WO2012065825A2

WO2012065825A2 - Nanostructured semiconductor materials, method for the manufacture thereof and current pulse generator for carrying out said method

Info

Publication number: WO2012065825A2
Application number: PCT/EP2011/068940
Authority: WO
Inventors: Yauhenyia Matveyeva; Yuri Makushok; Ester Lorena Pastor Galiano
Original assignee: Em-Silicon Nano-Technologies, S.L.
Priority date: 2010-10-29
Filing date: 2011-10-28
Publication date: 2012-05-24
Also published as: WO2012065825A3

Abstract

The present invention relates to a method for electrochemical treatments of a semiconductor material and the electrical pulse generator for said method used in the production of three-dimensional structures integrated in a wafer of semiconductor material or free standing nanostructured particles, characterized in that it comprises at least one step for the anodic formation of one or multiple porous layers etched in a semiconductor by means of applying electric current pulses with modulation of its intensity in time, a step of lifting the layers from the substrate by ultrasounds or mechanically and, optionally, a step of mechanical grinding.

Description

NANOSTRUCTURED SEMICONDUCTOR MATERIALS. METHOD FOR THE MANUFACTURE THEREOF AND CURRENT PULSE GENERATOR FOR CARRYING OUT SAID METHOD

Field of the Invention

The present invention is applied to the field of production of nanostructured semiconductors, it more specifically relates to a process and an apparatus that generates electric current of a modulated time profile used in obtaining said nanostructured material in the form of porous multilayers or free-standing nanoparticles having controlled shapes and sizes.

Background of the Invention

Nanostructured materials have particle size or sizes of other dimensional elements such as pores in the nanometer range. The dimensions of these elements are called nanostructuring parameters (each parameter being a different nanostructuring level) and determine the properties of the material. A widely used nanostructured material today is porous silicon.

Essentially two methods are currently used to prepare porous silicon: the chemical method and the electrochemical method. In both cases, porous silicon is formed on a crystalline silicon substrate in form of a layer attached to the substrate.

The electrochemical method consists of anodizing the silicon wafer in hydrofluoric acid solutions by means of applying a constant external current. Methods of these characteristics are described in some scientific publications, for example in Volker Lehmann, Electrochemistry of Silicon, Instrumentation, Science, Materials and Applications, Wiley-VCH Verlag GmbH, 2002, pp. 277. Obtaining the layers of porous silicon with different porosities by means of applying currents with different intensities is described in the publication by S. Lee et al. Optical Properties of Multilayered Porous-Si Formed by Current Pulse Modulation During Electrochemical Anodization", Journal of Korean Physical Society, 2005 Vol. 47 no. 5, pp. 876-879. By means of these processes, however, it is not possible to obtain free-standing particles of porous silicon.

To obtain the free-standing porous particles it is necessary first to separate the porous material from the wafer-substrate and second, grind it. Porous material can be separated manually or with the help of ultrasounds or by application of a strong short electrical pulse after formation of the porous layer.

On the other hand, in fabrication of porous material in form of particles, mechanical grinding applied in the step of particles production has no correlation with electrical regimes applied for formation of porous material and therefore the shape and sizes of the resulting particles are neither related nor controlled by the used electrical regimens. The distribution of resulting particles by sizes can be broad, which is an obstacle for many biomedical applications where the control of not only the size of the particles but also of their geometric shape is required.

In order to limit the distribution of particles by sizes and to make them as similar as possible, photolithographic methods are applied (WO 2008/134637 A1 ). Porous particles equal both in size and in shape defined by photolithographic masks used in the steps prior to anodizing of the silicon substrate are thus obtained. The drawback of this method is the scarce possibility to scale up the production: this method is limited to the use of only 2-5 microns of the entire thickness of the wafer of about 500 microns because the remaining material of the wafer is discarded. In other words, no more than 1 % of the material from the silicon substrate serves for the production of porous particles.

On the other hand, in complex electrochemical treatments that include not only formation of the pores but also separation the porous layer off the substrate, the used electrical equipment must provide the current densities and its time modulation broad and fast enough. Most existing electrical apparatuses have fixed output parameters. An example based on this concept is described in the US patent

US 2009/0166218 A1 . Consequently, existing electrical equipment can be very limited due to restriction of current modulation: the maximum current and the time response of the apparatus are not broad and/or fast enough. This drawback is especially critical when the method extends to the use of the commercial size silicon wafers (50, 100, 150, 200 mm in diameter), which multiplies needs for current.

Object of the Invention

The object of the invention is to palliate the technical problems in the fabrication of nanostructured materials mentioned in the preceding section. To do so the invention proposes using electrical regimes modulated in time which are programmed and formed with the help of a modular and expandable electrical apparatus - pulse generator.

One aspect of the invention relates to a method for the electrochemical treatment of a semiconductor material for production of layers or free-standing particles of nanostructured material, characterized in that it comprises the steps for the anodic formation of a porous layer on a conductor by means of applying periodic current pulses consisted of several levels of current, lifting the porous material off in an inert solvent and mechanical grinding the material. Each periodic pulse preferably comprises three levels of current during three periods of time, a first level of intensity 11 for the formation of an individual actual porous layer with the thickness and the porosity determined by time t1 , a second level of current I2 and time t2 for the pre-cutting of the porous layer from the substrate and a third level of current I3 and time t3 for relaxation of the system.

In the present invention, said three levels of current 11 , I2 and I3 can be of constant or variable current values. In the case that some of these three current values (11 , I2, and I3) are variable, they can fluctuate, for example, in the form of a rump, a sine-wave, or in any other form that enables obtaining the features required under each level of current.

