Inspection of microled arrays with noncontact field effect electroluminescense

Abstract

Systems and methods for inspection of microLED arrays with noncontact field effect electroluminescense are disclosed.

Description

[0001] INSPECTION OF MICROLED ARRAYS WITH NONCONTACT FIELD EFFECT ELECTROLUMINESCENSE

[0002] BACKGROUND

[0003] Micro light emitting diode (microLED) arrays are a flat panel display technology that utilizes arrays of microscope LEDs. Promising applications for this technology include wearable displays, such as those used for augmented reality (AR) and / or virtual reality (VR) applications.

[0004] To scale this technology to a commercial level, there is a need for efficient, nondestructive inspection of microLED arrays during fabrication. For example, there is a need to identify defective individual light emitting diodes (LEDs) or LED clusters in terms of electroluminescent (EL) intensity, emission wavelength, and halfwidth of emission peak. There is also a need to identify systematic changes in the emission characteristics, e.g., systematic intensity changes across an array (such as from a shadowing effect).

[0005] SUMMARY

[0006] Field effect electroluminescence (FEEL) refers to the stimulation of light emission by application of a sufficiently high electric field to a material or device. Systems and methods of using FEEL for inspection and characterization of light emitting diode arrays, such as microLED arrays are described. In particular, the use of “air couchant technology” (also referred to as the “air bearing” technique allows positioning of a probe close to, but not in contact with, a microLED array under test, to achieve repeatable and well-controlled capacitive coupling during FEEL inspection of the LED array.

[0007] Among other advantages, the disclosed techniques are contactless, meaning that the probe does not contact the test wafer during the measurement process. Contactless measurements are desirable for inspection and / or characterization during high volume manufacturing (HVM) because they typically result in less mechanical damage and / or contamination of parts under test, which can reduce yield, than using measurements where the wafers are contacted by the probe.

[0008] The disclosed techniques can be performed in ambient conditions, e.g., at ambient temperatures and atmospheres. For example, the disclosed techniques can be performed without heating or cooling the wafer under test, and / or without evacuating or purging air from the test environment. Performing inspections under ambient conditions can be faster than processes that involve temperature and / or atmosphere changes, improving manufacturing throughput.

[0009] Other features and advantages will be apparent from the drawings, the description below, and the claims.

[0010] DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is a schematic diagram showing an example of a system for inspecting a microLED array using field effect electroluminescence.

[0012] FIG. 2A and 2B are a schematic and plan view of an example probe for the system shown in FIG. 1. FIG. 2A shows a cross-sectional view of the example probe (which may be rotationally symmetric) and FIG. 2B shows a plan view of the bottom of the probe.

[0013] FIG. 3A is schematic diagram of an example of an optical measurement system for use in the system shown in FIG. 1.

[0014] FIG. 3B is a schematic diagram of another example of an optical measurement system for use in the system shown in FIG. 1.

[0015] FIG. 4 is a schematic diagram of another example of a system for inspecting a microLED array using field effect electroluminescence.

[0016] In the drawings, like reference numbers denote like elements.

[0017] DETAILED DESCRIPTION

[0018] This specification describes systems and methods of inspecting and characterizing light emitting diode (LED) arrays using field effect electroluminescence (FEEL) in which a transparent electrode is placed over, but separated from, the inspected LED array supported by a chuck to form a capacitive coupling with the LED array. A time varying voltage is applied to the transparent electrode to induce a forward current flow (J) though at least some of the LEDs, resulting in electroluminescence (EL) from those LEDs. The value of this current is proportional to the value of the capacitance (C) and the rate of the voltage change (dV / dt) according to the equation: dQ dV

[0019] J = — = C — . dt dt Generally, the maximum value of the voltage applied should not exceed the breakdown field of the LED structure, such as the breakdown of dielectric material in the LED structure and between the terminals forming the capacitor.

[0020] The capacitance, C, of the capacitive coupling depends on the distance between the LED array and the transparent electrode, and the dielectric constant (s) of the medium between transparent electrode and LED array, according to the equation: where A refers to the area of the capacitor an T is the thickness of the dielectric layer between the transparent electrode and the LED array.

[0021] It follows that to achieve a large current flowing in the LEDs, C should be as large as possible. One way to increase C is to reduce the thickness, T, of the dielectric layer between the transparent electrode and the LED array. Accordingly, in the example systems described below, a gap between the LED array and the electrode is in the range of tenths of microns to a few microns (e.g., from 0.1 microns to 10 microns, such as 5 microns or less, 3 microns or less, 2 microns or less, 1 micron or less, 0.8 microns or less, 0.5 microns or less, such as 0.1 microns or more, 0.2 microns or more, 0.3 microns or more).

[0022] Furthermore, the capacitive coupling between the electrode and each LED in the array should be identical, or as close as possible to identical, otherwise different currents can be induced in different LEDs. Alternatively, or additionally, if variations in T across the LED array are known, appropriate corrections can be applied during measurement analysis to account for resulting differences in the current values.

[0023] Referring to FIG. 1, an example system 100 for noncontact FEEL inspection of a microLED array includes a probe 110, a chuck 120 (e.g., electrically grounded or offset by a specific amount relative to ground) for supporting a wafer 101 with one or more microLED arrays and positioning it close to the probe with a surface of one of the LED arrays facing a transparent electrode 112 that is part of the probe. An optical measurement system 130 is positioned relative to the probe 110 and the chuck to image the microLED array 101 through the transparent electrode 112. A gas source 140 and gas line 142 is arranged to deliver a gas to a gap 105 between a top surface 102 of wafer 101 and the transparent electrode 112. An electronic controller 150 is connected to a signal generator 160, which generates and delivers electrical signals to the electrode 112. The electronic controller 150 is also connected to the imaging optical system 130. The electronic controller controls operation of the signal generator 160 and imaging optical system 130 during operation of the system, coordinating energizing the electrode 112 while acquiring images of the microLED array with the optical measurement system 130.

