OA17363A - Process for making spiral-conical micro or nanotholes on the surface of a silicon substrate. - Google Patents

Abstract

The object of the present invention is precisely to propose a process for producing micro or nanotracks of spiro-conical shape on the surface of a silicon substrate. More precisely, the method of the invention makes it possible to obtain nanostructural patterns on the surface of the silicon substrate. These patterns are micro or nanotholes of both conical and spiral shapes (spiroconic) made for the first time by a simple method.

Description

DESCRIPTION

at. The present invention relates to a Method for Making Micro or Nanotholes of Spiro-conical Shape on the Surface of a Substrate

Silicon.

Many nanotechnology applications use organized nanostructures, for example in microelectronics, optoelectronics, biomedical or biology. This specific conical and spiral shape is achieved for the first time by a simple technique.

b. State of the art

Silicon, in its natural elemental form, is not a light emitter; it must be transformed into porous silicon, a modified form of silicon.

The unique electronic, morphological and thermal properties of porous silicon make it useful for a range of applications. Porous silicon can even be a light emitter, which is useful in optoelectronics. In addition to potential applications in silicon-based optoelectronics, porous silicon has been used as an anti-reflective coating for solar cells.

Chemically modified porous silicon can be useful in chemical and biochemical detection; it can serve as an efficient matrix for the direct introduction of large mass biological macromolecules into mass spectrometry.

On the other hand, this unique nanostructure could have applications in perovskite solar cells using inorganic organic quantum wells as active materials in these microcavities. Indeed, the nanoscale arrangement of organic-inorganic compounds belonging to the perovskite family gives rise to special electronic and optical properties, which are neither the properties of the organic component alone nor the properties of the inorganic component alone.

In sum, porous silicon is useful in many applications and is likely to find many additional uses in the future.

Conventional methods for the production of porous silicon are often tedious, difficult or inefficient in producing stable porous silicon nanostructures. Equipment such as potentiostats and light sources are required in the etching processes of conventional porous silicon production processes.

Porous silicon is normally produced by anodic etching, with illumination (type n) or without illumination (type p). In the anodic etching process, the mobile holes are driven electrically at the silicon electrolyte interface where they participate in the dissolution by oxidation of the silicon atoms of the surface. Spatial anisotropy results from the potential barrier developed at the sharp points of evolving structures, which block the further transport of holes thus avoiding any further etching and giving rise to the porous structure. Porous silicones have also been produced without external polarization by chemical etching in HNO3 / HF solutions and by photochemical etching.

Overlap etching is generally slow (characterized by an induction period), therefore inconsistent, unreliable in producing light-emitting porous silicon, and is not readily ready for lateral structuring. Overlap etching is mainly used for the fabrication of very thin layers of porous silicon. Recently, it has been shown that the evaporation and annealing of 150-200 nm aluminum on silicon would allow faster overlap etching. However, the porous silicon produced by the improved Aluminum (Al) coating etching method is about ten times lower in luminescence than in the case of porous silicon obtained by anodic etching, of similar thickness, and the process always has an induction period before the start of the etching (see D. Dimova Malinovska et al., “Thin Solid Films”, 297, 9 - 12 (1997)). It has also been reported that Pt can be electrochemically deposited from a solution of Pt (IV) when etching silicon to produce porous silicon for light emission, but it has been found to be difficult to control the applied potential to affect both silicon etching and Pt deposition simultaneously (see, P. GOROSTIZA, R. Diaz, MA Kulandainathan, F. Sam, and JR Morante, J. Electroanal. Chem. 469, 48 (1999)). Thus, there is a need to find a more efficient process for the production of porous silicon. This is the goal of the invention of Li et al. (2004) to meet this need. His invention makes it possible to produce porous silicon (PSi) with tunable morphologies and light emitting properties. In the method of the invention, a discontinuous thin metallic layer is deposited on a silicon surface. Preferred metals are Pt for highly light-emitting PSi, Au for smooth-morphologically smooth PSi. It is important that the deposited layer is sufficiently thin that it forms a discontinuous film, which allows access of the etching species to the silicon surface in the zone where the metal has been deposited. The surface is then etched in a solution comprising HF and an oxidizer for a brief period, as little as 2 seconds up to 60 minutes. The preferred oxidizing agent is H2O2. The morphology and light-emitting properties of porous silicon can be selectively controlled depending on the nature of the metal deposited, the type of silicon doping, the level of silicon doping, and the etching time. The use of electrical energy is not necessary during the chemical attack of the invention. It is possible to manufacture this PSi by immersing the sample directly in the etching solution.

