JPH08306682A - Prevention of impurity auto-doping - Google Patents

Abstract

PURPOSE: To provide a method of preventing an impurity auto-doping, which can prevent impurities from being auto-doped to a silicon substrate without going through complicated different processes. CONSTITUTION: A BPSG film containing impurities (boron or phosphorus) is formed on a silicon base body 11 by a CVD method or the like and thereafter, the BPSG film is annealed at a temperature of about 850 deg.C or lower in an oxygen atmosphere. The diffusion coefficient of oxygen in the film 12 is very large, so when this film 12 is annealed in the oxygen atmosphere, a silicon oxide film (an SiO2 film) 13, which is used as an auto-doping preventive film, is formed on the base body 11. By the existence of this film 13, impurities, such as phosphorus or boron, in the film 12 are prevented from being diffused in the base body 11 by a BPSG film 12 flatterning process (The film 12 is annealed at about 900 deg.C in a nitrogen atmosphere.), which is performed after the formation of the film 13 and is performed by a reflow. By simply changing the temperature and the condition of an atmosphere, the formation of the auto- doping preventive film and a flatterning treatment of the reflowed film can be continuously performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used to prevent impurities from diffusing into a semiconductor substrate in a semiconductor manufacturing method including a step of forming a silicon oxide film containing impurities on a semiconductor substrate. The present invention relates to a method for preventing impurity autodoping.

[0002]

2. Description of the Related Art Conventionally, in the case of manufacturing a semiconductor device, a functional element such as a MOS transistor is formed on a P-type or N-type silicon substrate and then phosphorus, boron or the like is contained as an impurity. Silicon oxide film (PSG
(Phosphorus-silicate-glass) or BPSG (boron-phosphorus-silicate-glass), etc. is formed and heat-treated to obtain a flattening film by reflow. The silicon substrate is formed by this step. In some regions, the impurity-containing silicon oxide film such as BPSG may be directly formed on the silicon substrate. In such a portion, an autodoping phenomenon occurs in which impurities diffuse into the silicon substrate in the heat treatment step for reflow. This phenomenon is caused especially by the impurity concentration of the portion of the BPSG film in contact with the silicon substrate being 5 × 10 ²⁰ / cm 3.
^{When the number is 3} or more, it remarkably appears, and may be a factor that reduces the reliability and yield of the semiconductor device.

In order to solve this problem, BPS is conventionally used.
Before forming a silicon oxide film containing impurities such as G, a silicon oxide film (SiO ₂ ) or a silicon nitride film (Si ₃ N ₄ ) is formed in advance on a silicon substrate to prevent autodoping. .

[0004]

As described above, conventionally,
Since the auto-doping prevention film is formed on the silicon substrate in a separate process, there is a problem that the number of independent processes increases.

The present invention has been made in view of the above problems, and an object thereof is to provide a method of preventing impurity autodoping which can prevent autodoping of a silicon substrate without a complicated separate process. Especially.

[0006]

A method for preventing impurity autodoping according to the present invention comprises an oxide film forming step of forming a silicon oxide film containing impurities on a semiconductor substrate, and a step of forming the formed silicon oxide film with nitrogen. And a heat treatment step of performing heat treatment at a first temperature in an atmosphere,
After the silicon oxide film forming step and before the heat treatment step,
Heat treatment is performed in an oxygen atmosphere at a second temperature lower than the first temperature to prevent impurities in the silicon oxide film from diffusing into the semiconductor substrate at the interface between the semiconductor substrate and the silicon oxide film. A step of forming another silicon oxide film for the purpose. As a temperature condition, the first temperature is preferably 850 ° C or higher and the second temperature is preferably lower than 850 ° C. The silicon oxide film containing the impurities is, for example, a silicon oxide film containing phosphorus or boron or both as impurities, and is flattened by reflow by heat treatment at a first temperature in a nitrogen atmosphere. It is a chemical insulating film.

[0007]

In the method of preventing impurity auto-doping according to the present invention, after forming a silicon oxide film containing impurities on a silicon substrate, a second temperature lower than the first temperature (reflow temperature) is applied in an oxygen atmosphere. By performing the heat treatment, a silicon oxide film as an impurity autodoping prevention film is formed at the interface between the semiconductor substrate and the silicon oxide film.
Then, heat treatment is performed at a first temperature in a nitrogen atmosphere to perform planarization by reflow.

[0008]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 and FIG. 2 are for explaining a method for preventing impurity autodoping according to an embodiment of the present invention and show a cross section of the device. Here, a case where the present method is applied to, for example, a manufacturing process of an SRAM device will be described.

