Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
1999-05-25
2001-01-16
Kim, Robert (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S511000, C356S513000
Reexamination Certificate
active
06175421
ABSTRACT:
BACKGROUND
This invention relates to optical systems for measuring properties of a sample, e.g., the thickness of a thin film.
An optical measurement called four-wave mixing (FWM) has been used previously to measure a variety of different material properties, such as film thickness, and film delamination. In FWM measurements, two excitation laser beams are overlapped in time and space on a sample s surface to form a spatially varying optical interference pattern. The pattern consists of alternating “light” (i.e., constructive interference) and “dark” (i.e., destructive interference) regions; the spacing between these regions depends on the wavelength of the laser beams and the angle therebetween. In a class of FWM measurements called impulsive stimulated thermal scattering (ISTS), the excitation laser beam contains a series of short (e.g., a few hundred picoseconds) optical pulses. These pulses of radiation are absorbed by the sample in the light regions, but not in the dark regions, to excite a “transient grating”. This process heats and thermally expands the irradiated regions to launch coherent, counter-propagating acoustic waves whose wavelength and direction match those of the interference pattern. When ISTS is used to measure strongly absorbing films (e.g., metal films), the acoustic waves generate a time-dependent “ripple” on the film's surface that oscillates at the acoustic frequency. A probe beam then diffracts off the transient grating to form a series of signal beams, each of which represents a different diffracted order (e.g, the +/−1 and +/−2 orders). The signal beams oscillate in intensity at the acoustic frequency. One of the signal beams is detected and monitored to measure the properties of the sample.
Use of ISTS to measure film thickness and a variety of other properties is described, for example, in U.S. Pat. No. 5,633,711 (entitled MEASUREMENT OF MATERIAL PROPERTIES WITH OPTICALLY INDUCED PHONONS), U.S. Ser. No. 08/377,308 (entitled OPTICAL MEASUREMENT OF STRESS IN THIN FILM SAMPLES, filed Jan. 24, 1995), U.S. Ser. No. 08/783,046 (entitled METHOD AND DEVICE FOR MEASURING FILM THICKNESS, filed Jul. 15, 1996), and U.S. Ser. No. 08/885,786 (entitled METHOD AND APPARATUS FOR MEASURING THE CONCENTRATION OF IONS IMPLANTED IN SEMICONDUCTING MATERIALS, filed concurrently herewith), the contents of which are incorporated herein by reference.
There are several optical systems that generate the two excitation beams needed for ISTS. In a typical measurement, for example, a single excitation beam passes through a beam-splitter to form two excitation beams of roughly equal intensity. An imaging system (e.g., a lens) then collects these beams and then spatially overlaps them on the sample to form an optical interference pattern that includes the light and dark regions described above.
While useful for many types of FWM experiments, beam-delivery systems that generate two excitation beams, such as those using beam-splitters, suffer drawbacks. For example, these systems often have a relatively long depth of focus, i.e., the beams are overlapped for a relatively long distance. This makes it difficult to position the sample in the exact image plane of the imaging system. Another disadvantage is that the diffraction efficiency of the transient grating formed by two excitation beams is often quite small (e.g., on the order of 10
−4
−10
−5
). This means that the diffracted signal beam is often weak and difficult to measure. A weak signal beam, in turn, makes it difficult to precisely measure the acoustic frequency and any corresponding property (e.g., film thickness).
SUMMARY
The above-mentioned disadvantages are overcome in part using a method and apparatus that include improvements to beam-delivery systems that generate excitation beams for FWM measurements. These improvements are described with respect to several different optical systems.
In one improvement, a beam-delivery system includes a phase mask that generates three beams, rather than two, to form the transient grating in the sample. In a typical embodiment, the three beams are equally spaced in a linear fashion (i.e., the center, right, and left beams) prior to being imaged onto the sample. When focused with an imaging system, the center beam propagates down a central optical axis of the beam-delivery system, while the right and left beams converge toward the same spot at the same angle but on opposite sides of the center beam. The beams are overlapped on the sample so that two different transient gratings are simultaneously formed: (1) a grating is formed by the center beam and each of the right and left beams; and (2) a grating is formed by the right and left beams. The laser beams forming these gratings are separated by different angles (i.e., &thgr; and &thgr;/2), and thus they have different spatial frequencies and will simultaneously excite acoustic waves having different acoustic frequencies.
In another improvement, the excitation laser beam is collimated prior to irradiating the phase mask that generates the excitation beams. Collimating the beam in this way reduces variations in the distance separating the light and dark regions of the interference pattern. This consequently improves the precision to which the acoustic wave is measured in the sample. In another improvement, multiple diffracted signal beams, rather than just a single beam, are collected and imaged onto the photodetector. This increases the intensity of the measured signal and thus further improves the precision of the measurement.
In one aspect, the invention provides an apparatus for measuring a property (e.g., thickness) of a sample that includes: 1) an excitation laser that generates an excitation laser beam; 2) a beam-delivery system, aligned along an optical axis, that separates the excitation laser beam into at least three sub-beams; 3) an imaging system aligned along the optical axis that collects the sub-beams and focuses them onto the sample to form an optical interference pattern that generates a time-dependent response in the sample; 4) a probe laser that generates a probe laser beam oriented to diffract off the time-dependent response to form at least one signal beam; 5) a detector that detects at least one signal beam (and in some cases, two signal beams) and in response generates a radiation-induced electronic response; and 6) a processor that processes the radiation-induced electronic response to determine the property of the sample.
In a typical embodiment, the three sub-beams are generated with a transmissive phase mask, and one of the sub-beams propagates along the optical axis of the optical system. For example, the phase mask may contain a pattern that is oriented along the optical axis and partially diffracts the excitation laser beam to form at least two of the sub-beams, and partially transmits the excitation laser beam to form the remaining sub-beam. In this case, the sub-beam transmitted by the pattern is oriented along the optical axis of the optical system, and the diffracted sub-beams are oriented on each side of the transmitted sub-beam. Each of the sub-beams diffracted by the pattern has a roughly equal intensity that is typically between 20% and 40% of the excitation laser beam, and the sub-beam transmitted by the pattern has an intensity that is between 20% and 60% of the excitation laser beam.
In other embodiments, the interference pattern contains interference fringes separated by a distance of between 3 and 20 microns. Such an interference pattern is generated with a phase mask that contains multiple patterns, each of which generates sub-beams that diverge from the phase mask at a different angle.
In another aspect, an apparatus includes: 1) an excitation laser that generates an excitation laser beam; 2) a beam-delivery system, aligned along an optical axis, that separates the excitation laser beam into at least two sub-beams; 3) an imaging system aligned along the optical axis that collects the two sub-beams and focuses them onto the sample to form an optical
Banet Matthew J.
Fuchs Martin
Rogers John A.
Active Impulse Systems
Kim Robert
Piotrowski Tony E.
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