US3002821A

US3002821A - Means for continuous fabrication of graded junction transistors

Info

Publication number: US3002821A
Application number: US617330A
Authority: US
Inventors: Carl I Haron
Original assignee: Texas Instruments Inc
Current assignee: Texas Instruments Inc
Priority date: 1956-10-22
Filing date: 1956-10-22
Publication date: 1961-10-03
Anticipated expiration: 1978-10-03

Description

I. HARON MEANS FOR CONTINUOUS FABRICATION OF Oct. 3, 1961 c.

GRADED JUNCTION TRANSISTORS 2 Sheets-Sheet 1' Filed Oct. 22, 1956 I INVENT OR Gar/ Haron BY z ' ATTORNEYS Oct. 3, 1961 c. I. HARON A MEANS FOR commuous FABRICATION OF GRADED JUNCTION TRANSISTORS 2 Sheets-Sheet 2 Filed Oct. 22, 1956 x dumb m bmxa On kwllbl INVENTOR Gar/ Haron ATTORNEYS 3 002,921 BEANS FUR CONTENUOUS FABRICATION F GRADED JUNCTEON TRANSHSTQRS Carl E. Heron, Dallas, Ten, assignor to Texas Instruments incorporated, Dallas, Tera, a corporation of Delaware Filed Oct. 22, 1956, Ser. No. 617,330 9 Claims. (Cl. 23--273) This invention relates to the art of producing semiconductive bodies, and more particularly to a novel apparatus for use in conjunction therewith.

As is well known to those skilled in the art, semiconductor bodies which includes contiguous portions of differing electrical conductivity may be formed by withdrawing a partially immersed seed orystalfrom a molten mass of semiconductor material, such as silicon or germanium. Such semiconductor material is extremely sensitive to thermal or mechanical disturbances occurring during the solidification period which accompany such withdrawal from the molten melt. This sensitivity renders possible the fabrication of graded-junction type crystals.

More particularly, it has been discovered that a change in conditions, such as an abrupt increase in the tensile force on the crystalcolumn, or a sudden change in the growth rate will produce a sharp variation in the conductivity of the orystallizing material. This phenomena has been exploited in the present invention to form crystals having an interface, or junction with a conductivity different from the contiguous material. Moreover, by practicing the teaching of the present invention, graded-junction type semiconductor bodies can be fabricated in a substantially continuous process.

The graded junctions can be readily formed by a melt-back process, in which a minute portion of the crystallized material is dropped back or otherwise reimmersed in the molten material for a predetermined interval. Alternatively, the liquid level of the molten semiconductor material may be periodically raised by small amounts to achieve exactly the same result. In the manufacture of graded-junction germanium and silicon transisters according to the present invention, a melt-back process which included reimmersing the rod to a depth of .010" for approximately 30 seconds yielded excellent results.

After the formation of the desired junction by such a melt-back process, the gradual withdrawal of the crystalline rod is resumed, and the degree of conductivity along the ,rod reverts to its former value. The continuously alternated withdrawals and reimmersions contemplated by the invention provide an elongated rod-like'semiconductor with a series of graded junctions spaced along its length. graded-junction semiconductors continuously instead of by the use of the conventional batch process.

According to the continuous process disclosed in this specification, semiconductor crystals are grown from a continuously replenished supply of molten melt. replenishment of the melt by novel techniques to be disclosed eliminates many disadvantages associated with the depletion of the liquid phase or molten semiconductor which characterizes the conventional batch process. Many of the disadvantages which characterize prior art methods of'using the batch process are attributable to the depletion of the molten mass which occurs during withdrawal of the seed crystal. As the liquid phase of certain semiconductor materials, like germanium, is progressively de- It is an object of the invention to form the The.

pleted during the growth of the solid phase by the crystallization process, the concentration of impurities in the remaining liquid tends to increase. Consequently, the conductivity characteristics of a large crystal tends to vary progressively along the length of the crystal, unless the depletion of the molten batch is continually compensated for by an operator. In the present invention, the conembodiment of the invention.

idzgfi Patented Oct. 3, 19 61 tinually changing conditions occurring in the feed crucibles are eliminated as a limiting factor in the crystal growth.

Additionally, as earlier described, means are disclosed for continuously forming an elongate crystalline semiconductor provided with a series of graded junctions spaced at substantially equal intervals. The temperature and mass of the molten semiconductor material, as well as growth rate of crystallization, are maintained substantially constant and continuous. By providing a more or less continuous makeup to the feed crucibles, in order to avoid progressive variation in the conductivity between the graded junctions, a primary goal of the invention is accomplished. The manner in which this goal of crystal withdrawal from a constant mass is accomplished, in order to eliminate the undesirable changes in conductivity associated with prior art batch processes will become evident as the detailed description of the invention proceeds.