The modular pulse generator for application of such methods comprises of at least one power supply, one power amplifier and one precise measuring unit, all integrated at the level of their logical signals through a device - the interconnect bus, adopted for logically intercommunication of these modules one with another, safely and independently on the output parameters of each of them. The interconnect bus comprises in its turn of the system level bus and of at least one channel level bus.

The porous particles produced by the method according to this invention have a predetermined planar geometry that favors their use in pharmacological prolonged and controlled release systems and in other applications. The nanostructured material can additionally carry on its surface additives of different chemical compositions introduced during the anodizing step of the initial semiconductor material or the grinding step thereafter.

The invention seeks to reduce deficiencies in uniformity of each porous layer and to improve the quality of the interfaces in the fabricated nanostructured multilayers attached to the substrate or free-standing. In the production of free particles, the invention seeks to introduce correlations between electrical regimens and final particle sizes and shapes, to limit their distribution by sizes and to scale the manufacturing process up. All this is achieved with the help of a pulse generator that integrates in a single apparatus several pieces of electrical equipment and is controlled through the interconnect bus which enables secure communication between modules regardless of their output parameters. Using this pulse generator enables the scalable production of nanostructured materials both in form of porous multilayers attached to the substrate or free-standing and in form of planar free porous particles with sizes and shapes determined by the applied electrical regimen.

Brief Description of the Drawings

For the purpose of better understanding the features of the invention according to a preferred practical embodiment thereof, a set of drawings is attached to the following description in which the next has been depicted as an illustration:

Figure 1 shows two cases A and B of the standard regime of electrochemical treatments for pores formation under current of a single constant level 11 applied for a duration t1 . The values of these parameters determine the porosity and the thickness of the porous layer.

Figure 2 shows an example of the complex electrochemical regime applied in three periods/cycles, N=3, each consisted of two levels of constant current.

Figure 3 shows a preferred example according to this invention for the formation of sequential identical layers (N=3) of porous material in a complex regime with time modulation of current in three levels in each pulse. The layers are formed during application of currents of level 1 . Levels 2 (pre-cutting) and 3 (relaxation) are technological levels and serve to improve the quality of each layer - to homogenize the distribution of pores and their sizes with the depth of treatment and to further form a clear separation between these sequential layers.

Figure 4 shows a preferred example of a complex regime for the formation of porous material in different and periodic sequential layers. The time modulation of the current in each pulse is applied in several technological levels, three in this example. More than one pulse forms a period, two in this example.

Figure 5 shows the scheme of how electrical equipment for electrochemical treatments has been developing from simple power supplies (PS) which generate constant current and are used in the research, to complex generators composed of several components, or modules, that are integrated one with another with the help of electronic devices, or interconnect and intercommunication buses. The generator made up of several components can have different modular architectures, several output channels and different output parameters per each channel. In this figure, the scheme (1 ) shows the research level consisted of one power supply; the scheme (2) shows a simple technological system with only one output channel; and the scheme (3) shows an advanced technological scheme with the example of one of the possible modes of integrating of four power supplies and four amplifiers in a production line that possesses three output channels.

Figure 6 shows the traditional architecture of a generator for electrochemical applications consisted of one power amplifier, power supplies, measuring modules and a data acquisition system that allows communication between a generator and a computer. To prevent damage between the components, the entire structure is adapted to the fixed input and output parameters of each components/modules.

Figure 7 shows a schematic design of the pulse generator by this invention that allows integrating in a single apparatus the different components/modules with independence respect to their output parameters and without damaging them one by another.

Figure 8 shows some electronic photographs (transverse images) of porous nanostructured material formed: (A) under a standard constant current regime and (B) a complex periodic pulse regime with time modulation of current in pulses of three levels.

Figure 9 shows photographs of planar particles of porous nanostructured material formed in the sequential layers under a complex pulse regime with time modulation of current in pulses of three levels: (A) - optical photograph, (B) - electronic photograph. The layers have been lifted from the substrate, silicon wafer, by ultrasounds without applying the subsequent mechanical grinding.

Figure 10 shows the graph for the particle distribution by sizes in the porous material manufactured: (A) in a standard electrochemical constant current regime and (B) in a complex pulse regimen with time modulation of current in pulses of three levels. The porous material has been manually lifted from the substrate and mechanically ground after that.

Figure 11 shows additional photographs of ground particles manufactured in a complex three-level pulse electrical regime.

Detailed Description of the Invention

The level of the constant current applied in treatments for the formation of pores determines the size of the pores and the porosity of the formed structure. The process is developed in-depth of the substrate, silicon wafer, during the time the current is applied. Figure 1 shows this regime consisting of a single level of current for two cases that are differentiated by the levels of current applied and the duration thereof.

Another known method for generating nanostructured silicon by the electrochemical method uses a more complex electrical regime consisted of repetitive periods/cycles of the current of different levels. Figure 2 generally shows this regime. Two levels of the constant current are applied periodically (three times in the figure) and they form several periods of porous structures in the substrate differentiated by their porosity. The number of areas having different porosity coincides with the number of levels of current used, which are two in the case of Figure 2.

Applications of the periodic porous structures in many cases depend on the quality of the interface between each porous area and/or on the contrast in the porosity. The contrast is achieved by applying electrical currents that are well differentiated by their intensity, as shown in Figure 2.

