Method and apparatus for in-line oxide thickness determination in chemical-mechanical polishing

• US 6,020,264 A
• Filed: 01/31/1997
• Issued: 02/01/2000
• Est. Priority Date: 01/31/1997
• Status: Expired due to Term

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Lustig, Naftali Eliahu, Sandwick, Thomas E., Guthrie, William L.
• Primary Examiner(s): Dawson, Robert
• Assistant Examiner(s): Milstead, Mark W.
• Application Number:
  US08/792,082
• Time in Patent Office: 1,096 Days
• Field of Search: 156/345, 324/671, 438/692
• US Class Current: 438/692
• CPC Class Codes: B24B 37/013 Devices or means for detecting lapping completion
• CPC Class Codes: B24B 49/04 involving measurement of the workpiece at the place of grinding during grinding operation
• CPC Class Codes: G01B 7/085 for measuring thickness of coating
• CPC Class Codes: H01L 21/31053 involving a dielectric removal step
• CPC Class Codes: H01L 22/12 for structural parameters, e.g. thickness, line width, refractive index, temperature, warp, bond strength, defects, optical inspection, electrical measurement of structural dimensions, metallurgic measurement of diffusions electrical measurement of diffusions H01L22/14
• CPC Class Codes: H01L 22/20 Sequence of activities consisting of a plurality of measurements, corrections, marking or sorting steps

Abstract

In-line thickness measurement of a dielectric film layer on a surface of a workpiece subsequent to a polishing on a chemical-mechanical polishing machine in a polishing slurry is disclosed. The workpiece includes a given level of back-end-of-line (BEOL) structure including junctions. The measurement apparatus includes a platen and an electrode embedded within the platen. A positioning mechanism positions the workpiece above the electrode with the dielectric layer facing in a direction of the electrode. A slurry dam is used for maintaining a prescribed level of a conductive polishing slurry above the electrode, the prescribed level to ensure a desired slurry coverage of the workpiece. A capacitance sensor senses a system capacitance C in accordance with an RC equivalent circuit model, wherein the RC equivalent circuit includes a resistance R representative of the slurry and workpiece resistances and the system capacitance C representative of the dielectric material and junction capacitances. Lastly, a capacitance-to-thickness converter converts the sensed capacitance to a dielectric thickness in accordance with a prescribed system capacitance/optical thickness calibration, wherein the prescribed calibration corresponds to the given level of BEOL structure of the workpiece.

Claims

1. An apparatus for in-line thickness measurement of a dielectric film layer on a surface of a workpiece subsequent to a polishing of the dielectric film layer on a chemical-mechanical polishing machine in a polishing slurry, the workpiece having a given level of back-end-of-line (BEOL) structure including junctions, said apparatus comprising:

a measurement platen;

a measurement electrode embedded within said measurement platen;

means for positioning the workpiece above said measurement electrode with the dielectric layer facing in a direction of said measurement electrode;

means for maintaining a prescribed level of a conductive polishing slurry above said measurement electrode, the prescribed level to ensure a desired slurry coverage of the workpiece, wherein the slurry maintains a static electrical contact with the dielectric layer and the workpiece;

means for sensing a system capacitance C in accordance with an RC equivalent circuit model in which a resistance R is representative of the slurry and workpiece resistances and the system capacitance C is representative of the dielectric material and junction capacitances; and

means for converting the sensed capacitance to a dielectric thickness in accordance with a prescribed system capacitance/optical thickness calibration, wherein the prescribed calibration corresponds to the given level of BEOL structure of the workpiece.

2. The apparatus of claim 1, wherein

said measurement electrode includes a circular center electrode and a circular ring guard electrode surrounding and concentric with the center electrode, the center electrode and the guard electrode being electrically isolated from each other and from said measurement platen.

3. The apparatus of claim 1, wherein said capacitance sensing means includes (I) means for sensing an out-of-phase component of an alternating current flowing through the system at various preselected frequencies and constant applied voltage, (ii) a variable frequency alternating current source for providing the constant applied voltage at the various preselected frequencies, and (iii) current-to-capacitance converter means for converting the sensed out-of-phase component of the AC current at the various frequencies into a system capacitance in accordance with the RC equivalent circuit model.

4. The apparatus of claim 3, further wherein the various preselected frequencies include frequencies in the range of 1 to 50 kHz and the constant applied voltage includes a voltage of less than 0.5 volts.

5. The apparatus of claim 3, further wherein the current-to-capacitance converter means includes means for implementing a least-squares fit to the out-of-phase component of the current as a function of applied signal frequency for providing the sensed capacitance C.

6. The apparatus of claim 1, wherein

said capacitance-to-thickness converter means includes means for determining the dielectric thickness as a function of the sensed capacitance in accordance with the relationship given by ##EQU6## wherein C_measured is a measured capacitance as sensed by said capacitive sensing means, and α

and C₀ are constants determined in accordance with a calibration procedure with the use of a reference workpiece, the reference workpiece having a similar dielectric layer and given level of BEOL structure as the workpiece under dielectric thickness measurement.

