US 3619288 A
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United States Patent Inventor Erhard Sirtl Midland, Mich. App]. No. 886,946 Filed Dec. 22, 1969 Patented Nov. 9, 1971 Assignee Siemens Aktiengesellschaft Berlin, Germany Priority Jan. 2, 1969 Germany P 19 00 119.5
PROCESS FOR PRECIPITATING A HIGH MELTING METAL CONTACT LAYER AT LOW TEMPERATURES 15 Claims, 1 Drawing Fig.
US. Cl 117/227, l17/l07.2 R, 23/203 C Int. Cl C23c 11/02 Field of Search 23/203 C; l 17/ 107.2 R
semiconductor References Cited OTHER REFERENCES Zeitsclir'ift fur angewandte Chemie Vol. 79, No. l pages 27- 43, 1967 Primary Examiner-William L. Jarvis Attorneys curt M. Avery, Arthur E. Wilfond, Herbert L.
Lerner and Daniel J. Tick ABSTRACT: A method for precipitating a high melting metal contact layer, at low temperatures, through thermal dissociation of a gaseous compound of the high melting contact metal and precipitating the same upon a carrier body, preferably of material. The metal contact layer is precipitated upon the carrier body through thermal dissociation of the easily volatile trifluorophosphine or trifluorophosphine hydride of the respective metals.
PROCESS FOR PRECIPITATING A HIGH MELTING METAL CONTACT LAYER AT LOW TEMPERATURES My invention relates to a method for precipitating a highly melting metal contact layer, at low temperatures through thermal dissociation of a gaseous compound of the high melting contact metal and precipitating the same upon a carrier body preferably of semiconductor material.
Highly purified thin metal layers with extremely high melting temperature or extremely low moisture can be obtained by purely vapor depositing methods, only with great difiiculty. The heretofore used gas reactions invoke a reduction of fluorides or of chlorides with hydrogen or the pyrolitical dissociation of carbonyl compounds.
The known methods all have shortcomings which greatly influence the production of uniform metal contact layers. Thus, reaction of a fluoride brings about difficulties, caused by hydrofluoric acid formation during dissociation. Reduction of the chloride requires relatively high temperatures, for a substantial precipitation of the metal. Oxygen and carbon are formed during the dissociation of carbonyl become installed into the metal lattice or interfere with a homogeneous precipitation in the form of a foreign phase.
My invention overcomes these disadvantages by precipitating the metal contact layer through thermal dissociation of the readily volatile trifluorophosphine or trifluorophosphine hydride of the respective metals upon the carrier body. The advantages obtained thereby result from the fact that these compounds are:
l. easily volatile 2. substantially nonaggressive, hence to not entail difficulties associated with the work material.
3. easily dissociate, forming thereby relatively inert phosphorus trifluoride (Pl-" and 4. easily purified, which is of importance for the quality of the precipitated metal layer.
It is within the scope of the present invention to use trifluorophosphine compounds of the metals nickel, cobalt, iron, chromium, molybdenum, tungsten, niobium, tantalum, vanadium and/or metals of the platinum group.
The following table gives a picture regarding the pertinent metal complexes which were tested more closely by Th. Kruck in Zeitschrift fur angewandte Chemie, 79, 27 1967).
Sublimation or It was found to be most advantageous and to effect an improvement in the adhesiveness of the metal contact layer on the carrier body, to subject the surface of the carrier, prior to precipitating the metal contact layer, to a pretreatment through the action of sulphur hexafluoride (SR) or nitrogen trifluoride (NE), at elevated temperatures, preferably between 500 and l,000 C.
According to a preferred embodiment, hydrogen and/or noble gases are used as a carrier gas for the thermal dissociation of the trifluoro phosphine or trifluoro phosphine hydride of the respective metals. The variable decomposition of the PF; complex, due to the viscosity or the heat transfer differences of both carrier gas types, is to be taken into account at otherwise equal testing conditions.
According to another embodiment it is also possible to effect the thermal dissociation of the trifluorophosphine compound at a reduced pressure, preferably in a vacuum of 1 Torr. This can be done with or without a carrier gas. The reaction temperature must, of course, be adjusted to the pressure conditions. One can also operate within a flowing gas system.
The temperature range of 350 to 600 C. which is required for thermal dissociation of the trifluorophosphine'compounds is adjusted through indirect heating of a quartz table which is in thermal contact with the carrier body. It was found preferable, to use a slit molybdenum disc as a heater, which is rinsed by argon in order to avoid an oxidating effect caused by air.
The present invention also affords the opportunity of carrying out a selective and even an epitactic precipitation of metal layers. The thermal dissociation is so controlled that an additional energy source which acts from the outside, eg a selective UV radiation, limits the metal precipitation to specific regions of the carrier surface. ln this manner, all possible metal structures can be produced in a simple and rational manner, upon carrier bodies with and without masking layers.
In addition to semiconductor materials, such as germanium, silicon, or A"B" compounds; quartz or ceramic as well as metallic systems can also be used as materials for carrier bodies.
