PROCESS FOR DRY ETCHING A SILICON NITRIDE LAYER

Abstract: This process comprises the steps of: preparing a silicon bulk (11); oxidizing a surface of the bulk (11) to form a thin silicon dioxide layer (12) on the bulk (11); forming a silicon nitride layer (13) on the silicon dioxide layer (12); forming a resist layer (14) on the silicon nitride layer (13); patterning the resist layer (14) to expose a portion of the silicon nitride layer (13); dry etching selectively said exposed portion of the silicon nitride layer (13) using the patterned resist layer (14) as a mask, in which an etching gas comprising sulfur hexafluoride is used; and oxidizing selectively a portion of the silicon bulk (11) existing under the exposed portion of the silicon dioxide (12) using the selectively etched silicon nitride layer (13) as a mask, to form a locally oxidized silicon bulk.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for etching a silicon nitride layer. More specifically, it relates to a process for dry etching a silicon nitride layer on a silicon dioxide layer, useful in a so-called "local oxidation of silicon" (LOCOS) process. 2. Description of the Related Art A silicon nitride (Si₃N₄) 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 Si₃N₄ layer of, e.g., 100 nm thickness is formed on the oxidized surface of the silicon bulk. The Si₃N₄ layer is patterned with a mask of a patterned resist layer formed on the Si₃N₄ layer, the resist layer is removed, then the silicon bulk is locally oxidized by using the patterned Si₃N₄ layer as a mask. In this local oxidation process, there is the problem of formation of "bird beaks". 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 Si₃N₄ layer as mask but also horizontally toward a portion under the Si₃N₄ layer. These "bird beaks" 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 (SiO₂) layer formed on the surface of the silicon bulk and increasing the thickness of an Si₃N₄ layer or a mask layer for oxidation on the SiO₂ layer, it is possible to reduce the size of "bird beaks" formed during the local oxidation. However, no currently available etching process allows a desired combination of the thicknesses of the SiO₂ and Si₃N₄ layers. An etching gas comprising fluoromethane (freon), particularly tetrafluoromethane (CF₄), and oxygen (O₂) is typically used for dry etching an Si₃N₄ layer in the LOCOS process, which gas needs the above mentioned combination of thicknesses of the SiO₂ and Si₃N₄ layers of, e.g., 50 nm and 100 nm, respectively, since the above etching gas has a ratio of etching rate of Si₃N₄ to SiO₂ of about 2 to 3 at the highest. The above combination of the thicknesses of the two layers, i.e., 50 nm of the SiO₂ layer and 100 nm of the Si₃N₄ layers, not only requires severe control of the process conditions, but also results in "bird beaks" each having a width or lateral size of 0.3 to 0.4 µm. If the thickness of the SiO₂ layer is further reduced in the above prior art process, the SiO₂ layer may be over etched during the patterning or etching of an Si₃N₄ layer on the SiO₂ layer, which causes nonuniform oxidation of the silicon bulk existing under the SiO₂ layer. There are also known other dry etching gases, for example, a mixed gas of sulfur hexafluoride (SF₆) and chlorine (Cl₂) for etching silicon (Japanese Unexamined Patent Publication (Kokai) No. 58-7829); a gas of SF₆ , nitrogen trifluoride (NF₃), fluorine (F₂), or Cl₂ for etching silicon (Japanese Unexamined Patent Publication (Kokai) No. 59-214226); a gas of NF₃ for etching Si₃N₄ (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 SF₆ or CF₄ gas and a carbon-system gas such as ethylene (C₂H₄) or carbon tetrachloride (C Cl₄) for etching polycrystalline or crystalline silicon (Japanese Unexamined Patent Publication (Kokai) No. 59-51531). However, some of these publications, disclosing SF₆ as a dry etching gas, do not concern etching of Si₃N₄ and the others, disclosing a gas for dry etching Si₃N₄, 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 Si₃N₄. A further object of the present invention is to provide a dry etching process having a high ratio of etching rate of Si₃N₄/SiO₂ , useful for the LOCOS process. A still further object of the present invention is to reduce the size of "bird beaks" 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 Si₃N₄ layer with an etching gas comprising SF₆ . This invention is based on a finding that SF₆ has a high effectivity of etching Si₃N₄ and a high selectivity of etching Si₃N₄ in relation to SiO₂ or a photoresist. The etching gas further comprises fluoromethane such as CH₃F, CH₂F₂ , CHF₃ , and CF₄. A combination of SF₆ and CH₂F₂ is preferred. We further found that anode coupling reactive ion etching is preferable in a process for dry etching an Si₃N₄ layer on an SiO₂ layer with an etching gas comprising SF₆ and CH₂F₂ since it allows a higher etching rate ratio of Si₃N₄/SiO₂ , 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 Figure 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 Si₃N₄ SiO₂ , and resist when the flow rate of CH₂F₂ is varied; and Fig. 4 is a graph of the selectivities of etching of Si₃N₄ to SiO₂ and resist when the flow rate of CH₂F₂ is varied. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 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 3 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 4 is connected to a radio frequency (RF) source 7 of 13.56 MHz. A matching network 8 is inserted between the upper side electrode 4 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. 2A a silicon (Si) bulk or wafer 11 was prepared and a surface of the silicon bulk 11 was oxidized to form an SiO₂ layer 12 of 30 nm thickness. An Si₃N₄ layer 13 was deposited onto the SiO₂ layer 12 by sputtering, etc. The thickness of the Si₃N₄ layer 13 was 100 nm in this case, but could have also been made 150 nm. A resist layer 14 was coated on the Si₃N₄ 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 SF₆ and CH₂F₂ 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 33Pa (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 Si₃N₄ 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 Si₃N₄ layer 13 was obtained on the SiO₂ layer 12; the patterned Si₃N₄ 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 Si₃N₄ 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 Si₃N₄ layer exists was oxidized to form a thick SiO₂ layer 15 or a locally oxidized silicon. In this oxidation, "bird beaks" 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 "bird beaks" in the prior art. In the prior art, for example, using an etching gas comprising CF₄ and O₂ , as described before, the thickness of the SiO₂ layer had to be 50 nm if the Si₃N₄ layer had a thickness of 100 nm. This resulted in a width of the "bird beaks" of 0.3 µm to 0.4 µm after patterning the Si₃N₄ layer and oxidizing the silicon bulk under the SiO₂ layer using the Si₃N₄ layer as a mask. In another experiment, the thickness of an Si₃N₄ layer was made 150 nm while maintaining the thickness of an SiO₂ layer at 30 nm. Successful selective etching of the Si₃N₄ layer could be done, that is, the SiO₂ layer was not overetched, resulting in excellent local oxidation of a silicon bulk under the SiO₂ layer. Tests were made to ascertain preferable conditions of dry etching according to the present invention. First, the flow rate of CH₂F₂ was varied with the other conditions being fixed. The fixed conditions of the test were vacuum of 33Pa (0.25 Torr), RF power of 125 Watts, flow rate of the SF₆ of 50 SCCM, and temperature of 25°C. Thus, the etching rates of an Si₃N₄ layer, an SiO₂ layer, and a resist layer (novolak resincontaining positive-type photosensitive resist, "ONPR-800 HSl," produced by Tokyo Oka K.K., Japan) were determined. The obtained etching rates and the selectivities of Si₃N₄ to SiO₂ and resist or ratios of the etching rate of Si₃N₄ to SiO₂ 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 CH₂F₂ 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 Si₃N₄ to SiO₂. It should be noted that a selectivity of etching of Si₃N₄ to SiO₂ 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 Si₃N₄/resist was obtained and is also preferable. Thus, a mixing ratio by volume or mole of CH₂F₂:SF₆ 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 SF₆ 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 13 to 40 Pa (0.1 to 0.3 Torr), more preferably 27-40 Pa (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.

