Scanning systems for high resolution E-beam and X-ray lithography

Novel methods and apparatus for high resolution electron beam and X-ray lithography. For electron-beam lithography, a novel 1:1 imaging system is disclosed. For X-ray lithography, novel 1:1 imaging and n:1 reduction imaging systems are disclosed.

This invention pertains to high resolution lithography systems, 
particularly to scanning systems for X-ray and e-beam lithography, with 
either 1:1 imaging or n:1 reduction imaging. 
The linewidth of circuit elements in integrated circuits has decreased 
considerably in recent years with improvements in optical lithography 
techniques. It is expected that optical methods will soon reach their 
resolution limit. To produce even smaller linewidths will require other 
techniques, such as X-ray lithography or electron-beam ("e-beam") 
lithography. 
Although optical lithography is capable of patterning integrated circuits 
below 0.5 microns, it is clear that the wavelength of the illumination 
will eventually limit the resolution of the images. It is likely that 
X-ray lithography will become the method of choice when minimum feature 
sizes approach 0.1 micron, although e-beam lithography is a technique 
which should not be overlooked. 
Proximity printing as used in conventional X-ray lithography is limited by 
two effects: 
1) It is difficult to maintain the extremely small gaps between mask and 
wafer (about 5 microns) needed for high resolution; and 
2) It is difficult to manufacture, test, and repair the high resolution, 
high aspect ratio, low distortion masks needed by this 1:1 printing 
system. 
Projection X-ray lithography addresses the first of these issues and, when 
combined with image reduction, the second as well. However, while 
projection X-ray lithography has been demonstrated in principle, the 
technical requirements for useful versions of the proposed systems are 
beyond the present state of the art. 
(1) 1:1 Imaging with X-Rays. 
Fresnel zone plates for focusing X-rays are known in the art, as are 
methods for their fabrication. See, e.g., Vladimirsky et al., 
"High-Resolution Fresnel Zone Plates for Soft X-rays," J. Vac. Sci. 
Technol. B, Vol. 6, No. 1 (1988), the entire disclosure of which is 
incorporated by reference. Zone plates have severe chromatic aberration, 
in that their focal length is approximately proportional to the reciprocal 
of the wavelength. In addition, highly monochromatic X-ray sources are 
both weak and expensive. Consequently the use of a zone plate is 
effectively limited to a very small field. Therefore, the use of an array 
of zone plates has typically been proposed, as illustrated in FIG. 1. 
Because of image inversion by each zone plate 2, the boundaries of 
adjacent fields in the mask 4 do not align properly in the image 6, as 
illustrated by arrows 8 and 10. A past solution has been to use a second 
array of zone plates 12 for a second inversion, as illustrated in FIG. 2. 
Virtual imaging from the zone plates, and "crosstalk" between fields then 
results in unwanted background exposure. These effects may be reduced by 
limiting the field of each zone plate, spacing the zone plates relatively 
far apart, and using a third array of zone plates 14 as field lenses. The 
field lens zone plates direct the light from the first array 2 so that it 
passes more nearly through the center of the second array 12. Stops 16 
might also be used. Each of these strategies has the disadvantages of 
reducing the throughput of the system, and of increasing its complexity. 
(2) Reduction Imaging with X-Rays 
Reduction imaging with X-rays may be performed with Fresnel zone plate 
arrays, or with reflecting optical systems such as Schwarzschild lenses. 
Schwarzschild lenses have small usable fields, and are difficult and 
costly to manufacture. See H. Kinoshita et al., "Soft X-ray Reduction 
Lithography Using Multilayer Mirrors," J. Vac. Sci. Tech., Vol. B7, No. 6, 
pp. 1648-1651 (1989). Other proposed mirror systems would have larger 
fields, but would require very large components manufactured to accuracies 
that are currently unattainable in practice. 
(3) 1:1 Imaging with an e-beam 
Prior approaches to e-beam lithography have included (a) modulated scanning 
electron beam and aperture imaging direct write systems, which are 
inherently slow, and (b) 1:1 imaging systems, illustrated in FIG. 3, using 
parallel magnetic and electric fields between a mask 20 and a wafer 22. 
