Method for representing semiconductor doping by using light neutralization time constant of corona surface charge

Abstract

A method of characterizing semiconductor doping in a wide band gap semiconductor sample comprises: measuring an initial value V0 of a surface voltage at a region of a surface of the semiconductor sample in darkness; charging the region to deep depletion in darkness by depositing a defined corona charge at the region; measuring a surface voltage value at the area in darkness after charging; irradiating the charged region with light of a specific photon flux feff having a photon energy higher than the semiconductor band gap sufficient to generate free minority carriers in the semiconductor sample to cause photoneutralization of corona charges; using a non-contact time-resolved measurement of the surface voltage V (t) to monitor the relationship of light neutralization induced corona charge decay to the illumination time t at the area; analyzing the monitored time-resolved surface voltage decay data V (t) to determine a light neutralization time constant [tau] ph; and using the light neutralization time constant [tau] ph under the specific photon flux feff as a semiconductor doping index, and characterizing the semiconductor doping concentration at the region based on the value of the light neutralization time constant [tau] ph.

Description

[0001] This application is a divisional application of the patent application with the application number 202480001587.X (international application number PCT / US2024 / 019083), the application date of March 8, 2024, and the invention title "Method for Characterizing Semiconductor Doping Using the Photo-Neutralization Time Constant of Corona Surface Charge". Technical Field

[0002] This application generally relates to the characterization of semiconductor doping in wide-bandgap semiconductors, and more particularly, to methods for non-contact measurement of doping in epitaxial layers and systems for performing such measurements. Background Art

[0003] Semiconductors doped with donor or acceptor impurities change the electrical properties of the semiconductor and are key technical elements in semiconductor device manufacturing. Doping is typically accomplished by introducing impurity atoms that replace host atoms in the semiconductor lattice. The doping concentration is the concentration of doping atoms per volume of semiconductor, and its measurement is typically used for quality control in semiconductor device production.

[0004] For example, devices based on wide-bandgap semiconductors (including SiC, GaN, AlGaN, and AlGaN / GaN) are widely used in power electronics and high-frequency applications. Such devices typically include doped epitaxial layers, where strict doping control of the epitaxial wafers is necessary. To meet the demands of rapidly developing wide-bandgap semiconductor technologies and large-scale epi-wafer manufacturing, corresponding large-scale doping tests are needed. For this purpose, non-contact measurement techniques can be preferred. Non-contact metrology can eliminate the costs and time involved in fabricating test devices and can avoid the post-measurement wafer cleaning required in many conventional doping measurement methods (such as the Mercury Probe method).

[0005] An example of a preparation-free non-destructive doping measurement method for wide-bandgap semiconductors described in US10,969,370B2 can be performed using a commercially available corona non-contact capacitance-voltage (CnCV) tool manufactured by Semilab SDI. CnCV uses corona charging to electrically bias the semiconductor surface to deep depletion, similar to the voltage biasing of a metal Schottky barrier, yet achieved without metal contacting the semiconductor surface. In the CnCV method, the surface voltage response ΔV for a charge bias dose ΔQ C is monitored with a Kelvin probe. The differential capacitance C = ΔQ C / ΔV in sequential charge measurements gives the non-contact C-V characteristics. The doping concentration N D is given by 1 / C 2The slope of vs V is determined, which is a standard procedure in the C-V method. For a typical 12-site pattern, the CnCV measurement throughput using commercially available tools is currently limited to about 5 wafers per hour. SUMMARY OF THE INVENTION

[0006] Conventional CnCV techniques can be modified to employ a new doping-sensitive phenomenon that can be exploited to increase the test throughput. Conventional CnCV uses corona charge biasing to depletion, which is achieved through multiple incremental corona charge deposition steps. In response to the deposited charge, the surface voltage V changes and is measured using a non-contact vibrating Kelvin probe. Similar to mercury probes and Schottky junction devices, CnCV employs the slope of the 1 / C 2 -V characteristic to determine the doping concentration. CnCV is a quasi-static technique that has many relatively slow charge biasing steps involved in acquiring the C-V characteristic. This typically limits the measurement speed and throughput in multi-wafer monitoring, where each wafer typically has 9 or 12 test sites and a wafer map with 49 sites.

[0007] The deposited corona charge can be rapidly neutralized and removed from the bare surface of a wide-bandgap semiconductor by short-wavelength illumination. The corresponding time constant for corona photo-neutralization can be very short, enabling rapid sweeping of the depletion barrier. In measurements on differently doped epitaxial SiC, a direct relationship between the corona charge photo-neutralization time constant and the doping concentration was determined. This relationship was verified for other wide-bandgap semiconductors and is valid over a wide range of doping concentrations. The enhanced throughput doping measurement method in the present disclosure is based on this newly identified relationship.

