Method for evaluating semiconductor doping characteristics using the photoneutralization time constant of corona surface charge

Abstract

A method for characterizing semiconductor doping in a wide-bandgap semiconductor sample involves measuring the initial surface potential V0 of a region on the surface of the semiconductor sample in the dark, charging the region to a deep depletion state in the dark by applying a predetermined corona charge to the region, measuring the surface potential value of the region in the dark after charging, and then applying a specific photon flux f having a photon energy exceeding the semiconductor's bandgap, sufficient to generate free minority carriers in the semiconductor sample and cause photoneutralization of the corona charge. eff Irradiation with light, non-contact time-resolved measurement of surface potential V(t) to monitor corona charge decay induced by photoneutralization in the region as a result of light irradiation time t, and analysis of the monitored time-resolved surface potential decay data V(t) to determine the photoneutralization time constant τ ph To determine a specific photon flux f eff The photo-neutralization time constant τ in ph This includes using a semiconductor doping index and characterizing the semiconductor doping concentration in a region based on that value.

Description

[Technical Field]

[0001] This application relates, in general terms, to the characterization of semiconductor doping in wide-bandgap semiconductors, and more specifically, to a method and system for non-contact measurement of doping in an epitaxial layer. [Background technology]

[0002] Semiconductor doping using donor or acceptor impurities alters the electrical properties of semiconductors and is a crucial technical element in the manufacturing of semiconductor devices. Doping is generally performed by introducing impurity atoms that replace host atoms in the semiconductor lattice. Doping concentration is the concentration of doping atoms per unit volume of semiconductor, and its measurement is often used for quality control in the manufacturing of semiconductor devices.

[0003] Devices based on wide-bandgap semiconductors, including SiC, GaN, AlGaN, and AlGaN / GaN, are widely used in applications such as power electronics and high-frequency devices. Such devices often contain doped epitaxial layers, requiring strict doping control of the epitaxial wafer. To meet the demands of rapidly growing wide-bandgap semiconductor technology and mass production of epitaxial wafers, corresponding mass production testing of doping is necessary. For such purposes, non-contact measurement techniques may be preferable. Non-contact measurement can reduce the cost and time associated with manufacturing test devices and avoid the wafer cleaning required after measurement, which is necessary in many conventional doping measurement methods such as the mercury probe method.

[0004] An example of a preparation-free, non-destructive doping measurement method for wide-bandgap semiconductors, as described in Patent Document 1, can be performed using a commercially available corona noncontact capacitance potential (CnCV) tool manufactured by Semilab SDI. CnCV electrically biases the semiconductor surface into a deep depletion state using corona charging, which is similar to the potential bias of a metal Schottky barrier but is achieved without the metal contacting the semiconductor surface. In the CnCV method, the charge bias amount ΔQ C The surface potential response ΔV is monitored with a Kelvin probe. Differential capacitance C = ΔQ in sequential charge measurements. C / ΔV provides non-contact CV characteristics. Doping concentration N D This is the standard procedure in the CV method, 1 / C 2 It is determined by the slope relative to V. Currently, CnCV measurement throughput using commercially available tools is limited to approximately 5 wafers per hour for a typical 12-site pattern. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] U.S. Patent No. 10969370 [Overview of the project]

[0006] Conventional CnCV techniques can be modified to utilize novel doping sensitivity phenomena available to increase test throughput. Conventional CnCV uses a corona charging bias to a depletion state, achieved through multiple stepwise corona charge application steps. In response to the applied charge, the surface potential V changes, which is measured with a non-contact vibrating Kelvin probe. Similar to mercury probes and Schottky junction devices, CnCV is 1 / C in a depletion state. 2- The doping concentration is determined using the slope of the V characteristic. CnCV is a quasi-static technique that involves many relatively slow charge biasing steps to acquire the CV characteristic. This typically limits the measurement speed and throughput in multiple wafer monitoring with typical 9 or 12 test sites and 49 wafer mapping sites per wafer.

[0007] The imparted corona charge can be rapidly neutralized and removed from the bare surface of a wide-bandgap semiconductor by short-wavelength light irradiation. The corresponding corona photoneutralization time constant can be shortened, enabling rapid sweeping of the depletion barrier. In measurements of epitaxial SiC with different dopings, a direct relationship was confirmed between the corona charge photoneutralization time constant and the doping concentration. This relationship has also been confirmed for other wide-bandgap semiconductors and is valid over a wide range of doping concentrations. The high-throughput doping measurement method in this disclosure is based on this newly identified relationship.

[0008] Therefore, this disclosure describes a charge biasing method using a novel doping measurement principle that can improve measurement speed and throughput by up to 10 times compared to, for example, the conventional CnCV technology described above.

[0009] The method described herein allows a series of cumulative charging steps to be replaced with a single strong coronal charge to a deep depletion state. The charge then applied to the semiconductor surface is photoneutralized using near-ultraviolet light irradiation to generate excess carriers. The photoneutralization time constant is determined from time-resolved surface potential measurements, which are doping measurement parameters.

[0010] In the photo-neutralization process, minority carriers generated optically within the surface depletion layer induced by charges are active. They are guided to the surface by the depletion layer electric field, captured by corona ions of opposite polarity, neutralize the corona charge, and reduce the depletion width. This technique involves measuring and analyzing the corresponding change in surface potential with respect to the light irradiation time.

