JP4894104B2 - Method for measuring carrier concentration of silicon epitaxial layer - Google Patents

Description

[0001]
BACKGROUND 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 measuring the carrier concentration of an epitaxial layer with high reliability by a surface photovoltage method.
[0002]
[Prior art]
Silicon epitaxial wafers have long been used as wafers for manufacturing individual semiconductors, bipolar ICs and the like because of their excellent characteristics. 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 increasing in order to reduce the reliability failure of DRAMs caused by so-called Grown-in defects introduced during the production of silicon single crystals.
[0003]
In such a silicon epitaxial wafer, since the epitaxial layer directly becomes a device active region, accurately controlling and measuring the carrier concentration is extremely important for device operation.
[0004]
Conventionally, as a method of measuring the carrier concentration of an epitaxial layer of a silicon wafer, a method of forming a Schottky junction and measuring by a so-called CV method (Capacitance-Voltage method) has been performed. In order to form a Schottky junction, a metal electrode is generally deposited on the surface of the epitaxial layer. However, a method of forming a Schottky junction by bringing a Hg probe into contact with the surface of the epitaxial layer is also performed. It is.
[0005]
However, in the conventional CV method described above, the pretreatment for forming the Schottky junction by vacuum deposition requires about 1 hour, and it takes time to measure the carrier concentration. Therefore, there is a problem that the epitaxial growth must be interrupted during the measurement of the carrier concentration, and the productivity is lowered. Moreover, since metal 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 separate monitor wafer is required for inspection. there were.
[0006]
Therefore, in order to solve such a problem, a measurement method using a surface photovoltage method (SPV method) has recently been studied in measuring the carrier concentration. This measuring method, for example, takes only about 0.1 seconds per measuring point and can measure in a non-contact manner, so that the measuring method is excellent in both productivity and cost without contaminating or destroying the wafer. As expected.
[0007]
This carrier concentration measurement method uses the fact that the depletion layer width W can be measured by the surface photovoltage method, and obtains the carrier concentration from the depletion layer width W. Regarding the measurement of the depletion layer width W by the surface photovoltage method, see, for example, Voc. Sci. Technol. 20 (1982) p. 811 Eq. 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, Physics and technology of semiconductor devices (Jon Willy & Sons, Inc., New York, 1967). . It is shown at 270.
[0008]
As an apparatus for measuring the surface photovoltage, for example, Surface Charge Profiler (hereinafter abbreviated as SCP) manufactured by QC Solutions is known. The range of resistivity measurable by this SCP is usually 0.1 to 1000 Ωcm.
[0009]
The principle of SCP measurement will be described below. First, by irradiating the wafer surface in thermal equilibrium with light (hν) having energy equal to or greater than the Si band gap, 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 end of the depletion layer. The generated minority carriers (electrons e in the p-type semiconductor) change the surface barrier height by δV _s .
The potential δV _s at this time is called an SPV value.
[0010]
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)
Here, j is an imaginary unit, φ is the excitation light intensity, ω 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.
[0011]
In SCP, the measured depletion layer width W _d is assumed to be the maximum depletion layer width W _max, and 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)
Here, k is the Boltzmann constant, T is the absolute temperature, n _i represents the intrinsic free carrier concentration.
[0012]
By using an excitation light source with a shorter wavelength (for example, 450 nm) than that used in the normal surface photovoltage method, the light penetration depth can be reduced to 0.4 μm or less, and the carrier concentration only on the wafer surface layer (Strictly speaking, the dopant concentration) can be evaluated.
[0013]
As described above, the surface photovoltage method has an advantage that the measurement of the carrier concentration of the silicon epitaxial layer can be performed 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 due to differences in the cleaning method of the measurement wafer and other processing methods. was there.
