Surface potential measurement device

Abstract

The present invention has: a measurement probe (4) that is disposed in contact with a plurality of conductive field plates (13) provided along the surface of an element (1) to be measured; a dummy electrode (5) that is electrically connected to the measurement probe (4) via an electric wire (6); and a surface electrometer (8) that measures the surface potential of the dummy electrode (5) by means of a probe (7). A sealing material (2) is accommodated in a container (3) in which the element (1) to be measured is accommodated, so as to cover the element (1) to be measured and the tip portion of the measurement probe (4).

Description

Surface potential measuring device

[0001] The present disclosure relates to a surface potential measuring device.

[0002] In a surface potential measuring device, a detection electrode in a probe used in the device receives an electrostatic field strength from the object to be measured. The electrostatic field strength periodically changes due to the vibration of a vibrating electrode in the probe, causing a displacement current to flow from the detection electrode. By measuring this displacement current, the surface potential of the object to be measured can be obtained. Conventional surface potential measuring devices require miniaturization in order to measure surface potential with high spatial resolution. However, miniaturization of the device reduces the output of the displacement current, which presents a problem of reduced measurement accuracy.

[0003] Patent Document 1 discloses a method in which a minute conductive measurement probe with an insulating material attached is brought into contact with the object to be measured, the measurement probe and a dummy electrode are electrically connected, and the surface potential appearing on the dummy electrode is measured with the probe of a surface potential measuring device. This makes it possible to measure the surface potential of the object to be measured with high spatial resolution without having to miniaturize the device itself (see Patent Document 1).

[0004] JP 6-289083

[0005] According to the surface potential measuring device disclosed in Patent Document 1, a potential corresponding to the capacitance between the object to be measured and the measurement probe, the capacitance between the probe of the surface potential measuring device and the dummy electrode, and the surface potential of the object to be measured is generated in the dummy electrode. This potential is then measured by the probe of the surface potential measuring device. In this case, if the surface potential of the object to be measured is high, a high electric field is generated between the object to be measured and the measurement probe, or between the probe of the surface potential measuring device and the dummy electrode. These locations are exposed to the air. Therefore, if a high electric field is generated in these locations, aerial discharge may occur.

[0006] For example, if the object to be measured is a structure having a microstructure such as a power semiconductor element and to which a high voltage is applied, the surface potential measuring device shown in Patent Document 1 will generate air discharge and will not be able to measure the surface potential.

[0007] The present disclosure discloses a technique for solving the above-mentioned problems, and aims to provide a surface potential measuring device that measures the surface potential of a power semiconductor element to which a high voltage is applied without generating aerial discharge and with high spatial resolution.

[0008] A surface potential measuring device disclosed herein includes a measurement probe arranged in contact with a plurality of conductive field plates provided along the surface of a device under test, a dummy electrode electrically connected to the measurement probe, and a surface potential meter that measures the surface potential of the dummy electrode with the probe, and is provided with a sealing material that covers the device under test and the tip portion of the measurement probe. Another surface potential measuring device disclosed herein includes electric wires that are directly connected to a plurality of conductive field plates provided along the surface of the device under test and that connect the field plates and the dummy electrodes, and a surface potential meter that measures the surface potential of the dummy electrode with the probe, and is provided with a sealing material that covers the device under test and the tip portion of the electric wire.

[0009] According to the surface potential measuring device of the present disclosure, the surface potential of a power semiconductor element to which a high voltage is applied can be measured with high spatial resolution without generating air discharge.

[0010] Fig. 4A is a schematic cross-sectional view showing the configuration of a surface potential measuring device for a power semiconductor element according to embodiment 1. Fig. 4B is a schematic cross-sectional view showing the configuration of a surface potential measuring device for a power semiconductor element according to embodiment 1. Fig. 4C is a schematic cross-sectional view showing the configuration of a surface potential measuring device for a power semiconductor element according to embodiment 2. Fig. 4A, Fig. 4B and Fig. 4C are schematic cross-sectional views showing the joining process of electric wires in the surface potential measuring device for a power semiconductor element according to embodiment 2. Fig. 4C is a schematic cross-sectional view showing the configuration of a surface potential measuring device for a power semiconductor element according to embodiment 3. Fig. 4B is a schematic cross-sectional view showing the configuration of a surface potential measuring device for a power semiconductor element according to embodiment 3.