The temperature of the vaporizing vessel containing the trifluorophosphine compounds, is preferably from 20 to 100 C According to a preferred embodiment of the present invention, metal contact layers are precipitated at a thickness of approximately 1,000 A. These layers of metal contact are characterized through a particularly high purity and heat resistance, uniformity of the layer design and by a good electrical conductance.
The aforementioned qualities make the layers particularly well suited for the production of semiconductor device components, especially of metal base transistors and Schottky diodes. However, their use is not limited to semiconductor art, as can also be used with the same good results for producing frontal contact layers for electrical capacitors and resistors. Another application possibility associated with the device component industry, is plating radio tubes.
The single FIGURE of the drawing schematically shows a device suitable for carrying out the invention.
The invention will be described with reference to the precipitation of a tungsten contact metal layer upon a carrier body comprising a silicon semiconductor crystal using the apparatus of the FIGURE.
A quartz tube reaction chamber 1 holds a silicon crystal substrate 2 to be coated on a quartz table 3, into which a slit molybdenum wafer 4 is so installed as a heater that it can be rinsed during operation by a current of gaseous argon (flow rate 3 to 10 liter/hour), or other inert gas in order to flush out all air. The argon gas is blown in as indicated by arrow 5. The molybdenum wafter 4 is heated by the current leads 6 and 7 and the silicon crystal wafer 2, which is processed with pure aqueous HF, is first heated to a temperature of 750 C. The
nitrogen trifluoride (NF,,) taken from a storage container 22,
situated in a branch line 21, is thinned with argon (30 l./h.), with a mole ratio n(NF )/n(Ar) of 10 to 10", is passed for about 15 minutes through reaction chamber 1 which exposes the, pure silicon surface on the silicon crystal wafer 2. The flow meter 23, installed in the branch line 21 and the valves 24 and 25 are used to regulate the etching gas current. Hydrogen is subsequently passed via flow meter 26 through a gas supply line 8 with valve 27 open, at a flow rate of 30 l./h., with a cooling trap 9 (temperature bath -78 C.) and thence in a direction indicated by arrow 10, across a vaporization vessel 11. The hydrogen which acts as a carrier gas, thus becomes charged with the tungsten-trifluoride-phosphine (W(PF 12, contained in the vaporization vessel 11 maintained by a temperature bath 13, at 80 C. The compound, mixed with the carrier gas is then passed via a frit or screening plate 14, into the reaction chamber 1 and is dissociated at the gas-etched silicon carrier body 2 which is being maintained at 450 C. to precipitate tungsten. After about 30 minutes, an approximately 1000 A thick tungsten layer of high uniformity has formed on the silicon crystal wafer and is in tight contact with the silicon surface. With the aid of valves 15, 16 and 17, shown in the drawing, the reaction chamber can be charged, according to the position of the valves, with only pure carrier gas or with only trifluoridephosphine compounds. Valves 18 and 19 assure an exact adjustment of the flow rate of the carrier gas current. The residual gases and the volatile reaction products, leave the reaction chamber at the arrow 20.
The other complexes described above in the table, behave analogously.
1. A method of precipitating a high melting metal contact layer, at low temperatures, through thermal dissociation of a gaseous compound of the high melting contact metal and precipitating the same upon a carrier body, said contact metal layer is precipitated upon the carrier body through thermal dissociation of the easily volatile trifluorophosphine or trifluorophosphine hydride of the respective metal.
2. The method of claim 1, wherein a semiconductor is the carrier body.
3. The method of claim 2, wherein trifluorophosphine compounds of a metal selected from nickel, cobalt, iron, chromium, molybdenum, tungsten, niobium, tantalum, vanadium and metals of the platinum group, is used.
4. The method of claim 3, wherein the carrier surface is subjected, prior to precipitation of the contact metal layer, to a pretreatment of sulphur hexafluoride (SP or nitrogen trifluoride (NF at elevated temperatures.
5. The method of claim 3, wherein the temperature range is between 500 and l,000 C.
6. The method of claim 1, wherein hydrogen or a noble gas is used as a carrier gas during the thermal dissociation of trifluorophosphine compounds.
7. The method of claim 6, wherein the thermal dissociation is effected at reduced pressure, preferably in a dynamic vacuum of 10 to 1 Torr.
8. The method of claim 7 wherein the pressure is reduced 10 to 1 Torr.
9. The method of claim 1, wherein the carrier body is heated to a temperature required for thennal dissociation, through indirect heating of a quartz table in thermal contact therewith.
10. The method of claim 9, wherein a slit molybdenum wafer is used as a heater and rinsed with argon.
11. The method of claim 1, wherein the thermal dissociation is at a temperature of 350 to 600 C.
12. The method of claim 1, wherein a selective precipitation of the contact metal layer upon the carrier surface is produced by an energy source which acts from the outside.
13. The method of claim 1, wherein the carrier body is selected from quartz, ceramic or metal.
14. The method of claim 6, wherein the trifluorophosphine compound is vaporized at from 20 to C.
15. The method of claim 1, wherein the contact metal layer is precipitated in thickness of 1000 A.
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