 uses the Zerbst method. Based on the capacitance obtained in the high-frequency CV measurement, the depletion layer width is divided into minute regions and the lifetime is obtained for each region. It is a technique to obtain.

  In any case, it is an improvement of the Zerbst method based on the high-frequency CV method, and the time required for measurement may not basically change.

D. K. Schroder, Semiconductor Material and Device Characterization, New Jersey, 2006.

JP 2010-040793 A JP 2010-217184 A JP 11-186350 A Japanese Patent Publication No. 05-077336

  As described above, when the relationship with the actual device is considered, particularly when the depletion region near the surface is considered, the generation lifetime measurement is an option. At that time, by forming a MOS structure (forming an oxide film and an electrode on the semiconductor substrate: for example, forming a polysilicon electrode), measurement with a similar structure assuming an actual device process becomes possible, which is better than an actual device. It is conceivable that a relationship can be obtained. In this case, the measurement may be performed using the Zerbst method. However, when the Zerbst method is used, there is a problem that a long time is required for the measurement. It is not uncommon to actually spend more than 3 hours measuring a single MOS structure.

  The present invention has been made in view of such a problem, and an object of the present invention is to provide a semiconductor substrate evaluation method capable of measuring the generation lifetime in a short time.

To achieve the above object, according to the present invention, there is provided a method for evaluating a semiconductor substrate using a MOS structure,
A polarity in which an oxide film is formed on a semiconductor substrate, an electrode is formed on the oxide film, and a MOS structure is fabricated, and then a depletion layer is formed immediately below the oxide film on the semiconductor substrate by a corona charge method. A method for evaluating a semiconductor substrate, comprising: depositing a charge having a voltage on the electrode; measuring a change in voltage associated with the decay of the deposited charge amount; and determining a generation lifetime from the measured change in voltage. I will provide a.

  According to this method, in the method of evaluating a semiconductor substrate using a MOS structure, the generation lifetime can be obtained in a short time.

  At this time, the generated lifetime can be calculated using the following formula (1).

(Wherein, dV / dt is .q showing temporal changes in the measured voltage indicates the elementary electric charge, n _i denotes the intrinsic carrier concentration, W is .K _s indicating the width of the depletion layer dielectric of a semiconductor substrate Where ε ₀ is the vacuum dielectric constant, τ _g is the generation lifetime, and s _g is the surface recombination rate.)

Thus, the generation lifetime τ _g can be calculated using the above formula (1), and the semiconductor substrate can be evaluated.

  With the method for evaluating a semiconductor substrate of the present invention, the generation lifetime can be obtained in a short time in the method for evaluating a semiconductor substrate using a MOS structure. Therefore, for example, in-plane distribution data of a sample can be acquired in a short time. Thereby, it can also contribute to quality improvement of a semiconductor substrate.

It is process drawing which showed an example of the evaluation method of the semiconductor substrate of this invention. It is the schematic which showed a mode that a corona charge was carried out to the MOS structure formed in the semiconductor substrate of evaluation object. It is the schematic which showed a mode that the change of a voltage was measured after a corona charge. It is the graph of the generation | occurrence | production lifetime calculated | required in the Example and the comparative example. It is the schematic which showed a mode that the generation | occurrence | production lifetime was calculated | required by the conventional Zerbst method.

  Hereinafter, although an embodiment is described about the present invention, the present invention is not limited to this.

  As described above, when the Zerbst method is used to measure the occurrence lifetime, there is a problem that it takes a long time for the measurement.

  Therefore, the present inventor has intensively studied to solve such problems. As a result, an oxide film is formed on the semiconductor substrate, an electrode is formed on the oxide film, and a MOS structure is fabricated. Then, a depletion layer is formed immediately below the oxide film on the semiconductor substrate by a corona charge method. It has been found that a charge having polarity is deposited on an electrode, a change in voltage associated with the decay of the deposited charge is measured, and a generation lifetime is obtained from the change in the measured voltage. As a result, it has been found that the generation lifetime can be obtained in a short time in a method for evaluating a semiconductor substrate using a MOS structure. And the best form for implementing these was scrutinized and the present invention was completed.

  Hereinafter, the semiconductor substrate evaluation method of the present invention will be described in detail with reference to FIGS. 1, 2, and 3, but the present invention is not limited to this. FIG. 1 is a process diagram showing an example of a semiconductor substrate evaluation method of the present invention.

  As shown in FIG. 1, first, a MOS structure is fabricated on a semiconductor substrate to be evaluated, and thereafter, using this MOS structure, a change in voltage associated with attenuation of the amount of charge deposited by a corona charge method is measured. The generation lifetime, that is, the time from the generation of carriers to the recombination in the depletion layer is obtained from the change in voltage.

