MOLECULAR DOPING

Abstract

Method of doping a semiconductor sample in a uniform and carbon-free way, wherein said sample has a surface, comprising the following steps: A. removing oxides from at least part of the said surface; B. dip coating said at least part of the surface of the sample in a dopant based carbon-free solution of at least one dopant based carbon free substance diluted in water, wherein said at least one dopant based carbon free substance has a molecule comprising at least one dopant atom, wherein the dip coating is achieved by heating said dopant based carbon-free solution at a dip coating temperature from 65% to 100% of the boiling temperature of said dopant based carbon-free solution, thereby a self-assembled mono-layer including dopant atoms is formed; C. annealing said sample, wherein the annealing is configured to cause said dopant atoms included in said self-assembled mono-layer to be diffused into the sample.

Claims

1. Method of doping a semiconductor sample in a uniform and carbon-free way, wherein said sample has a surface, comprising the following steps: A. removing oxides from at least part of the said surface; B. dip coating said at least part of the surface of the sample in a dopant based carbon-free solution of at least one dopant based carbon free substance diluted in water, wherein said at least one dopant based carbon free substance has a molecule comprising at least one dopant atom, wherein the dip coating is achieved by heating said dopant based carbon-free solution at a dip coating temperature ranging from 65% to 100% of the boiling temperature of said dopant based carbon-free solution, thereby the dip coating is configured to form a self-assembled mono-layer including dopant atoms; C. annealing said sample, wherein the annealing is configured to cause said dopant atoms included in said self-assembled mono-layer to be diffused into the sample.

2. Method according to claim 1, wherein said dip coating temperature ranges from 80% to 100%, optionally from 90% to 100%, of the boiling temperature of said dopant based carbon-free solution, wherein said dip coating temperature is more optionally equal to the boiling temperature of said dopant based carbon-free solution.

3. Method according to claim 2, wherein, in step B, said dopant based carbon-free solution is boiled for a period of time ranging from 2 to 3 hours.

4. Method according to any one of the preceding claims, wherein, in step B, the dip coating step is configured to form one or more layers including dopant atoms on top of said self-assembled mono-layer.

5. Method according to any one of the preceding claims, wherein said at least one dopant atom comprised in said molecule is double linked to a corresponding oxygen atom.

6. Method according to any one of the preceding claims, wherein said at least one dopant atom is selected from the group comprising or consisting of phosphor and boron.

7. Method according to claim 6, wherein said at least one dopant based carbon free substance is selected from the group comprising or consisting of phosphoric acid, boric acid and meta boric acid.

8. Method according to any one of the preceding claims, wherein said semiconductor is selected from the group comprising or consisting of silicon (Si), gallium nitride (GaN), gallium arsenide (GaAs), germanium (Ge), silicon carbide (SIC), graphene, silicene, germanene, stanene and elemental two-dimensional materials.

9. Method according to any one of the preceding claims, wherein said dopant based carbon-free solution comprises a volume of phosphoric acid ranging from 15% and 30%, optionally between 17% and 25%, more optionally between 18% and 22%, even more optionally of 20% or a volume of boric acid and/or metaboric acid ranging from 15% and 30%, optionally between 17% and 25%, more optionally between 18% and 22%, even more optionally of 20%.

10. Method according to any one of the preceding claims, wherein said dopant based carbon-free solution comprises a volume of water ranging from 70% and 85%, optionally from 75% and 83%, more optionally from 78% and 82%, more optionally 80%.

11. Method according to any one of the preceding claims, wherein said step C is achieved by heating said sample at a temperature ranging from 1000° C. to 1100° C. for a period ranging from not less than 5 sec, optionally from not less than 20 sec, more optionally not more than 1000 sec, still more optionally not more than 500 sec.

12. Method according to any one of the preceding claims, wherein before step A, the following step is executed: D. cleaning said at least part of the surface to remove the organic and non-organic surface contaminants.

13. Method according to claim 12, wherein said step D is achieved by cleaning said at least part of the surface in ultrasonic in one or more cleaning solutions selected from the group comprising acetone, alcohol and water.

14. Method according to claim 12, wherein said step D is achieved by executing a cleaning sequence of a first sub-step of dipping said at least part of the surface in an acetone solution, a second sub-step of extraction from the solution and drying said at least part of the surface, a third sub-step of dipping said at least part of the surface in an alcohol solution, a fourth sub-step of extraction from the solution and drying said at least part of the surface, a fifth sub-step of dipping said at least part of the surface in an aqueous solution, and a sixth sub-step of extraction from the solution and drying said at least part of the surface.