7. The apparatus of claim 6, further comprising

means for calibrating said capacitance-to-thickness converter means, wherein said calibration means includes (I) input means for inputting at least three sets of calibration data obtained with the use of the reference workpiece, wherein each of the at least three sets of calibration data is obtained using the reference workpiece and further in which the reference workpiece dielectric layer has been polished to at least three different thicknesses, the calibration data including a sensed capacitance (C_measured) using the reference workpiece as sensed by said capacitive sensing means and a corresponding independent optically measured thickness (T_optical) of the dielectric layer of the reference workpiece for each of the at least three different reference workpiece dielectric thicknesses, and (ii) means for determining, from a plot of capacitance vs. 1/(optical thickness) obtained using the calibration data and with a suitable linear regression algorithm, the constants α

and C₀ in accordance with the relationship between thickness and capacitance as given by ##EQU7##

8. A chemical-mechanical polishing system comprising:

means for chemical-mechanical polishing of a dielectric layer on a surface of a workpiece in a polishing slurry, the workpiece having a given level of back-end-of-line (BEOL) structure including junctions;

means, separate from said chemical-mechanical polishing means, for measuring an in-line thickness measurement of the dielectric film layer subsequent to a prescribed polishing of the dielectric layer of the workpiece by said chemical-mechanical polishing means, said in-line thickness measurement means providing a thickness measurement output signal;

means for positioning the workpiece between the chemical-mechanical polishing means and said in-line thickness measurement means; and

means for controlling a polishing sequence of the workpiece by said chemical-mechanical polishing means in response to the thickness measurement output signal,

wherein said in-line thickness measurement means further includes

a measurement platen;

a measurement electrode embedded within said measurement platen, wherein said positioning means is further for positioning the workpiece above said measurement electrode with the dielectric layer facing in a direction of said measurement electrode;

means for maintaining a prescribed level of the polishing slurry above said measurement electrode, the prescribed level to ensure a desired slurry coverage of the workpiece, wherein the polishing slurry further maintains a static electrical contact with the dielectric layer and the workpiece;

means for sensing a system capacitance C in accordance with an RC equivalent circuit model in which a resistance R is representative of the slurry and workpiece resistances and the system capacitance C is representative of the dielectric material and junction capacitances; and

means for converting the sensed capacitance to a dielectric thickness in accordance with a prescribed system capacitance/optical thickness calibration, wherein the prescribed calibration corresponds to the given level of BEOL structure of the workpiece.

9. The apparatus of claim 8, wherein

said measurement electrode includes a circular center electrode and a circular ring guard electrode surrounding and concentric with the center electrode, the center electrode and the guard electrode being electrically isolated from each other and from said measurement platen.

10. The apparatus of claim 8, wherein

said capacitance sensing means includes (I) means for sensing an out-of-phase component of an alternating current flowing through the system at various preselected frequencies and constant applied voltage, (ii) a variable frequency alternating current source for providing the constant applied voltage at the various preselected frequencies, and (iii) current-to-capacitance converter means for converting the sensed out-of-phase component of the AC current at the various frequencies into a system capacitance in accordance with the RC equivalent circuit model.

11. The apparatus of claim 10, further wherein

the various preselected frequencies include frequencies in the range of 1 to 50 kHz and the constant applied voltage includes a voltage of less than 0.5 volts.

12. The apparatus of claim 10, further wherein

the current-to-capacitance converter means includes means for implementing a least-squares fit to the out-of-phase component of the current as a function of applied signal frequency for providing the sensed capacitance C.

13. The apparatus of claim 8, wherein

said capacitance-to-thickness converter means includes means for determining the dielectric thickness as a function of the sensed capacitance in accordance with the relationship given by ##EQU8## wherein C_measured is a measured capacitance as sensed by said capacitive sensing means, and α

and C₀ are constants determined in accordance with a calibration procedure with the use of a reference workpiece, the reference workpiece having a similar dielectric layer and given level of BEOL structure as the workpiece under dielectric thickness measurement.

14. The apparatus of claim 13, further comprising means for calibrating said capacitance-to-thickness converter means, wherein said calibration means includes (I) input means for inputting at least three sets of calibration data obtained with the use of the reference workpiece, wherein each of the at least three sets of calibration data is obtained using the reference workpiece and further in which the reference workpiece dielectric layer has been polished to at least three different thicknesses, the calibration data including a sensed capacitance (C_measured) using the reference workpiece as sensed by said capacitive sensing means and a corresponding independent optically measured thickness (T_optical) of the dielectric layer of the reference workpiece for each of the at least three different reference workpiece dielectric thicknesses, and (ii) means for determining, from a plot of capacitance vs. 1/(optical thickness) obtained using the calibration data and with a suitable linear regression algorithm, the constants α

and C₀ in accordance with the relationship between thickness and capacitance as given by ##EQU9##

