High resolution, multi-layer resist for microlithography and method therefor

A high resolution, multi-layer resist for use in microlithography and a method is disclosed. The resist consists of a planarized layer deposited onto a substrate and an active layer, consisting of arsenic sulfide and silver is deposited onto the planarized layer. Irradiation with light, or other source of irradiation causes the silver to ionically diffuse into the arsenic sulfide, thereby creating a non-phase separate ternary chalcogenide glass. Removal of either the reacted or unreacted ternary compound will provide a positive or negative mask which may be used in subsequent processing or left as an intermetal dielectric as part of the underlying circuitry.

BACKGROUND OF THE INVENTION 
The present invention relates generally to resist materials employed in the 
manufacture of integrated circuits and microcomponents. More particularly, 
the present invention relates to a high resolution, multi-layer resist for 
use in microlithography for the formation of microelectronic circuits. 
The rapid movement toward higher levels of integration in monolithic 
circuits has been made possible by increased component packing densities 
and smaller geometries within the devices and circuits. These advances are 
due principally as a result of advances in lithographic techniques and in 
resist technology, however limitations in the resists have become the 
resolution limiting factor. 
The minimum feature size is determined by the combination of the 
capabilities of the system optics and the contrast value of the 
photoresist. The photoresist contrast values for conventional optical 
resists are actually quite poor. Theoretical resolution of a typical 
lithography system must be two or more times greater than the feature size 
being defined. Thus, the equipment is theoretically capable of resolving 
features which are finer than those currently achieved. This 
characteristic suggests that current systems may be used to achieve higher 
resolution if higher contrast resists were available. 
The difficulties in achieving higher resolution may be met by providing 
thinner resist coatings. However, thinner coatings typically cannot 
withstand subsequent processing. Thin organic films produced by spin 
casting are prone to pin holes, which are remedied b compromising 
thickness. Conventional organic resists are developed with wet developers 
which often causes resist swelling which, in turn, causes resolution loss, 
bridging of adjacent features and snaking of narrow lines. Moreover, with 
wet development schemes, desired materials will not readily dissolve into 
small spaces due to surface tension exclusion effects. 
The problems of the current lithographic techniques are not met solely with 
higher resolution capabilities. Higher numerical aperture (NA) lenses may 
provide better resolution, but also result in decreased depth of focus and 
decreased field size. Reduced depth of focus is a serious deficiency when 
the systems are employed in modern integrated circuits having multiple 
levels of metalization. These types of circuits tend to have relatively 
large changes in surface elevation which may cause some regions to be out 
of focus. Additionally, conventional spin-on techniques tend to deposit a 
layer having varying thickness which is thinner at the edges of raised 
features. These thinner resist areas are then prone to overexposure and a 
change in line width occurs at the steps between the raised feature and 
its adjacent depression. The current tri-level resist schemes help to 
solve these problems but are non-ideal due to the complexity of the 
deposition and development processes. Tri-layer schemes also employ 
conventional organic photoresists and will, therefore, suffer from many of 
the aforementioned problems related to these materials. 
With conventional photoresists, reflections from the substrate may create 
standing waves which reduce photo speed and create an uneven exposure in 
the resist. If the underlying layer is highly reflective, incident light 
is reflected laterally into the resist where the layer passes over steps. 
This leads to a change of line width at the steps known as "reflective 
notching". 
Thus, the resist systems currently employed in microlithographic techniques 
suffer from a variety of problems which may be met by providing a high 
contrast resist system which can be dry developed, a resist material which 
is self-planarizing to avoid focus problems and which facilitates 
deposition of a uniformly thick photoactive layer, and which has a light 
absorbing photoactive region at the surface to guard against reflective 
notching. Additionally, the deficiencies of the current multi-layer 
resists dictate their solution by a new multi-layer resist scheme having 
enhanced high resolution capability and which will exhibit utility using a 
wide range of exposure methods including without limitation, light, x-ray, 
laser, ion beam and electron beam systems. 
