Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
2000-05-10
2001-02-06
Williams, Hezron (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C702S103000, C702S039000
Reexamination Certificate
active
06182510
ABSTRACT:
The present invention relates generally to ultrasonic transducers and particularly to pin transducers that use ultrasonic energy to determine a material's physical attributes, including temperature, thickness, density and the presence of defects.
BACKGROUND OF THE INVENTION
The present invention is an improvement of the system described in U.S. Pat. No. 5,469,742 (“Acoustic Temperature and Film Thickness Monitor and Method”). A goal of that prior art system was to monitor the temperature and/or thickness of a material during a semiconductor processing step without influencing the processing and with good survivability. The pin transducers described therein include an acoustic transducer at one end and a sharpened tip at the other end designed to contact a material to be monitored. In this system the transducer excites acoustic energy in the pin that is coupled by the tip into the material as Lamb or other acoustic waves. By monitoring the velocity of the Lamb waves in the material the thickness and/or temperature of the material can be determined (Lamb wave velocity varies with material thickness and temperature). The prior art system indirectly determines the Lamb wave velocity by measuring the time of flight of the Lamb waves between the (transmitting) pin that excited the Lamb waves and at least one receiving pin contacting the material.
This system measures the time between the occurrence of a predetermined zero crossing in the analog signal generated by an echo on the transmitting pin and the occurrence of pre-determined zero-crossing in the analog signal generated by the receiving transducer upon receiving acoustic energy excited in the receiving pin by a passing Lamb wave. The travel time of the acoustic energy in one of the pins is subtracted from the total time, yielding the time of flight between the transmitting pin and the particular receiving pin. Problems with this system include the need for sophisticated analog electronics to perform the necessary signal processing and errors due to differences between the pins.
Another problem with the prior art system described above and many other systems that use ultrasonic energy to monitor material characteristics is that they typically provide relative temperature and/or thickness measurements. I.e., these systems determine the change in a parameter and cannot absolutely determine a material parameter in practical situations.
SUMMARY OF THE INVENTION
In summary, the present invention is an apparatus and method for characterizing a semiconductor wafer during processing. In particular, the present invention includes a wafer characterization apparatus wherein one or more characteristics of a semiconductor wafer can be determined at one or more process times.
In the present invention, a set of measurements are made at an initial time when the one or more characteristics are known and a set of measurements are made at each of the process times, when at least one of the characteristics is not known. Perturbations in the one or more characteristics to be determined (i.e., changes from the known conditions at the initial time) are assumed to be related to the corresponding perturbations in the measurements via a known characterization sensitivity matrix.
In the preferred embodiment, each measurement is made by exciting acoustic waves in the wafer and then measuring indications of the waves' propagation in the wafer. The acoustic waves have propagation properties that vary depending on the wafer characteristics to be determined. Thus, the characterization sensitivity matrix describes the propagation properties of whichever type of acoustic waves are excited in the wafer.
In the preferred embodiment, the excited acoustic waves are Lamb waves, whose different frequency components have velocities that vary with characteristics of interest such as wafer temperature, thickness and structure or the states of films on the wafer. In this embodiment, the characterization sensitivity matrix describes how the different wafer and film characteristics change as a function of the frequency-dependent velocity. To ensure an accurate determination of one or more of the characteristics of interest at the process time a calibration procedure can be performed on each wafer being characterized when the conditions are known, to account for differences between wafers. To account for unknown aspects of the hardware used to make the measurements a setup procedure can also be performed wherein differences are determined in measurements made for known conditions. If the calibration and process measurements are performed with different hardware a hardware function is computed and then used to harmonize the measurements made during calibration and processing.
The present invention includes many different configurations for exciting and measuring the propagation of acoustic waves in a wafer or other test object. Most of the configurations employ at least one source transducer and at least one receiving transducer, where a source transducer excites acoustic waves in the wafer at an excitation point and the receiving transducer detects acoustic waves at a probe point. Specific configurations can include:
1. a SET configuration where there is exactly one source transducer and exactly one receiving transducer;
2. a DRP configuration where there is exactly one source transducer and at least two receiving transducers;
3. configurations where the source transducers and the receiving transducers are arranged so the excitation and probe points are on a diameter with respect to the surface of the wafer;
4. configurations where the receiving transducers and the source transducers are arranged so the probe points and the excitation points are collinear;
5. a SSRP configuration where there are two source transducers and two receiving transducers; and
6. configurations where at least one of.the transducers is both a source and a receiving transducer.
Combinations of these configurations are possible. Also, in many of the configurations, in addition to measuring the propagation of the acoustic waves transmitted directly from a source to a receiving transducer, the propagation of acoustic waves reflected from a wafer edge can also be measured. In any of these cases, the paths over which propagation of the waves are measured corresponds to regions of the wafer for which the wafer characteristics are determined.
The transducers can be acoustic pin transducers that excite and detect waves through direct contact with the wafer. Each pin transducer includes a pin with a probe end that contacts the wafer and a piezoelectric element that converts acoustic waves in the pin to electrical signals and vice-versa. A source transducer can also be a laser beam directed at the excitation point. The receiving transducer can also be a laser detector configured to detect the passage of acoustic waves at the probe point.
The present invention incorporates an apparatus and method for correcting measurements that are made when the wafers orientation is different from a preferred orientation. This is necessary to correct for velocity anisotropy in silicon wafers (i.e., the fact that the velocity of acoustic waves in silicon wafers varies with the angle between the measured propagation path and the preferred orientation. Corrections for anisotropy can be implemented in at least two ways. In a first preferred embodiment the characteristic is computed from an average of two measurements. In a second preferred embodiment, the orientation is estimated from two or more measurements and then the corrected characteristic is computed based on the estimated orientation of the wafer.
The present invention also teaches a system and method for measuring the temperature of a wafer using only a single transducer in contact with the wafer. In this system it is assumed that the temperature of the transducer is related to the temperature of the wafer. As a result, the temperature of the wafer can be determined by measuring the temperature of the transducer. A preferred transducer for this application in
Hasan Talat
Khuri-Yakub B. T.
Pham Hung
Stanke Fred E.
Flehr Hohbach Test Albritton & Herbert LLP
Miller Rose M.
Sensys Instruments Corporation
Williams Hezron
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