Abstract:
In a method for manufacturing a semiconductor device, a semiconductor substrate is provided. On the substrate, conductors spaced apart from one another are formed. Then, an insulating layer is formed on the conductors and the substrate. The insulating layer is formed by a chemical vapor deposition using tetramethylcyclotetrasiloxane as a source gas and oxygen as an adjunction gas. The chemical vapor deposition is performed while the substrate is irradiated by vacuum ultraviolet light. Finally, a part of the insulating layer is removed in a substantial uniform way to form a contact hole through the insulating film.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for manufacturing a semiconductor device such as a DRAM including a plurality of MOSFET, particularly to a method for forming a contact hole through an inter-layer insulating layer. 
     A semiconductor element, such as a Metal-Oxide-Semiconductor Field Effect transistor (“MOSFET”) formed on a semiconductor substrate, is covered by an inter-layer insulating layer such as a silicon oxide layer. The inter-layer insulating layer has a contact hole perforated in the direction of thickness of the inter-layer insulating layer. The contact hole is filled with conductive material that makes an electric contact of the semiconductor element with a conductive layer over the inter-layer insulating layer. 
     Conventional methods for forming the above-mentioned contact hole include so-called Self-Aligned Contact (“SAC”) method. According to an example of the conventional SAC method, a silicon nitride (SiN) layer is first formed to cover sidewall of the gate of the transistor. Then, an inter-layer insulating layer is formed on the silicon nitride layer. Because of the difference of etching rates, the silicon nitride layer functions as an etch-stopping layer. Therefore by etching the inter-layer insulating layer, a contact hole is formed on a portion of the inter-layer insulating layer under which the silicon nitride layers are not formed. 
     However, according to the conventional SAC method, it is required to perform process steps for forming the silicon nitride layer and patterning the silicon nitride layer. Therefore, forming a contact hole as a whole includes complicated process steps, and manufacturing cost for a semiconductor device is increased. 
     SUMMARY OF THE INVENTION 
     Therefore, the present invention provides a method for manufacturing a semiconductor device by forming a contact hole through simplified process steps. 
     The present invention may be achieved by using a low pressure chemical vapor deposition (“LPCVD”) method for forming an inter-layer insulating layer with tetramethylcyclotetrasiloxane ([SiH(CH 3 )] 4 O 4 ) as a source gas and oxygen gas as an annexation gas under vacuum ultraviolet light. 
     Basically, the method for manufacturing a semiconductor device of the present invention includes a process step for forming a contact hole, which perforates through an insulating layer in the direction of thickness of the insulating layer. The insulating layer is formed to cover conductors spaced apart from one another and formed on the surface of the semiconductor substrate. In order to form the insulating layer, tetramethylcyclotetrasiloxane ([SiH(CH 3 )] 4 O 4 ) as a source gas and an oxygen gas as an adjunction gas are provided into a reaction chamber of a LPCVD apparatus where the semiconductor substrate is placed. Then, the insulating layer is formed under vacuum ultraviolet light illuminating the semiconductor substrate. 
     By forming the insulating layer according to the method described above, the insulating layer has a surface profile corresponding to a convex and concave profile of the conductors on the semiconductor substrate. Further portions of the insulating layer, corresponding to the surface portions of the semiconductor substrate are formed to be relatively thin. Therefore, by etching the whole insulating layer, the relatively thin portions of the insulating layer is removed. That is, a contact hole is formed through the insulating layer without performing conventional process steps, such as forming a silicon nitride layer and patterning the silicon nitride layer. 
     The process step for etching the insulating layer may preferably be a step for performing a dry etching almost uniformly on the overall surface of the insulating layer, so that the relatively thin portions can be removed without damaging other portions. Further, as described above, since it is possible to perform a “blanket” dry etching, there is no need to use an etching mask for forming a contact hole. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A further understanding of the nature and advantage of the present invention will become apparent by reference to the remaining portions of the specification and drawings. 
     FIG. 1 is a schematic diagram showing a cross sectional view of a CVD apparatus related to a method according to an embodiment of the present invention. 
