US3649882A - Diffused alloyed emitter and the like and a method of manufacture thereof - Google Patents

Diffused alloyed emitter and the like and a method of manufacture thereof Download PDF

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US3649882A
US3649882A US36777A US3649882DA US3649882A US 3649882 A US3649882 A US 3649882A US 36777 A US36777 A US 36777A US 3649882D A US3649882D A US 3649882DA US 3649882 A US3649882 A US 3649882A
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aluminum
emitter
diffusion
diffused
junction
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Albert Louis Hoffman
Derek Elden Longstaff
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof  ; Multistep manufacturing processes therefor
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof  ; Multistep manufacturing processes therefor
    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/68—Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
    • H01L29/70—Bipolar devices
    • H01L29/72—Transistor-type devices, i.e. able to continuously respond to applied control signals
    • H01L29/73—Bipolar junction transistors
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S148/00—Metal treatment
    • Y10S148/033—Diffusion of aluminum

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  • This invention relates to a method for increasing the surface concentration of aluminum atoms in an aluminum diffusion forming a junction and it is an object of the invention to provide an improved method of this character.
  • the invention also relates to a junction device wherein the aluminum diffusion has its surface concentration of aluminum substantially increased. More particularly the invention relates to such a method and device wherein an aluminum emitter was selectively diffused from vapor phase using a patterned silicon dioxide mask into an N-diffused base region of germanium and it is a further object of the invention to provide an improved method and device of this character.
  • germanium is a much preferable material for making high-frequency diodes, transistors and other junction devices.
  • Ultra high frequency devices require ultrasmall geometry diffusions into germanium.
  • the lateral dimensions of the emitter and the depth of its diffusion into the base are critical for controlling narrow base widths which are important to satisfactory operations of ultrahigh frequency transistors of the mesa, planar, annular and other types.
  • the lateral dimensions of the emitter are of the order of several microns, the depth of the emitter diffusion may be a fraction of a micron and the base width even less.
  • Germanium with its much higher carrier mobility can operate at frequencies two to three times greater than silicon devices. Also, germanium devices have a much higher signal to noise ratio than silicon devices.
  • silicon dioxide has not been a good mask for the other P-type dopants of aluminum, gallium and indium whether used on a silicon or a germanium substrate. No other simple or satisfactory mask has been known for these dopants.
  • germanium technology lagged behind that of silicon despite the fact that germanium has superior qualities for high-frequency junction devices because of its substantially greater carrier mobility.
  • an alloyed aluminum emitter In some prior mesa transistors utilizing P-type germanium as a collector, and an N- (antimony) type diffused base, an alloyed aluminum emitter has been used. Such transistors required a relatively low concentration base diffusion to get necessary beta and thus required an adjacent high concentration contact diffusion of N+ material, arsenic, for example, to achieve a low R',, C, (the product of base resistance and collector capacitance) for high-frequency applications, as is well known. Moreover, the alloying of aluminum for the emitter was subject to usual process limitations resulting in a substantial number of devices having alloyed emitter punch-through, electrical emitter-collector punch-through and BV shorts (emitter-base breakdown voltage, collector open). Accordingly, it is an object of the invention to minimize these problems of such prior devices.
  • the localized aluminum emitter difiusions (patterned silicon dioxide mask) of the said Ashton et al. invention enables precisely controllable emitter diffusion thickness. This, together with the precisely controllable diffusions of high concentration bases of N-type material such as antimony and phosphorus, for example, permits very narrow base widths to be obtained.
  • the alloyed emitter punch-through and electrical emitter-collector punch-through have, in essence, been eliminated.
  • the aluminum emitter diffusion of the said Ashton et al. application has achieved a surface concentration of about 5X10" atoms per c.c. of aluminum whereas a high-efficiency emitter requires concentrations of 5X10 atoms per c.c. or higher. Accordingly, it is a further object of this invention to provide diffused aluminum (and other P-dopant) emitters having surface concentrations of the order stated.
  • Achievements include: Higher yield, lower cost, higher and more uniform beta (B) and current-gain-bandwidth (f, use of a lesser number of photolithographic masks, and improved reliability.
  • B beta
  • f current-gain-bandwidth
  • the higher frequency extension enabled by the use of germanium, along with the diffused alloyed process and planar technology with its many inherent advantages are more readily available to the art for RF. power germanium devices, germanium integrated circuits, field effect transistors and ultrasmall germanium devices.
  • a method of increasing the surface concentration of aluminum atoms diffused from a vapor phase into a selected area of diffused N-type germanium forming part of the P-type emitter of a germanium mesa transistor, said emitter forming a junction at a certain depth comprising the steps of: evaporating and alloying a film of aluminum on said diffusion of an area essentially no greater than the said area of diffusion and essentially aligned therewith, the thickness of said alloyed filrn being less than the ultimate depth of said diffusion, thus maintaining the integrity of said junction.
  • a mesa transistor comprising a collector of P-type germanium, a diffused base of N-type germanium, and an aluminum emitter having one portion of aluminum diffused into said base and having a surface concentration of aluminum atoms of about 5 atoms per cubic centimeter, and a film of aluminum evaporated over said diffused aluminum and alloyed thereto without destroying the integrity of the emitter base junction, for effecting a surface concentration of about 5X l O atoms per cubic centimeter.
  • FIG. I is an elevational view, somewhat diagrammatic, partially in section, and on a greatly enlarged scale, of a mesa transistor embodying the invention
  • FIG. 2 is an elevational view similar to FIG. 1 ofa modified form of device according to the invention.
  • FIG. 3 is an elevational view of a wafer of P-type germanium, one type ofstarting material, appropriate to the invention
  • FIGS. 4l3 are elevational views similar to FIG. 3, partially in section, illustrating subsequent operative steps according to the invention.
  • FIG. 14 is an elevational view similar to the preceding figures showing a mesa transistor formed on a wafer prior to scribing and breaking;
  • FIG. 15 is an elevational view, partially in section, of a wafer of P-type germanium, one type of starting material, leading to the form of the invention shown in FIG. 2;
  • FIGS. 16 and 17 are elevational views partially in section similar to FIG. 15 illustrating subsequent operative steps according to the invention.
