JP6020358B2 - Evaluation method of silicon single crystal wafer - Google Patents

Description

  The present invention relates to a method for evaluating a silicon single crystal wafer, and more particularly to a method for performing CV measurement of an n-type silicon single crystal wafer using a mercury electrode.

  Conventionally, as a method for measuring the resistivity of a silicon single crystal wafer, a method for measuring CV (Capacitance-Voltage) characteristics is known. In order to measure the CV characteristics, a depletion layer is formed inside the silicon single crystal wafer by forming a Schottky junction on the surface of the silicon single crystal wafer to be a sample and applying a reverse bias voltage continuously. To expand the capacity and change the capacity. For example, a mercury electrode is used to form a Schottky junction on the surface of the silicon single crystal wafer.

  When the sample silicon single crystal wafer is n-type, the surface of the wafer is oxidized in advance to form a thin silicon oxide film, and a mercury electrode is bonded onto the silicon oxide film to measure CV characteristics. be able to.

  A thin silicon oxide film is formed on the surface of a silicon single crystal wafer by rinsing and drying after immersing the wafer in a solution containing an oxidizing agent such as hydrogen peroxide for several minutes (non-patented). Reference 1). However, in this method, since it takes about 20 minutes from the oxidation treatment to drying, it is necessary to shorten the time.

  Further, the CV characteristics can be measured even when a relatively thick silicon oxide film of 5 nm or more is formed on the surface of the silicon single crystal wafer (Patent Document 1). However, in the method described in Patent Document 1, it is necessary to perform heat treatment for about 1 hour in order to convert the silica glass into silicon oxide, and the working efficiency is poor.

  Thus, a method has been proposed in which an n-type silicon single crystal wafer is exposed to ultraviolet light in an oxygen-containing atmosphere to oxidize the wafer surface with ozone (Patent Documents 2 and 3).

  When the silicon single crystal wafer is p-type, the wafer surface is prepared by treating the wafer with concentrated HF solution or diluted HF solution, washing with deionized water, and centrifugal drying in a nitrogen atmosphere. 2.

ASTM Standards F1392-02

JP 2012-138463 A Special table 2002-516486 gazette JP 2013-46030 A

  However, when the CV characteristic of a low resistivity n-type silicon single crystal wafer is measured, the measurement accuracy tends to deteriorate.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for evaluating a silicon single crystal wafer that can obtain measurement accuracy of CV characteristics that is improved as compared with the prior art. .

  In order to achieve the above object, the present invention provides a natural oxide film removing step of removing a natural oxide film by treating an n-type silicon single crystal wafer with hydrofluoric acid, and the n-type silicon single crystal from which the natural oxide film has been removed. An oxide film forming step for forming an oxide film on the surface of the crystal wafer, and a CV characteristic measurement for measuring a CV characteristic by bonding a mercury electrode to the surface of the n-type silicon single crystal wafer on which the oxide film is formed. And a method for evaluating a silicon single crystal wafer.

  In this way, since the natural oxide film that affects the CV characteristics of the n-type silicon single crystal wafer is removed in advance, it is possible to obtain CV characteristics that are closer to the true value than before, and gradually gradually from day to day. Therefore, the influence of the natural oxide film that becomes thicker is eliminated, so that the daily fluctuation of resistivity can be reduced.

Here, it is desirable that the resistivity of the n-type silicon single crystal wafer is 0.04 Ωcm or more and 1 Ωcm or less.
Since the influence of the natural oxide film increases as the resistivity decreases, the resistivity measurement accuracy is effectively improved when the resistivity of the n-type silicon single crystal wafer is 0.04 Ωcm or more and 1 Ωcm or less. Can be raised.

The resistivity of the n-type silicon single crystal wafer is more preferably 0.2 Ωcm or less.
Thus, when the resistivity of the n-type silicon single crystal wafer is 0.2 Ωcm or less, the measurement accuracy of the resistivity can be increased more effectively.

