Fiber optic scatterometer for measuring optical surface roughness

A method and apparatus for measuring scattered light reflectance. The apparatus comprises a plurality of individual light transmitting fibers having first receiving ends positioned at different angular locations about an object to be tested and second exiting ends positioned in a linear array. The apparatus can simultaneously receive different angular components of scattered light from the object being tested and convert the scattered light components into a linear array.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to measuring light reflectance and, more 
particularly, to a scatterometer for measuring surface scatter of light 
from a sample being tested and a method of doing the same. 
2. Prior Art 
Surface scatter of light from optical components, such as mirrors, 
beamsplitters, etc., is an important means of measuring the geometry of 
surface microstructure and thus the quality of fabrication of these 
optical components and the quality of the materials used in their 
fabrication. Generally, there are three standard methods of measuring 
surface scatter or the roughness of a surface; profilometry (both 
mechanical and optical), global type measurements (i.e: collection of all 
scattered light over a hemisphere in terms of the ratio of scattered light 
to the incident light) and bidirectional reflectance distribution function 
(BRDF) measurements. BRDF measures angular scatter or the amount of 
power/sterradian as a function of angular departure from the surface 
normal vector. BRDF is also usually a function of the incidence angle. 
In the past, BRDF has been used because it can produce more detailed 
information about the interaction of light with the surface being tested 
than profilometry or global type measurements. Generally, a detector would 
be provided which would swing about a sample 180.degree. and take 
sequential measurements of scattered power as a function of its angle in 
the plane of the incidence light beam. However, this process of moving a 
detector and taking sequential measurements is time consuming. In 
addition, the stability of the light source is critical in this sequential 
process to insure accurate measurements. 
It is therefore an object of the present invention to provide for accurate 
bidirectional reflectance distribution function measurements in a fast 
manner. 
It is a further object of the present invention to provide for parallel or 
simultaneous collection of data in a bidirectional reflectance 
distribution function measurement system. 
It is a further object of the present invention to reduce the critical 
reliance on the stability of the light source in bidirectional reflectance 
distribution function measurement systems. 
SUMMARY OF THE INVENTION 
The foregoing problems are overcome and other advantages are provided by an 
instrument and method for measuring scattered light reflectance from an 
object being tested comprising means for simultaneously receiving 
different angular components of scattered light at different angles of 
reflectance and means for converting the different angular components into 
a linear array. 
In accordance with one embodiment of the invention, a device for measuring 
scattered light reflectance from an object being tested is provided. The 
device comprises means for directing light at the object being tested, 
means for collecting reflected scattered light components from the object 
and a linear array detector means for reading the scattered light 
components and producing electrical output signals corresponding thereto. 
The collecting means includes a plurality of light transmitting fibers 
that can each receive a different angular component of the reflected 
scattered light at first ends and transmit the scattered light components 
to second ends arranged in a linear array. 
In accordance with one method of the invention, a method of measuring 
scattered light reflectance from an object being tested comprises the 
steps of directing light at the object, collecting different portions of 
scattered light reflected from the object simultaneously at different 
angular locations about the object, transmitting the collected light 
portions from first ends of light transmitting fibers proximate the 
collecting locations to second ends of the fibers, the second ends being 
aligned in a linear array and exiting the transmitted light portions from 
the second ends of the fibers for reading by a linear array detector such 
that the detector can produce electrical output signals corresponding to 
the linearly exited light.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIGS. 1 and 1A, there are shown schematic views of a 
bidirectional reflectance distribution function measuring system known in 
the prior art and its associated computer generated data readout, 
respectively. The device shown in FIG. 1 generally comprises a detector 2 
for measuring scattered light reflectance from a sample 4 which is being 
tested. The detector 2 is generally mounted on a movable arm 6. A light 
source 8 directs a beam of light C at the sample 4 and the detector 2 is 
capable of swinging about the sample 4 in a 180 degree arc and measures 
scattered power as a function of angle .theta. in the plane of incidence. 
The specular component of the reflected light is generally indicated by 
arrow A and the scattered light is generally indicated by the arrows B. 
The sample 4 is also suitably mounted for rotation as indicated by arrow 
D. The detector 2 generally takes different measurements at different 
angles and transmits signals to a suitable microprocessor or computer such 
that a display as shown in FIG. IA can be produced. 
Referring now to FIG. 2, there is shown one embodiment of the present 
invention. The scatterometer 9 shown basically eliminates the cumbersome 
rotating detector arm 6 and sequential data taking known in the prior art 
device of FIG. 1. The scatterometer generally comprises a plurality of 
optical fibers or fiber bundles 10, a linear array detector 12 and a 
microprocessor or computer 14. Each of the optical fibers 10 generally 
comprises a first end 16 and a second end 18. The first ends 16 of the 
optical fibers are arranged in a circular array as shown. The second ends 
18 of the optical fibers are arranged in a linear array proximate the 
linear array detector 12. In the embodiment shown, the first ends 16 of 
the optical fibers are generally provided with equal spacing between 
adjacent first ends 16 about the circumference of the circle, the center 
of which is intended to receive a suitable sample 4 to be tested. In a 
preferred embodiment of the invention all of the optical fibers are of 
equal length. Because the fibers 10 are of equal length and are 
substantially equidistant from the sample 4, scattered light reflected off 
of the sample 4 at different angles can nonetheless be collected by input 
couplings 30 (see FIG. 3) into the fibers 10 and simultaneously exited 
from the second ends 18. If the fibers 10 where of different length, then 
the angular components of the scattered light would not exit the second 
ends 18 simultaneously. In addition, because there is loss of light while 
the light is transmitted through the fibers 10, the losses due to the use 
of the fibers 10 is kept the same for each of the individual angular 
components because of the fibers 10 having the same length. If the fibers 
10 did not have the same length, then there would be inequitable losses in 
different angular components resulting in incorrect measurements at the 
detector 12. An incident beam of light C is directed at the sample 4 from 
a suitable light source such as a laser (not shown) such that the incident 
beam C of light will pass between two adjacent first ends 16 or input 
couplings 30. The light, after hitting the sample 4, reflects off of the 
sample 4 with two main components; the specular component A and multiple 
scattering components B. The sample 4 is suitably angled relative to the 
incident light beam C such that the specular component A can pass between 
two adjacent first ends 16 and their associated input couplings 30. 