However, despite the ease of implementation and production of silicon nano or micropores by this technique, it is impossible to control or produce nano or microcavities of particular shapes such as those we propose in this invention.

vs. Brief description

The object of the present invention is precisely to provide a process for the surface nanostructuring of a silicon substrate in a specific manner.

More precisely, the method of the invention makes it possible to obtain structural patterns on the surface of the silicon substrate. These patterns are micro or nanotholes of both conical and spiral (spiro-conical) shapes.

In the method of this invention, a wafer or a silicon sample is first cleaned following a standard procedure. Then it is introduced into the etching solution composed of HF / AgNC for a given time to create nanowires. An HF / H2O2 oxidizing solution is then used which, with a fairly long time, shaves practically all the nanowires and thus gives a porous surface (made up of nanopores). This is where fluid mechanics will come into play through a whirlwind (or vortex) game. In fact, the subsequent use of a solution consisting only of AgNO3 with the presence of residual HF / H2O2 involves the vortex phenomenon in the nanopores, following a movement of the vortex fluid, thus giving a unique spiroconic shape. Then the catalyst metal thus deposited is removed using a concentrated solution of HNO3. The cleaning of metal residues can be supplemented by the use of an HNO3 / HCl / H2O2 solution.

The production of a porous surface can be done by directly using a solution making it possible to directly obtain nanopores, then immerse the sample thus nanostructured in an oxidizing HF / H2O2 solution for a limited time and go directly to the phase of realization of the spiroconic holes by the use of AgNÛ3.

To have an array of nano or microholes ordered as desired with well-defined sizes, it is possible to use nanospheric polystyrene masks, thus making it possible to obtain well-defined shapes of networks and hole sizes upstream. The depth of the nano or microholes will depend on the etching time and the type of catalyst used for the etching.

d. Brief Description of Figures

The present invention will be better understood on reading the description of embodiments given, purely as an indication and in no way limiting, with reference to the accompanying drawings and images in which:

Figures IA to IC show different steps of an example of the method according to the invention ranging from a sample of the silicon wafer to the production of cylindrical nanotholes;

Figures 2A to 2C show steps of another example of the method of the invention ranging from a sample of the silicon wafer to the production of cylindrical nanotholes and whose distribution on the surface is controlled; Figure 3 shows a front view slanted slightly forward and a side view of spiral and conical micro or nanotholes made from the two examples of methods described above according to the invention;

FIGS. 4A to 4C represent the SEM images of several “spiro-conical” micro or nanotholes distributed on the nanostructured surface of oriented silicon <100> respectively for 1 kX, 2 kX and 5 kX of magnification;

Figures 4D and 4E represent an enlarged image of a selected microhole showing the perfect spiral and conical shape within which there are residues of silver molecules according to the invention;

Figures 4F and 4G show an enlarged image of another selected microhole showing the internal morphology and its spiral and conical shape free from any silver residue according to the invention.

e. Detailed description of the invention

Referring to Figures 1A to IC, we will now describe an example of the method according to the invention. Used for the first time by Peng et al. [1], the metal-assisted chemical method has since become one of the most widely used methods to develop silicon nanostructures. Peng et al. [1] have shown that the HF / AgNO3 mixture allows both the deposition of silver on the silicon surface without an electrode and the etching of silicon resulting in the formation of nanowires.

However, to develop the spiro-conical nano or microholes, an original process based on the combination of two phenomena: one chemical (Electrochemical reaction: chemical method assisted by the metal Silver) and the other physical (vortex phenomenon: Fluid Mechanics ) has been created. We have called this process the Vortex Etching Method (MGV) or Vortex Method.

Etching Method (VEM in English).

The substrate 10 used for the preparation of the spiro-conical micro or nanotholes is p-type monocrystalline silicon doped with Boron, oriented <100>, with a resistivity between 1-5 Q.cm ' ¹ , thickness 600 to 650 μm with a polished face 35, shown in FIG. IA. This substrate 10 is obtained by the Zochralski synthetic methods.

