Apparatus and method for in-situ endpoint detection for chemical mechanical polishing operations

• US 20040014395A1
• Filed: 03/25/2003
• Published: 01/22/2004
• Est. Priority Date: 03/28/1995
• Status: Active Grant

• Current Assignee: Applied Materials Incorporated
• Original Assignee: Applied Materials Incorporated
• Inventors: Birang, Manoocher, Gleason, Allan, Johansson, Nils
• Granted Patent: US 6,875,078 B2
• Time in Patent Office: Days
• US Class Current: 451/5
• CPC Class Codes: B24B 37/013 Devices or means for detecting lapping completion
• CPC Class Codes: B24B 37/205 provided with a window for inspecting the surface of the work being lapped
• CPC Class Codes: B24B 47/12 by mechanical gearing or electric power B24B47/16 takes precedence
• CPC Class Codes: B24B 49/04 involving measurement of the workpiece at the place of grinding during grinding operation
• CPC Class Codes: B24B 49/12 involving optical means
• CPC Class Codes: B24B 51/00 Arrangements for automatic control of a series of individual steps in grinding a workpiece if applicable to other machine tools, G05B takes precedence
• CPC Class Codes: G01B 11/0683 measurement during deposition or removal of the layer

Abstract

An apparatus and method of chemical mechanical polishing (CMP) of a wafer employing a device for determining, in-situ, during the CMP process, an endpoint where the process is to be terminated. This device includes a laser interferometer capable of generating a laser beam directed towards the wafer and detecting light reflected from the wafer, and a window disposed adjacent to a hole formed through a platen. The window provides a pathway for the laser beam during at least part of the time the wafer overlies the window.

Claims

1. An apparatus for chemical mechanical polishing (CMP) of a wafer, comprising:

(a) a rotatable polishing platen with an overlying polishing pad wetted with an abrasive slurry, the platen be rotatably mounted to a chassis;

(b) a rotatable polishing head for holding the wafer against the polishing pad, the wafer comprising a semiconductor substrate underlying an oxide layer; and

,

(c) an endpoint detector, comprising,

(c1) a laser interferometer capable of generating a laser beam directed towards the wafer and detecting light reflected from the wafer, and,

(c2) a window disposed adjacent to a hole formed through the platen, the window providing a pathway for the laser beam to impinge on the wafer at least during part of a period of time when the wafer overlies the window.

2. The apparatus of claim 1, wherein the window comprises:

an insert mounted within the platen hole, the insert being transmissive to the laser beam.

3. The apparatus of claim 2, wherein:

an upper surface of the insert protrudes above a surface of the platen and extends away from the platen a distance such that a gap is formed between the upper surface of the insert and the wafer whenever the wafer is held against the pad over the insert.

4. The apparatus of claim 3, wherein:

the gap between the wafer and the insert is made as small as possible without allowing the insert to touch the wafer, and is no more than approximately 1 mm.

5. The apparatus of claim 4, wherein:

the gap between the wafer and the insert is 250 μ

m.

6. The apparatus of claim 1, wherein the window comprises:

a portion of the polishing pad, the portion being at least partially transmissive to the laser beam.

7. The apparatus of claim 1, wherein the window comprises:

a plug formed in the pad, the plug being highly transmissive to the laser beam.

8. The apparatus of claim 1, wherein:

the hole through the platen, and the window, are circular in shape.

9. The apparatus of claim 1, wherein:

the hole through the platen, and the window, are arc-shaped having a radius with an origin coincident with a center of rotation of the platen.

10. The apparatus of claim 1, wherein:

the laser beam generated by the laser interferometer has a beam diameter at a point of impingement on the wafer which is significantly greater than the smallest diameter possible for the wavelength employed.

11. The apparatus of claim 1, wherein the endpoinnt detector further comprises:

a position sensor for sensing when the window is adjacent the wafer such that the laser beam generated by the laser interferometer can pass unblocked through the window and impinge on the wafer.

12. The apparatus of claim 11, wherein the position sensor comprises:

(a) a flag attached along a portion of the periphery of the platen and extending radially outward therefrom; and

,

(b) an optical interrupter-type sensor mounted to the chassis at the periphery of the platen and capable of producing an optical beam which causes a signal to be generated for as long as the optical beam is interrupted by the flag; and

wherein,

(c) the flag is attached to the periphery of the platen in a position such that the optical beam is interrupted by the flag thereby producing the signal from the sensor whenever the laser beam can be made to pass unblocked through the window and impinge on the wafer.

