The suggested invention relates to the means for X-ray contact lithography
used in microelectronics, namely for lithography devices, which use X-ray lenses
to form an X-ray quasi-parallel beam.
The first information about such devices relates to the end of eighties.
Some works, describing the application of X-ray lenses in lithography devices,
one can find in: "Book of Abstracts. 3rd All-Union Conference on Relativistic
Particles Radiation in Crystals. May 25-30, 1988". (Nalchik, 1988) . If a divergent
radiation source is used these devices comprise a lens, transforming the said radiation
to a quasi-parallel one directed through the mask on the resist, applied on the
substrate. The said lens comprises a set of channels for a radiation transporting,
curved along barrel-shaped generatrix. An effect of multiple total external reflections
from the walls of the channels is used when a radiation is transported. As a whole
a lens for a divergent radiation transforming to a quasi-parallel one is half barrel-shaped
and it is called a half lens (as against a full lens, which focuses a divergent
X-rays and is barrel-shaped).
More detailed information about the X-ray lithography devices including
all elements of such devices mentioned above one can find in the review "Multiple
reflection from surface X-ray optics" (M.A. Kumakhov and F.F. Komarov. PHYSICS
REPORTS. A review Section of Physics Letters, volume 191, number 5, August 1990.
North-Holland) , p. 345-348).
Nonuniform radiation intensity (decreasing toward a periphery of the
output face of a lens), falling on a mask, is a disadvantage of the described devices.
It makes necessary to use filters in order to absorb "extra" radiation in the central
section of the emergent beam of the lens. This solution was mentioned in source
, however a structure of the device, comprising the said filter, as a whole
is described in USA patent No. 5,175,755  (published 29.12.92). A placement
of the absorbing filter after the half lens (before or after the mask) is described
in this patent.
The second mentioned variant of an absorbing filter placing is obviously
poor, as according to this variant a radiation of extra intensity exposures on
the central part of the mask, being an expensive precise unit, what precipitates
the mask destroying. Besides, the influence of actual out-of-parallelism of emergent
radiation of a half lens appears in this variant to a greater extent. This radiation
divergence determines the more spreading of shadowgraph after the mask the more
is the distance after the mask. Placing the filter after the mask inevitably increases
The first variant (placing a filter between a half lens and a mask)
is more desirable. However according to this variant as well a radiation, which
extra intensity should be attenuated, passes through another expensive unit of
the device, an X-ray half lens, what precipitates its aging.
The influence of the aforesaid factors leads to refusal of usage of
absorbing filters, in particular if it is necessary "to smooth" great difference
between a radiation intensity in the central and peripheral parts of the cross-section
of the emergent beam, when a radiation intensity, being transported along the central
channels of the lens, exceeds a lot a radiation intensity after the filter. This,
in its turn, makes necessary to use rather "thin" lenses to make the said difference
small. Such lenses have a small capture angle of radiation, emerging from the source.
One more factor, fostering the usage of "thin" lenses with a rather
small capture angle in X-ray lithography, is a sharp drop of transmission coefficient
of a lens at a radiation rotation angle (when a half lens is used it is equal to
a half of a capture angle) exceeding some limit value. So, according to calculations
(see the results in source , p. 318), this value is equal to 0,3-0,4 radian.
As this effect is well known, it is considered that further increasing of a capture
angle will not increase an integral intensity of an emergent beam. The necessity
to suppress a radiation intensity in the center of the beam "to equalize" it with
a low peripheral level of intensity seems to be a very strong factor, making no
point in using a small capture angle in lithography.
So, for instance, in the work: M.A. Kumakhov. State and perspectives
of capillary Roentgen optics. Proceedings of SPIE - The International Society for
Optical Engineering. Volume 2011, 14-16 July 1993, San Diego, California  real
lithography devices with a capture angle of a half lens from 0,15 up to 0,3 radian
are described. Such half lenses capture not more than 1-2 % of an isotope source
Another result of using "thin" half lenses is a small cross-section
of the emergent beam, what makes possible to irradiate only small part of the substrate
area with a resist, applied on it. To process the whole area it is necessary to
use stepwise irradiation. In spite of using special high-precise devices for this
purpose, it is impossible to avoid errors, caused by errors in conjugation of neighboring
zones under exposure.
