US3112554A - Process of manufacturing field-effect transistors - Google Patents

Description

Dec. 3, 1963 S. TESZNER PROCESS OF MANUFACTURING FIELD-EFFECT TRANSISTORS Original Filed Feb. 15, 1956 /17 @/f/Wf ffii/baffi* S. TESZNER Dec. 3, 1963 PROCESS OF MANUFACTURING FIELD-EFFECT TRANSISTORS Original Filed Feb. 15, 1956 2 sheetg-snet 2 United States Patent O M' 3,112,554 PROCESS F MANUFACTURING FIELD-EFFECT TRANSISTORS Stanislas Teszner, 49 Rue de la Tour, Paris, France Griginal application Feb. 13, 1956, Ser. No. 565,231, now

Patent No. 2,987,659, dated June 6, 1961. Divided and this application Sept. 29, 1958, Ser. No. 764,105

Claims. (Cl. 29-25.3)

This application is a division of my' copending application Serial No. 565,231, filed, February 13, 1956, now Patent No. 2,987,659, issued lune 6, 196-1.

The subject matter of the present invention is a process for manufacturing field-effect transistors, and more particularly field-effect transistors of cylindrical configuration having metallic gates of the type disclosed in my above-mentioned application.

The field-effect transistors to be manufactured comprise a rod of `a semi-conductivev body, metallic layers :deposited on the end faces of said rod having ohmic contact therewith and forming the source and drain electrodes ofthe transistor, said rod comprising a thinned cylindrical portion of circular cross-section near the center of the rod land a metallic layer forming a gate electrode surrounding the thinned cylindrical portion along a length which is less than that of said thinned portion and having a rectifying contact therewith.

An insulating layer may be inserted between the semiconductive body and the metallic gate.

Generally speaking, the process of the invention comprises the following steps: attaching ohmic contacts to the two end surfaces of -a cylindrical rod of a semiconductive body having a circular cross section passing a controlling current between said ohmic electrodes, rotating said rod under the jet of a nozzle which projects an etching electrolyte, positively biasing said rod with respect to said etching electrolyte, said electrolyte sinking a neck shaped thinned portion in the rod, stopping the projection of the etching electrolyte when the controlling current falls below Aa certain value, which is effective to invert the polarity of the original positive biasing and thereby stop the jet etching under the control of the inversion of polarity, a-nd rotating the rod beneath the jet of a nozzle that is projecting a metallic salt electroplating electrolyte used for electroplating the metallic conductive gate adhering to and surrounding said thinned portion.

The fact that the semi-conductive rod is continuously rotated under the etching jet yand the electroplating jet resu-lts in favorable characteristics of the field-effect transistors manufactured by the process of the invention.

As is well known in the lart, it is necessary for obtaining good rectifying contacts that the electroplating step follow the etching step without any interruption. In prior art processes where etching and electroplating result from directing electrolytic jets onto stationary semi-conductive wafers, at least two jets opposite each other along a common yaxis are necessary for each step, and from the necessity of immediately passing from etching to electroplating results the obligation to choose -as electroly'tic etchant the mere electroplating electrolyte, i.e., a meallic salt solution of the metal for the gate, the step change being merely due o a reversal in polarity of the potential difference between the semi-conductive body and the electrolyte. This identity between the two electrolytes is detrimental for the reverse potential limit value of the manufactured transistors (see Journal of Applied Physics, vol. 26, No. 8 August 1955, page 102.13; FIG. 3 of the article of E. H. Borneman, R. F. Schwartz and J. l. Stickler) since it rresults from applicants experiments that, even in the case of identical electrolytes, the acidity of the etchant must be positively larger than the acidity 3,1 12,554 Patented Dec. 3, 1963 of the metal salt solution. Experimental values of the pH of the solutions will be given hereinafter.

