High resolution E-beam lithographic technique

A method of reproducing sub-micron images in a first imaging layer. A second imaging layer is deposited on an etch-stop film formed on the first layer, and the second imaging layer is exposed to an E-beam at low dose. The resulting standing wave exposure pattern is converted into a corresponding topology pattern having peaks and valleys by exposure to a wet developer. Ions are implanted through the second imaging layer into portions of the first imaging layer below the valley portions of the standing wave topology pattern. The second imaging layer is removed without appreciably attacking the etch-stop layer, and then the etch stop layer is removed without appreciably attacking the first imaging layer. The first imaging layer is anisotropically etched in an O.sub.2 RIE, the implanted regions serving as an etch mask. The process results in the formation of small images at high throughput.

BACKGROUND OF THE INVENTION 
Optical exposure systems are the current technology of choice for 
patterning photosensitive polymers in manufacturing applications. However, 
in the sub-micron world that looms on the horizon, exotic photoresist 
compositions and complex processing techniques will become increasingly 
necessary in order to prolong the viability of optical exposure systems. 
Accordingly, alternate photoresist exposure systems are being explored in 
the hope that they will fulfill the stringent manufacturing requirements 
of tomorrow's technology. 
One particularly promising technology is electron beam (E-beam) exposure. 
In these systems, beams of electrons are irradiated on a surface to be 
patterned. In a particular application referred to as "direct-write E-beam 
exposure," these electron beams are controlled by an imposed electric 
field to expose selected areas of a photoresist layer, rather than 
exposing selected areas of the photoresist through a metallic mask as in 
conventional optical exposure systems. Since these metallic masks are 
costly to design and produce, the combined advantages of printing images 
at tighter geometries and eliminating metallic masks make direct-write 
E-beam systems very attractive. 
An article by Kenty et al, entitled "Electron Beam Fabrication of High 
Resolution Masks," J. Vac. Sci. Tech., October-December 1983, pp. 
1211-1214 discusses a particular patterning method for E-beam exposed 
resists. A quartz plate coated with polymethyl methacrylate (PMMA) was 
exposed to a direct-write electron beam at 20 kev in order to form 0.5 m 
features upon wet development. The resist pattern was then ion implanted 
with silicon, and the resulting mask was found to work well as a photomask 
for optically exposing other photoresist materials. See also an article by 
Maclver, entitled "High Resolution Photomasks with Ion-Bombarded 
Polymethyl Methacrylate Masking Medium," J. Electrochem Soc.: Solid-State 
Sci. & Tech., April 1982 pp. 827-830. 
In an article by Iida et al, entitled "An Approach to Quarter-Micron E-Beam 
Lithography Using Optimized Double Layer Resist Process," IEDM Digest of 
Technical Papers 1983, Paper 25.6, pp, 562-565, an E-beam is used in a 
direct write mode to pattern an upper thin layer of photoresist. The 
patterned photoresist is in turn used to pattern an underlaying thicker 
polyimide layer in an O.sub.2 plasma. As shown in FIG. 2 of the paper, by 
patterning only the upper 0.4 .mu.m photoresist layer be direct-write 
E-beam techniques, a 20 kev acceleration voltage produces better results 
than patterning a single, 1.8 .mu.m photoresist layer at a 120 kev 
acceleration voltage. 
In an article by Ishii et al, entitled "A New Electron Beam Patterning 
Technology for 0.2 .mu.m VLSI," 1985 VLSI Symposium, May 14-16 1985, Kobe, 
Japan, paper VIII-1, pp. 70-71, a direct-write E-beam system (acceleration 
voltage of 30 kev) is used to partially expose and pattern a photoresist 
layer. That is, only the upper portion of the photoresist layer is 
patterned. Then a second layer is deposited on the photoresist layer. The 
second layer is etched back so that portions of the second layer remain in 
the pattern that was formed in the photoresist. Finally, the photoresist 
is etched under conditions that do not etch the remaining portions of the 
second layer, so that the exposed portions of the photoresist are removed. 
