1. Field of the Invention
The invention relates to a method of manufacturing a semiconductor device in which a large number of circuit element regions within a semiconductor substrate are insulated by an oxide.
2. Description of the Prior Art
In a semiconductor integrated circuit (hereinbelow, termed "IC"), a large number of circuit elements such as transistors, diodes and resistances are formed within a common semiconductor substrate. The circuit elements must be electrically isolated from one another by isolation regions. As one of expedients for the isolation, there has been proposed a method which uses an oxide and which is known as the so-called isoplanar technique. This method is such that a semiconductor oxide is formed in a region within a semiconductor substrate prearranged for the isolation by applying a local oxidation technique and it is employed as an isolation oxide film. The local oxidation technique is described in detail, for example, in the publication "Phillips Research Report," pages 118 - 132, issued by Philips Research Laboratory in April 1970.
FIGS. 1A to 1C illustrate an example in the case of manufacturing an IC by utilizing the isoplanar technique referred above.
First, as is shown in FIG. 1A, a P conductivity type silicon substrate 1 is prepared, an N.sup.+ - type buried layer 2 is formed in a surface portion of the substrate 1, and an N-type silicon layer 3 is thereafter formed by vapor growth. At a desired surface area of the layer 3, a film 4 of, for example, silicon nitride (Si.sub.3 N.sub.4) is formed as a film which is impervious to an etchant for silicon. In forming the Si.sub.3 N.sub.4 film 4, there can be employed a known method which exploits the vapor phase reaction between, for example, monosilane (SiH.sub.4) and ammonia (NH.sub.3). A silicon oxide (SiO.sub.2) film is effectively formed between the silicon layer 3 and the silicon nitride film 4 in advance so as to prevent the silicon surface from becoming rough. The Si.sub.3 N.sub.4 film 4 formed on the entire surface is selectively treated in an etchant such as phosphoric acid (H.sub.3 PO.sub.4). Thus, an Si.sub.3 N.sub.4 film 4 about 1,300A thick can be formed on only the desired parts as shown in FIG. 1A.
Subsequently, as shown in FIG. 1B, an etching treatment is carried out by immersing the resultant substrate in the etchant for silicon, for example, a mixed solution consisting of hydrofluoric acid (HF) and nitric acid (HNO.sub.3). The etchant etches at substantially equal rates for the vertical direction and the lateral direction of the silicon layer 3. As a result, a recess 5 is etched-out so as to include portions directly beneath end parts of the silicon nitride film 4, as illustrated in FIG. 1B.
Subsequently, a heat treatment is carried out in an oxidizing atmosphere. Thus, as shown in FIG. 1C, the etched parts of the silicon layer 3 are selectively oxidized and converted into silicon oxide (SiO.sub.2) 6, which electrically isolates a large number of circuit element regions 7 from one another.
During the oxidizing treatment, the oxide 6 rises at the end part of the silicon nitride film 4, and a protuberance 8, called a "bird head," is formed. The height of the protuberance becomes as large as about half of the thickness t of the oxide 6. In the later stages of manufacture, accordingly, there occur such problems that wiring located on the surface of the protuberance is disconnected and that the close contact of a mask for a photolithographic processing becomes unstable. A protuberance 9, called a "bird beak," is also formed. The width W of the circuit element region 7, isolated by the oxide 6, is sharply decreased in comparison with the width W.sub.o before the selective oxidation treatment on account of the protuberance 9. Usually, the relation W .apprxeq. W.sub.o - 2t holds. As to the lateral dimension of the element region, therefore, the amount of decrease must be preestimated. As a consequence, enhancement of the density of integration cannot be expected.