The suggested invention is aimed at obtaining a technical result,
implying that the usage of a source radiation increases simultaneously with enlarging
of the area of a plate under exposure and increasing a lens longevity. This technical
result can be obtained owing to the combination of two expedients: an absorbing
filter is placed between a source and a half lens and a half lens with larger capture
angle is used. For the latter an existence of an optimum value, considerably exceeding
the aforesaid limit values and depending on an energy of the used radiation and
material properties of reflecting surfaces of the channels of a half lens (i.e.
a material, the channels are made of if they are not applied, or a material applied
if exists), has revealed. In the energy range of a radiation being used from 0,6
keV up to, at least, 6 keV an optimum value of a capture angle depends only on
a radiation energy. A material, a reflecting surface of the channels is made of,
can be any one, feasible from the technological point of view for producing the
channels of an X-ray lens or for their inner surface applying on condition that
it contains only light elements (atomic number should be not more than 22).
The suggested device for contact lithography comprises as well as
the aforesaid more close to it known device according to USA patent No. 5,175,755
a source of a soft X-rays, a half lens for a divergent radiation of this source
transforming to a quasi-parallel one (the said lens includes a set of channels
for a radiation transporting with a total external reflection, and the said channels
are oriented along a generatrix of barrel-shaped surfaces), the means for placing
a mask and a substrate with a resist applied on it being located on the side of
an output face of a half lens, and an absorbing filter for smoothing a nonuniformity
of a beam intensity of an emergent radiation of a half lens, manifesting by intensity
decreasing from the center to a beam periphery.
As against the known one in the suggested device an absorbing filter
is placed between a radiation source and an input face of a half lens, and relationship
of the half lens cross sizes and a focal distance from the side of an input is
chosen for reasons of providing a capture angle of a source radiation in the following
where ψ is a capture angle [rad];
E is a radiation energy of a used source [keV], thus a material of a reflecting
surface of the channels for a radiation transporting includes the elements with
an atomic number not more than 22, and a radiation energy of a used source is from
0,6 up to 6 keV.
A relationship (1) is empirical; therefore to obtain a proper result
the values appear in it should be expressed in the aforesaid terms.
It is preferable to use an X-ray tube with a rotating anode as a source
of a soft X-rays.
The suggested invention is illustrated with figures:
- fig. 1 depicts a respective placing of the members of a device for lithography;
- fig. 2 depicts main constructive sizes of a half lens, used in the device.
An X-ray lithography device includes a source of a divergent soft
X-rays 1, which output aperture is placed in the focus of a half lens 2. A means
(it is not shown in the figure) for a mask 3 placing is located on the side of
an output (right one according to fig. 1) face of a half lens 2. The said means
is placed so that a plane of a mask 3 is parallel to an output face of a half lens
2, i.e. it should be perpendicular to a longitudinal axis of a half lens and to
a axial line of an emergent quasi-parallel radiation beam, formed by a half lens.
A means (it is not shown in the figure) for a substrate 4 placing
with a layer of resist 5 applied on it is located after the means for a mask placing.
The means for a substrate placing should be located so that a plane of a resist
is parallel to a plane of a mask and it is spaced from the said mask at a minimum
An absorbing filter 6 is placed between a source 1 and an input face
of a half lens 2. The absorbing filter 6 represents a product made of material,
absorbing an X-rays, as an axi-symmetric body, being relative to an axis combined
with a longitudinal axis of a half lens. A thickness of the absorbing filter 6
is minimum in its peripheral part and it becomes thicker as approaching its central
part, adjacent to an axis of symmetry. A law of changing of thickness of an absorbing
filter 6 as a function of a distance from the axis of symmetry (the said axis of
symmetry, being combined with a longitudinal axis of a lens 2 when a filter is
placed) is chosen so that to obtain a uniform intensity of a beam along a cross
section on the output of a lens. When the choice is executed a monitoring of radiation
intensity distribution along the cross section of emergent beam of a specific half
lens, intended for usage in a given lithography device, is realized by means of
one or other detecting means while producing of the device. The said law of changing
of thickness of an absorbing filter is of a character close to an exponential one.
