LED chip evaluation method based on dynamic capacitance test

Abstract

The invention provides an LED chip evaluation method based on a dynamic capacitance test, and belongs to the technical field of LED device detection. According to the method, the junction capacitance under different bias and frequencies is measured and analyzed as a parameter for screening and judging the LED chip, so that the quality diagnosis of the dynamic response capability of the LED device is realized, and a reliable basis is provided for the production and quality control of the LED chip. The method is simple, convenient and efficient to operate, high in accuracy, low in cost, high in practicability, capable of rapidly completing detection of the LED chip, high in equipment universality degree, low in detection cost and beneficial to large-scale application.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of LED device detection, and particularly relates to a method for evaluating an LED chip by testing a junction capacitance under dynamic working. BACKGROUND

[0002] Since GaN-based LEDs are widely used in the lighting field, an electrical test system mainly based on I-V (current-voltage relationship) characteristics is directly related to the luminous efficiency, reliability and lifetime of the LED as a traditional test method in the general lighting application scenario, and effectively meets the performance evaluation requirements of the general lighting LED device.

[0003] The LED test system currently applied in the industry selects several representative characteristic points on the I-V characteristic curve for analysis, and establishes corresponding quality standards as evaluation indexes for screening and grading, for example, the luminous efficiency and thermal stability of the device are evaluated by measuring the voltage drop under a specific current, such as the voltage VF corresponding to 1 μA and 1 mA forward current; the reliability and stability of the device are evaluated by measuring the reverse leakage current under a specific reverse current or voltage, such as the voltage VR corresponding to 10 μA reverse current or the reverse current IR under-5V. Therefore, the current commercial LED tester is a simplified test of the I-V electrical characteristics, which greatly reduces the difficulty of the test system and the time cost of the test, and effectively promotes the revolutionary application of LEDs in the general lighting field.

[0004] Beyond lighting, GaN-based LEDs have rapidly developed into high-efficiency light sources suitable for various scenarios. In recent years, miniaturized LEDs have opened up a series of new application scenarios for high-quality display systems with full-scale, high brightness, and high contrast. Due to the physical effects brought about by the miniaturization of device size, micron-sized LED devices can carry current densities far exceeding those of lighting LEDs, and exhibit excellent luminous efficiency and controllable spectral and angular emission. This makes micron-sized LEDs advantageous for constructing high-brightness, compact pixel units, which are more precisely matched to high-resolution real-time display systems. Examples include micro-displays that integrate micron-sized LEDs with driving circuits to achieve direct addressing and control of individual pixels; ultra-small display arrays for head-mounted AR and VR devices based on advanced packaging processes such as wafer bonding; and high-definition display solutions that precisely adjust pixel size and arrangement on a backplane based on transfer printing technology and RGB three-primary-color emission. Displays based on micron-sized LEDs are considered the most promising next-generation display technology, and they also demonstrate significant application value in various fields such as biomedicine and free-space communication. Currently, on the one hand, there is still a need to overcome technical bottlenecks in manufacturing, such as the yield of micro-nano fabrication and mass transfer. On the other hand, systems with high display quality requirements place new demands on testing, sorting, and quality assessment, particularly regarding the wavelength uniformity and dynamic stability and consistency of LED devices. Static methods using IV curves for evaluation have many shortcomings, necessitating a simple, efficient method and testing system that can reflect the characteristics of LED devices under operating conditions. Summary of the Invention

[0005] The present invention aims to provide a method for evaluating LED chips by testing the junction capacitance of LEDs under dynamic conditions such as different biases, current injections, and high frequencies.

[0006] The technical solution provided by this invention is as follows:

[0007] An LED chip evaluation method based on dynamic capacitance testing includes the following main steps:

[0008] 1) Perform IV testing on LED chips of the same batch, measure the turn-on voltage of the LED chips, and record it as the turn-on voltage Vj;

[0009] 2) The junction capacitance of the LED chip under static conditions is tested using a capacitance testing device. Specifically, the junction capacitance value of the LED is measured under high-frequency signal testing conditions at zero bias voltage and recorded as the static junction capacitance Cj1.

