Apparatus and method of rapidly measuring hemispherical scattered or radiated light

A portable scatterometer and/or an angular radiated light measurement instrument that uses a measurement head which includes a double tapered fiber optic bundle with a concave front face to simultaneously collect partial or full hemispherically scattered light reflected from a point on a surface illuminated by a depolarized, telescopically focused, laser diode source, the light rays being received by each fiber normal to its face. The image of the collected light beams is minified and coupled by the fiber optic bundle into an anti-blooming CID camera with an x-y scanning area array which converts the light beams to electrical signals. In a unique real time, computer-controlled, data acquisition and reconstruction process, a frame grabber and a unique algorithm are used to collect over 200,000 points of light, reconstruct the data into a 2D or 3D scatter profile and display the results, all within one second. Multiple embodiments of the tapered fiber optic bundle collector include a full hemispherical version with the laser source directed through the boule, a unitary boule fused from two tapered halves, a linear in-plane tapered bundle and a partial taper with imaging lens mode. Alignment mechanisms provide z-translation, azimuth, and focus adjustments. The measurement unit is in a compact, rugged aluminum housing which is adapted to be secured to a production line machine for a variety of services, such as, a quality control inspection device.

BACKGROUND OF THE INVENTION 
1. Fields of the Invention 
The present invention relates generally to an apparatus and method of 
measuring hemispherical light scattered or emitted from a source, and, 
more particularly, to a portable scatterometer which uses a double tapered 
fiber optic bundle with a concave spherical face, a CID camera, and a 
frame grabber to hemispherically collect scattered light reflected from a 
laser illuminated sample and a unique algorithm to rapidly reconstruct the 
scatter profile on a computer screen. 
2. Discussion of Background and Prior Art 
a. Scatterometers 
Scatter from optical components reduces signal power, limits resolution, 
produces noise and has appeared as an unexpected problem in more than one 
optical design. Stover, "Optical Scatter: Careful Measurement Of Optical 
Scatter Provides A Keen Diagnostic", Lasers & Optronics, August 1988. 
Optical designers and manufacturers require a precise and fast 
hemispherical light scatter measurement tool because many optical surfaces 
interact with light in unpredictable ways. 
A scatterometer is a widely used and extremely valuable tool for optical 
designers, measuring scattered light from test objects in order to 
determine the quality and characteristics of surfaces down to the angstrom 
level. 
In 1987 Breault Research Organization, Inc.("BRO") introduced a 
multi-wavelength, surface-scanning, fully automated scatterometer ("FASCAT 
360") system for use in research and development markets. The system could 
accommodate up to seven lasers and obtained full hemispherical 
measurements of the light reflected from or transmitted through a sample, 
but, only by rotating a photosensitive detector 360 degrees about the 
sample holder while allowing for three-axis (X,Y and Z) rotation and 
translation of the sample itself (all automated). The instrument is a 
Bi-Directional Reflectometer ("BDR") capable of measuring BiDirectional 
Reflectance Distribution Functions ("BRDF") and BiDirectional 
Transmittance Functions ("BTDF") both in-plane and out-of-plane. The 
system printed 2-D and 3-D plots of the data in real time and provided 
unparalleled versatility and dependability. 
Until recently, this now conventional technology has limited the range of 
applications of scatterometers because of its large size (a 4'.times.8' 
steel top table completely encased in a Center For Radiological Devices 
and Health Class 1 housing, called the "truck", with sample access through 
safety interlock doors and a 386SX computer/laser printer system), its 
high cost ($400,000-$600,000) and its long acquisition time (10 minutes to 
3 hours to measure, calculate and print a complete analysis of a sample). 
To overcome this size and cost disadvantage, in 1988 Toomay, Mathis & 
Associates, Inc. ("TMA") introduced a single laser, table mounted, 
Complete Angle Scatter Instrument ("CASI".TM.) Class 3 scatterometer, but 
at great sacrifice to versatility and dependability. While this instrument 
provided three axis rotation and translation (one of which was automated) 
of the sample and 360 degree "sweep" (a different technique than 360 
degree rotation about the sample used by FASCAT) of the detector, it could 
only provide 2-D plots of BSDF and still was relatively expensive 
($97,000-$166,000). 
Even more recently, still lower cost, hand-held, battery-powered, 
microprocessor-controlled scatterometers have been introduced using 
hand-held or bench-mounted measurement heads with up to 8 individual, 
non-movable detectors spaced about the sample which restricted versatility 
even more and which further sacrificed reliability and volume of data. 
While performing some useful function, these smaller units are restricted 
in that they assume the surface to be measured is homogeneous and that 
there is no interaction of light which prevents homogeneity. Thus, they 
are restricted in the number of measurements they can make. 
The above attempts to design smaller instruments led to instruments that 
had much less capability than full scale versions, and, as a result, 
important information was lost. Applicant has found a solution to this 
problem which minimizes the sacrifices made in capability to achieve the 
small size and yet can measure scattered light on un-nice, non-behaved, 
non-homogeneous surfaces as did full scale versions, and, not only, 
without sacrificing versatility and capability, but rather, increasing it. 
Moreover, none of the prior scatterometers are capable of spherically 
simultaneously measuring the angular gradation of the light scattered from 
a source because none of the light collection systems in any of the prior 
scatterometers were capable of spherically, simultaneously collecting the 
reflected light scattered from a spot on a surface of the sample. 
Applicant's unique collection system has solved this problem. 
b. Fiber Optic Bundles 
Fiber optic bundles have been known for many years. See, U.S. Pat. Nos. 
2,354,591 and 3,033,071 and Siegmund, "Fiber Optic Tapers In Electronic 
Imaging", Schott Fiber Optics. 
A modern fiber optic bundle comprises millions of individual fibers of 
glass which are first made by pouring pure raw glass of high index of 
refraction into a tube of lower index of refraction cladding glass, and 
which are then precisely aligned and fused together to form a solid fiber 
glass bundle ("boule"). Each fiber sees and carries one small portion of 
the image by the well known process of internally reflecting light rays 
emanating from the image. Through this process high resolution images may 
be efficiently transferred from one surface to another. 
During the manufacturing process it is also well known to twist, bend or 
taper the boule depending on the end function desired. The taper, for 
example, is made by heating the center and pulling the ends to produce an 
hour-glass shaped boule with the fibers essentially parallel at the larger 
diameter ends and smaller diameter center of the boule. During this 
process the outermost fibers are stretched more and are longer than the 
innermost fibers. The boule is then cut in half at the small diameter 
center to provide two identical tapered halves, each of which becomes a 
fiber optic magnifier/minifier. 
Faceplates serve as windows and transmit the image straight through without 
changing the size or orientation. Twisted bundles function as image 
inverters. Tapers serve as magnifiers or minifiers. The two end faces of 
the bundles are preferably parallel planes and may be flat or curved to a 
desired radius. It is well known to couple the small end of the taper to a 
self scanned array, such as, a charge coupled device ("CCD") to convert 
the light level in a group of fibers or "pixels" to a corresponding 
electrical signal which can be digitized and reconstructed graphically as 
an intensified image on a computer screen, for example in spectroscope, 
astronomical and medical applications. 
Fiber optic bundles have found wide use in such fields as x-ray image 
intensifiers and night vision goggles, for example. 
As disclosed in U.S. Pat. No. 3,033,071, it is known to further heat 
segments of the boule and pull the ends of the fibers to form a double 
tapered, onion shaped boule which is then cut at a point in the tapered 
portion to form concave surfaces in one or both ends for use as an image 
or field flattener. In this early device, however, the image is not of a 
point which is the focal point of the bundle, normal to all fibers and 
which is a light or a scattered light source that radiates light such that 
the radiated light strikes the bundle at 0.degree. incidence along the 
entire surface of the bundle. 
As disclosed in U.S. Pat. No. 4,991,971, it is known to have a bundle of 
equal length optical fibers each end of each of which is arranged in a 
circular array equidistant from the object being tested and the other ends 
of which are in a linear array whereby each fiber simultaneously receives 
a different angular component of the scattered light at the one end and 
transmits it to the other end such that the transmitted components exit 
the linear array end simultaneously and are detected and converted to 
electrical signals by a computer. The constraint of equal length fibers 
prevents use of a tapered fiber optic bundle in this system. Moreover, the 
device is limited to reading only that portion of globally scattered light 
that appears in the single plane in which the circularly spaced fibers are 
located. Thus, the collection and computation of a scatter profile for a 
spherical segment or a full hemisphere requires rotating the sample as in 
other prior schemes with resultant lengthy, slow construction of the 
scatter profile. 
