Power device

Abstract

To provide a power device capable of reducing unevenness of a cut surface of a semiconductor material or a substrate used for the power device, capable of reducing residual stress, and excellent in reliability, in scribe processing using the semiconductor material or a brittle material of the substrate used for the power device as a workpiece.SOLUTION: A scribing process is performed on a cutting edge of a single crystal diamond tool 5 mounted on a tip of a cone of a high-rigidity vibration-assisted holder in which a Langevin-type piezoelectric element 3, a horn 4, and the cone are fixed by an outer cylinder, by amplifying vibration-assisted vibration. To provide a power device using a semiconductor material or a substrate scribed by a vibration-assisted scribing device equipped with a vibration-assisted scriber unit for promoting extension of a microcrack in a vertical direction.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a mechanical processing method for cleaving workpieces, such as various substrates and semiconductor materials used in power devices. Conventionally, a carbide wheel is pressed against the surface of the workpiece, rolling to create a scribe line, causing a crack, and then increasing the indentation load to propagate the secondary crack. This method requires a large load and generates distortion in the substrate due to residual stress during scribing and cleaving, which can damage the workpiece and cause quality issues. Vibration-assisted vibrations are applied to the tool tip or the workpiece suction table, minimizing the load when drawing the scribe line and promoting the propagation of vertical cracks. This significantly reduces the cleaving load, enabling damage-free separation. This results in minimal unevenness on the cut surface of workpieces, such as various substrates and semiconductor materials used in power devices, and no residual stress, thereby improving the reliability of power devices using these materials. [Background technology]

[0002] Substrates used in power devices (DBC, direct-bonded copper) are fabricated by directly bonding a copper plate to a ceramic plate, forming a resist using photolithography, and then chemically etching to create electrode patterns. In the chemical etching method, chemical etching is performed using an etchant such as ferric chloride through the gaps between the patterns formed by photolithography. In addition to the gaps between the patterns formed by photolithography, the copper pattern width is formed to approximately twice the thickness of the copper. This makes it difficult to narrow the spacing between the copper patterns. However, if thick copper patterns can be formed in the desired shape, the degree of freedom in pattern design for power devices will increase. If highly productive screen printing can be applied, it is possible to use multiple printing masks to customize the desired copper film thickness and opening width for each pattern. This is expected to lead to unique electrode structure designs and reduced mass production costs. On the other hand, conventional laser scribing methods have the risk of molten material (spatter) scattering from the laser irradiated area and adhering to the pattern. Furthermore, thermal diffusion by the laser generates thermal stress between the ceramic and copper electrodes due to the difference in their thermal expansion coefficients. It is expected that these problems can be reduced by adopting the vibration-assisted diamond scribing method, which does not require the application of heat. The cutting method for glass and hard, brittle ceramic substrates for power semiconductors, as well as various compound semiconductors, is the so-called mechanical scribing method, in which a carbide wheel is applied with a load to the surface of the workpiece, drawing a line, and the stress of the scribed line on the surface causes cracks in the workpiece in both horizontal and vertical directions, dividing it. Alternatively, there is the so-called dicing method, in which a thin, doughnut-shaped sintered diamond grinding wheel called a blade (peripheral cutting blade) is rotated at high speed and cut while spraying water, and there is also the laser scribing method, in which a laser such as the aforementioned carbon dioxide laser is aimed at the workpiece, cutting it by continuously drilling tiny holes. However, both methods have problems that impair the quality of the substrate from which the workpiece is cut. Conventional mechanical scribing methods apply a load of several newtons to cleave the substrate, leaving residual stress damage in the substrate and causing defects in the circuitry on the substrate. Dicing generates kerf loss, equivalent to the thickness of the peripheral cutting edge. Cutting while spraying water creates environmental issues, such as wastewater treatment. Furthermore, spraying water can affect the electrical circuits on the substrate. Laser scribing methods suffer from the problem of low yields, as the high-temperature laser causes molten material to adhere to the substrate and nearby electrodes, which cannot be removed even by washing. This can lead to defects being discovered at the chip packaging stage. Therefore, the vibration-assisted scribing device proposed here offers a solution to these problems, and there is great hope for its practical application. Summary of the Invention [Problem to be solved by the invention]