[0011] In wide-bandgap semiconductors, photo-neutralization of corona ions is irreversible. The corona charge does not recover after photo-neutralization, and the change in surface potential induced by photo-neutralization does not reverse in the dark. This enables process monitoring by measuring the surface potential while irradiating the charged region with light or after successive light irradiation pulses. This disclosure encompasses both measurement configurations. Specifically, measurement while irradiating the charged region with light is employed in a system configuration involving light irradiation under a surface potential probe. Measurement after a light irradiation pulse is used in a system configuration involving light irradiation at a position away from the surface potential probe.

[0012] The measurement technique separates charge photo-neutralization from other surface photoelectric effects by utilizing the irreversible nature of the surface potential change induced by light irradiation. The corresponding test involves monitoring the surface potential in the dark after stopping light irradiation. Under good measurement conditions, dark recovery should be negligible, e.g., in the range of 0.1%.

[0013] In certain embodiments, the measurement technique extracts the charge photo-neutralization rate by utilizing the depletion layer Schottky barrier electrostatic charge-potential relationship and determines the photo-neutralization time constant τ ph from the logarithmic characteristics of surface potential versus light irradiation time. The doping concentration N D is determined based on the calibrated dependence of the time constant τ ph on τ ph versus N D . The time constant versus surface potential characteristic τ ph -V provides a means for evaluating the doping depth profile. Uniform doping results in a constant τ phRecognized by its value. Multilayer films with different dopings exhibit different τ values ​​for a given time segment in the logarithmic properties of their photoneutralization surface potential. ph The values ​​are shown. The corresponding surface potentials reflect different depths below the surface.

[0014] Generally, the measurement configuration includes three system components with the following functions: (a) Corona charging to charge the semiconductor surface into a deep depletion state. (b) Non-contact surface potential measurement.

[0015] (c) Irradiation with short-wavelength light having a photon energy exceeding the semiconductor energy gap. The semiconductor sample wafer can be placed on a conductive chuck equipped with a high-speed moving function to position the sample area (i.e., the test site on the wafer) to be tested under system components (a), (b), and (c) according to the measurement cycle.

[0016] A housing can be provided for the measurement system to prevent stray light (e.g., short-wavelength stray light) from reaching the wafer. Such stray light could otherwise cause uncontrolled neutralization of corona charges on the wafer surface.

[0017] time constant τ ph This can be used directly as a relative doping concentration index. For precise quantitative measurement of doping concentration, this method may include a calibration function for the time constant determined by measurements on a reference sample wafer with known doping concentrations. Alternatively or additionally, the photoneutralization time constant τ ph Wafer-specific calibration may be performed using corona CV doping measurements conducted in the same sample region (same wafer site) as the measurement.

[0018] In conventional corona charging methods used for characterizing semiconductors and dielectrics, the response to changes in the applied corona charge density is often monitored by measuring the corresponding change in surface potential. This technique also utilizes surface potential measurement, but the change in corona-applied charge density is achieved not by charge application, but by photoneutralization of the charge. Photoneutralization of the charge and measurement of the surface potential can be performed simultaneously or sequentially, depending on the configuration of the light source and surface potential probe. Light irradiation under the probe enables measurement during actual corona photoneutralization. This allows for light irradiation and measurement of sample sites without moving the wafer between the light irradiation and the measurement probe, improving measurement speed and throughput. Configurations with separate light sources can achieve higher uniformity of light irradiation than light irradiation under the surface potential probe. This can contribute to accuracy and tool-to-tool matching.

[0019] The disclosed technology can utilize multiple light sources having different configurations and wavelengths, which can be selected and optimized according to the requirements of the specific semiconductor wafer manufacturing process in which the technology is used.

[0020] In general, in one aspect, the present disclosure is characterized by a method for characterizing semiconductor doping in a wide-bandgap semiconductor sample, the method comprising: measuring an initial value V0 of the surface potential in a region of the semiconductor sample's surface in the dark; charging the region to a deep depletion state in the dark by imparting a predetermined corona charge to the region; measuring the surface potential value of the region in the dark after charging; and applying a specific photon flux f having a photon energy exceeding the semiconductor's bandgap, sufficient to generate free minority carriers in the semiconductor sample and cause photoneutralization of the corona charge. eff Irradiation with light, non-contact time-resolved measurement of surface potential V(t) to monitor corona charge decay induced by photoneutralization in the region as a result of light irradiation time t, and analysis of the monitored time-resolved surface potential decay data V(t) to determine the photoneutralization time constant τ ph To determine a specific photon flux f effThe photo-neutralization time constant τ in ph This includes using a semiconductor doping index and characterizing the semiconductor doping concentration in a region based on that value.

[0021] The implementation of this method may include one or more of the following features. This method uses a corona charge photoneutralization time constant τ, which is inherent to the surface potential. ph This may include characterizing the doping concentration depth profile of a semiconductor sample based on V.

[0022] This method averages τ over the surface potential range. ph This may include characterizing the doping of a single epitaxial layer based on the values. In some examples, τ ph It is determined according to the following formula.