These deficiencies are overcome in the present invention through the use of 
a tapered fiber optic bundle, each fiber elements of which receives light 
normal to its aperture from the light source which is at the focal point 
of the bundle, and which has never before been used in measuring light in 
a scatterometer. Moreover, a double tapered fiber optic bundle of 
applicant's unique design provides the remarkable advantages of 
instantaneous, spherically segmented or hemispherical collection of light 
from a scatter source and has not been heretofore known. 
c. Frame Grabber Algorithms 
A frame grabber, or image memory, has been used in the past as part of an 
image sensor processor. See, U.S. Pat. Nos. 5,040,116, 4,954,962 and 
4,843,565. Typically, the frame grabber is contained within a frame 
grabber pc-board, such as a type made by Coreco or Image Technologies, and 
is coupled to a data processing device, such as, an 80486 Intel 
microprocessor driven computer, which, accesses the image memory and, 
according to a predetermined algorithm, reconstructs the image on a 
cathode ray tube or other luminescent screen. A standard frame grabber is 
capable of resolving 256 shades of grey. Where the ambient light has to be 
eliminated or otherwise adversely influences the reconstruction of the 
subject image, it has also been known to use a technique of subtracting 
out the ambient light value from the data. See U.S. Pat. No. 4,991,971 
(4:56-64). 
Prior frame grabber algorithms have been slow and inefficient and have 
required the use of expensive CCD cameras. The advantage of applicant's 
unique algorithm is that its high level of efficiency enables the use of a 
low cost charge injection device ("CID") camera which eliminates the 
significant "blooming" problem experienced with CCD cameras when pixels 
become saturated and which prevents good scatter profile reconstruction. 
SUMMARY OF THE INVENTION 
Set forth below is a brief summary of the invention in order to achieve the 
forgoing and other benefits and advantages in accordance with the purposes 
of the present invention as embodied and broadly described herein. 
One aspect of the invention is an apparatus and process for collecting 
light which comprises a plurality of optical fibers the one ends of each 
of which are fixed in spaced relation to each other in a curved surface 
having substantially a radius of curvature normal to each fiber and the 
other end of each of which is arranged in an indexed array, and the 
longitudinal axis of each fiber at the one end substantially converges at 
the common point, whereby light radiating angularly from the point is 
received simultaneously at the one end by each fiber and is transmitted to 
the other end. 
Further features of this aspect of the invention include embodiments 
wherein the curved surface is a full hemisphere, a spherical segment or a 
linear segment. 
A second aspect of the invention is an apparatus and process for measuring 
light reflected from a surface including a housing, a laser diode light 
source for illuminating the surface and supported within the housing, a 
tapered fiber optic bundle supported within the housing and having a 
concave face on the tapered portion at one end, with the other end formed 
as a flat array adapted for transmitting light reflected from a point on 
the surface, and a CID camera supported within the housing and having a 
scannable area array for receiving the transmitted light beams and 
converting the beams to electrical signals. 
A further feature of this aspect of the invention is controlling the power 
of the laser by controlling its on time. 
A third aspect of the invention is an apparatus and process of acquiring 
with an x-y scannable array camera light reflected by a subject that may 
exceed the dynamic range of the camera including the steps of a) measuring 
the ambient light and storing the measurement in a reference frame, b) 
illuminating the subject with a laser diode light source for a 
predetermined time period, c) collecting the light beams reflected from 
the subject during the on period and transmitting the beams to the array, 
d) digitizing the collected data by x-y scanning the array and converting 
the light beam to electrical data, e) storing the digitized data in the 
next frame, and f) repeating steps (b) to (e) while increasing the on time 
of the laser diode by predetermined amounts (for example, one order of 
magnitude) during each repetition, whereby a reference frame and N data 
frames are collected and stored in N +1 sequential frames. 
A fourth aspect of the invention is an apparatus and process of 
reconstructing a single data profile from the data stored in the plurality 
of sequential x-y oriented memory frames of the computer frame grabber the 
first frame of which is a reference frame in which the data represents a 
factor common to all frames in the sequence, and the remaining frames of 
which are data frames including the steps of a) setting an x-y oriented 
profile array in memory and filling the array with zeros, b) computing a 
scale factor, c) subtracting the common factor data (i.e. ambient light) 
in the reference frame from the data in each of the data frames in the 
sequence, d) scaling the data in the first data frame by the scale factor 
and adding the scaled data to the profile array, e) scaling and adding the 
data stored in each x-y point in the next succeeding data frame to its 
corresponding x-y location in the profile array only if data has not been 
previously stored in that location, and if (1) the data already in that 
x-y point in the array is zero, and (2) the data in that x-y point in the 
current and any prior data frame is less than T, where T is a threshold 
level representing a level above which the data is invalid, and f) 
repeating step (e) separately in sequence for each subsequent data frame 
in the sequence. 
A fifth aspect of the invention is a scatterometer apparatus and process 
for measuring light reflected angularly from a point which includes a 
power controlled laser light source for illuminating the point, a fiber 
optic bundle focused on the point for collecting light beams reflected 
therefrom, a camera for converting the reflected light beams into 
electrical signals, the signal level of the collected light beams being 
raised above the dark current noise level of the camera by sequential 
increases in the on time of the laser by predetermined amounts to form 
sequential images stored in a frame grabber, and the frame grabber also 
sequentially digitizing the stored images and reconstructing therefrom a 
single scatter profile of the point while simultaneously scaling out the 
order of magnitude increases. 
A further feature of this aspect of the invention is that the scatter 
profile may be completely reconstructed within one second. 
A sixth aspect of the invention is the process of collecting light in a 
scatterometer to convert the light to electrical signals which includes 
the step of directing the light into a tapered fiber optic bundle. A 
further feature of this aspect of the invention is directing the optical 
fibers in the bundle such that they have a common field of view. 
A seventh aspect of the invention is the method of manufacturing which 
includes forming a tapered fused fiber optic bundle and cutting a concave 
face in the tapered portion normal to the bundle such that all of the 
fibers have a common field of view. 
The principal advantages of the scatterometer of the present invention are 
its compact size, ruggedness, speed, and hemispherical capability. 
This new, table top, portable instrument is the fastest, most powerful 
scatterometer on the market. It allows high resolution (0.125.degree.) 
which can be increased by using higher density CID arrays, partial or full 
measurement of hemispherical scatter data in less than a second with no 
moving parts from a very small 12".times.10".times.6" footprint. 
This new technology enables taking hemispherical, rather than curvilinear, 
data measurements with negligible change in cost, while providing greatly 
increased performance benefits. 
The fiber optic bundle scatterometer ("OMNISCATR".TM.; of the present 
invention has the ability to measure light scatter caused by everything 
from scratches, blemishes, bubbles, subsurface defects, and surface 
roughness ("RMS") to BRDF AND BTDF. It measures over 200,000 points over 
the hemisphere, two orders of magnitude more spatial data than any known 
scatterometer, thus, enabling detection of defects regardless of 
orientation, and determination of the orientation itself. 
Until now, scatterometers have been mainly used in the aerospace industry. 
The new generation scatterometer of the present invention, however, with 
its higher speed, smaller size and lower cost is available as a quality 
control device to many other industries that require assurance of high 
surface quality, such as, for computer screens, precision bearings, flat 
and power optics and specially coated or painted surfaces.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Table of Contents 
I. DEFINITIONS 
II. INTRODUCTION 
III. INSTRUMENT DESCRIPTION 
A. Overview 
B. Optical Layout 
1. Source Section 
a. Laser Diode 
b. Cornu Psuedo--DePolarizer 
c. Focusing Optics 
2. Collection System 
a. Tapered Fiber Fundle 
(1) Unitary Double Tapered Fiber Optic Bundle 
(2) Full Hemisphere Fiber Optic Bundle 
(3) Fused Unitary Double Tapered Fiber Optic Bundle 
(4) Partial Tapered Fiber Optic Bundle 
(5) Linear Array Fiber Optic Bundle 
b. Fiber Optic/Camera Assembly 
c. Optical Filter 
3. CID Camera 
4. Enclosure & Adjustments 
a. Housing and Mounting Plate 
b. Compactness of Measurement Head 
c. Protective Shutter 
d. Ruggedness of Measurement Head 
IV. ALIGNMENT 
A. Internal Alignment 
B. External Alignment 
V. DATA ACQUISITION HARDWARE AND SOFTWARE 
A. Computer Hardware Interface 
B. Software User Interface 
C. Data Acquisition and Data Reconstruction 
1. Ambient Light Compensation 
2. Scatter Measurement/Data Acquisition 
3. Data Reconstruction 
D. Data Display 
E. Data Analysis 
1. Normalization 
2. Calibration 
3. System Profile 
4. Volume of Information 
F. Data Storage 
G. Two Dimensional Plotting 
H. Three Dimensional Plotting 
I. Graphic Output 
VI. ADDITIONAL MEASUREMENT TECHNIQUES 
A. Rapid Surface Scan 
B. Processing and Surface Analysis 
APPENDIX A. TECHNICAL NOTES 
Detector Linearity 
Calibration 
RSS Calibration Method 
ABDM Calibration Method 
Stray Light Control 
APPENDIX B. CALCULATIONS 
______________________________________ 
DEFINITIONS 
TERM MEANING 
______________________________________ 
BRDF Bidirectional Reflective Distribution 
Function 
BTDF Bidirectional Transmission distribution 
Function 
BSDF Bidirectional Scatter Distribution Function 
TIS Total Integrated Scatter 
RSS Reference Sample Substitution method 
RMS Surface Roughness 
PSD Power Spectral Density 
ABDM Attenuated Input Beam Direct Measurement 
HgCdTe Mercury Cadmium Telluride 
NIST National Institute of Standards & 
Technology 
FASCAT Fully Automated Scatterometer 
Sample Detector 
Detector which intercepts scatter from the 
sample 
Reference Detector 
Detector which monitors laser fluctuations 
and attenuation 
SNR Signal-to-Noise Ratio 
LVND Linear Variable Neutral Density 
AR Anti-Reflective 
CCD Charge Coupled Device 
CID Charge Injection Device 
TTL Transistor To Transistor Logic 
Source Head The laser radiation source with beam 
shaping and collimation in an enclosed 
compact unit. 