The propagation of the porous layer deeply in the substrate involves the consumption of the electrolyte at the bottom of the formed pores where the electrolyte is not easily renewed and its composition is gradually altered thereof. This causes the changes in the porosity which corresponds to the determined level of the constant current and the entire porous layer loses its uniformity. As a result, the porosity of the structures changes with depth and the uniformity and the quality of the structures themselves are consequently deteriorated.

The invention proposes using different levels of current in the electrical pulse, whereby the quality of the formed structures is improved and the different nanostructuring parameters in the produced material are accurately controlled. The method further enables scaling the production up to using the app. 90% of the substrate in depth. This special pulse and periodic anodizing regime consisted of repetitive cycles of the current in several levels is described below.

The preferred regime is shown in Figure 3. The current is applied in pulses and each pulse consists of several levels, three in the example. Layers, which are identical in this case, are formed in the substrate by periodically repeating the complex pulse.

The described regime allows performing and controlling the different steps of formation of the porous material throughout each pulse. The first step is the formation of the actual layer with the thickness and the porosity determined by the time t1 and level 11 of the applied current.

The following step is the pre-cutting of the porous layer, and it takes place during the application of the current of level I2 which approximates to the level of silicon electropolishing during time t2. A very thin layer of highly porous material is introduced during the step of pre-cutting. In the case that the current I2 were very high and exceeded the level of silicon electropolishing, its application during a minimum time could separate the porous material from the substrate. In order the structures formed by the proposed method possess sufficient integrity, it is necessary to apply the very short technological pre-cutting level 12 without considerably dissolving the crystalline silicon material. As a result, in a preferred embodiment of this invention the porous material formed during the application of level 12 must have minimum thickness and high porosity, but it cannot be detached from the substrate.

The third step is the relaxation of the porous semiconductor after the electrical stress and consumption of the electrolyte taking place during the preceding steps (level 13 of the current applied during time t3).

The periodic application of equal pulses consisted of three levels allows forming multiple equal layers attached to the substrate. In a subsequent mechanical grinding step, these layers will be fragmented by the predefined and pre-cut planes, allowing thereof the control of the thickness of the resulting particles and their shape- planar, almost disc rather than sphere-shaped.

The porous sequential layers with different parameters will be formed under a pulse regime in which the different levels 11 are applied during different periods of time t1 in different pulses. Figure 4 shows a preferred example for the formation of the porous structure having two periods (N=2) each made up of two layers having different thicknesses and porosities, consisting a total of four layers. The regime applied is of two sequential cycles consisted of two pulses each made up of three levels of current. In each pulse the levels 11 are different and the levels I2 and I3 could be equal. Current level 1 is a technological level for the formation of the porous layer 1 . Current level 2 is a technological pre-cutting level of the pre-cutting step and current level I3 is a relaxation level of the relaxation step. Otherwise the known methods for the formation of multilayers, the introduction of different current levels in the same pulse allows to considerably improve the quality of the entire complex porous structure. Additional levels such as I2 and I3 in Figures 3 and 4 allow increasing the contrast between the sequential porous layers formed, and maintaining the homogeneity of the entire complex porous structure while electrical treatment is developed in-depth of the wafer.

In conclusion, in the proposed method, the appropriate time variation of the applied current allows controlling the interfaces between the sequential layers with different porosities. The quality and the properties of the entire structure are very important parameters in the production of photonic crystals or interferometer sensors based on the porous structures integrated (attached) in monocrystalline silicon or free-standing, and they are improved in the three-level pulse regime. Example of the invention

In an embodiment of the invention, the method applies the periodic pulse regime of three levels that comprise current densities from 0 to 500mA/cm² for the working level, from 300 to 2000mA/cm² for the cutting level and from -50 to 70mA/cm² for the relaxation level. Preferably, the periodic pulse regime of three levels comprises current densities of 40-150mA/cm² for the working or formation level, of 300-500mA/cm² for the cutting level and of -10-OmA/cm² for the relaxation level. While the load ratio between the working level and the cutting level [R(load)] is of 1000:1 to 1 :1 , preferably of 2:1 to 20:1 , the time ratio between the anodic level (working and cutting levels) and the relaxation level [R(a/r)] is of 1 :100 to 500:1 , preferably of 1 :10 to 20:1 , and the total duration of the pulse ranges from 0.001 to 5000 seconds, preferably from 2 to 101 seconds. The multiple layers of porous silicon are herein formed after repeating the cycles (pulses) during the necessary time.

Table 1 Example of the periodic pulse regime in three levels in each pulse for the anodic formation of the multiple identical porous layers on crystalline silicon

In another particular embodiment of the invention, the method can include the anodic formation of a porous layer on crystalline silicon under a time modulated electrochemical regime with the simultaneous deposition of metals on the formed porous layer or introduction on its surface of different functional groups coming from the additives used in the electrolyte (by the electrografting process).

The simultaneous deposition of metals is performed when the salts of determined metals, such as Cu, Ni, Co, Ag, Pd, Pt or others, are added to the standard electrolyte (hydrofluoric acid). During the relaxation time, these metals are deposited in the pores by the contact electroless plating.

The electrografting process can be included during the electrochemical treatment if the electrolyte has specific additives in its composition and if the relaxation step (13, t3 of the complex pulse) performs at negative current.

The deposition of metals in the pores can have several uses, for example as catalysts. The electrografting process chemically modifies the surface of the nanostructured material, which is usually achieved by using additional specific methods after the manufacturing the particles.

The scalability of the technology for production of nanostructured materials is a very important aspect when choosing the manufacturing method. The following table shows the technological requirements for the currents applied in pulse regimes during anodic formation of the nanostructured porous material as a function of the size of the electrochemically treated area.