[0014]
In Japanese Patent Laid-Open No. 10-270517, in the method of measuring the carrier concentration of the silicon epitaxial layer grown on the silicon wafer by the surface photovoltage method, the change with time of the carrier concentration is obtained in advance, and the wafer is manufactured from the epitaxial wafer manufacturing apparatus. Concentration measurement method for determining the true carrier concentration N ₀ from the measured value N _s of the carrier concentration and the change over time of the carrier concentration by measuring the time from when the carrier is taken out until the carrier concentration is measured by the surface photovoltage method Is disclosed.
[0015]
However, even if the carrier concentration of a silicon epitaxial wafer is measured using the method disclosed in Japanese Patent Laid-Open No. 10-270517, there may be large variations in daily fluctuation and reproducibility, and the reliability of the measured value is sufficient. However, it was difficult to use for inspection and quality assurance.
[0016]
[Problems to be solved by the invention]
The present invention has been made in view of the above problems, and an object of the present invention is a method for measuring the carrier concentration of a silicon epitaxial layer by a surface photovoltage method. Another object of the present invention is to provide a highly reliable carrier concentration measuring method that can be easily measured and has small variations such as daily fluctuation and reproducibility.
[0017]
[Means for Solving the Problems]
In order to solve the above problems, according to the present invention, there is provided a carrier concentration measurement method for measuring the carrier concentration of a silicon epitaxial layer grown on a silicon wafer by a surface photovoltage method, wherein the silicon epitaxial wafer is free of a dielectric. A carrier concentration measurement method is provided, wherein an electric field is applied to the surface of the substrate to form a depletion layer having a maximum depletion layer width, and the carrier concentration is obtained by measuring the maximum depletion layer width. ).
[0018]
Thus, when measuring the carrier concentration of the silicon epitaxial layer 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, and this maximum depletion layer width is measured. Thus, measurement can be performed in a non-contact, quick and simple manner, and since there is almost no variation in the depletion layer width, it is possible to perform highly reliable carrier concentration measurement with little variation in measured values.
[0019]
In this case, it is preferable that the electric field is applied to the surface of the silicon epitaxial wafer in which the dielectric does not exist by performing a corona discharge treatment on the surface of the silicon epitaxial wafer.
[0020]
In this way, by performing corona discharge treatment on the surface of the silicon epitaxial wafer, a depletion layer with the maximum depletion layer width can be formed in a non-contact and non-destructive manner, thereby accurately measuring the carrier concentration without contaminating the silicon wafer. It becomes possible to do.
In the corona discharge treatment, a high voltage of 6 to 10 kV is applied to a metal wire having a diameter of about 100 microns to cause a corona discharge to treat the surface of a target dielectric or the like. Widely used in processing.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, although an embodiment is described about the present invention, the present invention is not limited to these.
Conventionally, when the carrier concentration of a silicon epitaxial layer is measured by the surface photovoltage method, variations such as daily fluctuations and reproducibility are large, the measured values are not reliable, and are difficult to use for quality assurance.
In order to solve these problems, the inventors of the present invention have conducted extensive investigations and examinations on the causes of large variations in daily fluctuation and reproducibility in the carrier concentration measurement by the surface photovoltage method. It was estimated that the measured depletion layer width W _d was not necessarily equal to the maximum depletion layer width W _max used in calculating the carrier concentration N _s .
[0022]
For example, in a p-type silicon epitaxial wafer, when the positive charge density on the surface increases, the depletion layer spreads 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 and the carrier concentration N _s is calculated using the equation (2). Therefore, in order to accurately measure the carrier concentration, it is important that a sufficient amount of positive charges exist on the wafer surface and the width of the formed depletion layer reaches the maximum depletion layer width _Wmax .
[0023]
However, in the measurement of the carrier concentration by conventional surface photovoltage method, there is a case where the charge amount of the surface of the epitaxial layer is not sufficient enough to reach the maximum depletion layer width W _max, the width of the depletion layer is not necessarily equal to the maximum depletion layer width W _max This is thought to have caused an apparent change in carrier concentration. Therefore, it was expected that the variation such as the daily fluctuation in the carrier concentration measured by the conventional surface photovoltage method is caused by the change of the depletion layer width, that is, the change of the charge state of the wafer surface.
