JP2018149611A - Silicon nanowall structure and method of producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique for producing silicon nanowalls without allowing silicon walls to stick to each other.SOLUTION: According to the present invention, the transition from a process of forming a plurality of silicon nanowalls by wet-etching a silicon substrate in the depth direction thereof to a process of removing mask parts by wet-etching is carried out while keeping the silicon substrate in a state of being immersed in an etchant and a washing liquid. As a result, the side wall faces of silicon walls are kept in a state of contacting the etchant, the washing liquid, and an oxydation liquid, and are prevented from sticking to neighboring silicon walls. Hence, thinner silicon walls relative to conventional ones can be formed by etching. A passivation layer can be formed by carrying out wet oxidation in a liquid (oxidation by a chemical liquid), or a silicon wall having a thickness as thin as the thickness of an intended silicon nanowall (SNW) can be formed. Thus, a burden to be borne by a specimen and a subsequent process upon thinning the wall thickness by subsequent thermal oxidation can be reduced.SELECTED DRAWING: Figure 3

Description

  TECHNICAL FIELD The present invention relates to a technique for manufacturing a silicon nanowall structure, and more particularly, a silicon nanowall having a thickness and a high aspect ratio that can be designed within a preferable range for widening a band gap due to a quantum effect, suitable for manufacturing a solar cell. Concerning the structure.

  A structure (nanostructure) having a fine shape of nanometer size has a band gap that is widened by a quantum size effect and exhibits characteristics that are not found in bulk materials. This quantum size effect is due to the fact that when the quantum well width is narrowed (decreased) as the size is reduced, the energy level of the electron increases due to the requirement of the boundary condition at the quantum well end that the electron wave function should satisfy. It is understood.

  Regarding such a nanostructure, JP 2011-519730 A (Patent Document 1) discloses an invention of a segmented semiconductor nanowire including a superlattice / quantum well structure and a method for manufacturing the same. Such nanostructures have been studied for application to solar cells. For example, research on silicon nanowall structures has been underway.

FIG. 1 is a diagram for conceptually explaining a conventional manufacturing process of a silicon nanowall structure. First, a silicon substrate 100 is prepared (a). A silicon nitride film (Si ₃ N ₄ ) 110a that will act as a mask in a later step is formed on the main surface of the substrate 100 by LPCVD or the like (b), and then on the silicon nitride film 110a, A resist 120a for nanoimprinting is applied (c). Then, the nanoimprint patterning 120b is performed on the resist 120a using the pattern mold 130 (d). By this patterning, a wall having a width of 50 to 70 nm is obtained.

  The region of the silicon nitride film 110a exposed by the nanoimprint patterning 120b is removed by dry etching to obtain a patterned silicon nitride film 110b (e). Thereafter, the resist on the silicon nitride film 110b is removed (f), and the silicon substrate 100 is anisotropically etched in the depth direction using the patterned silicon nitride film 110b as a hard mask. For this anisotropic etching of silicon, for example, a chemical solution such as KOH can be used.

  The etching mainly removes silicon in the depth direction to form a silicon wall. However, since the etching also proceeds on the side wall surface of the silicon wall, the thickness before etching (50 to 70 nm) after etching. For example, a silicon wall 140 having a thickness of about 2 to 3 μm and a thickness of about 30 to 50 nm is formed (g).

  After the formation of the silicon wall 140, the silicon nitride film 110b remaining on the upper end of the silicon wall 140 is removed by etching using a chemical such as a high concentration hydrofluoric acid solution or phosphoric acid solution (h). And after performing pure water washing | cleaning etc., in order to make the thickness of the silicon wall 140 still thinner, thermal oxidation is performed. In this thermal oxidation, the side wall of the silicon wall 140 is mainly oxidized, and the thickness of the silicon wall 140 is reduced by the amount of oxidized silicon. By forming such an oxide film 150, a silicon nanowall structure having a thickness of several nanometers is obtained (i). For example, a silicon wall with a thickness of 2 nm has a band gap of about 1.7 eV. When the silicon crystal portion having a thickness t is oxidized, a silicon oxide film having a thickness of 2t is obtained.

