US6323951B1
Expired / Lapsed
US
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Apparatus and method for determining the active dopant profile in a semiconductor wafer
Assignee: Semiconductor Physics Labs Co Ltd
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Filing: 1999-03-22
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Granted: 2001-11-27
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Expires: 2019-03-22
Abstract
Abstract A method (1) creates charge carriers in a concentration that changes in a periodic manner (also called "modulation") only with respect to time, and (2) determines the number of charge carriers created in the carrier creation region by measuring an interference signal obtained by interference between a reference beam and a portion of a probe beam that is reflected by charge carriers at various depths of the semiconductor material, and comparing the measurement with corresponding values obtained by simulation (e.g. in graphs of such measurements for different junction depths). Various properties of the reflected portion of the probe beam (such as power and phase) are functions of the depth at which the reflection occurs, and can be measured to determine the depth of the junction, and the profile of active dopants. Therefore, the just-described reflected portion of the probe beam is interfered with a reference beam formed by a portion of probe beam reflected by the front surface of the semiconductor material, and phase and amplitude of the interference signal resulting therefrom are both measured. Alternatively, a phase difference between a first interference signal (obtained by interference of (1) a variable phase beam and (2) the portion of probe beam reflected by the front surface) and a second interference signal (obtained by interference of (1) the variable phase beam and (2) a portion of the probe beam reflected by charge carriers at various depths) indicates the junction depth.
Inventors
- 👤 Peter G. Borden
- 👤 Regina G. Nijmeijer
Bibliographic Details
Application Number
—
Jurisdiction
US
Data Source
google_patents
Last Synchronized
2026-11-09 20:45
CPC Classifications
G; G01; G01N; G01N21/00; G01N21/17; G01N21/1717
Claims Analysis
Total Claims: 28
Claim 1
Independent Claim
122 words
A method for performing a measurement in a region of a wafer having a plurality of background carriers, the method comprising: creating a plurality of excess carriers in the region, the number of excess carriers in the plurality being modulated in time at a predetermined frequency that is sufficiently small to ensure that a majority of carriers move out of the region by diffusion; and measuring amplitude and phase of an interference signal, the interference signal being obtained by interference between: a reference beam; and a portion of a probe beam of electromagnetic radiation reflected by the plurality of excess carriers in the region, the portion of the probe beam being modulated in phase with modulation of the plurality of excess carriers.
Claim 2
Independent Claim
122 words
A method for performing a measurement in a region of a wafer having a plurality of background carriers, the method comprising: creating a plurality of excess carriers in the region, the number of excess carriers in the plurality being modulated in time at a predetermined frequency that is sufficiently small to ensure that a majority of carriers move out of the region by diffusion; and measuring amplitude and phase of an interference signal, the interference signal being obtained by interference between: a reference beam; and a portion of a probe beam of electromagnetic radiation reflected by the plurality of excess carriers in the region, the portion of the probe beam being modulated in phase with modulation of the plurality of excess carriers.
Claim 3
Dependent Claim
28 words
The method of claim 1 wherein: the reference beam is formed by another portion of the probe beam that is reflected from a front surface of said substrate.
Claim 4
Dependent Claim
49 words
The method of claim 2 further comprising: using predetermined data to look up a junction depth corresponding to measured values of the amplitude and phase obtained by the “measuring amplitude and phase”; wherein the predetermined data relates the measured value to a known junction depth of a predetermined substrate.
Claim 5
Dependent Claim
35 words
The method of claim 3 further comprising: generating the predetermined data by operating a simulator to determine a profile of excess carriers as a function of depth for a given dopant profile in the substrate.
Claim 6
Dependent Claim
91 words
The method of claim 4 wherein said generating further comprises: multiplying: a derivative of the profile of excess carriers with respect to depth from the front surface; and cos(2knz), wherein z is the depth, k=2π/λ, λ is the wavelength of the probe beam, and n is the index of refraction of the substrate; integrating the product of the multiplying with respect to depth from the front surface; and multiplying a result of the integrating, with a constant to determine a simulated value of the amplitude and phase of the interference signal.
Claim 7
Dependent Claim
46 words
The method of claim 1 further comprising: repeating the measuring at a plurality of times during the modulation; and computing an average value of a property of the region from a plurality of measured values of the property obtained during the repeating, and during the measuring.
Claim 8
Dependent Claim
21 words
The method of claim 6 further comprising: using predetermined data to look up a junction depth corresponding to the measured value.
Claim 9
Dependent Claim
76 words
The method of claim 1 wherein the region is henceforth referred to as first region, the method further comprising: creating a plurality of excess carriers in a second region adjacent to the region; repeating the “measuring amplitude and phase” in the second region; and computing a difference between a second measured value in the second region obtained during the repeating and a first measured value in the first region obtained during the “measuring amplitude and phase.”
Claim 10
Dependent Claim
31 words
The method of claim 8 further comprising: comparing the difference with a predetermined limit; and changing a process parameter used in fabricating the substrate if the difference exceeds a predetermined limit.
Claim 11
Dependent Claim
21 words
The method of claim 1 further comprising: annealing the substrate to activate dopants; wherein the measuring is performed after the annealing.
Claim 12
Dependent Claim
86 words
The method of claim 1 further comprising: changing an average number of excess carriers in the plurality; repeating the “measuring amplitude and phase”; and using a plurality of predetermined curves fitted to the predetermined data to look up a junction depth that best fits a first measured value determined by the “measuring amplitude and phase” and a second measured value determined by the repeating; wherein each predetermined curve relates the amplitude and phase to one of a plurality of known junction depths of a predetermined substrate.
