Abstract:
A silicon nitride layer, particularly on a silicon dioxide layer, is dry etched successfully by using an etching gas including SF 6 , preferably also CH 2  F 2 . This dry etching provides a high selectivity of etching of Si 3  N 4  to SiO 2  and may be used for the LOCOS (local oxidation of silicon) process to reduce the width of so-called &#34;bird beaks.&#34;

Description:
This application is a continuation of application Ser. No. 287,564, filed on Dec. 19, 1988, now abandoned which is a continuation of Ser. No. 143,302, filed on Jan. 11, 1988, now abandoned, which is a continuation of Ser. No. 813,896, filed on Dec. 27, 1985, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a process for etching a silicon nitride layer. Move specifically, it relates to a process for dry etching a silicon nitride layer on a silicon dioxide layer, useful in a so-called &#34;local oxidation of silicon&#34; (LOCOS) process. 
     2. Description of the Related Art 
     A silicon nitride (Si 3  N 4 ) layer is used as a mask for locally oxidizing a semiconductor bulk, such as a silicon bulk. In this process (LOCOS), the overall surface of a silicon bulk is first oxidized to a thin thickness, e.g., 50 nm. Then, an Si 3  N 4  layer of, e.g., 100 nm thickness is formed on the oxidized surface of the silicon bulk. The Si 3  N 4  layer is patterned with a mask of a patterned resist layer formed on the Si 3  N 4  layer, the resist layer is removed, then the silicon bulk is locally oxidized by using the patterned Si 3  N 4  layer as a mask. 
     In this local oxidation process, there is the problem of formation of &#34;bird beaks&#34;. That is, oxidation of the silicon bulk occurs not only vertically downward from the top surface of a portion of the silicon bulk not covered with the Si 3  N 4  layer as mask but also horizontally toward a portion under the Si 3  N 4  layer. These &#34;bird beaks&#34; create undesired wasteful areas in microcircuit devices, preventing densification of integrated circuit (IC) devices. 
     It is known that by decreasing the thickness of a silicon dioxide (SiO 2 ) layer formed on the surface of the silicon bulk and increasing the thickness of an Si 3  N 4  layer or a mask layer for oxidation on the SiO 2  layer, it is possible to reduce the size of &#34;bird beaks&#34; formed during the local oxidation. However, no currently available etching process allows a desired combination of the thicknesses of the SiO 2  and Si 3  N 4  layers. An etching gas comprising fluoromethane (feron), particularly tetrafluoromethane (CF 4 ), and oxygen (O 2 ) is typically used for dry etching an Si 3  N 4  layer in the LOCOS process, which gas needs the above mentioned combination of thicknesses of the SiO 2  and Si 3  N 4  layers of e.g., 50 nm and 100 nm, respectively, since the above etching gas has a ratio of etching rate of Si 3  N 4  to SiO 2  of about 2 to 3 at the highest. The above combination of the thicknesses of the two layers, i.e., 50 nm of the SiO 2  layer and 100 nm of the Si 3  N 4  layers, not only requires severe control of the process conditions, but also results in &#34;bird beaks&#34; each having a width or lateral size of 0.3 to 0.4 μm. 
     If the thickness of the SiO 2  layer is further reduced in the above prior art process, the SiO 2  layer may be over etched during the patterning or etching of an Si 3  N 4  layer on the SiO 2  layer, which causes nonuniform oxidation of the silicon bulk existing under the SiO 2  layer. 
