X-ray lithography method for irradiating an object to form a pattern thereon

An x-ray lithography method for irradiating an object (14) to form a pattern thereon uses an x-ray mask (10) having a membrane (18). The membrane (18) has an open membrane surface (26), and x-ray radiation (16) is passed through the open membrane surface (26) to irradiate the object (14). During this irradiation, the open membrane surface (26) is substantially uniformly exposed to the x-ray radiation (16) so that stress-induced distortion of the membrane (18) is reduced.

BACKGROUND OF THE INVENTION 
The present invention relates, in general, to a lithography method and, 
more particularly, to an x-ray lithography method that reduces 
stress-induced distortion in an x-ray lithography mask. 
An x-ray lithography mask is typically used to form a pattern in a 
radiation-sensitive resist layer on a semiconductor wafer surface. The 
mask has a membrane on which a lithography mask pattern is disposed. X-ray 
radiation is passed through the membrane, and more particularly the mask 
pattern thereon, to impinge on the resist layer so that a copy of the mask 
pattern is formed in the resist layer. 
The conventional types of membrane materials used in an x-ray mask are 
typically sensitive to x-ray radiation and exhibit a change in stress with 
cumulative x-ray exposure. This change in stress adversely distorts the 
lithography pattern on the mask and leads to distortion of the pattern to 
be formed in the resist layer. Accordingly, it is desirable to have a 
method that reduces stress changes in the membrane material during x-ray 
lithography.

