Apparatus and methods for optically inspecting a sample for anomalies

• US 6,833,913 B1
• Filed: 06/24/2002
• Issued: 12/21/2004
• Est. Priority Date: 02/26/2002
• Status: Expired due to Term

• Current Assignee: Kla-Tencor Technologies Corporation (KLA Corporation)
• Original Assignee: Kla-Tencor Technologies Corporation (KLA Corporation)
• Inventors: Wolf, Ralph C., Greene, John D., Vo, Khiem D., Benitez, Eva L., Chen, Dongsheng Don, Sullivan, Jamie M., Vella, Eric N.
• Primary Examiner(s): Rosenberger, Richard A.
• Application Number:
  US10/180,807
• Time in Patent Office: 911 Days
• Field of Search: 356/237--23, 250/214.AG
• US Class Current: 356/237.2
• CPC Class Codes: G01N 21/47 Scattering, i.e. diffuse reflection G01N21/25, G01N21/41 take precedence G01N21/55 takes precedence
• CPC Class Codes: G01N 21/9501 Semiconductor wafers manufacturing processes per se of semiconductor devices implementing a measuring step H01L22/10
• CPC Class Codes: G01N 21/956 Inspecting patterns on the surface of objects contactless testing of electronic circuits G01R31/308; testing currency G07D; manufacturing processes per se of semiconductor devices implementing a measuring step H01L22/10

Abstract

Disclosed are methods and apparatus for detecting a relatively wide dynamic range of intensity values from a beam (e.g., scattered light, reflected light, or secondary electrons) originating from a sample, such as a semiconductor wafer. In other words, the inspection system provides detected output signals having wide dynamic ranges. The detected output signals may then be analyzed to determine whether defects are present on the sample. For example, the intensity values from a target die are compared to the intensity values from a corresponding portion of a reference die, where a significant intensity difference may be defined as a defect. In a specific embodiment, an inspection system for detecting defects on a sample is disclosed. The system includes a beam generator for directing an incident beam towards a sample surface and a detector positioned to detect a detected beam originating from the sample surface in response to the incident beam. The detector has a sensor for detecting the detected beam and generating a detected signal based on the detected beam and a non-linear component coupled to the sensor. The non-linear component is arranged to generate a non-linear detected signal based on the detected signal. The detector further includes a first analog-to-digital converter (ADC) coupled to the non-linear component. The first ADC is arranged to digitize the non-linear detected signal into a digitized detected signal.

Claims

1. An inspection system for detecting defects on a sample, the system comprising:

a beam generator for directing an incident beam towards a sample surface;

a detector positioned to detect a detected beam originating from the sample surface in response to the incident beam, wherein the detector comprises;

a sensor for detecting the detected beam and generating a detected signal based on the detected beam;

a non-linear component coupled to the sensor, the non-linear component being arranged to generate a non-linear detected signal based on the detected signal; and

a first analog-to-digital converter (ADC) coupled to the non-linear component, the first ADC binge arranged to digitize the non-linear detected signal into a first digitized detected signal;

a data processor for determining whether there is a defect present on the sample surface based on the first digitized detected signal,

wherein the non-linear component is arranged to match a dynamic range of the detected signal to at least a portion of a dynamic range of the first ADC;

a transformation mechanism for transforming the first digitized detected signal into a second digitized detected signal that compensates for noise variation associated with different intensity levels of the first detected output signal, and

wherein the data processor is further arranged to receive the second digitized signal and the step of determining whether there is a defect is based indirectly on the first digitized detected signal by being based directly on the second digitized detected signal.

2. An inspection system as recited in claim 1, wherein the transformation mechanism operates to cause a derivative of the second digitized detected signal to be equal to a normalization function which is an estimate of the inverse of the noise level or uncertainty in the measurement, and wherein the normalization function is computed by dividing an average of an envelope functioned by the envelope function itself, and the envelope function is calculated based on an observed repeatability of measurements of the first digitized detected signal.

