Optical waveguides – Optical fiber waveguide with cladding – Utilizing multiple core or cladding
Reexamination Certificate
2001-05-18
2004-08-10
Font, Frank G. (Department: 2877)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing multiple core or cladding
C385S018000, C250S484200
Reexamination Certificate
active
06775452
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an optical waveguide. In particular, a phosphor coated waveguide for efficient collection and detection of back-scattered electrons in an electron beam apparatus such as a scanning electron microscope is disclosed.
2. Description of Related Art
An electron beam apparatus that incorporates an electron beam microcolumn may be used in electron beam lithography as well as in electron microscopes such as a scanning electron microscope. Electron microscopes are often utilized to image and measure features on semiconductor wafers and can facilitate detection of contaminants. In an electron beam apparatus, a specimen to be examined, such as a semiconductor wafer, is scanned by an electron beam focused onto the specimen. Back-scattered and secondary electrons result from the electron beam impacting the specimen.
Backscattered and secondary electrons may be detected using scintillators. In a scintillator, electrons strike a phosphor coating on a surface of a waveguide and are converted to photons. The phosphor coating is usually deposited on a surface of the waveguide about an axis of the electron beam. The photons generated as a result of the electrons striking the phosphor are collected and directed through the waveguide to an end where an optical detector is placed. The waveguide is generally disposed such that the photons are directed along a length of the waveguide perpendicular to the electron beam axis toward the optical detector. The optical detector such as a photomultiplier tube (PMT) detects the photons that reach the end of the waveguide.
FIG. 1
is a schematic of a conventional electron beam microscope system
20
and
FIG. 2
is a top view of a conventional waveguide
30
utilized by the electron beam microscope system of FIG.
1
. As shown in
FIG. 1
, the electron beam system
20
includes an electron beam source
22
that generates and focuses an electron beam
24
through the waveguide
30
onto a specimen
26
to be examined. Back-scattered and secondary electrons
28
result from the electron beam
24
impacting the specimen
26
and are generally directed toward the waveguide
30
and/or a phosphor coated region
44
of the waveguide
30
. The waveguide is typically made of glass or plastic.
As shown in
FIGS. 1 and 2
, the waveguide
30
includes two side faces
32
as well as angled faces
34
extending between a top and a bottom face
36
,
38
, respectively. An optical detector (not shown) is located at an end
40
of the waveguide
30
. The waveguide defines a hole
42
about an axis of the electron beam through which the electron beam passes. In addition, the phosphor coated region
44
of the waveguide is typically an annular phosphor coating on portions of the angled faces
34
about the hole
42
.
As noted above, back-scattered and secondary electrons strike the phosphor coating
44
and are converted to photons that are ideally directed by the waveguide
30
toward the waveguide end
40
for detection by the optical detector. The angled faces
34
tend to reflect photons toward the end
40
either directly or off the side, top and/or bottom faces
32
,
36
,
38
, respectively.
However, conventional electron beam microscope systems such as the one shown and described with reference to
FIGS. 1 and 2
typically have low collection efficiency, thereby limiting the speed at which the conventional systems can be operated. As is well known in optics, an angle of incidence &thgr;
i
, i.e., measured relative to the normal of an interface or surface that the photons strike, greater than or equal to the critical angle achieves total internal reflection, i.e., no refraction. In contrast, at least a portion of the photons that strike a surface at an angle less than the critical angle is transmitted through the waveguide material, i.e., refracted. Refraction of the photons decreases the collection efficiency in that the refracted photons do not reach and thus are not detected by the detector.
The critical angle depends upon the relative refractive indexes of the two different materials through which light travels. Because electron beam microscope systems operate in vacuum (n
vacuum
=1), the critical angle is given by Arc sin (1
) where n is the refractive index of the waveguide material.
In addition, the photon collection efficiency may not be homogeneous in that the collection efficiency may be dependent upon where the electron strikes the phosphor. In the electron beam microscope system shown in
FIGS. 1 and 2
, electrons that strike the phosphor on the right side of the hole are more efficiently collected than those that strike the left side of the hole. The collection inhomogeneity leads to a reduced contrast depending upon how the electrons scatter from the specimen.
As advances in semiconductor fabrication technologies have enabled fabrication of smaller and smaller integrated circuits, it has become increasingly important to accurately, efficiently, and effectively detect contamination on the semiconductor wafers in a time-efficient manner. Thus, it is desirable to provide a waveguide that has an improved collection efficiency by providing a waveguide that results in greater portion of photons being detected by the optical detector. It is also desirable to provide a waveguide that has an improved collection efficiency homogeneity. It is further desirable to limit the size of the waveguide, e.g., to approximately 1.5 mm in thickness and/or approximately 6 mm in width, depending upon its application.
SUMMARY OF THE INVENTION
A phosphor coated waveguide for efficient collection and detection of back-scattered electrons in an electron beam apparatus such as a scanning electron microscope is disclosed. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, or a method. Several inventive embodiments of the present invention are described below.
According to one preferred embodiment, a waveguide for use in an electron microscope generally comprises a first waveguide portion having opposing first and second faces defining a beveled hole therebetween to allow an electron beam to pass therethrough, the beveled hole decreasing in cross-sectional size from the first to the second face. The second face has a phosphor coating around the beveled hole. The beveled hole includes a beveled portion and optionally a straight portion. The beveled portion defines a beveled surface preferably coated with a reflective material. Generally, the beveled surface may be at an angle between approximately 35° and 55°, and more preferably at 45°, relative to the first face. The waveguide optionally includes a second waveguide portion having opposing first and second ends, the first end being coupled to the first waveguide portion and the second end being larger than the first end and adapted to be coupled to an optical detector.
In another embodiment, a waveguide for use in an electron microscope generally comprises a first waveguide portion having opposing first and second faces defining a hole therebetween to allow an electron beam to pass therethrough, a phosphor coating on the second face disposed about the hole, and a second waveguide portion having opposing first and second ends, the first end being adapted to be coupled to the first waveguide portion and the second end being larger than the first end and adapted to be coupled to an optical detector.
The second waveguide portion has first and second sides that preferably taper at a taper angle relative to the first and second faces of the first waveguide portion, respectively. The taper angle is generally between approximately 7° and 15°, and more preferably approximately 10°. Alternatively the second side is non-tapered while the taper for the first side is increased to approximately 15° to 20°.
With regard to any of the waveguide embodiments, the first waveguide portion may further comprise opposing first and second ends, the second end bei
Applied Materials Inc.
Font Frank G.
Kianni Kevin C
Kuo Jung-hua
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