[0023]

number

[0024] Here, t is the light irradiation time, and V const This is the offset of the surface potential probe. This method uses a known doping concentration value N. D τ in one or more reference samples having ph Using the calibration measurement of the calibration function τ ph vs N D and, f cal The inverse calibration function N is expressed as D vs τ ph This may include determining the effective photon flux φ. eff The time constant τ measured in ph Calibrate photon beam φ cal against

[0025]

number

[0026] Normalizing as, the normalized time constant τ * ph Using equation N D =τ * ph ·f cal This includes determining the absolute doping concentration according to the following. This method involves τ ph This may include calibrating the semiconductor wafer sample, and the calibration may be specific to the semiconductor wafer sample and multi-site (e.g., 9 sites, 12 sites, or 49 sites) of the wafer sample. ph This is performed on one site of the wafer sample measured during the base characterization, and all τ ph Measurements are performed under the same light irradiation conditions. Calibration is performed using the corona charge photoneutralization time constant τ. ph Measurement of doping concentration N using non-contact coronal CV (CnCV) method D This method may include independent measurements of τ, both of which are performed sequentially at the same wafer site and within similar corona charge-induced depletion potential ranges. ph and N D Using N D =τ ph ·f cal wafer-specific inverse calibration function value f that satisfies this condition cal This could include making a decision.

[0027] In some examples, this method involves τ at each wafer site. ph Based on determining wafer-specific f determined at a single site cal This includes determining the doping concentration at all different wafer sites using the value of .

[0028] Capacitive probes can be non-contact vibrating Kelvin probes. The region can be irradiated with light simultaneously with the monitoring of the surface potential, or the region can be irradiated with light separately from the monitoring of the surface potential.

[0029] The semiconductor sample may include semiconductors selected from the group consisting of SiC, GaN, and AlGaN. This method measures V after corona charging but before light irradiation. Dark and dark attenuation rate ΔV Dark / Δt Dark This may include identifying defect regions in a semiconductor sample.

[0030] This method measures the magnitude of dark attenuation ΔV after light irradiation. Dark Based on this, it may be possible to identify the contribution to the monitored surface potential from surface photovoltaic effects other than corona photoneutralization.

[0031] Other features and advantages will become apparent from the drawings, description, and claims. [Brief explanation of the drawing]

[0032] [Figure 1A] Figure 1A is a schematic diagram of an exemplary system for measuring doping of a semiconductor wafer using charge-photoneutralization induced by photoirradiation following corona charge application and determination of the photoneutralization time constant from non-contact time-resolved surface photopotential. In this example, the configuration includes photoirradiation placed beneath a surface potential probe. [Figure 1B] Figure 1B is a schematic diagram of an alternative configuration of the exemplary system shown in Figure 1A, where the light irradiation is located away from the surface potential probe. [Figure 2] Figure 2 shows the steps in an exemplary measurement flow for determining the photoneutralization time constant from time-resolved surface potential decay. [Figure 3A] Figure 3A is a plot showing exemplary results of surface potential versus photoirradiation time measured during photoneutralization of corona charges. [Figure 3B] Figure 3B is an example plot used to determine the photoneutralization time constant corresponding to the transient response in Figure 3A. [Figure 3C] Figure 3C shows a plot of τph intrinsic to the surface potential, illustrating the case of a uniform doping depth profile. [Figure 4] Figure 4 is an example plot showing the corresponding logarithmic surface potential decay of an n-type SiC epitaxial wafer with three layers of different doping. [Figure 5] Figure 5 is a plot of exemplary doping calibration results showing the dependence of the photoneutralization time constant τph on doping concentration ND, measured on n-type 4H-SiC. The measurements were performed using a negative corona-charged density of -4E12q / cm2. Light irradiation at a wavelength of 325 nm with an incident photon flux of 1.2 × 10¹³ photons / cm2s was used. The doping concentration ND corresponds to the results from a CV mercury probe. [Figure 6] Figure 6 is an energy band diagram showing the photoneutralization of corona charges on an n-type wide-bandgap semiconductor. The threshold energy hν is the energy gap Eg. [Figure 7] Figure 7 is a plot of exemplary results showing the dependence of the photoneutralization time constant on the incident photon flux. The results are for n-type SiC with doping concentrations of ND=6.11e15cm⁻³ and ND=1.85e16cm⁻³. The ND values ​​are based on results from a CV mercury probe. [Modes for carrying out the invention]

[0033] Referring to Figure 1A, an exemplary apparatus 100 for evaluating the corona charging-potential photoneutralization characteristics of a semiconductor wafer sample 101 is schematically shown. The wafer sample includes a substrate 104 and an epitaxial layer 103 supported by the substrate 104. The apparatus 100 includes a corona band power supply module 120 for precise charge application to a test site 126, a surface potential measurement module 130 for surface potential measurement, and a light source module 140 for illuminating the wafer surface with light.

[0034] The wafer 101 is supported by a wafer chuck 110, which is positioned on a movable stage 112. The stage 112 can move the wafer chuck by translation and rotation. A coulomb meter 114 is connected to the wafer chuck 110.

[0035] The corona band power module includes a corona-charging electrode positioned to impart a corona charge 124 to a site 126 on the surface of the wafer 101. The surface potential measurement module 130 includes a Kelvin probe having an electrode 132 positioned to vibrate at a short distance (e.g., less than 1 millimeter) above the surface of the wafer 101. The example described here uses a Kelvin probe, but other capacitances can also be used.

