So far, a wide variety of vapor phase reaction methods aimed at the surface smoothing, etc., of electronic devices, have been developed and put into practical use. E.g., the method of smoothing a substrate surface shown in Patent Reference 1 irradiates a substrate surface at a low angle with ions of monomer atoms or molecules of Ar (argon) gas and so on, and smoothes it by sputtering.
Moreover, in recent years, solid surface smoothing methods using a gas cluster ion beam have gained attention for enabling little surface damage and very small surface roughness. E.g., in Patent Reference 2, there is disclosed a method of reducing surface roughness by irradiating a gas cluster ion beam on a solid surface. In this method, the gas cluster ions irradiated on the object being processed are broken down by collisions with the object being processed, on which occasion there arise many-body collisions between the constituent atoms or molecules of the cluster and the constituent atoms or molecules of the object being processed, and a movement in a horizontal direction with respect to the object being processed becomes noticeable, as a result of which cutting is performed in a transverse direction with respect to the surface of the object being processed. This is a phenomenon called “lateral sputtering”. By further movement of particles in a lateral direction on the surface of the object being processed, the apices of the surface are planed, the result being that atomic-size, ultra-accurate polishing is obtained. In addition, the energy held by the gas cluster ion beam is different from that of conventional ion etching in that, the energy being lower, no damage is inflicted on the surface of the object being processed, making possible the required ultra-accurate polishing. This means that solid surface smoothing method based on a gas cluster ion beam exhibits the advantage of there being less damage to the processed surface than the ion etching method shown in the aforementioned Patent Reference 1.
For smoothing based on a gas cluster ion beam, it is generally recognized that it is desirable for the direction of irradiation of the cluster ion beam on the surface of the object being processed to be one coming from a nearly perpendicular direction with respect to the surface being processed. This is to make maximum use of the effect of “surface smoothing based on lateral sputtering” described previously. However, in the aforementioned Patent Reference 2, it is described that, in case the surface being processed is a curved surface or the like, the irradiation may be in an oblique direction in response to that situation of the surface, but there is no mention regarding the effect in the case of irradiation be in an oblique direction. Consequently, in this Patent Reference 1, it comes about that the most efficient method for the smoothing of a solid surface is one where the beam is irradiated from a nearly perpendicular direction with respect to that surface.
Moreover, concerning the smoothing of a solid surface using a gas cluster ion beam, there is also an example in Patent Reference 3. There is no description in this Patent Reference 3 either of the relationship between the angle formed between the gas cluster ion beam and the solid surface, and the smoothing of the surface, so if one considers, from the disclosed description, that the “lateral sputtering” effect is used, one may consider that data for perpendicular irradiation are shown, in the same way as the previously indicated Patent Reference 2.
In addition, there is also an account pertaining to the smoothing of a solid surface based on gas cluster ion beam irradiation in Non-Patent Reference 1. Toyoda et al. carried out irradiating Ar cluster ions on surfaces of materials like Cu, SiC, and GaN and show a reduction in surface roughness. Even in this case, the work presented is irradiated by a gas cluster ion beam from a nearly perpendicular direction with respect to the surface.
Moreover, there are descriptions in Non-Patent Reference 2 regarding the changes in the roughness of a solid surface in the case of irradiating a gas cluster ion beam at various irradiation angles with respect to a solid surface. If the case of perpendicular irradiation with respect to the solid surface is taken to be 90° and the case of irradiation in parallel with the surface is taken to be 0°, it is shown that the sputtering rate, which is the speed at which the surface is etched, is the greatest for perpendicular irradiation and the etching rate decreases as the irradiation angle decreases. Regarding the relationship between surface roughness and irradiation angle, tests were performed by changing the irradiation angle to 90°, 75°, 60°, 45°, and 30°, and it was shown that the surface roughness increases as the irradiation angle decreases. No investigation was carried out experimentally for irradiation angles below 30°, but this may be thought to be due to the fact that it was judged to be of no use to carry out something like that, since surface roughness increases as the irradiation angle is decreased.
In addition, the majority of electronic devices such as integrated circuits and optical devices used in optical communications have concavo-convex patterns prepared by microshaping in solid surfaces or thin film material surfaces, but there is no account of using a gas cluster ion beam for the smoothing of the side wall surfaces of concave portions or convex portions in those concavo-convex patterns. This is because it was believed that it is difficult to irradiate a gas cluster ion beam nearly perpendicularly to the side wall surfaces of concave portions or convex portions or that the smoothing of side wall surfaces is not possible with the lateral sputtering mechanism.
As mentioned above, since, in the case of smoothing a solid surface by using a gas cluster ion beam, the surface roughness is the smallest when the irradiation angle of the gas cluster ion beam with respect to the solid surface is chosen to be 90°, and the surface roughness increases as the irradiation angle is decreased, it is not an exaggeration to say that no consideration has been given to cases other than making the irradiation angle nearly perpendicular.
Patent Reference 1: Japanese Patent Application Laid Open No. 1995-58089.
Patent Reference 2: Japanese Patent Application Laid Open No. 1996-120470.
Patent Reference 3: Japanese Patent Application Laid Open No. 1996-293483.
Non-Patent Reference 1: Japanese Journal of Applied Physics, Vol. 41 (2002), pp. 4287-4290.
Non-Patent Reference 2: Materials Science and Engineering, R34 (2001), pp. 231-295.