Active solid-state devices (e.g. – transistors – solid-state diode – Combined with electrical contact or lead – Of specified material other than unalloyed aluminum
Reexamination Certificate
2001-10-12
2004-09-07
Brewster, William M. (Department: 2823)
Active solid-state devices (e.g., transistors, solid-state diode
Combined with electrical contact or lead
Of specified material other than unalloyed aluminum
C257S774000, C257S775000
Reexamination Certificate
active
06787913
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor device and a method of forming the same, and more particularly to an improved contact plug having an ohmic contact with a silicon substrate and a method of forming the same.
As the advanced large scale integrated circuits have been developed to further increase the density of integratio, the requirement for size down of a contact plug has been on the increase, wherein the contact plug is provided in a contact hole formed in an inter-layer insulator over a silicon substrate so that an interconnection extending over the inter-layer insulator is electrically connected through the contact plug in the contact hole to a semiconductor device formed in an upper region of the silicon substrate. A thickness of the inter-layer insulator is not reduced despite scaling down of the semiconductor device and despite the increase in density of the integratio of the large scale integrated circuits. This means that the depth of the contact hole or the height of the contact plug is also not reduced. Notwithstanding, a diameter or a horizontal size of the contact hole or the contact plug is reduced upon scaling down of the semiconductor device and also upon the increase in density of the integratio of the large scale integrated circuits. An aspect ratio of the contact hole is defined to be a ratio of a depth of the contact hole to a diameter of the contact hole, wherein the depth of the contact hole corresponds to a thickness of the inter-layer insulator in which the contact hole is formed. The recent further increased requirement for scaling down of the semiconductor device and increase in the density of integratio of the large scale integrated circuit results in an increase in the aspect ratio of the contact hole. This increase in the aspect ratio of the contact hole makes it difficult to reduce a resistance of the contact plug in the contact hole. In general, in order to obtain a possible ohmic contact of the contact plug with a silicon substrate of both type, namely n-type and p-type, a thin titanium film is deposited on a bottom and a side wall of the contact hole prior to a further deposition of a thin titanium nitride film as a barrier metal on the thin titanium film to form laminations of the titanium film and the titanium nitride film, before a tungsten film is deposited by a chemical vapor deposition on the titanium nitride film as the barrier metal so as to deposit the tungsten film within the contact hole, whereby a contact plug is then formed in the contact plug, wherein the contact plug comprises the tungsten film and the laminations of the titanium nitride film and the titanium film. The tungsten film deposited by the chemical vapor deposition has a good contact coverage. It is required that each of the titanium film and the titanium nitride film has a thickness of not less than 10 nanometers on the bottom of the contact hole. If the thickness of the titanium film on the bottom of the contact hole is less than 10 nanometers, then a contact resistance of the plug with the silicon substrate is high. If the thickness of the titanium nitride film on the bottom of the contact hole is less than 10 nanometer, this extremely thin titanium nitride film is incapable of exhibiting a barrier property to tungsten hexa-fluoride as a source gas for carrying out the chemical vapor deposition to deposit the tungsten film, thereby allowing tungsten hexa-fluoride to be reacted with titanium nitride of the titanium nitride film and titanium of the titanium film, resulting in an increase in the contact resistance of the contact plug and a possible breaking of the p-n junction.
In the advanced semiconductor devices such as 64 Mb-DRAM, the aspect ratio of the contact hole exceeds 4. In this case, it is difficult to apply or use the normal sputtering method to deposit the titanium film or the titanium nitride film, both of which have a thickness of not less than 10 nanometers. In order to obtain a good contact coverage, it is required that sputtered particles fall in a direction just or almost vertical to a surface of the substrate or in a parallel direction to the vertical side wall of the contact hole. In order to respond to this requirements, a collimated sputtering method and a long distance sputtering method are available.
The collimated sputtering method is carried out by using a collimator plate between a target to be sputtered and a substrate on which sputtered particles are deposited, wherein the collimator plate has many openings through which sputtered particles pass. Only when the sputtered particles fall in the direction just or almost vertical to the surface of the substrate, then the sputtered particles are allowed to pass through the collimator plate. Otherwise, the sputtered particles are captured by the collimator plate. For those reasons, only the sputtered particles, which fall in the direction just or almost vertical to the surface of the substrate, will be deposited on the surface of the substrate or on the bottom of the contact hole having the high aspect ratio. This technique is, for example, disclosed in Japanese laid-open patent publication No. 5-299375.
The long distance sputtering method is carried out by setting such a large distance between a target and a substrate that only sputtered particles, which fall in the direction just or almost vertical to the surface of the substrate, can reach on the surface of the substrate, whilst other sputtered particles having a lateral velocity component will be captured by shielding plates.
It was also known in the art to which the present invention pertains that after the titanium film and the titanium nitride film have been deposited, then a heat treatment is carried out in a nitrogen atmosphere to cause a silicidation reaction of titanium with silicon on an interface between the titanium film and the silicon substrate, so that a titanium silicide film is formed on the interface of the silicon substrate with the titanium film, whereby a contact resistance between the plug and the silicon substrate is reduced. This heat treatment can concurrently cause a nitration reaction of unreacted titanium with nitrogen in the titanium nitride film, thereby increasing the barrier capability of the titanium nitride film as a barrier metal. This heat treatment may be carried out by using a furnace as disclosed in the above mentioned Japanese laid-open patent publication No. 5-299275 or by use of a lamp anneal system for rapid anneal in a short time and this lamp anneal system is suitable for a large size substrate.
The following descriptions will be directed to a conventional method of forming the contact plug in the contact hole of the high aspect ratio, wherein the collimated sputtering method is utilized.
Semiconductor devices are formed in a silicon substrate. A boro-phospho-silicate glass film as an inter-layer insulator is deposited by a chemical vapor deposition method on a surface of the silicon substrate. A contact hole is formed in the inter-layer insulator by use of a photo-lithography and a subsequent dry etching process so that the contact hole reaches a surface of the silicon substrate, whereby a part of the surface of the silicon substrate is exposed through the contact hole. The contact hole has a diameter of not larger than 0.35 micrometers.
The collimator sputtering method is carried out to deposit a titanium film on the top surface of the inter-layer insulator and on vertical side walls and a bottom of the contact hole, wherein the titanium film has a thickness in the range of 50-100 nanometers. Subsequently, a titanium nitride film is then deposited by the collimator sputtering method on the titanium film, wherein the titanium nitride film has a thickness in the range of 80-150 nanometers. This collimator sputtering method is carried out by use of a collimator plate which has an aspect ratio of 1.5, wherein the aspect ratio is defined to a ratio of a thickness of the collimator plate to a diameter of openings. The first collimator sputtering process for depo
Yamada Yoshiaki
Yokoyama Takashi
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