Collection Head 
All collection optics and the detector on 
a small base plate. 
Measurement head 
An enclosed unit to be mounted on a 
production machine containing the source 
head, 
collection head, external focus 
control, 
shutter, and mounting 
accommodations. 
______________________________________ 
II. INTRODUCTION 
The detailed description of the invention has been divided into several 
major sections. The first section, Instrument Description, contains a 
brief overview and describes the functional components of the 
scatterometer. The next section describes the internal and external 
alignment of the measurement head. The data acquisition and software 
section describe the control of the scatterometer, the analysis of the 
data and, by its nature, the use of the instrument. 
III. INSTRUMENT DESCRIPTION 
A. Overview 
An overview of the measurement unit 10 and the data acquisition and 
reconstruction 200 system of the new scatterometer design are shown in 
FIGS. 1 and 16, respectively. The scatterometer of the present invention 
incorporates two uniquely distinctive concepts. The first is the 
incorporation of a tapered fiber optic bundle 40 (FIGS. 2 and 4) to 
collect the scattered light from the sample 65. The second is a data 
acquisition algorithm 170, 180 (FIGS. 17, 18) that allows rapid collection 
of the five orders of magnitude of light required without using the 
traditional slow detector/lock-in amplifier pair technique used in current 
scatterometers. Some advantages offered by this design include: 
Hemispherical data acquisition in less than 1 second 
Resolutions of up to 0.125.degree. or higher 
Measurement of in-plane and out-of-plane scatter 
Using a collection of many fibers 44 (FIGS. 2, 4) fused together with a 
concave face 41 cut into the tapered section 48 of the bundle, scatter 
data can be collected and directed to a camera 130 for very fast data 
collection. This aspect of the invention allows all of the data to be 
collected simultaneously, eliminating many undesired repetitious 
measurements based on time related variables. To get the low signal levels 
required for at least 10.sup.-5 BRDF, the laser power and/or camera 
integration time will be controlled to raise the signal level above the 
dark current noise in the camera. A method 170 has been developed to 
control the laser power so that the data can be collected approximately 
one order of magnitude at a time. With a conventional frame grabber 
digitizing multiple camera images, each order of magnitude of data can be 
collected separately and used 180 to reconstruct the scatter profile. At 
30 frames/second collection rates, the time to collect the data is well 
under a second. 
The detailed description of the instrument section of the scatterometer is 
further divided into several sections to facilitate a complete system 
description. These sections are optical layout and specification summary, 
source optics 20, collection optics 30, CID camera 130, and the system 
enclosure and adjustments 140. 
Working examples of the scatterometer are set forth in Table 1 which sets 
forth a component summary of the measurement head 10 characteristics, 
Table 2 which shows the instrument's overall parameters, and Table 3 which 
describes the instruments data acquisition and software characteristics. 
More technical notes such as stray light control and CID camera 
calculations are included in Appendices A and B, respectively. 
B. Optical Layout 
The optical layout is shown in FIGS. 1, 2, 3, 4 and 13. As best seen in 
FIG. 1, the optical layout includes the laser diode 21 and controller 22, 
fold mirror 24, psuedo-depolarizer 25, focusing optics 26, shutter 
assembly 143, beam dump 31, and fiber optic/camera and camera controller 
assembly 30 with camera imaging array 131 (FIG. 7). 
1. Source Section 
a. Laser Diode 
To obtain ample power in a small package a conventional laser diode 21 is 
used as the laser source. The laser diode 21 is powered and controlled 
within the housing 141 by the laser connector 22. The laser beam is 
directed to and reflected by a fold mirror 24 supported in the housing 141 
at an angle. 
The particular diode and power chosen is based on the minimum BRDF of 
10.sup.-5 sr.sup.-1, optimal reception characteristics of the collection 
optics, and the camera sensitivity. There is a trade off in selecting 
laser diode power and wavelength. Visible camera sensitivity is at a peak 
around 670 nm, while diode power peaks around 840 nm or higher. As shown 
in the power calculations in Appendix B, the laser power and wavelength 
are optimal at 70 mW and 840 nm respectively. The diode unit 21 is 
complete with beam circularization (beam forming) and a collimated output 
beam of 7.5 mm. The diode unit 21 includes a driver (not shown) for power 
stabilization, thermoelectric cooling, and TTL/Analog modulation. The 
cooling is required since power output, which must be held constant, is 
temperature dependent. TTL modulation 160 is used to pulse the laser diode 
21 or turn it on for a specified time, thus controlling the amount of 
power on the sample 65 and detector array 131. The lifetime of the laser 
diode 21 is about 50,000 hours. 
In the preferred embodiment the laser diode controller controls the amount 
of power on the camera to five orders of magnitude. Laser diodes are 
typically only adjustable in power level over one to one and one half 
order of magnitude. So the alternative is to control the time that the 
power is incident on the camera. The controller 23 is set to modulate the 
time the laser is on from a range of 10 ns to continuously on. This 
capability yields several orders of magnitude of BRDF range above what is 
required. The rise-fall time of the laser power output is also in the 10 
ns range. This range is sufficient to allow control of the incident laser 
energy over more than the required five orders of magnitude. The laser 
timing is controlled through the use of a TTL 160 level initiate signal 
provided by the computer system 150 and loading the controller 23 with the 
desired power level through an IEEE-488 interface 159. 
The output power per pulse is controllable to a maximum specified for the 
laser. The controller 23 has the ability, through the use of a photodiode 
feedback loop (not shown), to both stabilize and measure the power output. 
Power and timing calculations are provided in Appendix B. 
A working example of a laser diode unit 21 is a Model 06 PLL 807 made by 
Melles Griote and has the following characteristics: 
833 nm beam wavelength (required visible to 3.0 microns) 
70 mw power output of the laser unit 
7.5 mm collimated circular beam (internal beam shaping) beam profile is 
R/e.sup.2 =3.4 mm 
3 part collimating lens--s p h e r i c a 1 a b e r r a t i o n correction 
and collimation 
cylindrical lens--astigmatism correction 
anamorphic prisms--circular beam shaping 
Thermoelectrically cooled 
External power supply/controller Melles Griote MOdel 103 allows full 
control of laser power, temperaturel and on/off time 
Pulse modulation up to 1.0 MHz (used to control power output) 
b. Cornu Pseudo-deoolarizer 
Due to the use of a laser diode 21 in the measurement head 10, the source 
section 20 starts with linearly polarized light. Since no true 
depolarizers exist, the laser light is spatially randomly "depolarized" 
through the use of a cornu pseudo-depolarizer 25. Careful attention to the 
beam size and intensity pattern is required to achieve a high degree of 
depolarization. Depolarization efficiencies of better than 95% are 
attained which is sufficient to achieve the desired accuracy of BRDF. 
It is sometimes useful to use polarized light, but in general BRDF 
measurements should be taken with unpolarized light, randomly circular 
polarized light, or, if need be, circular polarized light. Relating 
mathematically the s or p polarization sources to the scatter and the 
surface characteristics, and then compensating for the polarization 
effects in software is a complex problem. To avoid this problem a 
pseudo-depolarized laser beam is used for the measurements. 
Pseudo-depolarizers transform one polarization state into a continuum of 
states which may mimic the behavior of unpolarized radiation under certain 
conditions. 
A cornu pseudo-depolarizer (CPD) 25 employs optical activity in crystalline 
quartz. It operates on collimated beams of light, transforming linear 
polarization states into a complicated and spatially changing continuum of 
linear polarization states. 
c. Focusing Optics 
As shown in FIG. 1, the output of the laser/depolarizer assembly is 
directed onto the sample and focused on the collection optics with 
focusing lenses which function as an inverted telephoto system, i.e. a 
negative and positive lens system. 
The lens system 26 is designed to minimize the collection aberrations from 
the negative lens 27 and positive lens 28, each of which has a 
nonreflective coating. 
The lenses are a good quality achromat and are adjustable to control the 
focus of the beam with respect to the fiber optic bundle 40. The 
adjustment of the focus lenses 27, 28 allows for proper compensation for 
powered optical surfaces. However, the range of the adjustment may be 
restricted by the small size of the measurement head. 
Focus adjustment is performed manually by a micrometer 29. Approximately 3 
mm of micrometer travel is required for infinite (flat surface) to 600 mm 
(concave surface) radius of curvature. The focusing optics 26 produce 
approximately a 4 mm Gaussian spot size on the sample. Diffracted energy 
from the test sample not striking the fiber bundle is kept well within 
2.degree.. 