Claims: A process for dry etching a silicon nitride layer (13) with an etching gas comprising a combination of sulfur hexafluoride and fluoromethane. A process according to claim 1, wherein said silicon nitride layer (13) is on a silicon dioxide layer (12) and a portion of said silicon nitride layer is selectively etched. A process according to claim 1, wherein said fluoromethane is difluoromethane. A process according to claim 2, wherein said silicon dioxide layer (12) has a thickness of less than 50 nm and said silicon nitride layer (13) has a thickness of 100 nm to 200 nm. A process acoording to claim 4, wherein said silicon dioxide layer (12) has a thickness of about 30 nm and said silicon nitride layer (13) has a thickness of 100 nm to 150 nm. A process according to claim 1, wherein said etching is carried out under a vacuum of 13 Pa (0.1 Torr)or more. A process according to claim 6, wherein said vacuum is 27 to 40 (0.2 Torr to 0.3 Torr). A process according to claim 3, wherein a feeding ratio of sulfur hexafluoride to difluoromethane by volume is up to 35:100. A process according to claim 4, wherein the ratio of sulfur hexafluoride to difluoromethane is 8 - 24:100. A process according to any one of claims 1 to 9, wherein said dry etching is reactive ion etching. A process according to claim 10, wherein said reactive ion etching is carried out between electrodes (3,4) of anode coupled-type. A process for forming locally oxidized silicon, comprising the steps of : preparing a silicon bulk (11); oxidizing a surface of the silicon bulk (11) to form a thin silicon dioxide layer (12) on the silicon bulk (11); forming a silicon nitride layer (13) on the silicon dioxide layer (12); forming a resist layer (14) on the silicon nitride layer (13); patterning the resist layer (14) to expose a portion of the silicon nitride layer (13); dry etching selectively said exposed portion using the patterned resist layer (14) as a mask, in which an etching gas comprising sulfur hexafluoride and flouromethane is used; and oxidizing selectively a portion of the silicon bulk (11) existing under the exposed portion of the silicon dioxide (12) using the selectively etched silicon nitride layer (13) as a mask, to form a locally oxidized silicon bulk.

Inventors: SHIN, TESHIKI
Applicants: FUJITSU LIMITED
Application year: 1985
Main classification: H01L21
IPC codes: H01L21