Such a filter can be made, in particular, as a substrate, made of a light metal
(for instance, aluminum) and applied with a layer of more heavy metal (for instance,
copper or lead), with thickness, decreasing to a periphery.
The suggested X-ray lithography device works as follows.
A divergent radiation of a source 1 passes through an absorbing filter
2, which attenuates it in dependence on the deflection angle from the longitudinal
axis of a half lens 2 in inverse proportion to impending attenuation when the radiation
passes through the channels of a half lens. A half lens 2 transforms a divergent
input beam of radiation to a quasi-parallel one. Owing to aforesaid character of
the input radiation. attenuating the emergent radiation has an intensity distribution
along the cross section of a beam close to a uniform one (practical tolerable nonuniformity
is 5-10%). This radiation after passing through the transparent sections of the
mask 3 reaches the resist 2, applied on the surface of the substrate 4 to be irradiated.
As a result of an X-rays acting on the resist, sensitive to such radiation, "the
windows" (free of resist sections of the substrate surface under irradiation) appear,
the said windows form an image, repeating the image of the mask 3.
From the point of view of imaging precision an actual divergence of
the emergent quasi-parallel radiation of the half lens 2 and the distance between
the mask and the resist are of essential importance, as the image spreading is
of the following order:
δ ≈ d • Δ&thetas;,
where d is a distance between the mask 3 and the resist 5,
Δ&thetas; is a divergence angle of a quasi-parallel emergent radiation of
a half lens 2.
In what follows the substrate 4 is etched wherein an etchant acts
on the surface under processing through the "windows" in the resist 5 and it does
not act on the other sections, a layer of resist stable to the etchant retains
on. As a whole this stage of a technological process does not differ from a traditional
one (see, for instance, Encyclopedic dictionary "Electronics", Moscow, "Soviet
encyclopedia", 1991 , pp. 254-256).
As it was mentioned above in the invention description placing the
absorbing filter 6 on the path of radiations of source 1 to the lens 2 protects
a lens from an extra radiation. In experiments, carried out with the usage of the
power values of the source 1 discussed below, in the absence of an absorbing filter
before the input face of a half lens the said face softening was observed. An absorbing
filter is of no such fine structure as an X-ray half lens, and it is much more
thermal strong. Besides a resist is a simple and cheap unit, which can be periodically
The suggested placing of an absorbing filter makes possible to exclude
the action of a secondary scattered radiation, emerging in the said filter, on
the resist. If an absorbing filter is placed on the output of a half lens 2 this
radiation would cause much more spreading, than spreading defined according to
the formula (2), as this radiation is not quasi-parallel and its divergence is
not limited by a small angle Δ&thetas; and can reach 90°.
Let's make estimated calculations of possible coefficients and main
constructive parameters of the suggested X-ray lithography device.
As of now a diameter of plate-substrates about 30 cm (i.e. an area
is about 750 cm2) has been obtained. On the basis of desirable productivity
of processing of 10 such plates per an hour, the area to be processed will be 7500
cm2. A sensitivity of present X-ray resists is of 20 mJ/cm2
order. It means that 7500 &peseta; 20 = 150 000 mJ = 150 J of X-rays energy should
be delivered on the resist within 1 hour. A coefficient of power transformation,
consumed by an X-ray tube, to an X-rays is defined by a formula:
g = k(U-Uk)1-5,
where k ≈ 10-4 (for X-rays with the quantum energy within the range
Uk is an ionization potential of the characteristic radiation [kV] under
U is voltage across the tube.
