Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
  • G01N27/22—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating capacitance
  • G01N27/221—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating capacitance by investigating the dielectric properties
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
  • G01N33/0095—Semiconductive materials
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R29/00—Arrangements for measuring or indicating electric quantities not covered by groups G01R19/00 - G01R27/00
  • G01R29/12—Measuring electrostatic fields or voltage-potential

Definitions

• This disclosure relates to systems and methods for measuring a semiconductor doping concentration, in particular, using constant surface potential corona charging.
• Doping a semiconductor with donor or acceptor impurities changes the semiconductor electrical conductivity.
• electrical conductivity is generally an important characteristic of a semiconductor material, for quality control purposes, it can be beneficial to measure or monitor a semiconductor doping concentration (e.g., the concentration of doping atoms per volume of the semiconductor) during the production of semiconductor materials and devices.
• a manufacturing process can include steps for monitoring the doping concentration of semiconductor wafers sliced from semiconductor crystals, monitoring the doping concentration of thin epitaxial layers grown or deposited on bulk wafers, and monitoring the doping concentration of semiconductors after the performance of deliberate doping processes (e.g., after impurity diffusion or ion implantation processes).
• a method of characterizing a semiconductor sample includes measuring an initial value, V in , of a surface potential at a region of a surface of the semiconductor sample, biasing the semiconductor sample to have a target surface potential value (V 0 ) of TV or less, and depositing a monitored amount of corona charge (AQy) on the region of the surface after adjusting the surface potential to the target value.
• the corona charge AQ k can be corrected by a calibration function F(V, I) to obtain a charge density (AQ c ) under a probe used for measuring the surface potential.
• a quality of the calibration function can be determined using the capacitance charge factor C ck Q ck .
• the method can further include performing a charge density calibration using a semiconductor calibrating sample with a known doping density over a distance W under a surface of the semiconductor calibrating sample. The corona charge deposition and surface potential adjustment steps can be repeated until a target doping monitoring depth is reached.
• a polarity of the corona charging can be such that it creates a depletion surface space charge region in the semiconductor sample.
• the semiconductor sample can be an n-type semiconductor and the corona charge is deposited using a negative charging polarity.
• the semiconductor sample can be a p- type semiconductor and the corona charge is deposited using a positive charging polarity.
• characterizing the semiconductor sample can include determining a doping density and/or a doping profile.
• the semiconductor sample can include a wide bandgap semiconductor. In some implementations, the semiconductor sample can include a semiconductor selected from the group consisting of SiC, GaN, ZnO, ZnS, and ZnSe.
• the surface potential measurements can be performed using a non-contact vibrating probe.
• surface potential measurements can be performed using a capacitive electrode.
• V 0 can be 0.5 V or less.
• V 0 can be approximately 0 V.
• the deposited corona charge can be photo- neutralized and removed from the semiconductor surface using illumination with light of photon energy sufficient to generate excess carriers in the semiconductor sample.
• the method can further include repeating characterization of the semiconductor sample after photo-neutralizing the deposited corona charge prior to each repetition.
• a method in general, in another aspect, includes repeatedly depositing a corona charge at a region of a surface of a sample after biasing a surface potential at the region to have the same target value.
• embodiments may be used to measure the doping concentration of a semiconductor sample without fabricating any permanent or temporary metal-semiconductor diodes and without any invasive electrical contacts.
• measurements can be obtained relatively quickly, and the semiconductor wafer can be further processed or used after the doping measurement is performed.
• electrical charges can be deposited onto a surface of a semiconductor sample incrementally and according to a constant compensated surface potential value (e.g., biased to zero or some other constant value), such that the effects of electrostatic charge repulsion are reduced, or in some cases, practically eliminated.
• implementations of these techniques can be performed without the use of mercury, a hazardous material that is often subject to environment regulation or other usage restrictions.
• FIGS. 1A-C are schematic diagrams an example system for measuring doping concentrations of semiconductor samples.
• FIG. 5 shows example charge profiles for a semiconductor wafer after an initial deposition of charge, after a sequence of ten depositions of charge, and after a sequence of twenty depositions of charge.
• the present disclosure pertains to measuring doping concentrations of a semiconductor using accurate, fast-feedback, non-invasive, and non-contact techniques. Implementations of these techniques include measuring the surface potential at a test site of a semiconductor, applying a bias voltage to the
• the semiconductor in order to obtain a particular surface potential at the test site (e.g., zero, substantially zero, or some other constant value), and depositing a charge on the test site of the semiconductor surface (e.g., using corona discharge in air).