[0003] The problem that this invention aims to solve is the large unevenness and residual stress on the cut surface of the workpiece used in power devices. The semiconductor material used in power devices is a brittle material, and the cutting process increases the unevenness of the cut surface due to chipping and other factors. Furthermore, the cutting process leaves residual stress, especially tensile stress, which reduces the lifespan of the semiconductor material. Furthermore, the substrate on which the semiconductor material is mounted contains wiring circuits for supplying and routing power, signals, etc., and the difference in thermal expansion coefficients between the substrate material and the circuit material creates unevenness and warping, resulting in residual stress due to thermal stress. Therefore, depending on the cutting method, the cutting process can increase residual stress, potentially causing damage. Furthermore, the unevenness of the cut surface may reduce the positioning accuracy of the power device during assembly. [Means for solving the problem]

[0004] The present invention has been made to solve the above-mentioned problems, and by using a vibration-assisted scribing device consisting of a unit made up of a transmitter, a Langevin-type piezoelectric element, a horn, a load cell, a sensor for a contact-type surface profile scanner, a single-crystal diamond with a shaft, and a single-crystal diamond wheel, and a control system, semiconductor materials and substrates used in power devices can be scribed, reducing unevenness on the cut surface and allowing them to be used in a state where no residual stress remains. The vibration-assisted scribing device is equipped with a vibration-assisted scriber unit that amplifies vibrations generated by a Langevin type piezoelectric element using a horn and then amplifies the vibrations to a single crystal diamond tool attached to the tip of the horn. The above problem can be solved by using this vibration-assisted scribing device. That is, in the process of scribing workpieces such as brittle materials, a vibration-assisted scribing device equipped with a vibration-assisted scriber tool unit is used, which applies vertical vibration to a single crystal diamond tool to scribe, thereby making it possible to generate deeper vertical cracks required for dividing the workpiece. Its unique feature is that it applies vertical vibrations to a single crystal diamond or single crystal diamond wheel with an attached shaft and a tool tip shaped like a cone or pyramid, and marks straight or quadratic curved lines on the surface of workpieces such as glass, ceramic substrates, and compound semiconductor materials (various wafers, various substrates, electrode-formed products), thereby promoting the extension of microcracks in the vertical direction. The glass to be processed can be any of quartz glass, tempered glass, soda glass, and glass whose main component is silicon oxide. The ceramic substrate used as the workpiece can be any of the following: alumina substrate, sapphire (single crystal alumina) substrate, alumina-zirconia substrate, silicon nitride substrate, aluminum nitride substrate, and gallium arsenide single crystal substrate. Electrodes (circuit wiring) for mounting semiconductor materials are formed on these ceramic substrates. Furthermore, the semiconductor material to be processed can be any of silicon wafers, gallium arsenide wafers, gallium nitride wafers, gallium oxide wafers, aluminum nitride wafers, and silicon carbide wafers. Circuits for operating as power devices are formed on these semiconductor materials. Some workpieces processed with vibration-assisted scribing equipment have uneven surfaces that are not flat. In order to scribe on workpieces with uneven surfaces, the equipment is equipped with a load cell to detect the alignment function of the workpiece and the position of the workpiece surface, detects the position where a load is applied when the tip of the tool comes into contact with the workpiece, and controls the Z-axis position by tracking the indentation load to a constant load. Any of the glass, ceramic substrates, and semiconductor materials processed using the vibration-assisted diamond scribing device can be used in power devices. For glass, which is a workpiece processed using a vibration-assisted scribing device, it is important that the unevenness of the cut surface is less than 0.1 mm; for use in power devices, even less unevenness is preferable, with 0.05 mm or less being desirable. Furthermore, for ceramic substrates, which are workpieces processed using a vibration-assisted diamond scribing device, it is important that the unevenness of the cut surface is less than 0.2 mm; for use in power devices, even less unevenness is preferable, with 0.05 mm or less being desirable. Furthermore, for semiconductor materials, which are workpieces processed using a vibration-assisted diamond scribing device, it is important that the unevenness of the cut surface is less than 0.05 mm; for use in power devices, even less unevenness is preferable, with 0.02 mm or less being desirable. [Effects of the Invention]

[0005] The vibration-assisted scribing device of the present invention can extend vertical microcracks that affect post-scribing separation deeper, to approximately 70% of the plate thickness, compared to scribing using a diamond tool that does not employ vibration. Furthermore, the unevenness of the cut surface can be significantly reduced compared to conventional processing methods, leaving very little residual stress. [Brief explanation of the drawings]

