Dual ion beam deposition of dense films

A novel dual ion beam sputtering process for depositing thin films of high density is described. One of the ion beams contains relatively heavy sputtering ions, such as argon ions, for ejecting atoms from a target. The second ion beam is also directed at the target and contains ions having energies of at least 3 electron volts and less than 20 electron volts. The products of the beams are collected on a substrate as a thin film. High density, hydrogenated amorphous semiconductor films, oxide and nitride films, and other films, may be deposited according to the process. The films have densities nearly equal those observed for bulk samples of the same materials. Hydrogenated amorphous silicon films deposited by the process exhibit enhanced photoconductivity.

BACKGROUND 
The technique of thin film deposition by ion beam sputtering is well 
established. In the typical process an ion beam of relatively heavy ions 
is directed at a target to cause ejection of atomic particles. These 
particles are collected on a substrate to form a film. In some variations 
of the technique, two ion beam sources are used; usually a sputtering beam 
is directed at a target and the second beam is directed at the depositing 
film. For a general description of these techniques see Weissmantel, et 
al., Preparation of Hard Coatings By Ion Beam Methods, 63 Thin Film 
Solids, 315-325 (1979), which is incorporated herein by reference. 
An improved technique for dual beam deposition of thin films has recently 
been devised. In this technique, a sputtering ion beam is directed at a 
target and the second ion beam is also directed at the target, rather than 
at the depositing film. For example, the target may be silicon and the 
second beam may be hydrogen ions. Improved hydrogenated amorphous silicon 
films may be prepared by this process. See U.S. patent application Ser. 
No. 647,208 filed Sept. 4, 1984, assigned to the assignee of the present 
application, the disclosure of which is incorporated herein by reference. 
This new technique may also be used to deposit hydrogenated semiconductor 
alloys by employing two ion beam sources for each element of the alloy 
film. See U.S. patent application Ser. No. 653,168 filed Sept. 28, 1984 
assigned to the assignee of the present application, the disclosure of 
which is incorporated herein by reference. 
While the referenced techniques can provide electronic quality 
semiconductor films, it is also desirable to deposit high density films 
for use as diffusion masks in integrated circuit manufacture and for use 
as corrosion and wear resistant coatings on metals. Amorphous films of 
acceptable density for such applications are usually achieved only by high 
temperature post-deposition treatments. The temperatures necessary to 
improve film density can have a detrimental effect on the substrate, 
particularly if the substrate is an integrated circuit. Accordingly, there 
is a need for a low temperature technique for depositing dense amorphous 
films. 
SUMMARY OF THE INVENTION 
In the invention a novel dual ion beam process is used to deposit thin 
films, for example, amorphous hydrogenated silicon, silicon oxides, 
nitrides and other compounds, having densities very near the bulk values 
of the deposited materials, on substrates maintained at temperatures 
between 20.degree. C. and 400.degree. C. In the novel process a beam of 
relatively heavy sputtering ions, such as argon ions, is directed at a 
target. A second beam of ions of relatively low energy is also directed at 
the target. These low energy ions, may be hydrogen, oxygen, nitrogen or 
other ions and typically have energies less than about 20 electron volts, 
but not less than about 4 electron volts. 
The low energy ions combine with the target atoms or atoms sputtered from 
the target to form the deposition species either as atoms or molecules 
that are collected on a substrate. The deposition species impact the 
substrate with relatively low energy depositing a much denser film than 
otherwise observed in deposition by sputtering. 
In contrast to known dual ion beam processes, in the new process, the low 
arrival energy of the deposition species reduces structural disorder or 
defect formation in the deposited film producing the higher density. 
The resulting thin films show densities and dielectric constants at or very 
near the values usually measured only for bulk samples of the deposited 
materials. Hydrogenated amorphous silicon films deposited according to the 
process show increases in photoconductivity by about an order of magnitude 
or more over the best similar films deposited by previously known dual ion 
beam techniques.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
An arrangement for carrying out the novel process is shown in FIG. 1. A 
sputter target 1, which is gradually consumed in the sputtering process, 
is disposed opposite a substrate 3, on which the deposition species 
produced in the process are collected to form a film. A beam 5 of 
relatively heavy sputtering ions is directed at target 1 from a 
conventional ion gun, not shown. The ion gun may be of the Kaufman type in 
which ions are formed in crossed electrical and magnetic fields. Electrons 
emitted from a central cathode follow spiral paths through the magnetic 
field toward a circumferential anode. The electrons form ions from gaseous 
atoms introduced into the gun between cathode and anode. The ions formed 
are accelerated out of the gun by an electrically charged accelerating 
grid. Just inside the gun from the accelerating grid is a screen grid 
maintained at a slightly different potential than the accelerating grid to 
protect the accelerating grid from ion impacts. Typically the sputtering 
ions are argon ions supplied to the beam source as argon gas. Other noble 
gases can be used as ion sources as well. The voltage applied to the ions 
in the sputtering beam source ranges from 750 to 1500 volts. 
A second beam 7 of low energy ions is directed at target 1 from a different 
direction. To achieve the best results, I prefer that the low energy ions 
have energies not greater than about 20 electron volts and no less than 
about 3 to 4 electron volts. A conventional Kaufman ion source cannot 
produce such low energy ions while sustaining a desired beam current. 
