Method for Producing via Holes in Integrated Circuit Layers

This invention relates generally to integrated circuit processing and more particularly to methods for etching via holes in a dielectric layer of an integrated circuit.

Integrated circuits are formed on a semiconductor wafer by subjecting the wafer to a number of processing steps. These processing steps include the deposition of various types of layers, the formation of masks over such layers, the etching of the layers through the masks, the doping of layers, etc. After the integrated circuits are formed, the wafer is cut into a number of dies, each of which includes an integrated circuit, and the dies are then packaged to complete the integrated circuit assembly.

An important processing step is the formation of vias through a dielectric layer which separates two conductive and/or semiconductive layers. Via holes are formed by first covering the dielectric surface with a photoresist mask typically provided with a great number of openings and then etching the dielectric surface through the mask openings. A conductive material, such as aluminum, is then deposited over the surface and within the via holes. Finally, the conductive material over the surface is fully or partially removed to leave conductive vias through the dielectric surface.

A prior art method for forming vias will be discussed with reference to FIG. 1a which shows a substrate layer S.sub.a, a dielectric layer D.sub.a, and a via hole V.sub.a. A conductive layer L.sub.a is typically formed by a sputter deposition process within a commercially available plasma sputter system available from such companies as Varian Associates, Inc. of Palo Alto, Calif. and Applied Materials, Inc. of Santa Clara, Calif. The sputtering process causes ionized atoms of an inert gas, such as argon, to strike an aluminum target, thereby dislodging aluminum atoms from the surface of the target, some of which will settle over the surface of dielectric layer D.sub.a and within the via hole V.sub.a. As indicated by the arrows in FIG. 1a, the aluminum (Al) atoms can settle on the surface D.sub.a and within the via V.sub.a from a number of directions.

A problem encountered with this prior art method for forming vias is the so-called shadowing effect where the rim of the via hole V.sub.a can block angularly moving aluminum atoms from depositing within certain portions of the via holes. For example, in FIG. 1a, some aluminum atoms travelling angularly downwardly from left to right may be blocked by the left-side rim of the via hole V.sub.a, while some aluminum atoms travelling angularly downwardly from right to left may be blocked by the right-side rim of the via hole. This tends to cause uneven deposition within the via hole V.sub.a, creating a via having pronounced cusps C.sub.a. These cusps are weak points in the via and can fracture, breaking the electrical connection through the via. Furthermore, as the via fills and the sidewalls of the via grow together, a void may form within the via which can be a further cause of potential failure of the via.

A prior art structure which reduces the shadowing effect in via holes is illustrated in FIG. 1b. This prior art structure includes a substrate layer S.sub.b, a dielectric layer D.sub.b, a via hole V.sub.b, and a conductive layer L.sub.b, where the rim of the via hole V.sub.b is beveled or tapered as shown at T.sub.b so that fewer of the angularly moving aluminum atoms are shadowed by the rim of the via hole. This reduced shadowing results in much smaller cusps C.sub.b and more uniform deposition within the via hole V.sub.b.

A method for forming the prior art tapered rim via hole structure will be discussed with reference to FIG. 1c which shows a substrate layer S.sub.c, a dielectric layer D.sub.c and a masking layer M.sub.c. The masking layer M.sub.c includes a number of openings O.sub.c which correspond to the locations at which via holes are to be formed. As a first step, a recess R.sub.c is isotropically etched through the openings O.sub.c as illustrated in the left side of the figure. This isotropic etch is typically a "wet etch", i.e. is accomplished with a liquid acid such as hydrofluoric (HF) acid. Wet etching tends to be omnidirectional, and therefore causes undercutting of the mask layer M.sub.c to form tapered surfaces T.sub.c. This wet, isotropic etch is then followed by an anisotropic etch to form the main body of the via hole, as illustrated on the right side of FIG. 1c. This anisotropic etch is typically a "dry etch", i.e. is accomplished within a plasma etch system which creates the nearly vertical sidewalls of a via hole V.sub.c.

As the feature sizes of integrated circuits shrink, the diameters of via holes become correspondingly smaller, making it increasingly difficult to cause liquid acid to flow into the openings O.sub.c of the mask layer M.sub.c due to the surface tension of the liquid and due to residual air bubbles trapped within the openings O.sub.c. Therefore, the wet etch portion of the prior art process for forming via holes with tapered rims is poorly adapted for the latest generations of very large scale integrations (VLSI) and ultra large scale integration (ULSI).