  • FIG. 1 of the drawings there is shown a mesa transistor 20 embodying one form of the invention, the mesa comprising a collector 21 of P-type germanium, a phosphorus diffused N-base 22, a P-type emitter 23, an ohmic contact 24 to the base 22, an overlay 25 of silicon dioxide, and overlay pads 26 and 27, making ohmic contact with the emitter 23 and the base contact 24, respectively.
  • Making contact with the P- type germanium collector 21 is an indium-gold backing layer 28.
  • FIG. 3 there is shown a starting wafer of P-type germanium 210 about 150 microns thick which may have a background concentration of P-type dopant such, for example, as gallium, equal to about l l0 atoms per c.c., for ex ample, that is a resistivity of about 4 ohm-centimeters, out of which a large number of mesa transistors as shown in FIG. 1 ultimately may be formed.
  • a background concentration of P-type dopant such, for example, as gallium, equal to about l l0 atoms per c.c., for ex ample, that is a resistivity of about 4 ohm-centimeters, out of which a large number of mesa transistors as shown in FIG. 1 ultimately may be formed.
  • the wafer 21a is shown with a diffused layer 22a of phosphorus doping which, at this stage of the operation, may be about 0.3 of a micron thick, typically, and will form a large number of N-type bases 22 in the finished transistors.
  • the layer 220 may be formed in any well-known diffusion furnace through which flow phosphine gas (PI-I in a hydrogen ambient or carrier gas.
  • PI-I flow phosphine gas
  • the wafers are placed in the furnace for about ten minutes and their temperature is maintained at 800 C.
  • the vapor pressure of the phosphine gas is such that, at the end of the diffusion period, the surface concentration of the phosphorus is about 8X10 atoms per cubic centimeter.
  • FIG. 5 the wafer of FIG. 4 is shown with a layer 31 of silicon dioxide applied atop the phosphorus diffusion 22a.
  • the layer 31, typically, may be 0.3 to 0.4 ofa micron in thickness and may be formed in a well-known horizontal reactor or chamber to which are supplied silane SiI-I. and oxygen in an ambient of hydrogen or nitrogen at a temperature of about 380 C.
  • the wafer 21a is shown with windows 32 opened in the silicon dioxide layer 31 by well-known photolithographic techniques, the details of which are not believed necessary to be described here.
  • the windows 32 are of dimensions such as to correspond to the desired dimensions of the emitter 23.
  • the emitters may have lateral dimensions of 15 microns by 30 microns. As many windows 32 may be opened on the wafer 21:: as the dimensions thereof will permit.
  • FIG. 7 a two-window portion of the wafer of FIG. 6 is shown on a larger scale.
  • the windows 32 of FIG. 7 there are shown bubbles 33 which represent the taking place of a localized chemical reaction as described in the said copending Ashton et al. application, Ser. No. 36,310.
  • the localized reaction 33 taking place in the windows 32 comprises, in one form, the reaction of aluminum trichloride (AlCl with germanium in a hydrogen ambient at a temperature of 800 C. for about 45 minutes, depending on the depth of diffusion desired.
  • the silicon dioxide mask 31 confines this chemical reaction, which may be characterized as a displacement reaction, to the space within the windows.
  • the surface concentration of aluminum atoms in the windows 32 from the chemical reaction is typically between about 5 and 7.5 times 10 atoms per c.c.
  • the depth of the difiusions 23a into the base portion 22a to the junction 23b would be about 0.4 of a micron after the anneal step described below.
  • the aluminum diffusions 23a have spread slightly laterally underneath the mask portions 31, the extent of the lateral displacement being about equal to the depth of the diffusion.
  • junctions 23b are formed where the surface concentration of aluminum atoms (a P-type dopant) in atoms per cubic centimeter equals the surface concentration in atoms per c.c. of the phosphorus, an N-type dopant in the base region 22a.
  • the wafer is annealed for 24 hours at 500 C. in an atmosphere of nitrogen essentially but includes 5 percent of hydrogen. This step removes copper pickup and restores the initial background resistivity of the collector material.
  • FIG. 8 the wafer of FIG. 7 is shown after removal of the silicon dioxide layer 31 in the usual manner.
  • a layer of antimony-gold 24a is evaporated on to the surface of the layer 22a covering the aluminum diffu sions 23a.
  • the layer 24a may be of the order of 0.3 to 0.5 microns thick, the purpose being to form ohmic contacts to the base region 220.
  • FIG. 8 there are shown two mask portions 34 of photoresist material formed by the usual photolithographic techniques.
  • the mask portions 34 are left in the positions where the antimony-gold contacts are to be made to the base region 220.
  • a well known etch is appropriately applied to remove unwanted portions of the antimony-gold surface 34.
  • the mask portions 34 are removed and the wafer is then washed and cleaned to give the structure shown in FIG. 9, in which appear the aluminum diffusions 23a and the antimony-gold portions 24 which form contacts to the base region 22.
  • the wafer After the wafer has been cleaned following etching of the antimony-gold layer, the wafer, as shown in FIG. 9, is placed in a furnace and subjected to a temperature of about 400 C. which alloys the antimony-gold contacts 24 into the phosphorus diffused N-base region 22a. As shown in FIG. 10, the lowermost edges of the antimony-gold contacts 24 extend slightly below the surface of the phosphorus diffused layer Following the antimony-gold alloying step, a layer or film of aluminum 35 is evaporated over the whole surface of the wafer, FIG. 10, and would have a thickness of the order of 0.3 micron.
  • Portions 23c of the layer 35 are to be retained above the diffusions 23a.
  • the excess aluminum is removed by the well known photolithographic techniques, leaving portions 230 (FIG. 11).
  • Part of the photolithographic technique is the use of a photomask through which photoresist is exposed to form the patterned areas where the aluminum film is to be retained. That is, over the emitter diffusions 23a.
  • the photomask used with the photolithographic process to remove the excess aluminum may be the same size photomask that is used for masking the base layer 22a during formation of the emitter diffusions 23a in an earlier step.
