US3294583A - Process of coating a silicon semiconductor with indium using an ion beam - Google Patents

Description

L. FEDOWS-FEDOTOWSKY PROCESS OF COATING A SILICON SEMICOND Dec. 27, 1966 3,294,583

UCTOR WITH INDIUM USING AN ION BEAM IN VEN TOR F EDOTOWSKY Chi/M66; amd 244%,

Filed June 14, 1962 HIS ATTORNEYS HROCESS F (IDATING A SZLICGN SEMECGNDUC- TOR WETH lNDlUM USING AN IUN BEAM Leonid Fedows-Fedotowsky, North Adams, Mass, assignor to Sprague Electric Company, North Adams, Mass,

- p a corporation of Massachusetts Filed June 14, 1962, Ser- No. 2ii2,5t)4 1 Claim. (Cl. 117-227) This invention relates to the formation of contacts or electrodes and more particularly to the production of contacts or electrodes on semiconductor devices.

The bringing together of the contact material and a semiconductor is a critically sensitive operation. It is complicated by the design requirements of the product, the limitations of space caused by the minuteness of the devices. the characteristics of the materials used and the desirability of automatic operation.

The common prior art techniques for applying contacts to semiconductors include evaporative plating, chemical platin and electroplating'using local contacts. Evaporative plating requires cumbersome equipment utilizing interposed masks between the source and the surface to be contacted. This technique also is limited to materials having comparatively low vapor pressures, such as indium and aluminum. Materials such as phosphorus, arsenic and antimony have vapor pressures too high for this method. 'Chemi-plating has proved to be unsatisfactory in that the layers produced have typically been too thin. Electroplating, using local contacts, produces uneven plating because the local contacts, when coupled with the relatively poor conductivity of the semiconductor material, results in a non-uniform current density across the surface to be plated. Furthermore, whenever liquids are used during contact formation the likelihood of contamination is greatest.

It is an object of the present invention to overcome the foregoing and related problems.

Another object is to present a method for forming rectifying or ohmic contacts on a semiconductor.

Still another object is the formation of contacts of controlled shape and thickness.

Yet another object is the formation of contacts in the absence of mechanical pressure on the semiconductor.

A further object of the instant invention is the formationof alloys of metals and metalloids on a semiconductor.

' A still further object is the deposit of a dielectric on a semiconductor.

Another object is the deposit of a contact on a semiconductor without the aid of interposed masks.

Other objects and advantages of the present invention will be made obvious to those skilled in the art by the following description when considered in relation to the accompanying drawing, in which:

The sole figure is a sectional view of an apparatus employed in the process of this invention.

In general, this invention involves the ionization of a material in an ion source, (indium trimethyl, In(CH will be used for purposes of illustration), extraction of the ions from the ion source, acceleration of the ions, shape definition of the ion beam, separation of the non-metallic ions from the indium ions, correction of spherical aberration, deceleration of the ions and the simultaneous discharge of the indium ions and deposit of indium metal on the semiconductor target and collection of the separated lighter ions.

The overall aspects of this invention will be best understood by reference to the drawing which shows an embodiment of the apparatus employed to illustrate the process of this invention. This apparatus comprises a United States Patent 0 3,294,583 Patented Dec. 27, 1966 main chamber it in which the ions are produced, extracted, accelerated, defined, focused, separated and decelerated and a target chamber 11 in which the ions are discharged, the metal deposited and the lighter ions collected. The two chambers are maintained during operation under a vacuum of between 10- and 10 mm. Hg by means of vacuum pumps 20. The chambers are separated by a lock 21. This lock permits sealing of the main chamber it while the semiconductor target 22 is being replaced with a new target in the target chamber 11.

The main chamber 10 is made up of any suitable ion source 12, an ion extracting electrode 13, a shape defining electrode 14, an einzel lens 15, a magnetic deflector 16, an electric prism deflector 17 and an ion decelerator 18. The target chamber 11 comprises a chamber lock 21, a semiconductor target 22, a target holder 23, a microscope 24, a viewing window 25, a mirror 26 and a light ion or proton collector 27. The various components of the two chambers are energized by appropriate voltages transmitted through lead-wires 19.

In the interest of clarity, einzel lens 15 and the combination of magnetic deflector i6 and electric prism deflector 17 have been shown as separate components. It is to be understood, however, that in actual practice 16 and 17 would be combined with the third electrode of the einzel lens in order to conserve space.

A preferred ion source employed in the instant process is a capillary-arc ion source. This device comprises a tungsten cathode and an anode, each positioned in di ametrically opposed chambers. The chambers are connected by a capillary restriction, within which the ionization of gas molecules takes place, see A Handbook on Mass Spectroscopy, by Mark G. Inghram et al., Nuclear Science Series Report No. 14, National Academy of SciencesNational Research Council, Washington, DC. 1954, pages 29 and 30. The particular type of ion source 12 contemplated herein is not critical. Considerations of operational convenience generally will dictate which of the many literature-described ion sources can best be adapted to the present process, see e.g. Inghram et al., supra, pages 32 and 33 (FIG. 22) and Modern Mass Spectrometry, G. P. Barnard, The Institute of Physics, London 1%3, pages 56-58 (FIG. 23). Other types of ion sources employing high temperatures in evaporating crucibles, or high voltages in discharge chambers, or gas discharges in radio frequency fields, etc. may also be used.