13. The apparatus of claim 11, wherein:

(a) the laser interferometer comprises means for producing a detection signal whenever light reflected from the wafer is detected;

(b) the position sensor comprises means for outputting a sensing signal whenever the window is adjacent the wafer such that the laser beam generated by the laser interferometer can pass unblocked through the window and impinge on the wafer; and

,

(c) the endpoint detector further comprises data acquisition means connected to the laser interferometer and position sensor for sampling the detection signal from the laser interferometer for the duration of the sensing signal from the position sensor, the data acquisition means comprising a means for outputting a data signal representing the sampled detection signal.

14. The apparatus of claim 13, wherein the data acquisition means comprises:

(a) means for integrating the sampled detection signal from the laser interferometer over a predetermined period of time; and

wherein,

(b) the outputting means outputs a data signal representing the integrated samples of the detection signal.

15. The apparatus of claim 13, wherein the data acquisition means further comprises:

(a) means for sampling the detection signal output from the laser interferometer during each complete revolution of the platen, the detection signal being sampled in each complete revolution over a sample time;

(b) means for integrating each sample of the detection signal over the sample time to produce an integrated value corresponding to each sample;

(c) means for storing each integrated value;

(d) means for computing a cumulative sample time after each complete revolution of the platen, the cumulative sample time being the summation of the sample times associated with each sample of the detection signal;

(e) means for comparing the cumulative sample time to a desired minimum sample time; and

(f) means for transferring the stored integrated values from the storing means to means for calculating a summation thereof whenever the cumulative sample time equals or exceeds the predetermined minimum sample time; and

wherein,

(g) the outputting means comprises means for outputting a data signal representing a series of the integrated value summations from the summation means.

16. The apparatus of claim 13, wherein the data signal output by the outputting means is cyclical, and wherein the endpoint detector further comprises:

(a) means for counting a number of cycles exhibited by the data signal;

(b) means for computing a thickness of material removed during one cycle of the data signal from the wavelength of the laser beam and the index of refraction of the oxide layer of the wafer;

(c) means for comparing a desired thickness of material to be removed from the oxide layer, to a removed thickness comprising a product of the number of cycles exhibited by the data signal from the counting means and the thickness of material removed during one cycle from the computing means; and

,

(d) means for terminating the CMP whenever the removed thickness equals or exceeds the desired thickness of material to be removed.

17. The apparatus of claim 13, wherein the data signal output by the outputting means is cyclical, and wherein the endpoint detector further comprises:

(a) means for counting a number of occurrences of a portion of a cycle exhibited by the data signal;

(b) means for computing a thickness of material removed during the portion of the cycle of the data signal from the wavelength of the laser beam and the index of refraction of the oxide layer of the wafer;

(c) means for comparing a desired thickness of material to be removed from the oxide layer, to a removed thickness comprising a product of the number of occurrences of the portion of the cycle exhibited by the data signal from the counting means and the thickness of material removed during the portion of the cycle from the computing means; and

,

(d) means for terminating the CMP whenever the removed thickness equals or exceeds the desired thickness of material to be removed.

18. The apparatus of claim 13, wherein the data signal output by the outputting means is cyclical, and wherein the endpoint detector further comprises:

(a) means for measuring the time required for the data signal to complete one of (i) a prescribed number of cycles, or (ii) a prescribed portion of one cycle, after each occurrence thereof;

(b) means for computing a thickness of material removed during the time measured by the measuring means from the wavelength of the laser beam and the index of refraction of the oxide layer of the wafer;

(c) means for calculating a rate of removal, said calculating means dividing the thickness of material removed by the time measured obtained from the measuring means;

(d) means for ascertaining a remaining removal thickness, said ascertaining means subtracting a cumulative thickness of material removed from a desired thickness of material to be removed from the oxide layer, said cumulative thickness of material removed being provided by a means for summing the thickness of material removed computed by the computing means after each occurrence of the one of (i) a prescribed number of cycles, or (ii) a prescribed portion of one cycle;

(e) means for establishing a remaining CMP time, said establishing means dividing the remaining removal thickness by the rate of removal; and

,

(f) means for terminating the CMP after the expiration of the remaining CMP time.

