JP5706776B2 - Semiconductor substrate evaluation method - Google Patents

Description

  The technical field relates to a method for evaluating a semiconductor substrate.

  As a method for evaluating a silicon substrate having a layer structure, the lifetime (bulk lifetime) of minority carriers in the silicon substrate is obtained from the time change (decay curve) of the excess carrier density obtained by the μ-PCD (Microwave-Photo Conductive Decay) method. (Also referred to as Patent Document 1).

  In addition, a method is known in which two types of semiconductor substrates having different thicknesses are measured by the μ-PCD method, the lifetime of each primary mode is obtained, and the surface recombination velocity and the bulk lifetime are separated and evaluated. (Refer nonpatent literature 1).

  The bulk lifetime is one of the indices indicating the magnitude of defect levels in the semiconductor energy gap. When a defect level exists in a particularly deep position in energy, carriers are recombined (disappeared) through the defect level. The bulk lifetime is a measure of the time (lifetime) from the generation of carriers to the disappearance by recombination through the defect level. Since the frequency of recombination increases as the number of defect levels increases, the bulk lifetime decreases.

  The μ-PCD method is an effective method for measuring a bulk lifetime because it can measure a sample such as a semiconductor substrate in a non-destructive and non-contact manner. The principle will be described below.

  The surface of the sample is irradiated with laser and microwave (also referred to as μ wave) at the same time. Excess carriers are generated in the sample by laser irradiation. When the laser irradiation is stopped, the carrier density returns to a thermal equilibrium state after a certain time due to recombination via defect levels in the energy gap. Here, by utilizing the correlation between the carrier density and the reflectance of the μ wave and detecting the reflected microwave, it is possible to follow the time change of the carrier density during the attenuation.

JP 59-55013 A

Usami Akira, Tokuda Yutaka, "Semiconductor Device Process Evaluation Technology Lifetime, DLTS Evaluation and Center", Realize Inc., September 11, 1990, p. 121-129

However, in the evaluation method for measuring two types of semiconductor substrates by the μ-PCD method, a semiconductor substrate used for measurement and a semiconductor substrate having the same parameters except for the thickness differing from the semiconductor substrate are prepared. There must be.

  Furthermore, although it is a bulk lifetime inherent to the sample, it is preferable to use a sample separated from the same ingot because there is concern about variations between semiconductor substrates, but preparation for this is complicated.

  In view of this, an object of one embodiment of the present invention is to provide a method for separating and evaluating surface recombination velocity and bulk lifetime.

One aspect of the present invention includes a bulk lifetime tau _b, the surface recombination velocity S ₀ of the one _surface, a first semiconductor substrate is the surface recombination velocity S ₀ of the other surface, the bulk lifetime tau _b A surface recombination rate S ₀ on one surface, a surface recombination rate S ₁ on the other surface, a bulk semiconductor lifetime τ _b, and a surface recombination rate S on one surface _1, a third semiconductor substrate is the surface recombination velocity S ₁ of the other surface, with respect to the lifetime tau ₁₁ of each first-order _mode, tau _21, measured tau _31, the first semiconductor substrate and The relationship between the surface recombination velocity S ₀ and the bulk lifetime τ _b , and the surface recombination velocity S ₁ , using the lifetimes τ ₁₁ , τ _{31 of the} first-order mode and the following formula (1) for the third semiconductor substrate: bulk lifetime τ Obtains a first correlation curve and a second correlation curve from the relationship of the following equation with respect to the second semiconductor substrate (2), Equation (3), using the first correlation curve and a second correlation curve The third correlation curve is obtained from the relationship between the primary mode lifetime τ ₂₁ and the bulk lifetime τ _b , and the obtained primary mode lifetime τ ₂₁ is substituted into the third correlation curve to obtain the bulk lifetime τ. _{In the} semiconductor substrate evaluation method, _b is obtained, and the obtained bulk lifetime τ _b is substituted into the first correlation curve and the second correlation curve to obtain the surface recombination velocity S ₀ and the surface recombination velocity S _1. is there.

