Optics: measuring and testing – By polarized light examination – Of surface reflection
Reexamination Certificate
2001-07-27
2003-10-21
Fuller, Rodney (Department: 2851)
Optics: measuring and testing
By polarized light examination
Of surface reflection
C356S365000
Reexamination Certificate
active
06636309
ABSTRACT:
TECHNICAL FIELD
The present invention relates to application of Ellipsometry to in-situ real-time monitoring and/or process control; and more precisely to the monitoring and/or controlling of the fabrication of multiple high/low refractive index layer narrow passband optical filters using oblique angle-of-incidence spectroscopic ellipsometric DELTA determination over a wavelength range, around said narrow passband center wavelength, in which wavelength range reflectivity is essentially 100%. Said method is optionally enhanced by combination with essentially normal angle-of-incidence non-ellipsometric transmission extrema turning point data.
BACKGROUND
The use of Spectroscopic Ellipsometry (SE) to non-invasively characterize properties, (such as thickness, composition, morphology and optical constants), of thin films ex-situ is well known. And, while less common, application to real-time in-situ fabrication monitoring and control is also known, particularly in the semiconductor area. Further, it is known that said techniques are directly applicable to investigating sample systems comprised of multiple thin film layers.
Ellipsometry basically monitors a change in Polarization State of a beam of electromagnetism, which polarization state change occurs as a result of interaction with a sample system. Based upon said change in polarization state, sample system characterizing ellipsometric PSI (&psgr;) and ellipsometric DELTA (&Dgr;), which are defined by:
Tan(4)
e
i&Dgr;
=&rgr;=&Ggr;
p
/&Ggr;
s
where r
p
and r
s
can be complex Fresnel reflectivities for “p” and “s” polarized components, can be determined. It is noted that Rho (&rgr;) is a complex number defined as the ratio of the reflectivity of “p-polarized” to reflectivity of the “s-polarized” components of a beam of polarized electromagnetic radiation. In polar form, Tan(&psgr;) corresponds to the magnitude of the reflectivity ratio and (&Dgr;) corresponds to the phase angle introduced between “p” and “s” polarized components by interaction with the sample system. Further, it should be understood that said “p” component is defined as being in the plane of an incident and reflected beam of electromagnetic radiation, which plane also contains a normal to the surface of a the reflective surface of the sample system. And the “s” component is defined as being perpendicular to the direction of the “p” component and also parallel to said reflective surface of the sample system.
It should be appreciated that ellipsometry determines a ratio of “p” and “s” polarization component intensity values rather than an absolute intensity value, and that ellipsometry provides phase shift information, (ie. between said “p” and “s” components), which is not available from electromagnetic beam intensity reflection or transmission data, wherein change in “p” and “s” polarization states are not monitored. It is further to be appreciated that said phase shift information is generally very sensitive to properties, (and changes therein), associated with ultra-thin films.
It must also be appreciated that many types of Ellipsometer systems exist, many of which sequentially comprise a source of electromagnetic radiation, a polarizer means for setting a polarization state, a means for supporting a sample system, an analyzer means for selecting a polarization state, and a detector means for receiving electromagnetic radiation and producing a signal which is proportional to its intensity. Typically at least one element in the ellipsometer system is caused to rotate during data acquisition, and said rotating element can be the polarizer means or analyzer means. A problem in applying rotating polarizer or rotating analyzer ellipsometer systems, however, is that ellipsometric DELTA's of 0.0 or 180 degrees are impossible to measure therewith without use of means such as the J.A. Woollam Co. Autoretarder, (see U.S. Pat. Nos. 5,757,494 and 5,956,145). In that light it is disclosed that a relevant benefit exists where the polarizer means and analyzer means are both held essentially stationary during data acquisition, and instead an additional element, (ie. a compensator), is present and rotated during said data acquisition. The reason for this is that an important benefit is associated with rotating compensator ellipsometers in that ellipsometric DELTA's are measurable thereby over the entire range of 0.0-360 degrees. In addition, although not as important to the present invention, rotating compensator ellipsometor systems can still measure ellipsometric PSI's the entire range of 0.0 to 90 degrees. The J.A. Woollam Co. “M2000” (Reg. Trademark), Rotating Compensator Ellipsometer System is described in U.S. Pat. No. 5,872,630.
Continuing, of growing importance is the fabrication and application of Narrow Bandpass Optical Filters. Said Narrow Bandpass Optical Filters, which provide very sharp cutoff characteristics, (eg. said passbands providing a bandwidth of a nanometer or so with combined high and low transition to cutoff being less than a nm), are typically comprised of up to a hundred or more layers of alternating quarter-wavelength thick high, and quarter wavelength thick low, refractive index materials, said sequence being beneficially interspersed with half-wavelength thick cavities and/or coupling layers. Present manufacturing techniques typically control deposition of the layers of alternating quarter-wavelength high, and quarter wavelength low, optically thick refractive index materials utilizing transmission data, wherein a cyclic pattern of Transmission vs. Layer Number “extrema” turning points are used to determine when to change from depositing low to high, and vice-verse, refractive index material. A problem with this approach is that in some ranges said Transmission data can be relatively insensitive to change in thickness of deposited material.
In view of known prior art, there remains need for improved methodology for monitoring and/or controlling fabrication of multiple layer Narrow Bandpass Optical Filters.
DISCLOSURE OF THE INVENTION
The present invention is based in the discovery that, over a limited range of wavelengths surrounding the central passband wavelength of a Narrow Bandpass Optical Filter, ellipsometric DELTA's which are determined using data obtained by oblique angle spectroscopic ellipsometric investigation, are very well behaved and that high and low refractive index materials demonstrate easily differentiated ellipsometric DELTA vs. Wavelength plots. Further, ellipsometric DELTA's are related to optical thickness of the surface layer of a Narrow Bandpass Optical Filter during fabrication, with minimal influence on said DELTA's being effected by previously deposited layers.
It is noted that presently Transmission Intensity Extrema Turning Point vs. Layer Number data is typically utilized during fabrication of Narrow Bandpass Optical Filters, to provide generally good insight as to when to change from deposition of high to low, and low to high refractive index materials, when quarter-wavelength optically thick layers are being formed. However, where other than quarter-wavelength optical thickness layers are being formed said transmission extrema turning point data is not sufficient to provide reliable data upon which can be based said decision. It is in that light that the present invention teaches supplementing said conventional Transmission Intensity Extrema Turning Point vs. Layer Number data with ellipsometric DELTA vs. wavelength data to improve fabrication precision.
The present invention methodology can however, be applied alone, (ie. not in combination with conventional Transmission Intensity Extrema Turning Point vs. Layer Number data). The present invention then teaches use of ellipsometric DELTA vs. wavelength data alone to monitor and/or control deposition of quarter-wavelength, and non-quarter wavelength, optical thickness layers. This is enabled as said ellipsometric DELTA data provides insight to the optical thickess of a layer of material, be it a high or low refractive ind
Hale Jeffrey S.
Johs Blaine D.
Fuller Rodney
J.A. Woollam Co.
Welch James D.
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