[0024]
Therefore, the present inventor measured the silicon epitaxial wafer, in which a sufficient electric field was applied to the wafer surface until the depletion layer width reached the maximum depletion layer width W _max , by measuring the surface photovoltage method, such as daily fluctuation, reproducibility, etc. The inventors have come up with the idea that carrier concentration measurement with small variation and high reliability can be performed, and the present invention has been completed.
[0025]
That is, when the carrier concentration of the silicon epitaxial layer grown on the silicon wafer is measured by the surface photovoltage method, a depletion layer having a maximum depletion layer width W _max is formed by applying an electric field to the surface of the silicon epitaxial wafer. By measuring the maximum depletion layer width W _max , highly reliable carrier concentration measurement can be performed.
At this time, the electric field strength or surface charge density required to reach the maximum depletion layer width _Wmax can be obtained by calculation as a function of the carrier concentration.
[0026]
Formulas used for the calculation are, for example, Physics of Semiconductor Devices (WILEY-INTERSCIENCE, New York, 1969) p. The equation 16 of 431 may be used. FIG. 8 shows a calculation example of the relationship between the surface potential φ _S and the charge density Q _{S on} the semiconductor surface in the case of p-type 10 Ωcm (dopant concentration N _A = 1.34 × 10 ¹⁵ / cm ³ ). In order to reach the maximum depletion layer width W _max , it is known that the surface potential φ _S should be at least twice the Fermi potential φ _f . The charge density on the semiconductor surface in the case of φ _S = 2φ _f is 0.02 μC / cm ² from this figure. That is, if a positive charge of 0.02 μC / cm ² or more exists on the silicon epitaxial wafer, the maximum depletion layer width W _max is theoretically reached.
[0027]
However, when there is no dielectric such as an oxide film as in this case, positive charges do not exist stably on the silicon epitaxial wafer. For this reason, it is necessary to attach more positive charges. Therefore, it is desirable to experimentally determine the necessary charge amount or electric field strength so that the measured carrier concentration is saturated and constant.
Since the silicon epitaxial layer is formed so as to obtain a target carrier concentration (resistivity), an electric field may be applied with a certain margin with respect to the target carrier concentration.
[0028]
In addition, when measuring by SCP, the wafer surface after the growth of the epitaxial layer is usually positively charged. Therefore, for the evaluation of the p-type epitaxial layer, a process for charging a positive charge after the unprocessed or HF cleaning finish is effective. is there. On the other hand, since the n-type wafer proceeds in a direction in which the depletion layer disappears, 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 _2). It is desirable to perform a process of charging a negative charge after (washing with a mixed solution of / H ₂ O).
[0029]
Hereinafter, the present invention will be described more specifically with reference to the drawings. However, the present invention is not limited to these.
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 in which a p-type normal resistivity epitaxial layer is grown on a p-type low resistivity substrate, for example, is placed on a stage 1. A high voltage is applied between the metal wire electrode 3 disposed substantially above the center of the substrate and the stage 1 so that the metal wire electrode 3 becomes a positive electrode, thereby generating a corona discharge on the silicon wafer. Then, positive ions 4 are poured onto the surface of the silicon wafer on the negatively charged stage.
[0030]
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 _Wmax , 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 is calculated. It is preferable to adjust the time and apply the electric field.
The electrode may have any shape, and various shapes such as a needle shape (rod) or a wire shape (wire) can be used.
[0031]
In order to carry out the measurement method of the present invention, a device for performing corona discharge treatment and a measuring device for measuring the carrier concentration of the epilayer by the surface photovoltage method are prepared separately, and electric field application treatment and carrier concentration measurement are individually performed. However, as shown in FIG. 2, the corona discharge treatment device 5 and the carrier concentration measuring device 6 may be combined to form a highly reliable carrier concentration measuring device 7.
