1. Technical Field
This invention relates to a method for obtaining information on the microstructure of non-monocrystalline matter, and of using this information to control a manufacturing process.
2. Prior Art
Although semiconductor devices generally are based on monocrystalline material, such as, for instance, epitaxial silicon, they typically require also the presence of non-monocrystalline matter, e.g., amorphous or polycrystalline insulating, semiconducting, or conducting layers, and polycrystalline metal layers. This is especially true in the case of integrated circuits (IC) in the currently practiced form of large scale integration. See, for instance, the article by W. G. Oldham, "The Fabrication of Microelectronic Circuits," Scientific American, pp. 111-128 (September 1977). As an example, in recent years polycrystalline silicon, deposited typically by chemical vapor deposition (CVD), has become of importance in many semiconductor applications. For instance, sub-micron thick films of polycrystalline silicon are used in silicon-gate MOS integrated circuits, and high-resistivity polysilicon films are used to control the potential and fields in high-voltage devices. See T. I. Kamins, IEEE Transactions on Parts, Hybrids, and Packaging, PHP-10(4), pp. 221-229 (1974). Under these circumstances it is clearly desirable to have available a reliable, fast, convenient, and non-destructive method for determining at least those properties of non-monocrystalline films, especially polysilicon films and the like, that are known to affect processing behavior and perhaps also device performance.
Contact-free conductivity and Hall angle measurements can be carried out that give information on carrier mobility, defect density, carrier concentration, and the like, and that substantially fulfill the above criteria. Similarly, contact-free capacitance techniques can be used for characterizing oxide films and oxide-semiconductor interfaces. For a review of electrical diagnostic techniques, see, for instance, W. M. Bullis and F. G. Vieweg-Gutberlet, Semiconductor Silicon 1977, H. R. Huff and E. Sirth, ed., The Electrochemical Society, pp. 360-366 (1977). It is also possible to determine non-destructively, by optical methods, film thickness and refractive index, the latter being typically equivalent to a composition determination. See, for instance, R. J. Kutko, Solid State Technology, pp. 43, 44 and 47 (February 1978), D. I. Bilenko et al, Optics and Spectroscopy, (USSR), Volume 45, (1), pp. 58-62 (July 1978), and D. Davies and W. A. Popov, Microelectronics J. (GB), Vol. 9(3), pp. 26-30 (1979). However, it appears that no practical non-destructive method exists for determining such an important characteristic of polycrystalline films as their microstructure. Reflection x-ray scattering allows non-destructive determination of the average crystallite size, but it can not give information on the density of the reflecting material. It is typically not a thin-film characterization technique, since x-rays are usually highly penetrating, and is therefore of low accuracy in thin films. The usual current method of grain size determination is transmission electron microscopy, but x-ray or electron scattering in transmission are also possible methods, that require, however, typically a model calculation to get information on grain size. These three methods require extensive sample preparation and are typically slow and inconvenient, in addition to not being non-destructive. Also, it should be pointed out that methods involving direct observation of the structure within a small area are often unreliable since there exists no assurance that the observed area is representative of the whole. The recently developed technique of acoustic microscopy could, in principle, non-destructively provide information on the microstructure of polycrystalline films, however, in practice the resolution is insufficient. See C. F. Quate, Semiconductor Silicon 1977 (op. cit.) pp. 422-430.
In addition to crystallite size there is another parameter of interest in the determination of microstructure, namely, the density of the material, or, equivalently, the void fraction in the material. Currently, this parameter is typically determined by observing the reduction in thickness of a film after heat treatment. Not only is this method inconvenient and slow, and undesirable for actual IC wafers because of possible mass transport of dopants during the heat treatment, but the method is typically incapable of reliably determining the actual void fraction since there generally exists no independent assurance that the void fraction in the film after heat treatment is zero.
Thus, there appears to exist no prior art method that would allow accurate non-destructive routine determination of the volume fractions of material underlying a free surface that are respectively polycrystalline, amorphous, and void. Yet it would be highly desirable to have available a capability for determining these parameters, since it has been found that processing behavior often depends substantially on these fractions. For instance, the optical properties of polysilicon surfaces vary with the microstructure of the material, where by "microstructure" we mean the structure of the material on the scale of approximately 10 A-1 .mu.m. In practice, the microstructure of a material may contain amorphous, crystalline, and void components, or a mixture of these. For instance, during semiconductor processing it is typically required to expose a layer of photoresist on a polycrystalline layer to some appropriate radiation. It is easy to see that variations in microstructure between batches or within a batch of semiconductor wafers would, because of the dependence of the optical properties on microstructure, lead to undesirable variation in the optimum exposure time. This has been found in practice to be a serious problem for which no good solution exists, since accurate determination of reflectance is in practice a rather difficult measurement that cannot be carried out routinely on a large number of wafers. As another example, during IC fabrication it is typically necessary that parts of a previously deposited polycrystalline layer be removed by chemical etching. It has been found that the microstructure has a profound influence on etch rate. Not only is the etch rate of, for example, amorphous silicon only a small fraction of that of polycrystalline silicon, but it has been found that apparently there exists a threshold value of polycrystalline volume fraction at which etch rate appears to drop step-function-like. It probably does not require emphasis that knowledge of etch rates and, more generally, etching behavior, is crucial for instance for IC manufacture. And, as a last example, the conductivity of doped polysilicon depends strongly and nonlinearly on grain size, requiring the ability to control, and therefore to measure, grain size. See J. Y. W. Seto, ibid, pp. 241-252.
From the above, it is obvious that availability of a reliable, fast, and non-destructive method for determining the microstructure of matter would be of great advantage for instance in semiconductor fabrication, and can be expected to lead to improvements in yield, and, therefore, to more economical manufacture of devices of improved performance.