  In such an evaluation method of the present invention, as will be described later, it is possible to use conventional equipment for the fabrication of the MOS structure, the corona charge, and the voltage change measurement.

(Production of MOS structure: SP1 in FIG. 1)
First, a semiconductor substrate to be evaluated is prepared. Here, as shown in FIG. 2, the case where the silicon substrate 1 is used will be described. However, the present invention is not limited to this, and a semiconductor substrate to be evaluated can be appropriately prepared according to the purpose.

  Oxidation is performed on the silicon substrate 1 as described above to form an oxide film 2 (gate oxide film). The oxide film 2 can be easily formed, for example, by placing the silicon substrate 1 on a boat and placing it in a horizontal heat treatment furnace or a vertical heat treatment furnace and performing heat treatment in an oxygen atmosphere. The method for forming the oxide film 2 is not particularly limited. For example, a conventional heat treatment furnace can be used, and the heat treatment conditions and the like can be determined each time according to the evaluation purpose.

  Next, an electrode 3 is formed on the oxide film 2 formed on the silicon substrate 1. As a material for the electrode 3, for example, polysilicon (polycrystalline silicon) can be used. If polysilicon is selected as the material for the electrode 3 in this way, the processing and procurement thereof are easy, and the electrode 3 can be formed in the same manner as in the prior art, which is convenient. The material of this electrode 3 is not specifically limited, For example, other materials, such as Al, can also be selected. It can be determined appropriately according to the cost or the like.

  And the electrode 3 is formed using such polysilicon. The electrode 3 can be formed as follows, for example, but the method for forming the electrode 3 is not particularly limited.

  First, a polysilicon film is grown on the oxide film 2 on the surface of the silicon substrate 1. This polysilicon film can be grown, for example, by introducing the silicon substrate 1 taken out from the heat treatment furnace into a CVD apparatus and introducing a growth gas such as monosilane into the reaction vessel of the apparatus under reduced pressure or normal pressure. . Then, after depositing the polysilicon layer as described above, impurities such as phosphorus are doped into the polysilicon layer using a thermal diffusion method or an ion implantation method to form a polysilicon layer having a low resistivity. Note that a low-resistivity polysilicon layer can also be formed by doping impurities at the same time when the polysilicon layer is deposited (Doped Poly-Si method).

  Thereafter, the low resistivity polysilicon layer formed as described above is subjected to a series of photolithography processes, for example, resist coating, exposure, and development, and then an etching process, whereby a desired position on the oxide film 2 is obtained. A polysilicon electrode 3 can be formed. Thus, the MOS structure 4 (MOS capacitor) in which the oxide film 2 and the polysilicon electrode 3 are formed on the silicon substrate 1 can be manufactured.

  The MOS structure 4 is one of the basic structures in an actual device, and the characteristics obtained from the MOS structure 4 are expected to reflect the actual device.

(Corona charge: SP2 in Fig. 1)
After the MOS structure 4 is fabricated on the silicon substrate 1 as described above, the polarity is such that a depletion layer is formed directly below the oxide film 2 of the semiconductor substrate 1 by the corona charge method using the corona charge probe 5. A corona charge for depositing the charge on the electrode 3 is performed. In addition, the corona charge apparatus for performing the above corona charges is not specifically limited, A conventional thing can be used.

  The polarity (plus / minus) of the charge deposited on the electrode 3 by corona charge depends on the conductivity type of the silicon substrate 1. When the silicon substrate 1 is n-type, the polarity is negative, and the silicon substrate 1 is p. In the case of a mold, it can be a positive charge.

(Measurement of voltage change: SP3 in FIG. 1)
Then, as shown in FIG. 3, the tester 6 measures the change in voltage accompanying the attenuation of the amount of charge accumulated in SP2. The tester 6 may be any one that can measure a change in voltage, and a general one can be used. For this reason, it is not necessary to use a special probe.

(Determining the occurrence lifetime: SP4 in FIG. 1)
Next, the occurrence lifetime is obtained from the change in voltage measured at SP3.
Specifically, for example, the occurrence lifetime can be calculated using the following formula (1).

In the above formula (1), dV / dt represents the time change of the measured voltage. q is an elementary electric charge ^{(1.6 × 10 -19 C),} n i denotes the intrinsic carrier concentration, W is showing a depletion layer width. K _s indicates the relative dielectric constant of the semiconductor substrate, and ε ₀ indicates the vacuum dielectric constant. τ _g indicates the generation lifetime, and s _g indicates the surface recombination rate. This equation (1) can be derived from a theoretical equation of voltage decay in the depletion layer.

Thus, the generation lifetime τ _g can be calculated using the above formula (1), and the semiconductor substrate can be evaluated.

  With the semiconductor substrate evaluation method of the present invention as described above, the generation lifetime can be obtained in a short time in the method of evaluating a semiconductor substrate using a MOS structure. Therefore, for example, in-plane distribution data of a sample can be acquired in a short time. Thereby, it can also contribute to quality improvement of a semiconductor substrate.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples of the present invention, but the present invention is not limited to these examples.

(Example)
As a semiconductor substrate to be evaluated, a boron-doped P-type silicon wafer having a diameter of 200 mm was used. The resistivity of the silicon wafer was 10 Ω · cm. This silicon wafer was subjected to gate oxidation with a thickness of 25 nm in a dry atmosphere at 900 ° C. to form an oxide film. Thereafter, a polysilicon (Poly-Si) layer doped with phosphorus is deposited by CVD to a thickness of 300 nm, and the sheet resistance is 30 Ω / sq. The electrode was formed. This was subjected to photolithography to form an electrode pattern, and an electrode having an area of 1 mm ² was formed on the oxide film formed on the silicon wafer.

  Next, a corona charge device was prepared, and using a corona charge probe, a charge having a positive polarity so that a depletion layer was formed immediately below the oxide film of the silicon wafer was deposited on the electrode. After performing the corona charge, the amount of change in voltage accompanying the decay of the amount of deposited charge was monitored with time using a semiconductor parameter analyzer B1500 manufactured by Agilent Technologies.

From the measurement results, a total of three silicon wafers were measured for four generation lifetimes τ _g for each silicon wafer based on the above formula (1) to determine the generation lifetime τ _g . The result at this time is shown in FIG. The values on the horizontal axis in FIG. 4 are the present embodiment, and the shortest occurrence lifetime is indicated by ▲, the middle one by ◆, and the long one by ■. In addition, the vertical axis | shaft of FIG. 4 was made into the value calculated | required by the below-mentioned comparative example.

(Comparative example)
A voltage was applied to the MOS structure electrode fabricated in the example, and the measurement was performed until the subsequent change in capacitance with time became constant. The time change of the capacity was monitored using a semiconductor parameter analyzer B1500 manufactured by Agilent Technologies.

  Then, a so-called conventional Zerbst method was used in which the measured time change of the capacity was analyzed by the following equation (2), and the generated lifetime was obtained from this slope.

In the above formula (2), C represents the measured capacity, and dC / dt represents the time change of the measured capacity. N _A indicates the dopant concentration, and C _ox indicates the oxide film capacity. n _i , K _s , ε ₀ , W, τ _g , and s _g are the same as above.

The measurement point is the same measuring point as in Example, by four points for each wafer, by performing measurements on a total of three silicon wafers were determined generation lifetime tau _g. The results at this time are shown on the vertical axis in FIG. 4 as described above.

  As a result, as shown in FIG. 4, it can be seen that the same measurement result is obtained regardless of the evaluation of the example and the comparative example. On the other hand, since the time required for measurement was measured by the conventional Zerbst method in the comparative example, it took about 4 hours per point for the sample having the longest lifetime. On the other hand, in the Example, it was about 20 minutes per point. Thus, in the Example using the method of the present invention, the generation lifetime could be obtained in a shorter time than the comparative example.

  The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and the present invention has any configuration that has substantially the same configuration as the technical idea described in the claims of the present invention and that exhibits the same effects. Are included in the technical scope.

DESCRIPTION OF SYMBOLS 1 ... Silicon substrate, 2 ... Oxide film, 3 ... Electrode, 4 ... MOS structure,
5 ... Corona charge probe, 6 ... Tester.

Claims (2)

A method for evaluating a semiconductor substrate using a MOS structure,
A polarity in which an oxide film is formed on a semiconductor substrate, an electrode is formed on the oxide film, and a MOS structure is fabricated, and then a depletion layer is formed immediately below the oxide film on the semiconductor substrate by a corona charge method. A method for evaluating a semiconductor substrate, comprising: depositing a charge having a voltage on the electrode; measuring a change in voltage associated with the decay of the deposited charge amount; and determining a generation lifetime from the measured change in voltage. . The method for evaluating a semiconductor substrate according to claim 1, wherein the generation lifetime is calculated using the following formula (1).

(Wherein, dV / dt is .q showing temporal changes in the measured voltage indicates the elementary electric charge, n _i denotes the intrinsic carrier concentration, W is .K _s indicating the width of the depletion layer dielectric of a semiconductor substrate Where ε ₀ is the vacuum dielectric constant, τ _g is the generation lifetime, and s _g is the surface recombination rate.)

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