  By the way, in the conventional manufacturing process, anisotropic etching at the time of forming the silicon wall, etching for removing the silicon nitride film 110b remaining on the upper end portion of the silicon wall 140, cleaning with pure water after these etchings, and the like are separately performed. It is carried out in a liquid tank. Therefore, the silicon substrate on which the silicon wall is formed is once taken out from the liquid tank and transferred to the liquid tank in the next process. At that time, the surface (wall surface) of the silicon wall taken out to the outside is exposed to the outside air, and the thin silicon wall is easily bent and sticks to each other. Such sticking phenomenon is more likely to occur as the silicon wall is thinner and the surface is more hydrophobic.

  Although the final goal is to obtain a silicon nanowall structure having a thickness of several nanometers, the thickness of the silicon wall formed by etching should be relatively thick, about 30 to 40 nm. The reason why it cannot be obtained is that the level of deflection needs to be kept low in order to avoid mutual sticking due to surface tension with water as much as possible.

  FIG. 2 shows the state (a) of the silicon wall when it is taken out from the liquid tank after anisotropic etching at the time of forming the silicon wall, and the silicon nitride film 110b remaining on the upper end of the silicon wall. It is an example of the SEM image which shows the mode (b) of the silicon wall when it takes out outside from a liquid tank after an etching. Even when the silicon wall is formed after anisotropic etching at the time of forming the silicon wall, sticking has already occurred locally. Furthermore, the silicon wall is completely adhered in a state where the silicon nitride film remaining on the upper end portion of the silicon wall is removed from the liquid tank after etching.

  However, as described above, if the thickness at the time of forming the silicon wall is made relatively thick, the problem of sticking hardly occurs, but in that case, it is difficult to increase the wall aspect ratio or due to oxidation. The burden on the sample and process when thinning the wall is increased, and the controllability of the oxide film thickness is difficult. Specifically, the oxide film necessary for thinning the wall naturally becomes thick, and the oxidation temperature must be increased or the oxidation time must be lengthened.

  In addition, when a silicon crystal portion having a thickness t is oxidized, the portion does not become a silicon oxide film having a thickness t, but a silicon oxide film having a thickness 2t. In order to make it thin, it means that it is necessary to design a pattern by providing a space corresponding to the thickness to be thinned in advance.

Special table 2011-519730 gazette

  In view of such circumstances, the present inventors have conducted sincerity studies on a process for preventing the above-mentioned silicon walls from sticking to each other. The objective of this invention is providing the technique for manufacturing a silicon nanowall structure, without producing sticking of a silicon wall.

  In order to solve the above problems, a silicon nanowall structure according to the present invention is a structure in which a plurality of silicon nanowalls (SNW) are formed in an array at substantially equal intervals on a silicon substrate, An average value W of the SNW thickness is 20 nm or less, and a distance D between adjacent SNWs is 30 nm or less.

  Preferably, an aspect ratio (R = H / W) when the average value of the height of the SNW is H is 100 or more.

  Preferably, the conductivity type of the silicon substrate and the SNW is n-type or p-type.

  Further preferably, the main surface of the silicon substrate is a {110} plane.

  In one embodiment, a passivation film is formed on the side wall surfaces of the plurality of SNWs.

  Moreover, in a certain aspect, the area | region of the space | interval D of SNW which adjoins mutually is filled with insulators other than silicon oxide or silicon oxide.

    The method for producing a silicon nanowall structure according to the present invention includes a step A for forming a mask on a stripe pattern extending in parallel at substantially equal intervals on a silicon substrate, and a crystal region exposed from the mask. A step B of forming a plurality of silicon nanowalls (SNW) by chemical etching in the depth direction of the silicon substrate, and a step C of removing the mask portion by chemical etching. The transition to the step C is performed in a state where the silicon substrate is immersed in the liquid.

  Preferably, the step C is performed by a light-assisted etching method using light having a wavelength of energy larger than the band gap of the silicon crystal.