Accordingly, therefore, a primary object of this invention is to provide an apparatus for continuous formation of graded-junction type crystals, accompanied by a degree of replenishment of the molten semiconductor material which is adeouate to eliminate undesirable variations in the conductivity of the crystal during the growing stage.

Still another object of this invention is to teach a system for maintaining substantially constant the temperature and mass of the molten semiconductor material as well as the rate of crystallization in order to obviate adverse etiects caused by progressive depletion of impurities in the molten semiconductor material.

Another object of this invention is to provide a novel means for forming an extended semiconductor crystal with a plurality of graded type junctions transversely disposed along the long axis thereof.

A further object of this invention is to disclose a novel apparatus for continuously forming a series of graded ty-pe junctions along an elongated crystal while simultaneously replenishing depletions in the quantity of molten semiconductor material from which the crystal is formed.

These and other objects of the present invention will become evident by reference to the following detailed description and drawing, in which like numerals indicate like parts and in which:

FIGURE 1 shows a cross sectional view of a preferred FIGURE 2 shows diagrammatically a system for automatically replenishing the supply of molten semiconductor material.

Turrn'ng now to the drawing, and more particularly to FIGURE 1 thereof, the reference numeral 1 indicates generally a portion of the novel apparatus for accomplishing continuous growth of the elongated crystalline rod 2.

More particularly, a housing 3 is provided in order to provide a thermal barrier and inhibit heat loss from the molten semiconductor material The housing 3 may be composed of a plurality of continguous adjacent lamina of asbestos and metal. The heat resistant qualities of the asbestos are thus exploited in combination with the reflective properties of the metal laminations.

The metallic lamina within housing 3 also act as a shield for the oscillatory radiant energy which is generated by the radio frequency induction coils 5 provided within the housing. The coils 5 are employed to generate internal heat and reduce the semiconductor material 4 to a molten state.

In order to confine and channel the movement of the molten material 4, a feeding crucible 6 composed of graphite or other heat resistant material is provided within the housing 3. The upper walls of the feeding crucible 6 engage the walls of a growth crucible 7. The growth crucible 7 may be-fabricated of graphite or other equally suitable heat resistant material.

The passage of molten semiconductor 4 between the outer portion of the feeding crucible 6 and the inner portion which communicates with the growth crucible 7 is regulated by means of a pair of

plug valves

8A and 8B. These valves may be formed of graphite or the like, with a suitable feed orifice provided therethrough. Moreover, the

valves

8A and 8B are rotatably disposed within the walls of the feed crucible 6, and may be adjusted to control the rate at which molten semiconductor material enters the growth crucible. The arrangement is such that the portions of the feed crucible 6 without the

plug valves

8A and 8B can hold dilierent charge materials. Even in these circumstances it is possible to have an uninterrupted change in the basic constitutent material during the crystal growing process. There may be a dead pull during purging of one basic material by the other, but the changeover will be continuous.

At the outermost portions of the feed crucible 6, there are provided charge pots QA and 9B. A supply of solid granular semiconductor material is contained within the

charge pots

9A and 9B. The method and means by which the solid granular material in the charge pots is replenished will be explained in connection with FIGURE '2 of this specification. The rate at which this solid material is allowed to enter the interior of the feed crucible 'is governed by means of a pair of adjustable barrel valves identified by the reference characters 10A and MB, respectively. Each of the barrel valves 10A and 103 may be composed of stainless steel or other equally suitable wear resistant material. The valves are machined in cylindrical form, and are each provided with a pair of recesses or pockets diametrically disposed so that the valves NA and 1GB seal off the feed crucible even when introducing material into it.

Returning now to the constructional details of the apparatus adjacent the growth crucible 7, there is shown a cylindrical member 11 made of quartz concentrically enclosing the crucible and associated induction coils. At the upper boundary of the cylindrical member 11, there is provided a rubber end seal 13. Below the end seal 13, three separate cooling zones are defined by the transverse seals M-A, 14B and 14C, respectively. The

seals

14A, 14B and 140 may be composed of temperature resistant rubber, or like material, characterized by the ability to withstand the elevated temperatures associated with the withdrawal of the crystalline work product from a molten melt. In order to shield the transverse seal 14C from the elevated ambient temperatures existing directly therebeneath, ring member 15 composed of monel and acting as a radiant shield is disposed across the bore of the cylindrical member 11.