A silicon resin was used as the second layer and PMMA was used as the 
photoresist. Note that in order to adequately define the final pattern, 
the etchback of the second layer had to be continued until at least half 
of the patterned portion of the photoresist layer was removed. 
Both of the above Iida and Ishii articles are directed to the same general 
idea. In order to completely pattern single photoresist layers of a 
thickness of 1 .mu.m or greater, the acceleration voltage must be kept at 
a high level and the photoresist must be exposed to the E-beam for a 
longer period of time (in other words, the E-beam "dose" necessary to 
pattern a conventional photoresist layer is high). As the dose increases, 
the throughput of the exposure tool decreases. Thus, others such as Iida 
and Ishii have attempted to decrease the necessary dose (to decrease dwell 
time and hence increase throughput) by totally patterning a thin layer 
(Iida) or by partially patterning a thick layer (Ishii). 
Accordingly, there is a need to formulate other processes that increase the 
throughput of direct-write E-beam exposure systems. 
SUMMARY OF THE INVENTION 
It is thus an object of the present invention to increase the throughput of 
direct-write E-beam systems. 
It is another object of the invention to formulate a simple E-beam exposure 
process. 
It is yet another object of the invention to provide an E-beam exposure 
process that reliably prints sub-micron images at high throughput. 
The above and other objects of the invention are realized by a method of 
forming small images in a first imaging layer arranged on a substrate, 
comprising the steps of forming a second imaging layer on the first 
imaging layer; exposing the second imaging layer under conditions such 
that an exposure pattern is formed only in an upper portion of the second 
imaging layer; developing the second imaging layer to convert the exposure 
pattern into a topology pattern in the upper portion of the second imaging 
layer; implanting ions through the second imaging layer into portions of 
the first imaging layer, as a function of the topology pattern in the 
second imaging layer; removing the second imaging layer; and etching the 
first imaging layer under conditions that do not appreciably attack 
implanted portions thereof, so as to form a pattern in the first imaging 
layer. Because a low E-beam dose is sufficient to form the exposure 
pattern in the second imaging layer, a high throughput can be realized.

BEST MODE FOR CARRYING OUT THE INVENTON 
With reference to FIG. 1, two imaging layers 10 and 30 are disposed on a 
substrate 1. The substrate 1 is depicted as a unitary structure for the 
purpose of more clearly illustrating the invention. In practice substrate 
1 could be comprises of a silicon or gallium arsenide wafer that may have 
one or more layers disposed on it to be patterned by use of the 
lithographic process of the invention. Imaging layer 30 can be made of and 
positive-acting material that can be patterned by an electron beam, e.g. 
polymethyl methacylate, or "PMMA." Imaging layer 10 can be made of any 
material in which silicon ions have a diffusion coefficient that is within 
the range of the diffusion coefficient in imaging layer 30. A conventional 
photosensitive polymer or other organic resin such as one of the "AZ" 
series of photosensitive novolac resins sold by the AZ Photoresist 
Products Group of American Hoechst Corp. of Somerville, N.J. ("AZ" is a 
trademark of American Hoechst Corp.) would meet the above criterion. Both 
imaging layers 10 and 30 can be formed on substrate 1 by spin-application 
to the desired thickness (typically 0.5 microns .+-.0.2 microns). Note 
that the imaging layer 30 should not be appreciably thicker than imaging 
layer 10. The layers can be of substantially equal thickness as shown in 
FIG. 1, or the imaging layer 30 can be thinner than imaging layer 10. 
A layer of silicon oxide 20 is disposed between photoresist layers 10 and 
30. The purpose of this silicon oxide layer 20 is to insure that 
photoresist layer 30 can be stripped without removing photoresist layer 
10. The thickness of this silicon oxide layer should be on the order of 
several hundred Angstroms. The etch-stop function of silicon oxide layer 
20 will be described in more detail below. 
