Optical stress generator and detector

• US 5,748,318 A
• Filed: 08/06/1996
• Issued: 05/05/1998
• Est. Priority Date: 01/23/1996
• Status: Expired due to Term

• Current Assignee: Brown University
• Original Assignee: Brown University
• Inventors: Maris, Humphrey J., Stoner, Robert J.
• Primary Examiner(s): Nelms, David C.
• Assistant Examiner(s): Smith, Zandra V.
• Application Number:
  US08/689,287
• Time in Patent Office: 637 Days
• Field of Search: 364/562, 364/563, 364/578, 356/381
• US Class Current: 356/630
• CPC Class Codes: G01N 21/1702 with opto-acoustic detection, e.g. for gases or analysing solids
• CPC Class Codes: G01N 21/59 Transmissivity G01N21/25 takes precedence
• CPC Class Codes: G01N 2291/02827 Elastic parameters, strength or force
• CPC Class Codes: G01N 2291/02881 Temperature
• CPC Class Codes: G01N 2291/0426 Bulk waves, e.g. quartz crystal microbalance, torsional waves
• CPC Class Codes: G01N 2291/0427 Flexural waves, plate waves, e.g. Lamb waves, tuning fork, cantilever
• CPC Class Codes: G01N 29/0681 by acoustic microscopy, e.g. scanning acoustic microscopy
• CPC Class Codes: G01N 29/2418 using optoacoustic interaction with the material, e.g. laser radiation, photoacoustics photoacoustic cells G01N21/1702; measuring characteristics of vibrations by using radiation-sensitive means G01H9/00; acousto-optical conversion techniques for short-range imaging G01S15/8965; sound-producing devices using laser bundle G10K15/046

Abstract

Disclosed is a system for the characterization of thin films and interfaces between thin films through measurements of their mechanical and thermal properties. In the system light is absorbed in a thin film or in a structure made up of several thin films, and the change in optical transmission or reflection is measured and analyzed. The change in reflection or transmission is used to give information about the ultrasonic waves that are produced in the structure. The information that is obtained from the use of the measurement methods and apparatus of this invention can include: (a) a determination of the thickness of thin films with a speed and accuracy that is improved compared to earlier methods; (b) a determination of the thermal, elastic, and optical properties of thin films; (c) a determination of the stress in thin films; and (d) a characterization of the properties of interfaces, including the presence of roughness and defects.

Claims

1. A method for characterizing a structure, comprising the steps of:

applying first electromagnetic radiation to the structure for creating propagating stress pulses within the structure;

applying second electromagnetic radiation to the structure at a plurality of different incidence angles so as to intercept the propagating stress pulses;

sensing a reflection or transmission of the second electromagnetic radiation from the structure at the plurality of incidence angles;

associating a change in the reflection of the second electromagnetic radiation over time with a value of an optical characteristic of the structure, and determining in accordance with the value of the optical characteristic the velocities of the propagating stress pulses; and

optionally determining the elastic modulus of the structure in accordance with the determined velocities of the propagating stress pulses.

2. A method for characterizing a three dimensional sample comprised of a substrate and possibly one or more films deposited on said substrate together with at least one structure that is disposed upon or embedded within the substrate or one or more of the films, comprising the steps of:

simulating a mechanical response of the sample, at a plurality of discrete time steps, to the application of pulses of first electromagnetic radiation;

applying pulses of the first electromagnetic radiation to the sample for creating propagating stress pulses within the sample;

applying second electromagnetic radiation to the sample so as to intercept the propagating stress pulses;

sensing from a reflection of the second electromagnetic radiation from the sample at least one of a time-varying change in intensity, position, direction, polarization state, and optical phase of the second electromagnetic radiation; and

associating the sensed time-varying change with a property of interest of the sample in accordance with the simulated response of the sample.

3. A method as set forth in claim 2, wherein the property of interest includes a dimension that is other than a thickness of the at least one film.

4. A method for characterizing a structure, comprising the steps of:

simulating at predetermined time step increments, in accordance with one or more characteristics of the structure, a mechanical response of a simulated structure over an interval of time to an application of a first pulse of optical radiation by at least the steps of,

determining an initial stress distribution within the simulated structure,

determining a change over the interval of time in the stress and strain distribution in the simulated structure following an application of the first pulse of optical radiation, and

determining the transient optical response of the simulated structure by application of a second pulse of optical radiation within the interval of time;

applying the first pulse of optical radiation to the structure;

applying, during the interval of time, the second pulse of optical radiation to the structure;

comparing a measured transient response of the structure to the determined transient response for the simulated structure;

adjusting a value of the one or more characteristics of the simulated structure so as to bring the determined transient response into agreement with the measured transient response; and

associating the adjusted value of the one or more characteristics with a value of one or more actual characteristics of the structure.

