This invention relates to the formation of metallization layers during semiconductor processing.
As is known in the art, aluminum (Al) has long been the material of choice for back-end-of-the-line (BEOL) metallization. One approach for patterning aluminum is to use a photoresist mask to define individual features, such as lines and spaces. Then, portions of aluminum exposed by the photoresist mask are selectively removed by reactive ion etching (RIE). With shrinking device geometries, photoresist thicknesses have to be reduced to satisfy lithography requirements. At the same time, integration requirements tend to keep metallization layer thicknesses constant or even require an increase in the metal stack thickness to compensate for decreasing wire line width. These trends require improved etching methods.
More specifically, the etch chemistry for aluminum layers is typically based on chlorine, which creates a highly chemical etch environment. Upon completion of the aluminum etch, the photoresist is stripped. Sidewall polymers and fence-like residues (i.e., polymeric etch by-products formed on top of etched lines) sometimes remain. These sidewall polymers and fence-like residues are removed using an adequate wet clean, such as chromic phosphoric acid (CP), dilute sulfuric phosphoric acid (DSP), EKC (trade name of several solvents manufactured by EKC Technology, Inc.), or similar solvent.
A limitation of this patterning process employing photoresist arises for devices with sub-0.25 micron features which have metal stacks which are even higher than those used in previous generations. The photoresist will eventually not be able to withstand the etch processes required for higher metal stacks with shrinking device geometries.
Another limitation of photoresist-based etching processes is sidewall polymer. removal and corrosion control. During the etch, chlorine-containing polymer-like layers are formed on the sidewalls of etched features when chlorine reacts with the photoresist. Then, upon exposure to moisture, the chlorine on the wafer may give rise to serious corrosion problems.
An alternative approach to photoresist masks has been the implementation of a hard mask. Different dielectric materials have been suggested for a hard mask, such as SiO2 or Si3N4. However, these materials are not conductive. Thus, after the metal lines are defined, the dielectric hard masks must be removed to enable contact to subsequent metallization layers. Also, anti-reflective coatings must be used for patterning the photoresist, to counteract the reflectivity of the aluminum which interferes with the definition of fine lines and spaces.
In accordance with one embodiment of the invention, a method is provided which includes providing a layer which contains aluminum. A tungsten (W)-containing layer is provided over the aluminum-containing layer. The tungsten-containing layer is patterned to form an opening therein, with the opening exposing an underlying portion of the aluminum-containing layer. The patterned tungsten-containing layer is then exposed to an etch having a substantially higher etch rate of the aluminum-containing layer than of the tungsten-containing layer to remove the exposed portion of the aluminum-containing layer. In some embodiments, the etch is a dry etch.
In accordance with another aspect of the invention, a metal stack is provided which includes a first refractory metal layer, a layer comprising aluminum over the first refractory metal layer, and a second refractory metal layer over the aluminum layer. A tungsten-containing layer is provided in contact with the second refractory metal layer, with the second refractory metal layer being of a material different from the tungsten-containing layer. A mask is provided over a selected region of the tungsten-containing layer, with the mask exposing an unmasked portion of the tungsten-containing layer. The mask is exposed to an etch, with the etch removing the unmasked portion of the tungsten-containing layer. Thereby, the tungsten-containing layer is formed into a hard mask for the underlying metal stack layer, with the hard mask exposing an underlying portion of the metal stack. A dry etch is brought into contact with the hard mask to selectively remove exposed portions of the underlying metal stack while leaving the hard mask substantially unetched.
In another aspect of the invention, a method is provided for patterning an aluminum-containing layer. The method includes providing the aluminum-containing layer with a tungsten-containing layer thereon. A mask is formed over the tungsten-containing layer with an opening in the mask. This opening exposes an underlying portion of the tungsten-containing layer. The mask is exposed to an etch having a substantially higher etch rate of the tungsten-containing layer than of the aluminum-containing layer to remove the exposed portion of the tungsten-containing layer and to expose an underlying portion of the aluminum-containing layer. The exposed portion of the aluminum-containing layer is exposed to an etch having a substantially higher etch rate of the aluminum-containing layer than of the tungsten-containing layer to remove the exposed portion of the aluminum-containing layer.