[0024] Wafer 101 typically includes a number of microLED arrays, each composed of a two- dimensional array of microLEDs. Generally, the size (e.g., length, width, or diagonal) of a microLED is 100 pm or less (e.g., 50 pm or less, 30 pm or less, 20 pm or less, 15 pm or less, such as 3 pm or more, 5 pm or more, 10 pm or more). Arrays can be monochrome (i.e., each microLED emits light at the same color) or colored (e.g., the array is composed of pixels each containing three microLEDs of different color, such as red, green, blue or cyan, yellow, magenta). Moreover, while the array under test described here and below includes several arrays, the systems and methods described can be used to inspect a single array too (i.e., after it has been diced from a wafer). Generally, the methods and systems can be used to inspect microLED arrays at any stage in their manufacture or subsequent incorporation into larger systems, provided the LEDs are capable of emitting EL. This can include inspection of arrays during various stages of integrated circuit fabrication, packaging, or later.

[0025] As will be described in more detail below, during operation of the system 100, the electronic controller 150 causes the signal generator 160 to apply a voltage to the transparent electrode 112 sufficient to induce a current flow in at least some of the light emitting diodes of the microLED array on the wafer 101, where the current flow causes electroluminescent emission from the light emitting diode. Under the control of the controller 150, the system 100 acquires images of the microLED array using the electroluminescent emission and evaluate emission characteristics of the light emitting diodes. While inducing and detecting this electroluminescence, the system 100 maintains a small air gap between the lower surface of the transparent electrode 112 and the top surface of the wafer 101. Although the term “air gap” is used, maintaining this gap can include pumping a gas other than air - such as nitrogen or argon - into the gap to maintain the gap using an air cushion.

[0026] Referring to FIGS. 2 A and 2B, an example probe 200 for system 100 includes a chamber 210 bound by a lower plate 220, an upper plate, and a bellows 240. The lower plate 220 is attached to the bellows 240 by a collar 226. A gas line 242 delivers gas from the gas source to the chamber 210.

[0027] The lower plate 220 is composed of a porous annular disk 222 and a transparent electrode 212 attached to the disk 222 at a central aperture in the disk. The porous annular disk 222 can be made from a ceramic material, graphite or a perforated metal or alloy (e.g., a metallic ring with an array of small holes uniformly distributed across the ring). The transparent electrode can be composed of a transparent conducting oxide layer (e.g., indium tin oxide) on a transparent substrate (e.g., a glass or plastic substrate). The transparent electrode 212 is typically as large as or larger than the area of the microLED array 106 under inspection. In some examples, the transparent electrode has a lateral dimension (e.g., a diameter for a circular electrode) that is in a range from 2 mm to 50 mm (e.g., 5 mm or more, 10 mm or more, 15 mm or more, 20 mm or more, 25 mm or more, such as 40 mm or less).

[0028] A lower surface 213 of transparent electrode 212 is separated from the top surface 102 of wafer 101 by gap 105 having a thickness, T. Surface 213 is a planar surface and, when appropriately positioned with respect to the surface 102 of the array 106, the thickness T of the gap 105 is uniform across the area of the lower surface 213.

[0029] At least part of an optical measurement system 230 is situated in the probe 200, with its optical axis aligned with a central axis 232 of the disk 222.

[0030] During operation, the system pumps gas from the gas source into the chamber 210 via gas line 242, pressurizing the bellows 240. The gas in the overpressurized bellows (relative to ambient) escapes through the porous disk 222 into the gap 105 between the probe and the top surface 102 of wafer 101. The flow of the escaping gas is indicated in the Figure by curved arrows. The gas pressure is maintained at a level that supports a desired thickness, T, of the air gap. This thickness can be less than 10 microns (e.g., 5 microns or less, 3 microns or less, 2 microns or less, 1 micron or less, 0.5 micron or less, such as 0.1 micron or more). In some implementations, the thickness T is in a range from 0.1 micron to 1 micron. The thickness T can be uniformly maintained across the lower surface 213 of the transparent electrode 212. Uniformity of this gap can be important for maintaining a the same or close to the same capacitive couple between the transparent electrode and each of the light emitting diodes during a measurement. This can ensure that the induced current flowing through each light emitting diode is the same. In some implementations, the thickness across the lower surface 213 can vary by 5% or less (e.g., 3% or less, 3% or less, such as by 1% or more). In this manner, the probe 200 can be positioned very close to the surface of the wafer 101 without contacting the surface. In some implementations, the bellows 240 can be extended under operating conditions so that if the pressure in the chamber 210 drops, the bellows relaxes upward, drawing the electrode 212 away from the top surface of the wafer 101.

[0031] In certain implementations, uniformity of the gap thickness can be actively maintained using a measurement and actuation module that measures the gap thickness and adjusts the gap thickness to control the gap thickness. In some examples, the gap thickness is measured at multiple different locations across the probe and the system actuates the lower plate of the probe at one or more locations to adjust the piston position and / or tilt of the lower plate relative to the wafer surface. For example, in probe 200, three distance sensors 260 are housed in the lower plate 220 and three actuators 250 are housed in the collar 226 along common radial directions (e.g., R1 and R2, shown in FIG. 2B) as the distance sensors 260. In the present example, the radial directions of each sensor 260 are 120 degrees apart. The sensors and actuators are placed away from the transparent electrode, toward the periphery of the lower plate. In some implementations, the sensors 260 are non-contact capacitive sensors. The actuators 250 can be piezoelectric or electromagnetic actuators. The sensors can be capacitive sensors, interferometric sensors, or optical triangulation sensors.