15. Method according to any one of the preceding claims, wherein said step A is achieved by immersing said at least part of the surface in a hydrofluoric acid solution.

16. Method according to any one of the preceding claims, wherein the following step is performed before said step C: E. capping said at least part of the surface with a layer of oxide.

17. Method according to claim 16, wherein said step E is achieved by depositing a Spin on Glass procedure or by Chemical Vapor Deposition.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0069] The present invention will be now described, by way of illustration and not by way of limitation, according to its preferred embodiments, by particularly referring to the Figures of the annexed drawings, in which:

[0070] FIG. 1 shows a flow chart of the preferred embodiment of the doping method according to the invention;

[0071] FIG. 2 shows a schematic view of SRP profiles of samples doped according to the doping method of FIG. 1 by phosphoric acid and annealed at 1050° C. for 500 sec;

[0072] FIG. 3 shows a schematic view of a) Carrier dose and b) Sheet Resistance calculated from SRP profile of the samples doped by PA according to the doping method of FIG. 1 and by DPP according to a prior art method; and

[0073] FIG. 4 shows a schematic view of a SRP profile of the samples doped according to the doping method of FIG. 1 by phosphoric acid and annealed at 1050° C. for 20 sec.

[0074] In the Figures identical reference numerals will be used for alike elements.

DETAILED DESCRIPTION OF THE INVENTION

[0075] In the following of the present description, the invention will be disclosed with reference to silicon as semiconductor material of which the sample is made of and to phosphoric acid as a n-type molecular dopant precursor. However, it should be noted that the method according to the invention is also applicable to other types of semiconductor materials such as GaN, Ge, GaAs, SiC, graphene, silicene, germanene, stanene, other elemental two-dimensional materials and other dopant molecules may be used in case of p-type molecular doping such as boric acid and/or metaboric acid.

[0076] With reference to FIG. 1, the first embodiment of the method according to the invention dopes a silicon (Si) sample in a uniform and carbon-free way, wherein said sample has a surface, and comprises the following steps: [0077] A. removing (200) oxides from at least part of the said surface; [0078] B. dip coating (300) said at least part of the surface of the sample in a solution of phosphoric acid diluted in water; and [0079] C. annealing (400) said sample.

[0080] In step A, the preferred method for removing the native silicon dioxide (SiO.sub.2) naturally present on the Si surface includes immersing the sample in a hydrofluoric acid solution (HF).

[0081] In step B, the dip coating is configured to form a self-assembled mono-layer including dopant atoms, namely phosphorous atoms, and such dip coating is achieved by boiling in said solution said at least part of the surface of the sample.

[0082] Preferably, the sample is immersed in a 20% solution of phosphoric acid in water and boiled at a temperature of 120° C. for a period of time ranging from 2 to 3 hours, optionally substantially equal to 2,5 hours.

[0083] The deposition process takes place at the boiling point of the solution, guaranteeing two advantages: the formation of the chemical bond between the Si atoms and the self-controlled maintenance of the process temperature, since all the heat supplied is absorbed by the boiling phenomenon. The higher the similarity of the boiling temperatures of solvent and solute in the solution is, the better the control of the dopant distribution on the sample surface.

[0084] The boiling temperature of the solution, composed by the solvent and the solute, depends on the chosen solvent and solute and on the atmospheric pressure conditions. Under given atmospheric pressure, the boiling temperature of the solution depends on the relative concentration of the two components of the solution. In the specific case of water and phosphoric acid at 1 atm (1,01325 10.sup.5 Pascal), the boiling point of the solution, at the mentioned concentration of 20% of phosphoric acid in water, ranges between 120° C.-130° C. These values were predicted first by a calculation, and then also experimentally measured during the process. Bringing the solution to boiling, energy is supplied to the molecules which forms a covalent bond with the semiconductor substrate, silicon in the preferred embodiment of the method according to the invention, forming a first organized self-assembled monolayer on the sample surface.

[0085] In the phosphoric acid the phosphorus (P) is bound to an oxygen (O) atom through a double bond that is energetically favoured for the formation of the bond with the silicon (Si).

[0086] In order to have the formation of a monolayer, chemically bonded to the substrate, it is necessary to break the double bond P═O and simultaneously create a bond between P, O and Si. in this way, the molecules are directly linked to the substrate by chemical bonds, the molecules that will be located above the monolayer will be bound by physical bonds.

