Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2001-11-26
2002-11-19
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
Reexamination Certificate
active
06483594
ABSTRACT:
CROSS REFERENCE TO SOFTWARE APPENDIX
Appendix A, included herein as pages 54-62, is a listing of computer programs and related data for use with Visual Basic software version 5.0, 1997, available from Microsoft Corporation. The software may be loaded into a personal computer for implementing a method and apparatus as described below in reference to
FIGS. 4A-4F
in one illustrative embodiment of this invention.
A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever.
DISCUSSION OF THE 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. This anneal also causes implanted atoms to move from interstitial sites to substitutional sites in the crystal lattice (an atom must be in a substitutional site to be electrically active). 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 “measur[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
An apparatus and method in accordance with the invention stimulate a region of a semiconductor wafer (also called “semiconductor substrate”) that originally has a first number of charge carriers, so that there are a second number of charge carriers during the stimulation. The stimulation can be accomplished in any number of ways, including e.g. by use of a beam of electromagnetic radiation or by a beam of electrons. The apparatus and method use a measurement device (such as an interferometer in one embodiment) to obtain a measured value of a signal that is affected by the stimulation. In one embodiment, the affected signal is a probe beam that is reflected by the charge carriers, although other signals can be used in other embodiments.
The apparatus and method also operate a simulator (e.g. a personal computer programmed with simulation software) to generate a simulated value for the measured signal. The simulated value is based on: (i) conditions present during stimulation (as described above) and (ii) a predetermined profile of the concentration of active dopants in the region under stimulation. If the measured value matches the simulated value, then the predetermined profile used in simulation is used as a measure of the profile of active dopants in the region. The simulation may be repeated with a number of such predetermined profiles.
In one implementation, the simulations are repeated (prior to the stimulation) to obtain a set of such profiles, and the corresponding simulated values are used later to obtain a measure of the profile of active dopants in the region, e.g. by finding the closest simulated value to the measured value. In another implementation, one or more simulations are repeated after the stimulation only in case there is no match, until the simulated value and the measured value differ by less than a predetermined amount (e.g. less than 1%), and the corresponding predetermined profile is used as a measure of the profile of active dopants in the region.
The measured profile of active dopants can be used in a number of ways. In one embodiment, the measured profile is used to determine junction depth that is compared with specifications for acceptability of the wafer. If the junction depth falls within the specifications, the wafer is processed further (e.g. in a wafer processing unit to form another layer on the substrate, or in an annealer for heat treatment of the substrate), and otherwise the substrate is identified as unacceptable and placed in a bin of rejected substrates.
In one embodiment, the apparatus and method creates charge carriers in a region of the semiconductor material (also called “carrier creation region”) in a concentration that changes in a periodic manner (also called “modulation”) only with respect to time. Thereafter, the apparatus and method determine the number of charge carriers created in the carrier creation region by (1) measuring an interference signal obtained by interference between a reference beam and a portion of a probe beam that is reflected by the charge carriers, and (2) comparing the measurement with predetermined data (e.g. in a graph of such measurements plotted against junction depth).
Charge carriers that are created as described above (also called “excess carriers”) are in excess of a number of charge carriers (also called “background carriers”) that are normally present in the semiconductor material in the absence of illumination. The concentration of excess carriers is modulated in time at a frequency that is maintained sufficiently small to ensure that the variation in concentration is aperiodic (i.e. not oscillatory, e.g. decays exponentially or according to a monotonic function). Specifically, a profile of excess carrier concentration that is devoid of a wave (a
Borden Peter G.
Nijmeijer Regina G.
Boxer Cross, INC
Natividad Phil
Silicon Valley Patent & Group LLP
Suryadevara Omkar
LandOfFree
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