    In a certain aspect, the process of oxidizing the side wall surfaces of the plurality of SNWs in a chemical solution to form a passivation film, or the step of oxidizing the side wall surfaces of silicon nanowalls (SNW) with a chemical solution as a subsequent step of the step C The process D is provided, and the process from the process B to the process C or the process C to the process D is performed in a state where the silicon substrate is immersed in the liquid.

For example, the chemical solution used for the photo-assisted etching in the step C is a mixed acid of hydrofluoric acid (HF) and nitric acid (HNO ₃ ), or a mixed liquid of hydrofluoric acid (HF) and hydrogen peroxide (H ₂ O ₂ ). is there.

  For example, the acid concentration of the chemical used for the light-assisted etching in the step C is 1% or less.

  Moreover, in a certain aspect, the process E which thermally oxidizes the side wall surface of these SNW as a post process of the said process C or D is provided.

  In the conventional method, due to the above-described phenomenon of silicon wall sticking, the silicon wall formed by etching must be relatively thick, and the method of reducing the thickness by subsequent thermal oxidation has been used. On the other hand, in the present invention, the silicon substrate moves from the step of forming a plurality of silicon nanowalls by wet etching in the depth direction of the silicon substrate to the step of removing the mask portion by wet etching. Or the subsequent wet oxidation (oxidation with a chemical solution) step was performed in a state immersed in the solution.

  As a result, the side wall surface of the silicon wall is in contact with an etching solution, a cleaning solution, an oxidizing solution, and the like, and sticking to an adjacent silicon wall is prevented. Formation is possible. It is possible to form a passivation layer by wet oxidation as it is in the liquid, or to form a thin silicon nanowall (SNW). Subsequent thermal oxidation reduces the burden on the sample and process when reducing the thickness. By using light-assisted etching for wet etching of the mask portion, there is an effect of reducing the etchant concentration and aligning the SNW width.

It is a figure for demonstrating notionally the conventional manufacturing process of a silicon nanowall structure. The state (a) of the silicon wall when taken out from the liquid tank after anisotropic etching when forming the silicon wall, and the liquid tank after etching for removing the silicon nitride film 110b remaining on the upper end of the silicon wall It is a SEM image which shows the mode (b) of the silicon wall when taking out from the outside. It is sectional drawing for demonstrating the outline of an example of a structure of the silicon nanowall structure which concerns on this invention. It is a figure for demonstrating notionally an example of the manufacturing process of the silicon nanowall structure concerning this invention. In the method for producing a silicon nanowall structure, the state of the silicon wall before adding the photo-assisted etching method (a), the state of the silicon wall after removing the mask portion by adding the photo-assisted etching method (b), And it is a SEM image which shows the mode (c) of the silicon substrate surface vicinity which formed the silicon wall. When manufacturing a silicon nanowall structure, the distribution of the thickness of the wall in which the silicon wall was formed by the conventional method (wet etching) and the distribution of the thickness of the wall to which photo-assisted etching was added thereafter were shown in comparison. It is a graph. FIG. 7A is a schematic cross-sectional view illustrating an embodiment of an etching tank (liquid tank) used for carrying out the present invention, FIG. 7A is an example of a bathtub-type liquid tank, and FIG. 7B is a spin etcher type. This is an example of the liquid tank.

  Embodiments of the present invention will be described below with reference to the drawings. Hereinafter, a silicon wall having a thickness exceeding 10 nm may also be referred to as a silicon nanowall (SNW).

  FIG. 3 is a cross-sectional view for explaining an outline of an example of the configuration of the silicon nanowall structure according to the present invention.

  A plurality of silicon nanowalls (SNW) 20 are formed on the silicon substrate 10 in an array at substantially equal intervals.

  The conductivity type of the silicon substrate 10 may be p-type or n-type, but is preferably n-type. This is because, in photo-assisted etching, which will be described later, in n-type silicon in which majority carriers are electrons, even if the silicon wall has a thick region and a thin region, the nonuniformity of the wall width tends to be alleviated. Because it is big.