     FIG. 2 is a graph showing an FTIR analysis result of an insulating layer according to the present invention. 
     FIGS. 3 a  to  3   e  are cross sectional views of a semiconductor device showing process steps for forming contact holes according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, referring to attached drawings, embodiments of the present invention are described in detail. 
     Now, referring to FIG. 1, here is shown an LPCVD apparatus  10  for performing the method for manufacturing a semiconductor device according to the present invention. The LPCVD apparatus  10  is used for forming an insulating layer, for example a silicon nitride layer, on a semiconductor substrate. 
     As shown in FIG. 1, the LPCVD apparatus  10  includes a box type housing  12 , the inner space of which functions as a reaction chamber  11 , a negative pressure source  14 , such as a vacuum pump, coupled to the housing  12  through a pipe  13  for maintaining the reaction chamber under a low pressure condition, a susceptor  16  for holding and supporting a semiconductor wafer  15  made of, for example, silicon in the reaction chamber  11 , and a vacuum ultraviolet light source  17 , such as a Xe 2  excimer laser source. The vacuum ultraviolet light source  17  may be selected from various kinds of laser sources which can radiate short wavelength lights of 200 nm or less or ultraviolet lights having wavelengths in vacuum ultraviolet light range. 
     An experimental result according to the present invention, which can show the effect of the present invention, is described, hereinafter. 
     A plurality of wiring conductors  15   a  having width of 0.5 um and height of 0.5 um are formed on the surface of the semiconductor wafer  15 . Each of the wiring conductors  15   a  is spaced apart from the adjacent one by, for example, 0.7 um. The semiconductor wafer  15  is supported by the susceptor  16  with the surface, on which the wiring conductors  15   a  are formed, of the wafer  15  facing up. Temperature of the semiconductor wafer  15  is controlled by controlling temperature of the susceptor  16  and maintained to be the same as the room temperature of the chamber. 
     The vacuum ultraviolet light source  17  includes a quartz window  17   a  as a radiation window having a quartz plate of 20 mm in thickness. The light source  17  is supported the housing so that the quartz window  17   a  is located over the semiconductor wafer  15 . The vacuum ultraviolet light is radiated onto the semiconductor wafer  15  through the quartz window  17   a.    
     In the experiment, in order to form an insulating layer which covers the wiring conductors  15   a  on the semiconductor wafer  15 , tetramethylcyclotetrasiloxane ([SiH(CH 3 )] 4 O 4 ) (“TMCTS”) as a source gas and oxygen gas (O2) as an annexation gas are provided into a reaction chamber  11 . 
     The distance between the semiconductor wafer  15  supported by the susceptor  16  and the quartz window  17   a  of the vacuum ultraviolet light source  17  is maintained to be about 15 mm, and the illuminance of the vacuum ultraviolet light source is 10 mW/cm 2  directly under the quartz window  17   a . The semiconductor wafer  15  is irradiated by the vacuum ultraviolet lights having the illuminance 10 mW/cm2. Under these conditions, both of the TMCTS and the oxygen gas are provided into the chamber  11  at a same rate of 50 sccm. The reaction pressure of the chamber is 300 mTorr. 
     As described in detail below, it is preferable to heat the quartz window  17   a  to a temperature over the temperature of the vacuum ultraviolet light source  17  in order to prevent the insulating layer from growing on the surface of the quartz window  17   a  and resulting in degradation of the quartz window  17   a.    
     After about 15 minutes of operating the LPCVD apparatus  15  under the above-described conditions, an insulating layer having thickness of about 1.5 um is formed. Further, it is observed that the insulating layer has a “selective growth” characteristic. In other words, the insulating layer has a surface profile according to the convex and concave profile of the wiring conductors  15   a , and portions of the insulating layer between any pair of the wiring conductors  15   a  are extremely thinner than other portions of the insulating layer, which can be verified by using a Scan Electron Microscope (“SEM”). 