During passage through the coolant zones defined by the three transverse seals above described, heat may be abstracted from the elongated crystal 2 by means of convection contact with a suitable gas. For instance, a continuous flow of helium gas, or the like, may be directed into heat exchange relationship with the portion of the upwardly rising crystal enclosed by the transverse seals. Since conventional structures and components for gas cooling crystal growing apparatus are commercially available, no detailed exposition of these systems has been provided in the present specification.

The upward tractive force required to form a continuous elongated crystal is supplied by a first pneumatic clutch 16 shown diagrammatically directly above the cylindrical member 11. The clutch 16 is provided with a plurality of radially

displaceable jaws

16a, 16b and 16c.

Vertical rectilinear motion as well as rotary motion is imparted to the crystal 2 by means of the pneumatic clutch 16. Clutch 16 is utilized to subject the elongated crystal to an upwardly disposed tensile force as well as 'to'a spinning action, at the start of each growth cycle.

Directly above the first pneumatic clutch 16, a second 8,002,821 I, w r" I pneumatic clutch 17 is diagrammatically illustrated. The clutch 17 is provided withplural jaws 17a and 17b adapted to engage the crystal 2. ln general, the jaws 17a and 17b of the upper clutch will be retracted during the period that the first clutch 16 is lifting the crystal rod vertically upward. However, during those intervals when the first clutch 16 disengages and descends vertically in order to re-engage the crystal 2 for the next upward pull, the upper clutch 17 engages the rod and continues the spinning and whatever other motion is required until clutch 16 reengages the rod at which time clutch 1'7 is released.

In order to cut discrete semiconductor elements from the rod 2, a suitable saw blade 18 is provided. If desired, a second saw blade may be employed to cut from the opposite side simultaneously with the operation of the blade 18. During the spinning motion imparted to the rod 2, the blade 18 may be brought into cutting engagement therewith by conventional feed means (not shown). The upper clutch 1'7 is employed to grasp the rod dining these cut-oh intervals in each cycle. The blade 18 may consist of a diamond tipped implement, or other suitable means characterized by sufiicient hardness to provide a clean true cut across the long axis of the crystalline rod.

In severing segments of the rod according to the method of the present invention, the required cut-0E period using a single blade was found to be approximately 1 minute. By employing a pair of blades, as mentioned above, cutting times of less than thirty seconds are obtainable.

Turning to FIGURE 2, the system for replenishing the supply .of molten semiconductor material will now be explained. The manner in which this more or less continuous makeup to the feed crucibles eliminates the progressive variation in conductivity between the graded junctions has been discussed earlier in this specification.

In FIGURE 2 the successive withdrawal and solidification of the crystalline material causes the rod 2 to assume the characteristic elongated configuration. In order to compensate or makeup for the resulting depletion in the molten semiconductor material, there is provided a feedback system which includes a limit switch 19.

The switch 19 includes a pair of condition responsive contacts which are brought together when the lower clutch 16 reaches its maximum upward displacement. The attainment of this displacement, of course, signifies the loss of a predetermined volume of molten material from the feed crucible.

In order to initiate the required replenishment, the clutch 16 closes the contacts of limit switch 19. The momentary closure of the contacts of the limit switch results in the closure of the timer switch 20. The timer switch 20 may comprise a conventional relay which will close upon receipt of a current pulse, and remain closed for a predetermined adjustable time interval.

The timer switch 20 is interposed between the power supply 21 and the vibrator unit 22' of a conventional Syntron vibrator unit 22. Thus, power from the unit 21 is available to energize the Syntron feeder 22 only during theinterval that timer switch 20 remains closed.

The Syntron unit is provided with an inclined trough 22 which is mounted to receive a supply of granular semiconductor material from the hopper. The semiconductor material may comprise a mixture of semiconductor and dope in predetermined proportions. When power is supplied to the vibrator unit 22', via the timer switch 26, the resulting vibratory movement imparted to the trough causes semiconductor material drawnfrorn the hopper 23 to advance along the trough and drop into the charge pot 93. It will be appreciated that the charge pot 9A may receive semiconductor material from the hopper in the same manner. This replenishment process is carried out only during the intervals when the trough is subjected to vibratory movement by the unit 22.

