JP2838444B2 - Method of forming buried insulating film in silicon substrate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally relates to a method for forming a buried insulating film in a silicon substrate,
More specifically, the present invention relates to an improved method for planarizing an interface between a buried insulating film and a silicon layer existing on the buried insulating film.

[0002]

2. Description of the Related Art A silicon substrate having a buried insulating film has attracted attention as a substrate for a future VLSI.

FIG. 2 is a developed view of a buried oxide film showing the dose of oxygen ion implantation as a function,
This is a diagram schematically showing the crystal state. The diagram on the right side of FIG. 2 is a diagram showing a state of oxygen distribution when annealing is performed at a temperature of 1250 ° C. or more.

[0004] Referring to the diagram on the left side of FIG.
4.4 × 10 stoichiometric concentration^{twenty two}/ Cm^ThreeAbove which oxygen becomes
The silicon diffuses rapidly toward the bottom of the distribution,
Into SiO_TwoTo form In addition, the figure on the right side of FIG.
With reference to FIG. 2, when high-temperature annealing is performed, the Si layers 1a and 2
The oxygen inside is SiO_TwoIs diffused to the layer 3 side and deposited, _Two
The layer 3 increases in thickness, and the fineness of the Si layer 1a, 2
Small SiO_TwoThe precipitate disappears. As a result, the Si layer 1
a, 2 / SiO_TwoThe interface of layer 3 becomes steep.

[0005]

However, a large number of threading dislocations due to residual oxygen and implantation defects occur in the Si layer 2 during annealing, and these threading transitions remain even after annealing, and the crystal quality is reduced. Is affecting.

FIG. 3 is a diagram showing the relationship between the oxygen implantation amount and the transition density and the relationship between the oxygen implantation amount and the thickness of the buried oxide film. As is evident from FIG. 3, if the injection voltage is fixed, the occurrence state of the threading transition corresponds well to the injection amount.
As the injection amount decreases, the amount of threading dislocations tends to decrease. Utilizing this, multi-injection (multi-stage injection) has already been developed and reported. Referring to FIG. 4, for example, oxygen ions of about 0.7 to 1 × 10 ¹⁸ / cm ² are implanted into the surface of silicon substrate 1 at 200 KeV. After that, annealing is performed to recover defects and deposit the SiO ₂ layer 3, and then, ion implantation and annealing at the same voltage and at the same level are repeated to form a buried SiO 2 layer having few defects and having an Si layer 2 as an upper layer. Form _two structures. In this multi-injection method, the interface between the Si layer 2 and the SiO ₂ layer 3 is very steep,
An extremely high quality silicon substrate having a buried insulating layer having a transition density of 10 ³ / cm ² or less in the Si layer 2 can be obtained.

However, this multi-implantation method also has a problem that the interface between the Si layer 2 and the SiO ₂ layer 3 is smoothly wavy, and the thickness of the upper Si layer 2 becomes uneven, as shown in FIG. Was left. The non-uniformity of the thickness of the Si layer 2 causes a variation in characteristics when a MOS device is formed in this layer, which limits the circuit operation margin, which is a problem.

That is, in the case of a thin-film SOI / MOS transistor, in a completely depleted mode, at a threshold voltage _VA shown by the following equation, Qb changes according to the film thickness.
This change in film thickness contributes to the variation in _VA .

[0009]

(Equation 1)

Next, the reason why the thickness of the Si layer 2 becomes non-uniform in the above-described multiple implantation method will be described in more detail with reference to the drawings.

Referring to FIG. 5A, a silicon substrate 1
Then, the first stage ion implantation is performed. Immediately after the first-stage ion implantation, the oxygen concentration is less than the normal composition of SiO ₂ even at the peak position in the implantation region 10.
No iO ₂ is formed. However, with reference to FIG. 5B, at the time of heat treatment at 1300 ° C. or lower, precipitation of SiO ₂ (in the figure, 11 represents SiO ₂ precipitate) is promoted with defects and the like as nuclei. The precipitation nuclei of the SiO ₂ precipitate 11 are not always present at the peak position of the implanted oxygen, but are often present on the surface side having many defects during implantation.

Subsequently, referring to FIG.
When annealing at 2 ° C. for 2 hours, large precipitation nuclei are connected to each other, and a continuous SiO ₂ layer 3 is formed. At this time, the lack of oxygen is eliminated by incorporating a large number of Si islands 12 into the SiO ₂ layer. However, even though the SiO ₂ layer 3 is continuous, the center position of the original precipitation nucleus is different, so that the interface between the surface Si layer 2 and the SiO ₂ layer 3 undulates like an envelope of a large precipitation nucleus. Will be.

Referring to FIG. 5D, the next stage of ion implantation is performed. In this ion implantation, the amount of implantation is suppressed to the extent that defects are not increased. Therefore, the ion-implanted oxygen is consumed only for oxidizing the Si islands 12, and cannot be used to correct the irregularities at the interface between the Si layer 2 and the SiO ₂ layer 3. As a result, in the multiple implantation method, the interface between the Si layer 2 and the SiO ₂ layer 3 is not flattened.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has an improved buried insulating film in a silicon substrate in which the interface between the SiO ₂ layer and the Si layer is flattened. An object of the present invention is to provide a method for forming a film.

[0015]

SUMMARY OF THE INVENTION The present invention relates to a method for forming a buried insulating film in a silicon substrate.
First, first oxygen ions are implanted into a main surface of a silicon substrate at a first high energy that gives an oxygen concentration distribution that has a maximum value at a position below the main surface of the silicon substrate. The silicon substrate into which the first oxygen ions have been implanted is heat-treated to form a SiO ₂ layer in the silicon substrate. A second high energy that gives an oxygen concentration distribution at the interface between the SiO ₂ layer and the Si layer overlying the SiO ₂ layer on the main surface of the silicon substrate so as to have a maximum value;
Second oxygen ions are implanted. The silicon substrate into which the second oxygen ions have been implanted is heat-treated.

