Assembly for the deposition of silicon nanostructures

Abstract

An assembly for the deposition of silicon nanostructures comprising a deposition chamber, which is defined by a side wall and by two end walls; a microwave generator, which is adapted to generate microwaves inside the deposition chamber; an electromagnetic termination wall, made of a conductor material and reflecting the microwave radiation, which is such as to create a termination for a TE-mode waveguide and is housed inside the deposition chamber; and a substrate-carrier support, which is made of a dielectric material and on which the substrate is housed on which to perform the growth of silicon nanostructures. The substrate-carrier support is arranged inside the deposition chamber above the termination wall.

Claims

1. An assembly (1) for the deposition of silicon nanostructures comprising a deposition chamber (2), which is defined by a side wall (3) and by two end walls (4); said assembly (1) being characterized in that it comprises: a microwave generator (5), which is adapted to generate microwaves inside the deposition chamber (2), an electromagnetic termination wall (8), made of a conductor material and reflecting the microwave radiation, which is such as to create a termination for a TE-mode waveguide and is housed inside the deposition chamber (2), and a substrate-carrier support (9), which is made of a dielectric material and on which the substrate is housed on which to perform the growth of silicon nanostructures; said substrate-carrier support (9) being arranged inside the deposition chamber (2) above said termination wall (8); and a metal layer arranged on a substrate (S) where the deposition is going to take place; said metal layer being arranged so as to cover a surface of the substrate (S) where the growth of nanowires is not wanted.

2. The assembly (1) for the deposition of silicon nanostructures according to claim 1, characterized in that said termination wall (8) is made of a susceptor material.

3. The assembly (1) for the deposition of silicon nanostructures according to claim 2, characterized in that said termination wall (8) is made of graphite.

4. The assembly (1) for the deposition of silicon nanostructures according to claim 1, characterized in that said side wall (3) and said two end walls (4) are made of steel.

5. The assembly (1) for the deposition of silicon nanostructures according to claim 1, characterized in that in said side wall (3) two openings (6, 7) are obtained, which are adapted to be connected to a system to introduce gas into the deposition chamber (2) and to a vacuum pump, which is such as to generate a depression inside the deposition chamber (2) ranging from 0.1 to 100 mbar.

6. The assembly (1) for the deposition of silicon nanostructures according to claim 1, characterized in that said substrate-carrier support (9) is made of ceramic.

7. The assembly (1) for the deposition of silicon nanostructures according to claim 6, characterized in that said substrate-carrier support (9) is made of boron nitride.

8. The assembly (1) for the deposition of silicon nanostructures according to claim 1, characterized in that the conditions of deposition through microwave excitation ensure that the plasma does not ignite.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) An embodiment example is given below by way of non-limiting illustration with the help of the attached figures, in which:

(2) FIG. 1 is a schematic view of a preferred embodiment of the present invention;

(3) FIG. 2 is an enlargement of a detail of FIG. 1;

(4) FIG. 3 is a SEM image of a sample produced with the assembly of the present invention; and

(5) FIG. 4 is an image that shows evidence of the heating process of the termination wall made of a susceptor material obtained by means of image with pyrometer.

BEST MODE FOR CARRYING OUT THE INVENTION

(6) In FIG. 1, the number 1 indicates overall the assembly for the deposition of silicon nanostructures according to the present invention.

(7) The assembly 1 comprises a deposition chamber 2 defined by a cylindrically shaped side wall 3 and by two end walls 4a and 4b arranged to close the side wall 3. The side wall 3 and the two end walls 4a and 4b are made of steel. The assembly 1 comprises a microwave generator 5, for example a magnetron commercial generator at 2.45 GHz, illustrated only partially and useful for generation of the microwaves inside the deposition chamber 2. In particular, the microwave generator 5 faces the upper end wall 4a, which is made of quartz so as to allow the passage of the microwaves, at the same time ensuring the sealing of the vacuum in the deposition chamber 2.

(8) In the side wall 3 two openings 6 are obtained in use connected to a system (known and not described for the sake of simplicity) to introduce silicon precursor gas into the deposition chamber 2 and at least one opening 7 in use connected to a vacuum pump (known and not described for the sake of simplicity) to generate inside the deposition chamber 2 a depression ranging between 0.1 and 100 mbar.

(9) As is also illustrated in FIG. 2, the assembly 1 according to the present invention comprises an electromagnetic termination wall 8, made of conducting material, reflecting the microwave radiation and housed inside the deposition chamber 2. Here and below, by “electromagnetic termination wall” we mean a reflecting conductive wall such as to represent a termination of the TE-mode waveguide.

(10) Preferably, the electromagnetic termination wall 8 is made of a susceptor material such as, for example, graphite. In this case the termination wall 8 also has the function of representing also a first level heater of the sample. In fact, the graphite is heated by radiofrequency induction.

(11) The assembly 1 according to the present invention comprises a substrate-carrier support 9 made of dielectric material and on which the substrate S is housed (such as, for example, an electronic circuit) on which to perform the growth of the SiNWs. As illustrated in FIG. 2, the substrate-carrier support 9 is arranged above the electromagnetic termination wall 8 with a free surface facing the microwave generator 5.

(12) The substrate-carrier support 8, being made of dielectric material, is able to withstand the process temperatures, and must also be able to withstand the chemical reactivity of the silane.

(13) Preferably, the substrate-carrier support 9 can be made of ceramic, for example boron nitride.

(14) In particular, the substrate-carrier support 9 has the purpose of spacing the substrate, on which the droplets of catalyst metal are deposited, from the conductive termination wall 8 where the tangential electric field is annulled. The presence of an electric field other than zero, due to the presence of the dielectric material, allows heating of the metal droplets.