The quantity of material which is added is carefully calibrated to replace the volume withdrawn during the crystallization process occurring at the end of the rod 2. Where the melt-back is accomplished by raising the liquid level of the molten material, the volume of the material which is periodically added is great enough to raise the liquid level the required amount. The calibration for such volume is effected by adjusting the length of time that timer switch 20 allows the power supply 21 to energize the vibrator unit 22. Thus, by replacing a volume of material equal to that withdrawn, the temperature and mass of the molten semiconductor material, as well as the growth rate of crystallization is maintained substantially constant and continuous.

As an alternative technique, the feed can be arranged to make up molten semiconductor material as it is used. Thus, in place of adding an amount of semiconductor material after the formation of a predetermined portion of the crystal, it is continuously added through the barrel valves A and 1013 as it is used. An arrangement can be employed for this purpose which either estimates or senses the material used and governs the make-up or which determines or senses or estimates the rate of use and controls make-up in response thereto. in fact the make-up can be on a proportional, differential or integral basis or on any combination of these.

In practicing the continuous operation of the apparatus-for fabrication taught by the present invention, a seed crystal is immersed to a small depth in the molten melt of semiconductor material. The seed crystal may be mounted upon a suitable arbor, or mandrel (not shown) which extends vertically upward for engagement by the clutch to.

The seed crystal, during the initial period, must be withdrawn from the melt at a rate which permits the molten material adherent to the crystal to crystallize as rapidly as it is Withdrawn from the melt.

During this initial period, the rod 2 is gradually elevated by means of clutch 16 at a linear velocity of 1.68 mils per second, which corresponds to a formation of 2.89 grams per minute. It will be appreciated that the rod is simultaneously rotated during this interval.

Following the initial period of combined rotary and rectilinear displacement, a melt-back is required. This may consist of a descent by clutch 16 which efiects reimmersion to a depth of (say) .010, during which time no upward stress is applied to the crystal. Rotary motion of the crystal is continued during the melt-back interval. It will be recalled that the liquid level within the growth crucible may be raised by .010" in place of reimmersing the rod. It will be appreciated that regardless of which expedient is used for the melt-back process, the condition must prevail for a total interval of approximately 30 seconds.

Then, the cycle is resumed, and the gradual upward rotary motion of rod 2 is resumed. By continuing in this manner, 10 graded type junctions weighing about 90 grams were formed in approximately 31 minutes time. During the operation, the supply of pulverulent solid semiconductor material in the

charge pots

9A and 9B was replenished in the manner explained earlier, in order to prevent depletion of the supply of liquid phase semiconductor material, and avoid any undesirable increase in the concentration of impurities. As earlier explained, this replenishment of the supply of molten material minimizes variations in the conductivity of the portions of crystallized semiconductor material between the graded junctions. Thus at the end of this arbitrary period, 10 junctions had been formed and the system was in its original condition ready for further operation.

From the foregoing detailed description, it will be evident that I have disclosed my invention in full, clear and concise terms as required by the statute. However, it will be obvious that various modifications, substitutions and alterations may be made therein without departing in any manner from the spirit and scope of the appended claims.

What is claimed:

1. In combination, an enclosed means for containing a supply of molten semiconductor material, means for maintaining the supply of semiconductor material molten, means to introduce semiconductor material into the enclosed means, means to withdraw a solidifying mass from the surface of said molten semiconductor material and to remove the solidified mass already withdrawn from the surface of said molten material from the enclosed means while solidifying mass is being withdrawn from the surface of said molten material and sealing means cooperating with the solidified mass at its point of removal from the enclosed means to seal against gas leakage during removal of the solidified mass.

2. In combination, an enclosed means containing a supply of molten semiconductor material, means for maintaining the supply of semiconductor material molten, means including a barrel valve to introduce semiconductor material into the enclosed means, means to withdraw a solidifying mass from the surface of said molten semiconductor material and to remove the solidified mass from the enclosed means and sealing means cooperating with the solidified mass at its point of removal from the enclosed means to seal against gas leakage during removal of the solidified mass.

3. In combination, an enclosed means for containing a supply of molten semiconductor material, means for maintaining the supply of semiconductor material molten, means to introduce semiconductor material into the enclosed means, means to withdraw a solidifying mass from the surface of said molten semiconductor material and to remove the solidified mass already withdrawn from the surface of said molten material from the enclosed means while solidifying mass is being withdrawn from the surface of said molten material, sealing means cooperating with the solidified mass at its point of removal from the enclosed means to seal against gas leakage during removal of the solidified mass, and means to cool the solidified mass during removal thereof.