[0008] Accordingly, in the present invention, a charge biasing method with a novel doping measurement principle is described, which can increase the measurement speed and throughput, for example, by up to ten times compared to the conventional CnCV techniques described above.

[0009] The method described here can replace the sequence of cumulative charge steps with a single large corona charge to deep depletion. Thereafter, photo-neutralization of the charge deposited on the semiconductor surface is performed using near-UV illumination that generates excess carriers. The photo-neutralization time constant is determined by time-resolved surface voltage measurements, and this time constant is a doping measurement parameter.

[0010] During photo-neutralization, the minority carriers optically generated within the charge-induced surface depletion layer are mobile. They are guided to the surface by the depletion electric field and captured by corona ions of opposite polarity, thereby neutralizing the corona charge and reducing the depletion width. The technique involves measuring and analyzing the corresponding change in surface voltage with respect to the illumination time.

[0011] In wide-bandgap semiconductors, the photo-neutralization of corona ions is irreversible. The corona charge does not recover after photo-neutralization, and the surface voltage change induced by photo-neutralization does not reverse in the dark. This enables the monitoring of the process by measuring the surface voltage when irradiating the charging region or after successive irradiation pulses. This disclosure covers both measurement configurations. Specifically, it is employed in a system configuration for measurement when irradiating the charging region, where the irradiation is below the surface voltage probe. Measurement after irradiation pulses is used in a system configuration where the irradiation is at a location separated from the surface voltage probe.

[0012] This measurement technique utilizes the irreversible nature of the surface voltage change caused by light irradiation to separate charge photo-neutralization from other surface photovoltaic effects. The corresponding test involves monitoring the surface voltage in the dark after stopping irradiation. For good measurement conditions, the dark recovery should be negligible, e.g., within the range of 0.1%.

[0013] In some embodiments, the measurement technique employs the depletion-layer Schottky-barrier electrostatics charge-voltage relationship to extract the charge photo-neutralization rate and determines the photo-neutralization time constant τ from the logarithmic characteristic of the surface voltage versus irradiation time. ph . Based on the calibration correlation of the time constant τ ph and τ ph vs N D the doping concentration N is determined. The time constant vs surface voltage characteristic τ D -V provides a means for evaluating the doping depth profile. Uniform doping is identified by a constant τ ph value. In the logarithmic photo-neutralization surface voltage characteristic, multi-layers with different dopings will show different τ ph values over a certain time period. The corresponding surface voltage reflects different depths below the surface. ph value. The corresponding surface voltage reflects different depths below the surface.

[0014] Generally, the measurement device includes three system components with the following capabilities:

[0015] (a) - Corona charging for biasing the semiconductor surface charge to deep depletion.

[0016] (b) - Non-contact surface voltage measurement.

[0017] (c) - Short-wavelength irradiation with photon energy higher than the semiconductor bandgap.

[0018] The semiconductor sample wafer can be placed on a conductive chuck with the ability to move quickly for positioning the test sample area (i.e., the test site on the wafer) below the system components (a), (b), and (c) according to the measurement cycle.

[0019] A housing for a measurement system can be provided to prevent stray light (e.g., short-wavelength stray light) from reaching the wafer. Otherwise, such stray light can cause uncontrolled corona charge neutralization on the wafer surface.

[0020] Time constant τ ph It can be directly used as an index of relative doping concentration. For precise quantitative measurement of doping concentration, the method can include a calibration function of the time constant, which is determined by measurement of a reference sample wafer with a known doping concentration. Alternatively or additionally, wafer-specific calibration can be performed using corona C-V doping measurements performed on the same sample area (same wafer site) as the measurement of the photo-neutralization time constant τ ph of the measurement.

[0021] In a conventional corona charging method for semiconductor and dielectric characterization, the response to changes in the deposited corona charge density is typically monitored by measuring the corresponding change in surface voltage. In the present technique, measurement of surface voltage is also employed. However, the change in the charge density of the corona deposition is achieved by photo-neutralization of the charge rather than by charge deposition. The measurement of charge photo-neutralization and surface voltage can be performed simultaneously or sequentially, depending on the configuration of the light source and the surface voltage probe. Illumination under the probe enables measurement during actual corona photo-neutralization. This allows illumination and measurement of the sample site without transferring the wafer between the illumination and the measurement probe, which can be beneficial for the speed and throughput of the measurement. A configuration with a separate light source can promote higher illumination uniformity than illumination under the surface voltage probe. This can be beneficial for accuracy and tool-to-tool matching.