The nanowires 11, shown in Fig IB are obtained after using the Chemical Method of Metal-Assisted Etching. These nanowires 11 can be obtained for example as described in the patent application US6790785 40 B1-2000. In our case we used this method by following the steps below:

- Cleaning of the substrate using the standard method with solutions of Acetone, Ethanol, H2SO4 / H2O2 of varying volume proportions and finally deHF.

- Etching of the substrate with an HF / AgNCh solution of concentration 22.8 / 0.02 molar for 120 minutes and determined volume ratio. This concentration ratio as well as the etching time may vary. Nanowires 11 are thus obtained on the surface of the silicon for a first step. A mask can be placed on the face that we do not wish to engrave.

The electrochemical reactions involved during this etching step are as follows:

Ag + + e- = Ag Εθ = 0.79 V

Si + 2H ₂ O = SiO ₂ + 4H + + 4e- Εθ = 0.91 V

SiO ₂ + 6HF = SiF ₆ 2- + 2H ₂ O + 2H +

Overall reaction:

Si + 6HF + 4Ag + = 4Ag + SiF ₆ ² '+ 6H +

It is after obtaining these nanowires 11 that we use a solution Ί composed of HF / H ₂ O ₂ with a molar concentration ratio of 4.80 / 1.18x10 'for 30 minutes. This time can vary as well as the molar concentration ratio between HF and H ₂ O ₂ . During this step, the nanowires are practically shaved and there remains a nanostructured surface composed mainly of nanowires 12 as illustrated in FIG. 1C. The electrochemical reactions involved are as follows:

H ₂ O ₂ + 2H + -> 2H ₂ O + 2h + Εθ = 1.76 V

Si + 6HF + nht— * H ₂ SiF6 + nH + + [n / 2] H ₂

Overall reaction:

Si + 6HF + n / 2 H ₂ O ₂ -> H ₂ SiF ₆ + nH ₂ O + [2 - n / 2] H ₂ Finally, the silicon sample thus nanostructured with nanholes made randomly will be immersed in a new solution of AgNCh at a concentration of 0.1 M for 15 minutes (this time may vary as well as the concentration of AgNO ₃ ). Then two phenomena will take place:

- one is a physical phenomenon related to fluid mechanics. It is a kinetic mechanism of the movement of a liquid with a high flow rate entering a hole. This is the vortex or vortex movement.

- The other is rather an electrochemical reaction which is done as in the etching process. The Ag + ions will attack the silicon in order to oxidize it and dissolve it in the solution as detailed in the etching technique thanks to the presence of residues of the HF / H ₂ O _{2 solution} used. The use of this solution in the procedure seems fundamental because the presence of H ₂ O ₂ as oxidant at a percentage> 30 would increase the horizontal etching speed to the detriment of the vertical one by annihilating the gravity of the Ag + nanoparticles thus preventing them from descending. vertically.

The etching phenomenon linked to the mechanism of displacement of the AgNCh solution in a vortex gives the micro or nanotholes 31 of conical and spiral (spiro-conical) shape.

Finally, the silver deposit thus formed was removed by using a concentrated HNO3 solution for 60 minutes. Silver cleaning may be supplemented with a solution of HNO3 / HCl / H2O of volume ratio (v / v / v = 1: 1: 1) for at least 8 hours. Figure 3 is a representation of the results of the micro or nanotholes seen from a front slanted and sideways face (31).

In this procedure, the spatial distribution and the size of the upper diameters of the micro or nanotholes 41, 42 and 43 are not controlled and depend very much on the etching conditions (concentration of the solutions, concentration ratios, etching time, etc.) ( Fig 4A, 4B and 4C).

It is possible, alternatively, with reference to Figs. 2A to 2C, to describe an example of the process for having micro or nanotholes with a well-controlled spatial distribution and size of the diameters.

For this, we start with a substrate 10 used for the production which is p-type monocrystalline silicon doped with Boron, oriented <100>, with a resistivity between 1 - 5 Q.cm ' ¹ , thickness 600 to 650. pm with a polished face, shown in FIG. 2A.