4. In combination, an enclosed means for containing a supply of molten semiconductor material, means for maintaining the supply of semiconductor material molten, means to withdraw a solidifying mass from the surface of said molten semiconductor material and to remove the solidified mass already withdrawn from the surface of said molten material from the enclosed means while solidifying mass is being withdrawn from the surface of said molten material and sealing means cooperating with the solidified mass at its point of removal from the enclosed means to seal against gas leakage during removal of the solidified mass.

5. In combination, an enclosed means for containing a supply of molten semiconductor material, means for maintaining the supply of semiconductor material molten, means to withdraw a solidifying mass from the surface of said molten semiconductor material and to remove the solidified mass already withdrawn from the surface of said molten material from the enclosed means while solidifying mass is being withdrawn from the surface of said molten material, sealing means cooperating with the solidified mass at its point of removal from the enclosed means to seal against gas leakage during removal of the solidified mass and means to cool the solidified mass during removal.

6. In combination, an enclosed means for containing a supply of molten semiconductor material, means for maintaining the supply of semiconductor material molten, means to withdraw a solidifying mass from the surface of said molten semiconductor material and to remove the solidified mass already withdrawn from the surface of said molten material from the enclosed means while solidifying mass is being withdrawn from the surface of said molten material, sealing means cooperating with the solidified mass at its point of removal from the enclosed means to seal against gas leakage during removal of the solidified mass, means to cool the solidified mass ,during removal and-means to cut-ofi the solidified massindiscrete lengths.

maintaining the supply of semiconductor material molten,

a pair of clutches mounted outside the enclosed means to apply tractive effort to a column of semiconductor crystal undergoing crystallization in proximity to the surface of the molten semiconductor material and being removed from the enclosed means and sealing means cooperating with the solidified mass at its point of removal from the enclosed means to seal against gas leakage during removal of the solidified mass.

8. In combination, an enclosed means containing a supply of molten semiconductor material, means for maintaining the supply of semiconductor material molten, means to withdraw a solidifying mass from the surface of said molten semiconductor material and to remove the solidified mass from the enclosed means, sealing means cooperating with the solidified mass at its point of removal from the enclosed means to seal against gas leakage during removal of the solidified mass and means to introduce semiconductor material into the enclosed means in response to removal of the solidified mass from the enclosed means. a

9. In combination, an enclosed .means containing a supply of molten semiconductor material, means for maintaining the supply of semiconductor material molten, means to withdraw a solidifying mass from the surface of said molten semiconductor material and to remove the solidified mass from the enclosed means, sealing means cooperating with the solidified mass at its point of removal from the enclosed means to seal against gas leakage during removal of the solidified mass and means to introduce semiconductor material into the enclosed means in an amount correlated with removal of the solidified mass from the enclosed means.

References ited in the file of this patent UNITED STATES PATENTS 835,061 George Nov. 6, 1906 2,591,304 Schuller Apr. 1, 1952 2,686,212 Horn et a1. Aug. 10, 1954 2,686,864 Wroughton Aug. 17, 1954 2,727,839 Sparks Dec. 20, 1955 2,727,840 Teal Dec. 20, 1955 2,770,533 Kahmann Nov. 13, 1956 2,793,103 Emeis Mar. 21, 1957 2,809,136 Mortimer Oct. 8, 1957 2,876,147 Kniepkamp Mar. 3, 1959 FOREIGN PATENTS 1,127,036 France Aug. 6, 1956

Claims

1. IN COMBINATION, AN ENCLOSED MEANS FOR CONTAINING A SUPPLY OF MOLTEN SEMICONDUCTOR MATERIAL, MEANS FOR MAINTAINING THE SUPPLY OF SEMICONDUCTOR MATERIAL MOLTEN, MEANS TO INTRODUCE SEMICONDUCTOR MATERIAL INTO THE ENCLOSED MEANS, MEANS TO WITHDRAW A SOLIDIFYING MASS FROM THE SURFACE OF SAID MOLTEN SEMICONDUCTOR MATERIAL AND TO REMOVE THE SOLIDIFIED MASS ALREADY WITHDRAWIN FROM THE SURFACE OF SAID MOLTEN MATERIAL FROM THE ENCLOSED MEANS WHILE SOLIDIFYING MASS IS BEING WITHDRAWN FROM THE SURFACE OF SAID MOLTEN MATERIAL AND SEALING MEANS COOPERATING WITH THE SOLIDIFIED MASS AT ITS POINT OF REMOVAL FROM THE ENCLOSED MEANS TO SEAL AGAINST GAS LEAKAGE DURING REMOVAL OF THE SOLIDIFIED MASS.