As shown in FIG. 2, the upper imaging layer 30 is then exposed to a 
direct-write E-beam that defines a standing wave pattern in an upper 
portion 30A of layer 30. The E-beam acceleration energy should be on the 
order of 20-30 kev. At this acceleration energy and low dose, the 
throughput of the exposure system is high. As shown in the Iida et al 
article, these acceleration energies will result in a standing wave 
exposure pattern being formed in the upper surface of the exposed imaging 
layer 30. Imaging layer 30 is then exposed to a wet developer (e.g. IPA) 
that selectively removes the exposed portions 30A without removing the 
bulk film. The resulting standing wave topology pattern 35 may define a 
sub-0.5 .mu.m space peak-to-peak. 
Then, as shown in FIG. 3, ions are implanted through the upper imaging 
layer 30 to form implanted pits 10A in the lower imaging layer 10 as 
defined by the topology pattern 35. The implant energy/dose is a function 
of the thickness of imaging layer 30 as well as the diffusion coefficient 
of the particular ion specie through the particular imaging layer 
composition. The peak ion concentration should be at a point above the 
upper surface of imaging layer 10, such that the implanted pattern is 
centered about the interface between imaging layers 10 and 30. Note that 
the ions will penetrate through silicon oxide layer 20 without appreciably 
distorting the ion implant pattern. In this manner, only those portions of 
layer beneath the valleys defined by topology pattern 35 will be implanted 
with ions. The particular ion specie selected should provide a 
sufficiently high etch rate ratio between implanted and non-implanted 
portions of imaging layer 10, e.g. silicon or oxygen. The shape of the 
implanted pits 10A as well as the spacing between pits can also be 
controlled as a function of the implant energy (typically on the order of 
100 kev). 
Then, the upper imaging layer 30 is removed in an etchant that does not 
appreciably remove the silicon oxide layer 20. Such an etchant could be a 
solvent such as n-methyl pyrrollidone (NMP) or a dry etch such as an 
oxygen-based reactive ion etch (RIE). The dry etchant is preferably 
because it is more compatible with the subsequent processing described 
below. 
Then, the silicon oxide layer 20 is removed without appreciably removing 
the upper surface of the imaging layer 10. This is done by exposing the 
silicon oxide layer to a plasma etch in a CF.sub.4 --O.sub.2 gas 
combination, wherein the oxygen content is low (e.g., less than 25% of the 
total gas mixture by volume). At this low oxygen concentration, both the 
implanted pits 10A and the non-implanted portions of photoresist 10 will 
etch more slowly than the silicon oxide layer 20. While it is possible 
that the implanted pits 10A may be etched at a somewhat faster rate than 
the bulk photoresist 10, the overall etch rate difference between these 
materials and the silicon oxide layer 10 should dominate. The resulting 
structure is shown in FIG. 4. 
Then, as shown in FIG. 5, the lower imaging layer 10 is treated in an 
anisotropic etchant that is highly selective between the bulk film 10 and 
the implanted pits 10A. For example, if the implanted specie was silicon, 
an oxygen-based RIE will anisotropically etch the exposed portions of 
imaging layer 10 without appreciably etching the pits 10A. 
The resulting pattern can then be used to etch or define an ion implant 
with respect to the substrate 1 having none, one, or more layers thereon 
to be processed. Note that the pattern is formed by a process that 
utilizes a low E-beam dose, improving throughput. At the same time, due to 
its use of highly controllable implantation techniques, the process of the 
invention retains the narrow dimensions and other advantages present by 
direct write E-beam systems. 
Various modifications can be made to the invention as described above. For 
example, the silicon oxide layer 20 could be deleted. With the oxide layer 
removed, the upper photoresist layer 30 would be stripped in an O.sub.2 
plasma. The etch would have to be monitored so that it is terminated 
before an excessive amount of the underlaying photoresist layer 10 is 
removed. Silicon oxide layer 20 could be constituted from another 
material, such as silicon nitride or silicon oxynitride, or it could be 
constituted by layers of different materials, such as a lower layer of 
silicon nitride and an upper layer of silicon oxide. 
It is to be understood that the above teachings are not limitative to the 
invention per se. That is, various other modifications can be made to the 
best mode for carrying out the invention as described above without 
departing from the spirit and scope of the invention.