[0032] After levelling the probe 200 (e.g., through coarse adjustment using the bellows and micro adjustment with the actuators 250) the final distance measurements on each of the sensors 260 is performed and used by the electronic controller to map the final distance of each light emitting diode in the array 106 under test from the electrode. Using this mapping, the system introduces corrections into the capacitance values used in the calculation of the value of current flowing through each light emitting diode for a given voltage ramp on the transparent electrode 212.

[0033] In some cases, the system performs a final distance measurement with each of the sensors 260 without leveling of the probe. Based on these measurements, the system maps a final distance of each light emitting diode in the array 106 under test from the transparent electrode. Using this mapping, the system introduces corrections into the capacitance values used in the calculation of the value of current flowing through each light emitting diode for a specific voltage ramp on the transparent electrode 212. Alternatively, or additionally, the gap thickness is measured by measuring the pressure in the chamber 210 or gas flow rate into or out of the chamber. For example, a relationship between gas pressure and gap thickness can be established in a calibration process in which this distance is measured for a range of different gas pressures and other variables that can affect the gap thickness and / or dielectric properties of the gas in the gap. The system can then convert the gas pressure during operation to the gap thickness for purposes of determining capacitance and / or LED current.

[0034] While the foregoing example features a flat transparent electrode surface and a flat surface of the LED array, other implementations are possible. For example, in some implementations, the LED array can have a curved surface. In such cases, the probe can measure the curvature of the LED surface and the varying distance between the electrode surface and LED array surface can be used in a calculation of the capacitance between the electrode and LED array at each light emitting diode. The varying capacitance across the curved LED array can be accounted for during subsequent calculations of the light emitting diode’s emission characteristics.

[0035] In certain examples, a curved electrode surface can be used instead of a flat surface. The curvature of the electrode surface can match the curvature of the LED array surface so that a constant gap thickness is maintained across the LED array despite the curvature of the LED array surface.

[0036] Once any adjustment of the gap is complete, the system applies a time varying voltage to the transparent electrode while the optical measurement system records the electroluminescent emission from each pixel using a detector array. The optical measurement system can make spectral measurements allowing the system to determine parameters such as EL intensity, peak spectral position, and halfwidth at one or more voltage values during the voltage variation.

[0037] A variety of voltages change rates can be applied to induce a variety of different values of the current flowing thought the light emitting diodes. Generally, the voltage is varied at least to values sufficient to generate electroluminescence from the light emitting diodes, but less than a breakdown voltage of the light emitting diodes. Different voltage waveforms can be used including, for example, linear waveforms, sinusoidal waveforms, sawtooth waveforms, square waveforms. AC waveforms with a frequency in a range from about 1 kHz to about 10 MHz are possible. Voltages sufficient to induce diode currents in ranges from 0.0001 A / cm2to 0.5 A / cm2are possible (e.g., 0.0002 A / cm2or more, 0.005 A / cm2or more, 0.001 A / cm2or more, 0.002 A / cm2or more, 0.005 A / cm2or more, 0.01 A / cm2or more, 0.05 A / cm2or more, such as 0.5 A / cm2or less, 0.1 A / cm2or less, 0.05 A / cm2or less). In an example, the voltage is ramped linearly from zero to 600 V in 10 ps.

[0038] Having EL data for different current values allows the system to empirically determine a dependance between the emission characteristics of a light emitting diode and the current level. Based on the established relationship between current and emission, the emission intensity for each light emitting diode in the array value is recalculated for a common current value and the light emitting diodes are sorted based on desired criteria. Such criteria include, for example, a threshold emission intensity, a peak wavelength, a peak width, a wavelength distribution, and / or any other spectral parameter of the light emitted in response to the electric field.

[0039] As part of the sample measurement, the system can illuminate the LED array with light that improves certain measurement parameters. As such, the probe can include a light source, such as a light emitting diode, to illuminate the wafer on the chuck. Appropriate illumination can, for example, eliminate a contribution to capacitance from a space charge region on the LED array surface that may be present. Ultraviolet illumination with a wavelength suitably selected for the semiconductor material under inspection can generate minority carriers in the semiconductor materials and thus increase electroluminescent response of the sample on relatively low excitation voltages. For example, a sample composed of GaN can be illuminated with light having a wavelength in a range from 320 nm to 390 nm.

[0040] While the example probe 200 described above includes three distance sensors and three actuators, implementations with fewer or more than three of each of these components are possible. Moreover, the arrangement of sensors and actuators can vary. For example, the radial arrangement of the sensors can differ from the actuators. For instance, in some cases, the sensors can be offset from the actuators by 60 degrees. Further, the angular offset between the sensors (120 degrees in the probe 200) can differ. In some implementations, actuators are not included and the distance between the lower surface 213 and the top surface 102 of the wafer 101 is adjusted by changing the pressure in the chamber 210.

[0041] In general, optical measurement system 230 includes a multipixel detector (e.g., a CCD array or a CMOS array) for measuring light intensity at multiple locations on the wafer and optics for imaging the surface of the wafer onto the detector. An example of an optical measurement system suitable for use in the inspection systems described here is shown in FIG. 3A. Here, an optical measurement system 300 includes a barrel 310 that supports an objective lens 320 (e.g., composed of one or more lens elements) close to the wafer 101. The barrel is sized to fit into the inspection system’s probe, e.g., probe 200. Together with a tube lens 330, the objective lens 320 images the wafer 101 onto a multipixel detector 340. A beam splitter 322 is positioned to receive electroluminescent light from wafer 101 via the objective lens 320 and direct the light to a mirror 322, which directs the light to the multipixel detector 340. Generally, the beam splitter is configured to reflect at least a portion of the electroluminescence from the wafer towards the detector. This is typically light at wavelengths of 400 nm and above (e.g., up to about 700 nm). The beam splitter 322 can be a 50 / 50 beam splitter. In some implementations, the beam splitter can be a dichroic beam splitter, reflecting light only within a specific band or bands of wavelengths.

[0042] The tube lens 330, positioned in the optical path between the mirror 332 and detector 340, focuses the light onto the detector.