  • the aluminum remainders 230 are alloyed to the aluminum diffusions 23a by placing the wafer on a moving belt furnace, the wafer reaching an effective temperature of about 430 C. to 435 C.
  • the period of time in the furnace and the temperature thereof may be adjusted so that proper alloying takes place as determined by visual inspection.
  • the length of time in the furnace, its temperature, and the amount of aluminum in the spots 23c are such that the alloyed boundary between the alloyed spot 23c and the diffused spot 23a does not go deep enough to destroy the junction 23b that the aluminum diffusion 23a formed with the base region 220.
  • the aluminum in the spots 230 may short the aluminum diffusion 23a to the base 22a.
  • a new PN-junction 23d is formed between the alloyed aluminum spot 23c and the adjacent boundary or edge ofthe base region 22a. The formation of this new PN-junction 23d removes any emitter-base shorts formed by evaporation of aluminum.
  • the aluminum moves downward into the aluminum diffused region 23a but does not alloy beyond thejunction 23b.
  • the addition of the aluminum spots 23c alloyed to the aluminum diffusions 23a adds greatly to the number of carriers which are available at thejunction 23b, the surface concentration having been increased to about 5 l0 atoms per c.c. as compared with 5 l0 atoms per c.c.
  • the emitter 23 is thus substantially more efficient and is appropriate to the highfrequency application for which it is desired.
  • a silicon dioxide layer 36 is deposited over the whole wafer (FIG. 12) in the manner already described, the layer being about 0.4 of a micron in thickness. If desired, the growth of the layer 36 may take place before the alloying of the aluminum spots 230 to minimize spreading of aluminum alloyed portions 23d.
  • emitter contact windows 37 and base contact windows 38 are opened in the silicon dioxide layer 36 by the photolithographic techniques.
  • a film of aluminum 26a is then deposited by evaporation over the whole wafer (FIG. 13) including down and through the windows 37 and 38.
  • Ohmic contacts are thus made by virtue of the pegs of aluminum projecting into the windows and engaging the emitter portions 23c and the antimonygold base contacts 24.
  • Mesas are etched in the areas 39 in the usual manner and thereafter the wafer is turned upside down and a layer ofindi um-gold backing 41 is deposited on the back surface of the collector region 21a. The wafer is then scribed along l5-mil centerlines and broken to form the individual dice or mesa transistors. Thereafter each die is mounted on a transistor header and hermetically sealed by welding a metal cap thereon.
  • junction devices include not only mesa and moat devices but planar devices wherein isolation between adjacent units in the same plane is achieved by other well known means.
  • the aluminum emitter diffusions 23a localized by patterning a silicon dioxide mask are according to the said Ashton et al. invention.
  • the emitter diffusion thickness is precisely controllable in the nature of the diffusion process.
  • the high concentration bases of N-type material such as antimony and phosphorus, for example, are precisely controllable.
  • Narrow and accurate base widths of about 0.5 micron or less between junctions 23b and 22b have been obtained for ultrahigh frequency devices. Accurately alloying into a diffusion has eliminated alloying emitter punch-through. Control of the base width has in essence, eliminated electrical emitter-collector punch-through. Formation of the new PN-junction during alloying permits use of the same size mask for diffusing emitter and forming emitter contact.
  • the increased surface concentration in the diffused emitter according to the subject invention retains all of the advantages of the Ashton et al. invention. Additionally, surface concentrations of 5 10 aluminum atoms per c.c. are obtained. Commensurate high emitter efficiency and the higher values of beta needed for ultra high frequency devices result.
  • the improved beta permits use of a single high concentration base diffusion for low R C instead of double masking and diffusion steps required when using a lowmoncentration base diffusion and an adjacent high-concentration contact diffusion.
  • FIGS. 15, 16 and 17 relate to processing steps of a wafer which when combined with other process steps already described lead to the transistor 48 shown in FIG. 2 which has a collector 21, a base 49 and emitter 51 according to the invention, and an ohmic contact 52 to the base 49. There are also an overlay silicon dioxide layer 25 and overlay pads 26 and 27 similar to FIG. 1.
  • FIG. 15 there is shown a wafer 21a which may be of the same dimensions as wafer 21a of FIG. 3, may be of P-type germanium, and have the same background concentration P-doping.
  • a layer of silicon dioxide 53 has been deposited over the whole wafer. Windows 54 are opened in the layer 53, by the photolithographic techniques (FIG. 16).
  • the aluminum trichloride displacement reaction as referred to herein and described in the Ashton et al. application is carried on in the window spaces to form the aluminum emitter diffusions 51a. No junction is formed since a P-type dopant is diffused into a P-type material.
  • the halide reaction carried on in the window spaces 54 is localized by the silicon dioxide mask 53 which prevents the diffusions from spreading, except the slight usual lateral amount.
  • the wafer as shown in FIG. 17, with the mask 53 removed, is then placed into the second zone of a two-zone diffusion furnace, the first zone of which contains antimony metal and operates at a temperature of 540 C.
  • the temperature of the wafers in the second zone is 650 C. and the diffusion time is about 4 hours. Other temperatures and diffusion times may be used by those skilled in the art.
  • the ambient in the furnace at this stage is hydrogen gas.
  • the N-type dopant, antimony diffuses into the base region as shown by the arrows.
  • the wafer is treated by a hydrofluoric acid bath for about 20 minutes to remove excess antimony and thereafter is subjected for 1 hour to a temperature of 650 C. in a hydrogen ambient. In this period, antimony out diffuses some and drives in to its final form wherein the junction 55 is about 1.9 microns deep.
  • the junction 55 shifts slightly to form a junction 55a underneath the aluminum diffusion 510, because in this area the concentration of the P-type dopant, aluminum, increases the background concentration of the substrate.
  • the wafers are annealed for 24 hours at 500 C. in an atmosphere as already described.
  • the wafer as shown in this figure, is subjected to the process steps beginning with those of FIG. 8 as already described, and is carried through to the resultant as set out in connection with FIG. 14.
  • the final product accordingly is the mesa transistor shown in FIG. 2 wherein the base width between the base collector junction 55a and the emitter base junction 51b is about 0.5 ofa micron.
