Method For Depositing Refractory Metals

Abstract

A method for depositing a refractory metal on a semiconductor oxide coating, comprising simultaneously etching the oxide coating with the metallic hexafluoride and depositing a relatively thin layer of the refractory metal by reduction of the hexafluoride, and thereafter depositing a relatively thick layer of the refractory metal by reduction of the hexafluoride alone.

Description

BACKGROUND OF THE INVENTION

The present invention relates to an improved method for depositing a refractory metal on certain substrates and, more particularly, to an improved method for depositing tungsten on a substrate which comprises silicon dioxide.

An important advance in the ability to deposit refractory metals on silicon dioxide layers is disclosed in U.S. Pat. No. 3,477,872 to Amick. This method is a two-step process in which the oxide layer is first etched with the hexafluoride of the refractory metal, in an inert atmosphere. Thereafter, the refractory metal is deposited on the etched oxide layer by the hydrogen reduction of the hexafluoride. This process results in a refractory metal layer which adheres well to the oxide layer. However, it is often difficult to control the etching rate in a reproducible manner; thus, the oxide is often etched back beyond the PN junctions of the device which extend to the device surface, thereby exposing the junctions and making the device unusable.

SUMMARY OF THE INVENTION

The present invention comprises an improved process for depositing tungsten or molybdenum on a silicon dioxide layer. The process comprises the steps of heating the silicon dioxide layer in an enclosed chamber to a temperature of about 500.degree.C to 800.degree.C. An inert gas, such as nitrogen, and a reducing gas, such as hydrogen, are introduced into the chamber in the ratio of between about 20:1 to 40:1, respectively. The silicon dioxide layer is then treated with a vaporized substance selected from the group consisting of tungsten hexafluoride and molybdenum hexafluoride for a brief period, in order to etch the silicon dioxide layer and deposit a relatively thin layer of tungsten or molybdenum thereon. The chamber is then purged of all the unreacted hexafluoride. The inert gas flow is discontinued, and additional reducing gas and newly vaporized hexafluoride are mixed in the chamber to reduce the hexafluoride and deposit a relatively thick layer of the metal constituent thereof on the thin layer.

Treatment of the silicon oxide surface with the hexafluoride etches the oxide layer, modifying its properties such that a refractory metal layer adheres much better than without such treatment; additionally, the simultaneous reduction of a small amount of the hexafluoride controls the etching attack during the etching step, which protects the silicon dioxide over the PN junctions from further etching.

THE DRAWINGS

FIG. 1 is a cross-section of a semiconductor chip with circuit components already fabricated therein, and provided with a silicon oxide layer on which tungsten is to be deposited in accordance with the method of this invention.

FIG. 2 is a view like that of FIG. 1 showing further steps which may precede the carrying out of the present method.

FIG. 3 is a schematic diagram, partially in section, of apparatus that could be used in carrying out the method of the present invention.

FIG. 4 is a view like that of FIGS. 1 and 2 illustrating a first step in carrying out the present method.

FIG. 5 is a view like that of FIG. 4 showing a further processing step in carrying out the method of the present invention.

FIG. 6 is a view like that of FIG. 5 showing a further processing step that may be used in making a microcircuit.

DETAILED DESCRIPTION -- EXAMPLE

A specific example will now be given of how the method of the present invention can be utilized in making a microcircuit of the silicon monolithic type, but it will be understood that this is by way of example only and that the method is not limited to the manufacture of any particular product.

As shown in FIG. 1, the microcircuit being fabricated comprises a single crystal chip of silicon 2 having a transistor 4 and a diffused resistor 6 fabricated therein. The transistor 4 includes a base region 8 and an emitter region 10 made by diffusing suitable impurities into one surface of the chip 2. The transistor 4 also includes a collector region which is part of the chip 2 outside the base region 8. The resistor 6 is also made by diffusing suitable impurities into the chip 2. The entire surface of the chip 2, into which the circuit components 4 and 6 are fabricated, is covered with a silicon dioxide layer 12 which protects the PN junctions exposed at the surface of the silicon chip, and also serves as an insulating substrate for carrying metal interconnections.

The circuit components are to be interconnected with conductors made of tungsten. In order that interconnecting conductors can make contact with the circuit components, as shown in FIG. 2, suitable openings are made in the silicon dioxide layer 12 using conventional photoresist masking and etching techniques. By these techniques, an opening 14 in the layer 12 is made to expose a part of the base region 8, and an opening 16 is made to expose a part of the emitter region 10. Similar openings 18 and 20 are provided to expose portions of the opposite ends of the resistor 6.

In order to carry out the processing steps of the present method, the chip 2, as shown in FIG. 2, is placed within a quartz furnace tube 22 which is a part of the apparatus shown in FIG. 3. The assembly is supported on the top of a susceptor block 24 made of carbon coated with silicon carbide. The susceptor block rests on a tilted quartz support 26. The interior of the carbon block contains a thermocouple 28 connected by wires 29 to an RF generator, not shown. The furnace tube 22 is provided with an inlet tube 30 and a flow meter 32 which measures the flow rate of the incoming gas. The furnace tube is also provided with an outlet tube 34 so that exhaust gases may be passed off to a fume hood or other disposal means, not shown. A gas manifold 35 is connected to the flow meter 32 which measures the incoming flow of gas to the furnace tube 22. Connected to the manifold 35 is a flow meter 36 and an inlet tube 38 provided with suitable valves for admitting measured quantities of an inert gas to the system; in this example, nitrogen is used. Also connected to the gas manifold 35 is a second flow meter 40 and an inlet tube 42, provided with suitable valves, for admitting a reducing gas into the system; in this example, hydrogen is used. Included in the hydrogen inlet line is a palladium diffuser 44, which serves to purify the hydrogen gas. Also connected to the manifold 35 is a third flow meter 46 and an inlet tube 48 for admitting tungsten hexafluoride into the system. The gas inlet tube 48 is provided with a branch tube 50 which has a valve for admitting nitrogen into the line for purging purposes.

