Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2000-02-03
2003-04-22
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S497000
Reexamination Certificate
active
06552806
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 an automated method and apparatus for minimizing the optical path difference between the reference and the test arms of a Linnik white-light interference objective.
2. Description of the Related Art
White-light interferometric devices typically utilize microscope objectives for simultaneously imaging a sample surface and a reference surface onto a detector in order to produce interference fringes representative of the height of the sample. As illustrated in simple schematic form in
FIG. 1
, a typical Linnik interference microscope includes an objective
10
focused on a test surface S and incorporating an interferometer. The interferometer 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 toward 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 (OPD) between the reference and test beams. The light reflected by the test surface S interferes with the light reflected at the reference mirror R and, according to the principle of superposition, bright interference fringes are produced corresponding to all points on the reference mirror where the optical path difference of the light is equal to a multiple of its wavelength. In order for discernible fringes to appear, the OPD must be within the coherence length of the light source. Spectral filtering may be used to increase the coherence length of the light.
The light from the reference and sample surfaces is typically passed back through the interferometric microscope
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 the reference mirror R 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
18
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
20
connected to the processing unit
18
. As mentioned, a filter
32
may be provided to optionally narrow the bandwidth of the light beam W to a predetermined value.
In a Linnik interferometer, a smooth test surface S is typically profiled by phase-shifting interferometry (PSI), typically by scanning the reference arm of the microscope objective
10
. A piezoelectric translator or equivalent device (not shown in
FIG. 1
) is used in conventional manner to shift the reference optics
30
and mirror R together with respect to the sample surface S. Phase shifting is founded on the basic concept of varying the phase difference between two interfering beams in some known manner, such as by changing the optical path difference in discrete steps or linearly with time. Under such conditions, three or more measurements of the light intensity at a pixel of a receiving sensor array can be used to determine the relative phase of the light reflected from the point on a test surface corresponding to that pixel, with respect to the other points on the test surface. Based on such measurements at each pixel of coordinates x and y, a phase distribution map &PHgr;(x,y) can be determined for the test surface, from which very accurate height data h(x,y) are calculated in relation to the wavelength &lgr; of the light source used by the following general equation:
h
(
x
,
y
)
=
λ
4
π
Φ
(
x
,
y
)
.
(
1
)
Phase-shifting interferometry provides a vertical resolution in the order of {fraction (1/100)} of a wavelength or better. The technique is typically limited to measurements of smooth surfaces due to a height ambiguity encountered at phase steps greater than &pgr;.
Linnik interferometers are also used for vertical-scanning interferometry (VSI) of rough surfaces. VSI is a well-known technique where white light is used as the light source in an interferometer and the degree of modulation, or coherence, of interference fringes produced by the instrument is measured for various distances between the test surface and the reference surface of the interferometer (each corresponding to a different optical path difference) to determine surface height. The microscope objective is typically adapted for vertical movement (along the z coordinate). Thus, an interference-fringe map is generated by detecting the intensity of the light signal received in each cell of the light sensor as a function of the z-scan position. The position of the scanning mechanism corresponding to maximum interference at each pixel is determined and used, based on the distance from a reference point, to calculate the height of the surface at that pixel.
As shown in
FIG. 1
, the Linnik configuration of a microscope objective includes a white light source
22
and imaging optics
24
providing an illumination beam W to the system through a beam splitter or equivalent device
26
. The illumination beam is then focused, ideally, in the entrance pupils E and E′ of the reference and test imaging optics
30
and
28
, 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
30
through a process based on a visual determination of best focus produced by axially shifting the reference mirror with respect to its imaging optics
30
along a distance F, as one skilled in the art would readily understand. This is often accomplished by the introduction of a field stop in the illuminator (not shown in FIG.
1
). When the diameter of the field stop is reduced sufficiently to bring its edges into the field of view, a sharp image of the edges is formed on the camera when the reference surface R is in focus. Copending application Ser. No. 09/452,334 discloses a mechanism for focusing the reference mirror R automatically, which provides an opportunity for periodic recalibration of the instrument.
Prior to carrying out surface profiling by PSI measurements, the distance between the sample surface S and its focusing optics
28
is also adjusted to produce a sharp, in-focus image of the sample. This is normally achieved by moving the entire objective
10
vertically with respect to the sample stage (although the same result could obviously be achieved as well by moving the stage). The distance F′ (
FIG. 1
) between the lens
28
and the surface S is varied, either manually or automatically, until the sample surface is in focus. Typically, the optimal in-focus position of the sample surface is determined by visual observation of the best picture of the sample surface seen at the monitor
20
. Alternatively, an image of the field stop, as described above, can be used to set the optimal in-focus position. Similarly, for VSI measurements, fringes are present at a given scan position only for portions of the surface that are in focus.
When a light of short coherence length is used, such as produced by the white-light source
22
, interference between the return beams only occurs when the optical path difference between the test and the reference paths is within the coherence length of the system. The coherence length of a white-light source can be increased with spectral fil
Aziz David J.
Guenther Bryan W.
Swinford Richard W.
Unruh Paul R.
Connolly Patrick
Durando Antonio R.
Durando Birdwell & Janke PLC
Turner Samuel A.
Veeco Instruments Inc.
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