[0032]
Further, it is preferable to combine the corona discharge electrode and the measurement probe so that the carrier concentration can be measured after or during the corona discharge treatment on the same stage. For example, as shown in FIG. 3, the corona discharge is generated on the silicon wafer 2 using the corona discharge rod 8 to charge the positive charge 4, and then the depletion layer width is measured using the measurement probe 9. Alternatively, as shown in FIG. 4, the corona discharge treatment can be performed using the corona discharge wire 10 as an electrode, and measurement can be performed with a probe that has been waiting immediately thereafter. Further, as shown in FIG. 5, the depletion layer can be measured by the measurement probe 9 while charging the positive charge 4 on the wafer 2 from the corona discharge unit 11 through the tube 12.
[0033]
As described above, if 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 can be carried out extremely easily and quickly compared to the case of processing with each device. In addition, the charge state on the surface can be kept stable, and more reliable measurement can be performed.
[0034]
【Example】
EXAMPLES Hereinafter, although an Example and comparative example of this invention are given and this invention is demonstrated concretely, this invention is not limited to these.
(Example)
A so-called P-type epitaxial layer having a target resistivity level of 20, 10, 5 Ωcm and a thickness of 5 μm on a 0.008 Ωcm p-type silicon wafer in a single-wafer atmospheric pressure CVD apparatus. / P ⁺⁺ epitaxial wafer was produced. After removing the wafer from the CVD apparatus, corona discharge was generated in the corona discharge treatment, and positive charges were attached to the wafer surface. At this time, in the corona discharge treatment, KG101 CORONA CHARGE GENERATOR (manufactured by SEMILAB) was used, and 1 μC positive charge was attached to obtain the maximum depletion width. Immediately after the corona discharge treatment, the carrier concentration of the epitaxial layer was measured by SCP. Thereafter, using the same wafer, the same corona discharge treatment and carrier concentration measurement as described above were repeated for 10 days, and the daily fluctuation of the measured value was examined.
[0035]
FIG. 6 shows the result of measurement for each silicon epitaxial wafer having different resistivity. The daily fluctuation of the measured carrier concentration is 2.3% or less in any wafer, and it can be seen that the carrier concentration measuring method according to the present invention is a highly reliable measuring method with little variation. As for the absolute value, it is desirable to compare the measured value with the CV method or the like to obtain a correction formula.
[0036]
(Comparative example)
Next, silicon epitaxial wafers having three levels of resistivity were prepared as in Example 1, and carrier concentration measurement by SCP was repeated for 15 days without applying an electric field to each wafer. The result is shown in FIG. After the epitaxial wafer fabrication, the carrier concentration monotonously decreased until the 5th day in all wafers, and the daily fluctuation for 10 days after the 5th day showed a large value of about 4%.
[0037]
The present invention is not limited to the above embodiment. The above embodiment is merely an example, and the present invention has the same configuration as that of 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.
[0038]
【Effect of the invention】
As described above, according to the present invention, when the carrier concentration of the silicon epitaxial layer is measured by the surface photovoltage method, a depletion layer having the maximum depletion layer width is formed by applying an electric field to the surface of the silicon wafer. By performing the measurement, the carrier concentration of the silicon epitaxial layer with high reliability 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 great cost reduction effect can be obtained.
[Brief description of the 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 apparatus 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 apparatus in which a corona discharge unit and a measurement probe are combined.
FIG. 6 is a graph showing daily fluctuations in carrier concentration measured according to an example.
FIG. 7 is a graph showing daily fluctuations 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 ... Rod for corona discharge,
9 ... Probe for measurement, 10 ... Wire for corona discharge 11 ... Corona discharge unit, 12 ... Tube.

Claims (2)

A carrier concentration measurement method in which the carrier concentration of a silicon epitaxial layer grown on a silicon wafer is measured by a surface photovoltage method, and an electric field is applied to the surface of the silicon epitaxial wafer where no dielectric is present to deplete the maximum depletion layer width. A carrier concentration measuring method, wherein a carrier concentration is obtained by forming a layer and 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 in which the dielectric does not exist by performing a corona discharge treatment on the surface of the silicon epitaxial wafer.

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