[0006] [Figure 1] Schematic diagram of vibration-assisted diamond scribing device [Figure 2] Comparison of processing conditions due to differences in scribing methods DETAILED DESCRIPTION OF THE INVENTION

[0007] As shown in the figure, the scribing head 7 of this embodiment is composed of a transmitter 1, a cable 2, a Langevin-type piezoelectric element 3, a horn 4, a single-crystal diamond tool 5, and a load cell 6. The transmitter 1 applies a voltage to the Langevin-type piezoelectric element 3, generating vibration-assisted vibration. The generated vibration-assisted vibration is amplified by the horn 4, which is connected to the Langevin-type piezoelectric element 3, and transmitted to the single-crystal diamond tool 5 attached to the tip of the horn. The tip of the single-crystal diamond tool 5, to which the vibration-assisted vibration has been applied, is brought into contact with the top surface of a workpiece 9 fixed to a suction table 8, and while applying vibration-assisted vibration in the vertical direction, the tool is pressed against the uneven surface of the workpiece 9. The load cell 6 detects the load, and while performing feedback control of the Z axis, the single-crystal diamond tool 5 is moved horizontally to perform scribing in the scribing direction 10. [Example]

[0008] As shown in Figure 1, after the workpiece 8 is fixed to the suction table 7, the tip of the single crystal diamond tool 5 is lowered to the position of the top surface of the workpiece 8. The Z coordinate of the uneven surface at the separation position is scanned by a non-contact sensor, and the amount of engraving is set from that position, and the scriber head 7 is automatically programmed to control the engraving amount and movement in the X and Y directions. The amount of engraving in the Z direction and the movement speed in the X and Y directions are determined depending on the material of the workpiece 9. When scribing workpieces 9 that have little unevenness, such as glass (quartz glass, tempered glass, soda glass, glass whose main component is silicon oxide, etc.), ceramic substrates (sapphire (single crystal alumina) substrates, gallium arsenide single crystal substrates, etc.), and semiconductor materials (silicon wafers, gallium arsenide wafers, gallium nitride wafers, gallium oxide wafers, aluminum nitride wafers, silicon carbide wafers, etc.), the flatness of the suction table 8 that secures the workpiece 9 is important. If the equipment has sufficient parallelism, scribing is possible by setting the engraving amount (position) in the Z direction. On the other hand, sintered substrates such as ceramic substrates (alumina substrates, alumina-zirconia substrates, silicon nitride substrates, aluminum nitride substrates, silicon carbide substrates, etc.) used as the workpiece 9 have surface irregularities and warpage due to firing shrinkage during the substrate manufacturing process. Furthermore, when electrodes for mounting semiconductor materials and power / signal circuit wiring are formed on the ceramic substrate, warpage occurs due to the difference in thermal expansion coefficients between the ceramic substrate and the electrode material, so it is important to follow the irregularities in the Z direction. The system must be equipped with a load cell 6 for detecting the position of the irregularities on the surface of the workpiece 9, a single-crystal diamond tool 5 that contacts the workpiece 9, detects the position where the load is applied, and has a Z-axis position control function that tracks the indentation load so that a constant load is applied. By providing this Z-axis position control function, it is possible to scribe workpiece 9 with few irregularities even when the suction table 8 that fixes workpiece 9 is not sufficiently flat. The results of scribing various workpieces using the vibration-assisted scribing device used in the power device of the present invention are shown below. The workpiece 9 was a 0.2 mm thick glass plate, which was scribed using the vibration-assisted scribing device used in the present invention. The scribing conditions were a Z-direction engraving depth of 0.005 mm and a movement speed of 150 mm per second in the X and Y directions. Slight traces of the single-crystal diamond tool 5 were observed at the scribed area. Flaw detection ink was applied to the scribed area, and the progression of microcracks was observed after the cutting process. As a result, it was found that microcracks had developed to a depth of 0.14 mm, which corresponds to approximately 70% of the thickness of the glass plate. Shape measurement of the irregularities on the cut surface using a laser microscope revealed irregularities up to 0.035 mm. Furthermore, analysis of the Raman spectrum obtained from the cut surface by Raman spectroscopy revealed that no residual stress remained. With the conventional scribing method using a diamond wheel, the unevenness of the cut surface was large, about 0.1 mm, and analysis of the Raman spectrum obtained from the cut surface using Raman spectroscopy revealed that tensile stress remained. If tensile stress remains, microcracks may develop when stress is applied to the workpiece, leading to breakage. In other words, it was found that scribing using a vibration-assisted scribing device is effective and excellent in improving the reliability of power devices. The workpiece 9 was a 0.38 mm thick alumina substrate, which was scribed using the vibration-assisted scribing device used in the present invention. The scribing conditions were a Z-direction engraving depth of 0.015 mm and a movement speed of 150 mm per second in the X and Y directions. Slight traces of the single-crystal diamond tool 5 were observed at the scribed area. Flaw detection ink was applied to the scribed area, and the progression of microcracks was observed after the cutting process. As a result, it was found that microcracks had developed to a depth of 0.27 mm, which corresponds to approximately 70% of the thickness of the alumina substrate. Shape measurement of the cut surface irregularities using a laser microscope revealed irregularities up to 0.045 mm. Furthermore, analysis of the cut surface using Raman spectroscopy revealed that no residual stress remained. In the conventional scribing method using a diamond wheel, the unevenness of the cut surface, including the scratches made by the diamond wheel, was large, about 0.2 mm, and analysis of the Raman spectrum obtained from the cut surface using Raman spectroscopy revealed that tensile stress remained. If tensile stress remains, microcracks may develop when stress is applied to the workpiece, leading to breakage. In other words, it was found that scribing using a vibration-assisted scribing device is effective and excellent in improving the reliability of power devices. The workpiece 9 was a 0.3 mm thick silicon carbide wafer, and scribing was performed using the vibration-assisted scribing device used in the present invention. The scribing conditions were a Z-direction engraving depth