Method of Fabricating Metal Plug

This application claims the priority benefit of Taiwan application serial no. 86117571, filed Nov. 24, 1997, the full disclosure of which is incorporated herein by reference.

1. Field of the Invention

The invention relates to a method of fabricating multilevel interconnections of a semiconductor device, and more particularly to a method of fabricating a metal plug for multilevel interconnections.

2. Description of the Related Art

As the integration of semiconductors is enhanced, the dimensions of devices cannot supply enough area for interconnection. To match the requirements of the metal oxide semiconductor (MOS) devices with smaller dimensions, designs of multilevel interconnections are adapted in most of the integrated circuits (ICs). Normally, an inter-metal dielectric (IMD) layer is used to isolate two conductive layers. By the formation of a metal plug, the conductive layers are electrically connected. Moreover, to obtain a better dielectric characteristic, for example, the adhesion, a barrier layer made of a conductive material is formed between the metal plug and the conductive layer.

Titanium nitride is a very common material used as a barrier in a high density IC. To obtain a better ohmic contact between metal and silicon, a titanium nitride barrier layer is formed and assembled with a titanium layer as a titanium/titanium nitride layer. For example, for the process of fabricating a contact of an alloy, the whole metal structure is assemble by three different materials, titanium, titanium nitride, and alloys to decrease the work function, and the suppress the formation of spike and electromigration.

In a conventional process of forming a barrier and an interconnection structure, a titanium layer is formed. By physical vapour deposition, a titanium nitride layer having a thickness of about 800 .ANG. to 1200 .ANG. is formed on the titanium layer as a barrier layer. After a rapid thermal nitridation (RTN), a titanium nitride layer with a better contact with the titanium layer is formed. A refractory and conductive metal layer, such as a tunsteng layer, is formed. By performing an etching back process, a metal plug is formed. After cleaning the surface of the device, interconnection of another layer can be formed.

A conventional method is explained and shown on FIG. 1A to FIG. 1E.

Referring to FIG. 1A, on a substrate 100 having a MOS and metal layer formed thereon, a dielectric layer 102 is formed to isolate two metal layers. A better dielectric layer 102 is formed as a single-layered or double-layered spin-on-glass sandwich structured dielectric layer, which can prevent the voids formed during a pure chemical vapour deposition for forming a dielectric layer. Referring to FIG. 1B, using a mask to cover the dielectric layer 102, a photolithography process is performed to define a connecting channel for different metal layers. A contact window or a via hole is formed to expose the metal layer on the substrate 100. Referring to FIG. 1C, to enhance the adhesion between the tungsten plug formed subsequently and the other material, a titanium nitride layer or a tungsten nitride layer is formed before the formation of tungsten layer. Moreover, during the etching process, the titanium or tungsten nitride layer can be used as an etching stop layer due to the difference of plasma spectra between tungsten and titanium or tungsten nitride. Applying titanium nitride to the process metallization, titanium nitride is assembled with titanium as a form of titanium nitride/titanium layer. Before the formation of metal plug, a metal glue layer 106, such as a titanium layer, can be formed on the dielectric layer 102 and the circumference of contact window 104. By physical vapour deposition, a barrier layer 108 having a thickness of 800 .ANG. to 1200 .ANG. is formed on the metal glue layer 106. To improve the adhesion between the barrier layer 108 formed by titanium nitride and the metal layer 106, a rapid thermal nitridation is performed at a range of temperature between 550.degree. C. to 900.degree. C. Referring to FIG. 1D, a refractory and good conductive metal layer 110, for example, a tungsten layer, is formed on the barrier layer 108. The barrier layer 108 is covered by the metal layer 110, and the contact window 104 is filled by the metal layer 110. Referring to FIG. 1E, a part of the metal layer 110 is removed to form a metal plug 110a in the contact window 104. If the metal layer 110 is a tungsten layer, a gas source containing fluorine, such as a mixture of carbon fluoride and oxygen, a mixture of fluorine nitride and oxygen, or a mixture of fluorine sulfide and oxygen, is used as the etchant with the titanium nitride layer 106 as an etching stop layer. An etching back process is performed to form a tungsten plug 110a.