Table 2. Needs for electric currents and other parameters of interest in treatment of the samples of different sizes

It is understood that with the increase of the area to treat from 1 cm² - the dimension that is routinely used in research laboratories-, to industrial-sized silicon wafers of 50, 100 or 200 mm in diameter, the needs of the currents multiply. Therefore, the existence of the appropriate electric equipment determines the applicability of the electrical regimes of the invention.

Figure 5 schematically shows how the increasingly more complex electric apparatuses for electrochemical treatment have been developed. The power supplies (PS) are used to apply simple constant current regimes. Complex generators consisting of several components, or modules, integrated one with another with the help of an electronic device that provides special communication mechanism are necessary for more complex treatments. To achieve modularity, reconfigurability and scalability of such generators both, in the output parameters and in the number of output channels, a complex system for intercommunication between different modules has been implemented. In such a modular system this is vital, because without suitable intercommunication mechanism the modules could damage one another.

Figure 6 shows the traditional architecture of a generator for electrochemical applications. It consists of a power amplifier that provides the output signal, two power supplies that provide positive and negative feeding voltage, measuring modules (current/ voltage converter and electrometer) and a data acquisition system. The data acquisition system allows communication between the generator and a computer (from which the electrochemical process is controlled). In this structure each of the integrated elements has fixed input and output parameters making them compatible (preventing them from damaging one by another).

To meet the technical requirements in application of special periodic pulse regimes formulated in the present invention, a programmable and easily reconfigurable pulse generator is also proposed in this invention. It is assembled with the help of an interconnect and intercommunication bus which allows joining of the modules, at least one power supply, a power amplifier and a measuring module, in a standard and secure manner, regardless of the internal structure thereof and of the output parameters thereof. An open modular architecture of the generator will allow varying its configuration and creating new customized hardware from the available modules to cover technical needs. A generator based on the interconnect bus allows programming the electrical regimes with respect to the amount of levels of current and times of application of each of the pulses, their repetitions (cycles) or other necessary technological variations.

Figure 7 depicts the schematic design of the generator (1 ) with the power lines (2) of the power supplies (PS-,PS+) to the power amplifiers (3,3^'), which go to the electrochemical cells (4,4^') of each channel and to the proposed complex interconnect bus formed by a channel level bus A or B (6,6^') and a system level bus (5). For simplicity, the generator on Figure 7 has only two output channels (Channel A, Channel B); although it could have one or more channels with structure more or less complex than depicted.

The interconnect bus have the following functions: (i) integrating the modules of each individual channel and (ii) integrating the channels with the power supplies. These functions are implemented by two buses: a channel level bus A or B (6, 6^') and a system level bus (5).

Each bus consists of a set of signal lines to which modules are connected in parallel. Each bus has a specific controller which manages the signals generated by the modules and in turn generates control signals to all of them in order to comply with the operating requirements. The system bus (5) is thus controlled by the system bus controller (7), the channel bus A is controlled by the bus controller A (8), and the channel bus B is controlled by the bus controller B (8'). The two controllers (8, 8') communicate with the system bus (5).

The electrochemical cell A (4) is associated with a cell controller (10), which is in turn communicated with the channel bus A (6). Measuring modules (1 1 ) provide information on the state of the electrochemical cell A and are communicated with the channel bus A (6).

The person skilled in the art is aware of various electronic means available on the market, and particularly programmable electronic means, to implement the generator described above, therefore it is not considered necessary to extend the practical implementation thereof.

The channel level bus (channel bus) and its corresponding controller control the power amplifiers for protecting the measuring modules as well as personnel and external technological units.

The main functions of the channel bus (6) are as follows:

Limiting the amplitude of the output signal of the channel in the event that a module is at the limit of its operating range.

Disconnecting the loads connected to the corresponding channel and the power supplies of the power amplifier in the event of the occurrence of a critical error, stopping the operation of all the modules acting on this channel.

Protecting personnel from possible dangerous situations and allowing him/ her to disable the channel in case of emergency. Informing the host computer of a non-critical error, i.e. the treatment process has not been performed exactly as it had been programmed.

Informing the system bus that the limit of the operation range has been reached so that it could take the relevant measures.

Allowing working in two modes: one in which the modules work at the maximum limit of their operation range, and the other in which the operation range is always less than the maximum.

The channel level bus (6) operates the signals listed in Table 3.

Table 3. Description of the signals of the channel level bus

N Signal Origin of the Description Type of signal /

Signal Active state

1 Shutdown Bus controller Forces all the modules of the channel to switch to a state of maximum Digital / High security (loads disconnected, power amplifiers blocked)

Turbo / Safe Bus controller Turbo: allows all the modules to work at the maximum power permitted. Digital / Low Mode Safe: the modules must limit their power to a secure level (lower than

maximum).

Input control Bus controller Input to the power amplifiers. It is the result of the difference between the Analog signal of the external reference signal and the feedback signal provided by the

power amplifier, measuring modules for measuring the parameter to be stabilized (voltage

direct in the potentiostatic mode, current in the galvanostatic mode).

Input control Bus controller Inverted control signal of the power amplifier to control the H-bridge type Analog signal of the power amplifiers.

power amplifier,

inverse

Critical error Module Informs the bus controller about a critical error in one of the modules to Digital / Low switch the channel off.

Warning Module Informs the host computer about a non-critical error, i.e., the treatment Digital / Low process has not been performed exactly as it had been programmed.

7 Risky zone Module Informs that one of the modules is working within its operation range but Digital /Low

close to its technical limit.

