LUMINESCENCE METHOD FOR THE IN-LINE DETECTION OF ATOMIC SCALE DEFECTS DURING FABRICATION OF 4H-SIC DIODES

Abstract

A method of detecting atomic scale defects in semiconductors, comprising the steps of scanning the surface of the semiconductor with a field emission scanning electron microscope (SEM) to form an SEM image thereof; scanning the SEM image with a light detector and monochromator to obtain a cathodoluminescence (CL) spatial intensity map of the SEM image; determining the CL spectra, i.e. the CL intensity against photon energy for each integral CL intensity; and comparing the CL intensity to a threshold, whereby those semiconductors whose CL intensity is above the threshold are deemed to be defective

Claims

1. A method of detecting atomic scale defects in semiconductors, comprising the steps of scanning the surface of the semiconductor with a field emission scanning electron microscope (SEM) to form an SEM image thereof; scanning the SEM image with a light detector and monochromator to obtain a cathodoluminescence (CL) spatial intensity map of the SEM image; determining the CL spectra, i.e. the CL intensity against photon energy for each integral CL intensity; and comparing the CL intensity to a threshold, whereby those semiconductors whose CL intensity is above the threshold are deemed to be defective.

2. The method of detecting atomic scale defects according to claim 1 wherein the detection occurs on semiconductor wafers in-line during production of semiconductor devices.

3. The method of detecting atomic scale defects according to claim 2 wherein the detection occurs during fabrication of 4H-SiC diodes.

4. The method of detecting atomic scale defects according to claim 3 wherein the defects are the spatial distribution of carbon vacancies in SiC wafers, which indicates unacceptable leakage current for the SiC diodes.

5. The method of detecting atomic scale defects according to claim 4 wherein the field emission scanning electron microscope is a JEOL field emission scanning electron microscope model JSM-7001-F.

6. The method of detecting atomic scale defects according to claim 4 wherein the monochromator is a GATAN MonoCL3.

7. The method of detecting atomic scale defects according to claim 1 wherein the step of determining the CL spectra comprises the step of summing the photon energy for each integral CL intensity for the whole CL spatial mapping of the SEM image.

8. The method of detecting atomic scale defects according to claim 1 wherein the threshold or critical CL intensity for discrimination is obtained from a calibration curve based on the desired maximum value of the leakage current.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] This patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

[0013] The foregoing and other objects and advantages of the present invention will become more apparent when considered in connection with the following detailed description and appended drawings in which like designations denote like elements in the various views, and wherein:

[0014] FIG. 1A shows an SEM picture of a SiC diode having a low leakage current (low defects), FIG. 1B shows a corresponding CL mapping of the SiC diode having a low leakage current, FIG. 1C shows an SEM picture of a SiC diode having a high leakage current (high defects) and FIG. 1D shows a corresponding CL mapping of the SiC diode having a high leakage current;

[0015] FIG. 2 shows graphs of the CL signal intensity versus photon energy (eV) of 10 samples with leakage current Ir from 0.86 ?A to 139.00 ?A;

[0016] FIG. 3A shows electron hole excitation of 4H-SiC by an electron beam of the SEM and FIG. 3B shows the defect emission (DE) result of the transition from the carbon vacancy state (ED) to the valence band EV; and

[0017] FIG. 4 is a graph of the relationship between leakage current and CL intensity on the peak area of different commercial JBS diodes.

DETAILED DESCRIPTION OF THE INVENTION

[0018] In order to provide proof of the basic concept of the present invention, i.e., that CL intensity can be used as a parameter to screen out devices having too high a carbon vacancy and thus too high a leakage current, a study was carried out on ten 4H-SiC junction barrier diodes (JBS). The leakage currents of each of the diodes was measured by IV measurement. Typical SEM images for two of the devices after de-capsulation are shown in FIGS. 1A and 1C, respectively. They have leakage currents of 1.45 ?A (low) and 139 ?A (high), respectively, with a reverse bias of ?1700 V. The p-type region of each JBS diode was created by Al-ion implantation followed by post-implantation annealing at 1800? C. to activate the Al dopant. The implanted dopant region is illustrated by the dotted line in FIGS. 1A and 1C.