[0036] The light source module 140 includes a light source 142 positioned to illuminate the surface of the wafer 101 located directly beneath the Kelvin probe electrode. During operation, the light source 142 supplies light irradiation with a wavelength suitable for photoneutralizing corona charges on the wafer surface beneath the Kelvin probe electrode 132. The light source 142 may include, for example, a light-emitting diode (LED) or a near-ultraviolet laser. In the case of an LED, the light beam can be further monochromatized by passing it through a narrow-band pass filter.

[0037] Referring to Figure 1B, an alternative apparatus 100' is shown, in which the light source module 140' includes a light source 144 positioned to illuminate a site on the surface of wafer 101 different from the site below electrode 132. Similar to light source 142, light source 144 may include an LED or laser capable of emitting light with a wavelength suitable for photoneutralizing corona charges. Generally, the wavelengths of light irradiation provided by light sources 142 and 144 can be selected depending on the wafer being tested. Generally, the photon energy should exceed the semiconductor energy gap. For measurements on 4H-SiC with an energy gap of 3.26 eV, suitable light wavelengths include λ=355 nm or λ=325 nm, corresponding to photon energies hν=3.49 eV and hν=3.82 eV, respectively. Both photon energies exceed the SiC energy gap as required for the generation of free carriers (electron-holes) involved in the photoneutralization process of corona charges. For measurements on other semiconductors, the light wavelength can be selected based on the energy gap and absorption coefficient values. Shorter wavelength light irradiation at λ=315nm may be suitable for measuring doping in GaN and AlGaN materials, which have larger energy gaps than SiC. Depending on the application, other wavelengths of light below approximately 380nm can be used. Light sources 142 and 144 are positioned to provide uniform light irradiation within the surface area of ​​a corona-charged spot, for example, a circular spot with a diameter of approximately 10 mm or less (e.g., approximately 6 mm in diameter).

[0038] The computer controller 150 is used to control the operation of the apparatus and to perform data analysis to determine information regarding the doping of sample 101 using the measurement and analysis steps described below.

[0039] The exemplary apparatuses 100 and 100' shown in Figures 1A and 1B, respectively, illustrate two configurations of light irradiation: a light source 142 (LED1) for light irradiation below the vibrating electrode of the Kelvin probe electrode 132, and a light source 144 (LED2) for light irradiation separated from the Kelvin probe 132. A standard high-precision Kelvin probe can use an opaque gold electrode. If the Kelvin probe electrode 132 is opaque, low-angle light irradiation from the side can be used for the light source 142, as schematically shown in Figure 1A. In some implementations, transparent Kelvin probes can also be used, where the light irradiation passes through the electrode. In general, it may be easier to achieve high uniformity of light irradiation with separated light irradiation by the light source 144 shown in Figure 1B.

[0040] In the Kelvin probe method, a capacitive electrode vibrating less than 1 mm above the measurement surface generates an alternating current, which is zeroed out by compensating for a DC bias, providing a measure of the potential difference between the electrode and the semiconductor. In this application, accurate and fast-responding Kelvin probes are preferred, such as probes with a time constant of about 5 ms and an accuracy of 0.2 mV. For corona-charging biases that induce deep depletion of n-type (negative bias) and p-type (positive bias) semiconductors, a surface potential measurement range of -100 V to +100 V is typical. Electrode diameters of 1 mm or 2 mm are suitable for measuring the surface potential at the uniform center of a corona-charged and irradiated area.

[0041] Corona charging is achieved using a corona discharge in air generated by a high DC voltage applied from power supply 122 to discharge electrode 124. Negative polarity discharge in air is CO3 - It generates ions, and the positive discharge is (H2O) n H + Ions are generated. Corona discharge electrodes (needles for point charging or wires for whole-wafer charging) can be confined within a housing, and ion impartment is not driven by an electric field; instead, ions diffuse through openings onto the wafer surface. 10 in a room atmosphere -5Given the extremely short mean free path of cm, ions lose kinetic energy and are thermally equilibriumized before reaching the semiconductor surface. Such a configuration allows for non-destructive corona charging biasing. Precise corona charging is typically performed in a cleanroom environment with controlled humidity and temperature. Control of the imparted corona charge density is achieved by setting the high voltage, discharge current, and imparting time of the corona power supply 122. The imparting time can be approximately 1 second or less. The amount of imparted charge density can be monitored in situ using a coulomb meter 114.

[0042] Coronal charging into a deep depletion state is approximately 1 × 10⁻⁶ 12 q / cm 2 From approximately 1 x 10 13 q / cm 2 The charge extends to this extent, and charging can be achieved in a single charging step. The charge applied to the surface of test site 126 appears mirror-image as the opposite charge in the space charge region of the semiconductor surface, acting as an electrical bias. The amount of charge is much larger than the initially present surface charge, typically about 1 × 10⁻⁶. 11 q / cm 2 Therefore, the doping concentration N D Depending on the charge amount Q C By selecting, the Italian void width W D =-Q C / qN D You can obtain this.