Example Using indium as the metal to be deposited on the semiconductor target and using the above-described capillary discharge ion source the process is as follows: The metal is introduced into ion source 12 as a compound of the metal in vapor form. Indium trimethyl, which sublimes at room temperature, is the indium compound employed. The cathode ray ionizes the molecules into In+ ions and charged fragments of CH e.g. C-H fragments and I-I+ ions. For convenience the fragments of CH will be referred to as protons since a large proportion are H+ ions. An ion extracting electrode 13 removes the ions from the capillary restriction of the ion source and directs the ions to the shape-defining electrode 14. The apertures of the shape-defining electrode and the ion extracting electrode are in alignment. The instant ion-optical system will faithfully reproduce on the target but in reduced scale the shape of the aperture in the ion beam shape-defining electrode. The shape of the electrode desired will determine the shape of the aperture, e.g. circular, square, rectangular and any of countless odd shapes. The shape of the aperture in 14 of the drawing is square.

After passing through the shape-defining electrode the ion beam is focused by einzel lens 15. The design of the focusing lens is facilitated by the fact that the trajectories of the focused ions do not depend on the specific charge 6/111 of the ions, and hence a lens designed for focusing electron rays can be successfully used for focusing various ions. A size reduction in the order of up to about 10 times is required and this can be accomplished with a single lens. A three tube einzel lens (i.e., a single lens) has been selected as the preferred focusing lens 15. Lenses of this type are described by Klemperer in Elec tron Optics, University Press, Cambridge 1953, in his discussion of saddle-field lenses. The voltage V applied to the inner tube of the lens is equal to 0.2 of the voltage V applied to the outer tubes. The voltages applied to both outer tubes are equal. By changing the voltage V the focal length of the lens can be altered.

Next in sequence the protons are separated from the metal ions. This is accomplished by the simultaneous action of a magnetic deflector 16 and an electric prism deflector 17. By means of themagnetic field the lighter H+ ion beam is curved away from the target to a greater extent than the heavier metal ion beam. The resolution of 16 should not be so great as to cause separation of metal isotopes, as in mass spectroscopy, but only great enough to cause separation of the protons from the metal ions. This is readily accomplished because of the great difference in their respective masses. The magnetic deflector consists of two relatively small magnets positioned about a non-magnetic stainless-steel tube, which tube is part of electric prism deflector 17 in the instant illustration.

Since the magnetic field will also act upon the heavier metal ions, though not to the same extent as on the protons, this action must be compensated for in order to reduce spherical aberration. Compensation is brought about by the action of the electric prism deflector 17 which deflects the metal ion beam so that the metal ions strike the target at the same point they would have struck had the magnetic field not acted upon them.

In order to form an adherent metal deposit on the target it is necessary to decelerate the indium ions. The indium ions acquire an energy of several thousand electron volts during their passage from the ion source 12 through the magnetic deflector 16 and the electric prism deflector 17. This corresponds to velocities in the order of 10 cm./sec. At such ahigh velocity an adherent deposit will not be formed on the target. The particles will strike with such force as to rebound, taking away small pieces of the target surface in the process. To overcome this difliculty the ions are slowed down to thermal velocities of a few electron volts. This is accomplished by interposing tubes or rings 18 between the deflectors and the target. The rings have gradually diminishing potentials, with that of the last ring being close to the potential of the ion source 12. Ions passing through ion decelerator 18 are slowed down to a point where they will strike the target, discharge and adhere thereto. The deposition can be observed by means of micro-scope 24. The target 22 is connected by lead-wire 29 to an apparatus (not shown) for reading the ion current. Proton collector 27 is likewise connected to such an apparatus via lead-wire 29.

By the foregoing example an indium dot was formed having the dimensions 4 x 4 x 2 mils. This equals a volume of X A. The number of atoms in the dot are 1.7 x 10 The ions will discharge Q=N.e coulombs which equal 2.7 X 10 coulombs. The dot was formed in 10 seconds with a beam current of 270 microamperes, using a volume of 0.64 mrn. of indium trimethyl.

In addition to metals, metalloids such as phosphorus, arsenic, antimony, etc. and dielectric elements such as sulfur may also be employed. The following list of compounds is exemplary of those which may be used in the present process: A161 SiF PF SP Ni(CO) Ga(CH Ge(CH AsF Cd(CH SnCl SbCl Pb(CH3) etc.

The adherence of the electrode deposit to the semiconductor may be inhibited by the presence of an oxide film or some other contamination on the surface of the semiconductor. These unwanted surface layers can be removed by directing an electron beam or an ion beam of a neutral gas, e.g. helium, to the electrode area prior to deposition. The light ion or proton beam separated from the metal, metalloid or dielectric element ion beam may also be directed against the electrode area to clean the surface thereof. This may be accomplished by means of electric prism deflector 17. When using an electron or light ion beam for cleaning purposes the potential of the decelerating electrodes 18 is decreased in order to take advantage of the high speed of the particles. The electron or ion beam can also be employed to clean the rim or periphery of an electrode. This produces the same eflect as chemical etching, widely-known in the art.