8 Protection in Module Limits the output of the power amplifier in the positive voltage range to Analog/

positive polarity protect all the modules and loads. Transmits the minimum signal of all modules

9 Protection in Module Limits the output of the power amplifier in the negative voltage range to Analog/

negative polarity protect all the modules and loads. Transmits the

maximum signal of all modules

10 Channel output Security Enables or disables the power output of the channel to ensure the safety Digital /Low

enabled Button of the personnel.

NOTE:

1) Module: relates to any module connected to the control bus of said channel (there is one channel bus for each channel)

2) In Table 3 it is understood that bus controller relates to the channel bus controller.

The safe operation of the integrating modules in each channel is provided by handling the signals through a channel bus. The formation and operation of the signals is explained below:

1 . The difference between the signals "Input control signal of the power amplifier, direct" and "Input control signal of the power amplifier, inverse" controls all the power amplifiers of the channel and forces the generation of the programmed output signal. These two signals are produced by the bus controller as a result of the difference between the external reference signal and the feedback signal provided by the measuring modules (for measuring the parameter to be stabilized: voltage in the potentiostatic mode, current in the galvanostatic mode).

2. The analog "Protection in positive polarity" signal puts in the line of the bus the minimum voltage among all the voltages provided by all the modules connected to the bus. When any module detects that the output of the power amplifier in the positive range is too high for safe operation of this module, it reduces the voltage of the signal indicated above. In response, the bus controller reduces the signals "Input control signal of the power amplifier, direct" and "Input control signal of the power amplifier, inverse" until the output of the power amplifier reaches an acceptable level. The output of the power amplifier will thus never exceed the operating range regardless of the modules that are installed in the channel.

3. The analog "Protection in negative polarity" signal puts in the line of the bus the maximum voltage among all the voltages provided by all the modules connected to the bus. This signal generates the response similar to that described in the previous point but in negative polarity.

4. If the bus controller receives the "critical error" signal, it sets the "Shutdown" signal at the high level. In that moment all the modules of the channel are disconnected. The feeding lines of the power amplifiers are also disconnected. The bus controller maintains all the modules in this state until the operator resolves the problem, and the channel is restarted.

5. If any module generates the "Warning" signal, the bus controller will inform the operator and the host computer. The bus controller will keep this signal active even if the module that has generated the "warning" signal is no longer doing so. The operator or the host computer has to reset this signal before the next experiment or working cycle start.

6. If the "Channel output enabled" signal, controlled by a security button at the user interface or by an external setup, switches to a high level, the channel load is immediately disconnected regardless of the host computer instructions in order to protect the operator and the external units.

The system level signal bus (system bus) (5) with its corresponding controller (7) provides the mechanisms of safety and control of the power supplies to prevent overvoltage in the power amplifiers and to provide the complete disconnection of the system in case of overheating and other dangerous conditions.

The main functions of the system bus are:

Providing the control signal from the power supplies and obtaining the output voltage necessary for performing the desired regime.

Switching the system off (shutdown) and disconnecting the power supplies when a critical error occurs.

- Allowing the power amplifiers to modify the output voltage of the power supplies to respect the positive and negative feeding voltage limits and the difference between the positive and negative feeding voltage in the amplifiers.

Protecting the operator from possible dangerous situations and allowing him/her to disable all the channels in case of emergency.

The system level bus (5) uses the signals listed in Table 4.

Table 4. Description of the signals of the system level bus

N Signal Origin of the Description of the signal Type of signal / Active signal state

1 Shutdown Bus controller Switches off all the power supplies and forces all the channels to move Digital / High

on to the state of maximum security.

2 Turbo / safe Bus controller Turbo: allows all the power supplies to work at the maximum power Digital / Low

mode permitted. Safe: the power supplies must limit their power to a secure

output level (lower than normal).

3 Positive power Bus controller, Input control signal to the power supplies with positive voltage. This Analog / Transmits the supply control module signal is generated by the bus controller but any module can limit it to minimum signal of all prevent an overvoltage in positive feeding. the modules

Negative power Bus controller, Input control signal to the power supplies with negative voltage. This Analog / Transmits the supply control module signal is generated by the bus controller but any module can limit it to minimum signal of all prevent an overvoltage in negative feeding. the modules

Control of the Module With this signal any module can limit the difference between the positive Analog/ Transmits the difference and negative voltage of the power supplies. The bus controller uses this minimum signal of all between positive signal to modify the signals "Positive power supply control" and "Negative the modules and negative power supply control"

voltage of the

power supply

Critical error Module Informs the bus controller about a critical error in the module in order to Digital / Low force the entire system off.

Warning Module Informs the host computer and the user about a non-critfcaf problem in Digital / Low any power supply or in any channel. It means that the treatment process probably has not been performed exactly as it had been programmed.

Risky zone Module Informs that one of the power supplies or one of the modules of a Digital / Low channel is working within its operation range but close to its technical limit.

AH outputs Security button Enables or disables the power output of all the channels to assure the Digital / High disabled safety of the operator.

The safe operation of all the output channels and the power supplies is provided by signal managing through the system bus.

The generation and operation of the signals are explained below:

1 . The analog "Positive power supply control" and "Negative power supply control" signals from the modules and the bus controller have an output driver which sets in the line of the bus the minimum voltage among all the voltages provided by all the connected modules. The bus controller generates this signal based on the difference between the external reference signal and the feeding voltage feedback signal. When any module detects that the positive or negative supply voltage is too high this module reduces the voltage of the signals indicated above. As a response, the positive or negative power supplies will reduce the voltage until reaching an acceptable level. The outputs of the power supplies will thus never exceed the safe operating range regardless of the modules that are connected to the positive and negative supply lines.

