Apparatus and method for characterizing semiconductor wafers during processing

• US 6,182,510 B1
• Filed: 05/10/2000
• Issued: 02/06/2001
• Est. Priority Date: 04/30/1997
• Status: Expired due to Fees

• Current Assignee: Sensys Instruments Corporation (KLA Corporation)
• Original Assignee: Sensys Instruments Corporation (KLA Corporation)
• Inventors: Hasan, Talat, Khuri-Yakub, B. T., Pham, Hung, Stanke, Fred E.
• Primary Examiner(s): Williams, Hezron
• Assistant Examiner(s): MILLER, ROSE MARY
• Application Number:
  US09/568,659
• Time in Patent Office: 272 Days
• Field of Search: 735/97, 736/44, 736/43, 73D/IG.1, 374/119, 374/117, 374/137, 437/8, 437/9, 437/225, 702/39, 702/97, 702/103, 702/130, 702/171, 702/99
• US Class Current: 73/597
• CPC Class Codes: G01N 2291/02854 Length, thickness
• CPC Class Codes: G01N 2291/02881 Temperature
• CPC Class Codes: G01N 2291/0423 Surface waves, e.g. Rayleigh waves, Love waves
• CPC Class Codes: G01N 2291/0427 Flexural waves, plate waves, e.g. Lamb waves, tuning fork, cantilever
• CPC Class Codes: G01N 2291/044 Internal reflections (echoes), e.g. on walls or defects
• CPC Class Codes: G01N 2291/102 one emitter, one receiver
• CPC Class Codes: G01N 2291/2698 Other discrete objects, e.g. bricks
• CPC Class Codes: G01N 29/041 on the surface of the material, e.g. using Lamb, Rayleigh or shear waves
• CPC Class Codes: G01N 29/07 by measuring propagation velocity or propagation time of acoustic waves
• CPC Class Codes: G01N 29/449 Statistical methods not provided for in G01N29/4409, e.g. averaging, smoothing and interpolation
• CPC Class Codes: G01N 29/46 by spectral analysis, e.g. Fourier analysis or wavelet analysis spectral signal processing per se G06F17/14

Abstract

An apparatus and method are disclosed for characterizing semiconductor wafers or other test objects that can support acoustic waves. Source and receiving transducers are configured in various arrangements to respectively excite and detect acoustic waves (e.g., Lamb waves) in a wafer to be characterized. Signals representing the detected waves are digitally processed and used to compute a measurement set correlated with the waves'"'"' velocity in the wafer. A characterization sensitivity is provided that describes how different wafer characteristics of interest vary with changes in the propagation of the acoustic waves. Using the characterization sensitivity and measurement sets computed at a setup time when all wafer characteristics are known and one or more process times when at least one of the characteristics is not known the perturbation in wafer characteristics between the setup and the process times can be determined. Characterization accuracy is improved by a wafer calibration procedure wherein measurement offsets from known conditions are determined for each wafer being characterized. An apparatus and technique are disclosed for correcting for anisotropy of acoustic wave velocity due to the direction of wave propagation with respect to a preferred crystallographic axis of the wafer. An apparatus and technique are also described for measuring wafer temperature using a single transducer whose temperature is related to the temperature of the wafer and, optionally, resonator structures. For characterization steps that occur when the wafer is chucked, a chuck structure is described that reduces the likelihood of the chuck interfering with the waves in the wafer.

Claims

1. An apparatus for determining characteristics of a substrate, comprising:

an acquisition and processing module configured to perform a characterization measurement, wherein the acquisition module is configured to (1) inject acoustic waves into the substrate at least one excitation point; and

(2) generate a signal set representing a propagation state of the acoustic waves at respective interrogation points on the substrate; and

the processing module is configured to (3) compute from the signal set a measurement set related to propagation of the acoustic waves between respective subsets of the interrogation points using a function relating the sets;

the processing module being configured to use process functions to compute a respective process measurement set by performing the characterization measurement on the substrate at a respective process time and process location wherein process values of at least one characteristic associated with the substrate are unknown;

the processing module being configured to use calibration functions to compute a respective wafer calibration measurement set by performing the characterization measurement on the substrate at a respective calibration time and calibration location wherein calibration values of the at least one characteristic are known; and

the processing module being configured to compute one set of the process values of the at least one characteristic between each respective subset of the interrogation points based on corresponding calibration and process measurement sets using a characterization sensitivity and the calibration values of the at least one characteristic.