[0022] The disclosed technique can use multiple light sources with different configurations and wavelengths, which can be selected and optimized according to the requirements of the specific semiconductor wafer manufacturing using the technique.

[0023] Generally, in one aspect, the present invention features a method for characterizing semiconductor doping in a wide-bandgap semiconductor sample, the method comprising: measuring an initial value V of the surface voltage at a region of the surface of the semiconductor sample in the dark 0 ; charging the region to deep depletion in the dark by depositing a prescribed corona charge at the region; measuring the surface voltage value at the region in the dark after charging; irradiating the charged region with light having a specific photon flux f eff of the specific photon flux φ effHaving a photon energy above the semiconductor bandgap sufficient to generate free minority carriers in the semiconductor sample to cause photo-neutralization of the corona charge; monitoring the photo-neutralization-induced corona charge decay vs. irradiation time t at a region using non-contact time-resolved measurement of the surface voltage V(t); analyzing the monitored time-resolved surface voltage decay data V(t) to determine the photo-neutralization time constant τ ph ; and using the photo-neutralization time constant τ eff at a specific photon flux f ph as a semiconductor doping index and characterizing the semiconductor doping concentration at the region based on its value.

[0024] Embodiments of the method may include one or more of the following features.

[0025] The method may include characterizing the doping concentration depth profile of the semiconductor sample based on the surface voltage specific corona charge photo-neutralization time constant τ ph vs V.

[0026] The method may include characterizing the doping of a single epitaxial layer based on the average τ ph value within a surface voltage range.

[0027] In some examples, τ is determined according to the following equation ph :

[0028]

[0029] where t is the irradiation time and V const is the surface voltage probe offset. The method may include using calibration measurements of τ D for one or more reference samples having known doping concentration values N ph to determine the calibration function τ ph vs ND and the inverse calibration function N D vs τ ph (denoted as f cal ). In some examples, the method includes normalizing the time constant τ eff measured at an effective photon flux φ ph to a calibration photon flux φ eff , as follows: and determining the absolute doping concentration using the normalized time constant according to the following equation:

[0030] The method may include calibrating τ ph for doping concentration, where the calibration is specific to the semiconductor wafer sample and is based on τ ph at multiple sites (e.g., 9 sites, 12 sites, or 49 sites) of the wafer sample.Calibration is performed at a site on the measured wafer sample during the characterization, where all τ ph Measurements are performed under the same illumination conditions. Calibration may include measuring the corona charge photo-neutralization time constant τ using the corona non-contact C-V (CnCV) method ph and independently measuring the doping concentration N D , where the two measurements are performed one after the other at the same wafer site and within a similar corona charge-induced depletion voltage range. The method may include using τ ph and N D to determine the wafer-specific inverse calibration function value f that satisfies N D = τ ph ·f cal . cal .

[0031] In some examples, the method includes determining the doping concentration at all different wafer sites based on determining τ at each wafer site and using the wafer-specific value of f determined at a single site ph and using the wafer-specific value of f determined at a single site cal .

[0032] The capacitance probe can be a non-contact vibrating Kelvin probe

[0033] The region can be illuminated while monitoring the surface voltage, or the region can be illuminated separately from monitoring the surface voltage

[0034] The semiconductor sample can include a semiconductor selected from the group consisting of SiC, GaN, and AlGaN

[0035] The method can include identifying a defective region of the semiconductor sample based on V measured after corona charging but before illumination Dark and the dark decay rate ΔV Dark / Δt Dark .

[0036] The method can include identifying the contribution of the surface photovoltaic effect other than corona photo-neutralization to the monitored surface voltage based on the dark decay amplitude ΔV measured after illumination Dark .

[0037] Other features and advantages will be apparent from the drawings, the specification, and the claims BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1A is a schematic diagram of an exemplary system for measuring the doping of a semiconductor wafer that uses corona charge deposition followed by illumination-induced charge photo-neutralization and determination of the photo-neutralization time constant based on non-contact time-resolved surface photovoltage. In this example, the configuration includes illumination located below the surface voltage probe

[0039] Figure 1B is Figure 1A A schematic diagram of an alternative configuration of the example system shown, where the illumination is at a position separated from the surface voltage probe.

[0040] Figure 2 Is an illustration of the steps in an example measurement procedure for determining the photo-neutralization time constant from time-resolved surface voltage decay.

[0041] Figure 3A Is a graph showing an example result of the relationship between the surface voltage measured during corona charge photo-neutralization and the illumination time.

[0042] Figure 3B Is for determining the Figure 3A Example graph of the photo-neutralization time constant corresponding to the transient in

[0043] Figure 3C Is a graph of the surface voltage for a specific τ ph Showing the case of a uniform doping depth profile.