19. The apparatus of claim 13, wherein the wafer has an initially irregular surface topography and is planarized during the CMP, and the data signal output by the outputting means is cyclical only after the wafer surface is planarized, and wherein the endpoint detector further comprises:

(a) means for detecting a cyclic variation in the data signal; and

,

(b) means for terminating the CMP whenever the detecting means detects the cyclic variation in the data signal.

20. The apparatus of claim 19, wherein:

the detecting means is capable of detecting a cyclical variation in the data signal within at most one cycle of a beginning of the cyclical variation in the data signal.

21. The apparatus of claim 13, wherein the wafer has an initially irregular surface topography, and the data signal output by the outputting means is cyclical only after the wafer surface is planarized, and wherein the endpoint detector further comprises:

(a) means for filtering the data signal to pass only a component of the data signal having the predetermined frequency;

(b) means for counting a number of cycles exhibited by the filtered data signal;

(c) means for computing a thickness of material removed during one cycle of the filtered data signal from the wavelength of the laser beam and the index of refraction of the oxide layer of the wafer;

(d) means for comparing a desired thickness of material to be removed from the oxide layer, to a removed thickness comprising a product of the number of cycles exhibited by the filtered data signal from the counting means and the thickness of material removed during one cycle from the computing means; and

,

(e) means for terminating the CMP whenever the removed thickness equals or exceeds the desired thickness of material to be removed.

22. The apparatus of claim 13, wherein the wafer has an initially irregular surface topography, and the data signal output by the outputting means is cyclical only after the wafer surface is planarized, and wherein the endpoint detector further comprises:

(a) means for filtering the data signal to pass only a component of the data signal having the predetermined frequency;

(b) means for counting a number of occurrences of a portion of a cycle exhibited by the filtered data signal;

(c) means for computing a thickness of material removed during the portion of the cycle of the filtered data signal from the wavelength of the laser beam and the index of refraction of the oxide layer of the wafer;

(d) means for comparing a desired thickness of material to be removed from the oxide layer, to a removed thickness comprising a product of the number of occurrences of the portion of the cycle exhibited by the entered data signal from the counting means and the thickness of material removed during the portion of the cycle from the computing means; and

,

(e) means for terminating the CMP whenever the removed thickness equals or exceeds the desired thickness of material to be removed.

23. The apparatus of claim 13, wherein the wafer has an initially irregular surface topography, and the data signal output by the outputting means is cyclical only after the wafer surface is planarized, and wherein the endpoint detector further comprises:

(a) means for filtering the data signal to pass only a component of the data signal having the predetermined frequency;

(b) means for measuring the time required for the filtered data signal to complete one of (i) a prescribed number of cycles, or (ii) a prescribed portion of one cycle, after each occurrence thereof;

(c) means for computing a thickness of material removed during the time measured by the measuring means from the wavelength of the laser beam and the index of refraction of the oxide layer of the wafer;

(d) means for calculating a rate of removal, said calculating means dividing the thickness of material removed by the time measured obtained from the measuring means;

(e) means for ascertaining a remaining removal thickness, said ascertaining means subtracting a cumulative thickness of material removed from a desired thickness of material to be removed from the oxide layer, said cumulative thickness of material removed being provided by a means for summing the thickness of material removed computed by the computing means after each occurrence of the one of (i) a prescribed number of cycles, or (ii) a prescribed portion of one cycle;

(f) means for establishing a remaining CMP time, said establishing means dividing the remaining removal thickness by the rate of removal; and

,

(g) means for terminating the CMP after the expiration of the remaining CMP time.

24. The apparatus of claim 1, wherein:

the laser beam has a wavelength in approximately the red light range.

25. A method for chemical mechanical polishing (CMP) of a wafer comprising a semiconductor substrate underlying an oxide layer, the method comprising the steps of:

(a) holding the wafer in a rotatable polishing head against a polishing pad of an underlying rotatable polishing platen, the pad being wetted with an abrasive slurry;

(b) determining an endpoint whereat the CMP is terminated, the determining step comprising the steps of,

(b1) generating a laser beam directed towards the wafer, the laser beam passing through a window disposed adjacent a hole formed through the platen, the window providing a pathway for the laser beam to impinge on the wafer at least during part of a period of time when the wafer overlies the window, and,

(b2) detecting light reflected from the wafer.