Further, in the oxide film forming step, ozone gas is allowed to pass between the ultraviolet light source and the silicon single crystal wafer mounting portion. It is desirable to form a silicon oxide film on the surface of the n-type silicon single crystal wafer, and measure a CV characteristic by bonding a mercury electrode on the silicon oxide film in the CV characteristic measurement step.
In this way, an ozone gas is allowed to pass between the ultraviolet light source and the silicon single crystal wafer mounting portion, but a light-shielding plate that shields light is arranged, and the n-type silicon single crystal wafer is irradiated with ozone gas while shielding the ultraviolet light. By forming the silicon oxide film on the surface, the CV characteristics can be stably measured while shortening the working time.

  As described above, according to the present invention, since the natural oxide film that affects the CV characteristics of the n-type silicon single crystal wafer is removed in advance, a CV characteristic closer to the true value than before can be obtained. In addition, since the influence of the natural oxide film that gradually increases day by day is eliminated, daily fluctuations in resistivity can be reduced.

It is a schematic process drawing which shows an example of the evaluation method of the silicon single crystal wafer of this invention. It is the schematic which shows an example of a natural oxide film removal process. It is the schematic which shows an example of an ozone gas generator. It is a conceptual diagram which shows a mode that the static electricity electrically charged by the main surface of the n-type silicon single crystal wafer is removed. It is the schematic which shows an example of an electrostatic removal apparatus. It is a conceptual diagram which shows the capacity | capacitance formed in series when a reverse bias voltage is applied. It is a schematic explanatory drawing which shows the capacity | capacitance formed in series when a reverse bias voltage is applied. It is a graph which shows that a difference with a true capacity | capacitance becomes large, so that resistivity is small. It is a graph which shows the resistivity change of an Example. It is a graph which shows the resistivity change of a comparative example.

Hereinafter, the present invention will be described in detail as an example of an embodiment with reference to the drawings, but the present invention is not limited thereto.
FIG. 1 is a schematic process diagram showing an example of a silicon single crystal wafer evaluation method of the present invention.
First, an n-type silicon single crystal wafer, for example, an n-type silicon epitaxial wafer is prepared (FIG. 1A).
Next, the n-type silicon single crystal wafer is treated with hydrofluoric acid to remove the natural oxide film formed on the surface of the wafer (FIG. 1 (b) natural oxide film removing step).
Subsequently, a thin silicon oxide film is formed on the surface of the n-type silicon single crystal wafer (FIG. 1 (c) silicon oxide film forming step).
Finally, the CV characteristic at the desired position of the n-type silicon single crystal wafer is measured (FIG. 1 (d) CV characteristic measurement step).

Below, it demonstrates in more detail about each process of the evaluation method of the silicon single crystal wafer of this invention.
FIG. 2 is a schematic view showing an example of the natural oxide film removing step.
First, the n-type silicon single crystal wafer 12 is immersed in hydrofluoric acid 41, and the natural oxide film 42 formed on the surface is removed by etching (FIG. 2A). The concentration of hydrofluoric acid 41 is, for example, 25 to 50% by weight.

  Next, the n-type silicon single crystal wafer 12 is rinsed with deionized water 43 to sufficiently remove the hydrofluoric acid 41 (FIG. 2B). When rinsing with deionized water 43 is performed continuously in two or three rinsing tanks, hydrofluoric acid can be efficiently removed.

  Then, the n-type silicon single crystal wafer 12 taken out from the deionized water 43 is dried to remove moisture on the surface (FIG. 2 (c)). Examples of the drying method include spin drying, spraying compressed air or compressed nitrogen.

  Subsequently, a thin silicon oxide film 15 is formed on the main surface of the n-type silicon single crystal wafer 12 from which the natural oxide film has been removed using, for example, the ozone gas generator 10 shown in FIG. The ozone gas generator 10 includes a mercury lamp 11 that is an ultraviolet light source and a mounting portion 13 for a silicon single crystal wafer 12, and a light shielding plate 14 is disposed between the mercury lamp 11 and the mounting portion 13. ing.

  The ultraviolet ray irradiated from the mercury lamp 11 includes a wavelength of 184.95 nm. When ultraviolet rays are irradiated in the air, oxygen molecules are decomposed by light having a wavelength of 184.95 nm to generate ozone gas and atomic oxygen.

