Increased Uniformity of Silicon Needed for Critical Devices How Can We Improve the Float-Zone Process?
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Inc.
MATERIALS PROCESSING IN THE REDUCED GRAVITY ENVIRONMENT OF SPACE
Guy E. Rindone, editor
445
INCREASED UNIFORMITY OF SILICON NEEDED FOR CRITICAL DEVICES HOW CAN WE IMPROVE THE FLOAT-ZONE PROCESS?
EDWARD L. KERN Consultant, P.O.
Box 913,
Del Mar,
California
(92014)
USD
EXTENDED ABSTRACT INTRODUCTION The float-zone crystal growth process is used to grow silicon crystals for high performance devices, such as infra-red detector arrays, laser detectors, CCD imagers, high power rectifiers and radiation-hardened space solar cells. Recent developments have increased the purity and resistivity uniformity for high resistivity silicon (1). Device performance is now limited by microscopic resistivity nonuniformity and crystallographic defect densities. The objective of the NASA Float Zone Working activity is to understand the cause of these nonuniformities and to improve the process in microgravity (2). CRYSTAL NONUNIFORMITIES Knowing the magnitude of the inhomogeneities caused by melt transients will provide a reference for a) the magnitude of variation in device performance and b) the improvements to be made by better controlling the growth process. They can be characterized as a) resistivity striations, b) swirl and vacancy striations (for float zone) and c) variations in other impurities which may not lead to variations in the first two. Examples of the latter type of impurities are oxygen, which can be characterized by infra-red absorption (3) and carbon, which can be characterized by double crystal x-ray topography (4). Variations in swirls can be seen by etching (5) or by decorating with copper and observing with x-ray transmission topography (6) or infrared microscopes. Two techniques for observing striations are useful diagnostic tools for evaluating the effect of growth condition changes on all of the above inhomogeneities. The first is the spreading resistance measurement of resistivity which indicates fluctuations in electrically active dopants and in swirls (the correlation is developed in (7) ) and in oxygen (the correlation is developed in (3) ). The second is preferential etching, which gives surface topography differences at locations of greater chemical activity, which is indicative of where impurities and defects were incorporated more rapidly due to a higher instantaneous segreoation coefficient (keff). Both methods have resolutions in the 1-10 micron range. RESISTIVITY VARIATIONS Resistivity gradients can be evaluated on both the macroscopic scale (resolution of 2500 microns) and the microscopic scale (a few microns). As the segregation coeffecients decrease and the diameter increases, the microscopic variations increase markedly. An As-doped float zone crystal is profiled by spreading resistance in Figure 1. The crystal is doped to 1.5 x 1016 atoms cm-3 and grown in the (111) orientation at two inch diameter. The bottom curve is for a (111) slice, perpendicular to the growth axis, and shows a typical low resistivity region in the center. This is caused by supercooling of the melt above the
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