  • a high-efficiency aluminum-doped emitter region in an N-type germanium base device comprising an area of diffused aluminum forming a junction in said base and having a surface concentration of aluminum atoms of about X10 atoms per cubic centimeter, and a film of aluminum alloyed to said aluminum area for effecting a surface concentration of about 5X10 atoms per cubic centimeter, the extent of said aluminum alloy terminating short of said junction.
  • a mesa transistor comprising a collector of P-type germanium, a diffused base of N-type germanium, and an aluminum emitter having one portion of aluminum diffused into said base, forming a junction therewith, and having a surface concentration of aluminum atoms of about 5X10 atoms per cubic centimeter, and a second portion comprising a film of aluminum over said diffused aluminum and alloyed thereto, for effecting a surface concentration of about 5X10 atoms per cubic centimeter, the extent of said aluminum alloy terminating short of said junction.

Abstract

In a germanium transistor wherein an aluminum emitter is selectively diffused from an aluminum trichloride vapor phase chemical displacement reaction with germanium and is localized by a patterned or windowed silicon dioxide mask, the surface concentration of said emitter is increased substantially by alloying into said aluminum diffusion, in a nondestructive manner, a film of aluminum evaporated over said diffusion. The surface concentration of aluminum in the emitter can be increased from about 5 X 1019 atoms per cubic centimeter for the diffused emitter to 5 X 1020 atoms per cubic centimeter for the diffused alloyed emitter. Consequently, the efficiency of the aluminum diffused emitter is substantially increased by the subsequent alloying.

Description

waited States Patent Hoffman et a1. Man. 14, 11972 [54] DIFFUSED ALLOYED EMITTER AND 3,293,010 12/1966 Hackley ..29/l95 THE LIKE AND A METHOD OF 3,410,735 11/1968 Hackley.... MANUFACTURE THEREOF 3,453,724 7/1969 Gilbert 3,457,469 7/1969 Lawrence. Inventors! Albert Louis Hoffman, 6408 86th St, 3,341,377 9/1967 Wacker ..148/177 Scottsdale, Ariz, 85251; Derek Elden Longstaff, 2626 N. Foote St., Phoenix, primary Examiner |ohn w Hucken 85008 Assistant Examiner-Martin H. Edlow [22] Filed: May 13 1970 Att0rneyMueller and Aichele [21] Appl. N0.: 36,777 57 ABSTRACT In a germanium transistor wherein an aluminum emitter is [52] US. Cl. ..317/235 R, 317/235 AM, 317/235 J, selectively diff d f an aluminum m m vapor phase 317/235 T, 317/234 L, 148/179, 148/ 1 chemical displacement reaction with germanium and is local- [51 113E. Cl ..H0ll 7/44, H011 7/46 ized by a patterned or windowed Silicon dioxide mask the [58] new of Search "317/235 235 235 234 L; face concentration of said emitter is increased substantially by 148,177 29/589 alloying into said aluminum diffusion, in a nondestructive manner, a film of aluminum evaporated over said difiusion. [56] References Cited The surface concentration of aluminum in the emitter can be UNITED STATES PATENTS increased from about 5X10 atoms per cubic centimeter for the diffused emitter to 5X10 atoms per cubic centimeter for ldZlk, Jr. at the alloyed emitten Consequently the emciency of 10 4/1963 the aluminum diffused emitter is substantially increased by the 2,868,683 1/1959 subsequent alloying. 3,025,439 3/1962 3,462,829 8/ 1969 9Claims, 17 Drawing Figures YIII/IIIII; a,
54 54 s 53 53 \q-/ p 5| 5lb 5H0 P TYPE Ge P TYPE Ge Hg. /5 F/g /6 P TYPE Ge INVENTOR.
. Albert Lou/s Hoffman BY Derek Elder: Longslaff M MM DIFF USED ALLOYED EMITTER AND THE LIKE AND A METHOD OF MANUFACTURE THEREOF RELATED APPLICATIONS The invention of the subject application utilizes the fundamental process of the application Ser. No. 36,310, filed May 13, I970, entitled Method of Diffusing Impurities Into Selected Areas ofa Semiconductor;" Clark L. Ashton, Robert G. Hays and Ronald C. Pennell and assigned to the same assignee as the present invention.
BACKGROUND OF THE INVENTION This invention relates to a method for increasing the surface concentration of aluminum atoms in an aluminum diffusion forming a junction and it is an object of the invention to provide an improved method of this character. The invention also relates to a junction device wherein the aluminum diffusion has its surface concentration of aluminum substantially increased. More particularly the invention relates to such a method and device wherein an aluminum emitter was selectively diffused from vapor phase using a patterned silicon dioxide mask into an N-diffused base region of germanium and it is a further object of the invention to provide an improved method and device of this character.
Because of higher carrier mobilities in germanium semiconductors as compared with silicon semiconductors, germanium is a much preferable material for making high-frequency diodes, transistors and other junction devices. Ultra high frequency devices require ultrasmall geometry diffusions into germanium. Particularly the lateral dimensions of the emitter and the depth of its diffusion into the base are critical for controlling narrow base widths which are important to satisfactory operations of ultrahigh frequency transistors of the mesa, planar, annular and other types. In such devices the lateral dimensions of the emitter are of the order of several microns, the depth of the emitter diffusion may be a fraction of a micron and the base width even less.
In the case of silicon junction devices the use of a silicon dioxide mask formed on the silicon substrate, in combination with the use of photolithographic techniques, has enabled the selective diffusion of emitters and bases from any of the usual N-type dopants and including boron as a P-type dopant. Thus, high-frequency planar transistors and integrated circuits utilizing silicon as a starting material have become common.
However, the frequency response of silicon devices is limited by carrier mobility. Germanium with its much higher carrier mobility, can operate at frequencies two to three times greater than silicon devices. Also, germanium devices have a much higher signal to noise ratio than silicon devices.
Heretofore, silicon dioxide has not been a good mask for the other P-type dopants of aluminum, gallium and indium whether used on a silicon or a germanium substrate. No other simple or satisfactory mask has been known for these dopants.
Consequently, germanium technology lagged behind that of silicon despite the fact that germanium has superior qualities for high-frequency junction devices because of its substantially greater carrier mobility.