Dielectrics can also be deposited on semiconductors. Sulfur, for example, can be deposited on the semiconductor surface in order to sulfurize the surface. It is an excellent insulator, having a resistivity of 10 ohm-cm. Sulfur can be used in forming a space filling material over which interconnecting electrodes can be deposited as bridging conductors. The dielectric is deposited between two or more electrodes or contacts and then one or more strips or bridges are formed over the dielectric. In the case of sulfur it can be vaporized later, since its low melting point makes it unusable for semiconducting devices as a permanent insulating material.

Alloys can be deposited by introducing a mixture of I two or more gases into the ion source. In this modification a double magnetic deflection system is employed. The first magnetic deflector separates the lighter ions or pro tons from the two or more metal ion beams. The second magnetic deflector collects the metal ion beams on the target into one point or image. When employing this modification it is preferred to employ the electric prism deflector in conjunction with the two magnetic deflectors because, it is then possible to direct the mixed metal ion beam to any given location on the target. This mobility would be sacrificed by eliminating the electric prism deflector.

The contacts formed by the present process may be rectifying or ohmic depending upon the metal deposited and the character of the semiconductor. Alloys of a metal and a metalloid can be formed on the semiconductor by the successive deposition of the metal first, e.g. lead, and then over this deposit a deposit, slightly smaller in area, of a metalloid, e.g. arsenic. This is then followed by a heating step to alloy the two layers. In this modification either a single or a multiple ion source may be employed.

In forming contacts which are an alloy of two or more metals or of a metal and a metalloid, a heating unit can be employed within chamber 11 so that alloying can take place during or after deposition. In some instances the heat produced on the target due to the kinetice energy of the discharging ions will be high enough to cause alloying. It is, of course, also contemplated to alloy outside of the chamber.

By using an assembly such as that shown in chamber 10 on both sides of a semiconductor, it is possible to form transistor type devices. Such devices can also be formed by using a single assembly, as in the drawing, and after deposition on one side of the semiconductor target, the target can be turned or flipped over for deposition on the other side.

The present process is particularly effective in forming a good contact between a comparatively high melting metal and a semiconductor. Normally there is a considerable thermal expansion coeflicient differential between such materials which causes cracks to form in the contact. Two such materials are aluminum and silicon. In forming a strain-free contact between such material, aluminum is deposited on a silicon wafer, as in the example above, except that a heater means is employed to heat the silicon. This causes a surface alloying of the aluminum and the silicon. After cooling, a second layer of aluminum is deposited over the first, with heating to cause a blending of the two aluminum layers. Thereafter a small dot of tin or lead is deposited on the aluminum and a lead-wire soldered thereto. An electrode formed in this manner will not exhibit a tendency to crack because of a difference in thermal expansion.

While the foregoing description has been made with reference to the formation of contacts or electrodes on semiconductors, it is obvious that the process has utility in any instance where a thin film of the recited elements or alloys is required upon a solid substrate. It will be evident to those skilled in the art that many variations are possible within the spirit of the invention. There is no intention to limit the invention except as defined by the following claim.

What is claimed is:

A process for treating in a partial vacuum the surface of a silicon semiconductor body comprising:

(a) ionizing in an ion source an indium compound comprising indium and at least one other ion having a mass less than indium, in combined form therewith, whereby to form ions of indium and said lighter ions; 1

(b) drawing a beam of the ions out of the ion source by means of an ion extracting electrode;

(c) subsequently directing said beam through a shapedefining electrode so that said beam will assume the shape of the aperture thereof;

((1) subsequently passing the ion beam through a focusing lens designed to effect a size reduction in the crosssection of said beam of up to about ten times;

(e) subsequently subjecting said ion beam to the action of a magnetic deflector to separate the lighter ions from the heavier ions at an energy of several thousand electron volts and a velocity of the order of crn./sec.;

(f) subsequently subjecting the separated ion beams to the action of an electric prism deflector to compensate for the magnetic deflection 0f the heavier ion beam;

( and then passing the ion beams through an ion deoelerator; and, decreasing the ion velocity to thermal velocities of the order of a few electron volts and,

(h) finally impinging and discharging said heavier ion beam on said silicon semiconductor body to form an adherent layer thereon while simultaneously passing the separated lighter ions to a collector.

References Cited by the Examiner UNITED STATES PATENTS FOREIGN PATENTS 10/1958 Germany.

1/1957 Great Britain.

OTHER REFERENCES Powell et 'al.: Vapor Plating, pp. -46, 1955.

JOHN H. MACK, Primary Examiner.

MURRAY TILLMAN, WINSTON A. DOUGLAS,

Examiners.

G. E. BATTIST, R. MIHAL-EK, Assistant Examiners.

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