（ただし、数式（１）中、Ｓ_Ｌ（Ｌは、０または１）は表面再結合速度［ｃｍ／ｓｅｃ］、Ｄは少数キャリア拡散定数［ｃｍ^２／ｓｅｃ］、τ_ｎ１（ｎは、１または３）は一次モードのライフタイム［μｓｅｃ］、τ_ｂはバルクライフタイム［μｓｅｃ］、Ｗは半導体基板の厚さ［ｍｍ］をそれぞれ表す。）

(In the formula (1), S _L (L is 0 or 1) is the surface recombination rate [cm / sec], D is the minority carrier diffusion constant [cm ² / sec], τ _n1 (n is 1 or 3) the lifetime of the primary mode [μsec], τ _b is the bulk lifetime [μsec], W represents the thickness of the semiconductor substrate [mm], respectively.)

（ただし、数式（２）および数式（３）中、τ_２１は一次モードのライフタイム［μｓｅｃ］、τ_ｂはバルクライフタイム［μｓｅｃ］、Ｋ_２１は第２の半導体基板の１番目の固有モードの波数［ｃｍ^−１］、Ｄは少数キャリア拡散定数［ｃｍ^２／ｓｅｃ］、Ｗは半導体基板の厚さ［ｍｍ］をそれぞれ表す。ここで、Ｓ_Ｍ０≡Ｓ_０／Ｄ、Ｓ_Ｍ１≡Ｓ_１／Ｄと表し、Ｓ_０およびＳ_１は表面再結合速度［ｃｍ／ｓｅｃ］を表す。）

(However, in Equation (2) and Equation (3), τ ₂₁ is the primary mode lifetime [μsec], τ _b is the bulk lifetime [μsec], and K ₂₁ is the first eigenmode of the second semiconductor substrate. wavenumber ^[cm -1], D is the minority carrier diffusion constant ^[cm 2 / sec], W represents the thickness of the semiconductor substrate [mm], respectively. _{here, S M0 ≡S 0 / D,} S M1 ≡S ₁ / D, S ₀ and S ₁ represent surface recombination velocity [cm / sec].)

Another embodiment of the present invention, the first of the above surface-treated only on one surface of the semiconductor substrate a second semiconductor substrate, only the second surface of the semiconductor substrate surface recombination velocity S ₀ of The third semiconductor substrate may be manufactured by performing a surface treatment.

  In another embodiment of the present invention, a silicon substrate may be used for the first semiconductor substrate, the second semiconductor substrate, and the third semiconductor substrate.

In another embodiment of the present invention, the lifetimes τ ₁₁ , τ ₂₁ , and τ ₃₁ of the primary mode may be measured using the μ-PCD method.

In one embodiment of the present invention, the surface recombination velocity S ₀ , the surface recombination velocity S _1, and the bulk lifetime τ _b can be easily obtained by evaluating the above method using three types of semiconductor substrates.

  In another embodiment of the present invention, the lifetime of the primary mode does not depend on the number of incident photons and can be evaluated with high reliability.

The figure explaining an example of the evaluation method of a semiconductor substrate. The figure explaining an example of the evaluation method of a semiconductor substrate. The figure explaining an example of the evaluation method of a semiconductor substrate. The figure which shows an example of a semiconductor substrate. 10A and 10B illustrate an example of a method for manufacturing a semiconductor substrate. The figure which shows an example of an attenuation curve. 10A and 10B illustrate a method for manufacturing an SOI substrate.

  Embodiments of the present invention will be described below with reference to the drawings. However, the present invention is not limited to the following description, and it will be easily understood by those skilled in the art that modes and details can be variously changed without departing from the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited to the description of the embodiments below. Note that in the structures of the present invention described below, the same reference numerals are used in common in different drawings.

(Embodiment 1)
In this embodiment, an example of a method for evaluating a semiconductor substrate will be described. The evaluation is performed by the following steps A to E.

<Process A>
Comprises a bulk lifetime tau _b, the surface recombination velocity S ₀ of the one _surface, a first semiconductor substrate is the surface recombination velocity S ₀ of the other surface, comprising a bulk lifetime tau _b, of one surface A second semiconductor substrate having a surface recombination rate S ₀ , a surface recombination rate S ₁ on the other surface, and a bulk lifetime τ _b , a surface recombination rate S ₁ on one surface, a surface on the other surface Three types of third semiconductor substrates having a recombination speed S ₁ are prepared.

  The conditions other than the surface recombination rate on one surface and the surface recombination rate on the other surface of the three types of semiconductor substrates are all the same.

