The present invention relates to multilayer wafers, in particular, to ultra thin buried oxide (UTBOX) wafers designed for the fabrication of CMOS devices, and to the manufacture of improved shallow trench isolation (STI) structures in such wafers.
Because wafers with silicon on thin buried oxide layers (BOX) or on ultra thin buried oxide (UTBOX) layers are advantageously characterized by small variations of threshold-voltages, they are of growing interest in present and future CMOS technology. In particular, fully depleted CMOS technology allows for low-voltage and low-power circuits operating at high speeds. Moreover, fully depleted silicon-on-insulator (SOI) devices are considered as some of the most promising candidates for reducing short channel effects (SCE).
Silicon on Insulator (SOI) wafers and, in particular, UTBOX wafers, can form the basis for high-performance MOSFET and CMOS technologies. The control of SCE is mainly facilitated by the thinness of the active silicon layer formed above the insulator, i.e. above the buried oxide (BOX) layer. In order to reduce the coupling effect between source and drain and, further, to improve the scalability of thin film devices for future technologies, very thin BOX layers are mandatory. Control of threshold voltages also depends of the thinness of the BOX layers. Appropriate implantation of the substrate below the BOX layer allows for accurate adjustment of the threshold voltage by back biasing.
In the manufacture of SOI devices, in particular, of CMOS devices, trenches usually have to be formed, for example, in order to electrically isolate individual MOSFETs from each other. The formed trenches can be cleaned, slightly oxidized, and subsequently filled with an oxide liner and some insulator material above the liner, so as to form shallow trench isolation (STI) structures. However, in the manufacture of such trench structures, for example, STIs, a problem known as the so-called Bird's Beak Effect can arise. The Bird's Beak Effect in the context of the formation of trenches in SOI wafers is characterized by the increase in the thickness of the BOX layer in the areas close to the edges of a trench.
One of the reasons behind the Bird's Beak Effect is related to under-etching of the BOX layer, as can, for example, occur during a cleaning process that usually comprises hydrofluoric dipping. Another reason behind the Bird's Beak Effect is lateral oxidation of a not completely closed bonding interface within the BOX layer. Such bonding layers with interfaces can arise when the SOI wafer is manufactured by oxide-oxide bonding wherein, for example, a thin silicon layer is covered by an oxide layer, for example, an SiO2 layer, and transferred to a substrate that is also covered by an oxide layer of the same type.
Thus, there is a need for improved methods for the formation of trenches and corresponding isolator structures that alleviates the Bird's Beak Effect.
Accordingly, the present invention provides methods for the manufacture of trench structures in multilayer wafers comprising a substrate, an oxide layer on the substrate and a semiconductor layer on the oxide layer. In one embodiment, the method comprises:
Although the following description of the invention is largely in terms of substrates having such compositions, this invention is not limited to such substrates, and in other embodiments, is applicable more generally to semiconductor-on-insulator (SeOI) substrates. In view of the following description, one of ordinary skill in the art will be able to select parameters (e.g., thermal anneal atmospheres, temperatures and durations) appropriate to different semiconductors.
The thermal annealing step effectively reduces Bird's Beak effects that might arise during subsequent manufacturing steps using the formed trench. The semiconductor material overflowing the oxide exposed in the trench prevents under-etching of the semiconductor layer in subsequent cleaning processes, and also prevents unintended lateral oxidation of the previously exposed oxide layer when forming an oxide liner, for example, on the inner surfaces of the trench.
According to another embodiment, a cleaning treatment of the trench is performed after the thermal annealing. The cleaning treatment may comprise cleaning with a hydrofluoric acid solution, in particular, dipping in a hydrofluoric acid solution. No under-etching of the semiconductor layer is caused by the hydrofluoric acid solution due to the protective coating of the material of the semiconductor layer that has flowed over the exposed oxide surface during the thermal annealing.
After the thermal anneal and cleaning treatment, an oxide liner may be formed in the trench. The oxide liner may be formed by thermal oxidation of the wafer including the trench. The thickness of the formed oxide should be sufficient to consume the material that has flowed over the exposed oxide surface during the thermal annealing semiconductor material and, by so doing, to prevent an electrical short/connection between top Si layer and silicon substrate below the BOX layer. The method may, further, comprise filling the trench with an insulator material after the anneal treatment and after formation of the oxide liner.