[0011] Embodiment 1. This embodiment relates to a surface potential measuring device that measures the surface potential of a power semiconductor element with high spatial resolution. A surface potential measuring device 1000 for a power semiconductor element according to embodiment 1 will be described below with reference to the drawings. Fig. 1 is a schematic cross-sectional view showing the configuration of the surface potential measuring device for a power semiconductor element according to this embodiment.

[0012] As shown in Fig. 1, surface potential measuring apparatus 1000 for a power semiconductor element (element under test) according to this embodiment includes power semiconductor element 1, which is the object to be measured, sealing material 2, container 3, measurement probe 4, dummy electrode 5, electric wire 6, probe 7 of the surface potential measuring apparatus, surface electrometer 8, and power supply 9. As shown in Fig. 2, power semiconductor element 1 includes upper electrode 11 on the front surface of semiconductor wafer 10 and lower electrode 12 on the back surface, with a plurality of conductive field plates 13 mounted at intervals from the end of upper electrode 11 along the surface. An oxide film 14 is provided between two adjacent field plates 13.

[0013] The power semiconductor element 1 may be, for example, an insulated gate bipolar transistor (IGBT), a diode, or a reverse conducting IGBT made of silicon (Si). Alternatively, the power semiconductor element 1 may be a metal oxide semiconductor field effect transistor (MOSFET) or a Schottky diode made of a material with a larger band gap than Si, such as silicon carbide (SiC) or gallium nitride (GaN).

[0014] The upper electrode 11 and the lower electrode 12 are made of aluminum, the field plate 13 is made of aluminum or polysilicon, and the oxide film 14 is made of silicon dioxide. The encapsulant 2 is filled into the container 3 so as to cover the power semiconductor element 1 housed in the container 3 and the tip portion of the measurement probe 4 that is placed in contact with the field plate 13 of the power semiconductor element 1. Specifically, the encapsulant 2 is a low-elasticity solid or liquid insulating material such as silicone gel, Fluorinert, or silicone oil.

[0015] The container 3 is made of an insulating material, and a material with a heat resistance higher than the temperature of the measurement environment is selected. The measurement probe 4 is needle-shaped, and its tip is placed in contact with the field plate 13 of the power semiconductor element 1. The tip diameter of the measurement probe 4 is smaller than the width of the field plate 13, and the diameter is preferably 30 μm or less.

[0016] The dummy electrode 5 is formed in a plate shape, and the area of ​​the dummy electrode 5 is sufficiently larger than the measurement region of the probe 7 of the surface potential measuring device. The measurement probe 4 and the dummy electrode 5 are electrically connected via an electric wire 6, and the field plate 13 in contact with the measurement probe 4 and the dummy electrode 5 are at the same potential. The measurement probe 4, dummy electrode 5, and electric wire 6 are made of a metal material with excellent conductivity, such as aluminum, copper, tungsten, or platinum.

[0017] The probe 7 of the surface electrometer 8 is placed directly above the dummy electrode 5, and the electrostatic field caused by the surface potential of the dummy electrode 5 is detected by the probe 7 of the surface electrometer 8, and the surface potential of the dummy electrode 5 is output from the detected electrostatic field to the surface electrometer 8. The power supply 9 outputs a DC voltage or a square wave voltage, which is supplied to the lower electrode 12 of the power semiconductor element 1. The upper electrode 11 of the power semiconductor element 1 is at ground potential.

[0018] Next, a description will be given of the effects of surface potential measuring apparatus 1000 for power semiconductor element 1 according to embodiment 1. In surface potential measuring apparatus 1000 for power semiconductor element 1 according to embodiment 1, measurement probe 4 and field plate 13 are in contact with each other, and the potential difference between measurement probe 4 and field plate 13 is zero. Therefore, the electric field applied between measurement probe 4 and field plate 13 is zero.