[0010] 3) The turn-on junction capacitance of the LED chip is tested using a capacitance testing device. Specifically, the junction capacitance value of the LED is measured under intermediate frequency signal testing conditions when the bias voltage is the turn-on voltage Vj, and recorded as the turn-on junction capacitance Cj2.

[0011] 4) according to the amplitude of the static junction capacitance Cj1 and the amplitude of the opening junction capacitance Cj2 of the LED chip, whether the opening response ability of the LED chip is satisfied, the qualified LED chip is evaluated.

[0012] Further, the dynamic LED junction capacitance of the evaluated LED chip in normal working light emission is tested by using a capacitance testing device, specifically, the dynamic junction capacitance value of the LED chip is measured under the low-frequency signal testing condition when the bias current is the characteristic working current, recorded as the dynamic junction capacitance Cj3, according to whether the amplitude of the dynamic junction capacitance Cj3 satisfies the dynamic response ability of the LED chip under high-frequency modulation, the high-efficiency LED chip is evaluated.

[0013] Further, the capacitance testing device is an impedance analyzer LCR or a network analyzer with impedance option.

[0014] Further, the frequency range of the high-frequency test in step 2) is 1MHz-100MHz.

[0015] Further, the frequency range of the high-frequency test in step 3) is 1KHz-100kHz.

[0016] Further, the frequency range of the low-frequency test is 100Hz-1kHz.

[0017] Further, the capacitance testing device is calibrated before testing, specifically, the capacitance testing device is set to the same bias voltage or current and the same test frequency as the measured LED chip, and then the open circuit, short circuit and load calibration are performed respectively, so as to eliminate the influence of the clamp and the wire;

[0018] Advantages of the present application:

[0019] The present application starts from the electrical testing of the LED chip, which is different from the testing based on the I-V characteristics of the commercial LED tester, focuses on the improvement of the parallel testing channel in multiple dimensions to improve the throughput of the testing, the present application focuses on the measurement of the junction capacitance characteristics of the LED, provides an evaluation method for diagnosing high-performance LED chips, by measuring and analyzing the junction capacitance under different bias and frequency, as the parameter for screening and judging the LED chip, realizes the quality diagnosis of the dynamic response ability of the LED device. And provides a reliable basis for the production and quality control of the LED chip. The present application is simple and efficient, accurate, low in cost, practical, can quickly complete the detection of the LED chip, the equipment is universal, the detection cost is low, is conducive to large-scale application. DETAILED DESCRIPTION

[0020] The application is based on dynamic capacitance test. The capacitance of LED chip generally includes junction capacitance of PN junction and parasitic capacitance introduced by electrode and lead and packaging structure. The capacitance test of LED chip refers to junction capacitance. The parasitic capacitance is considered in high-frequency circuit design related to wiring after packaging. The junction capacitance is caused by depletion layer capacitance and diffusion capacitance. Since the I-V characteristic of LED is nonlinear, the junction capacitance test of LED is closely related to bias voltage and frequency.

[0021] The application analyzes the dynamic behavior of PN junction, i.e. minority carrier concentration and lifetime, by measuring the junction capacitance under a certain characteristic operating current, and then evaluates the quantum efficiency and responsivity of LED. The application is applied to important application scenarios of LED, such as display gray scale, linearity control and modulation bandwidth, and can quickly select high-standard LED chips.

[0022] The capacitance test device used in the embodiment of the application is specifically set as follows:

[0023] 1. The capacitance measurement device is an impedance analyzer LCR or a network analyzer with impedance option;

[0024] 2. A DC power supply or a built-in bias power supply of an advanced LCR is used;

[0025] 3. A test fixture, a probe station for COW chip level or a special SMD test fixture;

[0026] 4. The signal loading of two poles of the LED chip is HF+(AC hot end) and HF-(AC cold end) of the capacitance test device;

[0027] and DC Bias+(DC bias+) and DC Bias-(DC bias-) of the capacitance test device;

[0028] 5. Before the test of capacitance, the capacitance test device needs to be calibrated in open circuit, short circuit and load under test conditions.