2. The analog "Control of the difference between positive and negative voltage of the power supplies" signal from the modules has an output driver which sets in the line of the bus the minimum voltage among all the voltages provided by all the modules. When any module detects that the difference between the positive and negative supply voltages is too high, this module reduces the signal indicated above. As a response, the bus controller limits the "Positive power supply control" and/or "Negative power supply control" signals depending on the programmed strategy so that the positive or negative power supplies reduce their output voltage until reaching an acceptable level of difference between the feeding voltages. The required output parameters of the power supplies will thus never exceed safe operating levels of other integrating modules, regardless of their nominal parameters.

3. If the bus controller receives the "Critical error" signal, it sets the "Shutdown" signal at the high level. In that moment, all the power supplies are switched off and the bus controller maintains all the channels and modules in this state until the operator resolves the problem and restarts the system.

4. If the "Warning" signal is generated by any module, the bus controller will inform the operator and the host computer. The bus controller will keep the "Warning" signal active even the module generated it is no longer doing so.

The operator or the host computer must reset this signal before the next experiment or working cycle start.

5. If the "All outputs disabled" signal, controlled by a security button or external units, moves to the high level, the loads of all the channels are immediately disconnected regardless of what the host computer indicates in order to protect the operator or the external units.

Advantages of the pulse generator of the invention assembled with the help of the proposed interconnect bus with respect to known systems are:

Possibility of easy expansion of the power of the equipment while transferring the technological process from a laboratory scale to an industrial scale.

Easy reconfiguration of the equipment to adjust it to broad current and voltage ranges.

Programming the time shape (modulation) of the output signal in multiple levels for each of the treatment pulses according to the technological needs.

Electric energy savings due to optimization of its use by different modules.

- Application of single software for the control of all stages of manufacturing the nanostructured materials.

The generator can be used in two very different areas: in scientific research, which requires considerable versatility in nanostructuring parameters of material produced in reduced amounts (small area of electrochemical treatment), and in the industrial production of nanostructured materials with well defined nanostructuring parameters and greater amounts of material (treatments of industrial silicon wafers). The key feature of the proposed technological approach is its possibility to be easily scaled up. The transfer of the laboratory results achieved with experimental materials and therefore of a very small amount will spread more quickly to the industrial sector due to possibilities of easily amplifying the scale on the production thereof.

The nanostructured material produced according to the method object of the present invention, in two structural forms, multilayers and particles, has new, unique and improved properties with respect to the material manufactured by the previously disclosed electrochemical methods.

In the case of the production of nanostructured material to subsequently be lifted from the substrate and ground into particles, the prior formation of the layers having a determined thickness, pre-cut during the pore formation treatment, provides a predetermined fragmentation of the material and the obtaining of completely planar particles with thickness dimensions controlled by the electrochemical regimen. The control of a new nanostructuring parameter, the thickness of the pre-formed layers, i.e., one of the three dimensions of the particles produced during grinding, makes it possible to control the shape of the particle - planar particles with the thickness pre-determined by the used electrical regimen, rather than spheres, are obtained.

Figure 8 shows several transversal electronic photographs of porous nanostructured material formed under a standard constant current regimen (A) and under a complex pulse regimen with time modulation of current in pulses having three levels (B), object of the present invention. The interfaces between different porous layers which are very well defined can be seen. As a result, the quality of the photonic crystal formed by the proposed method is superior. In the standard method layers, equal or different, are not formed but rather a continuous material with a clear dependence between the porosity and the depth of the film.

Figure 9 shows photographs of the planar particles of porous nanostructured material formed in layers and lifted from the silicon wafer by ultrasounds, (A) is the optical photograph which allows appreciating that completely planar particles extend for tens of microns and (B) is the electronic photograph with a small aperture of the field of vision. Under the applied regime of three levels in a pulse, the produced material is extremely fragile and easily breaks into completely planar particles with the thickness determined by level 1 (11 , t1 ) in the applied complex electrical pulse.

The corresponding electronic photograph shows the pores, and the planar shape of the particles is well distinguished.

Figure 10 shows the graphs of a particle distribution by sizes in materials manufactured under a standard constant current regimen (A) and a pulse regimen having three levels (B). The analysis was performed after mechanically grinding the porous nanostructured material for 2 hours (A) and 15 minutes (B). In case of the standard regime, the distribution curve shows a single peak which corresponds with the preferred size of spherical particles. In the case of the pulse regime, the presence of two peaks in the distribution curve corresponds with two dimensions of the particles. A dimension of 400 nm has been predetermined as the thickness of the formed layer by the applied electrochemical regime having three levels in a pulse. Obtaining this correlation between the electrical regimen and grinding is interpreted as evidence of the control of an additional structuring parameter in the produced materials. The particles thus formed have an especially practical use in pharmacology as prolonged controlled release systems and in nanomedicine as particles that are easy to integrate with different living cells. The Table 5 provides a summary of the features of porous particles of silicon produced in two different regimes - standard and pulse - according to the invention.

Table 5 Comparison of nanostructuring parameters in materials manufactured by the standard methods and the proposed method

(1 ) Porous silicon materials obtained under standard regimes (constant current) and subsequent grinding

(2) Porous silicon materials obtained in the proposed method with the help of the pulse generator (pulse regimes) and subsequent grinding

^*) Increases due to the consumption of the electrolyte in the pore during prolonged treatment

^**) Controlled by the grinding regime applied

*^**) Controlled by the electrochemical regime

As it can be seen, application of pulse regimes provides material with better quality due to a more homogenous distribution of pores and the sizes thereof, and due to the introduction of a control of the dimensions and shapes of the particles by the electrochemical regime applied.

