1. Field of the Invention
The present invention relates generally to a process for detecting fine particles present on a surface of a semiconductor wafer. Specifically, the present invention relates to a process for optically detecting fine particles present on the surface of the semiconductor wafer. The present invention also relates to a process for removing such fine particles that were detected from the wafer surface.
2. Description of the Background Art
Recently, a remarkable advance has been made in the technology of semiconductor devices to provide a semiconductor integrate circuits with finer and multi-laying structures. As a fine processing of such circuits, various techniques, such as lithography and dry etching, have been well known. Cleaning technique to detect and remove fine particles which may be present on a surface of the circuit becomes more and more important according to the progress of the fine processing. In the near future, detecting accuracy of fine particles will be increased to the level such that 0.2 to 0.1 .mu.m or less of mean diameter thereof can be detected, though 0.3 .mu.m thereof is enough to obtain the cleaned wafer now.
Conventionally, particles having fine mean diameter present on the semiconductor wafer have been detected by laser beam irradiation. Referring to FIG. 7(A), a wafer 1, on which fine particles 2 may be present, is irradiated with laser beam 3 from one direction. Then, the laser beam 3 is reflected from the wafer surface 1. If fine particles 2 are present on the wafer surface 1, reflection of the laser beam 3 is scattered at the areas where particles 2 are present because of curved surfaces of particles. Sizes and distribution of particles can be detected from the scattering rate of the reflected beam.
However, because detecting sensitivity of the conventional process depends on the scattering rate of the laser beam, particles having very small diameters cannot be detected. For example, as shown in FIG. 7 (B), the laser beam 3 which irradiates such fine particles 2' are not specifically scattered. When an upper layer 4 is laid on the wafer surface 1, as shown in FIG. 7(C), the particle 2' is coated with the layer which causes an apparent diameter thereof to be enlarged. Therefore, during an integrating process of the wafer and layers to an integrated circuit, particles 2', which remains on the wafer surface 1 and which are not detected preliminarily, become accentuated. Particles not detected before the layer 4 formation are now found. These particles, which are called microdust, do not influence the article performance but deteriorate an outer appearance thereof.
Alternatively, deposition of an upper film on the wafer by means of chemical vapor deposition (CVD) can be done to make particles conspicuous. However, the upper film must be removed by mechanical techniques, such as etching, which tends to cause the wafer surface to be damaged. In addition, the process of forming the upper layer is a kind of a destructive test, therefore, a process for non-destructive detection has been required.
Fine particles on the wafer once detected must be removed before laying the upper layer thereon. Japanese Patent First Publication No. 3-152928, of which an inventor is the same as that of the present invention, discloses a process for removing fine particles which were detected. A wafer is chilled to freezing temperatures to freeze the wafer with fine particles present thereon. Then, the frozen particles are removed by applying physical force, such as jetting pressurized gas, onto the wafer surface. However, freezing of the wafer is done by taking advantage of moisture remains in a chilling system in which the wafer is placed. Therefore, when frozen moisture is evaporated after removing particles, contaminants which got mixed with the moisture still remaining on the wafer surface. Also, water marks, which marks formed by moisture evaporation, are frequently remain thereon.