Adjustable method for eliminating trench top corners

An adjustable method for making trenches for a semiconductor IC device having eliminated top corners is disclosed. The adjustable method includes forming a masking layer on the surface of the silicon nitride layer covering the device substrate that has openings corresponding to the openings of the trenches formed. Dimension of the masking layer opening is relatively greater than the dimension of the opening of the corresponding trench. An anisotropic etching procedure is then performed against the portions of the device substrate exposed out of the coverage of the masking layer, and the anisotropic etching shapes the trench sidewalls into sloped ones having larger dimension at the opening than at the surface of the filling material inside the trenches. This eliminates the top corners at the edges of the trench opening, charge accumulation and consequent leakage current can thus be prevented.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates in general to the fabrication of semiconductor IC 
devices and, in particular, to a method of making trenches without 
undesirable top comers. More particularly, this invention relates to an 
adjustable method for eliminating trench top corners for preventing 
electric charge accumulation and the consequent damaging leakage current 
discharge. 
2. Description of Related Art 
Trenching is a technique widely employed for the isolation of circuit 
elements in the circuitry of semiconductor IC devices. For example, DRAM 
is a specific category of IC devices that employs the trench configuration 
to provide electrical isolation between consecutive transistors in the 
arrays of memory cells. Trenching can provide electrical isolation for 
circuit elements that requires less space than required by regional local 
oxidation. In the case of DRAMs, in addition to providing electrical 
isolation, trenches can also be used to construct storage capacitors for 
the memory cell units in the array. Such trenches are normally formed in 
processes requiring the use of certain special gaseous mixtures. These 
gaseous mixtures are controlled by some hardware equipment so that device 
substrate can be properly etched to form the trenches. 
Dry etching is the traditional procedure widely used to make trenches in 
the device substrate. Plasma, rather than fluidic etching solution, is 
used to perform thin-film etching. One of the primary advantages of 
thin-film etching is that anisotropic etching can be achieved to have a 
greater etch consumption rate in the vertical direction than in the 
lateral. Since there is relatively much smaller etch consumption rate in 
the lateral orientation when compared with the vertical, the phenomenon of 
undercut is therefore avoided. Trenches obtained by anisotropic etching 
may therefore exhibit very straight trench sidewalls, with corners turning 
at an angle of nearly 90 degrees. 
FIGS. 1a and 1b are a cross-sectional views showing the selected cross 
sections of the structural configuration of a trench together with its 
filled material as obtained in a conventional chemical-mechanical 
polishing procedure. As is illustrated in FIG. 1a, oxide layer 11 and 
silicon nitride layer 12 are subsequently formed on the device substrate 
10. A photoresist layer is then formed covering the silicon nitride layer 
12 which is then defined with specific patterns. The patterned photoresist 
layer is then used as the protective mask for implementing an anisotropic 
etching procedure to form the trenches 13. Afterwards, the photoresist 
layer is then removed. 
Then, a layer of material 14 is formed covering the silicon nitride layer 
12 as well as being filled in the trenches 13. A chemical-mechanical 
polishing (CMP) procedure is then employed to completely remove the entire 
silicon nitride layer 12 and the layer 14 on top, leaving the filled 
material 14 inside the trenches 13. The oxide layer 11 is also removed in 
the CMP procedure, resulting in the structural configuration as shown in 
FIG. 1b. Surface of the structure of FIG. 1b does not present a true flat 
plane. 
Rather, shallow areas in the trench regions introduce top corners, as 
identified by reference numerals 15 in the drawing. These top corners 15 
have turning angles nearly 90 degrees that can easily result in electric 
charge accumulation when the device substrate is energized, and leakage 
currents arise in these top corner areas as the fabricated IC device is 
processed further. In the subsequent fabrication procedural steps, 
whenever there are electrically conductive materials striding across the 
region where substrate 10 and the layer 14 meet each other, such leakage 
currents incur short-circuiting to degrade device reliability. Fabrication 
yield rate is therefore deteriorated, a phenomenon known as the kink 
effect. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide an adjustable method 
of making trenches in semiconductor IC devices without the formation of 
top comers at the edges of the trench opening. 