It is known that the rate of electrolytic etching of semiconductive wafers is determined in large measure by the density of the etching current existing between the jet and the region of the semiconductive wafer impinged thereby. In the case of a parallellepipedic wafer, the etching rate is substantial-ly linear and proceeds at a relatively rapid rate throughout including the time just prior to perforation; in the case of a cylindrical rod the etching rate increases as the diameter of the rod decreases since, if one considers equal volumes of material etched in a unit time, the etched depth per second is constant if the volumes are rectangular cross sectional slices and said depth increases as the diameter of the rod decreases if the volumes are tubular portions. The proper time .for terminating the etching is therefore most critical and difficult to determine under varying conditions of wafer thickness.

It has been proposed to substantially arrest the rate of change of the depth of a depression in a semiconductive parallelepipedic wafer by means of a rectifying electrode deposited on the face of the wafer opposite that into which the depression yis su-nk and suitably reversebiased. Such an arrangement infers that the jet direction and. the reverse-biased electrode location are stationary and cannot be utilized in the case of the rotative rod. In accordance with the invention the reverse-bias control arrangement utilized for parallelepi-pedic wafers is replaced by a control current arrangement, the controlling current owing between ohmic electrodes provided to the end surfaces of the rod before jet etching initiation.

The process of the invention will now be described in detail with reference to the accompanying drawings in which:

FIG. 1 shows a unipolar field-effect transistor to be manufactured;

x FIG. 2 shows another unipolar field-effect transistor with insulating layer bene-ath the gate electrode; and FIGS. 3 and 4 relate to apparatus for manufacturing the iield-etect transistors.

In FIG. l, the transistor, denoted in its entirety by 1, is constituted by a semi-conductive body of the n-type, for example, by germanium of the n-type. It comprises a substantially cylindrical part 2 of a small diameter and two lateral parts 3 and 4 which are also substantially cylindrical and are of a greater diameter, on the end faces of which there are arranged two metallic electrodes 5 and 6 which are in ohmic contact with the semiconductive body. 5 is the source electrode which is equivalent to the cathode of a three-electrode thermionic tube and 6 is the drain electrode which is equivalent to the anode of such a tube. Round the narrow part controlling metallic electrode or gate is arranged. The nature of the metal used for making the gate does not constitute an essential factor, but certain metals are more suitable than others; for germanium of the n-type, indium, tin, zinc, gold or platinum are particularly suitable. The surface of the semi-conductive body should be clean and regular and, in the case of a semi-conductor of the n-type, the formation of a layer of oxide, which facilitates the attachment of a negative charge on the surface, should be promoted there. It is known that a layer of oxygen promotes this formation. The surface may therefore advantageously be treated with an aqueous solution of hydrogen peroxide having a concentration of the order of 5 to 20% by weight, preferably with a small addition, of a fraction of one percent by weight, of sodium carbonate. This treatment is carried out at an elevated temperature, at about 60 C. for example.

8 is a generator of signals to be amplified. 9 and 10 are two sources of direct current, the former being for the bias of the gate and the latter being for feeding the gate electrode, and 11 is a load resistance.

FIG. 2 shows a modified form of a held-effect transistor in which there interposed, between the metallic layer 39 that forms the gate electrode and the narrow part 40 of the semi-conductive body, an insulating film 41, for example a layer of epoxy resin such as that sold under the Registered Trademark Araldite D, which is liquid at room temperature. The thickness of the film may be of the order of .005 inch. t

Referring now to FIG. 3, a rod 18 of germanium is taken and it is soldered on a metallic rod 19 of nickel or of bronze, which is situated in its exact extension and which, when the transistor is linished, constitutes one of the electrodes. This rod is fixed in the jaw 22 of a rotating shaft 23 which rotates in a bearing 20 and is driven, through the intermediary of a pulley 21, by a motor (not shown). At the opposite end of the rod of germanium is soldered a loop 24 which consists of a flexible nickel wire of small diameter (.004 inch) and the free end of which is in the exact extension of the rod of germanium and rests on a droplet 25 of mercury which is contained in a small cup 26. The rod of germanium rotates inside a vat 27 into which there open two nozzles 28 and 29, the axes of which are situated very accurately in radial planes passing through the said rod. The nozzles are preferably horizontal; they have been represented as vertical in FIG. 3, only to simplify the drawing and in order for both to be seen. The nozzle 2S serves for the electrolytic etching of the narrow part and the nozzle 29 serves for the formation of the electroplated layer, of indium, for example, which constitutes the gate. The nozzle 28 is fed by a reservoir 30 of electrolyte in which an air pressure can be applied and into which the electrolyte, which has been used and collected at the bottom of the vat 27, is recycled. The nozzle 29 is fed by a reservoir 31 of electrolyte in which an air pressure can also be applied by means of a valve 32 and into which the electrolyte coming from the vat 27 is recycled.

