Method and apparatus for fabricating a device/circuit pattern by a converging atomic beam

Methods and apparatus for etching ultra fine lines of impurities on semiconductors and other materials. A cold diverging ion beam is generated, made to converge, encoded using a mask to correspond to an image, and then used to etch impurities on the substrate. An ECR plasma source is used to generate a warm plasma. A cooled neutral target gas is penetrated by the warm plasma ions so that the plasma ion charge is transferred to the cool target gas to provide cool ions, which are then extracted to provide a cryogenic ion beam. The ion beam is made converging and then encoded by the mask. The ion beam also may be transformed into an atom beam in a charge exchange cell.

FIELD OF THE INVENTION 
The present invention relates to the field of lithography, and more 
particularly to lithographic machines and techniques involving ion and 
atom beams for semiconductor and other materials. 
BACKGROUND OF THE INVENTION 
The desire for semiconductor chips with memory structures denser than 256 
megabytes has created the demand for machines and techniques which allow 
the etching of ultrafine lines of impurities in semiconductors. Some of 
the techniques currently used to etch such ultrafine lines are X-ray, 
electron beam, and ion beam lithography. 
Of the aforementioned techniques, the X-ray method is believed to be the 
most advanced. Compact synchrotrons are being tested and improved as X-ray 
beam sources for use in chip making. However, X-ray lithography only has a 
feasible resolution of about 0.2 microns. 
In order to overcome this resolution barrier, an approach using particles, 
such as atoms, electrons, or ions, is preferred to an approach using 
X-rays for several reasons. First, particles do not penetrate as far below 
a semiconductor substrate surface as X-ray photons. This results in the 
ability to etch relatively finer lines on a target. Second, shorter 
wavelengths, which also enable the etching of finer lines, can be more 
easily provided for particles than for X-ray photons. In this regard, the 
energy required to produce a particle beam of a given wavelength is far 
less than that required to produce an X-ray photon beam of the same 
wavelength and intensity. 
The ion beam lithographic method is preferable over both the X-ray and the 
electron beam method because the relatively heavier ion particles couple 
well with various resist materials, do not damage an underlying circuit 
layer, scatter weakly, and allow for image demagnification. 
However, there are at least two significant problems with the ion beam 
approach. One problem is that, in order to treat solid state surfaces at a 
high rate, an ion beam of high enough current density must be used. At 
high current densities, electrostatic repulsion of ions strongly distorts 
the image or the pattern etched. A second problem is that existing ion 
sources cause high ion temperatures which can result in the thermal 
distortion of the etched image. Therefore, in order to produce a heavy 
particle beam approach to lithography which is to be competitive or better 
than current X-ray approaches, one has to first overcome the problems of 
electrostatic and thermal distortion of the etched image while producing a 
sufficiently intense particle beam. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method and apparatus 
for particle beam lithography that does not thermally and 
electrostatically distort the etched image. 
It is another object of the present invention to provide a particle beam 
that is very cold (T.sub.1 &lt;0.1 eV, where T.sub.1 =kinetic temperature of 
ions) and sufficiently intense (on the order of 10 .mu.A) for use in 
lithography. 
It is another object of the present invention to provide for ultrafine 
etching of semiconductor chips, with such etching attaining a resolution 
of less than 0.2 microns. 
It is another object of the present invention to provide for an ion and 
atom beam lithographic method and apparatus wherein a cryogenic ion source 
is used to minimize thermal distortion of an etched image. 
It is another object of the present invention to provide for an ion and 
atom beam lithographic method and apparatus wherein an ion beam is 
transformed into an atom beam in order to minimize electrostatic 
distortion of an etched image. 
In accordance with a first aspect of the invention, an ion source is 
provided in which a warm Helium plasma is created, the warm Helium ions of 
this plasma are extracted and made to collide with and transfer their 
positive charges to cold Helium gas atoms in a cryogenic charge exchange 
chamber. Preferably, parasitic warm Helium ions are removed from the cold 
ionic beam emerging from the cryogenic charge exchange chamber by use of 
an ion energy monochromator. 
In accordance with a second aspect of the invention a charge exchange cell 
is further provided which converts a Helium ion beam into Helium atom 
beam. 
Advantageously, the present invention produces a high quality converging 
ion beam with extremely low ion temperature to minimize thermal distortion 
of an etched image or pattern. Furthermore, the cryogenic ion beam 
produced in accordance with the present invention may be further 
transformed, by an ion-atom charge exchange cell, into a converging atomic 
beam, thereby to minimize distortion of an etched pattern due to 
electrostatic repulsion of ions. Either the ion or the atom beam may be 
used to etch an image or other pattern in semiconductor materials, 
structures and devices. The present invention is thus designed to retain 
all the advantages of ion beam lithography while avoiding its deficiencies 
.

