Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2000-04-06
2002-12-03
Karlsen, Ernest (Department: 2829)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S754120, C324S765010, C356S432000, C356S445000
Reexamination Certificate
active
06489801
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the evaluation of a wafer of semiconductor material, and in particular to the measurement of a property of the semiconductor material.
2. Description of Related Art
In the processing of a semiconductor wafer to form integrated circuits, charged atoms or molecules are directly introduced into the wafer in a process called ion implantation. Ion implantation normally causes damage to the lattice structure of the wafer, and to remove the damage, the wafer is normally annealed at an elevated temperature, typically 600° C. to 1100° C. Prior to annealing, material properties at the surface of the wafer may be measured, specifically by using the damage caused by ion implantation.
For example, U.S. Pat. No. 4,579,463 granted to Rosencwaig et al. (that is incorporated herein by reference in its entirety) describes a method for measuring a change in reflectance caused by a periodic change in temperature of a wafer's surface (see column 1, lines 7-16). Specifically, the method uses “thermal waves [that] are created by generating a periodic localized heating at a spot on the surface of a sample” (column 3, lines 54-56) with “a radiation probe beam . . . directed on a portion of the periodically heated area on the sample surface,” and the method “measure[es] the intensity variations of the reflected radiation probe beam resulting from the periodic heating” (column 3, lines 52-66).
As another example, U.S. Pat. No. 4,854,710 to Opsal et al. (also incorporated herein by reference in its entirety) describes a method wherein the density variations of a diffusing electron-hole plasma are monitored to yield information about features in a semiconductor” (column 1, lines 61-63). Specifically, Opsal et al. state that “changes in the index of refraction, due to the variations in plasma density, can be detected by reflecting a probe beam off the surface of the sample within the area which has been excited” (column 2, lines 23-31) as described in “Picosecond Ellipsometry of Transient Electron-Hole Plasmas in Germanium,” by D. H. Auston et al., Physical Review Letters, Vol. 32, No. 20, May 20, 1974.
Opsal et al. further state (in column 5, lines 25-31 of U.S. Pat. No. 4,854,710): “The radiation probe will undergo changes in both intensity and phase. In the preferred embodiment, the changes in intensity, caused by changes in reflectivity of the sample, are monitored using a photodetector. It is possible to detect changes in phase through interferometric techniques or by monitoring the periodic angular deflections of the probe beam.”
A brochure entitled “TP-500: The next generation ion implant monitor” dated April, 1996 published by Therma-Wave, Inc., 1250 Reliance Way, Fremont, Calif. 94539, describes a measurement device TP-500 that requires “no post-implant processing” (column 1, lines 6-7, page 2) and that “measures lattice damage” (column 2, line 32, page 2). The TP-500 includes “[t] wo low-power lasers [that] provide a modulated reflectance signal that measures the subsurface damage to the silicon lattice created by implantation. As the dose increases, so does the damage and the strength of the TW signal. This non-contact technique has no harmful effect on production wafers” (columns 1 and 2 on page 2). According to the brochure, TP-500 can also be used after annealing, specifically to “optimize . . . system for annealing uniformity and assure good repeatability” (see bottom of column 2, on page 4).
SUMMARY
A method in accordance with this invention: (1) creates charge carriers in a concentration that changes in a cyclical manner (also called “modulation”) only with respect to time, in a region (also called “illuminated region”) of a semiconductor material, and preferably also (2) maintains the charge carriers at an average concentration that remains the same (or at least approximately the same e.g. varies less than 10%) before and during a measurement indicative of the number of charge carriers created in the illuminated region by act (1).
In one embodiment (also called “scanning embodiment”), one or more such measurements are compared each with the other, thereby to identify a sudden change in the measurements. In another embodiment (also called “measurement embodiment”), one or more measurements are compared with similar measurements on wafers (also called “reference wafers”) processed under known conditions and having known properties, thereby to determine one or more process conditions or properties of a wafer under fabrication.
In one implementation, an attribute derived from measurements on a wafer is interpolated with respect to corresponding attributes of wafers having a known material property (or process condition), thereby to determine a corresponding property (or condition) of the wafer under measurement. An example of a process condition is the temperature (also called “annealing temperature”) at which the wafer is annealed. Examples of material properties include surface concentration, mobility, junction depth, lifetime and defects that cause leakage current at the junction (when the junction is reversed biased).
The charge carriers (also called “excess carriers”) being created and measured as described above are in excess of a number of charge carriers (also called “background charge carriers”) that are normally present in the semiconductor material (e.g. due to dopant atoms) even in the absence of illumination. Therefore, in the first act described above, a number of excess carriers are created in the above-discussed region (also called “illuminated region”), e.g. by focusing thereon a laser beam or an electron beam. The concentration of excess carriers is modulated, both at the surface and in the bulk only as a function of time (e.g. by modulating the intensity of the just-described laser or electron beam that is also called “generation beam”).
The frequency of modulation of the concentration of excess carriers is deliberately selected to be sufficiently low to avoid modulation in space (i.e. avoid the creation of a wave of charge carriers). A carrier concentration that is devoid of a wave in space is created when at least a majority of the charge carriers (i.e. greater than 50%) move out of the illuminated region by diffusion. Such a temporal modulation under diffusive conditions (also called “diffusive modulation”) is used to measure the reflectance caused by excess carriers, e.g. by detection of the intensity of a beam (also called a “probe beam”) reflected by the illuminated region at the modulation frequency.
In the second act, an average concentration (e.g. root mean square average) of the excess carriers is determined from a measurement of the above-described reflectance over the time period of a modulation cycle. The average concentration is maintained the same (or approximately the same) prior to and during the measurement of reflectance. Specifically, the creation of new charge carriers (also called “measurement-related” carriers) in addition to the background charge carriers and the excess carriers is minimized or avoided during the reflectance measurement, thereby to maintain the total carrier concentration at or about the just-described average prior to the measurements.
An apparatus (also called “profiler”) that implements the above-described method includes, in one embodiment, a source that produces a probe beam formed of photons of energy lower than the bandgap energy (the energy necessary to generate conduction electrons) of the semiconductor material. Use of such a probe beam source eliminates the measurement-related carriers and the resulting errors that are otherwise created by a prior art apparatus, e.g. in measuring the reflectance with a probe beam that has photons of energy greater than the bandgap energy of silicon (such as the He—Ne laser probe beam described at column 15, line 56 of U.S. Pat. No. 4,854,710).
In addition to the above-described probe beam source, the profiler also includes a photosensitive element (such as
Borden Peter G.
Li Jiping
Nijmeijer Regina G.
Karlsen Ernest
Silicon Valley Patent & Group LLP
Suryadevara Omkar
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