VLSI Fabrication
`Principles
`
`Silicon and Gallium Arsenide
`
`Sorab K. Ghandhi
`Rensselaer Polytechnic Institute
`
`A Wiley-lnterscience Publication
`
`John Wiley & Sons
`New York Chichester Brisbane Toronto Singapore
`
`SAMSUNG-1011.001
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`

`
`Copyright© 1983 by John Wiley & Sons, Inc.
`
`All rights reserved. Published simultaneously in Canada.
`
`Reproduction or translation of any part of this work
`beyond that permitted by Section 107 or 108 of the
`1976 United States Copyright Act without the permission
`of the copyright owner is unlawful. Requests for
`permission or further information should be addressed to
`the Permissions Department, John Wiley & Sons, Inc.
`
`Library of Congress Cataloging in Publication Data:
`Ghandhi, Sorab Khushro, 1928-
`VLSI fabrication principles.
`
`"A Wiley-Interscience publication."
`Includes index.
`1. Integrated circuits-Very large scale integration.
`I. Title.
`2. Silicon. 3. Gallium arsenide.
`II. Title:
`V.L.S.I. fabrication principles.
`
`TK7874.G473 1982
`ISBN 0-471-86833-7
`
`621.381'71
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`82-10842
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`Printed'in the United States of America
`
`10 9 8 7 6 5 4 3 2 1
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`SAMSUNG-1011.002
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`WET CHEMICAL ETCHING
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`9.1.6.3 Mixed Oxides
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`495
`
`These include* borosilicate glass (BSG) as well as ZnO·Si02 and Sn02·Si02.
`All are useful as dopant sources. The BSG is used as a p-type doped oxide
`source for silicon, whereas ZnO·Si02 and Sn02·Si02 are used asp- and n(cid:173)
`type dopant sources for gallium arsenide, respectively. They are grown at
`350-450°C, by the simultaneous oxidation of silane and the appropriate
`dopant hydride or alkyl (diborane, diethylzinc, and tetramethyltin, respec(cid:173)
`tively). They can be readily etched in HF, or in BHF since Si02 is their
`primary constitutent. Again, etching proceeds by the dissolution of the Si02
`in the HF,"' rendering the structure porous and thus more susceptible to
`dissolution.
`Dilute BHF is commonly used with these doped oxides. Its dissolution
`of BSG has been investigated in some depth [ 42]. Typically, the etch rate
`is seen to increase monotonically with the B20 3 content. In some instances,
`however, anomalous characteristics have been observed during the etching
`of BSG films.
`A thin layer of BSG is also formed during the boron doping of masked
`silicon wafers. Selective etches, known as R-etch and S-etch, can be used
`[ 44] to remove this film while leaving the underlying Si02 unetched. Both
`etches are modifications of the P-etch formulation; typically they are about
`five to six times more rapid in their ability to remove the BSG layer.
`The etch characteristics of both ZnO·Si02 and Sn02·Si02 have not been
`studied. However, they are comparable to undoped Si02 grown by the silane
`process. Dilute BHF has been found useful for this purpose.
`
`9.1.6.4 Silicon Nitride
`
`Silicon nitride (commonly written as SiN) is an inert dense material which
`is an excellent diffusion barrier to both sodium and gallium. For this reason,
`it is widely used as a protective coating for silicon microcircuits, and also
`as a cap during the annealing of ion-implanted gallium arsenide. Its protective
`characteristics are generally superior to those of Si02 and PSG. However,
`its patterned removal is more difficult and is highly dependent on the growth
`technique.
`Both HF and BHF can be used to etch these films [ 45]. However, even
`at elevated temperatures, the etch rates are extremely slow, so that photo(cid:173)
`resist films are ruined during this process. Typically, the etch rate of CVD
`films (SiH4/NH3 process) is a function of the growth temperature [46]. The
`etch rate is concentrated HF is about 1000 A/min for films grown at 800°C,
`falling to as low as 140 A/min for films grown at 1100°C. The etch rate in
`BHF is considerably lower, about 5-15 A/min for films grown at l l00°C.
`
`* PSG belongs in this category as well. However, it has been treated separately because of its
`importance in microcircuit fabrication technology.
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`SAMSUNG-1011.003
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`496
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`ETCHING AND CLEANING
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`The problem of photoresist destruction can be avoided by depositing a
`molybdenum film between the nitride and the photoresist. This film can be
`readily etched with excellent edge definition by standard photolithographic
`techniques. Once patterned, it can be used as a mask for etching the under(cid:173)
`lying SiN film, after the resist has been stripped.
`Silicon nitride is often used as a cover layer over a film of silica. In these
`situations, neither HF nor BHF can be used, sirice this would result in deep
`undercutting of the Si02 layer, which is etched rapidly by these solutions.
`Instead, etching is carried out in boiling* H3 P04, and a reflux boiler ap(cid:173)
`paratus is used to avoid changes in the etchant composition during this
`process. Typically [47], the etch rate for CVD grown SiN films is about 100
`A/min, while the etch rate for thermally grown Si02 layers is about 10-20
`A/min. For these same conditions, the etch rate of silicon is under 3 A/min,
`so that this technique can be used in the presence of exposed silicon surfaces.
`A curve of comparative etch rates of these films is shown in Fig. 9.8.
`There are many situations in which consecutive layers of SiN and Si02
`must be etched at the same rate (the gate oxide of a metal-nitride-oxide
`semiconductor (MNOS) field effect transistor, for example). Here mixtures
`of HF and glycerol are used at 80-90°C, and experimentally adjusted to
`provide equal etch rates. Typically, a 1-3 mole/liter concentration of HF
`etches Si02 and CVD SiN films at equal rates of about 100 A/min at 80°C
`[48].
`The etch rate of SiN films is extremely sensitive to the incorporation of
`even trace amounts of oxygen in them. In general, the etch rate in HF and
`BHF increases with oxygen incorporation, whereas the etch rate in H3 P04
`decreases. Typically, the etch rate in concentrated HF varies [ 46] from 350
`A/min for SixOyNz films grown at 1000°C with 7% Si02 incorporated in
`them, to as high as 5000 A/min for films with 50% Si02 incorporation. Also,
`it should be noted. that SiN films, grown at low temperatures by plasma(cid:173)
`enhanced CVD (see Chapter 8), contain a large amount of incorporated
`hydrogen and have considerably faster etch rates than those grown by con(cid:173)
`ventional techniques.
`
`9.1.6.5 Polysilicon and Semi-Insulating Polysilicon
`
`The same chemical systems used for etching silicon can be used for both
`undoped and doped polysilicon. In general, their etch rate is considerably
`faster, so that their use results in films with poor edge definition. However,
`these etches can be modified to be suitable for polysilicon. This usually
`consists of greatly reducing the amount of HF, using large ratios of HN03 -
`to HF, and large amounts of diluent. Typical etch rates for these formulations
`are about 1500-7500 A/min for undoped films. Etch rates for doped films
`are strongly dependent on the crystallite size and the doping concentration.
`
`* The etching temperature is thus related to the concentration of the H3P04.
`
`SAMSUNG-1011.004
`
`

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`ISBN 0-471-86833-7
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