The latter approach has the disadvantages: (1) that if the wafer is not 
totally flat (e.g., if it has previously been printed), the electric field 
will be distorted near the wafer, causing a loss in resolution and/or 
local distortion; (2) that electrons hitting the wafer cause secondary 
electrons to scatter with a range of energies, after which the secondary 
electrons also hit the wafer, causing background exposure and a loss of 
resolution; (3) that it is difficult to create a magnetic field of 
sufficient uniformity (to the ppm level) over the volume needed; and (4) 
that no correction can be made for local wafer distortion. 
As illustrated in FIG. 4, problems (1) and (2) above, but not problems (3) 
and (4), have previously been reduced through the use of a conductive grid 
24 with many apertures, giving a zero electric field in the region between 
the grid 24 and the wafer 22. The grid is typically located at or near a 
first focus of the electrons. The grid must be moved during exposure to 
prevent the shadow of the grid from being imaged on the wafer. This 
approach does not use scanning, and has the disadvantages (a) of needing a 
relatively hard-to-produce, highly-uniform magnetic field over the large 
region between the mask and the grid, and (b) of not correcting local 
distortions. See, e.g., Ward et al., "A 1:1 Electron Stepper," J. Vac. 
Sci. Technol. B, Vol. 4, No. 1 (1986); Ward, U.S. Pat. Nos. 4,705,956 and 
4,695,732; and Elliston, U.S. Pat. No. 4,939,373. 
Feldman et al., U.S. Pat. No. 4,742,234 discloses apparatus for 
direct-write lithography with a charged particle beam, incorporating an 
electric field, a magnetic field, and a conductive plate having an 
elongated slit. Imaging from a mask is not discussed, the data being 
directly entered from a computer. 
In a "step-and-scan" system, different parts of the same circuit are imaged 
in different scans. Alignment, or overlay accuracy, on the boundaries 
between adjacent areas must be very precise; misalignment can be a source 
of circuit failure, because the effects of distortion are "concentrated" 
at these boundaries. 
Novel systems have been developed in the present invention for 
high-resolution lithography using either e-beams or X-rays. These systems 
variously permit either 1:1 imaging (i.e., imaging without reduction), or 
n:1 reduction imaging. 
(1) 1:1 imaging with X-rays is performed with two arrays of zone plates, 
with the mask at the focal plane of the first array. It is advantageous 
also to place plates with small apertures near one or both points of focus 
to block most virtual imaging and crosstalk. The aperture area is about 1% 
of the total plate area. The mask and wafer are locked together and 
scanned. Because the mask is at the focus of the first array, only one 
point is imaged by each zone plate at a given time (within the resolution 
of the optics). Because only points are imaged, no image inversion occurs. 
The zone plates are arranged in two-dimensional arrays such that after 
completion of scanning, every point on the mask is imaged to a 
corresponding point on the wafer. 
This 1:1 imaging system uses two zone plate arrays, two aperture plates, 
and a parallel beam of substantially monochromatic X-rays, such as may be 
generated by an undulator in a synchrotron. Because the zone plate arrays 
may be densely packed, relatively few X-rays are "lost" in interstices 
between plates, and the throughput is high. As with other zone plate 
systems, the focal length is selected to be relatively short, so that 
images over a reasonable spread of wavelengths remain within the depth of 
focus. 
To print a field on a wafer with the 1:1 X-ray projection system, the mask 
and wafer are first brought into registration with each other. They are 
then simultaneously scanned, while the X-ray beam and the zone plate 
arrays remain fixed. Alternatively, the X-ray beam and the zone plate 
arrays may be scanned with the mask and wafer fixed. 
The scan is similar to that used in conventional X-ray proximity printing 
on a storage ring beam line. The principal difference is that the zone 
plate arrays are positioned near the mask during the scan. In addition, 
the limited bandwidth and diffraction efficiency of the zone plates 
suggest that an undulator beam should better match the system requirements 
than would a bending magnet beam. 
(2) Reduction imaging with X-rays requires multiple scans, with accurate 
tracking of the mask and wafer positions. Mechanical tolerances are tight. 
However, they do not approach the tolerances of other proposed reduction 
projection X-ray systems based on multilayer coatings. 