2. Collection System 
As best shown in FIGS. 1, 3, 4 and 11 the collection optics 30 include a 
tapered fiber optic bundle and a camera. The tapered fiber bundle which 
collects the scattered light is directly connected to a solid state, CID 
camera. 
a. Tapered Fiber Bundle 
There are several embodiments for the fiber optic bundle of the present 
invention. 
(1) Unitary Double Tapered Fiber Optic Bundle 
As seen in FIG. 4, the preferred embodiment is a one piece double tapered 
fiber optic bundle 40 with a concave face 41 cut into the tapered portion 
48 at one end. Prior to the spherical cut forming the front face, the 
boule is shaped like an onion. The fiber bundle is a single piece of glass 
made of fused 25 um diameter fibers 44 tapered down to 3.8 um diameter by 
a well known process of heating, stretching and cutting. The bundle 40 
provides approximately 40% transmittance. The radius of curvature R on the 
front face 41 is required so that all fibers 44 have a common point of 
view and are, thus, viewing the same point 43 on the sample. This 
construction provides the enhanced functionality of partial or full 
hemispherical measurements depending on the type of fiber optic bundle 
used and the way in which the source light is applied. 
In the preferred form of the invention, a unitary double tapered fiber 
optic bundle 40, the bundle is stretched a first time to produce the well 
known hour-glass shape having a narrow portion in its center with wide 
portions at its ends as seen at 40a in FIG. 5. Then, each of the wide ends 
is stretched a second time to produce the new onion shape having a narrow 
section at each end and a wide section in the middle as seen at 40b in 
FIG. 5. Now, when the bundle is cut at the narrow sections 40c, 40d, what 
is left is a boule 40 which looks like an onion as shown in FIG. 4 in that 
the wide portion 47 of the taper is in the center and the narrow portions 
45, 46 of the taper are at each end. The spherical cut 41 is then placed 
at a point in the tapered portion 48 near the wide center 47 where the 
fibers 44 are converging to form the unitary double tapered shape 40 shown 
in FIG. 4. As further seen in dotted lines in FIG. 5, spherical cuts 41 
may be placed at other desired locations on the taper where the fibers are 
converging or diverging. The end 49 is cut in a flat planar surface 
forming an indexed array adapted to be mated to a camera for converting 
the transmitted light to electrical signals. 
As shown in FIGS. 4, 10, 11A and 13, the desired curvature is obtained 
automatically from the tapered fiber optic bundle by cutting the face 41 
at a selected radius R at the converging point 48 of the taper to form a 
spherical surface in the tapered bundle where the individual fibers are 
converging. The cut is made roughly perpendicular to each fiber 44 in the 
bundle 40 regardless of its position. The preferred radius on a 2" 
diameter boule gives a 40.degree. to 60.degree. three dimensional cone 
angle measurement range of the light scatter from the spot on the sample 
which is the focal point of the concave surface 41. All individual fibers 
44 within the bundle have a common light source 43 field of view. The 
longitudinal axis 43a (FIG. 6) of each fiber 44 is normal to the cut of 
the bundle 40. In addition each incident ray 44a (FIG. 7A) of light 
emanating from the focal point of the bundle and striking a fiber is 
normal to the cut of the bundle at the point it enters the fiber. While 
each fiber's surface normal may actually hit the sample surface at various 
locations within the illuminated spot, nevertheless, the effect of this 
slight deviation is averaged out, since each fiber 44 collects the scatter 
of the entire illuminated area only at its viewing angle. 
A working example for FIG. 11A is as follows: 
Outside diameter of bundle=2" 
Inside diameter at cut point 48=1.75" 
R=2.12" 
d=47.degree. 
A narrow (830 nm) waveband anti-reflective filter coating can be put on the 
back face 49 or front face 41 of the fiber bundle 40 which is very 
durable. This reduces unwanted light of other wavelengths. 
Some advantages of the fiber bundle 40 are that a high density of camera 
detector elements may be mated to the small end of the taper (minifier 
function) and three dimensional out-of-plane measurements may be taken at 
high speed. 
Stray light reflections from the faces of the fiber bundle 40 itself are 
controlled by the use of a good quality AR coatings. The specular 
reflection off of the polished AR coated face of the fiber bundle would 
create a scatter signal that would exceed the expected measured signal by 
orders of magnitude at the far angles. Even a black spot painted on the 
fiber bundle at the specular position would create appreciable stray 
light. For a specular measurement the light trap needs to be a 
sophisticated combination of a specular black surface and a diffuse 
absorber. For this reason the specular beam is excluded and a beam dump 31 
for the specular beam has been included. 
Currently the collection optics can be swung into the specular position and 
out again for separate specular and low scatter measurements. Future 
versions of the fiber shell may allow simultaneous measurement of near 
specular and far angles (low scatter) if a good solution is found for 
suppressing the specular beam. 
The collection system 30 is susceptible to ambient light. Ambient light is 
controlled through the use of a narrow band filter. It is recommended that 
fluorescent lights be used in the measurement room, since they have lower 
emission in the 0.670 .mu.m to 0.900 .mu.m region, which is an operating 
wavelength region of the scatterometer. Any remaining ambient light up to 
a certain threshold can be subtracted out of the data after it is acquired 
as discussed below in the data acquisition section. 
Analysis results have shown a worst case stray light condition of one order 
of magnitude above the desired scatter level when the specular beam is 
incident on the fiber bundle. When the specular reflection is directed 
into the beam dump 31, no stray light problems have been encountered. 
Transmission calculations are in Appendix B. 
(2) Full Hemisphere Fiber Optic Bundle 
Shown in FIG. 6 is yet another embodiment of the fiber optic bundle of the 
present invention. In this form of the invention a hemispherical dome 32 
which is one half of a full sphere is made of any rigid material and is 
pre-drilled with a plurality of radial holes 33 the common field of view 
of which is the center of the sphere 43. A fiber optic 44 is inserted and 
secured within each radial hole 33. The fibers are then bundled together 
and the loose ends are arranged into an array 95 such as is shown in FIG. 
12D. Any one of fibers 44 may function as a conduit for the laser diode 21 
source light to illuminate the point 43 on sample 65. Alternatively, a 
fiber 44 may be removed and replaced by a conduit for the light from laser 
diode 21. If stray light is not a problem, the specular beam is used for 
measurements. If stray light is a problem, a hole may be drilled in dome 
32 allowing the specular beam to pass through the dome thereby essentially 
eliminating the stray light. This form of the invention is used by placing 
the hemi-dome 32 over the point on the surface to be measured and 
illuminating the surface with the laser diode source 21. Full simultaneous 
hemispherical measurements of light scattered by the point sample are 
enabled because all fibers in a full hemisphere are have a common field of 
view at that point. 
(3) Modified Fused Unitary Double Tapered Fiber Optic Bundle 
A further embodiment of the double tapered fiber optic bundle 60 is shown 
in FIG. 7A. In this version of the bundle, the laser source beams are 
directed onto the sample through an enlarged fiber 61 or through a 
longitudinal hole in the fiber which begins at the small end 62 or through 
a side wall 63 of the bundle and terminates on the spherical face 64 cut 
into the converging portion of the large end of the bundle 60. In this 
form of the invention, the laser source beam is transmitted to the sample 
65 through the fiber bundle 60 and the specular beam 66 is reflected to a 
black spot 67 painted on the spherical face. Partial hemispherical 
measurements may be simultaneously made of the scattered light from the 
light source 43 which are then carried and reflected through the fibers 44 
to the small end 62 of the taper 60 which terminates in an indexed array 
as shown in FIG. 7A and is adapted to be mated to a CID array camera 130. 
The light reaching the small end of the taper is converted to electrical 
signals through the CID array 131 in the camera 130 and then is digitized 
in the frame grabber 152 and displayed by the computer 150. 
Still another embodiment for the fiber optic bundle 70 of the present 
invention is shown in FIG. 8. In this embodiment the fiber optic bundle 70 
comprises two-halves 71, 73. One-half 71 is a standard fiber optic bundle 
with flat faces at both ends. The other half is also a standard fiber 
optic bundle which has a hemispherical cut 74 in its front face in the 
converging portion of the taper. The other face (wide end) 75 is flat. The 
two flat faces 75 at the wide ends of the tapers are rotated 45.degree. 
relative to each other and are then bonded together using conventional 
bonding material to form a unitary double fiber optic taper. The purpose 
of the rotation is to eliminate any moire effect. 
(4) Partial Tapered Fiber Optic Bundle 
Shown in FIG. 9 is an alternate embodiment of the collection system 30. In 
this approach only a portion 73 (one half as shown in FIGS. 10 and 11A) of 
the tapered fiber optic bundle is used, and an imaging system is used to 
focus the light onto the camera detector array 131. 
In this embodiment the AR coated focusing lens 77 has a diameter larger 
than the outside diameter of fiber bundle 73 to collect all of the light 
emitted from the opposite end of the fiber bundle and focus it on the 
camera detector array 131 in camera 130. Since there are many more fibers 
44 than there are pixels it is possible to move the camera in and out to 
focus on portions of the image, thus achieving even higher resolution 
measurements, possibly up to 0.05.degree.. 