After the tungsten plug is formed, the subsequent processes, such as the deposition of another metal layer, photolithography and etching process, are performed to form the second level of interconnection.

In the above conventional process, the titanium nitride grain of the barrier layer is not uniform. Thus, during the deposition of tungsten, the reacting gas, tungsten fluoride (WF.sub.6) is reacted with titanium glue layer along the edge of the titanium nitride grain of the barrier layer to form titanium tri-fluoride (TiF.sub.3). An additional thermal process to remove the titanium tri-fluoride causes a volcano-like protrusion on the barrier layer. On the volcano-like protrusion, tungsten residue is left in the subsequent etching back process of tungsten.

In addition, the non-uniform titanium nitride grain further causes the deposited tungsten grain to be non-uniform. The larger size tungsten grain cannot be removed by the subsequent etching back process. The residue of tungsten is thus left.

It is therefore an object of the invention to provide a method of improving the structure of barrier structure. The titanium nitride grain is formed in an identical size. Consequently, the reaction of tungsten fluoride and the titanium glue layer along the edge of the titanium nitride grain of the barrier is suppressed. On the other hand, the tungsten is formed uniformly in an identical size. The tungsten residue is no longer left after the etching back process.

To achieve these objects and advantages, and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention is directed towards a method of forming a metal plug. A contact window is formed to penetrate through a dielectric layer on a substrate having a MOS formed thereon. A titanium glue layer is formed on the dielectric layer and the circumference of the contact window. A titanium barrier layer is formed on the titanium nitride layer. Using nitrogen plasma bombardment on the titanium nitride layer, the structure of the titanium nitride layer is transformed. The number of the nucleation seeds is increased, and the size of grains is reduced. A metal layer is formed on the titanium nitride layer and fills the contact window. A part of the metal layer is removed and a metal plug within the contact window is formed.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

FIG. 1A to FIG. 1E are cross sectional views of a conventional process of forming an interconnection of a semiconductor device; and

FIG. 2A to FIG. 2F are cross sectional views of the process of forming an interconnection of a semiconductor device in a preferred embodiment according to the invention.

Referring to FIG. 2A, a semiconductor substrate 200 having a MOS device and a metal layer formed thereon is provided. A dielectric layer 202 is formed over the substrate 200 as an inter-metal dielectric. A preferred dielectric layer comprises a single-layered or double layered spin-on-glass sandwich structured dielectric layer. This kind of structure prevents the formation of voids during chemical vapour deposition for forming the dielectric layer. The sandwich structured dielectric layer is formed by plasma enhanced chemical vapour deposition (PECVD), using tetra-ethyl-oxy-silane (TEOS) as precursor. A first dielectric layer is then formed by the deposition of silicon oxide. A spin-on-glass layer is formed on the first dielectric layer. Being baked and cured, a second dielectric layer is formed on the spin-on-glass layer, and the sandwich structured dielectric layer 202 is thus formed.

Referring FIG. 2B, using a mask to cover the dielectric layer 202, after photolithography and etching processes, the connecting channel, such as a via hole or a contact window 204, for connecting different metal layers is formed to expose the substrate 200.