Reduction (n:1) imaging with X-rays may be performed by replacing the 1:1 
imaging zone plates in FIG. 5 with n:1 imaging zone plates. The ratio of 
the mask scanning speed to the wafer scanning speed would be n. In 
addition, multiple scans from different parts of the mask would be 
required to completely cover the wafer with image points. 
Reduction (n:1) imaging with X-rays may also be performed with a line of 
reducing zone plates. The mask and wafer are scanned in opposite 
directions, each perpendicular to a line through the centers of the zone 
plates, and at a ratio of speeds equal to the reduction ratio n. The 
resulting image strips on the wafer will be smaller than those on the mask 
by a factor of n, and will be spaced farther apart by a factor of n 
relative to the strip separation on the mask. The mask and wafer are then 
stepped to fill in interleaving strips, etc., until the entire image is 
formed. A fixed aperture plate is advantageously added above the wafer to 
reduce background from virtual imaging. 
Because the images on the wafer will be inverted and separated relative to 
those on the mask, the layout on the mask will differ from that on the 
wafer. Thus "stitching" of adjacent areas will be needed, even if this 
system is used for 1:1 imaging, i.e., without reduction. 
Alignments must have close tolerances, but this mechanical problem is no 
greater than is the case with other systems. This novel system has the 
following advantages: (1) the X-ray optics are simple; (2) reduction of 
the mask also reduces errors in the mask (or in the stage); (3) a 
relatively small number of passes is required to expose the entire field, 
a number depending on the degree of reduction; and (4) the incident X-ray 
beam need not be as highly collimated. Generation of a zone plate array is 
within the current state of the art. X-ray projection lenses, by contrast, 
are beyond the current state of the art, or at least are very expensive. 
Furthermore, the throughput with an available X-ray projection lens would 
be slow, because a large number of passes would be needed due to the small 
field of view such a lens has. The present invention requires only a few 
passes to expose the entire field. 
Furthermore, the use of phase zone plates to increase zone plate efficiency 
is well known. Such zone plates may be made for X-ray lithography with 
thicknesses on the order of one micron of aluminum or silicon. In 
addition, blazed phase zone plates can be used to increase efficiency 
further, while simultaneously reducing the power of virtual images. See 
Leger, Holz, Swanson, and Veldkamp, Lincoln Lab Journal, Vol. 1, pp. 225 
et seq. (1988), the entire disclosure of which is incorporated by 
reference. 
The zone plate imaging systems of this invention are capable of a very flat 
field, and can have more than 4 microns depth of focus. The condensing and 
imaging zone plate arrays are essentially X-ray masks with relatively 
small fields, and can be made by conventional processing techniques. If 
they are distortion free, or at least matched to each other, then the 
imaging with a collimated X-ray beam is also distortion free. In addition, 
the on-axis imaging systems are telecentric. These optical properties are 
important to the lithography, and are difficult to achieve using 
conventional X-ray projection systems. 
An operating wavelength near 10 .ANG. is preferred for X-ray lithography 
based on considerations of resist characteristics and circuit damage. 
However, the techniques of this invention may be readily scaled and 
applied to other wavelengths. In contrast, multilayer projection systems 
appear to be constrained to longer wavelengths. 
Both the 1:1 and the n:1 reduction systems use conventional X-ray masks. 
Masks for an n:1 reduction system are easier to make, because the minimum 
feature size is correspondingly larger. However, the pattern is more 
complex than the image on the wafer. 
The techniques of this invention are capable of diffraction-limited 
projection X-ray lithography, with resolutions well below 0.1 micron. The 
mechanical requirements to maintain registration over the full scan are 
important, especially in the case of reduction imaging. However, they 
present no fundamental limitations, and in fact are similar to those of 
existing optical scanning systems. The system is based on existing X-ray 
mask technology, and all of the elements used have previously been 
demonstrated separately. 
(3) 1:1 imaging with e-beams is accomplished by focusing the beam with 
magnetic and electric fields, and passing the beam through a single narrow 
slit in a plate of conductive material. The conductive plate produces a 
zero electric field between the plate and the wafer, greatly reducing 
problems from scattered secondary electrons. 