of 0.002 mm and a movement speed in the XY directions of 150 mm per second. Slight traces of the single-crystal diamond tool 5 were observed at the scribed area. Flaw detection ink was applied to the scribed area, and the progression of microcracks was observed after the cutting process. As a result, it was found that microcracks had developed to a depth of 0.21 mm, which corresponds to approximately 70% of the thickness of the silicon carbide wafer. Shape measurement of the irregularities on the cut surface using a laser microscope revealed irregularities up to 0.01 mm. Furthermore, analysis of the Raman spectrum obtained by Raman spectroscopy on the cut surface revealed that no residual stress remained. In the conventional scribing method using a diamond wheel, the unevenness of the cut surface, including the scratches made by the diamond wheel, was large, about 0.2 mm, and analysis of the Raman spectrum obtained from the cut surface using Raman spectroscopy revealed that tensile stress remained. If tensile stress remains, microcracks may develop when stress is applied to the workpiece, leading to breakage. In other words, it was found that scribing using a vibration-assisted scribing device is effective and excellent in improving the reliability of power devices. The workpiece 9 was a 0.38 mm thick alumina substrate with copper electrodes formed on both sides. The substrate was scribed using the vibration-assisted scribing device used in the present invention. The copper electrodes caused a maximum warpage of approximately 2 mm, and fixation using the suction table 8 alone was not sufficient to achieve a flat surface. The scribing conditions were a Z-direction depth of 0.015 mm and an XY-direction movement speed of 150 mm / s. The scribed area was formed in the center between the electrodes using an image processing-based alignment mechanism. The impact marks made by the single-crystal diamond tool 5 were approximately the same size across the entire alumina substrate, and slight scribe marks were observed. Flaw detection ink was applied to the scribed area, and the progression of microcracks was observed after the cutting process. Microcracks were found to have developed to a depth of 0.27 mm, equivalent to approximately 70% of the thickness of the alumina substrate. Shape measurement of the cut surface using a laser microscope revealed irregularities of up to 0.05 mm. Furthermore, analysis of the cut surface using Raman spectroscopy revealed that no residual stress remained. With the conventional scribing method using a diamond wheel, the crack could not be propagated in one direction due to the influence of the heat treatment used to form the copper electrode, and good scribing could not be achieved. It was found that the scribing process using the vibration-assisted scribing device was extremely excellent. Furthermore, analysis of the Raman spectrum obtained by Raman spectroscopy on compound semiconductors scribed using the vibration-assisted scribing device used in this invention confirmed the absence of residual stress. Furthermore, although very slight compressive stress was observed in the scribed area, no tensile stress was observed, providing effective results for devices. Comparative Example: Figure 3 shows a comparative example of a scribing method for an alumina substrate. This example was compared with the laser scribing method, which has traditionally been the most widely used scribing method. The laser scribing method irradiates an alumina substrate with pulsed laser light, creating holes in the substrate like perforations. This melts the alumina where the laser light is irradiated, creating a raised shape in the laser-irradiated area, with some of the molten alumina scattering to the surrounding area. If this scattering (molten alumina) adheres to the electrode, it can interfere with the mounting of semiconductor materials, etc., and cause defects. Furthermore, because the laser-irradiated area is heavily gouged out, the cut surface will be very uneven. Figure 3 shows the results of scribing an alumina substrate with a thickness of 0.38 mm using the vibration-assisted scribing device of the present invention. The conditions for the vibration-assisted diamond scribing method were a digging depth of 0.01 mm in the Z direction and a movement speed of 150 mm per second in the X and Y directions. The load required to cut the sample scribed with the vibration-assisted scribing device of the present invention was approximately 0.1 N per mm of width. After cutting the sample scribed with the vibration-assisted diamond scribe, the shape of the vibration-assisted diamond scribe surface and the cut surface were measured. In the surface photograph after scribing, a mark made by the single crystal diamond tool 5 was formed in the vibration-assisted diamond scribe area in the form of a thin line approximately 0.01 mm wide in both length and width. In the surface photograph after cutting, it can be seen that the cut was made in an almost straight line. In the photograph of the cut surface after cutting, when the shape of the cut surface was measured after cutting the sample, it was found that microcracks had progressed to a depth of approximately two-thirds of the plate thickness. In addition, the unevenness of the cut surface was small, with a maximum of approximately 0.038 mm in the photographed area, which was very good. Figure 3 shows the results of an experiment using the laser irradiation scribing method on an alumina substrate with a thickness of 0.38 mm, with a laser output of 40 W, a laser head movement speed of 100 mm per second, and a laser irradiation pitch of 0.14 mm. After laser irradiation scribing, holes are formed in dots on the surface of the alumina substrate where the laser was irradiated. The holes are formed when the alumina melts due to the heat from the laser irradiation. The molten alumina is observed to have scattered over 1 mm around the laser irradiated area. Some products have an inter-electrode distance of 1 mm, which causes defects and cannot be used. The load required to cut the sample scribed using the laser irradiation scribing method was approximately 0.3 N per 1 mm of width. After cutting the sample scribed using laser irradiation, the shape of the laser-irradiated surface and the cut surface were measured. The photograph of the surface after cutting shows that the cut was made along the laser-irradiated area, but there is variation not only in the center of the laser irradiation but also in the cut area. As a result, there is significant unevenness. The photograph of the cut surface after cutting shows that the laser irradiation has created holes to a depth of approximately 0.13 mm to 0.17 mm, but even though the laser irradiation was performed at a constant output, there is significant variation in the depth of the holes created. Furthermore, because the alumina was melted by the laser irradiation, there was significant unevenness on the cut surface, measuring up to approximately 0.18 mm. [Explanation of symbols]

