Silicon-germanium transistor and associated methods

Details Silicon-germanium transistor and associated methods Silicon-germanium transistor and associated methods

1999-08-16

2001-05-22

Nelms, David (Department: 2818)

Semiconductor device manufacturing: process

Making field effect device having pair of active regions...

On insulating substrate or layer

C438S306000, C438S052000, C438S008000

Reexamination Certificate

active

06235560

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of semiconductor devices, and, more particularly, to a transistor fabrication method and a corresponding transistor produced thereby.
BACKGROUND OF THE INVENTION
Carrier mobility is the speed of movement of an electron or hole through the channel region of a transistor. This is an important property for high speed transistors. The higher the carrier mobility, the higher the operating frequency of the transistor. The addition of germanium dopant impurities in a silicon substrate increases channel mobility over that of pure silicon.
Conventional silicon transistor gates are formed by thermally oxidizing the silicon substrate. A thermally grown gate oxide is typically formed by exposing the silicon substrate surface to an oxygen containing ambient. The oxygen causes the silicon surface to be partially consumed and converted into the gate oxide. Unfortunately, a stable gate oxide can not be readily formed from a silicon-germanium substrate because germanium becomes unstable at high temperatures. In other words, the germanium begins to diffuse from the silicon at high temperatures, and consequently, does not retain the properties exhibited at low temperatures. Thus, oxide should not be present at the silicon-germanium interface because the germanium bond is not stable under normal operating conditions. The diffused germanium would create interface trap sites in the gate oxide which may severely limit the electrical performance of the transistor.
One approach to forming a stable gate oxide over a silicon-germanium substrate includes depositing a low temperature CVD oxide. However, such an oxide has a resulting undesirable higher surface state density. Another way of forming a stable gate oxide over a silicon-germanium substrate is by reoxidation of a silicon cap layer applied over the silicon-germanium substrate. Using a silicon cap layer results in a buried channel structure with an undesirably large effective gate oxide thickness.
Nonetheless, silicon-germanium transistors are desirable because of the higher operating speeds that can be achieved as compared to the operating speeds of silicon transistors.
SUMMARY OF THE INVENTION
In view of the foregoing background, it is therefore an object of the present invention to provide a silicon-germanium transistor fabrication method and a corresponding silicon-germanium transistor having a stable gate oxide that is relatively thin.
This and other objects, advantages and features in accordance with the present invention are provided by a method for making a transistor preferably comprising the steps of providing a silicon substrate having a silicon-germanium epitaxial layer, forming a masking implant layer on a channel region of the silicon-germanium epitaxial layer, and implanting dopants into the silicon-germanium epitaxial layer using the masking implant layer to define spaced apart source and drain regions adjacent the channel region.
The method further includes the steps of removing the masking implant layer after the implanting to expose the channel region, and forming a silicon epitaxial layer on the exposed channel region. At least a portion of the silicon epitaxial layer is preferably converted into silicon oxide to define a gate dielectric layer using either plasma enhanced chemical vapor deposition (PECVD) or high pressure oxidation. Both steps are performed at low temperatures to prevent the migration of germanium from the channel region to the gate dielectric layer when the silicon oxide is formed. A conductive gate is formed on an upper surface of the gate dielectric layer.
The source and drain regions are thus formed before the silicon oxide is formed. This is so since the method preferably further includes the step of subjecting the silicon substrate having the silicon-germanium epitaxial layer to high temperatures to activate the implanted ions in the source and drain regions, and to repair damage done during the implantation process. Otherwise, if the silicon oxide was formed before the source and drain regions are formed, germanium would become diffused from the silicon and migrate to the gate dielectric layer when the silicon substrate having the silicon-germanium epitaxial layer is subjected to the high temperatures. Oxidation of germanium would create an unstable gate dielectric layer.
In one embodiment, only a portion of the silicon epitaxial layer is preferably converted to silicon oxide to define the gate dielectric layer. Accordingly, the gate dielectric layer comprises a gate oxide layer and a silicon protection layer between the gate oxide layer and the channel region. The silicon protection layer advantageously prevents germanium from the channel region from migrating to the gate oxide layer under high temperatures.
Another aspect of the invention relates to a silicon-germanium transistor comprising a silicon substrate including a silicon-germanium epitaxial layer having a channel region, and a pair of spaced apart source and drain regions adjacent the channel region. A gate dielectric layer is on the channel region. The gate dielectric layer preferably includes a gate oxide layer adjacent the channel region, and a silicon protection layer between the gate oxide layer and the channel region. A conductive gate is on an upper surface of the gate oxide layer.


REFERENCES:
patent: 5296387 (1994-03-01), Aronowitz
patent: 5726459 (1998-03-01), Hsu et al.
patent: 5879996 (1999-03-01), Forbes

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