3. An inspection system as recited in claim 1, wherein the sensor is a photomultiplier tube (PMT).

4. An inspection system as recited in claim 1, wherein the sensor is selected from a group consisting of an electron multiplier tube, a micro-channel plate PMT, an avalanche photodiode, a metal channel dynode PMT, a wire mesh dynode PMT, and a PMT with explicit gate or grid electrodes and imaging arrays with programmable integration times.

5. An inspection system as recited in claim 1,

wherein the first feed back circuit comprises:

a variable voltage supply component coupled to the non-linear component and arranged to adjust a voltage level of the sensor gain based on non-linear detected signal or the detected signal, a voltage reference signal, and one or more control signal(s); and

an amplifier coupled to the variable voltage supply and arranged to amplify the season gain signal prior to it being input to the sensor.

6. An inspection system as recited in claim 5, wherein the variable voltage supply component is a proportional integral differential (PID) controller.

7. An inspection system as recited in claim 5, further comprising a compensation circuit for injecting a current which substantially cancels the current injected from the amplifier by the parasitic capacitances within the sensor.

8. An inspection system as recited in claim 7, wherein the compensation circuit includes a first and a second derivative compensation circuit.

9. An inspection system as recited in claim 5, further comprising

a second ADC for receiving the sensor gain, digitizing the sensor gain and outputting it as a digitized sensor gain signal;

a first transformation mechanism for calibrating the fast digitized detected signal into a calibrated detected signal;

a second transformation mechanisms for calibrating the digitized sensor gain signal into a calibrated gain signal; and

an arithmetic logic unit (ALU) arranged to subtract the calibrated gain signal from the calibrated detected signal to form a first detected output signal,

wherein the data processor is further arranged to receive the first detected output signal and the step of determining whether there is a defect is based indirectly on the first digitized detected signal by being based directly on the first detected output signal.

10. An inspection system as recited in claim 9, wherein the first and second transformation mechanisms take the form of a look-up table embodied within a memory device.

11. An inspection system as recited in claim 10, wherein the memory device is selected from a group consisting of an SRAM, DRAM, ROM, PROM, EPROM, EEPROM, non-volatile RAM, and flash memory.

12. An inspection system as recited in claim 9, further comprising

a third transformation mechanism for transforming the first detected output signal into a second detected output signal that compensates for noise variation associated with different intensity levels of the first detected output signal, and

wherein the data processor is further arranged to receive the second detected output signal and the step of determining whether there is a defect is based indirectly on the first digitized detected signal by being based directly on the second detected output signal.

13. An inspection system as recited in claim 12, wherein the transformation mechanism operates to cause a derivative of the second digitized detected signal to be equal to a normalization function which is an estimate of the inverse of the noise level or uncertainty in the measurement, and wherein the normalization function is computed by dividing an average of an envelope function by the envelope function itself and the envelope function is calculated based on an observed repeatability of measurements of the first detected output signal.

14. An inspection system as recited in claim 12, wherein the third transformation mechanism take the form of a look-up table embodied within a memory device.

15. An inspection system as recited in claim 9, further comprising an offset mechanism arranged to receive a user-selected sensor gain and offset the first detected output signal by a log of the user-selected gain to thereby emulate a programmable sensor gain, wherein the actual sensor gain is not altered.

16. An inspection system as recited in claim 15, wherein the ALU is further arranged to add the log user-selected sensor gain to the fast detected output signal.

17. An inspection system as recited in claim 9, further comprising a second logarithmic amplifier arranged to receive an illumination level of the incident beam and take the logarithmic value of the illumination level to produce a log illumination level.

18. An inspection system as recited in claim 17, wherein the ALU is flutter arranged to subtract the log illumination level from the first detected signal.

19. An inspection system as recited in claim 18, further comprising a second feed back circuit for automatically adjusting the illumination level based on the sensor gain, the non-linear detected signal or the first detected signal.