[0043] A band pass filter 334 can be positioned in the optical path downstream from the beam splitter 322, e.g., between the mirror 332 and the tube lens 330 as shown in FIG. 3A. The bandpass filter can block light at wavelengths outside of the range to be detected, reducing noise in the electroluminescence measurements. The bandpass filter 334 can be an interchangeable filter, allowing the system to be adapted to detect different wavelength bands of EL emission by replacing the filter with a filter having a different pass band.

[0044] A light source 350 is arranged to illuminate the wafer 101 through the objective lens 320. An EPI illumination lens 352 focuses light from the light source 350, which passes through the beam splitter 322 to the objective lens 320. In some implementations, the light source 350 provides illumination suitable for generating photoluminescence (PL) in the LED array, allowing the wafer to be inspected based on PL emission in addition to or instead of the EL emission. These sources of emission can be compared and / or contrasted to determine information about the LED array. Generally, PL measurements are faster per unit area than EL measurement and, therefore, can be used to map the whole surface of the wafer faster. EL measurements may provide more accurate results, e.g., on whether a light emitting diode will be operable or defective upon completion of the fabrication process. By comparing the PL and EL measurements for a certain type of sample, a correlation can be established between the PL response and the operable and / or defective light emitting diodes.

[0045] In some examples, measurements utilizing illumination from light source 350 can be used to monitor a property or properties of the wafer prior to or during fabrication of the LED array. For instance, a layer of an epitaxially grown material can be characterized prior to photolithographic patterning of the layer.

[0046] Alternatively, or additionally, light source 350 can be used to illuminate the LED array to generate photovoltage and thus create bias light illumination for the EL measurements. Generally, illumination at a suitable wavelength (e.g., UV) generates additional charge carriers in the sample. As a result of the increased charge carrier density, EL may be produced by the LED at a lower excitation voltage than would be needed in the absence of the illumination. Without the illumination bias, certain samples can produce EL only near the breakdown voltage and thus their FEEL measurement would be not possible and / or could damage the sample. Accordingly, the additional charge carriers generated by the illumination may allow the use of FEEL method on a wider range of samples.

[0047] The light source 350 can provide visible illumination to optically examine the sample surface for surface defects (e.g., particulates) and / or other surface features (e.g., so-called streets or dye corners, when the sample is so divided, or any other visible mark that is either an artifact of the manufacturing process or introduced specifically for facilitating precise positioning). Such inspection can be performed using bright field microscopy. Alternatively, or additionally, optical examination of the sample surface can be used to align the sensor with the sample, e.g., using pattern recognition to determine structural features or other fiducials with which to align the sensor.

[0048] The objective lens 320 and other imaging optics of the measurement optical system provide a field of view suitable for measuring the LED arrays under test at a magnification and resolution that resolves each light emitting diode in the array and enables inspecting a large number of light emitting diodes in a single step. For example, the field of view can have at least one dimension on the order of 0.2 mm or more (e.g., for micrometer scale resolution, e.g., at magnification of 100X or more). Larger fields (e.g., 1 mm or more, 2 mm or more, 10 mm or more, e.g., the diameter of the wafer) are possible for lower magnifications. Resolution can be 10 pm or better (e.g., 5 pm or better, 3 pm or better, 2 pm or better, such 1 pm, or even less than 1 pm). Magnification of the imaging system can be from IX to 100X, for example. In implementations where the LED array being inspected is larger than the field of view, the chuck can step the wafer sequentially through the field of view to allow the entire array to be inspected.

[0049] The objective lens 320 has a working distance suitable to accommodate the gap thickness T, while providing adequate resolution and field size. The working distance can be in a range from 1 mm to 10 mm (e.g., 2 mm or more, 3 mm or more, 5 mm or more).

[0050] The optical measurement system can be a compact system, capable of being partially or completely integrated into the inspection system probe. For example, the objective lens 320 can be formed from microoptics components, providing a compact form factor that can be incorporated into the probe without occupying a significant volume. The beam splitter 322 can be small enough to be incorporated into the barrel in the probe too. The detector 340 can be a board level device sized to fit within a common, compact housing for the optical measurement system along with the other components. The multipixel detector 340 can be a monochrome sensor or a multicolor sensor (e.g., RGB).

[0051] Another example of an optical measurement system 300’ is shown in FIG. 3B. This system is adapted to simultaneously detect emission in three different EL bands using three different detectors (340, 340’, and 340”). Each detector is located along a different optical path established by dichroic beam splitters 333 and 333’. Beam splitter 322 directs light from wafer 101 towards beam splitters 333 and 333’ as described above. Beam splitter 333 reflects light in a first wavelength band to detector 340. Beam splitter 333’ reflects light in a second, different wavelength band to a second detector 340’, while light in a third band (or bands) is reflected by mirror 332 to a third detector 340”. Each of the detectors thus detects EL light intensity in a corresponding different wavelength band simultaneously.

[0052] Emission characteristics from each LED being inspected can be characterized by one or more of a variety of measurement parameters. For example, the EL intensity at a particular wavelength or wavelength band can be measured for a series of current values. A comparison of each microLED’s current v. intensity can be used to identify defective microLEDs. In some cases, the EL emission intensity for each microLED can be compared at a common current value.

[0053] Other parameters that can be measured include EL emission intensity, peak wavelength and / or peak width, and / or changes of each of these parameters in response to changes of the excitation parameters, i.e., a functional dependence. Said functional dependence may be the dependence of the intensity, wavelength or wavelength-distribution (i.e., spectrum) of the emitted light based on a characteristic of the applied voltage, such as max voltage, ramp rate, frequency or waveform. Said functional dependence may be, e.g., an intensity-voltage or wavelength-voltage characteristic. In a preferred embodiment, a spectrograph is used as a detector for detecting the whole spectrum of the LEDs in the visible wavelength range for facilitating the determination of said functional dependence.