5. A method as set forth in claim 4, wherein the structure further comprises at least one layer disposed over a substrate, and wherein the step of determining an initial stress distribution within the simulated structure includes the steps of:

from optical constants and a thickness of the at least one layer, calculating an electric field, due to the first optical pulse, in the simulated structure in terms of an amplitude, angle of incidence, and polarization of the first pulse on a surface of the structure;

from the calculated electric field distribution, calculating an amount of energy absorbed in the simulated structure as a function of position;

determining an effect of thermal diffusion on the absorbed energy distribution;

calculating a temperature rise as a function of position within the simulated structure; and

calculating a stress within the simulated structure from the calculated temperature rise.

6. A method as set forth in claim 4, wherein the step of determining a change in stress and strain in the simulated structure includes the steps of:

selecting a time step τ

;

for each layer of the simulated structure, calculating a bin size b equal to the time step τ

multiplied by the sound velocity in the layer; and

dividing each layer into bins of the calculated bin size or bins smaller than the calculated bin size.

7. A method as set forth in claim 6, wherein the step of determining a change in stress and strain in the simulated structure further includes the steps of:

decomposing the stress in each bin into two components, one component initially propagating towards a free surface of the simulated structure and one component propagating away from the free surface of the simulated structure;

within each layer, stepping the two components forward from bin to bin in the appropriate direction, wherein for a bin adjacent to a boundary between two layers the stress propagating towards the boundary is stepped partly into the first bin on the other side of the boundary;

repeating the foregoing steps for a sufficient number of time steps τ

to determine the stress distribution for a period at least equal the interval of time; and

calculating the strain from the determined stress by division by an appropriate elastic coefficient.

8. A method as set forth in claim 4, wherein the structure further comprises at least one layer disposed over a substrate, and wherein the step of determining a transient response of the simulated structure to an application of a second pulse of optical radiation within the interval of time includes the steps of:

calculating changes Δ

n and Δ

κ

in optical constants of each layer from calculated strain distribution as a function of depth into the simulated structure; and

from the calculated changes Δ

n and Δ

κ

in the optical constants as a function of depth, and from unperturbed optical constants of the at least one layer, calculating at least one of the quantities Δ

R, Δ

T, Δ

P, Δ

.o slashed. and Δ

β

;

wherein Δ

R=a modulated chance in an intensity of a reflected portion of the second pulse, Δ

T=a chance in an intensity of a transmitted portion of the second pulse, Δ

P=a chance in a polarization of the reflected second pulse, Δ

.o slashed.=a change in an optical phase of the reflected second pulse, and Δ

β

=a chance in an angle of reflection of the second pulse.

9. A method as set forth in claim 8, wherein the step of comparing a measured transient response of the structure to the determined transient response compares the at least one of the calculated quantities Δ

R, Δ

T, Δ

P, Δ

.o slashed. and Δ

β

with a measured result.

10. A method as set forth in claim 4, wherein said plurality of discrete time steps are selected to be small compared to a time required for an acoustic wave to propagate through a thinnest layer that comprises the structure.

11. A non-destructive system for characterizing a sample, comprising:

means for generating an optical pump pulse and for focussing the pump pulse relative to a surface of the sample;

means for generating an optical probe pulse and for focussing the probe pulse relative to the surface of the sample;

means for measuring at least one transient response of the structure to the pump pulse by detecting a change in a reflected or transmitted portion of the probe pulse;

means for simulating an effect of an application of pump and probe pulses to an area on the surface of the sample; and

detector means for automatically adjusting the focus of at least one of the pump and probe pulses in response to reflected portions of at least one of the pump and probe pulses in order to stabilize an effective area of overlap of the pump and the probe pulses on the surface of the sample so as to be in agreement with the simulation.

12. A non-destructive system for characterizing a sample as set forth in claim 11, wherein said means for generating an optical pump pulse generates a train of pump pulses which are applied to a single location on the surface of the sample.

13. A non-destructive system for characterizing a sample as set forth in claim 11, wherein the pump pulse induces a stress pulse in the sample that propagates normal to the surface.

14. A non-destructive system for characterizing a sample, comprising:

means for a generating a sequence of optical pump pulses and for directing the sequence of pump pulses to an area of the surface of the sample;

means for generating a sequence of optical probe pulses, wherein a delay between individual ones of the probe pulses, with respect to an individual one of the pump pulses, is varied at a frequency f and for directing the sequence of delay varying probe pulses to a same or different area of the surface of the sample; and

means for synchronizing, as a function of f, the measurement of at least one transient response of the structure to the sequence of pump pulses by detecting a change in a characteristic of a reflected or transmitted portion of the sequence of delay varying probe pulses.