     There are also known other dry etching gases, for example, a mixed gas of sulfur hexafluoride (SF 6 ) and chlorine (Cl 2 ) for etching silicon (Japanese Unexamined Patent Publication (Kokai) No. 58-7829); a gas of SF 6 , nitrogen trifluoride (NF 3 ), fluorine (F 2 ), or Cl 2  for etching silicon (Japanese Unexamined Patent Publication (Kokai) No. 59-214226); a gas of NF 3  for etching Si 3  N 4  (Japanese Unexamined Patent Publication (Kokai) No. 60-20516); a reactive gas mixture comprising predominantly a fluorocarbon gas doped with carbon dioxide for etching an inorganic insulating layer comprising silicon as a main metal element (Japanese Unexamined Patent Publication (Kokai) No. 58-84113); and a combination of a fluorine-system gas such as SF 6  or CF 4  gas and a carbon-system gas such as ethylene (C 2  H 4 ) or carbon terachloride (C Cl 4 ) for etching polycrystalline or crystalline silicon (Japanese Unexamined Patent Publication (Kokai) No. 59-51531). However, some of these publications, disclosing SF 6  as a dry etching gas, do not concern etching of Si 3  N 4  and the others, disclosing a gas for dry etching Si 3  N 4 , are not very effective. 
     SUMMARY OF THE INVENTION 
     The main object of the present invention is to provide an effective dry etching process for etching Si 3  N 4 . 
     A further object of the present invention is to provide a dry etching process having a high ratio of etching rate of Si 3  N 4  /SiO 2 , useful for the LOCOS process. 
     A still further object of the present invention is to reduce the size of &#34;bird beaks&#34; formed in the LOCOS process in order to densify semiconductor IC devices. 
     These and other objects are attained by a process for dry etching an Si 3  N 4  layer with an etching gas comprising SF 6 . This invention is based on a finding that SF 6  has a high effectivity of etching Si 3  N 4  and a high selectivity of etching Si 3  N 4  in relation to SiO 2  or a photoresist. 
     The etching gas optionally further comprises, e.g., fluoromethane such as CH 3  F, CH 2  F 2 , CHF 3 , and CF 4 . A combination of SF 6  and CH 2  F 2  is preferred. 
     We further found that anode coupling reactive ion etching is preferable in a process for dry etching an Si 3  N 4  layer on an SiO 2  layer with an etching gas comprising SF 6  and optionally CH 2  F 2  since it allows a higher etching rate ratio of Si 3  N 4  /SiO 2 , although other types of dry etching, for example, cathode coupling type, reactive ion etching, and downflow type plasma etching, may have some advantages in a process according to the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic sectional view of an apparatus for dry etching used in a process according to the present invention; 
     FIGS. 2A, 2B, and 2C are sectional views of a silicon bulk to be locally oxidized in some steps of the LOCOS process according to the present invention; 
     FIG. 3 is a graph of the etching rates of Si 3  N 4 , SiO 2 , and resist when the flow rate of CH 2  F 2  is varied; and 
     FIG. 4 is a graph of the selectivities of etching of Si 3  N 4  to SiO 2  and resist when the flow rate of C 2  H 2  is varied. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an anode coupling type dry etching apparatus. In a reaction chamber 1 defined by a quartz tube 2, there are parallel electrodes 3 and 4. An upper side electrode 4 is made of aluminum (Al) and anodized on its surface, and a lower side electrode 4 is made of aluminum on which a wafer 5 to be treated is placed. The lower side electrode 3 is connected to the ground 6, and the upper side electrode 3 is connected to a radio frequency (RF) source 7 of 13.56 MHz. A matching network 8 is inserted between the upper side electrode 3 and the RF source 7. The reaction chamber 1 is evacuated by a vacuum pump (not shown). 
     Now, an example of the LOCOS process according to the present invention is described with reference to FIGS. 1, 2A, 2B, and 2C. In FIG. 1 a silicon (Si) bulk or wafer 11 was prepared and a surface of the silicon bulk 11 was oxidized to form an SiO 2  layer 12 of 30 nm thickness. An Si 3  N 4  layer 13 was deposited onto the SiO 2  layer 12 by sputtering, etc. The thickness of the Si 3  N 4  layer 13 was 100 nm in this case, but could have also been made 150 nm. A resist layer 14 was coated on the Si 3  N 4  layer 13 and patterned to leave a square or rectangular shape having a size L of 2.5 μm. 