26. The method of claim 25, wherein the step of generating a laser beam comprises the step of:

forming a beam diameter at a point of impingement on the wafer which is significantly greater than the smallest diameter possible for the wavelength of the laser beam employed.

27. The method of claim 25, wherein the determining step further comprises the step of:

sensing when the window is adjacent the wafer such that the laser beam can pass unblocked through the window and impinge on the wafer.

28. The method of claim 27, wherein:

(a) the detecting step comprises the step of producing a detection signal whenever light reflected from the wafer is detected;

(b) the sensing step comprises the step of outputting a sensing signal whenever the window is adjacent the wafer such that the laser beam generated by the laser interferometer can pass unblocked through the window and impinge on the wafer; and

,

(c) the determining step further comprises the step of data acquisition, the data acquisition step comprising,

(c1) sampling the detection signal from the laser interferometer for the duration of the sensing signal, and,

(c2) outputting a data signal representing the sampled detection signal.

29. The method of claim 28, wherein the data acquisition step further comprises:

(a) integrating the sampled detection signal over a predetermined period of time; and

wherein,

(b) the outputting step comprises outputting a data signal representing the integrated samples of the detection signal.

30. The method of claim 28, wherein the data acquisition step further comprises:

(a) sampling the detection signal output during each complete revolution of the platen, the detection signal being sampled in each complete revolution over a sample time;

(b) integrating each sample of the detection signal over the sample time to produce an integrated value corresponding to each sample;

(c) storing each integrated value;

(d) computing a cumulative sample time after each complete revolution of the platen, the cumulative sample time being the summation of the sample time associated with each sample of the detection signal;

(e) comparing the cumulative sample time to a desired minimum sample time; and

(f) transferring the stored integrated values whenever the cumulative sample time equals or exceeds the predetermined minimum sample time, and calculating a summation thereof; and

wherein,

(g) the outputting step comprises outputting a data signal representing a series of the integrated value summations.

31. The method of claim 28, wherein the data signal is cyclical, and wherein the determining step further comprises the steps of:

(a) counting a number of cycles exhibited by the data signal;

(b) computing a thickness of material removed during one cycle of the data signal from the wavelength of the laser beam and the index of refraction of the oxide layer of the wafer;

(c) comparing a desired thickness of material to be removed from the oxide layer, to a removed thickness comprising a product of the number of cycles exhibited by the data signal and the thickness of material removed during one cycle; and

,

(d) terminating the CMP whenever the removed thickness equals or exceeds the desired thickness of material to be removed.

32. The method of claim 28, wherein the data signal is cyclical, and wherein the determining step further comprises the steps of:

(a) counting a number of occurrences of a portion of a cycle exhibited by the data signal;

(b) computing a thickness of material removed during the portion of the cycle of the data signal from the wavelength of the laser beam and the index of refraction of the oxide layer of the wafer;

(c) comparing a desired thickness of material to be removed from the oxide layer, to a removed thickness comprising a product of the number occurrences of the portion of the cycle exhibited by the data signal and the thickness of material removed during the portion of the cycle; and

,

(d) terminating the CMP whenever the removed thickness equals or exceeds the desired thickness of material to be removed.

33. The method of claim 28, wherein the data signal is cyclical, and wherein the determining step further comprises the steps of:

(a) measuring the time required for the data signal to complete one of (i) a prescribed number of cycles, or (ii) a prescribed portion of one cycle;

(b) computing a thickness of material removed during the time measured from the wavelength of the laser beam and the index of refraction of the oxide layer of the wafer;

(c) calculating a rate of removal by dividing the thickness of material removed by the time measured;

(d) ascertaining a remaining removal thickness by subtracting the thickness of material removed from a desired thickness of material to be removed from the oxide layer;

(e) establishing a remaining CMP time by dividing the remaining removal thickness by the rate of removal; and

,

(f) terminating the CMP after the expiration of the remaining CMP time.

