CHAMFERED SILICON CARBIDE SUBSTRATE AND METHOD OF CHAMFERING

Abstract

The present invention relates to a chamfered silicon carbide substrate which is essentially monocrystalline, and to a corresponding method of chamfering a silicon carbide substrate. A silicon carbide substrate according to the invention comprises a main surface (102), wherein an orientation of said main surface (102) is such that a normal vector ({right arrow over (O)}) of the main surface (102) includes a tilt angle with a normal vector ({right arrow over (N)}) of a basal lattice plane (106) of the substrate, and a chamfered peripheral region (110), wherein a surface of the chamfered peripheral region includes a bevel angle with said main surface, wherein said bevel angle is chosen so that, in more than 75% of the peripheral region, normal vectors ({right arrow over (F)}_i) of the chamfered peripheral region (110) differ from the normal vector of the basal lattice plane by less than a difference between the normal vector of the main surface and the normal vector of the basal lattice plane of the substrate.

Claims

1-8. (canceled)

9. A method of chamfering an essentially monocrystalline silicon carbide substrate, so that a surface of a chamfered peripheral region includes a bevel angle with a main surface of the substrate, the method comprising the following steps: measuring an orientation of a basal lattice plane of the substrate with respect to the main surface of the substrate, determining said bevel angle so that, in more than 75% of the peripheral region, normal vectors of the chamfered peripheral region differ from a normal vector of the basal lattice plane by less than a measured difference between a normal vector of the main surface and the normal vector of the basal lattice plane of the substrate, working the substrate to form the chamfered peripheral region having the determined bevel angle.

10. The method according to claim 9, wherein the orientation of the basal lattice plane of the substrate with respect to the main surface of the substrate is measured in at least one location adjacent to the peripheral region that is to be chamfered.

11. The method according to claim 9, wherein the step of working the substrate includes a grinding step and/or a laser cutting step.

12. The method according to claim 9, wherein the step of measuring the orientation comprises: measuring an absolute value of a tilt angle in at least one location using an X-ray diffraction (XRD) measuring device, determining a minimal value of the measured tilt angle, and wherein the bevel angle is set to a value that is 0.5° less than the minimal value.

13. The method according to claim 9, wherein the substrate is worked using a grinding head which is mechanically connected to an X-ray diffraction (XRD) measuring device.

14. The method according to claim 9, wherein the tilt angle is measured at a plurality of locations around the circumference of the substrate and wherein the bevel angle is varied around the circumference of the substrate, depending on the measured tilt angle.

15. The method according to claim 9, wherein the silicon carbide substrate comprises a silicon side and a carbon side, and wherein the chamfered peripheral region with the determined bevel angle is arranged only on the silicon side of the substrate.

Description

[0040] The accompanying drawings are incorporated into the specification and form a part of the specification to illustrate several embodiments of the present invention. These drawings, together with the description serve to explain the principles of the invention. The drawings are merely for the purpose of illustrating the preferred and alternative examples of how the invention can be made and used, and are not to be construed as limiting the invention to only the illustrated and described embodiments. Furthermore, several aspects of the embodiments may form—individually or in different combinations—solutions according to the present invention. The following described embodiments thus can be considered either alone or in an arbitrary combination thereof. Further features and advantages will become apparent from the following more particular description of the various embodiments of the invention, as illustrated in the accompanying drawings, in which like references refer to like elements, and wherein:

[0041] FIG. 1 is a schematic representation of an SiC substrate according to a first embodiment of the present invention;

[0042] FIG. 2 is a schematic representation of an SiC substrate with an on-axis orientation of the chamfer;

[0043] FIG. 3 is a schematic representation of an SiC substrate with a too large bevel angle;

[0044] FIG. 4 is a schematic representation of an SiC substrate according to a further embodiment of the present invention;

[0045] FIG. 5 is a schematic representation of another SiC substrate with an on-axis orientation of the chamfer;

[0046] FIG. 6 is a schematic representation of another SiC substrate with a too large bevel angle;

[0047] FIG. 7 is a schematic representation of an SiC substrate according to a further embodiment;

[0048] FIG. 8 is a schematic representation of an SiC substrate according to a further embodiment;

[0049] FIG. 9 is a schematic representation of an SiC substrate according to a further embodiment;

[0050] FIG. 10 is a schematic representation of a measurement step;

[0051] FIG. 11 is a schematic representation of a grinding step;

[0052] FIG. 12 is a schematic representation of a combined measuring and machining unit.

[0053] The present invention will now be explained in more detail with reference to the Figures and firstly referring to FIG. 1.