In this embodiment, the parameters of the semiconductor substrate are as follows: the surface recombination speed S ₀ is 100 cm / sec, the surface recombination speed S ₁ is 1000 cm / sec, the bulk lifetime τ _b is 1000 μsec, and the minority carrier diffusion constant D is 30 cm ^2. / Sec, and the thickness W of the semiconductor substrate is assumed to be 2 mm.

  FIG. 4 shows a semiconductor substrate used in step A. 4A shows the first semiconductor substrate, FIG. 4B shows the second semiconductor substrate, and FIG. 4C shows the third semiconductor substrate.

The lifetimes of three types of semiconductor substrates are measured by experiments. The lifetimes τ ₁₁ , τ ₂₁ , and τ ₃₁ of the respective primary modes are measured for the first to third semiconductor substrates. In this embodiment, the mu-PCD method, the primary mode lifetime tau _11, tau _21, measured tau _31, the lifetime tau ₁₁ of the primary mode 526.8Myusec, lifetime tau ₂₁ of the primary modes 316 The lifetime τ _{31 of the} primary mode was 183.0 μsec.

Here, it is not always necessary to simultaneously prepare three types of semiconductor substrates that satisfy the above conditions. First, after adjusting the first semiconductor substrate, the lifetime τ ₁₁ of the primary mode is obtained, and then the first semiconductor substrate is obtained. A surface treatment for modifying the surface recombination velocity S ₀ to the surface recombination velocity S ₁ is applied to only one surface of the first semiconductor substrate to obtain a first mode lifetime τ ₂₁ . subjected to a surface treatment for modifying the surface recombination velocity S ₀ only on the surface of the surface recombination velocity S ₀ on the surface recombination velocity S ₁ and the third semiconductor substrate, a method of obtaining a lifetime tau ₃₁ of the primary mode There is.

  Hereinafter, a method of adjusting the semiconductor substrate in this embodiment will be described with reference to FIG.

The two first semiconductor substrates are bonded to each other so that one of the surfaces having a surface recombination velocity of S ₀ overlaps (FIG. 5A), and the surfaces of the two bonded first semiconductor substrates are hooked. The surface recombination velocity of the surface of the first semiconductor substrate that has not been bonded by treatment with an acid is defined as S ₁ (FIG. 5B).

  After that, the second semiconductor substrate can be obtained by peeling the semiconductor substrate treated with hydrofluoric acid (FIG. 5C).

Alternatively, the second semiconductor substrate may be obtained by treating only one of the surfaces of the first semiconductor substrate whose surface recombination rate is S ₀ with hydrofluoric acid.

Further, when the surface of the second semiconductor substrate is treated with hydrofluoric acid, the surface recombination velocity of both surfaces of the semiconductor substrate becomes S ₁ , and a third semiconductor substrate can be obtained (FIG. 5D).

  In the above method, since it is not necessary to prepare three types of semiconductor substrates at the same time, there is no variation between the semiconductor substrates, and highly reliable evaluation can be performed.

  In this embodiment, a silicon substrate is used as the semiconductor substrate, but the present invention is not limited to this. A compound semiconductor substrate such as a silicon germanium substrate or a silicon carbide substrate may be used. Note that a single crystal semiconductor, a polycrystalline semiconductor, or the like can be used.

FIG. 6 shows an attenuation curve of minority carriers at the center of the semiconductor substrate. In FIG. 6, the horizontal axis represents time [μsec], and the vertical axis represents excess carrier density [a. u. ], The curves in the figure are 1.5 × 10 ¹³ cm ⁻² , 1.0 × 10 ¹³ cm ⁻² , 5.0 × 10 ¹² cm ⁻² , 2.0 × 10 ¹² cm ⁻² , in order from the top. The attenuation curves when 1.1 × 10 ¹² cm ⁻² and 4.5 × 10 ¹¹ cm ⁻² photons are incident are shown.

  In this measurement method, only the primary mode lifetime information is obtained from the attenuation curve. The primary mode lifetime is in principle independent of the number of incident photons. Therefore, when the number of incident photons is increased as much as possible, the SN ratio (Signal to Noise ratio) is improved, so that highly reliable evaluation can be performed.