[0019] Furthermore, the power semiconductor element 1 and the tip of the measurement probe 4 are covered with an insulating sealing material 2. Therefore, even if a high voltage is supplied from the power supply 9 to the power semiconductor element 1 having a microstructure and a high potential is generated on the surface of the power semiconductor element 1, it is possible to suppress air discharge between the measurement probe 4 and the field plate 13. In this way, it is possible to measure the surface potential with high spatial resolution without generating air discharge.

[0020] Although the dummy electrode 5 and the probe 7 of the surface electrometer 8 are exposed to the air, their areas are sufficiently larger than those of the field plate 13 and the measurement probe 4, and they are also flat in shape, so a high electric field is not generated between the dummy electrode 5 and the probe 7 of the surface electrometer 8, and no air discharge occurs.

[0021] Furthermore, in the surface potential measuring device 1000 for the power semiconductor element 1 according to the first embodiment, the tip diameter of the measurement probe 4 is smaller than the width of the field plate 13, so that distortion of the potential distribution on the surface of the power semiconductor element 1 caused by bringing the measurement probe 4 close to the power semiconductor element 1 can be suppressed, and the surface potential of the power semiconductor element 1 can be accurately measured.

[0022] Furthermore, in surface potential measuring apparatus 1000 for power semiconductor element 1 according to embodiment 1, a low-elasticity solid or liquid insulating material such as silicone gel, Fluorinert, or silicone oil is used as encapsulant 2. Therefore, it is possible to change the position of measurement probe 4 within encapsulant 2, and by changing the position of measurement probe 4, it is possible to arbitrarily measure the surface potentials of multiple field plates 13 for one power semiconductor element 1 sample.

[0023] Embodiment 2. Figure 3 is a schematic cross-sectional view showing the configuration of a surface potential measuring device for a power semiconductor element according to embodiment 2. In embodiment 1, the configuration in which the measurement probe 4 and the dummy electrode 5 are connected via the electric wire 6 has been described, but in surface potential measuring device 1001 according to embodiment 2, as shown in Figure 3, there is no measurement probe 4, and the field plate 13 and the dummy electrode 5 are directly connected via the electric wire 6. The configuration is otherwise the same as in embodiment 1.

[0024] The electric wire 6 is ultrasonically bonded to the field plate 13. FIGS. 4A to 4C are schematic cross-sectional views showing the electric wire bonding process in the surface potential measuring device 1001 for a power semiconductor element according to the second embodiment. As shown in FIGS. 4A to 4C, the electric wire 6 is bonded to the field plate 13 using the bonding head 15 of the ultrasonic bonding device. That is, first, as shown in FIG. 4A, the electric wire 6 is ejected from the nozzle, and the tip is melted by sparks from the bonding head 15 to form a ball 6A. Next, as shown in FIG. 4B, the ball 6A and the electric wire 6 are heated and pressure-bonded to the field plate 13 while applying ultrasonic waves. Finally, as shown in FIG. 4C, the bonding head 15 is raised. The diameter of the electric wire 6 is smaller than the width of the field plate 13, and is preferably 30 μm or less.

[0025] Next, the effects of the surface potential measuring device 1001 for power semiconductor elements according to this embodiment will be described. In the surface potential measuring device 1001 for power semiconductor elements according to this embodiment, the measurement probe 4 is not used, and the electric wire 6 and the field plate 13 are directly connected by ultrasonic bonding. This provides high adhesive strength, preventing the electric wire 6 and the field plate 13 from losing contact due to external disturbances such as vibration. Furthermore, when the measurement probe 4 is used between the electric wire 6 and the field plate 13, there is a possibility that the contact between the measurement probe 4 and the field plate 13 may be lost due to external disturbances such as vibration. When this occurs, a voltage drop occurs between the measurement probe 4 and the field plate 13, causing the potential of the dummy electrode 5 to differ from the potential of the field plate 13, making it impossible to accurately measure the surface potential of the power semiconductor element 1. In contrast, by directly connecting the electric wire 6 and the field plate 13 by ultrasonic bonding, loss of contact can be prevented, and the surface potential of the power semiconductor element 1 can be accurately measured even when external disturbances occur.