DETAILED DESCRIPTION OF THE DRAWINGS 
Briefly stated, the present invention provides an x-ray lithography method 
for irradiating an object to form a pattern thereon using an x-ray mask 
having a membrane. The membrane has an open membrane surface, and x-ray 
radiation is passed through the open membrane surface to irradiate the 
object. During this irradiation, according to the method of the present 
invention, the open membrane surface is substantially uniformly exposed to 
the x-ray radiation so that stress-induced distortion of the membrane is 
reduced. 
The present invention can be more fully described with reference to FIGS. 
1-2. FIG. 1 illustrates a cross-section of an x-ray mask 10 disposed 
between an x-ray source 12 and an object 14 to be irradiated with x-ray 
radiation 16. X-ray mask 10 comprises a membrane 18 disposed on a support 
wafer 20 and a support ring 22 which provides a rigid base for support 
wafer 20. Support wafer 20 has an opening 24 therein which substantially 
defines an open membrane surface 26 of membrane 18. A lithography mask 
pattern 28 is disposed on a pattern surface 30 of membrane 18. Object 14 
may be, for example, a resist layer 32 disposed on a semiconductor wafer 
34 where resist layer 32 has a target surface 36 facing mask pattern 28. 
According to the present invention, x-ray radiation 16 from x-ray source 12 
is passed through opening 24 and lithography mask pattern 28 to irradiate 
target surface 36 of object 14. During this irradiation, it is important 
that open membrane surface 26 be substantially uniformly exposed to x-ray 
radiation 16 so that stress changes in the region of membrane 18 over 
opening 24 are minimized. An advantage thereby achieved is that distortion 
of mask pattern 28 is minimized so that a more accurate reproduction 
thereof may be transferred to resist layer 32. 
Typically, the present invention is used to form a repetitive pattern in 
resist layer 32 such as, for example, by stepper lithography which is 
known. According to a preferred embodiment of the present invention, the 
area of open membrane surface 26 is substantially equal to the area of the 
repetitive pattern to be copied to resist layer 32 so that open membrane 
surface 26 will be uniformly exposed to x-ray radiation 16. The resulting 
change in stress of membrane surface 26 will therefore be uniformly 
affected, resulting in minimal pattern distortion. The repetition of this 
pattern is used to provide for example a plurality of integrated circuits 
(not shown) from semiconductor wafer 34. 
In contrast to the present invention, a prior x-ray lithography method used 
aperture blades to cover a portion of the open membrane surface so that it 
was only partially exposed to x-ray radiation. This use of aperture blades 
prevented the uniform exposure of the open membrane surface because the 
portion of the surface covered by the aperture blades was not exposed to 
radiation. It has been discovered, however, that a non-uniform exposure of 
the open membrane surface as in this prior method leads to an unacceptably 
large distortion of the mask pattern due to stress changes in the membrane 
material. More specifically, the stress of a membrane surface exposed to 
x-ray radiation will be uniformly changed, whereas the stress in the area 
which has been covered by the aperture blades will remain unaffected. As a 
result, portions of the mask pattern located closest to the interface 
between exposed and unexposed areas of the membrane material will be 
significantly shifted from their original position, and thus distorted, 
due to the stress gradient across the interface. 
For example, according to the prior method above, an open membrane surface 
formed of silicon carbide having an area of 50.times.50 mm was irradiated 
over an area of only 46.times.46 mm, as limited by aperture blades which 
covered a portion of the opening in the support wafer. The portion of the 
open membrane surface not covered by the aperture blades was exposed to 
cumulative x-ray radiation of about 167 kJ/cm.sup.2. For this case, the 
silicon carbide became more compressive in the irradiated area, and as a 
result the membrane surface and the patterned area were displaced away 
from the center of the opening. For measurement purposes, this 
displacement is the difference between the original and final position of 
the mask pattern on the membrane surface. The maximum displacement 
occurred near the interface between the exposed and unexposed area of the 
membrane and had a value in either of orthogonal x or y directions of 55 
nm. It should be noted that circuits with 0.25 micron critical dimensions 
will require pattern placement accuracy on a target surface of less than 
50 nm. Thus, the displacement of 55 nm resulting from the use of aperture 
blades is undesirable. 
In contrast to the prior aperture blade method above, and according to the 
present invention, an open membrane surface having an area of 50.times.50 
mm exhibited a maximum displacement in either of orthogonal x or y 
directions (see FIG. 2) of only about 5 nm after the open membrane surface 
was exposed to cumulative x-ray radiation of about 167 kJ/cm.sup.2. Mask 
pattern 28 exhibited about the same displacement as exhibited by membrane 
18. In addition, after an exposure of this open membrane surface to 
cumulative x-ray radiation of about 500 kJ/cm.sup.2 mask pattern 28 and 
membrane 18 exhibited a maximum displacement in either the x or y 
directions of only about 15 nm. 
It is preferable that the maximum tensile stress of open membrane surface 
26 be kept at less than 5.times.10.sup.9 dynes/cm.sup.2, and more 
preferably between 0.5 and 2.times.10.sup.9 dynes/cm.sup.2. Membrane 
surfaces which are compressively stressed are wrinkled and not suitable 
for x-ray lithography. Also, membranes with stresses greater than 
5.times.10.sup.9 dynes/cm.sup.2 do not typically survive the mask-making 
process. When using the present invention, the average stress change over 
open membrane 26 is less than about six percent from an initial 
pre-radiation state to a state after an exposure of surface 26 to 
cumulative x-ray radiation of about 500 kJ/cm.sup.2. This cumulative dose 
represents the total exposure dose that a mask typically would receive in 
the manufacturing run of a particular integrated circuit such as an SRAM 
or a DRAM. 
Because membrane surface 26 is to be uniformly exposed, the intensity of 
the x-ray radiation impinging on membrane surface 26 preferably varies 
less than five percent over the full extent of surface 26. X-ray source 12 
may be a conventional x-ray apparatus such as a synchrotron ring or a 
laser-based system. 
X-ray mask 10 may be formed in many ways. As a specific example, support 
ring 22 may be formed of glass such as, for example, Pyrex brand glass. 
Membrane 18 is preferably silicon carbide, but may also be formed 
alternatively from boron-doped silicon, diamond, or silicon nitride as is 
known. Support wafer 20 is preferably a silicon wafer, and membrane 18 may 
have, for example, a thickness of about two microns. 
FIG. 2 illustrates a bottom view of x-ray mask 10 of FIG. 1. Common 
reference numbers from FIG. 1 are used for common elements. Open membrane 
surface 26 is shown having a rectangular area, but may have other shapes 
in alternative embodiments as one skilled in the art will recognize. The 
area of surface 26 is indicated by dimensions 40 and 42 of opening 24, and 
x and y directions are indicated on surface 26. As used herein, the 
primary significance of these x and y directions is their indication of 
orthogonal directions. 
The effects of radiation damage on membranes is further discussed in an 
article titled Accelerated Radiation Damage Testing of X-Ray Mask Membrane 
Materials, Seese et al., Electron-Beam, X-Ray, and Ion-Beam Submicrometer 
Lithographies for Manufacturing III, David O. Patterson, editor, Proc. 
Society of Photo-Optical Instrumentation Engineers, Vol. 1924, pp. 457-466 
(1993), which is hereby incorporated by reference in full. In this 
article, four different membrane materials which are used in x-ray 
lithography (silicon nitride, boron-doped silicon, diamond, and silicon 
carbide) were irradiated with x-ray radiation to determine if any of the 
materials were completely resistant to changes in stress. The material 
which was most resistant to changes in stress was silicon carbide. 
By now, it should be appreciated that there has been provided a novel 
method for x-ray lithography that reduces stress-induced distortion of an 
open membrane surface. The prior art used aperture blades to set the area 
of the membrane surface equal to the repetitive pattern on a silicon 
wafer. This induced distortion of the patterned surface that affected 
integrated circuit yield and eventually resulted in non-functioning 
integrated circuits. However, by using the present invention, it will be 
possible to make millions of integrated circuits, such as SRAMs or DRAMs 
using the same x-ray mask. Because the pattern distortion induced by the 
x-ray radiation is minimized, negative effects on yield will significantly 
be reduced.