     FIG. 2 shows a spectroscopic analysis result of the insulating layer by using a Fourier transform infrared spectroscopy. 
     In the graph, the horizontal axis is for wave numbers [cm −1 ], which is reciprocals of wavelengths, of the infrared lights irradiated onto the sample, the insulating layer, and the vertical axis is for absorbance [random unit]. 
     According to the Fourier transform infrared spectroscopy, infrared light of a wavelength corresponding to the characteristics of a sample is absorbed by the sample at a very high absorbance if the infrared light is irradiated on the sample while the wavelength of the infrared light is continuously shifted. Therefore, it is possible to identify the sample material by obtaining the wave number of the infrared light at which the absorbance is the highest. 
     According to the analysis result shown in FIG. 2, silicon dioxide SiO 2  and di-silicon oxide Si 2 O are shown to be main ingredients of the insulating layer. Both of these two ingredients are electrically nonconductive. Especially, since the di-silicon oxide Si 2 O is an organic material and has smaller dielectric constant than the silicon dioxide SiO 2 , the insulating layer including both materials has good characteristics as an inter-layer insulating layer of a semiconductor device. 
     Therefore, for example, by performing dry etching for removing the silicon dioxide almost uniformly on the overall portion of the insulating layer enough to remove the above described relatively thin portions, it is possible to form a hole perforating to the surface of the semiconductor wafer  15  without damaging the wiring conductors  15   a.    
     As described above, since this hole perforates to the surface of the semiconductor wafer  15  through the insulating layer, it is possible to use this hole as a contact hole. As widely known in the art, the contact hole is filled with conductive material, or a conductive film is formed on the insulating layer in order that an electric contact with the semiconductor wafer  15  can be established. 
     FIGS.  3 ( a ) to  3 ( e ) are cross sectional views of a MOSFET of a semiconductor device showing process steps for forming contact holes coupled to drain/source of the MOSFET according to an embodiment of the present invention. 
     As shown in FIG.  3 ( a ), a field oxide layer  19  is formed on a semiconductor substrate  18 , such as a silicon semiconductor substrate, according to the well-known LOCOS process. A gate oxide layer  21  made of silicon dioxide is formed on an active layer  20  defined by the field oxide layer  19 , according to, for example, a thermal oxidation process. A plurality of gates  22  are formed to be extended in parallel to each other on the gate oxide layer  21  according to a well-known photolithography process. 
     Each of the gates  22  is, for example, 0.18 um in width. The total thickness of the gate oxide  21  and the gate  22  is, for example, 0.3 um. Each of the gates  22  is formed to be spaced apart from the adjacent one by 0.2-0.7 um on the semiconductor substrate  18 . 
     The gate  22  may be made of metal material, such as tungsten, or metal alloy, such as Al—Si—Cu alloy. The gate  22  may also be made of a conductive polysilicon, well known in the art, including conductive impurities. Both sides of each of the gates  22  on the active area  20  of the semiconductor substrate  18  have impurity areas  23  for sources and/or drains formed by, for example, ion implantation process. 
     Further, as shown in the drawings, conductor lines  24 , which are extensions of the gates  22 , are formed on the field oxide layer  19 . 
     As shown in FIG.  3 ( b ), an insulating layer  25  is formed to cover the gates  22  and the conductor lines  24  on the semiconductor substrate  18 , where the gates  22  are formed on the active area  20  and the conductive lines are formed on the field oxide layer  19 . 
     In order to form the insulating layer  25 , the semiconductor substrate  18 , where the gates  22  and conductor lines  24  are formed as shown in FIG.  3 ( a ), is supported for the surface, on which the gates  22  and conductor lines  24  are formed, to face up by the susceptor  16  of the LPCVD apparatus  10 , shown in FIG.  1 . The insulating layer  25  is formed under the same conditions as described above. 
     As shown in FIG.  3 ( b ), the insulating layer  25  is mostly formed on the gates  22  and conductor lines  24 , which are made of conductive materials and constitute convex portions on the semiconductor substrate  18 , under the conditions as described above. Therefore, the insulating layer  25  has a wave-like surface profile according to the convex and concave profile of the convex portions, the gates  22  and conductor lines  24 . 
