X-ray microanalyzer for thin films

Details X-ray microanalyzer for thin films X-ray microanalyzer for thin films

1999-09-29

2002-04-30

Porta, David P. (Department: 2882)

X-ray or gamma ray systems or devices

Specific application

Fluorescence

C378S090000

Reexamination Certificate

active

06381303

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to analytical instruments, and specifically to instruments and methods for thin film analysis using X-rays.
BACKGROUND OF THE INVENTION
The National Technology Roadmap for Semiconductor (NTRS), published in 1997 by the Semiconductor Industry Association (SIA), indicates that metrology and testing is one of the “Grand Challenges” facing the semiconductor industry as it moves toward advanced technologies, such as 0.18 &mgr;m and 0.13 &mgr;m design rules. In the area of film thickness metrology, the required accuracy when measuring films with thickness less than 3 nm has stretched the capabilities of current film thickness equipment to its physical limits. In addition, the development of new materials, such as low-k dielectrics to accommodate the need for high speed devices and high-k dielectrics as a replacement for silicon dioxide, also require new capabilities for film thickness metrology.
Traditionally, films processed in semiconductor tabs are classed as transparent or opaque, requiring two distinct classes of film thickness metrology systems: optical equipment for thickness measurement of transparent films, and non-optical equipment for opaque films. Non-optical thickness measurement systems include laser acoustic devices, four-point probes, profilometers and X-ray fluorescent spectroscopy equipment.
Optical equipment, such as reflectometers and ellipsometers, have been widely used in thickness measurement and characterization of dielectric and other transparent films in semiconductor fabs. However, as the gate silicon dioxide thickness is reduced to less than 3 nm, the uncertainty in accuracy of these optical technologies, due to the difference in the assumed optical properties between bulk and thin film structures, is practically too large to be used for monitoring and controlling process equipment. For work with high-k and low-k dielectric films, additional film properties, such as composition and/or mass density, must also be known. These needs are not satisfied by available optical equipment.
Resistivity measurement based on four-point probe techniques has been used to deduced the thickness of metal films or other materials that are highly absorptive to optical radiation. Profilometers are also used to measure a step height at the surface of a sample, and the thickness of the top layer on the sample is deduced from the step height. These measurement technique are destructive, due to the contact between the measurement probes and the films under investigation. Furthermore, they are capable of measuring only the top surface layer, and the accuracy of thickness measurement is highly dependent on the condition of the measurement probes. In addition, the resistivity measurement of the four-point probe is highly susceptible to error when the materials under investigation vary in composition or density.
X-ray fluorescent spectroscopy has also been used to measure the thickness, composition and other properties metal and other films. This technique, however, is incapable of analyzing multilayer film structures that contain the same element in more than one layer, and it is ineffective when the elements to be analyzed are of low atomic number (Z). The high-power X-ray radiation used to excite the sample is destructive, and the measurement process is generally time-consuming and requires extensive calibration in order to obtain quantitative results.
Recently, the emergence of laser acoustic technologies has provided the capability of measuring multi-layer metal film structures. In operation, such techniques require layers of metal or low elastic constant materials in order to launch and sense an acoustic wave, which is used to probe the sample under investigation. The fundamental drawback of laser acoustic measurement is its inability to launch or sense an acoustic wave in dielectric films due to the high elastic constant of such films.
X-ray reflectometry (XRR) is a well-known technique for measuring the thickness, electron density and surface quality of thin film layers deposited on a substrate. Conventional X-ray reflectometers are sold by a number of companies, among them Technos (Osaka, Japan), Siemens (Munich, Germany) and Bede Scientific Instrument (Durham, UK). Such reflectometers typically operate by irradiating a sample with a beam of X-rays at grazing incidence, i.e., at a small angle relative to the surface of the sample, near the total external reflection angle of the sample material. Sequential measurement of X-ray intensity reflected from the sample as a function of angle gives a pattern of interference fringes, which is analyzed to determine the properties of the film layers responsible for creating the fringe pattern. The effectiveness of XRR is limited to layers that are less than 200 nm thick and have a surface roughness of no more than about 10 nm.
A method for performing the required analysis to determine film thickness from XRR data is described, for example, in U.S. Pat. No. 5,740,226, to Komiya et al., whose disclosure is incorporated herein by reference. Komiya et al. describe the application of their method to various types of thin films that are used in semiconductor electronic devices, including specifically SiO
2
, Ti and TiN.
U.S. Pat. No. 5,619,548, to Koppel, whose disclosure is likewise incorporated herein by reference, describes an X-ray thickness gauge based on reflectometric measurement. A curved, reflective X-ray monochromator is used to focus X-rays onto the surface of a sample. A position-sensitive detector, such as a photodiode detector array, senses the X-rays reflected from the surface and produces an intensity signal as a function of reflection angle. The angle-dependent signal is analyzed to determine properties of the structure of a thin film layer on the sample, including thickness, electron density and surface roughness. In order to determine the mass density of the layers, however, prior information is required regarding the composition of the analyzed layer, which cannot be determined based on XRR alone.
U.S. Pat. No. 5,923,720, to Barton et al., whose disclosure is incorporated herein by reference, also describes an X-ray spectrometer based on a curved crystal monochromator. The monochromator has the shape of a tapered logarithmic spiral, which is described as achieving a finer focal spot on a sample surface than prior art monochromators. Barton et al. calculate that the theoretical minimum spot size achievable by their monochromator is 5 nm. X-rays reflected or diffracted from the sample surface are received by a position-sensitive detector. It is suggested that the X-ray spectrometer may be used in surface mapping and film thickness measurements, with application to real-time in situ control of a film deposition system, such as systems used in MOCVD.
U.S. Pat. No. 5,003,569 to Okada et al., whose disclosure is incorporated herein by reference, describes a thickness determination method for organic films based on X-ray reflectometry. The organic film to be measured is irradiated with X-rays at a certain angle of incidence, and the angle is varied in order to find the angle of reflection at which the X-ray intensity reaches a peak. The peak is used to find the thickness of the film.
Another common method of X-ray reflectometric measurement is described, for example, in an article by Naudon et al., entitled “New Apparatus for Grazing X-ray Reflectometry in the Angle-Resolved Dispersive Mode,” in
Journal of Applied Crystallography
22 (1989), p. 460, which is incorporated herein by reference. A divergent beam of X-rays is directed toward the surface of a sample at grazing incidence, and a detector opposite the X-ray beam source collects reflected X-rays. A knife edge is placed close to the sample surface immediately above a measurement location in order to cut off the primary X-ray beam, so that only X-rays reflected from the measurement location reach the detector. A monochromator between the sample and the detector (rather than between the source and sample, as in U

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