[0029] Embodiment one: screening of high-quality LED chips for backlight display.

[0030] 1) I-V test is performed on LED chips of the same batch of 75um*150um, the turn-on voltage of the LED chip is measured, recorded as turn-on voltage Vj, and the chips with good uniformity of turn-on voltage are screened out, for example, the chips with turn-on voltage of 2.85±0.2V;

[0031] 2) The junction capacitance of the LED chip at static state is tested by the capacitance test device. Specifically, the junction capacitance value of the LED under the test conditions of zero bias voltage, high-frequency signal frequency of 1MHz and AC signal intensity of 10mVrms-100mVrms is measured, recorded as static junction capacitance Cj1;

[0032] 3) Test the opening junction capacitance of the LED chip by using the capacitance test equipment, specifically, when the bias voltage is the opening voltage Vj=2.85V, the intermediate frequency signal frequency is 10KHz, and the alternating current signal intensity is 1mVrms~50mVrms, ensure that the test is carried out in the linear interval, measure the junction capacitance value of the LED chip, and record it as the opening junction capacitance Cj2;

[0033] 4) Compare the amplitude of the static junction capacitance Cj1 and the amplitude of the opening junction capacitance Cj2 of the same batch of chips, evaluate the LED chip according to whether the amplitude of the static junction capacitance Cj1 and the amplitude of the opening junction capacitance Cj2 meet the opening response capability requirements of the LED chip, and select the chips with smaller and more uniform static junction capacitance Cj1 in 2~3pF and the chips with Cj2 in 10~50nF according to the typical amplitude of Cj1 in 1~5pF.

[0034] According to the above test steps, the dark state junction capacitance Cj1 and the junction opening capacitance Cj2 meet the opening response capability of the LED. Among them, the smaller the Cj1 and Cj2 values of the same batch of LED chips represent that the switching response speed of the chip is faster, and the smaller the Cj1 and Cj2 values represent that the consistency of the chip is better, which can better realize high-speed dimming. The application can evaluate LED chips of different grades with high consistency according to the high and low of the switching speed and low-frequency flicker performance of the device in different application scenarios, which greatly reduces the workload in subsequent applications.

[0035] Example two: screening of LED chips for visible light communication

[0036] 1) I-V test is performed on the same batch of 75um*150um LED chips, the opening voltage of the LED chip is measured, and is recorded as the opening voltage Vj. The chips with good uniformity of opening voltage, such as the chips with opening voltage in 2.85±0.5V, are selected;

[0037] 2) Test the junction capacitance of the LED chip at static state by using the capacitance test equipment, specifically, under the condition of zero bias voltage, intermediate frequency signal frequency of 1MHz, and alternating current signal intensity of 10mVrms~100mVrms, the junction capacitance value of the LED is tested, and is recorded as the static junction capacitance Cj1;

[0038] 3) Test the opening junction capacitance of the LED chip by using the capacitance test equipment, specifically, when the bias voltage is the opening voltage Vj=2.85V, the intermediate frequency signal frequency is 10KHz, and the alternating current signal intensity is 1mVrms~50mVrms, ensure that the test is carried out in the linear interval, measure the junction capacitance value of the LED, and record it as the opening junction capacitance Cj2,

[0039] 4) compare the amplitude of the static junction capacitance Cj1 and the amplitude of the opening junction capacitance Cj2 of the same batch of chips, and evaluate the LED chip according to whether the amplitude of the static junction capacitance Cj1 and the amplitude of the opening junction capacitance Cj2 meet the opening response capability requirements of the LED chip, and for the typical amplitude of Cj1 being 1-5pF, screen out the chips with small and uniform static junction capacitance Cj1 of 2-3pF and the chips with Cj2 of 10-50nF;