Referring to FIG. 2C, to improve the adhesion between the tungsten plug formed subsequently and other materials, before the deposition of tungsten metal, a layer of titanium nitride or tungsten nitride is formed. In addition to improving the adhesion, by the difference of the plasma spectra between tungsten and titanium nitride or tungsten nitride, the titanium nitride or tungsten nitride is also used as an etching stop layer. Applying titanium nitride to the process metallization, titanium nitride is assembled with titanium as a form of titanium nitride/titanium layer. Before the formation of a metal plug, a metal glue layer 206 is formed on the dielectric layer 202 and the circumference of the contact window 204. The metal layer 206 is titanium, for example. A barrier layer 208 is formed on the metal glue layer 206. A preferred metal glue layer 206 having a thickness of 200 .ANG. to 1500 .ANG. is formed by DC magnetron sputtering in an environment of argon. A preferred barrier layer 208 having a thickness of 800 .ANG. to 1200 .ANG. is formed by physical vapour deposition. The barrier layer 208 can normally be formed in two ways. One is using DC magnetron sputtering to form a titanium layer, and to put the device in an environment of nitrogen or ammonia. Through a rapid thermal process, the titanium layer is nitridized into a titanium nitride layer. The other way is to using reactive sputtering for the deposition of titanium nitride. Using a titanium target, and a mixture of argon and nitrogen as an reactive gas source, the titanium sputtered after ion bombardment is reacted with nitrogen produced by dissociation reaction from the plasma to form a titanium nitride layer. To improve the adhesion between the barrier layer 208 and the glue layer 206, after the formation of titanium nitride barrier layer, at a range of temperature between 550.degree. C. to 900.degree., a rapid thermal nitridation is performed.

Referring to FIG. 2D, by performing nitrogen plasma bombardment towards the titanium nitrogen barrier layer 208, the number of nucleation seeds of titanium nitride is increased. The size of the seeds is identical. The surface of the titanium nitride layer 208a is finer and more delicate, and therefore, the quality the titanium nitride barrier layer 208a is improved with a better adhesion. Due to the increased number of nucleation seeds, and the identical size of the seeds, the metal residue after etching back is improved.

Referring to FIG. 2E, a refractory and good conductive metal layer 210, for example, a tungsten layer, is formed to cover the titanium nitride layer 208a and fill the contact window 204. To obtain a better adhesion between the tungsten formed by chemical vapour deposition and other materials, a preferred process for the deposition of tungsten is using tungsten fluoride as a reactive gas. Under the pressure of 1 torr to 100 torr, and at the temperature of 300.degree. C. to 550.degree. C., two-step deposition is performed. A thin tungsten nucleation layer is formed on the barrier layer 208a. A tungsten layer having a thickness of 5000 .ANG. to 10000 .ANG. is then formed on the thin tungsten layer and fills the contact window. Due to the very fine and delicate surface of the titanium nitride layer 208a, the reaction of tungsten fluoride and the titanium glue layer along the edge of titanium nitride grain of the barrier layer 208 is suppressed. Moreover, due to the increasing number of the nucleation seeds, and the smaller and identical size of the grain, the nucleation seeds of the deposition of tungsten is increased. Therefore, the size of two grains is further more identical.

Referring to FIG. 2F, a part of metal layer 210 is removed to form a metal plug 210a within the contact window 204. If the metal layer is a tungsten layer, a gas source containing fluorine, such as a mixture of carbon fluoride and oxygen, a mixture of fluorine nitride and oxygen, or a mixture of fluorine sulfide and oxygen, is used as the etchant with the titanium nitride layer 206 as an etching stop layer. An etching back process is performed to form a tungsten plug 210a.

After the tungsten plug is formed, the subsequent processes, such as the deposition of another metal layer, photolithography and etching process, are performed to form the second level of interconnection.

Therefore, the characteristic of the invention is that after the formation of a titanium glue layer and a titanium nitride layer by a conventional method, using nitrogen plasma bombardment towards the surface of the titanium nitride layer, the electrical characteristics of the titanium nitride layer are improved. On the other hand, due to the increased number of nucleation seed of titanium nitride barrier layer, and the smaller and identical size of the titanium nitride grains, the titanium nitride has a finer and more delicate surface. Therefore, the adhesion and the quality of the titanium nitride layer is enhanced. Moreover, in the subsequent deposition process, the reaction of tungsten fluoride and the titanium glue layer along the edge of titanium nitride grain of the barrier layer is suppressed. Furthermore, the tungsten residue after the etching back process is no longer left.

Other embodiments of the invention will appear to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples to be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.