JPS597215B2 - Method of forming silicon oxide film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of forming a thick silicon oxide film embedded within a silicon substrate crystal.

As a manufacturing technique for semiconductor integrated circuits, it is often necessary to form a selective oxide film locally in a shape buried in the surface of a semiconductor substrate. It is generally understood that the selective oxidation method of the silicon substrate surface using a selective mask made of a nitride film, which is well known by the name LOCOS method, meets these requirements, but it is difficult to avoid local oxidation. The thickness of the oxide film formed is approximately 1 to 2 microns because the distortion due to the difference in expansion coefficient due to the use of a nitride film has an adverse effect on the crystal substrate, and the flatness of the surface is impaired. There are limits, and there are also limits in terms of microfabrication when it comes to increasing the density of semiconductor integrated circuits. On the other hand, in recent years, a technology has been proposed in which the surface area of a silicon crystal substrate is selectively made porous and then thermally oxidized to obtain a thick selective oxide film embedded in the substrate. Several applications have been attempted. However, in this porous oxidation method, in order to make a predetermined region of the silicon single crystal porous, the subsequent heat treatment causes volumetric shrinkage, which in turn causes the surface to become convex or concave due to volumetric expansion due to oxidation. Summer,
It was difficult to obtain a flat surface. In addition, since the porous region is chemically active, oxidation in deep porous regions is said to be rapid and a thick buried oxide film region can be obtained, but rapid thermal oxidation occurs at high temperatures. Then, the area near the surface is rapidly oxidized first, and the porous pores are filled with an oxide film, and the oxygen atoms necessary for oxidation of the deep part diffuse through this oxide film, so that the pores in the silicon single crystal are The oxidation rate will be close to that of the oxidation rate.

Therefore, in order to oxidize thick regions, methods such as oxidizing for a long time at a low temperature and then adding further oxidation at a high temperature are used, but this creates steps on the surface and requires In practice, it is quite difficult to selectively form a deep oxide film with a flat surface.

The present invention provides a method for forming a thick silicon oxide film embedded in a silicon crystal substrate, which improves upon these conventional drawbacks.

In the present invention, a predetermined area of the surface of a silicon crystal substrate is selectively made porous by anodization, and then 3 to 20 days to 90 minutes.
The substrate is heated and oxidized in a high-pressure oxygen atmosphere of /c-nt to form a deep buried silicon oxide film without forming a step on the surface.

For example, attempts to accelerate oxidation of silicon crystals in a high-pressure atmosphere are reported in Journal of Electrochemical Society 19.
It is reported on page 1409 in 1975.

However, what has been tried so far has only been silicon single crystals, and in order to actually have an effect with single crystals, a lot of water vapor is used instead of a dry oxygen atmosphere, which poses a great risk of contamination. In the case of dry oxygen, the pressure is extremely high, over 100 atmospheres, so it is dangerous to handle and the equipment is too large to be applied industrially. In the process of researching the oxidation of the porous region of silicon crystals, the inventors discovered that the chemical activity of the porous region was extremely strong, so that the state in the single crystal was different from that in the single crystal. It was discovered that this could be achieved by pressurizing several atmospheres rather than several hundred atmospheres. In other words, it was discovered that sufficient oxygen atoms can be supplied by applying pressure inside the porous region, and this makes it possible to stably oxidize deep porous regions without raising the temperature. This enabled porous oxidation with a flat surface without distortion.
Hereinafter, specific embodiments of the present invention will be described with reference to the drawings.

Fig. 1 shows a semiconductor substrate porosity processing apparatus according to the present invention, and Fig. 2 shows an implementation of the present invention. (The basic principle diagram of the pressure oxidation device for use is shown in each figure.)

In FIG. 1, a semiconductor substrate 1 to be made porous is mounted on a holder 2 made of a hydrofluoric acid-resistant resin, and similarly stored in a container 3 made of a hydrofluoric acid-resistant resin.

