Patent Application
20150155175
This description applies to the field of metalized porous structures and particularly porous structures based on a porous semiconducting material that has been filled with a metallic material.
It discloses a method of making a porous metalized structure with a better filling ratio and a more uniform distribution of the metallic material in the thickness of the structure.
It is known that a porous material can be metalized by an electrolytic deposition or electrodeposition technique that requires an input of electrical current or by a deposition method of the type commonly called “electroless” based on an auto-catalytic reaction.
The “Electrodeposition of metals into porous silicon” document by Jeske et al., Thin Solid films, 1995 describes different techniques for metallization of porous Si using Au, Cu or Ni.
This difficulty also becomes clear in the document “Different Behavior in immersion Plating of Nickel on Porous Silicon from Acidic and Alkaline Fluoride Media”, by Harraz et al., Journal of the Electrochemical Society, 2003, as shown in
The problem arises of finding a new method of metalizing a porous material by which the metal can penetrate through the entire thickness of the porous material and lead to a better distribution of the metal in the thickness of the porous material.
The invention firstly relates to a metallization method for a porous material by deposition of a metallic material in the liquid phase using a solution containing metal ions.
The invention requires a low deposition rate of a metallic material.
The conditions consisting of the deposition temperature, the pH of said solution and the concentration of metal ions in solution in which the porous material is dipped, are chosen jointly to result in a deposition rate of the metallic material equal to less than or equal to the order of 0.1 nm/min.
A deposition rate less than or equal to 0.1 nm/min can improve the filling of the porous material, and particularly can give a total filling of the porous layer or the porous substrate.
Such a deposition rate can also result in a uniform filling of the metallic material in the thickness of the porous material.
The use of a liquid phase deposition in particular has the advantage of being much less expensive than gas phase deposition processes.
The porous material that is metalized may be in the form of a substrate or a layer with a thickness of several hundred μm.
The porous material that is metalized may be a material with open porosity.
The porous material that is metalized may be a semiconducting material or a zeolithic type micro-porous mineral material
The porous material that is metalized may advantageously be Si.
The concentration of metal ions is preferably chosen to be less than or equal to a predetermined threshold concentration.
The molar concentration of metal ions in solution is preferably less than 0.1 M so as to achieve a deposition rate of the metallic material of less than 0.1 nm/min.
The temperature is preferably chosen to be less than or equal to a predetermined temperature threshold.
The temperature of the chemical bath used and containing the deposition solution may be chosen within the [0° C.; 25° C.] interval and will preferably be close to 0° C., to achieve a deposition rate of the metallic material of less than 0.1 nm/min.
The pH of the deposition solution is also preferably an acidic or low a pH.
The pH of the solution is adjusted as a function of the temperature and the concentration of metal ions while preventing dissolution of the porous material.
The pH of the deposition solution may be chosen to be less than 7 and preferably less than or equal to 6, and advantageously less than or equal to 4, so as to achieve a deposition rate of the metallic material of less than 0.1 nm/min.
The deposition solution may then be a solution containing Ni+ or Cu2+ or Au3+ or Ag+ ions in order to metalize the porous Si.
The deposition solution may include a surfactant so as to improve filling.
As a variant, the material to be metalized can be placed in a solution containing a surfactant before deposition of the metallic material.
This invention is applicable to metallization of porous structures in which the porosity is composed of pores or even cracks or trenches.
The invention also relates to a method of fabrication of a microelectronic device comprising a metallization method according to any one of the previous claims.
This invention will be better understood after reading the description of example embodiments given purely for information and in no way limitative with reference to the appended drawings in which:
As is usual in drawings of semiconducting structures, the various sectional views are not shown to scale. The various parts shown in the figures are not necessarily all at the same scale to make the figures more easily readable.
An example method according to the invention for metallization of a porous structure will now be disclosed.
