Methods of forming a barrier layer

Details Methods of forming a barrier layer Methods of forming a barrier layer

1997-11-21

2001-01-16

Nelms, David (Department: 2818)

Semiconductor device manufacturing: process

Coating of substrate containing semiconductor region or of...

Insulative material deposited upon semiconductive substrate

C438S663000, C438S664000, C438S701000, C438S760000

Reexamination Certificate

active

06174823

ABSTRACT:

This invention relates to methods for forming a barrier layer.
In the manufacture of semi conductor devices it is often necessary to deposit a barrier layer. This is frequently of Titanium or Titanium alloy and its purpose is generally to provide good adhesion to the surface layer and to avoid unintentional or undesirable alloying between the connection layer and the surface layer, in particular to avoid Aluminium alloy “spiking” through a contact junction. It is known that exposing the barrier layer to Oxygen can improve the mechanical qualities of the layer, but the Applicants have determined that this Oxygen is detrimental to the grain structure of Aluminium or its alloys when deposited on such a layer.
U.S. Pat. No. 5,552,341 describes an alternative process to a barrier layer to improve the subsequent Aluminium layer whereby a silylation layer is formed on the surface of the barrier layer by a plasma treatment using a silicon hydride. The barrier is improved by the known Oxygen treatment to stuff grain boundaries.
From one aspect the invention consists as a method of forming a barrier layer on a semiconductor surface comprising depositing a layer of Titanium Nitride, exposing the layer to Oxygen and nitriding at least the surface of the oxidised layer.
For convenience, for the purposes of this specification, such treatments of the barrier layer, whether or not they involve Oxygen will be known as reactivation.
The step of nitriding may be performed with a Nitrogen containing plasma. Additionally or alternatively, the barrier layer may be exposed to plasma-generated or U.V.atomic Hydrogen in the presence of Nitrogen to nitride any oxidised material in the barrier layer. Alternatively the removal of Oxygen by atomic Hydrogen and the nitriding step could be performed sequentially. With current apparatus, at least, nitriding in a pure Nitrogen atmosphere is difficult to achieve. The ratio of Hydrogen:Nitrogen may be between 1:10 and 3:1. The Hydrogen may be supplied in the form of NH
3
, in which case the NH
3
may supply at least some of the nitriding Nitrogen.
The barrier layer essentially contains atoms of Titanium and Nitrogen however Titanium Nitride forms columnar grain structures that present grain boundaries running from top to bottom. As the Titanium Nitride is acting as a physical barrier this is an inherent defect but it is well known that it can be mitigated by the addition of Oxygen during the formation of Titanium Nitride, or the exposure of Titanium Nitride to Oxygen (e.g. through exposure to air). The Oxygen is said to “stuff” the grain boundaries.
It is thus preferential to have Oxygen additions to the Titanium nitride (to improve the barrier quality), however this presents a problem if the top surface contains some atomic Oxygen.
The effect of Oxygen contamination on TIN is that under typical conditions there will be an oxidised surface, which consists of oxygen atoms bonded to the outer layers of Titanium, which will come into intimate contact with the Aluminium and which are likely to form a chemical bond with it, thus inhibiting the flow or drifting of material in certain processes.
The use of Oxygen is thought to improve the barrier by blocking diffusion paths at grain boundaries or defect sites by a process known as ‘oxygen stuffing’. This Oxygen may arrive at the barrier layer as part of a process step e.g. Nitrogen anneal where it is well known that some small quantity of Oxygen is present or an Oxygen plasma process or suchlike or as a secondary effect of exposure to atmosphere. The Nitrogen is essentially acting as a dilutant for Oxygen during the anneal process and does not usefully react with the layer during this process.
Only a very small quantity of Oxygen is required to effect some improvement in the mechanical properties of barrier layer. As Titanium Oxides are poor conductors the electrical qualities are not necessarily enhanced. Also the grain structure of the Aluminium is inferior if it deposited on a barrier containing oxygen at its surface.
From a further aspect the invention consists in a method of forming a barrier layer on the surface of a workpiece, for example a semi-conductor wafer, including depositing a layer of Titanium Nitride and subsequently exposing the surface of the layer to activated Nitrogen so that any free surface Titanium in the layer is nitrided to form Titanium Nitride.
Preferably the surface is exposed to energetic NH
3
and in particular to an NH
3
plasma. The TiN layer may be exposed to oxygen prior to the nitriding step and may additionally or alternatively be Oxygen annealed. There may be a vacuum break between the deposition of the Titanium Nitride layer and the nitriding step.
In particular, further experiments have determined that nitridation by an Ammonia plasma treatment improves the mechanical characteristics of the barrier even when no Oxygen exposure has taken place. Further as no Oxides are formed the process is not detrimental to the electrical characteristics.
This result is contrary to expectation and the teaching of the prior art.
The additional improvement of nitriding with Ammonia plasma treatments have been mentioned above.
The Applicants can only hypothesis at this time as to the precise mechanism which is involved. It has been observed that a TiN barrier is stronger than a TiN barrier with Ti deposited on top. A possible explanation is thus that the TiN barrier is weakened by having free Ti present. Oxygen exposure binds the free Ti forming Oxides of Titanium. Exposure of the barrier to Nitrogen heat treatment or an activated Nitrogen exposure, e.g. by Nitrogen containing plasma, appears to be as effective as Oxygen treatment because the free Ti has been bound as Titanium Oxides or Nitrides or TiN and not as previously taught by Oxygen 'stuffing grain boundaries' (see U.S. Pat. No. 5,552,341 e.g. column 4 lines 25 to 31 and lines 63 to 64 for an exposition of prior art to that invention and column 8 lines 26 to 31 showing that the use of an oxide in the barrier such that “the stuffing effect can be maintained” was still required for that invention).
The invention also includes a method of depositing a conductive layer, such as Aluminium, Aluminium alloy or Copper film, on a workpiece including initially depositing a barrier layer of Titanium Nitride, reactivating the barrier layer and depositing the conductive layer.
The barrier layer may be deposited using physical vapour deposition and may itself be laid down on a pure Titanium layer.
As has been set out in one aspect above, it has been found that barrier layers containing Oxygen can be made suitable for subsequent processing by replacing at least the surface Oxygen atoms with Nitrogen.
This has been achieved in a number of ways, in particular the use of Nitrogen and Hydrogen containing plasmas. An explanation of possible mechanisms is as follows.
The use of a Nitrogen containing plasma is probably explained by plasma generated species, possibly ion assisted, effectively nitriding the oxidised barrier surface.
TiO
2
+N
*
→TiN+O
2
This reaction is only slightly favourable above 625° C.
The use of a plasma—or Ultra Violet-generated atomic Hydrogen is more chemically favourable and does not require ion assistance:
TiO
2
+4H
*
→2H
2
O+Ti−485 KJ
If this reaction is carried out in the presence of Nitrogen, the final barrier layer surface would be simultaneously nitrided.
Chemical Vapour Deposited (CVD) TiN has superior characteristics than Physically Vapour Deposited (PVD) TiN. However, there can be a vacuum break as the wafers are transported from the CVD equipment to the PVD equipment for the metallisation processes.
The oxidised Titanium at the surface is re-nitrided in the presence of atomic Hydrogen and atomic Nitrogen either from a mix of gasses or a single gas containing both Nitrogen and Hydrogen (and possibly other elements). The re-nitridation by a mix of atomic Hydrogen and Nitrogen predominately takes place by an initial reduction of the Titanium oxide by the Hydrogen, and then a reacti

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