<Process B>
Regarding the first semiconductor substrate and the third semiconductor substrate, the surface recombination velocities on both surfaces of the first and third semiconductor substrates are equal, so that the lifetimes τ ₁₁ , τ _{31 of} the first-order mode obtained in step A and Using the following formula (1), the relationship between the surface recombination velocity S ₀ and the bulk lifetime τ _{b and} the relationship between the surface recombination velocity S ₁ and the bulk lifetime τ _b can be obtained.

（ただし、数式（１）中、Ｓ_Ｌ（Ｌは、０または１）は表面再結合速度［ｃｍ／ｓｅｃ］、Ｄは少数キャリア拡散定数［ｃｍ^２／ｓｅｃ］、τ_ｎ１（ｎは、１または３）は一次モードのライフタイム［μｓｅｃ］、τ_ｂはバルクライフタイム［μｓｅｃ］、Ｗは半導体基板の厚さ［ｍｍ］をそれぞれ表す。）

(In the formula (1), S _L (L is 0 or 1) is the surface recombination rate [cm / sec], D is the minority carrier diffusion constant [cm ² / sec], τ _n1 (n is 1 or 3) the lifetime of the primary mode [μsec], τ _b is the bulk lifetime [μsec], W represents the thickness of the semiconductor substrate [mm], respectively.)

Obtaining the first correlation curve and the second correlation curve as shown in FIG. 1 from the relationship between the surface recombination velocity S ₀ and the bulk lifetime τ _{b and} the relationship between the surface recombination velocity S ₁ and the bulk lifetime τ _b Can do. In FIG. 1, the horizontal axis represents the bulk lifetime τ _b [μsec], the vertical axis represents the surface recombination velocity S _L [cm / sec], the thick line represents the first correlation curve, and the thin line represents the second correlation curve.

<Process C>
The relationship between the primary mode lifetime τ ₂₁ and the bulk lifetime τ _b using the following formula (2), formula (3), the first correlation curve, and the second correlation curve for the second semiconductor substrate. From this, a third correlation curve can be obtained.

（ただし、数式（２）および数式（３）中、τ_２１は一次モードのライフタイム［μｓｅｃ］、τ_ｂはバルクライフタイム［μｓｅｃ］、Ｋ_２１は第２の半導体基板の１番目の固有モードの波数［ｃｍ^−１］、Ｄは少数キャリア拡散定数［ｃｍ^２／ｓｅｃ］、Ｗは半導体基板の厚さ［ｍｍ］をそれぞれ表す。ここでＳ_Ｍ０≡Ｓ_０／Ｄ、Ｓ_Ｍ１≡Ｓ_１／Ｄと表し、Ｓ_０およびＳ_１は表面再結合速度［ｃｍ／ｓｅｃ］を表す。）

(However, in Equation (2) and Equation (3), τ ₂₁ is the primary mode lifetime [μsec], τ _b is the bulk lifetime [μsec], and K ₂₁ is the first eigenmode of the second semiconductor substrate. wavenumber ^[cm -1], D is the minority carrier diffusion constant ^[cm 2 / sec], W represents the thickness of the semiconductor substrate [mm], respectively. here _{S M0 ≡S 0 / D, S} M1 ≡S 1 / D, S ₀ and S ₁ represent surface recombination velocity [cm / sec].)

From the relationship between the lifetime τ _{21 of the} primary mode and the bulk lifetime τ _b, a third correlation curve can be obtained as shown in FIG. The horizontal axis in FIG. 2 is the bulk lifetime τ _b [μsec], the first vertical axis is the surface recombination velocity S _L [cm / sec], the second vertical axis is the primary mode lifetime τ ₂₁ [μsec], The thick line represents the first correlation curve, the thin line represents the second correlation curve, and the thick dashed line represents the third correlation curve.

<Process D>
By substituting the lifetime τ ₂₁ of the first-order mode into the third correlation curve, the bulk lifetime τ _b can be obtained. In the present embodiment, the obtained bulk lifetime τ _b is 1000 μsec, which is the same value as the assumed bulk lifetime τ _b of the semiconductor substrate.

<Process E>
As shown in FIG. 3, the obtained bulk lifetime τ _b can be substituted into the first correlation curve and the second correlation curve to obtain the surface recombination velocity S ₀ and the surface recombination velocity S ₁ . In the present embodiment, the obtained surface recombination speed S ₀ is 100 cm / sec, the surface recombination speed S ₁ is 1000 cm / sec, and the assumed surface recombination speed S ₀ and surface recombination speed S of the semiconductor substrate are assumed. The value was the same as ₁ .

