Active solid-state devices (e.g. – transistors – solid-state diode – Integrated circuit structure with electrically isolated... – Including dielectric isolation means
Reexamination Certificate
1999-09-02
2001-05-15
Loke, Steven H. (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Integrated circuit structure with electrically isolated...
Including dielectric isolation means
C257S501000, C438S297000, C438S410000, C438S439000
Reexamination Certificate
active
06232644
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
A part of the fabrication process for semiconductor devices involves the formation of a field oxide to separate from each other the active regions in which the devices are to be fabricated. This part of the process generally involves the formation of a nitride layer mask over silicon with pad oxide thereon at what will later be the active region(s) with oxidation of the exposed (without nitride thereover) silicon to form the field oxide. A problem has been that some of the oxidizing species will migrate under the corner of the nitride layer and oxidize some of the silicon under the nitride layer, this phenomenon being known as “bird's beak” as well as by other names and generally not being controllable. In order to minimize the likelihood of such migration of an oxidizing species under the nitride layer to form additional silicon oxide with the underlying silicon, the prior art has removed a portion of the pad oxide at the corner formed by the silicon and nitride layer, regrown a thinner layer of oxide in the undercut region and then placed a sidewall nitride onto the nitride layer which also fills the remainder of the undercut region over the regrown oxide. The field oxide is then grown. There have been recent proposals with recessed isolation whereby the oxidant can be completely prevented from reaching the silicon surface under the nitride. For this case, it appears advantageous to provide for a small amount of oxidation at the corner to provide rounding for control of stress and field enhancement. Such “bird's beak” elimination is required as dimensions are scaled below 0.25 micron and become increasingly susceptible to stress and field emission leakages. In these approaches, it is difficult to balance the various steps to provide for optimum oxidation of the corner region while maintaining acceptable encroachment and an independently adjustable process is required to improve control of the process results.
SUMMARY OF THE INVENTION
In accordance with the present invention, a procedure is provided which enables greatly improved control of the oxidation profile at the corner of the nitride layer and the underlying silicon layer or “bird's beak” region.
Briefly, there is provided the nitride mask of the prior art which is not permeable to at least selected oxygen-bearing species. A path is then provided as a permanent part of the device being fabricated, which is used during device fabrication, which extends under the nitride mask and which is capable of directing the selected oxygen bearing-species or reactants to specific locations under the nitride mask. This permits an increase in oxide thickness during general growth of the thick oxide at preferred locations which are otherwise either not accessible or accessible without adequate oxide growth control. This path is preferably an oxide layer which is as thin as possible but still capable of performing its function. This path is provided as a sidewall to the nitride mask and extends to the corner formed by the nitride mask and the silicon layer thereunder with pad oxide. The thickness of this path, preferably a thin oxide layer, is critical only to the extent that it be capable of directing the oxygen bearing species therethrough in sufficient amount to provide the desired amount of oxide formation in the time available. The above described path is formed as a sidewall on the nitride mask and prior to formation of the subsequent sidewall nitride which is now provided as a sidewall on the path for oxygen-bearing species as described above.
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Hu Yin
Richardson William F.
Brady III Wade James
Garner Jacqueline J.
Loke Steven H.
Owens Douglas W.
Telecky , Jr. Frederick J.
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