For instance, for Kα radiation of aluminum (E ≈ 1,5 keV) at
U-Uk = 30 kV:
g= 1,65 &peseta; 10-2
i.e. a transformation coefficient is of 1% order. As of now X-rays tubes with power
consumption of 200 kW and more are produced. A tube not most high-power (30 kW)
with a rotating anode will be considered to be used. Such tubes can work without
repair within 10 000 hours, thus the required repair is of minor nature as the
operating experience implies. Their overall dimensions and weight are small. At
the said tube power it is possible to obtain the following X-rays power
30 &peseta;1,65 &peseta;10-2 = 0,495 kW ≈ 0,5 kW.
Thus it should be taken into account that at least half of this energy
is absorbed in the rotating anode. An additional 30 % falls on the hard part of
the radiation, which should be filtered (such a filtration is realized in the lens;
see, for instance: M.A. Kumakhov. Channeling particles radiation in crystals. Moscow,
Energoatomizdat, 10986 , p. 42). Altogether about 100 W from the said 0,5 kW
will represent useful power, being emitted by an X-ray tube with a rotating anode.
However this energy is emitted in a solid angle of 4 π.
The walls of the channels for a radiation transporting of a half lens
2 are made of light metals or their oxides, light glasses, and so forth materials
to provide good reflection and small absorption of used soft X-rays in the energy
range 1-5 keV (if the inner surface of the channels is coated the aforesaid refers
to the material of coating). Any materials, including elements with an atomic weight
not more than 22 and being acceptable for capillary lenses producing from the technological
point of view, are appropriate. A half lens 2 is made of polycapillaries or as
a monolithic lens according to the technology, described in the work: V.M. Andreevsky,
M.V. Gubarev, P.I. Zhidkin, M.A. Kumakhov, A.V. Noskin, I.Yu. Ponomarev, Kh.Z.
Ustok. X-ray waveguide system with a variable cross-section of the sections. The
IV-th All-Union Conference on Interaction of Radiation with Solids. Book of Abstracts
(May 15-19, 1990, Elbrus settlement, Kabardino-Balkarian ASSR, USSR, pp. 177-178)
If a radiation energy is E = 1 keV according to the formula (1) we
0,7 ≤ ψ ≤ 1,3.
Let's take ψ = 60° ≈ 1 radian. If ψ = 60° a half lens captures
about 10% of isotropicly diverging radiation of a tube, i.e. approximately 10 W.
If a radiation rotation angle is equal to 30° relevant to a respective capture
angle 60°, taking place in peripheral channels of a lens, only about 5% of energy
is transferred to a channel output. Excess of energy must be absorbed by a filter
6 before a radiation enters in the channels, placed more close to a longitudinal
axes of a half lens. So, it is necessary to recognize that only 5 % of energy,
captured by a half lens, reaches its output and forms approximately uniform quasi-parallel
beam at a cross-section.
It means that 5 &peseta;10-2 &peseta;10 J = 0,5 J = 500
mJ falls on the mask per a second. As it was described above it is necessary to
obtain 150 J per an hour, i.e. 40 mJ per a second. So, the studied system, including
a tube of 30 kW power and a half lens with 60° capture angle, meets accepted requirements
with safety from the point of view of energetics of lithographic process.
For main constructive sizes of a half lens (fig. 2) one can derive
the following formulas on the basis of relationships from the work: V.A. Arkadiev,
M.A. Kumakhov. Concentration of synchrotron radiation with capillary focusing systems.
Optic of beams, pp. 43-50. Institute for Roentgen Optical Systems. Moscow, 1993.
f = h/2tg(ψ/2),
h = H - 2L[1-cos(ψ/2)]/ sin(ψ/2),
R = L/sin(ψ2),
where ψ is a radiation capture angle;
f is a focal distance;
h is a an input diameter of a half lens;
H is an output diameter of a half lens;
L is a length of a half lens;
R is a curvature radius of a channel, being most distant from an optical axis of
a half lens.