DETAILED DESCRIPTION OF THE INVENTION 
An overall block diagram of an ion-atom etching apparatus 10 and method in 
accordance with the present invention is shown in FIG. 1. The ion-atom 
etching apparatus 10 includes a cryogenic ion source 12, a converging beam 
system 16, an ion-atom charge exchange cell 18, and an ion deflector 20. 
The cryogenic ion source 12 produces an ion beam through an opening 14, 
which may be an extraction orifice presenting a selected extraction 
potential, and is typically divergent. The divergent ion beam passes into 
converging beam system 16, which causes the beam to converge. System 16 
may include one or more cylindrical electrostatic or magnetic devices 17 
(two are shown) having the appropriate potential or field strength to 
cause the desired convergence. The convergent ion beam passes through a 
mask 19, which encodes the beam with the image or pattern to be etched. 
The encoded beam then may be passed into ion-atom charge exchange cell 18 
to convert most of the ions into neutral atoms. This produces a convergent 
beam of mostly neutral atoms exiting the charge exchange cell 18. The 
remaining ions may be removed by ion deflector 20. The image or pattern is 
then etched by the atoms upon a suitable target 22, e.g., a semiconductor 
22. 
FIG. 2 illustrates a detailed diagram of cryogenic ion source 12 in 
accordance with an embodiment of the present invention. Source 12 includes 
a UHF energy source 24, a plasma source chamber 28, a pumping section 34, 
a cryogenic charge exchange chamber 36, and an ion energy monochromator 
40. 
Source 24 injects UHF energy into the ion source 12 through a waveguide 26. 
The waveguide 26 is connected to a plasma source chamber 28 through a 
vacuum and electric insulator 30. Insulator 30 is preferably a dielectric 
material such as, for example, nylon or teflon, which is transparent to 
microwaves, electrically insulates chamber 28 from waveguide 26, and 
preserves a vacuum inside plasma discharge chamber 28. 
Plasma discharge chamber 28 is used for igniting and maintaining a plasma 
discharge under Electron Cyclotron Resonance (ECR) conditions. The chamber 
28 is made of a length of standard vacuumated waveguide 26, illustrated in 
FIG. 2 as 26', and a dielectric tube 32 which passes through the wall of 
chamber 28 and opens inside chamber 28. A plasma support gas (or working 
gas) is passed through tube 32 and a plasma is ignited inside tube 32 
under conditions of Electron Cyclotron Resonance. Preferably, the working 
gas pressure inside tube 32 is on the order of 10.sup.-4 Torr while the 
residual pressure is on the order of 10.sup.-6 Torr. Chamber 28 (ECR 
plasma source) has an open end, opposite insulator 30, for outputting a 
plasma flow. Any other type of plasma source may be used instead of the 
ECR plasma source 28 described. 
In the embodiment shown, each of source 24, waveguide 26, and plasma 
discharge chamber 28 are axially aligned along and about the first axis. 
Non symmetrical and non axially aligned structures also may be used, 
provided that adjustments are made in the magnetic fields as described 
below so that the desired dense plasma is provided in a stable manner for 
the purposes described herein. 
The plasma source chamber 28 is essentially disposed within a magnetic 
field that maintains the plasma discharge in tube 32 at a suitable 
density. The net magnetic field .vertline.B.vertline. along the first axis 
A is illustrated in FIG. 2. It is preferably comprised of a magnetic 
mirror having an axisymmetric magnetic profile. The field profile is 
initially established to facilitate creation of the plasma at the start of 
operation, and to form converging magnetic field lines so that the cold 
ECR plasma diffuses along the field lines on the first axis. 
The net magnetic field profile may be adjusted during operation for optimal 
operation. More specifically, the magnetic field profile is adjusted so 
that the value of the magnetic field strength inside chamber 28 during the 
start-up of operation can be varied around the ECR value for the 
UHF-microwave frequency used. As illustrated in FIG. 2, the magnetic field 
is at about the ECR value near the open end 32 of the dielectric tube 32. 
Referring to FIG. 2, in a preferred embodiment the axisymmetric magnetic 
field is generated by a magnetic device 50 and a series of magnetic 
devices 44, each of which may be an electromagnet, a permanent magnet or a 
solenoid, in any combination. Preferably, each magnetic device 50 and 44 
is a solenoid, and is hereafter referred to as a solenoid. 