The present invention achieves the above-identified objects by providing an 
adjustable method for making trenches for a semiconductor IC device having 
eliminated top comers. The adjustable method includes the steps of 
subsequently forming an oxide layer and a silicon nitride layer on the 
surface of the device substrate of the device. Trenches are then formed in 
the device substrate, and filling material is then formed inside the 
trenches, with the surface of the filling material being relatively lower 
than the opening of the trench, and further slightly lower than the top 
surface of the device substrate. A masking layer is then formed on the 
surface of the silicon nitride layer that has openings corresponding to 
the openings of the trenches. Dimension of the masking layer opening is 
relatively greater than the dimension of the opening of the corresponding 
trench. An anisotropic etching procedure is then performed against the 
portions of the device substrate exposed out of the coverage of the 
masking layer, and the anisotropic etching shapes the trench sidewalls 
into sloped ones having larger dimension at the opening than at the 
surface of the filling material inside the trenches.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Refer to FIGS. 2a-2d, a description of a fabrication process follows that 
can explain how the top corners can be eliminated for IC devices 
fabricated in accordance with the preferred embodiment of the invention. 
As is illustrated in FIG. 2a, an oxide layer 21 and a silicon nitride 
layer 22 are subsequently formed on the surface of the device substrate 
20. A photoresist layer is then formed covering the silicon nitride layer 
22 which is then defined with the specific pattern that is required for 
the fabrication of the trenches. The patterned photoresist layer is then 
used as the protective mask for implementing an etching procedure to form 
the trenches 23. Afterwards, the photoresist layer is then removed. 
Then, a layer of covering material 24 is formed covering the silicon 
nitride layer 22 as well as being filled into the trenches 23. This 
covering layer 24 may be formed of metal or oxide material in a process 
of, for example, deposition. The covering material 24 on the surface of 
the silicon nitride layer 22 is completely removed in a process of, for 
example, etching. During this process, the surface of the covering 
material 24 inside the trench 23 is controlled not only lower than the 
exposed surface of the silicon nitride layer 22, it is also slightly lower 
than the top surface of the device substrate 20 itself 
Then, as in FIG. 2b, a masking layer 25 such as a photoresist is formed on 
the surface of the device. This masking layer 25 is configured to have the 
corresponding patterning that reveals the opening of the trenches 23, but 
with the trench opening dimension larger than the opening of the trenches 
themselves. In general, the surface area for the trench opening in the 
masking layer is controlled to be about 110% that of the trench opening 
itself Note at this point that the edges 26a of the silicon nitride layer 
22 proximate to the opening of the trenches 23 are substantially 
right-angle corners. 
The structural configuration of FIG. 2b is now subject to an anisotropic 
etching procedure utilizing the masking layer 25 as the protective masking 
layer. All those portions not covered under the masking layer 25 are 
anisotropically etched simultaneously. These include the small portions of 
the top surface of the silicon nitride layer 22 exposed out of the 
coverage of the mask 25 and the sidewall of the trenches 23 themselves. 
When the anisotropic etching procedure concludes, a structure schematically 
shown in the cross-sectional view of FIG. 2c is resulted. Due to the 
nature of anisotropic etching, the right angles (26a in FIG. 2b) that 
would otherwise become the disadvantageous top corners are now eliminated, 
and the sidewalls of trenches 23 are shaped into sloped ones, as are 
identified in the drawing by reference numerals 26b. 
During this anisotropic etching processing, those portions of the oxide 
layer 21 as well as the device substrate 20 exposed in the plasma etching 
environment are also removed. Slope of the sidewalls 26b is adjustable 
under several factors. One factor is the ratio between the dimensions of 
the opening of the masking layer 25 and that of the trenches 23. Another 
factor is the height of the surface of the material 24 inside the trenches 
23. In general, the larger the dimensional ratio between the openings and 
the higher the top surface of the material 24 inside the trenches 23, the 
smaller the slope of the sidewall 26b is obtained, and the reverse is also 
true. This allows for the easy and convenient control over the slope of 
the sidewalls 26b. 
Then, as is illustrated in FIG. 2d, after the removal of the silicon 
nitride layer 22 and the oxide layer 21, trenches 23 may now have an edge 
27 with much easier contour than the right-angled top corners found in the 
prior-art technique. As mentioned above, this greatly reduces the 
possibility of producing the problem of electric charge accumulation and 
its consequent phenomenon of damaging leakage currents. The soft contour 
at the edge of opening of the trenches 23 as outlined in FIG. 2d can 
greatly improve the device quality in the post fabrication procedures 
after the structure shown is obtained. 
While the invention has been described by way of example and in terms of 
preferred embodiment, it is to be understood that the invention need not 
be limited to the disclosed embodiments. On the contrary, it is intended 
to cover various modifications and similar arrangements included within 
the spirit and scope of the appended claims, the scope of which should be 
accorded the broadest interpretation so as to encompass all such 
modifications and similar structures.