SPER Device for material working

A segmented plasma excitation and recombination (SPER) device is employed in a deposition scheme in which the plasma generated in the gap between adjacent electrodes is formed into a beam of ions/atoms by flowing a background gas through the gap. The beam strikes a workpiece and deposits a layer of the vaporized electrode material thereon. Also described are techniques where the ions react with workpiece or the background gas to form a layer, as well as where the ions bombard the workpiece to etch away a layer.

CROSS-REFERENCE TO RELATED APPLICATIONS 
This application is being concurrently filed with application Ser. No. 
389,779 entitled "CONTINUOUS WAVE SPER LASER" by J. J. Macklin, W. T. 
Silfvast and O. R. Wood II. 
BACKGROUND OF THE INVENTION 
This invention relates to techniques for material working including 
material deposition, reaction, and etching. 
Techniques for metal deposition span nearly two centuries of scientific 
exploration beginning with the discovery of gold electroplating in the 
early nineteenth century to recent demonstrations of single crystal 
aluminum growth utilizing molecular beam epitaxy. Many of these techniques 
are described in a book entitled, Thin Film Technology, D. Van Nostrand 
Company, Inc., Princeton, N.J. (1968) by R. W. Berry, et al. They 
categorize metal deposition techniques into three broad categories: 
(1) Chemical processes such as electrodeposition, chemical reduction, 
electroplating, and chemical vapor plating; 
(2) Evaporation which is accomplished by using vacuum pumps to reduce the 
pressure inside a deposition chamber and then heating the metal to be 
evaporated in a filament or boat made of a high-melting point material. 
The heat is supplied typically by resistant heating, radio frequency 
induction, or electron bombardment. Molecular beam epitaxy deposition, 
mentioned above, falls within this genus; and 
(3) Cathode sputtering which is similar to evaporation in that a partial 
vacuum is required but is dissimilar in that thermal energy is used in 
evaporation, whereas ion bombardment of the metal, causing ejection of 
electrons, is used for sputtering. 
Both evaporation and sputtering have been practiced in the presence of a 
glow discharge as described by Berry et al., supra, pages 156-157 and 
204-208. This variant of the evaporation technique has been termed "ion 
plating." Briefly, it entails establishing a glow discharge region between 
an evaporator filament and a cathode substrate. As the vapor atoms pass 
through the glow discharge, some are ionized and are accelerated to the 
substrate where metal deposition takes place. 
Each of these prior art techniques, however, suffers from one or more of 
the following disadvantages: inefficiency in that relatively high currents 
produced slow deposition rates and thereby increase both processing time 
and the likelihood of contamination; the need for maintaining relatively 
high vacuums and the concomitant cost of equipment to do so; lack of 
precise control as to deposition time (hence layer thickness) and 
deposition direction (hence layer patterning); contamination when chemical 
solutions contact the workpiece or from boats used to carry the evaporant; 
and difficulty in depositing relatively high-melting point (low vapor 
pressure) metals. 
SUMMARY OF THE INVENTION 
In the course of our experiments with segmented plasma excitation and 
recombination (SPER) lasers, we discovered that the plasma of metal ions 
generated in the gap between adjacent electrodes can be utilized to 
deposit a metal layer or coating on a workpiece. In a preferred 
embodiment, a background gas is rapidly flowed through an aperture in the 
gap so as to carry the plasma downstream, away from the electrodes. The 
plasma forms a collimated beam which strikes the workpiece and deposits a 
metal layer thereon. This technique is also applicable to the deposition 
of nonmetallic layers, for example semiconductor layers, provided that the 
requisite plasma can be generated. 
Due to the relatively low currents required to generate the plasma, our 
invention has the advantage of being able to efficiently deposit metal 
layers at reasonably high rates, thereby decreasing processing time and 
the likelihood of contamination. Because that current can readily be 
turned on and off in short periods of time, abrupt transitions can be made 
to precisely control layer thickness. Further control, in the form of 
directionality, arises from the fact that the beam-shaped plasma permits 
localized deposition. In addition, our technique does not require a high 
vacuum and is compatible with the deposition of relatively high-melting 
point materials such as copper and aluminum. 
