Method for thickness calibration and measuring thickness of material

• US 8,077,827 B2
• Filed: 02/23/2010
• Issued: 12/13/2011
• Est. Priority Date: 02/23/2010
• Status: Active Grant

• Current Assignee: Test Research Incorporated
• Original Assignee: Test Research Incorporated
• Inventors: Perng, Ming-Hwei
• Primary Examiner(s): SONG, HOON K
• Application Number:
  US12/711,230
• Publication Number:
  US 20110206182A1
• Time in Patent Office: 658 Days
• Field of Search: 378 51- 56, 378/207
• US Class Current: 378/56
• CPC Class Codes: G01B 15/025 by measuring absorption

Abstract

A method for measuring the thickness of a first absorbing material in the presence of a second absorbing material is provided. The method comprises the steps as follow. The thickness (t_S) of the first absorbing material is fixed and the thickness of the second absorbing material is varied to obtain a calibration standard. The intensity of the transmissive energy passing through the calibration standard is detected by acquiring multiple pairs of image data comprising a foreground value (log_n(I_c+s)) and a background value (log_n(I_c)). The thickness (t_Si) of the first absorbing material is changed and the above steps are repeated to obtain sets of image data. A fitting constant I_d is determined to describe each set of the intensity data as

$\frac{{\mu }_s}{\alpha }{t}_S={\log }_n\left(I_c+I_d\right)-{\log }_n\left(I_{c+s}+I_d\right).$
A best fit of the proportional constant

$\frac{{\mu }_s}{\alpha }$
is determined to further calculate an unknown thickness of the first absorbing material (t_s′) through the equation

$t_s^{\prime }=\frac{\alpha }{{\mu }_s}\left[\ln \left(I_c^{\prime }+I_d\right)-\ln \left(I_{c+s}^{\prime }+I_d\right)\right].$

Claims

1. A method for calibrating an transmissive energy imaging system for quantitatively determining the thickness of a first absorbing material in the presence of a second absorbing material where an incident transmissive energy having an incident intensity is transmitted through the first and second absorbing materials, wherein the method comprises the steps of:

fixing the thickness (t_S) of the first absorbing material and varying the thickness of the second absorbing material to obtain a calibration standard having a multiple combinations of thicknesses of the first and the second absorbing material;

exposing the calibration standard to the incident transmissive energy;

detecting the intensity of the incident transmissive energy which passes through the calibration standard by acquiring multiple pairs of image data which are representative of a portion of the transmissive energy which is measured after transmission through the first and the second absorbing materials, where a logarithm form of a foreground value (log_n(I_c+s)) in each pair of image data corresponds to a portion of the incident intensity which is transmitted through the known thickness of the first absorbing material in combination with one of the multiple thicknesses of the second absorbing material, and the logarithm form of a background value (log_n(I_c) in each pair of transmitted intensities corresponds to a portion of the incident intensity which is transmitted through only the corresponding thickness of the second absorbing material which was in combination with the first absorbing material when the foreground value was acquired;

changing the thickness (t_Si) of the first absorbing material to other fixed thicknesses and repeating the above steps to obtain a plurality sets of intensity data, wherein each set of the intensity data corresponds to a fixed thickness of the first absorbing material;

determining a fitting constant I_d such that a plurality of intercept values

2. The method of claim 1, further comprising the steps of:

measuring foreground (I_c+s′

) and background (I_c′

) values for a combination of the first and the second absorbing materials having unknown thicknesses; and

determining the corresponding unknown thickness of the first absorbing material (t_s′

) through the calculation of the following equation;

3. The method of claim 1, wherein the intensity of the transmissive energy which passing through the calibration standard is related to the gray level of the image data.

4. The method of claim 1, wherein the transmissive energy is from an X-ray beam.

5. The method of claim 4, wherein the intensity of the transmissive energy passing through the calibration standard is detected with an X-ray is detector.