[0019] Deep level transient spectroscopy (DLTS) is a probe for detecting electrically active defects having deep level states in the band gap. A DLTS signal called Z1/Z2 located at 0.56 eV below the conduction band was found in all the samples. This deep trap is usually associated with the carbon vacancy. See. Son, N. T., et al., Negative-U system of carbon vacancy in 4H-SiC, Phys Rev Lett, 2012. 109(18): p. 187603; and Carbon vacancy control in p+-n silicon carbide diodes for high voltage bipolar applications, Journal of Physics D: Applied Physics, 2021. 54(45).

[0020] The commercial JBS diodes were de-capsulated by wet chemical etching. The sample was etched by immersing in H.sub.2SO.sub.4/H.sub.2O.sub.2 for 10 minutes and HF for 10 minutes. After removing the surface electrodes, the sample was cleaned with deionized (DI) water. The sample was then scanned with the JEOL field emission scanning electron microscope (SEM) JSM-7001-F and a cathodoluminescence (CL) study was carried out with the attached monochromator GATAN MonoCL3. FIG. 1B is the corresponding cathodoluminescence (CL) spatial intensity mapping of the SEM image of the device in FIG. 1A, whereas FIG. 1D is the CL spatial intensity mapping of FIG. 1C. The images clearly show that the implanted region correlates with the region having high luminescence intensity in each sample.

[0021] To obtain the CL spectra (i.e., the CL intensity against photon energy), the photon energy for each integral CL intensity was summed up for the whole CL spatial mapping. The obtained CL spectra of the ten samples are shown in FIG. 2 with their corresponding leakage current. All of these spectra are characterized by a broad defect emission (DE) peaking at ?2.62 eV.

[0022] The origin of this DE is shown in the band diagram of FIG. 3A. FIG. 3A shows the electron-hole excitation by the electron beam of the SEM. Defect emission peaking at 2.62 eV can be understood as the electron transition from the defect state E.sub.D to the valance band E.sub.V with emission of a photon (FIG. 3B). As the photon energy is 2.62 eV and the band gap E.sub.g of 4H-SiC is 3.26 eV, E.sub.D is located at 0.64 eV, which coincides well with the VC energy state position as revealed by DLTS. The DE seen in the CL spectrum is thus associated with the carbon vacancy.

[0023] FIG. 4 shows the CL intensity of the DE against the log of the leakage current for the 10 samples, which demonstrates the positive correlation between the CL intensity and the leakage current. This implies that the leakage current increases with increasing carbon vacancy concentration, revealing that the leakage current is dominantly associated with defect assisted tunnelling. See, Grandidier, B., et al., Defect-assisted tunneling current: A revised interpretation of scanning tunneling spectroscopy measurements, Applied Physics Letters, 2000. 76(21): p. 3142-3144; and Mandurrino, M., et al., Physics-based modeling and experimental implications of trap-assisted tunneling in InGaN/GaN light-emitting diodes, Physica Status Solidi (a), 2015. 212(5): p. 947-953. The integrated DE intensity can thus serve as a performance merit parameter, and it can be used as the monitoring parameter to discriminate devices that will result in poor performance from the production line before the entire fabrication process is complete. The critical CL intensity for discrimination can be obtained from a calibration curve with the desired value of the leakage current.

[0024] The above is only the specific implementation mode of the invention and not intended to limit the scope of protection of the invention. Any modifications or substitutes apparent to those skilled in the art shall fall within the scope of protection of the invention. Therefore, the protected scope of the invention shall be subject to the scope of protection of the claims.

[0025] While the invention is explained in relation to certain embodiments, it is to be understood that various modifications thereof will become apparent to those skilled in the art upon reading the specification. Therefore, it is to be understood that the invention disclosed herein is intended to cover such modifications as fall within the scope of the appended claims.