Furthermore, our invention is not merely applicable to material deposition; 
that is, a layer can be formed on the workpiece not only by deposition but 
also by reaction of the beam with the workpiece or with the background 
gas. Moreover, a surface layer can be removed from the workpiece by 
utilizing an electric field to accelerate the ions toward the workpiece. 
These ions bombard the workpiece and thereby etch away the surface layer.

DETAILED DESCRIPTION 
With reference now th the FIGURE, there is shown a chamber 10 having a 
relatively narrow gas inlet 12 in one endplate 16 and a wider gas outlet 
14 in the opposite endplate 18. Typically inlet 12 is connected to a 
source of background gas (e.g., He, not shown) and outlet 14 is connected 
to a pump (also not shown). Gas introduced through inlet 12 flows through 
a narrow pipe 20 to an aperture 24 in a SPER device 22. A port 26 allows a 
workpiece 28, mounted on a holder 30, to be positioned in the chamber 10. 
The position of the workpiece is controlled by a manipulator 32 shown 
schematically as a mechanical device capable of both vertical and 
horizontal motion as shown by arrows 34. When a suitable electrical signal 
is applied across leads 36 and 38, SPER device 28 generates an ion plasma 
from vaporized material of its electrodes 101-102. This plasma takes the 
shape of a beam 40 which strikes workpiece 28 and deposits a layer of the 
vaporized material thereon. 
SPER device 22 is of the type described in four copending applications, all 
of which are assigned to the assignee hereof: Ser. No. 82,308 filed on 
Oct. 5, 1979 (W. T. Silfvast et al., now U.S. Pat. No. 4,336,506; Ser. No. 
367,216 filed on Apr. 9, 1982 (J. J. Macklin et al.); Ser. No. 367,092 
filed on Apr. 9, 1982 (W. T. Silfvast et al.); and Ser. No. 389,779 
concurrently filed herewith (J. J. Macklin et al.). These applications are 
incorporated by reference herein. SPER device 22 comprises at least two 
strip electrodes 101-102 positioned in tandem on an electrically 
insulating substrate 120 in such a manner as to leave a small gap between 
adjacent strips. This gap provides a discharge path between the strips 
when a suitable electrical signal is applied thereto. Aperture 24 is 
formed in the gap. This electrode arrangement is mounted in chamber 10. A 
high voltage supply 130 and a low voltage supply 132 are connectable in 
series across the electrodes via switches 134 and 136, respectively. The 
high voltage supply typically provides a high power pulse (e.g., a few kV 
at 20-50 mA for .about.1 msec) to pre-ionize the gap, after which low 
voltage supply 132 provides a lower power signal suitable for longer 
duration operation (e.g., 20 V D.C. at 3-6 A for .about.1 sec). A 
technique for eliminating the high voltage supply 130 and operating with 
low voltage supply 132 only is described in our application to be filed. 
For still longer duration operation, well-known cooling means (not shown) 
should be incorporated to prevent the electrodes from overheating and 
melting. This excitation produces a bright ion plasma of vaporized 
electrode material in the gap. The background gas flowing through aperture 
24 causes this plasma to take the shape of collimated beam 40 which 
strikes workpiece 28. Of course, the flowrate of the gas (e.g., 500 l/min) 
and the size of the aperture (e.g., a 0.25 mm diameter hole) determine the 
shape and length of beam 40. 
The entirety of each strip need not constitute a material which is 
vaporizable into a plasma. As described in our copending applications Ser. 
Nos. 367,092 and 367,216, it is sufficient if the cathode ends constitute 
such a material and that the anode ends constitute a nonvaporizable 
material under the operating condition of the device. Although the 
vaporizable material typically comprises a metal, other materials such as 
semiconductors are also suitable. Moreover, strips of different 
vaporizable materials can be mixed within a single device so as to yield a 
plasma comprising ions of more than one material. In this fashion a 
composite layer (e.g., an amalgam, alloy, or crystal) can be deposited on 
workpiece 28. To this end it may be desirable to heat the workpiece so 
that deposition takes place at an elevated temperature rather than at room 
temperature. 
