1. Field of the Invention
The present invention relates to a method and an apparatus for evaluating semiconductor layers formed on a substrate.
2. Description of the Related Art
In general, a nitride semiconductor, which is a generic term of mixed crystals expressed by a composition formula: AlxInyGa1-x-yN (0≦x≦1, 0≦y≦1, x+y<1), such as gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), is mechanically robust and chemically stable. In addition, the nitride semiconductors exhibit high thermal conductivity and excellent heat dissipation. Semiconductor devices composed of the nitride semiconductors, for example, HEMTs (high electron mobility transistors) composed of AlGaN/GaN layers and LDs (laser diodes) composed of InGaN/GaN layers, are suitable for high power operations.
Meanwhile, the nitride semiconductors have a remarkably high melting point. For example, AlN has a melting point of 3,273 K (Kelvin), GaN 2,000 K or more, and InN 1,373 K, respectively (document: S. Sakai, “III-nitride semiconductor”, edited by I. Akazaki, Chapter 1, published by Baifukan CO., LTD, 1999). Hence it is relatively difficult to grow nitride semiconductor layers with high crystallinity. In fact, it is known that cracks of the order of nanometers may be formed on a surface of the nitride semiconductor layer, depending on slight variations of growth condition. These cracks may increase gate leak current and degrade pulse response characteristics in HEMT devices. Accordingly, quantitative evaluation of crack density is quite important in manufacturing nitride semiconductor devices.
Conventional quantitative evaluation of a surface state, such as crack density, was performed mainly using AFM (atomic force microscope). The AFM can measure displacement of a cantilever by detecting reflected light from the cantilever when the cantilever is displaced based on atomic force between a probe fixed onto the tip of the cantilever and atoms on the surface of a sample. The cantilever or the sample is scanned and moved vertically so as to keep the displacement of the probe constant, in which conversion of the control signal into an image enables the surface state (concavity and convexity) of the sample to be measured at the atomic order.
The AFM has an advantage of directly evaluating the surface state, whereas it has a disadvantage of a low throughput in data acquisition. In addition, the AFM is also remarkably expensive and unsuitable for applying to mass production lines.
For another approach of directly evaluating a surface state, STM (scanning tunneling microscope) or KFM (Kelvin force microscope) is known but has the same problem as the AFM does.
Therefore, desired is a method for measuring a surface state quickly and sensitively with a relatively simple constitution.
Cracks existing in a semiconductor layer give a great influence on crystallinity of a surface. Hence by measuring parameters relatively sensitive to crystallinity among physical parameters of the semiconductor layer, the surface state of the semiconductor layer can be evaluated indirectly.
One parameter typically used among the parameters sensitive to crystallinity is a band width at half maximun of an X-ray diffraction pattern. The band width at half maximun is increased as crystallinity of the semiconductor layer is degraded; therefore it is relatively easy to measure. Thus, this parameter is often utilized for evaluating crystallinity of bulk crystals.
However, since the X-ray diffraction pattern is influenced not only by the surface of the crystal but also by an internal state thereof, a change of the band width at half maximun is strongly dominated by a change in the internal state of the crystal, consequently, not so sensitive to a change in the surface state of the crystal.
The band width at half maximun of the X-ray diffraction pattern, as described above, exhibits a physical value depending on the change in the surface state of the crystal as well as the surface of the crystal, hence, unsuitable for evaluating only the surface state of the crystal. Further, in case of a plurality of semiconductor layers, each having a different composition, being stacked on a substrate, the X-ray diffraction pattern may be influenced both by crystalline states of all the semiconductor layers and by a crystalline state of the substrate, thereby hardly separating only information regarding a particular semiconductor layer.
The related prior arts are listed as follows: Japanese Patent Unexamined Publications (kokai) JP-A-2-307046 (1990), JP-A-7-92236 (1995), and JP-A-2003-224171 (2003).