[0040] 5) test the dynamic LED junction capacitance of the LED chip under normal working light emission by using a capacitance test device, specifically, when the bias current is 1mA, the frequency signal frequency is 20Hz, and the alternating current signal strength is 1mVrms-50mVrms, ensure that the test is carried out in the linear interval, measure the dynamic junction capacitance value of the LED chip, record it as the dynamic junction capacitance Cj3, evaluate the LED chip according to whether the amplitude of the dynamic junction capacitance Cj3 of the LED chip meets the dynamic response capability requirements of the LED under high-frequency modulation, and screen out efficient LED chips, such as the chips with small and uniform dynamic junction capacitance Cj3 of 100-200nF.

[0041] The present application measures the dark state junction capacitance Cj1, the junction opening capacitance Cj2 and the dynamic junction capacitance Cj3 according to the above test steps, and the smaller the Cj1 and Cj2 values of the same batch of LED chips are, the faster the switching response speed of the chips is, and on this basis, further compare Cj3 to evaluate the LED chip suitable for visible light communication.

[0042] Although the present application has been disclosed as above with preferred embodiments, it is not intended to limit the present application. Any person skilled in the art can make many possible changes and modifications to the technical solutions of the present application, or modify equivalent embodiments with equivalent changes, by using the disclosed methods and technical contents, without departing from the scope of the technical solutions of the present application. Therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present application, without departing from the content of the technical solutions of the present application, still belongs to the scope of protection of the technical solutions of the present application.

Claims

1. A LED chip evaluation method based on dynamic capacitance test, characterized in that, The method comprises the following main steps: 1) I-V test is performed on a plurality of LED chips in the same batch to measure the turn-on voltage of the LED chips, which is recorded as the turn-on voltage Vj; 2) The junction capacitance of the LED chips in a static state is tested by using a capacitance testing device, specifically, the junction capacitance value of the LED is measured under high-frequency signal testing conditions at zero bias voltage, which is recorded as the static junction capacitance Cj1; 3) The turn-on junction capacitance of the LED chips is tested by using the capacitance testing device, specifically, the junction capacitance value of the LED is measured under medium-frequency signal testing conditions at a bias voltage of the turn-on voltage Vj, which is recorded as the turn-on junction capacitance Cj2; 4) The LED chips that meet the turn-on response capability are evaluated according to whether the amplitude of the static junction capacitance Cj1 and the amplitude of the turn-on junction capacitance Cj2 of the LED chips meet the turn-on response capability.

2. The LED chip evaluation method based on dynamic capacitance test according to claim 1, wherein, The dynamic LED junction capacitance of the LED chips that are evaluated is tested by using the capacitance testing device under normal working light-emitting conditions, specifically, the dynamic junction capacitance value of the LED is measured under low-frequency signal testing conditions at a bias current of the characteristic working current, which is recorded as the dynamic junction capacitance Cj3, and the LED chips that meet the dynamic response capability under high-frequency modulation are evaluated according to whether the amplitude of the dynamic junction capacitance Cj3 meets the dynamic response capability of the LED chips under high-frequency modulation.

3. The LED chip evaluation method based on dynamic capacitance test according to claim 1 or 2, characterized in that, The capacitance testing device is an impedance analyzer LCR or a network analyzer with impedance options.

4. The LED chip evaluation method based on dynamic capacitance test according to claim 1, wherein, The frequency range of the high-frequency test in step 2) is 1 MHz-100 MHz.

5. The LED chip evaluation method based on dynamic capacitance test according to claim 1, wherein, The frequency range of the medium-frequency test in step 3) is 1 kHz-100 kHz.

6. The LED chip evaluation method based on dynamic capacitance test according to claim 2, wherein, The frequency range of the low-frequency test is 100 Hz-1 kHz.

7. The LED chip evaluation method based on dynamic capacitance test according to claim 1 or 2, wherein, The capacitance testing device is calibrated before testing, specifically, the capacitance testing device is first set to the same bias voltage or current and the same test frequency as the LED chips to be tested, and then open circuit, short circuit and load calibration are performed.