Thin film process monitoring techniques using acoustic waves

• US 5,271,274 A
• Filed: 08/14/1991
• Issued: 12/21/1993
• Est. Priority Date: 08/14/1991
• Status: Expired due to Term

• Current Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford University)
• Original Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford University)
• Inventors: Bhardwaj, Sanjay, Saraswat, Krishna, Khuri-Yakub, B. T.
• Primary Examiner(s): Williams, Hezron E.
• Assistant Examiner(s): Finley, Rose M.
• Application Number:
  US07/745,005
• Time in Patent Office: 860 Days
• Field of Search: 73/597, 73/596, 73/598, 73/602, 73/620, 73/627, 73/628, 73/629, 73/643, 73/657, 73/655, 73/659, 73/661, 73/644
• US Class Current: 73/597
• CPC Class Codes: G01B 17/025 for measuring thickness of coating
• CPC Class Codes: G01N 2291/0231 Composite or layered materials
• CPC Class Codes: G01N 2291/0237 Thin materials, e.g. paper, membranes, thin films
• CPC Class Codes: G01N 2291/02854 Length, thickness
• CPC Class Codes: G01N 2291/044 Internal reflections (echoes), e.g. on walls or defects
• CPC Class Codes: G01N 2291/101 one transducer
• CPC Class Codes: G01N 29/07 by measuring propagation velocity or propagation time of acoustic waves
• CPC Class Codes: G01N 29/12 by measuring frequency or resonance of acoustic waves measuring frequency or resonant frequency of mechanical vibrations or acoustic waves in general G01H1/06, G01H3/04, G01H13/00; acoustic resonators G10K11/04; vibration or shock testing of structures G01M7/00

Abstract

A system and method is disclosed for measuring thickness of at least one film on a substrate by propagating an acoustic wave through the film on a substrate such that echo waves are generated and received by a transducer. An output signal is generated and processed to give a thickness value. The thickness valve is obtained from the time lapse between the propagated wave and receipt of the echo wave; by the frequency domain of the echo wave; or the phase of the echo wave.

Claims

1. A system for measuring thickness of at least one film on a substrate, such constituting a composite unit, said film and substrate having an interface, said system comprising:

means including a buffer rod having an end shaped to make Hertzian contact with said composite unit for propagating a high frequency acoustic wave through said composite unit, said acoustic wave generating reflected echo waves at the interfaces between the buffer rod and composite, between the composite and air, and between the film and substrate;

means for receiving said echo

waves and generating output signals; and

means for processing said output signals to give a film thickness value.

2. A system as recited in claim 1 wherein said means for propagating an acoustic wave through said composite unit makes Hertzian contact with the film side of the composite unit.

3. A system as recited in claim 1 wherein said means for propagating acoustic wave through said composite unit makes Hertzian contact with the substrate side of the composite unit.

4. A system as recited in claim 1 wherein said means for processing said output signals comprises:

means for determining the time separation between said echo waves; and

means for converting said time separation into thickness data.

5. A system as recited in claim 1 wherein said means for processing said output signals comprises:

means for determining the frequency domain of said output signals; and

means for converting said frequency domain signals into thickness data.

6. A system as recited in claim 1 wherein said system further comprises predetermined phase measurement data and wherein said means for processing said output signals comprises:

means for determining the phase of said echo waves with respect to said propagated wave; and

means for converting said determined phase into thickness data.

7. A system as recited in claim 1 wherein said propagated wave is generated by an ultrasonic transducer.

8. A system as recited in claim 1 wherein said propagated wave is generated by a pulse laser.

9. An apparatus for measuring the thickness of at least one film on a substrate, such constituting a composite unit, said film and substrate having an interface, said apparatus comprising:

a buffer rod having one end shaped to make a Hertzian contact with said composite unit;

a transmitting transducer in communication with the other end of said buffer rod for propagating an acoustic wave through said composite unit, said propagating acoustic wave reflecting off the interface between said buffer rod and composite unit and off of said interface between said film and said substrate to generate echo waves;

a receiving transducer for receiving said echo waves;

means for receiving said echo waves and generating an output signal representative between the time lapse between said echo waves; and

means for processing said output signal to give a thickness value.

10. A method for measuring the thickness of at least one film on a substrate, such constituting a composite unit, said film and substrate having an interface, said method comprising the steps of:

propagating a high frequency acoustic wave into and through said composite unit by a Hertzian contact, said acoustic wave reflecting off the Hertzian contact-composite unit interface, said interface between said film and said substrate, and the interface between the composite unit and air to generate echo waves;

receiving said echo waves and generating output signals; and

processing said output signals to give a film thickness value.

11. A method as recited in claim 10 wherein the step of propagating an acoustic wave propagates the wave first through the film of the composite unit.

12. A method as recited in claim 10 wherein the step of propagating an acoustic wave propagates the wave first through the substrate of the composite unit.

13. A method as recited in claim 10 wherein the step of processing said output signals comprises the steps of:

determining the time lapse between said echo waves; and

converting said time lapse into distance data.

14. A method as recited in claim 10 wherein the step of processing said output signals comprises the steps of:

determining the frequency domain of said output signals; and

converting said frequency domain of said signals into thickness data.

15. A method as recited in claim 10 wherein the step of processing said output signals comprises the steps of:

determining the phase of the reflection coefficient of said echo waves with respect to said propagated wave; and

converting said determined phase into thickness data.

16. A method as recited in claim 10 wherein said acoustic wave is generated by an ultrasonic transducer.

17. A method as recited in claim 10 wherein said acoustic wave is generated by a pulse laser.