Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
1999-11-30
2003-04-08
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
Reexamination Certificate
active
06545761
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is related in general to the field of interferometry and, in particular, to a method and apparatus for optimizing the focal-point calibration of an interferometric microscope.
2. Description of the Related Art
Microscope objectives are commonly used in interferometric devices for focusing a beam of light on a sample surface and a reference surface to produce interference fringes representative of the optical path difference (OPD) between the test and reference path. As illustrated in simple schematic form in
FIG. 1
, a typical interferometric microscope
10
consists of a microscope objective focused on a test surface S and incorporating an interferometer. The interferometer, shown in Linnik configuration for illustration, comprises a beam splitter
12
and a reference mirror R such that the light beam W directed to the sample surface S is split and also directed to the reference mirror. As is well understood by those skilled in the art, the light beams reflected from the reference mirror R and the test surface S (the reference and test beams, respectively) are combined to produce interference fringes as a result of the optical path difference between the reference mirror and the test surface S. The light is typically passed back through the interferometric microscope objective
10
and appropriate imaging optics
14
toward an imaging array
16
positioned in a camera in coaxial alignment with the objective. The imaging array usually consists of individual charge-coupled-device (CCD) cells or other sensing apparatus adapted to record a two-dimensional array of signals corresponding to interference effects produced by the interferometer as a result of light reflected at individual x-y coordinates or pixels in the surface S and received at corresponding individual cells in the array. Appropriate electronic hardware (not shown) is also provided to process the signals generated by each cell and transmit them to a computer
17
for further processing. Thus, an interference-fringe map is generated by detecting the intensity of the light signal received in each cell of the array. The map may be displayed on a monitor
19
connected to the processing unit
17
. Additional information about the test surface S can be gained by varying the OPD between the reference and test paths with a scanning device (not shown) shortening or lengthening either the reference path or the test path.
The present invention is directed at improving the focus calibration of the reference mirror R in such an interferometric microscope. As shown in
FIG. 1
, the Linnik configuration of a microscope objective includes a white light source
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and imaging optics
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providing an illumination beam W to the system through a beam splitter or equivalent device
22
. The illumination beam is then focused, ideally, in the entrance pupils E and E′ of the reference and test imaging optics
24
and
26
, respectively. This is known as Kohler illumination, and produces approximately uniform illumination on the sample surface S and the reference surface R. By way of calibration, the reference mirror R is set at the focal point of the imaging optics
26
through a process based on a visual determination of best focus produced by axially shifting the reference mirror with respect to its imaging optics
26
, as one skilled in the art would readily understand. One common way of determining best focus is by imaging a variable aperture, known as a field stop, onto the reference mirror such that the image of the edges of the aperture, upon reflection from the reference mirror, is sharply in focus at the imaging array
16
when the reference mirror is in focus. Once the optimal distance x between the lens
26
and the mirror R is found my manual manipulation, the position of the mirror within the microscope
10
may be fixed at the factory for a particular instrument. However, due to practical limitations on stability, it often must be left adjustable and set by the user during setup procedures.
Such manual setting of the reference-mirror focus suffers from two distinct limitations. First, it is based on an operator's visual observation of best focus, which is necessarily approximate because of the inability of the human eye to distinguish image variations produced by very small focal shifts. Thus, it is limited at best to a precision equal to the depth-of-focus of the objective (often considered to be +/−0.25 wavelengths of defocus in the converging wavefront). Second, the approach is inherently subjective, leading to different results from different operators.
Moreover, as interference microscopes have become standard quality-control tools in production environments, such as for testing the topography of magnetic heads, greater measurement precision and repeatability are required. Such interference microscopes are now capable of making measurements with sub-nanometer precision. Accordingly, the precise in-focus position of the reference mirror has become more and more critical. As is well known in the art, the depth of focus (DOF) of an objective is inversely proportional to the square of its numerical aperture. Thus, the depth of focus decreases rapidly with the objective's magnification. For example, while a Nikon® 5×/0.13NA objective has a depth of focus of about 30 microns, a Nikon® 100×/0.95NA objective has a DOF smaller than one micron. Therefore, minute shifts in the position of the reference surface R with respect to its imaging optics
26
are sufficient to cause the reference surface to be out of focus and produce incorrect profile measurements. Accordingly, it is now necessary to obtain extremely tight levels of control over the defocusing effects of the reference mirror and it has become desirable to periodically and routinely recalibrate interferometric microscopes.
A significant improvement toward these ends was achieved with the development of athermalized interference objectives to suppress the effects of environmental changes. As detailed in U.S. Pat. No. 5,978,086, incorporated herein by reference, in these objectives the reference mirror is held at the optimal focus location over a range of temperatures by adding structural components that offset the effects of temperature variations. Each component is coupled sequentially, such that the thermal response of the objective assembly is substantially the linear combination of the response of each component and is designed to cause a shift in the opposite direction to the shift produced by a temperature change in the unmodified device. The thermal characteristics and dimensions of the components are chosen empirically to minimize the shift of the reference mirror with respect to its in-focus position in the interferometric objective as a function of temperature.
In spite of this improvement, defocusing effects result from vibrations, wear, and other environmental forces that affect the reference mirror in addition to temperature changes. Thus, it remains desirable, and for some applications it has become necessary, to assure the current in-focus position of the reference surface of interferometric microscopes. This invention is directed at providing a solution to this problem. Furthermore, this invention readily allows focus of the reference mirror with a level of precision that cannot be obtained with a visual focus technique.
BRIEF SUMMARY OF THE INVENTION
One primary object of this invention is a method for setting the in-focus position of the reference surface of an interferometric microscope objective that is not operator-dependent.
Another object of the invention is a method that is suitable for automated implementation.
Still another objective is a method and corresponding implementing apparatus that are suitable for periodic calibration of an interferometric microscope objective while in service.
Another goal of the invention is a method and apparatus that are suitable for incorporation within existing instruments.
A final object is a procedure t
Aziz David J.
Guenther Bryan W.
Durando Antonio R.
Durando Birdwell & Janke PLC
Font Frank G.
Lee Andrew H.
Veeco Instruments Inc.
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