15. An method for in-line thickness measurement of a dielectric film layer on a surface of a workpiece subsequent to a polishing of the dielectric film layer on a chemical-mechanical polishing machine in a polishing slurry, the workpiece having a given level of back-end-of-line (BEOL) structure including junctions, said method comprising the steps of:

providing a measurement platen;

providing a measurement electrode embedded within the measurement platen;

positioning the workpiece above the measurement electrode with the dielectric layer facing in a direction of the measurement electrode;

maintaining a prescribed level of the polishing slurry above the measurement electrode, the prescribed level to ensure a desired slurry coverage of the workpiece, wherein the slurry maintains a static electrical contact with the dielectric layer and the workpiece;

sensing a system capacitance C in accordance with an RC equivalent circuit model in which a resistance R is representative of the slurry and workpiece resistances and the system capacitance C is representative of the dielectric material and junction capacitances; and

converting the sensed capacitance to a dielectric thickness in accordance with a prescribed system capacitance/optical thickness calibration, wherein the prescribed calibration corresponds to the given level of BEOL structure of the workpiece.

16. The method of claim 15, wherein

the measurement electrode includes a circular center electrode and a circular ring guard electrode surrounding and concentric with the center electrode, the center electrode and the guard electrode being electrically isolated from each other and from the measurement platen.

17. The method of claim 15, wherein

the step of sensing the capacitance includes (I) sensing an out-of-phase component of an alternating current flowing through the system at various preselected frequencies and constant applied voltage, (ii) using a variable frequency alternating current source for providing the constant applied voltage at the various preselected frequencies, and (iii) converting the sensed out-of-phase component of the AC current at the various frequencies into a system capacitance in accordance with the RC equivalent circuit model.

18. The method of claim 17, further wherein

the various preselected frequencies include frequencies in the range of 1 to 50 kHz and the constant applied voltage includes a voltage of less than 0.5 volts.

19. The method of claim 17, further wherein

said current-to-capacitance converting step further includes implementing a least-squares fit to the out-of-phase component of the current as a function of applied signal frequency for providing the sensed capacitance C.

20. The method of claim 15, wherein

said capacitance-to-thickness converting step further includes determining the dielectric thickness as a function of the sensed capacitance in accordance with the relationship given by ##EQU10## wherein C_measured is a measured capacitance as sensed by said capacitive sensing step, and α

and C₀ are constants determined in accordance with a calibration procedure with the use of a reference workpiece, the reference workpiece having a similar dielectric layer and given level of BEOL structure as the workpiece under dielectric thickness measurement.

21. The method of claim 20, further comprising the step of:

calibrating said capacitance-to-thickness converting step, wherein said calibration step includes (I) inputting at least three sets of calibration data obtained with the use of the reference workpiece, wherein each of the at least three sets of calibration data is obtained using the reference workpiece and further in which the reference workpiece dielectric layer has been polished to at least three different thicknesses, the calibration data including a sensed capacitance (C_measured) using the reference workpiece as sensed by said capacitive sensing step and a corresponding independent optically measured thickness (T_optical) of the dielectric layer of the reference workpiece for each of the at least three different reference workpiece dielectric thicknesses, and (ii) determining, from a plot of capacitance vs. 1/(optical thickness) obtained using the calibration data and with a suitable linear regression algorithm, the constants α and

C₀ in accordance with the relationship between thickness and capacitance as given by ##EQU11##

22. A method for chemical-mechanical polishing comprising the steps of:

chemical-mechanical polishing a dielectric layer on a surface of a workpiece in a polishing slurry, the workpiece having a given level of back-end-of-line (BEOL) structure including junctions;

measuring an in-line thickness of the dielectric film layer subsequent to a prescribed polishing of the dielectric layer of the workpiece by said chemical-mechanical polishing step, said in-line thickness measurement step providing a thickness measurement output signal;

positioning the workpiece as appropriate for polishing during said chemical-mechanical polishing step and for thickness measurement during said in-line thickness measurement step; and

controlling a polishing sequence of the workpiece by said chemical-mechanical polishing step in response to the thickness measurement output signal,

wherein said in-line thickness measurement step includes the steps of

providing a measurement platen;

providing a measurement electrode embedded within the measurement platen;

positioning the workpiece above the measurement electrode with the dielectric layer facing in a direction of the measurement electrode;

maintaining a prescribed level of the polishing slurry above the measurement electrode, the prescribed level to ensure a desired slurry coverage of the workpiece, wherein the slurry maintains a static electrical contact with the dielectric layer and the workpiece;

sensing a system capacitance C in accordance with an RC equivalent circuit model in which a resistance R is representative of the slurry and workpiece resistances and the system capacitance C is representative of the dielectric material and junction capacitances; and

converting the sensed capacitance to a dielectric thickness in accordance with a prescribed system capacitance/optical thickness calibration, wherein the prescribed calibration corresponds to the given level of BEOL structure of the workpiece.