     In a dry etching apparatus as in FIG. 1, the wafer 1 was placed on the lower side electrode 3. A reaction gas of a mixture of SF 6  and CH 2  F 2  with flow rates of 50 SCCM (standard cubic centimeter per minute) and 10 SCCM respectively, was introduced into the reaction chamber 1, which was evacuated to about 0.25 Torr. A radio frequency of 13.56 MHz was applied between the two electrodes 3 and 4 at a power of about 125 Watts. By reactive ion etching under these conditions, a portion of the Si 3  N 4  layer 13, which was not covered with the patterned resist layer 14, was selectively etched or removed. The resist layer 14 was then removed. Thus, a patterned Si 3  N 4  layer 13 was obtained on the SiO 2  layer 12; the patterned Si 3  N 4  layer 13 having a square or rectangular shape, as shown in FIG. 2B. 
     Referring to FIG. 2C, oxidation was carried out in which the patterned Si 3  N 4  layer 13 was used as a mask to locally oxidize the silicon bulk 11, that is, a portion of the silicon bulk 11 above which no Si 3  N 4  layer exists was oxidized to form a thick SiO 2  layer 14 or a locally oxidized silicon. In this oxidation, &#34;bird beaks&#34; b were formed but had the width B of only 0.2 μm to 0.3 μm which was reduced from the width of the &#34;bird beaks&#34; in the prior art. 
     In the prior art, for example, using an etching gas comprising CF 4  and O 2 , as described before, the thickness of the SiO 2  layer had to be 50 nm if the Si 3  N 4  layer had a thickness of 100 nm. This resulted in a width of the &#34;bird beaks&#34; of 0.3 μm to 0.4 μm after patterning the Si 3  N 4  layer and oxidizing the silicon bulk under the SiO 2  layer using the Si 3  N 4  layer as a mask. 
     In another experiment, the thickness of an Si 3  N 4  layer was made 150 nm while maintaining the thickness of an SiO 2  layer at 30 nm. Successful selective etching of the Si 3  N 4  layer could be done, that is, the SiO 2  layer was not overetched, resulting in excellent local oxidation of a silicon bulk under the SiO 2  layer. 
     Tests were made to ascertain preferable conditions of dry etching according to the present invention. 
     First, the flow rate of CH 2  F 2  was varied with the other conditions being fixed. The fixed conditions of the test were vacuum of 0.25 Torr, RF power of 125 Watts, flow rate of the SF 6  of 50 SCCM, and temperature of 25° C. Thus, the etching rates of an Si 3  N 4  layer, an SiO 2  layer, and a resist layer (novolak resin-containing positive-type photosensitive resist, &#34;ONPR-800 HS1,&#34; produced by Tokyo Oka K. K., Japan) were determined. The obtained etching rates and the selectivities of Si 3  N 4  to SiO 2  and resist or ratios of the etching rate of Si 3  N 4  to SiO 2  and resist, respectively, are summarized in the graphs in FIGS. 3 and 4. 
     As can be seen in FIGS. 3 and 4, a flow rate of CH 2  F 2  in a range of 17.5 SCCM or less, preferably in a range of 4 to 12 SCCM, results in successful selectivity of etching of Si 3  N 4  to SiO 2 . It should be noted that a selectivity of etching of Si 3  N 4  to SiO 2  of more than 3, particularly, more than 4, was obtained in a process according to the present invention while at the highest only a selectivity of 2 to 3 could be obtained in a process in the prior art. It should be also noted that a relatively high selectivity of etching of Si 3  N 4  /resist was obtained and is also preferable. 
     Thus, a mixing ratio by volume or mole of CH 2  F 2  :SF 6  of up to 35:100, more preferably 8-24:100, particularly 20:100, is preferably used in a process according to the present invention. This fact was ascertained in another test in which the flow-rate of SF 6  is varied with the other conditions being fixed. 
     A third test in which the degree of vacuum in a reaction chamber was varied and the other conditions were fixed was carried out. We found that a vacuum of 0.1 to 0.3 Torr, more preferably 0.2 to 0.3 Torr, is preferably used. 
     A fourth test in which the RF power was varied and the other conditions were fixed was carried out. We found that 50 Watts was sufficient and 100 to 200 Watts was preferable for a process according to the invention.