[0009] 1 transmitter 2 cables 3 Langevin type piezoelectric element 4 horns 5 Single crystal diamond tools 6 load cells 7 Scribe head 8 suction table 9Work material 10Scribe processing direction

Claims

1. In order to scribe uneven, non-flat surfaces of the workpieces (substrates and semiconductor elements) used in power devices, a vibration-assisted scribing device is used that is equipped with a load cell for detecting the surface position of the workpiece and a function for detecting the position where a load is applied when the tip of the tool comes into contact with the workpiece, and a function for controlling the Z-axis position by tracking the indentation load to a constant load, thereby enabling the use of substrates and semiconductor elements that do not have residual stress on the cut surface of the workpiece.

2. The power device according to claim 1 is characterized in that the cut surface is not polished and has an unevenness of less than 0.05 mm as a workpiece applied to electrodes for mounting semiconductor materials, substrates on which wiring circuits for supplying and routing power, signals, etc. are formed, and semiconductor elements.

3. The power device according to claim 1 and claim 2 uses any of the following substrates (glass, alumina substrate, alumina-zirconia substrate, aluminum nitride substrate, silicon nitride substrate, silicon carbide substrate) that are workpieces processed using a vibration-assisted scribing device, and semiconductor wafers (silicon wafer, gallium arsenide wafer, gallium nitride wafer, gallium oxide wafer, aluminum nitride wafer, silicon carbide wafer).

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