Utilizing the techniques of the invention of the said Ashton et al. application Ser. No. 36,310, aluminum and other P-type dopants can now be diffused, as emitters, into N-type germanium bases, for example, thereby making the advantages of a diffused emitter available along with the superior qualities of germanium as a material for high-frequency devices.
In some prior mesa transistors utilizing P-type germanium as a collector, and an N- (antimony) type diffused base, an alloyed aluminum emitter has been used. Such transistors required a relatively low concentration base diffusion to get necessary beta and thus required an adjacent high concentration contact diffusion of N+ material, arsenic, for example, to achieve a low R',, C, (the product of base resistance and collector capacitance) for high-frequency applications, as is well known. Moreover, the alloying of aluminum for the emitter was subject to usual process limitations resulting in a substantial number of devices having alloyed emitter punch-through, electrical emitter-collector punch-through and BV shorts (emitter-base breakdown voltage, collector open). Accordingly, it is an object of the invention to minimize these problems of such prior devices.
The localized aluminum emitter difiusions (patterned silicon dioxide mask) of the said Ashton et al. invention enables precisely controllable emitter diffusion thickness. This, together with the precisely controllable diffusions of high concentration bases of N-type material such as antimony and phosphorus, for example, permits very narrow base widths to be obtained. The alloyed emitter punch-through and electrical emitter-collector punch-through have, in essence, been eliminated.
However, the aluminum emitter diffusion of the said Ashton et al. application has achieved a surface concentration of about 5X10" atoms per c.c. of aluminum whereas a high-efficiency emitter requires concentrations of 5X10 atoms per c.c. or higher. Accordingly, it is a further object of this invention to provide diffused aluminum (and other P-dopant) emitters having surface concentrations of the order stated.
Alloying a film of aluminum to a difiused aluminum emitter under appropriate conditions, nondestructive of the aluminum diffusion as will be described herein, retains all of the advantages of the Ashton et al. invention and, in addition, achieves, in a nonforeseeable manner, a many fold increase in the concentration of aluminum atoms. Thus, there is achieved the desired high emitter efficiency giving high beta values even though base concentrations are high to achieve low R' C product as required for ultrahigh frequency devices. In addition, the separate high concentration contact diffusion of prior devices is eliminated. BV shorts are minimized by reducing the need for high-temperature alloying.
Achievements include: Higher yield, lower cost, higher and more uniform beta (B) and current-gain-bandwidth (f, use of a lesser number of photolithographic masks, and improved reliability. The higher frequency extension enabled by the use of germanium, along with the diffused alloyed process and planar technology with its many inherent advantages are more readily available to the art for RF. power germanium devices, germanium integrated circuits, field effect transistors and ultrasmall germanium devices.
DESCRIPTION OF THE PRIOR ART The prior art has combined various forms of depositing metals and alloying them to form emitters and bases. In U.S. Pat. No. 3,018,539, Taylor et al., U.S. Pat. No. 3,220,894, Ruchardt et al., and U.S. Pat. No. 3,341,377, Wacker, aluminum is vacuum deposited on diffused bases and alloyed thereto to form emitterjunctions. In U.S. Pat. No. 3,028,663, Iwerson et al., a gold layer is first vacuum deposited followed immediately by a silver layer and by a further gold layer before alloying. In U.S. Pat. No. 3,108,359, Moore et al., aluminum is vacuum deposited on a phosphorus diffused emitter, or on a boron diffused emitter, and alloyed thereto to form contacts to the emitter, the character of which is determined by that of the diffusion. In U.S. Pat. No. 3,397,450, Bittmann et al., one kind of an emitter is formed by ionic plating out a metal such as platinum, and a second kind of emitter is formed by vacuum depositing an aluminum film without subsequent alloying.
SUMMARY OF THE INVENTION In carrying out the invention according to one form, there is provided a method of increasing the surface concentration of aluminum atoms diffused from a vapor phase into a selected area of diffused N-type germanium forming part of the P-type emitter of a germanium mesa transistor, said emitter forming a junction at a certain depth comprising the steps of: evaporating and alloying a film of aluminum on said diffusion of an area essentially no greater than the said area of diffusion and essentially aligned therewith, the thickness of said alloyed filrn being less than the ultimate depth of said diffusion, thus maintaining the integrity of said junction.
According to another form of the invention, there is provided a mesa transistor comprising a collector of P-type germanium, a diffused base of N-type germanium, and an aluminum emitter having one portion of aluminum diffused into said base and having a surface concentration of aluminum atoms of about 5 atoms per cubic centimeter, and a film of aluminum evaporated over said diffused aluminum and alloyed thereto without destroying the integrity of the emitter base junction, for effecting a surface concentration of about 5X l O atoms per cubic centimeter.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is an elevational view, somewhat diagrammatic, partially in section, and on a greatly enlarged scale, of a mesa transistor embodying the invention;
FIG. 2 is an elevational view similar to FIG. 1 ofa modified form of device according to the invention;
FIG. 3 is an elevational view ofa wafer of P-type germanium, one type ofstarting material, appropriate to the invention;
FIGS. 4l3 are elevational views similar to FIG. 3, partially in section, illustrating subsequent operative steps according to the invention;
FIG. 14 is an elevational view similar to the preceding figures showing a mesa transistor formed on a wafer prior to scribing and breaking;
FIG. 15 is an elevational view, partially in section, of a wafer of P-type germanium, one type of starting material, leading to the form of the invention shown in FIG. 2; and
FIGS. 16 and 17 are elevational views partially in section similar to FIG. 15 illustrating subsequent operative steps according to the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 of the drawings, there is shown a mesa transistor 20 embodying one form of the invention, the mesa comprising a collector 21 of P-type germanium, a phosphorus diffused N-base 22, a P-type emitter 23, an ohmic contact 24 to the base 22, an overlay 25 of silicon dioxide, and overlay pads 26 and 27, making ohmic contact with the emitter 23 and the base contact 24, respectively. Making contact with the P- type germanium collector 21 is an indium-gold backing layer 28.
