US3193267A - Apparatus for continuous gas diffusion - Google Patents

Description

July 6, 1965 J. H. BECK APPARATUS FOR CONTINUOUS GAS DIFFUSION 4 Sheets-Sheet 1 Filed July 31, 1961 INVEN TOR.

J. HOWARD BECK ATTORNEYS July 6, 1965 J. H. BECK APPARATUS FOR CONTINUOUS GAS DIFFUSION 4 Sheets-Sheet 2 Filed July 31, 19.61

INVENTOR.

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July 6, 1965 Filed July 31, 1961 United States Patent 3,193,267 APPARATUS FOR CONTINUQUS GAS DIFFUSON Jacob Howard Beck, Waban, Mass, assignor to BTU Engineering Corporation, Waltham, Mass, a corporation of Delaware Filed July 31, 1961, Ser. No. 127,932 14 Claims. (Cl. 263-37) This invention relates to gas diffusion and more particularly to a gas diffusion furnace for use by the semiconductor industry.

One of the chief methods of making semiconductors is by gaseous difiusion of impurities. In this type of construction, a crystal of semiconductor material such as silicon and'a source, i.e., a mass of impurity such as antimony, are sealed together in a quartz tube and the complete assembly heated in a furnace to a very high temperature in the order of about 1200 C. At this temperature, the impurity is in the form of a gas which diffuses into the surface of the crystal, thereby forming P- or N-type layers. The reaction time is usually in the range of 16 to 36 hours. An advantage of this technique is the ability to form very large, fiat junctions, i.e., diffused layers, of controlled thicknesses. However, heretofore it has not been feasible to execute this technique on a continuous basis because of the inability to achieve strict diffusion control, a prerequisite to obtaining diffused layers of precise thicknesses. Basically, the extent of diffusion depends on three factors: (1) the concentra tion of impurity, (2) heating time, and (3) temperature. The first two variables are easily controlled, but temperature presents a problem. Not only does the rate of diffusion vary with temperature, but in addition, the rate of change is not linear but a complex exponential function. Hence a fiat temperature profile is required. Because of this temperature problem and also because of the belief that it is difficult to maintain the atmosphere free of undesired contaminants, continuous furnaces have been considered unfea-sible for gaseous diffusion and batch furnaces have been used instead. Some consideration has been given to making a continuous diffusion furnace wherein the quartz tube containing the semiconductor crystal and the source is moved along its own longitudinal axis through the furnace. However, this approach is not feasible. Proper diffusion requires that the concentration of the gaseous impurity be maintained at a predetermined value. This is achieved by precisely controlling the temperature to which the source is heated. If the temperature varies, the concentration of the gaseous impurity will go up or down and, therefore, the final product will be affected. An attempt to move the quartz tube axially through any sort of continuous furnace at one point or another will cause a change in the temperature of the source and crystal. Thus, for example, when the tube is entering the furnace, some vapor will enter the crystal before the crystal has attained full diffusion temperature, but the rate of diffusion at this point would be smaller than desired. On the other hand, as the tube progresses toward the exit end of the furnace, the source will be overheated. If the source is an alloy, overheating will decompose it. If the source is a single element, overheating will produce an excessive concentration of impurity vapor and also a difli'erent partial pressure. In either case, the overheating will cause uncontrolled and uncomputable diffusion.

Although batch furnaces do permit precise control of the thickness of a diffused layer, they have certain important drawbacks such as (1) low production, (2) excessive handling, (3) high labor requirements, and (4) furnace deterioration. In normal batch furnace operation, the furnace is cooled down between batches. This 3,l3,2? Patented July fi, 1965 repeated cooling causes rapid deterioration of the furnace, particularly of the heating elements. In addition to cost of replacement, heater deterioration causes uneven heating which in turn prevents normal operation.

Accordingly, the object of this invention is to provide a diffusion furnace which is capable of continuous operation with precise diffusion control.

A further object s to provide a novel method of diffusing impurities into the surface of semiconductor crystals.