[0087] The experiments show that during the deposition process for MD the formation of both a monolayer chemically bonded to the substrate and of the multilayers physically linked to it may possibly occur.

[0088] From the results of the experiments, it appears that the formation process of further upper multilayers, since they are weakly bound to the underlying molecules, does not depend on the boiling temperature of the solution, but on the process time. In particular, on top of the first SAM bonded to the substrate, it cannot be ensured that the other layers formed over the first one are SAMs as well because of the possible intervention of physical bonds along with chemical bonds.

[0089] When the deposited layer forms a single self-assembled layer, the molecules are spaced therein according to their steric bulk. As a result, their mutual distance is pre-determined and controlled by their size. This involves a very high intrinsic order and uniform distribution of the source atoms on the surface of the target allowing to control the amount of initial dose of dopant and of the finally incorporated dopant in the substrate that can be therefore pre-determined once the molecule is designed. During this process, the molecule bonds to the target surface with a self-limiting process ruled by its steric properties. The molecular footprint of the precursor directly governs the surface concentration of the dopants with larger molecules resulting in a lower dose.

[0090] It must be noted that other embodiments of the invention have that, in step B, a dip coating may also be carried out by heating the solution at a dip coating temperature lower than the solution boiling temperature. In general, according to the invention, in step B, such dip coating temperature ranges from 65% to 100% of the solution boiling temperature (i.e. from about 80° C. to about 125° C. for a 20% solution of phosphoric acid in water, and from about 68° C. to about 105° C. for a similar solution of boric acid in water); optionally, such dip coating temperature can range from 80% to 100% of the solution boiling temperature (i.e. from about 100° C. to about 125° C. for a 20% solution of phosphoric acid in water, and from about 84° C. to about 105° C. for a similar solution of boric acid in water); more optionally such dip coating temperature can range from 90% to 100% of the solution boiling temperature (i.e. from about 112° C. to about 125° C. for a 20% solution of phosphoric acid in water, and from about 95° C. to about 105° C. for a similar solution of boric acid in water). In particular, molecular doping also takes place when this dip coating temperature in step B is below the solution boiling temperature, although method optimization is achieved when such dip coating temperature is the solution boiling temperature.

[0091] The inventors have conducted experiments by also exploring the results obtained when the solution is kept at room temperature in order to verify possible effectiveness of this simplified condition, together with several other dilutions. By bringing the solution to boiling temperature, the process of adhesion of the molecule to the substrate and consequent formation of the chemical bond is effective, with respect to the case at room temperature. With regard to the solution concentrations, the experiments showed that the proposed doping method works also when the dip coating is achieved by boiling the sample in PA only.

[0092] However, further experiments showed that a better efficacy in terms of simplicity and low cost is achieved when the sample is dipped in an aqueous solution with a PA concentration between 15% and 30%, optionally between 17% and 25%, more optionally between 18% and 22%, even more optionally with a PA concentration of 20%.

[0093] In step C, the annealing is configured to cause said dopant atoms included in said self-assembled mono-layer to be diffused into the sample. In particular, the annealing step is achieved by heating said sample at a temperature ranging from 1000° C. to 1100° C. for a period ranging from not less than 5 sec, optionally from not less than 20 sec, more optionally not more than 1000 sec, even more optionally not more than 500 sec.

[0094] To test the efficacy of PA as a molecular precursor, the method has been tested on silicon, using methods similar to those of MD, to exploit the low cost, simplicity, speed of process and conformity to conventional technologies, without the need of using expensive equipment and materials which are dangerous for the environment and operators.

[0095] A characterization of the doped samples has been carried out as follows. In order to measure the concentration of electrically active carriers, which represents the yield of the method, the chosen methodology was the Spreading Resistance Profiling (SRP). Said measure allows an advanced carrier profiling as a function of the sample depth. FIGS. 2 and 4 show the results obtained on four different samples doped with phosphoric acid and processed at the same annealing temperature after the deposition. The curves are superimposed, which demonstrates a high repeatability of the method.

[0096] In the case of FIG. 2 the dose is about 6.5×10.sup.15 atoms /cm.sup.2 and the junction depth reaches more than 1.4 microns. The differences are remarkable by comparing these samples with those doped with DPP in mesitylene as shown in FIG. 3(a). Experimental results demonstrate outstanding doping efficiency with a concentration of peak electrical carriers of 1×10.sup.20 atoms/cm.sup.3. Indeed, in this case the dose values obtained with the PA doping method are more than one order of magnitude greater than the DPP method, moreover the junction depth values are about 5-6 times larger and the Rs (the layer or Sheet Resistance, see FIG. 3(b)) is less than one order of magnitude. Moreover, as can be seen in the FIG. 3(a), the samples doped with phosphoric acid have a lower error bar than DPP-doped samples, wherein the error bars have been quantified as the standard deviation calculated on the statistics of the several samples.

