Optics: measuring and testing – For light transmission or absorption
Reexamination Certificate
1999-04-09
2001-03-06
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
For light transmission or absorption
C356S243100, C356S329000, C374S161000
Reexamination Certificate
active
06198538
ABSTRACT:
BACKGROUND OF THE INVENTION
Semiconductor device fabrication involves a number of processes where temperature uniformity and control are critical. These processes include rapid thermal processing (RTP), chemical vapor deposition (CVD), physical vapor deposition (PVD), and plasma etch. The ability to measure and map the silicone wafer temperature during processing is an enabling technology for most modem processes, especially RTP.
Modern techniques for remote monitoring of semiconductor wafer temperature are generally ineffective. While thermocouples provide adequate precision, they require physical contact with the wafer for accurate measurement and hence, they disturb the temperature field and uniformity, and further provide a source of contamination. Optical pyrometry is a state-of-the-art technique for remote temperature analysis. However, pyrometers are inaccurate primarily because the temperature measurement is a strong function of emissivity which varies greatly with wafer coating, film growths, and depositions. Pyrometers are particularly ineffective at low temperatures. Furthermore, at high temperatures, heat lamps are commonly used during RTP. They generate bright light to heat the wafers with infrared radiation, which can cause false pyrometric readings.
Laser ultrasonics has recently been introduced as a means for remote temperature measurement of thin materials as described in U.S. Pat. No. 5,604,592, incorporated herein by reference. In this technique, a stimulus beam is incident on a portion of the silicon wafer.
The stimulus beam generates an ultrasonic, or elastic, stress wave which propagates along the body of the wafer. The elastic wave is remotely sensed by a sense beam at a sense location positioned at a known distance from the source location. The velocity of propagation between the stimulus location and the sense location is temperature-dependent. In this manner, the temperature of the wafer is determined as a function of propagation time, referred to in the art as “time of flight” (TOF) of the elastic wave. At present, the accuracy of laser ultrasound is limited to ±4° C. and therefore this technique is ineffective for modem RTP applications where an accuracy of ±3° C. is necessary and ±1° C. is desired. Furthermore, laser ultrasound is extremely sensitive to wafer thickness.
SUMMARY OF THE INVENTION
The present invention is directed to an apparatus and method for remote ultrasonic determination of thin material properties, for example temperature, using a match filter calibration technique. The present invention is especially amenable to determination of thin material properties in a manner which is independent of material thickness. In an embodiment adapted for determination of temperature, precision on the order of ±1° C. is achievable over a range of material thicknesses.
In a first aspect, the method of the present invention comprises a calibration technique for a system which determines a material property value of a thin material. For a plurality of known material property values and known material thicknesses, an elastic stress wave is generated in the material at a source location. The intensity of a signal generated by the elastic stress wave is sensed at a sense location positioned a known distance from the source location. A feature is selected from among the sensed signals which demonstrates minimal thickness dependence for the plurality of known material thicknesses. The selected feature is applied to the sensed signals to determine propagation time of the signals over the known distance. A calibration curve is then generated to characterize the relationship between signal propagation time and material property value for each material thickness.
In a preferred embodiment, the elastic stress wave is generated by launching a laser beam at a surface of the material. The elastic stress wave is preferably excited by point excitation, line excitation, arc excitation, semi-circle excitation, or ring excitation. The thin material preferably comprises a semiconductor wafer and the material property preferably comprises temperature.
The sensed signals used to generate the calibration curve preferably comprise discrete intensity samples of the signal as a function of time. The selected feature preferably comprises a feature in the signals having a defined starting time and a defined duration. Application of the selected feature to the sensed signals to determine propagation time preferably comprises a cross-correlation of the selected feature with each signal to generate correlation data corresponding to each signal. The propagation time of the signal is then determined as the time where the selected feature best correlates with the signal.
Cross-correlation preferably comprises, for each of a plurality of time positions, computation of a sum of products between the respective intensities of the selected feature and each signal to generate correlation data. A polynomial is fit to the correlation data for each signal. The relative peak of the polynomial is determined and the peak value is evaluated to determine a corresponding propagation time value.
Generation of the calibration curve preferably comprises fitting a polynomial to the calibration data to characterize the behavior of propagation time as a function of material property value for each material thickness. The polynomial is compared to the determined propagation time data at each material property value to determine a residual value at each data point. The standard deviation of the residual data is computed, characterizing the effectiveness of the feature.
In a preferred embodiment, the steps of selecting a feature, applying the selected feature, and generating a calibration curve are performed in an iterative process to determine an optimal selected feature for characterizing the sensed signals. In this embodiment, the standard deviation of the residual data is calculated at each iteration and an optimal feature is determined as the feature having the lowest standard deviation value. The optimal feature is preferably applied to the sensed signals to generate a calibration curve based on the optimal feature.
Following computation of the calibration curve, the curve and its associated selected feature can be applied to signal measurements generated in a material of unknown thickness and unknown material property value for determining the material property value, independent of material thickness. An elastic stress wave is generated in the material of unknown thickness and unknown material property value. The intensity of a measured signal generated by the elastic stress wave is sensed at a sense location positioned a known distance from the source location. The selected feature is applied to the measured signal to determine propagation time of the measured signal. The propagation time of the measured signal is, in turn, applied to the calibration curve to determine the corresponding material property value. Application of the selected feature to the measured signal to determine propagation time preferably comprises cross-correlation of the selected feature with the measured signal to generate correlation data corresponding to each signal and determination of the propagation time of the signal as the time where the selected feature best correlates with the signal.
The present invention may be applied to generate a calibration curve and selected feature for a plurality of material thickness ranges. Each signal feature and calibration curve combination is applicable to at least one of the thickness ranges.
In a second aspect, the method of the present invention is directed to a method for determining a material property value, for example temperature, of a thin material. An elastic stress wave is generated in a material of unknown thickness and unknown material property value at a source location. The intensity of a measured signal generated by the elastic stress wave is sensed at a sense location positioned a known distance from the source location. A selected featu
Klimek Daniel E.
Kotidis Petros A.
Pham Hoa Q.
Samuels , Gauthier & Stevens, LLP
Textron Systems Corporation
LandOfFree
Match filter apparatus and method for remote ultrasonic... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Match filter apparatus and method for remote ultrasonic..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Match filter apparatus and method for remote ultrasonic... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2453785