A more specific object of this invention is to provide a continuous diffusion furnace which provides quality control equal to that of a batch furnace yet is free of the disadvantages of .a batch furnace. In a continuous diffusion furnace embodying the present invention, diffusion is carried out on a precisely controlled temperature flat, thereby yielding diffused layers of precise thicknesses. This is achieved by providing separate heating zones for the source and crystal and moving them solely through their own zones. The source and crystal are supported in a heating tube which is mounted at to its path of movement, simultaneously through two heating zones. As viewed in cross-section, a furnace constructed according to the present invention comprises two separate furnace sections, appropriately described as source heater and diffuser heater, which are precisely controlled at different temperatures. A conveyor transports successive quartz tubes through the furnace, each quartz tube containing a source of impurity to be vaporized and a crystal which is to be doped by diffusion of the impurity. The impurity is vaporized in the source heater and diffuses into the crystal in the diffusion heater. In the practice of this invention, the quartz tubes may be closed off by a hermetic seal or they may be adapted for circulation of an inert carrier gas.

Other objects and many of the attendant advantages of the present invention will be readily appreciated as the invention becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a side View of a continuousdiffusion furnace embodying the present invention;

FIG. 2 is :an end view of the same furnace;

FIG. 3 is a cross-sectional view taken along line 3-3 of FIG. 1;

FIG. 4 is an enlarged fragmentary side view, partly in section, of the same furnace;

FIG. 5 is a schematic plan view of the same furnace as employed for closed tube operation;

FIG. 6 is a schematic plan view showing the invention as applied to open tube'operation; and

FIG. 7 is a schematic plan view showing how the invention is applied to a double diffusion process.

Referring now to FIGS. 1 and 2, the furnace is mounted on upper and lower metal supporting frames. The upper frame is made of uprights 2, crossbars 4, and longitudinal beams 6. The lower frame is made up of uprights 8, crossbars 10, and beams 12. The furnace itself comprises a top body section 16 attached to the undersides of crossbars 4 and a bottom body section 18 supported by a plurality of beams 20 mounted on crossbars 10.

As seen in FIGS. 3 and 4, each furnace section comprises an outer metal case 24 by which it is attached to the frame and a ceramic liner 26 attached to the metal case. The top and bottom furnace sections are mirror images of each other, having opposed side edge flanges 32, 34, 36 and 38 and opposed longitudinal dividing ribs 40 and 42, respectively. The two sections 16 and 18 deline a furnace interior which is divided by ribs 40 and 42 into two parallel, longitudinal extending heating chambers A and B. It is to be observed that the two body sections are in parallel spaced relation with each other so that heating chamber'A is open at edges 32 and 34 and heating chamber B is open at edges 36 and 38 and both chambers are open at their ends. In addition, chamber A communicates with chamber B. Mounted in both chambers at the top and bottom thereof are longitudinally extending electrical heater rods 46 and 48'which, when energized by a suitable source, serve to heat the chambers to desired temperatures. Chamber A and heater rods 46 together constitute a source heater. Chamber B and heater rods 48 together constitute a diffusion heater.

The edge flanges 32 and 34 are provided with identical grooves 50 which serve as guide tracks for a plurality of movable ceramic carriages 54. Edge flanges 36 and 38 have identical grooves 52 for a like plurality of identical movable ceramic carriages 56.- Each carriage is of rectangular block configuration having identical tongues 58 at two opposite sides, a similar tongue 60 at a third side, and a slot 62 at the fourth side. Each slot 62 is sized to receive a tongue 60 of another seal 54. The tongues 58 are sized to make a snug but slidable fit in grooves 56 and 52. The carriages 54 and 56 are arranged in pairs, being attached to opposite ends of a plurality of identical open 7 ceramic carrier tubes 66. The outer diameter of these tubes is slightly less than the distance between rigs and 42, being-just large enough to substantially block off any convection currents between chambers A and B but small enough to be movable longitudinally along the space between the ribs.

