Optics: measuring and testing – By polarized light examination – Of surface reflection
Reexamination Certificate
1999-10-18
2003-04-15
Smith, Zandra (Department: 2877)
Optics: measuring and testing
By polarized light examination
Of surface reflection
Reexamination Certificate
active
06549282
ABSTRACT:
TECHNICAL FIELD
The present invention relates to ellipsometry and polarimetry, and more particularly is a system for focusing an electromagnetic beam as a small spot over a large range of wavelengths including into the deep UV, and further is a method for evaluating parameters in parameterized equations for calculating retardance entered to orthogonal components in a beam of electromagnetic radiation, by multiple element input and output lenses, through which said beam of electromagnetic radiation is caused to pass.
BACKGROUND
The practice of ellipsometry is well established as a non-destructive approach to determining characteristics of sample systems, which can be practiced in real time. The topic is well described in a number of publication, one such publication being a review paper by Collins, titled “Automatic Rotating Element Ellipsometers: Calibration, Operation and Real-Time Applications”, Rev. Sci. Instrum, 61(8) (1990).
In general, the practice of ellipsometry typically involves causing a spectroscopic beam of electromagnetic radiation, in a known state of polarization, to interact with a sample system at some angle of incidence with respect to a normal to a surface thereof, in a plane of incidence. (Note, a plane of incidence contains both a normal to a surface of an investigated sample system and the locus of said beam of electromagnetic radiation). Changes in the polarization state of said beam of electromagnetic radiation which occur as a result of said interaction with said sample system are indicative of the structure and composition of said sample system. The practice of ellipsometry determines said changes in polarization state by proposing a mathematical model of the ellipsometer system and the sample system investigated by use thereof. Experimental data is then obtained by application of the ellipsometer system, and a square error reducing mathematical regression, (typically), is then applied to the end that parameters in the mathematical model which characterize the sample system are evaluated so that the obtained experimental data, and values calculated by use of the mathematical model are essentially identical.
A typical goal in ellipsometry is to obtain, for each wavelength in, and angle of incidence of said beam of electromagnetic radiation caused to interact with a sample system, sample system characterizing PSI and DELTA values, (where PSI is related to a change in a ratio of magnitudes of orthogonal components r
p
/r
s
in said beam of electromagnetic radiation, and wherein DELTA is related to a phase shift entered between said orthogonal components r
p
and r
s
, caused by interaction with said sample system;
PSI=|r
p
/r
s
|;
and
DELTA=∠
r
p
−∠r
s
)).
As alluded to, the practice of ellipsometry requires that a mathematical model be derived and provided for a sample system and for the ellipsometer system being applied. In that light it must be appreciated that an ellipsometer system which is applied to investigate a sample system is, generally, sequentially comprised of:
a. a Source of a beam electromagnetic radiation;
b. a Polarizer element;
c. optionally a compensator element;
d. (additional element(s));
e. a sample system;
f. (additional element(s));
g. optionally a compensator element;
h. an Analyzer element; and
i. a Detector System.
Each of said components b.-i. must be accurately represented by a mathematical model of the ellipsometer system along with a vector which represents a beam of electromagnetic radiation provided from said source of a beam electromagnetic radiation, Identified in a. above)
Various ellipsometer configurations provide that a Polarizer, Analyzer and/or Compensator(s) can be rotated during data acquisition, and are describe variously as Rotating Polarizer (RAE), Rotating Analyzer (RAE) and Rotating Compensator (RCE) Ellipsometer Systems.
Where an ellipsometer system is applied to investigate a small region of a sample system present, it must be appreciated that the beam of electromagnetic radiation can be entered thereto through an converging input lens, and exit via a diverging output lens. In effect this adds said converging input and diverging output lenses as elements in the ellipsometer system as “additional elements”, (eg. identified in d. and f. above), which additional elements must be accounted for in the mathematical model. If this is not done, sample system representing parameters determined by application of the ellipsometer system will have the effects of said converging input and diverging output lenses at least partially correlated thereinto, much as if the converging input and diverging output lenses were integrally a part of the sample system.
It is further noted that where two sequentially adjacent elements in an ellipsometer system are held in a static position with respect to one another while experimental ellipsometric data is acquired, said two sequentially adjacent elements generally appear to be a single element. Hence, a beam directing element adjacent to a lens can appear indistinguishable from said lens as regards the overall effect of said combination of elements. In that light it is to be understood that converging input and diverging output lenses are normally structurally fixedly positioned and are not rotatable with respect to a sample system present in use, thus preventing breaking correlation between parameters in equations for sequentially adjacent converging input and diverging output lenses and an investigated sample system by an element rotation technique. While correlation of parameters in mathematical equations which describe the effects of groupings of elements, (such as a compensator and an optional element(s)), can be tollerable, correlation between parameters in the mathematical model of an investigated sample system and other elements in the ellipsometer system must be broken to allow obtaining accurate sample system representing PSI and DELTA values, emphasis added. That is to say that correlation between parameters in a equations in a mathematical model which describe the effects of a stationary compensator and a sequentially next lens element, (eg. correllation between effects of elements c. and d. or between f. and g. identified above), in a beam of electromagnetic radiation might be tolerated to the extent that said correlation does not influence determination of sample system describing PSI and DElTA values, but the correlation between parameters in equations which describe the effects of ellipsometer system components (eg. a., b., c., d., f., g., h. and i.), and equations which describe the effects of a present sample system (eg. element e. above), absolutely must be broken to allow the ellipsometer system to provide accurate PSI and DELTA values for said sample system. In-situ application of ellipsometry to investigation of a sample system present can then present a challenge to users of ellipsometer systems in the form of providing a mathematical model for each of a converging input and diverging output lens, and providing a method by which the effects of said converging input and diverging output lenses can be separated from the effects of an investigated sample system.
Thus is identified an example of a specific problem, solution of which is the topic of the present invention.
One typical approach to overcomming the identified problem, where space considerations are not critical, and where ellipsometer system configuration can be easily modified, is to obtain multiple data sets with an ellipsometer system configured differently during at least two different data set acquisitions. For instance, a data set can be obtained with a sample system present and in which a beam of electromagnetic radiation is caused to interact with said sample system, and another data set can be obtained with the ellipsometer system configured in a straight-through configuration, where a beam of electromagnetic radiation is caused to pass straight through an ellipsometer system without interacting with a sample system. Simultaneous mathematical regr
Ciphardt Martin M.
He Ping
Herzinger Craig M.
Johs Blaine D.
J. A. Woollam Co. Inc.
Smith Zandra
Welch James D.
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