The mask and wafer are locked together and scanned, the apparatus is then 
stepped to image another strip, etc. Because the electron beam passes only 
through a narrow slit, only a relatively small region of the magnetic 
field must have high uniformity. In addition, small corrections based on 
local alignments may adjust the mask-to-wafer registration, and introduce 
small gradients in the magnetic field to correct for local distortions.

ZONE PLATES 
The resolution R of a zone plate is given by 
##EQU1## 
where .lambda. is the wavelength of the exposing light, and NA is the 
numerical aperture. For zone plates used at a large reduction ratio, R is 
also the approximate width of the narrowest, outer ring. Using zone plates 
patterned by e-beam lithography, resolutions well below 0.1 micron are 
routinely obtained. For example, X-ray microscopes based on zone plates 
are a well-established technology. 
To obtain the high flux needed for practical lithographic applications, the 
zone plate must function over a band of wavelengths, .DELTA..lambda.. 
Using a band of wavelengths introduces chromatic aberration, however, 
because the focal length of a zone plate is proportional to 1/.lambda.. We 
have 
##EQU2## 
.DELTA.f should not exceed the depth of focus, D, given by 
##EQU3## 
which leads to 
##EQU4## 
For example, with a 1% bandwidth X-ray beam centered at 10 .ANG. and R=0.1 
micron, we have 
EQU f.ltoreq.2mm, NA.apprxeq.0.005 [5] 
which implies a maximum zone plate diameter of about 20 microns. It is 
apparent that even with a scanning system it is impractical to image a 
large field with such a small zone plate. However, many zone plates can be 
combined into an array to produce a useful imaging system. For efficient 
utilization of the X-ray beam, it is desirable that the array fully cover 
the projected area of the X-ray beam. See FIGS. 6 and 7. Complete 
patterning of the wafer surface is obtained by scanning, just as in 
proximity printing with an electron storage ring beam. 
I. 1:1 X-RAY IMAGING 
The image formed by a lens or zone plate is inverted through the axis of 
the lens or zone plate. In scanning systems generally, the mask and wafer 
move in opposite directions so that conjugate points remain conjugate over 
the entire field of the lens. For 1:1 imaging, it is mechanically 
desirable to lock the mask to the wafer during scanning. In single lens 
scanning systems, folding mirrors may be used to obtain this result. 
The inverted images formed by adjacent zone plates in a two dimensional 
array bring together images of portions of the mask which were previously 
well separated, as illustrated in FIG. 1. This effect precludes 
replication of the mask by scanning unless one of two strategies is 
followed: 
1) A second array of zone plates is used to invert the image again, so that 
a noninverted final image is formed, as illustrated in FIG. 2. This 
approach probably requires a third array of zone plate field lenses 14, as 
well as various stops to minimize cross talk, and would be relatively 
complex. It is not discussed further here. 
2) As illustrated in FIG. 5, the field of each zone plate 30 is restricted 
to a single point 32 on mask 34 on axis. This result may be achieved with 
a corresponding array of zone plate condensing lenses and a collimated 
X-ray beam. An array of stops 38 near the wafer 40 minimizes cross talk 
and light from virtual images. This approach has the major advantage that 
mask 34 and wafer 40 can be mechanically locked together and scanned. For 
this reason it is the preferred method for 1:1 replication. An optional 
array of stops 42 near the mask can also reduce cross talk. 
Each condensing zone plate 30 and its corresponding imaging zone plate 36 
form a simple imaging system. The component values suggested in Table I 
easily satisfy equation [5], and are modest compared to those of zone 
plates that been fabricated for use with X-ray microscopes. Zone plates 
for such applications are often free standing to avoid absorption of soft 
X-rays in the substrate; at 10 .ANG. the zone plate arrays would use a 
thicker absorber on a substrate. They are, in fact, essentially X-ray 
masks. However, because the imaging zone plates are used at 1:1 
replication, they should have a minimum feature size of about half that of 
the features to be printed. 
TABLE I 
______________________________________ 
Design parameters for a 1:1 on-axis zone plate imaging system. 