(5) Linear Array Fiber Optic Bundle 
Shown in FIGS. 12A, B, C, D and E are several views of yet another 
embodiment of the fiber optic bundle of the present invention. In these 
FIGURES is shown a linear array type fiber optic bundle 90. As shown in 
FIG. 12A, a working example includes 133 400 micron diameter fibers 44 
orientated in the aperture blocks 92, 93 which have been pre-formed for 
the fibers 44 to be inserted at 0.25.degree. spacings such, that the 133 
total fibers 44 cover 33.degree. field of view directed at a single point 
43 (not shown) on the sample. The assembly is then bonded by potting 94 
compound which adheres the fibers to the aperture blocks as shown in FIGS. 
12B and 12C. 
The other end of the fibers 44 are brought together in an array 95 of 
smaller but thicker dimension as shown in FIG. 12D which can be readily 
mated to the CID camera. A working example of the small end array 95 is 
approximately 6 millimeters wide and 4 millimeters high and comprises 8 
rows of 15 fibers, numbers 1 96 to 120 97, with the bottom row containing 
13 fibers, numbers 121 98 through 133 99. As a further working example 
shown in the plan view FIG. 12E, the central axis 100 of the linear array 
90 of the fibers at its wide end is orientated f=28 to 29 degrees from the 
central axis of the small end of the fibers. The front face 101 of the 
linear array 90 is flat and perpendicular to the central axis of its plane 
of curvature. 
b. Fiber Optic/Camera Assembly 
Finally, as shown in FIG. 13, the double tapered fiber optic bundle 40 of 
the present invention is seen in its encapsulated form in its housing 
attached to the camera head unit 130 which forms the fiber optic camera 
assembly 110. The mated CID camera and fiber optic bundle assembly 110 
allow 90% transmittance. A fiber optic face plate 118 replaces the 
standard glass window used in most arrays. The small tapered end of the 
bundle 40 is spring loaded 115 against the fiber optic face plate 118 in 
the camera head unit. The camera head unit 130 is fixed to the housing 112 
which receives the fiber optic taper 40 and is mounted thereto by a 
mounting plate 114. The fiber optic taper is located within the 
encapsulation fixture 111 within the housing 112 with potting 13 filling 
the vacant areas and providing a seat for the fiber optic taper. A spring 
115 holds the encapsulation fixture firmly against the fiber optic face 
plate by having its other end abutting the spring retainer 116. 
c. Optical Filter 
An optical filter 120 (FIG. 9) is selected to match the source wavelength 
which can be coated on the front or back surface of a taper or be a 
separate filter in front of the camera or collection optics. This 
eliminates a large portion of any stray ambient light. 
3. CID Camera 
The measurement requirements place unusually high demands on the detection 
electronics. A single element detector is not feasible due to the required 
resolution and measurement time. The mechanical requirements for scanning 
the device are also prohibitive. Linear array detectors are readily 
available, but when compared to area array detectors they are just as 
expensive and require less convenient support electronics. An area array 
camera is selected for data acquisition as the best cost performance 
alternative since it is sold in a more competitive market, is available in 
a wide variety of formats, and provides larger data capacity. 
An area array CCD camera based on raw BRDF data requirements is available 
to measure and convert light beams to electrical signals. Such a camera 
has the sensitivity and dynamic range required to measure a BRDF range of 
10.sup.-1 down to 10.sup.-5 Sr.sup.-1, but its cost is prohibitive ($30 
k). In the preferred embodiment, however, the present invention uses a CID 
anti-blooming camera 130 (which is an order of magnitude lower in cost) to 
collect the data in a unique process involving collecting the data range 
segments (see software description for details). The scatter range segment 
and number of segments needed to reconstruct the data is determined by the 
particular CID camera specifications. The superior anti-blooming 
characteristics of the CID camera allows pixels to be saturated without 
affecting neighboring pixels. This tolerance is required in the unique 
reconstruction algorithm used in the present invention in which, during 
the data acquisition process, portions of the image are allowed to 
saturate in sequentially collected range segments. 
The appropriate laser power and camera features of variable time 
integration and anti-blooming allow the desired data to be collected in 
less than a second. As stated above, a standard fiber optic acts as a face 
plate interface between the small end of the fiber optic bundle and the 
front end of the detector array. 
A working example of a CID camera 130 is Model CIDTEC 2250 made by CIDTEC. 
This camera provides 512 pixels by 512 pixels resolution which provides 
0.125 degrees angular resolution. Each pixel is a 15 um.times.15 um 
square. The camera is impervious to magnetic fields, shock, and vibration 
and does not degrade in sensitivity over time. Included in the capability 
of the camera is asynchronous full-frame capture and multiple frame 
integration. The camera power supply allows full control of camera speed 
and integration time. 
Camera sensitivity and power requirements are in Appendix B. 
4. Enclosure & Adjustments 
a. Housing And Mounting Plate 
As shown in FIGS. 14 and 15, a strong, anodized, aluminum housing 141 is 
used to house all of the optics and electronics (except the computer). The 
measurement head housing 141 sits on top of a base mounting plate 142 or 
platform which is adapted for mounting onto a production line machine (not 
shown) and which contains the adjustment mechanisms required to perform 
the alignment procedure. These adjustments will include an axial position 
control 144 and an azimuth rotation control 145. 
A working example is as follows: 
The actual size of the measurement unit housing 141 is 11.50" 
long.times.9.125" wide.times.5.25" high, including the base mounting plate 
142 and adjustment mechanisms 144, 145. With the adjustment stages in 
operating position, the dimensions are 16.00" long.times.11.00" 
wide.times.9.50 high. With the adjustment stages fully extended, the 
dimensions are 22.00" long.times.11.00" wide.times.9.50" high. 
b. Compactness of Measurement Head 
The measurement head 10 (FIG. 1) itself is 9.5" long.times.8.5" 
wide.times.4.0" thick and contains the laser source 21, source optics 20, 
collection optics 30, and detector. These subsystems are integrated into a 
robust aluminum housing which is designed to withstand vibrations and G 
loads of harsh working environments. The measurement head 10 is a fully 
integrated unit that may also be hermetically sealed as described below. 
The adjustments required for sample to measurement head alignment require 
about 2-3 inches additional height. 
c. Protective Shutter 
The unit comes with a protective shutter 143 (FIG. 1), which is manually 
operated in the base instrument. This function may be automated as an 
optional feature. The shutter protects the internal optics from outside 
contaminants, especially when lubricants or other contaminants are 
present. If the shutter 143 is open and the sample under test is in harsh 
environments, applicant recommends precautions be made so that 
contaminants do not get onto the optics. 
Alternatively a permanent window may be substituted for the shutter 143 to 
protect the collection optics which makes the measurement head a 
hermetically sealed unit. If a window is used, the measurement head 10 
could actually be hermetically sealed and would become robust in the 
presence of cutting oils and other contaminates. However, the effects of 
such contaminants will strongly affect the measurements, particularly when 
sprayed onto the window. 
d. Ruggedness of The Measurement Head 
The measurement head 10 contains very few individual parts; these include 
the beam dump 31, diode head 21, source focus lens 26, fused fiber bundle 
40, collection lens 77, optical filter 120, and camera 130. These are 
mounted using vibration resistent mounts. Based on 0.5" aluminum, the 
measurement head will be built to withstand the vibrations of 0.2 g and 5 
khz. The only moving part inside of the measurement head is a focus 
control 29. Thus the lifetime of the instrument is exceedingly long, and 
it will require little or no maintenance other than occasional cleaning 
(environment dependent). The ruggedness is also highly desirable for many 
of the market environments anticipated for the instrument's use (factory 
floor, aircraft, etc.). 
IV. ALIGNMENT 
A. Internal Alignment 
The internal alignment of the scatterometer head is fairly simple. Internal 
alignment is required during assembly only, and is not required during 
use. The internal alignment of the measurement head is inherently defined 
by the mechanics of the system. Each of the subassemblies has sufficient 
individual alignment control. 
The source optics, including the laser diode 21 with heat sink, the fold 
mirror 24 mount, the pseudo-depolarizer 25 mount and the focus lens 26 
mount may be provided with their own internal tip and tilt adjustments 
relative to the collection optics. In this case, the internal alignment 
procedure is very simple. The collection optics 30 will be oriented within 
the measurement head 10. A kinematically mounted reference sample 146 
(FIG. 19) will be used to define the desired test sample location. The 
source subassembly is first internally aligned to the reference sample 146 
and beam dump 31. 
B. External Alignment 
The external alignment of the scatterometer to the part under test, or 
conversely, the alignment of the part under test to the scatterometer, is 
very simple. The alignment procedure is a two step process performed while 
watching a video screen. The user first controls the axial position 
(z-translation) of the part relative to the instrument and then controls 
the azimuth (which sets the incidence angle of the measurement head normal 
to the surface being measured) of the instrument relative to the point 
under test. Only a small azimuth adjustment (&lt;10.degree.) is required. 
(FIG. 3 shows a=b=7.degree.) Future expansion and automation of this 
adjustment may be developed to perform surface scans. 
Axial Adjustment: The axial position of the measurement head 10 is adjusted 
to maintain a specified distance from the test sample. Once this is done, 
use the angular control 147 (FIG. 1) to bring the specular beam from the 
laser into the fiber optic bundle for the following two adjustments. 