[0054] FIG. 1 shows a SiC substrate 100 according to a first advantageous embodiment of the present invention. It has to be noted, that the dimensions are not to scale in order to illustrate the idea according to the present invention. In particular, the thickness of the substrate is shown enlarged in comparison to the diameter. The SiC substrate has a first surface 102 and a second surface 104. In the following, the first surface 102 will be referred to as the main surface, where as the second surface 104 is also referred to as the bottom surface. Normally, the first surface 102 is the silicon surface of the SiC substrate 100, and the second surface 104 is the carbon surface of the substrate 100. The main surface 102 is the surface on which later epitaxial layers are deposited. The orientation of the main surface 102 is indicated by the normal vector {right arrow over (O)}.

[0055] In FIG. 1, the set of lattice basal planes 106 are shown to have a normal vector {right arrow over (N)}. As schematically indicated by the lattice planes 106, the SiC substrate 100 is offcut at an angle of 4°. In other words, the normal vectors {right arrow over (O)} and {right arrow over (N)} include a tilt angle of 4°. Due to the tilt angle, epitaxial growth is taking place as a step-flow growth. The step-flow growth direction is indicated in FIG. 1 by the arrow 108.

[0056] In order to mechanically stabilize the substrate, the peripheral regions of the essentially circular wafer have to be provided with a chamfered region 110. The orientation of the surface plane of the chamfered region at any given location i is described by a normal vector {right arrow over (F)}_i. The inventors of the present invention have recognized that in case that the normal vector {right arrow over (F)} coincides with the normal vector {right arrow over (N)} of the lattice planes, this would be a local on-axis orientation of the surface of the silicon carbide substrate 100. Such an on-axis orientation is disadvantageous because on on-axis oriented surfaces, a faceted growth takes place. Any faceted growth of an epitaxial layer is undesired because it causes enhanced forming of defects which may even reach into the area of the substrate main surface 102 where electronic components will be located. The quality of the epitaxial layer may be impaired and waste might be enhanced.

[0057] Furthermore, in case that the normal vector {right arrow over (F)}_i includes with the normal vector {right arrow over (N)} of the lattice planes a bevel angle which is larger than the tilt angle, step-flow growth would be induced that is directed against the direction 108. When the direction of the step-flow growth is reversed, an additional growth front is generated, which may collide with the main step-flow growth front growing along the direction 108 and thus cause defects in the layer formation.

[0058] Consequently, according to the present invention, it is ensured that a uniform step-flow growth of the epitaxial layers is induced not only for the substrate main surface 102 but also in the peripheral chamfered regions 110. In particular, the bevel angle which is included by the normal vector {right arrow over (F)}_i and normal vector {right arrow over (O)} of the main substrate surface 102 has to be smaller than the tilt angle which is included by the normal vector {right arrow over (O)} and the normal vector {right arrow over (N)} of the basal lattice planes.

[0059] The condition that has to be fulfilled is given by the following relation (1):

({right arrow over (F)}_i, {right arrow over (O)})<({right arrow over (N)}, {right arrow over (O)})   (1)

[0060] It could be shown that this condition has to be met for at least 75% of the total circumference of the silicon carbide substrate in order to effectively reduce undesired defects. Preferably, the chamfering according to the present invention is applied around at least 95% of the circumference.

[0061] In other words, the present invention provides an essentially monocrystalline silicon carbide substrate with a chamfered periphery, wherein the absolute value of the vector difference between the surface of the chamfered area and the lattice plane orientation has to be smaller than the vector difference between the orientation of the main surface and lattice plane orientation. Advantageously, this particular dimensioning of the chamfered region is only applied to the silicon side 102, while the carbon side 104 may be provided with an arbitrary chamfering. For instance, the silicon substrate 100 has a diameter of at least 150 mm and a thickness of not more than 1000 μm, at least 200 μm, preferably 350±25 μm. Advantageously, the substrate has a rough number of 10 nm or less on at least 80% of the chamfered surface 110. The polytype of the substrate may for instance be 4H or 6H, and preferably is 4H. The tilt angle (offcut angle) between the main surface and the lattice plane orientation preferably is 4° as shown in FIG. 1, but may take any value between 0.5° and 8°.

[0062] By way of comparison, FIGS. 2 and 3 show the undesired scenarios mentioned above for a silicon carbide substrate having a cutoff (or tilt) angle of 4°. In particular, FIG. 2 shows a silicon carbide substrate, where the normal vector {right arrow over (F)}_i coincides with the normal vector {right arrow over (N)} of the lattice claims, as given by equation (2):

({right arrow over (F)}_i, {right arrow over (O)})=({right arrow over (N)}, {right arrow over (O)})   (2)

Faceted growth will be induced when depositing the epitaxial layers.