In FIG. 3 there is shown a starting wafer of P-type germanium 210 about 150 microns thick which may have a background concentration of P-type dopant such, for example, as gallium, equal to about l l0 atoms per c.c., for ex ample, that is a resistivity of about 4 ohm-centimeters, out of which a large number of mesa transistors as shown in FIG. 1 ultimately may be formed.
In FIG. 4 the wafer 21a is shown with a diffused layer 22a of phosphorus doping which, at this stage of the operation, may be about 0.3 of a micron thick, typically, and will form a large number of N-type bases 22 in the finished transistors.
The layer 220 may be formed in any well-known diffusion furnace through which flow phosphine gas (PI-I in a hydrogen ambient or carrier gas. The wafers are placed in the furnace for about ten minutes and their temperature is maintained at 800 C. The vapor pressure of the phosphine gas is such that, at the end of the diffusion period, the surface concentration of the phosphorus is about 8X10 atoms per cubic centimeter.
In FIG. 5 the wafer of FIG. 4 is shown with a layer 31 of silicon dioxide applied atop the phosphorus diffusion 22a. The layer 31, typically, may be 0.3 to 0.4 ofa micron in thickness and may be formed in a well-known horizontal reactor or chamber to which are supplied silane SiI-I. and oxygen in an ambient of hydrogen or nitrogen at a temperature of about 380 C.
In FIG. 6 the wafer 21a is shown with windows 32 opened in the silicon dioxide layer 31 by well-known photolithographic techniques, the details of which are not believed necessary to be described here. The windows 32 are of dimensions such as to correspond to the desired dimensions of the emitter 23. For
high-frequency applications, the emitters may have lateral dimensions of 15 microns by 30 microns. As many windows 32 may be opened on the wafer 21:: as the dimensions thereof will permit.
In FIG. 7 a two-window portion of the wafer of FIG. 6 is shown on a larger scale. In the windows 32 of FIG. 7 there are shown bubbles 33 which represent the taking place of a localized chemical reaction as described in the said copending Ashton et al. application, Ser. No. 36,310. As described in the Ashton et al. application, the localized reaction 33 taking place in the windows 32 comprises, in one form, the reaction of aluminum trichloride (AlCl with germanium in a hydrogen ambient at a temperature of 800 C. for about 45 minutes, depending on the depth of diffusion desired. The silicon dioxide mask 31 confines this chemical reaction, which may be characterized as a displacement reaction, to the space within the windows.
As a result of this localized chemical reaction, aluminum diffusions 23:: take place into the body of the phosphorus diffused N-base material 220. The surface concentration of aluminum atoms in the windows 32 from the chemical reaction is typically between about 5 and 7.5 times 10 atoms per c.c. The depth of the difiusions 23a into the base portion 22a to the junction 23b would be about 0.4 of a micron after the anneal step described below.
As shown in FIG. 7, the aluminum diffusions 23a have spread slightly laterally underneath the mask portions 31, the extent of the lateral displacement being about equal to the depth of the diffusion.
As a result of the time and temperature existing in the reaction chamber while the aluminum diffusions 23a are made, and following the anneal step described below, the thickness or width of the base portion 220 increased from its initial 03 ofa micron to about 0.9 ofa micron, forming a junction 221;. In this process, the surface concentration of phosphorus decreased to about 4X10" atoms per c.c. As is well known, junctions 23b are formed where the surface concentration of aluminum atoms (a P-type dopant) in atoms per cubic centimeter equals the surface concentration in atoms per c.c. of the phosphorus, an N-type dopant in the base region 22a.
Following the aluminum diffusion step, the wafer is annealed for 24 hours at 500 C. in an atmosphere of nitrogen essentially but includes 5 percent of hydrogen. This step removes copper pickup and restores the initial background resistivity of the collector material.
It will be understood, through this specification, that other dimensions may be utilized by those skilled in the art to make devices according to particular requirements.
In FIG. 8 the wafer of FIG. 7 is shown after removal of the silicon dioxide layer 31 in the usual manner. After appropriate cleaning steps a layer of antimony-gold 24a is evaporated on to the surface of the layer 22a covering the aluminum diffu sions 23a. The layer 24a may be of the order of 0.3 to 0.5 microns thick, the purpose being to form ohmic contacts to the base region 220. In FIG. 8 there are shown two mask portions 34 of photoresist material formed by the usual photolithographic techniques. The mask portions 34 are left in the positions where the antimony-gold contacts are to be made to the base region 220. After the mask portions 34 have been formed as described, a well known etch is appropriately applied to remove unwanted portions of the antimony-gold surface 34. The mask portions 34 are removed and the wafer is then washed and cleaned to give the structure shown in FIG. 9, in which appear the aluminum diffusions 23a and the antimony-gold portions 24 which form contacts to the base region 22.
After the wafer has been cleaned following etching of the antimony-gold layer, the wafer, as shown in FIG. 9, is placed in a furnace and subjected to a temperature of about 400 C. which alloys the antimony-gold contacts 24 into the phosphorus diffused N-base region 22a. As shown in FIG. 10, the lowermost edges of the antimony-gold contacts 24 extend slightly below the surface of the phosphorus diffused layer Following the antimony-gold alloying step, a layer or film of aluminum 35 is evaporated over the whole surface of the wafer, FIG. 10, and would have a thickness of the order of 0.3 micron.
Portions 23c of the layer 35 are to be retained above the diffusions 23a. The excess aluminum is removed by the well known photolithographic techniques, leaving portions 230 (FIG. 11). Part of the photolithographic technique is the use of a photomask through which photoresist is exposed to form the patterned areas where the aluminum film is to be retained. That is, over the emitter diffusions 23a. The photomask used with the photolithographic process to remove the excess aluminum may be the same size photomask that is used for masking the base layer 22a during formation of the emitter diffusions 23a in an earlier step. Since the aluminum diffusions 23a are already present and have formed PN-junctions 23b with the base layer 23a, minor misalignments of the photomask in determining the amount of aluminum left above the diffusions 23a is not too critical. Minor misalignments may be permitted. The reason for this becomes evident in the following steps.