A third nozzle marked 29e is provided as shown in FIG. 3 for the purpose of delivering liquid epoxy resin to the necked-shaped thinned portion of the rod in the manufacture of the embodiment of modified field-effect transistor illustrated in FIG. 2. Effectively, the nozzle 29e, constituting the third nozzle as diagrammatically shown in FIG. 3 is fed by a reservoir 30e. containing epoxy resin in liquid form. The showing of the reservoir 30e does not include the means for mixing and delivering the liquid epoxy resin (Araldite D) and its hardening agent prior to the projection of the resin through the nozzle 29e since the mixing may be done by hand for the special purpose to achieve the desired coating thickness of 0.0005 inch after drying.

It will be appreciated that, although the showing in FIG. 3 apparatus herein of the housing is diagrammatic, this housing may be subjected to vacuum by means indicated only in the legend at the lower left of the housing in FIG. 3 whereby the turning of the vacuum on and off can aid in dispensing the epoxy resin from reservoir 30e through its associated nozzle 29E, the primary purpose of the vacuum being to subject the rod to vacuum conditions after the rod has been coated with the epoxy resin and before the gate is formed by electroplating.

Between the electrode 19 and the droplet 25 is inserted an electric controlling circuit comprising the source 33 and the relay 34 which is de-energized when the resistance of the rod of germanium becomes greater than a certain value.

The relay 34 controls, by its contacts 55, the applica-` ton, to the electrodes 35 and 36 respectively in contact with the electrolytes, of a voltage supplied by the source 37. The polarity of the electrolyte is negative in relation to that of the germanium during the etching and positive during the electroplating. During the etching operation, the voltage supplied diminishes progressively owing to the automatic displacement, in the direction of the arrow, of the cursor 56 on the potentiometer 57, while the voltage which controls the electroplating is constant. The applicant will now describe a speciiic method, chosen by way of example, of manufacturing a transistor.

Examples of manufacture of a unipolar transistor- (1) Nature of the semi-conductive body: germanium of the ntype having a resistivity of 3 to 30 ohm cms. or silicon of the p-type having a resistivity of l0 to 100 ohm cms.

(2) Dimensions of the assumed rod of germanium before treatment- Diameter: .02 inch.

Length: .1 inch.

(3) Etching electrolyte: decinormal H2804 having a pH value of 1.3.

Diameter of the outlet orifice of the nozzle 28: .01 inch.

Rate of electrolyte: 0.4 cc. per second.

Current passing through the rod of germanium: variable between 10 ma. and Z50/ra. from the commencement to the end of the operation. When the resistance of the rod of germanium reaches a certain value, which corresponds to the desired thinning (see below), the relay 34 is de-energized and actuates its contacts 55, and means solution of may be provided for automatically operating the valve 32.

Duration of operation: about 10 minutes.

Referring to FIG. 4, which gives the resistance of the rod of germanium as a function of the time, for germanium of the n-type having a resistivity of 30 ohm cms., it is seen that the operation is stopped at the end of 10 minutes when the resistance is about 20,000 ohms, the diameter of the neck then amounting to about .002 inch. At the end of 11 minutes, the resistance would exceed 30,000 ohms and there would be a breakage of the rod at a diameter of the neck of the order of .001 inch.

(4) Jet electroplating- Electrolyte used for deposition: a solution of In2(SO4)3 at the strength of 25 grams per litre of water, with addition of H2504, giving a resultant pH of 2.8.