In the present embodiment, the values of the surface recombination velocity S ₀ , the surface recombination velocity S _1, and the bulk lifetime τ _b using the lifetimes τ ₁₁ , τ ₂₁ , τ ₃₁ of the first-order modes obtained from the assumed semiconductor substrate. Although the values of the surface recombination velocity S ₀ , the surface recombination velocity S ₁ and the bulk lifetime τ _b are unknown, the surface recombination velocity S ₀ can be obtained by using three types of semiconductor substrates. Relational expression of bulk lifetime τ _b (first correlation curve), relational expression of surface recombination velocity S ₁ and bulk lifetime τ _b (second correlation curve), surface recombination velocity S ₀ and surface recombination velocity The surface recombination velocity S ₀ , the surface recombination velocity S _1, and the bulk lifetime τ _b can be determined by solving simultaneous equations of three relational expressions of S ₁ and bulk lifetime τ _b (third correlation curve). To find the value Kill.

  This embodiment can be implemented in appropriate combination with any of the other embodiments.

(Embodiment 2)
In this embodiment mode, a method for manufacturing an SOI (Silicon On Insulator) substrate using the semiconductor substrate evaluated by the method of Embodiment Mode 1 is described.

  First, the semiconductor substrate 101 is prepared (FIG. 7A). As the semiconductor substrate 101, the one evaluated by the method of the first embodiment is used.

  Next, ion irradiation 103 is performed on the semiconductor substrate 101 to form a damaged region 105 at a predetermined depth (FIG. 7B).

  The damaged region 105 can be formed by irradiating the semiconductor substrate 101 with ions (ion beams) 103 accelerated by an electric field and introducing the ions 103 to a predetermined depth from the surface of the semiconductor substrate 101.

  Irradiation with the ions 103 can be performed by ion doping or ion implantation using ions of hydrogen, an inert element (eg, helium), or halogen (eg, fluorine).

  The above-mentioned “ion doping method” means that the ionized gas generated from the source gas is not mass-separated, and is accelerated by an electric field as it is to irradiate the object, so that the ionized gas element is included in the object. Refers to the method. In addition, the above-mentioned “ion implantation method” means that a source gas is turned into plasma, ion species contained in the plasma are extracted, mass separation is performed, ion species having a predetermined mass is accelerated, and an ion beam is obtained. It is a method of injecting into an object.

  Next, the support substrate 107 is prepared (FIG. 7C).

  As the support substrate 107, a substrate made of an insulator such as glass, plastic, ceramic, quartz, or sapphire, a substrate made of a semiconductor such as silicon, or a substrate made of a conductor such as metal or stainless steel can be used.

  Next, the semiconductor substrate 101 and the supporting substrate 107 are attached to each other with the insulating layer 109 interposed therebetween (FIG. 7D). The bonding is performed using the side of the semiconductor substrate 101 where the damaged region 105 is formed as a bonding surface (also referred to as a bonding surface).

  The insulating layer 109 functions as a bonding layer for bonding two substrates, and may be formed over the semiconductor substrate 101 or the support substrate 107. In the case where the insulating layer 109 is formed over the semiconductor substrate 101, the insulating layer 109 may be formed before the ion 103 irradiation.

  The insulating layer 109 may be formed by a single layer or a stack of oxides, nitrides, or the like by a CVD method. Specific examples of the material include silicon oxide, silicon nitride, silicon oxynitride, and silicon nitride oxide.

  Note that in this specification, “oxynitride” such as silicon oxynitride indicates a composition having a higher oxygen content than nitrogen.

  Note that in this specification, “nitride oxide” such as silicon nitride oxide indicates a composition having a higher nitrogen content than oxygen.

  Here, the content is compared based on the measurement results of the Rutherford backscattering method and the hydrogen forward scattering method.

  Further, before bonding, surface treatment such as cleaning or plasma treatment may be performed on the bonding surfaces of the two substrates. By performing the surface treatment, hydrophilicity or cleanliness is improved, and the bonding strength at the time of bonding can be improved. Note that the surface treatment may be performed on at least one of the two substrates. In addition, when surface treatment is performed on the substrate over which the insulating layer 109 is formed, the surface treatment is performed on the surface of the insulating layer 109.