Solenoid 50 provides the axially symmetric magnetic mirror in discharge 
chamber 28. The mirror ratio is preferably in the range of 1.2 to 5.0, 
more preferably between 2 and 4. Solenoid 50 is positioned in axial 
alignment with the first axis A. It is spaced with its midplane a distance 
from the end of dielectric tube 32 to provide a magnetic field strength in 
tube 32 that would be equal during the start-up to the ECR value for the 
selected microwave frequency. For example, for a frequency of 2.45 GHz, 
the magnetic field strength for the ECR value is 875 gauss, while for the 
frequency 10 GHz it is 3.6 kilogauss. 
The initial magnetic profile is provided with a field strength in tube 32, 
on the side oriented toward source 24, that is lower than the ECR value, 
and a field strength on the other side higher than the ECR value. During 
operation, solenoid 50 may be mechanically and/or electrically adjusted to 
adjust the magnetic field .vertline.B.vertline. profile in chamber 28 to 
obtain the best condition of continued operation, namely, the creation and 
the contained maintenance of a stable plasma. 
As an example of one embodiment, if the microwave frequency of 2.45 GHz is 
used, the solenoid 50 is made of water cooled copper wire and produces a 
field strength of the order of from 0 to 0.3 Tesla, preferably 0.2 Tesla 
in its central plane. It may be made of an 8.times.8 mm copper conductor 
having a 4 mm inner diameter for cooling water. Solenoid 50 has 44 turns 
having an inner diameter of 250 mm, an outer diameter of 360 mm and a 
length of 80 mm. Solenoid 50 at start-up requires 8.9 volts and 600 amps. 
Chamber 28 is evacuated to a pressure on the order of 10.sup.-5 to 
10.sup.-6 Torrs to provide for plasma ignition under ECR conditions inside 
tube 32. A conventional pump (not shown), such as turbomolecular pump 
having a pumping capacity of 1,000 L/min, may be used to create the 
reduced pressure. It should be understood that the plasma also could be 
created in other ways, by other plasma sources, and that gases other than 
helium can be used as the support material for the plasma and for the ion 
and atom beam formed therefrom. 
Solenoids 44 are located downstream of the pumping section 34 and are 
spaced along the cryogenic charge exchange chamber 36. The net magnetic 
mirror profile provides two advantages. First, it causes perpendicularly 
energetic electrons to be reflected, so that they are not likely to pass 
into charge exchange chamber 36. As a result, less thermal energy is 
brought to chamber 36. In addition, the increasing magnetic field 
stabilizes the plasma against magnetohydrodynamic (MHD) turbulence, which 
can cause the plasma to move in an unstable manner. 
After being generated in tube 32, the plasma passes through pumping section 
34. Pumping section 34 is designed to pump out, through outlet 42, any 
warm neutral atoms of Helium remaining in the flow. The charged matter is 
restrained from flowing out outlet 42 by the net magnetic field and thus 
passes through pumping section 34. 
The plasma next diffuses through a first inlet 46 into the cryogenic charge 
exchange chamber 36. The cryogenic charge exchange chamber 36 is cooled by 
liquid Helium, illustrated in FIG. 2 as horizontal dashes in area 52, 
which is located outside of and in contact with the walls bounding the 
inside of chamber 36. The Helium can be kept at liquid temperature by any 
of a number of conventional processes. 
A second inlet 46 allows Helium gas to enter the chamber 36. This Helium 
gas is injected to maintain in equilibrium the helium atom concentration 
sufficient to provide a charge exchange target, as discussed below, and is 
cooled by the chamber 36. More particularly, it is cooled by passing 
through a tube bathed in the liquid helium and by the walls of chamber 36. 
The cooled helium gas is a target gas as follows. The warm Helium plasma 
from the pumping section 34 passes into chamber 36 and the warm Helium 
plasma ions collide with the cooled neutral Helium target gas molecules 
and atoms. The collisions result in a transfer of charge to the cooled 
Helium gas particles. After the warm Helium plasma ions have lost their 
charge, they become neutral particles. The neutral particles then fall, 
contact the cooled walls of chamber 36, and lose their thermal energy. 
The cooled Helium gas atoms thus become ions at liquid Helium temperature. 
