JP2003045926A - Method for measuring carrier concentration of silicon epitaxial layer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for measuring a carrier concentration of the silicon epitaxial layer by means of surface photovoltage technique in which the carrier concentration of a silicon epitaxial layer can be measured in a non-contact manner, speedily and easily, and further, the variation in fluctuation between days or in repeatability, etc., is small, for high reliability. SOLUTION: In the method for measuring the carrier concentration, where the carrier concentration of the silicon epitaxial layer grown on a silicon wafer is measured by means of the surface photovoltage technique, a depletion layer of maximum width is formed by applying an electric field on the surface of the silicon epitaxial wafer and the carrier concentration is obtained by measuring the maximum depletion layer width.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring the carrier concentration of a silicon epitaxial layer, and more particularly to a method for highly reliably measuring the carrier concentration of an epitaxial layer by a surface photovoltage method.

[0002]

2. Description of the Related Art Silicon epitaxial wafers have been used for a long time as wafers for widely manufacturing individual semiconductors, bipolar ICs, etc. because of their excellent characteristics. Further, MOS LSIs are also widely used in microprocessor units and flash memory devices because of their excellent soft error and latch-up characteristics. Furthermore, the demand for epitaxial wafers is ever increasing in order to reduce the reliability defects of DRAMs caused by so-called grown-in defects that are introduced during the production of silicon single crystals.

In such a silicon epitaxial wafer, since the epitaxial layer directly serves as a device active region, accurate control and measurement of its carrier concentration are extremely important for device operation.

Conventionally, as a method of measuring the carrier concentration of an epitaxial layer of a silicon wafer, a Schottky junction is generally formed and a so-called CV method (Capa method) is used.
Citance-Voltage method). In order to form a Schottky junction, a metal electrode is generally deposited on the surface of the epitaxial layer, but Hg is deposited on the surface of the epitaxial layer.
A method of forming a Schottky junction by bringing a probe into contact is also used.

However, in the conventional CV method described above,
The pretreatment for forming the Schottky junction by vacuum vapor deposition required about 1 hour, and it took time to measure the carrier concentration. Therefore, there is a problem in that the epitaxial growth must be interrupted during the measurement of the carrier concentration, which lowers the productivity. Further, since a metal vapor deposition or Hg probe is brought into contact with the wafer surface to form a Schottky junction, so-called destructive inspection is required, and a monitor wafer for inspection is additionally required, which causes a problem in terms of cost. there were.

Therefore, in order to solve such a problem, the surface photovoltage method (Su
rface Photovoltage method: SPV method)
In recent years, a measurement method using is investigated. This measuring method takes, for example, only about 0.1 seconds per measuring point, and since it can be measured in a non-contact manner, the measuring method is excellent in productivity and cost without contaminating or destroying the wafer. Is expected as.

This method of measuring the carrier concentration utilizes the fact that the depletion layer width W can be measured by the surface photovoltage method, and the carrier concentration is obtained from the depletion layer width W. Regarding the measurement of the depletion layer width W by the surface photovoltage method, see J.
Voc. Sci. Technol. 20 (1982)
p. 811 Eq. It is shown in 15. Regarding the relationship between the maximum depletion layer width W _max and the dopant concentration (carrier concentration) N _s in the depletion layer, for example, Physic
s and technology of semic
onducer devices (Jhon Wil
ley & Sons, Inc. , New York,
1967) p. Shown at 270.

As a device for measuring the surface photovoltage,
For example, Surface manufactured by QC Solutions
A Charge Profiler (hereinafter abbreviated as SCP) is known. The range of resistivity that can be measured by this SCP is usually 0.1 to 1000 Ωcm.

The measurement principle of SCP will be described below. First, a wafer in thermal equilibrium is irradiated with light (hν) having energy higher than the band gap of Si on the sample surface, and excess carriers are generated at a penetration depth corresponding to the wavelength of the irradiated light. The generated electrons move to the surface side and the holes move to the edge of the depletion layer. The generated minority carriers (electrons e in the p-type semiconductor) change the barrier height on the surface by δV _s . The potential δV _s at this time is called the SPV value.