[0044] Figure 4 Is a graph showing an example of the corresponding logarithmic surface voltage decay of an n-type SiC epitaxial wafer with 3 layers of different doping.

[0045] Figure 5 Is a graph of example doping calibration results showing the photo-neutralization time constant τ ph Versus the doping concentration N measured on n-type 4H-SiC D The measurements were made using a negative corona charging charge density of -4E12q / cm 2 Irradiation was performed at a wavelength of 325 nm with an incident photon flux of 1.2×10 13 Photons / cm 2 s. The doping concentration N D Corresponds to the C-V mercury probe results.

[0046] Figure 6 Is a band diagram showing the photo-neutralization of corona charge on an n-type wide-bandgap semiconductor. The threshold energy hv is the bandgap E g .

[0047] Figure 7 Is a graph showing an example result of the correlation between the photo-neutralization time constant and the incident photon flux. The results are for n-type SiC with doping concentrations of N D = 6.11e15cm -3 And N D = 1.85E16cm -3 The N D Values are based on the C-V mercury probe results. Detailed implementation manners

[0048] Reference Figure 1A , schematically shows an example device 100 for corona charge-voltage photo-neutralization characterization of a semiconductor wafer sample 101. The wafer sample includes a substrate 104 and an epitaxial layer 103 supported by the substrate 104. The device 100 includes a corona charge source module 120 for precise charge deposition at a test site 126, a surface voltage measurement module 130 for surface voltage measurement, and a light source module 140 for irradiating the wafer surface.

[0049] The wafer 101 is supported by a wafer chuck 110, and the wafer chuck 110 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.

[0050] The corona charge source module includes a corona charge electrode arranged to deposit a corona charge 124 at a site 126 on the surface of the wafer 101. The surface voltage measurement module 130 includes a Kelvin probe having an electrode 132 arranged to vibrate a small distance (e.g., one millimeter or less) above the surface of the wafer 101. Although the example described here uses a Kelvin probe, other capacitive probes can be used.

[0051] The light source module 140 includes a light source 142 arranged to irradiate the surface of the wafer 101 located directly below the Kelvin probe electrode. During operation, the light source 142 delivers irradiation having a wavelength suitable for photo-neutralization of the corona charge at the wafer surface below the Kelvin probe electrode 132. The light source 142 can include, for example, a light emitting diode (LED) or a near-UV laser. For an LED, the light beam can be further monochromatized by passing through a narrow bandpass filter.

[0052] Reference Figure 1B, shows an alternative device 100’, where the light source module 140’ includes a light source 144, and the light source 144 is positioned to irradiate a position on the surface of the wafer 101 different from the site under the electrode 132. Similar to the light source 142, the light source 144 can include an LED or a laser, which is capable of emitting light with a wavelength suitable for photo-neutralization of the corona charge. Generally, the irradiation wavelengths provided by the light sources 142 and 144 can be selected according to the wafer under test. Generally, the photon energy should be higher than the semiconductor bandgap. For the measurement of 4H-SiC with a bandgap of 3.26 eV, suitable light wavelengths include λ = 355 nm or λ = 325 nm corresponding to photon energies hv = 3.49 eV and hv = 3.82 eV respectively. Both photon energies exceed the SiC bandgap, which is required to generate free carriers (electrons - holes) involved in the photo-neutralization process of the corona charge. For the measurement of other semiconductors, the light wavelength can be selected based on the values of the bandgap and the absorption coefficient. Shorter wavelength λ = 315 nm irradiation can be applicable to the measurement of doping on GaN and AlGaN materials, which have a bandgap greater than that of SiC. Depending on the application, other wavelengths of light of about 380 nm or less can be used. The light sources 142 and 144 are arranged to provide uniform irradiation within the surface area of the corona charging point, such as a circular point with a diameter of about 10 mm or less (e.g., a diameter of about 6 mm).

[0053] The computer controller 150 is used to control the operation of the device and perform data analysis to determine information about the doping of the sample 101 using the measurement and analysis steps described below.

