Aluminum Corrosion - Dry etching

Aluminum Corrosion - Dry etching

Material Name: Aluminium
Record No.: 4
Primary Chemical Element in Material: Al
Sample Type: Layer
Uses: Etching
Etchant Name: None
Etching Method: Dry etching
Etchant (Electrolyte) Composition: No data
Procedure (Condition): No data
Note: Aluminum based alloys were the dominant conductors for interconnect in VLSI circuits before the introduction of copper, although other metals, such as tungsten, have been used occasionally. The aluminum alloys usually contain small amount of silicon and/or copper—the former to improve contact resistance when contact is made to the silicon substrate and the latter to control electromigration. Since copper does not form volatile compounds with the chemistries used for aluminum etching (at the standard operating temperatures), its concentration is limited to less than 5%. When the Al layer is “sandwiched” between TiN layers, the same chemistry is used to etch the TiN layers (and the Ti that is used as an adhesion layer below the lower TiN layer) with possible variation of process conditions tailored for each layer.

Al will etch spontaneously in Cl2 gas, but the presence of surface oxide inhibits etching. BCl3/Cl2 mixtures are commonly used for etching of Al, usually in a high-density plasma reactor, such as an ICP or ECR system. CCl4,288 SiCl4, and BBr3feed gases have also been used less frequently. The role of BCl3 is to remove the native oxide on the aluminum, scavenge any moisture in the chamber that may inhibit the etch process, and possibly to inhibit sidewall etching. The interaction of the plasma with photoresist leads to formation of a layer on the sidewall that will prevent lateral etch of the aluminum, but may lead to subsequent corrosion (discussed below). Sometimes an additive to the feed gas helps in the formation of sidewall passivation and can also be used to taper the metal lines. Typical additives are N2 and CHF3, but other polymer-forming additives have been used as well [e.g., CHCl3]. Although the additive may have a positive effect on the etched profile, it can cause particulate formation on the wafer due to the flaking off of deposits from the chamber walls. Therefore, the amount and type of the additives are important in establishing the optimum tradeoff between a desired profile and minimum wall deposits.

A hard mask can also be used for the pattern transfer. It has the advantage of minimized variation between isolated and nested aluminum lines, but since a major component of the etching process, the eroding photoresist, is absent, the etching process has to be modified. Etching in a low pressure (~2 mTorr) Cl2/HCl/N2 plasma has been used successfully to pattern an aluminum stack consisting of TiN/Al/TiN/Ti. Etching uniformity is one of the challenges in aluminum etching. Generally, the etch rate at the edge of the wafer is higher than the center and the metal is cleared in “bullseye” pattern. However, by process optimization and the use of focus-rings, the effect can be minimized.
Postetch corrosion is a major concern and it can be either purely chemical or galvanic. The chemical corrosion is associated with residual chlorides present on the wafer, especially on the sidewalls. Although during etching the wafer is heated to 50–70 C to help volatilize the etch by-products, some AlCl3 is embedded in the sidewall deposits, leading to chemical reactions with moisture in the air.

The process continues to corrode the aluminum, creating “worm”-like residues that are easily observed in an optical microscope (Fig. 1). The standard procedure to avoid chemical corrosion is to minimize sidewall deposits, heat the wafer during etching to the maximum temperature that will not reticulate the photoresist, and use a passivation step, combined with a partial or complete stripping of the photoresist. The passivation/strip is carried out by transferring the substrates in a load-lock, under vacuum, to a separate chamber designed specifically for that purpose (in earlier batch metal etchers, resist strip was carried out in-situ using an oxygen plasma with small amount of fluorine containing gas, with the intention to convert the chlorides to noncorrosive fluorides). The passivation step is intended to convert the residual chlorides to volatile HCl and is accomplished by hydrogen-containing plasmas, typically water vapor. Additional steps to reduce chemical corrosion are to heat the wafer after the passivation step on a hot stage in the etch tool, keeping the wafer under vacuum until the entire lot is etched, water rinse immediately after venting to dissolve any residues that are left, and chemical sidewall removal, followed by another resist ashing step to insure complete resist removal. Wafers should be kept in a dry-box before a capping oxide layer is deposited. The time interval between completion of the etch and the deposition step should be short, preferably less than 24 h.

Galvanic corrosion takes place when a galvanic cell is formed between two dissimilar metals in the presence of an electrolyte. The two metals in this case are the aluminum metal and the TiN layer(s) if used, and the electrolyte is the residual chlorides dissolved in water. This galvanic corrosion looks different than the chemical corrosion described above and is characterized by voids in the aluminum, while the TiN layer(s) is (are) intact. Usually, the TiN is oxidized, preventing the formation of the galvanic cell; however, if there are discontinuities in the film the cell can form, and often, galvanic corrosion is observed in a small number of sites. The galvanic corrosion is prevented by minimizing sidewall deposits, and a postetch rinse in an adequate amount of water: quick-dump-rinse or overflow-rinse are preferable to spin-rinse. The major component in prevention of the two type of corrosion is the minimization of sidewall deposits. However, these deposits are necessary for anisotropic etch. The optimized etch process has to take into account these diametrically opposite requirements: just enough sidewall deposits to prevent undercut, but not enough to trap large amounts of residual chlorides.
Reference: Vincent M. Donnellya and Avinoam Kornblit, Plasma etching: Yesterday, today, and tomorrow, J. Vac. Sci. Technol. A 31(5), Sep/Oct 2013, pp. 050825-1 - 050825-48.

Figure 1: Aluminum corrosion. The reaction by-products of the reaction between the chlorine-based residues, moisture, and aluminum are evident on top and the side of the etched metal lines. See text for details.