Referring also to FIG. 3, there is shown an enlarged view of an input 
coupling 30 to a first end 16 of an optical fiber 10. In the embodiment 
shown, a suitable collecting lens 20 is used to collect scattered light B 
and direct the collected light into the first end 16 of the fiber 10. In 
the embodiment in FIG. 2, a suitable lens 20 is provided for each of the 
first ends 16 of the fibers 10. The collecting lens 20 generally act as an 
input coupler such that collected scattered light can be properly focused 
such that it can enter the first end 16 of the fiber and travel through 
the fiber to its second end 18. The second ends or exit faces 18 of the 
fibers are arranged in a linear array 19 and monotonically with angle as 
per the position of the first ends or entrance faces 16. The linear array 
19 of fiber exit faces 18 is then coupled to a linear array detector 12. 
In the embodiment shown, an imaging lens 22 is used between the exit faces 
18 and the linear array detector 12 for indirect coupling. In an alternate 
embodiment, the exit faces 18 and the linear array detector 12 may be 
directly coupled to each other by butting them up against each other. 
The linear array detector 12 can generally receive light components from 
each of the exit faces 18 and produce electrical signals which are sent to 
the computer 14 such that the computer can produce a display of the 
scattering relative to the angle .theta.. With the embodiment shown, 
scattering data is generally collected in parallel or simultaneously, but 
read out of the detector sequentially. This procedure is generally several 
orders of magnitude faster than the sequential gathering of scattering 
data via a movable arm as shown in FIG. 1. In addition, since data is 
collected in parallel or simultaneously, the stability of the incident 
beam of light or light source is less critical in obtaining accurate data. 
In an alternate embodiment of the invention, a second data collecting arc 
or circle orthogonal to the plane of FIG. 2 and centered on the sample 
could be implemented thereby obtaining three-dimensional scattering 
information. 
Referring now to FIG. 5, there is shown an alternate embodiment of the 
invention. In the embodiment shown, the optical fibers 10 are polarization 
preserving fibers and a rotation analyzer 24 is provided between the 
linear array 19 of the second ends 18 of the fibers and the detector 12. 
With this embodiment, the incident beam of light C may be linearly 
polarized either in the plane of incidence or orthogonal to it. This 
embodiment can generally be provided to obtain data on polarized 
scattering. 
Referring to FIG. 6, there is shown another embodiment of the present 
invention. In the embodiment shown, a third generation image intensifier 
or channel plate amplifier 26 has been provided adjacent to and in front 
of the linear array detector 12. Because scattered light intensities from 
high quality optical surfaces are extremely weak, by providing the image 
intensifier 26 the signal levels to the computer 14 from the detector 12 
can be dramatically increased. In an alternate embodiment of the 
invention, the image intensifier 26 can be fabricated as part of a 
detector package 12. 
Referring now to FIG. 7, there is shown another alternate embodiment of the 
invention. Generally, the specular component A of the reflected beam is 
adjusted to fall between a pair of input couplers 30. This avoids 
saturating the detection system 12. Although there is no need to have a 
physical surface at a location where secondary scatter from the specular 
component could have an effect on the primary data of interest, the 
embodiment shown in FIG. 7 comprises an auxiliary detector 28 to provide a 
reflectant measurement of the specular component. In the embodiment shown, 
the auxiliary detector 28 is generally provided outside the scatterometer 
circle such that the interception of the specular component will not 
interfere with the collection of the primary data of interest; scattered 
light. Alternatively, an auxiliary detector may be comprised of a second 
linear array detector for measuring small angle scattering. 
As with all devices used to measure light reflectants, ambient light from 
operational instruments associated with tests can be compensated for. One 
method to compensate for ambient light is through the use of data 
subtraction. A data test run without the target or sample in place can be 
taken first and the data can later be subtracted from a data test run on 
the actual target or sample. This data subtraction can generally be 
accomplished by the use of the computer 14. In an alternate method of 
compensating for ambient light, the light source can be modulated and the 
detection system can provide for synchronous detection of the scattered 
light. The present invention may also be extended to the infrared 
wavelength ranges by providing optical fibers that can receive and 
transmit infrared light. In addition, the ends of the fibers may be coded 
with photo-luminescent material for detecting secondary radiation having 
wavelengths shorter or longer than the bandpass of the optical fibers. 
It should be understood that the foregoing description is only illustrative 
of the invention. Various alternatives and modifications can be devised by 
those skilled in the art without departing from the spirit of the 
invention. Accordingly, the present invention is intended to embrace all 
such alternatives, modifications, and variances which fall within the 
scope of the appended claims.