[0097] In order to verify whether the diffusion length of the charge carriers measured in samples doped with PA can be modulated through the annealing time, a second process was carried out by varying the annealing time from 500 seconds to 20 seconds. Also in this case, the samples were characterized by SRP and the profiles were shown in FIG. 4. Even in this case, the profiles obtained are overlapped between them and, by comparing them with the previous data, it is possible to note that the junction depth value is less (about 1 micron, against 1,4 microns in previous data), while the dose is about half of that obtained in the case of 500 sec (3×10.sup.15 atoms/cm.sup.2). Thus, the obtained data show that the heating condition at 1050° C. for 20 sec can be an alternative process.

[0098] The promising results obtained by PA can be attributed to the small size of the molecule of phosphoric acid with respect to the DPP, which allow a greater packing and consequently a higher surface density of dopant atoms.

[0099] As anticipated, the semiconductor is silicon, but the method is also applicable to other types of semiconductor materials such as GaN, Ge, GaAs, SIC, graphene, silicene, germanene, stanene and other elemental two-dimensional materials.

[0100] In other embodiments, said step B is carried out as follows. Once the native oxide has been removed from the surface of the sample, said sample is dip coated in a dopant carbon-free solution including a dopant phosphoric acid diluted in water wherein the concentration of said acid optionally ranges in volume from of 15% and 30%, optionally between 17% and 25%, more optionally between 18% and 22%, even more optionally of 20%.

[0101] In other embodiments of the method, optionally, the volume of water of said solution ranges from 70% and 85%, optionally from 75% and 83%, more optionally ranges from 78% and 82%, more optionally with a water concentration of 80%.

[0102] In further embodiments of the invention said at least one dopant is boron and the dopant based carbon free substance may be selected from the group comprising or consisting of boric acid or metaboric acid. Said precursors are chosen because their molecular structures, as in the PA case, are composed only of oxygen, hydrogen and boron, no carbon atom is present in order to avoid introduction of unwanted contaminants inside the substrate during the annealing process.

[0103] The concentration of said dopant boric and/or metaboric acid diluted in water, ranges in volume from of 15% and 30%, optionally between 17% and 25%, more optionally between 18% and 22%, even more optionally of 20%.

[0104] In the case of water and boric acid and/or metaboric acid, the boiling point of the solution, at the selected concentration, ranges from 102° C. to 110° C.

[0105] As shown in FIG. 1, before step A, the following optional step D may be executed: cleaning (100) said at least part of the surface to remove the organic and non-organic surface contaminants.

[0106] In some embodiments of the invention, said step D is carried out by cleaning said at least part of the surface in ultrasonic in one or more cleaning solutions selected from the group comprising acetone, alcohol and water.

[0107] According to further embodiments of the invention, step D is achieved by executing the following cleaning sequence: [0108] dipping said at least part of the surface in an acetone solution, [0109] extraction from the solution and drying said at least part of the surface, [0110] dipping said at least part of the surface in an alcohol solution, [0111] extraction from the solution and drying said at least part of the surface, [0112] dipping said at least part of the surface in an aqueous solution, [0113] extraction from the solution and drying said at least part of the surface.

[0114] However, semiconductor samples already cleaned can be provided, making it clear that the contaminants cleaning and the oxide removal is not an essential feature of the method according to the invention. Possibly, already cleaned samples and/or samples from which oxides have been already removed may be provided in order to have the doping steps of the method executed.

[0115] Furthermore, as shown in FIG. 1, a protective layer can be applied to the doped sample surface. In this case, the following step E is performed before step C: capping (500) said least part of the surface with a layer of oxide. Said capping layer prevents molecular components, such as phosphorus and boron, from evaporate from the surface and it also ensures the dopant atoms' diffusion towards the semiconductor. In this case, the evaporation is reduced and the diffusion phenomenon prevails. Said capping layer can be applied on the surface samples through method such as spin on glass deposition procedure or by chemical vapor deposition.

[0116] The preferred embodiments of this invention have been described and a number of variations have been suggested hereinbefore, but it should be understood that those skilled in the art can make variations and changes, without so departing from the scope of protection thereof, as defined by the attached claims.