In conclusion, the different nanostructuring parameters and properties of materials produced by means of the proposed method are different respect to the properties of the material manufactured by the traditional method. The manner of preparing layers, separating them from the wafer-substrate by a current pulse, and mechanically grinding the material allows producing different and varied nanostructures with multiple nanostructuring parameters in an easy, controllable and scalable way with the help of a versatile pulse generator which is reconfigurable and modular due to employing of the proposed intercommunication bus.

Claims

1 . - Electrochemical method for obtaining a nanostructured material, in which at least one porous layer of three-dimensional silicon is formed on a crystalline silicon wafer, characterized in that it comprises a step i) for the anodic formation of a porous layer of silicon on a crystalline silicon wafer by means of applying an electric current in the electrolyte which is modulated in intensity during the time of its application.

2. - Method according to claim 1 , characterized in that the modulation of the intensity of the current comprises forming at least one current pulse including: a first time period (t1 ) during which a constant or variable current (11 ) suitable for forming a porous layer of silicon is applied, followed by a second time period (t2) during which a second constant or variable current (I2) suitable for obtaining the pre-cutting of said porous layer of silicon is applied, and a third time period (t3) in which a third constant or variable current (I3) suitable for obtaining the relaxation of the layer is applied.

3. - Method according to claim 2, characterized in that a pulse regime is applied with time modulation of the current, consisted of a complex pulse with current densities of 0 to 300 mA/cm² for porous layer formation levels, of 300 to 2000 mA/cm² for pre-cutting levels and of -50 to 70 mA/cm² for relaxation levels.

4. - Method according to claim 2, characterized in that a pulse regime is applied with time modulation of the current, consisted of a complex pulse with current densities of 40 to 150 mA/cm² for porous layer formation levels, of 300 to 500 mA/cm² for pre-cutting levels and of -10 to 0 mA/cm² for relaxation levels.

5. - Method according to claims 3 or 4, characterized in that the time ratio between the duration of formation and relaxation levels is from 1 :100 to 500:1 and the total duration of the complex pulse is from 0.001 to 5000 seconds.

6. - Method according to claim 5, characterized in that the time ratio between the duration of formation and relaxation levels is from 1 :20 to 100:1 and the total duration of the complex pulse is from 2 to 101 s seconds.

7. - Method according to any of claims 2 to 6, characterized in that the anodic formation of one or several porous layers on crystalline silicon in pulse regime with time modulation of the current follows any of the pulse regimes shown in the following table: Duration of the level of Duration

Current density, mA/cm²

Regimen s Time ratio of the

Formation Relaxation Formation Relaxation pulse, s

1 40 -5 100 1 100:1 101

2 60 0 20 1 20:1 21

3 150 -10 0.5 10 1:20 10.5

4 300 70 0.1 5 1:50 5.1

8. - Method according to claim 7, characterized in that the load ratio between the formation and pre-cutting levels [R(load)] is of 1000:1 to 1 :1 , the time ratio between durations of anodic (work plus pre-cutting) and relaxation levels [R(a/r)] is of 1 :100 to 500:1 , and the total duration of the complex pulse is from 0.001 to 5000 seconds.

9. - Method according to claim 8, characterized in that the load ratio between the work and pre-cutting levels [R(load)] is of 2:1 to 20:1 , the time ratio between duration of anodic (work plus pre-cutting) and relaxation levels [R(a/r)] is of 1 :10 to 20:1 and the total duration of the complex pulse is from 2 to 101 seconds.

10. - Method according to any of the preceding claims, characterized in that several pulses are applied for the anodic formation to create multiple layers.

1 1 . - Method according to any of the preceding claims, characterized in that the anodic formation of one or several porous layers on crystalline silicon in pulse regime with time modulation of the current follows any of the pulse regimes shown in the following table:

Current density, Duration of the level

Time ratio

(mA/cm²) of (seconds) Pulse

Reg I

Pre- Pre- duration men Relax Relax R

Work cuttin Work cutti R(a r) (seconds) ation ation (load) g ng

2.67:

A 40 300 -10 50 2.5 5 1 1 .5:1 57.5

1

B 60 300 0 20 1 2 4:1 10.5:1 23

C 150 500 -10 1 0.1 2 3:1 1 .1 :1 3.1

D 300 500 70 2 0.1 5 2.1 :1 2.1 :5 7.1

E 100 2000 0 10 0.1 5 5:1 2.02:1 15.1 F 500 2000 0 2 0.1 5 5:1 0.42:1 7.1

12. - Method according to claim 1 , wherein the anodic formation of a porous layer on crystalline silicon under an electrochemical regime with time modulation of the level of the current is carried out with the simultaneous deposition of metals in the porous layer formed.

13. - Method according to claim 12, wherein the simultaneous deposition of metals is carried out in the electrolyte based on hydrofluoric acid to which the salts of noble, semi-noble or transition metals such as Cu, Ni, Co, Ag, Pd, Pt at a concentration from 10e-5 to 1 M are added.

14.- Method according to any of the preceding claims, characterized in that it further comprises at least one of the following steps:

ii) separating the formed porous material from the substrate in an inert solvent;

iii) mechanically grinding the porous material.

15. - Method according to claim 14, wherein after step i) and before steps ii) or iii), the porous material formed on the crystalline silicon is washed with mixed organic or inorganic solvents.