• This sequence of surface potential measurement, surface potential compensation, and charge deposition can be repeated multiple times in order to create a precisely controlled depletion surface space charge layer, similar to that of a Schottky barrier in metal contact diodes.
• the semiconductor depletion space charge capacitance C can then be measured to determine the doping concentration of the semiconductor in the depletion space charge layer, and to obtain doping depth profiles beneath the semiconductor surface.
• the product of the semiconductor depletion space charge capacitance and the total charge deposited at the test site, CQ can be used as a measurement quality factor for precise doping depth profiling.
• the depletion space charge layer can be formed without fabrication of any permanent or temporary metal- semiconductor diodes and without any invasive electrical contacts, measurements can be obtained relatively quickly, and the semiconductor wafer can be further processed or used after the doping measurement is performed. Further, in general,
• implementations of these techniques can be performed without the use of mercury, a hazardous material that is often subj ect to environment regulation or other usage restrictions.
• Implementations of these techniques can be used to measure doping concentrations of semiconductor materials capable of retaining corona deposited charge.
• semiconductor materials capable of retaining corona deposited charge.
• This can include, for example, wide bandgap semiconductors (e.g., SiC, GaN, or ZnO semiconductors that are not exposed to short wavelength illumination).
• This can also include, for example, semiconductors that have been coated with dielectric films that prevent corona charge neutralization (e.g., SiC , or S13N4 films).
• implementations of these techniques can be used to improve the manufacturing of semiconductor wafers used for a variety of applications.
• these techniques can extend the range of corona-induced depletion voltages to relatively high voltages (e.g., hundreds of volts), which can be advantageous for conducting doping profiling in SiC and GaN semiconductor wafers for high voltage power devices.
• these technique can be used to improve the manufacturing of semiconductor wafers that are used for optoelectronic applications.
• the described techniques also can be applied to other corona charge-based characterization processes, including the measurement of dielectrics and interface on wafers and micro-test sites.
• FIG. 1 A An example system 100 for measuring the doping concentration of a semiconductor sample is shown in FIG. 1 A.
• the system 100 includes a stage assembly 1 10, a Kelvin probe 120, a corona charging station 130, a voltage compensation assembly 140, and a processing module 150.
• a semiconductor sample 160 is placed on the stage assembly 110 and is positioned for examination by the Kelvin probe 120.
• the Kelvin probe 120 measures the surface potential of a test site 162 on the front surface of the semiconductor sample 160.
• the voltage assembly 140 then applies a bias voltage to the semiconductor sample 160 (e.g., at the back surface 164 of semiconductor sample 160) to compensate the surface potential of the test site to a particular value (e.g., zero volts, 0.5V or less, or some other value).
• the semiconductor sample 160 e.g., at the back surface 164 of semiconductor sample 160
• semiconductor sample 160 is then repositioned beneath the corona charging station 130, which deposits a charge on the test site 162 of the semiconductor sample 160. After the charge is deposited, surface potential measurement, surface potential compensation, and charge deposition are repeated one or more times in order to create a depletion space charge layer that is induced beneath the surface on the
• the processing module 150 determines the doping concentration of the semiconductor and/or doping depth profiles beneath the semiconductor surface.
• the semiconductor sample 160 can be shielded from stray illumination (e.g., by enclosing all or part of the system 100 in a light-shielded housing or facility).
• the stage assembly 110 supports and positions the semiconductor sample 160 during examination by the system 100.
• the stage assembly 110 includes a chuck 112 capable of conducting electric charge to and from the semiconductor sample (e.g., a semiconductor wafer).
• the chuck 112 can be composed of one or more electrically conductive materials.
• the chuck 112 can be composed of aluminum, and coated with titanium nitride. In practice, other conductive materials are also possible, depending on the implementation.
• the semiconductor sample 160 (e.g., semiconductor wafer) is secured to the chuck 112 such that electrical contact is provided between the semiconductor sample 160 (e.g., the back surface 164) and the chuck 112.
• the semiconductor sample 160 can be non-invasively secured to the chuck 112.
• the semiconductor sample 160 can be secured to the chuck 112 through suction applied by a vacuum 114.
• a vacuum 114 is shown in FIG. 1 A, in practice, other securing mechanisms (e.g., pins, brackets, and/or ties) are also possible, depending on the implementation.
• the stage assembly 1 10 can move along one or more axes, such that the semiconductor sample 160 can be translated in relation to the Kelvin probe 120 and the charging station 130.