[0063] On the other hand, FIG. 3 illustrates the undesired case where the bevel angle which is included by the normal vector {right arrow over (F)}_i and normal vector {right arrow over (0)} of the main substrate surface 102 is larger than the tilt angle, as given in relation (3) below:

({right arrow over (F)}_i, {right arrow over (O)})>({right arrow over (N)}, {right arrow over (O)})   (3)

[0064] In this case, step-flow growth of the epitaxial layer may occur along the reverse direction 112, causing colliding growth fronts.

[0065] FIGS. 4-6 illustrate the scenarios shown in FIGS. 1-3 for the case that the cutoff angle is 8°. In particular, for silicon carbide substrate 400 according to the present invention, relation (1) is fulfilled. For silicon carbide substrate 500, on the other hand, equation (2) is fulfilled. Faceted growth will be induced when depositing the epitaxial layers. For silicon carbide substrate 600, relation (3) is fulfilled. In this case, step-flow growth of the epitaxial layer may occur along the reverse direction 112, causing colliding growth fronts.

[0066] In order to ensure that the chamfered region is machined onto the silicon carbide substrate, first the actual tilt angle ({right arrow over (N)}, {right arrow over (O)}) has to be measured. In particular, it has to be considered at which location(s) such a measurement is performed. In the following, the various possible locations of where to measure the tilt angle will be discussed. Referring to FIG. 7 and additionally turning back to FIG. 1, a substrate 100 is shown, where the nominal tilt angle ({right arrow over (N)}, {right arrow over (O)}) at the main surface 102 is 4°. In this case, the lattice planes 106 have the same orientation in the peripheral region that will become the chamfered region 110 after machining. The peripheral region has a dimension d from an end face 114 of the substrate 100, the distance d representing the future bevel's breadth. In the case that the normal vector krim within the distance d coincides with the normal vector {right arrow over (N)} at the rest of the substrate, it is of no particular importance where the measurement is performed. The tilt angle may for instance be measured within the distance d or within twice the distance d from the end face 114. Alternatively, the location of the measurement may be expressed as a suitable fraction, for instance 90%, of the SiC substrate's radius r, which is counted from the center of the wafer.

[0067] A different situation is depicted in FIG. 8. According to this FIGURE, the lattice planes 106b are tilted against the lattice planes 106a in the bulk of the substrate 800. In other words, the marginal normal vector {right arrow over (N)}_rim within the distance d from the end face 114 includes an angle with the normal vector {right arrow over (N)} of the bulk material. This deviation of the lattice plane orientation in the marginal region compared to the center is caused by the PVT growth conditions when fabricating the single crystal boule (see for instance U.S. Pat. No. 9,590,046 B2). In order to take into account such local deviations, it is advantageous to measure the lattice plane orientation as close as possible to the later chamfered regions 110, preferably within the distance d from the end face 114.

[0068] The analogous considerations are of course also valid for the case where the orientation of the lattice planes 106b is tilted in the other direction, as this is illustrated in FIG. 9 for a substrate 900.

[0069] With reference to FIG. 10, a first embodiment of measuring the lattice plane orientation as a basis for choosing the bevel angle of the chamfered periphery will be explained.

[0070] Advantageously, the letters plane orientation of the substrate 100 is measured by means of an X-ray Diffraction (XRD) unit 116 with an energy dispersive detector.

[0071] Generally, the arranged atoms of a crystal form a series of parallel lattice planes separated from one another by a distance a, which varies according to the nature of the material. For any crystal, planes exist in a number of different orientations each with its own specific distance a. X-ray Diffraction (XRD) allows the identification of the orientation of these lattice planes with reference to a reference plane. The X-ray diffractometer 116 comprises an X-ray generator 118, a goniometer (angle-measuring device), a sample holder for the substrate 100, and an X-ray detector 120. The X-ray beam 122 hits the sample substrate 100 and the detector 120 records the X-ray intensity diffracted at the substrate. The distances a between the adjacent lattice planes are correlated with the radiation's wavelength X and the angle of reflection by applying Bragg's Law (equation (4)):

nλ=2a.Math.sin Θ  (4)

where n is the order of diffraction (0, 1, 2, 3, . . . ), λ is the wavelength of the incident X-ray beam, d is the distance between adjacent lattice planes, and Θ is the angle of incidence of the X-ray beam. The diffraction angle 20 is equal to twice the incident angle Θ. The goniometer is motorized and the detector 120 moves so that the angle 126 of the Ω axis varies through a range of the angle 2Θ. Each time the Bragg condition is satisfied, the detector 120 measures the intensity of the reflected radiation. As the lattice plane distance a is known, the actually measured value of the angle Θ allows calculating the angle under which the lattice planes are lying with respect to the upper surface 102 of the substrate 100. Advantageously, the detector 120 is energy dispersive, thus yielding very fast and accurate measurement results with a comparatively simple set-up.