______________________________________ 
Zone plate diameter 10 .mu.m 
focal length of zone plate 30 
1 mm 
object distance of zone plate 36 
1 mm 
image distance of zone plate 36 
1 mm 
separation of stops 38 from the wafer 
50 .mu.m 
40 
resolution 0.1 .mu.m 
______________________________________ 
During exposure each imaging zone plate 36 scans a line on the mask 34 onto 
the wafer 40. An array of 100 columns of zone plates, with adjacent 
columns offset by 0.1 micron from each other, as illustrated in FIG. 6, 
uniformly covers the wafer with 100 scan lines in every 10 microns. While 
this approach is satisfactory, it requires critical control of the angle 
between the zone plate arrays and the direction of scanning. For example, 
the loss of entire scan lines could be caused by an error of only 0.1 
mrad. A more satisfactory layout of the arrays is shown in FIG. 7. In this 
arrangement an error of 0.1 mrad modulates the average spacing between 
scan lines, resulting in an intensity variation of 1%, but with no gaps in 
the coverage. 
From Equation [3] the depth of focus is 5 microns, which is reduced to 
about 4 microns by a 1% wavelength spread. Accurate spacing should be 
maintained both from the condensing zone plate to the mask, and from the 
mask to the wafer. However, the locations of the imaging zone plate and 
the stop are less critical. 
The 1% bandwidth required for zone plate imaging may be obtained by 
filtering the spectrum of a conventional bending magnet X-ray lithography 
beam line. However, higher throughput should be possible by using the high 
brightness and narrow bandwidth of undulator beams. 
UNDULATOR BEAMS 
Undulator beams use a periodically varying magnetic field to concentrate 
synchrotron light into one (or several) narrow bands. A typical undulator 
beam at 10 .ANG. with a 1% bandwidth has a diameter on the order of 1 mm, 
and a divergence of less than 0.1 mrad. Undulator beam techniques are 
described, for example, in H. Maezawa et al., Appl. Optics, Vol. 25, pp 
3260 ff (1986), the entire disclosure of which is incorporated by 
reference. The beam can be shaped and collimated by means known in the 
art, so that there is a useable uniform rectangular cross section of 
dimensions w by h, where w is the width of the zone plate array and h is 
equal to both the height of the columns and the width of the field being 
scanned. The physical direction of scanning may either be vertical, as in 
conventional proximity printing, or horizontal, which has some mechanical 
advantages and is illustrated in FIGS. 6 and 7. 
In the h direction a high degree of uniformity is desirable for uniform 
exposure of the wafer, just as in conventional proximity printing. If the 
w direction is wide enough, multiple groups of zone plates may be used, 
each of which completely scans the mask. This permits interlacing and 
exposure compensation, leading to improved resolution and greater latitude 
in beam uniformity in this direction. 
High spectral purity of the undulator beam is desirable. For example, in 
FIG. 5 the third order harmonic which is diffracted by the condensing zone 
plate 30 and not by the imaging zone plate 36 is focused at the final 
image point. While this effect may be minimized by a stop 42 near the mask 
and/or a central obscuration of the imaging zone plate, spectral purity 
remains important. 
Scanning techniques are known in the art, as disclosed for example in U.S. 
Pat. No. 3,819,265, the entire disclosure of which is incorporated by 
reference. Fabrication techniques for zone plates are also known in the 
art, as disclosed for example in Vladimirsky et al., cited above, the 
entire disclosure of which is incorporated by reference. 
II. REDUCTION IMAGING WITH X-RAYS 
Single lens scanning reduction systems, which produce an inverted image, 
must scan the mask and wafer in opposite directions at velocities whose 
ratio is equal to the reduction ratio. The dynamic alignment required to 
maintain registration is technically difficult. However, an optical 
projection system utilizing this principle has been demonstrated and is 
commercially available. See J. D. Buckley, et al., J. Vac. Sci. Tech., 
Vol. B7, p. 1607 (1989), the entire disclosure of which is incorporated by 
reference. 
Maintaining this ratio of scan velocities is necessary, but is not alone a 
sufficient condition for reduction printing with an array of imaging 
elements, because the fields on the mask are physically larger than the 
fields imaged on the wafer. But the fields on the mask cannot be larger 
than the separation between the imaging elements. Therefore, only a 
fraction of the wafer is exposed in a single scan. Subsequent scans, using 
different portions of the mask, are required to fill in the gaps. The mask 
pattern is not an enlarged version of the pattern printed on the wafer. 