[0054] As an example, information about the spectral shape of a light emitting diode’s emission peak can be determined using optical system 300’ as follows. The EL from the LED array is directed by beam splitter 322 into semitransparent reflective mirrors 333, 333’ and mirror 332 which direct each of the beams of light to the narrow band optical filters 330, 330’ and 330”, then to detector 340, 340’ and 340”, respectively. The transmission wavelengths of each narrow band filter are slightly shifted (e.g., by 10 nm to 100 nm) relative to each other, centered around expected EL emission wavelength. The intensity of EL for these different wavelengths is measured by the detectors 340, 340’ and 340”. From the measured values of the emission intensity for the different wavelengths the shape of emission peak is reconstructed. Based on the shape of the emission peak, the system can determine values for the wavelength of maximum emission and the halfwidth of the peak.

[0055] Each of the detectors may be a multipixel detector (e.g., a CCD array or a CMOS array). When a single image does not cover the whole surface of the sample, multiple sequential images can be taken of different areas of the sample. These areas may be predetermined distinct regions of the sample or overlapping or neighboring areas so that the images cover a substantial part of the sample, preferably the whole area of interest, that is in some cases the whole area of a wafer. The multiple images may be stitched together to form a single combined image covering the whole area of interest.

[0056] In some examples, corrections can be applied to the multiple images and / or the combined image. The corrections may include, based on the type of detector and the circumstances of the image recording, dark current correction, flat field correction, affine transformation, correction of any further optical aberrations, and / or any combination thereof.

[0057] In some cases, the corrections are applied to each of the sequentially acquired images before the stitching. Alternatively, the images are stitched together first, and then corrections are applied to the combined image. Images (e.g., raw and / or corrected, individual and / or combined images) can be stored for further analysis.

[0058] The corrected individual images and / or the corrected combined image can be subjected to geometrical segmentation according to a segmentation grid, corresponding to a predetermined geometrical arrangement of the LEDs, e.g., according to a Cartesian grid, a rectilinear grid, a skewed grid and / or a triangular grid. The magnification of the optical measurement system defines a fixed ratio between the known physical dimensions of the LED structure and the apparent size of each LED in the image. The apparent size of each LED in the image is preferably more than one pixel, e.g., an area of 2 by 2 pixels or more, an area of 3 by 3 pixels or more, or an area of 4 by 4 pixels or more. The segmentation grid can be fitted to the corrected individual images or to the corrected combined image so that the image of a single LED is contained within each cell of the segmentation grid. Then, the intensity values of each pixel within a cell of the segmentation grid (and possible fractional intensity values of partial pixels) are summed or averaged for each cell of the segmentation grid to obtain a brightness value describing the intensity of light emitted by each LED in the wavelength range corresponding to the image. For example, a brightness value may be determined for a LED in the wavelength range corresponding to the red color, in the wavelength range corresponding to the green color, and / or in the wavelength range corresponding to the blue color.

[0059] Defective pixels may be identified based on too high or too low brightness in any of the wavelength ranges or any combination thereof. For example, a bottom threshold may be set for each of the different wavelength ranges, said bottom threshold may be the same for each of the wavelength ranges, or alternatively may be different for each wavelength range, and LEDs with a brightness value under said bottom threshold may be deemed defective. Alternatively, or additionally, a top threshold may be set for each of the different wavelength ranges, said top threshold may be the same for each of the wavelength ranges, or alternatively may be different for each wavelength range, and LEDs with a brightness value above said top threshold may be deemed defective.

[0060] Other implementations are possible. For example, in some cases, the optical measurement system the free space optics of the optical measurement system described above can be replaced with a micro imaging organized fiber, fitted to the sensor aperture. In some examples, a FEEL system can include a corona emission module for depositing corona charge on the surface 102 of the wafer. Referring to FIG. 4, a FEEL system 400 includes, in addition to those components described previously, a corona charging station 410 situated in proximity to the probe 110 and arranged to deposit a corona charge at one or more locations on the top surface of wafer 101. Corona charge is deposited by moving the chuck 120 with the wafer 101 under a point source corona gun or a wire corona bar included in corona charging station 410 and depositing a desired amount of corona charge (positive or negative). The chuck is then moved under the probe 110 for inspection as described above.

[0061] Generally, the relationship between the current flowing through a LED and its emission characteristics, e.g., the light intensity, depends on the presence or absence of bulk (e.g., crystallographic) defects in the crystalline layers of the LED, bulk defect density if present, and the surface passivation of the LED. Corona charge can be used to separate these two contributions to defectivity.

[0062] Use of corona charge for surface passivation during inspection can be useful at various stages of LED processing. For example, the LEDs arrays used in visible displays are generally wide band gap materials based on GaN. Corona charge deposited on a surface of a wide band gap material is generally stable on surfaces of semiconductor material layers. Accordingly, corona charge passivation can be useful for inspection during processing steps when a semiconductor layer (as opposed to a dielectric layer) is exposed.

[0063] Surface passivation using corona charging can also be useful for mitigating reduced electroluminescence due to depletion of injected carriers. Recombination of minority carriers is typically very efficient on surface in depletion. This means that for a LED array surface with depletion the number of injected carriers will be reduced, resulting in a reduction in electroluminescence. Sufficient positive corona charge deposited on such a surface can cause an accumulation in n-type carriers at the surface and an inversion in p-type carriers. Alternatively, sufficient negative corona charge deposited on the surface can cause an inversion of the surface in n-type and an accumulation in p-type carriers. Such surfaces are passivated, and minority carrier recombination that would otherwise reduce electroluminescence is avoided (or, at least, reduced).

[0064] Accordingly, in some implementations, inspection can include comparing electroluminescent emission from a LED array with and without passivation in order to differentiate crystallographic (bulk) defects from passivation defects. Bulk defects will generally affect emission with and without corona passivation. Passivation defects will manifest during inspection without corona charge deposition, but their effects should be reduced or absent upon inspection after corona deposition.