Rotational Adjustment: After the axial alignment is performed, the azimuth 
lock 148 is unlocked (FIG. 1) and the measurement head 10 is rotated until 
the specular reflection of the laser source is aligned to and blocked by 
the beam dump 31. Again, this alignment is made while watching the video 
monitor 161 until the specular reflection appears at a specific location 
on the monitor and takes only a few seconds. 
Focus Adjustment: After rotational adjustment the focus control 29 (FIG. 1) 
is used to compensate for powered optics by getting the spot on the 
monitor as small as possible. The collection optics are then again moved 
to the measurement position by using angular control 147. 
V. DATA ACQUISITION HARDWARE AND SOFTWARE 
The software section of the detailed description is divided into several 
sections which include computer hardware interface, software user 
interface, data acquisition and reconstruction, data display, data 
analysis, data storage, two dimensional plotting, three dimensional 
plotting, graphical output and measurement speed. 
A. Computer Hardware Interface 
A computer system 150 is used as the measurement controller. The 
computer-scatterometer data acquisition system is shown schematically in 
FIG. 16. The computer interface allows for complete automation of the data 
acquisition process, such as laser power control, camera integration time, 
ambient light monitoring, and data capture. The automation provides 
consistency between measurements and reduces chances for human error. 
Also, the hardware eases labor intensive tasks such as focusing and 
diagnostic checking by providing real time video and status information. 
In addition, the features of the fiber bundle are fully utilized to 
provide a set of powerful capabilities such as realtime video monitoring, 
higher density measurements 0.125.degree. or greater and measuring a large 
portion of the scatter hemisphere. These state-of-art capabilities use low 
cost readily available hardware. The design of the acquisition system has 
no moving parts, yet acquires large amounts of data in less than a second. 
The recommended computer is a 486 compatible PC 148 which has the 
capability to interface to all of the scatterometer components and 
requires one IEEE-488 card 149 for diode monitoring and a TTL card 160 for 
high speed diode modulation. 
A working example of a PC computer system is as follows: 
Gateway 486DX2, 50 MHz/8MB for 
high speed display of data, and 
high speed pulse modulation of laser 
640KB of RAM memory is sufficient for 50 sets of in-plane measurements 
without storage to disk. 
Hemispherical data requires storage to disk for each data set. 
1.2 MB Floppy 
1.44 MB 3.5" floppy for data transfer 
MS-DOS 5.0 
Windows 3.1 
200 MB Hard Drive for mass data storage 
VGA Graphics 800.times.600.times.256 for graphics display of data 
Multisync Monitor 
Frame Grabber to capture the video data from the camera 
IEEE-488 Card & cable for data transfer to/from the laser power supply 
TTL Digital Interface Card & cable for high speed pulse modulation of laser 
Hybrid Data Acquisition/Control & Analysis Software 
Abaton Laser Printer (optional) 
Print-A-Plot HPGL Laser converter (optional) 
The software and hardware developed will be compatible with 286/386 
systems. However, the data display rate on these systems may take more 
than one second. 
B. Software User Interface 
The user interface will be a Graphical User Interface (GUI) with the look 
and feel of Windows 3.0, but will not require Windows 3.0. High speed 
graphics will be used to display information on the screen in near real 
time (less than 2 seconds). Data entry will be by keyboard 153 and mouse 
154. Graphical Icons, axis scales, slide bars, push buttons, data entry 
windows with realtime error checking, and drop down menus will be used to 
provide intuitive and flexible interaction with the acquisition and data 
visualization process. 
C. Data Acquisition and Data Reconstruction 
As shown in FIG. 16 the acquisition system is composed of laser diode 21 
and controller 155, a fiber optic/CID camera assembly 110 and controller 
132, a 486 personal computer 148 and software. As far as the user is 
concerned, the measurement process consists of entering measurement 
parameters and then initiating the measurement sequence. The automatic 
measurement process consists of an ambient light test, scatter 
measurement, and data reconstruction. Once the data has been reconstructed 
it is displayed in the format previously specified by the user. The 
measurement and display process can be repeated once, a specified number 
of times, or continuously at a user selected interval (three seconds and 
up). After the data is collected it can be analyzed and stored. Detailed 
descriptions of these functions are in the following subsections. 
1. Ambient Light Compensation 
After all parameters have been selected by the user and before each and 
every scatter measurement the ambient light level will be measured. This 
measurement (laser off) will be continuous with a warning light on the 
computer screen until either the user aborts or the light level is dropped 
below an acceptable level. If the ambient measurement is terminated due to 
the light level being dropped to an acceptable level the scatter 
measurement will commence. If the ambient level is initially in the 
acceptable range, then the ambient and scatter measurements will both be 
accomplished in less than one second. Any mean ambient light within the 
acceptable threshold will automatically be subtracted out of the data. 
2. Scatter Measurement/Data Acquisition 
A camera and frame grabber which digitizes the video image will be used to 
collect the data. To collect data beyond the range of the camera, 
applicant has developed a unique algorithm to place multiple orders of 
magnitude of data one per frame (camera limit) in separate memory pages or 
frames on the frame grabber. The first page (frame) to be acquired has the 
power (.phi..sub.1) and time (t.sub.1) adjusted such that no pixels of the 
image are saturated. Each of the successive pages (frames) have the laser 
power incremented by an order of magnitude. Eventually some pixels close 
to the specular beam will saturate (without damage). This saturation is 
detectable and ignored since the relevant measurements for the saturated 
pixels were saved on a previous page (frame) when they were not saturated. 
The camera is anti-blooming, so saturating pixels do not offset 
neighboring pixels. 
The detailed data acquisition process 170 steps shown in FIG. 17 are 
described as follows (variables are in italics): 
______________________________________ 
Step Description 
______________________________________ 
1. Set variables a, b and p to start levels 171, where a 
and b represent predetermined exponential powers (for 
example, powers of ten) of on time of the laser as a 
function of a, b and p as follows: source-on-time = 
t = t.sub.i .times. 10.sup.(p-1)(a-b) where t.sub.i is the initial 
time for page 
1 such that no pixels are saturated given the laser 
power and p represents page number of the frame- 
grabber. As a working example, set a = -2; b = -1; 
page = 0; 
2. With the laser off measure ambient light and store in 
the first page of the frame grabber 172. (Page = 0). 
3. increment 173 page = page + 1. 
4. Turn the laser on for sufficient time for the camera 
to collect data between 10.sup.a to 10.sup.b in magnitude 174, 
and, then 
5. Turn laser off 174. 
6. Digitize image and store the data in the current page 
of the frame grabber 175. 
7. Decrement variables a, b by 1 at 176, a = a - 1; b = 
b - 1; (I.e., increase the diode on time by a 
predetermined amount (for example, one order of 
magnitude) 
8. Repeat steps 3 through 7 N times 177. 
______________________________________ 
3. Data Reconstruction 
The frame grabber now has N+1 images (data magnitude range dependent) 
stored on it, one per frame (page). The first image contains the ambient 
light measurement, which is a factor common to all of the frames in the 
sequence, and which is subtracted out of all pages leaving the reduced 
values in the remaining pages which comprise the scatter measurement data 
otherwise unchanged. These remaining frames of data contain information 
such as saturation, data not yet detectable, and greylevel. From this 
information a single scatter image can be constructed that represents the 
scatter profile. During the reconstruction process this scatter profile is 
normalized (scaled) relative to the varied incident power of the laser 
diode and relative to a systematic reference calibration measurement taken 
previously. Each frame can hold various pixel resolutions of information 
depending on the angular resolution chosen. Only the measurement range 
selected by the user will be processed. In this way the user can choose 
between less data at higher display speeds or high volume information at 
lower display rates. 
Once the N+1 data frames, i.e. one ambient reference frame and N data 
frames, have been collected, the detailed steps of the data reconstruction 
process 180 to reconstruct the scatter profile are shown in FIG. 18: 
1. A profile array (RDATA) is set up in computer memory and filled with 
zeros 181. 
2. A dual function scale factor is computed. The scale factor is a function 
of the source on time of the laser diode and a function of the systematic 
reference calibration measurement. As seen below, preferably both scale 
factors are applied simultaneously during the reconstruction process 
thereby shortening the reconstruction time period. However, optionally, 
the systematic calibration scale factor may be applied after the 
reconstruction is first completed. 
3. The ambient light (reference frame, page 0) is subtracted from the 
scatter data 182. This is assumed to be the same for all frames of scatter 
data since the camera integrated for the same time on each frame. It was 
the laser on-off time that changed. 
4. Set variable p=1 at 183 and scale the data in the first data frame (page 
1) and add the scaled data to the profile array RDATA at 184. 