The carriages 54 and 56 are partially encased in metallic collars 70 and 72 which are connected to a pair of endless chains 74 and 76 by identical bracket members 78 and'80, respectively. Chain 74.is mounted on suitable sprockets 84 and 86 attached to a pair of shafts 90 and 92 journaled at opposite ends of the frame. Although not fully shown, it is to be understood that'chain 76 is nace are two guide rails 104 and 106 which support the upper runs of chains 74 and 76. The vertical length of the brackets 78 and 80 is such that with the chains riding V on the guides 104 and 106, the carriages 54 and 56 are supported at the elevation required to allow their ribs 58 to ride in grooves and 52.

When the motor M is operating, the carriages and the ceramic carrier tubes 66 will be driven by brackets 78 and 80 in an endless path in the direction shown in FIGJ. Tubes 66 and carriages 54 and 56 will enter the furnace at one end and will exit at the opposite end. During the travel through the furnace, the tubes will be supported in a horizontal plane by the carriages 54 and 56. The

' carriages 54 (and also the carriages 56) will engage each other while they are in the furnace. In this connection, it is to be observed that the brackets 78 and'80 are spaced along the chains 74 and 76 a distance such that the tongues on one carriage will be fully disposed within the slot 62 of the preceding carriage While the tubes 66 are moving along within the furnace. When the ribs 60 are disposed within the slot 62, carriages 54 and 56 effectively seal off the spaces between flanges 32, 34 and 36, 38, respectively, thereby completing the side walls of the furnace. The side walls may be considered as made up of the flanges 32, 34, 36, and 38 and the carriages 54 'and 56. On leaving the interior of the furnace, the carriages move away from each other as they travel around sprockets 86 and re-engage each other after they have passed around the sprockets 86. The spacing movement g. ceramic carrier tubes 66 as described hereinafter. Loading and unloading is also facilitated by the speed at which the chains travel. The chain speed is relatively slow so as to allow tubes 66 to be within the furnace for a substantial interval, -e.g., 22m 36 hours, depending upon the particular diffusion process to be executed.

The ceramic tubes66 function as' carriers for quartz heating tubes of the kind used heretofore in difiusion processes. These quartz tubes may be of various types. For example, they may be fully sealed off, either permanently or by removable end caps for so-called close tube operation. The permanent type is useable only once since it must be broken open in order to remove its contents. Alternatively, the quartz tubes may also .be provided with end openings for so-called open tube operation wherein a gas is circulated through the tubes to promote diffusion or out-diffusion. The quartz tubes 110 shown 'in FIGS. 1-4 are of the latter type.

In practice, each quartz tube 110 functions to contain (l) a boat 112 within which is a source of impurity 114 which is to be diffused into a semiconductor material and (2) a boat .116 within which is stacked a plurality of thin wafers or disks118 of a crystal semiconductor. The boat 112 is disposed within the quartz tube 110 in a position such that when the quartz tube is transported through the furnace by a carrier tube 66, the source 114 will travel through chamber A. Boat 116 is positioned so that it will travel through chamber B.

At this point, it is to be observed that the heater rods 46 are controlled (by means not shown) so that chamber A will have a temperature sufficient to cause vaporiza: tion of the source 114. On the other hand, the heater rods 48 are controlled so as to produce in chamber B a temperature at which the vaporized impurity will diff se into the surfaces of the crystal wafers 118. In a typical case, chamber A will have a temperature of 300 C., and chamber 13 will have a temperature of 1200" C. As each quartz tube is transported along within the furnace, the impurity in boat 112 will vaporize, and then, due to its own vapor pressure or to the influence of a carrier gas, the vaporized impurity will envelope the crystalline wafers 118 and diffuse into. their surfaces.