20. An inspection system as recited in claim 19, the second feedback circuit comprising:

a power supply for supplying a power level to the beam generator for the incident beam; and

a variable voltage supply component for receiving the sensor gain, the non-linear detected signal or the detected signal and one or more control signal(s), the variable voltage supply component being me;

aged to adjust the power level supplied by the power supply so as to adjust the illumination level of the incident beam.

21. An inspection system as recited in claim 20, wherein the variable voltage supply component is a proportional integral differential (PID) controller.

22. An inspection system as recited in claim 19, wherein the beam generator comprises a deflector for scanning the incident beam across the sample, and wherein the second feedback circuit adjusts the illumination level by adjusting an efficiency of the deflector.

23. An inspection system as recited in claim 19, wherein the beam generator comprises a variable attenuator within a path of the incident beam, and wherein the second feedback circuit adjusts the illumination level by modulating the variable attenuator.

24. An inspection system as recited in claim 9, further comprising:

a third transformation mechanism for transforming the first detected output signal into a second detected output signal, the second detected output signal being a relinearized first detected output signal when a mode signal input to the third transformation mechanism indicates a first mode and the second detected output signal equaling the first detected output signal when the mode signal indicates a second mode, and

wherein the data processor is further arranged to receive the second detected output seal and the step of determining whether there is a defect is based indirectly on the first digitized detected signal by being based directly on the second detected output signal.

25. An inspection system as recited in claim 24, wherein the second detected output signal equals a noise compensating transformation of the first detected output signal when the mode signal indicates a third mode.

26. An inspection system as recited in claim 25, wherein the transformation mechanism operates to cause a derivative of the second digitized detected signal to be equal to a normalization function which is an estimate of the inverse of the noise level or uncertainty in the measurement, and wherein the normalization function is computed by dividing an average of an envelope function by the envelope function itself, and the envelope function is calculated based on an observed repeatability of measurements of the fist detected output signal.

27. An inspection system as recited in claim 9, wherein the first and second transformation mechanisms and the ALU have a higher resolution than the first and second ADCs, in order to avoid rounding errors in the transformations.

28. An inspection system for detecting defects on a sample, the system comprising:

a beam generator for directing an incident beam towards a sample surface;

a detector positioned to detect a detected beam originating from the sample surface in response to the incident beam, wherein the detector comprises;

a sensor for detecting the detected beam and generating a detected signal based on the detected beam;

a logarithmic amplifier coupled to the sensor and arranged to generate a logarithmic detected signal based on the detected signal;

a first analog to digital converter (ADC) coupled to the logarithmic amplifier, the first ADC being arranged to digitize the logarithmic detected signal into a digitized detected signal;

a first look-up table embodied in a first memory device and arranged to calibrate the digitized detected signal into a calibrated detected signal;

a feed back circuit for automatically adjusting a sensor gain of the sensor based on the logarithmic detected signal or the detected signal, the sensor gain being input to the sensor;

an amplifier arranged to amplify the sensor gain to an amplified sensor gain;

a second ADC coupled to the amplifier and arranged to digitize the amplified sensor gain into a digitized sensor gain;

a second look-up table embodied in a second memory device and arranged to calibrate the digitized sensor gain signal into a calibrated sensor gain arguer; and

an arithmetic logic unit (ALU) arranged to subtract the calibrated gain signal from the calibrated detected signal to form a first detected output signal.

29. An inspection system as recited in claim 28, further comprising:

a third lookup table embodied in a third memory device and arranged to transform the first detected output signal into a second detected output signal to facilitate data processing; and

a data processor arranged to analyze the second detected output signal to determine whether there is a defect on the sample surface.

30. An inspection system as recited in claim 29, wherein the second detected output signal is a relinearized first detected output signal when a mode signal input to the third look-up table indicates a fast mode and the second detected output signal equaling the first detected output signal whoa the mode signal indicates a second mode.

31. An inspection system as recited in claim 30, wherein the second detected output signal equals a noise compensating transformation of the first detected output signal when the mode signal indicates a third mode.