• the chuck 1 12 can move along the x, y, and/or z axes of a Cartesian coordinate system in order to move the semiconductor sample 160 along any of three dimensions in relation to the other components of the system 100.
• the stage assembly 110 initially positions the semiconductor sample 160 beneath the Kelvin probe 120, such that the Kelvin probe 120 can measure the surface potential of the test site 162. Measurements obtained by the Kelvin probe 120 are transmitted to the processing module 150 for analysis.
• the Kelvin probe 120 can measure the surface potential of the test site 162 without contacting the semiconductor sample 160.
• the surface potential can be obtained by measuring the constant potential difference (CPD) between a reference probe having a known work function, and the semiconductor surface having an unknown work function.
• CPD constant potential difference
• the probe is placed above the surface of the semiconductor test site and is vibrated, which causes a periodic variation in capacitance between the probe and the semiconductor test site and causes a time- varying current into the probe.
• This current may be measured and nullified by applying an opposite voltage on the probe or semiconductor surface.
• This voltage known as the backing potential, results in zero current when it is equal to the CPD between the probe and the semiconductor surface.
• This voltage can be determined, for example, using a measuring system 122.
• a vibrating Kelvin probe 120 is shown in FIG. 1A, in practice, other non-contact vibrating probes or measurement systems also can be used, depending on the implementation. For example, in some cases, a Monroe probe can be used instead of a Kelvin probe
• the vibrating probe 120 can have a particular diameter (e.g., 1 mm, 2 mm, or some other diameter).
• the Kelvin probe 120 can move with respect to the semiconductor sample (e.g., move laterally across the semiconductor sample) in order to measure the surface potential of several test sites on the semiconductor sample.
• the voltage compensation assembly 140 is configured to electrically ground the back surface 164 of the semiconductor sample 160.
• the voltage compensation assembly 140 includes a direct current (DC) voltage source 142 coupled to the chuck 112 through a switch 148a, and an input capacitor 146 coupled the chuck 112 through a switch 148b.
• DC direct current
• the switch 148a is open, and the switch 148b is in position A.
• the chuck 112 and the back surface 164 of the semiconductor sample 160 are both electrically grounded.
• the system 100 compensates the test site surface potential by applying a bias voltage to the back surface 164 of the semiconductor sample 160.
• this bias voltage is applied by electrically coupling the input capacitor 146 to the back surface 164 of the semiconductor sample 160, and charging the input capacitor 146 until a particular bias voltage is obtained across it.
• this can be performed by closing the switch 148a and toggling the switch 148b to position B.
• the DC voltage source 142 and the input capacitor 146 are each electrically coupled together and to the chuck 112.
• the DC voltage sources 142 electrically charges the input capacitor 146.
• the voltage across the input capacitor 146 biases the test site surface potential.
• the bias voltage of the input capacitor 146 can be monitored using a measuring system 170 in parallel with the input capacitor 146. Measurements obtained by the measuring system 170 can be transmitted to the processing module 150 for analysis.
• the DC voltage source 142 can be configured to charge the input capacitor 146 to a particular bias voltage in order to compensate the surface potential at the test site 162. In some cases, the DC voltage source 142 can be configured to achieve a zero, or substantially zero surface potential at the test site 162. For instance, the DC voltage source 142 can charge the input capacitor 146 such that the bias voltage across the input capacitor 146 is the opposite of the surface voltage that was measured by the Kelvin probe 120.
• the Kelvin probe 120 measured a surface voltage of V 0 while the back surface 164 was electrically grounded (e.g., while the switch 148a was open and the switch 148b was in position A), while the switch 148b is closed and the switch 148b is in position B, the DC voltage source 142 can apply a voltage of—V 0 across the input capacitor 146.
• the surface potential at the test site 162 is compensated to zero once the input capacitor 146 is charged and a bias voltage of—V 0 is obtained across it.
• an electrical charge is deposited onto the test site 162 according to the biased surface potential condition.
• the semiconductor sample 160 is repositioned by the stage assembly 110, such that the test site 162 is positioned beneath the corona charging station 130.
• the corona charging station 130 deposits a charge on the test site 162.
• the switch 148a is in an open position, and the switch 148b is in position B.
• the input capacitor 146 continues to bias the test site's surface potential during charge deposition.
• the corona charging station 130 deposits charge onto the test site 162 through corona discharge in air.
