Method for characterizing semiconductor doping using the light neutralization time constant of the corona surface charge

Abstract

Methods for characterizing semiconductor doping in wide-bandgap semiconductor samples include: measuring an initial value V0 of the surface voltage at a region on the surface of the semiconductor sample in darkness; charging the region to deep depletion in darkness by depositing a specified corona charge at the region; measuring the surface voltage value at the region in darkness after charging; and using a specific photon flux f. eff The light irradiates the charging region, and a specific photon flux f eff A photon energy above the semiconductor bandgap is used, sufficient to generate free minority carriers in the semiconductor sample, thereby causing photoneutralization of the corona charge. The relationship between the photoneutralization-induced corona charge decay and the irradiation time t at the region is monitored using non-contact time-resolved measurements of the surface voltage V(t). The monitored time-resolved surface voltage decay data V(t) is analyzed to determine the photoneutralization time constant τ. ph ; and using a specific photon flux f eff Light neutralization time constant τ ph It serves as a semiconductor doping index, and its value characterizes the semiconductor doping concentration in that region.

Description

[0001] This application is a divisional application of the application patent application with application number 202480001587.X (International application number PCT / US2024 / 019083), filing date 8 March 2024, and title “Method for characterization of semiconductor doping with light neutralization time constant of corona surface charge”. TECHNICAL FIELD

[0002] This application relates generally to characterization of semiconductor doping in wide bandgap semiconductors, and more particularly to a method for non-contact measurement of doping in epitaxial layers and a system for performing the measurement. BACKGROUND

[0003] Semiconductors doped with donor or acceptor impurities change the electrical properties of the semiconductor and are a key technological element in semiconductor device manufacturing. Doping is typically accomplished by introducing impurity atoms that replace host atoms in the semiconductor lattice. Doping concentration is the concentration of doped atoms per volume of semiconductor, and its measurement is commonly 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 wafer is necessary. To meet the demands of rapidly developing wide bandgap semiconductor technology and large-scale epi-wafer manufacturing, corresponding large-scale doping testing is required. For this purpose, non-contact measurement techniques can be preferred. Non-contact metrology can eliminate the costs and time involved in manufacturing test devices, and can avoid post-measurement wafer cleaning required in the case of 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 US 10,969,370 B2 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 bias of a metal Schottky barrier, however, achieved without a metal contact to the semiconductor surface. In the CnCV method, the surface voltage response AV is monitored with a Kelvin probe for a charge bias dose AQ C C The differential capacitance C = AQ D / AV in sequential charge measurements gives the non-contact C-V characteristic. The doping concentration N 2 ​The slope of the V vs V determination, 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

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

[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 of the corona charge photo-neutralization can be short, enabling a rapid depletion barrier sweep. In measurements on epitaxial SiC of different doping, a direct relationship between the corona charge photo-neutralization time constant and the doping concentration was determined. This relationship is verified for other wide bandgap semiconductors, and is valid over a wide doping concentration range. The enhanced throughput doping measurement method in this disclosure is based on this newly identified relationship.

[0008] Thus, in the present invention, a charge bias method with novel doping measurement principles is described that can increase the measurement speed and throughput, e.g., by up to a factor of ten compared to the conventional CnCV technique 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, the deposited charge on the semiconductor surface is photo-neutralized using near-UV illumination that generates excess carriers. The photo- neutralization time constant is determined from time-resolved surface voltage measurements, and this time constant is the doping measurement parameter.

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

[0011] In wide bandgap semiconductors, 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 monitoring of the process by measuring the surface voltage while the charged area is illuminated or after a continuous illumination pulse. The present disclosure encompasses both measurement configurations. Specifically, the measurement while the charged area is illuminated employs a system configuration in which the illumination is underneath the surface voltage probe. The measurement after an illumination pulse is used in a system configuration in which the illumination is at a location separate from the surface voltage probe.