32. An inspection system as recited in claim 31, wherein the transformation mechanism operates to cause a derivative of the second digitized detected signal to be equal to a normalization function which is an estimate of the inverse of the noise level or uncertainty in the measurement, and wherein the normalization function is computed by dividing an average of an envelope function by the envelope function itself and the envelope function is calculated based on an observed repeatability of measurements of the first detected output signal.

33. An inspection system as recited in claim 28, further comprising an offset mechanism arranged to receive a ma-selected sensor gain and offset the first detected output signal by a log of the user-selected gain to thereby emulate a programmable sensor gain, wherein the actual sensor gain is not altered.

34. A method for detecting defects on a sample, the method comprising:

directing an incident beam towards a first sample surface;

detecting a first detected beam and generating a first detected signal based on the first detected beam, wherein the first detected beam originates from the first sample surface in response to the incident beam;

generating a first non-linear detected signal based on the first detected signal;

digitizing the first non-linear detected signal into a first digitized detected signal;

analyzing the first digitized detected signal to determine whether it corresponds to a defect on the first sample surface;

automatically adjusting a first sensor gain of the sensor based on the first non-linear detected signal or the first detected signal, wherein the first non-linear detected signal is generated to match a dynamic range of the first detected signal to at least a portion of a dynamic range of the first digitized detected signal;

directing an incident beam towards a second sample surface;

detecting a second detected beam and generating a second detected signal based on the second detected beam, wherein the second detected beam originates from the second sample surface in response to the incident beam;

generating a second non-linear detected signal based on the second detected signal;

digitizing the second non-linear detected signal into a second digitized detected signal; and

automatically adjusting a second sensor gain of the sensor based on the second non-linear detected signal or the second detected signal,

wherein the first and second digitized detected signals are logarithmic values,

wherein analyzing the first digitized detected signal is accomplished by subtracting the second digitized detected signal, the subtraction resulting in a difference of a log of intensity values from the first and second sample surfaces, wherein it is determined that the first digitized detected signal corresponds to a defect on the first sample surface when the ice is above a predetermined threshold.

35. A method as recited in claim 34, further comprising:

calibrating the first and second digitized detected signals prior to analyzing the first digitized detected signal;

digitizing and calibrating the fuel and second sensor gains prior to analyzing the first digitized detected signal; and

prior to analyzing the first digitized detected signal and after the first and second senor gains are digitized, subtracting the first digitized and calibrated sensor gain from the first digitized and calibrated detected signal and subtracting the second digitized and calibrated sensor gain from the second digitized and calibrated detected signal.

36. An inspection system for detecting defects on a sample, the system comprising:

a beam splitter for directing an incident beam towards a sample surface;

a beam splitter for receiving a detected beam from the sample surface which is responsive to the incident beam and splitting the detected beam into a first fraction and a second fraction, wherein the first fraction is significantly larger than the second fraction;

a high gain sensor for receiving the first fraction of the detected beam and generating a first detected signal based on the fast fraction;

a low gain sensor for receiving the second fraction of the detected beam and generating a second detected signal based on the second fraction;

a control black coupled with the low gain sensor and operable to regulate a gain of the high gain sensor based on the second detected signal and to output a invalid signal indicative of a reliability factor of the first detected signal;

a first ADC for receiving the first detected signal, digitizing it, and outputting a first digital detected signal;

a second ADC for receiving the second detected signal, digitizing it, and outputting a second digital detected signal; and

a data processor for receiving the first and second digital detected signals and the invalid signal and determining whether there is a defect present on the sample surface, wherein the determination is based on the first digital detested signal when the Invalid signal indicates that the first digital detected signal is reliable and based on the second digital detected signal when the Invalid signal indicates that the first digital detected signal is unreliable.