SUMMARY OF THE INVENTION 
According to a broad aspect of the invention, there is provided a 
multi-layer resist system consisting principally of a planarized layer 
deposited upon a substrate and an active layer which is exposed to 
irradiation. The planarized layer may be spun on or otherwise deposited, 
and may be photoactive if desired according to a design, but photoactivity 
of the planarized layer is not essential. The planarized layer is thick 
enough to reduce the possibility of pin holes, and, if spun-on, is baked 
at a sufficient temperature to cause flow which seals the pin holes and 
removes the solvent to leave a film with few volatile components. The 
active layer is a thin film of chalcogenide glass, arsenic sulfide, topped 
with a coating of elemental silver. When exposed to light or other 
irradiation, the silver ionically diffuses into the arsenic sulfide to 
create a non-phase separated ternary chalcogenide glass compound which has 
very different material properties when compared to the arsenic sulfide 
alone. 
After exposure to irradiation and formation of the non-phase separated 
ternary chalcogenide glass, the unreacted arsenic sulfide is soluble in a 
CF.sub.4 plasma, and the ternary compound is extremely insoluble under the 
same conditions. The unreacted silver may be removed by sputtering or a 
wet chemical dip and the unreacted arsenic sulfide regions may be removed 
by reactive ion etching with CF.sub.4 to selectively expose the underlying 
planarized layer. The unremoved ternary compound may now be used to 
selectively protect the planarized layer during a subsequent etching step, 
in an O.sub.2 plasma or in NaOH, to yield a negative of the mask pattern. 
Alternatively, a positive image of the mask pattern may be produced by 
processing the exposed resist by reactive ion etching using a sulfur gas 
plasma to dissolve the ternary, but not completely dissolve the unexposed 
arsenic sulfide. Thus, either a negative or positive image may be obtained 
using the same resist scheme but using a different reactant during 
subsequent development. While the dry development schemes provide better 
resolution, wet development techniques may also be employed in the 
processing. 
The resulting structure may be used either as a surface mask or left as a 
patterned underlayer, such as an inter-metal dielectric, which remains as 
part of the finished circuit or structure. 
These and other objects, features and advantages of the present invention 
will become more apparent to those skilled in the art from the following, 
more detailed description of the invention with reference to its preferred 
embodiments and to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In accordance with the method of the present invention, and as illustrated 
with reference to FIG. 1 in the accompanying drawing, the process of 
forming the multi-layer resist according to the present invention is 
diagrammatically illustrated. The process assumes the provision of a 
substrate, such as a silicon wafer. The process consists generally of the 
following steps: 
1. A planarized layer is spun or deposited onto the substrate 10. The 
planarized layer may be made of virtually any material which is 
self-leveling by spin casting, and may consist of organic or inorganic 
materials. Alternatively, planarization may be achieved by deposition of a 
material and subsequent etching to obtain the planarized layer. Examples 
of such planarizing techniques are known in the art as Resist Etch Back 
(REB) or Bias Sputtered Quartz (BSQ). Organic self-planarizing materials, 
such as novolac resin, polyimide, or polymethyl methacrylate (PMMA), may 
be diluted with solvent to an appropriate viscosity to planarize the 
particular circuit topography. Inorganic materials, such as spin on glass 
formations may be used, although the development schemes for these 
substances is different from that used for the organic materials. 
2. If a spun on planarizing layer is employed, this layer is baked to 
remove the solvent and leave a film with few volatile components. The 
baking temperatures are within the range of about 50.degree. to about 
200.degree. C. for organics or about 100.degree. to about 500.degree. C. 
for inorganics, to cause flow which seals any pin holes in the planarized 
layer. 