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

[0013] In certain embodiments, the measurement technique employs the depletion layer Schottky barrier electrostatic charge-voltage relationship to extract the charge photo-neutralization rate and determine the photo-neutralization time constant τ ph from the logarithmic dependence of the surface voltage on the illumination time. ph The doping concentration N D is determined based on the calibrated dependence of the time constant τ ph vs N D . The time constant vs surface voltage characteristic τ ph -V provides a means for assessing the doping depth profile. A uniform doping is identified by a constant τ ph value. In the logarithmic photo-neutralization surface voltage characteristic, a multilayer film of different doping will show different τ ph values over a certain time period. The corresponding surface voltage reflects different depths underneath the surface.

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

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

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

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

[0018] The semiconductor sample wafer can be placed on a conductive chuck with fast movement capability for positioning the test sample area (i.e., test sites on the wafer) under the system components (a), (b), and (c) according to the measurement cycle.

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

[0020] time constant τ ph may be used directly as a relative doping concentration index. For accurate quantitative measurement of the doping concentration, the method can include a calibration function of the time constant, which is determined by measurements of a reference sample wafer with known doping concentration. Alternatively or additionally, wafer specific calibration can be performed with 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 .

[0021] In conventional corona charging methods for semiconductor and dielectric characterization, the response to deposited corona charge density variations is typically monitored by measuring the corresponding variation of the surface voltage. In the present technology, measurements of the surface voltage are also employed, however, the variation of the corona deposited charge density is achieved by photo-neutralization of the charges rather than by charge deposition. The charge photo-neutralization and the measurement of the 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 measurements during the actual corona photo-neutralization. This allows for illumination and measurement of the sample site without transferring the wafer between the illumination and the measurement probe, which can benefit the speed and throughput of the measurements. Configurations with separate light sources can facilitate higher illumination uniformity than illumination under the surface voltage probe. This can benefit the accuracy and tool-to-tool matching.

[0022] The disclosed technology 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 that uses the technology.

[0023] In general, in one aspect, the invention features a method of characterizing a semiconductor dopant in a wide bandgap semiconductor sample, the method comprising: measuring an initial value V0 of a surface voltage at a region of a surface of the semiconductor sample in the dark; charging the region to deep depletion in the dark by depositing a prescribed corona charge at the region; measuring a surface voltage value at the region after the charging in the dark; illuminating the charged region with light of a specific photon flux f eff having a photon energy higher than 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 illumination time t at the region using non-contact time resolved measurements of the surface voltage V(t); analyzing the monitored time resolved surface voltage decay data V(t) to determine a photo- neutralization time constant τ eff .ph ; and using a specific photon flux f eff Light neutralization time constant τ ph It serves as a semiconductor doping index, and its value characterizes the semiconductor doping concentration in that region.

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

[0025] This method may include a specific corona charge light neutralization time constant τ based on surface voltage. ph vs V is used to characterize the depth distribution of doping concentration in semiconductor samples.

[0026] This method may include average τ based on the surface voltage range ph The value is used to characterize the doping of a single epitaxial layer.

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

[0028]

[0029] Where t is the irradiation time, and V const This refers to surface voltage probe offset. The method may include using a probe with a known doping concentration value N. D τ of one or more reference samples ph The calibration measurement determines the calibration function τ. ph vs ND and inverse calibration function N D vsτ ph (represented as f) cal In some examples, the method includes adjusting the effective photon flux φ. eff The time constant τ of the measurement ph Normalized to calibrated photon flux φ eff ,as follows: And using the normalized time constant according to the following equation Determine the absolute doping concentration:

[0030] This method may include calibrating τ for doping concentration. ph The calibration is specific to the semiconductor wafer sample and is based on τ at multiple sites (e.g., 9 sites, 12 sites, or 49 sites) on the wafer sample. ph During characterization, calibration is performed at one site on the measured wafer sample, where all τ ph Measurements are performed under the same irradiation conditions. Calibration may include measuring the corona charge light neutralization time constant τ using a non-contact corona CV (CnCV) method. ph and independent measurement of doping concentration N DThe two measurements are performed sequentially, one after the other, at the same wafer site and within a similar corona charge-induced depletion voltage range. This method may include using τ ph and N D Determine if N is satisfied D =τ ph ·f cal The chip-specific inverse calibration function value f cal .