[0026] In this embodiment, since the electric wire 6 is bonded to the field plate 13, the position of the electric wire 6 cannot be changed within the encapsulant 2. Therefore, when measuring the surface potential of the field plate 13 at a different position, it is necessary to prepare a different sample in which the bonding position of the electric wire 6 is changed.

[0027] 5 and 6 are schematic cross-sectional views showing the configuration of a surface potential measuring device for a power semiconductor element according to a third embodiment. In a surface potential measuring device 1002 for a power semiconductor element according to this embodiment, as shown in FIG. 5, a dummy electrode 5 is disposed on top of a container 3, and a field plate 13 and the dummy electrode 5 are connected via a measurement probe 4. The measurement probe 4 is integrated with the dummy electrode 5. The other configurations are the same as those of the first and second embodiments.

[0028] Since the dummy electrode 5 is not fixed to the container 3, the position of the measurement probe 4 can be changed within the sealing material 2 by moving the dummy electrode 5 integrated with the measurement probe 4. In the surface potential measuring device 1003 of FIG. 6, the field plate 13 and the dummy electrode 5 are connected via an electric wire 6. The effect of the surface potential measuring devices 1002 and 1003 for power semiconductor elements according to this embodiment is that by arranging the dummy electrode 5 above the container 3, the measurement space region can be made smaller than in the first and second embodiments.

[0029] Although various exemplary embodiments and examples are described in this application, the various features, aspects, and functions described in one or more embodiments are not limited to the application of a particular embodiment, but may be applied to the embodiments alone or in various combinations. Therefore, countless variations not illustrated are contemplated within the scope of the technology disclosed in this specification. For example, this includes cases where at least one component is modified, added, or omitted, or where at least one component is extracted and combined with components of another embodiment.

[0030] REFERENCE SIGNS LIST 1 Power semiconductor element, 2 Sealing material, 3 Container, 4 Measurement probe, 5 Dummy electrode, 6 Electric wire, 7 Probe, 8 Surface electrometer, 9 Power supply, 10 Semiconductor wafer, 11 Upper electrode, 12 Lower electrode, 13 Field plate, 14 Oxide film, 15 Bonding head

Claims

1. A surface potential measuring device comprising: a measurement probe arranged in contact with a plurality of conductive field plates provided along the surface of a device under test; a dummy electrode electrically connected to said measurement probe; and a surface potential meter that measures the surface potential of said dummy electrode using the probe; and a sealing material that covers said device under test and the tip of said measurement probe.

2. The surface potential measuring device according to claim 1, wherein the measuring probe and the dummy electrode are electrically connected via an electric wire.

3. A surface potential measuring device according to claim 1, wherein the measuring probe is directly attached to the dummy electrode, and the dummy electrode is disposed above a container that houses the sealing material and the device under test.

4. A surface potential measuring device comprising: electric wires that are directly connected to a plurality of conductive field plates provided along the surface of a device under test, and that connect the field plates to dummy electrodes; a surface potential meter that measures the surface potential of the dummy electrodes using a probe; and a sealing material that covers the device under test and the tip of the electric wires.

5. The surface potential measuring device according to claim 4, wherein the dummy electrode is disposed above a container that houses the sealing material and the device to be measured.

6. A surface potential measuring device according to any one of claims 1 to 3, wherein the tip diameter of the measurement probe is smaller than the width of the field plate, and the diameter is 30 μm or less.

7. A surface potential measuring device according to claim 4 or 5, wherein the diameter of the electric wire is smaller than the width of the field plate, and the diameter is 30 μm or less.

8. A surface potential measuring device according to any one of claims 1 to 7, wherein the sealing material is a low-elasticity solid or liquid insulating material such as silicone gel, Fluorinert, or silicone oil.

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