The present invention relates to method for structuring a trench in a semiconductor substrate. More specifically, the present invention relates to a method for etching deep trenches such as those having aspect ratios of 30 and higher.
A method for etching trenches in a semiconductor substrate is described in the published European Patent Application EP 1 420 438. This method uses a plurality of processing cycles comprising plasma etch and deposition periods. During the etch periods the substrate bias power is pulsed. During the deposition periods a protective coating or layer is formed on the sidewalls of the trench. This protective layer protects the sidewalls and stops further etching thereon. Accordingly, just the trench's bottom is mainly etched and a high aspect ratio is obtained.
The known method uses a polymeric coating as protective layer. This coating is deposited “in situ”, i.e., inside the same process chamber where the etch periods take place. In order to obtain the targeted trench's depth as mentioned above, a plurality of plasma etch and deposition periods have to be applied.
In one aspect, the present invention provides a method for etching trenches that provides a large aspect ratio and smooth sidewalls. A further aspect of the invention is to provide a method that is easy to carry out.
The method according to the preferred embodiment of the present invention comprises a first etch cycle wherein the trench is etched to a first depth. Thereafter, a protective layer or protective liner comprising an inorganic material is deposited on at least the upper part of the trench's sidewalls. During at least one second etch cycle the trench is etched to its final depth. The terms “protective liner” and “protective layer” are hereinafter used as synonyms.
According to embodiments of the invention, the protective liner consists of inorganic material. Compared to prior art methods that use polymeric layers, the etch selectivity of the inventive protective liner is much larger. Accordingly, the second etch cycle may be carried out for a longer period of time and a larger etch depth is obtained. In consequence, less plasma etch and deposition periods are required to obtain the targeted trench's depth. It is even possible to reach the final depth in just one second etch cycle; in this case there is no need to deposit a second protective liner.
According to a preferred embodiment of the invention, the protective liner is deposited ex situ. This means that the substrate is removed from the etch chamber where the first etch cycle has been carried out. The protective liner is deposited during a separate process, which may be optimized individually; therefore, there is no need to consider process parameters that are necessary or advantageous with regard to the first etch cycle since the etch cycle and the depositing step are split up and totally separated from another.
The substrate's surface should be cleaned before the protective liner is deposited. Removing the etch byproduct from the first etch step improves the quality of the protective liner and maintains or enlarges the orifice of the trench opening. In contrast to prior art methods, such an additional cleaning step is easy to carry out as the substrate is removed from the process chamber thanks to the “ex situ”-process.
To minimize production costs, just two etch cycles should be applied to reach the trench's final depth. As mentioned above, two etch cycles are usually sufficient since an inorganic protective liner is used.
As an example, the protective liner may consist of or comprise an oxide, a nitride or an oxynitride such as a silicon oxide, a silicon nitride or a silicon oxynitride. Preferably however, the protective liner consists of or comprises a byproduct or a material chemically similar to a byproduct that is produced during the second etch cycle. In the latter case the protective liner will resist to the etch chemicals for quite a long period of time.
If, for instance, during the first or during the second etch cycle silicon oxide is produced as a byproduct, the protective liner preferably consists of a PECVD-oxide.
The first-etch cycle may be carried out using a gas mixture containing HBr, NF3 and O2.
The second etch cycle may be carried out using the same gas mixture as the first etch cycle (HBr, NF3 and O2).
The second etch cycle may as well be carried out using a gas mixture containing SF6 and C4F8. A so called “BOSCH” process may be applied as described in detail in European Patent Publication EP 0625285 and U.S. Pat. No. 5,501,893, which are incorporated herein by reference.
Alternatively, the second etch cycle may be carried out using a gas mixture containing SF6 and O2. Preferably a so-called “Kryo-process” is used as described in detail in “Dussart R. et al., J. Micromech Microeng. 12, p. 190-196, (2004)”, which is incorporated herein by reference.