[0054] Figure 1A and Figure 1B The example devices 100 and 100’ shown in Figure 1A respectively show two irradiation configurations: namely, the light source 142 (LED 1) for irradiating under the vibrating electrode of the Kelvin probe electrode 132, and the light source 144 (LED 2) for irradiation separated from the Kelvin probe 132. A standard high-precision Kelvin probe can use an opaque gold electrode. In the case where the Kelvin probe electrode 132 is opaque, low-angle irradiation from the side can be used, such as the light source 142 schematically shown in Figure 1B . In some embodiments, a transparent Kelvin probe that irradiates through the electrode can also be used. Generally, in the case of irradiating with the light source 144 alone, higher uniformity of irradiation can be more easily achieved, as shown in

[0055] In the Kelvin probe method, an AC current is generated by a capacitive electrode vibrating a fraction of a millimeter above the surface being measured. This AC current is nulled by compensating for the DC bias, thereby providing a measurement of the voltage difference between the electrode and the semiconductor. For the present application, an accurate and fast-responding Kelvin probe is preferred, such as a probe having a time constant of about 5 ms and an accuracy of 0.2 mV. For deeply depleted corona charges biased to n-type (negative bias) and p-type (positive bias) semiconductors, the surface voltage measurement range is typically from -100 V to +100 V. An electrode diameter of 1 mm or 2 mm is suitable for measuring the surface voltage in the uniform central region of the corona-charged and irradiated areas.

[0056] Corona charging is achieved using corona discharge in air generated by a high DC voltage applied from a power supply 122 to a discharge electrode 124. Negative polarity discharge in air generates ions, while positive polarity discharge generates (H 2 O) n H + ions. The corona discharge electrodes (a needle for point charging or a wire for whole-wafer charging) can be confined in a housing, and the ion deposition is not field-driven; instead, the ions diffuse through the hole openings to the wafer surface. Considering the very short mean free path of 10 -5 cm in an indoor atmosphere, the ions lose kinetic energy and become thermalized before reaching the semiconductor surface. Such a configuration enables non-invasive corona charge biasing. Precise corona charging is typically performed in a cleanroom environment with controlled humidity and temperature. Control of the deposited corona charge density is achieved by setting the high voltage, discharge current, and deposition time period of the corona power supply 122. The deposition time period can be about 1 second or shorter. The deposited charge density dose can be monitored in-situ with a coulomb meter 114.

[0057] Corona charging to deep depletion extends from about 1x10 12 q / cm 2 to about 1x10 13 q / cm 2 , and the charging can be achieved in a single charging step. The charge deposited on the surface at the test site 126 is mirrored as an opposite charge in the semiconductor surface space charge region and acts as an electrical bias. The charging dose is much larger than the surface charge initially present, typically about 1x10 11 q / cm 2 . Thus, depending on the doping concentration N D , the charging dose Q C can be selected to obtain the desired depletion width W D =-Q C / qN D .

[0058] Wide-bandgap semiconductor devices are mainly fabricated on n-type epitaxial layers, and 4H-SiC is a commonly used material. Accordingly, exemplary results for methods for n-type 4H-SiC and negative-polarity corona charging are presented in this disclosure. To measure such wafers, the method may also include stabilizing a new epitaxial 4H-SiC wafer using a UV pretreatment chamber, which may be part of apparatus 100. After the epitaxial growth process, the 4H-SiC may immediately exhibit rapid dissipation of the deposited corona charge. This is caused by surface diffusion of the corona ions. If present, this effect may interfere with the measurement of photoinduced charge photo-neutralization. The pretreatment to eliminate the surface diffusion effect may be performed simultaneously with the doping measurement of multiple sets of wafers. This capability may be added to an automated version of apparatus 100.

[0059] The stable bare surface of depleted SiC typically exhibits good corona charge stability in the dark. The large bandgap prevents the thermal generation of free minority carriers that can neutralize the corona charge. In this method, the charge stability in the dark after charging is verified by the corresponding stability of the surface voltage. For this purpose, the method may include a specified period of surface voltage measurement in the dark after charging but before illumination. Such a measurement is an example of a dark decay rate measurement and is based on the change in the dark surface voltage V dark as a function of time to produce a decay rate. A large dark surface voltage decay rate and a surface voltage amplitude below the nominal expected value of the deposited charge density may indicate defects that cause charge dissipation and thus interfere with the photo-neutralization measurement of the corona charge. In such cases, the measurement may be repeated at adjacent wafer sites that are defect-free and have a negligible dark decay rate. In the epitaxial 4H-SiC used as an example in this disclosure, the interfering defects are device-killing triangular defects, downfall defects, or carrot defects. These defects may have sub-millimeter dimensions, and shifting the measurement location by a distance greater than the Kelvin probe diameter (e.g., 3 mm to 5 mm in the case of a 2 mm diameter Kelvin probe) may be sufficient to eliminate the interference.

[0060] In some embodiments, additional verification steps may also be performed. For example, such a step may include a measurement of the surface voltage performed in the dark after turning off photoinduced charge photo-neutralization. This verification involves the irreversible nature of photo-neutralization and is intended to identify other surface photovoltaic effects in the surface depletion layer that are reversible.