Before introducing the microcircuit chip into the furnace tube 22, the silicon dioxide surface may first be suitably cleaned, e.g., by rinsing in Methanol, or any other suitable cleaning agent. This step is not essential and may be omitted if the silicon dioxide surface is already sufficiently clean.

After the microcircuit is placed within the furnace tube 22, the tube is closed off and then heated by means of an RF induction coil 52 from the RF generator previously mentioned but not shown, to a temperature of about 500.degree. C to 800.degree. C (preferably at about 700.degree.C). Meanwhile, nitrogen is admitted through the inlet tube 38 and the flow meter 36 so that it passes through the manifold 35, the flow meter 32 and the inlet tube 30, to enter the furnace tube 22 at a rate of about 30 cfh (cubic feet per hour).

Tungsten hexafluoride in gaseous form is then admitted through the inlet tube 48 and the flow meter 46 to join the nitrogen stream in the manifold 35 and pass into the furnace tube 22 at a rate of about 30 cc per minute (about 0.063 cfh). Hydrogen is also admitted through the inlet tube 42 and the flow meter 40 at a rate of about 1.0 cfh to join the nitrogen and tungsten hexafluoride streams in the manifold 35, and pass into the furnace tube 22. It is necessary to maintain the ratio of nitrogen to hydrogen between about 20:1 to 40:1, respectively, because it has been found that a ratio of significantly below 20:1 results in an undesirable amount of hydrogen reduction, while a ratio significantly above 40:1 results in an undesirably high etching rate. A ratio of about 30:1 nitrogen to hydrogen is optimum.

As the mixture of nitrogen, hydrogen, and tungsten hexafluoride passes over the microcircuit assembly, the silicon which is exposed at the bottom of the openings 14, 16, 18, and 20 in the silicon dioxide layer 12, reacts with the tungsten hexafluoride, and by a silicon replacement reaction, a thin layer of tungsten 54 is deposited in the openings and on the silicon (FIG. 4). At the same time, the tungsten hexafluoride gas etches the surface of the silicon dioxide layer 12 and roughens it slightly; simultaneously, the hydrogen reacts with the tungsten hexafluoride, and, by a hydrogen reduction process a thin layer 56 of tungsten, about 2,000A. thick, is deposited over the silicon dioxide layer 12, as shown in FIG. 4. The present step of treatment is permitted to continue for a few seconds.

At the conclusion of the etching and partial deposition step, the tungsten hexafluoride flow is terminated, and the tungsten hexafluoride is purged from the apparatus by permitting nitrogen to continue flowing into the system at a rate of about 4,000 cc per minute (about 8.4 cfh) for a time sufficient to sweep all the unreacted hexafluoride out of the furnace tube 22. The nitrogen flow is then discontinued.

The hydrogen flow is continued through the furnace tube 22 at a rate of about 2,000 cc per minute (about 4.2 cfh). Next, tungsten hexafluoride at a rate of about 30 cc per minute (about 0.063 cfh) is again admitted into the gaseous stream through the inlet tube 48 and the flow meter 46. The heating temperature of the assembly is the same as mentioned previously. Under these conditions, as shown in FIG. 5, the hydrogen reduces the tungsten hexafluoride and deposits tungsten on all of the heated surface. Thus, a relatively thick layer of tungsten is deposited on the entire surface of the thin layers of tungsten 54 and 56 over the silicon and the silicon dioxide. Suitably, the composite thick tungsten layer 56' is about 2.0 microns thick. At the conclusion of the tungsten deposition process, the tungsten hexafluoride and hydrogen flows are discontinued, and nitrogen is admitted through the branch line 50 to purge the apparatus of the corrosive tungsten hexafluoride to prevent attack of the apparatus walls.

In order to remove unwanted tungsten from the layer 56' and leave only the desired pattern of interconnections, excess tungsten can be removed with any suitable masking and etching technique, resulting in a microcircuit like that shown in FIG. 6.

Although the method has been illustrated with an example in which tungsten is the deposited metal, molybdenum can be similarly deposited from molybdenum hexafluoride.

Claims

I claim:

1. An improved process for depositing tungsten or molybdenum on a silicon dioxide layer, comprising the steps of:

a. heating said silicon dioxide layer in an enclosed chamber to a temperature of about 500.degree.C. to 800.degree.C.;

b. introducing an inert gas and a reducing gas into said chamber;

c. treating said silicon dioxide layer with a vaporized substance selected from the group consisting of tungsten hexafluoride and molybdenum hexafluoride for a brief period to etch said silicon dioxide layer and deposit a thin layer of tungsten or molybdenum thereon,

d. purging all of the unreacted hexafluoride from said chamber,

e. discontinuing the introduction of said inert gas into said chamber, and thereafter

f. mixing additional reducing gas with more of said vaporized substance in said chamber to reduce said substance and deposit a thick layer of the metal constituent thereof on said thin layer.

2. A process according to claim 1, wherein the ratio of inert gas to reducing gas is between about 20:1 to 40:1.

3. A process according to claim 2, wherein said reducing gas consists essentially of hydrogen.