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 xe2x80x9cspeciesxe2x80x9d or xe2x80x9catomic speciesxe2x80x9d 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(copyright) 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(copyright) 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(copyright) 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 H2+ 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 particlesxe2x80x94instead 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 xe2x80x9cfixxe2x80x9d 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(copyright) 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(copyright) 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(copyright) type are typically greater than 1016 atoms per square centimeter (cm2).
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 xe2x80x9cRutherford backscattering spectrometry (RBS)xe2x80x9d by Scott M. Baumann, published by Charles Evens and 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 is poorly adapted to characterizing uniformity of implantation. In addition, that method is very expensive to implement.
It can thus be seen that although various methods do indeed exist making it possible, to some extent, to characterize the dose of species that have been implanted or the uniformity of implantation, there nevertheless remains a need for a method that is fast and simple and that makes it possible characterize both aspects simultaneously, while avoiding the drawbacks mentioned above.
It is also specified that it has already been possible to show up the influence of various parameters of an implantation step on the structure of an implanted substrate. In this regard, there exists an article by L. J. Huang et al., xe2x80x9cModel for blistering of hydrogen implanted silicon and its application to silicon-on-quartzxe2x80x9d, Electrochemical Society Proceedings, Processing of 8th International Symposium on Semiconductor Silicon {Vol. 98-1, May 4-8, 1998, pp. 1373-1384}. However, that article does no more than observe a resultant effect on the substrate after implantation as a function of various different implantation parameters. Under no circumstances does that article suggest the converse, i.e., making use of an observation of said resultant effect to characterize the implanted dose.
It should also be observed that when it comes to characterizing implantation dose, that article discloses a method of observing the implanted substrate that is relatively expensive to implement (transmission electron microscope (TEM) type observation seeking to provide a section image through the depth of the substrate).
Other documents are also known which characterize to some extent the influence of an implantation dose on the characteristic of the implanted substrate. By way of example, mention can be made of an article by Shiettekatte et al. xe2x80x9cDose and implantation temperature influence extended defects nucleation in c-Sixe2x80x9d, Nuclear instruments and methods in physics research, section B: beam interactions with materials and atoms, North-Holland Publishing Company, Vol. 164-165, April 2000 (2000-04), pp. 425-430. However, in that case also, the article does no more than observed that effects exist that are the result of variations in various implantation parameters, and it does not suggest any way of making use of such observations for characterizing implantation parameters themselves. In addition, that method is likewise relatively expensive, being of the TEM type, and it is used to observe the implanted substrate: in the context of that article, it is xe2x80x9cextended defectsxe2x80x9d buried in the thickness of a substrate that are observed. Finally, it should be observed that that article sets out to characterize the influence of implantation temperature, and is not in any way focused on the influence of implantation dose.
Mention is also made of an article by Da Silva et al., xe2x80x9cThe effects of implantation temperature on He bubble formation in siliconxe2x80x9d, Nuclear instrument and methods in physics research, section B: beam interactions with materials and atoms, North-Holland Publishing"" Company, Amsterdam, NL, Vol. 175-177, April 2001 (2001-04), pp. 335-339. In that article also, there is no suggestion of making use of the observations performed to characterize implantation dose. Furthermore, the methods used for observing the in-depth structure of the implanted substrate are expensive, being of the TEM or the RBS type, and the article focuses solely on the influence of implantation temperature and does not consider aspects associated with dose. It should also be observed that the annealing to which the substrate is subjected in the context of that article is of the rapid thermal annealing (RTA) type, whereas, as explained below in the context of the present invention, it is desirable to avoid annealing temperatures that are too high.
It can thus be seen that the documents mentioned above seeking to observe the influence of various characteristics of an implantation step on an implanted substrate do not satisfy the above-mentioned need. The present invention now provide improvements over known methods in this area.
The invention relates to a method for characterizing a dose or dosage of implanted atomic species in a substrate. The method comprises annealing the substrate after implantation of the atomic species, with the anneal conducted at a temperature and for a time sufficient to cause the implanted atomic species to from blisters in a surface region of the substrate but below that which would cause a majority or at least a significant amount of the blisters to burst; imaging the surface region of the substrate to obtain a surface image; and processing the surface image to characterize the implant dose of the atomic species. This characterization can be performed on a qualitative or quantitative basis, as desired.
For example, the density and size of the blisters may be analyzed or calculated. In an embodiment, the surface image is obtained by a charge coupled device and the implant dose is characterized by a density parameter. In another embodiment, the blister area may be calculated. These calculations allow the implantation dose to be calibrated, prior to annealing, to obtain a desired density or size of blisters to be obtained in the substrate.
The dose of implanted atomic species may be calculated from blister density parameters or by comparing the processed surface image to images of known implanted doses of atomic species. Also, compensation factors may be established for implantation dose measurements by comparison of the processed image to reference implantation data. The compensation factor may be applied to an implanter to obtain improvements in subsequent implanted doses. Also, the compensation factor may also be determined by balancing implantation operations performed by different implanters that are used to implant the atomic species.
On the qualitative side, the spatial distribution of the blisters from the processed image may be analyzed to determine uniformity of implantation of the atomic species. Different blister measurements can be performed on different locations on the substrate so as to obtain a spatial distribution of the dose over the surface of the substrate. Furthermore, such measurements can be made on a plurality of substrates which have been annealed under the same conditions but with different orientations in order to determine local temperature effects. The processed image may also be observed to characterize the uniformity or thickness of the implanted dose of atomic species. Such uniformity may be determined by establishing regions of the substrate that have received a dose of atomic species per unit area that differ from a mean dose of atomic species that is received by the substrate.
Preferably, the atomic species that is implanted comprises hydrogen or helium and the implantation is conducted at a dose of greater than 1016 atoms per square centimeter, the substrate is a semiconductor material such as silicon and the annealing is conducted for a time of between about 5 and 20 minutes at a temperature of between 300 and 550xc2x0 C.