[0065] Inspection of microLED arrays using the systems and methods described above can be integrated into processes for manufacturing headsets for AR / VR applications. For example, in some cases, an inspection system can be included at a foundry that manufactures microLED arrays and, during or after manufacturing the array, it can be inspected using the system. The arrays can be qualified based on the inspection. For instance, if each light emitting diode in the array meets certain emission thresholds evaluated using the inspection, it can be qualified for sale or for further process steps. Arrays that fail inspection can be discarded or repaired. Those arrays that pass the inspection process can be incorporated into a headset for AR or VR applications during an assembly process of the headset.

[0066] Furthermore, while the foregoing description has focused on inspection of microLED arrays, more generally, the methods and systems described can be used for inspecting other devices as well. Generally, the methods and systems can be used to inspect any device that displays EL emission, including regular LEDs and other semiconductor devices.

[0067] NUMBERED PARAGRAPHS

[0068] In some embodiments, the present invention can be described by reference to the following numbered paragraphs.

[0069] Paragraph 1: A method, including: positioning a light emitting diode (LED) array on a chuck; while the LED array is on the chuck, positioning a transparent electrode relative to a surface of the LED array so that the transparent electrode and the surface of the LED array are separated by a gap having a thickness in a range from 0.1 microns to 100 microns, the gap containing a gas at ambient or greater pressure; applying a voltage to the transparent electrode sufficient to induce a current flow in at least some of the light emitting diodes of the LED array, the current flow causing electroluminescent emission from the light emitting diodes of the LED array; acquiring an image of the LED array using the electroluminescent emission; and evaluating emission characteristics of the at least some of the light emitting diodes in the LED array based on the acquired image.

[0070] Paragraph 2: The method of claim 1, further including flowing the gas into the gap while the transparent electrode is positioned relative to the surface of the LED array.

[0071] Paragraph 3: The method of paragraph 1 or paragraph 2, wherein the gas is nitrogen gas.

[0072] Paragraph 4: The method of any one of paragraphs 1 to 3, wherein the chuck is a grounded chuck while the voltage is applied.

[0073] Paragraph 5: The method of any one of paragraphs 1 to 3, wherein a potential of the chuck is offset relative to a ground potential while the voltage is applied.

[0074] Paragraph 6: The method of any one of the previous paragraphs, wherein applying the voltage includes varying the voltage between a minimum voltage to a maximum voltage.

[0075] Paragraph 7: The method of paragraph 5, wherein the maximum voltage during the varying is below a breakdown field of the gas in the gap.

[0076] Paragraph 8: The method of paragraph 5, wherein varying the voltage includes ramping the voltage at multiple different ramp rates sufficient to induce different current values through at least some of the light emitting diodes.

[0077] Paragraph 9. The method of paragraph 7, wherein evaluating the emission characteristics includes: determining a functional dependence between emission from the at least some of the light emitting diodes and a characteristic of the applied voltage; and determining an emission intensity for a preset current value for the at least some of the light emitting diodes based on the functional dependence.

[0078] Paragraph 10: The method of any one of the previous paragraphs, wherein positioning the transparent electrode relative to a surface of the LED array includes measuring a thickness of the gap at at least one measurement location and adjusting a relative position of the electrode relative to the surface of the LED array based on the thickness measurement.

[0079] Paragraph 11 : The method of paragraph 9, wherein the thickness is measured at multiple measurement locations.

[0080] Paragraph 12: The method of paragraph 9, wherein the relative position is adjusted using one or more piezoelectric actuators. Paragraph 13: The method of paragraph 9, wherein the relative position is adjusted by changing a pressure of the gas or a flow rate of the gas.

[0081] Paragraph 14: The method of paragraph 9, wherein adjusting the relative position improves a uniformity in the thickness of the gap across the LED array.

[0082] Paragraph 15: The method of paragraph 9, wherein the thickness is measured at three different measurement locations beyond an edge of the electrode and the relative position is adjusted at one or more of three different adjustment locations beyond the edge of the electrode.

[0083] Paragraph 16: The method of paragraph 14, wherein the electrode has a central axis corresponding to an optical axis of an optical measurement system that acquires the image of the LED array, each of the three measurement locations being along different radial directions from the central axis and each of the three adjustment locations being along a corresponding one of the radial directions at different radial distances from the central axis compared to the corresponding measurement location.

[0084] Paragraph 17: The method of paragraph 9, further including calculating a gap thickness for one or more of the light emitting diodes based on the thickness measurement and evaluating the emission characteristics includes accounting for the gap thickness for each of the at least some of the light emitting diodes.

[0085] Paragraph 18: The method of paragraph 9, wherein the gap thickness is measured based on a capacitance between the transparent electrode and the LED array.

[0086] Paragraph 19: The method of any one of the previous paragraphs, wherein the image is acquired through the transparent conducting electrode.

[0087] Paragraph20: The method of any one of the previous paragraphs, wherein the light emitting diodes are microLEDs.

[0088] Paragraph 21: The method of any one of the previous paragraphs, wherein the emission characteristics for each light emitting diode are selected from the group consisting of light intensity, emission wavelength, and halfwidth.

[0089] Paragraph 22: The method of any one of the previous paragraphs, further including passivating the at least some of the light emitting diodes by depositing a corona charge on the surface and identifying defective light emitting diodes due to a presence of bulk defects or defective passivation based on a comparison of emission from each of the at least some of the light emitting diodes with and without the corona passivation. Paragraph 23: The method of any one of the previous paragraphs, further including illuminating the LED array or a wafer used to form the LED array with light sufficient to cause photoluminescent emission from the LED array or from the wafer, detecting the photoluminescent emission, and evaluating the LED array or the wafer based on the detected photoluminescent emission.