[0043] Wide-bandgap semiconductor devices are primarily fabricated on n-type epitaxial layers, and 4H-SiC is a commonly used material. Therefore, this disclosure presents exemplary results of the method for n-type 4H-SiC and negatively corona-charged wafers. To measure such wafers, the method may further include a step of stabilizing a new epitaxial 4H-SiC wafer using a UV pretreatment chamber, which may be part of the apparatus 100. Immediately after the epitaxial growth process, 4H-SiC may exhibit rapid dissipation of the imparted corona charge. This is caused by surface diffusion of corona ions. If present, this effect may interfere with the measurement of photo-induced charge photoneutralization. Pretreatment to eliminate the surface diffusion effect can be performed concurrently with doping measurements on a set of multiple wafers. Such functionality can be added to an automated version of the apparatus 100.

[0044] A stabilized bare surface of depleted SiC typically exhibits good corona charge stability in the dark. A large energy gap prevents the thermal generation of free minority carriers that could neutralize the corona charge. In this method, the charge stability in the dark after charging is verified by the corresponding stability of the surface potential. For this purpose, the method may include measuring the surface potential in the dark for a predetermined period after charging but before light irradiation. Such a measurement is an example of measuring the decay rate in the dark, and the surface potential V in the dark as a function of time. DarkThe attenuation rate is calculated based on the change in the surface potential. A large surface potential attenuation rate in the dark, where the magnitude of the surface potential relative to the applied charge density is lower than nominally expected, may indicate a defect causing charge dissipation and thus interfere with corona charge photoneutralization measurements. In such cases, the measurement can be repeated on an adjacent defect-free wafer site with a negligible dark attenuation rate. In the epitaxial 4H-SiC used as an example in this disclosure, the defects causing interference are triangular defects, downfall defects, or carrot defects that are fatal to the device. These defects may have dimensions of sub-millimeters, and it may be sufficient to eliminate the interference by shifting the measurement position by a distance greater than the diameter of the Kelvin probe, for example, 3 mm to 5 mm in the case of a 2 mm diameter Kelvin probe.

[0045] In some implementations, additional verification steps may also be performed. For example, such steps may include measuring the surface potential performed in the dark after turning off photo-induced charge photoneutralization. This verification is related to the irreversible nature of photoneutralization and aims to recognize other reversible surface photovoltaic effects in the surface depletion layer.

[0046] Generally, measurements are performed under conditions where corona charge photoneutralization is dominant. This can be tested by measuring surface potential recovery in the dark immediately after light irradiation. Under optimized measurement conditions, the magnitude of such surface potential recovery, which exhibits other effects, should be negligible compared to the magnitude of corona charge photoneutralization, for example, in the range of about 100 mV or less compared to a corona charge photoneutralization magnitude of about 20 V or more. In SiC, such a negligible contribution is about 10 13 ~about 10 14 photon / cm 2 This can be achieved using a low effective photon flux in the range of s (number of photons per square centimeter per second).

[0047] The characterization of semiconductor doping is determined by the time constant τ of photoneutralization of corona charge under surface depletion conditions. ph This includes determining τph Once determined, it functions as a doping indicator. ph The value is the doping concentration N D =τ ph ·f cal It is converted to f cal This is the so-called inverse calibration function. The time constant τ ph and calibration function f cal The decision will be explained further below.

[0048] The steps included in the measurement flow that can be used to obtain time-resolved surface potential data for determining the photoneutralization time constant are shown in the table in Figure 2. The surface potential measured at each step is shown in the plot included in Figure 2. The surface potential measurement is characteristic of a typical n-type epitaxial wafer of 4H-SiC measured using apparatus 100 or 100' and the light irradiation configuration shown in Figure 1A. The wafer is held on a conductive chuck 110 by vacuum suction. During corona charging and measurement, the chuck 110 provides a back electrode capacitively coupled to the wafer. The non-contact characteristic of this method refers to the absence of physical contact with the front surface of the wafer, in this case the surface of the epitaxial layer 103. All light in the system is first turned off and the wafer is in the dark. The wafer chuck is movable and, in step 1 of Figure 2, the wafer test site is placed under the Kelvin probe electrode 132 to measure the initial surface potential V0, for example V0 = 0.8V. After measuring the initial surface potential, the test site 126 is moved to a position below the corona band power supply (as shown in Figures 1A and 1B), and in step 2, a predetermined charge density Q C For example, -1.5 × 10 12 q / cm 2 A negative charge having is applied to the surface of the epitaxial layer 103 at the test site 126. As a result of the charge application, the surface is biased to a deep depletion state. In step 3, the charged site is shifted to a position below the Kelvin probe electrode 132. Next, the surface potential of the deep depletion state after charging is measured, and the value after charging, for example V D=-59V is obtained. In step 4, the test site is irradiated with short-wavelength light having a photon energy exceeding the 3.26 eV energy gap of 4H-SiC, which is appropriate for causing photo-neutralization of the corona charge. For example, light irradiation with a wavelength of 325 nm (hν = 3.82 eV) by a near-ultraviolet LED can be used. In the example shown in Fig. 1A, light irradiation under the Kelvin probe electrode 132 using the light source 142 can be used, enabling continuous monitoring of the depletion layer surface potential V(t) while irradiating the charged region with light. In the case of the exemplary device 100’ shown in Fig. 1B, the light irradiation is at a position separate from the surface potential probe. Therefore, it is not possible to perform monitoring while irradiating the charged region with light. Instead, it is performed using a series of steps including a short light irradiation period (typically a duration of about 10 ms to 20 ms) and surface potential measurement. In this sequence, the charged region is shifted from a position under the surface potential measurement probe 132 to a position under the light irradiation source 144, then back to the surface potential probe, and so on, repeating in the same manner.