In accordance with another aspect of the invention, a method is provided for patterning an aluminum-containing layer, which includes depositing an aluminum-containing layer over a substrate. A tungsten-containing layer is deposited on the aluminum-containing layer. A mask is formed over the tungsten-containing layer with an opening in such mask exposing an underlying portion of the tungsten-containing layer. The mask is exposed to an etch which etches the tungsten-containing layer at a substantially higher etch rate than the aluminum-containing layer to pattern the tungsten-containing layer into a mask. This etch removes the exposed portion of the tungsten-containing layer and exposes an underlying portion of the aluminum-containing layer. Using the tungsten-containing mask, the exposed portion of the aluminum-containing layer is exposed to an etch having a substantially higher etch rate of the aluminum-containing layer than of the tungsten-containing mask to remove the exposed portion of the aluminum-containing layer.
Such a method provides one of more of the following advantages. A tungsten-containing mask makes possible the use of a thin initial masking layer, e.g. photoresist, because the latter needs to withstand only the etching of the tungsten-containing layer, and not the etching of the entire aluminum-containing layer. This method also allows one to etch thicker metal stacks, because one is not limited by the robustness of the initial masking layer, such as photoresist. Further, the tungsten-containing mask is conductive, so, unlike a dielectric hard mask, it does not need to be removed. Also, the tungsten-containing hard mask enables good electrical contact to subsequent layers. Moreover, improved reliability results are anticipated.
Embodiments may include one or more of the following. After an etch removes the unmasked portion of the tungsten-containing layer, thereby exposing an underlying portion of the metal stack, an exposed portion of the second refractory metal layer of the metal stack is removed with the same etch.
The tungsten-containing layer is etched in a fluorine-containing plasma. The fluorine-containing plasma contains SF6. The aluminum-containing layer is etched in a chlorine-containing plasma. This plasma readily etches the aluminum-containing layer, while being highly selective to the tungsten-containing layer. This selectivity enables one to use the tungsten-containing layer as a mask, thereby eliminating the need to rely on any other mask for etching the aluminum-containing layer. The chlorine-containing plasma contains BCl3. Forming a mask over the tungsten-containing layer includes forming a photoresist mask using photolithography. The photoresist mask is removed before the aluminum-containing layer is exposed to an etch. An advantage of removing the photoresist mask prior to the etch of the aluminum-containing layer is that the amount of organic polymers deposited during the aluminum etch is reduced, and fences are not formed. Thus, unlike with photoresist mask based etching of aluminum, control of sidewall-polymer-inducing gases such as N2 is not critical. Also, since the organic polymers formed during dry etching of aluminum with a photoresist mask incorporate chlorine, the passivation layer consisting of these polymers must be removed to prevent the chlorine from corroding the metal. By eliminating the photoresist mask, a heavy passivation layer is not formed and one can use milder post-etch corrosion prevention methods.
The aluminum-containing layer is a structure of a layer of aluminum and a first layer of refractory metal, which, in some embodiments, includes a layer of titanium nitride (TiN) and/or titanium (Ti). The aluminum-containing layer also includes a second layer of refractory metal, so that the aluminum layer is sandwiched between the first and the second layer of refractory metal. The second layer of refractory metal includes a layer of Ti and/or TiN. The aluminum-containing layer includes aluminum containing copper and/or silicon and/or titanium.
In another aspect of the invention, a metallization layer for an integrated circuit is provided, including a first layer of refractory metal and an aluminum-containing layer disposed on the first layer of refractory metal. A second layer of refractory metal is disposed on the aluminum-containing layer, and a layer of tungsten is disposed on the second layer of refractory metal.
Embodiments may include some of the following. The first layer of refractory metal includes TiN and/or Ti. The second layer of refractory metal includes TiN and/or Ti. The aluminum-containing layer includes aluminum containing copper, silicon, or titanium.
Further aspects, features, and advantages will be found in the following.