The container 3 is filled with a 45-49% hydrofluoric acid solution 4, and the anode 5 and cathode 6 made of platinum plates are immersed in the hydrofluoric acid container 3.
At this time, the main surface (front surface) 7 of the semiconductor substrate 1 to be subjected to the porous treatment is placed facing the cathode 6 . It is usually desirable to form a high concentration region of a conductive type on the opposite surface (back surface) 8 to the main surface 7 of the substrate with a thickness of several microns over the entire surface of the opposite surface. A power source 9, preferably a constant current stabilized power source, is connected between the anode and the cathode, and the current density is 10 to 50 mA depending on the surface area of the semiconductor substrate 1 to be made porous.
/ Perform anodization with ~. In order to selectively form a porous region, since the rate of porosity depends on the hole concentration, a P region and an N region are formed by taking advantage of the fast property of the P type region. A method in which only the P region is selectively made porous and a method in which a nitride film or a photoresist is used are known.

The substrate that has undergone the porous treatment is oxidized under pressure in a subsequent step.

FIG. 2 shows a state in which a substrate is installed in a pressure oxidation apparatus. The semiconductor substrate 1 is placed on a boat 12 in an inner tube 11 made of a stainless steel tube. The inner tube 11 is configured with a heating coil 13 that wraps around the tube on the outside, an inner lid 14 is fastened with thumbscrews 15 on the insertion port side of the inner tube 11, and the inner tube 11 itself is compressed on the outlet side. pores 16 are opened. The inner tube 11 is warped and the outer tube 17
The outer wall of the outer tube 17 is cooled by a cooling water pipe 18. A barometer 19 is attached to the outer tube 17, and the outer cover 20, like the inner cover 14, is tightened with thumbscrews 21. The high-pressure pipe 22 is connected to the pipe 2 of the inner cover 14 by a connector 23 of Boniljyo'' Indian type.
3, and pressurized oxygen gas is introduced into the inner tube 11. The pressure balance between the inner tube 11 and the outer tube 17 is maintained by the pores 16.
Adjusted by. 24 is an exhaust pipe.

FIGS. 3a to 3d are cross-sectional views illustrating the main steps when the present invention is applied to the formation of selectively oxidized regions of a semiconductor device.

First, a P-type (100) silicon substrate 1 is prepared as a starting material, and as described above, a p+ region 102 is formed on the entire back surface by diffusion, etc., and a predetermined region is left on the surface, and other regions are formed. Nitride film 103 on the surface as a mask for selective porosity
Form the adhesion. The thickness of the nitride film 103 varies depending on the conditions for making it porous, but it is usually between 1000 and 2000A.
is used. (FIG. 3a) Next, a porous treatment as explained in FIG. 1 is performed to selectively make the porous region 104.
(Fig. 3, 6).

The conditions for forming porosity are that the specific resistance of the substrate 1 is 1Ω-? In this case, the nitride film 103 is removed by applying current at a current density of 10 mA/C7i for about 85 minutes, and then immersing it in hydrofluoric acid for about 40 minutes without applying any current. The depth of the porous region 104 formed at this time from the surface is approximately 4 mt. Next, the porous region 104 is oxidized and transformed into a thick oxide region 105 by performing pressure oxidation at 9 using a pressure oxidation apparatus as described in FIG.

At this time, a thin oxide film 106 is also formed on the non-porous surface (FIG. 3C). The conditions for pressurized oxidation are dry oxygen 3~20K7/(-Flt) in the high pressure piping 22 in Figure 2.
The substrate 1 is heated and held at 700° C. for 10 minutes to obtain a silicon dioxide film of 4 microns as the thick oxide region 105. The thickness of the thin oxide film 106 at this time is approximately 1000λ. In addition, when performing pressure oxidation, 3K7A? A pressure smaller than 20K7/Cl will not produce the pressurizing effect, and a pressure larger than 20K7/Cl will require a large-scale pressurizing device and increase the risk of high pressure, making it unsuitable for achieving the purpose of the present invention. It is.

Thereafter, an N+ source, drain regions 107 and 108, and a gate oxide film 109 are formed using a conventionally known MOS manufacturing process, and a source electrode 110, a gate electrode 111, and a drain electrode 112 are formed by At vapor deposition (first step). Figure 3 d).