[0090] Paragraph 24: The method of any one of the previous paragraphs, further including imaging the surface of the LED array using bright field microscopy and identifying surface defects or other surface features based on the imaging.

[0091] Paragraph 25: The method of any one of the previous paragraphs, further including illuminating the LED array or a wafer used to form the LED array with light sufficient to generate photovoltage in the LED array or in the wafer.

[0092] Paragraph 26: A method for manufacturing a head mounted display, including: fabricating a microLED array; inspecting the microLED array using the method of any one of the previous paragraphs; qualifying the microLED array based on the inspection; and assembling the head mounted display incorporating the microLED array.

[0093] Paragraph 27: A system, including: a probe including a transparent electrode; a signal generator in electrical communication with the transparent electrode; a chuck situated relative to the transparent electrode to support a light emitting diode (LED) array with a surface of the LED array facing the transparent electrode and a gap between the surface and the transparent electrode, the gap having a thickness in a range from 0.1 microns to 100 microns; an optical measurement system situated relative to the probe and the chuck to image the LED array through the transparent electrode; a gas source and delivery module arranged to deliver a gas to the gap between the surface and the transparent electrode; and an electronic controller in communication with the signal generator and the optical measurement system, the electronic controller being programmed to: (i) cause the signal generator to apply a voltage to the transparent electrode sufficient to induce a current flow in at least some of the light emitting diodes of the LED array, the current flow causing electroluminescent emission from the light emitting diodes of the LED array, (ii) cause the optical measurement system to acquire an image of the LED array using the electroluminescent emission, and (iii) evaluate emission characteristics of the at least some of the light emitting diodes in the LED array based on the acquired image. Paragraph 28: The system of paragraph 27, further including a light source arranged to illuminate the LED array with illumination sufficient to generate photoluminescence in the LED array and an optical detector configured to detect the photoluminescence, wherein the electronic controller is further programmed to evaluate the LED array based on the detected photoluminescence.

[0094] Paragraph 29: A method, including: positioning a light emitting diode (LED) array on a chuck; while the LED array is on the chuck, positioning a transparent electrode relative to a surface of the LED array so that the transparent electrode and the surface of the LED array are separated by a gap containing a gas at ambient or greater pressure; leveling a surface of the transparent electrode relative to the surface of the LED array; applying a voltage to the transparent electrode sufficient to induce a current flow in at least some of the light emitting diodes of the LED array, the current flow causing electroluminescent emission from the light emitting diodes of the LED array; acquiring an image of the LED array using the electroluminescent emission; and evaluating emission characteristics of the at least some of the light emitting diodes in the LED array based on the acquired image.

[0095] Paragraph 30: A method, including: positioning a light emitting diode (LED) array on a chuck, wherein the chuck is grounded or offset by a specific voltage relative to a ground voltage; while the LED array is on the chuck, positioning a probe including a transparent electrode parallel to a surface of the LED array so that the transparent electrode and the surface of the LED array are separated by the gap having a thickness in range of 0.1 pm to 100 pm; controlling a thickness of the gap by flowing gas at a constant, predetermined flow velocity from the probe into the gap; measuring, using distance sensors at multiple different locations, a thickness of the gap at the edges of the transparent electrode to map capacitance changes between the probe and individual light emitting diodes of the LED array; applying a time-varying voltage to the transparent electrode sufficient to induce forward current flow in the light emitting diodes, the current flow causing electroluminescent emission from the light emitting diodes; acquiring an image of the LED array using the electroluminescent emission; evaluating emission characteristics of the light emitting diodes in the array based on the acquired image; measuring electroluminescent emission values for one or more voltage ramp rates of the time varying- voltage and / or different values of induced forward current flow; determining an empirical relationship between the emission characteristics and the values of induced forward current flow for each of the light emitting diodes; and calculating an emission characteristic for each of the light emitting diodes for a common current value based on the empirical relationship.

[0096] A number of embodiments have been described. Other embodiments are in the following claims.

Claims

What is claimed is:

1. A method, comprising: positioning a light emitting diode (LED) array on a chuck; while the LED array is on the chuck, positioning a transparent electrode relative to a surface of the LED array so that the transparent electrode and the surface of the LED array are separated by a gap having a thickness in a range from 0.1 microns to 100 microns, the gap containing a gas at ambient or greater pressure; applying a voltage to the transparent electrode sufficient to induce a current flow in at least some of the light emitting diodes of the LED array, the current flow causing electroluminescent emission from the light emitting diodes of the LED array; acquiring an image of the LED array using the electroluminescent emission; and evaluating emission characteristics of the at least some of the light emitting diodes in the LED array based on the acquired image.

2. The method of claim 1 , further comprising flowing the gas into the gap while the transparent electrode is positioned relative to the surface of the LED array.

3. The method of claim 1 or claim 2, wherein the gas is nitrogen gas.

4. The method of any one of claims 1 to 3, wherein the chuck is a grounded chuck while the voltage is applied.

5. The method of any one of claims 1 to 3, wherein a potential of the chuck is offset relative to a ground potential while the voltage is applied.

6. The method of any one of the previous claims, wherein applying the voltage comprises varying the voltage between a minimum voltage to a maximum voltage.

7. The method of claim 5, wherein the maximum voltage during the varying is below a breakdown field of the gas in the gap.

8. The method of claim 5, wherein varying the voltage comprises ramping the voltage at multiple different ramp rates sufficient to induce different current values through at least some of the light emitting diodes.

9. The method of claim 7, wherein evaluating the emission characteristics comprises: determining a functional dependence between emission from the at least some of the light emitting diodes and a characteristic of the applied voltage; and determining an emission intensity for a preset current value for the at least some of the light emitting diodes based on the functional dependence.

10. The method of any one of the previous claims, wherein positioning the transparent electrode relative to a surface of the LED array comprises measuring a thickness of the gap at at least one measurement location and adjusting a relative position of the electrode relative to the surface of the LED array based on the thickness measurement.

