Using implants to lower anneal temperatures

Details Using implants to lower anneal temperatures Using implants to lower anneal temperatures

1999-02-26

2001-07-17

Thomas, Tom (Department: 2811)

Active solid-state devices (e.g., transistors, solid-state diode

Combined with electrical contact or lead

Of specified material other than unalloyed aluminum

C257S384000, C257S456000, C257S486000, C257S758000, C438S627000, C438S630000, C438S649000

Reexamination Certificate

active

06262485

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to methods for forming semiconductor device interconnects and, in particular, to a method for forming refractory metal silicide at a metal/semiconductor interface.
BACKGROUND OF THE INVENTION
Contacts between metal and semiconductor active areas are currently improved by forming a refractory metal silicide at the interface between the two materials. Titanium silicide is the most commonly used silicide. Titanium is deposited or sputtered on the semiconductor active area and annealed to form titanium silicide. This provides a good, low resistance contact.
Borophosphosilicate glass (BPSG) is used as a device insulator, surrounding contact areas. Recently, in order to get BPSG to reflow at lower temperatures, the amount of dopants in the BPSG film is being increased. It is desirable to lower the reflow temperature of BPSG to avoid diffusion of dopants into undesired areas during high temperature steps. Normally, reflow temperatures exceed those required during annealing process steps. However, the addition of dopants causes BPSG to reflow even at the temperatures required for the titanium anneal process. This is undesirable because it causes titanium to buckle, resulting in degradation of contacts by increasing their resistance. This is counterproductive to the main reason for forming titanium silicide at semiconductor/metal interfaces—to improve contacts by lowering their resistance.
The addition of a refractory metal nitride layer at the surface of the refractory metal silicide provides both a barrier to diffusion into the contact, and helps with the adhesion of the refractory metal silicide to the metal, which may comprise tungsten, aluminum, and similar conductive metals. Conventionally, such layers are formed by annealing in a nitrogen-containing ambient simultaneously with forming refractory metal silicide because it is important that a barrier nitride layer form simultaneously with titanium silicide, so that both can be formed in one processing step. Typically, titanium silicide is used for the refractory metal silicide and titanium nitride is used for the barrier layer.
As devices are becoming smaller, there is a need for lowering the temperature at which the refractory metal anneal occurs when forming refractory metal silicide at semiconductor/metal interfaces. In smaller devices, the acceptable amount of thermal-induced dopant diffusion is lower. There is a further need for lowering the temperature at which refractory metal nitride is formed in order that the refractory metal nitride layer can be formed simultaneously with the refractory metal silicide layer. It is paramount that the temperature of these anneals be lowered so that low temperature reflow doped oxide insulator layers can be used.
SUMMARY OF THE INVENTION
A method for lowering the anneal temperature required to form a multicomponent material, such as refractory metal silicide, is described. The method is described with reference to the most common refractory metal silicide, titanium silicide, and a typical insulator material such as borophosphosilicate glass (BPSG) or other suitable insulative material. Insulator materials are deposited prior to depositing refractory metals and subsequently annealing to form silicide. Lowering the anneal temperature prevents the BPSG insulator film from reflowing at undesired times, which, if not prevented, causes titanium buckling. The invention is applicable to other semiconductor structures, which do not comprise BPSG as an insulator film, such as those using a doped oxide insulating film. One advantage of the invention is that lower processing temperatures prevent unwanted dopant diffusion at higher temperatures, which is a limiting factor in the manufacture of small devices.
In one embodiment of the invention, a shallow layer of a refractory metal, such as titanium, is implanted in the bottom of a defined contact area, such as a contact hole defined over a silicon substrate. The refractory metal is then deposited over the contact area and annealed, forming a refractory metal silicide in the area where the refractory metal is implanted. Due to the increased concentration of refractory metal in the contact area, annealing is performed at lower temperatures due to the decreased diffusion lengths of silicon/refractory metal required to form the contact.
In a second embodiment of the invention, after a contact area is defined, a refractory metal is deposited over the contact area Silicon is then implanted in the deposited refractory metal layer. Annealing causes refractory metal silicide to form in the region where the refractory metal is deposited, in contact with the underlying silicon substrate.
In a third embodiment of the invention, the previous two embodiments are combined. A shallow layer of a refractory metal, such as titanium, is implanted in the bottom of a defined contact area, such as a contact hole defined over a silicon substrate. The refractory metal is then deposited over the contact area. Silicon is then implanted in the deposited refractory metal layer. Annealing causes refractory metal silicide to form in the region where the refractory metal is deposited and implanted, in contact with the underlying silicon substrate.
In a further embodiment of the invention, an additional step comprises implanting nitrogen over the deposited refractory metal layer prior to the anneal step. Refractory metal nitride is then formed simultaneously with refractory metal silicide during the anneal step. In yet a further embodiment, nitrogen is implanted subsequent to annealing to form a refractory metal silicide. The nitrogen implant lowers the temperature at which the refractory metal nitride is formed by decreasing the nitrogen/refractory metal diffusion lengths required to form the nitride layer. Refractory metal nitride helps provide a barrier to unwanted diffusion into the contact.
In yet a further embodiment of the invention, an additional step comprises implanting an element selected from the group consisting of: cobalt, cesium, hydrogen, fluorine, and deuterium in the bottom of the contact area, prior to implanting a refractory metal. In still another embodiment, an element selected from the group consisting of: cobalt, cesium, hydrogen, fluorine, and deuterium is implanted in the deposited titanium layer, prior to implanting silicon therein. In still another embodiment, an element selected from the group consisting of: cobalt, cesium, hydrogen, fluorine, and deuterium is implanted in the refractory metal silicide layer subsequent to the anneal step and prior to subsequent process steps. The addition of such an implant further defines grain structure and lowers the temperature needed to form the refractory metal silicide layer when it is performed prior to the anneal step, which forms the silicide.


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