[0049] The last step 5 of the measurement cycle includes recording the surface potential after turning off the short-wavelength light. This value is called the dark potential V after in Fig. 2. A very small depletion potential recovery is used to confirm a negligible contribution from surface photovoltaic effects other than photo-neutralization of the corona charge.

[0050] An example of the measured V(t) is shown in Fig. 3A. The initial part of the transient response of the potential in the dark does not show attenuation before light irradiation. This dark part is an indicator of negligible charge dissipation due to charge neutralization related to surface diffusion or any defects. During light irradiation, the rapid photo-neutralization of the corona charge is evident as the corresponding attenuation of the surface potential. The time dependence of this recorded V(t) can be reliably analyzed using the Schottky barrier potential-charge equation, providing a means for determining the corona charge photo-neutralization time constant τ ph The quadratic relationship between the depletion surface barrier and the surface charge density (V D ~Q C 2) can be used for the derivation of the decay of the surface potential corresponding to the decay of the corona charge. In practice, τ ph is

[0051]

Number

[0052] can be determined as the reciprocal of the slope of the logarithmic surface potential time decay during photo-neutralization, where V0 is the initial value before charging including the offset related to the calibration of the Kelvin probe. This is shown in Figure 3B for the V(t) data in Figure 3A.

[0053] Such a fitting procedure gives an average τ ph value within a given surface potential range, in this case between -5V and -59V. The good linear fitting in Figure 3B indicates a constant τ ph value within this range. This is consistent with the uniform doping with respect to depth in this epitaxial layer.

[0054] For doping depth profiling, the time constant specific to the potential corresponding to a given depletion surface potential V on the photo-neutralization V vs. photo-irradiation time characteristic V(t) can be used. It is

[0055]

Number

[0056] calculated from the relative surface potential decay rate. Figure 3C shows the τ ph specific to the potential as a function of the depletion surface potential for the data in Figure 3A, indicating a constant τ ph value within this potential range consistent with a uniform doping depth profile in this epitaxial layer.

[0057] The example in Figure 4 shows the logarithmic surface potential time decay of an epitaxial wafer having three layers with different doping. Each layer has a τph1 , τ ph2 and τ ph3 It is characterized by the following: Layer 1, Layer 2, and Layer 3 are counted from the top of the epitaxial layer structure.

[0058] Figure 5 shows n-type 4H-SiC with a wavelength λ=325nm and effective incident photon flux φ eff = 1.2 × 10 13 photon / cm 2 The photo-neutralization time constant τ for light irradiation at s ph doping concentration N D It exhibits a dependence on a specific doping range N. D , and light irradiation conditions λ and φ eff The corresponding calibration function N D vs τ ph It is useful for making a decision.

[0059] Dopant concentration N D The photo-neutralization time constant τ increases with increasing ph The increase is typically due to the same corona charge Q C Surface depletion width W D =Q C / qN D This is a result of the becoming smaller. This results in less photocatalysis of holes in the depletion layer. As shown in Figure 6, free holes h generated in the depletion layer + It is active in photoneutralization and is guided to the surface by the depletion field. At the surface, negative corona ions trap the holes (Q - +h + →Q↑), the neutralized coronavirus species detaches from the surface.

[0060] As doping increases, hole generation decreases due to a reduction in depletion width, which is related to the incident photon flux φ. eff This can be overcome by increasing τ. As shown in Figure 7, ph It is inversely proportional to the incident photon flux. Therefore, by increasing the incident photon flux, a higher N DThis allows for rapid measurement. The speed of measurement is a major advantage of this method. In this regard, the time constant τ ph and the inverse relationship between the incident photon flux (τ ph ~1 / φ eff ) may be useful for optimizing the measurement. For example, if the photon flux is 10 14 photon / cm 2 By increasing it to the range of s, 10 16 cm -3 N in the range D The time constant τ is approximately 0.1 s. ph This can be achieved. 10 18 cm -3 High levels of doping, such as those mentioned above, can be rapidly measured by combining an increased photon flux with light of shorter wavelengths, such as λ=315nm or λ=300nm.

[0061] As mentioned above, calibration can be useful in converting the measured time constant to doping concentration. For this purpose, a specific doping range and (λ,φ) can be used. eff A specific family of calibration functions can be determined for the value of ). For accurate measurement, each specific calibration function is N D and φ eff The value can be limited to a subset of the entire range, for example, the exact calibration function is N D and φ eff It can cover a range of less than one order of magnitude. The calibration function can be determined using precise setting of measurement and light irradiation conditions for a reference wafer with known doping concentrations. For example, N pre-measured by mercury probe CV method D Wafers having the specified properties, or wafers previously measured by non-contact corona CV (CnCV), can be used. Then, the exact same light irradiation conditions can be used to measure the doping of epitaxial layers manufactured in the production environment.