As seen, in FIG. 3, each quartz tube 110 is provided at its opposite ends with removable end caps 124 and 126 having small tubular extensions to which may be connected flexible hoses 128 and 136, respectively. As seen in FIG. 1, hoses 128 are attached to a suitable inlet manifold 132 which is coupled by a tube 134 to as gas supply (not shown). Althoughnot shown, it is to be understood that flexible hoses 13% are connected to an outlet manifold of similar construction located on the opposite side of the furnace. The function ofhoses 128 and and the manifolds to which they are attached'is to permit an appropriate inert gas such as argon to be circulated through the quartz tubes (in the direction of the small arrows in FIG. 5) at a suitable rate for the purpose of facilitating diffusion. The gas promotes distribution of the vaporized impurity throughout the quartz tubes and also helps control its concentration, thereby helping to provide precise diffusion control. The hoses 128 and 130 are attached to the quartz tubes immediately after they are inserted in the carrier tubes 66 but before the tubes enter the furnace. As indicated in FIG. 5, the carrier gas is made to circulate as soon as the tubes enter the furnace and is cut off after they leave the furnace. The hoses are removed after the tubes have left the furnace. T he'quartz tubes are removed after they have been transported out of the furnace but before passing through any substantial angle about sprockets 86. Because of the slow speed of the conveyor chains, the quartz tubes have adequate time to cool before being removed from the carrier tubes.

Although the use of a carrier gas is most beneficial, the factor which is primarily responsible for attainment of precise diffusion control is the provision of the two parallel heater chambers A and B. With this arrangement,

the source is vaporized at a relatively constant rate since it is exposed only to the temperature in heater chamber A which is maintained substantially constant throughout its length. Moreover, diffusion of the impurity into the crystal proceeds at an even, predictable rate and for the time duration of the crystals travel through the furnace since the crystal is exposed only to the temperature in chamber B which is maintained substantially fixed throughout its length at the desired level. Accordingly, the furnace will operate equally with closed tubes, in which case the flexible hoses 12S and 130 are not required. In this connection, it is to be noted that in any closed tube operation, the partial pressure of the impurity itself promotes contact with the crystal wafers, the vaporized impurity being present in sufficient quantity to assure super saturation of the surfaces of the wafers. However, as a consequence of the super saturation condition, it is often necessary to transfer the wafers to another furnace to execute out-diffusion. The latter may be defined as the process of boiling off excess impurity from the wafers. In out-diffusion, it is customary to pass an inert carrier gas over the heated crystal for the purpose of flushing away excess impurity as it is released by heating of the crystal. A noteworthy advantage of the furnace shown in FIG. 1 is that it may also be used for a straight out-diffusion process. Of course, such use does not require operation of the source heater. For out-diffusion, hoses 128 and 130 will circulate the insert carrier gas through the quartz tubes in the same manner described previously in connection with straight diffusion.

FIGS. 6 and 7 illustrate still other variations of the same invention. In FIG. 6, gas is circulated through the quartz tubes 110A continuously while they are in the furnace. However, the source heater chamber A1 is illustratedas shorter than the diffusion heater chamber B1. At this point, it is to be noted that the two chambers may actually be of different lengths. Alternatively, they may have the same physical lengths, but the effective length of chamber A from a heating standpoint may be restricted by shortening the heater rods 46 so that they terminate a substantial distance from the end of the furnace. As a consequence, vaporization and diffusion of the impurity will be stoppedwhen the tubes reach a predetermined point along the length of chamber B, and thereafter, since the wafers are still being heated in chamber B, excess impurity absorbed by the crystal will diffuse out and be flushed away by the carrier gas. FIG. 7 illustrates how the furnace also may be used for a double diffusion process. Here the furnace is provided with two source heater chambers A2 and A3 and a single diffusion heat chamber B2. Each quartz tube 110B contains three boats, two of which contain two different sources of impurity 114A and 11413 and the third of which contains crystal wafers 118A to be doped. Because the effective upstream end of chamber A3 is downstream of the upstream ends of chambers A2 and B2, source 114A will vaporize and start to diffuse before source 114B, the latter will vaporize and start to diffuse before diffusion of impurity from source A2 has ceased. Out-diffusion occurs with the cooperation of the carrier gas for a prescribed period of time after diffusion of impurity from source 1143 has ended.