11. The method of claim 9, wherein the thickness is measured at multiple measurement locations.

12. The method of claim 9, wherein the relative position is adjusted using one or more piezoelectric actuators.

13. The method of claim 9, wherein the relative position is adjusted by changing a pressure of the gas or a flow rate of the gas.

14. The method of claim 9, wherein adjusting the relative position improves a uniformity in the thickness of the gap across the LED array.

15. The method of claim 9, wherein the thickness is measured at three different measurement locations beyond an edge of the electrode and the relative position is adjusted at one or more of three different adjustment locations beyond the edge of the electrode.

16. The method of claim 14, wherein the electrode has a central axis corresponding to an optical axis of an optical measurement system that acquires the image of the LED array, each of the three measurement locations being along different radial directions from the central axis and each of the three adjustment locations being along a corresponding one of the radial directions at different radial distances from the central axis compared to the corresponding measurement location.

17. The method of claim 9, further comprising calculating a gap thickness for one or more of the light emitting diodes based on the thickness measurement and evaluating the emission characteristics comprises accounting for the gap thickness for each of the at least some of the light emitting diodes.

18. The method of claim 9, wherein the gap thickness is measured based on a capacitance between the transparent electrode and the LED array.

19. The method of any one of the previous claims, wherein the image is acquired through the transparent conducting electrode.

20. The method of any one of the previous claims, wherein the light emitting diodes are microLEDs.

21. The method of any one of the previous claims, wherein the emission characteristics for each light emitting diode are selected from the group consisting of light intensity, emission wavelength, and halfwidth.

22. The method of any one of the previous claims, further comprising passivating the at least some of the light emitting diodes by depositing a corona charge on the surface andidentifying defective light emitting diodes due to a presence of bulk defects or defective passivation based on a comparison of emission from each of the at least some of the light emitting diodes with and without the corona passivation.

23. The method of any one of the previous claims, further comprising illuminating the LED array or a wafer used to form the LED array with light sufficient to cause photoluminescent emission from the LED array or from the wafer, detecting the photoluminescent emission, and evaluating the LED array or the wafer based on the detected photoluminescent emission.

24. The method of any one of the previous claims, further comprising imaging the surface of the LED array using bright field microscopy and identifying surface defects or other surface features based on the imaging.

25. The method of any one of the previous claims, further comprising illuminating the LED array or a wafer used to form the LED array with light sufficient to generate photovoltage in the LED array or in the wafer.

26. A method for manufacturing a head mounted display, comprising: fabricating a microLED array; inspecting the microLED array using the method of any one of the previous claims; qualifying the microLED array based on the inspection; and assembling the head mounted display incorporating the microLED array.

27. A system, comprising: a probe comprising a transparent electrode; a signal generator in electrical communication with the transparent electrode; a chuck situated relative to the transparent electrode to support a light emitting diode (LED) array with a surface of the LED array facing the transparent electrode and a gap between the surface and the transparent electrode, the gap having a thickness in a range from 0.1 microns to 100 microns;an optical measurement system situated relative to the probe and the chuck to image the LED array through the transparent electrode; a gas source and delivery module arranged to deliver a gas to the gap between the surface and the transparent electrode; and an electronic controller in communication with the signal generator and the optical measurement system, the electronic controller being programmed to:(i) cause the signal generator to apply a voltage to the transparent electrode sufficient to induce a current flow in at least some of the light emitting diodes of the LED array, the current flow causing electroluminescent emission from the light emitting diodes of the LED array,(ii) cause the optical measurement system to acquire an image of the LED array using the electroluminescent emission, and(iii) evaluate emission characteristics of the at least some of the light emitting diodes in the LED array based on the acquired image.

28. The system of claim 27, further comprising a light source arranged to illuminate the LED array with illumination sufficient to generate photoluminescence in the LED array and an optical detector configured to detect the photoluminescence, wherein the electronic controller is further programmed to evaluate the LED array based on the detected photoluminescence.

29. A method, comprising: positioning a light emitting diode (LED) array on a chuck; while the LED array is on the chuck, positioning a transparent electrode relative to a surface of the LED array so that the transparent electrode and the surface of the LED array are separated by a gap containing a gas at ambient or greater pressure; leveling a surface of the transparent electrode relative to the surface of the LED array; applying a voltage to the transparent electrode sufficient to induce a current flow in at least some of the light emitting diodes of the LED array, the current flow causing electroluminescent emission from the light emitting diodes of the LED array; acquiring an image of the LED array using the electroluminescent emission; andevaluating emission characteristics of the at least some of the light emitting diodes in the LED array based on the acquired image.

30. A method, comprising: positioning a light emitting diode (LED) array on a chuck, wherein the chuck is grounded or offset by a specific voltage relative to a ground voltage; while the LED array is on the chuck, positioning a probe comprising a transparent electrode parallel to a surface of the LED array so that the transparent electrode and the surface of the LED array are separated by the gap having a thickness in range of 0.1 pm to 100 pm; controlling a thickness of the gap by flowing gas at a constant, predetermined flow velocity from the probe into the gap; measuring, using distance sensors at multiple different locations, a thickness of the gap at the edges of the transparent electrode to map capacitance changes between the probe and individual light emitting diodes of the LED array; applying a time-varying voltage to the transparent electrode sufficient to induce forward current flow in the light emitting diodes, the current flow causing electroluminescent emission from the light emitting diodes; acquiring an image of the LED array using the electroluminescent emission; evaluating emission characteristics of the light emitting diodes in the array based on the acquired image; measuring electroluminescent emission values for one or more voltage ramp rates of the time varying-voltage and / or different values of induced forward current flow; determining an empirical relationship between the emission characteristics and the values of induced forward current flow for each of the light emitting diodes; and calculating an emission characteristic for each of the light emitting diodes for a common current value based on the empirical relationship.

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