Optics: measuring and testing – By configuration comparison – With photosensitive film or plate
Reexamination Certificate
1998-10-21
2001-02-13
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By configuration comparison
With photosensitive film or plate
C356S432000
Reexamination Certificate
active
06188478
ABSTRACT:
FIELD
This invention relates to a method and apparatus for measuring properties of a sample, e.g., the thickness of a thin film.
BACKGROUND
An all-optical measurement technique called Impulsive Stimulated Thermal Scattering (ISTS) measures a variety of different material properties, such as film thickness. In ISTS, two or more excitation laser beams from an excitation laser overlap in time and space on a surface of a sample to form a spatially varying optical interference pattern. The excitation laser beam consists of a series of short (e.g., a few hundred picoseconds) optical pulses having a wavelength within the absorption range of the sample. The excitation pattern features alternating “light” (i.e. constructive interference) and “dark” (i.e. destructive interference) elliptical regions with a spacing that depends on the wavelength of the laser beams and the angle between them. The light regions of the pattern heat the sample, causing it to thermally expand. This launches coherent, counter-propagating acoustic waves whose wavelength and direction match the pattern.
For opaque films (e.g., metal films), the acoustic waves generate a time-dependent “ripple” pattern on the film's surface that oscillates at one or more acoustic frequencies (typically a few hundred megahertz). The acoustic frequency depends on film properties such as thickness, density, and elastic moduli. A probe beam then diffracts off the ripple to form a series of signal beams, each representing at least one distinct diffraction order (e.g., the +1, −1, +2, or −2 orders). The signal beams oscillate in intensity at the acoustic frequency or a multiple thereof, or at sums or differences of acoustic frequencies if several are present. One or more 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 pending and issued U.S. Pat. No. 5,633,711 (entitled MEASUREMENT OF MATERIAL PROPERTIES WITH OPTICALLY INDICED PHONONS); U.S. Pat. No. 5,546,811 (entitled OPTICAL MEASUREMENT OF STRESS IN THIN FILM SAMPLES); and U.S. Ser. No. 08/783,046 (entitled METHOD AND DEVICE FOR MEASURING FILM THICKNESS, filed Jul. 15, 1996), the contents of which are incorporated herein by reference.
ISTS-measured film thickness can be used as a quality-control parameter during and/or after the manufacturing of microelectronic devices. In these devices, thin films of metals and metal alloys are deposited on silicon wafers and used as electrical conductors, adhesion-promoting layers, and diffusion barriers. For example, metal films of copper, tungsten, and aluminum are used as electrical conductors and interconnects; titanium and tantalum as adhesion-promoting layers; and titanium:nitride and tantalum:nitride as diffusion barriers. Thickness variations in the metal films can modify their electrical and mechanical properties, thereby affecting the performance of the devices in which they are used. To effectively monitor metal films in a fabrication process, the ISTS film-thickness measurement must therefore be highly repeatable, precise, and accurate.
SUMMARY
To address these needs, the invention provides a method and apparatus for improving the repeatability, precision, and, in some cases, accuracy of film thickness measurements made using ISTS. Each improvement is attributed to “dithering” an optical or mechanical element in the system used for ISTS. In this case, “dither” is defined as any movement or modulation of a component that changes a spatial phase of the excitation pattern. In the preferred embodiment, the pattern consists of alternating light and dark regions as described above that are roughly parallel. Changing the spatial phase of the excitation pattern means that the positions of the light and dark regions of the excitation pattern are moved in concert relative to the surface of the sample. The change of spatial phase as a result of dither is preferably in a direction perpendicular to the orientation of the long axis of the elliptical light and dark regions. The terms phase and spatial phase will be used interchangeably herein with respect to the excitation pattern.
In general, in one aspect, the invention provides a method for measuring a sample that includes the steps of: 1) irradiating a portion of the sample with an excitation pattern characterized by at least one spatial phase and spatial period; 2) diffracting a portion of a probe beam off a surface of the sample; 3) detecting the diffracted portion of the probe beam with an optical detector to generate a light-induced signal; 4) adjusting the phase of the excitation pattern; 5) repeating the irradiating, diffracting and detecting steps to generate an additional light-induced signal; and 6) processing the light-induced signals to determine a property of the sample.
In another aspect, the invention provides an apparatus for measuring a sample that includes: 1) a first light source that generates an optical excitation pulse; 2) an optical system aligned to receive the optical excitation pulse, separate it into at least two optical pulses, and focus at least one pulse onto a surface of the sample to form an excitation pattern characterized by at least one spatial phase and spatial period; 3) a phase-adjusting component that adjusts the spatial phase of excitation pattern; 4) a second light source that generates a probe beam that diffracts off the sample; 5) an optical detector that detects the diffracted portion of the probe beam to generate a light-induced signal; and 6) a processor configured to process the light-induced signal from the optical detector to determine a property of the sample.
Further advantageous embodiments of the invention are recited in the dependent claims.
The invention has many advantages. In general, using dithering to vary the phase of the excitation pattern during a measurement improves the precision of ISTS-based thickness measurements: even very rough films, or films with regions containing areas that scatter radiation, can be accurately measured. In this application, ISTS detects minor variations in the thickness of thin films that can affect their functions in microelectronic devices.
The improvement in precision is particularly evident in multi-point measurements. These measurements involve making thickness measurements from multiple points within a given area. Examples of multi-point measurements include: 1) “line scans” that measure more than one point along a line on a film's surface, e.g., along a film's diameter or edge; and 2) “contour maps” based on a two-dimensional array of points measured in an area (e.g., a circle, square, or rectangle) on the film. During these multi-point measurements, dithering a component of the optical system decreases the standard deviation of the thickness measured from each point, thereby increasing the overall precision of the measurement.
In a more general sense, the invention improves an all-optical, non-contact measurement technique that effectively measures the thickness of thin films in single and multi-layer structures, such as ISTS. The thickness values can then be used to control a fabrication process (e.g., fabrication of a microelectronic device). The apparatus features all the advantages of optical metrology: each measurement is non-contact, rapid (typically less than 1 or 2 seconds per point), remote (the optical system can be as far as 10 cm or more from the sample), and can be made over a small region (as small as about 20 microns). Other properties besides film thickness may also be measured more precisely through the use of dithering.
REFERENCES:
patent: 5377006 (1994-12-01), Nakata
patent: 5479259 (1995-12-01), Nakata et al.
patent: 5734470 (1998-03-01), Rogers et al.
patent: 5812261 (1998-09-01), Nelson et al.
patent: WO 98/03044 (1998-01-01), None
Banet Matt
Fuchs Martin
Joffe Michael A.
Font Frank G.
Philips Electronics North America Corporation
Piotrowski Tony E.
Smith Zandra V.
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