Patterned heat conducting photocathode for electron beam source

Details Patterned heat conducting photocathode for electron beam source Patterned heat conducting photocathode for electron beam source

1999-07-29

2002-04-23

Patel, Nimeshkumar D. (Department: 2879)

Electric lamp and discharge devices

Photosensitive

Having phosphor screen

C313S231010

Reexamination Certificate

active

06376984

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to electron beam sources and, more particularly, to photocathodes for the generation of electron beams.
BACKGROUND
Electron beam (e-beam) sources are used in several fields, including scanning electron microscopes, defect detection instruments, VLSI testing equipment and electron beam (e-beam) lithography. In general, e-beam systems include an electron beam source and electron optics. The electrons are accelerated from the source and focused to define an image at a target. These systems typically use a physically small electron source having a high brightness.
Improvements in optical lithography techniques in recent years have enabled a considerable decrease in the line widths of circuit elements in integrated circuits. Optical methods will soon reach their resolution limits. Production of integrated circuit elements with smaller line widths (i.e., those with line widths less than about 0.1 &mgr;m) will require new techniques such as X-ray or e-beam lithography, which can provide accompanying resolutions well below 1 &mgr;m because of the shorter wavelengths associated with X-rays or electrons.
In e-beam lithography, (also called charged particle beam lithography), a beam of e.g. electrons from an electron source is directed onto a substrate. The electrons expose a resist layer (in this case an electron sensitive resist) on the substrate surface. Electron beam lithography uses what are called “electron lenses” to focus the electron beam. These are not optical (light) lenses but are either electro-static or magnetic. Typically, electron beam lithography is used for making masks; however it can also be used for direct exposure of semiconductor wafers.
Since modem lithographic systems must achieve fast writing times (high throughput rates) in addition to high resolution, their electron beams must also have a high brightness, which in the case of electron beams requires a high current density. This property is especially important for so-called direct write applications in which the electron beam is rapidly scanned and modulated so as to effect a projection of the image of a highly complex circuit directly onto a semiconducting chip substrate.
The primary motivation for using multiple beams in an electron-beam lithography system is to increase the total current that can be delivered while minimizing space-charge effects in each beam.
FIG. 1
depicts in a side view one type of so-called “hybrid” multiple e-beam lithography, in which multiple electron beams are created by focusing an array of light beams, where each light beam's intensity can be independently regulated, onto a photocathode in transmission mode (wherein the photocathode is back-illuminated with the light beams which are focused on a photoemission layer). The electron beams emitted from the photoemission layer are then accelerated, focused, and scanned across the wafer or mask using a conventional electron-optical column.
A conventional mask
118
(reticle) of the type now used in photolithography is positioned on a conventional stage
124
which may or may not be movable along one or both of the depicted x and y axes, depending on the type of photolithography subsystem. A source of the light is, for instance, a conventional UV light source or a laser illumination system
114
of the type now used in photolithography, which provides a relatively large diameter beam
116
of, for instance, laser illumination light which passes through the transparent portions of the mask
118
. It is to be understood that the mask is a substrate transparent to the incident light
116
on which are located opaque areas. The transparent portions of the substrate define the image which is to be transferred by the mask
118
. Typically, one such mask includes the entire pattern of one layer of a single integrated circuit die. The mask is usually, in terms of its X, Y dimensions, some convenient multiple of the size of the actual die being imaged.
A light optical lens system
128
(which is actually a lens system including a large number of individual lens components) focuses the light
126
passed by the mask
118
. The light optical lens system
128
is either a 1:1 or demagnifying lens system which demagnifies by e.g. a factor of four or five the image
126
incident thereon to form image
130
, which in turn is incident onto the object. A 1:1 ratio is more advantageous when mask size is limited. In this case the object, rather than being a semiconductor substrate, is the photosensitive backside of a photoemission cathode
132
. The photoemission cathode
132
defines for instance a minimum feature size of 0.5 micrometers or less, the minimum feature size of course being dependent upon the parameters of the system. The photoemission cathode
132
is for example a thin gold (or other metal) layer deposited on a transparent substrate.
The photoemission cathode
132
(which like the other elements herein is shown in simplified fashion) includes a photoemission cathode layer
134
which absorbs the incident photons
126
and causes electrons present in the photoemission layer
134
to be excited above the vacuum level. Some portion of the electrons
138
which retain sufficient energy to escape from the photoemission layer
134
are emitted into the vacuum portion
140
of the photoemission cathode downstream from the photoemission layer
134
. An electric voltage (typically several kilovolts to tens of kilovolts) is applied to the extraction electrode
142
associated with the photoemission cathode
132
. Extraction electrode
142
extracts the electrons
138
which have escaped from the photoemission layer
134
and accelerates them. Thus the accelerated electrons
146
form a virtual image of the incident photons
130
. In effect then the photoemission cathode
132
and extraction electrode
142
form a divergent lens.
There may also be, immediately downstream of the extraction electrode
142
, a magnetic (or electrostatic) lens (not shown) to reduce aberrations. (A magnetic lens is conventionally a set of coils and magnetic pole pieces, and yokes which focus the electron beam.) Such an electron beam system has been found to offer resolution of below 10 nm. Immediately following (downstream of) this portion of the system is a conventional electron optical lens system
150
consisting of one or more electron lenses and alignment, deflection and blanking systems
152
(shown only schematically in FIG.
1
).
This lens system further demagnifies the virtual image
146
at the writing plane, which is the plane of the principal surface of the wafer
158
by a factor determined to achieve the desired minimum feature size. For instance, if a minimum feature size of 0.5 &mgr;m is resolved at the photoemission cathode, an electron beam demagnification factor of five times is needed for a 100 nanometer minimum feature size on the wafer
158
. This means that when a total area of approximately 1 mm×1 mm is exposed on the wafer
158
, a total illuminated area of 5 mm×5 mm is required on the photoemission cathode layer
134
. Correspondingly for a 4:1 demagnification ratio an area of 20 mm×20 mm is illuminated on the mask
18
, and a 5 mm×5 mm area is illuminated for a 1:1 ratio. Of course these are merely illustrative parameters.
The total demagnification factor and exposed wafer area can be varied to achieve the desired minimal feature size and throughput. The wafer
158
, including its electron beam resist layer
160
, is typically supported on a stage
164
which is movable in the x, y and z axes, as is conventional. Other elements of both the photo and the electron beam subsystems which are well known are not shown, but include positioning measurement systems using for instance laser interferometer to determine the exact location of the mask on its stage and the wafer on its stage, vacuum systems, air bearing supports for the stages, various vibration absorption and isolation mechanisms to reduce environmental effects, and suitable control systems, all of the type well kn

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