     Further, the insulating layer  25  is restrained from forming between the convex portions  22  and  24 . Therefore, the insulating layer  25  has relatively thin portions  25   a  between the convex portions  22  and  24 . 
     After forming the insulating layer  25  under the above-described conditions, a well-known dry etching for silicon dioxide is performed on the overall portions of the desired area of the insulating layer  25 . The relatively thin portions  25   a  of the desired area of the insulating layer  25  is completely removed. 
     In order to perform the dry etching selectively on the desired area of the insulating layer  25 , it is possible to use an etching mask. However, since the relatively thin portions  25   a  of the insulating layer  25  are easier to be removed than the other portions of the insulating layer  25 , the etching mask does not need to be aligned as precisely as conventionally done. 
     As shown in FIG.  3 ( c ), by performing the etching process, the relatively thin portion  25   a  of the desired area of the insulating layer  25  is removed, the impurity area  23  of the semiconductor substrate  18  is exposed and a contact hole  26  is formed. 
     As shown in FIG.  3 ( d ), after forming the contact hole  26 , in order to fill in the contact hole  26  and cover the other portions of the insulating layer  25 , for example, a conductive polysilicon  27  is formed. 
     Finally, as shown in FIG.  3 ( e ), unnecessary portions of the polysilicon  27  on the insulating layer is removed by photolithography and etching process. An electric contact with the impurity area  23  through the insulating layer  25  is established by the remaining polysilicon  27   a  in the contact hole  26  of the insulating layer  25 . 
     Over this contact  27   a , additional wiring layers (not shown) and insulating layers (not shown) may be formed. 
     According to the embodiment of the present invention, it is possible to form an insulating layer  25  having a surface profile corresponding to the convex and concave profile of the gates  22  and the conductor lines  24  on the semiconductor substrate. Further, the insulating layer  25  has a relatively thin portion  25   a  at a place corresponding to the place between any pair of the gates  22  or the conductor lines  24 . By performing etching process on overall portion of the insulating layer  25 , it is possible to form a contact hole perforating the insulating layer  25  by etching the thin portion  25   a  of the insulating layer  25  without performing processes of forming and patterning a nitride layer, which is required for a conventional SAC process. 
     The etching process is a dry etching process performed almost uniformly on overall portions of the desired area of the insulating layer. By this dry etching process, it is possible to remove only the relatively thin portions  25   a  without damaging the other portions than the relatively thin portions  25   a  of the insulating layer  25 . Therefore, it is possible to perform a “blanket” dry etching on the insulating layer. Further, since it is not needed to use highly precise masks, it is possible to form a contact hole relatively easily. 
     Since formation the relatively thin portions  25   a  of the insulating layer  25  is depend on the distance between adjacent convex portions  22  and/or  24 , it is possible to restrain the insulating layer  25  from forming by increasing the distance between adjacent convex portions  22  and/or  24 . Therefore, it is preferable to design a circuit layout so that the distance between adjacent convex portions  22  and/or  24  becomes relatively short. In other words, the density of gates  22  and/or conductor lines  24  becomes high on an area of the semiconductor substrate  18  where the contact hole is not needed. Further, distance between adjacent convex portions  22  and/or  24  becomes relatively long. In other words, the density of gates  22  and/or conductor lines  24  becomes low on an area of the semiconductor substrate  18  where the contact hole is needed. 
     In the above description, it is described to apply the method of the present invention to processes for manufacturing a MOSFET. The present invention, however, can also be applied to processes for forming a contact hole for a memory cell capacitor of a DRAM. 
     Further, the present invention can also be applied to processes for forming a transistor of the peripheral area of memory cells or other various kinds of semiconductor devices. 
     According to the present invention, it is possible to form a contact hole relatively easily without using a conventional SAC process. 
     Therefore, it is possible to simplify manufacturing processes for a semiconductor device and to decrease manufacturing costs.