Therefore, I have found it necessary to modify a 2.5 cm. Kaufman source to 
produce ions with the desired energy at an acceptable beam current. In 
order to increase the current of the beam at low voltage, the screen grid 
is removed. As a result, the width of the dark space in the plasma within 
the Kaufman gun determines the beam current. The width of the dark space 
varies in a complex way with the absolute value of the anode voltage, 
permitting the beam current to be maintained at a desired level, although 
the voltage on the anode is reduced to produce ion energies between 3 and 
20 electron volts. The ions in the second beam are also derived from a gas 
which may be elemental, such as hydrogen, oxygen or nitrogen, or a 
compound, such as silane and germane. 
In the arrangement of FIG. 1, substrate 1 may be heated, for example, by an 
electrical current flowing through a substrate holder or by heat lamps to 
aid the deposition process. Typically, the range of substrate temperatures 
used in the novel process are 20.degree. C. to 400.degree. C. The 
substrate may, for example, be glass, quartz, silicon, a metal or metal 
alloy. Typically, the vacuum in which the process is performed is 
maintained at about 0.13 to 1.33.times.10.sup.-3 pascal. 
In the arrangement of FIG. 1, the target may be silicon, germanium, 
molybdenum, nickel, etc. The ions in the low energy beam may be hydrogen, 
oxygen, nitrogen, fluorine, etc. By choosing the proper combinations from 
these exemplary lists, films of silicon oxides, hydrogenated silicon, 
nitrides, germanium-silicon alloys and other compounds and alloys can be 
produced. 
I have deposited numerous films of silicon oxides at room temperature by 
the novel process using argon ions as a sputtering beam, a silicon target 
and a low energy oxygen ion beam. Those oxide films have been analyzed by 
spectroscopic ellipsometry and show the same dielectric constant as does 
bulk silicon dioxide, even when the films are very thin. The same 
technique was used to measure the density of the films and demonstrated 
that the films had already reached their maximum density when only 2 
nanometers thick. Using x-ray spectroscopy techniques, it was determined 
that the film had a formula of SiO.sub.x where x equalled 2.0. 
Many hydrogenated amorphous silicon films have been deposited by the new 
process using an argon beam, a silicon target and low energy hydrogen 
beam. These films not only have higher densities than other dual ion beam 
sputtered hydrogenated silicon films, but show substantially improved 
photoconductivities. In FIG. 2, the measured dark conductivity (broken 
line) and photoconductivity (solid line) are plotted for films deposited 
according to the new process and by a known dual ion beam process in which 
both beams are directed at the target. The latter process is disclosed in 
U.S. patent application Ser. No. 647,208 filed Sept. 4, 1984 now abandoned 
in favor of U.S. patent application Ser. No. 814,837, filed Dec. 30, 1985. 
In that application, the hydrogen beam voltage employed was 100, 150 or 
200 volts and the deposited films showed an optimized quality at a beam 
voltage of 150 volts. Those films showed maximum logarithmic dark 
conductivities and photoconductivities of -11 and -6.8 respectively. Films 
prepared according to that earlier disclosed process are shown in FIG. 2 
for hydrogen ion beam voltages of about 100 volts or more. 
Films of hydrogenated amorphous silicon prepared according to the novel 
process are shown in FIG. 2 for the plotted data points corresponding to 
hydrogen beam voltages less than about 40 volts. (The logarithm of dark 
conductivity is displayed on the right ordinate and the logarithm of 
photoconductivity is displayed on the left ordinate). As is apparent from 
the graph, both dark current and photoconductivity increase dramatically, 
more than an order of magnitude, when the hydrogen beam voltage drops 
below about 20 volts, i.e. when the energy of the ions is below 20 
electron volts. As indicated on FIG. 2, the deposition results plotted 
there were all for substrate temperatures of 150.degree. C., sputtering 
beams of argon ions at 1000 volts and 30 milliampere currents and hydrogen 
beam currents of 10 milliamperes. The amount of hydrogen incorporated in 
the films decreases sharply as the hydrogen beam voltage is reduced to the 
range of the novel process. See FIG. 3. 
In order to optimize the improved results shown in FIG. 2, I find it 
important to avoid contamination of the depositing films by heavy metal 
ions. While the arrangement of FIG. 1 shows the sputtering and low energy 
ion beams striking only the target, in practice some of the ions in the 
beams miss the target and can strike the walls of the vacuum chamber in 
which the deposition takes place. Since the typical vacuum chamber may 
have a metal wall or base plate, the stray ions can sputter heavy metal 
ions into the chamber and onto the substrate. To avoid this source of 
contamination, I prefer to use a curtain of high purity silicon wafers to 
shroud the volume between the target and substrate. 
A hydrogenated silicon-germanium alloy film may be deposited by using a 
silicon target and a low energy beam of ions formed from germane gas. 
The precise mechanism resulting in the deposition of the high density 
amorphous films is not known. In the typical sputtering process a growing 
film is constantly bombarded with sputtered atoms or electrons of 
relatively high energy. These atoms may well cause some sputtering or 
structural damage the depositing film, such as the creation of voids, 
reducing its density. In addition, some of the sputtering gas atoms may be 
incorporated in the depositing film. In the novel process the low energy 
ions interact with the target and/or with atoms sputtered from the target 
to form the depositing species. The substrate is not bombarded with 
species having energies that may damage the film. In addition, the number 
of sputtering gas ions incorporated into films prepared according to the 
novel process is quite low. A typical secondary ion mass spectroscopy 
(SIMS) measurement of films deposited according to the invention shows 
about 10.sup.-3 percent of argon in a hydrogenated amorphous silicon film. 
The invention has been described with reference to certain preferred 
embodiments. Various modifications and additions without departing from 
the spirit of the invention will occur to those of skill in the art. 
Accordingly, the scope of the invention is limited solely by the following 
claims.