[0061] Typically, measurements are carried out under conditions where corona charge photo-neutralization predominates. This can be tested in the measurement of surface voltage recovery in the dark, immediately after irradiation. For optimized measurement conditions, the amplitude of this surface voltage recovery indicating other effects should be, i.e., not significant compared to the amplitude of corona charge photo-neutralization, e.g., in the range of about 100 mV or less for a corona charge photo-neutralization amplitude of about 20 V or higher. In SiC, this negligible contribution can be achieved using a low effective photon flux in the range of about 10 13 to about 10 14 photons / cm 2 s.

[0062] Characterization of semiconductor doping involves determining the time constant τ ph of corona charge photo-neutralization under surface depletion conditions. Once τ ph is determined, it is used as the doping index. The τ ph value is converted to a doping concentration of N D = τ ph ·f cal , where f cal is the so-called inverse calibration function. The determination of the time constant τ ph and the calibration function f cal is discussed further below.

[0063] Figure 2 The table in Figure 2 shows the steps involved in the measurement procedure that can be used to obtain time-resolved surface voltage data to determine the photo-neutralization time constant. The surface voltage measured at each step is shown in the graph included in Figure 1A . The surface voltage measurement is a characteristic of a typical n-type epitaxial wafer of 4H-SiC measured using the device 100 or 100’ and the irradiation configuration shown in . The wafer is held on the conductive chuck 110 by vacuum suction. During corona charging and measurement, the chuck 110 provides a backside electrode capacitively coupled to the wafer. The non-contact nature of this method means there is no 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 initially off, and the wafer is in the dark. The wafer chuck is movable, and for Figure 2 step 1, it positions the wafer test site under the Kelvin probe electrode 132 for measuring the initial surface voltage V 0 , e.g., V 0 = 0.8 V. After the initial surface voltage measurement, the test site 126 is transferred to a position under the corona charging source (as shown in Figure 1A and Figure 1B ), and in step 2, a corona charge with a specified charge density Q C (e.g., -1.5x1012 q / cm 2 ) The negative charges are deposited on the surface of the epitaxial layer 103 at the test site 126. As a result of the charge deposition, the surface is biased into deep depletion. For step 3, the charging site is shifted to a position under the Kelvin probe electrode 132. Then the surface voltage of the charged deep depletion is measured, giving a post-charge value, such as V D = -59V. In step 4, the test site is irradiated with short-wavelength light having a photon energy higher than the 3.26 eV bandgap of 4H-SiC, suitable for causing photo-neutralization of the corona charges. For example, irradiation with a 325 nm wavelength (hv = 3.82 eV) having a near-UV LED can be used. In Figure 1A the example shown, irradiation under the Kelvin probe electrode 132 with a light source 142 can be used, enabling continuous monitoring of the depletion layer surface voltage V(t) while irradiating the charged area. In Figure 1B the case of the exemplary device 100’ shown, the irradiation is at a position separate from the surface voltage probe. Therefore, monitoring cannot be performed while irradiating the charged area. Instead, it uses a sequence of steps including short irradiation periods (typically about 10 ms to 20 ms duration) and surface voltage measurements. In this sequence, the charged area is moved from a position under the surface voltage measurement probe 132 to a position under the irradiation source 144 and then back to the surface voltage probe... and so on.

[0064] The final step 5 of the measurement cycle includes recording the surface voltage after turning off the short-wavelength light. This value is called the dark voltage V Figure 2 in after . A very small depletion voltage recovery is used to confirm the negligible contribution of the surface photovoltaic effect other than the photo-neutralization of the corona charges.

[0065] An example of the measured V(t) is shown in Figure 3A . The initial part of the voltage transient in the dark does not show attenuation before irradiation. This dark part is an indication of negligible charge dissipation due to surface diffusion or any defect-related charge neutralization. During irradiation, the rapid photo-neutralization of the corona charges is evident as a corresponding surface voltage decay. This recorded V(t) time correlation can be reliably analyzed using the Schottky barrier voltage-charge equation, providing a means for determining the photo-neutralization time constant τ ph of the corona charges. The quadratic relationship between the depletion surface barrier and the surface charge density (V D ∼ Q C 2 ) can be used to derive the surface voltage decay corresponding to the decay of the corona charges. In practice, τ ph can be determined as the negative slope of the logarithmic surface voltage time decay during photo-neutralization as where V 0 is the initial pre-charge value, including the offset related to Kelvin probe calibration. For the Figure 3A V(t) data in Figure 3B is shown in

