JP3061973B2 - Method for making silicon substrate porous and method for forming porous silicon layer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for forming a porous silicon substrate and a method for forming a porous silicon layer. More specifically, the present invention relates to a method for selectively forming porous silicon.

[0002]

2. Description of the Related Art A single crystal silicon and a platinum plate are placed in a chemical conversion solution containing hydrofluoric acid as a main component, and the single crystal silicon is used as an anode and the platinum plate is used as a cathode. It is known that when the value is greater than a certain value, mirror-like electrolytic polishing occurs, but below that value, porous silicon is formed.

In general, it is known that when single crystal silicon is immersed in hydrofluoric acid, its surface is terminated with hydrogen atoms. The anodization reaction for forming porous silicon starts from this state, and the mechanism is shown in FIG. First, when holes reach the silicon surface due to the application of a voltage, F ^- ions attack Si-H bonds on the silicon surface to form Si-F bonds (step 1). Then, another F atom is influenced by the polarity of the F atom bonded thereto.
^- ions attack the Si-H bond to form with H ₂ molecule,
This causes injection of electrons into the electrodes (steps 2 and 3). Due to the polarization of the Si—F group, the electron density of the Si—Si bond is reduced, and the bond is weakened. The bond is attacked by HF or H ₂ O and detaches from the surface (steps 4 and 5). However, the remaining silicon surface is still terminated with hydrogen atoms.

Here, if silicon atoms are desorbed from a flat silicon surface at the atomic level by the above reaction, a depression of the atomic size remains. This geometrical change induces a non-uniform charge distribution on the surface, resulting in selective concentration of holes in the recessed area. By repeating the above reaction, the unevenness of the surface is emphasized, and porous silicon having a large number of pores is formed.
This reaction is restricted by holes, so that the porous silicon layer can be thickened as much as a voltage is applied.

[0005]

In a conventional method of forming porous silicon using anodization, crystal defects such as dislocations existing on the substrate surface are involved as reaction starting points. Is non-uniform and has a low distribution density, so that it is difficult to uniformly form a porous layer having a finer pore size, and there is a problem that distribution also occurs in the pore size in the depth direction.

In view of such a conventional problem, the present inventors have found a method for forming porous silicon having fine pores uniformly and with good control in the depth direction on a single crystal silicon substrate, and have reached the present invention.

[0007]

According to the present invention, high-energy ions are implanted into a silicon substrate to introduce defects uniformly in the depth direction in the silicon substrate. A method for making a silicon substrate porous is provided, wherein a porous silicon layer is formed on a surface layer of the silicon substrate.

That is, by implanting high-energy ions, Frenkel defects are introduced into the silicon substrate, and by selectively making the defects porous,
A porous silicon layer is formed.

The silicon substrate that can be used in the present invention may be p-type or n-type. In order to make the substrate p-type, a substrate doped with boron or the like in advance can be used. In order to obtain the n-type, a substrate doped with phosphorus, arsenic, antimony or the like in advance can be used.

Next, high energy ions are injected into the substrate.
Enter. The ions that can be used here include phosphorus and arsenic.
And antimony. The injection conditions are
Energy 1
３０30 MeV, injection amount 1 × 10 ¹³~ 1 × 10¹⁶Pieces / cm
^TwoAnd preferably 1 to 11 MeV, 1 × 10¹⁴~
1 × 10¹⁵Pieces / cm^TwoIt is. This implant allows
Frenkel defects are introduced into the substrate.

Next, the surface layer of the silicon substrate is transformed into a porous silicon layer based on the known anodization method. This anodization method can be performed as follows.
First, the silicon substrate is connected to the anode. Next, both electrodes are immersed in an electrolytic solution using platinum as a cathode. As an electrolytic solution that can be used at this time, hydrofluoric acid, hydrofluoric acid and ethyl alcohol, hydrofluoric acid and methyl alcohol, and the like can be used, but hydrofluoric acid is preferably used. More preferably, the composition of the hydrofluoric acid solution is HF: H ₂ O = 1: 1 (volume ratio).

Next, an electrochemical current is applied to the anode and the cathode at a current density of ² to 200 mA / cm ² for 1 to 100 minutes, depending on the thickness and pore size of the porous silicon layer to be obtained. A reaction occurs to form a porous silicon layer. In this reaction, the holes control the reaction, so that the layer thickness can be increased as much as the voltage is applied. Preferably it is 10 nm or less.

