Semiconductor device manufacturing: process – Introduction of conductivity modifying dopant into... – Ion implantation of dopant into semiconductor region
Reexamination Certificate
2004-03-26
2004-12-21
Lebentritt, Michael S. (Department: 2824)
Semiconductor device manufacturing: process
Introduction of conductivity modifying dopant into...
Ion implantation of dopant into semiconductor region
C438S514000
Reexamination Certificate
active
06833314
ABSTRACT:
BACKGROUND
The present invention relates to operations for characterizing the processing to which a substrate of material is subjected. More precisely, the invention relates to a method for characterizing dosage in a step of implanting one or more atomic species in a substrate. The substrate is generally a semiconductor material such as silicon.
The term “species” or “atomic species” as used herein means any type of ion or atom that can be implanted into a substrate. As explained below, in the most preferred application of the invention, the species is H
+
ions and/or hydrogen atoms H.
By way of example, one way of implanting species (ions or atoms) into a material substrate is to expose the surface of the substrate to bombardment by the species. As a function of the energy associated with the bombardment, and as a function of the nature of the species being implanted, the atomic species becomes implanted in the mass of the substrate with a distribution that presents a well-marked maximum at a given depth. This establishes a concentration maximum for the implanted species at a given depth in the substrate.
For any given species, it is possible to vary this implantation step by controlling implantation energy. An example of a method that implants atomic species in an implementation step is described in U.S. Pat. No. 5,374,564, where the implantation step is used for fabricating a thin film or layer of a semiconductor material. One such method according to the teaching of that patent is known as the SMART-CUT® method. In that method, the implantation step is intended to define a plane of weakness in a substrate typically made of a semiconductor material such as a silicon single crystal. A subsequent step in the method is a cleaving step for at least partially fracturing the plane of weakness as defined by the layer of implanted species.
Thus, in SMARTCUT® type methods, the implantation step defines the plane of weakness. Depending upon the implantation characteristics and in particular the implantation dose, cleavage can be achieved more or less easily. In addition, the implantation determines to some extent the roughness of the wafer surface after cleavage.
It has thus been observed in the context of the SMARTCUT® method, that it would be desirable to be able to characterize the dose of species implanted in a material substrate. This need also applies to implanting species in other contexts. In general, it would thus be desirable to be able to characterize two important parameters of implantation, namely:
the dose of species implanted in the substrate; and
the uniformity of implantation in the substrate, at different points over the surface of said substrate.
Methods and apparatuses are known which provide at least partial responses to this need. One method is known which consists in performing in situ measurements, i.e. measurements in real time during implantation, of the dose of species being implanted. For example, U.S. Pat. No. 4,743,767 discloses means for measuring an electric current that is representative of implantation. The method implemented in that patent is based on performing an electrical measurement on a beam of charged particles with which it is desired to implant substrates.
A first drawback of that method is that it does not make it possible to measure electrically neutral species that might be implanted in substrates. Unfortunately, even when implanting species that are initially charged (e.g., H
+
ions), at least some of the species can come into collision with residual elements present in the implantation chamber (atoms and/or molecules of oxygen or nitrogen, for example) and lose their electric charge. Such species that have become electrically neutral can nevertheless conserve sufficient energy to become implanted in the substrate, and the above-mentioned method does not enable them to be taken into account.
Similarly, that method does not make it possible to take account in representative manner of species in which the electric charge is transformed in some way. This applies for example to H
2
+
ions which, having a ratio of mass divided by electric charge double that of an H
+
ion, are each counted as being a single ion by such a method, whereas the actual dose that is implanted is twice that. In addition, such a method does not enable uniformity of implantation to be characterized.
There also exist in situ measurement methods which propose solutions to some of the above-mentioned drawbacks. For example, U.S. Pat. No. 4,751,393 describes a method enabling point measurements to be interpolated in order to provide at least partial information concerning uniformity of implantation. Furthermore, U.S. Pat. No. 5,998,798 proposes mitigating the absence of neutral particle measurement to some extent by compensation. However such attempts take account only of one of the above-mentioned drawbacks concerning in situ methods. In addition, the responses provided are imperfect (mere interpolation for uniformity, and a posteriori compensation for measuring neutral particles—instead of directly measuring the implantation of such neutral particles).
Another known method consists in measuring the characteristics of implantation ex situ, i.e., after the implantation step has been performed. A first method of this type consists in performing annealing after implantation, with the annealing parameters being controlled so as to “fix” the implanted species in the structure of the substrate. Following such annealing, the implanted substrate is characterized electrically in such a manner as to measure the implanted dose of species.
A major limitation of that type of method is that it is not suitable for measuring the implantation dose of lightweight species such as hydrogen (or indeed helium). That limitation is particularly penalizing for characterizing implantation by means of a light ion such as hydrogen, which corresponds to a preferred application of a SMARTCUT® type method.
In a second method of taking measurements ex situ, the surface layer of the implanted substrate is characterized optically. U.S. Pat. Nos. 5,834,364 and 4,807,994 are illustrations of such a method. However, in that case also, the method is adapted to measuring implantation with heavy ions such as phosphorus or boron, and is poorly adapted to measuring implantation with light ions such as hydrogen. Furthermore, implementing that method requires specific equipment (e.g., of the THERMAPROBE® type).
Also, U.S. Pat. No. 4,807,994 is also limited to measuring uniformity of implantation. Furthermore, that document discloses a method limited to characterizing relatively small implantation doses, whereas the implantation doses used in a method of the SMARTCUT® type are typically greater than 10
16
atoms per square centimeter (cm
2
).
In a third ex situ method of measurement, it is known to analyze the reflected portion of a single-energy beam of high energy particles directed against a previously-implanted substrate in order to establish a profile of implantation in a surface layer of the substrate. A description of such a method is to be found in the article “Rutherford backscattering spectrometry (RBS)” by Scott M. Baumann, published by Charles Evens & Associates, 810 Kifer Road, Sunnyvale, Calif., USA.
A first limitation of such a method is that it is ill-suited to characterizing uniformity of implantation. To do that it would be necessary to proceed with a multitude of point-by-point measurements, which would be tedious and expensive. In addition, the thickness of the substrate layer that can be characterized in that way remains limited. Finally, the precision of measurements obtained by that type of method is no better than to within 5%, which is not sufficient in certain applications.
Finally, a fourth ex situ method of measurement consists in using an energy beam to etch the surface of an implanted substrate and then analyzing the substrate as etched in depth. One such method is known as secondary ion mass spectrometry.
A first drawback of that type of method is that it too
Maleville Christophe
Schwarzenbach Walter
Lebentritt Michael S.
Owens Beth E.
S.O.I.Tec Silicon on Insulator Technologies S.A.
Winston & Strawn LLP
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