2. The apparatus of claim 1, wherein the processing module further comprises

a calibration database storing information about the at least one characteristic and the calibration measurement set;

a hardware database storing the calibration functions and the process functions; and

a sensitivity database storing the characterization sensitivity.

3. The apparatus of claim 1, wherein the at least one characteristic being measured is at least one of:

substrate temperature;

substrate thickness;

thickness of a film on the substrate; and

state of the film.

4. The apparatus of claim 1, wherein:

the acquisition module comprises a waveform digitizer configured to digitize waveforms representing the acoustic waves at the interrogation points; and

the processing module comprises a digital signal processor that is configured to compute the signal set from the digitized waveforms and to compute the measurement set from the signal set.

5. The apparatus of claim 4, wherein the signal set is selected from:

spectral phases of the waveforms;

times of zero crossings of the waveforms; and

arrival times of packets of energy carried by the acoustic waves.

6. The apparatus of claim 1, wherein the interrogation points and the excitation points are co-linear.

7. The apparatus of claim 6, wherein the co-linear points lie substantially on a diameter of the substrate.

8. The apparatus of claim 1, wherein the interrogation points comprise two distinct interrogation points.

9. The apparatus of claim 8, wherein the excitation points comprise at least two distinct excitation points.

10. The apparatus of claim 1, wherein the excitation points comprise at least two distinct excitation points.

11. The apparatus of claim 1, wherein at least one of the subsets of points comprises at least one point where the acoustic waves are excited and a signal representing a propagation state is generated.

12. The apparatus of claim 11, wherein the signal at a respective acoustic wave excitation point is selected from:

an electrical signal in the acquisition module that excites the acoustic waves at that excitation point;

or

an echo representing reception by the acquisition module of ultrasonic waves reflected from that excitation point upon injection thereat of the acoustic waves.

13. The apparatus of claim 1, wherein, for at least one of the subsets of the interrogation points, the propagation state of the acoustic waves at two of the points differs at least by a reflection from the edge of the substrate.

14. The apparatus of claim 13, wherein the two points are the same point at different times, such that the propagation state has changed by a reflection from the edge of the substrate.

15. The apparatus of claim 1, further comprising at least one pin transducer having:

a pin with a probe end configured to couple acoustic energy between the pin and the substrate at one of the points; and

a transducer end configured to convert acoustic energy in the pin to external electrical signals and vice-versa.

16. The apparatus of claim 1, further comprising at least one laser configured to direct a laser beam at one of the excitation points.

17. The apparatus of claim 1, further comprising at least one laser detector configured to detect the acoustic wave in the substrate at one of the interrogation points.

18. The apparatus of claim 1, further comprising

a first set of one or more source transducers configured to excite acoustic waves at the excitation points;

and

a second set of one or more receiving transducers, each being configured to detect acoustic waves in the wafer and to produce a transmitted electrical signal representing acoustic energy in the wafer at the interrogation points due to the excited acoustic waves;

wherein the first set and second set have a configuration that is at least one of;

(a) the first set includes exactly one source transducer and the second set includes exactly one receiving transducer;

(b) the first set includes exactly one source transducer and the second set includes at least two receiving transducers;

(c) the source transducers and the receiving transducers are configured such that the excitation and interrogation points are arranged diametrically with respect to the surface of the substrate;

(d) the interrogation points and the excitation points are collinear;

(e) the first set includes two source transducers and the second set includes two receiving transducers; and

(f) at least one of the pin transducers belongs to both of the sets.

19. The apparatus of claim 1, wherein the process location is identical to the calibration location.

20. The apparatus of claim 19, wherein, when a process step coinciding with the process time requires a high temperature plasma:

prior to the calibration time and after ignition of the plasma the power of the plasma is reduced; and

following computation of the calibration measurement the power of the plasma is increased for the process step.