The cooled Helium ions diffuse to the extraction orifice 38 with the same 
velocity as the plasma flow rate during entry into the cryogenic chamber 
36. This is because the electron kinetic temperature does not change 
significantly between these stages, and the diffusion of ions in a plasma 
is largely dependent on the electron kinetic temperature rather than on 
the ion kinetic temperature. The electron kinetic temperature does not 
vary significantly because collisions between electrons and cold Helium 
ions are not likely. Thus, the cooled helium target gas ions provide a 
cryogenic ion flow. 
The interrelationship between ion diffusion rate and electron kinetic 
temperature will now be explained. Plasma sets up a sheathlike electrical 
potential at a boundary which forces the loss rate of electrons and ions 
to be the same. This potential is known in the art as the "ambipolar 
potential" and exists because electrons have much greater mobility than 
ions, and will thus attempt to diffuse more quickly. Electrons with higher 
kinetic temperatures attempt to diffuse more rapidly than those with lower 
kinetic temperatures causing greater ambipolar potentials, which 
inevitably cause greater ion diffusion rates. The diffusion of electrons 
and ions in this situation is known in the art as ambipolar diffusion. Ion 
diffusion is based largely on the magnitude of the ambipolar potential 
which is based upon the electron kinetic temperature, rather than on the 
ion kinetic temperature. 
It is undesirable that the ambipolar electrical potential affect the 
perpendicular energy of the ion beam which emerges from the extraction 
orifice 38, e.g., the small end of a conventional conical electrode. 
Therefore, the extraction orifice 38 is made much smaller than the plasma 
diameter, i.e., the dimensions over which the radial ambipolar electrical 
potential acts. Thus, the radial ambipolar potential seen at the 
extraction orifice 38 will be a small fraction of the entire radial 
ambipolar potential. The significance of this is discussed below. 
The ion beam which comes out of the extraction orifice 38 passes through 
cylindrical electrodes 39 for profiling the beam. It may contain some 
parasitic warm ions in addition to the desired cryogenic ions. 
Accordingly, one aspect of this embodiment of the present invention also 
provides for passing the ion beam through an energy monochromator 40 which 
selectively passes the cryogenic component of the ion beam, but not the 
thermal component of the ion beam. Monochromator 40 may be any 
conventional magnetic, electrostatic or electromagnetic type device. 
The following are examples of parameters for an ion source such as the one 
previously described. The microwave power source 24 may have a frequency 
of 2.45 GHz. At this frequency a warm Helium ECR plasma (T.sub.e =10 eV, 
T.sub.1 =1 eV, n=1.times.10.sup.10 ions/cm.sup.3, where T.sub.e is the 
kinetic temperature of electrons and T.sub.1 is the kinetic temperature of 
ions) is usually produced at a Helium gas pressure of about 
5.times.10.sup.-4 Torr without difficulty. At this pressure and at room 
temperature the density of the neutral atoms of Helium remaining in the 
plasma source chamber 28 is 1.76.times.10.sup.13 (atoms/cm.sup.3). The 
mean free path of these neutral atoms is several meters, which is much 
greater than the plasma discharge chamber 28 diameter of about 10.0 cm. 
Under the aforementioned parameter conditions, the ratio of the density of 
neutral atoms in the plasma discharge chamber 28 and the density of 
neutral atoms in the charge exchange chamber 36 is inversely proportional 
to the square root of the ratio of the temperatures. The wall temperature 
in the chamber 36 is maintained at about 4.2.degree. K. However, due to 
possible parasitic heat penetration into the chamber 36, the temperature 
of the Helium gas could be higher, and 10.degree. K. is a more realistic 
value. Therefore the density of cold Helium neutral atoms inside the 
chamber 36 is n=9.6.times.10.sup.13 atoms/cm.sup.3. A gas temperature in 
the range of from 4.2 to 20.degree. K. is suitable for the present 
invention. 
In the energy range up to 20eV, the charge exchange cross section between 
warm Helium ions and cooled Helium atoms is .sigma..sub.ce 
=2.times.10.sup.-15 cm .sup.2. Therefore the charge exchange length in the 
cryogenic charge exchange chamber 36 is: 
EQU .lambda..sub.ce =1/ =i n.times..sigma..sub.ce =5.2 cm 
All the parasitic processes caused by electron-atom and ionatom collisions 
do not exceed the geometrical cross section of the atom of Helium which is 
.sigma..sub.g =3.5 .times.10.sup.-16 cm .sup.2. Thus, parasitic processes 
will only be significant if the length of the charge exchange chamber 
exceeds a characteristic length L, which is: 
EQU L=1/n.times..sigma..sub.g =30 cm 
A length must be chosen for the charge exchange chamber 36 so that charge 
exchange is obtained and the resulting ions do not become heated by 
parasitic processes. This means that the length 2O required to obtain 
satisfactory charge exchange chamber 36 is greater than 5.2 cm and the 
length required to avoid undesired heating is less than 30 cm. A length of 
about 10 cm has been found to be suitable. 