5. Scale and add the scatter data in the next succeeding data frame to the 
profile array according to the formula described below and shown 
graphically in FIG. 18 at 185-190. This is done on a pixel by pixel basis 
185, 186, 187, 188, 189 and 190, and for each data frame in sequence 191, 
storing the pixel data only if the pixel data has not been stored 
previously 188. That is, the pixel data from the data frame is stored in 
its spot in the profile array only if the data already in that spot in the 
profile array is zero and the data in that spot in the current and any 
prior data frame is less than the saturation level of the pixel. The 
reason for this double test as to saturation level is to negate the effect 
of oversaturation of a pixel which causes the pixel to reverse color and 
look like it is not saturated. Looking to the prior frame to confirm the 
pixel was saturated prevents the frame grabber from being fooled by the 
pixel color reversal due to oversaturation. Thus, if pixels in the current 
frame or previous frame are saturated or any pixels in any previous frame 
(not the current page) are non-zero then the pixel has been stored 
previously 188 and data in the current frame is ignored. Thus, data gets 
added to an x-y location in the profile array only once in the entire 
reconstruction process. If pixels in the profile array are still zero, it 
means the scatter was too low to measure. 
6. Step 5 is repeated for all frames until all frames 191, 185 have been 
processed. 
Now that the scatter profile has been constructed it can be displayed 
and/or stored to disk. 
D. Data Display 
The latest GUI technology, and high speed graphics are used to visualize 
the data as shown in the graphical output screen 193 in FIG. 20. Using 
graphical slide bars the user is able to select multiple segments (near, 
mid, far angles) of the BRDF curve. The values over the range of a segment 
can then be reported by peak value and/or average value. These values can 
be displayed in one window while the data is plotted in multiple formats 
in another window. Other values that can be displayed are: diode output 
power, average ambient light level, RMS, PSD, TIS, and auto-correlation. 
Also the camera system provides real time display of large sections of the 
scatter hemisphere. 
E. Data Analysis 
1. Normalization, 
The data is normalized to the laser output power. This factor is reflected 
in the scale factor in block 181 of FIG. 18. The diode controller 155 has 
a photodiode feedback circuit (not shown) that stabilizes the output power 
of the laser. This circuit can be sampled to read out the power output 
from the laser. The diode responsivity (milliamps/milliwatt) is a fixed 
known characteristic of the laser laser calibrated by the manufacturer. 
The photo diode current is read automatically through the IEEE-488 bus 
during the scatter measurement and is used to calculate the diode power. 
2. Calibration 
FIG. 19 shows a reference sample adapter plate 146 which easily snaps onto 
the front of the measurement head. This is used to calibrate scatter data 
using a NIST traceable Reference Standard. This factor is reflected in the 
scaling factor in block 181 of FIG. 18. If the data has not been 
normalized with respect to output power, it is recommended to use a 
reference measurement taken the same day as the measurement. This process 
maximizes accuracy of calibrated measurements, and minimizes error due to 
any possible power changes from one measurement to another. To eliminate 
this possible error the software may be put into an automatic mode where 
the sample and reference data are always normalized with respect to the 
output power which is measured and stored with each data set. 
3. System Profile 
The system profile is used to determine how close to specular measurements 
are valid. It also determines the lowest BRDF the instrument can detect. 
When the two measurements, system profile versus scatter measurement, are 
overlaid on a graph, any scatter data that overlaps the system profile 
should be considered to be invalid. 
A kinematically mounted reference sample consisting of a highly polished 
mirror is provided to allow measurement of the system profile. This 
configuration is preferred because of the potential interaction of stray 
light from the face of the fiber shell and the sample surface. Since 
reference samples are available that are significantly better in rms 
surface finish than most samples to be measured, this method of 
determining the system profile is sufficient. 
4. Volume of Information 
If all the data collected by a 512.times.512 camera were used, the image 
and data file would be over 256KB when stored in binary. The user can 
specify what region and resolution of the scatter hemisphere that is to be 
collected, analyzed, and stored. This region can range from a single plane 
or a two dimensional segment of the scatter hemisphere. A 512.times.512 
hemispherical measurement can be displayed under 10 seconds. 
F. Data Storage 
A resident or remote personal computer is preferred as the optimum 
controller. It allows data acquisition, analysis, and storage to be 
performed quickly and easily. In addition, software alterations to match 
increasing demands can be performed with less perturbations than using a 
micro-controller. Also large volumes of data can be stored on a hard disk 
or floppy depending their size. 
Data can be accumulated in memory and later stored to disk. The amount of 
data stored in memory will depend on the data acquisition size At a 
minimum for in-plane measurements with the minimum BRDF (2.degree. in 
plane, 0.125.degree. resolution), the number of data sets stored in memory 
will be fifty (50). 
G. Two Dimensional Plotting 
Conventional BRDF plotting software, such as SOFTSCAT-2D.TM. software 
package by BRO is available to plot all two-dimensional data taken by a 
scatterometer complying with the ASTM standard E1392-90. Also, the 
software can easily be configured to read other data formats. 
The data will be computed and plotted in a variety of ways, including: 
.beta.-.beta..sub.o vs BSDF (Harvey Shack) 
.theta.-.theta..sub.o or .theta. vs BSDF 
.theta. vs BSDF 
polar plots 
RMS surface roughness, RMS slope, and power spectral density 
linear, log-log, or semi-log format 
Some of SOFTSCAT-2D's features include: 
ability to "zoom in" on the data 
cross-hair positioning 
user-definable legends 
multiplying and/or adding constants to the data (for comparing different 
curve shapes independent of magnitude) 
H. Three Dimensional Plotting 
Conventional software, such as, SOFTSCAT-3D, by BRO plots all 
three-dimensional data, such as, hemispherical data and surface scans 
taken by a scatterometer complying with the ASTM standard E1392-90. Also, 
the software can easily be configured to read other data formats 
The data can be plotted in a variety of ways, including: 
color scaled perspective plots 
color maps 
contours 
cross-sectional plots of the three dimensional data 
Other features include: 
cubic splines 
plotting the log of the data 
cross-hair positioning 
automatic labeling 
I. Graphic Output 
Both SOFTSCAT-2D and SOFTSCAT-3D support the following: 
over 200 graphics cards 
Epson printer output 
HPGL plotter output 
HPGL file output for input to word processors, such as, Word Perfect 
publication quality output 
VI. ADDITIONAL MEASUREMENT TECHNIQUES 
A. Rapid Surface Scan 
Taking data at a few points on a test surface is not always adequate. There 
are times when an entire surface must be characterized. Very fast surface 
scans of a surface can be performed. Since the data is collected at near 
video rates, entire surfaces can be scanned very quickly. Instead of 
signal integration being the limiting factor, now the limitation is the 
speed of the x-y scanner. 
Note that there are now four dimensions to the BRDF data which include x, 
y, .alpha., .beta., where .alpha. and .beta. are the hemispherical angles 
in cosine space. Now the data needs a 4th display parameter which can be 
supplied by displaying the data in rapid succession, much like a movie. 
B. Processing and Surface Analysis 
A two dimensional grey scale image is produced with the hemispherical 
approach. From the patterns in the image a wide range of image processing 
and data analysis software can be developed to analyze the nature of the 
surface and its defects. 
A working example of a scatterometer of the present invention has the 
following features: 
TABLE 1 
______________________________________ 
MEASUREMENT HEAD COMPONENT 
CHARACTERISTICS: 
Item Characteristics 
______________________________________ 
Dimensions 9.5 .times. 8.5 .times. 4" without adjustments 
Shutter Removable cover 
Radiation Source 
Laser Diode, 70 mW, 830-840 nm 
collimated, circulated, wavefront 
corrected 
computer controlled with temperature & 
output stabilization 
Polarization Randomly polarized (Cornu pseudo- 
depolarizer) 
Source Focus Lens 
Adjustable for powered optics 
& Control 
Fiber Bundle Solid glass fiber bundle .apprxeq. 5000 fibers 
across dia. .apprxeq. 0.015.degree. resolution/fiber 
40-60.degree. angular measurement range 
Optical Filter 
Bandpass filter to reduce ambient light 
Beam Dump Traps specular reflection from 
measurement surface 
Camera Head CID detector array 
512 .times. 512 resolution 
15 um .times. 15 um pixel size 
11 mm diagonal array 
anti-blooming 
______________________________________ 
TABLE 2 
______________________________________ 
SCATTEROMETER AMETERS 
Parameter Range/Quantity 
______________________________________ 
BRDF Range 10.sup.-1 to 10.sup.-5 
Angular Resolution.sup.1 
0.25.degree. (256 .times. 256) 
0.125.degree. (512 .times. 512) 
Angular Range 2.degree. to .apprxeq.60.degree. 
Hemispherical Forward Scatter 40.degree.-60.degree. 
Measurement hemispherical circular 
section 
Wavelength 830.degree.-840.degree. 
Spot Size .apprxeq.4 mm 
Incident Angle 7.degree. 
Measurement Time &lt;1 sec 
Display Time &lt;2 sec 
______________________________________ 
.sup.1 Angular resolution is selectable under computer control and is 
defined by the spatial resolution digitized from the camera. 