Using this SPV value, the depletion layer width can be calculated according to the following equation (1). _{δV s = -j (δφ / ω} ) (1-R) q (W d / ε s) ··· (1) where, j is imaginary unit, phi excitation light intensity, omega is the angular frequency of the excitation light , R is the reflectance of the wafer surface, q is the unit charge amount, and ε _s is the dielectric constant of the semiconductor.

Then, in SCP, the measured depletion layer width W
Assuming that _d is the maximum depletion layer width W _max , the carrier concentration N _s can be calculated from the following equation (2). _{W max = [2ε s kTln (} N s / n i) / q 2 N s] 1/2 ··· (2) where, k is the Boltzmann constant, T is the absolute temperature, the _{n i} is the intrinsic free carrier concentration Represent

The excitation light source used for the measurement has a shorter wavelength (eg 450 nm) than that used in the usual surface photovoltage method.
When the excitation light source is used, the penetration depth of light is reduced to 0.
The thickness can be 4 μm or less, and the carrier concentration (strictly speaking, the dopant concentration) of only the wafer surface layer can be evaluated.

As described above, the surface photovoltage method has an advantage that the carrier concentration of the silicon epitaxial layer can be measured quickly, simply and in a non-contact manner, and in particular, the surface layer of the epitaxial layer can be evaluated. However, when the carrier concentration of the epitaxial layer is measured by this method, the measured value is not stable and changes over time, and the data changes depending on the cleaning method of the measurement wafer and other processing methods. was there.

Therefore, in Japanese Patent Laid-Open No. 10-270517, in the method of measuring the carrier concentration of a silicon epitaxial layer grown on a silicon wafer by the surface photovoltage method, the change over time of the carrier concentration is obtained in advance to manufacture an epitaxial wafer. By measuring the time from the removal of the wafer from the device to the measurement of the carrier concentration by the surface photovoltage method, the true carrier concentration N ₀ is obtained from the measured value N _s of the carrier concentration and the change with time of the carrier concentration. A method for measuring concentration is disclosed.

However, even if the carrier concentration of a silicon epitaxial wafer is measured using the method disclosed in Japanese Patent Application Laid-Open No. 10-270517, there are cases in which there are large variations over time and variations in reproducibility, and the reliability of measured values is high. However, it was difficult to use it for inspection and quality assurance.

[0016]

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 measuring the carrier concentration of a silicon epitaxial layer by the surface photovoltage method. Another object of the present invention is to provide a highly reliable carrier concentration measuring method which can measure the carrier concentration in a non-contact manner quickly and easily and has little variation in day-to-day variation and reproducibility.

[0017]

In order to solve the above problems, according to the present invention, there is provided a carrier concentration measuring method for measuring the carrier concentration of a silicon epitaxial layer grown on a silicon wafer by a surface photovoltage method. , A carrier concentration measuring method is characterized in that an electric field is applied to a surface of a silicon epitaxial wafer to form a depletion layer having a maximum depletion layer width, and the carrier concentration is determined by measuring the maximum depletion layer width. (Claim 1).

As described above, when the carrier concentration of the silicon epitaxial layer is measured by the surface photovoltage method, an electric field is applied to the surface of the silicon epitaxial wafer to form a depletion layer having the maximum depletion layer width. Can be measured in a non-contact manner quickly and easily, and since the depletion layer width hardly changes, the carrier concentration can be measured with high reliability and little variation in measured values.

In this case, it is preferable that the electric field is applied to the surface of the silicon epitaxial wafer by subjecting the surface of the silicon epitaxial wafer to corona discharge treatment (claim 2).