6. The method of claim 1, wherein the first and the second absorbing material is solder and copper respectively.

7. The method of claim 1, wherein the base n of the logarithm form is e, the logarithm form is a natural logarithm form.

8. A method for calibrating an transmissive energy imaging system for quantitatively determining the thickness of a first absorbing material in the presence of a second absorbing material where an incident transmissive energy having an incident intensity is transmitted through the first and second absorbing materials, wherein the method comprises the steps of:

fixing the thickness (t_S) of the first absorbing material and varying the thickness of the second absorbing material to obtain a calibration standard having a multiple combinations of thicknesses of the first and the second absorbing material;

exposing the calibration standard to the incident transmissive energy;

detecting the intensity of the incident transmissive energy which passes through the calibration standard by acquiring multiple pairs of image data with a detector, wherein the multiple pairs of image data are representative of a portion of the transmissive energy which is measured after transmission through the first and the second absorbing materials, where a logarithm form of a foreground value (log_n(I_c+s)) in each pair of image data corresponds to a portion of the incident intensity which is transmitted through the known thickness of the first absorbing material in combination with one of the multiple thicknesses of the second absorbing material, and the logarithm form of a background value (log_n(I_c)) in each pair of transmitted intensities corresponds to a portion of the incident intensity which is transmitted through only the corresponding thickness of the second absorbing material which was in combination with the first absorbing material when the foreground value was acquired;

changing the thickness (t_Si) of the first absorbing material to other fixed thicknesses and repeating the above steps to obtain a plurality sets of intensity data, wherein each set of the intensity data corresponds to a fixed thickness of the first absorbing material;

determining a constant I_d by performing experiments on the detector to find out the relation between an incident energy and a detected intensity;

determining a best fit of the proportional constant

9. The method of claim 8, wherein the relation between the incident energy and the detected intensity is described as the form of:

E=a(I+I_d)^α

where E is the incident energy, I is the detected intensity, a and α

are sensor parameters.

10. The method of claim 8, further comprising the steps of:

measuring foreground (I_c+s′

) and background (I_c′

) values for a combination of the first and the second absorbing materials having unknown thicknesses; and

determining the corresponding unknown thickness of the first absorbing material (t_s′

) through the calculation of the following equation;

11. The method of claim 8, wherein the intensity of the transmissive energy which passing through the calibration standard is related to the gray level of the image data.

12. The method of claim 8, wherein the transmissive energy is from an X-ray beam.

13. The method of claim 12, wherein the intensity of the transmissive energy passing through the calibration standard is detected with an X-ray detector.

14. The method of claim 8, wherein the first and the second absorbing material is solder and copper respectively.

15. The method of claim 8, wherein the base n of the logarithm form is e, the logarithm form is a natural logarithm form.

16. A method for measuring the thickness of a first absorbing material in the presence of a second absorbing material comprising the steps of:

fixing the thickness (t_S) of the first absorbing material and varying the thickness of the second absorbing material to obtain a calibration standard having a multiple combinations of thicknesses of the first and the second absorbing material;

exposing the calibration standard to a source of transmissive energy having an incident intensity;

detecting the intensity of the transmissive energy which passes through the calibration standard by acquiring multiple pairs of image data which are representative of a portion of the transmissive energy which is measured after transmission through the first and the second absorbing materials, where a logarithm form of a foreground value (log_n(I_c+s)) in each pair of image data corresponds to a portion of the incident intensity which is transmitted through the known thickness of the first absorbing material in combination with one of the multiple thicknesses of the second absorbing material, and the logarithm form of a background value (log_n(I_c)) in each pair of transmitted intensities corresponds to a portion of the incident intensity which is transmitted through only the corresponding thickness of the second absorbing material which was in combination with the first absorbing material when the foreground value was acquired;

changing the thickness (t_Si) of the first absorbing material to other fixed thicknesses and repeating the above steps to obtain a plurality sets of intensity data, wherein each set of the intensity data corresponds to a fixed thickness of the first absorbing material;

determining a fitting constant I_d such that a plurality of intercept values

17. The method of claim 16, wherein the transmissive energy is from an X-ray beam, the intensity of the transmissive energy passing through the calibration standard is detected with an X-ray detector.

18. The method of claim 16, wherein the first and the second absorbing material is solder and copper respectively.

19. The method of claim 16, wherein the intensity of the transmissive energy which passing through the calibration standard is related to the gray level of the image data.

20. The method of claim 16, wherein the base n of the logarithm form is e, the logarithm form is a natural logarithm form.