3. A thin film active layer, about 30 to about 160 nm, of chalcogenide 
glass, arsenic sulfide is deposited 14 and a subsequent thin layer of 
elemental silver, about 10-40 nm, is deposited onto the chalcogenide glass 
16. These materials may be deposited by a variety of methods, including, 
without limitation, spin casting, physical vapor deposition or chemical 
vapor deposition. Evaporation and sputtering are favored because of their 
cleanliness, thickness and stoichiometry control are good. 
4. The active layer is exposed to light or other irradiation 18 which 
causes the elemental silver to diffuse into the arsenic sulfide to create 
a ternary compound. The ternary compound has different material properties 
when compared to the arsenic sulfide. The total thickness of the active 
layer, i.e., arsenic sulfide plus silver, is typically less than about 200 
nm. This permits the elemental silver to substantially diffuse through the 
glass within a reasonable exposure time. The planarized layer acts to 
protect the substrate from arsenic contamination through the active layer. 
5. Any excess unreacted silver is sputtered off 20. 
6. 
(a) Unreacted arsenic sulfide is etched in CF.sub.4 plasma by RIE 22 to 
selectively expose the underlying planarized layer. RIE provides the best 
edge definition due to the anisotropic nature of the etch. The unremoved 
ternary compound, which is insoluble in CF.sub.4, selectively protects the 
planarized layer during the planarized etch step 24. The process yields a 
negative of the mask pattern because the exposed material remains after 
development; or 
(b) A positive image may also be produced by following a different 
development sequence. In order to produce a positive image, the resist is 
exposed to the irradiation 18. The first RIE etching development step 
employs a sulfur gas plasma which etches the ternary compound 23 but only 
partially etches the arsenic sulfide. An oxygen plasma is then used to 
etch the underlying planarized layer 24. Those skilled in the art will 
recognize that the oxygen plasma is employed with an organic planarized 
layer 24, however, other etchants may be employed for inorganic planarized 
layers 24. Thus, either a negative or positive image may be achieved using 
the same resist scheme but employing a different reactant during the 
development phase. 
7. The resulting negative or positive structures may then be used in the 
microlithography process as a surface mask during a subsequent ion 
implantation step, etch step, lift-off step, etc. 28, and then stripped 
afterwards in the same manner as conventional resists. The planarized 
layer may then be stripped 30 in an O.sub.2 plasma. Alternatively, removal 
of the surface ternary 32 leaves a patterned underlayer as, for example, 
an intermetal dielectric which will remain part of the finished circuit. 
FIG. 2 diagrammatically illustrates the photoresist at different processing 
stages. FIG. 2(a) illustrates the photoresist 50 consisting of the 
substrate 52, the planarized layer 54 which is deposited onto the 
substrate 52, the chalcogenide layer, arsenic sulfide 56 and the elemental 
silver layer 58. After exposure to irradiation 18 through mask 51, as 
illustrated in FIG. 2(b), the unreacted silver is removed by sputtering or 
by a wet chemical dip leaving the reacted silver/arsenic sulfide ternary 
55 and unreacted As-S regions 56. The unreacted arsenic sulfide regions 56 
are then removed by reactive ion etching, as is shown in FIG. 2(c), with 
CF.sub.4 to selectively expose the underlying planarized layer 54. The 
unremoved ternary 55 selectively protects the planarized layer 54 during 
the subsequent planarized layer etching step, FIG. 2(d), using the 
reactive ion etching to remove the exposed underlying material without 
attacking the ternary. Where the planarized layer is an organic material, 
the planarized layer etch is performed in an oxygen plasma, whereas if the 
planarized layer were an inorganic layer, the etch reactants are 
different. 
In accordance with the preferred embodiment of the present invention, a 
particular arsenic sulfide compound has been chosen due to its high 
resolution capability. As.sub.33 S.sub.67 forms a substantially 
homogeneous, non-phase separate ternary glass when combined with silver. 