As described above, by performing corona discharge treatment on the surface of the silicon epitaxial wafer, a depletion layer having a maximum depletion layer width can be formed in a non-contact and non-destructive manner, whereby a precise carrier can be obtained without contaminating the silicon wafer. It is possible to measure the concentration. In addition, the corona discharge treatment means that a metal wire having a diameter of about 100 μm is 6-1
It applies a high voltage of 0 kV to cause corona discharge to treat the surface of the intended dielectric or the like, and is widely used in the semiconductor industry for antistatic treatment and the like.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below, but the present invention is not limited thereto. Conventionally, when the carrier concentration of a silicon epitaxial layer is measured by the surface photovoltage method, variations in day-to-day variations and reproducibility are large, reliability of measured values is poor, and it is difficult to use it for quality assurance. In order to solve these problems, the inventors of the present invention have made earnest investigations and studies on the cause of large variations in day-to-day variation and reproducibility in the carrier concentration measurement by the surface photovoltage method, and as a result, the silicon epitaxial wafer surface being affected by the change in the charge state, the measured depletion width W _d is, the carrier concentration N _s
It was estimated that it does not always match the maximum depletion layer width W _max used when calculating

For example, in a p-type silicon epitaxial wafer, the depletion layer expands as the surface positive charge density increases, and eventually reaches a saturated state. At this time, when the carrier concentration is measured by the SCP, as described above, the measured depletion layer width W _d is assumed to be the maximum depletion layer width W _max ,
Since the carrier concentration N _s is calculated using the equation (2), in order to accurately measure the carrier concentration, there is a sufficient amount of positive charges on the wafer surface, and the width of the formed depletion layer is the maximum depletion. It is important that the layer width W _max is reached.

However, in the conventional measurement of the carrier concentration by the surface photovoltage method, the charge amount on the surface of the epitaxial layer may not be sufficient to reach the maximum depletion layer width W _max , and the depletion layer width is not necessarily the maximum depletion layer width W max. It is considered that the value did not match _max , which caused an apparent variation in carrier concentration. Therefore, it was expected that variations in the carrier concentration, such as daily variations, measured by the conventional surface photovoltage method were caused by changes in the depletion layer width, that is, changes in the state of charges on the wafer surface.

Therefore, the present inventor measured the silicon epitaxial wafer having a sufficient electric field applied to the wafer surface until the depletion layer width reaches the maximum depletion layer width W _max , by the surface photovoltage method. The present invention has been completed based on the idea that it is possible to perform highly reliable carrier concentration measurement with less variation in properties and the like.

That is, when the carrier concentration of the silicon epitaxial layer grown on the silicon wafer is measured by the surface photovoltage method, an electric field is applied to the surface of the silicon epitaxial wafer to obtain a depletion layer having a maximum depletion layer width W _max . Forming the maximum depletion layer width W
By measuring _max , highly reliable carrier concentration measurement can be performed. At this time, the maximum depletion layer width W
_The electric field strength or surface charge density required to reach _max can be calculated as a function of carrier concentration.

The mathematical formula for the calculation is, for example, Physi
cs of Semiconductor Device
es (WILEY-INTERSCIENCE, New
York, 1969) p. Equation 16 of 431 may be used. In FIG. 8, p-type 10 Ωcm (dopant concentration N _A =
A calculation example of the relationship between the surface potential φ _S and the charge density Q _S of the semiconductor surface in the case of 1.34 × 10 ¹⁵ / cm ³ ) will be shown. Up to reach the depletion layer width W _{m ax} is known that the surface potential phi _S may if more than twice the Fermi potential phi _f. The charge density on the semiconductor surface when φ _S = 2φ _f is 0.02 μC / cm from this figure.
It is ² . That is, if there is a positive charge of 0.02 μC / cm ² or more on the silicon epitaxial wafer,
Theoretically, the maximum depletion layer width W _max is reached.

