Optics: measuring and testing – By light interference – Having wavefront division
Reexamination Certificate
1998-05-28
2004-09-21
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Having wavefront division
Reexamination Certificate
active
06795198
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method and apparatus for measuring properties (e.g., thickness) of thin layers (e.g., metal films) contained in a structure.
During fabrication of microelectronic devices, thin films of metals and metal alloys are deposited on silicon wafers and used as electrical conductors, adhesion-promoting layers, and diffusion barriers. Microprocessors, for example, use metal films of copper, tungsten, and aluminum as electrical conductors and interconnects; titanium and tantalum as adhesion-promoting layers; and titanium:nitride and tantalum:nitride as diffusion barriers. Thickness variations in these films can modify their electrical and mechanical properties, thereby affecting the performance of the microprocessor. The target thickness values of metal films vary on their function: conductors and interconnects are typically 3000-10000 angstroms thick, while adhesion-promoting and diffusion-barrier layers are typically between 100-500 angstroms thick.
During fabrication of the microprocessor, films are deposited to have a thickness of within a few percent of their target value. Because of these rigid tolerances, film thickness is often measured as a quality-control parameter during and/or after the microprocessor's fabrication. Noncontact, nondestructive measurement techniques (e.g., optical techniques) are preferred because they can measure patterned “product” samples, rather than “monitor” samples. Measurement of product samples accurately indicates errors in fabrication processes and additionally reduces costs associated with monitor samples.
Optical methods for measuring thin, opaque films have been described. For example, U.S. Pat. No. 5,633,711 (entitled MEASUREMENT OF MATERIAL PROPERTIES WITH OPTICALLY INDUCED PHONONS), U.S. Pat. No. 5,546,811 (entitle OPTICAL MEASUREMENT OF STRESS IN THIN FILM SAMPLES), U.S. Pat. No. 5,672,330 (entitled MEASURING ANISOTROPIC MATERIALS IN THIN FILMS), and U.S. Ser. No. 08/783,046 (entitled METHOD AND DEVICE FOR MEASURING THE THICKNESS OF OPAQUE AND TRANSPARENT FILMS) describe an optical measurement technique called impulsive stimulated thermal scattering (“ISTS”). In ISTS two optical pulses are overlapped on a sample to form a spatially and temporally varying excitation pattern that launches counter-propagating acoustic waves. These patents and applications have the same inventors as this application, and are incorporated herein by reference. U.S. Pat. No. 5,394,413 (entitled PASSIVELY Q-SWITCHED PICOSECOND MICROLASERS) describes a small-scale “microlaser” that can be used to form the excitation pulses. U.S. Pat. No. 5,734,470 (entitled DEVICE AND METHOD FOR TIME-RESOLVED OPTICAL MEASUREMENTS) describes how a single pulse passes through a diffractive mask, e.g. a phase mask, to form the two optical pulses. These patents are also incorporated herein by reference.
In ISTS the acoustic waves a “transient grating” that includes an alternating series of peaks and nulls. A probe pulse irradiates the grating, and is diffracted to form a pair of signal beams. One or both of the signal beams are detected and analyzed to measure a property of the sample.
In another embodiment of ISTS, the two excitation beams are separated from a single beam by a partially reflecting mirror (e.g., a beamsplitter) and used to form the transient grating. The probe beam is then focused on a peak or null of the grating, where it is reflected, detected, and analyzed to determine a property of the sample; in this case the diffracted beam is not detected. Accuracy in this measurement requires the phase of the grating to be fixed, i.e., the position of the peaks and nulls must be stationary relative to the probe beam. The peaks and nulls are typically separated by a few microns, and thus even small fluctuations of the laser beam causes these components to move relative to the probe beam; over short periods of time this averages out any modulation (e.g., signal) mapped onto the probe beam. Since beams generated by conventional lasers typically undergo spatial fluctuations, active stabilization systems including optical detectors, closed-loop feedback systems, and electrooptic light modulators (or similar means) are typically used in the measurement. Such systems fix the position of the peaks and nulls, making it possible to accurately measure the reflected beam.
U.S. Pat. No. 4,522,510 describes another optical technique wherein a single excitation beam irradiates a sample and is absorbed to initiate a “thermal wave”. A probe beam reflects off the thermal wave and is analyzed to determine a property (e.g., concentration of implanted ions) of the sample.
SUMMARY OF THE INVENTION
In general, in one aspect, the invention provides both a method and apparatus that when used a “reflection mode” geometry measure a property of a structure that includes at least one layer. The apparatus features a laser (e.g., a microchip laser, described below) that generates an optical pulse, and a diffractive element that receives the optical pulse and diffracts it to generate at least two excitation pulses. An optical system, (e.g., an achromat lens pair) receives the optical pulses and spatially and temporally overlaps them on or in the structure to form an excitation pattern that launches an acoustic wave (or an electronic response, or a thermal response). The acoustic wave modulates a property of the structure, e.g., it generates a time-dependent “surface ripple” or modulates an optical property such as the sample's refractive index or absorption coefficient. Surface ripple is defined as a time-dependent change in the morphology of the surface; its peak-to-null amplitude is typically a few angstroms or less. The apparatus also includes a light source that produces a probe beam that reflects off an area of the structure containing the modulated property to generate a signal beam. An optical detection system receives the reflected signal beam and in response generates a light-induced electrical signal. An analyzer analyzes the signal to measure the property of the structure.
In embodiments, the diffractive element is a phase mask that includes an optically transparent substrate (e.g., a quartz plate or microscope slide). The substrate typically features one or more patterns characterized by a series of parallel trenches having a spatial periodicity of between 0.1 and 100 microns.
As described above, the laser is typically a microchip laser that is diode-pumped and passively Q-switched. For example, the laser can include Nd:YAG, titanium:sapphire, chromium:LISAF, analogs of these materials, or a fiber laser. In typical embodiments the laser features a Nd:YAG layer having a thickness of less than 5 mm. The laser used in the apparatus emits a pulse having a duration that is typically 1 nanosecond or less.
The acoustic waves typically modulate a structure's surface. When the acoustic waves generate a time-dependent ripple on the surface the probe beam is aligned to deflect off the ripple to form the signal beam. In this case, the optical detection system includes a detector (e.g., a bi-cell detector or photodiode, described below) that generates an electrical signal that changes with a deflection angle of the probe beam. Alternatively, the modulated property is an optical property of the structure, such as a refractive index or absorption coefficient. Here, the probe beam reflects off an area of the structure containing the modulated optical property, and the optical detection system is configured to detect a phase of the reflected signal beam. Here, the optical detection system includes an interferometer.
In other embodiments, the optical system includes at least one lens that collects and overlaps the excitation pulses on or in the structure. For example, the optical system can include a lens pair (e.g., an achromat pair) having a magnification ratio of about 1:1. “About 1:1” means between 0.8:1 and 1.2:1. The apparatus also typically includes a lens that focuses the probe laser beam onto the acoustic waves. For example, when the aco
Banet Matthew J.
Fuchs Martin
Nelson Keith A.
Rogers John A.
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