Prior to the introduction of the above-mentioned defects, an intermediate layer resistant or soluble in the electrolytic solution used in the anodization method is formed to form a porous silicon layer having a uniform thickness with higher accuracy. Can be. First, when, for example, hydrofluoric acid is used as the electrolytic solution, a solution-resistant intermediate layer can be formed as follows. 1-6 MeV of carbon ions on a silicon substrate, 1 × 10 ¹⁶ -1 × 10 ¹⁸ / c
Irradiation under the condition of m ² . Next, an annealing process is performed on the silicon substrate to form an intermediate layer made of silicon carbide in the silicon substrate. In addition, the processing is from 1200 to 1400
C. for 2 to 10 hours in an atmosphere of an inert gas such as Ar. Since the intermediate layer made of silicon carbide has resistance to hydrofluoric acid, only the silicon layer above the intermediate layer can be made porous, and the layer thickness can be controlled more precisely. Become.

Next, when, for example, hydrofluoric acid is used as the electrolytic solution, an intermediate layer soluble in the solution can be formed as follows. Oxygen ions on silicon substrate 1-5Me
V is applied under the conditions of 1 × 10 ¹⁷ to 1 × 10 ¹⁹ particles / cm ² . Next, an annealing process is performed on the silicon substrate to form an intermediate layer made of silicon oxide in the silicon substrate. The treatment can be performed at 900 to 1200 ° C. for 1 to 5 hours under an atmosphere of an inert gas such as Ar. The intermediate layer made of silicon oxide is soluble in hydrofluoric acid and is dissolved in hydrofluoric acid when it is made porous, so that the layer thickness can be controlled more precisely, and the porous layer Only the substrate can be separated from the substrate.

[0015]

The porous silicon layer obtained by the method of the present invention utilizes irradiation defects formed by the incident ion beam of the material. That is, by controlling the type, energy and implantation amount of the ion beam, porous silicon having a desired pore diameter can be formed. Furthermore, an extremely uniform porous silicon layer having no pore size distribution in the depth direction, which has conventionally been a problem, can be obtained. In addition, by combining the ion implantation technique and heat treatment to form a layer resistant or soluble to a hydrofluoric acid solution in the intermediate layer of the substrate, it is possible to obtain a porous silicon layer of a desired thickness with good controllability. Become.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. 1, 2, 3 and 4. FIG. The present invention is not limited to the following materials, steps, conditions, and the like.

Example 1 An 11-type silicon substrate 1 doped with boron was
Irradiation with MeV As ions 2 was performed at 2 × 10 ¹⁵ ions / cm ² to introduce defects (see FIG. 1A). Next, the surface layer of the silicon substrate 1 was transformed into a porous silicon layer 3 by anodization (see FIG. 1B).

This anodization is performed by the method shown in FIG.
Was. In a reaction cell 12, a positive electrode comprising a p-type silicon substrate 1 is provided.
HF: H_TwoO =
It was immersed in a 1: 1 (volume ratio) hydrofluoric acid solution 14. With the anode
Current density of 20 mA / cm on cathode ^TwoAnd let current flow for about 3 minutes
This causes an electrochemical reaction, resulting in a p-type silicon substrate
The porous silicon layer 3 having a thickness of 2 μm is formed on the surface layer
Was. The obtained porous silicon layer is converted to a high-resolution electron microscope.
When examined by mirror observation and X-ray two-crystal method, the depth direction
A porous silicon layer having a uniform thickness and a pore diameter of 5 nm has a layer thickness of 2
It was confirmed that it was obtained at μm.

Example 2 3M was formed on a p-type silicon substrate 4 doped with boron.
carbon ions 5 eV were irradiated with 1 × 10 ¹⁸ pieces / cm ² (see FIG. 2 (a)). Next, in an electric furnace, at 1350 ° C, about 3
Annealing was performed in an Ar gas atmosphere for a period of time to form a SiC layer 6 having a thickness of 1 μm in a region about 5 μm below the surface (see FIG. 2B).

Then, 2 × 10 As ions 2 of 11 MeV
Irradiation was performed at ¹⁵ particles / cm ² to introduce defects (see FIG. 2C). Next, the surface layer of the silicon substrate 1 was transformed into a porous silicon layer 7 by anodization (see FIG. 2D).