After the cleaning step detailed above, nanospheric polystyrene masks 21 are deposited uniformly on the surface of the sample as shown in FIG. 2B. A solution of HF / AgNO3 with a 22.8 / 0.02 molar deconcentration ratio is used for the etching. Thus we obtain nanotholes 22 of well-determined diameter and whose distribution and number can be chosen (Fig. 2C). Subsequently the sample will be immersed in the HF / H2O2 solution described above for a short time.

Finally, the silicon sample thus nanostructured with nanotholes produced will be immersed in a new solution of AgNCL of concentration 0.1 M for 15 minutes (this time may vary).

As explained above, the etching phenomenon linked to the mechanism of displacement of the AgNCh solution in a vortex gives the micro or nanotholes 31 of conical and spiral (spiro-conical) shape.

The advantage of this variant lies in the control of the dimensions and the distribution density of the micro or nanotholes on the silicon surface.

Figs. 4A, 4B, 4C show SEM images of spiro-conical shaped micro or nanotholes with non-uniform distribution as explained in the first procedure.

Figs. 4D and 4E show the internal morphology of a nanothole 44 chosen and whose uniformity and spiral and conical shape are well shown as well as the presence of silver nanoparticles 45 formed, after etching, to confirm the mechanism detailed above.

Figs. 4F and 4G are also SEM images of a nanhole46, 47 with the same geometric shape but in which any silver particle has been cleaned by the HNO3 solution.

Fig. 4H is an SEM image of a side view of the resulting nanotholes showing the various conical shapes and in some cases the prominent spiral grooves.

Claims (17)

1. Process for producing surface nanostructures of a silicon substrate (lO) of micro or nanotholes of spiral and conical (spiro-conical) shapes 5 without external energy input in that it comprises the following steps: - Realization of nanotholes (12) on one side of the silicon (10) using the first or the second method which will constitute the base width of the cone - Realization of spironconic micro or nanotholes (31,41,42, 43,44,46) by the combination of a physical mechanism and an electrochemical mechanism. 2. Method according to claim 1, characterized in that the nanostruction consists of conical and spiral shaped holes. 3. Method according to claim 1 or 2, characterized in that the realization of 15 Nanowires by chemical etching is carried out with a long time (120 min) and constitutes a step facilitating the creation of nanowires randomly on the surface of the substrate (10). 4. Method according to claim 1 or 2, characterized in that the use of a 20 HF / H2O2 oxidizing solution contributing to the domination of the horizontal etching speed over the vertical one. 5. Method according to claim 1, characterized in that the deposition of polystyrene nanospheres (21) contributing to the creation of cylindrical nanotholes (12) 25 distributed in an orderly manner on the surface of the silicon (10). 6. Method according to claim 5, characterized in that it comprises the production of cylindrical nanotholes (12) contributing to the vortex flow phenomenon. 7. Method according to claim 6, characterized in that the use of an HF / AgNO _{3 solution} allowing the production of cylindrical nanotholes (12). 8. Method according to claim 7, characterized in that the use of a 35 HF / H2O2 oxidizing solution contributing to the domination of the horizontal etching rate over the vertical one. 9. Method according to claims 4 or 8, characterized in that it comprises the use of an AgNO3 solution causing a second etching 40 chemical. 10. The method of claim 4 or 8, characterized in that it takes into account the creation of conditions necessary for the vortex phenomenon in the cylindrical nanotholes (12) with the flow of the AgNC ^ etching solution. 11. The method of claim 10, characterized in that the combination of a chemical phenomenon and a vortex flow phenomenon allows the realization of conical holes with spiral grooves inside these holes. 12. The method of claim 11, characterized in that the size of these holes can vary from nano to micrometer (nano or spiro-conical micro-holes) (31, 42, 43,44,46). 13. The method of claim 12, characterized in that these holes can form an array of nanotholes that can give a specific geometric shape (48). 14. The method of claim 9, characterized in that the species of the second etching is chosen from silver, gold or a species for etching silicon. 15. The method of claims 6 and 10, characterized in that it comprises two different stages of chemical etching assisted by metal catalyst combined with a physical phenomenon 16. The method of claim 15, characterized in that it constitutes a new and unique chemical procedure combined with a physical phenomenon of production of such a nanostructure. 17. The method of claim 16, characterized in that it is called Vortex Etching Method (MGV) or Vortex Etching Method (VEM in English).