JPS60258919A - Manufacture of semiconductor device - Google Patents

Abstract

PURPOSE:To level the surface of semiconductor device so as to enable minute wiring by combining the boron-doped polysilicon with selective etching utilizing a difference in concentration. CONSTITUTION:The first polysilicon layer 6 is formed on the silicon substrate 1 on which an oxide film 2 for element isolation is formed and boron is diffused to high concentration. The substrate is coated with the second polysilicon layer 7 and the desired part is selectively etched with SiO2 as a mask by using an etching solution of alkaline group which has different etching speeds according to p type impurity concentration. The first polysilicon 6 is exposed on the part except a base electrode 3 is completely made an oxide film by oxidizing the polysilicon. An oxide film 5 is formed also over the base electrode 3. The polysilicon doped with boron to high concentration can be oxidized at high speed and it is possible to form a thicker oxide film than the base electrode. That is effective for levelling.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a method of manufacturing a semiconductor device, and more particularly to planarization of a bipolar transistor having a structure in which a base electrode is extended by polysilicon onto an oxide film in an element isolation region.

[Background of the invention]

Traditionally, the base wiring of bipolar transistors was formed on the base diffusion layer, but recently, in order to reduce the device size and increase speed, the base electrode has been placed on a thick oxide film for device isolation using polysilicon (1). A method has been proposed in which wiring is then done using aluminum or the like.

As shown in FIG. 1, this method involves forming a thick oxide film 2 on a semiconductor substrate l, depositing polysilicon on the entire surface, and then forming a base polysilicon electrode 4 by photo-etching.
A part of it is oxidized to form an oxide film 5. This polysilicon is doped with the same impurities as the base region, and by heat treatment such as oxidation, a diffusion layer 3 is formed in the region where the substrate silicon and polysilicon are in contact.

In these methods, the step formed at the end of the polysilicon on the thick oxide film becomes large, and it is difficult to form a fine wiring pattern thereon because the wiring breaks.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for manufacturing a semiconductor device that can solve the above-mentioned conventional problems and flatten the surface of the semiconductor device to enable fine wiring.

[Summary of the invention]

(2) As mentioned above, drawing out the base electrode using polysilicon is effective in reducing the size of the active region of the transistor and increasing its speed, but if the polysilicon is simply etched and oxidized, the peripheral area will become larger. If a level difference occurs and wiring is performed on top of it, there is a possibility that a disconnection will occur.

In the present invention, a thin layer of polysilicon is left around the base electrode, and in the step of oxidizing the base electrode surface to form an oxide film, the remaining silicon is simultaneously oxidized, thereby reducing the occurrence of steps and at the same time shaping the shape of the steps. It is something that makes it gentle.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIGS. 2 to 4.

As shown in FIG. 2, a first polysilicon layer 6 is formed on a silicon substrate 1 on which a thick oxide film 2 for element isolation has been formed by selectively oxidizing the silicon substrate 1.
6・'"J'"7°yM# (QJI; Ht200nm, boron diffusion was carried out at 950°C for 20 minutes to achieve a high concentration of boron. Next, the second polysilicon (3) layer 7 was deposited. The film thickness is 500 nm.

The second polysilicon is then etched using a suitable mask. A 20 nm thick 5inQ film was used as a mask.

When etching polysilicon, it is difficult to remove only the second polysilicon because all the polysilicon is etched using ordinary hydrofluoric acid and nitric acid solutions or plasma etching using CF4 gas. be. For this purpose, an alkaline etchant whose etch rate varies depending on the P-type impurity concentration was used. In this liquid, silicon to which boron has been added up to the solid solution limit hardly dissolves, so that only the second polysilicon 7 can be removed.

In this example, the etchant was 40% potassium hydroxide.
Etching was performed at 25°C using an aqueous solution. The etch rate of polysilicon is 2 when no impurities are added.
5 nm/win, and when boron is added at a high concentration, the resistance is 1 to 2 nm/n.

Therefore, the etch rate ratio is 10:1 or more, and selective etching is possible due to the difference in impurity concentration.

(4) FIG. 3 shows a state in which a desired portion of the second polysilicon 7 is selectively etched using a 5inQ mask, and the StO film used as the mask is removed. Next, the polysilicon is oxidized. The oxide film thickness is 200 nm, which is the thickness at which the first polysilicon layer exposed in areas other than the base electrode part is completely oxidized.
A 450 nm thick oxide film was formed on the polysilicon. As a result of this oxidation, an oxide film 5 is also formed on the base electrode portion, completing the structure shown in FIG. Note that high-pressure oxidation, which can form a thick oxide film at a relatively low temperature, is effective because it suppresses the diffusion of boron to a shallow level. Further, polysilicon doped with boron at a high concentration has a fast oxidation rate, and can form an oxide film thicker than the base electrode portion, which is highly effective in terms of planarization.

〔Effect of the invention〕

According to the present invention, the level difference around the base electrode made of polysilicon can be reduced to about 1/2 or less, which is very effective in forming fine aluminum wiring.

The present invention combines boron-doped polysilicon with selective etching based on the concentration difference (5) and develops a method that leaves polysilicon with a uniform thickness, thereby making it possible to achieve good flattening of the shape at stepped portions. The effect is great.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing a conventional structure, and FIGS. 2 to 4 are process diagrams for explaining the present invention. 1... Silicon substrate, 2... Thick oxide film for element isolation, etc., 3... Diffusion layer for base contact, 4,6.7
... Polysilicon layer, 5... Oxidized polysilicon (6)

Claims (1)

[Claims] After forming a first polysilicon layer doped with boron on a semiconductor substrate, a second polysilicon layer substantially free of impurities is formed on the surface of the first polysilicon layer doped with boron.
1. A method of manufacturing a semiconductor device, comprising: forming polysilicon layers one on top of the other, selectively removing the second polysilicon layer, and further oxidizing the polysilicon layer to form an oxide film.