At this stage the aluminum remainders 230 are alloyed to the aluminum diffusions 23a by placing the wafer on a moving belt furnace, the wafer reaching an effective temperature of about 430 C. to 435 C. The period of time in the furnace and the temperature thereof may be adjusted so that proper alloying takes place as determined by visual inspection. The length of time in the furnace, its temperature, and the amount of aluminum in the spots 23c are such that the alloyed boundary between the alloyed spot 23c and the diffused spot 23a does not go deep enough to destroy the junction 23b that the aluminum diffusion 23a formed with the base region 220.
Prior to the alloying step the aluminum in the spots 230 may short the aluminum diffusion 23a to the base 22a. However, after the alloying step has taken place, a new PN-junction 23d is formed between the alloyed aluminum spot 23c and the adjacent boundary or edge ofthe base region 22a. The formation of this new PN-junction 23d removes any emitter-base shorts formed by evaporation of aluminum.
In the process of alloying the aluminum spots 230, the aluminum moves downward into the aluminum diffused region 23a but does not alloy beyond thejunction 23b.
The addition of the aluminum spots 23c alloyed to the aluminum diffusions 23a adds greatly to the number of carriers which are available at thejunction 23b, the surface concentration having been increased to about 5 l0 atoms per c.c. as compared with 5 l0 atoms per c.c. The emitter 23 is thus substantially more efficient and is appropriate to the highfrequency application for which it is desired.
Following the step as described for FIG. 11, a silicon dioxide layer 36 is deposited over the whole wafer (FIG. 12) in the manner already described, the layer being about 0.4 of a micron in thickness. If desired, the growth of the layer 36 may take place before the alloying of the aluminum spots 230 to minimize spreading of aluminum alloyed portions 23d.
Following the steps as indicated in FIG. 12, emitter contact windows 37 and base contact windows 38 are opened in the silicon dioxide layer 36 by the photolithographic techniques. A film of aluminum 26a is then deposited by evaporation over the whole wafer (FIG. 13) including down and through the windows 37 and 38. Ohmic contacts are thus made by virtue of the pegs of aluminum projecting into the windows and engaging the emitter portions 23c and the antimonygold base contacts 24.
Thereafter, the photolithographic techniques are again called upon to remove the excess aluminum leaving only the overlay pads 26 and 27 (FIG. 14) to which leads may be attached in a well-understood manner.
Mesas are etched in the areas 39 in the usual manner and thereafter the wafer is turned upside down and a layer ofindi um-gold backing 41 is deposited on the back surface of the collector region 21a. The wafer is then scribed along l5-mil centerlines and broken to form the individual dice or mesa transistors. Thereafter each die is mounted on a transistor header and hermetically sealed by welding a metal cap thereon.
While mesas have been shown, the emitters and the bases are essentially planar with each other and it is intended that the term planar types of junction devices include not only mesa and moat devices but planar devices wherein isolation between adjacent units in the same plane is achieved by other well known means.
The aluminum emitter diffusions 23a localized by patterning a silicon dioxide mask are according to the said Ashton et al. invention. The emitter diffusion thickness is precisely controllable in the nature of the diffusion process. Similarly, the high concentration bases of N-type material such as antimony and phosphorus, for example, are precisely controllable. Narrow and accurate base widths of about 0.5 micron or less between junctions 23b and 22b have been obtained for ultrahigh frequency devices. Accurately alloying into a diffusion has eliminated alloying emitter punch-through. Control of the base width has in essence, eliminated electrical emitter-collector punch-through. Formation of the new PN-junction during alloying permits use of the same size mask for diffusing emitter and forming emitter contact.
The increased surface concentration in the diffused emitter according to the subject invention retains all of the advantages of the Ashton et al. invention. Additionally, surface concentrations of 5 10 aluminum atoms per c.c. are obtained. Commensurate high emitter efficiency and the higher values of beta needed for ultra high frequency devices result. The improved beta permits use of a single high concentration base diffusion for low R C instead of double masking and diffusion steps required when using a lowmoncentration base diffusion and an adjacent high-concentration contact diffusion.
FIGS. 15, 16 and 17 relate to processing steps of a wafer which when combined with other process steps already described lead to the transistor 48 shown in FIG. 2 which has a collector 21, a base 49 and emitter 51 according to the invention, and an ohmic contact 52 to the base 49. There are also an overlay silicon dioxide layer 25 and overlay pads 26 and 27 similar to FIG. 1.
In FIG. 15 there is shown a wafer 21a which may be of the same dimensions as wafer 21a of FIG. 3, may be of P-type germanium, and have the same background concentration P-doping. As shown, a layer of silicon dioxide 53 has been deposited over the whole wafer. Windows 54 are opened in the layer 53, by the photolithographic techniques (FIG. 16).
After the windows 54 are opened, the aluminum trichloride displacement reaction as referred to herein and described in the Ashton et al. application is carried on in the window spaces to form the aluminum emitter diffusions 51a. No junction is formed since a P-type dopant is diffused into a P-type material. The halide reaction carried on in the window spaces 54 is localized by the silicon dioxide mask 53 which prevents the diffusions from spreading, except the slight usual lateral amount.
Thereafter the silicon dioxide layer 53 is stripped off. The wafer as shown in FIG. 17, with the mask 53 removed, is then placed into the second zone of a two-zone diffusion furnace, the first zone of which contains antimony metal and operates at a temperature of 540 C. The temperature of the wafers in the second zone is 650 C. and the diffusion time is about 4 hours. Other temperatures and diffusion times may be used by those skilled in the art. The ambient in the furnace at this stage is hydrogen gas. During this step, the N-type dopant, antimony, diffuses into the base region as shown by the arrows. The antimony in diffuses through the diffused aluminum emitter areas 510, continuing until the surface concentration of the antimony in atoms per cubic centimeter equals that of the collector material, about 1X10, at which point the junction 55 between the base region 49 and the collector region 21a is formed. Following the in diffusion step, the wafer is treated by a hydrofluoric acid bath for about 20 minutes to remove excess antimony and thereafter is subjected for 1 hour to a temperature of 650 C. in a hydrogen ambient. In this period, antimony out diffuses some and drives in to its final form wherein the junction 55 is about 1.9 microns deep.
