Sputtering

Sputtering machines, not only differ in their mechanical set-up, but also in their basic physical mode of operation. The quality of the films and the control over the deposition process vary with each type of machines. There are different sources, magnetron, planar, circular electrodes, linear electrodes, s-gun, rf, dc etc. all of which have an impact on the results. Thin films have properties that can differ significantly from the bulk materials because the morphology of the films can be very different. If the vacuum integrity is good, if the pumps are adequate and correctly sized, if an RGA is installed and properly placed, a great deal of the sputtering engineer efforts will be spent in finding stable operating parameters. Sputtering is a very repeatable process, given the right equipment and proper maintenance.

Because the morphology of a sputtered film is highly dependent on thermal conditions, a sputtering system needs to provide adequate heat control of the wafers. Polymers are sensitive to excessive thermal load and can breakdown with overheating. The rate of heat transfer to a wafer, depends on the heat of condensation of the particular material sputtered and on the deposition rate. Should the wafer have no mean of transferring heat away from it, energy will keep accumulating in the wafer. The amount of heat tolerable before failure depends on the heat resistance of the materials.

High electrical performance redistribution layers need a minimum metallization thickness of 2 microns. Copper will place a higher thermal load on a wafer than aluminum. In any case, cooling of the wafer is a must to consistently and reliably obtain this thickness without permanent damage to the wafer's circuitry or to the polymer.

Plasma Etching

Plasma etching, not to be confused with sputter etching, is an effective way of removing scumming, caused by incomplete or uneven development of many thicker polymers films. The bulk of the removal is done by oxidation of the polymer in an O2 plasma. Dilution of the gas with an inert carrier is often done to help controlling heat. A fluorine gas is added to prevent the formation of residues or to enhance clean etching of silicon bearing polymers. Adding H2 to CF4 improves the selectivity of SiO2 etching with respect to Silicon. Dry etching aluminum is not cost effective, given the coarse linewidth of wafer bumping geometries, where wet etching attainable resolution is quite sufficient.

Metal adhesion and diffusion barrier layers

Adhesion of a sputtered or plated metal film to its substrate requires a physical interlocking, interdiffusion of the films or a chemical bonding between film and substrate in order to work. The role of a diffusion barrier is to prevent or to retard the interdiffusion of two superposed films. Therefore, to be effective, a good diffusion barrier requires inertness with respect to adjacent films. To obtain good adhesion and a diffusion barrier simultaneously, the bonding between layers needs to come from a chemical reaction of limited range at its boundaries. Anticipated operating temperature excursions and life expectancy are critical parameters to select diffusion barriers materials. Materials providing good adhesion are not necessarily good diffusion barriers and vice-versa.

Conductors for WLP

Over the past half-century, many thin-film metal combinations have been evaluated for their adhesion or diffusion barrier properties. For WLP applications, the list can be shortened since practical conductors are limited to Al, Cu. Gold has only limited applications. Aluminum provides good conductivity, adhesion and reliability because of its oxygen reactivity and the self-passivation properties of its oxide. Copper also easily reacts with oxygen but its oxides have poor adhesion to its metal source. As for gold, its virtue relies in its inertness to common environment, and therefore it does not readily form chemical bonds. Copper and gold must include an adhesion layer / barrier metal combination over the substrate, with the addition of a protective layer on top of copper. In addition, precautions must be taken in handling since any diffusion of gold or copper will dope silicon and cause device failure. Yet , copper is a desirable material because of its superior conductivity to aluminum.

Titanium-Tungsten

Ti-W is a pseudo-alloy that has extensive applications as a barrier metal. The Titanium provides its excellent adhesion properties while the Tungsten contributes its dense barrier stability. The effectiveness of Ti-W as diffusion barrier can be verified by accelerated environmental testing. Today, there is a large accelerated aging database, accumulated over many years. In applications like TAB (Tape Automated Bonding), now defunct, Ti-W was placed between the aluminum bond pad surface and the gold capped copper bumps. Later, integrated circuits used Ti-W as aluminum and aluminum-copper alloys under-layer or as capping in via structures. The purpose, then, went beyond that of a diffusion barrier; it was also used to minimize hillocks formation, and to control the reflectivity and provide protection to chemical attacks during photolithography. One trade-off to consider in choosing a Wolfram to Titanium ratio is that adhesion properties resulting from an increase in Titanium content are obtained at the expense of an increase in sheet resistivity. Regardless, adhesion of Ti-W sputtered under proper conditions and clean surfaces is excellent. Molybdenum was used in the 60's as diffusion barrier for gold metallization but was quickly replaced by Ti-W because of its superior stability.

Chromium

Chromium has excellent adhesion to many materials because of its reactivity. Its affinity for oxygen forms a thin stable oxide coat which prevents further oxidation and provides inertness to corrosive environment. Chromium early popularity in thin films was due to its ease of deposition by sublimation in less than perfect evaporators, while retaining good adhesion. Its affinity for oxygen creates a strong Cr-O2 bond using oxygen that is always available on a substrate exposed to air. However, chromium is not necessarily a stable diffusion barrier. For instance, it is known to readily diffuse through overlaying gold films. Over the years, Cr has been used in a multitude of metallization combinations with varying degrees of long-term stability. Cr-Cu-Cr has been used extensively and successfully by RCA and IBM, because of the limited solid solubility of the metals. However, the real practical difficulty with these thin films resides in controlling etching during patterning due to the formation of strong electrochemical couples.

Titanium

Titanium is an excellent adhesion layer because it readily forms oxides nitrides or carbides with adjacent layers. Conversely, its excellent gettering properties require very low levels of residual water and oxygen in the deposition chamber if unreacted Titanium metal is expected to reach the substrate. We must keep in mind that Titanium superior gettering properties were used to create high vacuum in the getter vacuum pumps of years past. This is the reason why vacuum levels can often be observed to improve after a Titanium deposition run. Titanium is primarily used as adhesion layer, not as barrier metal, because the metal diffuses too rapidly in other metal such as Au, and to some extent Cu, for long term reliability at higher temperatures. Because of this, Ti has been used in conjunction with other intermediate metal layers such as NiCr, NiV, Pt, Mo, W. The stabilization of Titanium by adding a layer of Wolfram, quickly led IBM to the Ti-W pseudo-alloy in the very late sixties. Ti-Cu-Ni-Au was proposed, at one time, as a substitute for Ti-Pd-Au then used as a thin film hybrid and as a Microwave Integrated Circuit (MIC) metallization. In recent years, Ti-N, obtained by reactive sputtering, is favored over the deposition of two layers and provides superior diffusion stability.

Miscellaneous metallization

Ni, NiCr, TiN, Ta, Hf, Nb, Zr, V, W are a few of the metals combinations used to form adhesion and diffusion barriers with Cu. This list is by no means exhaustive. Much research was done on the subject, by Bell Labs and IBM among others, when the price of gold shot up beyond an order of magnitude in the 70's.