Of course, it is not necessary to carry out double diffusion precisely as described in connection with FIG. 7. Thus, for example, chambers A2 and A3 could be arranged so as to start vaporization of sources 114A and 1143 at spaced intervals or substantially simultaneously and to terminate them simultaneously or in prearranged time sequence.

It is to be noted that it is not necessary to fully seal off the sides of the heater chambers in the manner accomplished by the carriages 54 since the amount of heat which can be lost through the openings therein is not great. These openings are not large, the outer diameter of the carrier tubes 66 usually being in the order of 2 /2 inches. Moreover, simple inexpensive radiation shields may be invention is not limited positioned along the furnace sides to minimize heat loss.

The present invention has many important advantages. For one thing, it makes possible furnaces which can be used for single or multiple diffusion and also for out-diffusion, either as a separate operation or as the final phase of a diffusion process. With respect to diffusion, it is to be noted also that different quartz tubes may have different sources of impurities and also different kinds of semiconductor material, whereby different products may be produced in the same furnace. Moreover, furnaces constructed as described above are capable of producing junction layers of precisely controlled thicknesses, while at the same time achieving greater production than is possible in batch furnaces. Further advantages of the foregoing type of construction are that it is capable of being built with existing materials and utilizing conventional quartz tubes. Also important to note is that the life of the heating elements is much greater than it is in the case of a batch furnace since it is not necessary for furances of the type described to be repeatedly cooled down as are batch furnaces. It is also recognized that the invention may be utilized for processes wholly unrelated to the manufacture of semiconductors.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is to be understood, therefore, that the in its application to the details of operation or of construction and arrangement of parts specifically described or illustrated, and that within the scope of the appended claims, it may be practiced otherwisethan as specifically described or illustrated.

I claim:

1. A furnace adapted for use in the manufacture of semiconductors by the, process of gas diffusion comprising means defining two parallel elongated furnace chambers each open at both ends and also having a restricted opening at a side immediately adjacent to the other, means for'heating one chamber to a first temperature, means for heating the other chamber to a second temperature, a series of hollow heating tubes each adapted to hold a semiconductor material and a vaporizable impurity which is to be diffused into said semiconductor material, and means for advancing said series of hollow'heating tubes in succession. through both chambers simultaneously whereby said semiconductor material is heated in said one chamber and said impurity is vaporized in said other chamber.

2. A furnace as defined by claim 1 wherein said heating tubes are removably disposed in ceramic carrier tubes forming part of said advancing means, said carrier tubes substantially closing off the opening in said adjacent side of said each chamber as they pass through said furnace whereby to prevent transfer of heat from one chamber to another by convection currents.

3. A furnace as defined by claim 1 wherein said chambers are open at their outer sides and said hollow heating tubes have their ends projecting through said outer sides.

4. A furnace as defined by claim 1 further including means connected to said heating tubes for circulating a gas through each heating tube advanced through said chambers by said tube-advancing means.

5. A furnace as defined by claim 4 wherein said gas circulating means comprises a plurality of flexible hoses releasably connected to opposite ends of said each heating tube and stationary manifold means for directing gas into certain of said hoses and receiving gas from other of said hoses.

6. A furnace as defined by claim 1 wherein said lastmentioned means comprises a pair of conveyor chains, a plurality of carriages carried by each chain, and a series of ceramic carrier tubes extending between said chains, said carrier tubes attached at their opposite ends to carriers on each chain, each heating tube removably disposed in one of said carrier tubes with each carrier tube having at least one end open to permit a heating tube to be inserted therein, said carrier tubes substantially closing off the open adjacent sides of said chambers whereby to minimize transfer of heat from one chamber to another so as to maintain said chambers at said first and second temperatures. I

7. A continuous gas diffusion furnace comprising means defining two parallel elongated furnace chambers and a'passageway connecting said chambers for the full length thereof, means for heating said sections to different temperatures, a plurality of hollow heating tubes, means for supporting said plurality of heating tubes. in transverse relation to said elongated chambers, means for transporting said tube supporting means along the full length of said chambers whereby a portion of each of the,

tubes supported thereby will advance through one of said chambers while another portion of said each tube will advance through the other chamber and an intermediate portion of said each tube will advance along said passageway, and' means connected to said heating tubes for flowing a gas therethrough during transport thereof through said chambers.