The process shown in FIG. 3d is not the main point of the present invention, and the case of an At gate MOS is shown, but since Si gates and MO gates are also formed using well-known techniques, detailed explanations will be omitted. Similarly, the thin oxide film 10 formed in FIG.
Even if a new oxide film is formed by normal thermal oxidation after removing 6, the oxide film formed in FIG. In any case, this does not impede the gist of the present invention. Furthermore, after forming each element of the semiconductor device, such as the MOS transistor described in this embodiment, on the substrate 1, the process may proceed to steps B and c in FIG. In other words, the gist of the present invention is to form a thick insulator region by performing porous treatment and then high-pressure oxidation to form it in a short time at a temperature lower than the conventional thermal oxidation temperature normally used. The thick insulator region obtained by this method is characterized in that it exhibits superior properties compared to conventional high-temperature thermal oxide films even under such formation conditions.

That is, by pressurizing and oxidizing the porous region at a low temperature for a short time, a silicon dioxide film with little distortion, a flat surface, and excellent electrical properties can be obtained.

Figure 4 shows oxide film A obtained by oxidizing a substrate with a porous region to a depth of 3 microns in dry oxygen at normal pressure of 800°C for 60 minutes and oxidation film A obtained by oxidizing it in dry oxygen under pressure of 20K7/~ at 700°C for 60 minutes. FIG. 4 is a diagram comparing the etching rate of film B with an etching solution having a composition of NH4F:HF=10:1.

The oxide film C shown in the figure covers the surface of ordinary single crystal silicon at 1
It was formed by steam oxidation at 200°C. Oxide film A
, B, and C are represented by I and H, respectively. In the figure, the horizontal axis shows the etching time, and the vertical axis shows the etching amount in terms of etched depth.

As can be seen from the figure, oxide film A (curve 1) has an extremely high etching rate and is unsuitable for the actual semiconductor device manufacturing process as it is, but oxide film B (curve 1) according to the present invention
Curve) shows the same characteristics as the conventional high-temperature thermal oxide film C (curve) despite being oxidized at a low temperature, indicating excellent etching resistance. FIGS. 5A to 5C show values measured using a stylus-type surface roughness meter for surface steps of an oxide film selectively embedded in the substrate surface.

Figure A shows a cross section of the measuring section, with selective oxide film region 3 on substrate 1.
01 was formed. Figure B shows a porous material with a diameter of 3 microns after being oxidized with dry oxygen at 800°C for 15 minutes.
Curve 3 shows the surface level difference when steam oxidized at 050℃ for 15 minutes.
The curve C in the same figure is based on the present invention and shows the case where porous formation was performed under the same conditions and oxidation with dry oxygen was performed at 700°C for 60 minutes under a pressure of 20K7/d. 303.

Comparing B and C in the figure, the oxide film according to the present invention has much better flatness. Although the specific embodiments of the present invention described above are applied to selective oxidation, the present invention has the advantage of being capable of low-temperature oxidation and less inducing crystal defects. It is clear that this effect can also be achieved.

As explained above, the method for forming a silicon oxide film of the present invention is as follows:
After making a certain area of the silicon crystal substrate porous. , the silicon crystal substrate is 3K7/? The porous region is converted to silicon oxide by heating and oxidizing in a dry oxygen atmosphere pressurized to 20 K7/~, and according to the present invention, the thick silicon oxide film is free from distortion and contamination. It is extremely valuable industrially because it can be formed quickly, flatly, under low pressure, and at low temperatures.

[Brief explanation of drawings]

FIG. 1 is a diagram of the basic principle of a silicon substrate porosity processing apparatus according to the present invention, FIG. 2 is a diagram of the basic principle of a pressure oxidation apparatus according to the present invention, and FIGS. 3 a to 3 d are illustrations of an embodiment of the present invention. FIG. 4 is a comparison diagram of the etching speed of the present invention and the conventional example, and FIGS. 5A to 5C are diagrams of the comparison of surface steps between the present invention and the conventional example. 1... Semiconductor substrate, 103... Nitride film, 104... Porous region, 105, 106.
...Oxide region.

Claims (1)

[Claims] 1. After making a predetermined region of the silicon crystal substrate porous, the silicon crystal substrate is heated to 3Kg/cm^3 to 20Kg/cm^
3. A method for forming a silicon oxide film, characterized in that the porous region is converted into silicon oxide by heating and oxidizing in a dry oxygen atmosphere pressurized to 3.