Semiconductor device manufacturing: process – Coating of substrate containing semiconductor region or of... – Multiple layers
Reexamination Certificate
1998-11-06
2001-05-08
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Coating of substrate containing semiconductor region or of...
Multiple layers
C438S763000
Reexamination Certificate
active
06228779
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to semiconductor processing technology and more particularly the invention relates to dielectrics used in submicron devices and ULSI microelectronic circuits.
The metal-insulator-silicon (MIS) transistor including the metal-oxide-silicon (MOS) transistor is used in large scale integrated (LSI), very large scale integrated (VLSI), and ultra large scale integrated (ULSI) microelectronic circuits. The transistor has a current carrier source region formed in a surface of a semiconductor (e.g., silicon) body, a carrier drain region formed in the surface and spaced from the source, and between the source and drain is a channel region through which the current carriers flow. Overlying the channel region and aligned with edges of the source and drain is a gate electrode which is physically and electrically separated from the channel by a dielectric layer. Typically, the dielectric layer comprises a silicon dioxide (SiO
2
) and the gate comprises a doped polysilicon material.
To prevent migration of dopants such as boron into the silicon dioxide gate dielectric layer from the silicon substrate and from the doped polysilicon gate, nitride ions have been placed in the silicon oxide layer by ion implantation and by NH
3
(anhydrous ammonia) nitridation. U.S. Pat. No. 5,397,720 discloses a method of making a MOS transistor having an improved oxynitride dielectric in which high quality ultra thin gate oxides have nitrogen ions therein with a profile having a peak at the silicon oxide-silicon interface. U.S. Pat. No. 5,578,848 discloses a low pressure rapid-thermal reoxidation of silicon nitride films with a rapid thermal reoxidation being carried out in N
2
O or in O
2
ambient.
Typical high thermal-budget oxynitridation processes, such as with N
2
O or NO result in nitrogen incorporation at the silicon dioxide/silicon interface in relatively small amounts. Increasing nitrogen concentration improves reliability and the ability of the dielectric layer to suppress boron penetration, but increases fixed-charge and interface-trap density. This, in turn, degrades device performance by reducing the peak channel mobility and degrading transconductance of the MOS device. Furthermore, boron penetration into the silicon oxide with a diffusion barrier situated at the silicon dioxide-silicon interface can degrade the oxide reliability due to boron accumulation in the oxide. Ideally, therefore, it is desirable to have the nitrogen-rich layer located at the polysilicon/dielectric interface for an effective barrier to suppress boron diffusion from the gate without affecting the channel carrier mobility.
Heretofore, plasma nitridation has been used in the formation of 4 nm gate dielectric films with nitrogen at the top (gate electrode/dielectric) interface. The process consists of nitriding a previously formed thermal oxide with a remote, high density helicon-based nitrogen discharge at room-temperature for short durations on the order of a few seconds, followed by a high temperature post-nitridation anneal. Nitridation was performed at room-temperature by exposing the gate oxide to a short, high-density, remote helicon-based nitrogen discharge.
SUMMARY OF THE INVENTION
In accordance with the present invention, ultra thin MOS gate dielectric films having nitrogen located away from the silicon dioxide/silicon interface are fabricated using an oxygen reoxidation of nitrided silicon substrates.
Both the nitridation of silicon in a NO or a NH
3
ambient, for example, and the reoxidation of the nitrided silicon in N
2
O or O
2
are carried out at high pressure, (i.e., greater than 1 atm). Both process steps can be carried out in-situ.
The process in accordance with the invention has a number of advantages including a low thermal budget requirement. The processing temperature for oxygen reoxidation can be significantly lower using high pressure, thus allowing fabrication of ultra compact and ultra small devices.
The nitridation process is a self limited growth process since the incorporation of a significant amount of nitrogen during nitridation acts as a diffusion barrier and limits the growth process. Therefore, the control over a very thin nitride growth is facilitated.
The oxygen reoxidation at high pressure allows slow diffusion of oxygen through the nitride layer without consuming the nitride layer, resulting in an oxidation of the underlying silicon substrate and forming a high quality silicon dioxide layer between the nitride layer and the silicon substrate. Device performance is thus enhanced since mobility and transconductance are higher for a silicon dioxide/silicon interface without the presence of nitrogen atoms. The resulting structure is a nitride/oxide stacked layer.
By first forming the nitride layer and then forming the underlying ultra thin silicon dioxide layer, process control is enhanced over prior art processes in which the silicon dioxide layer is first formed, followed by either chemical vapor deposition of nitride or the nitridation of the silicon oxide surface. Further, the thermally grown films at high pressures are shown to have improved quality and density and stability as compared to chemical vapor deposited or plasma processing.
The invention and objects and features thereof will be more fully understood from the following detailed description and appended claims taken with the drawings.
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Acker Bruce T.
Bloom John A.
Evans Robert K.
Kwong Dim-Lee
Nelms David
Nhu David
Novellus Systems Inc.
Tso Roland
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