16. - Method according to claim 15, wherein the porous material formed on the crystalline silicon are washed with solvents selected from: pentane, hexane, heptane, CCI₄, CCI₃H, CCI₂H₂, CCIH₃, C₆H₆, methanol, ethanol, propanol or mixtures thereof with water and/or electrolytes.

17. - Method according to claim 14, wherein the porous material is separated mechanically.

18. - Method according to claim 14, wherein the porous material after its formation is separated by applying a cutting electrical pulse comprising a higher level of current and a greater anodic load than those applied in the complex pulse during the formation of the porous layer or layers.

19. - Method according to claim 14, wherein the porous material is separated by sonication which is carried out during 1 -3600 seconds and at a temperature of 0 to 60^eC.

20.- Free-standing nanoparticles of the porous silicon material obtained by a method according to any of the preceding claims 14 to 19.

21 . - Free-standing nanoparticles of the porous silicon material according to the preceding claim, characterized in that they have a substantially planar geometry.

22. - Use of the free-standing nanoparticles of the porous silicon material obtained according to any of claims 14 to 19 in light photosensitizers, in optical devices, or in prolonged controlled pharmacological release systems as carriers of drugs or other applications for similar purposes.

23. - Use of the porous structures formed in multiple layers according to the method described in claims 1 -19 in optical sensors integrated in the silicon wafer or free standing.

24. - Modular and reconfigurable electric current pulse generator for the application of the methods according to any of claims 1 to 19, characterized in that it comprises a system intercommunication bus (5) controlled by a bus controller (7), configured to intercommunicate at least one power supply (PS), at least one power amplifier (3) and at least one precision measuring unit (1 1 ) for measuring an electrochemical cell A (4), and in which the power amplifier (3) is connected with the electrochemical cell A (4) for applying the current pulses generated.

25. - Generator according to claim 24, characterized in that it comprises at least one channel bus (6) controlled by a channel bus controller (8), and in that the channel bus intercommunicates the power amplifier (3) with the measuring modules

(1 1 ).

26. - Modular generator according to claim 24, characterized in that the system level bus operates each of the following signals:

- "Shutdown", of the digital and "high" active state type, from the controller of the system level bus which forces all the integrated modules to the state of maximum security;

- "Turbo/safe mode", of the digital and "low" active state type, from the controller of the system level bus which allows all the power supplies to work at the maximum permitted power or to otherwise limit their power to a secure level;

- "Positive power supply control", the analog type signal, is the input to the power supplies with positive voltage from the controller of the system level bus, but any integrated module can limit it to prevent an overvoltage in the positive polarity;

- "Negative power supply control", the analog type signal, is the input to the power supplies with negative voltage from the controller of the system level bus, but any integrated module can limit it to prevent an overvoltage in the negative polarity;

- "Control of the difference between positive and negative voltage of the power supplies", the analog type signal, from any integrated module; with this signal the difference between the positive and negative voltage of the power supplies is changed. The controller of the system level bus uses this signal to modify the "Positive power supply control" and "Negative power supply control" signals;

- "Critical error", of the digital and "low" active state type, from the integrated module which informs the controller of the system level bus about a critical error in the module in order to force the entire system off;

- "Warning", of the digital and "low" active state type, from the integrated module which informs the host computer and the operator about a non-critical problem in any power supply or in any channel, and therefore that the treatment process has not been performed exactly as it had been programmed;

- "Risky zone", of the digital and "low" active state type, from a power supply or an integrated module which informs that the latter is working in the allowed zone but close to its technical limit and the generator needs special attention;

- "All the outputs disabled", of the digital and "high" active state type, from the security button at the user interface, which disables all the power outputs to assure the safety of the personnel.

27.- Modular generator according to claim 25, characterized in that the channel level bus operates each of the following signals:

- "Shutdown", of the digital and "high" active state type, from the controller of the channel level bus which forces all the modules of the channel to a state of maximum security;

- "Turbo / safe mode", of the digital and "low" active state type, from the controller of the channel level bus which allows all the channel modules to work at the maximum power permitted or to otherwise limit their power to a secure level;

- "Input control signal of the power amplifier, direct", the analog type signal from the controller of the channel level bus, is the input to the power amplifiers; it is the result of the difference between the external reference signal and the feedback signal provided by the measuring modules for measuring the parameter to be stabilized (voltage when it works as a potentiostat, current when it works as a galvanostat);

- "Input control signal of power amplifier, inverse", the analog type signal from the controller of the channel level bus, it is the inverted "Input control signal of power amplifier, direct" signal for using H-bridge type power amplifiers;

- "Critical error", of the digital and "low" active state type, from the channel module which informs the controller of the channel level bus about a critical error in the module to force the channel off;

- "Warning", of the digital and "low" active state type, from the channel module which informs the host computer and the operator about a non-critical problem, which means that the treatment process has not been performed exactly as it had been programmed;

- "Risky zone", of the digital and "low" active state type, from a channel module that informs that the latter is working in the permitted zone but close to its technical limit and generator needs special attention of the operator;

- "Protection in positive polarity", of the analog type signal with the function of transmitting the minimum voltage from all the modules in the channel, it limits the output of the amplifier in positive polarity to protect all the modules and loads;

- "Protection in negative polarity", of the analog type signal with the function of transmitting the maximum voltage from all the modules in the channel, it limits the output of the amplifier in negative polarity to protect all the modules and loads;

- "Channel output enabled", of the digital and "low" active state type, from the security button at the user interface, which enables or disables the power at the channel output to assure the safety of the personnel;