Diameter of outlet orifice of the nozzle 29: .006 inch.

Rate of electrolyte: 0.2 cc. per second.

Current passing through the rod: 1 ma.

Duration of the operation: 30 seconds to 1 minute.

(5) Speed of rotation of the rod of germanium during the treatment: 60 revs. per min. but this speed is not at all critical.

(6) Dimensions of the rod of germanium after treatment- Diameter of the narrow part: .002 to .006 inch.

Length of this narrow part: .004 to .016 inch.

Diameter of the etching nozzle corresponding to these limits: about .004 inch and .016 inch.

Length of the gate electrode: .003 to .015 inch.

Diameter of the electroplating nozzles corresponding to these limits: about .002 inch and .012 inch.

For manufacturing field-effect transistors of the type of PIG. 2, the etching process is carried in the same Way as in the case of the transistors of the type of FIG. l. Then a jet of epoxy resin is projected through a nozzle against the thinned part of the semi-conductive body at a temperature lower than C. T he transistor rod is then put in vacuo and a metallic alloy having a low melting point is evaporated and projected through a nozzle onto the insulating film while the rod is driven into rotation. As an alloy for forming the gate, one can take the alloy called Woods alloy the composition of which is the following: 50% Bi, 25% Ph,V 12.5% Sn and 12.5% Cd (melting point 65.5 C).

The insulating film in the structure of FIG. 2 has the effect of reducing the gate-drain and the gate-source capacities and to increase the service voltage of the transistor. The diameter of the neck may then be brought to about .012 inch. On the other hand, this hlm diminishes the etlciency of the modulating voltage and thus reduces the amplification factor.

I claim:

l. A method of manufacturing field-effect rod-shaped transistors having a thinned cylindrical rod portion consisting of a semiconductive material in a circular cross section, comprising the steps of providing a cylindrical rod of semiconductor material with ohrnic electrodes at the end surfaces of said rods, passing a controlling current between said electrodes through said rod and simultaneously rotating said rod under the jet action of an etching electrolyte in a direction towards the axis of said rod, positively biasing said rod with respect to said electrolyte whereby the jet etching action of said electrolyte sinks a necked-shaped thinned portion into the outer surface of said rod and increases the resistance in the thinned portion to reduce the value of said controlling current applied between the end electrodes, said controlling current being then reduced below a predetermined value which is eiiective to invert the polarity of the original positive biasing, stopping the jet etching under the control of the inversion of polarity and thereafter still rotating said rod and locating the necked portion below the jet action of a second electrolyte which is an electroplating electrolyte, projecting said electrolyte on to a gate location on the thinned portion of said rod and forming the gate as an annulus of metal conductor surrounding the necked p0rtion of said rod as a result of the electroplating action of said second electrolyte.

2. A method of manufacturing held-effect rod-shaped transistors having a thinned cylindrical gate region comprising the steps of providing ohmic electrodes to the end surfaces of a cylindrical semiconductor rod of circular cross section, passing a current between said electrodes through said rod, projecting a rst electrolytic etchant jet while continuously rotating said rod against a preselected region of the rod in which the gate is to be formed while positively biasing said rod with respect to said etchant, said current decreasing due to the increase in resistance caused by the thinning of said rod at said preselected gate region, said current decreasing to below a predetermined value at a point Where the thinned cross section remaining represents the precise necked-shaped dimension desired in the iinished field-effect transistor and thereby stopping said rst etching step when said desired dimension is obtained, continuing the rotation of said rod and Without interruption, projecting in jet action, a second metallic salt solution for electroplating a conducting metal annulus around said thinned portion While negatively biasing said rod with respect to the potential of Said metallic salt electroplating solution and thereby locating and forming said gate in the thinned region of said field-effect transistor.