The composition of the beam (i.e., the ratio of ionized species to neutral 
species in the plasma) is a function of distance from the gap. At points 
farther away from the gap (toward the workpiece) the ratio decreases, 
being illustratively about 30:1 at 1-2 cm from the gap and lower at, say, 
7-8 cm. At these larger distances some of the neutral species may even 
form clusters. Thus, by varying the distance between SPER device 22 and 
workpiece 28 it is possible to vary the character of the deposited layer. 
For example, for metal deposition it is possible to form a soot-like layer 
as well as a more metallic-like layer. This feature may be of particular 
importance in the fabrication of optical disc or video disc recording 
media. In addition, it should be noted that the beam composition can be 
controlled to varying degrees with other parameters (e.g., gas pressure in 
chamber 10 or in nozzle 24). 
As discussed in the aforementioned copending applications, ion plasmas have 
been generated in SPER devices utilizing a variety of electrode materials 
including, for example, Ag, Bi, C, Ca, Cd, Cu, In, Mg, Pb, Sn, Zn, Li, Al, 
and Ni. 
EXAMPLE 
This example describes the deposition of a metal layer on an insulating 
workpiece. Materials, dimensions, as well as other device parameters, and 
operating conditions are provided by way of illustrations only and, unless 
otherwise stated, are not intended to limit the scope of our invention. 
A Cd rod (0.75 in. long by 10 mm diameter) was spherically shaped at one 
end and then cut along the axis of the rod to form two electrodes 101 and 
102. These electrodes were mounted on a glass substrate 120 with their 
axes aligned. The hemispherical ends of the electrodes faced one another 
and were separated by a 2.5 mm gap. A 0.25 mm diameter hole was drilled in 
the gap to form aperture 24, which acted as a nozzle for He gas flowing in 
conduit 20. The workpiece 28 was a glass slide and was positioned between 
1 and 8 cm from SPER device 22. 
In operation, high voltage supply 130 provided a few kV at 20-50 mA for 
.about.1 msec to pre-ionize the gap, and then lower voltage supply 132 
provided about 20 V D.C. at about 3-6 A for 0.25-0.50 sec. With even this 
relatively low current level we were able to deposit 1000-2000 .ANG. thick 
Cd layers on the glass slide. This deposition was equivalent to about 
10.sup.-3 g/cm.sup.2 /sec. With higher currents, we expect that 
considerably faster deposition rates are possible. 
It is to be understood that the above-described arrangements are merely 
illustrative of the many possible specific embodiments which can be 
devised to represent application of the principles of the invention. 
Numerous and varied other arrangements can be devised in accordance with 
these principles by those skilled in the art without departing from the 
spirit and scope of the invention. In particular, the fact that our 
invention generates ions (rather than neutral species as in evaporation 
processes) implies that it may be possible to react two or more ions on 
the surface of a workpiece or even to react an ion with the workpiece 
itself (e.g., if the ion were a metal and the workpiece a semiconductor). 
Moreover, it may be possible to react the ions with the background gas to 
deposit compounds on the workpiece (e.g., use of Si electrodes and H.sub.2 
gas may make it possible to deposit hydrogenated polysilicon). 
Alternatively, the ions could be employed to etch the workpiece by 
employing an electric field to accelerate the ions toward the workpiece. 
The accelerated ions are thus provided with sufficient energy to effect 
etching. In this case, because the ions are positively charged, a voltage 
supply 140 is connected (via switch 142) across the workpiece 28 and SPER 
device 22 to place the former at a more negative potential with respect to 
the latter. In addition, selective etching of the workpiece can be 
realized by writing thereon with an electron beam. The electrons being 
negatively charged attract the positive ions in the beam causing etching 
to occur preferentially in the regions where the electron beam strikes the 
workpiece. Finally, because the beam 40 contains ions, conventional 
electric or magnetic field deflection techniques can be used to scan or 
focus the ions in the beam.