8. A furnace adapted for use in the manufacture of semiconductors by the process of gas diffusion comprising means defining two parallel elongatedfurnace chambers separated from one another by an intermediate wall member, each chamber open at both ends and communicating with the other by an elongated opening in said wall member, means for heating one chamber to a first temperature, means for heating the other chamber toa second temperature, a series oftceramic carrier tubes each having at least one end open, an endless conveyor having a run extending from one end to the other of said furnace, means securing said carrier tubes to said conveyor in transverse relation to said run with a portion of each tube aligned with one of said chambers and the remainder of each tube aligned with the other of said chambers, means for operating said conveyor to advance said tubes in succession through both chambers simultaneously with each carrier tube entering said chambers at one end and leaving said chambers at the other end, and a plurality of hollow heating tubes each removably disposed in one of said carrier tubes, suficient to simultaneously accommodate and transport a semiconductor material and a vaporizable impurity which is to be diffused into said semiconductor material with said semiconductor material positioned to be trans ported through said one chamber and said vaporizable said heating tubes having a length a impurity positioned to chamber. a

9. A furnace as defined by claim 8 wherein said carrier tubes substantially close off said elongated opening as they pass through said furnace whereby to prevent transfer of heat from one chamber to the other so as to maintain said chambers at said first and second temperatures. 10. A furnace as defined by claim 8 wherein said chambers are open at theirouter sides and said carrier tubes have their ends projecting through said outer sides, and further wherein said carrier tubes are connected to said conveyor at their projecting ends.

11. A furnace as defined by claim 8 wherein said conveyor comprises a pair of parallel conveyor chains disposed outside of said chambers and a plurality of like carriages carried by said chain having a counterpart on the other chain, and further wherein said carrier tubes are secured to said carriages. 12. A furnace as defined by claim '11 wherein said chambers are open at their outer sides, and further where- 'in said carriages travel along in the planes of said outer sides.

13. A furnace as defined by claim 12 wherein said carriages substantially seal oh the openings in said outer sides.

14. A furnace as defined by claim 8 further including means connected'to said heating tubes for circulating a gas through each heating tube as it advances through said chambers. 7

References Cited by the Examiner UNITED STATES PATENTS 1,005,335 10/11 Seigle 263-8 1,531,214 3/25 ODonovan 263-37 1,549,880 8/25 Johnson 263-37 1,738,039 12/29 ,Cope et a1 263-36 X 1,926,354 9/33 Spatta 263-8 2,588,141 3/52 McFarland et a1. 263-8 2,683,652 7/54 Martin 23-277 X 2,887,453 5/59 Billig et a1. 252-623 2,921,905 1/60 Chang 252-623 3,078,082 2/63 Hnilcka 266-19 CHARLES SUKALO, Primary Examiner. JULIUS GREENWALD, JOHN J. CAMBY,'Examiners.

be transported through said other chains, each carriage on one

Claims (1)

1. 7. A CONTINUOUS GAS DIFFUSION FURNACE COMPRISING MEANS DEFINING TWO PARALLEL ELONGATED FURNACE CHAMBERS AND A PASSAGEWAY CONNECTING SAID CHAMBERS FOR THE FULL LENGTH THEREOF, MEANS FOR HEATING SAID SECTIONS TO DIFFERENT TEMPERATURES, A PLURALITY OF HOLLOW HEATING TUBES, MEANS FOR SUPPORTING SAID PLURALITY OF HEATING TUBES IN TRANSVERSE RELATION TO SAID ELONGATED CHAMBERS, MEANS FOR TRANSPORTING SAID TUBE SUPPORTING MEANS ALONG THE FULL LENGTH OF SAID CHAMBERS WHEREBY A PORTION OF EACH OF THE

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