According to a preferred embodiment of the present invention, the maximum value of the first oxygen ions is set so as to be equal to or less than a concentration that does not reach the normal composition of SiO ₂ . Also,
The maximum concentration of the second oxygen ion is set to be equal to or lower than the concentration that does not reach the normal composition of SiO ₂ .

It is preferable that the first high energy E ₁ and the second high energy E ₂ have the following relationship.

[0018]

(Equation 2)

[0019]

SUMMARY OF] According to the present invention, the SiO ₂ layer, the SiO ₂
Since oxygen ions are implanted at the second high energy that gives an oxygen concentration distribution that has a maximum value at the interface with the Si layer existing on the layer, a flat Si / SiO ₂ interface is obtained.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view showing the steps of one embodiment of the present invention. With reference to FIG.
The first stage oxygen ion implantation is performed at 200 KeV for 1
The heating is performed in a state where the substrate is heated at 510 to 650 ° C. at × 10 ¹⁸ / cm ² . With this ion implantation amount, oxygen is supplied to the silicon substrate 1
The peak is distributed at about 4500 ° from the surface (in the figure,
10 indicates an oxygen implantation region), but the composition does not reach the normal composition, and no SiO ₂ is formed. To obtain a normal composition, about 1.35 × 10 ¹⁸ / cm ² is required.

Referring to FIG. 1B, when the silicon substrate 1 is annealed at 1325 ° C. in an Ar / O ₂ atmosphere for 2 hours, an SiO ₂ layer 3 is formed in the silicon substrate 1. At this time, the interface between the Si layer 2 and the SiO ₂ layer 3 fluctuates,
In addition, many Si islands 12 exist in the SiO ₂ layer 3. The average position of the interface between the Si layer 3 and the SiO ₂ layer 3 from the surface is about 3500 °.

Referring to FIG. 1C, the acceleration voltage of the second-stage oxygen ion implantation is set to 150 KeV so that the peak of the distribution substantially comes at 3500 ° from the surface of the silicon substrate. The injection amount is a critical injection amount of 1.05 × 10 ¹⁸ / c.
respect m ^2, is set to 7 × 10 ¹⁷ / cm ^2. In the second ion implantation, the independent implantation amount is not an implantation amount capable of forming SiO ₂ during implantation. However, in order to implant oxygen ions into the region where the SiO ₂ layer 3 has already been formed, the implanted oxygen moves quickly to the interface between the SiO ₂ layer 3 and the Si layer 2 and Reacts with Si to form SiO ₂ . As a result, Si / SiO ₂
Moves the interface to the surface side, SiO ₂ internal S
The i-island 12 can also be oxidized to form a sufficiently thick SiO ₂ layer 3.

Referring to FIG. 1D, oxygen in the surface Si layer 2 is also reduced by annealing at 1325 ° C. for 2 hours.
It moves to the O ₂ layer 3 side and precipitates as SiO ₂ at the interface.
Irregularities at the iO ₂ interface disappear, and are flattened.

In the above embodiment, the first injection voltage of oxygen ions was set to 200 KeV and the second injection voltage was set to 150 KeV. However, the present invention is not limited to this. By selecting an injection voltage having the following relational expression, appropriate processing can be performed. In the equation, E ₁ is the first acceleration voltage, and E ₂ is the second acceleration voltage.

[0026]

(Equation 3)

It is preferable that the implantation amount of oxygen ions satisfy the following equations.

[0028]

(Equation 4)

The heat treatment is preferably performed at a temperature of 1300 ° C. or higher. As an atmosphere, a mixed gas of argon and oxygen is used.

[0030]

As described above, according to the present invention, the second oxygen concentration distribution which gives the maximum value at the interface between the SiO ₂ layer and the Si layer existing on the SiO ₂ layer is provided. With high energy of oxygen ion implantation,
A flat Si / SiO ₂ interface is obtained. Further, since a surface Si layer having a uniform thickness is formed, a fully depleted SOI
The effect of improving the uniformity of the characteristics of / MOSFET and obtaining a stable circuit operation margin is exhibited.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing the steps of an embodiment of the present invention.

FIG. 2 is a diagram showing a crystal state at the time of oxygen ion implantation.

FIG. 3 is a diagram showing the dependence of the dislocation density of a surface Si layer and the thickness of a buried oxide film on the amount of injected oxygen.

FIG. 4 is a cross-sectional view of a silicon substrate on which a buried oxide film has been formed by a conventional multiple implantation method.

FIG. 5 is a sectional view showing steps of a method of forming a buried oxide film in a silicon substrate by a conventional multi-implantation method.

[Explanation of symbols]

1 silicon substrate 2 Si layer 3 SiO ₂ layer 10 oxygen injection region

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H01L 21/265 H01L 27/12

Claims (1)

(57) [Claims] A step of implanting first oxygen ions into the main surface of the silicon substrate at a first high energy to give an oxygen concentration distribution having a maximum value at a position below the main surface of the silicon substrate. Heat-treating the silicon substrate into which the first oxygen ions have been implanted, thereby forming SiO _{2 in the} silicon substrate.
Forming a layer; forming a SiO ₂ layer on the main surface of the silicon substrate;
implanting second oxygen ions at a second high energy that gives an oxygen concentration distribution that has a maximum value at the interface with the Si layer present on the iO ₂ layer; Heat treating the silicon substrate into which is implanted, a method of forming a buried insulating film in the silicon substrate.