The initial material for this method may be a solid substrate 10 based on a semiconducting material (
The substrate 10 may be doped with P type doping, with a doping level of between 1015 cm−3 and 1019cm−3. In this example, the doping level may for example be of the order of 1019cm−3. For this doping level, the porosity of the porous silicon layer may be between 20% and 85% and adjusted as a function of electrochemical conditions. The porosity of the substrate 10 may for example be of the order of 80% and its thickness may for example be of the order of 1 μm.
The diameter, also called the critical pore diameter, is between several nanometers and several tens of nanometers. In this example, the diameter of the pores may be greater than 5 nm, for example of the order of 10 nm. The method according to the invention is particularly applicable to filling small pores, i.e. with a critical dimension or diameter of less than 20 nm.
The porous material surface may be activated prior to metallization of the porous substrate 10.
This can be done by a first dip in an SnCl2 based solution, for example with a molar concentration of the order of 0.3M and a duration of the order of 2 minutes.
Rinsing and a second dip may be made in a solution possibly based on PdCl2, for example with a molar concentration of the order of 2*10−3 M, for example with a duration of the order of 1 minute.
These dipping steps help to modify and activate the surface of the porous material.
A surfactant may also be applied to the porous material before the metallization step in order to reduce the surface tension of the porous material and thus increase its wettability. The surfactant may be dispensed on the substrate 10 by means of a nozzle or it may be in the form of a bath in which the substrate 10 is immersed. The surfactant may for example be ethanol. Other surfactants such as methanol, acetic acid, sodium stearate, benzene sulfonate or an organic solvent may also be used.
Advantageously, the surfactant may be included in the deposition solution to metalize the porous material.
After immersion in the surfactant, the substrate 10 can then be rinsed without drying.
The next step is to deposit the metallic material in the liquid phase, the infill metallic material being in the form of metal ions present in a liquid deposition solution.
This deposition solution may for example include an NiSO4 based solution containing Ni+ metal ions when it is required to metalize the porous substrate using Nickel.
Other metals such as gold (Au), copper (Cu), silver (Ag) may be deposited by replacing the Ni salt (NiSO4) by a salt of the metal to be deposited. For example, metallization with Au can be done using a solution of Aucl3. Metallization with Cu may for example be done using a solution of CuSO4, whereas metallization with Ag can be done for example using a solution of Ag2SO4.
The molar concentration of Ni+ ions in this solution is preferably less than 0.1M and may for example be of the order of 0.05 M. A reducing agent, for example such as sodium hypophosphite Na2H2PO2 with a molar concentration for example of the order of 0.1 M may also be integrated into the deposition solution, as well as a complexing agent for example such as lactic acid CH3CHOHCO2 with a molar concentration of 0.2 M.
An acid, for example such as diluted sulfuric acid H2SO4, can be integrated in order to adapt the pH of the deposition solution and particularly to make this solution acidic.
Rinsing and drying steps can then be applied.
The inventors have discovered that in order to fill the porous substrate uniformly and preferably throughout its entire thickness, according to the invention, the metallic material is deposited at a very low deposition rate, particularly within a range of deposition rates less than or equal to 0.1 nm/min.
Such a deposition rate range is not easy to obtain, particularly in the liquid phase.
The inventors have discovered that such a range of deposition rate less than or equal to 0.1 nm/min can be achieved by making a combined adaptation of conditions consisting of the pH, solution temperature and concentration of metal ions in solution.
The temperature of the deposition solution is one of the parameters to be adapted to reach a low deposition rate, a low deposition temperature, i.e. at least less than 25° C. and close to 0° C. being conducive to a low deposition rate.
These curves C0 and C1 show a reduction in the deposition rate with the temperature.
To achieve a deposition rate of less than 0.1 nm/min, a concomitant adjustment of parameters other than temperature is thus required.
The pH is another of the parameters to be adapted to achieve a low deposition rate. A low pH is conducive to a low deposition rate.