The image is dissected, and it is precisely this dissection which permits 
the use of an array of imaging elements in the reduction system. See FIG. 
8. 
Methods of stepping and alignment are known in the art, and are shown for 
example in Buckley et al., cited above, and U.S. Pat. No. 4,924,257, the 
entire disclosures of both of which are incorporated by reference. 
Two specific examples of this aspect of the invention are discussed below: 
SINGLE POINT IMAGING 
Arrays of condensing and imaging zone plates are used to form simple 
imaging systems similar to those used for 1:1 printing, as illustrated in 
FIG. 5. A 4:1 reduction ratio has been chosen for purposes of 
illustration, and the feature size on the mask is correspondingly 4 times 
as large as that on the wafer. Typical component values are listed in 
Table II. The imaging zone plate is easier to fabricate than for the 1:1 
system, with a minimum linewidth of 0.08 microns for a final resolution of 
0.1 microns. 
TABLE II 
______________________________________ 
Design parameters of a 4:1 on-axis zone plate reduction 
imaging system. 
______________________________________ 
Zone plate diameter 10 .mu.m 
focal length of zone plate 30 
4 mm 
object distance of zone plate 36 
4 mm 
image distance of zone plate 36 
1 mm 
separation between stops 38 and the 
50 .mu.m 
wafer 40 
spot size on mask 0.4 .mu.m 
resolution 0.1 .mu.m 
______________________________________ 
The Zone Plates may be arrayed in either of the configurations shown for 
1:1 imaging (see, e.g., FIGS. 6 and 7). However, adjacent columns are 
offset four times as much. Consequently, only every 4th line is printed on 
the wafer, even through these lines are adjacent to each other on the 
mask. The mask scans 4 times as fast as the wafer; depending on the 
direction of patterning it may be scanned either in the same direction or 
in the opposite direction. Subsequently, the same area of the wafer is 
rescanned three more times to fill in the missing scan lines. The same 
zone plate arrays, offset appropriately, may be used for the scan. 
However, entirely different portions of the mask, storing the information 
for the remaining portions of the pattern, are used. Accurate positioning 
of the components is critical to ensure that the interdigitated pattern is 
correctly reconstructed. Such accurate positioning may be readily 
accomplished with an interferometrically-controlled stage, as is commonly 
used in optical step-and-repeat systems known in the art. 
STRIP IMAGING 
As illustrated in FIG. 8, reduction imaging may also be performed with a 
single column of zone plates 50, each of which images a field on the mask 
52 slightly smaller than the zone plate diameter. After scanning, a strip 
54 or 56 is printed on the wafer 58, with a strip width smaller by a 
factor equal to the reduction ratio. The mask and wafer scan in opposite 
directions. 
An advantage of this technique is that the mask pattern may consist of 
strips 54 and 56 which are separated from one other, relaxing the 
alignment tolerance between the mask and the imaging system. In addition, 
the X-ray beam need not be accurately collimated. A disadvantage is that 
more scans need to be performed to cover the area on the mask between 
patterned strips such as 54 and 56. 
Because the imaging array consists of only a single column of zone plates 
50, the area of the incident X-ray beam that is used for exposure is only 
about 10 microns wide. To match an undulator beam to this width requires 
the beam optics to demagnify by a factor of about 100 in the w direction. 
Although this produces a corresponding increase in the beam divergence, 
and therefore in the size of the mask field in the h direction, the zone 
plate imaging remains diffraction limited. Table III summarizes suggested 
operating parameters. 
TABLE III 
______________________________________ 
Design parameters of a 4:1 strip imaging zone plate reduction 
system. 
______________________________________ 
Zone plate diameter 10 .mu.m 
focal length of zone plate 50 
0.8 mm 
object distance of zone plate 50 
4 mm 
image distance of zone plate 50 
1 mm 
separation of aperture plate from 
50 .mu.m 
wafer 
mask field 8 .mu.m .times. 40 .mu.m 
wafer field 2 .mu.m .times. 10 .mu.m 
______________________________________ 
ZONE PLATE PERFORMANCE 
The resolution of arrays of state of the art zone plates is more than 
adequate for 0.1 micron lithography. Larger single zone plates have been 
used in microscopes with resolutions on the order of 0.03 microns. The 
diffraction efficiency of "amplitude" zone plates, formed by patterning an 
absorber, is theoretically limited to about 10%, and values within a 
factor of two of this limit have been reported in X-ray microscopy. 