TABLE 3 
______________________________________ 
DATA ACQUISITION AND SOFTWARE 
Parameter Description 
______________________________________ 
User Interface 
Graphical User Interface (GUI), 
keyboard, and mouse 
Measurement time &lt;1 second, at least 3 second 
intervals 
BRDF Range 10.sup.-1 to 10.sup.-5 
0.125.degree. or 0.25.degree. resolution 
in-plane BRDF (2.degree. to 40.degree. minimum) 
hemispherical area (out-of-plane) 
Display computer screen display time &lt;2 
seconds 
video display in realtime, 
near, mid, and far angle BRDF (peak 
and/or average) 
Analysis normalization, calibration 
TIS, RMS, PSD, AutoCorrelation, BRDF at 
other wavelengths 
Data Storage 
Selectively stores data in the ASTM 
1392-90 & binary formats 
Hard disk storage 
Graphical Output 
All acquired data can be plotted in 2 
or 3 dimensions 
Publication quality output 
______________________________________ 
The foregoing description of a preferred embodiment and best mode of the 
invention known to applicant at the time of filing the application has 
been presented for the purpose of illustration and description. It is not 
intended to be exhaustive or to limit the invention to the precise form 
disclosed, and obviously many modifications and variations are possible in 
the light of the above teaching. The embodiment was chosen and described 
in order to best explain the principles of the invention and its practical 
application to thereby enable others skilled in the art to best utilize 
the invention in various embodiments and with various modifications as are 
suited to the particular use contemplated. It is intended that the scope 
of the invention be defined by the claims appended hereto. 
APPENDIX A: TECHNICAL NOTES 
Detector Linearity 
As mentioned in Section 4.3 the CID Camera will be allowed to saturate. 
From experiments at BRO the camera can take two orders of magnitude of 
power beyond saturation before the pixels begin to invert. This inversion, 
as described by the manufacturer, is due to the electronics subtracting 
out what it believes to be fixed pattern noise. This inversion has no 
effect on the final data since the saturation is detected well before the 
inversion, and the data was collected before the saturation. Treating the 
camera in this way is quite common in the laser profiling market. And 
completely safe for the camera when the laser power is kept within a 
reasonable range, which is wavelength dependent. The power used for the 
proposed instrument is orders of magnitude within the safety limit. 
The laser output power can be controlled to within one (1) percent, through 
the use of a built in thermoelectric cooler and a feedback power control 
loop. 
Calibration 
The two methods of calibration which can be used for normalizing the BSDF 
data are the Reference Substitution Process method (RSS) and the 
Attenuated Input Beam Direct Measurement method (ABDM). 
These methods are described in detail as follows: 
RSS Calibration Method 
Currently, BRO scatterometers can automatically calibrate the data by 
measuring a gold calibration standard (which is traceable to a NIST 
standard). A gold reference works well with the large spectral range 
required. The equations are: 
EQU BRDF.sub.S =[BRDF.sub.R *cos(.theta..sub.R)*V.sub.S [/]cos(.theta..sub.1) 
*cos(.PHI..sub.i)*V.sub.R ] 
EQU BRDF.sub.R =P.sub.R /.pi.(Lambertian) 
where: 
BRDF.sub.S =BRDF of the sample 
BRDF.sub.R =BRDF of the reference standard 
P.sub.R =reflectivity of the Lambertian reference (known) 
.theta..sub.i,.PHI..sub.i =detector angles in and out of plane relative to 
the sample normal 
.theta..sub.R, .PHI..sub.R =detector angles in and out of plane when taking 
reference measurement 
V.sub.i =voltage from sample scatter at .theta..sub.i and .PHI..sub.i 
V.sub.R =voltage from reference scatter at .theta..sub.R and .PHI..sub.R 
(.PHI..sub.r =zero degrees) 
The errors in the RSS method are as follows: 
Error in the measured reflectivity of the standard itself (constant). 
The reference is measured at one detector angle (10 degrees from specular) 
with the assumption that the reference is diffuse and the BRDF curve is 
flat over a certain range of angles. In reality, there are no perfectly 
flat BRDF curves. Even from the most diffuse whites the BRDF can change 
.+-.30% or more. 
The contribution of these errors is constant; this means that they alter 
the level of the BSDF curve and not the shape of the curve. An advantage 
to this method is that some errors, or other factors in the data 
acquisition process common to taking data from the standard and the test 
sample, are normalized out. Past data taken on three different BRO 
scatterometers at the customer site with the same sample (Martin Black) 
show this calibration technique gives consistent results when implemented 
correctly. 
ABDM Calibration Method 
The Absolute method uses a reference mirror to direct the incident beam 
directly into the detector, or to catch the beam with no sample or 
reference mirror in place. The equations are: 
EQU BRDF.sub.S =V.sub.S A.sub.S R.sup.2 /[V.sub.T A.sub.T 
*cos(.theta..sub.1)*cos(.PHI..sub.1)*a*t] 
where 
.theta..sub.1,.PHI..sub.i =detector angles in and out of plane relative to 
the sample normal 
Vs=voltage output when measuring the test sample 
V.sub.T =voltage output when all radiation incident on sample enters the 
sample detector when measuring totaI power 
A.sub.T =attenuation required to allow all power on detector (positive 
exponent) 
a=aperture area 
A.sub.S =attenuation at .theta..sub.S, .PHI..sub.S 
R=distance from collecting optics to sample 
T=reflectivity of the reference mirror (T=1.0, for the straight through 
method). 
The errors in the ABDM method are as follows: 
error in normalizing out attenuation which is wavelength dependent 
(constant) 
error in measuring "R" and "a", which make up the solid angle 
error in determining that all the signal from the source is collected by 
the detector, and no significant additional stray light is present. 
linearity of the sample and reference detectors, even when the attenuators 
are in place 
For the laser sources, the RSS calibration method cited above will be used. 
Stray Light Control 
There are two sources of stray radiation in any scatterometer: the 
instrument profile, and the illumination beam. The instrument profile is a 
fixed noise produced by the interaction between the forward scatter from 
the illumination optics (chiefly the last optic before the sample) and the 
collection system's field-of-view. Its primary effect is to limit one's 
ability to make near-specular measurements. However, this is only a 
limitation when testing specular samples. When testing diffuse samples, 
the instrument profile is generally not a problem. 
There are two ways to reduce the instrument profile: decrease the source 
size or beam diameter, or decrease the collecting optic's field-of-view. 
Adjusting the source size or beam diameter changes the illuminated spot 
size on the primary mirror. A smaller collector FOV reduces the angle at 
which that spot is seen by the detector. The BRO scatterometers optimize 
these parameters in order to achieve an optimum spot size and a small 
instrument profile. 
The other source of stray light is the reflected beam. This arises when the 
beam strikes walls or support structures. If there are no walls, this 
source of stray light from internal sources is reduced. To avoid stray 
light from the support structure, it is best to underfill the sample. 
However, this is not always possible. In the "over-illumination" case, the 
beam spills off the sample onto the surrounding support structure. To 
minimize stray radiation from the support structure, a special sample 
holder has been designed. By eliminating most of the holder structure 
around the sample, controlling the overillumination beam size (focus 
adjust), and using beam dumps, BRO has been able to minimize all the 
instrument sources of stray light. 
APPENDIX B: CALCULATIONS 
The CID camera chosen has a full well capacity of 450,000 electrons, which 
is the saturation point. Thus the objective is when the BRDF is at lowest 
required to measure (10.sup.-5) it is desired that a pixel element 
accumulate on the order of 250,000 to 450,000 electrons. For a safety 
factor a goal of 450,000 electrons is assumed. 
At the minimum BRDF.sub.min the signal in electrons on the camera will be, 
##EQU1## 
where: t=frame (integration time) [sec] 
.phi..sub.1 =input laser power [watts] 
BRDF.sub.min =10.sup.-5 [steradians.sup.-1 ] 
.OMEGA.=solid angle [steradians] 
.tau.=throughput ratio of the fiber 
R=detector responsivity [amps/watts] 
q=charge of electron 1.6 E-19 [coulombs] 
N=number of collected electrons 
From among commercially available laser diodes two choices are made, at 
different locations in the available spectrum. Substituting representative 
values for these wavelengths: 
t=0.200 sec 
BRDF=1.times.10.sup.-5 
.OMEGA.=(0.25/57.3).sup.2 
.tau.=0.4 
q=1.6E-19 
.phi..sub.1 (840)=70 mw 
.phi..sub.1 (670)=3 mw 
R (840)=0.065 amps/watts 
R (670)=0.115 amps/watts 
1 amp=0.6 E.sup.19 electron/sec 
From the CID responsivity curve shown below, it is seen that the camera 
sensitivity peaks around 600 nm, while commercialized diodes have more 
power at higher wavelengths. 
Thus, from the above the product .phi..sub.i .multidot.R must be maximized. 
.lambda.=670 nm or .lambda.=840 nm 
q.sub.i =3 mW q.sub.i =70 mW 
From the CID responsivity curve shown below, the responsivities at these 
wavelengths are: 
R=f(.lambda. nm) 
R(670)=0.115 Amps/Watt 
R(840)=0.065 Amps/Watt 
using the above equation and values we get 
N(670)=32844 electrons 
N(840)=433160 electrons 
Previously it was assumed that 4.5.times.10.sup.5 electrons were desired. 
Thus, the 670 nm diode is not sufficient but, the 840 nm 70 mW is enough. 
Thus diode power can be controlled from a few micro-watts to 70 mW by 
controlling the diode on-off time (down to 10 ns, through the use of the 
diode controller).