[0062] Calibrated τ ph vs N D The quantitative determination of doping concentrations from the data is f cal The inverse function N shown byD vs τ ph The measurement is performed at the same wavelength as the calibration wavelength, for example, λ=325nm in Figures 5 and 7. The photon flux φ used during the measurement eff Since it is adjusted to take into account the measurement speed requirements, typically the photon flux φ used for calibration is cal This is different. The measured τ ph This is τ relative to the incident photon flux. ph It is normalized based on its dependency. Then the normalized value τ * ph =( φ eff / φ cal )·τ ph This is the inverse N D vs τ ph Used in a function, the doping concentration is N D =τ * ph ·f cal It is calculated as follows: In the example in Figure 5, τ ph The calibration of is φ cal =1.2e13 photons / cm 2 This was performed in s. However, when applied to different photon fluxes as shown in Figure 7, this calibration results in a correct N that matches within 0.5%. D A value was given.

[0063] An alternative method uses wafer-specific calibration performed during actual measurements of a given wafer. Apparatuses 100 and 100' shown in Figures 1A and 1B, respectively, are suitable for this purpose. Wafer-specific calibration includes: (1) Corona charge photoneutralization time constant τ ph Measurement of (2) doping concentration N D Another non-contact CV (CnCV) measurement. These two independent measurements are performed sequentially on the same wafer site. The result is N D =τ ph ·f cal Inverse calibration function f that satisfies the following condition cal Used to determine τ phk Only is measured (k is the site number). Doping N at different k sites on the wafer. D =τphk ·f cal The measured τ phk and wafer-specific function f cal It is determined using the same value.

[0064] Generally, measurements using wafer-specific calibration are performed under the same light irradiation conditions for all measurement sites. These conditions can be optimized for different doping ranges. However, this method is normalized τ with respect to the photon flux. * ph Rather, the measured τ ph Because it uses [a specific method], precise knowledge of the incident photon flux is not required. The above measurements can be performed at any number of sites on the wafer. A typical manufacturing line test includes measurements at 12 wafer sites or wafer mapping at 49 sites. Wafer-specific calibration adds a single-site CnCV doping measurement time, which is typically 40 seconds. τ at a single site ph Measurement times are shorter, for example, about 4 seconds. For a 12-site test with wafer-specific calibration, the measurement time per wafer can be about 90 seconds. This measurement rate can provide a throughput of about 30 wafers / hour, including wafer transport overhead time. For a 49-site wafer mapping with an additional 40 seconds of calibration time, the total measurement time is about 240 seconds, resulting in a throughput of about 13 wafers / hour, including wafer transport overhead.

[0065] Measurements using wafer-specific calibration can reduce uncertainty by reproducing the light irradiation conditions, and still provide very fast, high-throughput epitaxial SiC doping monitoring.

[0066] The photoneutralization method described above uses a specific definition of the photoneutralization time constant based on a logarithmic analysis of charge decay. However, it should be noted that, in general, the kinetic sensitivity of photoneutralization to the doping specified in this invention can use different definitions of time constants for doping measurement, such as the half-time of charge decay, the extended or compressed exponential time constant of charge decay, or others.

[0067] While a single application of photoneutralization has been described above, its technology and benefits are not limited to the monitoring of bare SiC doping. More generally, corona photoneutralization techniques can be applied to the characterization of other wide-bandgap materials and structures such as GaN and HEMTs. For example, this technique can be used with a pinch-off voltage (V p This can be applied to determining HEMT characteristics such as ) and 2DEG sheet charge.

[0068] Therefore, other embodiments are within the scope of the following claims.

Claims

1. A method for characterizing semiconductor doping in wide-bandgap semiconductor samples, Initial value V of the surface potential in the surface region of the semiconductor sample in the dark. ０ To measure, By applying a predetermined corona charge to the aforementioned region, the region is charged to a deep depletion state in the dark. After charging, measure the surface potential value in the area in a dark place. A specific photon beam φ having a photon energy exceeding the semiconductor's band gap is used to generate free minority carriers in the semiconductor sample and cause photoneutralization of the corona charge in the charged region. ｅｆｆ Irradiating with light, Using non-contact time-resolved measurement of the surface potential V(t), monitor the corona charge decay induced by photoneutralization in the region with respect to the light irradiation time t. The monitored time-resolved surface potential decay data V(t) was analyzed to determine the photoneutralization time constant τ ｐｈ To decide, A specific photon beam φ ｅｆｆ The optical neutralization time constant τ in ｐｈ Using this as a semiconductor doping index, the semiconductor doping concentration in the aforementioned region is characterized based on this value. Methods that include...

2. Corona charge photoneutralization time constant τ, which is intrinsic to the surface potential. ｐｈ The method according to claim 1, further comprising characterizing the doping concentration depth profile of the semiconductor sample based on V.

3. mean τ over the surface potential range ｐｈ The method according to claim 1, further comprising characterizing the doping of a single epitaxial layer based on a value.

4. τ ｐｈ However, the formula is: [Math 1] Determined according to, where t is the light irradiation time, and V ｃｏｎｓｔ is the offset of the surface potential probe, the method according to claim 1.

5. Known doping concentration values ​​N Ｄ τ in one or more reference samples having ｐｈ Using the calibration measurement of the calibration function τ ｐｈ vs N Ｄ and, f ｃａｌ The inverse calibration function N is expressed as Ｄ vs τ ｐｈ The method according to claim 4, further comprising determining the following.