Because many of the systems of this invention require two arrays of zone 
plates, it is important that individual zone plate efficiencies be high. 
Fortunately, phase zone plates may be made with much higher efficiencies. 
At 10 .ANG. the optimal thickness is approximately 1.6 microns of 
aluminum, or 1.7 microns of silicon, both of which lead to phase zone 
plates with theoretical efficiencies on the order of 30%. 
One of the factors limiting zone plate efficiency is the formation of a 
virtual image with as much power as the real image. The primary purpose of 
the stops shown in FIG. 5. is to prevent unfocused light from these 
virtual images from exposing the resist. However, the technique of "binary 
imaging," in which the zone plates are stepped to approximate blazed 
gratings, can be used to enhance the real image at the expense of the 
virtual image. Such a technique is described, for example, in J. Leger, et 
al., Lin. Lab. J., Vol. 1, p. 225ff (1988), the entire disclosure of which 
is incorporated by reference. Such zone plates are more difficult to 
fabricate because each binary step adds extra processing and reduces the 
minimum feature width on the zone plate by a factor of two. Nevertheless, 
the advantages of improving the diffraction efficiency while 
simultaneously reducing the background illumination make this technique 
highly attractive for use in this invention. 
III. 1:1 IMAGING WITH AN e-BEAM 
A 1:1 e-beam imaging system is illustrated in FIG. 9. Plate 70 is 
electrically conductive and nonmagnetic. The bulk of plate 70 is 
relatively electron-impermeable. Slit 72 is relatively electron-permeable, 
and may be, but is not necessarily, a gap or void in plate 70. Plate 70 
with slit 72 may be fabricated by means known in the art, and may include, 
for example: (1) a thin apertured copper sheet, (2) an apertured thin 
sheet of plastic coated with a conductive layer, (3) an apertured silicon 
wafer, (4) a glass channel plate, or (5) an apertured sheet of 
photo-etchable glass which is metallized on at least one surface. The 
relatively uniform electric and magnetic fields may be generated by means 
known in the art. 
The patterned electron beam from mask or source 74 is projected onto wafer 
76 to define a pattern. Because the velocity of electrons between slit 72 
and wafer 76 is substantially equal to twice their average velocity 
between mask or source 74 and slit 72, the spacing of wafer 76 from slit 
72 is preferably about twice the spacing of slit 72 from mask or source 
74. Mask or source 74 and wafer 76 are locked together and scanned. 
A "patterned electron beam," such as that emanating from mask or source 74, 
is one corresponding to the image desired on the wafer. There exist at 
least three different methods for generating such a patterned electron 
beam: (1) An external source (not shown) generates electrons, which pass 
through stencil mask 74. (2) Source 74 is a photocathode. Electrons are 
emitted where light strikes the photocathode, and light passing through a 
mask is imaged onto the photocathode. (3) Source 74 incorporates both a 
mask and a photocathode. 
If an external electron source is used, the electrons should be relatively 
monoenergetic. If a photocathode is used, the emitted electrons will have 
fairly low energies, so controlling their energy range is relatively 
simple. 
Where e-beams are used in the present invention, they may be replaced by 
ions or other charged particle beams. 
For any of the embodiments of this invention (X-ray or e-beam), suitable 
conditions for exposure (degree of vacuum, appropriate resists, etc.) are 
known in the art, as are methods for developing and processing the exposed 
resist. 
A "mask" means an object with a portion comprising a pattern relatively 
permeable to light (including X-rays) or e-beams (as appropriate in 
context), and with the remainder relatively impermeable to light or 
e-beams (as appropriate in context). 
It will be readily apparent to those skilled in the art that various 
modifications of the embodiments described are possible without departing 
from the spirit of the invention. For example, it is possible to move or 
scan components other than the ones specifically described here as moving, 
so long as the relative motion of the various components with respect to 
one another is preserved.