In order to achieve a deposition rate of less than 0.1 nm/min, in addition to adapting the temperature and the pH, the concentration of metal ions is adjusted concomitantly. In this example, the pH is preferably chosen to be less than 7, and preferably less than 6 so as to obtain a low deposition rate while preventing dissolution of the porous Si.
This curve C30 shows that the deposition rate remains constant above a threshold concentration that in this example is of the order of 0.1 M.
A large reduction in the deposition rate is observed below this threshold concentration.
With such conditions for the pH, concentrations of metal ions in the deposition solution and temperature, the deposition rate is of the order of 0.08 nm/min that is therefore less than 0.1 nm/min.
This metalized layer is of the type of substrate 10 described above with P+ type doping, 1 μm thick, with a porosity of the order of 80% and pore diameters of the order of 10 nm. The deposition of metallic material made under deposition conditions consisting of temperature 25° C., pH equal to 4, molar concentration of metal ions in the NiSO4 metal solution, adapted to obtain a deposition rate of less than 0.1 nm/min. The profile was measured after a stay of the Si layer in the deposition solution for approximately 2 hours.
The pores are completely filled after this duration of 2 h. The surface of the pores will be coated with metal for a shorter duration (i.e. less than 2 h).
Curve C50 shows infill of the Si layer by Ni over its entire thickness and a uniform Ni concentration throughout the entire thickness of the layer of metalized Si.
For comparison, curve C60 in
A variant example of the metallization method according to the invention may include the following sequence of steps:
Step 1: A porous substrate is immersed in a cold solution of ethanol, in other words at a temperature less than the ambient temperature that may be within the [0° C.; 25° C.] interval and preferably close to 0° C.;
Step 2: The porous substrate is rinsed in cold water, for example in water at a temperature within the [0° C.; 25° C.] interval and preferably close to 0° C.;
Step 3: The porous substrate is immersed in the deposition solution held at low temperature, i.e. within the [0° C.; 25° C.] interval and preferably approximately 0° C.
Step 4: The deposition solution is removed from the surface of the substrate 10 by centrifuging;
Step 5: the solution trapped inside the pores forms the source of the Ni deposition. The solution thus trapped in the pores may then be heated, by immersing the substrate in warm water, for example above 25° C.
With this possible embodiment, the material to be metalized is placed in the deposition solution at a low temperature close to 0° C. and under conditions in which a deposition can be initiated at a deposition rate of the metallic material less than or equal to 0.1 nm/min. Centrifuging is then done in order to remove the deposition solution from the surface of the porous material, while keeping this solution inside the pores.
The porous material is dipped once again in a bath at a temperature of more than 25° C. so that the deposition rate is faster in this case.
In this case, there is no risk of clogging the pore inlets. Since the solution is already inside the pores, the deposition is made uniformly throughout the thickness of the substrate.
This sequence of steps that has just been described may possibly be repeated one or several times.
As a variant of step 5, the substrate is placed on a heating support or a retaining plate (chuck) while water is poured on the substrate.
A method according to the invention is particularly applicable to porous Si but it may also be applied to filling of other types of porous materials, for example for which a porosity is as shown in
The aspect ratio H/d of cracks 11 may for example be at least 100, for example of the order of 1000.
The metallization method may in particular be applicable to metallization of structures obtained by lithography and etching as used in microelectronic systems or microsystems.
A low deposition rate of the metallic material, and particularly a deposition rate of less than or equal to 0.1 nm/min, is applied in each of the example methods described above.
Such a deposition rate may be determined for example by means of images obtained for example by scanning microscopy on which thicknesses of metallic material deposited at different instants are measured and are then related to a time elapsed between these instants.
Another method of measuring the deposition rate may for example be to weigh the porous structure on which the deposition is made at different instants, and then relating the difference in mass obtained to a time elapsed between these instants.
| Number | Date | Country | Kind |
|---|---|---|---|
| 13 61849 | Nov 2013 | FR | national |