3. A method of manufacturing held-eifect rod-shaped transistors having a thinned cylindrical gate region comprising the following stages: passing a controlling current through a cylindrical rod of a semi-conductive body having a circular cross-section, rotating said rod under the jet of a nozzle, projecting an etching electrolyte against said rod while said controlling current is being passed t'neretl'irough,l said electrolyte sinking a neck-shaped thinned portion in theprod, stopping the projection of the etching electrolyte when the current decreases to below a certain value, projecting through a nozzle an epoxy resin onto said thinned portion, putting the rod in vacuo, and while rotating it beneath the jet of another nozzle for electroplating a metal layer surrounding said thinned portion in the rod and serving at the gate, then projecting a metallic electropiating solution through said nozzle to form said gate.

4. A method of manufacturing field-effect transistors as claimed in claim 3 wherein said rod is positively biased in respect to said etching electrolyte whereby the jet etching action of the electrolyte in a direction toward the axis of the rod sinks a necked-shaped thinned portion into the outer surface of said rod and increases the resistance in the thinned portion to reduce the value of said controlling current applied, said controlling current being then reduced below a predetermined Value which is effective to invert the polarity of the original positive biasing, stopping the jet etching under the control of the inversion of polarity.

5. A method of manufacturing field-effect transistors as claimed in claim 4 wherein said cylindrical rod is provided with ohmic electrodes at each of its end surfaces prior to subjecting said rod to the action of said etching electrolyte and said controlling current is passed between said ohrnic electrodes at each of said end surfaces.

References Cited in the tile of this patent UNITED STATES PATENTS 2,741,594 Bowersett Apr. l0, 1956 2,778,926 Schneider Jan. 22, 1957 2,843,809 Varela July 15, 1958 2,846,346 Bradley Aug. 5, 1958 2,869,055 Noyce Ian. 13, 1959 2,885,571 Williams et al. May 5, 1959

Claims (1)

1. A METHOD OF MANUFACTURING FIELD-EFFECT ROD-SHAPED TRANSISTORS HAVING A THINNED CYLINDRICAL ROD PORTION CONSISTING OF A SEMICONDUCTIVE MATERIAL IN A CIRCULAR CROSS SECTION, COMPRISING THE STEPS OF PROVIDING A CYLINDRICAL ROD OF SEMICONDUCTOR MATERIAL WITH OHMIC ELECTRODES AT THE END SURFACES OF SAID RODS, PASSING A CONTROLLING CURRENT BETWEEN SAID ELECTRODES THROUGH SAID ROD AND SIMULTANEOUSLY ROTATING SAID ROD UNDER THE JET ACTION OF AN ETCHING ELECTROLYTE IN A DIRECTION TOWARDS THE AXIS OF SAID ROD, POSITIVELY BIASING SAID ROD WITH RESPECT TO SAID ELECTROLYTE WHEREBY THE JET ETCHING ACTION OF SAID ELECTROLYTE SINKS A NECKED-SHAPED THINNED PORTION INTO THE OUTER SURFACE OF SAID ROD AND INCREASES THE RESISTANCE IN THE THINNED PORTION TO REDUCE THE VALUE OF SAID CONTROLLING CURRENT APPLIED BETWEEN THE END ELECTRODES, SAID CONTROLLING CURRENT BEING THEN REDUCED BELOW A PREDETERMINED VALUE WHICH IS EFFECTIVE TO INVERT THE POLARITY OF THE ORIGINAL POSITIVE BIASING, STOPPING THE JET ETCHING UNDER THE CONTROL OF THE INVERSION OF POLARITY AND THEREAFTER STILL ROTATING SAID ROD AND LOCATING THE NECKED PORTION BELOW THE JET ACTION OF A SECOND ELECTROLYTE WHICH IS AN ELECTROPLATING ELECTROLYTE, PROJECTING SAID ELECTROLYTE ON TO A GATE LOCATION ON THE THINNED PORTION OF SAID ROD AND FORMING THE GATE AS AN ANNULUS OF METAL CONDUCTOR SURROUNDING THE NECKED PORTION OF SAID ROD AS A RESULT OF THE ELECTROPLATING ACTION OF SAID SECOND ELECTROLYTE.

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