GB2171360A - Etching aluminum/copper alloy films - Google Patents

Abstract

Films of an aluminum/copper alloy on selected substrates are etched by means of a chlorine-containing gaseous plasma in a reaction chamber while maintaining the gas pressure at between 300 and 1500 millitorrs and holding the temperature at between about -30 and +15 degrees C.

Description

SPECIFICATION Method of etching aluminum/copper alloy films This invention relates to the manufacture of integrated circuit and other solid state semiconductor devices having metal films as interconnection layers, and more particularly to an improved method of etching those metal films.

Background of the invention Commonly integrated circuit devices (IC devices) are formed on silicon or sapphire substrates which typically have a circular form and are called "wafers" or "chips". Virtually every IC fabrication process involves (1) a metallization step in which at least one film of a conductive metal such as aluminum or an aluminum alloy is formed as a discrete layer on the substrate, and (2) an etching step in which one or more patterns are created in the discrete metal layer according to a predetermined image. Each etched metal film serves as an interconnection layer for some or all of the devices on the substrate. The metal layers are imaged by the use of photosensitive resists which are processed to serve as etching masks. An alternative masking process used in etching aluminum and aluminum alloy films is disclosed in U.S.Patent 4314874 where selected areas of the aluminum or aluminum alloy film are irradiated by an oxygen ion beam to form oxygen ion implanted regions.

The implanted regions serve as a mask, whereby unimplanted areas of the aluminum film are etched by subjecting the aluminum film to plasma etching in a carbon tetrachloride gas atmosphere, or in an atmosphere containing other etching gases such as bromine bichloride (BrCI2) or trichloroethylene (C2HCl2). Metal film layers may be etched by means of either wet or dry etchants, e.g., by coating the exposed portions of the film with a liquid etchant solution, by reactive ion etching, and by plasma etching.

In recent years, plasma and reactive ion etching have been rapidly supplanting wet etching in IC production technology. Various different kinds of plasma and reaction ion etching systems for use in IC production are known, as illustrated by U.S. Patents Nos. 4,255,230; 4,261,762; 4,353,777; 4,357,195; 4,376,672; 4,405,406; and 4,422,897.

The etching of aluminum and its alloys is not understood as well as is the etching of silicon. It is known that plasma and reactive ion etching of aluminum may be conducted with various gaseous materials, including Cl2, Br2, HCI, HBr, CCI, and BCl3. However, aluminum films cannot be etched with fluorine based gases due to the fact that aluminum fluoride gas (the etching reaction product) has a relatively low vapor pressure that complicates its removal from plasma and reactive ion etching systems. Where chlorine-based gases are used for reactive ion etching or plasma etching of aluminum, gaseous aluminum chloride (AICI2) is formed as a volatile etching reaction product. Aluminum chloride gas has a relatively high vapor pressure that facilitates its removal from the etching system.Conse-quently chlorine-based gases are mostly used for plasma and reactive ion etching of aluminum. The most common plasma etching techniques for etching aluminum films on silicon IC devices employ a mixture of CCI, and Cl2 gases, usually in the presence of an inert gas such as argon or helium.

The prior art seems to indicate that significant ion bombardment is required to plasma etch aluminum and aluminum copper alloys. It is speculated that the significant ion bombardment serves to activate the aluminum/chlorine reaction or to remove the surface aluminum oxide coating. In any event, aluminum and aluminum/silicon are easier to etch than aluminum alloys containing copper, although alloys of aluminum and copper have been plasma etched using CCI, and Cl2.

In the plasma etching of aluminum/copper alloys, one of the etching reaction products is copper chloride (CuCI and/or CuCI2). Because copper chlorides have low volatility, a common practice is to heat the wafers to about 200 degrees C during the etching of copper-containing aluminum alloys.

However, etching of aluminum/copper alloys has been achieved without deliberate wafer heating. It also has been suggested that etching of aluminum/ copper alloys may be improved by a post treatment to remove copper residues. It has been proposed that the residue (after etching) on aluminum copper silicon metallized wafers should be removed by rinses in nitric acid, a wetting agent, and deionized water. Another proposal has been to follow a carbon tetrachloride-helium plasma etching method with an in situ oxygen plasma clean.

Presently the trend in the semiconductor industry is to use cassette-to cassette single wafer etching machines to etch films of aluminum or aluminum alloys on silicon wafers. Single wafer machines are plasma etching machines in which a single wafer at a time is subjected to plasma etching in a vacuum chamber, the single wafer machine including means for automatically (a) feeding wafers one at a time from an input cassette to the vacuum chamber, and (b) removing the wafers one at a time from the vacuum chamber and transporting them to an output cassette. Single wafer plasma etching machines have two mutually-confronting, vertically spaced electrodes in the vacuum chamber that are coupled to an RF power supply. The lower electrode is grounded and the machine typically include means for clamping the wafer to the ground electrode during the time that the wafer is undergoing etching.

Single wafer machines offer the advantage that the entire etching operation is automated and is conducted without the use of operator intervention. However, because aluminum/copper alloys are more difficult to etch than aluminum films, it has been virtually impossible to satisfactorily etch aluminum/copper alloy films in automatically controlled single wafer plasma etching machine so as to achieve a production rate comparable to the rate at which wafers containing aluminum films can be etched. Instead the industry has been using batch etching in which anywhere from 5 to 30 wafers at a time are etched.

Objects of the invention The primary object of the invention is to provide an improved method of etching aluminum/copper alloy films using plasma etching techniques.

A more specific object of the invention is to provide a method for etching aluminum/copper alloy films on semi-conductor wafers which is faster than prior plasma etching techniques.

These and other objects of the invention are achieved by conducting plasma etching of alumi num [ copper alloys at a relatively high pressure and a relatively low temperature in the etching chamber. Etching is achieved with a power density substantially higher than has been customary in plasma etching of aluminum/copper alloys.

Other features and many of the advantages of the invention are set forth in the following detailed description of the invention which is to be considered together with the accompanying drawings.

The drawings Figures lA-IFschematically illustrate a conventional aluminum etching sequence.

Detailed description of the invention The present invention is intended to be used to facilitate etching of aluminum/copper alloys in various semiconductor fabrication processes. Figures 1A to 1F illustrate the several steps of a typical aluminum metallizing and etching sequence used in the semiconductor industry, which may be improved upon by etching with the method of the present invention.

Referring to Figure 1A, a silicon substrate or chip 2 has portions of one surface coated with islands 4 of SiO2, and in turn portions of the SiO2 islands 4 are coated with a glass 6. In Figure 1B a layer 8 of aluminum has been deposited over the glass and the exposed silicon surfaces.

In Figure 1C a photoresist 10 has been deposited over the aluminum layers. In Figure 1D the photoresist has been imaged and developed, as a consequence of which some portions of the resist have been removed and other portions 10A of the resist remain.

In Figure 1E the exposed portions of the aluminum have been etched away. In Figure 1F the pho toresist has been removed, with the result that portions of glass layer 6 and aluminum layer 8 are exposed.

When aluminumicopper alloys are plasma etched using chlorine-containing gases as described above, the normal reaction products are CuCI and CuCI2. Both of these reaction products have a very low vapor pressure which complicates their removal from the reaction chamber. The problem of removing these reaction products has been attacked by the maneuver of using a lower pressure and a higher temperature in the etching chamber. Typically etching of aluminum/copper alloys using carbon tetrachloride and chlorine has been conducted at a temperature between 70 and 100 degrees C and a pressure of between 1 and 20 millitorrs. However, even under such conditions the etching of aluminum/copper alloys is relatively slow. Hence, as noted above, industry prefers to conduct the etching on a batch basis rather than by using an automated single wafer plasma etching machine.

We have discovered that plasma etching of alu minumicopper alloy films using chlorine-containing gases can be accomplished at a faster rate by (a) increasing the pressure in the vacuum chamber to a level substantially above 20 millitors, and (b) decreasing the temperature to a level substantially below 70 degrees C.

More specifically we have determined that improved etching rates can be achieved if aluminum copper alloy films are etched using a mixture of carbon tetrachloride and chlorine if the pressure within the vacuum chamber is maintained to between 300 and 1500 millitorrs, preferably between about 500 and about 900 millitorr, and the temperature in the vacuum chamber (i.e., at the surface of the metal film on the substrate) is maintained at a temperature between -30 degrees C and +15 degrees C, preferably between about -5 degrees C and about +10 degrees C. The foregoing conditions are suitable for etching aluminum/copper alloy films containing up to 4% copper (remainder aluminum). As a further departure from prior techniques, it is preferred that the etching method be conducted using maximum power densities in the range of from 1.5 to 3.5 watts per square centimeter.However, power densities down to about 1.0 wattes per square centimeter may be used where maximizing the rate of production is not critical.

Prior known methods have suggested maximum power densities of 1 watt per square centimeter.

The composition of the etching gas mixture may vary. Preferably the ratio of CCI, to Cl2 is in the range of from 3:1 to 10:1 by volume, and the ratio of those gases to the inert diluent (usually helium) ranges from about 1:1 to about 5:1 by volume. In a typical cassette to cassette single wafer etching machine, the active gases, i.e., the mixture of CCI, and Cl2, is introduced to the reaction chamber at flow rates between about 10 and 50 SCCM (standard cubic centimeters), preferably between about 25 and 35 SCCM, and the inflow of inert diluent gas, e.g., helium, is maintained between about 50 and 500 SCCM.

Following is a specific example illustrating the preferred method of practicing the invention. In this example, the etching method is carried out in a Balzers Model SWE 654 Cassette-To-Cassette Single Wafer Etching Machine made and sold by Balzers AG of Liechtenstein and Balzers of Hudson, NH 03051.

Example A plurality of silicon wafers coated with 1 micron thick films of an aluminum/copper alloy containing about 4% copper were placed in a sender cassette in the sender chamber of a Balzers SWE 654 single wafer plasma etching machine, and maintained there in an atmosphere of helium under a pressure of about 0.5 to about 1.5 Torr. Subsequently each wafer in turn was removed from the sender chamber and transported to the reaction vacuum chamber where it was subjected to plasma etching under the conditions hereinafter described, after which it was removed from the reaction chamber and transported to the receiver cassette in the receiver chamber.

On introduction to the reaction chamber, each wafer was clamped to the lower electrode and held there during the etching operation. The reaction chamber was maintained at a pressure of about 750 millitorr during the etching operation. Additionally a heat exchanger fluid was circulated continuously through a tortuous passageway in the lower electrode so as to keep the lower electrode at a temperature of about +5 degrees C. as long as the machine was in operation. Upon seating of a wafer on the lower electrode, a 13.56 MHz RF power supply coupled across the electrodes was energized so as to create an RF field between the two electrodes, and simultaneously a mixture of helium, CCI, and Cl2 was supplied to the reaction chamber via the upper electrode at a rate of 125 SCCM for helium, 25 CCM for CCI4, and 5 CCM for Cl2.The two electrodes were spaced apart a distance in the range of 7 to 14 millimeters and the power supply was operated at a power level of about 100 to 250 watts. That power level resulted in a power density of about 2.8-3.0 watts per square centimer of wafer. The power supply and gas flow into the reaction chamber were shut off after about 120 seconds, after which the wafer was unclamped from the lower electrode and transported into the receiver chamber where it was received by a receiver cassette. The power supply and the gas flow were turned on again after the next wafer had been introduced to the reaction chamber and clamped to the lower electrode. Each wafer was subjected to etching under same conditions and for the same time period of 120 seconds.

The receiver cassette was maintained in an atmosphere of helium under a pressure of about 0.5 to about 1.5 torr.

Subsequent inspection of the wafers in the receiver chamber revealed that the aluminum/copper alloy films on the wafers were completely etched through. Using the foregoing operating conditions it was possible to operate the machine so that a minimum of 30 wafers per hour were satisfactorily etched.

A variety of experimental runs have confirmed that the method of this invention will function equally well if the operating conditions are varied within the limits hereinabove set forth and the composition of the etching gas is varied within the limits noted above. Additionally, for example, the etching gas may comprise little or no Cl2 or may include a bromine-containing gas of the kind hereinabove mentioned, or ethylenetrichloride (C2HCl2). Of course, the helium gas, which functions as a carrier or diluent, may be omitted or replaced with another suitable non-reactive gas such as argon.

An advantage of the invention is that plasma etching of aluminum/copper alloy films on semiconductor substrates may be conducted with precision and under control at a faster rate than was previously possible, thereby facilitating the use of computer controlled single wafer machines as desired by the semiconductor industry.

It is to be understood further that the invention will function equally well if the composition of the aluminum/copper alloy is varied, and the proportion of copper in the alloy may be substantially less than the 4 wt.% of the alloy used in the foregoing Example. Thus, the invention may be used to etch aluminum/copper alloys that comprise 2% copper.

While the invention has been described as an etching method for films of aluminum/copper alloys, it is to be understood that the invention may be equally applicable to etching aluminum or other alloys of aluminum such as aluminum/silicon/copper alloys, aluminum/manganese alloys, and the like. In any event the invention may be used in the manufacture of a variety of semiconductor devices which may require patterned plasma or reactive ion etching of an aluminum alloy film.

Claims (12)

1. A method of etching an aluminum/copper alloy film on a semiconductor wafer comprising the steps of placing the semiconductor wafer in a plasma etching chamber between a pair of electrodes and exposing the wafer within the chamber to a chlorine-containing gas plasma at a pressure between 300 and 1500 millitorrs and a temperature between -30 C and +15 C for a time sufficient to etch the film to a desired depth.

2. A method as claimed in claim 1, in which gas plasma is formed by injecting a mixture of CCI, and Cl2 into a field of electrical energy.

3. A method as claimed in claim 1, in which aluminum/copper alloy contains up to 4% by weight of copper.

4. A method as claimed in claim 1, in which electrical energy if RF power supplied at a rate between 1.5 and 3.5 watts per centimeter squared.

5. A method as claimed in claim 2, in which the mixture includes an inert carrier gas.

6. A method as claimed in claim 1, in which the carrier gas is helium.

7. A method as claimed in claim 1, in which the pressure in the reaction chamber is maintained at about 500 to 900 millitorrs and the temperature in the chamber is maintained between about -5"C and +10 C.

8. A method as claimed in claim 1, in which CCI, and chlorine are supplied to the reaction chamber at combined rates in the range of 10 to 50 SCCM.

9. A method as claimed in claim 8, in which CCI, is supplied to the reaction chamber at a rate of about 25 SCCM.

10. A method as claimed in claim 8, in which Cl2 is supplied to the reaction chamber at a rate of about 5 SCCM.

11. A method as claimed in claim 1, in which helium is supplied to the reaction chamber in ad mixture with CCI, and Cl2.

12. A method of etching an aluminum/copper alloy film on a semiconductor wafer substantially as herein described.