First, an NMOS transistor for driver and access is formed on the N-type silicon substrate 11. Specifically, after a thick silicon oxide film is selectively formed on the surface of the silicon substrate 11 as an element isolation film, boron (B) is introduced into the silicon substrate 11 from the element active region surrounded by the element isolation film. ) Is selectively ion-implanted to form a P-type well region, and then an NMOS transistor is formed in the device active region. In the NMOS transistor, after a gate oxide film is formed on the silicon substrate 11, a polycrystal silicon layer and a silicide layer are sequentially deposited on the gate oxide film to form a polycide layer, and the polycide layer is patterned to form a gate electrode. Consistently N
^A -type impurity region is formed to form a source / drain region.

Next, CVD (Chemical Vapor Depositio)
BPS containing boron and phosphorus as impurities due to n)
The G film is formed to have a thickness of about 200 to 500 nm. This allows
As shown in FIG. 1, the BPSG film 12 is directly formed on the silicon substrate 11 in the portion other than the NMOS transistor formation region.

Next, as shown in FIG. 3, annealing is performed at a temperature of about 850 ° C. or less in an oxygen atmosphere. The BPSG film 12 formed in the above process has a very large diffusion coefficient of oxygen in the film. Therefore, when this is annealed in an oxygen atmosphere, as shown in FIG.
A silicon oxide film (Si
O ₂ ) 13 is formed. The annealing temperature is about 85
It is desirable that the temperature is 0 ° C. or lower, because the diffusion of impurities from the BPSG film 12 into the silicon substrate 11 is 850.
This is because it occurs at ° C or higher.

Next, as shown in FIG. 3, annealing is performed at about 900 ° C. in a nitrogen atmosphere. As a result, B
The surface of the PSG film 12 is flattened by reflow. At this time, since the silicon oxide film 13 is formed at the interface between the silicon substrate 11 and the BPSG film 12, the BPSG
Impurities such as phosphorus and boron in the film 12 are contained in the silicon substrate 11
Diffusion into the inside is effectively prevented. The annealing temperature in the case of reflow needs to be set to a temperature sufficient for flattening the BPSG film 12, but is generally 850 ° C.
A temperature of ~ 900 ° C is suitable.

Then, a thin film transistor (TFT) as a load transistor is formed on the flattened BPSG film 12.
Then, the contact with each layer, metal wiring, etc. are formed, and the manufacture of the SRAM device is completed.

As described above, in this embodiment, the formation of the impurity auto-doping film is performed at a slightly low temperature in an oxygen atmosphere before the annealing process for reflow performed in a nitrogen atmosphere. That is, two heat treatments can be continuously performed only by changing the temperature condition and the atmospheric condition.

[0016]

As described above, according to the method for preventing impurity auto-doping according to the present invention, after forming a silicon oxide film containing impurities on a silicon substrate, the first temperature (reflow) is performed in an oxygen atmosphere. Second lower than temperature)
Since the impurity auto-doping prevention film (silicon oxide film) is formed at the interface between the semiconductor substrate and the silicon oxide film by performing the heat treatment at the temperature of, the impurity auto-doping prevention film forming process which has been required separately in the past. Is unnecessary. That is, the formation of the impurity auto-doping prevention film and the subsequent heat treatment in a nitrogen atmosphere can be continuously performed only by changing the temperature condition and the atmosphere condition, and the number of steps can be reduced.

[Brief description of drawings]

FIG. 1 is a sectional view of an element showing one step of forming a silicon oxide film by a method for preventing impurity auto-doping according to an embodiment of the present invention.

FIG. 2 is an element cross-sectional view showing a step that follows the step of FIG.

FIG. 3 is an explanatory diagram showing temperature conditions for forming a silicon oxide film by the method for preventing impurity autodoping.

[Explanation of symbols]

 11 silicon substrate 12 BPSG film 13 silicon oxide film (impurity autodoping prevention film)

Claims (3)

[Claims] 1. A semiconductor comprising: an oxide film forming step of forming an impurity-containing silicon oxide film on a semiconductor substrate; and a heat treatment step of heat-treating the formed silicon oxide film at a first temperature in a nitrogen atmosphere. In the manufacturing method, after the silicon oxide film forming step and before the heat treatment step,
In order to prevent impurities in the silicon oxide film from diffusing into the semiconductor substrate at the interface between the semiconductor substrate and the silicon oxide film by performing heat treatment at a second temperature lower than the first temperature in an oxygen atmosphere. A method for preventing impurity auto-doping, comprising the step of forming another silicon oxide film of 1. 2. The method for preventing impurity autodoping according to claim 1, wherein the first temperature is 850 ° C. or higher and the second temperature is lower than 850 ° C. 3. The silicon oxide film is a film containing phosphorus or boron or both as impurities, and the heat treatment step at a first temperature in the nitrogen atmosphere flattens the silicon oxide film by reflowing. The method for preventing impurity autodoping according to claim 1, which is a step of