[0031] In some examples, the method includes determining τ at each wafer site. ph And using f determined at a single site cal The wafer-specific values ​​are used to determine the doping concentration at all different wafer sites.

[0032] Capacitive probes can be non-contact vibrating Kelvin probes.

[0033] The area can be irradiated while the surface voltage is being monitored, or the area can be irradiated separately from the surface voltage monitoring.

[0034] Semiconductor samples may include semiconductors selected from the group consisting of SiC, GaN, and AlGaN.

[0035] This method may include V measured after corona charging but before irradiation. Dark and dark decay rate ΔV Dark / Δt Dark To identify defect areas in semiconductor samples.

[0036] The method may include based on the dark attenuation magnitude ΔV measured after irradiation. Dark To identify the contribution of surface photovoltaic effects, other than corona light neutralization, to the monitored surface voltage.

[0037] Other features and advantages will be apparent from the accompanying drawings, description and claims. Attached Figure Description

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

[0039] Figure 1B yes Figure 1A The diagram shows an alternative configuration of the example system, where illumination is applied at a location separate from the surface voltage probe.

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

[0041] Figure 3A is a plot of example results showing surface voltage measured during corona charge photo-neutralization versus irradiation time.

[0042] Figure 3B is a plot of an example of photo-neutralization time constant corresponding to a transient in Figure 3A .

[0043] Figure 3C is a plot of surface voltage specific τ ph for a uniform doping depth profile.

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

[0045] Figure 5 is a plot of example doping calibration results showing photo-neutralization time constant τ ph versus doping concentration N D measured on n-type 4H-SiC. Measurements were made using a negative corona charging charge density of -4E12 q / cm 2 . A 325 nm wavelength irradiation with incident photon flux of 1.2 x 10 13 photons / cm 2 s was used. Doping concentration N D corresponds to C-V mercury probe results.

[0046] Figure 6 is a band diagram showing 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 plot of example results showing correlation of photo-neutralization time constant with incident photon flux. Results are for n-type SiC with doping concentration N D = 6.11e15 cm -3 and N D = 1.85E16 cm -3 . N D values are based on C-V mercury probe results. DETAILED DESCRIPTION

[0048] Reference is made to Figure 1A, schematically illustrates an example apparatus 100 for corona charge-voltage optical 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 apparatus 100 includes a corona charge source module 120 for precise charge deposition on a test site 126, a surface voltage measurement module 130 for surface voltage measurement, and a light source module 140 for illuminating the wafer surface.

[0049] The wafer 101 is supported by a wafer chuck 110, which is positioned on a moveable 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 on 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. While 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 illuminate the surface of the wafer 101 directly below the Kelvin probe electrode. During operation, the light source 142 delivers illumination having a wavelength suitable for corona charge optical neutralization at the wafer surface directly 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 additionally monochromated by passing through a narrow bandpass filter.

[0052] Reference Figure 1BAn alternative device 100' is shown, wherein the light source module 140' includes a light source 144 positioned to irradiate a location 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 corona charge neutralization. Typically, the irradiation wavelengths provided by light sources 142 and 144 can be selected depending on the wafer under test. Generally, the photon energy should be higher than the semiconductor bandgap. For measurements of 4H-SiC with a bandgap of 3.26 eV, suitable light wavelengths include 355 nm and 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 the free carriers (electron-hole) involved in the corona charge neutralization process. For measurements of other semiconductors, the light wavelength can be selected based on the values ​​of the bandgap and absorption coefficient. Shorter wavelength irradiation (315 nm) is suitable for measuring doping on GaN and AlGaN materials, which have a larger band gap than SiC. Depending on the application, other wavelengths of light, such as approximately 380 nm or less, can be used. Light sources 142 and 144 are arranged to provide uniform illumination over the surface region of the corona charging point, for example, a circular spot with a diameter of approximately 10 mm or less (e.g., approximately 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 sample 101 using the measurement and analysis steps described below.

[0054] Figure 1A and Figure 1B The example devices 100 and 100' shown illustrate two illumination configurations: a light source 142 (LED 1) for illumination below the vibrating electrode of the Kelvin probe electrode 132, and a light source 144 (LED 2) for illumination separate from the Kelvin probe 132. Standard high-precision Kelvin probes can use opaque gold electrodes. In the case of opaque Kelvin probe electrodes 132, low-angle illumination from the side can be used, such as... Figure 1A The light source 142 is schematically shown. In some embodiments, a transparent Kelvin probe illuminated by electrodes may also be used. Generally, when illuminating solely with the light source 144, high uniformity of illumination can be achieved more easily, such as... Figure 1B As shown.

[0055] In the Kelvin probe method, a capacitance electrode oscillating a fraction of a millimeter above the surface under test generates an alternating current that is zeroed by compensating for the DC bias, thereby providing a measure of the voltage difference between the electrode and the semiconductor. For the present application, a Kelvin probe that is accurate and fast responding is preferred, such as a probe with a time constant of about 5 ms and a 0.2 mV accuracy. The surface voltage measurement range is typically -100 V to +100 V for corona charges that are deeply depleted to n-type (negative bias) and p-type (positive bias) semiconductors. A 1 mm or 2 mm electrode diameter is suitable for measuring surface voltage in the uniform central region of the corona charging and illumination area.

[0056] Corona charging is achieved using a corona discharge in air generated by a high DC voltage applied to the discharge electrode 124 from the power supply 122. Negative polarity discharges in air generate ions, while positive polarity discharges generate (H2O) n H + ions. The corona discharge electrode (a needle for point charging or a wire for whole wafer charging) can be confined in an enclosure, and the ion deposition is not field driven; rather, the ions diffuse through the aperture opening to the wafer surface. Given a very short mean free path of 10 -5 cm in the room 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 clean room environment with controlled humidity and temperature. Control of the deposited corona charge density is achieved by setting the high voltage of the corona power supply 122, the discharge current, and the deposition time period. The deposition time period can be about 1 second or less. The deposited charge density dose can be monitored in situ with the coulomb meter 114.

[0057] Corona charging to deep depletion extends from about 1 x 10 12 q / cm 2 to about 1 x 10 13 q / cm 2 and can be achieved in a single charging step. The charge deposited on the surface at the test site 126 is mirrored as opposite charge in the semiconductor surface space-charge region and acts as an electrical bias. The charge dose is much greater than the initial existing surface charge, typically about 1 x 10 11 q / cm 2 . Thus, depending on the doping concentration N D , the charge dose Q C can be selected to obtain the desired depletion width W D = -Q C / qN D .

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

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

[0060] In some embodiments, additional verification steps can also be performed. For example, such a step can include a measurement of the surface voltage performed in the dark after turning off the photo-induced charge photoneutralization. This verification involves the irreversible nature of the photoneutralization, and is aimed at identifying other surface photovoltaic effects that are reversible in the surface depletion layer.

[0061] Typically, measurements are performed under conditions where corona charge neutralization is dominant. This can be tested in measurements of surface voltage recovery in the dark, immediately after irradiation. For optimized measurement conditions, the magnitude of this surface voltage recovery, indicating other effects, should be insignificant compared to the magnitude of corona charge neutralization, for example, in the range of about 100 mV or less for corona charge neutralization magnitudes of about 20 V or higher. In SiC, measurements are performed using approximately 10... 13 To about 10 14 photons / cm 2 This insignificant contribution can be achieved with low effective photon flux in the s range.

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

[0063] Figure 2 The table shows the steps involved in the measurement process that can be used to acquire time-resolved surface voltage data to determine the light neutralization time constant. Figure 2 The included graph shows the surface voltage measured at each step. Surface voltage measurement is performed using equipment 100 or 100' and Figure 1A The illumination configuration shown measures the characteristics of a typical n-type epitaxial wafer of 4H-SiC. The wafer is held on a conductive chuck 110 by vacuum suction. During corona charging and measurement, the chuck 110 provides a back-side electrode coupled to the wafer capacitance. The non-contact characteristic of this method means that 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 turned off, and the wafer is in darkness. The wafer chuck is movable and for... Figure 2 Step 1 of the process involves positioning the wafer test site below the Kelvin probe electrode 132 to measure the initial surface voltage V0 in darkness, for example, V0 = 0.8V. After the initial surface voltage measurement, the test site 126 is moved to a position below the corona charging source (e.g., ...). Figure 1A and Figure 1B As shown), and in step 2, a charge density Q is specified. C (e.g. -1.5x10) 12 q / cm 2The negative charge is deposited on the surface of the epitaxial layer 103 at test site 126. As a result of the charge deposition, the surface is biased to be deeply depleted. For step 3, the charging site is shifted to a position below the Kelvin probe electrode 132. The voltage of the deeply depleted surface after charging is then measured, giving a value after charging, such as V. D = -59V. In step 4, the test site is illuminated with short-wavelength light having a photon energy higher than the 3.26 eV band gap of 4H-SiC, suitable for photonic neutralization of corona charge. For example, a 325 nm wavelength (hv = 3.82 eV) from a near-UV LED can be used for illumination. Figure 1A In the example shown, illumination below the Kelvin probe electrode 132 with light source 142 can be used, enabling continuous monitoring of the depletion layer surface voltage V(t) while illuminating the charging region. Figure 1B In the case of the exemplary device 100' shown, the illumination is applied to a location separate from the surface voltage probe. Therefore, monitoring cannot be performed simultaneously with illuminating the charging region. Instead, it is accomplished using a sequence of steps including a short illumination period (typically about 10 ms to 20 ms in duration) and surface voltage measurement. In this sequence, the charging region moves from a position below the surface voltage measurement probe 132 to a position below the illumination source 144, and then back to the surface voltage probe, and so on.

[0064] The final step, step 5, of the measurement cycle involves recording the surface voltage after the short-wavelength light is turned off. This value is... Figure 2 The dark voltage V is referred to as V. after Very small depletion voltage recovery was used to confirm the negligible contribution of the surface photovoltaic effect, in addition to corona charge light neutralization.

[0065] An example of the measured V(t) is in Figure 3A As shown in the figure, the initial portion of the voltage transient in the dark shows no decay before illumination. This dark portion is an indication of negligible charge dissipation due to charge neutralization associated with surface diffusion or any defects. During illumination, the rapid optical neutralization of corona charge is evident as a corresponding decay of the surface voltage. This recorded V(t) time correlation can be reliably analyzed using the Schottky barrier voltage-charge equation, thus providing the time constant τ for determining the optical neutralization time of corona charge. ph The method involves depleting the surface potential barrier and surface charge density (V). D ~Q C 2 The quadratic relationship between τ and τ can be used to derive the surface voltage decay corresponding to the corona charge decay. In practice, τ ph It can be determined by the inverse slope of the logarithmic surface voltage time decay during the light neutralization period. Where V0 is the initial precharge value, including the offset related to Kelvin probe calibration. For Figure 3A The V(t) data in, which is in Figure 3B As shown in the image.

[0066] This fitting process yields the average τ within the specified surface voltage range. ph The value, in this case, is in the range of -5V and -59V. Figure 3B A good linear fit indicates a constant τ within that range. ph This value is consistent with the uniform doping depth in the epitaxial layer.

[0067] For the doping depth distribution, a voltage-specific time constant corresponding to a given depletion surface voltage V can be used, based on the light neutralization V vs irradiation time characteristic V(t). It is calculated from the relative surface voltage decay rate as follows: Figure 3C It shows that for Figure 3A The data in the figure are voltage-specific τ as a function of the depletion surface voltage. ph It indicates a constant τ within the voltage range that corresponds to a uniform doping depth distribution in the epitaxial layer. ph value.

[0068] Figure 4 The example illustrates the logarithmic surface voltage time decay of an epitaxial wafer with three differently doped layers. Each layer is characterized by a corresponding τ that is proportional to the doping value. ph1 τ ph2 and τ ph3 Layers 1, 2, and 3 are counted from the top of the epitaxial layer structure.

[0069] Figure 5 This illustrates the application of n-type 4H-SiC with wavelength λ = 325 nm and effective incident photon flux φ. eff =1.2x10 13 photons / cm 2 Light irradiation by s, light neutralization time constant τ ph With doping concentration N D The correlation. This correlation can be used to determine the calibration function N. D vsτ ph It corresponds to a specific doping range N D and irradiation conditions λ and

[0070] Light neutralization time constant τ ph With dopant concentration N D The increase is usually for the same corona charge Q. C The smaller surface exhausts the width W D =Q C / qN D The result is that this leads to the depletion of smaller photogenerated cells within the internal cavity. For example... Figure 6 As shown, the free holes h generated during depletion + In light and in the atmosphere, they are active and are guided to the surface by a depletion electric field. At the surface, negatively charged corona ions capture holes, Q... - +h + →Q↑ and the neutralized corona substance separates from the surface.

[0071] With increased doping, the smaller hole generation caused by the reduced depletion width can be increased by increasing the incident photon flux φ. eff To overcome. For example Figure 7 As shown, τ ph It is inversely proportional to the incident photon flux. Therefore, by increasing the incident photon flux, higher N values ​​can be achieved. D This method enables rapid measurement. Measurement speed is the primary advantage of this method. In this respect, the time constant τ... ph and incident photon flux (τ) ph ~1 / φ eff The inverse proportionality of can be used to optimize measurements. For example, for 10 16 cm -3 N within the range D By increasing the photon flux to 10 14 photons / cm 2 Within the range of s, a time constant τ of approximately 0.1s can be achieved. ph By combining increased photon flux with shorter wavelengths of light, such as λ = 315 nm or λ = 300 nm, it is possible to rapidly measure highly doped light, such as 10⁻⁶. 18 cm -3 .

[0072] As mentioned earlier, calibration can be used to convert the measured time constant into doping concentration. For this purpose, it can be performed for a specific doping range and (λ, φ) eff The value determines a specific family of calibration functions. For accurate measurements, a single specific calibration function can be limited to N. D and A subset of the entire range of values; for example, a precise calibration function can cover N. D and φ eff This is an order of magnitude or smaller. The calibration function can be determined using precise settings of measurement and irradiation conditions on a reference wafer with a known doping concentration. For example, it can be determined using N-type nanometers pre-measured using a mercury probe CV method. D The wafer can be pre-measured using a corona non-contact CV method (CnCV). The exact same irradiation conditions can then be used to measure the doping in the fabricated epitaxial layer in the production environment.

[0073] From the calibrated τ ph vs N D The data is used to quantitatively determine the doping concentration, expressed as f. cal The inverse function N D vsτ ph Perform the measurement at the same wavelength used for calibration, for example... Figure 5 and Figure 7 λ = 325 nm. The photon flux φ used during the measurement. eff Typically different from the photon flux φ used for calibration cal This is because it is adjusted to take into account the measurement speed requirements. Based on τ ph The correlation between the incident photon flux and the measured τ ph Perform normalization. Then normalize the values. For N D vsτ ph inverse function f cal In this context, the doping concentration is calculated as... exist Figure 5 In the example, in φ cal = 12e13 photons / cm 2 φ under s cal Calibration. However, when applied to, for example... Figure 7 The calibration gives the correct N for the different photon fluxes shown. D The value is within 0.5%.

[0074] An alternative method uses wafer-specific calibration performed during the actual measurement of a given wafer. (Separately in...) Figure 1A and Figure 1B The devices 100 and 100' shown are suitable for this purpose. Wafer-specific calibration involves: (1) the corona charge light neutralization time constant τ ph (2) Non-contact CV (CnCV) measurements with different doping concentrations of ND. These two independent measurements were performed one after the other on the same wafer site. The results were used to determine the inverse calibration value function f. cal Satisfying N D =τ ph .f cal At all other test locations on the wafer, only τ was measured. phk (k is the position number). Using the measured τ... phk and chip-specific function f cal The same value determines the doping N at different k positions on the wafer. Dk =τ phk .f cal .

[0075] Generally, the measurement with wafer-specific calibration is done for all measured sites with the same illumination conditions. This condition can be optimized for different doping ranges. However, no precise knowledge of the incident photon flux is required, since the method uses the measured τ ph Instead of normalizing to the photon flux The above described measurements can be performed at any number of sites on a wafer. A typical production line test involves 12 wafer site measurements or 49 site wafer mapping. The wafer-specific calibration adds a unit site CnCV doping measurement time of 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 40 seconds additional calibration time, resulting in a total measurement time of about 240 seconds and a throughput of about 13 wafers / hour with wafer handling overhead.

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

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

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

[0079] Other embodiments are therefore in the attached claims.

Claims

1. A system for characterizing semiconductor doping in a wide bandgap semiconductor sample, the system comprising: a surface voltage measurement probe; a corona charge source; a light source; a moveable wafer chuck configured to support the semiconductor sample relative to the surface voltage measurement probe, the corona charge source, and the light source; and a computer controller in communication with the surface voltage measurement probe, the corona charge source, the light source, and the moveable wafer chuck, the computer controller programmed to cause the system to: (i) measure an initial value of surface voltage V0 at a region of a surface of the semiconductor sample in the dark; (ii) charge the region to deep depletion in the dark by depositing a prescribed corona charge at the region; (iii) measure a value of surface voltage at the region after charging in the dark using the surface voltage measurement probe; (v) monitor the decay of photo-induced and induced corona charge at the region as a function of illumination time t using non-contact time-resolved measurements of surface voltage V(t) using the surface voltage measurement probe; (iv) Using a specific photon flux φ from the said light source eff The specific photon flux φ is generated when the area being charged is illuminated by light. eff It has photon energy above the semiconductor bandgap sufficient to generate free minority carriers in the semiconductor sample, thereby causing optical neutralization of the corona charge; the light source arranged to illuminate the region at a location different from a location beneath the surface voltage measurement probe. (vi) analyzing the monitored time-resolved surface voltage decay data V(t) to determine a photoneutralization time constant τ ph ; and (vii) the semiconductor doping concentration at the region is characterized based on a specific photon flux φ eff under the light and a time constant τ ph ​ 2. The system of claim 1, wherein, the computer controller programmed to cause the system to illuminate the region prior to measuring surface voltage at the region to monitor the decay of photo-induced and induced corona charge at the region.

3. The system of claim 2, wherein, 4. The system of claim 2, further comprising a moving stage supporting the wafer chuck, and the computer controller programmed to cause the stage to move the region between the surface voltage measurement probe and the light source. the light source arranged to illuminate the region while the region is beneath the surface voltage measurement probe.

5. The system of claim 1, wherein, the computer controller programmed to cause the system to illuminate the region while monitoring the decay of photo-induced and induced corona charge at the region.

6. The system of claim 5, wherein, the surface voltage measurement probe comprises a transparent electrode, and the light source arranged to direct light through the transparent electrode to the region.

7. The system of claim 5, wherein, the surface voltage measurement probe is a non-contact vibrating Kelvin probe.

8. The system of claim 1, wherein, the light source comprises a light emitting diode or a near-UV laser.

9. The system of claim 1, wherein, ​ 10. The system of claim 1, wherein, The system is calibrated based on calibration measurements of one or more reference semiconductor samples having known doping concentrations. ph of τ.

Images