Dual layer etch stop barrier

Details Dual layer etch stop barrier Dual layer etch stop barrier

1998-02-26

2002-07-16

Loke, Steven (Department: 1765)

Active solid-state devices (e.g., transistors, solid-state diode

With means to control surface effects

Insulating coating

C257S635000, C257S637000, C257S649000

Reexamination Certificate

active

06420777

DESCRIPTION:
The use of silicon nitride as an etch stop barrier is well known in the art especially for stopping the etch during reactive ion etching (RIE) of silicon dioxide disposed over a silicon or doped silicon substrate in the manufacture of integrated circuit chips. Reactive ion etching is used in chip manufacturing to form openings through the silicon dioxide so as to provide access to the substrate. Typically the opening will be filled with metal such as tungsten or other metal as is well known. In etching the silicon dioxide an etch stop layer is used so as to allow the etching to stop or essentially terminate once the etching has penetrated through the silicon dioxide layer. Expressed another way, when the etching has pierced the silicon dioxide layer it is desired that the etching not continue to any significant extent. The barrier layer of etch stop material is to ensure that the etch stops substantially uniformly at all the various locations being etched through the silicon dioxide. Thus, one of the principal requirements of the etch stop material is that it have a relatively high selectivity of etching with respect to the material which is intended to be etched i.e. silicon dioxide. Expressed another way, once the silicon dioxide has been etched it is desirable that there be very little etching taking place after that.
FIG. 1 shows the etching rate of Si x N y in nanometers per minute using an AME 5000 tool with Ar:CHF 3 atmosphere at various ratios of silicon to nitrogen in a silicon nitride (Si x N y ) barrier. As can be seen, when the ratio of silicon to nitrogen is 0.75 (which is the stoichiometric ratio) the etch rate is between 140 and 160 nanometers per minute, but as the ratio of silicon to nitrogen increases, this etch rate decreases dramatically to a point where when the ratio of Si to N is about 1.0 the etch rate has dropped down to about 20 nanometers per minute. With a ratio greater than 1.0 no improvement in the etch rate resistance is achieved. Thus, based on this particular characterization, in order to get the lowest etch rate of silicon nitride and thus the highest etch selectivity, it is desirable to have a ratio of silicon to nitrogen of at least about 1.0.
However, in subsequent processing during chip manufacture there can be generated positive mobile ions (PMI), in particular Na + and K + , principally from contamination in the SiO 2 layer. If these positive ions diffuse even in small amounts into the silicon substrate they can cause significant degradation of the substrate material in some structures. Thus, it is desirable and often even necessary that these ions be essentially excluded from penetrating the barrier and diffusing into the substrate. FIGS. 2 and 3 show the amount of diffusion of positive mobile ions especially sodium (Na + ) as measured by Vt Shift (mV) shown in FIG. 2 and ion density in 10 10 Ions/cm 2 shown in FIG. 3 in substrates with Si x N y nitride barriers having various ratios of Si to N in the silicon nitride. At a Si to N ratio of 1.05 there is a very high number of mobile ions passing through the silicon nitride barrier, and even at a ratio of 1.0 there is an appreciable amount of these ions penetrating; indeed even at a ratio of silicon to nitrogen of 0.8 there is a significant amount of PMIC (positive mobile ion contamination). It is not until the ratio of silicon to nitrogen is 0.75 (i.e. the stoichiometric ratio) that the PMIC is essentially eliminated.
Thus, if one were to design the barrier to maximize resistance to positive mobile ion penetration one would use a ratio of silicon to nitrogen of 0.75. However, as shown above, this would provide very poor etch selectivity. On the other hand, if one were to design for the best etch selectivity, one would design a nitride barrier having a ratio of silicon to nitrogen of 1.0 or greater; but this would provide poor resistance to positive mobile ion penetration.
According to the present invention, a barrier is provided which will achieve both high resistance to positive mobile ion penetration and very g

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