(15) In fact, at the microwave frequencies only the conduction on the surface of the metal materials is activated, according to the so-called “skin effect”. At the frequency of 2.45 GHz, for example, the conduction in aluminium occurs only in the first micron of depth from the surface exposed to the radiation.

(16) Furthermore, the walls of the chamber, thick and very conductive, behave, to a good approximation, like ideal metals, by annulling the tangential electric field of the radiation on the surface.

(17) In other words, the assembly subject of the present invention is structured to perform a deposition of SiNWs by conveying the microwaves into a cylindrical chamber made of steel, with controlled environment and precursor gas flow. In particular, as will be obvious and easy to implement for a person skilled in the art, the sizes of the deposition chamber 2 must be chosen so as to cause propagation of only the first electromagnetic TE mode, at the frequency of the generator microwaves and, therefore, ensure good uniformity of the field in all its sections.

(18) Lastly, the deposition conditions must ensure that the plasma does not ignite due to the microwaves. In fact, the plasma, producing ionization of the gas, would produce deposition of amorphous material, inhibiting or covering the growth of the crystalline nanostructures, in particular of the nanowires. Also this precaution is part of the normal practice of a person skilled in the art.

(19) The characterizing element of the deposition assembly according to the present invention consists in the presence of the substrate-carrier support 9 made of dielectric material and adapted to maintain the support on which to perform the growth of SiNWs in a position separate from the surface of the termination wall 8. In this way, due to the presence of the dielectric material, the layer of nanometric droplets of catalyst metal is spaced from the terminal conductive surface and the tangential electric field is not null. Under the action of the tangential electric field, the metal droplets absorb the EM energy and heat up while the substrate, made of silicon for example, having a lower conductivity than the metal droplets, heats up to a lesser extent.

(20) In principle, in an integrated circuit also the circuit substrate, made of silicon with normally moderate doping, can be considered at the frequencies of the microwaves a dielectric which, therefore, offers a separation between the layer of metal droplets and the electromagnetic termination wall 8. However, said separation is small due to the reduced thickness of the substrate usually used in integrated circuits, and therefore much less effective than the separation offered by the presence of the substrate-carrier support 9.

(21) The metal connection layers on the surface of the substrate (for example CMOS chip), on which the growth of the nanowires will be performed, can be screened by a thicker metal layer. This metal layer, made for example of silver, and with a thickness varying between 200-300 nm, is able to withstand the currents induced by the electromagnetic field, reflecting it, and therefore without heating up. The assembly according to the present invention therefore guarantees that heating of portions of substrate is avoided by covering it with thicker highly conductive metal layers, of silver for example. The presence of said metal layers, in fact, reduces absorption of the electromagnetic energy, which is reflected to a greater extent.

(22) In addition to avoiding damage to the underlying structure, the lesser heating avoids the growth on said areas, allowing accurate definition of the areas deposited.

(23) An embodiment example obtained with the assembly subject of the present invention is described below.

(24) Floating zone type silicon substrates were used measuring 1 cm.sup.2 with thickness 250 micron and orientation 1.0.0. The silicon wafers were cleaned with an RCA solution. Subsequently, the silicon substrates were oxidized with a wet process at 300° C.

(25) 5 nm of tin were deposited outside the deposition chamber.

(26) The substrate was then heated to 450° C. in a vacuum chamber with pressure 1×10.sup.−6 mbar using a rapid heating regime to ensure very small droplets. The final hardening was achieved in 8 minutes. The temperature was measured with a thermocouple in contact with the lower side of the substrate.

(27) After slow cooling (30 minutes) at 200° C., the surface of the substrate was heated by means of microwave irradiation using a power of 450 W. Due to this heating on the portion of termination wall covered with susceptor material, a temperature of 712° C. is reached while the substrate is at the temperature of 200° C. See FIG. 4.

(28) For this measurement the coefficient of emissivity was chosen equal to 0.3 obtained by comparing the T measured by the thermocouple and by the pyrometer in stationary conditions.

(29) After the cooling, the sample was exposed to a flow of pure SiH.sub.4 of 4.5 sccm with a pressure in the chamber of 3 mbar. The microwave generator was then switched on at a power of 250 W.

(30) The electromagnetic coupling was adjusted to increase the reflection coefficient to 75% ensuring that the plasma was not activated.

(31) In these conditions a droplet temperature of 485° C. was estimated higher than the eutectic point of the Si—Sn alloy.

(32) The effect of the microwaves was to cause heating of the droplets of Sn beyond the T of the eutectic and, consequently, triggering of the VLP mechanism of growth of the SiNWs. Figure shows the nanowires obtained with a power of 250 W and an estimated T of 450° C. This sample shows the growth of regular nanowires with a diameter of 10 nm and the uniform presence of metal droplets at the top of the Si wires. It has been found that by increasing the power, an excess of material is deposited which covers the droplets.

(33) As is evident from the above description, the assembly for the deposition of SiNWs according to the present invention allows the use of very widespread processes for the production of sensors with high production levels, and therefore limited costs.

(34) The availability of a technology for the production of sensors integrated in CMOS circuits allows the production of sensors directly connected to the control and reading circuits, if necessary also to the circuits for transmission of the information from the sensor towards a control radio base station.

(35) Said invention allows sensors to be obtained distributed in the environment, for monitoring climatic conditions or contaminants, or biological sensors in direct contact with the patient or implantable.

(36) In particular, the assembly of the present invention is advantageously applied in the production of CMOS technology integrated chemical and biological sensors, in the production of batteries and in the production of photovoltaic cells.