• the corona charging station 130 includes a high voltage source 132, and a corona charging gun 134 with a discharge needle electrode. High voltage creates corona discharge and the ions generated by the corona discharge are directed from the charging gun 134 towards the test site 162, and are deposited on the surface of the test site 162. As a result of the charge deposition, a slight change of voltage is induced across the input capacitor 146.
• This change in voltage can be measured using the measuring system 170 in parallel with the input capacitor 146. Measurements obtained by the measuring system 170 can be transmitted to the processing module 150 for analysis.
• the corona charging station 130 is configured to deposit charge within a particular region (e.g., a circular region having, e.g., a diameter of approximately 8-10 mm), and the Kelvin probe 120 can be configured to measure the surface potential of a smaller sub-region within the charge deposition region (e.g., a circular sub-region having a diameter of approximately 2 mm). This can be beneficial, for example, to ensure that charge is being uniformly deposited across the entirety of the region being measured by the Kelvin probe 120.
• a particular region e.g., a circular region having, e.g., a diameter of approximately 8-10 mm
• the Kelvin probe 120 can be configured to measure the surface potential of a smaller sub-region within the charge deposition region (e.g., a circular sub-region having a diameter of approximately 2 mm). This can be beneficial, for example, to ensure that charge is being uniformly deposited across the entirety of the region being measured by the Kelvin probe 120.
• the sequence of surface potential measurement, surface potential compensation, and charge deposition shown in FIGS. 1 A-C can be repeated one or more times in order to vary, in a precisely controlled manner, the surface depletion space charge layer at the test site 162.
• the system 100 can be returned to the configuration shown in FIG.
• the system 100 can again be switched to the configuration shown in FIG. IC, such that charge can again be deposited onto the test site 162 according to the biased surface potential condition. In this manner, charge can be incrementally deposited onto the test site 162 according to a consistent or substantially consistent surface potential. Further still, the voltage change that is induced during each charge deposition step can be observed and used as an in situ measure of the deposited charge.
• the processing module 150 can be implemented using digital electronic circuitry, or in computer software, firmware, or hardware, or in combinations of one or more of them.
• the processing module 150 can be implemented, at least in part, as one or more computer programs (e.g., one or more modules of computer program instructions, encoded on computer storage medium for execution by, or to control the operation of, a data processing apparatus).
• a computer storage medium can be, or can be included in, a computer- readable storage device, a computer-readable storage substrate, a random or serial access memory array or device, or a combination of one or more of them.
• processing apparatus encompasses all kinds of apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, a system on a chip, or multiple ones, or combinations, of the foregoing.
• the apparatus can include special purpose logic circuitry, e.g., an FPGA (field
• the apparatus can also include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, a cross-platform runtime environment, a virtual machine, or a combination of one or more of them.
• the apparatus and execution environment can realize various different computing model infrastructures, such as web services, distributed computing and grid computing infrastructures.
• the processing module 150 can also control the operation one or more of the other components of the system 100.
• the processing module 150 can be communicatively coupled to the stage assembly 110, the Kelvin probe 120, the corona charging station 130, and/or the voltage compensation assembly 140 in order to control each of these components.
• compensation, and charge deposition can be repeated any number of times in order to create a particular surface depletion layer.
• the sequence can be repeated for a prescribed number repetitions (e.g., 10 repetitions, 20 repetitions, 30 repetitions, or any other number of repetitions) or until a desired width of the surface depletion layer W is obtained.
• a larger number of incremental charging steps e.g., 10 or more, 20 or more
• an initial surface potential value V 0 is determined for a test site on the front surface of semiconductor sample, and the surface potential of the test site is compensated by applying a bias voltage (e.g.,—V 0 ) to the back surface of a semiconductor sample. A dose of charge having a charge density AQ t is then deposited at the test site.
• a bias voltage e.g.,—V 0
• the bias voltage is removed, and the surface potential value V 1 is determined for the test site.
• the surface potential of the test site is then compensated by applying a new bias voltage (e.g.,—V- ) to the semiconductor sample.
• a second dose of charge having a charge density AQ 2 is then deposited at the test site.
• the bias voltage is removed, and the surface potential value V 2 is determined for the test site.
• a series of consecutive monitored corona charge deposition steps can be performed, and the surface potential at the test site can be adjusted to a particular target value (e.g., zero volts, 0.5 V or less, or some other value) before each deposition step.
• the surface potential value can be measured after each deposition step, and the measurement results of each step can be used to determine
• the semiconductor sample can be characterized based on the values for V 0 , V k , AV k , Q k and C k .
• the deposited charge can be removed after the doping concentration of the semiconductor is determined.