However, when the dielectric such as the oxide film does not exist as in the present case, the positive charge does not exist stably on the silicon epitaxial wafer. For this reason,
More positive charge needs to be deposited. Therefore, it is desirable to experimentally obtain the necessary charge amount or electric field strength so that the measured carrier concentration becomes saturated and constant. Since the silicon epitaxial layer is formed so as to obtain a target carrier concentration (resistivity), the electric field may be applied with a certain margin with respect to the target carrier concentration.

When measured by SCP, the wafer surface after the growth of the epitaxial layer is usually positively charged, and therefore untreated or HF is used for the evaluation of the p-type epitaxial layer.
A treatment of charging positive charges after cleaning and finishing is effective.
On the other hand, since the n-type wafer progresses in the direction of eliminating the depletion layer, SC-1 cleaning (cleaning with a mixed solution of NH ₄ OH / H ₂ O ₂ / H ₂ O) or SC-2 cleaning (HCl / H ₂ O).
It is desirable to perform a process of charging negative charges after cleaning with a mixed solution of ₂ / H ₂ O).

Hereinafter, the present invention will be described more specifically with reference to the drawings, but the present invention is not limited thereto. A conceptual diagram of the corona discharge treatment used in the present invention is shown in FIG. As shown in FIG. 1, a P / P ⁺ silicon epitaxial wafer 2 obtained by growing a p-type low-resistivity substrate on which a p-type normal-resistivity epitaxial layer is grown is placed on the stage 1 as shown in FIG. A high voltage is applied between the metal wire electrode 3 and the stage 1 which are arranged substantially above the central part of the device so that the metal wire electrode 3 becomes a positive electrode, and corona discharge is generated on the silicon wafer. Then,
Positive ions 4 will be poured onto the surface of the silicon wafer on the negatively charged stage.

In this way, an electric field is applied to the P type silicon wafer 2 to form a depletion layer. At this time, since it is important that the depletion layer width reaches the maximum depletion layer width W _max , a certain margin is calculated according to the carrier concentration of the silicon epitaxial layer, and the distance between the wafer and the metal line electrode or the discharge It is preferable to adjust the time and apply the electric field. The shape of the electrode may be any shape, and various shapes such as a needle shape (rod) or a linear shape (wire) can be used.

In order to carry out the measuring method of the present invention,
An apparatus for performing corona discharge treatment and a measuring device for measuring the carrier concentration of the epilayer by the surface photovoltage method may be separately prepared, and the electric field application treatment and the carrier concentration measurement may be separately performed. The discharge treatment device 5 and the carrier concentration measuring device 6 may be combined to form a highly reliable carrier concentration measuring device 7.

Furthermore, the carrier concentration can be measured after or during the corona discharge treatment on the same stage.
It is preferable to combine a corona discharge electrode and a measurement probe. For example, as shown in FIG. 3, after the corona discharge is generated on the silicon wafer 2 using the corona discharge rod 8 to charge the positive charge 4, the depletion layer width is measured using the measurement probe 9. Alternatively, as shown in FIG. 4, it is also possible to perform corona discharge treatment using the corona discharge wire 10 as an electrode, and immediately thereafter measure with a probe that is on standby. Further, as shown in FIG. 5, while the positive charge 4 is being charged on the wafer 2 from the corona discharge unit 11 via the tube 12, the measurement probe 9
It is also possible to measure the depletion layer by.

As described above, when the means for applying the electric field to the surface of the silicon wafer and the means for measuring the carrier concentration are combined, the measurement is extremely simple and quick as compared with the case where each device is treated individually. In addition to being able to maintain the charge state of the surface stably, it is possible to perform more reliable measurement.

