Electrical resistors – With base extending along resistance element – Resistance element coated on base
Reexamination Certificate
2000-02-14
2002-06-04
Enad, Elvin (Department: 2832)
Electrical resistors
With base extending along resistance element
Resistance element coated on base
C338S309000, C438S210000, C438S239000
Reexamination Certificate
active
06400252
ABSTRACT:
TECHNICAL FIELD
The invention generally relates to electronic components, primarily electronic components which are included in electronic integrated circuits or are produced using the corresponding processing methods, and in particular to electric resistors of polycrystalline silicon, germanium or silicon-germanium, and to a method of manufacturing such resistors.
BACKGROUND OF THE INVENTION AND STATE OF THE ART
Resistors of polycrystalline silicon, also called polysilicon, have been used in the electronic circuit field for about thirty years. Methods of manufacturing polycrystalline silicon are well-known, as are methods of manufacturing resistors from polycrystalline silicon. It is also previously known how it is possible, by adding dopants, to control the resistivity of the polysilicon to a desired value. The general technology is described in the book “Poly Silicon for Integrated Circuit Applications” by T. Kamins, ISBN 0-89838-259-9, Kluver Academic Publishers, 1988.
In analog electronic circuits the requirements of stability of included resistors are extremely high: both the specifications in regard of largest allowable change of the absolute value of the resistance must be fulfilled, and possible changes of the resistances of resistors, which are matched to each other, must be such that the mutual relationship of the resistances of the resistors is all the time accurately maintained. This can only be done if the resistors are sufficiently stable during all of the time period when the circuit is used, i.e. if the resistances of the resistors maintain a sufficient constancy during all of this time period.
Previously known methods for influencing and particularly improving the long term stability of polysilicon resistors are described in the following documents: M. Rydberg and U. Smith, “Electrical Properties of Compensation-Doped Polysilicon Resistors”, May 1998, sent to IEEE Trans. Electron Devices, M. Rydberg and U. Smith, “The Effect of Fluorine on the Electrical Properties of Polysilicon IC-Resistors”, May 1998, sent to IEEE Trans. Electron Devices, M. Rydberg and U. Smith, “Improvement of the long-term stability of polysilicon integrated circuit resistors by fluorine doping”, Mat. Res. Symp. Proc. Vol. 472, (1997), M. Rydberg, U. Smith, A. Söderbärg and H. Hansson, “Compensation doping of polysilicon films for stable integrated circuit resistors”, Diffusion-and-Defect-Data-Part-B-(Solid-State-Phenomena), Vol. 51-52, pp. 561-566 (1996), the published International patent application WO 97/49103 which has inventors U. Smith and M. Rydberg and discloses a polysilicon resistor having a high long term stability resulting from a high doping with fluorine, and finally the published International patent application WO 97/10606 which has inventors U. Smith, M. Rydberg and H. Hansson and discloses a polysilicon resistor having an increased stability of its resistance because of such a high concentration of donors that charge carrier traps are blocked at the grain boundaries.
In U.S. Pat. No. 5,212,108 for M. S. Liu, G. A. Shaw and J. Yue, “Fabrication of stabilized polysilicon resistors for SEU control” stabilisation of resistors between and within different manufacturing batches is described. This patent is thus concerned with the statistical spread of resistance values in the manufacturing process whereas the problem discussed herein relates to the change of the resistance values in time.
In applications in which polysilicon resistors are used in critical portions of electronic circuits, the insufficient stability of the resistors is a known practical problem. The fact is that the resistors can, when being used, in an unforeseeable way change their resistance values. Such deviations from the value set by the designer, as well as deviations between the resistance values of resistors matched to each other, can jeopardize the operation of the electronic circuit in which such resistors are included. The cause of the instability is to search for in the unsaturated bonds existing in the grain boundaries in the material. The unsaturated bonds are formed in the boundaries between the individual monocrystalline grains in the polycrystalline material due to the fact that the periodic ordering of the silicon atoms in the shape of a crystal lattice does not exist there. The outermost silicon atoms in a monocrystalline grain therefore have not sufficiently many silicon atoms as close neighbours in order to be capable of forming the four bonds which are characteristic of the lattice of silicon crystals. The resulting unsaturated bonds act as traps for charge carriers and thereby bind charges to the grain boundaries what influences the capability of the material to transport charge carriers and thereby the resistivity of the material.
If the number of bonded charges would remain constant during the manufacture of the resistors and during all of the time when the resistor is used, no problems in regard of the stability of the resistors would exist. However, the number of traps can decrease if individual atoms can migrate to the grain boundaries and be attached to the unsaturated bonds and thereby prevent them from continuing to work as traps for the charge carriers. In the same way the number of traps can increase in the case where the atoms leave their positions at the grain boundaries and then each one leave a remaining unsaturated bond.
It is known that the unsaturated bonds can be blocked by hydrogen atoms in the grain boundary. Hydrogen can exist in a high concentration in layers deposited on a circuit containing a polysilicon resistor, for example in passivation layers of silicon dioxide and/or silicon nitride, what results from the special production thereof. The hydrogen atoms react with the unsaturated bonds in the polysilicon resistor and block them so that they cannot continue to work as traps. However, a problem associated with hydrogen atoms which have been bonded to the unsaturated bonds is that the bonding strength between hydrogen and silicon is low compared to for example the mutual bond between silicon atoms. The bonds between silicon and hydrogen can therefore be easily broken whereby the unsaturated bonds are again exposed. Since unsaturated bonds capture charge carriers this will result in that the value of the resistivity is changed. To the extent that the causes of the bonds being broken are known, they can be referred to a general increase of the temperature or to local temperature variations caused by an increased power production in critical points in the resistor. However, it can not be excluded that the bonds can also be broken because of kinetic or quantum-mechanical effects caused by the transport of charge carriers through the resistor.
Though the capability of the hydrogen atoms to block unsaturated bonds is what is primarily discussed in literature, it cannot be excluded that other atoms which happen to be placed in a grain boundary or leaves it in the manufacturing process and in the use of the resistor cause similar effects, if they have not the capability of being sufficiently strongly bonded to the silicon atoms of the grain boundary. Without indicating here the magnitude of the influence, it can be mentioned that it is also possible that dopant atoms which when using the resistor interact with the grain boundaries in a dynamic way can have the same influence on the resistivity as the hydrogen atoms. In the same way it cannot be excluded that also other kinds of atoms and unintentionally added impurities included in the resistor and/or in the circuit plate, of which the resistor normally is a part, can have the same influence.
SUMMARY OF THE INVENTION
It is an object of the invention to provide polysilicon resistors having a good long term stability, i.e. having a good constancy of their resistances, which in. a safe way can be used particularly in sensitive electronic circuits such as circuits of analog type, intended for example for measurements or intended to be part of sensors, in which the resistance values of the resistors for example included in
Rydberg Matts
Smith Ulf
Enad Elvin
Lee Kyung S.
Nixon & Vanderhye P.C.
Telefonaktiebolaget LM Ericsson
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