FIELD OF THE INVENTION
The present invention relates to a method for forming a semiconductor device; and, more particularly, to a method for forming a semiconductor device with low parasite capacitance by using an air gap and a self-aligned contact plug formed by a selective epitaxial growing method.
DESCRIPTION OF THE PRIOR ART
Generally, the refresh time, which is one of the most important features in semiconductor memory device, may be determined by a leakage current which is caused by a drain damage and this drain damage is often generated at the time of forming a charge storage node contact for electrically connecting the charge storage electrode to the drain of a transistor. The resolution of current lithography techniques may prevent a misconnection in 16M DRAM manufacturing processes or less, which may be caused by an undesired connection to other layers at sidewalls of a contact hole, but the space between the contact hole and adjacent conducting layers in an interlayer insulating layer becomes narrower with the development of the integrated circuits.
The contact is minimized in order to solve the abovementioned problem and this problem may be solved somewhat by modifying the exposure method in a step-and-repeat projection equipment (i.e., stepper), modifying mask to define the contact hole region, or using a self-aligned contact (hereinafter, referred to as SAC).
The most highlighted process is a nitride barrier SAC (hereinafter, referred to as NBSAC) process, in which a nitride layer is used as an etching barrier layer when an oxide layer is etched for forming the contact hole. The NBSAC process may be divided into two etching processes, i.e., oxide and nitride etching processes. In the oxide etching process, polymer-inducing gases, such as C3F8 and C4F8, are have been used in order to improve a selective etching rate to the nitride layer. The C3F8 and C4F8 gases induce a lot of polymer and then a high selective etching rate to the nitride layer may be obtain, but these gases may provoke a problem in that the oxide layer within the contact hole is not removed completely because the polymer may cause an etching stop of the oxide layer. Since the selective etching rate to the nitride layer and the etching stop of the oxide layer within the contact hole are contrary to each other, reappearance of the semiconductor devices may deteriorate and a process window of the contact hole may be narrow.
To make the process window broad, the processing conditions for etching the oxide layer is controlled by a polymer decreasing method. Accordingly, in the case where the oxide layer etching process is carried out by the polymer decreasing method, it requires that the nitride layer should be thick. However, the increase of the thickness of the nitride layer causes the contact area to be diminished and an isotropic etching process is applied to the nitride layer in order to make up for the diminution of the contact area.
Further, since the oxide layer under the nitride layer is used as an electrical insulating layer, the etching process requires that the oxide layer should be controlled by a high selective etching rate in order not to be damaged and a portion of a semiconductor substrate may be exposed at the time of etching an oxide layer in a periphery circuit area, the oxide etching process to minimize the loss of the exposed semiconductor substrate is required.
However, a conventional ion induced etcher to etch the nitride layer may not obtain such a high selective etching rate in the isotropic etching process. The recently highlighted radical etcher using the etchants, such as NF3 and CF4 and SF6, provides an isotropic etching process and a high selective etching rate to the oxide layer, but it does not provide a high selective etching rate to a silicon layer (i.e., silicon wafer) because the etching process uses the fluorine-bearing etchants. Also, the etching rate of the radical etching equipment is determined by the coherence of the oxide, nitride and silicon layers because these etchants may etch all of the oxide, nitride and silicon layers. That is, in the case where the oxide layer, nitride layer and silicon layer may be placed in the order of coherence, it is impossible to obtain a high selective etching rate to a silicon layer because the silicon layer is etched most rapidly.
As a result, the conventional NBSAC process is not sufficient for a high selective etching rate and it is more difficult to obtain a processing margin in etching the oxide, nitride and silicon layers with the decrease of the yield of the semiconductor devices.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a method for forming self-aligned contacts using a selective epitaxial growing method.
It is another object of the present invention to provide a method for improving electrical characteristics of a semiconductor device by reducing parasite capacitance.
In the present invention, a contact plug is formed on a contact area using a selective epitaxial growing method and a self-aligned contact which is formed by a selective etching rate between an epitaxial layer and an oxide layer.
In accordance with an aspect of the present invention, there is provided a method for forming a semiconductor comprising the steps of: forming word lines over a semiconductor substrate, wherein a plurality of contact areas are formed between the word lines; forming epitaxial layers for contact plugs on the contact areas, thereby forming a resulting structure; forming air gaps on non-contact areas on which the epitaxial layers is not formed, by depositing an interlayer insulation layer on the resulting structure; and patterning the interlayer insulation layer so as to expose the epitaxial layers.
After removing the barrier layer to the epitaxial growth, an interlayer insulation layer 25, an oxide layer, is formed on the resulting structure though the PECVD (Plasma Enhanced Chemical Vapor Deposition) method. When the interlayer insulation layer 25 is formed by the PECVD method, the non-contact areas between the word lines (the conduction layer 15 for a gate electrode) are filled with an air gap 21 because the PECVD method has a demerit in topology of the interlayer insulation layer 25. It is quite different from what is required in the general semiconductor processing techniques. Especially, in the PECVD method according to the present invention, the low power of RF bias is kept for poor topology of the interlayer insulation layer 25. The use of the air gap 21 may remove a gap filling process required in a conventional semiconductor processing method. In DRAM memory devices, this air gap 21 may reduce parasite capacitance. Typically, in the case where an oxide layer or a nitride layer, which has higher dielectric constant than the air, is used a gap filling materials, these materials may increase the parasite capacitance and require much more refresh operations of memory devives. However, since the present invention uses the air gap 21 as a gap filling materials, the parasite capacitance loaded on a bit line may be reduced and an additional gap filling process is not required.
On the other hand, the interlayer insulation layer 25 is etched by carbon and fluorine-bearing gases and particularly, C2F6, C3F8, C4F8, C5F8, C4F6 and their mixtures may be used for a high selective etching rate to the interlayer insulation layer 25 against the interlayer insulation layer 25. Also, C, H and F-bearing gases, such as CH3F, CH2F2, C2HF5, C3H2F6 and their mixtures, may be used to increase the selective etching rate to the interlayer insulation layer 25. In the preferred embodiment, an inert gas, such as Ar or He, may be contained to stabilize plasma while the interlayer insulation layer 25 is etched and the isotropic dry-etching process is applied to the interlayer insulation layer 25 to remove polymer which may cause the epitaxial layers 23 to be damaged.