15. A non-destructive system for characterizing a sample, comprising:

means for generating an optical pump pulse having a first wavelength and for directing the pump pulse to an area of the surface of the sample;

means for generating an optical probe pulse from the optical pump pulse and for directing the probe pulse to a same or different area of the surface of the sample so as to arrive after the pump pulse, the optical probe pulse being generated to have a second wavelength that is a harmonic of the first wavelength; and

means for measuring at least one transient response of the structure to the pump pulse by detecting a change in a characteristic of the reflected or transmitted portion of the probe pulse.

16. A non-destructive system for characterizing a sample, comprising:

means for generating an optical pump pulse and an optical probe pulse from an input pulse having a first wavelength, wherein the pump pulse has a wavelength that is a harmonic of the first wavelength and the probe pulse has a wavelength that is equal to the first wavelength;

means for directing the pump pulse to an area of the surface of the sample and for directing the probe pulse to a same or different area of the surface of the sample so as to arrive after the pump pulse; and

means for measuring at least one transient response of the structure to the pump pulse by detecting a change in a characteristic of the reflected or transmitted portion of the probe pulse.

17. A method for operating a non-destructive system for characterizing a sample, comprising the steps of:

generating an optical pump pulse and directing the pump pulse to an area of the surface of the sample;

for each generated optical pump pulse, generating an optical probe pulse and directing the probe pulse to the surface of the sample so as to arrive after the pump pulse, wherein some of the probe pulses are directed to the surface at a first angle relative to the surface, and others of the probe pulses are directed to the surface at a second angle relative to the surface; and

measuring at least one transient response of the structure to the pump pulses by detecting a change in a reflected portion of the probe pulses at each of the first and second angles.

18. A method for characterizing a structure comprised of a substrate and at least one layer that is an intentionally or a non-intentionally formed layer that is disposed over the substrate, comprising the steps of:

generating a reference data set of a transient optical response of the structure to an optical pump pulse, the reference data set being generated from at least one of (a) at least one reference sample or (b) a simulation of a mechanical motion of a simulated structure at predetermined time step increments selected to have a duration of less than one half of a time required for an acoustic pulse to propagate through a thinnest layer of the structure;

applying a sequence of optical pump pulses and optical probe pulses to the structure;

comparing a measured transient response of the structure to the reference data set;

adjusting a value of the one or more characteristics of the structure so as to bring the reference data set into agreement with the measured transient response; and

associating the adjusted value of the one or more characteristics with a value of one or more actual characteristics of the structure.

19. A method for characterizing a structure comprised of a substrate and at least one layer disposed on the substrate, comprising the steps of:

simulating a response of a model of the structure to an application of a first pulse of optical radiation followed by a transient response of the structure to an application of a second pulse of optical radiation within an interval of time;

applying the first pulse of optical radiation to the structure;

applying, during the interval of time, the second pulse of optical radiation to the structure;

comparing a measured transient response of the structure to the determined transient response;

adjusting one or more characteristics of the model of the structure so as to bring the determined transient response into agreement with the measured transient response; and

associating the adjusted one or more characteristics with one or more actual characteristics of the structure, wherein the step of adjusting adjusts at least one of a crystal orientation within the at least one layer, an interface roughness between the at least one layer and another layer or the substrate, a thermal diffusivity within the at least one layer;

an electronic diffusivity within the at least one layer, optical constants within the at least one layer, derivatives of optical constants with respect to stress or strain within the at least one layer, and a surface roughness of the sample.

20. A method as set forth in claim 19, wherein the step of adjusting further adjusts a static stress within the at least one layer.

21. A method as set forth in claim 19, wherein the step of adjusting further adjusts a presence of or a thickness of a region of intermixing between two layers or a layer and the substrate.

22. A method for characterizing a structure comprised of a substrate and at least one layer disposed on the substrate, comprising the steps of:

simulating a mechanical response of a model of the structure to an application of a first pulse of optical radiation by the steps of determining an initial stress distribution within the structure in response to the first pulse of optical radiation, calculating acoustical normal modes of the structure, decomposing the determined initial stress distribution into a sum over the calculated normal modes, and determining a change in a transient optical response of the structure, at a time of interest, to a second pulse of optical radiation by summing, for each calculated normal mode, a change in the transient optical response due to a spatial stress pattern associated with each normal mode;

applying the first pulse of optical radiation to the structure;

applying, at the time of interest, the second pulse of optical radiation to the structure;

comparing a measured transient optical response of the structure to the determined transient optical response;

adjusting one or more characteristics of the structure so as to bring the determined transient optical response into agreement with the measured transient optical response; and

associating the adjusted one or more characteristics with one or more actual characteristics of the structure.