6. Effective photon flux φ ｅｆｆ The time constant τ measured in ｐｈ Calibrate photon beam φ ｃａｌ against [Math 2] Normalize as Normalized time constant τ ＊ ｐｈ Using formula N Ｄ =τ ＊ ｐｈ f ｃａｌ The absolute doping concentration is determined according to the following: The method according to claim 5, further comprising:

7. τ ｐｈ This further includes calibrating the multi-site τ of the semiconductor wafer sample, wherein the calibration is specific to the semiconductor wafer sample and the multi-site τ of the wafer sample. ｐｈ This is performed on one site of the wafer sample that is measured during the base characterization, and all τ ｐｈ The method according to claim 4, wherein the measurement is performed under the same light irradiation conditions.

8. The aforementioned calibration is the corona charge photoneutralization time constant τ ｐｈ Measurement of doping concentration N using the non-contact corona C-V (CnCV) method. Ｄ The method according to claim 7, comprising an independent measurement of and both measurements being performed sequentially at the same wafer site and within similar corona charge-induced depletion potential ranges.

9. τ ｐｈ and N Ｄ Using N Ｄ =τ ｐｈ f ｃａｌ wafer-specific inverse calibration function value f that satisfies the following condition ｃａｌ The method according to claim 8, further comprising determining

10. τ ｐｈ Based on determining wafer-specific f determined at a single site ｃａｌ The method according to claim 7, further comprising determining the doping concentration at all different wafer sites using the value of .

11. The method according to claim 1, wherein the capacitance probe is a non-contact vibrating Kelvin probe.

12. The method according to claim 1, wherein the region is irradiated with light simultaneously with the monitoring of the surface potential.

13. The method according to claim 1, wherein the region is irradiated with light separately from the monitoring of the surface potential.

14. The method according to claim 1, wherein the semiconductor sample comprises a semiconductor selected from the group consisting of SiC, GaN, and AlGaN.

15. V measured before light irradiation, but after corona charging. Ｄａｒｋ and dark attenuation rate ΔV Ｄａｒｋ / Δt Ｄａｒｋ The method according to claim 1, further comprising identifying a defect region of the semiconductor sample based on the method.

16. The magnitude of dark attenuation ΔV measured after light irradiation. Ｄａｒｋ The method according to claim 1, further comprising identifying the contribution to the monitored surface potential from surface photovoltaic effects other than corona photoneutralization, based on the above.

17. A system for characterizing semiconductor doping in wide-bandgap semiconductor samples, Surface potential measurement probe, Corona period power supply, Light source and A movable wafer chuck configured to support the semiconductor sample with respect to the surface potential measurement probe, the corona band power supply, and the light source, The surface potential measurement probe, the corona band power supply, the light source, and the computer controller that communicates with the movable wafer chuck. The computer controller provides the following to the system: (i) Initial value V of the surface potential in the surface region of the semiconductor sample in the dark ０ To measure, (ii) By applying a predetermined corona charge to the region, the region is charged to a deep depletion state in the dark. (iii) After charging, measure the surface potential value in the region in the dark using the surface potential measuring probe. (iv) A specific photon beam φ from the light source having a photon energy exceeding the semiconductor band gap, sufficient to generate free minority carriers in the semiconductor sample and cause photoneutralization of the corona charge in the charged region. ｅｆｆ Irradiating with light, (v) Using non-contact time-resolved measurement of the surface potential V(t), monitor the corona charge decay induced by photoneutralization in the region with respect to the light irradiation time t using the surface potential measurement probe. (vi) Analyze the monitored time-resolved surface potential decay data V(t) to determine the photoneutralization time constant τ ｐｈ To decide, (vii) A specific photon beam φ ｅｆｆ The optical neutralization time constant τ in ｐｈ Based on this, the semiconductor doping concentration in the region is characterized. A system that is programmed to perform a certain action.

18. The system according to claim 17, wherein the light source is positioned to illuminate the region at a location different from the position below the surface potential measuring probe.

19. The system according to claim 18, wherein the computer controller is programmed to cause the system to irradiate the region with light before measuring the surface potential in the region, and to monitor the corona charge decay induced by photoneutralization in the region.

20. The system according to claim 18, further comprising a movable stage supporting the wafer chuck, wherein the computer controller is programmed to cause the stage to move the region between the surface potential measuring probe and the light source.

21. The system according to claim 17, wherein the light source is arranged to illuminate the region while the region is positioned beneath the surface potential measuring probe.

22. The system according to claim 21, wherein the computer controller is programmed to cause the system to irradiate the region with light and simultaneously monitor the corona charge decay induced by the photoneutralization in the region.

23. The system according to claim 21, wherein the surface potential measuring probe comprises a transparent electrode, and the light source is arranged to guide light to the region through the transparent electrode.

24. The system according to claim 17, wherein the surface potential measuring probe is a non-contact vibrating Kelvin probe.

25. The system according to claim 17, wherein the light source includes a light-emitting diode or a near-ultraviolet laser.

26. The system provides a τ of one or more reference semiconductor samples having known doping concentrations. ｐｈ The system according to claim 17, which is calibrated based on calibration measurements.

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