  Next, heat treatment is performed, and the semiconductor substrate 101 is separated (also referred to as division) in the damaged region 105 (FIG. 7E). By the separation, the insulating layer 109 and the semiconductor layer 111 including a part of the semiconductor substrate 101 can be sequentially provided over the supporting substrate 107. That is, the semiconductor layer 111 including a part of the semiconductor substrate 101 can be transferred onto the support substrate 107.

  Note that the heat treatment may be performed at a temperature higher than or equal to 300 ° C. and lower than the strain point of the support substrate 107.

  In this manner, the SOI substrate 113 can be manufactured.

  Note that an SOI substrate is a general term for a semiconductor substrate provided with an insulating layer on a supporting substrate, and is not limited to a substrate having a silicon layer.

  Then, a semiconductor device such as a transistor or a diode can be manufactured using this SOI substrate.

  This embodiment can be implemented in appropriate combination with any of the other embodiments.

101 Semiconductor substrate 103 Ion irradiation 105 Damaged region 107 Support substrate 109 Insulating layer 111 Semiconductor layer 113 SOI substrate

Claims (4)

Comprises a bulk lifetime tau _b, a first semiconductor substrate is the surface recombination velocity S ₀ of one of the surface recombination velocity S ₀ of the _surface, the other surface,
A second semiconductor substrate having a bulk lifetime τ _{b and} having a surface recombination rate S ₀ on one surface and a surface recombination rate S ₁ on the other surface;
Comprises a bulk lifetime tau _b, a third semiconductor substrate is the surface recombination velocity S ₁ of one of the surface recombination velocity S ₁ of the _surface, the other surface,
For each primary mode lifetime τ ₁₁ , τ ₂₁ , τ ₃₁ ,
For the first semiconductor substrate and the third semiconductor substrate, the surface recombination velocity S ₀ and the bulk lifetime τ using the first-order mode lifetimes τ ₁₁ and τ ₃₁ and the following formula (1): from _b of relation and the surface recombination velocity S ₁ and the relationship of the bulk lifetime tau _b obtains a first correlation curve and a second correlation curve,
Using the following formula (2), formula (3), the first correlation curve, and the second correlation curve for the second semiconductor substrate, the lifetime τ _{21 of the} primary mode and the bulk lifetime A third correlation curve is obtained from the relationship of τ _b ,
Substituting the obtained first-order mode lifetime τ ₂₁ into the third correlation curve to obtain the bulk lifetime τ _b ,
Semiconductor substrate evaluation method for obtaining the surface recombination velocity S ₀ and the surface recombination velocity S ₁ by substituting the obtained bulk lifetime τ _b into the first correlation curve and the second correlation curve .

(In the formula (1), S _L (L is 0 or 1) is the surface recombination rate [cm / sec], D is the minority carrier diffusion constant [cm ² / sec], τ _n1 (n is 1 or 3) the lifetime of the primary mode [μsec], τ _b is the bulk lifetime [μsec], W represents the thickness of the semiconductor substrate [mm], respectively.)

(However, in Equation (2) and Equation (3), τ ₂₁ is the primary mode lifetime [μsec], τ _b is the bulk lifetime [μsec], and K ₂₁ is the first eigenmode of the second semiconductor substrate. wavenumber ^[cm -1], D is the minority carrier diffusion constant ^[cm 2 / sec], W represents the thickness of the semiconductor substrate [mm], respectively. _{here, S M0 ≡S 0 / D,} S M1 ≡S ₁ / D, S ₀ and S ₁ represent surface recombination velocity [cm / sec].) The second semiconductor substrate is fabricated by subjecting only one surface of the first semiconductor substrate to surface treatment,
The method for evaluating a semiconductor substrate according to claim 1, wherein the third semiconductor substrate is manufactured by subjecting only the surface of the second semiconductor substrate having a surface recombination velocity S ₀ to surface treatment.   The semiconductor substrate evaluation method according to claim 1, wherein the first semiconductor substrate, the second semiconductor substrate, and the third semiconductor substrate are silicon substrates. 4. The semiconductor substrate evaluation method according to claim 1, wherein the lifetimes [tau] ₁₁ , [tau] ₂₁ , [tau] ₃₁ of the primary mode are measured using a [mu] -PCD method.

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