This anodization is carried out by the method shown in FIG.
Was. In a reaction cell 12, a positive electrode comprising a p-type silicon substrate 4 is formed.
HF: H_TwoO =
It was immersed in a 1: 1 (volume ratio) hydrofluoric acid solution 14. With the anode
Current density of 10 mA / cm on cathode ^TwoFor about 10 minutes.
Electrochemical reaction occurs by flowing, and p-type silicon base
2 μm thick porous silicon layer 7 is formed on the surface layer of plate 4
Was done. At this time, the intermediate layer of the p-type silicon substrate 4 is
Since the SiC layer 6 resistant to the
The porous silicon layer is formed only on the upper layer. Get
High-resolution electron microscopy and
According to the X-ray two-crystal method, it was found that it was uniform in the depth direction.
A porous silicon layer with a pore diameter of 6 nm was obtained with a layer thickness of 5 μm.
Was confirmed.

Embodiment 3 On a p-type silicon substrate 8 doped with boron, 2M
Irradiation with eV oxygen ions 9 was performed at 1 × 10 ¹⁸ ions / cm ² (see FIG. 3A). Next, in an electric furnace, at 1100 ° C, about 5
Annealing treatment was performed in an Ar gas atmosphere for 0 minutes to form an SiO ₂ layer 10 having a thickness of 1.5 μm in a region about 5 μm below the surface (see FIG. 3B).

Next, As ion 2 of 11 MeV was added to 2 × 10
Irradiation was performed at ¹⁵ particles / cm ² to introduce defects (see FIG. 3C). Next, the surface layer of the silicon substrate 1 was transformed into a porous silicon layer 7 by anodization (see FIG. 3D).

This anodization is performed by the method shown in FIG.
Was. In a reaction cell 12, a positive electrode comprising a p-type silicon substrate 8 is formed.
HF: H_TwoO =
It was immersed in a 1: 1 (volume ratio) hydrofluoric acid solution 14. With the anode
Current density of 50mA / cm on cathode ^TwoAnd let current flow for about 2 minutes
This causes an electrochemical reaction, resulting in a p-type silicon substrate
8 porous silicon layer 12 having a thickness of 5 μm is formed on the surface layer 8
Was done. At this time, the intermediate layer of the p-type silicon substrate 4 is
SiO that is soluble in_TwoSince the layer 10 is formed,
This layer dissolves during the electrochemical reaction with hydrofluoric acid.
As shown in FIG. 3D, only the porous silicon layer
Is separated from the silicon substrate. Many obtained
The porous silicon layer was observed with a high-resolution electron microscope and X-ray
According to the crystallization method, it was found that the pore size was uniform in the depth direction.
Confirmed that a 3 nm porous silicon layer was obtained.
did it.

[0025]

Since the porous silicon layer obtained according to the present invention utilizes irradiation defects caused by the incident ion beam of the material, the desired pore size can be obtained by controlling the type, energy and incident amount of the ion beam. Can be formed. Furthermore, it is possible to obtain an extremely uniform porous silicon layer without the distribution of pore diameter in the depth direction, which has conventionally been a problem.

[Brief description of the drawings]

FIG. 1 is a schematic process chart showing a method for forming a porous silicon layer of the present invention.

FIG. 2 is a schematic process diagram showing a method for forming a porous silicon layer of the present invention.

FIG. 3 is a schematic process chart showing a method for forming a porous silicon layer of the present invention.

FIG. 4 is a schematic explanatory view of a reaction cell used for an anodizing method.

FIG. 5 is a schematic explanatory view of a mechanism for forming a porous silicon layer by anodization.

[Description of Signs] 1 p-type silicon substrate 2 As ion 3 porous silicon layer 4 p-type silicon substrate 5 carbon ion 6 SiC layer 7 porous silicon layer 8 p-type silicon substrate 9 oxygen ion 10 SiO ₂ layer 11 porous silicon Layer 12 Anodization reaction cell 13 Platinum electrode 14 Hydrofluoric acid solution

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01L 21/31 H01L 21/205

Claims (4)

(57) [Claims] 1. After implanting high-energy ions into a silicon substrate to uniformly introduce defects in a depth direction in the silicon substrate, a porous silicon layer is formed on the surface layer of the silicon substrate by anodization. Forming a porous silicon substrate. 2. The silicon according to claim 1, wherein prior to the implantation of the high energy ions, carbon ions are implanted into the silicon substrate and an annealing process is performed to form a carbonized layer in the intermediate layer of the silicon substrate. A method for making a substrate porous. 3. The silicon according to claim 1, wherein prior to the implantation of the high energy ions, oxygen ions are implanted into the silicon substrate and annealing treatment is performed to form an oxide layer on the intermediate layer of the silicon substrate. A method for making a substrate porous. 4. Separating the porous silicon layer from the silicon substrate by removing the oxide layer of the silicon substrate having the oxide layer and the porous silicon layer obtained by the method according to claim 3 with hydrofluoric acid. A method for forming a porous silicon layer comprising: