A wide-gap semiconductor has a high breakdown voltage, and is, therefore, employed to enable a semiconductor device having a high output, a high breakdown voltage, and a low on-state resistance. It is known that the wide gap semiconductors include a nitride semiconductor, silicon carbide, diamond, etc. A high output, a high breakdown voltage, and a low on-state resistance of a semiconductor device are acquired using these semiconductors. A nitride semiconductor layer can be grown epitaxially on various kinds of substrates, thereby enabling it to epitaxially grow the nitride semiconductor layer on a Si layer and to provide a large-area and low-cost nitride semiconductor substrate.
A photolithography machine is needed to fabricate a semiconductor device, and a high-accuracy alignment is further required for recently advanced miniaturization of the device. Accordingly, it is important to employ short-wavelength light sources for lithographic exposure and substrate detection, i.e., for the miniaturization and the high-accuracy substrate alignment, respectively. Longer wavelength light has a tendency to more spatially spread, thereby making it difficult to perform the miniaturization and the high-accuracy substrate alignment. Therefore, visible light having a shorter wavelength than infrared light is preferably employed for, e.g., a pattern alignment in the lithographic exposure process of semiconductor manufacturing equipment. Detecting light is directed onto an alignment mark formed on a semiconductor substrate, and the detecting light reflected from the alignment mark is detected to perform the substrate alignment.
A substrate with a wide-gap semiconductor layer epitaxially grown thereon is employed to manufacture a power device or a radio-frequency device. The wide-gap semiconductor layer made of gallium nitride or aluminum nitride, which is transparent to not only infrared light but also visible light, was grown on a transparent sapphire or silicon carbide substrate. Furthermore, a GaN layer is grown epitaxially on a Si layer to provide a large-area and low-cost GaN substrate. In each case, carved concaves 62 are formed, e.g., on the surface of the GaN epitaxial layer to provide an alignment mark for the substrate alignment as shown in FIGS. 6A and 6B.
However, the carved concaves 62 are transparent to detecting light. Therefore, the detecting light reflected from edges of the carved concaves 62 is detected for the substrate alignment. However, the intensity of the detecting light reflected from the edges is so low that a proposal is made in which carved concaves are formed into two-step concave shapes in order to increase the intensity of detecting light reflected therefrom.
The substrate position and a prescribed point on the substrate cannot be accurately detected or pinpointed without detecting enough intensity of the reflected light from the alignment mark. Moreover, as shown in FIG. 6C, metal marks 63 made of a metal can be employed to reflect the detecting light, instead of the carved concaves 62 mentioned above. However, in case that the metal marks 63 are employed, extra steps for exclusive use are required so that a fabrication process of a device increases the number of steps included therein, thereby producing a risk that an insulating layer is contaminated by the metal employed for the metal marks 63. This has been a factor for complicating the fabrication process and reducing the reliability thereof, e.g., lowering a breakdown voltage of the device.
A wide-gap semiconductor is mostly transparent to visible light. Therefore, the intensity of the visible light reflected from an alignment mark to be formed by carving the wide-gap semiconductor substrate is too low to carry out a substrate alignment for the wide-gap semiconductor substrate using visible light of exposure equipment. When the alignment mark is prepared using, e.g., a metal on the wide-gap semiconductor substrate, the metal causes contamination in the subsequent steps of the device-manufacturing process. For example, the metal used for the alignment mark contaminates an apparatus for preparing an insulator film to be used subsequently to the preparation of the metal alignment mark, and reduces a break-down voltage of the semiconductor devices produced in the device-manufacturing process.
When Si is epitaxially grown on an alignment mark formed on a Si layer, the alignment mark thereon often deforms to reduce the accuracy of a substrate alignment. A method is disclosed as a means to prevent such a reduction in accuracy of the substrate alignment. The method employs detecting light with a wavelength of 1 μm or more, and provides a substrate alignment having a lower accuracy than a method employing visible light as detecting light.
There is a limit on reducing on-state resistances of the wide-gap semiconductor devices and controlling variations in the on-state resistances thereof, unless a more accurate substrate alignment can be carried out using detecting light, e.g., visible light.
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