In the process of diffusing antimony into the substrate 210, the junction 55 shifts slightly to form a junction 55a underneath the aluminum diffusion 510, because in this area the concentration of the P-type dopant, aluminum, increases the background concentration of the substrate.
After the antimony diffusion step, the wafers are annealed for 24 hours at 500 C. in an atmosphere as already described.
Following the steps as described in connection with FIG. 17, the wafer, as shown in this figure, is subjected to the process steps beginning with those of FIG. 8 as already described, and is carried through to the resultant as set out in connection with FIG. 14. The final product accordingly is the mesa transistor shown in FIG. 2 wherein the base width between the base collector junction 55a and the emitter base junction 51b is about 0.5 ofa micron.
While two embodiments have been shown, there are many others within the scope of this invention.
What is claimed is:
1. A high-efficiency aluminum-doped emitter region in an N-type germanium base device comprising an area of diffused aluminum forming a junction in said base and having a surface concentration of aluminum atoms of about X10 atoms per cubic centimeter, and a film of aluminum alloyed to said aluminum area for effecting a surface concentration of about 5X10 atoms per cubic centimeter, the extent of said aluminum alloy terminating short of said junction.
2. A mesa transistor comprising a collector of P-type germanium, a diffused base of N-type germanium, and an aluminum emitter having one portion of aluminum diffused into said base, forming a junction therewith, and having a surface concentration of aluminum atoms of about 5X10 atoms per cubic centimeter, and a second portion comprising a film of aluminum over said diffused aluminum and alloyed thereto, for effecting a surface concentration of about 5X10 atoms per cubic centimeter, the extent of said aluminum alloy terminating short of said junction.
3. The invention according to claim 2 wherein the film of aluminum is deposited by evaporation.
4. The method of increasing the surface concentration of aluminum atoms of a vapor phase aluminum diffusion having a certain area in N-type germanium, having formed a junction therein at a certain depth, having a certain area, and resulting from the localized chemical reaction of vaporous aluminum trichloride and germanium utilizing a silicon dioxide mask in a hydrogen ambient comprising the steps of:
depositing a film of aluminum on said diffusion and having an area essentially no greater than the said area of diffusion and essentially aligned therewith, and
alloying said film to said diffusion,
terminating the extent of said aluminum alloying short of said junction.
5. The method according to claim 4 wherein the film of aluminum is deposited by evaporation.
6. lna high beta germanium transistor, the method of forming a high-efficiency two-part emitter in a thin N-type base region comprising in combination the steps of:
forming a silicon dioxide mask over the thin base region;
forming an emitter window in said silicon diode mask;
carrying on in the emitter window a localized displacement chemical reaction of vaporous aluminum trichloride and the N-type germanium in a hydrogen ambient, and diffusing aluminum atoms from such reaction into said N-type germanium to a surface concentration of said aluminum atoms of about 5X 10 atoms per cc. to form ajunction in said base region at a certain depth as one part of said twopart emitter;
depositing a film of aluminum on said aluminum diffusion having an area essentially no greater than said area of the aluminum diffusion and essentially aligned therewith; alloying said film to said diffusion at a temperature substantially below the temperature of said diffusion to increase said surface concentration of aluminum to about 5 l0 atoms per cc; and terminating the extent of said aluminum alloying short of said junction as the second part of the two-part emitter.
UNITED STATES PATENT oTTTcE QETTMCATE I QGEQTE Patent No. 3&49 882 Dated March 14, 1972 Inventor s Albert Louis Hoffman. et al It is certified that error appears in the aboveidentified patent and that said Letters Patent are hereby corrected as shown below:
On the cover sheet, insert [73] Assignee Motorola, Inca, Franklin Park, Illinois.
Signed and sealed this 14th day of November 1972.
(SEAL) Attest:
EDWARD M.FLETCHER,JR.
ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents ORM PO-105O (10-69) USCOMM-DC 60376-1 69 a u 5. GOVERNMENT PRINTING OFFICE: 1959 0-366-334,

Claims (5)

  1. 2. A mesa transistor comprising a collector of P-type germanium, a diffused base of N-type germanium, and an aluminum emitter having one portion of aluminum diffused into said base, forming a junction therewith, and having a surface concentration of aluminum atoms of about 5 X 1019 atoms per cubic centimeter, and a second portion comprising a film of aluminum over said diffused aluminum and alloyed thereto, for effecting a surface concentration of about 5 X 1020 atoms per cubic centimeter, the extent of said aluminum alloy terminating short of said junction.
  2. 3. The invention according to claim 2 wherein the film of aluminum is deposited by evaporation.
  3. 4. The method of increasing the surface concentration of aluminum atoms of a vapor phase aluminum diffusion having a certain area in N-type germanium, having formed a junction therein at a certain depth, having a certain area, and resulting from the localized chemical reaction of vaporous aluminum trichloride and germanium utilizing a silicon dioxide mask in a hydrogen ambient comprising the steps of: depositing a film of aluminum on said diffusion and having an area essentially no greater than the said area of diffusion and essentially aligned therewith, and alloying said film to said diffusion, terminating the extent of said aluminum alloying short of said junction.
  4. 5. The method according to claim 4 wherein the film of aluminum is deposited by evaporation.
  5. 6. In a high beta germanium transistor, the method of forming a high-efficiency two-part emitter in a thin N-type base region comprising in combination the steps of: forming a silicon dioxide mask over the thin base region; forming an emitter window in said silicon diode mask; carrying on in the emitter window a localized displacement chemical reaction of vaporous aluminum trichloride and the N-type germanium in a hydrogen ambient, and diffusing aluminum atoms from such reaction into said N-type germanium to a surface concentration of said aluminum atoms of about 5 X 1019 atoms per c.c. to form a junction in said base region at a certain depth as one part of said two-part emitter; depositing a film of aluminum on said aluminum diffusion having an area essentially no greater than said area of the aluminum diffusion and essentially aligned therewith; alloying said film to said diffusion at a temperature substantially below the temperature of said diffusion to increase said surface concentration of aluminum to about 5 X 1020 atoms per c.c.; and terminating the extent of said aluminum alloying short of said junction as the second part of the two-part emitter.
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