Another problem encountered with the wet etch process is particulate contamination. Even the purest of liquid etchants include many fine particles which can adhere to portions of the integrated circuit and possibly destroy the functionality of the integrated circuit. Since these particles are very difficult to filter out, each wet etch process adds statistically to the failure rate of each integrated circuit die, thereby reducing the yield of functioning integrated circuits per wafer.

The wet etch process also requires a subsequent water rinse and dry cycle to remove the remaining acid. Since even the purest of distilled water contains some contaminants, the rinse and dry cycle also adds particulate contamination of the wafer with a subsequent reduction in the yield of functional integrated circuits per wafer.

In the present invention, an isotropic etch is performed utilizing an acidic vapor, such as a vapor of HF, rather than the liquid acids utilized in the prior art. The acidic vapor penetrates the mask openings to isotropically etch a recess in the dielectric layer with substantial undercutting of the mask layer around the mask openings. The acidic vapor isotropic etch is followed by an anisotropic etch to form substantially straight-walled via holes having tapered rims.

Since the isotropic etch step is performed with an acidic vapor, there are no surface tension or wetting problems to prevent the vapor from penetrating the mask openings. In consequence, the process of the present invention is well suited to the latest generations of VLSI and ULSI integrated circuit manufacture.

Furthermore, since the isotropic etchant is provided in vapor form the possibilities of particulate contamination can be greatly reduced because the particles are left behind in the liquid source for the acidic vapor. Also, no rinse/dry cycle is required after the isotropic etch step, again reducing the chance for particulate contamination. Therefore, the use of the method of the present invention can increase yield by reducing the number of non-functional integrated circuit die on a wafer.

These and other advantages of the present invention will become clear to those skilled in the art upon a study of the detailed description of the invention and of the several figures of the drawings.

FIG. 1a illustrates a first prior art via hole structure which is particularly vulnerable to the shadowing effect during sputter deposition;

FIG. 1b illustrates a second prior art via hole structure which is less vulnerable to the shadowing effect during sputter deposition;

FIG. 1c illustrates a prior art process for producing the via hole structure shown in FIG. 1b;

FIG. 2 is an improved method for producing the via hole structure shown in FIG. 1b; and

FIGS. 3a-3e successively illustrate an integrated circuit structure being processed by the method steps of FIG. 2.

Two prior art via hole structures were discussed with reference to FIGS. 1a and 1b. A prior art method for forming the via hole structure of FIG. 1b was discussed with reference to FIG. 1c. An improved process in accordance with the present invention for forming vias is illustrated in the flow diagram of FIG. 2.

As seen in FIG. 2, a first step 10 in accordance with the present invention is to cover a dielectric surface to be etched with a mask having a plurality of openings. The dielectric surface is then isotropically etched in a step 12 through the openings of the mask using an acidic vapor which preferably includes a vapor of hydrofluoric (HF) acid. The dielectric surface is next anisotropically etched in a step 14 through the mask openings to form the straight walls of the via holes and subsequently the mask is removed from the dielectric surface in a step 16. A conductive layer is then deposited over the dielectric surface and within the via holes in a step 18. The conductive layer can be subsequently partially or completely removed from the dielectric surface to leave conductive vias within the via holes.

The effects of these process steps are illustrated in FIGS. 3a-3e. In FIG. 3a, a multi-layer structure 20a includes a substrate 22, a dielectric layer 24, and a mask layer 26. The substrate 22 can be a semiconductor substrate, such as a silicon substrate, or it can be a layer formed over the semiconductor substrate. The dielectric layer 24 is typically an oxide or nitride, such as SiO.sub.2 or Si.sub.3 N.sub.4, or a boron or phosphorous doped oxide. The mask layer 26 is typically a photoresist material which is spun onto the dielectric layer 24 and photolithographically patterned to produce a number of openings 28 which correspond to the locations where the via holes are to be formed in the dielectric layer.

In FIG. 3b, the structure 20b is exposed to an acidic vapor 30 which penetrates the openings 28 of mask 26 to isotropically etch recesses 32 in the dielectric layer 24. Since the etch is isotropic, the mask layer 26 will be undercut around openings 28. The acidic vapor 30 is preferably substantially entirely in the gaseous state to promote diffusion into and through the openings 28 of the mask layer 26. However, the acidic vapor 30 can also include an aerosol of fine droplets which are similarly capable of entering the openings 28.