According to the aforesaid suggestions (i.e. a diameter of a plate-substrate
is 30 cm) an output diameter of a half lens should approximate 30 cm as well. Let's
accept, that a length of a half lens is 30 cm as well. With regard to that one
can calculate by the formulas (4) - (6) the values of the other sizes for a capture
angle ψ = 60°, defined above:
an input diameter of a lens h = 13,8 cm,
focal distance f= 11,9 cm,
a curvature radius of a channel, being most distant from an optical axis of a half
lens, R = 60 cm.
To estimate blurring of a mask image, transmitted on the resist, two
factors, which define a divergence of an emergent radiation of a half lens 2, should
be taken into account. The first factor is a radiation divergence on the input
of each channel of a half lens of the following order
where l0 is an aperture size of an X-ray source,
f is a focal distance of a half lens 2.
The second factor is an out-of-parallelism of initially parallel rays
after they were reflected from the curved wall of a channel. An influence of this
factor is maximum for the peripheral channels, being the most curved (minimum curvature
radius), and for input rays, being the most distanced from each other (i.e. this
distance is equal to a diameter d0 of a channel). A divergence, caused
by this factor, of an output radiation is equal to
where d0 is a diameter of a channel for a radiation transporting,
R is a curvature radius of a channel, being most distant from an optical axis of
a half lens.
To determine a resulting divergence, taking into account a random
and independent character of an influence of the said factors, let's add quadratically
divergences, defined by the formulas (7) and (8):
Δ&thetas;= [(Δ&thetas;1)2 + (Δ&thetas;2)2]1/2
= [(l0/f)2 + 2d0/R]1/2.
So, if a focal distance is f= 11,9 cm and a typical value 10 = 1 mm
for a diameter of the channels d0 = 5 micron, obtained above, a divergence
is of Δ&thetas; ≈10-2 radian order.
At such a divergence and a typical distance value between a mask 3
and a layer of a resist d = 20 micron an image blurring, defined by the formula
(2), is of δ = 0,2 micron order.
The obtained results testify that a submicron resolution using plate-substrates
of large (up to 30 cm) sizes without stepping and processing efficiency up to 10
plates per an hour can be realized by means of the suggested device.
The usage of radiation energy more than 1 keV makes possible to use
resists up to 1 mm thick, realizing spatial structures on the basis of LIGA-technology.
- 1. Book of Abstracts. 3rd All-Union Conference on Relativistic Particles
Radiation in Crystals. May 25-30, 1988. (Nalchik, 1988).
- 2. Multiple reflection from surface X-ray optics (M.A. Kumakhov and F.F. Komarov.
PHYSICS REPORTS. A Review Section of Physics Letters, volume 191, number 5, August
- 3. USA patent No. 5,175,755 (published 29.12.92).
- 4. M.A. Kumakhov. State and perspectives of capillary Roentgen optics. Proceedings
of SPIE - The International Society for Optical Engineering. Volume 2011, 14-16
July 1993, San Diego, California.
- 5. Encyclopedic dictionary "Electronics", Moscow, "Soviet encyclopedia", 1991,
- 6. M.A. Kumakhov. Channeling particles radiation in crystals. Moscow, Energoatomizdat,
- 7. V.M. Andreevsky, M.V. Guvarev, P.I. Zhidkin, M.A. Kumakhov, A.V. Noskin,
I.Yu. Ponomarev, Kh.Z. Ustok. X-ray waveguide system with a variable cross-section
of the sections. The IV-th All-Union Conference on Interaction of Radiation with
Solids. Book of Abstracts (May 15-19, 1990, Elbrus settlement, Kabardino-Balkarian
ASSR, USSR, pp. 177-178).
- 8. V.A. Arkadiev, M.A. Kumakhov. Concentration of synchrotron radiation with
capillary focusing systems. Optic of beams, pp. 43-50. Institute for Roentgen Optical
Systems. Moscow, 1993.