• corona deposited charge can be photo-neutralized and removed from the semiconductor surface using illumination with photon light energy sufficient to generate excess carriers in a semiconductor sample (e.g., blue light having photon energy above 3.3 eV for SiC, GaN or ZnO semiconductors).
• the doping concentration of a test site can be measured multiple times by removing deposited charges between each measurement, and repeating the deposition of charge.
• Measuring the doping concentration of a semiconductor often requires a particular depletion barrier voltage (e.g., 10V, 20V, 50V, 100V, 200V or more). Without compensation, high surface potentials can often hinder corona charge deposition due to the electrostatic forces that exist between the deposited and incoming corona ions. For example, in some cases, electrostatic repulsion between the deposited and incoming corona ions can limit the density of charge deposition. In addition, in many cases, the forces acting on corona ions can direct incoming ions away from the center of the charging site, causing non-uniform lateral distribution of charge density. This potentially can cause errors in the quantification of deposited charge dose density at the test site, and can cause corresponding errors in capacitance and doping density calculations. These limitations and errors can be mitigated or, in some cases, entirely eliminated by incrementally depositing charge onto a depletion barrier voltage (e.g., 10V, 20V, 50V, 100V, 200V or more). Without compensation, high surface
• semiconductor surface having a constant or substantially constant surface potential at the deposition site (e.g., in the manner described above).
• the surface depletion layer is created due to net electrical neutrality of the semiconductor surface and the underlying surface space charge region.
• an electric charge placed on the semiconductor surface is mirrored by the opposite charge in the surface space charge region.
• negative corona charge is deposited on an n-type semiconductor and positive corona charge is deposited on a p-type semiconductor.
• the depletion layer forms a capacitor with the capacitance C D per unit area:
• a doping measurement can be obtained based on the capacitance times charge factor CQ. This can used for two purposes: i) for calibration of corona deposited charge density Q, and ii) for determination of the doping concentration N.
• Corona charge calibration can be used to precisely determine the incremental charge density AQ deposited per unit surface area at the test site. By compensating the surface potential of a semiconductor test site and depositing charge at the test site according to constant compensated surface potential, the surface of the semiconductor can be charged more precisely and/or more uniformly.
• the calibration function F(V, I) can be determined empirically.
• a charge density calibration can be performed using a semiconductor calibrating sample having a known doping density with a constant doping depth profile over a known distance W under the surface.
• a 1/C 2 - V technique can be used.
• the calculation of doping density N can be made using a plot of 1/C 2 k versus the surface voltage V k based on the follow relationship, where the corrected space charge capacitance C ck can be used in place of C:
• the plot is a straight line and the doping density is determined from the slope of the line as:
• FIG. 2 illustrates an example 1/C 2 vs. V plot with and without corona charge areal density correction for a very uniformly doped p-type epitaxial SiC
• the cumulative total charge used in the Q 2 — V technique can be more sensitive to charge dose errors and to corresponding corona charge neutralization by leakage current, compared to a single charge increment AQ ck used in the capacitance measurement and the 1/C 2 - V technique. In cases of negligible leakage and proper correction of charge dose for electrostatic force effect, both techniques can provide similar results.
• the capacitance-charge product, CQ, of a semiconductor (or C ck Q ck if expressed according to the corrected space charge capacitance and the corrected corona charge increment) also can be used to determine the doping concentration N.
• the error in N is about 35% by not using corrected corona charge.
• the CQ factor can be a constant for a sample with a uniform dopant depth profile (e.g., as illustrated in FIG. 3), and in these cases, can be used as a self- consistent charge dose verification metric.
• semiconductors having nonuniform dopant depth profile can be distinguished by identifying semiconductors exhibiting a varying or non-constant CQ profile.
• FIG. 5 shows charge profiles for a semiconductor wafer after an initial deposition of charge (profile 510) nominally at a center location on a surface, after a sequence of ten depositions of charge (profile 520) at the same location, and after a sequence of twenty depositions of charge (profile 530) at the same location.
• the surface potential of the semiconductor wafer was not adjusted to a constant target value prior to each deposition of charge.
• system 100 can have other arrangements, depending on the implementation.
• the stage assembly 110 does not move during doping concentration measurement. Instead, one or more of the other components of the system 100 (e.g., the Kelvin probe 120 and/or the corona charging station 130) can move with respect to the semiconductor sample 160 in order to perform doping concentration measurements .
• the other components of the system 100 e.g., the Kelvin probe 120 and/or the corona charging station 130

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