  Briefly explaining this point, electron irradiation and electron-hole pairs are generated in the silicon crystal by light irradiation, but the amount of generation is relatively large in the region where the silicon wall width is thick and relatively small in the thin region. Very few. Although holes play an important role in the light-assisted etching, holes generated by light irradiation move toward the side wall surface of the silicon wall. When the etchant is hydrofluoric acid, the following reaction occurs on the side wall surface of the silicon wall, and etching proceeds.

Si + 6HF + 2 hole → H ₂ SiF ₆ + H ₂ + 2H ⁺

  Therefore, etching proceeds relatively fast in a region with a relatively large number of holes generated by light irradiation and a thick silicon wall width, whereas in a region with a small silicon wall width that has relatively few holes generated by light irradiation. Etching proceeds relatively slowly. As a result, the non-uniformity of the wall width is alleviated. If the conductivity type of the silicon substrate 10 is n-type, the SNW conductivity type is naturally n-type.

  The main surface of the silicon substrate 10 is preferably a {110} plane. This is because if the main surface of the silicon substrate 10 is a {110} plane, the {111} plane is a crystal plane perpendicular to the main plane (for example, the (1,1,0) plane and the (1, -1,1,1) ) Surface), but this {111} surface is a close-packed surface of silicon crystal and is suitable for selective etching. Therefore, if this surface can be used as a side wall surface of a silicon wall, a silicon nanowall with a high aspect ratio can be formed. This is because it is easy to form.

  In FIG. 3, what is indicated by reference numeral 30 is filled in a passivation film provided on the side wall surfaces of a plurality of SNWs, a silicon oxide having a thin SNW, or a region having a spacing D between adjacent SNWs. It is an insulator other than silicon oxide or silicon oxide. As such a thing, the alumina by atomic layer deposition (atomic layer deposition) can be illustrated.

FIG. 4 is a diagram for conceptually explaining an example of the manufacturing process of the silicon nanowall structure according to the present invention. Also in this process, the steps from the preparation (a) of the silicon substrate 10 to the patterning (b) of the hard mask 40 such as a silicon nitride film (Si ₃ N ₄ ) are the same as the above-described conventional ones.

  In the present invention, anisotropic etching (c) is performed in the depth direction of the silicon substrate 10 using the patterned hard mask 40, and the hard mask remaining on the upper end of the silicon wall after the silicon wall 20 is formed ( d) Further, the subsequent pure water cleaning, passivation film formation and the like (e) are performed consistently with the side wall of the silicon wall in contact with the chemical solution.

  As described above, in the conventional manufacturing process, the series of processes such as etching and pure water cleaning are performed in separate liquid tanks, so that the silicon substrate on which the silicon wall is formed is once taken out from the liquid tank. When this occurs, the side wall of the silicon wall is not in contact with the chemical. For this reason, the side wall surface of the silicon wall is exposed to the outside air, causing the above-described sticking phenomenon.

  However, even if a different etchant is required because the etching target is different, if the etchant is replaced while the side wall of the silicon wall is in contact with the chemical solution, some chemical solution is present between the side walls of the silicon wall. Since it exists, this acts as a buffer between the side walls, and the sticking phenomenon does not occur. Based on this idea, the inventors have made the present invention.

  That is, in the method for manufacturing a silicon nanowall structure according to the present invention, the step A for forming a mask in a stripe pattern extending in parallel at substantially equal intervals on the silicon substrate and the mask is exposed from the mask. A step B of wet-etching the crystal region in the depth direction of the silicon substrate to form a plurality of silicon nanowalls (SNW); and a step C of removing the mask portion by wet etching. C is performed by a light-assisted etching method using light having a wavelength of energy larger than the band gap of the silicon crystal, and the transition from the process B to the process C is performed in a liquid such as an etchant or a cleaning liquid. It is performed in an immersed state.

In the present invention, the photo-assisted etching method is adopted because the removal of the hard mask 40 such as the silicon nitride film (Si ₃ N ₄ ) of the silicon nanowall (SNW) formed up to the step B is performed with a low concentration acid. Even if etching is used, acceleration is achieved, and non-uniformity of the silicon nanowall (SNW) width can be reduced.