A vapor found to be particularly suitable for the isotropic etch step of the present invention is a vapor of hydrofluoric acid (HF). For the isotropic etch of an SiO.sub.2 layer through a mask having openings on the order of 0.8 microns, the following conditions have been discovered to be suitable:

Pressure: 760 Torr

Temperature: 25.degree. C.

N.sub.2 flow: 8 liters/minute

HF flow: 100 cc/minute

Under these conditions the etch rate of the SiO.sub.2 is approximately 1 micron per minute. For a typical 0.8 micron via hole, the process would continue for 3 minutes to provide a recess 32 in the surface of the dielectric layer 24 which is approximately 0.3 microns deep and 0.6 microns across at its base.

It should be noted that the process can be conveniently performed at standard temperatures and pressures, i.e. at ambient room temperature and pressure. The nitrogen, which serves as a carrier gas and diluent for the HF vapor, is semiconductor grade anhydrous nitrogen available from such sources as Air Products and Chemicals, Inc. of Allentown, Pa. and Linde Specialty Gasses, a division of Union Carbide Corporation, Somerset, N.J. Other inert or relatively inert gasses such as Argon can be used in addition to or exclusively of the nitrogen carrier gas. The HF vapor is formed from semiconductor grade liquid HF in commercially available apparatus which will be described subsequently.

The etch rate can be varied considerably from about 300 angstroms/minute to about 12,000 angstroms per minute by varying the flow rates of the nitrogen carrier gas and the HF vapor. In general, the flow rate of the N.sub.2 carrier gas is in the range of 1-30 liters/minute and the flow rate of the HF vapor is 0.01-1 liters/minute. The flow rates should be adjusted to prevent substantial condensation of the HF vapor on the various wafer surfaces or the formation of black silicon by the recombination of reactant byproducts with the wafer surfaces. If these conditions are met, no rinse/dry or other cleaning cycle will be required because the reactant byproducts will be carried away with the system's exhaust, leaving the wafer surfaces essentially dry and contamination free.

After the isotropic etch, the structure 20c is subjected to an anisotropic etch as illustrated in FIG. 3c. An anisotropic etch is typically performed in a plasma etch apparatus which causes positive ions 34 of an etchant gas to be accelerated substantially perpendicularly towards the structure 20c. This anisotropic etch, the process of which is well known to those skilled in the art, creates via holes 36 with substantially vertical sidewalls. Since the via holes 36 are formed through the recesses 32 which were formed by the isotropic etch the rim of the via holes 36 have the taper which is desired to reduce the shadowing effect during a subsequent metal deposition process.

In FIG. 3d, the structure 20d is shown with the mask 26 removed. Here the via holes 36 with their tapered upper rims can be clearly seen.

In FIG. 3e, the structure 20e includes the addition of a metal layer 38 which covers the dielectric layer 24 and fills the via holes 36 with vias 40. The tapered rim of the via holes permits the metal layer to substantially fill the via holes 36 to provide vias 40 which are substantially devoid of cusps and voids. The metal layer, which is preferably aluminum or tungsten, can be subsequently partially or completely removed from the surface of dielectric layer 24, leaving the vias 40 within the via holes 36.

The process of the present invention can be accomplished in commercially available machines. For example, Advantage Edge of Sunnyvale, Calif. produces the Advantage Edge 2000 Vapor Phase Oxide Etching System which is capable of supplying HF vapor from a liquid HF source and N.sub.2 from a nitrogen source into a reaction chamber to perform the process of the present invention. The Advantage Edge system has been found to be less than ideal for the process of the present invention, however, since it is a low-pressure which can cause contamination due to outgassing by the photoresist layer. A more suitable machine for process of the present invention is the Excalibur Gas Processing System made by FSI International of Chaska, Minn. The Excalibur system is capable of delivering HF vapor and N.sub.2 at substantially room temperatures and pressure into a process chamber which can accommodate wafers up to 200 mm in diameter. Both the Advantage Edge 2000 and the Excalibur are marketed as wafer surface cleaners and do not include a process for etching vias holes through a photoresist mask.

While this invention has been described in terms of several preferred embodiments, it is contemplated that various alterations and permutations thereof will become apparent to those skilled in the art. It is therefore intended that the appended claims include all such alterations and permutations as fall within the true spirit and scope of the present invention.