According to a further preferred embodiment of the invention, the protective liner is a non-conformal layer that covers exclusively the trench's sidewalls and leaves the trench's bottom uncovered. The protective liner may be deposited by a CVD process, a furnace process or a starved ALD (Atomic Layer Deposition) process. Alternatively the protective liner may be deposited by a Silanol-ALD process. Details are described in “Hausmann D. et al., Science, Vol. 298, p. 402-406 (2002)”, which is incorporated herein by reference.
Alternatively, the protective liner can be a conformal layer that covers the trench's sidewalls and the trench's bottom. Accordingly, the protective liner has to be removed from the trench's bottom first before the trench can be etched further down. However, the upper part of the trench's sidewalls should remain covered by the protective liner in order to achieve high aspect ratios while maintaining the sidewall profile from the first etch step.
The removal of the protective liner from the trench's bottom may be carried out during the second etch cycle. Alternatively, the protective liner may be removed from the bottom during an additional etch step that is carried out before the second etch cycle is started. This additional etch step may be carried out “in-situ” or “ex-situ”, meaning in the same etch reactor where the second etch step is carried out, or in a different etch reactor.
In order that the manner in which the above-recited and other advantages and objects of the invention are obtained will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof, which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:
The preferred embodiments of the present invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout.
It will be readily understood that the process steps of the present invention, as generally described and illustrated in the figures herein, could vary in a wide range of different process steps. Thus, the following more detailed description of the embodiments of the present invention, as represented in
The present invention provides a method for etching a trench in a semiconductor substrate. Referring to
In order to avoid the undesired etch rate in lateral direction and in order to increase the aspect ratio, the invention uses a protective liner at least on the upper part of the trench's sidewalls.
In
Then, the substrate 10 is removed from the etch chamber and cleaned. Later on, a protective liner 100 is deposited on the mask layer 20 and on the upper part 110 of the sidewalls 60. The deposition step is handled such that the protective liner 100 is non-conformal; accordingly, the lower part 120 of the sidewalls 60 and the bottom 130 of the trench 40 remain uncovered. For example, the protective liner may be deposited using one of the following preferred sets of deposition parameters; each set of parameters assures that the protective liner is non-conformal and consists of inorganic material:
The resulting trench 40 with covered sidewalls 60 is shown in
After completing the deposition of the protective liner 100 a second etch cycle is applied wherein the trench 40 is etched to its final depth d2. The second etch cycle is shown in
For example, the second etch cycle may be carried out using one of the following preferred sets of etch parameters:
According to the first exemplary embodiment of the invention, as explained above with reference to
Preferably, just two etch cycles are used to achieve the targeted etch depth d2 as shown in
In
During a first etch cycle a trench 40 is etched to a first depth d1 into the substrate 10 (see
The following explanations refer explicitly to parameter set 1. As can be seen in
Afterwards, the substrate 10 is cleaned and the re-deposited material 300 is removed. This is done ex-situ, i.e., outside the process chamber where the etch cycle has been carried out. The resulting structure is shown in
Later on, a protective liner 100 is deposited on the mask layer 20 and on the sidewalls 60. The deposition step is handled such that the protective liner 100 is a conformal layer; the upper part 110 and the lower part 120 of the sidewalls 60 as well as the bottom 130 of trench 40 are covered. For example, the protective liner 100 may be deposited using one of the following preferred sets of deposition parameters; each set of parameters assures that the protective liner 100 is a conformal layer and comprises inorganic material:
The trench 40 with covered sidewalls 60 is shown in
After completing the deposition of the protective liner 100 a second etch cycle is applied wherein the trench 40 is etched to its final depth d2. The second etch cycle is shown in
The following explanations refer explicitly to parameter set 1. As can be seen in
According to the described second exemplary embodiment of the invention, as explained above, the deposition of the protective liner 100 is carried out ex-situ. This allows to optimize the deposition step of the protective liner independently from both etch cycles.