The anneal treatment can be preferably performed, for example, at a temperature of at least 1100° C., in particular, at least 1150° C. and, more particularly, at least 1200° C. Moreover, the anneal treatment can be preferably performed in a hydrogen and/or argon atmosphere. The thermal anneal treatment can be preferably performed for a rather short time period, for example, for at most 4 minutes, in particular, at most 3 minutes and, more particularly, at most 2 minutes.
In the above-described embodiments of the invention, the BOX layer preferably can have a thickness in the range from 5 to 20 nm and the top semiconductor layer in an UTBOX wafer preferably can have a thickness in the range from 10 to 50 nm. Thus, a trench can be formed in a UTBOX wafer comprising a very thin BOX layer and a thin silicon layer intended to serve as an active layer for a semiconductor device, for example, an SOI CMOS device. The methods of the invention (and particularly the step of thermal annealing) can effectively suppress the Bird's Beak effect for thin BOX layers, even for BOX layers as thin as 2 to 20 nm. It is in such very thin BOX layers where the Bird's Beak effect is particularly problematic.
In further embodiments, the multilayer wafers in which trenches are formed by the methods of this invention can be manufactured by method comprising the steps of:
It is noted that in the case that the multilayer is manufactured as above, the thermal anneal step of the present invention advantageously also results in a consolidation of the bonding interfaces of the first and second oxide layers (that, when bonded together, form the BOX layer of the wafer). This consolidation is to be understood in the sense of a complete closing of the interface between the two oxide layers thereby resulting in an homogeneous BOX layer with few if any traces of the bonding interface. Due to the thus achieved homogeneity of the BOX layer, the Bird's Beak Effect caused by lateral oxidation (e.g., when forming by thermal oxidation an oxide liner at the inner surfaces of the trench) can be further significantly suppressed.
In further embodiments, the present invention also provides methods for the manufacture of semiconductor devices in a multilayer wafer, comprising the steps of
In further embodiments, the present invention also provides methods for the manufacture of a trench capacitor in a multilayer wafer, comprising the steps of
Additional features and advantages of the present invention will be described with reference to the drawings, which are meant to illustrate preferred embodiments of the invention. It is understood that such embodiments do not represent the full scope of the invention. Further aspects and details, and alternate combinations of the elements, of this invention that will be apparent from the following detailed description to one of ordinary skill in the art are also understood to be within the scope of this invention.
The present invention may be understood more fully by reference to the following detailed description of the preferred embodiments of the present invention, illustrative examples of specific embodiments of the invention and the appended figures in which:
a to 1f illustrate an embodiment of the methods for the manufacture of an STI in an SOI wafer according to the present invention; and
In the following, preferred embodiments of the methods of the invention for forming trench structures in multilayer wafers is described.
First, a wafer as shown in
Next, a trench 4 is formed in the wafer shown in
After the formation of the trench 4 in the multilayer wafer, a rapid thermal anneal process 100 is performed (see
Next, the trench 4 is cleaned 200 preferably by, e.g., a hydrofluoric acid solution as shown in
Again, the Bird's Beak Effect that might otherwise be caused by lateral oxidation of an imperfectly closed oxide-oxide interface within the oxide layer 2 (such as might result from the above-described manufacturing process comprising wafer transfer) is suppressed by the previously performed anneal treatment 100.
Next, the trench is filled with some insulator material 6, for example, some nitride or oxide material, to finish the formation of an STI (see
The above-described embodiment for forming the trench structure shown in
The above-described embodiment for forming trench structures, in particular, STIs, may also be applied as illustrated in
In the example shown in
The preferred embodiments of the invention described above do not limit the scope of the invention, since these embodiments are illustrations of several preferred aspects of the invention. Any equivalent embodiments are intended to be within the scope of this invention. Indeed, various modifications of the invention in addition to those shown and described herein, such as alternate useful combinations of the elements described, will become apparent to those skilled in the art from the subsequent description. Such modifications are also intended to fall within the scope of the appended claims.
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