The etchant used for the light-assisted etching in the above step C is, for example, a mixed acid of hydrofluoric acid (HF) and nitric acid (HNO ₃ ), or a mixture of hydrofluoric acid (HF) and hydrogen peroxide (H ₂ O ₂ ). It is a liquid.

  In the conventional method, due to the above-described phenomenon of silicon wall sticking, the silicon wall formed by etching must be relatively thick, and the method of reducing the thickness by subsequent thermal oxidation has been used. On the other hand, in the present invention, since the transition from the process B to the process C is performed in a state where the silicon substrate is immersed in a liquid such as an etching liquid or a cleaning liquid, the side wall surface of the silicon wall has an etching liquid or a cleaning liquid. Since it is in a state where it is in contact with this liquid and sticking to an adjacent silicon wall is prevented, a silicon wall thinner than the conventional one can be formed by etching. As a result, the burden on the sample and process when the thickness is subsequently reduced by thermal oxidation is reduced.

  FIG. 5 shows a state of a silicon wall (a) before adding a light-assisted etching method in a method of manufacturing a silicon nanowall structure, and a state of a silicon wall after removing a mask portion by adding a light-assisted etching method. It is a SEM image which shows (b) and the mode (c) of the silicon substrate surface vicinity which formed the silicon wall.

  FIG. 6 compares the wall thickness distribution obtained by forming a silicon wall by a conventional method (wet etching) and the wall thickness distribution obtained by adding photo-assisted etching after manufacturing the silicon nanowall structure. It is the graph shown.

  Due to the effect of light-assisted etching, the etching rate is relatively high when the silicon wall width is relatively thick, and the etching rate is relatively low when the silicon wall width is relatively thin. Therefore, if this effect is used, the thickness distribution (variation) in the stage before the light-assisted etching can be reduced, and the thickness uniformity can be improved. Note that the number of counted walls is 261 in all cases. If the width of the silicon wall is reduced by thermal oxidation, the effect of increasing the thickness uniformity cannot be obtained. Therefore, the thickness should be made uniform when it is required to reduce the width of the silicon wall to the nm level. It can be said that photo-assisted etching is a very effective technique.

When the photo-assisted etching method is used in Step C, the acid concentration of the etchant can be kept low. In the case of general wet etching, the acid concentration of HF or the like is about 20%. However, the optical assist etching method shortens the hard mask 40 such as a silicon nitride film (Si ₃ N ₄ ) even if the acid concentration is 1% or less. It can be removed in time, and it is advantageous when it is replaced with pure water after that.

  When performing the process of reducing the thickness of the silicon wall by thermal oxidation, it is necessary to take out the silicon substrate on which the silicon wall is formed from the liquid and put it in a heat treatment furnace. There is a risk of sticking. Therefore, in order to avoid this, wet oxidation (oxidation with a chemical solution) is performed to reduce the width of the silicon nanowall (SNW) in the liquid as a subsequent process of the above-described process C, or the silicon thin wall is made to the target thickness. You may make it provide the process D which forms a passivation film by oxidizing a side wall surface in a chemical | medical solution. Then, as a post-process of this process D, the side wall surface of the silicon wall is thermally oxidized to reduce the silicon nanowall (SNW) width to the desired thinness, and silicon oxide or You may make it provide the process E filled with insulators other than.

  The etching tank (liquid tank) used in carrying out the present invention can have various modes.

  FIG. 7 is a schematic cross-sectional view illustrating an embodiment of an etching tank (liquid tank) used in carrying out the present invention. FIG. 7A is an example of a bathtub-type liquid tank, and FIG. Is an example of a spin etcher type bath.

In the bathtub-type liquid tank shown in FIG. 7 (a), chemicals used for etching and rinsing (HF, HF + HNO ₃ , HF + H ₂ O ₂ , H ₂ O ₂ , HNO ₃ , H ₂ O, etc.) from the left side of the figure. ) Is introduced, and after etching is completed or oxidized, it is discharged from the right side of the figure. Note that light having a wavelength of energy larger than the band gap of the silicon crystal 10 is irradiated from above the liquid bath. In order to introduce the etchant used in the next process while discharging the etchant used in the previous process etching, the side wall surface of the silicon wall formed on the silicon substrate is always in contact with the chemical solution and sticks to each other. Is prevented.