The ion density needed from the charge exchange chamber 36, to obtain a 
current density of J=1 mA/cm.sup.2, can be calculated in the following 
manner. The ambipolar diffusion velocity at 10 eV electron temperature is: 
##EQU1## 
where k is the Boltzman constant, and M.sub.1 is the ion mass. The ion 
density necessary to obtain a current density of J=1mA/cm.sup.2 with the 
previously calculated ambipolar diffusion velocity is: 
EQU n=J/e.times.v=2.8.times.10.sup.9 (ions/cm.sup.3). 
This ion density is easily obtained from 2.45 GHz microwave source ECR 
plasma discharges. Specifically, to obtain a Helium ion flow with this ion 
density and velocity and having a diameter of 2 cm, the microwave power 
needed is: 
EQU P.sub..mu. =I.times.n.times.S.times.v=7.times.10.sup.-2 watts 
where I is the ionization potential for atomic Helium, and S is the cross 
section of the plasma flow. 
Due to losses for light emission, thermoconductivity, as well as other 
losses, the value of microwave power used is preferably raised to 
approximately 10 watts. 
At 1mA/cm.sup.2 of ion current density, the 10 .mu.A preferably supplied by 
the present invention can be provided with an extraction orifice radius of 
0.56 mm (560 .mu.m). 
In accordance with the present invention, a 0.56 mm radius orifice, taken 
together with the other parameters discussed above, will provide a 
resolution of less than 0.2 .mu.m, more preferably less than 0.1 .mu.m. 
The virtual emitter radius, R.sub.vs, which is 20.5 times the resolution 
value, can be solved from the following equation: 
##EQU2## 
where R.sub.s is the orifice radius, R.sub.ret /D.sub.rs is the 
geometrical parameter which is &lt;&lt;1, e is the ion charge, V is the 
extracting potential, k is the Boltzman constant, and T.sub.1 is the ion 
temperature. See Miller, J., Vac. Sci. Tech. B7 (5) pp. 1053-1063 
(Sept./Oct. 1989). Selecting R.sub.s =0.56 mm, V=50 kV, T.sub.1 
=10.degree. K. and R.sub.ret /D.sub.rs =0.1, we obtain R.sub.vs =0.73 
.mu.m. Dividing this number by a factor of 20.5 yields a resolution of 
0.036 .mu.m. 
The ion-atom charge exchange cell 18 is used to convert a converging ion 
beam into a converging atom beam. This is done to minimize electrostatic 
distortion of the encoded information in an ion beam. The information, 
which is the image or pattern to be etched, is coded in the momentum and 
position of each ion in a cross section of the converging ion beam. The 
coding preferably occurs by passing the beam through a mask 19 (or like 
object) containing an image or pattern. The mask may be positioned in the 
beam path by a suitable structure that holds the mask in the correct 
orientation and does not interfere with the beam. 
To minimize the distortion of the encoded information, the charge exchange 
that occurs between ions and neutral atoms during ion-atom collisions in 
the cell 18, should occur in such a way that minimizes energy and momentum 
exchange. Energy exchange may be minimized by an appropriate choice of the 
target neutral gas that is penetrated by the ion beam. Momentum exchange 
may be minimized by an appropriate choice of the target gas density and 
the length (depth) of the gas target in the cell 18. 
At least two mechanisms affect the energy and momentum exchange between a 
projectile ion and a neutral target atom. The first is the internal 
(potential) energy exchange which might be partially converted into 
kinetic energy. The second is the kinetic energy exchange that occurs as a 
result of the collision between the projectile and target. The kinetic 
energy exchange is connected with the momentum exchange, which causes 
scattering. 
An internal (potential) energy transfer takes place between a projectile 
ion and a target atom when the bound energy or ionization potential of the 
transferred electron is different in the projectile and the target 
particles. The charge exchange process can be expressed by the following 
equation: 
EQU X(+)+Y.fwdarw.X+Y(+)+E 
where X(+) is the projectile ion, Y is the target atom and E is the bound 
energy (ionization potentials) difference before and after the electron 
charge exchange. Potentially, E can be converted entirely or partially 
into the kinetic energy. 