34. The method of claim 33, wherein:

(a) steps (a) through (f) are repeated for each occurrence of the one of (i) a prescribed number of cycles, or (ii) a prescribed portion of one cycle;

(b) prior to step (d), the thickness of material removed after each occurrence of the one of (i) a prescribed number of cycles, or (ii) a prescribed portion of one cycle, is summed to produce a cumulative thickness of material removed; and

,

(c) the ascertaining step comprises ascertaining the remaining removal thickness by subtracting the cumulative thickness of material removed from the desired thickness of material to be removed from the oxide layer.

35. The method of claim 28, wherein the wafer has an initially irregular surface topography and is planarized during the CMP, and the data signal is cyclical only after the wafer surface is planarized, and wherein the determining step further comprises the steps of:

(a) searching for a cyclic variation in the data signal; and

,

(b) terminating the CMP whenever the cyclic variation in the data signal is discovered.

36. The method of claim 35, wherein the terminating step comprises the step of:

terminating the CMP within at most one cycle of a beginning of the cyclical variation in the data signal.

37. The method of claim 28, wherein the wafer has an initially irregular surface topography, and the data signal is cyclical only after the wafer surface is planarized, and wherein the determining step further comprises the steps of:

(a) filtering the data signal to pass only a component of the data signal having the predetermined frequency;

(b) counting a number of cycles exhibited by the filtered data signal;

(c) computing a thickness of material removed during one cycle of the filtered data signal from the wavelength of the laser beam and the index of refraction of the oxide layer of the wafer;

(d) comparing a desired thickness of material to be removed from the oxide layer, to a removed thickness comprising a product of the number of cycles exhibited by the filtered data signal and the thickness of material removed during one cycle; and

,

(e) terminating the CMP whenever the removed thickness equals or exceeds the desired thickness of material to be removed.

38. The method of claim 28, wherein the wafer has an initially irregular surface topography, and the data signal is cyclical only after the wafer surface is planarized, and wherein the determining step further comprises the steps of:

(a) filtering the data signal to pass only a component of the data signal having the predetermined frequency;

(b) counting a number of occurrences of a portion of a cycle exhibited by the filtered data signal;

(c) computing a thickness of material removed during the portion of the cycle of the filtered data signal from the wavelength of the laser beam and the index of refraction of the oxide layer of the wafer;

(d) comparing a desired thickness of material to be removed from the oxide layer, to a removed thickness comprising a product of the number occurrences of the portion of the cycle exhibited by the filtered data signal and the thickness of material removed during the portion of the cycle; and

,

(e) terminating the CMP whenever the removed thickness equals or exceeds the desired thickness of material to be removed.

39. The method of claim 28, wherein the wafer has an initially irregular surface topography, and the data signal is cyclical only after the wafer surface is planarized, and wherein the determining step further comprises the steps of:

(a) filtering the data signal to pass only a component of the data signal having the predetermined frequency;

(b) measuring the time required for the filtered data signal to complete one of (i) a prescribed number of cycles, or (ii) a prescribed portion of one cycle;

(c) computing a thickness of material removed during the time measured from the wavelength of the laser beam and the index of refraction of the oxide layer of the wafer;

(d) calculating a rate of removal by dividing the thickness of material removed by the time measured;

(e) ascertaining a remaining removal thickness by subtracting the thickness of material removed from a desired thickness of material to be removed from the oxide layer;

(f) establishing a remaining CMP time by dividing the remaining removal thickness by the rate of removal; and

,

(g) terminating the CMP after the expiration of the remaining CMP time.

40. The method of claim 39, wherein:

(a) steps (b) through (f) are repeated for each occurrence of the one of (i) a prescribed number of cycles, or (ii) a prescribed portion of one cycle;

(b) prior to step (e), the thickness of material removed after each occurrence of the one of (i) a prescribed number of cycles, or (ii) a prescribed portion of one cycle, is summed to produce a cumulative thickness of material removed; and

,

(c) the ascertaining step comprises ascertaining the remaining removal thickness by subtracting the cumulative thickness of material removed from the desired thickness of material to be removed from the oxide layer.

41. Thee method of claim 25, wherein the step of generating a laser beam comprises the step of:

employing a wavelength in approximately the red light range.