37. An inspection system as recited in claim 36, wherein the step of determining whether there is a defect present on the sample surface is accomplished by:

analyzing the first and second digital detected signals separately to determine whether there is a defect on the sample surface;

reporting any defect found during the analysis of the first digital detected signal when the Invalid signal indicates that the first digital detected signal is reliable for both the target and reference dies; and

reporting any defect found during the analysis of the second digital detected signal when the Invalid signal indicates that the first digital detected signal is unreliable in either the target or reference die.

38. An inspection system as recited in claim 36, wherein the step of determining whether there is a defect present on the sample surface is accomplished by:

selecting a first operating voltage for the high gain sensor and a second operating voltage for the low gain sensor so that a ratio of the effective gains of the high gain sensor and the low gain sensor is equal to the Mth power of two, where M is an integer;

forming an output data word from the first digital detected signal by padding the stir digital detected signal with M zeros on the moat significant bit side when the Invalid signal indicates that the first digital detected signal is reliable; and

forming an output data word from the second digital detected signal by shifting the second digital detected signal M bits toward the most significant bit and padding the shifted signal with M zeros on the least significant bit when the Invalid signal indicates that the first digital detected signal is unreliable.

39. An inspection system as recited in claim 38, wherein the integer M is selected from a group consisting of 3, 4, 5, 6, 7, and 8 and the number of bits of resolution of the first and second ADCs is selected from a group consisting of 8,10,12,14 and 16.

40. An inspection system as recited in claim 36, wherein the control block is operable to regulate the gain of the high gain sensor by automatically adjusting the gain of the sensor based on the second detected signal or the second fraction.

41. An inspection system as recited in claim 40, wherein the control block is operable to regulate the gain of the high gain sensor by quickly turning off the high gain sensor or indicating to the high gain sensor that it should turn off when the second detected signal or the second fraction rises above a predetermined threshold and turning the high gain sensor back on or indicating to the high gain sensor that is should turn back on when the second detected signal or the second fraction falls back below the predetermined threshold.

42. An inspection system as recited in claim 36, wherein the first digital signal is unreliable when the fast fraction of the detected beam has a higher intensity than the high gain sensor is capable of handling.

43. An inspection system as recited in claim 40, wherein the control block includes a high speed switch circuit having a rise and fall time between about 10 and about 100 nanoseconds.

44. An inspection system as recited in claim 36, further comprising:

a first amplifier coupled with the high gain sensor for receiving and amplifying the first detected signal; and

a second amplifier coupled with the low gain sensor for receiving and amplifying the second detected signal,

wherein the control block is coupled with an output of the second amplifier to receive the second detected signal.

45. An inspection system as recited in claim 44, wherein the control block comprises:

a comparator for receiving the second detected signal and a predetermined threshold value and outputting a Blank Request signal to indicate whether the second detected signal is lower than, or higher than the predetermined threshold value;

a high voltage power supply for supplying a voltage signal; and

a gain blocking circuit for receiving the Blank Request signal and the voltage signal and arranged to provide the voltage signal as the gain to the high gain sensor when the Blank Request signal indicates that the second detected signal is lower than the predetermined threshold value and to switch the high gain sensor off when the Blank Request signal indicates that the second detected signal is equal to or higher than the predetermined threshold value.

46. An inspection system as recited in claim 45, wherein the Blank Request signal is a logical zero when the second detected signal is lower than the predetermined threshold value and the Blank Request signal is a logical one when the second detected signal is equal to or higher than the predetermined threshold value.

47. An inspection system as recited in claim 46, wherein the gain blocking circuit is further arranged to output a Gain Unstable signal which indicates when the high gain sensor is off, is in the process of switching on or off or is suffering from signal artifacts generated by a recent gain transition.

48. An inspection system as recited in claim 47, wherein the control circuit further includes an OR component arranged to receive the Gain Unstable signal and the Blank Request signal and to output the Invalid signal.

49. An inspection system as recited in claim 45, wherein the Blank Request signal is output the Invalid signal.

50. An inspection system as recited in claim 36, wherein the second fraction is between about 5 and about 10 percent and the first fraction is equal to 1 minus a second fraction.