Optimally, to eliminate the energy difference E, the nature of the 
projectile gas particle must be the same as the nature of the target gas 
particle so that X=Y. This is known as a symmetric resonant charge 
exchange process. Further, for the symmetric resonant charge exchange, the 
cross-section for the He+.SIGMA.He at an energy of collision of 50 keV (a 
value of interest for the converging atomic beam lithography of the 
present invention) corresponds to 
EQU .sigma..sub.ce =1.2.times.10.sup.-15 cm.sup.2. 
This value is much larger than the corresponding elastic scattering 
cross-section which for light elements such as Helium, and at the energy 
of collision of 50 keV is 
EQU .sigma..sub.2 =4.pi.a.sub.o.sup.2 (I/I.sub.h).sup.-1 =2.times.10.sup.-16 
cm.sup.2 
where a.sub.o is the Bohr radius, I is the ionization potential of the 
given element (helium), and I.sub.h is the ionization potential of 
hydrogen, 13.6 eV. Thus, the free path for scattering is about six times 
longer than the free path for charge exchange. This provides for selecting 
the dimensions of the charge exchange cell 18 to provide for effective 
charge exchange without any significant exchange of energy and momentum. 
Accordingly, the physical structure of the charge exchange cell 18 can be 
designed in various ways in accordance with the present invention. 
However, the depth of the target gas as delimited by the physical 
structure of cell 18, should be smaller than the free path for scattering 
but larger than the free path for charge exchange. This relation allows 
charge exchange to occur without significant momentum and energy exchange 
or scattering which would cause loss of information during the ion-atom 
beam transformation. 
The charge exchange cell 18 could be a supersonic flow of target gas jet 
which is injected transverse to the flow of the converging ion beam. Such 
a flow is conventionally obtained by accelerating an ultrasonic gas flow 
using a conventional laval nozzle (not shown). Alternatively, the target 
gas could be placed in a separate chamber which has an orifice to allow 
the ion beam to enter the chamber. 
In one embodiment of the present invention the target gas density and depth 
or the physical structure which delimits the depth of the target gas are 
chosen as follows. The mean free path .lambda. for an ion-atom or an 
atom-atom collision is: 
EQU (.lambda.=.times.n.sigma.).sup.-1 
where n is the target gas density, and .sigma. is the cross section for a 
collision process. As stated previously, the present invention is designed 
to allow charge exchange without (or with minimal) energy and momentum 
exchange. Accordingly, the depth of the target, L, should be larger than 
the mean free path for charge exchange and smaller than the mean free path 
for scattering. Accordingly, the following restrictive conditions must be 
satisfied: 
EQU .sigma..sup.-1.sub.ce &lt;nL&lt;.sigma..sub.s.sup.-1 
This means than the gas target density n times the depth of the target L 
must be greater than the inverse of the charge exchange cross section and 
less than the inverse of the scattering cross section. 
For instance, in the case of a helium ion beam with an energy of 10 keV, 
the gas target should have the following parameters: 
EQU 0.83.times.10.sup.15 cm.sup.-2 &lt;nL&lt;5.times.10.sup.15 cm.sup.-2 
In this case, if the depth of the target helium gas is 10 cm, then the 
Helium target gas should have a density of 2.times.10.sup.14 
atoms/cm.sup.3 which is well within the limits specified by the above 
equation. A Helium density of 2.times.10.sup.14 cm.sup.3 corresponds to a 
pressure of 8.times.10.sup.-3 Torr. This pressure and density can be 
realistically reached by providing differential pumping in a conventional 
manner. 
Another aspect of the present invention concerns producing masks and other 
devices useful for ultra fine etching using a converging ion beam. In this 
embodiment, low energy ions should be used for etching desired patterns of 
lines and/or holes in the mask. 
The present invention can be used with conventional fine pattern processing 
which involves the use of resist which may be exposed and developed to 
produce a mask for semiconductor doping and removed following the doping. 
The present invention also can be advantageously used to etch a pattern 
without the use of pattern transfer media such as photoresist. For 
example, direct focussed low energy atomic beam milling as a selective 
ultrafine etching of a metal film, which has been deposited on the 
substrate for subsequent diffusion, could substantially minimize the 
number of processing steps for fabricating an integrated circuit. Thus the 
present invention is also a novel approach to high throughput submicron 
integrated circuit fabrication. 
One skilled in the art will appreciate that the present invention can be 
practiced by other than the described embodiments which are presented for 
purposes of illustration and not of limitation.