[0034]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples of the present invention, but the present invention is not limited thereto. (Example) 0.008 Ωc in a single wafer type atmospheric pressure CVD apparatus
A so-called P / P ⁺⁺ epitaxial wafer having a p-type epitaxial layer having a target resistivity level of 20, 10, 5 Ωcm and a film thickness of 5 μm was produced on a m-type p-type silicon wafer. After taking out the wafer from the CVD apparatus, corona discharge was generated in the corona discharge treatment, and a positive charge was attached to the surface of the wafer. At this time, for corona discharge treatment, KG101 CORONA
CHARGE GENERATOR (manufactured by SEMILAB) was used to set a positive charge of 1 μC to adhere to obtain the maximum depletion width. Immediately after the corona discharge treatment, the carrier concentration of the epitaxial layer was measured by SCP. Then, using the same wafer, the corona discharge treatment and the carrier concentration measurement similar to the above were repeatedly performed for 10 days, and the daily variation of the measured value was examined.

FIG. 6 shows the measurement results of silicon epitaxial wafers having different resistivities.
The daily variation of the measured carrier concentration is 2.3% or less in any of the wafers, which shows that the carrier concentration measuring method according to the present invention is a highly reliable measuring method with little variation. Regarding the absolute value, it is desirable to obtain a correction formula by comparing the measured value with the CV method or the like.

(Comparative Example) Next, silicon epitaxial wafers having three levels of resistivity were prepared as in Example 1, and S was applied to each wafer without applying an electric field.
The carrier concentration measurement by CP was repeated for 15 days. The result is shown in FIG. 7. After the epitaxial wafer was manufactured, the carrier concentration of each wafer monotonously decreased until the 5th day, and the daily fluctuation for 10 days after the 5th day showed a large value of about 4%.

The present invention is not limited to the above embodiment. The above-described embodiment is merely an example, and it has substantially the same configuration as the technical idea described in the scope of claims of the present invention, and has the same operational effect.
Anything is included in the technical scope of the present invention.

[0038]

As described above, according to the present invention,
When measuring the carrier concentration of the silicon epitaxial layer by the surface photovoltage method, a highly reliable silicon epitaxial layer can be obtained by forming a depletion layer with the maximum depletion layer width by applying an electric field to the surface of the silicon wafer. The carrier concentration of the layer can be measured. Further, since the corona discharge treatment and the measurement by the surface photovoltage method can be performed in a non-contact and non-destructive manner, a monitor wafer is not required and a large cost reduction effect can be obtained.

[Brief description of drawings]

FIG. 1 is a conceptual diagram of corona discharge treatment used in the present invention.

FIG. 2 is a conceptual diagram of a carrier concentration measuring device according to the present invention.

FIG. 3 is a conceptual diagram showing a measuring device in which a corona discharge rod and a measuring probe are combined.

FIG. 4 is a conceptual diagram showing a measuring device in which a corona discharge wire and a measuring probe are combined.

FIG. 5 is a conceptual diagram showing a measuring device in which a corona discharge unit and a measuring probe are combined.

FIG. 6 is a graph showing the daily variation of carrier concentration measured according to an example.

FIG. 7 is a graph showing daily variations in carrier concentration measured by a comparative example.

FIG. 8 is a graph showing the relationship between the surface potential φ _S and the charge density Q _{S on} the semiconductor surface.

[Explanation of symbols]

1 ... Stage, 2 ... Silicon wafer, 3 ... Metal wire electrode, 4 ... Positive charge, 5 ... Corona discharge treatment device, 6 ... Carrier concentration measuring device, 7 ... Carrier concentration measuring device, 8 ...
Corona discharge rod, 9 ... Measurement probe, 10 ... Corona discharge wire 11 ... Corona discharge unit, 12 ... Tube.

Claims (2)

[Claims] 1. A carrier concentration measuring method for measuring a carrier concentration of a silicon epitaxial layer grown on a silicon wafer by a surface photovoltage method, comprising depletion of a maximum depletion layer width by applying an electric field to the surface of the silicon epitaxial wafer. A carrier concentration measuring method comprising forming a layer and determining a carrier concentration by measuring the maximum depletion layer width. 2. The carrier concentration measuring method according to claim 1, wherein the electric field is applied to the surface of the silicon epitaxial wafer by subjecting the surface of the silicon epitaxial wafer to corona discharge treatment.