[0072] The measurement is performed at one or more measurement points 124 distributed around the Φ axis 128 by rotating the substrate 100 around the Φ axis 128. The exact location of the measurement point 124 is given as a particular ΔΦ value 130. for instance, the measurement is performed at a plurality of locations 124, and the minimum value of the measured local off-orientation angles is determined.

[0073] According to the present invention, these results are used in a second step which is depicted in FIG. 11. As shown in FIG. 11, the substrate undergoes a machining step in order to fabricate the chamfer. This is done for instance by means of a grinding head 132 which can be at adjusted regarding its chamfering angle 134. According to the present invention, the chamfering angle 134 of the grinding head 132 is chosen to be 0.5° smaller than the minimum measured value determined during the measuring step depicted in FIG. 10.

[0074] However, if the ΔΦ values 130 are recorded for each measurement point 124 and transmitted to the grinding head's control unit, the grinding head 132 may be positioned in its angle 134 depending on the actual measurement value of each location 124.

[0075] The present invention also relates to an integrated and continuous process for chamfering silicon carbide substrates 100. This process will be explained in more detail with reference to FIG. 12. Essentially, in FIG. 12 the units shown in FIGS. 10 and 11 are combined into one chamfering unit 136. The chamfering unit 136 comprises an XRT unit 116 and the grinding head 132. Moreover, a control unit (not shown in the Figures) performs control of the chamfering unit 136.

[0076] According to a first embodiment of the method according to the present invention, the XRD unit 116 determines by means of the an energy dispersive detector 120 the lattice plane off-orientation at a plurality of measurement points 124 which are located close to the margin of the substrate 100. The grinding head 132 is connected to the goniometer arm of the Ω axis of the XRD unit 116 with an adjustable angle. Again, the substrate 100 rotates around the Φ axis 128. In the shown embodiment, the grinding head 132 is distanced apart from the measurement point 124 by a defined angular ΔΦ distance 130.

[0077] The measurement is performed in the direction of the angle 126 of the Ω axis, the extreme value is determined and thereby the grinding angle 134 is determined.

[0078] According to a second embodiment of the single step process, the measurement is performed at Y measurement points 128, wherein Y is calculated as the ratio of the substrate circumference and the width of the grinding head's grinding disc. The grinding angle is adapted continuously according to the result of the measurement. Advantageously, the grinding angle can be adjusted automatically by a control unit allocated to the chamfering unit 136. Moreover, the one step method has the advantage that the substrate does not have to be relocated, thereby reducing the processing time and enhancing the accuracy of the chamfering process.

[0079] Exemplarily, the following advantageous chamfer angles may be chosen, depending on the particular substrate characteristics.

[0080] For a substrate with a nominal tilt angle of 8°±0.5° and a measured minimal value of the tilt angle in a marginal region of 7.8°, a bevel angle of 6.8° may be chosen when determining the bevel angle to be 1° smaller than the measured tilt angle.

[0081] Further, for another substrate with a nominal tilt angle of 4°±0.5° and a measured minimal value of the tilt angle in a marginal region of 3.5°, a bevel angle of 2.2° may be chosen when determining the bevel angle to be 1.3° smaller than the measured tilt angle.

[0082] Further, for the same substrate with a nominal tilt angle of 4°±0.5°, with a measured minimal value of the tilt angle in a marginal region of 4.5°, a bevel angle of 3.2° may be chosen when determining the bevel angle to be 1.3° smaller than the measured tilt angle.

[0083] For a substrate with a nominal tilt angle of 2°±0.5°, with a measured minimal value of the tilt angle in a marginal region of 1.9°, a bevel angle of 1.4° may be chosen when determining the bevel angle to be 0.5° smaller than the measured tilt angle.

REFERENCE NUMERALS

[0084]

TABLE-US-00001 Reference Numeral Description 100, 400 SiC substrate according to the invention 200, 300, 500, 600 SiC substrate with undesired bevel angle 102 Main surface; Si side 104 Bottom surface; C side 106, 106a, 106b Lattice planes 108 Direction of step-flow growth 110 Chamfered region 112 Direction of reverse step-flow growth 114 End face 116 XRD unit 118 X-ray generator 120 X-ray detector 122 X-ray beam 124 Measurement point 126 Angle of Ω axis 128 Φ axis 130 ΔΦ value 132 Grinding head 134 Chamfering angle 136 Chamfering unit {right arrow over (O)} Normal vector of main surface {right arrow over (N)} Normal vector of lattice planes {right arrow over (F)}_i Normal vector of chamfered surface {right arrow over (N)}_rim Normal vector of lattice planes in the peripheral region d Width of chamfered region r Radius of substrate