[0066] Such a fitting process gives the average τ ph value within a specified surface voltage range, in this case in the range of -5V to -59V. Figure 3B The good linear fit in ph indicates a constant τ

[0067] value within this range. This is consistent with the uniform doping vs depth in this epitaxial layer. Figure 3C For the doping depth profile, a voltage-specific time constant corresponding to a given depletion surface voltage V based on photo-neutralization V vs irradiation time characteristics V(t) can be used. It is calculated from the relative surface voltage decay rate as Figure 3A ph shows the voltage-specific τ ph as a function of the depletion surface voltage for the data in

[0068] Figure 4 which indicates a constant τ ph1 τ ph2 and τ ph3 value within this voltage range consistent with the uniform doping depth profile in this epitaxial layer. The layers 1, 2, and 3 are counted from the top of the epitaxial layer structure.

[0069] Figure 5 shows the correlation of the photo-neutralization time constant τ eff with the doping concentration N 13 for n-type 4H-SiC and photo-irradiation with wavelength λ = 325nm and effective incident photon flux φ 2 = 1.2x10 ph photons / cm D ² s. Such a correlation can be used to determine the calibration function N D vs τ ph corresponding to a specific doping range N D and irradiation conditions λ and

[0070] Figure 6 The photo-neutralization time constant τ ph increases with the increase of the dopant concentration N D usually for a smaller surface depletion width W C = Q​​C / qN D The result of this is a smaller photo-generation that depletes the internal holes. As Figure 6 shown, the free holes h generated in the depletion + are active in the photo and in the neutralization, and they are guided to the surface by the depletion electric field. At the surface, the negative corona ions capture the holes, Q - +h + →Q↑ and the neutralized corona material separates from the surface.

[0071] Under increased doping, the smaller hole generation caused by the reduced depletion width can be overcome by increasing the incident photon flux φ eff As Figure 7 shown, τ ph is inversely proportional to the incident photon flux. Therefore, by increasing the incident photon flux, fast measurements can be achieved at higher N D In this method, the measurement speed is the main advantage. In this regard, the time constant τ ph and the inverse proportionality of the incident photon flux (τ ph ~1 / φ eff ) can be used to optimize the measurement. For example, for N 16 in the range of 10 -3 cm D , by increasing the photon flux to 10 14 photons / cm 2 s range, a time constant τ ph of about 0.1 s can be achieved. Combining the increased photon flux and light of shorter wavelengths, such as λ = 315 nm or λ = 300 nm, high doping, such as 10 18 cm -3 , can be measured quickly.

[0072] As previously mentioned, calibration can be used to convert the measured time constant to the doping concentration. For this purpose, a family of specific calibration functions can be determined for a specific doping range and (λ, φ eff ) values. For precise measurement, an individual specific calibration function can be limited to a subset of the entire range of values of N D and , for example, the precise calibration function can cover a range of one order of magnitude or less of N D and φ eff . The calibration function can be determined using the measurement of a reference wafer with a known doping concentration and precise setting of the irradiation conditions. For example, wafers with N D previously measured using the mercury probe C-V method or wafers previously measured using the corona non-contact C-V method (CnCV) can be used. Then the exact same irradiation conditions can be used when measuring the doping in the fabricated epitaxial layer in a production environment.

[0073] From the calibrated τ ph vs N D Data quantitatively determines the doping concentration using the inverse function N expressed as f cal of D vs τ ph . Measurements are performed at the same wavelength as that used for calibration, e.g., Figure 5 and Figure 7 where λ = 325 nm in eff . The photon flux φ used during the measurement is usually different from the photon flux φ used for calibration cal , because it is adjusted considering the measurement speed requirements. Based on the correlation of τ ph with the incident photon flux, the measured τ ph is normalized. Then the normalized value is used in the inverse function f D of N ph vs τ cal to calculate the doping concentration as In the example of Figure 5 , φ is calibrated at φ cal = 12e13 photons / cm 2 s. However, when applied to different photon fluxes as shown in cal Figure 7 , this calibration gives the correct N D value, within 0.5%.

[0074] An alternative method uses wafer-specific calibration performed during the actual measurement of a given wafer. The apparatuses 100 and 100' shown in Figure 1A and Figure 1B are suitable for this purpose. Wafer-specific calibration involves: (1) measurement of the corona charge photo-neutralization time constant τ ph and (2) different non-contact C-V (CnCV) measurements of the doping concentration ND. These two independent measurements are performed one after another at the same wafer site. The results are used to determine the inverse calibration value function f cal such that N D = τ ph .f cal . At all other test positions on this wafer, only τ phk (k is the position number) is measured. The doping N at different k positions on the wafer is determined using the measured τ phk and the same value of the wafer-specific function f cal such that N Dk = τ phk .f cal .

