Semiconductor device manufacturing: process – With measuring or testing
Reexamination Certificate
1999-02-05
2001-01-23
Chaudhuri, Olik (Department: 2823)
Semiconductor device manufacturing: process
With measuring or testing
C438S401000, C438S973000, C438S733000, C216S002000, C216S039000
Reexamination Certificate
active
06177285
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of determining crystal orientation in a wafer made from a crystal with a zinc-blende crystal structure in which an etching mask having mask openings such as circular scale marks arranged one beside the other is applied in relation to a predefined marking of the wafer, and the wafer is etched anisotropically, producing etched-out channels, and the crystal orientation can be inferred from the etching structure of the channels. For example, silicon or germanium or gallium arsenide has a zinc-blende crystal structure.
BACKGROUND INFORMATION
A method of determining crystal orientation in a wafer is described in the publication “A latching accelerometer fabricated by the anisotropic etching of (110)-oriented silicon wafers” by Dino R. Ciarlo in the journal “Micromech. Microeng.,” 2, 1992, pages 10-13. According to the method described therein, rectangular mask openings such as circular scale marks are arranged one next to the other. The angle between two adjacent rectangular mask openings is 0.1°. Each rectangular mask opening is 8 &mgr;m wide and 3 mm long. During anisotropic etching of the wafer surface to which the etching mask has been applied, undercut zones, which are bounded by (111) planes, develop at the sides of the mask openings. The dimensions of the undercut zone will vary depending on the orientation of the mask opening relative to the crystal orientation which is to be found. The dimensions of a particular undercut zone are determined optically, and the location of the mask opening at the side of which the undercut zone is smallest is utilized for determining the crystal orientation. The dimension of the smallest undercut zone must be larger than the wavelength of the light used for measuring in order for it to be perceived; thus it must be larger than around 0.5 &mgr;m. This is ensured according to this method, among other things, by the dimensions selected for the rectangular mask openings. The achievable angle precision in determining crystal orientation using this method depends on the angle between two adjacent mask openings. For reasons related to the manufacture of masks, this angle cannot be made smaller than 0.1°. The best precision achievable in determining crystal orientation is thus 0.05°. The required etching time in the known method is around 30 hours, depending on the length of the rectangular mask openings. While a reduction of the length of the rectangular mask openings would result in a reduction in etching time, it would also affect the dimensions of the undercut zones causing them no longer to be measurable.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for determining crystal orientation in a wafer by which at least the same precision in determining crystal orientation can be realized as that of the conventional method in a comparably smaller amount of time.
This object is achieved in a method according to the present invention through the mask openings each containing two relatively short segments which in their longitudinal direction are almost parallel, with the first segment always being somewhat shorter than the second segment and each having a relatively long area which extends between each first and second segments forming a mask opening in the shape of a double T. The mask openings are arranged one beside the other each generating a modified double-T shaped mask opening, so that with equally large intervening distances between the areas, the first segments and the second segments are a predetermined distance from each other. The crystal orientation is determined with the distance of the two particular adjacent mask openings, whose intervening space is least undercut, from the preexisting marking.
An advantage of the method according to the present invention is that the length of the double-T mask openings can be selected to be smaller than the length of the rectangular mask openings in the known method by at least one order of magnitude and thus the required etching time is reduced accordingly. At the same time, the use of the double-T mask openings causes the dimensions of the undercut zones to remain easily measurable by optical means. The precision in the determination of crystal orientation is at least as good as with the conventional method.
In an advantageous embodiment of the method according to the present invention, the wafer is etched until at least one intervening space remains; the position of this intervening space is determined as the distance from the preexisting marking. An advantage of this procedure is that only a relatively small degree of measurement effort is required for determining the position of the intervening space. For example, it is possible to determine the position of this intervening space with the naked eye.
An additional advantageous embodiment of the method according to the present invention is achieved through the wafer being etched until an etched face, forming from the first segment or from the second segment of the mask openings along the area, extends over more than half of the length of the area and the sizes of all intervening spaces between each two etched-out channels being utilized for determining the crystal orientation.
An advantage of this further embodiment of the method according to the present invention is that on the basis of a plurality of determined sizes of intervening spaces, crystal orientation can be determined with a relatively high degree of precision.
An additional advantageous embodiment of the method according to the present invention is achieved through obtaining a series of intensity values from the optical measurement of intensities reflected from the wafer along a straight orientation line running parallel to the segments and intersecting the area approximately at its middle.
The series of intensity values is used in calculating the remaining sizes of the intervening spaces which are not undercut between each two adjacent etched-out channels in the direction of the orientation line. An inference on the crystal orientation is made on the basis of the distance between the channels associated with the largest determined intervening space sizes from the preexisting marking.
An advantage of this additional embodiment of the method according to the present invention is that the precise determination of crystal orientation can be performed automatically, i.e., it can be utilized in an automatic production unit.
In a further embodiment of the method according to the present invention, in a circular wafer a radius line parallel to a flat of the wafer is selected as the preexisting marking, and the crystal orientation is determined through determining the angle which in relation to the center of the circular wafer is formed by the distance of the mask openings with the least undercut intervening space from the radius line.
An advantage of this further embodiment of the method according to the present invention is that the wafer can be easily positioned in the direction of the crystal orientation after determination of the angle through appropriate rotation.
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Dino R. Ciarlo, “A latching accelerometer fabricated by the anisotropic etching of (110) oriented silicon wafers,” J. Micromech. Microeing. 2 (1992) 10-13.
Jantke Gabriele
Steckenborn Arno
Winkler Thoralf
Chaudhuri Olik
Coleman William David
Kenyon & Kenyon
Siemens Aktiengesellschaft
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