In the spin etcher type liquid tank shown in FIG. 7B, a silicon substrate is placed in a rotatable sample holder 50 provided in the liquid tank, and this sample holder serves as an etching tank. Also in this embodiment, chemicals used for etching and rinsing (HF, HF + HNO ₃ , HF + H ₂ O ₂ , H ₂ O ₂ , HNO ₃ , H ₂ O, etc.) are introduced into the sample holder from the left side of the figure, and etching is completed. Or after oxidation, it is discharged from the right side of the figure. Note that light having a wavelength of energy larger than the band gap of the silicon crystal is irradiated from above the liquid bath. In this case as well, the side wall surface of the silicon wall formed on the silicon substrate is always in contact with the chemical solution in order to introduce the etchant used in the next step while discharging the etchant used in the previous step etching. Mutual sticking is prevented.

  The present invention provides a technique for manufacturing a silicon nanowall structure without causing silicon wall sticking.

10, 100 Silicon substrate 20, 140 Silicon nanowall (SNW)
30, 150 Passivation film, silicon oxide 40, 110b Hard mask 50 Sample holder 120a Resist 130 Pattern mold

Claims (12)

A structure in which a plurality of silicon nanowalls (SNW) are formed in an array at substantially equal intervals on a silicon substrate,
The average value W of the SNW thickness is 20 nm or less, and the distance D between adjacent SNWs is 30 nm or less.
Silicon nanowall structure. The aspect ratio (R = H / W) when the average value of the SNW height is H is 100 or more.
The silicon nanowall structure according to claim 1. The conductivity type of the silicon substrate and the SNW is n-type or p-type,
The silicon nanowall structure according to claim 1 or 2. The main surface of the silicon substrate is a {110} surface,
The silicon nanowall structure according to any one of claims 1 to 3. A passivation film is formed on the side wall surfaces of the plurality of SNWs.
The silicon nanowall structure according to any one of claims 1 to 4. The region of the space D between adjacent SNWs is filled with an insulator other than silicon oxide or silicon oxide,
The silicon nanowall structure according to any one of claims 1 to 5. Forming a mask in a striped pattern extending in parallel at substantially equal intervals on the silicon substrate; and
A step B of forming a plurality of silicon nanowalls (SNW) by etching a crystal region exposed from the mask with a chemical in the depth direction of the silicon substrate;
And a step C of removing the mask portion by etching with a chemical solution,
The transition from the process B to the process C is performed in a state where the silicon substrate is immersed in a liquid.
A method for producing a silicon nanowall structure. The step C is performed by a light assisted etching method using light having a wavelength of energy larger than the band gap of the silicon crystal.
The method for producing a silicon nanowall structure according to claim 7. As a subsequent step of the step C, the step D is a step of oxidizing the sidewall surfaces of the plurality of SNWs in a chemical solution to form a passivation film, or a step of oxidizing the sidewall surfaces of silicon nanowalls (SNW) with a chemical solution. The process B to the process C or the process C to the process D is performed in a state in which the silicon substrate is immersed in a liquid.
A method for producing a silicon nanowall structure according to claim 7 or 8. The chemical solution used for the photo-assisted etching in the step C is a mixed acid of hydrofluoric acid (HF) and nitric acid (HNO ₃ ), or a mixed liquid of hydrofluoric acid (HF) and hydrogen peroxide (H ₂ O ₂ ).
The manufacturing method of the silicon nanowall structure of any one of Claims 8-9. The acid concentration of the chemical used for the light-assisted etching in the step C is 1% or less.
The manufacturing method of the silicon nanowall structure of any one of Claims 8-10. As a post-process of the process C or D, the process includes a process E of thermally oxidizing the side wall surfaces of the plurality of SNWs.
The method for producing a silicon nanowall structure according to any one of claims 9 to 11.