  The light shielding plate 14 is formed by overlapping a plurality of punching metals, and the positions of the holes of the upper and lower punching metals are shifted. Although ozone gas and atomic oxygen pass through the gaps and holes between the punching metals and reach the silicon single crystal wafer 12, the ultraviolet rays are almost shielded. The light shielding rate is 95% or more, more desirably 99% or more and less than 100%. If the light shielding rate is 95% or more, it is possible to suppress the occurrence of noise in the CV characteristics. Further, if the light shielding rate is less than 100%, ozone gas cannot pass through, and the phenomenon that the silicon oxide film is not formed on the main surface of the silicon single crystal wafer 12 can be suppressed.

  When ultraviolet rays are irradiated from the mercury lamp 11 for a predetermined time in the air, a silicon oxide film 15 having a thickness of about 0.3 nm is formed on the main surface of the n-type silicon single crystal wafer 12.

  When the silicon oxide film 15 is formed on the main surface of the silicon single crystal wafer 12 using the ozone gas generator 10, negative static electricity is charged on the main surface of the silicon single crystal wafer 12 as shown in FIG. When silicon single crystal wafer 12 is n-type, negative static electricity and n-type carriers repel each other, and depletion layer 32 is formed near the main surface. The width of the depletion layer 32 becomes narrower as static electricity is discharged over time.

  When the CV characteristic is measured in a state where the depletion layer 32 caused by negative static electricity is formed, the width of the depletion layer 32 becomes narrower along with the discharge of static electricity, so the CV characteristic cannot be measured stably. .

  Therefore, after the silicon oxide film 15 is formed on the main surface of the n-type silicon single crystal wafer 12 using the ozone gas generator 10, static elimination is performed (FIG. 4B). Then, since the static electricity charged on the main surface of the silicon single crystal wafer 12 is removed, the depletion layer 32 disappears at the same time, and the decrease in resistivity with time due to the discharge of static electricity is improved.

  For example, as shown in FIG. 5, the static eliminator 20 that performs static elimination includes an electrode needle 21 that generates + ions, an electrode needle 22 that generates − ions, a power source 23 that applies a DC voltage to the electrode needles 21 and 22, 24 and a ground electrode 25, and a corona discharge is generated by applying a high voltage to the electrode needle 21 and the electrode needle 22.

  When the corona discharge occurs, the air existing around the electrode needles 21 and 22 is electrically decomposed to generate ions, and the silicon single crystal wafer is electrically neutralized with the static electricity having the opposite polarity by the ions. 12 is removed.

  When a mercury electrode 31 is bonded onto the silicon oxide film 15 of the n-type silicon single crystal wafer 12 subjected to static elimination and applied while changing the reverse bias voltage continuously, the n-type silicon single crystal wafer is applied according to the reverse bias voltage. The depletion layer 33 formed in 12 spreads and is measured as a capacitance. When the relationship between the reverse bias voltage and the capacitance is plotted on a graph, CV characteristics can be obtained.

  At this time, as shown in FIGS. 6A and 7A, when the silicon oxide film 15 is formed on the natural oxide film 42 without removing the natural oxide film 42, a reverse bias voltage is applied. In addition, a capacitance C15 caused by the silicon oxide film 15, a capacitance C42 caused by the natural oxide film 42, and a capacitance C33 caused by the depletion layer 33 formed in the n-type silicon single crystal wafer 12 according to the reverse bias voltage are connected in series. Formed.

The capacitance C of the capacitor having the area A and the thickness d is expressed by equation (1).

C = A * ε ₀ * ε _{SiO 2} / d (1)
Here, ε ₀ is a vacuum dielectric constant of 8.85 × 10 ⁻¹² F / m, and ε _{SiO 2} is a relative dielectric constant of silicon oxide of 3.84. The area A is the area of the mercury electrode 31 and is 2 mm ² , for example.

When the thickness d of the silicon oxide film 15 formed by the ozone gas generator 10 is, for example, 0.3 nm, the capacitance C15 is

C15 = 2 * 10 ⁻⁶ * 8.85 × 10 ⁻¹² * 3.84 / 0.3 * 10 ⁻⁹
= 227 × 10 ⁻⁹ F (2)
It is.

Moreover, the thickness d of a general natural oxide film is 1 nm, and its capacity C42 is

C42 = 2 * 10 ⁻⁶ * 8.85 × 10 ⁻¹² * 3.84 / 1 * 10 ⁻⁹
= 68 × 10 ⁻⁹ F (3)
It is.

The capacitance C33 formed when a reverse bias voltage is applied to the n-type silicon single crystal wafer 12 is 1.6 × 10 ⁻⁹ F when the resistivity is 0.05 Ωcm, for example.

Since the capacitor C15, the capacitor C42, and the capacitor C33 are formed in series, if the entire capacity is C,

1 / C = 1 / C15 + 1 / C42 + 1 / C33
= 1/227 × 10 ⁻⁹ + ^1/68 × 10 ⁻⁹ + 1 / 1.6 × 10 ⁻⁹
= 0.644 × 10 ⁹ (4)
∴C = 1.55 × 10 ⁻⁹ F
It is. At this time, the difference C33-C between the true capacitance C33 and the measured capacitance C is:

(C33-C) / C33 × 100 = (1.6−1.55) /1.6×100
= 3.1% (5)
It is.

On the other hand, as shown in FIGS. 6B and 7B, when the natural oxide film 42 is removed by etching with, for example, hydrofluoric acid in advance, a capacitor C15 and a capacitor C33 are formed in series, and the overall capacitance is

1 / C = 1 / C15 + 1 / C33
= 1/227 × 10 ⁻⁹ + 1 / 1.6 × 10 ⁻⁹
= 0.629 × 10 ⁹ (6)
∴C = 1.59 × 10 ⁻⁹ F
It is. At this time, the difference C33-C between the true capacitance C33 and the measured capacitance C is:

(C33-C) / C33 × 100 = (1.6−1.59) /1.6×100
= 0.6% (7)
Thus, by removing the natural oxide film 42 in advance, the difference from the true capacitance C33 can be greatly reduced.

  Similarly, when the difference C33-C between the true capacitance C33 and the measured capacitance C is determined in the n-type resistivity range of 0.04 Ωcm to 37 Ωcm, the resistivity is 1 Ωcm or less as shown in FIG. It can be seen that the difference from the true capacity is remarkably increased in the region. Therefore, when the resistivity of the n-type silicon single crystal wafer 12 for measuring CV characteristics is 1 Ωcm or less, more desirably 0.2 Ωcm or less, the natural oxide film 42 is removed in advance, and then the silicon oxide film 15 is removed. It is desirable to form. When the resistivity is less than 0.04 Ωcm, it is difficult to obtain stable CV characteristics because the depletion layer 33 hardly expands even when a reverse bias voltage is applied.

When the natural oxide film 42 is removed in advance, the difference C33-C between the true capacitance C33 and the measured capacitance C is significantly reduced, so that a CV characteristic closer to the true value than before can be obtained.
When the reverse bias voltage value and the capacitance value obtained by the measurement are substituted into, for example, the expressions (8) and (9), the depth W of the n-type epitaxial layer and the dopant concentration N (W) at the depth W are calculated. Therefore, a dopant concentration profile in the depth direction in the epitaxial layer can be obtained.

W = Aε _Si / C (8)

N (W) = 2 / ( qε Si A 2) * {d (C -2) / dV} -1 ··· (9)
Where W is the width of the depletion layer corresponding to the depth in the epitaxial layer, A is the area of the mercury electrode (contact area when mercury is bonded to the surface of the epitaxial layer), ε _Si is the dielectric constant of silicon, and C is Capacitance capacitance, N (W) is a dopant concentration at a depth W in the epitaxial layer, q is an electron charge amount, and V is an applied voltage.

  If the measurement depth is specified in the profile of the dopant concentration in the depth direction in the epitaxial layer, the dopant concentration at that depth can be measured. Further, the dopant concentration can be converted into resistivity by converting the obtained dopant concentration by a conversion formula such as ASTM STANDARDDS F723.

  When the dopant concentration or resistivity obtained in this way is measured every day at the same location on the n-type silicon single crystal wafer 12, the influence of the gradually increasing natural oxide film is eliminated, so that the daily fluctuation of the resistivity is reduced.

  EXAMPLES Hereinafter, although an Example and a comparative example are shown and this invention is demonstrated more concretely, this invention is not limited to these.

(Example)
A silicon epitaxial wafer 12 having an n-type silicon epitaxial layer having a resistivity of 0.17 Ω · cm was prepared, treated with 25 wt% hydrofluoric acid, and the natural oxide film was removed by etching. After sufficiently rinsing with deionized water and drying, a silicon oxide film 15 having a thickness of about 0.3 nm was formed on the main surface of the silicon epitaxial wafer 12 by the ozone gas generator 10 using the light shielding plate 14.

  Subsequently, after the static electricity charged on the main surface of the silicon epitaxial wafer 12 is removed by the static eliminator 20, the mercury electrode is joined and the CV characteristic at the center of the silicon epitaxial wafer 12 is measured to determine the resistivity. Convert to. Measurement was carried out for 7 days by this procedure, and the results are shown in FIG. The resistivity was stable from the first day to the last day, and the resistivity increased during that period was 1.8%.

(Comparative example)
A silicon epitaxial wafer 12 having an n-type silicon epitaxial layer having a resistivity of 0.17 Ω · cm is prepared, and the main surface of the silicon epitaxial wafer 12 is obtained by the ozone gas generator 10 using the light shielding plate 14 without removing the natural oxide film by etching. A silicon oxide film 15 having a thickness of about 0.3 nm was formed on the surface.

  Subsequently, after the static electricity charged on the main surface of the silicon epitaxial wafer 12 is removed by the static eliminator 20, the mercury electrode is joined and the CV characteristic at the center of the silicon epitaxial wafer 12 is measured to determine the resistivity. Converted into On and after the second day, the CV characteristics were measured after removing static electricity without removing the natural oxide film and additionally forming the silicon oxide film 15. As a result, as shown in FIG. 10, the resistivity gradually increased from the first day to the last day, and increased by 9.4% within the period.

  According to the silicon single crystal wafer evaluation method of the present invention, the natural oxide film that affects the CV characteristics of the n-type silicon single crystal wafer is removed in advance, so that the CV characteristics closer to the true value than before are obtained. Can be obtained. Further, since the influence of the natural oxide film that gradually increases day by day is eliminated, the daily fluctuation of resistivity is reduced.

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

10 ... ozone gas generator, 11 ... mercury lamp,
12 ... n-type silicon single crystal wafer, 13 ... mounting part, 14 ... light shielding plate,
15 ... Silicon oxide film, 20 ... Static eliminator, 21, 22 ... Electrode needle,
23, 24 ... power source, 25 ... ground electrode, 31 ... mercury electrode,
32 ... depletion layer (depletion layer formed by static electricity),
33 ... depletion layer (depletion layer formed by reverse bias), 41 ... hydrofluoric acid,
42 ... Natural oxide film, 43 ... Deionized water.

Claims (4)

A natural oxide film removing step of treating the n-type silicon single crystal wafer with hydrofluoric acid to remove the natural oxide film;
An oxide film forming step of forming an oxide film on the surface of the n-type silicon single crystal wafer from which the natural oxide film has been removed;
Removing the static electricity charged on the main surface of the n-type silicon single crystal wafer by neutralizing the n-type silicon single crystal wafer on which the oxide film is formed;
And a CV characteristic measuring step of measuring a CV characteristic by bonding a mercury electrode to the surface of the n-type silicon single crystal wafer on which the oxide film has been formed and subjected to static elimination. Crystal wafer evaluation method.   2. The method for evaluating a silicon single crystal wafer according to claim 1, wherein the resistivity of the n-type silicon single crystal wafer is 0.04 Ωcm or more and 1 Ωcm or less.   The method for evaluating a silicon single crystal wafer according to claim 2, wherein the resistivity of the n-type silicon single crystal wafer is 0.2 Ωcm or less. In the oxide film forming step, ozone gas is allowed to pass between the ultraviolet light source and the silicon single crystal wafer mounting portion, but a light-shielding plate that shields light is disposed, and the n-type is formed using ozone gas while shielding ultraviolet light. A silicon oxide film is formed on the surface of a silicon single crystal wafer,
4. The silicon single unit according to claim 1, wherein in the CV characteristic measurement step, a CV characteristic is measured by bonding a mercury electrode on the silicon oxide film. 5. Crystal wafer evaluation method.

Images