Claim 13
Dependent Claim
33 words
The method of claim 1 wherein the predetermined frequency is in conformance with the formula: f ≦(1/2πτ) where f is the frequency, and τ is the lifetime of a carrier in the substrate.
Claim 14
Dependent Claim
122 words
The method of claim 1 wherein said plurality of excess carriers are created by a generation beam formed by a laser, the generation beam having a first wavelength λg and the probe beam having a second wavelength λp, the second wavelength λp being in conformance with the formula: λ g ≧[(10 αpPpλp )/(α gPg )][ w g /w p ] 2 wherein αp and αg are the absorption coefficients in the substrate of the probe beam and the generation beam respectively, Pp and Pg are the powers of the probe beam and the generation beam respectively, and w g and w p are radii of focal spots of the generation beam and the probe beam at a front surface of said wafer.
Claim 15
Dependent Claim
25 words
The method of claim 13 wherein: the generation beam includes a plurality of photons that have energy greater than the bandgap energy of the wafer.
Claim 16
Dependent Claim
34 words
The method of claim 13 wherein: the generation beam includes a plurality of photons that have energy greater than the bandgap energy of crystalline silicon but less than the bandgap energy of amorphous silicon.
Claim 17
Dependent Claim
26 words
The method of claim 1 wherein: the probe beam beam includes a plurality of photons that have energy lower than the bandgap energy of the wafer.
Claim 18
Dependent Claim
26 words
The method of claim 1 wherein: the probe beam includes a plurality of photons that have energy approximately equal to the bandgap energy of the wafer.
Claim 19
Dependent Claim
40 words
The method of claim 1 wherein: the probe beam is coherent; the reference beam is coherent with respect to the probe beam; and the reference beam has a phase that is variable independent of the phase of the probe beam.
Claim 20
Dependent Claim
18 words
The method of claim 18 further comprising: changing the phase of the reference beam; and repeating the measuring.
Claim 21
Dependent Claim
69 words
The method of claim 19 wherein: the measuring includes using a phase detector to detect the difference in phase between: a first interference signal obtained by interference of the reference beam and another portion of the probe beam reflected by the front surface; and a second interference signal obtained by interference of the reference beam and the portion of the probe beam reflected by the plurality of excess carriers.
Claim 22
Independent Claim
128 words
An apparatus for performing a measurement in a region of a wafer having a plurality of background carriers, said apparatus comprising: means for creating a plurality of excess carriers in a region of the substrate, the number of excess carriers in the plurality being modulated at a frequency that is sufficiently small to cause a majority of carriers moving out of the region to transfer by diffusion; a source of a probe beam of electromagnetic radiation; and an interferometer located in a path of a signal obtained by interference between a reference beam and a portion of the probe beam reflected by the plurality of excess carriers in the region, the portion of the probe beam being modulated in phase with modulation of the plurality of excess carriers.
Claim 23
Independent Claim
128 words
An apparatus for performing a measurement in a region of a wafer having a plurality of background carriers, said apparatus comprising: means for creating a plurality of excess carriers in a region of the substrate, the number of excess carriers in the plurality being modulated at a frequency that is sufficiently small to cause a majority of carriers moving out of the region to transfer by diffusion; a source of a probe beam of electromagnetic radiation; and an interferometer located in a path of a signal obtained by interference between a reference beam and a portion of the probe beam reflected by the plurality of excess carriers in the region, the portion of the probe beam being modulated in phase with modulation of the plurality of excess carriers.
Claim 24
Dependent Claim
52 words
The apparatus of claim 21 wherein the interferometer includes a lock-in amplifier that detects amplitude and phase of a signal obtained by interference between: the portion of the probe beam reflected by the plurality of excess carriers; and another portion of the probe beam reflected by a front surface of the substrate.
Claim 25
Dependent Claim
76 words
The apparatus of claim 21 further comprising: a detector coupled to the interferometer, wherein the detector is used to measure a difference in phase between: a first interference signal obtained by interference of the reference beam and another portion of probe beam reflected by a front surface of the substrate; and a second interference signal obtained by interference of the reference beam and the portion of the probe beam reflected by the plurality of excess carriers.
Claim 26
Dependent Claim
42 words
The apparatus of claim 21 further comprising: a computer coupled to the interferometer, the computer being programmed to use predetermined data to look up a junction depth that corresponds to a power of the portion of probe beam measured by the interferometer.
Claim 27
Dependent Claim
35 words
The apparatus of claim 21 wherein said frequency is in accordance with the formula: [ f ≦(10/2πτ)] f ≦(1/2πτ) where f is the frequency, and τ is the lifetime of one of the excess carriers.
Claim 28
Dependent Claim
119 words
The apparatus of claim 21 wherein: said means for creating includes a laser that generates a generation beam of electromagnetic radiation having a first wavelength λg; and the probe beam has a second wavelength λp, the second wavelength λp being in accordance with the formula: λ g ≧[(10 αpPpλp )/( αgPg )][ w g /w p ] 2 wherein αp and αg are the absorption coefficients in the substrate of the probe beam and the generation beam respectively, Pp and Pg are the powers of the probe beam and the generation beam respectively, and w g and w p are radii of focal spots of the generation beam and the probe beam at a front surface of said wafer.
References Cited (Backward Citations)
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