During exposure, the silver ionically diffuses into the arsenic sulfide 
with minimal lateral spread. The mechanics of the silver dissolution 
produce an edge-sharpening effect which enhances resolution beyond that 
which is predictable by diffraction-limited optics. The enhanced 
resolution provided by the edge-sharpening effect yields a practical 
contrast value greater than 12. Theoretical resolution is essentially the 
width of a few atoms because the material has no macromolecular elements 
as are characteristics of organic photoresists. 
Experiments using a conventional optical DSW machine utilizing the arsenic 
sulfide resist scheme have shown resolution to be better than a 
half-micron, which is about the theoretical limit for the exposure system. 
The As.sub.33 S.sub.67 material is extremely easy to deposit by evaporation 
with good control of layer stoichiometry, as revealed by EDXA. It is 
important to ensure that there is sufficient silver to dope at least 50% 
of the entire thickness of the As-S layer. It has been found that a 
thickness ratio of As-S:Ag in the range of about 5:1 to about 3:1 will 
provide sufficient photodoping of the silver into the As-S layer. When the 
Ag is fully diffused into the As-S layer, the final composition lies 
within the glass forming region of FIG. 3. While the Ag layer may be 
placed below the As-S, which configuration is necessary for thicker Ag 
layers so that radiation may readily reach the As-S:Ag interface, 
resolution under optical exposure tends to be less than satisfactory. For 
example, for 85 nm As.sub.33 S.sub.67 on 21 nm Ag, I-line (365 nm) 
illumination followed by wet development in an NaOH solution with IPA 
produce a contrast of less than 1 and poor sensitivity as measured by 
greater than 40 mJ/cm.sup.2. However, when using electron beam 
lithography, even with a relatively crude wet development step, a 70 nm 
pitch (35 nm line width) pattern has been produced. 
Plasma development using reactive ion etching with CF.sub.4 using a 35 nm 
Ag on 200 nm As.sub.33 S.sub.67 yielded a contrast of 5 with a sensitivity 
of 20 mJ/cm.sup.2. 30 nm Ag on 100 nm As.sub.33 S.sub.67 yielded a 
contrast of 12 with a similar sensitivity. A 1 micron layer of novalac 
based material, topped with 120 nm As.sub.33 S.sub.67 and 25 nm Ag, 
followed by a CF.sub.4 development and an O.sub.2 RIE step to etch the 
novalac, permitted the smallest features on the mask to be readily 
transferred as illustrated with reference to FIG. 4. 
It is important that the stoichiometry of the arsenic sulfide be 
approximately 33 at % As, 67 at % S, otherwise a grainy phase-separated 
film may form. Additionally, other chalcogenides, such as germanium 
selenide are characterized by greater lateral diffusion than arsenic 
sulfide and other diffusion materials, such as copper, tend to thoroughly 
diffuse with little or no applied radiation. 
By applying a thin silver coating onto the arsenic sulfide layer, the 
applied radiation penetrates the silver and is absorbed at or near the 
As-S/Ag interface. Because the arsenic sulfide layer is almost opaque for 
wavelengths shorter than 400 nm, the net result is that very little 
radiation will reach the underlying substrate to be reflected back to the 
active layers. This has the obvious benefit, in optical lithography, of 
eliminating reflective notching. The existence of the planarized layer 
also is important in electron-beam lithography in that a relatively low 
beam energy will greatly reduce the production of back scattered and 
secondary electrons from the substrate. 
An additional benefit of the present invention is that the active layer is 
sensitive to a wide range of radiation wavelengths, which facilitate 
combined exposure techniques on the same resist layer, e.g., optical and 
electron-beam, which may be combined to improve product throughput. 
Those skilled in the art will understand and appreciate that the foregoing 
described multi-layer resist scheme and method does not suffer from the 
problems of conventional resists and provides a number of added benefits. 
The multi-layer resist scheme and method of the present invention reduces 
the equipment need and facilitates high definition microlithography 
resists without complex processing.