[0075] ​Generally, measurements with wafer-specific calibration are done at the same illumination conditions for all measured sites. The conditions can be optimized for different doping ranges. However, precise knowledge of the incident photon flux is not required since the method uses the measured τ ph instead of being normalized to the photon flux The measurements described above can be performed at any number of sites on a wafer. Typical production line testing involves 12 wafer site measurements or 49 site wafer mapping. Wafer-specific calibration increases the unit site CnCV doping measurement time which is typically 40 seconds. The τ ph measurement time at a single site is shorter, e.g., about 4 seconds. For a 12-site test with wafer-specific calibration, the measurement time per wafer can be about 90 seconds. This measurement speed can give a throughput of about 30 wafers / hour, including wafer handling overhead time. For a 49-site wafer mapping with an additional 40 seconds calibration time, it results in a total measurement time of about 240 seconds and a throughput of about 13 wafers / hour with wafer handling overhead.

[0076] Measurements using wafer-specific calibration can reduce uncertainty by reproducing the illumination conditions and still provide very fast, high-throughput epitaxial SiC doping monitoring.

[0077] The above photo-neutralization method employs a specific photo-neutralization time constant definition based on logarithmic analysis of charge decay. However, it should be noted that generally, the sensitivity of photo-neutralization kinetics to doping determined in the present invention can use different time constant definitions for doping measurements, such as the half-life of charge decay, stretched or compressed exponential time constants of charge decay, etc.

[0078] Although a single application of the photo-neutralization method is described above, the techniques and benefits are not limited to bare SiC doping monitoring. More generally, the corona photo-neutralization technique can be applied to characterize other wide-bandgap materials and structures, such as GaN and HEMT. For example, the technique can be applied to determine HEMT characteristics, such as pinch-off voltage (V p th) and 2DEG sheet charge.

[0079] Accordingly, other embodiments are in the appended claims.

Claims

1. A system for characterizing semiconductor doping in a wide bandgap semiconductor sample, the system comprising: Surface voltage measurement probe; Corona charge source; light source; a movable wafer chuck configured to support the semiconductor sample relative to the surface voltage measurement probe, the corona charge source, and the light source; as well as a computer controller in communication with the surface voltage measurement probe, the corona charge source, the light source, and the movable wafer chuck, the computer controller being programmed to cause the system to: (i) measuring an initial value V0 of a surface voltage at a region of the surface of the semiconductor sample in darkness; (ii) charging the area to deep depletion in darkness by depositing a defined corona charge at the area; (iii) measuring a surface voltage value at the region in the dark after charging using a surface voltage measurement probe; (iv) using a specific photon flux φ from the light source eff The light irradiates the charged area, the specific photon flux φ eff having a photon energy above the semiconductor bandgap sufficient to generate free minority carriers in the semiconductor sample, thereby causing photoneutralization of the corona charge; (v) monitoring the photoneutralization-induced corona charge decay at the region as a function of the irradiation time t using a contactless time-resolved measurement of the surface voltage V(t) using a surface voltage measurement probe; (vi) Analysis of the monitored time-resolved surface voltage decay data V(t) to determine the photoneutralization time constant τ ph ;as well as (vii) Based on the specific photon flux φ eff The light neutralization time constant τ ph To characterize the semiconductor doping concentration in the region.

2. The system according to claim 1, wherein: The light source is arranged to illuminate an area at a position different from a position below the surface voltage measurement probe.

3. The system according to claim 2, wherein: The computer controller is programmed to cause the system to illuminate the area prior to measuring the surface voltage at the area to monitor photoneutralization-induced corona charge decay at the area.

4. The system of claim 2, further comprising a moving stage supporting the wafer chuck, and the computer controller is programmed to move the stage to the area between the surface voltage measurement probe and the light source.

5. The system according to claim 1, wherein: The light source is arranged to illuminate the area when the area is located below the surface voltage measurement probe.

6. The system according to claim 5, wherein: The computer controller is programmed to cause the system to illuminate the area while monitoring photoneutralization-induced corona charge decay at the area.

7. The system according to claim 5, wherein: The surface voltage measurement probe includes a transparent electrode, and the light source is arranged to direct light to the area through the transparent electrode.

8. The system according to claim 1, wherein: The surface voltage measurement probe is a non-contact vibrating Kelvin probe.

9. The system according to claim 1, wherein: The light source includes a light emitting diode or a near UV laser.

10. The system according to claim 1, wherein: Based on τ of one or more reference semiconductor samples with known doping concentrations ph The calibration measurements are used to calibrate the system.

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